Increasing Aromaticity in Selenium‐Embedded Hetero Ring Enables Stable Narrowband Multi‐Resonance TADF Emitter for Long‐Lifetime Pure‐Green OLEDs

The development of high-performance organic light-emitting diodes (OLEDs) that simultaneously achieve narrowband emission, superior efficiency, and extended operational stability remains a significant challenge for thermally activated delayed fluorescent (TADF) materials. Herein, a novel molecular architecture is reported that strategically integrates selenium (Se) into a rigid pentagonal aromatic sextet (dibenzoselenophene, DBSe) within a multi-resonance (MR) framework, addressing the intrinsic limitations of hexagonal Se-MR systems. The developed molecule DBSe-BN exhibits a rapid reverse intersystem crossing (RISC) rate in the order of 10⁶ s^-1 and narrow emission bandwidth of 21 nm, providing persuasive guarantee for high-performance pure-green emission. Notably, the DBSe-BN-based OLEDs demonstrate exceptional performance, including a high maximum external quantum efficiency (EQE_max) of 38.8%, suppressed efficiency roll-off (EQE₁₀₀₀ = 33.7%), and pure-green emission, characterized by Commission Internationale de l'Eclairage (CIE) coordinates of (0.18, 0.73). The devices exhibit remarkable operational stability, with greatly prolonged lifetime (LT₅₀) of 1341 h based on binary-emitting system and 5310 h based on the ternary device, at an initial luminance of 1000 cd m^-2. This work presents a feasible strategy for gathering high-performance MR-TADF emitters, achieving a multi-dimensional balanced OLED with enhanced FWHM, CIE, EQE, roll-off and lifetime promoted by Se-embedded aromatic sextet.