Study On Optical Nonlinearity Of Organic Conjugated Small Molecules By Charge Transfer In Excited States

The purpose of developing new nonlinear optical materials is meeting the requirements of larger nonlinear optical response,high transmittance,ultrafast response and high damage threshold for applications.Organic conjugated small molecules have been widely studied in the field of nonlinear optics due to their various molecular structures and abundant optical physical processes.In this dissertation,we systematically studied the nonlinear optical properties and ultrafast dynamics processes of several organic molecules by Z-scan and transient absorption spectroscopy（TAS）experiments.Combined with density functional theory,the relationship between the intramolecular charge transfer and the optical nonlinearity was discussed.Therefore,it provides reference for the design of organic molecular materials with more excellent nonlinear optical properties.This dissertation mainly carried out the following tasks:（1）Density functional theory was used to study the relationship between two-photon absorption（TPA）cross-section and molecular structures of PAHs.Combined with the sum over states formula of TPA,charge difference density（CDD）and charge transfer matrix（CTM）,the effects ofπbridge with different conjugate strength on the TPA cross-section and nonlinear optical properties of the PAHs were analyzed.The method of CTM in two-photon transition was proposed to study the characteristics of excited states,the path of charge transfer and the amount of charge transfer between atoms.The CTM was proposed to study the charge transfer in two-photon transition processes,which visually shows the charge transfer path and the amount of charge transfer between atoms.（2）Four acceptor groups were introduced into the PAHs to realize the intramolecular charge transfer and the regulation of nonlinear optical properties.The effects of excited state charge transfer on nonlinear optical properties and ultrafast dynamics were studied by using Z-scan and transient absorption spectroscopy.The two-photon absorption spectra were simulated by density functional theory and the sum over states formula。The relationship between intramolecular charge transfer and two-photon absorption cross-section were discussed by CDD and CTM.The nonlinear absorption and ultrafast dynamics parameters were also given.The four molecules have large two-photon absorption cross-sections and high damage threshold,among which DNTB has an optical limiting threshold of 5 mJ/cm² at 532nm,which is a kind of optical limiting material with application.（3）Three chalcone derivatives with different acceptor structures（furan,biphenyl,pyrenyl）were designed and synthesized.The nonlinear optical properties of chalcone derivatives were systematically analyzed by Z-scan,transient absorption spectroscopy and density functional theory.The intramolecular charge transfer process can be precisely regulated due to the cross-conjugated structure of acryl ketone which makes the donor group and acceptor group are not conjugated,while the acryl ketone structure can be conjugated with both at the same time.The combination of experimental data and theoretical simulation reveals the influence of the intramolecular charge transfer processes on the nonlinear optical properties.The charge transfer in excited states of the chalcone derivatives across the cross-conjugated structure enhances the absorption cross section of the two-photon induced excited state.The two-photon induced excited state absorption cross sections of the three chalcone derivatives reach～10^-20 m²,which is larger than that of most organic molecules reported at present.（4）The relationship between the nonlinear optical properties of DCDHF-2V and the solvent viscosity was studied due to the response characteristics of the DCDHF-2V excited state relaxation processes to the solvent viscosity.The physical mechanism of DCDHF-2V excited states absorption and refraction at 532 nm was explored by Z-scan and phase object pump probe technology.Experiments had found that the solvent viscosity limits the twist of DCDHF-2V excited state structure,and realizes the regulation of the nonlinear refraction properties of the molecule.At the same time,the femtosecond TAS results verify that the ultrafast kinetics of DCDHF-2V can be controlled by changing the solvent viscosity.