Synthesis and characterization of M(II) (M = Cd, Hg and Pb) complexes with naphthodiaza-crown macrocyclic ligand and study of metal ion recognition by fluorescence, 1H NMR spectroscopy, and DFT calculation

A Unique Sensitive and Highly Selective Fluorescent Naphthodiaza-Crown Macrocyclic Ligand Chemosensor for Hg2+ in Water

The noticeable enhancement in fluorescence emission of O₂N₂-donor naphthodiaza-crown macrocyclic ligand (L) in the presence of Hg²⁺ was observed in which the fluorescence quantum yield of free ligand L as well as L/Hg²⁺ complex were found to be as 0.29 and 0.49, respectively. The observed ultra-low limit of detection (LOD) for Hg²⁺ by L was determined as low as 1.0 × 10^-11 M in water. A 1:1 stoichiometry was also established for L/Hg²⁺ together with a binding constant K _BH = 66,543 by employing fluorescence spectrophotometry. The competition experiments on L/Hg²⁺ demonstrated highly selective detection of Hg²⁺ in the presence of the library cations. A two path mechanism for detection of metal ion in terms of coordination of metal ion to L and/or the formation of counter ion was proposed by using of ¹H NMR and fluorescence spectroscopy. Graphical Abstract pH dependence mechanism of interaction between Hg²⁺ and macrocyclic ligand L.