New Motions For Molecular Machinery

Macromolecules that can undergo a range of motions when stimulated by light or heat have been touted for several years as potential components of tiny machines in future nanoscale devices. One such molecular assembly, reported by Leonard R. MacGillivray, Anatoliy N. Sokolov, and Dale C. Swenson at the University of Iowa, is the first example of an organic solid in which the molecules can undergo three correlated motions, in this case functioning like a set of rack-and-pinion gears that convert linear motion into rotary motion (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.0706117105).

MacGillivray's group used its previously developed templating strategy to cocrystallize an alkyl phenol (4-hexylresorcinol) with a diene (all-trans-2,5-bis[4-ethenylpyridyl]thiophene). In this assembly, held together by four N∙∙∙H–O hydrogen bonds, two diene molecules are sandwiched between two phenol molecules.

Upon cooling, the assembly's molecules reorganize, which the researchers observed by X-ray crystallography. The hexyl groups, serving as the rack, expand and contract. There is a subsequent 180° crankshaft rotation about one of the dienes' double bonds, serving as the pinion gear. In combination with these motions, the assembly???s aromatic rings tilt. The collective molecular motions are reversible upon heating.

UCLA's Miguel A. Garcia-Garibay, also a molecular machinist, comments that "MacGillivray and coworkers illustrate how a temperature-induced transition between different crystal phases may offer a deep insight into the type of complex processes taking place within the bulk of single-crystalline specimens."

Future studies will likely address the rate of these motions as a function of temperature, and it might be possible to take advantage of small thermal fluctuations around a critical temperature to operate the machinery, Garcia-Garibay adds. "But more important, MacGillivray and coworkers provide a new perspective on crystalline molecular machinery, and we can expect that new developments may come at an accelerated pace," he says.