Solvent Dielectricity-Modulated Helical Assembly and Morphologic

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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, 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: [email protected]. 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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