Coupled Thermodynamic and Kinetic Changes in the Electrochemistry of Ferrocenyl Monolayers Induced by Light

We report on the light-induced systematic changes to the thermodynamics and kinetics of ferrocene units attached to a n-type silicon(100) photoelectrode. Both the reaction rate and the energetics of the charge transfer are simultaneously affected by changes in the intensity of the incident light. Cyclic voltammetry shows that increases in the intensity of illumination can drive the redox process toward less positive potentials, with a downhill shift in E_1/2 of ca. 160 mV by increasing the light intensity from 3 to 94 mW cm^-2. However, this thermodynamic shift is paralleled by an increase in the kinetics of the charge transfer. This latter observation-light-induced kinetic effects at monolayers on silicon electrodes-is made possible only by the stability of the surface chemistry construct. Furthermore, electrochemical impedance measurements showed that the electrodes exhibit faster electron-transfer kinetics under illumination than previously reported for ferrocene-terminated highly doped silicon (around 1 order of magnitude faster). An explanation for the kinetic effects is proposed on the basis of the consistent increase in photogenerated charge carriers inside silicon and the enlarged potential difference between the valence band of silicon and the surface-attached ferrocene.