Duan Ke, Wang Dan, Yang Ming, Liu Ziyang, Wang Chao, Tsuboi Taiju, Deng Chao, Zhang Qisheng
MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China.
Key Laboratory of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Department of Chemistry and Center for Flexible Electronics Technology, Tsinghua University, Beijing 100084, China.
ACS Appl Mater Interfaces. 2020 Jul 8;12(27):30591-30599. doi: 10.1021/acsami.0c02800. Epub 2020 Jun 10.
The development of host materials with high first-triplet state (T) energy and high charge mobility is a key to achieve efficient true-blue organic light-emitting diodes (OLEDs), employing phosphorescence and thermally activated delayed fluorescence (TADF). An ether-bridged double triphenylphosphine oxide (TPPO) compound, bis(2-(diphenylphosphino)phenyl)ether oxide (DPEPO), was reported to have a very high T energy of 3.3 eV but suffers from poor charge mobility. Here, five bridge-controlled multi-TPPO derivatives were studied through a combination of experiments and theory. We demonstrate that the push-pull electron capability of the bridge group governs the T energy and electron mobility of these materials. Replacing the ether bridge by a bis(trifluoromethyl)methylene group can reduce the energy barrier for intramolecular electron exchange and consequently enhance the electron mobility by two orders of magnitude without lowering the T energy. A blue TADF OLED employing this bis(trifluoromethyl)methylene-bridged compound achieves the same high external quantum efficiency but a much higher current density compared to the control device employing DPEPO. In contrast, a bridge group with strong electron-withdrawing capability, such as phosphine oxygen or sulfone, lowers the T energy of the compound by enhancing the electronic coupling between TPPO subunits and inhibits intermolecular electron transfer by trapping the electron charge around the bridge.
开发具有高第一三重态（T）能量和高电荷迁移率的主体材料是实现采用磷光和热激活延迟荧光（TADF）的高效真蓝光有机发光二极管（OLED）的关键。据报道，一种醚桥联双三苯基氧化膦（TPPO）化合物，双（2-（二苯基膦基）苯基）醚氧化物（DPEPO），具有3.3 eV的非常高的T能量，但电荷迁移率较差。在此，通过实验和理论相结合的方式研究了五种桥控多TPPO衍生物。我们证明，桥连基团的推拉电子能力决定了这些材料的T能量和电子迁移率。用双（三氟甲基）亚甲基取代醚桥可以降低分子内电子交换的能垒，从而在不降低T能量的情况下将电子迁移率提高两个数量级。与采用DPEPO的对照器件相比，采用这种双（三氟甲基）亚甲基桥连化合物的蓝色TADF OLED实现了相同的高外部量子效率，但电流密度要高得多。相比之下，具有强吸电子能力的桥连基团，如膦氧或砜，会通过增强TPPO亚基之间的电子耦合来降低化合物的T能量，并通过将电子电荷捕获在桥周围来抑制分子间电子转移。
