Color Packing Polymorphism in an Organic Chromophore

Optical materials represent a class of exotic materials in optics, lenses, lasers, optic fiber communication, and optical storage devices, etc. Organic optical materials are excellent alternatives to conventional inorganic optical materials because of their flexibility, low costs, environmentally friendly nature, etc. Here, we report color trimorphism (three polymorphic forms: orange, light-orange, and yellow, abbreviated as OLY) in an organic chromophore via systematic physicochemical characterizations. We focused on a N,N-dimethyl bromo substituted thienyl chalconea push–pull chromophore. We performed physicochemical characterizations using UV–vis, solid-state photoluminescence, powder and single-crystal X-ray diffraction, differential scanning calorimetry, thermogravimetric analysis, and hot-stage microscopy to highlight the similarities and differences in the OLY forms of the title compound. We also calculated the lattice and interaction energies and compared them to understand the energetic stabilities of the polymorphs. Further, based on electron densities calculated for the selected dimers extracted from the crystal structure, we quantified the intermolecular interactions and highlighted the differences in electron density distributions across the three polymorphs. The detailed analysis of the crystal structures suggests that this is a case of packing color polymorphism. The colors of these polymorphs correlate well with the results obtained from the spectroscopic and computational studies.