Thermally Activated Delayed Fluorescence Emitters Based on Rigid Lactam Acceptors: Simultaneously Achieving Desirable Emission Efficiency, Horizontal Orientation, and Reverse Intersystem Crossing

Thermally activated delayed fluorescence (TADF) materials that exhibit simultaneously high photoluminescence quantum yield (PLQY), rapid reverse intersystem crossing (RISC), and a high horizontal transition dipole ratio are highly desirable for realizing high‐performance organic light‐emitting diodes (OLEDs). However, achieving this goal remains a formidable challenge due to the stringent molecular design principles involved. Herein, three highly efficient TADF materials based on lactam‐type electron‐acceptors are reported. The inherent rigidity and planar structure of lactam units, along with the ordered molecular arrangement in solid states, contribute to the reduction of nonradiative decay and the high horizontal transition dipole ratio in the optimized TADF emitters. Moreover, through precise control of the alignment of the lowest excited states by adjusting the charge transfer strength, the rate constants for reverse intersystem crossing (kRISC) are dramatically boosted. Consequently, the two optimized emitters exhibit outstanding merits of ultra‐high PLQYs (98% and 99%), high horizontal transition dipole ratios (91% and 87%), and fast RISC (kRISC ≈ 1.7 × 106 s−1). Thanks to these merits, the doped OLEDs achieve excellent performance. The top‐performing device achieve a maximum external quantum efficiency of 34.3%, a peak luminance of 57376 cd m−2, and small efficiency roll‐off. Novel lactam electron‐acceptors are utilized to construct high‐performance TADF molecules. The rigid molecular skeletons, ordered molecular arrangements, and precise control of excited states contribute to achieving ultra‐high PLQYs (98%−99%), high horizontal dipole ratios (87%−91%), and rapid RISC (kRISC ≈ 1.7 × 106 s−1) simultaneously. The optimized OLEDs attain high EQEs of up to 34.3%, accompanied by ultra‐high luminances and small efficiency roll‐offs.