Superexchange seen in molecular electronic switch broadens design possibilities
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 arrays, or 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 unique molecular component is attached and, when activated, accepts and dissipates the energy, turning off the light emission.
To the surprise of the chemists, 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 characterized 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. Such a 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, activating 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. That is known as controlling the switch porphyrin's redox state. All these functions happen incredibly fast, on the order of picoseconds, a trillionth of a full second. Holten and Lammi studied the operation of the molecular switches in Holten's laboratory, a high-technology maze of ultra high-speed lasers, mirrors, lenses and machinery.
The porphyrin arrays were synthesized by John Lindsay, Ph.D., professor of chemistry, and his students at North Carolina State University, and key functions of the molecules also studied by David Bocian, Ph.D., professor of chemistry, and his students at the University of California, Riverside. "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."
The results were published in the May 11, 2001, Journal of Physical Chemistry, and Lammi will discuss the research and other results at the American Chemical Society's national meeting, held August 26-30, 2001, in Chicago. The collaborative research between the groups at Washington University, North Carolina State University and the University of California, Riverside, is sponsored by the National Science Foundation.
Now that researchers know that distant molecules in an array can communicate through superexchange, they are better able to experiment with designs that work more efficiently and bring about different functions. "The knowledge that our collaborative work has uncovered over the past several years gives a better understanding of molecular switching and 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.
An ultimate goal of this line of 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 colors 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."
Many of the molecular arrays that Holten, Lindsey, and Bocian create and test are photosynthesis analogs. In photosynthetic bacteria and plants, light is absorbed by arrays of antenna pigments and goes to special proteins called reaction centers where electrons move across membranes converting light energy to chemical energy.
Holten and his wife, Christine Kirmaier, Ph.D., research associate professor of chemistry at Washington University, have been working on photosynthesis for nearly 30 years to understand the mechanics of the photosynthetic system and the properties and function of porphyrins.
"We've expanded into molecular photonics through our terrific collaboration with Jon Lindsey and Dave Bocian in just the past six years," said Holten. "At this point in the research we've provided an infrastructure of different molecular architectures and an increased understanding of how the molecules work and can be tuned to have desired properties. Robin Lammi's studies show we understand the current molecular redox-based switches quite well now, and give us insights to advance the designs to the next stage."