Theoretical Study On Two-photon Absorption Properties Of New Factional Organic Molecules

The birth of the first ruby laser has opened a new era of nonlinear optics and made it quickly developed into an important branch of modern optics. In recent years, scientists have shown considerable interest in exploring new nonlinear optical materials, thanks to their widely use and attractive application foreground, such as, 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. Meanwhile, much emphasis has been put on organic nonlinear optical materials because they have many advantages which inorganic materials can?t pursue, such as wide response wave band, high optical damage threshold, well flexibility, low cost and easy combination and modification. Solvent effects are important factors to molecular nonlinear optical properties. Recently, experimental and theoretical results have shown that the aggregation effect among the polar molecules has important effect on two-photon absorption properties. Aggregation effect is thus a hot topic in the community.The thesis studies the linear and nonlinear optical properties of various newly synthesized organic molecules utilizing quantum chemical computational methods at ab initio level. Some structure-property relations are presented. Substitutions and conjugated bridge effect on the geometrical structures and optical properties of the molecules are investigated. The main work of this article is the study of one- and two-photon absorption (OPA and TPA) properties of a series of newly synthesized organic molecules in gas phase. The main contents and results are described as follows.1. The OPA and TPA properties of five newly synthesized compounds with pyrrole ring as a centre are calculated using response theory at density functional theory (DFT) level. Firstly, we optimize the molecular structures at B3LYP level. Then, we study OPA properties at DFT level and TPA properties using response functional method. The results show that, in low energy region, the optical properties of the molecules are related to the substitutions on the end, in which para-substituted and meta-substituted cases can effectively enhance the capacity of one-photon and two-photon absorption.2. The OPA and TPA properties of a newly synthesized series of diphenyl pyrrole core derivatives. At first, the molecular geometrical structures are optimized at HF level, and the main bodies of molecules hold good planarity. The time-dependent density functional theory (TDDFT) is applied to calculate the OPA properties of the compounds. The TPA calculations of the compounds are completed utilizing the response functional theory at DFT level. The calculated results show that, in the optical region, the series of molecules all display good OPA and TPA intensities. The two-photon absorption cross section is dramatically increased when the substitute is changed from the bromine to the diphenylamine or triphenylamine; in addition, the properties of OPA and TPA are great related to the planarity of molecules and conjugated bridge. The molecule with planarity and three-bond displays the better properties of TPA.The content of this thesis is as follows. There are six chapters. 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 quantum chemistry theories for studying Multi-electron system are introduced, these theories include the Born-Oppenheimer approximation, the Hartree-Fock approximation and the density functional theory. The different approaches developed recently for calculating molecular two-photon absorption properties of studying the OPA and TPA properties and aggregation effect on two-photon absorption properties are discussed in the third chapter, the different approaches include sum-over-state, few-state model, finite field and response functional theory methods. In the forth chapter, we discuss two-photon absorption properties of five kinds of compounds with pyrrole ring as a centre. One-photon and two-photon absorption properties of a series of diphenyl pyrrole derivatives are studied in the fifth chapter. The summary and prospect are presented in the last chapter.