Angew.Chem.Int.Ed.http://dx.doi.org/10.1002/anie.201107104(2011)

Molecules comprising mechanically interlocked components that are not covalently bonded to one another often form the basis of nanoscale switches and machines. Although constrained as part of a larger assembly, significant changes in the relative positions of the components can be achieved by modifying how they interact with each other. In rotaxanes, for example, the location of a macrocycle threaded on a linear chain can be altered by changing how strongly different sites along the chain bind to it.

Now, a team in Japan led by Kentaro Tanaka at Nagoya University have made a four-fold rotaxane that can be used to control the intramolecular distance and, therefore, the electronic coupling, between two copper(II) ions. The rotaxane is made up of two building blocks: a phthalocyanine ring with four crown-ether loops fixed around its edge and a porphyrin with four 'arms' that each thread through one of the loops. The structure is prevented from unravelling by bulky groups fixed to the end of each arm. The large porphyrin and phthalocyanine rings — each with a central copper(II) ion — stack on top of one another and their relative positions can be controlled by tuning the interactions between the crown-ether loops and the dialkylamine-containing arms.

Credit: © 2011 WILEY

When the dialkylamine groups are protonated, the crown ethers bind to them and the porphyrin and phthalocyanine rings are held too far apart for the copper(II) ions to interact. Once deprotonated, however, there is no thermodynamic driving force for the crown ethers to encircle the amine groups and the two large aromatic systems move close enough to enable antiferromagnetic coupling between the copper(II) ions.