Polymorphism Steered Thermochromism in a Sulfonamide

Color polymorphism is a rare but intriguing phenomenon, crucial for fundamental understanding and with broad implications for a number of industries. We report the facile preparation of six color polymorphs (forms I-VI) of a sulfonamide. All forms have been structurally characterized, and the studies attribute the color polymorphism to molecular rotations about the C═N-C-C dihedral (torsion 1). A definite correlation between color wavelength and value of torsion 1 has been established, indicating the tendency of red-shift with a decrease in values of torsion 1. Remarkable reversible (four forms) and irreversible (two forms) thermochromism arises due to controlled small and large angle molecular rotations, respectively. Conformational energy scans suggest that the six polymorphs are located in one of two energy basins. Computational crystal structure prediction (CSP) was performed, taking into account the molecular flexibility to correctly locate all polymorphs on the crystal form landscape. The temperature-induced enantiotropic phase transition between III and IV was rationalized using free energy calculations within the harmonic approximation. Overall, this work reports a record six crystal polymorphs of a simple molecular compound with striking chromism and thermochromic behavior and emphasizes the importance of a joint experimental and computational approach to understanding and controlling polymorphism in conformationally flexible organic molecules.