Rationally designed donor-acceptor dicyanoethylene derivatives functionalized with carbazole for obvious aggregation-induced emission and reversible high-contrast mechanofluorochromism

Abstract

Two mechanofluorochromic (MFC) molecules, which were labeled as 2-substituted carbazole (PY-CE-C2) and 3-substituted carbazole (PY-CE-C3), respectively, featuring substantially twisted molecular configurations, were synthesized by strategically combining electron-rich carbazole groups with electron-deficient dicyanoethylene and pyrene units. Both luminogens display representative intramolecular charge transfer (ICT) properties. Importantly, PY-CE-C2 and PY-CE-C3 display prominent aggregation-induced emission (AIE) with AIE coefficients of 2.3 and 14.9, respectively, and remarkable MFC behavior with excellent reversibility, which were regulated by the position of substitution of the carbazole unit. In their as-prepared solid state, they emit strong yellow and yellow-green fluorescence, with solid-state fluorescence efficiencies of 0.372 and 0.312, respectively. Upon grinding, the fluorescence colors shift to red and orange-red, exhibiting emission maxima shifts from 575 nm to 538 nm–632 nm and 610 nm, respectively. PXRD analysis indicated that the reversible MFC characteristics of PY-CE-C2 and PY-CE-C3 is primarily due to the interconversion between crystalline and amorphous states. The red shift in fluorescence spectra results from a reduced band gap, induced by expanded π-conjugation, enhanced PICT effects, strengthened π-π stacking, excitonic interactions, and increased orbital overlap between neighboring compounds.