RESUMO
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 (kRISC ) of MR-emitters (103 -105 â 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 kRISC . As illustrated examples, efficient MR-emitters have been developed with a sub-microsecond delayed lifetime and a high kRISC of 2.13×106 â s-1 , which enables to assemble high-performance OLEDs with a maximum external quantum efficiency (EQEmax ) as high as 32.5 % and an alleviated efficiency roll-off (EQE1000 : 22.9 %).
RESUMO
Three thermally activated delayed fluorescence (TADF) molecules, namely PQ1, PQ2, and PQ3, are composed of electron-accepting (A) tetrabenzo[a,c]phenazine (TBPZ) and electron-donating (D) phenoxazine (PXZ) units are designed and characterized. The combined effects of planar acceptor manipulation and high steric hindrance between D and A units endow high molecular rigidity that suppresses nonradiative decay of the excitons with improved photoluminescence quantum yields (PLQYs). Particularly, the well-aligned excited states involving a singlet and a triplet charge-transfer excited states and a localized excited triplet state in PQ3 enhances the reverse intersystem crossing rate constant (kRISC ) with a short delay lifetime (τd ). The orange-red OLED based on PQ3 displays a maximum external EL quantum efficiency (EQE) of 27.4 % with a well-suppressed EL efficiency roll-off owing to a completely horizontal orientation of the transition dipole moment in the film state.