Photoinduced electron transfer at liquid/liquid interfaces. Part III. Photoelectrochemical responses involving porphyrin ion pairs

The interfacial photoreactivity of water soluble porphyrin ion pairs was ex plored by photocurrent measurements at the water/1,2-dichloroethane interfa ce. The anionic zinc meso-tetrakis(p-sulfonatophenyl)- porphyrin and cation ic zinc tetrakis(N-methylpyridyl)porphyrin undergo spontaneous association in solution, leading to electrically neutral surface active heterodimer spe cies. Heterogeneous quenching of the porphyrin ion pair was observed in the presence of the electron donor decamethylferrocene and the electron accept or tetracyanoquinodimethane. No substantial photoresponses were detected in the presence of only one of the porphyrin species. Fundamental aspects on the dimer structure and photoreactivity were extracted from the photocurren t dependence on the concentration of both porphyrins. Photocurrent spectra as well as analysis of the association constant indicate that the porphyrin species are intimately associated as inner sphere ion pairs. Relevant issu es in connection to back electron transfer, heterodimer adsorption, and ori entation at the interface are also exposed.