Synthesis and characterization of a new iridium(III) complex with bulky trimethylsilylxylene and applications for efficient yellow-green emitting phosphorescent organic light emitting diodes

Article abstract of DOI:10.1016/j.dyepig.2011.04.017

We designed and synthesized a new iridium(III) complex with phenylpyridine ligands containing a bulky trimethylsilylxylene, 5-(2,5-dimethyl-4- (trimethylsilyl)phenyl)-2-phenylpyridine, by Suzuki coupling reaction and characterized using various spectrosco

Full text of DOI:10.1016/j.dyepig.2011.04.017

Dyes and Pigments 92 (2011) 603e609
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Synthesis and characterization of a new iridium(III) complex with bulky
trimethylsilylxylene and applications for efficient yellow-green emitting
phosphorescent organic light emitting diodes
,
,
Ki Ho So a 1, Ran Kim a 1, Hyuntae Park a, Il Kang b, K. Thangaraju b, Young Seo Park c, Jang Joo Kim c,
Soon-Ki Kwon b , Yun-Hi Kim
,
a,
*
*
a Department of Chemistry and Research Institute of Natural Sciences (RINS), Gyeongsang National University, Jinju 660 701, Republic of Korea
b School of Materials Science and Engineering, and Research Institute for Green Energy Convergence Technology (RIGEC), Gyeongsang National University, Jinju 660 701, South Korea
c OLED centre, Department of Materials Science and Engineering, Seoul National University, Seoul 151 744, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
We designed and synthesized a new iridium(III) complex with phenylpyridine ligands containing a bulky
trimethylsilylxylene, 5-(2,5-dimethyl-4-(trimethylsilyl)phenyl)-2-phenylpyridine, by Suzuki coupling
reaction and characterized using various spectroscopic studies. Nuclear magnetic resonance studies
show its structure to be that of a facial isomer. Thermogravimetric analysis and differential scanning
calorimetry studies show its higher thermal stability (ΔT5%) of 638 K with a glass transition temperature
of 425 K. It shows the photoluminescence emission at 532 nm in solution with the band gap energy of
2.56 eV. The new iridium(III) complex as dopant in phosphorescent organic light emitting diodes exhibits
the yellow-green emission at 532 nm as it effectively hinders aggregation formation in the solid state at
a dopant concentration of 6%, resulting in higher device efficiencies of 12.7% and 45.7 cd/A. The results
show that the new iridium complex could be useful in white organic light emitting diodes for the lighting
applications.
Received 2 February 2011
Received in revised form
26 April 2011
Accepted 30 April 2011
Available online 7 May 2011
Keywords:
Phosphorescent organic light emitting
diodes
Iridium(III) complex
Trimethylsilyl xylene substituted ligands
Yellow-green emission
Reduced aggregation formation
High efficiency
Ó 2011 Published by Elsevier Ltd.
1. Introduction
tuned by the introduction of substituent groups having different
electronic effects or leading to variations in the conjugation system
Since organic light emitting diodes (OLEDs) were demonstrated
first by Tang and Van Slyke, tremendous efforts have been made in
the field of OLEDs. OLEDs have many advantages such as low power
consumption, wide viewing angle, fast response time, compactness
and light weight for display and solid state lighting applications
[1e4]. Compared to the fluorescent OLEDs, the performance of the
phosphorescent OLEDs is significantly improved because both
singlet and triplet excitons generated can be harvested for light
emission [5]. Since then, phosphorescent OLED materials and
devices have been intensively studied for higher device perfor-
mances. Among the various phosphorescent materials, iridium
complexes remain as efficient phosphorescent emitters in OLEDs
because of their relatively short life-times and high quantum effi-
ciencies. The emissionwavelength of iridium complexes can be fine-
of ligands [6e13]. The highly efficient white OLEDs (WOLEDs) have
recently been demonstrated using three-colour (red, green and
blue) phosphorescent materials in which the most commonly used
green-emitting phosphorescent iridium complex, fac-tris(2-
phenylpyridine)iridium [Ir(ppy)3], was employed for the green
emission with peak emission around 510 nm [14,15]. WOLEDs
employing Ir(ppy)3 as a phosphorescent dopant emitter for the
green emission exhibit a lack of emission intensity in the yellow-
green region, which influences the colour purity of the devices
[14,15]. Fluorescent dyes, such as 5,6,11,12-tetraphenylnaphthacene
(rubrene) (peak at 560 nm) and 4-(dicyanomethylene)-2-t-butyl-6-
(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) (peak at
568 nm), which were sensitized by the phosphorescent Ir(ppy)3
complex, have been utilized for yellow-green emission in efficient
WOLEDs suitable for the lighting applications [16,17].
Phosphorescent emitter materials having high thermal stability,
high vapour pressure, good solubility, and a wide range of emission
colours are useful in both solution-processed- and vacuum-
deposited- OLED devices for display and lighting applications. The
* Corresponding authors. Tel.: þ82 55 751 5296; fax: þ82 55 753 6311.
E-mail addresses: skwon@gnu.ac.kr (S.-K. Kwon), ykim@gnu.ac.kr (Y.-H. Kim).
Ki Ho So and Ran Kim are equally contributed.
1
0143-7208/$ e see front matter Ó 2011 Published by Elsevier Ltd.
doi:10.1016/j.dyepig.2011.04.017

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K.H. So et al. / Dyes and Pigments 92 (2011) 603e609
substitution of methyl groups on the ppy ligands of green-emitting
Ir(ppy)3 has considerable influence on photophysical, electro-
chemical and electroluminescence properties, in particular colour
tuning up to yellow-green emission [9]. The substitution of Triar-
ylsilyl moieties showed high chemical stability and improved
solubility, resulting in improved device performances [18].
However, the triarylsilyl derivatives possess some drawbacks such
as lower yield, difficulties in synthesis and low vapour pressure. We
have reported that the trimethylsilyl functional groups confer such
attributes as higher vapour pressure, thermal stability (high Tg),
good solubility and steric bulk via higher volume [7]. Iridium(III)
complexes with trimethylsilyl groups also showed yellowish-green
electroluminescence (524 nm) with higher device efficiencies of
39.2 cd/A and 17.3 lm/W.
In this study, we designed and synthesized a new iridium(III)
complex with phenylpyridine ligands containing a bulky trime-
thylsilylxylene, 5-(2,5-dimethyl-4-(trimethylsilyl)phenyl)-2-phe-
nylpyridine [Ir(dmtppy)3], suitable for lighting applications. The
introduction of a bulky, twisted trimethylsilylxylene on pyridine
ring of Ir(ppy)3 is expected to effectively hinder the aggregation
formation as well as tune the emission colour, red-shifted, to
yellow-green, suitable for lighting applications.
tetrabutylammonium perchlorate (C16H36ClNO4) as supporting
electrolyte under nitrogen environment at a scan rate of 50 mV sꢁ1
.
2.3. Synthesis
2.3.1. (4-Bromo-2,5-dimethylphenyl)trimethylsilane (1)
A solution of 1,4-dibromo-2,5-dimethylbenzene (15 g, 0.43 mol)
in tetrahydrofuran (THF) (200 mL) was cooled to 195 K. n-BuLi
(27.3 mL, 0.68 mol, 2.5 M in hexane) was slowly added dropwise to
the above solution. After stirring for 1 h, the solution was cooled
down again to 195 K. Then, trimethylsilyl chloride (TMSCl, 8.97 mL,
0.74 mol) was added dropwise and the mixture was allowed to
warm to room temperature and stirred overnight. Then, 50 mL of
water was added to the above solution. The aqueous phase was
extracted three times with 150 mL of ether. The combined organic
phase was washed with water (3 ꢀ 150 mL) and then dried over
MgSO4. After removal of the solvents, using column chromatog-
raphy on silica with hexane, the pure product was obtained in 87%
yield as colourless oil (12.7 g). 1H NMR (300 MHz, CDCl3) ppm: 7.32
(s, 1H), 7.28 (s, 1H), 2.93 (s, 6H), 0.32 (s, 9H). IR (KBr, cmꢁ1): 3150,
1600, 1452, 1412, 730, 650.
2.3.2. 2,5-Dimethyl-4-(trimethylsilyl)phenylboronic acid (2)
An oven-dried, three-necked flask was loaded with (4-bromo-
2,5-dimethylphenyl)trimethylsilane (1) (11 g, 0.43 mol), in 150 mL
of THF under N2. After cooling down the solution to 195 K, 20.53 mL
(2.5 M solution in hexane) of n-BuLi (0.51 mol) was added dropwise
to the solution. The solution was then allowed to warm up to 263 K
and then cooled to 195 K again. 9.40 mL of triethylborate (0.56 mol)
was slowly added and the solution warmed up to room tempera-
ture and left under N2 overnight. Then, 50 mL of 2 N HCl was added
and the solution was stirred for 30 min before evaporation to
drying. The crude product was dissolved in dichloromethane and
washed with water. The aqueous phase was extracted three times
with 150 mL of dichloromethane. The combined organic layers
were evaporated to dryness. The crude product was washed with
hexane. The pure product was obtained in 70% yield as a white
powder (4g). m.p: 469e471 K. 1H NMR (300 MHz, CDCl3) ppm: 7.96
(s, 1H), 7.37 (s, 1H), 2.78 (s, 3H), 2.51 (s, 3H), 1.5 (s, 2H), 0.36 (s, 9H).
IR (KBr, cmꢁ1): 3250, 3150, 1600, 1450, 1412, 725.
2. Experimental
2.1. Materials
All experiments were performed under dry N2 atmosphere
using standard Schlenk technique. All solvents were freshly
distilled over appropriate drying reagents prior to use. All starting
materials were purchased from either Aldrich or TCI and were used
without further purification.
2.2. Measurements
1H and 13C nuclear magnetic resonance (NMR) spectra were
recorded using DRX 300 MHz Bruker spectrometer and chemical
shifts in spectra were reported in parts per million (ppm) units with
tetramethylsilane as internal standard. Infrared (IR) measurements
of the samples were carried out using a Genesis II FT-IR spec-
trometer. A Jeol JMS-700 mass spectrometer (MS) was used to
obtain the high-resolution mass (HR-Mass) spectra of the samples.
Thermogravimetric analysis (TGA) was performed under nitrogen
using a TA instrument 2025 thermogravimetric analyzer. The
sample was heated from 323 K to 1073 K with a heating rate of
283 K per minute. Differential scanning calorimeter (DSC) studies
were carried out under nitrogen atmosphere using a TA instrument
2100 differential scanning calorimeter. The sample was heated
from 303 K to 523 K with a heating rate of 283 K per minute.
UVevisible absorption and photoluminescence (PL) spectra were
recorded at room temperature using Perkin Elmer LAMBDA-900
UV/VIS/NIR spectrometer and LS-50B luminescence spectropho-
tometer, respectively. For the PL efficiency measurements, CBP thin
film of 50 nm doped with Ir(dmtppy)3 (6%) was deposited on the
quartz substrate and excited at 325 nm determined from the
excitation spectrum of Ir(dmtppy)3 in solution. Cyclic voltammo-
gram (CV) of the sample was recorded using a Epsilon E3 cyclic
voltammeter at room temperature using the conventional three
electrode configuration consisting of platinum working electrode,
a platinum wire as auxiliary electrode and Ag/AgCl wire as quasi
2.3.3. 5-Bromo-2-phenylpyridine (3)
2,5-Dibromopyridine (4.5 g, 0.19 mol), phenylboronic acid (2.8 g,
0.22 mol) and Pd(PPh3)4 (0.66 g, 3 mol%) were dissolved in toluene
(60 mL). A solution of 2M K2CO3 (20 mL) was added and the
mixture was refluxed with stirring for 24 h in the nitrogen atmo-
sphere. After it was cooled, the mixture was poured into 2N HCl and
extracted with ether. The organic layer was dried over MgSO4. The
solvent was removed under reduced pressure to give a yellow oil.
The crude product was purified by chromatography on silica gel
(dichloromethane/hexane, 1/6, v/v) to obtain a white powder
(3.07 g, 70%). m.p: 348e349 K. 1H NMR (300 MHz, CDCl3) ppm: 8.74
(s, 1H), 7.97e7.94 (d, 2H), 7.88e7.85 (d, 1H), 7.64e7.61 (d, 1H),
7.47e7.44 (q, 3H). IR (KBr, cmꢁ1): 3150, 1690, 1600, 1450, 1350, 625.
2.3.4. 5-(2,5-Dimethyl-4-(trimethylsilyl)phenyl)-2-
phenylpyridine (4)
5-Bromo-2-phenylpyridine (3) (1.5 g, 0.06 mol), 2,5-dimethyl-4-
(trimethylsilyl)phenylboronic acid (1.98 g, 0.07 mol) and Pd(pph3)4
(0.22 g, 3 mol%) were dissolved in toluene (20 mL). A solution of 2M
K2CO3 (5 mL) was added and the mixture was refluxed with stirring
for 24 h under nitrogen. After it was cooled, the mixture was
poured into 2N HCl and extracted with ether. The organic layer was
dried over MgSO4. The solvent was removed under reduced pres-
sure to give yellow oil. The crude product was purified by
reference
electrode.
4,40-bis[N-(1-nathyl)-N-phenyl-amino]
biphenyl (NPB) was used as an internal reference. The cyclic vol-
tammogram was made in one compartment glass cell in methylene
chloride containing 1 ꢀ 10ꢁ3 complex and 0.1 M solution of

K.H. So et al. / Dyes and Pigments 92 (2011) 603e609
605
Fig. 1. The synthetic scheme of Ir(dmtppy)3 complex.
chromatography on silica gel (EtOAc/hexane, 1/10, v/v) to obtain
2.4. Device fabrication
the yellow solid (2.05 g, 96%). m.p.: 350e352 K. 1H NMR (300 MHz,
CDCl3) ppm: 8.73 (s, 1H), 8.10e8.06 (d, 2H), 7.81e7.78 (q, 2H),
7.53e7.51 (m, 3H), 7.43 (s, 1H), 7.13 (s, 1H), 2.52 (s, 3H), 2.35 (s, 3H),
0.40 (s, 9H). IR (KBr, cmꢁ1): 3150, 1650, 1600, 1475, 1412, 725.
Phosphorescent organic light emitting diodes (PHOLEDs) based
on Ir(dmtppy)3 dopant were fabricated by a thermal evaporation
process using the following device configuration: indium-tin-oxide
(ITO)/1,1-bis[di-4-tolylamino]phenyl]cyclohexane (TAPC) as hole
transport layer (HTL) (40 nm)/(4,40-N, N0-dicarbazole)biphenyl
(CBP) host: x% of Ir(dmtppy)3 dopant as emissive layer (EML)
(30 nm)/2,9-dimethyl-4,7-dipheny-1,10-phenanthroline (BCP)
(10 nm)/tris-(8-hydroxyquinoline)aluminum (Alq3) (40 nm)/LiF
(1 nm)/Al (100 nm). The concentration of Ir(dmtppy)3 dopant in
CBP host was 6% for device A, 12% for device B and 18% for device C.
2.3.5. [(dmtppy)2IrCl]2 (5)
Iridium trichloride hydrate (0.54 g, 1.81 mmol) and 5-(2,5-
dimethyl-4-(trimethylsilyl)phenyl)-2-phenylpyridine (4) (1.5 g,
4.52 mmol) were dissolved in a mixture of 2-ethoxyethanol (30 mL)
and water (10 mL) and refluxed for 24 h. The solution was cooled to
room temperature and 100 mL of water was added to the cooled
solution. The resulting yellow precipitate was collected on a glass
filter. The precipitate was washed with water and filtered.
[(dmtppy)2IrCl]2 (1.6 g, 50%).
2.3.6. Ir(dmtppy)3 (6)
[(dmtppy)2IrCl]2 complex (5) (0.5 g, 0.28 mmol), 5-(2,5-
dimethyl-4-(trimethylsilyl)phenyl)-2-phenylpyridine (4) (0.2 g,
0.62 mmol) and K2CO3 (0.27 g, 1.96 mmol) were dissolved in
ethylene glycol (15 mL) and the mixture was heated to 453 K under
nitrogen for 24 h. The reaction mixture was then cooled to room
temperature. Then, 150 mL of water was added to the mixture and
the solution was extracted with 150 mL of chloroform. The organic
layer was dried over MgSO4. The solvent was removed under
reduced pressure, yielding a yellow powder. The crude product was
purified by chromatography on silica gel (ethyl acetate/hexane, 1/8,
v/v) to obtain a light brown powder (0.12 g, 36%). m.p: 552e554 K.
1H NMR (300 MHz, CDCl3) ppm: 7.92e7.89 (d, 3H), 7.69e7.65
(m, 6H), 7.58e7.55 (d, 3H), 7.21 (s, 3H), 6.97e6.89 (m, 9H), 6.81
(s, 1H), 2.34 (s, 9H), 1.89 (s, 9H), 0.32 (s, 27H). IR (KBr, cmꢁ1): 3150,
1640, 1600, 1475, 1412, 720. 13C NMR (300 MHz, CDCl3): 165.07,
161.14, 147.05, 143.53, 141.18, 138.33, 137.60, 137.16, 136.90, 139.70,
135.19, 131.31, 130.89, 129.85, 124.04, 119.84, 118.30, 22.30, 19.73,
- 0.18. HR Mass (FAB): m/z ¼ 1183.5 [Mþ].
Fig. 2. UVevisible absorption and photoluminescence spectra of Ir(dmtppy)3 complex
in chloroform at room temperature.

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K.H. So et al. / Dyes and Pigments 92 (2011) 603e609
Table 1
The photophysical and electrochemical properties of Ir(dmtppy)3 complex.
Absorptiona labs (nm)
Emission lPL (nm)
PLc efficiency (%)
Tdd/Tg (K)
HOMO (eV)
5.38
LUMO (eV)
2.82
Eg (eV)
b
e
f
300,390,470
532
53.1
638/425
2.56
a
The major UVevisible absorption peaks are listed.
b
c
d
e
f
Photoluminescence (PL) emission peak measured in chloroform at room temperature.
PL efficiency of Ir(dmtppy)3 complex for the dopant concentration of 6% in CBP thin film (50 nm) at an excitation wavelength of 325 nm.
d e Temperature corresponding to 5% weight loss in thermogravimetric analysis.
Tg e Glass transition temperature from differential scanning calorimetry (DSC) studies.
T
Eg e Band gap energy calculated from the equation Eg ¼ hc/
l
¼ 1241/
l, where
l
is the edge wavelength (nm) of UVevisible absorption spectrum.
BCP as hole blocking layer (HBL), Alq3 as electron transport layer
(ETL), LiF as electron injection layer and Al as cathode were
deposited under a vacuum of 6.6 ꢀ 10ꢁ5 Pa without breaking the
vacuum. Prior to the deposition of organic layers, ITO substrates
(anode) were degreased in acetone and IPA followed by the
UV eozone treatment for 10 min. The current densityevoltagee
luminescence (JeVeL) characteristics of PHOLEDs were carried out
using Keithley 2400 source meter and Spectra Colorimeter PR650.
The electroluminescence spectra were measured at a driving
3. Results and discussion
The cyclometalated ligand, 5-(2,5-dimethyl-4-(trimethylsilyl)
phenyl)-2-phenylpyridine, was conveniently prepared by Suzuki
coupling reaction. Tris-cyclometalated iridium(III) complexes can
be prepared from the cyclometalating ligand precursors as depicted
in Fig. 1. The tris-cyclometalated complex can be prepared from the
appropriate dichloro-bridged dimer, [(C^N)2Ir(m-Cl)2Ir(C^N)2], by
heating the iridium complex with 2e3 equiv of cyclometalating
ligand in ethylene glycol. The dichloro-bridged dimers and
compounds are easily prepared from IrCl3 ꢀ H2O. The obtained
yellow-green Ir(dmtppy)3 was characterized by 1H nuclear
magnetic resonance (NMR), 13C NMR, infrared (FT-IR) and high-
resolution mass (HR-Mass). From the 1H & 13C NMR studies, the
structure of Ir(dmtppy)3 was found to be fac-isomer.
current of 10 mA/cmꢁ2
.
Fig. 2 shows the absorption and photoluminescence spectra of
Ir(dmtppy)3 complex in chloroform (10ꢁ5 M) at room temperature.
In the UVevisible absorption spectrum of the trimethylsilylxylene-
substituted Ir(dmtppy)3 complex, the absorption below 320 nm is
attributed to spin-allowed ligand-centered
pe
p* transitions. The
absorption band with shoulders in the lower energy region span-
ning from 356 nm to 500 nm is attributed to spin-allowed and spin-
forbidden metaleligand charge transfer (MLCT) transitions of the
iridium(III) complex [19e21]. The spectrum also shows the pho-
toluminescence emission at 532 nm in solution. The photo-
luminescence efficiency of Ir(dmtppy)3 complex (6%) doped in CBP
thin film at an excitation wavelength of 325 nm was found to be
53.1%. The thermal properties of the Ir(dmtppy)3 complex were
determined by differential scanning calorimeter (DSC) and ther-
mogravimetric analysis (TGA) measurements under the nitrogen
atmosphere. TGA data revealed excellent thermal stability and 5%
Fig. 3. (a) Thermogravimeteric analysis (TGA) and (b) differential scanning calorimeter
(DSC) studies of Ir(dmtppy)3 complex.
Fig. 4. Cyclic voltammogram of Ir(dmtppy)3 complex.

K.H. So et al. / Dyes and Pigments 92 (2011) 603e609
607
Fig. 5. The device structure and molecular structures of the materials used in the devices.
weight loss at 638 K (Fig. 3a). DSC data of Ir(dmtppy)3 showed no
crystallization peak but a high glass transition temperature (Tg) of
425 K (Fig. 3b) and exists as an amorphous solid that is resistant to
crystallization. Amorphous films with high stability and efficiency
are usually desirable for OLED applications. The electrochemical
properties of the Ir(dmtppy)3 complex were investigated by cyclic
voltammetric (CV) studies, and the CV curve is shown in Fig. 4. The
highest occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO) energy levels of the Ir(dmtppy)3 complex
were determined from the electrochemical data and the edge
wavelength of the UVevisible absorption spectrum. It shows a band
gap energy of 2.56 eV. The photophysical and electrochemical data
are summarized in Table 1.
Phosphorescent organic light emitting diodes (PHOLEDs) based
on the Ir(dmtppy)3 dopant were fabricated using the device config-
uration: ITO/TAPC (40 nm)/CBP doped with a percentage of
Ir(dmtppy)3 (30 nm)/BCP (10 nm)/Alq3 (40 nm)/LiF/Al. The concen-
tration of Ir(dmtppy)3 dopant in the CBP host was 6% for device A,12%
for device B and 18% for device C. The molecular structures of
materials and the device structure are shown in Fig. 5. TAPC with
a higher LUMO energy level of 2.0 eV and BCP with a HOMO energy
level of 6.5 eV compared with those (HOMO: 5.9 eV; LUMO: 2.6 eV) of
the CBP host were used as a hole transport- and hole blocking- layer
respectively for the effective confinement of charge carriers within
the emissive layer (EML) of the device. The energy level diagram for
the materials used in the devices is shown in Fig. 6 [10,22,23]. The
current densityevoltageeluminescence (JeVeL) characteristics of
devices A, B and C are shown in Fig. 7. From JeVeL characteristics,
device A showed a maximum luminescence of 9189 cd/m2 at 9.6 V.
The turn-on voltage, which is defined as the applied voltage required
to produce a luminescence of 1 cd/m2, was observed to be 3.3 V for
device A. Fig. 8 shows the current- and external quantum efficiency
characteristics of devices A, B and C. Device A showed the maximum
external quantum efficiency of 12.7% and current efficiency of
45.7 cd/A. The electroluminescence spectra of the devices A, B and C
measured at 10 mA/cm2 are shown in Fig. 9. Device A exhibited an
electroluminescence (EL) emission peak at 532 nm with CIE colour
coordinates of (0.37, 0.60), which is the same as with the PL emission
Fig. 7. The current densityevoltageeluminescence (JeVeL) characteristics of devices.
The doping concentration of Ir(dmtppy)3 in CBP host as the emissive layer of the device
was 6% for device A, 12% for device B and 18% device C.
Fig. 6. The energy level diagram of the materials used in the devices.

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K.H. So et al. / Dyes and Pigments 92 (2011) 603e609
Fig. 9. The electroluminescence spectra of the devices. The doping concentration of
Ir(dmtppy)3 in CBP host as the emissive layer of the device was 6% for device A, 12% for
device B and 18% device C.
effective confinement of charge carriers and/or excitons and
complete energy transfer from CBP host to Ir(dmtppy)3 dopant
emitter in the EML of the device A.
Device B based on 12% Ir(dmtppy)3 doped in the CBP host
showed a slightly higher current density compared with that of
device
A (Fig. 7). It exhibited a maximum luminescence
of 10310 cd/m2 at 9.6 V. The maximum external quantum effi-
ciency of 7.5% and current efficiency of 26.6 cd/A were observed
for the device B. It showed an EL emission peak at 534 nm with CIE
colour coordinates of (0.39, 0.59). Device C with 18% Ir(dmtppy)3
dopant in CBP also showed slightly higher current density
compared with that of device B. The slightly increasing current
density for the increasing dopant concentration in the EML of the
device reveals the charge transporting ability of the Ir(dmtppy)3
complex in the device. A maximum luminescence of 9354 cd/m2
was observed for device C. It exhibited a maximum external
quantum efficiency of 5.5% and current efficiency of 19.3 cd/A.
Device C exhibited an EL emission peak at 538 nm with colour
coordinates of (0.40, 0.58). The device performances and EL
characteristics of the devices are summarized in Table 2. The
electroluminescence emission peak of device C with a dopant
concentration of 18% was red-shifted by 6 nm compared with that
(532 nm) of device A with a dopant concentration of 6%. The effect
of the doping concentration of Ir(dmtppy)3 molecules on the EL
spectra is attributed to the strong dipoleedipole interaction
between the dopant molecules due to aggregation formation with
an increase in the doping concentration [24]. The results show
that the new bulky trimethylsilylxylene- substituted Ir(dmtppy)3
complex exhibits yellow-green emission at 532 nm with higher
Fig. 8. (a) The current efficiency-current density- and (b) external quantum efficiency-
current density- characteristics of the devices. The doping concentration of
Ir(dmtppy)3 in CBP host as the emissive layer of the device was 6% for device A, 12% for
device B and 18% device C.
(532 nm) of the Ir(dmtppy)3 complex in solution. This indicates that
the yellow-green EL emission originates from the triplet excited
states of the Ir(dmtppy)3 dopant emitter. This also reveals that the
bulky twisted trimethylsilylxylene- substituted iridium(III) complex
effectively hinders aggregation formation in the solid state for the
dopant concentration of 6% in CBP. No emission from the host and/or
adjacent charge transporting layers was observed, indicating the
Table 2
The electroluminescence characteristics of the devices.a
Deviceb
Doping Concentration(%)
Turn-onc (V)
L (cd/m2)
hc (cd/A)
hext(%)
ELd(nm)
CIEe(x,y)
A
B
C
6
12
18
3.3
2.7
2.7
9189
10310
9354
45.7
26.6
19.3
12.7
7.5
5.5
532
534
538
(0.37, 0.60)
(0.39, 0.59)
(0.40, 0.58)
a
The data for luminescence (L), current efficiency (hc) and external quantum efficiency (hext) are the maximum values of the corresponding device.
The device structure is ITO/TAPC (40 nm)/CBP doped with concentration (x%) of Ir(dmtppy)3 as emissive layer (30 nm)/BCP (10 nm)/Alq3 (40 nm)/LiF/Al.
The turn-on voltage for all the devices was defined as voltage required to get the luminescence of 1 cd/m2.
The peak emission wavelength of the EL spectrum of corresponding device at 10 mA/cm2.
b
c
d
e
CIE colour coordinates of electroluminescence of the corresponding device.

K.H. So et al. / Dyes and Pigments 92 (2011) 603e609
609
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4. Conclusion
A new iridium(III) complex with phenylpyridine ligands con-
taining bulky trimethylsilylxylene, Ir(dmtppy)3, was synthesized by
a Suzuki coupling reaction and confirmed using various spectro-
scopic studies. The thermal, photophysical and electrochemical
properties of the Ir(dmtppy)3 complex were studied using ther-
mogravimetric analysis and differential scanning calorimetry,
UVevisible absorption, photoluminescence and cyclic voltameteric
studies. The phosphorescent organic light emitting diodes based on
Ir(dmtppy)3 complex doped in CBP host exhibits the yellow-green
emission at 532 nm with CIE colour coordinates of (0.37, 0.60),
which is consistent with PL emission (532 nm) of Ir(dmtppy)3
complex in solution. This reveals that the bulky, twisted trime-
thylsilylxylene- substituted iridium(III) complex effectively hinders
aggregation formation in the solid state at 6% doped in CBP,
resulting in a higher device external quantum efficiency of 12.7%
and current efficiency of 45.7 cd/A. The results show that
Ir(dmtppy)3 dopant could be useful in white organic light emitting
diodes for the lighting applications.
Acknowledgements
This research was financially supported by MKE and KIAT
through the Workforce Development Program in Strategic Tech-
nology, by technology innovation program (10030814) of MKE and
by Basic Science Research Program through the National Research
Foundation of Korea (NRF) funded by the Ministry of Education,
Science and Technology (2011-0000310).
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devices using a phosphorescent sensitizer. Applied Physics Letters 2003;82:
4224e6.
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host. Journal of Materials Chemistry 2006;16:4706e13.
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crowding in phosphorescent dendrimers. Macromolecules 2005;38:9564e70.
[20] Cho MJ, Jin JI, Choi DH, Kim YM, Park YW, Ju B-K. Phosphorescent, green-
emitting Ir(III) complexes with carbazolyl-substituted 2-phenylpyridine
ligands: effect of binding mode of the carbazole group on photoluminescence
and electrophosphorescence. Dyes and Pigments 2009;83:218e24.
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polymer light emitting diodes bearing green-emitting Ir(III) complexes: the
structural role of 9-((6-(4-fluorophenyl)pyridin-3-yl)methyl)-9H-carbazole
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