Optimization Of Excited State Absorption Property Of D-A-Typeπ-Conjugated Molecules And Its Application For Optical Limiting

The rapid development of laser technology has promoted the renewal of laser weapons and the popularization of civil laser industry.It is urgent to realize effective laser protection to avoid photodetector and human body from being harmed by laser.Optical limiting is one of the most important applications in nonlinear optics（NLO）and an effective means to solve the problem of laser protection.However,the development and application of optical limiting materials have been severely hindered by complex preparation technology and high research cost for decades.With low cost and well-defined structural flexibility,functional organic small molecules have broad application prospects in biological imaging,all-optical switching,data storage,photodynamic therapy and laser protection.In this dissertation,we select Donor-Acceptor（D-A）-typeπ-conjugated molecules as the object of study and use quantum chemistry calculation software to conduct molecular modeling and property prediction,and design and prepare a series of functional organic small molecule materials with excellent nonlinear optical properties.The NLO properties of these materials are measured and the photophysical mechanism is systematically analyzed through the nonlinear optical experiments including Z-scan,transient absorption spectrum（TAS）,optical limiting（OL）and degenerate pump-probe experiments.This dissertation analyzes the structure-property relationships of D-A-typeπ-conjugated molecules from the perspectives of molecular configuration,molecularπ-conjugated structure,and intramolecular charge transfer（ICT）.We gradually promote the excited state absorption（ESA）performance of these materials and achieve excellent OL materials suitable for 532 nm in the nanosecond time domain.The research work includes the following parts:（1）Reavel the enhancement of the effectiveπ-conjugated structure on two-photon absorption（TPA）of terthienyl-based chalcone derivatives.Extend theπ-conjugated length of chalcone molecule based on tridithiophene group and modulate the intramolecular charge transfer（ICT）intensity by linking different electron acceptor groups,three terthienyl-based chalcone derivatives（T3CT,T3CP2and T3CP3）are designed and synthesized.The ultrafast nonlinear optical response and transient dynamics of terthienyl-based chalcone derivatives are systematically studied by femtosecond multi-wavelength Z-scan technique and transient absorption spectrum.The electron transition characteristics are theoretically characterized by hole-electron analysis.It was found that TPA of these materials is significantly enhanced by extending the effectiveπ-conjugated structure.Compared with 650 and750 nm,the TPA cross section of T3CT at 580 nm is enhanced by about 20 times.In addition,the terthienyl-based chalone derivatives show obvious charge transfer state（CTS）based ESA signals.（2）In order to enhance the CTS-based ESA of chalone derivatives,we design and synthsize three D-A-type chalcone derivatives（1,2 and 3）with different electron push-pull systems.We visually describe the ICT processes by changing the solvent polarity in the femtosecond TAS and systematically analysize the performance difference of excited state dynamics of terthienyl-based chalcone derivatives and triphenylamine-based chalcone derivatives.The effects of ultrafast ICT and CTS on TPA and ESA are systematically studied by femtosecond and picosecond Z-scan experiments and hole-electron analysis.It is found that the introduction of triphenylamine group based on chalcone molecular skeleton is conducive to the establishment of stronger CTS-based ESA.（3）In order to enhance the ESA performance of chalcone derivatives and improve OL response of these materials in the nanosecond time-domain,two triphenylamine-based chalcone derivatives（T1 and T2）are designed and synthesized by introducing pyrenyl based on the molecular skeleton of triphenylamine-based chalcone to expand molecularπ-conjugated structure.Their OL responses at 532 nm in the nanosecond time domain are studied and compared through OL experiments and the measured OL threshold is low to 0.72 J cm^-2.The photophysical mechanism of OL response is systematically analyzed by femtosecond TAS,degenerate pump-probe experiment and nanosecond Z-scan technique.It is found that these triphenylamine-based chalcone derivatives are organic OL materials with simple molecular configuration and excellent OL response.In addition,we found that the introduction of polycyclic aromatic hydrocarbons（PAHs）groups to expand theπ-conjugated structure of triphenylamine-based chalcone derivatives is conducive to enlarging their ESA cross section.（4）In order to study the modulation effect ofπ-conjugated structure on the reverse saturation absorption（RSA）of D-A-type twistacene derivatives,we design and synthesize four twistacene derivatives（H1,H2,H3 and H4）containing different terminal torsionalπ-conjugated units.Use TAS and Z-scan experiments study their broadband ESA properties and RSA response in femtosecond,picosecond and nanosecond time domain,respectively.The electron transition characteristics of excited states are theoretically characterized by hole-electron analysis.It is found that the RSA properties of these PAHs can be significantly enhanced by extending molecularπ-conjugated structure.In addition,all of these twistacene derivatives show broadband ESA properties.By combining the structure of twistacenes with molecular skeleton of triphenylamine-based chalcone derivatives,the ESA performance of these materials are expected to be further improved.