Development of a panchromatic photosensitizer and its application to photocatalytic CO2 reduction.

We designed and synthesized a heteroleptic osmium(ii) complex with two different tridentate ligands, Os. Os can absorb the full wavelength range of visible light owing to S-T transitions, and this was supported by TD-DFT calculations. Excitation of Os 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 (τ em = 40 ns). Since excited Os could be reductively quenched by 1,3-dimethyl-2-(o-hydroxyphenyl)-2,3-dihydro-1H-benzo[d]imidazole, Os displays high potential as a panchromatic photosensitizer. Using a combination of Os and a ruthenium(ii) catalyst, CO2 was photocatalytically reduced to HCOOH via irradiation with 725 nm light, and the turnover number reached 81; irradiation with light at λ ex > 770 nm also photocatalytically induced HCOOH formation. These results clearly indicate that Os can function as a panchromatic redox photosensitizer.