Angewandte
Chemie
Light-Emitting Diodes
Triarylboron-Based Fluorescent Organic Light-Emitting Diodes with
External Quantum Efficiencies Exceeding 20%
Katsuaki Suzuki, Shosei Kubo, Katsuyuki Shizu, Tatsuya Fukushima, Atsushi Wakamiya,
Yasujiro Murata, Chihaya Adachi, and Hironori Kaji*
Abstract: Triarylboron compounds have attracted much
attention, and found wide use as functional materials because
of their electron-accepting properties arising from the vacant p
orbitals on the boron atoms. In this study, we design and
synthesize new donor–acceptor triarylboron emitters that show
thermally activated delayed fluorescence. These emitters dis-
play sky-blue to green emission and high photoluminescence
quantum yields of 87–100% in host matrices. Organic light-
emitting diodes using these emitting molecules as dopants
exhibit high external quantum efficiencies of 14.0–22.8%,
which originate from efficient up-conversion from triplet to
singlet states and subsequent efficient radiative decay from
singlet to ground states.
maximum hEQE of OLEDs with conventional fluorescent
emitters is limited to 5–7.5%. Thus, efficient conversion of
triplet excitons into light is needed to realize OLEDs with
high hEQE
.
To extract light from triplet excitons, phosphorescent
organometallic materials have been used as emitters in
[8]
OLEDs. Phosphorescent organometallic emitters can the-
oretically convert 100% of excitons to light because of strong
spin–orbit coupling, so they have been frequently used in
[8c]
OLEDs. Several TAB-based organometallic phosphores-
[
9]
cent emitters have been reported.
Thermally activated delayed fluorescence (TADF) has
recently been used as a different approach to acquire light
[10,11]
from triplet excitons.
TADF emitters, which often have
T
riarylboron (TAB) compounds have a vacant p-orbital on
D-A frameworks, can convert the lowest triplet excited state
(T ) to the lowest singlet excited state (S ) through reverse
intersystem crossing (RISC) by thermal activation.
Although early TADF-based OLEDs showed relatively low
the central boron atom so they possess attractive electron-
accepting properties. Donor-acceptor (D-A) systems with
a TAB acceptor and amine-based donor groups have received
considerable interest because of their strong intramolecular
1
1
[10,11]
[
10]
hEQE
,
recent TADF emitters possess outstanding perfor-
[
1–5]
[11]
charge transfer (ICT) properties.
TAB-based D-A compounds strongly influences their photo-
physical and photochemical properties, and makes them
useful for non-linear optics, anion sensing, hydrogen
activation and storage, and optoelectronics.
However, a limited number of TAB D-A compounds have
been used as emitters in organic light-emitting diodes
The ICT character of
mance. The number of highly efficient TADF emitters is
increasing rapidly at present, but is still small compared with
the multitude of phosphorescence emitters. In particular,
[
1]
[
2]
[3]
[12]
there are few reports of TAB-based TADF because it is still
[
4]
[5]
challenging to use versatile TAB-based D-A frameworks as
TADF emitters, even though the strong p-electron accepting
ability related to the p-p* conjugation of TAB materials
should be beneficial to realize TADF.
Herein, we report TAB-based TADF emitters 1–3 com-
posed of an electron-accepting trimesitylboron analogue
(
OLEDs) because the external quantum efficiency (hEQE) of
[5a–f]
such devices has been relatively low.
One origin of the low
[
6]
hEQE is spin statistics; conventional fluorescent materials can
convert only 25% of electrogenerated singlet excitons into
light, and the remaining 75% of generated triplet excitons are
deactivated as heat. Considering that the light out-coupling
(Mes B) and three amine-based electron-donating units (Fig-
3
ure 1a). OLEDs using 1–3 as emitting dopants exhibit high
hEQE of up to 22.8%, far exceeding the theoretical limit for
normal fluorescent emitters of 5–7.5%. Specifically, the hEQE
of 21.6% for a sky-blue OLED containing 2 is the highest
[
7]
efficiency of OLEDs is typically 20–30%, the theoretical
[13]
value for a TADF-based blue OLED, while 22.8% is the
highest value for an OLED with a TAB emitter.
[
*] Dr. K. Suzuki, S. Kubo, Dr. K. Shizu, Dr. T. Fukushima,
Prof. Dr. A. Wakamiya, Prof. Dr. Y. Murata, Prof. Dr. H. Kaji
Institute for Chemical Research Kyoto University
Uji, Kyoto 611-0011 (Japan)
To realize efficient TADF emission, a very small energy
[11]
gap between S and T (DE ) is required to promote RISC.
1
1
ST
E-mail: kaji@scl.kyoto-u.ac.jp
DEST generally decreases when the exchange interaction
between the highest occupied molecular orbital (HOMO) and
lowest unoccupied molecular orbital (LUMO) of a molecule
Prof. Dr. C. Adachi
Center for Organic Photonics and Electronics Research
Kyushu University
[14]
weakens. We selected the widely used Mes B unit as an
3
7
44 Motooka, Nishi, Fukuoka 819-0395 (Japan)
[1–5]
electron-accepting segment,
which has methyl groups at
and
the ortho position of CÀB bonds that protect the B atoms
from oxygen and water. As the electron-donating unit, we
used phenoxazine (PXZ), bis(diphenylamino)carbazole
(2DAC), and diphenylaminocarbazole (DAC) units aiming
to minimize DEST (Figure 1a).
Japan Science and Technology Agency (JST)
Exploratory Research for Advanced Technology (ERATO)
Adachi Molecular Exciton Engineering Project
7
44 Motooka, Nishi, Fukuoka 819-0395 (Japan)
Angew. Chem. Int. Ed. 2015, 54, 15231 –15235
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
15231