Tridentate Benzimidazole-Pyridine-Tetrazolates as Sensitizers of Europium Luminescence

We report on new anionic tridentate
benzimidazole-pyridine-tetrazolate ligands that form neutral 3:1 complexes
with trivalent lanthanides. The ligands are UV-absorbing chromophores
that sensitize the red luminescence of europium with energy-transfer
efficiency of 74–100%. The lifetime and quantum yield of the
sensitized europium luminescence increase from 0.5 ms and 12–13%
for the as-prepared solids to 2.8 ms and 41% for dichloromethane solution.
From analysis of the data, the as-prepared solids can be described
as aqua-complexes [Ln­(κ3-ligand)2(κ1-ligand)­(H2O)x] where
the coordinated water molecules are responsible for the strong quenching
of the europium luminescence. In solution, the coordinated water molecules
are replaced by the nitrogen atoms of the κ1-ligand
to give anhydrous complexes [Ln­(κ3-ligand)3] that exhibit efficient europium luminescence. X-ray structures
of the anhydrous complexes confirm that the lanthanide ion (LaIII, EuIII) is nine-coordinate in a distorted tricapped
trigonal prismatic environment and that coordination of the lanthanide
ion by tetrazolate is weaker than by carboxylate.