Stepwise self-organization of molecules into hexameric rosettes and then into nanotubes

05-Jan-2004

Tiny structures like nanoscopic tubes are highly desirable as potential components for optoelectronics, "intelligent" materials, or the basis for new systems for pharmaceutical transport in the body. Such mini tubes must have precisely defined dimensions and specific chemical properties, which is not at all easy to achieve. A Dutch-Belgian team led by Frans C. De Schryver and E. W. Meijer has now synthesized a type of molecule that aggregates into long tubes through stepwise self-organization. When the scientists created their molecules, two oligo para phenylvinylenes (OPV3 and OPV4), they were actually not even doing research with nanotubes in mind. Scanning tunneling microscopic images then revealed a surprise: having been deposited on a Graphite support, the molecules arranged themselves into a highly organized rosette-shaped structure. In solution, on the other hand, these rosettes stack themselves into long tubes. The secret behind these unusual aggregates lies in the special structure of the molecules, which consist of a "head", a stretched "backbone", "arms" protruding from the sides, and three long "tails". Six molecules at a time stick their heads together, and this social circle is held together by two bridging hydrogen bonds between each pair of neighboring heads. The arrangement of the bridges forces the heads into an angled position, so that the backbones don't stick out like rays, but rather form a rosette. The two-dimensional variation is fixed by the interlacing of the tails of neighboring rosettes. Like a spiral, rosettes can be "twisted" either clockwise or counterclockwise. Amazingly, OPV3 and OPV4 rosettes are twisted in opposite directions, despite the fact that they only differ in the length of their "backbone" and the number of arms (two or four). "The molecules are trying to use the available space as intensively as possible -- without having bits of molecule get in the way," explains Meijer. "Our OPVs are not completely symmetrical and the two possible directions of rotation for the rosettes are thus not equivalent. Which is preferred depends on the actual size and geometry of the OPV in question. In addition, interactions with the graphite structure of the support also play a role." Stacking of the rosettes in solution, which are held together by attractive forces between the flat aromatic rings of the OPV backbone, also involves optimal use of space. This results in very dense tubes with an inner diameter of about 1nm, which could be of interest as transport channels. Variation of the OPVs should make it possible to obtain specific tubes with different dimensions and properties.

https://www.chemeurope.com/en/news/33313/stepwise-self-organization-of-molecules-into-hexameric-rosettes-and-then-into-nanotubes.html