Synthesis, Characterization, and Photophysical Properties of Luminescent Gallium and Indium Complexes Constructed using Tridentate 6-Azolyl-2,2′-bipyridine Chelates

Three systematically functionalized 6-azolyl-2,2′-bipyridine ligands were prepared from reactions initiated by 6-cyano-2,2′-bipyridine. These ligands readily reacted with the metal alkyl reagents GaMe3 and InMe3 to afford the pentacoordinate complexes [(fpzbpy)MMe2] (1a, M = Ga; 1b, M = In), [(ftzbpy)MMe2] (2a, M = Ga; 2b, M = In), and [(N4bpy)MMe2] (3a, M = Ga; 3b, M = In), in which (fpzbpy)H, (ftzbpy)H, and (N4bpy)H denote 6-pyrazolyl-, 6-triazolyl-, and 6-tetrazolyl-substituted 2,2′-bipyridine, respectively. These complexes exhibited moderate blue-green emission ranging from 412 to 493 nm. On the other hand, treatment of the bidentate 2-pyridyl tetrazole ligand (pyN4)H with InMe3 afforded the bridged dimer [Me2In(pyN4)]2 (4). Calculation based on time-dependent density function theory (TDDFT) showed that the S1 state of complexes 1−3 is mainly attributed to an allowed intraligand π → π* electronic transition located at tridentate chelating moieties, together with a small contribution (<10%) of gallium (or indium) → π* (ligand) charge transfer transition. Accordingly, the corresponding emission properties of 1a−3a (or 1b−3b) can be rationalized using the correlation between the substituent effect of azolyl groups and the relative HOMO/LUMO energy level.

以6-氰基-2,2′-联吡啶为起始原料，通过反应制备了三种经系统性功能化修饰的6-唑基-2,2′-联吡啶配体。上述配体可与金属烷基试剂三甲基镓(GaMe3)和三甲基铟(InMe3)发生反应，得到五配位配合物[(fpzbpy)MMe2]（1a，M=Ga；1b，M=In）、[(ftzbpy)MMe2]（2a，M=Ga；2b，M=In）以及[(N4bpy)MMe2]（3a，M=Ga；3b，M=In）；其中(fpzbpy)H、(ftzbpy)H和(N4bpy)H分别代表6-吡唑基取代、6-三唑基取代和6-四唑基取代的2,2′-联吡啶。该系列配合物展现出波长范围为412~493 nm的中等强度蓝绿色发光性能。另一方面，将双齿配体2-吡啶基四唑(pyN4)H与三甲基铟(InMe3)反应，得到桥联二聚体[Me2In(pyN4)]2（4）。基于含时密度泛函理论(time-dependent density function theory, TDDFT)的计算结果表明，配合物1~3的第一单重激发态(S1)主要源自三齿螯合结构单元上的允许配体内π→π*电子跃迁，同时伴随占比不足10%的镓（或铟）→配体π*电荷转移跃迁贡献。据此，可通过唑基取代基的取代效应与最高占据分子轨道/最低未占据分子轨道(HOMO/LUMO)的相对能级之间的相关性，合理阐释1a~3a（或1b~3b）的发光性能。