Nanochemistry: Layers step out of line

COF structure - delicious simulation. Source: e-conversion / V. Hiendl

If you were to advertise so-called COFs in a commercial, the slogan would be something like: “The airy, light miracle sponge for all needs.” Because this porous material cleans, separates and stores gases and liquids. In addition, it acts as a catalyst for chemical reactions. COF stands for Covalent Organic Framework. The basic building blocks of the material are hexagons of organic molecules that arrange side by side to form a single layer. Stacked on top of one another, the layers form an extensive three-dimensional network that is very stable even at relatively high temperatures due to the covalent bonds between the molecules in the layer plane.

The main reason that COF materials are so popular is that chemists can adjust their properties during synthesis with a high level of control. These include the pore size, shape or distribution of functional groups. Only the stacking of the individual layers does not always go according to plan: They are often placed above one another not exactly, but with a slight offset. This frustrates the experts, because even small deviations unintentionally change the structure of the surface and channels and make access to functional groups more difficult.

Bettina Lotsch, Professor of Chemistry at LMU and Director at the Max Planck Institute of Solid State Research, and colleagues have now found that such staggered stacking occurs more often than previously thought. The reason is that the previous crystallographic analysis method is simply not detailed enough. The PhD student Alexander Pütz explains the new procedure: Common X-ray diffraction techniques reduce the image of our hexagonal structure to a symmetrical grid of determinable size and geometry. Thus, subtle details go unnoticed - such as the way the layers stack on top of each other. With the so-called pair distribution function analysis, we can differentiate between ordered and disordered structures, as well as different types of stacking. This shows that a material, which at first glance appears highly symmetric, actually has small, random shifts between neighboring layers.

But what influences the stacking? One possibility could be the temperature at which the reaction takes place. The chemists therefore synthesized COFs at 120 degrees Celsius and at room temperature. The influence was clearly visible thanks to the new analytical method: At a low synthesis temperature the disorder in the layered system was stronger. Bettina Lotsch and her team are already developing their method further to examine errors in more detail. Thereby the e-conversion scientists offer a promising tool for optimizing this exciting materials family.
Chemical Science, 2020