Enhancing Device Efficiency Through Subtle Substituent Tuning in DABNA‐Based Emitters

Blue multi‐resonance thermally activated delayed fluorescence (MR‐TADF) emitters are pivotal for expanding the color gamut of organic light‐emitting diodes (OLEDs), owing to their narrowband emission, high photoluminescence quantum yields, and rapid reverse intersystem crossing (RISC) characteristics. Among this, the rigid fused organoboron‐nitrogen (BN) framework serves as a foundational scaffold for MR emitters, extensively studied to advance OLED performance. However, strategies to optimize the BN core for enhanced device efficiency while maintaining emission wavelength stability remain underexplored. In this work, we designed two 2,12‐di‐ tert ‐butyl‐5,9‐bis(4‐( tert ‐butyl)phenyl)‐5,9‐dihydro‐5,9‐diaza‐13 b ‐boranaphtho[3,2,1‐ de ]anthracene ( t ‐DABNA) derivatives by introducing the methyl (Me)‐ and phenyl (Ph)‐substituent, named Me‐ t ‐DABNA and Ph‐ t ‐DABNA, respectively. Subtle substituent tuning enabled precise modulation of device performance. Notably, Me‐ t ‐DABNA retained the deep‐blue emission ( λ = 457 nm in toluene) and narrow full‐width at half‐maximum (FWHM = 22 nm) of the parent t ‐DABNA while significantly improving electroluminescence characteristics. The sensitized OLED based on Me‐ t ‐DABNA achieved a maximum external quantum efficiency (EQE) of 32.48%, with ultrapure blue emission ( λ = 461 nm, FWHM = 30 nm) and Commission Internationale de L'Eclairage (CIE) coordinates of (0.137, 0.123), outperforming the t ‐DABNA reference device. This work establishes methyl substitution as an effective approach for precise RISC enhancement in BN‐core MR‐TADF emitters, without compromising narrowband emission characteristics.