Color differences highlight concomitant polymorphism of chalcones

The meta- and para-nitro isomers of (E)-3'- dimethylamino-nitrochalcone (Gm8m and Gm8p) are shown to exhibit concomitant color polymorphism, with Gm8m appearing as yellow (P21/c) or orange (P1) crystals and Gm8p appearing as red (P21/n) or black (P21/c) crystals. Each of the polymorphs was characterized optically via UV-vis spectroscopy, and their thermal behavior was characterized via differential scanning calorimetry and low-temperature powder X-ray diffraction. To assess the effect of molecular configuration and crystal packing on the colors of crystals of the different polymorphs, time dependent density functional theory (wB97x) calculations were carried out on isolated molecules, dimers, stacks, and small clusters cut from the crystal structures of the four polymorphs. The calculated color comes from several excitations and is affected by conformation and most intermolecular contacts within the crystal, with the color differences between polymorphs mainly being due to the differences in the π-π stacking. The visual differences between these related polymorphic systems make them particularly useful for studying polymorph behavior such as phase transitions and concomitant polymorph growth.