An anionic iridium(iii) complex as a visible-light absorbing photosensitizer

Abstract

In this work, a new anionic Ir(III) complex (Ir2) was synthesized using coumarin 6 and orotate ligands, and its use as a photosensitizer in photochemical hydrogen (H₂) generation was demonstrated. For comparison purposes, we also prepared a known anionic Ir(III) complex (Ir1) bearing 2-phenylpyridine and orotate ligands, which was previously reported as an emitting material by another research group. The photophysical properties of complexes Ir1 and Ir2 were examined in several solvents including aqueous acetonitrile (CH₃CN–H₂O), which is frequently used in photocatalytic H₂-generating experiments. Ir1 was found to be weakly emissive in CH₃CN–H₂O, which is probably due to the effect of hydrogen bonding between the orotate ligands and H₂O molecules. In sharp contrast, Ir2 exhibited remarkable properties that include strong visible-light absorption (ε = 78 800 M⁻¹ cm⁻¹ at 462 nm), a high phosphorescence quantum yield (50%), and a long-lived excited state (15 μs), even in the same solvent. Actually, Ir2 functioned as a photosensitizer during visible-light-driven H₂ generation using sodium ascorbate and a cobalt(III) diglyoxime complex as the electron donor and water-reduction catalyst, respectively. To the best of our knowledge, this is the first example of the use of an anionic Ir(III) sensitizer in a photoinduced electron-transfer reaction. The results of this study suggest that Ir2 can be applied to future photochemical H₂-generating systems that are based on ion-paired photocatalysts.