| Building blocks could construct supramolecular assemblies through supramolecular forces,because the supramolecular forces are non-covalent,and the interaction between building blocks will change under different conditions,resulting in the supramolecular assemblies could respond to the external stimuli.So that the assembly could change from the assembled state black to the disassembled state.Therefore,supramolecular assemblies play an important role in the fields of flexible porous materials,disease diagnosis and treatment,selfhealing materials and so on.Besides,ordered supramolecular assemblies have more application scenarios than disordered supramolecular assemblies and more popular,such as using in selective catalysis,size separation applications and so on.A prerequisite for the construction of stable ordered supramolecular assemblies is that the binding force intensity is high and the direction is fixed between the adjacent building blocks,and the position between the building blocks is ordered and controllable,the ordered supramolecular assembly can be fabricated orderly only when the building blocks extended orderly.Cyclodextrins,are a class of supramolecular macrocycle host molecules,could bind with a variety of guests through hostguest interaction,and play an important role in directionally connecting different structural building blocks.Such directionally binding would bring more ordered structures and more diverse functions with the supramolecular assembly construction.Polyoxometalates are a kind of inorganic cluster molecules,could also be used as good construction building blocks,and meanwhile,polyoxometalates have very good properties in catalysis,and furthermore they could connect the guest molecules with both covalent modification and the electrostatic interaction.When polyoxometalates and cyclodextrins get together,ordered supramolecular assemblies could be established and the catalytic ability of polyoxometalates could be the applications of the supramolecular assembly.In this study,our starting point is constructing ordered supramolecular assemblies through host-guest interactions with both polyoxometalates and cyclodextrins.We designed two kinds of bridged ?-cyclodextrin molecules with different lengths with three branches at 120 degrees to each other as the host assembly building blocks.At the same time,several guest building blocks were also designed.We hope that the host-guest building blocks could be extended orderly in the plane through the host-guest interaction,by a mode of "3+2" or "3+3" to form ordered two-dimensional supramolecular assemblies,so as to exert the catalytic performance of polyoxometalates.We first synthesized a three-arm bridging ?-cyclodextrin named tri-CD-1 with a planar triphenylbenzene as the molecular core and the linker was a triazole group formed by a "click reaction" with Cu? catalyzed.The angle of the branches are 120° between three-arms bridging cyclodextrin between each other.Another three-arm bridging ?-cyclodextrins named tri-CD-2 was also synthesized with a planar benzene as the molecular core and the linker was a triazole group.The angle of the branches is 120° between three-arms bridging cyclodextrin between each other.These two three-arm bridging cyclodextrins could work as host building blocks.As for the guests,we synthesized:two kinds of linear double-side covalently connecting with guest groups on the polyoxometalate;two kinds of guests covalent modified with different numbers of hydrophilic groups on the three branches with the angles at 120 degrees to each other;and two kinds of the 120°three branched electrostatic complexes with different lengths of cationguest group.We studied the structure of the supramolecular assembly formed by two kinds of threearm bridging cyclodextrins with different guests.Through comparison,we believe that the hosts and the guests both have a great influence on the final structure of the supramolecular assemblies.One of the three-arm bridging cyclodextrin tri-CD-1 has a self-inclusion problem in water due to its long linking group in molecular structure.It is found that in water,tri-CD-1 was with one of the three cyclodextrins in self-inclusion conformation and the other two of the three cyclodextrins in non-inclusion conformation.When in solution,binding with the guests,tri-CD-1 partially unbinds itself,therefore,some of tri-CD-1 binds with two guest groups and some of tri-CD-1 binds with three guest groups,so that tri-CD-1 cannot be used as a stable planar three-branch node nor a two-branch node to construct the supramolecular assemblies.The host-guest building blocks cannot be extended and assembled in an orderly manner,and as a whole,the supramolecular assembly is disordered.With the studies,we found that the other three-arm bridging cyclodextrin tri-CD-2,which did not self-include in water,formed more ordered supramolecular assemblies with the guests,which was more advantageous as the host building blocks of supramolecular assembly.We also found that the guests effect the structure of assemblies very much that the guests with different structure,molecular flexibility,hydrophilicity and hydrophobicity.For two kinds of guests with the similar structure,the assembly structure reveals different greatly,such as the two guests,which molecular skeleton structures are the same,but with different number of hydrophilic triethylene glycol monomethyl ether groups.Different hydrophilic groups bring different overall guest hydrophilicity.Both of the two guests could be solubilized by tri-CD-2 and completely soluble in water,but the supramolecular assembly formed by the more hydrophilic guest with triethylene glycol monomethyl ether group precipitates out of the aqueous solution as droplets,while the supramolecular assembly formed by less hydrophilic guest dissolves in water.And comparing the two electrostatic complex guests with different lengths of three branches adamantane group to polyoxometalate,the two electrostatic complex guests are poorly soluble in water,and after interacting with tri-CD-2,the forming supramolecular assemblies are slightly soluble in water,the difference of the guests is the length of the hydrophobic groups between adamantane group and cationic quaternary ammonium salt head,the longer linkers,the more hydrophobic assembly,lead to the assemble associate too fast,in the assembly process,there are more wrong combinations between the host-guest,so that the assembly is less ordered..By contrast,the supramolecular assemblies constructed by two polyoxometalate clusters covalently modified with adamantane group and azobenzene group and through host-guest interacting with tri-CD-2 respectively,and the structures of the formed assemblies were also quite different.Azobenzene guest formed a relatively ordered two-dimensional supramolecular assembly,while adamantane guest formed a disordered assembly.The 2D supramolecular assembly formed by tri-CD-2 as the host and the covalently azobenzene group double-side modified on the polyoxometalate cluster as the guest,though the organization in the 2D supramolecular assembly layer is likely not ordered enough,due to the host-guest building blocks'direction control is not perfect.But its growth in plane is relatively intact,because of the highly dispersion and the sustained fixation of the inorganic cluster locating in the edge of the porous assembly structure.The porous network assembly shows good stability in both aqueous and hydrophobic organic phases.And the constructed polyoxometalate containing supramolecular assembly has more than the surface of the negatively charged clusters and the porous structure of 2D supramolecular assembly promotes the catalytic performance.So that the supramolecular assembly could degrade dyes in aqueous solution and in organic solution,under the mild condition of no extra energy input,which is the absence of strong oxidizer and without UV/Visible light irradiation at room temperature.We also tried to flow the organic solution of dyes through the assembly material,and achieved the degradation of dye in the flow process,which also had a good degradation effect.In addition,it was found that the degradation rate of Malachite Green mixed with Rhodamine B was significantly accelerated in both aqueous solution and dichloromethane.We believed that the size of the dye aggregate had an impact on the degradation rate,and we believe this phenomenon is the auxiliary degradation effect of the dye.The research results of this paper extend the preparation method of supramolecular materials and provide a new idea for dye degradation through the method of constructing supramolecular assembly with the interaction between the host and guest of the bridging cyclodextrin and the bridging guest. |
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