Polymer films on electrodes. 17. The application of simultaneous electrochemical and electron spin resonance techniques for the study of two viologen-based chemically modified electrodes

Simultaneous electrochemical and electron spin resonance (SEESR) techniques were used to study the environment and rates of electron transfer in two viologen-based chemically modified electrodes. Electrodes derivatized with N,N'-bis[3-(trimethoxysilyl)propyl]-4,4'-bipyridinium, where the redox sites are covalently anchored to the polymer backbone, exhibit broad and featureless spectra upon reduction, suggesting that the spin sites are restrained and immobile. The ESR signal intensity as a function of the extent of film reduction indicates that the rate on electron transfer (self-exchange) is high, >lo6 M-l s-'. Electrodes covered with the perfluorosulfonate polymer Nafion in which MV2+ (MV2+ = methyl viologen) has been incorporated display ESR spectra upon reduction that are similar to those of MV+ in solution, suggesting that the radicals are free to tumble on the ESR time scale. The rates of electron exchange are smaller in the Nafion modified electrodes (58 x 105 M-1 s-1 ); this is consistent with physical diffusion of the redox species contributing substantially, if not completely, to charge transport in these films. This paper concerns the application of electron spin resonance (ESR) spectroscopic techniques to polymer electrodes and a comparison of the behavior of a cation radical (viologen) in two different types of polymer films on an electrode surface: one, where the radical ion is attached to the polymer backbone, and the other, a polyelectrolyte, to which the species is held by electrostatic binding. Chemically modified electrodes have been characterized by a variety of techniques. Electrochemical methods such as cyclic voltammetry, chronoamperometry, chronopotentiometry, and rotating disk voltammetry have been used to determine the redox, charge transport, and catalytic properties of these e1ectrodes.l Spectroscopic techniques, e.g., X-ray photoelectron, Auger, UVvis, photoacoustic, photothermal, and reflectance spectroscopy, have also provided information about the morphology of the modifying layer.' Electron spin resonance spectroscopy has recently been introduced in conjunction with electrochemical techniques to characterize polymer modified electrodes, e.g., in the study tetracyancquinodimethane (TCNQ)-modified Pt electrodes.2 Albery et al. have developed an electrochemical cell for ESR studies based on convective diffusion and used it to study polymer films of poly(nitrostyrene) and poly(vinylanthraq~inone).~ (1) Murray, R. W. 'Electroanalytical Chemistry"; Bard, A. J., Ed.; Dek(2) Izelt, G.; Day, R. W.; Kinstle, J. F.; Chambers, J. Q. J . Phys. Chem. ker: New York, NY, 1984; Vol. 13, p 191 and references therein. 1983.87, 4592-4598. In both previous studies the electrogenerated radical ions were attached to the polymer backbone and the ESR signal was a single line. ESR measurements are particularly attractive for the study of modified electrodes because information concerning the environment and mobility of surface-confined redox couples, as well as some insight into the mechanism of charge transport, can be obtained. Viologens have been extensively studied in recent years in their role as electron acceptors in p h o t ~ c h e m i c a l ~ ~ ~ and photoelectrochemical6,' schemes. The reversibility of the 2+/1+ couple and the large visible spectral changes that accompany the oxidation/reduction process have made the viologens especially useful in modified electrode studies as Cyclic voltammetric (3) Albery, W. J.; Compton, R. G.; Jones, C. C. J . Am. Chem. SOC. 1984, (4) Young, R. C.; Meyer, T. J.; Whitten, D. G. J . Am. Chem. SOC. 1975, (5) (a) Moradpur, A.; Amouyal, E.; Keller, P.; Kagan, H. Nouu. J . Chim. 1978, 2, 547-549. (b) Kiwi, J.; Gratzel, M. J. J . Am. Chem. SOC. 1979, 101, ( 6 ) (a) Bookbinder, D. C.; Bruce, J. A,; Dominey, R. N.; Lewis, N. S.; Wrighton, M. S. Proc. Natl. Acad. Sci. U.S.A. 1980, 77, 6280-6289. (b) Dominey, R. N.; Lewis, N. S.; Bruce, J. A,; Bookbinder, D. C.; Wrighton, M. S. J . Am. Chem. SOC. 1982, 104, 467-482. (7) (a) Reichman, B.; Fan, F-R. F.; Bard, A. J. J . Elecrrochem. SOC. 1980, 127, 333-338. (b) Ward, M. D.; White, J. R.; Bard, A. J. J . Am. Chem. SOC. 106, 469-473.