A TINY light switch has been made from a single molecule by chemists in
Italy. It works in a different way to other molecular switches and could be a
step on the road to building a chemical computer.
鈥淪witching mechanisms like these could be used in computers with
molecule-sized logic gates,鈥� says Luigi Fabbrizzi, who led the team at the
University of Pavia. 鈥淢olecules like ours could one day form the basis of a
`wet鈥� computer鈥攐ne whose circuit elements are chemicals in solution.鈥�
Fabbrizzi鈥檚 light switch is an example of a bistable molecule: it can exist
in two different but stable forms. In some bistable molecules, the change occurs
when a metal atom bound inside the molecule loses electrons (is oxidised) or
gains electrons (is reduced).
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But in Fabbrizzi鈥檚 molecule it is acidity that matters. In an acidic
environment, the molecule fluoresces, but when its surroundings are made
alkaline, a nickel atom within the molecule changes position and the light
switches off.
The switch works because the molecule has two 鈥渃ompartments鈥� that can hold
the nickel atom. One of the these compartments has attached to it an anthracene
group, which fluoresces blue when struck by ultraviolet light.
At a near neutral pH of 7.5, the nickel atom sits some distance away
from the anthracene group. If the group is struck by a photon of ultraviolet
light, one of its loosely-attached electrons absorbs the photon鈥檚 energy and
moves to an excited state. The electron then releases this energy by emitting a
photon of blue light.
Increase the pH to 9.5, however, and the nickel moves to an
alternative binding site closer to the anthracene. In this position, the nickel
ion can trap the excited electron, preventing the anthracene emitting blue
light. The result is that the blue fluorescence is switched off.
Reducing the pH back to 7.5, returns the nickel to its original
site, freeing up the electrons and allowing the fluorescence to kick in again,
Fabbrizzi and his colleagues report in a recent edition of the Royal Society of
Chemistry鈥檚 Dalton Transactions (p 185).
Chemists hope that molecules such as Fabbrizzi鈥檚, which can be switched
between two stable states, could form the basis of a molecular machine that
could act, for example, as a tiny switching device. Most molecular switches
designed so far have no moving parts and depend for their action on simple
colour differences between the two states. Fabbrizzi鈥檚 molecule is different in
that conversion between its two states depends on the movement of the nickel
within the molecule.
A wet computer is almost certainly a long way off, however. Some major
obstacles still have to be overcome before simple logic gates based on layers of
these molecules can be built. For example, the switching rate of Fabbrizzi鈥檚
molecule is painfully slow. 鈥淎t the moment, fluorescence quenching takes about
12 seconds and to turn it back on again takes over a minute,鈥� says Fabbrizzi.
鈥淭hat鈥檚 simply not fast enough for a practical device鈥攂ut we鈥檙e working on it.鈥�
