Tetrathienylsilane as a precursor of highly conducting electrogenerated polythiophene

277

J. Electroanal. Chem., 312 (1991) 211-283 Elsevier Sequoia S.A., Lausanne

JEC 01530 Short communication

Tetrathienylsilane as a precursor of highly conducting electrogenerated polythiophene Jean Roncali Loboratoire

Alain

des Matkiaux

Mol&ulaires,

Guy, Marc Lemaire,

Laboratoire

C.N.R.S.

Robert

de Chimie Organique, CNAM,

ER 241 2 rue Henri Dunant, 94320 Thiais (France)

Garreau

and Huynh

Anh Hoa

(/A 1103, 292 rue Saint-Martin,

75141 Paris Cedex 03 (France)

(Received 15 February 1991)

INTRODUCTION

The unique combination of the electronic, electrical and electrochemical properties of conducting polymers originates from the existence of an electronic r system delocalized over a large number of recurrent monomer units. As a consequence, the extent of this conjugated system constitutes the essential structural parameter which controls the magnitude of the energy bandgap, conductivity and electroactivity of these materials. Among the numerous conjugated polymers that have been synthesized and characterized over the past decade, poly(thiophene) derivatives are still subject to intensive studies owing to their high conductivity [l] and their structural versatility, which has led to the development of new classes of functional electroactive materials with original properties [2-91. In the absence of distortion of the polymeric chain which may result from the substitution of the thiophene ring, the effective mean conjugation length of poly(thiophene) is controlled by the stereoregularity of the LY--0~’couplings of monomer units during the electropolymerization. Although the higher reactivity of the (ILpositions leads mainly to the formation of (Y-_(y’linkages, the non-negligible reactivity of the /I positions allows the formation of a significant number of cu-/3’ couplings that disrupt the rr electron delocalization along the polymer chains and therefore shorten the mean conjugation length [lo]. Various strategies have been proposed to increase the stereoselectivity of the electropolymerization reaction. The most immediate involves optimization of the electrosynthesis conditions in order to improve the molecular definition and thus the mean conjugation length and conductivity of poly(thiophenes) [1,11,12]. An 0022-0728/91/$03.50

0 1991 - Elsevier Sequoia S.A.

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alternative method consists in preventing the formation of (Y-P’ couplings by the disubstitution of the p,/3’ positions. However, this approach is limited severely by the distortions of the conjugated backbone resulting from the steric interactions between substituents grafted on adjacent monomer units [13-151. A third possible approach involves activation of the (Y positions in order to increase their relative reactivity. Thus, we have shown recently that highly conducting and electroactive poly(thiophenes) can be obtained by electropolymerization of thiophene monomers with one or both OLpositions activated by trimethylsilyl groups [16]. However, whereas the electropolymerization of 2,5-bistrimethylsilylthiophene ((TMS),T) leads only to a limited improvement of the electrical and electrochemical properties compared with the polymer prepared from thiophene itself, the comparison of the conductivity and electroactivity of the polymers obtained from 2,5’-bistrimethylsilylbithiophene (TMS),BT) and bithiophene (BT) reveals a much larger relative improvement of these properties. Thus, instead of the powdery deposits obtained from BT, the electropolymerization of (TMS),BT produces compact free-standing films with a conductivity increased by ca. one order of magnitude, a doping level 50% larger and a 20 mV decrease of the oxidation potential, changes that are indicative of a significant increase of the mean conjugation length. The stronger effects of the activation of the a: positions observed with bithiophene can be attributed to two causes. On the one hand, theoretical and experimental work on pyrrole and thiophene oligomers has shown that increasing the length of the oligomer leads to (i) a decrease of the overall reactivity of the corresponding radical, which is expected to limit the progress of the electropolymerization reaction and hence the degree of polymerization; and (ii) a decrease of the relative reactivity of the (Y positions, which increases the probability of (Y--P’ couplings and thus shortens the mean conjugation length [17,18]. These combined effects explain the shorter mean conjugation length and lower conductivity of the polymer synthesized from BT [18]. As a consequence of this lower reactivity and (r-a’ selectivity of the BT radical, the activation of the (Y positions of the starting molecule by TMS groups is expected to produce larger relative improvements of the stereoregularity of the polymer in the case of the dimer than in the case of the monomer. On the other hand, elimination of the bulky trimethylsilyl group during electropolymerization can affect the morphology of the resulting polymer and hence its electrochemical and electrical properties. In this respect, a lower TMS/monomer ratio in (TMS),BT appears more favourable. These considerations lead to the conclusion that the ratio TMS/monomer unit has to be kept as low as possible and that a further reduction of this ratio could improve the properties of the resulting poly(thiophene). Taking into account the good results obtained with 3-chloro-2-trimethylsilylthiophene in which only one a! position was activated [16], tetrathienylsilane (T4Si), in which the TMS/monomer ratio is reduced to its minimal value, has been synthesized as a precursor for the electrosynthesis of poly(thiophene). It is shown that the use of T,Si as a substrate for the electrosynthesis of poly(thiophene) allows the electropolymerization to be performed with very low

219

T@i monomer concentrations electrochemical properties

and leads to further of poly(thiophene).

improvements

in the electrical

and

EXPERIMENTAL

T,Si was prepared by reaction of 2-thienyllithium with tetrachlorosilane in THF solution and recrystallized from cyclohexane, mp 136 o C. The oxidation potential of T,Si (1 x 10e4 M) was determined in acetonitrile containing 5 X 10-l M LiClO, by cyclic voltammetry at 500 mV/s. Electropolymerization was carried out according to the already described procedure [ll] from a synthesis medium involving 2.5 X lop3 M T,Si (1O-2 M equivalent thiophene) and 5 x 10e2 M Bu,NPF, in distilled nitrobenzene. The solution was degassed by argon bubbling prior to electropolymerization, which was performed under an argon atmosphere, either in galvanostatic or potentiostatic conditions. Films for electrochemical characterization were deposited on polished Pt disk electrodes of 0.07 cm* area, using a Pt wire as the counter-electrode and a saturated calomel electrode (SCE) as the reference. After film deposition, the polymer-coated electrodes were rinsed with acetone, dried and immersed in 1 X 10-l M LiClO, in dry acetonitrile. Electrochemistry was performed with a PAR 273 potentiostat-galvanostat and four-probe conductivities were measured on films deposited on a 2 x 2 cm massive platinum plate that was polished before each experiment. RESULTS

AND

DISCUSSION

The oxidation potential of T,Si was found to be 2.05 V vs. SCE, a value very close to that of thiophene itself. A 20 mV/s potential sweep performed in the synthesis medium shows that the current increases sharply at 1.68 V vs. SCE, as is observed with thiophene. The successive cyclic voltammograms resulting from recurrent potential sweeps performed in the synthesis medium (Fig. 1) show that the electropolymerization occurs steadily and that the resulting voltammograms exhibit a smaller peak separation than in the case of thiophene [19], suggesting faster charge and/or mass transport.

I

I

I* E/VGCE)

I

I

t

E/V(SCE:

Fig. 1. Successive cyclic voltammograms between -0.2 and 1.8 V vs. SCE corresponding to the electropolymerization of T,Si. Scan rate: 100 mV/s. Monomer 2.4X lOI M, Bu,NPF, 5 X10-’ M + nitrobenzene. Fig. 2. Cyclic voltammogram of poly T,Si, 100 mC/cm’ on Pt. in 1 x IO-’ h4 LiClO, +CH,CN.

Scan

rate: 20 mV/s.

It has been shown that no polymer can be formed when th~ophene concentrations as low as 2 X IO-* M are used [20]. It is therefore surprising that the electropolymerization reaction of T4Si occurs str~ghtfo~ardly with a substrate concentration of 2.5 x lOa M, which corresponds to 1 X lo-’ M equivalent thiophene. This result suggests that the low monomer concentration is counterbalanced by the particular structure of T,Si which allows the rapid coupling of neighbouring thiophene rings linked to the silicon atom. We have already shown that the electrical conditions applied for the electropolymerization of thiophene derivatives exert a determining effect on the structure and the electrical and electrochemical properties of the resulting polymer [ll,lZ]. Furthermore, we have also shown that the variation of the doping level, calculated from the amount of charge exchanged reversibly during voItammet~~ cycles, constitutes a reliable criterion for the optimization of the electrosynthesis conditions [19], There-

281 TABLE

1

Electrochemical data for poly(thiophene) films electrosynthesized on Pt from T,Si in galvanostatic and potentiostatic conditions

Jappl /M 2 3 5 10

cm ’

E/V(SCE)

Qd/mC cm-*

E,, /V(SCB)

Q,/mC

1.88 2.00 2.20 3.00

100.9 101.43 100.00 100.00

0.86 0.84 0.83 0.82

8.70 9.23 9.48 10.83

18.9 20 21 24.3

100.00 102.57 101.14 102.57 103.71 105.86

0.86 0.83 0.82 0.84 0.84 0.85

7.71 10.03 11.26 10.87 10.90 10.51

16.7 21.7 25 23.7 23.5 22

1.68 2.50 2.80 3.00 3.20 3.50

a a a a = a

cm-’

Y/S

fore, in order to define the optimal electrosynthesis conditions, the electropolymerization was carried out using both galvanostatic and potentiostatic conditions with increasing values of the applied current density or potential, and the resulting polymer films were characterized by cyclic voltammetry. The results listed in Table 1 show that when the current density is increased from 2 to 10 mA/cm*, the doping level (v) increases from 18.9 to 24% while the potential of the anodic current peak (Ep4) shifts from 0.86 to 0.82 V vs. SCE. The results obtained in potentiostatic condrtrons are consistent with the previous ones and show that both the maximum doping level (25%) and the less positive value of E,, (0.82 V) are obtained with the films prepared between 2.8 and 3 V. A further increase of the applied potential leads to a decrease of y and an increase of Epa, possibly because of the degradation of the deposited material or of that of the electrolyte or solvent. Such differences between the oxidation potential of the substrate and the optimum electrosynthesis potential have already been observed with other thiophene derivatives [ll-13,191. Comparison of the electrochemical data of the polymer synthesized from T,Si with those of the poly(thiophenes) prepared from thiophene and (TMS),T (Table 2) shows that the use of T,Si as a precursor leads simultaneously to a further increase of y from 21 to 25% and to a 30 mV decrease of E,,. These results suggest that the

TABLE 2 Electrochemical data for polymers prepared from thiophene (T), (TMS)xT and T,Si Substrate

%, /V(SCE)

Y/W

T (TMS);T T,Si

0.87 0.85 0.82

20 21 25

282

reduction of the silicon/monomer ratio in the activated precursor leads to a polymer with a more regular and more conjugated structure. The high definition of the cyclic voltammogram of the polymer prepared in optimal conditions (Fig. 2) gives further support to this conclusion. Attempts to confirm this extension of the mean conjugation length by UV-visible absorption spectroscopy were unsuccessful owing to the low polymer yields obtained when using transparent indium-tin oxide electrodes (ITO). Although further work is required to clarify this point, a possible explanation could involve the poor efficiency of the adsorption of T,Si on ITO. As a matter of fact, it has been shown that the electropolyme~zation of thiophene is preceded by the adsorption of the monomer onto the electrode surface and that the initial step in the electropolymerization involves the formation of a monolayer of polymer growing two dimensionally parallel to the electrode surface [21J. In this context, it seems reasonable to assume that the tetrahedral structure of T,Si is less favourable for efficient adsorption than the planar structure of thiophene. Conductivity measurements on films deposited on massive platinum yielded values exceeding 200 S/cm. These results, which rank among the highest conductivities obtained for poly(thiophene) [22], appear to be consistent with the improved molecular definition of the polymer structure already indicated by the electrochemical data. In summ~, these preliminary results confirm that the electr~esilylation of monomers activated at the LYposition constitutes an interesting method of electrosynthesis of conducting polymers. Furthermore, the particular structure of T,Si allows the electrosynthesis to be performed at a very low substrate concentration while the reduction of the silicon/monomer ratio to its minimum value leads to further improvements in the electrical and electrochemical properties of electrogenerated poly(thiophene). REFERENCES 1 J. Roncah, A. Yassar and F. Gamier, J. Chem. Sot., Chem. Commun., (1988) 581. 2 M. Lemaire, D. Delaboughse, R. Garreau, A. Guy and J. Roncaii, J. Chem. Sot., Chem. Commun., (1988) 658. 3 D. Kotkar, V. Joshi and K. Gosh, J. Chem. Sot., Chem. C’ommun., (1988) 917. 4 R. Mirrazaei, D. Parker and H.S. Munro, Synth. Met., 30 (1989) 265. 5 J. Roncali. R. Garreau, D. Delabouglise, F. Gamier and M. Lemaire, J. Chem. Sot.. Chem. Commun., (1989) 679. 6 J. Grimshaw and SD. Perera, J. Electroanal. Chem., 278 (1990) 287. 7 J. Roncah, H. Korri Youssoufi. R. Garreau, F. Gamier and M. Lemaire, J. Chem. Sot., Chem. Commun., (1990) 414. 8 P. Bauerle and K.W. Gaudl, Adv. Mater., 2 (1990) 18.5. 9 J. Roncali, R. Garreau and M. Lemaire, J. Electroanal. Chem., 278 (1990) 373. 10 D. Delabouglise, R. Garreau, M. Lemaire and J. Roncah, New J. Chem.. 12 (1988) 155. 11 J. Roncali, R. Garreau, A. Yassar, P. Marque, F. Garnier and M. Lemaire, J. Phys. Chem.. 91 (1987) 6706. 12 A. Yassar, J. Roncah and F. Gamier, Macromol~ules, 22 (1989) 804.

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