Angewandte
Communications
Chemie
Organic Light-Emitting Diodes
Aggregation-Induced Delayed Fluorescence Based on Donor/
Acceptor-Tethered Janus Carborane Triads: Unique Photophysical
Properties of Nondoped OLEDs
Ryuhei Furue+, Takuro Nishimoto+, In Seob Park, Jiyoung Lee, and Takuma Yasuda*
Abstract: Luminescent materials consisting of boron clusters,
such as carboranes, have attracted immense interest in recent
years. In this study, luminescent organic–inorganic conjugated
systems based on o-carboranes directly bonded to electron-
donating and electron-accepting p-conjugated units were
elaborated as novel optoelectronic materials. These o-carbor-
ane derivatives simultaneously possessed aggregation-induced
emission (AIE) and thermally activated delayed fluorescence
(TADF) capabilities, and showed strong yellow-to-red emis-
sions with high photoluminescence quantum efficiencies of up
to 97% in their aggregated states or in solid neat films. Organic
light-emitting diodes utilizing these o-carborane derivatives as
a nondoped emission layer exhibited maximum external
electroluminescence quantum efficiencies as high as 11%,
originating from TADF.
annihilation in the devices at high doping concentrations
and high current densities. To simplify the device structures
and fabrication processes, it is desirable to construct highly
efficient OLEDs with nondoped (neat) emission layers.
In 2001, Tang and co-workers discovered an intriguing
photophysical phenomenon termed aggregation-induced
emission (AIE), which has been proved to be an effective
approach to suppress concentration quenching and hence to
attain efficient solid-state luminescence.[2] Thus far, a series of
AIE-active fluorophores, including siloles,[2,3] cyanostil-
benes,[4] tetraphenylethenes,[5] and o-carborane derivatives,[6]
has been reported. These molecules are essentially non-
luminescent in dilute solutions but become highly emissive
upon formation of aggregates. On this basis, some AIE-active
fluorophores have been developed as emitters for efficient
nondoped OLEDs.[5,7] However, these fluorescent devices
still utilize only the singlet (S1) excitons for electrolumines-
cence (EL); these excitons correspond to about 25% of the
total electro-generated excitons, whereas employing the
remaining unharvested 75% triplet (T1) excitons could
potentially lead to molecular systems with enhanced EL
properties.
o-Carborane (1,2-closo-dicarbadodecaborane (C2B10H12))
is known as an electron-deficient icosahedral boron cluster
consisting of three-center two-electron bonds, and possesses
highly polarizable s-aromatic character.[8] This unique feature
allows the carborene cage to interact electronically with p-
conjugated systems attached through the carbon atoms. Chujo
and Kokado reported a series of luminescent o-carborane
derivatives substituted with various electron-donating and
electron-withdrawing aromatic groups that were applied to
color-tunable AIE-active systems.[6a] Research interest has
also focused on the use of o-carborane cages as building
blocks to explore new optoelectronic functional materials,
including organic small molecules,[6a–c,9] polymers,[6d–f, 10] and
metal complexes.[11] However, the detailed photophysical
properties of carborane-based AIE-active molecules and
their potential applicability as emitters in OLED devices
have not been fully elucidated to date.
O
rganic materials with efficient solid-state emission are
essential for various optoelectronic applications, especially
for organic light-emitting diodes (OLEDs).[1] However, the
efficient luminescence detected for most organic fluoro-
phores in dilute solutions is generally weakened or quenched
when the molecules aggregate in their condensed solid states.
This effect is because of aggregation-caused emission quench-
ing (so-called concentration quenching) and is attributed to
nonradiative deactivation processes, such as excitonic cou-
pling, excimer formation, and intermolecular excitation
energy transfer. Therefore, for OLEDs, organic fluorophores
are generally used as a dopant (guest emitter) dispersed in an
appropriate host matrix, whereas the application of nondoped
materials is restricted. Moreover, efficacious doping often
requires precise control of the doping concentration in order
to suppress serious efficiency roll-off caused by exciton
[*] R. Furue,[+] I. S. Park, J. Lee, Prof. Dr. T. Yasuda
INAMORI Frontier Research Center (IFRC), Kyushu University
744 Motooka, Nishi-ku, Fukuoka 819-0395 (Japan)
E-mail: yasuda@ifrc.kyushu-u.ac.jp
tronics/
R. Furue,[+] J. Lee, Prof. Dr. T. Yasuda
Department of Automotive Science, Graduate School of Integrated
Frontier Science, Kyushu University
744 Motooka, Nishi-ku, Fukuoka 819-0395 (Japan)
T. Nishimoto,[+] I. S. Park, Prof. Dr. T. Yasuda
Department of Applied Chemistry
Graduate School of Engineering, Kyushu University
744 Motooka, Nishi-ku, Fukuoka 819-0395 (Japan)
In this Communication, we report a novel class of AIE-
active organic–inorganic conjugated systems based on o-
carboranes tethered with donor (D) and acceptor (A)
moieties (denoted “Janus” carboranes 1–3; Figure 1). It is
found that these triad molecules can harvest both S1 and T1
excitons and display thermally activated delayed fluores-
cence[12] through upconversion from nonradiative T1 excitons
into emissive S1 excitons in their aggregated states. As
a consequence of such unique aggregation-induced delayed
fluorescence (AIDF) ability,[13] high external EL quantum
[+] These authors are contributed equally to this work.
Supporting information for this article can be found under:
Angew. Chem. Int. Ed. 2016, 55, 7171 –7175
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
7171