Self-RegulatingPhotoactuation of Molecular CrystalsBased on Halogenstyrylquinoxalines

Herein, we synthesized (<i>E</i>)-2-chloro-3-styrylquinoxaline (<b>4HQXL</b>) and its halogen-substituted derivatives (<i>E</i>)-2-chloro-3-(4-fluorostyryl)­quinoxaline (<b>4FQXL</b>), (<i>E</i>)-2-chloro-3-(4-chlorostyryl)­quinoxaline (<b>4ClQXL</b>), and (<i>E</i>)-2-chloro-3-(4-bromostyryl)­quinoxaline (<b>4BrQXL</b>), which can undergo [2 + 2] cycloaddition upon 365 nm irradiation. Remarkably, the obtained needle-like crystals of <b>4FQXL</b>, <b>4ClQXL</b>, and <b>4BrQXL</b> exhibited photoinduced bending followed by rotating under UV irradiation, and such photomechanical moving can constantly turn the backlight surface of the crystal toward the light source. Hence, the innovative mechanical motion optimized the conversion of light energy into chemical and mechanical energy. In the case of the crystal of <b>4HQXL</b>, a remarkable photosalient behavior was observed upon exposure to UV light. The crystalline structures of the halogenstyrylquinoxalines illustrated that the substitution of halogen atoms significantly affected the molecular packing mode, which exerted a profound influence on the photomechanical properties of the crystals.