Synthesis and luminescence properties of iridium complexes chelated with coumarin ligands

Article abstract of DOI:10.1166/jnn.2013.7257

According to a recent report, the organic light-emitting diodes (OLEDs) using the iridium complexes of coumarin derivatives as emissive dopants are highly efficient and stable. Unlike the other Ir(III) phopsphorescent dopants, these coumarin-based Ir(III)

Full text of DOI:10.1166/jnn.2013.7257

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Journal of
Nanoscience and Nanotechnology
Vol. 13, 3441–3445, 2013
Synthesis and Luminescence Properties of Iridium
Complexes Chelated with Coumarin Ligands
Hye Rim Park, Bo Young Kim, Young Kwan Kim, and Yunkyoung Ha∗
Department of Information Display, Hongik University, Seoul 121-791, Korea
According to a recent report, the organic light-emitting diodes (OLEDs) using the iridium complexes
of coumarin derivatives as emissive dopants are highly efficient and stable. Unlike the other Ir(III)
phopsphorescent dopants, these coumarin-based Ir(III) complexes can effectively trap and trans-
port electrons in the emissive layer.1 We have prepared a series of phosphorescent cyclometalated
Ir(III) complexes containing 3-(2-pyridinyl)coumarin (pc) as an ancillary ligand. The new heterolep-
tic iridium complexes, Ir(C∧N)2(pc) (C∧N = 2-(2,4-difluorophenyl)pyridine (F2-ppy), 2-phenylpyridine
(ppy) and 2-phenylquinoline (pq)) were characterized by1H NMR and mass spectrometer. As main
ligands, F2-ppy, ppy and pq were employed, which should have the drastically different ligand molec-
ular orbital energy levels. The iridium complexes showed various emission ranges from 560 to
610 nm, depending upon the relative energy levels of their main and ancillary ligands. The pho-
toabsorption, photoluminescence and electroluminescence of the complexes were studied. We also
investigated the electrochemical properties of the iridium complexes to compare the HOMO and
LUMO energy levels of these phosphorescent materials.
Keywords: Coumarin, Iridium Complexes, Organic Light-Emitting Diodes (OLED), Dopant
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Materials.
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Copyright: American Scientific Publishers
1. INTRODUCTION
injection and transport ability of phosphorescent materials.8
The studies of coumarin derivatives demonstrated that they
have excellent thermal stability and outstanding optical
properties including an extended spectral response, high
quantum yields, and superior photostability.9–10 We expect
that chelation of a coumarin ligand to the iridium com-
plexes might allow emitting color tuning and luminous effi-
ciency increase.
Herein, we described the results of our investigation on
the preparation, characterization, electrochemical behavior,
and luminescent properties of the iridium complexes con-
taining 3-(2-pyridinyl)coumarin (pc) as an ancillary ligand.
We analyzed the emission properties of the new iridium
complexes to find how the different main ligands have
influence on the device performance and emisson wave-
length of the coumarin chelated iridium complexes.
The organic electroluminescent materials have been in-
tensely applied in the past two decades. As the most potent
luminescent material of all, iridium(III) complexes have
attached much attention owing to their manifold advan-
tages. Their excellent phosphorescent quantum yields and
easy tuning of emission color by ligand molecular struc-
tures have led to great development of phosphorescent
organic light-emitting diodes (OLEDs). Furthermore, there
is a potential ability of these materials for the use of the
flat-panel displays and solid state lighting.2–5
Intensive and extensive studies with the synthetic Ir
complexes have been carried out for developing phospho-
rescent materials with improved their characteristics, phos-
phorescent quantum yields, and efficient phosphorescence
color tuning.6 The iridium complexes are still currently
under active investigation with the aim of achieving full-
color displays.7
2. EXPERIMENTAL DETAILS
To obtain the various color emitting materials of good
color purity and high efficiency, we prepared coumarin-
based heteroleptic iridium complexes in this study. Accord-
ing to the previous report, charge transporting groups have
been incorporated onto key ligands to improve charge
All of the chemical reagents were purchased from Aldrich
and Strem Co., and used without further purification. The
reactions were carried out under an argon atmosphere. The
solvents were dried by standard procedures. The column
chromatography was performed with the use of silica gel
(230-mesh, Merck).
∗Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2013, Vol. 13, No. 5
1533-4880/2013/13/3441/005
doi:10.1166/jnn.2013.7257
3441

Synthesis and Luminescence Properties of Iridium Complexes Chelated with Coumarin Ligands
Park et al.
2.1. Synthesis of Ligands and Iridium Complexes
Glass substrate coated with a 180-nm-thick indium tin
oxide (ITO) layer had a sheet resistance of 12 ꢃ/sq.
All organic layers were sequentially deposited onto the
substrate without breaking vacuum at a pressure of about
5 × 10−7 torr, using the thermal evaporation equipment.
The organic materials were deposited as the following
structure: 4,4ꢀ-bis[N-(naphthyl)-N-phenylamino]biphenyl
(NPB, 50 nm) as hole transporting layer (HTL)/4,4ꢀ,4ꢀꢀ-
tris(N-carbazolyl)triphenylamine (TCTA, 10 nm) as buffer
layer/iridium complex phosphors (8%) doped in
4,4ꢀ-bis(9-carbazolyl)-1,1ꢀ-biphenyl (CBP, 30 nm) as
emitting layer/2,2ꢀ,2ꢀꢀ-(1,3,5-benzinetriyl)-tris(1-phenyl-1-
H-benzimidazole) (TPBi, 40 nm) as electron transporting
layer (ETL)/lithium quinolate (Liq, 2 nm) as electron
injection layer (EIL). The deposition rates were 1.0–1.1
and 0.1 Å/sec for organic materials and lithium quino-
late (Liq), respectively. Finally, the aluminum cathode
was deposited at a rate of 10 Å/sec. The devices were
encapsulated in a glove box with O2 and H2O at concen-
trations below 1 ppm. The devices had emission areas of
3 × 3 mm2. The electrical characteristics of devices and
the EL spectra were measured and immediately recorded
with a Chromameter CS-1000A (Minolta). The current
and voltage were controlled with a measurement unit
(model 236, Keithely).
Synthesis of F2-ppy. 2,4-difluoro-2-phenylpyridine was
obtained from the reaction of 2-chloropyridine and
2,4-difluorophenylboronic acid by Suzuki coupling.11
Yield: 65%
Synthesis of pc. 3-(2-pyridinyl)coumarin was obtained
from the reaction of salicylaldehyde (0.1 mol), 2-pyridyl
acetonitrile (0.1 mol) and piperidine (0.1 mol).12
Yield: 71%
Synthesis of Ir(C∧N)2(pc) · [C∧N = ppy, F2-ppy, pq]
First, the cyclometallated Ir(III) ꢀ-chloro-bridged dimer,
(C∧N)2Ir(ꢀ–Cl)2Ir(C∧N)2 (0.5 mmol), was prepared
according to the Nonoyama method.13 Second, the result-
ing dimer and 3-(2-pyridinyl)coumarin (1.75 mmol) and
K2CO3 were refluxed in 2-ethoxyethanol overnight. After
cooling to room temperature, a precipitate was filtered
off, and washed with water and methanol, and dried
with MgSO4. The crude product was redissolved in
dichloromethane and purified by silica gel chromatography
by using CH2Cl2. Ir(C∧N)2(pc) was obtained as a solid
after recrystallization in CH2Cl2/methanol.
[Ir(F2-ppy)2(pc)]: A yellow powder (Yield: 69%). FAB-
MS: calculated 794; found 795. 1H NMR (400 MHz,
DMSO-d6ꢁ: ꢂ9.08 (d, 1H) 8.26 (d, 1H) 8.19 (d, 1H) 8.13
(s, 1H) 7.94 (m, 2H) 7.86 (t, 1H) 7.72 (s, 1H) 7.44 (d, 1H)
7.35 (t, 1H) 7.24 (t, 3H) 7.13 (d, 2H) 6.80 (m, 3H) 5.69
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(d, 1H) 5.55 (d, 1H).
3. RESULTS AND DISCUSSION
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[Ir(ppy)2(pc)]: A light orange powder (Yield: 67%).
FAB-MS: calculated 722; found 723. 1H NMR (400 MHz,
DMSO-d6): ꢂ 9.06 (d, 1H) 8.16 (d, 1H) 8.10 (d, 1H)
8.04 (d, 1H) 7.85 (m, 4H) 7.74 (t, 2H) 7.44 (d, 1H) 7.35
(s, 1H) 7.30 (t, 1H) 7.16 (m, 2H) 7.03 (d, 2H) 6.96 (t, 1H)
6.88 (t, 2H) 6.80 (t, 1H) 6.59 (t, 1H) 6.29 (d, 1H) 6.08
(d, 1H).
Copyright: American Scientific Publishers
The synthesis of the heteroleptic iridium complexes,
Ir(C∧N)2(pc), (C∧N = F2-ppy, ppy, qc) were straight-
forward and Ir(F2-ppy)2(pc), Ir(ppy)2(pc) and Ir(pq)2(qc)
were prepared according to the reported procedures.11–13
The yields of these complexes were in the range of
60–70%. The complexes were characterized with 1H NMR
and FAB-MS. The overall synthetic schemes are illustrated
in Figure 1.
[Ir(pq)2(pc)]: A red powder (Yield: 59%). 1H NMR
(400 MHz, DMSO-d6ꢁ: ꢂ 9.15 (d, 1H) 8.71 (t, 1H) 8. (d,
4H) 8.31 (t, 3H) 8.13 (d, 2H) 7.96 (t, 3H) 7.85 (t, 2H)
7.80 (s, 2H) 7.65 (d, 2H) 7.52 (d, 2H) 7.41 (m, 4H) 7.19
(t, 2H).
The UV-Vis absorption spectra of the iridium complexes
were obtained and compared, as shown in Figure 2. The
spectra of Ir(F2-ppy)2(pc), Ir(ppy)2(pc) and Ir(pq)2(pc) have
the strong absorption bands, appearing in the ultraviolet
2.2. Optical and Electro Chemical Measurement
Cl
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
10−5 M dilute CH2Cl2 solution. Cyclic voltammograms
were obtained with Electrochemical Analyser (CHI600C)
at scan rate of 100 mV/s, and tetrabutylammonium hex-
afluorophosphate was added as an electrolyte in CH2Cl2
solution.
C^N
2-ethoxyethanol:H2O
LX
CH2Cl2/EtOH
Ir(C^N)2
(IrC^N)2(LX)
IrCl3nH2O
(IrC^N)2
Cl
LX =
C^N=
N
N
N
O
O
F
N
,
,
2.3. Device Fabrication
F
ppy
pq
F2-ppy
pc
The OLEDs containing the heteroleptic iridium com-
plexes as a dopant in emitting layers were fabricated.
Fig. 1. The synthesis of Ir(C∧N)2(pc), and their iridium complexes.
J. Nanosci. Nanotechnol. 13, 3441–3445, 2013
3442

Park et al.
Synthesis and Luminescence Properties of Iridium Complexes Chelated with Coumarin Ligands
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Ir(F2-ppy)2(pc)
Ir(ppy)2(pc)
Ir(pq)2(pc)
1.0
0.8
0.6
0.4
0.2
0.0
Ir(F2-ppy)2(pc)
Ir(ppy)2(pc)
250
300
350
400
450
Wavelength (nm)
400
450
500
550
600
650
700
750
Fig. 2. The UV-Vis absorption spectra of the heteroleptic iridium com-
Wavelength (nm)
plexes of Ir(C∧N)2(pc) in a 10−5 M CH2Cl2 solution.
Fig. 4. The EL spectra of the heteroleptic iridium complexes of
Ir(C∧N)2(pc).
region between 230 and 310 nm. These bands are assigned
1
to the spin-allowed ꢄ–ꢄ∗ transitions of the ligands. The
attributed to effective mixing of these charge-transfer
transitions with high lying spin-allowed transitions on
the cyclometalating ligand.14ꢈ15 The absorption patterns
of Ir(F2-ppy)2(pc) and Ir(ppy)2(pc) are similar, but the
absorption maxima of Ir(F2-ppy)2(pc) occur at the shorter
wavelength, presumably due to the bigger energy gap of
F2-ppy ligand in the complex.
1ꢄ–ꢄ∗ bands of the complexes are accompanied by weaker
and lower energy features extending into the region of
longer wavelengths. These extended absorption bands of
Ir(F2-ppy)2(pc) are observed in the region from 300 to
350 nm and those of Ir(ppy)2(pc) and Ir(pq)2(pc) are
shifted to the longer wavelength from 310 to 385 nm.
These absorption bands are assigned to the spin-allo
The PL spectra of the iridium complexes in CH2Cl2
solution are shown in Figure 3. The emission wavelength
1
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metal charge transfer ( MLCT) bands. The absorption
IP: 79.168.200.28 On: Tue, 22 Mar 2016 07:57:01
of Ir(F2-ppy)2(pc) is also observed around 350–415 nm
of Ir(F -ppy) (pc), Ir(ppy) (pc) and Ir(pq) (pc) appear at
Copyright: American Scientifi2c Pub2lishers
2
2
and those of Ir(ppy)2(pc) and Ir(pq)2(pc) are observed at
the longer wavelength around 380–420 nm. The weaker
absorption bands can be attributed to the spin-forbidden
560, 610 and 590 nm, respectively. The PL maxima of
Ir(F2-ppy)2(pc) are observed at the shortest wavelength,
as expected from the biggest energy gap of F2-ppy lig-
and in the complexes. According to these trends, we
expected that the emission of Ir(pq)2(pc) should be shown
at the longest wavelength since the extended ꢄ conjuga-
tion length of pq main ligand could lead to the smallest
energy gap of the iridium complex, among the main lig-
ands we employed in this study. However, Ir(pq)2(pc) in
3
3MLCT and spin-orbit coupling enhanced ꢄ–ꢄ∗ transi-
tion. The high intensity of the MLCT bands has been
Ir(F2-ppy)2(pc)
Ir(ppy)2(pc)
Ir(pq)2(pc)
1.0
0.8
0.6
0.4
0.2
0.0
32
30
Ir(F2-ppy)2(pc)
Ir(ppy)2(pc)
28
26
24
22
20
18
16
14
12
10
8
400
500
600
700
Wavelength (nm)
Fig. 3. The PL spectra of the heteroleptic iridium complexes of
Ir(C∧N)2(pc) in a 10−5 M CH2Cl2 solution.
6
4
2
Table I. Electrochemical data for the iridium complexes.
0
1E-4
Ir complex
ꢅem/nma Eox/Vb HOMO/eVc LUMO/eVd ꢆE/eVd
1E-3
0.01
0.1
1
10
100
Current density (mA/cm2)
Ir(F2-ppy)2(pc)
Ir(ppy)2(pc)
Ir(pq)2(pc)
560
610
590
0.74
0.42
0.59
−5ꢇ54
−5.22
−5.39
−2ꢇ36
−2ꢇ43
−2ꢇ57
3.18
2.79
2.82
Fig. 5. The luminous efficiency versus current density of devices with
iridium complexes.
J. Nanosci. Nanotechnol. 13, 3441–3445, 2013
3443

Synthesis and Luminescence Properties of Iridium Complexes Chelated with Coumarin Ligands
Park et al.
Table II. Electroluminescence data for the iridium complexes.
Current density
(mA/cm2)
Luminous efficiency
(cd/A)
Power efficiency
(lm/W)
Dopant
Voltage (V)
Quantum efficiency (%)
CIE (xꢈy)
Ir(F2-ppy)2(pc)
Ir(ppy)2(pc)
5.50
5.50
0.43
0.32
30.19
21.32
17.25
12.18
8.83
9.60
(0ꢇ36ꢈ0ꢇ59)
(0ꢇ51ꢈ0ꢇ48)
dichloromethane solutions exhibits orange photolumines-
cence at 590 nm, the shorter wavelengths compared to the
emission shown by Ir(ppy)2(pc) at 610 nm. It is noted that
the emission peak of Ir(ppy)2(pc) at 610 nm shows a sig-
nificant red shift compared to that of previously reported
Ir(ppy)2(acac) at 525 nm. We attribute such red shift exhib-
ited by Ir(ppy)2(pc) to the energy gap decrease of the
complex due to involvement of 3-(2-pyridinyl)coumarin
ancillary ligand with the main ppy ligand by inter-ligand
energy transfer (ILET) in Ir(ppy)2(pc).
respectively. We expected that the complexes synthesized
in this study might exhibit the improved device perfor-
mance owing to outstanding quantum yield and good opti-
cal properties of pc ancillary ligand in the complexes. The
luminous efficiencies of the iridium complexes were in fact
high, reflecting the pc ligand effect.
4. CONCLUSION
We reported the synthesis of the new heteroleptic
iridium(III) complexes coordinated with 3-(2-pyridinyl)
coumarin, pc, as an ancillary ligand. The iridium com-
plexes, Ir(F2-ppy)2(pc), Ir(ppy)2(pc) and Ir(pq)2(pc), pre-
pared in this study exhibited from the green emission to
orange at 560, 610 and 590 nm, respectively. F2-ppy was
chosen as one of the C∧N ligands on the basis of its high
triplet energy and large energy gap, while pq ligand had
a low energy gap between HOMO and LUMO compared
to ppy. We unexpectedly found that Ir(ppy)2(pc) which
was supposed to have the medium energy gap exhibited
We investigated electrochemical properties of the Ir
complexes with the cyclic voltammetry (CV) as shown
in Table I, which could reveal their positions of the
HOMOs and LUMOs.16ꢈ17 The oxidation potential which
indicates the HOMOs of Ir(F2-ppy)2(pc), Ir(ppy)2(pc) and
Ir(pq)2(pc) were reversible at 0.42 and 0.74 V relative
to an internal ferrocenium/ferrocene reference (Fc+/Fc),
respectively. The LUMOs of the complexes were estimated
from their respective absorption spectra and electrochem-
ical data, using the optical edge and band gap equation
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(ꢆE = Eox − Ered). Their estimated band gaps between
the most bathchromic shift among the iridium complexes
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highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) were 3.18, 2.79 and
2.82 eV for Ir(F2-ppy)2(pc), Ir(ppy)2(pc) and Ir(pq)2(pc),
respectively. The electrochemical data of Ir(ppy)2(pc) is
consistent with its PL maxima, reflecting the unexpected
small band gap shown by Ir(ppy)2(pc). The band gap of
Ir(ppy)2(pc) was comparable to that of Ir(pq)2(pc), which
supports the red PL shift of Ir(ppy)2(pc). The detailed CV
data of the heteroleptic iridium complexes were summa-
rized in Table I.
Copyright: American Scientific Publishers
synthesized in this study. The ELs of Ir(F2-ppy)2(pc) and
Ir(ppy)2(pc) revealed rather high luminous efficiencies of
30.19 and 21.81 cd/A, which was attributed to high quan-
tum yield and good optical properties of the coumarin-
based ancillary ligand in the complexes. Our study of the
Ir(III) complexes containing a coumarin-based ancillary
ligand could be useful for the future design of Ir complexes
as further modified functional materials in OLEDs.
Acknowledgment: This research was supported by the
The electroluminescence (EL) properties of the com-
plexes were also investigated for their OLED appli-
cation. The configuration of the EL devices with
Ir(F2-ppy)2(pc) and Ir(ppy)2(pc) used as a dopant was
ITO/NPB/TCTA/CBP:DOPANT (8%)/TPBi/Liq/Al. The
attempts to fabricate the EL device containing Ir(pq)2(pc)
as a dopant were failed since Ir(pq)2(pc) was not sub-
limable for vacuum deposition. The EL peaks of devices
with Ir(F2-ppy)2(pc) and Ir(ppy)2(pc) were observed at 521
(559(sh)) and 587 nm, respectively, as shown in Figure 4.
The EL spectral patterns of the Ir complexes were mostly
consistent with their PL and indicated that emissions were
originated from the iridium complexes in the emitting lay-
ers. Figure 5 and Table II shows the luminance efficiency
versus current density characteristics of the devices involv-
ing the iridium complexes as emissive dopants. The maxi-
mum luminous efficiencies of device with Ir(F2-ppy)2(pc)
and Ir(ppy)2(pc) were 30.19 and 21.32 cd/A at 5.50 V,
Korea Research Foundation. (No. 2011-0003765).
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Received: 5 November 2011. Accepted: 12 February 2012.
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Copyright: American Scientific Publishers
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3445