ELECTROCHEMISTRY AT OMEGA-HYDROXY THIOL COATED ELECTRODES .3. VOLTAGE INDEPENDENCE OF THE ELECTRON-TUNNELING BARRIER AND MEASUREMENTS OF REDOX KINETICS AT LARGE OVERPOTENTIALS

Self-assembled monolayers of omega-hydroxy thiols on Au electrodes are investigated as electron tunneling barriers allowing the measurement of heterogeneous electron kinetics of solution species over a wide range of electrode potentials without mass transport limitations. From the dependence of the electron-transfer rate of a series of redox couples on the thickness of the monolayer film, a more precise tunneling coefficient, beta, of 1.08 +/- 0.20 per methylene unit in the omega-hydroxy thiol was measured independent of the redox couple and was found to be nearly independent of the electrode potential. The heterogeneous electron-transfer rates for a series of facile redox couples measured at omega-hydroxy thiol monolayer coated Au electrodes display a pronounced sigmoidal dependence on the electrode overpotential, which is predicted by the Marcus theory. Reorganization energies and preexponential factors for a series of redox couples are extracted from current/voltage curves. The level of defects within these omega-hydroxy thiol monolayers is probed by several electrochemical measurements, which indicate that defects do not significantly perturb the kinetic measurements.