Two-photon absorption in V-type chromophores with electron-rich heterocyclevinylene bridges

Article abstract of DOI:10.1007/s11426-011-4248-4

A series of V-type chromophores with electron-rich heterocyclevinylene bridges have been synthesized via Wittig-Horner-Emmons and Vilsmeier reactions. All the target chromophores showed strong one-photon and two-photon excited emission, and their electron properties could be tuned by using different heterocycles such as furan, thiophene and pyrrole moieties in I-III. The maximum two-photon absorption (TPA) cross sections occurred at 760 nm and were measured to be in the range of 400-800 GM.

Full text of DOI:10.1007/s11426-011-4248-4

SCIENCE CHINA
Chemistry
April 2011 Vol.54 No.4: 625–630
doi: 10.1007/s11426-011-4248-4
• ARTICLES •
Two-photon absorption in V-type chromophores with
electron-rich heterocyclevinylene bridges
LI QianQian1, ZHONG AoShu1, LIU HaiJie1, PENG Ming1, LIU Jun2, PEI ZhiGuo2,
HUANG ZhenLi2, QIN JinGui1 & LI Zhen1*
1Department of Chemistry, Hubei Key Laboratory on Organic and Polymeric Opto-Electronic Materials, Wuhan University, Wuhan 430072, China
2Key Laboratory of Biomedical Photonics of Ministry of Education; Huazhong University of Science and Technology, Wuhan 430074, China
Received December 12, 2010; accepted January 17, 2011
A series of V-type chromophores with electron-rich heterocyclevinylene bridges have been synthesized via Wittig-Horner-
Emmons and Vilsmeier reactions. All the target chromophores showed strong one-photon and two-photon excited emission,
and their electron properties could be tuned by using different heterocycles such as furan, thiophene and pyrrole moieties in
I–III. The maximum two-photon absorption (TPA) cross sections occurred at 760 nm and were measured to be in the range of
400–800 GM.
two-photon absorption, electron-rich, heterocycle, chromophore
end groups can vary the charge distribution, and the prop-
1 Introduction
erty and length of the conjugated electron bridge can modu-
late the electronic communication between the D or A
moieties.
There is much interest in the development of organic con-
jugated materials with large two-photon absorption (TPA)
cross sections () as promising candidates for potential ap-
plications in a number of new areas, including the fluores-
cence imaging of biological samples, optical limiting,
photodynamic therapy, three-dimensional optical data stor-
age and micro-fabrication [1–5]. Organic conjugated mole-
cules generally provide large electron delocalization of the
 electrons throughout the whole molecule, which would be
beneficial to the enhancement of the  value. Many reports
proposed various design strategies for efficient TPA mole-
cules by a systematic variation of chromophores with vari-
ous electron-donor (D) and electron-acceptor (A) moieties,
which were attached symmetrically or unsymmetrically to a
conjugated electron bridge () [6–12]. The effects of vary-
ing the electron-donating or electron-acceptor strength of
Among the various conjugated bridges, the extensively
utilized -centers were benzene, biphenyl, fluorene, and
dihydrophenathrene, with simple functionalities such as OR,
NR2, NO2, and CN bonded to them. Their electron proper-
ties could be tuned, and the -centers with different elec-
tronic properties played an important role in enhancing the
charge transfer in the TPA chromophores. Also, various
heterocycles with electron-rich or electron-deficient proper-
ties were employed, which act as efficient auxiliary donor
or acceptor moieties [13–16]. Furthermore, the intrinsic
tunable nature of the heterocycles rings made these systems
particularly appropriate to finely control the electronic and
optical properties. The electron-rich heterocycles, such as
furan, pyrrole and thiophene, have frequently been incorpo-
rated into the conjugated bridges of TPA chromophores.
Albert et al. pointed out that electron-rich heterocycles ex-
hibited a lower tendency to deplete charge from donor sub-
*Corresponding author (email: lizhen@whu.edu.cn)
© Science China Press and Springer-Verlag Berlin Heidelberg 2011
chem.scichina.com
www.springerlink.com

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Li QQ, et al. Sci China Chem April (2011) Vol.54 No.4
stituents and hence increase the donor ability [17]. Marder’s
group reported that the TPA chromophores in which vi-
nylene units and either pyrrole or dialkoxythiophene groups
form the bridge between two diarylaminophenyl groups
with D--D type exhibited high two-photon cross-sections
[18].
In this work, we extended the study on the V-type TPA
chromophores with the electron-rich five-member hetercy-
cles furan, pyrrole and thiophene incorporated as the conju-
gated bridges (Chart 1), and triphenylamine moieties as the
core or the donor group. Compared with the linear chromo-
phores with plane configuration, the V-type chromophores
which incorporated triphenylamine with propeller-shape
and bulky volume as the core bridge, would benefit to the
prevention of possible aggregation. The details of the syn-
thesis, characterization, and optical properties were pre-
sented. The results suggested that the heterocycles with
electron-rich properties as the conjugated bridges would be
beneficial to the enhancement of the TPA cross section,
however, the trend did not always accord well with the en-
hancement of the electron-rich properties.
Varian Mercury 300 spectrometer using tetramethylsilane
(TMS;  = 0 ppm) as internal standard. UV-vis spectra were
obtained using a Shimadzu UV-2550 spectrometer. Photo-
luminescence spectra were performed on a Hitachi F-4500
fluorescence spectrophotometer. Matrix-assisted laser de-
sorption ionization time-of-flight mass spectra were meas-
ured on a Voyager-DESTR MALDI-TOF mass spectrome-
ter (MALDI-TOF MS; ABI, American) equipped with a
337 nm nitrogen laser and a 1.2 m linear flight path in posi-
tive ion mode, or a GCT premier CAB048 mass spectrome-
ter. A mode-locked Ti:sapphire laser (Mai Tai, Spectra-
Physics Inc.) was used as the excitation source. The average
output power, pulse width, and repetition rate were 1.5 W,
100 fs, and 80 MHz, respectively. After passing through a
circular variable neutral density filter that was used to con-
trol the power of the laser, the laser was focused into the
cell (polished on all sides) by a focusing lens (f=6 cm). The
emission light was collected by an objective lens (10, NA=
0.3, Olympus, Japan) and then was focused by another ob-
jective lens (10, NA = 0.25, DHC, China) into a fiber optic
spectrometer (USB2000, Ocean Optics Inc.), which was
used to record the fluorescent spectra. Fluorescein in water
was chosen as the reference standard.
2 Experimental
Synthesis of compound 2
2.1 Materials
A solution of compound 1 (4.97 g, 16.5 mmol) and NaBH4
(1.25 g, 33.0 mmol) was suspended in the mixture solvents
of ethanol (10 mL) and THF (20 mL) at 40 °C overnight.
The mixture was poured into ice cold water and extracted
with chloroform for several times, the organic fractions
Tetrahydrofuran (THF) was dried over and distilled from
K-Na alloys under an atmosphere of dry argon. N,N-Di-
methylformamide (DMF) was dried over and distilled from
CaH2 under an atmosphere of dry argon. 1,2-Dichloroethane
was dried over and distilled from phosphorus pentoxide.
Phosphorus oxychloride was freshly distilled before use.
Ethanol was dried over and distilled from sodium. 4-(Di-
phenylamino)benzaldehyde, diethyl 4-(diphenylamino)benzyl-
phosphonate and N-hexylpyrrole-2-carbaldehyde were pre-
pared following the procedure reported in refs. [19–20]. All
other reagents were used as received.
were collected and dried over Na
2SO4. The solvent was re-
moved under reduced pressure to yield pure compound 2
1
(5.03 g, 100%). H NMR (CDCl3)  (ppm): 7.23–7.17 (m,
ArH, 5H), 7.06–6.98 (m, ArH, 8H), 4.59(s, 4H, CH2).
Synthesis of compound 3
Compound 2 (5.03 g, 16.5 mmol) was dissolved in triethyl
phosphate (39.15 g, 214.5 mmol), and the solution was
cooled to 0–5 °C in an ice bath. Then iodine (8.38 g, 33.0
mmol) was added in batches. The resultant mixture was
stirred at room temperature for 48 h. After the residual
triethyl phosphate was removed under reduced pressure, the
mixture was washed with water and extracted with chloro-
form for several times, the organic fractions were collected
and dried over Na2SO4. The crude product was purified by
column chromatography on silica gel using petroleum/ethyl
acetate (6/1) as eluent to afford a white solid 3 (5.89 g,
65.4%). 1H NMR (CDCl3)  (ppm): 7.22–7.16 (m, ArH, 5H),
7.04–6.98 (m, ArH, 8H), 4.08–3.99 (m, OCH2, 8H), 3.09
(d, 4H, J = 21.3 Hz, CH2), 1.25 (t, 12H, J = 7.5 Hz,
CH3).
2.2 Instruments
1H and 13C NMR spectroscopy study was conducted with a
General procedure for the synthesis of compounds 4–6
Compound 3 (1 equiv) was suspended in anhydrous tetra-
Chart 1

Li QQ, et al. Sci China Chem April (2011) Vol.54 No.4
627
hydrofuran under an atmosphere of dry argon, t-BuOK (2
ArH), 6.20 (s, 1H, ArH).
equiv) was added directly as a solid and the resultant mix-
ture was stirred at room temperature for 10 min. After the
addition of the solution of the required aldehyde (2 equiv) in
anhydrous tetrahydrofuran dropwise, the reaction mixture
was stirred at room temperature overnight, then poured into
water, The organic product was extracted with chloroform
for several times, and dried over anhydrous Na2SO4. After
removing the solvent, the crude products were purified
through a silica gel chromatography column by using pe-
troleum/ethyl acetate (10/1) as eluent.
Compound 8: Compound 5 (0.54 g, 1.17 mmol), POCl3
(0.43 g, 2.81 mmol) and DMF (0.26 g, 3.51 mmol). Yellow
1
green solid (0.33 g, 55.0%). H NMR (CDCl3)  (ppm):
9.84 (s, 1H, CHO), 7.65 (d, 1H, J = 6.6 Hz, ArH), 7.39 (d,
2H, J = 2.1Hz, ArH), 7.36 (d, 2H, J = 2.4 Hz, ArH),
7.33–7.26 (m, 2H, ArH), 7.16 (d, 3H, J = 6.6 Hz, ArH),
7.12–7.05 (m, 10H, ArH and CH=CH), 7.00 (t, 1H, J =
4.5 Hz, ArH), 6.89 (d, 1H, J = 16.2 Hz, CH=CH).
Compound 9: Compound 6 (0.75 g, 1.26 mmol), POCl3
(0.46 g, 3.02 mmol) and DMF (0.28 g, 3.78 mmol). Yellow
green solid (0.37 g, 45.3%). 1H NMR (CDCl3)  (ppm): 9.45
(s, 1H, CHO), 7.39 (d, 4H, J = 8.4 Hz, ArH), 7.34–7.26 (m,
3H, ArH), 7.16–7.07(m, 8H, ArH and CH=CH), 6.92 (d,
2H, J = 4.5 Hz, ArH), 6.86 (d, 2H, J=16.2 Hz, CH=CH),
6.55 (d, 2H, J = 3.6 Hz, ArH), 4.45 (t, 4H, J = 8.4 Hz,
NCH2), 1.75 (s, br, 8H, CH2), 1.31 (s, br, 12 H,
CH2), 0.86 (s, br, 6H, CH3).
Compound 4: Compound 3 (1.10 g, 2.00 mmol), furan-2-
carbaldehyde (0.39 g, 4.00 mmol) and t-BuOK (0.45 g, 4.00
1
mmol). Yellow oil (0.59 g, 68.7%). H NMR (CDCl3) 
(ppm): 7.38–7.26 (m, 13H, ArH), 7.12 (d, 1H, J = 7.2 Hz,
ArH), 7.05 (d, 3H, J = 8.7 Hz, ArH), 6.98 (d, 2H, J = 15.9
Hz, CH=CH), 6.79 (d, 2H, J = 16.8 Hz, CH=CH), 6.41
(s, 1H, ArH), 6.31 (d, 2H, J = 3.0 Hz, ArH).
Compound 5: Compound 3 (1.10 g, 2.00 mmol), thio-
phene-2-carbaldehyde (0.45 g, 4.00 mmol) and t-BuOK
General procedure for the synthesis of compound I–III
1
(0.45 g, 4.00 mmol). Yellow oil (0.54 g, 58.6%). H NMR
4-(Diphenylamino)benzylphosphonate (1.0 equiv) was sus-
pended in anhydrous tetrahydrofuran under an atmosphere
of dry argon, t-BuOK (2.0 equiv) was added directly as a
solid and the resultant mixture was stirred at room tempera-
ture for 10 min. After the addition of the solution of one of
compound 7–9 (2.0 equiv) in anhydrous tetrahydrofuran
dropwise, the reaction mixture was stirred at room tem-
perature overnight, then poured into water. The organic
product was extracted with chloroform for several times,
and dried over anhydrous Na2SO4. After removing the sol-
vent, the crude products were purified through a silica gel
chromatography column by using petroleum/ethyl acetate
(10/1) as eluent.
Compound I: Compound 7 (0.24 g, 0.50 mmol), 4-(di-
phenylamino)benzylphosphonate (0.40 g, 1.00 mmol),
t-BuOK (0.11 g, 1.00 mmol). Yellow solid (0.10 g, 21.0%).
1H NMR (CDCl3)  (ppm): 7.36 (d, 15H, J = 8.1 Hz, ArH),
7.13–7.04 (m, 28H, ArH), 6.76 (d, 4H, J = 16.2 Hz, ArH),
6.57 (s, 2H, ArH), 6.37 (d, 4H, J = 15.6 Hz, ArH). 13C NMR
(CDCl3)  (ppm): 147.7, 132.7, 129.5, 127.5, 125.6, 124.8,
123.6, 123.3, 122.6, 114.9, 111.0. MS (MALDI-TOF), m/z
[M+]: 967.20, calcd, 967.42.
(CDCl3)  (ppm): 7.35 (d, 4H, J = 8.7 Hz, ArH), 7.31–7.26
(m, 3H, ArH), 7.18–7.11 (m, 5H, ArH), 7.07–6.98 (m, 9H,
ArH and CH=CH), 6.88 (d, 2H, J = 16.2 Hz, CH=CH).
Compound 6: Compound 3 (1.10 g, 2.00 mmol), N-
hexylpyrrole-2-carbaldehyde (0.72 g, 4.00 mmol) and
t-BuOK (0.45 g, 4.00 mmol). Yellow oil (0.75 g, 63.2%).
1H NMR (CDCl3)  (ppm): 7.35–7.15 (m, 8H, ArH and
CH=CH), 7.11–7.01(m, 9H, ArH and CH=CH), 6.84
(s, 5H, ArH), 6.16 (s, 1H, ArH), 3.95 (t, 4H, J = 6.6 Hz,
NCH2), 1.75 (s, br, 4H, CH2), 1.31 (s, br, 12H,
CH2), 0.88 (s, br, 6H, CH3).
General procedure for the synthesis of compound 79
DMF (3.0 equiv) was added to freshly distilled POCl3 (2.4
equiv) under an atmosphere of dry argon at 0 °C, and the
resultant solution was stirred until its complete conversion
into a glassy solid. After the addition of a yellow solution of
one of compound 46 (1.0 equiv) in 1,2-dichloroethane
dropwise at 0 °C, the mixture was stirred at room tempera-
ture overnight, then poured into an aqueous solution of so-
dium acetate (1 M), and stirred for another 2 h. The mixture
was extracted with chloroform for several times, the organic
fractions were collected and dried over Na2SO4. After re-
moving the solvent under reduced pressure, the crude prod-
ucts were purified through a silica gel chromatography
column.
Compound II: Compound 8 (0.21 g, 0.40 mmol), 4-(di-
phenylamino)benzylphosphonate (0.32 g, 0.80 mmol),
t-BuOK (0.09 g, 0.80 mmol). Yellow solid (78.3 mg,
1
19.6%). H NMR (CDCl3)  (ppm): 7.33 (t, 8H, J = 6.6 Hz,
ArH), 7.25 (t, 8H, J = 7.2 Hz, ArH), 7.15–6.99 (m, 27H,
ArH and CH=CH), 6.90–6.81 (m, 10H, ArH and
CH=CH). 13C NMR (CDCl3)  (ppm): 148.6, 148.3,
148.1, 144.4, 143.1, 143.0, 132.7, 132.2, 130.5, 129.1,
128.9, 128.8, 128.4, 127.9, 126.8, 126.0, 125.7, 125.1,
124.6, 124.3, 121.6, 121.4.
Compound 7: Compound 4 (0.59 g, 1.37 mmol), POCl3
(0.50 g, 3.29 mmol) and DMF (0.30 g, 4.11 mmol). Yellow
1
green solid (0.28 g, 43.0%). H NMR (CDCl3)  (ppm):
9.57 (s, 1H, CHO), 7.37–7.24 (m, 7H, ArH), 7.20–6.92 (m,
7H, ArH), 6.81 (d, 2H, J=15.9 Hz, CH=CH), 6.68 (d, 2H,
J = 16.2 Hz, CH=CH), 6.50 (s, br, 2H, ArH), 6.28 (s, 1H,
Compound III: Compound 9 (0.20 g, 0.31 mmol),

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Li QQ, et al. Sci China Chem April (2011) Vol.54 No.4
4-(diphenylamino)benzylphosphonate (0.30 g, 0.75 mmol),
3.2 One-photon physical properties
t-BuOK (0.07 g, 0.62 mmol). Yellow solid (66.4 mg,
18.9%). H NMR (CDCl3)  (ppm): 7.31–7.25 (m, 21H,
The photophysical properties of I–III were investigated in
dilute THF solutions, with their normalized absorption
spectra depicted in Figure 1, and the corresponding photo-
physical data summarized in Table 1. All the TPA chromo-
phores displayed an intense absorption in the UV-vis region,
and the maximum of the absorption spectra could be tuned
by the nature of heterocycles as the conjugated bridges.
When the conjugated bridges changed from furylvinylene to
thienylvinylene, chromophore II demonstrated much longer
1
ArH and CH=CH), 7.12–6.86 (m, 28H, ArH and
CH=CH), 6.53 (s, br, 4H, ArH), 3.96 (s, br, 4H,
NCH2), 1.66 (m, 4H, CH2), 1.50 (s, br, 4H, CH2),
1.25–1.03 (m, 8H, CH2), 0.79 (s, br, 6H, CH3). 13C
NMR (CDCl3)  (ppm): 147.8, 133.3, 132.5, 129.4, 126.9,
125.9, 124.5, 124.3, 124.1, 123.1, 115.6, 107.4, 31.6, 29.9,
26.7, 22.7, 14.2. MS (MALDI-TOF), m/z [M+]: 1133.1464,
calcd, 1133.6335.

max, about 12 nm red-shifted than that of chromophore I,
then changing the thiophene moiety with pyrrole one, the
max of chromophore III further red-shifted a little. Thus,
the red-shift of the max was along with the increasement of
the electron-rich properties of the heterocycles, indicating
that the electron-rich heterocycles as the conjugated bridges
would be beneficial to the charge transfer in the whole
molecules. Perhaps, the richer the electron lied in the Con-
jugated system, the better the charge transfer could take
place. In addition, for the chromophores of V-type, there
3 Results and discussion
3.1 Synthesis and characterization
The synthetic route and chemical structures of the TPA
chromophores (I–III) are shown in Scheme 1. For the
unique configuration of triphenylamine, compound 3 was
synthesized as the -center of the V-type chromophores in
two-step sequences: the deoxidization of the aldehyde group
in compound 1, and then followed by the reaction with
triethyl phosphate. The heterocyclevinylene-containing
building blocks of 4–6 were obtained by means of a double
Wittig-Horner-Emmons olefination of compound 3 and the
required aldehydes in the presence of t-BuOK in THF with
satisfactory yields from 58.6% to 68.7%. The conversion of
compound 4–6 to the corresponding bisaldehydes 7–9 was
achieved by the Vilsmeier reaction. In the case of this reac-
tion, it was possible that the aldehyde group could be intro-
duced to triphenylamine moieties, thus leading to their rela-
tively low yields. Finally, the TPA chromophores were
yielded by the double Wittig-Horner-Emmons olefination of
compound 7–9 and 4-(diphenylamino)benzylphosphonate.
All the chromophores were soluble in common organic sol-
vents such as THF, chloroform, DMF and DMSO and cha-
racterized by 1H and 13C NMR.
Figure 1 UV-vis spectra of the TPA chromophores I–III in THF solution.
Concentration: 1.0 × 106 M.
Scheme 1

Li QQ, et al. Sci China Chem April (2011) Vol.54 No.4
629
Table 1 Some photophysical data of chromophores
mabax (nm) a)
memax (nm) b)
mtpax (nm) c)
 (%) d)
66.3
 (GM) e)
625
I
429 (5.5×104)
441 (6.2×104)
444 (5.0×104)
487
503
499
760
760
760
II
50.8
777
III
76.7
441
a) 1.0×106 M in THF, wavelength of maximum absorbance and the value of molar absorptivity (, cm1·mol1·L1) is given in parentheses. b) 1.0×106 M in
THF, wavelength of maximum intensity. c) 2.0 × 105 M in THF, wavelength of maximum TPA cross sections. d) Quantum yields in THF solution using
fluorescein in water (= 90%, pH 11) as a standard. e)TPA cross sections, 1 GM (Göppert-Mayer) = 1050 cm4·s·photon1, the experimental uncertainty on
max is of the order of 10%–15%.
were double charge transfers from the donor groups to the
-center, resulting in the broad absorption and high molar
extinction coefficients.
One-photon emission spectra of the TPA chromophores
were demonstrated in Figure 2, with the data summarized in
Table 1. The max of the emission spectra shifted from 487
nm for chromophore I to 503 nm for chromophore II and
499 nm for chromophore III, respectively. The little
blue-shifted emission wavelength of III in comparison with
that of II was a little strange, indicating the special elec-
tronic properties of pyrrole moieties. In Figure 2, the dif-
ferent emission peak heights of the three chromophores re-
flected their different fluorescent emission quantum yields
(), as calibrated by using dye molecules with well estab-
lished fluorescent emission quantum yields such as fluo-
rescein. All of these three chromophores emitted green
fluorescence and their  values ranged from 50.8% to 76.7%.
Figure 3 The dependence of logarithmic output fluorescence intensity
(up-conversion signal) on logarithmic input laser power under the excited
wavelength of 800 nm for chromophore III. Concentration: 2.0 × 105 M.
Typical two-photon absorption upconverted fluorescence
signatures for chromophores are shown in Figure 4, and
they exhibited strong two-photon excited emission. The
two-photon-induced fluorescence signal was collected at the
same detection wavelength for both the reference and sam-
ple compounds. The chromophores were stable under the
experimental conditions described here, and no bleaching of
the solutions was observed at the end of the measurement.
The corresponding two-photon excitation spectra of com-
pounds I–III in THF solution for excitation wavelengths
ranging from 730 to 900 nm are shown in Figure 5. All the
3.3 Two-photon absorption properties
The TPA cross sections () were determined with the
two-photon-induced fluorescence measurement technique
by using femto-second laser pulses, which can avoid possi-
ble complications due to the excited-state excitation. In all
cases, the output intensity of two-photon excited fluores-
cence was linearly dependent on the square of the input la-
ser intensity, thereby confirming the TPA process (Figure 3).
Figure 2 One-photon emission spectra of the TPA chromophores I–III in
Figure 4 Two-photon-absorption-induced fluorescence emission spectra
of chromophores in THF.
THF solution. Concentration: 1.0 × 106 M.

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Figure 5 Two-photon absorption cross section () of chromophores in THF.
chromophores exhibited modest to larger two-photon cross
sections ranging from 441 to 777 GM at 760 nm. It was
surprised that chromophore III with the most electron- rich
hetercycle, pyrrole, as the conjugated bridge, exhibited the
lowest  value, not in accordance with the phenomena re-
ported by Marder’s group [18], that was, in the symmetrical
linear D--D type chromophores, compared with the chro-
mophores bearing pyrrole or dialkoxythiophene groups as
the conjugated bridge between two diarylaminophenyl
groups, the more electron-rich pyrrole examples showed the
higher  value, disclosing that the final properties of the
chromophores were heavily dependent on the cooperation
of each part of the whole molecule, but not some special
part. The obtained results of the V-type and linear chromo-
phores indicated that the charge transfer through the whole
molecule was different. In the V-type chromophores, be-
cause of the nonplanar structure of the triphenylamine, the
conjugated system did not well extend as that of the
chromophores with the planar -center, therefore, the effect
of the electron-rich properties was not obvious as that in
linear chromophores. Therefore, the interplay of torsional
conformation is an important factor in the design of TPA
chromophores with large two-photon cross sections.
7
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4 Conclusions
In summary, we have introduced different electron-rich het-
erocyclevinylene bridges into the V-type chromophores, in
which the triphenylamine moieties acted as the core or the
donor group, and different heterocycles such as furan, thio-
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We thank the National Natural Science Foundation of China (21002075
and 21034006) for financial support.