Molecular-level electronics are here to play. We can now make molecular wires that can carry electrons and electronic energy. We can also make molecular-level logic gates. But Plug 'n' Play (mechanical switches) has been an elusive goal. Now the Italian team of Balzana and Mandoline has demonstrated a molecular plug-and-socket device.

A large cyclic molecule (a crown ether) resembling the molecular equivalent of a bicycle tire without the spokes and hub formed the socket. Attached to it was an ultraviolet-absorbing molecule called naphthalene. If you irradiate this socket with ultraviolet light, the absorbed energy is re-emitted as light or heat but doesn't go any further.

However, if you make a suitable plug, you can make the absorbed energy jump onto the inserted molecules, and the plug and socket pair is complete. Such a plug can be obtained by taking ammonia and replacing two of the hydrogens. One of the hydrogens is replaced by a simple hydrocarbon molecule; the other is attached to an anthracene molecule, which is capable of absorbing the energy you push into the naphthalene part of the socket.

Recall from studying weak bases that ammonia, methyl amine, and other amines are weak bases. By altering the pH, you can change them into the protonated (acidic) form or change them back to the basic form. By changing the solution from acid to base and back, the plug can be inserted into the socket or withdrawn. When the plug is inserted into the socket, the energy supplied to the naphthalene on the socket is able to be transferred to the anthracene piece on the plug. The situation is completely reversible and controllable by the acidity/basicity of the solution.

This is a molecular switch of the light fantastic. Consider the implications of attaching these two pieces next to each other on a surface. This is a mechanical switch at the molecular level for a light-based computer. The underlying mechanism is that studied in this chapter--namely, acid and base equilibria.

Reference: Chemistry, A European Journal, vol. 5, 1999, pp. 984–989.