Depression of the Melting Point in Naturally Grown Circular Crystals

The melting point is one of the most fundamental properties of crystalline materials and has been commonly used to determine the chemical and phase purity of organic compounds. Here, we report a significant depression in the melting point of about 1.4 K in naturally grown crystals with a circular shape obtained for one polymorph of the tetradecamorphic compound 5-methyl-2-((2-nitrophenyl)-amino)-3-thiophenecarbonitrile (ROY). Crystals of ROY grown by microspace sublimation have peculiar habits, with irregular, elongated, occasionally bent, or curled habits, and with one or both of their ends closed into loops. In contrast to the straight crystals, the Mueller matrix microscopic analysis suggested the continuous reorientation of electrical dipole moments in the curled crystals. When heated, these curly crystals often start to melt at the kink and exhibit a lower melting onset point and a broader endothermic peak in the thermal (DSC) fingerprint compared with the regular crystals. The decrease in the melting point was found to be proportional to the deformation expressed as the curvature of the crystals; it is also inversely proportional to the crystal width for narrow crystals, but it is independent of the width for wider crystals. The bent section of the crystals is mechanically softer than the straight part, and both the stiffness and hardness are inversely proportional to the degree of curling, presumably due to defects. In the three-dimensional reciprocal space, the curled crystals show diffuse diffraction or streaks due to lattice distortion. Nanoinfrared spectroscopic signatures indicate that the lattice distortion is related to the conformational changes of the molecule. The results highlight the dependence of the extent of crystal deformation on melting properties, which may have broad implications for modulating properties of pharmaceutical crystals.