4592
J. Phys. Chem. 1983, 87, 4592-4598
Electrochemlstry and Electron Spin Resonance of Tetracyanoquinodlmethane Modified Electrodes. Evidence for Mixed-Valence Radical Anions in the Reduction Process Gyorgy Inzelt,' Roger W. Day, James F. Klnstle,' and James 0. Chambers. hpartment of Chemistry, UnlVefSRy of Tennessee, Kmxvillle, Tennessee 37996- 1600 (Received: January 17, 1983)
Tetracyanoquinodimethane (TCNQ) modified electrodes, adsorbed films of (TCNQ), oligomers on platinum substrates, have been studied by voltammetric, spectroelectrochemical, and electron spin resonance (ESR) techniques. A maximum in the ESR signal, interpreted as arising from a mixed-valence species, occurs at potentials in the region of the foot of the first reduction wave for electrolyses in aqueous electrolytes. An equilibrium mixture of radical anion sites, (TCNQ-e),, and a dimeric complex, (TCNQZ2-),,is formed in fully reduced films. The correspondence between the calculated fractional distribution coefficients, the potential dependence of the ESR signal, and the absorption bands due to the radical anion and dimer dianion species is used to extract the following parameters from the results: the degree of dissociation in a fully reduced film, CY = 0.27; the dimer dianion formation constant, KD = 1.4 dm3/mol; and the formal potentials for the (TCNQ),/(TCNQ-e), and (TCNQ2-.),/(TCNQ22-),couples, 0.030 and -0.015 V vs. SCE in contact with 0.5 M LiC104. Slow electrochemicalsteps revealed by the ESR experiments are attributed to unfavorable dimerization kinetics on reduction, and isolation of unpaired electron sites in the bulk of the film upon oxidation.
Electron spin resonance (ESR) spectroscopy has been used routinely in the last 20 years for detection and identification of electrochemicallygenerated radical It is well recognized that the high sensitivity and the molecular structure information in the hyperfine splitting pattern of the spectra make the ESR technique a powerful tool for the study of organic electrode reactions. Herein we report on an ESR study of electroactive tetracyanoquinodimethane (TCNQ) polymers adsorbed on platinum electrodes using classical ESR-electrochemical techniques. The results support and amplify mechanistic conclusions of complementary voltammetric and spectroelectrochemical studies of these TCNQ modified electrodes. In spite of intense recent activity in the area of polymer modified electrodes, there have been no studies reported in which the dependence of an ESR signal on electrochemical parameters (electrodepotential, current) has been used to obtain information concerning electrode reactions at a polymer film electrode. One reason for this void is the nature of the most widely studied polymer film materials. For example, the technique pioneered by Anson and co-~orkers,6*~ in which electroactive ions, often inorganic species such as Fe(CN)63-/4-,are exchanged into polymeric ion-exchange resin films on electrodes, does not lend itself to ESR spectroscopic characterization. Similar comments can be made about the bipyridyl metal ion complexes which have been studied as polymeric films on electrodes by Murray and co-workers.s Other systems, such as poly (nitrostyrene): involve radical ions which are extremely air and moisture sensitive, and therefore difficult to study. However, for electroactive polymers possessing electron donors or acceptors that can form stable radical ions, electron spin resonance spectroscopy can provide valuable (1)Department of Physical Chemistry and Radiology, L. Edtvds University, Budapest, Hungary. (2)Geske, D. H., Maki, A. H . J. Am. Chem. SOC.1960,82,1971. (3)Piette, L.H.;Ludwig, P.;A d m , R. N. Anal. Chem. 1962,34,916. (4)Goldberg, I. R.; Bard, A. J. J.Phys. Chem. 1971,75, 3281. (5)McKinney, T.M.;In "Electroanalytical Chemistry"; Bard, A. J., Ed.; Marcel Dekker: New York, 1977;Vol. 10,_DP _ 97-278 and references cited therein. (6)Ovama, N.;Anson, F. C. J. Am. Chem. SOC. 1979.101.739.3450. (7)Oyama, N.; Anson, F. C. J. Electrochem. SOC. 1980,127, 640. (8)Abruiia, H.D.; Denisevich, P.; Umaiia, M.; Meyer, T. J.; Murray, R. W. J. Am. Chem. SOC.1981,103, 1. (9)Van De Mark, M. R.; Miller, L. L. J . Am. Chem. SOC.1978,100, 3223. 0022-3654/83l2087-4592$01.50/0
insight into the nature of the polymer film electrolysis process. This is the situation for electroactive polymers based on the tetrathiafulvalene (TTF)electron donor and the tetracyanoquinodimethane(TCNQ) electron acceptor. Previous papers have reported on the electrochemistryand spectroelectrochemistry of electrodes modified with polymeric coatings of these materia1s."'-'l4 For the TTF polymer electrodes, the spectral evidence indicated extensive aggregation of the TTF+cations in the 1+ oxidation state and the existence of a mixed-valence 0.5+ oxidation state at potentials in the region of the Eo' value for the first electron-transfer step.1°J3 While the TCNQ electrodes have not been as thoroughly studied as the 'M'F electrodes, spectral evidence also indicates dimerization of the TCNQ-. radical anion sites in reduced films in contact with aqueous electr01ytes.l~ As described below, the aggregation steps in this electrode process can be followed by using ESR techniques which readily detect species possessing unpaired spins.
Experimental Section Chemicals. The TCNQ polymer used in this study was synthesized from 2,5-bis(2-hydroxyethoxy)-7,7,8,8-tetracyanoquinodimethane (1) which was prepared following
go-
NCYcN HO-0
NC+N
1
the procedure of Hertler.16 This was employed as a co(10)Kaufman, F. B.;Schroeder, A. H.; Engler, E. M.; Kramer, S. R.; Chambers, J. Q.J. Am. Chem. SOC.1980,102,493. (11)Schroeder, A. H.; Kaufman, F. B.; Patel, V. V.; Engler, E. M.; J . ElectroanaL Chem. 1980,113,193. (12)Schroeder, A. H.; Kaufman, F. B., J. Electroanal. Chem. 1980, 113,209. (13)Chambers, J. Q.;Kaufman, F. B.; Nichols, K. H. J.Electroanal. Chem. 1982,142,277. (14)Day, R. W.;Inzelt, G.; Kinstle, J. F.; Chambers, J. Q.J. Am. Chem. SOC.1982,104,6804. (15)Inzelt, G.; Day, R. W.; Chambers, J. Q.J. Electroanal. Chem., manuscript accepted for publication. (16)Hertler, W. R. J. Org. Chem. 1976,41, 1412.
0 1983 American Chemical Society
Tetracyanoqulnodimethane Modified Electrodes
monomer in a low-temperature-solution polycondensation reaction with adipoyl ch10ride.l~ The molecular weight (A&.) of the light purple (TCNQ), polyester was estimated to be ca. 2200 by gel permeation chromatography following the approach of Benoit and co-workers.l8 Full details of the synthetic procedure will be published elsewhere. Spectral-grade acetonitrile (Aldrich), LiC104,NaC104 (G. Frederick Smith) were used without further purification. Tetrabutylammonium perchlorate (TBAP (Eastman)) was recrystallized from acetone/H20 and dried under vacuum prior to use. Tetrahydrofuran (THF) for film preparation was refluxed over (C$IS)&ONa under nitrogen and freshly distilled prior to use. The acetonitrile was stored over 3-A molecular sieves and bubbled with dry nitrogen prior to use. Instrumentation. Electrochemical measurements were performed with a PAR Model 173 potentiostat in conjunction with a PAR Model 175 programmer and Hewlett-Packard Model 7046A X-Y recorder. In aqueous solution a saturated calomel electrode was used, and in acetonitrile either a Ag/AgCl electrode immersed in the same supporting electrolyte or a Ag/AgN03 reference electrode was used. The ESR measurements were conducted with a Varian ES-109 spectrometer and a Varian E-101 microwave bridge equipped with a rectangular cavity, TEIo2mode. For low-temperature work a Varian variable-temperature controller was used and the cavity was purged with dry nitrogen. The g values were determined by reference to the spectra of either DPPH (g = 2.0036) or Fremy's salt (g = 2.0057) which were recorded simultaneously with the spectrum of interest.lg In the usual fashion, these standards were contained in a small glass vial which was either taped to the outside of the thin portion of the ESR cell or placed inside a quartz sample tube that contained the solution of interest. An all-Teflon cell, mounted in the sample compartment of a Cary 171 spectrometer, was used for simultaneous measurement of the voltammetric current and the film absorbance in the UV-vis-near-IR region. An optically transparent substrate, Pt on q u a r t ~ , ~ O was J ~ employed in the transmission mode. Procedures. The electrochemical and spin-coating procedures for polymer film measurements have been described previ0us1y.l~'~ The platinum grid electrode for the ESR thin cell was prepared by dip-coating in a THF polymer solution for a short time ( E > Eo2corresponds to the mixed-valence species and the broader line (AHpp= 7.5 G) to the radical anion. Note that the abscissas in Figures 7 and 8 are referenced to Eo1for the (A)x/(A-*)zcouple. The values of CT and KD used in the calculation of Figures 7 and 8 were chosen carefully. The total concentration of TCNQ sites can be estimated in two independent ways both of which give r = Cd where d is the film thickness and I' is the "surface concentration". First, the total charge accepted by the film gives r = QT/FA,where A is the electrode area and n is set equal to unity. Second, the absorbance of the neutral film at 432 nm can be used to estimate the surface concentration: I' = Abs432/~432, where 6 is the molar absorptivity of (TCNQ),. In the latter estimation a measured value of 43 000 M-' cm-' was used for c432, which is in agreement with literature values for TCNQ2' and 1.16 Both of these estimates of r contain uncertainties relating to assumptions of the surface (27) Acker, D. S.; Hertler, W. R. J. Am. Chem. SOC.1962, 84, 3370.
The Journal of Physical Chemistry, Vol. 87, No. 23, 1983 4597
Tetracyanoquinodimethane Modified Electrodes
0.3
' I
c
s!
0.2
\
.-
c a .)
W
w
0.1
0.0
- 0.05
-0.10
-0.15
AE/Volt
Flgure 9. Variation of 6€m(see text) with A€ = E o 2 - E o , .
roughness in the latter case, and the full electroactivity of the (TCNQ), f h in the former case. In combination with the measured film thickness for the dry reduced film, these measurements gave approximately the same value for CT, 3.6 mol/dm3. Interestingly, the thickness of a dry reduced film is found to be somewhat less than the thickness of the corresponding dry neutral film. In contact with aqueous solution, however, the films are likely to be swollen; thus this estimate of the concentration is an upper limit. The value of KD (1.4 dm3/mol) used in the calculation was estimated in a similar fashion using the absorbances of the fully reduced film and the literature absorptivities of TCNQ-- and TCNQ:-.z3 It is necessary to use the monomer absorptivities because it is impossible to vary the total concentration of acceptor sites for a given (TCNQ), matetial to obtain the absorptivities at the appropriate extrapolation limits. Of course, the degree of dissociation, a,the dimer formation constant, and the total concentration are linked by the relation KD = (1- a)/2a2C. Equation 6 allows the estimation of a from absorbance a = {tlD(Abs2)- tzD(Absl))/{(Absl)X ( 2 ~ -2 tzD) ~ - (Abs2)(2tlM- tlD)l (6) measurements, Absl and Absz, at two wavelengths where enM and tnD are the absorptivities of the monomer and dimer, respectively, at wavelength A,. The overlap of the monomer and dimer bands made estimation of the terms in parentheses in eq 6 difficult and caused a further uncertainty in the estimate of KD. However, a is dimensionless and, consequently, uncertainties in the total concentration and film thickness will tend to cancel when eq 6 is used to estimate a. Measurements made at Absl = 804 nm (where tlD N 0) and Abs2 = 656 nm on the dimer band gave a = 0.27 for the degree of dimer dissociation in the fully reduced film. The values of Eo1and Eo2for the reactions of eq 1 and 3 were extracted from the spectroelectrochemical data in the following fashion. Inspection of Figures 7-9 reveals that 6EsPi",the peak width of the maximum in the spin concentration due to the mixed-valence species, is directly linked to the value of AE = Eoz- Eo1. Strictly, 6EP"was obtained by measuring the peak width at S = (S" SIim)/2, where Slimis the limiting spin concentration at E E" made estimation of 6EsPindifficult. Nonetheless, the experimental ratio of the maximum in the ESR signal to the value at -0.2 V was in the range of 1.4-1.8, in agreement with Figure 8. As discussed below, the peak was established more rapidly on the positive-going than on the negative-going pulse sequence, and, for this reason, the former sequence was used to determine AE. In combination with the spectroelectrochemicalmeasurements, we estimate Eo = 0.030 V and Eo2= -0.015 V vs. SCE in 0.5 M LiC104. Work in progress indicates that these values are somewhat dependent on the supporting electrolyte. No evidence could be found of an absorption band for the mixed-valence species in the visible or near-infrared spectral region. A broad band at 1055 nm is found in the reduced films which can be assigned to the dimer dianion species on the basis of its potential dependence. This band, which has not been reported previously, is also observed in the solution spectra of M+1-.. I t is likely that additional weak absorption bands for the (A,-.), sites lie underneath the strong overtone water bands which are located in the near-infrared r e g i ~ n . ~ Owing * ~ ~ ~ to the absorption of energy by the aqueous solvent/electrolyte it is impossible to make absorbance measurements for wavelengths greater than 1350 nm with our spectroelectrochemical cell. It can be noted that no evidence for mixed-valence bands in solution was found by Melby et aL30 in their classic study of TCNQ complexes. The structure of the dimeric anions in the reduced films is of course not revealed by these studies. It seems likely, however, the intermolecular stacking in a manner similar to that found in conducting and semiconducting TCNQ salts occurs. It is possible that the reduced films have some degree of crystallinity and are best described as a (TCNQ-M+),/((TCNQ)z~,(M+)z), mixed salt. This picture finds analogy in the recent work of Henning et al.,31who demonstrated that Nafon-TTF bromide polymers formed nonelectroactive crystalline aggregates upon electrochemical cycling. Mechanistic Implications. These results have some interesting mechanistic and kinetic implications concerning the electrochemistry of fixed-site polymer films. Little, if any, cathodic current is observed in cyclic voltammograms in the potential region where the mixed-valence species is stable. Appreciable cathodic current is not observed until the electrode potential is negative enough to (28) Buijs, K.; Choppin, G. R. J. Chem. Phys. 1963,39,2035. Choppin, G. R.; Buijs, K. Ibid. 1963, 39, 2042. (29) Subramanian, S.; Fisher, H. F. J. Phys. Chem. 1972, 76, 84. (30) Melby, L. R.; Harder, R. J.; Hertler, W. R.; Mahler, W.; Benson, R. E.; Mochel, W. E. J. Am. Chem. SOC.1962,84, 3374. (31) Henning, T. P.; White, H. S.; Bard, A. J. J.Am. Chem. SOC.1982, 104, 5862.
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The Journal of Physical Chemistry, Vol. 87,No. 23, 1983
reduce the acceptor sites to the radical anion state. This implies that the slow cathodic formation of the (A,-.), species observed in the ESR experiments is a consequence of the low concentration of (A-e), sites in this potential region and the resulting slow formation of the dimeric complex. In this view (see the reduction scheme) reduction scheme
2A
+ 2e- + 2A-. A2-.
overall 2A
+ e-
+ slow
e-
slow
A,-*
the mixed-valencespecies is formed via an electron-transfer reaction between neutral acceptor sites and the dimer dianion in the bulk of the polymer film phase, eq 7. Eo = -AE = +0.045 V (7) A?A + A; A--
+
+
Overall, this results in a kinetically slow formation of (A,-.), sites. Once the film is fully reduced, however, oxidation of the dimer dianion leads to the dimer radical anion directly via eq 3. Accordingly, formation of the mixed-valence species is more rapid for anodic electrolysis of the reduced film than for the reduction process. This kinetic aspect of A2-. formation accounts for the difference between the ESR signal potential dependence on the forward and reverse sequences shown in Figures 4 and 5. In addition, complete oxidation of the reduced films is a very slow process as demonstrated by the slow decrease in the ESR signal when the film is oxidized at 0.2 V. This decrease is paralleled by a somewhat more rapid decrease of the A-e absorption bands at 735 and 804 nm under similar conditions in the spectroelectrochemical cell. Evidently a sizable fraction of the acceptor sites are trapped in the film in the dimer radical or radical anion state such that they are not associated with a rapid charge-transfer process. The slow oxidation may result when the (Ap), sites become isolated by neutral film which will not support rapid charge transport, because of either slow electron hopping or counterion migration through the neutral film. Furthermore, this picture may explain the stability of the reduced polymer films in the laboratory atmosphere as compared
Inzelt et at.
to the oxygen sensitivity of the monomeric radical anions of 1 in aqueous solutions. The nature of these processes is under further study, the results of which will be published separately.
Summary The combination of electrochemical and ESR methods provides a detailed insight into the film electrochemistry of TCNQ modified electrodes. The existence of a mixedvalence state, (TCNQ,-.),, is indicated, although this species is invisible in the visible-near-infrared spectral region and its formation is electrochemically silent owing to slow dimerization kinetics in the electrochemical reduction process. However, the mixed-valence sites are formed more rapidly upon electrochemical oxidation of fully reduced films which contain an appreciable fraction of dimeric dianion sites. Analysis of the potential dependence of the ESR signal and the absorption bands of the reduced (TCNQ), f h permits the following estimates to be made: Eo1= 0.030 V vs. SCE for the TCNQ/TCNQ-. couple and Eo2 = -0.015 V vs. SCE for the TCNQ2-./ TCNQt- couple in the polymer film state in the contact with 0.5 M LiC104. Electrochemical charge injection into a polymer film provides a convenient method for preparation of samples for ESR studies and provides a method for study of electrodimerization equilibria involving paramagnetic species. The method outlined above for extraction of the potential difference, A E = Eo2 - Eo1,from the experimental results should have some generality outside of the polymer film electrochemistry experiment. Finally, close correspondence is found between the solution electrochemistry of 2,5-bis(2-hydroxyethoxy)7,7,8,&tetracyanoquinodimethane (1)and its radical anion and the electrochemistry and spectroscopy of the (TCNQ), polymer film. Acknowledgment. This research was supported by the US. Army Research Office (Project No. P-17715-C) and the University of Tennessee. We thank Marty Mansfield, who wrote PASCALprograms for calculation of the distribution coefficients. Registry NO. 1,58268-29-4;I-., ~7115-ag-7;(TCNQ), (polyester), 83462-96-8; Pt, 7440-06-4.
