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

Abstract

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 dichloromethane 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 characteristics of XA-CE-CAR and XA-CE-BCAR. The reduced gap between bands resulting from the planar intramolecular charge transfer as well as an increase in π–π interactions, exciton coupling, and orbital overlap between neighboring molecules are the reasons for the redshifts in the photoluminescence spectra following grinding.