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 irans-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.