Theoretical Study On Second-order Nonlinear Optical Properties Of Organic Conjugated Molecules With Pyrazine Group

Recently, organic conjugated molecules with pyrazine group as nonlinear optical(NLO) materials are being attracted considerable attention. It is mainly because that pyrazine group is a typical nitrogen-rich heterocyclic which is with a rigid stable plane conjugate structure. When other conjugated systems are introduced in pyrazine groups, the π-electron conjugate is improved effectively and the intra molecular charge transfer degree is enhanced in the structures. Meanwhile, pyrazine compounds have high thermal stability and better optical transparency.In this thesis, quantum chemistry methods are employed to calculate and analyze the second-order NLO properties of a series of organic conjugated molecules with pyrazine group. The following is the main contents.1. Density functional theory(DFT) B3LYP/6-311G**//FF+TD-DFT methods were used to investigate the geometrical structures, the second order NLO properties and electronic spectra of a series of organic conjugated molecules with a bipyrazine core. The results show that the molecules are distorted planar configuration and the electronic structures belong to D-π-A-π-A type. The βtot values of all moleculars are improved significantly and the maximum absorption wavelengths λmax are reduced with the enhancement of the push-pull electronic strength of the substituent groups at both ends of the molecules and the introduction of azaheterocycle(pyrrole etc). They would relieve the conflicts of “nonlinear-transparency tradeoff”.2. On the level of B3LYP/6-311G**//FF+TD-DFT, DFT methods were adopted to calculate the geometrical structures, the second-order NLO properties and electronic spectra of a series of organic aromatic molecules with hexaazatriphenylene(HAT) chromophores. It is shown that the second-order NLO coefficient βtot values of all molecules are influenced greatly by the change of the push-pull electronic ability or the relative position of the substituent groups on both sides of the molecules or the introduction of the azaheterocycle. When substituent groups on both sides are-NO2 and-C(NH2)3 respectively and the benzene ring is replaced by pyrrole heterocycle, the molecule would exhibit the maximum βtot value and good transparency. The electron transitions between the shallow orbits have an important contribution to the electronic spectra of the molecules.3. DFT B3LYP/6-311G**//CAM-FF-TD/B3LYP/6-311G** methods were employed to calculate the second-order NLO properties and the electronic spectra of a series of pyrazine derivatives with carbazole group. It could be concluded that there is a corresponding relationship between polarizability α and the second-order NLO coefficient βtot values of moleculars. When the pull electronic tricyanostyrene and the push electronic propyl are in the left and right sides of the molecule respectively and the benzene ring is replaced by pyrrole heterocycle, the molecule can get optimal combination, thus getting the largest βtot value. Moreover, according to the analysis of electronic spectrum and the corresponding molecular orbit composition, the contribution to the second-order NLO coefficient mainly comes from the electronic transitions between the frontier molecular orbits and shallow orbits.4. The second order NLO properties and electronic spectra of a series of organic conjugated molecules with hexaazatriphenylene core were investigated by DFT methods. According to the results, the second-order NLO activity of molecules could be effectively improved by the push-pull electronic strength and the relative number of the substituent groups or the properties of conjugated bridge. The changing rule of the second-order NLO coefficient βtot values agrees with that of the polarizability α. B2 molecule with a pair of methylamino donor and two pairs of tricyanoethenyl acceptors and vinyl conjugated bridge would exhibit the optimization of the largest second-order NLO coefficient and good transparency in research systems. Spectroscopic data show that the electron transitions between the shallow orbits have an important contribution to the second-order NLO activity. Among them, the mainly electronic transitions of A3 and B2 molecules which have the biggest βtot values in the two series of molecules are HOMO�'LUMO+3 and HOMO�'LUMO+6 respectively and the electron transfer modes of two molecules are also similar that electrons diffuser from the local to the holistic.