Color Polymorphs of ROY-ol

Abstract

Color polymorphism is a fascinating property exhibited by compounds that have polymorphs of different colors. The most famous family of substances forming molecular organic color polymorphs is the ROY family of compounds. ROY, which states for the red, orange, and yellow colors of the polymorphs of 5-methyl-2-((2-nitrophenyl)amino)thiophene-3-carbonitrile, is currently the compound exhibiting the largest number of known polymorphs (12), all of them with crystallographic structures determined. In this study, the color polymorphism exhibited by a novel member of the ROY family, 2-((4-hydroxy-2-nitrophenyl)- amino)thiophene-3-carbonitrile (ROY-ol), has been investigated. Four color polymorphs of ROY-ol were identified (one brightburgundy and the others dark-burgundy), from which the crystal structures of two (polymorphs 1 and 2) were solved by single-crystal X-ray diffraction, and the intermolecular interactions in these crystals being subsequently evaluated using the Hirshfeld analysis method. These two polymorphs were also characterized vibrationally by infrared and Raman spectroscopies, supported by fully periodic density functional theory (DFT) calculations. The performed thermal analysis studies, undertaken by differential scanning calorimetry (DSC), polarized light thermomicroscopy (PLTM), and temperature-variation Raman spectroscopy, allowed us to identify two additional polymorphs of ROY-ol (polymorphs 3 and 4). Investigation of the conformational space of the ROY-ol isolated molecule within the DFT framework (together with similar calculations on similar molecules, including ROY) allowed to establish a series of correlations between the conformational landscapes of the molecules and some relevant characteristics of the polymorphism the compounds exhibit. As a whole, a detailed description of the color polymorphism exhibited by ROY-ol has been achieved, which is an additional contribution to the understanding of this phenomenon.