Supramolecular Assembly And Functionalization Of Polyoxometalate Hybrid Reverse Micelle In Silica Matrix

Polyoxmetalates (POMs) are a class of well defined early transition metal (V, Mo, W etc.) oxygen clusters with sizes ranging from one to several nanometers. POMs exhibit various topologies and extensive chemical and electronic properties, leading to applications in catalysis, electro-optics, magnetics, medicine, and biology. However, POMs are often synthesized and existed as crystals or raw powders, which exhibit high crystal energy, low pH stability and water solubility. From the view point of material science, POMs are not processable. Because of the poor processability of pure POMs, practical POM-based materials or devices are quite limited. The POM chemistry has been experiencing an unparalleled development of rapid creation of new compounds and slow development of POM-based materials and devices.The encapsulation of surfactants provides an effective method to protect POMs and improve their surface chemical properties. Through electro static interactions, the cationic surfactants can replace the countercations of POMs, forming surfactant-encapsulated POMs complexes (SEP complexes). The structure of SEPs is similar to reverse micelles, the hydrophilic POMs locate at the center of the complex, and the hydrophobic alkyl chains of surfactants locate at the periphery of POMs. Because of the defined chemical composition, size and tunable surface physical chemical property, SEPs are normally used as functional nano-building blocks to fabricate molecular devices or functional hybrid materials. On the other side, because of its transparency, porous structure, chemical inactivity and plasticity, silica sol-gel is an ideal candidate to be used as matrix to assemble POMs and exhibits their extraordinary photo-physical property and catalytic activity etc. In this dissertation, we attempt to combine the encapsulation method of POMs and the sol-gel technique, and provide a novel method to realize controllable supramolecular assembly and functionalization of SEPs in silica matrix. Through the cooperation of the functionality of organic surfactants and inorganic POMs, the obtained SEP/SiO2 hybrid exhibit excellent luminescent, photochromic, reductive and catalytic properties.Firstly, we have designed and synthesized a functional cationic surfactant with two hydroxyl terminal groups: di(11-hydroxyundecyl)dimethylammonium bromide (DOHDA). Through the electrostatic interaction, cationic DOHDA and luminescent anionic POM [EuW10O36]9- could combine with each other forming SEP-1. The extraordinary alcohol-like surface makes SEP-1 soluble in the mixed solvent of ethanol and water and the following sol-gel reaction can be put in effect. After the condensation with sol-gel precursor tetraethyl orthosilicate (TEOS), and the following aging and drying process, a transparent and stable POM based organic-inorganic hybrid monolith, SEP-1/SiO2 hybrid, can be ultimately constructed. The introduction of SEPs separates the fractal structure of the silica matrix greatly, and makes the packing of silica matrix much more loosen. Because SEP-1 distributed uniformly in the silica matrix with covalent linkage, SEP-1/SiO2 hybrid is transparent. From the systemically investigation on the luminescent property of SEP-1 and SEP-1/SiO2 hybrid, it can be found that they all keep the strong luminescent property of [EuW10O36]9-, and the quantum yield of them is all above 50 %. With the increasing of SEP-1 content in SEP-1/SiO2 hybrid, the I(5D0→7F2)/I(5D0→7F1) value, radioactive rate kr, life timeτand quantum yieldηexhibit regular changes, indicating the regular change of the micro-environment of POMs. The alkyl chains of DOHDA protect [EuW10O36]9- from outside etching efficiently, which is favorable in practical applications. From this part, we have confirmed the basic structure of SEP/SiO2 hybrid, which is important for my following work. The method we used has provided a common approach for the fabrication of multifunctional POM-based silica materials suitable for various requirements.Secondly, with the organic modification of surfactant, we have designed and synthesized cationic surfactant 11-hydroxylundecyldimethylamine hydro bromide (HUDAH), which possesses both–OH reactive site and an active H atom at the amine head. Through the same method as mentioned above, we herein introduced [EuP5W30O110]12-, which exhibits strong photochromic and redox ability, into the silica matrix with HUDAH as a bridge to construct stable SEP-2/SiO2 hybrid film. SEP-2/SiO2 hybrid film is transparent and crack free, which should be attributed to the introduction of flexible alkyl chains of surfactants. The hybrid film exhibits reversible and stable photochromic property, which should be attributed to the special structure of the surfactant HUDAH that was applied to modify POMs and the designed artificial microenvironment in SEP-1 grafted onto silica matrix. The active H atom at the ammonium head of the surfactant can provide a proton to [EuP5W30O110]12-, which stabilizes the reduced state of [EuP5W30O110]12- and makes the transition between original and reduced state reversible. This process exhibits the cooperation between organic part of surfactant and inorganic part of POMs, which provides a reference for fabricating multifunctional hybrid materials. Because of the reversible and stable photochromic property, SEP-2/SiO2 hybrid film may have potential applications in the field of photo-switch and memory.Further more, utilizing the reductive property of POMs, we can in-situ synthesize Ag or Au nanoparticles in SEP-2/SiO2 hybrid film with two different methods. Because of the modification of POMs with HUDAH bearing amine groups and the mono-dispersion of POMs in silica matrix, the obtained metal nanoparticles present spherical morphology and considerably uniform distribution in SEP-2/SiO2 hybrid film. More significantly, through adjusting the reduction conditions (photo-irradiation time, photo-irradiation location and adsorption time), we can control the formation of nanoparticles at a preassigned location and tune the size of nanoparticles in a certain extent, which may be used in the field of the construction of patterned metal nanoparticles. The hybrid films containing POMs and metal nanoparticles may have potential applications in the areas of optical materials, antibacterial coatings, as well as in redox catalysts, etc. In this part, we have realized the inorganic reductive reaction of metal ions into nanoparticles utilizing the redox property of POMs existing in SEP/SiO2 hybrid, and it is the foundation for realizing the catalytic oxidation of organic compounds.At last, the oxidation catalytic property of SEP/SiO2 hybrid has been eventually realized. Keggin type of [PW12O40]3- with well redox catalytic property is encapsulated by DOHDA. The structure of obtained SEP-3 is similar to reverse micelle systems. Through the sol-gel condensation with TEOS, SEP-3 with hydroxyl groups at the periphery can be covalently anchored into the silica gel matrix with porous structure, forming immobilized supramolecular hybrid catalyst (SHC-3) with confined hydrophobic nano-environment. The supramolecular interactions do not only exist in the constructing process of the hybrid catalyst, but also play a crucial role in promoting the activity of the catalyst during the reactions. The defined hydrophobic nano-environments around POMs in SHC-3 tend to capture the weak polar substrates and release the strong polar oxidized products in the catalytic oxidation process. This property promotes the compatibility between POM catalytic center and organic substrate in the maximal extent and improves the catalytic activity effectively. Through monitoring the reactions, we can observe that the oxidative reactions catalyzed by SHC-3 fitted well to pseudo first order kinetics, and the reaction rate is close to that of homogeneous catalytic oxidation systems and superior to that of the immobilized POM catalysts, which exhibit the high catalytic activity of SHC-3. SHC-3 does not show any selectivity for reaction solvent, the well performance of SHC-3 in acetonitrile, acetone, chloroform, hexane and dodecane provides the potential applications in different conditions. The polarity of the solvent influences the reaction kinetics regularly, which further confirms the existence of the supramolecular synergy during the reaction process. After the reaction finished, SHC-3 could be easily recovered by simple filtration. The recovered SHC-3 possesses the same structure as that of the freshly prepared one. More significantly, the recovered SHC-3 can be reused directly without any special treatments, which is much more economical and suitable for practical applications. The catalytic activity and efficiency of the recovered SHC-3 are examined as high as that of the freshly prepared one and no trace of the catalytic activity decrease has been found in five cycles. SHC-3 performed effectively in the selective catalytic oxidation of several different kinds of organic compounds, such as alkenes, alcohols and sulfides, and the main products are corresponding epoxides, ketones, sulfoxides and sulfones respectively. Because of the high catalytic activity and selectivity of SHC-3 for the oxidation reactions and the easy recovery process, we believed that this kind of supramolecular hybrid catalyst may have great potentials in organic or biological synthesis, as well as in petroleum industry.In conclusion, with supramolecular assemble and function integration as guiding ideas, combining surfactant modification to POMs and silica sol-gel technique, we have successfully realized the controllable supramolecular assembly and functionalization of polyoxometalate hybrid reverse micelle in silica matrix. SEP complex dispersed uniformly in silica matrix with covalent linkage, which could overcome the shortcoming bringing by traditional physical blending, such as aggregation of POMs, phase separation, instability and so on. Further more, utilizing the cooperation of the functionality of POMs and surfactants, the hybrid materials exhibit excellent photo-luminescence, photo-chromic, reduction, and oxidation catalytic propertys.