Carbazol-3-olate photosensitizers enable photocatalytic hydrodefluorination and Birch-type reduction reactions†

Despite significant recent advances in the development of organic photosensitizers (PSs), designing a PS with both long-wavelength absorption and high reducing ability remains challenging. In this study, we introduce carbazol-3-olates as a novel class of PSs that exhibit these desirable properties. Compared to their neutral carbazole counterparts, anionic carbazol-3-olates exhibited a 70 nm red-shift in absorption and a 0.54 V negative shift in the excited-state reduction potential. The use of carbazol-3-olates as PSs enabled photocatalysis of hydrodefluorination and Birch-type reduction reactions under long-wavelength visible-light irradiation. Furthermore, the Birch reduction reactions exhibited tolerance toward aqueous environments when conducted using our PS, thus expanding their industrial applicability. Mechanistic studies revealed electron transfer from the excited carbazol-3-olates to inert substrates, such as fluoroarenes, highlighting the high catalytic efficiency of carbazol-3-olates as PSs for reduction reactions. Therefore, with their transition-metal-free nature and facile synthesis process, we expect carbazol-3-olates to be industrially advantageous PSs for catalyzing challenging photochemical reduction reactions.