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

The heterogeneous photoreduction of benzoquinone by hydrophilic porphyrin s pecies was studied at a polarised water \ 1,2-dichloroethane interface. The dynamics of photoinduced electron injection and back charge transfer proce sses were analysed by periodic on-off light intensity perturbation and inte nsity modulated photocurrent spectroscopy (IMPS). The flux of electron inje ction increased and the phenomenological rate constant of back charge trans fer decreased upon addition of the hexacyanoferrate redox couple used as a supersensitiser. The dynamics of the photoresponses as a function of the su persensitiser concentration suggest that the hexacyanoferrate(II) species i njects an electron into the porphyrin excited state, generating an intermed iate, which can subsequently reduce the benzoquinone. Despite the decrease in the back electron transfer rate, the steady-state quantum efficiency rem ains rather low for all concentrations of the supersensitiser. These phenom ena suggest a rather complex mechanism where other processes may be involve d in the millisecond time scale. (C) 2000 Elsevier Science S.A. All rights reserved.