Two-Electron Oxidation of Water Through One-Photon Excitation of Aluminium Porphyrins: Molecular Mechanism and Detection of Key Intermediates

The reaction mechanism of photochemical water oxidation catalyzed by Al(III)porphyrins ((AlTMPyP)-M-III(OH)(2)) covalently adsorbed through the coordination of one axial OH group on the surface of TiO2 particles was investigated by time-resolved nanosecond laser flash photolysis (NLFP). Three types of transient species with completely different absorption and decay characteristics were detected by means of NFLP, demonstrating that the photochemical production of H2O2 from water is initiated only by one-photon excitation of the catalyst followed by stepwise two-electron conversion processes, providing a promising model for photochemical water oxidation. Laser flash excitation of (AlTMPyP)-T-III adsorbed on a transparent TiO2 film leads to the formation of the AlTMPyP(OH) radical cation through one-electron injection from the excited singlet state of the catalyst to the conduction band of TiO2. The radical cation is subsequently deprotonated into AlTMPyP(O-.), which further reacts with OH-/H2O to form a key intermediate, the radical anion of AlTMPyP(O-OH), having a characteristic O-O bond at the axial position within a delay time of 2-3s. Thereafter the radical anion efficiently transfers a second electron to TiO2 leading to the formation of (AlTMPyP)-M-III(O-OH) (approximate to 67s), which reacts further with OH-/H2O to liberate free hydrogen peroxide as an end-product with regeneration of the starting catalyst, (AlTMPyP)-M-III(OH), (approximate to 500s).