Straight Route To Nanorings

NANOSCALE RINGS have been a challenging architecture to synthesize. Now, Caltech and University of California, Berkeley, chemists report a direct catalytic route for making these rings, which could have applications in drug delivery or organic photovoltaic devices (J. Am. Chem. Soc., DOI: 10.1021/ja901658c).

C. Grant Willson, a chemistry and chemical engineering professor at the University of Texas, Austin, says the new catalytic method exemplifies "the ability of chemists to control not only the sequence connectivity of atoms in molecules, but the way in which the resulting molecules form themselves into three-dimensional shapes."

Traditional approaches to making cyclic polymers primarily focused on coupling the reactive ends of a linear polymer but often resulted in dimers and trimers rather than rings. The new rings "demonstrate an ability to construct complex nanostructures of well-defined size and shape in a simple, straightforward manner and in reasonable quantities," says Robert H. Grubbs, a chemistry professor at Caltech.

Grubbs, UC Berkeley chemistry professor Jean M. J. Fréchet, Caltech postdoc Andrew J. Boydston, and colleagues used ring-expansion metathesis polymerization with ruthenium-carbene catalysts to generate toroidal structures from dendrimers. They then examined the topology of the rings with atomic force microscopy (AFM).

Craig Hawker, a polymer materials chemist at UC Santa Barbara, says, "The AFM images are simply gorgeous," noting the rings' almost perfect doughnut shapes and nearly uniform features.

Shape and size are critical for many biological interactions and are becoming increasingly important in devices that rely on synthetic materials, Hawker says. Adding rings to the palette of nanoscale objects that can be tailor-made for these specific applications will open up new vistas of research, he adds.