Redox kinetics in monolayers on electrodes: Electron transfer is sluggish for ferrocene groups buried within the monolayer interior

A study was undertaken of the redox chemistry of ferrocene groups in alkanethiolate-based monolayer films on gold electrodes. The monolayers are structured so as to position the ferrocene groups within the interior portion of the monolayer. The study focused on the difference in the redox thermodynamics (oxidation-reduction potential) and kinetics (standard electron-transfer rate constant) for ferrocene groups at the monolayer-solution interface and those in the monolayer interior. Ferrocene groups that were buried in the monolayer interior exhibited oxidation-reduction potentials that were strongly shifted to positive potentials relative to the potentials found for similar monolayers in which the ferrocene groups are exposed to electrolyte solution, in agreement with prior work on similar systems. The principal new finding from this study is that the redox kinetics associated with ferrocene oxidation/reduction were much more sluggish when the ferrocene groups were buried within the monolayer interior when compared with monolayers in which the ferrocene groups were exposed to electrolyte. For example, the standard electron-transfer rate constant for ferrocene oxidation/reduction in a monolayer of ferrocenyl-decanethiol coadsorbed with decanethiol was found to be approximately 40 000 s(-1), whereas that for a monolayer of ferrocenyl-decanethiol coadsorbed with 1-mercaptoeicoscane (C20H41SH) was approximately 200 s(-1). The differences in behavior for "buried" and "exposed" ferrocene groups were found to depend critically upon the electrolyte anion size, i.e., ferrocene oxidation/reduction was almost completely inhibited when sodium poly(p-styrene sulfonate) was used as the electrolyte. These results are interpreted in terms of a reaction involving coupled electron- and ion-transfer, i.e., electron-transfer from ferrocene to the electrode much occur concomitantly with anion transfer from the electrolyte to the oxidized ferricenium site in the monolayer, and the rate of electron transfer can depend critically upon the rate of ion transfer into the monolayer.