Proton coupled electron transfer of galvinol in self-assembled monolayers

Proton coupled electron transfer (PCET) half-reactions play a vital role in a variety of biological and chemical processes. A previous kinetic analysis of proton coupled redox couples on electrodes assumes that electron and proton transfers proceed stepwise and that the transfer coefficient for electron transfer steps remains constant at all overpotentials. This work explores the consequences of the stepwise model with a potential-dependent transfer coefficient using a surface-attached 1-electron/1-proton-redox couple. The potential dependence of the transfer coefficient is based on the Marcus density-of-states model for heterogeneous electron transfer. Galvinol is attached to a self-assembled monolayer on a gold electrode and its voltammetry investigated from pH 2 to 14. The formal potential, standard rate constant and pK(a) for galvinoxyl/deprotonated galvinol/protonated galvinol are obtained from the pH dependence of the apparent formal potential and apparent standard rate constant. The standard rate constant for galvinoxyl/deprotonated galvinol (5600 s(-1)) suggests that the reorganization energy of the galvinol system is comparable to or higher than the reorganization energy of Ru(NH3)(5)(pyridine) complex (0.8 eV). Deviations from the predicted pH dependence of the apparent standard constant are seen at pH less than 9. The deviations suggest that the assumption of stepwise electron and proton transfer should be questioned. (C) 2002 Elsevier Science B.V. All rights reserved.