Hierarchical self-assembly of surfactant-encapsulated and organically grafted polyoxometalate complexes

Article abstract of DOI:10.1002/chem.201002298

A series of surfactant-encapsulated and organically grafted polyoxometalates (SEOPs) were prepared through a co-precipitation procedure. Through a rational selection of the molecular components in the structure of the complex, SEOP complexes self-assemble into ordered aggregates with two different hierarchical self-assembled structures in an organic solvent mixture of dichloromethane and methanol in different volume ratios. FTIR, ¹H NMR, and X-ray photoelectron spectroscopy were used to characterize the self-assembly process and the involved driving forces. In a weakly polar solvent, SEOPs aggregated into fibers with a lamellar structure. When the solvent polarity was increased, SEOPs formed ribbonlike aggregates with a tetragonal structure. The change of the hierarchical self-assembled structure was deduced in regard to the arrangement of alkyl chains, electrostatic interactions, and hydrogen bonding between the pyridyl groups and terminal oxygen atoms of the polyoxometalates. The ribbonlike aggregates exhibit birefringence due to the ordered arrangement of SEOPs in the microstructure. Copyright

Full text of DOI:10.1002/chem.201002298

FULL PAPER
DOI: 10.1002/chem.201002298
Hierarchical Self-Assembly of Surfactant-Encapsulated and Organically
Grafted Polyoxometalate Complexes
[
a]
Huanbing Wang, Yi Yan, Bao Li, Lihua Bi, and Lixin Wu*
1
Abstract: A series of surfactant-encap-
sulated and organically grafted poly-
oxometalates (SEOPs) were prepared
through a co-precipitation procedure.
Through a rational selection of the mo-
lecular components in the structure of
the complex, SEOP complexes self-as-
semble into ordered aggregates with
two different hierarchical self-assem-
bled structures in an organic solvent
mixture of dichloromethane and meth-
anol in different volume ratios. FTIR,
H NMR, and X-ray photoelectron
spectroscopy were used to characterize
the self-assembly process and the in-
volved driving forces. In a weakly polar
solvent, SEOPs aggregated into fibers
with a lamellar structure. When the sol-
vent polarity was increased, SEOPs
formed ribbonlike aggregates with a
tetragonal structure. The change of the
hierarchical self-assembled structure
was deduced in regard to the arrange-
ment of alkyl chains, electrostatic inter-
actions, and hydrogen bonding between
the pyridyl groups and terminal oxygen
atoms of the polyoxometalates. The
ribbonlike aggregates exhibit birefrin-
gence due to the ordered arrangement
of SEOPs in the microstructure.
Keywords: birefringence · polyoxo-
metalates · ribbonlike aggregates ·
self-assembly · tetragonal structures
Introduction
could be integrated conveniently according to the initial
design.
The fabrication of multiscaled organic–inorganic hybrid
structures has attracted much attention recently owing to
both fundamental interest and potential applications for
Polyoxometalates (POMs) are an intriguing class of inor-
ganic cluster that exhibit structural, chemical, and electronic
versatilities, thus showing features as functional materials
[1]
[5]
functional materials and devices. Molecular self-assembly,
as a promising strategy, has been widely used to achieve this
goal, in which predesigned building blocks are allowed to
form various organized aggregations. In general, the forma-
tion process of the self-assembly is governed by noncovalent
interactions, such as electrostatic interactions, hydrogen
bonding, p–p stacking, van der Waals forces, and so forth,
which occur among the components that serve as coopera-
for catalysis, biomedicine, and magnetism. The incorpora-
tion of POMs into ordered structures has attracted increas-
ing interest recently to exploit POM-based self-assembly
[6]
systems. Among the methods proposed for the organiza-
tion of POMs into nanostructured systems, a powerful ap-
proach is to modify the surface of POMs with cationic or-
ganic molecules through electrostatic interactions, thus
yielding organic–inorganic hybrid complexes. Many kinds of
[2]
[6b,c,7]
tive building blocks. By manipulating the equilibrium of
these weak interactions, various self-assembled structures
organic molecules, such as typical cationic surfactants,
[8]
[9]
surfactants with functional groups, peptides, dendrim-
[3]
[10]
[6g,11]
from zero- to three-dimensional order have been obtained.
ers, and polymers,
have been employed for this pur-
Rational molecular-structure design of supramolecular
building blocks and adjustment of the chemical microenvir-
onment have been proven to be efficient approaches to con-
trol the self-assembly process and structure. Following this
strategy, various intra-/intermolecular interactions can be si-
multaneously endowed to govern a self-assembly process
over distinct multilevels. Furthermore, through a hierarchi-
cal self-assembly approach, well-defined aggregated struc-
tures composed of incompatible molecular components
pose. These hybrid complexes exhibit interesting self-assem-
bly behavior in solution and solid state by controlling the
equilibrium among the intra-/intermolecular interactions.
[12]
However, due to the nondirectional nature of the binding
sites of electrostatic interactions, it is difficult to control the
arrangement of POMs on the nanoscale. Fortunately, a
novel series of organically grafted POMs have been success-
fully developed by covalently anchoring functional groups
[4]
[13]
onto the surface of POMs. Owing to the convenient sub-
stitution of various functional groups, the covalently organi-
cally grafted POMs provide a chance to control the arrange-
[14]
[
a] H. Wang, Y. Yan, Dr. B. Li, Prof. L. Bi, Prof. L. Wu
State Key Laboratory of Supramolecular Structure and Materials
Jilin University, Changchun 130012 (P.R. China)
Fax : (+86)431-8519-3421
ment of POMs in an oriented manner.
We applied both strategies of organic encapsulation and
covalent modification to the POM surface to fabricate de-
fined hybrid self-assemblies precisely and to control the ar-
rangement of the POMs directly. Because of the planar
structure and possibility of covalent modification through
E-mail: wulx@jlu.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201002298.
Chem. Eur. J. 2011, 17, 4273 – 4282
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4273

the exchange of hydroxy groups, as a typical representative,
Anderson-type POMs automatically became a perfect candi-
tively. The counterions of these SEOPs were further re-
placed by the double alkyl-chain cationic surfactants diocta-
decyldimethylammonium (DODA), dihexadecyldimethylam-
monium (DHDA), and didodecyldimethylammonium
[14b,15]
date for the present purpose.
Due to the convenient
synthesis, high yield, and the certain replaceable charge, an
III
Anderson-type POM with the Mn heteroatom was chosen
(DDDA), thus giving SEOP-2–SEOP-4 and SEOP-6, re-
III
II
II
[14b]
rather than Fe -, Zn -, and Ni -substituted POMs. Differ-
ent organic functional groups can be anchored to the Ander-
son-type POM surface, for example, long alkyl chains, for
which aggregation morphologic changes have been investi-
gated and echinus-like spherical aggregates were ob-
spectively.
The synthetic procedures and chemical-struc-
ture characterizations are described in the Experimental
Section and Supporting Information (Figures S1–S9). For
SEOP-2–SEOP-4 and SEOP-6, the characteristic vibrational
bands of the terminal Mo=O mode at around n˜ =943, 912,
[16]
ꢀ1
tained. Herein, pyridyl- and amido-grafted Anderson-type
POMs were used and their counterions were further ex-
changed by cationic surfactants with long alkyl chains, thus
yielding surfactant-encapsulated organically grafted POMs
and 905 cm and the bridging Mo-O-Mo mode at about n˜ =
ꢀ
1
670 cm in the FTIR spectra, as observed in the spectra of
SEOP-1 and SEOP-5, confirm that the frame structure of
the organically grafted Anderson-type POMs is not affected
by the exchange of counterions. Meanwhile, as shown in the
(
SEOPs; Figure 1). By adjusting the solvent polarity, the
1
H NMR spectra, the disappearance of the signals of TBA
ions accompanied by the appearance of signals of double
alkyl-chain surfactants demonstrates that the counterions
are entirely substituted. By combining these results with ele-
mental analysis, the exact chemical formulas of SEOP-2–
SEOP-4 and SEOP-6 can be assigned (DODA) ACHTUNGRTNEUNG[ MnMo O -
3 6 18
A
H
U
G
R
N
N
{(OCH ) CNHCOC H N} ],
(DHDA)
(DDDA) ACHTUNGNTERNUNG[ MnMo O -
3 6 18
ACHTUNGTRNENUG[ MnMo O -
2
2
2
2
3
3
3
3
5
5
5
4
4
4
2
2
2
3 6 18
A
H
U
G
R
N
U
G
A
H
U
G
R
N
U
G
A
H
U
G
R
N
N
[MnMo O -
3
6
18
A
H
U
G
R
N
U
G
2
2
According to previous reports on surfactant-encapsulated
[6b,7d,12]
complexes,
solvent polarity plays a crucial role in the
formation of self-assembled aggregates. Weakly polar sol-
vents, such as chloroform and dichloromethane, and the
polar-solvent acetone are often used in the surfactant-encap-
sulated POM system. The addition of highly polar solvents,
such as methanol and other alcohols, can promote hydro-
phobic interactions between alkyl chains and is favorable
for stabilizing the aggregate morphologies of the complexes
in solution. In the present study, we chose the mixed solvent
of CH Cl /CH OH (2:1, v/v) due to the appropriate polarity
and similar evaporation rate. With SEOP-2 as an example,
the preparation steps are: SEOP-2 (1.0 mg) was added to
the solvent mixture (1.0 mL), and a clear solution was ob-
tained after sonication for 5 min at room temperature. The
reaction mixture was aged for another 10 min to give
enough time for a self-assembling equilibrium to be reached.
The Tyndall phenomenon was obviously seen, thus suggest-
ing that the self-assembly process of SEOP-2 occurs in the
mixture solution (Figure S10 in the Supporting Information).
The obtained assemblies were transferred onto substrates
for further characterization. The aggregates of other SEOPs
were obtained by using the same route.
Figure 1. Chemical structures of organically grafted Anderson-type
POMs and surfactants employed for the preparation of SEOPs.
prepared SEOPs, bearing different electrostatically bound
surfactants and covalently bound pyridyl groups, can self-as-
semble into one-dimensional well-defined aggregates, which
2
2
3
[6e,f,9a]
were, to the best of our knowledge, rarely reported.
Moreover, because the pyridyl group was directly anchored
onto the POM surface, additional hydrogen bonding could
be anticipated, which can be easily controlled by adjusting
the solvent polarity. Therefore, the present investigation
confirms an effective way to fabricate a new self-assembled
hierarchical hybrid structure based on POM complexes on
the nanoscale through a synergetic combination of electro-
static, hydrogen-bonding, and van der Waals interactions as
well as molecular-structure modification and solvent-envi-
ronment regulation.
Hierarchical self-assembly and aggregation structure of
SEOP-2: Ribbonlike aggregates that are several microme-
ters long and hundreds of nanometers wide are distinctly ob-
served from the SEM image of SEOP-2 (Figure 2a). The
average thickness of the ribbonlike aggregates reaches to
approximately 100 nm according to atomic force microscope
(AFM) measurements (Figure 2b). The thickness of the rib-
bonlike aggregates obtained from a cross-section TEM
Results and Discussion
Synthesis and aggregate-preparation of SEOPs: Herein, the
precursors, that is, the pyridyl- and amino-grafted Ander-
son-type POMs, were prepared and characterized as tetra-
butylammonium (TBA) salts SEOP-1 and SEOP-5, respec-
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Chem. Eur. J. 2011, 17, 4273 – 4282

Fabrication Polyoxometalate Aggregates
FULL PAPER
be confirmed but needs to be recognized. It is known that
the only difference between the two possible structural
types is the length of the unit cell axes, in which a=b=c for
the cubic structure and a=b¼
6
c for the tetragonal structure,
whereas the angles of the unit cell are both a=b=g=908.
The analysis of the possible structural pattern can be per-
formed according to the Equation (1),
2
2
2
2
1
h þ k
l
c
¼
þ
ð1Þ
2
2
d
a
hkl
where d=l/2 sinq (q=diffraction angle, l=wavelength
of the X-ray beam), a and c are the unit cell axes, and h, k,
and l are related to the Miller indices of the diffraction
[18]
planes. According to this equation, the XRD pattern of
the ribbonlike aggregates can be indexed (Table 1), which
Figure 2. a) SEM, b) AFM, c) high-resolution-TEM and (inset: SAED
pattern), and d) cross-section TEM images of ribbonlike aggregates of
SEOP-2 prepared in mixed solvent CH Cl /CH OH (2:1 in v/v).
2 2 3
Table 1. Summary of Miller indices and comparison of observed diffrac-
tion patterns of the ribbonlike structure with calculated diffractions from
the tetragonal structure.
Measured
Predicted
Fitted Miller indices
image is in good agreement with the AFM results. Interest-
ingly, a very fine parallel strip substructure with regular
spacing of approximately 3.0 nm can be exactly observed
through TEM measurement (Figure 2c). Because transition-
metal atoms bear a high electronic contrast, we assigned the
dark region to the POM domain and the light region to the
organic component (i.e., DODAs). The lamellar substruc-
ture can also be clearly predicted from the cross-section of
the aggregates in the TEM images (Figure 2d). Surprisingly,
the selected-area electron diffraction (SAED) pattern dis-
plays a higher level order that exists in the inner layer, that
is, a square symmetry (inset in Figure 2c), which indicates a
typical pattern of a cubic or tetragonal structure rather than
diffraction [ꢁ]
diffraction [ꢁ]
h
k
l
29.65
14.88
29.80
14.90
13.33
13.00
11.92
11.06
10.54
9.93
1
2
2
0
1
1
2
3
4
0
0
0
1
0
0
1
2
0
2
0
0
0
0
1
1
1
0
0
0
2
1
1
1
1
3.64
3.00
1.91
1.08
10.73
9
6
6
.89
.81
.50
6.66
6.50
are in perfect agreement with the fitted tetragonal structure
with the lattice parameters a=b=29.80 and c=13.00 ꢁ and
a=b=g=908. Combined with the XRD results, the diffrac-
tion spots in the SAED pattern can be well indexed as
(200), (020), and (220) reflections (inset of Figure 2c).
[17]
a simple lamellar structure.
XRD measurements were performed to identify further
the packing mode of SEOP-2 in the ribbonlike aggregates
(
Figure 3). The diffraction pattern proves that, in addition to
the layered substructure, there is indeed a more ordered
structure located in the inner layer, which is consistent with
the SAED result. Thus, the cubic or tetragonal structure can
Intermolecular interactions in the ribbonlike aggregates: Be-
cause of the flexibility of the long alkyl chains in SEOP-2, it
is difficult to get a suitable crystal for single-crystal structure
analysis. Meanwhile, due to the anisotropy of Anderson-
type POMs, the detailed arrangement of SEOP-2 on the mo-
lecular scale can not be given just by using one-dimensional
XRD and two-dimensional SAED patterns. However, the
crystalline-like order of the ribbonlike aggregates reveal dis-
tinctly different self-assembled structures from those of sur-
factant-encapsulated polyoxometalate (SEP) complexes
without organically grafted POMs, such as spherical
[7d]
[12c]
[12a]
[8e]
shape, stripe,
cone, tube,
and hexagonal columns.
In the case of SEPs, it is considered that the rearrangement
of the alkyl chains is the main driving force that contributed
to the self-assembly of SEPs in solution. When the POM
core of SEPs is changed to the organically grafted core, in
addition to van der Waals forces, other intermolecular inter-
actions must be considered in the self-assembly process.
Figure 3. XRD pattern of ribbonlike aggregates of SEOP-2 prepared
from the mixed solvent CH Cl /CH OH (2:1 in v/v).
2 2 3
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4275

L. Wu et al.
Therefore, a detailed analysis of the driving forces is essen-
tial to understand the particular tetragonal structure of
SEOP-2.
SEOPs that possess short alkyl chains are not strong enough
to force the SEOPs to organize into ribbonlike aggregates
with a tetragonal structure. It is clear that the alkyl chains
can be applied to adjust the self-assembled structure by
changing the hydrophobic interactions.
First, the molecular structure was considered to under-
stand the factors behind the driving forces in the formation
of SEOP-2 aggregates. As analogues of SEOP-2, other
SEOPs with the same inorganic core but different surfactant
cations bearing shorter alkyl chains (i.e., SEOP-3 and
SEOP-4 with cetyl and dodecyl groups, respectively) were
employed to compare the aggregate features. It is seen from
SEM and TEM images that SEOP-3 and SEOP-4 can not
form ribbonlike aggregates but some random structures
under the same preparation conditions used for SEOP-2
However, it should be noticed that the van der Waals
force is an inherently weak and nondirectional interaction,
which hardly leads to periodic structures without the help of
[19]
other oriented forces such as hydrogen bonding. From the
SEM image of SEOP-6 aggregates, obtained from the same
mixed solvent of CH Cl /CH OH (2:1, v/v), we can see simi-
2
2
3
lar ribbonlike aggregates, although their size becomes short-
er than the aggregates of SEOP-2 (Figure 4c). However,
only a lamellar substructure with a layer spacing of 3.71 nm
was found, as clearly shown in the TEM and XRD results
(
Figures 4a,b and S11 in the Supporting Information). Fur-
(
Figured 4d and 5). As a contrast complex, SEOP-6 is en-
wrapped by the same surfactants, that is, DODA, used in
SEOP-2, but has a different inorganic core. Because both
complexes form one-dimensional ribbonlike aggregates, we
believe that the alkyl chains have the same effect as in the
mixed solvent. Therefore, the reason that SEOP-2 bearing
pyridyl groups self-assembles into a tetragonal structure
whereas SEOP-6 bearing amino groups on the Anderson-
type POM can only form the lamellar structure can be ex-
plained as the difference between the two grafted groups.
The additional interaction between the pyridyl groups
rather than between amino groups can be predicted, which
directs SEOP-2 to form an extra innerlayer periodic struc-
ture in the ribbonlike aggregates.
In addition to the influence of the molecular components
contained in the complexes, solvent polarity is demonstrated
to play a key role in the self-assembly process due to the
amphiphilicity of SEOPs. When CH Cl is employed as a
single solvent, SEOP-2 only forms bundled fibrous aggre-
gates (Figure 6a), and the TEM results show parallel strips
Figure 4. SEM images of a) SEOP-3, b) SEOP-4, and c) SEOP-6 aggre-
gates obtained from the mixed solvent CH Cl /CH OH (2:1, v/v). d) Cor-
responding TEM image of SEOP-6 aggregates.
2
2
2
2
3
thermore, only lamellar structures with layer spacings of
.05 and 2.61 nm existed in those aggregates according to
3
the XRD patterns (Figure 5). Evidently, the length of the
alkyl chains takes part in a close relationship with the
strength of the van der Waals forces; that is, the van der
Waals forces between the alkyl chains in the aggregates of
Figure 6. a) SEM and b) TEM images of SEOP-2 aggregates obtained
from the single solvent CH Cl .
2 2
with a regular spacing of 3.0 nm (Figure 6b). However, the
order level is obviously less than in the ribbonlike aggre-
gates. According to the XRD results, only a lamellar sub-
structure exists in the fibrous aggregates. Therefore, it can
be deduced that the addition of CH OH could induce
3
SEOP-2 to arrange into a more ordered state. On increasing
the CH OH content gradually, the diffraction intensities at-
Figure 5. XRD patterns of SEOP-3, SEOP-4, and SEOP-6 aggregates
3
prepared from the mixed solvent CH
2
Cl
2
/CH
3
OH (2:1, v/v).
tributed to the tetragonal structure in the low-angle region
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Fabrication Polyoxometalate Aggregates
FULL PAPER
2 2
Figure 7. XRD patterns of SEOP-2 aggregates obtained from CH Cl ac-
companied by the addition of CH
inset) regions.
3
OH in the low-angle and high-angle
(
of the XRD patterns increase simultaneously (Figure 7). In
addition, the intensity of the diffraction peak at 19.58 (d=
4.52 ꢁ), attributed to the crystalline packing of the alkyl
chains, increases sharply with an increase in the amount of
CH OH (inset of Figure 7). Apparently, the increase in the
3
solvent polarity promotes the rearrangement of alkyl chains
from a relatively disordered structure to a more ordered
[20]
state. Owing to the electrostatic interactions between the
surfactant cation and POM, the rearrangement of the alkyl
chains supports the formation of a much more ordered
structure of POMs in the innerlayer of the SEOP-2 aggre-
Figure 8. a) FTIR spectra of SEOP-2 aggregates obtained from CH
2
Cl
OH (2:1, v/v) (bottom). b) H NMR spectra of
OD with gradually increasing volumes of CD OD.
2
1
(
top) and CH
2
Cl
2
/CH
/CD
3
SEOP-2 in CDCl
3
3
3
[21]
gates. As is known, methanol is also a protic solvent. Sev-
eral highly polar but nonprotic and weakly polar but protic
solvents were added to the solution of SEOP-2 to evaluate
if methanol associates to the grafted organic component
through hydrogen bonding. The SEM and XRD results (Fig-
ure S12 in the Supporting Information) clearly show that
SEOP-2 can form “ribbonlike” aggregates with a tetragonal
structure in solution with CH Cl /CH CN (2:1, v/v). Mean-
almost all the alkyl chains in the SEOP-2 aggregates are in
the trans conformation, which is favorable for generating
[22]
strong van der Waals forces. Meanwhile, slightly less or-
dered (gauche conformation) alkyl chains in the sample pre-
pared from the single solvent CH Cl reveal that the in-
2
2
crease in solvent polarity promotes the tetragonal structure
in the ribbonlike aggregates.
2
2
3
while, when a protic but weakly polar solvent, such as etha-
nol, isopropanol or n-butyl alcohol, was added to the solu-
tion, only lamellar and cross-linked aggregates were found.
Therefore, we believe that solvent polarity plays an impor-
tant role in aggregate formation and change of SEOP-2. We
also investigated the influence of solvent polarity on the ag-
gregation of SEOP-2 by dispersion in CH Cl and then addi-
More importantly, the obvious difference in the FTIR
spectra comes from the amide I band (Figure 8a). The
ꢀ
1
amide I band of compound 1 appears at n˜ =1650 cm ,
which is indicative of the existence of hydrogen bonding be-
[23]
tween amide groups. Comparably, SEOP-2 aggregates ob-
tained from CH Cl display the amide I band at n˜ =
2
2
2
2
ꢀ
1
tion of CH OH. Similar results were observed to when we
1656 cm , which indicates hydrogen bonding between
amide and pyridyl rings.
3
[14c]
prepared the “ribbonlike” aggregates in the solvent mixture
of CH Cl and CH OH (Figure S13 in the Supporting Infor-
In contrast to this outcome,
when SEOP-2 aggregates are obtained from the mixed sol-
2
2
3
ꢀ
1
mation), thus indicating that it is the solvent polarity that
drives the aggregation of SEOP-2 from a layered structure
to a tetragonal structure.
vent, the intensity of amide I band at n˜ =1656 cm decreas-
ꢀ
1
es and a new band appears at n˜ =1671 cm . The shift of the
band to a high wavenumber implies that the addition of
methanol, to some extent, destroys the hydrogen bonding
between the amide and pyridyl groups in SEOP-2 (see Fig-
FTIR spectroscopic characterization was performed for
the self-assemblies prepared from solvents of CH Cl and
2
2
[14c,24]
CH Cl /CH OH (2:1, v/v) on CaF plates to reveal any addi-
ure 10).
However, the breakage of the hydrogen bond-
2
2
3
2
tional interactions (Figure 8a). The characteristic frequen-
ing is normally the source of disordered aggregation but not
the driving force of ordered aggregation of SEOP-2 due to
the saturation and direction of hydrogen bonds. Thus, other
driving forces that guide SEOP-2 into a tetragonal structure
should be considered.
cies of CH antisymmetric and symmetric stretching bands
2
ꢀ1
appear at n˜ =2917 and 2849 cm for the aggregates pre-
pared from CH Cl and at n˜ =2916 and 2850 cm for the
ꢀ
1
2
2
aggregates from the mixed solvent. This result indicates that
Chem. Eur. J. 2011, 17, 4273 – 4282
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4277

L. Wu et al.
1
H NMR spectroscopic analysis was employed to monitor
the change in proton signals in the formation process of the
tetragonal structure of SEOP-2 by increasing the amount of
methanol in the mixed solvent (Figure 8b and Figure S14 in
the Supporting Information). On increasing the volume
ratio of CDCl /CD OD, the proton signals of the alkyl
3
3
chains ꢀ(CH ) ꢀCH do not change obviously. Meanwhile,
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
2
n
3
the broad proton peak of the ammonium head splits into
two peaks, in which one is ascribed to the methyl group in
+
(
CH ) N ꢀ and the other is attributed to the methylene
3
2
+
group in ꢀN (CH ) ꢀ. The most obvious change comes
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
2
2
from the signals of the pyridyl group, which is also in ac-
cordance with the analysis of the FTIR results. We can see
in Figure 8b that the signal of H at d=7.49 ppm shifts to
b
d=7.63 ppm on increasing the volume ratio of CD OD. Be-
3
cause deuterated solvents were used, the chemical-shift
change that arises from the hydrogen bonding between the
methanol and pyridyl group could be excluded. Further-
1
more, comparison of the H NMR spectra of SEOP-2 in
CDCl /CD CN and CDCl /CD OD mixed solvents supports
3
3
3
3
this conclusion (Figure S15 in the Supporting Information).
On the basis of the FTIR results, if the hydrogen bonding
between the O=C amide bond and H ꢀC pyridyl bond had
b
been destroyed, the H signal should shift upfield due to its
b
change to a free state. Therefore, the continuous shifting of
this signal downfield implies a much stronger interaction
Figure 9. XPS spectra and peak fitting of the O1s signal for SEOP-2 ag-
gregates obtained from a) CH Cl and b) CH Cl /CH OH (2:1, v/v).
2 2 2 2 3
occurs at the H proton in the pyridyl groups than the hy-
b
drogen bonding between amide and pyridyl groups. Mean-
while, a slight upfield shift of the H peak means that proton
a
H changes to a relatively free state because of the change
hydrogen bonds should have formed between the H ꢀC pyr-
a
b
in the interactions after the addition of CD OD.
idyl bond and terminal O=Mo bonds of the Anderson-type
POM upon the addition of methanol, as reported in other
organically grafted POMs. Therefore, increasing solvent po-
larity drives the alkyl-chain packing into a more ordered
state, which results in the change of hydrogen bonding from
between pyridyl and amide groups to between pryidyl
groups and the POM. Evidently, a suitable adjustment of hy-
drogen-bonding mode is favorable for the tight packing of
the alkyl chains and the formation of a tetragonal structure
of SEOP-2.
3
In viewing the molecular structure and crystal data of
SEOP-1, we definitely see that the active proton of the pyr-
idyl group forms a hydrogen bond with the terminal O=Mo
[25]
bond of the POM.
To identify that such an interaction
exists in SEOP-2 aggregates prepared from the mixed sol-
vent, XPS was used to detect the change with respect to the
inorganic components. Compared with the XPS spectra of
the fiber and ribbonlike aggregates of SEOP-2, only the
binding energy of the oxygen atoms in the POM changes
while the other atoms keep invariant (Figure S16 in the Sup-
porting Information). According to previously reported re-
sults, detailed peak fitting was performed (XPSPEAK Ver-
Based on the above discussion, the detailed hierarchical
self-assembly process of SEOP-2 can be described as follows
(Figure 10): First, the surfactants cover the surface of the
pyridyl-grafted Anderson-type POM through electrostatic
interactions, thus forming SEOP complexes. Subsequently,
the SEOP complexes self-assemble into a lamellar structure
driven by van der Waals forces between alkyl chains in a
weakly polar solvent. Although there is hydrogen bonding
between the amide and pyridyl groups, the loose packing of
the alkyl chains can not propel the SEOPs into a more or-
dered structure. When dissolved in a relatively highly polar
solvent, the alkyl chains of SEOP-2 rearrange into a highly
ordered structure with a lateral packing distance of 0.45 nm.
Meanwhile, the hydrophobic-interaction-driven tight pack-
ing of the alkyl chains in SEOP-2 destroys the hydrogen
bonding between the pyridyl and amide groups and pro-
motes the formation of new hydrogen bonding between the
[
26]
sion 4; Figure 9).
present in the POM: -CONH- (532.6 eV, change ratio=2/26,
8%); terminal Mo=O bond (531.5 eV, change ratio=12/
There are four types of oxygen atom
ꢁ
2
6, ꢁ46%); bridging Mo-O-Mo unit (530.5 eV, change
ratio=6/26, ꢁ23%); bridging Mo-O-Mn unit connected to
the organic component (529.6 eV, change ratio=6/26,
ꢁ
23%). Upon the addition of methanol, the intensity of the
terminal oxygen atoms decreases and a new peak at
5
2
29.8 eV appears with a change ratio of about 14% (i.e., 4/
6, ꢁ15%). This change implies that about four terminal
oxygen atoms accepted electrons from active hydrogen
atoms, thus forming hydrogen bonds. By combining the
FTIR and H NMR spectroscopic analysis of SEOP-2 aggre-
1
gates obtained from different solvents, we believe that new
4278
www.chemeurj.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 4273 – 4282

Fabrication Polyoxometalate Aggregates
FULL PAPER
and 3158 alternatively. This phenomenon clearly reveals the
birefringence property of SEOP-2 under polarized light and
indicates that the ribbonlike aggregates have a pronounced
directional order.
Conclusion
We have successfully fabricated a well-ordered hybrid struc-
ture of POMs through hierarchical self-assembly of SEOPs
in solution. By rational design of the molecular structure
and adjustment of the solvent polarity, the SEOP with long
alkyl chains and a pyridyl functional group can self-assemble
into one-dimensional ribbonlike aggregates with a tetrago-
nal structure on the molecular level. From the molecular-
structure point of view, it is clear that when the alkyl-chain
length of the surfactants is shorter (less than 18 carbon
atoms), the van der Waals forces are not strong enough to
make SEOPs arrange into one-dimensional ribbonlike ag-
gregates, as they just align in a lamellar structure on the
nanometer scale. Meanwhile, by grafting a pyridyl group
onto the surface of the POMs, the formed SEOPs are pro-
pelled into arranging a more ordered tetragonal structure
on the nanoscale. In addition to the influence of the organi-
cally modified POM structure, solvent polarity also plays a
critical role in the aggregation. In a low-polarity solvent,
weak alkyl-chain packing is not beneficial enough to create
a more-ordered structure in the inner-lamellae. Accompa-
nied by an increase in the solvent polarity, the alkyl chains
become more ordered, thus leading to SEOPs arranging in a
tetragonal structure with the assistance of hydrogen bond-
ing. Therefore, we demonstrate that the self-assembly of
SEOPs is a hierarchical process governed by electrostatic in-
teractions between the surfactant and cluster, van der Waals
forces between the alkyl chains, and the weak CꢀH···O=Mo
Figure 10. Schematic drawing of the nanostructure of one-dimensional
ribbonlike aggregates of SEOP-2 prepared by hierarchical self-assembly
in the mixed solvent of CH Cl /CH OH.
2 2 3
HꢀC pyridyl bond and the terminal O=Mo bond of the
POM. Finally, the synergy of the van der Waals forces and
hydrogen bonding and the space fitting of component struc-
tures lead SEOP-2 to form ribbonlike aggregates with a tet-
ragonal substructure in solution.
Anisotropic properties of ribbonlike aggregates: Owing to
the arrangement of SEOP-2 in a highly ordered state on the
molecular scale, the ribbonlike aggregates exhibit birefrin-
gence under a polarized optical microscope. It is known that
when the optical axis of birefringence materials is parallel
or vertical to the incident light beam, the materials become
dark under the polarized light. As the optical axis of bire-
fringence materials deviates from the incident light beam,
[27]
the materials appear bright. In the present system, only
when the ribbon object is aligned at 458 to the direction of
the polarizer does the birefringence anisotropy become
maximized. Figure 11 shows the consecutive rotating photo
microscopic images of a ribbon under crossed polarization.
At the polarized angles of 0, 90, 180, and 2708, the ribbons
of SEOP-2 appear dark and become bright at 45, 135, 225,
hydrogen bonds between the pyridyl rings and the terminal
oxygen atoms of the POMs. Moreover, by tuning the solvent
polarity, the arrangement of the SEOP can be controlled in
the self-assembled aggregates. In addition, the ribbonlike ag-
gregates exhibit anisotropic properties due to the ordered
arrangement of SEOP-2 on the nanometer scale.
Experimental Section
Materials: Unless specified, all the chemicals were analytical-grade re-
agents and used without further purification. 4-Pyridinecarboxylic acid
and tris(hydroxymethyl)aminomethane (Tris) were purchased from Sino-
pharm Chemical Reagent Co. Ltd. and dioctadecyldimethylammonium
bromide
(DODA·Br),
dihexadecyldimethylammonium
bromide
(
DHDA·Br), and didodecyldimethylammonium bromide (DDDA·Br)
were purchased from Acros Organics. [N
pared according to a reported procedure.
were purchased from the Beijing Chemical Reagent Industry. CH
A
H
U
G
R
N
U
G
4
28]
H
9
)
4
]
4
A
T
N
R
N
U
G
8
O
26] was pre-
[
The remaining chemicals
OH
3
2 2 2
was freshly dried from Mg and CH Cl was distilled from CaH .
1
Figure 11. Polarized microscopic images of ribbonlike aggregates of
Characterization: H NMR spectra were recorded on a Bruker Avance
SEOP-2 prepared from the mixed solvent CH
2
Cl
2
/CH
3
OH (2:1, v/v).
6 3
500 MHz instrument using [D ]DMSO or CDCl as the solvent and tri-
These images were taken as the sample rotated in a plane between a
crossed polarizer.
methylsilane (TMS) as an internal reference. FTIR spectroscopic data
were obtained on a Bruker Vertex 80v spectrometer equipped with a
Chem. Eur. J. 2011, 17, 4273 – 4282
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
4279

L. Wu et al.
DTGS detector (64 scans) with the films on a CaF
2
substrate or a dried
7.0 Hz, 18H; -CH
3
); FTIR (KBr): n˜ =3330, 3032, 2958, 2924, 2852, 1678,
ꢀ
1
sample pressed into KBr pellets. ESI-MS was recorded on a Thermo Fin-
nigan LCQ LC-MS/MS system with acetonitrile as the solvent. XRD pat-
terns were carried out on a Rigaku D/MAX-2500 with CuKa radiation
with a wavelength of 1.542 ꢁ; elemental analysis (C, H, N) was per-
formed on a Flash EA1112 analyzer from Thermo Quest Italia SPA.
SEM images were performed on a JEOL JSM-6700F field-emission scan-
ning electron microscope by dropping the sample on a silicon wafer.
AFM images were obtained by using the tapping mode on a commercial
instrument (Digital Instrument, Nanoscope III, and Dimension 2000) at
room temperature in air. TEM images were obtained on Hitachi H8100
electron microscope with an accelerating voltage of 200 kV without stain-
ing, and high-resolution images were obtained with a JEOL 3010 high-so-
lution TEM operating at 300 kV. The sample used for the cross-section
TEM was cut on a LKB III ultra-microtome and was put on a carbon-
coated copper grid for the measurement. The sample for the cross-section
TEM was prepared by dropping a solution of SEOP-2 onto a glass slide.
After solvent evaporation, a white film formed, which was scaled off and
embedded in epoxy resin. The epoxy resin was placed at 608C for 24 h to
polymerize and model, cut into nanometers sections by microtome, and
placed onto clean copper grids for the experiment. XPS was carried out
on Thermo VG Scientific ESCALAB 250 spectrometer with a monochro-
matic AlKa X-ray source (1486.6 eV). The birefringent phenomena were
studied on a Zeiss Axioskop 40 polarizing microscope.
1666, 1597, 1537, 1487, 1095, 1065, 1028, 941, 916, 899, 661 cm ; elemen-
tal analysis (%) for (DODA) [MnMo {(OCH CNHCOC N} ]: C
53.32, H 8.75, N 3.24; found: C 53.37, H 8.55, N 2.78.
(DHDA) [MnMo {(OCH CNHCOC N} ] (SEOP-3): The synthe-
3
A
H
U
G
E
N
N
6
O
18
A
H
U
G
E
N
N
2
)
3
5
H
4
2
A
H
U
G
R
N
U
G
3
A
H
U
G
R
N
U
G
6
O
18
A
H
U
G
R
N
U
G
2
)
3
5
H
4
2
sis of SEOP-3 is similar to that of SEOP-2, except that DHDA is used as
the surfactant instead of DODA. SEOP-1 (0.5 g, 0.2 mmol) and DHDA
(4.1 g, 7.0 mmol) were used to prepare SEOP-3 (0.51 g, 75%). H NMR
1
(500 MHz, CDCl +CD OD, 258C, TMS): d=8.70 (brs, 4H; Py-CH),
3
3
+
3
2
2 2
7.62 (s, 4H; Py-CH), 3.22 (br, 28H; (CH ) N ACHTUNGTRNEUNG( CH ) ), 1.71 (br, 12H;
-CH -), 1.40–1.24 (brm, 162H; -(CH ) -), 0.87 ppm (t, 18H; -CH ); FTIR
2
2
n
3
(KBr): n˜ =3334, 3032, 2958, 2923, 2852, 1677–1664, 1597, 1535, 1487,
1466, 1377, 1095, 1028, 941, 916, 899, 663 cm ; elemental analysis (%)
ꢀ
1
for (DHDA)
3
A
H
U
G
R
N
U
G
6
18
A
H
U
G
R
N
U
G
2
3
5
4
2
3.44; found: C 51.09, H 8.10, N 3.32.
A
H
U
G
R
N
U
G
3
A
H
U
G
R
N
U
G
6
O
18
A
H
U
G
R
N
N
2
)
3
5
H
4
N}
2
] (SEOP-4): The synthe-
sis of SEOP-4 is similar to that of SEOP-2, except DDDA is used as the
surfactant instead of DODA. SEOP-1 (0.39 g, 0.2 mmol) and DDDA
(
1.72 g, 4.0 mmol) was used to prepared SEOP-4 (0. 28 g, 60%).
1
H NMR (500 MHz, CDCl
Py-CH), 7.54 (s, 4H; Py-CH), 3.18 (br, 28H; (CH
2H; -CH -), 1.41–1.26 (brm, 107H;-(CH -), 0.88 ppm (t, 18H; -CH
FTIR (KBr): n˜ =3334, 3033, 2954, 2924, 2854, 1678, 1664, 1599, 1537,
3 3
+CD OD, 258C, TMS): d=8.65 (brs, 4H;
+
3
)
2
N
A
H
U
G
R
N
N
(CH
2
)
2
), 1.72 (br,
);
1
2
2
)
n
3
ꢀ1
1487, 1466, 1377, 1095, 1028, 941, 916, 899, 661 cm ; elemental analysis
(%) for (DDDA) [MnMo {(OCH CNHCOC N} ]: C 46.83,
7.62, N 3.90; found: C 46.57, H 7.82, N 3.41.
[N(C H ) ]3 A[ MnMo O {(OCH ) CNH } ] (SEOP-5): The synthesis of
SEOP-5 is similar to that of SEOP-1, except that Tris was used instead of
1. [N(C H ) ]4 A[ a-Mo O ] (3.00 g, 1.4 mmol), Mn(OAc) (0.56 g,
Synthesis of
C
5
H
4
NCONHC
N; 10.00 g, 81.3 mmol) was dissolved in dry CH
150 mL) and several drops of 98 wt% H SO was added as the catalyst.
A
H
U
G
R
N
N
(CH
2
OH)
3
(1): 4-Pyridinecarboxylic acid
3
A
H
U
G
R
N
U
G
6
O
18
A
T
N
T
E
N
G
2
)
3
5
H
4
2
H
(
HOOCC
5
H
4
3
OH
(
2
4
A
H
U
G
R
N
U
G
C
U
G
R
N
U
G
18
ACHTUNGTRENNUNG
4
9
4
6
2
3
2 2
The reaction mixture was heated to reflux for 12 h. After cooling to
room temperature, the reaction solution was adjusted to approximately
A
H
U
G
R
N
U
G
4
9
4
C
U
G
R
N
U
G
8
26
ACHTUNGTRENNUNG
3
pH 7 with saturated aqueous Na
ester was extracted with CHCl
2
CO
3
. Pyridine-4-carboxylic acid methyl
2.1 mmol), and Tris (0.6 g, 4.9 mmol) were mixed together and stirred in
3
and obtained as a light-yellow solution
CH CN (100 mL) at 858C for 24 h in a N atmosphere. The solution was
3
2
and used in the next reaction without further purification. The obtained
pyridine-4-carboxylic acid methyl ester (4.00 g, 26.5 mmol) and Tris
buffer (3.21 g, 26.5 mmol) were mixed and stirred at room temperature
cooled to room temperature and the precipitate was filtered off. The
orange filtrate was exposed to diethyl ether vapor for several days. An
orange crystal was obtained after 1 day and dried under vacuum (2.90 g,
1
in dry dimethyl sulfoxide (DMSO; 50 mL) with anhydrous
K
2
CO
3
73%). H NMR (500 MHz, [D
6
]DMSO, 258C, TMS): d=61.1 (br;
-), 1.57 (m, 24H; -CH -), 1.31 (m, 24H;
-), 0.93 ppm (t, J=7.4 Hz, 36H; -CH ); FTIR (KBr): n˜ =2960, 2937,
(
3.66 g, 36.5 mmol) for 30 h in a N atmosphere. The insoluble product
2
-CH
-CH
2
O-), 3.18 (m, 24H; -CH
2
2
was filtered off, and the filtrate was evaporated under vacuum. Com-
2
3
ꢀ1
pound 1 was obtained by column chromatography on silica gel (eluent:
2873, 1479, 1381, 1041, 939, 920, 903, 827, 793, 754, 663 cm ; elemental
analysis (%) for (TBA) [MnMo {(OCH CNH ]: C 35.74, H 6.64, N
3.72; found: C 35.52, H 6.83, N 3.56.
(DODA) [MnMo O {(OCH ) CNH } ] (SEOP-6): The synthesis of
1
CH
2
Cl
2
/CH
3
OH, 10:1) in a yield of 3.70 g (60.9%). H NMR (500 MHz,
3
A
H
U
G
E
N
N
6
O
18
A
H
U
G
R
N
U
G
2
)
3
2 2
}
[
(
-
3
1
D
6
]DMSO, 258C, TMS): d=8.70 (d, J (H,H)=4.5 Hz, 2H; Py-CH), 7.70
d, J=4.5 Hz, 2H; Py-CH), 7.49 (s, 1H; -NH-), 4.66 (t, J=5.5 Hz, 3H;
OH), 3.70 ppm (t, J=6.0 Hz, 6H; -CH O-); FTIR (KBr): n˜ =3483, 3394,
186, 3074, 2981, 2879, 1651, 1606, 1556, 1543, 1502, 1469, 1462, 1421,
A
H
U
G
R
N
N
3
A
H
N
T
E
N
G
6
18
ACHTUNGTRENNUNG
2
3
2 2
2
SEOP-6 is similar to that of SEOP-2, except that SEOP-5 is used as the
organically grafted POM instead of SEOP-1. SEOP-5 (0.15 g, 0.08 mmol)
and DODA (2.51 g, 4.0 mmol) were used to prepare SEOP-6 (0.18 g,
ꢀ
1
411, 1333, 1294, 1259, 1240, 1178, 1140, 1082, 1045, 1016, 978, 941 cm
(C [MnMo {(OCH CNHCOC N} (SEOP-1): [N-
[a-Mo 26] (3.00 g, 1.4 mmol), Mn(OAc) (0.56 g, 2.1 mmol),
and 1 (1.10 g, 4.9 mmol) were mixed and stirred in CH CN (100 mL) at
58C for 24 h in a N atmosphere. The reaction solution was cooled to
.
1
[
N
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
4
H
9
)
4
]
3
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
6
O
18
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
2
)
3
5
H
4
2
]
80%). H NMR (500 MHz, CDCl
3
, 258C, TMS): d=60.8 (br; -CH
), 1.62 (m, 12H; -CH -), 1.20 (m, 174H;
-), 0.82 ppm (t, 18H; -CH ); FTIR (KBr): n˜ =3026, 2953, 2914,
850, 1471, 1381, 1034, 939, 913, 898, 661 cm ; elemental analysis (%)
[MnMo {(OCH CNH ]: C 52.18, H 9.19, N 2.49;
found: C 52.03, H 8.96, N 2.61.
2
O-),
+
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(C
4
H
9
)
4
]
4
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
8
O
A
H
U
T
E
U
G
3
3.09 (m, 30H; (CH
3
)
2
N
A
H
U
T
N
U
G
2
)
2
2
-
(CH
2
)
n
3
3
ꢀ
1
2
8
2
for (DODA)
3
A
H
U
G
R
N
U
G
6
O
18
A
H
U
G
R
N
U
G
2
)
3
2 2
}
room temperature and the precipitate was filtered off. The orange filtrate
was exposed to diethyl ether vapor for several days to give large-orange
1
crystals (2.90 g, 88%). H NMR (500 MHz, [D
6
]DMSO, 258C, TMS): d=
6
4.6 (br; -CH
2
O-), 8.89 (brs, 4H; Py-CH), 7.99 (brs, 2H; -NH-), 7.67
-), 1.60–1.54 (m, 24H; -CH
-), 0.93 ppm (t, J=7.4 Hz, 36H; -CH ); FTIR
(
)
(
1
brs, 4H; Py-CH), 3.18–3.15 (m, 24H; -CH
, 1.35–1.27 (m, 24H; -CH
KBr): n˜ =3275, 3049, 2960, 2937, 2873, 1670, 1599, 1541, 1483, 1408,
2
2
-
2
3
Acknowledgements
ꢀ
1
381, 1026, 941, 920, 903, 669 cm ; ESI-MS (negative mode, CH
3
CN): m/
] ; elemental analy-
This work was financially supported by the National Basic Research Pro-
gram of China (2007CB808003), National Natural Science Foundation of
China (20973082, 20921003, 20703019), and 111 Project (B06009) for the
visit and fruitful discussions with Prof. U. Kortz at Jacobs University and
Open Project of State Key Laboratory of Polymer Physics and Chemistry
of Chinese Academy of Science.
ꢀ
z: 1849 [(TBA)
sis (%) for (TBA)
.26, N 4.69; found: C 39.21, H 6.46, N 4.28.
(DODA) [MnMo {(OCH CNHCOC
0.39 g, 0.2 mmol) in CH CN (30 mL) was slowly added to the solution of
DODA (2.5 g, 4.0 mmol) dissolved in CH CN/CHCl (3:1 v/v; 100 mL).
2
MnMo
6
O
18
A
H
U
G
R
N
U
G
2
)
3
CNHCOC
5
H
4
N}
2
3
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[MnMo
6
O
18
A
H
U
G
R
N
N
2
)
3
CNHCOC
5 4 2
H N} ]: C 39.03, H
6
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
3
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
6
O
18
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
2
)
3
5
H
4
N}
2
]
(SEOP-2): SEOP-1
(
3
3
3
The solution immediately became cloudy, was stirred for another 2 min,
and was isolated by filtration. An orange precipitate was obtained,
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washed in turns with H
2
O and diethyl ether, and dried under vacuum
1
(
0.47 g, 84%). H NMR (500 MHz, CDCl , 258C, TMS): d=8.76 (brs,
3
+
4
H; Py-CH), 7.50 (s, 4H; Py-CH), 3.25 (br, 28H; (CH
3
)
2
N
A
H
U
G
R
N
U
G
2 2
) ), 1.71
(
br, 12H; -CH
2
2 n
-), 1.41–1.26 (brm, 192H; -(CH )
4280
www.chemeurj.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 4273 – 4282

Fabrication Polyoxometalate Aggregates
FULL PAPER
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