Colorimetric Fluoride Ion Sensing by Boron-Containing π-Electron Systems

The boron-containing π-conjugated systems, including tri(9-anthryl)borane (1) and tris[(10-dimesitylboryl)-9-anthryl]borane (2), have been investigated as a new type of fluoride chemosensor. Upon complexation of 1 with a fluoride ion, a significant color change from orange to colorless was observed and, in the UV−visible absorption spectra, the characteristic band of 1 at 470 nm disappeared and new bands around 360−400 nm assignable to π−π* transitions of the anthryl moieties were observed. This change can be rationalized as a result of the interruption of the π-conjugation extended through the vacant p-orbital of the boron atom by the formation of the corresponding fluoroborate. The binding constant of compound 1 with the fluoride ion was quite high [(2.8 ± 0.3) × 105 M-1], whereas 1 only showed small binding constants with AcO- and OH- of around 103 M-1 and no sensitivity to other halide ions such as Cl-, Br-, and I-, thus demonstrating its selective sensing ability to the fluoride ion. In contrast to the monoboron system 1, compound 2 having four boron atoms showed multistage changes in the absorption spectra by the stepwise complexation with fluoride ions.