| As an artificial synthetic new carbon-based material,graphite phase carbon nitride（g-C3N4）has the advantages of tunable electronic structure,moderate band gap,good visible light response,high chemical stability,etc,which has broad application prospects in optical and optoelectronics fields,such as photocatalysis,sensors and solar cells.Especially,g-C3N4 shows stronger saturated absorption performance in the visible light band than in the near infrared band,which makes it a new favorite in the field of optical limiting.However,the disadvantages of g-C3N4,such as narrow light absorption range and poor dispersion in organic reagents,which have been the biggest obstacles to its application in nonlinear optical devices.How to solve these problems has become the bottleneck of the practical application of g-C3N4 in the field of nonlinear optics.From Molecular design perspective,three polymers structure types has been constructed in the present study,and covalently modified g-C3N4 to prepare porphyrin-based polymer covalently modified g-C3N4 complex,respectively.The disadvantages of g-C3N4 are improved through the photoinduced electron transfer/energy transfer effect between the porphyrin group and the g-C3N4 group in the compounds,enabling it to exhibit good nonlinear optical properties.The main research work is divided into the following three aspects:1.Three kinds of poly（methyl methacrylate）with porphyrin terminal group（Por-PMMA）were designed and synthesized by atom transfer radical polymerization（ATRP）method.On this basis,Por-PMMA-g-C3N4 compounds was prepared by etherification of Por-PMMAn and g-C3N4-BO.The nonlinear optical properties of Por-PMMAn-g-C3N4 compounds were investigated by Z-scan technique.And the results showed that the third-order nonlinear optical properties of Por-PMMAn-g-C3N4 compounds were significantly improved compared with those of Por-PMMAn and g-C3N4-BO,which was attributed to the synergistic effect of the accumulation of the unique advantages of the nonlinear optical properties of porphyrin and g-C3N4,and photoinduced electronic energy transfer（PET/ET）between Por-PMMAn group and g-C3N4 group.It is also found that the PET/ET effect between porphyrin group and g-C3N4 group in the compounds is affected by the molecular weight of Por-PMMAn,so the nonlinear optical properties of the complex can be controlled by changing the molecular weight of Por-PMMAn.Among them,the nonlinear absorption coefficient of the compound Por-PMMA100-g-C3N4 is 4.5×10-9 m/W.Por-PMMAn-g-C3N4/PMMA optical plastic was prepared,the third-order nonlinear absorption coefficient is 4.7×10-9 m/W,linear transmittance 67%,limiting threshold 3.15 J/cm2,dynamic range 0.3,which can be used to make optical limiting devices.2.A series of comb-type porphyrin-based polymers（PPorn-C≡CH）with different molecular weights were designed and synthesized by reversible addition-fracture chain transfer polymerization with porphyrin as monomer,polymethacrylate as main chain and alkynyl groups as terminal group of main chain.On this basis,PPorn-MWCNT,PPorn-GO,PPorn-g-C3N4 compounds were prepared by clicking chemical reaction between PPorn-C≡CH and azido multi-walled carbon nanotubes（MWCNT-N3）,azido graphene oxide（GO-N3）and azido graphite phase carbon nitride（g-C3N4-N3）.Through covalent combination with different carbon-based materials,the influence of the microstructure and energy band structure of carbon-based materials on the PET/ET effect between the PPorn group and the carbon-based material group in the compounds were studied,and then the influence on the nonlinear optical properties of the compound was investigated.It is found that the smaller band gap and appropriate microstructure of carbon-based materials will not only increase the content of porphyrin group in the composites,but also play a positive effect in the PET/ET effect between porphyrin group and carbon-based materials,thus improving the nonlinear optical properties of the composites.Among them,the third-order nonlinear absorption coefficient of composite PPor5-g-C3N4 is as high as 5.17×10-9 m/W.PPor5-g-C3N4/PMMA optical plastic with excellent NLO performance was prepared by doping PPor5-g-C3N4 in PMMA,and its third-order nonlinear absorption coefficient was7.2×10-9 m/W,linear transmittance is 67%,limiting threshold is 1.71 J/cm2,minimum transmittance is 8%,and dynamic range is 1.09.3.A series of porphyrin-based conjugated organic polymers（PCOP）were prepared by liquid phase synthesis,using TAPP and 1,4-Phthalaldehyde as monomers and changing the ratio of TAPP and 1,4-Phthalaldehyde.On this basis,a series of PCOPx-g-C3N4（x=1,2,3）composites were prepared by covalently linking the graphite phase carbon nitride covalently modified with benzoic acid via acylation reaction.The photophysical study found that due to the difference in the structural order of PCOPx,the conjugation degree of PCOPx was different,resulting the fluorescence quenching degree of g-C3N4 was changed,which verified that the conjugation of PCOPx was the key factor to control the PET/ET effect between the PCOPx group and the g-C3N4 group in the complex PCOPx-g-C3N4.The performance of the composite was studied by Z-scan system.The results showed that due to the PET/ET effect between the g-C3N4group and the PCOPx group in the compounds and the accumulation of the nonlinear optical properties of PCOPx and g-C3N4,the NLO performance of PCOPx-g-C3N4 was significantly improved compared with the PCOPx and g-C3N4-BA,and the nonlinear optical properties of the compounds PCOPx-g-C3N4 could be controlled by changing the conjugation of the PCOPx group.Those results will provide new ideas for the design and development of the application of composites in the field of nonlinear optics and provides valuable theoretical and data references.To improve the practicability of the compounds,PCOP1-g-C3N4/PMMA optical plastic has been prepared,the nonlinear absorption coefficient is 10.8×10-9 m/W,limiting threshold 1.82 J/cm2,minimum transmittance 7.3%,dynamic range 1.13. |
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