Colored polymorphs:: Thermochemical and structural features of N-picryl-p-toluidine polymorphs and solvates

An intriguing example of conformational and color polymorphism is observed in p-tolyl-(2,4,6-trinitrophenyl)-amine (picryltoluidine, PT), which forms two crystalline modifications, the red form I-R and the orange form IIo. Solvated crystals with a PT/solvate ratio of 1: 1 (pyridine) or 2:1 (benzene, chlorobenzene, toluene, or xylene) were obtained only from aromatic solvents, albeit with some difficulty, from aromatic solvents. The crystallization from all other tested solvents produced either a highly pure phase or a mixture of the two anhydrous forms. Single crystal structure determinations were carried out on the seven solid forms, along with a characterization by thermal analysis (hot stage microscopy, DSC, TGA), vibrational spectroscopy (IR, Raman) and X-ray powder diffraction. The PT molecules in all structures exhibit intramolecular N-H center dot center dot center dot O bonds. The different colors are attributed to intramolecular electronic effects due to an increased delocalization of the secondary amino nitrogen lone-pair electrons in the aromatic ring and nitro groups, rather than to differences in the intermolecular excitation. The relationships between the seven crystal structures were investigated using the program XPac. They are all composed of the same one-dimensional stack of PT molecules. All 2:1 solvates in this study are isostructural. The desolvation behavior of the pyridine solvate is explained by its very close two-dimensional packing similarity to form IR. The molecular aggregation in the solid forms of PT is discussed in terms of similar packing fragments and weak interactions (C-H center dot center dot center dot O, C-H center dot center dot center dot pi contacts, and stacking).