Dihydrophenazine-derived thermally activated delayed fluorescence emitters for highly efficient orange and red organic light-emitting diodes

Thermally activated delayed fluorescence (TADF) emitters in the long wavelength region usually suffer from severe nonradiative decay according to the energy-gap law, and show relatively large energy split (ΔEST) between the lowest singlet and triplet excited states which compromise efficient up-conversion. Here, we report three orange and red TADF emitters comprising 5, 10-disubstituted phenazine electron-donor and quinoxaline-derived electron-acceptors, which show excellent TADF properties and yield efficient organic light-emitting diodes (OLEDs). These symmetrical acceptor-donor-acceptor (A-D-A) type and rodlike molecules concurrently exhibit high photoluminescence quantum yields up to 85% and fast reverse intersystem crossing. Experimental and theoretical studies reveal that these excellent photophysical properties are attributed the synergistic effect of structural rigidity, steric hindrance and strength of the donor and acceptor units. Doped OLEDs based on these emitters achieved high maximum external electroluminescence quantum efficiencies up to 19.9%, and maximum luminance surpassing 31240 cd/m2.