Boosting electroluminescence efficiency based on TADF emitters featuring multiple resonance segment integrated with a co-host exciplex

In the field of organic light-emitting diodes (OLEDs), the development of thermally activated delayed fluorescence (TADF) materials has emerged as a promising strategy for achieving high internal quantum efficiency by effectively utilizing both singlet and triplet excitons. In this study, we explore the design and synthesis of three TADF emitters, DPA-QAO, PhDPA-QAO and MeODPA-QAO, featuring a unique multi-resonance quinolino[3,2,1-de]acridine-5,9-dione (QAO) segment. The structural design emphasizes a non-planar conformation that promotes the formation of a twisted excited state, effectively facilitating charge transfer (CT). This conformation also leads to intermolecular π-π stacking interactions, particularly observed in DPA-QAO and PhDPA-QAO, which enhance exciton energy transfer and maintain high photoluminescent quantum yields (PLQYs) of up to 90%, the radiative transition constant rate (k_r) up to 3.41 × 10⁷ s⁻¹. These structural features enable the MR-TADF emitters with a low energy gap (ΔE_ST) of 0.10 eV and fast reverse intersystem cross (RISC) rate. When integrated into an exciplex co-host OLED devices, maximum external quantum efficiency (EQE) of 23.83%, 26.01%, and 11.34% have been achieved, respectively. The work highlights the significant role of molecular twisting and π-π interactions in optimizing the photophysical properties of MR-TADF materials, offering a pathway to the development of highly efficient OLEDs with broad application.