Modified Intramolecular‐Lock Strategy Enables Efficient Thermally Activated Delayed Fluorescence Emitters for Non‐Doped OLEDs

The development of intramolecular-lock strategy is an appealing task for designing efficient thermally activated delayed fluorescence (TADF) molecules, but only limited examples have been reported so far. Herein we present a "medium ring"-lock strategy to develop TADF emitters for improving the efficiency of organic light-emitting diodes (OLEDs). The installation of an electron-deficient heptagonal diimide lock onto a highly rotatable biphenyl-based emitter not only enhances electron-withdrawing ability of acceptor that decreases singlet-triplet energy gap (ΔEST ), but also endows the skeleton with modest rigidity and flexibility that increases photoluminescence quantum yield (PLQY) in neat film. In particular, the integration of the diimide lock also leads to an increase in horizontal orientation factor (Θ// ) from 69 % to 83 %. Consequently, this modified intramolecular-lock strategy enables an efficient TADF emitter to assemble high-performance non-doped OLEDs with a high external quantum efficiency of 26.2 % and a power efficiency of 76.6 lm W-1 .