Fused π-Extended Multiple-Resonance Induced Thermally Activated Delayed Fluorescence Materials for High-Efficiency and Narrowband OLEDs with Low Efficiency Roll-Off

The simultaneous achievement of multiple-resonance thermally activated delayed fluorescence (MR-TADF) materials with strong narrowband emission and efficient reverse intersystem crossing (RISC) process can further promote the advancement of organic light-emitting diodes (OLEDs). Herein, a new strategy is proposed to achieve two pi-extended MR-TADF emitters (NBO and NBNP) peaking at 487 and 500 nm via fusing conjugated high-triplet-energy units (carbazole, dibenzofuran) into boron-nitrogen (B/N) framework, aiming to increase charge transfer delocalization of the B/N skeleton and minimize singlet-triplet energy gap ( increment E-ST). This strategy endows the two emitters with full width at half maximum of 27 and 29 nm, and high photoluminescence efficiencies above 90% in doped films, respectively. Additionally, considerable rate constants of RISC are obtained due to the small increment E-ST (0.12 and 0.09 eV) and large spin-orbital coupling values. Consequently, the OLEDs based on NBO and NBNP show the maximum external electroluminescence quantum efficiency of up to 26.1% and 28.0%, respectively, accompanied by low-efficiency roll-off. These results provide a feasible design strategy to construct efficient MR-TADF materials for OLEDs with suppressed efficiency roll-off.