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

Abstract

Abstract 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 M –1 cm –1 at 483 nm, which is an unprecedentedly high value relative to those of other recently reported Ir‐based dyes. Ir sensitizers 2 – 4 were tested for visible‐light‐driven hydrogen generation by using an Ir–Co 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.