Aggregation induced emission and reversible mechanofluorochromism active carbazole–anthracene conjugated cyanostilbenes with different terminal substitutions

Abstract

Three novel carbazole–anthracene–cyanostilbene based D–A conjugates were designed and synthesized with slight structural modification of the acceptor unit (cyanostilbene; OMe is 5a; H is 5b; Cl is 5c) to understand the effect of the change of the optical properties. The photophysical characteristics of the compounds in various solvents and in the solid state, and aggregation emission behaviour were extensively examined. The fluorescence spectrum of compound 5a was completely different from those of the other two compounds (5b and 5c), which exhibited reverse solvatochromism on increasing the polarity of solvents, whereas the other two exhibited positive solvatochromism. The compounds displayed AIE characteristics in a solution of acetonitrile-H₂O mixture, could be attributed to the non-planar/twisted geometry of cyanostilbene and the restriction of intramolecular motion (RIM) in aggregated states and formation of aggregates was confirmed by FE-SEM and AFM images. Remarkably, all compounds exhibited reversible mechanofluorochromic (MFC) characteristics in which the color of compound 5a changed from bluish green to green color, while the other compounds showed greenish yellow to yellow color in the solid state, which was further supported by powder XRD and solid-state emission data. The optimized ground state structures of compounds 5a–c were obtained using theoretical investigations. The current work presents an extensive evaluation of the optical, electronic, AIE, and MFC properties of carbazole–anthracene–cyanostilbene derivatives caused by structural modifications of the acceptor unit (cyanostilbene).