Solid-state molecular motions (SSMM) play a critical role in adjusting behaviors and properties of materials. However, research on SSMM, especially for multicomponent systems, suffers from various problems and is rarely explored. Herein, through collaboration with cocrystal engineering, visualization and manipulation of SSMM in two-component systems, namely, FSBO ((E)-2-(4-fluorostyryl)benzo[d]oxazole)/TCB (1,2,4,5-tetracyanobenzene) and PVBO ((E)-2-(2-(pyridin-4-yl)vinyl)benzo[d]oxazole)/ TCB, were realized. The obtained yellow-emissive F/T (FSBO/TCB) cocrystal displayed turn-on fluorescence, and the green-emissive P/T (PVBO/TCB) cocrystal presented redder emission, both of which exhibited an aggregation-induced emission property. At varied pressure and temperature, the grinding mixtures of FSBO/TCB and PVBO/TCB displayed different molecular motions that were readily observed through the fluorescence signal. Notably, even without grinding, FSBO and TCB molecules could move over for 4 mm in a 1D tube. The unique emission changes induced by SSMM were applied in information storage and dynamic anticounterfeiting. This work not only visualized and manipulated SSMM but offered more insights for multicomponent study in aggregate science.

The fluorine-free chalcones and the chalcones bearing different numbers of fluorine atoms have been synthesized. It is found that fluorine can tune the reactivity of the photo-induced [2+2] cycloaddition reactions...

The polymorphs of 2ClChMe-4 in Form I (ribbonlike 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.

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,βunsaturated 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.

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 wave-length range from UV to the visible region, might have important applications in biomimetic systems, biomedicine, artificial muscles, and smart microrobotics. The rapid photoinduced 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.