Strategic Acceleration of Reverse Intersystem Crossing in Multi-Resonance TADF Emitters

Advanced multi-resonance-induced thermally activated delayed fluorescence (MR-TADF) materials offer intrinsically narrowband emissions and excellent luminescent efficiencies, making them promising emitters for next-generation organic light-emitting diodes (OLEDs). However, their development remains hindered by slow reverse intersystem crossing (RISC) rates, which cause severe efficiency roll-off at high luminance and limit their application in high-performance OLEDs. Here, we propose an effective approach for designing blue MR-TADF molecules by integrating a crumpled and asymmetric heptagonal dibenzodiazepine building block, rather than conventional planar donors, which not only promotes efficient triplet-to-singlet RISC processes but also preserves narrowband emission. The proof-of-concept emitter AzBN2 exhibits a bright deep-blue emission centered at 462 nm with a full-width at half-maximum of 19 nm and CIE coordinates of (0.13, 0.067), accompanied by a more than twofold increase in the RISC rate. As a result, TADF OLEDs based on AzBN2 achieved a record maximum external quantum efficiency (EQE) of up to 35.7%. Moreover, ascribed to the improved RISC rates of AzBN2, the device exhibits an extremely low efficiency roll-off; notably, the EQE remains at 29.8% under a high luminance of 1000 cd m^-2, representing state-of-the-art performance for MR-TADF OLEDs.