Covalent Organic Frameworks: Chemistry of Pore Interface and Wall Surface Perturbation and Impact on Functions

Abstract

ConspectusPorous structures are ubiquitous, discovered in nature ranging from biological channels in animals and plants to various pores in sediments and minerals and playing vital roles in maintaining biological activities and ecosystems. Synthetic pores have been known for over 100 years and currently continue to be a central subject in the fields of chemistry, physics, materials science, and technology. A fundamental key issue is how to develop specific functions with pores. Pores are determined by three parameters, including pore shape, size, and environment; how to design these parameters in a controlled manner is a key subject. Covalent organic frameworks (COFs) are a distinct class of crystalline porous polymers as they combine covalent and noncovalent chemistries to result in long-range-ordered polygonal skeletons and discrete pores. The topological diagram─the principle for designing COFs─enables the predesign of not only skeletons but also pores, offering a powerful molecular platform for constructing tailor-made organic/polymeric materials. Over the past decade, progress in chemistry has greatly enhanced our capability to synthesize COFs to achieve different structures. Especially, the pores in COFs are constructed with lightweight elements, covalent bonds, and organic components, which offer numerous combinations to design and synthesize pore shapes, sizes, and interfaces. These parameters control the interplay with guest molecules and ions to determine the property and function of pores. Among various synthetic porous materials, COFs are unique in that these pore parameters are topologically designable and synthetically controllable. Two complementary strategies, i.e., topology diagram and pore surface engineering, have been developed for pore chemistry, in which the first one emphasizes the in situ approach to design and control pores and the second one highlights the postsynthetic way to tune the pore structures finely yet precisely. These two different chemistries offer individual ways to explore different pores, properties, and functional materials and systems. Looking at the features of COFs, a basic common structure of the pores is the well-defined pore interface, which is constituted by aligned surface atoms and side units at a proximate distance along the pore long-axial direction and distributed periodically over the pores. These features of pore interfaces are specific to COFs and predetermine their functions. In this Account, we scrutinize the chemistry of pore interfaces by focusing on the pore shape, pore size, and aligned atoms and units on pore walls to design unique properties and functions. We highlight pore wall perturbation with surface atoms and side units to demonstrate their decisive roles in structural formation and functional expression. We summarize perspectives, key issues to be addressed, and opportunities with the aim of showing a promising way to next-stage materials and functional designs.