Crystal structure and photoisomerism of 1-phenyl-3-methyl-4-(4-fluorobenzal)-5-pyrazolone 4-methylthiosemicarbazone in the solid state

A new photoisomerization compound containing a pyrazolone-ring unit, 1-phenyl-3-methyl-4-(4-fluorobenzal)-5-pyrazolone 4-methylthiosemicarbazone (PM4FBP-MTSC), was synthesized and characterized by elemental analyses, MS, IR and 1H NMR spectra. The photoisomerization properties of PM4FBP-MTSC in the solid state have been studied by UV spectra and the reaction rate constant was obtained from the first-order kinetics curve. Based on its spectra data, theoretical calculations and single crystal structural analyses, the title compound undergoes a photoisomerism process from the enol-form (Ι) to the keto-form (III) by the intra- and intermolecular double-proton transfer upon UV light irradiation.