Multiple charge-transfer excited state induced efficient and stable thermally activated delayed fluorescence

Abstract

The development of efficient and stable electroluminescent devices is one of the formidable challenges in the study of organic light-emitting diodes (OLEDs). Herein, three donor (D) and acceptor (A) connection arrangements in dual-D–A thioxanthone derivatives (ADAD, ADDA, DAAD) are investigated to parse their different intramolecular charge transfer (ICT) excited state features and their effects on intersystem crossing (ISC)–reverse intersystem crossing (RISC) as well as radiative transition processes. Compared to two others, the alternate-D–A structure (ADAD) affords nearly isoenergetic singlet and triplet states with weak ICT characters and strong spin–orbit couplings, suppressing the non-radiative transition and inducing multiple ISC–RISC cycles to achieve the highest photoluminescence quantum yield and RISC rate constant. Logically, OLED devices based on this emitter enable ultra-high exciton utilization efficiency of nearly 100%, an external quantum efficiency of 28.93%, and excellent device stability (with an estimated LT₅₀ of about 2303 h at 1000 cd m⁻²). These findings could open a wide opportunity for a new class of organic light-emitting diodes for various applications.