Spectroscopic Characterization and Mechanistic Studies on Visible Light Photoredox Carbon-Carbon Bond Formation by Bis(arylimino)acenaphthene Copper Photosensitizers

Currently, the most popular molecular photosensitizers used for synthetic organic chemistry and energy applications are still the noble-metal-based ruthenium and iridium complexes that usually require expensive metal and ligand precursors. In contrast, bis(arylimino)acenaphthene (Ar-BIAN) is an established redox noninnocent π-accepting ligand that is easily assembled in one condensation step from affordable and commercially available precursors. Herein, we have developed a series of Ar-BIAN Cu^I complexes as visible-light-harvesting photosensitizers. Notably, one of these panchromatic, homoleptic Ar-BIAN Cu^I complexes exhibits a radiative recombination lifetime that is longer than diffusion-controlled reactions, as observed by time-correlated single-photon counting spectroscopy. Ar-BIAN Cu^I facilitates visible-light-promoted atom-transfer radical addition reactions via carbon-carbon bond formation with CBr₃ radicals in good yields of up to 75%. Steady-state and transient absorption spectroscopic measurements, together with spectroelectrochemical experiments and intermediate isolation studies, were performed to obtain insights into this photoredox catalysis and provide guidelines for the general deployment of Ar-BIAN Cu^I photosensitizers in synthetic organic chemistry and renewable energy applications.