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 (F(-)) with no affinity for other anions, i.e. (-)OAc, Br(-), I(-), HSO(4)(-), SO(4)(2-), PO(4)(3-), ClO(3)(-), ClO(4)(-), CN(-) and SCN(-). Further, upon the gradual addition of a fluoride anion (F(-)) source (tetrabutylammonium fluoride), a well-defined change in colour of the solution of probes 3a-c was observed. The anion-sensing process was studied in detail via UV-visible absorption, fluorescence and (1)H-NMR experiments. Moreover, during the synthesis of acridine probes 3a-c nickel fluoride (NiF(2)), a rarely explored transition metal fluoride salt, was used as the catalyst. Theoretical studies via density functional theory were also carried out to further investigate the sensing and anion (F(-)) selectivity pattern of these probes.