The boron-containing 7-conjugated systems, including Lri(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 I 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 rim disappeared and new bands around 360-400 nm assignable to pi-pi* transitions of the anthryl moieties were observed. This change can be rationalized as a result of the interruption of the pi -conjugation extended through the vacant p-orbital of the boron atom by the formation of the corresponding fluoroborate. The binding constant of compound I with the fluoride ion was quite high [(2.8 +/- 0.3) x 10(5) M-1], whereas 1 only showed small binding constants with AcO- and OH- of around 10(3) 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.