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Journal of Electroanalytical Chemistry 587 (2006) 308–313

ElectroanalyticalChemistry

Electrosynthesis of soluble polythiophene derivatives with highthermal stability

Zhuo Zhang, Hong Tang, Xiao Liang, Gaoquan Shi *

Department of Chemistry and Key Laboratory of Organic Optical Electronics and Molecular Engineering, Tsinghua University, Beijing 100084, PR China

Received 27 July 2005; accepted 27 November 2005Available online 4 January 2006

Abstract

Poly(3-[4-(4-pentyl-cyclohexyl)-phenyl]-thiophene) (PPeChPhT) and poly(3-[2-fluoro-4-(4-pentyl-cyclohexyl)-phenyl]-thiophene)(PFPeChPhT) films were electrosynthesized by direct oxidation of the corresponding monomers in the mixed electrolytes of boron tri-fluoride diethyl etherate and ethyl ether (4:1, by volume). As grown PPeChPhT and PFPeChPhT films are in doped states and have con-ductivities of 0.10 and 0.11 S cm�1, respectively. These two polymers in dedoped states are soluble in usual organic solvents such astetrahydrofuran and chloroform, etc. The molar masses of the polymers were measured to be about 3400 g with dispersity of 1.32(PPeChPhT) and 9300 g and dispersity of 1.51 (PFPeChPhT). Fluorescent spectral results showed that the polymers were strong yel-low-green light emitters. Thermal degradation analysis results indicated that these polymers had high thermal stability with decompo-sition temperatures of about 500 �C.� 2005 Elsevier B.V. All rights reserved.

Keywords: Polythiophene derivatives; Boron trifluoride diethyl etherate; Electrochemical polymerization; Conjugated polymer; Thermal stability

1. Introduction

Conjugated polymers have been widely applied in fabri-cation of microelectronics and optoelectronics, such asorganic transistors [1], light emitting diodes (LEDs) [2],solar cells [3], actuators [4], and electrochromic windows[5]. Among them, polythiophene (PT) and its derivativeshave received a great deal attention due to their easinessfor processing, structural versatility and environmental sta-bility [6–8]. Poly (3-phenylthiophene) and poly (3-alkylthi-ophene) are the most studied polythiophene derivatives.The phenyl groups introduced into the backbone of PT sta-bilize the conjugated p-bonds and the gratified alkyl groupsimprove the solubility of the polymers. However, the ther-mal stability of polythiophene and its derivatives is usuallylow and their decomposition temperatures are reported tobe about 300 �C [9,10].

0022-0728/$ - see front matter � 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.jelechem.2005.11.027

* Corresponding author.E-mail address: gshi@mail.tsinghua.edu.cn (G. Shi).

In this paper, we desire to report electrochemicalpolymerization of 3-[4-(4-pentyl-cyclohexyl)-phenyl]-thio-phene (PeChPhT) and 3-[2-fluoro-4-(4-pentyl-cyclohexyl)-phenyl]-thiophene (FPeChPhT) in the mixed electrolyteof boron trifluoride diethyl etherate (BFEE) and ethyl ether(EE). The resulting polymers in dedoped state are solublein usual organic solvents such as tetrahydrofuran and haveexcellent high thermal stability with decomposition temper-atures of about 500 �C. Here, we chose boron trifluoridediethyl etherate (BFEE) as the electrolyte because of themonomers can be polymerized at low oxidation potentialsin this medium [11,12].

2. Experimental

2.1. Materials

The monomers were synthesized in our lab by Suzukicoupling reaction [13] with Pd(PPh3)4 as the catalyst(Scheme 1). 3-Bromothiophene (>95%, Shanghai Chem.

Scheme 1. Synthesis of 3-[4-(4-pentyl-cyclohexyl)-phenyl]-thiophene and 3-[2-fluoro-4-(4-pentyl-cyclohexyl)-phenyl]-thiophene.

Z. Zhang et al. / Journal of Electroanalytical Chemistry 587 (2006) 308–313 309

Reg. Co., Shanghai, China), BFEE (Beijing ChangyangZhenxing Chem. Co. Ltd., Beijing, China) and ethyl ether(Beijing Chemical Plant, Beijing, China) were purified bydistillation before use. Tetrabutylammonium tetrafluoro-brate (TBA- TFB, >99%) was purchased from AcrosOrganics and used directly.

2.2. Monomer synthesis

2.2.1. 3-[4-(4-Pentyl-cyclohexyl)-phenyl]-thiophene

(PeChPhT)

About 4.7 g (28.8 mmol) 3-bromothiophene solved in50 mL benzene, 9 g (84.9 mmol) Na2CO3 solved in 45 mLH2O, 9.3 g (33.9 mmol) 4-(4-pentyl-cyclohexyl)-phenyl-boric acid solved in 60 mL alcohol, were added into a200 mL flask. After deaerated by dry nitrogen bubblingfor 5 min, 1 g catalyst was added into the mixture andheated with reflux for 24 h under nitrogen. After coolingto room temperature, the reaction mixture was diluted withbenzene. The water phase was extracted by CH2Cl2. Theorganic phase was washed with saturated aqueous NaClsolution, dried over Na2SO4 and concentrated. The roughproduct of PeChPhT was purified by column chromatogra-phy (silica gel, mineral ether) and re-crystallization to give4.8 g (15.4 mmol, 51% yield) pure product.

GC–MS: 99.9%, m/z: 312 (100); 186(85).1H NMR (CDCl3, ppm): d 0.90(3H, t); 1.0–1.5 5(13H,

m), 1.90(4H, s), 2.48(1H, t), 7.22–7.53(7H, m).

2.2.2. 3-[2-Fluoro-4-(4-pentyl-cyclohexyl)-phenyl]-thiophene (FPeChPhT)

FPeChPhT was also synthesized through the sameexperimental procedures adopted for PeChPhT and 3-[2-fluoro-4-(4-pentyl-cyclohexyl)-phenyl]-thiophene was usedas one of the starting materials, 67% yield.

GC–MS: 99.9% , m/z: 330(87); 204(100).1H NMR (CDCl3, ppm): d 0.89(3H, t); 0.99–1.77(13, m);

1.89(4H, t); 2.48(1H, t, t); 7.00(1H, t); 7.26–7.55(5H,m).

2.3. Electrochemical polymerization

The electrochemical polymerization and examinationswere performed in a one-compartment cell by the use of

a Model 273 potentiosat–galvanostat (EG&G PrincetonApplied Research) under computer control. In the processof electrochemical polymerization of PeChPhT, platinumsheet was used as the working electrode and a stainless steelsheet (AISI 304) was used as the counter electrode. The dis-tance between these two electrodes was kept to be 10 mm.The surface areas of the working and counter electrodeswere 2 and 3 cm2, respectively. The typical electrolyte wasa mixture of BFEE and diethyl ether (4:1, by volume).The concentration of monomer was 7.5 mmol/L. All thesolutions were deaerated by dry nitrogen bubbling for5 min and a slight nitrogen overpressure was maintainedduring the experiments.

All potentials were referred to an Ag/AgCl electrodeimmersed in the solution directly. The thickness of the filmswas controlled by the total charge density passed throughthe electrochemical cell. After polymerization, the filmswere washed repeatedly with ethyl ether to remove the elec-trolyte and monomer. Polymer films were dedoped with25% ammonia for 3 days, and then washed repeatedly withdistilled water, alcohol and acetone. Finally, they weredried under vacuum at 70 �C for 24 h.

2.4. Characterizations

The infrared (IR) spectra were recorded on a GX FT-infrared spectrometer (Perkin–Elmer Company) by usingthe KBr pellets of the polymers. Raman spectra were car-ried out by using a RM2000 microscopic confocal Ramanspectrometer (Renishaw PLC, England) employing a633 nm laser beam and a charge coupled detector (CCD)with 4 cm�1 resolution. The spectra were recorded by usinga 20· objective and the laser power was kept very low(�0.1 mW) to avoid destruction of the polymers.

UV–Vis spectra were taken out by using an ultra-spec 4000 spectrometer (Biotech). Fluorescence spectrawere performed by the use of a LS 55 luminescencespectrometer (Perkin–Elmer). Scanning electron micro-graphs were taken out by using a KY2800 electron micr-ographer (Scientific Instrumental Plant of ChineseAcademy of Sciences, China). Thermal tests were carriedout on a TGA2050 thermal degradation analyzer (TAInstruments, N2, 20 �C/min). The molar masses of the

310 Z. Zhang et al. / Journal of Electroanalytical Chemistry 587 (2006) 308–313

polymers were measured by using a HP1100 gel perme-ation chromatography at 30 �C and polystyrene was usedas the standard. The conductivity of the as-grownpolymer films was measured by conventional four-probetechnique.

3. Results and discussion

3.1. Electrochemical polymerization

Fig. 1A shows the cyclic voltammograms (CVs)of 7.5 mmol/L 3-[4-(4-pentyl-cyclohexyl)-phenyl]-thiophene(PeChPhT) in the mixed electrolyte at stainless steel elec-trode. As can be seen from the first cycle of the CVs, theonset of PeChPhT oxidation was initiated at 1.2 V. Duringthe process of CV scanning, the color of the electrolyte nearthe working electrode surface changed gradually from paleyellow to dark blue because of part of the monomer wasoxidized into oligomers, which were dissolved or dispersedin the solution. Meanwhile, a polymer film was depositedon the working electrode surface (grey violet to black asthe film thickens). The polymer was reduced and oxidizedin the potential scale of 0.3–1.2 V. The increase of redoxwave currents indicated the accretion of the deposits onthe electrode. The potential shift of the wave current max-imum provides the information about the increases of theelectrical resistance in the polymer film and the over-poten-tial required for overcome the resistance [14]. The cyclicvoltammograms (CVs) of 7.5 mmol/L 3-[2-fluoro-4-(4-pen-tyl-cyclohexyl)-phenyl]-thiophene (FPeChPhT) in themixed electrolyte at stainless steel electrode are shown inFig. 1B. The oxidation potential of the monomer was mea-sured to be at 1.35 V. This value is 0.15 V higher than thatof PeChPhT, mainly due to the fluorine group decreasedthe electron density of the monomer.

After electrochemical polymerization of PeChPhT at aconstant current density of 0.52 mA/cm2 for 9.36 C/cm2,

Fig. 1. (A) Cyclic voltammograms of 7.5 mmol/L PeChPhT and (B) cyclic volscan rate of 50 mV/s.

a polymer film with a thickness of 42 lm was achieved.The conductivity of the film was measured to be0.10 S/cm. The PFPeChPhT film electrosynthesizedpotentiostatically at 1.6 V for 18,000 s was measured tobe 45 lm thick and its conductivity was measured tobe 0.11 S/cm. Accordingly, their conductivity was about10 times that of poly(3-phenylthiophene) (�0.01 S/cm)[15].

3.2. Electrochemistry of polymer films

The electrochemical properties of the as-grownpolymer films were studied in a monomer free electrolyteof acetonitrile +0.1 mol/L TBATB (Fig. 2A). ThePPeChPhT film was stably reduced in the potential of0.3 V without obvious oxidation wave, while a coupleof redox waves were observed in the potential scale of0.8–1.5 V for a PFPeChPhT film. As can be seen fromFig. 2B, the CV of PFPeChPhT film is not stable. Thisis mainly due to that the polymer in dedoped state is sol-uble in the solvent.

3.3. Spectral studies

The FT-IR spectrum of a dedoped PPeChPhT film ispresented in Fig. 3A. The bands at 3056 and 3023 cm�1

are attributed to the vibrations of the thiophene rings.The bands at 2918 and 2846 cm�1 are assigned to the C–Hstretching vibrations of the methylene groups in alkylchains, while the 2952 and 2869 cm�1 bands are ascribedto the C–H stretching vibration of the terminal methyl.The bands of 1612, 1506 and 1446 cm�1 are typical frame-work vibrations of the phenyl ring. The band around823 cm�1 is due to the C–H out-of-plane bending vibra-tions of a 2,3,5,-trisubstituted thiophene ring [16]. TheFT-IR bands of Fig. 3A also can be found in Fig. 3B,the IR spectrum of PFPeChPhT. Furthermore, a band

tammograms of 7.5 mmol/L FPeChPhT in BFEE + EE (4:1) solution at a

Fig. 2. Cyclic voltammograms of polymer film in acetonitrile +0.1 mol/L TBATFB: (A) a PPeChPhT film coated Pt electrode; (B) a PFPeChPhT filmcoated stainless steel electrode, with a potential scan rate of 25 mV/s.

Fig. 3. FT-IR spectra of depoded PPeChPhT (A) and (B) depodedPFPeChPhT; Raman spectra of: (C) a PPeChPhT film and (D) aPFPeChPhT film.

ig. 4. UV–Vis spectrum of dedoped: (A) PPeChPhT and (B)FPeChPhT dissolved in THF. The concentrations of PPeChPhT and

PFPeChPhT are 0.12 and 0.11 g/L, respectively.

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associated with C–F vibration at 1242 cm�1 is alsoobserved in Fig. 3B.

Figs. 3C and D illustrate the Raman spectra ofPPeChPhT and PFPeChPhT films, respectively, by excita-tion at 633 nm. Both spectra have similar spectral features.As can be seen from this figure that the band at ca.1611 cm�1 is assigned to the C@C stretching vibration ofphenyl rings, the bands at 1546 and 1443 cm�1 are attrib-uted to the stretching modes of m(Ca@Cb) vibration ofquinoid unit of the polymer chain. The band at 1346

cm�1 is associated with the Cb–Cb ring stretching. Andthe Ca–Ca in-plane stretching band is appeared at1201 cm�1. The sharp band at around 1108 cm�1 is origi-nated from the totally symmetric in-plane wag of Cb–Hin the neutral state. The infrared and Raman spectralresults described above confirm that the polymers had beensuccessfully electrosynthesized [15].

Fig. 4 shows the UV spectra of dedoped PPeChPhT (A)and PFPeChPhT (B). As can be seen from this figure,PPeChPhT has a broad absorption band with maximum atapproximately 450 nm and that of PFPeChPhT is approxi-mately at 470 nm. GPC results indicated that the numberaverage molecular weight of PPeChPhT was 3400 g/molwith a dispersity of 1.23 and that of PFPeChPhT was9300 g/mnol with a dispersity of 1.51.

Fig. 5 is the fluorescence spectra of the THF solutions ofthe dedoped polymers. The excitation spectra of these two

FP

Fig. 5. Fluorescence spectrum of: (A) dedoped PPeChPhT and (B) dedoped PFPeChPhT dissolved in THF. The concentrations of PPeChPhT andPFPeChPhT are 0.12 and 0.11 g/L, respectively.

Fig. 6. TGA curve of: (A) PeChPhT and a PPeChPhT film; (B) FPeChPhT and a PFPeChPhT film.

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polymers are similar to their UV–Visible spectra. The fluo-rescence spectrum of PPeChPhT solution represents astrong and broad emission band at 566 nm, while the emis-sion band of PFPeChPhT solution is at 571 nm. The fluo-rescence spectral results indicate that the two polymers areyellow-green light emitter.

3.4. Thermal Properties

Fig. 6A shows me TGA curves of PeChPhT monomerand a PPeChPhT film. It is clear from this figure that themonomer decomposes at the temperatures higher than200 �C. In contrast, the polymer does not decompose evenit is heated up to 500 �C. The decomposition temperatureof the polymer is about 200 �C higher than that ofpoly(3-phenylthiophene) [15] and even higher than that ofpoly(p-phenylene) [17]. The TGA curve of a PFPeChPhTfilm also indicates that the polymer has an excellentthermal stability (Fig. 6B). The high decomposition

temperatures of these polythiophene derivatives indicatethey can be applied in a wide temperature scale.

4. Conclusion

Free-standing PPeChPhT and PFPeChPhT films withconductivity of ca. 0.1 S/cm were obtained by direct oxida-tion the corresponding monomers in the mixed electrolyteof boron trifluoride diethyl etherate and ethyl ether. Thededoped polymers are soluble in usual organic solventsand their solutions can emit strong yellow-green lights.The polymers exhibit excellent thermal stability with decom-position temperatures of about 500 �C, and this value isabout 200 �C higher than that of poly (3-phenylthiophene).

Acknowledgements

This research was financially supported by NationalNatural Science Foundation of China (Nos. 90401011,

Z. Zhang et al. / Journal of Electroanalytical Chemistry 587 (2006) 308–313 313

20374034, and 50225311), the National 863 Program ofChina (Contract No. 2003AA311100).

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