Design for a Pure-Blue-Emissive Polymer Film through the Selective Perturbation of the Energy Level of the Highest Occupied Molecular Orbital in a Boron Complex

The logical design for obtaining a blue-luminescent film based on a conjugated polymer is demonstrated. We recently found solid-state luminescent boron complexes with fused ketoiminate ligands (FBKIs). From quantum chemical calculations, it was proposed that there is a skeletal carbon having isolated highest occupied molecular orbital (HOMO) where the lobe in HOMO and the node in lowest unoccupied molecular orbital (LUMO) exist. From these results, we presumed that only the HOMO energy level could be selectively lowered by aza-substitution at this carbon atom. Based on this idea, we designed and synthesized the aza-substituted FBKI (azaFBKI). From the optical and electrochemical measurements, it was shown that azaFBKIs had solid-state luminescent properties and especially the wider energy gaps originating from the lower HOMO levels than the corresponding FBKIs. Next, the conjugated homopolymer including azaFKBI as a repeating unit was designed and obtained. Finally, it was observed that the synthesized polymer exhibited purely blue luminescence in film (λem = 440 nm, ΦPL,film = 0.48, CIE; 0.19, 0.18).

本研究展示了基于共轭聚合物制备蓝色发光薄膜的逻辑设计方案。我们近期发现了一类含稠合酮亚胺配体的固态发光硼配合物（fused ketoiminate boron complexes，FBKIs）。通过量子化学计算，我们提出存在一类骨架碳原子，其具有孤立的最高占据分子轨道（highest occupied molecular orbital，HOMO），该轨道同时具备HOMO的波瓣与最低未占据分子轨道（lowest unoccupied molecular orbital，LUMO）的节点。基于上述结果，我们推测仅通过对该碳原子进行氮杂取代，便可选择性降低其HOMO能级。基于这一思路，我们设计并合成了氮杂取代型FBKI（azaFBKI）。光学与电化学测试结果表明，azaFBKIs具备固态发光性能；相较于对应未取代的FBKIs，其HOMO能级更低，因而具有更宽的能带隙。随后，我们设计并合成了以azaFBKI为重复单元的共轭均聚物。最终我们观察到，所合成的聚合物在薄膜状态下展现出纯蓝色发光（发射波长λem=440 nm，薄膜光致发光量子产率ΦPL,film=0.48，国际照明委员会色坐标（Commission Internationale de l'Éclairage，CIE）：0.19, 0.18）。