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J. Electroanal. Chem., 318 (1991) 235-246 Elsevier Sequoia S.A., Lausanne

JEC 01746

Charge transfer studies of pyrrole copolymers substituted by anthraquinone, phenothiazine or anthracene moieties

Claude P. Andrieux l , Pierre Audebert and Clarisse Salou

Laboratoire d’Electrochimie Moltfculaire, U.R.A. CNRS 438, Universite’ de Paris VII, 2 PI. Jussiey 75005 Paris (France)

(Received 14 March 1991: in revised form 19 July 1991)

Charge transfer to the functionalized group has been investigated in several functionalized homopolymers and copolymers of pyrrole. The cases of anthraquinone, phenothiazine and anthracene moieties with the substituent linked to the nitrogen (N-polymers) and of anthraquinone and anthracene with the substituent linked to the 3-position of pyrrole (3-polymers) have been examined. As previously described, the copolymers were synthesized “at random” from binary solutions of monomers. The apparent diffusion coefficients of the electrons, determined for all the N-polymers over a large dilution range, showed significant variations with the relative proportion of the substituted group in the polymer and a strong dependence on the nature of the electroactive group. In contrast, there were only small variations with the nature of the supporting electrolyte and none at all with its concentration. Fewer results were obtained on the 3-polymers because of the unexpected reactivity of the pyrrole radical cation with the anthracene moieties which led to a very poor polymerization yield.

INTRODUCTION

The preparation of modified electrodes based on covalently functionalized polypyrrole or copolymers is attracting increasing attention [l-12]. In the course of our current research on derivatized polypyrrole modified electrodes [5-81, we became particularly interested in examining the kinetic transport phenomena in such electrode coatings. However, to separate the effects of the polymer backbone from those due to the specific characteristics and immediate surroundings of the sites, it was necessary to study polymers where these characteristics can be changed independently, i.e. to be able to modify both the kind and the dilution of the derivatized redox centres without affecting the general polymer structure or,

l To whom correspondence should be addressed.

0022-0728/92/$05.00 0 1992 - Elsevier Sequoia S.A. All rights reserved

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0

SOT 0- ‘aqz-

PHAS PHNPH

&-” &&~o-w6~

0 PHANC 3-PHAS

Scheme 1

3-PHANC

conversely, to modify the polymer structure without affecting the proportion and number of derivatized centres. In previous studies [13,141, we have demonstrated that this can be achieved using the following basal molecules (Scheme 1): (l-(pyrrol-1-yl)-hex-6-yl)-9,10-anthraquinone sulphonate (PI-US), (l-(pyrrol-1-yl)-hex-6- yl)-9-anthracene carboxylate (PHANC) and (1-(pyrrol-1-yl)-hex-6-yl)-N-phenothiazine (PHNPH).

These pyrroles, functionalized by an anthraquinone (AQ), a phenothiazine (PH) and an anthracene moiety respectively, allow the preparation and study of homopolymers and copolymers which satisfy the conditions detailed above. This is due to the very similar steric hindrance associated with the different electroactivity range of the two anthraquinone and phenothiazine substituents, while anthracene acts only as a diluent for the other moieties. This approach is rather similar to the work done in classical polymers with osmium and ruthenium sites in various concentrations [15l. In our previous study, we investigated the behaviour of the copolymers by means of cyclic voltammetry allied with other physico-chemical data; in this paper, our interest will be focused mainly on the charge transfer in N-polymers (measured using chronoamperometry) in relation to both the kind of functionalized moiety and to its dilution inside the polymer. Almost all possible dilutions in the N-copolymers were obtained and examined. In a similar way we checked the influence of the nature and concentration of the electrolyte salt used in electrochemical experiments on films.

An important aim of this analysis was to determine whether the conductive backbone of the polymer participates in the conduction between the redox sites, or if these polymers simply behave like classical polymers where the conduction occurs by exchange of electrons between neighbouring sites. In the latter case the

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kinetic law depicting the charge propagation is, to a first approximation, a diffusion law in which the diffusion coefficient D, can be expressed as [16,17]

D, = k&Y

where k, is the volume rate constant of electron transfer between adjacent redox sites, 6 is the average optimal electron hopping range and co is the total concentration of redox sites. The effects of migration and ion pairing are required to improve the theoretical treatment, but it has been shown [l&19] that Cottrell’s law remains valid in the framework of potentiostatic step experiments.

In addition to this complete study of the N-polymers, we briefly examined the behaviour of some 3-copolymers obtained with 3-PIUS and either 3-PHANC (see Scheme 1) or simple pyrrole (according to the following scheme, 3-PHAS and 3-PHANC are the homologues of PHAS and PHANC respectively, but with the substituent linked to the 3-position of the pyrrole ring). In fact, the homopolymer poly(3-PHAS) is characterized by a more efficient electron transfer indicated by an increase of about two orders of magnitude in the measured diffusion coefficient. This result was not fully understood, and so a study of copolymers including 3-PI-LAS was expected to clarify the reason for such an increase when compared with N-polymers. As previously stated, all the copolymers were prepared by random electropolymerization which was demonstrated to be a suitable method of producing relatively homogeneous copolymers where the two redox systems of the anthraquinone and the phenothiazine could be observed separately in their electroactivity range [14].

EXPERIMENTAL

Monomer synthesis

The synthesis of the three monomers PI-LAS, PHANC and PHNPH (starting from N-pyrrole-w-hexanol) has been described previously [14]. The synthesis of the 3-functionalized monomers has been reported elsewhere [20].

Polymer synthesis and electrochemical study

As reported previously [14], all the polymers, including the 3-polymers, were prepared by controlled potential oxidation, at + 1.06 V versus a standard calomel electrode (SCE) (no difference was noticed between the optimal polymerization potential of 3-polymers and N-polymers), of 2-4 X 10m3 M solutions of pure monomers or monomer mixtures in acetonitrile with 10-l M LiClO, as the electrolyte salt. The electrolyte solvent was obtained by standing HPLC grade acetonitrile (Merck) on activated alumina (Woelm; dried for 48 h at 150°C under vacuum) for 1 h. A platinum electrode 2 mm in diameter was used throughout the whole analytical study, and typical polymerization charges were in the lop4 C range (for more precision, see the captions to the figures). For copolymer synthe-

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sis, solutions of the two monomers were obtained starting from a 2 x lop3 M solution of the more concentrated compound in acetonitrile containing LiClO,, to which were added suitable amounts of the other compound in the solid state. After synthesis, films were thoroughly rinsed before being transferred into a clean electrolyte solution. The behaviour of the phenothiazine redox system was studied in 0.1 M LiClO,-acetonitrile, and the behaviour of the anthraquinone system was studied in dimethyl sulphoxide (DMSO) containing either 0.3 M tetrabutylammo- niumhexafluorophosphate (TBAFP) or 0.3 M LiClO,. In the case of the experiments to evaluate the influence of the electrolyte salt, the concentration of the salt was as stated in the figure captions. The choice of acetonitrile to observe the behaviour of the phenothiazine system was justified by the observation of the degradation of the phenothiazine-substituted films upon cycling in DMSO.

The electrochemical method applied was the single potential step. As expected, the final potential value had little influence on the i(t) curves provided that it was chosen far enough from the redox potential of the system observed, and therefore values of - 0.8 V and + 0.8 V were chosen for the anthraquinone and phenothiazine systems respectively. In constrast, we have checked that in the case of anthraquinone electrochemistry the starting potential E, of the step had an influence on the curve in such a way that a largely positive E, (vs. E”), namely 0 V, introduced a delay in the response of the system, identified by the appearance of a peak at very short times (ranging from a fraction of a second to a few microsec- onds) on the chronoamperometric curves. However, we found that such an effect could be avoided by selecting Es at the very beginning of the voltammetric wave, and such values (generally close to -0.5 V) were selected in all experiments dealing with anthraquinone-substituted polymers and copolymers; in the case of phenothiazine-substituted polymers such an effect was not observed and E, = 0 V was retained. In such cases, the curves obtained are typical of those shown in Fig. 1, where Fig. l(a) illustrates the as-obtained i(t) curve and Fig. l(b) illustrates the i(t-‘I*) transform with the same current scale. The full curve shows the exact

Fig. 1. Result of a typical chronoamperometry experiment for a poly(PHNPH) film prepared using 2X lo-* C/cm* in acetonitrile: (a) as obtained transient current, i(t); (b) i(t-1/2) transform (full curve, real plot; broken line, theoretical linear plot).

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transform which is in good agreement with Cottrell’s law (broken line) on an average time-scale with the classical distortions; currents are slightly smaller at short times, because the remaining ohmic drop makes the potential applied slightly different from the real electrode potential, and at long times, because the film thickness sets a limit to the hypothesis of semi-infinite diffusion.

The electrochemical apparatus consisted of a home-made potentiostat, fitted with a PAR 175 pilot, a Nicolet digital oscilloscope and an Ifelec plotter. In several cases, the conversion of the i(t) curves into i(t-‘/2> curves was performed on-line by an Apple II computer using a home-made procedure. All electrolytic solutions were degassed with nitrogen flushing. The determination of D values was based on the previously demonstrated assumption [14] that the relative concentrations of the functionalized moieties were always in good agreement with the proportions of the corresponding monomers in the polymerization feed.

RESULTS AND DISCUSSION

Homopolymers

The results for poly(PHAS) have been described previously [5-71: the charge transfer follows a diffusion law with coefficients of the order of lo-” cm2/s. Poly(PHNPH) also shows typically Cottrellian behaviour (Fig. l), with only negligi- ble deviation due to the capacitive current produced by oxidation of the polypyrrole chain, and this is generally outranked by the response of the funtionalized phenothiazines. Apparently, and surprisingly, the electron diffusion coefficient obtained with pure poly(PHNPH) was found to have almost the same value as that of poly(PHAS), which to a first approximation would exclude a mechanism involving chain participation in the process; an opinion which should probably be modified in the light of the results obtained for copolymers.

As previously stated [8], the diffusion coefficient obtained for poly(3-PHAS) is very high in the range 1O-9-1O-8 cm2/s), but in a region where the polymer is insulating this result can be explained by the existence of a highiy ordered structure inside the polymer.

N-copolymers

Three kinds of N-copolymers can be prepared from our starting monomers, which will be designated poly(PHAS-PHANC), poly(PHNPH-PHANC) and poly(PHAS-PHNPH) respectively. The last-named family of copolymers has the interesting advantage of providing information through both the AQ-AQ--system (insulating polypyrrole matrix) and the PH-PH+ system (possibly a conducting matrix). As in the case of the homopolymers, it was found that the copolymers also exhibit Cottrellian behaviour (Fig. 2) and therefore it was possible to plot the diffusion coefficient values against the relative concentration of the electroactive species examined in the functionalized polymer. Of course, it was checked that the

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I/mA A,A:PHhtl+c~u@.

l ,o I Aa/Aa’ COUPI.

lo (a)

/

5 10

PNA5tl PNANCd2S to)

Fig. 2. Typical Cottrell plots obtained with films 2X lo-* C/cm* thick in 0.1 M LiCIO,-DMSO. (a) Plots for poly(PHAS-PHNPH) copolymers: curve 1, 50% PEAS-50% PHNPH; curve 2, 80% PEAS- 20% PHNPH (PH-PH+ system in 0.1 M LiCIO,-acetonitrile). (b) Similar plots for poly(PHAS- PHANC) copolymers (proportions as stated).

i(t) curves were approximately the same, whatever the film thickness between lo-* and 5 x lo-* C/cm*. The results obtained are summarized in Figs. 3 and 4. It can be seen that, while the homopolymers exhibit similar values of the electron diffusion coefficients, the variation with concentration is completely different depending on the type of functionalized polymer being considered.

Behaviour of the phenothiazine couple It is possible to prepare two kinds of copolymers incorporating the phenothia-

zine moiety, poly(PHAS/PHNPH) and poly(PHANC/PHNPH), which retain the electroactivity of the PH moieties. It is worth noting that when attention is focused on the electrochemistry of the substituted phenothiazines, the current due to the polypyrrole chains itself can be seen underlying the remainder, and this gradually becomes more important relative to the phenothiazine signal as the dilution of the phenothiazine increases. Unfortunately this reduces to 20% (in PH versus diluent moieties), the dilution limit where chronoamperometric measurements are still reasonably accurate. Provided that this precaution is taken, it is possible to observe the behaviour of the electron diffusion coefficient which varies depending on the kind of polymer studied. Figure 3 shows the experimental dependence on the

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log (C/C”) I

-1 -QS

Fig. 3. Variation in the diffusion coefficients with the relative concentration of redox centres for several copolymers (c/c” where co is the concentration in the pure polymer) in the PH-PH+ system in 0.1 M LiCIO,-acetonitrile: curve 1, poly(PHNPH-PHAS); curve 2, poly(PHNPH-PHANC).

Fig. 4. Variation in the diffusion coefficients with the relative concentration of redox centres for several copolymers in the AQ-AQ system in DMSO: curve 1, poly(PHAS-PHNPH) copolymers (the electrolyte salt was 0.1 M TBAPP); curve 2, poly(PHAS-PHANC) copolymers (the electrolyte salt was (a) 0.1 M LiCIO,, (b) 0.1 M TBAFP).

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relative proportion of phenothiazines in the films. It can be seen that a slight continuous rise is obtained in the case of poly(PI-IAS-PHNPH) copolymers whereas, in contrast, a slight continuous decrease is seen in poly(PHANC- PHNPH). Therefore the diffusion coefficients do not vary in the same way in polymers that should be very similar, which suggests that there is no intervention of polymeric chains in this case. It is difficult to explain the observed increase in the kinetics with dilution obtained for poly(PHAS-PHNPH) copolymers, but it must be pointed out here that the behaviour of our polymers is rather different from that of the classical redox polymers (e.g. redox Nafions [21,221, probably because of the much more compact structure of the films used in this work.

Behaviour of the anthraquinone couple The electrochemical behaviour of anthraquinone can be observed again in two

parallel kinds of copolymers, poly(PHAS-PHNPH) and poly(PHANC-PHAS). In the potential range where AQ electroactivity can be observed there is no interfer- ence by the polymer electroactivity, and therefore copolymers with dilutions as low as 10% in AQ moieties can still be examined. Figure 4 shows the variation of the diffusion coefficient of the charge transfer, like Fig. 3 but now focusing on the electroconversion of the AQ moieties in the films. Again, different behaviour is observed depending on the kind of polymer studied. In poly(PHNPH-PI-IAS) copolymers a slight increase is observed (similar to that shown in Fig. 3 for PH conversion in the same kind of copolymer), followed by a sharper decrease at low dilutions of the AQ moieties. In contrast, in poly(PHANC-PHAS) copolymers the D values drop drastically as soon as the dilution takes place, at percentages as high as 90% of AQ moieties versus the homopolymer content. Such a result is reminiscent of those reported by Murray and coworkers for redox films [15,231, except that in this case the concentration range is quite different from the range found in classical derivatized polymers. In fact, it is much more probable that a favourable arrangement of the anthraquinones exists inside the homopolymer poly(PHAS), which is very quickly lost as soon as the AQ moieties are diluted by anthracene moieties. It should be pointed out that in this case the polymer backbone is insulating and therefore the charge transfer certainly occurs via exchange between redox sites, with the anthraquinones probably being situated in a suitable position in pure poly(PI-IAS) to lower the activation energy of the exchange process. At low dilutions of the AQ moieties, whatever the type of polymer examined, the diffusion coefficient drops to low values (sometimes below lo-‘* cm’/s> where the proposed mechanism is probably drastically slowed down by the distance between the active sites.

Influence of the counter-ions An interesting point to examine in our analysis is the extent to which the charge

transfer kinetics could be affected by the nature or concentration of the supporting electrolyte salt. Figure 4 shows the D value dependence recorded for the same type of polymer (PI-IAS-PHANC in this case) with either Li+ or, in contrast, the

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Fig. 5. Variation in the diffusion coefficients with electrolyte concentration for poly(PHAS) films (prepared at 2X 10W2 C/cm*): curve 1, TBAFP-DMSO; curve 2, LiCIO,-DMSO.

bulky NBu: cation. It can be seen that there is only a very small difference, and the qualitative behaviour of the polymer remains the same.

We also checked the influence of the concentration of the supporting electrolyte on the same type of polymer. Figure 5 shows the variation in the diffusion coefficients in the range 0.02-2 M for poly(PHAS). Almost no effect is observed, and in any case the small difference between the two cations may be due to the complexation of the AQ radical anions by the lithium which could influence the self-exchange process between two neighbouring sites. It should be noticed that the absence of variation observed with poly(PHAS) contrasts with what has been suggested [24] for the case of simple polypyrrole, although the difference observed could be within the allowed experimental error. Since it has been demonstrated, using formal theoretical considerations [241, that counter-ion diffusion is necessar- ily the limiting process in polypyrrole, it is probable that in our case the limiting process is of a different nature.

3-finctionalized polymers

We have recently found that poly(3-PHAS), the 3-functionalized homologue polymer of poly(PHAS), exhibited a very fast charge transfer. Therefore 3-PHANC, the homologue of PHANC, was synthesized and copolymerized with 3-PHAS. However, as soon as the experiments started, it was observed that the polymerization yields (see appendix to ref. 14) obtained during the preparation of the 3-copolymers dropped sharply from almost quantitative values to 10%-Z% of these values even after addition of as little as 1% 3-PHANC (versus PHAS) in the polymerization feed. Such an effect merited further investigation, and so propyl-9- anthracene carboxylate (which has the same anthracenic structure as 3-PHANC

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without the pyrrole substitution) was added to a 3-PHAS solution in order to check whether a quenching phenomenon was involved. As expected, the drop in yield was observed again and so we concluded that there was an unexpected reactivity of the 3-substituted pyrrole radical cation towards the anthracene moieties. There- fore the quality of the films was expected to be completely different and so, unfortunately, few conclusions could be drawn from the chronoamperometry experiments. Confirmation of this assessment was provided by the observation that the range of the diffusion coefficients of the poly(3-PHAS) films prepared in the presence of any kind of anthracene addition dropped by more than one order of magnitude and therefore that the quality of the films was certainly strongly affected by this. This quenching reaction was all the more unexpected in that it does not occur in the case of N-functionalized polymers. Such an observation enhances our appreciation of the difference that actually exists between the reactivity of the 3-substituted pyrrole radical cations and the N-substituted cations.

The influence of the chain length was also investigated by preparing and polymerizing 3-PPAS, the analogue of 3-PHAS bearing a five-carbon alkyl link instead of a six-carbon link between the pyrrole ring and the anthraquinone. Surprisingly, it was found that the diffusion coefficient of the charge transfer in poly(3-PPAS) had a value of (l-3) x 10-‘” cm2/s, much lower than the poly(3- PHAS) value but of the same order as the poly(PHAS) value.

Copolymerization with pyrrole

In order to overcome the difficulties described above, we prepared and studied copolymers of 3-PHAS and unsubstituted pyrrole. It was found that pyrrole copolymerized very easily with 3-PI-LAS to give electroactive copolymers with high polymerization yields, as shown by the voltammograms obtained (Fig. 6). As expected, it was possible to determine the diffusion coefficients as a function of

Fig. 6. Cyclic voltammograms of poly(3-PHAS-3-PHANC) copolymers in 0.1 M TBAFP-DMSO: curve 1, 85% 3-PHAS; curve 2, 50% 3-PHAS.

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TABLE 1

D values in 3-PHAS-pyrrole copolymers

[3-PHASI cAQ in the in solution/% film/m01 I- ’

x lo-” D/ cm2 s-l

100 3.22 27 97 3.17 0.8 95 3.15 2.5 90 3.09 3.0 85 3.05 1.2 70 2.87 1.3

The D values for PHAS-pyrrole and 3-PPAS-pyrrole copolymers are all between lo-” and 3 x 10-‘” cm*/s. cAQ, relative concentration of AQ moieties in the film determined by integration of low scan rate cyclic voltammograms.

the proportion of the AQ moieties in the film. However, as can be seen clearly in Table 1, as soon as some unsubstituted pyrrole moieties are inserted into the polymer backbone, the D values drop by roughly an order of magnitude (compared with the pure poly(3-PHAS)) to a stabilized value close to the value generally obtained with classical poly(PHAS) and probably typical of a less ordered polymer. We then prepared copolymers of pyrrole and PHAS, and of pyrrole and PPAS. In all cases, in the accessible dilution range the diffusion coefficient had a value almost independent of the dilution which was about the same as that of either pure poly(PHAS) or pure poly(3-PPAS) and probably typical of a disordered polymer.

DISCUSSION AND CONCLUSION

These results show that in amost all cases Cotrellian behaviour is obtained for the pseudo-diffusion of electrons inside functionalized polypyrrole polymers whatever the nature or the dilution of the functionalized moiety. Unfortunately, at least in the case of polymers functionalized by nitrogen, the relatively low D values associated with the similar dependence found for bifunctionalized poly(PHAS- PHNPH) (for the conversion of either AQ or PH moieties) attest to the fact that there is no intervention of chains in such cases, even in the case when the chain is in its oxidized state. However, we should not conclude hastily that the charge transfer mechanism is exactly similar to that observed with classical polymers. The dependence on the dilution of the electroactive moieties is far too large and is anomalous compared with previous results, particularly in the case of the poly(PHAS-PHANC) copolymers. In fact, even though the invariance with the electrolyte salt concentration proves that we are dealing with fully saturated structures, these structures are probably much more rigid than the ionomers and probably more rigid than those of the polyvinyl type. Therefore viscoelastic effects could account for the significant differences observed both between relatively similar dilutions in the same type of polymer, and between two closely related but

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slightly different polymers (e.g. poly(PHAS-PHANC) and poly(PHAS-PHNPH) while observing AQ electroconversion).

It was believed that a parallel study of the 3-substituted analogues would bring more light to bear on this problem, but its effectiveness was substantially reduced by the impossibility of preparing satisfactory copolymers with 3-PHANC. However, it is very striking that copolymerization with unsubstituted pyrrole, which should modify at least the arrangement of the anthraquinones inside the film, substantially reduces the high D values obtained in pure poly(3-PHAS) while it has very little influence on the lower values obtained with poly(PHAS). Therefore it should probably be assumed that, rather than the intervention of the chains, it is a specific highly ordered structure (possibly helicoIda as has been demonstrated for poly- thiophenes [251 which accounts for the fast diffusion in poly(3-PI-LAS). As soon as this structure is disrupted, whether in poly(PHAS) or in any kind of copolymer, the D value drops to a value characteristic of a “disordered” functionalized polypyrrole which has previously been found to lie between lo-” and 10F” cm*/s [6].

The final conclusion of this study is that the interest of the functionalized polypyrroles lies mainly in the ease of the polymerization reaction and the wide range of synthetic opportunities, but not in the conductive character of the chain which has almost no effect on the charge transfer kinetics.

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