Illustration realized in the framework of a collaboration between the Image/Recit option of the HEAD (Haute École d'Art et de Design) - Genève and the Faculty of Sciences of the University of Geneva.

Supramolecular polymers are an alternative to traditional plastics, where monomers or building blocks are held together by weak, reversible interactions. These materials are becoming increasingly relevant due to their valuable properties of recyclability, self-healing or good processability amongst others, and they present numerous promising applications in optoelectronics or biomedicine. The best-known example is DNA, where two strands are held together by hydrogen bonds, forming the classic double helix structure—a feat not possible with conventional plastics. 

 

But how can we control and tune the different factors that lead to an assembly? How can we increase their stability and dictate their final structure? Often this is done by changing the type of interactions between monomers or by applying stimuli such as temperature or light. We, instead, seek to know how the shape of a building block, or more specifically the curvature of a molecule, can affect the outcome of a growing polymer. 

 

Three simplified cases of shaping a molecule are possible: as a disc or flat surface, a bowl, and a saddle. If one thinks of two discs stacked on top of each other, these can rotate and translate off one another. Multiply this a millionfold to the typical length of a classical plastic fiber, this quickly becomes a mess. If we now consider a bowl sitting on top of another bowl, these can still rotate off one another but they cannot translate, giving less freedom to the assembly and thus a better-defined polymer. We have not changed the nature of the interaction between the building blocks, but simply how these systems can come together architecturally. This effect is even more pronounced when we use saddles instead of bowls. We now prevent rotation and translation, just like in a stack of the famous branded potato chips. 

 

With this approach in mind, we have recently introduced a design principle that we named ‘Shape-Assisted Self-Assembly’. We demonstrate that monomers of an appropriate curvature, saddles, are able to form supramolecular polymers without the necessity of incorporating the strong “glue” often used to keep units together. By using a saddle-shaped porphyrinoid derivative that we have termed ‘carpyridine’, we were able to grow two-dimensional supramolecular polymers by following this principle. We found that these molecules aggregate vertically through weak supramolecular interactions, forming loose columns that then associate laterally to form a 2D surface. 

 

In our most recent work, we were able to control the dimensionality of the grown assembly by blocking the lateral association between columns of stacked monomers. As a result, we have been able to form a one-dimensional supramolecular polymer grown from our molecules, just like potato chips packed inside a tube can. We were able to elucidate the mechanism for this process, in which the molecules stack on top of each other, constantly elongating the polymer. These structures, that could be confirmed under the microscope, open the door for studying direct applications such as migration of charge or light inside these columns. 

 

Simply put, we have been able to gain an additional degree of control over the dimensionality of a supramolecular polymer, underlining the power of curvature. Learning how molecules interact and order themselves is key to understanding how large systems like our cells work. We unravel it, one chip at a time.