Stimuli-responsive fluorescence switching: Aggregation-induced emission (ATE), protonation effect and reversible mechanofluochromism of tetraphenylethene hydrazone-based dyes

Two new simple and easily prepared TPE-based hydrazone derivatives of isoniazid (TBIH) and benzoyl hydrazide (TBBH) have been designed and synthesized, which showed AIE behavior and remarkable and reversible mechanofluorochromic (MFC) properties. The spectroscopic properties and luminescence color change in the solid state were reversible upon grinding and dichloromethane vapor (DCM) fuming or heating. The crystal structures showed slight disorder that was caused by mechanical grinding, which could be restored by DCM vapor fuming or heating. In addition, the results of powder X-ray diffraction (PXRD), differential scanning calorimetry (DSC) and field emission scanning electron microscopy (FESEM) revealed that the MFC mechanism was the transformation between ordered crystalline and amorphous states upon external stimuli. Meanwhile, the extension in molecular conjugation caused by planarization of molecular conformation and subsequent planar intramolecular charge transfer (PICT) process were responsible for the red-shifts in the fluorescence spectra. Interestingly, protonation-deprotonation of the pyridine moiety in TBIH had a significant effect on the frontier molecular orbitals as well as very distinctive emission characteristics upon acid and base stimuli. The dual-response performance and the ease of its preparation and renewal endow the material with potential applications in pressure and acid/alkali fluorescence sensing.