Ac voltammetry studies of electron transfer kinetics for a redox couple attached via short alkanethiols to a gold electrode

Ac voltammetry (ACV) is used to determine the chain length dependence of electron transfer kinetics between gold bead electrodes and ruthenium redox centers attached to the electrode surface via short alkanethiols. The pentaamminepyridine ruthenium redox centers are attached to pre-assembled monolayers of mercaptoalkanecarboxylic acids (HS(CH₂)_mCOOH, m=5, 7, 10). Equations for faradaic admittance of strongly adsorbed, non-interacting electroactive species are fitted to the experimental faradaic admittance data. Kinetic heterogeneity is suggested to be a probable reason for the apparent increase in the standard rate constant (k_s) and decrease in the total coverage of redox centers (θ_total) as the perturbation frequency increases. Because of the frequency dependence of k_s and because of limitations in the correction for uncompensated resistance, faradaic admittance at a fixed phase angle of 70° is chosen to compare k_s for different chain lengths. The plot of ln(k_s) versus m (the number of methylene units) is linear with a slope of −1.2±0.1 per CH₂.