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Article
Solvent Dielectricity-Modulated Helical Assembly and Morphologic
Transformation of Achiral Surfactant-Inorganic Cluster Ionic Complexes
Jing Zhang, Xiaofei Chen, Wen Li, Bao Li, and Lixin Wu
Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b01259 • Publication Date (Web): 19 Oct 2017
Downloaded from http://pubs.acs.org on October 26, 2017
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Solvent Dielectricity-Modulated Helical Assembly
and Morphologic Transformation of Achiral
Surfactant-Inorganic Cluster Ionic Complexes
Jing Zhang,‡,§ Xiaofei Chen,‡ Wen Li,† Bao Li,*,† Lixin Wu‡
†
State Key Laboratory of Supramolecular Structure and Materials, Institute of Theoretical
Chemistry, Jilin University, Changchun 130012, China
‡
State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin
University, Changchun 130012, China
§
Institute of Applied Chemistry, Shanxi University, Taiyuan 030006, China
ABSTRACT Ionic complexes comprising of single/double chain cationic surfactant and
Lindqvist-type polyoxomolybdate anionic cluster were used for controlled self-assembly in
organic solutions. In the solvent with low dielectric constant the complexes self-assembled into
flat ribbon like lamellar aggregations with an inverse bilayer substructure where the cluster
located at the middle. Under the condition of increased dielectric constant, the solvent triggered
the formation of helical self-assemblies, which finally transformed from helical ribbons to the
flower-like assemblies due to the bilayer becoming excessive twisting. The self-assembled
morphology and the substructure were characterized by SEM, TEM and XRD. The solvent
dielectricity-controlled morphologic transformations modulated by the variation of electrostatic
interactions between organic cations and inorganic polyanions were demonstrated by 1H NMR
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and IR spectra. The strategy in this work represents an effective route in targeting the chiralitydirected functionalization of inorganic clusters by combining controllable and helical assemblies
of achiral polyoxometalate complexes in one system.
INTRODUCTION
Constructing controllable supramolecular self-assemblies is of growing interest for
both understanding the nature of molecular self-organization and developing nanoscaled
functional materials as well as devices based on single or multiple molecular
components.1,2 The assembling process manipulated by noncovalent interactions offers
more opportunities relevant to the properties on reversibility and dynamic self-adaptation
under the external stimulus.3−5 The insertion of asymmetric element into the molecular
assemblies leads to rich structural and functional information that could be regulated in a
suitable way.6−13 The integrated characteristics allow potentials in optical switches,14,15
template-assisted self-assembly,16,17 chiral liquid crystals18 and asymmetric catalysts,19
and so forth. Among those known multiple molecular component systems, the
combination of organic and inorganic components with precise structure feature and
definite interaction relationship represents a typical approach and has been employed for
synergistic physicochemical properties.20 Polyoxometalates (POMs), as a type of
structurally well-defined metal oxide clusters with versatile uniform architectures,21−23
have been demonstrated to be applicable candidates for functional soft materials.
The smart structure transformation of POM complexes in assemblies through tuning
external environments have been reported recently,24−26 in which the size and charge
density of POMs, the structure of organic moieties, and the nature of solvents were found
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to play a key role.27−30 Wang and his coworkers obtained rich nano-architectures of POM
complexes through simply changing solvents.31 Liu’s group reported that the blackberry
size of the wheel-shaped POM can be modulated via the change of solvent polarity.32 Bu
and his co-workers found that the monodispersed star-shaped POM supramolecular
polymers organized into vesicles upon changing solvents.33 Zhang and Wan et al. also
used the solvent environment to control the luminescence of europium substituted POM
in polymer matrixes.34 Most recently, Wang at Nankai developed a series of amphiphilic
POM-POSS bola hammers for bilayer lamellar and vesicular structures via adjusting the
solvent.35 In addition, some other interesting results dealing with POM complexes
comprising of ionic liquid and amino acid have been reported for the modulation of
luminescence and gel-sol transformation.36−38
Among those POM complexes, especially those without chiral centers, however,
asymmetric self-assemblies are still quite limited39,40 although there are series of typical
examples on the asymmetric assemblies of organic molecules without chiral center.41 The
rigid framework, spherical shape and hard covalent grafting are unfavorable for the chiral
functionalization of achiral POMs in solutions, even though the POM-based helical
assemblies were observed in crystalline and gel states via the introduction of chiral
organic ligands or POM clusters.42−44 Alternatively, it is also a convenient strategy
following a supramolecular helical assembly of achiral building blocks of organic
molecules and metal complexes.16,41 By dealing the POM cluster as the counterion of
cationic surfactants, we got cluster-surfactant ionic complexes according to the route of
early phase transfer for POMs.30 Herein we would like to report the helical self-assembly
and its modulation of the ionic complexes without chiral centres, as shown in Scheme 1.
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The symmetrical linear structural feature makes the formed complexes suitable building
blocks for self-assemblies.45 Correspondingly, the self-assembly of the complexes alters
from flat ribbons to helical ribbons and finally to flower-like architectures via a simple
modulation of electrostatic interaction distance between inorganic anionic clusters. The
observed results provide a robust example for simple surfactant and inorganic clusters
without chiral element to build supramolecular chirality via controlling solvent dielectric
constant. Meanwhile, the strategy in this work can be directed to the combination of
controllable and helical assemblies in one system for the future functionalization of POM
complexes.
Scheme1 The schematic chemical structures of (DODA)2[Mo6O19], (ODTA)2[Mo6O19], and the
layered packing in self-assembly.
EXPERIMENTAL SECTION
Materials. All organic starting materials were obtained from commercial suppliers. The
surfactant dioctadecyldimethylammonium bromide (DODA·Br) was purchased from TCI
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without further purification. The surfactant octadecyltrimethylammonium bromide
(ODTA·Br) from Aladdin was used as received. The solvents were commercial products
from Sinopharm Chemical Reagent Co. Ltd. The cluster (TBA)2[Mo6O19] (TBA:
tetrabutylammonium) was prepared in a crystalline powder according to the reported
procedures.46 The POM complex (DODA)2[Mo6O19] was synthesized according to
published procedures.10
Characterizations. 1H NMR spectra were performed on a Bruker Avance 600 MHz
instrument with deuterated dimethyl sulfoxide (DMSO-d6), CDCl3, CD3OD, deuterated
tetrahydrofuran (THF-d8) or ethanol-d6 as the solvent and the tetramethylsilane (TMS) as
an internal reference. Elemental analysis (C, H, N) were conducted on a Flash EA1112
from ThermoQuest Italia S.P.A. Thermogravimetric analysis (TGA) measurements was
recorded on a Q500 Thermal Analyzer (New Castle TA Instruments) under a flowing air
with a heating rate of 10 °C min–1. Fourier transform infrared (FT-IR) spectra were collected
on a Bruker Optics VERTEX 80v Fourier transformation infrared spectrometer equipped
with a DTGS detector in pressed KBr pellets. A resolution of 4 cm–1 was chosen and 32
scans were signal-averaged. For X-ray diffraction (XRD) measurement, a Bruker AXS
D8 ADVANCE X-Ray diffractometer using Cu Kα radiation of a wavelength of 1.54 Å
with a mri Physikalische Geräte GmbH TC–Basic temperature chamber was used.
Scanning electron microscope (SEM) images were obtained on a JEOL FESEM 6700F
electron microscope. High resolution transmission electron microscopic (TEM) images
were acquired on JEOL JEM 2010 under an accelerating voltage of 200 kV. Circular
dichroism (CD) spectra were carried out on a Bio-Logic MOS-450 spectropolarimeter
with a step size of 1 nm at a speed of 5 s nm–1. High angle annular dark field scanning
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transmission electron microscopic (HAADF-STEM) images, energy-dispersive X-ray
analysis (EDX), and elemental mapping were collected on a FEI Tecnai F20 microscope
operating at an accelerating voltage of 200 KV. The surface morphology was examined
on an atomic force microscopy (AFM) (SPA-300HV) using tapping mode. Dynamic light
scattering (DLS) measurement was conducted on a Zetasizer Nano-ZS (Malvern
Instruments).
Preparation of (ODTA)2[Mo6O19] complex. The synthesis of (ODTA)2[Mo6O19]
complex was carried out following our previous method.10 Crystalline powder of
(TBA)2[Mo6O19] (0.27 g, 0.2 mmol) was dissolved in 20 mL of acetonitrile and then the
solution was added dropwise to a clear solution of a bit excess ODTA·Br (3.10 g, 7.9
mmol) dissolving in 40 mL of acetonitrile. Upon mixing the two solutions, a greenishyellow flocculate formed immediately. The yielded precipitate was centrifuged and
washed with cooled acetonitrile, and dried under vacuum overnight, giving the product in
yield of 82% (0.24 g). The complex is immiscible in water, but easily dissolves in polar
organic solvents such as THF, acetone, DMSO and N,N-dimethylformamide (DMF).
(TBA)2[Mo6O19]. 1H NMR (600 MHz; DMSO-d6, ppm): 0.936 (triplet, J = 7.2 Hz,
12H), 1.282–1.343 (sextuplet, 8H), 1.569 (quintupet, 8H), 3.163 (triplet, J = 8.4 Hz, 8H).
Elemental analysis (%): Anal. Calcd for (TBA)2[Mo6O19] (C32H72N2Mo6O19, 1364.6 g
mol–1): C 28.17, H 5.32, N 2.05; Found: C 28.42, H 5.30, N 2.17. IR (KBr, cm–1): ν =
2962 (s), 2931 (m), 2873 (s), 1468 (s), 1378 (m), 956 (s), 798 (s), 597 (w), and 438 cm–1
(w).
(DODA)2[Mo6O19]. 1H NMR (600 MHz, CDCl3, 30oC, ppm): 0.877 (triplet, J = 6 Hz,
6H), 1.253–1.306 (multiplet, 52H), 1.357 (quintuplet, 4H), 1.440 (quintuplet, 4H), 1.759
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(quintuplet, 4H), 3.229 (singlet, 6H), 3.349 (triplet, J = 6 Hz, 4H). MALDI-TOF MS
(m/z): 550.6, corresponding to molecular mass of [C38H80N]+ ion. Elemental analysis (%):
Anal. Calcd for (DODA)2[Mo6O19] (C76H160N2Mo6O19, 1981.7 g mol–1): C 46.06, H 8.14,
N 1.42; Found: C 46.39, H 8.21, N 1.36. IR (KBr, cm–1): ν = 2954 (m), 2918 (s), 2850 (s),
1483 (w), 1470 (w), 1379 (w), 960 (s), 910 (s), 795 (s), 598 (w), and 440 cm–1 (w).
(ODTA)2[Mo6O19]. 1H NMR (500 MHz, DMSO-d6, 30 oC, ppm): 0.855 (triplet, J = 6
Hz, 3H), 1.169–1.276 (multiplet, 32H), 1.674 (quintuplet, 2H), 3.031 (singlet, 9H), 3.270
(triplet, J = 6 Hz, 2H). Elemental analysis (%): Anal. Calcd for (ODTA)2[Mo6O19]
(C42H92N2Mo6O19, 1504.8 gmol–1): C 33.52, H 6.16, N 1.86; Found: C 33.47, H 6.09, N
1.82. IR (KBr, cm–1): ν = 3030 (w), 2954 (m), 2920 (s), 2850 (s), 1485 (w), 1467 (w),
1388 (w), 962 (s), 906 (s), 802 (s), 605 (w), and 439 cm–1 (w). Assuming that the organic
component has decomposed completely and all inorganic residuals are MoO3 at 700oC
(MoO3 may sublimate after the temperature), the measured residue of 56.7wt% in total
from TGA is in perfect agreement with the calculated value of 57.4wt% from the given
(ODTA)2[Mo6O19] formula. The structure characterizations are summarized in Figure S1–
S4 and Tables S1–S2. Due to the charge neutralization and reduced polarity, the prepared
complexes maintain soluble in organic media such as polar and weak polar solvents.
Sample preparation for self-assemblies. By taking (DODA)2[Mo6O19] as an example, the
steps of preparation are as follows. 1.0 mg of (DODA)2[Mo6O19] was added to a 1.0 mL
of the mixture solvent of dichloromethane and methanol in different volume ratios, which
giving a transparent solution (Figure S5a). After 5 min of sonication at 30°C, the obtained
mixture solution was allowed to stand for another 10 min to reach the self-assembly
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equilibrium. The Tyndall scattering can be observed obviously when laser light penetrates
through the clear solution. The result indicates that the self-assembly process of
(DODA)2[Mo6O19] occurs in the mixture solvent (Figure S5b). The obtained selfassemblies were transferred onto silica or copper grid substrates for further
characterizations. The samples preparation of (ODTA)2[Mo6O19] follows the same
procedure.
RESULTS AND DISCUSSION
Construction of helical self-assembly structure and morphology. Because of the polarity
difference between organic and inorganic components, the hydrophilic cluster and the
hydrophobic alkyl chain in the electrostatic complexes intend to segregate each other in
both polar and weak polar environments, driven by the interfacial energy. As a result, the
as-prepared amphiphilic complexes were found to form self-assemblies in organic media,
triggered by the phase separation.24 To investigate the self-assembled characteristics of the
ionic complexes in solution, the aggregation morphologies were examined firstly by SEM
and TEM. In weak polar dichloromethane, the (DODA)2[Mo6O19] complex self-assembles
into flat straight ribbons at a scale of 5−10 µm in width and over 100 µm in length but
very thin thickness, as seen from Figure 1a. The average thickness of the flat straight
ribbons reaches to ca. 85 nm according to AFM measurements (Figure S6), which
suggests that the flat ribbons are in multi-layered structure. Due to the lateral hydrophobic
interaction between DODA moieties, the (DODA)2[Mo6O19] stacks into layered packing
structure following the direction of strips spontaneously, as demonstrated in TEM image
(Figure1b). A very fine substructure parallel to the strip with regular spacing of ca. 3.0
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nm can be well interpreted as the alternate organic and inorganic layers (Scheme 1c) due
to the strong contrast between inorganic clusters (dark) and organic surfactants (bright)
under the condition without staining.
Figure 1. (a) SEM and (b) TEM images of (DODA)2[Mo6O19] self-assemblies in
dichloromethane under the concentration of 1.0 mg mL−1 at 30°C.
The solvent polarity plays an important role in the formation of self-assembled
morphologies
of
surfactants-encapsulated
POM
complexes.47
The
hydrophobic
interaction between alkyl chains can be enhanced upon addition of a polar solvent, which
is favorable for stabilizing the assembled morphologies of the complexes in solution
because the alkyl chains become frozen gradually with the increase of solvent polarity. In
contrast to all known POM complexes unexpectedly, the asymmetric morphologic
transformation takes place when adding a polar solvent such as methanol into the sample
dichloromethane solution. The straight ribbons become twisted and the helical strip-like
self-assemblies are observed in the mixed solvents starting from a volume ratio of
methanol over 30:1 (dichloromethane/methanol), as seen in Figure 2a and 2b. The
average width of these helical ribbons is about 0.7−1.2 µm. Since the average pitch of the
helical strips reaches around 1.0 µm as revealed by SEM image, it can be inferred that the
complexes are arranged next to each other with a very slight twist. Owing to the absence
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of chiral center in the (DODA)2[Mo6O19] complex, both right-handed and left-handed
helices appear in almost equal quantities, resulting in overall racemic mixtures during the
formation of twisted self-assemblies. The silence of Cotton signals in the CD spectrum
confirms the racemization (Figure S7). The handedness generated from the helical
superstructure is apparently triggered by the initial randomly twisted assembling of the
first several POM complexes under equal probability, which induce the subsequent
complexes tailoring along the twisted direction consistently. The fabrication of helical
architectures built up by the molecules without the introduction of chiral species is
generally known as spontaneous symmetry breaking.48,49 Therefore, the observed
phenomenon can be explained to derive from the packing restriction between different
segments of building blocks. The enantiomeric excess in supramolecular gels and
polymer assemblies deriving from symmetry breaking was observed in some publications
especially under the induction of external stimuli such as chiral seed, mechanic shearing
and/or other physical factors. We have tried the physical and chemical stimuli including
sheering force modulation, the use of chiral solvent, and the introduction of chiral seeds,
but in the present system, little influence on the helical direction control was observed.
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Figure 2. (a) SEM image of (DODA)2[Mo6O19] in the mixed solution of dichloromethane and
methanol (30:1 in v/v) with a concentration of 1 mg mL−1 at 30°C and (b) local amplification,
and (c) corresponding TEM image and (d) its local amplification at focused area marked in (c).
The helical characteristic is further confirmed by TEM images (Figure 2c) and the
lower electron transmission following the rolling direction in the twisted region than in
the flat area is observed definitely. In addition, the fine curved parallel dark and bright
stripes with a regular spacing ca. 3.0 nm shown in the magnified TEM image (Figure 2d)
demonstrates the similar packing substructure of the complex in the helical assembly.
EDX proves the existence of both carbon and molybdenum elements in the same helical
strips, confirming that the helical self-assemblies are composed of the organic–inorganic
ionic complex (Figure S8). DLS measurement gives a monodispersed size distribution
with an average hydrodynamic diameter (DH) of 255 nm (Figure S9), which is much
smaller than those from SEM and TEM observations. We further carry out the SEM
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measurement by quickly sucking the excess solvent, but no definite evidence on helical
structure could be concluded in solution because of the non-spherical assembly.
To identify if the substructure follows the morphology evolution, XRD patterns (Figure
3) of (DODA)2[Mo6O19] in the mixed solvent of dichloromethane and methanol (30:1
v/v) were carried out, which provide a further understanding for the incidental symmetry
breaking of self-assembled substructure. A lamellar structure with a layer spacing of ca.
3.16 nm is calculated from equidistant diffractions at 2θ = 2.83, 5.74, 8.64, and 11.55°,
which is in perfect agreement with the width estimated from TEM image (Figure 1b).
This value is also much close to the layered spacing of 3.0 nm of the complex in its
crystalline state. The complex has been proved to be in a lamellar structure, in which
DODA cations locate on both sides of [Mo6O19]2– cluster with a tilted angle (31°) to the
anion line rather than adopting interdigitated packing state between two POM layers.50
Because the layer spacing does not change with increasing the solvent polarity, the
preferential orientation of the complex and the layered structure should maintain even
after the formation of helical structure in a solvent with increased polarity. The CH2
antisymmetric and symmetric stretching modes of alkyl chains in (DODA)2[Mo6O19]
emerge at 2918 and 2850 cm–1 in FT-IR spectra (Figure S10), confirming that the long
alkyl chains in the helical structure are still in highly ordered packing state. Thus, it is
believable that the directed twisting of stacking plane perpendicular to the POM axis of
the complexes contributes the helical structure.
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Figure 3. XRD patterns of cast film of (a) (DODA)2[Mo6O19] and (b) (OTDA)2[Mo6O19]
prepared from the mixed solvent of dichloromethane and methanol with the same concentration
1 mg mL−1 under different volume ratios.
The complex (ODTA)2[Mo6O19] that carries two surfactants with single long alkyl chain
displays consistent phenomena when dissolving in different solvents. Because of the increased
hydrophilicity and the decreased solubility, (ODTA)2[Mo6O19] does not dissolve in
dichloromethane well. Thus, we used a mixture solvent of THF and ethanol that has stronger
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polarity to examine the self-assemble process. Similar to that of (DODA)2[Mo6O19], the complex
(ODTA)2[Mo6O19] forms flat ribbon like assembly in pure THF solution, as seen in Figure 4a.
With the addition of ethanol, the asymmetric morphology transformation (Figure 4b) occurs and
the helical structure is observed when the volume ratio of THF to ethanol reaches about 4:1.
Therefore, it could be concluded that the ionic POM complexes in hydrophilic-hydrophobic
separation state with symmetric linear structure are favourable for the formation of helical selfassemblies once the symmetry between inorganic and organic moieties is broken by simply
changing their polarity environment. TEM image further illustrates the ribbon and helical
structures of (ODTA)2[Mo6O19] in different solvent environments. As shown in Figure 4c, the
(ODTA)2[Mo6O19] assembly in THF solution has a similar layered structure with that in
(DODA)2[Mo6O19] and the estimated layer spacing between the light and dark streak
representing packing organic and inorganic components is ca. 2.7 nm. But this value is smaller
than that of (DODA)2[Mo6O19] though both complexes have similar ideal complex length, which
implies the partial interdigitation or greater tilting of alkyl chains. While in the mixed solution of
THF/ethanol with the volume ratio 4/1, a helical structure is observed in Figure 4d, which has an
identical spacing, showing the unchanged packing substructure. The XRD results further prove
the substructure in the ribbon and helical state, as shown in Figure 3b. The diffractions of five
samples prepared from the mixed solution with different volume ratios display consistent
patterns. The calculation to the diffraction peaks at 2θ = 3.29, 6.58, and 9.86° shows a layer
spacing distance of 2.68 nm, very close to the value estimated from TEM images, showing the
maintained substructure in both assembly states.
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Figure 4. SEM images of (ODTA)2[Mo6O19] (1.0 mg mL−1) in (a) THF, (b) THF/ethanol (4:1 in
v/v) solution and their corresponding TEM images in (c) THF, and (d) THF/ethanol (4:1 in v/v)
solution.
Solvent-triggered
transformation
of
self-assembled
morphologies.
The
twisted
characteristics of (DODA)2[Mo6O19] were maintained with increasing the solvent polarity by
adding methanol to 20:1 (v/v) of dichloromethane to methanol (Figure S11). After the volume
ratio further increases to 10:1, the long helical ribbons become short and the wrecking structure
appears as the main state although the twisted characteristics still exist as displayed in Figure 5.
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Figure 5. (a) SEM image of (DODA)2[Mo6O19] (1.0 mg mL−1) in the solution of
dichloromethane/methanol (10:1 v/v) at 30oC, and (b) the local magnification at selected area
marked in (a).
Interestingly, the uniform flower-like architecture of (DODA)2[Mo6O19] complex in
diameter ca. 1.0 µm is observed when the volume ratio of methanol in the mixed solvent
of dichloromethane increases to 4:1, as shown in Figure 6. The magnified SEM image
indicates that the flakes of petals derive from the debris of shortened ribbons maintaining
the helical feature, which comprise the round flower like assemblies. An evident contrast
between the central and peripheral segments was observed from TEM image, revealing
that the central part is less occupied due to the curvature and the size of the segments. The
magnified TEM image shows that the petals still have well-defined lamellar nanostructure.
The XRD pattern in Figure 3 confirms the same layered structures of the petals at the
state of flower-like assemblies as that in flattened and helical ribbon structure. The
intermediate morphologies at different time intervals were investigated to figure out the
assembly process. For the species after a sonication for 5 min and then aging for 5 min,
the formed flower like architecture (Figure S12) is in close to the samples undergoing a
quick or a slow preparation, revealing that the flower-like structure forms quickly in the
polar solvent. Further increasing the polarity of mixture solvent through adding methanol
to dichloromethane solution up to the volume ratio 3:1 does not lead to a continuous
transformation of the self-assembly architecture but instead result in a fusion of the petals.
Finally, the highly fused spherical assemblies with rough surface emerge at a volume
ratio of 3:1 while the substructure changes very little (Figure S13).
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Figure 6. (a) SEM image of (DODA)2[Mo6O19] (1.0 mg mL−1) in mixture solution of
dichloromethane/methanol (4:1 in v/v) at 30oC and (b) its magnification marked at arrow site,
and (c) corresponding TEM image and (d) its magnification at the position pointed by arrow in
(c).
Formation mechanism of helical structure and morphologic transformation. Several
factors can affect the self-assembly structure and morphology of the prepared ionic complexes in
solution due to the dependence of ionic interaction in the complex on the environment.
Considering that there is no special binding interaction from the solvents and no water in the
solution, the predominant factors correlating with the self-assembled structure of the complexes
could be outlined in Figure 7, which include (1) the packing order or the hydrophobic interaction,
(2) the attraction between organic head and inorganic cluster, and (3) the repulsion between
organic cationic heads and between inorganic anionic clusters. The stacking interaction of
hydrophobic alkyl chains is highly affected by the polarity change of solvent.51 An increased
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polar environment is normally favorable for the ordered alignment and structural frozen during
solvent evaporation when ex-situ measurement is required. Therefore, the interaction between
alkyl chains should be weaker in dichloromethane than in the condition of increased polarity.
The weak polar condition makes the organic component become a little bit flexible so as to adapt
easily to the volume mismatch deriving from ionic head’s packing. As a result, a flat reverse
bilayer in the ribbon like architecture in dichloromethane is observed as the main existing state.
In contrast, the alkyl chains get rigid in an increased polarity environment, which is unfavorable
for a self-adjustment during assembling. However, considering the sustained alkyl chain
conformation and the constant layer spacing, the change of solvent polarity should have no
rigorous influence on the self-assembled structure and morphology. Combining the consistence
between published results and the present results,52,53 the interactions between alkyl chains of
DODA should have very little contribution to the helical structure. However, in comparison to
the hydrophobic interaction, the ionic interaction is more sensitive to the polarity change in
various solutions. Based on the Coulomb’s law, the force interaction between any two point
charges can be described as:
|F|=|q1q2|/4πεr2
(1)
Where q1 and q2 represent the two point charges, r denotes the distance between charges,
and ε is dielectric constant of the media between the charges. From this equation, one can
see that the electrostatic force between ionic POM clusters is inversely proportional to the
dielectric constant value of the solution, and the higher interaction force leads to
shortened distance between opposite charges due to the charge attraction and the
elongated distance between same charges because of the charge repulsion.
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Figure 7. The proposed mechanism for the solvent-dielectricity controlled self-assembly and the
stepped structural transformations of the prepared complexes in the mixture solvents with
different dielectric constants from flat to helical ribbon like assemblies to the flower like
structures composed of broken petals.
For the electrostatic interaction, the attractive binding force between POM and DODA
is perpendicular to the axial packing along the ribbon like assembly, and therefore this
interaction contributes little to the observed twisting along the packing direction of the
complexes in the assemblies and could be ignored rationally. Obviously, the electrostatic
repulsion between POM clusters along the packing direction plays a decisive role for the
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formation of helical architecture and the following transformation to flower like structure
with the addition of other solvents. Since the electrostatic interaction between organic
cationic heads shows similar tendency on the self-assembly of the complexes to that
occurs between inorganic anionic clusters, we combine them together for the discussion
on the helical self-assembly.
Because the POM cluster is not a point charge, Coulomb’s formula could not be
directly applied to the force’s calculation quantitatively. Therefore, we only use the
dielectricity to describe the solvent effect for the formation of helical self-assembly and
the morphology change. In dichloromethane with a smaller dielectric constant (ε = 9.1,
25oC), the size matching between cationic surfactant and [Mo6O19]2− cluster can be
visualized due to the existence of flat reverse bilayer self-assembly. Meanwhile, the less
changed hydrophobic interaction between alkyl chains and the balanced force between the
increased charge repulsion in two clusters and the enhanced compression by increased
interfacial energy make the good fitting of the two components in the planar ribbon like
self-assembly. But the space matching is easily broken by altering solvent polarity
because of the different responses from ionic and non-ionic components in the complex
under the external agitation. In addition to the lateral hydrophobic interaction between
alkyl chains, the electrostatic interaction becomes weakened with the addition of
methanol (ε = 32.6, 25oC) due to the increased dielectric constant value of the mixed
solvent. The reduced repulsive interactions result in the occupied space of the ionic
clusters shrinking. Thus, the decreased distance between clusters corresponding to the less
compressed packing of alkyl chains yields a space mismatch of ionic head to non-ionic
tail in the POM complexes, and as a result, a symmetry breaking take places and results in
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the helical packing of alkyl chains around the decreased cluster space at a tight packing
state. For a 1.0 µm of average pitch in twisted structure, the length of helical curve should
be equal to that in straight ribbon structure. According to the geometrical model shown in
Figure 8, the calculated shrinkage in two POM clusters is only about 0.05 nm in one pitch
and this means that a very small compression along the direction of cluster packing axis
leads to the helical morphology with a twist angle ca. 0.3º between two adjacent alkyl
chains. The gradually decreased average helical pitch with increased solvent polarity (ε
value) indicates the larger space mismatch. The over twisting breaks the ribbon like
assemblies into pieces and finally form a flower like structure (Figure 7).
Figure 8. Schematic model for illustration of helical assembly of (DODA)2[Mo6O19], with the
pitch ca. 1000 nm based on SEM image and the radius ca. 1.6 nm from XRD data, the length
sum of one cluster and one surfactant molecule from crystal structure.
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The self-assembled structure change of the complex (ODTA)2[Mo6O19] versus the
modulation of the solvent polarity follows the similar process. The single alkyl chain
surfactant ODTA occupies a smaller lateral area than DODA. But the solvent THF used
has a lower dielectric constant (ε = 6.7, 25oC) in comparison to dichloromethane, causing
a much less packing density to that of DODA. Both of which direct to the late appearance
helical structure during the addition of ethanol (ε = 24.3, 25oC) up to the volume ratio of
4:1 (THF:ethanol) with an equivalent dielectric constant of ε = 9.1.
The self-assemblies were further evaluated versus the gradual change of solvent
environment. With the addition of methanol to the dichloromethane solution of
(DODA)2[Mo6O19], the XRD patterns of the casting film show equidistant diffractions all
the time (Figure 3), suggesting that the layered structure with a constant d-spacing does
not change with formation of helical assembly. FT-IR spectra provide robust evidence on
the conformation order of alkyl chains by monitoring anti-symmetric and symmetric
vibrations of CH2 groups. With increasing the solvent dielectric constant by adding
methanol, these stretching vibrations show no obvious change, indicating the stable transconformation of alkyl chains and the high order in the helical state (Figure S14).
The XRD and FT-IR results confirm that the reverse bilayered packing structure has
little change though the assembled morphologies show evident transformation. To
demonstrate the helical assembly is closely associated with the change of Coulomb
interaction between clusters, 1H NMR spectra (Figure 9) were employed as the chemical
shifts of head groups of DODA are proportional to the alteration of electrostatic
interaction between the organic cations and [Mo6O19]2− cluster. By gradually adding
CD3OD to the original CDCl3 solution of (DODA)2[Mo6O19], the chemical shifts of N-
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methyl (Ha) and N-methylene (Hb) locating at the cationic head move to the high field
independently while other chemical shifts belonging to CH2 and CH3 tail groups hold the
line. Since the up-field shifting of protons is sourced from the shielding effect of
neighboring groups, the chemical shift moving of Ha and Hb can be explained as the
weakened electrostatic attraction between organic cation and inorganic clusters in the
mixture solution with increased polarity. Simultaneously, this also indicates the decreased
electrostatic repulsion between clusters in the assembly under a higher volume ratio of
methanol.32 The 1H NMR spectra of (ODTA)2[Mo6O19] in the mixture solvents of THF
and ethanol under different volume ratios are also investigated. With the addition of
ethanol into the sample THF solution, those chemical shifts of both Ha and Hb protons
move to the high field due to the increased dielectricity in the mixed solvents though their
moving becomes smaller obviously.
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Figure 9. 1H NMR spectra of (A) (DODA)2[Mo6O19] under fixed concentration of 1.0 mg mL−1
in mixture solvents of CDCl3 and CD3OD at different volume ratios, and (B) (ODTA)2[Mo6O19]
under fixed concentration of 3.0 mg mL−1 in mixture solvents of deuterated THF and ethanol at
different volume ratios, where s1 and s2 denote the protons from solvent.
The electrostatic repulsive interaction between ionic groups and the hydrophobic effect
between non-ionic moieties are proved to be the major governing forces in the selfassemblies of linear amphiphiles.54 Based on above results and analysis, we propose a
process for the assembly structure evaluation modulated by the increase of polarity in
solution. The enhanced electrostatic repulsive interaction between polyanions results in a
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space balance between organic and inorganic part and the consequent reverse bilayer
packing in the ribbon like assembly. With increasing the polarity in solution, the
electrostatic repulsion decreases so that the ionic inorganic clusters can get closer and that
leads to a space mismatch. The packing twisting triggers the helical assembly. Further
increasing solvent polarity lead to a frustrated growth and hence to the formation of
emanative strip like assemblies with shorter scale. Because the lateral interaction between
hydrophobic alkyl chains is enhanced during the addition of methanol with high dielectric
constant, the reverse bilayer still retains even if the long ribbons break into fragments.
Hence, the broken helical structures comprise a circular assembly at much higher
percentage of methanol.
CONCLUSIONS
By using an anionic inorganic cluster as the counterion of two surfactants, we realized
a novel asymmetric self-assembly and structure evolution of ionic complexes,
(ODTA)2[Mo6O19] and (DODA)2[Mo6O19] by modulating solvent environment. Due to
the hydrophobic/hydrophilic separation, the complexes maintained their linear central
symmetric state in reverse bilayer substructure while the morphology changed from flat
ribbon to helical ribbon and to flower-like self-assembly. Quite different from those wellknown adjusting methods for intermolecular interaction such as hydrophobic force, π-π
interaction, hydrogen bonding, recognition and so forth, the dielectricity of solvent also
drives the change of interaction between inorganic clusters in the linear packing state
based on Coulomb’s Law. The distance change between clusters dominated the twisting
stacking of hydrophobic alkyl chains in the complexes due to the space mismatch
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between organic and inorganic components when both parts packed along a lateral
direction in a reverse bilayer style. The present study also points a strategy to modulate
the helical alignment of ionic surfactants or complexes in solution or on solid surface. In
the present stage, the out of control for the helical direction is understandable because of
the non-chiral center existing in the complexes. We are trying to introduce a chiral agent
to govern the chirality of self-assemblies so that it can be attached to the selected
asymmetric catalysis of inorganic clusters.
ASSOCIATED CONTENT
Supporting Information.
The Supporting Information is available free of charge on the ACS Publications website at DOI:
10.1021/acs.lang-muir.xxxxxx.
Further characterizations (Synthesis route, 1H NMR spectra, IR spectra, elemental analysis, TGA
curves, CD spectra, EDX-mapping image, SEM, TEM, AFM, DLS, optical photographs) (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: libao@jlu.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
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We acknowledge the financial support from National Basic Research Program (2013CB834503),
National Natural Science Foundation of China (21773090, 21574057, 21502107), Changbaishan
Distinguished Professor Funding of Jilin Province, China. Dr. J. Zhang thanks Shanxi Province
Natural Science Fund (2014021019-5), Research Foundation for Talented Scholars of Shanxi
University (020451801001), and Scientific & Technological Innovation Programs of Higher
Education Institutions in Shanxi (2016118).
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