| Nonlinear optics (NLO) develops rapidly since the birth of the laser. As an important embranchment of the field of modern optics, nonlinear optics has more and more attracted one's attention. 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 wide response wave band, good flexibility, high optical damage threshold, low cost and easy combination and modification. Rencently, metal complexs are reported to be excellent candidates for NLO materials, since their high damage threshold and fast respons time in comparison to organic compounds, and these compounds have a large variety of structures and diverse electronic properties that can be tuned by virtue of the coordinated metal. Moreover, metal complexs can extend theπ-conjugated length, and their NLO properties can be enhanced by the introduction of ligand→ligand, metal→ligand and ligand→metal charge-transfer states, which play an important role in increasing the two-photon absorption (TPA) cross section. Thus, as a new kind of NLO materials, metal complexs are caused one's attention largely.The thesis studies the linear and nonlinear optical properties of a variety of newly synthesized organic 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, and investigates solvent effects on the molecular geometry structures and optical properties. The whole works contain three parts: one part is numerical simulation of one-photon and two-photon absorption properties of 2,5-bis[4-(2-arylvinyl)phenyl]-1,3,4-oxadiazoles in gas;the second part is about the solvent effects on the molecular structures and optical properties; the last part is the theoretical studies on one-photon and two-photon absorption properties of platinum acetylide complexes. The main contents and results are presented as follows.I. The one- and two-photon absorption properties of organic molecules in gasThe one- and two-photon absorption properties of 2,5-bis[4-(2-arylvinyl)phenyl]- 1,3,4-oxadiazoles synthesized recently are investigated using response theory at DFT level. First, we optimized the molecular structures by the DFT/B3LYP and HF methods. It is interesting to observe that the molecular geometry is sensitive to the choice of the computational method. At the B3LYP level, the backbones of both two molecules lie practically on the same plane, whereas the HF optimization yields a nonplanar structure. The excitation energies agree very well with the experimental results based on the HF optimized geometry, while results based on the B3LYP optimized geometry have a large discrepancy. The HF geometry is demonstrated to be more realistic. It is found that both one- and two-photon absorptions are extremely sensitive to geometry of molecules, and the TPA cross section can be dramatically reduced by conformation distortion. The simulation shows that the two compounds have strong TPA characteristics and are thus good candidates as nonlinear optical materials.II. The solvent effects on the molecular structures and optical propertiesMany experimental measurements of nonlinear optical materials of molecules are taken place in solvents. When solute molecules are solvated in the solvents, the solute molecular charge distribution will polarize 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.The solvent effects on the geometrical structures, as well as one photon absorption process of two serials of (alkyne and alkene)π-bridging molecules are investigated by use of the polarized continuum model (PCM). Solvent is found to affect the geometrical structures of the molecules, and leads to a geometric distortion measured by the bond-length-alternation (BLA) parameter and dihedral angles. The one-photon absorption peak is red-shifted going from gas phase to solvent, and the oscillator strength is higher in solvent, but the measured solvatochromic shift of the molecule exhibits a nonmonotonic behavior with respect to the polarity of the solvents. The solvent also has influence on the TPA cross section, which is slightly enhanced upon solvation.III. Theoretical studies on one-photon and TP absorption properties of platinum acetylide complexes. Metal complexs are reported to be excellent candidates for NLO materials. The one-photon and two-photon absorption properties of platinum acetylide complexes are investigated by use of the analytic response theory at DFT level. We found that there is a greater extent of conjugation in the A ?π?A compound than that in the D ?π?D complexs. For the studied molecules, the transition to the first excited state dominates the one-photon absorption (OPA) spectrum. As the platinum insert into the ligands, the maximum OPA wavelength of the molecules is red-shifted, and the one- and two-photon absorption properties are significantly enhanced. The charge transfers of metal→ligand, ligand→metal and ligand→ligand all have contribution to the enhancement of the NLO properties. The simulation shows that the platinum acetylide complexes have strong TPA characteristics and are thus good candidates as nonlinear optical materials.The content of this thesis is organized 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, two-photon absorption theory and numerical methods for calculating two-photon absorption cross sections are introdued. The fundamental ways of representing the solvent environment are discussed. In the third chaper, the numerical simulation of one-photon and two-photon absorption properties of 2,5-bis[4-(2-arylvinyl)phenyl] -1,3,4-oxadiazoles is discussed in details. The solvent effects on alkyne and alkene chromophores are investigated in the forth chapter. In the fifth chaper, the one-photon and two-photon absorption properties of platinum acetylide complexes are studied. In the last chapter, the summery and the prospect are presented.
|
PDF Full Text Request