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

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 H-1 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 the [CTC-F-1] complex underwent an intracomplex proton transfer from N-H fragment of the host (CTC) to the anion producing a distinctly emissive (cyan colored fluorescence) receptor [CTC](2-). (C) 2015 Elsevier B.V. All rights reserved.