Acetylated phenylpyridine derived iridium (III) complexes for orange-red electrophosphorescent light-emitting diodes

Article abstract of DOI:10.1080/15421406.2011.590369

The photophysical characteristics of (Actpy)2Ir(acac) and (Acpsf)2Ir(acac) were investigated to determine their suitability as candidates for red light-emitting materials for organic light emitting diodes (OLEDs) [where Actpy = 3-acetyl-6-(4-tert-butylphenyl)pyridine, acac = acetoylacetonate, and Acpsf = 3-acetyl-6-(9,9'-spirobi[fluorene]-2-yl)pyridine]. To explore the electroluminescent properties of these materials, multilayered OLEDs were fabricated the device structure of ITO/NPB/CBP: 8% Ir(III) complexes/BCP/Alq3/ Liq/Al. Particularly, device 1 using complex (Actpy)2Ir(acac) as a dopant material furnished a maximum luminance of 41,800 cdm^-2 at 15 V, and luminous efficiency and power efficiency of 5.70 cdA^-1 and 2.70 lmW^-1 at 20 mAcm^-2, respectively. Additionally, this device showed orange-red emission with CIE coordinates of (0.57, 0.42) at 10.0 V. Copyright Taylor &Francis Group, LLC.

Full text of DOI:10.1080/15421406.2011.590369

This article was downloaded by: [University of Kent]
On: 11 November 2014, At: 16:56
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered
office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Molecular Crystals and Liquid Crystals
Publication details, including instructions for authors and
subscription information:
http://www.tandfonline.com/loi/gmcl20
Acetylated Phenylpyridine Derived
Iridium (III) Complexes for Orange–Red
Electrophosphorescent Light-Emitting
Diodes
Kum Hee Lee a , Hyun Ju Kang a , Seok Jaelee b , Ji Hyun Seo b ,
Young Kwan Kim b & Seung Soo Yoon a
a Department of Chemistry , Sungkyunkwan University , Suwon,
Republic of Korea
b Department of Information Display , Hongik University , Seoul,
Republic of Korea
Published online: 07 Oct 2011.
To cite this article: Kum Hee Lee , Hyun Ju Kang , Seok Jaelee , Ji Hyun Seo , Young Kwan Kim &
Seung Soo Yoon (2011) Acetylated Phenylpyridine Derived Iridium (III) Complexes for Orange–Red
Electrophosphorescent Light-Emitting Diodes, Molecular Crystals and Liquid Crystals, 548:1, 256-264,
DOI: 10.1080/15421406.2011.590369
To link to this article: http://dx.doi.org/10.1080/15421406.2011.590369
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the information (the
“Content”) contained in the publications on our platform. However, Taylor & Francis,
our agents, and our licensors make no representations or warranties whatsoever as to
the accuracy, completeness, or suitability for any purpose of the Content. Any opinions
and views expressed in this publication are the opinions and views of the authors,
and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content
should not be relied upon and should be independently verified with primary sources
of information. Taylor and Francis shall not be liable for any losses, actions, claims,
proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or
howsoever caused arising directly or indirectly in connection with, in relation to or arising
out of the use of the Content.
This article may be used for research, teaching, and private study purposes. Any
substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,
systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &

Conditions of access and use can be found at http://www.tandfonline.com/page/terms-
and-conditions

Mol. Cryst. Liq. Cryst., Vol. 548: pp. 256–264, 2011
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2011.590369
Acetylated Phenylpyridine Derived Iridium (III)
Complexes for Orange–Red Electrophosphorescent
Light-Emitting Diodes
KUM HEE LEE,1 HYUN JU KANG,1 SEOK JAELEE,2 JI HYUN
SEO,2 YOUNG KWAN KIM,2,∗ AND SEUNG SOO YOON1,∗
1Department of Chemistry, Sungkyunkwan University, Suwon, Republic of
Korea
2Department of Information Display, Hongik University, Seoul Republic of
Korea
The photophysical characteristics of (Actpy)2Ir(acac) and (Acpsf)2Ir(acac) were
investigated to determine their suitability as candidates for red light-emitting ma-
terials for organic light emitting diodes (OLEDs) [where Actpy = 3-acetyl-6-
(4-tert-butylphenyl)pyridine, acac = acetoylacetonate, and Acpsf = 3-acetyl-6-
(9,9’-spirobi[fluorene]-2-yl)pyridine]. To explore the electroluminescent properties
of these materials, multilayered OLEDs were fabricated the device structure of
ITO/NPB/CBP: 8% Ir(III) complexes/BCP/Alq3/Liq/Al. Particularly, device 1 using
complex (Actpy)2Ir(acac) as a dopant material furnished a maximum luminance of
41,800 cdm−2 at 15 V, and luminous efficiency and power efficiency of 5.70 cdA−1 and
2.70 lmW−1 at 20 mAcm−2, respectively. Additionally, this device showed orange–red
emission with CIE coordinates of (0.57, 0.42) at 10.0 V.
Keywords AcetylatedIr(III)complexes; phosphorescent OLEDs; spirobifluorene; tert-
butyl group
1. Introduction
Since Forrest and Thompson’s group reported the first example of electrophosphorescence
at room temperature [1], PHOLED materials and devices have been intensively studied
because of high external quantum efficiencies (EQEs) in their electroluminescence devices
[2,3]. The Ir(ppy)3 derivatives have shown a number of advantages such as easy tun-
ing emission energy by functionalized ppy ligand with electron donating and withdrawing
moieties. In this way, a variety of red Ir(III) complexes have been developed as electrophos-
phorescent emitter [4–6]. Among those are Ir(III) complexes based on the carbonylated
phenyl–pyridine ligands [7–9]. For example, we have reported orange–red emitters based
on carbonylated phenyl–pyridine derivative-Ir(III) complexes, (Acppy)2Ir(acac) in organic
∗Address correspondence to Young Kwan Kim, Department of Information Display, Hongik
University, Mapo-gu, Sangsu-dong, Seoul, 121-791 Republic of Korea. Tel.: (+82)2-320-1646; Fax:
(+82)2-3141-8928. E-mail: kimyk@hongik.ac.kr
∗Address correspondence to Seung Soo Yoon, Department of Chemistry, SungKyunKwan Uni-
versity, Cheoncheon-dong, Jangan-gu, Suwon, 440-746 Republic of Korea. Tel.: (+82)31-290-5971;
Fax: (+82)31-290-7075. E-mail: ssyoon@skku.edu
256

Acetylated Ir (III) Complexes for Orange–Red Electrophosphorescent
257
light emitting diodes (OLEDs) with high luminous efficiency and brightness were reported
[10]. Also, in our recent work, a highly efficient red OLED was obtained by adopting
5-benzoyl-2-phenylpyridine derived Ir(III) complex with tert-butyl moiety in benzoyl unit
[11].
In this article, as a continuing effort to develope the efficient phosphorescent
iridium complexes based on the carbonylated phenylpyridine ligand for OLED, two
orange–red phosphorescent Ir(III) complexes based on acetylated phenylpyridine ligand,
(Actpy)2Ir(acac) and (Acpsf)2Ir(acac) are synthesized and their electrophosphrescent
properties are investigated. Also, in (Actpy)2Ir(acac) and (Acpsf)2Ir(acac), the differ-
ent bulky groups of electron-donating tert-butyl and spirobifluorene were introduced in
the ligands of Ir(III) complexes to study the substituent effect on the electroluminescent
properties. As will be seen in below, the electroluminescent properties of devices using
(Actpy)2Ir(acac) and (Acpsf)2Ir(acac) as dopants are highly sensitive to the structural
features of the orange–red emitters.
2. Experimental Details
5-Acetyl-2-bromopyridine, 4-tert-buthylphenylboronic acid, spirofluorenyl-2-boronic acid,
and acetylacetone were used as received from Aldroch or TCI. All reactions were performed
under nitrogen. Solvents were carefully dried and distilled from appropriate drying agents
prior to use. Commercially, available reagents were used without further purification unless
1
otherwise stated. H- and 13C-NMR spectra were recorded using a Varian (unity Inova
300Nb or 500NB) spectrometer at 300 or 500 MHz. FT-IR spectra were recorded using
a VERTEX 70 spectrometer. Low- and high-resolution mass spectra were recorded using
either a Jeol JMS-AX505WA spectrometer in FAB mode or a Jeol JMS-600 spectrometer
in EI mode. The UV-vis absorption measurements of these blue-emitting materials in
dichloromethane (10−5 M) were acquired with a Sinco S-3100 in a quartz cuvette (1.0 cm
path). The photoluminescence (PL) spectra were measured on an Amincobrowman series
2 luminescence spectrometer. The phosphorescence quantum yields were determined in
CH2Cl2 solutions at 293 K against Ir(ppy)3 as a reference (ꢀ = 0.40) [12]. The HOMO
(highest occupied molecular orbital) energy levels were determined using a low energy
photoelectron spectrometer (Riken-Keiki, AC-2). The energy band gaps were determined
from the intersection of the absorption and PL spectra. The lowest unoccupied molecular
orbital (LUMO) energy levels were calculated by subtracting the corresponding optical
band gap energies from the HOMO energy values.
2.1. Synthesis and Characterization
2.1.1. General Procedure for the Synthesis of Ir(III) Complexes. Actppy (0.88 g, 4.46
mmol) was dissolved in 2-ethoxyethanol (18.0 mL) in a 50 mL round-bottom flask.
IrCl3·3H2O (0.60 g, 2.01 mmol) and water (6.0 mL) were then added to the flask. The
mixture was stirred under nitrogen at 120◦C for 24 h and cooled to room temperature. The
precipitate formed in the mixture was collected and washed with methanol, hexane, and
dried in vacuum to give the corresponding cyclometalated Ir(III)-µ-chloro-bridge dimer.
In a 50 mL flask, the dimer complex, acetylacetone (0.31 mL, 3.02 mmol) and Na2CO3
(0.64 mg, 6.03 mmol) were mixed with 2-ethoxyethanol (20 mL) and mixture was heated
at 100◦C for 6h. After cooling at room temperature, the solid precipitate was collected by
filtration, washed with ethanol and hexane. The residue was dissolved in dichloromethane

258
K. H. Lee et al.
and the solid was filtered off. The solution was concentrated in vacuo and the residue was
purified on a silica gel column using dichloromethane and methanol as eluent. The prod-
uct was obtained after recrystallization from dichloromethane/ethanol. Similar procedures
were also employed for the synthesis of other Ir(III) complexes. The yields and spectral
data of all Ir(III) complexes are as follows.
Complex 1 (Actppy)2Ir(acac) (32% Yield). 1H NMR (300 MHz, CDCl3): δ ppm 9.09
(d, J = 2.0 Hz, 2H), 8.24 (dd, J = 2.0, 8.6 Hz, 2H), 7.87 (d, J = 8.5 Hz, 2H), 7.54 (d, J =
8.3 Hz, 2H), 6.89 (dd, J = 1.9, 8.3 Hz, 2H), 6.23 (d, J = 1.9 Hz, 2H), 5.27 (s, 1H), 2.60
(s, 6H), 1.85 (s, 6H), 1.03 (s, 18H). 13C NMR (125 MHz, CD2Cl2): δ ppm 194.3, 185.5,
172.4, 153.6, 149.9, 149.4, 141.5, 136.8, 130.4, 129.6, 125.7, 119.5, 118.0, 101.1, 34.6,
30.9, 30.8, 28.4, 26.4. IR (KBr): ν = 2959, 2866, 1687, 1584, 1517, 1480, 1454, 1387,
1358, 1265, 1113, 1061, 815, 756 cm−1. FAB-MS (m/z): 796 [M+]. HRMS-FAB+ calcd
for C39H43IrN2O4: 796.2855, found: 796.2852.
Complex 2 (Acpsf)2Ir(acac) (44% Yield). 1H NMR (300 MHz, CDCl3): δ ppm 9.11
(d, J = 2.0 Hz, 2H), 8.16 (dd, J = 2.0, 8.3 Hz, 2H), 7.84 (d, J = 7.7 Hz, 4H), 7.68 (d, J = 8.7
Hz, 2H), 7.38–7.31 (m, 6H), 7.17 (td, J = 1.0, 7.5 Hz, 2H), 7.13–7.02 (m, 6H), 6.98 (td, J
= 1.0, 7.5 Hz, 2H), 6.79 (d, J = 7.6 Hz, 2H), 6.70 (s, 2H), 6.65 (d, 7.6 Hz, 2H), 6.59 (d, J =
7.6 Hz, 2H), 5.32 (s, 1H), 2.64 (s, 6H), 1.87 (s, 6H). 13C NMR (125 MHz, CD2Cl2): δ ppm
194.4, 185.6, 171.7, 150.1, 149.9, 149.4, 149.3, 149.1, 144.2, 143.9, 142.4, 142.0, 141.9,
141.2, 136.8, 130.6, 128.6, 128.2, 128.0, 127.9, 127.7, 125.4, 124.3, 123.9, 123.8, 121.1,
121.0, 120.4, 120.3, 118.8, 101.1, 65.8, 28.5, 26.5. IR (KBr): ν = 3057, 1688, 1591, 1518,
1488, 1439, 1396, 955, 754, 733 cm−1. HRMS-FAB+ calcd for C69H47IrN2O4: 1160.3165,
found: 1160.3167.
2.1.2. Synthesis of Actppy. A mixture of 5-acetly-2-bromopyridine (0.95 g, 4.75 mmol),
4-tert-butylphenylbronic acid (0.93 g, 5.23 mmol), tetrakis(triphenylphosphine)palladium
(0.22 g, 0.19 mmol), toluene (20 mL), ethanol (10 mL), and 2M Na2CO3 (10 mL) was
refluxed at 120◦C for 5 h. After reaction mixture was cooled at room temperature, it was
poured into water and extracted with ethyl acetate. The combined organic layer was dried
with anhydrous MgSO4 and evaporated to dryness. The crude product was further purified
by silica gel column chromatography. The product was a white solid with an isolated yield
of 74% (0.89 g). 1H NMR (300 MHz, CDCl3): δ ppm 9.22 (dd, J = 0.8, 2.3 Hz, 1H), 8.28
(dd, J = 2.3, 8.4 Hz, 1H), 8.02 (d, J = 8.4 Hz, 2H), 7.83 (dd, J = 0.8, 8.4 Hz, 1H), 7.53 (d,
J = 8.4 Hz, 2H), 2.66 (s, 3H), 1.37 (s, 9H). 13C NMR (125 MHz, CDCl3): δ ppm 196.7,
161.2, 153.8, 150.4, 136.5, 135.6, 130.6, 127.4, 126.2, 120.0, 35.1, 31.5, 26.9. IR (KBr):
ν = 2961, 2868, 1680, 1458, 1414, 1384, 1359, 1117, 1019, 953, 830, 765, 735 cm−1
.
HRMS-EI+ calcd for C17H19ON: 253.1467, found: 253.1466.
2.1.3. Synthesis of Acpsf. The reaction described for the synthesis of Actppy was car-
ried out starting with 5-acetly-2-bromopyridine (0.60 g, 3.00 mmol) and spirofluorenyl-2-
boronic acid (1.19 g, 3.30 mmol). The product was a white solid with an isolated yield of
87% (1.14 g). 1H NMR (300 MHz, CDCl3): δ ppm 9.10 (d, J = 2.1 Hz, 1H), 8.15 (td, J =
7.8, 8.6 Hz, 2H), 7.97 (d, J = 7.8 Hz, 1H), 7.91–7.86 (m, 3H), 7.64 (d, J = 8.7 Hz, 1H),
7.42–7.35 (m, 4H), 7.14–7.08 (m, 3H), 6.78–6.72 (m, 3H), 2.58 (s, 3H). 13C NMR (125
MHz, CDCl3): δ ppm 196.6, 160.9, 150.2, 149.9, 149.8, 148.6, 144.2, 142.1, 141.1, 137.9,
136.4, 130.6, 128.8, 128.2, 128.1, 127.8, 124.4, 124.3, 123.2, 120.8, 120.7, 120.4, 66.3,
26.9. IR (KBr): ν = 3061, 1683, 1585, 1447, 1375, 1273, 909, 755, 730 cm−1. HRMS-EI+
calcd for C32H21ON: 435.1623, found: 435.1623.

Acetylated Ir (III) Complexes for Orange–Red Electrophosphorescent
259
2.2. Fabrication and Characterization of Device
OLED fabrication of the indium-tin-oxide (ITO) thin films coated on glass substrates
were used, which were 12 Ω sq−1 (emitting area was 3 mm2 × 3 mm2) of the sheet re-
sistivity and a 1000 Å of thickness. The ITO-coated glass was cleaned in an ultrasonic
bath by the following sequence: acetone, methyl alcohol, distilled water, followed by
storage in isopropyl alcohol for 20 min and drying with a N2 gas gun. The substrates
were treated with O2 plasma argon environment. Organic layers were deposited by ther-
mal evaporation from resistively heated alumina crucibles onto the substrate at a rate of
1.0 Å s−1. All organic materials and metal were deposited under high vacuum (5.0 × 10−7
Torr). The OLEDs were fabricated in the following sequence: ITO/bis[N-(1-(naphthyl)-N-
phenyl-amino]biphenyl (NPB) (65 nm)/4,40,N,N0-dicarbazolebiphenyl (CBP): 8% Ir(III)
complexes/bathocuproine (BCP) (10 nm)/tris-(8-hydroxyquinoline) aluminum (Alq3)
(25 nm)/lithium quinolate (Liq) (2 nm)/Al. The current density (J), luminance (L), luminous
efficiency (LE), and the CIE chromaticity coordinates of the OLEDs were measured with
Keithly 2400, Chroma meter CS-1000A. Electroluminance was measured using a Roper
Scientific Pro 300i.
3. Result and Discussion
Scheme 1 outlines the structures and synthetic processes of the Ir(III) complexes
(Actppy)2Ir(acac) and (Acpsf)2Ir(acac). The Ir(III) complexes were obtained by cy-
clometalation of the ligands with IrCl3 and subsequent reaction with acetylacetone [13].
After conventional purification, such as column chromatography and recrystallization,
these synthesized orange–red emitting materials (Actppy)2Ir(acac) and (Acpsf)2Ir(acac)
were purified further by train sublimation at reduced pressure, below 10−3 torr, and fully
1
characterized with H- and 13C-NMR, infrared (IR), and low- and high-resolution mass
spectrometry.
The UV-vis absorption and PL spectra of (Actppy)2Ir(acac) and (Acpsf)2Ir(acac)
were shown in Fig. 1, and their data summarized in Table 1. The absorption spectra of
Ir(III) complexes (Actppy)2Ir(acac) and (Acpsf)2Ir(acac) below 360 nm can assigned
1
to the spin-allowed π-π∗ transition involving Actppy and Acpsf ligands, and the band
around 400 nm to a spin-allowed metal charge transfer (1MLCT) band. These orange–red
phosphors show outstanding strong absorption band 450–550 nm regions, which can be
assigned to spin-forbidden (3MLCT) band. The maximum emission wavelength (λmax
)
of (Actppy)2Ir(acac) and (Acpsf)2Ir(acac) appeared at 600, and 612 nm, respectively.
Especially, compared with the PL spectra of (Acppy)2Ir(acac), the maximum emission
wavelength of complex (Acpsf)2Ir(acac) with spirobifluornyl moiety in the phenyl unit was
red-shifted (ca. 9 nm), respectively, due to increasing of π-conjugation length of ligand. The
emission quantum yields of complexes (Actppy)2Ir(acac) and (Acpsf)2Ir(acac) were 0.42
and 0.19, respectively, as determined by using Ir(ppy)3 (0.40) used as the reference [12].
Interestingly, compound (Actppy)2Ir(acac), bearing bulky tert-butyl groups in the phenyl
unit, had quantum yields higher than that of Ir(ppy)3. The HOMO and LUMO energy levels
of complexes (Actppy)2Ir(acac) and (Acpsf)2Ir(acac) are shown in Table 1. The respective
HOMO energy levels of (Actppy)2Ir(acac) and (Acpsf)2Ir(acac) were measured at −5.51
and −5.54 eV by a low-energy photoelectron spectrometer (Riken-Keiki AC-2). The optical
energy band gaps (Eg) of (Actppy)2Ir(acac) and (Acpsf)2Ir(acac) were 2.24 and 2.17 eV,
as determined from the absorption spectra and emission spectra. The LUMO energy levels
calculated by subtraction of the optical energy band gap from the HOMO levels were

260
K. H. Lee et al.
Scheme 1. Synthetic route to new Ir(III) complexes (Actppy)2Ir(acac) and (Acpsf)2Ir(acac).
−3.27 eV and −3.37 eV, respectively. In case of (Acpsf)2Ir(acac) with spirobifluornyl
group, the HOMO and LUMO energy levels lower than those of (Actppy)2Ir(acac). The
energy diagrams for complexes (Actppy)2Ir(acac) and (Acpsf)2Ir(acac), as well as for
devices using other materials, are shown in Fig. 2.
Figure 1. (a) The absorption, and (b) emission spectra of complexes (Actppy)2Ir(acac) and
(Acpsf)2Ir(acac).

Acetylated Ir (III) Complexes for Orange–Red Electrophosphorescent
261
Table 1. Photophysical properties
Egb
(eV) Q.Yc
a
a
Absorption (293K) λabs
λem
(nm)
HOMO/LUMO
(eV)
Complexes
(nm)
(Actppy)2
Ir(acac)
(Acpsf)2
Ir(acac)
289(3.37), 322(3.78),
513(0.47)
300(4.36), 311(4.25),
362(4.81), 530(0.67)
600
−5.51/−3.27
−5.54/−3.37
2.24
2.17
0.42
0.19
612
aMeasured in CH2Cl2 at a concentration of 10−5 M and molar extinction coefficient (ε) values
(104 M−1 cm−1) are shown in parentheses.
bBand-gap energies estimated from the red edge of absorption spectra and emission spectra.
cDegassed CH2Cl2 relative to fac-[Ir(ppy)3] (Фp = 0.40), λex = 400 nm.
The electroluminescent properties of complexes (Actppy)2Ir(acac) and
(Acpsf)2Ir(acac) were explored by complexes as dopant materials in the fabrica-
tion of multilayered OLEDs. The device configuration was ITO/NPB (65 nm)/CBP:
8% Ir(III) complexes/BCP (10 nm)/Alq3 (25 nm)/Liq (2 nm)/Al (100 nm). The EL
characteristics of devices 1 and 2 are summarized in Table 2. Figure 3 shows the EL
spectra of the devices 1 and 2 exhibited orange–red emissions with maximum peaks of
578 nm and 604 nm, respectively. The CIE coordinates versus voltage of the devices
based on complexes (Actppy)2Ir(acac): (0.57, 0.42), and (Acpsf)2Ir(acac): (0.57, 0.37)
are presented in Fig. 4. This trend of the CIE coordinates of devices is well compatible to
the PL spectra of Ir(III) complexes (Actppy)2Ir(acac) and (Acpsf)2Ir(acac). Compared
to device 1 using (Actppy)2Ir(acac), the bathochromic shifted CIE coordinate of device 2
using (Acpsf)2Ir(acac) can be attributed mainly to the increasing of π-conjugation length
of dopant (Acpsf)2Ir(acac). The current density-voltage-luminance (J-V-L) characteristics
and luminous efficiencies of devices 1 and 2 are shown in Fig. 4(a) and (b). The luminance
Figure 2. Energy diagram of devices 1 and 2.

262
K. H. Lee et al.
Table 2. EL properties of devices
Lb at
15V
LEc, d
PEd CIEe
(lm W−1) (x, y)
EL
a
Dopant λmax
(%)
Von
Device
(nm) (cd m−2) (cd m−2
)
(cd A−1
)
1
(Actppy)2 578
Ir(acac)
4.0
3.6
41,800 6.88(4V )/5.70
17,600 6.06(5.5V )/5.83
2.7
(0.57, 0.42)
(8%)
2
(Acpsf)2
Ir(acac)
(8%)
604
2.5
(0.57, 0.37)
aTurn-on voltage at 1 cd m−2
bMaximum luminance.
.
cMaximum values. Values in parentheses are the voltages at which the maximum values were
obtained.
dAt 20 mA cm−2
.
e
´
Commission Internationale d’Enclairage (CIE) coordinates at a 10 V.
Figure 3. EL spectra of devices 1 and 2.
Figure 4. (a) J-V-L curve, and (b) luminous efficiencies of OLED the devices 1 and 2.

Acetylated Ir (III) Complexes for Orange–Red Electrophosphorescent
263
were 41,800 cd m−2 and 17,600 cd m−2 at 15 V, and the turn-on voltages were 4.0 V and
3.6 V for devices 1 using (Actppy)2Ir(acac) and 2 using (Acpsf)2Ir(acac), respectively.
The power efficiencies of devices 1 and 2 were 2.7 lm W−1 and 2.5 lm W−1 at 20 mA
cm−2, and the luminous efficiencies of devices 1 and 2 were 5.70 cd A−1 and 5.83 cd A−1
at 20 mAcm−2
.
Interestingly, compared with device 1 using (Actppy)2Ir(acac), device 2 using
(Acpsf)2Ir(acac) exhibits, similarly in EL performance, even if (Acpsf)2Ir(acac) (ꢀ =
0.19) has quantum yields lower than that of (Actppy)2Ir(acac) (ꢀ = 0.42). This ob-
servation imply that spirobifluorene group of iridium complex (Acpsf)2Ir(acac) leads
to the suitable energy transfer from host material in device, which improves the EL
efficiencies of device using (Acpsf)2Ir(acac). Although color purity of a red OLED
using (Actppy)2Ir(acac) and (Acpsf)2Ir(acac) are inadequate for full-color displays,
(Actppy)2Ir(acac) and (Acpsf)2Ir(acac) could be applied to fabricate the efficient white
organic light-emitting diodes (WOLEDs) with a combination of suitable sky blue emitters
[14].
4. Conclusion
A series of orange–red phosphorescent Ir(III) complexes based on acetylated pheyl pyridine
derivatives was synthesized by cyclometallation of the ligands with IrCl3 and subsequent
reaction with acetylacetone. An OLED using the orange–red dopant (Actppy)2Ir(acac)
with CBP as the host exhibited the maximum luminance of 41800 cd m−2 at 15 V. The
luminous and power efficiencies were 5.70 cd A−1 and 2.7 lm W−1 at 20 mA cm−2
,
respectively. The CIEx,y-coordinates of (0.57, 0.42) at 10 V. Also, an orange–red OLED
using dopant (Acpsf)2Ir(acac) with the CIE coordinate (0.57, 0.37) exhibited the maximum
luminance of 17,600 cd m−2 at 9 V. The luminous and power efficiencies were 5.80 cd A−1
and 2.5 lm W−1 at 20 mA cm−2, respectively. These results indicated that the electrical
properties of the OLED devices were very sensitive to the structure of dopant in the emitting
layer.
Acknowledgments
This research was supported by Basic Science Research Program through the National
Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and
Technology (20100007370).
References
[1] Baldo, M. A., O’Brian, D. F., You, Y., Shoustikov, A., Sibley, S., Thompson, M. E., & Forrest,
S. R. (1998). Nature, 395, 151.
[2] Huang, W. S., Lin, C. W., Lin, J. T., Huang, J. S., Chu, C. W., Wu, Y. H., & Lin, H. C. (2009).
Org. Electron., 10, 594.
[3] Rayabarapu, D. K., Paulose, B. M. J. S., Duan, J. P., & Cheng, C. H. (2005). Adv. Mater., 17,
349.
[4] Lee, K. H., Seo, J. H., Kim, Y. K., & Yoon, S. S. (2009). J. Nanosci. Nanotechnol., 9, 7099.
[5] Lee, K. H., Lee, W.-C., Kim, M. K., Seo, J. H., Kim, Y. K., & Yoon, S. S. (2010). Thin Solid
Films, 518, 3972.
[6] Kim, J. H., Nam, E. J., Hong, S. Y., Kim, B. O., Kim, S. M., Yoon, S. S., Suh, J. H., Ha, Y., &
Kim, Y. K. (2004). Mat. Sci. Eng. C, 24, 167.
[7] Lo, K. K.-W., Chung, C.-K., & Zhu, N. (2005). Chem. Eur. J., 9, 475.

264
K. H. Lee et al.
[8] Lee, K. H., Kim, J. H., Seo, J. H., Kim, Y. K., & Yoon, S. S. (2010). J. Nanosci. Nanotechnol.,
10, 3193.
[9] Lee, K. H., Kang, H. J., Kim, S. O., Lee, S. J., Seo, J. H., Kim, Y. K., & Yoon, S. S. (2010).
Mol. Cryst. LIQ. Ctyst., 530, 30.
[10] Lee, K. H., Park, J. K., You, J. N., Seo, J. H., Kim, Y. K., & Yoon, S. S. (2009). IMID DIGEST,
762–766.
[11] Lee, K. H., Kang, H. J., Park, J. K., Seo, J. H., Kim, Y. K., & Yoon, S. S. (2010). Thin Solid
Films, 518, 6188.
[12] King, K. A., Spellane, P. J., & Watts, R.-J. (1985). J. Am. Chem. Soc., 107, 1431.
[13] Xie, H. Z., Liu, M. W., Wang, O. Y., Zhang, X. H., Lee, C. S., Hung, L. S., Lee, S. T., Teng, P.
F., Kwong, H. L., Zheng, H., & Che, C. M. (2001). Adv. Mater., 13, 1245.
[14] Lee, K. H., Kang, L. K., Lee, J. Y., Kang, S., Jeon, S. O., Yook, K. S., Lee, J. Y., & Yoon, S. S.
(2010). Adv. Func. Mater., 20, 1345.