Molecular switches modelled on photosynthesis

Using photosynthesis as their model, chemists from three US Universities have tested molecular electronic switches that turn the flow of light energy on and off.

Using photosynthesis as their model, chemists at Washington University in St. Louis, North Carolina State University, and the University of California, Riverside have tested molecular electronic switches that turn the flow of light energy on and off.

Taking molecules called porphyrins that are related to the green chorophyll pigments of photosynthesis; the chemists have studied many different alignments of molecules.

In molecular electronic wires, light energy absorbed by an input molecule at one end is transmitted from one molecule to another until the final output molecule emits light.

To make a molecular optoelectronic switch, a molecular component is attached and, when activated, accepts and dissipates the energy, turning off the light emission.

The chemists 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.

The Washington University chemists characterised the speeds of the various processes involved and found that the key to the operation is efficient communication between molecules that are distant from one another in the device.

The process is known as superexchange and has been known for some time in charge transfer, but its role in excited-state energy transfer is less well studied.

Dewey Holten, Ph.D., professor of chemistry at Washington University, and Robin Lammi, Washington University doctoral candidate, removed an electron from the switch molecule, a magnesium porphyrin, and activated it to rapidly accept energy.

At this juncture the energy in the wire has been blunted, or quenched, and instead of light going out, heat is released in a process known as controlling the switch porphyrin’s redox state.

‘It has been a big mystery why the T gate arrangement works as well as the linear arrangement,’ said Holten. ‘Now, we’ve been able to show that the T gate functions efficiently in both the on and off states because the molecules are able to communicate distantly through the array, namely between the switch and output molecules, even if removed from one another.’

One of the ultimate goals of this research is to create molecular arrays and building blocks for use in molecular photonics, solar energy conversion, and nanotechnology. In one near term project, the chemists want to extend the operation of their molecular switches.

Currently, the experiments use electrochemistry involving electrodes to activate the switching action in solution, but a goal is to develop optoelectronic switches that respond directly to light and can operate in the solid state.

‘With the correct design, we can control this process with two different colours of light rather than with electrochemistry,’ said Holten. ‘The next generation of these switches will use, for example, blue light to initiate energy flow along the wire to cause red light output at the other end, and green light to activate the switching function and turn the output off.’