Higher Excited-State Reactivity of a Charge-Neutral Organic Radical

While the reactivity of excited radical anions and cations has received considerable attention in modern photoredox catalysis, the photochemistry of charge-neutral organic radicals remains underexplored. Here, we present direct laser spectroscopic evidence showing that a neutral boryl radical remains unreactive in the lowest electronically excited state formed after red light excitation. However, it can initiate photoreduction from an upper excited state reached using blue light. This represents an important deviation from typical photochemical behavior, in which only the lowest excited state of a given spin multiplicity is expected to be reactive. Transient absorption spectroscopy provides further insight into the reactivity between charge-neutral radical species and substrates, particularly regarding their propensity to undergo in-cage charge recombination events that influence the quantum yield of overall photoreactions, an aspect that differs conceptually from previously investigated radical cations and anions, where light typically induces charge shift rather than charge separation processes. These insights are complemented by a detailed mechanistic picture describing how boryl radicals regenerate from boronium cations, enabling catalytic turnover in synthetically relevant borylation reactions. The gained mechanistic insights are broadly relevant to photoredox catalysis and carry important implications for synthetic photochemistry, artificial photosynthesis, and the photodegradation of environmentally harmful substances.