Recent Development of Photodeformable Crystals: From Materials to Mechanisms

Abstract

Photodeformable materials are a class of molecules that can convert photon energy into mechanical energy, which have attracted tremendous attention in the last few decades. Owing to their unique photoinduced deformable properties, including fast light-response and diverse mechanical behaviors, photodeformable materials have exhibited great potential in many practical applications such as actuators, photoswitches, artificial muscles, and bioimaging. In this review, we sort out the current state of photodeformable crystals and classify them into six categories by molecular structures: diarylethenes, azobenzenes, anthracenes, olefins, triarylethylenes, and other systems. Three distinct light-responsive mechanisms, photocyclization, trans-cis isomerization, and photodimerization, are revealed to play significant roles in the molecular photodeformation. Their corresponding photodeformable behaviors such as twisting, bending, hopping, bursting, and curling, as well as the potential applications, are also discussed. Furthermore, the challenges and prospective development directions of photodeformable crystals are highlighted.

Mechanical properties and peculiarities of molecular crystals

“…Over the last twenty years, crystals composed of photoreactive molecules have emerged as a new frontier in the development of photomechanical materials, prompting several reviews on photomechanical, photodeformable, or photoresponsive molecular crystals. 29,[162][163][164][165][166][167][168] 3.1.2. Molecular tunability.…”

An Elastic Organic Crystal Enables Macroscopic Photoinduced Crystal Elongation

“…Mechanically adaptive molecular crystals, which deform but do not break when they are subjected to stress, are promising materials for applications in compliant organic electronics, artificial muscles, and actuating devices. − While new elastic and plastic molecular crystals continue to be reported, − the criteria and scope in molecular configuration, crystal packing mode, and nonbonded interactions toward crystal elasticity or plasticity remain to be established. − In addition, the phenomenon of photochemical-reaction-triggered crystal deformation is well-documented, − but examples of photoinduced crystal elongation are very rare and are limited to microcrystals, − because of the ease of crystal disintegration (e.g., fragmentation and splitting) during the expansion process. The increased tendency for larger or thicker crystals to break − ,− indicates that strain-induced molecular reconfiguration in the interior bulk of crystals is much less efficient than that near the crystal surface.…”

Intersectional Effects of Crystal Features on the Actuation Performance of Dynamic Molecular Crystals

“…The molecular transformations driving the actuation also determine the amount of induced strain in the single crystal, its directionality, and whether it is a surface phenomenon or a single-crystal-to-single-crystal transformation. When strain is gradient-like and only accumulates on the surface of the crystal, the deformation occurs as bending or twisting. , Diffusion-free strains induced homogenously throughout the crystal structure produce anisotropic expansions and contractions. ,, Depending on the kinetics and intensity of the chemical reaction and the mode of dissipation of the resulting elastic energy, the response might be in the form of motility, such as jumping, or even disintegration, costing the crystal its structural integrity and causing it to shatter. − Given the different mechanics involved, the type of deformation inevitably has a direct effect on the amount of force that molecular crystals can generate (Figure E). Finally, comparing thermoresponsive and photoresponsive crystals indicates that the former produce more mechanical work and recover faster (Figure F,G).…”