The Theoretical Study Of Organic Molecules Multiple-Photon Absorption

Nonlinear optics develops rapidly since the birth of the laser. As an importantembranchment of the field of modern optics, nonlinear optics has more and moreattracted people?s attention. In recent years, scientists have shown considerable interestin exploring new nonlinear optical materials, due to their widely use and attractiveapplication foreground, for example, in laser frequency doubling, laser frequencymixing, parametric amplification and oscillation, integrated optics, opticalcommunication, beam steering, beam distortion removing, image multiplication andtransformation, optical limiting and threshold monitoring, complete optical connecting,optical computer and other fields. Much emphasis has been put on organic nonlinearoptical materials because they have many advantages which inorganic materials can?tpursue, such as wide response wave band, good flexibility, high optical damagethreshold, low cost and easy combination and modification.This thesis studies the optical properties of a series of newly synthesized organicmolecules utilizing different theoretical, computational approaches and solvent modelson the basis of the ab initio level. We discuss the solvent effects on the moleculargeometry structures and optical properties, investigate the relationship of the molecularstructures and optical properties. The main contents and results are represented asfollows.The nonlinear optical properties of one-dimension nonsymmetrical D-Ï€ -Acharge-transfer (CT) molecule BPYPA in gas phase and different solvents have beenstudied on the base of time dependent density functional theory. First, the geometry ofBPYPA molecule in gas phase is optimized on the base of density functional theory;thepolarized continuum model is used when studying the solvent effects, and the geometryoptimization of the molecule in each solvent is redo by using the polarized continuummodel on the base of density functional theory, the solvent effects on the geometry ofBPYPA molecule has been investigated. Then the nonlinear optical properties ofBPYPA molecule in gas phase and different solvents are studied using few-state modelapproach on the base of time dependent density functional theory. In previous study,only one charge-transfer state exists in the low frequency extent for the one-dimensionD-Ï€ -A nonsymmetrical CT molecules, and the two-state model approach can exactlydescribe the maximum two-photon absorption (TPA) properties of the lowest excitedstates, the results show that with BPYPA molecule , there exists two CT states in thelow frequency extent, so we use three-state model approach to calculate the TPA crosssection of this molecule and find its strong TPA properties. This is mainly due to whenthe molecule simultaneously absorbs two photon transits from the ground state to themaximum TPA state, there are two scattering processes, and the interference termbetween the two scattering channels has displayed a great contribution to the TPA crosssection. The TPA cross sections of the compound are enhanced in the solvents, this canbe explained by the fact that when the solute molecules are in solvents, the excitationenergies of the molecules are decreased, the transition dipole moments and the dipolemoment difference among the ground state and the first excited state are increased. It isalso observed the nonmonotonic behavior for the nonlinear optical properties withrespect to the polarity of solvents. In this paper, the hydrogen-bonding effect on thegeometrical structures and solvatochromic shifts has been studied, it is shown thehydrogen-bonding strongly affecting the nonlinear optical properties, so in order toprecisely describe the interaction between the solute compound and solvents, thehydrogen-bonding effect should not be neglected.The few-state model approach based on density functional theory and the responsetheory approach based on Hartree-Fock theory are adopted in previous studies of theTPA properties of molecules, however, the contributions of all intermediate states andthe electronic correlated energy haven?t being taken into account, and this two aspectseffect on the nonlinear optical properties of molecules are considered moderately byusing the response theory approach on the base of density functional theory. In the paper,two D-Ï€ -D symmetrical charge-transfer molecules are studied by using this approach,the large effects of the electronic correlated energy on the TPA cross sections of themolecules are observed. The numeric values of the TPA cross sections are in agreementwith the experimental results.The nonlinear optical properties in the gas phase of a series of multi-branchedorganic molecules have been studied in this paper. First, the geometry of thesecompounds are optimized on the base of density functional theory, then the TPAproperties of these molecules are studied by using the response theory approach basedon the Hartree-Fock theory. The results show that the TPA intensities are enhancedeffectively by augmenting the dimension of the molecules and by modifying the centerswhich connect conjugated chains. The TPA cross sections of the molecule1-(4-bromobenzal)-3,5-bis(4-((E)-2-(pyridin-4-yl)vinyl)phenyl)benzene in differentsolvents are calculated by using Onsager response theory approach, and the resultsindicate that, with the increase of polarity of the solvent, the excited energies arered-shifted and the two-photon absorption cross sections augment, which is consistentwith the conclusion of Luo et al.The content of this thesis is as follows. The first chapter gives a brief introductionof the developing situation of the nonlinear optics and the nonlinear optical materials.In the second chapter, some quantum chemistry theories of studying the nonlinearproperties are introduced, including the Born-Oppenheimer approximation, theHartree-Fock approximation, the density functional theory and the time-dependentperturbation theory. In the third chapter, the different approaches developed recentlyfor calculating the nonlinear optical properties of molecules are discussed. Theresearch process and the results are given particularly and the results are discussed andcompared with the experimental outcomes in the fourth chapter. The summary and theprospect are represented in the last chapter.