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

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 (τTADF = 0.93 µs), a reverse intersystem crossing rate of 2.6 × 106 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.