Molecular pop-up toy: A molecular machine based on folding/unfolding motion of alkyl chains bound to a host

Abstract

We have designed and synthesized a new type of molecular machine based on the folding/unfolding motion of an alkyl chain bound to a host, triggered by a redox stimulus. A guest molecule containing a viologen unit with a long alkyl chain 12+ and its one-electron reduced species 1+C form very stable 1:1 host–guest complexes 22+ and 2+C, respectively, with cucurbit[8]uril (CB[8]), where the long alkyl chain of the guests is in a folded conformation inside the host cavity. Upon addition of 2,6-dihydroxynaphthalene as an electron donor, the binary complex 22+ turns into a ternary complex 32+ through host-stabilized charge-transfer complex formation with the alkyl chain extended into the solution outside the host cavity. The ternary complex behaves like a molecular machine reminiscent of a pop-up toy, as it shows reversible folding/unfolding motion of the alkyl chain of the guest in response to a redox stimulus. For example, one-electron reduction of 32+ results in the rapid generation of the 2+C complex, accompanied by a dramatic conformational change of the alkyl chain from an extended to a folded conformation, and the process can be reversed by oxidation.