Amidinate-ligated iridium(iii) bis(2-pyridyl)phenyl complex as an excellent phosphorescent material for electroluminescence devices

Article abstract of DOI:10.1039/b902807b

Highly efficient, low driving-voltage, and emitter concentration insensitive phosphorescent EL devices are established for the first time by using an amidinate-ligated iridium(iii) complex as an emitting component.

Full text of DOI:10.1039/b902807b

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Chemical Communications
www.rsc.org/chemcomm
Number 25 | 7 July 2009 | Pages 3641–3812
ISSN 1359-7345
COMMUNICATION
Zhaomin Hou et al.
FEATURE ARTICLE
Martyn P. Coles
Amidinate-ligated iridium(III) bis
(2-pyridyl)phenyl complex as an
excellent phosphorescent material for
electroluminescence devices
Bicyclic-guanidines, -guanidinates
and -guanidinium salts: wide ranging
applications from a simple family of
molecules
1359-7345(2009)25;1-Z

COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Amidinate-ligated iridium(III) bis(2-pyridyl)phenyl complex as an
excellent phosphorescent material for electroluminescence devicesw
Yu Liu,ab Kaiqi Ye,b Yan Fan,b Weifeng Song,b Yue Wangb and Zhaomin Hou*a
Received (in Cambridge, UK) 10th February 2009, Accepted 11th March 2009
First published as an Advance Article on the web 26th March 2009
DOI: 10.1039/b902807b
Highly efficient, low driving-voltage, and emitter concentration
insensitive phosphorescent EL devices are established for the
first time by using an amidinate-ligated iridium(III) complex as
an emitting component.
of the iridium(III) bis(pyridylphenyl)/amidinate complex
(ppy)2Ir(dipba). This new complex showed highly efficient
electroluminescence not only in a non-doped device but also
in doped devices in a wide range of doping concentrations.
Scheme 1 illustrates the chemical structure and synthetic
route of the amidinate-ligated iridium complex (ppy)2Ir(dipba).
The one-pot reaction of bromobenzene with nBuLi and
N,N0-diisopropylcarbodiimide, followed by refluxing with
Organic light-emitting devices (OLEDs) using phosphorescent
metal complexes as emitting materials have received much
current interest, because their emission efficiency is higher than
that of the fluorescent counterparts.1,2 Unfortunately, however,
high-performance phosphorescent OLEDs are usually fabricated
by doping the emitters into a charge-transporting host matrix
in a rather low and narrow concentration range, because of the
poor carrier mobility or luminescence self-quenching of
the neat phosphorescent dopants. Consistent control of the
doping process to keep a constant, low dopant concentration
(less than 10%) is a challenging task for the reproducibility
of mass production. Recently, considerable efforts have
been devoted to the development of phosphorescent OLEDs
with a non-doped emitting layer or with higher dopant
concentrations.3 However, the emitting materials reported so
far usually required complicated synthetic processes and the
resulting EL devices generally showed rather high turn-on
voltages (44 V) and moderate efficiency (o7 lm Wꢀ1).
The search for new emitting materials suitable for low
driving-voltage, high performance OLEDs with a non-doped
emitting layer or that are insensitive to dopant concentration
is, therefore, of much importance and interest.
5
[(ppy)2Ir(m-Cl)]2 in hexane afforded (ppy)2Ir(dipba) in 74%
isolated yield.
An X-ray diffraction studyz revealed that the distance
between two nearest parallel ppy planes is as long as
3.9 A and there is no overlap between these two planes in
(ppy)2Ir(dipba) (Fig. 1 (left)). These results suggest that no
significant intermolecular p–p interaction between the ppy
groups could be present, which could help prevent undesired
self-quenching problems in the solid state of this compound.
In agreement with this structure feature, (ppy)2Ir(dipba)
displayed brilliant yellowish-green photoluminescence in both
neat thin film and powder even in the air (Fig. 1 (right)). The
emission maxima of (ppy)2Ir(dipba) in neat thin film appeared
at 553 nm, which was blue-shifted in dichloromethane to
543 nm (see ESIw). Its phosphorescence quantum yield (Fp)
in solution is 0.30, which is comparable to that of the
well-known Ir(ppy)3 complex (0.4).3b The phosphorescence
lifetime of (ppy)2Ir(dipba) under ambient temperature conditions
was found to be 0.35 ms.
Amidinates [R0NC(R)NR0] have been widely used as
ancillary ligands for various transition metal complexes in
coordination chemistry.4 However, the use of an amidinate
ligand for a phosphorescent metal complex has not been
reported previously. We envisioned that a sterically hindered
chelating amidinate ligand might help to solve the self-quenching
problems encountered previously with phosphorescent metal
complexes and thus lead to formation of more efficient
phosphorescent OLEDs. To examine the usefulness of an
amidinate unit as an ancillary ligand for phosphorescent metal
complexes, we chose N,N0-diisopropylbenzamidinate (dipba)
as an ancillary ligand to combine with the well-known
phosphorescent ‘‘(ppy)2Ir’’ species (ppy = o-(2-pyridyl)phenyl).
We report here the synthesis and novel luminescence properties
To ascertain the influence of the amidinate ligand on
the luminescence properties of the iridium complex, DFT
calculations on (ppy)2Ir(dipba) were carried out. The LUMO
of (ppy)2Ir(dipba) is located largely on the ppy ligand (Fig. 2),
which is similar to those of the well-known, analogous Ir
complexes Ir(ppy)3 and (ppy)2Ir(acac).6 However, the HOMO
of (ppy)2Ir(dipba) consists principally of a mixture of the N
atoms of the dipba ligand (49.1%) and the d-orbitals of the Ir
a Organometallic Chemistry Laboratory, RIKEN Advanced Science
Institute, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan.
E-mail: houz@riken.jp
b State Key Laboratory of Supramolecular Structure and Materials,
Jilin University, Changchun 130012, People’s Republic of China
w Electronic supplementary information (ESI) available: Experimental
details. CCDC 714889. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/b902807b
Scheme 1 Synthetic route for the Ir complex (ppy)2Ir(dipba).
Chem. Commun., 2009, 3699–3701 | 3699
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This journal is The Royal Society of Chemistry 2009

Fig. 1 Crystal packing diagrams (left) and fluorescence images (right)
of solid film and powder of (ppy)2Ir(dipba).
Fig. 3 Brightness–voltage curves of devices I–III. Inset: EL spectra of
devices I–III at 6 V.
which has a low doping concentration (7 mol%). The
maximum emission of device II, which has a higher dopant
concentration (25 mol%), was almost the same as that of the
non-doped device I.
The non-doped device I and the high-concentration doped
device II exhibited rather low turn-on voltages (2.3 cd mꢀ2 at
2.4 V and 1 cd mꢀ2 at 2.5 V, respectively), which are even
lower than that of the low-concentration doped device III
(3.6 V) (Fig. 3). Moreover, devices I and II showed very low
driving voltages at the practical brightness of 100 cd mꢀ2 and
1000 cd mꢀ2. In particular, the driving voltages for 100 cd mꢀ2
and 1000 cd mꢀ2 in the non-doped device I are as low as 3.15 V
and 4.30 V, respectively, which are near the best values
reported for Ir(ppy)3-doped devices which possessed further
sophisticatedly modified device structures.7 The lowering of
the driving voltages in devices I and II could result from
the narrow HOMO–LUMO energy gap of (ppy)2Ir(dipba).
Moreover, because the HOMO level of (ppy)2Ir(dipba)
(ꢀ4.78 eV) is higher than that of CBP (ꢀ6.1 eV),8c the direct
injection of holes from NPB (HOMO: ꢀ5.4 eV)8c into the Ir
complex in the non-doped device I and the high-concentration
doped device II should be energetically more favorable
than that in low-concentration doped device III. These
results suggest that the direct hole-transfer between the hole-
transporting layer and the triplet emitter is obviously important
to guarantee an efficient carrier recombination on the
phosphor.8
Fig. 2 The contour plots of LUMO (left) and HOMO (right) in
(ppy)2Ir(dipba) from DFT calculations.
atom (29.6%). This is different from those of Ir(ppy)3 and
(ppy)2Ir(acac) which are distributed on the p orbitals of the ppy
ligands in addition to the Ir d-orbitals. These results can be
ascribed in part to the p-bonding ability of the dipba ligand
being stronger than that of the acac ligand or the ppy ligand,6
and are consistent with the fact that the HOMO energy level of
(ppy)2Ir(dipba) (ꢀ4.78 eV) is higher than those of Ir(ppy)3
(ꢀ5.2 eV)7a and (ppy)2Ir(acac) (ꢀ5.0 eV)8c and that the
HOMO–LUMO energy gap of (ppy)2Ir(dipba) (ꢀ2.29 eV) is
significantly smaller than those of Ir(ppy)3 (ꢀ2.4 eV) and
(ppy)2Ir(acac) (ꢀ2.4 eV). These data also suggest that the
amidinate-ligated iridium complex should have hole-transporting
ability better than those of Ir(ppy)3 and (ppy)2Ir(acac).9
To examine the electrophosphorescence properties of the
present iridium complex, three devices (I, II and III) were
fabricated with the same configuration of ITO/NPB
(30 nm)/emitter (35 nm)/BCP (10 nm)/AlQ (25 nm)/LiF
(0.5 nm)/Al (NPB = 4,4-bis(N-(1-naphthyl)-N-phenylamino)-
biphenyl, BCP = 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline,
AlQ = tris(8-hydroxyquinoline)aluminium). Device I used
(ppy)2Ir(dipba) as a non-doped emitter, while devices II and
III employed (ppy)2Ir(dipba) doped in CBP (4,40-N,N0-
dicarbazolylbiphenyl) with different doping levels (25 mol%
in II and 7 mol% in III). Here, NPB, BCP and AlQ serve as
hole-transporting, hole-blocking and electron-transporting
layers, respectively, while CBP acts as a host material.
In addition to their better brightness/driving-voltage
performance, devices I and II also exhibited power efficiency
a little higher than that of device III in the whole current
density range (Fig. 4). Device I showed the maximum power
efficiency of 32.5 lm Wꢀ1, which, as far as we are aware,
represents the most efficient non-doped phosphorescent
OLED ever reported.3 The maximum power efficiency of
device II reached 29.5 lm Wꢀ1, which is the highest ever
reported for an OLED at a similar doping concentration level
(ca. 25 mol%).3b,d,f It is also noteworthy that at the practical
brightness of 100 cd mꢀ2 and 1000 cd mꢀ2, the power
efficiencieꢀs1of device I remained as high as 28 lm Wꢀ1 and
22 lm W , respectively, and those of device II as high as
28.5 lm Wꢀ1 and 23.5 lm Wꢀ1, respectively. The pronounced
EL performance enhancement could partly profit from the
very low driving voltages, and on the other hand, may also
suggest that the emitting iridium complex (ppy)2Ir(dipba) can
All devices fabricated above exhibited bright yellowish-green
emission, independent of the applied voltages in the range of
3 V to 16 V. The electroluminescence (EL) spectra of devices
I–III at a driving voltage of 6 V are shown in Fig. 3 (inset). The
maximum EL emission of the non-doped device I appeared at
552 nm. It was blue-shifted to 544 nm in the doped device III
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This journal is The Royal Society of Chemistry 2009
3700 | Chem. Commun., 2009, 3699–3701

the c rotation scan mode. The structure determination was performed by
direct methods using SHELXTL 5.01v and refinements with full-matrix
least squares on F2.
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Fig. 4 Power efficiency–current density curves of devices I–III.
efficiently transport charges without the need for a host.
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emitting layer in the non-doped device.7a
In summary, we have demonstrated for the first time that
the use of a sterically demanding amidinate group as an
ancillary ligand for a bis-cyclometalated (C4N) phosphorescent
iridium species can lead to
a significant reduction in
self-quenching and a dramatic improvement in the electro-
luminescence properties of the complex, thus enabling the
successful fabrication of high efficiency, low driving-voltage
phosphorescent OLEDs in a wide range of doping concentrations
or even without the requirement of doping the emitter into a
host matrix. The insensitivity of EL properties to emitter-
doping concentration would make a sophisticated control of
the doping process unnecessary and thus lead to more efficient,
reproducible fabrication of high performance EL devices.
Because of the excellent performance and the ease of synthesis,
amidinate-ligated phosphorescent metal complexes should
have high potential in practical applications such as flat-panel
displays and organic lighting. Further studies on other phos-
phorescent cyclometalated (C4N) metal complexes bearing
amidinate and related ancillary ligands are in progress.
We are grateful to the Japan Society for the Promotion of
Science (JSPS) for a Postdoctoral Fellowship for Yu Liu. This
work was partly supported by the National Basic Research
Program of China (973 Program, 2009CB623600), the Natural
Science Foundation of China (50733002) and Jilin Provincial
Science and Technology Bureau of Jilin Province (20070107).
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z Single crystals suitable for X-ray structural analysis were obtained by
vacuum sublimation. Diffraction data were collected on a Rigaku R-Axis
Rapid diffractometer (Mo Ka radiation, graphite monochromator) in
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Chem. Commun., 2009, 3699–3701 | 3701