Cascade Photoreaction in Crystals: A Phase Change Caused by a Dewar Benzene Quantum Chain Triggers a Topochemical [2 + 2] Photodimerization

Crystals undergoing tandem reactions where the first transformation enables the second one are rare. Using photoreactive Dewar benzene 3,4,5,6-tetramethyl-1,2-dicarboxylic diacid (<b>DB</b>) as a hydrogen-bonding template for the [2π+2π] photodimerization of <i>trans</i>-4,4'-bipyridyl-ethenes (<b>BPE</b>), we obtained crystals <b>DB-BPE-NT</b> with a <b>DB:BPE</b> = 2:1 stoichiometry with double bonds at a nonreactive distance of 5.957 Å. Exposure to UV light resulted in valence bond isomerization to Hückel benzene 3,4,5,6-tetramethyl-1,2-dicarboxylic acid (<b>HB</b>) by a quantum chain reaction that triggered the sought-after topochemical [2π+2π] photodimerization reaction. Notably, crystals <b>HB:BPE-T</b> with the Hückel benzene isomer and <b>BPE</b> adopted a 2:2 stoichiometry and displayed an efficient topochemically templated photodimerization reaction.