Isatin-3-thiosemicarbazone as Chromogenic Sensor for the Selective Detection of Fluoride Anion

Abstract In this study, we describe the anion recognition ability of isatin-3-thiosemicarbazone 2, which contains two different anion recognition units i.e. isatin NH and the thiourea moiety. Both have the ability to act as proton donors. Most importantly, a significant colour change of 2 was observed (from light yellow to reddish orange) in organic medium only after the addition of the F– anion. No such colour change could be observed for any other anions including Cl–, Br–, I–, H2PO4−, ${{\text{H}}_{2}}\text{P}{{\text{O}}_{4}}^{-},$ NO2−, $\text{N}{{\text{O}}_{\text{2}}}^{-},$ PF4− $\text{P}{{\text{F}}_{4}}^{-}$etc. The UV-Vis spectroscopic studies also indicate the potential of this compound for selective detection of fluoride anions. 1H-NMR titrations clearly indicate the formation of the 2.F– anionic complex. The Density-functional theory (DFT) calculations are also performed to get further insights on the formation of 2.F– complex.