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rsc.li/polymers

Polymer Chemistry

www.rsc.org/polymers

ISSN 1759-9954

PAPERMunju Goh et al.Enhancement of the crosslink density, glass transition temperature, and strength of epoxy resin by using functionalized graphene oxide co-curing agents

Volume 7 Number 1 7 January 2016 Pages 1–246

Polymer Chemistry

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Wang, Y. Lin, C. Tang, J. Dou, H. Tan, Q. Zheng, C. Ma and Z. Cui, Polym. Chem., 2017, DOI:

10.1039/C6PY02161A.

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a. Printable Electronics Research Center, Suzhou Institute of Nano-Tech and Nano-

Bionics, Chinese Academy of Sciences, Collaborative Innovation Center of Suzhou

Nano Science and Technology, 398 Ruoshui Road, SEID SIP, Suzhou, Jiangsu

215123, PR China. Email: [email protected] b. College of Chemistry, Beijing Normal University, No. 19, XinJieKouWai St., HaiDian

District, Beijing, 100875, PR China. c. Department of Chemistry, Xi’an Jiaotong Liverpool University, 111 Ren Ai Road,

SEID SIP, Suzhou, 215123, P. R. China. d. State Key Laboratory of Structure Chemistry, Fujian Institute of Research on the

Structure of Matter, Chinese Academy of Sciences, 155 Yangqiao West Road,

Fuzhou, Fujian 350002, P. R. China e. University of Chinese Academy of Sciences, Beijing, 100049, P. R. China.

† Footnotes rela)ng to the )tle and/or authors should appear here.

Electronic Supplementary Information (ESI) available: [details of any

supplementary information available should be included here]. See

DOI: 10.1039/x0xx00000x

Received 00th January 20xx,

Accepted 00th January 20xx

DOI: 10.1039/x0xx00000x

www.rsc.org/

Peripherally Diketopyrrolopyrrole-Functionalized Dendritic

Oligothiophenes – Synthesis, Molecular Structure, Properties and

Applications

Wei Gao,a,e

Qun Luo,a Junkai Wang,

b Yi Lin,

c Changquan Tang,

d Junyan Dou,

a Hongwei Tan,*

b

Qingdong Zheng,d

Chang-Qi Ma,*a

Zheng Cui*a

Three-dimensional π-conjugated dendrimers are a class of structure defined macromolecules for use in organic

electronics. Herein, a new family of dendritic oligothiophenes (DOT-p-DPPs) that functionalized with diketopyrrolopyrrole

group at the periphery were synthesized by a precise stepwise approach. The chemical structure and the monodispersed

nature of these DOT-p-DPPs were confirmed by NMR, MALDI-TOF MS, HR MS, and GPC measurements. UV-vis absorption

and fluorescence spectra, and cyclic voltammetry of these compounds were also measured. Small band gaps (~ 1.8 eV)

and almost identical HOMO/LUMO energy levels (-5.2/-3.5 eV) were measured for these DOT-p-DPPs independent to the

molecular size. However, the molecular molar extinction coefficient (ε) of DOT-p-DPP was found to be linearly correlated

with the number of terminal DPP units, and high ε of 3.6×105 cm

-1·L·mol

-1 was measured for the bigger molecules. These

results in combination with the theoratical calculation results confirm that the frontier molecular orbitals are mostly

localized over the periphery DPP units. Applications of DOT-p-DPPs in organic solar cells as the electron donor are

presented. However, unfavorable nanophase separation and lower DOT-p-DPP content in the blended films led to poor

device performance. Two photon absorption cross section of these DPP decorated dendrimers was measured, and high

cross section values of over 2000 GM were measured for these dendritic molecules, among which, the G1 dendrimer 6T-p-

DPP with a high TPA cross section value close to 7000 GM was achieved.

1. Introduction

Conjugated polymers and small molecules are among the

most widely investigated organic semiconducting materials for

use in organic electronics.1 Among which, conjugated polymers

are considered having advantages of superior film-forming

ability, good processability, and high device performance.

However, polymeric materials possess its inherent drawbacks,

including wide molecular weight distributions, poor

regioregularity control, and undesired end group effects,

which usually affect the optical and electronic properties of

the materials a lot, and consequently lead to a poor batch-to-

batch reproducibility in device application.2 Conjugated small

molecules, on the other hand, possess well-defined and

monodispersed molecular structure, which are considered

having advantages of good reproducibility in device

application.3 In addition, for their controllable and defined

molecular structure, conjugated small molecules are also often

used as model compounds for structure–property-

performance relationship studies.4 However, conjugated small

molecules have also drawbacks of strong crystallization

tendency in solid films, which brings difficulties in thin film

deposition and poor thermal stability in device application.2a, 2c

Conjugated dendrimers that are composed of conjugation

moieties over the molecular skeleton are a new type of

polymers with three-dimensional hyperbranched molecular

structures.5 Unlike conjugated polymers, conjugated

dendrimers have defined and controllable molecular structure

owing to their precise step-by-step synthesis approach.6 In

addition, high-generation conjugated dendrimers could have

molecular weight as high as 10,000 Da.7 Therefore, conjugated

dendrimers are a kind of structure defined macromolecules,

which is expected to have both advantages of good

processibility of polymers and good reproducibility of small

molecules.3a, 8

Till now, conjugated dendrimers based on

benzene,9 thiophene

10 and carbazole unit

11 have been

synthesized and investigated, among which thiophene based

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ARTICLE Journal Name

2 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx



ones, named also dendritic oligothiophenes (DOTs) are highly

interesting in organic electronics for their unique optical and

electronic properties originating from the extended π-

conjugation chain.12

Not only the all-thiophene dendrons and

dendrimers have been synthesized and characterized,6b, 10c

dendritic oligothiophenes functionalized at the core,13

the

branching π-bridge,14

and at the periphery7, 15

were also

developed. Various functional groups, including porphyrin,13a

dibenzo-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (dibenzo-

BODIPY),16

pyridine or methylpyridinium,13b

perylenediimide

derivatives (PDI),17

benzothiadiazole (BT),13c

bisthienoimidazoles,18

pyrazino[2,3-g]quinoxaline,19

squaraine

(SQ),20

fluorenyl hexa-peri-hexabenzocoronene (FHBC),21

metal

complex,22

anchoring groups,23

or electron donating and

withdrawing groups24

have been introduced for such

molecular modification. Application of these functionalized

dendritic macromolecules in nonlinear optical devices,25

organic electroluminescence devices,15a

organic field effect

transistors,26

organic solar cells,12b

metal nanoparticle

synthesis,23

and in biosensoring15b

have been reported,

demonstrating versatile properties and application aspects of

these thiophene dendrons and dendrimers.

Especially, the all-thiophene dendrons and dendrimers

exhibit quite promising photovoltaic performance in organic

solar cells (OSCs). A power conversion efficiency (PCE) of 1.7%

was achieved for the 42T:PC61BM based device, which was

further improved to 3.5% when using PC71BM as the electron

acceptor.12b, 27

The organic solar cell using branched

quinquethiophene based dendrimer showed also highest PCE

of 3.1% when blended with PC71BM as the acceptor.15d

Knowing that the maximum absorption wavelength of these

all-thiophene dendrimers is around 420 nm, a moderate PCE

of more than 3% confirms that 3D conjugated dendrimers

could be an excellent molecular skeleton for constructing

conjugated molecules for OSCs application. On the other hand,

Zheng et. al first reported that π-conjugated dendritic

structure is able to enhance the two-photon absorption (TPA)

of organic chromophores, and TPA cross-section of 1412 GM

(1GM = 10−50

cm4

s photon−1

) was achieved for a six branched

dendrimers, which is much larger than the conventional single

branched chromophores.28

Since then, various dendrictic

molecules were developed for TPA application, and high TPA cross

section values over 104 GM were reported in the literature.

29

Inspired by the good photovoltaic performances of low band

gap polymers, in this paper, a new family of thiophene

dendrimers decorated with diketopyrrolopyrrole (DPP) groups

at the periphery was synthesized and characterized. Such a

chemical structure modification greatly reduces the optical

band gap and increases the light harvesting ability of the

thiophene dendrimers. The chemical structures of these

molecules were proved by NMR spectroscopic, MALDI-TOF MS

and HR MS. The monodisperse nature of these molecules was

investigated by MALDI-TOF MS and GPC analysis. Optical and

electrochemical properties will be also discussed. Finally,

photovoltaic as well as two-photon absorption properties of

these novel molecules were investigated in detail to obtain a

better understanding of these materials properties.

2. Experimental

2.1. Materials

Chemicals were purchased and used without further

purification: 2,5-bis(2-octyldodecyl)-3,6-bis(thiophen-2-

yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DPP-H) (Suna Tech

Inc), n-butyl lithium (n-BuLi, 2.4 M in n-hexane, J&K Scientific

LTD), [Ir(OMe)(COD)]2 (COD=1,5-cyclooctadiene) (Strem

Chemicals), 4,4'-Bis(di-tert-butyl)-2,2'-bipyridine (dtbpy)

(Sigma-Aldrich), Pd2(dba)3·CHCl3 (Sigma-Aldrich), HP(t-Bu)3·BF4

(Sigma-Aldrich), N-bromosuccinimide (NBS, Alfa Aesar), K2CO3

(Enox®), K3PO4 (Enox

®), 4,4,5,5-Tetramethyl-1,3,2-

dioxaborolane (HBpin) (Sigma-Aldrich). Solvents were purified

and dried by usual methods prior to use and typically used

under inert gas atmosphere.30

PC61BM was purchased from

Solarmer Energy, Inc. (Beijing).

2.2. Instruments and measurements

Nuclear magnetic resonance (NMR) spectra were

recorded on a Bruker Avance III 400 spectrometer (1H NMR:

400 MHz, 13

C NMR: 100 MHz). Chemical shifts are denoted in δ

(ppm), and were referenced to tetramethylsilane (TMS) via the

residual non-deuterated solvent peaks (CDCl3: 1H NMR: 7.26

ppm, 13

C NMR: 77.0 ppm; C2D2Cl4: 1H NMR: 6.00 ppm,

13C

NMR: 74.0 ppm) as internal standard. The splitting patterns

are designated as follows: singlet (s), doublet (d), triplet (t),

and multiplet (m). Matrix assisted laser desorption/ionization

time-of-flight mass spectrometry (MALDI-TOF MS) were

performed on a Bruker Autoflex Speed using trans-2-[3-(4-tert-

butylphenyl)-2-methyl-2-propenylidene] malononitrile (DCTB)

as the matrix. High resolution mass spectrometry (HR MS) was

performed on a Bruker ultrafleXtreme MALDI-TOF/TOF

(operation mode: MALDI; matrix: DCTB). Gel permeation

chromatography (GPC) measurement was performed on a

Waters Breeze separations module apparatus with THF as the

eluent (flow rate 1mL min-1

, 35 ºC). Number-average molecular

weight (Mn) and polydispersity index (Mw/Mn) of compounds

were determined by GPC analysis relative to polystyrene

standards. UV-vis spectra of these new materials in chloroform

(CHCl3) solution and thin film were recorded on a Perkin Elmer

Lambda 750 UV-vis Spectrophotometer. For UV-vis absorption

spectrum measurement in solution, three concentrated

solutions (around 10-4

mol·L-1

) were prepared independently,

each of which were further diluted to get three diluted

solutions (with concentration around 10-7

-10-6

mol·L-1

) for UV-

vis absorption measurement. The absorption spectra of the

dilute solutions were recorded, and the data points of the

absorbance at a certain wavelength vs. concentration were

then plotted. A good linear relationship was found for all these

compounds, suggesting no obvious intermolecular interaction

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Journal Name ARTICLE




was found in such a concentration range. The molecular molar

extinction coefficient (ε) was obtained from the slope of the

best-fit line over the above mentioned data points according

to the Beer–Lambert’s Law equation, A = ε·l·c. Thin film

samples for UV-vis spectra measurement were prepared by

spin-casting a chloroform solution on quartz glasses. The

photoluminescence (PL) spectra of the DOT-c-BTs in

chloroform solution were obtained with an F-4600

fluorescence spectrophotometer.

Cyclic voltammetry (CV) experiments were performed

with a RST-3000 Electrochemistry Workstation (Suzhou

Risetech Instrument co., ltd). All CV measurements were

carried out at room temperature with a conventional three-

electrode configuration under nitrogen atmosphere. The

electrochemical cyclic voltammetry was performed in a 0.1 M

tetrabutylammonium hexafluorophosphate (TBAPF6)/dichloro-

methane (DCM) solution with a scan speed of 100mV s−1

. A Pt

disk (ϕ = 1 mm) embedded in Teflon was used as the working

electrode. The surface was polished before use. A Pt sheet (~ 1

cm2) and Ag/AgCl were used as the counter and reference

electrodes, respectively. After the measurement, small

amount of ferrocene was added in the solution and the

ferrocene/ferrocenium (Fc/Fc+) redox couple was measured as

an internal standard. Transmission electron microscopy (TEM)

tests were performed on a Tecnai G2 F20 S-Twin 200 kV field-

emission electron microscope (FEI). Specimens for the TEM

experiments were obtained by transferring the floated blend

films from the water onto the 200 mesh copper grid. Atomic

force microscopy (AFM) measurements were performed by

using a Scanning Probe Microscope-Dimension 3100 in tapping

mode.

For nonlinear optical experiments, the excitation pulse (1

KHz, 240–2600 nm, pulse-width < 120 fs) was generated from

an optical parametric amplifier (TOPAS-F-UV2, Spectra-Physics)

pumped by a regeneratively amplified femtosecond Ti-

sapphire laser system (800 nm, 1 KHz, pulse energy 4 mJ,

pulse-width < 120 fs, Spitfire Pro-F1KXP, Spectra-Physics),

which is seeded by femtosecond Ti-sapphire oscillator (80

MHz, pulse-width < 70 fs, 710–920 nm, Maitai XF-1, Spectra-

Physics). The data were obtained by the two-photon excited

fluorescence method with fluorescein (80 μM in water, pH =

11) as a reference. Two-photon excited fluorescence spectra of

the samples were recorded in THF solution at room

temperature by using an Ocean Optics s2000 spectrometer in

conjunction with a fiber coupler head. The laser beam was

focused by an f = 10 cm lens, and the solution sample in a 1 cm

fluorimeter cuvette (four optically clear windows) was placed

at a fixed distance of ~13.5 cm from the focusing lens.

2.3. Synthesis

3-(5-Bromothiophen-2-yl)-2,5-bis(2-octyldodecyl)-6-

(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DPP-Br):

A solution of N-bromosuccinimide (NBS) (870 mg, 4.89 mmol) in

chloroform (50 mL) was added to DPP-H (4 g, 4.65 mmol) dissolved

in well-degassed chloroform (150 mL) at room temperature under

nitrogen protection. The reaction mixture was protected from light

and stirred at 0 ºC for 4 h, then allowed to warm to room

temperature, and stirred overnight. The reaction mixture was

concentrated to 50 mL, water (100 mL) was added and the mixture

was extracted with chloroform (100 mL x3). The organic layer was

separated and dried over anhydrous sodium sulfate. After the

solvent was removed under reduced pressure, the crude product

was purified by column chromatography on silica gel with

dichloromethane:hexane from 1:9 to 2:3 (v/v). DPP-Br was obtained

as a purple-black solid powder (2.4 g, 55%). 1H NMR (CD2Cl2,

400MHz): δ = 8.85 (d, J = 3.04 Hz; 1H), 8.59 (d, J = 3.64 Hz; 1H), 7.68

(d, J = 4.52 Hz; 1H), 7.28 (dd, J1= 3.96 Hz, J2= 4.96 Hz; 1H), 7.25 (d, J

= 4.2 Hz; 1H), 3.99 (d, J = 7.72 Hz; 2H), 3.92 (d, J = 7.76 Hz; 2H), 1.86

(br; 2H), 1.23-1.29 (m, 64H), 0.84-0.89 ppm (m, 12H).

2-(5-Hexyl-2-thienyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane

(B-T): n-BuLi (40 mL, 2.4 M in hexane, 96.0 mmol) was added

dropwise to a solution of 2-hexylthiophene (13.5 g, 80.4 mmol) in

THF (120 mL) at –78 ºC under nitrogen protection, and the reaction

mixture was stirred for 2 h at –78 ºC. 2-isopropyloxy-4,4,5,5-

tetramethyl-1,3,2-dioxaborolane (18 g, 96.8 mmol) was added

dropwise by a syringe and the reaction mixture was allowed to

warm to room temperature slowly and stirred overnight. The

reaction was quenched with saturated aqueous NH4Cl (100 mL) and

the organic layer was separated. The aqueous phase was extracted

with diethyl ether. The combined organic extracts were dried over

Na2SO4. The solvent was removed under reduced pressure to give

the residue, the crude product was purified by column

chromatography on silica gel with hexane to give B-T as yellow oil

(22.5 g, 90%). 1H NMR (CDCl3, 400 MHz): δ = 7.47 (d, J = 3.44 Hz; 1H),

6.86 (d, J = 3.40 Hz; 1H), 2.85 (t, J = 7.48 Hz, 2H), 1.68(quint, J = 7.48

Hz, 2H), 1.33 (s, 12H), 1.27-1.33 (m, 6H), 0.86-0.90 ppm (m, 3H).

3-(5'-Hexyl-(2,2'-bithiophene-5-yl))-2,5-bis(2-

octyldodecyl)-6-(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-

1,4(2H,5H)-dione (DPP-T): Compound DPP-Br (2.0 g, 2.13 mmol),

B-T (1.5 g, 5.10 mmol), [Pd2(dba)3]·CHCl3 (44 mg, 43 μmol), and

HP(tBu)3·BF4 (25 mg, 86 μmol) were dissolved into well-degassed

THF (150 mL). The reaction mixture was bubbled with nitrogen and

a well-degassed aqueous solution of K2CO3 (1M, 12 mL, 12 mmol)

was added dropwise. The reaction mixture was stirred overnight at

room temperature and then poured into water (150 mL) with some

drops of 2 M HCl. The organic layer was separated and the aqueous

layer was extracted with dichloromethane. The combined organic

extracts dried over Na2SO4. The solvent was removed by rotary

evaporation. The residue was filtered through a short column of

silical gel to remove inorganic salts. The filtrate was concentrated

and the resident was purified by SEC column chromatography

eluting with THF to provide DPP-T as purple-dark solid (2.1 g,

94%).1H NMR (CDCl3, 400 MHz): δ = 8.92 (d, J = 4.04 Hz, 1H), 8.84 (d,

J = 3.04 Hz, 1H), 7.60 (d, J = 4.40 Hz, 1H), 7.24-7.27 (dd, 1H), 7.23 (d,

J = 4.12 Hz,1H), 7.14 (d, J = 3.44 Hz, 1H), 6.74(d, J = 3.60 Hz, 1H),

4.03 (d, J = 2.08 Hz, 2H), 4.01 (d, J = 2.00 Hz, 2H), 2.80-2.84 (t,J =

7.40 Hz, 2H), 1.91-1.96 (br, 2H), 1.66-1.74 (quint, J = 7.47 Hz, 2H),

1.21-1.34 (m; 70H), 0.83-0.92 ppm (m,15H); 13

C NMR (CDCl3, 100

MHz): δ = 161.83, 161.56, 147.80, 143.61, 140.30, 139.50, 136.96,

134.91, 133.58, 130.12, 129.97, 128.33, 127.32, 125.32, 124.98,

123.95, 108.20, 107.84, 46.24, 37.90, 37.73, 31.92, 31.88, 31.53,

31.51, 31.32, 31.18, 30.29, 30.05, 30.00, 29.64, 29.55, 29.49, 29.35,


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29.29, 28.74, 26.35, 26.20, 22.68, 22.65, 22.55, 14.10, 14.05 ppm;

MALDI-TOF MS: m/z calcd for C64H102N2O2S3 ： 1026.7 ；

found:1026.6.

3-(5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)thienyl)-2,5-

bis(2-octyldodecyl)-6-(5'-hexyl-[2,2'-bithiophen]-5yl)pyrrolo[3,4-

c]pyrrole-1,4(2H,5H)-dione (B-DPP-T): In glove box, a 5 mL flask was

charged with [Ir(OMe)(COD)]2 (62 mg, 94 μmol, 4%). HBPin (600 mg,

4.68 mmol, 2 eq) was added to the [Ir(OMe)(COD)]2 flask. The

mixture was stirred for 1 min to give a yellow solution. To the

[Ir(OMe)(COD)]2-HBPin mixture was added dtbpy (50 mg, 186 μmol,

8%), and the mixture was stirred for 3 min to give a claret-red

solution. Another 100 mL flask was charged with DPP-T (2.4 g, 2.34

mmol). Dried THF (50 mL) was added to the flask to dissolve the

DPP-T. To the DPP-T solution was added the claret-red catalyst

mixture, and the reaction mixture was stirred at 50 ºC for 30 min.

After the reaction mixture was cooled to room temperature, the

volatile solvent was removed by rotary evaporation. The crude

product was run through a short pad of silica gel column (first

eluting with 3:1 hexane:dichloromethane to remove starting

material, and then eluting with THF to give the pure product B-DPP-

T as deep purple solid (2.3 g, 85%). 1H NMR (CDCl3, 400 MHz): δ =

8.96 (d, J = 4.16 Hz, 1H), 8.85 (d, J = 3.88 Hz, 1H), 7.70 (d, J = 3.84 Hz,

1H), 7.23 (d, J = 4.16 Hz, 1H), 7.14 (d, J = 3.60 Hz, 1H), 6.74 (d, J =

3.60 Hz, 1H), 4.06 (d, J = 7.60 Hz, 2H), 4.02 (d, J = 7.72 Hz, 2H), 2.82

(t, J= 7.56 Hz, 2H), 1.89-1.96 (m, 2H), 1.70 (quint, J= 7.47 Hz, 2H),

1.37 (s, 12H), 1.21-1.34 (m; 70H), 0.83-0.92 ppm (m,15H); 13

CNMR

(CDCl3, 100MHz): δ = 161.84, 161.56, 147.90, 143.88, 140.81,

139.07, 137.62, 137.28, 135.89, 135.60, 133.56, 127.26, 125.34,

125.06, 124.01, 108.84, 108.00, 84.51, 46.28, 46.24, 37.90, 37.76,

31.91, 31.87, 31.52, 31.50, 31.33, 31.26, 30.29, 30.04, 30.02, 30.01,

29.63, 29.58, 29.55, 29.51, 29.35, 29.28, 28.73, 26.36, 26.31, 24.75,

22.68, 22.65, 22.54, 14.10, 14.05 ppm; MALDI-TOF MS: m/z calcd

for C70H113BN2O4S3:1151.8; found: 1151.6.

3T-p-DPP: Compound 3T-I (100 mg, 200 μmol), B-DPP-T (530

mg, 460 μmol), [Pd2(dba)3]·CHCl3 (10 mg, 10 μmol), and

HP(tBu)3·BF4 (8 mg, 27 μmol) were dissolved into well-degassed THF

(10 mL).The reaction mixture was bubbled with nitrogen and a well-

degassed aqueous solution of K2CO3 (1 M, 1.2 mL, 1.2 mmol ) was

added dropwise. The reaction mixture was stirred overnight at

room temperature and then poured into ice water (50 mL) with

some drops of 2 M HCl. The organic layer was separated and the

aqueous layer was extracted with CH2Cl2. The combined organic

extracts were dried over Na2SO4. The solvent was removed by

rotary evaporation to give residue. The residue was purified by

column chromatography on silica gel with chloroform: hexane from

1:4 to 2:3 (v/v). to give the blue solid product (368 mg, 80%). 1H

NMR (CDCl3, 400MHz): δ = 8.94 (d, J = 2.92 Hz, 1H), 8.93 (d, J = 2.96

Hz, 1H), 8.89 (t, J = 4.48 Hz, 2H), 7.34 (d, J = 5.28 Hz, 1H), 7.27 (d, J =

1.04 Hz, 1H), 7.26 (d, 1H), 7.23 (d, J = 3.80 Hz, 1H), 7.21 (d, J = 4.24

Hz, 3H), 7.19 (d, J = 5.28 Hz, 1H), 7.13 (d, J = 3.56 Hz, 2H), 7.11 (d, J =

3.84 Hz, 1H), 7.05 (d, J = 3.84 Hz, 1H), 6.73 (d, J = 3.64 Hz, 2H),

4.02(d, J = 7.00 Hz, 8H), 2.81 (t, J = 7.56 Hz, 4H), 1.96 (br, 4H), 1.70

(quint, J = 7.50 Hz, 4H), 1.21-1.41 (m; 140H), 0.81-0.92 ppm (m,

30H); 13

C NMR (C2D2Cl4, 100MHz): δ = 161.58, 161.48, 148.13,

143.72, 142.24, 141.80, 139.83, 139.68, 138.88, 138.75, 138.16,

137.24, 137.04, 136.98, 136.53, 136.42, 136.03, 135.52, 133.47,

131.59, 131.17, 130.08, 129.10, 128.52, 128.25,127.86, 127.38,

127.36, 125.90, 125.54, 125.24, 124.96, 124.74,123.97, 108.63,

108.51, 108.14, 46.31, 37.96, 32.03, 31.993, 31.987, 31.63, 31.58,

31.47, 30.38, 30.17, 29.78, 29.74, 29.67, 29.49,29.42, 28.89, 26.49,

22.83, 22.80, 22.70, 14.32, 14.28 ppm; MALDI-TOF MS: m/z calcd

for C140H208N4O4S9:2297.4; found: 2297.0; HR MS: m/z calcd for

C140H208N4O4S9: 2297.3682; found: 2297.3665.

B-3T-p-DPP: In glove box, a 5 mL flask was charged with

[Ir(OMe)(COD)]2 (30.0 mg, 45.0 μmol, 4%). HBPin (2.70 g, 2.10 mmol,

2 eq) was added to the [Ir(OMe)(COD)]2 flask. The mixture was

stirred for 1 min to give a yellow solution. To the [Ir(OMe)(COD)]2

and HBPin mixture was added dtbpy (25.0 mg, 93.0 μmol, 8%), the

mixture was stirred for 3 min to give a claret-red solution. In

another 250 mL flask was charged with 3T-DPP (2.48 g, 1.08 mmol),

dried THF (100 mL) was added to the flask in order to dissolve the

3T-p-DPP. To the 3T-p-DPP solution was added the resulting claret-

red catalyst mixture, and the reaction mixture was stirred at 50 ºC

for 30 min. After the reaction mixture was cooled to room

temperature, the volatile solvent was removed by rotary

evaporation. The crude product was run through a short pad of

silica gel column (first eluting with DCM/hexane (1:3) to remove

starting material 3T-p-DPP, and then eluting with THF to give the

product as a blue solid (2.30 g, 88%).1H NMR (CDCl3, 400 MHz): δ =

8.94 (d, J = 2.76 Hz, 1H), 8.93 (d, J = 2.76 Hz, 1H), 8.90 (d, J = 4.16 Hz,

1H), 8.88 (d, J = 4.16 Hz, 1H), 7.68 (s, 1H), 7.28 (d, J = 1.96Hz, 1H),

7.27 (d, J = 1.96 Hz, 1H), 7.21-7.23 (m, 4H), 7.13-7.14 (m, 3H), 7.06

(d, J = 3.80 Hz, 1H), 6.73(d, J = 3.56 Hz, 2H), 4.03 (d, J = 7.16 Hz, 8H),

2.81 (t, J = 7.54 Hz, 4H), 1.96 (br, 4H), 1.66-1.73 (quint, J = 7.45 Hz,

4H), 1.38 (s, 12H), 1.21-1.33 (m; 140H), 0.81-0.92 ppm (m, 30H); 13

C

NMR (CDCl3, 100 MHz): δ = 161.60, 161.52, 161.48, 147.74, 147.71,

143.55, 143.45, 142.14, 141.58, 139.85, 139.68, 138.95, 138.73,

138.01, 137.81, 137.56, 136.97, 136.87, 136.48, 136.32, 135.44,

133.62, 132.47, 128.76, 128.60, 128.24, 127.81, 127.41, 127.37,

125.29, 124.99, 124.92, 124.90, 124.68, 123.94, 108.57, 108.43,

108.09, 108.06, 84.49, 46.28, 37.89, 31.90, 31.87, 31.85, 31.52,

31.48, 31.32, 31.28, 30.27, 30.03, 29.63, 29.54, 29.35, 29.28, 28.73,

26.34, 26.31, 24.74, 22.67, 22.64, 22.54, 14.09, 14.04 ppm; MALDI-

TOF MS: m/z calcd for C146H219BN4O6S9:2422.5; found: 2423.0.

6T-p-DPP: Compound B-DPP-T (630 mg, 546 μmol), 6T-Br (100

mg, 124 μmol), [Pd2(dba)3]·CHCl3 (10 mg, 10 μmol), and


(25.0 mL).The reaction mixture was bubbled with nitrogen and a

well-degassed aqueous solution of K2CO3 (1 M, 2 mL, 2 mmol ) was




layer was extracted with CH2Cl2. The combined organic extracts

dried over Na2SO4. The solvent was removed by rotary evaporation.

The residue was filtered through a short column of silical gel to

remove any inorganic salts. The filtrate was concentrated and

purified by SEC column chromatography eluting with THF to give

the product 6T-p-DPP as a deep blue solid (460 mg, 81%).1H NMR

(CDCl3, 400 MHz): δ = 8.93-8.74 (m, 8H), 7.21 (d, J = 3.52 Hz, 2H),

7.13-7.18 (m, 8H), 7.08 (s, 2H), 7.03 (d, J = 3.04 Hz, 6H), 6.96 (br, 4H),

6.69-6.73 (m, 4H), 3.99 (br, 16H), 2.82 (t, J = 7.58 Hz, 8H), 1.92 (br,

8H), 1.67-1.75 (quint, J = 7.49 Hz, 8H), 1.21-1.41 (m; 280H), 0.80-


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0.94 ppm (m, 60H); 1H NMR (C2D2Cl4, 400 MHz): δ =8.90 (d, J = 3.56

Hz, 4H), δ =8.87 (d, J = 4.16 Hz, 2H), δ =8.84 (d, J = 4.24 Hz, 2H),7.24-

7.28 (m, 10H),7.16-7.18 (m, 8H), 7.13 (d, J = 3.12 Hz, 4H), 6.75 (d, J =

3.40 Hz, 4H), 3.99 (br, 16H), 2.82 (t, J = 7.70 Hz, 8H), 1.94 (br, 8H),

1.67-1.74 (quint, J = 7.50 Hz, 8H), 1.21-1.42 (m; 280H), 0.80-0.94

ppm (m, 60H); 13

C NMR (CDCl3, 100 MHz): δ = 161.15, 160.97,

147.29, 147.27, 143.40, 143.34, 141.57, 141.30, 139.60, 139.46,

138.39, 138.18, 137.57, 137.22, 137.15, 136.75, 136.61, 136.58,

136.37, 136.34, 136.32, 135.33, 134.91, 133.96, 131.37, 130.57,

130.54, 128.75, 128.54, 128.32, 127.80, 127.77, 127.46, 127.45,

126.76, 126.68, 125.25, 125.22, 125.14, 124.82, 124.72, 123.74,

108.23, 108.15, 107.64, 107.62, 46.31, 37.86, 31.96, 31.93, 31.91,

31.62, 31.56, 31.51, 31.39, 30.36, 30.15, 29.74, 29.70, 29.64, 29.61,

29.41, 29.38, 28.89, 26.54, 26.43, 22.70, 22.64, 14.14, 14.11 ppm;

MALDI-TOF MS: m/z calcd for C280H414N8O8S18: 4592.7; found:

4592.6; HR MS: m/z calcd for C280H414N8O8S18: 4592.7202; found:

4592.7496.

9T-p-DPP. Compound 9T-I (50 mg, 40 μmol), B-DPP-T (204 mg,

177 μmol), [Pd2(dba)3]·CHCl3 (5 mg, 5 μmol), and HP(tBu)3·BF4 (4 mg,

14 μmol) were dissolved into well-degassed THF (10 mL). The

reaction mixture was bubbled with nitrogen and a well-degassed

aqueous solution of K3PO4 (1 M, 1 mL, 1 mmol) was added dropwise.

The reaction mixture was stirred overnight at room temperature

and then poured into ice water (10 mL) with some drops of 2 M HCl.

The organic layer was separated and the aqueous layer was

extracted with CH2Cl2. The combined organic extracts dried over

Na2SO4. The solvent was removed by rotary evaporation. The

residue was filtered through a short column of silical gel to remove

any inorganic salts. The filtrate was concentrated and purified by

SEC column chromatography eluting with THF to give the product as

a deep blue solid (126 mg, 62%). 1H NMR (CDCl3, 400 MHz): δ =8.92

(d, J = 4.08 Hz, 4H), 8.89 (d, J = 4.12 Hz, 2H), 8.87 (d, J = 4.16 Hz, 2H),

7.33 (d, J = 5.24 Hz, 1H), 7.22 (s, J = 4.08 Hz, 2H), 7.13-7.20 (m, 12H),

7.03-7.12 (m, 13H), 6.70-6.72 (m, 4H), 3.99 (br, 16H), 2.78-2.82 (m,

8H), 1.94 (br, 8H), 1.65-1.73 (m, 8H), 1.20-1.32 (m; 280H), 0.79-0.92

ppm (m, 60H); 13

C NMR (CDCl3, 100 MHz): δ =161.46, 161.34,

147.62, 147.60, 143.46, 143.40, 141.85, 141.83, 141.50, 141.46,

139.67, 139.60, 138.69, 138.57, 137.45, 137.36, 137.28, 137.23,

137.06, 137.02, 136.60, 136.56, 136.48, 136.27, 136.01, 135.11,

135.01, 134.61, 133.74, 131.77, 131.72, 131.71, 131.20, 130.20,

129.94, 129.86, 128.79, 128.62, 128.59, 128.52, 128.42, 128.02,

127.42, 126.36, 126.12, 125.25, 124.84, 124.72, 124.61, 123.89,

108.48, 108.40, 108.00, 46.29, 37.97, 31.95, 31.92, 31.58, 31.51,

31.39, 31.34, 30.32, 30.11, 29.68, 29.60, 29.40, 29.35, 28.82, 26.42,

26.38, 22.71, 22.70, 22.60, 14.14, 14.08 ppm; MALDI-TOF MS: m/z

calcd for C292H420N8O8S21: 4838.7; found: 4838.8, HR MS: m/z calcd.

for C292H420N8O8S21: (100% abundance) 4842.6906; found:

4842.6917.

18T-p-DPP. Compound B-3T-p-DPP (600 mg, 248 μmol), 6T-Br

(45 mg, 56 μmol), [Pd2(dba)3]·CHCl3 (10 mg, 10 μmol), and


(20 mL).The reaction mixture was bubbled with nitrogen and a well-

degassed aqueous solution of K3PO4 (1 M, 1.5 mL, 1.5 mmol ) was




layer was extracted with CH2Cl2. The combined organic extracts

dried over Na2SO4. The solvent was removed by rotary evaporation.

The residue was filtered through a short column of silical gel to

remove any inorganic salts. The filtrate was concentrated and

purified by SEC column chromatography eluting with THF to give

the product as a deep blue solid (351 mg, 65%). 1H NMR (C2D2Cl4,

400 MHz): δ = 8.90 (br, 8H), 8.86 (br, 8H), 7.15-7.34 (m, 54H), 6.76

(br, 8H), 3.99 (br, 32H), 2.82 (br, 16H), 1.93 (br, 16H), 1.70 (m, 16H),

1.21-1.32 (m; 560H), 0.82-0.91 ppm (m, 120H); 13

C NMR (CDCl3, 100

MHz): δ = 161.17, 147.27, 143.26, 141.74, 141.40, 139.40, 138.44,

137.36, 137.19, 136.63, 136.07, 135.05, 133.92, 131.52, 128.60,

128.39, 127.47, 125.12, 124.65, 123.76, 108.20, 107.82, 46.32,

37.99, 31.99, 31.96, 31.63, 31.54, 31.51, 31.41, 30.35,30.18, 29.73,

29.67, 29.44, 29.41,28.92, 26.51, 26.43, 22.75, 22.65, 14.17, 14.11

ppm; MALDI-TOF MS: m/z calcd for C584H838N16O16S42: 9685.4; found:

9684.8, HR MS: m/z calcd for C584H838N16O16S42: (100% abundance)

9685.3664; found: 9685.3677.

21T-p-DPP. Compound B-3T-p-DPP (235mg, 248 μmol), 9T-I (25

mg, 20.1 μmol), [Pd2(dba)3]·CHCl3 (5 mg, 5 μmol), and HP(tBu)3·BF4

(4 mg, 13 μmol) were dissolved into well-degassed THF (20 mL). The

reaction mixture was bubbled with nitrogen and a well-degassed

aqueous solution of K3PO4 (1 M, 1.5 mL, 1.5 mmol) was added

dropwise. The reaction mixture was stirred overnight at room

temperature and then poured into water (50 mL) with some drops

of 2 M HCl. The organic layer was separated and the aqueous layer

was extracted with CH2Cl2. The combined organic extracts dried

over Na2SO4. The solvent was removed by rotary evaporation. The

residue was filtered through a short column of silical gel to remove

any inorganic salts. The filtrate was concentrated and purified by

SEC column chromatography eluting with THF to give the product as

a deep blue solid (123 mg, 63%). 1H NMR (C2D2Cl4, 400 MHz): δ =

8.83 (br, 8H), 8.80 (br, 8H), 7.18-7.40 (m, 60H), 6.79 (br, 8H), 4.04

(br, 32H), 2.87 (br, 16H), 2.01 (br, 16H), 1.77 (m, 16H), 1.21-1.32 (m;

560H), 0.82-0.91 ppm (m, 120H); MALDI-TOF MS: m/z calcd for

C596H844N16O16S45: 9931.3; found:9931.7; HR MS: m/z calcd for

C584H838N16O16S42: (100% abundance) 9931.3293; found: 9931.2081.

2.4. Fabrication and characterization of the organic solar cells

Photovoltaic devices were fabricated with the

configuration of ITO/PEDOT:PSS/DOTs:PC61BM/LiF/Al. The

indium tin oxide (ITO) coated glass substrates were cleaned

sequentially by ultrasonic in detergent, deionized water,

acetone and isopropanol for 30 min each. After an additional

treatment for 30 minutes in ultraviolet-ozone chamber, a thin

layer (30-40 nm) of PEDOT:PSS (Clevios P VP AI 4083, filtered

through 0.45 μm filter) was spin-coated onto each substrate at

3500 rpm for 1 min, the substrates were transferred into a

nitrogen-filled glove box (<0.1 ppm O2 and H2O). After

annealing in a glove box at 124 or 10 minutes, the substrates

was cooled to room temperature. Compound DOTs and

PC60BM were dissolved in chloroform (CF), then stirred for 4h.

The substrates were spin-coated with the solution of DOTs:

PC61BM in chloroform to form the photoactive layer. The

cathode made of LiF (1 nm) and aluminum (100 nm) was

sequentially deposited by vacuum evaporation under high

vacuum (<10-4

pa). The effective area of the devices was 0.09

cm2.


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The current density–voltage (J–V) characteristics was

measured in a N2-filled glove box using a Keithley 2400 source

meter under a simulated AM 1.5G (100 mW cm-2

) sun light

generated by white light from halogen tungsten lamp, filtered

by a Schott GG385 UV filter and a Hoya LB120 daylight filter.

External quantum efficiencies (EQE) were measured under

simulated one sun operation condition using bias light from a

532 nm solid state laser (Changchun New Industries, MGL-III-

532). Light from a 150 W tungsten halogen lamp (Osram

64610) was used as probe light and modulated with a

mechanical chopper before passing the monochromator (Zolix,

Omni-λ300) to select the wavelength. The response was

recorded as the voltage by an I-V converter (DNR-IV Convertor,

Suzhou D&R Instruments), using a lock-in amplifier (Stanford

Research Systems SR 830). A calibrated Si cell was used as

reference. The device for EQE measurement was kept behind a

quartz window in a nitrogen filled container.

3. Results and discussion

3.1 Synthesis and Structure Characterization

Synthesis

2,5-bis(2-octyldodecyl)-3,6-bis(thiophen-2-yl)pyrrolo[3,4-

c]pyrrole-1,4(2H,5H)-dione (DPP-H, scheme S1) is the most

widely used starting materials for synthesizing DPP based

materials. However, DPP-H is not suitable for peripheral

functionalization of DOTs, since DPP-H has two reactive

terminal C-H positions. To solve this problem, α-hexyl-thienyl

group was selected as the terminal blocking group. To ensure

necessary solubility and film-forming properties of the target

dendritic oligothiophenes, 2-octyldodecyl was selected as the

solubilizing group at the N-position of the DPP unit.

Scheme S1 depicts the synthetic route to the key

precursor B-DPP-T (Supporting information Scheme S1). 3-(5-

Bromothiophen-2-yl)-2,5-bis(2-octyldodecyl)-6-(thiophen-2-

yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DPP-Br) was

synthesized by bromination of DPP-H using 1.1 equivalent N-

bromosuccinimide (NBS), which gave a 55% yield (Scheme S1,

reaction i).31

Reaction of lithiated 2-hexylthiophene (T) with 2-

isopropyloxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane gave

corresponding boronic ester B-T in 90% yield (Scheme S1,

reaction ii). Suzuki coupling of DPP-Br with B-T by using

[Pd2(dba)3]·CHCl3/HP(tBu)3BF4 as catalyst gave asymmetrical

DPP derivative DPP-T in 94% yield (Scheme S1, reaction iii).

Iridium-catalyzed borylation of DPP-T by using

[Ir(OMe)(COD)]2/dtbpy as catalyst gave the key precursor B-

DPP-T in 85% yield (Scheme S1, reaction iv).32

Scheme 1 depicts the synthetic route to the first

generation dendron 3T-p-DPP and dendrimer 6T-p-DPP.

Palladium-catalyzed Suzuki coupling of diiodinated

terthiophene 3T-I6b

and B-DPP-T (1:2 mol/mol) gave the first

generation 1st

dendron 3T-p-DPP in 80% yield (Scheme 1,

reaction i). Side reactions within the Suzuki coupling reaction

include, 1) homocoupling of B-DPP-T to yield a dimer T-DPP-

DPP-T; 2) homocoupling of 3T-I with a following up Suzuki

coupling with B-DPP-T to give higher order oligomer T-DPP-3T-

3T-DPP-T, which are confirmed by mass-spectrometric analysis

on the crude products (Supporting information Scheme S2 and

Figure S1). Palladium-catalyzed Suzuki coupling of the

tetrabrominated sexithiophene core 6T-Br6b

with four

equivalents of B-DPP-T under the same coupling condition

gave the first generation 1st

DPP-terminated thiophene

dendrimer 6T-p-DPP in a yield of 81% (Scheme 1, reaction ii).

Both 3T-p-DPP and 6T-p-DPP were purified by preparative size-

exclusion chromatography (SEC, Bio-Rad Beads SX1) eluting

with THF. One step further, iridium-catalyzed borylation of 3T-

p-DPP selectively gave the corresponding boronic ester B-3T-p-

DPP (Scheme 1, reaction iii), which is a basic building block for

the construction of higher generational dendrons and

dendrimers.

The 2nd

generation DPP-terminated dendron 9T-p-DPP was

synthesized by Suzuki coupling of the tetraiodinated G2 dendron

9T-I6b

and four equivalents of boroinc ester B-DPP-T in a yield of

62% (Scheme 2, reaction i). The side products were separated

successfully by SEC with THF as eluent. Alternative synthetic route

to 9T-p-DPP was Suzuki coupling of the diiodinated terthiophene

3T-I and the G1 boronic ester B-3T-p-DPP (Scheme 2, reaction ii).

However, side products including 6T-p-DPP and (3T-3T-p-DPP)2

formed (Supporting information Scheme S3) originating from the

homocoupling of B-3T-p-DPP and homocoupling of 3T-I with a

following up Suzuki coupling with B-3T-p-DPP, respectively (Scheme

S3 and Figure S2 in supporting information). Owing to the similar

hydrodynamic molecular volume for these products, it is difficult to

purify the final compound by SEC purification. Only after multiple

Scheme 1. Synthesis of first-generation dendrons (3T-p-DPP, B-

3T-p-DPP) and dendrimer (6T-p-DPP). Reagents and conditions: i)

and ii) [Pd2(dba)3]·CHCl3, HP(tBu)3BF4, K2CO3, THF; iii)

[Ir(OMe)(COD)]2, dtbpy, HBpin, THF.


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SEC column chromatography separation, pure 9T-p-DPP was

obtained in a yield of 64%. The G2 DPP-terminated dendrimer 18T-

p-DPP was synthesized by Suzuki cross coupling of the

tetrabrominated G1 dendrimer 6T-Br with four equivalents of the

G1 boronic ester B-3T-p-DPP in 65% yield (Scheme 2, reaction iii).

Purification of 18T-p-DPP by preparative SEC was found to be much

easier than that of 9T-p-DPP, which was ascribed to large

hydrodynamic molecular volume difference for the product and

side-products.

The 3rd

generation DPP-terminated dendron 21T-p-DPP

was synthesizedby Suzuki cross coupling of the tetraiodinated

G2 dendron 9T-I and four equivalents of the G1 boronic ester

B-3T-p-DPP in 63% yield (Scheme 3). Purification of 21T-p-DPP

was found to be more difficult than that of 18T-p-DPP. On one

hand, the molecular weight of 21T-p-DPP is close to 10,000 Da,

reaching the separation limit for the preparative SEC column

we used. The uncompleted coupled side-products of this

reaction, on the other hand, have similar hydrodynamic

molecular volumes as the product. Nevertheless, the final

compound 21T-p-DPP can be purified by multiple SEC column

chromatography, which was confirmed by the mass

spectrometry (vide infra).

Structural characterization

All these compounds are highly soluble in organic solvent for

their hyperbranched structure and the multiple alkyl side chains on

the DPP units. The chemical structures of the synthesized

compounds were then characterized by 1H NMR,

13C NMR, MALDI-

TOF MS and high resolution MS. Owing to the unsymmetrical

molecular structure of these dendrimers, 1H NMR spectra of these

compounds are rather complicated, especially for the higher-

generation dendrimers. However, characteristic 1H resonance

peaks can be clearly identified for the small molecules. Figure S3

(supporting information) shows the full 1H NMR spectra of DOT-p-

DPPs. As can be seen from Figure S3, all these DOT-p-DPP showed

Scheme 2. Synthesis of second-generationdendron (9T-p-DPP) and dendrimer (18T-p-DPP). Reagents and conditions: i),

ii) and iii) [Pd2(dba)3]·CHCl3, HP(tBu)3BF4, K2CO3, THF.


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characteristic 1H resonance peaks at 4.0 and 1.9 ppm (protons of -

N-CH2-CH-R1(R2) on the DPP unit), 2.8 ppm (protons of Th-CH2-R on

the terminal thiophene unit), 8.9 ppm (Ha and Hb, β-H of the core

thiophene unit next to the DPP units, see more details in

supporting information Figure S3), confirming the successfully

attaching DPP units to the DOT periphery. Except for these

characteristic proton resonances of the terminal DPP units, 1H

resonance peaks of the core dendritic thiophene can be also clearly

seen. Figure S3b depicts the 1H NMR spectra of the small molecules

(3T-p-DPP, 6T-p-DPP, 9T-p-DPP) in aromatic region. For 3T-p-DPP,

except for the characteristic 1H resonance peaks at 8.9 ppm (Ha

and Hb, for the β-H of the core thiophene unit next to the DPP),

characteristic doublet peak couples at δ =7.34, 7.05 ppm (Hc and

Hd) can be clearly distinguished, which are assigned to the α,β-

protons of the central thiophene unit. Such a doublet peak couple

is changed to a single peak at 7.08 ppm for the 6T-p-DPP molecule,

which is reasonable since the central thiophene unit was

homocoupled. Interestingly, all the resonance peaks up-field

shifted for the 6T-p-DPP when compared with 3T-p-DPP, which

could be due to the shielding effect of the conjugated wedge-shape

dendron unit. It is worth noting that 1H NMR of 6T-p-DPP tends to

be broader and featureless, which was ascribed to more intensive

intermolecular interaction. Checking 1H NMR with other deuterated

solvent, such as THF-d8, CD2Cl2, C2D2Cl4, or diluting the solution

could not get a better resolution results. For 9T-p-DPP, a doublet

peak at 7.34 ppm can be also clearly seen, which is ascribed to the

α-proton of the central thiophene unit, confirming a wedge-shape

dendron molecular structure of the 9T-p-DPP. The H resonance

peaks become broader and featureless with the increase of

molecular size (Figure S3d in supporting information), which was

on one hand ascribed to more complicated molecular structure,

and on the other hand to the increased intermolecular interactions.

Nevertheless, 1H NMR peaks for the small molecules confirmed

defined molecular structure of them. In addition, it is fully

confirmed that DPP units were successfully attached to the

periphery of the dendritic oligothiophene core.

To further confirm the chemical structure of these compounds,

mass spectrometry of these compounds was measured by MALDI-

TOF MS and HR MS measurements. Figure 1a shows the MALDI-TOF

MS results of target DPP-functionalized thiophene dendrons and

dendrimers with defined isotopic patterns, and the HR MS results

are shown in supporting information. The obtained MS data of

these compounds are listed in Table 1. For small molecules (3T-p-

DPP, 6T-p-DPP, 9T-p-DPP), the isotopic patterns fit very well to the

simulated spectrometry, both for the MALDI-TOF MS and HR MS

results (See supporting information). For 18T-p-DPP and 21T-p-DPP,

the HR-MS showed broad and featureless spectra, which was

ascribed to the high molecular weight of these two compounds,

which is close to the measurement limit of the instrument.

However, the MALDI-TOF MS clearly showed the isotopic patterns

of these two compounds, which fits very well to the simulated one

(See supporting information). Nevertheless, the MS and NMR

results unambiguously confirmed the chemical structures of the

S

S

S

N

N

O

O

S S

C10H21

C8H17

C10H21

C8H17

S

C6H13

N

N O

O

S

S

C10H21

C8H17

C10H21

C8H17

SC6H13

BO

O

S

S

S

S

S

S

SS

S

I

I

I

I

9T-I

S

S

S

S

S

S

S

SS

N

NO

O

S

S

SC6H13

S

S

S

SS

SS

S

S

S

SS

C10H21

C8H17

C10H21

C8H17 N

N

O

OS

S

S

C6H13

C10H21C8H17

C10 H

21 C8

H17

N

N

O

OS

S

S

C6H13

C10H21C8H17

C10H21 C 8H 17

N

N

O

O

S

S SC6H13

C10H21

C8H17

C10H21

C8H17

N

N

O

O

S S

S

C6H13

C10H21

C8H17

C10H21C8H17

N

NO

O

S

S

SC6H13

C10H21

C8H17

C10H21

C8H17

N

N O

O

S

S

SC6H13

C10H21

C8H17

C10H21

C8H17

N

N

O

OS

S

S

C6H13

C10H21 C 8H 17

C10H21C8H17

B-3T-p-DPP

21T-p-DPP

i 63%

Scheme 3. Synthesis of third-generation dendron (21T-p-DPP). Reagents and conditions: i) [Pd2(dba)3]·CHCl3, HP(tBu)3BF4,

K2CO3, THF.


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synthesized macromolecules. In addition, all these compounds

show one predominant molecular ion peak signal (Figure 1a),

demonstrating an excellent monodispersity of these dendrons and

dendrimers.

Molecular weight properties and polydispersities of these

peripherally DPP-functionalized dendritic oligothiophenes were

further studied by gel permeation chromatography (GPC). The GPC

traces of these compounds are depicted in Figure 1b, and the

calculated average molecular weights as well as the polydispersity

of these compounds are listed in Table 1. Interestingly, wedge-

shaped thiophene dendrons exhibit smaller hydrodynamic volumes

than the corresponding dendrimers with similar molecular weight.

For example, the G2 DPP-terminated dendron 9T-p-DPP has a larger

25.0 27.5 30.0 32.5 35.0 37.5

Inte

nsit

y

Retention Volume(mL)

3T-p-DPP

6T-p-DPP

9T-p-DPP

18T-p-DPP

21T-p-DPP

b)

Figure 1. a) MALDI-TOF mass spectra (matrix: DCTB) of

DOT-p-DPPs, the insertion showed the isotopic patterns

for each compound, which corresponds well to the

simulated one (see supporting information for detail

comparison);b) GPC profile of dendrons and dendrimers

eluting with THF as the solvent.

HOMO LUMO

3T-p-DPP

6T-p-DPP

9T-p-DPP

18T-p-DPP

21T-p-DPP

Figure 2. DFT calculated optimal molecular conformations and

molecular orbital surfaces of the HOMO and LUMO levels of

conjugated dendrimers.

Table 1. HR MS and GPC analysis data of the synthesized DOTs

Compound Formula [M]+

calcd [M]+

founda GPC Analysis Results

Mnb Mw PDI

3T-p-DPP C140H208N4O4S9 2297.3682c 2297.3665

c 2512 2539 1.01

6T-p-DPP C280H414N8O8S18 4592.7207c 4592.7496

c 5405 5529 1.02

9T-p-DPP C292H420N8O8S21 4842.6906d 4842.6917

d 4683 4770 1.02

18T-p-DPP C584H838N16O16S42 9685.3664d 9685.3677

d 10436 11119 1.07

21T-p-DPP C596H844N16O16S45 9931.3293d 9931.2088

d 8577 9458 1.10

aBy HR MS, using DCTB as matrix;

bNumber average molecular weight, calculated against polystyrene standards;

cMonoisotopic

peak; d100% abundance peak.


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retention volume than 6T-p-DPP, although 9T-p-DPP has only three

thiophene units more than 6T-p-DPP, demonstrating a smaller

hydrodynamic volume of 9T-p-DPP. A similar trend was also found

in the TMS-protected all-thiophene dendrons and dendrimers,6b

and this could be ascribed to more condensed molecular structure

wedge-shaped dendrons. For smaller derivatives (3T-p-DPP, 6T-p-

DPP, 9T-p-DPP) investigated by GPC, the PDI indices were almost

constant and ranged from 1.01 to 1.02, indicating a high degree of

defined and monodisperse structures. Much broader GPC traces

were found for 18T-p-DPP and 21T-p-DPP, leading to slightly higher

PDI values of 1.07 and 1.10, respectively. Such a broader GPC trace

was ascribed to formation of aggregates of dendritic molecules,

which was also found in the all-thiophene dendrimers.6b

Molecular simulation

To investigate the molecular geometries and electronic

structures of DOT-p-DPPs, density functional theory (DFT)

calculations were carried out by using the Gaussian 09 program

with B3LYP/6-31G approach. Calculated optimal molecular

conformations and molecular orbital surfaces of the frontier

molecular orbitals (FMOs), including the highest occupied molecular

orbital (HOMO) and the lowest unoccupied molecular orbital

(LUMO) are shown in Figure 2 and the related data are listed in

Table 2. As can be seen in Figure 2, for DOT-p-DPPs, the electron

density distributions of the HOMO and LUMO levels are both

located on the peripheral DPP groups, which is in good accordance

with the cyclic voltammetry results, where both 1st

oxidation and

reduction processes are ascribed to the oxidation and reduction of

DPP units (vide infra). In addition, these results are also in good

agreement with the UV-vis absorption results where the low energy

absorption bands are ascribed to the DPP units. Slightly reduced

optical band gaps were found for the bigger molecules, which could

be attributed to slightly increased π-conjugation systems. The

molecular dimensionality of few nanometers was calculated for all

these compounds. Specially, 18T-p-DPP and 21T-p-DPP have a

molecular dimension of 6-7 nm, respectively, demonstrating

synthesized single molecular organic nanoparticle (supporting

information Figure S4).

3.2 UV-vis absorption and fluorescence spectroscopy

300 400 500 600 700 8000

1

2

3

4

ε

ε

ε

ε / 1

05

cm

L m

ol-

1

Wavelength(nm)

T-DPP-T

3T-p-DPP

6T-p-DPP

9T-p-DPP

18T-p-DPP

21T-p-DPP

a)

0 5 10 15 200

1

2

3

4

5

0 1 2 3 4 5 6 7 8 90

1

2

3

4

21

T-p

-DP

P

18T

-p-D

PP

9T

-p-D

PP

6T

-p-D

PP

3T

-p-D

PP

358 nm

416 nm

636 nm

ε

ε

ε ε / 1

05 c

m L

mo

l-1

Number of thiophene units of the dendritic core

b)

21T

-p-D

PP

18T

-p-D

PP

9T

-p-D

PP

6T

-p-D

PP

636 nm

ε

ε

ε ε / 1

05 c

m L

mo

l-1

Total DPP units

T-D

PP

-T

3T

-p-D

PP

300 400 500 600 700 800 900

0.0

0.2

0.4

0.6

0.8

1.0 T-DPP-T

3T-p-DPP

6T-p-DPP

9T-p-DPP

18T-p-DPP

21T-p-DPP

No

rmalized

Ab

so

rban

ce

Wavelength(nm)

c)

600 650 700 750 800 8500.0

0.2

0.4

0.6

0.8

1.0

Fluorescence Intensity(a.u.)

Wavelength(nm)

T-DPP-T

3T-p-DPP

6T-p-DPP

9T-p-DPP

18T-p-DPP

21T-p-DPP

d)

Figure 3. a) UV-vis spectra of DOT-p-DPPs in chloroform solution (1×10

−6 mol L

−1); b) linear correlation between the

molar excitation coefficients and the number of thiophene units or DPP units in the synthesized dendrons and

dendrimers; c) Normalized UV−Vis absorption spectra ofDOT-p-DPPs thin films; d) normalized fluorescence emission

spectra of DOT-p-DPPs.


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The UV-vis absorption and fluorescence spectroscopy of these

DOT-p-DPPs and the reference compound T-DPP-T were measured,

and the optical data are listed in Table 2. Figure 3a shows the UV-

Vis absorption of these compounds in chloroform solution. As can

be seen from Figure 3a, all compounds exhibit similar absorption

pattern, which can be divided into two parts, a high-energy

absorption band from 300 to 500 nm and a low-energy band from

500 to750 nm. The absorption band of these compounds in short

wavelength range (300-500 nm) was ascribed to the

superimposition of the absorption of the conjugated oligothiphene

unit with different conjugation length, supported by the linear

correlation between the molar extinction coefficient and the total

thiophene number of the dendritic oligothiophene core (Figure 3b),

which is similar to that in all-thiophene dendrons and dendrimers.6b

On the other hand, the molar extinction coefficient of the long

wavelength band (500-750 nm) was also found linearly correlated

to the number of the DPP units rather than the number of the

thiophene units (Figure 3b, insertion). Therefore, such a low-energy

absorption band was ascribed to the absorption of the periphery

DPP units. Owing to the multiple DPP units for the bigger molecules,

high molar extinction coefficient was obtained for these

compounds. Nevertheless, an molar extinction coefficient of 3.6 ×

105 cm

-1 L mol

-1 for both 18T-p-DPP and 21T-p-DPP is a high value

for a structure defined macromolecule. Figure 3c shows the

absorption spectra of the DOT-p-DPPs in thin solid film. In

comparison with the corresponding spectrum in chloroform

solution, the absorption spectrum in thin solid film was found be

broader and red-shifted (Figure 3c), which can be ascribed to the

intensive intermolecular interaction of these molecules in solid

film33

. Optical band gap (Eg) of these compounds determined from

the absorption onset λabsonset

decreases slightly with the increase of

molecular size (Table 2), indicating a slightly extended π-

conjugation chain length of the DPP unit and the α-oligothiophene

chain.7

Fluorescence emission spectra of the peripherally DPP-

functionalized dendritic oligothiophenes in chloroform are

displayed in Figure 3d. As can be seen from here, the reference

compound T-DPP-T showed an emission band peaking at 652 nm.

The emission band for the DOT-p-DPPs red-shifted slightly with the

increase of molecular size, with peak wavelength in the range of

670-680 nm, suggesting a slightly increased π-conjugation system in

these molecules, which is in consistence with the optical band gap

results. Interestingly, the emission wavelength of these dendrimers

was found to be independent of the excitation wavelength, i.e.

excitation on the dendritic thiophene core also leads to the

emission of the DPP units (Figure S5, in supporting information).

These results indicate that the excited energy is more delocalized

over the periphery DPP units rather than within the dendritic

oligothiophene core, suggesting an energy anti-funnel effect within

these molecules. This is different to the behavior of all-thiophene

dendrimers,10c, 25b

where excited state energy is more localized in

the core part of the molecules.

3.3 Redox Properties and Molecular Energy Levels

The electrochemical properties of the DOT-p-DPPs were

investigated by means of cyclic voltammetry (CV) and differential

pulse voltammetry (DPV). Figure 4a shows cyclic voltammograms of

these DOT-p-DPPs, and the DPV curves of these compounds are

shown in Figure S6 in supporting information. The redox potentials

data are listed in Table 2 for comparison. Interestingly, all these

Table 2. Photophysical and electrochemical data of DPP-functionalized dendritic oligothiophenes

λmaxsol

(nm)a

λonsetsol

(nm)a

λmaxem

(nm)b

εsol

[cm-1

L

mol-1

]c

Egopt(sol)

(eV)d

λmaxfilm

(nm)e

λonsetfilm

(nm)e

Egopt(film)

(eV)e,f

Eox10

(V)g,h

Eox20

(V)g,h

Eoxonset

(V) g

Ered0

(V)g,h

Eredonset

(V)g

EHOMO

(eV)i

ELUMO

(eV)j

Egcv

(eV) k

EHOMOcal

(eV)l

ELUMOcal

(eV)l

Egcal

(eV)m

T-DPP-T 623 665 652 58,260 1.86 586 719 1.72 0.30 0.56 0.22 -1.58 -1.50 -5.32 -3.60 1.72 — — —

3T-p-DPP 634 683 670 118,820 1.82 595 752 1.65 0.20 0.30 0.15 -1.63 -1.54 -5.25 -3.56 1.69 -4.81 -2.97 1.84

6T-p-DPP 635 695 681 215,500 1.78 609 766 1.62 0.20 0.34 0.10 -1.60 -1.57 -5.20 -3.53 1.67 -4.79 -3.09 1.70

9T-p-DPP 598 693 683 213,013 1.79 610 756 1.64 0.19 0.31 0.09 -1.59 -1.54 -5.19 -3.56 1.63 -4.90 -3.14 1.76

18T-p-DPP 602 702 685 359,140 1.77 613 758 1.64 0.25n 0.46

n 0.07

n -1.63

n -1.61

n -5.17 -3.49 1.68 -4.75 -3.03 1.72

21T-p-DPP 603 704 686 358,490 1.76 605 738 1.68 0.27o 0.45

o 0.08

o -1.60

o -1.57

o -5.18 -3.53 1.65 -4.76 -3.04 1.72

PCBMp — — — — — — — — — — — — — -5.91 -4.01 — — — —

aIn CHCl3 solution (10

−6mol L

−1);

bThe maxima emission peak with the excitation wavelength;

cExtinction coefficient ε

sol [cm

-1 L mol

-1] in solution was obtained by linear fitting

absorbance vs. concentration at 633 nm; dOptical band gap estimated from the absorption edge in solution, Eg

opt(sol) (eV) = 1240/λonset

sol[nm];

eSpin-coated from CHCl3 solution onto

the quartz;fOptical band gap in solid film, Eg

opt(film) (eV) = 1240/λonset

film [nm];

gCH2Cl2, [M]=1×10

-3 mol L

-1, TBAPF6 (0.1 M), 295 K, V = 100 mV s

-1, versus (Fc

+/Fc);

hdetermined by DPV

measurements; iCalculated from the cyclic voltammograms, ELUMO = –[Eox

onset + 5.10] (eV), where the vacuum energy level of Fc

+/Fc was set as -5.10 eV;

jCalculated from the cyclic

voltammograms, ELUMO = –[Eredonset

+ 5.10] (eV), where the vacuum energy level of Fc+/Fc was set as -5.10 eV;

k Eg

cv = Eox

onset ﹣ Ered

onset (eV);

lB3LYP/6-31G** energies by DFT;

mB3LYP/6-31G** HOMO-LUMO energy gaps;

nCH2Cl2, [M] = 5×10

-4 mol L

-1, TBAPF6 (0.1 M), 295 K, V=100 mV s

-1, versus (Fc

+/Fc);

oCH2Cl2, [M] = 2.5×10

-4 mol L

-1, TBAPF6 (0.1 M), 295 K,

V = 100 mV s-1

, versus (Fc+/Fc);

pfrom ref 35.


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compounds showed one reversible reduction process over the

negative potential range (0 - -2.0 V). The standard reduction

potentials were determined to be around -1.6 V vs. Fc+/Fc, which is

closed to that of the reference compound T-DPP-T. Therefore, such

a reversible reduction process was ascribed to the reduction of the

DPP units. Although the bigger molecules have more DPP units at

the periphery, the anode or cathode peak currents (ipa and ip

c) for

the reduction of DPP normalized to the DOT-p-DPP concentration

were found to be almost identical for all these compounds. This

result suggests that only one of these DPP units was reduced in

such a negative potential range.

The oxidation potentials of these compounds, on the other

hand, were found to be dependent on the molecular structure. For

example, the 1st

generation dendron, 3T-p-DPP displays three

reversible oxidation processes (Eox0 = 0.20, 0.30, and 0.55 V,

respectively) during the positive potential sweeping. For

comparison, the reference compound T-DPP-T showed two

reversible oxidation processes with Eox0 of 0.30 and 0.56 V. For 6T-

p-DPP, two oxidation processes were measured with Eox0 = 0.20,

0.30 V, respectively, which is quite similar to that of 3T-p-DPP.

Knowing that the 1st

oxidation potentials for trimethylsilane

terminated thiophenedendron (3T-Si) and dendrimer (6T-Si) are

0.82 and 0.53 V, respectively,6b

the measured 1st

oxidation

processes was then ascribed to the oxidation of periphery DPP unit

as well. For larger dendrons and dendrimer (9T-p-DPP, 18T-p-DPP

and 21T-p-DPP), the oxidation waves become broader, which could

be due to the fact that these compounds are composed of multiple

π-conjugated moieties. Nevertheless, the first oxidation potential

could be obtained by DPV measurement, which was found to be

negatively shifted with increasing generation of the dendrons and

dendrimers (See supporting information Figure S6). This was

ascribed to the increased π-conjugation length with the increase of

molecular size, which is in accordance with the shift trend of the

UV-Vis absorption band. It worth also to note that, although there is

one reactive α-H at the core thiophene ring in the dendrons (3T-p-

DPP, 9T-p-DPP and 21-p-DPP), neither dimerization nor

electropolymerization was found during positive potential sweeping,

which is unlike the all-thiophene dendrons where electrochemical

dimerization was found for these compounds during CV sweeping.6b

These results further confirmed that oxidation occurred on the

periphery DPP unit, which could not undergo dimerization or

polymerization.

HOMO/LUMO energy levels of these compounds were

determined from the onset oxidation (Eoxonset

) and the onset

reduction potentials (Eredonset

) according to Equation (1) and (2),

EHOMO = - (Eoxonset

+ 5.10) (eV) (1)

ELUMO = - (Eredonset

+ 5.10) (eV) (2)

EgCV

= Eoxonset

- Eredonset

(eV) (3)

whereby Eoxonset

and Eredonset

represent the onset oxidation and the

onset reduction potential value relative to the

ferrocene/ferricenium couple, respectively, for which an energy

level of -5.10 eV versus vacuum was taken.34

Energy levels of these

compounds are listed in Figure 4b. As can be seen here, all these

compounds show similar HOMO-LUMO energy levels almost

independent of the molecular structure. Again, this was ascribed to

the fact the frontier molecular orbitals are mostly localized over the

periphery DPP units.

In comparison with PC61BM,35

all these DOT-p-DPPs showed

higher HOMO and LUMO energy levels. The higher LUMO energy

levels of DOT-p-DPP would enables electron transfer from DOT-p-

DPPs to PCBM after photoexcitation. All these DOT-c-DPPs show

low-lying HOMO levels of -5.1 eV, which could be beneficial for

achieving high VOC when used in organic solar cells.

3.4 Application of these compounds as electron donor in organic

solar cells

Solution-processed bulk heterojunction solar cells with the

conventional device configuration of ITO/PEDOT:PSS/DOT-p-

DPP:PC61BM/LiF/Al (ITO:indium tin oxide; PEDOT:PSS: poly(3,4-

ethylenedioxythiophene):poly(styrenesulfonate); PC61BM: [6,6]-

phenyl-C61-butyric acid methyl ester) were fabricated and

characterized, where the DOT-p-DPPs and PC61BM were used as the

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0

18T-p-DPP

I /

µµ µµA

E / V (vs. Fc+/Fc)

21T-p-DPP

3T-p-DPP

6T-p-DPP

9T-p-DPP

a) T-DPP-T

-6

-5

-4

-3

-5.18

21

T-p

-DP

P

-3.53

En

erg

y L

evel(

eV

)

3T

-p-D

PP

6T

-p-D

PP

9T

-p-D

PP

18

T-p

-DP

P

PC

61B

M-3.56 -3.53 -3.56 -3.49

-5.25 -5.20 -5.19 -5.17

-4.01

-5.95

b)

T-D

PP

-T

-3.60

-5.32

Figure 4. a) Cyclic voltammograms of T-DPP-T, 3T-p-DPP, 6T-p-DPP, 9T-p-

DPP (1×10-3

mol L-1

in CH2Cl2), 18T-p-DPP (5×10-4

mol L-1

in CH2Cl2), 21T-p-

DPP (2.5×10-4

mol L-1

in CH2Cl2), tetrabutylammonium hexafluorophosphate

(TBAPF6, 0.1 M), scan rate = 100 mV s-1

versus ferrocene/ferrocenium

(Fc+/Fc); b). Representation of HOMO–LUMO energy levels of selected

dendritic compounds obtained by electrochemical data. The values for

PC61BM are also given for comparison and were taken from reference 35.


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electron donor and acceptor, respectively. The DOT-p-DPP:PC61BM

blended ratio (w/w) was optimized for each compound (see

supporting information for details), and the optimized photovoltaic

performance of the optimized devices are given in Table 3. The

current–voltage (J–V) curves under light illumination and EQE

response spectra of the optimized devices are shown in Figure 5a

and Figure 5b, respectively.

As can be seen from the Table 3, the open circuit voltages(VOC)

for these DOT-p-DPP based devices are in the range of 0.75-0.81 V,

which are slightly higher when compared with the DPP based

polymer based devices.36

The VOC was found to be slightly

decreased with the increase of molecular size, which may be

ascribed to the slightly higher HOMO energy level for the larger

molecules (vide supra). Surprisingly, except for 3T-p-DPP based

device which showed a relative low fill factor (FF) of 0.41, all the

other DOT-p-DPP based devices showed high FF values with a

highest number of 0.69. The high FF was also found for DPP

polymer based devices.36a, 37

However, owing to the low short

circuit current (JSC), all these DPP based devices showed relative low

power conversion efficiency of around 1.0%. EQE spectra reflect

also the low incident photon-to-current efficiency for these cells.

To better understand the reason for the low device

performance, transmission electron microscope (TEM) images of

the blended film were measured. Figure 6 presents TEM images of

the DOT-p-DPPs:PC61BM films fabricated under the optimal

conditions. As can be seen, all blend films displayed obviously large

phase separation. Such an unusual nanophase separation was also

found for DPP polymer:fullerene blended films.38

Such unfavorable

nanophase separation is adverse for the JSC, since photogenerated

excitons cannot be dissociated efficiently and the electrons

generated in the photoactive layer cannot be collected by the

electrode efficiently. It worth also to note that the optimized DOT-

p-DPP:PC61BM blend ratio was found to be 1:4 or 1:5 (except for 3T-

p-DPP, which has an optimized ratio of 2:1), indicating an extreme

low electron donor content in this blended film. Increase the DOT-

p-DPP concentration led to poorer device performance (Supporting

information, Figure S8-S11, Table S2-S5). Such a low electron donor

contents lower the light absorption ability of the photoactive layer,

Table 3. Characterization of bulk heterojunction solar cells containing DPP-functionalized dendritic oligothiophenes as the

donor material and PC61BM as the acceptora.

Donor D:Ab EQEmax

c

(@wavelength[nm])

JSCd

[mA·cm-2

]

VOC

[V]

FF PCEe

[%]

3T-p-DPP 2:1 0.24 (576) 3.77 0.80 0.41 1.24(1.17±0.083)

6T-p-DPPf 1:5 0.17 (605) 2.28 0.81 0.67 1.24(1.18±0.075)

9T-p-DPP 1:4 0.10 (615) 1.75 0.76 0.65 0.86(0.83±0.043)

18T-p-DPP 1:5 0.13 (471) 2.04 0.77 0.59 0.93(0.87±0.058)

21T-p-DPP 1:4 0.14 (460) 1.97 0.75 0.69 1.02 (0.97±0.048)

aWith a standard device structure of ITO/PEDOT:PSS/DOT-p-DPP:PC61BM/LiF/Al;

bD: DOT-p-DPPs, A: PC61BM;

cExternal

quantum efficiencies (EQE) were measured under simulated one sun operation condition using bias light from a 532 nm

solid state laser, dJSC determined by integrating the EQE spectrum with the AM1.5 G spectrum; eAverage from over 8

devices. Tested under illumination of AM 1.5G 100 mW cm−2

; fThermal annealing at 90 °C.

0.0 0.5 1.0

-4

-3

-2

-1

0

1

Cu

rre

nt

de

ns

ity

(m

A/c

m2)

Voltage(V)

3T-p-DPP

6T-p-DPP

9T-p-DPP

18T-p-DPP

21T-p-DPP

a)

400 500 600 700 8000.00

0.05

0.10

0.15

0.20

0.25 3T-p-DPP

6T-p-DPP

9T-p-DPP

18T-p-DPP

21T-p-DPP

EQ

E

Wavelength(nm)

b)

Figure 5. a) J-V curves of the optimized DOT-p-DPP:PC61BM based BHJ

solar cells illuminated under standard AM1.5G conditions (100 mWcm-2

);

b) EQE spectra of the corresponding devices.


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which could be the 2nd

reason for the low JSC of the devices. Surface

morphology of the DOT-p-DPP:PC61BM films were measured by

atomic force microscope, and the results are shown in Figure S12 in

supporting information. For the 3T-p-DPP, 6T-p-DPP and 18T-p-DPP

based films, rather smooth surfaces were measured, suggesting

that morphology on the surface of these films might be different to

that in the bulk, whereas the 9T-p-DPP and 21T-p-DPP films show

large aggregates on the surface, demonstrating a molecular shape

related nanophase separation, which are in good accordance with

the TEM results. Nevertheless, the preliminary results showed that

DOT-p-DPPs could be used as electron donor in organic solar cells.

Further optimization the D:A nanomorphology are still needed to

achieve high device performance.

3.5 Two-Photon Absorption (TPA)

Organic optical materials with large TPA cross section have

attracted increasing attention owing to their applications in two-

photon bio-imaging and two-photon biosensing.29a

Conjugated

dendrimers are recognized as a special type of high performance

TPA materials for its intriguing chemical structures.29a

In

comparison with linear materials, conjugated dendrimers has larger

TPA cross section values owing to a higher density of effective

chromophores in an ordered and confined geometry, strong

interchromophore interactions and efficient intramolecular charge

transfer.39

Therefore, TPA behaviors of the synthesized DPP

decorated thiophene dendrimers were then measured and tested.

TPA cross-sections of the various molecules as a function of

excitation wavelength are shown in Figure 7. For comparison, the

TPA spectra of the reference molecules T-DPP-T and DPP-3T-DPP

with linear structures (see chemical structures in supporting

information, Figure S13) are also shown in Figure7. As shown in

Figure 7, all linear molecules (T-DPP-T and DPP-3T-DPP) have a

small TPA cross-section value of less than 1000 GM. In comparison

with the linear oligothiophenes, DPP-based dendritic

oligothiophenes show significant enhancement in TPA cross-section

values, which could be ascribed to the cooperative enhancement

effect derive from the 3D special dendritic structures,29a with a high

density of effective chromophores as well as stronger

intramolecular and intermolecular interaction among the branched

multichromophores.39, 40

For example, 3T-p-DPP and DPP-3T-DPP

are two configurational isomers with the same molecular weight.

Figure 6. Bright-field TEM images of DOT-p-DPPs:PC61BM blend films. a) 3T-p-DPP; b) 6T-p-DPP; c) 9T-p-DPP; d) 18T-p-DPP; e) 21T-p-DPP.

800 850 900 9500

1000

2000

3000

4000

5000

6000

7000

TP

A C

ros

s-s

ecti

on

s/G

M

Wavelength/nm

T-DPP-T

DPP-3T-DPP

3T-p-DPP

6T-p-DPP

9T-p-DPP

18T-p-DPP

Figure 7. TPA spectra of DPP-functionalized dendritic

oligothiophenes. TPA of the linear model compound T-DPP-T and

3T-DPP-3T were also added for comparison.


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However, 3T-p-DPP with branched structures has substantially

215% larger TPA cross-section values than the corresponding linear

isomer DPP-3T-DPP, which is in accordance with the previous

report.28

The first generation dendron 3T-p-DPP has a TPA cross section

peak value of 2499 GM at λ=810 nm, and the corresponding first

generation dendrimer 6T-p-DPP has a TPA cross section peak value

of 6984 GM, which is 180% larger than the value for 3T-p-DPP. The

increased TPA cross section peak value for 6T-p-DPP, in comparison

with 3T-p-DPP is mainly attributed to the extended conjugation

length. Surprisingly, the TPA cross section peak values have not

increased with increasing generation of molecules with wavelength

in the range from 780 nm to 880 nm. The second generation

dendron 9T-p-DPP has a TPA cross section peak value of 3252 GM

at λ=810 nm, and the corresponding second generation dendrimer

18T-p-DPP has a TPA cross section peak value of 3672 GM. The

phenomenon may be related to the increased steric hindrance

which decreased the orbital overlap of the α−α linkage around

the thiophene chain.41

It should be noted that a large TPA is

attributed to an extended π-conjuated system as well as an

increased intramolecule charge transfer. However, the TPA cross

section peak values have increased gradually with increasing the

generation of DOT-p-DPPs in the wavelength range from 880 nm to

980 nm. 9T-p-DPP has a TPA cross section peak value of 1978 GM at

λ=930 nm, and 18T-p-DPP has a TPA cross section peak value of

2110 GM. The diketopyrrolopyrrole group may be mainly

responsible for the TPA band in the longer wavelength range

considering the relative lower bandgap of the unit. Thus larger

generation of DOT-p-DPPs would exhibit a larger TPA value with the

increasing number of diketopyrrolopyrrole building blocks provided

that their steric hindrance is relatively insignificant. These initial

results clearly demonstrate that peripherally diketopyrrolopyrrole-

functionalized dendritic oligothiophenes are a potentially suitable

class of two-photon absorbing materials.

Conclusion

In summary, a new series of three dimensional

conjugated thiophene dendrons and dendrimers with

peripheral DPP units were synthesized by a step by step

synthesis approach that involves Ir-catalyzed boronation, and

palladium-catalyzed Suzuki cross coupling reaction. All these

peripherally DPP-functionalized DOTs are highly soluble in

common organic solvents, thus facilitating the full structure

characterization, including NMR, MALDI-TOF MS and HR-MS.

Both MS and GPC analysis showed the monodisperse nature of

these molecules. Optical and electrochemical properties

investigation, as well as the theoretical simulation results

confirmed that the frontier molecular orbitals are mostly

localized on the periphery DPP units, which leads to similar

optical band gaps and HOMO/LUMO energy levels for all these

functionalized dendrimers. Applications of these DPP-

functionalized DOTs in organic photovoltaic devices showed a

low performance with power conversion efficiencies around

1%. High FF of more than 0.6 was measured for these DPP

dendrimers, which is similar to that of DPP based polymers.

Unfavorable nanophase separation and low DOT-p-DPP

concentration in the blend films were the main reason for the

low JSC of these devices. In addition, these branched DPP

dendrimers showed very high two photon absorption cross

section, and a clear dendritic effect can be seen from these

results.

Acknowledgements

The authors greatly appreciate the financial supports from the

National Natural Science Foundation of China (Grant No.

21274163, 21573020, 21303252) and Natural Science

Foundation of Jiangsu Province (Grant No. BK20130352). W.G.

and C.-Q.M. would like to thank Prof. Peter Bäuerle and Dr.

Makus Wunderlin (Ulm University, Germany) for their helpful

discussion and HR-MALDI-MS measurement of 3T-p-DPP and

6T-p-DPP. W.G. and C.-Q.M.and J. D. would like to thank Dr.

Ying Wu and Prof. Zhengping Liu (Beijing Normal University,

China) for their help in GPC analysis.

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Graphical Abstract

Structure defined DPP functionalized conjugated thiophene dendrimers with narrow optical band

gap and high TPA cross section are reported.

For Table of Contents use only

Submit the graphic at the actual size to be used for the TOC so that it will fit in an

area no larger than 3.25 inches by 1.75 inches (approx. 8.5 cm by 4.75 cm).


Pol

ymer

Che

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try

Acc

epte

dM

anus

crip

t

Publ

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d on

10

Janu

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2017

. Dow

nloa

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by U

nive

rsity

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nia

- Sa

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10/0

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17 1

5:42

:39.



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