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.