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 1(2+) and its one-electron reduced species 1+. form very stable 1:1 host-guest complexes 2(2+) and 2+., 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 2(2+) turns into a ternary complex 3(2+) 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 3(2+) results in the rapid generation of the 2+. 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.