High-Performance Orange-Red Organic Light-Emitting Diodes with External Quantum Efficiencies Reaching 33.5% based on Carbonyl-Containing Delayed Fluorescence Molecules.

Developing orange to red purely organic luminescent materials having external quantum efficiencies (η ext s) exceeding 30% is challenging because it generally requires strong intramolecular charge transfer, efficient reverse intersystem crossing (RISC), high photoluminescence quantum yield (Φ PL ), and large optical outcoupling efficiency (Φ out ) simultaneously. Herein, by introducing benzoyl to dibenzo[a,c]phenazine acceptor, a stronger electron acceptor, dibenzo[a,c]phenazin-11-yl(phenyl)methanone, is created and employed for constructing orange-red delayed fluorescence molecules with various acridine-based electron donors. The incorporation of benzoyl leads to red-shifted photoluminescence with accelerated RISC, reduced delayed lifetimes, and increased Φ PL s, and the adoption of spiro-structured acridine donors promotes horizontal dipole orientation and thus renders high Φ out s. Consequently, the state-of-the-art orange-red organic light-emitting diodes are achieved, providing record-high electroluminescence (EL) efficiencies of 33.5%, 95.3 cd A -1 , and 93.5 lm W -1 . By referring the control molecule without benzoyl, it is demonstrated that the presence of benzoyl can exert significant positive effect over improving delayed fluorescence and enhancing EL efficiencies, which can be a feasible design for robust organic luminescent materials.