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 II O . 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···O bonds. The different colors are attributed to intramolecular electronic effects due to an increased delocalization of the secondary amino nitrogen lone-pair el...