Tetrabenzo[a,c]phenazine Backbone for Highly Efficient Orange-Red Thermally Activated Delayed Fluorescence with Completely Horizontal Molecular Orientation

Three thermally activated delayed fluorescence (TADF) molecules, namely PQ1, PQ2, and PQ3, are composed of electron-accepting (A) tetrabenzo[a,c]phenazine (TBPZ) and electron-donating (D) phenoxazine (PXZ) units are designed and characterized. The combined effects of planar acceptor manipulation and high steric hindrance between D and A units endow high molecular rigidity that suppresses nonradiative decay of the excitons with improved photoluminescence quantum yields (PLQYs). Particularly, the well-aligned excited states involving a singlet and a triplet charge-transfer excited states and a localized excited triplet state in PQ3 enhances the reverse intersystem crossing rate constant (k(RISC)) with a short delay lifetime (tau(d)). The orange-red OLED based on PQ3 displays a maximum external EL quantum efficiency (EQE) of 27.4% with a well-suppressed EL efficiency roll-off owing to a completely horizontal orientation of the transition dipole moment in the film state.

Automated summary

Three TADF molecules PQ1, PQ2, and PQ3, featuring planar acceptor manipulation and high steric hindrance, exhibit high molecular rigidity that suppresses nonradiative decay of excitons, leading to improved photoluminescence quantum yields. PQ3, with well-aligned excited states, enhances reverse intersystem crossing rate constant and displays a high external EL quantum efficiency in the orange-red OLED.