Space-Confined Donor-Acceptor Strategy Enables Fast Spin-Flip of Multiple Resonance Emitters for Suppressing Efficiency Roll-Off.

Multi-resonance boron-nitrogen-containing thermally activated delayed fluorescence (MR-TADF) emitters have experienced great success in assembling narrowband organic light-emitting diodes (OLEDs). However, the slow reverse intersystem crossing rate (k RISC ) of MR-emitters (10 3 -10 5  s -1 ) that will lead to severe device efficiency roll-off has received extensive attention and remains a challenging issue. Herein, we put forward a "space-confined donor-acceptor (SCDA)" strategy to accelerate RISC process. The introduction of SCDA units onto the MR-skeleton induces intermediate triplet states, which leads to a multichannel RISC process and thus increases k RISC . As illustrated examples, efficient MR-emitters have been developed with a sub-microsecond delayed lifetime and a high k RISC of 2.13×10 6  s -1 , which enables to assemble high-performance OLEDs with a maximum external quantum efficiency (EQE max ) as high as 32.5 % and an alleviated efficiency roll-off (EQE 1000 : 22.9 %).