Polyoxometalete-Based Supramolecular Complexes:Gel And Magnetic Resonance Imaging Contrast Agent

Polyoxometalates (POMs) are a kind of inorganic nanoclusters with abundant chemical composition and topologies. They are molecular clusters built from the connection of{MOX} polyhedra, and M is a d-block element in high oxidation state. Due to their great diversity of applications, such as catalysis, redox, electrochemisty, photochromism, magnetism and medicine properties, POMs are attracting building blocks for new materials. However, the development and application of materials based on POMs are greatly restricted because of their high crystal energy, poor processability and low pH stability. The supramolecular self-assembly is an effective methodology to control the combination between POMs and organic building block. This method can not only improve the processability of POMs and provide various supramulecular self-assembly made from inorganic-organic hybrid based on POMs, but also optimize the properties of POMs through the synergy of organic/inorganic building blocks and aggregate structures. Especially, amphiphilic molecules, with distinct structures and properties, can direct POMs into different supramolecular assemblies through various fabrication methods. For example, surfactant-encapsulated polyoxometalates (SEP) complexes, as amphiphilic building blocks, have developed several organic-inorganic hybrid soft materials. In this dissertation, taking the concept of supramolecular chemistry as the guide, we employed organic amphiphilic molecules and inorganic POMs as building blocks to prepare several new organic-inorganic supramolecular self-assembly structures, by different fabrication methods. And we further utilized the magnetic properties of inorganic clusters to realize the functionalization of hybrid aggregates as magnetic resonance imaging (MRI) contrast agents.Firstly, taking surfactant-encapsulated polyoxometalate (SEP) hybrid complexes as building blocks, we utilized synergetic self-assembling between organic and inorganic components to obtain a new type of hybrid supramolecular gels. We compared systematically gel behaviors, critical gelation temperature and gel-sol transition temperature of eleven complexes with different surfactants and POM types. The proper alkyl chain density, alkyl chain length and shape of POM are proven to be favorable for fabricating stable SEP gels. And among the general organic solvents we employed, some of SEPs readily gel with nonpolar solvents. In addition, the structure of POM influences the aggregation morphologies of SEPs, SEPs with sphere and twin-sphere shaped POMs form stripes and spheres type aggregates in gel phases, respectively. Despite of different aggregate morphologies, X-ray diffractions results reveal that all the SEPs possess similar lamellar aggregation structures in molecular scale, and each complex layer is composed of POM layer inside and alkyl chain bilayers shielding from the nonpolar solvent on both sides. The phase separation of SEP is proved to be the original driving force of gel formation. And the synergic effect of electrostatic interaction, van der Waals interaction and solvent effect induces SEPs to form gels structure in nonpolar solvents.Secondarily, we extended the SEP supramolecular self-assembly system from organic phase to aqueous solution. We encapsulated paramagnetic Gd3+-containing polyoxomatalates (Gd-POM) by cationic amphiphile containing poly(ethylene oxide) chains through electrostatic interaction, obtaining encapsulated gadolinium-polyoxometalate hybrid (EGdPH). The resulted complexes can dissolve in water readily because of the outside PEO layer. The stable combination of two parts through electrostatic interaction and the core-shell structure of EGdPH still remain in aqueous solution due to the designed suitable hydrophobic environment inside EGdPH. EGdPH act as effective supramolecular building blocks because of their amphiphilic property, and self-assembly into multilayered vesicular structure with diameter of 100-200 nm and layer spacing of 6.8 nm. Meanwhile, EGdPH can serve as a new type of MRI T1contrast agent due to the paramagnetic property of Gd-POM, and accelerate the water-proton relaxation. More important, EGdPH present different longitudinal relaxivity (r1) values in different aggregation states. Comparing with that of Gd-POM alone (9.2 mM-1s-1), the increase of rotational correlation time, resulted from the encapsulation by organic component, is responsible for the significant r1 enhancement (61 mM-1s-1,500 MHz and 25℃) at low concentration. However, the phase-separation and self-assembly of EGdPH at high concentration induce more hydrophobic environment around Gd-POM, reducing r1to 2.3 mM-1s-1. Moreover, in vitro MRI and reasonable stability of EGdPH in physiological condition provide potential magnetic resonance imaging application for paramagnetic POM-based complexes.Thirdly, our research focused on a kinds of POMs with special structure and property, Mn12O12(OOCR)16 (Mn12) single-molecule magnets. According to the structural properties of the Mn12, we utilized a simple emulsion-assisted self-assembly method to construct hybrid supramolecular aggregates, taking stearic acid modified Mn12 (Mn12-C18) as inorganic building blocks and nonionic surfactant C18H37(OCH2CH2)10OH (C18EO10) as organic building blocks. The diameter of resulting Mn12-C18/C18EO10 aggregates is 75±30 nm. In this progress, C18EO10 acts not only the emulsifier but also the stabilizing agents of Mn12-C18 aggregates. Due to the protection of C18EO10 and the alkyl chains of Mn12-C18, the structure stability of Mn12 cluster in water is greatly improved. Moreover, Mn12-C18/C18EO10 complexes present the magnetic hysteresis phenomenon, which means Mn12-C18 remain their superparamagnetic property as single-molecule magnets. These make water-dispersed Mn12-C18/C18EO10 aggregates show potential as effective MRI T2 contrast agent. More important, the aggregation of Mn12-C18 and the protection effect of C18EO10 to the aggregates induce the enhanced transverse relaxivity r2, compared with the results of Mn12 coordinated with acetic acid in previouse literature. And the free PEO chains, decorating the aggregate surface, ensure the aggregate stability of Mn12-C18/C18EO10 aggregates in different environment of aqueouse solution and the long-time r2 stability.In conclusion, we focused researche on the hybrid supramolecular self-assembly of amphiphile-polyoxometalate complex systems. Using suitable supramolecular fabrication mothodologies, we have constructed suprmolecular gel, multilamellar vesicle and spherical aggregates structures based on POMs. We investigated the effect of organic/inorganic building block properties and the interaction between them to the hybrid supramolecular self-assembly morphology. Furthermore, utilizing the synergic effect between supramolecular structure and magnetic properties of inorganic clusters, we realized the functionalization of self-assembly systems as MRI contrast agents. We believe that these researches not only provide a new strategy and theoretic guide for constructing distinct organic-inorganic hybrid self-assembly of POMs, but also play an important role in developing low-toxic, sensitive or target POMs contrast agents systems by introducing functional groups.