Sulfur‐Atom Incorporated Multi‐Resonance Emitters via an Isomerization Strategy Enabling High‐Performance Narrowband OLEDs With Suppressed Efficiency Roll‐Off

ABSTRACT Organic light‐emitting diodes (OLEDs) based on multiple‐resonance thermally activated delayed fluorescence (MR‐TADF) emitters are promising for achieving high efficiency and narrowband emission. However, most of devices suffer from severe efficiency roll‐off due to long triplet exciton lifetime, and slow reverse intersystem crossing (RISC) rate. Herein, we present a molecular design strategy that bridges dual sulfur atoms at different peripheral positions of a BN‐based MR core, yielding three isomeric MR‐TADF emitters 3,4‐TECzBN, 2,3‐TECzBN, and 1,2‐TECzBN. This design retains the rigidity of the MR framework while exploiting the heavy‐atom effect of sulfur to enhance spin–orbit coupling (SOC) and accelerate RISC. All emitters show narrowband emissions with full‐width at half maximum of 24–30 nm in solutions. Also, high k RISC of 4.80 × 10 5 and 2.30 × 10 5 s −1 are achieved for 3,4‐TECzBN and 2,3‐TECzBN in doped films. Consequently, OLED devices utilizing 3,4‐TECzBN and 2,3‐TECzBN as emitters exhibit high maximum external quantum efficiencies (EQEs) of 27.2% and 30.4%, and suppressed efficiency roll‐off at high luminance with EQEs of 18.6% and 20.8% at 1000 cd m −2 , indicating the effective peripheral sulfur bridging strategy for advancing high‐performance MR‐TADF emitters.