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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]

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]

The photomechanical effects of crystals of halogen‐substituted 9‐anthracenecarbonitrile were investigated. The needle‐like crystals of 10‐fluoroanthracene‐9‐carbonitrile (FCNA) bend rapidly under irradiation with 365 nm, 410 nm light and even white light, and the bent crystal could almost recover to its original shape upon exposure to 254 nm light or heating. The reversible photo‐induced mechanical behaviors originated from [4+4] cycloaddition and the reverse reaction. This type of photoactuator, in response to light irradiation over a broad wavelength range from UV to the visible region, might have important applications in biomimetic systems, biomedicine, artificial muscles, and smart microrobotics. The rapid photo‐induced mechanical movements of the crystals inspired us to fabricate a hybrid polymer film of polyvinylidene difluoride incorporated with the microcrystals of FCNA, which not only showed bending toward or away from the light source depending on the initial bending direction of the film, but also could lift small objects induced by remote light. [ABSTRACT FROM AUTHOR]

The polymorphs of 2ClChMe‐4 in Form I (ribbon‐like crystal) and Form II (block‐like crystal) were prepared, and they exhibited curling/flipping and expansion upon heating on account of single‐crystal‐to‐single‐crystal transformations. The irreversible phase transformations occurred separately at 53.2 °C and 57.8 °C for the crystals in Form I and Form II, during which the molecular conformation of 2ClChMe‐4 changed and the molecules slipped along the (100) plane. Movement at the molecular level resulted in changes of cell parameters, which in turn led to macroscopic motions of the crystals upon heating. Additionally, the ribbon‐like crystals of 2ClChMe‐4 showed photo‐induced bending driven by [2+2] cycloaddition. Accordingly, an actuator showing reversible bending behavior was fabricated triggered by light and heat successively. Like biomimetic self‐actuators, such multi‐stimuli mechanical responsive molecular crystals might have potential applications in soft robots, artificial muscles and microfluidic systems. [ABSTRACT FROM AUTHOR]

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]

Herein, three new fluorostyrylindazoles bearing one, three and five fluorine atoms (FPIZ , TFPIZ and PFPIZ) have been synthesized. Interestingly, the crystal of PFPIZ bent back away from the lighht source at the initial stage of 365 nm light irradiation, and then it showed phototropic bending when prolong the irradiation time without changing the irradiation direction. It was found that the photo-induced [2 + 2] cycloaddition could lead to the expansion and contraction of the unit cell in different directions, and in turn trigger the crystal to exhibit macroscopic mechanical moving. Meanwhile, the photochemical reaction was reasonable for the photochromism of the PFPIZ , which is helpful for the visual observation of the photomechanical effects. For instance, the fluorescence of the crystals of PFPIZ was changed from blue-purple to white under 365 nm irradiation. Besides, TFA (trifluoroacetic acid) could quench the emission of the synthesized fluorostyrylindazoles to different degree in solutions as well as in the films. The acidochromism of the styrylindazole derivatives was originated from the protonation of indazole by acid. The design of multi-stimuli response crystalline materials would be helpful for the fabrication of intelligent photoactuators and fluorescence sensors. [Display omitted] • The molecular crystals based on (E)-1-methyl-3-(2-(perfluorophenyl)vinyl)-1 H -indazole exhibited photomechanical effects. • Stereospecific photo-induced [2 + 2] cycloaddition underwent in the crystal of PFPIZ. • The fluorescence of the crystals of PFPIZ was changed from blue-purple to white on account of the photodimerization. • The films of fluorostyrylindazoles could probe gaseous trifluoroacetic acid rapidly and sensitively. [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]

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]

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]