Importance of the Molecular Orientation of an Iridium(III)-Heteroleptic Photosensitizer Immobilized on TiO2 Nanoparticles

To elucidate the effect of the molecular orientation of a photosensitizing (PS) dye molecule on photoinduced interfacial electron transfer to a semiconductor substrate, we have synthesized two new Ir­(III) heteroleptic complexes each comprising two phosphonic acid groups: [Ir­(ppy)2(CPbpy)]+ and [Ir­(CPppy)2(bpy)]+ (1B and 1P, respectively; Hppy = 2-phenylpyridine, bpy = 2,2′-bipyridine, CPbpy = 4,4′-bis­(methylphosphonic acid)-2,2′-bipyridine, and CPppy = 4-(methylphosphonic acid)-2-phenylpyridine). Both Ir­(III) complexes exhibit similar UV–vis absorption spectra and quasi-reversible Ir­(IV)/Ir­(III) redox behavior at a potential of 1.67 V vs NHE. On the other hand, the triplet metal-to-ligand charge-transfer (3MLCT) phosphorescence energy of 1B was ∼0.12 eV higher than that of 1P. This difference was attributed to the electron-donating methyl phosphonate groups attached to the bpy ligand that destabilize the 3MLCT excited state in which the photoexcited electron is localized in the bpy moiety. Both Ir­(III) PS dyes were immobilized onto the surface of the Pt-co-catalyst-loaded TiO2 nanoparticles (1B@Pt-TiO2 and 1P@Pt-TiO2). Immobilization was comparable, suggesting that the effect of the positions of the methyl phosphonate groups on the immobilization behavior was negligible. On the other hand, the photocatalytic H2 evolution activity of 1B@Pt-TiO2 was about 6-fold higher than that of 1P@Pt-TiO2, indicating the importance of the methyl phosphonate anchoring group position in regulating not only the redox potentials but also the orientation of the molecular photosensitizer on the semiconductor substrate.