Synthesis and Solid-State Photochromism of 1,3-Diphenyl-4- (2-chlorobenzal)-5-hydroxypyrazole 4-Methylthiosemicarbazone

A new photochromic compound containing a pyrazole-ring unit, 1,3-diphenyl-4-(2-chlorobenzal)-5-hydroxypyrazole 4-methylthiosemicarbazone,
was synthesized. Its structure, photochromic properties, and photochemical kinetics were characterized. The results show that the title compound
exhibits reversible enol−keto photoisomerization, excellent photostability, and high fatigue resistance. An intra- and intermolecular proton-transfer mechanism is proposed.