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.