Solid-state photochemistry of cis-cinnamic acids: a competition between [2 + 2] addition and cis–trans isomerization

With three examples, we have established that cis-cinnamic acids can dimerize via a diradical intermediate in the crystalline state provided that the intermolecular distance is less than 4.2 A. In the excited state of cis-isomers, C–C bond formation with an adjacent molecule competes with geometric isomerization.