Luminescence comparison of iridium(III) complexes containing symmetric vs. asymmetric quinolinate ligands

Article abstract of DOI:10.1080/15421406.2011.563645

We have focused our research on development of the iridium complexes, especially with respect to blue and red emission for OLED. Previously, we prepared bis-[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(acetylacetonate) [Ir(2,3-dpqx-F₂)₂

Full text of DOI:10.1080/15421406.2011.563645

Mol. Cryst. Liq. Cryst., Vol. 538: pp. 67–74, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421406.2011.563645
Luminescence Comparison of Iridium(III)
Complexes Containing Symmetric vs. Asymmetric
Quinolinate Ligands
HYE RIM PARK AND YUNKYOUNG HA
Department of Information Display Engineering, Hongik University,
Seoul, Korea
We have focused our research on development of the iridium complexes, especially
with respect to blue and red emission for OLED. Previously, we prepared bis-[2,3-
bis(4-fluorophenyl)quinoxalinato]iridium(acetylacetonate)
acac)] where the main ligand, 2,3-dpqx-F , itself is symmetric. The luminescence
spectra of Ir(2,3-dpqx-F ) (acac) revealed red emission with the peak at approxi-
2 2
[Ir(2,3-dpqx-F ) -
(
2
2
2
mately 640 nm with the CIE coordinates of (0.684, 0.311). Recently, Johannes et al.
reported the iridium complexes containing fluorophenylmethylquinoxaline deriva-
tives (fpmqx) as asymmetric main ligands and investigated their luminescence
properties. Herein, we synthesized symmetric Ir(2,3-dpqx-F ) (AL) and asymmet-
2
2
ric Ir(fpmqx)
accetylacetonate (acac) and 2-(5-(trifluoromethyl)-2H-1,2,4-triazol-3-yl)pyridine
trzl-CH ) were introduced as ancillary ligands(AL). We investigated the emission
2
(AL) and compared their emission patterns and characteristics where
(
3
properties of Ir(2,3-dpqx-F ) (AL) and Ir(fpmqx) (AL), and found that asym-
2
2
2
metric Ir(fpmqx)
symmetric Ir(2,3-dpqx-F
2
(AL) showed blue-shifted photoluminescence compared with
(AL). We also estimated the energy gap between the
2 2
)
HOMO and LUMO of the complexes with electrochemical properties measured
by cyclic voltammetry.
Keywords Asymmetric vs. symmetric ligand; iridium complex; organic
light-emitting diode; red phosphorescence
Introduction
As phosphorescent materials of organic light emitting devices (OLEDs), iridium(III)
complexes are attracting much attention because of their high efficiency and chemical
stability. These complexes are easy to process as small molecules in ultra-high vacuum
processes, which make them good candidates for OLED applications [1–5]. In
addition, they have ability to harvest both singlet and triplet excitons, achieving
nearly 100% internal quantum efficiency [6–8]. Such high efficiency is due to strong
spin orbit coupling by the heavy metal iridium center, which allows fast energy
transfer from singlet to triplet states and also removes the forbidden character of
radiative transition to ground states, leading to efficient phosphorescence for these
Address correspondence to Yunkyoung Ha, Department of Information Display Engin-
eering, Hongik University, Sangsu-dong, Mapo-gu, Seoul 121-791, Korea (ROK). Tel.:
(
þ82)2-320-1490; Fax: (þ82)2-3142-0335; E-mail: ykha@hongik.ac.kr
67

6
8
H. R. Park and Y. Ha
complexes [9]. However, it is difficult to the goal of achieving three primary colors
with high color purity. Compared to green phosphorescent iridium complexes, devel-
opment of the orange- and red-emitting complexes presents a particular challenge
because their luminescence quantum yield tends to be low due to the smaller energy
gap [10,11].
Recently, Johannes et al. reported the iridium complexes containing fluoro-
phenylmethylquinoxaline derivatives (fpmqx) as asymmetric main ligands and inves-
tigated their luminescence properties [2]. In this study, we prepared the iridium
complexes, Ir(2,3-dpqx-F ) (AL) and Ir(fpmqx) (AL), where symmetric 2,3-
2
2
2
dpqx-F , asymmetric fpmqx, and AL (acac and trzl-CH ) represent the ligands of
2
3
2
,3-bis(4-fluorophenyl)quinoxaline, 2-(4-fluorophenyl)-3-methylquinoxaline, acety-
lacetonate and 2-(5-methyl-4H-1,2,4-triazol-3-yl-pyridine, respectively. We com-
pared the effect of the symmetric and asymmetric C^N ligands on luminescence of
their complexes. Electrochemical characterization of iridium complexes was also per-
formed to investigate their luminescence color and energy gap between HOMO and
LUMO. It was found that the asymmetric complexes exhibit the shorter emission
wavelengths than the symmetric ones. Furthermore, introduction of two different
ancillary ligands to the iridium complexes revealed their effects to photoabsorption,
photoemission and electrochemical properties and thereby the emission color tuning
of the complexes.
Experimental
All reagents were purchased from Aldrich Co. and Strem 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 with the use of silica gel (230-mesh, Merck).
Synthesis of Ligands
2
according to Friedlander Reaction [12] with the corresponding precursor, 2-phenyl-
,3-dpqx-F . The symmetric 2,3-bis-(4-fluorophenyl)quinoxaline was obtained
2
0
acetophenone (1.96 g, 10.0 mmol) and 4,4 -difluorobenzil (2.46 g, 10.0 mmol).
Yield: 70%.
Fpmqx. The asymmetric 2-(4-fluorophenyl)-3-methylquinoxaline was obtained
from the reaction of 1-(4-fluorophenyl)propane-1,2-dione and 1,2-diaminobenzene.
The synthesis of 1-(4-Fluorophenyl)propane-1,2-dione (fppd) was as follows: First,
to a 100 ml round bottom flask containing 4-fluoropropiophenone (4.0 g, 26.3 mmol)
and aluminum chloride (52 mg, 0.4 mmol) in 20 ml dry diethyl ether, bromine
ꢀ
(3.2 ml, 63.1 mmol) was added dropwise at 0 C over 10 min. The dark red suspension
was heated to reflux with stirring for 21 h. After cooling to room temperature, the
solvent was removed under reduced pressure. The resulting oil was mixed with a sol-
ution of sodium methoxide (3.83 g, 71.0 mmol) in 30 ml of dry methanol, and the
ꢀ
mixture was stirred at 0 C over 15 min. Followed by addition of 8.2 ml of concen-
trated hydrochloric acid, the overall mixture was warmed slowly to room tempera-
ture and stirred for 1.5 h. The precipitate was filtered off and washed repeatedly with
diethyl ether. The combined filtrate was concentrated and extracted with chloro-
form. The organic phases were dried with anhydrous magnesium sulfate and the sol-
vent was removed under reduced pressure. The crude product was purified by flash

Luminescence Comparison of Iridium(III) Complexes
69
chromatography to obtain yellow oil. The resulting 1-(4-fluorophenyl)propane-
,2-dione (2.44 g, 14.7 mol) was reacted with 1,2-diaminobenzene (1.91 g, 17.6 mol)
1
and iodine (0.37 g, 1.5 mmol) in 15 ml of acetonitrile. The reaction mixture was stir-
red for 10 min at room temperature. The solvent was removed under reduced press-
ure, and the crude product was purified by flash chromatography to yield a yellow
solid (88%).
Synthesis of Iridium Complexes
The iridium complexes with an ancillary ligand, acetylacetonate (acac) or
2
-(5-methyl-4H-1,2,4-triazol-3-yl)pyridine (trzl-CH ) was prepared from the reaction
3
of the cyclometalated iridium l-chloro-bridged dimer, (C^N) Ir(l-Cl) Ir(C^N) ,
2
2
2
with the corresponding ancillary ligand, according to the method reported by Non-
oyama with slight modification [13].
Ir(2,3-dpqx-F ) (AL) (AL = acac, trzl-CH3). (2,3-dpqx-F ) Ir(l-Cl) Ir(2,3-
2
2
2 2
2
dpqx-F ) (1.73 g, 1.9 mmol) and an excess of the ancillary ligands were mixed with
Na CO (500 mg) in 2-ethoxyethanol (30 mL). The mixture was refluxed for 2 h. The
2
2
2
3
solution was cooled to room temperature and the red solid was filtered. The obtained
product was chromatographed on silica gel column with dichloromethane and
purified by recrystallization.
Ir(2,3-dpqx-F ) (acac). Yield: 69%. FAB-MS: calculated 926; found 926.
2
2
1
HNMR(CDCl , 400 MHz) : d 8.20–6.04 (m, 22H, aromatic C-H); 5.3, 4.7 (S, 2H,
3
acac-CH3); 1.6 (S, 1H, acac-H). Anal. Calc. for C H F IrN O : C, 58.37; H,
3
4
5
29
4
4
2
.16; N, 6.05. Found: C, 56.97; H, 3.18; N, 5.87%.
Ir(2,3-dpqx-F ) (trzl-CH ). Yield: 62%. FAB-MS: calculated 986; found 986.
2
2
3
1
HNMR(CDCl , 400 MHz) : d 8.09–6.36 (m, 24H, aromatic C-H); 1.20 (S, trzl-CH ,
3
3
3
H). Anal. Calc. for C H F IrN : C, 58.47; H, 2.96; N, 11.36. Found: C, 56.76; H,
48 29 4 8
3
.18; N, 10.15%.
Ir(fpmqx) (AL) (AL = acac, trzl-CH3). (fpmqx) Ir(l-Cl) Ir(fpmqx) (0.80 g,
2
2
2
2
0
1
.57 mmol), an excess of the ancillary ligands and potassium tert-butoxide (0.16 g,
.42 mmol) in 20 ml of dry dichloromethane and 6 ml of dry ethanol was stirred at
room temperature for 20 h. The solvent was removed under reduced pressure and
the crude product purified by flash chromatography [2].
Ir(fpmqx) (acac). Yield: 78%. FAB-MS: calculated 766; found 766.
2
1
HNMR(CDCl , 400 MHz): d 8.24–6.14 (m, 14H, aromatic C-H); 4.55 (s, 1H,
3
CH3); 3.32, 3.14 (s, 2H, acac-CH3); 1.47 (d,1H, acac-H). Anal. Calc. for
C H F IrN O : C, 54.89; H, 3.55; N, 7.32. Found: C, 50.47; H, 3.52; N, 6.67%.
3
5
27
2
4
2
Ir(fpmqx) (trzl-CH ). Yield: 77%. FAB-MS: calculated 826; found 826.
2
3
1
HNMR(CDCl , 400 MHz) : d 8.46–6.36 (m, 16H, aromatic C-H); 4.83 (s, 1H,
3
CH3); 2.38 (t, 1H, trzl-CH3). Anal. Calc. for C H F IrN : C, 55.26; H, 3.30; N,
1
3
8
27
2
8
3.57. Found: C, 48.75; H, 3.36; N, 11.22%.
Measurements
UV-visible absorption spectra were obtained from Hewlett Packard 8425A spec-
trometer. Photoluminescence (PL) spectra were measured on a Perkin Elmer LS
5
1
0B spectrometer. UV-Vis and PL spectra of iridium complexes were measured in
ꢁ
5
0
M dilute CH Cl solution. Cyclic voltammograms were obtained at scan rate
2 2
1
of 100 mV=s with Electrochemical Analyzer of CH Instruments. HNMR spectra

7
0
H. R. Park and Y. Ha
were obtained from a 400 MHz NMR at Sogang University, and mass spectra were
determined on JEOL, JMS-AX505WA, HP 5890 Series II Hewlett-Packard 5890A
(capillary column) at Seoul National University in Korea.
Results and Discussion
For the new iridium complexes we prepared two different main ligands, 2,3-dpqx-F2
which is symmetric itself and fpmqx which is asymmetric, according to the modified
Suzuki coupling method [14]. We also introduced two different ancillary ligands,
acac and trzl-CH to the iridium complexes. The ancillary ligand, trzl-CH , which
3
3
was previously developed for fine tuning of the emission color and luminescence
efficiency [15] was prepared from the reaction of amidrazone precursor with acetyl
chloride, according to the reported procedure [15,16]. The iridium complexes were
synthesized via two steps, as reported by Nonoyama [13]. The overall synthetic
schemes are illustrated in Figure 1.
The UV-Vis absorption spectra of the complexes in CH Cl are shown in
2
2
Figure 2. The absorption spectra of iridium complexes, Ir(2,3-dpqx-F ) (acac) and
2
2
Ir(2,3-dpqx-F ) (trzl-CH ), containing symmetric main ligands have resemblance
3
2
2
in the absorption patterns. Those of iridium complexes, Ir(fpmqx) (acac) and
2
Ir(fpmqx) (trzl-CH ), containing asymmetric fpmqx ligands are also similar each
2
3
other. First, Ir(2,3-dpqx-F ) (acac) and Ir(2,3-dpqx-F ) (trzl-CH ) have strong
2
2
2 2
3
absorption appearing in the ultraviolet region of the spectrum between 230 and
1
ꢂ
3
10 nm. These bands have been assigned to the spin-allowed (p ! p ) transitions
1
ꢂ
of the main ligands. The (p ! p ) bands are accompanied by weaker and lower
energy features extending into the visible region from 320 to 420 nm. These absorp-
tion bands have been assigned to the allowed MLCT transitions. The high intensity
of the MLCT bands has been attributed to effective mixing of these charge-transfer
transitions with high lying spin-allowed transitions on the cyclometalating ligand
[
tions at the longer wavelengths than 420 nm are presumably due to the forbidden
17]. MLCT is important with respect to the emission efficiency. The weak absorp-
3
MLCT transitions. Secondly, the absorption spectra of Ir(fpmqx) (acac) and
2
Ir(fpmqx) (trzl-CH ) also have almost identical patterns each other. The spin-
3
2
1
ꢂ
allowed (p ! p ) transitions of the ligands appeared below 310 nm. The MLCT
bands of the Ir complexes containing 2-(5-methyl-4H-1,2,4,-triazol-3-yl)pyridine
(trzl-CH ) shown from 310 to 400 nm have higher absorption intensity than those
3
of the Ir acetylacetonate complexes. Relatively strong absorption in that region
could lead to improvement of emission efficiency in the iridium complexes.
ꢁ5
The photoluminescence (PL) spectra of the Ir complexes in 10 M CH Cl sol-
2
2
ution are shown in Figure 3. Ir(2,3-dpqx-F ) (acac), Ir(2,3-dpqx-F ) (trzl-CH ),
2
2
2 2
3
Ir(fpmqx) (acac) and Ir(fpmqx) (trzl-CH ) exhibited the emission maxima at 650,
2
2
3
6
ligands, Ir(fpmqx)Ir(fpmqx) (trzl-CH ), showed PL emission at shorter wavelengths.
20 and 631, 615 nm, respectively. The complexes containing asymmetric main
2
3
We attribute such hypsochromic shift by the complexes of fpmqx in PL partly to the
less conjugation in the main ligands. Though the fluorophenyl group in 2,3-dpqx-F2
ligand can partially be tilted from the main plane of the ligand, it may still contribute
to the elongation of conjugation, resulting in lower energy gap of the iridium
complexes, as pointed out by Johannes, et al. [2]. Furthermore, substitution of the
fluorophenyl group with the methyl group can reduce the rotational movement
of the fluorophenyl group which may cause nonradiative pathway of the iridium

Luminescence Comparison of Iridium(III) Complexes
71
Figure 1. The synthesis of main ligands, ancillary ligands and their iridium complexes.
complexes. The effect of ancillary ligands on the luminescence characteristics of their
iridium complexes was also investigated. The blue shifts of 16ꢃ30 nm by the com-
plexes containing the trzl-CH ancillary ligand were observed. Such hypsochromic
3
shift by the ancillary ligand can be attributed to involvement of electron withdrawing
N atom to the ancillary ligand skeleton. In fact, this blue shift by ancillary ligand
substitution is more than expected. The ancillary ligands are usually known to
involve in fine tuning of emission color by their complexes. However, up to 30 nm
shift upon ancillary ligand change from acac to trzl-CH3 was observed in
Ir(2,3-dpqx-F ) (AL).
2
2
We investigated electrochemical properties of the Ir complexes by the cyclic vol-
tammetry (CV), which reveal their positions of the HOMO=LUMO [11,12]. The
detailed CV data were summarized in Table 1. The oxidation potentials which indi-
cate the HOMOs of the complexes were irreversible in the range of 0.45–0.78 V

7
2
H. R. Park and Y. Ha
Figure 2. UV-Vis absorption spectra of (a) Ir(2,3-dqpx-F
solution (AL ¼ acac, trzl-CH
2
)
2 2
(AL) and (b) Ir(fpmqx) (AL) in a
ꢁ5
10
M CH
2
Cl
2
3
).
þ
relative to an internal ferrocenium=ferrocene reference (Fc =Fc). The reduction
potentials of the complexes in this study were calculated to be between ꢁ2.53 and
ꢁ
2.93 eV. Using the equation HOMO ¼ ꢁ4.8 ꢁ E , the HOMO of Ir(fpmqx) -
ox
2
(
trzl-CH ) was calculated to be ꢁ5.52 eV, that has the largest energy gap (DE) of
3
2
.85 eV, which consists with emission spectra having the shortest emission wave-
length in PL.
Further studies are undergoing on device fabrication using these complexes as
red phosphorescent materials to compare electroluminescence characteristics such

Luminescence Comparison of Iridium(III) Complexes
73
ꢁ
5
Figure 3. PL spectra of Ir(2,3-dqpx-F ) (AL) and Ir(fpmqx) (AL) in a 10 M CH Cl sol-
2
2
2
2
2
ution (AL ¼ acac, trzl-CH
3
).
Table 1. Physical parameters for the complexes
a
b
c
d
d
Ir complex
kem=nm E =V HOMO=eV LUMO=eV DE=eV
ox
Ir(2,3-dpqx-F ) (acac)
2
650
620
631
615
0.62
0.45
0.5
ꢁ5.42
ꢁ5.25
ꢁ5.30
ꢁ5.58
ꢁ2.83
ꢁ2.53
ꢁ2.61
ꢁ2.75
2.59
2.72
2.69
2.83
2
Ir(2,3-dpqx-F ) (trzl-CH )
2
2
3
Ir(fpmqx) (acac)
2
Ir(fpmqx) (trzl-CH )
2
0.78
3
a
b
Measured in CH
Scan rate: 100 mV=s, Electrolyte: tetrabutylammonium hexafluorophosphate. The poten-
2
Cl
2
solution.
tials are quoted against the internal ferrocene standard.
Deduced from the equation HOMO ¼ ꢁ4.8 ꢁ Eox
c
.
d
Calculated from the optical edge DE ¼ LUMO ꢁ HOMO.
as CIE coordinates and luminescence efficiencies of the iridium complexes contain-
ing symmetric vs. asymmetric main ligands.
Conclusions
We reported the detailed synthesis and photophysical properties of the phos-
phorescent iridium complexes having symmetric 2,3-dpqx-F2 ligands and an
asymmetric fpmqx ligands, and directly compared their luminescence properties.
We also employed two different ancillary ligands and investigated the effect of
the ancillary ligand on the luminescence of their complexes. The iridium com-
plexes, Ir(2,3-dpqx-F ) (acac), Ir(2,3-dpqx-F ) (trzl-CH ) and Ir(fpmqx) (acac)
2
2
2 2
3
2
Ir(fpmqx) (trzl-CH ), prepared in this study exhibited the red emission at 650,
2
3
6
20, 631 and 615 nm respectively.
We expected that the iridium complexes which contain asymmetric main ligands
might exhibit the blue shift compared to the iridium complexes containing the

7
4
H. R. Park and Y. Ha
symmetric ones. As a matter of fact, the hypsochromic shift by the complexes
with asymmetric fpmqx ligands was observed and such blue shift can be explained
by substitution of the conjugated phenyl ring with a non-conjugated methyl frag-
ment. The ancillary ligand also served an important role in determining the lumi-
nescence wavelength. Ir(fpmqx) (trzl-CH ) showed the most blue shift on emission
2
3
and had the largest energy gap due to the trzl-CH ancillary ligand having the
3
electron-withdrawing ability.
Acknowledgment
This work was supported by National Research Foundation of Korea Grant funded
by the Korean Government (NO. 2010-0017189).
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