Playing with Isomerism: Cocrystallization of Isomeric N‑Salicylideneaminopyridines with Perfluorinated Compounds as Halogen Bond Donors and Its Impact on Photochromism

In N-salicylideneaniline derivatives, photochromism occurs by a two-step isomerization mechanism. Photochromism has been reported to be closely related to the molecular conformation of the N-salicylideneanilines (described by the dihedral angle Φ) and to the free available space (V_free) in the crystal. In this contribution, we focus on cocrystals of isomeric N-salicylideneaminopyridines with perfluorinated halogen bond donors as coformers. The advantage of working with isomers is that they have a similar (if not equal) molecular volume. This aspect means that the evaluation and comparison of the free available spaces within cocrystals is not affected by the differences in the molecular volumes of their constituents. Cocrystals were synthesized by a one-pot procedure, which allowed concomitant formation of covalent (imine) and noncovalent (halogen) bonds. Each cocrystal was characterized by X-ray diffraction, and its photochromism was assessed by UV–vis diffuse reflectance. Our results suggest that the Φ/V_free model grossly oversimplifies the complexity of the dynamic phenomenon behind the photoisomerization of N-salicylideneanilines. For the above-mentioned reasons, careful examination of intermolecular interactions and crystals packing (localization of voids, evaluation of intermolecular interactions and stacking modes) should be taken into account and used to supplement the current proposed model based on Φ and V_free.