Antiaromatic Azepino[3,2,1‐<i>jk</i>]Carbazole‐Based π‐Extended Molecular Engineering for High‐Efficiency and Stable BT.2020 Green OLEDs

Abstract Multiple‐resonance thermally activated delayed fluorescence (MR‐TADF) materials have shown great potential as contenders for next‐generation displays with wider color gamut and ultra‐high definition (UHD) display, attributed to its narrowband emission and ability to utilize triplet excitons. However, achieving MR‐TADF materials and corresponding devices with high efficiency, long operational lifetime, and ultra‐pure green emission that meet the Broadcast Service Television 2020 (BT.2020) standard is a major challenge in the field of organic light‐emitting diodes (OLEDs). Herein, a π‐extended molecular engineering strategy is proposed for developing superior green emitters, exemplified by ACz‐BN x ( x = 1, 2, 3, 4), which integrates an antiaromatic azepino[3,2,1‐ jk ]carbazole (ACz) fragment into a well‐established boron/nitrogen‐based MR framework. The proof‐of‐concept compounds exhibit near‐unity photoluminescent quantum yields and high molecular rigidity, along with exceptional electroluminescent stability. Corresponding green top‐emission OLED devices achieve remarkable operational stability, with an LT 95 lifetime of 434.9 h at an initial luminance of 1000 cd m −2 as well as high color purity that closely aligns the green BT.2020 standard and outstanding external quantum efficiencies of 38.8–52.2%.