D-A type dicyanoethylenes modified by carbazole: Obvious aggregation-induced emission enhancement and high-contrast mechanofluorochromism

Two novel luminophores, namely CDCEEC-1 and CDCEEC-2, were synthesized from dicyanoethylene segments and D-A-type carbazoles. Decoration of dicyanoethylene with carbazole causes them to exhibit significant aggregation-induced emission enhancement (AIEE) behavior. Hence, while CDCEEC-1 and CDCEEC-2 exhibit extremely weak luminescence in common solvents, they show intense emissions because they possess highly twisted molecular structures once the molecules aggregate as nanoparticles or as solid powders. As a result of their intramolecular charge transfer (ICT) behavior, both luminophores exhibit a bathochromic effect. The combination of ICT and AIEE features not only enables CDCEEC-1 and CDCEEC-2 to have intense solid-state yellow and orange-yellow fluorescent emissions with quantum efficiencies of 0.363 and 0.328, respectively, but also imparts them with reversible and high-contrast mechanofluorochromic (MFC) behavior as a result of phase transition between crystalline and amorphous states. This behavior resulted in the emission peaks shifting from 550 nm (yellow) to 621 nm (orange-red), and from 574 nm (orange-yellow) to 648 nm (red), respectively. Our research has shown that it is feasible to produce high-efficiency ICT-AIEE-MFC luminogens with tunable emissions by using a suitable combination and modulation of donors and/or acceptors. This provides novel insights into the use of these luminogens in optical applications.