Many scientists have focused in recent years on the search for artificial molecular machines. A team of researchers from the University of Basel, ETH Zurich and the Paul Scherrer Institute (PSI) has now succeeded in creating a network of nano-sized molecular switches under particular conditions, and in operating these individually. The paper was published on Friday 25 May 2007 in the new international edition of the scientific journal “Angewandte Chemie”. (1)

The researchers achieved this by evaporating porphyrin molecules onto a copper substrate in ultra-high vacuum, whereupon the molecules aggregate together to form a regularly ordered network with hexagonal pores. Each porphyrin molecule borders on two neighbouring pores. Using a scanning tunnelling microscope, the researchers also discovered that at a temperature of –196°C there was an additional particle resting on top of some of the pores, which they again interpreted as a porphyrin molecule since they had not applied any other substances to the copper.

Because the pore shape is hexagonal, this unexpected “guest” can take up that same number of orientations. Three of them can be distinguished with the scanning tunnelling microscope. The higher the temperature, the faster the guest molecules change their alignment. At room temperature this takes place so quickly that the molecule appears to rotate. Under the scanning tunnelling microscope the pore seems to be filled.

Because the superimposed porphyrin molecules are embedded in a network of nanopores, at temperatures below –160 degrees Celsius the pores can be actuated individually. The superimposed molecule can be moved into a different position by using the tip of the scanning tunnelling microscope. As a result, a unit of this kind acts as a kind of supramolecular switch. However, the researchers were unable to specify the direction in which the porphyrin rotates.

The two co-authors of the study, François Diederich, ETH Zurich Professor for Organic Chemistry, and Paul Scherrer Institute researcher Thomas Jung from the Laboratory for Micro- and Nanotechnology, see in this discovery hitherto unknown opportunities to build novel materials, switching elements or data storage devices. Diederich says “The development of semiconductors in micro- and nano-technology will come up against physical and technical limits in the medium term. The supramolecular structures demonstrated here for the first time open up entirely new opportunities to overcome these obstacles.”

Under the scanning tunnelling microscope the supramolecular structures of a particular porphyrin molecule are recognisable as hexagons resembling water wheels (left). The white dots are individual superimposed porphyrin molecules that can be moved into a different alignment by using the microscope tip (centre and r.).

However, he says this structure is merely an initial demonstration of this type of switch. Nevertheless its architecture has enormous potential, namely to pave the way for molecular switches on a nanometre scale by combining synthetic and supramolecular chemistry with physical techniques. This potential was also confirmed by the “Information Technology Roadmap for Semiconductors”, a strategy paper published regularly by the semiconductor manufacturing industry. Jung adds that “What we have achieved in the laboratory today is a breakthrough that can change the technology of computers, data storage devices and sensors enormously in the next 10 years.”

The amalgamation of many individual structures and their integration into current technology – via wires and contacts – remains a big problem. Another obstacle are the low temperatures without which this molecular complex cannot function as a switch.

However, Diederich and Jung are convinced that future research and development will reveal ways to enable these switching processes at ideal temperatures. For example this includes researching different molecules and materials that show a comparable supramolecular architecture but with a larger bonding force between the “rotor”, the superimposed molecule, and the “stator”, the porous network.

Since the eighties scientists have used the principles of the self-organisation of molecules to create individually selectable supramolecular structures as the basis for technical applications. The first structure of this kind was created by the French chemist Jean-Marie Lehn, who received the Nobel Prize for it in 1987.