Hydroxychalcone dyes that serve as color indicators for pH and fluoride ions

A chalcone, which is composed of two aromatic rings bridged by an α,β-unsaturated carbonyl group, exhibits a variety of biological activities. With an objective to develop a novel chalcone-based functional dye, we have synthesized a chalcone diol CLN1, bearing two OH groups at the 2-positions on both phenyl rings, as well as reference compounds CLN2-6, and found that it serves as color indicators for pH and fluoride ions. CLN1 showed a vivid color change from colorless to yellow (halochromism) in water at pH ≥ 10. Furthermore, it presented a selective color change from colorless to red upon the addition of TBAF in an organic solvent such as CH₃CN. CLN1 provided a strong red-shifted absorption band in the visible region under alkaline conditions in water and upon the addition of TBAF in CH₃CN. The absorption spectral study together with TD-DFT calculations and X-ray crystallographic analysis revealed that the characteristic π-resonant structures of CLN1 caused by the ionization or OH-F^- interactions and the planar conformation due to its intramolecular hydrogen bonding may provide a strong charge transfer (CT) absorption in the visible region.