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Structurally ordered soft materials that respond to complementary stimuli are susceptible to control over their spatial and temporal morphostructural configurations by intersectional or combined effects such as gating, feedback, shape‐memory, or programming. In the absence of general and robust design and prediction strategies for their mechanical properties, at present, combined chemical and crystal engineering approaches could provide useful guidelines to identify effectors that determine both the magnitude and time of their response. Here, we capitalize on the purported ability of soft intermolecular interactions to instigate mechanical compliance by using halogenation to elicit both mechanical and photochemical activity of organic crystals. Starting from (E)‐1,4‐diphenylbut‐2‐ene‐1,4‐dione, whose crystals are brittle and photoinert, we use double and quadruple halogenation to introduce halogen‐bonded planes that become interfaces for molecular gliding, rendering the material mechanically and photochemically plastic. Fluorination diversifies the mechanical effects further, and crystals of the tetrafluoro derivative are not only elastic but also motile, displaying the rare photosalient effect. [ABSTRACT FROM AUTHOR]

Generally, the potential reactive "olefin pairs" in the molecular crystals satisfying Schmidt's criteria could undergo topological [2+2] cycloaddition. In this study, another factor that affects the photodimerization reactivity of chalcone analogues was found. The cyclic chalcone analogues of (E)‐2‐(2,4‐dichlorobenzylidene)‐2,3‐dihydro‐1H‐inden‐1‐one (BIO), (E)‐2‐(naphthalen‐2‐ylmethylene)‐2,3‐dihydro‐1H‐inden‐1‐one (NIO), (Z)‐2‐(2,4‐dichlorobenzylidene)benzofuran‐3(2H)‐one (BFO), and (Z)‐2‐(2,4‐dichlorobenzylidene)benzo[b]thiophen‐3(2H)‐one (BTO) have been synthesized. While the geometrical parameters for the molecular packing of the above four compounds did not exceed Schmidt's criteria, [2+2] cycloaddition did not occur in the crystals of BIO and BTO. The single crystal structures and Hirshfeld surface analyses revealed that interactions of C=O⋅⋅⋅H (CH2) existed between adjacent molecules in the crystal of BIO. Therefore, the carbonyl and methylene groups linked with one carbon atom in carbon‐carbon double bond were tightly confined in the lattice, acting as a tweezer to inhibit free movement of the double bond and suppressing [2+2] cycloaddition. In the crystal of BTO, similar interactions of Cl⋅⋅⋅S and C=O⋅⋅⋅H (C6H4) prevented free movement of the double bond. In contrast, the intermolecular interaction of C=O⋅⋅⋅H only exists around the carbonyl group in the crystals of BFO and NIO, leaving the C=C double bonds to move freely and allowing the occurrence of [2+2] cycloaddition. Driven by photodimerization, the needle‐like crystals of BFO and NIO displayed evident photo‐induced bending behavior. This work demonstrates that the intermolecular interactions around carbon‐carbon double bond affect the [2+2] cycloaddition reactivity except for Schmidt's criteria. These findings provide valuable insights into the design of photomechanical molecular crystalline materials. [ABSTRACT FROM AUTHOR]

Three groups of chalcone derivatives and their analogues involving halogen atoms (X=F, Cl, Br) have been synthesized. Firstly, the nearly planar acyclic chalcone derivatives were inclined to undergo photo‐induced stereospecific [2+2] cycloaddition, which triggered the crystals to exhibit macroscopic motions of bending or cracking. In particular, the single‐crystal‐to‐single‐crystal transformation happened upon UV irradiation of the crystals, which was helpful for the understanding photomechanical effects. Cyclic 3,4‐dihydronaphthalene‐based chalcone analogues possess a more twisted conformation, and they tend to undergo trans‐cis isomerization. No photomechanical effect was observed for the crystals of the cyclic chalcone analogues due to the lower isomerization rate. The twist degree of chroman‐based molecules was in between of the first two, [2+2] cycloaddition and trans‐cis isomerization simultaneously took place in crystals. Photo‐induced bending and twisting were observed for the crystals of chroman‐based chalcone analogues. Therefore, the differences in molecular dihedral angles in α,β‐unsaturated ketones were responsible for their photochemical characters and in turn to tune the photomechanical effects. In this work, a bridge between the molecular structures and solid‐state photochemical reactions triggered photomechanical crystals is built. [ABSTRACT FROM AUTHOR]

We report interesting photomechanical behaviors of the dynamic molecular crystals of ( E)-2-(2,4-dichlorostyryl)benzo[d]oxazole ( BOACl24). The photosalient effect of the rod-like crystal based on a metal-free olefin driven by photodimerization is observed. Moreover, the needle-like crystals of BOACl24 exhibit a reversible bending away from a UV light source. The nanofibers curl easily under UV irradiation in an organogel, in which the photo-induced rolling of a small slice occurs. This suggests that the rapid release of the accumulated strain during photodimerization may lead to a photosalient effect, and the bending or curling happens when the strain is released slowly. Notably, [2+2] cycloaddition takes place between two different conformational isomers of BOACl24 on account of the rotation of the benzoxazole ring around the C−C bond in an excited state before photodimerization. Such topo-photochemical reaction has not been reported elsewhere. [ABSTRACT FROM AUTHOR]