Cationic iridium complexes coordinated with coumarin dyes - Sensitizers for visible-light-driven hydrogen generation

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. Cationic iridium(III) complexes with Coumarin 6 ligands were prepared and used as sensitizers for visible-light-driven hydrogen generation. Intense absorption in the visible region was observed, and high turnover numbers in hydrogen generation were achieved. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.