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f. Eieetroanal. Chem., 219 (1987) 165-181 Elsevier Sequoia S.A., Lausme - Printed in The Netherlands

~EHAV~OUR~FSOME:P~LY(PYRROLE-A~Q~~NE)FILM~ IN DMSO ELECTBOLYTES

P. AUDEBERT, G. BIDAN and hi. LAPKOWSKI

Luboratoires de Chimie- Equipe d~Electr~h~mie Mo~~~aire, ~~~ternent de ~echerc~e F~ndame~taie, Centre d’Etudes Nucl&ires, 85 X, 38041 Grenoble Cedex (France)

(Received 1st September 1986; in revised form 16th October 1986)

ABSTRACT

Several films of ~ly~y~ole-~~r~uinone) have been prepared by the electropolymerization in organic media of anthraquinones adequately substituted by some N-alkylpyrroles. These films have been studied mainly by electrochemical methods in DMSO with either TEAFB or LiClO, as the supporting salt. Bonded anthraquinones can be reversibly reduced to their radical anions or dianions with a fair yield, except on slow or prolonged cycling where the protonation effect modifies this behaviour. Spectroe1ectrochemka.l studies confirm these facts and additionally allow the probable complex forma- tions in fully reduced films to be detected.

INTRODUCTION

Considerable interest has been shown recently in the preparation of electrodes modified by quinoidal units for catalytic purposes [I]. One of the most promising ways consists of coating the electrode with q&one loaded polymer films of appropriate thicknesses. In a preliminary communication, we described the synthe- sis of a polypyrrole film modified with anthraquinone pendant groups by electro- polymerization of an iv-substituted pyrrole with a 2-anthraquinone alkylsulfonyl group [ZJ. This film exhibited all the electrochemical characteristics of the anthra- quinone (AQ) moieties in a thin layer, as previously noted for analogous polypyrrole films substituted by the ferrocene [3], paraquat [4], or nitroxide [5] groups. Continu- ing the same approach, we prepared the four monomers listed below.

They can all be polymerized in a similar way on a Pt electrode. The electrochem- ical behaviour of the films produced has been explored in DMSO with either tetrae~yl~o~um tetra~uoroborate (TEAFB) or LiClO, as supporting salts, using cyclic voltammetry, chronoamperometry and coulometry. Comparison be- tween the four polymers rests mainly on site accessibility and doping level and is

discussed in relation to the relative anthraquinone content per polymer&d pyrrolic

0022-0728/87/$03.50 @ 1987 Elsevier Sequoia S.A.

H2C-N ; / 3 RE -CH \ WAS RZ -CH

Hi-N 3 3 \ . 6PAD “C--N / 3

R E -_(CH2)a-N > 3 PHAS PHAD

moiety. Comparative electrochemical and speetroelectrochemical data for both monomers and the subsequent polymers are discussed. Stability of the films was investigated by repetitive voltammetric cycles in relation to the cycling conditions.

EXPERIMENTAL

Synthesis of the monomers

All the monomers were prepared by condensation of the lithium salt of the appropriate hydroxyalkyl pyrrole [6] with either 9,10-anthraquinone-2-sulfonyl chlo- ride or 2,7disulfonyl chloride* in a dry box. A typical procedure, exemplified by PHAS preparation, is:

5 x 10s3 mol (825 nag) of l-p~ol~l-yl-hex~-6~1 is dissolved in 20 ml of absolutely anhydrous THF and converted to the lithium salt by 5 x 10m3 mol of butyllithium (Janssen, 1.6 A4 in hexane) (3.1 cd). The solution is then added to an equimolar amount of 9,10-anthraquinone-2-sulfonyl chloride (1532 mg) dissolved in 50 crns of anhydrous benzene. After 5 mm stirring, the mixture is taken out of the box, evaporated to dryness, triturated with 100 ems of dichloromethane and filtered to remove the insoluble lithium chloride. The solvents are evaporated and the final product is recrystallized from a dichloromethane/hexane mixture. Full analytical data for all the monomers are listed in Table 1.

* Since the literature procedure (71 was found not to work, we achieved the reaction by using a SO/SO SOCi,/DMF mixture in twice as much dry benzene. A five fold excess of reagent was used in both cases and the resulting 9,l~~~~~one-2-s~f~yl chloride and 9,10-an~~uinone-2,?-disulfonyl chloride were extracted and rectystallized in dry benzene. The former compound was obtained with a 75% yield and the latter with a 65% yield.

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TABLE 1

Analytical data for the pyrrole-anthraquinone monomers ’

Compound Name Yield Analysis ‘H NMR (I/ppm) b m.p.

/w talc. Found Aromatic Ahphatic /“’

An&m- Pyrrotes auinone

PHAS [I-@yrrol-l-yl)-hex- 85 -6-yl]-9,1O_anthraqui- none-Zsulfonate

BPAS [1,3-b~~ol-l-yl)- 83 -prop-2-ylj-9,lOanthra- quinone-Zsulfonate

PHAD di-[l-(pyrrol-l-yl)-hex- 68 -6yl]-9.1~anthraqui- non-2,7-disulfonate

BPAD di-[I,3-bis(pyrrol-l-yl)- 65 -prop-2-yl]-9,10-anthra- quinone-2,7disulfonate

C 63.02 C 63.70 8.79 (s) H 5.03 H 5.17 8.47 (d) N 3.07 N 3.03 8.30 (m) 0 17.00 0 18.14 7.85 (m) S 7.00 S 7.25

C 65.21 C 64.95 8.55 (d) H 4.35 H 4.54 8.30 (q) N 6.09 N 5.63 7.88 (m) 0 17.39 15.41 S 6.95 S 6.62

C 61.17 C 58.98 8.81 (d) H 5.85 H 5.54 8.52 (d) N 4.20 H 3.91 8.32 (dd) 0 19.19 0 19.24 S 9.60 S 10.59

C 60.66 C 61.47 8.55 (d) H 4.53 H 4.51 8.32 (d) N 7.86 N 7.24 7.95 (m) 0 17.96 0 16.73 S 9.00 S 9.58

6.58 (s) 6.07 (s)

6.49 (t) 5.95 (t)

6.56 (s) 6.07 (s)

6.49 (t) 4.09 (m) 215 5.96 (t) 1.38 (m)

4.13 (t) 92 3.82 (t) 1.71 (m) 1.31 (m)

4.18 (d) 170 4.08 (m) 1.58 (m)

4.15 (t) 124 3.82 (t) 1.72 (m) 1.31 (m)

’ IR data are not given in the Table for clarity and show similar features for all compounds differing only by the relative intensities of the bands (cm-‘): anthraquinone: 1680, 3100; pyrrole: 3080; alkyl: 2860, 2950; sulfonic, SO,: 1290; sulfonyl ester, S&-O-: 1180. b CDCI, used as a solvent; TMS as internal standard.

Synthesis of the polymers All polymer films were grown by electropolymerization onto a platinum electrode

in a two compartment cell at a controlled potential of: + 0.78 V for poly(BPAS) and poly(BPAD) and +0.8 V for poly(PHAS) and poly(PHAD) (all potentials in this paper refer to an Ag/lO-’ M Agf reference). The electrolyte was acetonitrile with 0.1 M LiClO, and the concentration of substrate was 2 X 10m3 44, except for BPAD where a saturated solution (5 x low4 M) was used because of its low solubility.

Polymer electrochemistry and eiectrochemical device Film studies were performed in a dry box in a cell similar to that used for their

synthesis, but in DMSO with LiClO, or TEAFB as supporting salts at a 0.1 1M concentration. DMSO (Prolabo, HPLC grade) and acetonitrile (Merck, HPLC grade) were stirred and stored on neutral ahnnina (Woelm, Act. Grade 0) in the dry

168

box. LiClO, (Fed. Chem Co,) was dried for 2 days at 1WC under vacuum and TEAFB (Fluka) was dried for 2 days at 110°C under vacuum prior to use. The electrochemical device was made of PAR 173 and PAR 175 units connected to a SEFRAM TGM recorder.

SpeetroelectrochemicaI measurements were made and ~~~perornet~ ex- periments performed in the tight cell described previously [8] with a 0.4 M concentration of supporting electrolyte. For chronoamperometry, the counter elec- trode faced the working electrode in order to minimize the ohmic drop. A Nicolet digital storage scope was used for these experiments. Cell filling and film prepara- tions were effected inside the dry box.

RESULTS AND DISCUSSION

(I) Monomer voltammeiric behaviour in DMSO

The classical electrochemical response of the ant~~~o~e system is well-known to follow two successive one-electron transfers in aprotic solvents f9]:

AQ F1: AQ’- P AQ=- However, complexation of the anionic species AQ”- and AQ2- with alkali

metals cations often modifies this ideal behaviour, especially when acetonitrile is the solvent [9b]. Such considerations made us turn towards DMSO to observe the two step reduction of our monomers, while the pyrrole oxidation peak should stiB be observed in acetonitrile because of the DMSO limit at positive potentials.

Figure 1 shows the cyclic voltammogram of BPAS in 0.1 M TEAFB f DMSO. As expected, it exhibits two monoelectronic peaks according to the formation of the AQ’- and AQ2- species. Table 2 gives E;, values for all the monomers according to the electrolyte. ft should be noted that, while potential ~&es remain roughly constant for the AQ/AQ’- couple, with both Lii and TEA+ cations the values for the AQ’-/AQ2- couple are shifted by an average of 200 mV towards negative potentials when passing from LiCIO, to TEAFB, thus showing a probably strong complexation of the AQ2- species with the Li+ cation [9d]. On the other hand, it is also noticeable that the presence of two sulfonic esters, instead of one, on the ~t~a~~~one base shifts the redox potentials positively, while only little infhzenee

of the esterifying alcohol occurs, as could reasonably be expected. The shifting effect of the second sulfonyl group seems relatively independent of the electrolyte and may be evaluated at ca. 250 mV and the AQ/AQ’- couple and 150 mV on the AQ’-/AQ2- couple.

It is likely that the pyrroh+anthraquinone monomers polymerize in the same way as pyrrole itself, by radical-cation coupling [lo] Polymer films are strongly adherent

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Fig. 1. Cyclic vuft~mogr~ of a 2X10e3 M BPAS solution at 50 mV s-l. Full line: negative sweep in 0.1 M TBAFB i DMSO, dashed.line: positive sweep in 0.1 M LiClO, acetordtrile.

to the Pt electrode and have color which varies from yellow for thin films to black for thick films. As with classical polypyrrole, the film growth can be checked by coutometry before rinsing and tr~fe~ng the film into the cycling electrolyte.

Cyclic voltammetry of thin polymer fihns at 50 mV s-l exhibits quasi-ideal behaviour which compares with the reports of Funt et al. [ld,e] whatever the nature

TABLE 2

Reduction po&~~tials (E, - (Ep4 t E&/2) of the ~~r~~a~~ monomers in DMSO with 0.1 M of the electratyte salts listed. The last tine gives the oxidation peak potential of the pyrrole units in these monomers in 0.1 M LICK& + CR&N

BPAII BPAS PHAD PWAS

LiClO,

TEAFB

Pyrrok units in LiClO, + CH,CN

G/v -0.820 - 1.080 -0.790 - O.%S EO2P - 1.200 - 1.370 - 1.215 - 1.365

G/v - 0.820 - 1.080 - 0.800 - l.Q6Q &U/v - I.365 - 1.590 - 1.415 - 1.600

EP,iy f 0.980 + 0.97tI +1.000 + 0.970

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TABLE 3

Equilibrium rcdox potentials of the anthraquinone in polymer films according to the electrolyte sait (in DMSO). Measurements were made on films prepared using 5 X 10T5 C on a 2 mm 0 Pt electrode

Electrolyte salt Polymer

polY(BPAW P~IY(BPAS) poly(PHADj polY(PHASI

LicIo, EOl/v -0.830 - 1.040 - 0.880 - 1.040 &2rv - 1.2OO - 1.330 - I.190 - 1.310

TEAFB EOlfv - 0.850 -1.060 - 0.893 - 1.080 Eo2D' - 1.420 - 1.530 - 1.430 - 1.520

of the starting monomer. ~~b~urn redox potentials for the different polymers when deposited in thin films are listed in Table 3 and are very dose to the values reported for the monomers; moreover, the substitue~ts induce similar potential shifts. With thicker films however, the ideal&y disappears, side peaks are formed while the relative size of the AQ’-/AQ*- peaks decreases (Fig. 2). Although the appearance of prepeaks may be related to protonation reactions, the distortion of the AQ’-/AQ’- peaks is due to the slow kinetics of the dianion formation, since well-shaped peaks are observable at low sweep rates (5 mV s-r) in most cases. However, the dianion appears to be unstable under these slow cycling conditions leading to degradation of the film (Fig. 3a). Prolonged dipping in co&dine does not

Fig. 2. Cyclic voltammograms of different poIy(BpAS) Nms in 0.1 M TEAFB + DMSO at 50 mV s-l. (a) Full line, film prepared using 1.7~10-~ C cm-*, x = 0.4 PA. (b) Dashed line, film prepared using 6.5X10~3Ccm~2,x-1~A.(c)Dash-dotline,fi~preparedusing1.5X10~2Ccm~S,x-2pA.

171

iC* f

a

Fig. 3. (a) Cyclic ~01~~~~ at 5 mV s-l of a poly(PHAS) film prepared using 3.3 x 10Y3 C crne2 in 0.1 h4 l%AFB+DMSO. Full line, 1st scan; dashed line, 2nd scan. (b) Same as (a), but with co&dine dipping of the film before cycling (dash-dot line, 3rd scan).

reactivate the film after cycling. However, when the film is dipped in collidine before slow cycling, reoxidation of the AQ2- species is observed and the degrada- tion process is considerably slowed (Fig. 3b). Moreover, it has been shown previ- ously that proton effects were likely to occur in some substituted polypyrroles [5], therefore the modification of the redox activity of our polymers involves irreversible protonation reactions, at least partially, under slow cycling conditions, as could be expected with anthraquinone groups.

Nevertheless, AQ’- species are fairly stable under the working conditions and ideal behaviour is obtained at low sweep rates whatever the thickness of the film when the potential range is limited to the radical-anion conversion.

On the other hand, the voltammograms of the films show peaks of the polypyr- role couple in the range -0.1 to + 0.4 V (noted as Pp on Fig. 2), as previously reported for this hind of compound. There is Iittle difference in the voltammograms of the films when the starting monomer changes, except in the relative importance of the polymer peaks, which grow in relation with the number of pyrrolic cycles per anthraquinone in the monomer. Some peak broadening is also noticeable with

172

Pig. 4. Variation of AQ reduction peak currfmts with the scan rate for different pofy(PHAS) films prepared using respectively 2.5, 5, 10, 20 and 50 X 10e5 C on a 2 mm 0 pfatimzm electrode in 0.1 M LiC104 + DMSO.

poly(BPAD) and poly(BPAS), possibly indicating some changes in the kinetics of the electronic and/or ionic motion inside the film. It is likewise seen that substitu- tion by the 1,3-b~p~ol-l-yl)-prop-2-yl group instead of the long 1-(py~ol-l-yl)- hexad-yl chain makes the polymer more compact for ionic motion. Peak currents vary with the sweep rate for these polymer films, as expected for highly adsorbed species (Fig. 4), with a deviation from linearity occurring at high sweep rates and for thick films, the dependence changing progressively from D towards v’/* as diffusion becomes a limiting step in the electrochemical process.

Figure 5 shows two CottrelI plots obtained for the same film of poly (PHAS) for the AQ/AQ’- system. The experiments were performed with LiC10, and TEAFB as supporting electrolyte.

The similarity exhibited by the plots leads to the same apparent diffusion coefficient of 0.86 x 10-t’ s-i cm2 using the CottrelI equation [ll], the film thickness being estimated after coulometry as discussed below. This relatively low

173

ia

Fig. 5. Cottrell plot for a poly(PHAS) film prepared using 2 X 10e2 C on a 1 cd Pt electrode. (a) In 0.4 M LiClO, + DMSO, (b) in 0.4 A4 TEAFB + DMSO (potential step: 0 to - 1.2 V).

D app value irrespective of the nature of the cation involv& in the electrochemical process tends to show that, even in this poorly conducting film, it is charge motion which remains the limiting kinetic factor of the electrochemical process. The same experiment performed with poly(BPAS) showed the slightly lower values of 0.4 and 0.2 X lo-” s-l cm2 for D app with TEAFB and LiClO~, respectively, as supporting electrolyte. The fact that ‘).pp changes with the electrolyte is probably due to an ionic contribution to the -2)app value in poly(BPAS). Such a phenomenon may be explained by a decrease in the ionic conductivity of this polymer due to its more rigid structure.

We performed coulometric experiments on our polymer films by measuring the charge Q, passed for the fihn synthesis, the charge Qr, recovered by discharging the

174

polypyrrole couple (from its synthesis potential of 0.8 V to -0.2 V) and the charges Q, and Qd recovered respectively from the radical anion (AQ + AQ’-) and the subsequent dianion (AQ’- + AQ*-) formation.

If p is the polymerization yield for a given monomer, 6 the doping level of the subsequent polymer, r1 and 7* the conversion ratios of the anthraquinones, respec- tively to their AQ’- and AQ*- forms in the polymer and a the number of pyrrolic cycles per ~t~aq~~one in the starting monomer, we can write the following relations between these factors and the measured charges Q,, Qpt Q, and Qd. If n moles of monomers are polymerized, with all the pyrrole units taking part in the polymer structure, one has:

pQs = nFa(2 + S) (1)

QP = nFa8 (21

Q* = nFr, (3) Qd = ~FT~ (4

Eliminating anF, we find;

6 QP 2+s =PQ,

i.e. S = 2QP

PQ, - Qp However, eqn. (5) still contains two undetermined factors, S and p. We therefore

chose p arbitrarily to be equal to lOO%, since no coloration of the electrolyte allowed us to suppose that there was oligomer formation during the film synthesis and in addition, such a hypothesis leads to the 6 values listed in Table 4, which are in very good agreement with reported values for both polypyrrole [12] and poly(sub- stituted pyrroles) [12,13]. it and 7* values can be extracted from eqns. (3) and (4):

~10 = QJQp, 0 = QdQ,- TV and +r2 values are presented in Table 3 for different polymer films. It is

noticeable that in the case of poly(PHAS), poly(BPAS) and poly(PHAD) pi is in the range of literature reports [ld,e], while an abnormal value is found in the case of poly((BPAD).

TABLE 4

8,7, I’ and e values for polymer fibs prepared passing 3 X 1K3 C cm-’ in 0.1 M LiClO, + acetonitrile. Coulometric experiments were performed in 0.1 &4 LiCiO, + DM!SO

Polymer poly(P=W poly(BPAS) pofY(PHAD) poly(BPAD)

OL 1 2 2 4 a/% 23 23 21 22 71 0.6 0.84 0.84 1.47 72 0.42 0.72 0.41 0.14 10s r,/mol cm-’ 1.40 0.70 0.71 0.35 10s r,/mol cm-’ 0.84 0.6 0.6 0.52 e/urn 46 24 34 26 ’

’ In this case, e was calculated from r, instead of r,.

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It is likewise evident that, when there is more than one polyme~sable moiety in a monomer, some unpolymexized pyrrole rings may remain in the final polymer structure, introducing an error factor in eqns. (1) and (2) where a should be replaced by a' -z a to indicate the distinction between the number of pyrrole rings per monomer and the number of polymerized pyrrole rings per monomer.

The generally low values obtained for r2 may probably be explained by changes in the accessibility of sites caused by the first reduction step of the polymer inducing repulsive coulombic interactions.

We also estimated the surface coverages, I‘, and the film thicknesses, e, either on the basis of Q, or Q, values, leading to two different values, I’, and I’,. Taking again the previous notations, if S represents the electrode surface:

I?, = n/S = Q$(2 + S) aFS

I’, = n,,/S i= QJFS i.e. I’, = rtr,

Film thicknesses, e, were calculated from I’, for poly (PHAS), (BPAS) and (PHAD). Using eqn. (6), e = MrJd, where M is the molar mass of a given monomer and d the polymer density. However, in the case of poly(BPAD), we calculated only the apparent thickness of the film by using Ir instead of P, in eqn. (6), because I’, had been shown to lead to an overestimation of the anthraquinone content of this polymer (see discussion above). For the e calculations, the same density as dry polypyrrole (i.e. 1.5 g cmM3 [14]) was assumed for all polymers, so that e represents the thickness that the dry film would have, thus being the only accessible parameter.

The variations of Q,, Q, and Qd are plotted in Fig. 6 as a function of Q, for various poly(PHAS) films. All charges show a linear variation for thin films up to 1.5 X 10m4 C which then curves slowly for higher Q,. Therefore, S, 7t and TV values are stable in thin films and slowly decrease, as expected, when the film behaviour deviates from idea&y. In the case of poly(PHAS), the ratio 7,/S is equal to QJQ, and can be extracted from Fig. 6. This ratio remains roughly constant, thus showing the same accessibility for the quinonic and polypyrrolic sites in this polymer.

Spectroelectrochemical study

All the polymers show similar spectroelectrochemical features of well defined bands due to the formation of the AQ’- and A@- species, as previously shown for a poly(PHAS) film [2] (Fig. 7).

Further investigations were therefore made choosing poly(PHAS) as a test compound. Figure 7 shows the comparison of absorption spectra of poly(PHAS) using LiClO, or TEAFB electrolytes. It is first noticeable that changing the electrolyte cation introduces an irn~rt~t shift in most absorption wavelengths of both charged species. However, it is difficult to assign some peaks unequivocally because of the possible absorption of AQH- or AQHz species at low levels in the film. In addition, with LiClO, a band appears at 473 nm which has no equivalent

Fig. 6. Variations of Q,, Qd and Q, versus Q, for different poly(PHAS) films in 0.1 M LiClO., +DMSO preparad on a 2 mm 0 Pt electrode.

with TFUPB. These considerations, together with the electrochemical data (Table 3) confirm that the AQ*- dianion probably assumes a complexed state AQ*- . . . Li+ inside the film which is responsible for the 473 nm band.

To complete this analysis, chronoabsorptometric experiments have been per- formed on poly(PHAS) films with LiC10, as supporting electrolyte by stepping the potential from 0 V to either -1.0 V (AQ’- existence region) or -1.4 V (AQ*- existence region). Absorption was observed both at 565 nm (AQ’- absorption} {Fig. 8) and 4’73 mn (AQ*- . . . Li+ absorption) (Pig. 9).

The reduction of poly(PHAS) to - 1.4 V results in the rapid formation of AQ‘- with concommitant slower reduction of these latter species into the Li+ complexed AQ2- dianions. The curve registered at 565 run (Fig. 8a) shows a very weak maximum followed by a slow decrease in absorbance, indicating that the kinetically limiting step is the AQ *- formation as expected. Conflation of this is provided by Fig. 9a which show the slow increase of the absorption at 473 nm, which may be due to the complex AQ*- . . . Li+. The disappearance of the AQ’- absorption is not very marked on Fig. 8a, since AQ’- reduction is slow and its lack of absorbance is

177

Pig. 7. Absmbance versus the potential appiied of a ~~P~~ film dePosited using 2 X10-’ G 0x1 a 1 cm2 IT0 &ctrode. Top curve, in 0.4 N LiCIO, 4 DIMSO; bottom curve, in 0.4 M TENFB + DMSO. Applied potential values were respectively (a) - 0.88 V; (b) -0.99 V; (c) - 1.02 V; (d) - 1.12 V; (e) -1.2% (f) -1.3 V, (8) -1.45; (h) -1SV; (i) -1.6 v; G) -1.7 v; (k) -1.8 V; (I) -1.9V.

partly balanced by the AQ2- . ..Li+ absorption which is not negligible at this wavelength.

When the potential step is limited at - 1 V, AQ’- fo~tio~ is slowed (Fig. Sb), but reaches a higher level. Absorption at 473 nm (Fig. 9b) is also mainly due to the complex, AQ2- . . _Li+, whose presence in the me may be expkimxl by the existence of a dispraportionation reaction:

2 AQ- # AQ2- + AQ

178

,b

4, a

565 nm

L..___ so

time&

Fig. 8. Chronoabsorptogram of the poly(PHAS) film referred to in Fig. 7 in 0.4 N LiClO, + DMSO at 565 nm (AQ’- absorption). For curve (a) the potential was stepped at - 1.4 V and for curve (b) at - 1.0 V.

Potential data show that the disproportionation is not thermodynamically favoured, but a high AQ’- concentration exists inside the film and the complex formation shifts the equilibrium towards the right.

! AU 03

Fig. 9. Same as Fig. 8, but the recording was made at 473 urn (AQ*- absorption).

179

The stability of the substituted polymers has been studied by rapid scan cyclic voltammetry.

All polymers exhibit rather stable behaviour on cycling in DMSO electrolytes on the polymer and the AQ/AQ’- system. For example, the observed decrease in peak integration was 10% over 400 cycles with poly(BPAS) with TEAFB and poly(PHAS) with LiClQ4, respectively, as supporting salts. However, when the cycles were effected up to -1.7 V over the AQ’-/AQ2- system, the observed decrease was much faster, reaching an average 50% over a hundred cycles.

Qn cycling a poly(BPAS) film precisely over the AQ/AQ’- couple a progressive decrease was noticed, going up to the q~~i-di~p~~~ of the signal after ~

Fig. 10. Cyclic vokumnogr~s (scan rate 50 mV s-I) of 8 poly(BFAS) film prepared using 3.3 x 10m3 C cm-’ in 0.1 M TEAFB+DMSCZ. (a) 2nd cycle, (b) 200th cycle, (c) 400th cycle, (d) 4000th cycle, (e) 4001st cycle.

180

cycles. However, cycling the film again on the polymer couple resulted in an intense prepeak at -0.3 V followed by a regeneration of about 80% of the anthraquinone activity (Fig. 10).

If the same experiment was repeated dipping the film into 1 M collidine instead of cycling on the polypyrrole couple, the film activity was not regenerated; however if the film was dipped into collidine before cycling, the signal was much more stable under identical cycling conditions, with only a 50% decrease after 4000 cycles. Such experiments prove again that some protonation occurs slowly on the anthraquinone reduced species in the course of prolonged cycling. The intense reoxidation peak observed in the first experiment is probably provided by the reoxidation of AQH, species at a higher potential mainly due to the disproportionation of transient AQH’ species resulting on slow AQ’- protonation according to:

AQH’+ AQ- --, AQH- + AQ + AQHz + AQ

This peak is reminiscent of the additional anodic peak P’ observed in Fig. 2 with a thick film.

On the other hand, even after collidine dipping, some loss of electroactivity occurs which cannot be regenerated, and is probably due to irreversible chemical reactions of the AQ’- species, but a 50% loss over 4000 cycles nevertheless shows fairly good stability for this type of polymer_

CONCLUSION

In this paper, we have shown that functioning pyrrole monom~ with anthraquinones is a convenient way to generate polymers having the electrochemical properties of anthraquinone in organic media.

However, it seems that, even in this kind of polymer, charge transfer remains the limiting factor of the film activity and can seriously perturb the polymer properties.

On the other hand studies are currently under way to determine the film activity in aqueous media and to determine the reduction of solvated dioxygen, which will be the scope of another paper.

ACKNOWLEDGEMENTS

The authors thank CNRS, PIRSEM and AFME (ATP “Preparations &ctrochimiques et g&rateurs &ctrochimiques”) for partial financial support. M.L. thanks the French government (CIES) for financial support during his eight month’s stay in the Equipe d’Electrocbimie Mok%ulaire, Laboratoires de Chimie.

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