Aggregation-induced emission and reversible mechanochromic luminescence of carbazole-based triphenylacrylonitrile derivatives†
Novel carbazole-based triphenylacrylonitrile derivatives (Cz1-TPAN and Cz2-TPAN) have been carefully prepared via the Heck coupling reaction. Both compounds exhibited typical intramolecular change transfer (ICT) and aggregation-induced emission (AIE) characteristics with high solid-state efficiency. For example, the ΦF of Cz1-TPAN in as-synthesized crystals reached 75.6%, which was more than 84 times that in THF. The single crystal structure of Cz1-TPAN revealed that the intermolecular interactions of C–H⋯π (2.242 Å, 2.889 Å), would lock the molecular conformation, thus yielding highly emissive behaviors in the crystals. Interestingly, Cz1-TPAN also exhibited mechanochromic behavior. The as-prepared crystals of Cz1-TPAN giving yellowish green fluorescence could be transformed into the powders emitting orange yellow light upon grinding, and the emission could be recovered when the ground powders were heated or fumed with DCM. This suggests that the reversible mechanochromism originates from the transformation between crystalline and amorphous states. It should be noted that Cz2-TPAN did not show mechanochromic properties. We deduced that compared with Cz1-TPAN, Cz2-TPAN had more a planar conjugated skeleton, which could give it compact intermolecular stacking and strong π–π interactions in the solid state, thus it exhibited no morphology change upon grinding.