Solvent effect on electron transfer through alkanethiols

The data on the rate constants for ferrocene/ferrocenium (Fc/Fc+) electron transfer in ionic liquid 1-butyl-3-methyl imidazolium tetrafluoroborate ([bmim][BF4]) are reported for a series of -S-(CH2)n-Fc adlayers (n = 8…16) on polycrystalline gold. The values and trends are compared to the previously published data for similar series in aqueous medium, as well as to the results for 'free' Fc at -S-(CH2)n − 1-CH3 modified gold in the same ionic liquid. For the latter case, the trend is opposite as compared to that in the available aqueous series (attached and 'free' ruthenium ammine complex): the ratio of rate constants for attached and 'free' reactants decreases with electrode-reactant distance in the former case, and increases in the latter. The presented comparison discovers the role of reactant and product work terms, which are essential for attached reactants as well because of conformational diversity. Corresponding contributions are estimated from molecular modeling and considered in terms of reaction volume.