Influence of Symmetry on the Luminescence and Radiative Lifetime of Nine-Coordinate Europium Complexes

Homoleptic mononuclear nine-coordinate lanthanum(III) and europium(III) tris-complexes [Ln(N^N^O)3]·nH2O with two tridentate N-benzyl-benzimidazole pyridine-2-carboxylates exhibit a rare C3-symmetry of the lanthanide coordination polyhedron in the solid state, as confirmed by luminescence spectroscopy and by X-ray crystallography (the three N^N^O ligands are arranged "up-up-up" around the lanthanide ion). The symmetry, however, is changed to the more-common C1 upon dissolution of the complexes in dichloromethane, as revealed by luminescence spectroscopy (the three ligands are arranged "up-up-down"). The new europium complexes emit efficient ligand-sensitized metal-centered luminescence with excited-state lifetimes of 1.56-2.18 ms and quantum yields of 25-41% in the solid and in the solution. The change of the symmetry from (a higher) C3 to (a lower) C1 alters the luminescence spectrum, shortens the radiative lifetime, and increases the luminescence efficiency of the europium complexes