Thermal-induced dynamic self-assembly of adenine-grafted polyoxometalate complexes

Article abstract of DOI:10.1039/c2dt30421j

A new kind of organic-inorganic hybrid complexes based on polyoxometalate were synthesized through symmetrically grafting two adeninyl groups onto Anderson-type MnMo₆ clusters and encapsulating the clusters by organic surfactants. The resultant complexes exhibited thermal-induced dynamic self-assembly behaviors which greatly depended on the ambient temperature and the chain length of cationic surfactants. With the encapsulation of a short surfactant tetrabutyl ammonium, the complex assembled into fibrous, rod-like, and tubular architectures respectively upon heating; while for the case of using a long surfactant dimethyldioctadecyl ammonium as counter ions, the assemblies of the complex transformed from fibers to spheres with the increased temperature. Moreover, the two types of transformations were both reversible during a cooling process. The related mechanism was investigated by combining multiple characterization methods including X-ray crystallography, XPS, FT-IR and temperature-dependent ¹H NMR, which indicated that such a thermal-induced morphological transformation resulted from a synergy effect of the variation of the multiple hydrogen bonds among the complexes and the rearrangement of the surfactants surrounding the MnMo₆ clusters. These results demonstrated a new concept that hydrogen bonds can be rationally employed as the driving force for the fabrication of polyoxometalate-based materials with smart responsive properties.

Full text of DOI:10.1039/c2dt30421j

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This paper is published as part of a Dalton Transactions themed issue entitled:
Polyoxometalates
Guest Editors: De-Liang Long and Leroy Cronin
Published in issue 33, 2012 of Dalton Transactions
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Articles published in this issue include:
Polyoxometalates as efficient catalysts for transformations of cellulose into platform
chemicals
Weiping Deng, Qinghong Zhang and Ye Wang
Dalton Trans., 2012, DOI: 10.1039/C2DT30637A
Surfactant-encapsulated polyoxometalate building blocks: controlled assembly and
their catalytic properties
Amjad Nisar and Xun Wang
Dalton Trans., 2012, DOI: 10.1039/C2DT30470H
A dodecanuclear Zn cluster sandwiched by polyoxometalate ligands
Guibo Zhu, Yurii V. Geletii, Chongchao Zhao, Djamaladdin G. Musaev, Jie Song and
Craig L. Hill
Dalton Trans., 2012, DOI: 10.1039/C2DT30733B
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PAPER
Thermal-induced dynamic self-assembly of adenine-grafted polyoxometalate
complexes†
Zhenfeng He, Yi Yan, Bao Li, Hui Ai, Huanbing Wang, Haolong Li* and Lixin Wu
Received 22nd February 2012, Accepted 30th April 2012
DOI: 10.1039/c2dt30421j
A new kind of organic–inorganic hybrid complexes based on polyoxometalate were synthesized through
symmetrically grafting two adeninyl groups onto Anderson-type MnMo clusters and encapsulating the
6
clusters by organic surfactants. The resultant complexes exhibited thermal-induced dynamic self-assembly
behaviors which greatly depended on the ambient temperature and the chain length of cationic
surfactants. With the encapsulation of a short surfactant tetrabutyl ammonium, the complex assembled
into fibrous, rod-like, and tubular architectures respectively upon heating; while for the case of using a
long surfactant dimethyldioctadecyl ammonium as counter ions, the assemblies of the complex
transformed from fibers to spheres with the increased temperature. Moreover, the two types of
transformations were both reversible during a cooling process. The related mechanism was investigated by
combining multiple characterization methods including X-ray crystallography, XPS, FT-IR and
1
temperature-dependent H NMR, which indicated that such a thermal-induced morphological
transformation resulted from a synergy effect of the variation of the multiple hydrogen bonds among the
complexes and the rearrangement of the surfactants surrounding the MnMo clusters. These results
6
demonstrated a new concept that hydrogen bonds can be rationally employed as the driving force for the
fabrication of polyoxometalate-based materials with smart responsive properties.
Introduction
length scales and multiple dimensional orders have been con-
structed. In principle, the organic groups bearing interactive
7
The fabrication of organic–inorganic hybrid structures and
materials has attracted enormous attention in recent years due to
its potential applications in optical devices, molecular elec-
sites and organizable properties are usually modified onto the
inorganic functional moieties, such as nanoparticles and
nanoclusters, to direct the assembly of these inorganic
1
–3
8,10
tronics, energy materials, and biomimetic materials.
assembly, as a “bottom-up” approach, is widely used to construct
Self-
components.
Polyoxometalates (POMs) are intriguing inorganic clusters
due to their molecularly well-defined structures, and versatile
properties in chemical, physical and medical fields, which make
POMs potential modular building blocks for the construction of
4,5
hybrid materials. In general, non-covalent interactions act as
the predominant driving forces in a self-assembly process, such
as electrostatic adsorption, hydrogen bond, π–π stacking, and
van der Waals force, which can facilitate the assembled struc-
tures away from thermodynamic equilibrium and adapt to
11–14
hybrid materials.
Currently, two representative modification
strategies have been developed to integrate POMs into organic
matrices. The first one is covalent functionalization, which pro-
6
–9
ambient environments. By rationally designing the molecular
structure of building blocks and modulating the weak inter-
actions among them, various hybrid materials with multiple
vides
components.
a robust connection between POMs and organic
15–17
According to this strategy, a series of organic
functional groups, such as conjugated ligands, dendrimers, rec-
ognition groups, alkyl chains, and polymer chains, have been
State Key Laboratory of Supramolecular Structure and Materials,
College of Chemistry, Jilin University, Changchun 130012, China.
E-mail: hl_li@jlu.edu.cn; Fax: +86-(0)431-85193421;
Tel: +86-(0)431-85168499
18
grafted onto POM frameworks, leading to porous materials,
1
9–21
22,23
hybrid gels,
assemblies,
chiral materials,
micellar and vesicular
2
4–26
and so on. The second strategy is electrostatic
†
Electronic supplementary information (ESI) available: Preparation of
encapsulation, in which the inorganic counter cations on POM
ethyl (9-adeninyl) acetate and N-tris (hydroxymethyl) methy (9-adeni-
1
surfaces are replaced by organic cations, forming organic–inor-
nyl) acetamide; H NMR, FT-IR, and TGA data of SEOPS; ESI-MS data
2
7,28
ganic hybrid complexes.
Different kinds of cationic organic
of SEOP-1; FT-IR data of SEOP-1 at different temperature; Tyndall
effect of the SEOPs solutions; Single crystal structure of SEOP-1; TEM
images of spherical and fibrous SEOP-2 assemblies; SEM images of
SEOP-2 assemblies upon cooling. CCDC reference number 875849. For
ESI and crystallographic data in CIF or other electronic format see DOI:
29–34
35,36
molecules
including
and polymers
surfactants,
peptides,
37–39
40,41
dendrimers,
have been employed for this
purpose. Until now, although a fair number of POM-hybrid
materials have been fabricated by these strategies, few of them
1
0.1039/c2dt30421j
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Dalton Trans., 2012, 41, 10043–10051 | 10043

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exhibited dynamic transition under external inducements, such
4
2–44
as pH, optical and redox stimulus.
Therefore, it is necessary
to carry out an in-depth investigation of POM-hybrid responsive
materials.
Recently, we realized a reversible adjustment of POM-hybrid
architecture by using an Anderson-type cluster MnMo which
6
was systemically grafted by azobenzene (Azo) groups and elec-
4
3
trostatically encapsulated by long chain surfactants. We found
that the trans to cis photoisomerization of Azo groups can
induce a fibrous to spherical transition of the Azo-grafted
MnMo assemblies. This finding demonstrated that the combi-
6
nation of covalent functionalization and electrostatic encapsula-
tion is an advisable principle to construct POM-hybrid materials
with a dynamically controllable structure. In this situation, the
response of grafted groups can be sensitively conducted to POM
clusters via the robust covalent bonds; and meanwhile, the cat-
ionic molecules easily rearrange on POM surfaces due to the
3
1
electrostatic adsorption, which can provide a flexible micro-
environment for the structural transition of POM-hybrid assem-
blies. In the present work, we intend to employ this principle to
exploit a new kind of POM-hybrid materials with thermal-
responsive properties. Considering that the strength of hydrogen
Fig. 1 Synthetic route of SEOP-1.
characteristic vibration bands of Anderson-type clusters in which
the terminal MovO mode appeared at around 941, 914, and
4
5
bonds highly depends on the temperature, we synthesized a
POM-based hydrogen-bonding synthon through systematically
−
1
822 cm , and the bridging Mo–O–Mo mode appeared at about
673 cm− , confirming the existence of MnMo
1
moiety in
6
grafting two adeninyl groups onto a MnMo cluster via a “tris”
6
1
linker. Further, we systemically studied the thermal-induced self-
assembly behavior of the as-prepared hybrid complexes which
were encapsulated by two different surfactants: tetrabutyl
ammonium (TBA) and dimethyldioctadecyl ammonium
SEOP-1 (Fig. S1†). Comparing the H NMR spectra of A-Tris
with SEOP-1, two distinct changes were observed: the proton
signal of the three hydroxyl groups of A-Tris at 4.61 ppm disap-
peared in SEOP-1; meanwhile, the signal of the three hydroxy-
methyl groups of A-Tris shifted from 3.56 to 64.22 ppm. These
changes indicated a successful esterification reaction in which
the three hydroxymethyl groups of A-Tris were covalently
(DODA). Two types of hierarchical self-assembled architectures
were obtained. For the case of using TBA as counter cations, the
complex formed fibrous, rod-like, and tubular assemblies
respectively when the ambient temperature was increased;
however, for the case of using DODA, the assemblies of MnMo6
complexes transform from fibrous to spherical morphology upon
heating. Moreover, the two types of morphological transform-
ations are both reversible in a cooling process. Detailed investi-
gations showed that this thermal-induced dynamic self-assembly
behaviour resulted from a synergy of the variation of the multiple
grafted to MnMo
protons to the terminal methyl protons of TBAs demonstrated
that each adenine-grafted MnMo cluster was averagely
clusters. The 1 : 3 integral ratio of the adeninyl
6
6
accompanied by three TBA counterions (Fig. S2†). The results
of elemental analysis and TGA also implied the 1 : 3 ratio of
TBA to MnMo cluster in SEOP-1. Moreover, the ESI-MS spec-
6
trum of SEOP-1 exhibited a dominant peak at m/z of 1993,
which corresponded to an ionized SEOP-1 with two TBA
counter ions and confirmed the expected molecular composition
of SEOP-1 (Fig. S3†).
To evaluate the influence of surfactant size on the self-assem-
bly behaviour of MnMo complexes, we employed DODA to
6
intermolecular hydrogen bonds among the MnMo complexes
6
and the redistribution of the surfactants surrounding the com-
plexes. This work provides a new paradigm for the family of
POM-hybrid responsive materials, which should also be instruc-
tive for the further fabrication of POM-integrated smart devices
46
and nanosystems.
replace the TBA counter ions on SEOP-1 through an ion
exchange reaction, yielding SEOP-2 (detailed procedures are
shown in the Experimental section). The characteristic vibration
Results and discussion
bands of MnMo cluster appearing in the FT-IR spectrum of
6
−1
SEOP-2 at 941, 912, 897, and 669 cm (Fig. S4†), indicated
Synthesis and assemblies-preparation of SEOPs
the well-retained Anderson-type structure of MnMo cluster after
6
The adenine-grafted MnMo6 complexes were synthesized by
adopting the widely used tris(hydroxymethyl)aminomethane
the ion exchange process. The proton signals of TBA disap-
peared in the H NMR spectrum of SEOP-2 and replaced by the
1
(
Tris) functionalization method. As shown in Fig. 1, the adeninyl
signals of DODA, which implied a complete ion exchange
(Fig. S5†). Furthermore, the results of elemental analysis and
TGA also confirmed the existence of three DODA on SEOP-2
(see Experimental section).
The encapsulation by different counter ions endows SEOP-1
and SEOP-2 distinct solubilities. According to this, acetonitrile
and dimethylformamide (DMF) were chosen as the solvents for
groups were firstly connected to the Tris ligand via amide bond,
and then the resultant adenine-Tris derivatives (A-Tris) were sys-
temically grafted onto the Anderson-type MnMo6 clusters
through an esterification reaction. The TBA salts of adenine-
grafted MnMo complex was directly synthesized and named
SEOP-1. The FT-IR spectrum of SEOP-1 showed the
6
10044 | Dalton Trans., 2012, 41, 10043–10051
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SEOP-1 and SEOP-2, respectively. Both SEOPs can be comple-
tely dissolved in 5 min. Although the formed solutions were
clear, they all exhibited obvious Tyndall phenomena, indicating
the formation of large SEOPs assemblies in the solutions
that the tubes were formed through the adhesion of the rods.
More importantly, when we cooled down a hot solution of
SEOP-1 to the initial temperature 5 °C, the long fibers reformed
again, which performed a reversible self-assembly process.
Moreover, upon calcining at 650 °C in air for 1 h, the organic
part of the fiber was decomposed and the fibrous structures trans-
formed into linearly arranged nanoparticles of the oxide of Mn
and Mo, which implied the potential application of the hybrid
assemblies as a template for the fabrication of metal–oxide
nanostructures (Fig. S9†).
(Fig. S6†). The assemblies were transferred onto substrates for
further measurements. It should be noted that the sample transfer
process was performed at the same condition to the solution
preparation, such as the same temperature and the solvent atmos-
phere, so that the initial self-assembled morphologies of SEOPs
can be kept.
The SEOP-2 assemblies formed at room temperature are
similar to the initial fibrous SEOP-1 assemblies prepared at low
temperature, which were several micrometers in length and about
Morphology of SEOPs assemblies
2
00 nm in width, as shown in Fig. 3a and 3b. The magnified
TEM image indicated that the fiber possessed a lamellar structure
Fig. S10†). However, SEOP-2 displayed a distinct thermal-
Scanning electron microscopy (SEM) was employed to study the
morphology of SEOP-1assemblies. The sample prepared at a
low temperature of 5 °C exhibited a mass of cross-linked fibrous
architectures in a large scale of several micrometers in length, as
show in Fig. 2a. Energy-dispersive X-ray spectroscopic (EDX)
analysis showed that both carbon and molybdenum elements
exist in the fibers, revealing that the fibers are indeed formed by
SEOP-1 (Fig. S7†). In a magnified SEM image (Fig. 2b), the
diameter of the fibers was found to be about 100–200 nm. When
the temperature was increased to 15 °C, the fibers became much
shorter and the morphology seemed more rigid, forming a
unique flower-like structure composed of rods (Fig. 2c, 2d). The
diameter of an individual rod was about 200–300 nm. Interest-
ingly, upon further increasing the temperature to 45 °C, the rods
transformed into tubes which are seldom reported for organic–
inorganic hybrid architectures (Fig. 2e, 2f). The thickness of the
tube wall matched the diameter of the rods (Fig. S8†), implying
(
induced structural transition from SEOP-1. Upon increasing the
ambient temperature to 35 °C, the fibers tended to integrate and
aggregate together, while their linear morphology was still pre-
served (Fig. 3c). Continually heating the sample solutions to
4
5 °C induced the fibers to aggregate more seriously, leading to
the formation of willow-leaf-like assemblies (Fig. 3d). More
interestingly, when further increasing the temperature, the leaf-
like assembles underwent an unordered transition state at 55 °C
(Fig. 3e) and finally transformed into spherical assemblies with a
diameter of about 500 nm (Fig. 3f and S11†). At that time, the
initial linear structure of SEOP-2 assemblies was completely
destructed. Moreover, such a fiber-to-sphere transition is also
reversible upon cooling.
Fig. 2 SEM images of the hybrid assemblies formed from the aceto-
nitrile solution of SEOP-1 at different temperatures: (a) and (b) 5, (c)
and (d) 15, and (e) and (f) 45 °C, where the white arrow in (f) points at
the mouth of the tubes.
Fig. 3 SEM images of the hybrid assemblies formed from the DMF
solution of SEOP-2 at different temperatures: (a) and (b) 25, (c) 35,
(d) 45, (e) 55, and (f) 65 °C.
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Internal structure of SEOP-1 assemblies
The molecular stacking states and the intermolecular interactions
of the SEOPs are important for their self-assembly behaviors.
Fortunately, a single crystal of SEOP-1 was incubated at room
temperature and its structure can reflect the stacking information
of SEOP-1 rods because the rods were prepared at the same
temperature of 25 °C. As shown in Fig. S12†, the adjacent
SEOP-1 are linked by N–H⋯N double hydrogen bonds between
the adeninyl groups grafted on the two sides of MnMo clusters,
6
which leads to the formation of a one-dimensional supramolecu-
lar polymer chain based on SEOP-1. Moreover, the chains are
further cross-linked by the multiple lateral N–H⋯O hydrogen
bonds between the MovO and the amide groups of the MnMo6
clusters belonging to different SEOP-1 chains. Herein, we pro-
posed that the axial hydrogen bonds along the SEOP-1 chains
dominate the formation of the linear rod structure, and the lateral
hydrogen bonds across the SEOP-1 chains influence the aggrega-
tion extent of the chains, thus determining the stability and the
rigidity of the SEOP-1 rods. As is well known, hydrogen bonds
are very sensitive to temperature change. Therefore, we presume
that the multiple hydrogen bonds among SEOP-1 chains are the
driving force for the structural transformation of SEOP-1 assem-
blies. To get the in situ variation information of the hydrogen
bonds in SEOP-1 assemblies, we employed temperature-depen-
1
dent H NMR spectroscopy to investigate the SEOP-1 solutions.
As shown in Fig. 4A and Table 1, when the temperature was
increased from 5 to 65 °C (a higher temperature is not suitable
because the boiling point of acetonitrile is 82 °C), the character-
istic signal of the amide groups on adenine-cycle H gradually
c
shifted from 5.91 to 5.73 ppm. As a hydrogen bond donor, the
upfield shifting signal of H indicated the strength of the hydro-
c
gen bonds between H and the nitrogen atom at the position of
1
c
Fig. 4 H NMR spectra of (A) the adenine part and (B) the TBA part
the adenine-cycle became weak at high temperature. This change
lowered the axial intermolecular interaction along the SEOP-1
chains, and thus the initial long fibers could not be kept but
formed short rods. However, we think the hydrogen bonds were
−1
3
of SEOP-1 in CD CN with the concentration of 1 mg mL during a
heating process (curve a to d) and a cooling process (curve d to g).
1
not broken completely because the upfield shift of H signal did
Table 1 H chemical shifts of amino groups on SEOP-1 at different
c
not display a drastic change even when the temperature reached
temperature
6
5 °C, which should be the reason that linear structure was main-
a
T/°C
δ/ppm
Δ/ppm
tained all through from the fibrous, rod-like, to tubular assem-
blies of SEOP-1.
5
15
5.90
5.87
5.84
5.81
5.78
5.75
—
From the SEM images of SEOP-1 assemblies (Fig. 2), it can
be observed that when the initial fibers transformed to rods, their
diameters increased from 100–200 nm to 200–300 nm, which
implied that the lateral aggregation of SEOP-1 chains in the rod-
like assemblies became stronger than that in the fibrous assem-
blies. According to the single crystal structure, the lateral N–
H⋯O hydrogen bonds existed between the polymer chains
under the support of MovO and amide, so we presumed that the
lateral hydrogen bonds drive the aggregation of SEOP-1 chains
and caused the fibers to became thick. The temperature-depen-
0.03
0.03
0.03
0.03
0.03
2
3
4
5
5
5
55
a
The Δ value corresponds to the difference of two adjacent δ values.
which facilitates the formation of the lateral N–H⋯O hydrogen
bonds between SEOP-1 chains due to the weakened steric hin-
drance of TBA. Therefore, the lateral aggregation of SEOP-1
chains should be more intense in the rod-like assemblies, that is
why the diameter of the rods is larger than that of the fibers.
The redistribution of TBA was further confirmed by XPS
measurement. The O 1s binding energy of the oxygen atoms of
MnMo clusters is sensitive to the microenvironment of MnMo .
1
dent H NMR spectra of the TBA part of SEOP-1 supported our
viewpoint, as shown in Fig. 4B. When the temperature was
increased, the proton signals of the –N–CH3 and –N–CH2–
groups on TBA shifted to downfield obviously, indicative of a
weaker electrostatic interaction between the TBA and the
MnMo clusters at high temperature. In this situation, the TBA
6
6
6
cations should distribute farther way from the MnMo clusters,
In general, a low binding energy reflects a relatively low ambient
6
10046 | Dalton Trans., 2012, 41, 10043–10051
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electronegativity; while the binding energy changing to a high
value can be attributed to the increased electronegativity of the
microenvironment. In the XPS spectra of SEOP-1 assemblies,
two types of binding energy were observed, which appeared at
about 530 and 532 eV, respectively (Fig. 5). We assigned the
evaporated faster than the internal solvent, which created a capil-
lary effect and drove the rods to stack. Meanwhile, the lateral
N–H⋯O bonds between SEOP-1 chains can favour the “inte-
gration” of the rod edges to form a hollow tubular structure.
5
30 eV to a state of MnMo clusters with a high encapsulating
6
Internal structure of SEOP-2 assemblies
density of TBA, because the cationic TBA can reduce the
ambient electronegativity around MnMo clusters; correspond-
ingly, the 532 eV should indicate a low TBA density. Therefore,
the intensity ratio of the two binding energies (530/532) can
6
Through the analysis of the self-assembly mechanism of
SEOP-1, we have confirmed that the hydrogen bonds are the
driving force for the morphological transformation. Therefore,
we further studied the self-assembly of SEOP-2 by analyzing the
change of hydrogen bonds in this system. From the temperature-
dependent NMR spectra of SEOP-2, we found that the character-
istic peaks of the amino groups on adenine-cycle shifted to
upfield from 7.20 to 6.88 ppm upon heating (Fig. 6 and Table 2).
This phenomenon indicated that axial hydrogen bonds exist
between the adjacent adeninyl groups of SEOP-2 at low tempera-
ture, similar to the case of SEOP-1, which should cause the for-
mation of the linear fibrous assemblies of SEOP-2 at a relatively
low temperature 25 °C. However, different from SEOP-1, the
amino signal of SEOP-2 exhibited a drastic upfield shift in the
range from 35 to 45 °C (Table 2). This change can be attributed
to a drastic weakening of the axial hydrogen bonds. IN this situ-
ation, the one-dimensional structure of SEOP-2 chains cannot be
maintained, which led to the destruction of the linear fibers and
the formation of new SEOP-2 assemblies with curvature, such as
willow-leaf-like and spherical structures (Fig. 3). The different
self-assembly behavior of SEOP-2 compared with SEOP-1
should result from the different counter cations encapsulating on
clearly reflect the redistribution of TBA on MnMo cluster. We
6
found that the ratios were 1.01 and 0.41 respectively for the
fibrous and rod-like assemblies, implying that the TBA density
on MnMo clusters was decreased when the assemblies trans-
6
formed from fibers to rods. However, the similar ratios for the
rods and the tubes meant that the TBA encapsulating state did
not change during the transition between them. The distribution
of TBA to far away from the MoMo clusters can facilitate the
6
formation of the lateral N–H⋯O hydrogen bonds between
SEOP-1 chains due to the weakened steric hindrance of TBA.
Therefore, the lateral aggregation of SEOP-1 chains should be
more intense in the rod-like assemblies, that is why the rods are
thicker than the fibers. In addition, the characteristic vibration
bands of the amide groups in fibrous, rod-like, and tubular
SEOP-1 assemblies appeared at 3307, 3294, and 3291 cm−
respectively in FT-IR spectra (Fig. S13†). Such a shift to low
wavenumber region also reflected the enhanced hydrogen bonds
1
47
of amide. Therefore, the NMR, XPS, and FT-IR results are
well consistent with each other, demonstrating the same internal
transition process in SEOP-1 system upon the increased ambient
temperature: the axial hydrogen bonds along the SEOP-1 chains
became weak, but the lateral hydrogen bonds between the
SEOP-1 chains were enhanced.
In the SEM images of Fig. 2, the thickness of the tube wall is
consistent with the diameter of the rods. Moreover, the similar
XPS and FT-IR results of the rod-like and the tubular assemblies
of SEOP-1 indicated that the two assemblies possessed a similar
internal hydrogen bonding state. Therefore, we believe that the
tubes were directly stacked and fused by the rods, without an
internal structure change. At high temperature, the solvent
outside of the bunched rods in the flower-like structure
1
Fig. 6 H NMR spectra of the adenine part of SEOP-2 in DMSO-d
6
−1
with the concentration of 1 mg mL during a heating process (curve a
to e) and a cooling process (curve e to f). The assignment of proton
a d
signals (H to H ) refers to Fig. 4A.
1
Table 2 H chemical shifts of amino groups on SEOP-2 at different
temperature
a
T/°C
δ/ppm
Δ/ppm
25
35
45
55
65
7.20
7.13
7.00
6.94
6.88
—
0.07
0.13
0.06
0.06
a
Fig. 5 O 1s XPS spectra of the SEOP-1 assembly morphologies: (a)
fiber, (b) rod, (c) tube.
The Δ value corresponds to the difference of two adjacent δ values.
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effect of the variable multiple hydrogen bonds and the rearrange-
able distribution of surfactants in the two SEOPs systems. Par-
ticularly for SEOP-2, when the strength of the axial hydrogen
bonds along the SEOP-1 chains was weakened to a critical state,
the van der Waals interaction among DODA would replace the
dominating role of the hydrogen bonds. At that time, SEOP-2
would follow the aggregation style of DODA, forming spherical
assemblies. Based on the above analysis, we describe a whole
self-assembly mechanism for both SEOP-1 and SEOP-2, as
shown in Scheme 1.
Conclusion
In conclusion, we have successfully synthesized two novel
POM-hybrid complexes, through covalently grafting the adeni-
Fig. 7 XRD patterns of SEOP-2 assemblies which were prepared
during a heating process (curves a to e) and a cooling process (curves
e to f).
nyl groups onto Anderson-type MnMo clusters and encapsulat-
6
ing the clusters by TBA and DODA respectively. Axial
hydrogen bonds were found to exist between the adeninyl
groups of adjacent SEOPs, leading to the formation of SEOP-
based supramolecular polymer chains. Besides, lateral hydrogen
bonds were also found to cross-link the SEOP chains. The mul-
tiple hydrogen bonds among SEOPs drove them to form linear
fibrous assemblies at low temperature. Upon heating, the axial
hydrogen bonds were weakened and the fibrous assemblies
became less sable, which induced a morphological transition of
the assemblies. Interestingly, the relatively weak van der Waals
interaction among TBA did not affect the linear structure of
SEOP-1 assemblies; however, the strong van der Waals inter-
action among DODA were able to replace the dominant role of
the axial hydrogen bonds, and thus the linear fibrous structure of
SEOP-2 assemblies were destroyed at high temperature, forming
a DODA-dominated spherical structure. Therefore, we believed
that the thermal-induced dynamic self-assembly behavior of the
SEOPs is attributed to a synergic effect of the varied multiple
hydrogen bonds and the rearranged distribution of surfactants in
the two SEOPs systems. The present work demonstrated that the
incorporation of the active groups with hydrogen bonding and
electrostatic interacting abilities is an efficient strategy to endow
POM-hybrid materials dynamic features under thermal or other
kinds of stimulus. Our further work will focus on the exploita-
tion of more supramolecular interaction styles as the driving
force for constructing dynamic POM hybrids, and the application
research of the obtained POM hybrids in template synthesis of
metal-oxide nanostructures and as nanoreactors for POM cata-
lysed reactions.
the two complexes. In contrast to TBA, the alkyl chains of
DODA are much longer and thus can create much stronger van
der Waals interactions. It has been widely reported that DODA
tended to form vesicles in solution. Therefore, we presume that
when the axial hydrogen bonds along SEOP-2 chains were
weakened enough, the van der Waals interactions among DODA
would dominate the self-assembly of SEOP-2, and consequently
induced the curvature creation in SEOP-2 assemblies.
As aforementioned, the TEM image of the fibrous SEOP-2
assemblies exhibited lamellar structures (Fig. S10†), which was
also confirmed by the XRD patterns of SEOP-2 assemblies
(Fig. 7). The lamellar structure should arise from the stacking of
alkyl chains of DODA. Although the SEOP-2 assemblies trans-
formed from fiber to sphere during the heating process, their
lamellar structure were maintained, except for the interlayer
spacing showing a mutation from 2.71 to 2.87 nm when the
temperature exceeded 45 °C. This transition point is consistent
with the structural transition point of SEOP-2 assemblies from a
linear to a spherical structure, and it is also close to the drastic
weakening point of axial hydrogen bonds in the temperature-
dependent NMR spectra. These results revealed a fact: the varied
axial hydrogen bonds affected the lamellar stacking of SEOP-2,
and finally influenced the morphology of SEOP-2 assemblies.
Furthermore, the interlayer spacing could recover to 2.75 nm
when the temperature was decreased to 25 °C again. The revers-
ible transition of the internal structure should lead to the revers-
ible transformation of bulk morphologies of SEOP-2 assemblies
(Fig. S14†).
Experimental section
Materials
Self-assembly mechanism of SEOPs
Unless otherwise noted, the chemicals were all obtained from
commercial supplies and used without further purification.
Adenine was purchased from Taizhou Xingming pharmacy Co,
Ltd. Ethylbromoacetate and tris(hydroxymethyl)aminomethane
(Tris) were purchased from Sinopharm Chemical Reagent Co,
Ltd. [N(C H ) ] [α-Mo O ] was synthesized according to the
Comparing the self-assembly behaviours of SEOP-1 and
SEOP-2, sameness and difference both exist. The same place is
that their assemblies both exhibited a reversible thermal-induced
morphological transformation; the different phenomenon is that
the linear structure of SEOP-1 assemblies was maintained but
that of SEOP-2 assemblies was destroyed and changed to a
spherical structure. By summarizing these results, we present a
general mechanism: the transition is dominated by a synergy
4
9 4 4
8 26
4
8
literature. Acetonitrile were distilled over P O before use, and
2
5
N,N′-dimethylformamide (DMF) and dimethyl sulfoxide
(DMSO) was both dried with CaH2 before use. And the
10048 | Dalton Trans., 2012, 41, 10043–10051
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Scheme 1 Schematic illustration of thermal-induced dynamic self-assembly process of SEOP-1 and SEOP-2.
synthesis of N-tris(hydroxymethyl)methy(9-adeninyl) acetamide
can be found in the ESI.†
all the inorganic residuals are MoO and Mn O at 800 C (the
3 2 3
MoO3 may sublime after 800 °C), the measured residue of
8.88% in total from TGA (Fig. S15†) is in agreement with the
3
calculated value of 42.22% from the given formula of SEOP-1.
IR (KBr): ν = 3290 (s), 3246 (s), 3086 (s), 2960 (s), 2937 (sh),
Synthesis of SEOP-1
2
873 (s), 1697 (s), 1643 (s), 1596 (s), 1573 (sh), 1512 (s), 1483
18,49
SEOP-1 was synthesized according to the literature.
mixture of N-tris(hydroxymethyl)methy (9-adeninyl) acetamide
A
(s), 1469 (sh), 1421 (s), 1380 (s), 1359 (s), 1323 (s), 1307 (s),
1272 (sh), 1244 (s), 1155 (s), 1109 (s), 1037 (s), 941 (s), 914 (s),
822 (s), 800 (s), 673 cm− (s).
1
(0.24 g, 0.813 mmol), MnOAc ·2H O (0.09 g, 0.35 mmol), and
3 2
[
N(C H ) ] [α-Mo O ] (0.50 g, 0.232 mmol) were stirred at
4 9 4 4 8 26
8
5 °C in the mixture solvent of DMF and CH CN for 24 h in a
3
N atmosphere. The reaction solution was cooled to room temp-
erature and the precipitate was removed by filtration. The orange
filtrate was exposed to diethyl ether vapour after several days
2
Synthesis of SEOP-2
A mixture of SEOP-1 (50 mg, 0.0223 mmol), and DODA·Br
(42.2 mg, 0.0669 mmol) were sonicated in the mixture solvent
(8 mL) of chloroform and deionized water (1 : 1 v/v), and then
the orange precipitates were separated by centrifugation and
dried under vacuum overnight. Yield: 50% (0.035 g). H NMR
(500 MHz; DMSO-d , TMS, ppm): 0.85 (18H, triplet), 1.23
(180H, m), 1.61 (12H, m), 2.96 (18H, singlet), 5.18 (4H,
singlet), 7.14 (4H, singlet), 8.05 (2H, singlet), 8.13 (2H, singlet),
64.28 (12H, broad). Elemental analysis calcd (%) for
and orange needle like crystals were obtained. Yield: 19%
1
(
0.26 g, based on Mo). H NMR (500 MHz; DMSO-d , TMS,
6
ppm): 0.93 (36H, triplet), 1.32 (24H, m), 1.57 (24H, m), 3.16
24H, triplet), 5.18 (4H, singlet), 7.14 (4H, singlet), 8.05 (2H,
singlet), 8.14 (2H, singlet), 64.64 (12H, broad). Anal. Cald for
1
(
6
−1
C H N O MnMo (2232.5 g mol ): C 37.66, H 6.05, N
7
0
134 15 26
6
9
.41; found C 37.55, H 6.07, N 9.35; ESI-MS (negative mode,
−1
−
DMF) 1990.0 g mol ([M − 2TBA] , see Fig. S3†); by assum-
ing that the organic component has decomposed completely and
−1
C136H266N O MnMo (3158.2 g mol ): C 51.72, H 8.49, N
15
26
6
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6
.65; found C 51.35, H 8.26. N 6.70. By assuming that the
organic component has decomposed completely and all the in-
organic residuals are MoO and Mn O at 800 °C, the measured
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residue of 28.28 wt% in total from TGA (Fig. S16†) is in agree-
ment with the calculated value of 29.84 wt% from the given
6
B. A. Grzybowski, C. E. Wilmer, J. Kim, K. P. Browne and
K. J. M. Bishop, Soft Matter, 2009, 5, 1110–1128.
−1
SEOP-1 formula. FT-IR (KBr, cm ): ν = 3323 (s), 3347 (s),
085 (s), 3032 (s), 2920 (s), 2850 (s), 1697 (s), 1631 (s), 1568
s), 1468 (sh), 1417 (s), 1377 (s), 1355 (s), 1323 (s), 1309 (s),
298 (sh), 1271 (s), 1244 (s), 1149 (s), 1110 (s), 1036 (s), 941
7
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(
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−1
(s), 912 (s), 822 (s), 802 (s), and 669 cm (s).
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Sample preparation for measurements
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In order to characterize the true aggregation states of the SEOPs
in solutions at different temperatures, all the samples for SEM,
XRD, and XPS measurements were prepared through direct
casting the sample solutions onto silicon wafers at the same
temperature with the same solvent atmosphere to the samples.
For the TEM measurements, the sample solutions were dropped
onto a carbon-coated copper grid, and then the grid was dried
quickly through adsorbing the excess solution by a filter paper.
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FT-IR spectra were carried out on a Bruker Vertex 80v FT-IR
spectrometer equipped with a DTGS detector (32 scans) with a
resolution of cm− . H NMR spectra were recorded on a Bruker
AVANCE 500 MHz spectrometer. X-ray photoelectron spec-
troscopy (XPS) were carried out on an ESCALAB 250 spec-
trometer with a monochromic X-ray source (Al Kα line, 1486.6
eV) and the charging shift was corrected by the binding energy
of C 1s at 284.6 eV. X-ray diffraction (XRD) data were recorded
on a Rigaku X-ray diffractometer using Cu Kα radiation at a
wavelength of 1.542 Å. Organic elemental analysis (C H N) was
performed on a Flash EA1112 from ThermoQuest Italia S.P.A.
Thermal gravimetric analysis (TGA) measurements were per-
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3
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005, 17, 2688–2692.
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This work was financially supported by the National Basic
Research Program (2007CB808003), National Natural Science
Foundation of China (20973082, 20921003), and Open Project
of State Key Laboratory of Polymer Physics and Chemistry,
CAS.
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Dalton Trans., 2012, 41, 10043–10051 | 10051