Halogen-modulated mechanical force-induced luminescence enhancement and chromism of AIE-active tetraphenylethylene-based chalcone derivatives

In this paper, four new tetraphenylethylene-based chalcone derivatives (E)-1-phenyl-3-(4-(1,2,2-triphenylvinyl) phenyl)prop-2-en-1-one (PLPE), (E)-1-(4-fluorophenyl)-3-(4-(1,2,2-triphenylvinyl)phenyl)prop-2-en-1-one (PLPE-F), (E)-1-(4-chlorophenyl)-3-(4-(1,2,2-triphenylvinyl)phenyl)prop-2-en-1-one (PLPE-Cl), (E)-1-(4-bromophenyl)3-(4-(1,2,2-triphenylvinyl)phenyl)prop-2-en-1-one (PLPE-Br) with different halogen substitutions were reported, and the effects of halogens on their photophysical properties, aggregation-induced emission behaviors and mechano-responsive luminescent properties were systematically investigated. Interestingly, aggregation-induced emission enhancement (AIEE) behaviors were observed for PLPE, PLPE-F, PLPE-Cl, and PLPE-Br. Moreover, upon grinding, all of the four compounds displayed mechanical force-induced luminescence enhancement and redshifted behaviors, which was found to be reversible in the case of fuming with dichloromethane (DCM) vapors or heat treatment. In particular, upon grinding, PLPE-F showed the highest contrast fluorescence response behaviors (red-shifted 49 nm and solid fluorescence quantum yield increased 20-fold). Furthermore, the results of powder X-ray diffraction (PXRD), differential scanning calorimetry (DSC) and field emission scanning electron microscopy (FESEM) measurements indicated that reversible phase transitions between the ordered crystalline state and the disordered amorphous form was crucial to their interesting mechanical force-induced luminescence enhancement and chromism. And the obvious red shifts of fluorescence emission are ascribed to the increased molecular conjugations via the planarization of molecular conformations. Moreover, halogen-substituted PLPE-F, PLPE-Cl, PLPE-Br have more twisted conformations than PLPE, which are responsible for their higher contrast mechanical force-induced luminescence chromism. Hence, the present study provides a new strategy to construct new advanced high-contrast mechanical force-induced luminescence enhancement and chromism luminescent materials with the introduction of halogen atoms.