Tuning Excited‐State Reactivity by Proton‐Coupled Electron Transfer

The reactivity, and even reaction pathway, of excited states can be tuned by proton‐coupled electron transfer (PCET). The triplet state of benzophenone functionalized with a Brønsted acid (³*BP‐COOH) showed a more powerful oxidation capability over the simple triplet state of benzophenone (³*BP). ³*BP‐COOH could remove an electron from benzene at the rate of 8.0×10⁵ m ^?1 s^?1, in contrast to the reactivity of ³*BP which was inactive towards benzene oxidation. The origin of this great enhancement on the ability of the excited states to remove electrons from substrates is attributed to the intramolecular Brønsted acid, which enables the reductive quenching of ³*BP by concerted electron–proton transfer.