Theoretical Study Of Organic Boron-based Compounds Nonlinear Optical Properties

It has become an important project to find new nonlinear optical materials with strong nonlinear optical properties and quick response behavior due to their attractive applications foreground, 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 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 study on optical properties of the organic molecular material has made great progress recently. Many organic molecular materials with strong nonlinear optical properties have been synthesized and their structure-property relations have been explored. As a new nonlinear optical material, the excellent nonlinear optical properties of organic Boron-based compounds have been discovered. The special structure-property relation of organic Boron-based compounds needs to be theoretically studied.The thesis studies the linear and nonlinear optical properties of a variety of newly synthesized organic Boron-based compounds utilizing theoretical and computational approaches on the base of the ab initio level, represents the relation of the molecular structures and properties, investigates the solvent effects on the molecular geometry structures and optical properties, and discusses the relation of the charge-transfer activity and molecular properties by analyzing the molecular charge-transfer process. The whole works contain three parts: one part is the study of the one- and two-photon absorption (OPA and TPA) properties of a series of one- and multi-branched organic molecules in gas; the other part is about the solvent effects on the molecular structures and nonlinear optical properties; the last part is about molecular charge-transfer in the one- and two- photon absorption process. The main contents and results are represented as follows.一,The one- and two-photon absorption properties of organic molecules in gas.1. The one- and two-photon absorption properties of one-dimensional organic molecules.At the hybrid density functional theory (DFT), the one- and two-photon absorption cross sections of a series of organic molecules with D -π- A, D -π- D, A -π- A types are studied by employing few-state model method. It is well known that the maximum two-photon absorption cross sections of the lowest excited states can be exactly described by only considering a few electronic states for 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 pushing and pulling electrons, the molecular symmetry. The molecular conjugated length in Boron-based compounds plays a more obvious role than that in Nitrogen-based compounds. When the molecular conjugated length of Boron-based compound is lengthened, the TPA activity is highly increased. To the molecules with similarπcenter, the D -π- A type molecule with dimesitylboryl as acceptors and with diphenylamino as donors posses weaker one-photon absorption intensity than the D -π- D and A -π- A analogues. But it has stronger two-photon absorption properties than its analogues. Then, we analyze the microcosmic mechanism of increasing the molecular TPA cross sections. The TPA cross sections can be enhanced by increasing the dipole moment difference between the lowest excited and the ground states and the transition dipole moments between the states, and by making the directions of transition dipole moment vectors parallel or antiparallel. In addition, we also study the TPA cross sections of the one-dimensionality organic molecules by using the response theory approach where the contributions of all intermediate states being taken into account, and attain the analytic values of computation results. The result shows that the general trend is similar for the both of theoretical methods by analyzing the TPA activities of a series of organic molecules. Otherwise, the availability of the few-state model method is testified by the response theory approach.2. The two-photon cross sections of multi-branched organic molecules.Being differ from one-dimensionality molecules, the few-state model method is not suitable for computing the TPA cross sections of the multi-branched molecules because they have more charge-transfer states, thus, we apply the response theory approach to attribute the TPA properties of multi-branched molecules. By analyzing the TPA actions of a series 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 two-photo absorption cross sections of multi-branched Boron-based molecules are smaller than that of multi-branched Nitrogen-based molecules with similar structure.二,The solvent effects on the molecular structures and optical properties.Now, many experiments measurements of the TPA cross sections of organic 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.1. Solvent effects on the molecular structure, one- and two-photon absorption properties.The structure and one-photon absorption properties of many molecules in solvents are computed by use of the polarized continuum model (PCM). The theoretical studies show that solvent can change the molecular structure, decrease the excitation energies of the molecules, enhance the one-photon absorption intensities and make wavelengths of the excited states take red-shifted. However, the nonmonotonic behavior for the enhancement of the one-photon absorption intensities with respect to the polarity of solvents is also observed. Onsager reaction field model is used to study the two-photon absorption cross sections of the molecules in solvent. The results indicate that the two-photon absorption cross sections of the molecules have a relatively large enhancement in solutions compared to those in gas.2. Hydrogen-bonding effects.In solvent effects, it is well known that the long range interaction between the solvents and the solute molecule can largely change the molecular geometrical structure and optical properties. Furthermore the short range interaction, namely hydrogen-bonding, also has a largely effect on the molecular geometrical structure and optical properties. In this thesis, the hydrogen-bonding effect on the geometrical structures and one-photon absorption properties of 3, 6-Bis [2-(4-pyridyl) ethenyl]-9- ethylcarbazole molecule is studied in detail. It is shown that the geometrical structure of molecule has been greatly changed in solvents, because theπconjugate electrons are very easily induced by the reaction field. When the hydrogen-bonding interaction is considered, the one-photon absorption intensities of the molecule become strong. So the hydrogen-bonding effect should not be neglect in order to precisely describing the interaction between solvents and the solute molecule.三,The charge-transfer activity of molecule.The charge-transfer activity of molecule will influence the one- and two-photon absorption properties when the molecule is excited from the ground state to charge-transfer state. We calculate the charge variation for the one- and multi-dimension molecules in the one- and two-photon absorption process. It can be seen that in the one-photon absorption, the charges of one-dimensional D -π- D type Nitrogen-based compounds transfer from the two end to the center of molecule, the charges of one-dimensional A -π- A type Boron-based compounds transfer from the center to the two ends, and the charges of D -π- A type compound transfer from the donor to acceptor. When theπcenter is lengthened, the acceptor group for Boron-based compound can easily get more electrons from theπcenter, but the donor group for Nitrogen-based compound is difficult for donating electron to theπcenter. The charges of multi-branched compounds transfer from one branch to the other two branches in the excited process. The center atom N of Nitrogen-based compound loses electrons and the center atom B of Boron-based compound obtains electrons. The different charge-transfer characteristic is expected to result in the different dependence of optical property on length ofπcenter for Boron-based and Nitrogen-based compounds.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, finite field, analytic derivative and response theory methods. The fundamental ways of representing the solvent environment are discussed in the forth chapter, at the same time, the solvent influence on molecular polarizabilities is studied. In the fifth chapter, the results of a series of one-dimensional organic molecules are given in detail. The one- and two-photon absorption properties of multi-dimensional organic molecules are studied in the sixth chapter. In the seventh chapter, the solvent effects on molecular TPA cross sections are investigated by two theoretical models, including polarized continuum model and Onsager reaction field model. The charge-transfer activity is discussed in the eighth chapter when the molecule is excited from the ground state to the charge-transfer state. In the last chapter, the summary and the prospect are represented.