Thiosemicarbazone-based bifunctional chemosensors for simultaneous detection of inorganic cations and fluoride anion

Abstract Two thiosemicarbazone-based bifunctional chemosensors (L1 and L2) have been synthesized and characterized by various spectroscopic techniques. Sensing behavior of the chemosensors were initially tested by colorimetric signalling, followed by absorption, emission, NMR and CV techniques, and found to behave as bifunctional molecules. These bifunctional chemosensors recognize selected cations (Mn2+, Co2+, Ni2+, Cu2+ and Zn2+) and F¯ anion among the studied cations (Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Ag+, Hg2+ and Pb2+) and anions (F¯, Cl¯, Br¯, I¯ and HSO4¯). Both the sensor molecules displayed remarkable colorimetric and fluorimetric responses in the presence of cations and fluoride anion with a rapid response through color change of the solution. Nitrogen and thione donors of the receptor molecules are liable for cation binding, whereas –NH protons of thiosemicarbazone moiety was found to be responsible for selective fluoride ion sensing. Binding ability of the chemosensors toward cations and F¯ anion were determined with spectrophotometric data by calibration plot, and the binding stoichiometry of the complexes was studied by Job’s plot. Electrochemical measurements showed that the addition of anion to the chemosensors pretreated with cations resulted in greater changes in reduction potentials compared to the addition of anion to the receptors in the absence of cations. Computational studies using density functional theory were also carried out, whereupon comparison of the interactions of copper cation and fluoride anion with the chemosensors explains the selective detection towards respective ions.