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

Abstract

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 x 10(4) M-1 that is much greater than that of a neutral borane 2a (K = 4.2 x 10(2) M-1). DFT calculation results suggest that the absorption process in the fluoroborate (2F) is involved with pi(Mes)->pi*(phenylbenzothiazolium) intramolecular charge transfer and the greater elevation of the pi(Mes) donor level is responsible for the turn-on colorimetric response of [2](+). (C) 2012 Elsevier B.V. All rights reserved.