Photoinduced reversible gel-sol transitions of dicholesterol-linked azobenzene derivatives through breaking and reforming of van der Waals interactions

Article abstract of DOI:10.1039/c0sm00330a

A series of new symmetric dicholesterol-linked azobenzene gelators with different spacer lengths have been synthesized. The compounds with spacers of zero, two or six methylene units are denoted as DCAZO0, DCAZO2 and DCAZO6, respectively. A gelation test reveals that a subtle change in the length of the spacer can produce a dramatic change in the gelation behavior of the compounds. DCAZO2 obtains the minimum gelation concentration among the three gelators. For cyclopentanone gel of DCAZO2, the reversible gel-sol transitions by irradiation with UV and visible light are investigated by UV-vis absorption and circular dichroism (CD) spectra, SEM, TEM and XRD analyses. Upon UV irradiation of the gel, trans-cis photoisomerization of the azobenzene groups occurs, the change in molecular polarity leads to the breaking of van der Waals interactions, resulting in the gel-sol transition. The gel can be recovered by the reverse cis-trans photoisomerization after the exposure to visible light. SEM, TEM and XRD studies reveal that the gelator molecules self-assemble into one-dimensional fibers with diameters 50-100 nm in an anticlockwise direction, which further crossed-linked to form three-dimensional networks.

Full text of DOI:10.1039/c0sm00330a

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PAPER
www.rsc.org/softmatter | Soft Matter
Photoinduced reversible gel–sol transitions of dicholesterol-linked azobenzene
derivatives through breaking and reforming of van der Waals interactions†
*
Yeping Wu, Si Wu, Xiujie Tian, Xin Wang, Wenxuan Wu, Gang Zou and Qijin Zhang
Received 5th May 2010, Accepted 27th September 2010
DOI: 10.1039/c0sm00330a
A series of new symmetric dicholesterol-linked azobenzene gelators with different spacer lengths have
been synthesized. The compounds with spacers of zero, two or six methylene units are denoted as
DCAZO0, DCAZO2 and DCAZO6, respectively. A gelation test reveals that a subtle change in the
length of the spacer can produce a dramatic change in the gelation behavior of the compounds.
DCAZO2 obtains the minimum gelation concentration among the three gelators. For cyclopentanone
gel of DCAZO2, the reversible gel–sol transitions by irradiation with UV and visible light are
investigated by UV-vis absorption and circular dichroism (CD) spectra, SEM, TEM and XRD
analyses. Upon UV irradiation of the gel, trans–cis photoisomerization of the azobenzene groups
occurs, the change in molecular polarity leads to the breaking of van der Waals interactions, resulting in
the gel–sol transition. The gel can be recovered by the reverse cis–trans photoisomerization after the
exposure to visible light. SEM, TEM and XRD studies reveal that the gelator molecules self-assemble
into one-dimensional fibers with diameters 50–100 nm in an anticlockwise direction, which further
crossed-linked to form three-dimensional networks.
and it shows that the aromatic unit (A) plays a critical role in the
relationship of structure and property. Recently, dimeric choles-
terol-based derivatives, which were denoted as A(LS)2 gelators,
are reported as a new class of LMOGs. The changes in the
molecular structure can bring about new potentialities for the
construction of novel cholesterol-based gelators.16,17
Introduction
Molecular self-assembly through specific interactions into
supramolecular structures is a powerful approach to the devel-
opment of new functional materials.1 Low-molecular-weight
organogelators (LMOGs) have attracted increasing attention in
the past few decades2 not only for basic self-assembly behavior but
also for various potential applications such as photovoltaics,3 light
harvesting,4 templates,5 drug controlled release,6 energy transfer,7
etc. Organogels are made of a small amount of an organogelator
and relatively much organic liquid, which self-organize into three-
dimensional supramolecular networks. The driving forces for the
self-assembling process could be weak non-covalent interactions
such as hydrogen bonding,8 p–p stacking,9 dipole–dipole,10 and
van der Waals interactions.11 However, H-bonding takes respon-
sibility for most reported gelators; only few gelators are found
without H-bonding but weaker interactions.12
Besides natural thermal stimuli, the gel–sol transition of
organogels can be controlled reversibly or irreversibly by other
stimuli such as photochemical,18 chemical,19 or mechanochemi-
cal.20Among these stimuli, light can be used as a precise stimulus
by selecting suitable wavelengths, polarization directions, and
intensity, allowing non-contact control.21 Azobenzene is one
of the smartest molecules among all known photochromic
compounds because of its efficient and reversible trans–cis pho-
toisomerization that leads to large changes in molecular geom-
etry and polarity.22 Although organogelators containing
azobenzene groups are a large family,23–25 it is still not very clear
about the relationship between the structure and the gelation
ability of the gelators. And how does the trans–cis photo-
isomerization of azobenzene groups induces the gel–sol transi-
tion needs further study.
Among all the known LMOGs, cholesterol-based gelators form
a versatile category that has been systematic studied because
cholesteryl groups can easily self-assemble through weak van der
Waals interactions.13,14 The first series of cholesterol-based
LMOGs were reported by Weiss and coworkers15 which were
denoted as ALS gelators (abbreviation of aromatic-linker-steroid)
and displayed effective and somewhat predictable gelation abili-
ties. Since then, plenty of such ALS gelators have been reported
Herein we report the synthesis and investigation of a series of
new symmetric A(LS)2 organogelators DCAZO (Scheme 1),
consisting of an azobenzene core linked on both sides to cho-
lesteryl units, directly or via flexible alkyl chains. The gelation
properties of these organogelators vary dramatically with the
spacer length between cholesteryl and azobenzene units. Spec-
troscopic studies indicated that the driving force for molecular
aggregation should be relatively weak van der Waals interac-
tions between cholesteryl units and asymmetrical aggregation
of these gelators is induced by the presence of their chiral
cholesteryl units. Photoisomerization of azobenzene units
changes molecular polarity and thus results in the change in
CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer
Science and Engineering, University of Science and Technology of China,
Key Laboratory of Optoelectronic Science and Technology in Anhui
Province, Hefei, Anhui, 230026, P. R. China. E-mail: zqjm@ustc.edu.cn;
Fax: +86 551 3601704
† Electronic Supplementary Information (ESI) available: Pictures and
CD spectra showing the reversibility of the gel. See DOI:
10.1039/c0sm00330a/
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2.40(s, 4H, alkyl), 2.03–0.68(m, 82H, cholesteryl protons). FT-IR
(KBr), nmax/cmꢁ1: 2950, 2870 (–CH2–), 1744 (C]O), 1239 (C–O).
ꢀ
1
DCAZO6: Yield 30%. Mp 208–209 C. H NMR (400 MHz;
CDCl3) dH (ppm): 7.85 (d, 4H, ArH), 6.98 (d, 4H, ArH), 5.39
(s, 2H, vinylic), 4.47 (m, 2H, oxyalkyl), 4.14 (t, 4H, oxyalkyl),
4.03 (t, 4H, oxyalkyl) 2.40(s, 4H, alkyl), 2.02–0.67(m, 98H,
cholesteryl and alkyl protons). FT-IR (KBr), nmax/cmꢁ1: 2945,
2868 (–CH2–), 1740 (C]O), 1253 (C–O).
General methods
1H NMR spectra were recorded on a Bruker AVANCE III
400 MHz spectrometer. FT-IR spectra were recorded using
a Nicolet 8700 spectrometer. UV-vis absorption spectra were
measured on a Shimadzu UV-2550 PC spectrophotometer.
Circular dichroism (CD) spectra were recorded in a Jasco J-810
spectropolarimeter. Optical cells (0.13 mm or 1 cm optical path
length) were used for UV-vis and CD measurements. Photo-
irradiation was carried out with a high-pressure mercury lamp
(Hamamatsu LC6) through a UV or visible filter. The intensity of
the UV light (365 nm) was about 1.9 mW/cmꢁ2. X-Ray diffraction
(XRD) patterns were obtained on a Philips X’Pert PRO diffrac-
Scheme 1 Synthetic route for the dicholesterol-linked azobenzene
derivatives.
affinity between DCAZO2 and the solvent, which triggers
breaking and reforming the van der Waals interactions between
cholesteryl units, leading to gel–sol transitions of the system.
ꢀ
tometer using Cu Ka (l ¼ 1.5418 A). Scanning electron micros-
copy (SEM) was performed on a FEI Sirion200 system. The
samples for SEM measurements were prepared by drop casting of
the sample solution on silicon wafers and leaving them dry in air.
Finally, the samples are coated with gold. Transmission electron
microscopy (TEM) images were obtained using a Hitachi H-7650
system at an accelerating voltage of 100 kV. The sample for TEM
measurements was prepared by applying a drop of sample solution
on a copper grid and leaving it dry in air.
Experimental
Materials
Benzene, pyridine and N,N-dimethylformamide (DMF) were
purified before use. Benzene was purified by shaking with
concentrated sulfuric acid for three times to free from thiophene,
and then with dilute Na2CO3 solution and water, followed by
drying with CaCl2, and distillation. Pyridine was dried with solid
KOH and then purified by distillation. DMF was dried with
MgSO4 and then distilled under reduced pressure. 6-Chloro-1-
hexanol (Aldrich), 2-chloroethanol and cholesteryl chlor-
oformate (Alfa Aesar) were used without further purification.
Other reagents and solvents were commercially available and
used as received.
Gelation test
A weighted amount of gelator and a measured volume of selected
pure organic solvent were placed into a sealed glass bottle and the
solution was heated until the solid was dissolved. The solution
was allowed to stand at room temperature for 12 h, and the state
of the mixture was evaluated by the ‘‘stable to inversion of a test
tube’’ method.27
Preparation of compound 1, 2a, 2b: synthesis, purification,
and characterization of these compounds were reported in our
previous work.26
Preparation of the DCAZO series: 1 (or 2a, 2b) (3 mmol) was
dissolved in benzene (100 ml) and pyridine (10 ml). Then cho-
lesteryl chloroformate (6 mmol) dissolved in benzene (30 ml) was
added drop wise and the mixture was refluxed for 24 h at 80 ꢀC.
After the reaction, the mixture was distilled and the obtained
crude product was dissolved in chloroform and then precipitated
in methanol. For DCAZO0 and DCAZO6, the product was
purified by column chromatography (petroleum ether (60–90
ꢀC)/chloroform¼ 1/9) to afford a yellow powder. For DCAZO2,
the product was recrystallized twice from petroleum ether (60–90
ꢀC)/ethyl acetate ¼ 4/1.
Results and discussion
Molecular design and gelation behavior
The synthetic route for the symmetric dicholesterol-linked azo-
benzene gelators is shown in Scheme 1. The gelators consist of
three parts, two cholesteryl units (S), two spacer units (L), and
one aromatic unit (A). It is known that cholesteryl groups (S) can
easily self-assemble through weak van der Waals interactions.
However, H-bonding takes responsibility for most reported
A(LS)2 gelators. It is still unclear about the role of the spacer
length (L) when gel forms in a system without H-bonding. The
introducing of azobenzene unit as aromatic unit (A) can bring
photosensitivity and thus make the gel controllable.
ꢀ
1
DCAZO0: Yield 51%. Mp 257–259 C. H NMR (400 MHz;
CDCl3) dH (ppm): 7.94 (d, 4H, ArH), 7.34 (d, 4H, ArH), 5.43
(s, 2H, vinylic), 4.61 (m, 2H, oxyalkyl), 2.50(s, 4H, alkyl), 2.04–
0.69(m, 82H, cholesteryl protons). FT-IR (KBr), nmax/cmꢁ1
:
The gelation abilities of the DCAZO series were tested in
various organic solvents at room temperature and the results are
summarized in Table 1. DCAZO0 can gelate polar cyclic solvents
such as cyclopentanone, cyclohexanone, pyridine and 1,4-dioxane,
but precipitates in nonpolar cyclic solvents such as benzene,
2950, 2868 (–CH2–), 1761 (C]O), 1225 (C–O).
ꢀ
1
DCAZO2: Yield 44%. Mp 228–230 C. H NMR (400 MHz;
CDCl3) dH (ppm): 7.88 (d, 4H, ArH), 7.01 (d, 4H, ArH), 5.39
(s, 2H, vinylic), 4.51 (m, 6H, oxyalkyl), 4.27 (t, 4H, oxyalkyl),
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Table 1 Gelation properties of DCAZO series at room temperaturea
increasing length of the alkylene spacers.28 However, the
minimum gelation concentration of all tested solvents is obtained
by DCAZO2 in cyclopentanone and cyclohexanone (1% w/v),
the formed gel is transparent and stable for several months. The
gelation process may be based on a subtle balance between
solubility and insolubility, which requires to possess both
moderate affinity with solvent molecules and the moderate self-
aggregation ability.14,23 In the case of cyclopentanone and
cyclohexanone, DCAZO0 with no spacer is relatively more
soluble so that although it can form gels but the critical gelation
concentrations are much higher. DCAZO6 with 6 methylene
units is relatively less soluble and tend to self-aggregation. And
DCAZO2 with 2 methylene units achieves a subtle balance
between them, so it obtains the minimum gelation concentration.
Solvent
DCAZO0
DCAZO2
DCAZO6
cyclopentanone
cyclohexanone
1-butanol
benzene
toluene
pyridine
G(2)
G(3)
P
P
P
G(4)
TG(1)
TG(1)
P
P
S
S
S
I
P
P
P
P
P
P
P
I
1,4-dioxane
n-hexane
G(3)
I
cyclohexane
DMF
DMSO
acetone
ethyl acetate
P
P
I
I
I
P
G(1.5)
P
P
P
P
P
I
I
I
a
G: turbid gel; TG: transparent gel; P: precipitate; S: solution I:
insoluble; for gels, the minimum gelation concentrations at room
temperature are shown in parentheses (% w/v).
UV-vis absorption and circular dichroism (CD) spectra
To investigate the photoresponsiveness of the gel, the cyclo-
pentanone gel of DCAZO2 was irradiated with a high-pressure
mercury lamp through appropriate filters, and the change of the
configuration of the azobenzene units was followed by UV-vis
spectroscopy. Fig. 1a shows the UV-vis absorption spectra of the
gel–sol transition upon irradiation with UV light at 365 nm in
different time. The p–p* transition band of trans isomers with
toluene and cyclohexane. DCAZO2 can only gelate cyclo-
pentanone, cyclohexanone and DMF, while DCAZO6 cannot
gelate any solvent tested herein. It can be easily seen that the
length of the spacer between cholesteryl and azobenzene units
has a profound effect upon the gelation abilities of the
compounds studied. If we only consider the numbers of solvents
gelated by the gelators, DCAZO0 is more efficient than
DCAZO2 and DCAZO6. This is in accordance with a series of
L-lysine based gemini organogelators reported by Suzuki and
coworkers, whose gelation abilities tended to decrease with
lmax at 358 nm decreases remarkably and, at the same time, the
n–p* transition band of cis isomers at around 448 nm increases
slightly, together with the macroscopic gel to sol transition. After
irradiation for 120 s, the UV-vis spectra do not change any more,
Fig. 1 UV-vis spectral changes of the cyclopentanone gel of DCAZO2 (1% w/v, 0.13 mm path length): (a) irradiation of the gel at 365 nm and (b)
irradiation at 435 nm of the sol obtained after irradiation at 365 nm for 120 s. (c) Normalized UV-vis spectra of the cyclopentanone gel of DCAZO2 (1% w/v,
0.13 mm path length) (solid line) and its dilute cyclopentanone solution (0.01% w/v, 1 cm path length) (dashed line). (d) Dependence of ln[(AN-At)/(AN-A0)]
on irradiation time (at 358 nm) of the cyclopentanone gel of DCAZO2 (-) and its dilute cyclopentanone solution (,).
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indicating azobenzene groups reach to a cis saturated state
(ca. 96%). The cis saturated DCAZO2 could not form gel after
kept in dark overnight. Visible light (435 nm) irradiation on the cis
saturated solution recovered the original spectrum by the reverse
cis–trans isomerization in 300 s (Fig. 1b). Gel was reformed after
kept in dark overnight (see ESI,† Fig. S1 and S2). The two same
isosbestic point at 317 and 424 nm in Fig. 1a and 1b suggests that
only two absorbing species, namely trans and cis isomers of the
azobenzene groups exist in the reversible gel-to-sol and sol-to-gel
transition. Unlike most of the reported photoresponsive azo-
benzene-containing organogel, in which azobenzene groups were
aligned in H-type aggregation, the p–p* absorption spectra of the
cyclopentanone gel of DCAZO2 and its dilute solution are nearly
the same in the near UV range, indicating that there is no strong
aggregation between azobenzene groups. The corresponding first-
order rate constants of p–p* transition (trans to cis isomer, k) are
determined by fitting the experimental data to29
AN ꢁ At
ln
¼ ꢁkt
ANꢁA0
where At, A0, and AN are absorbance at 358 nm at time t, time
zero, and infinite time, respectively. After careful comparison of
the plots of the cyclopentanone gel of DCAZO2 and its dilute
solution, it is found that the plots of the solution can be well-
fitted as a linear relationship and the corresponding first-order
rate constant ks is determined to be 0.075 ꢂ 0.004 sꢁ1. However,
the plots of the gel can be divided into two parts. In the first 30 s,
the first-order rate constant (kg1 ¼ 0.029 ꢂ 0.004 sꢁ1) is much
slower than in solution, suggesting that there are much stronger
intermolecular interactions in the gel state. In the second stage,
from 40 to 80 s, the first-order rate constant (kg2 ¼ 0.082 ꢂ 0.002
sꢁ1) is very close to the rate constant of the solution. Based on the
above results, the trans–cis photoisomerization of the gel can be
comprehended as follows: in the first stage, while most of the
azobenzene groups are in the gel state, the isomerization process
is hindered by the strong intermolecular interactions between
the gelators; when the isomerization process reaches a specific
degree (trans–cis isomerization degree 57.7%, calculated from the
intersection point of the two fitted lines), the gel collapses and the
intermolecular interactions of the gelators are much weakened,
so the following isomerization process is similar to that in dilute
solution. The large conversion ratio from trans to cis isomer
(ca. 96%) and no strong aggregation suggest that there is suffi-
cient free volume in the gel state for the photoisomerization of
the azobenzene groups to take place.
Fig. 2 (a) CD spectral changes of the cyclopentanone gel of DCAZO2
(1% w/v, 0.13 mm path length) under the irradiation of UV light at
365 nm; (b) intensities of UV-vis absorbance at 358 nm (-) and the CD
signal at 368 nm (,) as a function of time during the UV light (365 nm)
irradiation of the cyclopentanone gel of DCAZO2 (1% w/v). Both
experiments were performed in same conditions.
supramolecular chirality in the gel state, rather than the inherent
chirality of the gelators.
Fig. 2b shows the absorption intensities of the p–p* absorption
maximum (358 nm) and the CD signal at 368 nm against the
irradiation time. It is found that the decrease of CD intensity is
slower than the trans–cis photoisomerization of azobenzene
groups, which reveals that the breaking of van der Waals interac-
tions happens after photoisomerization of azobenzene groups. The
mechanisms for most of the reported photoresponsive azobenzene-
containing organogel are based on the large changes in molecular
geometry caused by the photoisomerization of the azobenzene
groups. As already discussed above, in our system, there is suffi-
cient free volume in the gel state for the photoisomerization of the
azobenzene groups to take place, so it is not the same case to what
have been reported by other groups.18,25,30 Upon UV light radia-
tion, the less polar trans isomers convert to more polar cis isomers,
which is easier to dissolve in polar solvent as cyclopentanone. The
change in affinity between DCAZO2 and the solvent triggers
breaking and reforming the van der Waals interactions between
cholesteryl units, leading to gel–sol transitions of the system. This is
similar to a photoresponsive H-bonding azobenzene-containing
organogel reported by Matsuzawa and Tamaoki very recently.31
To obtain further insight into the nature of the self-assembled
structures, circular dichroism (CD) studies were performed. The
cyclopentanone gel of DCAZO2 shows a negative exciton CD
spectrum which consists of the negative first Cotton effect and
the positive second Cotton effect, a sign of (S)-chirality (Fig. 2a).
It is known that azobenzene-based cholesterol gelators with
natural (S) C-3 configuration tend to give a negative sign for the
first Cotton effect, indicating that the dipole moments of azo-
benzene chromophores tend to orient into the anticlockwise
direction.14,16 Upon UV irradiation (365 nm), the intensity of the
CD signal decreases gradually and eventually disappears as the
gel-to-sol transition proceeds. And the dilute solution of
DCAZO2 shows no CD signal. Since they are only observed in
the gel state, the CD signals should be attributed to the
Morphology and structure studies
In order to obtain a visual insight into the morphologies of
the molecular aggregation mode, the microstructures of the
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reversible system are examined with SEM and TEM. As shown
in Fig. 3a, b, c, the cyclopentanone gel of DCAZO2 forms
fibers of 50–100 nm in width and tens of micrometers in length
and these fibers form three-dimensional networks so that the
macroscopical flow of the solution stops to induce the forma-
tion of a solid-like gel. Fig. 3d shows that the fibers observed
in gel before UV irradiation disappear in the dried sol formed
by UV irradiation. The cis isomers of azobenzene groups
induced by UV irradiation tend to dissolve in the solvent and
thus the fibrous network are dissociated. And the recovered
gel by irradiation of visible light shows self-assembled fibers
again.
To reveal the detailed changes in molecular packing modes
of the system, XRD analyses are conducted and the results
are shown in Fig. 4. The XRD pattern of the xerogel of
DCAZO2 (Fig. 4a) shows three reflection peaks, indicating
periods of 4.50, 2.23 and 1.49 nm, which are almost exactly in
the ratio of 1 : 1/2 : 1/3, thus indicating a lamellar organiza-
tion within the gel with an interlayer distance of 4.50 nm.32 The
molecular length for the extended form is estimated to be
4.50 nm, which is in accordance with the long period of the
lamellar structure in the gel phase of DCAZO2 (Fig. 5a).
However, the three reflection peaks disappears and changes
to a single peak of 5.13 nm in the dried sol (Fig. 4b). This
indicates that the order structure changes to less regular
aggregates. After visible light irradiation to the sol state, the
original XRD pattern is recovered, suggesting that the
lamellar organization.
Fig. 3 SEM images (a, c) and TEM image (b) of the cyclopentanone
xerogel of DCAZO2 (1% w/v), SEM image of the dried sol (d), and
photographs of the gel (e) before and (f) after UV irradiation.
Fig. 5 illustrates the photoinduced reversible gel–sol transi-
tions of the system. One-dimensional superstructures of the
gelators are driven by the van der Waals interactions between
the cholesteryl units and thus self-assemble into thin fibers, which
are further aggregated into thick fibers of 50–100 nm. Upon UV
irradiation, trans–cis photoisomerization of azobenzene groups
occurs first, leading to increasing in affinity between DCAZO2
and the solvent, thus triggers the breaking of van der Waals
interactions between the cholesteryl units. In contrast, upon
visible light irradiation, the molecules precipitate from the
solvent after the cis-trans photoisomerization, and subsequently
self-assemble into fibrous network through reforming of the van
der Waals interactions.
Fig. 4 XRD patterns of (a) the cyclopentanone xerogel of DCAZO2, (b)
the dried sol obtained by UV irradiation to the gel, and (c) the dried gel
obtained by irradiating the UV-exposed sol with visible light.
Fig. 5 Schematic representation of photoinduced reversible gel–sol transitions of the cyclopentanone gel of DCAZO2. (a) The energy minimized
molecular structure of DCAZO2 has reappeared.
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Conclusions
In summary, a series of new symmetric dicholesterol-linked
azobenzene gelators have been synthesized. The difference in the
spacer length between cholesteryl and azobenzene units can
produce a dramatic change in the gelation behavior. DCAZO0
can gelate polar cyclic solvents such as cyclopentanone, cyclo-
hexanone, pyridine and 1,4-dioxane. DCAZO2 can only gelate
cyclopentanone, cyclohexanone and DMF, while DCAZO6 can
not gelate any solvent tested herein. And the minimum gelation
concentration of all tested solvents is obtained by DCAZO2 in
cyclopentanone and cyclohexanone (1% w/v). UV-vis absorption
and CD spectroscopy reveals that the gel–sol transition through
breaking van der Waals interactions occurs after photo-
isomerization of azobenzene groups. Considering there is suffi-
cient free volume in the gel state for the photoisomerization of
the azobenzene groups, the change in solvent affinity resulted
from the change in polarity after photoisomerization, triggers the
breaking and reforming of the gel system. Based on the results of
SEM, TEM and XRD studies, it is found that the gelator
molecules self-assemble into one-dimensional fibers with diam-
eters of 50–100 nm in an anticlockwise direction, which further
crossed-linked to form three-dimensional networks. These
results afford useful information for the development of new
versatile low molecular mass gelators and soft matter.
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This work is supported by National Natural Science Foundation
of China (No. 50703038, 50773075 and 21074123) and the Chinese
Academy of Sciences (kjcx3.sywH02 and kjcx2-yw-m11).
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