Rigidification with B‐π‐N Scaffolds Through Four Bridging Atoms Toward High Efficiency and Color‐Tunable Electroluminescence

Abstract Boron‐based emitters exhibiting thermally activated delayed fluorescence (TADF) nature have emerged as promising guests for organic light‐emitting diodes (OLEDs). However, balancing the trade‐off between the small singlet‐triplet energy gap (Δ E ST ) and the high oscillator strength ( f ) remains a significant challenge. This study demonstrates a rational molecular design strategy for high‐performance TADF emitters by integrating BOO/BOS/BSS acceptors and a benzo[5,6][1,4]oxazino[2,3,4‐kl]phenoxazine (NO) donor into a rigid framework. Systematic modulation of bridging atoms and peripheral tert‐butyl functionalization fine‐tuned the excited‐state properties, yielding broad emission color tenability from blue‐green (498 nm) to orange (572 nm). Theoretical calculations reveal effective spatial separation of frontier molecular orbitals with maintained overlap on the central benzene ring, balancing a small Δ E ST (0.07–0.23 eV) and a high ƒ (0.1728–0.1911). The introduction of tert‐butyl groups increases the horizontal dipole ratio from 61% in BNO4 to 77–78% in the other three derivatives. Among the designed emitters, TBNSO3 exhibits the smallest Δ E ST (0.07 eV) due to symmetry‐breaking charge transfer, facilitating efficient reverse intersystem crossing. The corresponding OLED achieves a maximum external quantum efficiency (EQE max ) of 35.1%, representing state‐of‐the‐art performance among non‐sensitized boron‐based devices.