Crystal Engineering of Photochromic and Thermochromic Co-Crystals

Salicylidene aniline derivatives represent an important family of compounds, which have shown thermochromic and photochromic behavior in the solid state. Photo- and thermochromic materials can be used in a broad range of applications, i.e. in electronic display systems, in optical switching devices, in sensors or in molecular rotors. Derivatives of salicylidene aniline can show (i) a thermal tautomeric equilibrium between the cis-keto form and the enol form, which accounts for their thermochromism and (ii) photo-isomerisation between the cis-keto form and the trans-keto form, induced by irradiation. Two main structural features responsible for the cis-trans photo-isomerisation were defined: the planar character of the anil molecule and the crystal packing [1]. In this study we report on the relationship between structural and optical properties of salicylidene aniline derivatives, obtained via solid-state synthesis [2]. We have used a crystal engineering approach to modify the optical behavior of these molecules, in particular photochromism. The compounds synthesized in the solid-state were co-crystallized with a halogenated coformer. The preparation of co-crystals via mechanochemistry combines the quest for clean and green processes with the investigation of multicomponent new materials [3], and has shown to be a promising tool to tune the optical properties of organic compounds [4]. The thermochromic and photochromic behavior of the compounds in the solid-state has been investigated at RT and low temperature via single crystal and powder X-ray diffraction, combined with a comprehensive photophysical characterization.