Engineering Solid‐State Molecular Switches: N‐Salicylidene N‐Heterocycle Derivatives

A supramolecular engineering approach has been developed for a novel family of N‐salicylidene aniline derivatives to control their thermo‐ and photochromic behaviours. Hsaltrz, Habs, Hsalphen, Hsaltz and Ksaltz are versatile molecules built from N‐heterocycles, which drive the molecular arrangement to form a controlled crystal packing with predesigned optical properties. A complete structural, optical and computational study of powders of these new molecular nanoswitches is presented. An N‐salicylidene aniline derivative possessing no thermo‐ or photoinduced chromic properties thanks to a specific molecular geometry was sought, and Habs and Hsalphen were designed to enhance π–π stacking interactions. A theoretical study of the crystal packing, combined with time‐dependent diffuse reflectance studies of Habs, have confirmed the appearance of photochromism at room temperature completed with an unprecedented “spring‐type effect” observed during the photochemical relaxation of the metastable trans‐keto form. The absence of thermochromism in an N‐salicylidene aniline derivative, induced by the absence of cis‐keto form, is a unique behaviour, firstly identified and explained for these types of compounds. Finally, Hsaltz and Ksaltz are the result of a molecular derivation of Hsaltrz which allow enhancing the amplitude of thermochromism of these functional materials.