New Phosphorescent Polynuclear Cu(I) Compounds Based on Linear and Star-Shaped 2-(2‘-Pyridyl)benzimidazolyl Derivatives: Syntheses, Structures, Luminescence, and Electroluminescence

Four dinuclear and trinuclear Cu(I) complexes that contain 2-(2‘-pyridyl)benzimidazolyl derivative ligands including
1,4-bis[2-(2‘-pyridyl)benzimidazolyl]benzene (1,4-bmb), 1,3-bis[2-(2‘-pyridyl)benzimidazolyl]benzene (1,3-bmb), 1,3,5-tris[2-(2‘-pyridyl)benzimidazolyl]benzene (tmb), and 4,4‘-bis[2-(2‘-pyridyl)benzimidazolyl]biphenyl (bmbp) have been
synthesized. The formulas of these complexes are [Cu2(1,4-bmb)(PPh3)4][BF4]2 (1), [Cu2(1,3-bmb)(PPh3)4][BF4]2
(2), [Cu3(tmb)(PPh3)6][BF4]3 (3), and [Cu2(bmbp)(PPh3)4][BF4]2 (4), respectively. The crystal structures of 2−4 have
been determined by single-crystal X-ray diffraction analyses. The Cu(I) ions in the complexes have a distorted
tetrahedral geometry. For 3, two structural isomers (syn and anti) resulted from two different orientations of the
three 2-(2‘-pyridyl)benzimidazolyl chelating units were observed in the crystal lattice. Variable-temperature 1H NMR
experiments established the presence of syn and anti isomers for 1−3 in solution which interconvert at ambient
temperature. Complexes 1−4 have a weak MLCT absorption band in the 350−450 nm region and display a yellow-orange emission when irradiated by UV light. One unexpected finding is that the yellow-orange emission of complexes
1−4 has a very long decay lifetime (∼200 μs) at 77 K. An electroluminescent (EL) device using 4 as the emitter
and PVK as the host was fabricated. However, the long decay lifetime of the copper complexes may limit their
applications as phosphorescent emitters in EL devices.

本研究成功合成了4种含2-(2'-吡啶基)苯并咪唑衍生物配体的双核与三核铜(I)配合物，所使用的配体包括1,4-双[2-(2'-吡啶基)苯并咪唑基]苯（1,4-bmb）、1,3-双[2-(2'-吡啶基)苯并咪唑基]苯（1,3-bmb）、1,3,5-三[2-(2'-吡啶基)苯并咪唑基]苯（tmb）以及4,4'-双[2-(2'-吡啶基)苯并咪唑基]联苯（bmbp）。上述配合物的分子式依次为[Cu₂(1,4-bmb)(PPh₃)₄][BF₄]₂（编号1）、[Cu₂(1,3-bmb)(PPh₃)₄][BF₄]₂（编号2）、[Cu₃(tmb)(PPh₃)₆][BF₄]₃（编号3）以及[Cu₂(bmbp)(PPh₃)₄][BF₄]₂（编号4）。通过单晶X射线衍射分析解析了配合物2、3、4的晶体结构。配合物中的铜(I)离子均呈现畸变四面体配位几何构型。针对配合物3，其晶体晶格中存在两种结构异构体：由三个2-(2'-吡啶基)苯并咪唑螯合单元的不同取向所形成的顺式（syn）与反式（anti）异构体。变温¹H核磁共振实验证实，配合物1、2、3在溶液中存在顺式与反式异构体，且二者在室温下可相互转化。配合物1至4在350~450 nm波长范围内均存在弱的金属到配体电荷转移（Metal-to-Ligand Charge Transfer, MLCT）吸收带，且在紫外光激发下可发射橙黄色光。一项意外的研究发现是，配合物1至4的橙黄色发射光在77 K下具有极长的衰减寿命（约200 μs）。研究人员制备了以配合物4为发光掺杂剂、聚乙烯基咔唑（PVK）为主体材料的电致发光（Electroluminescent, EL）器件。然而，该类铜(I)配合物较长的衰减寿命可能限制其作为磷光发射材料在电致发光器件中的应用。