5698
J. Phys. Chem. B 1997, 101, 5698-5706
Redox Chemistry of Bipyrroles: Further Insights into the Oxidative Polymerization Mechanism of Pyrrole and Oligopyrroles Laurent Guyard,1a Philippe Hapiot,*,1b and Pedatsur Neta1c Laboratoire de Chimie et Electrochimie Mole´ culaire, UniVersite´ de Franche Comte´ , La Bouloie, Route de Gray, 25030 Besanc¸ on Cedex, France, Laboratoire d’Electrochimie Mole´ culaire de l’UniVersite´ Denis Diderot (Paris 7), Unite´ de Recherche Associe´ e au CNRS No. 438, 2 place Jussieu, 75251 Paris Cedex 05, France, and Physical and Chemical Properties DiVision, National Institute of Standards and Technology, Gaithersburg, Maryland, 20889 ReceiVed: February 18, 1997; In Final Form: May 6, 1997X
The oxidation of 2,2′-bipyrrole, 5-methyl-2,2′-bipyrrole, and 5,5′-dimethyl-2,2′-bipyrrole has been investigated by means of electrochemistry, flash photolysis, and pulse radiolysis. The bipyrrole cation radical was found to give polypyrrole or oligopyrrole under electrochemical and chemical oxidation and also under UV-light irradiation of the solution in the presence of CCl4 as an electron acceptor. The cation radicals have been characterized by their optical absorption spectra, and their decay processes have been followed. In all processes (chemical, electrochemical, and photochemical), the first step involves the reaction between two cation radicals. The cation radical does not react on starting bipyrrole nor on pyrrole monomer. Depending on pH, the cation radical can deprotonate to form a neutral radical. It was found that only the cation radicals, but not the neutral radicals, produce higher oligomers, which explains the inhibition of polymerization by strong bases.
Introduction Organic conducting polymers have attracted considerable and increasing attention over the past decade; two of the most widely studied polymers were polythiophenes2 and polypyrroles.2 Despite this intense activity, many questions remain concerning the mechanisms involved in the polymerization reaction, although some key steps have been elucidated.3 The polymerization mechanism involves several consecutive electrochemical and chemical steps, i.e., heterogeneous and homogeneous electron-transfer processes, carbon-carbon bond formation, and deprotonations. Several possibilities have been considered for the carbon-carbon bond formation,3 i.e., coupling between two cation radicals (CR-CR coupling)4 or two neutral radicals (R-R coupling)5 and also the reaction between the cation radical and the starting monomer (CR-S coupling).6,7 The R-R mechanism differs from the other two mechanisms in the order of the different reaction steps, i.e., deprotonation is prior to the carbon-carbon bond formation. 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.3b 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 series of thiophenes, the oxidation mechanism of the monomer was difficult to elucidate because of the high reactivity of the cation radical and its high oxidation potential, but several detailed kinetic studies were performed with classes of oligothiophenes for which the cation radical was more stable8 (quaterthiophenes, quinquethiophenes, silyloligothiophenes). It was found that the carbon-carbon bond formation involved coupling between two cation radicals followed by deprotonation of the dimer. However, only dimer formation is generally observed with these compounds because the cation radicals of the produced dimer X
Abstract published in AdVance ACS Abstracts, June 15, 1997.
S1089-5647(97)00608-1 CCC: $14.00
are stable and cannot react with another molecule to permit the polymerization. In view of this active research on substituted thiophene oligomers, it seems interesting to investigate the behavior of pyrrole oligomers toward substitution and chainlength effects. Little is known about the properties of oligopyrrole cation radicals toward coupling reactions or deprotonation. Recently, the synthesis of several unprotected oligopyrroles has been described9 and cyclic voltammograms of oligopyrroles have been reported.10 The standard potentials,10 E°, and lifetimes10d of the cation radicals in acetonitrile have been determined at low10 and high scan rates.10d Since the aromaticity energy is higher for thiophenes (29 kcal/mol) than for pyrroles (23 kcal/mol), the behavior of pyrrole oligomers is not expected to be entirely parallel with that of the thiophene oligomers.11 In particular, the pyrrolic cation radicals probably have a lower tendency to rearomatize than the corresponding thiophenes, which may lead to a wider variety of mechanisms. Besides the nature of the coupling step involved in the carbon-carbon bond formation, the role of acids and bases remains speculative. Since the origins of the electropolymerization of pyrrole, it has been noted that small concentrations of water were required to observe the polymerization12a,b and that high concentrations lead to loss of conductivity that can be related to defects in the film.12b Several hypotheses have been proposed to explain the role of water in the electropolymerization of pyrrole: water can act as a scavenger to prevent the protonation of the starting pyrrole by the proton released during the polymerization;12c water can increase the rate of the coupling step between two cation radicals by decreasing the electrostatic repulsion in the transition state,12d and even a new mechanism involving protonated pyrrole was suggested.12e It was also proposed that the use of weakly basic compounds during pyrrole polymerization should improve the polymer properties by limiting the condensation between protonated pyrrole or oligomers.12f Similarly, it was noticed that the polymer was not formed by oxidation of bipyrrole in dry acetonitrile and that polymerization required a minimal con© 1997 American Chemical Society
Bipyrroles centration of water.10a In contrast, in the case of thiophene, addition of water or base impedes formation of the film.13 The electrochemical techniques used in these previous studies are powerful tools for kinetic studies, but they suffer from difficulties when the electrode surface is modified during the oxidation process as is the case during electropolymerization. Thus, it is important to have additional independent methods that allow extension to higher rate constants and verification of the earlier results. Laser flash photolysis and pulse radiolysis offer such possibilities,14 although the chemistry required to produce the radicals, and thus the assignment of the spectra, is rarely straightforward. In this context, photochemical generation of the cation radical of oligothiophenes was achieved with terthiophenes15 by the quenching of their triplet state15a with a good electron acceptor or by direct photoionization of the neutral molecule.15c This suggests that photochemical or radiolytic oxidation should be possible with pyrroles as well. In preliminary experiments, we found that it was not possible to achieve such oxidations with pyrrole itself clearly because of the high oxidation potential. In the present paper, we report on the electrochemical and photochemical properties of bipyrroles toward polymerization reactions in water and organic solvents. Bipyrroles are suitable compounds for such studies because (a) they are key intermediates in the overall polymerization process and yet their behavior and reactivities appear to be close to those of the pyrrole monomer and (b) their oxidation potentials are at least 700 mV10 lower than those of pyrroles,3a and thus, it is possible to produce their radical cations more readily by photochemical or radiolytical oxidation. Experimental Section16 Chemicals. Acetonitrile was of Uvasol quality (Merck) and was used as received. CCl4 and FeCl3 were from Aldrich. The supporting electrolyte, NEt4BF4, was from Fluka (puriss). The synthesis of 2,2′-bipyrrole (BP), 5-methyl-2,2′-bipyrrole (MBP), and 5,5′-dimethyl-2,2′-bipyrrole (DMBP) were described in a previous study.10d Quaterpyrrole (2,2′:5′,2′′:5′′,2′′′-quaterpyrrole) was prepared from the N-tert-butoxycarbonyl-protected compound (N,N′,N′′,N′′′-tetra-tert-butoxycarbonyl-2,2′:5′,2′′: 5′′,2′′′-quaterpyrrole) just before experiments.10d N,N′,N′′,N′′′-
J. Phys. Chem. B, Vol. 101, No. 29, 1997 5699 SCN- dosimetry.18 All experiments were carried out at room temperature, 20 ( 1 °C. Flash Photolysis Experiments. Irradiations were performed with a Questek Laser 2048 (100 mJ, 20-50 ns) using a XeCl mixture (λ ) 308 nm). The kinetic spectrophotometric detection system consisted of a 150 W xenon lamp, a 1.5 cm optical path length irradiation cell, a Jobin-Yvon high-intensity monochromator, and a Hammamatsu photomultiplier. The signals were digitized with a Nicolet 450 digital oscilloscope and analyzed by a PC. Electrochemical Apparatus and Procedure. All the cyclic voltammetry experiments were carried out at 20 ( 0.1 °C with a three-electrode setup using a cell equipped with a jacket allowing circulation of water from the thermostat. The counter electrode was a Pt wire and the reference electrode an aqueous saturated calomel electrode (E°/SCE ) E°/NHE - 0.2412 V) with a salt bridge containing the supporting electrolyte. The SCE electrode was checked against the ferrocene/ferricinium couple (E° ) +0.405 V/SCE) before and after each experiment. For low scan rate cyclic voltammetry (0.05-500 V s-1), the working electrode was either a glassy carbon disk (0.8 mm diameter Tokai Corp.) or a 1 mm diameter gold or platinum disk. They were carefully polished before each set of voltammograms with 1 µm diamond paste and ultrasonically rinsed in absolute ethanol. Electrochemical instrumentation consisted of a PAR Model 175 Universal programmer and a home-built potentiostat equipped with a positive feedback compensation device.19 The data were acquired with a 310 Nicolet oscilloscope. For high scan rate cyclic voltammetry, the ultramicroelectrode was a gold or platinum wire (10 µm diameter) sealed in soft glass.20 The signal generator was a HewlettPackard 3314A, and the curves were recorded with a 4094C Nicolet oscilloscope with a minimum acquisition time of 5 ns/ point. Simulations were made with the BAS Digisim Simulator 2. Spectroelectrochemical measurements were achived with a diode array spectrophotometer (J. and M. Analytische Messund Regeltechnik Gmbh, Aalen, Germany) and a platinum anode inserted in the 0.05 quartz cell.20c Results
tetra-tert-butoxycarbonyl-2,2′:5′,2′′:5′′,2′′′-quaterpyrrole was a gift from Dr. Groenendaal and E. W. Mejer (Eindhoven University of Technology) and was synthesized according to a previously published procedure.9b Solutions of the quaterpyrrole were prepared inside a glovebox with a very low concentration of oxygen. Radiolytic Experiments. Fresh solutions of the bipyrroles ((5-8) × 10-4 mol L-1) were prepared by bubbling N2O in water (purified with a Millipore Super-Q system) containing 0.1 mol L-1 of NaN3 or NaBr. The pH was adjusted by using KOH or phosphate buffer. The pulse radiolysis apparatus has been described before.17 It utilizes 50 ns pulses of 2 MeV electrons from a Febetron 705 accelerator. The dose per pulse was varied between 8 and 80 Gy to produce between 5 × 10-6 and 5 × 10-5 mol L-1 of oxidizing radicals, as determined by
Electrochemical Oxidation of Bipyrroles. As observed for the oxidation of pyrrole, repetitive cyclic voltammetry at low scan rates of a solution of bipyrrole (BP) leads to formation of polypyrrole on the electrode.10a,12d,g On a clean electrode surface and for an initial concentration in the mmol L-1 range, BP exhibits an irreversible oxidative wave (Ep ) 0.8 V/SCE at 0.2 V s-1) corresponding to a one-electron process10a,12d,g in dry acetonitrile. Under our experimental conditions, we found a slightly higher value, around 1.2 electron per mole of bipyrrole (at 0.2 V s-1), by comparison with the reversible ferrocene/ ferricinium wave, which may be related to a larger concentration of residual water in our experiments. When increasing the scan rate above 10 000 V s-1, a partial reversibility of the oxidation wave of BP appeared, indicating that the lifetime of the cation radical is in the range of several microseconds.10d For detailed kinetics studies it is desirable to control the coupling reactions that follow the initial quaterpyrrole formation. For this purpose, we focused our attention on the oxidation of 5-methyl-2,2′-bipyrrole (MBP), where only one of the two R-terminal positions is likely to participate in dimer formation. As for the unsubstituted bipyrrole, MBP exhibits at low scan rate an irreversible wave, which was found to correspond to the exchange of one electron per mole of MBP. A reversible wave is visible at a lower potential (when the oxidation wave
5700 J. Phys. Chem. B, Vol. 101, No. 29, 1997
Guyard et al.
Figure 1. Cyclic voltammetry of a solution of monomethylbipyrrole (MBP) (C°) 2 10-3 mol L-1) on a 1 mm diameter platinum electrode in acetonitrile (+0.2 M NEt4BF4): (a) scan rate ) 0.2 V s-1; (b) variation of the oxidation peak potential with the scan rate.
TABLE 1: Electrochemical Characteristics, Absorption Spectra, and Decay Kinetics of the Bipyrrole Cation Radicals acetonitrile solvent a
cyclic
flash photolysis
d
water solvent voltammetryb
pulse radiolysisc
λmax (nm)
2kdim (L mol-1 s-1)
E° (V/SCE)
ks (cm s-1)
BP•+
580
1.2 × 109
0.600
0.5
MBP•+
590
1.2 × 109
0.460
1
8 × 108
DMBP•+
600
e
0.300
2
4 × 108
compd
2kdim (L mol-1 s-1) 109
λmax (nm)
(L mol-1 cm-1)
2kdim (L mol-1 s-1)
360 580 365 590 370 600
27000 6200 26000 7400 25000 7900
7.8 × 108 1.0 × 109 8.6 × 108
a
Using the molar absorptivities from pulse radiolysis in water. The standard uncertainties in the decay rate constants are estimated to be (20%. The uncertainties in the decay rate constants estimated from cyclic voltammetry are around a factor of 2 (CR-CR mechanism). c The uncertainties in the molar absorptivities and the decay rate constants are estimated to be (20%. d From ref 10d. e Not a second-order decay.
b
of the monomethylbipyrrole has been scanned before), which can be ascribed to the first oxidation wave of the dimethylquaterpyrrole (Figure 1a). When the scan rate is increased, a partial reversibility is observed for scan rates higher than 10 000 V s-1, indicating that the lifetime of the cation radical was in the same range as that of the cation radical of the unsubstituted bipyrrole (BP). Simultaneously, the wave ascribed to the electrogenerated dimethylquaterpyrrole disappears. To determine the nature of the kinetics following the electron transfer, it is necessary to examine the variation of the peak potential as a function of the scan rate (V) and the initial concentration (C°) of MBP. After repetitive experiments, we found that the oxidation peak potential varies linearly as a function of V with a slope close to 19 mV per 10-fold increase of V (Figure 1b) as it was previously observed for the oxidation of BP in dry acetonitrile.12d Unfortunately, we failed to obtain reliable results concerning the variation of the peak potential with the C°. Even if the variation of Ep with the scan rate does not allow us to reject completely the reaction of the cation radical with the starting MBP, this behavior is consistent with a rate-determining coupling of two cation radicals formed upon fast electron transfer from MBP (mechanism CR-CR):22
Considering this mechanism for the oxidation of bipyrrole and monomethylbipyrrole, we can estimate the dimerization rate
constants, kdim, by comparing the partially reversible voltammograms recorded at high scan rates with simulated curves (Table 1). The values measured (around 109 L mol-1 s-1) are in the same range as the lifetimes of the pyrrole monomer cation radical.3 The standard rate constants, ks, for the heterogeneous electron transfers from the electrode to the bipyrroles were estimated from the separation between the cathodic and anodic peak potentials to be in the range 0.5-1 cm s-1, corresponding to a fast electron transfer. The electrochemical oxidation of MBP in acetonitrile was also examined by spectroelectrochemistry in a thin layer cell using a platinum grid as an anode. This technique20c permits the observation of the absorption spectra of the compounds formed upon oxidation on a time scale longer than that of cyclic voltammetry (on the order of 30 s to several minutes). Upon oxidation at +0.5 V/SCE (i.e., at a potential more positive than the oxidation peak observed by voltammetry), we can observe first the formation of the dimethylquaterpyrrole (peak at 350 nm) and its cation radical (532 nm and absorption in the 1000 nm region), and later the dication (peak at 685 nm) (Figure 2). For comparison with these two bipyrroles, the oxidation of 5,5′-dimethyl-2,2′-bipyrrole (DMBP) was also examined. For fast cyclic voltammetry experiments, the reversibility for the oxidation of the dimethylbipyrrole appears at scan rates slightly lower (3000-5000 V s-1 for a millimolar concentration) than the values needed for the two other bipyrroles, indicating that the introduction of a second methyl group did not add appreciable stabilization to the cation radical. However, at low scan rates (0.2 V s-1), no new peak could be ascribed to the formation of a dimer at lower potentials. It is noticeable that for scan rates in the range 1-10 V s-1, an apparent reversibility of the oxidation wave of DMBP is visible;10d this reverse peak disappears when increasing or decreasing the scan rate. As
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J. Phys. Chem. B, Vol. 101, No. 29, 1997 5701
Figure 2. Spectroelectrochemistry showing the variation of the UVvisible spectra upon continuous electrochemical oxidation of 5.5 × 10-4 mol L-1 of methylbipyrrole (MBP) at 0.5 V/SCE in acetonitrile + 0.1 mol L-1 of NEt4BF4. Cell is 0.5 mm in length. Absorbances correspond to those for dimethylquaterpyrrole (DMQ), dimethylquaterpyrrole cation radical (DMQ•+), and dimethylquaterpyrrole dication (DMQ2+).
discussed before,10d this cathodic peak does not correspond to the reduction of the generated cation radical but to the reduction of another intermediate with a stability on the order of some tenths of a second. Pulse Radiolysis Experiments. Exposure of water to ionizing radiation leads to production of e-aq, H•, and OH•. The e-aq and OH• radical can be converted into purely oxidant species, such as N3• or Br2•- radicals, in N2O-saturated solutions containing NaN3 or NaBr. Oxidation of bipyrroles then occurs via reaction with N3• or Br2•-:
H2O f e-aq, H•, OH•, H+, H2, H2O2 eaq- + N2O f N2 + OH- + OH• OH• + N3- f OH- + N3• N3• + BP f BP•+ + N3or
OH• + Br- f OH- + Br• Br• + Br- f Br2•Br2•- + BP f 2Br- + BP•+ At pH 6, the absorption spectra observed in the pulse radiolysis of the bipyrroles, monitored several microseconds after the pulse (Figure 3) exhibit a sharp peak around 360 nm and a broad band centered at 560-600 nm. The spectrum was practically identical whether Br2•- or N3• were used as the oxidant. The rate constants for the oxidation reactions with methylbipyrrole (MBP) were measured by monitoring the formation rate at 600 nm as a function of MBP concentration. The value was found to be close to the diffusion-controlled limit, kox ) (8 ( 2) × 109 L mol-1 s-1 and kox ) (4.2 ( 1.0) × 109 L mol-1 s-1 for N3• and Br2•-, respectively. These values are not surprising, since the oxidation potential of the couple N3•/N3- (1.09 V/SCE)23 is 0.5 V higher than that of the BP/BP•+ couple, and the oxidation potential of the couple Br2•-/2Br- is even higher (1.37 V/SCE).23 For both oxidants, the formation of the cation
Figure 3. Pulse radiolysis showing differential absorption spectra of methylbipyrrole cation radical MBP•+ (-) (pH 6) and methylbipyrrole radical MBP• (- - -) (pH 11) in N2O-saturated aqueous solutions containing 0.1 mol L-1 NaN3 and 10-3 mol L-1 methylbipyrrole recorded 4 µs after the pulse.
TABLE 2: Absorption Spectra and Decay Kinetics of the Bipyrrole Radicals acetonitrilea
waterb
compd
λmax 2kdim λmax 2kdim (nm) (L mol-1 s-1) (nm) (L mol-1 cm-1) (L mol-1 s-1)
BP• MBP• DMBP•
540 530 550
1.0 × 109 6.8 × 108 3.8 × 108
540 530 560
5800 6800 7400
1.4 × 109 1.4 × 109 1.0 × 109
a Using the extinction coefficient from pulse radiolysis in water. The standard uncertainties in the decay rate constants are estimated to be (20%. b The uncertainties in the molar absorptivities and the decay rate constants are estimated to be (20%.
radical was sufficiently rapid to allow the study of subsequent reactions of the radicals without interference by the formation process. In alkaline solutions the spectrum was different from that observed at pH 6; the peak around 600 nm shifts to lower wavelengths and the peak at 360 nm shifts to 340 nm and its intensity decreases (Figure 3). The transient spectra recorded at pH 6 and pH 11 can be ascribed to the cation radical and the neutral radical of the bipyrroles, respectively, the neutral radical arising from the deprotonation of the cation radical.24 By comparing these results with those in the literature, we find that similar spectral changes were observed in the case of the cation radical of indole in water,24b,c confirming the assignment of the absorbances in the case of bipyrroles. The spectral characteristics of the neutral and the cation radicals are summarized in Tables 1 and 2. In the case of MBP, we monitored the variation of the absorbance at 600 nm as a function of pH to derive the pKa value for the cation radical. The absorbance was monitored at 20 µs after the pulse after the acid-base equilibrium was reached. A plot of the experimental data shows good agreement with the curve calculated for pKa ) 8.7 (Figure 4). The decay kinetics of the cation radicals and the neutral radicals were monitored at the 500-600 nm peak at various pH values for the three bipyrroles. The decay kinetics were recorded with different initial radical concentrations (different dose per pulse) and were found to fit a second-order rate law. This kinetics behavior is in agreement with the electrochemical experiments in acetonitrile, where the peak potential variation against the scan rate indicates that the cation radical decays by reaction with another cation radical (mechanism CR-CR).25 The rate constants were found to be 2k ) (7.8 ( 2) × 108 L mol-1 s-1 and (1.0 ( 0.2) × 109 L mol-1 s-1, respectively, for the bipyrrole and the methylbipyrrole cation radicals, about the same as the values determined by cyclic voltammetry in acetonitrile.
5702 J. Phys. Chem. B, Vol. 101, No. 29, 1997
Figure 4. Pulse radiolysis of MBP showing the variation of the optical absorption at 600 nm as a function of pH. Solutions are as in Figure 3. The absorbance is monitored 20 µs after the pulse.
To rule out the existence of a CR-S mechanism in the case of BP and MBP, we examined whether the decay rates were dependent on the concentration of the bipyrrole. The upper limit estimated for addition of BP•+ (or MBP•+) to a neutral BP (or MBP) molecule is k ≈ 2 × 106 L mol-1 s-1. However, during an exhaustive polymerization reaction, different types of oligomers (dimer, trimer, tetramer, etc.) are formed and are present in solution with the monomer. Thus, one could envisage that cross-coupling reactions between a cation radical of one type of oligomer and a different neutral oligomer may change the nature of the chemical process. Because the monomeric pyrrole is not oxidized by the N3• radical, we could check the possibility of reaction between BP•+ and the pyrrole monomer by monitoring the change of the decay rate upon addition of large concentrations of pyrrole. We found that the decay of BP•+ was not increased by the addition of 2 × 10-2 mol L-1 of pyrrole. From this, we estimate an upper limit of k ≈ 1 × 105 L mol-1 s-1 for the reaction of BP•+ with pyrrole monomer. Aqueous solutions of bipyrroles containing NaN3 and N2O were also irradiated in a Gammacell with various doses, and the spectra of the final products were monitored after each irradiation. At low pH, broad absorptions were formed over the whole range monitored, from 300 to 800 nm. Although broad maxima appeared around 360 and 700 nm for BP and around 600 nm for MBP, the main feature is the increasing absorbance at all wavelengths, indicating scattering by small particles that are formed by the radiolysis. Photolysis Experiments. We found that continuous irradiation of a solution of bipyrrole by an UV-lamp in acetonitrile in the presence of 10-2 mol L-1 CCl4 (acting as an irreversible electron acceptor) results first in a change of color of the solution and then in the appearance of a black precipitate. To study in more detail this photochemical process, we irradiated the same solution with a short pulse of an excimer laser (λ ) 308 nm) and recorded the spectra of the generated stable products after one and then several pulses to obtain the spectra at long times. As seen from Figure 5, these spectra show the formation of stable products (stable for at least several minutes) and display a large absorption with a maximum at 345 nm and a broader absorption in the 600 nm region. These absorptions are indicative of the formation of the dimer quaterpyrrole and its dication (see Experimental Section). After several pulses, increasing absorbance at all wavelengths was noticed, indicating scattering by small particles (at least insoluble oligomers). After several more pulses, the solutions contained a black solid. The same experiments performed with the methylbipyrrole resulted in changes at 350 nm and at 535 and 980 nm, which are indicative of the formation of a mixture of dimethylquaterpyrrole
Guyard et al.
Figure 5. Flash photolysis showing spectral changes of a solution containing 1.5 × 10-3 mol L-1 bipyrrole (BP) in acetonitrile + 0.1% CCl4, in a 2.5 × 10-2 mol L-1 acidic buffer 1/1(3-chloropyridinium/ 3-chloropyridine) after one and then several pulses at 308 nm. The number on each curve is the total number of pulses.
and its cation radical, respectively.26 All these spectra are very similar to those obtained after electrochemical oxidation (electrolysis) or after homogeneous oxidation by FeCl3 and suggest that the same chemical steps are involved in photochemical and electrochemical oxidation. It should be emphasized that bipyrrole derivatives are oxidized and dimerized, leading to formation of the quaterpyrrole and to longer oligomers, not only by electrochemical oxidation but also by UV irradiation. Differential absorption spectra of bipyrroles in acetonitrile solutions containing 0.1% CCl4 were recorded immediately after laser pulse irradiation (Figure 6). Two absorptions peaks were observed: a very intense peak in the 350 nm range and a broader weaker absorption in the 500-650 nm range. Comparison with the spectra recorded by pulse radiolysis reveals that these absorptions correspond to a mixture of bipyrrole cation radical and neutral radical. The formation of these intermediates can proceed through either a biphotonic or a monophotonic process, involving either the excited singlet or triplet states of the bipyrrole.15 Analysis of the maximum absorption as a function of the energy of the excitation pulse shows a linear dependence, which allows one to conclude that mainly a monophotonic process is involved. In a similar situation concerning the photochemical production of the cation radical of the unsubstituted terthiophene in dichloromethane, it was concluded that the process did not involve the triplet state but a monophotonic photoionization of the terthiophene.15c Because bipyrroles are easier to oxidize than the unsubstituted terthiophene (E° ) 1.11V/SCE),27 a monophotonic photoionization is highly possible with bipyrroles (E° ) 0.6 V/SCE). This low oxidation potential may explain some of our preliminary experiments, where we irradiated solutions of the pyrrole monomer containing the same electron acceptor. In this case, we observed some modifications of the spectra of the solution but we did not find any evidence for the formation of bipyrrole, suggesting a low yield for cation radical formation. We think that this difference in behavior may be related to the much lower oxidation potential for bipyrrole compared with pyrrole (E° ) 1.31 V/SCE).3a Nevertheless, in both mechanisms, generation of the cation radical of BP will also result in the formation of the neutral radical CCl3• and of the chloride ion in equal concentrations. In oxygenated solutions, CCl3• reacts rapidly with O2 (k ) 3.3 × 109 L mol-1 s-1) to form the peroxyl radical.28 This radical is a strong oxidant29 able to react with BP to form another BP•+ and CCl3O2- at a rate constant close to the diffusion-controlled limit. Therefore, to decrease the influence of reactions with the species arising from the reduction of CCl4 on the kinetics
Bipyrroles
J. Phys. Chem. B, Vol. 101, No. 29, 1997 5703
Figure 7. Flash photolysis showing the variation of the optical absorption at 600 nm as a function of pH for acetonitrile solution containing 0.1% CCl4 and 6.9 × 10-3 mol L-1 of methylbipyrrole (MBP) 400 µs after the pulse.
Figure 6. Transient absorption spectra in acetonitrile + 0.1% CCl4, 4 µs after the laser pulse, in a 2.5 × 10-2 mol L-1 acidic buffer 1/1 (3-chloropyridinium/3-chloropyridine) (-) and in presence of 2 × 10-2 mol L-1 of tert-butylamine (- - -) containing (a) 1.5 × 10-3 mol L-1 of bipyrrole (BP), (b) 7 × 10-5 mol L-1 of methylbipyrrole (MBP), and (c) 6.3 × 10-5 mol L-1 of dimethylbipyrrole (DMBP).
studies, we chose to work in the presence of oxygen (because peroxyl radicals have a lower tendency to add to aromatic molecules than alkyl radicals) and also in buffered solutions (because CCl3O2- is a strong base able to deprotonate BP•+, which explains the formation of a mixture of cation radical and neutral radical in experiments performed in unbuffered acetonitrile). Thus, we can summarize the first part of the mechanism by the general scheme: hν
BP 98 BP•+ + CCl3• + ClCCl3• + O2 f CCl3O2• CCl3O2• + BP f CCl3O2- + BP•+ As we found in the pulse radiolysis experiments, the spectra are different in acidic and basic media. In acidic solution buffered with 3-chloropyridine/3-chloropyridinium (pKa ) 9.0)14b buffer, an absorbance with a maximum at 580 nm is observed. When the same experiment is performed in a solution containing 10-3 mol L-1 of a strong base, such as tertbutylamine, the absorption shifts to a lower wavelength (λmax ) 540 nm) and the peak at 360 nm decreases (Figure 6). Comparison with the pulse radiolysis results allows us to unambiguously ascribe these spectra to the cation radical BP•+ and the neutral radical BP•, respectively. Their properties are
summarized in Tables 1 and 2. To obtain more information about the acid-base properties of bipyrroles in acetonitrile, we investigated the photooxidation of MBP as a function of the pH and the nature of the base. We measured the absorption at a long time after the pulse (400 µs) with high concentrations of buffer to reach the equilibrium between the cation radical and the neutral radical. Under these experimental conditions, we found that the concentration of cation radical mainly depends on the pH and not on the nature of the base (Figure 7). By fitting the data to the theoretical curve, we found a pKa value of 16.6, indicating that the cation radical of MBP is a weak acid in acetonitrile, as it is in water. At pH values above the pKa of the cation radical, its deprotonation to the neutral radical can be observed at short times. This decay was measured against the base concentration, and we found values of k ) 2.2 × 109 L mol-1 s-1 and k ) 1.1 × 106 L mol-1 s-1 for the reactions with tert-butylamine (pKa ) 18.1)14b and 2,6-lutidine (pKa ) 15.4),14b respectively, which confirms that proton exchange is rapid even for bases with a pKa value lower than that of MBP•+. The decay kinetics of the cation radicals and the neutral radicals of the bipyrroles were monitored at the 600 nm peak in acidic conditions (3Cl-pyridine/3Cl-pyridium 1/1 buffer) and in the presence of base (tert-butylamine) at 540-550 nm. The decay processes were recorded with different radical concentrations (different energies of the excitation pulse) and different initial concentrations of bipyrrole (7 × 10-5 to 7 × 10-4 mol L-1). In the case of BP and MBP, a second-order decay was found for the cation radical that was not influenced by the initial concentration of neutral bipyrrole. As discussed above, this behavior is in agreement with the electrochemical experiments in acetonitrile and the pulse radiolysis results and confirms the occurrence of a coupling between two cation radicals in the carbon-carbon bond formation. Assuming the same molar absorptivity, , for the cation radicals of bipyrrole in water and in acetonitrile, we derived the value of the rate constants of the carbon-carbon bond formation from the decay. A good agreement was found between the estimations from cyclic voltammetry (electrochemistry) and the flash photolysis measurements. In basic media, second-order decays for the neutral radicals were also observed with slightly different rate constants. However, the differential spectra recorded a long time after the pulse (1 ms) were very different in acidic and basic media. Figure 8 shows that a new absorbance with λmax ) 370 nm is formed in acidic media (corresponding to the compound formed after the coupling of two MBP•+) and that, in contrast, only a broad absorption with a maximum below 340 nm is found
5704 J. Phys. Chem. B, Vol. 101, No. 29, 1997
Figure 8. Flash photolysis showing spectral changes of a solution containing 7 × 10-5 mol L-1 methylbipyrrole (MBP) in acetonitrile + 0.1% CCl4, 1 ms after the pulse, in a 2.5 × 10-2 mol L-1 acidic buffer 1/1 (3-chloropyridinium/3-chloropyridine) (-) and in the presence of 2 × 10-2 mol L-1 of tert-butylamine (- - -).
in basic media. Moreover, examination of the solution spectra after one and then several pulses reveals that dimethylquaterpyrrole was formed in acidic media but not in basic media. These results demonstrate that only the cation radical, but not the neutral radical, of bipyrrole leads to the formation of quaterpyrrole and further to longer oligomers or polymers. For the dimethylbipyrrole, DMBP, the lifetime of the cation radical appears to be slightly longer than for the two other bipyrroles. However, the decay of DMBP•+ appears more complicated with a combination between a first- and a secondorder rate law, indicating a different mechanism. This is clearly due to the fact that the coupling reaction on the R-terminal positions is hindered by the two methyl groups. Discussion Carbon-Carbon Bond Formation. The results presented above show that the photochemical or electrochemical oxidations of bipyrrole and monomethylbipyrrole lead first to the cation radical. The cation radical of bipyrrole, BP•+, can be characterized by its redox potential in cyclic voltammetry and displays the same UV-visible spectra in flash photolysis experiments in acetonitrile and in pulse radiolysis studies in water. The same kinetics behavior is observed by all techniques in both solvents; BP•+ decays by a second-order process. This reaction corresponds to the coupling between two BP•+ to form a protonated quaterpyrrole (CR-CR mechanism), which has been identified by comparison with authentic samples. The kinetics constants measured in both solvents by the different methods are of the same magnitude (109 L mol-1 s-1) and indicate a fast coupling step. However, the bipyrrole cation radical does not react with the starting bipyrrole or added monomeric pyrrole. Influence of π-Dimerization on the Coupling Reaction. As found by Meijer et al. concerning the oxidation of end-capped oligopyrroles, cation radicals of oligopyrrole can exist under a diamagnetic association of two cation radicals (“π-dimer”).10c In previous publications, it has been proposed that carboncarbon bond formation can occur, not directly between two free cation radicals but inside the π-dimer that would serve as a preformed complex.30 However, if we consider the equilibrium constant for the π-dimerization of the quaterpyrrole cation radical (K ) 70 L mol-1; see Supporting Information section), we find a value at least 2 orders of magnitude lower than the values reported for the π-dimer of quaterthiophene cation radicals.31 This shows that under our experimental conditions at room temperature, the π-dimer is not the major species for the quaterpyrrole cation radical. As was found in the thiophene
Guyard et al. series, it is likely that the equilibrium constant for π-dimerization decreases when going to smaller oligomers21,31 and that this constant is very small for the bipyrrole cation radical, meaning that the formation of the π-dimer is thermodynamically unfavorable. Thus, a mechanism involving the passage through the π-dimer prior to carbon-carbon bond formation would be in total disagreement with the high rate constants found for the reaction between two BP•+. Deprotonation Reactions. After the carbon-carbon bond formation reaction, a deprotonation reaction will follow to produce the final quaterpyrrole, which was clearly observed after oxidation. This reaction is an important step in the real polymerization reaction, where the dication must eliminate two protons to permit the polymer to grow. Although the effects of base and water in this polymerization process have received considerable attention, their exact role has not yet been established.12,13 Pyrrole and oligopyrroles are weak bases that undergo protonation at the 2- and 3-positions, which under acid conditions can react with another pyrrole to form a nonconjugated dimer (2,2′-(1′-pyrrolinyl)pyrrole) and other nonconducting polymers.32 It has been suggested that the defects observed in polypyrrole can be related to the formation of such nonconjugated oligomers.12f We noticed that quaterpyrrole was not stable in acetonitrile at pH below 7-8, and it is likely that a protonated quaterpyrrole may undergo the same type of condensation reactions. So to limit the formation of defects in the polymer, it is better to deprotonate the generated protonated oligopyrroles (here (BP)22+ ) as fast as possible and also to trap the released protons to avoid the protonations of the other pyrrole species (the starting bipyrrole, for example). The exact mechanism of the two C-H bonds cleavage is kH
difficult to study, and only the global kinetics ((BP)22+ 98 quaterpyrrole + 2H+) can be easily investigated. To study the deprotonation step, one should take into account that the oxidation potential of the oligopyrrole decreases with increasing size of the oligomer. For example, we found that quaterpyrrole was more readily oxidized than bipyrrole, which is more readily oxidized than pyrrole.3a Consequently, during the dimerization process, the quaterpyrrole formed after the deprotonation reaction will immediately be oxidized to its cation radical (n ) 1) or to its dication (n ) 2) depending of the value of the oxidation potential. As a result, the apparent number of electrons per monomer varies from 1 to (2 + n)/2 as the deprotonation of the quaterpyrrole becomes more and more rapid:
BP - e- f BP•+ kdim
2BP•+ 98 (BP)22+ kH
(BP)22+ 98 quaterpyrrole + 2H+ quaterpyrrole + BP•+ f quaterpyrrole•+ + BP In our present study in neutral acetonitrile, cyclic voltammetric experiments performed at a low scan rate (0.2 V s-1) show that the oxidation wave of MBP corresponds to 1 electron/mol. The peak current for the generated dimer was also quite small (less than half of the oxidation peak of MBP), indicating that a low concentration of dimer was formed during the scan. It has been suggested that the low number of electrons is due to the fact that the released protons attack half of the parent pyrrole, resulting in an overall one-electron oxidation.12c However, the possible inhibition of half of the starting MBP does not explain the low concentration of formed quaterpyrrole during the time
Bipyrroles
J. Phys. Chem. B, Vol. 101, No. 29, 1997 5705
of the voltammetric scan.33 However, as found by Cava et al. for the oxidation of mixed thiophene-pyrrole oligomers,8b this behavior is in agreement with a slow deprotonation rate constant, kH, following the fast irreversible coupling between two cation radicals. At the lowest scan rates, the number of electrons (measured by comparison with the reversible wave of the ferrocene at the same scan rate) increases just a little (1.05 at 50 mV s-1, 1.07 at 20 mV s-1). From that, we estimated that the deprotonation rate constant, kH, was lower than 0.1 s-1 in acetonitrile. Cava et al. have found that the deprotonation of the dimer dication ranges from 0.1 to 2 s-1.8b These low values confirm that the deprotonation of (BP)22+ occurs a long time after the carbon-carbon bond formation. This slow acidity can be compared to the fast acid-base equilibrium found for the MBP•+/MBP• couple. The nature of the deprotonation is different in the two cases in the sense that it involves the cleavage of an N-H bond in one case24 and the cleavage of C-H bonds in the other. Low values for the deprotonation rates are generally found for this second class of acids even when the acids are strong, and several experimental situations have already been reported.34 At this point one might wonder whether it would be possible to make the deprotonation reaction more rapid and or at least to improve the quality of the polymer by adding bases stronger than acetonitrile or residual water.35 When we added strong bases to the solution 3,5-lutidine or tert-butylamine, the number of electrons increased considerably (1.6e- for 10-2 mol L-1 of lutidine at 0.2 V s-1), but at the same time the oxidation peak of the generated quaterpyrrole disappeared, confirming the inhibition of the coupling by bases12,13 and in agreement with our flash photolysis and spectroelectrochemical experiments. We found that under these conditions, MBP•+ lost a proton to form MBP•. By analogy with the behavior of aromatic cation radicals in the presence of base,36 it is likely that the neutral radical MBP• is easier to oxidize than the starting bipyrrole. This second oxidation would explain the large increase of the electron number for oxidation of the bipyrrole when the quaterpyrrole is not produced, revealing the occurrence of a different ECE mechanism in this case:
MBP - e- f MBP•+ MBP•+ + base f MBP• + base H+ MBP•+ + MBP• f MBP + product Thus, we may propose an explanation to the effect of base against the polymerization. A low basicity of the medium will help the deprotonation step and can also act as a proton scavenger to impede the protonation of the parent molecules. In contrast, if deprotonation occurs before the coupling step, the neutral radical will be formed and defects will be created in the polymer. Conclusions The coupling of bipyrrole can be achieved not only by chemical and electrochemical oxidation but also by photochemical oxidation (UV-light irradiation of the solution containing CCl4 acting as an irreversible electron acceptor). These oxidation processes involved the formation of the bipyrrole cation radical. Convergent results obtained by cyclic voltammetry, flash photolysis, and pulse radiolysis provide the first direct evidence that only the cation radical leads to the formation of longer oligomers or polypyrrole, whereas the neutral radical does not (i.e., that deprotonation 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 carbon-carbon bond formation, in agreement with 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 creates a new ECE mechanism. A proton 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 BP•+ begins to compete with the coupling reaction. The overall mechanism can be summarized in the following general scheme:
Acknowledgment. We thank Professor P. Audebert (Universite´ de Besanc¸ on, France) for helpful discussions concerning the reactivity of pyrroles and Dr. P. Guiriec for his help in spectroelectrochemical experiments. We also thank Dr. E. W. Meijer and Dr. L. Groenendaal (University of Eindhoven, Netherlands) for the gift of a sample of N-(tert-butoxycarbonyl)quaterpyrrole and helpful discussions concerning the π-dimer of stable oligopyrrole cation radicals. Supporting Information Available: Discussion, table, and figure of UV-visible spectral data for cation radicals and dications of quaterpyrrole and dimethylquaterpyrrole (2 pages). Ordering information is given on any current masthead page. References and Notes (1) (a) Universite´ de Franche Comte´. (b) Universite´ Denis Diderot (Paris 7). (c) National Institute of Standards and Technology. (2) (a) Ba¨uerle, P. AdV. Mater. 1993, 5, 879. (b) Tour, J.-M. Chem. ReV. (Washington, D.C.) 1996, 96, 537. (3) (a) Andrieux, C. P.; Audebert, P.; Hapiot, P.; Save´ant, J.-M. J. Am. Chem. Soc. 1990, 112, 2439. (b) Andrieux, C. P.; Audebert, P.; Hapiot, P.; Save´ant, J.-M. J. Phys. Chem. 1991, 95, 10158. (4) Ge´nies, E. M.; Bidan, G.; Diaz, A. F. J. Electroanal. Chem. 1983, 149, 101. (5) (a) Schichiri, T.; Toriumi, M.; Tanaka, K.; Yamabe, T.; Yamauchi, J.; Deguchi, Y. Synth. Met. 1989, 33, 389. (b) Lowen, S. V.; Van Dyke, J. D. J. Polym. Sci., Part A: Polym. Chem. 1990, 28, 451. (6) (a) Satoh, M.; Imanishi, K.; Yoshino, K. J. Electroanal. Chem. 1991, 317, 139. (b) Wei, Y.; Chan, C. C.; Tian, J.; Jang, G. W.; Hsueh, K. F. Chem. Mater. 1991, 3, 888. (7) We can distinguish two limiting subcases in the Cr-S coupling. One will be denoted Cr-Sirr, in which the coupling step is rate determining, and a second one denoted CR-Srev, in which the coupling step acts as a preequilibrium versus the following rate-determining electron-transfer step.3b (8) (a) Zotti, G.; Schiavon, G.; Berlin, A.; Pagani, G. Chem. Mater. 1993, 5, 430. (b) Niziurski-Mann, R. E.; Scordilis-Kelley, C.; Liu, T.-L.; Cava, M. P.; Carlin, R. T. J. Am. Chem. Soc. 1993, 115, 887. (c) Audebert, P.; Catel, J.-M.; Le Coustumer, G.; Duchenet, V.; Hapiot, P. J. Phys. Chem.
5706 J. Phys. Chem. B, Vol. 101, No. 29, 1997 1995, 99, 11923. (d) Hapiot, P.; Gaillon, L.; Audebert, P.; Moreau, J. J. E.; Le`re-Porte, J.-P.; Wong Chi Man, M. J. Electroanal. Chem., in press. (9) (a) Martina, S.; Elkelmann, V.; Schlu¨ter, A.-D.; Wegner, G. Synth. Met. 1992, 51, 299. (b) Groenendaal, L.; Peerlings, H. W. I.; Van Dongen, J. L. J.; Havinga, E. E.; Vekemans, J. A. J. M.; Meijer, E. W. Macromolecules 1995, 28, 116. (c) Groenendaal, L.; Peerlings, H. W. I.; Havinga, E. E.; Vekemans, J. A. J. M.; Meijer, E. W. Synth. Met. 1995, 69, 467. (10) (a) Zotti, G.; Martina, S.; Wegner, G.; Schlu¨ter, A.-D. AdV. Mater. 1992, 4, 798. (b) 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. (c) Van Haare, J. A. E. H.; Groenendaal, L.; Havinga, E. E.; Janssen, R. A. J.; Meijer, E. W. Angew. Chem., Int. Ed. Engl. 1996, 35, 638. (d) Andrieux, C. P.; Hapiot, P.; Audebert, P.; Guyard L.; Nguyen Dinh An, M.; Groenendaal, L.; Meijer, E. W. Chem. Mater. 1997, 9, 723. (11) Jones, R. A. The Chemistry of Pyrroles; Academic Press: New York, 1977; p 11. (12) (a) Diaz, A. F.; Kanazawa, K. K.; Gardini, G. P. J. Chem. Soc., Chem. Commun. 1979, 635. (b) Downard, A. J.; Pletcher, D. J. Electroanal. Chem. 1986, 206, 139. (c) Zotti, G.; Schiavon, G.; Berlin, A.; Pagani, G. Electrochim. Acta 1989, 34, 881. (d) Beck, F.; Oberst, M.; Janssen, R. Electrochim. Acta 1990, 35, 1841. (e) Qian, R.; Pei, Q.; Huang, Z. Makromol. Chem. 1991, 192, 1263. (f) Raymond, D. E.; Harrison, D. J. J. Electroanal. Chem. 1993, 355, 115. (13) (a) Downard, A. J.; Pletcher, D. J. Electroanal. Chem. 1986, 206, 147. (b) Visy, C.; Lukkari, J.; Kankare, J. Synth. Met. 1994, 66, 61. (14) (a) Anne, A.; Hapiot, P.; Moiroux, J.; Neta, P.; Save´ant, J.-M. J. Phys. Chem. 1991, 95, 2370. (b) Anne, A.; Fraoua, S.; Hapiot, P.; Moiroux, J.; Save´ant, J.-M. J. Am. Chem. Soc. 1995, 117, 7412. (15) (a) Evans, C. H.; Scaiano, J. C. J. Am. Chem. Soc. 1990, 112, 2694. (b) Zinger, B.; Mann, K. R.; Hill, M. G.; Miller, L. L. Chem. Mater. 1992, 4, 1113. (c) Wintgens, V.; Valat, P.; Garnier, F. J. Phys. Chem. 1994, 98, 228. (16) The mention of commercial equipment or material does not imply recognition or endorsement by the National Institute of Standards and Technology nor does it imply that the material or equipment identified are necessarily the best available for the purpose. (17) Neta, P.; Huie, R. E. J. Phys. Chem. 1985, 89, 1783. (18) Schuler, R. H.; Patterson, L. K.; Janata, E. J. Phys. Chem. 1980, 84, 2088. (19) Garreau, D.; Save´ant, J.-M. J. Electroanal. Chem. 1972, 35, 309. (20) (a) Andrieux, C. P.; Garreau, D.; Hapiot, P.; Pinson, J.; Save´ant, J.-M. J. Electroanal. Chem. 1988, 243, 321. (b) Andrieux, C. P.; Hapiot, P.; Save´ant, J.-M. Chem. ReV. (Washington, D.C.) 1990, 90, 723. (c) Lexa, D.; Save´ant, J.-M.; Zickler, J. J. Am. Chem. Soc. 1977, 99, 2786. (21) Miller, L. L.; Mann, K. R. Acc. Chem. Res. 1996, 29, 417. (22) (a) Theoretical slopes for the variation of the peak potential against the scan rate and the initial concentration of bipyrrole are at 20 °C: 19.4
Guyard et al. and -19.4 mV/decade for CR-CR, 29.1 and -29.1 mV/decade for CRSirr, and 19.4 and -38.7 mV for CR-Srev.22b,c (b) Andrieux, C. P.; Nadjo, L.; Save´ant, J.-M. J. Electroanal Chem. 1973, 42, 223. (c) Nadjo, L.; Save´ant, J.-M. J. Electroanal. Chem. 1973, 48, 113. (23) Stanbury, D. M. AdV. Inorg. Chem. 1989, 33, 69. (24) (a) In the case of indole and tryptophan,24b,c it was found that the acidity involves the N-H bond, in agreement with other aromatic N-heterocycle compounds.24d (b) Bent, D. V.; Hayon, E. J. Am. Chem. Soc. 1975, 97, 2612. (c) Shen, X.; Lind, J.; Mere´nyi, G. J. Phys. Chem. 1987, 91, 4403. (d) Bent, D. V.; Hayon, E.; Moorthy, P. N. Chem. Phys. Lett. 1974, 27, 544. (25) A first-order decay is expected for a mechanism involving the reaction between the cation radical and the starting bipyrrole because the bipyrrole is in large excess compared to the quantity of generated cation radical during the pulse (some 10-6 mol L-1) (mechanism CR-S). (26) It should be noted that, because of the low oxidation potential of quaterpyrroles, it is difficult to make conclusions if the oxidation of the dimer is related to the photochemical process or to subsequent oxidation under air. (27) Audebert, P.; Catel, J.-M.; Le Coustumer, G.; Duchenet, V.; Hapiot, P. In preparation. (28) Mo¨ning, J.; Bahnemann, D.; Asmus, K. D. Chem. Biol. Interact. 1983, 47, 15. (29) (a) E° for CCl3O2•/CCl3O2- was estimated as 1.15 V/NHE.29b (b) Mere´nyi, G.; Lind, J.; Engman, L. J. Chem. Soc., Perkin Trans. 2 1994, 2551. (30) Ba¨uerle, P.; Segelbacher, U.; Maier, A.; Mehring, M. J. Am. Chem. Soc. 1993, 115, 10217. (31) (a) Zotti, G.; Martina, S.; Wegner, G.; Schlu¨ter, A.-D. AdV. Mater. 1992, 4, 798. (b) Hapiot, P.; Audebert, P.; Monnier, K.; Pernaut, J.-M.; Garcia, P. Chem. Mater. 1994, 6, 1549. (32) (a) Potts, H. A.; Smith, G. F. J. Chem. Soc. 1957, 4018. (b) Naqvi, N.; Fernando, Q. J. Org. Chem. 1960, 25, 551. (c) Chiang, Y.; Whipple, E. B. J. Am. Chem. Soc. 1963, 85, 26. (d) Chiang, Y.; Whipple, E. B. J. Am. Chem. Soc. 1963, 85, 2763. (e) Smith, G. F. AdV. Heterocycl. Chem. 1963, 2, 287. (33) Spectroelectrochemical experiments indicate that the yield of produce dimethylquaterpyrrole is higher than 70% (see Figure 2). (34) (a) See, for example, refs 14b and 34b and references therein. (b) Hapiot, P.; Lorcy, D.; Tallec, A.; Carlier, R.; Robert, A. J. Phys. Chem. 1996, 100, 14823. (35) When 1% of water was added to the solution, little change was found in the voltammogram of MBP, just a small increase of the wave of the produced dimethylquaterpyrrole. (36) (a) See, for example, ref 36b and references therein. (b) Schlesener, C. J.; Amatore, C.; Kochi, J. K. J. Am. Chem. Soc. 1984, 106, 7472.
