Mechanism and kinetic analysis of the photo-induced electron transfer mediated by a stacked metallotriporphyrin in planar lipid bilayers

The photo-induced transmembrane charge transfer across planar lipid bilayers sensitized by a stacked ZnCuZn triporphyrin and enclosed between two aqueous phases is shown to occur via monomers spanning the membrane. The first step of the proposed mechanism involves the oxidation of the excited state of the porphyrin ring which faces the aqueous oxidizing phase. The positive charge then shifts from the resulting radical cation toward the porphyrin ring facing the reducing phase. This mechanism for the intramolecular electron transfer is preferred to the one based on the formation of a charge-separated state such as P⁺—P^?—P. The reaction scheme selected is supported by fluorescence and UV—visible absorption data. The kinetic equations derived fit the experimental time course of the photocurrents and photopotentials. A numerical simulation provides a lower estimation of the rate constant of the photo-induced intramolecular electron-transfer step.