COMPLETELY by chance, Californian chemists have discovered that
ring-shaped molecules containing gold can emit ghostly flashes of yellow light.
The glowing compound could one day be used to detect hazardous solvents such as
chloroform and dichloromethane, say the researchers. And it might even lead to
unconventional kinds of battery.
While studying the chemistry of gold, Alan Balch of the University of
California at Davis made a compound in which three gold atoms and three organic
units form a ring-shaped molecule. This 鈥渙rganogold鈥� compound emerges from
solution as tiny colourless crystals. When his colleague Ella Fung filtered off
the solvent and washed the crystals with chloroform, she noticed eerie flashes
of intense yellow light.
鈥淓lla came to my office, somewhat shaken by her observation,鈥� says Balch. 鈥淪o
we called a halt to the experiments, and with Jess Vickery we set about finding
out what was going on.鈥� Balch鈥檚 hunch was that the organogold crystals must have
absorbed ultraviolet light from the overhead fluorescent lamps in the laboratory
and that the solvent had somehow liberated the energy.
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To test this theory, the team prepared the crystals again and deliberately
gave them a flash of light from a UV lamp. This time when they poured on solvent
they saw a much brighter glow (Angewandte Chemie, vol 36, p 1179). They
also discovered that the glow was very strong with chloroform and
dichloromethane, but very weak with water.
To find out exactly why the crystals glow, Balch鈥檚 colleague Marilyn Ormstead
used X-ray crystallography to work out the precise structure of the crystals. It
turns out that the gold rings stack together like a pile of coins.
However, each crystal has tiny imperfections or 鈥渉oles鈥�. Balch says it is
likely that the UV light ejects electrons from certain atoms, and that the
electrons then become trapped in the holes. But when the electrons come into
contact with a solvent, they can flow through the crystallised stacks and are
recaptured by the atoms, releasing energy as photons of visible light.
Some crystals glow when they are squeezed, and others emit light after
exposure to gamma rays. But this is the first time chemists have triggered
fluorescence using a solvent. Balch says this peculiar effect could be put to
use in detectors for potentially hazardous solvents. Prasanna de Silva, an
expert on sensors at Queen鈥檚 University in Belfast is fascinated by the finding.
鈥淭he phenomenon is very interesting indeed, and has potential for sensing
solvent vapours where a power supply cannot by used for reasons of isolation or
hazard,鈥� he says.
Balch also believes that eventually, the compound could act as a
鈥渞echargeable鈥� energy storage system. Energy could be stored by repeatedly
shining UV light on the compound, and then released by adding a suitable
solvent. 鈥淚 certainly expect small-scale devices,鈥� says Balch, 鈥渂ut we are far
from them at this early stage.鈥�
