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

Abstract

ABSTRACT 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.