Macrocyclic Eu³⁺ chelates show selective luminescence responses to anions

A series of lanthanide-containing macrocycles, Eu(2)-Eu(5), exhibited unique luminescent responses in the presence of strong hydrogen-bond-accepting anions (F^-, CH₃COO^-, and H₂PO ₄^-) in dimethyl sulfoxide. The macrocycles examined herein were designed to include a lanthanide chelate, aromatic spacers that function as antennae, thiourea groups as anion-binding units, and an alkyl or aryl linker between the thioureas that tailors the size and rigidity of the macrocycle. The anion-induced change in the emission intensity (λ_exc = 272 nm; λ_em = 614 nm) varied across the series of macrocycles and was dependent on the basicity of the anion. The largest luminescence response was observed in Eu(2), whereby the emission increased 77% upon the addition of 8 equiv of fluoride. A change in luminescence was not observed when exciting Eu³⁺ directly (λ_exc = 395 nm) over the course of anion titration experiments with all of the anions studied. These macrocycles contain only slight variations in structure, and insights into the mechanism of the anion interaction have been gained through monitoring of anion titrations via luminescence, absorbance, and luminescence lifetime measurements. In addition, model compounds (2-5) lacking the Eu³⁺ moiety were synthesized to study the binding pockets of Eu(2)-Eu(5) using absorbance and ¹H NMR spectroscopy. These studies indicate that the anions interact with the thiourea moiety of Eu(2)-Eu(5), and the luminescent response is controlled by changes in the morphology of the macrocycle binding pocket.