Polymorphism Steered\nThermochromism in a Sulfonamide

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