Theoretical Study On The Nonlinear Optical Properties Of Organic Molecules

For their very large nonlinear optical responses,Organic materials , especiallyπconjugated organic materials with electron donors (D) and electron acceptors (A),have been extensively investigated. in this thesis,4-amino-4′-nitro stilbene(ANS) has been selected as a representative D-π-A nonlinear optical organic molecules.The thesis studies nonlinear optical properties of a variety of substituted organic molecules ANS utilizing Density Function Theory based on the first principle calculation. The contents is organized as follows.First of all, static first hyperpolarizabilities of a series of Stilbene derivatives have been calculated at the CPHF/6-311G(d) level. the molecules which possess the electron push-pull pair and delocalized conjugated bridge structure have large first hyperpolarizabilities because of the intramolecular charge transfer. The influence of substitutional positions, strength of electron donating or withdrawing and number of donors and acceptors, planarity and asymmetry of molecules is discussed. The results indicate that molecules with more stronger and more donors and acceptors, large asymmetry of HOMO and LUMO, large planarity and acentricity can possess large first hyperpolarizabilities. two-level model, which shows that transition energies between the ground state and first excited state owe to charge-transfer mainly determine the variations inβvalues, can explain the trend above very well.Secondly, we present the calculation of the static first hyperpolarizabilities of the push-pullπconjugated molecule ANS within the framework of the polarization continuum model in different Solvents. The effects of solvents effect on the molecular structure, spatial distribution of electron, charge transfer excited states and nonlinear optical properties are discussed. The solvent effect leads to charge separation and a geometrical distortion of molecules measured by the bond-length-alternation (BLA). The first hyperpolarizabilities of ANS molecules are all enhanced with the increase of the polarity of solvents. However, the change of polarizabilities with respect to the polarity of solvents is nonmonotonic due to the hydrogen-bonding effect. Moreover, two-level model is also introduced to explain the nonmonotony between polarity of solvents and polarizabilities quantitatively.Last, the dynamic first hyperpolarizabilities of ANS molecule have been calculated at the CPHF/6-311G(d) level. The first hyperpolarizabilities of ANS molecules are all enhanced with the increase of the external light frequency in infrared region. When the frequency is close to the molecular resonance frequency, the first hyperpolarizabilities of ANS at all are exceptionally large when its absorption is also large, which decreases the transparency of the ANS material. Thus, We should avoid this situation.All this works dedicate to provide theoretical references for research and development of novel nonlinear optical molecules.