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

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 (ΔE_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 ΔE_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 ΔE_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.