Development of a panchromatic photosensitizer and its application to photocatalytic CO reduction.

We designed and synthesized a heteroleptic osmium(ii) complex with two different tridentate ligands, . can absorb the full wavelength range of visible light owing to S-T transitions, and this was supported by TD-DFT calculations. Excitation of using visible light of any wavelength generates the same lowest triplet metal-to-ligand charge-transfer excited state, the lifetime of which is relatively long ( = 40 ns). Since excited could be reductively quenched by 1,3-dimethyl-2-(-hydroxyphenyl)-2,3-dihydro-1-benzo[]imidazole, displays high potential as a panchromatic photosensitizer. Using a combination of and a ruthenium(ii) catalyst, CO was photocatalytically reduced to HCOOH irradiation with 725 nm light, and the turnover number reached 81; irradiation with light at > 770 nm also photocatalytically induced HCOOH formation. These results clearly indicate that can function as a panchromatic redox photosensitizer.