Unlocking Cryogenic Flexibility in Organic Crystals via Photoinduced E→Z Isomerization.

Dynamic tuning of mechanical properties in organic crystals remains a major challenge due to their intrinsic brittleness and rigidity, especially at cryogenic temperatures. Herein, we report the first example of light-induced cryogenic elasticity in organic molecular crystals via photoactivated E→Z isomerization. Crystals of a photoresponsive acylhydrazone derivative (NPH) undergo surface-localized isomerization upon 365 nm UV irradiation, inducing partial surface melting while maintaining internal crystallinity. This remote, postsynthetic modulation dramatically enhances mechanical compliance, enabling irradiated crystals to exhibit elastic deformation and photomechanical bending even in liquid nitrogen (77 K)-whereas nonirradiated crystals fracture upon immersion, with only the fragments retaining photomechanical activity. This work represents a breakthrough in mechanically reconfigurable molecular crystals and establishes a generalizable strategy for designing stimuli-responsive materials operable in extreme environments. These findings open new avenues for the development of cryogenic actuators, adaptive optical systems, and low-temperature flexible electronics.