Charge Transfer Dynamics in Dye-Sensitized Photocatalysts Using Metal Complex Sensitizers with Long-Wavelength Visible Light Absorption Based on Singlet-Triplet Excitation.

An Os-(II) polypyridyl complex was applied as a photosensitizer in dye-sensitized photocatalyst systems based on Pt-intercalated HCa2Nb3O10 and Pt-loaded TiO2. The Os-(II) complex exhibits a spin-forbidden but partially allowed triplet metal-to-ligand charge transfer (3MLCT) transition, enabling broad visible light absorption up to 800 nm, which surpasses that of conventional Ru-(II)-based dyes. Despite its shorter excited-state lifetime compared to Ru-(II) complexes, efficient electron injection from the excited Os-(II) dye into the semiconductor was confirmed. Under visible-light irradiation, the Os-(II)-sensitized photocatalysts showed higher H2 evolution activity than the Ru-(II)-sensitized photocatalysts when sodium ascorbate was used as an electron donor, demonstrating effective utilization of long-wavelength visible light. In contrast, negligible H2 evolution was observed when NaI was employed as a redox mediator for Z-scheme water splitting. Transient absorption spectroscopy revealed that the lack of activity stemmed from inefficient electron transfer from I- to oxidized Os-(II). These findings highlight the importance of selecting appropriate redox mediators to fully exploit long-wavelength dyes for overall water splitting under visible light.