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 boolean AND N boolean AND O)(3)]center dot nH(2)o with two tridentate N-benzylbenzimidazole pyridine-2-carboxylates exhibit a rare C-3-symmetry of the lanthanide coordination polyhedron in the solid state, as confirmed by luminescence spectroscopy and by X-ray crystallography (the three N boolean AND N boolean AND O ligands are arranged up-up-up around the lanthanide ion). The symmetry, however, is changed to the more common C-1 upon dissolution of the complexes in dichloromethane, as revealed by luminescence spectroscopy (the three ligands are likely to be 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 solution. The change of the symmetry from (a higher) C-3 to (a lower) C-1 alters the luminescence spectrum, shortens the radiative lifetime, and increases the luminescence efficiency of the europium complexes.