Heavy Atom Effect of Bromine Significantly Enhances Exciton Utilization of Delayed Fluorescence Luminogens

Raising triplet exciton
utilization of pure organic luminescent materials is of significant
importance for efficiency advancement of organic light-emitting diodes
(OLEDs). Herein, by introducing bromine atom(s) onto a typical molecule
(bis­(carbazol-9-yl)-4,5-dicyanobenzene) with thermally activated delayed
fluorescence, we demonstrate that the heavy atom effect of bromine
can increase spin–orbit coupling and promote the reverse intersystem
crossing, which endow the molecules with more distinct delayed fluorescence.
In consequence, the triplet exciton utilization is improved greatly
with the increase of bromine atoms, affording apparently advanced
external quantum efficiencies of OLEDs. Utilizing the enhancement
effect of bromine atoms on delayed fluorescence should be a simple
and promising design concept for efficient organic luminogens with
high exciton utilization.