Tellurophene‐Induced Triplet–Singlet Spin‐Flip Acceleration: An Advanced Design for Narrowband Organoboron Emitters with Fast Reverse Intersystem Crossing

Abstract Organoboron narrowband emitters leveraging the multi‐resonance (MR) effect have emerged as key material components for wide‐color‐gamut organic light‐emitting diodes (OLEDs) that offer high efficiency and excellent color reproducibility. However, conventional MR‐type thermally activated delayed fluorescence (MR‐TADF) emitters suffer from severe efficiency roll‐off in OLEDs operated at high brightness due to slow reverse intersystem crossing (RISC). Here, we report the first MR‐TADF emitters incorporating tellurium (Te)—the heaviest stable chalcogen—to accelerate the spin‐flip RISC process through enhanced spin–orbit coupling. Our design strategy involves tethering dibenzo[ b , d ]tellurophene as a RISC booster to a representative MR skeleton, thereby promoting efficient delayed fluorescence. The resulting Te‐containing MR‐TADF emitters exhibit fast RISC rates of up to 2.8 × 10 6 s −1 and narrow emission bandwidths of 23–25 nm in solution. OLEDs fabricated with these emitters achieve exceptional high external quantum efficiencies (EQEs) of up to 35.3%, with significantly reduced efficiency roll‐off, maintaining EQEs of 32.2% and 25.4% even at very high brightness levels of 10 3 and 10 4 cd m −2 , respectively. This study demonstrates the potential of Te‐containing MR‐TADF systems for next‐generation OLEDs and highlights their functional impact on device performance.