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Topological molecular nanocarbons: All-benzene catenane and trefoil knot

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Science  19 Jul 2019:
Vol. 365, Issue 6450, pp. 272-276
DOI: 10.1126/science.aav5021

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Carbon catenation

Preparing interlocked rings and knots at the molecular scale traditionally relies on preorientation of the building blocks by nitrogen or oxygen substituents. Segawa et al. devised a distinct strategy to synthesize catenane and trefoil structures composed exclusively of carbon and hydrogen (see the Perspective by Van Raden and Jasti). They linked phenyl rings end to end into macrocycles that met in the middle at a silicon center. Excision of the silicon with fluoride then yielded the interlocked products.

Science, this issue p. 272; see also p. 216

Abstract

The generation of topologically complex nanocarbons can spur developments in science and technology. However, conventional synthetic routes to interlocked molecules require heteroatoms. We report the synthesis of catenanes and a molecular trefoil knot consisting solely of para-connected benzene rings. Characteristic fluorescence of a heterocatenane associated with fast energy transfer between two rings was observed, and the topological chirality of the all-benzene knot was confirmed by enantiomer separation and circular dichroism spectroscopy. The seemingly rigid all-benzene knot has rapid vortex-like motion in solution even at –95°C, resulting in averaged nuclear magnetic resonance signals for all hydrogen atoms. This interesting dynamic behavior of the knot was theoretically predicted and could stimulate deeper understanding and applications of these previously untapped classes of topological molecular nanocarbons.

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