New blue phosphorescent iridium complexes containing phenylpyridine and triazole ligands: Synthesis and luminescence studies

Article abstract of DOI:10.1166/jnn.2011.3656

The synthesis and luminescence of iridium(III) complexes containing new phenylpyridine (C∧N) ligands, 4-Me-4'-F-ppy, 4-Me-4'-CF₃-ppy and 4-OMe-4'-CF₃-ppy, were studied. These ligands were designed for development of the blue light-em

Full text of DOI:10.1166/jnn.2011.3656

Copyright © 2011 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 11, 4414–4418, 2011
New Blue Phosphorescent Iridium Complexes
Containing Phenylpyridine and Triazole Ligands:
Synthesis and Luminescence Studies
So Youn Ahn, Hyun Shin Lee, and Yunkyoung Ha∗
Department of Information Display Engineering, Hongik University, Seoul, 121-791, Korea
The synthesis and luminescence of iridium(III) complexes containing new phenylpyridine (C∧N)
ligands, 4-Me-4ꢀ-F-ppy, 4-Me-4ꢀ-CF3-ppy and 4-OMe-4ꢀ-CF3-ppy, were studied. These ligands
were designed for development of the blue light-emitting iridium complexes by introducing the
electron-withdrawing group (F, CF3ꢀ and the electron-donating group (Me, OMe) at the para posi-
tions of the phenyl and pyridine ligand rings, respectively. As an ancillary ligand, trzl-CMe3 was
employed where trzl-CMe3 represents 2-(5-tert-butyl-2H-1,2,4-triazol-3-yl)pyridine. The resulting
iridium complexes, Ir(4-Me-4ꢀ-F-ppy)2(trzl-CMe3ꢀ, Ir(4-OMe-4ꢀ-CF3-ppy)2(trzl-CMe3ꢀ and Ir(4-Me-4ꢀ-
CF3-ppy)2(trzl-CMe3ꢀ exhibited the blue emission at 472, 484 and 494 nm in CH2Cl2 solution,
respectively. Ir(4-Me-4ꢀ-F-ppy)2(trzl-CMe3ꢀ showed the most hypsochromic shift in photolumines-
cence (PL) among the complexes prepared herein. In the electroluminescence (EL) spectra, Ir(4-
Me-4ꢀ-F-ppy)2(trzl-CMe3ꢀ and Ir(4-Me-4ꢀ-CF3-ppy)2(trzl-CMe3ꢀ exhibited the luminescence peak at
437 nm and 496 nm, respectively. In the aspect of blue emission color purity, Ir(4-Me-4ꢀ-F-ppy)2(trzl-
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CMe3ꢀ had the CIE coordinates of (0.176, 0.143), very close to the saturated standard blue emission.
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Keywords: Iridium Complex, Blue Phosphorescence, OLED, ppy Ligand Derivatives.
Copyright: American Scientific Publishers
1. INTRODUCTION
on the ligand design: increasing LUMO and lowering
HOMO energy levels of a ligand with respect to ppy,
yielding the energy gap increase of the complex. First,
withdrawing groups, F and CF3, were involved in the
phenyl fragment of the phenylpyridine moiety to decrease
HOMO levels. Secondly, methoxy and methyl groups were
included at the pyridine ring to raise the LUMO energy
levels. As a result, the energy gap of the iridium com-
plexes may increase and emission can be closer to true
blue color. Considering these strategies, we synthesized
the new ppy ligand derivatives and prepared their irid-
ium complexes. For a charge balance, triazole (trzl) was
introduced as an ancillary ligand for blue phosphorescent
iridium complexes.5ꢀ6 The photoabsorption and lumines-
cence properties of the iridium complexes, Ir(4-Me-4-
F-ppy)2(trzl-CMe3ꢁ, Ir(4-Me-4-CF3-ppy)2(trzl-CMe3ꢁ and
Ir(4-OMe-4-CF3-ppy)2(trzl-CMe3ꢁ, were investigated.
Development of red, green and blue emitting materials with
good efficiency and pure chromacity is the most important
issue for the advance of full-color display. Theoretically,
the internal quantum efficiency of phosphorescent emitters
can approach 100% while that of fluorescent molecules
is limited to 25%.1ꢀ2 Thus, OLEDs that involve phos-
phorescent compounds as an emitting material have been
developed with high efficiency.3ꢀ4 Compared to the green
phosphorescent compounds, development of blue and red
phosphorescent dopants has difficulties, mostly due to the
problems in color purity and efficiency. Ir(F2-ppy)2(trzl),
a blue phosphorescent emitter, was recently reported to
have emission peaks at 461 nm and 492 nm, where F2-
ppy and trzl represent 4,6-difluorophenylpyridine and 2-(5-
methyl-2H-1,2,4-triazol-3-yl)pyridine ligands, respectively.
Its electroluminescence (EL) showed a maximum effi-
ciency of 13.4 cd/A, a power efficiency at 300 cd/m2 of
5.5 lm/W and color point of (0.168, 0.281).5
2. EXPERIMENTAL DETAILS
To obtain the blue phosphorescent material of good
color purity and efficiency, we proposed two strategies
All reagents were purchased from Aldrich Co., except
Ir(III) trichloride hydrate (IrCl3 · H2O), which was
purchased from Strem Co. and used without further
∗Author to whom correspondence should be addressed.
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1533-4880/2011/11/4414/005
doi:10.1166/jnn.2011.3656

Ahn et al.
New Blue Phosphorescent Iridium Complexes Containing Phenylpyridine and Triazole Ligands
purification. All reactions were carried out under a nitro-
gen or argon atmosphere. Solvents were dried by stan-
dard procedures. Column chromatography was performed
with the use of silica gel (230-mesh, Merck Co). 1H
NMR spectra were obtained from an NMR spectrome-
ter of 600 MHz at Seoul National University. Mass spec-
tra were determined on JEOL, JMS-AX505WA, HP 5890
Series II Hewlett-Packard 5890A (capillary column) at
Seoul National University, Korea.
2.2. Synthesis of Iridium(III) Complexes
2.2.1. Synthesis of Ir(C∧N)2(Trzl-CMe3)
To a flask containing IrCl3 · H2O (1.49 g, 5 mmol) and a
cyclometallating ligand (C∧N = 4-Me-4ꢀ-F-ppy, 4-Me-4ꢀ-
CF3-ppy or 4-OMe-4ꢀ-CF3-ppy, 12.5 mmol (2.5 eq)) was
added a 3:1 mixture of 2-ethoxyethanol and water. The
mixture was refluxed for 16 hr, cooled to room temperature
and slowly evaporated. The resulting yellow solid was fil-
tered and washed with ethanol to give the chloride-bridged
dimer, (C∧N)2Ir(ꢃ-Cl)2Ir(C∧N)2. The dimer (2 mmol)
was then placed in a 50 ml two-neck flask filled with
2-ethoxyethanol (30 mL). 2-(5-tert-butyl-2H-1,2,4-triazol-
3-yl)pyridine (6.8 mmol (3.4 eq)) was added and the reac-
2.1. Synthesis of Ligands
2.1.1. Synthesis of 4-Me-4ꢀ-CF3-ppy
ꢁ
2-(4ꢀ-(trifluoromethyl)phenyl)-4-methylpyridine
ligand
tion mixture was refluxed for 4 hr at 135 C. The solution
was cooled to room temperature and poured into 10 ml of
water. The precipitate was filtered, washed with methanol
and acetone and dried under vacuum.
was obtained from the reaction of 2chloro-4-methyl-
pyridine with 4-trifluoromethyl-phenylboronic acid by
Suzuki coupling. 2chloro-4-methylpyridine (0.968 g,
11 mmol), 4-fluorophenyl-boronic acid (1.40 g, 10 mmol)
and tetrakistriphenylphospine palladium(0) (0.196 g,
0.17 mmol) were placed in a mixture of toluene (20 ml),
ethanol (10 ml) and 2 N sodium carbonate aqueous solu-
tion (20 ml). The reaction mixture was heated to reflux
for 8 hr at 135 ꢁC. The mixture was cooled to room
temperature and extracted with 20 ml of ethyl acetate. The
organic fraction was dried over anhydrous MgSO4. After
removal of MgSO4, the solvent was evaporated and the
residue was chromatographed on a silicagel column with
ethyl acetate/hexane (1:3). The product was collected and
dried in vacuum to yield a light yellow liquid with 57%
yield.
Ir(4-Me-4ꢀ-CF3-ppy)2(trzl-CMe3ꢁ, Yield: 47%. FAB-
MS: calculated 866.2; found 867. 1H NMR (DMSO,
600 MHz): ꢂ8.26–6.37 (m, 16H, aromatic Hsꢀ); 2.54, 2.51
(s, 3H each, CH3ꢁ; 1.25 (s, 9H, C(CH3)3ꢁ ppm.
Ir(4-Me-4ꢀ-Fppy)2(trzl-CMe3ꢁ, Yield: 52%. FAB-MS:
calculated 766.2; found 767. 1H NMR (DMSO, 600 MHz):
ꢂ8.07–5.75 (m, 16H, aromatic Hsꢀ); 2.47, 2.45 (s, 3H each,
CH3ꢁ; 1.26 (s, 9H, C(CH3)3ꢁ ppm.
Ir(4-OMe-4ꢀ-CF ppy) (trzl-CMe ꢁ, Yield: 49%. FAB-
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3
2
3
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ꢀ
Copyright: American Scientific Publishers
600 MHz): ꢂ8.11–6.43 (m, 16H, aromatic Hs ); 3.99, 3.96
(s, 3H each, OCH3ꢁ; 1.26 (s, 9H, C(CH3)3ꢁ ppm.
2.3. Optical Measurements
UV-Vis absorption spectra were measured on a Hewlett
Packard 8425A spectrometer. PL spectra were measured on
a Perkin Elmer LS 55 spectrometer. UV-Vis and PL spectra
of the iridium complexes were measured in 10−5 M dilute
CH2Cl2 solution and in the PMMA film. The PMMA film
was fabricated by the spin-coating onto the glass substrate
with 10 wt% Ir complexes of PMMA in 1,2-dichloroetane
solution and following solvent evaporation.
2.1.2. Synthesis of 4-Me-4ꢀ-F-ppy
2-(4ꢀ-fluorophenyl)-4-methylpyridine ligand was synthe-
sized from the reaction of 4-fluorophenylboronic acid
with 2chloro-4-methylpyridine, according to the procedure
mentioned above. Yield: 60%.
2.1.3. Synthesis of 4-OMe-4ꢀ-CF3-ppy
2.4. Fabrication of the EL Devices
2-(4ꢀ-(trifluoromethyl) phenyl)-4-methoxypyridine ligand
was synthesized from the reaction of 4-trifluoromethyl-
phenylboronic acid with 2-chloro-4-methoxypyridine,
according to the procedure mentioned above.
Yield: 60%.
The configuration of the devices was ITO/4,4ꢀbis[N-
(naphthyl)-N-phenyl-amino]biphenyl (NPB) (50 nm)/8%
Ir complex in 4,4,N,Nꢀ-dicarbazolebiphenyl (CBP) (30
nm)/4,7-diphenyl-1,10-phenanthroline (Bphen) (30 nm)/
lithium quinolate (Liq) (2 nm)/Al (100 nm). The OLEDs
were fabricated by high vacuum (5 × 10−7 torr) ther-
mal deposition of the materials onto an indium tin oxide
(ITO)-coated glass substrate. The current density–voltage
(J–V ) characteristics of the OLEDs were measured with a
source measure unit (Kiethley 236). Their luminance and
CIE chromaticity coordinates were measured using a chro-
mameter (MINOLTA CS-100A). The EL spectra of the
2.1.4. Synthesis of Trzl-CMe3
The ancillary tetrazolone ligand was prepared accord-
1
ing to the literature.5 Yield: 70%. H NMR (DMSO-d6,
300 MHz): ꢂ 8.57, 8.08, 7.88, 7.44 (m, 1H each, aromatic
Hsꢀ); 1.21(s, 9H, C(CH3)3ꢁ ppm.
J. Nanosci. Nanotechnol. 11, 4414–4418, 2011
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New Blue Phosphorescent Iridium Complexes Containing Phenylpyridine and Triazole Ligands
Ahn et al.
devices were measured by IVL-200 of JBS international
Co., Ltd. All measurements were performed in ambient
conditions under DC voltage bias.
as illustrated in Figure 1(a). The ancillary ligand, trzl-
CMe3, was prepared from the reaction of amidrazone
precursor with trimethylacetyl chloride, according to
the reported procedure.5 Ir(C∧N)2(trzl-CMe3ꢁ were syn-
thesized via the chloride bridged dimer, according to
Nonoyama method.8 The overall synthetic schemes are
illustrated in Figure 1.
The UV-Vis absorption spectra of the complexes in
CH2Cl2 are shown in Figure 2. The absorption spectra
of the complexes have intense bands, appearing at the
3. RESULTS AND DISCUSSION
The synthesis of the C∧N ligand, an ancillary lig-
and and their iridium complexes was straightforward.
The ligands (C∧N), 4-Me-4ꢀ-CF3-ppy, 4-Me-4ꢀ-F-ppy and
4-OMe-4ꢀ-CF3-ppy, were prepared by the Suzuki coupling,
(a)
X
X
OH
Pd(PPh3)
B
+
HO
reflux
N
N
Cl
Y
Y
X = CH3, Y= CF3 : 4-Me-4′-CF3-ppy
X = CH3, Y= F : 4-Me-4′-F-ppy
X = OMe, Y= CF3 : 4-OMe-4′-CF3-ppy
(b)
N
O
N
Cl
CMe
3
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N
H2N-NH2
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Copyright: American Scientific Publishers
EtOH
1) Na2CO3, DMAA
2) Ethylene glycol, 200 °C
HN
N
NH
HN
NH2
N
CN
CMe3
Cl
(c)
C^N
(N^C)2Ir
Ir(C^N)2
IrCl3 nH2O
reflux
Cl
CMe3
N
N
N
N
H
N
(N^C)2Ir
EtOH, CH2Cl2
N
N
N
CMe3
C^N = 4-Me-4′-CF3-ppy
4-Me-4′-F-ppy
4-OMe-4′-CF3-ppy
Fig. 1. The synthesis of substituted ppy ligands, triazole and their iridium complexes.
4416
J. Nanosci. Nanotechnol. 11, 4414–4418, 2011

Ahn et al.
New Blue Phosphorescent Iridium Complexes Containing Phenylpyridine and Triazole Ligands
Ir(4-Me-4′-CF3-ppy)2(trzl-CMe3) : 494, 513 nm
Ir(4-Me-4′-F-ppy)2(trzl-CMe3) : 471, 497 nm
Ir(4-Me-4′-CF3-ppy)2(trzl-CMe3)
Ir(4-Me-4′-F-ppy)2(trzl-CMe3)
1.0
0.8
0.6
0.4
0.2
0.0
Ir(4-OMe-4′-CF3-ppy)2(trzl-CMe3) : 492, 511 nm
0.3
Ir(4-OMe-4′-CF3-ppy)2(trzl-CMe3)
0.2
0.1
0.0
400
500
600
700
Wavelength (nm)
300
400
500
600
700
800
Fig. 4. PL spectra of the iridium complexes in a PMMA film.
Wavelength (nm)
Fig. 2. UV-Vis absorption spectra of the iridium complexes.
respectively, which cause green tint in blue phospho-
rescence of the complexes. Ir(4-Me-4ꢀ-F-ppy)2(trzl-CMe3ꢁ
shows the most hypsochromic shift in emission among
the Ir(III) complexes prepared herein. The complex having
the electron-withdrawing fluorine (F) substituents in phenyl
ring experiences more blue-shift in emission than the com-
plex containing trifluoromethyl (CF3ꢁ substituents. It indi-
cates that the fluorinated phenyl ring contributes more to
the energy gap increase by lowering the HOMO levels of
the complex than the trifluoromethyl susbstituted analog.11
Both methyl (Me) and methoxy (OMe) substituents have
ultraviolet region of the spectrum between 200 and
310 nm. These bands are assigned to the spin-allowed
1ꢄꢅ → ꢅ∗) transitions of the ligands in the complexes.
The weak bands from 320 nm extended to 400 nm can be
assigned to the spin-allowed metal-ligand charge transfer
band (1MLCT), and the weaker absorption bands at the
longer wavelengths than 450 nm are attributed to the spin-
3
forbidden MLCT.9 The intensities of the MLCT bands
were lower than those of red or green phosphorescent
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emitters because the large energy gap causes reduction of
electron-donating properties, and the emission peak of Ir(4-
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the MLCT transitions.10
ꢀ
Copyright: American Scientific Publishers
OMe-4 -CF3-ppy)2(trzl-CMe3ꢁ is blue-shifted, compared
with that of Ir(4-Me-4ꢀ-CF3-ppy)2(trzl-CMe3ꢁ. It could be
attributed to the difference of resonance and inductive
effects by the substituents.12 The methoxy substituent has a
resonance effect with the ꢅ-system, leading to easy trans-
fer of the electrons while the methyl substituent has only
an inductive effect on the ꢆ-system.
The photoluminescence (PL) spectra of the iridium
complexes in 10−5 M CH2Cl2 solution are shown in
Figure 3. The PL patterns of the complexes are similar,
but the emission maxima occur at somewhat different
wavelengths. The PL peaks for Ir(4-Me-4ꢀ-CF3-ppy)2(trzl-
CMe3ꢁ, Ir(4-Me-4ꢀ-F-ppy)2(trzl-CMe3ꢁ and Ir(4-OMe-4ꢀ-
CF3-ppy)2(trzl-CMe3ꢁ appeared at 494, 472 and 484 nm.
The less intense shoulder peaks at the longer wave-
lengths were also observed at 514, 493 and 505 nm,
We also attempted to investigate their PL in PMMA
(poly(methylmethacrylate)) film for the polymer light-
emitting device (PLED) fabrication. The films were
Ir(4-Me-4′-CF3-ppy)2(trzl-CMe3) : 494, 514 nm
Ir(4-Me-4′-F-ppy)2(trzl-CMe3) : 472, 493 nm
Ir(4-Me-4'-CF -ppy) (trzl-CMe ) : 496, 526(sh)nm
3
2
3
1.0
1.0
0.8
0.6
0.4
0.2
0.0
Ir(4-OMe-4′-CF3-ppy)2(trzl-CMe3) : 484, 505 nm
Ir(4-Me-4'-F-ppy) (trzl-CMe ) : 437, 468(sh)nm
2
3
0.8
0.6
0.4
0.2
0.0
400
500
600
700
800
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
Fig. 3. PL spectra of the iridium complexes in a 10−5 M CH2Cl2
solution.
Fig. 5. EL spectra of the iridium complexes.
J. Nanosci. Nanotechnol. 11, 4414–4418, 2011
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New Blue Phosphorescent Iridium Complexes Containing Phenylpyridine and Triazole Ligands
Ahn et al.
Table I. Characteristics of OLED devices of the iridium complexes.
EL ꢇmax (nm)
of devices
Max.
luminance (cd/m2ꢁ
Max. Luminous
efficiency (cd/A)
Max. Power
efficiency (lm/W)
CIE
coordinates
Dopant
Ir(4-Me-4ꢀ-CF3-ppy)2(trzl-CMe3ꢁ
Ir(4-Me-4ꢀ-F-ppy)2(trzl-CMe3ꢁ
496/526(sh)
437/468(sh)
10850
870.7
12ꢈ49
1ꢈ09
7.79
0.86
(0.259, 0.526)
(0.176, 0.143)
made by the spin-coating method with 1,2-dichloroethane
solution of the complexes and PMMA mixture. PMMA
was chosen as a host because its non-emitting property
within the visible range could make the emission peak of
the iridium complex shown only. Ir(4-Me-4ꢀ-F-ppy)2(trzl-
CMe3ꢁ and Ir(4-Me-4ꢀ-CF3-ppy)2(trzl-CMe3ꢁ in PMMA
exhibited PL maxima at 471/497(sh) and 494/513(sh) nm,
respectively (Fig. 4). The emission range of these com-
plexes in the PMMA film were somewhat broader than
those in solution, but both the film and the solution states
showed the similar aspects in emission maxima. It means
that the emission process in both the solution and the film
is similar without the significant quenching. On the other
hand, the PL maxima of the PMMA film of Ir(4-OMe-4ꢀ-
CF3-ppy)2(trzl-CMe3ꢁ occurred at 492/511(sh) nm, com-
pared with that of its solution at 484/505(sh) nm. The
peaks of the films became broader and underwent some-
what red-shift from its solution PL, presumably due to
quenching by aggregation.
ancillary ligand. The iridium complexes, Ir(4-Me-4ꢀ-CF3-
ppy)2(trzl-CMe3ꢁ and Ir(4-OMe-4ꢀ-CF3-ppy)2(trzl-CMe3ꢁ
exhibited the blue-green emission at 494 and 484 nm
with the shoulder at 514 and 505 nm, respectively. The
emission of Ir(4-Me-4ꢀ-F-ppy)2(trzl-CMe3ꢁ appeared at
472 nm with a shoulder peak at 493 nm in its solu-
tion PL, which was blue-shifted most among the com-
plexes in this study. Although there were some differences
between the PL of the solutions and of the films, the PLs
had similar emission patterns. EL emission of Ir(4-Me-4ꢀ-
CF3-ppy)2(trzl-CMe3ꢁ and Ir(4-Me-4ꢀ-F-ppy)2(trzl-CMe3ꢁ
appeared at 496/526(sh) and 437/468(sh) nm, respectively,
especially, the CIE coordinate of Ir(4-Me-4ꢀ-F-ppy)2(trzl-
CMe3ꢁ was (0.176, 0.143), very close to that of the satu-
rated standard blue emission.
Acknowledgment: This work was supported by the
Korea Research Foundation (KRF-2008-531-C00036).
The EL devices were fabricated and their EL perfor-
mances of the new complexes were investigated. The
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References and Notes
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EL spectra of Ir(4-Me-4ꢀ-CF3-ppy)2(trzl-CMe3ꢁ and Ir(4-
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Copyright: American Scientific Publishers
Me-4ꢀ-F-ppy)2(trzl-CMe3ꢁ showed the emission peaks at
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496/526(sh) nm and 437/468(sh) nm, respectively, as
shown in Figure 5. The CIE coordinate of the devices con-
taining Ir(4-Me-4ꢀ-F-ppy)2(trzl-CMe3ꢁ as a light-emitting
dopant was (0.176, 0.143), being shifted toward the sat-
urated blue emission, compared with (0.259, 0.526) of
Ir(4-Me-4ꢀ-CF3-ppy)2(trzl-CMe3ꢁ. However, the maximum
efficiency of device with Ir(4-Me-4ꢀ-F-ppy)2(trzl-CMe3ꢁ
was much lower than that of Ir(4-Me-4ꢀ-CF3-ppy)2(trzl-
CMe3ꢁ. Because the increase of energy gap means that
decrease of MLCT transition, we presume that decrease
of MLCT transition due to energy gap increase leads to
reduction in the T1 → S0 transition and thereby causes a
low emission quantum efficiency.10ꢀ13 The electrical and
optical characteristics are summarized in Table I.
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4. CONCLUSION
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Herein, we report the detailed syntheses and the lumines-
cence properties of the phosphorescent iridium(III) com-
plexes containing the substituted ppy ligands and a trzl
Received: 23 May 2009. Accepted: 20 November 2009.
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J. Nanosci. Nanotechnol. 11, 4414–4418, 2011