A Cl− anion-responsive luminescent Eu3+ complex with a chiral tripod: ligand substituent effects on ternary complex stoichiometry and anion sensing selectivity

Abstract

A new series of N₃,O-mixed donor tripods was prepared for luminescent Eu³⁺ complexes, in which the soft quinoline nitrogen, tertiary amine nitrogen, and hard amide oxygen donors were cooperatively involved. The mixed donor tripods formed more stable 1 : 1 complexes with Eu(NO₃)₃, La(NO₃)₃ and Tb(NO₃)₃ than the corresponding N₄ donor tripods, and their Eu³⁺ complexes particularly exhibited anion-responsive luminescence properties. NMR, UV, and luminescence spectroscopic characterizations revealed that –CH₃ substitution on the tripod skeleton remarkably altered the preferred stoichiometry of the “tripod–Eu³⁺–anion” ternary complex and gave anion-dependent europium luminescence. Although the disubstituted tripod preferred to form non-luminescent 2 : 1 (tripod : Eu³⁺) complexes with Eu(NO₃)₃ and other salts, it formed a luminescent 1 : 1 complex with EuCl₃. Thus, this type of tripod offered Cl⁻ anion-selective luminescence enhancement that was easily observed by the naked eye.