Construction Of Novel Covalent Organic Frameworks And Structural Transformation

As a class of crystalline porous organic polymers,covalent organic frameworks(COFs)can be assembled by connecting organic building units via covalent bonds to form periodically extended one-dimensional,two-dimensional or three-dimensional framework structures.Due to their high specific surface areas,permeable channels,stable structures and adjustable functional characteristics,COFs have been proven to be multi-functional materials and successfully applied in many fields including gas capture and separation,energy storage and conversion,drug delivery,sensing,catalystis,environmental remediation and photoelectric devices.However,the structural diversity and novelty of COFs reported so far are still not enough to meet all requirements of different fields.This contradiction is more prominent in some fields such as photoelectricity and catalystis for which high performance materials are stringently required.In this context,designing COFs with specific structures to meet targeted applications is highly desired.Heteropore COFs and one-dimensional COFs,as the new research directions in the field of COFs,have attracted more and more attention due to their unique framework structures.On the other hand,using facilely accessible COFs as precursors,the COF-to-COF transformation could be a promising strategy to construct COFs which are difficult to synthesize through direct condensation of monomers.This new synthetic strategy has also been developed rapidly in recent years.In the first chapter,the origin and development of COFs are introduced.The general situation and current developments of COFs are summarized from five aspects:the design principles of topological structures,the types of covalent bonds,the synthetic methods and strategies,the applications,and the COF-to-COF transformation strategy.In the second chapter,we have established a model system based on the combination of two C_2v-symmetric building blocks.By changing the connection sequences of the two C_2vbuilding units,the assembly of two C_2v-symmetric building blocks would result in four different types of framework structures:two single-pore structures(SP-r and SP-h),one dual-hole structure(DP)and one three-pore structure(TP).This concept has been verified by the targeted synthesis of three proposed COFs.Their structures are elucidated by X-ray powder diffraction and the nitrogen adsorption experiments.This work reveals the influence of connection sequences of building units on the formation of the framework structures of COFs.In Chapter 3,two dual-pore COFs are synthesized as precursors for the study of the COF-to-COF transformation based on exchange of monomers with different symmetries.In the experiments of COF structure conversion,the excess C₃symmetric monomers are used to in situ replace the C_2vsymmetric monomers in the precursor COFs and the two-pore COF is converted to the single-pore COF successfully.As a result,the conversion between two COFs with distinct pore hierarchy is realized.This COF-to-COF transformation strategy can not only be applied to amino building units with different symmetries,but also to aldehyde monomers,which demonstrates the generality of this transformation strategy.This research expands the application scope of the building-unit-exchange-based COF-to-COF transformation.In the fourth chapter,polycondensation between a D_2hsymmetric monomer with four connection sites and a C_2vsymmetric monomer with two connection sites is carried out to construct a one-dimensional COF.The one-dimensional COF not only can be synthesized in a variety of single solvents,but also can maintain a high crystallinity under harsh conditions such as long-term solvent immersion and ultrasonic dispersion,indicating a high stability of the COF.In addition,the one-dimensional COF shows good capabilities of dispersion and self-assembly.It can assemble into nanoribbons,nanotubes or nanosheets by adjusting the type of solvents,demonstrating the unique properties endowed by the unique structure of the one-dimensional COF.In Chapter 5,based on the strategy established in the previous chapter,another new one-dimensional COF is synthesized through the polycondensation of a D_2hsymmetric monomer and a C_2vsymmetric monomer.Terpyridine units are introduced to the main chain of the one-dimensional COF by modifying the structure of the C_2vsymmetric monomer.The incorporation of the terpyridine segments expands the application potential of the COF.Due to the strong coordination capability of the terpyridine units evenly distributed on the nanoribbon skeleton,the COF can be embellished with metal ions on the designated sites.And it also provides the possibility to weave the one-dimensional COF into the high-dimensional polymers driven by tripyridine-metal coordination interactions.