Optical Properties, Thermochromism, and Crystal Structures of the Dimorphs of 2-Iodoanilinium Picrate

The crystals of 2-iodoanilinium picrate have three polymorphic forms. The crystal structures of the monoclinic form (form I) and the triclinic form (form II) have been determined by the X-ray diffraction method. The crystals of the third form (form III) are unstable. In forms I and II, 2-iodoanilinium cation and picrate anion form a nearly planar pair of these ions with similar geometry of N–H···O hydrogen bond. In form I the pairs related by a translation are stacked to form the segregated columns, in which the same kinds of ions are stacked with each other, while in form II the anions and cations are alternately stacked to form the continuous columns. The thermochromism of these crystals was studied by measuring the polarized visible and IR absorption spectra and the differential scanning calorimetry.