2,4,6-Trinitro-N-(m-tolyl)aniline: A New Polymorphic Material Exhibiting Different Colors

Abstract

2,4,6-Trinitro-N-(m-tolyl)aniline or N-picryl-m-toluidine (abbreviated as TMA), exhibiting color polymorphism, was synthesized and characterized structurally, spectroscopically and thermodynamically. The studies consider both the isolated molecule of the compound (studied theoretically at the DFT(B3LYP)/6-311++G(d,p) level, and experimentally by matrix isolation infrared spectroscopy) and its polymorphic crystalline phases. The investigations on the isolated molecule allowed to evaluate the major intramolecular interactions determining the conformational preferences of the compound, and characterize it in detail from the vibrational point of view. Two conformers were found (A and B), which differ in the relative orientation of the ring moieties of the molecule. Polymorph screening, by recrystallization of the compound using different solvents, allowed to identify three different polymorphs, exhibiting red, orange and yellow colors, which were subsequently characterized structurally by single crystal X-ray diffraction, vibrationally, by infrared and Raman spectroscopies (complemented by fully-periodic DFT calculations on the crystals), electronically, using solid state ultraviolet-visible absorption spectroscopy, and thermodynamically, by differential scanning calorimetry and polarized light thermomicroscopy. The yellow polymorph was found to be a conformational polymorph of the orange and red ones, while the last two are packing polymorphs. Mechanistic insights on the causes of the different colors of the polymorphs are presented. On the whole, the reported investigation constitutes a comprehensive structural (for both the isolated molecule and crystalline phases), spectroscopic and thermal analysis of the newly prepared compound, with particular emphasis on the rare color polymorphism it exhibits.