Recently, various interlocked molecules, such as rotaxanes and catenanes, have been investigated as both chemosensors and molecular machines. These interlocked molecules can bind specific guest molecules, including alkali metal cations and chloride and sulfate anions, inside three-dimensional cavities. Herein, we describe a highly sensitive alkali metal cation-switchable pseudorotaxane, newly designed and synthesized by our research group, formed from the inclusion complex of a bipyridinium salt with a cryptand. The threading�dethreading molecular motion of this pseudorotaxane has been characterized by studies of the binding between the cryptand and bipyridinium salt based on UV-Vis and 1H NMR spectral changes.  A new type of cryptand-based pseudorotaxane was prepared, utilizing a novel cryptand incorporating two aromatic rings as a wheel component, combined with various bipyridinium salts as the axle component. These pseudorotaxanes exhibited charge-transfer absorption at approximately 380 nm due to p-electron interactions between the cryptand and the bipyridinium salt. Upon addition of one equivalent of sodium ion to the pseudorotaxane, this absorption was observed to disappear as a result of displacement of the bipyridinium salt by the sodium.  This work accomplished both the design and synthesis of a cryptand containing two electron-rich aromatic rings and able to bind bipyridinium derivatives. This cryptand structure has been shown to exhibit complexation behavior towards various bipyridinium salts by p-electron interactions and the resulting pseudorotaxane is capable of sensing alkali metal cations via a displacement mechanism. As a result, we have developed a simple synthetic strategy for a pseudorotaxane employing bipyridinium salts and have demonstrated the functioning of this pseudorotaxane as a chemosensor for alkali metal cations through the use of displacement assays.

Molecular switch by a cryptand-containing rotaxane

246th National Meeting (2013)

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