Polyoxometalate cluster-contained hybrid gelator and hybrid organogel: a new concept of softenization of polyoxometalate clusters

Abstract

Newly designed and synthesized polyoxometalate cluster-contained hybrid molecules with two dendritic poly(urethane amide) wings are interesting as gelators that form hybrid organogels of self-assembled hybrid nano-ribbons.

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Supramolecular gels, as an important class of functional soft materials, have attracted the considerable attention of scientists and engineers with different backgrounds due to their self-assembly features and potential applications in diverse fields such as sensors, optical technologies, electronic devices, biomaterials, and organic–inorganic hybrid materials etc.¹ In recent years, a variety of ways to design and synthesize self-assembling molecules leading to functional gelator systems have been developed.² Metallo-gelators are a class of metal-containing gelators which are prepared by incorporation of metal anions within conventional gelators mainly through ionic interactions to form discrete metal complexes or coordination polymers.³ It should be noted that additional functionalities can be added into these metallogels to widen their application range.

Polyoxometalates (POMs), a type of anionic metal-oxygen clusters with a diverse range of properties, are attractive units as building blocks with potential applications in medicines, catalysts, analytical chemistry, and materials science.⁴ The study of POM-containing supramolecular gels, however, is relatively rare. The first POM-containing organogel, which exhibits optical anisotropy, was found in the acetonitrile solution of a coordination polymer of a 4,4′-bipyridine modified polyoxomolybdate and Pd^II.⁵ Another important finding is that a chiral POM-containing hybrid forms supramolecular organogels in some solvents through hydrogen bonding between POM clusters to generate twisted gel fibers.⁶ Furthermore, some surfactant-encapsulated POM complexes are another route to generate POM-containing supramolecular organogels.⁷

Because of the unique molecular shape, controllable functionality and various supramolecular interactions, dendrons and dendrimers can easily self-assemble into highly ordered nano-objects with attractive functions for potential applications.⁸ The dendron or dendrimers which contain cations in their body, apex or periphery have also been used to encapsulate POMs to develop new functional materials.⁹ Recently, it has been reported that covalently linking POMs with dendrimers is an efficient route to construct novel organic–inorganic hybrids in which POM clusters can be protonated.¹⁰ Very recently, we reported our findings concerning manipulation of nanostructures of protonated polyoxometalate cluster in the organic matrix through controllable self-assembly of the dendron-POM-dendron hybrids.¹¹ Herein we report a new type of gelator of dendron-POM-dendron hybrids in which two poly(urethane amide) (PUA) dendrons covalently bond to a protonated Anderson-type POM. The special molecular architecture and the multiple hydrogen bonds drive the hybrid molecules in organic solvents to assemble into single-molecular-layer hybrid ribbons in which protonated POM clusters are sandwiched by two dendron layers, which is the key supramolecular structure for the formation of the hybrid supramolecular gel.¹² Our study demonstrates a new concept of how to soften POM clusters through utilization of synergistic effects of organic and inorganic components within the hybrid molecule to contribute to the self-assembly of the hybrid molecule into a highly ordered supramolecular structure that can further organize into hybrid organogels.

Fig. 1 presents our synthetic strategy based on the amidation of the g₂-PUA-COOH¹² dendron and (Bu₄N⁺)₃{(MnMo₆O₁₈) ³⁻[(OCH₂)₃CNH₂]₂} cluster (denoted as tris-POM-tris).¹³ The three negative charges of anionic tris-POM-tris were neutralized by three tetrabutyl ammonium cations (Bu₄N⁺). After the PUA dendrons have been covalently linked to the tris-POM-tris, further purification using silica gel column chromatography resulted in fully protonated POMs from the neutralized POMs. In this work, the protonated hybrid molecules are denoted as g₂-PUA-H₃POM-g₂-PUA. The compound thus obtained was fully characterized using FT-IR spectroscopy, ESI-MS spectroscopy, and ¹H NMR spectroscopy (see Fig. S4 to S5, ESI†). In the FT-IR spectrum, the characteristic bands of –OC–NH– linkage at 3321 (N–H stretching), 1693 (C=O stretching) and 1539 (N–H bending) and 1269 (C–N stretching) cm⁻¹ and the important characteristic bands at 1049, 941, 922, 904, 667 and 567 cm⁻¹ for the Mn-Anderson POM clusters after amidation were noted.^13b The ESI-MS spectrum in Fig. 1 shows a mass centered around 1527.22 m/z which corresponds to the compound with three charges and fully coincides with the calculated value (1527.40 m/z, see Fig. S4, ESI†), revealing that the hybrid contains one tris-POM-tris linked together by two PUA dendrons. More significantly, the spectrum illustrated a high purity of the product. Finally, we could not see any signals of Bu₄N⁺ in the ¹H NMR spectrum (see Fig. S5, ESI†). This indicates that we have successfully synthesized the hybrid containing a protonated POM cluster inserted between two PUA dendrons and the structural integrity of the POM cluster is preserved in the whole process. Characteristically, the protonated POM cluster is hydrophilic, whereas the PUA dendron containing four alkyl chains is hydrophobic, so that this hybrid is amphiphilic. More importantly, the hydrophilic cluster is located in the middle of two hydrophobic dendrons. As such the molecular architecture is totally different from conventional amphiphiles with a head-tail type.

Fig. 1 Synthesis of g₂-PUA-POM-g₂-PUA and corresponding ESI-MS.

In our experiment we found that the hybrid molecule can be dissolved in N,N-dimethylformamide (DMF) in a concentration of 10 g L⁻¹ at 70 °C and the solution turns to a transparent and orange organogel when cooled to 10 °C (see Fig. 2a1 and 2a2). Interestingly, bundles of uniform ribbon-like objects were observed in the dried sample of the gel-phase by bright field transmission electron microscopy (BF-TEM) as clearly shown in Fig. 2b. The ribbons are fairly straight and with only seldom bending or winding. They randomly overlap similar to other organogels of PUA derived dendrons.^14a,c The ribbon length is longer than a couple of micrometres, much larger than its width. Fig. 2c shows an AFM image of the dried sample of the gel-phase on a silicon wafer to further confirm the ribbon-like feature. The height profiles across the fiber width (Fig. 2d) show that these ribbons have a uniform thickness of 7.8 ± 0.5 nm and the thickness in areas where two ribbons overlap is 15.6 ± 0.5 nm, approximately twice the thickness measured for that of a single ribbon.

Fig. 2 (a1) and (a2): Hybrid solution and hybrid organogel in DMF which is confirmed through inverting the vial. (b): BF-TEM image showing features of ribbons in the gel. (c): AFM image showing the ribbons with uniform height. (d): Height profiles of the ribbons and measured heights. (e): EDX spectrum of an ∼1 μm² area with aggregated ribbons showing presence of molybdenum (Mo) and manganese (Mn) in the ribbons. (f): HAADF-STEM image of the ribbons showing finer threads. (g): ED pattern of a 10 μm diameter area of ribbons with diffraction signals at 0.47, 0.39 and 0.24 nm representing the monoclinic packing of the alkyl chains.

The significance of this work is to confirm that the supramolecular aggregates are synergistically formed by the organic dendrons and inorganic clusters in the hybrid molecules. To confirm the presence of the POM cluster in the fibers, we used energy-dispersive X-ray spectroscopic (EDX) analysis as shown in Fig. 2e.¹⁵ The signals at the characteristic energy of molybdenum (Mo) and manganese (Mn), reveal the existence of the POM clusters in the ribbons (the Cu peak comes from the copper grid). So, we conclude that the ribbons are likely to be composed of the hybridized Anderson-type POMs and PUA dendrons. The image in Fig. 2f was obtained using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Fine threads can be seen as pointed by the arrows. Their width is ca. 5–6 nm (see the inset), similar to the finer perimary ribbons found in our previous work.^14bFig. 2g depicts a selected-area electron diffraction (SAED) pattern presenting ordered structures existing in the ribbons. In this pattern we can see three diffraction signals corresponding to d-spacings of d = 0.47, 0.39 and 0.24 nm. These are the typical diffraction spacings observed for the crystalline structure of alkyl chains in the PUA dendron.^14b

Because of the specific chemical structure and properties of the two building blocks in this molecule, it is important to have a deep comprehension how the building blocks with totally different architecture and properties synergistically arrange in the ordered ribbons. It is the special molecular architecture and multiple interactions of our hybrid gelator to promote self-assembly into highly ordered ribbons. It is well known that POM clusters normally crystallize with a three-dimensionally ordered structure. However, when PUA dendrons are introduced on two sides of the Anderson-type cluster, the large dendrons restrict the interaction between POM clusters and protons to a two-dimensional space. Therefore, protonated POM clusters in these hybrids prefer to spontaneously self-assemble into two-dimensionally ordered nanostructures. Furthermore, the multiple hydrogen bonds within the plate-shaped PUA blocks are the main driving force for the anisotropic growth of the aggregates with lengths in the micrometre range.¹⁴ Owing to this synergistic effect, the hybrid gelator assembles into nanoribbons in which the POM clusters are sandwiched by two PUA dendron layers, as schematically shown in Step A in Fig. 3. In this ribbon model the distance between adjacent dendrons remains in the range of hydrogen bond length of amides. These long ribbons can constitute a three-dimensional network through interactions between alkyl chains on the ribbon surface leading to the formation of the organogel phase (i.e. Step B in Fig. 3). We applied the ChemDraw® software (MM2 force field) to carry out a simple estimation of the sizes of the two dendrons as well as the Diamond® software to calculate the POM diameter from the reported crystallographic data (see Fig. S6, ESI†). The molecular dimension is ca. 9.08 nm, close to the ribbon thickness (ca. 8.0 nm). So the ribbon is a single molecular layer.

Fig. 3 Schematic representation of the self-assembly of the POM cluster-containing gelator into single-molecular-layer ribbons which further constitute a three-dimensional network for gelation.

In summary, we have successfully grafted two PUA dendrons through covalent bonds to the organically modified Anderson-type POM cluster with the aim to develop robust POM-containing hybrid gelator. After protonation of POMs clusters the gelator turns to a hybrid amphiphile composed of one hydrophilic POM cluster and two hydrophobic PUA dendrons. As such the molecular architecture and the multiple interactions synergistically promote self-assembly of the hybrid gelator in DMF into highly ordered and single-molecular-layer ribbons in which the POM layer is sandwiched by two dendron layers. The hybrid ribbons can constitute a three-dimensional network which further leads to the formation of a stable organogel phase. The significance of our finding justifies marking one step on the way to soften polyoxometalate clusters in order to prepare POM-containing functional materials.

Acknowledgements

The Nankai group is grateful for the financial support given by the National Science Foundation of China (NSFC Grant NOS. 20734001 and 20974057) and State Key Laboratory of Polymer Physics and Chemistry. W. Wang thanks Dr X. L. Feng and Dr D. Q. Wu of Max-Planck-Institute for Polymer Research for help with the ESI-MS measurements. B. Liu and J. Yang thank the “100 Projects” of creative research for the undergraduates of Nankai University. Finally, support by the Karlsruhe Nano Micro Facility (KNMF) for the electron microcopy characterization is gratefully acknowledged.

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Footnote

† Electronic supplementary information (ESI) available: Synthesis and characterization of the product. See DOI: 10.1039/c1sm05032j

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