Tunable emission of polymer light emitting diodes bearing green-emitting Ir(III) complexes: The structural role of 9-((6-(4-fluorophenyl)pyridin-3-yl)methyl)-9H-carbazole ligands

Article abstract of DOI:10.1016/j.dyepig.2009.10.017

9-((6-Phenylpyridin-3-yl)methyl)-9H-carbazole and 9-((6-(4-fluorophenyl)pyridin-3-yl)methyl)-9H-carbazole were synthesized as ligands by attaching a carbazolyl group to the pyridine in 2-phenylpyridine and 2-(4-fluorophenyl)pyridine, respectively. Four different Ir(III) complexes were prepared using a simple procedure, the solubility of which was significantly greater than that of conventional green-emitting Ir(ppy)₃. Among the many devices fabricated, that which contained 10% of Ir(Cz-ppy)₁(Cz-Fppy)₂ in PVK/PBD (70:30?wt%), exhibited an external quantum efficiency of 6.80%, luminous efficiency of 20.23?cd/A, and maximum brightness of 17520?cd/m². In particular, the electronic property of the new 9-((6-(4-fluorophenyl)pyridin-3-yl)methyl)-9H-carbazole ligand can manipulate the triplet energy level of the Ir(III) complex to finely tune the emission spectra.

Full text of DOI:10.1016/j.dyepig.2009.10.017

Dyes and Pigments 85 (2010) 143e151
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Tunable emission of polymer light emitting diodes bearing green-emitting
Ir(III) complexes: The structural role of 9-((6-(4-fluorophenyl)pyridin-3-yl)
methyl)-9H-carbazole ligands
,
a
a
Min Ju Cho a, Jung-Il Jin a, Dong Hoon Choi a , Jung Hee Yoon , Chang Seop Hong ,
*
Young Min Kim b, Young Wook Park b, Byeong-Kwon Ju b
a Department of Chemistry, Advanced Materials Chemistry Research Center, Korea University, 5 Anam-dong, Sungbuk-gu, Seoul 136-701, Republic of Korea
b Display and Nanosystem Laboratory, College of Engineering, Korea University, 5 Anam-dong, Sungbuk-gu, Seoul 136-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
9-((6-Phenylpyridin-3-yl)methyl)-9H-carbazole and 9-((6-(4-fluorophenyl)pyridin-3-yl)methyl)-9H-car-
bazole were synthesized as ligands by attaching a carbazolyl group to the pyridine in 2-phenylpyridine
and 2-(4-fluorophenyl)pyridine, respectively. Four different Ir(III) complexes were prepared using
a simple procedure, the solubility of which was significantly greater than that of conventional green-
emitting Ir(ppy)3. Among the many devices fabricated, that which contained 10% of Ir(Cz-ppy)1(Cz-
Fppy)2 in PVK/PBD (70:30 wt%), exhibited an external quantum efficiency of 6.80%, luminous efficiency of
20.23 cd/A, and maximum brightness of 17520 cd/m2. In particular, the electronic property of the new
9-((6-(4-fluorophenyl)pyridin-3-yl)methyl)-9H-carbazole ligand can manipulate the triplet energy level
of the Ir(III) complex to finely tune the emission spectra.
Received 14 September 2009
Received in revised form
24 October 2009
Accepted 28 October 2009
Available online 31 October 2009
PACS:
78.60.Fi
78.66-w
78.55
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Iridium(III) complex
Ligand
Solubility
Photoluminescence
Triplet energy transfer
Electrophosphorescence
1. Introduction
architecture improves both solubility and the site-isolation effect
by employing various peripheral groups around the emission
Of the various types of light-emitting organic materials avail-
able, phosphorescent materials have been recognized as superior
candidates because both singlet and triplet excitons can generate
unique light emissions, with a theoretical internal quantum effi-
ciency of 100% [1e3]. In particular, cyclometalated Ir(III) complexes
show high phosphorescent efficiencies and are one of the most
promising classes of phosphorescent dyes used in organic light-
emitting diodes (OLEDs) [4e11]. Along with advancements in
processing solutions for the fabrication of OLEDs, novel, highly
soluble materials displaying phosphorescence have been reported
recently [12e14]. An extensively used technique for enhancing the
solubility of Ir(III) complexes is via the attachment of lengthy alkyl
moieties or their introduction into dendrimers [15e18]. A dendritic
center [19,20].
In addition to the above methods, a very promising Ir(III)
complex containing the tetraphenylsilyl group has been reported to
exhibit excellent luminous efficiency using a conventional poly-
mer-based [poly(N-vinyl carbazole), PVK] OLED. The design and
synthesis of a highly phosphorescent tris-cyclometallated homo-
leptic Ir(III) complex [Ir(TPSppy)3] (TPSppy ¼ 2-(40-(triphenylsilyl)
biphenyl-3-yl)pyridine) with a silane-based dendritic substituent
have been demonstrated. It has also been suggested that aryl silane
was quite effective at emitting UV light for improving device effi-
ciency compared to a long alkyl substituent [21].
Liu et al. reported PVK based light-emitting diodes fabricated
with pinene-substituted Ir(III) phosphorescent dopants; three
different Ir(III) complex dyes were demonstrated for which a pho-
toluminescent spectral shift was observed with ligand modifica-
tion. In addition, the pinene substitution induced steric hindrance
to molecular structure of the dopant that aided suppression of
* Corresponding author. Tel.: þ82 2 3290 3140; fax: þ82 2 924 3141.
E-mail address: dhchoi8803@korea.ac.kr (D.H. Choi).
0143-7208/$ e see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2009.10.017

144
M.J. Cho et al. / Dyes and Pigments 85 (2010) 143e151
tripletetriplet annihilation between Ir(III) dyes and consequently
improved device performance [22]. These findings suggested that
by substituting an electron withdrawing fluorine atom within the
ligand, the HOMO energy levels could be tuned, thereby increasing
the bandgap energy.
water, 50 mL of hexane, and 50 mL of ethyl ether. The crude product
was purified by silica-gel column chromatography (CH2Cl2) to give
a yellow powder (4a) in 71% yield (3.87 g, 2.14 mmol).
To a 2-ethoxyethanol (15 mL) solution of 4a (1.82 g, 1.01 mmol)
were added 3b (0.78 g, 2.21 mmol) and excess K2CO3. The reaction
mixture was heated to 120 ꢀC for 12 h. After cooling the mixture to
room temperature, a dark yellowish precipitate could be filtered off
and was washed with 200 mL of methanol and 100 mL of ethyl
ether. The crude product was purified by silica-gel column chro-
matography (hexane: CH2Cl2 ¼ 1:4) to give a yellow powder in 48%
The objective of this work was to explore a method for tuning
emission color by combining novel ligand design and good
solubility and miscibility of the host polymer. This paper deals
with three different highly soluble Ir(III) complexes bearing the
9-((6-phenylpyridin-3-yl)methyl)-9H-carbazole and 9-((6-(4-fluo-
rophenyl)pyridin-3-yl)methyl)-9H-carbazole ligands. Green elec-
trophosphorescence polymer light-emitting diodes (PLEDs) were
fabricated by doping Ir(Cz-ppy)2(Cz-Fppy)1, Ir(Cz-ppy)1(Cz-Fppy)2,
or Ir(Cz-Fppy)3 into PVK blended with 5-4-tert-butylphenyl-1,3,4-
oxadiazole (PBD), an electron transport molecule. For reference, the
Ir(Cz-ppy)3 was previously reported as being employed to compare
the performances of photoluminescence (PL) spectrum and elec-
trophosphorescent devices fabricated under identical conditions
[23]. Two different ligands were systematically tethered to form
a complex for investigating the effect of an electron withdrawing
substituent in the ligand on the photophysical and electro-
phosphorescent properties.
yield (1.17 g, 0.98 mmol). 1H NMR (400 MHz, CDCl3)
d 8.10e8.14 (m,
6H), 7.29e7.38 (m, 12H), 7.13e7.20 (m, 6H), 7.10 (d, J ¼ 8.0 Hz, 1H),
7.00e7.04 (m, 2H), 6.74e6.81 (m, 6H), 6.60e6.70 (m, 6H),
6.33e6.38 (m, 1H), 6.24e6.28 (m, 3H), 6.15 (s, 1H), 4.71e4.93 (m,
6H). Exact mass (MALDI-TOF) for [MH]þ calcd for C72H50FIrN6:
1210.3710, Found: 1210.2440. Anal. Calcd for C72H50FIrN6: C, 71.44;
H, 4.16; N, 6.94. Found: C, 71.48; H, 4.15; N, 6.77.
Ir(Cz-ppy)1(Cz-Fppy)2 (5c): To
a solution of 3b (2.33 g,
6.6 mmol) in 2-ethoxyethanol:H2O (3:1, 60 mL) was added Iridium
(III) Chloride Trihydrate (IrCl3$3H2O; caution: avoid metals,
hydroxides, carbonates, cyanides, sulfides, sulfites, formaldehyde;
incompatible with strong oxidizing agents). (1.06 g, 3.0 mmol) and
the reaction mixture was heated to 120 ꢀC for 24 h. The resulting
solution was concentrated at 55 ꢀC and the crude solid was
collected. It was washed with 100 mL of water and 100 mL of
hexane. The crude product was purified by silica-gel column
chromatography (CH2Cl2) to give a yellow powder (4b) in 65% yield
(1.76 g, 0.98 mmol).
2. Experimental
2.1. Materials
All commercially available starting materials and solvents were
purchased from Aldrich, TCI, and ACROS Co. and used without
further purification unless otherwise stated. HPLC grade dime-
thylformamide (DMF) and methylene chloride (MC) werepurchased
from Samchun chemical and distilled from CaH2 immediately before
use. All reactions were performed under an argon atmosphere
unless otherwise stated. 9-(6-Chloro-pyridin-3-ylmethyl)-9H-
carbazole (1), 9-(6-phenyl-pyridin-3-ylmethyl)-9H-carbazole (3a),
and Ir(Cz-ppy)3 (5a) were synthesized by following the method we
reported previously [23].
To a 2-ethoxyethanol (15 mL) solution of 4b (1.24 g, 0.67 mmol)
was added 3a (0.49 g, 1.48 mmol) and excess K2CO3. The reaction
mixture was heated to 120 ꢀC for 12 h. After cooling the mixture to
room temperature, a dark yellow precipitate was filtered off and
washed with 200 mL of methanol and 100 mL of ethyl ether. The
crude product was purified by silica-gel column chromatography
(hexane: CH2Cl2 ¼ 1:4) to give a yellow powder in 41% yield (0.66 g,
0.56 mmol). 1H NMR (400 MHz, CDCl3)
d 8.08e8.12 (m, 6H),
7.30e7.37 (m, 12H), 7.06e7.15 (m, 4H), 6.92e6.98 (m, 3H),
6.69e6.80 (m, 7H), 6.62 (s, 2H), 6.32e6.37 (m, 2H), 6.19e6.28 (m,
3H), 6.10 (s, 1H), 6.00 (d, J ¼ 8.0 Hz, 2H) 5.88 (s, 1H), 4.74e4.90 (m,
6H). Exact mass (MALDI-TOF) for [MH]þ calcd for C72H49F2IrN6:
1228.3616, Found: 1228.1944. Anal. Calcd for C72H49F2IrN6: C,
70.40; H, 4.02; N, 6.84. Found: C, 70.45; H, 4.00; N, 6.68.
2.2. Synthesis
9-((6-(4-Fluorophenyl)pyridin-3-yl)methyl)-9H-carbazole
(3b): Compound 1 (4.68 g, 16 mmol) and tetrakis(triphenylphos-
phine)palladium(0) (Pd(PPh3)4, (caution: air sensitive; light sensi-
tive; incompatible with water, strong oxidants) 0.56 g, 0.48 mmol)
were dissolved in toluene (50 mL) followed by treating with
a degassed solution of K2CO3 (4.53 g, 32 mmol) in H2O (16 mL).
Then, the solution of 4-fluorophenylboronic acid (2.79 g, 20 mmol)
in EtOH (12 mL) was added dropwise into the mother solution. The
mixture was stirred at 85 ꢀC for overnight under Ar gas. After
cooling the reaction mixture, it was poured into 100 mL of water
and the mixture was extracted with toluene. The combined organic
layers were washed with brine and dried over Na2SO4. Removal of
the solvent under a reduced pressure gave a crude product, which
was purified by silica-gel column chromatography (ethyl acetate:
chloroform ¼ 1:15) to give a white powder in 90% yield (5.07 g,
Ir(Cz-Fppy)3 (5d): To a 2-ethoxyethanol (15 mL) solution of 4b
(1.49 g, 0.80 mmol) were added 3b (0.62 g, 1.78 mmol) and excess
K2CO3. The reaction mixture was heated to 120 ꢀC for 12 h. After
cooling it to room temperature, a dark yellowish precipitate was
filtered off and washed with 200 mL of methanol and 100 mL of
ether. The crude product was purified by silica-gel column chro-
matography (hexane: CH2Cl2 ¼ 1:4) to give a yellow powder in 56%
yield (1.08 g, 0.91 mmol). 1H NMR (400 MHz, CDCl3)
d 8.09e8.14 (m,
6H), 7.32e7.39 (m, 12H), 7.14 (d, J ¼ 8.0 Hz, 3H), 6.92e6.97 (m, 6H),
6.69e6.74 (m, 6H), 6.32e6.39 (m, 3H), 6.20 (d, J ¼ 8.0 Hz, 3H), 5.87
(s, 3H), 4.75e4.88 (m, 6H). Exact mass (MALDI-TOF) for [MH]þ calcd
for C72H48F3Ir-N6: 1246.3522, Found: 1246.2828. Anal. Calcd for
C72H48F3Ir-N6: C, 72.52; H, 4.31; N, 7.05. Found: C, 72.39; H, 4.31; N,
6.98.
14.4 mmol). 1H NMR (400 MHz, CDCl3)
d
8.66 (s, 1H), 8.16 (d, J ¼ 8.0
Hz, 2H), 7.89e7.93 (m, 2H), 7.50 (d, J ¼ 8.0 Hz, 1H), 7.47 (t, J ¼ 8.0 Hz,
2H), 7.40 (d, J ¼ 8.0 Hz, 2H), 7.36 (d, J ¼ 8.0 Hz,1H), 7.29 (t, J ¼ 8.0 Hz,
2H), 7.12 (t, J ¼ 8.0 Hz, 2H), 5.56 (s, 2H). Anal. Calcd for C24H17N2F: C,
81.80; H, 4.86; N, 7.96. Found: C, 81.92; H, 4.85; N, 7.69.
2.3. Characterization
1H NMR spectra were recorded on a Varian Mercury NMR
400 Hz spectrometer using deuterated chloroform purchased from
Cambridge Isotope Laboratories, Inc. Elemental analysis was per-
formed by using an EA1112 (Thermo Electron Corp.) elemental
analyzer. Mass analysis was performed on a JMS-AX505WA (JEOL)
mass spectrometer. The redox properties of the Ir(III) complexes
Ir(Cz-ppy)2(Cz-Fppy)1 (5b): To
a solution of 3a (4.41 g,
13.2 mmol) in 2-ethoxyethanol:H2O (3:1, 60 mL) was added
IrCl3$3H2O (2.12 g, 6.0 mmol) and the reaction mixture was heated
to 120 ꢀC for 24 h. The resulting solution was concentrated at 55 ꢀC
and the crude solid was collected. It was washed with 100 mL of

M.J. Cho et al. / Dyes and Pigments 85 (2010) 143e151
145
were examined by using cyclic voltammetry (Model: EA161 eDAQ).
The electrolyte solution employed was 0.10 M tetrabutylammo-
nium hexafluorophosphate (Bu4NPF6) in a freshly dried MC. The
Ag/AgCl and Pt wire (0.5 mm in diameter) electrodes were utilized
as reference and counter electrodes, respectively. The scan rate was
at 50 mV/s. Absorption spectra of chloroform solutions were
obtained using a UVevis spectrometer (HP 8453, PDA type) in the
wavelength range of 190e1100 nm. PL spectra were recorded with
a Hitachi's F-7000 FL Spectrophotometer.
methyl)-9H-carbazole ligand was newly synthesized for preparing
the heteroleptic and homoleptic Ir(III) complexes (see Figs. 1e3).
For a new ligand, 9-((6-chloropyridin-3-yl)methyl)-9H-carbazole
and 4-fluorophenylboronic acid were reacted via the Suzuki
coupling method to yield 9-((6-(4-fluorophenyl)pyridin-3-yl)
methyl)-9H-carbazole (3b) in a moderately good yield (>90%) (see
Fig. 1).
The structures of Ir(Cz-ppy)3 (5a), Ir(Cz-ppy)2(Cz-Fppy)1 (5b), Ir
(Cz-ppy)1(Cz-Fppy)2 (5c), and Ir(Cz-Fppy)3 (5d) are illustrated in
Figs. 2 and 3. As a reference, in Ir(Cz-ppy)3 [20], the carbazolyl
group was attached to the 5-position of the pyridine ring to yield
the carbazolyl phenylpyridine ligand (3a).
2.4. Crystallography
Following the Nonoyama reaction, IrCl3$3H2O was treated with
an excess of ligands (3a or 3b) in a mixed solvent of 2-ethox-
yethanol and water (3:1 v/v) to form an iridium dimer (4a or 4b)
having chloride bridges. For a ligand exchange reaction, 4a was
treated with a stochiometric amount of 9-((6-(4-fluorophenyl)
pyridin-3-yl)methyl)-9H-carbazole to yield 5b. The exchange
reaction between the two ligands was successfully performed in
this study. The compound 4b was also reacted with 9-((6-phenyl-
pyridin-3-yl)methyl)-9H-carbazole, and the Ir(III) complex, 5c [Ir
(Cz-ppy)1(Cz-Fppy)2], was carefully collected from the mixture by
repeated column chromatography. A significant improvement in
the solubility of the iridium complex was observed only on intro-
ducing the carbazolyl moiety into the conventional ligand in Ir
(ppy)3. We examined the solubility difference between conven-
tional Ir(ppy)3 and new Ir(III) complexes. Monochlorobenzene was
selected as 4.5 mg, 5.1 mg, 4.0 mg, and 3.8 mg of Ir(Cz-ppy)3, Ir(Cz-
ppy)2(Cz-Fppy)1, Ir(Cz-ppy)1(Cz-Fppy)2, and Ir(Cz-Fppy)3 were
dissolved in 1 g of monochlorobenzene at room temperature,
respectively while only 1 mg of Ir(ppy)3 was barely soluble in hot
monochlorobenzene (1 g) under sonication for 1 h. The increased
solubility is very beneficial for solution-based device fabrication.
X-ray data for 5b and 5c were collected on a Bruker SMART
APEXII diffractometer equipped with graphite monochromated
ꢀ
MoKa
radiation (
l
¼ 0.71073 A). Preliminary orientation matrix and
cell parameters were determined from three sets of
u/4 scans at
different starting angles. Data frames were obtained at scan inter-
vals of 0.5ꢀ with an exposure time of 10 s per frame. The reflection
data were corrected for Lorentz and polarization factors. Absorp-
tion corrections were carried out using SADABS (Sheldrick,
G. M. SADABS, A program for area detector absorption corrections,
€
University of Gottingen, Germany, 1994.). The structures were
solved by direct methods and refined by full-matrix least-squares
analysis using anisotropic thermal parameters for non-hydrogen
atoms with the SHELXTL (Sheldrick, G. M. SHELXTL, version 5,
Bruker AXS: Madison, Wisconsin, 1995.) program. All hydrogen
atoms were calculated at idealized positions and refined with the
riding models.
2.5. Electroluminescence measurement
The multi-layer diodes have a structure of ITO/PEDOT:PSS
(40 nm)/PVK:Ir(III) complexor PVK:PBD:Ir(III) complex (40 nm)/BCP
(5 nm)/Alq3(35 nm)/LiF(1 nm)/Al(100 nm). The conducting PEDOT:
PSS layer was spin-coated onto the ITO-coated glasses in an argon
atmosphere. The emitting layer then was spin-coated onto the
thoroughly dried PEDOT layer using the solution (conc: 2.0 wt%) in
monochlorobenzene.
For multi-layer devices, 2,9-dimethyl-4,7-diphenyl-1,10-phe-
nanthroline (BCP) and tris(8-hydroxyquinoline) aluminium (Alq3)
layer were vacuum-deposited onto the emitting polymer layer.
Finally, LiF (1 nm)/Al (100 nm) electrodes were deposited onto the
Alq3 layer. Current densityevoltage characteristics were measured
with a Keithley 2400 source meter. The brightness and electrolu-
minescence spectra of the devices were measured with Spectra
Colorimeter PR-650.
3.2. Structure of Ir(Cz-ppy)1(Cz-Fppy)2 (5c) and Ir(Cz-Fppy)3 (5d)
A very interesting aspect of the newly synthesized Ir(III)
complexes, 5c and 5d, involves the determination of their crystal
structures. A single crystal was prepared by using a hyper-
concentrated monochlorobenzene solution. It was quite intriguing
to grow Ir(Cz-ppy)1(Cz-Fppy)2(5c) and Ir(Cz-Fppy)3(5d) as large
single crystals in a solution system. In our previous study, the
crystal structure of 5a was analyzed and it was seen that the three C
atoms of the three ligating groups are positioned in a facial
configuration. The principal axis of the molecule lies on the pseudo
C3 symmetry [24].
The molecular structures of 5c and 5d were determined by X-ray
crystallography and are illustrated in Fig. 4 [24,25]. Both complexes
have distorted octahedral coordination environments and three
bulky cyclometalating ligands encircle the Ir centers. As expected,
5c comprises an unsymmetric core with one Cz-ppy and two
F-substituted Cz-ppy, while in 5d, three Cz-Fppy ligands coordinate
to an Ir metal ion. The isomeric arrangements for the central Ir
atoms for 5c and 5d can be viewed as facial configurations, which is
3. Results and discussion
3.1. Materials synthesis
We demonstrated the synthetic route to the Ir(III) complex Ir
(Cz-ppy)3, which contains a carbazolyl substituent, in a previous
report [23]. In this study, the 9-((6-(4-fluorophenyl)pyridin-3-yl)
Fig. 1. Synthetic procedure: i) Pd(PPh3)4, K2CO3,Toluene/H2O/EtOH, 80 ꢀC.

146
M.J. Cho et al. / Dyes and Pigments 85 (2010) 143e151
Fig. 2. Synthetic procedure: i) IrCl3$3H2O, 2-ethoxyethanol, H2O, 120 ꢀC, ii) K2CO3, 2-ethoxyethanol, 120 ꢀC.
ꢀ
also found in 5a constructed from three Cz-ppy ligands without F
groups [20]. The Ir-C and Ir-N lengths in 5c range from 2.007(6) to
(tpy)3 [Ir-N ¼ 2.132(5) A] [26]. The observation of the significantly
long Ir-N bond length was definitely associated with the trans effect
of the phenyl groups. For 5d, the mean Ir-C and Ir-N bond distances
ꢀ
ꢀ
2.021(6) A and from 2.129 to 2.132 A, respectively, which are similar
to those in 5a. These metrical parameters are more scattered than
those of fac-Ir(tpy)3 [tpy ¼ 2-(p-tolyl)pyridyl], and bear no long or
structurally congested side groups [26]. This may be related to the
steric effect stemming from the bulky Cz-ppy ligands. The averaged
ꢀ
were equal to 2.007(5) and 2.113(6) A, respectively, which were
slightly shorter than those observed in 5a, 5c, and tpy-based Ir
complexes [26].
The electron-withdrawing F group is placed in a meta position
with respect to the C atom bound to Ir. Hence, the electronically
inductive effect of F is not strong, which may be responsible for the
small reduction in Ir-C and Ir-N for 5d.
ꢀ
lengths for 5a and 5c were determined as follows; Ir-C ¼ 2.018(9) A
ꢀ
ꢀ
ꢀ
for 5a and 2.016(8) A for 5c; Ir-N ¼ 2.130(8) A for 5a and 2.310(1) A
for 5c. The length of Ir-N for 5c is a little longer than that for fac-Ir
Fig. 3. Synthetic procedure: i) IrCl3$3H2O, 2-ethoxyethanol, H2O, 120 ꢀC, ii) K2CO3, 2-ethoxyethanol, 120 ꢀC.

M.J. Cho et al. / Dyes and Pigments 85 (2010) 143e151
147
Fig. 4. ORTEP diagrams of 5c and 5d. The thermal ellipsoid for the image represents a 50% probability limit. (a) 5c, (b) 5d.
3.3. UVevis absorption and the photoluminescent properties
of iridium complexes
(Cz-Fppy)3 had emission maximums at 510 and 495 nm, respectively.
Tethering the fluorine in the phenylpyridine ligand structure had an
effect on the optical and photophysical properties.
The absorption and PL spectra of the solution samples of 5a, 5b, 5c,
and 5d are shown in Fig. 5. The spectral shapes were similar to that
observed for Ir(ppy)3, except for a spectral shift. The absorption bands
of the spectra of the four carbazolyl substituted phenylpyridine Ir(III)
3.4. Electrochemical analysis
The electrochemical characterizations of these complexes showed
that their oxidationwas reversible, which is necessary information to
determine their electronic energy levels (see Fig. 6). Cyclic voltam-
mograms were recorded on film samples and the potentials were
obtained relative to an internal ferrocene reference (Fc/Fcþ). These CV
scans in the ꢁ2.0 to þ2.0 V (vs Ag/AgCl) range showed reversible
oxidation peaks. Unfortunately, the reduction behaviors were irre-
versible; therefore, we were unable to accurately estimate their
HOMO and LUMO energies. To determine the LUMO levels, we
combinedtheCVoxidationpotentialwiththeopticalenergybandgap
(Eogpt) resulting from the absorption edge in the absorption spectrum.
Voltammograms of 5a, 5b, 5c, and 5d in their solution states showed
that their lowest oxidative waves were at þ0.74, þ0.75, þ0.84, and
þ0.90 V, respectively. As the density of the fluorine substituted phe-
nylpyridine ligand, 3b, increased, the oxidative stability improved. As
shown in Table 1, 5a, 5b, 5c, and 5d had HOMO levels of ꢁ5.14, ꢁ5.15,
ꢁ5.24, and ꢁ5.30 eV, respectively. The HOMO energy levels in 5b, 5c,
and 5d became deeper due to the presence of an electron with-
drawing substituent in the ligand structure.
complexes below 320 nm are ascribed to the intra-ligand
p
p* tran-
ꢁ
sitions originating from the Ir-complex, while the absorption at
around 342e343 nm is due to the carbazole moieties. In a lower
energy region spanning from 380 to 520 nm, we could observe aweak
and broad absorption band with shoulders, which can be attributed to
spin-allowed and spin-forbidden metal-to-ligand charge transfer
(MLCT) transitions of the Ir(III) complexes. The absorption edge was
shifted to a lower wavelength with the number of the fluorinated
ligand (3b), which isduetothevariation in the electronicproperties of
the three ligands. The PL spectra of the Ir(III) complexes in CHCl3
solutions are shown in Fig. 5. They show a spectral shift with ligand
modification according to the slight shift in the absorption spectrum.
It should be noticed that introducing one 3b instead of 3a yields
a hypsochromic shift of 5 nm in the PL spectrum of Ir(Cz-ppy)2
(Cz-Fppy)1, relative to that of the green-emitting Ir(Cz-ppy)3,
(
lem ¼ 520nm). Itcanbeconsideredthattheslightlydissimilarexcited
andgroundstateswereinvolvedinthephosphorescenttransitionofIr
(Cz-ppy)2(Cz-Fppy)1. When we added one and two more fluorine
substituted ligands, the PL spectra of Ir(Cz-ppy)1(Cz-Fppy)2, and Ir
Fig. 5. UVevis absorption and PL spectra of (a) 5a, (b) 5b, (c) 5c, and (d) 5d in the
solution state (solvent: chloroform 4 ꢂ 10ꢁ6 M).
Fig. 6. Cyclic voltammograms of (a) 5a, (b) 5b, (c) 5c, and (d) 5d in the solution state
(solvent: methylene chloride).

148
M.J. Cho et al. / Dyes and Pigments 85 (2010) 143e151
Table 1
The current densityevoltage luminance curves of the four
Physical properties of the four different Ir(III) complexes.
devices are shown in Fig. 8. The turn-on voltages for these devices
were typical for Ir(III) complex doped PLEDs, falling in the 4.5e5.0 V
range.
½
Ir(III)-complex Absorption
lem
Eox
HOMO LUMO Eg
Wavelength (nm) (nm) (V)a
(eV)b
(eV)c
2.74
(eV)d
2.40
Ir(Cz-ppy)3
343(Cz), 384,
412, 461, 493
342(Cz), 374,
408, 455, 487
342(Cz), 367,
401, 441, 478
342(Cz), 361,
395, 438, 473
520
515
511
496
0.74 5.14
0.75 5.15
0.84 5.24
0.90 5.30
The devices were fabricated under identical conditions during
the same period and at least five devices were characterized to
provide reproducible performance data, which are tabulated in
Table 2. The maximum brightness values of the LEDs were around
20,250 cd/m2 (at 2018.7 A/m2) for device A, 25,520 cd/m2 (at
2978.2 A/m2) for device B, 17520 cd/m2 (at 1816.3 A/m2) for device
C, and 9501 cd/m2 (at 2154.5 A/m2) for device D. These brightness
levels could not be compared and explained in an appropriate way
since the emission color coordinates were somewhat different from
each other. However, an enhancement of the brightness was clearly
observed when PBD was added to the PVK. Compared to device A,
the other three devices (B, C, D) showed higher current flows under
an identical bias voltage, which might be attributed to a lessening
of the charge trapping effect arising from the deeper HOMO levels.
Fig. 9 displays the dependence of the luminous efficiency and
external quantum efficiency on the current density for the four
electrophosphorescent devices. The maximum luminous efficien-
cies of devices A, B, C, and D were determined to be 19.24 cd/A (at
14.0 A/m2, hEQE ¼ 8.68%), 16.69 cd/A (at 34.1 A/m2, hEQE ¼ 5.72%),
20.25 cd/A (at 101.1 A/m2, hEQE ¼ 6.80%), and 13.87 cd/A (at 35.1
A/m2, hEQE ¼ 5.77%), respectively. It should be pointed out that the
triplet state of the PVK (T1 ¼ 2.50 eV) was higher in energy than that
of the Ir(III) complexes (T1 ¼ 2.40e2.46 eV), except for Ir(Cz-Fppy)3
(T1 ¼ 2.53 eV). There were subtle differences between the effi-
ciencies of the three samples, which cannot be explained using the
molecular energy levels. However, according to the literature [14],
the addition of an electron withdrawing substituent into the ligand
raises the LUMO energy level of the Ir(III) complex and less efficient
electron trapping in the PVK host might occur due to this shallow
LUMO energy level. The triplet energy of Ir(Cz-Fppy)3 was 2.53 eV,
which was close to that of PVK, which may have undergone an
endothermic tripletetriplet energy transfer from the PVK host to
the Ir(III) complex [14].
Ir(Cz-ppy)2
(Cz-Fppy)1
Ir(Cz-ppy)1
(Cz-Fppy)2
Ir(Cz-Fppy)3
2.70
2.78
2.77
2.45
2.46
2.53
a
Oxidation potentials measured by cyclic voltammetry.
HOMO ¼ Eox þ 4.4 eV.
b
c
LUMO ¼ HOMO ꢁ Eg.
d
Eg estimated from the UVevis absorption spectra.
3.5. Electrophosphorescent properties
The multi-layer diodes had a structure of ITO/PEDOT:PSS
(40 nm)/PVK:PBD with the Ir(III) complex (40 nm)/BCP (5 nm)/Alq3
(35 nm)/LiF (1 nm)/Al (100 nm), respectively (see Fig. 7). PVK
(Mww90,000, ACROS Co.) was selected as the polymer host, as is
usual. We fabricated four different multilayered devices. Devices A,
B, C, and D were fabricated using 5a, 5b, 5c, and 5d (10 wt%) doped
into PVK:PBD host. Due to the presence of steric hindrance via the
carbazolyl groups, the complexes can be doped at a high concen-
tration (10 wt%) into PVK, for electrophosphorescent devices,
without exhibiting significant emission quenching behavior [27].
There was a good overlap between the PL spectrum of PVK:PBD
(30 wt%) and the metaleligand charge transfer MLCT absorption
bands of the iridium complexes [8,22]. This overlap should enable
efficient energy transfer from the singlet-excited state in the host to
the MLCT band of the guest.
To emphasize the effect of the PBD on the electro-
phosphorescence, we also added the performance data of the
devices (e.g. devices 1, 2, 3, and 4) without PBD to Table 2.
Fig. 7. Molecular structures of Ir(III) complexes, PVK, and PBD. The device configuration of the electrophosphorescent PLED.

M.J. Cho et al. / Dyes and Pigments 85 (2010) 143e151
149
Table 2
Measured parameters of the electrophosphorescent devices.
Device
Turn-on (V)
Lmm/cd mꢁ2 (J)a
LEmax/cd Aꢁ1 (J)a
PEmax/lm Wꢁ1 (J)a
EQEmax/%(J)a
EL lmaxb (nm)
CIE coordinateb (x, y)
Device lc
Device 2c
Device 3c
Device 4c
Device A
Device B
Device C
Device D
4
4
12,820 (1665.4)
10,060 (1670.4)
7605 (1665.1)
5983 (1764.2)
20,250 (2018.7)
25,520 (2978.2)
17,520 (1816.3)
9501 (2154.5)
15.53 (2.39)
11.33 (9.41)
7.96 (12.86)
4.55 (37.06)
19.24 (1.40)
16.69 (3.41)
20.25 (10.11)
13.87 (3.51)
4.64 (2.39)
3.95 (9.41)
3.05 (7.67)
1.80 (12.61)
7.55 (1.40)
6.99 (3.41)
7.22 (3.37)
6.22 (3.51)
4.90 (2.39)
3.89 (9.41)
2.72 (7.67)
1.83 (12.61)
8.68 (1.40)
5.72 (3.41)
6.80 (10.11)
5.77 (3.51)
528
518
516
494
520
512
512
492
0.37, 0.58
0.32, 0.59
0.30, 0.59
0.25, 0.53
0.35, 0.60
0.31, 0.60
0.30, 0.60
0.25, 0.54
4.5
4.5
5
4.5
4.5
4.5
a
Corresponding J(A/m2).
At maximum luminance.
ITO/PEDOT:PSS(40 nm)/PVK:Ir(III) complex(40 nm)/BCP(5 nm)/Alq3(35 nm)//LiF/Al.
b
c
It is well known that PVK can be blended with PBD to improve
spectra of each Ir(III) complex (see Fig. 5). This suggests that the
same excited state specie was responsible for both the PL and EL
emissions, resulting from the triplet emission due to the Ir(III)
complex. The EL emission was dominated by an Ir(III) complex
emission peak at around 520e492 nm. No host emission was
observed in the fabricated devices. This seems to indicate that
energy transfer from the PVK:PBD host to the four Ir(III) complexes
was quite efficient at the optimized dopant concentration of the
experiment.
The effect of the 9-((6-(4-fluorophenyl)pyridin-3-yl)methyl)-
9H-carbazole ligand on the spectral shift to a shorter wavelength
induced a variation in the green emission color. The synthetic
strategy to add the fluorine substituted phenylpyridine ligand to
the balance of the electron and hole transporting properties
[28e30]. A well-balanced charge-carrier injection and the transport
and confinement of the emissive triplet excitons within the emis-
sion layer could be achieved, resulting in a significant improvement
in the stability of the device efficiency. Due to the presence of
a carbazolyl peripheral moiety around the iridium core, the present
device had a much slower decrease in efficiency with an increase in
current density, thereby suppressing tripletetriplet annihilation.
The relatively poor stability of the efficiency in device D was also
attributed to a possible backward energy transfer from the PVK host
to Ir(Cz-Fppy)3, which is not usual in other devices [14].
Fig. 10 shows the EL emission spectra of the four devices. The
spectra of the four devices were quite similar to the PL solution
Fig. 8. Dependence of current density and luminance on the applied voltage. Open
symbol: Luminance; Filled symbol: Current density.
Fig. 10. EL spectra of the four electrophosphorescent devices.
Fig. 9. Dependence of luminous efficiency and external quantum efficiency on the current density.

150
M.J. Cho et al. / Dyes and Pigments 85 (2010) 143e151
References
[1] Yersin H. Highly efficient OLEDs with phosphorescent materials. Weinheim
Germany: Wiley-VCH; 2007.
[2] Adachi C, Baldo MA, Thompson ME, Forrest SR. Nearly 100% internal phos-
phorescence efficiency in an organic light-emitting device. Journal of Applied
Physics 2001;90:5048e51.
[3] Baldo MA, O'Brien DF, You Y, Shoustikov A, Sibley S, Thompson ME, et al.
Highly efficient phosphorescent emission from organic electroluminescent
devices. Nature 1998;395:151e4.
[4] Tsutsui T, Yang M-J, Yahiro M, Nakamura K, Watanabe T, Tsuji T, et al. High
quantum efficiency in organic light-emitting devices with iridium-complex as
a triplet emissive center. Japanese Journal of Applied Physics 1999;38:L1502e4.
[5] Ostrowski JC, Robinson MR, Heeger AJ, Bazan GC. Amorphous iridium
complexes for electrophosphorescent light emitting devices. Chemical
Communications 2002:784e5.
[6] Duan JP, Sun PP, Cheng CH. New iridium complexes as highly efficient orange-
red emitters in organic light-emitting diodes. Advanced Materials
2003;15:224e8.
[7] Beeby A, Bettington S, Samuel IDW, Wang Z. Tuning the emission of cyclo-
metalated iridium complexes by simple ligand modification. Journal of
Materials Chemistry 2003;13:80e3.
[8] Gong X, Robinson MR, Ostrowski JC, Moses D, Bazan GC, Heeger AJ. High-
efficiency polymer-based electrophosphorescent devices. Advanced Materials
2002;14:581e5.
Fig. 11. Stability of the chromaticity depicted by CIE coordination.
[9] Ikai M, Tokito S, Sakamoto Y, Suzuki T, Taga Y. Highly efficient phosphores-
cence from organic light-emitting devices with an exciton-block layer. Applied
Physics Letters 2001;79:156e8.
the Ir(III) complex successfully tuned the color coordinate precisely,
which was evidenced by the CIE coordinates (see Fig. 10 and
Table 2). Thus, it can be explained in greater detail by using the
differences in the electronic properties of the 9-((6-phenylpyridin-
3-yl)methyl)-9H-carbazole and 9-((6-(4-fluorophenyl)pyridin-
3-yl)methyl)-9H-carbazole ligands.
Fig. 11 shows the current density dependence of the chroma-
ticity, which was used to evaluate its stability. When the EL spec-
trum was converted into chromaticity coordinates on the CIE 1931
diagram, we could observe the evident stability of the chromaticity
with an increase in the applied voltage, concomitantly with an
increase in the current density. Therefore, the stability of the color
coordinate was also improved by introducing the electron transport
molecule.
[10] Xia Z-Y, Xiao X, Su J-H, Chang C-S, Chen CH, Li D-L, et al. Low driving voltage
and efficient orange-red phosphorescent organic light-emitting devices based
on a benzotriazole iridium complex. Synthetic Metals 2009;159:1782e5.
[11] Xu Z, Li Y, Ma X, Gao X, Tian H. Synthesis and properties of iridium complex
based 1,3,4-oxadiazoles derivatives. Tetrahedron 2008;64:1860e7.
[12] Huang W-S, Lin JT, Lin H-C. Green Phosphorescent iridium dendrimers
containing dendronized benzoimidazole-based ligands for OLEDs. Organic
Electronics 2008;9:557e68.
[13] Bettington S, Tavasli M, Bryce MR, Beeby A, Al-Attar H, Monkman AP. Tris-
cyclometalated iridium(III) complexes of carbazole(fluorenyl)pyridine ligands:
synthesis, redox and photophysical properties, and electrophosphorescent
light-emitting diodes. Chemistry-A European Journal 2007;13:1423e31.
[14] Liu HeM, Wang PeF, He J, Zheng C, Zhang XeH, Chew SeL, et al. High-effi-
ciency endothermic energy transfer in polymeric light-emitting devices based
on cyclometalated Ir complexes. Applied Physics Letters 2008;92:023301.
[15] Anthopoulos TD, Frampton MJ, Namdas EB, Burn PL, Samuel IDW. Solution-
processable red phosphorescent dendrimers for light-emitting device appli-
cations. Advanced Materials 2004;16:557e60.
4. Conclusions
[16] King SM, AleAttar HA, Evans RJ, Congreve A, Beeby A, Monkman AP. The use of
substituted iridium complexes in doped polymer electrophosphorescent
devices: the influence of triplet transfer and other factors on enhancing device
performance. Advanced Functional Materials 2006;16:1043e50.
[17] Lo SC, Richards GJ, Markham JPJ, Namdas EB, Sharma S, Burn PL, et al.
The new soluble iridium complexes, Ir(Cz-ppy)2(Cz-Fppy)1, Ir
(Cz-ppy)1(Cz-Fppy)2, and Ir(Cz-Fppy)3 were successfully synthe-
sized, and their electrophosphorescence device performances were
evaluated using a PVK:PBD blend host. The Ir(III) complexes had
good solubility over Ir(ppy)3, thus possessing greater processability.
It was intriguing that introducing one 3b instead of 3a yielded
a hypsochromic shift of 5 nm in the PL spectrum of Ir(Cz-ppy)2
(Cz-Fppy)1, relative to that of the green-emitting Ir(Cz-ppy)3,
A
light-blue phosphorescent dendrimer for efficient solution-processed
lighteemitting diodes. Advanced Functional Materials 2005;15:1451e8.
[18] Xie HZ, Liu MW, Wang OY, Zhang XH, Lee CS, Hung LS, et al. Reduction of self-
quenching effect in organic electrophosphorescence emitting devices via the
use of sterically hindered spacers in phosphorescence molecules. Advanced
Materials 2001;13:1245e8.
[19] Wong WY, Ho CL, Gao ZQ, Mi BX, Chen CH, Cheah KW, et al. Multifunctional
iridium complexes based on carbazole modules as highly efficient electro-
phosphors. Angewandte Chemie International Edition 2006;45:7800e3.
[20] Wong WY, Zhou GJ, Yu XM, Kwok HS, Tang BZ. Amorphous diphenylamino-
fluorene-functionalized iridium complexes for high-efficiency electro-
phosphorescent light-emitting diodes. Advanced FunctionalMaterials2006;16:
838e46.
[21] You Y, An CeG, Lee DeS, Kim J-J, Park SY. Silicon-containing dendritic tris-
cyclometalated Ir(III) complex and its electrophosphorescence in a polymer
host. Journal of Materials Chemistry 2006;16:4706e13.
[22] Liu HeM, He J, Wang PeF, Xie HeZ, Zhang XeH, Lee CeS, et al. High-efficiency
polymer electrophosphorescent diodes based on an Ir (III) complex. Applied
Physics Letters 2005;87:221103.
(lem ¼ 520 nm). When we added one and two more fluorine
substituted ligands, the PL spectra of Ir(Cz-ppy)1(Cz-Fppy)2 and Ir
(Cz-Fppy)3 had emission maximums at 510 and 495 nm, respectively.
The efficiencies of all of the devices were shown to be quite
promising compared to other similar, conventional devices made of
Ir(ppy)3. In particular, with the variation of the ligand composition,
the PL emission and electrophosphorescence displayed quite
unique characteristics. The color coordinate of the device was finely
tuned by adding the fluorine substituted phenylpyridine ligand into
the Ir(III) complex. Our work unambiguously demonstrated that
new highly soluble Ir(III) complexes that take advantage of efficient
excited energy transfer can be fully utilized for tuning the emission
properties of phosphorescent EL devices.
[23] Cho MJ, Kim JI, Hong CS, Kim YM, Park YW, Ju BeK, et al. Highly soluble green-
emitting Ir(III) complexes with 9-(6-phenyl-pyridin-3-ylmethyl)-9 H-carba-
zole ligands and their application to polymer light-emitting diodes. Journal of
Polymer Science Polymer Chemistry Edition 2008;46:7419e28.
[24] Crystal data for Ir(Cz-ppy)1Ir(Cz-Fppy)2: C72H49F2IrN6, Fw.¼.1228.36, mono-
ꢀ
clinic, space group P21/c, a ¼ 15.7313(14) b ¼ 13.9434(12) c ¼ 25.414(2) A,
ꢀ
3
ꢀ
b
¼ 99.563(3) , V ¼ 5497.10(8) A , Z ¼ 4, Dcalc ¼ 1.484.g.cmꢁ3, 82143 reflec-
Acknowledgments
tions collected, 13547 unique (Rint ¼ 0.1232), R1 ¼ 0.0493, wR2.¼.0.0938,
Final R indices [I>2sigma(I)]. CCDC 709485.
[25] Crystal data for Ir(Cz-Fppy)3: C72H48F3Ir-N6, Fw ¼ 1246.35, monoclinic, space
This research work was supported by LG display (2009e2010).
Particularly, Prof. D. H. Choi thanks the financial support by the
Seoul R&BD Program (2008e2009), and second stage of the Brain
Korea 21 Project in 2009 (Korea Research Foundation)
b
106.434(2)ꢀ,
ꢀ
group P21/n, a ¼ 15.8583(4) b ¼ 15.8707(5) c ¼ 25.5745(9) A, ¼
3
V ¼ 6173.7(3) A , Z ¼ 4, Dcalc ¼ 1.462 g cmꢁ3, 61540 reflections collected,
15306 unique (Rint ¼ 0.0344), R1 ¼ 0.0279, wR2 ¼ 0.0588, Final R indices
[I > 2sigma(I)]. CCDC 709486.
ꢀ

M.J. Cho et al. / Dyes and Pigments 85 (2010) 143e151
151
[26] Tamayo AB, Alleyne BD, Djurovich PI, Lamansky S, Tsyba I, Ho NN, et al.
Synthesis and characterization of facial and meridional tris-cyclometalated
iridium(III) complexes. Journal of the American Chemical Society
2003;125:7377e87.
[27] Chew SL, Lee CS, Lee ST, Wang PF, He J, Li W, et al. Photoluminescence and
electroluminescence of a new blue-emitting homoleptic iridium complex.
Applied Physics Letters 2006;88:093510.
[28] Pai DM, Yanus JF, Stolka M. Trap-controlled hopping transport. Journal of
Physical Chemistry 1984;88:4714e7.
[29] Yang XH, Neher D. Polymer electrophosphorescence devices with high
power conversion efficiencies. Applied Physics Letters 2004;84:2476e8.
€
[30] Yang XH, Neher D, Hertel D, Daubler TK. Highly efficient single-layer
polymer electrophosphorescent devices. Advanced Materials 2004;16:
161e6.