Fully space-confined donor-acceptor interaction for highly efficient thermally activated delayed fluorescence emitters

The emerging through-space charge transfers (TSCT) emitters with thermally activated delayed fluorescence (TADF) have drawn increasing interest. However, the low luminescence efficiency and slow reverse intersystem crossing (RISC) always limit their device performances. A new molecular design strategy for TSCT-TADF emitters with a fully space-confined donor/acceptor conformation that integrates high luminescence efficiency and fast RISC is proposed herein. Two new TSCT-TADF emitters, TRZ-STFMe and TRZ-STFPh, are developed by intro-ducing the methyl and phenyl substituents at the C2 of the rigid fluorene scaffold, respectively. The repulsive interaction between methyl and the acceptor induces a close and face-to-face donor-acceptor stacking to generate highly efficient TADF and suppress structural changes during the RISC process in TRZ-STFMe. In contrast, TRZ-STFPh shows relatively low luminescence efficiency and slow RISC because of its conformationally unstable phenyl substituent. With a high luminescence efficiency of 97 % and a significant RISC rate of 5.23 x 105 s- 1, TRZ-STFMe demonstrates a high electroluminescence performance with a maximum external quantum efficiency (EQE) of 29.6 % and an EQE of 26.5 % at 1000 cd m- 2.