Synthesis,photoinitiated Behavior And Applications Of Novel Flavonoid Photoinitiators

Photopolymerization is the most economical,environmentally friendly,and effective method for synthesizing polymer materials with light energy and is widely used in 3D printing,microelectronic processing,coatings,inks,and adhesives.Photoinitiators are core components that absorb light energy and generate active species in photopolymerization technology,directly affecting the light source used and polymerization process.The banning of mercury lamps is due to the problems of traditional photoinitiators with regard to biosafety,such as short absorption wavelength,high biotoxicity,and initiator residues,which greatly limit the application scope of photopolymerization technology.Novel photoinitiators need to have the characteristics of low biotoxicity,compatibility with LED light sources,and multicomponent synergistic initiation ability.Flavonoids are a class of natural products that not only have good biological activity,but also have good photochemical reaction properties.When flavonoid molecules absorb light energy,a series of photophysics and photochemical processes,such as intramolecular proton transfer(ESIPT)process,intermolecular electron transfer,and radiation/nonradiation inactivation can occur.In this paper,in order to explore the initiation performance of flavonoids as LED photoinitiators,twenty-three flavonoids were designed and synthesized.First,the photophysical properties of the flavonoids and their photochemical properties in four photopolymerization initiation systems(cleavage initiation,hydrogen abstraction initiation,dye-sensitized initiation,and redox initiation systems)were systematically discussed.Then,the effects of electronic effects,internal filtration,and ESIPT on the initiation performance of flavonoids as initiators were discussed.Finally,the optimal formulation was selected and successfully applied to 3D printing and shape memory polymers based on the initiation properties of flavonoids.In addition,the effects of essential properties,such as flavonoids enone tautomerism and internal filtration,on the resolution of 3D printing and the stimulus-responsive properties of the polymers were elucidated.Overall,this study provides a theoretical foundation for the application of flavonoids in photopolymerization.The main work of this thesis is as follows:(1)Synthesis and photophysical properties of flavonoidsIn order to explore the effects of electronic effects,molecular structure,and ESIPT on the initiation performance of flavonoids,four types of flavonoids were designed and synthesized: flavonol derivatives(3HF-HSubs)containing methyl and halogen substituents;flavonol derivatives containing triphenylamine(SBA),carbazole(CZ),phenothiazine(PTZ)and N,N-dimethylaniline structures(3HF-Ar-Am);flavonol ester derivatives(3HF-OH-Chl);and aurone derivatives(CT-Ar-Am).The structures of all intermediates and flavonoids were characterized through NMR,IR,and HRMS.The UV-Vis absorption spectroscopy results showed that the absorption wavelengths of 3HF-Ar-Am,3HF-OH-Chl and CTAr-Am can be extended to more than 400 nm.Among them,the aurone derivatives containing PTZ group(CTPTZ)can extend the absorption wavelength to the red range.Fluorescence spectroscopy results showed that 3HF-H-Subs and 3HF-Ar-Am had rapid ESIPT processes,and their fluorescence intensity decreased when 3HF-OH-Chl inhibited the ESIPT process of 3HF-Ar-Am.The molecular electrostatic potential map revealed that flavonoids contained reactive sites that interact electrostatically with electron donors or acceptors.(2)Study on photochemical properties of the flavonoids in LED photopolymerization initiation systemsIt was found that 3HF-H-Subs,3HF-Ar-Am and 3HF-OH-Chl underwent photodegradation under LED light illumination through the self-photolysis reaction.Further research found that: 3HF-OH-Chl had low S-O bond dissociation energy and photoacid generation performance,so the 3HF-OH-Chl itself can undergo a cracking reaction to generate free radicals.The aurone compound(CTBJQ)and aurone derivatives containing CZ group(CTCZ)underwent cis-trans isomerization of alkene,While aurone derivatives containing SBA,PTZ and N,N-dimethylaniline groups(CTSBA,CTPTZ and CTN)had certain photostability under the LED light source.Through the photolysis reaction between flavonoids and triethanolamine(TEOA),it was found that 3HF-H-Subs,3HF-Ar-Am and 3HF-OH-Chl can undergo hydrogen abstraction reaction with TEOA to generate free radicals.Through the photolysis reaction between flavonoids and iodonium salt(ONI),it was found that 3HF-H-Subs,3HFAr-Am,3HF-OH-Chl and CT-Ar-Am can undergo electron transfer reaction with ONI to generate free radicals.(3)Study on the photoinitiated behavior of the flavonoids as photoinitiatorsUsing 385 nm LED as the light source,the photoinitiated activities of the 3HF-H-Subs hydrogen abstraction initiation and dye-sensitized initiation systems in free radical photopolymerization were investigated.It was found that the electron-donating substituent reduced the photoinitiated activities of the 3HF-H-Subs hydrogen abstraction initiation system,and improved the photoinitiated activities of the 3HF-H-Subs dye-sensitized initiation system.Using 405 nm and 460 nm LEDs as light sources,the photoinitiated activities of the 3HF-OH-Chl cleavage initiation,hydrogen abstraction initiation,dye-sensitized initiation,and redox initiation systems in free radical and cationic photopolymerization were investigated.It was found that the internal filtration of 3HF-OH-Chl greatly affected its initiation efficiency,and the initiation efficiency can be improved by reducing the amount of 3HF-OH-Chl,which also reduces the biological toxicity of the initiation systems.At the same time,3HF-OH-Chl inhibits the ESIPT process of 3HF-Ar-Am,reduces the radiation inactivation of3HF-OH-Chl,and can significantly improve its photoinitiated activities.Using 405,460,520,590 and 620 nm LEDs as light sources,the photoinitiated activity of CT-Ar-Am dye-sensitized initiation system in free radical and cationic photopolymerization were investigated.The use of longer wavelength LED light sources can significantly reduce the effect of the internal filtering of the yellow pigment on its initiation efficiency.(4)Application of the flavonoids in photopolymerizationUsing a photopolymerization DLP-3D printer,the application of3HF-OH-Chl redox initiation system in DLP-3D printing was investigated.In the 3HF-OH-Chl redox initiation system,3HF-OH-Chl could not only act as a photoactive absorber to generate active species to initiate polymerization,but also undergo classic enone tautomer conversion to mitigate light dispersion,generating 3D-printed polymers with high resolution.At the same time,the 3HF-OH-Chl dye-sensitized initiation system can rapidly initiate the cationic polymerization of DVE-3 monomer under low light intensity,which can also be applied to high-resolution 3D printing.Based on the fact that the internal filtration of 3HF-OH-Chl has a great influence on its initiation efficiency,the effect of the internal filtration of 3HF-OH-Chl on the stimuli-responsiveness of 3D printed samples and thick polymer samples was investigated.It was found that the internal filtration of 3HF-OH-Chl had little effect on the stimuliresponsiveness of the 3D-printed samples,but had a large effect on the stimuli-responsiveness of the thick polymer samples.The transformation of static polymers to shape memory polymers can be achieved by different water absorption/dehydration methods.Further research found that the 3D printed samples still had strong fluorescence after the stimuliresponsiveness,which is promising for bioluminescent materials.