Immobilization And Potential Applications Of Organic-Based Molecular Switches In Metal-Organic Frameworks

Organic-based molecular switches(OMS),which can be reversibly transferred between two or more stable states,are the smallest component or unit for the construction of novel smart materials.In order to realize real-world applications,it is essential to immobilize OMS into a solid state from solution state.However,after being immobilized from the solution state,OMS in solid state may face the problems of lower transfer efficiency or even no transfer behavior as the strong interaction and spatial confinement resulted from the decrease of the instance between them.Metal-organic frameworks(MOFs)are a kind of infinite frameworks self-assembled from metal nodes and organic linkers.As a novel kind of organic-inorganic hybrid porous materials,MOFs were thought to be a kind of ideal platform for immobilizing OMS.There are several reasons that support this.First,OMS can be easily designed to the organic liker and further efficiently immobilized to the frameworks of MOFs.Second,by judicious design and topology selection,MOFs can be designed and synthesized to provide enough space for the free access of reactants to activate OMS and the conformation changes of OMS.Third,some stable frameworks of MOFs can tolerate the drastic changes during the transformation of OMS,thus provide the possibility for the reversible conversion of OMS under triggers.What's more,the crystalline structure of MOFs may also provide crystallographic characterization for the transfer of OMS,which would deepen our understanding for the structure-function relationships at the molecular level and provide important information for the design and construction of novel OMS and stimuli-responsive materials for applications.In this thesis,a brief introduction of OMS and the immobilization of OMS with the problems faced were firstly discussed.The thesis also gave a brief introduction to MOFs and the advantages of immobilizing OMS into MOFs.After that,a detailed literature review was made on the stimuli-responsive MOFs based on OMS and the problems facing stimuli-responsive MOFs were clarified.In order to solve these problems,the thesis put forward the strategy to enable the efficient conversion of OMS in solid state by immobilizing OMS into the frameworks of MOFs as one of their organic components.Therefore,the main content of the thesis is to immobilize OMS into the frameworks of MOFs to study their efficient switching behavior and explore their potential applications.The specific contents are as follows:(1)Constructed the crystalline Zr-MOF named UiO-68-OH by immobilizing hydroquinone into the stable zirconium metal-organic framework(Zr-MOF)via an organic linker containing hydroquinone unit.After many trials and errors about exploring the redox conditions,the thesis constructed a redox triggered sngle-crystal-to-single-crystal transformation system based on the quantitative redox reactions of hydroquinone in the frameworks of MOFs while keeping the frameworks intact.What's more,from the tracking studies of the redox reaction in the frameworks by recording the crystal photo images,1H NMR and solid state 13C NMR in different reacting times,the thesis found the redox reactions were procceded smoothly in the framework from the outsurface to the inner,which demonstrated it is an efficient strategy to enable the efficient conversion of OMS in the solid state by immobilizing OMS into the frameworks of MOFs.(2)Combining the strategy to enable the efficient conversion of OMS in the solid state with supramolecular chemistry,the thesis constructed a water-stable Zr-MOF named UiO-68-azo after immobilizing the photo-responsive azobenen units into the framework as dangling groups.Then,UiO-68-azo was used to construct on-command cargo release system for rhodamine B(RhB).After loading RhB into the pores of UiO-68-azo,the MOF were rinsed with ?-cyclodetrin(?-CD)aqueous solution.Owing to the supramolecular reaction of the trans-azobenzene units in the outer surface of UiO-68-azo with ?-CD,a supramolecular complex was formed in the outer surface of the MOF and stablized the RhB molecules in the inner pores.What's more,the supramolecular complex can be disassembled under the trigger of UV light or the competition regeant of amantadine,thus release the loaded RhB in the pores in a controlled manner.As a result,a simple but efficient method to build on-command cargo release system was constructed,which could be used to construct on-command drug delivery in the future.(3)The thesis immobilized both donor(anthracene)and acceptor(maleimide)of the fluorescent photoinduced electron transfer(PET)into the same framework of a MOF through multivariate MOFs strategy and constructed the crystalline bifunctional Zr-MOF named UiO-68-An/Ma.Interestingly,unlike the regular'fluorophore—spacer-receptor' format of fluorescent PET switches/sensors,the close arrangement of fluorescent and acceptor groups in the confined nanopores of MOFs can also favor the pseudo-intermolecular PET process.What's more,by tuning the electron accepting ability of maleimide via Diels-Alder reaction or thiol-ene reaction,the fluorescent PET process in the framework can be efficiently modulated.As a result,a thermal-responsive solid-state fluorescent switch and a selective fluorescent turn-on sensor were constructed based on D-A and thiol-ene reaction respectively.Furthermore,UiO-68-An/Ma shows fast and convenient response to 50 ?mol/L biothiols within 5 min from the sensing studies toward different biothiols and represntive native amino acids.These studies further investigated the potential applications of OMS and provided a novel strategy for constructing solid state fluorescent PET switch/probe.