Colossal Thermal Expansion and Continuous Rolling Locomotion of a Robust Flexible Molecular Single Crystal.

Amplifying microscopic or molecular perturbations to induce macroscopic mechanical effects in well-ordered molecular crystals is the foundation of the newly recognized potential of organic crystalline smart materials for soft organic electronics, optics, actuators, switches, and robots. Diverse molecular crystal actuators that transform external energy into mechanical motions have been prepared in the past decade, yet their spatiotemporal operational capability, adaptability, reversibility, and durability have not been fully explored. In this study, we present adaptive molecular single crystals that can respond to force, heat, and light, demonstrating mechanical flexibility, reversible expansion, and complex movements, including rolling and climbing locomotion. These crystals exhibit significant anisotropic thermal expansion within the temperature range from 303 to 413 K, expanding by approximately 4.8% along their longest axis. The exceptional flexibility and thermal expandability of this material enable quick and sustained locomotion of the single crystals when exposed to ultraviolet light. Our findings highlight the considerable yet underexplored potential of adaptive organic crystals that can be used as lightweight thermomechanical and photomechanical actuators.