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 wavelengths in the visible-light region, a self-photosensitized, mononuclear 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.