Synthesis and luminescence study of red phosphorescent Ir(III) pyrazolonate complexes for OLED

Article abstract of DOI:10.1016/j.jpcs.2007.10.110

A pyrazolone-based ligand, 4-acetyl-5-methyl-2-phenyl-2,4-dihydro-pyrazol-3-one (pyzl), was prepared and introduced as a new ancillary ligand, replacing acac in the iridium complexes for high efficient OLED. The purpose of this study was that modification

Full text of DOI:10.1016/j.jpcs.2007.10.110

ARTICLE IN PRESS
Journal of Physics and Chemistry of Solids 69 (2008) 1305–1309
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Synthesis and luminescence study of red phosphorescent Ir(III)
pyrazolonate complexes for OLED
Ã
Hyun-Shin Leea, Ji-Hyun Seob, Moon Kyu Choia, Young-Kwan Kimb, Yunkyoung Hab,
aDepartment of Chemical Engineering, Hongik University, Seoul 121-791, Republic of Korea
bDepartment of Information Display Engineering, Hongik University, Seoul 121-791, Republic of Korea
Received 25 June 2007; received in revised form 23 October 2007; accepted 30 October 2007
Abstract
A pyrazolone-based ligand, 4-acetyl-5-methyl-2-phenyl-2,4-dihydro-pyrazol-3-one (pyzl), was prepared and introduced as a new
ancillary ligand, replacing acac in the iridium complexes for high efficient OLED. The purpose of this study was that modification
of the ancillary ligand might allow luminous efficiency increase by reducing T–T annihilation. The przl ligand in the iridium
complex might also lead to fine tuning of the emission color by affecting the energy level of Ir d-orbitals. Previously, we reported
that the iridium complexes, Ir(4-Me-2,3-dpq)2(acac) and Ir(2,3-dpqx-F2)2(acac), exhibited their electroluminescence (EL) emission at 603
and 630 nm with luminous efficiency of 8.10 and 2.75 cd/A, respectively, where 6-Me-2,3-dpq, 2,3-dpqx-F2 and acac represent 4-methyl-
2,3-diphenylquinoline and 2,3-bis-(4-fluorophenyl)quinoxaline and acetylacetonate, respectively. In this study, we synthesized a new
series of iridium complexes with the pyrazolonate ligand, Ir(6-Me-2,3-dpq)2(przl) and Ir(2,3-dpqx-F2)2(przl), and investigated their
luminescence properties with respect to the involvement of a new bulky ancillary ligand, przl, compared with the complexes containing
the ligand acac.
r 2007 Elsevier Ltd. All rights reserved.
Keywords: A. Electronic materials; B. Chemical synthesis; D. Luminescence
1. Introduction
luminance efficiency of OLEDs can be improved by
reducing T–T annihilation. In addition, we expect that
the przl ligand in the iridium complex may also lead to fine
tuning of the emission color by mixing with the energy level
of Ir d-orbitals. Thus, the luminescence properties of the
iridium complexes containing przl were investigated and
compared with the ones containing acac.
Phosphorescent organic light-emitting devices (OLEDs)
have received much of attention owing to their high
emission efficiency compared with the fluorescent ones
[1–6]. The heavy metal iridium complexes were known to
have high photoluminescence (PL) efficiency and relatively
short excited emissive states [7,8]. Thus, the green phos-
phorescent iridium complexes have been utilized as
dopants in the emitting layer of the phosphorescent
OLEDs [9,10]. However, the application of red phosphor-
escent dopants has difficulties, mostly due to the problems
in red color purity and in the low efficiency.
2. Experimental section
All reactions were carried out under a dry argon
atmosphere.
Herein, we prepared a pyrazolone derivative (przl) as
an ancillary ligand and synthesized its iridium complexes,
Ir(4-Me-2,3-dpq)2(przl) and Ir(2,3-dpqx-F2)2(przl). Since
the ligand przl is bulkier than acetylacetonate (acac), the
2.1. Synthesis of ligands (L=przl, 2,3-dpqx-F2, 4-Me-2,
3-dpq)
2.1.1. Pyrazolone (przl)
The ancillary pyrazolone ligand was prepared according
to the literature [11].
Ã
Corresponding author. Tel.: +82 2 320 1490; fax: +82 2 3142 0335.
E-mail address: ykha@hongik.ac.kr (Y. Ha).
0022-3697/$ - see front matter r 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2007.10.110

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H.-S. Lee et al. / Journal of Physics and Chemistry of Solids 69 (2008) 1305–1309
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1
2.1.2. 4-Me-2,3-dpq and 2,3-dpqx-F2
FAB-MS: calculated 996; found 996. H NMR (ppm): d
8.52–6.40 (aromatic H’s, 18H); 2.56, 2.45 (CH3’s of przl,
3H each); 2.02, 1.97 (CH3’s of 4-Me, 3H each).
The dpq and dpqx ligand derivatives were obtained
according to Friedlander eaction with the corresponding
precursors (2-aminoacetophenone (1.214 ml, 10.0 mmol)
and 2-phenyl-acetophenone (1.96 g, 10.0 mmol)) and (1,2-
diaminobenzene (1.081 g, 10.0 mmol) and 4,40-difluoroben-
zil (2.46 g, 10.0 mmol)), respectively.
2.2.2. Ir((2,3-dpqx-F2)2(przl)
This complex was prepared from the reaction of 2,
3-dpqx-F2, with IrCl3 ꢀ H2O and then with the pyrazolone,
similar to the procedure described above. Ir(2,3-dpqx-
F2)2(przl) was obtained as a bright red powder. FAB-MS:
calculated 1042; found 1043. H NMR (ppm): d 8.45–5.97
(aromatic H’s, 16H); 2.17, 2.05 (CH3’s of przl, 3H each).
2.2. Synthesis of complexes
1
2.2.1. Ir(4-Me-2,3-dpq)2(przl)
First, the cyclometalated Ir(III) m-chloro-bridged
dimer, (4-Me-2,3-dpq)2Ir(m-Cl)2Ir(4-Me-2,3-dpq)2 (1.73 g,
1.9 mmol), was prepared according to the Nonoyama
method. Second, the resulting dimer and pyrazolone
(1.41 g, 6.5 mmol) were mixed with Na2CO3 (500 mg) in
2-ethoxyethanol (30 ml). The mixture was refluxed for 2 h
and the red solid was filtered after cooling. Ir(4-Me-2,3-
dpq)2(przl) was purified by chromatography on silica
gel column with dichloromethane and recrystallization.
3. Results and discussion
The replacement of acac with przl in the Ir complexes
might improve the emissive efficiency of the complex and
lead to fine tuning of the emissive wavelength. According
to the literature [11], the pyrazolone ligand as an ancillary
ligand in the Ir complexes not only has suitable triplet
energy levels matching 5d-orbital of Ir but also has good
CH
3
CH
N
3
N
Cl
Ca(OH)2/Ba(OH)2
N
+
N
O
O
O
O
O
O
N
CH3COOH , H2SO4
reflux
+
N
H2N
F
NH2
F
F
F
O
CH COOH , H SO
reflux
+
N
O
NH2
N
N
L=
N
F
Cl
Cl
,
F
acac or przl
reflux
L
(L) Ir(przl)
or
Ir
(L) Ir(acac)
IrCl nH O
(L)
(L) Ir
reflux
acac =
przl =
O
O
N
O
N
,
O
Fig. 1. Synthetic route of the ligands, 4-Me-2,3-dpq and 2,3-dpqx, and their iridium complexes.

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1307
carrier transporting properties. The 4-acetyl-5-methyl-2-
phenyl-2,4-dihydro-pyrazol-3-one (przl) ligand was pre-
pared as shown in Fig. 1(a).
The other ligands (L), 2,3-dpqx-F2 and 4-Me-2,3-dpq,
were synthesized according to the Friedlander reaction, as
illustrated in Fig. 1(b). The synthetic method of iridium
complexes Ir(L)2(przl) involved two steps: formation of the
dimer and the following coordination of przl as summar-
ized in Fig. 1(c).
The UV–vis absorption spectra of the complexes in
CH2Cl2 are shown in Fig. 2. The absorption patterns
of these complexes are similar. The weak bands between
400 and 500 nm in the visible region can be assigned to the
spin-allowed metal-to-ligand charge transfer band
(1MLCT), and the weaker absorption band at the longer
wavelength can be attributed to the spin-forbidden
3MLCT and spin-orbit coupling enhanced 3p–p* transition.
The formally spin-forbidden 3MLCT gains the inten-
sity through the strong spin-orbit coupling on the Ir
center.
The PL spectra of the Ir complexes in 10ꢁ5 M CH2Cl2
solution are shown in Fig. 3. The emission maxima for Ir(4-
Me-2,3-dpq)2(acac), Ir(4-Me-2,3-dpq)2(przl), Ir(2,3-dpqx-
F2)2(acac) and Ir(2,3-dpqx-F2)2(przl) appeared at 604, 602,
631 and 630 nm, respectively. PL peak wavelengths were
not shifted upon the change of the ancillary ligand in the
complexes within the spectra errors.
However, the electroluminescence (EL) of the complexes
exhibited the expected bathochromic shift in emission. In
the EL spectra, the EL peaks of Ir(4-Me-2,3-dpq)2(przl)
were observed at 608 nm, compared to 603 nm in the case
of Ir(4-Me-2,3-dpq)2(acac). More bathochromic shift was
observed in the case of Ir(2,3-dpqx-F2)2(przl) (641 nm) as
compared to the case of Ir(2,3-dpqx-F2)2(acac) (630 nm).
The luminances and/or luminance efficiencies were also
improved upon replacement of acac with przl in the Ir
(4-Me-2,3-dpq)2(ancillary ligand) complex. The luminances
of Ir(4-Me-2,3-dpq)2(acac) and Ir(4-Me-2,3-dpq)2(przl)
were 12,800 and 17,900 cd/m2 at 14 V, respectively.
The luminous efficiency maxima of the device containing
(4-Me-2,3-dpq)2Ir(acac)
(4-Me-2,3-dpq)2Ir(acac)
0.8
1.0
(4-Me-2,3-dpq)2Ir(przl)
(4-Me-2,3-dpq)2Ir(przl)
604 nm
602 nm
0.8
0.6
0.4
0.2
0.0
0.6
0.4
0.2
0.0
200
400
600
400
500
600
700
wavelength (nm)
wavelength (nm)
0.8
(2,3-dpqx-F2)2Ir(acac)
(2,3-dpqx-F2)2Ir(przl)
(2,3-dpqx-F2)2Ir(acac)
(2,3-dpqx-F2)2Ir(pzrl)
1.0
0.8
0.6
0.4
0.2
0.0
631 nm
630 nm
0.6
0.4
0.2
0.0
200
400
600
400
500
600
700
wavelength (nm)
wavelength (nm)
Fig. 2. UV–vis absorption spectra of Ir(4-Me-2,3-dpq)2(przl) and Ir(2,3-
dpqx)2(przl) compared to their acac complexes.
Fig. 3. PL spectra of Ir(4-Me-2,3-dpq)2(przl) and Ir(2,3-dpqx)2(przl).

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H.-S. Lee et al. / Journal of Physics and Chemistry of Solids 69 (2008) 1305–1309
1308
20000
18000
16000
14000
12000
10000
8000
6000
4000
2000
0
14
Ir(4-Me-2,3-dpq)2(przl)
Ir(4-Me-2,3-dpq)2(przl)
12
10
8
-2000
-20
0
20 40 60 80 100 120 140 160 180 200 220 240 260
Current density [mA/cm2]
-20
0
20 40 60 80 100 120 140 160 180 200 220 240 260
Current density [mA/cm2]
2500
2000
1500
1000
500
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
Ir(2,3-dpqx-F2)2(przl)
Ir(2,3-dpqx-F2)2(przl)
0
0
20
40
60
80
100
120
0
20
40
60
80
100
120
Current density [mA/cm2]
Voltage [v]
Fig. 4. Luminance and luminance efficiency of Ir(4-Me-2,3-dpq)2(przl) and Ir(2,3-dpqx)2(przl).
Table 1
Characteristics of OLED devices of Ir(4-Me-2,3-dpq)2(przl) and Ir(2,3-dpqx)2(przl)
Ir complexes
PL lmax in CH2Cl2
(nm)
EL lmax of devices
(nm)
Luminance (cd/m2)
Luminance efficiency
(cd/A)
CIE
Ir(4-Me-2,3-
dpq)2(acac)
Ir(4-Me-2,3-
dpq)2(przl)
Ir(2,3-dpqx-
F2)2(acac)
604
602
631
630
603
608
630
641
12800
17900
3570
8.10
13.0
2.75
3.46
(0.644, 0.352)
(0.625, 0.370)
(0.684, 0.311)
(0.685, 0.311)
Ir(2,3-dpqx-
F2)2(przl)
2100
Ir(4-Me-2,3-dpq)2(acac) and Ir(4-Me-2,3-dpq)2(przl) were
8.10 and 13.0 cd/A and those of Ir(2,3-dpqx-F2)2(acac)
and Ir(2,3-dpqx-F2)2(przl) were 2.75 and 3.46 cd/A at
20 mA/cm2, respectively, as illustrated in Fig. 4. Their
performances are summarized in Table 1. The Commission
Internationale de L’Eclairage (CIE) coordinates of Ir
(4-Me-2,3-dpq)2(przl) and Ir(4-Me-2,3-dpq)2(acac) were
(0.625, 0.370) and (0.644, 0.352), respectively, being shifted
to purer red luminescence, while those of Ir(2,3-dpqx-
F2)2(przl) and Ir(2,3-dpqx-F2)2(acac) were quite similar.
4. Conclusion
The new iridium complexes with the pyrazolonate ligand
(przl), Ir(6-Me-2,3-dpq)2(przl) and Ir(2,3-dpqx-F2)2(przl),
were prepared for the application to OLEDs. We expected

ARTICLE IN PRESS
H.-S. Lee et al. / Journal of Physics and Chemistry of Solids 69 (2008) 1305–1309
1309
that the bulky przl ligand might lead to affect the emission
color and device performance. The experiment results
proved that the modification of an ancillary ligand from
acac to przl alters the emissive wavelength toward true red
emission, and improves the emissive efficiency of its iridium
complexes as well.
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Acknowledgment
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This work was supported by the Korea Research
Foundation (KRF-2006-531-C00031).
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