Preparation Of Functional Two-dimensional Polymers Based On Covalent Self-assembly

Two-dimensional materials,for example,graphene,metal chalcogenides,and boron nitride have attracted considerable attention due to their unique properties.These materials with internal building blocks ordered arrangement prompt scientists to use synthetic methods to prepare similar two-dimensional polymers.To date,the preparation,properties research and applications of two-dimensional polymers have gradually become an important branch of modern materials chemistry.A two-dimensional polymer refers to a polymer sheet in which planar monomers are cross-linked by covalent bonds or non-covalent bonds and periodically arranged in long-range order.Two-dimensional polymers generally exist in three forms:layered crystals,multilayer structures or monolayer structures.In order to better study and utilize the properties of two-dimensional polymers,people have been trying to obtain monolayer structures.In general,two-dimensional polymers can be prepared through non-covalent bonds or covalent bonds.Non-covalent interactions are more beneficial for the ordered arrangement of building blocks,through which monomers can adapt to each other to achieve long-range order.However,the reversibility of non-covalent bonds leads to the poor stability of the ordered structures,which limits the application of two-dimensional polymers based on non-covalent interactions.Owing to the good stability,covalent bonds are also utilized to prepare two-dimensional polymers.When covalent bonds are used to construct two-dimensional polymers,in order to obtain single layer or few layer polymers,people usually exfoliate the multilayer structures,or introduce a solid surface to assist the growth of single layer.People also realize the goal of preparing ultrathin two-dimensional polymers by accurately and rationally designing and synthesizing molecules with specific structures as building blocks.These methods undoubtedly increase the complexity of work.Covalent self-assembly is an approach with the advantages of simple operation and one-step synthesis that has been discovered in recent years for fabricating two-dimensional polymers.It requires the building blocks should be rigid,flat molecules with symmetrical groups at the periphery,which can be directly linked into two dimensional polymers by flexible linkers.As a novel method for fabricating two-dimensional polymers,covalent self-assembly still remains further explored in many aspects,for example,can all the flat and rigid monomers with symmetrical groups be used for covalent self-assembly?What effects chemical bonds have on covalent self-assembly?And how to further functionalize the assemblies?Based on these considerations,we respectively synthesized C₃ symmetrical molecules with three chemical groups and C₄ symmetrical molecules with four chemical groups as building blocks,and creatively employed non-covalent interactions for the preparation of monomers.Through studying their covalent self-assembly behaviors and functionalizing the covalent assemblies,we explored the mechanism of covalent self-assembly from molecular level and provided ideas for the application of covalently self-assembled nanostructures.The research results are as follows:1.Construction of two-dimensional polymers based on covalent self-assemblyIn this part of the work,based on the methodological study of covalent self-assembly,we respectively prepared monomers for covalent self-assembly through employing covalent and non-covalent interactions,and studied their covalent self-assembly behaviors.We firstly prepared a C₃ symmetrical monomer?Tta?with three chemical groups,and constructed different polymer nanostructures based on Tta as building blocks and Schiff bonds as linkages,and realized the control on the size of nanocapsules,proving that Schiff bonds can be utilized for covalent self-assembly.In order to study the mechanism of covalent self-assembly from the molecular level,and directly observe the ordered arrangement of building blocks,we synthesized C₄symmetrical molecules?Thpp and Fe-Thpp?with four chemical groups as building blocks which had a larger molecular size.The constructed building blocks can be directly linked into nanocapsules by the reaction of phenolic hydroxyl groups and bromine.We successfully observed lattice-like structures from the nanosheets in the formation process of covalently assembled nanocapsules,which suggested that the monomers were orderly arranged.Thus,we provided an evidence for the mechanism of covalent self-assembly from the molecular level.The above research results indicated that the rigid,flat monomers were crucial for covalent self-assembly.Therefore,we creatively employed the metal coordination interaction for fabricating the building blocks for covalent self-assembly and prepared rigid,symmetrical Ni²⁺-Imidazole-4-carbaldehyde complexes.We successfully obtained different nanostructures through covalent self-assembly based on the prepared metal complexes as building blocks by changing the solvents used for self-assembly,which indicated the feasibility of using non-covalent interactions for fabricating covalent self-assembly building blocks.Through the work of this part,we have a deeper understanding for covalent self-assembly,which works well for rigid,flat monomers and has no special requirements for chemical bonds.Except for organic synthesis for building blocks,non-covalent interactions can also be used for preparing covalent self-assembly building blocks,which greatly expands the preparation of building blocks for covalent self-assembly.2.Construction of artificial light-harvesting systems based on covalently self-assembled nanocapsulesLight harvesting systems are widespread in nature and play important roles in photosynthesis.In the natural light-harvesting systems,antenna chromophores are orderly arranged on the thylakoid membrane in a very close distance.Considering for this,two dimensional polymers with long-range ordered monomers may be perfect candidates for fabricating artificial light-harvesting systems.Covalent self-assembly has been proved to be a feasible method for preparing ultrathin two dimensional polymers,however,it is still unknown that whether donor molecules and receptor molecules with different shapes can covalently self-assemble into polymer nanostructures or not.Herein,we successfully prepared polymer nanocapsules with Tta donors?C₃ symmetrical molecules with three reactive sites?and Tapp acceptors?C₄ symmetrical molecules with four reactive sites?together as buiding blocks and glutaraldehyde as linkers through covalent self-assembly,which indicated that monomers with different shapes can covalently coassemble into polymer nanostructures.Fluorescence spectroscopy showed that energy was efficiently transferred from Tta donors to Tapp acceptors.This phenomenon suggested that the covalently assembled nanostructures can serve as the platforms for light-harvesting.In this part of the work,we firstly covalently assembled a polymer nanocapsule based on two building blocks with different shapes,and developed it into a novel artificial light-harvesting system.This study greatly expands the choice of building blocks for covalent self-assembly,in addition,we provided a novel strategy for constructing artificial light-harvesting systems.3.Construction of artificial peroxidase mimics based on covalently self-assembled nanocapsulesPeroxidase,whose active site is mainly a kind of iron-porphyrin derivative,is very common in nature,can effectively catalyze the substrates oxidation reaction by hydrogen peroxide or other peroxide.Inspired by this,a variety of peroxidase mimics have been developed,however,nanocapsules as peroxidase mimics have been rarely reported.An ultrathin polymer nanocapsule whose shell is nearly one monomer thick,mainly composed of covalently linked iron-porphyrins may be a perfect candidate for mimicking the natural peroxidase.Herein,we prepared a polymer nanocapsule with Fe-TPyP?C₄ symmetrical molecules with four reactive sites?as building blocks and 1,6-dibromhexan as linkers,obtaining a peroxidase mimic equipped with multiple catalytic centers.Due to the shell mainly composed of iron-porphyrins and nearly one monomer thick,the peroxidase mimic constructed here had the advantages of rapid mass transfer,high catalytic activity and good substrate affinity.Based on the peroxidase-like activity of the covalently assembled nanocapsules,a highly sensitive colorimetric sensor for glucose was fabricated.In this part of the work,we firstly constructed an artificial enzyme based on a covalently self-assembled nanostructure,providing a novel strategy for mimicking natural enzymes.