Halogen substitution modulation on triphenylamine-1,1-dicyanoethylene AIEgens: tunable reversible bathochromic mechanofluorochromism and applications in anti-counterfeiting

In this work, four new AIE-active 1,1-dicyanoethylene derivatives decorated with triphenylamine and different halogen substituents ( E )-2-(1-(4-(diphenylamino)phenyl)-3-phenylallylidene)malononitrile ( DPPM ), ( E )-2-(1-(4-(diphenylamino)phenyl)-3-(4-fluorophenyl)allylidene)malononitrile ( DPPM-F ), ( E )-2-(1-(4-(diphenylamino)phenyl)-3-(4-chlorophenyl)allylidene)malononitrile ( DPPM-Cl ), and ( E )-2-(1-(4-(diphenylamino)phenyl)-3-(4-bromophenyl)allylidene)malononitrile ( DPPM-Br ) were rationally designed and synthesized via Knoevenagel condensation. Their photophysical properties, AIE behavior, solvatochromism and mechano-responsive luminescence characteristics were systematically investigated. It was found that all the target compounds exhibited remarkable solvatochromic effects and obvious AIE features. Notably, the four compounds displayed reversible high-contrast mechanofluorochromism (MFC). The pristine crystalline solids displayed orange fluorescence and mechanical grinding induced a pronounced bathochromic shift of 38–80 nm, resulting in red emission. The initial luminescence can be completely regenerated via dichloromethane vapor fuming, demonstrating excellent reversibility of the MFC effect. Powder X-ray diffraction (PXRD), scanning electron microscopy (SEM) and single-crystal analysis revealed that the MFC mechanism was attributed to the crystalline-to-amorphous phase transition and molecular conformation planarization induced by mechanical force, accompanied by enhanced intramolecular charge transfer and π-conjugation effect. Halogen substituents effectively regulate the intermolecular interactions and twisting degree of molecular conformation, thereby modulating the fluorescence response contrast and spectral shift range. Furthermore, the reversible luminescence switching behavior endowed these molecules with promising application in rewritable information encryption and anti-counterfeiting. This work provided a feasible strategy for constructing high-performance MFC materials by halogen modulation, and expanded the application scope of 1,1-dicyanoethylene-based AIE luminogens.