Theoretical Studies On The Nonlinear Optical Properties Of Lindqvist-type Polyoxometalates/carbon Nanotube Composites

Polyoxometalates （POMs） are a rich class of inorganic cluster systems and exhibitremarkeable chemical and physical properties, which have been applied to a variety offields, such as chemistry, analytical chemistry and materials science. With thedevelopment of research, quantum chemistry became the effective method to investigatethe electronic structures and properties of POMs. The quantum chemistry method explainsome phenomenon at microscopic level that can not be explained in experiment. However,the POMs containing many metal （transition） atoms and high negative charges are verycomputationally demanding. With the development of the density functional theory （DFT）and computer technology, the computational precision and speed for the POMs havegreat improvement.Carbon nanotubes （CNT）, are a unique calss of materials that have received anenormous amount of attention over recent years because of their remarkable mechanical,thermal, electronic, and optical properties. However, a number of practical aspects suchas purity, solubility and processability still hinder their widespread exploitations. Greateffort has recently been exerted with regards to their chemical treatment andfunctionalization to overcome these problems and the production of more manageablemultifunctional materials. Depending on the means of structural modificationfunctionalizations of CNT can be categorized in three general groups （1） sidewallcovalent attachment of chemical groups （2） noncovalent adsorption （wrapping） of variousfunctional molecules and （3） filling of the inner empty cavity. The experimental andtheoretical researches on nanotubes have achieved good improvement.DFT methods have been performed to study the electronic properties andsecond-order nonlinear optical coefficients for POMs/CNT complexes. Moerover, thebonding feature and optical properties of the Lindqvist-type [Mo₆O₁₉]^2-derivatives thatcontain photoisomerized chromophore DTE have been studied.The first part of the thesis is a simple overview of the basic properties of POMs andnanotubes, and the synopsis of DFT. The second part is the main contents of the thesis,which contains the following three aspects:1. The nonlinear optical （NLO） properties of [Mo₆O₁₉]^2-decorated single wallcarbon nanotubes （SWCNT） have been investigated by density functional theory （DFT） method. These special organic-inorganic hybrids possess a large second-order NLOresponse. It is noteworthy that the second-order NLO behaviors can be switched by thevariable angle between [Mo₆O₁₉]^2-and CNT. The characteristic of the charge-transfertransition corresponding to the dominant contributions to the β_vecvalues indicates that thevariable anchoring angles of [Mo₆O₁₉]^2-influence on the intramolecular donor oracceptor character. When the anchoring angle of [Mo₆O₁₉]^2-becomes30°, the complexexhibits the largest static second-order polarizibility (β_vec); In addition, thepolyoxometalate cluster acts as the acceptor and the CNT as the donor. Furthermore, theincorporation of an electron donor （the amido group） leads to a larger β_vecvalue at the endof the CNT segment.2. The confinement effect of the hydrogenated （n, n） armchair single-walled CNTswith n=9,10,11on the second-order NLO properties of Lindqvist-type [[M₆O₁₉]^2-（M=Mo, W） anion inside and outside CNTs have been investigated by DFT method. Theresults reveal that the electron of POMs were redistributed and the electron transfer from[[M₆O₁₉]^2-to CNTs. In addition, the second-order NLO properties are dependent on thediameter of CNTs and the position of POM. Furthermore, the second-order NLOcoefficients are increased with increasing diameter of CNTs, and the positions of POMsinfluence it seriously. When the [Mo₆O₁₉]^2-is outside （11,11） CNTs, the system exhibitsthe largest β_vecvalue is-727.70×10^-30esu.3. The switchable second-order nonlinear optical （NLO） responses of thephotoisomerized chromophore dithienylperfluorocyclopentene （DTE） derivatives,organic-inorganic systems of [Mo₆O₁₉]^2-, have been investigated by tuning open-ring andclosed-ring form. In present paper, we performed density functional theory （DFT）combined with finite field （FF） methods to calculate the second-order NLO coefficientsfor these organic-inorganic compounds. The calculations with three functionals confirmthe switching behavior on NLO properties by the photoisomerization reaction. The β_totvalue of system2c （closed-ring form） is10times larger than that of its open-ring form（system2o）. And the other two pairs of systems also show good tuning properties. Theampliative ratio on second-order NLO coefficients between systems2o and2c (β_2c/β_2o) is13times as large as that of DTE (β_DTEc/β_DTEo). It suggests that the introduction of[Mo₆O₁₉]^2-and organic groups to the DTE monomer effectively improve the conversionratio of second-order NLO coefficients between the open-ring and closed-ring forms.