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.
In previous reports, the solid-state
photo-induced [2 + 2] cycloaddition
of styrylpyridine or styrylquinoline derivatives took place only assisted
by templates. Herein, (E)-2-(2,4-difluorostyryl)quinoline
(24FQL) and (E)-2-(3,5-difluorostyryl)quinoline
(35FQL) have been synthesized and can undergo photodimerization
in the pristine crystals. Interestingly, the photomechanical effects
of the plate-like crystals are triggered by the photochemical reaction.
Furthermore, the cocrystal approach was employed to harmonize the
photochemical reactivity of 24FQL and 35FQL as well as the photoactuation performance of the molecular crystals.
In the cocrystals of 24FQL/TFBA and 24FQL/TFPTA, the [2 + 2] cycloaddition reactivity
of 24FQL decreased compared with that in a one-component
crystal. However, the cocrystals showed more significant photoactuation.
It was deduced that during the [2 + 2] cycloaddition, more intermolecular
interactions would be broken in cocrystals, which might weaken the
reactivity but enhance the photomechanical performance due to more
strain being accumulated. On the contrary, the photochemical conversion
of 35FQL was improved significantly in cocrystals of 35FQL/TFBA and 35FQL/TFPTA, leading to obvious photo-induced movement. Therefore, the cocrystal
strategy can be a useful tool to harmonize the photochemical reactivity
and photomechanical behaviors of the potentially photoactive molecular
crystals.
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 (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 (CH 2 ) 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 (C 6 H 4 ) 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 carboncarbon 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.
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