A small, weakly diffracting electrochemical switch

Rotaxanes and catenanes (right) incorporating the tetracationic cyclophane cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺) have over the last few years become one of the most important types of interlocked molecules for the development of molecular devices and machines. In recent years, to facilitate future applications, interest has been focused towards the development of techniques to transfer these supramolecular assemblies from solution to the solid-state, where their ability to function coherently has allowed the fabrication of surface confined molecular machines and devices. For the majority of these systems, this transferal has been achieved by the self-assembly of appropriately functionalized systems onto solid substrates as self-assembled monolayers. Electropolymerization of the components onto surfaces, on the other hand, not only provides a versatile method for surface deposition, but also provides a conjugated backbone with interesting electrical and optical properties that could electronically link the components of the polymer film.

An example of a catenane containing this group has been synthesized and its structure determined by X-ray diffraction. This has then been immobilized onto a platinum working electrode surface by electropolymerization of the pyrrole unit. 

The crystals of the catenane were quite small and very weakly diffracting, principally due to a large amount of disordered solvent present in the lattice. Additional difficulties are caused by a significant amount of thermal motion in the PF₆ counterions. This motion in the most strongly diffracting atom types in the structure dramatically weakens the higher angle data; in this structure the high angle data was virtually non-existent. Despite these problems, careful refinement of the structure using data collected on the RAPID II (35 minutes per image) proved that the compound synthesized was a catenane. Without the sensitivity of this detector, data collections would have required a much stronger X-ray source, such as station 9.8 at 9 Daresbury.

Crystal structure of the catenane, with the molecules coloured by atom type (left), and by chain (right). It is clear from the figure on the right how the two chains interlink.

Tags:  hydrogen bonding, variable temperature studies, disorder