Synergetic Multiple Charge-Transfer Excited States for Anti-Quenching and Rapid Spin-Flip Multi-Resonance Thermally Activated Delayed Fluorescence Emitter

Abstract

The development of multiple resonances thermally activated delayed fluorescence (MR-TADF) emitters exhibiting high efficiency, narrowband emission, rapid reverse intersystem crossing rate (k_RISC), and suppressed concentration quenching simultaneously is of great significance yet a formidable challenge. Herein, an effective strategy is presented to realize the above target by synergizing multiple charge-transfer excited states, including short-range charge transfer (SRCT), through-bond charge transfer (TBCT), and through-space charge transfer (TSCT). The proof-of-concept emitter 4tCz2B exhibits a bright green emission with a narrow full width at half maximum (FWHM) of 21 nm (0.10 eV) in solution, high photoluminescence quantum yield of 97%, fast k_RISC of 7.8 × 10⁵ s⁻¹ and significantly suppressed concentration quenching in film state. As a result, the sensitizer-free organic light-emitting diodes (OLEDs) achieve maximum external quantum efficiencies (EQE_maxS) of over 34.5% together with an unaltered emission peak at 508 nm and FWHM of 26 nm at doping concentrations ranging from 3 to 20 wt.%. Even at a doping ratio of 50 wt.%, EQE_max is still as high as 25.5%. More importantly, the non-sensitized devices exhibit significantly reduced efficiency roll-offs, with a minimum value of 13.4% at a brightness of 1000 cd m⁻².