Scanning Electrochemical Microscopy (SECM) of Photoinduced Electron Transfer Kinetics at Liquid/Liquid Interfaces

The kinetics of the photoinduced electron transfer (ET) reaction between the aqueous photosensitizer tris(2,2-bipyridyl) ruthenium(II) (Ru(bipy)32+) and the organic quencher 7,7,8,8-tetracyanoquinodimethane (TCNQ) in 1,2-dichloroethane (DCE) is investigated with a scanning electrochemical microscopy (SECM) photoelectrochemical setup. In this system, an optical fiber placed in one phase is employed to illuminate the interface between two immiscible electrolyte solutions (ITIES), while an ultramicroelectrode (UME) tip in the other phase, positioned with high precision close to the end of the fiber, is used to detect the products of the photoelectrochemical process at the interface. The possible transfer of reactants and products, Ru(bipy)32+, Ru(bipy)33+, TCNQ, and TCNQ•–, across the interface is elucidated by extraction experiments, UV–visible spectroscopy, and SECM-double potential step chronoamperometry (SECM-DPSC) and verified to have no detectable effect on the interfacial photoinduced ET processes in this system. By applying a step function in the light flux through the optical fiber (off–on), the effects of key experimental variables on the photocurrent magnitude are determined. The resulting photochronoamperometric behavior depends on the distance between the UME and the interface, the light intensity, and the concentration of TCNQ (for a fixed concentration of Ru(bipy)32+). A numerical model is developed and used successfully to quantitatively analyze several aspects of the photoelectrochemical process. Values of rate constants measured at different driving forces are in reasonable agreement with theoretical values calculated by the Marcus thick layer model, but the possibility of some interfacial concentration effects, which could result in a decrease of rate constant with increasing driving force, is also discussed.