Polymer Journal, Vol. 27, No. 8, pp 773~779 (1995)

Synthesis of Fluoroalkoxy Substituted Polythiophene under Well-Controlled Electrosynthetic Conditions

Xuqing ZHANG, Xueming SHEN, Shiyong YANG, and Jingyun ZHANG

Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Feng/in Road, Shanghai 200032, People's Republic of China

(Received July 22, 1994)

ABSTRACT: Poly[3-(2,2,3,3-tetrafluoropropoxy)thiophene] was prepared by electrochemical polymerization under various conditions. Analyses of conductivity, electrochemical and spectroscopic properties as functions of current density (J) in the electrosynthesis showed that mean conjugation of polymer chains was noticeably extended at low current density. The above properties of the polymer also varied with different solvents and working electrodes. The electrosynthetic conditions determine to a large extent the molecular structure and physicochemical properties of the resulting polymer. KEY WORDS Polythiophene / Fluoroalkoxy Substituted Polythiophene / Current Density / Mean Conjugation Length /

Achieving practical applications of conduct­ electrosynthetic conditions is generally known ing polythiophenes (PTs) in molecular elec­ to be another strategy in the control of polymer tronic devices, solid-state batteries, chemically properties. modified electrodes, and sensors requires solu­ PTs substituted by alkoxy groups have been tion to some serious problems such as incon­ investigated by several research groups because venient processability and environmentally of their good solubility and low oxidation instable conductivity. 1 - 4 Advances in film potential, 13•14 However, due to the negative casting were made in the last few years through electronic effects of alkoxy side chains attached the manipulation of the monomer structure by to the thiophene ring, the conductivity and covalent grafting of various side chains. Thus electrochemical stability of the resulting poly­ 3-substituted thiophenes with alkyl, alkoxy, mer are significantly lower than those of un­ alkylsulphonate, and polyether chains and substituted polythiophene. Hence, it is neces­ their corresponding polymers have been de­ sary to offset the electronic effect of oxygen scribed in recent literature aiming at improving atom linked to the thiophene nucleus in order the solution processibility. 5 - 8 More recently, to achieve both the polymerizability of the polythiophenes 3-substituted by polyfluoro­ monomer and the electronic properties of the alkyl groups have also been reported with polymer obtained. It is anticipated that the improved environmental stability.9 •10 introduction of fluorine atoms to the alkoxy Furthermore, as a consequence of the di­ chain on the 3-position of thiophene will lead versity of the electropolymerization factors, to a new interesting material. In our previous which involves solvent, reagents, anode mate­ communication, 15 we described a novel poly­ rials and applied electrical conditions, and (thiophene) containing polyfluoroalkoxy sub­ complexity of the polymerization mechanism, stituent, namely poly[3-(2,2,3,3-tetrafluoro­ electrosynthetic conditions affect to a large propoxy)thiophene] (PTFPT), which incor­ extent the structure and properties of the porates the specifies of alkoxy and fluoroalkyl resulting polymers. 11•12 The optimization of groups and possesses good stability and solu-

773 X. ZHANG et al.

tion processability. We describe here the effects graphy (SiO 2/petroleum benzine, ethyl acetate) of the electrochemical factors of polymeriza­ and distillation. 3-(2,2,3,3-tetrafluoropro­ tion on the properties of PTFPT and optimiza­ poxy)thiophene-yield: 35%. Anal. Calcd for: tion of preparation conditions. C, 39.25%; H, 2.80%; S, 14.05%; F, 35.51 %. Found for: C, 39.26%; H, 2.84%; S, 14.94%; OCH2 CF2 CF2 H 1 F, 35.85%. H NMR (200 MHz, CDC1 3 , TMS, ppm): 4.30 (2H, t); 6.28 t, 6.02 t, 5.76 t, IH, -PZ)-1·- 6.34 (IH, dd, J= 1.5, 2.0), 6.78 (IH, dd, J= 1.5, (PTFPT) 3.0), 7.20 (lH, dd, J=3.0, 5.0). 19F NMR (60 MHz, CDC1 3 , CFC1 3 , ppm): -138 (2F, d); -124 (2F, d). IR (cm- 1); 3100w, 2900m, EXPERIMENT AL 1500 s, 1100--1230 s, 950 s, 850 s. MS: m/z, 214; Materials CsH5 SO +, 113; C4 H 3 SO +, 99; C4 H 5 S +, 85. UV-VIS: Amax, 310nm; log~, 2.57. The solvents, nitrobenzene (PhNO2), aceto­ nitrile (CH 3 CN), benzonitrile (PhCN), and Preparation of Polymer nitromethane (CH3NO2) were purified accord­ The electrosynthesized polymer was prepar­ ing to the previously described procedures. 16 ed in a one-compartment cell containing 0.1 1 The electrolytes, tetrabutylammonium tetra­ mo11- 1 monomer and 0.05 mo11- Bu4 NPF 6 fluoroborate (Bu4 NBF 4 ), tetrabutylammoni­ in various solvents. The solutions were de­ um perchlorate (Bu4 NC1O4 ), and tetrabutyl­ gassed by argon bubbling prior to polym­ ammonium trifluoromethanesulfonate (Bu4 - erization. Electropolymerization was per­ NCF 3 SO 3) were prepared using the reaction formed at ambient temperature under argon of tetrabutylammonium hydroxide and corre­ atmosphere at constant current of 0.5~10 sponding acids. After synthesis, the products mAcm- 2 • After polymerization, the film was were recrystalized twice. Lithium Perchlorate rinsed with acetone or n-hexane, dried m a (LiC1O 4) and tetrabutyl ammonium hexafluo­ nitrogen flow and stored in the dark. rophosphate (Bu4 NPF 6 ) were used as received from Tokyo Kasai Chemical Industry Co., Ltd. Physical Measurement Electrosynthesis and cyclic voltammetric All electrolytes were dried under vacuum were conducted using an before use. (CV) experiments EG&G PAR (Model 273) potentiostat/galva­ Monomer nostat equipped with a PAR 270 universal 3-(2,2, 3,3-tetrafluoropropoxy )thiophene programmer. UV-VIS spectroscopy was re­ was prepared from 3-bromothiophene and corded with a Perkin-Elmer Lambda 5 spectro­ sodium fluoro-propoxide in the presence of photometer. FTIR spectroscopy was per­ copper oxide and potassium iodide, 1 7 under formed on a Perkin Elemer 983 spectropho­ the following conditions: 100 mmol 3-bro­ tometer. Conductivities were determined by mothiophene was added to a suspension of the standard four-probe technique. 150 mmol of sodium fluoro-propoxide, 4.0 g of The polymers for CV experiment were grown copper oxide and 0.1 g of potassium iodide in on platinum electrodes (surface area= 0.1 cm2) 50 ml of toluene. The resulting mixture was using a deposition charge of 100 mC cm - 2 • CV then refluxed for 120 hours. After cooling the experiments were performed in a three-elec­ suspension was treated with I N HCI and ether. trode cell containing 0.1 mol 1- 1 LiClO 4 in dry

The organic phase was washed with water until CH3 CN. All potentials refer to a saturated neutral, dried and evaporated under vacuum. calomel electrode (SCE). Films for conduc­ The crude products were purified by chromato- tivity measurement have been prepared on

774 Polym. J., Vol. 27, No. 8, 1995 Synthesis of Fluoroalkoxy Substituted Polythiophene platinum flag electrodes (surface area= 2-3 Es.------~

2 2 cm ) using deposition charge of 7 .2 C cm - . A ~7 platinum foil was used as cathode. All platinum electrodes were cleaned in a H 20 2-H2 S04 (1: 4, v/v) solution and treated with an oxidizing flame before use. The polymers for UV-VIS spectroscopy were prepared under the 0 above conditions on platinum flag electrodes, rinsed with acetone, electrochemically undoped 4 6 8 ,o 12 by reversing the direction of the current flow current density mA/cm 2 2 (0.01 mA cm - ) until the potential of the Figure 1. Dependence of conductivity on current density working electrode was constant at about 1.1 V (deposition charge, 7.2Ccm- 2 ). vs. SCE, indicating the polymer was no longer being reduced and then dried under nitrogen current density, linkage defects in the polymer flow and dissolved in N,N-dimethylformamide chains greatly increased and could generate (DMF). distortions in other chains. 20 Moreover, the early formed polymers on the electrode could RESULTS AND DISCUSSION be degraded at overhigh current density. The correlation between the current density and the Effect of Electrosynthetic Current Density on conductivity of PTFPT films implies that an the Properties of the Polymer regular structure leading to highly conjugated Previous studies on electrosynthesized PTs planar conformation of the polymer chains is show that the current density (J) in polymeriza­ a critical factor for increased of conductivity tion exerts a strong effect on the properties of observed in polymer films. Another possible the polymers such as Epa (anodic peak potential cause for high conductivities may be modifica­ for the oxidation reaction), absorption maxi­ tion of the intramolecular structure along mum value (Amax) and conductivity. 18 But the polymer chains and particularly, longer mean relationships between electrosynthetic condi­ conjugation in the polymer films at low current tions and structure and properties of the density of polymerization. polymer have been scarely considered. 19 The Table I shows the electrochemical properties properties of the polymer PTFPT film prepared of PTFPT films synthesized under various at various current densities have been examined current densities. The data in Table I indicate at various current densities to get more detailed that lowering the current density leads to less information on structure-property relations. positive Epa· Such a conclusion is consistent Figure 1 shows the dependence of the con­ with previous observations reported for poly(3- ductivity on the current density (J). The highest methylthiophene). 21 Many authors show that conductivity (about 0.1 s cm - l) was reached by the oxidation potential, the position of the using 0.5mAcm- 2 . Below 0.5mAcm- 2 , there absorption maximum (Amax) and the electric were no polymers growing onto the electrodes. conductivity are closely related to the extent This suggests that the corresponding radicals of conjugation. 22 In Table I, the shift of Epa of monomers generated on Pt surface can toward less positive values when reducing the diffuse away from the electrode to form soluble current density appears corresponding to oligmers in solution when the applied current moving toward longer mean conjugations. density is quite low. It was also observed that Figure 2 depicts the FTIR spectrum of polymer higher current density led to more brittle film PTFPT electrosynthesized under various cur­ with lower conductivity. Beyond the optimal rent densities. The polymer synthesized under

Polym. J., Vol. 27, No. 8, 1995 775 X. ZHANG et al.

Table I. Electrochemical properties of PTFPT chains was mainly determined by the regio­ films synthesized in nitrobenzene at regularity of the polymer, i.e. the ratio of various current densities• a-a' /a-fJ linkages of thiophene units. It is well Current densi tyh known that the reactivity of the a position in Ecellc £pa Epcd the thiophene monomer is much higher than (V vs. SCE) (V VS. SCE) (V VS. SCE) mAcm- 2 that of the fJ position. The overhigh current density, which decreases the difference of the 0.5 2.65 1.05 0.94 1.0 2.79 1.07 0.92 reactivity between the a and fJ positions, leads 2.0 3.14 1.12 0.88 to a statistical increase of the number of a-fJ 5.0 4.07 1.15 0.84 couplings and hence to a decrease of the 10.0 4.58 1.20 0.82 regioregularity in the polymer. In curves (b ),

• Synthesized polymers using 0.1 mol 1- 1 monomer and (c), and (d), when the current density used for 1 0.05 mol l- Bu4 NPF 6 • b Cyclic voltammogram of PTFPT synthesizing PFTPT was turned up, a band 2 2 (deposition charge, 100 mC cm - on 0.1 cm Pt; electrolytic at 720 cm - l assigned to the o:-C-H out-of­ 1 medium, 0.1 mol 1- LiC1O4 /CH3CN; scanning rate, plane bending absorptions appeared and grew 20 m Vs- 1 . 'Cell potential measured by chronoamper­ metry. d Cathodic current peak. stronger. In the meantime, the band at 840 cm - 1 grew weeker. All these data indicate that there were a few /J-/J' or a-fJ defects in the polymer chain when the polymer was grown at relatively high current density. It is indicated that the presence of a larger number of the defects when applied overhigh current density could generate distortions both intramolecu­ larly and intermolecularly. The absorption spectra of the undoped PTFPT in N,N-dimethylformamide (DMF) as a function current density are given in Figure

4000 3000 2000 1000 400 3. These spectra and Table II show that the

wavenumber (cm-1) Amax shifted bathochromically from 367 nm to 529 nm when the current density decreases from Figure 2. FTIR spectra of neutral PTFPT synthesized at 2 • various current densities (J). a, 1=0.5mAcm- 2; b, l= 10 to 0.5 mA cm - Such a displacement of the 2.0mAcm- 2 ; c, 1=5.0mAcm- 2 ; d, l= 10.0mAcm- 2 . Amax indicates that the average length of con­ jugation on which the electron delocalization 0.5mAcm- 2 [curve (a)] had one band at occurs is considerably extended in films at low 3060 cm - l attributable to the /J( 4)-C-H bond current density. Furthermore, the color of the in the region of the aromatic C-H stretching polymer in DMF changes from pale yellow to vibration and no band attributable to o:(2 or amethyst violet (Table II) when the current 5)-C-H bond at 3110 cm - 1 . The spectrum also density is decreased. This indicates that various showed a band at 840 cm - 1 assigned to the lengths of conjugated segments exist at different C-H deformation vibration of a 2,3,5-trisub­ current densities. stituted thiophene ring. There did not appear to be any significant o:(2 or 5)-C-H out-of­ Effects of Solvents on the Properties of the plane bending absorptions in the spectrum. Polymer These results demonstrated that the polymer The solvent of the electrolytic medium exerts chain had a highly defined a-a' structure. The a strong effect on the structure and proper­ average conjugation length along the polymer ties of PTs films. The solvent must simultane-

776 Polym. J., Vol. 27, No. 8, 1995 Synthesis of Fluoroalkoxy Substituted Polythiophene

Table III. Properties of PTFPT synthesized in various solvents

b Epaa Epc Amax Conductivity Solvent (V vs. (V vs. SCE) SCE) nm scm- 1

2 PhNO2 I.OS 0.94 528 7.0xl0- 4 CH3CN 1.08 0.94 430 6.0 X 10- PhCN 1.10 0.94 420 _d _d CH3 NO2 1.14 0.92 398 4 PhNO2 0.61 0.51 525 9.0x 10-

• Cyclic voltammogram of PTFPT (deposition charge, I 00 mC cm - 2 on 0.1 cm2 Pt; electrolytic medium, 0.1 1 1 300 500 700 900 moll- LiC1O4 /CH3CN; scanning rate, 20mvs- ). >.. max (nm) b Polymer disolved in DMF. c Poly(3-propoxy)thiophene. d Film too brittle to be measured. Figure 3. Effects of current density (J) on the UV-VIS spectra ofundoped PTFPT in DMF under various current trochemical properties and UV-VIS spectro­ densities. a, J=0.SmAcm- 2 ; b, J= l.0mAcm- 2 ; c, scopic data of PTFPT synthesized in various J=2.0mAcm- 2 ; d, J=S.0mAcm- 2 ; e, J=7.5mAcm- 2 ; solutions. Nitrobenzene allowed the obtain­ f, J=I0.0mAcm- 2 • ment of compact and flexible films, with 1 Table II. UV-VIS spectroscopic data of PTFPT conductivity of about 0.1 s cm - . Acetonitrile in DMF synthesized in nitrobenzene under led to powdery deposits with poor conductivity various current densities• (6 x 10-4 scm- 1). Nitromethane and benzoni­ trile afforded slack deposits. The conductivities Current densityh Amaxc Color of these deposits could not be measured by the (Neutral state) mAcm- 2 nm four-probe method.

0.5 529 Amethyst violet Effects of Working Electrodes on the Properties 1.0 509 Amethyst violet 2.0 484 Brick red of the Polymer 5.0 378 Mineral yellow The physicochemical properties of the sur­ 7.5 369 Pale yellow face of the working electrode determine the 10.0 367 Pale yellow nature and strength of the bond between the • Electrochemical undoped by reversing the direction polymer and electrode, which can affect both of current flow (0.0 I mA cm - 2). b Deposition charge 7 .2 the polymerization process and the properties Ccm- 2 on 3cm2 Pt. cpoJymers dissolved in DMF. of the resulting polymer. Platinum, on which PTs are effectively electrosynthesized, is studied ously possesses a high dielectric constant to en­ in the literatures. 24 However, PTs have been sure the ionic conductivity of the electrolytic also deposited on other surfaces such as medium and an adequate stability to allow titanium or iron. Glass carbon, usually used the electropolymerization of the monomer. 2 3 to synthesize some organic compounds not Some rigorously anhydrous solvents of high applied to synthesize PTs before. Another dielectric constant and low nucleophilicity such electrode, nickel is always used to synthesize as acetonitrile, benzonitrile, nitrobenzene, ni­ fluoro-compounds. In the present study, we tromethane, and propylene carbonate, which investigated the influence of the anodes on the offer highest current efficiency, are usually used structures of the polymers. Figure 4 compares in electropolymerization of thiophene mono­ the CV of PTFPT films synthesized on Pt and mer and derivatives. Table III lists elec- glass carbon in acetonitrile. These curves and

Polym. J., Vol. 27, No. 8, 1995 777 X. ZHANG et al.

The properties of the surfaces of the electrodes exerted different influence on the polymeriza­ tion process. Platinum allowed instantaneous creation of initial nucleation sites, which contributed to the compactness and thus conductivity of the polymer. The formation of nucleation sites in the early steps of elec­ tropolymerization on the surface of the electrode has dramatic consequences for the overall stereoregularity and stacking order of the polymer chains. Platinum, which favors chain-growth process of electropolymerization, o 0.2 o.4 o.s o.s 1.2 1.4 allowed more ordered and longer conjugated V (vs.SCE) polymer chains. Compared with the polymer Figure 4. Cyclic voltammogram of PTFPT in 0.1 mo! I - 1 synthesized on the glass carbon, the polymer LiCIO4 /CH3CN (deposition charge l00mCcm- 2 ; scan synthesized on the platinum decreased the rate 20mCcm- 2). a, on platinum; b, on glass carbon. oxidation potential of the polymer and a bathochromic shift of the Amax and increased Table IV. Properties of PTFPT synthesized in conductivity. This appears consistent with the nitrobenzene with various working electrodes results of previous works on ITO surface. 22•26 a b Conductivity The initial potential is the determining factor Working EP• £pc Amax (V vs. (V vs. electrode controlling the density of initial nucleation sites scm- 1 SCE) SCE) nm and hence the cohesion and the conductivity Pt 1.08 0.94 528 7.0x 10- 2 of resulting films. We also obtained a PTFPT Glass carbon 1.30 I.I 1 - C - C film on nickel. But we could not get the Ni _d d 472 3.0 X J0- 4 electrochemical data because of the compli­ cated surface of nickel electrode. • Synthesized polymer in CH3CN (deposition charge, 100 mC cm - 2 ). h Synthesized polymer in PhNO2 and then dissolved in DMF. ° Film is too brittle. d Surface of Ni is Effects of Dopants on the Conductivity of the complicated. Polymer The nature of the dopant strongly affects the the data in Table IV show that Pt electrode morphology and conductivity of PTs. Highly yielded a sharpening of anodic wave, together conducting PTs are generally electrosynthe­ with a 220 m V of shift the anodic current peak sized using small anions from strong acids such (Epa) toward less positive potentials. It has as Cl04, PF6, BF4, and CF3 S03. We been shown that the electropolymerization of compared the effects of these four anions on thiophene is initiated by the adsorption of the properties of polymer PTFPT here. Table monomer onto the electrode surface. 25 The V shows the conductivities of PTFPT with initial step in the electropolymerization consists various dopants. The polymers were synthe­ of the formation of a monolayer of polymer sized under the optimal electrosynthetic current growing two-dimensionally, parallel to the density (J = 0.5 mA cm - 2 ) in PhN02 on Pt. We electrode surface. Thus, the differences found found that free-standing films could be only between glass carbon and platium anodes may obtained using Bu4 NBF4 and Bu4 NPF6 . Slack reflect that thiophene adsorbed more strongly and brittle films were deposited on the electrode on Pt than on glass carbon because Platinum using Bu4 NC104 and Bu4 NCF 3 S03 . From 22 has a larger number of potentially active sites. data in Table V Bu4 NBF 4 and Bu4 NPF 6 lead

778 Polym. J., Vol. 27, No. 8, 1995 Synthesis of Fluoroalkoxy Substituted Polythiophene

Table V. Conductivity of PTFPT with REFERENCES various dopants•

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