Reversible fluorescent switching properties of pyrene-substituted acylhydrazone derivatives toward mechanical force and acid vapor with aggregation-induced emission

Four pyrene-substituted acylhydrazone derivatives named PMBH ((E)-N'-(pyren-1-ylmethylene)benzohydrazide), PMPH ((E)-N'-(pyren-1-ylmethylene)picolinohydrazide), PMNH ((E)-N'-(pyren-1-ylmethylene)nicotinohydrazide) and PMIH ((E)-N'-(pyren-1-ylmethylene)isonicotinohydrazide) by functionalizing the phenyl ring and pyridyl ring with pyrene-substituted acylhydrazone moiety have been designed and synthesized, whose properties were investigated. All the four synthesized compounds exhibited favorable aggregation induced emission (AIE) features and reversible mechanofluorochromism (MFC) behaviour. Especially, the intense cyan emission of PMIH changed into orange-yellow emission with a 101 nm red-shift upon grinding. The MFC behavior was associated with the crystalline-amorphous phase transformation between the pristine and ground states, which was confirmed by the investigations of the power X-ray diffraction (PXRD), the differential scanning calorimetry (DSC) and field emission scanning electron microscopy (FESEM). Specifically, PMPH, PMNH and PMIH demonstrated remarkable and reversible acidofluorochromic response due to the presence of free nitrogen atom of pyridyl groups at the terminal position. The chosen approaches showed a new possibility for generating multi-stimuli responsive materials with modulated solid state properties.