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

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 (kRISC) 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 kRISC, 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 kRISC of 4.2 × 106 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 CIEy coordinates exceeding 0.70, high maximum external quantum efficiencies (EQEmaxs) 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 EQEmaxs of 30.7%-33.1% with virtually unchanged emission spectra.