Aggregation-induced emission, photochromism and self-assembly of pyrazolone phenlysemicarbazones

Four pyrazolone phenylsemicarbazone derivatives, 1,3-diphenyl-4-(3-substituent)-5-hydroxypyrazole 4-phenylsemicarbazone [substituent = chlorolbenzal (1), fluorobenzal (2), bromobenzal (3) and benzal (4)], were synthesized. On the one hand, the four compounds exhibit good solid-state photochromic behaviors upon the irradiation of UV light and heat, and high fatigue resistance. On the other hand, they have aggregation-induced emission (AIE) characteristics. The emission could also be greatly enhanced by cooling solutions, suggesting that the aggregation-induced emission is caused by the restricted intramolecular rotations of the peripheral aromatic rings or other substituents around the central pyrazole ring. In addition, with the increase of the aging time, aggregates of these compounds can self-assemble into 1D nanostructures from 0D nanoballs by the reprecipitation, leading to the red-shift of emission spectra. Thus, it paves the way for preparing multifunctional materials with photochromic and AIE behaviors.