Specific Bis[2 + 2] Cycloadditions and Photomechanical Behaviors of Halogen-Substituted Dienone Crystals

Topochemical [2 + 2] cycloaddition offers an efficient, regioselective, and stereospecific route to bioactive cyclobutane-containing compounds and serves as a valuable platform for light–mechanical energy conversion, making it highly relevant for the design of photomechanical crystalline materials. Diene compounds possess dual reactive sites, enabling controlled [2 + 2] cycloaddition through modulation of molecular packing, which can lead to photoproducts containing either one or two cyclobutyl rings. Such transformations often involve substantial atomic displacements and thus hold potential for photoactuation behaviors. In this study, halogen-substituted phenyl 1,4-pentadiene-3-one derivatives containing two C═C bonds were designed and synthesized. The dienone microcrystals of DIEO-35F, DIEO-35Br, and DIEO-24Br could simultaneously undergo topochemical [2 + 2] cycloadditions at both olefin pairs with stereospecificity and regiospecificity. Molecular crystals of DIEO-35F, DIEO-35Cl, DIEO-35Br, DIEO-24F, DIEO-24Cl, and DIEO-26F exhibited characteristics of photoinduced cracking or bending behaviors driven by photoreaction, and DIEO-24Cl demonstrated elastic behavior when subjected to mechanical forces. The effects of halogen species and substitution positions on the solid-state photoreactivity and photoinduced mechanical responses of these crystals in relation to their molecular packing in the respective crystals were also investigated.