Communication
terials is comparatively lower than that of small-molecule-
[
11,12]
Abstract: A thiophene-phenylquinoline-based homoleptic
based OLEDs.
In contrast, the design and synthesis of
III
Ir complex, [Ir(Th-PQ) ], has been synthesised by a simple
3
highly soluble small molecular phosphorescent emitter for so-
lution-processed PhOLEDs is very scarce. Very recently, Wu
et al. reported the solution processed small molecule PhOLEDs
with the highest external quantum efficiency (EQE) of 15%.
route and utilised as a dopant in solution-processed phos-
phorescent organic light-emitting diodes (PhOLEDs). It
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1
shows the current efficiency of approximately 26 cdA
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and the external quantum efficiency of about 21%, which
are the highest values reported to date for PhOLEDs pre-
pared by solution-process.
However, they achieved the homoleptic Ir complex by multi-
[13]
step synthetic routes.
In this work, we have synthesised tris(4-phenyl-2-(thiophen-
2
-yl)quinoline)iridium, [Ir(Th-PQ) ], and utilised it as a dopant
3
III
for solution-processed PhOLEDs. The ligand of this Ir complex
was synthesised from two commercially available starting ma-
terials.
Phosphorescent organic light-emitting diodes (PhOLEDs) have
potential applications in flat-panel displays and solid-state
[
1]
lighting sources. The use of phosphorescent dopants can
greatly increase the PhOLED efficiency due to harvesting both
singlet and triplet excitons; this can provide nearly 100% inter-
Introduction of electron-donating thiophene unit into the
ligand frame of [Ir(Th-PQ) ] improves the quantum yield, and
3
red shifts the photoluminance (PL) maximum compared to
their benzene counterparts due to the decrease in the ligand’s
[
2]
nal quantum efficiency. Among the phosphorescent emitters,
III
[14]
Ir complexes have been considered as the most efficient trip-
triplet energy with increasing p conjugation. On the other
let dopant in high efficiency PhOLEDs due to their excellent
colour tunability and relatively short phosphorescence life-
hand, the strong electron-accepting character of the quinoline
3
group can effectively reduce the MLCT exited energy of the
[
3]
times. When compared to the green phosphors, the design
and synthesis of red- and near-infrared emitting phosphors are
more complicated because the luminescence quantum yields
tend to decrease with longer wavelengths according to the
[Ir(Th-PQ) ]. The simple solution-processed PhOLEDs were fab-
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ricated using [Ir(Th-PQ) ] as a dopant and achieved the lumi-
3
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1
nous current efficiency of approximately 26 cdA and the EQE
of about 21%, which are the highest values reported to date
for PhOLEDs prepared by the solution-process.
[
4]
energy-gap law. Therefore, the design and synthesis of red
phosphors for highly efficient PhOLEDs remain to be further
developed.
[Ir(Th-PQ) ] was synthesised according to the route shown in
3
1
13
Scheme 1 and its structure was confirmed by H, C NMR,
HRMS spectral techniques and single-crystal X-ray analysis. The
synthetic details are provided in the Supporting Information.
The single crystal was grown by the diffusion of hexane into
The general approach to increase quantum efficiency of par-
ticular phosphorescent dopants is to replace electron-donating
or electron-withdrawing groups with heteroatoms in the cyclo-
[
5]
metalated ligand systems. However, ligands containing two
heteroatoms that increase the efficiency and luminance half-
a dichloromethane solution of [Ir(Th-PQ) ]. The ORTEP repre-
3
sentation of the [Ir(Th-PQ) ] is shown in Figure 1a. It has tet-
3
III
[6]
lifetime of Ir complexes have seldom been reported. For this
purpose, quinolone-based compounds have been investigated
because of their elevated electron affinities in opto-electronic
ragonal geometry with three Th-PQ main ligands around the
metal centre.
III
The thermal stability of the Ir complex was investigated by
[
7]
materials. These compounds are stable at high current densi-
ty and possess short phosphorescence lifetimes to suppress
triplet–triplet annihilation, thereby improving device quantum
thermal gravimetric analysis (TGA; Figure S1 in the Supporting
Information). The onset decomposition temperature of [Ir(Th-
PQ) ] was observed at 4098C indicating its high thermal stabili-
3
[
8,9]
efficiency.
ty. The UV–visible absorption spectrum of [Ir(Th-PQ) ] in chloro-
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Fabrication of PhOLEDs by the solution process has more
advantages, such as low cost fabrication and large area appli-
cations, than their highly expensive vacuum-deposition coun-
form solution is shown in Figure 1b. It shows two absorption
bands at 312 and 421 nm corresponding to the spin-allowed
1
p!p* transition of the cyclometalated ligands and a well-re-
[
10]
terparts. The luminous efficiency achieved in polymeric ma-
solved shoulder at 459 nm originating from the singlet metal-
to-ligand charge transfer (MLCT) transitions. A structure-less
+
weak band at 578 nm was attributed to the spin-forbidden
[
a] T. Giridhar, W. Cho, Dr. C. Saravanan, Prof. Dr. S.-H. Jin
3
Department of Chemistry Education
MLCT transition. The PL spectrum of [Ir(Th-PQ) ] is shown in
3
Graduate Department of Chemical Materials and
Institute for Plastic Information and Energy Materials
Pusan National University, Busan, 609-735 (Republic of Korea)
Figure 1b. It shows an emission maximum at 601 nm in chloro-
form solution due to the presence of donor–acceptor interac-
tions between the electron-donor thiophene and electron-ac-
ceptor quinoline (Th-PQ) groups in the cyclometalated ligand.
The donor–acceptor character is caused by the interaction be-
tween electron-rich thiophene and electron-deficient quinoline
E-mail: shjin@pusan.ac.kr
+
[
b] T.-H. Han, Prof. Dr. T.-W. Lee
Department of Materials Science and Engineering
Pohang University of Science and Technology (POSTECH)
San 31 Hyoja-dong, Nam-gu, Pohang
Gyungbuk, 790-784 (Republic of Korea)
E-mail: twlee@postech.ac.kr
[14]
units.
The electrochemical properties of [Ir(Th-PQ) ] were studied
3
+
using cyclic voltammetry and the corresponding cyclic voltam-
mogram is shown in Figure 2a. Appearance of good redox
waves confirms the presence of both anionic and cationic radi-
[
] These authors contributed equally to this work
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201400243.
Chem. Eur. J. 2014, 20, 8260 – 8264
8261
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim