Self-reversible mechanofluorochromic AIEgens with tunable solid-state fluorescence: effect of acceptors and intermolecular interactions

Exploring the organic molecular structural features and supramolecular interactions provide basis for developing stimuli-responsive organic solid-state fluorescent materials. Herein, we have synthesized methoxyphenyl-substituted carbazole-based pi-conjugated derivatives (Cz-MN, Cz-ECA, Cz-CA, Cz-CAA and Cz-NBA) with different acceptor units and investigated their influence on solid-state fluorescence. All five compounds showed aggregation-induced emission (AIE) in the solid state with acceptor-dependent tunable fluorescence from 488 to 602 nm (quantum yield (Phi f) = 7.2 to 1.9%). Cz-ECA exhibited blue-shifted fluorescence, whereas Cz-NBA showed red-shifted fluorescence. Cz-MN, Cz-ECA and Cz-CA revealed relatively strong solid-state fluorescence compared to Cz-CAA and Cz-NBA. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) calculations suggested intramolecular charge transfer (ICT) from the carbazole donor to the acceptor unit. Mechanofluorochromic studies of Cz-MN and Cz-ECA showed self-reversible fluorescence switching but the other three AIEgens did not show any stimuli-response. Single crystal structural analysis of Cz-MN, Cz-ECA and Cz-CA showed twisted molecular conformations with different intermolecular interactions. Both Cz-MN and Cz-ECA did not show any strong intermolecular interactions in the crystal lattice. In contrast, Cz-CA exhibited strong amide-amide complementary H-bonding and an extended network structure. Hence, the twisted molecular conformation and the absence of strong intermolecular interactions in Cz-MN and Cz-ECA contributed to self-reversible mechanofluorochromism (MFC). Thus, the present study is expected to provide further insight to design AIEgens with stimuli-responsive self-reversible fluorescent materials. The intermolecular interacting functionality in the carbazole-based pi-conjugated fluorophores controlled mechano-responsive fluorescence switching.