Knotting is very common in daily life, such as tying a shoelace when going out, and the earphone wires will get entangled in the pocket, and another example is the more distant "knotting notes"-knotting even participated in the early civilization process of human beings. Knotting is actually related to the concept of geometric topology. In mathematics, it refers more to the study of geometric or spatial relationships between things.

At the microscopic level, molecules can also form mechanically interlocked structures like knots. This is the concept of molecular knots. In nature, "knots" are ubiquitous, such as proteins and DNA contain naturally occurring molecular knots. You have to thank these "knots", otherwise our life code may be messed up.

Molecular knots can not only produce a certain degree of elasticity in macromolecules, but also because of their extremely small nanoscale, they are a class of materials with great potential. Scientists have been exploring how to artificially design and regulate the topology of molecular knots like building blocks.

Recently, on the basis of molecular-strain engineering, Zhichang Liu's team at Westlake University invented a brand new “nested contra-helices” strategy for one-pot self-assembly to efficiently construct topological molecular trefoil knots with different strains, and achieved chiral self-sorting, as well as intramolecular thermally induced spin-crossover tuned by topomechanical strain. The research results were published in Nature Synthesis ( https://doi.org/10.1038/s44160-022-00173-7) entitled “Synthesis of contra-helical trefoil knots with mechanically tuneable spin-crossover properties”.

In this study, the team constructed a series of nested contra-helical trefoil knot molecules with different intramolecular strains through "one-pot self-assembly" using Fe(II) ions as templates, including the longest trefoil molecule ever synthesized contains a ~11 nm-long closed-loop structure composed of 111 atoms. Quite uniquely, they found that the spin-crossover properties of the two Fe(II) ion centers in each knot can be modulated in two opposite directions by changes in the intramolecular topological mechanical strain. In addition, the team also discovered the chiral self-sorting behavior of one of the molecular junctions during crystallization, and successfully determined the structure of the reductively demetallated pure organic trefoil knot using single crystal X-ray diffraction.

Compared with previous synthetic methods, the nested contra-helices strategy developed in this study can construct more complex torus knots and molecular links in a relatively simplified and more efficient manner, and the regulation of the topological strain of molecular knots may lead to emergent properties of materials, thereby enhancing their stimulus responsiveness. Therefore, this achievement opens up a new path for the synthesis of other complex molecular topological structures and the use of topological strain to regulate the properties of molecules, and also lays a foundation for in-depth exploration of the properties and applications of topological molecular knots.