Mechanochromic luminescence of AIEE-active tetraphenylethene-containing cruciform luminophores

Abstract

Two donor–acceptor cruciform luminophores MB-CTPEEB and DFB-CTPEEB with twisted molecular conformations have been prepared. It was found that both cruciforms showed unique intramolecular charge transfer (ICT) emission due to the existence of tetraphenylethene with electron-donating ability and dicyanovinylbenzene with electron-withdrawing ability. Moreover, MB-CTPEEB and DFB-CTPEEB bear obvious aggregation-induced enhanced emission (AIEE) properties and good solid state fluorescence emission with high solid state luminescence efficiency of 0.633 and 0.783, respectively. In particular, both luminophores exhibited substituent-dependent reversible mechanofluorochromic (MFC) behavior. Under external force stimuli, the emission color of DFB-CTPEEB powders changes from initial bright yellowish green (517 nm) to final orange (583 nm), thus large spectral shift of 66 nm, which originates from the planarization of the molecular conformation and subsequent planar intramolecular charge transfer (PICT) caused by destruction of the crystals in the as-prepared samples under external force, is obtained. By sharp contrast, MB-CTPEEB exhibits no MFC behavior. The reason is that MB-CTPEEB possessed less twisted molecular conformation and weaker ICT degree than that of DFB-CTPEEB, which endows the MB-CTPEEB molecules with strong intermolecular forces and close packing, the small deformability and relative weak PICT ability under external force.