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

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 × 10⁶ 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 10³ and 10⁴ 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.