Introduction of new ancillary ligands to the iridium complexes having 2,3-diphenylquinolinato ligands for OLED

Article abstract of DOI:10.1016/j.jorganchem.2009.06.003

We investigated the effect of an ancillary ligand (AL) on the emission color and luminous efficiencies of its complex, Ir(4-Me-2,3-dpq)₂(AL), where 4-Me-2,3-dpq represents 4-methyl-2,3-diphenylquinolinato ligand. We expected that ancillary ligand modification by introduction of the bulky substituent to the complexes might allow luminous efficiency increase by reduction of T-T annihilation. Furthermore, some ancillary ligands may contribute to fine-tuning of their complex emission colors by influencing the energy level of Ir d-orbitals upon the orbital mixing. As new ancillary ligands substituting for acac which is a typical AL in the iridium complexes, pyrazolone-based ligands, 4-R-5-methyl-2-phenyl-2,4-dihydro-pyrazol-3-one series (przl-R), were prepared, where R represents C₆H₅, C₆H₄CH₃ and C₆H₄Cl. These ligands were chelated to the iridium center to yield a new series of the iridium complexes, Ir(4-Me-2,3-dpq)₂(przl-R). The X-ray crystal structure of Ir(4-Me-2,3-dpq)₂(przl-C₆H₄Cl) was determined. The electrochemical and luminescence properties of the iridium complexes were investigated. The effect of the przl-substituents on the emission colors of the complexes was not significant. On the other hand, the luminous efficiencies of Ir(4-Me-2,3-dpq)₂(przl-C₆H₅) and Ir(4-Me-2,3-dpq)₂(przl-C₆H₄CH₃) were higher than that of Ir(4-Me-2,3-dpq)₂(acac).

Full text of DOI:10.1016/j.jorganchem.2009.06.003

Journal of Organometallic Chemistry 694 (2009) 3325–3330
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Introduction of new ancillary ligands to the iridium complexes having
2,3-diphenylquinolinato ligands for OLED
Hyun Shin Lee a, So Youn Ahn a, Hyun Sue Huh b, Yunkyoung Ha a,
*
a Department of Information Display, Hongik University, 72-1 Mapo-gu Sangsoo-dong, Seoul 121-791, Republic of Korea
b Department of Chemistry, Texas A&M University, College Station, TX 77843, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
We investigated the effect of an ancillary ligand (AL) on the emission color and luminous efficiencies of its
complex, Ir(4-Me-2,3-dpq)2(AL), where 4-Me-2,3-dpq represents 4-methyl-2,3-diphenylquinolinato
ligand. We expected that ancillary ligand modification by introduction of the bulky substituent to the
complexes might allow luminous efficiency increase by reduction of T–T annihilation. Furthermore, some
ancillary ligands may contribute to fine-tuning of their complex emission colors by influencing the
energy level of Ir d-orbitals upon the orbital mixing. As new ancillary ligands substituting for acac which
is a typical AL in the iridium complexes, pyrazolone-based ligands, 4-R-5-methyl-2-phenyl-2,4-dihydro-
pyrazol-3-one series (przl-R), were prepared, where R represents C6H5, C6H4CH3 and C6H4Cl. These
ligands were chelated to the iridium center to yield a new series of the iridium complexes, Ir(4-Me-
2,3-dpq)2(przl-R). The X-ray crystal structure of Ir(4-Me-2,3-dpq)2(przl-C6H4Cl) was determined. The
electrochemical and luminescence properties of the iridium complexes were investigated. The effect of
the przl-substituents on the emission colors of the complexes was not significant. On the other hand,
the luminous efficiencies of Ir(4-Me-2,3-dpq)2(przl-C6H5) and Ir(4-Me-2,3-dpq)2(przl-C6H4CH3) were
higher than that of Ir(4-Me-2,3-dpq)2(acac).
Received 21 April 2009
Received in revised form 28 May 2009
Accepted 2 June 2009
Available online 7 June 2009
Keywords:
Pyrazolone-based ancillary ligands
(przl-R series)
Iridium complex
2,3-Diphenylquinolinato ligand
Red phosphorescence
OLED
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
of T–T annihilation. We have focused on development of efficient
orange–red and blue–green dopant combination for white OLEDs,
The heavy transition metal complexes usually have high phos-
phorescence efficiencies with large population of the emitting trip-
let states by strong spin–orbit coupling induced by the metal
center. Among these complexes, the iridium(III) complexes were
particularly known to have high quantum efficiencies and rela-
tively short excited emissive states [1–6]. Thus, the phosphores-
cent iridium complexes have been used as dopants for organic
light-emitting devices (OLEDs) owing to utilization of both singlet
and triplet excited states for emission without their significant loss
by none-emissive pathways [7,8].
There are, however, some limitations in the application of phos-
phorescent materials to OLEDs. Compared with the short emission
life-time of the fluorescent materials, the relatively long phospho-
rescence life-time of the iridium complexes may cause triplet–trip-
let (T–T) annihilation at high currents, which can be one of the
factors in lowering the efficiency [9].
and recently reported that the iridium complexes, Ir(4-Me-2,3-
dpq)2(acac), exhibited orange–red electroluminescence (EL) at
603 nm with luminous efficiency of 8.10 cd/A [12]. Modification
of the ancillary ligand with the bulky ones in bis(2,3-diphenylqui-
nolinato)iridium complexes might also allow luminous efficiency
increase. In addition, fine-tuning of its emission color can be ex-
pected with the przl-based ligands in the iridium complex by their
orbital mixing with the d-orbitals of the iridium center.
Thus, 4-R-5-methyl-2-phenyl-2,4-dihydro-pyrazol-3-one (przl-
R) series were prepared as new ancillary ligand substitutes for acac
in the iridium complexes, and their iridium complexes, Ir(4-Me-
2,3-dpq)2(przl-R), were synthesized, where R represents C6H5,
C6H4CH3 and C6H4Cl at 4-position of the pyrazolone. The crystal
structure of Ir(4-Me-2,3-dpq)2(przl-C6H4Cl) was determined and
this is the first structure of the iridium complexes which contain
4-Me-2,3-dpq as a main ligand. We investigated the photoabsorp-
tion properties of the complexes with their UV–Vis spectra and
studied the electrochemical characteristics with their cyclic vol-
tammetric diagrams. We compared the photoluminescence (PL)
and electroluminescence (EL) properties of Ir(4-Me-2,3-dpq)2(przl)
with those of the previously reported Ir(4-Me-2,3-dpq)2(acac) to
find how the przl ancillary ligands have an influence on the
luminous efficiency and emission wavelength of the iridium com-
plexes.
According to the literature [10,11], introduction of a new bulky
ancillary ligand, pyrazolonate, to red-emitting Eu(III) and Ir(III)
complexes resulted in improvement of their luminous efficiency.
The bulky environment of the emitting complexes might prevent
quenching between the excited states, contributing to reduction
* Corresponding author. Tel.: +82 2 320 1490; fax: +82 2 3142 0335.
E-mail address: ykha@hongik.ac.kr (Y. Ha).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.06.003

3326
H.S. Lee et al. / Journal of Organometallic Chemistry 694 (2009) 3325–3330
absorption with SADABS [15]. The calculations were carried out with
2. Experimental
SHELXTL programs [16]. The structures were solved by direct meth-
ods. The non-hydrogen atoms were refined anisotropically. All
the hydrogen atoms were generated in idealized positions and re-
fined using the riding model. Details on crystal data, intensity col-
lection, and refinement details are given in Ref. [17].
2.1. Synthesis and characterization
Iridium trichloride hydrate (IrCl3ꢀH2O) was purchased from
Strem Co. All other reagents were purchased from Aldrich Co.
and used without further purification. All reactions were carried
out under an argon atmosphere. Solvents were dried by standard
procedures. All column chromatography was performed on silica
gel (230-mesh, Merck Co.) column. 1H NMR spectra were recorded
with a Varian Mercury 300 MHz spectrometer at Sungkyunkwan
University. FAB-MS and elemental analysis were performed with
JMS-AX505WA and EA1110, respectively, at Seoul National Univer-
sity in Korea.
2.2. Optical and electrochemical measurements
UV–Vis absorption spectra were measured with a Hewlett Pack-
ard 8425A spectrometer. PL spectra were measured on a Perkin El-
mer LS 50B spectrometer. The UV–Vis and PL spectra of iridium
complexes in the solution state were measured in 10ꢁ5 M CH2Cl2
solution. Cyclic voltammograms were obtained at scan rate of
100 mV/s, and tetrabutylammonium hexafluorophosphate was
added as an electrolyte in CH2Cl2 and DMF solution.
2.1.1. Synthesis of ligands and iridium complexes
Prazolone ligands (przl-R series). 1-Phenyl-3-methyl-5-prazolone
(1.75 g, 10.0 mmol) in 40 ml of 1,4-dioxane were placed in a
100 ml flask, and calcium hydroxide (1.78 g) and barium hydroxide
(1.03 g) were added. The corresponding acyl chloride (benzoyl
chloride, 4-methylbenzoyl chloride or 4-chlorobenzoyl chloride
(11.6 mmol)) was dripped slowly to the solution mixture under
stirring. After the reflux for 24 h, the reaction mixture was poured
into a 70 ml solution of ice-cold hydrochloric acid [HCl:H2O = 3:7
(v/v)], and the resulting yellow precipitate was filtered and recrys-
tallized from acetone and water. Yield: 60%.
4-Me-2,3-dpq. 4-Me-2,3-dpq Ligand [12] were obtained accord-
ing to Friedlander reaction [13] with the corresponding precursors,
2-aminoacetophenone (1.35 g, 1.21 ml, 10.0 mmol) and deoxy-
benzoin (1.96 g, 10.0 mmol). Yield: 75%.
Ir(4-Me-2,3-dpq)2(acac). Ir(4-Me-2,3-dpq)2(acac) was prepared
according to the procedure previously reported [12]. Yield: 62%.
Anal. Calc. for C49H39IrN2O2: C, 66.72; H, 4.69; N, 3.18. Found: C,
65.54; H, 4.77; N, 2.90%.
2.3. Device fabrication and measurements
The OLEDs containing the iridium complexes as a red dopant in
emitting layers were fabricated. The ITO glass was cleaned with ace-
tone, methanol, distillated water and isopropyl alcohol. The organic
materials used as carrier transport, carrier injection and host mate-
rials were supplied by Gracel Display Incorporation in Korea. The
OLED was fabricated by high vacuum (5 ꢂ 10ꢁ7 torr) thermal depo-
sition of the organic materials onto the surface of an indium tin oxide
(ITO, 30
X/h, 80 nm) coated glass substrate. The organic materials
were deposited in the following sequence: 60 nm of 4,40,400-tris-
[2-naphthylphenylamino]triphenylamine (2-TNATA) and 20 nm of
4,40-bis[N-(naphthyl)-N-phenylamino]biphenyl (NPB) were applied
as a hole injection layer (HIL) and a hole transporting layer (HTL),
respectively, followed by a 30 nm thick emissive layer (EML) of the
iridium complex phosphor doped in 4,4,N,N0-dicarbazolebiphenyl
(CBP). The doping ratio of the phosphor was 10%. Bathocuproine
(BCP, 10 nm), tris-(8-hydroxyquinolinato)aluminum (Alq3, 20 nm)
and lithium quinolate (Liq, 2 nm) were deposited as an exciton
blocking layer, as an electron transporting layer (ETL) and as an elec-
tron injection layer (EIL), respectively. The typical organic deposi-
tion rate was 0.1 nm/s. Finally, 100 nm of Al was deposited as a
cathode. The active area of the OLEDs was 0.09 cm2. After the fabri-
cation, the current density–voltage (J–V) characteristics of the
OLEDs were measured with a source measure unit (Kiethley 236).
The luminance and CIE chromaticity coordinates of the fabricated
devices were measured using a chromameter (MINOLTA CS-100A).
All measurements were performed in ambient conditions under
DC voltage bias.
Ir(4-Me-2,3-dpq)2(przl-R). First, the cyclometallated Ir(III)
l-chloro-bridged dimer, (4-Me-2,3-dpq)2Ir(l-Cl)2Ir(4-Me-2,3-
dpq)2 (3.10 g, 1.90 mmol), was prepared according to Nonoyama
method [14]. Second, the resulting dimer and a przl-R (6.50 mmol)
were mixed with Na2CO3 (500 mg) in 2-ethoxyethanol (30 ml).
The mixture was refluxed for 2 h and the resulting red solid was
filtered after cooling. Ir(4-Me-2,3-dpq)2(przl-R) was purified by
chromatography on silica gel column with CH2Cl2 and by recrystal-
lization from CH2Cl2/hexane.
Ir(4-Me-2,3-dpq)2(przl-C6H5 ). A red powder (yield: 65%). MS
(FAB): m/z 1058 (M+) (4.69%); 781 (100%); 482 (7.77%); 296
1
0
(4.05%). H NMR(ppm): d 8.46–6.40 (aromatic H0s 36H); 1.21 (CH3 s
0
of przl, 3H); 2.53, 2.51 (CH3 s of 4-Me-2,3-dpq, 3H each).
Ir(4-Me-2,3-dpq)2(przl-C6H4CH3). A red powder (yield: 60%). MS
(FAB): m/z 1072 (M+) (9.70%); 781 (100%); 482 (6.28%); 296
(3.32%). 1H NMR(ppm): d 8.49–6.43 (aromatic H0s 35H); 1.28,
1.21 (CH3 s of przl, 3H each); 2.54, 2.53 (CH3 s of 4-Me-2,3-dpq,
3H each). Anal. Calc. for C62H47IrN4O2: C, 69.45; H, 4.42; N, 5.23.
Found: C, 68.72; H, 4.37; N, 5.07%.
3. Results and discussion
0
0
Pyrazolone series as ancillary ligands in the Ir complexes have
not only suitable triplet energy levels matching 5d orbital of Ir, but
also good carrier transporting properties. The energy level matching
may lead to better metal-to-ligand charge transfer (MLCT), resulting
in high luminous efficiency of the complex and improvement of car-
rier transport can lead to better performance of the device. Our ear-
lier results [18] indicated that ancillary ligand replacement of acac
with przl in the iridium complexes containing diphenylquinoxaline
(dpqx) ligands as main ligands could improve the emission effi-
ciency and result in fine-tuning of the emission wavelength. How-
ever, the iridium complexes containing dpqx as a main ligand
suffer from poor efficiency in spite of good chromacity. On the other
hand, we found that the device containing Ir(4-Me-2,3-dpq)2(acac)
showed better efficiency than that of Ir(dpqx)2(acac). Therefore,
the iridium complexes coordinated with 4-Me-dpq and przl-R li-
Ir(4-Me-2,3-dpq)2(przl-C6H4 Cl). A red powder (yield: 55%). MS
(FAB): m/z 1092 (M+) (12.42%); 781 (66.82%); 307 (23.75%); 289
(11.96%); 154 (100%); 136 (65.06%). 1H NMR(ppm): d 8.45–6.41
0
0
(aromatic H0s 35H); 1.25 (CH3 s of przl); 2.53, 2.51 (CH3 s of 4-
Me, 3H each). Anal. Calc. for C61H44ClIrN4O2: C, 67.05; H, 4.06; N,
5.13. Found: C, 67.23; H, 4.10; N, 5.03%.
2.1.2. X-ray crystal structure determination
The suitable single crystal of Ir(4-Me-2,3-dpq)2(przl-C6H4Cl)
was recrystallized from CH2Cl2–hexane. The X-ray data was col-
lected with a Bruker Smart APEX2 diffractometer equipped with
a Mo X-ray tube. Intensity data were empirically corrected for

H.S. Lee et al. / Journal of Organometallic Chemistry 694 (2009) 3325–3330
3327
a
b
O
reflux
+
N
- 2H O
2
O
NH2
CH3
R
CH3
N
N
Cl
Ca(OH)2/Ba(OH)2
N
N
+
R
-HCl
O
O
O
O
R =
Cl
,
,
c
R
Cl
Cl
O
przl-R
LH
Ir
(L)2
(L)
2
N
Ir
IrCl3nH2O
(L)
2Ir
O
N
reflux
LH =
R =
Cl
N
,
,
Fig. 1. Synthetic scheme of the ligands and the iridium complexes.
gands were synthesized and investigated as red phosphors in OLEDs
in this study.
structure with very similar Ir–N, Ir–C, and Ir–O bond lengths has
been reported with Ir(NAPQ)2(acac) showing the Ir–O bond lengths
of 2.175 Å where NAPQ represents 2-(naphtha-1-yl)-4-phenylquin-
The main ligand, 4-Me-2,3-dpq, which determines the emission
color of the complexes was synthesized according to Friedlander
reaction, as illustrated in Fig. 1a. A series of przl ancillary ligands,
4-R-5-methyl-2-phenyl-2,4-dihydro-pyrazol-3-one (R = C6H5, p-
C6H4CH3 and p-C6H4Cl) were prepared as shown in Fig. 1b. The R
groups were chosen to study the electronic effects of the R at a przl
ligand on the luminescence properties of their complexes. The syn-
thesis of iridium complexes Ir(4-Me-2,3-dpq)2(przl-R) involved
two steps, formation of the iridium dimer and the following coor-
dination of przl-R, as summarized in Fig. 1c. As a reference, Ir(4-
Me-2,3-dpq)2(acac) was also prepared, according to the procedure
reported previously [12]. The iridium complexes prepared herein
were characterized by 1H NMR, FAB-MS and elemental analysis.
We have obtained single crystal to elucidate the structure of the
iridium complexes containing 4-Me-2,3-dpq. Among them, the suit-
able single crystal of Ir(4-Me-2,3-dpq)2(przl-C6H4Cl) was recrystal-
lized from layer diffusion of CH2Cl2–hexane and structurally
characterized by X-ray diffraction. There are two crystallographi-
cally distinct molecules in an asymmetric unit, which are geometric
isomers. The structures of these two molecules are given in Figs. 2a
and 2b. Each displays two chelating 4-Me-2,3-dpq and one przl-
C6H4Cl ligands, with the difference of przl chelate disposition to
the iridium center. The iridium center is 6-coordinated with a dis-
torted octahedral [IrN2C2O2] core, retaining the cis-C,C- and trans-
N,N-chelate disposition. The Ir–N, Ir–O, and Ir–C bond lengths are
in the range of 2.060(5)–2.077(4), 2.175(4)–2.186(4), and
1.969(6)–2.009(5) Å, respectively. The Ir–O bonds in the complex
are especially longer than the mean value of 2.088 Å reported in
the Cambridge Crystallographic Database [19]. Such a distorted
Fig. 2a. ORTEP drawing of one molecule Ir(4-Me-2,3-dpq)2(przl-C6H4Cl) in an
asymmetric unit showing the atom-labeling scheme and 30% probability thermal
ellipsoids. Selected bond distances (Å) and bond angles (°): Ir1–N1 2.070(4), Ir1–N2
2.077(4), Ir1–O1 2.182(4), Ir1–O2 2.186(4), Ir1–C1 1.971(5), Ir1–C23 1.969(6); N1–
Ir1–C1 78.9(2), N1–Ir1–O1 81.7(2), N2–Ir1–C23 79.5(2), N2–Ir1–O2 100.1(2), N1–
Ir1–N2 176.9(2), O1–Ir1–C1 176.0(4), O2–Ir1–C23 176.3(2).

3328
H.S. Lee et al. / Journal of Organometallic Chemistry 694 (2009) 3325–3330
oline [20]. These observations support the large steric hindrance
caused by the bulky przl ligand as well as the main ligands.
The UV–Vis absorption spectra of the complexes in CH2Cl2 are
shown in Fig. 3. The strong absorption bands between 200 and
400 nm in the ultraviolet region are assigned to the spin-allowed
1p p* transition of the cyclometalated 4-Me-2,3-dpq ligand in
–
the complexes. The weak bands between 400 and 460 nm in the
visible region are assigned to the spin-allowed metal-to-ligand
charge transfer band (1MLCT), and the weaker absorption bands
at the longer wavelengths can be attributed to the spin-forbidden
3MLCT and spin–orbit coupling enhanced 3 * transition. The for-
p–p
mally spin-forbidden 3MLCT gains the intensity by mixing with the
higher-lying 1MLCT transition through the strong spin–orbit cou-
pling on the iridium center [21]. The absorption patterns of the
complexes prepared in this study are similar, suggesting that
change of the ancillary ligand does not make significant contribu-
tion to the absorption process of the complex.
We investigated the electrochemical properties of the com-
plexes, using cyclic voltammetry (CV). The HOMO and the LUMO
energies of Ir(4-Me-2,3-dpq)2(AL) where AL represents acac, przl-
C6H5, przl-C6H4CH3 or C6H4Cl were estimated. The oxidation which
indicates the HOMO of the complexes was reversible. The oxida-
tion potential of Ir(4-Me-2,3-dpq)2(przl-C6H4Cl) was 5.20 eV, the
highest among the complexes studied herein, but the overall
HOMO differences among the complexes were less than 0.1 eV
only. The reduction potentials (LUMO) of the complexes did not
Fig. 2b. ORTEP drawing of the other molecule Ir(4-Me-2,3-dpq)2(przl-C6H4Cl).
Selected bond distances (Å) and bond angles (°): Ir2–N5 2.063(5), Ir2–N6
2.060(5),Ir2–O3 2.182(4), Ir2–O4 2.175(4), Ir2–C62 1.979(6), Ir2–C84 2.009(5);
N5–Ir2–C62 79.2(2), N5–Ir2–O3 80.2(2), N6–Ir2–C84 79.1(2), N6–Ir2–O4 82.6(2),
N5–Ir2–N6 177.8(2), O3–Ir2–C84 176.3(4), O4–Ir2–C62 176.2(2).
1.0
0.8
0.6
0.4
0.2
0.0
Ir(4-Me-2,3-dpq)2(acac)
Ir(4-Me-2,3-dpq)2(przl-C6H5)
Ir(4-Me-2,3-dpq)2(przl-C6H4CH3)
Ir(4-Me-2,3-dpq)2(przl-C6H4Cl)
250
300
350
400
450
500
550
600
650
Wavelength(nm)
Fig. 3. UV–Vis absorption spectra of the iridium complexes.
Table 1
Physical parameters for the complexes.
Complex
kabs (nma)
kem (nma)
Eox (Vb)
Ered (Vb)
HOMO (eVc)
LUMO (eVc)
D
E (eVd)
Ir(4-Me-2,3-dpq)2(acac)
279, 431, 466
604
601
599
601
0.30
0.37
0.37
0.40
1.82
1.79
1.89
1.77
5.10
5.17
5.17
5.20
2.98
3.01
2.91
3.03
2.12
2.16
2.26
2.17
Ir(4-Me-2,3-dpq)2(przl-C6H5)
Ir(4-Me-2,3-dpq)2(przl-C6H4CH3)
Ir(4-Me-2,3-dpq)2(przl-C6H4-Cl)
281, 350, 431, 465
280, 351, 430, 465
278, 348, 430, 465
a
Measured in CH2Cl2 solution.
b
Scan rate: 100 mV/s, electrolyte: tetrabutylammonium hexafluorophosphate. The potentials are quoted against the internal ferrocene standard.
c
Deduced from the equation HOMO = 4.8 + Eox, LUMO = 4.8ꢁEred
.
d
Calculated from the equation
D
E = HOMOꢁLUMO [21].

H.S. Lee et al. / Journal of Organometallic Chemistry 694 (2009) 3325–3330
3329
show the reversible wave. Ir(4-Me-2,3-dpq)2(przl-C6H4CH3) and
Ir(4-Me-2,3-dpq)2(przl-C6H4Cl) had the lowest LUMO at 2.91 eV
and the highest LUMO at 3.03 eV, respectively. Again, the differ-
ences were not conspicuous change across the complexes. Thus,
it was concluded that there was no evident electronic effect of
the ancillary ligand substituents on the electrochemical properties
of the complexes in this study. The detailed CV data were summa-
The PL spectra of the iridium complexes in 10ꢁ5 M CH2Cl2 solu-
tion are shown in Fig. 4. The emission maxima for Ir(4-Me-2,3-
dpq)2(acac),
Ir(4-Me-2,3-dpq)2(przl-C6H5),
Ir(4-Me-2,3-
dpq)2(przl-C6H4CH3) and Ir(4-Me-2,3-dpq)2(przl-C6H4Cl) appeared
at 604, 601, 599 and 601 nm, respectively. The wavelengths of PL
peaks were not shifted significantly upon change of the ancillary
ligand in the complexes. Such similar PL data are consistent with
both the absorption spectral results. Therefore, the ancillary li-
gands, either acac or przl-R series, do not seem to change the band
gaps of their complexes significantly. It thereby indicates that the
main ligand, 4-Me-2,3-dpq, is the important factor in determining
the emission color of its complexes.
rized in Table 1 [22]. Resulting
D
Es (EoxꢁEred) of the complexes
were calculated to show subtle changes in the range of 2.12–
2.26 eV. Though these band gaps somewhat correlate with their
PL wavelengths, vide infra, it does not seem to have meaning within
the experimental errors of their CVs and PLs.
Ir(4-Me-2,3-dpq)2(acac): 604nm
Ir(4-Me-2,3-dpq)2(przl-C6H5): 601nm
Ir(4-Me-2,3-dpq)2(przl-C6H4CH3): 599nm
Ir(4-Me-2,3-dpq)2(przl-C6H4Cl): 601 nm
1.0
0.8
0.6
0.4
0.2
0.0
400
500
600
700
800
Wavelength(nm)
Fig. 4. PL spectra of the iridium complexes.
Ir(4-Me-2,3-dpq)2(acac): 603nm
Ir(4-Me-2,3-dpq)2(przl-C6H5): 604nm
Ir(4-Me-2,3-dpq)2(przl-C6H4CH3): 611nm
Ir(4-Me-2,3-dpq)2(przl-C6H4Cl): 607nm
1.0
0.8
0.6
0.4
0.2
0.0
400
500
600
700
800
Wavelength (nm)
Fig. 5. EL spectra of the iridium complexes.

3330
H.S. Lee et al. / Journal of Organometallic Chemistry 694 (2009) 3325–3330
Table 2
Characteristics of OLED devices of the iridium complexes.
Ir complex
EL kmax (nm)
Luminance (cd/m2)
Luminance efficiency (cd/A)
CIE
Ir(4-Me-2,3-dpq)2(acac)
603
604
611
607
12 800
11 920
6550
8.10
(0.644, 0.352)
(0.629, 0.370)
(0.635, 0.380)
(0.629, 0.367)
Ir(4-Me-2,3-dpq)2(przl-C6H5)
Ir(4-Me-2,3-dpq)2(przl-C6H4CH3)
Ir(4-Me-2,3-dpq)2(przl-C6H4Cl)
11.38
10.10
5.38
2980
The electroluminescence properties of the iridium complexes
were also investigated. The configuration of the EL device with
Ir(4-Me-2,3-dpq)2(przl-R) was ITO/2-TNATA/NPB/CBP: 10% dop-
ant/BCP/Alq3/Liq/Al. The EL of the complexes containing przl ligands
exhibited the red emission around 604–611 nm, similar to their PL
bands (Fig. 5). Similarity of PL and EL supports that the red EL is orig-
inated from the iridium complex dopant in the emitting layer of the
device. The detailed EL properties of the przl complexes were found
to be different from those of Ir(4-Me-2,3-dpq)2(acac). As shown in
Fig. 5, the FWHM (Full Width at Half Maximum) of the complex
containing przl was broader than that of the complex containing
acac, leading to the lower color purity toward red emission. The
Commission Internationale de L’Eclairage (CIE) coordinates of
Ir(4-Me-2,3-dpq)2(acac), Ir(4-Me-2,3-dpq)2(przl-C6H5), Ir(4-Me-
2,3-dpq)2(przl-C6H4CH3) and 4-Me-2,3-dpq)2(przl-C6H4Cl) were
(0.644, 0.352), (0.629, 0.370), (0.635, 0.380) and (0.629, 0.367),
respectively.
On the other hand, the luminous efficiencies were improved
upon change of an ancillary ligand to the bulky przl, as we suggested.
The luminous efficiency maxima of the device containing Ir(4-Me-
2,3-dpq)2(przl-C6H5) and Ir(4-Me-2,3-dpq)2(przl-C6H4CH3) were
11.38 and 10.10 cd/A, respectively, compared to 8.10 cd/A at the
device of Ir(4-Me-2,3-dpq)2(acac). In the case of Ir(4-Me-2,3-
dpq)2(przl-C6H4Cl), the efficiency was the lowest among the
complexes prepared herein. We attribute such low efficiency to
involvement of Cl substituent which might cause the excited state
of its complex to lose the energy by a non-radiative pathway. The
overall electroluminescence properties of the iridium complexes
were summarized in Table 2.
gands in the complex could contribute to reduction of T–T-annihi-
lation. This finding indicates that there is no visible charge transfer
between the ancillary ligand and the cyclometalating ligand, and
therefore, the ancillary ligands, acac and przls, did not change
the photophysical properties of their complexes significantly.
Acknowledgement
This work was supported by the Korea Research Foundation
(KRF-2008-531-C00036).
Appendix A. Supplementary material
CCDC 723680 contains the supplementary crystallographic data
for compound Ir(4-Me-2,3-dpq)2(przl-C6H4Cl). These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary
data associated with this article can be found, in the online version,
at doi:10.1016/j.jorganchem.2009.06.003.
References
[1] B.A. Baldo, D.F. O’Brian, Y. You, A. Shoustikov, S. Sibley, M.E. Thompson, S.R.
Forrest, Nature 395 (1998) 151.
[2] B.A. Baldo, S. Lamansky, P.E. Burrows, M.E. Thompson, S.R. Forrest, Appl. Phys.
Lett. 75 (1999) 4.
[3] M.A. Baldo, M.E. Thompson, S.R. Forrest, Nature 403 (2000) 750.
[4] M. Ikai, S. Tokito, Y. Sakamoto, T. Suzuki, Y. Taga, Appl. Phys. Lett. 79 (2001)
156.
[5] F.C. Chen, Y. Yang, M.E. Thompson, J. Kido, Appl. Phys. Lett. 80 (2002)
2308.
We also prepared Ir(4-Me-2,3-dpq)2(przl-C6N4NMe2) and tried
to compare the EL efficiencies. However, the attempts to fabricate
the EL device containing Ir(4-Me-2,3-dpq)2(przl-C6H4NMe2) as a
dopant were failed. The failure of device fabrication could be
attributed to the relatively large molecular weight of this complex.
It has more than 1000 g/mol of the molecular weights and thus
may be too non-volatile to be sublimed.
[6] Y. You, K.S. Kim, T.K. Ahn, D. Kim, S.Y. Park, J. Phys. Chem. C 111 (2007)
4052.
[7] A. Kohler, J.S. Wilson, R.H. Friend, Adv. Mater. 14 (2002) 701.
[8] A.B. Tamayo, B.D. Alleyne, P.I. Djurovich, S. Lamansky, I. Tsyba, N.N. Ho, R. Bau,
M.E. Thompson, J. Am. Chem. Soc. 125 (2003) 7377.
[9] S.-J. Yeh, M.-F. Wu, C.-T. Chen, Y.-H. Song, Y. Chi, M.-h. Ho, S.-F. Hsu, C.H. Chen,
Adv. Mater. 17 (2005) 285.
[10] M. Shi, F. Li, T. Yi, D. Zhang, H. Hu, C. Huang, Inorg. Chem. 44 (2005) 8929.
[11] Q. ZhaO, C.Y. Jiang, M. Shi, F.Y. Li, T. Yi, Y. Cao, C.H. Huang, Organometallics 259
(2006) 3631.
[12] G.Y. Park, Y. Ha, Synth. Met. 158 (2008) 120.
[13] E.C. Riesgo, X. Jin, R.P. Thummel, J. Org. Chem. 61 (1966) 3017.
[14] M. Nonoyama, J. Oranomet. Chem. 86 (1975) 263.
4. Conclusion
We studied the effect of the new ancillary pyrazoline-based li-
gands on the photophysical, electrochemical, electroluminescent
properties of their iridium complexes. We focused on the question
whether the iridium complex involving a bulky przl ligand might
lead to affect the emission color and device performance. The PL
peaks of the complexes synthesized in this study appeared around
600 nm, regardless of the ancillary ligand. The UV and electro-
chemical patterns of the complexes of acac and of pyrazolone-
based ligands were also similar, indicating that the new ancillary
ligands and their substituents did not make significant change in
the absorption and emission process of their complexes. The de-
vices of Ir(4-Me-2,3-dpq)2(przl-C6H5) and Ir(4-Me-2,3-dpq)2(przl-
C6H4CH3) exhibited the better electroluminescent performance
with a luminous efficiency of 11.38 and 10.10 cd/A than that of
Ir(4-Me-2,3-dpq)2(acac), supporting that the bulky ancillary li-
[15] G.M. Sheldrick, SADABS
Göttingen, 1996.
, Program for Absorption Correction, University of
[16] Bruker, SHELXTL, Structure Determination Software Programs, Bruker Analytical
X-ray Instruments Inc., Madison, Wisconsin, USA, 1997.
[17] Crystallographic data for Ir(4-Me-2,3-dpq)2(przl-C6H4Cl): C61H44ClIrN4O2,
FW = 1092.65, T = 296(2) K, orthorhombic, Pca21, purple, 0.28 ꢂ 0.24 ꢂ
0.20 mm, block, a = 29.8646(8) Å, b = 14.8162(4) Å, c = 24.1281(6) Å, V =
10676.2(5) Å3, Z = 8, Dcal = 1.360 g cmꢁ3
,
l
= 2.596 mmꢁ1
,
F(0 0 0) = 4384,
Tmin = 0.5301, Tmax = 0.6247, No. of reflections measured = 113 762, No. of
unique reflections = 16 489, No. of reflections with I > 2 (I) = 14 056, No. of
r
parameters refined = 1202, GOF = 1.055, R1 = 0.0311, wR2 = 0.0685.
[18] H.S. Lee, J.H. Seo, M.K. Choi, Y.K. Kim, Y. Ha, J. Phys. Chem. Solids 69 (2008)
1305.
[19] F.H. Allen,J.E. Davies, J.J. Galloy, O. Johnson, O. Kennard, C.F. Macrae, E.M. Mitchell,
G.F. Mitchell, J.M. Smith, D.G. Watson, J. Chem. Inf. Comput. Sci. 31 (1991) 187.
[20] J. Ding, J. Gao, Q. Fu, Y. Cheng, D. Ma, L. Wang, Synth. Met. 155 (2005) 539.
[21] B. Schmid, F.O. Garces, J.R. Watts, Inorg. Chem. 33 (1994) 33.
[22] K.R.J. Thomas, M. Velusamy, J.T. Lin, C. Chien, Y. Tao, Y.S. Wen, Y. Hu, P. Chou,
Inorg. Chem. 44 (2005) 5677.