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Chinese Chemical Letters 22 (2011) 1001–1004

Synthesis and properties of two novel copolymers based on

squaraine and fluorene units for solar cell materials

Zheng Wang, Wei Zhang, Feng Tao, Kai Ge Meng, Long Yi Xi, Ying Li *, Qing Jiang

College of Chemistry, Sichuan University, Chengdu 610064, China

Received 26 October 2010

Available online 18 May 2011

Abstract

Two novel copolymers based on squaraine and fluorine units have been synthesized through palladium catalyzed Suzuki

coupling reaction and Sonogashira coupling reaction, respectively. The structures and properties of the two copolymers were

characterized by FT-IR, NMR, UV–vis absorbance (Abs), gel permeation chromatography (GPC), thermal gravimetric analysis

(TGA), differential scanning calorimetry (DSC) and cyclic voltammetry (CV). The solution absorption spectrums of P1 and P2

show two distinct absorption bands, one locates at 300–500 nm and the other at 600–800 nm. The absorption spectrums of P1 and P2

in films are broadened obviously and the spectral responses are extended up to 900 nm. Thermal gravimetric analysis demonstrates

that the polymers are stable. Cyclic voltammetry experiment shows that the band gaps of the copolymers are 1.65 eV and 1.67 eV,

respectively, suggesting their potential for applications as solar cells materials.

# 2011 Ying Li. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.

Keywords: Squaraine; Polymer; Synthesis; Organic solar cell materials

Harvesting energy directly from sunlight using photovoltaic technology is one of the most important ways to

address the growing global energy needs. Polymer solar cells (PSCs) are becoming a promising alternative for clean

and renewable energy due to their unique advantages, such as light weight, low cost and processability [1,2]. The

photoconversion process in PSCs materials is connected with light absorption and charge transfer between those

polymers. In order to harvest as much as possible of photo flux and achieve high power conversion efficiencies (PCEs),

the most powerful strategy is to design a low band gap conjugated polymer which incorporate electron-rich donor and

electron-deficient acceptor segments in the polymer backbone [3–5].

Squaraines, a class of 1,3-derivatives of squaric acid in which the substituents are highly electron-donating

aromatic groups, display strong absorption in the visible and near infrared regions, have been found to be important

application in PSCs [6,7]. In this work, we have studied and synthesized two novel polymers based on squaraine and

dihexylfluorene. First we prepare symmetrical squaraine monomer M consisting of electron-deficient squaric acid and

electron-rich styrylpyrrole units, then we synthesize two novel polymers through the palladium-catalyzed cross-

coupling reaction. The long alky chains on the central core could enhance the solubility of the copolymers, the donor–

acceptor route could enhance energy transfer between those polymers. Herein, we report our initial results of

synthesis, characterization, thermal, optical and electrochemical properties of the two novel copolymers.

* Corresponding author.

E-mail address: profliying@sina.com (Y. Li).

1001-8417/$ – see front matter # 2011 Ying Li. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.

doi:10.1016/j.cclet.2010.12.052

Z. Wang et al. / Chinese Chemical Letters 22 (2011) 1001–10041002

1. Experimental

Reagents and chemicals were purchased from Aldrich or Acros Chemical Companies and were used without

further purification. All the solvents were purified according to the standard methods prior to use. 4-Bromo-2,5-

dioctyloxybenzyldiethylphosphonate 1 [8], hexadecylpyrrole-2-carbaldehyde 2 [9], 2,7-dioxaborolan- 9,90-dihexylfluorene [10] and 2,7-diethynyl-9,90-dihexylfluorene [11] were prepared according to the similar procedures

reported.

Scheme 1 showed the synthesis route of the intermediates and polymers. Monomer M was synthesized in a solvent

mixture of 1-butanol and toluene (v/v, 2/1). P1 was synthesized through Suzuki palladium-catalyzed coupling reaction.

P2 was synthesized by the Sonogashira palladium-catalyzed coupling reaction.

M: Dark green solid. Yield: 65%. 1H NMR (400 MHz, CDCl3): d 0.86–0.93 (m, 18H), 1.09–1.58 (m, 92H), 1.79–

1.93 (m, 12H), 4.03 (m, 8H), 4.82 (t, 4H, J = 12.8 Hz), 6.91 (d, 2H, J = 4.8 Hz), 7.01 (s, 2H), 7.13 (s, 2H), 7.27 (d, 2H,

J = 16 Hz), 7.44 (d, 2H, J = 16 Hz), 7.87 (d, 2H, J = 2.8 Hz).

P1: Dark green solid. Yield: 62%. IR (KBr pellet, cm�1): 3426, 2923, 2853, 1620, 1505, 1463, 1380, 1261, 1204,

1099, 1026, 803, 722. 1H NMR (400 MHz, CDCl3): d 0.69–0.80 (m, 24H), 1.17–1.97 (m, 120H), 3.88–4.04 (m, 8H),

4.78 (s, 4H), 6.78–7.81 (m, 18H).

P2: Dark green solid. Yield: 57%. IR (KBr pellet, cm�1): 3426, 2923, 2852, 2203, 2137, 1624, 1501, 1463, 1384,

1264, 1203, 1109, 958, 818, 720. 1H NMR (400 MHz, CDCl3): d 0.80–0.98 (m, 24H), 1.07–1.96 (m, 120H), 3.93–4.04

(m, 8H), 4.82 (s, 4H), 6.86–7.14 (m, 4H), 7.28–7.32 (m, 2H), 7.46–7.58 (m, 6H), 7.56–7.68 (m, 4H), 7.86 (s, 2H).

[()TD$FIG]

OR1

R1O

Br CH2PO(OEt)2N

R2

OHC

OR1

R1O

Br N

R2

OH

O

O

OH

OR1

R1O

Br N

R2 O

OR1O

OR1

BrN

R2

M

M

R3R3

BO

OBO

O

R3R3

OR1

R1O

N

R2 O

O R1O

OR1N

R2R3 R3

M

P1

P2

OR1

R1O

N

R2 O

OR1O

OR1N

R2R3 R3

+

1 2 3

n

n

R1 =

R2 =

R1 =

R2 =

R3 =

R1 =

R2 =

R3 =

a

b

c

d

Scheme 1. Synthesis of intermediates and polymers. Reagents and conditions: (a) NaH/THF, reflux 24 h; (b) 1-butanol/toluene (v/v, 2/1), reflux 4 h;

(c) Pd(PPh3)4, toluene/K2CO3, 90 8C, 48 h; and (d) Pd(PPh3)2Cl2/CuI/PPh3, toluene/K2CO3, 90 8C, 48 h.

Z. Wang et al. / Chinese Chemical Letters 22 (2011) 1001–1004 1003

Table 1

The properties of P1 and P2.

Mw (g/mol) PDI Td (8C) lmax, sol lmax, film HOMO (eV) LUMO (eV) Eg (eV)

P1 64,200 4.4 216 730, 350 746, 361 �5.53 �3.97 1.65

P2 22,200 1.9 379 735, 396 748, 677, 398 �5.49 �3.91 1.67

2. Results and discussion

The copolymers exhibit good solubility in common organic solvents. Molecular weights of the polymers are

determined by GPC, thermal properties of the polymers are measured by thermogravimetric analysis (TGA) and

differential scanning calorimetry (DSC) under a nitrogen atmosphere. The weight-average molecular weights (Mw) of P1,

P2 are 64,200 g/mol with polydispersity indice (Mw/Mn) of 4.4 and 22,200 g/mol with polydispersity indice (Mw/Mn) of

1.9, respectively. Fig. 1 shows that the thermal decomposition temperatures (Td, 95 wt% residue) of P1 and P2 are 216 8Cand 379 8C, respectively. The relative low thermal decomposition temperature (Td) of P1 shows that squaraine monomer

M and fluorine linked by single-bond is less stable than triple-bonds. Differential scanning calorimetry (DSC) of P2 shows

that distinct glass transition temperatures (Tg) at 158 8C, whereas P1 has no obvious Tg before the weight loss. Fig. 2 shows

the cyclic voltammetry of P1 and P2. The HOMO, LUMO and the band gap (Eg) of the copolymer P1, P2 are estimated to be

�5.56 eV,�3.91 eV, 1.65 eVand�5.63 eV,�3.96 eV, 1.67 eVaccording to the following empirical relationships [12]:

Ip ¼ �HOMO ¼ �eðEonset; ox þ 4:39Þ ðeVÞ; Ea ¼ �LUMO ¼ �ðEonset; red þ 4:39Þ ðeVÞ;Eg ¼ Eonset; ox � Eonset; red ðeVÞ:

The absorption spectrums of the monomer M and the polymers P1, P2 are shown in Fig. 3. It could be seen from the

figure that the solution absorption spectrums of P1 and P2 show two distinct absorption bands, one locates at 300–

500 nm and the other at 600–800 nm. The absorption peak at short wavelength (300–500 nm) originates from p! p*

transition of fluorine units, while the absorption peak at long wavelength (600–800 nm) is due to p! p* transition of

the squaraine M units. The absorption peaks at long wavelength of P1 and P2 are red shift by 23 nm and 28 nm as

compared with monomer M (lmax = 707 nm), respectively, this behavior can be attributed to the longer conjugation

length of the low-band units of the new polymers. The absorption maximum of P1 and P2 films in visible region are red

shift by 16 nm and 13 nm in comparison with their solutions. This is attributed to more aggregated configuration

formed in solid state. In addition, the absorption spectrums of P1 and P2 in films are broadened obviously and the

spectral responses are extended up to 900 nm. All the results are presented in Table 1.

In conclusion, we have synthesized two novel copolymers based on squaraine and fluorine units. These polymers

have low band-gaps, good overlap with solar spectra and relative high thermal decomposition temperatures. The

LUMO energy levels of the copolymers are estimated to be �3.97 eV and �3.91 eV, which is very close to that of

PCBM (�3.91 eV) [13], suggesting their potential for applications as an n-type solar cells materials. Intensive studies

on the photovoltaic properties of these polymers devices are in progress.[()TD$FIG]

Fig. 1. Thermal properties of the polymers.

Z. Wang et al. / Chinese Chemical Letters 22 (2011) 1001–10041004[()TD$FIG]

Fig. 2. Cyclic voltammogram of the polymers.[()TD$FIG]

Fig. 3. UV–vis absorption spectrums for monomer M and polymers P1, P2 in CHCl3 solution and in the solid state.

Acknowledgments

This work was supported by the Key Foundation of Education Ministry of China (No. 20070610053), Sichuan

Province Foundation for Youths (No. 2008JY0050) and Technology Research Support Program (No. 09H1016). The

authors also acknowledge the Analytical & Testing Center of Sichuan University for the NMR measurements.

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