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.