Artificial photosynthesis at liquid | liquid interfaces: photoreduction of benzoquinone by water soluble porphyrin species

The heterogeneous photoreduction of benzoquinone by hydrophilic porphyrin species was studied at a polarised water | 1,2-dichloroethane interface. The dynamics of photoinduced electron injection and back charge transfer processes were analysed by periodic on–off light intensity perturbation and intensity modulated photocurrent spectroscopy (IMPS). The flux of electron injection increased and the phenomenological rate constant of back charge transfer decreased upon addition of the hexacyanoferrate redox couple used as a supersensitiser. The dynamics of the photoresponses as a function of the supersensitiser concentration suggest that the hexacyanoferrate(II) species injects an electron into the porphyrin excited state, generating an intermediate, which can subsequently reduce the benzoquinone. Despite the decrease in the back electron transfer rate, the steady-state quantum efficiency remains rather low for all concentrations of the supersensitiser. These phenomena suggest a rather complex mechanism where other processes may be involved in the millisecond time scale.