Study On Light And Thermal Oxidation Mechanism Of Oxazolidine Molecular Switches And Application Of Polymer DASA Molecular Switches

Organic molecular switches,when stimulated by external environment factors,such as light,heat,electricity,force,acid/base or solvent,the molecular structure or conformation will undergo reversible changes,resulting in changes in color,volume and polarity.It has important application value in many fields（such as nonlinear optics,information storage,sensing detection,repeatable printing and intelligent color changing materials）.However,an important factor that restricts its widespread use in actual production and life,is that when small organic molecular switches are exposed to external stimuli for a long time（such as light,heat,oxidants and metal ions）,oxidative degradation occurs,and the switching property is lost.Although there are many kinds of organic molecular switches,few works involved the mechanism of oxidative degradation,mainly focusing on molecular switches such as spiropyran,spirooxazine and triarylethene.Therefore,people need to explore the oxidative degradation mechanism of more kinds of organic molecular switches.In addition,it is worth noting that mostly organic molecular switches are dyes with good conjugation,so some non-fractured degradation products may also produce new properties,which deserve further study.In this thesis,the oxazolidine molecular switches were studied,and the photooxidation mechanism in solution was investigated first.After demonstrating that the photooxidation product formed a rigid seven-membered ring,we further quantitatively synthesized such materials by thermal oxidation,and studied the light properties and potential applications of these novel dyes.In addition,we further explored whether the application of organic molecules attached to polymers can improve the stability of its application and expand the scope of application.In the second chapter,we studied the photooxidation process of oxazoline molecular switches in solution and detected a new oxidation product.Combined with various characterization techniques（such as mass spectrometry,infrared spectroscopy,two-dimensional nuclear magnetic resonance spectroscopy）,it is inferred to be a non-cleaved oxidation product with a seven-membered ring.By studying the wavelength band of light,intermediates,solvents and oxygen species involved in the photooxidation process,as well as photooxidation of comparative molecules without flexible ethoxyalkyl chains,the mystery of the photooxidation mechanism was finally unveiled.That is,the hydrogen ion generated by solvent photolysis first induces the ring opening of the oxazoline molecule to form a flexible ethoxy group,and then the ethoxy group attacks adjacent carbon-carbon double bonds under the action of singlet oxygen formed under the ultraviolet sensitization of the dye,to form a seven-membered ring.This photooxidation mechanism is also generally applicable to oxazoline molecular switches with other substituents or conjugated structures.Although the photo-oxidation product may have attractive fluorescent properties due to its unique conjugated structure,the conditions of strong ultraviolet light generation and a large amount of oxidation by-products make the target product difficult to separate.This greatly limits the exploration of the properties and applications of such novel dyes.In the third chapter,we describe an efficient synthesis of seven-membered ring products by one-step synthesis using heat and O₂（and CH₃I）,I₂ or NIS as oxidants.Further,the reaction process was monitored by HPLC-MS and nuclear magnetic resonance spectroscopy to explore the important mechanism of thermal oxidation.In addition,we have conducted a detailed study of the basic properties of these new dyes,including absorption and fluorescence spectra in solution and polymer films,molar absorptivity,fluorescence quantum efficiency,fluorescence lifetime,and the effect of molecular concentration in the polymer on fluorescence.Comparing these dyes with the ring-open form of oxazolidine with a flexible conjugated structure,it was found that the fluorescence of the target product was greatly improved.In particular,the products 2e and 2f showed extremely strong red fluorescence in methylene chloride with fluorescence quantum efficiencies of0.92 and 0.62,respectively,which were very rare in conventional dyes.Theoretical calculations reveal the relationship between the strong fluorescence of the product and the enhanced structural plane.Finally,possible applications of these novel dyes as fluorescent inks,anti-counterfeiting and bio-label materials are demonstrated.In the fourth chapter,in order to improve the stability of molecular switch applications and expand the scope of application,we try to integrate the organic molecules into the polymer material.Considering that the main stimuli of oxazolidine molecular switches is water or acid,there are certain application limitations to its attachment to the polymer.We selected low-cost and expandable production of photo-/thermo-chromic DASA molecular switches and their precursors,active furans,to explore the stability and new functions of their attachment to polymers.The results indicate that the polymer active furan can detect amines in aqueous solution and is very stable in the absence of dye leakage or degradation problems as small molecules.In addition,the system has an easily tunable polymer matrix,which provides the possibility to study the fundamental structure and properties of DASA-polymers.It is worth noting that by adjusting the polymer glass transition temperature（Tg）,we can achieve a reversible switching ring of DASA molecules at different temperatures,which provides the possibility for temperature sensing.In summary,the photooxidation and thermal-oxidation mechanisms of oxazoline molecular switches were studied in this paper,and the fluorescence properties and possible applications of quantitatively synthesized oxidation products were investigated.By connecting the DASA-like molecular switch and its precursor to the polymer whose matrix is easy to adjust,stable visualization sensing of the ammonia in the water and temperature sensing are realized respectively.This work provides references for the oxidative degradation of spiro-type organic molecular switches,and also provides strategies for expanding the application of organic molecular switch attached to polymers.