Coupled electron/proton transfer of galvinol attached to SAMs on gold electrodes

The theory for a 1-electron, 1-proton redox couple with potential-dependent transfer coefficients is tested using a self-assembled monolayer with attached galvinol/galvinoxyl redox couples on a gold electrode. The potential dependence of the transfer coefficients is derived from the Marcus density-of-states model of heterogeneous electron transfer. The pK(a) of surface-attached galvinol is 11.3. The predicted asymmetry in kinetically controlled cyclic voltammograms is observed at pHs lower than It. Apparent standard rate constants between pH 10 and 13 are fitted to the theory to yield a standard rate constant of 4.5 x 10(3) s(-1) for the deprotonated galvinol/galvinoxyl radical redox couple; this standard rate constant is comparable to the standard rate constant for ferrocene/ferricenium with the same number of atoms linking the redox center to the electrode. Anomalous behavior is seen for the apparent standard rate constants at pHs between 7 and 9.