Turn-on colorimetric sensing of fluoride ions by a cationic triarylborane bearing benzothiazolium

A cationic triarylborane Lewis acid bearing benzothiazolium moiety, 2-(4′-dimesitylborylphenyl)-3-methylbenzo[d]thiazol-3-ium ([2]+) was prepared from a neutral borane (2a) and their crystal structures were determined from X-ray diffraction studies. While 2a shows a blue-shift with a small decrease in the UV–vis absorption band upon fluoride binding to the boron atom, [2]+ undergoes a red-shift of the absorption band which tails over visible region, giving rise to the color change of the solution from colorless to yellow. The fluoride binding constant in THF/H2O (9/1, v/v) is calculated to be 1.3 × 104 M−1 that is much greater than that of a neutral borane 2a (K = 4.2 × 102 M−1). DFT calculation results suggest that the absorption process in the fluoroborate (2F) is involved with π(Mes)→π*(phenylbenzothiazolium) intramolecular charge transfer and the greater elevation of the π(Mes) donor level is responsible for the turn-on colorimetric response of [2]+.