Theoretical Study Of Nonlinear Optical Properties On Dodecavanadate And Lindqvist-type Polyoxometalate Derivatives

Nonlinear optical (NLO) materials have the potential application in the field of opticalmemory, optical modulation, optical information processing, molecular switching, andfrequency doubling. Therefore, the design and synthesis of compounds with large nonlinearoptical response are still focuses for many scientists. Polyoxometalates (POMs) and theirderivatives with special physical and chemical properties have attracted extensive attention inrecent years. In particular, Su and co-workers have shown great interests in the nonlinearoptical properties of a series of organic-inorganic polyoxometalate derivatives. The resultsshow that the proper modification for POM can effectively improve the molecular nonlinearoptical response.In this thesis, density functional theory (DFT) calculations have been performed toinvestigate the electronic spectra and second-order NLO response for [V12O32]4-andLindqvist-type POM derivatives. The present work has been focused on the following threeaspects:1. The geometry structures and NLO properties of [V12O32]4-,[V12O32M]3-(M=Li, Na,K) and the protonated derivatives have been investigated by DFT method. The results showthat the alkali metal cation M+has little effect on the geometry structures and the firsthyperpolarizabilities (β0) of the systems. However, the different positions and differentorientations of the protons have great impact on the β0value. The β0values of systems withthe central bridging oxygen atoms protonated, exhibit a significant increase than that of[V12O32]4-.2. The effect of the position for tricarbonyl metal ligand and different metals on theelectronic absorption spectra and second-order NLO properties have been calculated for theLindqvist-type polyoxometalate complexes [Nb2W4O19M(CO)3]3-/2-(M=MnI, TcI, ReI, FeII,RuII, OsII) by DFT method. Among the three isomers of [Nb2W4O19Mn(CO)3]3-(system1),isomer a, the binding sites between M(CO)3and POMs is formed by an ONb2oxygen and twoONbW oxygens; in isomer b, two ONbW oxygens and an OW2oxygen are involved; and inisomer c, three OW2oxygens form the binding sites. The β0value of isomer1ais the largest,which is1.4times and2.3times larger than that of isomer1band1c, respectively. The β0values also depends on the metals M, the β0values show an order of β0(Ru)> β0(Re)> β0(Fe)> β0(Os)> β0(Mn)> β0(Tc). In addition,[Nb2W4O19Ru(CO)3]2-has the largest hyperpolarizability β0value,16.6×10-30esu. The analysis on main transition orbitals show thatthe tricarbonyl metal ligand acts as an electron donor in all complexes except[Nb2W4O19Fe(CO)3]2-, in which [Fe(CO)3]2+ligand acts as the electron acceptor.3. The electronic structures and NLO properties of [Mo6O17(NC8H8R1)(NC10H6R2)]2-(R1=H, NO2; R2=H, CH3, NH2, NH(CH3), N(CH3)2) have been investigated by DFT method.The β0values largely depend on the electron-donating ability of the substituent group. The β0value of [Mo6O17(NC8H8NO2)(NC10H6N(CH3)2)]2-is3times larger than that of[Mo6O17(NC8H9)(NC10H7)]2-, showing that the modification of electron-donating groupincreases the NLO responses of the investigated systems. By analyzing the frontier molecularorbitals of all systems, we found that the highest occupied molecular orbital (HOMO) islargely localized on the naphthalene and HOMO-1is largely localized on thedimethylbenzene. Furthermore, the results show that the charge transfer (CT) from thenaphthalene and dimethylbenzene to the hexamolybdate cluster made mainly contributes tothe NLO properties of all systems.