Theoretical Studies On The Electronic Structures, The Nonlinear Optical Properties, And The Ecd Spectra Of Chiral Lindqvist-type Polyoxomolybdate And Divacant Keggin-type Polyoxotungstate Derivatives

Polyoxometalates （POMs） are a kind of inorganic clusters composed of early transition metal oxides in their high valent states （usually W^Ⅵ, Mo^Ⅵ, and V^Ⅴ）. Due to the various molecular structures and marvelous physicochemical properties, POMs have been applied in many scientific research fields, such as chemical catalysis, electromagnetic materials, nano-materials, chiral materials, nonlinear optical （NLO） materials, and biomedicine area. In rencent years, organometallic frameworks based on POMs become the hot spots in POM science. People continuously exploit and take advantages of the high quanlities of POMs. In the mean time, the development of quantum mechanical methods aids the studies of POM-based complexes. Density functional theory （DFT） method has been the representative for calculating POMs and their derivatives. Theoretical calculations have been successfully applied to study the electronic properties, redoxation, catalysis mechanisms, and NLO properties. More recently, POM-based chiral molecules and crystal materials are successfully designed and synthesized, which provokes a new research field about POM science. However, the theoretical studies on the chiralities of POM-based complex are relative fresh and seldom.In this thesis, quantum chemical calculations have been used to investigate the electronic properties, the second-order polarizabilities, and the electronic circular dichroism （ECD） spectra of a series of organic group substituted Lindqvist-type molybdates and phosphonate substituted divacant Keggin-type silicon tungstates. The work focuses on the following several aspects:1. DFT methods have been used to investigate the electronic structures, the NLO properties, and the bonding features of pentamethylcyclopentadienyl （Cp^*） substitutied Lindqvist-type molybdates. The results show that the high orbital mixing effects stabilize the cis-isomer of Cp₂^*Mo₆ anion. In Cp^*Mo₆, Cp₂^*Mo₆, and Cp^*TiMo₅ anions, the center oxygen atoms deviate much from the molecular center, bond length alternations, and extra surface charges tend to extend to trans-site terminal oxygen atoms, which are explained well by trans-influence model. The Cp^* fragments modify the HOMOs of Cp^*Mo₆ and Cp₂^*Mo₆ anions. Owning to the double-electron occupiedσ-bonding andσ_-^*antibonding orbitals, the bond order of Cp_-^*Mo is ascribed to twoπbonds. In addition, Cp^*Mo₆ and Cp₂^*Mo₆ anions have smaller second-order polarizabilities than [Mo₆O₁₈NPh]^2- due to the weaker intramolecular charge transfers. Our work is helpful to understand the stabilities and reactivities of Cp^* substituted POMs in the experimental processes. The bonding features of Cp_-^*Mo are disclosed, which promotes the studies on novel POM-based organic-inorganic hybrid materials.2. The long-range corrected hybrid functional CAM-B3LYP is employed to calculate the electronic properties and the ECD spectra of bisarylimido bearing o-alkoxy chain substituted Lindqvist-type molybdates. The ECD spectra of POM-based complexes are firstly studied by using time-dependent density functional theory （TDDFT）. The simulated and the experimental ECD spectra agree well with each other after the predicted spectra are blue-shifted by 0.8 eV. The o-alkoxy chain affects the signs and magnitudes of rotatory strengths, and slightly affects the absorption sites of the ECD spectrum. The low-lying ECD spectra of studied systems are assigned to charge-transfer transitions fromπ-conjugated fragments to POM cages, while the high-lying parts due to the combinations of charge-transfer transitions fromπ-conjugated fragments to POM cages and those located on POM cages. The o-alkoxy chain acts as scaffold for generating chirality, rather than contributes to the rotatory absorptions. Chirality transfers are observed in these complexes for the rotatory absorptions corresponding to the electronic transitions completely localized on POM cages. CAM-B3LYP functional proves generally well to predict the excitation energies of POM derivatives. Provided enough computational resource, basis sets that are larger than LANL2DZ are suggested to be used in ECD calculations for POMs.3. The static first hyperpolarizabilities and the ECD spectra ofΛ-type chiral compounds which are composed of Lindqvist-type polyanions linked by 1,1’-binaphthyl throughπ-bridge arylimido are investigated by using TDDFT method. The structure-property relationship of these compounds is presented. The main contributions are as follows:ⅰ) The smaller torsion angle between two naphthyl rings enhances the second-order NLO responses of studied compounds as the torsion angle affect the delocalized degree of compound. Comparing with electron acceptor, electron donor significantly enlarges the first hyperpolarizabilities of studied compounds.ⅱ) The substituents on 2,2’-site of 1,1’-binaphthyl two-fold control the second-order NLO responses of studied compounds by the nature of electron acceptor or donor and constraining the torsion angle between two naphthyl rings. An outstanding NLO compound is obtained when the substituent is ferrocene. Further, theseΛ-type compounds are potentially 2D-NLO materials owing to their large "in-plane nonlinear anisotropy".ⅲ) The ECD calculations show that the simulated ECD spectra are reliable for having an excellent comparison with the experimental CD spectra. For the bonding of POM clusters to 1,1’-binaphthyl-based precursors, additional ECD bands are observed and assigned to the CT transitions from 1,1’-binaphthyl andπ-bridge fragments to POM cages. Some different electron transition origins for the counterpart ECD bands of compounds 1 and 2 are found, which are responsible for their different ECD spectra observed in experiment. Our work highlights a novel multifunctional organic-inorganic POM hybrid, which are promising in NLO material researches due to their controllable chiralities, electronic transition properties, and second-order NLO responses.4. The optical activities and the NLO properties of divacant Keggin-type polyoxotungstate bearing chiral organophosphonate are theoretically investigated by using TDDFT method, which presents a synchronously chiroptical and NLO switch triggered by multistep redox. The ECD calculations on Boltzmann weighted conformations of three oxidation states of this chiral POM clearly present a chiroptical switching process. The electronic transition and the bond-length alternation studies show that the chirality transfer from chiral carbon atoms to POM cage increases as the polyanion is reduced. Simultaneously, the static first hyperpolarizability of studied chiral POM increases by more than two times from the oxidized state to the 1e-reduced state, and is further doubled to the 2e-reduced state, which is mainly due to the increasing electronic-dipole-allowed d-d charge transfer transitions in POM cage. This work firstly reproduces the ECD spectrum of chiral POM with high accuracy and proves the possibility for confirming the molecular conformations of flexible chiral POMs in solution by the aid of ECD calculations. Most importantly, a sensitive diplex switch based on chiral POM is predicted in theory, which helps the experimentalists design novel POM-based switches.