Unique twisted donor-acceptor cruciform π-architectures exhibiting aggregation-induced emission and stimuli-response behaviors

Two twisted pi-conjugated cruciform luminophores MDCS-TPA and FMDCS-TPA containing two conjugated donor and acceptor groups intersecting at the central benzene core have been rationally designed and successfully prepared. The two compounds display unique intramolecular charge-transfer (ICT) process from electron-rich triphenylamine and electron-poor cyanostyryl units, which are evidenced by theoretical calculations and spectral analysis. MDCS-TPA and FMDCS-TPA exhibit AIE effects with high fluorescence quantum yields of 0.321 and 0.312 in the solid state. Interestingly, MDCS-TPA and FMDCS-TPA possess the distinct mechanofluorochromic (MFC) behavior. The as-prepared powders of the two luminophores could emit strong green (491 nm) and yellow-green fluorescence (529 nm), and the fluorescence color changed into yellow (542 nm) and orange (576 nm) after grinding, respectively, giving the red shifts of 51 nm and 47 nm. Such mechanochromism is reversible by repeating both the grinding-fuming and grinding-annealing processes. The MFC behavior of MDCS-TPA and FMDCS-TPA originates from the transition between the crystalline and amorphous states and the red-shifts induced by grinding is the result of the planarization of the molecular conformation and subsequent planar intramolecular charge transfer (PICT) under the external force stimuli.