Cationic Iridium Complexes Coordinated with Coumarin Dyes - Sensitizers for Visible-Light-Driven Hydrogen Generation

Cationic iridium(III) complexes comprising two cyclometalating 3-(2-benzothiazolyl)-7-(diethylamino)coumarin (Coumarin 6) ligands and one diimine ancillary ligand (N?N) were prepared [N?N = 2,2'-bipyridyl (2), 1,10-phenanthroline (3), 2,2'-biquinoline (4)] and examined for their photophysical properties. The synergic effect of the intraligand charge-transfer (ILCT) transition in the chromophoric Coumarin 6 moiety and the metal-ligand-to-ligand CT (MLLCT) and/or ligand-to-ligand CT (LLCT) transitions associated with the diimine ligand rendered these complexes strongly absorptive in the visible region. Their enhanced absorptivity is significant when compared to the corresponding electroneutral complex 1 with acetylacetonate. Specifically, the molar extinction coefficient of complex 2 reached 129000 M1?cm1 at 483 nm, which is an unprecedentedly high value relative to those of other recently reported Ir-based dyes. Ir sensitizers 24 were tested for visible-light-driven hydrogen generation by using an IrCo photocatalytic system, where triethylamine and tris(2,2'-bipyridyl)dichloridocobalt were employed as a sacrificial electron donor and a water-reduction catalyst, respectively. As a result, complexes 2 and 3 were found to be highly effective sensitizers displaying turnover numbers up to 1500.