Molecular Twisting Affects the Solid‐State Photochemical Reactions of Unsaturated Ketones and the Photomechanical Effects of Molecular Crystals

Abstract

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.

Cocrystal Strategy to Harmonize the Photochemical Reactivity of Styrylquinolines and the Photoactuation Performance of the Molecular Crystals

“…It has been known that the photo-induced [2 + 2] cycloaddition has been found to be the driving force for the photoactuation of the photomechanical crystals. 53,54 Herein, it was found that the photo-induced cracking behaviors were observed for the crystals of 24FQL and 35FQL. As depicted in Figure 5a and Video S1, the platelike crystal of 24FQL was colorless and transparent before UV irradiation.…”

Halogen effect in photomechanical molecular crystals

“…7a), which underwent disparate photochemical reactions in crystals under 365 nm irradiation. 103 For instance, the crystals based on the series of 15a–c exhibited bending or cracking on account of the stereospecific photo-induced [2+2] cycloaddition. No mechanical effects were observed in the crystals based on the series of 16a–c due to their low isomerization reactivity.…”

“…Photomechanical effects of the crystals of 17a (b), 17b (c), 17c (d), 15a (e), 15b (f) and 15c (g) under UV (365 nm) light. Reproduced with permission from The Royal Society of Chemistry 2020 103.…”

Three-in-One: Dye-Volatile Cocrystals Exhibiting Intensity-Dependent Photochromic, Photomechanical, and Photocarving Response

“…Modulating the optical or photoresponsive properties of solid-state materials by light, i.e., inducing changes in shape, , reversible or irreversible photomechanical motion, − or optical properties, is of great interest for applications including molecular robotics, nanoactuators, and light harvesting . Control over optical properties of crystalline materials has attracted attention in the context of organic semiconductors, organic light-emitting diodes (OLEDs), elastic materials, or waveguides. , Photoresponsive behavior in organic solids has been accomplished through a wide range of transformations, including diverse isomerization, , polymerization, , and dimerization reactions. − The formation of multicomponent crystals (cocrystals) is a powerful crystal engineering and supramolecular strategy to tune the solid-state environment of an organic molecule, enabling the optimization of solid-state properties or even the development of materials with entirely new properties. , Notable applications of cocrystal formation include the development of pharmaceutical solid forms, , the design of mechano-, photo-, or thermoresponsive materials, , organic semiconducting solids, optical materials, and more .…”