The Structure Optimization of Indenoquinoxalinone‐Based Novel Deep‐Red Emitters for OLEDs With External Quantum Efficiency Achieving 16%

ABSTRACT Developing efficient deep‐red thermally activated delayed fluorescence (TADF) emitters remains challenging. Herein, using 11H‐indeno[1,2‐b]quinoxalin‐11‐one (IQ) as the electron acceptor and triphenylamine (TPA) as the electron donor, two isomeric emitters, Y‐shaped 23‐IQ and T‐shaped 14‐IQ were constructed by introducing TPAs at the 2,3‐ and 1,4‐positions of IQ, respectively, enabling a systematic comparison of the influence of molecular structure on photoelectric properties. Both emitters exhibit small singlet‐triplet energy gaps (∼0.13 eV); however, 14‐IQ uniquely possesses a high‐lying locally excited triplet ( 3 LE) state that facilitates faster reverse intersystem crossing (RISC). Consequently, device based on 14‐IQ achieved a maximum external quantum efficiency (EQE) of 16.0% with electroluminescent peak at 633 nm and a maximum luminance of 5736 cd m −2 , substantially outperforming 23‐IQ (EQE = 9.6%). This work underscores that the T‐shaped 1,4‐substitution pattern on the IQ core represents a promising molecular design strategy for developing high‐performance deep‐red TADF emitters.