Covalent organic frameworks

The last 10 years have seen an explosion of interest in crystalline porous frameworks, in particular metal–organic frameworks (MOFs) and porous coordination polymers (PCPs). MOFs and PCPs exploit reversible metal–coordination chemistry to form extended, crystalline solids. In principle, an analogous set of porous materials should exist based on reversible covalent organic bond forming reactions instead of metal coordination. However, there are significantly fewer examples of crystalline porous organic solids in comparison with the burgeoning number of MOFs and PCPs.

As exemplified by the articles in this themed issue, that balance is now beginning to shift. Starting with an important contribution from the Yaghi group,¹ the synthesis of ‘covalent organic frameworks’ (COFs) and associated organic materials has become a vibrant area. The first COFs exploited reversible boronate ester formation, which has proved to be quite versatile and which is represented by a number of articles in this issue. Excitingly, this chemistry is now being used to produce ordered crystalline solids with physical properties, such as photoconductivity, that transcend simple porosity. There are also strong parallels between the preparation of crystalline COFs and the formation of 2-D organic assemblies on surfaces, as covered, for example, by a previous themed issue of this journal.²

Covalent strategies for porous organic materials are not limited, however, to boronate ester chemistry, nor are porous organic solids restricted to extended networks. For example, irreversible covalent bond forming reactions can be used to produce organic polymer networks that have little or no long-range order, but which can exhibit very high pore volumes and excellent physicochemical stability. Despite their lack of long-range order, studies have shown that the porous properties for such polymers can be fine-tuned by synthesis. This raises an associated challenge in terms of generating structure–property relationships and informative structural models to rationalize porous materials that cannot be characterized by X-ray crystallography.

There has also been a recent resurgence of interest in porous molecular crystals that are not interconnected by extended covalent bonding. Such materials are related to COFs, but, unlike frameworks, they can be solution processable. Materials in this class are represented here in articles relating to porous dipeptide crystals and organic cage molecules.

Given the large worldwide research effort in porous solids, it is pertinent to ask whether these new classes of porous organic solids are adding usefully to the portfolio, or whether they are merely replicating physical properties that are already well catered for by MOFs, PCPs, or other porous solids, such as zeolites. The studies included in this issue contribute to a growing argument that porous organic solids can indeed compete with metal–organic or inorganic equivalents for certain applications, and that in some cases there might be unique properties, such as solution processability. I hope that readers find inspiration in this collection of articles, and I thank the editorial team for their work in putting this issue together.

References

1. A. P. Côté, A. Benin, N. W. Ockwig, A. J. Matzger, M. O. Keeffe and O. M. Yaghi, Science, 2005, 310, 1166.
2. Themed issue entitled “2D Crystal Engineering”: CrystEngComm, 2011, 13, 5521–5598.

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