Although molecular knots are attracting attention on account of their topological intricacies and potential uses, efficient and versatile strategies for tying molecular knots with different topologies on larger length scales remain a challenge, not to mention harnessing their topomechanical characteristics in order to modulate their properties. Here, we report a general approach to constructing torus knots from two coaxially nested multistranded contra-helices. As a proof of concept, a series of contra-helical trefoil knots have been synthesized near-quantitatively in one step. One of these molecular knots features the longest trefoil knot made so far, namely a 111-atom closed loop of ~11 nm long. The thermally induced spin-crossover in each knot can be modulated in opposing directions by changes in the intramolecular topomechanical strain. The concise synthesis of molecular knots with topomechanically tunable emergent properties offers a transformative approach to unleashing their stimuli-responsive multifunctionalities. Unexpectedly, one of these molecular knots exhibits, during crystallization, narcissistic self-sorting, enabling the manual separation of both enantiomers in a manner which is reminiscent of the manual separation in 1848 of sodium ammonium tartrate crystals by Pasteur. This separation is a textbook example of chiral self-sorting by crystallization of racemic knots having only topological handedness and devoid of any chiral stereogenic centers. A purely organic trefoil knot, obtained by reductive demetallation of its metalated precursor, has also been characterized unambiguously by X-ray crystallography.

Topologically chiral contra-helical trefoil knots exhibiting topomechanically modulable spin-crossover properties

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