Synthesis and charaterization of 1,3,4-oxadiazoles carrying triazolopyridinone and carbazole as the potential bluish-green electroluminescent materials for the single layer device

Article abstract of DOI:10.3987/COM-06-10758

1,3,4-Oxadiazoles carrying triazolopyridinone and carbazole were synthesized and characterized for the electroluminescent materials in single layer devices. Carbazole and triazolopyridinone were introduced into 2,5-centers of 1,3,4-oxadiazole core to assist the control of fundamental photolytic process due to their electron-donating nature, excellent photo-conductivity, and flexible structure properties. Study from spectrometry and cyclic voltammetry revealed that the 1,3,4-oxadiazole derivatives carrying these two heterocyles are able to be used as the potential bluish-green electroluminescent materials.{A figure is presented}.

Full text of DOI:10.3987/COM-06-10758

HETEROCYCLES, Vol. 68, No. 8, 2006
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HETEROCYCLES, Vol. 68, No. 8, 2006, pp. 1585 - 1594. © The Japan Institute of Heterocyclic Chemistry
Received, 6th April, 2006, Accepted, 30th May, 2006, Published online, 1st June, 2006. COM-06-10758
SYNTHESIS AND CHARATERIZATION OF 1,3,4-OXADIAZOLES
CARRYING TRIAZOLOPYRIDINONE AND CARBAZOLE AS THE
POTENTIAL BLUISH-GREEN ELECTROLUMINESCENT MATERIALS
FOR THE SINGLE LAYER DEVICE
Hung-Te Chang,c Kuo-Chen Chiang,a Fung Fuh Wong,＊,a,b and
,c,d
Mou-Yung Yeh＊
aSustainable Environment Research Center, National Cheng Kung University, No.
500, Sec. 3, An-ming Rd., Tainan City, Taiwan 709, R.O.C.
bGraduate Institute of Pharmaceutical Chemistry, China Medical University, No.
91 Hsueh-Shih Rd. Taichung, Taiwan 40402, R.O.C.
cDepartment of Chemistry, National Cheng Kung University, No. 1, Ta Hsueh Rd.,
Tainan, Taiwan 70101, R.O.C.
dNan Jeon Institute of Technology, No.178, Chaocin Rd., Yanshuei Township,
Tainan County 737, Taiwan R. O. C.
*Corresponding Author. Tel.: +886 6 384 0136 ext. 212; Fax.: +886 6 384 0960.
E-mail address: wongungfuh@yahoo.com.tw (F. F. Wong).
Abstract – 1,3,4-Oxadiazoles carrying triazolopyridinone and carbazole were
synthesized and characterized for the electroluminescent materials in single layer
devices. Carbazole and triazolopyridinone were introduced into 2,5-centers of
1,3,4-oxadiazole core to assist the control of fundamental photolytic process due
to their electron-donating nature, excellent photo-conductivity, and flexible
structure properties. Study from spectrometry and cyclic voltammetry revealed
that the 1,3,4-oxadiazole derivatives carrying these two heterocyles are able to be
used as the potential bluish-green electroluminescent materials.
INTRODUCTION
Thin-film organic light-emitting diodes (LEDS) based on small molecules are of great interest due to their
potential application in the large-screen emissive flat-panel color displays or as white back-lights for
liquid crystal displays.1 To accomplish optimal efficiency and device lifetime, the injection and the
transport of holes and electrons must be balanced, such that similar densities of the two carriers are

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HETEROCYCLES, Vol. 68, No. 8, 2006
achieved. As a result, the efficient mixture of hole- and electron-transporting materials into a blend,2,3 or
the copolymerization of hole- and electron-transporting groups in the simple single-layer device was
investigated to meet the approach. To fulfill such requirement, easily synthesized materials of
1,3,4-oxadiazole carrying triazolopyridinone and carbozole moieties with good film-forming properties
and temporal stability are studied in this work.
Carbazole,4 triazolopyridinone5,6 and their derivatives can be easily functionalized and are beneficial for
raising the glass-transition temperature and thermal stability. In particular, carbazole and its derivatives
have been used extensively for the construction of functional materials such as photorefractive materials,7
photoconductors,8
nonlinear optical materials,9 light-emitting materials,10 and hole-transporting
materials,11 as flexible building blocks due to their inherent electron-donating nature, excellent photo
conductivity, and unique nonlinear optical property. Triazolopyridinones own the electron-donating
properties and are investigated to improve the photolytic properties.6 Pyridine, being an electron-deficient
heterocycle, was introduced to the main chain of a conjugated system to improve the electron-transporting
of 1,3,4-oxadiazole.12 In this paper, we first report the 1,3,4-oxadiazole-triazolopyridinone-carbozole
hybrids as new potential blue and bluish-green electroluminescent materials for single layer devices.
From the structural point of view, carbazole, pyridine and triazolopyridinone moieties were introduced to
1,3,4-oxadiazole molecule structure as the chromophores to emphasize the conjugation and expect to tune
electroluminescent efficiencies and hole- or electron-transport property.
RESULTS AND DISCUSSION
Scheme 1 shows the synthesis of the 1,3,4-oxadiazole-pyridine-carbazole (3). Isonicotinohydrazide (1)
was prepared in 83% yield as a white solid by the reaction of ethyl isonicotinate with hydrazine
monohydrhydrate at reflux in EtOH overnight.13 Isonicotinohydrazide (1) was treated with
9-hexyl-9H-carbazole-3-carbaldehyde to give the acyl hydrazone (2) as a yellow solid in 80 % yield.
Upon reflux with KMnO4 in acetone, 2 provided 2-carbazol-2-yl-5-(4-pyridyl)-1,3,4-oxadiazole (3) in
58% isolated yield by oxidative cyclization.
We treated the 1,3,4-oxadiazole (3) with N-aminochloro-N-arylamide hydrochlorides (4a–4c)14 at 80 C
in i-PrOH in the presence of tribulylamine for 2 h,13 to yield the crude triazolopyridinone derivatives
(5a–5c). The crude products (5a–5c) were purified by recystallization from CH2Cl2 to give the pure
products (5a–5c) as light yellow solid in 55–67 % yields (Scheme 2).

HETEROCYCLES, Vol. 68, No. 8, 2006
1587
9-Hexyl-9H-carbazole-
O
O
NH2NH2. H2O
EtOH, 83%
3-carbaldehyde
N
N
N
OEt
EtOH, 80%
NHNH2
1
O
H
KMnO4
NHN
N
NR1
O
acteone, 50%
NR1
N N
3. R1 = n-hexyl
2. R1 = n-hexy
Scheme 1
O
N
Cl
n-Bu3N
i-PrOH
N
O
+
R2
NR1
NH2 HCl
N N
4a. R2 = H, 4b. R2 = Me,
4c. R2 = OEt
3. R1 = n-hexyl
O
5a. R1 = n-hexyl, R2 = H, 55%
5b. R1 = n-hexyl, R2 = Me, 60%
5c. R1 = n-hexyl, R2 = OMe, 67%
N
O
N
NR1
N
R2
N N
Scheme 2
The UV-vis spectra of the 1,3,4-oxadiazole derivative (3) and their triazolopyridinone derivatives
(5a–5c) measured in CH2Cl2 are almost identical, which show four peaks at 242, and ~315 nm in CH2Cl2.
The main absorption band at 242 nm is contributed from carbazole and 1,3,4-oxadiazole moieties,15 and
the λmax values of pyridine in the range 296–300 nm in CH2Cl2. When the pyridine was modified to the
triazolopyridinone, it brought about a slightly red shift to 306 nm. The chromophore effects of pyridine
and triazolopyridinone are resembled. Figure 1 shows the photoluminescence (PL) spectra of 3 and 5a–5c
in CHCl3. The excitation wavelengths for 3 are between 380 and 450 nm and the λmaxs of PL is ~ 430 nm
and has the intense blue fluorescenc. When the structures (5a–5c) own the triazolopyridinone moieties,
they have obviously red-shift and exhibit the intense bluish-green fluorescence in CHCl3 (λmaxs of PL is
478–486 nm, see Figure 1). The solution fluorescence quantum yields (Φf) of 5a–5c, all of which fall in
the range 0.68–0.71, were determined relative to that of 2-phenyl-5-(4-biphenyl)-1,3,4-oxzdiazole in
benzene (Φf = 0.80, see Table 1).16 The PL spectrum of the vacuum evaporatedfilms 5c on quartz
substrates, with a landa maximum at 503 nm, showed a red-shift (~20 nm) with respect to its solution
spectrum (Figure 2). Most of the published analogues of 3 and 5 were estimated as a potential bluish

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electroluminescent
materials.
At
any
rate,
we
successfully
treated
the
2-carbozolyl-5-pyridyl-1,3,4-oxadiazole (3) with the acid chloride (4) to form the triazolopyridinone core
by means of the dehydration–cyclization reaction. The triazolopyridinone moiety efficiently conjugate
and connect two chromophores (oxadiazole and pyridine) to provide the bluish-green electroluminescent
materials.
Table 1. UV-Vis absorption maxima and photoluminescence peak wavelength of the 1,3,4-oxadiazole
derivatives (3 and 5a–5c)
a
Compound
R1
λ
max /nm of UV-Vis
λ
max /nm of PL
CHCl3
430
Φf
Substrate
R2
-
CH2Cl2
314
3
n-hexyl
n-hexyl
n-hexyl
n-hexyl
-
5a
5b
5c
H
322
478
0.68
0.67
0.71
Me
OMe
312
479
308
486
a
Φf: Fluorescence quantum efficiency, relative to 2-phenyl-5-(4-biphenyl)-1,3,4-oxadiazole in benzene (Φf = 0.8).
Figure 1. The fluorescence spectra of 3 and 4a–4c in chloroform solution

HETEROCYCLES, Vol. 68, No. 8, 2006
1589
Figure 2. Normalized photoluminescence spectra of 5c (vacuum evaporated film)
The electrochemical behaviors of 5a–5c were investigated by cyclic voltammetry in solution. The
measurements were carried out at a platinum electrode using millimolar solution in using CH2Cl2
containing 0.1 M of the supporting electrolyte, tetrabutylammonium hexafluorophosphate (TBAPF6), in a
three electrode cell an potentiostat assembly. The potential was measured against Ag/AgCl as reference
electrode and each measurement was calibrated with an internal standard, ferrocene/ferrocenium (Fc)
redox system.17 The data were tabulated in Table 2. Upon the anodic sweep, 5a–5c showed irreversible
reduction processes. The bandgap energies of 1,3,4-oxadiazole-triazolopyridinone-carbazole derivatives
(5a–5c) were estimated from the onset wavelength (λonset) of the UV-vis absorption.18 From the high
electron affinities, 5a–5c owned the potential of electron-transporting and highly efficient bluish-green
electroluminescent materials.
Table 2. Electrochemical properties of 5a–5c
Eonseta E’onsetb Ipc = EHOMO Egd,e = Bandgap Eaf = ELUMO
g
Compound
Φf
(v)
(v)
(eV)
energy (eV)
3.21
(eV)
5a
5b
5c
1.35
1.16
–5.96
–2.75
0.74
0.76
0.69
1.44
1.24
1.25
1.05
–6.05
–5.85
3.21
3.21
–2.84
–2.64
a Measured vs. ferrocene/ferrocenium.
bE’onset = Eonset – 0.19 eV (Measured vs. Ag/AgCl)
cIp = - (E’onset + 4.8)
dEg: the bandgap energy estimated from the onset wavelength of UV-vis absorption
dThe bandgap energy estimated based on the value 386 nm
fEa = Ip + Eg
g
Φf: Fluorescence quantum efficiency, relative to 2-phenyl-5-(4-biphenyl)-1,3,4-oxadiazole in benzene

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HETEROCYCLES, Vol. 68, No. 8, 2006
We are successful to prepare a series of the 5-triazolopyridinone-2-carbazolyl-1,3,4-oxadiazole
derivatives (5a–5c) by treating the 2-carbazolyl-5-pyridyl-1,3,4-oxadiazole with
N-aminochloro-N-arylamide hydrochloride (4a–4c). Carbazole, pyridine and triazolopyridinone moieties
in 5a–5c obviously play an excellent assistant role in controlling fundamental photolytic process. The
1,3,4-oxadiazole derivatives (5a–5c) could be as the new potential bluish-green electroluminescent
materials.
EXPERIMENTAL
General. Isonicotinohydrazide5 and N-aminochloro-N-arylamide hydrochloride14 were synthesized
according to literature procedures. All chemicals were reagent grade and used as purchased. All reactions
were carried out under nitrogen atmosphere and monitored by TLC analysis. Flash column
chromatography was carried out on silica gel (230-400 mesh). Commercially available reagents were
used without further purification unless otherwise noted.
Analytical thin-layer chromatography (TLC) performed on precoated plates (silica gel 60 F-254) was
purchased from Merck Inc. Mixtures of ethyl acetate and hexanes were used as eluants. Purification by
gravity column chromatography was carried out by use of Merck Reagents Silica Gel 60 (particle size
0.063–0.200 mm, 70–230 mesh ASTM). Infrared (IR) spectra were measured on a Bomem Michelson
-1
Series FT-IR spectrometer. The wavenumbers reported are referenced to the polystyrene 1601 cm
absorption. Absorption intensities are recorded by the following abbreviations: s, strong; m, medium; w,
weak. UV-visible spectra were measured with a HP 8452A diode-array spectrophotometer.
Photoluminescence (PL) spectra were obtained on a Perkin-Elemer fluorescence spectrophotometer (LS
1
55). H-NMR spectra were obtained on a Bruker (200 MHz) spectrometer by use of CDCl as solvent.
3
13
C-NMR spectra were obtained on a a Bruker (50 MHz) spectrometer by use of chloroform-d as solvent.
C chemical shifts are referenced to the center of the CDCl triplet (δ 77.0 ppm). Multiplicities are
13
3
recorded by the following abbreviations: s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet; J,
coupling constant (hertz). Elemental analyses were carried out on a Heraeus CHN–O RAPID element
analyzer.
Cyclic Voltammetry Measurements: Cyclic voltammetry (CV) and differential pulse voltammetry
(DPV) measurements were performed on a PGSTAT 20 electrochemical analyzer. The oxidation and
reduction measurements were carried out, respectively, in anhydrous CH2Cl2 and anhydrous THF
containing 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as the supporting electrolyte at a

HETEROCYCLES, Vol. 68, No. 8, 2006
1591
scan rate of 50 mV s−1. The potentials were measured against an Ag/Ag+ (0.01 M AgCl) reference
electrode using ferrocene as the internal standard. The onset potentials were determined from the
intersection of two tangents drawn at the rising current and background current of the cyclic
voltammogram.17
9-Hexyl-9H-carbazole.6 A solution of 9H-carbazole (16.7 g, 0.10 mole, 1.0 equiv.), and KOH (8.40 g,
0.15 mole, 1.5 equiv.) were mixed and stirred in H2O (40 mL) solution at rt for 15 min.
Tetrabutylammonium bromide (1.0 g, 10.3 mmol, 0.11 equiv.) was added dropwise into the reaction
mixture, then 1-bromohexane (24.8 g, 0.15 mol, 1.5 equiv.) was added dropwise into the reaction mixture
and heat-up to 60 °C for 3 h. After the reaction was completed, the reaction mixture was extracted with
EtOAc (150 mL × 2). The combined EtOAc solutions were washed with water and saturated aqueous
NaCl. The combined organic extracts were dried over MgSO4, filtered, and concentrated under reduced
pressure. EtOH (100 mL) was added to the residue and precipitated the desired product. The wet cake was
dried in vacuum oven overnight to give the 9-hexyl-9H-carbazole as white powder in 80% yield (20.0 g,
1
80.0 mmol): mp 56–57 °C6; H NMR (DMSO-d6, 200 MHz) δ 0.78 (t, J = 7.2 Hz, 3H, CH3), 1.05–1.30
(m, 6H, CH2), 1.73 (m, 2H, CH2), 4.36 (t, J = 6.9 Hz, 2H, CH2), 7.17 (dd, , J = 6.8, 8.2 Hz, 2H, ArH),
7.56 (d , J = 8.2 Hz, 2H, ArH), 8.09 (dd, J = 6.8, 8.2 Hz, 2H, ArH), 8.14 (d , J = 8.2 Hz, 2H, ArH).
9-Hexyl-9H-carbazole-3-carbaldehyde A solution of cold POCl3 (130 mL) was added dropwise into
DMF solution (60 mL) in ice-bath and warmed-up to rt for 1 h. The reaction mixture was stirred at rt for 2
h. 9-Hexyl-9H-carbazole (25.1 g, 0.10 mol, 0.10 equiv.) was added into the reaction mixture in ice-bath
overnight. After the reaction completed, the reaction mixture was quenched with NaHCO3 aqueous
solution (100 mL) and stirred for 1 h. The precipitation was filtered and washed with cold EtOH (50
mL). The wet cake was dried in vacuum oven overnight to give the 9-hexyl-9H-carbazole-3-carbaldehyde
1
as white powders in 72% yield (20.1 g, 72.1 mmol): mp 58–59 °C; H NMR (DMSO-d6, 200 MHz) δ
0.78 (t, J = 7.2 Hz, 3H, CH3), 1.10–1.25 (m, 6H, CH2), 1.71–1.74 (m, 2H, CH2), 4.41 (t, J = 6.9 Hz, 2H,
CH2), 7.28 (dd, J = 6.7, 7.8 Hz, 1H, Ar-H), 7.51 (dd, J = 6.7, 8.0 Hz, 1H, ArH), 7.65 (d, J = 9.0 Hz, 1H,
ArH), 7.72 (d, J = 8.0 Hz, 1H, ArH), 7.95 (d, J = 7.8 Hz, 1H, ArH), 8.26 (d, J = 9.1 Hz, 1H, ArH), 8.73 (s,
1H, Ar-H), 10.06 (s, 1H, CHO).
N-[1-Aza-2-(9-hexylcarbazol-3-yl)vinyl]-4-pyridylcarboxamide (2) A solution of isonicotinohydrazide
(6.00 g, 4.37 mmole, 1.1 equiv.) and 9-hexyl-9H-carbazole-3-carbaldehyde (1.08 g, 4.30 mmole, 1.0
equiv.) in EtOH (50 mL) was stirred at 40 °C for 6 h. After the reaction completed, the reaction mixture
was concentrated under reduced pressure to remove EtOH. The residue was added with cold EtOH (25
mL), filtrated and washed with cold EtOH (15 mL). The wet cake was dried in vacuum oven overnight to
give the isonicotinic acid[(9-hexyl-9H-carbazol-3-yl)methylene] hydrazide (2) as yellow powders in 80%

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HETEROCYCLES, Vol. 68, No. 8, 2006
yield (1.37 g, 3.43 mmol): 1H NMR (DMSO-d6, 200 MHz) δ 0.79 (t, J = 6.7 Hz, 3H, CH3), 1.16–1.35 (m,
6H, CH2), 1.69–1.85 (m, 2H, CH2), 4.42 (t, J = 6.8 Hz, 2H, CH2), 7.22 (t, J = 7.3Hz, 1H, ArH), 7.45 (t, J
= 7.3 Hz, 1H, ArH), 7.82–7.86 (m, 2H, ArH), 7.82–7.91 (m, 2H, ArH), 8.24 (d, J = 7.6 Hz, 1H, ArH),
8.50–8.61 (m, 2H, ArH), 8.78 (d, J = 5.7 Hz, 2H, ArH), 11.98 (s, 1H, NH).
(9-Hexyl-carbazol-2-yl)-5-(4-pyridyl)-1,3,4-oxidiazole (3). A solution of the hydrazone 2 (4.00 g, 10.1
mmole, 1.0 equiv.) and KMnO4 (4.00 g) in acetone (45 mL) was stirred at 50 °C for 4 h. After the
reaction completed, the reaction mixture was concentrated under reduced pressure to remove acetone. The
residue was added with a saturated Na2SO3 aqueous solution (30 mL), and extracted with CH2Cl2 (30
mL). the reaction mixture was extracted with EtOAc (120 × 2 mL). The combined EtOAc solutions were
washed with water and saturated aqueous NaCl. The combined organic extracts were dried over MgSO4,
filtered, and concentrated under reduced pressure. The wet cake was dried in vacuum oven overnight to
1
give the oxadiazole 3 as yellow powders in 50% yield (10.0 g, 40.0 mmol): mp 142–143 °C; H NMR
(DMSO-d6, 200 MHz) δ 0.79 (t, J = 6.6 Hz, 3H, CH3), 1.16–1.27 (m, 6H, CH2), 1.74–1.79 (m, 2H,
CH2), 4.45 (t, J = 6.9 Hz, 2H, CH2), 7.29 (t, J = 7.4 Hz, 1H, ArH), 7.53 (t, J = 7.4 Hz, 1H, ArH), 7.68 (d,
J = 8.2 Hz, 1H, ArH), 7.83 (d, J = 8.7 Hz, 1H, ArH), 8.10 (d, J = 4.3 Hz, 2H, ArH), 8.23 (dd, J = 1.2, 7.6
Hz, 1H, ArH), 8.34 (d, J = 7.6 Hz, 1H, ArH), 8.87 (d, J = 4.3 Hz, 2H, ArH), 8.99 (d, J = 1.2 Hz, 1H,
ArH); FABMS m/z (relative intensity) 399 (M+1, 19), 398 (M, 55). Anal. Calcd for C25H26N4O: C, 75.35;
H, 6.58; N, 14.06. Found: C, 75.26; H, 6.61; N, 14.14.
Standard Procedure for the synthesis of 5-triazopyridinone-2-carbazolyl-1,3,4-oxadiazole
derivatives (5a–5c): A solution of carbazolylpyridyl-1,3,4-oxadiazole (3, 1.62 mmole, 1.0 equiv) was
stirred with n-tributylamine (1 mL) in i-PrOH (15 mL). The reaction mixture was heated up to 80 °C and
the acid chloride (4, 1.78 mmole, 1.1 equiv) was added into the reaction mixture. After the reaction
completed, the separating-product was collected by filtration while hot and washed with cold EtOH (10
mL) to give the crude product. The crude product was dried and crystallized from CH2Cl2 to give a pure
(4a–4c) as light yellow solid in 55–67% yields.
1
Compound (4a): mp 252–254 °C; H NMR (CDCl3, 200 MHz) δ 0.90 (t, J = 6.8 Hz, 3H, CH3),
1.18–1.25 (m, 6H , CH2), 1.88–1.92 (m, 2H , CH2), 4.36 (t, J = 6.9 Hz, 2H , CH2), 7.33–7.56 (m, 8H),
-1
7.93–8.01 (m, 2H), 8.14–8.23 (m, 4H), 8.88 (d, J = 1.2 Hz, 1H); IR (KBr) 1715 (m, C=O) cm ; FABMS
m/z (relative intensity) 529 (M+1, 29), 528 (M, 61). Anal. Calcd for C32H28N6O2: C, 72.71; H, 5.34; N,
15.90. Found: C, 72.65; H, 5.46; N, 15.79.
1
Compound (4b): mp 259–261 °C; H NMR (CDCl3, 200 MHz) δ 0.91 (t, J = 6.8 Hz , 3H , CH3),
1.18–1.26 (m, 6H, CH2), 1.88–1.91 (m, 2H, CH2), 2.35 (s, 3H, CH3), 4.36 (t, J = 6.7 Hz, 2H, CH2),

HETEROCYCLES, Vol. 68, No. 8, 2006
1593
7.26–7.33 (m, 5H), 7.48–7.55 (m, 4H), 7.92–8.04 (m, 2H), 8.18–8.22 (m, 2H), 8.86 (d , J = 1.2 Hz, 1H);
IR (KBr) 1714 (m, C=O) cm-1; FABMS m/z (relative intensity) 543 (M+1, 19), 542 (M, 31). Anal. Calcd
for C33H30N6O2: C, 73.04; H, 5.57; N, 15.49. Found: C, 73.11; H, 5.60; N, 15.51.
1
Compound (4c): mp 228–230 °C; H NMR (CDCl3, 200 MHz) δ 0.89 (t, J = 6.8 Hz, 3H, CH3),
1.18–1.28 (m, 6H), 1.89–1.92 (m, 2H), 4.36 (t, J = 6.8 Hz, 2H, CH2), 7.03 (d, J = 8.9 Hz, 2H), 7.26–7.40
(m, 2H), 7.45–7.56 (m, 3H), 7.95–8.06 (m, 4H), 8.17–8.27 (m, 2H), 8.88 (d , J = 1.3 Hz, 1H); IR (KBr)
1710 (m, C=O) cm-1; FABMS m/z (relative intensity) 559 (M+1, 36), 558 (M, 66). Anal. Calcd for
C33H30N6O3: C, 70.95; H, 5.41; N, 15.04. Found: C, 70.87; H, 5.55; N, 15.12.
ACKNOWLEDGEMENTS
We are grateful to the National Science Council of Republic of China for financial support.
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