Credit: © 2009 NAS, USA

Catenanes — molecules comprising interlocked rings not covalently bonded to each other — initially attracted attention for their unusual structures, but are now being used in molecular electronic devices. Their synthesis, which has traditionally required pre-organization of the precursors via non-covalent interactions, has often been achieved by interlocking a ring containing π-donor groups with one containing π-acceptor groups.

Rather than relying on irreversible ring-closing steps, thermodynamically controlled syntheses, where building blocks reversibly self-assemble in a variety of interconverting structures, have also been undertaken. Now, Jeremy Sanders and co-workers, at the University of Cambridge, have used this dynamic combinatorial approach to make1 an unusual two-ring catenane in which each ring features both a π-donor and a π-acceptor unit. The synthesis uses two acyclic dithiol precursors; one that contains an electron-rich aromatic system (π-donor) and the other an electron-poor system (π-acceptor).

When mixed in an aqueous solution, these building blocks react together to form disulfide bonds to give various macrocycles as well as a two-ring catenane in the which aromatic units are stacked in a donor–acceptor–acceptor–donor sequence. Moreover, this equilibrium can be displaced to favour catenane formation by adding an electron-rich template that intercalates between the two electron-deficient units.