Redox chemistry of thiophene, pyrrole and thiophene-pyrrole-thiophene oligomers

ELSEVIER

Synthetic Metals 101 (1999) 642-645

Redox chemistry of thiophene, pyrrole and thiophene-pyrrole-thiophene oligomers P. Audebert”; J.-M Catelb, V. Duchenetb, L. Guyarda; P. Hapiotcs*; G. Le CoustumeP aLaboraroire de Chimie et Electrochuimie Mol&ulaire,

UniversifP de Franche ComtP + La Bouloie, Route de Gray, 2.5030 Besanfon Cede?t,France.

bLuboratoire de Chimie MolPculaire et Thioorganique, Unit6 Mixte de Recherche No 6507 CNRS. Universit.? de Caen, 6 Bd du Markhal Juin, 14050 Caen, France CLaboratoire d’Electrochimie MolPculaire de I’Universik! Denis Diderot (Paris 7), UnitC Mixte de Recherche Universit& Paris7-CNRS N” 7591, 2 place Jussieu, Case Courrier no 7107, 75251 Paris Cedex OS,France.

Abstract Electrochemical properties of oligopyrroles and thiophene-pyrrole-thiophene oligomers, which are key-step intermediates in electropolymerization processes, have been investigated by means of fast electrochemistry, flash photolysis and pulse radiolysis techniques. From this studies, conclusions have been drawn concerning the reactions involved in the polymer formation, x-dimerization, carbon-carbon bond formation, deprotonation and the nature of coupling positions. Keywords: Electrochemical polymerization, Polythiophene, Polypyrrole.

1. Introduction Small oligomers (oligothiophenes or oligopyrroles) are well structured molecules, which ailows the rationalization of structural effects on chemical reactivities of the corresponding cation radicals. They are key-step intermediates in the electropolymerization reactions and their study brings a better understanding of the electropolymerization mechanism [l]. The polymerization mechanism involves several consecutive electrochemical and chemical steps, i.e., heterogeneous and homogeneous electron transfer processes, carbon-carbon bond formation, and deprotonations. In the literature, several possibilities have been considered for the carbon-carbon bond formation, i.e., coupling between two cation-radicals (CR-CR coupling) or two neutral radicals (R-R coupling) and also the reaction between the cation-radical and the starting monomer (CR-S coupling). We found that in the case of the oxidation of substituted-pyrroles, the first step involves formation of the cation-radical of pyrrole, followed by coupling between two cation-radicals, and then deprotonation, whereas coupling between the cation-radical and the starting molecule is unimportant [Z]. It is not clear, however, whether the same mechanism is valid with the longer oligomers formed during the polymerization process, or in solutions containing different chain-length oligomers and monomer. In the present contribution, we report on the electrochemical properties of several thiophene-pyrrole-tophene oligomers and oligopyrroles. Their properties have been compared to the corresponding oligothiophenes. The variations of the standard oxidation potentials, reactivity of the electrogenerated cation-radicals with the chemical substitution have been investigated by means of fast electrochemistry, flash photolysis and pulse radiolysis techniques and analyzed in terms of electronic and steric substituent effects. Steric effects can be considerable when the substituent introduces a distortion of the planarity of the oligomer, especially between the internal &substituents. These results have been compared and supported by ab-initio and DIT calculations. * Correspondingauthor.

2. Oxidation of unsubstituted oligopyrroles Thanks to the use of ultramicroelectrodes (electrodes with a diameter in pm range), cations radicals of unstable-oligomers can be observed. The scan rate for which the voltammograms become reversible gives a direct estimation of the life-time of the produced cation-radicals. In a preceding work, it has been found that the part% reversibility of the oxidation wave of the monomeric pyrrole was reached around 30,000 V.s-t [Z]. In the case of the unsubstituted bipyrrole BP, a partial reversibility was observed around 10,000 V.s-I, indicating a small increase of the stability of the cation-radical of the bipyrrole in comparison with the one from the pyrrole monomer 13,4]. At low scan rate (O.lV.s-1), the oxidation wave of I3P is irreversible and leads to the formation of polymer when cycling in traces of water are present [4,.5]. More surprisingly, passing from bipyrrole to terpyrrole TP does not bring appreciable further stabilization (only a factor of 2-3). On the contrary, a large change for the stability of the cation-radical takes place when we consider the oxidation of the quaterpyrrole In this case, not only the first oxidation wave was found to be reversible at slow scan rate (O.lV.s-‘), but also the second wave which corresponds to the formation of the dication. This behavior shows a large stabilization of the cation-radical of the quaterpyrrole QT. Its lifetime increases by at least four orders of magnitude in comparison with the data obtained for the terpyrrole TP. It is clear that the weak difference between the E” values of the terpyrrole and of the quaterpyrrole does not account for the large reactivity difference observed between the two corresponding cation-radicals. These results indicate that, first, there is only a slight reactivity difference between the cation-radical and the dication in large oligopyrroles like quaterpyrrolc, as it has also been found for longer oligopyrroles and secondly, that oligopyrrole cationradicals require at least 4 rings to be stabilized, in agreement with the doping level values usually observed in polypyrroles.

Email: hapiot9paris7.jussieu.fr

0379-6779/99/$ - see front matter 0 1999 Elsevier Science S.A. All rights reserved. PII: SO379-6779(98)0031 1-7

P. Hapiot

et al. i Syrzthetic Metals

101 (1999) 642-645

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Table 1. Electrochemical Characteristicsof unsubstituted oligopyrroles a Compound

E”N(SCE)

ScanrateN.s-* b

BP

0.600

s,ooo- 10,000

TP

0.2go

2,000-4,000

0.160c

reversible

‘: po

6

H

QT

0.435c

a from reference 3. b Lowest scan rate for the observation of a partial reversibility for an initial 10m3mol.L-’ concentration

The electrochemical behavior of the terpyrrole TP is more complicated (see Fig. 1). By examining the voltammograms at a concentration of 10e3 mol.L-‘, the following changes can be observed as the scan rate was increased: at 0.1V.st the voltammogram was almost completely irreversible and a cathodic peak IIIc was visible at a much lower potential (O.lV/SCE) after the peak Ia has been scanned beforehand, at 1V.s1 the voltammogram appearedas reversible (peak IIc), at higher scan ratesthe reversibility decreasedand at 5OV.s’ the voltammogram was completely irreversible. At still higher scan rates (v>2,OOOV.s~‘),the reversibility reappeared (peak II’c), then increased again. From these experiments, it is clear that the reversibility observed at low scan rate (1V.s’) did not correspond to the direct reduction of the cation-radical which was only visible at a much higher scan rate (2,000 V.S’). Moreover, it has been found that the electrolysis of the terpyrrole led as a final product to the dimeric cation-radical (sexypyrrole). The peak 111~ visible at 0.1 VX* (Fig. la) can be ascribed to the reduction of the sexypyrrole cation-radical in agreement with its redox potential (see below). Thus, the apparent reversibility (peak 11~) observed between 0.5-2OV.~~ correspondsto the reduction of an electrogeneratcd species, which is different from the cationradical and also from the final sexypyrrole cation-radical. This behavior seemsindicative of a reversible dimerisation following the electron transfer as observed previously during oxidation of the tetramethylbithiophene [6]. TP __) 2 TP’+ =

of TP. For these compounds, the values reported in the table were measured at high scan rates (4,OOOV.s’). From the comparison of the unsubstituted bipyrrole, with the mono and dimethylbipyrrole , one can seethat the substitution of one or two methyl groups on the a-position decreasesthe oxidation potential as expected for a donor substituent. However, life-times of cation-radicals of the mono- or dimethylated bipyrroles are not increasedby the substitution. (a)

lb)

la

----ii

IJJc

-. -0.1

03

(cl

la ,A

0;

I----A -0.1

0.1 .‘-

0.3

) (4

TP” + e-

0

protonated dimer

02

0.4

1 -0.1

02 “-

03

Fig. 1 Cyclic voltametry of terpyrrole in acetonitrile(C”=2mM).

3. Oxidation of the ~1,&-substituted pyrroles and bipyrroles

Scan rates= (a) 0.1, (b) 1, (c) 50, (d) 13000 V.s’.

In the case of oligothiophenes, it is well known that the properties of oligomers (redox potential, stability of the cationradical) can be changed in a large extend by substituting the ctterminal positions of the thiophene backbone. We have investigated the effect of substitution by methyl and rert-butyl groups on Or-terminalpositions of pyrrole and bipyrrole on the oxidation potential and of the stability of the corresponding cation-radicals. The results are summarized in Table 3. Indications of a reversible dimerisation were also found for the 5,5’-dimethyl-2,2’-bipyrrole and the oxidation of 5-methyl,Srert-butyl-2,2’-bipyrrole as we found previously for the oxidation

4. Oxidation mechanism of bipyrrole. The coupling of bipyrrole can be achieved by chemical, photochemical or electrochemical oxidation. Photochemical oxidation is obtained when bipyrrole is irradiated by UV light in presenceof an irreversible electron acceptorlike Ccl, [4]. BP + CC&, hv, CClj’ + 02 --+ CCl,O,

+ BP __)

BP’+ + CC13’ + ClCC1302’ CCl,O,- + BP ‘f

644

R Hapiot et al. J Synthetic Metals 101 (1999) 642-645 N

R?

~~q--/x

Table 2 ElectrochemicalCharacteristicsof a-Substituted F’yrrolesand Bipyrrolesa.

Rl

Rz

H

CH3

CH3

CH3

E”N(SCE)b

Scanrate/V.s-l c

0.460 0.300 0.315 0.330

5,000-10,000 5,000-10,000

(CH,),C totally reversibleat IO,OOOV.S-~.~ CH3 l-2 (CH,),C (CH,),C H (CH,),C irrrv&ble at ~,OOOV.S-~ a From reference3. b ReferenceWE electrodewas calibrated against the ferrocene/ferricinium couple 0.405 V(vs SCE). Error k 0.01 V.C Lowest scanrate for the observation of a partial reversibility for an initial 10W3 mol.L*’ concentration. c E” measuredat 4,000-10,000 VS’, it was not possible to estimatethe lowest scanrate to observedthe chemical stability of the cation-radical. Table 3 ElectrochemicalCharacteristics,Absorption Spectraand DecayKinetics of the Bipyrrole Cation-Radicalsa.

Method Compound

Water

Acetonitrile

Solvent Flash Photolysis h,, mG

Pulse Radiolysis

Cyclic Voltammetry

2 kdim iL.mol-l s-l)

ks -11 (ems 1

&E)

BP’+

2 k&m (L m&l s-l)

580

1.2 x 109

0.600

0.5

109

590

1.2x 109

0.460

1

8x lo*

600

sb

0.300

2

4x 108

MBP’f DMBP’+

h CZ 360 580 365 590 370 600

E 27,000 6200 26,000 7400 25,000 7900

2 k&m (L mol-l s-l) 7.8 x 108 LOX 109 8.6 x 108

a from ref 4. enot a secondorder decay. Scheme1

k,m H

n disproportionation

+ H

+

I Ox&ion

products

2H+ !-

During radiolysis experiments,the oxidant (Ns’) is generatedby irradiation of water by-iotiizing radiation [4]. In both techniques, the cation-radicals of the bipyrrole is monitored by its UV absorption and kinetics rafes constants are derived. Convergent results obtained by flash photolysis, pulse radiolysis and electrochemistry techniques provide the first direct evidence that only the cation-radical leads to the formation of longer oligotiers or polypyrrole, whereas the neutral radical does not (i.e. that deprotonarion does not occur before the coupling step). The dimerization involves reaction between two cation-radicals to form a protonated dimer, whereas the coupling between the bipyrrole cation radical and the starting bipyrrole or added monomeric pyrrole is negligible. Deprotonation of the dimer is much slower than the carbon-carbon bond formation, m agreementwith previous results concerning mixed thiophenepyrrole oligomers. Moreover, when a base is added in order to accelerate the deprotonation, the cation-radical can easily be deprotonated,but in this case, the neutral radical does not lead to the formation of the dimer but createsa new ECE mechanism.The pKa of the cation radical has been measuredand found to be 8.7 and 16.6 in water and acetonitrile respectively [4]. A proton

t? Hupior

er (11. / Synrlwric

acceptor is required to effect the deprotonation of the dimer and the rearomatization step, but if a strong base is present, the deprotonation of the cation-radical of bipyrrole begins to compete with the coupling reaction. The overall mechanism can be summarizedin schemel. 5. Oxidation of thiophene-pyrrole-thiophene oligomers

Formal oxidation potentials, E”, and life-times of the electrogeneratedcation-radicals have been measured at low or high scan rates cyclic voltammetry for the substituted pyrrolethiophene-pyrrole oligomers and terthiophenes shown in table 4 [7,8]. As expected, substitution by a methyl groups induces a lowering of the oxidation potentials except when the methyl is introduced on the p’ positions of the mixed oligomers which impedes the cation-radical to achieve a planar configuration as confirmed by molecular modelization. A very good agreementis found between the variations of the experimental E” and of the

Metals

101 (1999) 642-645

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differences of energiesAEn (energy of neutral oligomers - energy of cation radical) calculatedby DFT and ab-initio methods[S]. 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 [8,9]. By detailed studies using cyclic voltammetry and double-step chronoamperometry,we show that a CR-CR mechanism (coupling between two cation-radical) is encountered for the cr-unsubstituted oligomers but also for the a,&-dibrominated oligomers, but in this last case,no polymers are formed [S]. Steric effects, comparison with the electrochemical behavior of other a,cc’-disubstituted oligothiophenes and oligopyrroles, and spin densities calculation in the cation-radicals suggeststhat the reaction involves the hindered coupling between two o-positions of the cc@-dibrominated oligomers. All these results indictes that the couplings by one u-position with one of the fi-positions (or 0’ or B”) or between two of the P-positions (or 0’ or B”) are negligible for these triheterocycle cation-radicals. However, the situation may be completely different during real polymerization process, where 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 fi position of a longer polymeric chain and thus start a new branched chain.

Compounds BfwBr

z,,,

R

;;;’

5000-10000 vc

;:;

E”ox2b

0.2-0.5

2104 104

1.302 1.326

DSP

1.238

50-100

Bw

RwR

Br

NC4H9 NC6H13

H

0.882 0.887

NC6H13

CH3

0.835

0.2-0.5

104e 3 104

S

H

2-3 lo8

CH3

1.11 1.055

15000

s

5000-10000

2 108

s NCH3

C6H13

5000-10000 20-50

2 108

NCH3

CH3

1.06 0.76 0.735

500-1000

2 107

NCH3

C6H13

NC4H5

H

0.72 0.794

200-500 10-20

4 105

1.280

NC6H13

H

0.805

10-20

4 105

1.277

S NCH3

CH3 CH3

l.025 0.880

4-8000 50-100

l-2 108 d 2 106d

1.251

H

106

1.264

1 107

a from reference 8. b in Volt versus SCE. c lowest scan rate to observed partial reversibility in VS t d dimerization rate in l.mol.‘.s’. eDouble potential step chronoamperometry,cyclic voltammetry experimentselsewhere. References [l] H.S. Nalwa (ed.), Handbook of Conducting Molecules and Polymers, John Wiley and Son, New-York, 1997. [2] C.P. Andrieux, P. Audebert, P. Hapiot, J.-M. Saveant,J. Phys. Chem. 95 (1991) 10158. [3] C.P. Andrieux, P. Hapiot, P. Audebert, L. Guyard, M. Nguyen Dinh An, L. Goenendaal, E.W. Meijer, Chem. Mater. 9 (1997) 723.

141L. Guyard, P. Hapiot, P. Neta, J. Phys. Chem. B. 101 (1997) 5698.

[5] G. Zotti, S. Martina, G. Wegner, A.-D. Schltiter, Adv. Mater. 4 (1992) 798. [6] P. Tschuncky, J. Heinze, A. Smie, G. Engelmann, G. KoBmehl, J .Electroanal. them. 433 (1997) 225. [7] P. Audebert, J.-M. Catel, G. Lecoustumer, V. Duchenet, P. Hapiot, J. Phys. Chem. 99 (1995) 11923. [8] P. Audebert, J.-M. Catel, G. Lecoustumer, V. Duchenet, P. Hapiot, J. Phys. Chem. B. (199Q submitted. [9] R.E. Niziurski-Mann, C. Scordilis-Kelley, T.-L. Liu, M.P. Cava, R.T. Carlin, 1. Am. Chem. Sot. 115 (1993) 887,