Strong ligand field effects of blue phosphorescent iridium(III) complexes

Article abstract of DOI:10.1080/15421400903582857

In this study, new deep blue emitting mixed ligand iridium(III) complexes such as Ir(dFppy)(PPhMe₂)₂(H)(Cl), [Ir(dFppy)(PPhMe ₂)₂(H)(NCMe)]⁺ and Ir(dFppy)(PPhMe ₂)₂(H)(CN), [dFpp

Full text of DOI:10.1080/15421400903582857

Mol. Cryst. Liq. Cryst., Vol. 520: pp. 97=[373]–107=[383], 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400903582857
Strong Ligand Field Effects of Blue
Phosphorescent Iridium(III) Complexes
HO WAN HAM1 AND YOUNG SIK KIM1,2
1Department of Information Display, Hongik University, Seoul, Korea
2Department of Science, Hongik University, Seoul, Korea
In this study, new deep blue emitting mixed ligand iridium(III) complexes such
as Ir(dFppy)(PPhMe2)2(H)(Cl), [Ir(dFppy)(PPhMe2)2(H)(NCMe)]þ and
Ir(dFppy)(PPhMe2)2(H)(CN), [dFppy ¼ 2-(20,40-difluoro-phenyl)pyridine] were
synthesized and studied to tune the phosphorescence wavelength to the deep blue
region and to enhance the luminescence efficiencies. These iridium complexes com-
prise one cyclometalating, two phosphines trans to each other and two cis-ancillary
ligands. We investigate the electron-withdrawing capabilities of ancillary ligands
using the DFT and TD-DFT calculations on the ground and excited states of the
three complexes to gain insight into the factors responsible for the emission color
change and the different luminescence efficiency. Reducing the molecular weight
of phosphine ligand with PPhMe2 leads to a strategy of the efficient deep blue
organic light-emitting devices (OLED) by thermal processing instead of the solution
processing. The electron-withdrawing difluoro group substituted on the phenyl ring
and the cyano strong field ancillary ligand in the trans-position to the carbon
atom of phenyl ring increased HOMO-LUMO gap and achieved the hypsochromic
shift in emission color. As a result, the maximum emission spectra of Ir(dFp-
py)(PPhMe2)2(H)(Cl), [Ir(dFppy) (PPhMe2)2(H)(NCMe)]þ and Ir(dFppy)
(PPhMe2)2(H)(CN) were in the ranges of 452, 443, 442 nm, respectively.
Keywords Blue; iridium complex; mixed ancillary ligands; OLED materials;
phosphorescence
1. Introduction
In the last few years, phosphorescent materials and devices have been extensively
studied in order to achieve a high efficiency for organic light-emitting diodes
(OLEDs) since Thompson et al. reported OLEDs with phosphorescent heavy metal
complexes [1–3]. Heavy metals present in the complexes, such as iridium or platinum,
are known to induce an intersystem crossing by strong spin-orbit coupling. This
crossing leads to a mixing of the singlet and triplet excited states [4–8]. Radiative
relaxation of the spin-forbidden nature from the triplet excited state is then allowed,
resulting in high phosphorescence efficiencies. Thus, heavy metal complexes can
serve as efficient phosphors in OLEDs. Unfortunately, most phosphorescent emit-
ters have a long radiative lifetime, which leads to the dominant triplet-triplet
Address correspondence to Young Sik Kim, Department of Information Display, Hongik
University, Seoul 121-791, Korea. E-mail: youngkim@wow.hongik.ac.kr
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H. W. Ham and Y. S. Kim
(T-T) annihilation at high currents. The occurrence of T-T annihilation diminishes
the performance of phosphorescent material, particularly its maximum brightness
and luminescence efficiency at high currents [9,10].
Until now, the development of a green and red emitter in fluorescent and
phosphorescent displays has achieved in great success. However, pure blue-emitting
complex dopants are not available. Hence, we have focused current research efforts
on the syntheses of blue-emitting iridium complexes. Some phosphorescent
blue-emitter materials have been synthesized and the results reported; particularly
2-phenylpyridine with added electro-withdrawing groups to the phenyl ring (fluoro,
cyano group and others), and electro-donating groups added to the pyridyl ring
(methyl group and others) exhibited good blue-emission characteristics [11,12].
In this study, new deep blue-emitting mixed-ligand iridium(III) complexes
such as Ir(dFppy)(PPhMe2)2(H)(Cl), Ir(dFppy)(PPhMe2)2(H)(NCMe)þ and Ir(dFp-
py)(PPhMe2)2(H)(CN), [dFppy ¼ 2-(20,40-difluoro-phenyl)pyridine] were synthesized
and studied to tune the phosphorescence wavelength to the deep blue region and to
enhance its luminescence. These iridium complexes comprise of one cyclometalating
and two phosphines trans to each other, and two cis-ancillary ligands. We investi-
gated the strong field effects of ancillary ligands to gain insight into the factors
responsible for the emission color change and the different luminescence efficiency.
To analyze the color tuning by the effects of trans ligands to dFppy ligand on the
luminescent property, we have measured the UV-absorption and photoluminescence
(PL) spectra of iridium complexes, and have calculated these complexes theoretically
by the density functional theory (DFT) Method.
2. Experimental Details
2.1. Synthesis
All ligands and reagents were purchased from Aldrich Co., except for Ir(III) trichlo-
ride hydrate (IrCl3 ꢀ H2O), which was purchased from Strem Co. and used without
further purification. All reactions were carried out under a nitrogen or argon atmos-
phere. Solvents were dried using standard procedures. All column chromatography
was performed with the use of silica gel (230-Mesh, Merck). Iridium complex,
[Ir(COD)Cl]2 was synthesized by the literature methods.
2.1.1. Synthesis of Ligand: dFppy. dFppy was obtained from the reaction of
2-Chloropyridine with the corresponding 2,4-difluorophenyl-boronic acid by Suzuki
coupling. 2-Chloropyridine (0.56 g, 5 mmol), 2,4-difluoro-phenylboronic acid (0.79 g,
5 mmol) and tetrakis-(triphenylphospine)palladium(0) (0.196 g, 0.17 mmol) were
dissolved in 20 ml of toluene, 10 ml of ethanol and 20 ml of 2 N sodium carbonate
aqueous solution. The mixture was refluxed at 110ꢁC under the nitrogen atmosphere
for eight hours. The mixture was cooled to room temperature and extracted with
20 ml of ethyl acetate. The organic fraction was dried over anhydrous MgSO4, then
filtered off and dried under vacuum. The residue was purified by silica gel
chromatography using ethyl acetate=hexane (1:3). The product was collected and
dried under vacuum.dfppy – Yield 60%, bright orange oil, MW: 192.2.
2.1.2. Synthesis
of
Complex:
Ir(dFppy)(PPhMe2)2(H)(Cl). Chloro(1,5-
cyclooctadiene)iridium(I) dimmer [Ir(COD)Cl]2 (0.2 g, 0.28 mmol), dimethylphenyl-
phospine (PPhMe2) (0.16 ml, 1.12 mmol) and dFppy (0.11 g, 0.56 mmol) were

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dissolved in 20 ml of 2-ethoxyethanol in a 50 ml flask. The mixture was refluxed at 90ꢁC
under the nitrogen atmosphere for six hours and then cooled to room temperature.
The product portion was collected and washed with 20 ml of methanol, recrystallized
in chloroform=n-pentane, and dried under vacuum.
Ir(dFppy)(PPhMe2)2(H)(Cl) – Yield 78%, pale-yellowish crystal.
2.1.3. Synthesis of Complex: [Ir(dFppy)(PPhMe2)2(H)(NCMe)](OTf). Ir(dFppy)
(PPh2Me)2(H)(Cl) (0.082 g, 0.1 mmol), silver trifluoromethane sulfonate (AgOTf)
(0.026 g, 0.1 mmol) and acetonitrile anhydrous (MeCN) (1 ml, 20 mmol) were
dissolved in 10 ml of chloroform in a 50 ml flask. The mixture was stirred under
the nitrogen atmosphere for two hours at normal temperature. After two hours of
stirring, white powder of AgCl was removed by filtration and 20 mmol of MeCN
was added once more into the product solution, and stirred under the nitrogen
atmosphere for an extra two hours at normal temperature. 30 ml of n-pentane was
added to yield pale-yellowish concentrated oil and residue were collected and
washed with methanol and then dried under vacuum.
[Ir(dFppy)(PPhMe2)2(H)(NCMe)](OTf) – Yield 65%, yellowish concentrated oil.
2.1.4. Synthesis of Complex: Ir(dFppy)(PPhMe2)2(H)(CN). [Ir(dFppy)(PPhMe2)2
(H)(NCMe)](OTf) (0.9 g, 0.1 mmol) and tetrabutylammonium cyanide (0.027 g,
0.1 mmol) were dissolved in 10 ml of methylene chloride in a 50 ml flask. The
mixture was stirred under the nitrogen atmosphere for three hours at normal
temperature. After six hours of stirring, 20 ml of n-pentane was added to yield
pale-yellowish, concentrated oil. The residue was collected and washed with
methanol and dried under vacuum.
Ir(dFMeppy)(PPhMe2)2(H)(CN) – Yield 63%, yellowish concentrated oil.
2.2. Optical Measurements
The UV-Vis absorption spectra were measured on Hewlett Packard 8425A spec-
trometer. The PL spectra were obtained on Perkin Elmer LS 55 spectrometer.
UV-Vis and PL spectra of Ir(dFppy)(PPhMe2)2(H)(Cl), Ir(dFppy)(PPhMe2)2
(H)(NCMe)þ and Ir(dFppy)(PPhMe2)2(H)(CN) were measured in CH2Cl2 at room
temperature.
2.3. Theoretical Calculations
Computationally, the electronic ground states of Ir(dFppy)(PPhMe2)2(H)(Cl),
Ir(dFppy)-(PPhMe2)2(H)(NCMe)þ and Ir(dFppy)(PPhMe2)2(H)(CN) were calcu-
lated using the B3LYP density functional theory (DFT). LANL2DZ [13] and
6-31G(d) [14] basis sets were employed for Ir and the other atoms, respectively.
For the calculated ground state geometries, the electronic structures were examined
in terms of the highest occupied molecular orbitals (HOMOs) and the lowest unoc-
cupied molecular orbitals (LUMOs). Moreover, the electronic populations on the
central atom were calculated to show the significant admixture of ligand p character
with the amount of metal 5d character in the occupied molecular orbitals related to
those metal-to-ligand charge-transfer (MLCT) transitions. To obtain the vertical
excitation energies of the low-lying singlet and triplet excited states of the complexes,
time-dependent density functional theory (TD-DFT) calculations using the B3LYP

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H. W. Ham and Y. S. Kim
functional were performed at the respective ground-state geometry, where the basis
set of ligands was changed to 6-31þG(d).
3. Results and Discussion
The colors of phosphorescent complexes used in OLEDs are tuned by the variation
of both cyclometalating and ancillary ligands. Several small molecular and polymeric
blue-emitting phosphorescent complexes have been reported, but the realization of
highly efficient deep blue-emission remains a challenge. For the common 2-phenyl-
pyridine (ppy) cyclometalating ligand, the highest occupied molecular orbital
(HOMO) is localized on phenyl p and iridium 5d orbitals, while the lowest unoccu-
pied molecular orbital (LUMO) is localized on the pꢂ orbitals of the pyridyl ring. To
achieve deep blue-emission: (1) substituting fluoro groups on the 20,40-position of the
phenyl ring of ppy ligand as electron withdrawing group to stabilize the HOMO and
increase HOMO-LUMO gap, (2) changing ancillary ligands coordinated to iridium
atom to phosphine and cyano groups. Both phosphine and cyano groups are known
as very strong field ligands, and their inclusion in the coordination sphere can
increase the HOMO-LUMO gap to achieve the hypsochromic shift in emission
color, and (3) reducing the molecular weight of phosphine ligand for fabrication
method by thermal processing instead of the solution processing.
The main ligands, dFppy, were prepared by the Suzuki coupling, as illustrated in
Figure 1(a). The final iridium complex was obtained via several steps. In the first
step, [Ir(COD)Cl]2, PPhMe2 and dFppy ligand were synthesized, as illustrated in
Figure 1(b). In the second step, chlorine ligand of Ir(F2Meppy)(PPh2Me)2(H)(Cl)
was replaced with MeCN ligand by precipitation reaction of Ag and Cl ions, as
illustrated in Figure 1(c). Finally, in the third step, MeCN ligand was substituted
with CN ligand, as illustrated in Figure 1(d). The overall reaction yields were
approximately 60 to 78%, respectively.
The solution UV–Vis absorption of complexes has been measured, as shown in
Figure 2. The interesting characteristics of the UV absorption spectra for
[Ir(dFppy)(PPhMe2)2(H)(NCMe)]þ resemble those of Ir(dFppy)(PPhMe2)2(H)(CN).
The highest energy absorptions are due to transitions centered on the dFppy ligands,
and shoulder at ꢃ270 nm is attributed to transitions centered on the phosphine
ligand. Weaker levels at lower energy (k > 330 nm) are due to spin-allowed singlet-
to-singlet metal-to-ligand charge-transfer (1MLCT) and spin-forbidden singlet-
to-triplet 3MLCT transitions. MLCT transition has been known for complexes
containing heavy metal, such as iridium, in which spin-orbit coupling is strong
[14,15]. It is slightly difficult to classify the 1MLCT and 3MLCT transitions in
these compounds. However [Ir(dFppy) (PPhMe2)2(H)(NCMe)þ and Ir(dFppy)-
(PPhMe2)2(H)(CN) compounds display more blue shifted p-pꢂ and MLCT transition
1
3
than that of the Ir(dFppy)(PPhMe2)2(H)(Cl) compound. The MLCT and MLCT
absorption peaks of [Ir(dFppy)(PPhMe2)2(H)(NCMe)þ and Ir(dFppy)(PPhMe2)2
(H)(CN) are observed at 340ꢃ402 nm, whereas absorption peaks of Ir(dFppy)(PPh-
Me2)2(H)(Cl) are observed at 357ꢃ411 nm. This was due to the strong field ancillary
ligand, such as CNꢄ, lowering the HOMO energy level more than the weak field
ancillary ligand, Clꢄ. Ancillary ligands alter the MLCT energy, mainly by changing
the HOMO energy level. The HOMO energy level may then be lowered by
strong-field ancillary ligands, which causes a large d-orbital energy splitting. It is also

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Figure 1. (a) Synthesis of dFppy ligand; (b) Synthesis of Ir(dFppy)(PPhMe2)2(H)(Cl); (c) Syn-
thesis of [Ir(dFppy)(PPhMe2)2(H)(NCMe)]þ; (d) Synthesis of Ir(dFppy)(PPhMe2)2(H)(CN).
likely that strong field ancillary ligands lengthen the bond lengths of Ir–C (ppy) trans
to the ancillary ligand to lower the energy levels of the dp-orbitals of the metal, as
strong field ligands are known to show a high trans effect. Medium filed ancillary
ligand, MeCN, is located between them. Fluoro groups substituted on the
20,40-position of phenyl ring acted as the electron withdrawing group and increased
the HOMO-LUMO energy gap and tune of the MLCT absorption peak to bluish.
Figure 3 shows the calculated HOMO and LUMO energy levels and energy
gap of iridium complexes on the optimized molecular structure of Ir(dFppy)
(PPhMe2)2(H)(Cl) and Ir(dFppy) (PPhMe2)2(H)(CN) using DFT. The HOMO
of Ir(dFppy)(PPhMe2)2(H)(CN) is stabilized compared to that of Ir(dFppy)
(PPhMe2)2(H)(Cl) because of a high trans effect of the strong-field ancillary ligand,
CNꢄ, which causes a significant blue shifted emission.
The PL spectra of iridium complexes in CH2Cl2 at room temperature are shown
in Figure 4. The maximum emission spectra of Ir(dFppy)(PPhMe2)2(H)(Cl),

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H. W. Ham and Y. S. Kim
Figure 2. UV–Vis absorption spectra of Ir(dFppy)(PPhMe2)2(H)(Cl), [Ir(dFppy)(PPhMe2)2
(H)(MeCN)]þ and Ir(dFppy)(PPhMe2)2(H)(CN) in CH2Cl2 solution at room temperature.
[Ir(dFppy)(PPhMe2)2(H)(NCMe)]þ and Ir(dFppy)(PPhMe2)2(H)(CN) were 452,
443, and 442 nm, respectively. The PL spectra of iridium complexes are compared
with those of similar iridium complexes such as Ir(dFppy)(PPh3)2(H)(Cl), [Ir(dFppy)
(PPh3)2(H)(NCMe)]þ and Ir(dFppy)(PPh3)2(H)(CN) at 448, 441 and 441 nm,
respectively [16]. Their PL spectra of Ir complexes are nicely matched with those
of Ir complexes with PPhMe2. Knowing that d-orbitals of iridium are involved in
the HOMO of C^N complexes, such as Ir(C^N)3 and Ir(C^N)2LL0, one could expect
the HOMO energy level to be lowered by strong field ligands more than by weak
field ligands. A significantly longer wavelength emission band is measured for the
Figure 3. HOMO and LUMO energy levels of Ir(dFppy)(PPhMe2)2(H)(Cl), and Ir(dFppy)
(PPhMe2)2-(H)(CN).

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Figure 4. Relative PL emission spectrum of Ir(dFppy)(PPhMe2)2(H)(Cl), [Ir(dFppy)
(PPhMe2)2(H)-(MeCN)]þ and Ir(dFppy)(PPhMe2)2(H)(CN) in CH2Cl2 solution at room
temperature.
complex that contains a chlorine ligand trans to the carbon of the C^N ligand,
whereas the emission band is measured at much shorter wavelengths for all other
complexes with a strong field ligand, such as CNꢄ. The trans and axial ligand
PPhMe2 is not involved in the HOMO of iridium complexes.
In order to examine which ligand mainly contributes to MLCT transition pro-
cess of these complexes, the d-orbital characteristics of HOMOs and LUMOs were
investigated after geometry optimization of the molecular structure of these com-
plexes using DFT. Contour plots of the three highest HOMOs and three lowest
LUMOs of Ir(dFppy)(PPhMe2)2(H)(CN) are shown in Figure 5. These orbitals
Figure 5. Contour plots of HOMOs and LUMOs of Ir(dFppy)(PPhMe2)2(H)(CN).

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H. W. Ham and Y. S. Kim
Table 2. Comparison of calculated bond lengths of Ir-C, Ir-N, and Ir-ancillary(A) for
Ir(dFppy)-(PPhMe2)2(H)(Cl), [Ir(dFppy)(PPhMe2)2(H)(NCMe)]þ, and Ir(dFppy)
(PPhMe2)2(H)(CN)
˚
Ir-C (A)
˚
Ir-N (A)
˚
Ir-A (A)
Complex
Ir(dFppy)(PPhMe2)2(H)(Cl)
Ir(dFppy)(PPhMe2)2(H)(NCMe)þ
Ir(dFppy)(PPhMe2)2(H)(CN)
2.098
2.106
2.103
2.060
2.074
2.133
2.470
2.000
1.980
are important because dominant excitations and emissions mainly occur by the
electronic transition among these orbitals. In the HOMO side, most of the electron
population of Ir(dFppy)(PPhMe2)2(H)(CN) is localized at iridium atom and the
ancillary ligand CNꢄ. This effect can destabilize the MLCT state, leading to the
luminescence wavelength in the blue region.
TD-DFT calculations were employed to examine the low-lying singlet and triplet
states of Ir(dFppy)(PPhMe2)2(H)(Cl), [Ir(dFppy)(PPhMe2)2(H)(NCMe)]þ and
Ir(dFppy)(PPhMe2)2(H) (CN). Calculated excitation energies, dominant orbital exci-
tation obtained, and oscillator strengths are shown in Table 1. The excitation energy
of Ir(dFppy)(PPhMe2)2(H)(Cl), [Ir(dFppy)(PPhMe2)2(H)(NCMe)]þ became greater
than that of Ir(dFppy)(PPhMe2)2(H)(Cl), with a difference of 0.069ꢃ0.072 eV, which
is a hypsochromic shift of about 10 nm in wavelength. Strong field ancillary ligands
alter the MLCT energy mainly by changing the HOMO energy level. The HOMO
energy level may then be lowered by strong-field ancillary ligands, which causes sig-
nificant d-orbital energy splitting. It is also likely that strong-field ancillary ligands
lengthen the bond lengths of Ir–C (ppy) trans to the ancillary ligand, which lowers
the energy levels of the dp-orbitals of the metal. Comparison of calculated bond
lengths for Ir(dFppy)(PPhMe2)2(H)(Cl), [Ir(dFppy)(PPhMe2)2(H)(NCMe)]þ and
Ir(dFppy)(PPhMe2)2-(H)(CN) are summarized in Table 2. Therefore, it can be said
that the ancillary ligand of CN induced the blue shift.
As a result, the PL spectrum of Ir(dFppy)(PPhMe2)2(H)(CN) shows a significant
blue shifted phosphorescence emission, due to the substitution of fluoro groups in
the phenyl ring. The fluoro groups in the phenyl ring pulls the HOMO level of
iridium complexes and the strong field ancillary such as CNꢄ, which helps enlarge
HOMO-LUMO energy gap to achieve the hypsochromic shift in emission color.
4. Conclusions
We have synthesized and studied photophysical properties of phosphorescent
mono-cyclometalated iridium (III) complexes, having one cyclometalating and two
phosphines trans to each other and two cis-ancillary ligands, such as Ir(dFppy)
(PPhMe2)2(H)(Cl), Ir(dFppy)(PPhMe2)2(H)(NCMe)þ and Ir(dFppy)(PPhMe2)2(H)
(CN). The purpose was to find an efficient blue-emitting material for the OLED with
thermal processing by tuning the ancillary ligand and reducing the molecular weight
of phosphine ligands. We also investigated the electron-withdrawing capabilities of
ancillary ligands using the DFT and TD-DFT calculations on the ground and
excited states of the three complexes to gain insight into the factors responsible
for the emission color change and the different luminescence efficiency.

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The PL spectra of Ir(dFppy)(PPhMe2)2(H)(CN) shows strong field effects of
ancillary ligands, leading to a significant blue shifted phosphorescence, as well as
higher luminescence efficiency due to replacing chlorine with the CN ligand. This
study shows that the new type of mixed iridium complex, Ir(dFppy)
(PPhMe2)2(H)(CN) could act as a good radiation characteristic blue dopant for
the fabrication of thermal processing in new phosphorescent OLEDs.
Acknowledgment
This work was supported by Hongik University Sabbatical Research Funds.
References
[1] Baldo, M. A., Thompson, M. E., & Forrest, S. R. (2000). Nature, 403, 750.
[2] Baldo, M. A., Lamansky, S., Burrows, P. E., Tompson, M. E., & Forrest, S. R. (1999).
Appl. Phys. Lett., 75, 4.
[3] Park, Y. H. & Kim, Y. S. (2007). Thin Solid Films, 515, 5084.
[4] Baldo, M. A., Thompson, M. E., & Forrest, S. R. (1999). Pure Appl. Chem., 71, 2095.
[5] Brooks, J., Babaya, T., Lamansky, S., Djurovich, P. I., Tsyba, I., Bau, R., & Thompson,
M. E. (2002). Inorg. Chem., 41, 3055.
[6] Kohler, A., Wilson, J. S., & Friend, R. H. (2002). Adv. Mater., 14, 701.
[7] Tamayo, A. B., Alleyne, B. D., Djurovich, P. I., Lamansky, S., Tsyba, I., Ho, N. N., Bau,
R., & Thompson, M. E. (2003). J. Am. Chem. Soc., 125, 7377.
[8] Dedeian, K., Shi, J., Shepherd, N., Forsythe, E., & Morton, D. C. (2005). Inorg. Chem.,
44, 4445.
[9] Kohler, A., Wilson, J. S., & Friend, R. H. (2002). Adv. Mater., 14, 701.
[10] Dedeian, K., Shi, J., Shepherd, N., Forsythe, E., & Morton, D. C. (2005). Inorg. Chem.,
44, 4445.
[11] Park, N. G., Choi, G. C., Lee, Y. H., & Kim, Y. S. (2006). Curr. Appl. Phys., 6, 620.
[12] Takizawa, S., Nishida, J., Tsuzuki, T., Tokito, S., & Yamashita, Y. (2007). Inorg. Chem.,
46, 4308.
[13] Hay, P. J. & Wadt, W. R. (1985). J. Chem. Phys., 82, 270.
[14] Hehre, W. J., Ditchfield, R., & People, J. A. (1972). J. Chem. Phys., 56, 2257.
[15] Seo, J. H., Kim, I. J., Kim, Y. K., & Kim, Y. S. (2008). Jpn. J. Appl. Phys., 47, 6987.
[16] Eum, M., Chin, C. S., Kim, S. y., Kim, C., Kang, S. K., Hur, N. H., Seo, J. H., Kim,
G. Y., & Kim, Y. K. (2008). Inorg. Chem., 47, 6289.

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