Tridentate Benzimidazole-Pyridine-Tetrazolates as Sensitizers of Europium Luminescence
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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.
