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 di ferrocenylethane are studied at the water/1,2-dichloroethane interface. Bas ic features of the photocurrent transients are analyzed within the framewor k of classical photoinduced electron-transfer mechanisms. The potential dep endence of the photocurrent at various light intensities and porphyrin conc entrations provides a quantitative analysis of photoinduced heterogeneous e lectron-transfer kinetics at interfaces between two immiscible electrolyte solutions (ITIES). The dependence of the photocurrent intensity upon porphy rin concentration indicates that most of the photoresponses arise from sens itizer adsorbed at the liquid/liquid junction. The adsorption of porphyrins as well as the formation of interfacial ion pairs are confirmed by capacit ance measurements. The rate of electron transfer was found to be of the sam e 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 pertur bs the potential distribution across the liquid/liquid interface. This phen omenon is reflected in the potential dependence of the surface coverage. Th e potential dependence of the electron-transfer rate constant is briefly di scussed in terms of the existing models for ITIES. The possibility of novel solar energy conversion devices where the photoinduced electron transfer d oes not intimately involve a solid electrode is also envisaged.