Application of Visible-to-UV Photon Upconversion to Photoredox Catalysis: The Activation of Aryl Bromides

The activation of aryl–Br bonds was achieved by sequential combination of a triplet–triplet annihilation process of the organic dyes, butane‐2,3‐dione and 2,5‐diphenyloxazole, with a single‐electron‐transfer activation of aryl bromides. The photophysical and chemical steps were studied by time‐resolved transient fluorescence and absorption spectroscopy with a pulsed laser, quenching experiments, and DFT calculations. Two‐photon process: The combination of a visible‐to‐UV photon upconversion with a chemical electron‐transfer reaction mechanism has been applied to the reductive aryl–Br σ‐bond activation. Spectroscopic, preparative, and theoretical studies support the operation of a triplet–triplet annihilation process and sequential electron transfer and H‐atom‐transfer steps (see figure).