Theoretical Study Of Electronic Properties In Keggin-type Polyoxometalate Derivatives

Polyoxometalates (POMs) are a rich and diverse family of metal-oxygen clusters made up of early transitional metals with unique photonic, electronic, and magnetic properties and chemical reactivity that has promised dramatic applications in quite diverse disciplines, including catalysis, medicine, and materials sciences. Following the development of the computer technology, quantum chemistry calculation as a theoretical study method has been introduced to the field of POMs. In early time, the ab initio Hartree-Fock (HF) approximation provided a reasonable starting point for understanding structures and electronic properties of POMs. However, it is necessary to analyze systematically molecular and electronic structures, redox, catalysis and optics of POMs, consequently these could direct synthesis and material preparation. In the last ten years, a few groups have been especially active and have made important progress in describing and rationalising the electronic and magnetic properties of POMs. But a combination of three factors—the large size of polyoxoanions, the presence of transition metal ions and the high negative charge—produce, in practice, important computational limitations. The ab initio and density-functional theory (DFT) modeling of polyoxoanions is still incipient.In this thesis, quantum chemistry calculations have been carried out to investigate electronic properties, stability, protonation, redox, bond character, and the second-order nonlinear optical (NLO) properties of a series of Keggin-type POM derivatives. The present work has been focus on the following five aspects:1. The electronic properties, redox properties, protonation, and stability of five [α-PTi2W10O40]7– isomers have been investigated employing DFT method. The results reveal that the stability of [α-1,2-PTi2W10O40]7– and [α-1,6-PTi2W10O40]7– is weaker and the redox ability is higher among five isomers, while for [α-1,5-PTi2W10O40]7–, [α-1,4-PTi2W10O40]7–, and [α-1,11-PTi2W10O40]7– the stability is higher, but the redox ability is weaker. At the same time, Ti-substituted systems are preferentially reduced in the tungsten centers. An analysis of molecular electrostatic potential maps of [α-PTi2W10O40]7– isomers suggests that the preferred protonation site corresponds to bridging oxygens (OTi2 and OTiW) and terminal oxygens (OTi), especially bridging oxygens bonded to titaniums (OTi2) in [α-1,2-PTi2W10O40]7– and [α-1,6-PTi2W10O40]7– with Cs symmetry. By means of total bonding energies of [α-PTi2W10O40]7– isomers, the relative sequence of stability has then been shown to be [α-1,4-PTi2W10O40]7– > [α-1,5-PTi2W10O40]7– > [α-1,11-PTi2W10O40]7– > [α-1,2- PTi2W10O40]7– > [α-1,6-PTi2W10O40]7–. In addition, the one-electron-reduced species of [α-PTi2W10O40]7– are also discussed. 2. DFT calculations were carried out to characterize the optimal site of the protons and the precise protonation state in the POM anions [V13O40]15– and [H12V13O40]3–. Six kinds of possible protonated stable isomers with the whole Keggin anion units are discussed. The calculations reveal that the preferred protonation site corresponds to bridging oxygens that belong to the same trimetallic group (isomers B and C). Both isomers B and C are comparatively stable in the gas phase, but only isomer B could exist stably in aqueous solution because of being stabilized by the electrostatic interaction. The solvent effects and protonation are also discussed.3. Systematic DFT calculations have been carried out on the lacunaryα-Keggin POM derivatives [PW11O39]7–, [XW9O34]n– (X = AlIII, SiIV, GeIV, PV, AsV, and SbV), [XW9M2O39]n–, and [XW9M3O40]n– (X = PV and SiIV, M = MoVI, VV, NbV, and TaV) to investigate the geometric structure and element substitution effects on the molecular NLO response. Analysis of the computed static second-order polarizability (β0) predicts that the molecular NLO activity of lacunary Keggin POM derivatives can be modified by replacing the central heteroatom and the addenda metal atom. Substitution of the central Al atom or the addenda V atom causes significant enhancement in the molecular nonlinearity. Moreover, theβ0 values are substantially dependent on the defect structures. This class of inorganic complexes possesses remarkably large molecular optical nonlinearity, especially for the partial substitution complex [SiW9Nb2O39]10–, which has a computedβ0 value of 2071.0 a.u.4. The dipole polarizabilities, second-order polarizabilities, and origin of second-order NLO properties of trisorganotin-substitutedβ-Keggin POM [XW9O37(SnR)3](11–n)– (X = P, Si, Ge, R = Ph; X = Si, R = PhNO2, PhC≡CPh) have been investigated by using time-dependent density functional response theory. This class of organic-inorganic hybrid complexes possesses a remarkably large molecular second-order NLO response, especially for [SiW9O37(SnPhC≡CPh)3]7– with the static second-order polarizability (βvec) computed to be 1569.66×10–30 esu. Thus, these complexes have the possibility to be excellent second-order NLO materials. Analysis of the major contributions to theβvec value suggests that the charge transfer from the heteropolyanion to the organic segment along the z-axis plays the key role in the NLO response of [XW9O37(SnR)3](11–n)–. The computedβvec values increase as a heavy central heteroatom changes in the order Ge > Si > P. Furthermore, nitro substitution on the aryl segment and the lengthening of organostannicπ-conjugation are more important in enhancing the optical nonlinearity, especially for the latter factor.5. In this work, the relationship between the reversible redox properties and the second-order NLO responses for [PW11O39(ReN)]n– (n = 3–7) has been systematically investigated by using TDDFT method combined with the sum-over-states (SOS) formalism. The results reveal that the successive reduction processes of five PW11ReN redox states should be PW11ReVII→PW11ReVI→PW11ReV→PW11ReV1e→PW11ReV2e. Furthermore, their electrochemical properties have been reproduced successfully. It is noteworthy that the second-order NLO behaviors can be switched by reversible redox for the present studied complexes. Full oxidation constitutes a convenient way to switch off the second-order polarizability. The incorporation of extra electrons causes significant enhancement in the second-order NLO activity, especially for the third reduced state, whose static second-order polarizability (βvec) is about 144 times larger than that of fully oxidized 1. The characteristic of the charge-transfer transition corresponded to the dominant contributions to theβvec values indicates that metal-centered redox processes influence the intra-molecular donor or acceptor character. Therefore, this kind of complexes with the facile and reversible redox states could become excellent switchable NLO materials.