A simple comparison of interfacial electron-transfer rates for surface-attached and bulk solution-dissolved redox moieties

A relationship is developed that facilitates a comparison between the standard electron-transfer rate constant (k0) of a redox couple covalently attached (as a constituent of a self-assembled monolayer (SAM)) to an electrode and the k0 of a similar redox couple dissolved in an electrolyte solution in contact with a similar (but not electroactive) electrode/SAM assembly. Such comparisons performed for ferrocene and ruthenium redox couples demonstrate that the (normalized) rate of electron-transfer (and, also, the electronic coupling) for a solution-dissolved couple through an alkanethiol SAM/aqueous electrolyte solution interface is, within one order of magnitude, equal to that for a covalently attached redox species. A related comparison involving the Marcus theory limiting rate constants for solution-dissolved couples and the Arrhenius preexponential factors for attached couples supplies additional evidence for this surprising result.