Bright mono-aqua europium complexes based on triazacyclononane that bind anions reversibly and permeate cells efficiently

A series of five europium(III) complexes has been prepared from heptadentate N₅O₂ ligands that possess a brightness of more than 10 mM^-1 cm^-1 in water, following excitation over the range λ=330-355 nm. Binding of several oxy anions has been assessed by emission spectral titrimetric analysis, with the binding of simple carboxylates, lactate and citrate involving a common ligation mode following displacement of the coordinated water. Selectivity for bicarbonate allows the rapid determination of this anion in human serum, with K_d=37 mM (295 K). The complexes are internalised quickly into mammalian cells and exhibit a mitochondrial localisation at early time points, migrating after a few hours to reveal a predominant lysosomal distribution. Herein, we report the synthesis and complexation behaviour of strongly emissive europium (III) complexes that bind oxy-anions in aqueous media with an affinity and selectivity profile that is distinctively different from previously studied systems. Let there be light! A new class of luminescent anion-responsive europium complexes, which enter mammalian cells and accumulate in the lysosomes, has been synthesized. Each Eu^III complex is based on a triazacyclononane core with two alkynyl-aryl sensitizers and one coordinated water molecule (see figure).