Helicity-Biased Light-Driven Rolling of Twisted Crystals

Achieving dynamic motions in molecular crystals with both long-range displacement and precise control remains a central challenge. Herein, we report a light-driven rolling motion in twisted single crystals of 9-cyanoanthracene, representing a new motility paradigm that combines structural asymmetry with directional actuation. Under UV irradiation, straight crystals exhibit limited bending due to anisotropic lattice expansion from localized [4 + 4] photodimerization. However, when twisted into helices, they roll rapidly and directionally toward the light source. Systematic investigations reveal that rolling is driven by a transient, light-induced shift in the center of mass, which generates torque through the misalignment of gravitational and normal forces. The rolling velocity can be finely tuned through external parameters including light intensity and incidence angle, as well as internal structural features including crystal length, width, and helical pitch. While the handedness of helicity does not affect rolling velocity under unconstrained conditions, introducing a lateral constraint with a fine wire reveals a distinct helicity-dependent deflection in the rolling trajectory. Specifically, when rolling toward the light, left-handed helices consistently deviate to the right, whereas right-handed helices deviate to the left. This helicity-biased rolling arises from asymmetric contact forces during rolling and highlights the role of contact mechanics in translating structural chirality into directional motion. This work establishes rolling as a conceptually novel mode of crystal actuation, demonstrating how structural chirality and photoreactivity can be synergistically harnessed to impart directionality to dynamic motion. Our findings lay the foundation for developing advanced smart materials with complex, programmable functionalities based on molecular crystals.