Reversible mechanofluorochromism and acidochromism using a cyanostyrylbenzimidazole derivative with aggregation-induced emission

Abstract

A novel D–π–A structure triphenylamine functionalized benzimidazole derivative TBM was designed and synthesized. It was found that the compound TBM exhibited typical aggregation-induced emission (AIE) characteristic. It was proved by the fact that the aggregates emitted strong yellow light in THF/water with high water fraction, while it was almost non-emissive in THF. More interestingly, the TBM showed multi-stimuli responsive fluorescent behaviors. First, TBM showed significant solvatochromic effect, with emission color tuning from green to orange by changing the solvent from nonpolar hexane to polar dimethyl sulfoxide. Second, TBM exhibited reversible mechanofluorochromism. The pristine crystal of TBM emitting yellow fluorescence could be transformed into powder emitting an orange emission upon grinding, and it could be restored when the ground powder were fumed with methanol vapor. XRD and DSC results demonstrated that the reversible mechanofluorochromism was due to the transformation between the crystalline and amorphous states, which could be ascribed to its different molecular stacking mode in the solid state. Finally, we found that the TBM revealed remarkable acid-induced fluorescence quenching effect in solution and film, and the emission of TBM in CHCl₃ could be quenched by trifluoroacetic acid (TFA) significantly. This work offered cyanostyrylbenzimidazole derivative that could be used as sensitive fluorescent indicator for mechanical sensors and organic acids.