Ruthenium Picolinate Complex as a Redox Photosensitizer With Wide-Band Absorption

Ruthenium(II) picolinate complex, [Ru(dmb)(2)(pic)](+) (Ru(pic); dmb = 4,4′-dimethyl-2,2′-bipyridine; Hpic = picolinic acid) was newly synthesized as a potential redox photosensitizer with a wider wavelength range of visible-light absorption compared with [Ru(N(∧)N)(3)](2+) (N(∧)N = diimine ligand), which is the most widely used redox photosensitizer. Based on our investigation of its photophysical and electrochemical properties, Ru(pic) was found to display certain advantageous characteristics of wide-band absorption of visible light (λ(abs) < 670 nm) and stronger reduction ability in a one-electron reduced state ([Formula: see text] = −1.86 V vs. Ag/AgNO(3)), which should function favorably in photon-absorption and electron transfer to the catalyst, respectively. Performing photocatalysis using Ru(pic) as a redox photosensitizer combined with a Re(I) catalyst reduced CO(2) to CO under red-light irradiation (λ(ex) > 600 nm). TON(CO) reached 235 and Φ(CO) was 8.0%. Under these conditions, [Ru(dmb)(3)](2+) (Ru(dmb)) is not capable of working as a redox photosensitizer because it does not absorb light at λ > 560 nm. Even in irradiation conditions where both Ru(pic) and Ru(dmb) absorb light (λ(ex) > 500 nm), using Ru(pic) demonstrated faster CO formation (TOF(CO) = 6.7 min(−1)) and larger TON(CO) (2347) than Ru(dmb) (TOF(CO) = 3.6 min(−1); TON(CO) = 2100). These results indicate that Ru(pic) is a superior redox photosensitizer over a wider wavelength range of visible-light absorption.