A collaboration of research groups from Japan, Italy, Switzerland and the UK has succeeded in forming and studying structures comprising mechanically interlocked self-assembled rings, made solely from one molecular ingredient.
In 2016, the Nobel Prize in Chemistry was shared by Jean-Pierre Sauvage for the design and synthesis of molecular machines, after he managed to connect two ring-shaped molecules into what is called a "catenane". Unlike ordinary chemical bonds, the molecules in catenanes are linked like a chain, and so the links can move relative to each other. Both the synthesis and characterisation of such structures are notoriously difficult, particularly when the rings themselves are not held together by strong covalent bonds.
This work, published in Nature, is the first report of synthesis of nano-poly[n]catenanes via molecular self-assembly, without using an additional molecular template. By altering the self-assembly conditions, the group were able to create intricate structures, including a nano-catenane with interlocked rings in a linear arrangement, which has been coined “nanolympiadane" in homage to the catenane system “olympiadane" first reported by Fraser Stoddart and colleagues in 1994, and the well-known symbol of the Olympic games. The size of component ring (30 nm) means that the group were able to see these impressive structures using atomic force microscopy.
Each component ring comprises around 600 identical small molecules (monomers). These monomers first assemble into 6-membered flat “rosettes", which then collectively stack to form a ring. The group designed methods to purify the rings, removing any material that hadn't assembled as desired, and found that the addition of such rings to the hot monomer solution facilitates the formation of new assemblies on the surface of the rings, a process known as secondary nucleation. Based on this finding, they applied the sequential addition of monomers, and were able to create poly[n]catenanes with up to 22 rings.
Small-angle scattering and multiscale molecular simulations were instrumental in the characterisation of the nano-rings and in identifying where the secondary nucleation took place, and the mechanism behind it. X-ray data obtained at Diamond Light Source, and neutron data obtained through an Xpress measurement on the new Zoom beamline at ISIS, were analysed together to unveil a structure made of a compact core surrounded by an open fatty shell.
The open shell would be easily accessible to newly added monomers or partially grown rosettes. This was supported by multiscale molecular simulations. The results indicate that their poor solubility in the solvent could be what causes the monomers and the rosettes to go to the surface of pre-formed rings, causing the growth of other rings.
Self-assembly of a barbituric acid based molecule into nano-poly[n]catenanes.
The understanding of this mechanism led the group to adopt the sequential addition method. As the interlocking process occurs in a stepwise fashion, they thought, correctly, that stepwise addition would improve the extent of interlocking.
Given the size of these structures (already hundreds of nanometres), it is possible that only minor changes to the sequential addition process may build structures that are sufficiently large for purification by simple filtration. This would allow for in-depth study of the unique physical properties that a structure made up of miniscule interlocked chain links may have, and their potential for creating molecular machines.
The full paper can be found online at: https://www.nature.com/articles/s41586-020-2445-z
The study has also been covered in Nature's "News and Views", online here: https://www.nature.com/articles/d41586-020-02007-y