| Summary: | Interfacial electron injection from a photoexcited surface-immobilized dye to a semiconductor substrate is a key reaction for dye-sensitized photocatalysts. We previously reported that the molecular orientation of heteroleptic Ir(III) photosensitizer on the TiO<sub>2</sub> nanoparticle surface was important for efficient interfacial electron injection. In this work, to overcome the weak light absorption ability of heteroleptic Ir(III) photosensitizer and to improve the photoinduced charge-separation efficiency at the dye–semiconductor interface, we synthesized two heteroleptic Ir(III) complexes with different coumarin dyes, [Ir(C6)<sub>2</sub>(H<sub>4</sub>CPbpy)]Cl and [Ir(C30)<sub>2</sub>(H<sub>4</sub>CPbpy)]Cl [Ir-CX; X = 6 or 30; HC6 = 3-(2-enzothiazolyl)-7-(diethylamino)coumarin, HC30 = 3-(2-<i>N</i>-methylbenzimidazolyl)-7-<i>N</i>,<i>N</i>-diethylaminocoumarin, H<sub>4</sub>CPbpy = 4,4′-bis(methylphosphonic acid)-2,2′-bipyridine], as the cyclometalated ligands and immobilized them on the surface of Pt-cocatalyst-loaded TiO<sub>2</sub> nanoparticles. Ultraviolet-visible absorption and emission spectroscopy revealed that the singlet ligand-centered (<sup>1</sup>LC) absorption and triplet <sup>3</sup>LC emission bands of Ir-C30 occurred at shorter wavelengths than those of Ir-C6, while time-dependent density-functional-theory data suggested that the ligand-to-ligand charge transfer (LLCT) excited states of the two complexes were comparable. The photocatalytic H<sub>2</sub> evolution activity of the Ir-C6-sensitized Pt-TiO<sub>2</sub> nanoparticles (Ir-C6@Pt-TiO<sub>2</sub>) under visible light irradiation (λ > 420 nm) was higher than that of Ir-C30@Pt-TiO<sub>2</sub>. In contrast, their activities were comparable under irradiation with monochromatic light (λ = 450 ± 10 nm), which is absorbed comparably by both Ir-CX complexes. These results suggest that the internal conversion from the higher-lying LC state to the LLCT state effectively occurs in both Ir-CX complexes to trigger electron injection to TiO<sub>2</sub>.
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