Halogen effect on enhanced mechanofluorochromic properties of AIE-active tetraphenylethylene-based acylhydrazone luminophores

Abstract

Four tetraphenylethylene-based acylhydrazone derivatives TPEBH-H, TPEBH-F, TPEBH-Cl and TPEBH-Br which displayed aggregation-induced emission (AIE) properties were designed and synthesized to study the influence of halogen atoms on their mechanofluorochromic (MFC) properties. These compounds. Interestingly, the pristine samples of TPEBH-H, TPEBH-F, TPEBH-Cl and TPEBH-Br indicated a red-shift of 55, 76, 69 and 73 nm, respectively, in their fluorescence spectra upon grinding, which revealed MFC characteristics. And the luminescence colors of ground samples could return to the original blue upon fuming with dichloromethane or heating. The power X-ray diffraction (PXRD), the differential scanning calorimetry (DSC) and field emission scanning electron microscopy (FESEM) results indicated the MFC properties could be contributed to the crystalline-to-amorphous transition. Moreover, the red shifts of emission wavelengths in the fluorescence spectra should be mainly attributed to the extension of the molecular conjugation after grinding. This work indicated that the introduction of halogen atoms to a acylhydrazone derivative could be an useful and feasible method for constructing high contrast MFC-active materials.