Mechanofluorochromism of difluoroboron β-ketoiminate boron complexes functionalized with benzoxazole and benzothiazole

New difluoroboron beta-ketoiminate boron complexes bearing benzoxazole (CBO) and benzothiazole (CBS) have been synthesized. It was found that CBO was almost non-emissive in THF, and the emission could be intensified significantly when great amount of H2O was added, illustrating AIE (aggregation-induced emission) property. CBS could not show AIE property in THF/H2O system, but its emission in solid state was also strong. The single crystal structure of CBS suggested that pi-pi interactions and the hydrogen bonds of C(Ar) - F center dot center dot center dot H, C(Ar) - H center dot center dot center dot S and C(Ar) - H center dot center dot center dot Cl would suppress the rotation of single bonds, resulting into obvious emission enhancement. It is interesting that the as-synthesized crystals of CBO and CBS both emitted azure light, and the grinding made their emitting colors to change into cyan and green, which could be recovered under fuming with DCM or heating. The reversible mechanofluorochromism was due to the transformation between crystalline and amorphous states, which could be confirmed from the results of absorption spectra, XRD patterns and DSC curves in different solid states. Additionally, the high-contrast mechanofluorochromism of CBS compared with CBO might be originated from the loose packing in crystalline state and the better pi-electron delocalization.