Li Wei, Li Mengke, Li Wenqi, Xu Zhida, Gan Lin, Liu Kunkun, Zheng Nan, Ning Chengyun, Chen Dongcheng, Wu Yuan-Chun, Su Shi-Jian
State Key Laboratory of Luminescent Materials and Devices and Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Wushan Road 381, Guangzhou 510640, Guangdong Province, P. R. China.
Key Laboratory of Biomedical Engineering of Guangdong Province, South China University of Technology, Wushan Road 381, Guangzhou 510640, Guangdong Province, P. R. China.
ACS Appl Mater Interfaces. 2021 Feb 3;13(4):5302-5311. doi: 10.1021/acsami.0c19302. Epub 2021 Jan 20.
Thermally activated delayed fluorescence (TADF) emitters with a spiral donor show tremendous potential toward high-level efficient blue organic light-emitting diodes (OLEDs). However, the underlying design strategy of the spiral donor used for blue TADF emitters remains unclear. As a consequence, researchers often do "try and error" work in the development of new functional spiral donor fragments, making it slow and inefficient. Herein, we demonstrate that the energy level relationships between the spiral donor and the luminophore lead to a significant effect on the photoluminescent quantum yields (PLQYs) of the target materials. In addition, a method involving quantum chemistry simulations that can accurately predict the aforementioned energy level relationships by simulating the spin density distributions of the triplet excited states of the spiral donor and corresponding TADF emitters and the triplet excited natural transition orbitals of the TADF emitters is established. Moreover, it also revealed that the steric hindrance in this series of molecules can form a nearly unchanged singlet (S) state geometry, leading to a reduced nonradiative decay and high PLQY, while a moderated donor-acceptor (D-A) torsion in the triplet (T) state can induce a strong vibronic coupling between the charge-transfer triplet (CT) state and the local triplet (LE) state, achieving an effective reverse intersystem crossing (RISC) process. Furthermore, an electric-magnetic coupling is formed between the high-lying LE state and the charge-transfer singlet (CT) state, which may open another RISC channel. Remarkably, in company with the optimized molecular structure and energy alignment, the pivotal TADF emitter DspiroS-TRZ achieved 99.9% PLQY, an external quantum efficiency (EQE) of 38.4%, which is the highest among all blue TADF emitters reported to date.
具有螺旋供体的热激活延迟荧光（TADF）发光体在高效蓝色有机发光二极管（OLED）方面展现出巨大潜力。然而，用于蓝色TADF发光体的螺旋供体的潜在设计策略仍不明确。因此，研究人员在开发新的功能性螺旋供体片段时常常进行“试错”工作，导致进展缓慢且效率低下。在此，我们证明螺旋供体与发光体之间的能级关系对目标材料的光致发光量子产率（PLQY）有显著影响。此外，建立了一种量子化学模拟方法，通过模拟螺旋供体和相应TADF发光体的三重激发态的自旋密度分布以及TADF发光体的三重激发自然跃迁轨道，能够准确预测上述能级关系。而且，研究还表明，这一系列分子中的空间位阻可形成几乎不变的单重态（S）几何结构，导致非辐射衰减降低和PLQY提高，而三重态（T）中的适度供体 - 受体（D - A）扭转可诱导电荷转移三重态（CT）态与局域三重态（LE）态之间产生强振动耦合，实现有效的反向系间窜越（RISC）过程。此外，在高能级LE态和电荷转移单重态（CT）态之间形成了电磁耦合，这可能开辟另一条RISC通道。值得注意的是，伴随着优化的分子结构和能量排列，关键的TADF发光体DspiroS - TRZ实现了99.9%的PLQY和38.4%的外量子效率（EQE），这是迄今为止报道的所有蓝色TADF发光体中最高的。
