Giant Single‐Crystal Shape Transformation with Wide Thermal Hysteresis Actuated by Synergistic Motions of Molecular Cations and Anions

Manipulating the collective molecular movements to implement macroscopic mechanical response of bulk material is attractive and challenging. Here, we synthesize an organic-inorganic hybrid single crystal that exhibits a giant macroscopic shape transformation with a remarkable thermal hysteretic feature. The colossal anisotropic shape change, which manifests as an abrupt elongation of ca. 9% along crystallographic c-axis and a concomitant contraction of ca. 9% in a perpendicular direction, is induced by a significant reorientation of imidazoliums accompanied with a substantial configurational variation in CuBr 4 2- complex anions. The synergistic motions of both the molecular cations and anions engender a remarkable large thermal hysteresis (> 30 K) in the shape transformation of single crystal, implying this material may play a role in alternating memory media. Furthermore, due to the stable crystal lattice, a single crystal that demonstrates naked-eye detectable large shape transformation was used as a thermal actuator to spontaneously control an electric circuit by temperature variation.