Moisture‐Gated Synergistic Rapid Crystal‐to‐Liquid Transition in Pyridinium Halide Crystals via [2 + 2] Photocycloaddition

Stimuli‐responsive dynamic crystals, particularly those that respond to light, offer tremendous potential for developing intelligent materials. Whereas specialized photoresponsive behaviors can be achieved through subtle optical modulation, understanding how these crystals interact with other ambient factors remains equally critical but not fully elucidated. Herein, we show crystals based on methylated pyridinium halide derivatives (MX, X═Cl, Br, I) that undergo moisture‐gated photoinduced crystal‐to‐liquid transition (PCLT) via [2+2] photocycloaddition at the vinyl structure. Under visible‐light irradiation and high local humidity, MX crystals melt rapidly within 1 min (20–50 s). In contrast, under drier conditions, the crystals display vigorous photomechanical motions, including jumping, expanding, and splintering instead. Ambient water molecules surrounding crystal surfaces are found to significantly promote photocycloaddition and stabilize the generated photodimers, leading to a crystal that fully melts outward to inward when local humidity exceeds 60% under light. This process also enables the instant recrystallization growth of photodimer single crystals via a solvent‐free approach. Furthermore, we develop a facile sensor system for real‐time monitoring of local humidity and for quantifying trace moisture in water‐miscible solvents, such as tetrahydrofuran (THF). Our work highlights the significance of synergistic environmental conditions and light stimuli in unlocking advanced functionalities in crystalline materials.