Achieving 34.3% External Quantum Efficiency for Red Thermally Activated Delayed Fluorescence Organic Light‐Emitting Diode by Molecular Isomer Engineering

Abstract The development of red organic emissive materials with high‐efficiency and low‐cost is of great significance but formidable challenge for organic light‐emitting diodes (OLEDs). Herein, through isomer engineering, a pair of red thermally activated delayed fluorescence (TADF) isomers with Y‐shape and cruciform structures, namely TPA‐APQDCN‐Y and TPA‐APQDCN‐C, are developed by integrating two triphenylamine (TPA) donor moieties into different positions of rigid planar acenaphtho[1,2‐b]quinoxaline‐9,10‐dicarbonitrile (APQDCN) acceptor core. Compared to common Y‐shape structure, the unique cruciform structure can result in the formation of intramolecular H‐bonding, the limited molecular packing, the balance of the contradiction between the small spatial overlap of frontier molecular orbitals, and high oscillator strength, contributing to a higher photoluminescence quantum yield of 95%, a smaller singlet–triplet energy split of 0.24 eV and a larger horizontal emitting dipole ratio of 86%. An external quantum efficiency of 34.3% with an emission peak at 610 nm is achieved for TPA‐APQDCN‐C based red electroluminescent device, which is the highest value for red TADF‐OLEDs with emission maximum beyond 600 nm ever reported.