Essay: Covalent Organic Frameworks

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## 1.1 Covalent Organic Frameworks
The development of porous materials has accompanied the demands of modern society. A large number of porous materials have been designed and synthesized in the past half century. Because of the intrinsic properties of large specific surface area, high chemical stability, and low skeleton density, porous organic polymers (POPs) have garnered considerable attention from both the academic and the industrial communities. In general, POPs are categorized into several classes, such as hyper-cross-linked polymers (HCPs),1 polymers of intrinsic microporosity (PIMs), covalent organic frameworks (COFs), and conjugated microporous polymers (CMPs), which exhibit potential applications in gas storage, gas separation, and heterogeneous catalysis.
Covalent organic frameworks (COFs) are a class of crystalline porous polymers, which are atomic precisely constructed with well-designed organic building blocks via covalent bonds. In 2005, Yaghi and co-workers demonstrated the utility of the topological design principle in their synthesis of porous organic polymers connected via reversible bonds, which produced the first successful examples of Covalent organic frameworks (COFs).Since this landmark discovery, the chemical synthesis of COFs has attracted tremendous interest and progressed significantly, and they show great potential for functional exploration.
Because COFs are composed of lightweight elements linked by strong covalent bonds, they have low mass densities, possess high thermal stabilities, and provide permanent porosity. Depending on the building block dimensions, COFs can be categorized into either two- (2D) or three-dimensional (3D) COFs. In 2D COFs, the building blocks for the vertices and edges are covalently linked to form extended 2D polygon sheets that stack to constitute layered frameworks, giving rise to two structural characters, i.e., periodic ?? arrays and ordered one-dimensional (1D) channels. This periodic ?? arrays structure provides a unique means to construct ordered ?? systems that are difficult to create via conventional covalent and/or non-covalent approaches. The ordered columns in 2D COFs could facilitate charge carrier transport in the stacking direction, which implies that 2D COFs have potential for developing new type ??-electronic and photo-functional materials for optoelectronics and photovoltaics. In contrast, 3D COFs, which extend this framework three-dimensionally through a building block containing an sp3 carbon or silane atom, characteristically possess high specific surface areas, numerous open sites, and low densities. These features make 3D COFs ideal candidates for gas storage.
In this chapter, we will discuss the COFs from the design and synthesis, functions and properties, and characterization.
## 1.2 Design and Synthesis
One significant feature of COFs is that they allow for the topological design of structures. The skeletons and pores of COFs can be controlled by design the size and geometry of building blocks. Therefore, they are a new class of crystalline polymers that enable the molecular design of structures. A variety of different COFs have been developed using different linkages and building blocks. They all have discrete crystal and porous structures.
### 1.2.1 Design concerns
As a class of porous crystalline materials, the basic design concern of COFs is to obtain new materials with high porosity, high crystallinity and specific functions. Because COFs are similar to another class of well developed porous crystalline materials, metal-organic frameworks (MOFs) which are consisting of metal ions coordinated to organic molecules, the principles of reticular chemistry as well as much experience has been obtained for the MOF systems could be adapted to the COFs design as well. However, constructing building blocks via reversible covalent bonds to form crystalline COFs is much more difficult than that of MOFs which are constructed via coordination chemistry.
#### Porosity
As for any porous materials, the issue of porosity is the first basic concern for constructing COF materials. In this notion, some design strategies that applied to prepare other porous polymers might be applicable to the COF synthesis. One applicable strategy is to utilize the rigid building units to create the porous structures. A variety of different porous polymers have been synthesized by polymerization of rigid monomers via coupling reactions.

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