Theoretical Study On Two-photon Absorption Properties Of Double-conjugated-segment Organic Molecular

Nonlinear optics is the study of the interaction of intense laser light with matter, and it is a rapidly developed subject since the birth of the laser. As the important embranchment of modern optics, nonlinear optics has more and more attracted one's considerable attention. It has become an important project to find new nonlinear optical materials with strong nonlinear optical properties and quick response velocity due to their attractive applications, for instance, in modern laser technology, optical communication, data storage, optical information processing, 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 wave band, good flexibility, high optical damage threshold, low cost, and easy combination and modification.The thesis studies the linear and nonlinear optical properties of a variety of newly synthesized organic molecules utilizing theoretical and computational approaches on the base of the ab initio level, discusses the relation of the molecular structures and properties, and investigates solvent effects on the molecular geometry structures and optical properties. The whole works contain three parts: one part is the study of the one- and two-photon absorption (OPA and TPA) properties of a double-conjugated-segment molecule (BSBAB) and two single-conjugated-segment molecules which compose BSBAB in gas; another part is about the solvent effects on the molecular structures and optical properties; the last part is the nonlinear optical properties of two triphenylamine derivatives (PVMB and DPVMB). The main contents and results are represented as follows.一,The one- and two-photon absorption properties of organic molecules in gas On the base of density functional theory, we study the nonlinear properties of a double-conjugated-segment molecule (BSBAB) and two single-conjugated-segment molecules which compose BSBAB in gas. First, the geometry of BSBAB and two one-dimensional molecules in gas phase is optimized on the base of density functional theory. The results indicate that the one-dimensional molecules are well complanate, while the two single-conjugated-segment parts in molecule BSBAB hold their own coplanarity and are nearly perpendicular with each other. Then we use ZINDO-SOS, a few states and response functions methods to calculate the two-photon absorption cross section of a one-dimensional D -π-D molecules. The results indicate that the maximum one- and two-photon transitions all take place in the first excited state for one-dimensional D -π- A type asymmetry charge-transfer molecules, but it is different for one-dimensional D -π- D type symmetry charge-transfer molecules where the maximum one- and two-photon absorptions do not appear at the same excited state. There are two charge transfer states for the double-conjugated-segment molecule BSBAB, which are from the two single-conjugated-segment molecules respectively. The two-photon absorption cross section of BSBAB is calculated using a few states method. It is found that this molecular has three TPA peaks in lower energy region. So the strong TPA and broadband optical power limiting properties in the near infrared region have been demonstrated. The numerical values of the TPA cross sections are in agreement with the experimental results.二,The solvent effects on the molecular structures and optical properties Now, many experiments measurements of the TPA cross sections of organic molecules take place in solvents. When solute molecules are solvated in the solvents, the solute molecular charge distribution will polarization solvents around it and this gives rise of a reaction field which acts back on the molecules, then, the molecular geometry structure and optical properties will be modified. So the solvent should be considered in order to realizing the good agreement between the numerical simulation and the experimental results.Time-dependent hybrid density functional theory in combination with Onsager model and hydrogen bonding effects has been applied to study the solvent effects on the geometrical and electronic structures, as well as one/two-photon absorption processes, of 4-(N-(2-hydroxyethyl)-N-methyl)-amino-4'-nitroazobenzene. It is found that geometrical and electronic properties of the solute molecule show great changes, and red-shift of one/two-photon absorption is caused by solvent effects. Solvent effects reduce the bond length alternation and the charge variation for the donor and acceptor in compounds between the charge transfer state and ground state.三,Theoretical studies on nonlinear optical properties of triphenylamine derivatives PVMB and DPVMBThe one-photon and two-photon absorption properties of 1-{(1E)-2-[4- (diphenylamino)phenyl]vinyl}-4-[4-N,N-di-methylamino]benzene(PVMB) and1-[(1E)-2-(4-{[4-((1E)-2-{4-[4-N,N-di-methylamino]pheny}vinyl)phenyl]phenylamino}phenyl)vinyi]-4[4-N,N-di-methyl-amino]benzene(DPVMB) is investigated by use of the analytic response theory at DFT level. The calculations show that two molecules have strong two-photon absorption cross-sections in the visible light region. The two-branch molecule DPVMB has more charge-transfer states and larger TPA cross sections than one-branch molecule PVMB. DPVMB is a broadband strong two-photon absorption chromophore. The charge-transfer process for the charge-transfer states is visualized.The theoretical results are in good agreement with the available measurements.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 quantum chemistry theories of studying the nonlinear properties are introduced, including the Born-Oppenheimer approximation, the Hartree-Fock approximation, the density functional theory and the time-dependent perturbation theory. The fundamental ways of representing the solvent environment are discussed in the third chapter. In the fourth chapter, the different approaches developed recently for calculating molecular nonlinear optical properties are summarized, including sum-over-state, few-state model, finite field, analytic derivative and response theory methods. The fundamental ways of representing the solvent environment are discussed in the fifth chapter. The research process and the results are given particularly and the results are discussed and compared with the experimental results in the sixth chapter. The summary and prospect are represented in the last chapter.