Abstract

Photoacoustic spectroscopy has allowed to show that physisorbed Co^IITPP is ground state monooxidized into (Co^IIITPP)⁺ when adsorbed on Nb₂O₅, TiO₂, SrTiO₃, ZnO, SnO₂. A double oxidation is observed when the porphyrin is adsorbed on WO₃. On the contrary when it is adsorbed on ZrO₂ and SiO₂, oxidation only occurs if the porphyrin has been driven by irradiation in its excited state. It remains in the reduced form when adsorbed on Ta₂O₅. This behavior has been correlated with the band positions of these various oxides and has allowed to roughly position the oxidation potential of CoTPP in adsorbed state ∼0.8 V higher than in solution (0.5 V/SCE or 5.3 V/vacuum) that is 4.5 V/vacuum.