Shuttling Motion in a Host–Guest Complex Triggered by Spiropyran to Merocyanine Reversible Chemical Transformation

A stimulus-responsive guest-containing
spiropyran and viologen
unit assembles with a 24-membered crown ether into a stable host–guest
complex displaying a partially threaded geometry. Acid addition induces
guest transformation to a merocyanine species activating a second
recognition site, suitable for the formation of a pseudorotaxane.
The simultaneous presence of two recognition sites produces a small-amplitude
macrocycle shuttling motion, from the viologen to the merocyanine
moiety. Base addition returns the guest to its spiropyran form, and
concurrently the translation motion stops.