Eu(III) Complexes of Tetradentate Ligands Related to 2,9-Di(pyrid-2′-yl)-1,10-phenanthroline and 2,2′-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)]n+ (n = 2, 3) where S = H2O or CF3SO3−. Two of the ligands, 2,9-di(pyrid-2′-yl)-1,10-phenanthroline (4) and its dipyridophenazine analogue (6) are symmetrical around a central phenanthroline ring. The other four ligands are 2,2′-bi-1,10-phenanthroline and its 3,3′-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)](ClO4)3 is shorter than for [Eu(4)2(CF3SO3)](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.

本研究以6种四齿聚吡啶类配体（tetradentate polypyridine-type ligands，简称L）为原料，制备了对应的铕(III)配合物[Eu(L)₂(S)]ⁿ⁺（n=2、3），其中S为H₂O或三氟甲磺酸根（CF₃SO₃⁻）。其中两种配体分别为2,9-二(2'-吡啶基)-1,10-菲咯啉（化合物4）及其联吡啶吩嗪类似物（化合物6），二者均以中心菲咯啉环为对称轴呈对称结构。剩余四种配体为2,2'-联-1,10-菲咯啉及其3,3'-亚甲基、三亚甲基、四亚甲基桥联类似物（5a-d），这类配体的构象由聚亚甲基桥链的长度决定。通过核磁共振氢谱（¹H NMR）与X射线衍射分析可知，所有目标配合物均具有C₂v点群对称性。联菲咯啉系列配体的两个半部分之间的共轭程度由桥链长度调控，其与铕(III)配合物的吸收及发射性能存在显著相关性。采用扭曲程度最高的四亚甲基桥联配体的配合物，其π-π*高能吸收峰强度较弱，且敏化效率较低。化合物4对应的配合物的发光衰减为单指数过程；而化合物5对应的配合物的发光衰减则为单指数或双指数过程，具体取决于溶液浓度与桥链长度。相较于化合物5对应的配合物，化合物4的配合物具有更长的发光寿命、更高的总量子产率以及更优的敏化效率；而化合物6的配合物发光强度极弱。配合物[Eu(4)₂(H₂O)](ClO₄)₃的Eu(⁵D₀)发光寿命短于[Eu(4)₂(CF₃SO₃)](CF₃SO₃)₂，这体现了配位水的影响。本研究对配合物在水环境中的稳定性进行了测试，结果显示即使在大量水存在的条件下，配合物仍能保留大部分发光性能。