Polyoxometalate-Based Functional Organic-Inorganic Hybrids:Design, Synthesis, And Property Characterization

Polyoxometalates (POMs) stand for a large class of discrete metal-oxo anions of early transition metals such as V, Nb, Mo, W. The structural diversity and intriguing physical and chemical properties of POMs give rise to many potential applications in the fields of catalysis, biomedicine, and material science. Recently, tremendous attentions have been paid to the elaborating of POM-based functional organic-inorganic hybrids by utilizing modified POMs as building blocks. Combining the properties of nano-scaled inorganic polyanions and organic functions, POM-based hybrid materials show great perspectives in the design and synthesis of highly complex frame-works and also in processing advanced functional materials and devices.In this dissertation, we have successfully synthesized a series of POM-based functional organic-inorganic hybrids. The common problems existing in the development of POM-based materials have been well solved. Controllable assembly, thermotropic liquid crystal, and fluorescence enhancement of POM-based hybrids have been achieved by tuning molecular structures and compositions. The correlation between molecular structures and properties of such hybrids has also been studied in details. Meanwhile, a novel covalent modification method of fabricating symmetric and asymmetric POM assemblies has been established by utilizing a step-by-step hydrothermal technique. This study paves a new way for the further covalent functionalization of POM clusters. The main results of this dissertation are listed as follows:1. In order to investigate the relationship between structural compositions and aggregate morphologies of POM-based amphiphiles, we synthesized three Anderson-type POM-based hybrids, (TBA)3[MnMo6O18-{(OCH2)3CNHCOC6H2(OC8H17)3}2], (TBA)3[MnMo6O18{(OCH2)3CNHC-OC6H3(OC8H17)2}2], and (TBA)3[MnMo6018{(OCH2)3CNHCOC6H40-C8H17}2] (TBA=(n-C4H9)4N), by adopting the pre-modification method. This study demonstrated that it was possible to control the aggregate morphologies of amphiphiles by a pre-designed molecular structure. Moreover, our studies provided a better understanding of the spontaneous self-organization process. By increasing the length of hydrophobic carbon chains and exchanging the Keggin-type POMs as hydrophilic head groups, we prepared another two classes of POM-based amphiphiles. We found that Anderson-type amphiphiles tended to form reverse vesicles in CHC13/ n-hexane mixed solvents, while Keggin-type amphiphiles prefered lamellar structures. The thermotropic liquid crystal studies demonstrated that it was unsuccessful to construct POM-based liquid crystalline materials by covalently tethering dendritic organic molecules onto POM frameworks.2. Based on the above mentioned studies, we modified the molecular structure by introducing azobenzene as mesogenic groups, and successfully prepared the first type of POM-containing thermotropic liquid-crystalline nanomaterials with a general formula of (TBA)4[(SiW11O39)O{Si(CH2)3-NHCOC6H2[O(CH2)nOC6H4N2C6H5]3}2] (TBA=(n-C4H9)4N, n=8,10,12). These hybrid materials were found to be able to self-organize into a well-defined smectic lamellar structures, as confirmed by SAXS and HR-TEM measurements. This work provided fascinating perspectives in the design and elaboration of novel POMs-containing liquid-crystalline nanomaterials. We then developed two similar POM-based organic-inorganic hybrids, (TBA)4[(SiW11O39)O{Si(CH2)3NHCOC6H4N=NC6H4OC8H17}2] and (TBA)4[(SiW11O39)O{Si(CH2)3NHCOC6H3[O(CH2)12OC6H4N=NC6H4OC8 H17]2}2] (TBA=(n-C4H9)4N) with a bent-like and dendritic-like structure, respectively. The bent-like POM hybrid was found to stabilize BP Ⅰ phase, in which the widest temperature range reached 20.5℃, while dendritic-like POM hybrid was able to stabilize BP Ⅱ. Additionally, the optical switching of BP Ⅱ was achieved. Our study might give rise to a new strategy for both the stabilization of BPs and optical switching of Bragg reflection of BPs.3. A novel system based on POM-anthracene dyad was developed to tackle the quenching process of POM-chromophore dyads and to enhance the fluorescence of such systems. The POM-anthracene hybrid was fully characterized by various spectroscopic techniques including single-crystal X-ray diffraction, ESI-MS, etc. The fluorescence features of the dyad were studied in details. A dramatic emission enhancement could be observed by either changing the microenvironment or the amphiphilic property of the dyad or suppressing the intrinsic photo-induced electron transfer quenching process. Moreover, the photo-polymerization process of the dyad was initially investigated and carried out. As such, we demonstrated a new strategy for solving the quenching process of POM-chromophore dyads, and our work provided perspectives for the development of POM-based fluorescent sensors or switches.4. In the respect of covalent modification of POMs, we demonstrated that it was feasible to covalently functionalize POM clusters through a pre-designed synthetic strategy using a controllable hydrothermal approach. This led us to develop a series of organic-inorganic POMs including the mono-substituted, symmetrical bi-substituted and asymmetrical bi-substituted ones. This work was believed to bring the opportunity of the development of robust organic-POM hybrids one step closer.