2D Flexible Supramolecular-Ionic Framework For Precise Membrane Separation Of Small Nanoparticles

Polyoxometalates?POMs?are a class of polyanion clusters with nanoscale precise size and rich topologies.They not only exhibit strong functionality,but also can be a series of excellent assembly blocks.Diverse POM complexes can be prepared via covalent decoration and ionic encapsulation,which could form rich structures and morphologies when compounded with other building blocks in solution.Due to the delocalization of the charges on the surface of POM and the non-directional nature of electrostatic interaction,the POM based electrostatic complexes owe good flexibility.However,under the influence of hydrophilic and hydrophobic effects,the POM complexes themselves tend to aggregate to reduce the interface energy,the flexibility is thus difficult to show.Two-dimensional framework materials have ultra-high specific surface area and uniformly distributed functional sites,and its excellent performance in many aspects has attracted people's attention.At present,the two-dimensional covalent framework?COF?and two-dimensional metal organic framework?MOF?,which have good development state,have a regular and ordered pore structure and are widely used in many fields,but often exist in the form of crystalline powder resulting in the excessive rigidity and poor processability.In particular,the development in the direction of device manufacture and film formation is therefore limited.Introducing the POM electrostatic complexes into the two-dimensional framework and participating in the construction of the framework,and using electrostatic interaction and host-guest interaction to build a supramolecular-ionic framework?SIF?can solve the problems of traditional frameworks to some extent.The electrostatic interaction provides the necessary flexibility for framework and the host-guest interaction can bring stimulate respondence to the system.With the synergy of the two,SIF obtains good flexibility and processability.In this paper,on the one hand,we use electrostatic interaction and host-guest interaction to improve flexibility,on the other hand,we choose a guest molecule with a rigid core to support the entire structure to ensure the strength of the framework.First,we design and synthesize a double-charges Anderson-Evans type polyoxometalate and electrostatically encapsulate it with a pillar[5]arene based cationic surfactant.The two species undergo ionic exchange reactions to afford linear electrostatic complex host.We have performed detailed structural characterization of it via ¹H NMR,MALDI-TOF,FT-IR and elementary analysis test.Then we synthesize a porphyrin-based guest molecule with the four branches modified with cyano group.Furthermore,the two species form a supramolecular-ionic framework through host-guest recognition.We use ¹H NMR spectra to characterize the existence of host-guest interaction and confirm the binding mode of one cyano group recognizing one pillar[5]arene cavity,we also use NOESY test to obtain more precise host-guest position relationships.Second,we observe through transmission electron microscopy?TEM?and atomic force microscopy?AFM?that the two species are assembled in chloroform to form a micron-scale two-dimensional sheet-like structure,indicating that the internal assembly elements are highly cross-linked and tend to grow in two dimensions,the thickness is about four monolayers which meet our needs for two-dimensional structures.However,the assembly obtained in pure chloroform has certain defects and its lateral size can be further increased.Therefore,we choose the guest molecule n-hexane with weaker binding ability with pillar[5]arene,larger assemblies is expected to be obtained under the condition of competitive slow assembly.The result turns out that the assembly can indeed become larger,but at the same time its thickness is doubled,and because thechloroform volatilizes too quickly,the problems that easily cause cracks and defects in the assembly remain unresolved,which is contrary to our original intention.In order to avoid the above problems,the assembly powder obtained in pure chloroform solution was evenly dispersed in a mixed solvent of higher boiling point H₂O/DMSO to obtain ideal two-dimensional nanosheets with large size,small thickness,good flexibility and few defects,which lay the foundation for the next functionalization of nanosheets.We use powder X-ray diffraction and CO₂ sorption to characterize the pore structure,the nanoscale pore size and good processability of the nanosheets,making them useful in the field of nanoscale separation membranes.Third,with the support of a commercial polytetrafluoroethylene porous membrane,the dispersion liquid can be made into a separation membrane by simple filtration,and the membrane is stable in water system,no membrane leakage could be detected in the filtration process.After being treated by H₂O/DMSO,the membrane changes from hydrophobic to hydrophilic,which owes an ultra-high water flux,and can be used for precise separation of various aqueous nanoparticles.In short,the starting point of this paper is to construct a flexible two-dimensional framework.We use electrostatic interaction and host-guest interaction to achieve this goal synergistically,and successfully accomplish the construction of flexible two-dimensional supramolecular-ionic framework,furthermore the nanosheets are functionalized under these conditions for precise nanoscale separation.The work of this paper provides new ideas for design and construction of a new flexible two-dimensional framework structure,and we believe that multiple supramolecular forces will play an important role in the preparation of flexible membrane materials.