A small-molecular europium complex with anion sensing sensitivity

Abstract

A novel small-molecular europium(III) complex with quinoline derivative has been designed and prepared for anion recognition. This Eu³⁺ complex displays striking emission changes at 616 nm in response to NO₃⁻ and Cl⁻. What's more, the anion-selective luminescence enhancement is easily detected by the naked eye. UV, luminescence titration experiments and the Job plot analysis revealed that the ligand can form stable 1 : 1 complexes with Eu(NO₃)₃ or EuCl₃ in CH₃CN; log K = 7.10 and 6.76 (295 K), respectively. The crystal structure shows that three oxygen atoms and one nitrogen atom of the ligand are approximately coplanar and wrap around the Eu³⁺ ion through a semicircle-shaped coordination structure and some unoccupied sites of the central europium ions are available for guest anions in the complexes. Thus, three nitrate ions are located in the gap of the semicircle. It is supposed that the mechanism of luminescence enhancement is of NO₃⁻ or Cl⁻ anions squeezing out solvent molecules from the inner coordination sphere.