Highly efficient red phosphorescent organic light-emitting devices using iridium(III) complexes based on 2-(biphenyl-3-yl)quinoline derived ligands

Article abstract of DOI:10.1166/jnn.2012.5664

Red phosphorescent emitters were synthesized based on Ir(III) phenylquinoline complexes for applications to OLEDs. Ir(III) complexes 1 - 3 were based on 2-(biphenyl-3-yl)-quinoline units connected to various phenyl groups such as 5-phenyl, 5-(4-fluorophenyl), and 6-phenyl groups. The EL efficiencies were quite sensitive to the structural features of the dopants in the emitting layers. In particular, a high-efficiency red OLED was fabricated using complex 1 as the dopant in the emitting layer. This OLED showed a maximum luminance, luminous efficiency, power efficiency, external quantum efficiency and CIE_x,y coordinates of 21,600 cd/m² at 16 V, 11.80 cd/A at 20 mA/cm², 3.57 μm/W at 20 mA/cm², 10.90% at 20 mA/cm², and (x = 0.63, y = 0.32) at 12 V, respectively.

Full text of DOI:10.1166/jnn.2012.5664

Copyright © 2012 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 12, 3455–3459, 2012
Highly Efficient Red Phosphorescent Organic
Light-Emitting Devices Using Iridium(III) Complexes
Based on 2-(biphenyl-3-yl)Quinoline Derived Ligands
Hyun Ju Kang1, Kum Hee Lee1, Heo Min Kim2, Ji Hyun Seo2,
Young Kwan Kim2ꢀ∗, and Seung Soo Yoon1ꢀ∗
1Department of Chemistry, Sungkyunkwan University, Suwon, 440-746, Republic of Korea
2Department of Information Display, Hongik University, Seoul 121-791, Republic of Korea
Red phosphorescent emitters were synthesized based on Ir(III) phenylquinoline complexes for appli-
cations to OLEDs. Ir(III) complexes 1–3 were based on 2-(biphenyl-3-yl)-quinoline units connected
to various phenyl groups such as 5-phenyl, 5-(4-fluorophenyl), and 6-phenyl groups. The EL efficien-
cies were quite sensitive to the structural features of the dopants in the emitting layers. In particular,
a high-efficiency red OLED was fabricated using complex 1 as the dopant in the emitting layer.
This OLED showed a maximum luminance, luminous efficiency, power efficiency, external quantum
efficiency and CIExꢀy coordinates of 21,600 cd/m2 at 16 V, 11.80 cd/A at 20 mA/cm2, 3.57 lm/W at
20 mA/cm2, 10.90% at 20 mA/cm2, and (x = 0ꢁ63, y = 0ꢁ32) at 12 V, respectively.
Keywords: Phosphorescent OLED, Ir(III) Complexes, Phenylquinoline Ligand.
Delivered by Ingenta to: Rice University
IP: 79.110.28.125 On: Mon, 13 Jun 2016 08:04:48
Copyright: American Scientific Publishers
1. INTRODUCTION
gap between the HOMO and LUMO states can be reduced
effectively to provide emitters with a saturated red color.
For this reason, a new class of red emitters 1–3 were
designed and synthesized based on Ir(III) phenylquino-
line complexes. The new Ir(III) complexes 1–3 were based
on 2-(biphenyl-3-yl)-quinoline units connected to various
phenyl groups, such as 5-phenyl, 5-(4-fluorophenyl), and
6-phenyl groups. Ir(III) complexes 1–3 are expected to
have improved electroluminescent properties. Highly effi-
cient red OLEDs were fabricated using these new Ir(III)
complexes. Furthermore, their electroluminescent proper-
ties were highly sensitive to the structural features of the
red emitters.
Phosphorescent organic light-emitting devices (PhOLEDs)
have attracted considerable attention for their potential
use in general illumination and flat panel displays.1
Phosphorescent OLEDs are of great interest because it is
possible, in principle, to increase their internal quantum
efficiency to 100%, corresponding to an external quan-
tum efficiency of more than 20%.2ꢀ3 For full-color display
applications, RGB emitting materials with sufficient lumi-
nous efficiency and proper chromaticity must be devel-
oped. The design and synthesis of efficient red emitters
are intrinsically difficult because their luminance quantum
yields tend to decrease with increasing emission wave-
length in accordance with the energy gap law.4 Therefore,
a variety of red Ir(III) complexes were recently devel-
oped as electrophosphorescent emitters.5–8 More recently,
2-phenylquinoline, has attracted increasing attention to
achieve good red-emitting Ir(III) complexes by ligand
modification.9
2. EXPERIMENTAL DETAILS
2.1. Synthesis and Characterization
2.1.1. General Procedure of (C∧Nꢂ2 Ir(acac) Complexes
In a 50 mL flask, the ꢃ-chloride-bridged dimer Ir(III)
complex, the acac lgand, and K2CO3 were mixed with
2-ethoxyethanol (30 mL), and the mixture was stirred
under reflux for 2–12 h. After cooling to room temper-
ature, 2-ethoxyethanol was removed under reduced pres-
sure. The crude product was dissolved in dichloromethane
(20 mL), and the obtained solid was filtered off.
The focus of this study was the molecular design
of highly efficient red phosphorescent emitters based on
Ir(III) phenylquinoline complexes for OLEDs. By extend-
ing the ꢁ-conjugation of the aromatic ligand, the energy
∗Authors to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2012, Vol. 12, No. 4
1533-4880/2012/12/3455/005
doi:10.1166/jnn.2012.5664
3455

Highly Efficient Red PhOLEDs Using Iridium(III) Complexes Based on 2-(biphenyl-3-yl)Quinoline Derived Ligands
Kang et al.
The dichloromethane solution (20 mL) was concentrated
under reduced pressure. Sbsequently, n-hexanes were uti-
lized in order to precipitate the complexes, and the
optained solid was filtered off. The residue was purified
by silica colum chromatography.
[ATR]: ꢆ = 3029, 2970, 1738, 1582, 1518, 1361, 979, 772,
723 cm−1. FAB-MS (m/z): 1040 [M+], HRMS-EI+ calcd
for C59H41F2IrN2O2: 1040.2765, found: 1040.2760.
1
Complex 3: Yield of 0.3 g (33%). H NMR (300 MHz,
CDCl3ꢂ:ꢄ ppm 8.63 (d, J = 9ꢅ3 Hz, 2 H), 8.25 (t, J = 7ꢅ1
Hz, 4 H), 8.05–7.98 (m, 5 H), 7.71 (d, J = 7ꢅ8 Hz, 7 H),
7.59 (d, J = 7ꢅ8 Hz, 4 H), 7.45 (d, J = 7ꢅ8 Hz, 5 H), 7.40–
7.38 (m, 5 H), 6.92 (d, J = 6ꢅ1 Hz, 2 H), 6.68 (d, J = 8ꢅ1
Hz, 2 H), 4.74 (s, 1 H), 1.57 (s, 6 H).13C NMR (75 MHz,
CDCl3ꢂ:ꢄ ppm 185.9, 150.5, 149.1, 147.8, 142.3, 140.1,
138.7, 138.5, 136.6, 134.3, 130.2, 129.5, 129.2, 128.8,
128.2, 127.9, 127.8, 127.5, 126.8, 126.4, 125.6, 124.8,
117.3, 116.1, 31.2, 28.6. FT-IR [ATR]: ꢆ = 3052, 1688,
1580, 1521, 966, 755 cm−1. FAB-MS (m/z): 1004 [M+],
HRMS-EI+ calcd for C59H43IrN2O2: 1004.2954, found:
1004.2948.
1
Complex 1: Yield of 0.2 g (46%). H NMR (300 MHz,
CDCl3):ꢄ ppm 8.59 (d, J = 8ꢅ4 Hz, 2 H), 8.31 (d, J =
9ꢅ0 Hz, 2 H), 8.10 (d, J = 9ꢅ3 Hz, 3 H), 8.01 (s, 2 H),
7.59–7.56 (m, 13 H), 7.47–7.55 (m, 5 H), 7.37 (t, J =
7ꢅ7 Hz, 5 H), 6.92 (dd, J = 1ꢅ8, 8.1 Hz, 2 H), 6.70
(d, J = 8ꢅ1 Hz, 2 H), 4.76 (s, 1 H), 1.58 (s, 6 H).13C
NMR (75 MHz, CDCl3ꢂ:ꢄ ppm 185.9, 170.3, 150.4, 149.9,
147.9, 142.3, 140.6, 139.7, 136.8, 136.5, 134.2, 130.5,
130.0, 128.8, 128.0, 127.2, 126.8, 126.4, 125.7, 124.8,
116.8, 100.6, 28.6. FT-IR [ATR]: ꢆ = 3016, 2970, 1738,
1580, 1366, 817, 758 cm−1. FAB-MS (m/z): 1004 [M+]
HRMS-EI+ calcd for C59H43IrN2O2: 1004.2954, found:
1004.2951.
2.2. Device Fabrication and Measurements
Complex 2: Yield of 0.16 g (58%). 1H NMR (300 MHz,
CDCl3ꢂ:ꢄ ppm 8.58 (d, J = 8ꢅ7 Hz, 2 H), 8.24 (d, J =
9ꢅ0 Hz, 2 H), 8.11 (d, J = 9ꢅ0 Hz, 2 H), 8.01 (s, 2 H),
7.58–7.46 (m, 10 H), 7.38 (dd, J = 6ꢅ6, 7.8 Hz, 6
H), 7.27–7.22 (m, 6 H), 6.91 (d, J = 7ꢅ4 Hz, 2 H),
6.68 (d, J = 7ꢅ8 Hz, 2 H), 4.76 (s, 1 H), 1.58 (s, 6 H).
13C NMR (125 MHz, CDCl3ꢂ:ꢄ ppm 185.9, 185.8, 183.4,
170.3, 150.5, 149.9, 147.8, 137.9 (d, J = 243 Hz), 136.8,
135.6, 134.3, 132.0 (d, J = 8ꢅ2 Hz), 130.0, 128.8, 127.2,
126.8, 126.5 (d, J = 12ꢅ6 Hz), 125.7, 124.9, 116.8, 115.8
OLEDs using red dopants in the emitting layers were fab-
ricated by vacuum (107 torr) thermal evaporation onto pre-
cleaned ITO coated glass substrates. The structure was as
follows: ITO/2-TNATA (60 nm)/NPB (20 nm)/CBP: 8%
Ir(III) complexes (30 nm)/BCP (10 nm)/Alq3 (20 nm)/Liq
(2 nm)/Al (100 nm). All of the optical and electrical prop-
erties of OLEDs such as the current density, luminance,
luminous efficiency, and CIE coordinate characteristics
Delivered by Ingenta to: Rice University
were measured with Keithley 236, LS-50B, and MINOLTA
IP: 79.110.28.125 On: Mon, 13 Jun 2016 08:04:48
CS-100A, respectively.
Copyright: American Scientific Publishers
(d, J = 21ꢅ5 Hz), 114.2, 91.3, 31.0, 28.6, 25.2. FT-IR
Scheme 1. Synthetic route to new Ir(III) complexes 1–3.
3456
J. Nanosci. Nanotechnol. 12, 3455–3459, 2012

Kang et al.
Highly Efficient Red PhOLEDs Using Iridium(III) Complexes Based on 2-(biphenyl-3-yl)Quinoline Derived Ligands
Table II. EL performances of devices 1–4.
(a)
1
2
3
4
Voan [V]
4.0
612
4.0
615
3.5
613
4.0
602
EL
ꢇ
[nm]
max
Lb [cd/m2]
LEc/d [cd/A]
PEc/d [lm/W]
QEc/d [%]
CIEeꢊxꢀyꢂ
21600
19200
20000
35700
14.6/11.8
10.5/5.22
13.8/10.9
14.1/11.5
9.67/3.33
13.5/9.92
11.8/9.41
7.74/3.08
11.2/7.78
17.9/16.0
10.9/5.22
18.4/11.3
(0.63, 0.37) (0.64, 0.36) (0.64, 0.36) (0.61, 0.39)
Notes: aTurn-on voltage at 1 cd/m2; bMaximum luminance. Values were obtained at
16 V; cMaximum value; dAt 20 mA/cm2; eCommission Internationale d’Énclairage
(CIE) coordinates at a 12.0 V.
in the ligands. The maximum emission wavelength (ꢇmax
ꢂ
(b)
of complexes 1–3 ranged from 604 to 607 nm in the red
region of the visible spectrum. Compared to (pq)2Ir(acac)
with an emission wavelength at 596 nm, complexes 1–3
lead to a bathochromic shift of ca. 8–11 nm in the emission
peak wavelength, which can be rationalized qualitatively
by considering the decrease in the HOMO/LUMO energy
level, which results from an increase in the ꢁ-conjugation
length of the quinoline/phenyl moieties of the ligands. The
FWHMs of the emission spectra of complexes 1–3 were
quite narrow (62–66 nm), and the complexes showed high
PL quantum yields of 0.42–0.49. This suggests that the
resulting OLED devices should have highly efficient elec-
troluminescent properties. The highest occupied molecular
Fig. 1. (a) The absorption and (b) emission spectra of Ir(III) complexes.
Delivered by Ingenta to: Rice University
orbital (HOMO) and lowest unoccupied molecular orbital
IP: 79.110.28.125 On: Mon, 13 Jun 2016 08:04:48
(LUMO) energy levels of complexes 1–3 are shown in
3. RESULTS AND DISCUSSION
Copyright: American Scientific Publishers
Table I.
Scheme 1 outlines the structures and synthetic processes
of the Ir(III) complexes (1–3). The Ir(III) complexes were
obtained by cyclometalation of the ligands with IrCl3 and
subsequent reaction with acetylacetone.10
The HOMO energy levels were measured with an AC-2
photoelectron spectrometer and the LUMO levels calcu-
lated by subtracting the corresponding optical band gap
energies from the HOMO values. The HOMO/LUMO
energy levels of complexes 1–3 are −5ꢅ20/−2ꢅ93,
−5ꢅ55/−3ꢅ30, and −5ꢅ29/−3ꢅ05, respectively. Interest-
ingly, compared to complex 1 with 5-phenyl moiety, the
HOMO/LUMO energy levels of complex 2 with 5-(4-
fluorophenyl) moiety became lower than 0.32 and 0.37 eV,
respectively, due to the introduction of the electron-
withdrawing fluorine group in the ligand. To explore
the electroluminescent properties of these molecules,
devices 1–3 were fabricated with the same configura-
tions by employing 8% Ir(III) complexes 1–3 as a dopant
within a CBP host in the emitting layers, and their
Figure 1 shows the UV-vis and PL spectra of these new
phosphors in dilute dichloromethane solutions. A sum-
mary of the optical properties data of the three red
emitters are given in Table I. By comparing the absorp-
tion spectra of the free ligands, the absorption band
below and above 400 nm was assigned to the spin-
1
allowed ꢁ–ꢁ∗ transitions of the cyclometallated ligands
1
3
and the spin-allowed MLCT and spin-forbidden MLCT
band transitions, respectively. Relative to the absorption
band of (pq)2Ir(acac), complexes 1–3 showed a signifi-
cant bathochromic shift due to the extended ꢁ-conjugation
Table I. Optical properties of Ir(III) complexes 1–3.
ꢇAbs
[nm] (ꢀ)
ꢇEm
[nm]a
FWHM
[nm]
HOMO/LUMO
[eV]c
Complexes
ꢈpbh
1
2
3
288 (24.9), 428 (1.41), 479 (1.21)
289 (17.1), 429 (0.95), 481 (0.81)
289 (16.9), 411 (0.76), 484 (1.04)
270 (10.6), 425 (0.93), 464 (0.89)
604
604
607
596
0.43
0.42
0.49
0.86
66
65
62
66
5.20/2.93
5.55/3.30
5.29/3.05
5.11/2.50
(pq)2Ir(acac)
Notes: aMeasured in CH2Cl2 at a concentration of 10−5 M and molar extinction coefficient (ꢉꢂ values (104 M−1 cm−1ꢂ are shown in parentheses; bDegassed CH2Cl2 relative
to fac-[Ir(ppy)3] (ꢈp = 0ꢅ40), ꢇex = 470 nm; cThe energy levels were measured with a low-energy photo-electron spectrometer (Riken-Keiki, AC-2).
J. Nanosci. Nanotechnol. 12, 3455–3459, 2012
3457

Highly Efficient Red PhOLEDs Using Iridium(III) Complexes Based on 2-(biphenyl-3-yl)Quinoline Derived Ligands
Kang et al.
19200, 20000, and 35700 cd/m2 at 16 V, respectively. The
turn-on voltages for devices 1–4 were 4.0, 4.0, 3.5, and
4.0 V, respectively, which indicate that carrier injection
into the red emitting layer of device 3 was more effec-
tive than in devices 1, 2, and 4. Intriguingly, the EL effi-
ciencies of devices 1–3 using new Ir(III) complexes are
inferior to those of devices 4 using (pq)2Ir(acac). Presum-
ably, the lower quantum yields of Ir(III) complexes 1–3
than (pq)2Ir(acac) might explain the reduced EL efficien-
cies of devices 1–3. In addition, the lower HOMO energy
levels of Ir(III) complexes 1–3 than (pq)2Ir(acac) would
prevent the effective hole-trapping processes in devices
1–3 and reduce the EL efficiencies of devices 1–3. How-
ever, it is possible that other factors, such as the degree
of carrier balances and carrier transporting properties in
the emitting layer, would play important roles in the dif-
ferences in the EL performances of devices 1–4. Among
devices 1–3, device 1 showed the highest luminance
and displayed excellent EL performance with maximum
luminous, power, and quantum efficiencies of 14.6 cd/A,
10.5 lm/W, and 10.6%, respectively.
Fig. 2. EL spectra of devices 1–4.
electroluminescent performance characteristics of devices
are summarized in Table II. Figure 2 shows the EL spec-
tra of the red emissions of devices 1–3 with peaks at
612–615 nm, respectively. These EL spectra matched well
with the corresponding solution PL spectra, indicating that
all the emissions originated from the triplet excited states
of the complexes.11 The respective CIE coordinates of
devices 1–3 were (0.63, 0.37), (0.64, 0.36), and (0.64,
0.36). All the devices showed red shifts in the EL spec-
tra compared to device 4 using (pq)2Ir(acac) because the
introduction of an aromatic group in the dopants leads to
a decrease in the energy gap.
Interestingly, the structural change in the ligands of the
Ir(III) complexes leads to differences in the EL efficiency
of the devices using them as dopants. For example, com-
pared to device 1, using complex 1 (with 5-phenyl group),
the luminous, power and quantum efficiencies of device 3
using complex 3 (with 6-phenyl group) were 20, 41, and
29% lower at 20 mA/cm2, respectively. In addition, the
luminous, power, and quantum efficiencies of device 2
Delivered by Ingenta to: Rice University
The current density–voltage–luminance (J–V –L), lumi-
IP: 79.110.28.125 On: Mon, 13 Jun 2016 08:04:48
nance, power, and quantum efficiencies with respect to
the current density of devices 1–3 are shown in Figure 3.
The maximum luminance of devices 1–4 were 21600,
Copyright: American Scientific Publishers
using complex 2 (with 5-(4-fluorophenyl) group) were 2.5,
36 and 9% at 20 mA/cm2 lower, respectively. This study
Fig. 3. (a) J–V –L, (b) luminous efficiency, (c) power efficiency, and (d) external quantum efficiency versus current density relationship of devices 1–4.
J. Nanosci. Nanotechnol. 12, 3455–3459, 2012
3458

Kang et al.
Highly Efficient Red PhOLEDs Using Iridium(III) Complexes Based on 2-(biphenyl-3-yl)Quinoline Derived Ligands
demonstrates that structural modification in the ligand of
the phosphorescent Ir(III) complexes can help improve the
EL performance of OLEDs when used as dopants to tune
the HOMO/LUMO energy levels and the other physical
properties of the emitting materials in the devices. Fur-
thermore, this study demonstrated the excellent properties
of red phosphorescent Ir(III) complex 1 for applications to
red emitting materials in OLEDs. In particular, this highly
efficient red emitting dopant 1 can be used as a phospho-
rescent red emitter in WOLEDs.12
Acknowledgment: This research was supported by
Basic Science Research Program through the National
Research Foundation of South Korea (NRF) funded
by the Ministry of Education, Science and Technology
(20100007370).
References and Notes
1. M. A. Baldo, D. F. O’Brien, Y. You, A. Shoustikov, S. Sibely, M. E.
Thomson, and S. R. Forrest, Nature 395, 151 (1998).
2. L. S. Hung and C. H. Chen, Mater. Sci. Eng.
(2002).
3. F. I. Wu, H. J. Su, L. Luo, W. G. Diau, C. H. Cheng, J. P. Duan,
and G. H. Lee, J. Mater. Chem. 15, 1035 (2005).
4. S. D. Cummings and R. Eisenberg, J. Am. Chem. Soc. 118, 1949
(1996).
5. K. H. Lee, J. H. Seo, Y. K. Kim, and S. S. Yoon, J. Nanosci. Nano-
technol. 9, 7099 (2009).
6. K. H. Lee, J. H. Kim, J. H. Seo, Y. K. Kim, and S. S. Yoon,
J. Nanosci. Nanotechnol. 10, 3193 (2010).
7. K. H. Lee, W.-C. Lee, M. K. Kim, J. H. Seo, Y. K. Kim, and S. S.
Yoon, Thin Solid Films 518, 3972 (2010).
8. K. H. Lee, H. J. Kang, J. K. Park, J. H. Seo, Y. K. Kim, and S. S.
Yoon, Thin Solid Films 518, 6188 (2010).
9. F. L. Wu, H. J. Su, C. F. Shu, L. Luo, W. G. Diau, C. H.
Cheng, J. P. Duan, and G. H. Lee, J. Mater. Chem. 15, 1035
(2005).
R 39, 143
4. CONCLUSION
A series of red phosphorescent Ir(III) complexes based
on 2-(biphenyl-3-yl)-quinoline units connected to various
phenyl groups, such as 5-phenyl, 5-(4-fluorophenyl), and
6-phenyl groups, was synthesized, and their electrolumi-
nescent properties were investigated. An OLED using red
dopant 1 with CBP as the host exhibited a maximum
luminance of 21600 cd/m2 at 16 V. The luminous, power
and quantum efficiencies were 11.8 cd/A, 5.22 lm/W,
10.9% at 20 mA/cm2, respectively, with CIE coordinates
of (0.63, 0.37) at 12 V. Furthermore, the device using com-
plex 2 as a dopant exhibited efficient red emissions with
a maximum luminance, luminous, power, quantum effi-
ciencies and CIE coordinates of 19200 cd/m2 at 16 V,
11.5 cd/A at 20 mA/cm2, 3.33 lm/W at 20 mA/cm2, 9.92%
10. S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, H. E. Lee,
C. Adachi, P. E. Burrows, S. R. Forrest, and M. E. Thompson, J. Am.
Chem. Soc. 123, 4304 (2001).
Delivered by Ingenta to: Rice University
2
IP: 79.110.28.125 On: Mon, 13 Jun 2016 08:04:48
at 20 mA/cm , and (0.64, 0.36) at 12 V, respectively. This
Copyright: American S1c1ie. nCt.i-fLic. HPou, Wbl.i-sYh. Werosng, Z. Q. Gao, C. H. Chen, K. W. Cheah, B. Yao,
study demonstrates that phosphorescent Ir(III) complexes
Z. Xie, Q. Wang, D. Ma, L. Wang, X. M. Yu, H. S. Kwok, and
Z. Lin, Adv. Funct. Mater. 18, 319 (2008).
with ligands based on 2-(biphenyl-3-yl)quinoline deriva-
tives possess excellent properties for red emitting materials
in OLEDs.
12. J. H. Seo, I. H. Park, G. Y. Kim, K. H. Lee, M. K. Kim, S. S. Yoon,
and Y. K. Kim, Appl. Phys. Lett. 92, 183303 (2008).
Received: 28 October 2010. Accepted: 22 January 2011.
J. Nanosci. Nanotechnol. 12, 3455–3459, 2012
3459