Donor or Acceptor: Molecular Engineering Based on dibenzo[a,c]phenazine Backbone for Highly Efficient Thermally-Activated Delayed Fluorescence Organic Light-Emitting Diodes

Thermally-activated delayed fluorescence (TADF) emitters are usually constructed with twisted donor-acceptor (D-A) frameworks, while studies on the relationship between diverse D-A structures are still in high demand to achieve high-performance emitters. Herein, by adopting triphenylamine as electron donor and dibenzo[a,c]phenazine as electron acceptor, three TADF molecules are reported with different frameworks of D-A (TPZ), D-A-D (DPZ) and D-A-A (APZ). Theoretical and experimental results demonstrate that different D-A frameworks play significant effects on photophysical, horizontal dipole ratio, and electroluminescence properties of the TADF molecules. In comparison, the APZ-OLED device achieves the best performance with a maximum external quantum efficiency of 27.5%, resulting from its low energy gap between the singlet and triplet, effective reverse intersystem crossing, high photoluminescence quantum yield, and horizontal dipole ratio. This work provides an insight into the relationship between efficient TADF emitters and rational molecular design engineering.