Conjugation Effect Contributes to the CO₂-to-CO Conversion Driven by Visible-Light

Structural modification of a ligand is an effective way to improve the catalytic activity of molecular catalysts for photocatalytic CO2 reduction. In this study, we designed and synthesized three tripodal ligands with different conjugate groups (L1 = tris{2-[(9′ - anthrylmethyl)amino)ethyl}amine, L2 = tris{2-[(1′-naphthylmethyl)amino]ethyl}amine, L3 = tris[2-(benzylamino)ethyl]amine), and their corresponding mononuclear cobalt complexes, [CoL1(OH)]ClO4 (1), [CoL2(OH)]ClO4 (2), and [CoL3(OH)]ClO4 (3). Control experiments showed that 1 and 2 possess higher efficiency than 3 for the photocatalytic CO2-to-CO conversion, with TON and TOF for CO of 58 000 and 1.61 s–1 for 1, and 49 200 and 1.37 s–1 for 2, respectively, greatly higher than those of 3. Compounds 1 and 2 also display higher CO selectivity (≥97%) than 3. Control experiments and DFT calculations revealed that the excellent catalytic performances of 1 and 2 can be ascribed to the extended conjugation substituent in L1 and L2, which endows the CoII catalytic center with low reduction potential, accelerates the intermolecular electron transfer, and thus dramatically boosts the CO2-to-CO conversion. This study demonstrates that the improvement of the electron transfer between photosensitizer and catalysts is the key for enhancing the activity of catalyst for CO2-to-CO conversion.