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

Abstract 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 10 6 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 (EQE 1000 = 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 (LT 50 ) 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.