Crystal Packing–Trajectory Correlation in Topochemical Photoisomerization

Abstract Solid‐state photoisomerization is often associated with loosely packed molecular arrangements that allow conformational flexibility. In this study, we explore this premise by synthesizing a library of salicylhydrazone derivatives, 16 of which were successfully crystallized and analyzed. Notably, several planarly close‐packed crystals exhibited clear photochromic responses when their local environments provided sufficient steric freedom near the reactive bonds. By systematically examining π–π stacking motifs and quantifying accessible volume along the isomerization trajectory, we reveal a robust correlation between local free volume and photoresponse. From these insights, we develop a crystal‐structure based molecular descriptor that condenses the steric environment into a single‐parameter, the “pedal space” and identify a critical threshold for solid‐state photoreactivity. We term this phenomenon topochemical photoisomerization , highlighting how subtle variations in molecular packing, such as slipped versus co‐facial stacking in N ‐salicylideneaniline derivatives, dictate the resulting photochemical pathways. This study establishes a structural framework for the rational design of solid‐state photoswitchable materials through crystal‐engineering principles in silico, offering a route toward rapid, and cost‐effective development of responsive optoelectronic systems.