Polymorphism and Phase Behaviors of 2-(Phenylamino)nicotinic Acid

The discovery and phase transition studies of four polymorphs of 2-(phenylamino)nicotinic acid are described in this report. The four polymorphs were crystallized from solution under different conditions. All polymorphs differ in the degree of conjugation of the two aromatic rings and thus are of conformational polymorphism. Three of the four modifications have more than one conformer in the asymmetric unit. Due to the difference in conformation, all polymorphs display distinctive colors as well as different hydrogen-bonding arrangements. Phase behaviors of the four forms were studied by differential scanning calorimetry and hot-stage microscopy, indicating that metastable forms underwent phase transition to the stable one initiated by mechanical perturbation in solution or assisted by heating in the solid state. As such, this polymorphic system exhibits polychromism, conformational isomorphism, thermochromism, and mechanochromism. It may be valuable for further solid-state structure−property relationship studies.