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 explored by photocurrent measurements at the water/1,2-dichloroethane interface. The anionic zinc meso-tetrakis(p-sulfonatophenyl)- porphyrin and cationic zinc tetrakis(N-methylpyridyl)porphyrin undergo spontaneous association in solution, leading to electrically neutral surface active heterodimer species. Heterogeneous quenching of the porphyrin ion pair was observed in the presence of the electron donor decamethylferrocene and the electron acceptor 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 photocurrent 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 issues in connection to back electron transfer, heterodimer adsorption, and orientation at the interface are also exposed.