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NOVEL FUNCTIONAL 1D AND 2D CONJUGATED
POLYMERS: DESIGN, SYNTHESIS AND
CHARACTERIZATION
LI HAIRONG
NATIONAL UNIVERSITY OF SINGAPORE
2008
NOVEL FUNCTIONAL 1D AND 2D CONJUGATED
POLYMERS: DESIGN, SYNTHESIS AND
CHARACTERIZATION
LI HAIRONG
(M. Sc. Singapore-MIT Alliance, NUS, Singapore.
B. Eng. Zhejiang University, PRC)
A THESIS SUBMITTED
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
DEPARTMENT OF CHEMISTRY
NATIONAL UNIVERSITY OF SINGAPORE
2008
NAME: LI HAIRONG
DEGREE: DOCTOR OF PHILOSOPHY
DEPARTMENT: CHEMISTRY
THESIS TITLE: NOVEL FUNCTIONAL 1D AND 2D CONJUGATED POLYMERS: DESIGN, SYNTHESIS AND CHARACTERIZATION
AAbbssttrraacctt
Design and characterization of novel conjugated polymers are of great importance
in understanding the intrinsic properties to realize the practical applications in many
aspects. Among the various conjugated polymers, poly(p-phenylene)s (PPPs) and its
derivatives are of considerable interests due to their solvent tractability, high thermal
and chemical stabilities, high quantum yield and versatile synthetic strategies. Our
efforts focused on investigating the structure-property relations of rationally designed
PPPs, seeking the potential sensor and biological applications of water soluble PPPs.
Continuous endeavor to chemical modification of PPPs involved an introduction of
conjugated side chain onto PPP backbone, with specific highlights of crystalline
nature, aggregation phenomenon, unique photophysical and self-assembly properties
arisen from extended conjugation and strong π-π interaction. Structure-property
relations were further explored in cross-conjugated cruciform system and preliminary
work was carried out on cross conjugated polyphenols for antioxidant and toxicity
studies.
Keywords: poly(p-phenylene), water soluble, biomineralization, cross-conjugated,
sensor, photophysical properties, self-assembly, aggregation, crystal, cruciform,
polyphenol, antioxidant, toxicity.
ACKNOWLEDGEMENTS
I am deeply grateful to a lot of people, who have supported this work by different
means.
Above all, I would like to express my sincere gratitude to my supervisor Prof.
Suresh Valiyaveettil and co-supervisor Prof. Lim Chwee Teck for their constructive
guidance, full support and constant encouragement.
I sincerely thank all the current and former members of the group for their
cordiality and friendship. I thank Akhila, Balaji, Colin, Elena, Hairyu, Gayathri,
Jegadesan, Renu, Santosh, Sindhu, Sivamurugan, Shaowen, Sheeja, Shirley and Yean
Nee for all the good times in the lab and helping exchange knowledge skills. I am
grateful to Dr. Vetrichelvan for his sincere guidance and help in the beginning of my
research. I also appreciate the assistance from Nurmawati in OM, TEM, SEM and
Fathima in XRD, Nanofiber fabrication.
Technical assistance provided by the staffs of the various laboratories at the
Faculty of Science is gratefully acknowledged.
I thank National University of Singapore Nanoscience and Nanotechnology
Initiative for scholarship. Financial supports from Agency for Science and Technology
Research and National University of Singapore are also acknowledged.
Words cannot express my deepest gratitude to my beloved parents and
grandparents. I wholeheartedly thank them for their understanding, moral support and
encouragement.
i
TABLE OF CONTENTS Acknowledgments
i
Table of contents
ii
Summary
vii
Abbreviations and Symbols
x
List of Tables
xiv
List of Figures
xvii
List of Schemes
xxiii
1 Linear and Cross Conjugated Poly(p-
phenylenes) and Oligo(p-phenylenes)---Introduction
1
1.1 Conjugated polymers---What and Why? 2
1.2 Conjugated polymers---An overview 3
1.3 Poly(para-pheneylenes) (PPPs)---An important class of CPs 4
1.3.1 Overview of PPPs 4
1.3.2 Synthetic strategies of PPP and derivatives 5
1.3.3 Important photophysical properties---Brief introduction 10
1.3.3.1 Absorption and emission 10
1.3.3.2 Fluorescence lifetimes and quantum yields 12
1.3.3.3 Two photon absorption 14
1.3.4 Photophysical properties of typical PPPs 15
1.3.4.1 Linear PPPs 15
1.3.4.2 Cross conjugated macromolecules 33
ii
1.4 Scope and outline of the thesis 55
1.5 References 57
2 Synthesis and Comparison of Structure-Property Relationship of Symmetric and Asymmteric Water Soluble Poly (para-phenylenes)
68
2.1 Introduction 69
2.2 Experimental 70
2.2.1 Measurements 70
2.2.2 Synthesis 71
2.3 Results and discussion 75
2.3.1 Characterization of polymers 75
2.3.2. Thermal properties 76
2.3.3. Optical properties 76
2.3.4. Aggregation properties of polymers 78
2.3.5. Complexation studies 85
2.3.6. Packing structure in solid state 90
2.3.7. Polymer as template for controlled biomineralization mimics 92
2.4 Conclusion 97
2.5 References 97
iii
3 Water Soluble Multifunctional Cross Conjugated Poly(para-phenylenes) as Stimuli Responsive Materials: Design, Synthesis, and Characterization
100
3.1 Introduction 101
3.2 Experimental 102
3.2.1 Measurements 102
3.2.2 Synthesis 103
3.3 Results and discussion 109
3.3.1. Characterization 109
3.3.2. Thermal properities 110
3.3.3. Absorption and emission spectra 111
3.3.4. Titration studies 112
3.3.5 Aggregation properties 122
3.3.6. Thin film and nanofiber 124
3.4 References 126
3.5 Conclusion 127
4 Synthesis and Structure-Property Investigation of Novel Poly(p-phenylenes) with Conjugated Side Chain
130
4.1 Introduction 131
4.2 Experimental 132
4.3 Synthesis 133
4.4 Results and discussion 138
iv
4.4.1 Characterization 138
4.4.2 Powder X-ray diffraction 139
4.4.3 Absorption and emission 140
4.4.4 Quantum yield and two photon absorption (TPA) 146
4.4.5. Time-correlated single-photon counting 147
4.4.6. Solid phase self-assembly and morphology 150
4.5 Conclusion 152
4.6 References 153
5 Synthesis and Characterization of Cross Conjugated Cruciforms with Varied Functional Groups
160
5.1 Introduction 161
5.2 Experimental 162
5.2.1 Synthesis 162
5.2.2 Characterization 165
5.3 Results and discussion 170
5.3.1. Thermal properties 170
5.3.2. Absorption and emission properties 170
5.3.3 X-ray diffraction studies 172
5.3.3.1 Powder XRD studies 172
5.3.3.2 Single crystal XRD studies 174
5.3.4 Liquid crystal study 175
5.4 Conclusion 176
v
5.5 References 177
5.6 Appendix 179
6 Synthesis and Characterization of Cross Conjugated Polyphenols
185
6.1 Introduction 186
6.2 Experimental 188
6.3 Results and discussion 190
6.3.1 Synthesis 190
6.3.2 Characterization 200
6.3.3 Absorption and emission properties 201
6.3.4 Antioxidant properties 204
6.3.5 Bioactivity studies 207
6.4 Conclusion 213
6.5 References 213
7 Design, Synthesis and Characterization of Pyrene Derivatives with Conjugated Arms
216
7.1 Introduction 217
7.2 Synthesis and characterization 220
7.3 UV-Vis absorption and emission studies 224
6.4 Conclusion 229
6.5 References 229
List of publications 232
vi
SSuummmmaarryy
Chapter 1 is a literature review on history and recent development of conjugated
macromolecules, with major efforts on linear poly(para-phenylenes) and cross-conjugated
polymers and cruciforms.
My research work started from the structure-property comparison of two sets of
symmetrical and asymmetrical sulfonate water soluble poly(p-phenylenes) (PPPs)
(Chapter 2). The polymers aggregated in water/tetrahydrofuran (THF) mixture through
microphase separation of polar (water) and nonpolar (THF) groups into appropriate
solvents and strong intermolecular interactions. The fluorescence of the polymers was
quenched in the presence of analytes including viologen derivatives, cytochrome-C (Cyt-C)
and metal ions in water with Stern-Volmer constant in order of 106 M-1, indicating the
potential application in sensors. Light-emitting iso-oriented calcite crystals were
synthesized by controlled crystallization in presence of the water soluble PPPs. The nature
of the functional groups on the polymer backbone and their ordered pack played a crucial
role for the selective morphogenesis of the crystals with controlled particle shape, size and
orientation.
The polymers in Chapter 2 had only one acceptor, which limited their versatility.
Therefore, further efforts were made towards water soluble cross-conjugated PPPs
(Chapter 3). Polymers with two different acceptors and extended conjugation in two-
dimensions were able to respond to different kinds of analytes in trace amount via
fluorescence quenching combined with a blue or red shift of UV-Vis absorption maximum
vii
depending on structure of the polymers and quenchers. All polymers were able to form
smooth, flexible and uniform thin films with strong blue fluorescence. Aligned nanofibers
were also made successfully, thanks to the strong intermolecular electrostatic and π-π
interactions. These results provided a novel way to extend the capability in chemo- and
bio-sensor applications.
It was found that the polymers in Chapter 3 had strong aggregation in aqueous
solution, driven by electrostatic, hydrophobic-hydrophobic and π-π interactions. We were
interested in eliciting some information on how the π-π interaction affected the properties
of the polymers. Hence in Chapter 4, a series of organo-soluble cross-conjugated
polymers were designed and synthesized. Absorption, excitation and emission results
indicated a highly concentration dependent relationship due to strong intermolecular
energy transfer. Large two photon absorption cross sections were derived from the
extended conjugation along side chains and aligned “push-push” and “push-pull” structure.
Extended conjugation and strong intermolecular interaction were further confirmed by
time-correlated single-photon counting. Because of their unique structures, interesting
self-assembly properties were discovered simply by drop casting.
Since we had confirmed the extended π-electron delocalization existing in cross-
conjugated PPPs in Chapter 4, it was necessary to understand how the different kinds of
conjugated segment contributed to the overall properties, therefore we shift to new target
with well-defined structures call cross-conjugated cruciform. A series of cross conjugated
cruciforms with varied functional groups substituted at two different segments were
discussed in Chapter 5. It was concluded that the photophysical properties were highly
influenced by the extent to which conjugated segment participated in conjugation. Strong
viii
π-π interaction and ordered packing were confirmed by single crystal X-ray diffraction
(XRD). Liquid crystal behavior was observed for some cruciforms, which was originated
from strong intermolecular interaction between mesogenic cores and coordination of
flexible alkyl chains.
In Chapter 6, a novel class of cross conjugated polyphenols was synthesized and
characterized. The design of the molecules was targeted at achieving a high antioxidant
property by varying the number of hydroxyl groups on the numbers and the length of the
linear conjugation. Trolox equivalent antioxidant capacity (TEAC) assay has revealed that
more extended conjugation and larger number of phenolic hydroxyl groups contributed to
higher antioxidant property via lowering the dissociation energy of the phenolic O-H bond
and increasing the stability of resulting phenolate radicals.
In Chapter 7, pyrene derivatives with conjugated segment were reported, unlike the
previous chapters, we’d like to extend the core size and investigate the effect of
conjugation. Absorption and emissions studies were performed and comparisons made
amongst them and their precursors and the cruciforms reported in Chapter 5. It was noted
that all compounds had absorption and emission maxima in the range of 426-488 nm and
508-541 nm respectively. The HOMO-LUMO energy gap of the derivatives is in the range
of 2.30-2.58 eV. However, the band gap tuning was largely limited on 2, 5, 7, 10-
positions due to meta-link. Therefore, the photophysical properties of pyrene derivatives
were limited by less effective conjugation between segments, unlike the cruciforms in
Chapter 5 & 6 where the conjugation was limited by highly twisted para-phenylene
segment.
ix
ABBREVIATIONS AND SYMBOLS
1D One Dimensional
2D Two Dimensional
3D Three Dimensional
1H-NMR Proton Nuclear Magnetic Resonance
13C-NMR Carbon Nuclear Magnetic Resonance
Å Anstrom(s)
δ Chemical shift (in NMR spectroscopy)
Φ Fluorescence quantum yield
τ Measured fluorescence lifetime
τnrad Non-radiative lifetime
Γ Rate constant for fluorescence decay
τrad Radiative lifetime
λ Wavelength
θ Diffraction angle
ca. About
AFM Atomic Force Microscopy
b Broad
BuLi Butyllithium
CHCl3 Chloroform
CLSM Confocal Laser Scanning Microscope
conc. Concentrated
CV Cyclic Voltammetry
CP Conjugated Polymer
x
d Doublet
dd Double of Doublet
DCM Dichloromethane
DLS Dynamic Light Scattering
DP Degree of Polymerization
DMF Dimethylformamide
DMSO Dimethylsulfide
D2O Deuterium Oxide
EA Ethyl Acetate; Elemental Analysis
EI-MS Electron Impact Mass Spectrum
ESIPT Excited State Intramolecular Proton Transfer
EtOH Ethanol
FS Femtosecond
GPC Gel Permeation Chromatography
HBC Hexa-peri-hexaBenzoCoronene
Hz Hertz
FT-IR Infrared Fourier Transform
hr Hour(s)
i.e. That is (Latin id est)
J Coupling constant
KBr Potassium Bromide
K2CO3 Potassium Carbonate
Ksv Stern-Volmer constant
LED Light Emitting Diode
xi
LB Langmuir-Blodgett
m Multiplet
m/z Mass/Charge
MeOH Methanol
mg Milligram(s)
mN/m milliNewton/meter
mL Milliliter(s)
mmol Millimol
MW Molecular Weight
NBS N-bromosuccinimide
nm nanometer
OBn O-Benzyloxy
OM Optical Microscope
OPA One Photon Absorption
OPP Oligo(para-phenylene)
PA Polyacetylene
PANI Polyaniline
Pd(0)P(Ph3)4 Tetrakis(triphenylphosphine)Palladium(0)
Pd/C Palladium on Carbon
PDI PolyDispersity Index
PF Polyfluorene
PL Photoluminescence
PPE Polyphenyleneacetylene
PPP Poly(p-phenylene)
xii
PPS Poly(phenylene sulphide)
PPV Polyphenylenevinylene
PPY Polypyrrole
PT Polythiophene
ps Picosecond
QY Quantum Yield
q Quartet
ROS Reactive Oxygen Species
RT Room Temperature
s Singlet; Second
SEM Scanning Electronic Microscope
t Triplet
tt Transit time
TCSPC Time-Correlated Single-Photon Counting
TEAC Trolox Equivalent Antioxidant Capacity
TEM Transmission Electron Microscope
TGA Thermo Gravimetric Analyzer
TMS Tetramethylsilane
THF Tetrahydrofuran
TLC Thin Layer Chromatogharphy
TPA Two Photon Absorption
UV-Vis Ultra-Violet Visible spectroscopy
WSP Water Soluble Polymer
XRD X-Ray Diffraction
xiii
Table No.
LIST OF TABLES
Page No.
Chapter 1
Table 1.1 Representative linear PPPs 16
Table 1.2 Representative cross conjugated molecules 34
Chapter 2
Table 2.1 Absorption and emission wavelength for the polymers P1-P5 in water
77
Table 2.2 Stern-Volmer constants for the polymers upon addition of the quencher
89
Chapter 3
Table 3.1 GPC, TGA, UV-Vis absorption and emission maxima of P1-P6 111
Table 3.2 Stern-Volmer constant Ksv for P1-P6 with titration of five quenchers in water
121
Table 3.3 Changes in emission maxima in presence of added quenchers in water
121
Table 3.4 Ksv for P1-P6 with titration of five quenchers in 20 mM NaCl solution
123
Table 3.5 Ksv for P1-P6 with titration of five quenchers in 100 mM NaCl solution
123
Chapter 4
xiv
Table 4.1 Molecular weight and TG value of P1-P5 139
Table 4.2 Absorption maxima of the polymers in THF, fluorescence maxima at low (PL), high (PH) concentrations in THF, thin film (PN) and excitation maxima (PE)
141
Table 4.3 Absorption maxima of the polymers in toluene, fluorescence peaks at low (PL,), high (PH,) concentrations in toluene and excitation maxima (PE)
144
Table 4.4 Quantum yield, absolute and relative values of TPA cross section for P1-P5 in THF and toluene, polarity index is given in parenthesis, rhodamine B was the standard
147
Table 4.5 Life time (τ) and amplitude (a) for emission decay at concentration of 10-7 M
148
Chapter 5
Table 5.1 TGA results of O1-O10 (°C) 170
Table 5.2. Table 5.2. UV-Vis absorption and fluorescence properties of O1-O10
171
Table 5.3 Crystal data and structure refinement for O5 179
Table 5.4 Atomic coordinates ( x 104) and equivalent isotropic displacement parameters (Å2 x 103) for O5. U(eq) is defined as one third of the trace of the orthogonalized Uij tensor
180
Table 5.5 Bond lengths [Å] and angles [°] for O5 181
Table 5.6 Anisotropic displacement parameters (Å2 x 103) for O5 183
Table 5.7 Hydrogen coordinates ( x 104) and isotropic displacement parameters (Å2 x 103) for O5
184
xv
Chapter 6
Table 6.1 Absorption and emission maxima of 8b – e, 9b – e, 10b – e, 1b – e, 2b – e in THF (nm)
203
Table 6.2 TEAC values of the synthesized compounds and typical antioxidants
206
Chapter 7
Table 7.1 Summary of absorption and emission peak maxima and calculated band gap
225
xvi
Figure No. LIST OF FIGURES
Page No.
Chapter 1
Figure 1.1 Chemical structures of some of the typical conjugated polymers. 2
Figure 1.2 Chemical structure of unsubstituted PPP. 5
Figure 1.3 Representative ladder-type PPP structures. 8
Figure 1.4 Energetic processes on absorption of a photon in the Jablonski diagram.
10
Chapter 2
Figure 2.1 Molecular structure of the polymers. 73
Figure 2.2 Absorption (A) and fluorescence (B) spectra of polymers P1 – P5 in water.
77
Figure 2.3 (A) UV-Vis spectra and plot of absorption maxima v.s. THF concentration (inset) for polymer P1 in water and water/THF mixtures; (B) The fluorescence spectra of polymer P1 in water and water/THF mixtures.
80
Figure 2.4 Changes in the emission spectra of P2 at different water/THF mixtures. Inset: The emission intensity of P2 with the % of THF.
80
Figure 2.5 Changes in the UV-Vis spectra of P5 at different water/THF mixtures. Inset: Absorption maxima of P5 versus the % of THF.
81
Figure 2.6 Changes in the emission spectra of P5 at different water/THF mixtures. Inset: The emission intensity of P5 with the % of THF.
82
Figure 2.7 The hydrodynamic diameter (Dh) profile obtained from DLS studies for the polymer P1 in 100 % water (A), 25 % THF (B), 50 % THF (C) and 75 % THF (D)
82
xvii
Figure 2.8 AFM of polymer P1 in 100 % water (left) and in 25 % THF
(right). 83
Figure 2.9 Cartoon representing the aggregation of polymers by the addition of tetrahydrofuran to a water solution).
84
Figure 2.10 Changes in the emission spectra of P1 (A, C, E) and P5 (B, D, F) in water at different concentrations of viologens.
87
Figure 2.11 Stern – Volmer plots of the polymer P1 and P5 quenched by the viologen derivatives.
88
Figure 2.12 Changes in the emission spectra of P1(A) and P5 (B) in water at different concentrations of cytochrome-C.
88
Figure 2.13 (A) XRD power patterns of the polymers P1-P5. A cartoon representing a solid-state packing model of the polymers of P1-P3 (B) and P4-P5 (C).
91
Figure 2.14 SEM and CLSM image of the CaCO3 crystals grown in the presence of P1 (1mg/mL); XRD patterns of the calcite crystals grown in presence of 1 mg/mL of P1; Schematic representation of a possible binding of P1 with Ca2+ ions in the crystallization medium for the formation of oriented calcite crystals; a unit of {104} plane of the calcite crystal lattice is also shown.
94
Figure 2.15 XRD patterns of the calcite crystals grown in presence of 1 mg/mL of P1 (a) and in the absence of polymer (b). Inset shows the electron micrographs of the representative crystals.
96
Figure 2.16 SEM images of CaCO3 grown on a glass substrate, in presence of P2, with concentrations of 100 μg/mL (a), 500 μg/mL (b) and 1 mg/mL (c). XRD pattern of the crystals and composites formed in presence of P2 for different concentrations (d). Representative CLSM image shows the fluorescent nature of the calcium carbonate polymer composite formed (e).
96
Chapter 3
Figure 3.1 A cartoonistic representation of various pathways for electronic 101
xviii
conjugations in 2D polymers, X and Y represent donor-acceptor type functional groups.
Figure 3.2 Molecular structures of target polymers. 102
Figure 3.3 Normalized absorption (A) and emission (B) spectra of P1 (■), P2 (●), P3 (▲), P4 (□), P5 (○) and P6 (△) in water (20 mg/L).
111
Figure 3.4 Fluorescence spectra of P1 with titration of five different quenchers in water solution with concentration of 20 mg/L. Direction of intensity changes is indicated by the arrow. Quencher concentrations are indicated in each figure and parenthesis.
116
Figure 3.5 Fluorescence spectra of P2 with titration of five different quenchers in water solution with concentration of 20 mg/L. Direction of intensity changes is indicated by the arrow. Quencher concentrations are indicated in each figure and parenthesis.
117
Figure 3.6 Fluorescence spectra of P3 with titration of five different quenchers in water solution with concentration of 20 mg/L. Direction of intensity changes is indicated by the arrow. Quencher concentrations are indicated in each figure and parenthesis.
118
Figure 3.7 Fluorescence spectra of P4 with titration of five different quenchers in water solution with concentration of 20 mg/L. Direction of intensity changes is indicated by the arrow. Quencher concentrations are indicated in each figure and parenthesis.
119
Figure 3.8 Fluorescence spectra of P5 and P6 with titration five different quenchers in water solution with concentration of 20 mg/L. Direction of intensity changes is indicated by the arrow. Quencher concentrations are indicated in each figure and parenthesis.
120
Figure 3.9 Titration curves of P1 (○), P2 (●), P3 (▲), P4 (□) and P6 (■) (F) in presence of potassium hexacyanoferrate(III). Polymer concentration was 20 mg/L.
121
Figure 3.10 (A) Hydrodynamic diameter profile obtained from DLS for P2 with titration of 9,10-anthraquinone-2,6-disulfonic acid
124
xix
disodium salt (AD) in water. (B) UV-Vis absorption spectra of P2 with titration of AD in water. Direction of peak shift is indicated by the arrow. Polymer concentration was 20 mg/L.
Figure 3.11 SEM micrograph of the thin film (A) indicating the thickness of the film (B), confocal and optical (inset) micrographs of thin films of P1 prepared from a water solution with a concentration of 10mg/mL on a glass plate (C), SEM and confocal (inset) micrographs of nanofibers made from P1 with 5% vinyl alcohol as cross-linker.
125
Chapter 4
Figure 4.1 Chemical structures of five polymers.
132
Figure 4.2 Left: Powder XRD spectra for P1 to P5; Right: schematic diagrams of possible packing structures for P2 (a, double degeneracy) and P3 (b, c).
140
Figure 4.3 Normalized absorption (a) and emission at a low concentration in THF (0.002 g/L) (b), emission at high concentration in THF (0.2 g/L) (c), and emission of spin coated thin film (d), for P1-P5; Excitation spectra (EX) of P1-P5 monitored at the emission peaks at a low concentration.
142
Figure 4.4 Normalized absorption (a) and emission at a low concentration in toluene (0.002 g/L) (b), emission at high concentration in toluene (0.2 g/L) (c), for P1-P5; Excitation spectra (EX) of P1-P5 monitored at the emission peaks at a low concentration.
145
Figure 4.5 Fluorescence decay curves of P1-P5 in THF monitored at 450 nm (10-7 M--■; 10-5 M--□).
149
Figure 4.6 SEM images of P1 drop-casted from solutions of different concentrations onto glass plate: 0.5 mg/mL (A); 0.2 mg/mL (B); 0.05 mg/mL (C). Schematic view of the hierarchical self-assembly of P1 into left-helix supramolecular structures (D).
151
Figure 4.7 HRTEM images and electron diffraction patterns at crystalline domains of P3 thin film.
152
xx
Chapter 5
Figure 5.1 Molecular structures of the target compounds. 161
Figure 5.2 Normalized absorption spectra for O1-O10, recorded in THF solution at concentration of 10 mg/L.
172
Figure 5.3 Normalized emission spectra for O1-O10, recorded in THF solution at concentration of 10 mg/L.
172
Figure 5.4 Powder XRD results of O1-O10. 173
Figure 5.5 Molecular structure and packing of O5 solved by single crystal XRD.
174
Figure 5.6 POM graph of the textures observed on cooling of O2 (up) and O3 (mid) at a cooling rate of 0.5 °C /min and DSC spectra (bottom).
175
Chapter 6
Figure 6.1 Synthesized cross conjugated polyphenols. 187
Figure 6.2 Normalized absorption spectra of 8b – e, 9b – e, 10b – e, 1b – e, 2b – e in THF.
202
Figure 6.3 Normalized emmision spectra of 8b – e, 9b – e, 10b – e, 1b – e, 2b – e in THF.
203
Figure 6.4 (A) The differential interphase contrast (DIC) images of control cells IMR-90, (B) treated cells (IMR-90) which showed irregular cell morphology due to cell death (C), Control cells (U251) and (D) treated cells (U251).
208
Figure 6.5 The effect of the chemicals on U251 and IMR-90 expressed as % of viable cells after the treatment. At higher concentrations of the polyphenol a drop in viability was observed. X axis represents the cell line and polyphenol employed. Y-axis represents % of viable cells. Legends indicate the concentration
209
xxi
of polymer used.
Figure 6.6 The effect of polyphenols in controlling the mitochondrial activity inside the cells. X axis represents the cell line and polyphenol employed. Y-axis represents % of metabolically active cells. Legends indicate the concentration of polyphenol used.
209
Figure 6.7 Cytotoxicity of the polyphenols on different cells in terms of their potency to result in cell lysis. X axis represents the cell line and polyphenol employed. Y-axis represents % of release of LDH as calculated form the cell culture supernantent. Legends indicate the concentration of polyphenol used. A concentration dependant increase in LDH leakage was observed which indicates cell membrane damage.
210
Figure 6.8 (A) The cancer cells without any polyphenol added, (B) control cells, IMR-90 without any polyphenol (C) and (D) 2c treated cancer cells and fibroblasts respectively; (E) and (F) represents 2d treated cancer cells and normal cells respectively. (G) and (H) represents 2b treated cells cancer cells and normal cells respectively. The % of cells is indicated with each histogram. The cells are treated with a fixed concentration of the polyphenol, 60 μg/ml for the analysis.
212
Figure 6.9 Graphical representation of % of cells in different phases of cells cycle, subG1 represents percentage cells as indicated by M1 marker in histogram (Figure 6.8); similarly G1 represents M2; S represents M3; G2/M represents M4.
212
Chapter 7
Figure 7.1 Structure and labeled positions of pyrene. 218
Figure 7.2 UV-Vis absorption (left) and emission (right) of pyrene (T0), T1, T2, T3 and precursors 7 and 8.
224
Figure 7.3 Molecular structures of cruciform O1-O10 reported in Chapter 5.
225
xxii
Scheme No.
LIST OF SCHEMES Page No.
Chapter 1
Scheme 1.1
Precursor route synthesis of PPP using Ni(0)-catalyzed couplings.
6
Scheme 1.2
Synthesis of unsubstituted PPP by the polymerization of 5,6-dibromocyclohexa-1,3-diene.
7
Scheme 1.3
Precursor route to unsubstituted PPP from 5,6-diacetoxycyclohexa-1,3-diene.
7
Scheme 1.4
Chiral nematic LC induced by the addition of chiral dopant. 9
Scheme 1.5
Contribution of quinoid ground-state resonance structures fro PIF and PDIN.
53
Chapter 2
Scheme 2.1
Synthesis of polymers P1-P3. 73
Scheme 2.2
Synthesis of polymers P4 and P5. 74
Scheme 2.3 Molecular structure of the viologen derivatives used for complexation study.
85
Chapter 3
Scheme 3.1
Synthetic routes for P1-P6. 104
Chapter 4
xxiii
Scheme 4.1 Synthetic routes for P1-P5. 137
Chapter 5
Scheme 5.1 Synthetic routes for O1-O10. 163
Chapter 6
Scheme 6.1 Synthetic routes for ten target polyphenols (1a-1e, 2a-2e). 200
Chapter 7
Scheme 7.1 Synthetic routes of target compounds (T1-T3). 220
xxiv
Li Hairong National University of Singapore
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pphheennyylleenneess))------IInnttrroodduuccttiioonn
CChhaapptteerr 11
1
Linear and Cross-conjugated Poly(p-phenylene)s and Oligo(p-phenylene)s---Introduction
1.1. Conjugated polymers---What and Why?
Conjugated polymers (CPs) are distinguished by semi-conducting or metallic organic
macromolecules which consist of a backbone with alternating single and multiple bonds
(Figure 1.1). This system results in a general delocalization of the electrons across all of
the parallelly aligned p-orbitals of the atoms through mesomerization. CPs have gained
unprecedented attentions and investigations propelled by their unique physical and
chemical properties and potential substitutes for traditional semiconductor products as the
latter are approaching the bottom line of the current techniques. Researchers has witnessed
the dynamic evolution of the field of CPs, from simple-minded pictures of bond
alternation defects1 and applications to the development of light weight batteries in early
stage,2 to applications in electroluminescence,3 photorefractivity,4 electrochromism,5
electrochemistry,6 sensing7 and more recently molecular electronics.8
S
NH
HN
n n
n
nnn
n
Polyacetylene (PA) Polypyrrole (PPy)
Polyaniline (PANI)
Poly(phenylenevinylene) (PPV)Polythiophene (PT)
Poly(p-phenylene) (PPP)
Poly(phenyleneethynylene) (PPE)
n
Polyfluorene (PF)
Sn
Poly(phenylene sulphide) (PPS)
Figure 1.1. Chemical structures of some of the typical conjugated polymers.
2
Li Hairong National University of Singapore
1.2. Conjugated polymers---An overview
Historically, many CPs were well known in their nonconducting forms much before
their conductivity and other features of interests were discovered. For instance, poly(p-
phenylene sulfide) has been commercially produced for thermoplastics applications under
the brand name Ryton by Phillips Chemical Company since the early 1970’s. Chemical
oxidative polymerization of aniline was described by Letheby in 1862.1 Primary efforts on
chemical polymerization of polypyrrole were made in some details in 1916.9 Since 1957,
studies of electrochemical oxidation of aromatic monomers have been reported under
various descriptions such as “electron-organic preparations” and “electron-oxidations”.10-
12 Based on this, electrically conducting polymers from pyrrole, thiophene, furan and
aniline were first prepared in the late 1960s.13-15 The developments of importance that
focused attention on CPs as potential novel materials with highly promising conductivity
and other properties however started with the serendipitous discovery that poly(acetylene)
exposed to iodine vapors develops very high conductivities.16,17 Poly (acetylene) was the
most studied CP for both scientific and practical applications, however, due to its high
chemical instability in air and processing problems, interests were largely confined to its
scientific aspects. Poly(aniline), poly(pyrrole) and poly(thiophene) were studied
extensively from both scientific and practical or commercial points of view. In recent
years, poly(p-phenylenes), poly(p-phenylenevinylenes) (PPVs) and poly(p-phenylene-
ethynylenes) (PPEs), as well as other less well-known CPs such as poly(azulenes),
poly(quinolines), poly(acene), poly(azomethine) and poly(oxadiazoles) etc., have been
synthesized and well studied due to their interesting electroluminescence,18 photoinduced
electron-transfer properties19 and its possibility for functionalization which has overcome
3
Linear and Cross-conjugated Poly(p-phenylene)s and Oligo(p-phenylene)s---Introduction
the problems of intractability and insolubility. Other attractive aspects such as flexible and
low-cost processability, light weight, color-tunability, doping flexibility and thermal
stability also played a major role in their development.20,21 The discovery and
development of conducting polymers were recognized and Nobel Prize for chemistry in
2000 was awarded to Heeger, MacDiarmid, and Shirakawa.22-24
1.3. Poly(para-pheneylenes) (PPPs)---An important class of CPs
1.3.1. Overview of PPPs
Poly(p-phenylene)s constitute the prototype of rigid-rod polymers.25,26 One of the
first syntheses of PPP was reported in 1966 by Kovacic et al.27 In the early stage, major
efforts were made in exploring the synthesis of this simplest aromatic conjugated polymer
with new and novel molecular architectures (Figure 1.2),28 simply due to the key
advantages of PPPs arising from their conceptually simple and appealing molecular
structure, high chemical stability compared with the classical polyacetylenes, and
interesting physical properties. However poor solubility of the unfunctionalized PPPs
limited the processability for characterizations and device fabrications. Therefore,
different new synthetic strategies and functionalization methods have been being
developed, resulting in huge varieties of PPP derivatives with interesting properties. They
have become the active components in modern devices, involving (a) Li/CP batteries; (b)
light emitting diodes (LEDs); (c) sensor with variety of detecting modes (potentiometric,
amperometric, voltammetric, gravimetric and optical etc.); (d) electro-optic and optical
devices like waveguides, light modulators, CP-based lasers, non-linear optics,
photovoltaic cells etc.; (e) catalysis and drug/chemical delivery; (f) membranes; (g)
4
Li Hairong National University of Singapore
molecular electronics such as conducting wires, FETs, actuators, switches, rectifiers, logic
gates and memories, etc.29
n
Figure 1.2. Chemical structure of unsubstituted PPP.
1.3.2. Synthetic strategies of PPP and derivatives
Synthetic strategies of PPPs can be classified as either direct or indirect methods.29 In
the direct method, the phenylene moiety in the monomers will become the repeating unit
of the final polymer. While in the indirect method, a precursor polymer is first
synthesized, from which PPP is then formed by thermal treatment, elimination or
intramolecular reaction. For most of the direct synthesis in the early stage, the reaction
conditions are too harsh for a regiospecific coupling reaction to take place. Thus, linkages
between wrong sites, cross linking, and other side reactions occur. The molecular weights
of the polymers synthesized are usually low, which could be due to solubility problems.
Indirect method is superior to direct methods in terms of achieving high molecular
weights. But a serious limitation of this method is that the structural irregularities
contained in the precursor are inevitably transplanted into the target polymer. The
conversion of the precursor polymer does not proceed as cleanly as desired and it is either
incomplete or leads to chain fracture. Moreover, there are only a small number of suitable
precursor polymers available and it is imperative to develop new synthetic methods to
circumvent such limitations.
5
Linear and Cross-conjugated Poly(p-phenylene)s and Oligo(p-phenylene)s---Introduction
Typical direct methods for PPP synthesis are: 1) oxidative condensation of benzene
derivatives which requires strong reaction condition and usually results in intractable
products;27 2) transition metal-mediated couplings such as Ullmann coupling30 and
Grignard coupling,31 but these methods generally result in cross-linking and oxidative
condensation to more highly condensed, aromatic hydrocarbon building blocks as side
reactions; 3) organometallic approaches, which are by far the most important methods in
developing large variety of novel PPPs including homo-coupling reaction promoted by
Ni(0) complexes (Yamamoto coupling),32-35 cross-coupling reaction of boron derivatives
(Suzuki-Miyaura coupling),36 arylation of alkenes (Heck reaction),37 arylation of alkynes
(Cassar-Heck-Sonogashira reaction),38 cross-coupling reaction of tin derivatives (Stille
coupling),39 transition metal catalyzed cross-coupling reactions with organomagnesium
(Kumada-Corriu coupling) or organozinc (Negishi coupling) reagents,40 and metathesis
reactions,41 etc.
Indirect methods are less common due to the harsh reaction condition, intractability
and short of suitable precursors. Kaeriyama et al. reported the synthesis of PPP using
Ni(0)-catalyzed coupling via precursor route (Scheme 1.1).42 This method solved
problems of solubility and processability of the precursor polymer.
BrBr
COOMe COOMe
n
COOMe
n
FeCl3
CuO
n
OH-
BrBr
COOMe COOMe
n
COOMe
n
FeCl3
CuO
n
OH-
Scheme 1.1. Precursor route synthesis of PPP using Ni(0)-catalyzed couplings.
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Li Hairong National University of Singapore
Marvel et al. reported the polymerization of 5,6-dibromocyclohexa-1,3-diene to
poly(5,6-dibromo-1,4-cyclohex-2-ene) followed by a thermally induced, solid state
elimination of HBr to form PPP (Scheme 1.2).43 However the obtained products were not
defect free and showed several types of structural defects (incomplete cyclization, cross
linking etc.).
Br Br Br Brn n
Br Br Br Brn n
Scheme 1.2. Synthesis of unsubstituted PPP by the polymerization of 5,6-dibromocyclohexa-1,3-diene.
Another route started from 5,6-diacetoxycyclohexa-1,3-diene.44 The soluble
precursor polymer was aromatized thermally into unsubstituted PPP via elimination of
two acetic acid unit from each unit (Scheme 1.3). However, polymerization did not
proceed as a uniform 1,4-addition, ca. 10% of 1,2-linkages were also formed.
MeOCO OCOMe MeOCO OCOMe
n n
MeOCO OCOMe MeOCO OCOMe
n n
Scheme 1.3. Precursor route to PPP from 5,6-diacetoxycyclohexa-1,3-diene.
In recent years, new trends were developed towards different functional polymers for
real applications. It is necessary to highlight a few important progresses. First, Müllen,
Scherf and Tour et al. have made dedicated efforts in developing ladder-type PPPs
(Figure 1.3), which are of great interest as they overcome the problem of phenylene-
phenylene torsion. There are mainly four classes of ladder-type PPPs: 1) PPPs with
methane bridges;45 2) PPPs with ethene bridges;46 3) PPPs with ethane bridges;47 4) PPPs
7
Linear and Cross-conjugated Poly(p-phenylene)s and Oligo(p-phenylene)s---Introduction
with aza bridges.48 Based on these, variety of ladder and stepladder type PPPs were
synthesized and maximize their potential as active materials in LEDs and polymer lasers.
NRRRR1 R1
R2R2R R
Figure 1.3. Representative ladder-type PPP structures.
Traditionally, electrochemical synthesis of PPP was limited by severe chemical and
anodic conditions; furthermore the resulting polymers were randomly oriented with strong
consequences on energetic disorder and charge transport properties. The novel method
involves the electrolysis of biphenyl which has lower oxidation potential as starting
material, with existence of surfactant and/or membrane template.49-50 Uniform layers with
linear chain structure and higher doping level in contrast to chemical synthesis were
prepared. PPP films exhibited the reversible electrochemical behavior in organic and
inorganic media. This method is especially useful for those biphenyl units with electron
rich groups. Electrochemical synthesis of PPPs in ionic liquid is a novel method. The key
advantage is that it is possible to obtain free-standing thin film under mild, less toxic
conditions.51
Another important development is the synthesis of PPPs with controlled molecular
weight (MW) and polydispersity (PDI). Although, we have no problem in synthesizing
soluble and stable PPPs nowadays as the robust methods have already been well
established, researchers are still exploring the reliable way to control the MW and PDI as
both are intrinsically related to the properties. Yokozawa’s reported that monomer 1-
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Li Hairong National University of Singapore
bromo-4-chloromagnesio-2,5-dihexyloxybenzene was polymerized with Ni(dppe)Cl2 in
the presence of LiCl to give PPP with a narrow polydispersity close to 1.52 LiCl might
break the aggregation of Grignard type monomer, which was effective in the halogen-
magnesium exchange reaction between Grignard reagent and electron-rich aromatic
halides. In addition, the coupling was confirmed to proceed via step-wise growth. The
MW can be controlled by controlling the ratio of prepolymer and monomer. It is an
interesting result which makes researchers re-examine the Grignard method which was
hardly used before, since Suzuki coupling was still limited by uncontrollable MW and
PDI.
Last but not least, researches are interested in modifying conjugated polymer chains
into a helical self-assembly, generally there are two methods: one is the modification of
monomer with chiral group followed by normal coupling; the other polymerization of
non-chiral unit in liquid crystal medium with existence of chiral dopant. The latter
obviously has much wider prospect as it theoretically applies to all candidates. It was
reported that PPP synthesized in chiral nematic liquid crystal (LC) showed circularly
polarized fluorescence spectra and chiral self-assembly structures. This could be
promising for chiral magnetic and luminescent applications.53
Scheme 1.4. Chiral nematic LC induced by the addition of chiral dopant (A copy from Handbook of Conducting Polymers, 3rd ed., 2007).
9
Linear and Cross-conjugated Poly(p-phenylene)s and Oligo(p-phenylene)s---Introduction
1.3.3. Important photophysical properties---Brief introduction
This section gives a brief introduction to several important photophysical concepts
which are highly related to my work.
1.3.3.1. Absorption and emission54-55
When a molecule is excited by absorbing energy in the form of electromagnetic
radiation, there are a number of routes by which it can return to ground state.
Luminescence is one of the process in which energy is emitted from a material at a
different wavelength from that at which it is absorbed (excitation energy). This internal
conversion followed by emission is always complicated with other processes such as
intersystem crossing. The main energetic transitions are summarized in the Jablonski
diagram (Figure 1.4). The singlet ground, first and second electronic states are depicted
by S0, S1 and S2, respectively. Typically, absorption occurs from molecules with the
lowest vibrational energy and end up in higher energy states.
S2
S1
S0
abso
rptio
n
hν hνflu
ores
cenc
e
Internal conversionIntersystemcrossing
hν phosphorescence
T1
210
Jablonski diagram
S2
S1
S0
abso
rptio
n
hν hνflu
ores
cenc
e
Internal conversionIntersystemcrossing
hν phosphorescence
T1
210
S2
S1
S0
abso
rptio
n
hν hνflu
ores
cenc
e
Internal conversionIntersystemcrossing
hν phosphorescence
T1
210
Jablonski diagram
Figure 1.4. Energetic processes on absorption of a photon in the Jablonski diagram.
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Li Hairong National University of Singapore
An electron is usually excited to higher vibrational levels of either S1 or S2. After the
excitation, most of the molecules in condensed phases rapidly relax to the lowest
vibrational level of S1. This process is called internal conversion and occurs in 10-12 s or
less. Since fluorescence lifetimes are typically near 10-8 s, internal conversion is usually
complete prior to emission. Thus the fluorescence emission results from a thermally
equilibrated excited state, i. e. the lowest-energy vibrational state of S1. Molecules in the
S1 state can also undergo a spin conversion to the first triplet state T1. Emission from T1 is
termed phosphorescence and is generally shifted to longer wavelengths relative to the
fluorescence. Conversion of S1 to T1 is called intersystem crossing. Transition from T1 to
S1 state is forbidden and the rate constants for triplet emission are several orders of
magnitude smaller than those for fluorescence. Molecules containing heavy atoms such as
bromine and iodine are often phosphorescent (heavy atom effect).56-57
A few other processes such as internal quenching and external quenching are not
shown in the Jablonski diagram. Internal quenching occurs due to non-radiative energy
transfer to defects or traps (energetically favorable energy levels within the band gap
created by impurities, such as oxygen). External quenching or dissociation is due to
charge transfer to another material with a different electron affinity such as an electrode,
polymer, C60, or other external quenchers, which is the key working principle behind the
fluorescence quenching sensor.
From the Jablonski diagram it is clear that the energy of the emission is typically less
than that of absorption. Fluorescence typically occurs at lower energies or longer
wavelengths. This wavelength separation between the peak of absorption and emission of
a material is known as Stokes’ shift, which is important in determining the usefulness of a
11
Linear and Cross-conjugated Poly(p-phenylene)s and Oligo(p-phenylene)s---Introduction
material for various applications in terms of energy loss through heating of an excited
electron upon emission as well as the probability of an emitted photon to be reabsorbed.
Small Stokes’ shift is desired in most cases.
The excited states of chromophores (excitons) can also be generated by applying a
bias potential to an emissive polymer or a semiconductor sandwiched between an anode
and cathode due to the recombination of holes from the anode and electrons from cathode.
This process of radiative recombination of electrons and holes to generate light usually in
a semiconductor is called electroluminescence.
1.3.3.2. Fluorescence lifetimes and quantum yields58
The fluorescence lifetimes and quantum yields are the key characteristics in the
emission processes. The quantum yield (Φ) is the ratio of the number of emitted photons
to the number of absorbed photons. Lifetime (τ) determines the time available for the
fluorophore to interact with or diffuse in its environment. The fluorescence lifetime is the
average time that a molecule remains in an excited state prior to returning to the ground
state. It is an indicator of the time available for information to be gathered from the
emission profile. During the excited state lifetime, a fluorophore can undergo
conformational changes, interact with other molecules and diffuse through the local
environment. The decay of fluorescence intensity as a function of time in a uniform
population of molecules excited with a brief pulse of light is described by an exponential
function:
)/(0II(t) τte −=
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Li Hairong National University of Singapore
where I(t) is the fluorescence intensity measured at time t, I0 is the initial intensity
observed immediately after excitation, and τ is the fluorescence lifetime. Several
processes can compete with fluorescence emission in relaxation of excited electrons to
ground state, including internal conversion, intersystem crossing, non-radiative
recombination, quenching and phosphorescence .
All non-fluorescent processes that compete for deactivation of excited state electrons
combined into a single rate constant, termed as the non-radiative rate constant and denoted
by the variable k(nr). The non-radiative rate constant usually ignores any contribution
from vibrational relaxation owing to the rapid speeds (picoseconds) of these conversions
are several orders of magnitude faster than slower deactivation (nanoseconds) transitions.
Thus, the quantum yield is expressed in terms of rate constants as
rnrk
ττ /=+ΓΓ
=Φ
Where Γ is the rate constant for fluorescence decay, τ is the measured lifetime, i.e. the
combination of the intrinsic lifetime (radiative lifetime) and competing non-radiative
relaxation mechanisms. Intrinsic lifetime (τr) is defined as the lifetime of the excited state
in the absence of non-radiative processes. In practice, the excited state lifetime is
shortened by non-radiative processes, resulting in a measured lifetime (τ) that is a
combination of the intrinsic/radiative lifetime and competing non-radiative relaxation
mechanisms. Since the measured lifetime is always less than the intrinsic lifetime, the
quantum yield never exceeds a value of unity.
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Linear and Cross-conjugated Poly(p-phenylene)s and Oligo(p-phenylene)s---Introduction
Fluorescence measurements can be broadly classified into two types, steady-state and
time-resolved. Steady state measurement is the most common fluorescence measurement
and performed with constant illumination and observation, information quantum yield
could be obtained. Time resolved measurements are used for measuring intensity decay
pattern and lifetime. Herein the sample is exposed to a pulse of light, where the pulse
width is typically shorter than the decay time of the sample. This intensity decay is
recorded with a high-speed detection system that permits the intensity to be measured on
the femto- or nanosecond timescale.
1.3.3.3. Two-photon absorption
Two-photon absorption (TPA) is the simultaneous absorption of two photons of
identical or different frequencies in order to excite a molecule from its ground state to an
excited state. TPA is a third-order nonlinear optical process.59 The "nonlinear" in the
description of this process means that the strength of the interaction increases faster than
the linear trend with the electric field of the light.59 Under ideal conditions, the rate of
TPA is proportional to the square of the field intensity. This dependence can be derived
quantum mechanically, but is intuitively obvious when one considers that it requires two
photons to coincide in time and space. The imaginary part of the third-order nonlinear
susceptibility is related to the extent of TPA in a given molecule. The selection rules for
TPA are therefore different than for one-photon absorption (OPA), which is dependant on
the first-order susceptibility. For example, in a centrosymmetric molecule, one- and two-
photon allowed transitions are mutually exclusive. In quantum mechanical terms, this
difference results from the need to conserve spin. One-photon absorption requires
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Li Hairong National University of Singapore
excitation to involve an electron changing its molecular orbital to one with a spin different
by ±1, while two-photon absorption requires a change of +2, 0, or −2. Therefore, the
Beer’s Law for TPA should be modified as:
0
0
1)(
cxII
xIβ+
=
where I(x) is the intensity after passing through the material, I0 is initial intensity, c is the
concentration, x denotes distance of light travelling, β is called TPA cross section with
unit GM equal to 10-50cm4.s.photon-1molecule-1.
Two-photon absorption can be measured by several techniques. Typical two of them
are two-photon excited fluorescence (TPEF) and nonlinear transmission (NLT). Tunable
pulsed monochromatic lasers (typically Nd:YAG laser and Ti:Sapphire laser ) are most
often used because TPA is a third-order nonlinear optical process, and therefore is most
efficient at very high intensities in order to “bridge” an energy gap by two photons.
1.3.4. Photophysical properties of typical PPPs
1.3.4.1. Linear PPPs
Polymers with aromatic or heterocyclic units generally absorb light with wavelengths
in the range from 300 to 500 nm due to π−π∗ transitions. The chemical structures, MWs,
absorption and emission maxima of representative PPPs are summarized in Table 1.1.
15
Linear and Cross-conjugated Poly(p-phenylene)s and Oligo(p-phenylene)s---Introduction
Table 1.1. Representative linear PPPs
Absorption (nm)
Emission (nm)
Polymer Mw/Mn
Solution Solution
Ref/ Comments
OR
ROn
R = -C4H9R = -C8H17R = -C12H25R = -iC5H11
SP1-P4
Pn = 32 Pn = 21 Pn = 10 Pn = 24
336 400 60
O
On
SP5
336 404 61
O
O
n
*
*
SP6
22500/ 9700
336 62
O
On
SP7
332 411 63
O
On
O
OR
R SP8
64
O
O
RH
R H R Hn
SP9
12000-13000
64
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Li Hairong National University of Singapore
O
O
O
On
SP10
12700/ 3000
332 415 65
OR1
R1Om
R1 = -C4H9; R2 = -C8H17R1 = -C4H9: R2 = -C12H25n
OR2
R2O
SP11, SP12
60
nRO
OR
R=C4H9R=C8H17
SP13, SP14
6245-12880/ 5990-11430
66
NO2
C8H17O n
CN
C8H17O n SP15 SP16
8921/ 7160 8430/ 7203
66
C6H13O
OC6H13
n
SP17
350, 360 420 67
OC6H13
C6H13ON
n SP18
4600/ 4000
364 366(f)
408 416(f)
68
n
C6H13O
OC6H13
S
SP19
5800/ 3800
441 501 70
n
C8H17O
OC8H17
S
O O
SP20
13490/ 5110 7-14
kD/3.5-6kD
447 460(f),495(f
) 530(f)
512 71
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Linear and Cross-conjugated Poly(p-phenylene)s and Oligo(p-phenylene)s---Introduction
OR
RON n
Pa: R = -C4H9Pb: R = -C8H17Pc: R = -C12H25Pd: R = -C16H33
SP21-SP24
2737/ 2300
31680/ 22000 28200/ 23700 23595/ 19500
375, 383(f)
378, 387(f)
378, 388(f)
378, 389(f)
434, 440(f)
434, 442(f)
434, 442(f)
434, 500(f)
69
N n
O
O
SP25
12000/ 7400
72
14830/ 332 438, 447(f) 5320 206, 332(f)
73
N n
R
R
C6H13
R = H, -OCH3
20400/ 11300
305, 395 429, 438(f) 203, 285,
348(f)
SP26-SP27 350 420, 540 74
O
O
n
SP28
15700/ 9900
373 416 75
C8H17C8H17
C12H25O
OC12H25
n
SP29
21300/ 9130
359 415 76
N
Si
CH3
C8H17C8H17OC6H13
C6H13O
n
SP30
10700/ 8700
275, 375 480 77 OC6H13
C6H13O N
S
OC6H13
n
OC6H13
C6H13O N
S
OC16H33
n
17000/ 11800
SP31 SP32
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Li Hairong National University of Singapore
11500/ 4792
304, 503, 509(f)
640 78 OC5H11
C5H11O
N
N O
O
C6H13
C6H13
n SP33
3745/ 1950
79
N
OC12H25
C12H25OnN
SP34
14000/ 7000
380 426, 434 80
N N n
OC12H25
C12H25O SP35
15600/ 358 426 81 OCH3
ON N
O
n12000 444(f)
SP36
15800/ 7900
312, 367 444 82
O
NN
O
O
n
SP37
26600/ 16000
312, 374 475 82
NO
NN
NO
O
n
SP38
41580 210, 335 83
O
O
O(CH2)6OH
O(CH2)6OH
x yx = 90%
y = 10%
/19800
SP39
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Linear and Cross-conjugated Poly(p-phenylene)s and Oligo(p-phenylene)s---Introduction
155000 /6900
336 410 83 O
O
O(CH2)6O
O(CH2)6O
x yx = 90%
y = 10%
O
O
SP40
14688/ 9850
329, 338(f) 393,399(f),424(f)
84
R = -(CH2)5CH3
R = -CH2CH(CH2CH3)(CH2)3CH3
R = -(CH2)11CH3
NR
n
OC12H25
C12H25O
32412/ 23117
329, 338(f) 394, 397(f), 421(f)
24437/ 18581
328, 337(f) 395 395(f), 418(f)
SP41- SP43
31820/ 18500
408 486, 498(f) 85
NN
OC6H13
C6H13On
3865/ 2100
420 483, 502(f) SP44
NN
n
C6H13C6H13
SP45
4788/ 3600
362/370(f) 460(THF), 490(DCM), 512 (DMF), 470 (f)
86
NN
N
OC6H13
C6H13O
R
n
R = HR = OMe
5800/ 4000
362/370(f) 457(THF), 487(DCM), 511 (DMF), 47 (f)
SP46, SP47
13630/ 5800
277 457 87
n
OH3C CH3
C10H21
C10H21 SP48
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Li Hairong National University of Singapore
18720/ 7800
88
OHRO
OC6H13
OC6H13
nR = -H or -C6H13
17040/ 7100
SP49
22347/ 5944
230, 269, 335
404 89
C6H13O
OC6H13
C6H13O OC6H13
n
SP50
9900/ 8100
90, 91
C6H13O
OC6H13
O O
n
N N
SP51
18500/ 9000
90, 91
C6H13O
OC6H13
OH OH
n
SP52
~10500/~7000
310, 334, 424
554 92 OC12H25
C12H25On
NSe
N
309,
342,429(f) 538(f)
SP53 6975/ 4500
410 548 93
SS
C12H25O
OC12H25
O O
n
Calixarane
SP54
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Linear and Cross-conjugated Poly(p-phenylene)s and Oligo(p-phenylene)s---Introduction
238, 300, 337
403 94
N
N
But tBu
C12H25O
OC12H25
n
SP55
14800/ 274, 344 447 95 OC12H25
C12H25O
N Nn
5300 300(f)
SP56
14757/ 13063
96
nC8H17O
OC8H17 F
F SP57
13620/ 11592
96
C8H17O
OC8H17
S n
SP58
12900/ 5400
97 OR
RO
S
S S
S
n
R = -C6H13 and -C12H25
36700/ 16800
SP59, P60 35500/ 20300
334 98
n
OR
RO
OR
RO RO
OR
R = -C4H9, -C6H13
54500/ 29300
334
SP61, SP62 99
nm
R
R
m
SP63
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100
N
N
N
Ru2+ N
N
N
H3C(OH2CH2C)3O
O(CH2CH2O)3CH3 2PF6-
n
SP64
100
N
N
N
Ru2+ N
N
N
H3C(OH2CH2C)3O
O(CH2CH2O)3CH3 2PF6-
nNO
S SO
O-O O-
O
O
SP65
14879/ 11816
357, 382(f) 432 84
N y
OC12H25
C12H25O
OC12H25
C12H25O
P4, R = -CH2CH(CH2CH3)(CH2)3CH3
x
NR
SP66
440(f),464(f)
30943/ 21965
347, 359(f) 415, 421(f), 437(f)
84 R1R1
OC12H25
C12H25O
OC12H25
C12H25Oy
P5, R = -CH2CH(CH2CH3)(CH2)3CH3, R1 = -(CH2)5CH3
x
NR
SP67
14300 / 8600
370 415 75 R1O
OR1C8H17C8H17
R1O
OR1
SSOPO OPOSS
0.1 0.9
R1 = Octyl group
SP68
PX: 2700/
553, 620(f) 662, 101 S S
NS
N
n
R1 R1
R2
R2
PX; R1 = H, R2 = -OC8H17PY: R1 = -C8H17, R2 = -OC8H17PZ: R1 = -C8H17, R2 = -H
507, 522(f) 639, 642(f) 1500 506, 535(f) 630, 691(f) PY:
65300/37500 PZ:
6900/ SP69- SP71
3500
23
Linear and Cross-conjugated Poly(p-phenylene)s and Oligo(p-phenylene)s---Introduction
27140/ 11800
385, 390(f) 102 OC12H25
C12H25O
OC12H25
C12H25O
O
C12H25O
OC12H25
N
N
N
O
m
n
SP72
7940/ 5090
330 518 103 OC6H13
OC6H13
O
N
O O
N
OC6H13
OC6H13HO
yx
SP73
43kD-9kD/29kD-6kD
372 416, 440 104
N N
O
OH HO
OR2
R2O
OR2
R2OR1 R1
x 1-x
N N
O
OH
OR2
R2O
OR2
R2OR1 R1
x 1-x
OH
HOR1 R1
R1 = -C8H17R2 = -C4H9n
x = 0, 0.2, 0.4, 0.6 and 1
x = 0.2, 0.4, 0.6 and 1
300, 355 417 298, 353 398, 422 296, 349 440
294 423 299, 356 417 296, 351 434 295, 350 427
295 425 356 418, 436
SP74-SP76 14095/ 459 496 105
S S
OO
O
OOO
n m
O
O
O
O
(CH2)3CH3
H3C(H2C)3
10440 458 490 10515/ 455 490 9560 452 488 8880/ 7105
m/n = 1/50, 1/20, 1/10, 1/5 8795/ SP77-80 6065
3477/ 438 105
S S
OC8H17
C8H17ON N n
3104 517(f)
SP81
430 106 50kD 215, 335 COOH
HOOCn
O H2
SP82
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Li Hairong National University of Singapore
107
C6H13
C6H13O
O
HOOC
COOH
n
SP83
108
X(H2C)6
(CH2)6X
n
X = NEt3+ or pyridinium SP84-SP85
108 C6H13
C6H13X(H2C)6
(CH2)6X
n
X = NEt3+ or pyridinium SP86-SP87
109
N(H2C)6
(CH2)6N
n
N
N
SP88
1.9kD 244, 286, 338
410, 430 110
n
NaO3S(H2C)3O
O(CH2)3SO3Na SP89
4795/ 305 398 111
n
NaO3S(H2C)3O
NaO3S(H2C)3O RO
OR
R = -C6H13, -C12H25, -C6H5CH2
4765 296 361 7720/ 299 395 7230
11955/ SP90-SP92 9880
30-17kD/ 9-6kD
112
n
RO
ORR = -(CH2CH2O)xCH3 (x = 2 to 6)
SP93-SP98
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Linear and Cross-conjugated Poly(p-phenylene)s and Oligo(p-phenylene)s---Introduction
4700 343 (MeOH),
354(DMSO)
409(MeOH) 113
O
O
N
N
C6H13 C6H13
n
419(DMSO)
SP99
14.4kD/12.4kD
290, 330 408 114
O
O
N+
N+
Br-
Br-
n
SP100
49.2kD/28.5kD
339 440 115, 116
n
O
O
P45, n = 0P46, n = 2
OOO
O
S
O
OHO
HO
HOOH
OOO
S
O
OHO
HO
HOOH
On
n
339 439
SP101-SP102
23520/ 335 415 117 OC6H13
HO
n10450
SP103
7250/ 335 395 118 OC12H25
HOC11H22O
n5545
SP104
29k/ 119 C6H13
H3CO(H2CH2CO)3H2C
n16500
SP105
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Li Hairong National University of Singapore
7-44kD/ 120 C6H13
HO(H2CH2CO)5H2C
n9-
130kD
SP106 7-11kD 233, 322 121 COOH
n SP107
52-79kD/ 150-
230kD
122 CH3
C12H25
SO3R
RO3Sn
R=1,3-di-t-butyl-benzene SP108
4320/ 312 370 123
n
OCF3 2700
SP109
124
n
O
X
O
O
n'
X = F; Y = -SO3H
X = O Y =
Y
-SO3H
X = F; Y = -CF2CF2SO3H SP110
48.4k/ 125 OC12H25
n
O
O
O OO
CH3
CH33
3
29.1k
SP111
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Linear and Cross-conjugated Poly(p-phenylene)s and Oligo(p-phenylene)s---Introduction
18.2k/ 125
n
OO
O
O
OCH3
CH3
3
3
8.6k
SP112
3883/ 383 (THF) 425 (CHCl
503 126 C12H25O
OHN n
3265 ) 3
SP113
9290/ 330 445 127
n
N N
OHC12H25O
6135
SP114
4950/ 307 396, 437 128
NN
N
nOH
C12H25
n
4125
SP115
11210/ 401 581 129
N N
OC12H25
HO C12H25O
OC12H25
n5900
SP116
3610/ 371 422 130 O
OCH3
(CH2)9N N
C6H13 C6H13
n
2300
SP117
Homopolymers: Among the homopolymers (SP1-SP7), there were no significant
changes in absorption as well as in emission maxima when the side chain of the polymer
backbone was changed from C6 to C18 alkyl chains (λ = 332 to 336 nm and λmax emi = 400
to 415 nm). However the absorption coefficients were different for polymers with
different alkyl chains. The homopolymer SP5 and related series of oligomers were
28
Li Hairong National University of Singapore
synthesized and it was found that the effective conjugation length for the PPPs was about
8-12 repeat units.61 The optical properties of these oligomers and conjugated polymers
were mainly dependent on the conjugation length and torsion angle of the phenylene units
along the polymer backbone. The inclusion of chiral carbon in the PPP backbone did not
influence the optical properties (SP6).62 Usually polymers in thin film state gave slightly
higher bathochromic shift of its absorption maximum which was due to the aggregation of
the polymers or packing or orientation in thin film state (denoted by “(f)” in Table 1.1).
The emission spectra of the polymers were also in the similar ranges for the
dialkyl/dialkoxy PPPs.
Copolymers: 2,5-pyridine incorporated PPP copolymers (SP18, SP21-SP27, etc.)
usually showed larger absorption maxima than the dialkoxy phenylene homopolymers
(λmax = 378 nm and λ = 434 nm) due to donor/acceptor nature of the polymers.69-73emi But
changing the alkoxy group on phenylene rings did not influence the optical properties
significantly. These series of polymers usually showed solvatochromism properties, a
specific example was the addition of CF3COOH. Due to the protonation of pyridine rings
along the polymer chains, the absorption and emission maxima were shifted to higher
wavelengths. The bandgap was affected by the protonation of pyridine rings which caused
the distortion of the inter-ring bonds ascribing for the steric effect. Phenanthroline PPPs
(SP35) showed a red-shift of emission maxima compared to pyridine-PPPs due to higher
planarity.80 Thiophene/PEDOT incorporated copolymers (SP19, SP20) showed larger
values of absorption/emission maxima as compared to the dialkoxy benzene
copolymers.70,71 Fluorene incorporated PPPs were also widely explored. The absorption
and emission maxima were in between those of the homopolymers of diakoxy benzene
29
Linear and Cross-conjugated Poly(p-phenylene)s and Oligo(p-phenylene)s---Introduction
and the pyridine-containing copolymers. Phenazasiline alternating with dialkoxy benzene
polymers (359 nm) showed similar absorption maxima with the phenazasiline
homopolymers.76 Both SP31, SP32 polymers exhibited similar λ and λmax em irrespective of
the alkoxy chain on phenothiazine substituents, which means the phenothiazine unit
played a key role.77 78 So did the diketopyrrole incorporated PPPs. The oxadiazole
containing PPPs showed λmax about 367 and 371 nm for phenylene and pyridine systems
(SP37 and SP38) which was slightly blue shifted compared to the poly(pyridine)
homopolymer.82 The copolymer incorporated with N-alkylated carbazole-dialkoxy
benzene exhibited optical properties in between the homopolymers of dialkoxy benzene
and carbazole.84 There was no significant changes in the λmax with different alkyl
substitution on N-carbazole. Pyrazoline is an interesting group since it contains both the
electron donating aromatic and accepting imine groups. When incorporated into PPP
backbone, the polymer gave the blue-green photoluminescence.85 The absorption and
emission maxima were red shifted as compared to the homopolymers. Benzoselenadiazole
containing PPPs showed higher red shift of absorption at 424 nm and the emission at 554
nm, which was again due to the donor-acceptor charge transfer nature.92 Binapthaol,
quinolone containing PPPs were also explored and it was found that there was no
significant change in the optical properties compared to homopolymer of dialkoxy
benzene.88-91,95 The calix[4]arene-dialkoxy benzene copolymer showed higher red-shift in
the absorption of about 410 nm due to charge-transfer nature of the polymer.93
Multiple-component PPPs: The multiple-component PPPs contain more than two
monomer units in the backbone, such as tetrathiofluvaline related PPPs97 and Ru-
terpyridine containing PPPs98, but with no optical properties were investigated. In the
30
Li Hairong National University of Singapore
fluorene incorporated copolymers, the presence of dialkylated fluorene unit was
responsible for the enhanced steric effect between the dialkoxy benzene and fluorence
backbone,104 which lowered the effective conjugation length in SP67 compared to SP66.84
There was an increase in quantum yield of this polymer SP68 as compared to the
homopolymers of dialkoxy benzene or dialkylated fluorene.75 Benzothiadazole, thiophene
and dialkoxy benzene incorpated PPPs showed significant red-shift in both absorption
(530 nm) and emission (630 nm).101 PPPs with pyrenylpyrazine units (SP72) were
investigated by changing the stoichiometric ratio of dialkoxy benzene, benzene and
pyrenyl triazine units and the results showed that there was no significant change in λmax
for different stoicihiometric polymers but the emission maxima were significantly
changed.102 In the case of PPPs with benzoxazole, the presence of both phenolic -OH and
benzoxazole-N, planarization of the polymer backbone was reflected in its fluorescence
spectrum.103 PPPs containing phenol substituted oxadiazole moieties were synthesized and
explored for sensing fluoride ion (SP74-SP76).104 The absorption and emission maxima
ranged from 349-356 nm and 398-400 nm, depending on the content of the oxadiazole
moieties. The photoluminescence quantum yields were very much affected by the
oxadiazole units. J. J. E. Moreau et al. synthesized [thienylene-p-(2,5-dialkoxy)phenylene]
copolymers which contain chelating units (dibromodibenzo-18-crown-6) on the polymer
backbone (SP77-SP80).169 Another polymer SP81 was also synthesized by replacing 18-
crown-6 with the bipyridyl group and it was found that the λmax was blue shifted due to the
reduced conformational changes along the backbone. All these polymers are highly
sensitive to the presence of transition metal ions.169
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Linear and Cross-conjugated Poly(p-phenylene)s and Oligo(p-phenylene)s---Introduction
Water soluble PPPs: Water-soluble conjugated polymers can be easily synthesized
by functionalization of ionic groups on the pendant chains of the polymer backbone such
as sulfonate (SO - -3 ), carboxylate (CO2 ), phosphonate (PO4
2-) or ammonium (NR3+) groups.
Novak et al. synthesized the first PPP-type conjugated polyelectrolyte with carboxylic
acid groups by means of Suzuki coupling reaction (SP82).106 After that Rau and Rehahn
developed a hydrophobic polymer precursor approach to synthesize a water-soluble PPP
SP83.107 Rehahn et al. also synthesized a series of PPPs with the cationic side chains
SP84-SP88.108-109 However the optical properties of these polymers were not investigated
in detail. Reynolds et al. synthesized a series of sulfonatopropoxy substituted poly(para
phenylene)110 and found the λmax of 338 nm. When the polymer backbone decorated with
alkoxy groups,111 the λmax was blue-shifted by 8-10 nm, which might be due to the
disorder created by the alkoxy group (e.g., steric hindrance) in the polymer lattice,
resulting in the reduced planarity of the polymer backbone. A similar trend was observed
in the emission spectra as well. Aggregation properties of SP90 were investigated by
dynamic light scattering method (DLS) and atomic force microscopy (AFM).111
Oligoethylene glycol incorporated PPPs were shown to be water soluble but optical and
aggregation properties were not studied.112 Such polymers were shown to have potential
application in rechargeable lithium ion batteries as polyelectrolyte separators. Optical
properties of quarternized dialkylamino ethoxy substituted PPP114 and PF
(polyfluorene)113 were investigated in different solvents (H2O, MeOH, DMSO) and pHs
and it was found that there was a significant change in optical properties. H. Liu et al.
synthesized water-soluble glucose functionalized PPPs (SP101-SP102) bearing α-
mannopyranosyl and β-glucopyranosyl.115,116 The polymers were highly soluble in water
32
Li Hairong National University of Singapore
and phosphate buffer even without ionic groups, exhibiting λ at 339 nm and λmax em at 440
nm. These polymers were useful in the applications in biological systems such as
detection of proteins and bacteria.
Asymmetric PPPs: Compared to the corresponding dilakylated or benzylated
polymers (SP1-SP5), the λmax of asymmetric polymers (SP103) are significantly red
shifted. It may be attributed to the interchain hydrogen bonding of hydroxyl groups on the
polymer backbone. These polymers were also sensitive to bases due to the presence of
phenolic–OH groups. The presence of terpyridine moieties on the polymer backbone of
PPPs result in blue shift of their λmax due to the kink nature of the group (SP115). The
polymers exhibited a strong hypsochromic shift in λmax to the blue region on addition of
bases due to the formation of phenolate anions on the polymer backbone. The emission
spectra of the polymers were also red-shifted upon addition of bases at a large extent.
Aggregation and solid-state behavior of such polymers (SP107 and SP108) were also
significantly different as compared with symmetric ones (SP90-SP92) due to the interplay
between hydrophobic and hydrophilic interactions.111 When electron rich units like
pyridine, bipyridine, terpyridine incorporated into the polymer backbone with asymmetric
phenolic –OH groups, the absorption wavelengths were shifted to higher values and
formed LB monolayer on water/air interface (e.g. SP113-SP116).126-129 SP117 was
reported to have metal sensing capability due to presence of bipyridine unit as
complexation center.130
1.3.4.2. Cross-conjugated macromolecules
33
Linear and Cross-conjugated Poly(p-phenylene)s and Oligo(p-phenylene)s---Introduction
A cross-conjugated molecule is defined as a molecule with three unsaturated groups,
two of which, although conjugated to a third unsaturated center, are not conjugated to each
other.131 In order to facilitate the interchain charge transport through a hopping mechanism,
strong π-π interaction between adjacent chains is highly desired and can be achieved by
extended conjugation. Although many cross-conjugated oligomers and polymers have
been reported, their characterization and application are much less explored compared to
linear conjugated materials. As cross-conjugated frameworks provide a novel platform for
extended conjugated systems, many interesting properties can be expected. Therefore
studies on investigating the structural, photophysical, electronic and mechanical properties
have been dramatically sprouting in recent years and potential applications in PLED, TFT,
liquid crystal, photovoltaic device and cross-conjugated systems can generally be divided
into two major categories based on the backbone structure: acyclic (labeled as ACCM)
and cyclic (labeled as CCM) cross-conjugated systems, the latter also includes the
coexistance of aryl and vinyl/alkyne groups in the backbone. An overview of the typical
example is given in Table 1.2.
Table 1.2. Representative cross-conjugated molecules
Category Example Reference
n
ACCM1
132 Dendralene
n
ACCM2Et3Si SiEt3
133 Polydiacetylene
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Li Hairong National University of Singapore
ACCM3
SiEt3
R R
R R
R
Et3Si
R
134 Polytriacetylene
n
ACCM4
Sii-Pr3
Sii-Pr3
t-Bu
t-Bu
t-Bu
t-BuPolypentaacetylene 135
CCM5
SeSe
S S S S
R RR R
CCM6
S
S
S
S
Tetrathiafulvalene 136, 137
CCM7-10
Radialene 138
CCM11
Macrocycle 139
35
Linear and Cross-conjugated Poly(p-phenylene)s and Oligo(p-phenylene)s---Introduction
CCM12
Pt
Pt
PEt3
PEt3
PEt3
PEt3
R
R
R
RR
R
R
R
140
CCM13
RR
RR
RR
RR
141
C12H25
m
n
CCM14
144 OPV
CCM15
OPV 145
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Li Hairong National University of Singapore
CCM16
R
R
R
R
R
R
OPV 146, 147
CCM17
OC8H17
C8H17O
OC8H17
C8H17OOC8H17
C8H17O
C8H17O
OC8H17
OPV 148, 149
CCM18
OC8H17
C8H17O
NCCN
CN NC
OC8H17
C8H17O
C8H17O
OC8H17
OC8H17
C8H17O
148, 149 OPV
37
Linear and Cross-conjugated Poly(p-phenylene)s and Oligo(p-phenylene)s---Introduction
CCM19 N N
O
OC8H17
OC8H17
NN
O
H17C8O
OC8H17
OPV 148, 149
CCM20
NN
O
N N
O
H17C8O
OC8H17
OC8H17
OC8H17
OPV 148, 149
CCM21
OCH3
H3CO
OCH3H3CO 150-152 OPV, OPE
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Li Hairong National University of Singapore
CCM22
NR2
R2N
CF3
CF3
F3C
F3C
150-153 OPV, OPE
CCM23
NBu2
Bu2N
154
CCM24
NN 154
CCM25
NBu2
Bu2N
NN 154
39
Linear and Cross-conjugated Poly(p-phenylene)s and Oligo(p-phenylene)s---Introduction
CCM26
N
N
NN
OONa
ONaO
O
O
NaO
NaO
155
CCM27
NN
HS
SH
156
CCM28
OPV, OPP 157
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Li Hairong National University of Singapore
CCM29
O
NO
N
OR
OR
YX
a) X= Y= CHOb) X= Y= NH2c) X= SH, Y= CHOd) X= SBoc, Y= CHOe) X= Y= CN
OPP 158-160
CCM30
Sii-Pr3i-Pr3Si
SS
SS
SS
SS
CO2MeMeO2C
CO2MeMeO2C
CO2MeMeO2CMeO2C
CO2Me
OPE,
Tetrathiafulvalene 161
CCM31
S
S
S
S
S
S
S
S
S
S
S
S
OR
RO
OR
OR
162 OPV, Thiophene
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Linear and Cross-conjugated Poly(p-phenylene)s and Oligo(p-phenylene)s---Introduction
CCM32
C12H25OOC12H25
C12H25O
OC12H25
OC12H25
OC12H25 OC12H25OC12H25
OC12H25
OC12H25OC12H25
C12H25O
OC12H25OC12H25
C12H25O
C12H25O
C12H25OOC12H25OPP, OPV, OPE 163
CCM33
OR
RO
NNO
NN O n
PPV 164
CCM34
R
R
I I
R
R
n
N CO2Me
OMe NMe2OMe
MeO
MeO
a) b) c)
d) e) f)
PPE 165
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Li Hairong National University of Singapore
CCM35a CCM35b
n n
PF 166-167
CCM36
(C4H9)2N
N(C4H9)2
n
PPP 168
CCM37
C12H25C12H25
m
n
NN
O
NN
O
C12H25C12H25
169 PPP
43
Linear and Cross-conjugated Poly(p-phenylene)s and Oligo(p-phenylene)s---Introduction
CCM38
NHN
NNH
C8H17
C8H17
C8H17
C8H17
n
NN
NN
C8H17
C8H17
C8H17
C8H17
n
CCM39
PPP 170
Photophysical studies of ACCM1 indicated that the conjugation did not increase as a
function of chain length (or the degree of polymerization) probably due to the non-
planarity nature.132 Many polymers were thermally labile and highly insoluble, thus
hampering the characterization. ACCM2 with a more planar structure showed a red shift
in absorption maxima as the chain length was increased up to n = 9.133 Emission intensity
also increased as a function of chain length. ACCM3 was reported to be air and light
stable.134 A consistent drop in the HOMO-LUMO gap as the degree of polymerization
increases was confirmed by UV-Vis, suggesting a contribution from extended conjugation.
The molecule was soluble in common organic solvents due to the presence of terminal
solublizing groups.
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Li Hairong National University of Singapore
Extended tetrathiafulvalenes (e.g. CCM5 and CCM6) can be viewed as 1,3-dithiol
modified dendralenes. It is not restricted to thiol; selium and tellurium are the alternative
substituents owning to higher polarizability.136-137 These series of molecules have
interesting electrochemical behavior. Radialenes are the cyclic analogues of dendralenes;
many derivatives based on these structures (CCM7-10) have been reported,138 with
emphasis on the synthesis aspect, typical characterizations include CV, UV-Vis, single
crystal XRD, etc. Macrocycles can be considered as expanded radialenes with different
kinds of spacers. Despite the number of spacers, many macrocycles were reported to have
very weak cross conjugation (e.g. CCM11) probably because of non-planar structure.139
The first macrocycle with metal spacer (CCM12) has Pt atom to mediate electron
communication, leading to delocalization over the entire π-system.140 CCM13 takes a near
planar structure due to energy favored hexagonal structure.141
The above mentioned cross-conjugated systems have been reviewed in detail by
Tykwinski.142 We would like to emphasis more on cross-conjugated PPP (including PPV
and PPE) and cruciform systems which are highly related to our work.
PPP, PPV and PPE are good candidates for OLED applications. Efficient and
balanced charge transport is essential to the optimization of properties of molecular and
polymeric light-emitting diodes.143 In oligomeric and polymeric systems, charge transport
occurs via two routes. In intrachain transport, charges are delocalized across the polymer’s
linear conjugated backbone. In the process of interchain transport, charges move from
chain to chain through a hopping mechanism. In some of these oligomer and polymer
systems, two-dimensional charge delocalization can be achieved through π-π stacking
between adjacent chains. Furthermore, it also facilitates the self-assembly of the 2D
45
Linear and Cross-conjugated Poly(p-phenylene)s and Oligo(p-phenylene)s---Introduction
molecules to ordered packing as well as liquid crystal behavior with careful molecular
design. On the other hand, this interaction also increases their tendency to crystallize,
leading to the formation of excimers, which quench PL in films.
OPV based cruciforms (CCM14-CCM20): Cruciform CCM14 reported by
Bazan’s group is comprised of a ladder-type tri(p-phenylene) (LPP) and oligo
(phenylenevinylene) (OPV) with no conjugation between the OPV and the LPP moieties.
Although absorption showed features of both the LPP and OPV segments, the emission
spectrum showed only emission from OPV, indicating a certain degree of intrinsic 2D
charge delocalization without a conjugated linker,144 probably due to strong energy
transfer. Compound CCM15 has four conjugated arms attached to a non-conjugated
central sp3 hybridized carbon core, therefore there is no conjugation between the arms. It
exhibited a red shift in maximum emission as compared to linear polymer analogs,
indicating charge delocalization through the sp3 carbon core.145 Dendrimer CCM16 can be
regarded as core-antenna structure where the energy is transferred from the OPV arms
towards the benzene core, which acts as a trap and primary emissive species due to the
breakage of conjugation by the meta-linkage.146 Therefore, changing the core can easily
tune the emission color. A perfect stacking pattern was also observed by scanning
tunneling microscope (STM). Yu’s group reported the synthesis of the dendrimer
(CCM16) using an orthogonal approach.147 This synthetic route avoids the need for
protection chemistry, which is needed almost in all dendrimer syntheses. This approach is
also very versatile for the syntheses of different dendritic structures, which can be
achieved through variation of the building molecules. The strategy enables one to
construct phenylene vinylene dendrimers with high molecular weights; the resulting
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Li Hairong National University of Singapore
dendrimers maintain the uniformity of the structure in terms of linking units involved
between the generations. The reaction conditions were mild and could tolerate a large
variety of functional groups. Galvin’s group synthesized a series of CCM17-20
derivatives,148-149 with cross-conjugated structure involving OPV arms attached to a
central benzene core; the arms were chemically modified by different donor (alkoxy) and
acceptor (-CN) groups and arranged in either central (CCM17-18) or mirror (CCM19-20)
symmetry. Unlike CCM16, the HOMO and LUMO of this model compound can be easily
tuned by introducing electron donating or withdrawing groups such as alkoxy, oxadizaole
and cyano groups. Significant π-π interactions were observed in concentrated solution and
powder samples, explaining the red shift in emission maxima. X-ray diffraction studies
performed on powdered samples indicated that these X-shaped molecules form stacks
with a stacking distance of 3.5 Å, which provided pathways for charge transport along the
stacking direction. Pulse radiolysis with time-resolved Vis/NIR spectroscopy was used to
measure transient optical absorption spectra (in the wavelength range of 500-1600 nm
(0.8-2.5 eV) of charges on X-shaped phenylene vinylene oligomers produced by irridation
with high-energy electron pulses. The experimental spectra of the X-mer cations and
anions exhibited a strong absorption band near 1.6-1.7 eV. For the CCM18 cation an
additional weak absorption was found at low energy. Optical absorption spectra of
charged X-mers were calculated using the INDO/s-CIS method with a geometry obtained
by density functional theory (DFT). The calculations reproduced the measured optical
absorption band near 1.6-1.7 eV. The distribution of positive and negative charges on the
cruciforms was investigated using DFT. It was found that the excess positive charge was
mostly localized on the methoxy-substituted phenylene units, while the excess negative
charge was delocalized over the entire oligomer. Charge carrier mobility on static stacks
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Linear and Cross-conjugated Poly(p-phenylene)s and Oligo(p-phenylene)s---Introduction
of CCM20 oligomers was investigated using parameters obtained from DFT calculations.
It was found that the charge carrier mobility depended on the twist angle between adjacent
groups in the stack. The mobilities of electrons are higher than those of holes and exhibit a
somewhat different dependence on the twist angle. Molecular dynamics simulations
showed that the most favorable conformation of the CCM20 dimer was reached for a
twist angle of 10º, where the calculated hole mobility was 4.1 cm2V-1 -1S and the electron
mobility was 8.3 cm2 -1V S-1. According to the charge-transport calculations the mobility
can be significantly enhanced by reducing the twist angle. The high flexibility of color
tuning and high-mobility obtained from the calculations make these materials attractive
candidates for application in optoelectronic devices and emissive applications
Cross-conjugated cruciforms based on OPE, OPV and OPP hybrids (CCM21-
30): Bunz’s group reported the synthesis and characterization of oligo(p-phenylene
ethynylene) (OPE) and OPV hybrid oligomers arranged orthogonally to each other.150-152
Using the Horner–Sonogashira reactions, the two-dimensional molecules were
synthesized with varying terminal groups, two of which are highlighted here (CCM21 and
CCM22). For molecule CCM21, high solution fluorescence quantum yields of 78 % and
88 % were observed in both hexanes and chloroform, respectively. However, for CCM22,
the fluorescence was dramatically reduced to 53 % in hexanes and 14 % in chloroform.
Ground and excited state charge delocalizations were also altered by the nature of the side
groups. For CCM22, the HOMO was calculated to reside only on the OPV arms, the
LUMO was located on the OPE segments. Conversely, in molecule CCM21, this
separation between orbitals was not seen in the excited state, rather the charge is
delocalized across the entire molecule. Single crystal studies showed CCM22 and its
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analogs possessed a twisting of the OPE arms with rotations of approximately 30°.
Additionally, the styryl groups also showed a departure from planarity with a twisting of
approximately 15°.153 Meanwhile, three other cruciforms (CCM23-24) with different
sensing groups such as pyridine and aniline at the end of each conjugated arms were
investigated with respect to their metal sensing properties.154 Upon addition of metal
cations to these cruciforms, either a bathochromic or hypsochromic shift in emission and
absorption maxima was observed. The shift depended on whether the metal coordinates
preferentially to the pyridine or to the dibutylaniline branches of the cruciforms, which
was probably due to metal-coordination induced molecular deformation. The system could
discern calcium from magnesium cations and silver from mercury or lithium cations,
which was probably attributed to complete spatial separation of HOMO and LUMO upon
specific metal ion binding. Water soluble cruciform CCM26 features the attachment of
the carboxylate units to the aniline nitrogens, which influenced the photophysics and the
sensory responses, as the combined effect of steric and charge repulsion led to a blue shift
in absorption maximum as compared to that of CCM23. While the fluorescence of the
water-soluble CCM26 is sensitive toward metal cations, the mode of sensing action was
based upon the breakup of excimeric species rather than the specific binding of the zinc
cations to the lone pair of the aniline nitrogen of CCM23.155 Mayor’s group reported a
new switching concept based on the electrochemically triggered reversible coordination
between a single molecule and the surfaces of electrodes. The cruciform structures
consisting of pyridine functionalized OPE and OPV comprising terminal sulfur anchor
groups on both ends to hold the molecule in an electrode junction (CCM27) acted as a
model compound to investigate the proposed single molecular switching mechanism.156
Immobilization of the cruciform in a mechanically controlled break junction in a liquid
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Linear and Cross-conjugated Poly(p-phenylene)s and Oligo(p-phenylene)s---Introduction
environment was successful. It was observed that not only the sulfur-terminated OPV rod
acted as the bridging structure between both electrodes, but also that molecules
comprising additional functional properties could be immobilized on a single molecule
level in the MCBJ (Mechanically Controllable Break Junction) setup. The hypothesized
switching mechanism should arise from the electrochemical potential dependent
coordination of the pyridine unit to the electrode surface. Duan et al. synthesized a
distyrylbenzene derivative CCM28 for LED studies; this compound had a fishbone like
structure with OPP as the backbone and OPV as the side chain. The characteristic
electroluminescent devices have been carefully studied by judicious choice of material and
architecture of devices, stable blue EL with a peak at 460 nm and a maximum current
efficiency of 4.88 cd/A was obtained. The observed color coordinates of CIE 1931
chromaticity diagram were within pure blue region; maximum brightness has been
dramatically improved up to 15830 cd/m2, which is high for blue OLEDs. These
enhancements in EL performance were likely due to the better ability of hole injection in
bulk as well as efficient charge transfer through highly ordered structure by controlled
vapor deposition.157 However, further studies were limited due to the poor solubility. A
series of CCM29 derivatives were reported to address the problem that the orientation of
simple aromatic molecules in general and thiols in particular is often parallel to the
surface.158-159 The synthesis employed a method based on a double Staudinger cyclization,
where an aryl azide is converted to a nitrogen-based ylide using polystyrene-bound
triphenyl phosphine. These compounds (CCM29a-e) showed an upright conformation of
the molecules in the self-assembly monolayer (SAM), especially those with terminal thiol
groups to attach to metal surface (e.g. CCM29c). Moreover, strong π-π interaction tended
to facilitate the formation of closely packed monolayers. The presence of terminal
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Li Hairong National University of Singapore
functional groups like aldehyde and aniline-NH2 provide an untested route for the high-
throughput formation and evaluation of a number of different end-groups at different
length in molecular electronics.160 The absorption spectra showed a red-shift and an
increase in the molar extinction coefficient as the length of the molecule increases, which
was attributed to the push-pull electronic structure of the bis-benzoxazole and the long
path of delocalization up to planarized NH group. 2 Vestergaard et al. reported OPE based
cruciform conjugated molecules with tetrathiafulvalene,161 prepared from a variety of
acetylenic precursors. The synthetic protocols offer an easy access to aromatic building
blocks containing both triflate and ethynyl functional groups allowing for stepwise
acetylenic scaffolding via metal-catalyzed cross-coupling reactions. The redox and optical
properties of the cruciform molecule CCM30 were investigated by cyclic and differential
pulse voltammetries and UV-Vis absorption spectroscopy. CV showed that the
characteristic redox of tetrathiafulvalene was largely maintained but significantly shifted
because of conjugation to OPE backbone. The access to multiple redox states rendered the
molecules attractive candidates as wires, or possibly transistors, for molecular electronics.
Cross-conjugated dendrimer (CCM31-32): Hwan Kin synthesized a conjugated
molecule with oligothiophene as four arms connected to phenylenevinylene core.162 It was
intrinsically crystalline, as confirmed by well-defined X-ray diffraction patterns from
uniform orientations of molecules. Intermolecular interaction could be further enhanced to
affect the carrier transport phenomena. After annealing at 160 ºC, the carrier mobility was
as high as (3.7±0.5)×10−4 cm2 V−1 s−1 3 with an on/off ratio of about 1×10 . However, CV
spectra of CCM31 did not vary much from the oligothiophene analogs, it was explained
that the strong aromacity of the benzene core “blocked” the connection between the more
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Linear and Cross-conjugated Poly(p-phenylene)s and Oligo(p-phenylene)s---Introduction
“quinoid-like” oligothiophene segments. Dendrimer CCM32 and analogs were a class of
interesting molecules with OPP cores and alternating phenyleneethynylene and
phenylenevinylene units in the dendritic arms.163 They had been effectively synthesized up
to the fourth generation using orthogonal and convergent synthetic routes that combine
Sonogashira and Horner-Wadsworth-Emmons reactions. The choice of the appropriate
iterative route afforded specific control over the placement of double and triple bonds
within the interior of the dendrimers. The meta-substitution pattern causes all
chromophores to be independent and all molecules showed good transparency in the
visible region and blue-luminescence. The quantum yields for the higher generation
dendrimers were substantially lower than that from the lower generations at the same
molar concentration. This phenomenon had been explained because the through-space
interactions between the fluorescent units become more significant with increasing
molecular size, providing additional fluorescence quenching pathways.
Cross-conjugated PPE, PPV and PPP (CCM33-39): Cross-conjugated PPV with
oxadiazole segments (CCM33) was reported by Xu et al.164 It was found that, due to the
electron-withdrawing nature oxadizole arm, the LUMO state of the oxadiazole segments
was lowered to such an extent that it had a lower energy than the LUMO state of the PPV
backbone. Consequently, a charge transfer (CT) excited state became the lowest-lying
excited state in these polymers, thus exhibiting dramatically different fluorescence
behavior and much lower PL quantum efficiencies. This might not be suitable for LED
applications, but they could be highly photoconductive. In 2002, Bunz’s group
communicated a series of cross-conjugated PPEs, with different electron donating (alkoxy,
phenylanime) and withdrawing groups (pyridine) on the conjugated side chains.150-153
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Band-gaps of the molecules in solid state were investigated by CV, ranging from 2.00 eV
(CCM34f) to 3.40 eV (CCM34d).165 Scherf et al. reported two interesting low band gap
polymers: poly(indenofluorene) (PIF, CCM35a) and poly(diindenonaphthalene) (PDIN,
CCM35b) based on their pioneering work.166-167 These polymers possessed highly planar
backbones via sp2 carbon on fluorene unit, which dramatically extended the effective
conjugation length. The absorption maxima for PIF and PDIN were 799 nm and 724 nm,
respectively. The large absorption difference was due to the large energy difference
between the benzoid and quinoid state because of the higher aromatic resonance energy of
naphthalene. The larger energy difference between the benzoid and quinoid resonance
structure in 2,6-naphthylene in relation to 1,4-phenylene resulted in a decreased
contribution of quinoid resonance states to the ground state electronic configuration of
PIDN (Scheme 1.5.). The decreased quinoid character then caused the distinct blue shift
of the absorption.
n n n n
PIF PDIN
Scheme 1.5. Contribution of quinoid ground-state resonance structures fro PIF and PDIN.
A cross-conjugated polyfluorene copolymer (CCM36) was synthesized for studying
their responses in metal ion sensing by attaching phenylamine group on the OPV side
chain.168 This polymer showed totally different characteristics when it binds with metal
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Linear and Cross-conjugated Poly(p-phenylene)s and Oligo(p-phenylene)s---Introduction
ions compared to those observed for traditional linear conjugated polymers. It showed
significant changes in its fluorescence intensity and emission peak shift upon coordinating
with different metal ions, Zn2+ induced a significantly enhanced blue-shifted emission,
whereas Mg2+ and Ca2+ only showed quenched or/and red-shifted emission, due to their
weaker binding strength than Zn2+. The control experiments based on using a small
molecule indicated that the conjugation of CCM36 played an important role in enhancing
the selectivity for metal ion sensing. The donor-π-donor conjugated side chains of
CCM36 endowed it with large TPA cross section. However, once the sensing group was
converted to quaternary ammonium salts, the selectivity vanished. A similar polyfluorene
derivative was reported by Huang Wei’s group, where they presented a p-n diblock type of
cross-conjugated polyfluorene (CCM37).169 For linear conjugated p-n systems, the n-
block inserted into the p-type polymer mainchain will partially act as a hole-blocking unit
due to its high electron deficiency. Vice versa, the hole-transporting unit will lower the
electron mobility. Therefore, it is difficult to further improve the charge injecting and
transporting properties. On the other hand, the energy levels of the HOMO and LUMO of
most cruciform p-systems are spatially separated and the independent properties of each
branch mostly maintained. These unique properties are crucial for molecular electronic
applications, e.g., adding donor (e.g. oligofluorene in CCM37) and/or acceptor (e.g.
oxadizaole in CCM37) substituents to cross-shaped chromophores at suitable positions
can lead to independent electronic shifts in the LUMO and HOMO as well as increase of
carrier mobility. Variation of the length of polyfluorenes affords the possibility of tuning
redox behavior and the emissive color. Müllen’s group reported a novel PPP with a 2,2’-
(p-phenylene)-bis(4,5-diphenylimidazole) as an additional orthogonal chromophore
(CCM38). This polymer showed in solution and in the solid state, a blue-green emission
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owning to the presence of second bisimidazole-based chromophore. UV-Vis spectroscopy,
and cyclic voltammetry revealed that the optical and electronic properties of CCM38 were
fully determined by the incorporated 2,2’-(1,4-phenylene)bisimidazole structure. In
addition, the oxidation of CCM38 with potassium ferricyanide resulted in new quinoid
type cross-conjugated PPP (CCM39) with a low bandgap of 1.6 eV. Broad absorption
over entire UV-Vis region also made CCM39 suitable as light absorbing material in solar
cells.170
1.4. Scope and outline of the thesis
My research work started from understanding the structural-properties relation of
water soluble PPPs. In Chapter 2, a series of structurally symmetric and asymmetric PPPs
with sulfonate and alkoxy groups were reported and fully characterized by NMR, UV-Vis,
TGA, IR, EA and XRD. Aggregation studies were done by means of UV-Vis, DLS and
AFM in order to manifest the role of symmetry in self-assembly. Complexation studies
with metal ions, cytochrome C, violegens were carried out in aqueous solution to test its
sensor capability.
The water soluble system was further extended to two-dimensional framework.
Chapter 3 introduced the cross-conjugated water soluble PPPs in which the main focus
was to make more versatile polymer sensors by introducing multiple functional groups.
Meanwhile, extended conjugation was also expected to have interesting self-assembly
properties.
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Linear and Cross-conjugated Poly(p-phenylene)s and Oligo(p-phenylene)s---Introduction
In order to avoid the involvement of electrostatic and hydrophobic interactions in
aqueous medium, we designed and synthesized a new series of cross-conjugated polymers
soluble in common organic solvents. In Chapter 4, cross-conjugated PPPs with different
backbone structures were investigated comprehensively by XRD, UV-Vis, fluorescence,
quantum yield (QY), two photon absorption (TPA) and time-correlated single-photon
counting (TCSPC). The studies focused on how the strong π-π interaction between
polymer backbones affected the overall photophysical properties and self-assembly.
However, it was difficult to explore the role of each conjugated arms in the whole
system due to large size of molecules and uncertainty of the degree of polymerization and
polydispersity. Cross-conjugated oligomers (cruciform) were therefore the ideal
candidates. Different electron donating and withdrawing groups were introduced at
different positions; a deep insight on how those functional groups affected the overall
photophysical properties was achieved by structural-property studies. The potential crystal
growth provided more information on the structural-property relation at molecular level.
The work was summarized in Chapter 5.
Since cross-conjugated structure can largely delocalize the π-electron, this idea can
be applied to design a radical scavenger system, e.g. polyphenols. Cross-conjugated
polyphenols have unique properties as oxygen anion can be stabilized by delocalization,
so they are more prone to give protons. In Chapter 6, a series of cross-conjugated
polyphenols were designed, with different number of hydroxyl groups at different
positions, antioxidant property and bioactivity were studied.
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Finally, in Chapter 7, we briefly compared the photophysical properties of the
cruciforms introduced in Chapter 5 with pyrene derivatives with different donor-acceptor
arms. Although the latter, by definition, were not classified as cross-conjugated molecules,
we expected to see how the pyrene core and conjugated arms affected the photophysical
properties to highlight the significance of cross conjugation and pinpoint the potential
limitations of both systems.
1.5. References
1. Letheby, H. J. Chem. Soc. 1862, 15, 161.
2. Nigrey, P. J., MacDiarmid, A.G., Heeger, A. J. J. Chem. Soc. Chem. Commun.
1979, 594.
3. Burroughes, J. H., Bradley, D. D. C., Brown, A. R., Marks, R. N., Friend, R. H.,
Burn, P. L., Holmes, A. B. Nature 1990, 347, 539.
4. Stegeman, G. I., Zanoni, R., Seaton, C. T. “Nonlinear Organic Materials in
Integrated Optics Structures”, 1988, Pottsburg, Pennsylvania, p. 53.
5. Mori, T., Kato, M., Uemura, T. U.S. Patent 5, 073, 011, 1991.
6. Otero, T. F., Rodriguez, J. “Electrochemomechanical and Eletrochemopositioning
Devices: Artificial Muscles”, 1993, p.179.
7. Hoa, D. T., Kumar, T. N. S., Punekar, N. S., Srinivasa, R. S., Lal, R., Contractor,
A. Q. Anal. Chem. 1992, 64, 2645.
8. Paul, E. W., Ricco, A. J., Wrighton, M. S. J. Phys. Chem. 1985, 89, 1441.
9. Angeli, A. Gazz. Chim. Ital. 1916, 46, 279.
10. Lund, H. Acta Chem. Scand. 1957, 11, 1323.
57
Linear and Cross-conjugated Poly(p-phenylene)s and Oligo(p-phenylene)s---Introduction
11. Peover, M. E.; White, B.S. J.Electroanal. Chem. 1967, 13, 93.
12. Osa, T.; Yildiz, A.; Kuwana, T. J. Am. Chem. Soc. 1969, 91, 3994.
13. Armour, M.; Davies, A. G.; Upadhyay, J.; Wasserman, A. J. Polym. Sci. 1967, A1,
1527.
14. Jozefowicz, M.; Yu, L. T.; Belorgey, G.; Buvet, R. J. Polym. Sci. Part C 1969, 16,
2943.
15. Dall’Ollio, A.; Dascola, Y.; Varacca, V.; Bocchi, V. Comptes Rendus 1968, C267,
433.
16. Chiang, C. K.; Fincher, C. R.; Park, Y. W.; Heeger, A. J.; Shirakawa, H.; Louis,
E.J.; Gau, S. C.; MacDiarmid, A. G. Phys. Rev. Lett. 1977, 39, 1089.
17. Genies, W. M.; Bidan, G.; Diaz, A. J. Electroanal. Chem. 1983, 149, 101.
18. Burn, P. L.; Holmes, A. B.; Kraft, A.; Bradley, D. D. C. Brown, A. R.; Friend, R.
H.; Gymer, R. W. Nature 1992, 356, 47.
19. Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Science 1992, 258, 1474.
20. Hadziioannou, G.; van Hutten, P. F. (Eds) Semiconducting polymer: chemistry,
physics and engineering, Weinheim; Wiley-VCH, 2000.
21. Forrest, S. R. Nature 2004, 428, 911.
22. Shirakawa, H. Angew. Chem. Int. Ed. Eng. 2001, 40, 2574.
23. MacDiarmid, A. G. Angew. Chem. Int. Ed. Eng. 2001, 40, 2581.
24. Heeger, A. J. Angew. Chem. Int. Ed. Eng. 2001, 40, 2591.
25. Ivory, D. M.; Miller, G. G.; Sowa, J. M.; Shacklette, L. W.; Chance, R. R.;
Baughman, R. H. J. Chem. Phys. 1979, 71, 1506.
26. Schlüter, A. D. Handbook of Conducting polymers, (Eds. Skotheim, T. A.,
Elsenbaumer, R. L., Reynolds, J. R.) Marcel Dekker, Inc., 1998, 209.
58
Li Hairong National University of Singapore
27. Kovacic, P.; Marchionna, V. J.; Koch, F. W.; Oziomek, J. J. Am. Chem. Soc. 1966,
31, 2467.
28. Schlüter, A. D. J. Poly. Sci. Part A Poly. Chem. 2001, 39, 1533.
29. Skotheim T. A.; Elsenbaumer R. L.; Reynolds J. R. Handbook of conducting
polymers 2 , New York, Marcel Dekker Inc., 1998. ed
Ullmann, F.; Bielecki, J. Chemische Berichte 1901, 34, 2174. 30.
Grignard, V. Compt. Rend. 1900, 130, 1322.31.
32. Kanbara, T.; Saito, N.; Yamamoto, T.; Kubota, K. Macromolecules 1991, 24,
5883.
33. Rehahn, M.; Schlüter, A. –D.; Wegner, G. Feast, W. J. Polymer 1989, 30, 1054.
34. Yamamoto, T.; Morita, A.; Miyazaki, Y.; Maruyama, T.; Wakayama, H.; Zhou, Z.;
Nakumura, Y.; Kanbara, T.; Sasaki, S.; Kubota, K. Macromolecules 1992, 25,
1214.
35. Percec, V.; Okita, S.; Weiss, R. Macromolecules 1992, 25, 1816.
36. Miyaura, N.; Suzuki, A. Chem. Commun. 1979, 866.
37. Mizoroki, T.; Mori, K.; Ozaki, A. Bull. Chem. Soc. Jap. 1971, 44, 581.
Sonogashira, K.; Tohda, Y.; Hagihara, N. . 1975, 16, 4467. Tetrahedron Lett38.
39. Milstein, D.; Stille, J. K. J. Am. Chem. Soc. 1978, 100, 3636.
40. King, A. O., Okukado, N.; Negishi, E. I. Chem. Comm. 1977, 683.
41. Thorn-Csany E. and Kraxner P., Macromol. Chem. Phys., 1997, 198, 3827.
42. Chaturvedi, V.; Tanaka, S.; Kaeriyama, K. Macromolecules 1993, 26, 2607.
43. Marvel, C. S.; Hartzell, G. E. J. Am Chem. Soc. 1959, 81, 448.
44. Ballard, D, G. H.; Courtis, A.; Shirley, I. M.; Tailor, S. C. J. Chem. Soc., Chem.
Comm. 1983, 954.
59
Linear and Cross-conjugated Poly(p-phenylene)s and Oligo(p-phenylene)s---Introduction
45. Scherf, U.; Müllen, K. Macromolecules, 1992, 25, 3546.
46. Forster, M.; Scherf, U. Macromol. Rapid. Commin. 2000, 21, 810.
47. Chmil, K.; Scherf, U. Acta Polym. 1997, 48, 208.
48. Lambda, J. J. S.; Tour, J. M. J. Am. Chem. Soc. 1994, 116, 11723.
49. Aksimentyeva, O. L. ; Bodnaryuk-Lupshak, N. Revue Roumaine de Chimie 2002,
46, 175.
50. Fu, M.; Chen, F.; Zhang, J.; Shi, G. J. Mater. Chem. 2002, 12, 2331.
51. Geetha, S.; Trivedi, D. C. Synth. Met. 2005, 148, 187.
52. Miyakoshi, R.; Shimono, K.; Yokoyama, A.; Yokozawa, T. J. Am. Chem. Soc.
2006, 128, 16012
53. Tour, J. M., Adv. Mater. 1994, 6, 190.
nd54. Robinson, J. W. Atomic spectroscopy New York: Marcel Dekker, 2 ed. 1996,
p65.
55. Higson, Séamus P. J. Analytical chemistry Oxford: Oxford University Press, 2003,
p105.
56. Fleming, G. R.; Knight, A. W.; Morris, J. M.; Morrison, R. J. S.; Robinson, G. W.
J. Am. Chem. Soc. 1977, 99, 4306.
57. Dreeskamp, H.; Koch, E.; Zander, M. Chem. Phys. Lett. 1975, 31, 251.
58. Vahlenkamp, T.; Wegner, G. Macromol Chem Phys. 1994, 195, 1933. (e) Huber,
J.; Scherf, U. Macromol Rapid Commun. 1994, 15, 879.
59. Remmers, M.; Schulze, M.; Wegner, G. Macromol Rapid Commun. 1996, 17, 239.
60. Vahlenkamp, T.; Wegner, G. Macromol Chem Phys. 1994, 195, 1933.
61. Remmers, M.; Müller, B.; Martin, K.; Räder, H. J. Macromolecules 1999, 32,
1073.
60
Li Hairong National University of Singapore
62. Fiesel, R.; Scherf, U. Acta. Poly. 1998, 49, 445.
63. Fiesel, R.; Huber, J.; Apel, U.; Enkelmann, U.; Hentschke, R.; Scherf, U. Macro.
Chem. Phys. 1997, 198, 2663.
64. Fiesel, R.; Huber, J.; Scherf, U. Angew. Chem. Int. Ed. Engl. 1996, 35, 2111.
65. Huber, J.; Scherf, U. Macromol Rapid Commun. 1994, 15, 879.
66. Lightowler, S.; Hird, M.; Chem. Mater. 2004, 16, 3693.
67. Shin, S. H.; Park, J. S.; Park, J. W.; Kim, H. K. Synth. Met. 1999, 102, 1060.
68. Na, J. G.; Kim, Y. S.; Park, W. H.; Lee, T. S. J. Poly. Sci. A. Poly. Chem. 2004, 42,
2444.
69. Ng, S. C.; Lu, H. F.; Chan, H. S. O.; Fujii, T. L.; Yoshino, K. Macromolecules
2001, 34, 6895.
70. Xu, M. H.; Zhang, H. C.; Pu, L. Macromolecules 2003, 36, 2689.
71. Aubert, P. H.; Knipper, M.; Growenendaal, L.; Lutsen, L.; Manca, J.;
Vanderzande, D. Macromolecules 2004, 37, 4087.
72. Monkman, A. P.; Pälsson, L. O.; Higgins, R. W. T.; Wang, C.; Bryce, M. R.;
Batsanov, A. S.; Howard, J. A. K. J. Am. Chem. Soc. 2002, 124, 6049.
73. Wang, C.; Kilitziraki, M.; MacBride, J. A. H.; Bryce, M. R.; Horsburgh, L. E.;
Sheridam, A. K.; Monkman, A. P.; Samuel, I. D. W. Adv. Mater. 2000, 12, 217.
74. Pälsson, L. O.; Wang, C.; Monkman, A. P.; Bryce, M. R.; Rumbles, G.; Samuel, I.
D. Synth. Met. 2001, 199, 627.
75. Lee, J. G.; Cho, H. J.; Cho, N. S.; Hwang, D. H.; Kang, J. M.; Lim, E.; Lee, J. I.;
Shim, H. K. J. Poly. Sci. A. Poly. Chem. 2006, 44, 2943.
76. Hayashi, H.; Yamamoto, T.; Sakamoto, M.; Iida, K.; Nakao, H.; Hirano, K.;
Kawaguchi, M.; Hibino, T.; Takenaka, S.; Hattori, T. Polymer, J. 2005, 37, 52.
61
Linear and Cross-conjugated Poly(p-phenylene)s and Oligo(p-phenylene)s---Introduction
77. Tu, G. L.; Zhange, Y. G.; Su, G. P.; Cheng, Y. X.; Ma, D. G.; Wang, L. X.; Jing,
X. B.; Wang, F. S. Synth. Met. 2003, 137, 1117.
78. Beyerlein, T.; Tieke, B.; Lenger, S. F.; Brütting, W. Synth. Met. 2002, 130, 115.
79. Kato, S. I.; Takagi, K.; Suzuki, K.; Kinoshita, T.; Yuki, Y. J. Poly. Sci. A. Poly.
Chem. 2004, 42, 2631.
80. Yasuda, T.; Yamaguchi, I.; Yamamoto, T. Adv. Mater. 2003, 15, 293.
81. Shin, D. C.; Ahn, J. H.; Kim, Y. H.; Kwon, S. K. J. Poly. Sci. A. Poly. Chem. 2000,
33, 3086.
82. Wang, C.; Kilitziraki, M.; Pälsson, L. O.; Bryce, M. R.; Monkman, A. P.; Samuel,
I. D. W. Adv. Fun. Mater. 2001, 11, 47.
83. Remmers, M.; Neher, D.; Wegner, G. Macromol. Chem. Phys. 1997, 198, 2551.
84. Vetrichelvan. M.; Nagarajan, S.; Valiyaveettil, S. Macromolecules 2006, 39, 8303.
85. Fang, Q.; Jiang, B.; Xu, B.; Cao, A. Macro. Rapid. Commun. 2004, 25, 1856.
86. Ren, S.; Fang, Q.; Yu, F.; Bu, D. J. Poly. Sci. A. Poly. Chem. 2005, 43, 6554.
87. Nishioka, N.; Takagi, K.; Kinoshita, T.; Kunisada, H.; Yuki, Y. J. Poly. Sci. A.
Poly. Chem. 2004, 42, 1208.
88. Koeckelberghs, G.; Sioncke, S.; Verbiest, T.; Persoons, A.; Samyn, C. Polymer
2003, 44, 3785.
89. Wu, X.; Liu, Y.; Zhu, D. Synth. Met. 2001, 121, 1699.
90. Yu, H. B.; Zheng, X. F.; Lin, Z. M.; Hu, Q. S.; Huang, W. S.; Pu, L. J. Org. Chem.
1999, 64, 8149.
91. Hu, Q. S.; Huang, W. S.; Vithrana, D.; Zheng, X. F.; Pu, L. J. Am. Chem. Soc.
1997, 119, 12454.
92. Yasuda, T.; Imase, T.; Yamamoto, T. Macromolecules 2005, 38, 7378.
62
Li Hairong National University of Singapore
93. Crawford, K. B.; Goldfinger, M. B.; Swager, T. M. J. Am. Chem. Soc. 1998, 120,
5187.
94. Chen, Y. C.; Huang, G. S.; Hsiao, C. C.; Chen, S. A. J. Am. Chem. Soc. 2006, 128,
8549.
95. Economopoulos, S. P.; Andreopoulou, A. K.; Gregoriou, V. G.; Kallitsis, J. K. J.
Macromol. Sci. A. Pure and App. Chem. 2006, 43, 977.
96. Lightowler, S.; Hird, M.; Chem. Mater. 2004, 16, 3693.
97. Tamura, H.; Watanabe, T.; Imanishi, K.; Sawada. Synth. Met. 1999, 107, 19.
98. Fáber, R.; Staśko, A.; Nuyken, O. Macromol. Chem. Phys. 2001, 202, 2321.
99. Fáber, R.; Staśko, A.; Nuyken, O. Macromol. Chem. Phys. 2000, 201, 2257.
100. Hinderberger, D.; Schmelz, O.; Rehahn, M.; Jeschke, G. Angew. Chem. Int. Ed.
Eng. 2004, 43, 4616.
101. Jayakannan, M.; Hal, P. A. V.; Janssen, R. A. J. J. Poly. Sci. A. Poly. Chem. 2002,
40, 2360.
102. Mikroyannidis, J. A.; Persephonis, P. G.; Giannetas, V. G. Synth. Met. 2005, 148,
293.
103. Lee, J. K.; Lee, T. S. J. Poly. Sci. A. Poly. Chem. 2005, 43, 1397.
104. Zhou, G.; Cheng, Y.; Wang, L.; Jing, X.; Wang, F. Macromolecules 2005, 38,
2148.
105. Bouachrine, M.; Lere-Porte, J. P.; Moreau, J. J. E.; Serein-Spirau, F.; Torreilles, C.
J. Mater. Chem. 2000, 10, 263.
106. Wallow, T. I.; Novak, B. M. J. Am. Chem. Soc. 1991, 113, 7411.
107. Rau, I. U.; Rehahn, M. Polymer. 1993, 34, 2889.
63
Linear and Cross-conjugated Poly(p-phenylene)s and Oligo(p-phenylene)s---Introduction
108. Brodowski, G.; Horvath, A.; Ballauff, M.; Rehahn, M. Macromolecules 1996, 29,
6962.
109. Wittemann, M.; Rehahn, M. Chem. Commun. 1998, 623.
110. Child, A. D.; Reynolds, J. R. Macromolecules 1994, 27, 1975.
111. Vetrichelvan, M.; Li, H. R.; Renu, R.; Valiyaveettil, S. J. Poly. Sci. A. Poly. Chem.
2006, 44, 3763.
112. Lauter, U.; Meyer, W. H.; Wegner, G. Macromolecules 1997, 30, 2092.
113. Liu, B.; Yu, W. L.; Lai, Y. H.; Huang. W. Chem. Commun. 2000, 551.
114. Balanda, P. B.; Ramey, M. B.; Reynolds, J. R. Macromolecules 1999, 32, 3970.
115. Xue, C.; Jog, S. P.; Murthy, P.; Liu, H. Biomacromolecules 2006, 7, 2470.
116. Xue, C.; Donuru, V. R. R.; Liu, H. Macromolecules 2006, 39, 5747.
117. Baskar, C. ; Lai, Y. H.; Valiyaveettil, S. Macromolecules 2001, 34, 6255.
118. Renu, R.; Ajikumar, P. K.; Nurmawati, B. M. H.; Knoll, W.; Valiyaveettil, S.
Chem. Mater. 2006, 18, 1213.
119. Fütterer, T.; Hellweg, T.; Findenegg, G. H.; Frahn, J.; Schlüter, A. D.; Böttcher, C.
Langmuir 2003, 19, 6537.
120. Zhange, C.; Schlaad, H.; Schluter, A. D. J. Poly. Sci. A. Poly. Chem. 2003, 41,
2879.
121. Chaturvedi, V.; Tanaka, S.; Kaeriyama, K. Macromolecules 1993, 26, 2607.
122. Vanhee, S.; Rulkens, R.; Lehmann, U.; Rosenauer, C.; Schulze, M.; Köhler, W.;
Wegner, G. Macromolecules 1996, 29, 5136.
123. Nakazawa, M.; Han, Y. K.; Fu, H.; Matsuoka, S.; Kwei, T. K.; Okamoto, Y.
Macromolecules 2001, 34, 5975.
64
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124. Ninivin, C. L.; Longeau, A. B.; Demattei, D.; Palmas, P.; Saillard, J.; Coutanceau,
C.; Lamy, C.; Léger, J. M. J. App. Poly. Sci. 2006, 101, 944.
125. Kandre, R. M.; Kutzner, F.; Schlaad, H.; Schlüter, A. D. Macro. Rapid.
Commun. 2005, 206, 1610.
126. Vetrichelvan, M.; Valiyaveettil, S. Chem. Eur. J. 2005, 11, 5889.
127. Vetrichelvan, M.; Valiyaveettil, S. Submitted.
128. Vetrichelvan, M.; Loan, N.; Valiyaveettil, S. Submitted.
129. Yasuda. T; Yamamoto, T. Macromolecules 2003, 36, 7513.
130. Pei, J.; Liu, X. L.; Yu, W. L.; Lai, Y. H.; Niu, Y. H.; Cao, Y. Macromolecules
2002, 35, 7274.
131. Phelan, N. F.; Orchin, M. J. Chem. Educ. 1968, 45, 633.
132. Swager, T. M.; Grubbs, R. H. J. Am . Chem. Soc. 1987, 109, 894.
133. Eilser, S.; Tykwinski, R. R. Angew. Chem., Int. Ed. 1999, 38, 1940.
134. Boldi, A. M.; Anthony, J.; Gramlich, V.; Knobler, C. B.; Boudon, C.; Gisselbrecht,
J. –P; Gross, M.; Diederich, F. Helv. Chim. Acta 1995, 78, 779.
135. Martin, R. E.; Diederich, F. Angew. Chem. Int. Ed. 1999, 38, 1350.
136. Amaresh, R. R.; Liu, D.; Konovalova, T.; Lakshmikantham, M. V. Cava, M. P.;
Kispert, L. D. J. Org. Chem. 2001, 66, 7757.
137. Naraso Nishida, J.; Ando, S.; Yamaguchi, J.; Itaka, K.; Koinuma, H.; Tada, H.;
Tokito, S.; Yamashita, Y. J. Am. Chem. Soc. 2005, 127, 10142.
138. Weltin, E.; Gerson, F.; Murrell, J. N.; Heilbronner, E. Helv. Chim. Acta 1961, 44,
1400.
139. Boldi, A. M.; Diederich, F. Angew. Chem., Int. Ed. Engl. 1994, 33. 468.
140. Faust, R.; Diederich, F.; Gramlich, V.; Seiler, P. Chem. -Eur. J. 1995, 1, 111.
65
Linear and Cross-conjugated Poly(p-phenylene)s and Oligo(p-phenylene)s---Introduction
141. Campbell, K.; McDonald, R.; Tykwinski, R. R. J. Org. Chem. 2002, 67, 1133.
142. Gholami, M.; Tykwinski, R. R. Chem. Rev. 2006, 106, 4997.
143. Friend, R. H.; Gymer, R. W.; Homes, A. B.; Burroughes, J. H.; Marks, R. N.;
Taliani, C.; Bradley, D. D. C.; Dos Santos, D. A. Nature, 1999, 397, 121.
144. Peng, Z.; Galvin, M. E. Acta Polymerica 2001, 49, 244.
145. Oldham, W. J.; Lachicotte, R. J.; Bazan, G. J. Am. Chem. Soc. 1998, 120, 2987.
146. Pillow, J. N. G.; Burn, P. L.; Samuel, I. D. W.; Halim, M. Synth. Met. 1999, 102,
1468.
147. Deb, S. K.; Maddux, T. M.; Yu, L. P. J. Am. Chem. Soc. 1997, 119, 9079.
148. Niazimbetova, Z. I.; Christian, H. Y.; Bhandari, Y. J.; Beyer, F. L.; Galvin, M. E.
J. Phys. Chem. B 2004, 108, 8673.
149. Fratilou, S.; Senthilkumar, K.; Grozema, F. C.; Christian-Pandya, H.;
Niazimbetova, Z. I.; Bjandari, Y. J.; Galvin, M. E.; Siebbeles, L. D. A. Chem.
Mater. 2006, 18, 2118.
150. Wilson, J. N.; Josowicz, M.; Wang, Y. Q.; Bunz, U. H. F. Chem. Commun. 2004,
24, 2962.
151. Zucchero, A. J.; Wilson, J. N.; Bunz, U. H. F. J. Am. Chem. Soc. 2006, 128, 11872.
152. Zucchero, A. J.; Wilson, J. N.; South, C. R.; Bunz, U. H. F.; Weck, M. Chem.
Commun. 2006, 20, 2141.
153. Wilson, J. N.; Smith, M. D.; Enkelmann, V.; Bunz, U. H. F. Chem. Commun. 2004,
15, 1700.
154. Wilson, J. N.; Bunz, U. H. F. J. Am. Chem. Soc. 2005, 127, 4124.
155. Tolosa, J.; Zucchero, A. J.; Bunz, U. H. F. J. Am. Chem. Soc. 2008, 130, 6498.
156. Grunder, S.; Huber, R.; Horhoiu, V.; Gonzalez, M. T.; Schonenberger, C.; Calame,
66
Li Hairong National University of Singapore
M.; Mayor, M. J. Org. Chem. 2007, 72, 8337.
157. Duan, Y.; Zhao, Y.; Chen, P.; Li, J.; Liu, S.; He, F.; Ma, Y. G. Appl. Phys. Lett.
2006, 88, 263503.
158. Schreiber, F. Prog. Surf. Sci. 2000, 65, 151.
159. Ulman, A. Acc. Chem. Res. 2001, 34, 855.
160. Klare, J. E.; Tulevski, G. S.; Nuckolls, C. Langmuir, 2004, 20, 10068.
161. Vestergaard, M.; Jennum, K.; Sorensen, J. K. Kilsa, K.; Nielsen, M. B. J. Org.
Chem. 2008, 73, 3175.
162. Kim, K. H.; Chi, Z.; Cho, M. J.; Jin, J.-I.; Cho, M. Y.; Kim, S. J.; Joo, J. S.; Choi,
D. H. Synth. Met. 2007, 157, 497.
163. Diez-Barra, E.; Garcia-Martinez, J. C.; Rodriguez-Lopez, J. J. Org. Chem. 2003,
68, 832.
164. Xu, B. B.; Pan, Y. C.; Zhang, J. H.; Peng, Z. H. Synth. Met. 2000, 114, 337.
165. Wilson, J. N.; Windscheif, P. M.; Evans, U.; Myrick, M. L.; Bunz, U. H. F.
Macromolecules, 2002, 35, 8681.
166. Reisch, H.; Wiesler, U.; Scherf, U.; Tuytuylkov, N. Macromolecules 1996, 29,
8204.
167. Preis, E.; Shcerf, U. Macromol. Rapid Commum. 2006, 27, 1105.
168. Huang, F.; Tian, Y.; Chen, C. Y.; Cheng, Y. J.; Yong, A. C.; Yen, A. K. Y. J. Phys.
Chem. C 2007, 111, 10673.
169. Wang, H. Y.; Feng, J. C.; Wen, G. A.; Jiang, H. J.; Wan, J. H.; Zhu, R.; Wang, C.
M.; Wei, W.; Huang, W. New J. Chem. 2006, 30, 667.
170. Dierschke, F.; Müllen, K. Macromol. Chem. Phys. 2007, 208, 37.
67
Li Hairong National University of Singapore
Vetrichelvan, M.; Li, Hairong; Valiyaveettil, S. Poly. Mater. Sci. Eng. 2005, 46, 104. Vetrichelvan, M.; Li, Hairong; Renu, R.; Valiyaveettil, S. J. Poly. Sci.: Part A: Poly. Chem. 2006, 44, 3763. Sindhu, S.; Jegadesan, S.; Li, Hairong; Vetrichelvan, M.; Valiyaveettil, S. Adv. Funct. Mater. 2007, 17, 1698.
CChhaapptteerr 22
SSyynntthheessiiss aanndd CCoommppaarriissoonn ooff SSttrruuccttuurree--PPrrooppeerrttyy RReellaattiioonnsshhiipp ooff SSyymmmmeettrriicc aanndd AAssyymmmmtteerriicc
WWaatteerr SSoolluubbllee PPoollyy ((pp--pphheennyylleenneess))
68
Synthesis and Comparison of Structure-Property Relationship of Symmetric and Asymmteric Water Soluble PPPs
2.1. Introduction
Recently, water-soluble conjugated polymers (WSCP) have attracted much attention
owing to their applications towards chemical, ionic and biosensors.1-10 The sensoring
principle is based on the photon induced fast electron transfer to the trapping site formed
by electrostatic complexation between polymer and analyte, whereby the excitation is
effectively deactivated, thus the polymer fluorescence is quenched.11 A key advantage of
CP-based sensors over small molecules is the potential of the CP to exhibit additive
properties that are sensitive to very tiny perturbations. In particular, the CP’s electron
transport properties, electrical conductivity or rate of energy migration provide amplified
sensitivity.12 CP-based sensors have been fabricated based on a few different signal
transducers such as conductometric sensors, potentinometric sensor, colorimetric sensors
and fluorescence sensors. Fluorescence is a widely used and rapidly expanding method in
sensing. This method offers diverse transduction schemes based upon changes in
intensity, energy transfer, wavelength (excitation and emission), and lifetime. There are
advantages to using CPs in fluorescent sensory schemes due to amplification resulting
from efficient energy migration. A greatly enhanced sensitivity of the CPs over the
monomeric compounds is originated from the fact that complete quenching doesn’t
require all the acceptors on CPs to be occupied by analytes, as opposite to the small
molecules.13,14 The combination of the amplification and sensitivity in CP-based sensors
is evolving to produce new systems of unparalleled sensitivity. Owing to their
amphipolar nature (hydrophobic backbone and water soluble side chain), it is also
interesting to explore the aggregation behavior in aqueous solution.
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Li Hairong National University of Singapore
2.2. Experimental
2.2.1. Measurements
All reactions were carried out with dry, freshly distilled solvents under anhydrous
conditions or in an inert atmosphere. The NMR spectra were collected on a Bruker ACF
300 spectrometer in chloroform-d, D2O or DMSO-d6 as solvents and tetramethylsilane as
internal standard. Snake skin pleated dialysis tubing with MW cutoff value of 3500 was
purchased from Pierce. FT-IR spectra were recorded on a Bio-Rad FTS 165
spectrophotometer using KBr matrix. UV-Vis spectra were measured on a Shimadzu
3101 PC spectrophotometer and fluorescence measurements were carried out on a RF-
5301PC Shimadzu spectrofluorophotometer, the excitation width and emission width
were all set to 1.5 nm. Thermogravimetric analyses were carried out using TA
instruments SDT 2960 at a heating rate of 10 °C/min up to 1000 ºC under nitrogen
atmosphere. Elemental analyses were performed using a Perkin-Elmer CHNS
autoanalyzer. Gel permeation chromatography (GPC) was used to obtain the molecular
weight of the polymers with reference to polystyrene standards and THF or DMF as
eluant with flow rate of 0.8 mL/min. Phenogel 10 linear columns (S/N GP3673, GP42637)
were used and hyphenated with a Waters 410 differential refractometer and Waters 515
HPLC pump. X-ray powder diffraction patterns were obtained using a D5005 Siemens X-
ray diffractometer with Cu-Kα (1.54 Å) radiation (40 kV, 40 mA). Samples were
mounted on a sample holder and scanned between 2θ = 1.5° to 40° with a step size of 2θ
= 0.02°. Dynamic light scattering (DLS) studies were carried out with a 5-watt 514 nm
argon ion laser (Brookhaven Instruments) and a BI-200SM goniometer coupled to BI-
70
Synthesis and Comparison of Structure-Property Relationship of Symmetric and Asymmteric Water Soluble PPPs
9000AT digital correlators. The power of the laser was varied from 50 to 500 mW
depending upon the polymer concentration. The data were collected at a scattering angle
of 90° relative to the incident laser beam with a pulse time of 0.8 μs. The refractive
indices of the solvent mixtures were adjusted prior to the DLS experiments. The polymer
concentration was kept constant at 0.625 mg/mL. The CONTIN method was used to
analyze the first order filed correlation function g(1) (τ) as determined by the DLS. AFM
(Dimension 3100, Digital Instruments) was working in tapping mode with a silicon tip
having a force constant of 40 N/m. The polymer solution in appropriate solvent mixture
was dropcasted onto the freshly cleaved mica surface and dried in air. All measurements
were performed under ambient conditions (humidity 74 %, 23 °C) with the scan rate of 1
Hz, and images were obtained in the topographic mode. To eliminate imaging artifacts,
the scan directions were varied to ensure that a true image was obtained. Images were
obtained from at least five macroscopically separated areas on each sample. All images
were processed using procedures for plane fitting and flattening using the Nanoscope IV
software version 5.0 (Digital Instruments, Santa Barbara, CA).
2.2.2. Synthesis
The target polymers are shown in Figure 2.1. Synthetic details for the monomers
and polymers P1-P3 are given in Scheme 2.1. 2,5-dibromohydroquinone (2), 2,5-
dibromo-1,4-dialkoxybenzene (3), 1,4-dialkoxyphenyl-2,5-bis(boronic acid) (4), 1,4-
dibromo-2,5-bis(3-sulfonatopropoxy) benzene sodium salt (5), precursor polymers P6
and P7 were synthesized using a standard procedure reported in the literature.15-19
Polymers P4 and P5 were prepared using a route shown in Scheme 2.2.
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Li Hairong National University of Singapore
General procedure for polymerization of P1, P2 and P3. Dialkoxyboronic acid, 4
(1.0 g, 2.73 mmol), disulfonated compound, 5 (0.76 g, 2.73 mmol) were mixed in 35 mL
of DMF. 50 mL of Na2CO3 solution (2M) was added to the mixture followed by
degassing for about 8 hrs. After adding the catalyst [tetrakis(triphenylphosphino)
palladium(0), 3 mol% with respect to monomers], the mixture was heated up to 85 °C for
72 hrs, concentrated under reduced pressure, dissolved in H2O, dialyzed for 48 hrs with a
3500 g mol-1 cutoff membrane and vacuum dried to get a tan solid with an yield of 60 %.
P1, Yield: 68 %. 1H NMR (DMSO, δppm): 7.06 (b, H-Ar), 3.93 (b, PhOCH2-), 2.87 (b,
PhOCH2-SO3Na), 2.04 (b, -CH2CH2SO3Na), 1.57 (b, -(CH2)3CH2), 1.16 (b, -CH2CH3 ),
0.87 (b, -CH3). 13C NMR (DMSO, δppm): 147.5, 121.4, 106.8, 68.6, 48.0, 30.9, 28.9,
25.1, 22.0, 13, 8. FT-IR (KBr, cm-1): 2932, 2867, 2352, 1630, 1470, 1388, 1210, 1044,
796, 730, 612. Mn = 4764, Mw = 4793, PDI = 1.01.
P2, Yield: 61 %. 1H NMR (DMSO, δppm): 7.04 (b, H-Ar), 3.92 (b, PhOCH2-), 2.81 (b,
PhOCH2-SO3Na), 1.97 (b, -CH2CH2SO3Na), 1.55 (b, -(CH2)10CH2), 1.14 (b, CH2CH3)
0.80 (b, -CH3). 13C NMR (DMSO, δppm): 149.5, 118.2, 110.5, 68.8, 47.8, 30.6, 29.0,
25.5, 22.5, 14.0. FT-IR (KBr, cm-1): 2932, 2878, 2323, 1624, 1494, 1192, 1054, 802, 623.
Mn = 7227, Mw = 7720, PDI = 1.07.
P3, Yield: 63 %. 1H NMR (DMSO, δppm): 7.23 (b, Ar-H), 7.06 (b, aromatic), 4.97 (b, -
OCH2Ph), 3.92 (b, -PhOCH2CH2-SO3Na), 2.51 (b, CH2CH2SO3Na), 1.97 (b,
CH2CH2SO3Na). 13C NMR (DMSO, δppm): 147.1, 128.1, 119.2, 113.2, 68.8, 48.4, 26.9.
FT-IR (KBr, cm-1): 3068, 2920, 2855, 2364, 1635, 1482, 1441, 1163, 1048, 789, 742, 689.
Mn = 9878, Mw = 11955, PDI = 1.20.
72
Synthesis and Comparison of Structure-Property Relationship of Symmetric and Asymmteric Water Soluble PPPs
OR
ROO
O
n
P1, R = -C6H13P2, R = -C12H25P3, R = -CH2C6H5
O
RORO
O
n
P4, R = -C6H13P5, R = -C12H25
SO3Na
NaO3S
SO3Na SO3Na
Figure 2.1. Molecular structure of the polymers
OH
HO
OH
HO
Br Br
OR
RO
Br Br(ii)(i)
1
2 3
4
(iii)(iv)
OR
RO
(HO)2B B(OH)2
O(CH2)3SO3Na
NaO3S(H2C)3O
Br Br
RO
ORO
O
SO3Na
NaO3S
n
5
(v)
P1, R = CH3(CH2)5-
P2, R = CH3(CH2)11-
P3, R = C6H5CH2-
Scheme 2.1. Synthesis of polymers P1-P3: (i) Br2/AcOH, 80 %; (ii) NaOH in abs. EtOH, RBr, 45-50° C, 10 h, 65 %; (iii) 1.6 M soln of n-BuLi in hexane, THF at –78 °C, tri-isopropyl borate, stirred at RT for 10 h, 60 %; (iv) NaOH, EtOH, 1,3-propane sultone, 24 h (v) 2M Na2CO3 solution, DMF, 3.0 mol % Pd(PPh3)4, reflux for 72 h.
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Li Hairong National University of Singapore
OR
HO
n
OR
NaO3S(H2C)3On
(i)
P4, R = CH3(CH2)5-
P5, R = CH3(CH2)11-
P6, R = CH3(CH2)5-
P7, R = CH3(CH2)11-
Scheme 2.2. Synthesis of polymers P4 and P5: (i) NaOH in EtOH, 1,3-propane sultone,
50° C, 24 h.
General synthetic procedure for the polymer P4 and P5. The precursor polymer
P6 (Mn = 10,400, Mw = 23,500) was dissolved in 20 mL of THF and stirred with the
excess ethanolic solution of NaOH. About 10 equivalents of 1,3-propanesultone in 20 mL
dioxane was added at 50 °C and the resultant solution was stirred for about 24 hrs and
concentrated, the solid product was extracted with water, purified using semi-permeable
membrane to give the polymer P4. Similar procedure was also used for the synthesis of
polymer P5 from precursor polymer P7 (Mn = 5543, Mw = 7248). Molecular weight of
the polymers P4 and P5 could not be determined due to strong aggregation in solution.
P4, Yield: 71 %. 1H NMR (DMSO, δppm): 7.01 (b, Ar-H), 4.03 (b, PhOCH2-), 3.45 (m, -
CH2SO3Na), 2.54 (m, -CH2CH2SO3Na), 1.81 (b, PhOCH2CH2-), 1.24 (b, -(CH2)3CH3),
0.83 (b, -CH3). FT-IR (KBr, cm-1): 2957, 2866, 2748, 1647, 1469, 1382, 1207, 1105,
1040, 932, 795, 736, 613, 527.
P5, Yield: 68 %. 1H NMR (DMSO, δppm): 6.86 (b, Ar-H), 3.66 (b, PhOCH2-), 3.42 (b,
CH2SO3Na), 2.04 (b, -CH2CH2SO3Na), 1.69 (b, PhOCH2CH2-), 1.15 (b, -(CH2)9CH3),
0.74 (b, -CH3). FT-IR (KBr, cm-1): 2921, 2852, 1638, 1492, 1469, 1378, 1204, 1042, 866,
796, 721, 611, 529.
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Synthesis and Comparison of Structure-Property Relationship of Symmetric and Asymmteric Water Soluble PPPs
2.3. Results and discussion
2.3.1. Characterization of polymers
All polymers [P1-P5] were characterized using 1H-NMR, 13C-NMR, FT-IR, GPC,
TGA and X-ray diffraction studies. The molecular weights of the water soluble
conjugated polymers, especially P4 and P5 were difficult to evaluate due to strong
aggregation and polyelectrolyte nature.18 The polymers appeared to adhere to the GPC
column and did not elute after a long time. A few research groups8,19 reported that the
GPC results using polystyrene standards might not be fully reliable due to the higher
rigidity of the PPP backbone, presence of polar functional groups and aggregation of the
polymers in solution. Besides, monomers’s solubilities in organic and aqueous phases
also affect the coupling. All polymers gave broad signals in the 1H-NMR spectra. For P1,
the peaks at 7.06 ppm for phenylene protons, 3.93, 2.87, 2.04 ppm for the –CH2CH2SO3-
− groups and 1.57, 1.16, 0.87 ppm for the dialkyl chain groups were observed. In 13C-
NMR spectrum, there were three distinct sets of broad peaks; 147.5, 121.4 and 106.8 ppm
for phenylene carbons, 68.6, 48.0 and 25.1 ppm for propoxysulfonate carbons and 30.9,
28.9, 22.0 and 13.8 ppm for the long alkyl chain groups. In the FT-IR spectra, the
symmetric S=O stretching of the sulfonate groups for the polymers P1 - P5 were
observed at 1044, 1054, 1048, 1039 and 1042 cm-1, and the asymmetric S=O stretching
of the sulfonate groups for the polymers were observed at 1210, 1192, 1163, 1207 and
1204 cm-1, respectively. The aromatic C=C and C-H stretching were observed at 1630-
1640 and 3032-3050 cm-1. The obtained polymers are soluble in water in the
concentration range of 20-25 mg/mL.
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Li Hairong National University of Singapore
2.3.2. Thermal properties
The thermal properties of the polymers were investigated by thermo gravimetric
analyses at a heating rate of 10 °C/min under nitrogen atmosphere. Polymers P1-P3
showed good thermal stability and the onset degradation temperatures in the range of 250
– 271 °C. This decomposition corresponds to the degradation of the alkyl groups on the
polymer backbone. Second onset temperatures at 400–500 °C correspond to the
decomposition of the polymer backbone. Similarly for polymers P4 and P5, the first and
second decompositions start at 200 and 400 °C, which correspond to the decomposition
of alkyl and the sulfonatopropoxy chains, respectively.
2.3.3. Optical properties
The absorption and emission properties of the sodium salts of the polymers P1 - P5
in water are summarized in Table 2.1. The absorption spectra of the polymers are shown
in Figure 2.2-A. The optical properties of the polymers in water solution were dependent
on the side chains of the polymers. The absorption spectra of polymers showed maxima
(λmax) located in the range of 296 – 335 nm, indicating that the differences in the side
chain contribute much to the overall planarization of the polymer backbone. The λmax for
P1, P2 and P3 are 305 nm, 296 nm and 299 nm, respectively. In the case of polymers
with hexyloxy (P1) and benzyloxy (P3) side chains, a higher λmax was observed as
compared to polymer (P2) with longer dodecyloxy side chain. This might be due to
disorder created by the dodecyloxy group (e.g. steric hindrance) on the polymer backbone
and thereby decreasing its planarity.
76
Synthesis and Comparison of Structure-Property Relationship of Symmetric and Asymmteric Water Soluble PPPs
Table 2.1. Absorption and emission wavelength for the polymers P1-P5 in water
Polymer λmax (nm)
εmax (x 104 cm-1)
Band Gap (eV)
λem (nm)
P1 305 1.75 3.5 398 P2 296 2.63 3.87 361 P3 299 1.71 3.47 395 P4 329 1.76 3.38 399 P5 335 1.85 3.32 401
ε = absorption coefficient max
300 350 400 4500.0
0.5
1.0
P5
P4
P3P1
P2
Abs
orba
nce
Wavelength nm350 400 450 500 550
0
20
40
60
P5
P4
P3P2
P1
Inte
nsity
Wavelength nm
A B
300 350 400 4500.0
0.5
1.0
P5
P4
P3P1
P2
Abs
orba
nce
Wavelength nm350 400 450 500 550
0
20
40
60
P5
P4
P3P2
P1
Inte
nsity
Wavelength nm
A B
Figure 2.2. Absorption (A) and fluorescence (B) spectra of polymers P1 – P5 in water. Concentration of P1, P2, P3, P4 and P5 were 5.9 x 10-5 -6 -6 -6, 3.0 x 10 , 4.7 x 10 , 6.4 x 10 , and 5.4 x 10-6 M, respectively.
Conversely, in the case of the polymers P4 and P5, the λmax of polymer with short
hexyloxy group (P4) was lower (329 nm) than the polymer (P5) with longer dodecyl
group (335 nm). By comparing the two sets of water soluble polymers, the one with
asymmetric structure (P4, P5) showed higher λmax than the polymers with symmetrically
functionalized phenylene rings on the polymer backbone (P1 - P3). This could be due to
enhanced planarization of the polymer backbone of P4 and P5. A similar trend was
observed in the case of emission maxima as well. In water, all polymers were highly
77
Li Hairong National University of Singapore
fluorescent. The emission maximum (λem) for the polymers P1, P2 and P3 were 398 nm,
361 nm and 395 nm, respectively, and for the asymmetrical P4 and P5 399 nm and 401
nm, respectively. The fluorescence spectra for the polymers P1-P5 are shown in Figure
2.2-B.
2.3.4. Aggregation properties of polymers
The aggregation behavior of the polymers, P1-P5 was studied using UV-Vis and
fluorescence spectroscopy by titrating the polymer solution in water (good solvent) with a
poor solvent (THF), followed by monitoring the changes in absorption and emission
spectra.20-22 The UV-Vis spectrum of the polymer P1 in water (306 nm) and the mixture
of solvents (water/THF, 316 nm) are shown in Figure 2.3-A. By titrating with a poor
solvent (THF) for the polymer, absorption maximum shifted to longer wavelength, ∆λmax
= 10 nm after the addition of 40% THF (Figure 2.3-A). As expected, the polymers
tended to aggregate in water/THF mixture through microphase separation of polar (water)
and nonpolar (THF) groups into appropriate solvents and strong intermolecular
interactions. The value of ∆λmax was almost constant up to the addition of 60 % THF. By
using a water/THF mixture with 60 % or more THF led to a shift in the absorption
maximum to the blue region (in 70 % THF, λ = 310 nm and in 90 % THF, λmax max = 308
nm in the case of P1). This might be due to disaggregation of polymer chains to a less
ordered state. Such behavior was expected owing to the higher solubility of the
hydrophobic segments (e.g. polymer backbone and alkyl chains) of the polymers to be
more soluble in THF and the polar –SO3Na stays in the water rich regions. Similar
absorption behaviors were observed for the polymers P2 and P3. By addition of 60 %
78
Synthesis and Comparison of Structure-Property Relationship of Symmetric and Asymmteric Water Soluble PPPs
THF in the aqueous polymer solutions, the ∆λmax of ca 5 nm, 8 nm, 8 nm and 6 nm were
observed for polymers P2, P3, P4 and P5, respectively.
The fluorescence intensity of P1 increased with increase in THF content of about 50
% and then decreased (see the inset of Figure 2.3-B). As seen in the case of absorption
spectra, the aggregation could be facilitated by both ionic interactions and hydrophobic
interactions in water/THF mixture. The initial increase in fluorescence emission was due
to change in polarity of the solvent mixture as well as reorientation of the polymer chains.
Increasing the amount of THF 50 % or more, the polymer chains reorganized to form
more dispersed loosely aggregated structures in THF rich environment. This led to the
decrease in emission through dynamic quenching processes. P3 gave similar results. In
the case of polymer P2, the fluorescence intensity was increased with increase in the
amount of THF along with the appearance of multiple peak maxima (Figure 2.4). It is
anticipated that the long dodecyl groups on alternating phenylene rings create
conformational changes on the polymer backbone which leads to disorder in the lattice.
79
Li Hairong National University of Singapore
300 400 500
0
20
40
60
80
100 1,7,6,5,4,2,3
Abs
orba
nce
(a.u
)
Wavelength(nm)
0 20 40 60 80 100306308310312314316
Wav
elen
gth
(nm
)
% of THF
400 500 600
0
100
200
300
400
500
7
6
54
3
2
1
Inte
nsity
(a.u
)
Wavelength (nm)
0 20 40 60 80 100200
300
400
500
Inte
nsity
(a.u
)
% of THF
A B
300 400 500
0
20
40
60
80
100 1,7,6,5,4,2,3
Abs
orba
nce
(a.u
)
Wavelength(nm)
0 20 40 60 80 100306308310312314316
Wav
elen
gth
(nm
)
% of THF
400 500 600
0
100
200
300
400
500
7
6
54
3
2
1
Inte
nsity
(a.u
)
Wavelength (nm)
0 20 40 60 80 100200
300
400
500
Inte
nsity
(a.u
)
% of THF
400 500 600
0
100
200
300
400
500
7
6
54
3
2
1
Inte
nsity
(a.u
)
Wavelength (nm)
0 20 40 60 80 100200
300
400
500
Inte
nsity
(a.u
)
% of THF
A B
Figure 2.3. (A) UV-Vis spectra and plot of absorption maxima v.s. THF concentration (inset) for polymer P1 in water and water/THF mixtures, (1) 100 % water, (2) 20 % THF, (3) 33 % THF (4) 43 % THF (5) 50 % (6) 71 % (7) 90 % THF. (B) The fluorescence spectra of polymer P1 in water and water/THF mixtures (1) 100 % water (2) 5 % THF (3) 10 % THF (4) 46 % THF (5) 65 % THF (6) 86 % THF (7) 90 % THF. The intensity profile with the % of added THF is given in inset.
0 20 40 60
120
160
200
240
Inte
nsity
% of THF
400 500
0
200
4008
1
Inte
nsity
Wavelength (nm)
0 20 40 60
120
160
200
240
Inte
nsity
% of THF
400 500
0
200
4008
1
Inte
nsity
Wavelength (nm)
Figure 2.4. Changes in the emission spectra of P2 at different water/THF mixtures (1) 0 (2) 5 (3) 17 (4) 31 (5) 49 (6) 67 (7) 71 (8) 80 % THF. Concentration of P2 was 3.0 x 10-6
M per repeating unit. Inset: The emission intensity of P2 with the % of THF.
The aggregates from asymmetric polymers (P4, P5) seemed to be more stable in
THF (the plateau in UV-Vis in the inset of Figure 2.5) as compared to the symmetrically
80
Synthesis and Comparison of Structure-Property Relationship of Symmetric and Asymmteric Water Soluble PPPs
functionalized polymers (P1 - P3). Such high stability range may be due to their
amphiphilic structure. There was also a noticeable disaggregation of these polymers due
to increased solubility in THF/water mixture with more than 70 % THF. The fluorescence
titration data supported the aggregation-disaggregation process in water/THF mixture
(Figure 2.6). The value of λemis did not change (< 5 nm) significantly with the addition of
THF, but the emission intensity was increased initially to a maximum and decreased with
increase in concentration of THF. It is speculated that the structure might be different at
the low concentration (30%, micelles) and high concentration (70%, reverse micelle) of
THF. The decrease in emission intensity at high concentration of THF (> 80%) is due to
the precipitation of polymer from the medium and it was clearly visible at end the end of
titration.
300 400 500
1, 5, 4, 2, 3
Abso
rban
ce (N
orm
aliz
ed)
Wavelength (nm)
0 20 40 60 80
330
332
334
336
338
340
Wav
elen
gth
(nm
)
% of THF
300 400 500
1, 5, 4, 2, 3
Abso
rban
ce (N
orm
aliz
ed)
Wavelength (nm)
0 20 40 60 80
330
332
334
336
338
340
Wav
elen
gth
(nm
)
% of THF
Figure 2.5. Changes in the UV-Vis spectra of P5 at different water/THF mixtures (1) 0 (2) 20 (3) 33 (4) 50 (5) 67 % THF. Concentration of P5 was 5.4 x 10-6 M per repeating unit. Inset: Absorption maxima of P5 versus the % of THF.
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Li Hairong National University of Singapore
400 500 600
0
200
400
600
7
6
5
4
3
2
1In
tens
ity
Wavelength (nm)
0 20 40 60 80 1000
100
200
300
400
500
600
Inte
nsity
% of THF
400 500 600
0
200
400
600
7
6
5
4
3
2
1In
tens
ity
Wavelength (nm)
0 20 40 60 80 1000
100
200
300
400
500
600
Inte
nsity
% of THF
Figure 2.6. Changes in the emission spectra of P5 at different water/THF mixtures (1) 0 (2) 5 (3) 23 (4) 52 (5) 78 (6) 88 (7) 94 % THF. Concentration of P5 was 5.4 x 10-6 M per repeating unit. Inset: The emission intensity of P5 with the % of THF.
0.1 1 10 100 1000Size, nm
0
100
G(d
)
A
100% D = 94.76 nmIn Water
0.1 1 10 100 1000Size, nm
0
100
G(d
)
100% D = 178 nm25% THF
B
0.1 12.2 1500Size, nm
0
100
G(d
)
C
100% D = 655 nm50% THF
0.1 1 14 168 2000Size, nm
0
100
G(d
)
D
100% D = 770 nm75% THF
0.1 1 10 100 1000Size, nm
0
100
G(d
)
A
100% D = 94.76 nmIn Water
0.1 1 10 100 1000Size, nm
0
100
G(d
)
100% D = 178 nm25% THF
B
0.1 12.2 1500Size, nm
0
100
G(d
)
C
100% D = 655 nm50% THF
0.1 12.2 1500Size, nm
0
100
G(d
)
C
100% D = 655 nm50% THF
0.1 1 14 168 2000Size, nm
0
100
G(d
)
D
100% D = 770 nm75% THF
Figure 2.7. The hydrodynamic diameter (Dh) profile obtained from DLS studies for the polymer P1 in 100 % water (A), 25 % THF (B), 50 % THF (C) and 75 % THF (D).
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Synthesis and Comparison of Structure-Property Relationship of Symmetric and Asymmteric Water Soluble PPPs
The aggregation behavior of the polymer P1 was studied as a model compound in
aqueous solution and in water/tetrahydrofuran (THF) mixtures by DLS and AFM. The
polymer concentration was kept constant as 0.625 mg/mL. The particle size of P1 in pure
water solution showed a bimodal distribution and the monodispersed particles with the
mean hydrodynamic diameter of ca. Dh = 95 nm (Figure 2.7-A) and a few particles with
a size distribution around <12 nm diameter particles were observed. The addition of THF
into the aqueous polymer solution resulted in the formation of aggregates with large
hydrodynamic diameter which indicated that the polymer chains were in aggregated
forms. In the case of polymer P1, monomodal distribution was obtained with the mean
hydrodynamic diameters of ca. D = 178 nm and ca. Dh h = 650 nm for 25 % and 50 %
THF solution, respectively (Figure 2.7-B, C). Further addition of THF (75 %) into the
aqueous solution of the polymer P1 resulted in the bimodal distribution of polymer
aggregates with a hydrodynamic diameter of ca. Dh = 770 nm (Figure 2.7-D). Further
dilution with the THF solvent hindered the DLS measurements due to precipitation of
polymer aggregates.
Figure 2.8. AFM of polymer P1 in 100 % water (left) and in 25 % THF (right).
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Li Hairong National University of Singapore
Figure 2.9. Cartoon representing the aggregation of polymers by the addition of tetrahydrofuran to a water solution.
In addition to DLS studies, the similar aggregation behavior of P1 was also observed
on a mica surface using Atomic Force Microscopy (AFM). Figure 2.8 shows AFM
images of the aggregated polymer structures obtained through the evaporation of a drop
of aqueous soultion (A) and 25 % THF solution (B) of P1 on a clean freshly cleaved mica
surface. The particles obtained from pure water solution showed a size of 80-90 nm,
whereas the size increased to 180-200 nm upon the addition of 25% THF into the
aqueous polymer solution of P1. In 50 % and 75 % THF solution of the polymer P1, the
size of the aggregates were further increased to 610-630 nm and 780-800 nm,
respectively, which is consistent with the data obtained from DLS studies. The
aggregation process is described schematically in Figure 2.9. As the ratio of THF
increases, the aggregation undergoes micelle and reverse-micelle formations according to
the hydrophobicity of different functional groups, until complete precipitation when THF
dominates since P1 is insoluble in THF.
84
Synthesis and Comparison of Structure-Property Relationship of Symmetric and Asymmteric Water Soluble PPPs
2.3.5. Complexation studies
The aggregation of water soluble polymers is can be enhanced by the addition of the
ionic quenchers.23 The fluorescence spectra of the polymers P1 - P5 in presence of
viologen derivatives (MV2+, EV2+, BV2+) were recorded in water.
N N+ +
N N+ +
N N+ +
MV2+ EV2+
BV2+
Scheme 2.3. Molecular structure of the viologen derivatives (MV= methyl viologen, EV = ethyl viologen, BV = benzyl viologen) used for complexation study.
The Stern-Volmer equation states that I0/I = 1 + K [Quencher], where ISV 0 is the
intensity of the fluorescence from the polymer solution in the absence of quencher, I is
the intensity in presence of the quencher and [Quencher] is the concentration of quencher.
KSV is the Stern-Volmer constant which provides the quantitative measure of the
quenching. The quenching effects of the polymer fluorescence by the addition of
viologen derivatives and the Cyt-C have been investigated in detail. The fluorescence
spectra of the polymer P1 and P5 in presence of viologens are shown in Figure 2.10. It is
obvious that the quenching effect was increased by the increase in the concentration of
the viologen derivatives and complete quenching for P1 was observed by the addition of
10 μM solution of viologens (Figure 2.10-A, C, E). In a very dilute condition, the
quenching efficiency increases and I0/I Vs the concentration of the quencher gives a
linear plot (Figure 2.11). Hence, under low concentrations, it was confirmed that the
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Li Hairong National University of Singapore
static quenching through the complex formation between polymer and quencher
occurred.24 At higher concentration of the quenchers, the plots showed an upward
curvature deviating from linearity, which means that both static and dynamic quenching
mechanisms were involved, the situation becomes more complicated and Stern-Volmer
equation does not apply any more. Figure 2.12 shows the quenching effect of the
polymer fluorescence (P1 and P5) in presence of Cyt-C. A complete quenching of the
fluorescence was observed at a concentration of as low as 4 μM. When the low
concentrations of the viologen derivatives and Cyt-C (< 1 μM) were used, all the
polymers gave a linear relationship of I /I vs [Quencher]. 0
86
Synthesis and Comparison of Structure-Property Relationship of Symmetric and Asymmteric Water Soluble PPPs
D
B
FE
C
A
350 400 450 500 550
0
10
20
30
40
12,P5 + BV2+
1,P5
Inte
nsity
Wavelength (nm)
350 400 450 500 550
0
10
20
30
40
12,P5 + EV2+
1,P5
Inte
nsity
Wavelength (nm)
350 400 450 500 5500
10
20
30
40
50
13,P5 + MV2+
1, P5
Inte
nsity
Wavelength (nm)
350 400 450 500 5500
10
20
30
40
50
60
70C
10, P1 + EV2+
1, P1
Inte
nsity
Wavelength nm
350 400 450 500 5500
10
20
30
40
50
60
70
E
10, P1 + BV2+
1, P1
Inte
nsity
Wavelength nm
350 400 450 500 5500
10
20
30
40
50
60
70
10, P1 + MV2+
1, P1
Inte
nsity
Wavelength nm
Figure 2.10. Changes in the emission spectra of P1 (A, C, E) and P5 (B, D, F) in water at different concentrations of viologens (MV= methyl viologen, EV = ethyl viologen, BV = benzyl viologen), the arrows indicate decrease in fluorescence with increase in concentration of viologen. (1) 0 μM, (2) 0.1 μM, (3) 0.2 μM, (4) 0.5 μM, (5) 1.0 μM, (6) 2.5 μM, (7) 5.0 μM, (8) 7.5 μM, (9) 10.0 μM (10) 12.5 μM, (11) 25.0 μM, (12) 37.5 μM, (13) 50.0 μM. Concentration of P1 and P5 were 5.9 x 10-5 -6 and 5.4 x 10 M, respectively.
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0
2
4
6
8
10
12
0.00E+00 2.00E-07 4.00E-07 6.00E-07 8.00E-07 1.00E-06
Quencher (M)
I0/I MV2+
EV2+
BV2+P5P1
0
1
2
3
4
0.00E+00 2.00E-07 4.00E-07 6.00E-07 8.00E-07 1.00E-06
Quencher (M)
I0/I
MV2+
EV2+
BV2+P1
0
2
4
6
8
10
12
0.00E+00 2.00E-07 4.00E-07 6.00E-07 8.00E-07 1.00E-06
Quencher (M)
I0/I MV2+
EV2+
BV2+P5
0
2
4
6
8
10
12
0.00E+00 2.00E-07 4.00E-07 6.00E-07 8.00E-07 1.00E-06
Quencher (M)
I0/I MV2+
EV2+
BV2+
0
2
4
6
8
10
12
0.00E+00 2.00E-07 4.00E-07 6.00E-07 8.00E-07 1.00E-06
Quencher (M)
I0/I MV2+
EV2+
BV2+P5P1
0
1
2
3
4
0.00E+00 2.00E-07 4.00E-07 6.00E-07 8.00E-07 1.00E-06
Quencher (M)
I0/I
MV2+
EV2+
BV2+P1P1
0
1
2
3
4
0.00E+00 2.00E-07 4.00E-07 6.00E-07 8.00E-07 1.00E-06
Quencher (M)
I0/I
MV2+
EV2+
BV2+P1
Figure 2.11. Stern – Volmer plots of the polymer P1 and P5 quenched by the viologen derivatives; MV2+ - methyl viologen, EV2+ - ethyl viologen, BV2+ - benzyl viologen.
350 400 450 500 550
0
20
40
60
10, P1 + Cyt-C
1, P1
Inte
nsity
Wavelength nm350 400 450 500 550
0
10
20
30
40
50
11, P5 + Cyt-C
1, P5In
tens
ity
Wavelength nm
A B
Figure 2.12. Changes in the emission spectra of P1(A) and P5 (B) in water at different concentrations of cytochrome-C (1) 0 μM, (2) 0.1 μM, (3) 0.5 μM, (4) 1.0 μM, (5) 1.5 μM, (6) 2.0 μM, (7) 2.5 μM, (8) 3.0 μM, (9) 3.5 μM, (10) 4.0 μM. Concentration of P1 and P5 were 5.9 x 10-5 -6 and 5.4 x 10 M, respectively.
The other polymers showed the same general behavior and the constant KSV for all
the polymers are tabulated in Table 2.2. Among the polymers P1-P5, the highest Ksv
values were obtained for P5 by the addition of Cyt-C (3.0 x 106) and viologens
derivatives. Owing to the asymmetric structure of P4 and P5, the interaction of the
viologen derivatives with the polymer may be higher in P5, which resulted in efficient
88
Synthesis and Comparison of Structure-Property Relationship of Symmetric and Asymmteric Water Soluble PPPs
quenching of the fluorescence. In the case of symmetrically functionalized PPPs (P1-P3)
the values of Stern-Volmer constant didn’t change significantly with the change in length
of the side chains on the polymer back bone.
Table 2.2. Stern-Volmer constants for the polymers upon addition of the quencher
Ksv Polymer MV EV BV Cyt-C
P1 5.1 x 105 6.5 x 105 2.0 x 106 1.0 x 106
P2 4.7 x 105 6.2 x 105 8.1 x 105 0.8 x 106
P3 1.1 x 106 2.0 x 105 9.4 x 105 2.0 x 106
P4 5.8 x 105 8.8 x 105 1.0 x 106 2.0 x 106
P5 6.0 x 106 7.0 x 106 9.0 x 106 3.0 x 106
The quenching efficiencies also increased with the size of the side chain on viologen
molecule. Among the three viologen derivatives, the benzyl viologen quenches
fluorescence with significantly higher value of Ksv, which can also be straightforwardly
seen from the Stern-Volmer plots (Figure 2.12). In all polymers (P1 - P5), a trend in the
magnitude of the values of Ksv (BV2+) > Ksv (EV2+) > Ksv (MV2+) was observed (Table
2.2). The reason for this observation may be the increase in hydrophobicity of the
viologen derivatives with a change in substitution. This may facilitate stronger interaction
with polymer backbone and similar behavior was also reported for PPV derivatives.24-26
In addition, metal ions were also added to investigate the influence of optical
properties of the polymers. Among the metal ions (Co2+ 2+, Cu , Mg2+ + + 2+, Ag , Li , Cd , Gd3+,
Al3+ 3+, Pr , Zn2+, Mn2+ 3+ 3+ and Fe ), only Fe metal ion quenched the polymer fluorescence
significantly. It was found that the order of cations in ion-exchange resin containing
sulfonate groups is: Fe3+ 3+ 2+ 2+>Al >Cd >Cu >Co2+ >Mg2+ >Zn2+ >Mn2+ + + >Ag >Na >H+
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Li Hairong National University of Singapore
+>Li .27 3+ Therefore, Fe ion could probably bind to sulfonate groups stronger than the rest
in the aqoueous solution as well.
2.3.6. Packing structure in solid state
The powder X-ray diffraction studies on polymers, P1 - P5 were done to understand
the packing behavior of polymer chains in the solid-state. All XRD patterns for the
polymers P1 - P5 are shown in Figure 2.13-A. For polymer P1, there was a broad peak at
d = 4.1 Å, which may be due to the side-to-side distance between loosely packed polymer
chains. In XRD patterns, there were four sharp peaks at 8.8, 5.9, 4.4 and 3.8 Å for P2 and
7.4, 5.3, 4.1 and 3.7 Å for P3, which indicated the crystalline nature of the polymer
lattice. The peak appearing on the small angle region (d > 10 Å) might be assigned to the
interlayer distances separated by the side chains. The polymer backbone of P2 is
incorporated with dodecyl groups and propoxy sulfonyl groups on alternating benzene
rings, which makes the self-assembly of polymer chains very difficult. It is expected that
the polar groups and nonpolar groups will interact among themselves and organize in the
lattice causing conformational changes to the polymer backbone. As shown in Figure
2.13-B, this is only possible for the alkyl chains to organize perpendicular to the plane of
the polymer chain. Such packing was reported earlier for poly(p-phenylene-ethyenylene)
compounds (PPEs).28 The appearance of broad peaks at the wide angle region for
polymers P1 and P2 may be due to the disorder in the packing of polymer chains inside
the polymer lattice. Thus, polymers P1-P3 might adopt the face-on packing as shown in
Figure 2.13-B.
90
Synthesis and Comparison of Structure-Property Relationship of Symmetric and Asymmteric Water Soluble PPPs
Two major peaks were observed in the XRD pattern of P4 and P5, one peak at the
low angle region at 17.0 Å for P4 and at 24.1 Å for P5 and another one at wide angle
region at 4.1 Å (P4) and 4.2 Å (P5). These distances are illustrated in Figure 10A. The
peak at the low angle regions corresponds to the distance between the conjugated main
chains separated by the side chain, whereas d-spacing of ~ 4.1 Å corresponds to the
interplane distance of the face-to-face stacked assembly of the polymer chains. A
schematic representation of the polymer lattice of P4 and P5 is given in Figure 2.13-C.
0 10 20 302 theta value
4.1
8.8
4.224.1
7.4
4.4 3.8
3.7
17.0
4.1Polymer P4
Polymer P5
Polymer P3
Polymer P2
Polymer P1O
O O
O
NaO3S
NaO3SO
O O
O
NaO3S
NaO3S
O
O O
O
NaO3S
NaO3S
O
O O
O
NaO3S
NaO3S
O
OO
O
SO3Na
SO3NaO
OO
O
SO3Na
SO3Na
O
OO
O
SO3Na
SO3Na
O
OO
O
SO3Na
SO3Na
O O
O
NaO3S
O O
O
NaO3S
O
O O
O
NaO3S
O
O O
NaO3SNaO3SNaO3SNaO3SNaO3S
O
OO
O
SO3Na
OO
O
SO3Na
O
OO
O
SO3Na
O
OO
SO3Na SO3Na SO3Na SO3Na SO3Na
O
O O
O
NaO3S
O O
O
NaO3S
O
O O
O
NaO3S
O
O O
NaO3SNaO3SNaO3SNaO3SNaO3S
O
OO
O
SO3Na
OO
O
SO3Na
O
OO
O
SO3Na
O
OO
SO3Na SO3Na SO3Na SO3Na SO3Na
O
NaO3S
O
OO
O
SO3Na
OO
O
SO3Na
O
OO
O
SO3Na
O
OO
O O
O
NaO3S
O
O O
O
NaO3S
O
O O
NaO3SNaO3SNaO3SNaO3SNaO3S
O
OO
O
SO3Na
OO
O
SO3Na
O
OO
O
SO3Na
O
OO
SO3Na SO3Na SO3Na SO3Na SO3Na
O
C
B
A
0 10 20 302 theta value
4.1
8.8
4.224.1
7.4
4.4 3.8
3.7
17.0
4.1Polymer P4
Polymer P5
Polymer P3
Polymer P2
Polymer P1
0 10 20 302 theta value
4.1
8.8
4.224.1
7.4
4.4 3.8
3.7
17.0
4.1Polymer P4
Polymer P5
Polymer P3
Polymer P2
Polymer P1O
O O
O
NaO3S
NaO3SO
O O
O
NaO3S
NaO3S
O
O O
O
NaO3S
NaO3S
O
O O
O
NaO3S
NaO3S
O
OO
O
SO3Na
SO3NaO
OO
O
SO3Na
SO3Na
O
OO
O
SO3Na
SO3Na
O
OO
O
SO3Na
SO3Na
O
O O
O
NaO3S
NaO3SO
O O
O
NaO3S
NaO3S
O
O O
O
NaO3S
NaO3S
O
O O
O
O O
O
NaO3S
NaO3SO
O O
O
NaO3S
NaO3S
O
O O
O
NaO3S
NaO3S
O
O O
O
NaO3S
NaO3S
O
OO
O
SO3Na
SO3NaO
OO
O
SO3Na
SO3Na
O
OO
O
SO3Na
SO
O
NaO3S
NaO3S
O
OO
O
SO3Na
SO3NaO
OO
O
SO3Na
SO3Na
O
OO
O
SO3Na
SO3Na
O
OO
O
SO3Na
SO3Na
O O
O
NaO3S
O O
O
NaO3S
O
O O
O
NaO3S
O
O O
NaO3SNaO3SNaO3SNaO3SNaO3S
O
OO
O
SO3Na
OO
O
SO3Na
O
OO
O
SO3Na
O
OO
SO3Na SO3Na SO3Na SO3Na SO3Na
O
O O
O
NaO3S
O O
O
NaO3S
O
O O
O
NaO3S
O
O O
NaO3SNaO3SNaO3SNaO3SNaO3S
O
OO
O
SO3Na
OO
O
SO3Na
O
OO
O
SO3Na
O
OO
SO3Na SO3Na SO3Na SO3Na SO3Na
O
NaO3S
O
OO
O
SO3Na
OO
O
SO3Na
O
OO
O
SO3Na
O
OO
O O
O
NaO3S
O
O O
O
NaO3S
O
O O
NaO3SNaO3SNaO3SNaO3SNaO3S
O
OO
O
SO3Na
OO
O
SO3Na
O
OO
O
SO3Na
O
OO
SO3Na SO3Na SO3Na SO3Na SO3Na
O
O O
O
NaO3S
O O
O
NaO3S
O
O O
O
NaO3S
O
O O
NaO3SNaO3SNaO3SNaO3SNaO3S
O
OO
O
SO3Na
OO
O
SO3Na
O
OO
O
SO3Na
O
OO
SO3Na SO3Na SO3Na SO3Na SO3Na
O
O O
O
NaO3S
O O
O
NaO3S
O
O O
O
NaO3S
O
O O
NaO3SNaO3SNaO3SNaO3SNaO3S
O
OO
O
SO3Na
OO
O
SO3Na
O
OO
O
SO3Na
O
OO
SO3Na SO3Na SO3Na SO3Na SO3Na
O
O O
O
NaO3S
O O
O
NaO3S
O
O O
O
NaO3S
O
O O
NaO3SNaO3SNaO3SNaO3S
O O
O
NaO3S
O O
O
NaO3S
O
O O
O
NaO3S
O
O O
NaO3SNaO3SNaO3SNaO3SNaO3S
O
OO
O
SO3Na
OO
O
SO3Na
O
OO
O
SO3Na
O
OO
NaO3S
O
OO
O
SO3Na
OO
O
SO3Na
O
OO
O
SO3Na
O
OO
SO3Na SO3Na SO3Na SO3Na SO3Na
O
O O
O
NaO3SSO3Na SO3Na SO3Na SO3Na SO3Na
O
O O
O
NaO3S
O O
O
NaO3S
O
O O
O
NaO3S
O
O O
NaO3SNaO
O O
O
NaO3S
O
O O
O
NaO3S
O
O O
NaO3SNaO3SNaO3SNaO3SNaO3S
O
OO
O
SO3Na
3SNaO3SNaO3SNaO3S
O
OO
O
SO3Na
OO
O
SO3Na
O
OO
O
SO3Na
O
OO
SO3Na
OO
O
SO3Na
O
OO
O
SO3Na
O
OO
SO3Na SO3Na SO3Na SO3Na SO3Na
O
NaO3S
O
OO
O
SO3Na
OO
O
SO3Na
O
OO
O
SO3Na
O
OO
NaO3S
O
OO
O
SO3Na
OO
O
SO3Na
O
OO
O
SO3Na
O
OO
O O
O
NaO3S
O
O O
O
NaO3S
O
O O
NaO3SNaO
O O
O
NaO3S
O
O O
O
NaO3S
O
O O
NaO3SNaO3SNaO3SNaO3SNaO3S
O
OO
O
SO3Na
3SNaO3SNaO3SNaO3S
O
OO
O
SO3Na
OO
O
SO3Na
O
OO
O
SO3Na
O
OO
SO3Na
OO
O
SO3Na
O
OO
O
SO3Na
O
OO
SO3Na SO3Na SO3Na SO3Na SO3Na
O
C
B
A
Figure 2.13. (A) XRD power patterns of the polymers P1-P5, all peak values are given in Å. A cartoon representing idealized solid-state packing models of the polymers of P1-P3 (B) and P4-P5 (C).
2.3.7. Polymer as template for controlled biomineralization mimics
Nature employs specialized macromolecules to produce highly complex structures
and understanding of the role of these macromolecules allows us to develop novel
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materials with interesting properties.29 Bio-inspired mineralization is a unique,
environmentally friendly approach that can create inorganic or functional materials with
exceptional, ordered and refined shapes, regulated by specific macromolecules.30 An
advantage of bio-inspired approach is that ceramic materials can be deposited on a solid
substrate in a two-dimensionally ordered manner. The structure-function relationship of
these materials (morphology, shape, polymorph selectivity, mechanical strength and
functional role) are controlled by the cooperative interaction of the inorganic ions and
functional biomacromolecules.31,32
In Nature, calcium salts (e.g. CaCO3, CaPO x H4 2O, CaC2O4), are deposited in
presence of acidic biomolecules scuh as proteins and proteoglycans, which bind Ca2+ ions
and induce crystal nucleation and growth.33,34 As proteins are often difficult to analysis in
this process due to complicated structure, tiny amount and non-fluorescence, we herein
used water soluble PPPs as a template for nucleation of calcium salts in order to understand
the role of various organic additives in the crystallization process. One important reason is
that so far there is little information on the location and identification of macromolecules
inside the inorganic matrix due to the extremely low concentration of the organic
macromoelcules and unavialbility of techniques with moelcualr resolution for in situ
detection of such macromoelcules. Highly fluorescent WSPPPs could solve the problem.
To perform the crystallization experiments, polymer solutions of P1 and P2 in
different concentrations were prepared by dissolving aliquots of polymers in 7.5 mM
calcium chloride solution to get a series of solutions with polymer concentrations in the
range of 100 μg/mL, 500 μg/mL and 1 mg/mL. Solutions (1 mL) were introduced into the
92
Synthesis and Comparison of Structure-Property Relationship of Symmetric and Asymmteric Water Soluble PPPs
crystallization wells containing glass cover slips. The whole setup was covered with an
aluminum foil with a few pinholes at the top, and kept inside a closed desiccator containing
ammonium carbonate for different time intervals. Crystals formed after 48 hours showed
the influence of added P1 towards nucleation, growth and orientation of crystals. The
formation of iso-oriented calcite crystals and increase in crystal density with increase in
polymer concentration points out that the polymer provides nucleating sites for crystal
growth through complexation of Ca2+ by the -SO3- on the backbone and acts as a structure-
directing agent (Figure 2.14). The adsorption of polymer on the crystal surfaces was
evident from the confocal microscopy images which showed intense blue light emitting
behavior characteristic of PPP (Figure 2.14).
The structural analysis of the calcitic crystals were done using X-ray diffraction
studies. For a range of polymer concentrations from 100 μg/mL, 500 μg/mL to 1 mg/mL,
highly oriented calcite crystals with orientation along the {104} plane were observed, while
crystallization along all other planes was inhibited (Figure 2.15). Lower polymer
concentartion also gave {104} oriented caclite crystals. The formation of iso-oriented
crystals indicate that P1 provides a specific organization of sulfonyl groups, which is
responsible for the nucleation and growth of crystals with {104} faces. Based on the SEM
images and the XRD results of calcite crystals, a possible mecahnism was proposed to get
iso-oriented calcite crystals and shown schematically in Figure 2.14 (left). Here, the
organization of P1 results in a distance of ~ 5 Å between adjacent -SO3- groups along the
polymer chain and ~ 4.05 Å distance between the -SO3- groups of the adjacent parallaly
packed polymer chains. The distance between the sulfonrate group matches the location of
calcium cations on the {104} faces of calcite crystal lattice.35 This mechanism gave insight
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into the hypothesis that the polymer is attached to the lattice planes and act as a structure
directing agent.
Figure 2.14. SEM and CLSM image of the CaCO3 crystals grown in the presence of P1 (1mg/mL); XRD patterns of the calcite crystals grown in presence of 1 mg/mL of P1; Schematic representation of a possible binding of P1 with Ca2+ ions in the crystallization medium for the formation of oriented calcite crystals; a unit of {104} plane of the calcite crystal lattice is also shown.
The behavior of the polymer P2 in the crystallization medium was found to be
different from that of P1 owing to their structural differences. A polymer concentration of
100 μg/mL in the crystallization medium gave distorted calcitic crystals (Figure 2.16a),
indicating strong interactions of the polymer with the crystal planes and the electron
micrographs showed a layered arrangement of the crystals (see inset of Figure 2.16a). An
increase in polymer concentration from 100 μg/mL to 500 μg/mL and further to 1 mg/mL,
94
Synthesis and Comparison of Structure-Property Relationship of Symmetric and Asymmteric Water Soluble PPPs
gave cauliflower like vaterite crystals (Figure 2.16b-c). The aggregation behaviour of P2
in solution is more complex due to the presence of alkyl chains on the alternating benzene
rings on the polymer backbone, which influence the organization of sulfonate gorups in
solution to produce crystals with qualiflower type morphology. The polymorphic nature of
the crystals nucelated at 100 μg/mL, 500 μg/mL and 1 mg/mL was confirmed using X-ray
diffraction pattern (Figure 2.16d). Calcite crystals were nucleated at a polymer
concentration of 100 μg/mL, but at higher polymer concentrations (500 μg/mL and 1
mg/mL), the XRD spectra shows a mixture of vaterite and calcite peaks together with the
amorphous hallow indicating the formation of amorphous phase in the sample (Figure
2.16d). The amount of calcite crystals nucleated in presence of 500 μg/mL and 1 mg/mL
of polymer was less than 5%. SEM micrographs did not show the presence of calcitic
crystals together with vaterite qualiflowers owing to the low amount of calcite crystals
nucleated. But the presence of amorphous hallow in the XRD indicate the formation of
amorphos hybrid film on the substrate. The composite particles and film showed blue
emission due to the incorporation of PPP (Figure 2.16e). The aggregation behaviour of P2
is rather complicated and need further experiments to confirm the structure. But it is very
clear that small change on the chemical structure significantly affect the whole
mineralization process, which is again confirmed to be controlled very accurately.
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Li Hairong National University of Singapore
20 30 40 50 60 702 θ
[104]
(a)
(b)
20 30 40 50 60 702 θ
[104]
(a)
(b)
Figure 2.15. XRD patterns of the calcite crystals grown in presence of 1 mg/mL of P1 (a) and in the absence of polymer (b). Inset shows the electron micrographs of the representative crystals.
Figure 2.16. SEM images of CaCO3 grown on a glass substrate, in presence of P2, with concentrations of 100 μg/mL (a), 500 μg/mL (b) and 1 mg/mL (c). XRD pattern of the crystals and composites formed in presence of P2 for different concentrations (d). Representative CLSM image shows the fluorescent nature of the calcium carbonate polymer composite formed (e).
20 30 40 50 60 70
C
CCCCC
2θ
VVV
VVV
VV
C
C
C
1 mg/mL
500 μg/mL
100 μg/mL
(d
96
Synthesis and Comparison of Structure-Property Relationship of Symmetric and Asymmteric Water Soluble PPPs
2.4. Conclusion
Two sets of water soluble poly(p-phenylene)s with sulfonatopropoxy side chains
were synthesized, characterized and their thermal, optical properties and lattice structures
were established. The structure-property comparison of these polymers was investigated
in detail. The polymers tended to aggregate in water/THF mixture through microphase
separation of polar (water) and nonpolar (THF) groups into appropriate solvents and
strong intermolecular interactions. The fluorescence emission from polymers was
quenched in presence of added viologen derivatives or Cyt-C or metal ions in water, with
Stern-Volmer constant up to 106 M-1, indicating that they may be suitable for potential
applications in biosensors. Light-emitting iso-oriented calcite crystals, porous calcium
carbonate spherules were synthesized by controlling crystallization in presence of the
water soluble PPPs. Polymer on the crystal surface was successfully detected due to high
fluorescence. The nature and orientation of the side chain functional groups on the
polymer backbone plays a crucial role for the selective morphogenesis of the crystals
with controlled particle shape, size and orientation.
2.5. References
1. Hong, J. W.; Benmansour, H.; Bazan, G. C. Chem. Eur. J. 2003, 9, 3186.
2. Gaylord, B. S.; A. J. Heeger, Bazan, G. C. J. Am. Chem. Soc. 2003, 125, 896.
3. Ho, H, -A.; Leclerc, M, J. Am. Chem. Soc. 2004, 126, 1384.
4. Kim, S.; Jackie, J.; Robinson, E.; Schanze, K. S.; Reynolds, J. R.; Baur, J.;
Rubner, M. F.; Boils, D. Macromolecules 1998, 31, 964.
97
Li Hairong National University of Singapore
5. Yin, Y.; Fang, J.; Watari, T.; Tanaka, K.; Kita, H.; Okamoto, K. J. Mat. Chem.
2004, 14, 1062.
6. Zotti, G.; Zecchin, S.; Schiavon, G.; Groenedaal, L. B. Macromol. Chem. Phys.
2002, 203, 1958.
7. Harrison, B. S.; Ramey, M. B.; Reynolds, J. R.; Schanze, K. S. J. Am. Chem. Soc.
2000, 122, 8561.
8. Baur, J. W.; Kim, S.; Balanda, P. B.; Reynolds, J. R.; Rubner, M. F. Adv. Mater.
1998, 10, 1452.
9. Gaylord, B. S.; Wang, S.; Heeger, A. J.; Bazan, G. C. J. Am. Chem. Soc. 2001,
123, 6417.
10. Pinto, M. R.; Schanze, K. S. Synthesis 2002, 9, 1293.
11. Sonogashira K., in Metal-catalyzed Cross-coupling Reactions, ed. F. Diederich
and P. J. Stang, Wiley-VCH, Weinheim, 1998, p. 203.
12. Swager, T. M. Acc. Chem. Res. 1998, 31, 201.
13. Zhou, Q.; Swager, T. M. J. Am. Chem. Soc. 1995, 117, 7017.
14. Zhou, Q.; Swager, T. M. J. Am. Chem. Soc. 1995, 117, 12593.
15. Baskar, C.; Lai, Y, -H.; Valiyaveettil, S. Macromolecules 2001, 34, 6255.
16. Ji, W.; Izacc, H.; He, J, Fitrilawati, F.; Baskar, C, Valiyaveettil, S, Knoll, W. J.
Phys. Chem. B 2003, 107, 11043.
17. Renu, R.; Valiyaveettil, S.; Knoll, W. MRS Proceedings 2003, 776, 201.
18. Kim, S.; Jackie, J.; Robinson, E.; Schanze, K. S.; Reynolds, J. R.; Baur, J.;
Rubner, M. F.; Boils, D. Macromolecules 1998, 31, 964
19. Vetrichelvan, M.; Valiyaveettil, S. Chem. Eur. J. 2005, 5889.
98
Synthesis and Comparison of Structure-Property Relationship of Symmetric and Asymmteric Water Soluble PPPs
20. Tan, C.; Atas, E.; Muller, J. C. ; Pinto, M. R. ; Kleiman, V. D.; Schanze, K. S. J.
Am. Chem. Soc. 2004, 126, 13685.
21. Breitenkamp, R. B.; Tew, G. N. Macromolecules 2004, 37, 1163.
22. Xia, C.; Locklin, J.; Youk, J. H. Fulghum, T.; Advincula, R. C. Langmuir 2002,
18, 955.
23. Fan, C. H.; Hirasa, T.; Plaxco, K. W.; Heeger, A. J. Langmuir 2003, 19, 3554.
24. Wang, J.; Wang, D.; Miller, E. K.; Moses, D.; Bazan, G. C.; Heeger, A. J.
Macromolecules 2000, 35, 5153.
25. Wang, W.; Gong, X.; Heeger, P. S.; Rininsland, F.; Bazan, G. C.; Heeger, A. J.
Proc. Natl. Acad. Sci. 2002, 99, 49.
26. Kumpumpu-Kalemba, L.; Leclerc, M. Chem. Commun. 2000, 1847.
27. Braithwaite, A.; Smith, F. J. Chromtagraphic Methods 4th ed. London: Chapman
& Hall Ltd, 1985.
28. Kim, J.; Swager, T. M. Nature 2001, 411, 1030.
29. Mann, S.; Webb, J.; Williams, R. J. P. Biomineralization: Chemical and
Biochemical Perspectives; VCH Publishers, New York 1989.
30. Meldrum, F. C. Int. Mater. Rev. 2003, 48, 187.
31. Aksay, I. A.; Trau, M. Mann, S.; Honma, I.; Yao, N.; Zhou, L.; Fenter, P.;
Eisenberger, P. M.; Gruner, S. M. Science 1996, 273, 892.
32. Addadi, L.; Weiner, S. Angew. Chem. Int. Ed. 1992, 31, 153.
33. Addadi, L.; Weiner, S. Proc. Natl. Acad. Sci. USA 1985, 82, 4110. b)
34. Falini, G.; Albeck, S.; Weiner, S.; Addadi, L. Science 1996, 271, 67.
35. Kotachi, A.; Miura, T.; Imai, H. Chem. Mater. 2004, 16, 3191.
99
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Li, Hairong; Valiyaveettil, S. Macromolecules 2007, 40, 6057. Li, Hairong; Valiyaveettil, S. Poly. Mater. Sci. Eng. 2007, 96, 747.
CChhaapptteerr 33
WWaatteerr SSoolluubbllee MMuullttiiffuunnccttiioonnaall CCrroossss--ccoonnjjuuggaatteedd PPoollyy((ppaarraa--
pphheennyylleenneess)) aass SSttiimmuullii RReessppoonnssiivvee MMaatteerriiaallss:: DDeessiiggnn,, SSyynntthheessiiss,, aanndd
CChhaarraacctteerriizzaattiioonn
100
Water Soluble Multifunctional Cross-conjugated Poly(para-phenylenes) as Stimuli Responsive Materials ---Design, Synthesis, and Characterization
3.1. Introduction
As we have obtained some interesting results from the previous work on linear water
soluble PPPs, we made further effort to expand the system to a two-dimensional (2D)
framework. Chapter 1 has detailedly summarized the advantages of PPPs and special
merits of water soluble CPs; it was also mentioned that so far not much work has been
done on exploring the properties of cross-conjugated systems (oligomer, polymer,
dentrimer, macrocyle, etc.). Therefore we had an idea of incorporating wate-soluble
groups into cross-conjugated PPP backbone. The polymers were designed with the
intention to expand the conjugation and to facilitate electron/charge transport across the
2D architecture as shown by a cartoon in Figure 3.1. Another important reason is that 2D
structures provide more accessable sites for multiple functionalizations. The target was to
make the polymers not only novel, but also more versatile. The molecular structures of
the target polymers are given in Figure 3.2.
Figure 3.1. A cartoonistic representation of various pathways for electronic conjugations in 2D polymers, X and Y represent donor-acceptor type functional groups.
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Li Hairong National University of Singapore
N
N
O3S(CH2)3O
O(CH2)3SO3
n
O
O
SO3
SO3
O
O
N
N
n
KO3S(CH2)3O
O(CH2)3SO3K
n
P1
P5
P2
O
O
N
N
n
P6
N
O
O
N
N
n
P4
N
N
n
N
N
P3
II
I
I
I
I
Figure 3.2. Molecular structures of target polymers.
3.2 Experimental
3.2.1. Measurements
102
Water Soluble Multifunctional Cross-conjugated Poly(para-phenylenes) as Stimuli Responsive Materials ---Design, Synthesis, and Characterization
EM images were taken with a JEOL JSM 6700 scanning electron microscope (SEM).
The samples were carefully mounted on copper stubs with a double-sided conducting
carbon tape and sputter coated with 2 nm platinum before examination. For the
fluorescence imaging, a Carl Zeiss LSM 510 laser scanning microscope with an
excitation wavelength of 365±12 nm was used. Other instruments used in this work have
been introduced in the Section 2.2.1 of Chapter 2.
3.2.2. Synthesis
Polymers P1-P6 contain two different functional groups with polar water soluble
groups, which are structurally different from linear water soluble polymers reported
earlier.1-12 The synthetic routes for all polymers are given in Scheme 1. 4-(3-
sulfonatopropoxy) benzaldehyde (1) and 4-(2-diethylaminoethoxy) benzaldehyde (2)
were synthesized from 4-hydroxybenzaldehyde13 and 4-pyridyl acrolein 3 was purchased
from Lancaster. Bromination of 1,4-dibromo-p-xylene using NBS in CCl4 under reflux
gave 1,4-dibromo-2,5-bisbromomethyl benzene (4).14 2,7-Dibromo-9-bis(3-N,N-
dimethylaminopropyl)-9H-fluorene (8) was synthesized from 2,7-dibromofluorene and
N,N-dimethylaminopropylbromide in DMF with 50 % NaOH aqueous solution at 60 °C
for 6 hr.15 Two boronic acids 916 and 1017 were synthesized from the corresponding
dibromides using n-BuLi in THF solution at -78 ºC followed by addition of
triisopropylborate. 2,5-bis (4,4,5,5-tetramethyl-[1,3,2]-dioxaborolan-2-yl)-pyridine (11)
was synthesized according to reported procedures.18
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Li Hairong National University of Singapore
CHOKSO3(CH2)3O CHO(CH3CH2)2N(CH2)2O
BrBr
CH2Br
BrCH2
BrBr
O(CH2)3SO3K
KSO3(CH2)3O
BrBr
O(CH2)2N(CH2CH3)2
(CH3CH2)2N(CH2)2O
BrBr
N
N
NCHO
NBB
O
OO
O
BrBr
NN
B(OH)2(HO)2B
NN
(1) (2) (3)
(4)
BrBr
CH2Br
BrCH2
(4)
BrBr
CH2Br
BrCH2
(4)
(5) (6) (7)
(9)(8)
B(OH)2(HO)2B
O
O
(5) or (7)
CH3IP1 or P3
(6) Pd/C CH3I
OS
O O
P2
(6)
(10)
(11)
P4CH3I
B(OH)2(HO)2B(5) or (6)
CH3IP5 or P6
1.2.
2.
1.
1. 1.2.2.
BuLi
Pd(0) H2
(i-PrO)3B
Scheme 3.1. Synthetic routes for P1-P6.
1,4-Dibromo-2,5-bis[2-(4-(3-sulfonatopropoxy)-phenyl)-vinyl]benzene (5). A
mixture of compound 4 (2 g, 4.8 mmol) and triethylphosphine (3.3 mL, 19.2 mmol) was
heated to reflux for 6 hr without solvent, excess triethylphosphine was removed under
104
Water Soluble Multifunctional Cross-conjugated Poly(para-phenylenes) as Stimuli Responsive Materials ---Design, Synthesis, and Characterization
reduced pressure. The obtained ylid was mixed with DMF, potassium t-butoxide (1.8 g,
19.2 mmol) and compound (1) (5.4 g, 19.2 mmol) at 0 ºC and stirred overnight at room
temperature, poured into acetone, filtered and the crude solid was recrystalized from
methanol to obtain a yellow product in 75% yield. 1H NMR (DMSO-d6, δ ppm): 8.09 (s,
2H, central Ar-H), 7.56 (d, J = 9 Hz, 4H terminal Ar-H), 7.37 (d, J = 17 Hz, 2H CH =
CH), 7.17 (d, J = 12 Hz, 2H CH = CH), 6.98 (d, J = 9 Hz, 4H, H2 and H6 Ar-H), 4.10 (t,
J = 6 Hz, 4H CH2O), 2.56 (t, J = 6Hz, 4H CH2N), 2.01 (q ,J = 6 Hz, 4H –CH2–). 13C
NMR (DMSO-d6, δ ppm): 162.5, 136.2, 132.2, 129.2, 128.2, 127.8, 121.8, 121.5, 114.3,
67.2, 47.2, 24.7. FTIR (KBr, cm-1): 3036, 2940, 2869, 1671, 1604, 1473, 1366, 1253,
1202, 1050, 955, 816, 733, 619, 584, 530. Elemental analysis: C: 42.43, H: 3.31. Found:
C: 42.14, H: 3.38.
1,4-Dibromo-2,5-bis-[2-(4-(2-N,N-diethylaminoethoxy)-phenyl)-vinyl]benzene
(6). 1H NMR (CDCl3, δ ppm): 8.01 (s, 2H, central Ar-H), 7.48 (d, J = 9 Hz, 4H, H3 and
H5 terminal Ar-H), 7.09 (d, J = 12 Hz, 2H CH = CH), 6.91 (d, J = 12 Hz, 2H CH = CH),
6.84 (d, J = 9 Hz, 4H, terminal Ar- H), 3.99 (t, J = 4 Hz, 4H CH2O), 2.87 (t, J = 4 Hz, 4H
CH2N), 2.64 (q, J = 4Hz, 4H, N(CH2CH3)2), 1.05 (t, J = 5 Hz, 6H, N(CH2CH3)2 ). 13C
NMR (CDCl3, δ ppm): 158.7, 137.2, 131.5, 129.9, 128.2, 123.6, 122.7, 114.8, 68.7, 51.5,
47.7, 11.6. FTIR (KBr, cm-1): 2964, 2935, 2817, 1628, 1602, 1512, 1458, 1367, 1292,
1254, 1176, 1046, 958, 851, 805, 606, 553, 523. Elemental analysis: C: 60.90, H: 6.31.
Found: C: 60.78, H: 6.27.
1,4-Dibromo-2,5-bis(4-pyridyl-buta-1,3-dienyl)benzene (7). 1H NMR (CDCl3, δ
ppm): 8.49 (d, J = 5 Hz, 4H, Py-H), 8.05 (s, 2H central Ar-H), 7.52 (d, J = 7 Hz, 4H, Py-
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Li Hairong National University of Singapore
H), 7.14 (d, J = 17 Hz, 2H, CH = CH – CH = CH), 7.02 (d, J = 15 Hz, 2H, CH = CH –
CH = CH), 6.75 (d, J = 15 Hz, 2H, CH = CH – CH = CH), 6.68 (d, J = 14 Hz, 2H, CH =
CH – CH = CH). 13C NMR (CDCl3, δ ppm): 150.2, 142.9, 137.7, 135.5, 133.2, 131.5,
127.8, 126.9, 123.9, 107.7. FTIR (KBr, cm-1): 2962, 2925, 2853, 1707, 1634, 1474, 1416,
1364, 1260, 1056, 1025, 966, 804, 706, 619, 525. Elemental analysis: C: 58.33, H: 3.67.
Found: C: 58.15, H: 3.69.
Poly{2,5-bis-[2-(4-(3-sulfonatopropoxy)-phenyl)-vinyl]-1,4-benzene-alt-9-bis(3-
N,N,N-tri methylaminopropyl)-9H-2,7-fluorene} (P1). Compounds 5 (0.93 g, 1.18
mmol) and 9 (0.5 g, 1.18 mmol) were dissolved in 30 mL DMF. K2CO3 solution (2 M, 20
mL) was added to this mixture followed by tetrakis(triphenylphosphino)-palladium(0) (3
mol%) as catalyst and cetyltrimethyl ammonium bromide (30 mol%) as phase transfer
agent to increase the solubility. The mixture was stirred at 80 ºC for 3 days, concentrated
under reduced pressure and precipitated twice from methanol; the crude product was
directly treated with excess CH3I in DMSO for 2 days; 250 mL H2O was added followed
by filtration, the filtrate was dialyzed using snake skin pleated dialysis tubing with MW
cutoff value of 3500 for 3 days with H2O changed twice per day. The concentrated
polymer solution was passed through a syringe filter with pore diameter of 0.25 μm and
then lyophilized to get a dark brown solid in 40 % yield. 1H NMR (D2O, δ ppm): 7.92 (b,
Ar-H), 7.53 (b, Ar-H), 7.24 (b, Ar-H, CH=CH), 6.96 (b, Ar-H), 6.77 (b, CH=CH), 3.60
(b, CH2), 2.94 (b, CH2), 2.85 (b, CH2), 2.17 (b, CH3), 1.98 (b, CH2). 13C NMR (D2O, δ
ppm): 162.1, 150.1, 141.8, 140.4, 140.2, 140.0, 137.7, 135.0, 130.2, 129.6, 128.9, 128.5,
128.3, 128.0, 127.7, 127.3, 124.6, 116.0, 115.5, 114.4, 68.6, 66.7, 62.3, 56.4, 53.4, 47.9,
38.4, 33.0, 26.6, 22.2. FTIR (KBr, cm-1): 2969, 2884, 2786, 2451, 2361, 2273, 2204,
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Water Soluble Multifunctional Cross-conjugated Poly(para-phenylenes) as Stimuli Responsive Materials ---Design, Synthesis, and Characterization
2074, 1964, 1681, 1605, 1512, 1473, 1410, 1342, 1255, 1024, 960, 831, 793, 765, 615,
526. Elemental analysis: C: 63.82, H: 6.27. Found: C: 63.03, H: 6.42.
Poly{2,5-bis-[2-(4-(2-N,N,N-diethylmethylaminoethoxy)-phenyl)-vinyl]-1,4-ben-
zene-alt-2,5-bis-(3-sulfonatopropoxy)-1,4-benzene} (P2). After Suzuki coupling
described above, the copolymer coupled by monomer (10) and (6) was deprotected using
hydrogen gas and Pd/C as catalyst in THF solution for two weeks (crude NMR was taken
periodically to monitor the deprotection). The solution was passed through celite filter gel
to remove the catalyst, and concentrated to get a solid. The solid was dissolved in DMSO
mixed with excess 1,3-propanesultone at 55 ºC and reaction was stirred for another 3
days. The mixture was cooled to room temperature and excess CH3I was added for
quarternization. The solution was stirred for 3 more days, followed by purification
methods as discribed for P1. 1H NMR (D2O, δ ppm): 7.44 (b, Ar-H, CH=CH), 7.05 (b,
Ar-H), 6.92 (b, Ar-H), 6.85 (b, CH=CH), 3.98 (b, CH2), 3.75(b, CH2), 3.00 (b, CH2),
2.81 (b, CH2), 2.11 (b, CH3), 2.00 (b, CH2), 1.16 (b, CH3). 13C NMR (D2O, δ ppm):
152.7, 152.5, 149.6, 136.5, 134.9, 131.8, 130.9, 129.1, 128.3, 125.9, 117.2, 116.0, 116.3,
68.5, 67.4, 48.2, 47.8, 39.5, 39.2, 38.7, 26.5, 24.5, 7.24. FTIR (KBr, cm-1): 2963, 2881,
2449, 2363, 2270, 2201, 2071, 1961, 1647, 1610, 1509, 1407, 1385, 1341, 1238, 1031,
957, 798, 762, 735, 617, 529. Elemental analysis: C: 59.48, H: 6.45. Found: C: 58.23, H:
6.65.
Poly{2,5-bis(4-pyridyl-buta-1,3-dienyl)-1,4-benzene-alt-9-bis(3-N,N,N-trimethyl
aminopropyl)-9H-2,7-fluorene} (P3). The polymer was obtained following the same
procedure as for P1. 1H NMR (D2O, δ ppm): 8.49 (b, Py-H), 7.88 (b, Py-H), 7.61 (b, Py-
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H), 7.50 (b, Ar-H, Py-H), 7.28 (b, Ar-H, CH=CH-CH=CH), 6.70 (b, Ar-H, CH=CH-
CH=CH), 2.95 (b, CH2), 2.72 (b, CH2), 2.45 (b, CH2), 2.30 (b, CH3). 13C NMR (D2O, δ
ppm): 150.1,144.9, 144.1, 143.3, 141.3, 141.0, 136.7, 136.3, 133.3, 132.9, 132.7, 131.8,
130.6, 130.3, 129.3, 128.7, 128.2, 127.7, 127.1, 124.9, 122.9, 122.6, 120.2, 116.9, 111.7,
68.3, 52.4, 48.0, 39.4, 38.5, 22.3. FTIR (KBr, cm-1): 3013, 2951, 2925, 2864, 1617, 1475,
1411, 1385, 1237, 1054, 1025, 966, 911, 818, 789, 705, 672, 617, 538. Elemental
analysis: C: 84.20, H: 7.79. Found: C: 83.37, H: 8.01.
Poly{2,5-bis-[2-(4-(2-N,N,N-diethylmethylaminoethoxy)-phenyl)-vinyl]-1,4-benz
ene-alt-2,5-pyridine} (P4). The polymer was obtained following the same procedure as
for P1. 1H NMR (DMSO-d6, δ ppm): 8.40 (b, Py-H), 7.97 (b, Ar-H), 7.70 (b, Ar-H), 7.68
(b, Ar-H), 7.52 (b, Ar-H), 7.34 (b, Ar-H, CH=CH), 6.89 (b, CH=CH), 4.30 (b, CH2),
3.74 (b, CH2), 3.48 (b, CH2), 2.25 (b, CH3), 1.27 (b, CH3). 13C NMR (DMSO-d6, δ ppm):
158.1, 151.4, 142.4, 140.3, 137.1, 132.6, 130.6, 130.3, 129.0, 128.7, 123.1, 122.8, 120.4,
115.5, 61.9, 59.0, 57.0, 26.5, 8.24. FTIR (KBr, cm-1): 2964, 2936, 2818, 1631, 1603,
1513, 1458, 1389, 1367, 1348, 1293, 1255, 1176, 1046, 958, 852, 824, 607, 543, 523.
Elemental analysis: C: 79.70, H: 8.32. Found: C: 78.91, H: 8.47.
Poly{2,5-bis-[2-(4-(3-sulfonatopropoxy)-phenyl)-vinyl]-1,4-benzene-alt-1,4-benz
ene} (P5). The polymer was obtained following the same procedure as for P1. 1H NMR
(D2O, δ ppm): 7.85 (b, Ar-H), 7.75 (b, Ar-H), 7.70 (b, Ar-H), 7.66 (b, Ar-H), 7.57 (b,
Ar-H), 7.33 (b, Ar-H, CH=CH), 6.91 (b, CH=CH), 3.61 (b, CH2), 2.92 (b, CH2), 1.98 (b,
CH2). 13C NMR (D2O, δ ppm): 163.5, 158.9, 140.5, 132.2, 131.7, 130.4, 129.8, 129.6,
128.4, 127.8, 127.4, 127.2, 114.9, 102.1, 68.6, 65.6, 47.8, 46.0, 28.6, 24.5. FTIR (KBr,
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Water Soluble Multifunctional Cross-conjugated Poly(para-phenylenes) as Stimuli Responsive Materials ---Design, Synthesis, and Characterization
cm-1): 3036, 2941, 2869, 1670, 1604, 1574, 1512, 1473, 1388, 1366, 1254, 1206, 1050,
956, 889, 850, 814, 737, 619, 585, 530. Elemental analysis: C: 57.60, H: 4.27. Found: C:
56.71, H: 4.40.
Poly{2,5-bis-[2-(4-(2-N,N,N-diethylmethylaminoethoxy)-phenyl)-vinyl]-1,4-benz
ene-alt-1,4-benzene} (P6). The polymer was obtained following the same procedure as
for P1. 1H NMR (DMSO-d6, δ ppm): 7.57-7.61 (b, Ar-H), 7.32 (b, Ar-H), 7.44 (b, Ar-H),
7.26 (b, Ar-H, CH=CH), 6.94 (b, CH=CH), 4.40 (b, CH2), 3.41 (b, CH2), 3.03 (b, CH2),
2.81 (b, CH3), 1.25 (b, CH3). 13C NMR (DMSO-d6, δ ppm): 157.0, 139.2, 135.6, 132.0,
131.5, 131.1, 130.8, 129.6, 129.4, 129.0, 128.8, 127.8, 127.6, 114.9, 61.2, 58.6, 56.4, 47.2,
11.7, 7.6. FTIR (KBr, cm-1): 2975, 2927, 1701, 1654, 1604, 1510, 1465, 1395, 1304,
1241, 1176, 1115, 1059, 1013, 961, 833, 704, 619, 534. Elemental analysis: C: 81.78, H:
8.50. Found: C: 81.51, H: 8.59.
3.3. Results and discussion
3.3.1. Characterization
All polymers were soluble in water, partially soluble in dimethylformamide (DMF)
and dimethylsulfoxide (DMSO), except P4 and P6, which were readily soluble in all
three solvents. Molecular weights of the polymers were determined using gel permeation
chromatography with reference to polyethylene glycol standards and DMF as eluent and
the observed values were within the range of 7440 to 37640 (Table 3.1). Owing to the
polar nature and presence of many ligating groups on the polymer backbone, increasing
the molecular weight was difficult. P1 for example, had the lowest molecular weight with
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the number of repeating unit of ca. 8. The solubility of one of the intermediate 5 was poor
in DMF while the other intermediate 9 was readily soluble, which caused significant
difference in the effective concentrations of both compounds in DMF solution. This may
explain why the values of the molecular weights were not so ideal. On the other hand, P1
and P2 showed considerably narrow PDIs, which was deviated from ideal value 2 for
polycondensation. This is most likely due to the removal of the most and least soluble
fractions, as we used dialysis with MWCO 3.5 K to remove the low MW fractions and
syringe filtration with pore size of 0.25 μm to remove potential high MW aggregates.
Other solvents (e.g. piperidine) and phase transfer catalysts (tetrabutylammonium
bromide) were also tested with no obvious improvement in the molecular weight. Many
conjugated polymers were reported to have structural defects which disrupt the effective
conjugation length.19-26
3.3.2. Thermal properities
Thermogravimetric analysis (TGA) was done using a heating rate of 10 ºC/min up to
1000 ºC under N2 atmosphere. All polymers showed reasonably good thermal stability
(Table 3.1). It is interesting to note that P1, P2, P3, and P6 showed similar thermal
stability; P4 showed lowest stability close to P2 and P6, while P5 was the most stable
one among the polymers. In view of the structures, P1 and P3 are incorporated with 3-
(N,N,N-trimethylamino)propyl-9H-2,7-fluorene, and P2, P4, and P6 contain 2-(N,N-
diethylmethylamino)ethoxy groups along the polymer backbone. Therefore, these groups
decompose/degrade readily at high temperatures. P1-P6 showed similar thermal stability
compared with previously reported linear PPP polymers.8,10,27
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Water Soluble Multifunctional Cross-conjugated Poly(para-phenylenes) as Stimuli Responsive Materials ---Design, Synthesis, and Characterization
Table 3.1. GPC, TGA, UV-Vis absorption and emission maxima of P1-P6
Polymer Mn Mw PDI Tdecomp (ºC)
Absorption Max. (nm)
Emission Max. (nm)
P1 7400 8000 1.08 237 326 454 P2 8900 10000 1.12 290 313 449 P3 9800 15100 1.54 238 359 407, 428 P4 9200 17300 1.88 212 349 435 P5 37600 62300 1.66 377 277 468 P6 11200 21900 1.96 285 283 and 324 448
250 300 350 400 450 500
Abso
rben
ce (a
.u.)
Wavelength (nm)350 400 450 500 550 600 650
Inte
nsity
(a.u
.)
Wavelength (nm)
A B
Figure 3.3. Normalized absorption (A) and emission (B) spectra of P1 (■), P2 (●), P3 (▲), P4 (□), P5 (○) and P6 ( ) in water (20 mg/L)△
3.3.3. Absorption and emission spectra
As seen in the case of TGA results, polymers having similar functional groups
showed comparable absorption and emission properties. UV-vis absorption and emission
spectra of P1-P6 are shown in Figure 3.3, and peak maxima are summarized in Table
3.1. All characterizations were done in aqueous solution with a polymer concentration of
20 mg/L. All polymers showed absorption maxima between 325 and 360 nm, which
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corresponds to a π-π* transition and a red shift in λmax (Δλmax ~ 25 nm), as compared with
linear water-soluble PPPs without conjugated side chain.28 P2 and P6 with a 2-(N,N-
diethyl-N-methylamino)ethoxy group showed absorption maxima in the range of 280-320
nm; P3 and P4 with pyridyl groups exhibit absorption maxima at 355 nm, which is red-
shifted as compared to other linear PPP polymers. This may be due to the strong electron-
withdrawing effect of the pyridine ring, lowering the LUMO energy and band gap.
Absorption spectra of P5 and P6 were broad indicating possible aggregation in solution.
P1 showed a blue shift in the emission maximum (452 nm) compared with that of P5
(468 nm), indicating its lack of planarity caused by the bulky 3-(N,N,N-
trimethylamino)propyl-9H-2,7-fluorene unit. P3 showed two emission maxima (407 and
428 nm), while the emission maximum for P4 was at 434 nm. Compared with the linear
water-soluble sulfonated PPPs18 which showed emission maxima around 400 nm, a
significant red shift of 25-65 nm was observed for all polymers owing to the extended π-
conjugation. Compound 5 with the phenylene vinylene unit (Scheme 3.1) showed a blue
shift in emission maximum (396 nm) as compared with the corresponding polymers P1
(452 nm) and P5 (468 nm), possibly due to reduced conjugation. However, the formation
of excimer could not be neglected in our studies.29 The emission measurements were also
done in dilute solution, but no significant peak shift was observed.
3.3.4. Titration studies
Target polymers functionalized with different binding groups should possess sensing
behavior for various charged molecules. Five commercially available quenchers were
used in these studies, including acid blue 45 (50% dye content (AB)), 9,10-
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Water Soluble Multifunctional Cross-conjugated Poly(para-phenylenes) as Stimuli Responsive Materials ---Design, Synthesis, and Characterization
anthraquinone-2,6-disulfonic acid disodium salt (AD), potassium hexacyanoferrate(III)
(PF), benzyl viologen (BV), and ferrous chloride (FC). The first three are anionic, and the
rest are cationic in nature. All fluorescence quenching studies with different quenchers
were carried out in aqueous solution with a polymer concentration of 20 mg/L. The
concentrations of the quenchers varied in micromolar level based on the sensitivity of
each polymer. The fluorescence spectra were recorded for titration of quenchers into
polymer aqueous solutions, as illustrated in Figure 3.4–3.8. The static quenching through
the formation of a complex is assumed to be the only way in our system due to strong
electrostatic interactions between the polymer and small molecular quenchers; the
quenching efficiency was expressed by the following Stern-Volmer equation.30
][10 QKFF sv+=
Some of the data collected from titration experiments showed upward curves at high
concentrations of the quenchers (Figure 3.9); the values of Ksv calculated were based on
the linear region at low concentrations. Theoretically, binding of one quencher molecule
to one polymer chain is sufficient for quenching the fluorescence of that chain.31
However, if we take into account the aggregation induced self-quenching, which is quite
significant in the case of P1-P6 (see discussion below), the Ksv constants reported in
Table 3.2 could be underestimated. Polymers P1 and P2 have been functionalized with
both cationic and anionic groups which enhance the sensing properties through
electrostatic interactions. P3 and P4 are interesting because of the presence of pyridyl
groups on the polymer backbone which is highly sensitive to metal ions. However, the
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pyridyl moiety alone did not enhance solubility of polymers in water; hence, quaternary
ammonium groups were incorporated on the side chains.
All polymers showed good sensitivity to the quenchers with an average value of Ksv
around 105 (Table 3.2). P1 and P2 showed lower sensitivities, which was explained using
two factors. First, the low molecular weights might reduce the effective conjugation
length, depressing the signal amplification; it is also possible that the zwitterionic
character of the polymers results in competing electrostatic interactions: attraction and
repulsion. The repulsion force inevitably prevents the analytes from approaching closer to
a donor-acceptor distance (r) to the functional groups participating in the complexation.
According to Forster theory of dipole-dipole interaction,32 the rate of energy transfer (k)
is inversely proportional to r6, expressed as the following equation:
)(1 26 λκ J
rk ××∝
The electrostatic attraction might not be as "strong" as the case in other polymers.
P3 and P4 showed different sensitivities to anionic and cationic species. During the
titration experiments of P3 with Fe2+, the coordination of Fe2+ to nitrogen atom of the
pyridyl group occured on a fast time scale. Both P3 and P4 showed low response to
organic cations such as benzyl viologen as compared with inorganic Fe2+ ion. This may
be due to several competing factors such as steric hindrance of benzyl groups of benzyl
viologen, hydrophobicity, and high degree of solvation of viologen moiety.33
Complexation also induced a significant shift in the fluorescence maxima (λemi)
along with quenching. In the case of P2, titration of acid blue 45 (AB) and benzyl
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Water Soluble Multifunctional Cross-conjugated Poly(para-phenylenes) as Stimuli Responsive Materials ---Design, Synthesis, and Characterization
viologen (BV) induced blue shift, while titration of potassium hexacyanoferrate (PF)
showed a red shift in λemi along with decrease in intensity. Similar results observed for
other polymers and the values are summarized in Table 3.3. From the above studies, it is
possible to summarize the following: (i) the acid blue 45 induced a blue shift in λemi for
all polymers; (ii) only potassium hexacyanoferrate induced a red shift in λemi for a few
polymers (P1, P2, and P4); (iii) ferrous chloride induced no shift in λemi, except P5; (iv)
complexation with pyridine did not affect the λemi, but the emission of P3 changed to a
single broad peak in presence of benzyl viologen; (v) P5 and P6 with single functional
groups showed a blue shift in the λemi.
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Figure 3.4. Fluorescence spectra of P1 with titration of AB (A, 0, 5, 10, 16, 33, 66, 132, 200, 333 μM), AD (B, 0, 5, 10, 16, 33, 66, 132, 200, 333 μM), PF (C, 0, 5, 10, 16, 33, 66, 132, 200, 333 μM), BV (D, 0, 1.3, 2.5, 4, 8.4, 16.8, 33.6, 50, 84 μM), FC (E, 0, 2.5, 5, 8, 17, 33, 67, 100, 167, μM) in water solution with concentration of 20 mg/L. Direction of intensity changes is indicated by the arrow. Quencher concentrations are indicated in each figure and parenthesis.
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Water Soluble Multifunctional Cross-conjugated Poly(para-phenylenes) as Stimuli Responsive Materials ---Design, Synthesis, and Characterization
Figure 3.5. Fluorescence spectra of P2 with titration of AB (A, 0, 0.5, 1, 1.6, 3.3, 6.6, 13.2, 20, 33 μM), AD (B, 0, 0.5, 1, 1.6, 3.3, 6.6, 13.2, 20, 33 μM), PF (C, 0, 0.5, 1, 1.6, 3.3, 6.6, 13.2, 20, 33 μM), BV (D, 0, 0.6, 1.3, 2.5, 4, 8.4, 16.8, 33.6, 50, 84 μM), FC (E, 0, 1.3, 2.5, 5, 8, 17, 33, 67, 100, 167, μM) in water solution with concentration of 20 mg/L. Direction of intensity changes is indicated by the arrow. Quencher concentrations are indicated in each figure and parenthesis.
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Figure 3.6. Fluorescence spectra of P3 with titration of AB (A, 0, 0.05, 0.1, 0.16, 0.33, 0.66, 1.32, 2, 3.3 μM), AD (B, 0, 0.05, 0.1, 0.16, 0.33, 0.66, 1.32, 2, 3.3 μM), PF (C, 0, 0.05, 0.1, 0.16, 0.33, 0.66, 1.32, 2, 3.3 μM), BV (D, 0, 5, 10, 16, 33, 66, 132, 200, 333 μM), FC (E, 0, 1, 2, 3.2, 6.6, 13.2, 26.4, 40, 67 μM) in water solution with concentration of 20 mg/L. Direction of intensity changes is indicated by the arrow. Quencher concentrations are indicated in each figure and parenthesis.
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Water Soluble Multifunctional Cross-conjugated Poly(para-phenylenes) as Stimuli Responsive Materials ---Design, Synthesis, and Characterization
Figure 3.7. Fluorescence spectra of P4 with titration of AB (A, 0, 0.5, 1, 1.6, 3.3, 6.6, 13.2, 20, 33 μM), AD (B, 0, 0.5, 1, 1.6, 3.3, 6.6, 13.2, 20, 33 μM), PF (C, 0, 0.5, 1, 1.6, 3.3, 6.6, 13.2, 20, 33 μM), BV (D, 0, 5, 10, 16, 33, 66, 132, 200, 333 μM), FC (E, 0, 1.3, 2.5, 5, 8, 17, 33, 67, 100, 167, μM) in water solution with concentration of 20 mg/L. Direction of intensity changes is indicated by the arrow. Quencher concentrations are indicated in each figure and parenthesis.
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Figure 3.8. Fluorescence spectra of P5 with titration of BV (A, 0, 1, 2, 3.2, 6.6, 13.2, 26.4, 40, 67 μM), FC (B, 0, 1, 2, 3.2, 6.6, 13.2, 26.4, 40, 67 μM), and P6 with titration of AB (C, 0, 0.5, 1, 1.6, 3.3, 6.6, 13.2, 20, 33 μM), AD (D, 0, 0.25, 0.5, 0.84, 1.67, 3.3, 6.6, 10, 16.7 μM), PF (E, 0, 0.5, 1, 1.6, 3.3, 6.6, 13.2, 20, 33 μM) in water solution with concentration of 20 mg/L. Direction of intensity changes is indicated by the arrow. Quencher concentrations are indicated in each figure and parenthesis.
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Water Soluble Multifunctional Cross-conjugated Poly(para-phenylenes) as Stimuli Responsive Materials ---Design, Synthesis, and Characterization
Figure 3.9. Titration curves of P1 (○), P2 (●), P3 (▲), P4 (□) and P6 (■) (F) in presence of potassium hexacyanoferrate(III). Polymer concentration was 20 mg/L.
Table 3.2. Stern-Volmer constant Ksv for P1-P6 with titration of five quenchers in water
Ksv(M-1) Quenchers P1 P2 P3 P4 P5 P6
AB 6.4×104 1.6×105 5.5×106 7.0×105 N/A 3.7×105
AD 4.0×104 7.2×105 4.5×106 3.1×105 N/A 9.6×105
PF 5.4×104 3.6×105 6.3×106 2.2×105 N/A 3.4×105
BV 2.2×105 4.6×105 6.2×104 8.0×104 5.4×105 N/A
FC 1.5×105 1.6×105 2.5×105 1.2×105 4.8×105 N/A
Table 3.3. Changes in emission maxima in presence of added quenchers in water
Quenchers P1 P2 P3 P4 P5 P6 AB B(26nm) B(23nm) B(16nm) B(20nm) N/A B(10nm) AD N N B(14nm) N N/A B(10nm) PF R(22nm) R(24nm) B(16nm) R(30nm) N/A N BV N B(10nm) M N B(19nm) N/A FC N N N N B(12nm) N/A
B: blue shift R: red shift N: no change M: merging of peaks. Shifting values after complete quenching were indicated in parenthesis.
3.3.5. Aggregation properties
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Water-soluble CPs show aggregation in water,28 which should also be the case with
our zwitterionic cross-conjugated systems. Furthermore, there are two interesting but
contradictory phenomena: first, the aggregation of polymer chains was confirmed by the
blue shift in λemi in the presence of charged quenchers (usually organic) which may act as
cross-links between polymer chains. It may also disrupt the order of the stacked polymer
chains. Second, aggregation of polymer chains with certain degree of order is also
possible (red shift in λemi) with the addition of the highly charged quenchers like
[Fe(CN)6]3- which reduce the repulsion of the similar functional groups along the
polymer chain. Thus, the preliminary data collected here shall imply an alternative way
for fine-tuning photophysical properties of the cross-conjugated polymers.
Titration studies in aqueous solution with 20 mM NaCl and 100 mM NaCl were
carried out to examine the role of ionic strength in fluorescent quenching. It was observed
that with increasing ionic strength the sensitivity, i.e., value of Stern-Volmer constant,
was decreased. A significant decrease in Ksv was observed in the case of P2 with addition
of AB in 100 mM saline compared with the values obtained from pure water (Table 3.4
and 3.5). This is because the presence of salt leads to disaggregation or decomplexation
of the polymer in solution, which become stronger with increase in ionic strength. The
titration study could not be conducted in buffer solutions such as PBS or SSC due to
strong quenching by phosphate and citrate anions present in the buffer solution.
Table 3.4. Ksv for P1-P6 with titration of five quenchers in 20 mM NaCl solution
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Water Soluble Multifunctional Cross-conjugated Poly(para-phenylenes) as Stimuli Responsive Materials ---Design, Synthesis, and Characterization
Ksv (M-1)
Quenchers
P1 P2 P3 P4 P5 P6
AB 2.9×104 0.5×105 2.4×106 2.7×105 N/A 1.3×105
AD 1.8×104 3.0×105 1.9×106 3.1×105 N/A 4.7×105
PF 2.4×104 2.1×105 2.0×106 0.7×105 N/A 2.4×105
BV 1.4×105 3.2×105 2.8×104 5.4×104 2.9×105 N/A
FC 0.7×105 0.9×105 1.0×104 0.4×105 2.8×105 N/A
Table 3.5. Ksv for P1-P6 with titration of five quenchers in 100 mM NaCl solution
Ksv (M-1)
Quenchers P1 P2 P3 P4 P5 P6
AB 1.8×104 0.3×105 1.5×106 2.0×105 N/A 0.7×105
AD 1.2×104 2.1×105 1.3×106 1.8×105 N/A 3.1×105
PF 1.3×104 1.0×105 1.4×106 0.4×105 N/A 1.3×105
BV 0.9×105 2.6×105 2.1×104 4.1×104 1.4×105 N/A 5FC 0.5×10 0.4×105 0.7×105 0.3×105 1.6×105 N/A
The aggregation behavior of P2 induced by the sodium salt of 9,10-anthraquinone-
2,6-disulfonic acid (AD) was studied using DLS and UV-Vis in water with polymer
concentration of 20 mg/L (Figure 3.10). The pure polymer solution showed a mean
hydrodynamic diameter of around 118 nm, indicating the aggregation of polymer chains.
With increasing the concentration of quencher, the effective diameter increased almost
exponentially, indicating significant aggregation. Notably the hydrodynamic diameter of
the aggregates was increased to 4 μm when the quencher concentration was 43.3 μM, and
the solution became turbid. A UV-Vis study showed that with increasing concentration of
the quencher, the peak gradually shifted to longer wavelength indicating the aggregation,
but when the concentration of the quencher went beyond a critical poinit, the curve
suddenely collapsed, which should correspond to the precipitation of aggregates from
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solution. We also found that, unlike fluorescence quenching, the aggregation process was
not instantaneous; therefore, the solutions were mixed well and incubated for 3 min
before each measurement. Aggregation often reduces the quantum efficiency of the
emission via strong intermolecular charge transfer, which should provide added
quenching efficiency. However, the values of Stern-Volmer constants were close to
values reported for sulfonated PPPs,28 probably due to partial self-quenching among the
polymer chains.
Figure 3.10. (A) Hydrodynamic diameter profile obtained from DLS for P2 with titration of 9,10-anthraquinone-2,6-disulfonic acid disodium salt (AD) in water. (B) UV-Vis absorption spectra of P2 with titration of AD (0, 0.5, 1, 1.6, 3.3, 6.6, 13.2, 20 μM) in water. Direction of peak shift is indicated by the arrow. Polymer concentration was 20 mg/L.
3.3.6. Thin film and nanofiber
For the device fabrications, it is important that the polymer should be soluble and
processable. Enhancement of performance may be realized in solid state due to close
proximity of the polymer chains, facilitating the exciton diffusion.65
0 20 40 60 80 100 120 1400
1000
2000
3000
4000
430 nm
Effe
ctiv
e di
amet
er (n
m)
Titrition Volume (µL)
118 nm 229 nm
1109 nm
4039 nm A
350 400 4500.0
0.2
0.4
0.6
0.8
1.0
1.2
Abs
orbe
nce
(a.u
.)
Wavelength (nm)
B
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Water Soluble Multifunctional Cross-conjugated Poly(para-phenylenes) as Stimuli Responsive Materials ---Design, Synthesis, and Characterization
C D
10 μm
Figure 3.11. SEM micrograph of the thin film (A) indicating the thickness of the film (B), confocal and optical (inset) micrographs of thin films of P1 prepared from a water solution with a concentration of 10mg/mL on a glass plate (C), SEM and confocal (inset) micrographs of nanofibers made from P1 with 5% vinyl alcohol as cross-linker.
Our water soluble polymers were capable of forming thin films simply via
controlled evaporation of the aqueous polymer solutions. A self supporting films with an
average thickness around 10 μm is shown in Figure 3.11-A and B. The thickness of the
films can be varied by varying the concentration of the polymer solution and fabrication
methodologies. The film had smooth and uniform morphology and intense blue emission
after exposure to UV light (Figure 3.11-C). The films prepared from P3, P4 and P6 were
highly flexible and further improvement in film quality can be achieved by blending the
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polymers with other polymers such as poly(vinyl alcohol) (PVA). Therefore, it is possible
to use the polymers either in solution or in the solid state. Removal of water after casting
the film under different concentrations, temperatures and pressures were investigated, but
no significant changes in optical or morphological properties of the film were observed.
Nanofibers were also successfully fabricated by electrospinning (5 wt%, 30 kV AC). The
fibers were highly fluorescent as well but no more water soluble due to crosslinking of
PVA. This could be quite important in real application in aqueous solution since we don’t
want the polymer to be leached out.
3.4. Conclusion
A series of water-soluble cross-conjugated PPP derivatives have been synthesized
and fully characterized. All polymers incorporated with multiple functional groups on the
backbone and conjugated side chain are thermally stable. Optical studies indicated an
extended conjugated π-electron system with collective response arising from the
respective chromophores. Cross-conjugated structures offer possible exciton migration
pathways in the presence of a quencher. Titration studies with various organic and
inorganic compounds showed good sensitivity to the analytes at the micromolar
concentration, as indicated by fluorescence quenching. The aggregation of the polymer
was investigated using dynamic light scattering technique. Complexation also induced
blue or red shift of λmax, depending on concentration of the quencher, structure of the
polymer, and added quencher molecules. All polymers formed thin, smooth, and uniform
films with high emission characteristics. These results contribute a novel way to extend
the capability of CPs in chemo- and biosensor applications.
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Water Soluble Multifunctional Cross-conjugated Poly(para-phenylenes) as Stimuli Responsive Materials ---Design, Synthesis, and Characterization
3.5. References
1. Wallow, T. I.; Novak, B. M. J. Am. Chem. Soc. 1991, 113, 7411.
2. Kim, I. -B.; Dunkhorst, A.; Gilbert, J.; Bunz, U. H. F. Macromolecules 2005, 38,
4560.
3. Ksenijia, H. G.; Pinto, M. R.; Tan, C. Y.; Schanze, K. S. J. Am. Chem. Soc. 2004,
126, 14964.
4. Vetrichelvan, M.; Li, H. R.; Renu, R; Valiyaveettil, S.; J. Polym. Sci. Part A:
Polym. Chem. 2006, 44, 3763.
5. Child, A. D.; Reynolds, J. R.; Macromolecules 1994, 27, 1975.
6. Chem, L.; McBranch, D. W.; Wang, H.; Helgeson, R.; Wudl, F.; Whitten, D. G.
Proc. Natl. Acad. Sci. U. S. A. 1999, 96, 12287.
7. Pinto, M. R.; Kristal, B. M.; Schanze, K. S. Langmuir 2003, 19, 6523.
8. Fan, Q. L.; Lu, S.; Lai, Y. H. Macromolecules 2003, 36, 6976.
9. Harrison, B. S.; Ramey, M. B.; Reynolds, J. R.; Schanze, K. S. J. Am. Chem. Soc.
2000, 122, 8561.
10. Balanda, P. B.; Ramey, M. B.; Reynolds, J. R. Macromolecules 1999, 32, 3970.
11. Patil, A. O.; Ikeniue, Y.; Wudl, F.; Heeger, A. J. J. Am. Chem. Soc. 1987, 109,
1858.
12. Pickup, P. J. Electroanal. Chem. 1987, 225, 273.
13. Campbell, L. J.; Borges, L. F.; Heldrich, F. J.; Bioorg. Med. Chem. Lett. 1994, 4,
2627.
14. Otsubo, T.; Kohda, T.; Misumi, S. Bull. Chem. Soc. Jp. 1980, 53, 512.
127
Li Hairong National University of Singapore
15. Shen, H. L.; Huang, F.; Hou, L. T.; Wu, H. B.; Cao, W.; Yang, W.; Cao, Y. Synth.
Met. 2005, 152, 257.
16. Ma, W. L.; Iyer, P. K.; Gong, X.; Liu, B.; Moses, D.; Bazan, G. C.; Heeger, A. J.
Adv. Mater. 2005, 17, 274.
17. Baskar, C.; Lai, Y. H.; Valiyaveettil, S. Macromolecules 2001, 34, 6255.
18. Mandolesi, S. D.; Vaillard, S. E.; Podesta, J. C.; Rossi, R. A. Organometallics
2002, 21, 4886.
19. Franco, I.; Tretiak, S. Chem. Phys. Lett. 2003, 372, 403.
20. Chen, Y. S.; Meng, H. F. Phys. Rev. B: Conden. Matt. Mater. Phys. 2002, 66,
035202/1.
21. Grozema, F. C.; van Duijnen, P. Th.; Berlin, Y. A.; Ratner, M. A.; Siebbeles, L.
D. A. J. Phys. Chem. B 2002, 106, 7791.
22. Freo, L. D.; Painelli, A.; Girlando, A.; Soos, Z. G. Synth. Met. 2001, 116, 259.
23. Yurtsever, E.; Yurtsever, M. Synth. Met. 1999, 101, 335.
24. Baughman, R. H.; Shacklette, L. W. J. Chem. Phys. 1989, 90, 7492.
25. Kürti, J.; Kuzmany, H.; Nagele, G. J. Mol. Elec. 1987, 3, 135.
26. Baughman, R. H.; Chance, R. R. J. Appl. Phys. 1976, 47, 4295.
27. Huang, F.; Wu, H. B.; Wang, D. L.; Yang, W.; Cao, Y. Chem. Mater. 2004, 16,
708.
28. Vetrichelvan, M.; Li, H. R.; Renu, R; Valiyaveettil, S.; J. Polym. Sci. Part A:
Polym. Chem. 2006, 44, 3763.
29. Ding, L.; Egbe, D. A. M.; Karasz, F. E. Macromolecules 2004, 37, 6124.
128
Water Soluble Multifunctional Cross-conjugated Poly(para-phenylenes) as Stimuli Responsive Materials ---Design, Synthesis, and Characterization
30. Pesce, A. J. In Fluorescence Spectroscopy, Pesce, A. J., Rosen, C. G., Pasby, T.
L., Eds.; Marcel Dekker Inc.: New York, 1971.
31. Thomas III, S. W.; Joly, G. D.; Swager, T. M. Chem. Rev. 2007, 107, 1339.
32. Förster, Th. Naturwissenschaften 1946, 33, 166.
33. Fan, C. H.; Hirasa, T.; Plaxco, K. W.; Heeger, A. J. Langmuir 2003, 19, 3554.
34. Nesterov, E. E; Zhu, Z. Swager, T. M. J. Am. Chem. Soc. 2005, 127, 10083.
129
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Li, Hairong; Manoj, P.; Nurmawati, M. H.; Xu, Q. H.; Valiyaveettil, S. Macromolecules 2008, 41, 8473. Li, Hairong; Nurmawati, M. H.; Valiyaveettil, S. Poly. Mater. Sci. Eng. 2007, 97, 617.
CChhaapptteerr 44
SSyynntthheessiiss aanndd SSttrruuccttuurree--PPrrooppeerrttyy SSttuuddiieess ooff NNoovveell PPoollyy((pp--pphheennyylleenneess)) wwiitthh
CCoonnjjuuggaatteedd SSiiddee CChhaaiinnss
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Synthesis and Structure-Property Studies of Novel Poly(para-phenylenes) with Conjugated Side Chains
4.1. Introduction
Besides the commonly studied linear conjugated polymers and oligomers, molecules
based on cross-conjugated platforms has been gaining great interests in the past few
years.1-5 They are usually divided into two classes: acyclic systems such as dendralenes6-
10, iso-polydiacetylenes11-12, iso-polytriacetylenes13-15, poly(phenylene vinylidene)s16-18,
and cyclic systems mainly referred to radialene derivatives19-30. In Chapter 3, we
discussed a novel series of water soluble cross-conjugated PPPs with interesting
aggregation behavior in water driven by electrostatic and π-π interactions31. Specifically,
our interest lies in understanding how and to what extent does the π-π interaction affect
the photophysical properties of cross-conjugated polymers. In order to avoid the
involvement of electrostatic and hydrophobic interactions in aqueous medium, we have
designed and synthesized a new series of organic soluble polymers with cross-conjugated
structures (Figure 4.1). This strategy provides a route for tuning the optical properties of
the polymers by extending the conjugation length. It is anticipated that the structure of
the polymer backbone should possess strong π-π interactions among the polymer chains,
which provide added effect on tuning photophysical properties and favored self-assembly
in solid state. Furthermore, useful information could be elicited through investigation of
relationship between rigid backbone and flexible bisdiene side chain. Our strategy offers
an alternative approach for introducing different conjugated side chains onto backbone
for tuning optical properties with an added flexibility to incorporate multiple functional
groups such as electron donating/withdrawing groups on the conjugated side chains.
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OR
RO
C12H25 C12H25
P1P2, R=C6H13P3, R=C12H25
P5
S
N
n n
n
n
P4
Figure 4.1. Chemical structures of five polymers.
4.2. Experimental
SEM images were taken with a JEOL JSM 6700 scanning electron microscope.
HRTEM images were taken using a JEOL 3010 at an accelerating voltage of 300 kV. The
samples were carefully mounted on copper stubs with a double-sided conducting carbon
tape and sputter coated with platinum (2 nm) before examination. Fluorescence lifetimes
were measured using time-correlated single-photon counting technique (PicoQuant,
PicoHarp 300). The frequency-doubled output of a mode-locked Ti:Sapphire laser
(Tsunami, Spectra-Physics) was used for excitation of the sample at 400 nm. The output
pulses from Ti:Sapphire centered at 800 nm had a duration of 40 fs with a repetition rate
of 80 MHz. The Ti:Sapphire laser was pumped by 5 W output of a frequency doubled
diode Nd:YVO4 laser (Millennia Pro, Spectra-Physics). For life time measurements, the
132
Synthesis and Structure-Property Studies of Novel Poly(para-phenylenes) with Conjugated Side Chains
fluorescence intensity was collected by an optical fiber which is directed to the detector.
An avalanche photodiode (APD) was used as detector. Outputs of the APD (start pulse)
and a fast photodiode (stop pulse) were processed by the PicoHarp 300 module. The
width of the instrument response function was 100 ps.
4.3. Synthesis
The synthetic routes to the polymers are given in Scheme 4.1. 2,5-Dibromo-p-
xylene, hydroquinone, 2,7-dibromofluorene and carbazole were used as starting materials.
1,4-Dibromo-2,5-bis-bromomethyl-benzene (2),32 2,5-dibromohydroquinone (6),33 2,5-
dibromo-1,4-dialkoxy benzene (7),34 2,7-dibromo-9,9-bisdodecyl-9H-fluorene (10),35
3,6-dibromocarbazole (13),36 3,6-dibromo-9-hexyl-9H-carbazole (14)37 and all the
corresponding diboronic acids (8, 11, 15)38-39 were synthesized according to the reported
procedures.
1,4-Dibromo-2,5-bis-(4-phenyl-buta-1,3-dienyl)-benzene (4). A mixture of
compounds 2 (2 g, 4.8 mmol) and triethylphosphite (3.3 mL, 19.2 mmol) was heated to
reflux for 6 hr and the excess triethylphosphite was removed under reduced pressure. The
obtained phosphate (3) was dissolved in DMF (25 mL) and mixed with potassium t-
butoxide (1.8 g, 19.2 mmol) and cinnamaldehyde (3.6 mL, 28.8 mmol). The mixture was
stirred overnight at 50 ºC, poured into water, filtered and the crude solid was
recrystalized from MeOH, giving a yellow product with 75% yield.
1HNMR (THF-d8, δ ppm): 7.57-7.70 (m, 6H central Ar-H, terminal Ar-H), 7.30 (t, J =
7.5 Hz, 4H terminal Ar-H), 7.18 (t, J = 7.2 Hz, 2H, on terminal Ar-H), 6.90-6.96 (m, 4H,
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diene-H), 6.85 (d, J = 15.1 Hz, 2H, diene-H), 6.78 (d, J = 15.3 Hz, 2H, diene-H). 13C
NMR (THF-d8, δ ppm): 138.1, 136.0, 134.1, 130.8, (Ar-C), 129.8, 129.3, 128.6, 128.0,
127.4, 126.9, 126.5, (Ar-C and diene-C). FTIR (KBr, cm-1): 3022, 2924, 1753, 1610,
1458, 1446, 1365, 1053, 983, 887, 856, 821, 746, 684, 503. Elemental analysis: C: 63.44,
H: 4.10, Br 32.47. Found: C: 63.04, H: 4.41, Br 32.16.
Poly (2, 5-bis-(4-phenyl-buta-1, 3-dienyl)-1, 4-phenylene-alt-1, 4-thiophene) (P1):
Thiophene-2, 5- diboronic acid (0.41g, 2.34 mmol) and compound 4 (1g, 2.34 mmol)
were dissolved in 30 mL THF. K2CO3 solution (2 M, 20 mL) was added to the mixture
followed by tetrakis(triphenylphosphino) palladium (0) (3 mol %) as catalyst and
cetyltrimethyl ammonium bromide (30 mol %) as phase transfer agent. The mixture was
stirred at 70 ºC for 3 days, concentrated and precipitated twice from methanol and the
crude product was further purified by filtration through silica gel using dichloromethane
(DCM).
(P1): 1H NMR (CDCl3, δ ppm): 7.73 (b, 2H, central Ar-H), 7.38-7.45 (b, 6H,
terminal Ar-H, Thio-H), 7.31 (b, 4H, terminal Ar-H), 7.10-7.18 (b, 4H, terminal Ar-H,
Th-H), 6.89-6.96 (b, 6H, diene-H), 6.73 (b, 2H, diene-H). 13C NMR (CDCl3, δ ppm):
142.0, (Th-C), 137.2, 134.0, 133.2., 133.0, 131.6, 130.9, 130.4, (Th-C and Ar-C), 129.3,
128.6, 128.1, 127.7, 126.3, 125.9, (Ar-C, Th-C and diene-C). FTIR (KBr, cm-1): 3021,
2954, 2920, 2855, 1724, 1629, 1599, 1479, 1434, 1384, 1360, 1241, 1091, 1022, 983, 872,
805, 746, 690, 622, 509. Elemental analysis: C: 86.5, H: 6.09, S: 7.45. Found: C: 85.9, H:
6.63, S: 7.90.
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Synthesis and Structure-Property Studies of Novel Poly(para-phenylenes) with Conjugated Side Chains
(P2): 1H NMR (CDCl3, δ ppm): 7.62-7.70 (b, 6H, central Ar-H, terminal Ar-H),
7.55 (b, 2H, OAr-H), 7.30 (b, 4H, terminal Ar-H), 7.19 (b, 2H, terminal Ar-H), 6.90-6.99
(b, 6H, diene-H), 6.65 (b, 2H, diene-H)), 3.90 (b, ArOCH2CH2), 1.82 (b, ArOCH2CH2),
1.26 (b, (CH2)3CH3), 0.90 (b, CH3). 13C NMR (CDCl3, δ ppm): 150.9, 140.5, (OAr-C),
138.4, 136.2, 134.7, 132.0, 130.2, 130.1, (Ar-C and OAr-C), 129.2, 128.7, 127.9, 126.5,
125.4, 116.4, (Ar-C, OAr-C and diene-C), 69.5, 31.4, 29.3, 25.5, 22.5, 14.7, (Alk-C)
FTIR (KBr, cm-1): 3025, 2953, 2926, 2856, 1729, 1593, 1498, 1467, 1378, 1261, 989,
866, 800, 748, 690, 621, 507. Elemental analysis: C: 85.5, H: 8.23; Found: C: 84.85, H:
8.74.
(P3): 1H NMR (CDCl3, δ ppm): 7.58-7.73 (b, 8H, central Ar-H, terminal Ar-H,
OAr-H), 7.29 (b, 4H, terminal Ar-H), 7.18 (b, 2H, terminal Ar-H), 6.93 (b, 6H, diene-H),
6.69 (b, 2H, diene-H), 3.90 (b ArOCH2CH2), 1.80 (b, ArOCH2CH2), 1.24 (b, (CH2)3CH3),
0.88(b, CH3). 13C NMR (CDCl3, δ ppm): 151.9, 140.7, (OAr-C), 138.0, 136.1, 133.6,
132.4, 130.5, 130.1, (Ar-C and OAr-C), 129.3, 128.5, 127.5, 126.2, 125.4, 118.2, (Ar-C,
OAr-C and diene-C), 68.4, 31.8, 29.6, 27.2, 25.7, 22.5, 14.7, (Alk-C). FTIR (KBr, cm-1):
3025, 2922, 2852, 1729, 1592, 1484, 1468, 1378, 1261, 1214, 1028, 990, 868, 803, 784,
721, 690, 618, 506. Elemental analysis: C: 86.3, H: 9.54. Found: C: 85.71, H: 9.89.
(P4): 1H NMR (CDCl3, δ ppm): 7.70-7.84 (b, 6H, central Ar-H, Fl-H), 7.38-7.50 (b,
12H, Fl-H), 7.20-7.29 (b, terminal Ar-H, CHCl3), 6.90-7.02 (b, 6H, diene-H), 6.74 (b,2H,
diene-H), 3.60 (b, CH2 connected to sp3 carbon on fluorene), 2.00 (b, CH2 next to CH2
connected to sp3 carbon on Fl-H), 1.25 (b, (CH2)9CH3), 0.87 (b, CH3). 13C NMR (CDCl3,
δ ppm): 150.5, 141.5, (Fl-C), 140.7, 139.2, 137.2, 134.4, 131.0, 130.2, (Fl-C and Ar-C),
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129.3, 128.5, 128.2, 127.4, 126.2, 125.4, 124.7, 123.6, 119.7, (Fl-C, diene-C and Ar-C),
40.2, 31.5, 29.7, 23.8, 22.5, 14.7, (Alk-C). FTIR (KBr, cm-1): 3022, 2922, 2850, 1726,
1592, 1458, 1373, 1260, 1118, 1025, 983, 893, 823, 747, 688, 621, 536, 505. Elemental
analysis: C: 90.50, H: 9.49. Found: C: 90.05, H: 9.72.
(P5): 1H NMR (CDCl3, δ ppm): 8.21 (b, 2H, Cz-H), 7.82 (b, 2H, central Ar-H),
7.47-7.60 (m, 4H, Cz-H), 7.24-7.33 (b, terminal Ar-H, CHCl3), 7.16 (b, 2H, terminal Ar-
H), 6.85-6.99 (b, 6H, diene-H), 6.62 (b, 2H, diene-H), 4.37 (b, NCH2), 1.95 (b,
NCH2CH2), 1.36 (b, (CH2)3CH3), 0.91 (b, CH2CH3). 13C NMR (CDCl3, δ ppm): 140.8,
139.6, 134.5, 132.2, 131.6, 130.4, (Cbz-C and Ar-C), 129.8, 129.2, 128.4, 127.2, 126.2,
122.7, 121.4, 120.5, 118.9, 108.7, (Cbz-C, Ar-C and diene-C), 44.2, 32.5, 29.0, 27.0, 23.2,
14.7, (Alk-C). FTIR (KBr, cm-1): 3023, 2923, 2852, 1858, 1726, 1683, 1597, 1474, 1378,
1348, 1260, 1216, 1150, 1065, 887, 805, 747, 691, 624, 505. Elemental analysis: C: 90.4,
H: 7.25, N: 2.34. Found: 89.89, H: 7.75: N: 2.47.
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Synthesis and Structure-Property Studies of Novel Poly(para-phenylenes) with Conjugated Side Chains
OH
HO
OH
HO
BrBr
OR
RO
BrBr
OR
RO
B(OH)2(HO)2B
BrBr BrBr
CH2Br
BrH2C
BrBr
P(O)(OEt)2
(EtO)2(O)P
BrBr
BrBr BrBr
C12H25C12H25
B(OH)2(HO)2B
n-C12H25n-C12H25
HN
HN N N
Br Br Br Br (HO)2B B(OH)2
11
15
1 2 3 4
5 6 8a, R=C6H138b, R=C12H25
9 10
12 13 14
BrBr
4
SB(OH)2(HO)2B
8a, 8b
11
15
P1
P2, P3
P4
P5
7a, R=C6H137b, R=C12H25
Scheme 4.1. Synthetic routes for P1-P5.
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4.4. Results and discussion
4.4.1. Characterization
All polymers showed good solubility in common organic solvents such as THF,
dichloromethane (DCM), dimethylformamide (DMF) and toluene. But monomer 4
(Scheme 4.1) was less soluble in the above mentioned solvents. By incorporating a
second soluble monomer (i.e. boronic acids), soluble target polymers were obtained.
Molecular weights of all polymers were determined using gel permeation
chromatography with reference to polystyrene standards using THF as solvent. The
obtained values were in the range of 5.7 to 8.8 × 103 (Mn) with polydispersity index (PDI)
of 1.27 to 1.79 (Table 4.1).
Thermogravimetric anaysis (TGA) was done using a heating rate of 10 ºC/min up to
1000 ºC under N2 atmosphere. All polymers showed reasonably good thermal stability
(Table 4.1), up to 250 ºC. The decomposition temperatures varied among polymers and
P1 showed lowest stability due to the presence of thiophene units. All polymers
decomposed through a one step process except the P4 which showed a two-stage
decomposition, this may be due to the initial decomposition of long dodecyl alkyl chains
on fluorene.
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Synthesis and Structure-Property Studies of Novel Poly(para-phenylenes) with Conjugated Side Chains
Table 4.1. Molecular weight and TG value of P1-P5
Polymer Mn Mw
Mw/Mn (PDI)
TG (ºC)
P1 5900 7500 1.27 250 P2 6200 9600 1.55 325 P3 8800 11700 1.33 337 P4 6700 10800 1.61 285 P5 5700 10200 1.79 370
4.4.2 Powder X-ray diffraction
Polymer P2 and P3 possess organized packing structures as evident from XRDs
shown in Figure 4.2. The strong peaks in the low-angle region seem to correspond to a
distance between the polymer backbone separated by the side chain, indicating a long
range order.34,38 There is a sharp peak at 2θ = 4.45º (d = 19.2 Å) for P2, while P3 has two
peaks: 2θ = 2.59º (d = 26.0 Å) and 3.33º (d = 33.9 Å); it appears that there are two
packing structures. The presence of alternative side chains with different length on the
polymer backbone of P3 led to two sharp peaks in the low angle region, one involves
partial interdigitation and the other one comprises of end-to-end packing of side chains
(Figure 4.2-b, c).39 The broad peak in the high angle region (20-30º) for all polymers
were originated from the face-to-face distance between loosely packed side chains.40-41
Note that the difference in peak positions (33.9 Å and 26.0 Å) in the XRD pattern of P3
is about 7.9 Å, which can be assigned to the length of six C-C bonds in all-trans-
conformation. This is consistent to our proposed packing model.
139
Li Hairong National Uiversity of Singapore
O O O O
OOOO
O O O O
OOOO
O O O O
OOOO
0 10 20 30 40
2θ
P2
P3
P4
4.15 Å
P5
P1
4.23 Å3.73 Å
19.2 Å
26.0 Å33.9 Å
a
b
c
19.2 Å
26.0 Å
33.9 Å
O O O O
OOOO
O O O O
OOOO
O O O O
OOOO
0 10 20 30 40
2θ
P2
P3
P4
4.15 Å
P5
P1
4.23 Å3.73 Å
19.2 Å
26.0 Å33.9 Å
a
b
c
O O O O
OOOO
O O O O
OOOO
O O O O
OOOO
0 10 20 30 40
2θ
P2
P3
P4
4.15 Å
P5
P1
4.23 Å3.73 Å
19.2 Å
26.0 Å33.9 Å
0 10 20 30 40
2θ
P2
P3
P4
4.15 Å
P5
P1
4.23 Å3.73 Å
19.2 Å
26.0 Å33.9 Å
a
b
c
19.2 Å
26.0 Å
33.9 Å
Figure 4.2. Left: Powder XRD spectra for P1 to P5; Right: schematic diagrams of possible packing structures for P2 (a, double degeneracy) and P3 (b, c)
4.4.3. Absorption and emission
Absorption spectra of all polymers were measured in THF solution at a
concentration of 2 mg/L (Figure 4.3 and Table 4.2). All UV-Vis spectra showed broad
absorption maxima at 374 nm and 394 nm, indicating different conjugated species, which
could be assigned to π → π* transition of phenylene backbone and diene side chains,
respectively. Compared to conventional PPPs, which show an absorption maximum
approximately at ca. 340 nm,34,42-44 the absorption maxima of P1-P5 showed significant
bathochromic shift, indicating extended conjugation (Table 4.2). This implies a more
planar structure due to the cross-conjugation.45 The absorption maxima were similar for
P1, P2, P3 and P5 (Table 4.2), while P4 showed blue shift in absorption maximum with
a third absorption peak at 348 nm. This is due to the presence of many alkyl chains and a
140
Synthesis and Structure-Property Studies of Novel Poly(para-phenylenes) with Conjugated Side Chains
steric hindrance of adjacent side chains leads to a non-planar conformation for the
polymer backbone of P4. Such observation was also reported for other polymers.46-47
When comparing the excitation spectra with UV-Vis absorption spectra, it’s obvious that
the maximum absorption peaks for the polymers didn’t correspond to optimal excitation
wavelength. P2, P3 and P5 showed excitation peak around 340 nm, about 50 nm shorter
than corresponding UV-Vis absorption maxima (Table 2-PE). It was anticipated that the
backbone (PPP) was highly twisted relative to planar conjugated side chain, which
reflected the presence of two orthogonal chromophores weakly interacting with each
other. Excitation was initiated in the side chain chromophore with higher band gap,
which was then transferred to the one with lower band gap. Because of less effective
conjugation between side chain and backbone, it might result in disjoint nature of
HOMOs and LUMOs of side chain from backbone, the shorter transition could be largely
forbidden, while the emission from this state is due to the vibrational relaxation. 48
Table 4.2. Absorption maxima of the polymers in THF, fluorescence maxima at low (PL), high (PH) concentrations in THF, thin film (PN) and excitation maxima (PE)
Polymer P1 P2 P3 P4 P5 1st local ABSmax (nm) 394 394 393 387 396 2nd local ABSmax (nm) 374 374 372 370, 340 374
PL (nm) 433 422 414 418 438 PH (nm) 486 451 447, 475 456 469 PN (nm) 542 511 509 503 508 PE (nm) 353, 373 337 334 355, 373 337
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Li Hairong National Uiversity of Singapore
300 350 400 450 500 550 600 650
Abs
orbe
nce
(a.u
.)
dcba
Inte
nsity
(a.u
.)
Wavelength (nm)
P1
300 350 400 450 500 550 600 650
dcba
Abs
orbe
nce
(a.u
)
Inte
nsity
(a.u
)
Wavelength (nm)
P2
300 350 400 450 500 550 600 650
dcba
Inte
nsity
(a.u
.)
Abs
orbe
nce
(a.u
.)
Wavelength (nm)
P4
300 350 400 450 500 550 600 650
dcba
Inte
nsity
(a.u
.)
Abs
orbe
nce
(a.u
.)
Wavelength (nm)
P5 EX
250 300 350 400 450 500
P3P2
P4
P5
P1
Inte
nsity
(a.u
)
Wavelength (nm)
300 350 400 450 500 550 600 650
dcba
Inte
nsity
(a.u
.)
Abs
orbe
nce
(a.u
.)
Wavelength (nm)
P3
300 350 400 450 500 550 600 650
Abs
orbe
nce
(a.u
.)
dcba
Inte
nsity
(a.u
.)
Wavelength (nm)
P1
300 350 400 450 500 550 600 650
Abs
orbe
nce
(a.u
.)
dcba
Inte
nsity
(a.u
.)
Wavelength (nm)300 350 400 450 500 550 600 650
Abs
orbe
nce
(a.u
.)
dcba
Inte
nsity
(a.u
.)
Wavelength (nm)
P1
300 350 400 450 500 550 600 650
dcba
Abs
orbe
nce
(a.u
)
Inte
nsity
(a.u
)
Wavelength (nm)
P2
300 350 400 450 500 550 600 650
dcba
Abs
orbe
nce
(a.u
)
Inte
nsity
(a.u
)
Wavelength (nm)300 350 400 450 500 550 600 650
dcba
Abs
orbe
nce
(a.u
)
Inte
nsity
(a.u
)
Wavelength (nm)
P2
300 350 400 450 500 550 600 650
dcba
Inte
nsity
(a.u
.)
Abs
orbe
nce
(a.u
.)
Wavelength (nm)
P4
300 350 400 450 500 550 600 650
dcba
Inte
nsity
(a.u
.)
Abs
orbe
nce
(a.u
.)
Wavelength (nm)300 350 400 450 500 550 600 650
dcba
Inte
nsity
(a.u
.)
Abs
orbe
nce
(a.u
.)
Wavelength (nm)
P4
300 350 400 450 500 550 600 650
dcba
Inte
nsity
(a.u
.)
Abs
orbe
nce
(a.u
.)
Wavelength (nm)
P5
300 350 400 450 500 550 600 650
dcba
Inte
nsity
(a.u
.)
Abs
orbe
nce
(a.u
.)
Wavelength (nm)300 350 400 450 500 550 600 650
dcba
Inte
nsity
(a.u
.)
Abs
orbe
nce
(a.u
.)
Wavelength (nm)
P5 EX
250 300 350 400 450 500
P3P2
P4
P5
P1
Inte
nsity
(a.u
)
Wavelength (nm)
EX
250 300 350 400 450 500
P3P2
P4
P5
P1
Inte
nsity
(a.u
)
Wavelength (nm)250 300 350 400 450 500
P3P2
P4
P5
P1
Inte
nsity
(a.u
)
Wavelength (nm)
300 350 400 450 500 550 600 650
dcba
Inte
nsity
(a.u
.)
Abs
orbe
nce
(a.u
.)
Wavelength (nm)
P3
300 350 400 450 500 550 600 650
dcba
Inte
nsity
(a.u
.)
Abs
orbe
nce
(a.u
.)
Wavelength (nm)300 350 400 450 500 550 600 650
dcba
Inte
nsity
(a.u
.)
Abs
orbe
nce
(a.u
.)
Wavelength (nm)
P3
Figure 4.3. Normalized absorption (a) and emission at a low concentration in THF (0.002 g/L) (b), emission at high concentration in THF (0.2 g/L) (c), and emission of spin coated thin film (d), for P1-P5; Excitation spectra (EX) of P1-P5 monitored at the emission peaks at a low concentration
142
Synthesis and Structure-Property Studies of Novel Poly(para-phenylenes) with Conjugated Side Chains
The emission characteristics of the polymers were established at a low (0.002 g/L)
and a high (0.2 g/L) concentration in THF (Figure 4.3 and Table 2). At a low
concentration, all polymers showed emission peaks between 414 - 438 nm, with
significant overlaps with their respective absorption peaks. However, the excitation
spectra for all polymers showed that the optimal excitation wavelength, which gives
highest emission intensity, was averaged around 350 nm rather than 390 nm, where the
absorption intensity was maximum. This is not surprising for polymers with a strong
energy transfer character.44,49
It is well known that in polymer solids50-52 and polymer blends,53-54 the
intermolecular interactions via photon-induced electron transfer, formation of excimer,
exciplexes or aggregates become more efficient. This leads to red-shift or generation of a
new peak on the longer wavelength region and reduces the quantum efficiency of
fluorescence. At a high concentration of 0.2 g/L, P1 showed a single peak at 486 nm
(Table 4.2) with a red shift of more than 50 nm as compared with that in low
concentration (0.002 g/L). This should be attributed to the formation of the
intermolecular transition states of the polymer, i.e. emission from isolated polymer chains
to aggregates.55-56 Dramatic red shift of more than 100 nm in emission maximum (542
nm) was observed in the solid state due to further enhanced intermolecular energy
transfer to more ordered and compact aggregates,49 compared with that in dilute solution
(433 nm). All polymers showed similar results. P3 showed two emission peaks at high
concentration (447 and 475 nm, Figure 4.3), which might be due to the non-planar
structures as well as the coexistence of two intermolecular charge transfer pathways
resembling its powder XRD pattern (P3 in Figure 4.2).
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In order to examine the solvent influence on aggregation, UV-Vis, fluorescence and
excitation spectra of the polymers were recorded in non-polar toluene (Figure 4.4 and
Table 3) and compared with the data obtained in polar THF solution. In UV-Vis spectra,
absorption peaks of all polymers showed a blue shift of 5-10 nm in toluene indicating a
relatively lower degree of intermolecular interaction and/or solvatohromic effect. For
fluorescence spectra, all polymers showed well defined peaks at low concentrations in
toluene as compared to those in THF solution with no significant changes in emission
maxima. At high concentration in toluene, where the aggregation was expected, all
polymers showed a red shift of the emission maxima, but to less extent than those in THF
due to the change in polarity.57 For example P1 showed a less red shift of 10 nm, P2 and
P5 gave a less red shift of 6 - 12 nm. P3 showed less change and P4 didn’t show any
change in the emission maximum (Table 4.2 and 4.3) probably due to relatively longer
solubilizing alkyl chains. The most obvious change was the excitation spectra, where the
peak maxima of P2, P3 and P5 showed significant red shifts (between 356 and 375 nm,
Table 4.3) in toluene as compared with the values in THF. This indicated that non-polar
toluene molecules had a low dipole colliding relaxation effect on excited states and
disjoint nature of frontier molecular orbits (FMOs) of less conjugated chromophores.
Table 4.3. Absorption maxima of the polymers in toluene, fluorescence peaks at low (PL,), high (PH,) concentrations in toluene and excitation maxima (PE)
Polymer P1 P2 P3 P4 P5 1st local ABSmax (nm) 384 387 386 387 391 2nd local ABSmax (nm) 374 368 367 374, 342 373
PL (nm) 427 432 423 430 444 PH (nm) 476 445 443 455 457 PE (nm) 377 357 356, 375 357, 376 356, 375
144
Synthesis and Structure-Property Studies of Novel Poly(para-phenylenes) with Conjugated Side Chains
300 350 400 450 500 550 600 650
dcba
Inte
nsity
(a.u
.)
Abs
orbe
nce
(a.u
.)
Wavelength (nm)
P1
300 350 400 450 500 550 600 650
dcba
Inte
nsity
(a.u
.)
Abs
orbe
nce
(a.u
.)
Wavelength (nm)
P2
300 350 400 450 500 550 600
b ca
Inte
nsity
(a.u
.)
Abso
rben
ce(a
.u.)
Wavelength (nm)
P3
300 350 400 450 500
P5
P4
P3
P2
P1
Inte
nsity
(a.u
.)
Wavelength (nm)
EX
300 350 400 450 500 550 600 650 700
b c da
Inte
nsity
(a.u
.)
Abs
orbe
nce
(a.u
.)
Wavelength (nm)
P4
300 350 400 450 500 550 600 650 700
dcba
Inte
nsity
(a.u
.)
Abs
orbe
nce
(a.u
.)
Wavelength (nm)
P5
300 350 400 450 500 550 600 650
dcba
Inte
nsity
(a.u
.)
Abs
orbe
nce
(a.u
.)
Wavelength (nm)
P1
300 350 400 450 500 550 600 650
dcba
Inte
nsity
(a.u
.)
Abs
orbe
nce
(a.u
.)
Wavelength (nm)
300 350 400 450 500 550 600 650
dcba
Inte
nsity
(a.u
.)
Abs
orbe
nce
(a.u
.)
Wavelength (nm)
P1
300 350 400 450 500 550 600 650
dcba
Inte
nsity
(a.u
.)
Abs
orbe
nce
(a.u
.)
Wavelength (nm)
P2
300 350 400 450 500 550 600 650
dcba
Inte
nsity
(a.u
.)
Abs
orbe
nce
(a.u
.)
Wavelength (nm)
300 350 400 450 500 550 600 650
dcba
Inte
nsity
(a.u
.)
Abs
orbe
nce
(a.u
.)
Wavelength (nm)
P2
300 350 400 450 500 550 600
b ca
Inte
nsity
(a.u
.)
Abso
rben
ce(a
.u.)
Wavelength (nm)
P3
300 350 400 450 500 550 600
b ca
Inte
nsity
(a.u
.)
Abso
rben
ce(a
.u.)
Wavelength (nm)
300 350 400 450 500 550 600
b ca
Inte
nsity
(a.u
.)
Abso
rben
ce(a
.u.)
Wavelength (nm)
P3
300 350 400 450 500
P5
P4
P3
P2
P1
Inte
nsity
(a.u
.)
Wavelength (nm)
EX
300 350 400 450 500
P5
P4
P3
P2
P1
Inte
nsity
(a.u
.)
Wavelength (nm)
300 350 400 450 500
P5
P4
P3
P2
P1
Inte
nsity
(a.u
.)
Wavelength (nm)
EX
300 350 400 450 500 550 600 650 700
b c da
Inte
nsity
(a.u
.)
Abs
orbe
nce
(a.u
.)
Wavelength (nm)
P4
300 350 400 450 500 550 600 650 700
b c da
Inte
nsity
(a.u
.)
Abs
orbe
nce
(a.u
.)
Wavelength (nm)
300 350 400 450 500 550 600 650 700
b c da
Inte
nsity
(a.u
.)
Abs
orbe
nce
(a.u
.)
Wavelength (nm)
P4
300 350 400 450 500 550 600 650 700
dcba
Inte
nsity
(a.u
.)
Abs
orbe
nce
(a.u
.)
Wavelength (nm)
P5
300 350 400 450 500 550 600 650 700
dcba
Inte
nsity
(a.u
.)
Abs
orbe
nce
(a.u
.)
Wavelength (nm)
300 350 400 450 500 550 600 650 700
dcba
Inte
nsity
(a.u
.)
Abs
orbe
nce
(a.u
.)
Wavelength (nm)
P5
Figure 4.4. Normalized absorption (a) and emission at a low concentration in toluene (0.002 g/L) (b), emission at high concentration in toluene (0.2 g/L) (c), for P1-P5; Excitation spectra (EX) of P1-P5 monitored at the emission peaks at a low concentration
145
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4.4.4. Quantum yield and two photon absorption (TPA)
Fluorescence quantum yield measurements were carried out in toluene and THF,
quinine sulfate was used as standard (Table 4.4). The quantum yields of all polymers
were very low mainly due to several reasons. First, the existence of disjoint nature of
FMOs promoted the vibrational relaxation to lower state before emission, energy was lost
during inner transfer as explained by excitation spectra. Second, the polymers were
generally low in solubility even in good solvent, which probably induce the cluster
formation to minimize the exposure of unfavorable aromatic part, as is always the case of
water soluble conjugated PPPs,31 thus the intermolecular interaction further dissipated the
energy.58 Furthermore, although the polymer backbones are rigid in nature, trans-diene
side chains could undergo dramatic conformational changes upon excitation, migration of
excited state to “soft” chromophore followed by relaxation should account for the low
quantum efficiency.59 The TPA cross-section was measured by two photon induced
fluorescence (TPIF) and calculated using the equation given in the literature.60 In spite of
low quantum yield, they exhibited exceptionally high TPA cross sections (Table 4.4)
compared with the standard rhodamine B in ethanol (quantum yield = 70 %, δ = 150 ± 50
×10-50 cm4s/photon61-62). The two-photon absorption property of materials depends on at
least three factors. First of all, the polymers obey the “push-push” quadrupolar charge
transfer structure where the two electron donors (thiophene, alkoxylphenylene, carbazole,
etc.) were linked to the rigid center, which showed intense TPA.63-64 All these donors
were stringed by the rigid PPP backbone which provided additive effect on TPA. Second,
extended conjugation through bisdiene side chain offers large polarization which led to
increase in TPA.65-67 The system could be described as the coexistence of “push-push”
146
Synthesis and Structure-Property Studies of Novel Poly(para-phenylenes) with Conjugated Side Chains
along backbone and “push-pull” from backbone to side chain, which indicates that further
improvement could be done by introducing strong electron withdrawing group on the
conjugated side chain. It was also observed that the polarity of the solvent has a
significant effect on the TPA because increasing the polarity of the solvent further
increased the charge separation in both ground and excited states. Based on “push-pull”
framework from backbone to sidechain, the polar solvent tends to modulate the geometry
of the polymer structure from neutral to charge separated state. Therefore, such strong
dipole-dipole (induced dipole) interaction enlarged the TPA cross section.68-70 Materials
with large two-photon absorption cross sections are of interest in the application of
optical limiting,71 3D optical data storage,72 microfabrication64 and photodynamic
therapy,73 etc.
Table 4.4. Quantum yield, absolute and relative values of TPA cross section for P1-P5 in THF and toluene, polarity index is given in parenthesis, rhodamine B was the standard
Solvent Polymer
δ/10-50 cm4
s/photon at 800 nm
δ/δRhQY (%) Solvent Polymer
δ/10-50 cm4
s/photon at 800 nm
δ/δRhQY (%)
P1 1640 10.9 2.5 P1 1215 8.1 2.0 P2 877 5.8 6.3 P2 1121 7.5 4.7 P3 1159 7.7 8.6 P3 1698 11.3 5.6 P4 2196 14.6 3.3 P4 3210 21.4 2.3
Toluene (2.4)
P5 753 5.0 8.1
THF (4.0)
P5 630 4.2 7.2
4.4.5. Time-correlated single-photon counting
The excited state lifetimes of these polymers (P1-P5) were measured using time
correlated single photon counting technique (TCSPC). The decay profile was measured in
two solvents, toluene and THF. The detection wavelength was chosen at 450 nm where
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Li Hairong National Uiversity of Singapore
the emission of monomeric and aggregated systems overlapped. Table 4.5 summarizes
the fitting data (decay time and amplitude) of the fluorescence decay curves for all
polymers in toluene and THF. It was observed that at a low polymer concentration (10-7
M), P3 showed a mono-exponential decay in both solvents indicating monomeric
emission. For P1, P4 and P5, the excited state decay cannot be simply fit with a single
exponential component. At least two exponential components are needed to fit the data,
indicating formation of aggregate/excimer. Formation of aggregate/excimer usually
results in shortening of the excited state decay, usually a faster initial followed by a
slower decay compared to monomer.74 The fast decay is usually ascribed to energy
transfer from single excited fluorophores to aggregates. Polarity of solvent had strong
effect on the decay profile as shown in Figure 4.5. P3 in THF has faster decay probably
due to long alkyl chain, while the opposite is true for P5 with electron active carbazole
group attributed to strong dipole interaction with polar solvent.
Table 4.5. Life time (τ) and amplitude (a) for emission decay at concentration of 10-7 M
Polymer Toluene THF a(1) τ(1)* a(2) τ(2) a(1) τ(1) a(2) τ(2)
P1 0.41 461.24 0.59 1033.64 0.17 714.6 0.83 1221.99P2 1 1325.28 - - 1 930.30 - - P3 1 1211.69 - - 0.56 146.22 0.44 1226.31P4 0.85 117.77 0.15 996.27 0.6 138.71 0.4 1163.48P5 0.28 794.99 0.72 1397.03 0.12 952.43 0.88 1283.11
* All in picosecond a – Amplitude τ – lifetime
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Synthesis and Structure-Property Studies of Novel Poly(para-phenylenes) with Conjugated Side Chains
Figure 4.5. Fluorescence decay curves of P1-P5 in THF monitored at 450 nm (10-7 M--■; 10-5 M--□)
-1 0 1 2 3 4 5 6
0.0
0.2
0.4
0.6
0.8
1.0
Inte
nsity
(a.u
.)
Time (ns)
P1
-1 0 1 2 3 4 5 6
0.0
0.2
0.4
0.6
0.8
1.0
Inte
nsity
(a.u
.)
Time (ns)
P2
-1 0 1 2 3 4 5 6
0.0
0.2
0.4
0.6
0.8
1.0
Inte
nsity
(a.u
.)
Time (ns)
P4
-1 0 1 2 3 4 5 6
0.0
0.2
0.4
0.6
0.8
1.0
Inte
nsity
(a.u
.)
Time (ns)
P3
-1 0 1 2 3 4 5 6
0.0
0.2
0.4
0.6
0.8
1.0
Inte
nsity
(a.u
.)
Time (ns)
P5
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Li Hairong National Uiversity of Singapore
4.4.6. Solid phase self-assembly and morphology
The polymers have very good lateral packing in the molecular level, due to the
strong stacking force. At high concentration (2 mg/mL), thin films were formed; while at
moderate concentration (0.5 mg/mL) the polymer P1 forms nanofibers with diameter
around 20 nm (Figure 4.6-A). All fiber exhibited helical structure with a left hand
orientation (inset). At lower concentration of 0.2 mg/mL, the fiber tended to interweave
together forming ring shape structures, i.e. coexisting of both nanofiber and nano-rings
(Figure 4.6-B). While at a very dilute concentration (0.05 mg/mL), helical and well
dispersed nano-rings with different morphologies were observed (Figure 4.6-C). Drop
casting from THF, CHCl3 and toluene gave similar results, which indicated that the
driving force came from the polymer itself. Helical fibers have been reported with
oligothiophenes75-77 and oligo(p-phenylenevinylene)s78-81 with chiral center and/or
hydrogen bonding. In our case, neither chiral center nor hydrogen bonding exists in P1.
First of all, the polymer backbone is non-linear because of the 2,5 positions of thiophene
linker for P1. Second, the side-chain is conjugated, long and rigid in nature, which
confined the helical structures as shown in the (Figure 4.6-D). Any other possible
molecular structures would cause steric hindrance. So the rigid side-chain probably
unifies the molecular structure of the polymer, making it bend only in one direction.
Third, the strong stacking force further enhanced the maintaining of the helical structures;
obviously the more ordered packing provided more accessible intermolecular interaction
in this case. Last but not least, P1 had no alkyl chain, which promoted the aggregation.
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Synthesis and Structure-Property Studies of Novel Poly(para-phenylenes) with Conjugated Side Chains
Figure 4.6. SEM images of P1 drop-casted from solutions of different concentrations onto glass plate: 0.5 mg/mL (A); 0.2 mg/mL (B); 0.05 mg/mL (C). Schematic view of the hierarchical self-assembly of P1 into left-helix supramolecular structures (D)
Due to the rigid backbone structure of the conjugated polymer and the non-polar
nature of the side chains, it is very difficult to obtain highly ordered packing of the
conjugated polymers. To our knowledge, so far no direct evidence of the crystalline
structure of the PPP was reported. High-resolution transmission electron microscopy
(HRTEM) results showed that thin film of P3 contained nano-crystalline domains simply
by drop casting the polymer solution onto copper grid, as shown in Figure 4.7. The
domain size varied from several to tens of nanometers. Electron diffraction study further
S
S
S
SS
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
SS S
S
S
S
S
S
S
S
D
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Li Hairong National Uiversity of Singapore
confirmed its crystalline structure (inset). Initially, it is believed that this could be due to
the presence of residual palladium catalyst as an impurity, but the EDX result excluded
this possibility. Since the side chain of the polymer is non-polar and the backbone highly
rigid, intermolecular π-π stacking could be the only driving force. The d-spacing values
calculated from the diffraction pattern is about 2.6-3.1 nm, which was a bit deviated from
the proposed inter-planar spacing value obtained in powder XRD (3.7-4.2 nm). This may
be either due to the different sample preparations, as the packing of the polymer chains
could be complicated and might not be unique. Further detailed morphology studies of
the polymer and its nanocomposites are being done in our lab and will be presented in
separate contribution.
Figure 4.7. HRTEM images and electron diffraction patterns at crystalline domains of P3 thin film
4.5. Conclusion
A series of cross-conjugated polymers were synthesized and fully characterized. All
polymers are readily soluble in common organic solvents, and thermally stable up to 250
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Synthesis and Structure-Property Studies of Novel Poly(para-phenylenes) with Conjugated Side Chains
ºC. Powder XRD patterns showed an ordered packing structure for P2 and P3. Especially,
P3 showed two peaks probably due to the different length of the alternative sidechains.
UV-Vis spectra indicated the existence of absorption of intermolecular ground state
complex. Emission maxima showed red shift with increasing concentration due to
excimer formation. Quantum yields of polymers revealed the relationship between “soft”
bisdiene side chain and rigid backbone. Large two photon absorption (TPA) cross
sections were supposed to derive from extended conjugation and aligned “push-push”
and “push-pull” structure. Intermolecular interaction was further confirmed by time-
correlated single-photon counting (TCSPC). Solid phase studies showed that different
self-assembly patterns were formed with different concentrations. Surprisingly, P3 was
able to form nano-crystalline phases by π-π stacking force driven self-assembly.
4.6. References
1. Hopf, H. Angew. Chem., Int. Ed. Engl. 1984, 23, 948.
2. Hopf, H.; Maas, G. Angew. Chem., Int. Ed. Engl. 1992, 31, 931.
3. Iyoda, M.; Hasegawa, M.; Miyake, Y. Chem. Rev. 2004, 104,5085.
4. Gorgues, A.; Hudhomme, P.; Salle, M. Chem. Rev. 2004, 104, 5151.
5. Nielsen, M. B.; Diederich, F. Chem. Rev. 2005, 105, 1837.
6. Buchan, C. M.; Cadogan, J. I. G.; Gosney, I.; Henry, W. J. J. Chem. Soc., Chem.
Commun. 1985, 1785.
7. Fielder, S.; Rowan, D. D.; Sherburn, M. S. Angew. Chem., Int. Ed. 2000, 39, 4331.
8. van Walree, C, A.; Kaats-Richters, V. E. M.; Veen, S. J.; Wieczorek, B.; van der
Wiel, J. H.; van der Wiel, B. C. Eur. J. Org. Biomol. Chem. 2004, 2, 3432.
153
Li Hairong National Uiversity of Singapore
9. Misaki, Y.; Matsumura, Y.; Sugimoto, T.; Yoshida, Z. Tetrahedron Lett. 1989, 30,
5289.
10. Moore, A. J.; Bryce, M. R.; Skabara, P. J.; Batsanov, A. S.; Goldenberg, L. M.;
Howard, J. A. K. J. Chem. Soc., Perkin. Trans. 1 1997, 3443.
11. Zhao, Y.; Tykwinski, R. R. J. Am. Chem. Soc. 1999, 121, 458.
12. Ciulei, S. C.; Tykwinski, R. R. Org. Lett. 2000, 2, 3607.
13. Boldi, A. M.; Anthony, J.; Gramlich, V.; Knobler, C. B.; Boudon, C.;
Gisselbrecht, J. –P.; Gross, M.; Diederich, F. Helv. Chim. Acta 1995, 78, 779.
14. Zhao, Y.; McDonald, R.; Tykwinski, R. R. J. Org. Chem. 2002, 67, 2805.
15. Moonen, N. N. P.; Pomerantz, W. C.; Gist, R.; Boudon, C.; Gisselbrecht, J. –P.;
Kawai, T.; Kishioka, A.; Gross, M.; Irie, M.; Diederich, F. Chem. -Eur. J. 2005,
11, 3325.
16. Klokkenburg, M.; Lutz, M.; Spek, A. L.; van der Maas, J. H.; van Walree, C. A.
Chem. –Eur. J. 2003, 9, 3544.
17. Mao, S. S. H.; Tilley, T. D. J. Organomet. Chem. 1996, 521, 425.
18. Hörhold, H. –H.; Helbig, M.; Raabe, D.; Opfermann, J.; Scherf, U.; Stockmann,
R.; Weiss, D. Z. Chem. 1987, 27, 126.
19. Eisler, S.; Tykwinski, R. R. Angew. Chem., Int. Ed. 1999, 38, 1940.
20. Hopff, H.; Wick, A. K. Helv. Chim. Acta 1961, 44, 19.
21. Köbrich, G.; Heinemann, H. Angew. Chem., Int. Ed. Engl. 1965, 4, 594.
22. Iyoda, M.; Otani, H.; Oda, M.; Kai, Y.; Baba, Y.; Kasai, N. J. Chem. Soc., Chem.
Commun. 1986, 1794.
23. Kozhushkov, S. I.; Leonov, A.; de Meijere, A. Synthesis 2003, 956.
154
Synthesis and Structure-Property Studies of Novel Poly(para-phenylenes) with Conjugated Side Chains
24. Sakurai, H. Pure Appl. Chem. 1996, 68, 327.
25. Matsuo, T.; Fure, H.; Sekiguchi, A. Chem. Lett. 1998, 1101.
26. Boldi, A. M.; Diederich, F. Angew. Chem., Int. Ed. Engl. 1994, 33, 468.
27. Mitzel, F.; Boudon, C.; Gisselbrecht, J. –P.; Seiler, P.; Gross, M.; Diederich, F.
Chem. Commun. 2003, 1634.
28. Hascoat, P.; Lorcy, D.; Robert, A.; Carlier, R.; Tallec, A.; Boubekeur, K.; Batail,
P. J. Org. Chem. 1997, 62, 6086.
29. Kawase, T.; Darabi, H. R.; Uchimiya, R.; Oda, M. Chem. Lett. 1995, 499.
30. Gleiter, R.; Röckel, H.; Nuber, B. Tetrahedron Lett. 1994, 35, 8779.
31. Li, H.; Valiyaveettil, S. Macromolecules 2007, 40, 6057.
32. Otsubo, T.; Kohda, T.; Misumi, S. Bull. Chem. Soc. Jp. 1980, 53, 512.
33. Tietze, L. F. Reactions and Synthesis in the Organic Chemistry Laboratory;
University Science: Mill Valley, CA. 1989, 253.
34. Baskar, C.; Lai, Y. H.; Valiyaveettil, S. Macromolecules 2001, 34, 6255.
35. Wong, W. Y.; Choi, K. H.; Lu, G. L.; Shi, J. X.; Lai, P. Y.; Chan, S. M.; Lin, Z. Y.
Organometallics 2001, 20, 5446.
36. Smith, K.; James, D. M.; Mistry, A. G..; Bye, M. R.; Faulkner, D. J. Tetrahedron
1992, 48, 7479.
37. Paliulis, O.; Ostrauskaite, J.; Gaidelis, V.; Jankauskas, V.; Strohriegl, P.
Macromol. Chem. and Phys. 2003, 204, 1706.
38. Yasuda, T.; Yamamoto, T. Macromolecules 2003, 36, 7513.
39. Yamamoto, T.; Saitoh, Y.; Anzai, K.; Fukumoto, H.; Yasuda, T.; Fujiwara, Y.;
Choi, B. K.; Kubota, K.; Miyamae, T. Macromolecules 2003, 36, 6722.
155
Li Hairong National Uiversity of Singapore
40. Jordan, E. F., Jr.; Feldeisen, D. W.; Wrigley, A. N. J. Polym. Sci., Part A-1 1971,
9, 1835.
41. Hsieh, H. W.; Post, B.; Morawetz, H. J. Polym. Sci., Polym. Phys. Ed. 1976, 14,
1241.
42. Liu, B.; Yu, W. L.; Lai, Y. H.; Huang, W. Macromolecules 2002, 35, 4975.
43. Balanda, P. B.; Ramey, M. B.; Reynolds, J. R. Macromolecules 1999, 32, 3970.
44. Lee, S. A.; Hotta, S.; Nakanish, F. J. Phys. Chem. A 2000, 104, 1827.
45. Lamba, J. J. S.; Tour, J. M. J. Am. Chem. Soc. 1994, 116, 11723.
46. Huang, W. Y.; Gao, W.; Kwei, T. K.; Okamoto, Y. Macromolecules 2001, 34,
1570.
47. Shotwell, S.; Windscheif, P. M.; Smith, M.D.; Bunz, U. H. F. Org. Lett. 2004, 6,
4151.
48. Skoog, D. A.; Holler, F. J.; Nieman, T. A. Principle of Instrumental Analysis (5th
Edition), 1998, 359.
49. Peng, K. Y.; Chen, S. A.; Fann, W. S. J. Am. Chem. Soc. 2001, 123, 11388.
50. Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Science 1992, 258, 1474.
51. Theander, M.; Yartsev, A.; Zigmantas, D.; Sundstrom, V.; Mammo, W.;
Andersson, M. R.; Inganas, O. Phys. Rev. B 2000, 61, 12957.
52. Jenekhe, S. A.; Osaheni, J. Science 1994, 265, 765.
53. Halls, J. J. M.; Cornil, J.; dos Santos, D. A.; Silbey, R.; Hwang, D. H.; Holems, A.
B.; Bredas, J. L.; Friend, R. H. Phys. Rev. B 1999, 60, 5721.
54. Onoda, M.; Tada, K.; Zakhidov, A. A.; Yoshino, L. Thin Solid Films 1998, 331,
76.
156
Synthesis and Structure-Property Studies of Novel Poly(para-phenylenes) with Conjugated Side Chains
55. Yan, M.; Rothberg, L. J.; Kwock, E. K.; Miller, T. M. Phys. Rev. Lett. 1995, 75,
1992.
56. Samuel, I. D. W.; Rumbles, G.; Collison, C. J. Phys. Rev. B 1995, 52, 11573.
57. Joseph, R. L. Principles of Fluorescence Spctroscopy (2nd Edition), 1999, 187.
58. Padmanaban, G.; Ramakrishnan, S. J. Am. Chem. Soc. 2000, 122, 2244.
59. Siling, S. A.; Ronova, I. A.; Madii, V. A.; Lozinskaja, E. I.; Borisevich, Ju. E.;
Vinogradova, S. V.; Kizel, V. A.; Reznichenko, A. V.; Kokin, V. N. Synthesis and
Properties of Heterocyclic Compounds; Huntington, N.Y. : Nova Science
Publishers, 2001, p.115.
60. Wang, C; Liu, Li; Ma, W; Zhou, Z; Wang, G. Y.; Xu, Z. Z. Optik, 2005, 116, 75.
61. Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991.
62. Xu, C.; Webb, W. W. J. Opt. Soc. Am. B 1996, 13, 481.
63. Albota, M.; Beljonne, D.; Brédas, J. –L. ; Ehrlich, J. E.; Fu, J. –Y.; Heikal, A. A.;
Hess, S. E.; Kogej, T.; Levin, M. D.; Marder, S. R.; McCord-Maughon, D.; Perry,
J. W.; Röckel, H.; Rumi, M.; Subramaniam, G.; Webb, W. W.; Wu, X. L.; Xu, C.
Science 1998, 281, 1653.
64. Cumpston, B. H.; Ananthavel, S. P.; Barlow, S.; Dyer, D. L.; Ehrlich, J. E.;
Erskine, L. L.; Heikal, A. A.; Kuebler, S. M.; Sandy Lee, I. –Y.; McCord-
Maughon, D.; Qin, J.; Röckel, H.; Rumi, M.; Wu, X. –L.; Marder, S. R.; Perry, J.
W. Nature 1999, 398, 51.
65. Tanihara, J.; Ogawa, K.; Kobuke, Y. J. Photochem. Photobio. A Chem. 2006, 178,
140.
66. Drobizhev, M.; Karotki, A.; Kruk, M.; Rebane, A. Chem. Phys. Lett. 2002, 355,
157
Li Hairong National Uiversity of Singapore
175.
67. Taylor, P. N.; Wylie, A. P.; Huuskonen, J.; Anderson, H. L. Angew. Chem. Int. Ed.
1998, 37, 986.
68. Meyer, F.; Marder, S. R.; Pierce, B. M. Bredas, J. L. J. Am. Chem. Soc. 1994, 116,
10703.
69. Luo, Y.; Norman, P.; Macak, P. Ågren, H. J. Phys. Chem. A 2000, 104, 4718.
70. Wang, C. –K.; Zhao, K.; Su, Y.; Ren, Y.; Zhao X.; Luo, Y. J. Chem. Phys. 2003,
119, 1208.
71. Nalwa, H. S.; Miyata, S. Nonlinear Optics of Organic Molecules and Polymers,
CRC Press, Boca Raton, FL, 1997.
72. Parthenopoulos, D. A. Rentzepis, P. M. Science 1989, 245, 843.
73. Bhawalkar, J. D.; Kumar, N. D.; Zhao, C. F., Prasad, P. N. J. Clin. Laser Med.
Surg. 1997, 15, 201.
74. Peng, K. –Y.; Chen, S. –A.; Fann, W. –S. J. Am. Chem. Soc. 2001, 123, 11388.
75. Kilbinger, A. F. M.; Schenning, A. P. H. J.; Goldoni, F.; Feast, W. J.; Meijer, E.
W. J. Am. Chem. Soc. 2000, 122, 1820.
76. Schenning, A. P. H. J.; Kilbinger, A. F. M.; Biscarini, F.; Cavalini, M.; Cooper, H.
J.; Derrick, P. J.; Feast, W. J.; Lazzaroni, R.; Leclère, Ph.; McDonnell, L. A.;
Meijer, E. W.; Meskers, S. J. Am. Chem. Soc. 2002, 124, 1269.
77. Leclère, Ph.; Surin, M.; Jonkheijm, P. ; Henze, O.; Schenning, A. P. H. J.;
Biscarini, F.; Grimsdale, A. C.; Feast, W. J.; Meijer, E. W.; Müllen, K.; Bredas, J.
L.; Lazzaroni, R. Eur. Polym. J. 2004, 40, 885.
78. El-ghayoury, A.; Schenning, A. P. H. J.; van Hal, P. A.; van Duren, J. K. J.;
158
Synthesis and Structure-Property Studies of Novel Poly(para-phenylenes) with Conjugated Side Chains
Janssen, R. A. J.; Meijer, E. W. Angew. Chem., Int. Ed. 2001, 40, 3660.
79. Schenning, A. P. H. J.; Jonkheijm, P.; Peeters, E.; Meijer, E. W. J .Am. Chem. Soc.
2001, 123, 409.
80. Ajayaghosh, A.; George, S. J. J. Am. Chem. Soc. 2001, 123, 5148.
81. Ajayaghosh, A.; Praveen, V. K. Acc. Chem. Res. 2007, 40, 644.
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CChhaapptteerr 55
SSyynntthheessiiss aanndd CChhaarraacctteerriizzaattiioonn ooff CCrroossss--ccoonnjjuuggaatteedd CCrruucciiffoorrmmss wwiitthh VVaarriieedd FFuunnccttiioonnaall GGrroouuppss
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Synthesis and Characterization of Cross-conjugated Cruciforms with Varied Functional Groups
5.1. Introduction
We have discussed the interesting properties of the conjugated polymers with cross-
conjugated structure in Chapter 4. It was noticed that most unique properties we studied
were caused by the extended conjugation. It is also important to investigate the effects of
the different conjugated side chains. However, conjugated polymers often suffer from the
inevitable defects which interrupt the effective conjugation length. Furthermore, due to
strong aggregation, it could be difficult to elicit accurate information regarding the role of
individual conjugated side chain as it could be masked by the whole polymeric systems.
Hence, we are interested in designing structurally well-defined OPP based cross-
conjugated molecules with perpendicular conjugating segments participating in the whole
π electron system. It is interesting to study the effects of different functional groups
introduced on different segments on the overall photophysical properties of the
molecule.1-14 Herein, design, synthesis and characterization of ten different oligomers
with different electron donating and withdrawing groups on both segments are reported.
RR RR
N
N
SOC6H13
NO2
N
R =
Group 1: O1-O5 Group 2: O6-O10
O1, O6 O2, O7 O3, O8 O4, O9 O5, O10
RR RR
N
N
SOC6H13
NO2
N
R =
Group 1: O1-O5 Group 2: O6-O10
O1, O6 O2, O7 O3, O8 O4, O9 O5, O10
Figure 5.1. Molecular structures of the target compounds
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5.2. Experimental
5.2.1. Synthesis
Ten cross-conjugated cruciforms were synthesized (Figure 5.1). They were devided
into two groups. The only difference is the terminal pyridine unit in Group 2 in stead of
phenylene unit in Group 1. Different electron active groups were attached along the
para-phenylene segment.
The synthetic route was illustrated in Scheme 5.1. Bisdiene segment was first
obtained via Wittig reaction, where trans-cinnamaldehyde was purchased and 2-pyridyl
acrolein was synthesized according to literature.15 All boronic acids were synthesized by
the same procedure as described in previous Chapters. The Pd(0)-catalyzed Suzuki
coupling of 4 or 5 with boronic acids (compounds 6-10) in DMF/H2O solvent mixture
afforded oligomer O1-O10 with low yields. We found that protonation of pyridyl group by
equal amount of TFA can make 5 soluble in DMF, the weaker base (NaHCO3 instead of
K2CO3 in synthesizing O1-O5) was chosen to minimize the deprotonation. By this way we
were able to obtain our target compounds O6-O10 with low yields still, in fact the starting
material (5) was completely reacted and the main side products were the half coupled ones.
THF/H2O system also gave similar yields but toluene/H2O system failed to afford products.
162
Synthesis and Characterization of Cross-conjugated Cruciforms with Varied Functional Groups
BrBr BrBr
Br
Br
BrBr
PO(OEt)2
(EtO)2OP
NO
BrBr
X
XX= CH (4), N (5)
1 2
3
N
O
32 Oor
NBS, CCl4reflux, 2 h
P(OEt)3150 C, 3 h°
40% 95%
Ph3P=CHCHO
54 %
t-BuOK, DMFr.t. 6 h
70-80%
S(HO)2B (HO)2B OC6H13
C6H13C6H13
(HO)2B
NC6H13
(HO)2B
NO2
(HO)2B
6
109
87
4 6 10
DMF/H2O, K2CO3,70 C, 48 h°
O1-O5
5 6 10
DMF/TFA/H2O, NaHCO3,70 C, 48 h°
O6-O10
Scheme 5.1. Synthetic routes for O1-O10.
General procedure for Wittig reaction. Mixture of acrolein (5 mmol) and
triethylphosphine (20 mmol) were refluxed for 6 hr without solvent under dry condition,
the excess triethylphosphine was removed under reduced pressure, the obtained ylid was
immediately mixed with 30 mL DMF, potassium t-butoxide (15 mmol) and acrolein (20
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mmol). The mixture was stirred overnight at room temperature followed by addition of
water; the precipitate was filtered and thoroughly washed with water then MeOH, giving
a yellow solid with 70-80% yield.
General procedure to synthesize O1-O5. 4-Dibromo-2,5-bis-(4-phenyl-buta-1,3-
dienyl)-benzene (0.3 g, 0.61 mmol) and mono boronic acid (2.5 equivalent) were
dissolved in 30 mL THF. A K2CO3 aqueous solution (2 M, 20 mL) was added to this
mixture followed by tetrakis (triphenylphosphino) palladium (0) (3 mol%) and
cetyltrimethyl ammonium bromide (30 mol%) as catalysts. After degassing, the mixture
was stirred at 70 ºC for 2 days and precipitated twice from methanol; the product was
further purified by column chromatography through silica gel using DCM : Hexane = 1:1
(for O1, O5) or DCM : hexane = 1:3 (for O2 and O3) or DCM : hexane = 1:2 (for O4).
General procedure to synthesize O6-O10. 4-Dibromo-2,5-bis-(4-(3-pyridyl)-buta-
1,3-dienyl)- benzene (0.3 g, 0.61 mmol) was protonated by slightly excess of TFA. After
complete protonation 30 mL THF and mono boronic acid (2.5 equivalents) was added, a
clear THF solution was formed. A NaHCO3 aqueous solution (2 M, 20 mL) was added
carefully to this mixture, no phase transfer catalyst was added to make sure the two layers
remain to avoid fast deprotonation. Tetrakis (triphenylphosphino) palladium (0) (3 mol%)
was added as catalyst. After degassing, the mixture was stirred at 70 ºC for 2 days. The
reaction mixture was poured into water and extracted using DCM. The crude product was
purified by column chromatography through neutral alumina using DCM : EA : Et3N =
50:45:5 (for O6) or DCM : EA : Et3N = 60:35:5 (for O7-O9) or DCM : EA : Et3N =
30:65:5 (for O10).
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Synthesis and Characterization of Cross-conjugated Cruciforms with Varied Functional Groups
5.2.2 Characterization
1, 4-Dibromo-2, 5-bis-(4-(2-pyridyl)-buta-1, 3-dienyl)-benzene (5). 1H NMR (TFA-d,
δ ppm): 8.82 (d, 2H, J=5.8 Hz, Py-H), 8.69 (d, J=8.5 Hz, 2H, Py-H), 8.61 (d, J=5.7 Hz,
2H, Py-H), 8.04 (dd, J=8.2 Hz, 2H, Py-H), 7.91 (s, 2H, Ar-H), 7.27-7.43 (m, 4H, diene-
H), 7.04 (dd, J=15.3 Hz, 2H, diene-H), 6.84 (d, J=15.6 Hz, 2H, diene-H). 13C NMR
(TFA-d, δ ppm): 145.0, 141.4, 140.6, 140.3, 139.53, 139.47, 137.9, 132.8, 132.2, 129.5,
126.3, 125.2. Anal. Calcd. for C24H18Br2N2: C 58.33; H 3.67; N5.67; Br 32.34. Found: C
58.17 ; H 3.69; N 5.79; Br 32.07.
O1. Yield 39 %. 1H NMR (CDCl3, δ ppm): 7.73 (s, 2H, central Ar-H), 7.40-7.43 (m, 6H,
terminal Ar-H, Thio-H), 7.31 (t, J=7.2 Hz, 4H, terminal Ar-H), 7.14-7.24 (m, 6H, Thio-H,
terminal Ar-H), 6.89-6.99 (m, 6H, diene-H), 6.67 (d, J=15.4 Hz, 2H, diene-H). 13C NMR
(CDCl3, δ ppm): 142.3, 137.9, 135.7, 133.9, 133.7, 131.6, 131.2, 130.1, 129.3, 128.9,
128.5, 128.3, 128.1, 127.1, 126.6. FT-IR (KBr, cm-1): 1724, 1591, 1481, 1447, 1265,
1072, 985, 898, 837, 748, 690, 506. HRMS (EI) m/z Calcd. for C34H26S2: 498.1476.
Found: 498.1466. Anal. Calcd.: C 81.89; H 5.25; S 12.86. Found: C 82.11; H 5.22; S
12.65.
O2. Yield 60 %. 1H NMR (CDCl3, δ ppm): 7.70 (s, 2H, central Ar-H), 7.64 (t, J=8.5 Hz,
8H, Ar-Ar(H)-Ar), 7.51 (d, J=8.2 Hz, 4H, OAr-H), 7.39 (d, J=8.5 Hz, 4H, terminal Ar-H),
7.19-7.31 (m, 6H, terminal Ar-H), 7.03 (d, J=8.7 Hz, 4H, OAr-H), 6.84-6.98 (m, 4H,
diene-H), 6.78 (d, J=15.0 Hz, 2H, diene-H), 6.68 (d, J=15.0 Hz, 2H, diene-H), 4.03 (t,
J=6.5 Hz, 4H, OCH2), 1.83 (m, 4H, OCH2CH2), 1.24-1.53 (b, 12H, CH2), 0.93 (t, J=6.9
Hz, 6H, CH3). 13C NMR (CDCl3, δ ppm): 159.6, 140.7, 140.5, 139.5, 138.0, 135.1, 133.6,
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133.4, 131.7, 130.9, 130.8, 130.3, 129.3, 128.7, 128.2, 127.9, 127.1, 127.0, 115.6, 68.8,
32.3, 29.9, 26.4, 23.3, 14.7. FT-IR (KBr, cm-1): 3025, 2920, 2851, 1729, 1606, 1502,
1473, 1388, 1286, 1246, 1177, 1036, 981, 821, 747, 689, 522. HRMS (EI) m/z Calcd. for
C62H62O2: 838.4750. Found: 838.4742. Anal. Calcd.: C 88.74; H 7.45. Found: C 88.85; H
7.43.
O3. Yield 50 %. 1H NMR (CDCl3, δ ppm): 7.76-7.85 (m, 6H, central-H, FL-H), 7.34-
7.47 (m, 14H, FL-H (10H), terminal Ar-H (4H)), 7.24-7.29 (m (overlap with CHCl3
peak), 4H, terminal Ar-H), 7.20 (d, J=7.2 Hz, 2H, terminal Ar-H), 7.02 (dd, J=13.2 Hz,
2H, diene-H), 6.73-6.85 (m, 4H, diene-H), 6.64 (d, J=15.4 Hz, 2H, diene-H), 2.02 (t,
J=8.1 Hz, 8H, FL-CH2), 1.10-1.20 (b, 28H, CH2), 0.76 (b, 16H, CH2CH3). 13C NMR
(CDCl3, δ ppm): 151.7, 151.3, 141.5, 141.4, 141.0, 139.9, 137.9, 135.1, 133.6, 131.8,
130.9, 130.1, 129.2, 129.0, 128.2, 128.0, 127.8, 127.5, 127.0, 125.4, 123.6, 120.4, 120.2,
55.8, 41.0, 32.2, 30.5, 24.6, 23.2, 14.6. FT-IR (KBr, cm-1): 3023, 2924, 2852, 1727, 1593,
1451, 1377, 1261, 1124, 1070, 986, 895, 833, 744, 690, 504. HRMS (EI) m/z Calcd. for
C76H86: 998.6730. Found: 998.6705. Anal. Calcd.: C 91.33; H 8.67. Found: C 91.12; H
8.74.
O4. Yield 38 %. 1H NMR (CDCl3, δ ppm): 8.21 (s, 2H, Cz-H), 8.17 (d, J=7.7 Hz, 2H,
Cz-H), 7.82 (s, 2H, central Ar-H), 7.47-7.60 (m, 6H, Cz-H), 7.35 (d, J=7.4 Hz, 4H,
terminal Ar-H), 7.15-7.29 (m (overlap with CHCl3 peak), 8H, terminal Ar-H, Cz-H), 7.04
(dd, J=15.2 Hz, 2H, diene-H), 6.79-6.88 (m, 4H, diene-H), 6.60 (d, J=15.7 Hz, 2H, diene-
H), 4.39 (t, J=7.3 Hz, 4H, Cz-CH2), 1.97 (m, 4H, Cz-CH2CH2), 1.34-1.53 (b, 12H, CH2),
0.92 (t, J=6.9 Hz, 6H, CH3). 13C NMR (CDCl3, δ ppm): 141.6, 140.4, 138.1, 135.2, 133.0,
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Synthesis and Characterization of Cross-conjugated Cruciforms with Varied Functional Groups
132.3, 132.0, 130.6, 130.4, 129.3, 129.2, 128.5, 128.3, 128.0, 127.0, 126.5, 123.5, 122.2,
121.2, 119.6, 109.5, 109.0, 44.0, 32.3, 29.7, 27.7, 23.2, 14.7. FT-IR (KBr, cm-1): 3020,
2924, 2853, 1726, 1597, 1465, 1381, 1340, 1240, 1068, 986, 892, 804, 746, 690, 520.
HRMS (EI) m/z Calcd. for C62H60N2: 832.4756. Found: 832.4738. Anal. Calcd.: C 89.38;
H 7.26; N 3.36. Found: C 89.77; H 7.50; N 3.37.
O5. Yield 43 %. 1H NMR (CDCl3, δ ppm): 8.34 (s, 2H, NO2Ar-H), 8.33 (d, J=7.6 Hz, 2H,
NO2Ar-H), 7.79 (d, J=7.5 Hz, 2H, NO2Ar-H), 7.67 (d, J=7.8 Hz, 2H, NO2Ar-H), 7.65 (s,
2H, central Ar-H), 7.39 (d, J=7.2 Hz, 4H, terminal Ar-H), 7.20-7.32 (m (overlap with
CHCl3 peak), m, 8H, terminal Ar-H), 6.99 (dd, J=15.3 Hz, 2H, diene-H), 6.82 (dd, J=15.3
Hz, 2H, diene-H), 6.69 (d, J=15.3 Hz, 2H, diene-H), 6.53 (d, J=15.3 Hz, 2H, diene-H).
13C NMR (CDCl3, δ ppm): 149.0, 142.7, 139.2, 137.6, 136.6, 135.3, 134.9, 132.5, 129.9,
129.6, 129.5, 129.3, 128.6, 128.0, 127.2, 125.1, 123.1. FT-IR (KBr, cm-1): 3022, 2958,
2926, 1734, 1653, 1560, 1531, 1492, 1346, 1271, 1122, 1074, 987, 893, 808, 740, 690,
501. HRMS (EI) m/z Calcd. for C38H28N2O4: 576.2049. Found: 576.2042. Anal. Calcd.:
C 79.15; H 4.89; N 4.86. Found: C 79.03; H 4.87; N 5.01.
O6. Yield 30 %. 1H NMR (CDCl3, δ ppm): 8.63 (d, J =3.2 Hz, Py-H), 8.45 (dd, J=4.8 Hz,
2H, Py-H), 7.74 (s, 2H, central Ar-H), 7.73 (d, J=8.2 Hz, 2H, Py-H), 7.44 (dd, J=6.2 Hz,
2H, Thio-H), 7.14-7.24 (m, 6H, Thio-H, Py-H), 6.93-6.98 (m, 6H, diene-H), 6.64 (d,
J=15.4 Hz, 2H, diene-H). 13C NMR (CDCl3, δ ppm): 149.2, 142.0, 135.7, 134.0, 133.6,
133.1, 132.4, 132.0, 131.1, 130.0, 129.0, 128.5, 128.1, 126.8, 124.1. FT-IR (KBr, cm-1):
1722, 1654, 1560, 1467, 1436, 1417, 1386, 1242, 1186, 1118, 993, 844, 754, 700, 542,
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500. HRMS (EI) m/z Calcd. for C32H24N2S2: 500.1381. Found: 500.1371. Anal. Calcd.: C
76.76; H 4.83; N 5.60; S 12.81. Found: C 77.00; H 4.89; N 5.68; S 12.62.
O7. Yield 48 %. 1H NMR (CDCl3, δ ppm): 8.60 (d, J=2.9 Hz, 2H, Py-H), 8.43 (dd, J=4.6
Hz, 2H, Py-H), 7.61-7.71 (m, 12H, central Ar-H, Py-H, Ar-Ar(H)-Ar), 7.50 (d, J=8.2 Hz,
4H, OAr-H), 7.21 (dd, J=7.9 Hz, 2H, Py-H), 7.03 (d, J=8.7 Hz, 4H, OAr-H), 6.88-6.96
(m, 4H, diene-H), 6.82 (d, J=15.0 Hz, 2H, diene-H), 6.59 (d, J=14.6 Hz, 2H, diene-H),
4.03 (t, J=6.6 Hz, 4H, OCH2), 1.83 (m, 4H, OCH2CH2), 1.26-1.53 (b, 12H, CH2), 0.93 (t,
J=6.9 Hz, 6H, CH3). 13C NMR (CDCl3, δ ppm): 159.6, 149.1, 141.0, 140.6, 139.3, 135.1,
133.7, 133.5, 133.0, 132.2, 130.9, 130.4, 129.5, 128.7, 128.1, 127.2, 124.1, 115.6, 68.8,
32.3, 29.9, 26.4, 23.3, 14.7. FT-IR (KBr, cm-1): 3028, 2926, 2862, 1724, 1654, 1604,
1577, 1562, 1525, 1502, 1473, 1382, 1288, 1247, 1201, 1176, 985, 825, 704, 526. HRMS
(EI) m/z Calcd. for C60H60N2O2: 840.4655. Found: 840.4633. Anal. Calcd.: C 85.68; H
7.19; N 3.33. Found: C 85.85; H 7.12; N 3.23.
O8. Yield 42 %. 1H NMR (CDCl3, δ ppm): 8.54 (d, 2H, Py-H), 8.41 (dd, J=4.5 Hz, 2H,
Py-H), 7.75-7.85 (m, 6H, central Ar-H, FL-H), 7.65 (d, J=8.1 Hz, 2H, Py-H), 7.37-7.46
(m, 10H, FL-H), 7.17 (dd, J=8.0 Hz, 2H, Py-H), 7.02 (dd, J=13.6 Hz, 2H, diene-H), 6.76-
6.88 (m, 4H, diene-H), 6.59 (d, J=15.2 Hz, 2H, diene-H), 2.00 (t, J=8.2 Hz, 8H, FL-CH2),
1.08-1.25 (b, 28H, CH2), 0.74 (b, 16H, CH2CH3). 13C NMR (CDCl3, δ ppm): 151. 7,
151.4, 149.2, 149.1, 141.7, 141.3, 141.1, 139.7, 135.2, 133.6, 133.2, 132.7, 132.0, 130.4,
129.6, 129.0, 128.2, 127.9, 127.6, 125.4, 124.0, 123.6, 120.5, 120.2, 55.8, 41.0, 32.2, 30.5,
24.6, 23.2, 14.6. FT-IR (KBr, cm-1): 3030, 2926, 2852, 1726, 1654, 1620, 1560, 1456,
1415, 1379, 1267, 1159, 1122, 987, 896, 835, 744, 704, 538. HRMS (EI) m/z Calcd. for
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Synthesis and Characterization of Cross-conjugated Cruciforms with Varied Functional Groups
C74H84N2: 1000.6634. Found: 1000.6588. Anal. Calcd.: C 88.75; H 8.45; N 2.80. Found:
C 88.53; H 8.60; N 2.80.
O9. Yield 35 %. 1H NMR (CDCl3, δ ppm): 8.55 (d, J=3.2 Hz, 2H, Py-H), 8.39 (d, J=3.8
Hz, 2H, Py-H), 8.19 (s, 2H, Cz-H), 8.16 (d, J=7.7 Hz, 2H, Cz-H), 7.83 (s, 2H, central Ar-
H), 7.64 (d, J=8.0 Hz, 2H, Py-H), 7.46-7.58 (m, 8H, Cz-H), 7.25-7.29 (m (overlap with
CHCl3 peak), 2H, Cz-H), 7.17 (dd, J=8.0 Hz, 2H, Py-H), 7.07 (dd, J=10 Hz, 2H, diene-H),
6.83-6.92 (m, 4H, diene-H), 6.57 (d, J= 15.6 Hz, diene-H), 4.39 (t, J=7.2 Hz, 4H, Cz-
CH2), 1.96 (m, 4H, Cz-CH2CH2), 1.26-1.49 (b, 12H, CH2), 0.92 (t, J=6.9 Hz, 6H, CH3).
13C NMR (CDCl3, δ ppm): 149. 1, 148.9, 141.9, 141.6, 140.4, 135.2, 133.7, 133.6, 132.9,
132.4, 131.8, 129.9, 129.0, 128.5, 128.4, 126.6, 124.1, 123.5, 123.4, 122.2, 121.2, 119.7,
109.5, 109.0, 44.0, 32.3, 29.7, 27.7, 23.2, 14.7. FT-IR (KBr, cm-1): 3039, 2926, 2854,
1726, 1597, 1564, 1471, 1413, 1386, 1348, 1242, 1122, 983, 893, 808, 744, 704, 613, 500.
HRMS (EI) m/z Calcd. for C60H58N4: 834.4661. Found: 834.4663. Anal. Calcd.: C 86.29;
H 7.00; N 6.71. Found: C 86.15; H 6.93; N 6.84.
O10. Yield 27 %. 1H NMR (CDCl3, δ ppm): 8.60 (s, 2H, NO2Ar-H), 8.46 (d, J=3.6 Hz,
2H, Py-H), 8.34-8.32 (m, 4H, Py-H, NO2Ar-H), 7.78 (d, J=7.6 Hz, 2H, NO2Ar-H), 7.68-
7.73 (m, 4H, Py-H, NO2Ar-H), 7.66 (s, 2H, central Ar-H), 7.21-7.24 (dd, J=6.3 Hz, Py-
H), 7.00 (dd, J=15.1 Hz, 2H, diene-H), 6.87 (dd, J=15.2 Hz, 2H, diene-H), 6.65 (d,
J=15.4 Hz, 2H, diene-H), 6.58 (d, J=15.2 Hz, 2H, diene-H). 13C NMR (CDCl3, δ ppm):
149.4, 149.2, 149.1, 142.5, 139.5, 136.5, 135.3, 133.3, 133.2, 132.0, 131.4, 130.9, 130.8,
130.1, 128.2, 125.1, 124.2, 123.3. FT-IR (KBr, cm-1): 3032, 2926, 2854, 1730, 1564,
1527, 1483, 1436, 1413, 1346, 1267, 1190, 1118, 987, 898, 804, 742, 689, 543, 500.
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HRMS (EI) m/z Calcd. for C36H26N4O4: 578.1954. Found: 578.1939. Anal. Calcd.: C
74.73; H 4.53; N 9.68. Found: C 75.06; H 4.40; N 9.70.
All oligomers were readliy soluble in common organic solvents such as toluene,
DCM, CHCl3, THF, DMF but not in Hexane, MeOH and EtOH. Bisdienes were all in
trans conformation as were confirmed by NMR and single cyrstall XRD.
5.3. Results and discussion
5.3.1. Thermal properties
All compounds were thermally stable upto 310 ºC (Table 5.1). O10 had the lowest
thermal stability, followed by O5, O1 and O6, which indicated that the electron
withdrawing nitro and pyridine groups tended to slightly decrease the stability probably
due to the lower bandgap. O2, O3, O7 and O8 had higher stability simply due to the
presence of robust biphenylene and fluorene units.
Table 5.1. TGA results of O1-O10 (°C)
O1 O2 O3 O4 O5 O6 O7 O8 O9 O10 270 280 289 310 239 243 282 288 253 229
5.3.2. Absorption and emission properties
UV-Vis (Figure 5.2) and fluorescence (Figure 5.3) spectra were recorded for all
oligomers and the data are summarized in Table 5.2. The strong absorption between 388
- 405 nm was observed for O1-O5 and 412 - 426 nm for O6 - O10. This indicates that
electron withdrawing pyridyl group reduced the bandgap by lowering LUMO level.16-20
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Synthesis and Characterization of Cross-conjugated Cruciforms with Varied Functional Groups
However, the effect of five different functional groups on absorption maxima was limited.
The fluorescence spectra showed that O1 - O5 had emission maxima between 442 - 469
nm, while peak maxima of O6 - O10 were between 488 - 510 nm. The later showed
around 40 nm red shift. We can only attribute this to the existence of electron
withdrawing pyridyl group. O1 - O5 showed blue emssion range while the rest gave near
green emission.
These results indicate that different functional groups along para-phenylene of the
compounds have only a moderate effect on the overall photophysical properties. O7 with
4’-alkoxy biphenyl group showed relatively larger emission maxima probably due to
extended conjugation, less steric hindrance as well as electron donating effect from
alkoxy groups. O3, O4, O8 and O9 with bulky alkyl chain probably should influence the
molecular structure and affected the conjugation with a blue shift in emission maxima.
Since the two sets of compounds only differ in the structure of bisdiene segment, it is
anticipated that the functionalization of the bisdiene segment had more significant
influence on photophysical properties than that of para-phenylene segment.
Table 5.2. UV-Vis absorption and fluorescence properties of O1-O10
Entry O1 O2 O3 O4 O5 O6 O7 O8 O9 O10
λabs/nm 394 405 388 392 399 415 426 412 414 425
λemi/nm 467 469 447 442 454 510 509 501 488 498
BG/eV 3.14 3.06 3.19 3.16 3.10 2.98 2.91 3.00 2.99 2.91
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300 350 400 450 5000.0
0.2
0.4
0.6
0.8
1.0
1.2
Abs
orbe
nce
(a.u
.)
Wavelength (nm)
O1 O2 O3 O4 O5
300 350 400 450 5000.0
0.2
0.4
0.6
0.8
1.0
1.2
Abs
orbe
nce
(a.u
.)
Wavelength (nm)
O6 O7 O8 O9 O10
300 350 400 450 5000.0
0.2
0.4
0.6
0.8
1.0
1.2
Abs
orbe
nce
(a.u
.)
Wavelength (nm)
O1 O2 O3 O4 O5
300 350 400 450 5000.0
0.2
0.4
0.6
0.8
1.0
1.2
Abs
orbe
nce
(a.u
.)
Wavelength (nm)
O6 O7 O8 O9 O10
Figure 5.2. Normalized absorption spectra for O1-O10, recorded in THF solution at concentration of 10 mg/L.
400 450 500 550 600 6500.0
0.2
0.4
0.6
0.8
1.0
1.2 O1 O2 O3 O4 O5 O6 O7 O8 O9 O10
Inte
nsity
(a.u
.)
Wavelength (nm)400 450 500 550 600 650
0.0
0.2
0.4
0.6
0.8
1.0
1.2 O1 O2 O3 O4 O5 O6 O7 O8 O9 O10
Inte
nsity
(a.u
.)
Wavelength (nm)
Figure 5.3. Normalized emission spectra for O1-O10, recorded in THF solution at concentration of 10 mg/L.
5.3.3. X-ray diffraction studies
5.3.3.1. Powder XRD studies
Powder XRD measurements revealed that all oligomers possessed highly crystalline
nature in solid state (Figure 5.4). It was surprising that O3 and O8 bearing bishexyl
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Synthesis and Characterization of Cross-conjugated Cruciforms with Varied Functional Groups
group also showed ordered packing. O4 and O9 bearing hexylcarbazole group were less
organized in powder state and the XRD peaks were not well resolved, probably due
special flexibility of hexyl group. O7 and O8 (O2 and O3 as well) showed a sharp peak
at low angle (6-7º), which was assigned to the intermolecular distance separated by alkyl
chain, indicating the existence of long range order.
5 10 15 20 25 30 35 40
2θ
O1
O2
O3
O4
O5
O6
O7
O10
O8
O9
5 10 15 20 25 30 35 40
2θ
5 10 15 20 25 30 35 40
2θ
O1
O2
O3
O4
O5
O6
O7
O10
O8
O9
Figure 5.4. Powder XRD results of O1-O10
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5.3.3.2. Single crystal XRD studies
Understanding the molecular structures of the oligomers is imperative for
investigating the structure-property relationship. Single crystal XRD of O5 showed that it
was in cruciform shape with trans-bisdiene bond which was completely in the plane of
the central phenyl ring, while the oligo-phenylene unit was highly twisted (>60º), the
twist was larger than that of adjacent phenylene rings in normal PPPs ( as a result of
ortho-H repulsion between neighboring phenyl rings) (Figure 5.5) The twisting
inevitably affects the effective conjugation along the OPP. Overall, these cross-
conjugated oligomers were shown to have lower bandgap compared with linear OPV and
alkoxy substituted OPP,21-22 which should be contributed from both segments, although
to different extent.
Figure 5.5. Molecular structure and packing of O5 solved by single crystal XRD
The packing structure shows that segments of the same kind are facing parallelly to
each other, with an interplanar distance of 3.7 Å, indicating that the strong intermolecular
π-π interaction could be the main driving force for ordered packing. Also we mentioned
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Synthesis and Characterization of Cross-conjugated Cruciforms with Varied Functional Groups
-30 -10 10 30 50 70 90 110 130 150 170 190
in Chapter 4 that cross-conjugated polymer P3 formed nanocrystalline domains simply
by drop-casting.23 From the single crystal XRD results, it might be assumed that the
domains were orinigated from the ordered packing of at least low MW polymer chains. It
was also found that the solubilities (based on mass) of these cruciforms were lower than
the similar cross conjugated polymer (e.g. P1 in Chapter 4 v. s. O1).
5.3.4. Liquid crystal study
Figure 5.6. POM graph of the textures observed on cooling of O2 (left) and O3 (right) at a cooling rate of 0.5 °C /min and DSC spectra (bottom).
Differential scanning calorimetry (DSC) studies showed that cruciforms O2 and O3
had obvious glass transition behavior and the mesophases were also identified by a
polarized optical microscope (POM). Nematic phase of O2 (Schlieren texture) and
O2 O3
Tg = 83.4 oC
Tg = 49.7 oC
Tiso = 137.1 oC
Tiso = 139.8 oC
Temperature (oC)
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smectic-A phase of O3 were observed using a polarized optical microscope (POM)24-25
(Figure 5.6).
The results indicated that O3 formed a lamellar layer structure in the mesophase.
This is not suprising as we can refer to the packing structure of O5 in Figure 5.5. The
rigid mesogenic units not only force the molecule to take on an extended conformation
but also organize close to the backbone and form part of the rigid core by strong π-π
interaction. The microseparation between rigid core and the flexible alkyl chains is the
main driving force for the molecules to pack in a more ordered pattern.26-27 O3 had much
ordered structure probably because the steric crowding of bisalkyl chain on fluorene unit
restricted itself into a more rigid molecular layout.
5.4. Conlusion
We synthesized and characterized ten cross-conjugated oligomers with different
functional groups substituted at each segment. UV-Vis and fluorescence showed that O6-
O10 with electron withdrawing pyridyl group at the end of bisdiene segment showed
significant red shifts compared with phenyl substitured O1-O5. While functionalization
along para-phenylene segment showed less effect on the properties due to highly twist of
C-C bond between phenyl rings. Strong π-π interaction was directly confirmed by single
crystal XRD. Liquid crystal study indicated the Nematic phase for O2 and Smectic-A
phase for O3, which was originated from strong intermolecular interaction between
mesogenic cores and coordination of flexible alkyl chains.
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Synthesis and Characterization of Cross-conjugated Cruciforms with Varied Functional Groups
5.5. References
1. Koren, A. B.; Curtis, M. D.; Francis, A. H.; Kampf, J. W. J. Am. Chem. Soc. 2003,
125, 5040.
2. Sirringhars, H.; Tessler, N., and Friend, R. H. Science 1998, 280, 1741.
3. Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Marks, T. N.;
Taliani, C. Nature 1999, 397, 121.
4. McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. Rev. 2000, 100, 2537.
5. Meier, H. Angew. Chem., Int. Ed. 2005, 44, 2482.
6. Yu, G.; Heeger, A. J. J. Appl. Phys. 1995, 78, 4510.
7. Gholami, M.; Tykwinski, R. R. Chem. Rev. 2006, 106, 4997.
8. Oldham, W. J.; Lachicotte, R. J.; Bazan, G. C. J. Am. Chem. Soc. 1998, 120,
2987.
9. Robinson, M. R.; Wang, S. J.; Heeger, A. J.; Bazan, G. C. Adv, Funct. Mater.
2001, 11, 413.
10. Deb, S. K.; Maddux, T. M.; Yu, L. P. J. Am. Chem. Soc. 1997, 119, 9079.
11. Maddux, T. M.; Li, W. J.; Yu, L. P. J. Am. Chem. Soc. 1997, 119, 844.
12. Niazimbetova, Z. I.; Christian, H. Y.; Bhandari, Y. J.; Beyer, F. L.; Galvin, M. E.
J. Phys. Chem. B 2004, 108, 8673.
13. Fratilou, S.; Senthilkumar, K.; Grozema, F. C.; Christian-Pandya, H.;
Niazimbetova, Z. I.; Bjandari, Y. J.; Galvin, M. E.; Siebbeles, L. D. A. Chem.
Mater. 2006, 18, 2118.
14. Wilson, J. N., Josowicz, M.; Wang, Y. Q.; Bunz, U. H. F. Chem. Commun. 2004,
24, 2962.
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15. Hagedron, L.; Hohler, W. Angewandte Chemie 1975, 87, 486.
16. Roncali, J. Chem. Rev. 1992, 92, 711.
17. Sugimoto, R.; Takeda, S.; Gu, H. B.; Yoshino, K. Chem. Express 1986, 1, 635.
18. Kitamura, C.; Tanaka, S.; Yamashita, Y. J. Chem. Soc., Chem. Commun. 1994,
1585.
19. Karikomi, M.; Kitamura, C.; Tanaka, S.; Yamashita, Y. J. Am. Chem. Soc. 1995,
117, 6791.
20. Tamao, K.; Yamaguchi, S.; Ito, Y. J. Chem. Soc., Chem. Commun. 1994, 229.
21. Barashkov, N. N.; Guerrero, D. J.; Olivos, H. J.; Ferraris, J. P. Synth. Met. 1995,
75, 153.
22. Konstandakopoulou, F. D.; Gravalos, K. G.; Kallitsis, J. K. Macromolecules
1998, 31, 5264.
23. Li, H.; Parameswaran, M.; Murmawati, M. H.; Xu, Q.; Valiyaveettil, S.
Macromolecules. Submitted
24. Collings, P.; Hird, M. Introduction to Liquid Crystals: Chemistry and Physics;
Taylor and Francis, 1997.
25. Dierking, I. Textures of Liquid Crystals; Wiley-VCH, Weinheim, 1978.
26. Tschierske, C. J. Mater. Chem. 1998, 8, 1485.
27. Chen, W.; Wunderlich, B. Macromol. Chem. Phys. 1999, 200, 28
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Synthesis and Characterization of Cross-conjugated Cruciforms with Varied Functional Groups
5.6. Appendix
Table 5.3. Crystal data and structure refinement for O5
Empirical formula C38 H28 N2 O4
Formula weight 576.62
Temperature 295(2) K
Wavelength 0.71073 Å
Crystal system Monoclinic
Space group P2(1)/c
Unit cell dimensions a = 10.350(2) Å = 90°.
b = 21.674(5) Å = 107.287(6)°.
c = 6.7538(14) Å = 90°.
Volume 1446.6(5) Å3
Z 2
Density (calculated) 1.324 Mg/m3
Absorption coefficient 0.086 mm-1
F(000) 604
Crystal size 0.34 x 0.34 x 0.04 mm3
Theta range for data collection 1.88 to 26.00°.
Index ranges -9<=h<=12, -26<=k<=26, -8<=l<=7
Reflections collected 8960
Independent reflections 2825 [R(int) = 0.0366]
Completeness to theta = 26.00° 99.3 %
Absorption correction Sadabs, (Sheldrick 2001)
Max. and min. transmission 0.9966 and 0.9713
Refinement method Full-matrix least-squares on F2
Data / restraints / parameters 2825 / 0 / 199
Goodness-of-fit on F2 1.031
Final R indices [I>2sigma(I)] R1 = 0.0642, wR2 = 0.1398
R indices (all data) R1 = 0.1017, wR2 = 0.1572
Largest diff. peak and hole 0.200 and -0.191 e.Å-3
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Table 5.4. Atomic coordinates ( x 104) and equivalent isotropic displacement parameters (Å2x 103) for O5. U(eq) is defined as one third of the trace of the orthogonalized Uij tensor. ________________________________________________________________________________
x y z U(eq)
________________________________________________________________________________ O(1) 6038(3) 1689(1) 9107(3) 88(1)
O(2) 4244(3) 2204(1) 8906(5) 108(1)
N(1) 5440(3) 2173(1) 9049(3) 66(1)
C(1) 4536(2) 4702(1) 8054(3) 42(1)
C(2) 3988(2) 5256(1) 8446(3) 45(1)
C(3) 4072(2) 4406(1) 6015(3) 51(1)
C(4) 3080(2) 4571(1) 4380(3) 50(1)
C(5) 2708(2) 4233(1) 2473(3) 49(1)
C(6) 1723(3) 4363(1) 771(4) 54(1)
C(7) 1316(2) 4014(1) -1160(3) 47(1)
C(8) 1794(2) 3430(1) -1325(4) 56(1)
C(9) 1395(3) 3115(2) -3173(5) 76(1)
C(10) 510(3) 3368(2) -4883(5) 80(1)
C(11) 9(3) 3938(2) -4755(4) 79(1)
C(12) 408(3) 4264(1) -2915(4) 66(1)
C(13) 5580(2) 4443(1) 9668(3) 42(1)
C(14) 6238(2) 3847(1) 9454(3) 43(1)
C(15) 7593(3) 3819(1) 9585(4) 55(1)
C(16) 8233(3) 3265(1) 9508(4) 63(1)
C(17) 7534(3) 2723(1) 9339(4) 58(1)
C(18) 6197(3) 2750(1) 9195(3) 48(1)
C(19) 5533(2) 3298(1) 9224(3) 47(1)
________________________________________________________________________________
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Table 5.5. Bond lengths [Å] and angles [°] for O5
_____________________________________________________
O(1)-N(1) 1.213(3)
O(2)-N(1) 1.214(3)
N(1)-C(18) 1.462(3)
C(1)-C(2) 1.388(3)
C(1)-C(13) 1.404(3)
C(1)-C(3) 1.464(3)
C(2)-C(13)#1 1.381(3)
C(3)-C(4) 1.314(3)
C(4)-C(5) 1.431(3)
C(5)-C(6) 1.320(3)
C(6)-C(7) 1.458(3)
C(7)-C(8) 1.375(3)
C(7)-C(12) 1.386(3)
C(8)-C(9) 1.375(3)
C(9)-C(10) 1.358(4)
C(10)-C(11) 1.353(4)
C(11)-C(12) 1.382(4)
C(13)-C(2)#1 1.381(3)
C(13)-C(14) 1.487(3)
C(14)-C(19) 1.380(3)
C(14)-C(15) 1.380(3)
C(15)-C(16) 1.380(3)
C(16)-C(17) 1.365(4)
C(17)-C(18) 1.359(3)
C(18)-C(19) 1.376(3)
O(1)-N(1)-O(2) 123.2(3)
O(1)-N(1)-C(18) 118.6(3)
O(2)-N(1)-C(18) 118.1(2)
C(2)-C(1)-C(13) 117.5(2)
C(2)-C(1)-C(3) 121.3(2)
C(13)-C(1)-C(3) 121.2(2)
C(13)#1-C(2)-C(1) 123.1(2)
C(4)-C(3)-C(1) 128.9(2)
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C(3)-C(4)-C(5) 123.5(2)
C(6)-C(5)-C(4) 127.3(2)
C(5)-C(6)-C(7) 127.7(2)
C(8)-C(7)-C(12) 117.4(2)
C(8)-C(7)-C(6) 122.4(2)
C(12)-C(7)-C(6) 120.2(2)
C(9)-C(8)-C(7) 120.8(3)
C(10)-C(9)-C(8) 121.1(3)
C(11)-C(10)-C(9) 119.3(3)
C(10)-C(11)-C(12) 120.5(3)
C(11)-C(12)-C(7) 120.9(3)
C(2)#1-C(13)-C(1) 119.4(2)
C(2)#1-C(13)-C(14) 117.9(2)
C(1)-C(13)-C(14) 122.70(19)
C(19)-C(14)-C(15) 117.6(2)
C(19)-C(14)-C(13) 121.2(2)
C(15)-C(14)-C(13) 121.2(2)
C(16)-C(15)-C(14) 121.7(2)
C(17)-C(16)-C(15) 120.2(3)
C(18)-C(17)-C(16) 118.2(2)
C(17)-C(18)-C(19) 122.5(2)
C(17)-C(18)-N(1) 118.8(2)
C(19)-C(18)-N(1) 118.6(2)
C(18)-C(19)-C(14) 119.8(2)
_____________________________________________________________
Symmetry transformations used to generate equivalent atoms:
#1 -x+1,-y+1,-z+2
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Synthesis and Characterization of Cross-conjugated Cruciforms with Varied Functional Groups
Table 5.6. Anisotropic displacement parameters (Å2x 103) for O5 ______________________________________________________________________________
U11 U22 U33 U23 U13 U12
______________________________________________________________________________
O(1) 143(2) 41(1) 87(2) -3(1) 45(1) 5(1)
O(2) 81(2) 66(2) 167(3) 5(1) 22(2) -22(1)
N(1) 93(2) 45(1) 57(1) -1(1) 16(1) -10(1)
C(1) 46(1) 41(1) 37(1) -4(1) 9(1) -4(1)
C(2) 49(1) 44(1) 39(1) -1(1) 5(1) -1(1)
C(3) 60(2) 43(1) 45(1) -6(1) 8(1) 5(1)
C(4) 57(2) 44(1) 45(1) -6(1) 9(1) 1(1)
C(5) 54(1) 43(1) 45(1) -4(1) 9(1) 1(1)
C(6) 60(2) 50(2) 49(2) -4(1) 9(1) 6(1)
C(7) 45(1) 54(2) 41(1) -1(1) 9(1) -6(1)
C(8) 51(2) 60(2) 52(2) -8(1) 8(1) -3(1)
C(9) 79(2) 72(2) 72(2) -28(2) 14(2) -6(2)
C(10) 75(2) 105(3) 54(2) -27(2) 12(2) -20(2)
C(11) 70(2) 106(3) 46(2) -1(2) -4(1) -5(2)
C(12) 66(2) 71(2) 52(2) 1(1) 4(1) 3(2)
C(13) 45(1) 39(1) 43(1) -3(1) 14(1) -5(1)
C(14) 49(1) 43(1) 33(1) -3(1) 8(1) -2(1)
C(15) 53(2) 51(2) 59(2) -5(1) 14(1) -3(1)
C(16) 50(2) 68(2) 72(2) -2(2) 17(1) 8(1)
C(17) 71(2) 53(2) 50(2) 0(1) 17(1) 14(1)
C(18) 62(2) 41(1) 37(1) -1(1) 9(1) -4(1)
C(19) 50(1) 48(1) 41(1) -2(1) 9(1) -1(1)
______________________________________________________________________________
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Table 5.7. Hydrogen coordinates ( x 104) and isotropic displacement parameters (Å2x 10 3) for O5 ________________________________________________________________________________
x y z U(eq)
________________________________________________________________________________
H(2A) 3296 5433 7390 54
H(3) 4543 4053 5857 61
H(4) 2595 4925 4466 60
H(5) 3218 3883 2426 59
H(6) 1231 4720 809 65
H(8) 2396 3246 -171 67
H(9) 1736 2722 -3254 91
H(10) 251 3152 -6128 95
H(11) -609 4111 -5913 95
H(12) 64 4658 -2854 79
H(15) 8088 4183 9728 66
H(16) 9143 3260 9572 76
H(17) 7961 2347 9322 70
H(19) 4612 3298 9089 57
________________________________________________________________________________
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CChhaapptteerr 66
SSyynntthheessiiss aanndd CChhaarraacctteerriizzaattiioonn ooff CCrroossss--ccoonnjjuuggaatteedd PPoollyypphheennoollss
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Synthesis and Characterization of Cross-conjugated Polyphenols
6.1. Introduction
Polyphenolic compounds usually found in food and beverages attract much attention
because multiple studies have shown that they play an important role in the prevention of
many diseases.1,2 In particular, Alzheimer’s and Parkinson’s diseases, cancer,
cardiovascular diseases and multiple sclerosis that is the result of cell damage due to
severe oxidative stress.3 Oxidative stress is caused by reactive oxygen species (ROS) that
include free radicals such as hydroxyl radicals and non radical compounds such as ozone.
Our body combats this oxidative stress with antioxidants that can react with these free
radicals to give stable product before vital organs are damaged.4 Although, there are
some enzyme systems, like superoxide dismutase (SOD) and small molecules, in our
body capable of scavenging these radicals, the more effective principal antioxidants
Vitamin C, Vitamin E, β-carotene and other polyphenols cannot be produced in our
system and hence must be supplied in our diets.5 The total dietary intake of polyphenols
(1 g/day) is approximately 10 times higher than the intake of Vitamin C and 100 times
higher than the intake of Vitamin E and β-carotene.6,7 These polyphenols are also able to
complex with many proteins and polysaccharides to aid in the healing of wounds, burns
and inflammations.8
One of the better known polyphenol representative is trans-Resveratrol that has
received wide attention owing to its chemopreventive, cardioprotective, anti-
inflammatory activities apart from its antioxidant properties.9 Other important
polyphenols also include phenolic acids, flavonols, monomeric catechins and
anthocyanidins.10, 11 The antioxidant ability of trans-Resveratrol may be attributed to the
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structural conformations that result in the low bond dissociation energy of the phenolic –
OH in the para position where the proton can be easily abstracted by free radicals
forming stable products.11 The resultant phenolate radical can then be stabilized by the
extensive linear π-conjugation across the two phenyl rings and the ethylene bridge. To
date, the cross conjugation effect in organic compounds has not been fully explored and
little is known about the extend of π-conjugation in such systems.12,13 Moreover, studies
have mainly been focused on polyphenols that have linear limited conjugation length.14
Hence, to our interest, we will like to investigate the antioxidant and optical properties of
cross-conjugated polyphenols having phenolic rings separated based on extended
conjugation framework. We successfully synthesized and characterized ten novel cross-
conjugated polyphenols (Figure 6.1). In this chapter, we discuss how the combined
structural factors (number of hydroxyl groups, ethylene bonds and the cross conjugation
effect) influence the antioxidant properties of these polyphenolic compounds.
HO OH(OH)n
(OH
)n
1a, n = 01b, n = 1, o-hydroxy1c, n = 1, m-hydroxy1d, n = 1, p-hydroxy1e, n = 2, 3,4-dihydroxy
HO OH
(OH)n
(OH)n
2a, n = 02b, n = 1, o-hydroxy2c, n = 1, m-hydroxy2d, n = 1, p-hydroxy2e, n = 2, 3,4-dihydroxy
Figure 6.1. Synthesized cross-conjugated polyphenols
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Synthesis and Characterization of Cross-conjugated Polyphenols
6.2. Experimental
TEAC Assay: Trolox (6- hydro-2,5,7,8-tetramethylchroman-2-carboxylic acid) was
used as an antioxidant standard. Trolox (2.5 mM) was prepared in ethanol as the stock
solution. The ABTS+ radical was generated by the addition of potassium persulphate
(2.45 mM final concentration) to 2, 2’-azinobis (3-ethylbenzothiazoline-6-sulfonic acid)
diammonium salt (ABTS+●, 7mM final concentration) in 25 mL of deionised water. The
mixture was allowed to stand at room temperature for about 12 -16 hours before use. The
solution was stable for usage for 2 days from the time the solution was prepared. The
working ABTS+● solution was diluted 100 times with ethanol in the 3 mL UV quartz cell,
having an absorbance of 1.500 at 414 nm. The decrease in the absorbance of the solution
was monitored using a UV spectrophotometer. Various amounts (0, 2.5, 5.0, 7.5, 10.0,
12.5 μL) of Trolox were added to the working ABTS+● solution to obtain the decrease in
absorbance for different concentrations. The decrease in absorbance after a fixed period
of time was plotted against the concentration of the antioxidant. Each sample to be tested
(1a-e, 2a-e in Figure 6.1) was made up to 5 mL of solution having a concentration of
1mM. 10 μL of the solution was then added to the working ABTS+● working solution to
measure the decrease in the absorbance. The antioxidant testing for these samples are
repeated for 2 times and the average decrease in absorbance for each sample was used to
determine the TEAC value.
Cell Morphology: The morphology of the cells was analyzed using Olympus CK 40
microscope after the incubation period and pictures were taken using Olympus C7070WZ
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camera. The differential interphase contrast (DIC) images were captured using Zeiss
Axiovert 200M equipped with Axiocam HRc.
Cell Viability: The percentage of viable cells were determined as a measure of ATP
content in the cells using CellTiter Glow Luminescent cell viability kit 15 (Promega). The
cells and reagents were brought to room temperature 30 minutes before the assay. Equal
volume of the reagent is added to the wells and cell lysis was induced by shaking for 2
minutes in an orbital shaker, followed by 10 minutes incubation at room temperature. The
luminescence was measured by transferring 100 μL of the suspension a white opaque
walled 96 well plate (costar) and read using TECAN Genios Plus Luminometer. The
viability (%) was calculated as % of viability = Mean (test)/ Mean (control) *100.
Mitochondrial activity: Mitochondrial activity was measured using CellTiter 96
Aqueous one solution assay 16,17 (MTS Assay, Promega). The assay was performed as
described by the supplier with slight modifications. The medium was removed from the
wells after the incubation period and 40 μL of reagent was added in to the wells along
with 200 μL of warm medium. The plates are incubated at 37 °C for 2 hours and
absorbance was measured spectrophotometrically at 490 nm. The values were expressed
as MTS reduction (% of control) = Absorbance (test)/Absorbance (control) *100.
Membrane Integrity: Cytotoxicity was measured in terms of membrane integrity,
using Cytotox 96 Non-Radioactive cytotoxicity assay 18,19 (Promega) that is a quantitative
measurement of the Lactate dehydrogenase (LDH) activity in the medium. LDH is a
cytosolic enzyme, which is released in to the medium upon cell lysis. The assay was
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Synthesis and Characterization of Cross-conjugated Polyphenols
performed as described by Laura et.al. The LDH leakage (%) was calculated as % of
LDH leakage = Absorbance (test)/Absorbance (control) * 100.
Cell cycle analysis: The cells (0.8 million) were plated in to 6 cm culture dishes and
incubated overnite. The chemical treatment was carried out as described earlier. After the
incubation period, the cells were harvested, fixed in 70 % ethanol and stained with
Propidium iodide (PI) : RNAse (40 μg/mL PI : 100 μg/ml RNAse).20 Incubated overnite
at 4°C. The cells were brought to room temperature 30 minutes before the Flow
cytometry analysis (Epics Altra, Beckman and coulter). The relative % of cells at G0/G1,
G2/M and S phases were determined using WIN MDI 2.8.
Statistical analysis: The data are expressed as “mean ± standard deviation” of three
independent experiments.
6.3. Results and discussion
6.3.1. Synthesis
The synthetic route that was taken is summarized in Scheme 6.1. 1,4-Di(4-
methoxyphenyl)-2,5-bis-(4-phenyl-buta-1,3-dienyl)-benzene (9a). Dibromo compound
8a (0.25 g, 0.51 mmol) and boronic acid 6a (0.17 g, 1.11 mmol) were mixed in 25 mL of
DMF under a nitrogen atmosphere. A 20 mL aliquot of Na2CO3 solution (2 M) was added
to this mixture and the entire setup was degassed for 6 hours.
Tetrakis(triphenylphosphino)palladium (3 mol % with respect to dibromo compound 8a)
as catalyst was added and the reaction solution was stirred at 80°C for 40 hours then
poured into 500 mL of deionised water. The mixture was extracted with DCM (50 mL ×
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4), concentrated in vacuo and purified using column chromatography with n-hexane:
DCM: EA = 4: 2 : 1 as the eluent to afford 9a (bright yellow solid) in 80 % yield (0.22 g).
1H NMR (CDCl3, δ ppm): 7.61 (s, 2H, Ar-H), 7.40 – 7.36 (m, 8H, Ar-H), 7.31 (d, J = 7.8
Hz, 4H, Ar-H), 7.21 (d, J = 7.1 Hz, 2H, Ar-H), 7.03 (d, J = 8.7 Hz, 4H, Ar-H), 6.95 –
6.81 (m, 4H, diene-H), 6.70 (d, J = 14.8 Hz, diene-H), 6.61 (d, J = 14.8 Hz, 2H, diene-H),
3.90 (s, 6H, OMe). 13C NMR (CDCl3, δ ppm): 159.6, 140.4, 138.0, 135.1, 133.6, 133.2,
131.9, 131.6, 130.6, 130.4, 129.3, 128.1, 127.9, 127.0, 114.3, 56.0. EI-MS m/z (%): 546.0
(100.0), 547.1 (40.2), 548.1 (10.3). Elemental analysis calcd for C40H34O2: C, 89.36; H,
6.06. Found C, 89.70; H, 5.93.
1,4-Di(4-methoxyphenyl)-2,5-bis-[4-(2-methoxyphenyl)-buta-1,3-dienyl-benzene
(9b). A similar procedure was adopted for the synthesis of 9b as in the case of 9a
excepted that 25 mL of toluene was used instead of DMF and 2 %
cetyltrimethylammonium bromide as phase transfer catalyst was added to the reaction
mixture, with 75 % yield (0.21 g). 1H NMR (CDCl3, δ ppm): 7.61 (s, 2H, Ar-H), 7.45 (d,
J = 7.5 Hz, 2H, Ar-H), 7.37 (d, J = 8.7 Hz, 4H, Ar-H), 7.02 (d, J = 8.7 Hz, 4H, Ar-H),
6.95-6.84 (m, 12H, Ar-H, diene-H), 6.68 (d, J = 15.8 Hz, 2H, diene-H), 3.90 (s, 6H,
OMe), 3.85 (s, 6H, OMe). 13C NMR (CDCl3, δ ppm): 159.6, 157.4, 140.3, 135.1, 133.8,
131.6, 131.5, 131.3, 130.9, 129.4, 129.1, 128.1, 127.9, 127.0, 121.3, 114.3, 111.6, 56.1,
56.0. EI-MS m/z (%): 606.4 (100.0), 607.4 (45.7), 608.4 (12.0). Elemental analysis calcd
for C42H38O4: C, 83.14; H, 6.13. Found C, 82.90; H, 6.22.
1,4-Di(4-methoxyphenyl)-2,5-bis-[4-(3-methoxyphenyl)-buta-1,3-dienyl]-
benzene (9c). A similar procedure was adopted for the synthesis of 9c as in the case of
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Synthesis and Characterization of Cross-conjugated Polyphenols
9b to afford the product (yellow solid) in 70 % yield (0.19 g). 1H NMR (CDCl3, δ ppm):
7.61 (s, 2H, Ar-H), 7.36 (d, J = 8.7 Hz, 4H, Ar-H), 7.20 (d, J = 7.8 Hz, 2H, Ar-H), 7.03 (d,
J = 8.7 Hz, 4H, Ar-H), 6.99-6.67 (m, 16H, Ar-H, diene-H), 6.58 (d, J = 14.3 Hz, 2H,
diene-H), 3.90 (s, 6H, OMe), 3.81 (s, 6H, OMe). 13C NMR (CDCl3, δ ppm): 160.56,
159.6, 140.5, 139.5, 135.1, 133.6, 133.1, 132.1, 131.6, 130.7, 130.7, 130.5, 130.2, 127.9,
119.8, 114.4, 114.1, 111.9, 56.0, 55.9. EI-MS m/z (%): 606.4 (100.0), 607.4 (45.1), 608.4
(10.9). Elemental analysis calcd for C42H38O4: C, 83.14; H, 6.13. Found C, 82.95; H, 6.18.
1,4-Di(4-methoxyphenyl)-2,5-bis-[4-(4-methoxyphenyl)-buta-1,3-dienyl]-
benzene (9d). A similar procedure was adopted for the synthesis of 9d as in the case of
9b to afford the product (orange yellow solid) in 72 % yield (0.20 g). 1H NMR (CDCl3, δ
ppm): 7.63 (2H, s), 7.41 (d, J = 8.7 Hz, 4H, Ar-H), 7.36 (d, J = 8.7 Hz, 4H, Ar-H), 7.06 (d,
J = 8.7 Hz, 4H, Ar-H), 6.88 (d, J = 8.7 Hz, 4H, Ar-H), 6.81 – 6.66 (m, 6H, diene-H), 6.60
(d, J = 15.2 Hz, 2H, diene-H), 3.95 (s, 6H, OMe), 3.85 (s, 6H, OMe). 13C NMR (CDCl3, δ
ppm): 159.9, 159.6, 140.2, 135.0, 133.8, 132.8, 131.6, 130.9, 130.8, 130.7, 128.4, 128.2,
127.8, 114.8, 114.3, 56.06, 55.99. EI-MS m/z (%): 606.4 (100.0), 607.4 (43.8), 608.5
(37.2), 610.3 (15.0). Elemental analysis calcd for C42H38O4: C, 83.14; H, 6.13. Found C,
82.76; H, 6.04.
1,4-Di(4-methoxyphenyl)-2,5-bis-[4-(3,4-dimethoxyphenyl)-buta-1,3-dienyl]-
benzene (9e). A similar procedure was adopted for the synthesis of 9e as in the case of
9b. The crude compound was purified using column chromatography with n-hexane:
DCM: EA = 1: 2: 1 as the eluent to afford 9e (yellow solid) in 66 % yield (0.21 g). 1H
NMR (CDCl3, δ ppm): 7.60 (s, 2H, Ar-H), 7.38 (d, J = 8.6Hz, 4H, Ar-H), 7.03 (d, J =
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8.6Hz, 4H, Ar-H), 6.95 – 6.88 (m, 6H, Ar-H), 6.81 – 6.63 (m, 6H, diene-H), 6.55 (d, J =
15.5 Hz, 2H, diene-H), 3.90 (s, 12H, OMe), 3.88 (s, 6H, OMe). 13C NMR (CDCl3, δ
ppm): 159.6, 149.8, 149.6, 140.3, 135.0, 133.8, 133.0, 131.6, 131.2, 130.9, 130.6, 129.4,
128.6, 127.8, 120.6, 114.3, 111.8, 109.0, 56.6, 56.5, 56.0. EI-MS m/z (%): 666.3 (100.0),
667.3 (49.8), 640.3 (20.1), 668.30 (12.9). Elemental analysis calcd for C44H42O6: C,
79.25; H, 6.35. Found C, 78.88; H, 6.47.
1,4-Di(4-methoxybiphenyl)-2,5-bis-(4-biphenyl-buta-1,3-dienyl)-benzene (10a).
A similar procedure was adopted for the synthesis of 10a as in the case of 9a using 6b
(0.25 g, 1.11 mmol) instead of 6a to afford the product (yellow solid) in 60 % yield (0.35
g). 1H NMR (CDCl3, δ ppm): 7.70 – 7.63 (m, 10H, Ar-H), 7.52 (d, J = 8.1 Hz, 4H, Ar-H),
7.39 (d, J = 7.6 Hz, 4H, Ar-H), 7.30 (d, J = 7.1 Hz, 4H, Ar-H), 7.21 (d, J = 7.4 Hz, 2H,
Ar-H), 7.04 (d, J = 8.7 Hz, 4H, Ar-H), 6.99 – 6.95 (m, 4H, diene-H), 6.77 (d, J = 15.1 Hz,
2H, diene-H), 6.63 (d, J = 15.0 Hz, 2H, diene-H), 3.89 (s, 6H, OMe). 13C NMR (CDCl3, δ
ppm): 160.0, 140.7, 139.6, 138.0, 135.1, 133.9, 133.4, 131.7, 131.0, 130.9, 130.3, 129.3,
128.8, 128.2, 127.9, 127.3, 127.2, 127.0, 115.0, 56.0. EI-MS m/z (%): 698.3 (100.0),
699.3 (54.5), 700.3 (13.6). Elemental analysis calcd for C52H42O2: C, 89.36; H, 6.06.
Found C, 89.04; H, 6.30.
1,4-Di(4-methoxybiphenyl)-2,5-bis-[4-(2-methoxybiphenyl)-buta-1,3-dienyl]-
benzene (10b). A similar procedure was adopted for the synthesis of 10b as in the case of
9b to afford the product (dark yellow solid) in 80 % yield (0.27 g). 1H NMR (CDCl3, δ
ppm): 7.69 (d, J = 10.5 Hz, 4H, Ar-H), 7.68 (d, J = 10.2 Hz, 4H, Ar-H), 7.63 (s, 2H, Ar-
H), 7.52 (d, J = 6.2 Hz, 4H, Ar-H), 7.46 (dd, J = 6.6 Hz, 2H, Ar-H), 7.17 (d, J = 7.4 Hz,
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Synthesis and Characterization of Cross-conjugated Polyphenols
2H, Ar-H), 7.04 (d, J = 6.6 Hz, 4H, Ar-H), 6.99 – 6.84 (m, 10H, Ar-H, diene-H), 6.75 (d,
J = 15.3 Hz, 2H, diene-H), 3.89 (s, 6H, OMe), 3.84 (s, 6H, OMe). 13C NMR (CDCl3, δ
ppm): 159.9, 57.4, 140.6, 140.3, 139.7, 134.0, 131.7, 131.1, 131.0, 130.9, 130.5, 129.2,
128.8, 127.9, 127.18, 127.12, 127.0, 121.3, 115.0, 111.6, 56.1, 56.0. EI-MS m/z (%):
758.7 (100.0), 759.7 (55.6), 760.7 (11.1). Elemental analysis calcd for C54H46O4: C,
85.46; H, 6.11. Found C, 85.27; H, 6.27.
1,4-Di(4-methoxybiphenyl)-2,5-bis-[4-(3-methoxybiphenyl)-buta-1,3-dienyl]-
benzene (10c). A similar procedure was adopted for the synthesis of 10c as in the case of
9b to afford the product (orange yellow solid) in 76 % yield (0.26 g). 1H NMR (CDCl3, δ
ppm): 7.70 – 7.66 (m, 8H, Ar-H), 7.63 (s, 2H, Ar-H), 7.53 – 7.50 (m, 6H, Ar-H), 7.20 (dd,
J = 7.6 Hz, 2H, Ar-H), 7.04 (d, J = 8.7 Hz, 4H, Ar-H), 6.99 – 6.75 (m, 10H, Ar-H, diene-
H), 6.59 (d, J = 15.1 Hz, 2H, diene-H), 3.89 (s, 6H, OMe), 3.80 (s, 6H, OMe). 13C NMR
(CDCl3, δ ppm): 160.5, 160.0, 140.7, 140.4, 139.6, 139.4, 135.1, 133.9, 133.3, 131.9,
131.5, 130.9, 130.2, 129.5, 128.8, 127.9, 127.2, 119.9, 115.0, 114.2, 111.9, 56.0, 55.9.
EI-MS m/z (%): 758.6 (100.0), 759.7 (56.5), 760.6 (13.0), 503.4 (13.0), 515.4 (8.7),
637.5 (4.3). Elemental analysis calcd for C54H46O4: C, 85.46; H, 6.11. Found C, 85.21; H,
6.17.
1,4-Di(4-methoxybiphenyl)-2,5-bis-[4-(4-methoxybiphenyl)-buta-1,3-dienyl]-
benzene (10d). A similar procedure was adopted for the synthesis of 10d as in the case of
9b to afford the product (orange yellow solid) in 65 % yield (0.22 g). 1H NMR (CDCl3, δ
ppm): 7.73 – 7.70 (m, 8H, Ar-H), 7.58 – 7.54 (m, 6H, Ar-H), 7.37 (d, J = 8.7 Hz, 4H, Ar-
H), 7.08 (d, J = 8.9 Hz, 4H, Ar-H), 6.97 (dd, J = 15 Hz, 2H, diene-H), 6.87 (d, J = 8.7 Hz,
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Li Hairong National University of Singapore
4H, Ar-H), 6.79 – 6.73 (m, 4H, diene-H), 6.62(d, J = 15.4 Hz, 2H, diene-H), 3.93(6H, s),
3.84 (6H, s). 13C NMR (CDCl3, δ ppm): 161.6, 160.8, 137.2, 133.1, 132.9, 131.5, 130.9,
130.6, 129.6, 129.4, 128.8, 128.7, 127.1, 125.6, 126.5, 124.2, 122.6, 115.0, 109.6, 56.2,
56.1. EI-MS m/z (%): 758.7 (100.0), 759.7 (51.9), 760.7 (7.4). Elemental analysis calcd
for C54H46O4: C, 85.46; H, 6.11. Found C, 84.32; H, 6.13.
1,4-Di(4-methoxybiphenyl)-2,5-bis-[4-(3,4-dimethoxybiphenyl)-buta-1,3-dienyl]
-benzene (10e). A similar procedure was adopted for the synthesis of 10e as in the case
of 9e to afford the product (yellow solid) in 73 % yield (0.24 g). 1H NMR (CDCl3, δ
ppm): 7.71 – 7.66 (m, 8H, Ar-H), 7.63 (s, 2H, Ar-H), 7.52 (d, J = 8.5 Hz, 4H, Ar-H), 7.04
(d, J = 8.7Hz, 4H, Ar-H), 6.96 – 6.90 (m, 6H, Ar-H), 6.81 – 6.71 (m, 6H, diene-H), 6.57
(d, J = 15.3Hz, 2H, diene-H), 3.89 (s, 12H, OMe), 3.87 (s, 6H, OMe). 13C NMR (CDCl3,
δ ppm): 160.0, 149.8, 149.6, 140.6, 140.3, 139.8, 135.1, 133.8, 133.2, 131.2, 131.0, 130.7,
128.7, 128.5, 127.8, 127.1, 127.0, 120.7, 115.0, 111.8, 109.0, 56.6, 56.5, 56.0. EI-MS m/z
(%): 818.7 (100.0), 819.8 (53.8), 820.8 (7.7). Elemental analysis calcd for C56H50O6: C,
82.13; H, 6.15. Found C, 82.47; H, 6.09
1,4-Di(4-hydroxyphenyl)-2,5-bis-(4-phenyl-buta-1,3-dienyl)-benzene (1a). A
solution of BBr3 (0.92 mL, 0.92 mmol) in 5 mL of DCM was added dropwisely to a
solution 9a (0.10 g, 0.18 mmol) in 10 mL of DCM/MeOH mixture for 2 hours at room
temperature. The mixture was then poured into 200 mL of deionised water and extracted
with diethyl ether (25 mL × 4). The organic layer was dried over Na2SO4, concentrated
under reduced pressure and washed with excess DCM to afford 1a (dark brown solid) in
80 % yield (0.076 g). 1H NMR (MeOH-d4, δ ppm): 7.70 – 7.65 (2H, m), 7.5 – 7.0 (12H,
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Synthesis and Characterization of Cross-conjugated Polyphenols
m), 7.08 – 6.73 (10H, m), 6.76 – 6.62 (4H, m). 13C NMR (MeOH-d4, δ ppm): 158.4,
143.1, 134.7, 133.9, 132.5, 132.1, 131.5, 130.5, 130.2, 129.8, 128.8, 128.2, 127.3, 126.9,
116.1. Elemental analysis calcd for C38H30O2: C, 88.00; H, 5.83. Found C, 87.60; H, 6.03.
1,4-Di(4-hydroxyphenyl)-2,5-bis-[4-(2-hydroxyphenyl)-buta-1,3-dienyl-benzene
(1b). A similar procedure was adopted for the synthesis of 1b as in the case of 1a to
afford 1b (dark brown solid) in 80 % yield (0.073 g). 1H NMR (MeOH-d4, δ ppm): 7.75
(2H, s), 7.61 - 7.39 (8H, m), 7.30 - 7.01 (12H, m), 6.97 - 6.68 (6H, m). 13C NMR
(MeOH-d4, δ ppm): 158.3, 157.4, 143.1, 134.0, 133.2, 132.3, 132.0, 131.8, 130.7, 129.7,
129.0, 128.4, 127.8, 126.1, 125.3, 116.4, 115.9. Elemental analysis calcd for C38H30O4: C,
82.89; H, 5.49. Found C, 82.56; H, 5.74.
1,4-Di(4-hydroxyphenyl)-2,5-bis-[4-(3-hydroxyphenyl)-buta-1,3-dienyl]-
benzene (1c). A similar procedure was adopted for the synthesis of 1c as in the case of 1a
to afford 1c (dark brown solid) in 84 % yield (0.076 g). 1H NMR (MeOH-d4, δ ppm):
7.72 (2H, s), 7.68– 7.45 (6H, m), 7.38 – 7.00 (12 H), 6.96 - 6.50 (6H, m). 13C NMR
(MeOH-d4, δ ppm): 158.6, 157.7, 143.1, 135.8, 134.5, 134.0, 133.5, 132.0, 131.7, 131.2,
130.6, 130.0, 129.0, 126.7, 119.2, 116.8, 116.1. Elemental analysis calcd for C38H30O4: C,
82.89; H, 5.49. Found C, 83.18; H, 5.23.
1,4-Di(4-hydroxyphenyl)-2,5-bis-[4-(4-hydroxyphenyl)-buta-1,3-dienyl]-
benzene (1d). A similar procedure was adopted for the synthesis of 1d as in the case of
1a to afford 1d (dark brown solid) in 76 % yield (0.069 g). 1H NMR (MeOH-d4, δ ppm):
7.74 (2H, s), 7.70 – 7.50 (6H, m), 7.41 – 7.04 (12H, m), 7.00 – 6.51 (6H, m). 13C NMR
(MeOH-d4, δ ppm): 157.3, 156.7, 143.0, 136.8, 133.9, 132.0, 131.3, 130.9, 130.0, 129.8,
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Li Hairong National University of Singapore
128.4, 127.4, 126.8, 116.4, 115.9. Elemental analysis calcd for C38H30O4: C, 82.89; H,
5.49. Found C, 82.52; H, 5.35.
1,4-Di(4-hydroxyphenyl)-2,5-bis-[4-(3,4-dihydroxyphenyl)-buta-1,3-dienyl]-
benzene (1e). A similar procedure was adopted for the synthesis of 1e as in the case of 1a
to afford 11e (dark brown solid) in 79 % yield (0.072 g). 1H NMR (MeOH-d4, δ ppm):
7.75 (2H, s), 7.60 – 7.49 (6H, m), 7.41 – 7.21 (4H, m), 7.12 – 6.88 (8H, m), 6.78 - 6.57
(4H, m). 13C NMR (MeOH-d4, δ ppm): 157.2, 148.3, 147.4, 141.3, 136.8, 133.8, 132.3,
130.4, 129.8, 129.1, 128.6, 127.9, 127.4, 126.2, 117.0, 115.6, 114.9. Elemental analysis
calcd for C38H30O6: C, 78.33; H, 5.19. Found C, 78.71; H, 5.02.
1,4-Di(4-hydroxybiphenyl)-2,5-bis-(4-biphenyl-buta-1,3-dienyl)-benzene (2a). A
similar procedure was adopted for the synthesis of 2a as in the case of 1a to afford 2a
(dark brown solid) in 69 % yield (0.066 g). 1H NMR (MeOH-d4, δ ppm): 7.70 (2H, s),
7.57 –7.37 (8H, m), 7.33 - 7.15 (14H, m), 7.09 – 6.99 (6H, m), 6.92 - 6.68 (6H, m). 13C
NMR (MeOH-d4, δ ppm): 157.9, 141.3, 139.0, 138.4, 135.0, 133.9, 133.6, 132.6, 131.6,
130.8, 130.2, 129.9, 129.3, 128.9, 128.8, 128.7, 127.4, 125.1, 115.8. Elemental analysis
calcd for C50H38O2: C, 89.52; H, 5.71. Found C, 89.18; H, 5.41.
1,4-Di(4-hydroxybiphenyl)-2,5-bis-[4-(2-hydroxybiphenyl)-buta-1,3-dienyl]-
benzene (2b). A similar procedure was adopted for the synthesis of 1e as in the case of
1a to afford 1e (dark brown solid) in 76 % yield (0.070 g). 1H NMR (MeOH-d4, δ ppm):
7.74 – 7.64 (6H, m), 7.57 – 7.25 (16H, m), 7.20 – 7.02 (6H, m), 6.96 - 6.67 (6H, m). 13C
NMR (MeOH-d4, δ ppm): 157.9, 157.2, 140.9, 139.4, 138.6, 133.5, 132.2, 131.0, 130.3,
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Synthesis and Characterization of Cross-conjugated Polyphenols
129.5, 129.2, 128.6, 128.3, 127.9, 127.4, 127.0, 126.7, 126.0, 125.4, 117.1, 116.4.
Elemental analysis calcd for C50H38O4: C, 85.44; H, 5.45. Found C, 85.12; H, 5.60.
1,4-Di(4-hydroxybiphenyl)-2,5-bis-[4-(3-hydroxybiphenyl)-buta-1,3-dienyl]-
benzene (2c). A similar procedure was adopted for the synthesis of 1e as in the case of 1a
to afford 1e (dark brown solid) in 77 % yield (0.071 g). 1H NMR (acetone-d6, δ ppm):
7.76 (2H, s), 7.62 – 7.52 (6H, m), 7.45 – 7.15 (10H, m), 7.10 – 6.97 (6H), 6.93 - 6.63
(10H, m). 13C NMR (MeOH-d4, δ ppm): 157.8, 156.9, 140.9, 139.5, 138.8, 133.3, 132.2,
131.1, 130.4, 129.7, 129.5, 128.9, 128.8, 128.4, 128.0, 127.4, 127.1, 126.6, 125.7, 117.8,
116.6. Elemental analysis calcd for C50H38O4: C, 85.44; H, 5.45. Found C, 85.05; H, 5.77.
1,4-Di(4-hydroxybiphenyl)-2,5-bis-[4-(4-hydroxybiphenyl)-buta-1,3-dienyl]-
benzene (2d). A similar procedure was adopted for the synthesis of 1e as in the case of
1a to afford 1e (dark brown solid) in 61 % yield (0.056 g). 1H NMR (MeOH-d4, δ ppm):
7.70 (2H, s), 7.63 – 7.43 (8H, m), 7.38 – 7.31 (4H, m), 7.24 – 7.12 (8H, m), 7.05 – 6.82
(8H, m), 6.74 – 6.60 (4H, m). 13C NMR (MeOH-d4, δ ppm): 157.8, 157.0, 140.6, 138.9,
137.3, 133.9, 132.4, 132.0, 130.9, 129.8, 129.2, 129.0, 128.79, 128.73, 128.6, 128.6,
128.4, 116.9, 116.4. Elemental analysis calcd for C50H38O4: C, 85.44; H, 5.45. Found C,
85.09; H, 5.18.
1,4-Di(4-hydroxybiphenyl)-2,5-bis-[4-(3,4-dihydroxybiphenyl)-buta-1,3-dienyl]-
benzene (2e). A similar procedure was adopted for the synthesis of 1e as in the case of 1a
to afford 1e (dark brown solid) in 53 % yield (0.048 g). 1H NMR (MeOH-d4, δ ppm):
7.75 (2H, s), 7.56 – 7.40 (6H, m), 7.35 – 7.18 (8H, m), 7.07 - 6.91 (12H, m), 6.80 – 6.64
(4H, m). 13C NMR (MeOH-d4, δ ppm): 156.6, 149.3, 148.4, 140.5, 137.3, 133.4, 131.8,
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130.9, 130.7, 130.6, 130.2, 129.4, 128.8, 128.3, 127.96, 127.93, 127.4, 127.2, 117.9,
115.0, 114.2. Elemental analysis calcd for C50H38O6: C, 81.72; H, 5.21. Found C, 82.01;
H, 5.04.
CHO(OH)n
CH2=CHCO2C2H5, K2CO3
MeCN, reflux, 40 hrs (OH)n
CHO MeI, K2CO3
12 hrs (OM
e)n
CHO
3a, n = 03b, n = 1, o-hydroxy3c, n = 1, m-hydroxy3d, n = 1, p-hydroxy3e, n = 2, 3,4-dihydroxy
4a, n = 04b, n = 1, o-methoxy4c, n = 1, m-methoxy4d, n = 1, p-methoxy4e, n = 2, 3,4-dihydroxy
R Br R Br
R = OH, OH
MeI, K2CO3
12 hrs5a, R = OMe5b, R = OMe
R B(OH)2
6a, R = OMe6b, R = OMe
Br
Br
Br
Br
CH2Br
BrH2C
NBS, CCl4Reflux, 4 hrs
BuLi, THF, -78oCTriisopropylborate
1. P(OEt)3, 4 hrs
2. Cinnamaldehydes, t-BuOK, DMF, r.t.
BrBr
(OM
e)n
(OM
e)n
8a, n = 08b, n = 1, o-methoxy8c, n = 1, m-methoxy8d, n = 1, p-methoxy8e, n = 2, 3,4-dimethoxy6a, toluene,
K2CO3 (2M),Pd(PPh3)4
7
MeO OMe
(OMe)n
(OM
e)n
6b, toluene,K2CO3 (2M),Pd(PPh3)4
9a, n = 09b, n = 1, o-methoxy9c, n = 1, m-methoxy9d, n = 1, p-methoxy9e, n = 2, 3,4-dimethoxy
MeO OMe
(OM
e)n
(OMe)n
10a, n = 010b, n = 1, o-methoxy10c, n = 1, m-methoxy10d, n = 1, p-methoxy10e, n = 2, 3,4-dimethoxy
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Synthesis and Characterization of Cross-conjugated Polyphenols
HO OH
(OH)n
(OH
)n
1a, n = 01b, n = 1, o-hydroxy1c, n = 1, m-hydroxy1d, n = 1, p-hydroxy1e, n = 2, 3,4-dihydroxy
BBr3, CH2Cl2rt, 2 hrs
BBr3, CH2Cl2rt, 2 hrs
HO OH
(OH)n
(OH)n
2a, n = 02b, n = 1, o-hydroxy2c, n = 1, m-hydroxy2d, n = 1, p-hydroxy2e, n = 2, 3,4-dihydroxy
Scheme 6.1. Synthetic routes for ten target polyphenols (1a-1e, 2a-2e).
6.3.2. Characterization
The series of compound (8a – e, 9a – e, 10a – e) synthesized was generally soluble in a
number of solvents such as tetrahydrofuran (THF), chloroform, DMSO and DMF.
However, the deprotected compounds (1a – e, 2a – e) were only mainly soluble in polar
solvents like THF, ethanol, methanol, DMSO, DMF and acetone. A number of
characterization methods such as 1H, 13C, EI-MS and elemental analysis were carried out
to confirm the identity of the compounds. However, the final polyphenols were sensitive
in the air in a prolonged period as the color was gradually changed to dark, probably due
to the oxidation towards quinones. Therefore, all polyphenols were kept in cold and dark
condition and the thermal properties could not be invesitigated.
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6.3.3. Absorption and emission properties
All the absorption studies were done in THF solution (Figure 6.2 and Table 6.1).
From the absorption studies, 8b, 9b and 10b has the same absorption maximum at about
405 nm. It shows that the addition of more phenyl rings did not affect the absorption much.
However, when the compounds are deprotected to form 1b and 2b, there is a significant
blue shift to about 291 nm. The loss of an electron donating group (-OCH3) has caused this
large decrease. A similar trend was observed for the other compounds. Also the
intermolecular hydrogen bonding and solvatochromic effect played an important role.
Before deprotection, the compounds are non-polar, the dipole-dipole and dipole- induced
dipole interactions between solute and solvent are neglectable; After protection the exposed
hydroxyl groups make compounds highly polar, thus the strong dipole interactions provide
large stabilization energy for the ground state as well as the excited state of the
compound.24 From the results we infer that the stabilization energy in the ground state
should be much larger than that in the excited state, thus HOMO-LUMO gap could be
larger, inducing large blue shift.
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Synthesis and Characterization of Cross-conjugated Polyphenols
250 300 350 400 450 5000.0
0.2
0.4
0.6
0.8
1.0
Figure 6.2. Normalized absorption spectra of 8b – e, 9b – e, 10b – e, 1b – e, 2b – e in THF.
Abs
orba
nce
(a.u
.)
Wavelength (nm)
8b 9b 10b 1b 2b
250 300 350 400 450 5000.0
0.2
0.4
0.6
0.8
1.0
Abs
orba
nce
(a.u
.)
Wavelength (nm)
8c 9c 10c 1c 2c
250 300 350 400 450 5000.0
0.2
0.4
0.6
0.8
1.0
Abs
orba
nce
(a.u
.)
Wavelength (nm)
8d 9d 10d 1d 2d
250 300 350 400 450 5000.0
0.2
0.4
0.6
0.8
1.0
Abs
orba
nce
(a.u
.)
Wavelength (nm)
8e 9e 10e 1e 2e
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Figure 6.3. Normalized emmision spectra of 8b – e, 9b – e, 10b – e, 1b – e, 2b – e in THF
Table 6.1. Absorption and emission maxima of 8b – e, 9b – e, 10b – e, 1b – e, 2b – e in THF (nm)
UV-Vis Emission UV-Vis Emission
8b 407 475 8d 411 505
9b 402 451 9d 402 447
10b 403 442 10d 406 443
1b 290 383 1d 292 382
2b 292 391 2d 289 390
8c 401 472 8e 405 501
9c 399 459 9e 410 478
10c 402 458 10e 407 482
1c 293 386 1e 294 419
2c 291 391 2e 291 392
350 400 450 500 550 6000.0
0.2
0.4
0.6
0.8
1.0
Inte
nsity
(a.u
.)
Wavelength (nm)
8d 9d 10d 1d 2d
350 400 450 500 550 6000.0
0.2
0.4
0.6
0.8
1.0
Inte
nsity
(a.u
.)
Wavelength (nm)
8b 9b 10b 1b 2b
300 350 400 450 500 550 6000.0
0.2
0.4
0.6
0.8
1.0
Inte
nsity
(a.u
.)
Wavelength (nm)
8c 9c 10c 1c 2c
350 400 450 500 550 6000.0
0.2
0.4
0.6
0.8
1.0
Inte
nsity
(a.u
.)
Wavelength (nm)
8e 9e 10e 1e 2e
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Synthesis and Characterization of Cross-conjugated Polyphenols
Fluorescence spectra showed emission maxima between 450 to 550 nm, which
indicates a significant red shift comparing with normal linear PPPs (Figure 6.3).25-27
These results are consistent with the fact that cross-conjugated structure extends the
delocolization of π electrons. After deprotection, significant blue shifts of the emission
maxima were observed for all compounds, which is consistent with the similar trend of
absorption spectra, and can be explained by the lowering of the ground state energy
stabilized by dipole interaction. Conformational change may be another reason for that.
6.3.4. Antioxidant properties
The Trolox Equivalent Antioxidant Capacity (TEAC) value is an arbitrary value
given to a compound that has the same antioxidant properties as 1 mM of Trolox. The
reaction between the ABTS+ radical and Trolox was rather fast as the decrease in
absorbance had stopped at around 5 minutes. However, when our synthesized compound
was added to the working ABTS+ solution, the decrease in absorbance continued until
approximately 10 minutes from the start of the reaction. Henceforth, a time frame of 10
minutes was given for each run to ensure that the results obtained were accurate. The
experiment to determine the decrease in absorbance of the synthesized compound and
ABTS+ solution was carried out twice and the average decrease in absorbance was used
to calculate the TEAC value of the compounds. Summarized TEAC values for
compounds 1a – e and 2a – e were given in Table 6.2.
It is clear that 2e has the highest antioxidant properties (TEAC value = 2.14) among
the other followed by 1e having a TEAC value of 2.08. Both of these compounds have 6
phenolic-OH groups and the only difference between 1e and 2e is the length of
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conjugation across the phenyl rings. However the difference was not significant, which is
because the phenyl ring is highly twisted due to ortho-H repulsion, reducing the effective
conjugation. The high antioxidant property of these compounds was also attributed to
having a catechol unit where there will be H-bonding between the 2–OH groups. This
bonding weakened the O-H bond and thus lowered the bond dissociation energy that
results in the ease of proton donation to the radical.28,29 Upon formation of the phenolate
radical, it can be well stabilized by the extensive π-delocalised across the phenyl rings
and ethylene bridges. Moreover, these two compounds gave extra stability to the
phenolate owing to the larger number of phenyl rings allowing for more resonance
structures.
The TEAC values of the next set 2d and 1d were 1.85 and 1.82 respectively, which
were much lower than 2e and 1e, indicating the significance of the number of OH groups
in scavenging free radicals. Compounds 2b, 1b, 2d and 1d had a TEAC value much
larger than that of 2c and 1c. This could be due to the fact that the radical formed at the
para and ortho position can be delocalized entirely throughout the structure, i.e. more
resonant forms, but the radical formed from the deprotonated of –OH in the meta position
did not have this added stablizing effect. It was expected that compounds with ortho-OH
should have higher antioxidant activity than para and meta ones, as the ortho position of
phenol has higher electron density due to conjugation and inductive effects (chemical
shift of NMR for ortho-H, para-H and meta-H -H are 6.70, 6.81 and 7.14 respectively).
In the case of 1c, 2c, the higher electron density of para-H position is effectively
delocalized, thus lowering the dissociation energy of phenolic-OH. Therefore, steric
hindrance could be the main factor accounted for the lower antioxidant activity at ortho-
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Synthesis and Characterization of Cross-conjugated Polyphenols
OH as the ABTS+ radical is bulky in size. Ranking last will be 2a, 1a where there are
only two hydroxy groups on the molecule.
Therefore, we summarized that the number of hydroxyl groups on the molecules
will directly affect the antioxidant properties of the compound. The position of the
hydroxyl group was also found out to be an important factor in determining the properties
of the compounds. But the effect of adding two more phenyl groups along para-
phenylene was limited due to less effective conjugation.
Comparing our results with some other natural of synthetic antioxidants (Table 6.2)
gave promising results. All the 10 compounds tested had a TEAC value far larger than
ascorbic acid, α-tocopherol and even β-carotene. 1e and 2e had an antioxidant property
close to cyanidin. Nevertheless, as all the compounds are not soluble in water, they are
not suitable for practical applications. But having been inspired from these primary
results, multiple hyroxy groups and water-soluble groups (carboxyl group should be an
ideal choice) incorporpated conjugated phenols should be the right direction to the next
stage.
Table 6.2. TEAC values of the synthesized compounds and typical antioxidants
Compound TEAC Value Compound TEAC
Value Compound TEAC Value
1a 1.41 2c 1.63 Ascorbic acid 0.99
2a 1.45 1d 1.82 α-Tocopherol 0.97 1b 1.75 2d 1.85 β-carotene 1.9 2b 1.80 1e 2.08 Cyanidin 2.30 1c 1.57 2e 2.14 Gallic acid 3.11
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6.3.5. Bioactivity studies
The bioactivity studies were carried out with 2b-d for parallel comparison purpose.
The microscopic observations of the treated cells showed unhealthy cells at higher
concentration (Figure 6.4 and 6.5). The cells were small in size compared to the control.
The viability and metabolic activity data showed that all the three chemicals were equally
reactive, with IMR-90 being more susceptible (Figure 6.6). At 60 μg/mL the viability
was below 50% for all the chemicals. However the lower concentrations (5 μg, 10 μg)
did not shown much cell death except 2d in cancer cells. The mitochondrial function
diminished with increase in concentration (Figure 6.7). The mitochondrial activity can be
considered as an indicator of the viability as the viable cells only have active
mitochondrial functions. Therefore the low mitochondrial activity could be the result of
increased cell death. The LDH assay showed that there was a concentration dependant
increase in the concentration of LDH in the tissue culture medium suggesting that the
chemical was damaging the cell membrane. The 2d showed low levels of LDH in culture
medium (Figure 6.8). The cell cycle analysis proved that the cell death was not a result
of apoptosis, as there was no significant subG1 peak seen. Taken together the data from
LDH release and cell cycle, the cell death was a possible result of necrosis than apoptosis.
Additionally, 2c induced G1 arrest in U 251 cells where as 2b and 2d induced S/G2/M
arrest in cancer cells 2d had S/G2/M phase arrest in IMR-90 (Figure 6.9). In summary,
all the three chemicals are affecting the cells at the concentrations employed, 10 μg/mL
being the safer concentration for bio-application. The present study indicates that the
chemicals can be employed in bio-application strictly below a concentration of 10 μg/mL,
which is less toxic for all the three chemicals tested. The Confocal images of the cells
207
Synthesis and Characterization of Cross-conjugated Polyphenols
showed fluorescent specks, which were less distinguishable (Data not included). The fact
that the LDH is released in to the culture medium in a concentration dependant manner
proves that the chemical penetration in the cells results in lysis.
A
B
D C
Figure 6.4. (A) The differential interphase contrast (DIC) images of control cells IMR-90, (B) treated cells (IMR-90) which showed irregular cell morphology due to cell death (C), Control cells (U251) and (D) treated cells (U251)
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0%
20%
40%
60%
80%
100%
120%
140%
U251 IMR-90 U251 IMR-90 U251 IMR 90
2c 2d 2b
Viab
ility
(%)
0 5 ug/ml 10ug/ml 20 ug/ml 40 ug/ml 60 ug/ml
Figure 6.5. The effect of the chemicals on U251 and IMR-90 expressed as % of viable cells after the treatment. At higher concentrations of the polyphenol a drop in viability was observed. X axis represents the cell line and polyphenol employed. Y-axis represents % of viable cells. Legends indicate the concentration of polymer used.
0%
20%
40%
60%
80%
100%
120%
140%
U251 IMR 90 U251 IMR 90 U251 IMR 90
2c 2d 2b
% o
f met
abol
itica
lly a
ctiv
e ce
lls
0 5ug/ml 10ug/ml 20ug/ml 40ug/ml 60ug/ml
Figure 6.6. The effect of polyphenols in controlling the mitochondrial activity inside the cells. X axis represents the cell line and polyphenol employed. Y-axis represents % of metabolically active cells. Legends indicate the concentration of polyphenol used.
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Synthesis and Characterization of Cross-conjugated Polyphenols
-20%
0%
20%
40%
60%
80%
U 251 IMR-90 U251 IMR-90 U251 IMR-90
2c 2d 2b
% o
f rel
ease
of L
DH
0 5ug/ml 10ug/ml 20ug/ml 40ug/ml 60ug/ml
Figure 6.7. Cytotoxicity of the polyphenols on different cells in terms of their potency to result in cell lysis. X axis represents the cell line and polyphenol employed. Y-axis represents % of release of LDH as calculated form the cell culture supernantent. Legends indicate the concentration of polyphenol used. A concentration dependant increase in LDH leakage was observed which indicates cell membrane damage.
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211
A B
C D
E F
Synthesis and Characterization of Cross-conjugated Polyphenols
G H
Figure 6.8. (A) The cancer cells without any polyphenol added, (B) control cells, IMR-90 without any polyphenol (C) and (D) 2c treated cancer cells and fibroblasts respectively; (E) and (F) represents 2d treated cancer cells and normal cells respectively. (G) and (H) represents 2b treated cells cancer cells and normal cells respectively. The % of cells is indicated with each histogram. The cells are treated with a fixed concentration of the polyphenol, 60 μg/ml for the analysis.
0%
25%
50%
75%
100%
% of Cells
Con
trol 2c 2d 2b
Con
trol 2c 2d 2b
U251 IMR-90
G2/MSG1sub G1
Figure 6.9. Graphical representation of % of cells in different phases of cells cycle, subG1 represents percentage cells as indicated by M1 marker in histogram (Figure 6.8); similarly G1 represents M2; S represents M3; G2/M represents M4.
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6.4. Conclusion
A novel class of polyphenols has been synthesized and characterized. The design of
the molecules is targeted at achieving a high antioxidant property by varying the number
of hydroxyl groups on the numbers and the length of the linear conjugation. TEAC assay
has revealed that 2e has the highest antioxidant properties due to the dihydroxy structures
giving rise to H-bonding. This lowers the bond dissociation energy of the O-H bond that
results in the ease of donation to the radicals. Further works towards high antioxidant
activity water soluble conjugated polyphenols are being carried on in our lab. The
photophysical and bioactive properties were investigated as well.
6.5. References
1. Birt, D. F.; Hendrich, S.; Wang, W. Pharmacol. Thera. 2001, 90, 157.
2. Frémont, L. Life Science 2000, 66, 663.
3. Orellana, M.; Rodrigo, R. ; Thielemann, L.; Guajardo, V. Comp. Biochem. Physi.,
Part C. 2000, 126, 105.
4. Kaizer, J.; Pap, J.; Speier, G. Food Chem. 2005, 93, 425.
5. Ley, J. P.; Bertram, H. -J. Tetrahedron 2001, 57, 1277.
6. Manach, C.; Scalbert, A.; Morand, C.; Rémésy, C.; Jiménez, L. Am. J. Clin. Nutr.
2004, 79, 727.
7. Scalbert, A.; Williamson, G. J. Nutri. 2000, 130, 2073S.
8. Haslam, E. J. Nat. Prod. 1996, 59, 205.
9. Cai, Y. J.; Wei, Q. Y.; Fang, J. G.; Yang, L.; Liu, Z. L.; Wyahe, J. H.; Han, Z. Y.
Anticancer Res. 2004, 24, 999.
213
Synthesis and Characterization of Cross-conjugated Polyphenols
10. Katalinic, V.; Milos, M.; Modun, D.; Music, I.; Boban, M. Food Chem. 2004, 86,
593.
11. Rice-Evans, C. A.; Miller, N. J.; Paganga, G. Free Rad. Biol. Med. 1996, 20, 933.
12. Belfield, K. D.; Morales, A. R.; Hales, J. M.; Hagan, D. J.; Van Stryland, E. W.;
Chapela, V. M.; Percino, J. Chem. Mater. 2004, 16, 2267.
13. Ciulei, S. C. C.; Tykwinski, R. R. Org. Lett. 2000, 2, 3607.
14. Pinelo, M.; Manzocco, L.; Nuñez, M. J.; Nicoli, M. C. Food. Chem. 2004, 88, 201.
15. Lövborg, H.; Wojciechowski, J.; Larsson, R.; Józefa Wesierska-Gadek, J. Cancer
Res. 2002, 62, 4206.
16. Koziara, J. M.; Lockman, P. R.; Allen, D. D.; Mumper, R. J. J. Control. Release
2004, 99, 259.
17. Pandha, H.; Birchall, L.; Meyer, B.; Wilson, N.; Relph, K.; Anderson, C.;
Harrington, K. J. Urol. 2006, 176, 2255.
18. Braydich-Stolle, L.; Hussain, S.; Schlager, J. J.; Hofmann, M. C. Tox. Sci. 2005,
88, 412.
19. Bringmann, G.; Feineis, D.; Munchbach, M.; God, R.; Peters, K.; Peters, E. M.;
Mossner, R.; Lesch, K. P. Z Naturforsch [C], 2006, 61, 601.
20. Jiang, H.; Zhang, L.; Kuo J.; Kuo, K.; Gautam, S. C.; Groc, L.; Rodriguez, A. I.;
Koubi, D.; Hunter, T. J.; Corcoran, G. B.; Seidman, M. D.; Levine R. A. Mol.
Cancer Ther. 2005, 4, 554.
21. Mahata, P. K.; Barum, O.; Ila, H.; Junjappa, H. Syn. lett. 2000, 9, 1345.
22. Bellassoued, M.; Majidi, A. J. Org. Chem. 1993, 58, 2517.
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Li Hairong National University of Singapore
23. Kim, J. H.; Lee, S. K.; Kwon M. G.; Park, Y. S.; Choi S. K.; Kwon, B. M. Syn.
Comm. 2004, 34, 1223.
24. Suppan, P. J. Photochem. Photobiol., A: Chem. 1990, 50, 293.
25. Lucarini, M.; Mugnaini, V.; Pedulli, G. F. J. Org. Chem. 2002, 67, 928.
26. Zhang, H. Y.; Sun, Y. M.; Wang, X. L. J. Org. Chem. 2002, 67, 2709.
27. Liu, B.; Yu, W. L.; Lai, Y. H.; Huang, Wei. Macromolecules 2002, 35, 4975.
28. Balanda, P. B.; Ramey, M. B.; Reynolds, J.R. Macromolecules 1999, 32, 3970.
29. Lee, S. A.; Hotta, S.; Nakanish, F. J. Phys. Chem. A 2000, 104, 1827.
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D
CChhaapptteerr 77
Deessiiggnn,, SSyynntthheessiiss aanndd CChhaarraacctteerriizzaattiioonn ooff PPyyrreennee
DDeerriivvaattiivveess wwiitthh CCoonnjjuuggaatteedd AArrmmss
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Design, Synthesis and Characterization of Pyrene Derivatives with Conjugated Arms
7.1. Introduction
Since the very first publication of efficient organic light emitting diodes (OLEDs) by
Tang and co-workers in the late 1980s,1 the research into the development of organic
materials as electroluminescent devices has been extensive. The widespread research into
OLEDs is a result of the many advantages it possesses over the inorganic light emitting
devices. The organic compounds are much more economical to synthesize and its
processability into thin films are higher. The ease of its tunability to the desired emissive
colour by structural modifications makes it more convenient in synthesis than the
inorganic materials. Moreover, inorganic materials require the use of costly and
complicated semiconductor technologies compared to the easier techniques such as
coating methods.2
Amongst the different classes of materials, the polymers are generally of lower
purity which affects their device lifetimes and efficiency. However, they are also the
most promising candidates for fabrication of large full colour displays at economical
rates. Small molecules, on the other hand, provide high purity and easy vacuum
deposition condition that is favorable for their device efficiency. The main drawback is
the limitation on size of the displays that can be accomplished. Oligomers, with
molecular weight lying in the range of 1000-10,000g/mol, provides for the best of both
worlds, as they incorporate properties intermediate of polymers and small molecules.3-5
Various aromatic compounds are used as core compounds. Pyrene is one potential
candidate, pyrene is a tetracyclic aromatic hydrocarbon and belongs to the D2h point
group.6 It has intense UV-Vis absorption in 240-310nm and emission at 360-380 nm. Due
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to the four fused aromatic rings, it is highly planar with strong π-delocalization energy.7
One main drawback of its planar structure is its tendency to form π-aggregates that would
decrease its luminescence efficiency. However, it can be overcome by inclusion of
attachments which disrupt the overall planarity of the molecule and hence decrease
aggregation effects. It is known to be a good electron acceptor, thus is able to serve as a
good electron transporter in OLEDs.8
1
2
109
3
8
45
6
7
Figure 7.1. Structure and ‘C’ atom labeling of pyrene.
Bo-Cheng Wang et al.’s theoretical investigations on substitutions on pyrene
revealed that the HOMO level is raised as an electron donating substituent is included.
On the other hand, an electron acceptor attached lowers the LUMO of pyrene. They also
noted that the combination of effects from both electron donating and electron
withdrawing groups on pyrene results in an overall decrease in the energy gap between
HOMO and LUMO. This decrease is greater than an inclusion of two electron
withdrawing or electron donating groups on pyrene. It is also calculated that the greatest
red shifts in absorption and emission wavelengths can be expected when the substituents
are attached to positions 1,6 as compared to positions 2,7.7
Many different types of substituents have been attached to the pyrene core to
enhance their use as optoelectronic devices. There are three main roles that the
substituents play which are important to the overall structure of the designed target
218
Design, Synthesis and Characterization of Pyrene Derivatives with Conjugated Arms
molecules. Firstly, of the utmost importance is the increase in π-conjugation of the parent
core.9-15 This increase in conjugation will bring about the decrease in the HOMO-LUMO
energy gap which could be used to fine-tune the desired emissive colour. Most common
substituents used for this purpose are multiple bonds like acetylenes and double bonds.
They provide for linear extension of conjugation that helps shift absorption and emission
wavelengths. Phenyl rings are also adapted for the same role as it possesses a planar ring
of π-electrons that are delocalized. Secondly, substituents can also change the distribution
of the electron density on the parent core which helps fine-tune the HOMO-LUMO
energy level.12-20 The addition of electron-withdrawing groups would decrease the energy
levels of HOMO and LUMO but greater decrease is observed for LUMO. On the other
hand, an electron-donating group increases the electron density of both orbitals but
greater increases are observed on the HOMO. In both cases, they give rise to a decrease
in the overall energy gap between HOMO and LUMO. Last but not least, substituents
also help prevent the formation of aggregates.18-26 Phenyl rings not only extend π-
conjugation, but its bulky nature results in steric hindrance on the overall compound.
Meanwhile, the long alkyl chains enable the material to be soluble in most organic
solvents and hence more convenience is achieved during fabrication of the material as
thin films.
We are interested in synthesizing novel pyrene derivatives and investigating their
photo-physical properties. These target molecules were designed to possess attachments
which extend their conjugation and thus change the photo-physical properties. The photo-
physical properties by introducing different donor and acceptors onto pyrene core were
investigated.
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7.2. Synthesis and characterization
C8H17O Si(CH3)3 C8H17O
C8H17O I C8H17O
C8H17O I
O2N I
(i) (ii)
(iii)
O2N
C8H17O
C8H17O
OC8H17
OC8H17
II
BrI
Br Br
Br I6a
6b T1
(vi)
II
I
I5a
5b
(v)
(iv)
21
3
4
3
Br
Br
C8H17O
OC8H17C8H17O
OC8H17
NO2
O2N
6a/6b
7
2 4
(vii) (viii)
T2
C8H17O
OC8H17
C8H17O
OC8H17O2N
NO2
7
IO2N
8T3
(ix) (x) (xi)
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Design, Synthesis and Characterization of Pyrene Derivatives with Conjugated Arms
Scheme 7.1. Synthetic routes of target compounds (T1-T3). (i) TMSA (2 eq.), 10 % Pd(OAc)2, 20 % PPh3, 20 % CuI, THF/Et3N, Yield: 85 %. (ii) K2CO3 (6 eq.), MeOH, Yield: 96 %. (iii) Tributylvinyltin (1.2 eq.), 10 % Pd(OAc)2, 20 % PPh3, 20 %CuI, THF/Et3N. Yield: 82 % (3), 70 % (4). (iv) I2 (1 eq.), KIO3 (0.4 eq.), 4mL H2SO4, CH3COOH/10 % H2O, 40 oC, Yield: 36.1 %, (v) Br2 (2 eq.), CCl4, 60oC, Yield: 59.3 % (overall). (vi) 3 (6 eq.), 20 % Pd(OAc)2, 50 % PPh3, 40 % CuI, THF/Et3N. Yield: 45.3 %. (vii) 2 (3 eq.), 20 % Pd(OAc)2, 50 % PPh3, 20 % CuI, THF/Et3N, Yield: 39.5 %. (viii) 4 (3 eq.), 20 % Pd(OAc)2, 50 % PPh3, 40 % CuI, THF/Et3N, Yield: 46.3 %. (ix) TMSA (5 eq.), 20% Pd(OAc)2, 50% PPh3, 20% CuI, THF/Et3N, Yield: 48.1% (x) K2CO3 (6 eq.), MeOH, Yield: 45.7 %. (xi) 4-Nitro-1-iodobenzene (2.2 equiv), 20 % Pd(OAc)2, 50 % PPh3, 40 % CuI, THF/Et3N, Yield: 37.9%.
Compounds (2-(4-(2-ethylhexyloxyl-3,5-dimethylphenyl)ethynyl)trimethylsilane
(1),27 5-ethynyl-1,3-dimethyl-2(2-ethylhexyloxy)benzene (2),27 1,3-dimethyl-2-(2-
ethylhexyloxy)-5-vinylbenzene (3),28 1-nitro-4-vinylbenzene (4),29 Diiodopyrene (5a and
5b)30 and Dibromo-diiodopyrene (6a and 6b)30 were synthesized according to literatures.
1,6-dibromo-3,8-bis(2-(3,5-dimethyl-4-(octyloxy)phenyl)ethynyl)pyrene (7): 615
mg (1.01mmol) of mixture 6a and 6b was dissolved in a mixture of 30 mL freshly
distilled THF and 20 mL of TEA. The mixture was stirred and purged in N2. After two
hours, 45.1 mg (0.2 mmol, 20 % eq.) of palladium acetate, 76.6 mg (0.402 mmol, 40 %
eq.) of CuI and 132 mg (0.503 mmol, 50 % eq.) of triphenylphosphine were added. After
another hour of stirring, 1.3 g (5.03 mmol, 5 eq.) of compound 2 was added dropwisely
into the reaction flask over a period of 30 mins. The reaction was kept overnight and
performed under N2 atmosphere. The crude product was obtained by extraction with
DCM and washed with saturated NH4Cl, brine and water. The purified compound was
obtained using column chromatography using hexane/DCM as the eluen to afford a
yellow liquid. Yield: 45%. 1H NMR (CDCl3, δ ppm): 8.41-8.59 (d, 2H, pyrene-H), 8.43-
8.57 (d, 2H, pyrene-H), 8.35 (s, 1H, pyrene-H), 7.37 (s, 4H, Ar-H), 3.69-3.72 (d, 4H,
OCH2), 2.34 (s, 12H, Ar-CH3), 0.84-1.77 (m, 30H, Alk-H). 13C NMR (CDCl3, δ ppm):
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Li Hairong National University of Singapore
157.1, 156.6, 133.9, 133.2, 132.6, 131.1, 131.0, 130.0, 125.7, 123.1, 119.5, 118.0, 116.7
(Ar-C, Vinyl-C)), 96.8, 95.9 (ethynyl-C), 74.6 (OCH2), 40.7, 30.3, 29.6, 24.2, 23.6, 23.0,
14.0, 11.2 (Alk-C). Mass(EI): 872.5 (m/z). Elemental anaylsis: C: 71.56, H: 7.47; Found
C: 71.73, H: 7.35.
1,6-diethynyl-3,8-bis(2-(3,5-dimethyl-4-(octyloxy)phenyl)ethynyl)pyrene (8):
760 mg (0.871 mmol) of compound 7 was dissolved in a mixture of 30mL of freshly
distilled THF and 20 mL of TEA. The mixture was stirred and purged in N2. After 2
hours, 39 mg (0.174 mmol, 20 % eq.) of palladium acetate, 66.2 mg (0.348 mmol, 40 %
eq.) of CuI and 113.9 mg (0.435 mmol, 50 % eq.) of triphenylphosphine were added. The
reaction mixture was then heated to 70 oC. After 30 min, 1.20 mL of TMSA (0.23 mL, 10
eq.) was added dropwise over a period of 30 min into the reaction mixture. The reaction
mixture was kept stirring under N2 overnight at 70 oC. The crude product was obtained by
extraction with DCM and washed with saturated NH4Cl and brine. The crude product was
directly added into a two neck round bottom flask containing 45.6 mg of K2CO3 (0.331
mmol, 10 eq.) and 10 mL of MeOH. The reaction was left to stir under N2 and kept
overnight. The purified compound was obtained using column chromatography using
hexane/DCM as the eluent to afford yellow solid. Yield: 60%. 1H NMR (CDCl3, δ ppm):
8.63-8.74 (d, 2H, pyrene-H), 8.68-8.70 (d, 2H, pyrene-H), 8.36 (s, 2H, pyrene-H), 7.39 (s,
4H, phenyl H), 3.69-3.71 (d, 4H, OCH2), 3.64 (s, 2H, alkyne-H), 2.34 (s, 12H, Ar-CH3),
0.83-1.79 (m, 30H, Alk-H). 13C NMR (CDCl3, δ ppm): 156.9, 134.0, 132.2, 131.8, 131.4,
131.2, 130.5, 129.4, 126.9, 123.6, 119.7, 117.8 (Ar-C), 96.3, 93.1, 83.3, 81.7 (ethynyl-C),
74.8 (OCH2), 40.7, 31.5, 29.1, 23.7, 22.5, 16.2, 14.0, 11.2 (Alk-C). Mass (FAB): 762.3
(m/z). Elemental analysis: C: 88.15, H: 7.66; Found: C: 88.35, H: 7.49.
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Design, Synthesis and Characterization of Pyrene Derivatives with Conjugated Arms
1,3,6,8-Tetrakis-{2-[4-(2-ethyl-hexyloxy)-phenyl]-vinyl}-pyrene (T1): A similar
procedure was adopted for the synthesis of T1 as in the case of 8. Yield: 83 %. 1H NMR
(CDCl3, δ ppm): 8.52 (s, 2H), 8.04 (d, J = 8.4 Hz, 4H), 7.63 (d, J = 7.5 Hz, 8H), 7.35 (d, J
= 7.5 Hz, 8H), 6.98 (m, 8H), 3.92 (d, J = 5.5 Hz, 8H), 2.34 (s, 24H), 1.74 (m, 4H), 1.36-
1.26 (m, 32H), 0.92 (m, 24H). 13C NMR (CDCl3, δ ppm): 160.1, 132.5, 132.1, 131.0,
128.7, 128.5, 128.4, 127.8, 126.6, 124.4, 124.2, 124.1, 123.9, 123.5, 114.7 (Ar-C, vinyl-
C), 71.3 (OCH2), 40.1, 31.2, 29.8, 24.6, 23.7 (Alk-C), 16.1 (CH3), 14.8, 11.8 (Alk-C).
Mass (FAB): 1234.7 (m/z). Elemental analysis: C: 85.52, H: 9.30; Found: C: 85.97, H:
9.26.
1,6-Bis-[4-(2-ethyl-hexyloxy)-phenylethynyl]-3,8-bis-[2-(4-nitro-phenyl)-vinyl]-
pyrene (T2): A similar procedure was adopted for the synthesis of T2 as in the case of 8.
Yield: 38 %. 1H NMR (CDCl3, δ ppm): 8.59 (d, J = 8.4 Hz, 2H), 8.42 (d, J = 8.4 Hz, 2H),
8.38 (s, 2H), 8.17 (d, J = 8.7 Hz, 4H), 7.53 (d, J = 8.4 Hz, 4H), 7.38 (s, 4H), 6.98 (m, 8H),
6.95 – 6.73 (m, 4H), 3.72 (d, J = 5.5 Hz, 4H), 1.74 (m, 2H), 1.36-1.26 (m, 16H), 0.92 (m,
12H). 13C NMR (CDCl3, δ ppm): 160.5, 144.5, 135.6, 131.0, 129.6, 129.0, 128.8, 127.8,
127.5, 126.5, 125.7, 124.9, 124.6, 124.2, 123.5, 123.4, 121.9, 119.2, 114.7 (Ar-C, vinyl-
C), 93.4, 92.5 (ethynyl-C), 71.3 (OCH2), 40.1, 31.2, 29.8, 24.6, 23.7, 14.8, 11.8 (Alk-C).
Mass (EI): 1008.5 (m/z). Elemental analysis: C: 80.92, H: 6.79; Found: C: 81.44, H: 6.76.
1,6-bis(2-(3,5-dimethyl-4-(octyloxy)phenyl)ethynyl)-3,8-bis(2-(4-nitrophenyl)
ethynyl)pyrene (T3): A similar procedure was adopted for the synthesis of T3 as in the
case of 8. Yield: 70 %. 1H NMR (CDCl3, δ ppm): 8.61 (d, 2H, pyrene-H), 8.44 (d, 2H,
pyrene-H), 8.40 (d, 4H, Ar-H), 7.59 (d, 4H, Ar-H), 7.41 (s, 4H, Ar-H), 3.68-3.71 (d, 4H,
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OCH2), 2.34 (s, 12H, Ar-CH3), 0.83-1.79 (m, 30H). 13C NMR (CDCl3, δ ppm): 156.9,
145.7, 133.9, 132.3, 131.7, 131.4, 129.2, 128.5, 126.8, 125.5, 123.7, 119.0, 118.5, 118.1
(Ar-C), 103.0, 101.2, 96.3, 89.6 (ethynyl-C), 71.4 (OCH2), 40.8, 30.4, 29.7, 23.7, 23.1,
16.3, 14.1, 11.2 (Alk-C). Mass (FAB): 1004.5 (m/z). Elemental analysis: C: 81.25, H:
6.42; Found: C: 81.67; H: 6.38.
7.3. UV-Vis absorption and emission studies
Absorption and emission spectra for T1, T2, T3, pyrene and two precursors 7 and 8
were recorded in THF solution with a concentration of 1 mg/mL. The results are shown
in Figure 7.2 and summarized in Table 7.1.
Figure 7.2. UV-Vis absorption (left) and emission (right) of pyrene (T0), T1, T2, T3 and
precursors 7 and 8.
300 350 400 450 500 550 600 0.0
0.2
0.4
0.6
0.8
1.0 T0 T1 T2 T3 7 8
Wavelength (nm)
350 400 450 500 550 600 6500.0
0.2
0.4
0.6
0.8
1.0 T0 T1 T2 T3 7 8
Wavelength (nm)
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Design, Synthesis and Characterization of Pyrene Derivatives with Conjugated Arms
Table 7.1. Summary of absorption and emission peak maxima and calculated band gap
Compound Absorption (nm) Emission (nm) ΔE (eV)
Pyrene (T0) 321 and 337 374, 385, 394 and
414 3.62
T1 366, 427, 457 and 484 508 2.40
T2 255, 336, 433 and 457 475, 504 2.58
T3 236, 257, 344, 433 and 488 521, 541 2.30
7 217, 317, 416, 437 460 and 479 2.84
8 254, 314, 327, 428 and 452 474 and 500 2.62
O1 394 467 3.14
O2 405 469 3.06
O3 388 447 3.19
O4 392 442 3.16
O5 399 454 3.10
O6 415 510 2.98
O7 426 509 2.91
O8 412 501 3.00
O9 414 488 2.99
O10 425 498 2.91
SOC6H13
C6H13C6H13N
C6H13 NO2
RR
X
X
R=
X = CH (O1-O5)
X = N (O6-O10)
O1, O6 O2, O7
O3, O8 O4, O9 O5, O10
Figure 7.3. Molecular structures of cruciform O1-O10 reported in Chapter 5.
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From Figure 7.2 and Table 7.1, it is evident that the absorption and emission
wavelengths for the three target molecules were red shifted compared to pyrene (T0)
itself. Red shifts of 89-151nm in absorption maxima and 109-147nm in emission maxima
were observed. The HOMO-LUMO energy gap decreased from 3.62 to 2.30-2.58 eV. To
compare within the target molecules, T3 had the longest wavelength of absorption
maxima and hence the lowest HOMO-LUMO energy gap.
A close look at the fluorescence spectra of T2 and T3 showed that both compounds
exhibited similar pattern, with one strong peak followed by a smaller peak. This could be
due to their similar structures. The only difference in the structure lies in the spacers used.
T2 has both alkyne and vinyl spacers while T3 has only alkyne spacers. As such, it is
possible to draw a conclusion that alkyne provided better conjugation and showed a
larger shift in emission maximum. Unlike T2 and T3, the emission spectrum of T1 is
vastly different. The most probable explanation could be due to the structural differences.
T1 is homo-substituted while T2 and T3 are substituted with both electron donating and
electron withdrawing moieties.
It is also evident that there is an increase of absorption and emission maxima, as well as
a decrease in the HOMO-LUMO energy gap for T2 as compared with precursor 7. The
absorption and emission maxima of 7 were smaller than T2. This could be due to the
presence of two bromide groups on the pyrene core which resulted in the decrease in electron
density due to electron-withdrawing effect. With the addition of vinylbenzene moiety, the
absorption maximum was increased by 20 nm while the emission increased by 24 nm. This
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Design, Synthesis and Characterization of Pyrene Derivatives with Conjugated Arms
increase was due to the extension of conjugation by the phenyl ring and the vinyl groups.
The spectra of the two compounds were of similar pattern due to the similarity in structures.
If we compare the absorption and emission among T3, 7 and 8, it is evident that
there is a gradual increase in both the absorption and emission maxima from 7, 8 to T3.
This was due to the increase in conjugation length from one molecule to another through
structural modifications. It can be observed in the absorption spectra that the significant
increase is in T3. This is due to the inclusion of two phenyl rings to the structure. The
increase from 7 to 8 is not as great as the modification involves only an exchange of two
bromide groups for two acetylene bonds.
It was our interest to compare the photophysical properties of these pyrene
derivatives with that of cross-conjugated oligomers in Chapter 5 (molecular structures
and absorption and emission maxima were recapped in Figure 7.3 and Table 7.1). It was
obvious that pyrene derivatives had significantly lower band gap. The bandgap of O2
with alkoxy groups was 3.06 eV as compared with the value of 2.40 eV for T1 and 2.84
eV for compound 7. It was confirmed that the band gaps decrease in the order: benzene >
naphthalene > anthracene > pyrene.31 This is mainly due to confinement effect. The
variations of the frontier orbital energies and band gaps were correlated with the increase
of both Coulomb and resonance integral; furthermore the confinement effect is associated
with the conjugated system of the aromatic molecules. In other words, confining organic
molecules in larger cavities was sufficient to alter their electronic properties as a
consequence of the increase in the molecular orbital energies and decrease of the band
gaps. O6 and O7 with donor-acceptor structures had bandgaps at 2.98 and 2.91 eV,
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respectively, which is higher than T2 (2.58 eV) and T3 (2.30 eV) with similar structure.
The smaller bond twisted angle at pyerene compared to the OPP unit of O1-O10
(confirmed by single crystal studies of O5 in Chapter 5) permits the pyrene derivatives
to deviate more from the rigid rod conformation and effectively adopt a coil like
conformation. This particular conformation causes more π-stacking between the pyrene
repeating units resulting in the red shift of the absorption maxima.32-33 Therefore the
donor-accepter effect of pyrene derivatives might not be as significant as it seems. A
good example is the comparison of T2 and compound 8, both have similar dimensions in
conjugation but the former having extra acceptor group, yet the band gap difference was
only 0.04 eV. In fact, both donor and acceptor segment should affect the overall
conjugation but this is not additive. Close look at the structure of T1-T3 gives possible
explanation that all segments linked to pyrene core is in meta-position relative to each
other and delocalization of electrons is affected, which reduces the overall conjugation as
compared to O1-O10 having ortho-connection. Therefore, there pyrene derivatives are
not classified as cross-conjugated molecules by definition. More important positions are
1, 3, 6 and 8 as they are positions where the electron population of the HOMO level is
localized (Figure 7.1).34 Hence, variations on these sites are crucial in manipulating the
HOMO-LUMO energy gap.6 All in all, the photophysical properties of pyrene derivatives
were restricted by no conjugation between arms while the cruciforms O1-O10 were
restricted by highly twisted OPP segment affecting the effective conjugation.
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Design, Synthesis and Characterization of Pyrene Derivatives with Conjugated Arms
7.4. Conclusion
Pyrene derivatives T1, T2 and T3 were designed, synthesized and characterized.
Absorption and emissions studies were investigated in detail. It was noted that all are
blue-emitting compounds with absorption and emission wavelengths in the range of 426-
488 nm, and 508-541 nm, respectively. The HOMO-LUMO energy gap of the derivatives
is in the range of 2.30-2.58 eV. A direct comparison with cross-conjugated cruciforms in
Chapter 5 was also given.
7.5. References
1. Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913.
2. Mitschke, U.; Bäüerle, P. J. Mater. Chem. 2000, 10, 1471.
3. Bolink, H. J.; Barea, E.; Costa, R. D.; Coronado, E.; Sudhakar, S.; Zhen, C.;
Sellinger, A. Org. Elec. 2008, 9, 155.
4. Mitschke, U.; Bäüerle, P. J. Mater. Chem. 2000, 10, 1471.
5. Wu, K. C.; Ku, P. J.; Lin, C. S.; Shih, H. T.; Wu, F. I.; Huang, M. J.; Lin, J. J.;
Chen, I. C.; Cheng, C. H. Adv. Funct. Mater., 2008, 18, 67.
6. Park, Y. H.; Lee, Y. H.; Park, G. Y.; Park, N. G.; Young, S. K.; Mol. Cryst. Liq.
2006, 444, 177.
7. Wang, B. C.; Chang, J. C.; Tso, H. C.; Hsu, H. F.; Cheng, C. Y., Theochem. 2003,
629, 11.
8. Xing, Y.; Xu, X.; Zhang, P.; Tian, W.; Yu, G.; Lu, P.; Liu, Y.; Zhu, D. Chem.
Phys. Lett. 2005, 408, 169.
229
Li Hairong National University of Singapore
9. Moorthy, J. N.; Natarajan, P.; Venkatakrishnan, P.; Huang, D. F.; Chow, T. J.
Org. Lett. 2007, 9, 5215.
10. Ohshita, J.; Yoshimoto, K.; Tada, Y.; Harima, Y.; Kunai, A.; Kunugi, Y.;
Yamashita, K., J. Organomet. Chem., 2003, 678, 33.
11. Beinhoff, M.; Weigel, W.; Jurczok, M. Rettig, W. Modrakowski, C.; Brudgam, I.;
Hartl, H.; Schluter, A. D. Eur. J. Org. Chem. 2001, 35, 3819.
12. Soujanya, T.; Philippon, A.; Leroy, S.; Vallier, M.; Fages, F., J. Phys. Chem. A.
2000, 104, 9408.
13. Shimizu, H.; Fujimoto, K.; Furusyo, M.; Maeda, H.; Nanai, Y.; Mizumo, K.;
Inouye, M. J. Org. Chem. 2007, 72, 1530.
14. Mikroyannidis, J. A., Synth. Met. 2005, 155, 125.
15. Xing, Y.; Xu, X.; Zhang, P.; Tian, W.; Yu, G.; Lu, P.; Liu, Y.; Zhu, D. Chem.
Phys. Lett. 2005, 408, 169.
16. Leroy-Lhez, Parker, A.; Lapouyade, P.; Belin, C.; Ducasse, L.; Oberle, J.; Fages,
F., Photochem. Photobiol. Sci. 2004, 3, 949.
17. Maeda, H.; Maeda, T.; Mizuno, K.; Fujimoto, K.; Shimizu, H.; Inouye, M. Chem.
Eur. J. 2006, 12, 824.
18. Shymala, T.; Sankararaman, S.; Mishra, A. K, Chem. Phys. 2006, 330, 469.
19. Connor, D. M.; Collard, D. M.; Liotta, C. L.; Schiraldi, D. A. Dyes and Pigments,
1999, 43, 203.
20. Venkataramana, G.; Sankararaman, S., Org. Lett., 2006, 8, 2739.
21. Nagatoishi, S.; Nojima, T.; Juskowiak, B.; Takenaka, S. Angew. Chem., Int. Ed.
2005, 44, 5067.
230
Design, Synthesis and Characterization of Pyrene Derivatives with Conjugated Arms
22. Zhang, H.; Wang, Y.; Shao, K.; Liu,Y.; Chen, S.; Qiu,W.; Sun, X.; Qi, T.; Ma,
Y.; Yu, G.; Su, Z.; Zhu, D. Chem. Commun. 2006, 755.
23. Ramon, M. M.; Felix, S. Coordination Chem. Rev. 2006, 250, 3081.
24. Sagara, Y.; Mutai, T.; Yoshikawa, I.; Araki, K. J. Am. Chem. Soc. 2007, 129,
1520.
25. Kamikawa, Y.; Kato, T. Langmuir, 2007, 23, 274.
26. Tang, C.; Liu, F.; Xia, Y. J.; Lin, J.; Xie, L. H.; Zhong, G. Y.; Fan, Q. L.; Huang,
W. Organic Electronics, 2006, 7, 155.
27. Chang, J. Y.; Y, J. R.; Shin, Y. S.; Han, M. J.; Hong, S. -K. Chem. Mater. 2000,
12, 1076.
28. Meier, H.; Holst, H. C. Adv. Synth. & Catal. 2003, 345, 1005.
29. Chretien, J. -M.; Mallinger, A.; Zammattio, F.; Le Grognec, E.; Paris, M.;
Montavon, G.; Quintard, J. –P. Tetrahedron Lett. 2007, 48, 1781.
30. Xiao, J.; Xu, J.; Cui, S.; Liu, H.; Wang, S.; Li, Y. Org. Lett. 2008, 10, 645.
31. Zhang, L. Z.; Cheng, P. Phys. Chem. Comm. 2003, 6, 62.
32. Wilson, J. N.; Bunz, U. H. F. J. Am. Chem. Soc. 2005, 127, 4124.
33. Fratiloiu, S. Senthilkumar, K.; Grozema, F. C.; Christian-Pandya, H.;
Niazimbetova, Z. I. ; Bhandari, Y. J. ; Galvin, M. E. ; Siebbeles, L. D. A. Chem.
Mater. 2006, 18, 2118.
34. Phelan, N. F.; Orchin, M. J. Chem. Educ. 1968, 45, 633.
231
LLIISSTT OOFF PPUUBBLLIICCAATTIIOONNSS 1. Li, Hairong; Ng, D. C.; Asharani P. V. N.; Valiyaveettil, S. Synthesis and
characterization of cross conjugated polyphenols. Submitted
2. Li, Hairong; Valiyaveettil, S. Synthesis and characterization of cross conjugated cruciforms with varied functional groups. Tetrahedron Lett. Submitted.
3. Nurmawati, M. H.; Ajikumar, P. K.; Heng, L.; Li, Hairong; Valiyaveettil, S. Cross-conjugated poly(p-phenylene)-aided supramolecular self-organization of fullerene nanocrystallites. Chem. Commun. 2008, 4945
4. Li, Hairong; Manoj, P.; Nurmawati, M. H.; Xu, Q. H.; Valiyaveettil, S. Synthesis and structure-property investigation of novel poly(p-phenylenes) with conjugated side chains. Macromolecules 2008, 41, 8473. Poster presentation at International Conference on Materials for Advanced Technologies (ICMAT), Singapore, 2007.
5. Sindhu, S.; Jegadesan, S.; Li, Hairong; Ajikumar, P. K.; Vetrichelvan, M; Valiyaveettil, S. Synthesis and patterning of luminescent CaCO3-Poly(p-phenylene) hybrid materials and thin films. Adv. Func. Mater. 2007, 17, 1698.
6. Li, Hairong; Nurmawati M. H.; Valiyaveettil, S. Cross conjugated poly(p-phenylenes) with strong π-π stacking force: synthesis and characterization. Polym. Mater. Sci. Eng. 2007, 97, 617. Poster presentation at 234th ACS National Meeting, Boston, USA.
7. Li, Hairong; Valiyaveettil, S. Water-soluble multifunctional cross-conjugated poly(p-phenylenes) as stimuli-responsive materials: design, synthesis, and characterization. Macromolecules 2007, 40, 6057. (Featured on the journal’s Most-Accessed Articles, 2007).
8. Li, Hairong; Valiyaveettil, S. Interesting sensory molecules based on cross conjugated water soluble poly(p-phenylenes). Polym. Mater. Sci. Eng. 2007, 96, 747. Oral presentation at 233rd ACS National Meeting, Chicago, USA.
9. Vetrichelvan, M.; Li, Hairong; Ravindranath, R.; Valiyaveettil, S. Synthesis and comparison of the structure-property relationships of symmetric and asymmetric water-soluble poly(para-phenylenes). J. Polym. Sci, Part A: Polym. Chem. 2006, 44, 3763.
10. Sindhu, S.; Jegadesan, S.; Li Hairong.; Valiyaveettil, S. Calcium rich biocomposites with tuned optical properties: a polymer driven approach. Polym. Prep. 2006, 47, 326.
11. Li, Hairong; Vetrichelvan, M.; Valiyaveettil, S. Synthesis of water soluble conjugated poly(p-phenylene)s for the biosensor applications. Polym. Prep. 2005, 46, 104. Poster presentation at 1st Nano-Engineering and Nano-Science Congress 2004, Singapore.
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