Dynamic Pseudorotaxane Crystals Containing Metallocene Complexes

Molecular machines and switches composed of flexible pseudorotaxanes respond to external stimuli, transducing incident energy into mechanical motions. This study presents thermo- and photoresponsive dynamic pseudorotaxane crystals composed of axle molecules containing ferrocene or ruthenocene groups threaded through dibenzo[24]crown-8 ether rings. The ruthenocene-containing pseudorotaxane exhibits a crystal-to-crystal thermal phase transition at 86 °C, which is much lower than that of the ferrocene-containing pseudorotaxane (128 °C). Single-crystal X-ray crystallography at various temperatures reveals the details of the structural changes, and shows that the bulky ruthenocene provides distortion in the pseudorotaxane structure to facilitate twisting of the axle molecule. A mixed ferrocene and ruthenocene pseudorotaxane crystal is applied to photomechanical conversion under 405 nm laser irradiation at 85 °C and provides a lifting force 6,400-times the weight of the crystal itself upon phase transition.