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 <i>E</i>-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 <i>Z</i>-hydrazone thread affords no rotaxane under similar conditions. However, the <i>E</i>-rotaxane can be smoothly converted into the <i>Z</i>-rotaxane in 98% yield under UV irradiation. The X-ray crystal structures of the <i>E</i>- and <i>Z</i>-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