Covalent organic frameworks (COFs) are crystalline polymers constructed from organic building blocks with highly ordered structures and tunable porosity. In their landmark paper, Adrien Côté, Omar Yaghi and co-workers set out the synthesis of “covalent frameworks that could be functionalized into lightweight materials optimized for gas storage, photonic and catalytic applications.”  Looking back on 15 years of research on COFs we can say that COFs quickly developed as an attractive material for application in many different fields, such as adsorption and separation, energy storage and sensing.
Initially, the crystallization of 2D or 3D covalent frameworks was thought as too difficult or even impossible to realize, also known as the crystallization problem. Using dynamic covalent chemistry, a concept which allows reversible covalent bond formation under thermodynamic control, Côté, Yaghi and co-workers obtained microcrystalline products of a 2D extended framework through self-condensation of a diboronic acid and condensation of a diboronic acid with a diol.  Two years after publication of 2D COFs, the first 3D COF was reported using the same chemistry.  That was the starting point of intense research work on expanding the structure motifs, synthetic protocols, functionalization and exploring different applications of COFs.
Although dynamic covalent chemistry allows to overcome the crystallization problem, products are typically isolated as microcrystalline powders and structure solutions are only accessible through powder XRD or electron crystallography. Using aniline as moderator to inhibit nucleation, the first single crystal COFs were reported recently.  However, a general method for single crystal growth of COFs is still lacking.
Different synthetic routes which enable dynamic bond formation and bond breaking were applied in the synthesis of COFs during the past 15 years. Under them boronic acid condensation, imine condensation, imide and azine formation, just to name a few, have extended our scope of building blocks and led to now topologies. The first report of a luminescent and semiconducting COF in 2008  as well as developing efficient methods to functionalize COFs fueled research on the application side. As a result, COFs emerged in research areas which were previously dominated by inorganic or composite materials. Benefiting from their low density and large surface area they quickly became interesting as electrode and catalyst materials. Relatively new, however, is their application as biomaterial such as in drug carrier and in drug delivery systems.
Besides great research efforts on the academic level, the uptake of COFs in industry and in large scale production remains sparse. Important to gain foothold in industry will be the identification of unique key applications of COFs which go beyond traditional fields of application such as gas storage and which clearly outperform other materials candidates [5,6,7]. Additionally, poor solution processability hampers application in optoelectronics and device fabrication, one of the emerging areas in COF research. [5,6,8] In order to unleash the full potential of COFs, we need to expand the structural diversity as well as understand the structure-property correlation in detail. Crucial will be therefore the development of synthetic strategies leading to single crystals as well as to new structural motifs. [6,8] In order to expand possible fields of application, understanding of the chemical and physical events taking place on the surface as well as in the pores is important, while developing better ways to grow large area COFs with less defects will facilitate uptake from the device fabrication community. [6,7]
On the occasion of 15 years in COF research we have put together a collection showcasing some of the contributions that were published in journals of the Nature Research portfolio. We hope you enjoy reading them and we are looking forward to many more successful years in COF research to come.
 Côté, A. P. Benin, A. I. Ockwig, N. W. O’Keeffe, M. Matzger, A. J. Yaghi, O. M. Porous, Crystalline, Covalent Organic Frameworks. Science (2005) 310, 1166-1170
 Hani M. El-Kaderi et al. Designed Synthesis of 3D Covalent Organic Frameworks. Science (2007) 316, 268-272
 Ma, T. et al. Single-crystal x-ray diffraction structures of covalent organic frameworks. Science (2018) 361, 48-52
 Wan, S. Guo, J. Kim, J. Ihee, H. Jiang, D. A Belt-Shaped, Blue Luminescent, and Semiconducting Covalent Organic Framework. Angew. Chem. Int. Ed. (2008) 47, 8826-8830
 Xiaodong Zou answers questions about 15 years of research on covalent organic frameworks. Nat. Commun. (2020) 11, 5330, DOI: 10.1038/s41467-020-19300-z
 Yi Liu answers questions about 15 years of research on covalent organic frameworks. Nat. Commun. (2020) 11, 5333, DOI: 10.1038/s41467-020-19301-y
 Donglin Jiang answers questions about 15 years of research on covalent organic frameworks. Nat. Commun. (2020) 11, 5336 DOI: 10.1038/s41467-020-19302-x
 Natalia Shustova answers questions about 15 years of research on covalent organic frameworks. Nat. Commun. (2020) 11, 5329, DOI: 10.1038/s41467-020-19299-3
Image Credit: 10.1038/s42004-020-0278-1