Strategy of Modulating Nonradiative Decay for Approaching Efficient Thermally Activated Delayed Fluorescent Emitters

The photoluminescence quantum yield (PLQY) of an emitter
reflects
the extent of the nonradiative process and significantly influences
the performance of the electroluminescent devices. This study presents
a new strategy for designing thermally activated delayed fluorescence
(TADF) materials based on suppressing nonradiative decay. The substituents
and their positions can modulate exciton behavior in a molecule. We
modified the diboraanthranene framework with various alkyl substituents
as models to explore the strategy for designing donor–acceptor–donor
(D–A–D)-type TADF emitters. Experimental results and
theoretical simulations indicate that suppressing nonradiative decay
can enhance PLQYs and the delayed component (Φd)
of PLQY. Based on the theory of nonradiative decay, our findings suggest
that the local C–H stretching modes originating from the alkyl
groups are the main contributors to the nonradiative decay rates of
the first excited singlet state (knr,S). Notably, our simulations demonstrate that the PLQYs of the five
emitters are increased by alkyl deuteration and can be nearly optimized
to 100% through perdeuteration. MECzDBA- and EECzDBA-based green organic
light-emitting diodes (OLEDs) achieve superior external quantum efficiencies
of 29.4 and 28.6%, respectively, with reduced efficiency roll-off
of only 5.1 and 5.2% at 1000 cd m–2. The outcomes
from our study suggest a molecular design strategy that successfully
counteracts the nonradiative decay in TADF materials, providing a
new direction for creating high-efficiency organic emitters.