New Motions For Molecular Machinery

Macromolecules that can undergo a range of motions when stimulated by light or heat have been touted as potential components of tiny machines in nanoscale devices. One such assembly, reported by Leonard R. MacGillivray and coworkers 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 to 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 (trans-2,5-bis[4-ethenylpyridyl]thiophene). In this assembly, held together by hydrogen bonds, two diene molecules are sandwiched between two phenol molecules. Upon cooling, the terminal hexyl groups, serving as the rack, expand and contract. There is a subsequent 180?? rotation about one of the dienes' double bonds, which serves as a pinion gear. In combination with these motions, the assembly's aromatic rings tilt. The collective molecular motions are reversible upon heating.