Science 337, 1521–1526 (2012)

Tubular structures are ubiquitous in nature and recently, analogous synthetic structures have been synthesized using supramolecular chemistry. However, developing synthetic nanoscale tubules with stimuli-responsive capabilities remains a significant challenge because such structures are typically either too strong to be responsive or too flimsy to remain intact as environmental conditions change. Myongsoo Lee and colleagues at Seoul National University, Nagoya University and Harbin Institute of Technology have now created a supramolecular nanotubule that can reversibly change its diameter in response to changes in temperature.

The researchers start with a molecule with a pyridine centre that has two hydrophobic p-phenylene chains arranged in a 120°-bent structure on one side of it, and an alkyl ether dendron on the other. In aqueous solution, the molecule can self-assemble into a hexameric ring and then into a stacked architecture with a hollow interior. The tubule shrinks on heating to 60 °C and swells again on cooling, creating a breathing motion

Two features make this system functional. First, the p-phenylene units in the hexameric ring can slide along one another allowing the entire structure to tolerate changes in diameter. Second, the large diameter of the tubules can be stabilized because the pyridine centre nucleates water molecules through hydrogen bonding in the interior of the tubule. On heating, the water cluster is disrupted and the water molecules are squeezed out of the tubule. Subsequently, the aromatic rings slide on top of each other to maximize hydrophobic interactions and shrink the tubule by as much as 47%.