• fluorescence;
  • kinetics;
  • photochromism;
  • pyrazolones;
  • semicarbazides


Two novel photochromic compounds, 1,3-diphenyl-4-benzal-5-hydroxypyrazole 4-phenylsemicarbazone (1 a) and 1,3-diphenyl-4-(4-nitrobenzal)-5-hydroxypyrazole 4-phenylsemicarbazone (2 a), are synthesized and characterized by elemental analysis, mass spectrometry, FTIR spectroscopy, and 1H NMR spectroscopy. Their properties, including photochromic behavior, fluorescence properties, and thermal bleaching kinetics, are investigated. The results show that the two compounds exhibit improved photochromic performance in coloration and thermal bleaching rates, excellent photostability, high fatigue resistance, and reversible fluorescence switching properties in the solid state in comparison to reported pyrazolone thiosemicarbazones. The thermal bleaching process obeys first-order kinetics. Bleaching of powders at 130 °C is completed within 90 s for 1 b (the colored isomer of 1 a) and 150 s for 2 b (the colored isomer of 2 a). The activation energy for the thermal bleaching process is determined to be 69 and 95 kJ mol−1, with frequency factors of 9.5×107 and 9.4×1010 s−1 for 1 b and 2 b, respectively.