Rotaxanes are composed of macrocyclic molecules threaded by dumbbell shaped molecules (the axle) forming a unique interlocked system. They are often designed as molecular shuttles, driven by chemical stimuli. 

Kohtaro Osakada and colleagues at the Tokyo Institute of Technology present recent developments of rotaxanes and pseudorotaxanes which contain iron, palladium or platinum axles. There are two main types -  one in which the macrocycle contains the metal, the second where the metal is part of the axle itself.   Osakda's group focus their research efforts on the latter. The metals can be incorporated either as bulky end group to prevent 'dethreading' or within the axle rather than at its end.

In one example, single crystals of a ferrocene-containing pseudorotaxane undergo a thermal phase transition which results in altered optical properties - as seen by a green to orange colour change of the crystal at around 128 degrees Centigrade. Ferrocene groups are used as they have redox properties which can be employed in the electrochemical synthesis of these systems. 

The authors say that their results provide a protocol for the design of crystalline devices composed of discrete molecular components. 

This article is featured on the outside front cover of issue 36, Dalton Transactions 2008.