Study On Third-Order Nonlinear Optical Properties Of Organic Molecules With Strong Donor Units

By using the nonlinear process of nonlinear optical(NLO)materials,the light with a frequency different from that of the incident laser can be obtained,so as to achieve the purpose of optical frequency transformation.NLO materials have been widely used in many fields such as biological imaging,photodynamic therapy,optical information processing,optical fiber communication,etc.The discovery and preparation of better NLO materials to facilitate our production and life is a hot issue in current research.This thesis first aimed at a group of organic molecules containing vinyl quinolinone chromophores,using(time-dependent)density functional theory(DFT,TD-DFT)to simulate molecular energy levels,absorption spectra,charge layout analysis,dipole moment,hyperpolarizability,two-photon absorption(TPA)cross-section were simulated using density functional theory(DFT)to explore the relationship between the modification of the acceptor groups and NLO properties.In addition,the few-state model was used to determine the dominant two-photon transition channel,and the internal factors that affect the TPA cross-section and hyperpolarizability were analyzed in detail.Results show that changing the acceptor group has an obvious effect on the NLO properties of chromophores.The addition of the five-membered heterocyclic ring in the acceptor group has a significant effect on improving the static hyperpolarizability and TPA cross-section.Secondly,for another type of triphenylamine derivatives as photosensitizers,visualization methods(charge density difference and transition density matrix)are used to explore the excited state charge transfer mechanism during the one photon absorption(OPA)and TPA process,and it is found that the polymerization effect has a significant impact on the charge transfer in the OPA and TPA process.With the increase of the degree of polymerization n,electrons are mainly transferred between the middle area of the molecular chain and the side where the end of the molecular chain is the donor.Results also show that the energy gap(excitated energy)has a linear relationship with the reciprocal of the degree of polymerization,and the polymerization effect can effectively improve the electron transport capacity,TPA performance and second hyperpolarizability.Therefore,reasonable receptor modification and polymerization effect can effectively improve the intramolecular charge transfer(ICT),NLO coefficient and TPA cross-section,and provide reasonable guidance for the design and performance optimization of NLO materials.