Photocatalytic CO2 Reduction Using an Osmium Complex as a Panchromatic Self‐Photosensitized Catalyst: Utilization of Blue, Green, and Red Light

The photocatalytic reduction of carbon dioxide (CO2) represents an attractive approach for solar-energy storage and leads to the production of renewable fuels and valuable chemicals. Although some osmium (Os) photosensitizers absorb long wave-lengths in the visible-light region, a self-photosensitized, mono-nuclear Os catalyst for red-light-driven CO2 reduction has not yet been exploited. Here, we discovered that the introduction of an Os metal to a PNNP-type tetradentate ligand resulted in the absorption of light with longer-wavelength (350-700 nm) and that can be applied to a panchromatic self-photosensitized catalyst for CO2 reduction to give mainly carbon monoxide (CO) with a total turnover number (TON) of 625 under photoirradiation (λ ≥ 400 nm). CO2 photoreduction also proceeded under irradiation with blue (λ0 = 405 nm), green (λ0 = 525 nm), or red (λ0 = 630 nm) light to give CO with >90% selectivity. The quantum efficiency using red light was determined to be 12% for the generation of CO. A catalytic mechanism is proposed based on the detection of intermediates using various spectroscopic techniques, including transient absorption, electron paramagnetic resonance, and UV-vis spectroscopy.