Three-photon Absorption In Two Novel Fluorene-based Derivatives

Nonlinear optics is a new subject between basic and applied, and the nonlinear optical materialsinterested people so much when laser appeared in1960s and the nonlinear characteristic was found in1964.With the development of nonlinear optics, it has been applied in many fields.With the deep investigation of theory and application, people only know a few of its nature. A lot ofresearch shows that nonlinear materials with large three-photon absorption cross-section have potential inmany fields such as three-dimensional storage and fluorescence imaging. But the mechanism of threephoton absorption has not been established, we need more research.We synthesized two novel fluorene derivatives,2,7-bis（2-（4-methoxyphenyl）ethynyl）-9,9-dioctyl-9H-fluorene （A） and2-bromo-7-（2-（4-methoxyphenyl）ethynyl）-9,9-dioctyl-9H-fluorene （B）. Weinvestigated fifth-order nonlinear optical properties and analyzed the essence of the three-photonabsorption with combination of the calculation of quantum.Firstly, the two compounds were synthesized with a efficient palladium catalyzed coupling reaction,and we confirmed their structures with1H NMR、13C NMR and IR spectrum.Secondly, we tested their ultraviolet absorption and fluorescence spectrum, found that the absorptionbands of A and B are275～400nm and280～380nm, respectively, and compound A and B had a strongblue fluorescence emission in band375～475nm and350～450nm, respectively. We tested the effect oftemperature on fluorescence intensity of compound A and calculated the fluorescence quantum efficiency.Thirdly, we tested their nonlinear characteristics and found that they had obvious optical limitingeffect. We estimated their three-photon absorption coefficient to be5.2×10^-20and1.1×10^-20cm³/W²throughfitting the curves, respectively. Their three-photon absorption cross-section are （6.03±0.6）×10^-76and （4.25±0.4）×10^-76cm⁶s²/photon².Fourthly, Gaussian03software has carried on the structure optimization. We calculated their excitedstates and molecular frontier orbital, and analyzed the reason of difference for three photon absorptionbetween the two compounds.