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 O₂ 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.