Thermochromism of a novel organic compound in the solid state via crystal-to-crystal transformation

A novel thermochromic organic compound, 1,5-bis(hydroxyethylamino)-2,4-dinitrobenzene (BDB), was synthesized and characterized using X-ray diffraction, differential scanning calorimetry, visible and infrared spectroscopy. It was observed that BDB exists in two different crystal structures of A- and B-forms at room temperature. In each crystal structure, there exist intra- and intermolecular hydrogen bonds between hydroxy, amino, and nitro groups. As the temperature is increased, the color of BDB crystalline powder changes from yellow to orange. The yellow color was recovered for a specific period of time when it is cooled again, indicating that thermochromism of BDB in the solid state is reversible. Both crystals of A- and B-forms are transformed to the C-form crystal at the transition temperature. From the results of X-ray diffraction, thermal data, and infrared spectra, it is suggested that the thermochromism of BDB originates from the crystal-to-crystal transition accompanying with the configurational transformation between nitro-form and acid-form via intramolecular hydrogen transfer.