synthesis and characterisation - core · cordiality and friendship. i thank akhila, balaji, colin,...

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


3.3.1. 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



4.3 Synthesis 133


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



5.2.1 Synthesis 162



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




6.3.1 Synthesis 190


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


7.2 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


117


118


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

LLiinneeaarr aanndd CCrroossss--ccoonnjjuuggaatteedd PPoollyy((pp--pphheennyylleenneess)) aanndd OOlliiggoo((pp--

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

http://en.wikipedia.org/wiki/Delocalization

http://en.wikipedia.org/wiki/Electron


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


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


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


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.

6


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


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-

8


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


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


hν phosphorescence

T1

210

S2

S1

S0

abso

rptio

n

hν hνflu

ores

cenc

e


hν phosphorescence

T1

210

Jablonski diagram

Figure 1.4. Energetic processes on absorption of a photon in the Jablonski diagram.

10


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


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 −=

12


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.

13


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

14


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


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

16


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

17


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

18


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

19


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

20


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

21


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

22


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


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

24


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

25


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

26


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

27


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


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


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


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

31


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


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


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

34


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


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

36


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


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

38


CCM22

NR2

R2N

CF3

CF3

F3C

F3C

150-153 OPV, OPE

CCM23

NBu2

Bu2N

154

CCM24

NN 154

CCM25

NBu2

Bu2N

NN 154

39


CCM26

N

N

NN

OONa

ONaO

O

O

NaO

NaO

155

CCM27

NN

HS

SH

156

CCM28

OPV, OPP 157

40


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

41


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

42


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


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.

44


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


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

46


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

47


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

48


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

49


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

50


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

51


“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

52


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

53


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

54


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.

55


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.

56


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.

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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.

69


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


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.

71


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


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.

73


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.

74


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.

75


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


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


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


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


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


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.

81


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).

82


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).

83


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


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

85


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


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.

87


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


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+

89


+>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


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


O

OO

O

SO3Na

OO

O

SO3Na

O

OO

O

SO3Na

O

OO


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


O

OO

O

SO3Na

OO

O

SO3Na

O

OO

O

SO3Na

O

OO


O

C

B

A


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


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


O

OO

O

SO3Na

OO

O

SO3Na

O

OO

O

SO3Na

O

OO


O

O O

O

NaO3S

O O

O

NaO3S

O

O O

O

NaO3S

O

O O


O

OO

O

SO3Na

OO

O

SO3Na

O

OO

O

SO3Na

O

OO


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


O

OO

O

SO3Na

OO

O

SO3Na

O

OO

O

SO3Na

O

OO


O

O O

O

NaO3S

O O

O

NaO3S

O

O O

O

NaO3S

O

O O


O

OO

O

SO3Na

OO

O

SO3Na

O

OO

O

SO3Na

O

OO


O

O O

O

NaO3S

O O

O

NaO3S

O

O O

O

NaO3S

O

O O


O

OO

O

SO3Na

OO

O

SO3Na

O

OO

O

SO3Na

O

OO


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


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

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


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


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


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


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

91


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


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

93


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


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.

95


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


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.

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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.

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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.

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99


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.

101


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


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

103


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


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-

105


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,

106


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-

107


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,

108


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.


3.3.1. 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

110


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

111


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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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

113


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

114


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.

115


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.

116


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.

117


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.

118


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.

119


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.

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.

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

121


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

122


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

123


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

124


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

125


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.

126


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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.

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24. Baughman, R. H.; Shacklette, L. W. J. Chem. Phys. 1989, 90, 7492.

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129

Li Hairong National Uiversity of Singapore

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

130

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.

131


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


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,

133


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.

134


(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),

135


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.

136


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.

137




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.

138


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


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


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

141


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


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).

143


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


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


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


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

147


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

148


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

149


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.

150


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

151


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

152


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.

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159


CChhaapptteerr 55

SSyynntthheessiiss aanndd CChhaarraacctteerriizzaattiioonn ooff CCrroossss--ccoonnjjuuggaatteedd CCrruucciiffoorrmmss wwiitthh VVaarriieedd FFuunnccttiioonnaall GGrroouuppss

160

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

161


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


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

163


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).

164


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,

165


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,


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,

166


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,

167


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,


168


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.

169


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.


5.3.1. 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

170


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

171


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

172


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

173


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

174


-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)

175


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.

176


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.

177


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

178


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

179


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)

________________________________________________________________________________

180


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)

181


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

182


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)

______________________________________________________________________________

183


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

________________________________________________________________________________

184


CChhaapptteerr 66

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185

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

186


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


Figure 6.1. Synthesized cross-conjugated polyphenols

187


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

188


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

189


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.


6.3.1. 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 ×

190


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

191


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 =

192


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,

193


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.


benzene (10c). A similar procedure was adopted for the synthesis of 10c as in the case of


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.




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,

194


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,

195


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]-


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,

196


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,

197


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.


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


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.


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


198


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


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

199


HO OH

(OH)n

(OH

)n


BBr3, CH2Cl2rt, 2 hrs

BBr3, CH2Cl2rt, 2 hrs

HO OH

(OH)n

(OH)n


Scheme 6.1. Synthetic routes for ten target polyphenols (1a-1e, 2a-2e).


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.

200


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.

201


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

202


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

203


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

204


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-

205


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

206


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


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)

208


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.

209


-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.

210


211

A B

C D

E F


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.

212


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


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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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.

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215


D

CChhaapptteerr 77

Deessiiggnn,, SSyynntthheessiiss aanndd CChhaarraacctteerriizzaattiioonn ooff PPyyrreennee

DDeerriivvaattiivveess wwiitthh CCoonnjjuuggaatteedd AArrmmss

216

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

217


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


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.

219


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)

220


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):

221


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.

222


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,

223


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)

224


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.

225


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

226


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,

227


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.

228


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.

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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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