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

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 (³LE) 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.