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.