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

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 H-1 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.