Theoretical Study On Two-photon Absorption Properties And Aggregation Of Molecules

In the 1960s, the birth of laser promotes the rapid development of non-linear optical and makes it to be an important branch of modern optics. Nonlinear optics has more and more attracted people?s attention. 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 that 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 computional methods on the base of the ab initio level. Some structure-property relation is presented and the solvent effect on the geometrical structures and optical properties of the molecules are investigated. The research contains two parts: one is the study of the one- and two-photon absorption (OPA and TPA) properties of a series of newly synthesized organic molecules in gas phase, the other part is related to the aggregation effect on TPA properties of one-dimensional polarized molecules. The main contents and results are described as follows. I. The one- and two-photon absorption properties of organic molecules in gas1. The OPA and TPA properties of two newly synthesized compounds with fluorene asπcentre (noted as SK-G1 and NT-G1) are calculated using response theory at density functional theory (DFT) level. Firstly, we optimize the molecular structures at Hartree-Fock (HF) level. Then, we study OPA properties at DFT level and TPA properties using response functional method. The results show that, in low energy region, molecules have good OPA and TPA intensities, which are related to intramolecular charge transfer process.2. The OPA and TPA properties of a newly synthesized series of pyrene-core derivatives. At first, the molecular geometrical structures are optimized at HF level, and compounds display good coplanarity. 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.II. Aggregation effect on OPA and TPA properties of molecules In the experiment, nonlinear optical properties of molecules are measured in solution. Some organic polar molecules that contain stronger electronegativity atoms may have hydrogen bonds among them, which have influence on the structures and optical properties of molecules. The aggregation effect on OPA and TPA properties should be considered.We take one-dimensional asymmetric polar molecules as our research system. Firstly, we design the geometric structures of dimers and trimers formed through aggregation effect, then, TDDFT is applied to calculate the OPA properties. Finally, the TPA properties are studied utilizing the response functional theory at DFT level.The content of this thesis is as follows. There are eight 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 of studying the nonlinear properties are introduced, including the Born-Oppenheimer approximation, the Hartree-Fock approximation and the density functional theory. In the third chapter, the different approaches developed recently for calculating molecular two-photon absorption properties are summarized, including sum-over-state, few-state model, finite field and response functional theory methods. The fundamental ways of dealing with the solvent environment are discussed in the forth chapter. In the fifth chapter, we discuss two-photon absorption properties of two kinds of compounds with fluorene as centre. One-photon and two-photon absorption properties of a series of pyrene-core derivatives are studied in the sixth chapte.In the seventh chapter, aggregation effect on two-photon absorption properties of the asymmetric polar molecules are studied. The summary and prospect are presented in the last chapter.