Synthesis and characterization of red-emitting iridium complexes of 5′-substituted 2,4-diphenylquinolines

Article abstract of DOI:10.1080/15421400903065770

To examine the substituent effect, 5′-substituted 2,4-diphenylquinoline (dpq) ligands and their iridium complexes, Ir(dpq) ₂(acac), Ir(dpq-5F)₂(acac), Ir(dpq-5CH₃) ₂(acac) and Ir(dpq-5OCH₃)₂

Full text of DOI:10.1080/15421400903065770

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Synthesis and Characterization
of Red-emitting Iridium
Complexes of 5′-Substituted
2,4-diphenylquinolines
Seung-Chan Lee a & Young Sik Kim a b
a Department of Information Display, Hongik
University, Seoul, Korea
b Department of Science, Hongik University, Seoul,
Korea
Available online: 05 Oct 2009
To cite this article: Seung-Chan Lee & Young Sik Kim (2009): Synthesis and
Characterization of Red-emitting Iridium Complexes of 5′-Substituted 2,4-
diphenylquinolines, Molecular Crystals and Liquid Crystals, 509:1, 292/[1034]-299/
[1041]
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Mol. Cryst. Liq. Cryst., Vol. 509, pp. 292=[1034]–299=[1041], 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400903065770
Synthesis and Characterization of Red-emitting Iridium
Complexes of 50-Substituted 2,4-diphenylquinolines
Seung-Chan Lee1 and Young Sik Kim1,2
1Department of Information Display, Hongik University, Seoul, Korea
2Department of Science, Hongik University, Seoul, Korea
To examine the substituent effect, 50-substituted 2,4-diphenylquinoline (dpq)
ligands and their iridium complexes, Ir(dpq)2(acac), Ir(dpq-5F)2(acac), Ir(dpq-
5CH3)2(acac) and Ir(dpq-5OCH3)2(acac), were synthesized and prepared, and the
photophysical properties were investigated for red electro-phosphorescent materi-
als. The 50-substituted phenyl ring generated 30-positon and 50-position isomers.
However, the 50-position of the phenyl ring is more stable than the 30-position
due to steric effects. The maximum emission wavelengths of Ir(dpq)2(acac),
Ir(dpq-5 F)2(acac), Ir(dpq-5CH3)2(acac) and Ir(dpq-5OCH3)2(acac) were found at
612, 606, 616, 606 nm, respectively. The emission spectrum of Ir(dpq-5CH3)2(acac)
was red shifted compared to that of Ir(dpq)2(acac) because the methyl group sub-
stituted on the 50-position of the phenyl ring is an electron donating group.
Ir(dpq-5 F)2(acac) shows highly efficient and blue-shifted luminescence because
the electron withdrawing group (-F) substitute on the 50-position of the phenyl
ring lowered the HOMO level slightly. Hence MLCT coupling is increased and
blue shifted luminescence by 6 nm was observed compared to Ir(dpq)2(acac).
Especially, Ir(dpq-5OCH3)2(acac) shows blue-shifted phosphorescence due to the
effect of the electron withdrawing methoxyphenyl group onto the iridium atom.
This destabilized the MLCT state and leads to an increased in the emission
energy.
Keywords: iridium complex; luminescence; OLED; phosphorescence; red
This work was supported by Korea Research Foundation Grant funded by Korean
Government (MOEHRD) (KRF-2007-313-D00290).
Address correspondence to Young Sik Kim, Department of Science and Department
of Information Display, Hongik University, Seoul 121-791, Korea. E-mail: youngkim@
hongik.ac.kr
292=[1034]

Characterization of Red-emitting Iridium Complexes
293/[1035]
1. INTRODUCTION
Electroluminescent (EL) organic materials have received much of
attention due to their potential application in flat-panel displays and
to the rapid progress in material design and device fabrication in recent
years [1–5]. Thompson et al. have developed electrophosphorescent
OLEDs with a high efficiency approaching 100% of the internal quan-
tum efficiency, which utilize both singlet and triplet excitons produced
at the emitting layer doped with the phosphorescent dopants [6,7].
Heavy metals such as iridium or platinum in their complex forms are
known to induce intersystem crossing by strong spin-orbit coupling,
leading to mixing of the singlet and triplet excited states. Thus, iridium
complexes are known to have high photoluminescence (PL) efficiency
and a relatively short excited state lifetime, which minimizes quench-
ing of triplet emissive states [8,9]. Recently, Ir(dpq)2(acac), a red phos-
phorescent emitter, has been reported to have an emission peak at
606 nm, and the EL properties were shown to include a maximum
brightness of 17457 cd=m2 and a maximum quantum efficiency as high
as 6.1% with CIE coordinates of x ¼ 0.61 and y ¼ 0.30 [10].
In this article, we have designed and synthesized iridium complexes,
Ir(dpq)2(acac), Ir(dpq-5 F)2(acac), Ir(dpq-5CH3)2(acac) and Ir(dpq-
5OCH3)2(acac)andinvestigatedthephotophysicalpropertiestoexamine
the 50-substituent effect of 2,4-diphenylquinoline (dpq) ligands.
2. EXPERIMENTAL DETAILS
2.1. Synthesis
All reagents were purchased from Aldrich Co., except Ir(III) trichlor-
ide hydrate (IrCl3 ꢀ H2O), which was purchased from Strem Co. and
used without further purification. All reactions were carried out under
a nitrogen or argon atmosphere. Solvents were dried by standard pro-
cedures. All column chromatography was performed with the use of
silica gel (230-mesh, Merck Co).
2.1.1. Synthesis of Ligands (dpq, dpq-5F, dpq-5OCH3 and
dpq-5CH3)
These ligands were obtained from the Friedlander Reaction as
illustrated in Figure 1 [11,12]. 2-aminobenzophenone, 5-methyl-2-
aminobenzophenone or 5-metoxy-2-aminobenzophenone (10.0 mmol)
was mixed with acetophenone (10.0 mmol) in 30 ml of glacial acetic
acid according to the procedure described above. (Yield: dpq-5CH3
2.42 g, 74%, dpq-5OCH3 2.37 g, 69%).

294/[1036]
S.-C. Lee and Y. S. Kim
FIGURE 1 Synthesis of C^N ligands and their iridium complexes
[Ir(C^N)2(acac)].
2.1.2. Synthesis of Complexes: Ir(dpq-X)2(acac).
(X ¼ H, F, –CH3 and –OCH3)
The cyclometalated Ir(III) m-chloro-bridged dimer, (dpq-X)2Ir
(m-Cl)2Ir(dpq-X)2 was synthesized by the method reported by Non-
oyama with slight modification as illustrated in Figure 1 [13].
IrCl3 ꢀ H2O (0.418 g, 1.4 mmol) and H2O (10 ml) were added to a solu-
tion of dpq-X (3.5 mmol) in 2-ethoxyethanol (30 ml). The mixture was
refluxed at 120ꢁC under argon for 30 hr and then cooled to room tem-
perature. The solution mixture was slowly evaporated under vacuum
to obtain the crude dimer, (dpq-X)2Ir(m-Cl)2Ir(dpq-X)2. The resultant
precipitate was dissolved in dichloromethane and was chromato-
graphed on silica gel column with dichloromethane. The dimer product
was collected and dried in vacuum. (dpq-X)2Ir(m-Cl)2Ir(dpq-X)2
(0.50 mmol) and 2,4-pentanedione (0.17 ml, d ¼ 0.975, 1.73 mmol) were
mixed with Na2CO3 (250 mg) in 2-ethoxyethanol (30 mL). The mixture
was refluxed for 2 hr. The solution was cooled to room temperature
and the red solid was filtered. Ir(dpq-X)2(acac) was obtained after
chromatography on silica gel column with dichloromethane and
solvent evaporation.
2.2. Optical Measurements
UV-Vis absorption spectra were measured on a Hewlett Packard
8425A spectrometer. PL spectra were measured on a Perkin Elmer
LS 50B spectrometer. UV-Vis and PL spectra of iridium complexes
were measured in 10ꢂ5 M dilute CH2Cl2 solution.

Characterization of Red-emitting Iridium Complexes
295/[1037]
2.3. Theoretical Calculations
Calculations on the electronic ground states of Ir(dpq)2(acac), Ir(dpq-
5CH3)2(acac), Ir(dpq-5OCH3)2(acac), Ir(dpq-5 F)2(acac) were carried
out using Gaussian 98. The basis set of ligands was changed to
6–31 þ G(d). LANL2DZ and 6–31G(d) basis sets were employed for Ir
and the other atoms, respectively. To obtain the vertical excitation
energies of the low-lying singlet and triplet excited states of the
complexes, time-dependent density functional theory calculations
using the B3LYP functional were performed at the respective
ground-state geometry.
3. RESULTS AND DISCUSSION
Stable structures were calculated in Gaussian 98 and 03, as shown in
Figure 2. And the relative energies of the iridium complex are shown
in Table 1. The 50-substituted phenyl ring generated 30-positon and
50-position isomers. However, the Fꢂ, CH3–, and OCH3– substituted
ligands give the 50-position isomers lower energy in them. As
FIGURE 2 Calculated most stable geometry structure of Ir(dpq)2(acac),
Ir(dpq-5 F)2(acac), Ir(dpq-5CH3)2(acac), and Ir(dpq-5OCH3)2(acac).

296/[1038]
S.-C. Lee and Y. S. Kim
TABLE 1 Calculated Relative Energies, HOMO and LUMO Energies, Band
Gaps, Measured T1 Energies and Relative Quantum Yields of Ir(dpq)2(acac)
and Ir(dpq-5 F)2(acac), Ir(dpq-5CH3)2(acac), and Ir(dpq-5OCH3)2(acac)
Erel
(KJ=mol)a
HOMO LUMO Band gap PL (exp)
Upl
Complex
(eV)b
(eV)b
(eV)b
(eV)c
(rel)d
Ir(dpq)2(acac)
ꢂ4.754 ꢂ1.783
ꢂ4.957 ꢂ1.883
ꢂ4.975 ꢂ1.856
ꢂ4.716 ꢂ1.823
ꢂ4.642 ꢂ1.750
ꢂ4.442 ꢂ1.440
ꢂ4.445 ꢂ1.464
2.971
3.047
3.118
2.893
2.892
2.982
2.989
2.029
–
2.049
–
2.017
–
2.049
0.14
–
0.36
–
0.13
–
0.07
Ir(dpq-3 F)2(acac)
Ir(dqp-5 F)2(acac)
Ir(dpq-3CH3)2(acac)
Ir(dpq-5CH3)2(acac)
Ir(dpq-3OCH3)2(acac)
Ir(dpq-5OCH3)2(acac)
5.25
0.0
26.25
0.0
10.50
0.0
aRelative energies are the difference between the total energies of the 3-position and
5-position isomers, with the lower energy set to zero.
bCalculated by Gausian 98=6–31 þ G(d). LANL2DZ and 6–31G(d) basis sets.
cMeasured in 10ꢂ5M dilute CH2Cl2 solution.
dRelative photoluminescence quantum yield are compare Ir(ppy)3 (U ¼ 0.40) with
another Ir complexes.
presented in Table 1, the 30-position isomer give 5.25, 26.25, and
10.5 KJ=mol higher energy due to steric effects. To protect the Ir com-
plexes from isomerization and impurities, we use a method of recrys-
tallization and repeated column chromatography. GC=MS data of
the Ir(dpq)2acac and Ir(dpq-5OCH3)2(acac) are shown in Figure 3. In
Figure 3, the highest mass spectrums are 753.32 and 813.40 molecular
weight respectively. (Calculated molecular weight is 852.02 and
912.06). Due to the GC=MS measurement method, acac structure is
easily taken off from the iridium complex. (Calculated acac molecular
weight is 99.11). All products compare the calculated function of
the UV-Vis absorption peak to the measured UV-Vis absorption
spectra. The UV-Vis absorption spectra of the Ir(dpq)2(acac), Ir(dpq-
5CH3)2(acac), Ir(dpq-5OCH3)2(acac), Ir(dpq-5 F)2(acac) solution are
shown in Figure 4. The MLCT absorption peaks of Ir(dpq)2(acac)
and Ir(dpq-5CH3)2(acac) are observed at 470, 512, and 550 nm. Overall
profiles of the absorption spectrum and the peak position of the MLCT
absorption of Ir(dpq-5CH3)2(acac) are very similar to those of Ir(dp-
q)2(acac). However, the MLCT absorption peaks for Ir(dpq-5 F)2(acac)
appeared at 465, 489, and 548 nm, and for Ir(dpq-5OCH3)2(acac) were
shown at 460, 502, and 550 nm. In addition, the intensity of absorption
spectrum of Ir(ppy)2(dpq-5 F) is greater than that of Ir(dpq)2(acac).
The strong MLCT transition occurs because of the strong coupling
between the 5d-orbital of the iridium atom and the HOMO of the
dpq-5 F ligand.

Characterization of Red-emitting Iridium Complexes
297/[1039]
FIGURE 3 GC=MS data of the Ir(dpq)2acac and Ir(dpq-5OCH3)2(acac).
The emission spectra of the complexes, calculated relative energies,
HOMO and LUMO energies, band gaps, measured T1 energies and
relative quantum yields are shown in Figure 5 and Table 1. The max-
imum emission wavelength of Ir(dpq-5 F)2(acac), Ir(dpq-5CH3)2(acac),
and Ir(dpq-5OCH3)2(acac) in CH2Cl2 was recorded at 606, 616, and
606 nm, and measured T1 energies of Ir(dpq-5 F)2(acac), Ir(dpq-
5CH3)2(acac), and Ir(dpq-5OCH3)2(acac) were 2.029, 2.049, 2.017, and
2.049 electron volt, respectively. For comparison, the PL spectrum of
Ir(dpq)2(acac) shows an emission peak at 612 nm in CH2Cl2. It was
found that the PL spectrum of Ir(dpq-5CH3)2(acac) was red shifted
compared to that of Ir(dpq)2(acac) because the methyl group substi-
tuted on the 50-position of the phenyl ring is an electron donating
group. So the methyl group strongly affected the HOMO more
than the LUMO. The emission spectrum of Ir(dpq-5 F)2(acac) shows
efficient blue-shifted luminescence spectra because the electron
withdrawing group (-F) on the 50-position of the phenyl ring lowered

298/[1040]
S.-C. Lee and Y. S. Kim
FIGURE 4 UV-Vis absorption spectra of Ir(dpq)2(acac), Ir(dpq-5 F)2(acac),
Ir(dpq-5CH3)2(acac), and Ir(dpq-5OCH3)2(acac).
the HOMO level slightly, increasing the MLCT coupling and blue-
shifted luminescence by 6 nm compared to that of Ir(dpq)2(acac).
And, Although the methyl and methoxy group on the 50-position of
the phenyl ring of Ir(dpq-5CH3)2(acac) and Ir(dpq-5OCH3)2(acac)
shows blue-shifted phosphorescence wavelength. It is because that
FIGURE 5 PL spectra of Ir(dpq)2(acac), Ir(dpq-5 F)2(acac), Ir(dpq-5CH3)2(acac),
and Ir(dpq-5OCH3)2(acac).

Characterization of Red-emitting Iridium Complexes
299/[1041]
although the methoxy and methyl group increased the HOMO energy
level, it strongly increased the LUMO more than the HOMO. Espe-
cially, Ir(dpq-5OCH3)2(acac) shows blue-shifted phosphorescence
wavelength even though the methoxy group (–OCH3) substituted on
the 50-position of the phenyl ring is an electron donating group. It is
because the methoxy group on the 50-position raises both the LUMO
and HOMO level. Moreover, the methoxyphenyl group onto the iri-
dium atom destabilized the MLCT state and leads to an increase in
the emission energy. Thus, the emission wavelength is blue-shifted.
4. CONCLUSIONS
We have studied a 5’substitutent effect of Ir(dpq)2(acac) for the mate-
rials in red emissive OLEDs. We found that substitution on phenyl
ring has influence on the emission wavelength and efficiency. The
maximum emission wavelength of Ir(dpq-5 F)2(acac), Ir(dpq-5CH3)2
(acac), and Ir(dpq-5OCH3)2(acac) in CH2Cl2 was recorded at 606,
616, and 606 nm, respectively. While the substitution of EWG (ꢂ5 F)
caused blue-shift of emission wavelength from 612 nm to 606 nm and
improved the efficiency withdrawing group (-F) on the 50-position of
the phenyl ring lowered the HOMO level slightly increased PL
efficiency by decreasing the HOMO energy level.
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