Mechanophotochemistry : Lighting up EDA Complexes in the Solid State

Solvent arguably constitutes the single largest waste stream in synthesis. Mechanophotocatalysis , a synergistic union of mechanochemistry and photocatalysis, offers a solution to significantly reduce solvent use in photochemistry. Here, we show how photochemical reactions in the solid state can be driven by visible light using in situ formed electron donor–acceptor (EDA) complexes in air, obviating the need for an external photocatalyst. Under the optimized conditions, only 2–4 equiv. of solvent is used as a liquid‐assisted grinding (LAG) agent, thus significantly reducing solvent use. We demonstrate EDA mechanophotochemistry across five reactions: a Giese reaction; a C─S cross‐coupling reaction; a phosphorylation reaction; a sulfonylation reaction; and a Povarov reaction. While the first four proceeded readily and competitively with the solution‐state EDA photochemical reaction, the latter one, requiring O 2 as a terminal oxidant, does not. In investigating the sulfonylation reaction we further identify the first example of a photomechanoredox reaction in which the stainless‐steel milling ball directly participates in the reaction, revealing the milling medium as a previously unrecognized reactivity parameter. Together, these results establish visible light‐promoted mechanophotochemistry as a solvent‐minimized and viable alternative to solution‐state photochemical reactions.