Light control of stoichiometry and motion in pseudorotaxanes comprising a cucurbit[7]uril wheel and an azobenzene-bipyridinium axle

Pseudorotaxanes are the simplest prototypes for the construction of molecular machines based on threaded species. Investigation on molecular motions in these model systems is a necessary action for an efficient design of working molecular machines and motors. Herein we report on photoactive pseudorotaxanes based on the interaction between bipyridinium and cucurbit[7]uril (CB7). The molecular axle is composed of a central bipyridinium unit and two azobenzene moieties at the extremities. CB7 can form two different complexes with this molecule: a [2]pseudorotaxane, in which the macrocycle shuttles fast along the length of the axle, and a [3]pseudorotaxane, in which two CB7s are confined at the extremities of the axle. Upon trans to cis isomerization of the azobenzene moieties, the [3]pseudorotaxane is destabilized, and only one CB7 resides on the axle, surrounding the bipyridinium unit. The system was successfully inserted into the core of liposomes, and preliminary investigations confirmed that it maintains its switching ability.