Electrochemistry and Polymerization Mechanisms of Thiophene

Ulises Páramo-García , Bernardo A. Frontana-Uribe , Patricia Guadarrama , and Victor M. Ugalde-Saldívar. The Journal of Organic Chemistry 2010 75 (21)...
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J. Phys. Chem. B 1998, 102, 8661-8669

8661

Electrochemistry and Polymerization Mechanisms of Thiophene-Pyrrole-Thiophene Oligomers and Terthiophenes. Experimental and Theoretical Modeling Studies P. Audebert,1a J.-M. Catel,1b G. Le Coustumer,1b V. Duchenet,1b and P. Hapiot*,1c Laboratoire de Chimie Organique, UniVersite´ de Franche Comte´ , La Bouloie, Route de Gray, 25030 Besanc¸ on, France; Laboratoire de Chimie Mole´ culaire et Thioorganique, Unite´ Mixte de Recherche Nο. 6507 CNRS, UniVersite´ de Caen, 6 Bd du Mare´ chal Juin, 14050 Caen, France; and Laboratoire d’Electrochimie Mole´ culaire de l’UniVersite´ Denis Diderot (Paris 7), Unite´ Mixte de Recherche CNRS-UniVersite´ No. 7591, 2 place Jussieu, 75251 Paris Cedex 05, France ReceiVed: December 5, 1997; In Final Form: June 26, 1998

The electrochemical properties of several substituted terheterocycles thiophene-pyrrole-thiophene oligomers have been studied by cyclic voltammetry and double-potential-step chronoamperometry and then compared with the behavior of the corresponding pure terthiophenes. E° and lifetimes of the cation radicals have been measured. Ab initio and DFT calculations show that the cation radicals of thiophene-pyrrole-thiophene oligomers (N-alkyl-2,5-bis(thien-2-yl)pyrroles) are twisted in contrast to the terthiophene case where cation radicals are planar. These geometries explain the variations of the oxidation potentials with the substituents positions. When the R-positions are free, the cation radical undergoes a fast coupling reaction in solution, leading to either a dimer or a polymer involving the coupling between two cation radicals. The same cation radical-cation radical coupling reaction is involved when the R-terminal positions are substituted by bromides. We propose that this reaction involves a hindered R-R′ coupling followed by a nucleophilic attack of the protonated dimer and not the coupling with one of the β-positions (or β′ or β′′). This view is supported by comparison with the electrochemical behavior of other oligothiophenes and oligopyrroles.

Introduction Polythiophenes and polypyrroles are certainly two of the most widely studied organic conducting polymers due to both their good conductivity and stability.2 Small oligomers (oligothiophenes or oligopyrroles) are well-structured molecules, which allow the rationalization of structural effects on electronic properties of their parent polymers and are interesting materials for applications in electronics and optoelectroelectronics.3,4 Thus, one can expect to draw direct relationships between molecular oligomers and materials properties. A large amount of work has been dedicated to the oxidation of oligothiophenes for which the properties can easily been modified using chemical substituents, pointing out the large potential offered by these modifications.5 Less has been done about the oligomer containing one or several pyrrole units, but it is known that polymers can be prepared by oxidation of triheterocycles such as 2,5-bis(2thienyl)pyrrole.6-11 More recently, the electrochemical properties (and of the corresponding cation radicals) of mixed pyrrolethiophene oligomers12-16 or pure oligopyrroles have been described.17-21 All these oligomers are also key-step intermediates in the electropolymerization reactions. In this field, their study allows a better understanding of the global mechanism even if the nature of the coupling step has been determined in the case of monomeric pyrroles,22,23 substituted bithiophene,24 bipyrroles,18,21 terthiophenes,25,26 and pentathiophenes.15 For both oligopyrroles17,20 and oligothiophenes,27 the reactivity of oligomers decreases upon increasing the ring number. This adds interest to investigating the electrochemical behavior of small length oligomers since their solubility is still high enough in common solvents while the reactivity of the electrogenerated cation radical is low enough to allow a precise mechanistic

study. In principle, both linear (R-R coupling) or branched structures (involving at least one of the β- or β′-positions) can be expected from early theoretical calculations predicting that the unpaired electron density of the radical intermediate can be almost as high in the β-positions as in the R-positions.28 However, until now, only R-R couplings have been clearly identified as the major process during the electrochemical oxidation of oligomer with free R-positions (for example, see for bipyrroles,18,21 quinquethiophenes,15 and sexithiophenes29), suggesting that R-R coupling is faster than branched coupling. In the case of oligothiophenes, this conclusion is also supported by well-known results where the substitution of the two R-terminal positions allows a large stabilization (increase of its lifetime) of the corresponding cation radicals.27 This question is of great importance because the formation of branched C-C bonds when they occur during the polymerization creates defects in the conjugation of the parent polymer and a decrease of its conductivity.28,30 Reactivity studies of these oligomers as a function of the substituent position and of its nature permit useful information about the relative kinetics rate constants involving the different reactive positions to be obtained. However, a great care should be taken in such investigations, because the substitution of one hydrogen by another chemical group or atom in these conjugated molecules has generally not only steric but also electronic effects. We describe hereafter the synthesis and reactivity of several triheterocycles (terthiophenes and thiophenepyrrole-thiophene oligomer), bearing alkyl and bromide substituents on various β-positions of the terminal rings. The variations of the standard oxidation potentials and reactivity of the electrogenerated cation radicals with the chemical substitution have been determined by cyclic voltammetry and analyzed in terms of electronic and steric substituent effects. Steric effects

10.1021/jp9804289 CCC: $15.00 © 1998 American Chemical Society Published on Web 10/09/1998

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SCHEME 1a

a 1, X ) S; 2, X ) NMe; 3, X ) NBu, R ) H; 4, X ) NHex, R ) H; 5, X ) NHex, R ) Me; 6, X ) S, R ) H; 7, X ) S, R ) Me; 8, X ) S, R ) Hex; 9, X ) NMe, R ) H; 10, X ) NMe, R ) Me; 11, X ) NMe, R ) Hex; 12, X ) NBu, R ) H; 13, X ) NHex, R ) H; 14, X ) S, R ) Me; 15, X ) NMe, R ) Me.

can be considerable when the substituent introduces a distortion of the planarity of the oligomer (due to steric interactions), especially with the internal β-substituents. These results have been compared and supported by ab initio and DFT calculations. The properties of the coupling products have been investigated, which are shown to be the dimer or a polymer. In some cases, simple examination of the cyclic voltammograms allows one to choose among the two possibilities. Experimental Part and Procedures Syntheses. NMR spectra were determined on a Bruker AC 250 spectrometer in CDCl3. Mass spectra (IE) were obtained on a Nermag Riber R10 10H. A general method for the synthesis of the thiophenepyrrole-thiophene terheterocycle which is difficult to obtain by other classical coupling methods has been recently described.31 The method is based on the cyclization of dibrominated 1,4-bis(thien-2-yl)-1,4-butanediones and is also well adapted to the synthesis of thiophene-pyrroles oligomers. In this work, we have extended this method to the synthesis of N-alkyl-2,5-bis(thien-2-yl)pyrroles β or β′ disubstituted. The principles of the synthesis are shown in Scheme 1. The terthiophenes obtained by cyclization from butane-1,4-diones were described in a previous paper,31 except the known terthiophene 14.32 Formation of compounds involving a Nsubstituted pyrrole ring was achieved by acid cyclization with a primary amine based on the method of refs 33 and 34. N-Methyl-2,5-bis(4-bromothien-2-yl)pyrrole 2: yellow powder (78%); mp 108 °C; 1H NMR δ ) 3.64 (s, 3H); 6.27 (s, 2H); 6.88 (d, J3-5 ) J3′′-5′′ ) 1.89 Hz, 2H); 7.19 (d, J3-5 ) J3′′-5′′ ) 1.89 Hz, 2H); 13C NMR δ ) 33.8; 110.1; 110.8; 122.4; 127.8; 129.3; 136.1. MS m/e (relative intensity): 405 (M + 2, 55); 403 (M+, 100); 401 (44); 390 (8); 388 (15); 386 (4); 242

(11); 241 (11); 241 (37); 240 (23); 227 (20); 226 (16); 225 (6); 163 (16); 162 (32); 161 (23); 146 (25); 79 (13); 64 (12). Anal. Calcd for C13H9S2NBr2: C, 38.7; H, 2.3; S, 15.9; N, 3.5. Found: C, 38.5; H, 2.7; S, 15.7; N, 3.4. N-Methyl-2,5-bis(5-bromo-4-methylthien-2-yl)pyrrole: salmoncolor powder (43%); mp 137 °C; 1H NMR δ ) 2.22 (s, 6H); 3.68 (s, 3H); 6.27 (s, 2H); 6.73 (s, 2H); 13C NMR δ ) 15.4; 33.6; 108.7; 110.4; 127.7; 128.9; 134,2; 137.7; MS m/e (relative intensity): 433 (M + 2, 12); 431 (M+, 100); 429 (31); 416 (16); 414 (11); 273 (8); 149 (10). Anal. Calcd for C15H13S2NBr2: C, 41.8; H, 3.0; N, 3.2. Found: C, 42.1; H, 3.0; N, 3.3. N-Hexyl-2,5-bis(5-bromothien-2-yl)pyrrole 4: yellow oil (72%); 1H NMR δ ) 0.82 (t, J ) 6.75 Hz, 3H); 1.14 (m, 6H); 1.56 (m, 2H); 4.05 (t, J ) 7.81 Hz, 2H); 6.28 (s, 2H); 6.79 (d, J3-4 ) J3′′-4′′ ) 3.8 Hz, 2H); 6.81 (d, J3-4 ) J3′′-4′′ ) 3.8 Hz, 2H); 13C NMR δ ) 13.9; 22.4; 26.0; 31.1; 31.2; 45.2; 111.3; 111.6; 126.4; 127.7; 130.2; 136.3. MS m/e (relative intensity): 475 (M + 2, 30); 473 (M+, 54); 471 (30); 391 (13); 390 (23); 389 (21); 388 (38); 387 (14); 386 (17); 309 (20); 308 (12); 307 (18); 229 (14); 228 (14); 184 (13); 57 (11); 55 (17); 45 (12); 43 (98); 41 (100). Anal. Calcd for C18H19S2NBr2: C, 45.9; H, 4.1; S, 3.0; N, 13.6; Br, 33.5. Found: C, 46.3; H, 4.2; S, 2.9; N, 14.1; Br, 33.5. N-Hexyl-2,5-bis(5-bromo-4-methylthien-2-yl)pyrrole 5: yellow pale powder (74%); mp 80 °C; 1H NMR δ ) 0.82 (t, J ) 6.7 Hz, 3H); 1.15 (m, 6H); 1.5 (m, 2H); 2.20 (s, 6H); 4.06 (t, J ) 7.8 Hz, 2H); 6.25 (s, 2H); 6.72 (s, 2H). 13C NMR δ ) 14.0; 15.4; 22.5; 26.1; 31.15; 31.2; 45.3; 108.8; 111.1; 128.0; 128.1; 134.3; 137.5. MS m/e (relative intensity): 503 (M + 2, 57); 501 (M+, 100); 499 (50); 418 ([10); 416 (19); 414 (10). Anal. Calcd for C20H23S2NBr2: C, 47.9; H, 4.6; S, 12.8; N, 2.8; Br, 31.9. Found: C, 48.1; H, 4.6; S, 13.5; N, 2.6; Br, 31.5.

Thiophene-Pyrrole-Thiophene and Terthiophenes N-Methyl-2,5-bis(4-methylthien-2-yl)pyrrole 10: obtained by reduction of N-methyl-2,5-bis(5-bromo-4-methylthien-2-yl)pyrrole according to ref 31.Yellow pale powder (87%); mp 94 °C; 1H NMR δ ) 2.28 (s, 6H); 3.72 (s, 3H); 6.3 (s, 2H); 7.85 (s, 4H). 13C NMR δ ) 16.0; 33.8; 109.9; 120.4; 127.8; 128.9; 134.2; 137.8. MS m/e (relative intensity): 274 (M + 1, 3); 273 (M+, 10); 270 (35); 269 (91); 268 (100); 258 (9); 253 (13); 224 (13); 193 (17); 189 (16); 179 (13); 138 (17); 137 (20). Anal. Calcd for C15H15S2N : C, 65.9; H, 5.5; S, 23.4; N, 5.1; Found: C, 66.0; H, 5.6; S, 23.3; N, 5.3. N-Methyl-2,5-bis(4-hexylthien-2-yl)pyrrole 11: obtained by alkylation of 2 according to ref 31.Yellow pale needles (72%); mp 46 °C; 1H NMR δ ) 0.89 (t, J ) 6.7 Hz, 6H); 1.32 (m, 12H); 1.64 (m, 4H); 7.61 (t, J ) 7.65 Hz, 4H); 3.73 (s, 3H); 6.31 (s, 2H); 6.88 (d, J ) 3.13 Hz, 4H). 13C NMR δ ) 14.2; 27.7; 29.1; 30.5; 30.7; 31.8; 33.8; 109.8; 119.6; 127; 129.6; 134.7; 143.7. MS m/e (relative intensity): 414 (M + 1, 89); 413 (M+, 100); 343 (22); 316 (43); 315 (69); 258 (52). Anal. Calcd for C25H35S2N : C, 72.6; H, 8.5; S, 15.5. Found: C, 72.9; H, 8.6; S, 15.6. N-Methyl-2,5-bis(3-methylthien-2-yl)pyrrole 15 was synthesized by acid cyclization of the corresponding dione, obtained according to ref 35, with methylamine. Yellow pale oil (76%); 1H NMR δ ) 2.19 (s, 6H); 3.34 (s, 3H); 6.27 (s, 2H); 6.93 (d, J4-5 ) J4′′-5′′ ) 5.16 Hz, 2H); 7.24 (d, J4-5 ) J4′′-5′′ ) 5.16 Hz, 2H). 13C NMR δ ) 14.9; 32.5; 110.9; 125.0; 127.1; 129.0; 129.9; 136.8. MS m/e (relative intensity): 274 (M + 1, 2); 273 (M+, 9); 270 (30); 269 (70); 268 (100); 258 (12); 253 (13); 224 (12); 193 (16); 189 (12); 138 (11); 137 (11). Electrochemical Apparatus and Procedure. Electrochemistry experiments were performed with a three-electrode setup using a platinum counter electrode and a calomel reference electrode. Acetonitrile (Merck, Uvasol, less than 0.01% water) was used with tetraethylammonium tetrafluoroborate electrolyte (Fluka, puriss., used as received). Solutions were kept under argon as a safety procedure. For low scan rate cyclic voltammetry using millimetric electrodes (1 mm diameter gold or vitreous carbon disk electrodes), electrochemical instrumentation consisted of a PAR model 175 Universal programmer and of home-built potentiostat equipped with a positive feedback compensation device.36 The data were acquired with 310 Nicolet oscilloscope. Fast electrochemical experiments were performed using 10 µm diameter disk platinum ultramicroelectrodes. Equipment and procedures were the same as previously published.37 The potential values reported are averaged out of several reproducible experiments; all potentials were internally calibrated against the ferrocene/ferricinium couple (E ) 0.405 V vs SCE) for each experiment. Simulations of the cyclic voltammograms were performed using Bas DigiSim 2.0 (Bioanalytical Systems, Inc., West Lafayette, IN 47906). Theoretical Modeling. The calculations were performed using the PC-Spartan software 1.1 (Wavefunction Inc, Irvine, CA) and the Gaussian 94 package38 for HF calculations and Gaussian 94 package for DFT calculations. Geometries were calculated by full optimization of the conformations using the 6-31G* basis set39 at the RHF level for neutral molecules and the UHF level for the cation radicals. The choice of this basis set was justified by previous published studies where reasonable agreements were found between calculated geometries and experimental data for thiophene and bithiophene40,41 and for sterically hindered bithiophene (biisothianaphthene).41 Electron correlation effects in the cation radical were tested by reoptimizing the geometry of compound 9 using DFT theory with the B3LYP functional42 using the 6-31G* base. Differences

J. Phys. Chem. B, Vol. 102, No. 44, 1998 8663 SCHEME 2: General Structure of the Studied Triheterocycle Oligomers (s-trans Conformer)

less than 1° were found for the torsion angles in the cation radical of the thiophene-pyrrole-thiophene oligomer 9 between UHF and B3LYP levels. The s-trans conformation (as presented in Scheme 2) were found to be the most stable conformer, in agreement with previous theoretical published studies40,41 and X-ray structures (see refs 43 and 44 and references therein). Results As generally observed for short oligothiophenes in acetonitrile (see for example refs 2, 4, 15, and 17), several oxidation steps are observed in our series of oligomers. The first peak corresponds to the formation of the cation radical.4,15,17 For all the studied compounds, this first peak is irreversible at low scan rate (and at a concentration on the order of 10-3 mol L-1). This makes it difficult to conclude only from these experiments whether the further oxidation processes are related to the dication formation or to the oxidation of subsequent products. When increasing the scan rate, the first oxidation wave becomes at least partially reversible. (The scan rate required to observe the reversibility of the process is directly related to the lifetime of the electrogenerated cation radical.) The standard potentials E° for the first monoelectronic transfers can be immediately derived as the midpoint between the forward and backward scan peak potentials. For the most stable cation radical, it was also possible to study the further oxidation steps. Generally, we found that the second wave becomes reversible at a slightly higher scan rate. In these conditions, the two reversible waves can be ascribed unambiguously to the formation of the cation radical and to its oxidation to the dication, and both standard oxidation potentials have been determined. These values are summarized in Table 1. Reactivity of the Cation Radicals. During oxidation, oligothiophenes and mixed pyrrole-thiophene oligomers undergo coupling reactions (oligomerization or polymerization). These processes are very sensitive to the structure of the oligomers (nature, substituent, position, number, etc.) and also to the replacement of one sulfur of the thiophene ring by a different heteroatom. Three different types of general behavior have been observed: (i) With the mixed pyrrole-thiophene oligomers 11, 12, and 13, the formation of coupling products is clearly identifiable by a new reversible electrochemical system at potentials about 100-200 mV less positive than the monomer oxidation. In the case of compounds 12 and 13, the peaks ascribable to the coupling products have the classical behavior and shapes of species in solution (nonsymmetric shape, peak separation about 60 mV), indicating that the products are at least partially soluble (see Figure 1) and that they are certainly dimers.45 We derived an oxidation standard potential for the produced dimer equal to E° ) 0.58 V/SCE. Similarly in the case of the other compound 11, the thin shape of the peaks for the coupling product suggests that mainly one type of dimer is formed, but the dimer is now precipitated onto the electrode (Figure 2). In these cases, the peak currents grow during successive scans, indicating an accumulation of the products. (ii) For the other oligomers 1, 6, 7, 9, 14, and 15 broad peaks

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TABLE 1 compound

a

no.

X

R

E°1a

Vb

1.31 0.957

5000-10000 50-100

2 × 108 5 × 106

104 2 × 104 104 d 3 × 104

kdimc

1 2

S NCH3

3 4

NC4H9 NC6H13

H H

0.887 0.882

5

NC6H13

CH3

0.835

0.2-0.5 0.2-0.5 DSP 0.2-0.5

6 7 8 9 10 11 12 13

S S S NCH3 NCH3 NCH3 NC4H5 NC6H13

H CH3 C6H13 H CH3 C6H13 H H

1.11 1.055 1.06 0.76 0.735 0.72 0.794 0.805

15000 5000-10000 5000-10000 20-50 500-1000 200-500 10-20 10-20

(2-3) × 108 2 × 108 2 × 108 106 2 × 107 1 × 107 4 × 105 4 × 105

14 15

S NCH3

CH3 CH3

1.025 0.880

4-8000 50-100

(1-2) × 108d 2 × 106d

E°2a

1.326 1.302 1.238

1.264 1.280 1.277

1.251

In volts versus SCE. b In V s-1. c In L mol-1 s-1. d Double-potential-step chronoamperometry, cyclic voltammetry elsewhere (see text).

Figure 1. Cyclic voltammetry of the oxidation of 12 on a 1 mm diameter platinum disk electrode in acetonitrile (+0.2 mol L-1 of NEt4BF4). Scan rate V ) 0.2 V s-1. C0 ) 1.8 × 10-3 mol L-1.

appear and grow at lower potentials (see Figure 3), probably ascribable to the formation of an electroactive polymer or at least a distribution of higher oligomers, as with 15. (iii) In contrast to the two previous cases, after the oxidation of the R-bromine-substituted oligomers (3, 4, and 5) no new peak appears upon the reverse reduction sweep at less positive potentials (as for the other oligomers), and no modification of the electrochemical response is observed during successive voltammetric scans. On the contrary, a reversible system corresponding to the formation of a product is visible at low scan rates and at a more positive potential (E° ) 1.10 V/SCE). This indicates that the reaction decay of the cation radical leads to a different type of product than the cation radicals of the R-unsubstituted oligomers and that no layer of polymer is made on the electrode surface.

Figure 2. Repetitive scans cyclic voltammetry of the oxidation of 11 on a 1 mm diameter platinum disk electrode. Scan rate V ) 0.2 V s-1. C0 ) 1.3 × 10-3 mol L-1.

The lowest scan rate required to observe a partially reversible wave is a measurement of the cation radical lifetime, and if the mechanism is known, these values can be converted to kinetics rates by comparison with simulated voltammograms. As expected, compounds 3, 4, and 5, for which the R-terminal positions are substituted, display partially reversible voltammograms at the lowest scan rates (lower than 1 V s-1). Figure 4 shows the cyclic voltammograms when the first oxidation peak is irreversible (a) and reversible (b). In the reversible conditions, the second oxidation step corresponding to the quasi-reversible formation of the dication is observable. The dication is slightly

Thiophene-Pyrrole-Thiophene and Terthiophenes

J. Phys. Chem. B, Vol. 102, No. 44, 1998 8665

Figure 5. Cyclic voltammetry of the oxidation of 9 on a 10 µm diameter platinum disk ultramicroelectrode in acetonitrile (+0.2 mol L-1 of NEt4BF4). Scan rate V ) 3000 V s-1. C0) 2 × 10-3 mol L-1.

Figure 3. Repetitive scans cyclic voltammetry of the oxidation of 9 on a 1 mm diameter platinum disk electrode. Scan rate V ) 0.2 V s-1. C0 ) 2 × 10-3 mol L-1.

example, the lifetimes of the β-dibrominated cation radical 1 of the β- or β′-dimethylated 7 and 14 are identical to the value found for the unsubstituted terthiophene 6. The same trend is observed when substituting the β′-positions of the thiophenepyrrole-thiophene oligomer (compare 9 with 15). On the contrary, substitution on the β-positions decreases the lifetime of the corresponding cation radicals (compare 9 with 2, 10, and 11). Little effect is also observed when increasing the size of the N-substituent when passing from a methyl to butyl group both on the oxidation potentials or reactivity of the produced cation radical (compare 9 with 12 and 13). Discussion

Figure 4. Cyclic voltammetry of the oxidation of 4 on a 1 mm diameter platinum disk electrode in acetonitrile (+0.2 mol L-1 of NEt4BF4). Scan rate (a) V ) 0.2 V s-1, (b) V ) 10 V s-1. C0 ) 2 × 10-3 mol L-1.

less stable than the cation radical by a factor of 2-4, as seen by the lower reversibility of the second oxidation wave. All other terthiophenes display irreversible voltammograms at much higher scan rates, and the use of ultramicroelectrodes is required to reach the domain where the cation radical formation is reversible (5000-10 000 V s-1). When changing the central pyrrole ring with a thiophene, the first oxidation wave becomes much more irreversible, and scan rates higher by 3 orders of magnitude are required to observe a first oxidation reversible wave (see Figure 5). Table 1 shows the scan rates required to observe at least a partial reversibility for all the compounds. The first conclusion is that pure thiophene oligomer cation radicals are always more reactive than the cation radicals from oligomers with a central pyrrole moiety. If we compare the two unsubstituted molecules 6 and 9, a stability increase of a factor 1000 can be observed. This general behavior can be expected by the fact than pyrrole is more electron-rich than thiophene. This assumption is confirmed by the difference in redox potentials around 350 mV. Similar decreases of the oxidation potential have already been observed and reported for a series of R,R′-disubstituted thiophene-pyrrole oligomers.16 The substituents on the β- or β′-positions have weak effects on the reactivities of the terthiophene cation radicals; for

Variation of the Redox Potentials with the Substituents. Geometries of the Cation Radicals. From the reversible cyclic voltammetric experiments, the standard oxidation potentials have been determined (see Table 1). As expected, the substitution of a hydrogen on R- or β-positions by a withdrawing group (Br) increases the oxidation potential, in agreement with the values of the Brown coefficient σ+ for these substituents.46 Similarly, substitution on a β-position by a donor group (CH3 or C6H13) decreases the oxidation potentials. The effects are in the same range for the pure terthiophenes and for the thiophene-pyrrole-thiophene terheterocycles. As previously reported,15 substitution on the internal β′-positions of the oligomer backbones by methyl groups creates inverse variations in the order of the oxidation potentials as can be observed for the pyrrole-thiophene oligomer 15. This compound has a more positive oxidation than 10 (when the methyl groups are on the β-position) and is also more difficult to oxidize than the unsubstituted compounds. On the contrary, no such effects are visible between the dimethylterthiophene 14 and the unsubstituted terthiophene 6. To clarify the observed behaviors and to obtain information about the geometries of the cation radicals, we performed several molecular modelizations with ab initio calculations.47,48 The geometries of the neutral oligomers and of their cation radicals were optimized at the RHF/6-31G* and UHF/6-31G* level, respectively. The most stable conformations for the neutral molecules were always the twisted rotamers,49 and the dihedral angles (θ) between the terminal and the central ring are reported in Table 2. The θ values for neutral molecules were generally larger for thiophene-pyrrole-thiophene oligomer than for the terthiophenes, indicating a steric hindrance due to the central N-methyl group.6,7 θ values also increase when methyl groups are present on the β′-positions for both types of oligomers. This increase is confirmed by the differences between the experimental UV-vis spectra where the variations of the λmax values bring information about the extent of the oligomer π-system.27b,c

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TABLE 2: UV-Vis, Electrochemical Characteristics, and Theoretical Modeling of Terheterocycles at the HF/6-31G* Level

neutral molecule 6 7 14 9 10 15

cation radical

X

R1

R2

θa

Ehomob

λmaxf

θa

Ehomob

∆End

E°oxe

S S S NCH3 NCH3 NCH3

H CH3 H H CH3 H

H H CH3 H H CH3

31 30 58 46 45 70

-7.49 -7.43 -7.89 -7.19 -7.13 -7.62

353 (3.51) 360 (3.44) 341 (3.78) 322 (3.85) 326 (3.80) 292 (4.25)

0 0 0 19 19 33

-4.14 -4.03 -3.85 -3.62 -3.53 -3.67

5.63 5.54 5.55 5.23 5.17 5.42

1.11 1.06 1.03 0.76 0.74 0.88

a Dihedral angle between thiophene and central rings in degrees. b Energies in electronvolts. c First unoccupied β-orbital in UHF calculation. Energy differences between the neutral molecule and its cation radical in electronvolts. e In volts versus SCE. f From experimental UV-vis spectra in acetonitrile in nanometers. Corresponding transition energies in electronvolts.

d

A red shift is visible between the unsubstituted oligomers and the β-subtituted oligomers as generally observed with alkyl substituents (donating effect).27b,c On the contrary, a blue shift to higher energies is visible when the methyl groups are introduced on the β′-positions due to the distortion of the π-system due to higher values of θ. All the terthiophene cation radicals were found to be planar, in agreement with X-ray structure of stable cation radical.44 The values of θ for all the cation radicals were almost unchanged by the introduction of methyl groups on the outside β-positions. On the contrary, the cation radicals of the thiophene-pyrrole-thiophene terheterocycle were twisted, and the N-methyl group was outside the pyrrole plane due to the steric interaction with the thiophene rings. When methyl groups are introduced on the β′-positions of these oligomers, the torsion angles in the cation radical increase a lot (steric interactions between three methyl groups).6,7 Similar variations with the substitution are observed between the experimental values of the standard oxidation potentials, E°, and the differences of energies between the neutral molecule and its cation radical, ∆En. The variations of the HOMO energy of the neutral molecules and of the LUMO energy of the cation radical indicate that the β′-substitution affects both the neutral molecule and the cation radical of the pyrrole-thiophene oligomer 15 and only the neutral molecule in the case of the terthiophene 14,50 suggesting that the difference in behavior is mainly related to the destabilization of the cation radical of 15. It is clear that solvation will change the values of molecular orbitals energies in the liquid phase and that the MO energy variations have just to be considered as general trends, but it is noticeable that the calculations reflect the variation of the experimental standard potentials. The inverse variation of E° with the β′-substitution in the thiophene-pyrrole-thiophene oligomer is due to an increase of the torsion angle θ, leading to a decrease of the conjugation. Mechanisms of Dimerization. It has now been demonstrated in several instances (for the electrochemical oxidation of pyrroles,23 bipyrroles,18,21 substituted oligothiophenes15,24-26) that the dimerization involves the reaction between two cation radicals to form a protonated dimer whereas the coupling between the cation radical and the starting oligomer is negligible.21 The produced dimer has still to lose two protons to allow the growing of the polymer, and it was shown for several examples that this deprotonation step can be much slower than the carbon-carbon bond formation.12,21 In view of these numerous results, it is likely that a similar type of coupling between two cation radicals (CR-CR mechanism) is involved

Figure 6. Conformations of the cation radicals of 14 (a) and 15 (b) at the UHF/6-31G* level.

during the R-R-dimerization of thiophene-pyrrole-thiophene oligomers with free R-terminal positions. Inside this series, we choose one of our terheterocycles (N-methyl-2,5-bis(4-bromothien-2-yl)pyrrole, 2) to check that this assumption was valid. For this oligomer, the surface modifications due to the oxidative polymerization during one cyclic voltammetry experiment are limited. This allows detailed kinetics study of the process based on the variation of the peak potential with both the scan rate and the monomer concentration. We found a linear variation with a slope of 20 mV/log(V) (variation with the scan rate) and slope close to -17 mV/log(C0) (variation with the initial concentration of oligomer). These two values are in agreement with the theoretical behavior for a radical-radical coupling mechanism where the coupling step is the rate-determining step. (Theoretical variations of 19.4 mV/log(V) and -19.4 mV/log(C0) are expected for a CR-CR mechanism.51) Considering this mechanism valid for the oxidation of the other terheterocycles, we have measured the dimerization rate constants, kdim, by comparing the partially reversible voltammograms recorded at high scan rates with simulated curves (see Table 1). For the dimerization of the cation radical from the unsubstituted terthiophene 6, we found a dimerization rate constant of 2 × 108 L mol-1 s-1. In previous studies of the oxidation of 6 by flash photolysis where the cation radical was produced by UV irradiation, it was found that this cation radical decays by a second-order kinetics law with a kinetics rate constant similar to the one measured from cyclic voltammetric measurements.52 This agreement between the two techniques confirms the nature of coupling step, i.e., reaction between two cation radicals.53 Let us now consider the electrochemical behavior for the oxidation of the R,R′-brominated oligomers (3, 4, and 5). As

Thiophene-Pyrrole-Thiophene and Terthiophenes

Figure 7. Double-potential-step chronoamperometry of the oxidation of 4 on a 1 mm diameter platinum disk electrode in acetonitrile (+0.2 mol L-1 of NEt4BF4). C0 ) 10-3 mol L-1 (9), 3 × 10-3 mol L-1 (2), and 11 × 10-3 mol L-1 (b). The curve is the theoretical variation of R for a CR-CR mechanism (see text).

noticed previously, the reactivity of the corresponding cation radicals is much lower than for the cation radicals of the unsubstituted oligomers. A quantitative analysis of the oxidation mechanism was achieved using double-potential-step chronoamperometry experiments (DPSC).54 After the first potential step, the potential is stepped back to its initial value at time τ. The anodic current I(τ) is measured at time τ and the cathodic current at time 2τ. The ratio I(2τ)/I(τ) is normalized to the value [I(2τ)/I(τ)]diff that it would have in the absence of follow-up chemical reactions by calculating the quantity R ) [I(2τ)/I(τ)]/ [I(2τ)/I(τ)]diff. This technique has the advantage of providing kinetic information on the chemical steps that follow the initial electron transfer with a limited number of parameters. For dimerization mechanisms, R depends on a single parameter λ ) kC0τ.15,23 DPSC experiments have been performed for compound 4 for concentration in the range 10-3-10-2 mol L-1 and are displayed in Figure 7. The three brominated pyrrolethiophene oligomers present in cyclic voltammetry the same behavior, and the reversibility of the first oxidation process was observed in the same range of scan rates, indicating similar lifetimes for the cation radicals. As could be noticed, a very SCHEME 3

J. Phys. Chem. B, Vol. 102, No. 44, 1998 8667 good agreement is obtained between the experimental variations of R as a function of C0τ and the theoretical behavior for the CR-CR mechanism. These results show a similar nature for the coupling step involved in the oxidation of unsubstituted and of R,R′-brominated oligomers. The next question that arises concerns the reactive positions on the oligomer which are involved in the dimerization between the two cation radicals, leading to linear or branched coupling. As explained in the Introduction, it can be envisaged that the coupling reactions take place between two R-terminal positions leading to a linear polymer and also involve one of the β′- or β′′-positions leading to branched couplings (R-β′, R-β′′, β′β′, β′-β′′, β′′-β′′). If it is clear that when one of the R-positions of the oligomer is free, the major coupling will take place on these positions,15,21,29 but what will be the behavior when the two R-positions are blocked by groups such as methyl or bromide? In a previous published investigation, assuming a CR-CR mechanism for the oxidation of R,R′-bromo or R,R′dimethyloligomers (similar to our study), the decay reaction was ascribed to the coupling of two cation radicals by the β′′positions.12 If the possibility of such coupling cannot be completely excluded, in view of more recent results, another interpretation of the electrochemical behavior can be proposed. The difficulties in the interpretation of these results come from the fact that substitution modifies the accessibility of the reactive sites but also can change the electronic density in the cation radical and in the produced diprotonated dimer and, sometimes, other geometric factors such as torsion angles. Two related questions are risen by this problem: first, is the substitution of the hydrogens on the two R-terminal positions by Br or CH3 really able to impede the coupling on these positions, and second, if we assumed a branched coupling with one of the β-positions, which positions will be involved in the reaction (β, β′, or β′′)? As explained before, from the resonance structures, the three possibilities exist because the unpaired electron can be localized on the R-, β′-, and β′′-positions. To get a better representation of the cation radical of the thiophenepyrrole-thiophene oligomers, we calculated the spin densities in the cation radicals of 9 and for comparison with its oligothiophene analogue 6 cation radical. We used the DFT method B3LYP/6-31G* to take into account the correlation

8668 J. Phys. Chem. B, Vol. 102, No. 44, 1998 effects which can have considerable effects in open-shell systems.55 We found values of 0.184, 0.135, and 0.059 respectively for the R-, β′-, and β′′-positions for the cation radical of the pyrrole-thiophene oligomer 9 (0.208, 0.135, and 0.058 for the pure terthiophene 6). If we just consider these results, we found that the unpaired electron is more localized on the β′-position than on the β′′-positions and thus that coupling reactions are more likely to occur with the β′-positions if the R-terminal positions are not available,56 in agreement with the early calculations with INDO methods. From the experimental results of Table 2, it is noticeable that the dimerization rate constants of the cation radicals of the thiophene-pyrrolethiophene oligomers are not lowered by the substitution on the β- or β′-positions. On the contrary, these rates increase by a factor of 10 in the case of β-substitution by methyl group and only slightly in the case of β′-substitution.57 However, no considerable effects are observed for cation radicals of pure thiophene oligomers upon β- or β′-methyl or Br substitutions. Indeed, these results indicate that the dimerization on the β or β′ does not play a major role, suggesting that the R-R-coupling is the major process even in the substituted compound58 and that branched coupling are negligible. For the second question, recent results obtained with substituted oligothiophenes24or oligopyrroles20,21 cation radicals have shown that the substitution of the two R-terminal positions by methyl groups increases the reversibility of the oxidation wave but also that this does not mean that the R,R-coupling between two of these substituted cation radicals does not exist. On the contrary, when the R-terminal positions of short oligothiophenes were substituted by two bulky SiMe3 groups,59 the linear polymer obtained after oxidation of the oligomer has a better quality with less defects.26 In this case, the oxidation mechanism involved the hindered coupling between two silylated cation radicals followed by nucleophilic assisted elimination of two “SiMe3+”.25 When the two substituents are methyl groups, the protonated dimer produced after the coupling step does not easily lead to the corresponding dimer, and the C-C bond can break to give the cation radical monomer back, leading to a reversible dimerization behavior (i.e., existence of an equilibrium between the cation radical A•+ and the protonated dimer (A)22+;24 see Scheme 3). However, on long time scale, (A)22+ certainly decays by nucleophilic attack of the impurities contained in the solvent.20,24 The forward dimerization reaction for the 5,5′dimethyl-2,2′-bipyrrole was found to be fast, around 4 × 108 L mol-1 s-1,21 and only the substitution by two bulky tert-butyl groups allows a considerable decrease of this reaction.20 There are less results concerning the substitutions with bromide, but based on the respective sizes of bromide and methyl group, it is likely that similar reversible coupling can exist between the two corresponding cation radicals on their R-positions. In view of these results, we proposed that the behavior observed during the oxidation of the R,R′-dibrominated oligomers 3, 4, and 5 is mainly due to a reversible (R-R-coupling) between two cation radicals followed by a nucleophilic attack of the protonated dimer. Conclusion Formal oxidation potentials, E°, and lifetimes of the electrogenerated cation radicals have been measured at low or high scan rate cyclic voltammetry for several substituted pyrrolethiophene-pyrrole oligomers and terthiophenes. As expected, substitution by a methyl groups induces a lowering of the oxidation potentials except when the methyl is introduced on the β′-positions of the mixed oligomers which impedes the

Audebert et al. cation radical to achieve a planar configuration as confirmed by molecular modelization. A very good agreement is found between the variations of the experimental E° and of the differences of energies ∆En (energy of neutral oligomers energy of cation radical) calculated by ab initio methods. For all the studied oligothiophenes or pyrrole-thiophene oligomers, the cation radical undergoes a coupling reaction in solution, leading to either a dimer or a polymer according to the starting monomer. By detailed studies using cyclic voltammetry or double-step chronoamperometry, we show that a CR-CR mechanism (coupling between two cation radicals) is encountered for the R-unsubstituted oligomers but also for the R,R′-dibrominated oligomers, but in this last case, no polymers were formed. Steric effects, comparison with the electrochemical behavior of other R,R′-disubstituted oligothiophenes and oligopyrroles, and spin densities calculation in the cation radicals indicate that the reaction involves the hindered coupling between two R-positions of the R,R′-dibrominated oligomers. All these results suggest that the coupling by one R-position with one of the β-positions (or β′ or β′′) or by two of the β-positions (or β′ or β′′) is negligible between these triheterocycle cation radicals. However, the situation is completely different during the real polymerization process when different sizes of oligomers and large concentration of monomers (pyrrole or thiophene) are present together in the solution or on the electrode and where small monomers can easily react on the β-position of a longer polymeric chain and thus start a new branched chain. Acknowledgment. We are grateful to Dr. J.-C. Lacroix (ITODYS, Universite´ Denis Diderot, Paris 7) for his help and advice in the theoretical modeling calculations. Supporting Information Available: Spin densities calculated at the B3LYP/6-31G* level for the N-methyl-2,5-bis(thien2yl)pyrrole cation radical and the terthiophene cation radical (Scheme S1); calculated energies of compounds 6, 7, 9, 10, 14, and 15 calculated at the HF/6-31G* level (Table T1) (2 pages). See any current masthead for ordering and Internet access instructions. References and Notes (1) (a) Universite´ de Besanc¸ on. (b). Universite´ de Caen. (c) Universite´ Denis Diderot (Paris 7). (2) For a recent general reviews about conducting polymers and related oligomers see for example: Handbook of Organic ConductiVe Molecules and Polymers; Nalwa, H. S., Ed.; John Wiley and Sons: New York, 1997. (3) Garnier, F. Angew. Chem., Int. Ed. Engl. 1989, 28, 513. (4) Roncali, J. Chem. ReV. 1992, 92, 711. (5) For reviews see for example: Roncali, J. Chem. ReV. 1997, 97, 173 and references therein or ref 2. (6) Ferraris, J. P.; Andrus, R. G.; Hrncir, D. C. J. Chem. Soc., Chem. Commun. 1989, 1318. (7) Ferraris, J. P.; Hanlon, T. R. Polymer 1989, 30, 1319. (8) Niziurski-Mann, R. E.; Cava, M. P. AdV. Mater. 1993, 5, 547. (9) Kowalik, J.; Tolbert, L.; Ding, Y.; Bottomley, L. A. Synth. Met. 1993, 55-57, 1171. (10) Ro¨ckel, H.; Huber, J.; Gleiter, R.; Schuhmann, W. AdV. Mater. 1994, 6, 568. (11) Otero, T. F.; Carrasco, J.; Figueras, A.; Brillas, E. Synth. Met. 1996, 83, 193 and references therein. (12) Niziurski-Mann, R. E.; Scordilis-Kelley, C.; Liu, T.-L.; Cava, M. P.; Carlin, R. T. J. Am. Chem. Soc. 1993, 115, 887. (13) Parakka, J. P.; Jeevarajan, J. A.; Jeevarajan, A. S.; Kispert, L. D.; Cava, M. P. AdV. Mater. 1996, 8, 54. (14) Kozaki, M.; Parakka, J. P.; Cava, M. P. J. Org. Chem. 1996, 61, 3657. (15) Audebert, P.; Catel, J.-M.; Le Coustumer, G.; Duchenet, V.; Hapiot, P. J. Phys. Chem. 1995, 99, 11923. (16) Van Haare, J. A. E. H.; Groenendaal, L.; Peerlings, H. W. I.; Havinga, E. E.; Vekemans, J. A. J. M.; Janssen, R. A. J.; Meijer, E. W. Chem. Mater. 1995, 7, 1984.

Thiophene-Pyrrole-Thiophene and Terthiophenes (17) Zotti, G.; Martina, S.; Wegner, G.; Schlu¨ter, A.-D. AdV. Mater. 1992, 4, 798. (18) Zotti, G.; Schiavon, G.; Zecchin, S.; Sannicolo, F.; Brenna, E. Chem. Mater. 1995, 7, 1464. (19) Van Haare, J. A. E. H.; Groenendaal, L.; Havinga, E. E.; Janssen, R. A.; Meijer E. W. Angew. Chem., Int. Ed. Engl. 1996, 35, 638. (20) Andrieux, C. P.; Hapiot, P.; Audebert, P.; Guyard, L.; Nguyen Dinh An, M.; Groenendaal, L.; Meijer, E. W. Chem. Mater. 1997, 9, 723. (21) Guyard, L.; Hapiot, P.; Neta, P. J. Phys. Chem. B 1997, 101, 5698. (22) Andrieux, C. P.; Audebert, P.; Hapiot, P.; Save´ant, J.-M. J. Am. Chem. Soc. 1990, 112, 2439. (23) Andrieux, C. P.; Audebert, P.; Hapiot, P.; Save´ant, J.-M. J. Phys. Chem. 1991, 95, 10158. (24) Tschuncky, P.; Heinze, J.; Smie, A.; Engelmann, G. Kossmehl, G. J. Electroanal. Chem. 1997, 433, 225. (25) Wintgens, V.; Valat, P.; Garnier, F. J. Phys. Chem. 1994, 98, 228. (26) Hapiot, P.; Gaillon, L.; Audebert, P.; Moreau, J. J. E.; Le`re-Porte, J.-P.; Wong Chi Man, M. J. Electroanal. Chem. 1997, 435, 85. (27) (a) The stabilization of the cation radical of oligothiophene (increase of its lifetime) with the chain length or the R-substituent is a well-known phenomena which has been observed by several groups. See for example refs 27b,d. (b) Guay, J.; Kasai, P.; Diaz, A.; Ruilan, W.; Tour, J. M.; Dao, L. H. Chem. Mater. 1992, 4, 1097. (c) Garcia, P.; Pernaut, J.-M.; Hapiot, P.; Vintgens, V.; Valat, P.; Delabouglise, D.; Garnier, F. J. Phys. Chem. 1993, 97, 513. (d) Ba¨uerle, P.; Segelbacher, U.; Maier, A.; Mehring, M. J. Am. Chem. Soc. 1993, 115, 10217. (28) Waltman, R. J.; Bargon J. Tetrahedron 1984, 40, 3963. (b) Waltman, R. J.; Bargon, J. Can. J. Chem. 1986, 64, 76. (29) Demanze, F.; Godillot, P.; Garnier, F.; Hapiot, P. J. Electroanal. Chem. 1996, 414, 61. (30) Dos Santos, D. A.; Bre´das, J.-L. J. Chem. Phys. 1991, 95, 6567. (31) Duchenet, V.; Andrieu, C. G.; Catel J.-M.; Le Coustumer, G. Phosphorus, Sulfur Silicon 1996, 118, 117. (32) Zimmer, H.; Shabana, R.; Galal, A.; Mark, H. B.; Gronowitz, S.; H. Phosphorus, Sulfur Silicon 1989, 42, 171. (33) Ferraris, J. P.; Skiles, G. D. Polymer 1987, 28, 179. (34) Sorensen, A. R.; Overgaard, L.; Johannsen, I. Synth. Met. 1993, 55, 1626. (35) Moriarty, R. M.; Penmasta, R.; Prakash, J. Tetrahedron Lett. 1987, 28, 873. (36) Garreau, D.; Save´ant, J.-M. J. Electroanal. Chem. 1972, 35, 309. (37) Andrieux, C. P.; Garreau, D.; Hapiot, P.; Pinson, J.; Save´ant, J.M. J. Electroanal. Chem. 1988, 243, 321. (38) Gaussian 94, Revision E.1: Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; Petersson, G. A.; Montgomery, J. A. Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; Head-Gordon, M. Gonzalez, C.; Pople, J. A. Gaussian, Inc., Pittsburgh, PA, 1995. (39) Hariharan, P. C.; Pople, J. A. Chem. Phys. Lett. 1972, 16, 217. (40) Orti, E.; Viruela, P. M.; Sa´nchez-Marı´n, J.; Toma´s, F. J. Phys. Chem. 1995, 99, 4955. (41) Viruela, P. M.; Viruela, R.; Orti, E.; Bre´das, J.-L. J. Am. Chem. Soc. 1997, 119, 1360. (42) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.

J. Phys. Chem. B, Vol. 102, No. 44, 1998 8669 (43) Yassar, A.; Garnier. F. AdV. Mater. 1994, 6, 660. (44) Graf, D. D.; Duan, R. G.; Campbell, J. P.; Miller, L. L.; Mann, K. R. J. Am. Chem. Soc. 1997, 119, 5888. (45) It is unlikely that trimer (9 heterocycle units) or higher oligomers of our studied triheterocycles can be soluble at concentration around 10-3 mol L-1 in acetonitrile. (46) Hansch, C.; Leo, A.; Taft, R. W. Chem. ReV. 1991, 91, 165. (47) In the case of bithiophenes, ab initio calculations at the HF/6-31G* have been shown to give reasonable precision for the geometry of the most stable conformer (see ref 48 and references therein). (48) Beljonne, D.; Cornil, J.; Friend. R. H.; Janssen, R. A. J.; Bre´das, J. L. J. Am. Chem. Soc. 1996, 118, 6453. (49) Except in the case of the hindered oligomers, the values of the rotation barrier are small for neutral oligomers, and thus it is difficult to get accurate values of torsion angles in such cases.40,41 Torsion of the rings along the chain axis is more likely to occur in solution than in gas phase.48 (50) The oxidation potentials are related both to the energy of the HOMO of the neutral oligomer and to the energy of the LUMO of the cation radical (lowest unoccupied β-orbital in UHF calculations). (51) Andrieux, C. P.; Nadjo, L.; Save´ant, J.-M. J. Electroanal. Chem. 1973, 42, 242. (52) Wintgens, V.; Valat, P.; Garnier, F. J. Phys. Chem. 1994, 98, 228. (53) A first-order decay is expected in the case of coupling between the cation radical and the neutral oligomer because this last one is in large excess (concentration in the millimolar range) compared to the produced cation radical (concentrations in the range of few 10-6 mol L-1) in the experimental conditions of flash photolysis experiments.21,52 (54) (a) In this case, the reaction kinetics was too slow to be investigated by the variation of the oxidation peak potential in cyclic voltammetry.54b (b) Andrieux, C. P.; Save´ant, J.-M. Electrocemical Reactions. In InVestigation of Rates and Mechanisms of Reactions; Bernasconi, C. F., Ed.; Wiley: New York, 1986; Vol. 6, 4/E, Part 2 pp 305-390. (55) We found that the dihedral angle θ for the optimized geometries of the cation radical of 9 at the B3LYP/6-31G* level (θ ) 18.9°) was the same than at the UHF/6-31G* level (θ ) 19.1°). (56) It is noticeable that the reaction sites for bromination which are related to the electron density of the neutral oligomer can be different from the spin density on the cation radical. From HF/6-31G* calculations on the neutral oligomer, we obtained that the carbons on the R- and β′′-positions present the highest electronic density, in agreement with experimental results. (57) (a) Similar differences of reactivity toward the position of the substituent (β or β′) have already been found for cation radical of oligothiophenes with stronger donnor groups such as methoxy. The β′-substituted oligomers cation radicals were always more stable than the β-substituted ones certainly because of a better electronic interaction with the R-reactive position.57b,c (b) Zotti, G.; Gallazzi, M. C.; Zerbi, G.; Meille, S. V. J. Electroanal. Chem. 1995, 73, 217. (c) Yu, Y.; Gunic, E.; Zinger, B.; Miller, L. L. J. Am. Chem. Soc. 1996, 118, 1013. (58) Previous published results have reported that the Br substitution on the two β′′-positions of an R,R′-disubstituted thiophene-pyrrolethiophene oligomer increases considerably the lifetime of its cation radical.12 Simultaneously, the oxidation potential increased by 300 mV, which is a huge increase compared to the expected change of 30-40 mV expected by Br introduced on oligothiophenes.27c From the high oxidation potential of the β′′,β′′-dibrominated oligomer, it is likely that the observed stabilization is mainly due to electronic and not to steric hindrance effects. (59) Several groups have reported the synthesis of polythiophene from silylated monomers or oligomers. See ref 26 and references therein.