Light-Driven Vibration and Liquefaction of Flexible and Naturally Bent Single Crystals

Single crystals obtained by cocrystallization of 4,4'-azopyridine with long-chain fatty acids, viz. lauric acid, myristic acid, and palmitic acid, exhibit a variety of light-responsive properties. The molecular arrangement of the cocrystal components in the crystal structure, coupled with the flexibility of the long alkyl chains and the presence of the azo group, enables the elastic and plastically deformable crystals to demonstrate light responsiveness, which manifests as bending, vibration, and liquefaction. The photoresponses arise from two primary mechanisms: the photoisomerization of azopyridine groups within the crystal lattice and the photothermal effect that generates localized heat under UV light. Consequently, the single crystals display reversible bending and vibrational motion in response to on/off light cycles, as well as temporary liquefaction of the crystalline materials when exposed to higher-intensity UV light. The arrangement of the long alkyl chains and the presence of weak interactions within the crystal structure impart deformability to the single crystals while lowering melting points. This work illustrates how adaptive crystalline materials can be developed by modulating the crystal structure of light-sensitive molecular components through non-covalent synthesis.