Pyridyl-Acyl Hydrazone Rotaxanes and Molecular Shuttles

We report on rotaxanes featuring
a pyridyl-acyl hydrazone moiety
on the axle as a photo/thermal-switchable macrocycle binding site.
The pyridyl-acyl E-hydrazone acts as a hydrogen bonding
template that directs the assembly of a benzylic amide macrocycle
around the axle to form [2]­rotaxanes in up to 85% yield; the corresponding Z-hydrazone thread affords no rotaxane under similar conditions.
However, the E-rotaxane can be smoothly converted
into the Z-rotaxane in 98% yield under UV irradiation.
The X-ray crystal structures of the E- and Z-rotaxanes show different intercomponent hydrogen bonding
patterns. In molecular shuttles containing pyridyl-acyl hydrazone
and succinic amide ester binding sites, the change of position of
the macrocycle on the thread can be achieved through a series of light
irradiation and heating cycles with excellent positional integrity
(>95%) and switching fidelity (98%) in each state.