Aggregation-induced emission and mechanofluorochromism of tetraphenylbutadiene modified beta-ketoiminate boron complexes

Two new benzo[d]oxazole- and benzo[d]thiazole-fused beta-ketoiminate boron complexes (BF2-TBO and BF2-TBT) modified by tetraphenylbutadiene have been designed and successfully prepared, and their intramolecular charge-transfer (ICT), aggregation-induced emission (AIE) and mechanofluorochromic (MFC) properties were investigated. The results showed that the two D-pi-A type compounds exhibited typical ICT emission, evident AIE characteristics (alpha(AIE) = 89 and 33, respectively), and high state emission intensity (up to 0.445 and 0.367, respectively). More importantly, BF2-TBO and BF2-TBT showed the reversible MFC behavior with their emitting colors changing from yellowish green (located at 528 and 530 nm) to yellow and orange (located at 552 and 572 nm) upon grinding, respectively, with red-shifts of 24 and 42 nm were obtained. The X-ray diffraction analyses confirmed that the MFC properties of BF2-TBO and BF2-TBT should be derived from a phase transition between crystalline and amorphous states. It was noticed that benzo[d]thiazole-fused BF2-TBT showed more contrasting MFC behavior relative to benzo[d]oxazole-fused BF2-TBO. The reason is that the existence of sulfur atom in BF2-TBT could lead to a larger degree of ICT in the excited state than that of BF2-TBO, which endowed it with greater planar intramolecular charge transfer (PICT) after grinding, resulting in a larger red-shift in the PL spectrum.