Modulated Intermolecular Interactions in Ferrocenylalkanethiolate Self-Assembled Monolayers on Gold

Abstract

Redox-labeled self-assembled monolayers (SAMs) on gold are excellent model systems for the study of long-range electron transfer processes at electrolyte–electrode interfaces, particularly the distance and reorganization energy dependences. In this work, we have shown that the intermolecular interaction among redox centers is in fact a crucial factor in the overall, nonideal electrochemical response of ferrocenylalkanethiolate SAMs on gold. In both single-component and high-ratio binary monolayers of 11-ferrocenyl-1-undecanethiol (FcC11SH), the two distinct pairs of redox peaks are corresponding to rather moderate differences in the packing densities of the two structural domains. We have discovered that the redox peak at lower potential becomes narrower and higher when organic solvents (nitrobenzene or octanol) are added to the aqueous electrolyte, while the peak at the higher potentials is barely influenced. On the basis of the Frumkin isotherm, we have obtained the intermolecular interaction parameters in the different structural domains of the monolayers by fitting the experimental data. The results showed that the intermolecular interaction in the FcC11S–Au SAMs can change from repulsion to attraction upon adding organic solvent in the aqueous electrolyte. It is suggested that the solvent perturbation to the SAM structure at monolayer/electrolyte interface induces remarkable change in the intermolecular interactions and therefore modulates the observed electrochemical responses from nonideal to nearly ideal.