The Theoretical Study On Nonlinear Optical Properties Of Keggin-type Polyoxotungstate And Anderson-type Polyoxomolybdate Derivatives

olyoxometalates(POMs), which are made up of transition metals, such as Mo6+, V5+, W6+, and linked by oxygen bridges. Due to the variety, excellent chemical stability and unique electronic structure, they show great promise in a diverse range of fields such as medicine, catalysis, electronics and optics. Moreover, the covalent attachments of organic or organometallic groups to the POMs via linkages constitute an extremely interesting class of organic-inorganic hybrid polyoxoanions, which combine the advantages of organic materials so as to realize the so-called “value-added properties”, and have promised dramatic applications in nonlinear optical(NLO). It has been received increasing attention. Importantly, the core issues of NLO materials are searching methods to enhance and switch the molecular NLO response. Quantum chemical calculations would be very helpful to establish the structure-property relationships as well as the design of novel POM-based materials with functional properties by exploiting the unique electronic and structural characteristics of these complexes.In this thesis, the density functional theory(DFT) and time dependent DFT(TDDFT) methods have been carried out to explore the electronic properties and the second-order NLO properties based on Keggin-type polyoxotungstates and Anderson-type polyoxomolybdates. The purpose of present thesis is that:(1) to understand the electronic structures of POMs.(2) to analyze the relationship between transition properties and second-order NLO responses.(3) to investigate the effects of substituted atom or group and obtain the switchable NLO molecules based on redox properties, and stimulate experimental research in the field of NLO materials for obtaining excellent switchable NLO materials.The first part of this thesis is a review of electronic structure, and related experimental and theoretical studies for POMs. The second part is a brief introduction of computational methods for calculating NLO coefficients. The third part is the main body of this thesis, and the present works include the following five aspects:1. The electronic properties, redox properties and second-order NLO properties of organic-inorganic hybrid polyoxoanion [LNb OEMe3]3-, [LNb OEPh3]3-(L = α-{PW11O39}7-, E = Si, Ge, Sn, Pb) were investigated by DFT and TDDFT. Geometric structure and element substitution effects on the molecular nonlinear response have been analyzed, and the computed β0 values depend on both E and the organic groups connected with E. Furthermore, the character of charge transfer transition indicates that the Keggin-type POM acts as an electron acceptor and the organic group acts as an electron donor. Our results show that this kind of organic-inorganic hybrid compound possesses remarkably large molecular second-order NLO polarizability and might be an excellent second-order NLO material.2. The static second-order NLO responses of a series of lacunary γ-Keggin polyanion derivatives, [XM2W10O38(μ-OH)2]n–(X = SiIV, GeIV, PV, AsV, M = AlIII, TiIV, VV; X = ZnII, VV, M = AlIII), were investigated by using DFT methods. The results show that this class of inorganic compounds has evidently large static second-order polarizability, especially for [γ-As Al2W10O38(μ-OH)2]7-, which has a computed β0 value of 47206 a.u.. Furthermore, the molecular NLO activity of Keggin POM derivatives can be modified by replacing the central heteroatom(X) and the substituted addenda metal atom(M). When the central heteroatom X is same, the replacement of M with three elements(Al, Ti, V) significantly alters the β0 values, which decrease as M changes in the order Al > Ti > V; when M is Al and X is from the same row of the periodic table, the β0 values decrease with the increasing atomic radius and increase with the increasing electronegativity. For polyanions [XM2W10O38(μ-OH)2]n–,(M = Ti and V), the replacement of X slightly affects the β0 values.3. The second-order NLO properties of a series of hybrid complexes via coordination interaction between porphyrin and Keggin-type α-[MSi W11O39]3-(M = NbV or VV), were investigated by TDDFT. The calculated results show that this kind of organic-inorganic hybrid compound possesses remarkably large molecular second-order NLO polarizability, especially for the Zn P3P-C≡C-4-Py-[VSi W11O39]3-, which has a computed β0 value of 261410 a.u., and might be an excellent second-order NLO material. Furthermore, the effects of the substituted metal atom(M), the metalloporphyrin, and the Ï€-conjugation on the NLO response were analyzed, the metalloporphyrin and the lengthening of conjugation are helpful in enhancing the optical nonlinearity, which reveals the general rules to design the systems with large optical nonlinearity.4. The quantum chemical study on the relationship between reversible redox properties and second-order NLO responses has been firstly performed on photochromic organic-inorganic hybrid complex by using TDDFT method. The results reveal that the successive redox process of system SP is Mored-MnII-spirored(SP3+)??®???MoVI-MnII-spirored(SP2+)??®???MoVI-MnII-spiro(SP1+)??®???MoVI-MnIII-spiro(SP)??®???MoVI-MnIV-spiro(SP1-)??®???MoVI-MnIV-spiroox(SP2-), which well agrees with the experimental and theoretical forecast by structure change. Furthermore, it is noticeable that the second-order NLO behaviors can be switched by the reversible redox of studied systems. The β0 value of two-electron-oxidized system, SP2-, is largest and is ~48 times as large as that of system SP. The absorption bands possess remarkably shift with the reversible and manipulable redox states. The dominant electron transitions for studied systems are also significantly changed by reversible redox. Therefore, this kind of complex with the facile and reversible redox states might become excellent switchable NLO material.5. We have designed a series of metalloporphyrin and polyoxometalates complexes to obtain novel high performance dyes for p-DSSCs. The electronic properties, absorption spectra, electronic transition properties and photovoltaic performance of designed dyes have been discussed by TDDFT methods. The results show that the designed dyes 2-6 are p-type dyes as their HOMO energies are lower than the VB of Ni O, and their LUMO energies are higher than I-/I3-. Owing to the charge transfer form organic group to POM cluster, the absorption spectra of dyes 2-6 exhibit stronger and broader absorptions in the UV/Vis region of the solar spectra compared with dye 1. The LHE of dyes 2-6 are similar with the range of 0.98-0.99, and higher than other reported dyes. The present work is supposed to be helpful for the design of metalloporphyrin and POM dyes with target properties to improve the performance of dye-sensitized solar cells.