Sterically Shielded Heavy‐Atom Strategy for Constructing Narrowband Pure‐Green MR‐TADF Emitters With Concentration‐Quenching Resistance and Fast Reverse Intersystem Crossing

ABSTRACT Multiple resonance thermally activated delayed fluorescence (MR‐TADF) emitters with superior color purity and high photoluminescence quantum yields (PLQYs) hold great promise for organic light‐emitting diodes (OLEDs). Nevertheless, the rigid planar structures and intrinsically slow reverse intersystem crossing rates ( k RISC ) of these materials lead to pronounced aggregation‐caused quenching and significant efficiency roll‐off. Here, we propose a sterically shielded heavy‐atom strategy that simultaneously suppresses detrimental π–π stacking, accelerates k RISC , and reinforces the rigidity of the conjugated framework to retain narrowband emission. The proof‐of‐concept emitters BN‐FLS, BN‐FLSe, and BN‐FLSSe show the green emission at 508–517 nm with the narrow full width at half maximums (FWHMs) of 25–26 nm in solution. Notably, BN‐FLSSe achieves rapid k RISC of 4.2 × 10 6 s −1 , maintains high PLQYs above 95%, and stable emission profiles over 1–30 wt% broad doping range. Non‐sensitized OLEDs based on BN‐FLS, BN‐FLSe, and BN‐FLSSe show pure‐green electroluminescence with CIE y coordinates exceeding 0.70, high maximum external quantum efficiencies (EQE max s) of 41.7%, 41.1%, and 38.5%, and well‐suppressed efficiency roll‐offs, maintaining EQEs of 21.9%, 30.9%, and 32.7% at 1000 cd m −2 , respectively. Impressively, even at high doping concentration of 30 wt%, these devices retain high EQE max s of 30.7%–33.1% with virtually unchanged emission spectra.