Enhancement of\nCharge Separation in the Photochemical\nTwo-Electron Water Oxidation by Sn-Porphyrins Covalently Bound through\nSilylation with Flexible Bridging Moieties on SnO<sub>2</sub> Nanoparticles

As one of the most promising molecular catalyst sensitized\nsystems\n(MCSS) for an artificial photosynthesis, photochemical two-electron\nwater oxidation into hydrogen peroxide catalyzed by tetra-pyridylporphyrinatetin<sup>IV</sup> (SnTPyP) adsorbed on SnO<sub>2</sub> nanoparticles fabricated\nas a photoanode upon one-photon visible-light excitation was examined.\nUnimolecular SnTPyP<sub>axial</sub>/SnO<sub>2</sub> covalently bound\nthrough an axial ligand O atom (<b>1</b>), cationic unimolecular\n(SnTPyP(PrBr)<sub>4</sub>)<sub>axial</sub>/SnO<sub>2</sub> covalently\nbound through an axial ligand O atom (<b>2</b>), cationic unimolecular\n(SnTPyP(PrBr)<sub>4</sub>)<sub>ionic</sub>/SnO<sub>2</sub> bound through\nionic interaction (<b>3</b>), cationic oligomer ((SnTPyP(PrBr)<sub>4</sub>)<sub>ionic</sub>)<sub>4</sub>/SnTPyP<sub>axial</sub>/SnO<sub>2</sub> with only the central SnTPyP covalently bound through axial\ncoordination (<b>4</b>), cationic oligomer ((SnTPyP(PrBr)<sub>4</sub>)<sub>axial</sub>)<sub>4</sub>/SnTPyP<sub>axial</sub>/SnO<sub>2</sub> with all five SnTPyPs covalently bound through axial coordination\n(<b>5</b>), cationic oligomer ((SnTPyP(PrBr)<sub>4</sub>)<sub>ionic</sub>)<sub>3</sub>/SnTPyP<sub>silyl</sub><b>/</b>SnO<sub>2</sub> covalently bound through silylation of the SnO<sub>2</sub> surface (<b>6</b>), and unimolecular SnTPyP<sub>silyl</sub>/SnO<sub>2</sub> covalently bound through silylation of the SnO<sub>2</sub> surface (<b>7</b>) were fabricated as the photoanodes,\nand their photoelectrochemical behavior was examined to characterize\nfive factors: the absorbed photon to current efficiency (APCE), light\nharvesting efficiency (LHE), durability as the time period for 90%\ndecay from the initial photocurrent (<i>T</i><sub>90%</sub>), amount of H<sub>2</sub>O<sub>2</sub> accumulation, and Faradaic\nyield of H<sub>2</sub>O<sub>2</sub> formation. The unimolecular silylated\nSnTPyP<sub>silyl</sub>/SnO<sub>2</sub> (<b>7</b>) exhibited\nthe highest APCE (45%), which could be caused by a swing motion of\npropylsilyloxyl bridging moieties to make one-electron oxidized SnTPyP\naway from the SnO<sub>2</sub> surface just after an electron injection\nfrom the excited SnTPyP into the conduction band of SnO<sub>2</sub> and to facilitate the charge separation by retarding the charge\nrecombination process. On the other hand, the unimolecular SnTPyP\ncovalently bound through coordination of an axial ligand atom (<b>1</b>) exhibited a modest APCE (11%), which faces rather intimately\nwith the SnO<sub>2</sub> surface to undergo substantial charge recombination.\nThe modest reactivity was similarly observed for oligomeric SnTPyPs\n(<b>5</b>: APCE 12%) with all five porphyrins being covalently\nbound to the SnO<sub>2</sub> surface suffering enhanced charge recombination.\nOther oligomeric SnTPyPs, <b>4</b> (APCE 22%) and <b>6</b> (APCE 29%), showed moderate reactivity owing to a presumed hole\nhopping among SnTPyPs within oligomers enabling the better charge\nseparation. All the oligomers (<b>4</b>, <b>5</b>, and <b>6</b>) with strongly light absorptive SnTPyPs exhibited better\nLHE than unimolecular systems and better durability (<i>T</i><sub>90%</sub>) by hanging over the SnO<sub>2</sub> surface to protect\nfrom an attack of hydronium ion (H<sub>3</sub>O<sup>+</sup>) leading\nto a substantial desorption of SnTPyPs under the acidified conditions\ncaused by a microscopic pH jumping effect in the narrow gap space\nbetween SnO<sub>2</sub> nanoparticles, emphasizing the privileged\nsituation for the oligomeric systems.