Multicolored fluorescence variation of a new carbazole-based AIEE molecule by external stimuli

In this article, we design and synthesize a new carbazole-based molecule, Cz2CN, with a twisted D-A structure, using the carbazole group as the donor and a dicyanoethylene fragment as the acceptor. Such a twisted D-A structure endows Cz2CN with two characteristic emission bands, LE emission and ICT emission. Cz2CN exhibits AIEE, solvatochromism, and different responses to anisotropic shearing force and isotropic hydrostatic pressure, due to the sensitivity of ICT emission to aggregation, solvent polarity and mechanical force. Aggregation benefits the ICT emission, leading to the AIEE of Cz2CN in the THF/water system. Polar solvents can help stabilize the ICT-excited states and make the ICT emission red shift. The original crystalline powder is strongly fluorescent with a high quantum yield of 40.4%. A single crystal of Cz2CN is obtained and dimers without π-π interactions among carbazole groups contribute to the strong emission. Anisotropic shearing alters the emission of Cz2CN powder from sky blue (474 nm) to green (520 nm). The single crystal of Cz2CN undergoes a distinct multicolored variation from sky blue (476 nm) to green (510 nm) and further to orange (590 nm) upon isotropic hydrostatic pressure. The emission of the original powder/crystal of Cz2CN is dominated by LE (locally excited state) emission in HLCT (intercrossed hybridized local and charge transfer excited state). To the best of our knowledge, Cz2CN is an interesting carbazole-based molecule that shows simultaneous AIEE, solvatochromism and force-induced multicolored variation.