Synthesis and properties of polyfluorene copolymers

bearing thiophene and porphyrin

Xing Xu, Hao Chen, Xian Rong Cai, Ying Li *, Qing Jiang

College of Chemistry, Sichuan University, Chengdu 610064, China

Received 2 March 2007

Abstract

To search for more wider absorption and higher charge carriers mobilities materials of polymer solar cell, a series of soluble

alternating polyfluorene copolymers were synthesized by palladium-catalyzed Suzuki coupling reaction. Their structures were

determined by 1H NMR, IR and UV–vis. And their UV–vis absorption spectra indicated that they had strong absorption over

600 nm spectral range and nearly cover 400–700 nm visible region. The band gaps of copolymers calculated according to cyclic

voltammetry (CV) were between 1.96 and 2.03 eV. Polymer:TiO2 bulk-heterojunction films were made from mixtures of polymer

and titanium isopropoxide, a precursor for TiO2, via hydrolysis in air overnight. The photoluminescence at 380–800 nm of the blend

film of PT-TPP20 (5 mg/mL):Ti(OC3H7)4 (80 mL/mL) (20% volume fraction) was significantly quenched in the 50% Ti(OC3H7)4

blend film, indicating that rapid and efficient separation of photoinduced electron–hole pairs.

# 2007 Ying Li. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.

Keywords: Polyfluorene copolymers; Synthesis; Absorption; Bulk-heterojunction

In the past years conjugated polymers solar cells had made some progress [1–4]. But the main defects of polymers

solar cells to their application were narrow spectral coverage, poor mobility of charge carriers and instability of

polymer materials, etc.

Polyfluorenes had emerged as organic optoelectronic materials because of their good thermal and chemical stability,

and hole-transporting properties. Porphyrin and its derivatives had excellent absorption at 400–700 nm visible region.

People had synthesized polymers containing porphyrin. Morgado et al. [5] prepared polymers bearing porphyrin in the

side chains of PPV. Hou et al. [6] synthesized red electrophosphorescence polymers containing porphyrin. Thiophene

with rich-electrons could enhance the charge transfer, and its polymers had been used for solar cell [3].

In this paper, we report the synthesis and properties of polyfluorenes containing tetraphenylporphyrin (TPP) and

thiophene units in main chain. We expected that TPP unit introduced to the main chain of polyfluorene could extend

the spectral coverage of copolymers and make the most of solar spectrum, thiophene unit could improve the hole

accepting and transporting of copolymers, and fluorene unit with good thermal and chemical stability could favor the

stability of copolymers. The hybrid donor–acceptor bulk-heterojunctions based on the hole accepting and transporting

properties of copolymer and the electron transporting properties of TiO2 could lead to an efficient charge transfer and

separation.

www.elsevier.com/locate/cclet

Chinese Chemical Letters 18 (2007) 879–882

* Corresponding author.

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

1001-8417/$ – see front matter # 2007 Ying Li. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.

doi:10.1016/j.cclet.2007.05.040

Scheme 1 showed the synthesis routes of the monomers and copolymers. Three monomers, 1 [7,8] and 4 [9] were

synthesized according to literatures, and trans-substituted porphyrin 3 [10] was prepared by the reaction of meso-

substituted dipyrromethane 2 with benzaldehyde. Yield: 15%. 1H NMR (CDCl3, 400 MHz, ppm) d 8.86 (m, 8H), 8.22–

7.74 (m, 16H, ArH),�2.84 (s, 2H, NH). Polyfluorene copolymers derived from monomers 1, 3 and 4 were synthesized

via the palladium catalyzed Suzuki coupling reaction [2]. The comonomer feed molar ratios of thiophene to porphyrin

were 99:1 (PT-TPP1), 95:5 (PT-TPP5), 90:10 (PT-TPP10) and 80:20 (PT-TPP20). Yield: 70–75%. The resulting

copolymers had good solubility in common organic solvents, such as: CH2Cl2, CHCl3, THF. The weight-average

molecular weights (Mw) and polydispersity indices of copolymers were 5300(1.65) (PT-TPP20), 6000(1.66) (PT-

TPP10), 8800(1.67) (PT-TPP5), 12,000(1.7) (PT-TPP1). NMR results: for PT-TPP20: 1H NMR (CDCl3, 400 MHz,

ppm) d 8.84–8.81 (m, 8H), 7.74–7.50 (m, 30H, ArH), 7.30 (s, 1H), 1.80–0.50 (m, 77H),�2.87 (s, 2H). IR (KBr, cm�1):

3473 (N–H), 3058 (ph–H), 2900 (C–H), 1400 (C–H), 750 (ph–H).

Bulk-heterojunction films of PT-TPP20 and TiO2 were made by spin-casting a solution of PT-TPP20 (5 mg/mL)

and Ti(OC3H7)4 (80 mL/mL) (v:v = 1:0, 1:0.25, 1:1) in dry THF on the quartz substrate. Ti(OC3H7)4 reacted with

moisture, and turned to TiO2, then the samples were kept in the dark overnight [4].

X. Xu et al. / Chinese Chemical Letters 18 (2007) 879–882880

Scheme 1. Synthetic routes of monomers and polyfluorene copolymers. Reagents and conditions: (a) C12H25Br, Mg, (CH3CH2)2O, Ni(dppp)2Cl2;

(b) NBS, CHCl3:CH3COOH (v:v = 1:1); (c) pyrrole, CF3COOH, room temperature; (d) benzaldehyde, BF3�O(Et)2, DDQ, room temperature; (e) 1,

3, Pd(PPh3)4, 2 mol/L K2CO3, toluene, reflux for 72 h.

Fig. 1. UV–vis absorption spectra of polyfluorene copolymers and TPP monomer.

Fig. 1 showed the UV–vis absorption spectra of polyfluorene copolymers and TPP monomer. Polyfluorene

copolymers had strong absorption at 400–450 nm region for the Soret-band of TPP and absorption at 516, 552, 590 and

647 nm region for the Q-band of TPP. The absorption peaks of fluorene and thiophene around 400 nm, so polymers

appeared redshifted absorption in comparison with fluorene and thiophene obviously, and nearly cover 400–700 nm

visible region. The spectra of copolymers were similar to that of TPP above 450 nm, indicating that TPP unit had been

introduced into the main chains of copolymers, and fluorene and thiophene units in the polymers had no influence on

the absorption of the TPP unit. It was worth noting that with the increase of TPP feed composition in the copolymers,

the absorption peaks were not different obviously, but the intensity of absorption increased significantly.

The electrochemical behaviors of the copolymers were investigated by cyclic voltammetry as shown in Fig. 2.

According to empirical formulas (EHOMO = IP = �e(Eoxonset + 4.4) eV and ELUMO = EA = �e(Eredonset + 4.4) eV)

X. Xu et al. / Chinese Chemical Letters 18 (2007) 879–882 881

Fig. 2. Cyclic voltammograms of polyfluorene copolymers.

Table 1

Electrochemical parameters of the polyfluorene copolymers

Polymer Eredonset (V) Eoxonset (V) LUMO (eV) HOMO (eV) Eg (eV)

PT-TPP1 �1.16 0.87 �3.24 �5.27 2.03

PT-TPP5 �1.12 0.89 �3.28 �5.29 2.01

PT-TPP10 �1.11 0.87 �3.29 �5.27 1.98

PT-TPP20 �1.11 0.85 �3.29 �5.25 1.96

Fig. 3. PL spectra of PT-TPP20:TiO2 bulk-heterojunctions with varying amounts of Ti(OC3H7)4. a: VPT-TPP20:VTiðOC3H7Þ4 = 1:0;

b: VPT-TPP20:VTiðOC3H7Þ4 = 1:0.25; c: VPT-TPP20:VTiðOC3H7Þ4 = 1:1.

[11], HOMO and LUMO levels calculated were shown in Table 1. The band gaps of copolymers calculated decreased,

respectively, with the increasing of TPP content in the copolymers.

The PL spectra of PT-TPP20:TiO2 bulk-heterojunctions with varying amounts of Ti(OC3H7)4 were displayed in

Fig. 3. Because the emission of polyfluorene was in the blue light region, the emission of the fluorene segment was

completely quenched for the copolymer. It indicated that the efficiency of energy transfer from fluorene unit to the

thiophene unit. So the emission peaks at 508 and 660 nm with a weak vibronic shoulder at 720 nm of the PL spectrum

of PT-TPP20 ‘a’ could be attributed to the emission of thiophene unit and TPP unit, respectively. The emission

intensity of thiophene unit was weaker than that of TPP unit. It was due to the result of energy transfer from thiophene

unit to the TPP unit. The PL spectrum of ‘b’ exhibited similar emission peaks in comparison to ‘a’, but it gave PL

spectrum with weaker intensity in 380–800 nm. The PL of bulk-heterojunction decreased with increasing amount of

Ti(OC3H7)4 compared to that of a pristine copolymer film, and was significantly quenched in the 50% blend. The PL of

polymer:TiO2 was quenched in the blend, indicative of rapid and efficient separation of photoinduced electron-hole

pairs with electrons on the TiO2 and holes on the polymer [12]. Further studies on the application as the photovoltaic

materials are now in progress.

Acknowledgment

This work was supported by the Key Foundation of Education Ministry of China (No. 105142).

References

[1] K.J. Jiang, Y.L. Sun, K.F. Shao, J.F. Wang, et al. Chin. Chem. Lett. 14 (10) (2003) 1093.

[2] F. Wang, J. Luo, X. Yang, et al. Macromolecules 38 (6) (2005) 2253.

[3] C.L. Huisman, A. Goossens, J. Schoonman, Synth. Met. 138 (2003) 237.

[4] P.A. Hal, M.M. Wienk, J.M. Kroon, et al. Adv. Mater. 15 (2) (2003) 118.

[5] J. Morgado, F. Cacialli, R.H. Friend, et al. Chem. Phys. Lett. 325 (2000) 552.

[6] Q. Hou, Y. Zhang, F.Y. Li, J.B. Peng, Y. Cao, Organometallics 24 (19) (2005) 4509.

[7] L.G. Tong, X.G. Jian, et al. Youjihuaxue (Chin. J. Org. Chem., Chinese) 24 (1) (2004) 67.

[8] R.S. Loewe, R.D. McCullough, Chem. Mater. 12 (10) (2000) 3214.

[9] Q. Peng, Z.Y. Lu, Y. Huang, M.G. Xie, et al. Macromolecules 37 (2) (2004) 260.

[10] C.J. Raymond, M.T. James, Org. Lett. 2 (2) (2000) 111.

[11] A.K. Agrawal, S.A. Jenekhe, Chem. Mater. 8 (2) (1996) 579.

[12] G. Yu, A.J. Heeger, J. Appl. Phys. 78 (7) (1995) 4510.

X. Xu et al. / Chinese Chemical Letters 18 (2007) 879–882882