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David Bradley ISSUE #63
March 2007
Chemists Go Round the Bend
Markus Hoffmann

Chemists often think of molecular wires as "shape-persistent" rods with limited flexibility, so says Oxford University's Harry Anderson, and he should know, having worked with the inflexible nanoscopic objects known as molecules since the early 1990s. However, he and his colleagues Markus Hoffmann, Craig Wilson, and Barbara Odell were not entirely convinced of this empirical inflexibility and have now shown that molecular wires can be bent into ring shapes. The discovery could revolutionize the way some chemical reactions are carried out and lead molecular architects down entirely new avenues of exploration.

Harry Anderson

"It is easy to bend an ordinary piece of wire," Anderson told Reactive Reports, "but it can be difficult to grab hold of a piece of wire when it is only a few nanometers long." He and his colleagues in Oxford's state-of-the-art Chemical Research Laboratory have found a way to grab hold of those ends using metal coordination chemistry. Coordination chemistry involves the formation of bonds between metal ions and chemical species known as ligands. A ligand can be something as simple as a water molecule or a complex organic molecule.

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Anderson's team chose to work with a coordination compound formed between zinc ions and a natural molecule known as a porphyrin. Porphyrins are flat almost square molecules found at the heart of the photosynthetic unit in plants, chlorophyll. The team made a zinc-porphyrin octamer wire containing, as the name would suggest, eight Zn-porphyrin units. They then grabbed on to this linear octamer using a wheel-shaped template molecule with eight biting groups—it is a cyclic octadentate molecule in other words—arranged equally around its perimeter.

With the zinc-porphyrin complex bent around the template, the team then used a palladium-catalyzed coupling reaction to chemically "weld" the ends together to prevent it from springing back into its linear form once the template was removed. The final step is to add the strong organic solvent pyridine to dissolve the template leaving behind the zinc-porphyrin molecular hoop. This step is actually quite tough as the template has a very strong affinity for the zinc-porphyrin complex. "The resulting molecule has a circular belt-like shape," Anderson told us, "with eightfold symmetry and a diameter of 3 nanometers."

Anderson explains how many scientists, and particularly physicists, have long been fascinated by the fact that persistent ring-currents and quantized magnetization can be observed in small loops of wire. He suggests that it will be interesting to discover whether similar effects emerge on the molecular scale in his team's nanoscopic rings. "The synthesis of these wire rings could herald the development of molecular solenoids and induction coils with unusual optical and magnetic characteristics, leading to applications in nanoelectronics and nonlinear optics," he told us.

"Many large cyclic porphyrin molecules have been reported before," Anderson adds, "but this is the first fully conjugated example, in which the closed loop should behave like a ring of molecular wire."