Analysis of Anion Binding Effects on the Sensitized Luminescence of Macrocyclic Europium(III) Complexes

Abstract

Four triazamacrocyclic and two tetraazamacrocyclic ligands carrying two and three coordinating donor groups, respectively, and a carbostyril light-harvesting antenna were synthesized. The antenna was linked to a macrocycle either via secondary or tertiary amide. Complexation with europium (Eu), gadolinium (Gd), terbium (Tb), and ytterbium (Yb) yielded overall + 1 or + 2 charged species. Paramagnetic 1 H NMR, and steady-state and time-resolved luminescence spectroscopies showed that the complexes had 1-3 inner-sphere water molecules and displayed a variety of coordination geometries in solution. The antennae sensitized Eu(III) and Tb(III) emission with quantum yields of 0.3-4.3 % and 9.9-24.5 %, respectively. The addition of excess fluoride or cyanide to buffered Eu(III) complex solutions resulted in anion-dependent changes. Fluoride addition increased the Eu(III) luminescence intensity by displacing all inner-sphere water molecules and stabilizing the + 3 oxidation state of Eu. Eu(III) luminescence increased up to 25-fold for one emitter. Cyanide quenched Eu(III) luminescence in all cases despite partial water ligand displacement.

A Primer on Luminescence Sensing

“…In particular, Eu(III) and Tb(III) have attracted great interest, and five articles in the Special Collection describe the development and sensing application of such lanthanide complexes. Borbas et al [20] . synthesized and characterized various lanthanide complexes based on structurally diverse macrocyclic ligands that complex europium (Eu), gadolinium (Gd), terbium (Tb), and ytterbium (Yb) ions.…”

“…In particular, Eu(III) and Tb(III) have attracted great interest, and five articles in the Special Collection describe the development and sensing application of such lanthanide complexes. Borbas et al [20] synthesized and characterized various lanthanide complexes based on structurally diverse macrocyclic ligands that complex europium (Eu), gadolinium (Gd), terbium (Tb), and ytterbium (Yb) ions. In addition to an in-depth photophysical characterization of the Eu and Tb complexes, the authors found that these lanthanide complexes can serve both as labels for sensor development and as direct probes, in which fluoride ions strongly enhanced Eu luminescence, whereas cyanide ions resulted in significant luminescence quenching.…”