Crystal engineering of thermo- and photochromic materials

Salicylidene aniline derivatives represent an important family of organic compounds, characterized by thermo- or photochromic behavior in the solid state. 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, induced by variation of temperature and (ii) photo-isomerisation between the cis-keto form and the trans-keto form, induced by irradiation. Thermochromism is a common process in this class of compounds, whereas photochromism is rarely observed, since specific conditions are required; in particular, the molecule needs to possess an adequate reaction volume, in the crystal packing, for the isomerization process to occur. In this study we investigate the relationship between crystal packing and thermo/photochromic behaviour.