Kinetic Separation of Faradaic Currents: Binary Monolayers as Model Systems

Abstract

Binary monolayers have been formed by simultaneous coadsorption of [Os(bpy) 2 Cl(p2p)] + and [Ru(bpy) 2 Cl-(p2p)] + , where bpy is 2,2'-dipyridyl and p2p is 1,2-bis(4-pyridyl)ethane. The electrochemical responses of these mixed monolayers are unusually ideal. Microsecond time scale chronoamperometry reveals that the heterogeneous electron transfer rate constants k for both the Os 2+/2+ and Ru 2+/3+ redox reactions are constant as the ratio of osmium to ruthenium sites is varied from 0.9 to 0.1. Identical surface coverages are obtained using cyclic voltammetry and high-speed chronoamperometry. Adsorption of [Os(bpy) 2 Cl(p2p)] + or [Os(bpy) 2 Cl(p3p)] + , where p3p is 4,4'-trimethylenedipyridine, follows a Langmuir isotherm, and a free energy of adsorption of 37.9± 2.2 kJ mol -1 is observed for both complexes. Dense monolayers containing both redox centers have been prepared by first immobilizing a partial monolayer of one component and then depositing the second component to backfill the assembly. These binary monolayers exhibit only one voltammetric peak for the Os 2+/3+ redox reaction. However, the heterogeneous electron transfer rate constants for the two redox centers are not identical because of the different bridging ligand lengths. In short-time scale potential step experiments, three single exponential current decays, corresponding to double layer charging and heterogeneous electron transfer to the two types of redox centers, are separated on a low microsecond time scale. We have exploited these differences in kinetic behavior to determine the individual surface coverages of the two redox centers despite their identical formal potentials. The ability to determine the bulk concentrations of species having identical formal potentials by combining information about heterogeneous kinetics and adsorption thermodynamics is demonstrated