Faced with the new stage of water oxidation by molecular catalysts (MCs) in artificial photosynthesis to overcome the bottle neck issue, the "Photon-flux density problem of sunlight," a two-electron oxidation process forming H2O2 in place of the conventional four-electron oxidation evolving O2 has attracted much attention. The molecular characteristics of tin(IV)-tetrapyridylporphyrin (SnTPyP), as one of the most promising MCs for the two-electron water oxidation, has been studied in detail. The protolytic equilibria among nine species of SnTPyP, with eight pKa values on the axial ligands' water molecules and peripheral pyridyl nitrogen atoms in both the ground and excited states, have been clarified through the measurements of UV-vis, fluorescence, 1H NMR, and dynamic fluorescence decay behaviour. The oxidation potentials in the Pourbaix diagram and spin densities by DFT calculation of the one-electron oxidized form of each nine species have predicted that the fully deprotonated species ([SnTPyP(O-)2]2-) and the singly deprotonated one ([SnTPyP(OH)(O-)]-) serve as the most favourable MCs for visible light-induced two-electron water oxidation when they are adsorbed on TiO2 for H2 formation or SnO2 for Z-scheme CO2 reduction in the molecular catalyst sensitized system of artificial photosynthesis.
Keywords: acid-base equilibrium; artificial photosynthesis; axial ligand; hydrogen peroxide; molecular catalyst; tin-porphyrins.