Credit: © 2010 AAAS

Biological electron-transfer processes are often controlled by external cofactors, which are usually small ions. The fast turnover of water oxidation in photosystem II, for example, is activated by calcium and chloride ions. In cyctochrome c oxidase, a key enzyme for respiration, anions bind to a positive protein surface and trigger structural changes that affect the electron-transfer processes. In spite of this widespread use in nature, chemists have been slow to exploit the potential of using ions to control electron transfer in supramolecular donor–acceptor systems.

Now, Jung Su Park and colleagues have devised1 a system where the addition of simple anions or cations controls electron transfer and allows the isolation of separate radicals and a supramolecular intermediate. The system consists of a tetrathiafulvalene calix[4]pyrrole (TTF-C4P) donor that can act as a bowl-shaped ligand for a suitable acceptor, in this case a dicationic bisimidazolium quinone (BIQ2+). The acceptor is large and a strong electron acceptor, making it an ideal candidate to fit into the bowl. Furthermore, its dicationic nature means that it could accept electron transfer from the bowl.

If certain salts of the quinone are used, such as bromide or chloride, then a capsule-like donor–acceptor complex is formed. Electron transfer rapidly follows to form an encapsulated product before giving individual radical species [TTF-C4P]+ and BIQ+. However, if other counteranion salts are used, such as tetrafluoroborate, no transfer occurs. Furthermore, adding a tetraethylammonium cation to the donor–acceptor complex gives back electron-transfer, splitting the pair and restoring their original oxidation states.