Near-Infrared Luminescence of Nine-Coordinate Neodymium Complexes with Benzimidazole-Substituted 8-Hydroxyquinolines

A new class of benzimidazole-substituted 8-hydroxyquinoline ligands has been prepared that contain a monoanionic tridentate N,N,O-coordinating unit. These ligands form charge-neutral lanthanide complexes of the type [Ln(L-R)3]·nH2O or [Ln2(L2)3]·nH2O (n = 1−3) with early lanthanides from LaIII to GdIII inclusive. Crystallographic characterization was carried out for 11 complexes with 6 different ligands. In all of these structures, the lanthanide ion was found to be nine-coordinated by three ligands arranged in an “up−up−down” fashion around the metal center. The coordination environment can be described as a tricapped trigonal prism, with N atoms of quinoline rings occupying capping positions. Upon deprotonation of the ligand and complex formation, a new absorption band appears in the visible range of the spectrum with a maximum in the range of 466−483 nm and molar absorption coefficient of (7.2 − 18) × 103 M−1 cm−1. Its origin is likely to be an intraligand phenolate-to-pyridyl charge transfer transition centered on the 8-hydroxyquinolate chromophore. Upon excitation in ligand absorption bands, new NdIII complexes display characteristic metal-centered luminescence in the near-infrared region from 850 to 1450 nm with quantum yields and lifetimes in solid state at room temperature as high as 0.34% and 1.2 μs, respectively.

本研究制备了一类新型苯并咪唑取代的8-羟基喹啉配体，该类配体带有单阴离子三齿N,N,O配位单元。此类配体可与从三价镧（LaIII）到三价钆（GdIII，包含钆在内）的轻稀土形成电荷中性的稀土配合物，配合物类型为[Ln(L-R)3]·nH2O或[Ln2(L2)3]·nH2O（n = 1−3）。研究对6种不同配体对应的11种配合物开展了晶体结构表征。在所有上述配合物的晶体结构中，稀土离子均为九配位，三个配体围绕金属中心以“上−上−下”的空间构型排布。其配位环境可被描述为三帽三角棱柱结构，喹啉环的氮原子占据帽位。当配体发生去质子化并形成配合物后，光谱的可见光区域会出现新的吸收带，其吸收峰波长介于466−483 nm范围内，摩尔消光系数为(7.2 − 18) × 103 M−1 cm−1。该吸收带的起源大概率为以8-羟基喹啉发色团为中心的配体内酚盐到吡啶基电荷转移跃迁。在配体吸收波段激发后，新型三价钕（NdIII）配合物可在850至1450 nm的近红外区域展现出特征的金属中心发光；室温固态条件下，该类配合物的量子产率最高可达0.34%，荧光寿命最高可达1.2 μs。