| Polyoxometalates (POMs or metal-oxygen clusters) are a rich family of polynuclear compounds which are made up of early transitional metals with d0 electron configuration linked by oxygen atoms, and their significant units {MO4} and {MO6} can build kinds of polyanions with different function and structure by edge-shared, angle-shared or face-shared approach. POMs has promised dramatic applications in many areas including optics, electrics, magnetics, catalysis, biology and medicine due to their thermostability and novel chemical, physical and biological properties. Recently, the synthesis of POMs has come into the molecular assembling which can be designed at the molecular level to synthesize POM complexes with optical, electrical and magnetical properties. Moreover, the investigation tends to focus on the compounds with metastability and variable valence with supermolecules instead of oxidated species. The futher research has been prohibited due to the present deficient characterization appoarch, so the quantumn chemistry calculation as a theoretical method has been introduced into the field of POMs and investigated the structure-property relationship at the micro-level. In early time, the ab initio Hartree–Fock (HF) approximation presented reasonable achievment for studying the structural and electronic properties of POMs. Moreover, the electronic, redox, catalytic and optical properties of the POM derivatives which can guide synthesis and material preparation have attracted more attention. In this thesis, quantum chemistry calculations have been performed to investigate redox properties and second-order nonlinear optical (NLO) response for a series of organic-inorganic hybrid tetranuclear endohedral vanadate and Lindqvist-type POM derivatives. The present work has been focused on the following four aspects:1. Density Functional Theory (DFT) calculations were carried out to investigate the polarization and redox-switching properties of tetranuclear endohedral vanadate. It indicated that the bowl-shaped vanadate with large polarizability can attach different guest anions, and polarize them to evoke electron transition originated from the guest to the host bowl. The introduction of different guest anions can generate distinct redox potentials, which enables the host-guest vanadate to be promisng redox-swicthing materials based on various guest anoins.2. The redox property and second-order NLO response for organoimido derivatives of Lindqvist-type molybdate has been investigated by DFT method. It was indicated that organic conjugated groups were much more sensitive to the redox property and nonlinear optical response than non–conjugated ones. The increment of organic conjugated group, that is the greater net charge donation between organic segment and inorganic polyanion cluster, drove the first reduction potential dramatically negative. In contrast, the lengthening of conjugated chain resulted in remarkable improvement on second-order nonlinear optical response than in reduction potentials.3. DFT method has been performed to character the family of 2-D dipolar NLO phenanthroline-POMs based on redox-switchable transition metal centers in POM segment. It is therefore apparent that the electron transition in phenanthroline-POMs exhibits largeβzyy tensor along y-axis to confirm better 2-D character with sizable anisotropy values. Interestingly, the addition of electrons into high-valence metal Mo in POM acceptor evokes dramatic enhancement in NLO response for reduction state by contrast with that for corresponding oxidation state, and compound IIared exhibits nearly 1019 times in second-order NLO response as much as compound IIa owing to the addition of the Ni atom. Therefore, the NLO activity of such compounds can be reversibly and effectively switched via the transition metal MoVI/V redox couple in hybrid redox-active phenanthroline-POMs, and these compounds have turned out to be very promising candidates for redox-switching NLO application and novel optoelectronic applications. |
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