Improving Efficiency of Red Thermally Activated Delayed Fluorescence Emitter by Introducing Quasi‐Degenerate Orbital Distribution

Abstract

Comprehensive SummaryRed thermally activated delayed fluorescence (TADF) emitters are essential for the development of organic light‐emitting diodes (OLEDs). To better understand and develop red TADF emitters, herein, the effect of quasi‐degenerate orbital distribution was investigated on their performance. Two red TADF emitters 10,10'‐(11,12‐difluorodibenzo[a,c]phenazine‐3,6‐diyl)bis(10H‐phenoxazine) (DPXZ‐BPF) and 10,10',10”‐(12‐fluorodibenzo[a,c]phenazine‐3,6,11‐triyl)tris(10H‐phenoxazine) (TPXZ‐BPF) were constructed by combining two or three phenoxazine (PXZ) donor units with a rigid coplanar acceptor core. As expected, a couple of PXZ units can induce quasi‐degenerate orbital distribution by virtue of the high steric hindrance between each donor unit and acceptor core, which enables multiple excited states engaged in the reverse intersystem crossing (RISC) process. Consequently, DPXZ‐BPF and TPXZ‐BPF exhibit fast RISC rates (kRISCs) of 1.07 × 106 s–1 and 1.29 × 106 s–1 and high PLQYs of 74.6% and 81.1%, respectively. And the higher kRISC and PLQY of TPXZ‐BPF are mainly attributed to the additional quasi‐degenerate orbitals, which further enhance the triplet RISC process and the singlet radiative process. As a result, TPXZ‐BPF‐based OLEDs achieved improved efficiencies with a red‐shifted emission compared with DPXZ‐BPF‐based OLEDs. These results demonstrated the tremendous potential of introducing quasi‐degenerate orbital distribution in developing red TADF emitters.