POM–Organic–POSS Cocluster - American Chemical Society

Apr 16, 2013 - first dumbbell-shaped hybrid molecule is a POM−organic−POSS cocluster produced by covalently coupling a POM cluster and a. POSS clu...
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POM−Organic−POSS Cocluster: Creating A Dumbbell-Shaped Hybrid Molecule for Programming Hierarchical Supramolecular Nanostructures Min-Biao Hu, Zhan-Yao Hou, Wei-Qiong Hao, Yu Xiao, Wei Yu, Chi Ma, Li-Jun Ren, Ping Zheng, and Wei Wang* Center for Synthetic Soft Materials, The Key Laboratory of Functional Polymer Materials of Ministry of Education and Institute of Polymer Chemistry, Nankai University, Tianjin 300071, China S Supporting Information *

ABSTRACT: We report the construction of dumbbell-shaped hybrid molecules for programming their hierarchical supramolecular nanostructures through a synergetic self-assembly. Our first dumbbell-shaped hybrid molecule is a POM−organic−POSS cocluster produced by covalently coupling a POM cluster and a POSS cluster together through an organic tether. Structural analyses demonstrated a highly ordered lamellar morphology with a 4.9 nm periodicity, indicating a strong thermodynamic force driving a nanoscale phase separation of the POM and POSS blocks. The POM clusters were arranged in an orderly fashion within the POM-containing layer with a 1.38 nm periodicity because of fixed shape and size of the cluster. This investigation provides in-depth understanding of how to construct hierarchical supramolecular nanostructures at a nanoscale less than 5 nm by manipulating and controlling the topological shape of hybrid molecules.

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INTRODUCTION

molecular structures and properties of the generated nanomaterials or nano-objects have been the highly topical subject. POSS is an inorganic, cagelike, and cubic-shaped hybrid molecule of silicon and oxygen with similarities to both silica and silicone. Its diagonal length is about 0.53 nm, but the actual size is greater depending on the size of the side groups.33 Importantly, POSS and its derivatives are novel candidates for creating POSS-based functional materials.34,35 POMs are a class of molecularly defined, discrete, polyatomic anions that consist of several early transition metal oxyanions linked together by shared oxygen atoms.36−40 The hydrophilic nature of POM macroions can be altered by using cationic surfactants. Their various structures, shapes, and elemental compositions contribute to diverse properties with a wide range of applications. Covalent linkage with organic species has created a large number of advanced POM-containing hybrids.41−43 The

Many different building blocks have been used to construct nanomaterials or nano-objects with well-defined structures or architectures.1−4 These building blocks include block copolymers (or polymeric amphiphiles)5−8 and surfactants (or lowmolecular-mass amphiphiles),9,10 which play a critical role in mimicking natural self-assemblies. The symmetrical and asymmetrical topological shapes of amphiphilic molecules are important in understanding the induced three-dimensional (3D) structures of the building blocks and the supramolecular assembly architectures.11−14 Polyoxometalates (POMs),15−27 polyhedral oligomeric silsesquioxane (POSS),28−31 and fullerene (C60)31,32 are the three building blocks most commonly used for this purpose. These materials have been used to produce shape amphiphiles by covalently linking them together with a tether.15−17,30,31 They have been also used to create shape amphiphiles with anisotropic topology by using covalent conjugation with linear polymers, oligomers, or short chains.18−29,32 The supra© 2013 American Chemical Society

Received: March 2, 2013 Revised: April 9, 2013 Published: April 16, 2013 5714

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Scheme 1. (A) Three-Dimensional Chemical Structure of POM−Organic−POSS, a Dumbbell-Shaped Hybrid Molecule;a (B) General Synthetic Route of POM−Organic−POSS

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The six cationic (Bu4N+)s used for encapsulating the anionic POM are not shown. added. The reaction mixture was stirred at room temperature under argon for 24 h, and the CH2Cl2 solution was washed with hydrochloric acid (HCl) solution (pH = 2) (50 mL × 3) and distilled water (50 mL × 3). Then the organic portion was collected and dried over anhydrous sodium sulfate overnight. After removing the solvent, the product was obtained as white powders and used without further purification (yield: 0.95 g, 1.03 mmol, 85.2%). 1H NMR (400 MHz, CDCl3): δ = 5.74 (s, 1H, −CONH−), 3.30−3.26 (m, 2H, −CONHCH2−), 2.70 (t, J = 6.7 Hz, 2H, HOOCCH2−), 2.53 (t, J = 5.7 Hz, 2H, HOOCCH2CH2−), 1.86 (m, 7H, −SiCH2CHC2H6), 1.62 (m, 2H, −CONHCH2CH2−), 0.96 (d, J = 6.6 Hz, 42 H, −SiCH2CHC2H6), 0.61 (d, J = 6.9 Hz, 16H, −SiCH2CHC2H6 + −CONHCH2CH2CH2Si−). 13C NMR (100 MHz, CDCl3): δ = 176.1, 172.1, 42.2, 30.7, 29.9, 25.7, 23.9, 22.8, 22.5, 9.5. Synthesis of Compound 3 (POSS−3OH). To a 250 mL roundbottomed flask equipped with a condenser and a magnetic stirrer was added 2 (0.50 g, 0.51 mmol), EEDQ (0.15 g, 0.61 mmol, 1.2 equiv), and Tris (0.07 g, 0.58 mmol, 1.1 equiv), followed by addition of fresh distilled 25 mL of acetonitrile (CH3CN) and 50 mL of CH2Cl2 to fully dissolved all the samples. The solution was stirred and refluxed at 85 °C under argon for 24 h. The reaction was stopped by cooling to room temperature, and after CH2Cl2 solvent partially evaporated, the compound POSS−3OH precipitated in the solution and was then filtered to get a white powder. The crude product was purified by flash chromatography [CH2Cl2:methanol (25:1)] (yield: 0.46 g, 0.43 mmol, 83.2%). 1 H NMR (400 MHz, CDCl 3 ): δ = 6.91 (s, 1H, −(CH2)3CNHCO−), 5.71 (s, 1H, −CH2CONH−), 3.71 (s, 6H, −(CH2)3CNHCO−), 3.23 (m, 2H, −CH2CONHCH2−), 2.63 (t, J = 5.7 Hz, 2H, −(CH2)3CNHCOCH2−), 2.50 (t, J = 5.8 Hz, 2H, −NHCOCH2CH2NHCO−), 1.85 (m, 7H, −SiCH2CHC2H6), 1.58 (m, 2H, −CONHCH2CH2CH2−), 0.96 (d, J = 6.5 Hz, 42 Hz, −SiCH2CHC2H6), 0.61 (m, 16H, −SiCH2CHC2H6 + −CONHCH2CH2CH2Si−). 13C NMR (100 MHz, CDCl3): δ = 174.4, 172.0, 64.8, 61.3, 42.0, 32.1, 31.6, 25.7, 23.9, 22.8, 22.5, 9.5. Synthesis of Block Cocluster 4 (POM−Organic−POSS). A solution of (Bu4N+)6H3[P2W15V3O62]9− (1.00 g, 0.18 mmol) and 3 (0.24 g, 0.22 mmol, 1.2 equiv) in dry N,N-dimethylformamide (DMF)

diverse characteristics of POSS and POM clusters suggested their use in the development of new block coclusters, i.e., dumbbell-shaped hybrid molecules to be used for programming their hierarchical supramolecular nanostructures. In this work we report our new design and facile synthesis of a dumbbellshaped hybrid molecule constructed by covalently coupling a POM cluster and a POSS derivative together through a short organic tether. This hybrid molecule self-assembles into a nanosized lamellar morphology composed of alternatively arranged POM and POSS layers. The formation of this supramolecular structure is associated with a nanoscale phase separation of the POM and POSS blocks. The characteristic 5 nm thickness of the lamellae reflects the success in controlling the size of the two building blocks. The POM cluster was arranged within the POM-containing layer in an orderly fashion with a 1.38 nm periodicity. These findings help us better understand the impact of manipulating and controlling the topological shape of hybrid molecules on programming hierarchical supramolecular nanostructures at nanoscales less than 5 nm.

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EXPERIMENTAL SECTION

Materials. 1-Aminopropyl-3,5,7,9,11,13,15-heptaisobutyl-POSS was purchased from Hybrid Plastics as crystalline powders and used as received. Succinic anhydride and tris(hydroxymethy1)aminomethane (Tris) were purchased from Aldrich, and 2-ethoxy-1ethoxycarbonyl-1,2-dihydroquinoline (EEDQ) was purchased from Alfa Aesar. Other reagents were purchased from major chemical supplies and used as received unless otherwise noted. (Bu4N+)6H3[P2W15V3O62]9− was synthesized according to the literature procedure.44 Synthesis of Compound 2 (POSS−COOH). To a solution of POSS−NH2 (1.00 g, 1.14 mmol) in 100 mL of fresh distilled dichloromethane (CH2Cl2), succine anhydride (0.14 g, 1.40 mmol, 1.2 equiv) and triethylamine (Et3N) (0.14 g, 1.39 mmol, 1.2 equiv) were 5715

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Figure 1. (A) 1H NMR spectra of POSS−3OH 3 and POM−organic−POSS 4. Δppm = 1.79 ppm shows a signal shift of the methylene in Tris after conjugation. (B) 31P NMR spectra of the POM cluster and POM−organic−POSS 4. The two signals correspond to the phosphorus atoms in the POM cluster. A Δppm = 0.33 ppm shift of the signal of phosphorus near the trivanadium substitutes attributes to the conjugation of Tris and POM cluster. (C) ESI-MS spectrum of POM−organic−POSS 4.

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(50 mL) was placed into the 100 mL round-bottomed flask with a magnetic stirrer under argon. After heating the solution to 70 °C for 6 days, it was cooled and condensed. The condensed solution was then added in a dropwise manner to stirred diethyl ether (100 mL) to precipitate the product and remove the excess reactant 3, and the resulting yellow solid was collected and redissolved in a minimum volume of CH3CN to reprecipitate into diethyl ether. This procedure was repeated twice, and the resulting yellow powders were washed with diethyl ether and dried under vacuum at 50 °C for 24 h (yield: 1.05 g, 0.17 mmol, 88.6%). 1H NMR (400 MHz, DMSO-d6): δ = 7.80 (s, 1H, −(CH2)3CNHCO−), 7.58 (s, 1H, −CH2CONH−), 5.51 (s, 6H, −(CH2)3CNHCO−), 3.18 (t, J = 8.3 Hz, 48H, NCH2CH2−), 3.00 (m, 2H, −CH 2 CONHCH 2 −), 2.32−2.78 (m, 4H, −(CH2)3CNHCOCH2CH2−), 1.81 (m, 7H, −SiCH2CHC2H6), 1.58 (m, 48H, NCH2CH2−), 1.43 (m, 2H, −CONHCH2CH2CH2−), 1.33 (m, 48H, NCH2CH2CH2−), 0.94 (t, J = 7.2 Hz, 114 Hz, −SiCH 2 CHC 2 H 6 + NCH 2 CH 2 CH 2 CH 3 ), 0.59 (m, 16H, −SiCH2CHC2H6 + −CONHCH2CH2CH2Si−). 13C NMR (100 MHz, DMSO-d6): δ = 172.0, 171.0, 85.9, 57.5, 53.8, 40.9, 31.5, 31.0, 25.3, 23.4, 23.1, 22.4, 21.9, 19.2, 13.5, 8.9. 31P NMR (121.5 MHz, DMSO-d6): δ = −7.0 (s, PW6V3), −13.1 (s, PW9). 29Si NMR (79.4 MHz, CDCl3) δ = −67.5, −67.8. EA: (Bu4N)6[(P2W15V3O59)(OCH2)3CNHCOCH2CH2CONH(C31H69Si8O12)], % found (calculated values in parentheses): C 24.95 (25.15), H 4.78 (4.73), N 1.85 (1.71). Characterization. 1H NMR, 13C NMR, and 29Si NMR spectra were recorded on a Varian UNITY plus-400 spectrometer in CDCl3 and dimethyl-d6 sulfoxide (DMSO-d6), and chemical shifts are given in ppm, referenced to the residual resonances of the solvents (δ = 7.26 ppm for CDCl3 and δ = 2.50 ppm for DMSO-d6). 31P (121.5 MHz) NMR spectra were obtained by use of a Varian Mercury Vx-300 spectrometer in DMSO-d6 at a concentration of 60 mg/mL. The molecular mass of compound POM−organic−POSS was taken on an electrospray ionization time-of-flight (ESI-TOF) mass spectrometer (X7ICP-MS). Transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), energy-dispersive X-ray spectroscopy (EDX), and electron energy-loss spectroscopy (EELS) were performed by using a field emission transmission electron microscopy FEI Tecnai G2 F20 operating at an acceleration voltage of 200 kV. X-ray diffraction (XRD) experiment was conducted with a Rigaku D/Max-2500 X-ray diffractometer, equipped with a Cu Kα radiation (λ = 0.154 nm) source operated at 40 kV/100 mA. Atomic force microscopy (AFM) images were recorded on a multimodel atomic force microscope (Digital Instrumental Nanscope IV) using glass slide as substrate at room temperature performed in tapping mode. Elemental analysis: carbon, nitrogen, and hydrogen contents were determined by using a Vario EL CUBE elemental analyzer.

RESULTS AND DISCUSSION Design and Synthesis of the POM−Organic−POSS Cocluster. The unique 3D structure of the cocluster is shown in Scheme 1A. We selected a POM cluster and a POSS cluster to construct the dumbbell-shaped cocluster. The selected POM cluster was a trivanadium-substituted derivative of a Wells− Dawson-type polyoxotungstate, with two phosphorus heteroatoms in the center. Its chemical structure is (Bu4N+)6H3[P2W15V3O62]9−, and its molecular weight is 5422 Da. This POM cluster is encapsulated by six cationic tetrabutylammoniums (Bu4N+)s, facilitating its solubility in organic solvents like DMF. Trivanadium-substituted polyoxotungstate can be covalently modified in DMF. The three vanadium atoms are linked with the organic compound tris(hydroxymethyl)aminomethane (denoted as Tris).45 Another cluster used is 1-aminopropyl-3,5,7,9,11,13,15-heptaisobutyl-POSS (aminopropylisobutyl-POSS, molecular weight 874 Da). The two clusters are covalently linked by a short tether to form the cocluster. Hereafter, the cocluster is denoted as POM−organic−POSS. The POM cluster without counterions has an ellipsoidal shape with a long axis of 1.2 nm and short axis of 1.0 nm.46 The diagonal length of the cubic POSS cluster with its eight groups is about 1.3 nm.47 The tether length is about 1.0 nm, as estimated by ChemDraw. The estimated characteristic length of POM−organic−POSS is about 3.5 nm. The synthetic route is shown in Scheme 1B. We used the commercially available POSS derivative, aminopropylisobutylPOSS, to construct this cocluster. The starting compounds were aminopropylisobutyl-POSS (1: POSS−NH2) and succinic anhydride (organic tether). The amino group in compound 1 was reacted with succinic anhydride to give its carboxyl groupcontaining derivative (2: POSS−COOH). Amidation of 2 with Tris was carried out with 2-ethoxy-1-(ethoxycarbonyl)-1,2dihydroquinoline (EEDQ) as an activator to produce a trihydroxyl group-containing derivative (3: POSS−3OH). Compound 3 was conjugated with the POM cluster (Bu4N+)6H3[P2W15V3O62]9− in DMF at 70 °C for 6 days.45 The product obtained was characterized by NMR, ESI-MS, and elemental analyses, confirming the presence of the block cocluster, POM−organic−POSS 4. The characterization results are summarized in Figures S1−9 of the Supporting Information. The synthesis and purification of 2 and 3 were straightforward. The conjugation of 3 and the trivanadium5716

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substituted POM cluster was the key step in creating the cocluster 4. Figure 1A shows the 1H NMR spectra of 3 and 4. The individual spectral resonance signals are assigned to the corresponding hydrogen atoms (see Experimental Section). We see a 1.79 ppm shift of the methylene signal in Tris after conjugation. This shift reflects the methylenes of Tris in 3 being directly linked with the trivanadium in the POM to form 4. Figure 1B displays the 31P NMR spectra of the POM cluster and 4. The two 31P signals originate from the different situations of two phosphorus atoms in the trivanadium substituted cluster. After conjugation, the signal of the phosphorus atom near the trivanadium substitutes shifts 0.33 ppm. The spectrum indicates an intact POM cluster being in 4. Figure 1C is an ESI-MS spectrum of 4 showing one characteristic peak with a normal distribution. The peak at 1664.6 is assigned to the cocluster anions H3[(P2W15V3O59)(OCH2)3CNHCOCH2CH2CONH(C31H69Si8O12)]6−, and the full spectrum of ESI-MS is given in Figure S9 to depict the complete situation of cocluster anions. These evidences confirm the synthesis of the POM−organic−POSS. Characteristics and Property of the Cocluster. The 3D fixed shape, nanoscale size, and diverse properties or functions of the two clusters award this hybrid molecule a fascinating hybrid-dumbbell topological characteristic, stemming from the similarity in geometry and size but the vast inconsistencies in physical and chemical properties. The cocluster has a dumbbell topology, which has rarely been reported. The molecular weight is 6445 Da, similar to block copolymers but much larger than surfactants. Its 3.5 nm molecular length is close to that of surfactants but much shorter than the contour length of block copolymers. This unique structure stems from the spheroid shape of the two clusters, each with an about 1.0 nm diameter. Furthermore, the rigid 3D structure of POM−organic−POSS may accelerate the self-assembly process due to its characteristic without intermolecular entanglement. The POM and POSS clusters have different solubility in organic solvents. The POSS derivative dissolves in weakly polar solvents, such as toluene and CH2Cl2. The POM cluster, encapsulated by six (Bu4N+)s, dissolves in strongly polar solvents, such as acetonitrile and DMF. POM−organic−POSS dissolves in some solvents used to dissolve POM or POSS only, such as acetonitrile and CH2Cl2. This is important for sample preparation. Since we see no crystals formed in our structural studies no matter how we cultivated the crystalline samples from its solution according to some standard protocols of crystal growths, we consider that its crystallization ability is greatly restricted because of the loss of symmetry and the flexible linker between the two clusters. XRD Characterization of Ordered Supramolecular Structures. Although 4 had no ability to crystallize, the strong incompatibility of the two building blocks led to self-assemble into ordered supramolecular structures. Figure 2 displays the Xray diffraction (XRD) pattern obtained from a thin film specimen on a glass slide surface. The methods of sample preparation are presented in the following section. In Figure 2, the reflection peaks marked by black arrows appear at 2θ = 1.79°, 3.54°, 5.40°, and 7.20°. The corresponding d-spacings are d = 4.92, 2.50, 1.63, and 1.23 nm. The ratio of 1/d1:1/d2:1/ d3:1/d4 is 1:2:3:4, and the first peak at d1 = 4.92 nm indicates the highly ordered lamellar structure with a 4.92 nm periodicity in the specimen.48 The second peak at 2θ = 3.54° is extremely weak, almost invisible. This may be due to the form factor being zero at this particular position. Three weak reflection

Figure 2. XRD pattern obtained from a thin film specimen prepared by casting the acetonitrile solution of the POM−organic−POSS on a water surface and then annealing for 24 h at room temperature.

peaks at 4.0° < 2θ < 9.0° are seen in the inset of Figure 2 (white arrows). The most intense peak is at 2θ = 6.39°, corresponding to a d-spacing of d = 1.38 nm. This contributed to the orderly arrangement of the POM clusters encapsulated by six (Bu4N+)s, as the XRD characterization of pure POM cluster showed a sharp peak at the same position (see Figure S10 in the Supporting Information). Observation and Identification of Supramolecular Structures. TEM was used to characterize the ordered supramolecular structures. Thin specimens were prepared for TEM characterization. An acetonitrile solution of POM− organic−POSS (concentration of 5 mg/mL) was cast in a dropwise manner on a deionized water surface using a syringe. The solution drops spread on the water surface to form a transparent film, which then became opaque. The change from transparent to opaque occurred as the acetonitrile diffused into the water, producing the solid film. The thin film floating on the water surface annealed for 24 h at room temperature. Films were then carefully transferred onto copper grids for TEM characterization or on a glass slide surface for XRD and AFM. Figure 3A shows an AFM height image presenting singleand double-layer films on the glass surface. The thickness of a single layer is 4.8−4.9 nm, an ideal thickness for TEM characterization. Figure 3B is a typical wide-view bright-field TEM image obtained from the thin film specimen showing the lamellar structure. Actually, the lamellar or hexagonal structures constructed by POM-containing complexes or hybrids have been observed by TEM.25,49−53 But fine structures of POM clusters within lamellar or hexagonal structures have rarely been characterized, which will be discussed in the following section. This 1 × 1 μm image provides the morphological characteristics of the POM−organic−POSS film. The brightness difference in the upper part and the lower part of the image reflected differences in thickness of the film (see our AFM height image). There are the regions with ordered and disordered structures, as highlighted by the circle and the square, respectively. EDX was used to find the chemical composition of the film. The characteristic X-ray energies of tungsten, vanadium, and silicon elements were identified (see Figure S11). In Figure 4, the three TEM images obtained at a higher magnification display the details of the supramolecular 5717

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Figure 3. (A) An AFM height image of a thin film specimen showing the film thickness. (B) A bright-field TEM image showing fine ordered structures.

Figure 4. Three bright-field TEM images obtained at higher magnification, displaying the physical structure within a disordered region (A), a transition from ordered to disordered structure (B), and an ordered structure (C).

Figure 5. (A) A dark-field STEM image showing a lamellar structure. The EELS spectrum image was obtained along the white line. (B) Line contrast image. (C) Silicon contents.

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Figure 6. (A) A bright-field TEM image obtained at higher magnification showing a fine structure within the lamellae. (B) FFT pattern corresponding to (A). From the center outward we see two sharp dots: a wide halo and two semimoon-shaped arcs. (C) I-FFT image shows a clusters-on-a-string feature.

structures within a disordered region (A), transition region from an ordered to a disordered structure (B), and a region of an ordered structure (C). In Figure 4A, there is not any specific structural feature except for randomly distributed dark dots with a size of about 2−3 nm. In Figure 4B, there are ordered lamellae and disordered dark dots, representing the gradual transition from ordered to disordered structure. Figure 4C shows a typical ordered structure with a lamellar feature, shown as alternatively arranged bright and dark streaks. The periodicity of the lamellae is about 5.0 nm, similar to the dspacing obtained in the XRD characterization. This lamellar morphology, formed by the POM−organic−POSS, was directly observed using TEM in the nonstaining thin specimens under bright-field conditions. The POM cluster used contains 15 tungsten atoms and 3 vanadium atoms and is able to scatter more electrons than the organic tether and the POSS derivative. This difference in electron density causes the structural characteristics seen under bright-field conditions, which makes the supramolecular structures be observed easily. The POM clusters present in the dark streaks and POSS clusters present in the bright streaks. Furthermore, we combined STEM with EELS to chemically map the two-dimensional distribution of silicon in the specimen. In Figure 5A, the dark-field STEM image displays the lamellar structural feature, with alternatively arranged bright and dark streaks under dark-field conditions. The dark streaks indicate silicon enrichment within the streaks, opposite to the bright-field TEM images. The white line in Figure 5A is the scan direction to obtain the contrast (Figure 5B) and silicon contents (Figure 5C) under the condition normal to the lamellae. The peaks in the contrast profile (Figure 5B) correspond to the peaks in the silicon content (Figure 5C), reconfirming that silicon (or POSS) is enriched in the dark streaks. On the basis of these findings, we conclude that both the POM-containing layer and the POSS-containing layer jointly constitute the ordered lamellar structure. This means that the POM and POSS blocks in POM−organic−POSS can form a lamellar morphology through a nanoscale phase separation. Fine Supramolecular Structure and Suggested Structural Model. TEM observations at higher magnification were performed to better characterize the fine structure of the POM or POSS clusters within the lamellae. A specimen prepared by casting the solution on a glass surface was examined. A number

of dark dots in a string were arranged in an orderly fashion along the POM-containing layer (Figure 6A). Figure 6B shows the corresponding fast Fourier transform (FFT) pattern. From the center outward we see two sharp dots, a wide halo, and two semimoon-shaped arcs. The two sharp dots (arrow a) correspond to the highly ordered lamellar structure consisting of the POM- and POSS-containing layers. These layers had a ca. 5 nm periodicity. A wide halo (arrow b) was thought to be an artifact caused by instrument undulation at high magnification. We found a similar halo in all the FFT patterns obtained from TEM images including those taken with a blank carbon film. The two semimoon-shaped arcs (arrow c) correspond to the fine structure existing within the POMcontaining layer. Its periodicity is ca. 1.39 nm, similar to the 1.38 nm d-spacing found in the XRD characterization. In contrast to the sharp dots, the arcs are wider and have an arc angle of 80°. These indicate that the arrangement of the POM blocks within the POM-containing layer is not as uniform as in the lamellar structure. Figure 6C shows an image obtained using inverse FFT (I-FFT). Individual clusters are strung together to form the POM-containing layer with a clusters-ona-string feature. In this image we see that most POM clusters were arranged in a zigzag manner (highlighted in area a in Figure 6C). We also found many defects due to misorientation and misarrangement of the POM clusters. Two of these are highlighted in areas b and c of Figure 6C. The existence of these defects contributed to the wider arcs. In order to help propose a hypothetical model to describe the hierarchical supramolecular nanostructures, in Figure 7A we display an enlarged TEM image selected from Figure 6C. The lamellar structure and clusters-on-a-string feature in this image is the model of the hierarchical supramolecular nanostructures, as presented in Figure 7B. The ca. 5 nm lamellar structure was composed of the POM and POSS layers. This means a nanoscale phase separation occurred between the POM and POSS building blocks. The difference between the physical and chemical properties of the two blocks (for instance, the totally different solubility) is the thermodynamic force driving the phase separation. The special topological shape and 3.5 nm molecular length of this dumbbell-shaped hybrid molecule are attributed to the fixed shape and size of the POM and POSS clusters. These two key factors contributed to the nanoscale phase separation of the POM and POSS building blocks and the highly ordered lamellar structure with ca. 5 nm periodicity. 5719

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casted by the POM−organic−POSS solution on water surface; enlarged image of Figure 3B. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (W.W.); Fax (+86) 2223498126. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We appreciate the financial support of the National Natural Science Foundation of China for grants (Grants NSFC 20974057 and 21274069), Research Fund for the Doctoral Program of Higher Education of China (20100031110015), and Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry.

Figure 7. (A) TEM image clearly showing orderly POM clusters within the POM-containing layer. (B) Hypothetical model to describe the hierarchical supramolecular nanostructures.

The POM clusters arranged in an orderly fashion in the POMcontaining layer, as reflected by the two semimoon-shaped arcs in the FFT pattern (Figure 6B). The 4.9 nm lamellar periodicity was larger than the 3.5 nm characteristic length of POM−organic−POSS but less than twice the length (i.e., 7.0 nm). This indicates that POM− organic−POSS cannot adopt a simple “head-to-tail”, “head-tohead”, or “tail-to-tail” arrangement to assemble into the lamellae. In Figure 7B, the hypothetical model describes the arrangement of the POM (and/or POSS) clusters within the lamellae as a zigzag pattern. It is important to emphasize that the hierarchical nanostructures form through a synergetic selfassembly. The nanoscale phase separation of the POM and POSS building blocks and the arrangement of the POM (and/ or POSS) clusters occur simultaneously.

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CONCLUSION We have successfully designed and synthesized a novel dumbbell-shaped hybrid molecule, POM−organic−POSS, by covalently binding the POM and POSS clusters together with a short organic tether. Characterization of the supramolecular nanostructures demonstrated a nanostructural hierarchy forming via a synergetic self-assembly. The hybrid molecule self-assembled into a highly ordered lamellar morphology composed of POM- and POSS-containing layers with a 4.9 nm periodicity. This demonstrated the strong thermodynamic forces driving the nanoscale phase separation of the POM and POSS blocks. Within the POM-containing layer, the POM clusters were arranged in an orderly fashion with a 1.38 nm periodicity and a clusters-on-a-string feature. The strong thermodynamic driving force for the phase separation came from the difference in the physical and chemical properties of the POM and POSS clusters. The clusters-on-a-string arrangement stemmed from the fixed shape and size of the cluster. This investigation provides guidance as to how to construct nanostructures on a scale less than 5 nm. Manipulating and controlling the topological shape of hybrid molecules is the key to this procedure.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Detailed 1H NMR and 13C NMR spectra of compounds 2−4; 31 P NMR, 29Si NMR, and ESI-MS spectra of compound 4; XRD profile of pure POM cluster; EDX result of a thin film 5720

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