Theoretical Study Of The Regulation Of The Second-Order Nonlinear Optical Response Of D-A Systems By Different Substituents

With the rapid development of Nonlinear Optical（NLO）,organic NLO materials have been widely used in data storage,optical regulation and optoelectronic communication due to their large optical range,high transmittance and large second-order NLO response.However,the development of organic NLO materials has been seriously hampered by the difficulties in synthesis,low yields and high measurement costs.Therefore,calculating the second-order NLO response of organic materials and predicting their optical properties through quantum chemistry is essential to reduce the cost of research and development.The purpose of this paper is to use density functional theory（DFT）to calculate on organic molecules with different electron-donating（D）and electron-absorbing（A）groups and analyse the response of their second-order NLO so as to provide some theoretical guidance for the further design and synthesis of new organic NLO materials in the D-A system.The specific research contents are as follows:1.In this paper,theoretical calculations are designed for the closed D₃-A series based on dibenzo[a,c]phenazine and the open D₃-A series based on 2,3-diphenylquinoxaline.In this paper,the DFT method is used to optimize the structures of the D₃-A series molecules,and on this basis,the static first hyperpolarizability(β_tot),dynamic first hyperpolarizability(β_HRS)and excited states are calculated,and their calculated results are compared and analyzed.Firstly,the closed conformation has a higher value ofβ_tot,which means their second-order NLO response is greater.Secondly,the replacement of the donor from carbazole to phenoxazine resulted in a reduction in theβ_totvalue of the FPQVPO molecule,which means the type and volume of substituent had an effect on its second order NLO response.Finally,the FPQVCZ molecule with carbazole as the electron donor group,dibenzo[f,h]pyrido[2,3-b]quinoxaline as the acceptor and the F atom as the co-acceptor has the largest first hyperpolarizability value(35.0×10^-30esu).2.Based on the previous studies,a theoretical calculation was carried out on a closed D₃-A system molecule with dibenzo[f,h]pyrido[2,3-b]quinoxaline as the acceptor and 9,9-dimethylacridine as the donor.The analysis of the variation pattern ofβ_totvalues of the D₃-A system molecule showed that,firstly,hydrogen bonding could improve the second-order NLO response;secondly,the D₃-A molecule exhibited a larger second-order NLO response when the three electron-donating groups were different;finally,the substituent group with a V-shaped structure had a larger first hyperpolarizability value.The results showed that it has the largestβ_totvalue(68.18×10^-30esu)when the PQVACCN molecule with 9,9-dimethylacridine and cyanide groups is as electron donating groups and dibenzo[f,h]pyrido[2,3-b]quinoxaline as the electron acceptor group.3.Based on the previous work,a theoretical study of the D-D’-A system molecule with 2,4-diphenyl-1,3,5-triazine as the acceptor,N-phenylcarbazole and triphenylamine as the donor was carried out.With a face-to-face structure of linking D and A attached to carbazole bridge,there are intra-molecular van der Waals forces,so the weak mutual forces of D-D’-A system molecules were analysed.It is shown that not only can the type of substituent attached to the carbazole bridge be changed to improve the NLO response,but the face-to-face structure of the donor and acceptor is expected to enhance the charge transfer through space to improve the second-order NLO response.Compared to the usual planar charge transfer,space charge transfer offers more possibilities for the development and synthesis of new NLO materials and higher second-order NLO responses.