The mechanical bond has experienced a great success in recent years in the chemical world and beyond. Indeed, it has been incorporated into artificial molecular machines (AMMs), such as molecular muscles and molecular pumps. Although the synthesis of mechanically interlocked molecules (MIMs) represents a challenge, many approaches have been reported in the literature to template their formation. In particular, the use of radical templation allows us to construct rotaxanes and catenanes in a convenient and efficient manner. While many MIMs incorporate the radical-pairing binding motif between 4,4’-bipyridinium units and the cyclobis(paraquat-p-phenylene) ring, only a few examples have been reported employing the cyclobis(paraquat-4,4'-biphenylene) ring (MS). Indeed, this larger ring has been only recently reported to form an inclusion complex with the cyclobis(paraquat-m-phenylene) ring with a high affinity. Motivated by the use of this strong affinity in the construction of a new ring-in-ring [2]rotaxane, we designed a redox-switchable MIM incorporating this binding motif which can contract and expand in a muscle-like fashion. This molecular muscle which can be also related to a molecular zipper, is based on the mechanical motion of the MS ring (zipper slider) between two recognition sites; one driven by radical pairing in its reduced state and the other consisting in π-donor/π-acceptor interactions with two electron-rich stations (dihydroxynaphthalene units), each located on a different PEGylated chain in its oxidized state. This rotaxane has been obtained by radical templation and in situ Huisgen cycloaddition. The ring sliding along the dumbbell has been characterized by NMR and UV-vis spectroscopy and by cyclic voltammetry. Ongoing studies of the rotaxane include AFM imaging and molecular modelling in order to demonstrate the topological change from linear to cyclic PEGylated chains upon quantitative cyclization by a ring shuttling mechanism. Such a system incorporating long polymeric chains could find applications in materials science.

Design, synthesis and characterization of an artificial molecular zipper

256th National Meeting (2018)

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