Photoinduced electron transfer at liquid/liquid interfaces.: I.: Photocurrent measurements associated with heterogeneous quenching of zinc porphyrins

Photocurrent responses associated with the heterogeneous quenching of water-soluble zinc tetrakis(carboxyphenyl)porphyrin (ZnTPPC) by ferrocene and diferrocenylethane are studied at the water/1,2-dichloroethane interface. Basic features of the photocurrent transients are analyzed within the framework of classical photoinduced electron-transfer mechanisms. The potential dependence of the photocurrent at various light intensities and porphyrin concentrations provides a quantitative analysis of photoinduced heterogeneous electron-transfer kinetics at interfaces between two immiscible electrolyte solutions (ITIES). The dependence of the photocurrent intensity upon porphyrin concentration indicates that most of the photoresponses arise from sensitizer adsorbed at the liquid/liquid junction. The adsorption of porphyrins as well as the formation of interfacial ion pairs are confirmed by capacitance measurements. The rate of electron transfer was found to be of the same order as the lifetime of the excited state. The maximum quantum yield was estimated to be 60% for the photooxidation of ferrocene. The high surface charge introduced by the specific adsorption of nonprotonated ZnTPPC perturbs the potential distribution across the liquid/liquid interface. This phenomenon is reflected in the potential dependence of the surface coverage. The potential dependence of the electron-transfer rate constant is briefly discussed in terms of the existing models for ITIES. The possibility of novel solar energy conversion devices where the photoinduced electron transfer does not intimately involve a solid electrode is also envisaged.