Theoretical Study On Second-order NLO Properties Of Organic Conjugated Molecules With Carbazole Group

In recent years, organic conjugated molecules with carbazole group as nonlinearoptical (NLO) materials have aroused wide public concern. On one hand, it has beenfound that carbazole compounds have larger conjugated system and strongintramolecular electron transfer. On the other hand, it is more conductive to thecarbazole ring with a variety of functional groups. This is helpful to molecular design.So organic conjugated molecules with carbazole group become hot research areas ofNLO materials.In this thesis, quantum chemistry density functional theory (DFT) method areused to investigate second-order NLO properties of a series of organic molecules withcarbazole group and obtain some important information about the research system.These results may provide elementary theoretical evidence for further experimentalresearch. The following is the main results,1. DFT B3LYP/6-31G*method was employed to optimize the geometricalstructures of a series of heteroaromatic molecules with carbazole chromophores. Then,NLO properties and electronic spectra were studied by finite field (FF) andtime-dependent DFT (TD-DFT) methods at the6-311G**level. The results show thatthe polarizability α and the second-order NLO coefficient β values of all systems aresensitive to push-pull strength change of carbazole substituent groups and theintroduction of heteroaromatic. When the pull electronic nitro and the push electronichydroxyl were linked by carbazole substituent groups respectively and the furanheterocycle was introduced, the β values decreased (blue-shifted) with an increase inthe maximum absorption wavelengths λmaxof the molecules.2. Density functional theory DFT B3LYP/6-31G*and FF and TD-DFT methodswere employed to theoretically investigate a series of dibenzothienocarbazole-coredorganic conjugated molecules. The results show that dibenzothienocarbazole-coredhas different charge characteristics in different systems. However, due to the smallrelatively absolute charge values, so they play a role of charge transfer in two types ofsystems. As the furan ring is introduced into dibenzothienocarbazole-cored molecules,the second-order NLO coefficient values of systems would increase. When themolecules on both sides of the benzene ring with cyano, cyanide vinyl substituents would make the second-order NLO coefficient β decrease (blue-shifted) with theincrease of the maximum absorption wavelengths λmaxof the molecules. They wouldrelieve the conflicts of “nonlinear-transparency tradeoff”.3. DFT B3LYP/6-311G**//FF+TD-DFT methods were employed to optimize thegeometrical structures of indolo [3,2-b]carbazole-cored organic conjugated molecules,Then, NLO properties and electronic spectrum of molecules were studied. The resultsshow that the introduction of C=Cand C≡Cconjugated bridge as well as furan andthiophene heterocycles instead of the benzene ring could effectively improve the NLOactivity of molecules. The electronic transitions from shallow occupied orbital tolowest unoccupied orbital would make an important contribution to absorption spectra.Electronic transitions between the frontier molecular orbital also have some impact.The research system has a large second-order NLO coefficient values and themaximum absorption wavelengths are in the UV-visible region. The A2molecular hasgood NLO activity and appropriate transparency.4. DFT B3LYP/6-311G**//FF+TD-DFT methods were used to calculate theelectronic properties and the effect of second-order NLO of a series of triindole-coredpseudo octupolar molecules. It can be shown that the polarizability α, second-orderNLO coefficient β of the molecules increase with increasing of electron-acceptorability of the substituent groups. The molecule with tricyanostyrene acceptor and C=Cdouble bond conjugated bridge would exhibit the optimization of the largesecond-order NLO coefficient and good transparency in research systems. The“nonlinear-transparency tradeoff” conflict is effectively relieved because of themultiple charge transfers of pseudo octupolar molecules. These pseudo octupolarmolecules might become candidates in the development of NLO materials.