Novel acridine-based thiosemicarbazones as “turn-on” chemosensors for selective recognition of fluoride anion: a spectroscopic and theoretical study

New thiosemicarbazide linked acridines 3a-c were prepared and investigated as chemosensors for the detection of biologically and environmentally important anions. The compounds 3a-c were found selective for fluoride (Fˉ) with no affinity for other anions i.e. ˉOAc, Brˉ, Iˉ, HSO4ˉ, SO4ˉ2, PO4ˉ3, ClO3ˉ, ClO4ˉ, CNˉ, and SCNˉ. Further, upon the gradual addition of fluoride anion (Fˉ) source (TBAF), a well-defined change in color of solution of probes 3a-c was observed. The anion sensing process was studied in detailed via UV-Vis absorption, fluorescence and 1H NMR experiments. Moreover, during the synthesis of acridine probes 3a-c nickel fluoride (NiF2), a rarely explored transition metal fluoride salt was used as catalyst. Theoretical studies via density functional theory (DFT) were also carried out to further investigate the sensing and anion (Fˉ) selectivity pattern of these probes.