Beyond Frustration

Unexpected discovery of a new type of covalent organic framework.

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Jun 19, 2019
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“Why not?”

Depending on whom you ask within the scientific community, you might encounter reservation or surprised looks when you come up with seemingly unconventional ideas, illustrating the habits in or the limitations of your field of research. However, it can be rewarding to go beyond the usual and established, trying something unorthodox once in a while.

In the context of covalent organic frameworks (COFs), some initial difficulties have already been overcome. Since their discovery in 2005, COFs have emerged as a stable class of crystalline porous organic materials with various potential applications. As they are composed of small organic building blocks, selective variations of these linkers can be used to tune the polymers’ bulk properties such as crystallinity, porosity and stability. Continuous extension of the COF chemist’s toolbox has led to the colossal versatility in COFs we have to date. Suitable linkers, combined in the right proportions, can quite easily form a fully condensed organic network, which makes COF design fairly intuitive. At that point, we were asking ourselves: Why not go beyond intuition? Why not combine linkers in a way that does not obviously make sense?

Fully condensed vs. frustrated 2D networks

To be specific, the unconventionality of our approach lied in not combining tritopic linkers with di- or tritopic counterparts as it is usually done, but to instead add a tetratopic building block. This combination is opposed to traditional design principles, since no ordered network is expected to form; there is just no way to imagine an arrangement with all linkers connected in an orderly manner. Nevertheless, when combining triangular and rectangular linkers in an equimolar ratio, a highly crystalline and porous COF is obtained. How is that possible?

Since this sub-stoichiometric approach cannot lead to fully condensed networks by any means, we had to widen our scope of structural models considered. As illustrated above, the tetratopic linker was found to be both bi- and tetradentate, which enables connection to the tritopic building blocks in an ordered fashion. Such a two-dimensional COF with bex topology has not been reported previously.

This new type of framework is not only structurally interesting, though. Since it is not fully condensed, the COF exhibits inherent free amine functionalities decorating the pore walls. Free functional groups are nothing new in the field but could usually only be achieved using cumbersome post‑synthetic functionalization or a linker-defect approach. The uncondensed amine groups are of special interest in terms of potential applications. We exemplarily show their use for carbon dioxide sorption, organic catalysis, and the possibility of further derivatization.

With this example, we want to demonstrate how creative and uncommon thinking can pave the way for exciting new discoveries in science. Because, why not?

Financial support by the 'COFLeaf' ERC starting grant 639233 is kindly acknowledged.

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Stefan Trenker

PhD Student, Max Planck Institute for Solid State Research

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