Synthesis and the luminescent study of the iridium complexes containing 2,3-diphenylquinoline derivatives and the new ancillary ligand for OLED

Article abstract of DOI:10.1080/15421400903217546

We previously reported that Ir(6-F-2,3-dpqx-(OMe)₂) ₂(acac) (6-F-2,3-dpqx-(OMe)₂=6-fluoro-2,3-bis(4- methoxylphenyl)quinoxaline) exhibited photoluminescence(PL) and electroluminescence (EL) emission at 645 and 667nm, respectively. To modulate the emission maxima toward the saturated red chromacity, a new main ligand and its iridium complexes were prepared, and their photophysical properties were investigated. As a main ligand, 6-OMe-2,3-dpqx-(OMe)₂ (6-OMe-2,3-dpqx-(OMe)₂=6-methoxy-2,3-bis(4-methoxylphenyl) quinoxaline), were designed and synthesized. Acetylacetonate (acac) and pyrazolonates (przls) were also chelated to the iridium center as an ancillary ligand. The resulting complexes prepared herein were Ir(6-OMe-2,3-dpqx-(OMe) ₂)₂(acac), Ir(6-OMe-2,3-dpqx-(OMe)₂) ₂(przl1) and Ir(6-OMe-2,3-dpqx-(OMe)₂)₂(przl2). The photoluminescence (PL) of these complexes were observed around 660nm, and their electroluminescence maxima were observed around 650nm. The PL investigation of the complexes in PMMA (PMMA=poly(methylmetacrylate)) for the polymer solution process revealed that the emission peaks became broader with the range of 600～850nm than those of the complexes only.

Full text of DOI:10.1080/15421400903217546

Mol. Cryst. Liq. Cryst., Vol. 514, pp. 14=[344]–24=[354], 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400903217546
Synthesis and the Luminescent Study of the Iridium
Complexes Containing 2,3-Diphenylquinoline
Derivatives and the New Ancillary Ligand for OLED
Hyun-Shin Lee, So Youn Ahn, and Yunkyoung Ha
Department of Information Display Engineering, Hongik University,
Seoul, Korea
We previously reported that Ir(6-F-2,3-dpqx-(OMe)
-fluoro-2,3-bis(4-methoxylphenyl)quinoxaline)
PL) and electroluminescence (EL) emission at 645 and 667 nm, respectively. To
2
)
2
(acac) (6-F-2,3-dpqx-(OMe)
2
¼
6
(
exhibited photoluminescence
modulate the emission maxima toward the saturated red chromacity, a new main
ligand and its iridium complexes were prepared, and their photophysical properties
were investigated. As
dpqx-(OMe)
and synthesized. Acetylacetonate (acac) and pyrazolonates (przls) were also che-
lated to the iridium center as an ancillary ligand. The resulting complexes prepared
2
a main ligand, 6-OMe-2,3-dpqx-(OMe) (6-OMe-2,3-
2
¼ 6-methoxy-2,3-bis(4-methoxylphenyl)quinoxaline), were designed
2 2 2 2
herein were Ir(6-OMe-2,3-dpqx-(OMe) ) (acac), Ir(6-OMe-2,3-dpqx-(OMe) ) (przl1)
and Ir(6-OMe-2,3-dpqx-(OMe) (przl2). The photoluminescence (PL) of these
2 2
)
complexes were observed around 660 nm, and their electroluminescence maxima
were observed around 650 nm. The PL investigation of the complexes in PMMA
(
PMMA ¼ poly(methylmetacrylate)) for the polymer solution process revealed that
the emission peaks became broader with the range of 600ꢀ850 nm than those of
the complexes only.
Keywords: iridium complex; OLED; pyrazolone-based ancillary ligands (przl series);
red phosphorescence
1. INTRODUCTION
Organic light-emitting diodes (OLEDs) involving phosphorescent
compounds as an emitting dopant have been developed because
they generally exhibit higher efficiencies than those containing
This work was supported by the Korea Research Foundation (KRF-2007-531-C00031).
Address correspondence to Prof. Yunkyoung Ha, Department of Science, College of
Engineering, Hongik University, 72-1 Sangsu-Dong, Mapo-Ku, Seoul 121-791, Korea
(ROK). E-mail: ykha@hongik.ac.kr
1
4=[344]

Synthesis and the Luminescent Study of the Iridium Complexes
15/[345]
conventional fluorescent materials [1–7]. The heavy transition metal
complexes can provide large population of the emitting triplet state
by strong spin-orbit coupling induced by the metal center, resulting
in high efficiency. The iridium(III) complexes with high luminescence
efficiency were also known to have relatively short excited emissive
states, suitable for a phosphorescent dopant in OLEDs [8–9]. The
cyclometalating ligands in the iridium complex were reported to deter-
mine the major color of phosphorescence with control of the energy
gap between the highest occupied molecular orbitals (HOMOs) and
the lowest unoccupied molecular orbitals (LUMOs) [10–11]. Thus,
modification of cyclometalating ligands by adding an electron-
donating or -withdrawing group should lead to emission color tuning
of their complexes.
The ancillary ligands for the iridium complexes tend to influence
the perturbation degree of the metal-to-ligand charge transfer (MLCT)
transition by the excited-state interaction with the cyclometalating
ligand centered (LC) state, causing the minor change in emission
wavelengths [12]. As recently reported [13–14], introduction of the
new bulky ancillary ligand, pyrazolate, to red-emitting Eu(III) and
Ir(III) complexes could result in the luminous efficiency improvement
by their orbital mixing with the Ir d-orbitals. Our previous results also
indicated that replacement of acac with przl in the iridium complexes
could improve the emission efficiency and lead to fine-tuning of the
emission wavelength [15]. Pyrazolone based ligands as ancillary
ligands in the Ir complexes have not only suitable triplet energy levels
matching 5d orbital of Ir but also good carrier transporting properties.
According to our earlier report [16] an iridium complex, Ir(6-F-2,3-
dpqx-(OMe) ) (acac), exhibited its electroluminescence (EL) emission
2
2
at 667 nm with luminous efficiency of 2.39 cd=A. This luminescence
was in rather deep red region, and thus increase of the band gap
was required for the saturated red emission by controlling the energy
levels of the HOMOs and=or LUMOs. Therefore, another cyclometal-
lating ligand, 6-methoxy-2,3-bis(4-methoxyphenyl)quinoxaline, was
prepared to tune the emission color of its iridium complexes toward
the saturated red region. Additionally, for fine tuning of the emission
color, the ancillary ligand, acetylacetonate (acac) was replaced with
the pyrazolone-based ligands, 4-R-5-methyl-2-phenyl-2,4-dihydro-
pyrazol-3-one (przl) series, where the przl ligand series have different
substituents at the 4-position of the ring. As a result, new iridium
complexes, Ir(6-OMe-2,3-dpqx-(OMe) ) (acac) and Ir(6-OMe-2,3-dpqx-
2
2
(
OMe) ) (przls), were synthesized and their PL properties were
2 2
compared. Fabrication of OLEDs was attempted with the complexes
prepared herein as an emitting dopant, and the preliminary EL

1
6/[346]
H.-S. Lee et al.
spectra were obtained. Furthermore, the PMMA film spin coated
with iridium complexes were fabricated and their film PLs were also
studied.
2. EXPERIMENTAL DETAILS
All reagents were purchased from Aldrich Co. and Strem Co. and used
without further purification. All reactions were carried out under an
argon atmosphere. Solvents were dried by standard procedures. All
column chromatography was performed with the use of silica gel
(
230-mesh, Merck Co.). Mass spectra were determined on JEOL,
JMS-AX505WA, HP 5890 Series II Hewlett-Packard 5890A (capillary
column) at Seoul National University, Korea.
2
.1. Synthesis of Ligands
Synthesis of Prazolone Ligands (Przl1 and Przl2)
-phenyl-3-methyl-5-pyrazolone (3.5 g, 10 mmol) was dissolved in
0 ml of dried 1,4-dioxane in a 100 ml flask. Calcium hydroxide
1
4
(
(
3.552 g) and barium hydroxide (2.064 g) were added. Acylchloride
acetyl chloride or benzoyl chloride) (11.6 mmol, 1.64 ml=1.345 ml,
respectively) were dripped slowly to the solution mixture under stir-
ring. After the reflux for 24 h, the reaction mixture was cooled and
poured into a 70 ml solution of ice-cold hydrochloric acid solution
[
HCl:H O ¼ 3:7 (v=v)]. The product was extracted with dichloro-
2
methane and recrystallized from acetone and water [13–14]. Yield: 60%.
6
(
-methoxy-2,3-bis(4-methoxyphenyl)quinoxaline
6-OMe-2,3-dpqx-(OMe)2)
-Methoxy-o-phenylenediamine dihydrochloride (2.1 g, 10.0 mmol)
4
with NaOH(0.48 g, 12 mmol) in 30 ml ethanol. The solution was heated
to reflux for 5 h. The mixture was cooled at room temperature and
removed water and salt. The dark purple solution was evaporated at
vacuum. This step was neutralization reaction before synthesize the
ligand. This ligand for emission was obtained according to Friedlander
reaction [17] of neutralized 4-Methoxy-o-phenylenediamine dihy-
0
drochloride (1.38 g, 10.0 mmol) with 4,4 -dimethoxybenzil (2.70 g,
1
0.0 mmol) in 30 ml of glacial acetic acid. The solution was heated to
ꢁ
reflux for 6 h at 110 C. The reaction mixture was then cooled and
dripped slowly with stirring into an ice-cold solution of 50% ammonium
hydroxide (160 ml). The resulting yellow precipitate was collected by
filtration. The solid was column chromatographed on a silica gel col-
umn with dichloromethane and dried to obtain the product. Yield: 68%.

Synthesis and the Luminescent Study of the Iridium Complexes
17/[347]
2.2. Synthesis of Iridium(III) Complexes
Synthesis of Ir(6-OMe-2,3-dpqx-(OMe) ) (acac)
2
2
First, the cyclometallated Ir(III) l-chloro-bridged dimer, (6-OMe-2,
-dpqx-(OMe) ) Ir(l-Cl) Ir(6-OMe-2,3-dpqx-(OMe) ) (3.88 g, 2 mmol),
3
2
2
2
2 2
was prepared according to the Nonoyama method [18]. Second, the
resulting dimer and 2,4-pentandione (0.698 ml, 6.8 mmol) were mixed
with Na CO (500 mg) in 2-ethoxyethanol (30 mL). The mixture was
2
3
refluxed for 2 h and the resulting red solid was filtered after cooling.
Ir(6-OMe-2,3-dpqx-(OMe) ) (acac) was purified by chromatography
2
2
on silica gel column with dichloromethane and recrystallization from
dichlorometane=hexane. Yield: 63%. FAB-MS: calculated: 1034.3;
found: 1034.
Synthesis of Ir(6-OMe-2,3-dpqx-(OMe) ) (przl)
2
2
Ir(6-OMe-2,3-dpqx-(OMe) ) (przl) were prepared according to the
2
2
method as described above. Yield: 60%.
Ir(6-OMe-2,3-dpqx-(OMe) ) (przl1) : Yield 59% FAB-MS: calculated:
2
2
1
1
150.3; found: 1150.
Ir(6-OMe-2,3-dpqx-(OMe) ) (przl2) : Yield 61% FAB-MS: calculated:
212.3; found: 1212.
2
2
2
.3. Optical Measurements
UV-Vis absorption spectra were measured on a Hewlett Packard
425A spectrometer. PL spectra were measured on a Perkin Elmer
8
LS 50B spectrometer. UV-Vis and PL spectra of iridium complexes
ꢂ5
were measured in 10 M dilute CH Cl solution and in the PMMA
2
2
film. The film was prepared by spin-coating of PMMA solution
containing 10 wt% Ir complexes in 1,2-dichloroetane onto the glass
substrate and the following solvent evaporation.
3. RESULTS AND DISCUSSION
The main ligand, 6-OMe-2,3-dpqx-(OMe) , was synthesized according to
2
Friedlander Reaction, as illustrated in Figure 1(a). A series of przl, 4-R-5-
methyl-2-phenyl-2,4-dihydro-pyrazol-3-one (R¼ methyl (przl1), phenyl
(
przl2)) were prepared as shown in Figure 1(b). Two kinds of R groups
were chosen to study the electronic and steric effects of the R at przl in
the iridium complexes on the luminescence properties. The synthetic
method of the iridium complexes, Ir(6-OMe-2,3-dpqx-(OMe) ) (acac or
2
2
przl), involved two steps, formation of the dimer and the following coor-
dination of przl-based ligands, as summarized in Figure 1(c).

1
8/[348]
H.-S. Lee et al.
FIGURE 1 Synthetic scheme of the ligands and iridium complexes.
The UV-Vis absorption spectra of the complexes in CH Cl2 are
2
shown in Figure 2. The strong absorption bands between 200 and
4
00 nm in the ultraviolet region are assigned to the spin-allowed

Synthesis and the Luminescent Study of the Iridium Complexes
19/[349]
ꢂ5
FIGURE 2 UV-Vis absorption spectra of the iridium complexes in a 10
M
CH
2 2
Cl solution.
1
ꢃ
p-p transition of the cyclometalating 6-OMe-2,3-dpqx-(OMe) ligand
2
in the complexes. The weak bands between 400 and 450 nm in the
visible region can be assigned to the spin-allowed metal-to-ligand
1
charge transfer band ( MLCT), and the weaker absorption bands at
3
the longer wavelengths can be attributed to the spin-forbidden MLCT
3
ꢃ
and spin-orbit coupling enhanced p-p transition. The formally spin-
3
forbidden MLCT gains the intensity by mixing with the higher-lying
MLCT transition through the strong spin-orbit coupling by the iridium
1
center. The absorption patterns of these complexes are similar with few
differences in the peak wavelengths, suggesting that change of the ancil-
lary ligand does not make significant contribution to the absorption pro-
cess. The absorption spectra of those in PMMA film have similar patterns
and the absorption peaks appeared at 425, 430, and 480 nm (Fig. 3).
ꢂ5
The photoluminescence (PL) spectra of the Ir complexes in 10
M
CH Cl solution are shown in Figure 4. The emission maxima for
2
2
Ir(6-OMe-2,3-dpqx-(OMe) ) (acac), Ir(6-OMe-2,3-dpqx-(OMe) ) (przl1)
2
2
2 2
and Ir(6-OMe-2,3-dpqx-(OMe) ) (przl2) appeared at 667, 661, and
2
2
6
58 nm, respectively. There is no substantial difference in PL emission
by changing the ancillary ligands in the complexes. The PL spectra are
only slightly shifted within 10 nm upon the ancillary ligand variation
in the complexes.
Such PL peaks of Ir(6-OMe-2,3-dpqx-(OMe) ) (acac or przl) around
2
2
6
60 nm should be noted. According to our previous result, the emission

2
0/[350]
H.-S. Lee et al.
FIGURE 3 UV-Vis absorption spectra of PMMA film containing the Ir
complexes.
maxima of Ir(6-F-2,3-dpqx-(OMe) ) (acac) occurs at 645 nm.
2
2
Substitution of an electron withdrawing F for an electron donating
OMe group at 6-position in the LUMO site of the complex is supposed
ꢂ5
FIGURE 4 PL spectra of the iridium complexes in a 10 M CH
2
Cl
2
solution.

Synthesis and the Luminescent Study of the Iridium Complexes
21/[351]
to cause band gap increase by raising the LUMO levels, resulting in
hypsochromic shift. However, the results indicate otherwise. The PL
of the complexes containing 6-OMe substituted ligands exhibit slightly
red-shifted peaks. On the other hand, comparison of EL spectra of
these complexes reveals that Ir(6-OMe-dpqx-(OMe) ) (acac or przl)
2
2
has the maxima around 650 nm, blue-shifted from the EL peaks of
Ir(6-F-dpqx-(OMe) ) (acac) at 660 nm as expected, vide infra. There-
2
2
fore, we conclude that the substituents at 6-position of the dpqx ligand
in the complex do not influence the emission wavelengths signifi-
cantly, contrary to our prediction.
Their electroluminescence (EL) properties are also investigated
(
Fig. 6). The configuration of the EL device is ITO= NPB= CBP:Ir
complex 8%=BPhen=Liq=Al. The maximum EL wavelengths of
Ir(6-OMe-2,3-dpqx-(OMe) ) (acac) and Ir(6-OMe-2,3-dpqx-(OMe)2)2
2
2
(
przl2) are 655 and 646 nm, blue-shifted from their PLs and that of
r(6-F-2,3-dpqx-(OMe) ) (acac) as mentioned above. Nonetheless, the
2
2
EL patterns of the device for Ir(6-OMe-2,3-dpqx-(OMe) ) (acac) are
2
2
consistent with the corresponding PL patterns in solution, suggesting
that the same exciting states are generated during photo- and electrical
excitation. Detailed EL investigation was not possible due to the low
volatility of the iridium complexes containing 6-OMe-2,3-dpqx-(OMe)
and przl ligands.
FIGURE 5 PL spectra of PMMA film containing the Ir complexes.

2
2/[352]
H.-S. Lee et al.
FIGURE 6 EL spectra of the Ir complexes.
Instead, we attempted to investigate their PLs in PMMA(poly
(
(
methylmethacrylate)) film for the polymer light-emitting device
PLED) application. PMMA is chosen as a host because its non-
emitting property within the visible range could provide the PL of
the iridium complex only. The PMMA film is fabricated by spin-
coating of 10 wt% Ir complexes in PMMA onto the glass substrate.
As depicted in Figure 5, The PL maxima of the PMMA film for
Ir(6-OMe-2,3-dpqx-(OMe) ) (acac), Ir(6-OMe-2,3-dpqx-(OMe) ) (przl1)
2
2
2 2
and Ir(6-OMe-2,3-dpqx-(OMe) ) (przl2) appear at 657=719(sh) nm,
2
2
6
65=713(sh) nm and 657=704(sh) nm, respectively. PL spectra in
PMMA film of the complexes show broader emission than those of
the complexs in solution with the shoulder peaks in longer wave-
lengths as listed above. We attribute such emission at the longer wave-
lengths to formation of excimers. The interaction between the dopants
may cause excimer formation through some extension of dopant
dimeric units with strong p-p interaction. The emission from the
excimers could be shown in the longer wavelengths than that of the
complex itself, yielding the red-shifted and broadened the PL emission
[
19–20]. Further investigation of the complexes in other polymeric
films such as PVK is in progress for the PLED application.
4. CONCLUSION
We designed and prepared the iridium complexes involving the ligand
of dpqx derivatives and an acac or a przl as an ancillary ligand.

Synthesis and the Luminescent Study of the Iridium Complexes
23/[353]
The UV-Vis spectra of the complexes containing acac and przl-based
ligands were similar, indicating that variation of ancillary ligands
did not make significant contribution to absorption process. The PL
emission in solution of the iridium complex containing acac as an
ancillary ligand were slightly shifted within 10 nm, from those of
pyrazolone-based iridium complexes. Such shift, however, was not sig-
nificant, and thus the effect by the ancillary ligands was found to be
negligible within the experimental errors. The PL spectra of the com-
plexes dispersed in the PMMA film were red-shifted and broadened
with the shoulder peaks, compared with their corresponding solution
spectra, suggesting formation of dopant excimers in the film. The EL
devices of Ir(6-OMe-2,3-dpqx-(OMe) ) (acac) and Ir(6-OMe-2,3-dpqx-
2
2
(
OMe) ) (przl2) were fabricated and their EL spectra showed the
2 2
peaks around 650 nm. Overall, Ir(6-OMe-dpqx-(OMe) ) (acac or przl)
2
2
studied herein and previously reported Ir(6-F-dpqx-(OMe) ) (acac)
2
2
did not show substantial difference in both PL and EL spectra. Thus,
it was concluded that substituent variation at 6-position of the dpqx
ligand and change of ancillary ligands in the iridium complexes did
not significantly influence the emission wavelengths of the complexes.
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