Pentacyclic Ladder-Heteraborin Emitters Exhibiting High-Efficiency Blue Thermally Activated Delayed Fluorescence with an Ultrashort Emission Lifetime

Abstract

As one of the three primary colors, blue is significant for display and lighting applications and the development of blueemitting molecules with high quantum efficiency, and color purity for organic light-emitting diodes (OLEDs) remains a key challenge. Herein, a new series of blue thermally activated delayed fluorescence (TADF) materials featuring π-extended ladderoxaborin and thiaborin acceptors has been developed. Steadystate and time-resolved photophysical measurements revealed the advantages of the ladder-oxaborin emitter, including a tiny singlet− triplet energy splitting of 10 meV, an ideal photoluminescence quantum yield of 100%, and an ultrashort TADF lifetime of 780 ns. The TADF-OLEDs incorporating the ladder-oxaborin (deep-blue) and thiaborin (sky-blue) emitters achieved significantly high external electroluminescence quantum efficiencies of up to 20.1% and 25.9%, respectively, accompanied by suppressed efficiency roll-offs.

Thermal equilibration between singlet and triplet excited states in organic fluorophore for submicrosecond delayed fluorescence

“…This requires us to consider the involvement of 3 LE as an intermediate excited state in RISC, which can provide an effective SOC between the 1 CT and 3 LE states with different orbital symmetries necessary for spin flipping in accordance with the El-Sayed rule ( 19 – 25 ). Very recently, we revealed that in heteraborin-based TADF emitters, the incorporation of a heavier sulfur atom instead of oxygen and nitrogen lead to an intensified SOC between the 1 CT and 3 LE states and, thus, a remarkably high k RISC approaching 10 7 s −1 , which is comparable to k r ( 26 – 28 ). The introduction of heteroatoms with lone-pair electrons or empty orbitals results in an effective perturbation to the excited-state electronic configurations for accelerating RISC.…”

Recent Advancement in Boron-Based Efficient and Pure Blue Thermally Activated Delayed Fluorescence Materials for Organic Light-Emitting Diodes

“…Later, Agou et al (2020) reported the pentacyclic ladderheteraborin blue TADF emitters. Many researchers have investigated the boron-containing π-electron system because of its fascinating properties in terms of TADF emitter.…”

19‐1: Invited Paper: Stable Pure‐Blue Hyperfluorescence OLEDs

“…In fact, sky-blue (or green) TADF emitters exhibiting larger kRISCs with one order of magnitude higher than that of HDT-1 have been realized through a strict control of the excited state alignment between a CT state and a LE state. 22 The concept presented here can be applied to other advanced TADF materials to realize more stable pure-blue OLEDs. In addition, we should note that a suppression of direct carrier trapping on a terminal dopant can also improve the operational lifetime.…”