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)_<i>x</i>] 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 (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