Eu(III) Complexes of Tetradentate Ligands Related to 2,9-Di(pyrid-20-yl)-1,10-phenanthroline and 2,20-Bi-1,10-phenanthroline

A series of six tetradentate polypyridine-type ligands (L) have been used to prepare the corresponding Eu(III) complexes [Eu(L)2(S)]nþ (n = 2, 3) where S = H2O or CF3SO3 -. Two of the ligands, 2,9-di(pyrid-20-yl)-1,10- phenanthroline (4) and its dipyridophenazine analogue (6) are symmetrical around a central phenanthroline ring. The other four ligands are 2,20-bi-1,10-phenanthroline and its 3,30-di-, tri-, and tetramethylene-bridged analogues (5a-d) whose conformations are governed by the length of the polymethylene bridge. 1H NMR and X-ray analysis indicate that all of the complexes have a C2v symmetry. The biphenanthroline series shows a strong correlation of the conjugation between the two halves of the ligand, as governed by the bridge, with the absorption and emission properties of the Eu(III) complex. The complex having the most distorted, tetramethylene-bridged ligand exhibits a weak, high energy π-π* absorption and low sensitization efficiency. The luminescence decays are monoexponential for complexes of 4 and either monoexponential or biexponential for the complexes of 5 depending on its solution concentration and the length of the bridge. The complexes of 4 exhibit much longer lifetime, higher overall quantum yield, and higher sensitization efficiency than complexes of 5 while the complex of 6 emits very weakly. The Eu(5D0) lifetime for Eu(4)2(H2O)3 is shorter than for Eu(4)2(CF3SO3)2, reflecting the effect of the coordinated water. The complexes are examined for stability in the presence of water and found to retain most of their luminescent properties even in the presence of a large excess of water.