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 H-1 and F-19 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.