Photochromism in solid state and acidichromism in solution of 1-phenyl-3-methyl-4-(4′-methylbenzal)-pyrazolone-5 thiosemicarbazone

1-Phenyl-3-methyl-4-(4′-methylbenzal)-pyrazolone-5 thiosemicarbazone (PM4MBP-TSC) has been synthesized and its single crystal has also been developed. Its photochromism is investigated in the solid state through the analyses of its UV–vis reflection spectra and the mechanism is proved to be intermolecular proton transfer by the structure analysis. The acidichromic properties were also studied from the absorption and fluorescence spectra by adding acetic acid to the methanol solution of the compound. The observed phenomena can be explained by the solute–solvent complex of PM4MBP-TSC and the acetic acid constituted in the solution that destroyed the former conjugated structure.