Chemical switches control light

17 Jun 2002

Novel molecular alignments offer promise for designing wires, gates and light-harvesting arrays.

Chemists at Washington University in St Louis, North Carolina State University and the University of California, Riverside, all in the US, have found a new design for molecular electronic switches and have identified the key process that determines their efficiency.

Molecular electronic switches are arrays, or alignments, of covalently-linked molecules that, when stimulated, allow hole/electron transfer along the array, thereby transmitting energy - and that also allow the energy transmission to be switched on and off. If the molecules have the right pigmentation the arrays can respond to light as the stimulating energy source, and become units for light input, output and switching.

The scientists focused on molecules called porphyrins - related to the green chlorophyll pigments used in photosynthesis - and studied different arrays to optimize their characteristics. They made wires, in which light energy is absorbed by an input molecule at one end and transmitted along the array until the final output molecule emits light. They also made switches, in which a particular molecular component attached to the wire can be activated to accept and dissipate the energy. In this case they removed an electron from a magnesium porphyrin molecule, activating the molecule to rapidly accept energy so that heat was released and light emission was turned off.

The scientists found that a "T" arrangement, in which the switching molecule is located perpendicular to one of the transmission molecules in the wire, works just as effectively as a linear arrangement where the switch molecule is attached directly to the output component. They characterized the speeds of the processes involved and found that efficient communication between molecules that are distant from one another in the device to be key to the operation. This process is known as superexchange.

"It has been a big mystery why the T-gate arrangement works as well as the linear arrangement," said Dewey Holten, professor of chemistry at Washington University. "We've shown that the T-gate functions efficiently because the molecules are able to communicate distantly through the array, namely between the switch and output molecules, even if the molecules are separated."

With this knowledge, the researchers can optimize their designs to bring about different functions. Possible applications include molecular arrays for use in molecular photonics, optoelectronic switches that respond directly to different colours of light, solar energy conversion, and nanotechnology.

"[This improved] understanding of molecular switching enables us and others to tailor molecular design for better flow of energy and charge in order make novel wires, gates and light-harvesting arrays," Holten added.