Molecular Characteristics of Water-Insoluble Tin-Porphyrins for Designing the One-Photon-Induced Two-Electron Oxidation of Water in Artificial Photosynthesis.

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 HO in place of the conventional four-electron oxidation evolving O 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 p 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, H 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)]) 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 TiO for H formation or SnO for Z-scheme CO reduction in the molecular catalyst sensitized system of artificial photosynthesis.