A colorimetric and ratiometric fluorescent chemosensor for fluoride based on proton transfer

N-Phenyl-N'-(3-quinolinyl)urea (1) has been developed as a highly selective colorimetric and ratiometric fluorescent chemosensor for fluoride ion based on a proton transfer mechanism. Evidences for the mechanism were provided by UV-vis and fluorescence titration and especially ¹H and ¹⁹F NMR experiments. The sensor gave the largest ratiometric fluorescent response reported so far (R _max/R _min=2620) to fluoride. Taking H ⁺ as the "recovering reagent", the sensor can be reversibly "used" and "recovered" for several cycles with only a slight decay of the response ability.