| Materials with strong two-photon absorption (TPA) are currently attracting considerable attention in the chemistry, photonics and biology communities because of their potential applications in optical power limiting, upconverted lasering , three-dimensional datastorage ,nondestructiveimaging and photodynamic therapy . However , the development of two-photon technology is confined because of the small TPA cross sections of materials. Recently, with the development of experiments, numerous organic materials that exhibit large TPA cross sections have been synthesized in different laboratories. The results of experiments show that the most efficient TPA materials seem to be the ones with various electron-donor (D) and electron-acceptor (A) moieties attached symmetrically or asymmetrically to a π center.In order to understand the structure-property relationship of materials with large TPA cross sections and examine the TPA properties of these molecules, a large number of theoretical models are developed to calculate TPA cross sections of molecules . Theoretical calculations can not only predict the two-photon absorption intensity but also provide guideline and help for synthesis of new materials with large TPA cross sections. We have calculated one- and two-photon absorption cross sections of the lowest excited states of a series of molecules combined with benzene, stilben, thiophene as π center attached with amine, diphenylamine, diethylamine as electron-donor and nitryl as electron-acceptor; the effects of molecular length, n center and electron-donor on two-photon absorption cross sections have been studied and all calculations have been carried out using the density functional theory at an ab initio level .It is found that the molecular length and the one-photon absorption intensity are quite strongly correlated factors, and that a corresponding correlation for the two-photon absorption is decreasing .It is also found that a most crucial role for the two-photon absorption is played by the n center. For molecules with a given n center strong donors can result in the increasing of TPA cross sections. By caculation the D-A-A-Dmolecule which is constituted of bipyridine inserting in the middle of stilbene as π center exhibits a maximum TPA cross section .Nowadays, many experimental measurements of TPA cross sections of organic molecules take place in solutions. When molecules are solvated in the solution, the solute molecule polarize solvent molecules around it and this gives rise to a field reaction field acting back-OB-the molecule ,which will have effects on the structure of solvated molecules and their optical properties. So ,in order to explain the experimental results and design strong two-photon absorption molecules , solvent effects must be considered. New theoretical models for calculations of solvent effects must be developed.The solvent effects are carried out by the self-consistent reaction field theory with the polarizable continuum model in this work. The results show that the solutions hardly change the structure of molecules, but have a significant effect on the two-photon absorption of molecules. Furthermore ,with the increase of the polarity of solvent, the excited energies is decreased and the TPA cross section is increased. The stronger the electron-donor is, the more significant the solvent effect is for molecules with a given π center.The structure of this thesis is as follows. In the first chapter, nonlinear optics and the research of two-photon absorption materials are introduced. In the second chapter, we introduce the nonlinear optical properties of molecules. We also introduced the time dependent perturbation theory in this chapter in order to understand the nonlinear optical properties of molecules. When the molecule is solvated in the solution ,the solutions have effects on the structure and optical properties of the molecule. The solvent effect is introduced in the third chapter. In the fourth chapter, we introduce the Density Functional Theory. Reseacher steps...
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