Theoretical Study On Two-photon Absorption Properties Of Organic Conjugated Molecules

In recent years, scientists have shown considerable interest in exploring new nonlinear optical materials, due to their widely use and attractive application foreground, for example, in laser frequency doubling, laser frequency mixing, parametric amplification and oscillation, integrated optics, optical communication, beam steering, beam distortion removing, image multiplication and transformation, optical limiting and threshold monitoring, complete optical connecting, optical computer and other fields. Much emphasis has been put on organic nonlinear optical materials because they have many advantages, such as large nonlinear optical coefficient, wide response waveband, good flexibility, high optical damage threshold, low cost, and easy combination and modification. The rapid development of modern quantum chemical theory at ab initio level has also led to the rapid development of theoretical study on nonlinear optical properties of molecular materials. Theoretical studies not only can forecast the nonlinear optical properties of the already existed molecular materials, but also can easily design many new molecular materials with exceptional properties and provide guide lines for experimental syntheses.The thesis studies the linear and nonlinear optical properties of a variety of newly synthesized organic conjugated molecules and platinum acetylide complexes utilizing theoretical and computational approaches on the base of the ab initio level, discusses the relation of the molecular structures and properties.The whole works contain two parts: one part is the opetical studies on nonlinear optical properties of the series of new (E)-4, 4 '- bis[diphenyl-amino] styrene (BDPAS)derivatives . The second part is the study of two-photon absorption properties of triphenylamine-branching chromophore in gas.The main contents and results are represented as follows. 一,The one- and two-photon absorption properties of series of BDPAS derivatives in gas. At the hybrid density functional theory (DFT), the one- and two-photon absorption cross sections of series of BDPAS derivatives by employing few-state model method.1. The one- and two-photon absorption properties of one-dimensional organic molecules. The results indicate that the TPA activities are related to some factors including molecular conjugated length, the strengths of electron-donor (D) and electron-acceptor (A) for one-dimensional organic molecules When the molecular conjugated length is lengthened, the TPA activity is highly increased.2. The two-photon cross sections of multi-branched organic molecules. By analyzing the TPA actions of multi-branched molecules, one can see that the length of branch, the structure of the branch and the center atom of molecule have a largely effect on the one- and two-photo absorption activities. The charger-transferred processes of the charge-transferred states are displayed in order to understand their maximal two-photon absorption cross sections.二,The study of two-photon absorption properties of triphenylamine-branching chromophore in gas.The simulation shows that the two compounds have strong TPA characteristics and are thus good candidates as nonlinear optical materials. And the two-photon absorption cross section of multi-branched organic molecule is stronger than that of molecule. The numerical calculations are of good agreement with the measurement.The work is done on DALTON and GAUSSIAN-98 program packs.The content of this thesis is as follows. The first chapter gives a brief introduction of nonlinear optics and also the developing process of the nonlinear optics and nonlinear optical materials. In the second chapter, some basic of quantum chemical theory is introduced, which mainly focuses on Hartree-Fock approximation and density functional theory. The different approaches developed recently for calculating molecular nonlinear optical properties are summarized in the third chapter, including sum-over-state, few-state model response theory methods. The research process and the results of the series of new (E)-4, 4 '- bis [diphenyl-amino] styrene (BDPAS) are given particularly and the results are discussed and compared with the experimental results in the forth chapter. Nonlinear optics properties are studied triphenylamine-branching chromophore in the fifth chapter. The summary and prospect are represented in the last chapter.