ELECTRON-TRANSFER KINETICS OF FERROCENE ALKANETHIOLATE MONOLAYERS IN ETHER AND POLYETHER SOLVENTS

The electron-transfer kinetics of self-assembled mixed monolayers of 1 6-(ferrocenylcarbonyloxy)hexadecanethiol and hexadecanethiol have been compared in a monomeric ether solvent (dimethoxyethane) and two oligo meric polyether solvents of molecular mass 400 and 1000. The rate meas urements of the ferrocene-ferrocenium electron transfer, which presuma bly occurs by electron tunnelling through the alkane chain linker, wer e made using cyclic voltammetry at Au electrodes, as a function of pot ential sweep rate and temperature. The electron-transfer rate constant s 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 solvent s. 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 fa ctor of two of the 27-fold difference predicted on the basis of the so lvent dynamics model and given the longitudinal relaxation times, tau( L), of the two solvents. Solvent dynamics is proposed to be at least i n 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 indicati ve of a kinetic dispersion in the ferrocene electron-transfer kinetics ; this dispersity increases in the polymer solvents and at lowered tem perature. The ferrocene kinetics are faster in DME solvent than in aqu eous HClO4 which is rationalized based on differences in dielectric pr operties and a consequent lower outer sphere reorganizational barrier in DME.