Synthesis and electropolymerization of 3

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the single coupling of a terminal chlorosilane group. Despite the presence of a .... Distillation afforded 4.56 g (80%) of a colorless liquid,. Eb,, 140°C n;": 1.4125, ...
ADVANCED MATERIALS

Communications

Synthesis and Electropolyrnerization of 3-(3,3,4,4,5,5,6,6,6-Nonafluorohexyl1-dimethylsilyl thiophene): Reduction of Steric Hindrance in 3-Substituted Poly(thiophenes) by Fluorophilic Interactions

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By Christine Thobie-Gautiev, Alain Guy, Alain Gorgues, Michel Jubault, and Jean Roncali* The controlled modification of the structure and properties of conducting polymers in view of specific applications has attracted much research effort."] In this context, conjugated poly(thiophenes), PTs, have been the focus of particular attention owing to their environmental stability and to the structural versatility of the thiophene ring, which allows the design of tailored conducting polymers combining the electronic and electrochemical properties of the 71-conjugated system to specific functions.['] From this viewpoint, fluoropolymers are of special interest. As a matter of fact, these materials are generally known for their low coefficient of friction, high thermal and chemical stability, and hydro- and lipoph~bicity.[~l Therefore, the association of these properties with those of conjugated PTs could lead to novel materials of potential fundamental and technological interest. Although the first examples of poly[3-(polyfluoroalkyl thiophenes)] have been recently d e ~ c r i b e d ,unlike ~ ~ ] 3-alkylthiophenes, the preparation of fluorinated thiophene monomers is not straightforward, and requires multistep syntheses.[51 We now report the synthesis of a new 3-polyfluoroalkylthiophene monomer in which a polyfluoroalkyl chain is attached at the 3-position of the thiophene ring by means of the single coupling of a terminal chlorosilane group. Despite the presence of a bulky dimethylsilyl group at its 3-position, this monomer can be readily electropolymerized, leading to a hydrophobic conjugated polymer with doping level and conductivity comparable to those of its alkyl-substituted analogue. 3-(3,3,4,4,5,5,6,6,6-nonafluorohexyl-l-dimethy~silyl thiophene), 1, was prepared by the reaction of dimethyl(3,3,4,4,5,5,6,6,6-nonafluorohexyl)chlorosilane, 2, with 3thienyllithium obtained by reaction of n-butyllithium with 3-bromothiophene (Scheme 1). M Single scan voltammetry at 100 mVs-' in a 2 x solution of I in MeCN shows an oxidation wave peaking at 2.15V versus a standard calomel electrode (SCE). The slight anodic shift of this value compared to that of thiophene contrasts with the small + I effect of the silyl group on the

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1 Scheme 1. The synthesis of 1.

oxidation potential.[61Such a slight anodic shift, which has been previously observed when increasing the length of the alkyl chain in poly(3-aIkylthiophene~),~'~ may result from the lower diffusion coefficent of thiophenes containing bulky substituents.['' Figure 1 shows the cyclic voltammograms resulting from the application of recurrent potential scans in a 0.1 M solution of 1 in nitrobenzene. Upon repetitive cycling, a new redox system, with a broad anodic wave in the 0.70-1.50V region and a reduction wave in the 1.50-0.30V range, progressively emerges, which corresponds to the doping-undoping process of the deposited material. With the develop-

[*] Dr. J. Roncali Laboratoire des Materiaux MolBculaires, CNRS UPR 241 2 rue Henry Dunant, F-94320 Thiais (France) Prof. A. Guy Laboratoire de Chimie Organique, CNAM, UA 1103 292 rue Saint-Martin F-75003 Paris (France)

Dr. C. Thobie-Gautier, Prof. A. Gorgues, Prof. M. Jubault Laboratoire de Chimie Organique Fondamentale et Appliquee Universite d' Angers 2 Bd. Lavoisier, F-49045 Angers Cedex (France)

Adv. Muter. 1993, 5, N o . 9

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Fig. 1. Electropolymerization of 1 by recurrent potential scans between -0.30 and 2.20 VjSCE. Electrolytic medium 0.1 M 1 0.1 M Bu,NPF,/nitrobenzene.

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VCH Verlagsgesellschujt mbH, 0-69469 Weinheim, 1YY3

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ADVANCED MATERIALS ment of this new system, a small reduction wave assigned to the reduction of the protons resulting from the coupling of the cation radicals of 1 appears around 0.0 V. The intensity of this wave decreases then with progressive coverage of the entire platinum surface by the as-grown polymer. The similarity of these electrodeposition curves with those recorded during the electropolyinerization of thiophene under the same condition^^^] indicates that the conductivity of the growing polymer (poly-lj is high enough to sustain the electropolyinerization process. Figure2 shows the cyclic voltammograms of a film of poly-1 deposited potentiostatically on Pt at the oxidation potential of the monomer. The symmetry of the anodic and = 1.I 8) and the small peak separation cathodic waves (Ipa/lpc ( A E , = 60 mV) indicate a highly reversible redox system.

Communications

similar or even larger than that of its alkyl-substituted analogue. Unfortunately, all attempts to confirm this hypothesis by UV-visible absorption spectroscopy remained unsuccessful. Electrodeposition on I T 0 or Sn0,-coated electrodes using galvanostatic or potentiostatic conditions, potential scans or potential pulses, produces uneven deposits growing on preferential nucleation sites (essentially on the edges of the electrodes). Such problems which have been previously encountered with polyfluorinated 3-alkyithiophene~,[~’ could result from the strong hydrophobicity of these substrates which renders their grafting onto an hydrophilic surface difficult. The highly hydrophobic character of poly-1 is confirmed by the comparison of the voltammograms recorded in both organic and aqueous media. Thus, as shown in Figure 3, replacing the MeCN electrolytic solution by an aqueous medium leads to a complete loss of electroactivity.

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Fig. 3. Cyclic voltammograms of poly-I. Deposition charge 500 mCcrn-’ on Pt. scan rate 50 mVs-‘; a) in 0.1 M LiCIO,/MeCN; b) in 0.1 M LiClO,/H,O.

E V/SCE

Fig. 2. Cyclic voltammograms of poly-l in 0.1 M LiClOJMeCN. Deposition charge jOOrnCcrn-’ on Pt. scan rate 20 Lo 1SOmVs-’ by increments of 20 mVs- ’

For both the anodic and cathodic waves, the peak current scales linearly with scan rate as expected for a surface reaction not limited by diffusion. Coulometric measurement of the amount of charge reversibly exchanged upon redox cycling led to a calculated doping level (yj of 0.1 5, a value close to that of 0.1 6 obtained for poly(3-heptylthiophene) under the same conditions.[’] The curves recorded at low scan rates show that the anodic wave involves two sub-components at 0.62 and 0.80 V. The Fact that these potentials are slightly less anodic than those of poly(3-heptylthiophene) together with the similarity of the J’ values suggest that the extent of conjugation in poly-1 is

Although the quality of the deposits was insufficient for spectroscopic experiments or for the determination of the four probe conductivity, preliminary studies of conductivity were made on film fragments with a two probe apparatus. Taking into account the fact that two-probe measurements give generally lower values than the four probe technique, the obtained values, (lo-’ to lo-’ Scm-‘), indicate that poly-1 exhibits a rather high level of conductivity, which is consistent with the extended conjugation suggested by electrochemical data. Although these results demonstrate that 1 can be easily electropolyrnerized leading to a polymer with properties comparable to those of its alkyl-substituted analogues, they appear rather intriguing in the light of previous results on sterically constrained 3-substituted poly(thiophenes).[’] As a matter of fact, a comparative analysis of the electropolymerization of 3-isoamyl-, 3-isobutyl- and 3-isopropyl-thio-

Communications

phenes has shown that decreasing the distance between the thiophene ring and the branched alkyl chain leads to a progressive increase of steric interactions and hence of the torsion angle between adjacent thiophene rings in the polymer.[71Thus, whereas poly(3-isoamylthiophene) and poly(3-pentylthiophene) have comparable Epaand y values,['* the oxidation potential of poly(3-isobutylthiophene) is 150 mV higher than for poly(3-butylthiophene) while y decreases from 0.19 to 0.07. A further decrease of the distance between the thiophene ring and the secondary carbon inhibits the electropolymerization reaction as shown by the unsuccessful attempts to electropolymerize 3-isopropylthio~ h e n e , [and ~ ] more recently 3-cyclohexylthiophene.[' '1 In view of the even larger size of the substituted silyl group in 1 compared to an isopropyl group, a complete inhibition of electropolymerization could have been expected, and the straightforward electropolymerization of 1 constitutes a quite surprising result. In addition to the fact that the S i c bond is significantly longer than the C-C bond, a possible explanation of this unexpected behavior could resort to the specific properties of perfluoroalkyl chains. As a matter of fact, although perfluoroalkyl compounds are simultaneously hydrophobic and lipophobic, they exhibit a strong affinity for them~elves.~~] In the case of poly-1, this fluorophilicity might induce interactions among the perfluoroalkyl side chains strong enough to counterbalance the steric hindrance between substituted silyl groups, thus forcing the polymer chain to adopt a planar conformation. In conclusion, the first example of polythiophene 3-substituted by a polyfluorinated alkylsilyl chain has been described and it has been shown that the use of a chlorosilane end group constitutes a convenient synthetic method of preparation of polyfluorinated 3-alkylthiophenes. Furthermore, fluorophilic interactions among the side chains allow the neutralization of the steric hindrance expected from the size of such substituents and thus the straightforward electrosynthesis of an extensively conjugated, highly doped conducting polymer.

nitrogen atmosphere. Films for electrochemical characterization were deposited on Pt in potentiostatic conditions at the oxidation potential of the monomer using a deposition charge of 500 rnCcm-'. The polymer films were then rinsed with acetone and placed in another cell containing 0.1 M LiCIO, (Fluka puriss) in either MeCN (HPLC grade) or deionized water. Received: April 19, 1993 Final version. May 22. 1993 [I] a) Proc. ICSM '88, Synth. Met. 1989, 28. b) Proc. ICSM '90, Svnth. Mel. 1991, 41-44, [2] J. Roncali, Chem. Rev. 1992, 92, 711. [3] M.R.C. Gerstenberger, A. Haas, Angew. Chem. Int. Ed. EngI. 1981, 20, 647. [4]W. Biichner, R. Garreau, M. Lemaire, J. Roncali, J. Elecfroanul. Chem. 1990, 277, 355. [5] W. Biichner, R. Garreau, J. Roncali, M. Lemaire, J. Fluorine Chem. 1992, 59, 301. [6] a) J. Roncali, A. Guy, M. Lemaire, R. Garreau. A.H. Huynh, J. Elertrounul. Chem. 1991,312,277.b) J. Guay, P. Kasai, A. Diaz, R. Wu, 1M. Tour, L.H. Dao, Chem. Muter. 1992, 4, 1097. [71 J. Roncali, A. Yassar, P. Marque, R. Garreau, F. Garnier, M. Lemaire, J. Phys. Chem. 1987, 91, 6706. [XI A.J. Bard, L.R. Faulkner. Electrochemicul Merhods. Fundumentul.s und Applications, Wiley, New York 1980. [91 H.S. Li, J. Roncali, F. Garnier, J. Electrounul. Chem. 1989, 263, 155. [lo] J. Roncah. P. Marque, R. Garreau, F. Garnier. M. Lemaire, M u m + molecules, 1990, 23, 1347. [I 11 W.A. Goedel, N.S. Somanathan, V. Enkelmann, G. Wegner, Makwmol. Chem. 1992, 193. 1195.

Modeling the Elementary Steps of Low-Pressure Diamond Deposition By Jiirgen Biener, Uwe A . Schubert, Angelika Schenk. Bernd Winter, Carsten Lutterloh, and Jiirgen Kiippers*

Since the invention of diamond deposition from low-pressure hydrocarbon/hydrogen gas-phase mixtures,[' the elementary steps of the deposition process and, in particular, its rate-determining steps have been of primary interest. In the pioneering work of Deryagin and Spitsyn['] and AngusL3I the importance of the role of atomic hydrogen has been Experimental stressed as an important means to remove graphitic con(3,3,4,4,5,5,6,6,6-nonafluorohexyl-l-dimethylsilyl thiophene): To a stirred stituents of the deposit. It was assumed that graphite was solution of 3-bromothiophene (2.4 g, 14.7 mmol) in 30 ml of dry Et,O, n-BuLi (1.6 M in hexane, 10 ml, 16 mmol) was added dropwise at -70°C under nitroetched preferentially over diamond, thus giving the "kinetic gen. The reaction mixture was stirred 15 min at -70°C and dimethyl(3advantage" to diamond growth. However, only vague reac(3,3,4,4,5,5,6,6,6-nonafluorohexyl)chloro~ilane (Fluka) (5 g, 14.7 mmol) was tion pathways have been proposed by which the preferential added and allowed to react for 3 h. The reaction vessel was then allowed to warm slowly to room temperature and 5 nil of deionized water were added. The removal of the graphitic deposit was plausible. mixture was extracted with Et,O. The organic phase was washed with water, In the present study, model systems consisting of C:H dried and concentrated. Distillation afforded 4.56 g (80%) of a colorless liquid, Eb,, 140°C n;": 1.4125, 'H-NMR (CDCI,) 6 [ppm]: 7.54, dd, 1H; 4 J = films, a few-monolayers thick, ion-beam deposited onto a 2.55Hz,"J= 1.11Hz,l.49,dd,1H;3J=4.70Hz,4J=2.55H~;7.24,dd,1H; carrier, were used to identify elementary steps which lead to 'J = 4.69 Hz,4J = 1.12 Hz;2.10, m,2H; 1.07, m,2H; 0.40, m, 6H. "C-NMR preferential erosion of the graphitic constituents of a hydro(CDCI,) 6 [ppm]: 137.9; 135-102, m, CF,-CF,-CF,-CF,; 132.2; 131.0; 25.71 'JCF= 23.6 Hz; 5.48; -3.2 (two carbons). carbon deposit. It is found that sp2 hybridized carbon at the Electrochemistry was performed with a PAR 273 potentiostat-galvanostat, in surface of a C: H film gets hydrogenated to sp3 by thermal H a three-electrode single-compartment cell equipped with platinum microeleccm2area, a platinum wire counter electrode and a saturattrodes of 7.85 x ed calomel reference electrode. Electropolymerizations were carried out in nitrobenzene (Aldrich ACS reagent) containing 0.1 M tetrabutylammonium hexafluorophosphate (Fluka puriss) and 0.1 M monomer. Solutions were degassed by nitrogen bubbling prior to each experiment which was run under a Adv. Muter. 1993, 5, No.9

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[*I Prof. J. Kiippers, J. Biener, U.A. Schubert, A. Schenk, Dr. B. Winter, C. Lutterloh Max-Planck-Institut fur Plasmaphysik, Euratom-Association Boltzmannstrasse 2, D-85748 Garching (FRG)

Weinheirn, 1993

0935-9648/93/0909-0639 $5.00+.25/0

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