4861
J. Phys. Chem. 1994,98, 4861-4864
Ion Exchange in the Electrochemical Switching of Polypyrroles in Acetonitrile by the Electrochemical Quartz Crystal Microbalance. Electrolyte Incorporation by Hydrogen Bonding of Anions to Pyrrole Gilbert0 Schiavon,' Gianni Zotti, and Nicola Comisso Istituto di Polarografia ed Elettrochimica Preparativa, Comiglio Nazionale delle Ricerche, c.0 Stati Uniti 4, 35020 Padova, Italy Anna Berlin and Giorgio Pagani Dipartimento di Chimica Organica e Industriale dell'Universitb e Centro CNR, Speciali Sintesi Organiche, via C. Golgi 19, 20133 Milano, Italy Received: August 30, 1993; In Final Form: February 7, 1994"
Ion movement in the dedoping-doping process of polypyrroles in acetonitrile has been investigated by the electrochemical quartz crystal microbalance on selected polypyrroles with and without N-alkyl substitution. The dedoping process involves reversible anion explusion for the former but irreversible cation incorporation for the latter. Electrolyte uptake by the polymer promoted by the polar pyrrole moieties accounts for the behavior of the N-unsubstituted polypyrroles. 1. Introduction
SCHEME 1
The process of doping-dedoping of polyconjugated polymers, namely, the switchingbetween the neutral and the oxidized state, is accompanied by mass (electrolyte and solvent) exchange. This phenomenon has been proposed for some important applications such as actuators, operating on the basis of the volume change (muscles)' or of the release of anions upon dedoping (microtitrators).2-s The process, which is best monitored by the electrochemical quartz crystal microbalance (EQCM), was formerly assumed to involve the simple exchange of the counteranion, but it was soon clear6 that the cation also was, in some instances involved. This result is particularly evidenced by polypyrroles (PP) with polymeric counteranions (anionic polyelectrolytes such as poly(viny1 sulfonate)) but is found also with smaller anions such as tosylate and even perchlorate.' On the contrary, polythiopheneappears to display only anion exchange.* In any case, the situation with polypyrroles is not that clear, as for instance poly(N-methylpyrrole) is reported to involve anion exchange ~ o l e l y . A ~ major difficulty with polypyrrole is its reactivity as neutral toward oxygen, which prevents the relevant ex-situ measurements from being performed. The reactivity to oxygen of N-alkyl-substituted polypyrroles is in fact acceptably low, but this polymer is difficult to obtain in a reasonably defectfree form, as pointed out by former investigations,lO due to its high oxidative potential. We therefore applied the EQCM technique to polypyrroles with redox potentialsintermediate between that of the air-sensitive polypyrrole and that of the defective poly(N-methylpyrrole), namely the polymers from some dipyrroles (1,2, and 3 in Scheme 1) which we recently investigated.11.12 Dipyrrole 1 was selected because it yields a good quality N-alkyl-substituted polypyrrole whereas dipyrroles 2 and 3 were selected because they produce isomeric polypyrroles with and without N-alkyl substitution. This paper reports on the EQCM investigation of the ionexchangeprocessesin poly(l), poly(2), and poly(3) in acetonitrile along with a comparison with the exchange process in polypyrrole itself, in order to correlate the permselective behavior of these materials with their molecular structure. Abstract published in Advance ACS Abstracts, March 15, 1994.
0022-36S4/94/2098-4861$04.50/0
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(3) 2. Experimental Section
Chemical and Reagents. The dipyrroles dihydrodipyrrolopyrazine (l), N,N'-dimethylbenzodipyrrole (2), and 3,3'-dimethylbenzodipyrrole (3) were prepared as previously reported.11.12 Acetonitrile (AN) wasdistilled twice over CaH2. The supporting electrolytes tetraethylammonium perchlorate (TEAP) and tosylate (TEATos) were previously dried under vacuum at 70 OC. Pyrrole and all other chemicals were reagent grade and used as received. Apparatusand Procedure. Experiments were performed under nitrogen at 25 OC in three electrode cells. The counterelectrode was platinum, and the reference electrode was a silver/O.l M silver perchlorate in AN (0.34 V vs SCE). EQCM analysis was performed with a gold-coated AT-cut quartz electrode (0.35 cm2), resonating at 6 MHz, onto which the polymers were deposited. The oscillator circuit was homemade, and the frequency counter was Hewlett-Packard Model 5316B. Calibration of the quartz crystal microbalance was performed with silverdeposition from a 10-2 M solution of AgN03 0 1994 American Chemical Society
4862 The Journal of Physical Chemistry, Vol. 98, No. 18, 1994
Schiavon et al.
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Q/mC Figure 1. Mass change Am vs deposition charge Q for PP as perchlorate measured in solution (AN + 0.1 M TEAP) (0)and in air (0). in AN + 0.1 M TEAP. Thevoltammetricapparatus was AMEL (Italy) and included a 551 potentiostat modulated by a 568 programmable function generator and coupled to a 731 digital integrator. Data were collected by a microcomputer with a homemade analyzing software by which frequency changes (Au) were monitored as mass changes (Am). Proceduresof PolymerDeposition and Dopiag-Moping. Thin (0.03-0.1pm) films were deposited either potentiostatically or potentiodynamically (at 50 mV/s) from 0.05 M solutions of the appropriate monomer in 0.1 M TEAP or TEATos AN 1% H20. The amount of deposited polymer was determined when the reactivity to oxygen was low enough (Le., in all cases but polypyrrole), after reduction of the polymer to the neutral state followed by washing in AN and drying to constant weight. The mass change upon immersion in the solvent allowed, after correction for the solvent shift of the bare electrode, the determination of the solvent content (swelling) of the wet materials. Unless otherwisestated, the dedoping-doping process was investigated in AN containing the electrolyte used for polymerization (e.g., TEATos for the tosylate salt of the polymer) in 0.1 M concentration. Conditions for optimal polymer deposition, with frequency responses free of complications from nonrigid behavior and roughness effects, were checked with polypyrrole deposited potentiostatically at 0.5 V in perchlorate solution (Figure 1). Deposition occurs with a linear Am vs Q curve up to 10 mC with a slope corresponding to the expected content of 0.25anion per monomeric unit.1° For highercharges, the Amvs Qcurvedeviates positively from linearity but keeps its linearity if the mass measurements are made in the air. We attribute this behavior to roughness of the polymer because micrographs of the deposits show rough surfaces, as found with surface profiling of the polymer from tetrafluoroborate vs that from tosylate,I3 and because mechanical smoothing of the polymer surfaces makes the relationship linear both in air and in solution. Therefore, we may assume that a positive deviation from linearity in polypyrrole deposition is an indication of rough surfaces, which must be avoided for a correct interpretation of the results.
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3. Results and Discussion
Polymer Deposition. Poly(1) was deposited from tosylate solution potentiodynamically (between -0.3 and 0.6 V) since potentiostatic deposition resulted in low yields due to loss of oxidized oligomers. The resulting Am vs Q curve is linear up to a deposition charge (in the oxidized state) of 30 mC/cm2 with a slope (as FAm/Q) of 63 g/mol. Since the expected FAm/Q
Figure 2. Mass change Am vs redox charge Q for poly(1) as tosylatesalt in AN 0.1 M TEATos. Weight of the polymer (neutral in air): 4.3
+
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value for doping with 0.5 anion per monomeric unit" is 95.8 g/mol, the value indicates that deposition occurs with a 60% yield. The linearity of the deposition curve indicates that the deposit has a rigid behavior. The Am vs Q curve for PP deposition in tosylate solution is perfectly linear up to a deposition charge of 90 mC/cmz with a FAmlQvalue of 54.5glmol. Since the expected value for doping with 0.4 tosylate anion per monomeric unitI4 is 55 g/mol, the value indicates that the deposition yield is 100% and that the deposit has a rigid behavior, as reported in the 1iterat~re.l~ The deposition of poly(dimethylbenzodipyrro1e)s occurs with yields lower than unity, as for poly(1). Potentiodynamicdeposition as perchlorate of the N-substituted poly(2) between -0.6 and 0.3 V occurs with a yield of ca. 40%; for the N-unsubstituted poly(3) the yield in potentiostatic deposition at 0.6 V in tosylate solution is 60%. In both cases the Am vs Q relationship is linear up to 60 mC/cm2. Following these results, polymer films for EQCM analysis were typically produced with deposition charges of 30 mC/cm2. Polymer Moping-Doping. Dedoping of poly(1) as tosylate salt produces a net mass decrease. Comparison of the polymer weight in air in the neutral and oxidized states shows that 0.5 anion per monomeric unit is present in the oxidized material, in agreement with the literature." In solution the oxidized polymer does not contain solvent whereas in the neutral state it is swelled with ca.one solvent molecule per monomeric unit. The relationship of Am vs Q during dedoping is linear (Figure 2), and in the reverse process the mass returns to its initial value following the same linear path. The value of FAm/Q is 82 g/mol, suggesting the reversible loss of one tosylate anion (171 g/mol) and the uptakeofca. two acetonitrile(41 g/mol) molecules in thededoping process. This result is in agreement with the above mentioned absence of solvent in the oxidized polymer and the measured solvent content of the neutral polymer. The dedoping process of PP as tosylate salt is completely different, as it is accompanied by an increase of weight (Figure 3), in accordance with previous observations.7 In the course of the first dedoping step, the mass increases linearly with charge with a slope of 132 g/mol, corresponding to the uptake of one tetralkylammonium cation (MW = 130) per injected electron. In the backward step the mass does not decrease, yet it keeps on increasing with further cycling. Thus, it appears that neutral PP incorporates electrolyte, in agreement with the literature.16-1* Electrolyte loading at the end of cycling is over equilibrium as the weight of the neutral and oxidized polymer decreases with time, attaining a steady value only after a long delay. In a cyclic
The Journal of Physical Chemistry, Vol. 98, No. 18, 1994 4863
Ion Movement in Polypyrroles 4.0
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