HBT-based chemosensors for the detection of fluoride through deprotonation process: experimental and DFT studies

Abstract

When searching to develop fluoride chemosensors based on O–H⋯F, we discovered that an HBT-based fluorophore containing a hydroxyl group was easily synthesized and displayed excellent fluorescence properties. 4-(benzothiazol-2-yl)-phenol (L¹H) was found to facilitate the monitoring of fluoride and showed ratiometric fluorescence changes. It is worth noting that an aldehyde group in conjugation with the HBT-based fluorophore core at an adjacent position to the hydroxyl group (i.e. 5-(benzothiazol-2-yl)-2-hydroxybenzaldehyde, L²H) would elevate the sensitivity towards fluoride immensely. Spectroscopic studies indicated that L¹H and L²H interacted with a fluoride anion, which involved a two-step reaction: hydrogen bond formation and deprotonation. Deprotonation of the chemosensors by a fluoride anion enhanced the electron-donating ability of the phenolic O⁻ to the HBT core acceptor and facilitated an intramolecular charge transfer process, resulting in a red shift in both UV-vis absorption and fluorescence spectra. The mechanism of L²H binding with fluoride was confirmed by ¹H NMR titration experiments and DFT computational calculations.