Five‐Boron Multiple‐Resonance TADF Emitter With Accelerated Reverse Intersystem Crossing for High‐Performance Narrowband Green OLEDs

ABSTRACT Multiple‐resonance thermally activated delayed fluorescence (MR‐TADF) emitters are promising candidates for organic light‐emitting diodes owing to their narrowband emission and high exciton utilization. While π‐conjugated framework expansion has been extensively explored for tuning excited‐state properties, five‐boron MR‐TADF emitters have not yet been realized. Herein, we report the first five‐boron MR‐TADF emitter, M‐DABNA‐Mes, featuring an unprecedented odd‐numbered boron‐rich multiple‐resonance framework. Theoretical investigations of one‐, three‐, and five‐boron MR frameworks reveal that incorporation of the fifth boron atom reduces the singlet–triplet energy gap, preserves spin–orbit coupling, and reduces the total reorganization energy while maintaining favorable narrowband emission characteristics. Consistent with these predictions, M‐DABNA‐Mes doped in a polystyrene film exhibits a sub‐microsecond delayed fluorescence lifetime ( τ TADF = 0.93 µs), a reverse intersystem crossing rate of 2.6 × 10 6 s −1 , and narrow green emission at 512 nm with a full width at half maximum of 19 nm. OLEDs based on M‐DABNA‐Mes achieve a maximum external quantum efficiency of 34.3% with remarkably low efficiency roll‐off (32.3% at 10,000 cd m −2 ) and a maximum luminance of 275,800 cd m −2 . These results demonstrate that the five‐boron MR framework enables rapid spin conversion, high spectral purity, and excellent OLED performance, providing a new platform for high‐performance MR‐TADF emitters.