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-benzylbenzimidazole
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 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) C3 to (a lower) C1 alters the luminescence
spectrum, shortens the radiative lifetime, and increases the luminescence
efficiency of the europium complexes.