Theoretical Study on the Nonlinear Optical Properties of Organic Molecules
|Course||Condensed Matter Physics|
|Keywords||Stilbene Nonlinear optics First hyperpolarizability Substituents Spatial effects Solvent effect|
Donor (Donor) - electronic push-pull conjugated π bridge - acceptor (Acceptor) (D-π-A) structure of conjugated organic molecular materials show unusually large nonlinear optical polarizability receive attention and a wide range of theoretical and experimental research. The papers selected representative 4 - amino - 4'-nitro stilbene (ANS) molecules as a research object. This thesis is based on first-principles density functional theory, and nonlinear optical properties of a series of organic molecules formed by ANS. The thesis is organized as follows. First, the coupling - perturbative Hartree-Fock method (CPHF), calculated series of derivatives of stilbene static first hyperpolarizability. The study found that organic molecules with delocalized conjugated system and electronic push-pull structure, conducive to the intramolecular charge transfer, and thus has a large first hyperpolarizability β. Further studied the position of the substituent in the system, the number of donor and acceptor, and trade-offs in electron the coplanarity and symmetry of the molecule on the molecular first hyperpolarizability, and results showed that the organic molecules have more quantity and stronger cons electron-donating ability of the donor and acceptor, or has the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the spatial distribution of asymmetry, or having a higher atomic coplanarity and more low molecular center of symmetry, can significantly increase the first hyperpolarizability. In the two-level model, the charge-transfer excitation can determine the molecular first hyperpolarizability. Lead the charge transfer excitation energy changes due to changes in the molecular structure, which greatly affects the molecular first hyperpolarizability. Then, using the polarized continuum model (PCM), select ANS molecular static first hyperpolarizability were calculated in different solvents. Discussion of the solvent effect on the structure of organic molecules, the charge spatial distribution of the state as well as non-linear optical properties of the charge transfer excitation. The calculation results show that the solvent lead to organic molecules having a greater degree of charge separation, the organic molecules conjugated to the chain adjacent to the average value of the difference of the carbon-carbon single bond and double bond length (BLA) is reduced, the first hyperpolarizabilities rate increases. However, due to changes in hydrogen bonding effect, the polarity of the solvent, the first hyperpolarization rate relationship presents a non-monotonic. Uses the same two-level model to make a quantitative explanation of its non-monotonic. Finally, the level of CPHF/6-311G (d) calculate the the ANS molecules of dynamic first hyperpolarizability. In the infrared region, the molecular first hyperpolarizability increases with the increase in the frequency of the external light field. The outer light field frequency is close to the molecular excitation frequency produces resonance enhanced molecular first hyperpolarizability is unusually large, but the resonance absorption of organic molecules is also the largest impact of the transparency of the material should be avoided. The results of this thesis is conducive to provide a theoretical basis for the design of new nonlinear optical organic molecular materials, synthetic.