Photoresponsive single crystals of organic small molecules

Photoresponsive organic single crystals have emerged as dynamic materials capable of converting light energy into versatile chemical, optical, and mechanical responses. This review systematically explores the interplay between molecular design, crystal engineering, and external stimuli in governing photochromic, photomechanical, photothermal, and photosalient behaviors. Key advancements include the development of dual-mode photochromic-fluorescent systems, rational design of diarylethene derivatives for visible-light switching, and the application of topochemical principles to achieve strain-driven mechanical motions. Structural flexibility in organic crystals, coupled with precise control over molecular packing and intermolecular interactions, enables functionalities such as reversible fluorescence switching, circularly polarized luminescence, and programmable actuation. Recent progress in integrating photothermal effects and hybrid materials has expanded the operational wavelength range and mechanical durability of these systems. Challenges remain in understanding cooperative dynamics between photochemical reactions and lattice strain, improving fatigue resistance, and scaling up practical applications. Future opportunities lie in multifunctional crystal design, machine-learning-guided crystal engineering, and applications in soft robotics and adaptive technologies. Bridging molecular-scale motions with macroscopic performance through interdisciplinary approaches will drive the transition of these materials from laboratory curiosities to cornerstone components in next-generation optoelectronic and photomechanical devices.