Twisted Donor−π–Acceptor Carbazole Luminophores with Substituent-Dependent Properties of Aggregated Behavior (Aggregation-Caused Quenching to Aggregation-Enhanced Emission) and Mechanoresponsive Luminescence

To get insight on how the donor (D)/acceptor (A) substitutions and their substitution pattern affect optical properties in solution and in the solid state, four novel carbazole derivatives C1–C4 with different terminal groups (N­(CH₃)₂, H, Br, and CN) have been employed. Their single-crystal X-ray structures confirm that the twisted structures, intermolecular interactions, and loose molecular packing are functions of the terminal groups. Interestingly, they displayed substituent-dependent aggregation-induced behaviors and mechanochromic (MC) properties. C1 showed aggregation-caused quenching because of the strong intermolecular π–π interaction, whereas C2, C3, and C4 exhibit strong aggregation-enhanced emission characteristics, which contributed to the formation of J-aggregates. Different twisted conformations and molecular arrangements induce different mechanostimuli response behaviors for C1–C4 between green-yellow to red color along with large red-shifts of 63, 30, 56, and 49 nm, respectively. C1, C3, and C4 displayed obvious MC characteristics, but C2 displayed irreversible mechanostimulus behavior. Powder X-ray diffraction and single-crystal X-ray analysis revealed that planarization of the twisted structure extended the π-conjugation and the destruction of the crystalline structure accounted for the tuned solid-state fluorescence.