Crystal Packing–Trajectory Correlation in Topochemical Photoisomerization

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.