Red phosphorescent iridium(III) complexes containing 5-benzoyl-2- phenylpyridine derived ligands with electron-donating/-withdrawing moieties for organic light-emitting diodes

Article abstract of DOI:10.1080/15421406.2010.495891

A series of red-phosphorescent Ir(III) complexes based on 5-benzoyl-2-phenylpyridine derivatives were synthesized. To explore their electroluminescent properties, multilayered OLEDs were fabricated the device structure of ITO/2-TNATA/NPB/Ir(III) complexes

Full text of DOI:10.1080/15421406.2010.495891

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Red Phosphorescent Iridium(III)
Complexes Containing 5-Benzoyl-2-
phenylpyridine Derived Ligands with
Electron-Donating/-Withdrawing Moieties
for Organic Light-Emitting Diodes
Kum Hee Lee a , Hyun Ju Kang a , Seul Ong Kim a , Suk Jae Lee b , Ji
Hyun Seo b , Young Kwan Kim b & Seung Soo Yoon a
a Department of Chemistry , Sungkyunkwan University , Suwon,
Gyeonggi-do, Korea
b Department of Information Display , Hongik University , Seoul,
Korea
Published online: 14 Nov 2010.
To cite this article: Kum Hee Lee , Hyun Ju Kang , Seul Ong Kim , Suk Jae Lee , Ji Hyun Seo , Young
Kwan Kim & Seung Soo Yoon (2010) Red Phosphorescent Iridium(III) Complexes Containing 5-Benzoyl-2-
phenylpyridine Derived Ligands with Electron-Donating/-Withdrawing Moieties for Organic Light-
Emitting Diodes, Molecular Crystals and Liquid Crystals, 530:1, 30/[186]-39/[195]
To link to this article: http://dx.doi.org/10.1080/15421406.2010.495891
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Mol. Cryst. Liq. Cryst., Vol. 530: pp. 30=[186]–39=[195], 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421406.2010.495891
Red Phosphorescent Iridium(III) Complexes
Containing 5-Benzoyl-2-phenylpyridine Derived
Ligands with Electron-Donating/-Withdrawing
Moieties for Organic Light-Emitting Diodes
KUM HEE LEE,1 HYUN JU KANG,1 SEUL ONG KIM,1
SUK JAE LEE,2 JI HYUN SEO,2 YOUNG KWAN KIM,2
AND SEUNG SOO YOON1
1Department of Chemistry, Sungkyunkwan University, Suwon,
Gyeonggi-do, Korea
2Department of Information Display, Hongik University, Seoul, Korea
A series of red-phosphorescent Ir(III) complexes based on 5-benzoyl-2-phenylpyridine
derivatives were synthesized. To explore their electroluminescent properties, multi-
layered OLEDs were fabricated the device structure of ITO=2-TNATA=NPB=Ir(III)
complexes (8%) doped in CBP=BCP=Alq3=Liq=Al. The device 2 displays the excel-
lent EL performances; its LE, PE, and EQE reach 30.4 cd=A, 10.1 lm=W, and 8.23%
at 20mA=cm2, respectively. The maximum electroluminescence spectrum peak was
571 nm, with the CIE coordinates of (0.486, 0.512) at 12.0V, and the device also
showed a stable color chromaticity with various voltages.
Keywords 5-Benzoyl-2-phenylpyridine ligand; electroluminescence; iridium
complex; red phosphorescence
Introduction
Iridium(III) organometallic complexes are explored for a multitude of interest in a
variety of photonic applications. Especially, the charged compounds have been
intensely studied for use as dopants in light-emitting electrochemical cells (LEECs)
[1,2] and the neutral ones in organic=organometallic light-emitting diodes (OLEDs)
[1–3]. In contrast with the fluorescent emission, the electrophosphorescence is easily
harnessed from both singlet and triplet excited states. Thus, their internal quantum
efficiency can reach a theoretical level of unity rather than the 25% inherent upper
limit imposed by the formation of singlet excitons for the respective fluorescent
materials [4]. Fortunately, this upper limit can be overcome by using phosphorescent
complexes as a dopant in OLEDs to harvest both singlet and triplet excitons. This
Address correspondence to Prof. Seung Soo Yoon, Department of Chemistry,
Sungkyunkwan University, Cheoncheon-dong, Jangan-gu, Suwon, Gyeonggi-do, 440-746
Korea (ROK). Tel.: (þ82)31-290-7071; Fax: (þ82)31-290-5971; E-mail: ssyoon@skku.edu
30=[186]

Red Phosphorescent Iridium(III) Complexes
31=[187]
phenomenon increases the internal quantum efficiency to that of almost 100% [5–7].
The highly efficient phosphorescence emission is attributed to the strong spin-orbit
coupling of the electronic state by the heavy-atom effect of the metal, which can
facilitate the singlet-triplet spin crossover [8,9].
In this paper, a series of red phosphorescent Ir(III) complexes based on benzoyl-
ated phenylpyridine ligands were synthesized and their electrophosphorescent
properties were investigated. The electron-donating group (-OMe) and electron-
withdrawing group (-CF3) were introduced to tune the band gap of iridium com-
plexes by the control HOMO or LUMO energy levels [10–14]. Multilayered organic
light-emitting diodes (OLEDs) were fabricated to explore their electroluminescent
properties by employing the complexes as dopant and revealed that these new Ir(III)
complexes (1–3) had highly efficient electroluminescent properties.
Experimental
General Procedure for the Synthesis of Ir(III) Complexes
L1 [15] (0.68 g, 2.2 mmol) was dissolved in 2-ethoxyethanol (9 mL) in a 30 mL
round-bottom flask. IrCl3 ꢀ 3H2O (0.30 g, 1.0 mmol) and water (3 mL) were then
added to the flask. The mixture was stirred under nitrogen at 120ꢁC for 24 h
and cooled to room temperature. The precipitate formed in the mixture was col-
lected and washed with methanol, hexane and dried in vacuum to give the corre-
sponding cyclometalated Ir(III)-l-chloro-bridge dimer. In a 20 mL flask, the dimer
complex, acetylacetone (1.5 mL, 1.5 mmol) and Na2CO3 (0.32 g, 3.0 mmol) were
mixed with 2-ethoxyethanol (10 mL) and mixture was heated at 100ꢁC for 6 h.
After cooling to room temperature, the precipitation solid was collected by fil-
tration, washed with ethanol and hexane. The residue was dissolved in dichloro-
methane, and then the solid was filtered off. The solution was concentrated in
vacuo and the residue was purified on a silica gel column suing dichloromethane
and hexane as eluent. The product was obtained after recrystallization from
dichloromethane=ethanol. Similar procedures were also employed for the synthesis
of other iridium complexes. The yields and spectral data of all iridium complexes
are as follows.
1
Compound 1 (Bzppy)2Ir(acac). The yield of 39%. H-NMR (300 MHz, CDCl3): d
8.89 (d, J ¼ 1.2 Hz, 2H), 8.30 (dd, J ¼ 1.8 Hz, 8.4 Hz, 2H), 7.98 (d, J ¼ 8.7 Hz, 2H),
7.83–7.80 (m, 4H), 7.65–7.58 (m, 4H) 7.53–7.48 (m, 4H), 6.86 (td, J ¼ 1.2 Hz,
7.5 Hz, 2H), 6.74 (td, J ¼ 1.5 Hz, 7.5 Hz, 2H), 6.30 (dd, J ¼ 1.2 Hz, 7.8 Hz, 2H),
5.29 (s, 1H), 1.55 (s, 6H); 13C-NMR (125 MHz, CDCl3): d 192.9, 185.2, 172.5,
151.0, 150.6, 143.6, 138.8, 137.2, 133.7, 133.0, 130.8, 130.6, 130.1, 128.9, 125.9,
121.5, 118.4, 100.9, 28.8; IR (KBr): 3054, 2999, 1659, 1597, 1577, 1515, 1434,
1399, 1315, 1284, 1265, 947, 739, 712, 661 cmꢂ1; FAB-MS (m=z): 808 [Mþ].
HRMS-FABþ calcd for C41H31IrN2O4 808.1913: found, 808.1910.
Compound 2 (Bzppy-OMe)2Ir(acac). The yield of 65%. 1H-NMR (300 MHz,
CDCl3): d ppm 8.80 (d, J ¼ 2.0 Hz, 2H), 8.25 (dd, J ¼ 2.0, 8.6 Hz, 2H), 7.84 (d,
J ¼ 8.7, 2H), 7.79 (d, J ¼ 7.7 Hz, 4H), 7.61 (d, J ¼ 8.7 Hz, 4H), 7.53–7.48 (m, 4H),
6.46 (dd, J ¼ 2.5 Hz, 8.6 Hz, 2H), 5.79 (d, J ¼ 2.5 Hz, 2H), 5.28 (s, 1H), 3.56 (s,
6H), 1.55 (s, 6H). 13C-NMR (125 MHz, CDCl3: d ppm 192.9, 185.1, 171.9, 161.4,
152.8, 151.2, 138.5, 137.3, 136.6, 132.9, 130.0, 129.2, 128.9, 127.7, 118.4, 117.6,

32=[188]
K. H. Lee et al.
107.6, 101.0, 54.9, 28.9. FT-IR (KBr): n ¼ 3058, 2936, 2834, 1656, 1581, 1543, 1516,
1398, 1314, 1275, 1214, 1176, 1039, 729 cmꢂ1. FAB-MS (m=z): 868 [Mþ].
HRMS-FABþ calcd for C43H35IrN2O6: 868.2125. found: 868.2139.
Compound 3 (Bzppy-CF3)2Ir(acac). The yield of 50%. 1H-NMR (300 MHz,
CDCl3): d ppm 8.85 (d, J ¼ 1.9 Hz, 2H), 8.36 (dd, J ¼ 2.0, 8.4 Hz, 2H), 8.08 (d,
J ¼ 8.6 Hz, 2H), 7.82–7.79 (m, 4H), 7.74 (d, J ¼ 8.1 Hz, 2H), 7.63–7.60 (m, 2H),
7.54–7.49 (m, 4H), 7.14 (d, J ¼ 7.7 Hz, 2H), 6.50 (s, 2H), 5.23 (s, 1H), 1.53 (s, 6H).
13C-NMR (125 MHz, CDCl3): d ppm 192.4, 185.5, 170.7, 150.7, 149.4, 147.2,
139.4, 136.7, 133.4, 131.9, 131.3 (d, J ¼ 31.0 Hz), 130.1, 129.4 (d, J ¼ 3.7 Hz),
129.0, 125.5, 122.9, 119.4, 118.5 (d, J ¼ 3.7 Hz), 101.2, 28.7. FT-IR (KBr):
n ¼ 3064, 1663, 1600, 1564, 1516, 1397, 1379, 1315, 1284, 1265, 1163, 1122, 1073,
949, 910, 733, 714 cmꢂ1. FAB-MS (m=z): 944 [Mþ]. HRMS-FABþ calcd for
C43H29F6IrN2O4 944.1661: found, 944.1659.
Synthesis of L2. A mixture of 5-benzoly-2-bromopyridine (0.5 g, 1.91 mmol),
4-methoxyphenylbronic acid (0.32 g, 2.10 mmol), tetrakis(triphenylphosphine)-
palladium (0.09 g, 0.076 mmol), toluene (15 mL), ethanol (5 mL), and 2 M Na2CO3
(2.0 g, 19.1 mmol) was refluxed at 120ꢁC for 5 h. After reaction mixture was
cooled to 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 with 5:1 ethyl acetate:hexane as eluent. The product was
1
a white solid with an isolated yield of 98% (0.54 g). H-NMR (300 MHz, CDCl3):
d ppm 9.04 (d, J ¼ 2.2 Hz, 1H), 8.19 (dd, J ¼ 2.2, 8.3 Hz, 1H), 8.07 (d, J ¼ 8.9 Hz,
2H), 7.84 (t, J ¼ 8.3 Hz, 3H), 7.67–7.62 (m, 1H), 7.55–7.50 (m, 2H), 7.04 (d,
J ¼ 8.9 Hz, 2H), 3.89 (s, 3H). 13C-NMR (125 MHz, CDCl3): d ppm 195.01, 161.61,
160.25, 151.59, 138.42, 137.43, 133.15, 131.00, 130.76, 130.20, 129.05, 128.81,
119.26, 114.59, 55.68. FT-IR (KBr): n ¼ 2959, 1642, 1586, 1450, 1251, 1043, 1022,
836, 798, 767, 704 cmꢂ1. EI-MS (m=z): 289 [Mþ]. HRMS-EIþ calcd for
C19H15O2N 289.1103: found, 289.1103.
Synthesis of L3. The reaction described for the synthesis of L1 was carried out starting
with 5-benzoly-2-bromopyridine (0.4 g, 1.53 mmol) and 4-(trifluoromethyl)-
phenylbronic acid (0.35 g, 1.83 mmol). After purification by chromatography on
silica gel with 2:1 dichloromethane:hexane as eluent. The product was a white
1
solid with an isolated yield of 96% (0.48 g). H-NMR (300 MHz, CDCl3): d ppm
9.10 (dd, J ¼ 0.8, 2.2 Hz, 1H), 8.26 (dd, J ¼ 2.2, 8.3 Hz, 1H), 8.22 (d, J ¼ 8.2 Hz,
2H), 7.93 (dd, J ¼ 0.8, 8.3 Hz, 1H), 7.88–7.85 (m, 2H), 7.78 (d, J ¼ 8.2 Hz, 2H),
7.67–7.64 (m, 1H), 7.57–7.52 (m, 2H). 13C-NMR (125 MHz, CDCl3): d ppm 194.7,
158.9, 151.5, 141.7 (d, J ¼ 1.2 Hz), 138.7, 137.0, 133.5, 132.3, 131.9 (d, J ¼ 32.8 Hz),
130.3, 128.9, 127.9, 126.1 (q, J ¼ 3.7 Hz), 124.3 (d, J ¼ 273.Hz), 120.5. FT-IR
(KBr): n ¼ 1639, 1586, 1336, 1316, 1287, 1170, 1114, 1072, 1015, 841, 700 cmꢂ1
.
EI-MS (m=z): 327 [Mþ]. HRMS-EIþ calcd for C19H12 F3NO 327.0871: found,
327.0870.
Materials and Measurements
Phenylboronic acid, 2,5-Dibromopyridine, and N,N-Dimethylbenzamide were used
as received from Aldrich or TCI. 5-Benzoyl-2-bromopyridine [16] were synthesized

Red Phosphorescent Iridium(III) Complexes
33=[189]
as reported previously. The solvent were dried using standard procedures. All
reagents were used as received from commercial sources, unless otherwise stated.
All reactions were performed under N2 atmosphere. 1H- and 13C-NMR were
obtained using a Varian (Unity Inova 300Nb) or Varian (Unity Inova 500NB) spec-
trometer at 300 MHz and 500 MHz, respectively. FT-IR spectra were recorded using
a Thermo Nicolet Avatar 320 FT-IR spectrometer. Low- and high-resolution mass
spectra were recorded using a Jeol JMS-AX505WA spectrometer in FAB mode or
a Jeol JMS-600 spectrometer in EI mode.
Fabrication of OLEDs
The OLEDs using a red dopant in the emitting layers were fabricated by high vac-
uum (5.0 ꢃ 10ꢂ7 Torr) thermal deposition of organic materials onto the surface of
an indium thin oxide (ITO, 30 X=sq, 80 nm)-coated glass substrate. The ITO glass
was first cleaned with acetone, methanol, distillated water, and isopropyl alcohol.
The organic materials were deposited in the following sequence: 65 nm of 4,4-
bis[N-(1-(naphthyl)-N-phenyl-amino]biphenyl (NPB) was applied as a hole trans-
porting layer (HTL), followed by a 30 nm thick emissive layer of the IrIII complex
doped in 4,40,N,N0-dicarbazolebiphenyl (CBP). The doping rate of the phosphor, an
IrIII complex, was 8%. Next, 10 nm thick bathocuproine (BCP), 25 nm thick tris-
(8-hydroxyquinoline) aluminum (Alq3), and 2 nm thick lithium quinolate (Liq) were
deposited as a hole blocking layer, an electron transporting layer, and an electron
injection layer, respectively. The typical organic deposition rate was 0.1 nm=s.
Finally, 100 nm of Al was deposited as a cathode. The active areas of the OLEDs
were 0.09 cm2. After fabrication, the current-voltage (J-V) characteristics of the
OLEDs were measured with a source measure unit (Keithley 236). The luminance
and CIE chromaticity coordinates of the fabricated devices were measured using
a chromameter (MINOLTA CS-100A). Electroluminance was measured using
LS-50B. All measurements were performed in ambient conditions under DC
voltage bias.
Table 1. Optical properties of compounds
a
kmax FWHM
HOMO=
LUMOb [eV] Eg
a
Compound
kmax [nm]
[nm]
[nm]
Uc
(Bzppy)2Ir(acac)
289 (0.431), 619
330 (0.430),
522 (0.020)
54
ꢂ5.56=ꢂ3.34 2.22 0.29
ꢂ5.56=ꢂ3.23 2.37 0.55
ꢂ5.93=ꢂ3.59 2.34 0.57
(Bzppy-OMe)2Ir(acac) 280 (3.311), 599
345 (0.370),
494 (0.064)
82
76
(Bzppy-CF3)2Ir(acac)
280 (3.374), 600
506 (0.045)
aMaximum absorption or emission wavelength, measured in CH2Cl2 solution.
bObtained from AC-2 and UV-vis absorption measurements.
cUsing fac-[Ir(ppy)3] as a standard; kex ¼ 400 nm (Up ¼ 0.40 in CH2Cl2).

34=[190]
K. H. Lee et al.
Table 2. EL performance characteristic of the doped-devices
Lb
[cd=m2]
Jb [mA=
cm2]
LEc=d
[cd=A]
PEc=d
[lm=W]
QEd
[%]
CIEe
(x,y)
a
EL
Device
(compound)
Von
[V]
k
max
[nm]
1
2
3
7.0
606
5994
34700
15300
71
194
183
17.7=
15.3
40.5=
30.4
15.6=
12.6
11.8=
5.93
29.9=
10.1
9.23=
4.17
6.55
8.23
6.86
(0.610,
0.380)
(0.486,
0.512)
(0.570,
0.428)
6.0
6.3
571
593
aTurn-on voltage at 1 cd=m2.
bMaximum luminance and current density.
cMaximum value.
dAt 20 mA=cm2.
Commission Internationale d’Enclairage (CIE) coordinates at a 12.0 V.
e
´
Figure 1. (a) The absorption and (b) emission spectra of compounds 1–3.

Red Phosphorescent Iridium(III) Complexes
Results and Discussion
35=[191]
The optical properties of the red Ir(III) complexes ((Bzppy)2Ir(acac), (Bzppy-OMe)2-
Ir(acac), and (Bzppy-CF3)2Ir(acac)) were presented in Table 1. The HOMO energy
levels were measured by a low-energy photo-electron spectrometer (Riken-Keiki
AC-2). The HOMO=LUMO energy levels of (Bzppy)2Ir(acac), (Bzppy-OMe)2-
Ir(acac), and (Bzppy-CF3)2Ir(acac) were ꢂ5.56=ꢂ3.34, ꢂ5.56=ꢂ3.23, and ꢂ5.93=
ꢂ3.59, respectively. In case of 3 with electro-withdrawing group, the HOMO=
LUMO energy levels lower than those of 1.
The absorption and emission spectra of the Ir(III) complexes are shown in
Figure 1. In the absorption spectra, the intense absorption bands located at 250
1
and 400 nm were assigned the spin-allowed p-pꢄ transition of the cyclometalated
ligands in the Ir(III) complexes and the weak and broad absorption bands at
400–470 nm assigned to spin-allowed metal-to-ligand charge transfer 1MLCT transi-
tions [17,18]. In addition, all complexes showed weaker absorptions longer than
3
470 nm, attributed to spin forbidden MLCT=3p-pꢄ transitions from the spin-orbit
coupling of Ir(III), according to the band position and size [17]. This result revealed
the strong 3MLCT energy bands, with corresponding kmax values of 520 nm for 1–3.
The maximum emission wavelength (kmax) of 1–3 appeared at 619, 599, and 600 nm,
respectively. Compared to complex 1, complex 2 and 3 with methoxy (-OMe) group
or trifluoromethyl (-CF3) group in the phenyl unit was blue-shifted (ca. 19–20 nm) in
the PL spectrum. Their emission quantum yield were 0.29, 0.55, and 0.57, respect-
ively, as determined with Ir(ppy)3 (0.40), used as a reference [19]. Interestingly, com-
pounds 2 and 3, bearing, electro-donating (-OMe), and electro-withdrawing (-CF3)
groups in the phenyl unit, had quantum yields higher than that of Ir(ppy)3.
The electroluminescent properties of complexes 1–3 were explored by employing
the complexes as a dopant material in the fabrication of multilayered organic
light-emitting diodes (OLEDs). As shown in Figure 3, devices 1–3 emitted in the
red-orange region at 606, 571, and 593 nm, respectively. Compared to 1, both 2 with
electro-donating and 3 with electro-withdrawing groups were blue shifted similarly
Figure 2. Energy-level diagram of OLEDs.

36=[192]
K. H. Lee et al.
Figure 3. EL spectra of OLED devices 1–3.
Figure 4. (a) Current density and (b) Luminance vs. voltage curres of OLED the devices 1–3.

Red Phosphorescent Iridium(III) Complexes
37=[193]
in PL and EL spectra. However, complex 2 with electro-donating group in more blue
shifted than with electro-withdrawing group complex 3 in EL spectra. The CIE coor-
dinates of the devices were (0.610, 0.380), (0.486, 0.512), and (0.570, 0.428) at 12.0 V,
respectively. The current density-voltage-luminance (J-V-L) characteristics are
shown in Figure 4. The maximum luminances were 5994, 34700, and 15300 cd=m2
at 14.0 V, respectively, with device 2 showing the highest luminance at this operating
voltage. The luminous, and power efficiencies with respect to current density of the
devices 1–3 were shown in Figure 5. The luminous efficiencies were 11.0, 30.4, and
12.6 cd=A at 20 mA=cm2, respectively. The devices 1–3 showed power efficiencies
of 3.1, 10.1, and 4.17 lm=W at 20 mA=cm2, respectively. The EL performances were
sensitive to the dopant structure within the emitting layer. In particular, an OLED
device employing 2 as a dopant exhibited the best performance with a maximum
Figure 5. (a) Luminance, and (b) Power efficiency vs. current density curres of OLED
device 1–3.

38=[194]
K. H. Lee et al.
Scheme 1. Synthetic route to new iridium complexes 1–3. Conditions: a) Pd(PPh3)4, 2 M
Na2CO3, EtOH, toluene, 2 h, reflux. b) IrCl3 ꢀ H2O, H2O, 2-ethoxyethanol, 120ꢁC, 24 h. c)
2,4-pentanedione, Na2CO3, 2-ethoxyethanol, 100ꢁC, 6 h.
luminance of 34700 cd=m2 at 14.0 V and a luminous efficiency of 30.4 cd=A at
20 mA=cm2, respectively. In addition, this device showed orange emissions with
CIE coordinates of (0.486, 0.512) at 12.0 V. This study demonstrates that phosphor-
escent Ir(III) complexes with benzoylated phenylpyridine possess excellent properties
for red-orange emitting materials for OLEDs. Presumably, the electron-donating
(-OMe) groups on the phosphorescent Ir(III) complexes could contribute to
improved OLED efficiency by tuning the energy levels and charge transporting
properties of the emitting materials in the solid state device. This highly efficient
orange-red emitting iridium complexes could be used in the WOLED. Although
white emission in OLED can be achieved by a mixture of the three primary colors
(red, green, blue), highly efficient WOLEDs by fabrication devices composed of
two emitters with complementary colors such as sky-blue and orange-red have been
demonstrated [20,21]. Thereby, the highly efficient orange-red emitting material 2
can be combined with suitable sky-blue emitting materials to fabricate efficient
WOLEDs.
Conclusion
We have synthesized the efficient red phosphorescent Ir(III) complexes with benzoyl-
ated phenylpyridine ligands. The OLED device employing 2 as a dopant exhibits the
best performance with a maximum luminance of 34700 cd=m2 at 14.0 V, and a lumi-
nous efficiency of 30.40 cd=A at 20 mA=cm2, respectively. The device showed orange-
red emission with CIE coordinates of (0.486, 0.512) at 12.0 V. This study shows that
the slightly structural difference of ligands affect the electroluminescent properties of
the Ir(III) complexes in OLEDs.
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