Symmetry Breaking Assisted Fast Reverse Intersystem Crossing for Efficient TADF Materials

Reverse intersystem crossing (RISC) process is critical for thermally activated delayed fluorescence (TADF) materials to realize spin-flip of triplet excitons in organic light-emitting diodes (OLEDs), but the RISC processes of most TADF materials are not fast enough, undermining electroluminescence (EL) efficiency stability and operational lifetime. Herein, a symmetry breaking strategy to accelerate RISC processes is proposed. By designing asymmetric electron-withdrawing backbone consisting of benzonitrile and xanthone/thioxanthone groups, two new asymmetric TADF molecules, 4tCzCN-pXT and 4tCzCN-pTXT, with multiple 3,6-di-tert-butylcarbazole donors are successfully developed. They own increased molecular vibrations, which promote intrinsic RISC process and enable multi-channel transitions via vibronic coupling of high-lying triplet states. Consequently, they exhibit fast RISC rates of up to 1.24 × 107 s-1, being one order of magnitude higher than that of the symmetric control molecule. They can perform as luminescent materials in OLEDs, providing outstanding external quantum efficiencies (EQEs) of up to 31.2% and 35.8% in non-doped and doped devices, respectively, with very small roll-offs. The OLEDs using them as sensitizers for multi-resonance emitters achieve remarkable EQEs over 40%, and extraordinary operational stability with LT90 of 24974 h at 1000 cd m-2, demonstrating their great potentials in OLEDs.