Multi-resonance (MR) emitters have emerged as one of the most attractive materials for OLEDs in both scientific community and industry. However, MR emitter-based OLEDs (MR-OLEDs) generally display severe efficiency roll-off due to the relatively slow reverse intersystem crossing (RISC) rate (kRISC) as well as the planar structure of MR emitters that will lead to severe aggregation-caused quenching (ACQ) effect. Therefore, reducing device efficiency roll-off of MR-OLEDs has received extensive attention and remains challenging issues.
To address this issue, Jingsong You/Zhengyang Bin et al. in the College of Chemistry, Sichuan University, put forward a medium-ring strategy to design efficient MR emitters that feature heptagonal tribenzo[b,d,f]azepine (TBA) donors. The highly twisted conformation enlarges the intermolecular distances between the MR-emitting cores, and thus suppresses ACQ effect. Meanwhile, the introduction of heptagonal donors enhances spin-orbital coupling, so as to accelerate RISC process.
The medium-ring strategy developed herein gives rise to the first example of blue MR emitter with kRISC as fast as 106 s-1 and kR as fast as 108 s-1. Accordingly, DTBA-B2N3 enables to assemble high-performance binary-EML blue MR-OLEDs with maximum external quantum efficiency (EQEmax) of 30.9% and alleviated efficiency roll-off (EQE1000: 20.5%), representing one of the best results for binary-EML blue MR-OLEDs. In contrast, the EQEmax of planar B2-based OLEDs is only 17.2% and the EQE1000 is decreased dramatically to 8.1%.
This work is published on Angewandte Chemie International Edition, titled as “Medium-Ring Strategy Enables Multiple Resonance Emitters with Twisted Geometry and Fast Spin-Flip to Suppress Efficiency Roll-Off”. Sichuan University is top institute in the address list. Prof. Jingsong You and Prof. Zhengyang Bin are corresponding authors, and Mr. Bowen Lei is the first author. This work is financially supported by financial support from the National NSF of China and the Sichuan Science and Technology Program as well as the Institutional Research Fund from Sichuan University. Thanks for Dr. Jing Li (Sichuan University) for assistance with NMR measurements.
Links to: https://doi.org/10.1002/anie.202218405