Tuning the Photocatalytic CO₂ Reduction through para-Substituents in Bipyridyl Rhenium Complexes

Six fac-[Re-(NN)-(CO)₃Cl] complexes, where NN is a 2,2'-bipyridine ligand with systematically varied electron-donating or -withdrawing groups at the 4,4' positions, were explored as photocatalysts for CO₂ reduction to CO. The light absorption, redox potentials, and excited-state dynamics of the complexes, as well as the reactivity and stability of key catalysis intermediates, were correlated with the Hammett constants (σ_p) of the substituents, revealing their impact on the photocatalytic activity. Electron-donating substituents, such as -OCH₃ and -CH₃, resulted in slower excited-state quenching by sacrificial electron donors in comparison to the unsubstituted complex, but their corresponding one-electron-reduced species (OERS) reacted quickly with CO₂. On the other hand, electron-withdrawing substituents, such as -Br, -COOH, and -CO₂CH₃, resulted in more favorable and faster reductive quenching, but at the cost of slower reactivity of the OERS. A comparison of the catalytic activity of the complexes employing two different sacrificial electron donors displayed how the interplay of these factors affects the CO₂ reduction performance and how photocatalysis can be controlled by strategic manipulation of both the ligand environment and the reaction conditions.