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

In recent years, organic conjugated molecules with thiazole group as nonlinear optical(NLO) materials have been paid more and more attention. It is mainly because that thiazolehas non-centrosymmetric structure, so it can be formed larger asymmetric conjugationsystem. Meanwhile, thiazole compounds have lower π-electron delocalization energy, highthermal stability and better optical transparency.In this thesis, density functional theory(DFT) method are emploryed to investigate thesecond-order NLO properties of a series of organic conjugated molecules with thiazolegroup. The following is the main results.1. DFT B3LYP/6-311G**//FF+TD-DFT method was employed to investigate thesecond-order NLO properties and the spectral characteristics of a series of thiazolederivatives with Schiff base. The results showed that the second-order NLO coefficients βvalues of all moleculars were improved significantly with the enhancement of thepush-pull electronic strength of the substituent groups, and the increase in the number ofthiazole rings and the introuction of Schiff base. Spectral data showed that the wavelengthof maximum absorption λmaxof all moleculars were in the visible region. The electrontransition between the shallow orbits had an important contribution to the electronicspectra of the molecules.2. On the level of6-311G**, DFT method was adopted to calculate the molecularstructure, the second-order NLO properties and the electronic spectra of a series ofaromatic organic molecules with benzothiazole group. The calculated results showed thatthe ground state structure of all moleculars were near-planar conjugated structure. In thesystem, the changes of the relative strength and position of the two substituents had notableinfluence on the polarizability and the second-order NLO coefficients of all molecules,however, the introduction of different azaheterocycle had little impact on it.3. DFT B3LYP/6-311G**//FF+TD-DFT method was employed to calculate thesecond-order NLO properties and the electronic spectra of a series of thiazolothiazolederivatives with different donor/acceptor electronic substituents. The calculated resultsshowed that the changes of the charging characteristics, strength and position of thesubstituent in the conjugated structure had a significant impact on the NLO properties ofthe system. NLO activities of molecules were improved significantly with the introduction of the suitable push-pull electronic substituent groups. Meanwhile, good opticaltransparency was obtained.4. DFT B3LYP/6-311G**//FF+TD-DFT method was adopted to investigate thesecond-order NLO properties and the spectral characteristics of a series of bi-thiazolesubstituted thiazolothiazoles derivatives. According to the results, the introduction of thestrong push-pull electronic strength of the substituent groups’ or pyrrole and thiopheneheterocycles substitute for the benzene ring could effectively improve the second-orderNLO activity of molecules, moreover, the latter’s role was more visible. Spectral datashowed that the electron transition between the frontier orbits had a great contribution tothe electronic spectra of the molecules.