Review

The atom, the molecule, and the covalent organic framework

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Science  03 Mar 2017:
Vol. 355, Issue 6328, eaal1585
DOI: 10.1126/science.aal1585

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A framework for molecular assembly

Covalent molecular frameworks are crystalline microporous materials assembled from organic molecules through strong covalent bonds in a process termed reticular synthesis. Diercks and Yaghi review developments in this area, noting the parallels between framework assembly and the covalent assembly of atoms into molecules, as described just over a century ago by Lewis. Emerging challenges include functionalization of existing frameworks and the creation of flexible materials through the design of woven structures.

Science, this issue p. eaal1585

Structured Abstract

BACKGROUND

Just over a century ago, Gilbert N. Lewis published his seminal work “The Atom and the Molecule” on what later became known as the covalent bond. Since then, organic chemists have systematically developed synthetic methodologies for covalent molecular chemistry, and this has led to the art and science of total synthesis. Extending these organic reactions beyond the molecule to making covalently linked two- and three-dimensional (2D and 3D) organic structures has been a long-standing objective. Recently, this has been realized in the reticular synthesis of covalent organic frameworks (COFs)—extended porous structures entirely composed of light elements and held together by strong covalent bonds. COFs have robust architectures endowed by high porosity and thermal and chemical stability, which have allowed organic and inorganic reactions to be carried out on these frameworks without losing their porosity or crystallinity. This has given rise to the “chemistry of the framework,” where Lewis’ concept of the atom and the molecule is extended to the framework in which matter can be further controlled and manipulated.

ADVANCES

The ability to design COFs and to adjust their pore metrics using the principles of reticular synthesis has given rise to frameworks with ultralow densities (0.17 g cm−3), high surface areas (4210 m2 g−1), large pore sizes (up to 4.7 nm), and high charge-carrier mobility (8.1 cm2 V−1 s−1). The combination of adjustable pore metrics with the backbone functionalization of the framework by means of pre- and postsynthetic modification has successfully been used to tailor COFs for a plethora of applications in areas such as gas separation, energy storage, catalysis, and electronics. Recently, the union between the covalent and the mechanical bond in the context of the chemistry of the framework has resulted in the first implementation of the concept of molecular weaving. The added degrees of flexibility in woven COFs have the potential to combine dynamics with resilience in solids. At present, this methodology is being applied to the design of frameworks with different modes of entanglement, such as interpenetration of 2D and 3D networks or formation of extended structures based on the interlocking of discrete (0D) rings to make molecular chain mail.

OUTLOOK

Historically, the field of chemistry has flourished as our ability to control matter on the molecular level has improved. COFs are the first examples of controlling the covalent bond beyond molecules and demonstrate how this control results in expansion of the scope of covalent organic solids and their properties. Organic chemists study the chemistry of new organic molecules, and similarly we expect the study of the basic structure and reactivity of COFs and the investigation of their properties to continue. However, this emerging chemistry of the framework is already pointing to several new directions, such as the ability to work with atomically well-defined interfaces. The traditional view that interfaces are 2D is not strictly applicable in the framework, where substrates form boundaries with 2D as well as 3D frameworks, which are atomically and metrically well defined. By virtue of the framework chemistry outlined in this contribution, such 3D interfaces can be chemically functionalized and metrically altered. In this way, heterogeneously arranged functionalities can be arranged within well-defined distances to operate in a manner akin to active sites of enzymes. The extension of Lewis’ original concept from atoms to molecules and now to covalent organic frameworks adds pore space into the realm of synthetic chemists’ ability to control matter.

The atom, the molecule, and the covalent organic framework.

Since the discussion of strong chemical interactions between atoms by Gilbert N. Lewis in 1916, covalent organic chemistry has progressed from discrete molecules to porous covalent organic frameworks such as COF-1, the first COF, illustrated here in the traditional Lewis dot structure.

Abstract

Just over a century ago, Lewis published his seminal work on what became known as the covalent bond, which has since occupied a central role in the theory of making organic molecules. With the advent of covalent organic frameworks (COFs), the chemistry of the covalent bond was extended to two- and three-dimensional frameworks. Here, organic molecules are linked by covalent bonds to yield crystalline, porous COFs from light elements (boron, carbon, nitrogen, oxygen, and silicon) that are characterized by high architectural and chemical robustness. This discovery paved the way for carrying out chemistry on frameworks without losing their porosity or crystallinity, and in turn achieving designed properties in materials. The recent union of the covalent and the mechanical bond in the COF provides the opportunity for making woven structures that incorporate flexibility and dynamics into frameworks.

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