Three-Dimensional Covalent Organic Frameworks: From Topology Design to Applications

ConspectusCovalent organic frameworks (COFs) represent a novel type of crystalline porous polymers with potential applications in many areas. Considering their covalent connectivity in different dimensions, COFs are classified as two-dimensional (2D) layered structures or three-dimensional (3D) networks. In particular, 3D COFs have gained increasing attention recently because of their remarkably large surface areas (>5000 m/g), hierarchical nanopores and numerous open sites. However, it has been proven to be a major challenge to construct 3D COFs, as the main driving force for their synthesis comes from the formation of covalent bonds. In addition, there are several stones on the roads blocking the development of 3D COFs. First, the successful topology design strategies of 3D COFs have been limited to [4 + 2] or [4 + 3] condensation reactions of the tetrahedral molecules with linear or triangular building blocks in the first decade, which led to only three available topologies (ctn, bor, and dia) and strongly restricted the incorporation of some important functional units. Next, as it is very challenging to obtain large-size single crystals of 3D COFs and the same building blocks may yield many possible structures that are quite difficult to identify from simulations, their structure determination has been considered a major issue. Last, the building blocks utilized to synthesize 3D COFs are very limited, which further affects their functionalization and applications. Therefore, since it was first announced in 2007, research studies regarding 3D COFs have been underexplored for many years, and very few examples have been reported.To confront these obstacles in 3D COFs, we started contributing to this field in 2016. Considering that many interesting quadrilateral molecules (e.g., pyrene and porphyrin) cannot be easily derivatized into linear or triangular motifs, we developed a novel topology design strategy to construct 3D COFs via [4 + 4] condensation reactions of tetrahedral and quadrilateral building blocks. After many trials, we found that this is a general synthetic strategy to build 3D COFs with the new pts topology. In addition, we explored the structure determination of polycrystalline 3D COFs prepared by our developed strategy via a 3D electron diffraction technique. Moreover, we expanded the toolbox of molecular building blocks for creating 3D COFs and successfully demonstrated the functionalization of 3D COFs with characteristic properties and applications. In this Account, we summarize our above ongoing research contributions, including (i) a novel topology design strategy for the synthesis of 3D COFs; (ii) attempts to determine the crystal structure of polycrystalline 3D COFs with atomic resolution; and (iii) the diversification of building blocks and applications of functionalized 3D COFs. Overall, our studies not only offer a new paradigm of expansion in the topology design strategy and building block families of 3D COFs, but also provide an idea of future opportunities for relevant researchers in this field.