![3D COFs with the dia topology. a,b) Schematic representation of COF‐300 with dia‐c5 topology. Reproduced with permission.[⁴⁹] Copyright 2009, American Chemical Society. c) Schematic representation of the aging strategy in the synthesis of COF‐300. Reproduced with permission.[⁵⁰] Copyright 2018, American Chemical Society. d) Schematic representation of PI‐COF‐4 and PI‐COF‐5; e) 3D structural representations of PI‐COF‐4 (above) and PI‐COF‐5 (below). d,e) Reproduced with permission.[⁵¹] Copyright 2009, American Chemical Society.](https://www.researchgate.net/publication/377811914/figure/fig4/AS:11431281241859511@1715263461373/3D-COFs-with-the-dia-topology-a-b-Schematic-representation-of-COF-300-with-dia-c5_Q320.jpg)
![3D COFs with the ctn and bor topology. a) Schematic representation of COF‐102, 103, 105 and 108 with ctn and bor topology. Adapted with permission.[⁴⁸] Copyright 2007, The American Association for the Advancement of Science. b) Schematic representation of DBA‐3D‐COF with bor topology. Reproduced with permission.[⁶⁸] Copyright 2016, American Chemical Society.](https://www.researchgate.net/publication/377811914/figure/fig5/AS:11431281241859512@1715263461596/3D-COFs-with-the-ctn-and-bor-topology-a-Schematic-representation-of-COF-102-103-105_Q320.jpg)
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Topological Analysis and Structural Determination of 3D
Covalent Organic Frameworks
Zi’ang Guo, Zeyue Zhang, and Junliang Sun*
3D covalent organic frameworks (3D COFs) constitute a new type of
crystalline materials that consist of a range of porous structures with
numerous applications in the fields of adsorption, separation, and catalysis.
However, because of the complexity of the three-periodic net structure, it is
desirable to develop a thorough structural comprehension, along with a
means to precisely determine the actual structure. Indeed, such
advancements would considerably contribute to the rational design and
application of 3D COFs. In this review, the reported topologies of 3D COFs are
introduced and categorized according to the configurations of their building
blocks, and a comprehensive overview of diffraction-based structural
determination methods is provided. The current challenges and future
prospects for these materials will also be discussed.
1. Introduction
Covalent organic frameworks (COFs) are a new class of porous
crystalline materials composed of light elements connected
by covalent bonds.[1]Due to their composition and porous
nature, COFs exhibit a number of desirable properties, in-
cluding low densities, high surface areas, and designable
structures.[2]In addition, they have demonstrated excellent
performances in various application fields, including adsorp-
tion and separation,[3–6]energy storage,[7–10 ]catalysis,[11–17]mass
transfer,[18–22]sensing,[23–27]and film devices.[28–30]Based on their
dimensional characteristics, COFs can be classified into 2D struc-
tures with 𝜋–𝜋stacking and 1D channels, or 3D structures, con-
taining interpenetrated frameworks and covalent bonds that ex-
tend across the skeleton.[31]Among these, the 3D COFs, which
exhibit more extensive structures, tend to contain more abundant
channels, greater numbers of active sites, lower densities, and
larger specific surface areas, thereby indicating their broader ap-
plication prospects.[31–35]
However, 3D COFs have received less research attention than
2D COFs owing to the challenges associated with their struc-
tures. For example, due to the fact that COFs are connected by
Z. Guo, Z. Zhang, J. Sun
College of Chemistry and Molecular Engineering
Beijing National Laboratory of Molecular Sciences
Peking University
Beijing 100871, P. R. China
E-mail: junliang.sun@pku.edu.cn
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adma.202312889
DOI: 10.1002/adma.202312889
irreversible covalent bonds, it is extremely
difficult to obtain single crystals that are
large enough for structural determina-
tion by single-crystal X-ray diffraction (SC-
XRD). Therefore, their structures are usu-
ally verified by structural modeling and
refinement of the powder X-ray diffrac-
tion (PXRD) data.[36]In the case of 2D
COFs, the possible topologies are lim-
ited, and therefore the topology of a cer-
tain COF can be predicted according to
the geometric features of the building
blocks, and can subsequently be verified
by PXRD measurements. However, 3D
COFs, which are generated by the conden-
sation of nonplanar building blocks, pos-
sess more topological possibilities than 2D
COFs, thereby rendering it more difficult to
rationally design and predict their topologies, especially in the
case of 3D COFs prepared from highly connected building
blocks.[37]It is therefore important to obtain an improved under-
standing of the topologies of 3D COFs and develop appropriate
structural determination techniques to promote further research
into these materials. Thus, in this review, we clearly describe the
three-periodic net structures and topologies that may exist in 3D
COF structures, and further analyze the symmetric relationship
between the monomers and topologies of the target 3D COFs,
with inference on the underlying design regularities of 3D COFs.
Subsequently, we introduce useful diffraction techniques for de-
termining the structures of 3D COFs in practical cases. Finally,
we present prospects for future development in this area.
2. What Is Topology?
The synthesis of COFs can be regarded as a type of crystal engi-
neering that involves the rational synthesis of target structures
from specific monomers under suitable reaction conditions.[38]
In practice, 3D COFs are built from organic monomers, which
are also referred to as building blocks or building units. Two-
connected (2-c) monomers are often referred to as linkers, and
according to reticular chemistry,[39–47]building units and linkers
can be considered as nodes (i.e., vertices) and links (i.e., edges) re-
spectively. Accordingly, 3D COFs can be described as three-period
nets.[37]These nets are characterized mainly by the types of ver-
tices, edges, faces, and tilings, from 0D to 3D, and are widely
known as topologies. Such topologies are easily differentiated
based on the formation and numbers of these four indices.[39,40]
Vertices with different coordination numbers (CN) are the ba-
sic elements of topologies, while edges are often characterized by
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