Issue Date: October 25, 2004
BEATING A PATH TO SMART MATERIALS
With their elegant designs and ability to move on command, synthetic molecular machines and motors have captured the imagination of chemists and other researchers, especially those working on molecular electronics. But as beautiful and clever as they are, molecular machines have taken their lumps too.
Skeptics contend that these molecules don't actually do any work and therefore aren't machines at all. About a year ago, Science published a story [302, 556 (2003)] that voiced the doubts of scientists who say it's unrealistic to expect that molecular switches will find their way into commercial products anytime soon--if ever.
In September, about 100 scientists--including a number of chemists working on molecular machines and switches--gathered in Tomar, Portugal, to present their work and to discuss how they could move their systems from benchtop curiosities to real components in useful devices.
The meeting, "From Clever Molecules to Smart Materials," was the fourth in the Chemistry & Physics of Multifunctional Materials series begun in 1996 by Patrick W. Fowler of England's University of Exeter. The series has been run under the aegis of the European Research Conferences. EURESCO meetings were conceived of as a European version of the Gordon Research Conferences--small, tightly focused meetings that encourage frank discussions and collaborations. They are organized by the European Science Foundation's association of research councils and academies and sponsored by the High-Level Scientific Conferences Activity of the European Commission. Most speakers come from European Community countries, although other nations are represented as well.
Meeting Chairman David A. Leigh, a chemistry professor at the University of Edinburgh, in Scotland, said that he invited speakers with the hope of sparking a discussion. Synthetic chemists, he said, have developed a number of clever molecular machines, but now they are faced with some important questions: "How can we get useful work out of these ingenious systems? How can we harness their molecular motion?"
These are questions that J. Fraser Stoddart, a chemistry professor at the University of California, Los Angeles, and director of the California NanoSystems Institute, has been asking himself for a decade. Stoddart has made a career out of developing elegant interlocked molecules that switch in solution between two states. "We can do beautiful experiments," he said of his earlier work, "but you cannot endlessly go on studying exotic chemical compounds for their sheer beauty. What I felt back in the mid-nineties was that this science was going nowhere unless we can put these two-state molecules into devices."
CONSEQUENTLY, Stoddart and his collaborators have been working in earnest to develop interlocked rotaxane and catenane molecules into switches for electronic devices. They've made progress, but they've also taken their share of criticism. So, one could not help but detect a note of triumph in Stoddart's voice as he presented his recent work, which will appear in an upcoming issue of Angewandte Chemie. Working in collaboration with California Institute of Technology's James R. Heath, Stoddart and colleagues have managed to use a certain rotaxane as an electrochromic switch in a solid-state polymer electrolyte. With this molecular machine embedded in the polymer, the researchers can change the material's color by applying a voltage that switches the machine between its ground state and its metastable state. "You don't need to be Einstein to see that the molecule is switching," Stoddart said. "You just need to not be color-blind."
Furthermore, in response to their critics, the group found that a common mechanistic picture describes the electrochemically driven system in both solution and devices.
Another demonstration of a practical application of molecular motors came from Ben Feringa, a chemistry professor at the University of Groningen, in the Netherlands. Feringa has doped liquid-crystal films with the light-driven molecular motor that he developed a few years ago. Irradiating these films with light makes the motor molecules spin. These molecules, in turn, "stir" the molecules that make up the liquid-crystal phase. As the liquid-crystal phase reorganizes, it changes colors.
THIS COLOR CHANGE demonstrates that a molecular motor can induce a visibly observable macroscopic change in a material, Feringa said. Also, the color of the film can be tuned by controlling the rotation of the motor. This effect could be used to make color pixels, he explained. Feringa has begun to push the applications of his molecular motor systems in other directions as well. "Now we have to get onto the solid phase. We have to work at interfaces," he said. Feringa added that his group is working to anchor molecular motors onto surfaces, and they are also focusing on larger systems. Specifically, he said, they are trying to build a molecular car.
Before they see these molecular machines move into commercial products, chemists such as Leigh acknowledge that they have to reach out to colleagues in other fields, such as physics and engineering. "It can be a scary environment for organic chemists," Leigh half-joked. He said that one of his goals in organizing the meeting was to bring researchers from the areas of physics and chemistry together. "It's a very, very exciting time in the field of molecular machines because chemists are starting to listen seriously to physicists when designing molecules that exploit motion," he noted.
University of Maine physics professor R. Dean Astumian was one participant who tried to bring a physical perspective to the chemistry of molecular motors. "If you look in the pages of Nature or Science, you will find it filled with descriptions of judo throws, of molecules flying apart, and even of protein turbines," Astumian said. "From a physical point of view, molecules don't work like that. With the mechanical chemistry, we use analogies from macroscopic physics, but the environment, and hence the mechanism of movement, is very different."
Learning to see things from a similar perspective, Astumian argued, is becoming increasingly important in interdisciplinary fields like nanotechnology where chemists, physicists, and biologists are trying to talk to one another.
While molecular motors were one focus of the Tomar meeting, many of the invited speakers presented work in other areas of supramolecular chemistry. "This is an area of very lively activity," said Maurizio Prato, vice chair of the meeting and a chemistry professor at Italy's University of Trieste. "I think the meeting's discourse was at an excellent scientific level," he added.
By design, about half of the meeting's participants were students from universities all over Europe. As chemistry professor A. Prasanna de Silva of Queen's University, Belfast, put it, "We have the heaviest of hitters, but we also have the freshest of minds." The meeting organizers encouraged the students to ask questions and even awarded bottles of port for the queries deemed most insightful.
The young scientists also had a chance to discuss their own research with the "heavy hitters" during evening poster sessions. "I thought that the posters were the best that I have seen at any small meeting ever," Astumian raved.
Feringa concurred. "The people who presented posters were outstanding," he said, adding that he managed to recruit a handful of students to work in his lab as postdocs.
Several young up-and-coming faculty members also gave talks at the meeting. "I was really proud when my name appeared with Peter Day and Jean-Marie Lehn and Fraser Stoddart and all the other speakers on the program," Lee Cronin of the University of Glasgow, in Scotland, told C&EN a few weeks after the meeting.
"So many things have happened as a result of that meeting," he reflected. Cronin established a collaboration with another young researcher who spoke at the meeting, the University of Cambridge's Sijbren Otto. Cronin was so inspired by the talks that he spent the flight back to Scotland designing new supramolecular systems with his students.
As a chemist working on inorganic clusters, Cronin might have seemed a bit out of place among the many organic chemists in attendance. "I knew I wanted to make inorganic devices, but I wanted to find out what had been done in organic devices," he said. "I thought, if I went along and saw what they were doing, I might get some ideas for more inorganic analogs. I wanted to see if I could take ideas from the carbon-based systems and put them in the less-well-behaved inorganic systems."
Throughout the week, the participants commented on how collaborative the event felt. In many ways, it seemed more like a gathering of old friends than a formal scientific conference. Relaxed by Portugal's pleasant late-summer weather, most of the speakers dressed casually and joked with and teased their colleagues during the presentations.
The collegial atmosphere and enthusiastic discussions inspired the event's first speaker, Jean-Marie Lehn of Louis Pasteur University in Strasbourg, France, to give an impromptu second talk about his recent work on helices that extend and contract based on the presence of a metal. "This meeting has really charged up my batteries again," he said.
Lehn's talk led the participants into a particularly lively philosophical discussion about the ultimate goal of the elegant supramolecular systems discussed at the meeting: Are chemists pursuing them for the sake of beauty and basic science, or is the work driven toward applications?
Playing devil's advocate, Stoddart pointed out that if they were working in the U.S., many of the European chemists would have a hard time finding funding for beautiful chemistry where the applications seem pretty far off.
Lehn, taking the contrarian view, responded: "If, in science, we lose this possibility for 'blue sky' research, then I might as well go off and become a lawyer."
While this impromptu discussion proved to be one of the highlights of the meeting, it was the discussion that didn't take place that proved to be the greatest disappointment. The final day of talks was scheduled to end with a discussion on the future of the Chemistry & Physics of Multifunctional Materials series. However, at dinner the evening before, Prato--who as vice chair of the Tomar meeting would have been the chair of the next meeting in the series--announced that because of funding changes, the series would probably not be continuing.
IN THE PAST, the conferences have been largely financed, as the Tomar meeting was, by grants from the European Commission. It was fairly typical for EURESCO to continue series that are popular without the organizer having to formally submit an entirely new proposal. Caroline Hirst, acting head of the EURESCO conferences, said that the EC wants to direct its funding toward meetings and courses that place a stronger emphasis on the training and education of young scientists, leaving the current slate of EURESCO meetings without its largest financier after 2006. "We're at a crossroads," Hirst explained. "We have to seek alternative means of financing."
This change makes the future of the series uncertain. "Theoretically, they could continue," Hirst said. EURESCO is working to develop a new funding scheme for similar meetings that will be called ESF Research Conferences. Although these new meetings will continue to promote cutting-edge science and focus on interdisciplinary research, there will be some changes. Hirst reckoned that the size of the meetings will grow and that they'll probably be limited to a smaller number of locations. Currently, ESF is working out the finer details of these new conferences, and then they hope to announce a call for proposals in the next few months.
"We very much hope that some of the series we've already launched will find a home under this new funding scheme," Hirst said. But there's no guarantee that the series that were formerly EURESCO conferences will be included in the lineup of ESF Research Conferences.
Naturally, that's disappointing for the scientists who have participated in these meetings in the past. "The demand is very great. The scientists like these meetings very much," Hirst said. "It's a real pity that after 15 years of such a program, we can't continue the current scheme. We are confident, however, that we will be able to develop a new mechanism of proposing a coherent program like this."
Prato expressed measured optimism about continuing the series in some way under the new funding scheme. "It looks like there's a very pale possibility that there might be a new conference," he said.
Until then, many of the participants are finding they have plenty of ideas from the Tomar meeting to sustain them for the moment. Prato said that he and Leigh have had numerous "thank-you" e-mails since returning from Tomar. "Many people said it was the best conference they had been to in years."
COURTESY OF AMAR FLOOD
View the machine in action<br > (COURTESY OF SCOTT VIGNON)
POLYCHROMIC POLYMER Transparent polymer electrolyte gel changes color when an oxidizing +1-V pulse is applied at the working electrode seen at the top of each image. The original color returns after one second. The reference electrode (bottom left) and counter electrode (bottom right) are also shown. COURTESY OF AMAR FLOOD
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