Iridium(III) complexes with orthometalated quinoxaline ligands: Subtle tuning of emission to the saturated red color

Article abstract of DOI:10.1021/ic0489443

Rational design and syntheses of four iridium complexes (1-4) bearing two substituted quinoxalines and an additional 5-(2-pyridyl) pyrazolate or triazolate as the third coordinating ligand are reported. Single-crystal X-ray diffraction studies of 1 reveal a distorted octahedral geometry, in which two dpqx ligands adopt an eclipse configuration, for which the quinoxaline N atoms and the C atoms of orthometalated phenyl groups are located at the mutual trans-and cis-positions, respectively. The lowest absorption band for all complexes consists of a mixture of heavy-atom Ir(III)-enhanced ³MLCT and ³ππ* transitions, and the phosphorescent peak wavelength can be fine-tuned to cover the spectral range of 622-649 nm with high quantum efficiencies. The cyclic voltammetry was measured, showing a reversible, metal-centered oxidation with potentials at 0.76-1.03 V, as well as two reversible reduction waves with potentials ranging from -1.61 to -2.06 V, attributed to the sequential addition of two electrons to the more electron-accepting heterocyclic portion of two distinctive cyclometalated C∧N ligands. Complex 1 was used as the representative example to fabricate the red-emitting PLEDs by blending it into a PVK-PBD polymer mixture. The devices exhibited the characteristic emission profile of 1 with peak maxima located at 640 nm. The maximum external quantum efficiency was 3.15% ph/el with a brightness of 1751 cd/m² at a current density of 67.4 mA/cm ², and the maximum brightness of 7750 cd/m² was achieved at the applied voltage of 21 V and with CIE coordinates of (0.64, 0.31).

Full text of DOI:10.1021/ic0489443

Inorg. Chem. 2005, 44, 1344−1353
Iridium(III) Complexes with Orthometalated Quinoxaline Ligands: Subtle
Tuning of Emission to the Saturated Red Color
Fu-Ming Hwang,† Hsing-Yi Chen,† Po-Shen Chen,† Chao-Shiuan Liu,† Yun Chi,*,† Ching-Fong Shu,*,‡
Fang-Iy Wu,‡ Pi-Tai Chou,*,§ Shie-Ming Peng,§ and Gene-Hsiang Lee§
Department of Chemistry, National Tsing Hua UniVersity, Hsinchu 300, Taiwan, Department of
Applied Chemistry, National Chiao Tung UniVersity, Hsinchu 300, Taiwan, and Department of
Chemistry and Instrumentation Center, National Taiwan UniVersity, Taipei 106, Taiwan
Received August 3, 2004
Rational design and syntheses of four iridium complexes (1−4) bearing two substituted quinoxalines and an additional
5-(2-pyridyl) pyrazolate or triazolate as the third coordinating ligand are reported. Single-crystal X-ray diffraction
studies of 1 reveal a distorted octahedral geometry, in which two dpqx ligands adopt an eclipse configuration, for
which the quinoxaline N atoms and the C atoms of orthometalated phenyl groups are located at the mutual trans-
and cis-positions, respectively. The lowest absorption band for all complexes consists of a mixture of heavy-atom
3
Ir(III)-enhanced MLCT and 3ππ* transitions, and the phosphorescent peak wavelength can be fine-tuned to cover
the spectral range of 622
a reversible, metal-centered oxidation with potentials at 0.76
with potentials ranging from 1.61 to 2.06 V, attributed to the sequential addition of two electrons to the more
electron-accepting heterocyclic portion of two distinctive cyclometalated C N ligands. Complex 1 was used as the
representative example to fabricate the red-emitting PLEDs by blending it into a PVK PBD polymer mixture. The
−649 nm with high quantum efficiencies. The cyclic voltammetry was measured, showing
−1.03 V, as well as two reversible reduction waves
−
−
∧
−
devices exhibited the characteristic emission profile of 1 with peak maxima located at 640 nm. The maximum
external quantum efficiency was 3.15% ph/el with a brightness of 1751 cd/m2 at a current density of 67.4 mA/cm2,
and the maximum brightness of 7750 cd/m2 was achieved at the applied voltage of 21 V and with CIE coordinates
of (0.64, 0.31).
Organometallic complexes possessing a third-row transi-
tion-metal element are crucial for the fabrication of highly
efficient organic light-emitting diodes (OLEDs).1 The strong
spin-orbit coupling induced by a heavy-metal ion such as
iridium(III) promotes an efficient intersystem crossing from
the singlet to the triplet excited-state manifold, which then
facilitates strong electroluminescence by the harnessing of
both singlet and triplet excitons after the initial charge
recombination. Because internal phosphorescence quantum
efficiency (hint) of as high as ∼100% could theoretically be
achieved, these heavy-metal-containing emitters would be
superior to their fluorescent counterparts in future OLED
applications.2 As a result, there is a continuous trend of
shifting research endeavors to these heavy transition metal
complexes.
Moreover, since the manufacture of a full color display
requires the usage of emitters with all three primary colors,
i.e., blue, green, and red, rationally tuning the emission
wavelength of heavy-metal phosphorescent emitters over the
(2) (a) Adachi, C.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. J. Appl.
Phys. 2001, 90, 5048. (b) Ikai, M.; Tokito, S.; Sakamoto, Y.;
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S.; Iijima, T.; Tsuzuki, T.; Sato, F. Appl. Phys. Lett. 2003, 83, 2459.
(d) Lo, S.-C.; Namdas, E. B.; Burn, P. L.; Samuel, I. D. W.
Macromolecules 2003, 36, 9721. (e) Kwong, R. C.; Nugent, M. R.;
Michalski, L.; Ngo, T.; Rajan, K.; Tung, Y.-J.; Weaver, M. S.; Zhou,
T. X.; Hack, M.; Thompson, M. E.; Forrest, S. R.; Brown, J. J. Appl.
Phys. Lett. 2002, 81, 162. (f) He, G.; Chang, S.-C.; Chen, F.-C.; Li,
Y.; Yang, Y. Appl. Phys. Lett. 2002, 81, 1509. (g) Lee, C.-L.; Lee, K.
B.; Kim, J.-J. Appl. Phys. Lett. 2000, 77, 2280. (h) Chen, C.-T. Chem.
Mater. 2004, 16, 4389.
* Authors to whom correspondence should be addressed. E-mail:
ychi@mx.nthu.edu.tw (Y.C.); shu@cc.nctu.edu.tw (C.-F.S.); chop@ntu.edu.tw
(P.-T.C.).
† National Tsing Hua University.
‡ National Chiao Tung University.
§ National Taiwan University.
(1) (a) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.;
Thompson, M. E.; Forrest, S. R. Nature 1998, 395, 151. (b) Baldo,
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1344 Inorganic Chemistry, Vol. 44, No. 5, 2005
10.1021/ic0489443 CCC: $30.25
© 2005 American Chemical Society
Published on Web 02/04/2005

Ir(III) Complexes with Quinoxaline Ligands
were recorded on a Varian INOVA-500 instrument; chemical shifts
entire visible range has emerged as an important ongoing
research task.3 In contrast to the well-developed green
phosphorescent materials, the design and synthesis of the
red emitters is intrinsically more difficult because their
luminescent quantum yields tend to be lower due to the
smaller energy gap, implying that the red emitters could be
prone to poor illumination efficiencies.2 Despite this limita-
tion, reports on the red-emitting complexes with the Os(II)4
and Pt(II)5 elements have been documented in the literature.
Unfortunately, to our knowledge, more generalized, system-
atic approaches, which allow a lucid prediction of the
emission color and illumination quantum efficiency, have
not yet been well established. Progress, to a certain extent,
still relies on precedent experiences or certain serendipitous
discoveries.
In this paper, we report a systematic design, synthesis,
and characterization of red-emitting Ir(III)-containing com-
plexes via judicious variation of ligands, for which, on one
hand, the high rigidity of the ligand framework would
significantly reduce the nonradiative transition. On the other
hand, the energies of ππ* or MLCT manifolds, i.e., the
energy gap between the ground and the lowest excited states,
can be effectively reduced either by substituting the nitrogen
atom with a less electronegative carbon atom or extending
the π-electron delocalization of the aromatic ligand chro-
mophore, giving emitters with the saturated red color.
1
are quoted with respect to the internal standard TMS for H and
13C NMR data. Elemental analysis was carried out with a Heraeus
CHN-O Rapid Elementary Analyzer.
Spectroscopic and CV Measurements. Steady-state absorption
and emission spectra were recorded with a Hitachi (U-3310)
spectrophotometer and an Edinburgh (FS920) fluorometer, respec-
tively. Both the wavelength-dependent excitation and emission
response of the fluorometer had been calibrated. Lifetime studies
were performed with an Edinburgh FL 900 photon-counting system
with a hydrogen-filled/or a nitrogen lamp as the excitation source.
Data were analyzed using the nonlinear least-squares procedure in
combination with an iterative convolution method. The emission
decays were analyzed by the sum of exponential functions, which
allows partial removal of the instrument time broadening and
consequently renders a temporal resolution of ∼200 ps. DCM was
used as a reference, assuming a quantum yield Φf ) 0.44.8 to
determine luminescence quantum yields of the studied compounds
in solution. Solution samples were degassed by three freeze-
pump-thaw cycles. The resulting luminescence was acquired by
an intensified charge-coupled detector. Cyclic voltammetry (CV)
and differential pulse voltammetry (DPV) measurements were
performed using a BAS 100 B/W electrochemical analyzer. The
oxidation and reduction measurements were recorded using Pt wire
and Au disk coated with Hg as working electrodes, respectively,
in anhydrous CH2Cl2 and anhydrous THF containing 0.1 M TBAPF6
as the supporting electrolyte, at the scan rate of 50 mV s-1. The
potentials were measured against an Ag/Ag+ (0.01 M AgNO3)
reference electrode with ferrocene as the internal standard.
Experimental Section
Preparation of [(dpqx)2IrCl]2. The chloride-bridged complex
[(dpqx)2IrCl]2 was synthesized according to the method reported
by Nonoyama,9 involving treatment of IrCl3‚3H2O (1.0 g, 2.84
mmol) with 2,3-diphenylquinoxaline, (dpqx)H (2.4 g, 8.51 mmol),
in 2-methoxyethanol (30 mL) for 48 h. After the cooling of the
reaction mixture, the solution was treated with water (30 mL) to
induce precipitation of a brick red solid. The solid was then filtered
off, washed with diethyl ether, and dried under vacuum (1.93 g,
1.22 mmol, 86%).
General Experiments. 2,3-Bis(4-fluorophenyl)quinoxaline,
(bfqx)H, was obtained from direct condensation of 1,2-phenylene-
diamine and 4,4′-difluorobenzil in refluxing ethanol,6 while 3-(tri-
fluoromethyl)-5-(2-pyridyl)pyrazole, (fppz)H, 3-tert-butyl-5-(2-
pyridyl)pyrazole, (bppz)H, and 3-(trifluoromethyl)-5-(2-pyridyl)-
triazole, (fptz)H, were prepared using methods described in the
literature.7 The reactions were monitored by TLC with Merck
precoated glass plates (0.20 mm with fluorescent indicator UV254).
Compounds were visualized with UV light irradiation at 254 and
365 nm. Column chromatography was carried out using silica gel
from Merck (230-400 mesh). Mass spectra were obtained on a
JEOL SX-102A instrument operating in electron impact (EI) mode
or fast atom bombardment (FAB) mode. 1H and 13C NMR spectra
Preparation of [(dpqx)2Ir(fppz)] (1). A mixture of [(dpqx)2IrCl]2
(350 mg, 0.22 mmol), 3-(trifluoromethyl)-5-(2-pyridyl)pyrazole
((fppz)H, 130 mg, 0.61 mmol), and Na2CO3 (120 mg, 1.13 mmol)
in 2-methoxyethanol (30 mL) was heated to reflux for 12 h. Excess
of water was added after cooling the solution to RT (room
temperature). The precipitate was collected by filtration and washed
with anhydrous ethanol (10 mL), followed by diethyl ether (10 mL).
Black crystals of [(dpqx)2Ir(fppz)] (1) were obtained from repeated
recrystallization using a mixture of CH2Cl2 and methanol at room
temperature (180 mg, 0.19 mmol, 43%). Solubility in CH2Cl2 at
28 °C: 26 mg/mL.
(3) Grushin, V. V.; Herron, N.; LeCloux, D. D.; Marshall, W. J.; Petrov,
V. A.; Wang, Y. Chem. Commun. 2001, 1494.
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Y.-L.; Wu, P.-C.; Liu, C.-S.; Chi, Y.; Yu, J.-K.; Hu, Y.-H.; Chou,
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F.-I. Organometallics 2004, 23, 3745. (e) Su, H.-J.; Wu, F.-I.; Shu,
C.-F.; Tung, Y.-L.; Chi, Y.; Lee, G.-H. J. Polym. Sci., Part A: Polym.
Chem. 2005, in press. (f) Tung, Y.-L.; Lee, S.-W.; Chi, Y.; Tao, Y.-
T.; Chien, C.-H.; Cheng, Y.-M.; Chou, P.-T.; Peng, S.-M.; Liu, C.-S.
J. Mater. Chem. 2005, 15, 460.
(5) (a) Kwong, R. C.; Sibley, S.; Dubovoy, T.; Baldo, M.; Forrest, S. R.;
Thompson, M. E. Chem. Mater. 1999, 11, 3709. (b) Che, C.-M.; Hou,
Y.-J.; Chan, M. C. W.; Guo, J.; Liu, Y.; Wang, Y. J. Mater. Chem.
2003, 13, 1362.
(6) Steel, P. J.; Caygill, G. B. J. Organomet. Chem. 1990, 395, 359.
(7) (a) Thiel, W. R.; Eppinger, J. Chem.sEur. J. 1997, 3, 696. (b) Singh,
S. P.; Kumar, D.; Jones, B. G.; Threadgill, M. D. J. Fluorine Chem.
1999, 94, 199. (c) Kubota, S.; Uda, M.; Nakagawa, T. J. Heterocycl.
Chem. 1975, 12, 855. (d) Funabiki, K.; Noma, N.; Kuzuya, G.; Matsui,
M.; Shibata, K. J. Chem. Res., Miniprint 1999, 1301.
1
Spectral data for 1: MS (FAB, 193Ir) m/z 967 (M+); H NMR
(500 MHz, CDCl3, 294 K) δ 8.69 (s, 1H), 8.62 (s, 1H), 8.52 (s,
1H), 8.42 (d, JHH ) 8.5 Hz, 1H), 8.39 (d, JHH ) 9.0 Hz, 1H), 8.32
(s, 1H), 7.75 (d, JHH ) 8.0 Hz, 1H), 7.71 (d, JHH ) 8.5 Hz, 1H),
7.69-7.40 (m, 2H); 13C NMR (125 MHz, CDCl3, 294 K) δ 165.3,
163.8, 160.1, 155.2, 154.4, 153.0, 152.8, 149.7, 147.0, 146.4, 145.4,
144.4 (q, JCF ) 35 Hz), 141.6, 140.4, 140.2, 140.0, 139.9, 139.7,
138.3, 136.9, 134.5, 131.2, 130.8, 130.3, 130.0, 129.9 (2C), 129.8
(2C), 129.7, 129.3, 129.2 (br, 2C), 129.1, 129.0 (2C), 128.8 (br,
2C), 128.7, 128.6, 127.2, 124.6, 122.5 (q, JCF ) 266 Hz), 122.1,
(8) Darke, J. M.; Lesiecki, M. L.; Camaioni, D. M. Chem. Phys. Lett.
1985, 113, 530.
(9) Nonoyama, M. J. Organomet. Chem. 1975, 86, 263.
Inorganic Chemistry, Vol. 44, No. 5, 2005 1345

Hwang et al.
121.4 (2C), 121.1, 119.6, 102.3; 19F NMR (470 MHz, CDCl3, 294
K) δ -60.0 (3F). Anal. Calcd for C50H33Cl2F3IrN7: N, 9.32; C,
57.09; H, 3.16. Found: N, 9.30; C, 57.09 H, 3.55.
Preparation of [(dfqx)2Ir(bppz)] (4). Procedures identical with
that of 1 were followed, using 300 mg of [(dfqx)2IrCl]2 (0.174
mmol), 87 mg of pyrazole (bppz)H (0.43 mmol), and 180 mg of
Na2CO3 (1.7 mmol) in 30 mL of 2-methoxyethanol. Dark red
crystals of [(dfqx)2Ir(bppz)] (4) were obtained from column
chromatography using hexane and EA ) 2:1 and crystallization
from a mixture of CHCl3 and methanol at RT (124 mg, 0.12 mmol,
35%). Solubility in CH2Cl2 at 28 °C: 18 mg/mL.
Preparation of [(dfqx)2IrCl]2. On the basis of a procedure
similar to that described for [(dpqx)2IrCl]2, the second chloride-
bridged complex [(dfqx)2IrCl]2 was synthesized in a yield of ∼85%.
Preparation of [(dfqx)2Ir(fppz)] (2). Procedures identical with
that of 1 were followed, using 200 mg of [(dfqx)2IrCl]2 (0.116
mmol), 61 mg of pyrazole (fppz)H (0.28 mmol), and 120 mg of
Na2CO3 (1.13 mmol) in 20 mL of 2-methoxyethanol. Dark red
crystals of [(dfqx)2Ir(fppz)] (2) were obtained from column chro-
matography using hexane and ethyl acetate (EA) ) 3:1 and
crystallization from a mixture of CHCl3 and hexane at RT (122
mg, 0.117 mmol, 51%). Solubility in CH2Cl2 at 28 °C: 6.5 mg/
mL.
1
Spectral data for 4: MS (FAB, 193Ir) m/z 1027 (M+); H NMR
(400 MHz, CDCl3, 294 K) δ 8.32 (d, JHH ) 5.2 Hz, 1H), 8.07 (d,
JHH ) 9.0 Hz, 1H), 7.96 (d, JHH ) 7.4 Hz, 1H), 7.88 (d, JHH ) 7.4
Hz, 1H), 7.83 (br, 2H), 7.54 (d, JHH ) 9.0 Hz, 1H), 7.53-7.44 (m,
3H), 7.33-7.19 (m, 7H), 7.15-7.05 (m, 5H), 6.47 (td, JHH ) 9.0,
2.6 Hz, 1H), 6.40 (ddd, JHH ) 9.0, 8.4, 2.6 Hz, 1H), 6.31 (dd, JHH
) 9.0, 2.6 Hz, 1H), 6.14 (s, 1H), 5.87 (dd, JHH ) 9.0, 2.6 Hz, 1H),
1.27 (s, 9H); 13C NMR (125 MHz, CDCl3, 294 K) δ 165.2, 164.5,
1
Spectral data for 2: MS (FAB, 193Ir) m/z 1039 (M+); H NMR
164.1 (d, JCF ) 243 Hz), 163.8 (d, JCF ) 243 Hz), 162.9 (d, JCF
)
(400 MHz, CDCl3, 294 K) δ 8.44 (d, JHH ) 5.2 Hz, 1H), 8.00 (d,
JHH ) 5.2 Hz, 1H), 7.95 (dd, JHH ) 6.8, 1.2 Hz, 1H), 7.91 (dd, JHH
) 6.8, 1.2 Hz, 1H), 7.83 (br, 2H), 7.63-7.60 (m, 1H), 7.52 (ddd,
243 Hz), 162.8, 162.1 (d, JCF ) 243 Hz), 156.5 (br, 2C), 152.6,
151.5, 148.7, 146.5, 142.4, 141.7, 141.6, 140.7, 140.1, 140.0, 138.0,
135.9, 135.6, 132.7 (d, JCF ) 9 Hz), 132.0 (d, JCF ) 9 Hz), 131.2,
131.1, 130.0, 129.7 (2C), 129.6, 128.9 (2C), 128.5 (2C), 127.2,
124.5, 122.2 (d, JCF ) 17 Hz), 120.7, 120.1 (d, JCF ) 17 Hz), 119.2,
116.1 (br, 4C), 109.4 (d, 2C, JCF ) 22 Hz), 108.7 (d, 2C, JCF ) 22
Hz), 32.3, 31.4 (3C); 19F NMR (470 MHz, CDCl3, 294 K) δ
-108.6, -109.7, -110.4, -110.8. Anal. Calcd for C52H36F4IrN7:
C, 60.81; H, 3.53; N, 9.55. Found: C, 60.39; H, 3.93; N, 9.47.
X-ray Structural Analysis. Single-crystal X-ray diffraction data
were measured on a Bruker Smart ApexCCD diffractometer using
λ(Mo KR) radiation (λ ) 0.710 73 Å). The data collection was
executed using the SMART program. Cell refinement and data
reduction were made with the SAINT program. The structure was
determined using the SHELXTL/PC program and refined using full-
matrix least squares. All non-hydrogen atoms were refined aniso-
tropically, whereas hydrogen atoms were placed at the calculated
positions and included in the final stage of refinements with fixed
parameters.
JHH ) 6.8, 5.2, 1.2 Hz, 1H), 7.48-7.44 (m, 1H). 7.42 (d, JHH
)
7.2 Hz, 1H), 7.34-7.23 (m, 9H), 7.18-7.15 (m, 2H), 7.07-7.04
(m, 1H), 6.53 (s, 1H), 6.50 (ddd, JHH ) 9.2, 8.8, 2.4 Hz, 1H), 6.44
(ddd, JHH ) 9.2, 8.8, 2.4 Hz, 1H), 6.37 (dd, JHH ) 9.2, 2.4 Hz,
1H), 5.92 (dd, JHH ) 8.8, 2.4 Hz, 1H); 13C NMR (125 MHz, CDCl3,
294 K) δ 164.2, 164.1 (d, JCF ) 234 Hz), 163.8 (d, JCF ) 234 Hz),
163.1, 163.0 (d, JCF ) 234 Hz), 162.7, 162.0 (d, JCF ) 234 Hz),
155.2, 155.0, 153.0, 151.7, 150.0, 146.9, 144.6 (q, JCF ) 36 Hz),
142.5, 141.4 (2C), 140.1 (2C), 139.8, 138.6, 135.8, 135.5, 132.9
(d, JCF ) 10 Hz), 132.2 (d, JCF ) 10 Hz), 131.6, 131.0 (br, 2C),
130.2, 129.8, 129.7, 129.0, 128.8, 126.6, 124.0, 122.6 (d, JCF ) 17
Hz), 122.3 (q, JCF ) 267 Hz), 122.2, 120.3 (d, J ) 17 Hz), 119.9,
116.5 (d, 2C, JCF ) 22 Hz), 116.0 (d, 2C JCF ) 22 Hz), 109.7 (d,
2C JCF ) 22 Hz), 109.4 (d, 2C JCF ) 22 Hz), 102.5; 19F NMR
(470 MHz, CDCl3, 294 K) δ -60.0 (3F), -108.0, -109.1, -110.2,
-110.8. Anal. Calcd for C49H27F7IrN7: C, 56.64; H, 2.62; N, 9.44;
Found: C, 56.82; H, 2.79. N, 8.99.
Selected crystal data for 1: C49H31F3N7Ir‚CH2Cl2, M ) 1051.93;
triclinic, space group P1h; a ) 11.1500(5), b ) 12.5173(5), c )
17.0626(7) Å; R ) 100.4154(8), â ) 92.6985(9), γ ) 114.9504(8)°;
V ) 2103.2(2) Å3, Z ) 2; Fcalcd ) 1.661 g cm-3; F(000) ) 1040;
crystal size ) 0.35 × 0.35 × 0.12 mm3; λ(Mo KR) ) 0.7107 Å;
Preparation of [(dfqx)2Ir(fptz)] (3). Procedures identical with
that of 1 were followed, using 330 mg of [(dfqx)2IrCl]2 (0.19 mmol),
102 mg of triazole (fptz)H (0.48 mmol), and 200 mg of Na2CO3
(1.9 mmol) in 30 mL of 2-methoxyethanol. Orange red crystals of
[(dfqx)2Ir(fptz)] (3) were obtained from column chromatography
using hexane and EA ) 2:1 and crystallization from a mixture of
CHCl3 and methanol at RT (191 mg, 0.183 mmol, 48%). Solubility
in CH2Cl2 at 28 °C: 22 mg/mL.
T ) 295(2) K; µ ) 3.361 mm-1; 9651 reflections collected (Rint
)
0.0365), maximum and minimum transmission ) 0.6885 and
0.3859; GOF on F2 ) 1.049; final R1[I > 2σ(I)] ) 0.0304 and
wR2[I > 2σ(I)] ) 0.0724; final R1(all data) ) 0.0352 and wR2(all
data) ) 0.0767; largest difference peak and hole ) 1.375 and
-0.955 e/Å3.
1
Spectral data for 3: MS (FAB, 193Ir) m/z 1040 (M+); H NMR
(500 MHz, CDCl3, 294 K) δ 8.54 (d, JHH ) 5.0 Hz, 1H), 8.26 (br,
2H) 7.97 (d, JHH ) 8.0 Hz, 1H), 7.92 (d, JHH ) 8.0, 1H), 7.90-
7.87 (m, 2H), 7.83 (br, 2H), 7.79-7.75 (m, 1H), 7.55 (t, JHH ) 7.8
Hz, 1H), 7.49-7.45 (m, 2H), 7.35-7.28 (m, 6H), 7.20-7.15 (m,
2H), 7.05 (t, JHH ) 7.8, Hz, 1H), 6.54 (ddd, JHH ) 9, 8.5, 2.5 Hz,
1H), 6.47 (td, JHH ) 9, 2.5 Hz, 1H), 6.38 (dd, JHH ) 9, 2.5 Hz,
1H), 5.95 (dd, JHH ) 9.0, 2.5 Hz, 1H); 13C NMR (125 MHz, CDCl3,
294 K) δ 164.1 (d, JCF ) 226 Hz), 163.9 (d, JCF ) 226 Hz), 163.8,
163.0 (d, JCF ) 226 Hz), 162.9, 162.5, 162.0 (d, JCF ) 226 Hz),
160.8, 156.6 (q, JCF ) 36 Hz), 152.9 (2C), 151.8, 151.0, 147.0,
142.3, 141.2, 140.8, 140.2, 140.1, 139.8, 139.4, 135.5, 135.4, 133.0
(d, JCF ) 9 Hz), 132.3 (d, J ) 9 Hz), 131.9, 131.0 (br, 2C), 130.4,
130.2, 130.1, 129.3, 129.2, 126.2, 124.8, 123.8, 122.7 (d, JCF ) 17
Hz), 121.9, 120.7 (d, JCF ) 17 Hz), 120.1 (q, JCF ) 268 Hz), 116.6
(d, 2C, JCF ) 21 Hz), 116.2 (d, 2C, JCF ) 21 Hz), 110.0 (m, 4C);
19F NMR (470 MHz, CDCl3, 294 K) δ -63.7 (3F), -107.3, -108.3,
-109.8, -110.4. Anal. Calcd for C48H26F7IrN8: C, 55.44; H, 2.52;
N, 10.77. Found: C, 54.83; H, 2.66; N, 10.47.
Device Fabrication. The light-emitting devices with the con-
figuration of ITO/poly(styrenesulfonate)-doped poly(3,4-ethylene-
dioxythiophene) (PEDOT:PSS) (35 nm)/doped emitting layer (50-
60 nm)/TPBI (30 nm)/Mg:Ag (100 nm)/Ag(100 nm) were fabricated.
The PEDOT:PSS was spin coated onto the ITO glass and dried at
80 °C for 12 h under vacuum to improve the smoothness. Films of
poly(vinylcarbazole) (PVK), consisting of 2-(4-biphenylyl)-5-(4-
tert-butylphenyl)-1,3,4-oxadiazole (PBD) (30 wt %) and different
amounts of complex 1 (0.18, 1.8, and 5.3 wt %, respectively) were
spin coated on top of the PEDOT:PSS layer using chlorobenzene
as the solvent, followed by drying at 60 °C for 3 h under vacuum.
Prior to film casting, the polymer solution was filtered through a
Teflon filter (0.45 µm) to remove the larger particulates. The 2,2′,2′′-
(1,3,5-benzenetriyl)tris[1-phenyl-1H-benimidazole] (TPBI) layer
was grown by thermal sublimation in a vacuum of 3 × 10-6 Torr
to use as the electron-transport layer, which would block holes and
1346 Inorganic Chemistry, Vol. 44, No. 5, 2005

Ir(III) Complexes with Quinoxaline Ligands
Scheme 1
excitons.10 The cathode Mg:Ag alloy (10:1, 100 nm) was deposited
onto the TPBI layer by coevaporation. An additional layer of Ag
(100 nm) was deposited onto the cathode as a protective coating.
The current-voltage-luminance was measured in ambient condi-
tions with a Keithley 2400 source meter and a Newport 1835C
optical meter equipped with 818ST silicon photodiode.
Figure 1. ORTEP diagram of 1 with thermal ellipsoids shown at the 30%
probability level. Bond distances (Å): Ir-C(10) ) 1.996(3), Ir-C(30) )
1.992(3), Ir-N(1) ) 2.211(3), Ir-N(2) ) 2.128(3), Ir-N(4) ) 2.059(3),
Ir-N(6) ) 2.064(3) Å. Nonbonding distances (Å): C(25)‚‚‚N(2) ) 3.12,
C(25)‚‚‚N(3) ) 3.23, C(25)‚‚‚C(6) ) 3.80, C(45)‚‚‚N(1) ) 3.28, C(45)‚‚
‚C(1) ) 3.48, C(45)‚‚‚C(5) ) 4.05 Å. Bond angles (deg): C(10)-Ir-N(4)
) 79.0(1), C(30)-Ir-N(6) ) 79.3(1), C(30)-Ir-C(10) ) 93.2(1), N(2)-
Ir-N(1) ) 75.2(1), N(4)-Ir-N(6) ) 174.3(2).
Results
Synthesis and Characterization. The reaction of (dpqx)H
and IrCl3‚nH2O in refluxing methoxyethanol afforded chloride-
bridged complex [(dpqx)2IrCl]2. The monomeric complex
[(dpqx)2Ir(fppz)] (1) was obtained by the treatment of
[(dpqx)2IrCl]2 with pyrazole (fppz)H in the presence of
Na2CO3, where (fppz)H ) 3-(trifluoromethyl)-5-(2-pyridyl)-
pyrazole. Moreover, a second quinoxaline ligand, (dfqx)H,
with two 4-fluorophenyl substituents was also synthesized
for the purpose of subtle tuning of the emission wavelength.
Thus, condensation of 1,2-phenylenediamine and 4,4′-
difluorobenzil gave the corresponding 4-fluorophenyl-
substituted quinoxaline in a high yield. Subsequently, the
reactions were executed using the identical metal reagent
IrCl3‚nH2O, followed by treatment with the pyrazolate or
triazolate chelate ligands such as (fppz)H, (fptz)H, and
(bptz)H to afford three additional complexes, i.e., [(dfqx)2Ir-
(fppz)] (2), [(dfqx)2Ir(fptz)] (3), and [(dfqx)2Ir(bppz)] (4);
their molecular structures are shown in Scheme 1. All iridium
complexes 1-4 are highly soluble in chlorinated solvents
and show negligible decomposition in solution within a
period of 24 h. Detailed characterizations were conducted
using MS, NMR, and elemental analysis (see Experimental
Section), whereas the parent dpqx complex 1 was further
identified using single-crystal X-ray analysis to establish their
three-dimensional structures.
eclipse configuration with their coordinated nitrogen atoms
N(4) and N(6) and carbon atoms C(10) and C(30) being in
a trans and a cis orientation, respectively, while the fppz
pyrazolate is located at a unique position opposite to the
carbon atoms of the dpqx ligands. This ligand arrangement
is akin to those of the parent phenylpyridine ligands in
chloride-bridged dimer complex [(ppy)2Ir(µ-Cl)]2,11 diketo-
nate complexes such as (ppy)2Ir(acac),12 and even the
associated derivatives with pyrazolyl ancillary ligands,13
suggesting that the pyrazolate ligand in our case is attached
to the metal via a simple replacement of both chloride
ligands. In addition, both quinoxaline ring skeletons exhibited
large distortion with respect to the cyclometalated phenyl
fragment. This abnormal phenomenon is evidenced by the
large deformation angles of 35.3 and 36° calculated by using
the least-squares planes of the cyclometalated phenyl group
and the outer C6 hexagon of the adjacent quinoxaline
fragment. It appears to us that the nonbonding repulsion
between C(25) and C(45) atoms of the dpqx ligands and the
perpendicularly arranged fppz pyrazolate ligands is the main
driving force to induce the large distortion. This viewpoint
is supported by the interligand nonbonding contacts of ca.
3.5 Å between C-C and C-N, the distance of which is
As depicted in Figure 1, complex 1 reveals a distorted
octahedral geometry around iridium, consisting of two
cyclometalated dpqx quinoxaline ligands and one fppz
pyrazolate ligand. The quinoxaline ligands adopt a mutual
(11) Graces, F. O.; King, K. A.; Watts, R. J. Inorg. Chem. 1988, 27, 3464.
(12) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Lee, H.-
E.; Adachi, C.; Burrows, P. E.; Forrest, S. R.; Thompson, M. E. J.
Am. Chem. Soc. 2001, 123, 4304.
(13) Li, J.; Djurovich, P. I.; Alleyne, B. D.; Tsyba, I.; Ho, N. N.; Bau, R.;
Thompson, M. E. Polyhedron 2004, 23, 419.
(10) Vaeth, K. M.; Tang, C. W. J. Appl. Phys. 2002, 92, 3447.
Inorganic Chemistry, Vol. 44, No. 5, 2005 1347

Hwang et al.
Figure 2. UV-vis absorption and normalized emission spectra of
complexes 1 (9), 2 (O), 3 (2), and 4 (4) in CH2Cl2 at room temperature.
Table 1. Photophysical Properties of Iridium Complexes 1-4 at Room
Temperature
max
λem
τdegassed
(µs)
λabsmax (ꢀ, M-1 cm-1
)
(nm) Φdegassed
Figure 3. Cyclic voltammograms of (a) 1, (b) 2, (c) 3, and (d) 4. Inset:
Differential pulse voltammetry in the reduction region.
1
2
3
4
381 (23 000), 458 (6100)
642
630
622
649
0.40
0.83
0.85
0.62
1.9
2.8
3.3
1.9
359 (17 500), 384 (19 000), 453 (6700)
357 (22 500), 382 (25 400), 450 (8800)
359 (16 300), 469 (5100)
Table 2. Electrochemical Properties of Complexes 1-4
E1/2ox (V)a
E1/2red (V)a
1
2
3
4
0.76
0.96
1.03
0.77
-1.73, -2.06
-1.65, -2.00
-1.61, -1.95
-1.71, -2.04
consistent with the interplanar separation between aromatic
fragments showing substantial π-π stacking interaction.14
Photophysical Data. The absorption and luminescence
spectra of complexes 1-4 in CH2Cl2 are depicted in Figure
2. The strong absorption bands in the UV region with distinct
a Potential values referenced vs Fc/Fc+.
1
nm hypsochromic shift in the emission peak wavelength and
can qualitatively be rationalized by a decrease of π* MO
energy level due to the stronger electron-withdrawing
character of the fluorine atom at a para position. Moreover,
with respect to 2, altering the third chelate ligand from fppz
to fptz (3) and bppz (4) results in an 8 nm blue shift and a
19 nm red shift in emission peak maxima λmax, respectively.
This observation is in accordance with the π-accepting nature
of the triazolate ligand in 3 as well as the expected electron-
donating property of the tert-butyl group in 4, which affects
the energy level of ππ* excited states (vide infra). Table 1
lists the corresponding photophysical data for the studied
complexes in solution phase at room temperature. The
observed lifetimes of ca. 1.9-3.3 µs, together with high
quantum yields of 0.4-0.8, in degassed CH2Cl2 for all
studied complexes lead us to deduce the radiative lifetimes
of 3.0-4.8 µs, which are considerably shorter than those of
most reported red-emitting Ir(III) complexes.
Electrochemistry. The electrochemical behavior of these
Ir metal complexes was investigated by cyclic voltammetry
using ferrocene as the internal standard. The results are
displayed in Figure 3, with the respective redox data listed
in Table 2. During the anodic scan in CH2Cl2, all iridium
metal complexes exhibited a reversible oxidation with
potentials in the region 0.76-1.03 V. Upon the switch to
the cathodic sweep in THF, two reversible reduction
processes, with potentials ranging from -1.61 to -2.06 V,
vibronic features are assigned to the spin-allowed ππ*
transition of the cyclometalated quinoxaline ligands. The next
lower energy absorption with peak wavelengths in the region
of 440-475 nm can be ascribed to a typical spin-allowed
metal to ligand charge-transfer (1MLCT) transition, while
their extinction coefficients at peak wavelengths are in the
range 5100-8800 M-1 cm-1 and tend to decrease in the order
of 3 > 2 > 1 > 4. Finally, the weak shoulder extending
into the visible region are believed to be associated with both
spin-orbit coupling enhanced 3ππ* and 3MLCT transitions.
Highly intensive luminescence was observed for 1-4 in
degassed CH2Cl2 with λmax located at 642 nm, 630, 622, and
649 nm, respectively. The entire emission band originating
from a triplet state manifold was ascertained by the O2
quenching rate constant of as high as (1.5-2.0) × 109 M-1
s-1 for all samples. The significant overlap of the 0-0 onsets
between emission signal and the lowest energy absorption
band, in combination with a broad, structureless spectral
feature, leads us to conclude that the phosphorescence
originates primarily from the 3MLCT state, together perhaps
3
with a lesser contribution from the ππ excited states. In
comparison to 1 with the cyclometalated phenyl group,
complex 2 bearing the 4-fluorophenyl group reveals an ∼12
(14) (a) Chan, S.-C.; Chan, M. C. W.; Wang, Y.; Che, C.-M.; Cheung,
K.-K.; Zhu, N. Chem.sEur. J. 2001, 7, 4180. (b) Kato, M.; Takahashi,
J.; Sugimoto, Y.; Kosuge, C.; Kishi, S.; Yano, S. J. Chem. Soc., Dalton
Trans. 2001, 747.
1348 Inorganic Chemistry, Vol. 44, No. 5, 2005

Ir(III) Complexes with Quinoxaline Ligands
were detected for all of them. Additional data provided by
differential pulse voltammetry (DPV) also showed two
distinct reduction processes. As revealed previously by
electrochemical studies and theoretical calculations,15,16 the
oxidation occurred mainly at the Ir metal cationic site,
together with a minor contribution from the cyclometalated
phenyl fragment, whereas the double reversible reductions
occur primarily on the stronger electron-accepting hetero-
cyclic portion of the cyclometalated C∧N ligands. Accord-
ingly, replacing the phenyl fragment with a 4-fluorophenyl
moiety would significantly affect the electrochemical proper-
ties of these metal complexes. This was first demonstrated
by the greater oxidation potential of 2 (0.96 V) vs that of
parent complex 1 (0.76 V). At the same time, the reduction
potentials of 2 are also shifted from those of the complex 1
(-1.73 and -2.06 V) to a less negative value of -1.65 and
-2.00 V, respectively. Our data are also in good agreement
with those of the related Ir cyclometalated complexes
documented in literature,17 concluding that the addition of
the fluorine substituent on the phenyl rings of cyclometalated
ligands would notably increase the oxidation potential and
concurrently decrease the reduction potential.
Figure 4. (a) Current density versus applied voltage for the doped devices
showing dopant concentrations of 1 at 0.18, 1.8, and 5.3 wt % level. (b)
Plots of luminance versus applied voltage.
Similarly, the redox potentials of these Ir complexes are
also affected by the ancillary LX ligand. This was revealed
by the fact that substitution of the fppz ligand by the more
electron-donating bppz ligand markedly decreased the oxida-
tive potential from 0.96 V of 2 to 0.77 V of 4, while
replacement of the fppz ligand with a more electron-deficient
fptz led to an anodical shift from 0.96 V for 2 to 1.03 V
observed in 3. It should be noted that minor changes at the
reduction potential were also observed and were mainly
associated with the cyclometalated ligands, to which the
ancillary LX ligand can still transmit its electronic nature
via an inductive effect passing through the central metal
atom. The reduction potentials shown in Table 2 are fully
consistent with this delineation; however, the variation of
the data is far less significant than that imposed on the
respective oxidative potential.
Electroluminescent Devices. To demonstrate the basic
electrophosphorescent properties, we selected complex 1,
which exhibits a saturated red emission, for the fabrication
of PLED and testing of their performances. The devices
consist of a multilayer configuration ITO/PEDOT:PSS/
PVK-PBD (30 wt %):x wt % of 1/TPBI/Mg:Ag/Ag (100
nm), where ITO, PEDOT:PSS, PVK, PBD, TPBI, and
Mg:Ag denote indium tin oxide, poly(styrenesulfonate)-
doped poly(3,4-ethylenedioxythiophene), poly(vinylcarba-
zole), 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadia-
zole, 2,2′,2′′-(benzene-1,3,5-triyl)tris[1-phenyl-1H-benzimid-
azole], and magnesium:silver alloy (ca. 10:1), respectively.
A thin film of PEDOT was spin-cast onto a precleaned ITO
surface and then baked at 80 °C for 12 h under vacuum to
improve hole injection and to increase substrate smoothness.
The emitting layer, PVK-PBD (30 wt %) with different dop-
ing concentrations of complex 1 (0.18, 1.8, and 5.3 wt %,
respectively; corresponding to complex 1/PVK monomer unit
molar ratios of 0.05:100, 0.50:100, and 1.5:100), was then
spin-cast onto the surface of PEDOT. A typical film thickness
of the emitting layer was 50-60 nm. Through the blend with
PBD, the electron transport in the emitting layer was
facilitated. A layer of TPBI electron-injection/transport layer
was also used at the cathode for the hole-blocking as well
as the exciton confinement.
Figure 4a shows the current-voltage curves of the
electroluminescent devices for 1 at various doping concentra-
tions. The driving voltages of these devices increase with
dopant concentrations, consistent with earlier reports that
iridium complexes behave as carrier traps in devices.18 At
the lowest doping concentration of 0.18 wt %, the EL
spectrum mainly originates from the triplet emission of
complex 1, accompanied by a minor contribution from the
host emission (not shown here). Among various blends with
complex 1, the best device performance was achieved at 1.8
wt % (Figure 4b), which rendered a turn-on voltage of 9.4
V (at 1 cd/m2) and reached the maximum brightness of 7750
cd/m2 at a driving voltage of 21 V and with CIE coordinates
of (0.64, 0.31). Figure 5 shows external quantum efficiency
and luminous efficiency, respectively, as a function of the
current density of the device at a doping concentration of
1.8 wt %. The maximum external quantum efficiency was
shown to be 3.15% ph/el at a current density of 67.4 mA/
cm2 with a brightness of 1751 cd/m2. The device quantum
efficiency remained at a relatively high value up to a driving
current of 370 mA/cm2. A further increase in doping
concentration to 5.3 wt % led to a decrease in the external
(15) (a) Ohsawa, Y.; Sprouse, S.; King, K. A.; DeArmond, M. K.; Hanck,
K. W.; Watts, R. J. J. Phys. Chem. 1987, 91, 1047. (b) Tamayo, A.
B.; Alleyne, B. D.; Djurovich, P. I.; Lamansky, S.; Tsyba, I.; Ho, N.
N.; Bau, R.; Thompson, M. E. J. Am. Chem. Soc. 2003, 125, 7377.
(16) Hay, P. J. J. Phys. Chem. A. 2002, 106, 1634.
(17) Brooks, J.; Babayan, Y.; Lamansky, S.; Djurovich, P. I.; Tsyba, I.;
Bau, R.; Thompson, M. E. Inorg. Chem. 2002, 41, 3055.
(18) Chen, F.-C.; Chang, S. C.; He, G.; Pyo, S.; Yang, Y.; Kurotaki, M.;
Kido, J. J. Polym. Sci., Part B: Polym. Phys. 2003, 41, 2681.
Inorganic Chemistry, Vol. 44, No. 5, 2005 1349

Hwang et al.
Scheme 2
luminescent devices,21 while the strong emission occurring
at λmax 514 nm is believed to originate from the triplet excited
states possessing both intraligand ππ* and metal-to-ligand
charge-transfer characters (MLCT).22 It is thus anticipated
that tuning the emission to the red can be qualitatively
achieved by lowering the energy level of either ππ* or
MLCT states of the iridium derivatives. The results of MO
calculations on group 3 metal-8-quinolinato chelates23 as
well as the boron pyridyl benzimidazole complexes24 lead
us to propose that LUMO and HOMO of the cyclometalated
ligands in the Ir(ppy)3 type of complexes are mainly located
at the neutral part and the negatively charged side of the
ligands, respectively. This delineation is also supported by
the recently published, time-dependent density functional
theory (TDDFT) calculation on Ir cyclometalated com-
plexes.25 Accordingly, as shown in Scheme 2, replacement
of one CH group at the pyridyl fragment by an electroneg-
ative nitrogen atom would give a pyrazine-substituted ligand
fragment and display a significantly decreased LUMO energy
level. Conversely, the HOMO of this complex, which is
mainly located at the relatively electron rich phenyl group
of the cyclometalated ligand or the dπ states of the iridium
atom, remains unchanged. Thus, such an atom substitution
at the cyclometalated ligand will cause an appreciable
Figure 5. External quantum efficiency and luminous efficiency versus
current density of the device with 1.8 wt % of 1 as the dopant emitter.
Inset: PL and EL spectra recorded at a driving voltage of 21 V.
quantum efficiency (2.11% ph/el) and brightness (1102 cd/
m2 at 67.4 mA/cm2). This decrease might result from the
concentration quenching and triplet-triplet annihilation.1a,19
We also studied PL spectra of the polymer blends. At the
doping concentration of 1.8 wt %, there was still a significant
contribution of emission from the host material (the inset of
Figure 5). At a doping concentration of 5.3 wt %, the energy
transfer was nearly complete and the PL was mainly from
complex 1; however, the PL intensity was slightly decreased
comparing with that at a concentration of 1.8 wt %. This
result is consistent with the concentration quenching20 and
again indicates that the decrease in the external quantum
efficiency at a doping concentration of 5.3 wt % might be a
result of concentration quenching.
Since the PVK-PBD host exhibits an emission maximum
at 420 nm, there is a substantial overlap between the emission
of the host and the spin-allowed MLCT absorption of 1.
Accordingly, rapid Forster energy transfer between host and
complex 1 is expected. The inset of Figure 5 shows the
corresponding PL and EL spectra at the doping concentration
of 1.8 wt % and a driving voltage of 21 V. It should be
noted that, despite the contribution of the PL spectrum from
the host material, the corresponding EL spectrum was
dominated by the triplet excited states of 1 with an emission
peak wavelength at 640 nm. The result strongly supports
the supposition that both Forster energy transfer and direct
charge-trapping/recombination at the iridium metal centers
are responsible for the observed EL.20
3
reduction of the energy gap for the expected ππ* and
3MLCT emission. Further attachment of an extra aromatic
ring into the ligand π framework, e.g. using quinoxaline to
replace the hypothetical pyrazine fragment, should elongate
the overall π-conjugation, giving a further decrease of the
energy gap. Therefore, continuously tuning the emission to
the red can be rationalized.
In fact, the dpqx-based complex 1 shows a dark red
emission at λmax 642 nm, which has been tuned much closer
(21) (a) Baldo, M. A.; Lamansky, S.; Burrows, P. E.; Thompson, M. E.;
Forrest, S. R. Appl. Phys. Lett. 1999, 75, 4. (b) Tsutsui, T.; Yang,
M.-J.; Yahiro, M.; Nakamura, K.; Watanabe, T.; Tsuji, T.; Fukuda,
Y.; Wakimoto, T.; Miyaguchi, S. Jpn. J. Appl. Phys. 1999, 38, L1502.
(c) Adachi, C.; Baldo, M. A.; Forrest, S. R.; Thompson, M. E. Appl.
Phys. Lett. 2000, 77, 904. (d) Bruce, D.; Richter, M. M. Anal. Chem.
2002, 74, 1340.
Discussion
(22) (a) Sprouse, S.; King, K. A.; Spellane, P. J.; Watts, R. J. J. Am. Chem.
Soc. 1984, 106, 6647. (b) Wilde, A. P.; King, K. A.; Watts, R. J. J.
Phys. Chem. 1991, 95, 629. (c) Colombo, M. G.; Brunold, T. C.;
Riedener, T.; Guedel, H. U.; Fortsch, M.; Buergi, H.-B. Inorg. Chem.
1994, 33, 545.
(23) (a) Chen, C. H.; Shi, J. Coord. Chem. ReV. 1998, 171, 161. (b) Curioni,
A.; Boero, M.; Andreoni, W. Chem. Phys. Lett. 1998, 294, 263. (c)
Anderson, S.; Weaver, M. S.; Hudson, A. J. Synth. Met. 2000, 111-
112, 459. (d) Sapochak, L. S.; Benincasa, F. E.; Schofield, R. S.; Baker,
J. L.; Riccio, K. K. C.; Fogarty, D.; Kohlmann, H.; Ferris, K. F.;
Burrows, P. E. J. Am. Chem. Soc. 2002, 124, 6119.
Tuning of the Emission Properties. Having gathered the
above information, in combination with the reports in the
literature (vide infra), in this section, we elaborate on the
strategies of tuning the emission property in a systematic
manner.
Cyclometalated Ir(ppy)3 has been extensively applied in
fabricating green organic light-emitting and electrochemi-
(24) (a) Liu, S.-F.; Wu, Q.; Schmider, H. L.; Aziz, H.; Hu, N.-X.; Popovic,
Z.; Wang, S. J. Am. Chem. Soc. 2000, 122, 3671. (b) Liu, Q.; Mudadu,
M. S.; Schmider, H.; Thummel, R.; Tao, Y.; Wang, S. Organometallics
2002, 21, 4743.
(19) Gong, X.; Ostrowski, J. C.; Bazan, G. C.; Moses, D.; Heeger, A. J.;
Liu, M. S.; Jen, A. K.-Y. AdV. Mater. 2003, 15, 45.
(20) Guo, T.-F.; Chang, S.-C.; Yang, Y.; Kwong, R. C.; Thompson, M. E.
Org. Electron. 2000, 1, 15.
(25) Hay, P. J. J. Phys. Chem. A 2002, 106, 1634.
1350 Inorganic Chemistry, Vol. 44, No. 5, 2005

Ir(III) Complexes with Quinoxaline Ligands
to the dark red region. To counterbalance the excessive red
shift, a rational design is to lower the HOMO of the
cyclometalated ligand by replacing the phenyl fragment with
a 4-fluorophenyl substituent. As predicted, the corresponding
4-fluorophenyl-substituted dfqx complex 2 exhibited a red-
orange emission at λmax ) 630 nm, supporting our proposed
principle. Moreover, it has been reported that the replacement
of one ppy ligand in the trisubstituted derivative Ir(ppy)3 with
a bidentate ancillary ligand (LX) afforded mixed-ligand
complexes (ppy)2Ir(LX), where LX ) acetylacetonate (acac),
N-methylsalicyliminate (sal), and picolinate (pic).26 In these
mixed-ligand complexes, despite the emissions retaining the
main characteristics of parent complex Ir(ppy)3, a minor
dependence on the emission peak wavelength was also noted,
which varies according to the nature of the ancillary ligand
LX. For instance, the observed emission λmax changes
according to an order of pic < sal ∼ acac, which is
proportional to their electron donor strengths, resulting in a
reduction of the energy gap. Accordingly, we can select the
third ancillary ligands from the electron-deficient pyrazole
(fppz)H and triazole (fppz)H or the electron-rich pyrazole
(bppz)H,27 in an attempt to reach a saturated red emission.
As the donor strengths are expected to follow a qualitative
trend of fptz < fppz < bppz, we are not surprised that
complex 3 displays the most blue-shifted emission signal.
Conversely, complexes 1 and 4 show the best performance
toward the saturated red among all the iridium metal
complexes applied in this study.
Figure 6. Basic molecular design and emission λmax of iridium complexes
(C∧N)2Ir(acac), where symbol [Ir] indicates the missing molecular fragment
(C∧N)Ir(acac). The ligand abbreviations are named according to those that
first appeared in the literature.
Moreover, albeit not so well engineered, a qualitative trend
of color tuning can sporadically also be found in the
literature. Thompson and co-workers have reported a series
of Ir complexes (C∧N)2Ir(LX), where C∧N denotes various
cyclometalated ligands such as phenyl pyridine (ppy),
benzothiazole (bt), and benoxazole (bo) and LX is the
â-diketonate ligand (Figure 6).17 The respective emission data
make us to conclude that substitution of the parent phe-
nylpyridine ligand (ppy) with the phenylbenzothiazole (bt)
ligand caused a bathochromic shift of emission wavelengths
from 516 to 557 nm due to the higher polarizibility and
basicity of the sulfur atom.28 Certainly, the contribution from
the extended aromatic π system on the benzothiazole ligand
cannot be completely ignored in accounting for the observed
red shifting. Further increasing the degree of ligand π
conjugation can possibly be done by replacing the phenyl
with a 1-naphthyl group, as observed in assembling the Rbsn
ligand. This manipulation is expected to shift the emission
wavelength further toward the red, giving a strong PL
emission at 606 nm. It is also notable that the 2-naphthyl
group in âsbsn acts less effectively than the 1-naphthyl group
in Rbsn, resulting in an orange emission with λmax at 594
nm, accompanied by a slight decrease in the luminescent
quantum efficiency from 0.22 to 0.16. Although the exact
nature of the ligand orientation effect remains unclear at this
stage, the results imply that the orientation of π conjugation
would exert a large effect on the photophysical properties
of the phosphorescent complexes. On the other hand, an
electron-donating thiophene group (thp) can be selected to
replace the phenyl substituent of ppy ligand to induce a
similar red shift to 562 nm. Further extending the π
conjugation to the thp ligand as well as the insertion of an
extra electron withdrawing CF3 group at the 4-position of
the pyridyl group would result an additional red shift, giving
rise to emission peak wavelengths at 612 and 617 nm,29 as
observed for the btp and fbtp complexes shown in Figure 6.
Finally, although it was not explicitly shown by the available
data, these sulfur-containing materials may show slightly
lower emission quantum yields related to complexes pos-
sessing no sulfur substituent. This result, to a certain extent,
could be attributed to the thermal population of the proximal
nπ* excited state, which tends to give a less effective
radiative transition probability.
Researchers have also directed their attention to fine-tuning
the emission color of another class of iridium complexes,
(26) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Kwong,
R.; Tsyba, I.; Bortz, M.; Mui, B.; Bau, R.; Thompson, M. E. Inorg.
Chem. 2001, 40, 1704.
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Wu, P.-C.; Song, Y.-H.; Chi, Y. Chem. Commun. 2003, 2628. (b) Wu,
P.-C.; Yu, J.-K.; Song, Y.-H.; Chi, Y.; Chou, P.-T.; Peng, S.-M.; Lee,
G.-H. Organometallics 2003, 22, 4938.
(28) Zollinger, H. Color Chemistry: Syntheses, Properties and Applications
of Organic Dyes and Pigments, 2nd ed.; VSH: Weinheim, Germany,
1991.
(29) (a) Okada, S.; Iwawaki, H.; Furugori, M.; Kamatani, J.; Igawa, S.;
Moriyama, T.; Miura, S.; Tsuboyama, A.; Takiguchi, T.; Mitutani.
H. SID Symp. Dig. 2002, 1360. (b) Chen, X.; Liao, J.-L.; Liang, Y.;
Ahmed, M. O.; Tseng, H. E.; Chen, S. A. J. Am. Chem. Soc. 2003,
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Inorganic Chemistry, Vol. 44, No. 5, 2005 1351

Hwang et al.
complexes possessing the F-pbq ligand.32 This manipulation
is consistent with a two-step synthetic scheme incorporating
an extension of π-conjugation, followed by lowering of the
HOMO energy level using an electronegative F atom.
Moreover, when the isoquinoline fragment is targeted to link
with a 1-naphthyl or 2-naphthyl substituent (route B3), red
and dark-red emission at 633 and 680 nm were eventually
observed due to the extension of π conjugation.33 The large
difference of 47 nm in these emission peaks is rationalized
by the presence of two distinctive naphthyl ligand orienta-
tions, in which the electronic transition dipole moment could
be markedly different. An additional example involves the
conversion from isoquinoline to quinazoline (route B4), for
which the extra nitrogen atom on the ligand skeleton is
expected to exert a large bathochromic effect on the emission
wavelength via lowering of the LUMO energy level.34
Phenylquinazoline appears to be an accessible and highly
reactive cyclometalated ligand, which gives rise to required
iridium complexes with a red emission (625-638 nm) after
incorporating a 2-pyridylpyrazolate type of ligand, which is
comparably more electron deficient compared with diketo-
nate chelating ligands while completing the octahedral ligand
arrangement required for Ir metal complexes.
Other color-tuning principles have also been documented
in the literature. For example, routes C1 and C2 indicate
that substitution of pyrimidine for pyridine exerts a similar
effect of lowering the LUMO. A further red shift is observed
when employing a 1-naphthyl group to replace the phenyl
fragment, of which the highest energy emission λmax occurs
at 618 nm, suitable for fabricating the saturated red color
OLEDs. Note that further red shifting is commonly observed
after mixing with the polymer host matrix.35 In contrast, route
D represents a unique strategy, in which introducing an
electronegative nitrogen atom and enlarging the π delocal-
ization across the whole ligand π system have been achieved
in one step using an aromatic heterocycle, dibenzo[f,h]-
quinoxaline.36 Devices based on this Ir complex have been
made with orange red emission at ∼610 nm, accompanied
by an exceedingly high brightness of 58100 cdm-2 and
excellent current and power illumination efficiencies of 26.2
cd A-1 and 13.7 lm W-1, respectively.
Figure 7. Basic molecular design and emission λmax of iridium complexes
(C∧N)3Ir or (C∧N)2Ir(LX), where symbol [Ir] indicates the missing
molecular fragment (C∧N)2Ir or (C∧N)Ir(LX). The ligand abbreviations
are named according to those that first appeared in the literature.
for which the cyclometalated C∧N ligands of Ir(C∧N)3 or
Ir(C∧N)2(LX) show no sulfur substituents and LX is
â-diketonate or other nitrogen-containing chelate ligand.30
On one hand, as indicated in Figure 7, one simple strategy
is to replace the pyridine fragment of ppy ligand with a
2-quinolinyl group. Orange emission at 597 nm is thus
observed by extending the π conjugation (see route A in
Figure 7).12 On the other hand, a more effective approach is
to replace the pyridine fragment of the ppy ligand with a
1-isoquinoline group. Using this methodology, complexes
of formula Ir(piq)3 or Ir(piq)2(acac) were synthesized, both
revealing intense red emissions centered at ∼622 nm (route
B1).31 Further extending the π system via selecting the
benzoquinoline yields the corresponding pbq complex with
a rather dark red color (route B2). As a result, application
of 4-FC6H4 group is necessary to blue shift the emission
wavelength toward 631 nm, as has been demonstrated in
Nevertheless, several less successful examples of color
tuning have also been documented in the literature. Parts of
these articles described the use of aldehyde-,37 perfluorophen-
yl-,38 and even fluorene-modified phenylpyridine-based
(32) Su, Y.-J.; Huang, H.-L.; Li, C.-L.; Chien, C.-H.; Tao, Y.-T.; Chou,
P.-T.; Datta, S.; Liu, R.-S. AdV. Mater. 2003, 15, 884.
(33) Yang, C.-H.; Tai, C.-C.; Sun, I.-W. J. Mater. Chem. 2004, 14, 947.
(34) Song, Y.-H.; Yeh, S.-J.; Chen, C.-T.; Chi, Y.; Liu, C.-S.; Yu, J.-K.;
Hu, Y.-H.; Chou, P.-T.; Peng, S.-M.; Lee, G.-H. AdV. Funct. Mater.
2004, 14, 1221.
(35) Niu, Y.-H.; Chen, B.; Liu, S.; Yip, H.; Bardecker, J.; Jen, A. K.-Y.;
Kavitha, J.; Chi, Y.; Shu, C.-F.; Tseng, Y.-H.; Chien, C.-H. Appl. Phys.
Lett. 2004, 85, 1619.
(30) Wang, Y.; Herron, N.; Grushin, V. V.; LeCloux, D.; Petrov, V. Appl.
Phys. Lett. 2001, 79, 449.
(31) (a) Tsuboyama, A.; Iwawaki, H.; Furugori, M.; Mukaide, T.; Kamatani,
J.; Igawa, S.; Moriyama, T.; Miura, S.; Takiguchi, T.; Okada, S.;
Hoshino, M.; Ueno, K. J. Am. Chem. Soc. 2003, 125, 12971. (b) Jaing,
C.; Yang, W.; Peng, J.; Xiao, S.; Cao, Y. AdV. Mater. 2004, 16, 537.
(36) Duan, J.-P.; Sun, P.-P.; Cheng, C.-H. AdV. Mater. 2003, 15, 224.
(37) Beeby, A.; Bettington, S.; Samuel, I. D. W.; Wang, Z. J. Mater. Chem.
2003, 13, 80.
(38) Tsuzuki, T.; Shirasawa, N.; Suzuki, T.; Tokito, S. AdV. Mater. 2003,
15, 1455.
1352 Inorganic Chemistry, Vol. 44, No. 5, 2005

Ir(III) Complexes with Quinoxaline Ligands
ligands.19,39 It is notable that all these functional groups are
unable to exert further reduction in the HOMO/LUMO gap
to fulfill the saturated red emission. As a result, the
corresponding emission wavelength spans only a narrow
range between yellow and orange, despite some of them
showing rather interesting and marvelous characteristics, such
as the enhancement of thermal stability, improved electron-
transporting properties, or even a higher glass transition
temperature so that the smooth film can be directly obtained
from the amorphous state.
and OLED applications. By executing our design strategy,
we have gained knowledge and developed a semiquantitative
method of achieving the much-needed red emission. This
method incorporates the utilization of two cyclometalated
ligands with extended π system and one extra electron
negative nitrogen atom to counterbalance the energy gap
suited to the saturated red emission, which is complementary
to the results first reported by Thompson and co-workers.12
Simple PLED devices were also prepared to evaluate their
practical potentials, for which good device performance has
been successfully achieved using complex 1 at lower dopant
concentrations, demonstrating a rare example of highly
efficient Ir(III)-based light-emitting devices with saturated
red emission.
Conclusion
In summary, we report detailed syntheses and electro-
chemical and photophysical properties of four red-emitting
iridium metal complexes using the cyclometalated quinoxa-
line ligands as well as one nitrogen chelate ligand, such as
the pyridylpyrazolate or -triazolate. These complexes exhibit
high emission quantum yields and short phosphorescence
radiative lifetimes in the range of several microseconds that
are suitable for serving as red-emitting materials for PLED
Acknowledgment. We thank the National Science Coun-
cil for the support (Grants NSC 93-2113-M-007-012 and
NSC 93-ET-7-007-003). Special thanks go to Prof. C.-H.
Cheng and Dr. J.-P. Duan for their assistance on preparation
of PLED devices and recording the electroluminescent data.
Supporting Information Available: An X-ray crystallographic
data file (CIF) for complex 1. This material is available free of
charge via the Internet at http://pubs.acs.org.
(39) (a) Ostrowski, J. C.; Robinson, M. R.; Heeger, A. J.; Bazan, G. C.
Chem. Commun. 2002, 784. (b) Gong, X.; Lim, S.-H.; Ostrowski, J.
C.; Moses, D.; Bardeen, C. J.; Bazan, G. C. J. Appl. Phys. 2004, 95,
948.
IC0489443
Inorganic Chemistry, Vol. 44, No. 5, 2005 1353