Polymorphism in N-salicylideneaniline reconsidered

The aromatic Schiff base N-salicylideneaniline (SA) represents the aristotype of a whole family of chromic derivatives. SA forms two photochromic polymorphs, alpha(1) and alpha(2), both of which feature non-coplanar phenyl rings. Since many details of its reversible photoreactivity are still unexplained, we report the structure of a planar, thermochromic polymorph, named beta, and propose a new model for the structure of the alpha(1)-polymorph. Finally, we discuss structure-property relations between the polymorphs and the beta to alpha(1) phase transition.