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Fluorine-free chalcones and chalcones bearing different numbers of fluorine atoms have been synthesized. It is found that fluorine can tune the reactivity of photo-induced [2 + 2] cycloaddition reactions in crystals. The higher the number of fluorine atoms, the higher the reactivity of the photodimerization. The single crystal structures and Hirshfeld surface analysis illustrated that the introduction of fluorine not only increased the molecular planarity but could also steer the potentially reactive double bonds in appropriate positions of the crystal lattices to meet Schmidt's criteria. Therefore, the stereospecific [2 + 2] cycloaddition reactions took place to afford one diastereoisomer from the reactive chalcones except for (E)-3-(4-fluorophenyl)-1-phenylprop-2-en-1-one (1FChH). Additionally, the chalcone-based molecular crystals exhibited various photomechanical behaviors, such as bending toward or away from the light source, swinging, cracking and jumping, driven by topo-photoreactions. As expected, the more efficient photo-induced [2 + 2] cycloaddition reactions in crystalline states led to more significant motions of the molecular crystals. Powder X-ray diffraction results suggested that the solid state [2 + 2] cycloaddition reactions changed the unit cell of the single crystals. The photo-induced bending toward or away from the light source for the needle-like crystals originated from the contraction or the expansion of the phototropic surface. Hence, the robust balancer fluorine in chalcones might play an important role in the crystalline packing. This provides a facile approach in crystal engineering to fabricate photo-induced mechanically responsive crystalline materials. [ABSTRACT FROM AUTHOR]

Abstract: New diarylethene derivatives containing benzoxazole (NBO) and benzothiazole (NBT) have been synthesized. Light‐induced trans–cis isomerization of NBO and NBT took place in crystals, and only induced the needle‐like crystals of NBO to bend backwards away from the UV light source. The movement of the atoms was deemed to take place during the isomerization of NBO; hence, strain would be produced and accumulated rapidly in the surface of crystals exposed to UV light. The uniform release of strain led to the bending of needle‐like crystals. The light‐induced trans–cis isomerization efficiency of NBT was too low to drive the motion of crystals, which might have originated from the large repulsion between naphthyl and benzothiazole. These results provide a new platform for the transformation of light energy into mechanical energy in molecular crystals through the unimolecular photochemical reaction of diarylethene derivatives. [ABSTRACT FROM AUTHOR]