Enhancement of Charge Separation in the Photochemical Two-Electron Water Oxidation by Sn-Porphyrins Covalently Bound through Silylation with Flexible Bridging Moieties on SnO₂ Nanoparticles

As one of the most promising molecular catalyst sensitized systems (MCSS) for an artificial photosynthesis, photochemical two-electron water oxidation into hydrogen peroxide catalyzed by tetra-pyridylporphyrinatetinIV (SnTPyP) adsorbed on SnO2 nanoparticles fabricated as a photoanode upon one-photon visible-light excitation was examined. Unimolecular SnTPyPaxial/SnO2 covalently bound through an axial ligand O atom (1), cationic unimolecular (SnTPyP(PrBr)4)axial/SnO2 covalently bound through an axial ligand O atom (2), cationic unimolecular (SnTPyP(PrBr)4)ionic/SnO2 bound through ionic interaction (3), cationic oligomer ((SnTPyP(PrBr)4)ionic)4/SnTPyPaxial/SnO2 with only the central SnTPyP covalently bound through axial coordination (4), cationic oligomer ((SnTPyP(PrBr)4)axial)4/SnTPyPaxial/SnO2 with all five SnTPyPs covalently bound through axial coordination (5), cationic oligomer ((SnTPyP(PrBr)4)ionic)3/SnTPyPsilyl/SnO2 covalently bound through silylation of the SnO2 surface (6), and unimolecular SnTPyPsilyl/SnO2 covalently bound through silylation of the SnO2 surface (7) were fabricated as the photoanodes, and their photoelectrochemical behavior was examined to characterize five factors: the absorbed photon to current efficiency (APCE), light harvesting efficiency (LHE), durability as the time period for 90% decay from the initial photocurrent (T90%), amount of H2O2 accumulation, and Faradaic yield of H2O2 formation. The unimolecular silylated SnTPyPsilyl/SnO2 (7) exhibited the highest APCE (45%), which could be caused by a swing motion of propylsilyloxyl bridging moieties to make one-electron oxidized SnTPyP away from the SnO2 surface just after an electron injection from the excited SnTPyP into the conduction band of SnO2 and to facilitate the charge separation by retarding the charge recombination process. On the other hand, the unimolecular SnTPyP covalently bound through coordination of an axial ligand atom (1) exhibited a modest APCE (11%), which faces rather intimately with the SnO2 surface to undergo substantial charge recombination. The modest reactivity was similarly observed for oligomeric SnTPyPs (5: APCE 12%) with all five porphyrins being covalently bound to the SnO2 surface suffering enhanced charge recombination. Other oligomeric SnTPyPs, 4 (APCE 22%) and 6 (APCE 29%), showed moderate reactivity owing to a presumed hole hopping among SnTPyPs within oligomers enabling the better charge separation. All the oligomers (4, 5, and 6) with strongly light absorptive SnTPyPs exhibited better LHE than unimolecular systems and better durability (T90%) by hanging over the SnO2 surface to protect from an attack of hydronium ion (H3O+) leading to a substantial desorption of SnTPyPs under the acidified conditions caused by a microscopic pH jumping effect in the narrow gap space between SnO2 nanoparticles, emphasizing the privileged situation for the oligomeric systems.