A chelation enhanced selective fluorescence sensing of Hg²⁺ by a simple quinoline substituted tripodal amide receptor

Abstract

Two tris(2-aminoethyl)amine (tren) based tripodal amide fluoroionophores, 1 and 2, functionalized with quinoline (chelating fluorophore) and naphthalene (non-chelating fluorophore) respectively, are synthesized in good yields. Fluoroionophore 1 shows a selective UV-Vis spectral shift in the case of Hg²⁺ in acetonitrile among different metal ions like Li⁺, Na⁺, Ca²⁺, Mg²⁺, Cr²⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Hg²⁺, Pb²⁺, and Ag⁺. On the other hand, fluoroionophore 2 shows no selectivity towards any of the above metal ions in the UV-Vis study. Furthermore, 1 shows a selective chelation induced fluorescence enhancement in the presence of Hg²⁺ whereas 2 shows the enhancement of fluorescence with most of the metal ions via a photoinduced charge transfer mechanism. The naked eye detection of Hg²⁺ in an acetonitrile solution of 1 shows a greenish fluorescence upon UV light irradiation. The isolated Hg²⁺ complex of 1, 3, shows a similar UV-Vis and fluorescence spectral output as observed from in situ spectroscopic studies of 1 in the presence of Hg²⁺. Infra-red (IR) and ¹H- NMR studies also reveal the interaction of Hg²⁺ with the quinoline nitrogen atoms as well as with the amide functionality.