KINETICS OF ELECTRON-TRANSFER TO ATTACHED REDOX CENTERS ON GOLD ELECTRODES IN NONAQUEOUS ELECTROLYTES

Self-assembled monolayers with attached redox centers are characterize d by their reversible behavior and their electron-transfer kinetics in a range of nonaqueous solvents: methanol, ethanol, propanol, butanol, acetonitrile, N,N-dimethylformamide, dimethylsulfoxide, propylene car bonate, acetone, and tetrahydrofuran. The monolayers are formed by coa dsorbing the thiols HS(CH2)(n)CONHCH(2)pyRu(NH3)(5)(PF6)(2) and HS(CH2 )(n)COOH (n = 10 or 15) on gold electrodes. The monolayer-coated elect rodes are examined by cyclic voltammetry and chronoamperometry first i n the nonaqueous electrolyte and then in an aqueous electrolyte. The r eversible CV's (n = 10) indicate the presence of strong ion-pairing an d the relatively disordered structure of the monolayers in the nonaque ous solvents. A method is introduced for the correction of iR drop dis tortion when the chronoamperometry data are analyzed for rate constant s. Apparent rate constants (n = 15) are obtained as a function of the percent conversion of the redox centers from the initial to the final oxidation state and of the iR drop-corrected overpotential. Apparent s tandard rate constants and reorganization energies are obtained by fit ting the Tafel plots with Marcus theory. For most of the more polar so lvents, the apparent reorganization energies are nearly identical to t he values obtained in water (0.9 eV for oxidation and 0.7-0.8 eV for r eduction of the redox centers); the least polar solvents yield lower a pparent reorganization energies. Propylene carbonate data deviate mark edly from the pattern of the other polar solvents. The standard rate c onstants in water are reproducibly close to 1 s(-1). The apparent stan dard rate constants in the nonaqueous solvents show a considerably gre ater heterogeneity and are generally faster by up to a factor of 10 th an the corresponding aqueous standard rate constants; however, the sta ndard rate constants do not correlate with solvent relaxation times. T he dominant factors which control the kinetic parameters of the monola yers in nonaqueous solvents appear to be monolayer disorder and the lo cal water concentration.