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(k(3)-ligand)(2)(K-1-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 K-1-ligand to give anhydrous complexes [Ln(K-3-ligand)(3)] that exhibit efficient europium luminescence. X-ray structures of the anhydrous complexes confirm that the lanthanide ion (La-III, Eu-III) is nine-coordinate in a distorted tricapped trigonal prismatic environment and that coordination of the lanthanide ion by tetrazolate is weaker than by carboxylate.