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

Self-assembled monolayers with attached redox centers are characterized 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 carbonate, acetone, and tetrahydrofuran. The monolayers are formed by coadsorbing 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 electrodes are examined by cyclic voltammetry and chronoamperometry first in the nonaqueous electrolyte and then in an aqueous electrolyte. The reversible CV's (n = 10) indicate the presence of strong ion-pairing and the relatively disordered structure of the monolayers in the nonaqueous solvents. A method is introduced for the correction of iR drop distortion when the chronoamperometry data are analyzed for rate constants. 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 standard rate constants and reorganization energies are obtained by fitting the Tafel plots with Marcus theory. For most of the more polar solvents, the apparent reorganization energies are nearly identical to the values obtained in water (0.9 eV for oxidation and 0.7-0.8 eV for reduction of the redox centers); the least polar solvents yield lower apparent reorganization energies. Propylene carbonate data deviate markedly from the pattern of the other polar solvents. The standard rate constants in water are reproducibly close to 1 s(-1). The apparent standard rate constants in the nonaqueous solvents show a considerably greater heterogeneity and are generally faster by up to a factor of 10 than the corresponding aqueous standard rate constants; however, the standard rate constants do not correlate with solvent relaxation times. The dominant factors which control the kinetic parameters of the monolayers in nonaqueous solvents appear to be monolayer disorder and the local water concentration.