Two Dimensional Polyoxometalate-based Ionic Organic Frameworks Constructed By Electrostatic Assembly

Polyoxometalates are a kind of nano-size polyanionic clusters with abundant topological structure and strong functional properties.It can form various structures with cationic surfactants via electrostatic interactions in solution which makes it an excellent supramolecular building blocks.Due to the lack of directivity in electrostatic interaction,it is hard to form regular arrangement for polyanionic clusters assembly which is always aggregated into balls or layers affected by the interfacial energy.Two-dimensional supramolecular organic framework is a new type of porous material developed from two-dimensional coordination framework and two-dimensional covalent framework.To a certain extent,it solve the weakness of traditional frameworks which always form into crystals or powders and have poor mechanical strength.On the other hand,it introduce the advantages of flexibility and smart response.However,relative to the vigorous development of the 2D MOF and COF,the supramolecular organic framework still develop slowly,which is related to its single binding mode.As we all know,many supramolecular interaction have no direction which is difficult to be used for building ordered frameworks.The combination of polyoxometalates and organic frameworks will improve the functional properties of POMs by using the high specific surface area of the organic frameworks.Now there are two ways,one is to use the coordination bond to link two POMs,the other way is to use the POMs as a template.It is still hard to form organic framework by electrostatic directly.In this dissertation,I successfully restrict the free sliding of the cation on the POMs by using the space matching among the POMs and the large cation heads,to realize the ordered arrangement of the building blocks and achieve the polyoxometales-based ionic organic frameworks.In the next I will study the the structural fratures of the POM-based two-dimensional ionic organic framework in two parts.First,in order to construct a flexible hydrophilic organic framework material,we designed and synthesized a series of bola-cationic surfactants with different PEG chain length.On the each side of the surfactant contain a pyridine group and an azobenzene group.We characterized the bola-surfactant by ¹H NMR,ESI-MS and elemental analysis.We expect to inhibit the hydrophobic interaction between the azobenzene groups and get homogeneous solution by adding alpha cyclodextrin.At the very beginning,we investigated the host-guest interaction between alpha cyclodextrin and azobenzene group by a series 1H NMR results with different molar ratio.We found that the host-guest complexation exhibit slow chemical exchange on the NMR time scale and calculate the binding the ratio,1/2,by Job-plot curve fitting.Further,by 2D NOESY,we accurately know that the small side of the cyclodextrin is near the pyridine group?outer side of the surfactant?and the large side of the cyclodextrin is near the inner side.We also studied the thermodynamic changes in this process by ITC.After adding the CDs into the bola-cationic surfactants,the host-guest complex appear spontaneously with heat release.From the ITC results we know that the binding ratio is 1/2 and the binding constant is 10⁴ M^-1.The increase of ionic strength will increase the host-guest binding constant.We can also calculate the binding ratio by circular dichrosim titration.Second,by mixing the bola-cationic surfactant obtained in the previous work and different Keggin-type POMs we get novel assemblies based on electrostatic interaction.At first through observing the Tydall phenomenon and the DLS data we know that micron size assemblies appear in the aqueous solution.And we ensure the host-guest pseudorotaxane still exist after adding POM by comparing the ¹H NMR results before and after assembling.By using water film,soak and lift,suction filter methods,we successful transfer the assemblies onto the carbon film,mica and porous PC film.Then we investigated the assemblies'structure by using TEM,AFM,and PXRD.And it can be judged that the shape of the assemblies are one molecule thickness thin sheets and the height of the sheet is uniform.The radius of the sheets are positively correlated with the concentration of the solution,usually ranging from one micron to more than ten microns.Then we simulated the possible assembly structure by molecular fitting and found that four CDs can match well with one POMs on size and the nearest distance between two CDs is short enough to form hydrogen bond.At last we verify the structure model by the square lattice of the POMs on the sheets by HRTEM.In brief,in order to build a two-dimensional supramolecular framework by electrostatic interactions,we use size matching and synergistic assembly as methods to limit the directionless of the electrostatic force.First we synthesized a bola-cationic surfactant,and study the process of the recognition between the azobenzene group and alpha cyclodextrin,confirm the binding ratio and constant;then by the electrostatic interaction between the bola-cationic pseudorotaxane and Keggin polyoxometalates in aqueous solution we get a new kind of 2D polyoxometalates-based ionic organic framework,and its structure was analyzed and fitting later.This work not only provides a new idea for constructing the supramolecular organic framework,but also realizes the ordered assembly of POMs in solution,...