Simple and efficient coumarin-based colorimetric and fluorescent chemosensor for F− detection: An ON1–OFF–ON2 fluorescent assay

Abstract

New, simple and inexpensive receptors based on coumarin-thiocarbohydrazone (CTC) and coumarin-carbohydrazone (CC) were synthesized for fluoride ion in CH3CN/DMSO. Receptor CTC showed specific selectivity and sensitivity toward fluoride giving rise to an ON1–OFF–ON2 fluorescent response and visual colorimetric change. The recognition mechanism was investigated using UV–vis, fluorescence spectroscopy, and 1H NMR titration experiments. The Job's plot analysis and Benesi–Hildebrand equation revealed that the binding between the CTC or CC with F− is in 1:1 stoichiometry. Formation of such 1:1 complex led to the quenching of green fluorescence of CTC and upon excess addition of F−, the [CTC–F−] complex underwent an intracomplex proton transfer from NH fragment of the host (CTC) to the anion producing a distinctly emissive (cyan colored fluorescence) receptor [CTC]2−.