Electron-transfer kinetics of ferrocene alkanethiolate monolayers in ether and polyether solvents

Abstract

The electron-transfer kinetics of self-assembled mixed monolayers of 16-(ferrocenylcarbonyloxy)hexadecanethiol and hexadecanethiol have been compared in a monomeric ether solvent (dimethoxyethane) and two oligomeric polyether solvents of molecular mass 400 and 1000. The rate measurements of the ferrocene–ferrocenium electron transfer, which presumably occurs by electron tunnelling through the alkane chain linker, were made using cyclic voltammetry at Au electrodes, as a function of potential sweep rate and temperature. The electron-transfer rate constants are larger in dimethoxyethane (DME) than in the two polyethers, and slowest in the longer chain polyether, whereas the activation barriers for electron transfer are approximately the same in the three solvents. The difference in rate constant between DME and the poly(ethylene) oxide of molecular mass 400 solvents at 298 K was 14-fold, within a factor of two of the 27-fold difference predicted on the basis of the solvent dynamics model and given the longitudinal relaxation times, τ_L, of the two solvents. Solvent dynamics is proposed to be at least in part the origin of the kinetic difference between the monomeric and polymeric solvents. The behaviour of voltammetric peak broadening and apparent rate constant variation with potential sweep rate is indicative of a kinetic dispersion in the ferrocene electron-transfer kinetics; this dispersity increases in the polymer solvents and at lowered temperature. The ferrocene kinetics are faster in DME solvent than in aqueous HClO₄ which is rationalized based on differences in dielectric properties and a consequent lower outer sphere reorganizational barrier in DME.