Wu Yimin, Liu Xiaoyu, Liu Junjie, Yang Ge, Deng Yayin, Bin Zhengyang, You Jingsong
Key Laboratory of Green Chemistry and Technology of Ministry of Education, College of Chemistry, Sichuan University, 29 Wangjiang Road, Chengdu 610064, People's Republic of China.
J Am Chem Soc. 2024 Jun 12;146(23):15977-15985. doi: 10.1021/jacs.4c02872. Epub 2024 May 7.
Unveiling innovative mechanisms to design new highly efficient fluorescent materials and, thereby, fabricate high-performance organic light-emitting diodes (OLEDs) is a concerted endeavor in both academic and industrial circles. Polycyclic aromatic hydrocarbons (PAHs) have been widely used as fluorescent emitters in blue OLEDs, but device performances are far from satisfactory. In response, we propose the concept of "nitrogen effects" endowed by doping electron-withdrawing nitrogen atoms into PAH fluorescence emitters. The presence of the n orbital on the imine nitrogen is conducive to promoting electron coupling, which leads to increased molar absorptivity and an accelerated radiative decay rate of emitters, thereby facilitating the Förster energy transfer (FET) process in the OLEDs. Additionally, electronically withdrawing nitrogen atoms enhances host-guest interactions, thereby positively affecting the FET process and the horizontal orientation factor of the emitting layer. To validate the "nitrogen effects" concept, cobalt-catalyzed multiple C-H annulation has been utilized to incorporate alkynes into the imine-based frameworks, which enables various imine-embedded PAH (IE-PAH) fluorescence emitters. The cyclization demonstrates notable regioselectivity, thereby offering a practical tool to precisely introduce peripheral groups at desired positions with bulky alkyl units positioned adjacent to the nitrogen atoms, which were previously beyond reach through the Friedel-Crafts reaction. Blue OLEDs fabricated with IE-PAHs exhibit outstanding performance with a maximum external quantum efficiency (EQE) of 32.7%. This achievement sets a groundbreaking record for conventional blue PAH-based fluorescent emitters, which have an EQE of 24.0%.
揭示设计新型高效荧光材料的创新机制，并借此制造高性能有机发光二极管（OLED），是学术界和工业界的共同努力。多环芳烃（PAHs）已被广泛用作蓝色OLED中的荧光发射体，但器件性能远不能令人满意。对此，我们提出了“氮效应”的概念，即通过将吸电子氮原子掺杂到PAH荧光发射体中赋予其该效应。亚胺氮上n轨道的存在有利于促进电子耦合，这导致发射体的摩尔吸光率增加和辐射衰减速率加快，从而促进了OLED中的Förster能量转移（FET）过程。此外，吸电子氮原子增强了主体-客体相互作用，从而对FET过程和发射层的水平取向因子产生积极影响。为了验证“氮效应”的概念，利用钴催化的多C-H环化反应将炔烃引入基于亚胺的骨架中，从而得到各种嵌入亚胺的PAH（IE-PAH）荧光发射体。环化反应表现出显著的区域选择性，从而提供了一种实用工具，可在所需位置精确引入外围基团，且使庞大的烷基单元位于氮原子相邻位置，而这些位置以前通过Friedel-Crafts反应是无法实现的。用IE-PAHs制造的蓝色OLED表现出出色的性能，最大外量子效率（EQE）为32.7%。这一成就为传统的基于蓝色PAH的荧光发射体创造了一个开创性的记录，其EQE为24.0%。
