Aggregation-induced emission and reversible high-contrast mechanofluorochromic behavior of dicyanoethylenes modified by xanthene and carbazole

Two novel donor-acceptor type molecules, XA-CE-CAR and XA-CE-BCAR, were created and synthesized using xanthene, carbazole and dicyanoethylene. The two luminophores possessed effective emissions in the solid-state with the respective fluorescence efficiencies of 0.256 and 0.238 and showed distinct aggregation-induced emissions. Particularly, XA-CE-CAR and XA-CE-BCAR exhibited high-contrast mechanofluorochromic (MFC) characteristics. When the as-prepared powders of the two compounds underwent grinding, the colors of emission shifted from yellow-green to yellow and then orange-yellow, respectively. Accordingly, the emission peaks of the two samples were red-shifted from 500 to 505 nm to 563 and 581 nm, respectively. Moreover, after dichloro-methane vapor fuming, the emission colors and wavelengths of the ground powders were restored to conditions that were similar to those of the as-prepared powders. Powder X-ray diffraction revealed that the transformation between the crystalline and amorphous states following external stimuli was responsible for the MFC charac-teristics of XA-CE-CAR and XA-CE-BCAR. The reduced gap between bands resulting from the planar intra-molecular charge transfer as well as an increase in pi-pi interactions, exciton coupling, and orbital overlap between neighboring molecules are the reasons for the redshifts in the photoluminescence spectra following grinding.