An international research group led by Chiba University Professor Shiki Yagai has for the first time succeeded in forming self-assembled "polycatenanes," which are structures comprised of mechanically interlocked small molecule rings. The research group also succeeded in observing the geometric structure of the polycatenanes by using atomic force microscopy (AFM). This work, published in the journal Nature, is the first to achieve synthesis of nano-polycatenanes through molecular self-assembly without using additional molecular templates. Yagai, a professor of applied chemistry and biotechnology at Chiba University, sees this as the first vital step in technological innovation for creating nanometer-sized topological structures.
Catenane synthesis has been widely researched, especially since Jean-Pierre Sauvage devised a metal-templated strategy to synthesize a catenane. In recognition of their pioneering work, Sauvage and two other researchers were awarded the Nobel Prize in Chemistry for the design and synthesis of molecular machines in 2016. As the molecules in catenanes are linked together into a chain, the links can move relative to one another. This makes synthesis and characterization of the structure very difficult, especially when the rings are not held together by strong covalent bonds.
By modifying the self-assembly protocol with a templated strategy, the research group from Japan, Italy, Switzerland, and the UK were able to create polycatenanes including complex structures made up of five interlocked rings in a linear arrangement similar to the Olympic Games symbol, which were large enough to be observed by atomic force microscopy. While searching for methods to purify the nano-rings, the research group found that adding the rings to 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, the research group examined optimal conditions for secondary nucleation and successfully created polycatenane made up of as many as 22 connected rings. By observing this polycatenane through atomic force microscopy, it was confirmed that the structure reached up to 500 nm in length.
"The innovative finding of this research lies in the utilization of the self-assembly characteristic of the molecules," says Professor Yagai. "We were able to create intricate geometric structures in mesoscale without using complex synthetic methods. This paves the way to creating even more complex geometric compounds such as "rotaxane" and "trefoil knot" in a similar scale. As the molecular assemblies used in this research are made up of molecules which react to light and electricity, this finding can potentially be applied to organic electronics and photonics, and other molecular machines."