Selective colorimetric and fluorometric sensing of Cu(II) by iminocoumarin derivative in aqueous buffer

Abstract

A new iminocoumarin based receptor L (C₂₇H₂₆N₄OS) is synthesized with pyridyl and benzothiazolyl functionality. Synthesis of L is easy and it is isolated in good yield. L shows a selective and distinct color change from yellow to orange with Cu²⁺ over Li⁺, Na⁺, Ca²⁺, Mg²⁺, Cr²⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Pb²⁺, and Ag⁺ whereas a slight change in color is also observed in the case of Hg²⁺ but L shows selective fluorescent quenching only in the presence of Cu²⁺ in aqueous HEPES buffer (pH 7.0). The naked eye detection limit of Cu²⁺ is determined at 2 μM whereas an emission experiment shows a lower detection limit at 200 nM. Selectivity studies of L in presence of 50 equivalents of other ion(s) by emission experiment show no interference toward the detection of 1 equivalent of Cu²⁺. Both UV-Vis and fluorescence studies in the presence of Cu²⁺-salts of different counter anions with various sizes and shapes (Cl⁻, ClO₄⁻, NO₃⁻, CF₃SO₃⁻, SO₄²⁻ and BF₄⁻) show almost similar spectral output in buffer media irrespective of the nature of the counter anions. The detailed UV-Vis and fluorescence titration experiments suggest the existence of both 1 : 1 and 2 : 1 (L : Cu²⁺) complexation stoichiometry and EPR study shows dx²_-y² ground state of the Cu(II) centre in the complex. Furthermore the formation of a mononuclear [Cu(L)(CH₃CN)].2ClO₄ complex and the flexible conformation of L in the solid state are confirmed by the single-crystal X-ray structural study.