Abstract
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.
Highlights
Molecular machines and switches are of significant interest because they can be designed to transduce chemical, electrical and photochemical energy into controllable mechanical motion[1,2,3,4,5,6,7,8,9,10]
Complex 2 was synthesised by mixing the ruthenocene-containing axle molecule and dibenzo[24]crown-8 ether (DB24C8) in dichloromethane, followed by crystallisation by diffusion of diethyl ether vapour into the pseudorotaxane solution (Fig. 1, Supplementary Figs S1–S9)
We have developed metallocene-containing dynamic pseudorotaxane crystals that respond to thermo- and photostimuli, transducing the incident energy into mechanical motions
Summary
Molecular machines and switches are of significant interest because they can be designed to transduce chemical, electrical and photochemical energy into controllable mechanical motion[1,2,3,4,5,6,7,8,9,10]. The dynamics of these crystals include bending, twisting, rotation and salient effects such as hopping and flipping[17,18,19,20,21,22,23,24,25,26], allowing their potential application as actuators[23,24,25,26,27,28,29,30,31,32] These dynamics are accompanied by molecular structural changes induced by thermal stimulus[17,18,19,33,34] or photochemical reactions such as photoaddition[35,36,37], photoisomerisation[22,38,39], or photocyclisation[40] in the crystal state. The ferrocenyl group in the axle molecule of the pseudorotaxane acts as a photosensitiser This ferrocene-containing pseudorotaxane is the only example reported to date of a pseudorotaxane capable of reversible switching accompanied by crystal-to-crystal phase transitions as directly observed by single-crystal X-ray crystallography[41,42,43]. It is essential to develop further unique pseudorotaxanes with different functional groups that allow significant dynamic motion in the single-crystal state
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