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.

提升纯有机发光材料的三重态激子（triplet exciton）利用率，对有机发光二极管（organic light-emitting diodes, OLEDs）的效率提升具有重要意义。本文中，我们将溴原子引入具有热活化延迟荧光（thermally activated delayed fluorescence）特性的典型分子双(咔唑-9-基)-4,5-二氰基苯（bis(carbazol-9-yl)-4,5-dicyanobenzene）中，研究发现溴的重原子效应（heavy atom effect）可增强自旋轨道耦合（spin–orbit coupling），促进反向系间窜越（reverse intersystem crossing），进而使该类分子展现出更为显著的延迟荧光特性。随着溴原子数目增加，三重态激子利用率得到大幅提升，使得OLEDs的外量子效率（external quantum efficiencies）显著提升。利用溴原子对延迟荧光的增强效应，可为开发兼具高激子利用率的高效有机发光体（organic luminogens）提供一种简便且极具应用前景的设计思路。