Factors Affecting the Electrochemical Responses of Metal Complexes at Pyrolytic Graphite Electrodes Coated with Films of Poly(4-Vinylpyridine)

Elec t rochemical responses f rom the reduct ion of Ru m (edta) coordinated to films of high molecular weight poly (4 -v iny lpyr id ine ) on pyro ly t ic g raphi te e lectrodes were s tudied as functions of film thickness, t empera tu re , suppor t ing e lec t ro ly te composition, and solvent. Responses at f i lmed electrodes f rom meta l complexes tha t do not coordinate to the films were also examined. With films th icker than ca. 1000A, the cur ren t responses are l imi ted by the ra tes of molecu la r motions wi th in the films. Pene t ra t ion of counterions, segmenta l mot ion of sections of the po lymer chains, and jux tapos i t ion ing of pairs of a t tached meta l complexes to faci l i ta te in te rcomplex e lect ron t ransfe r wi th in the film or combinat ions of the three are suggested as l ike ly cur ren t l imi t ing proces se s . There has been considerable cur ren t in teres t in th e prepa ra t ion and proper t ies of po lymer -coa t ed elect rode surfaces (1-12). Examples of po lymer coatings tha t have been descr ibed include nonelec t roact ive po lymers (1, 6), po lymers bear ing repea t ing e lec t roact ive groups (2, 3, 7, 8, 11), and e lec t roinact ive po ly meric coatings tha t become e lec t roact ive when meta l complexes are a t tached to them (4, 5, 7, 9). Among the questions tha t awai t definit ive answers for such coatings is how elect ron t ransfe r is accomplished be tween the e lect rode surface and e lec t roact ive sites p resen t in the polymer ic films. Kau fman and Engler (8) have recen t ly proposed that electrons are conducted th rough ra the r th ick films of a po lymer ic pyrazol ine by e lec t ron t ransfe r be tween the e lect roact ive groups wi th in the polymer . They proposed tha t such t ransfe r requ i red tha t the sites of e lec t roac t iv i ty be mobi le enough to achieve appropr ia t e jux tapos i t ions of ne ighbor ing sites in o rder for e lec t ron t ransfe r to occur. In addit ion, the po lymer film must be porous enough to a l low pene t ra t ion by ions of the suppor t ing e lec t ro ly te to main ta in e lec t roneu t ra l i ty wi th in the film. In the presen t r epor t we address this and re la ted issues for films of po ly (4 -v iny lpy r id ine ) (PVP) a t tached to pyro ly t ic g raphi te electrodes. The versa t i l i ty of this po lymer ic l igand in anchor ing t rans i t ion meta l complexes to e lec t rode surfaces ~was descr ibed re cent ly (9) and i t seemed des i rable to obta in more informat ion on the e lect ron t ransfe r proper t ies of these films which have e lect roact ive groups (meta l complexes) in t roduced into them af te r they have been a t tached to the graphi te surface.