Taiwanese researchers have demonstrated a novel synthetic molecular brake , one of the components in the arsenal of molecular machines (Jye-Shane Yang et al. 2008 DOI), also see the earlier post on molecular hinges .
The earliest such system (Kelly et al. 1994 DOI, article aptly called a molecular break) is based on a trypticene rotator and a bipyridine brake which can reversibly lock and unlock by a complexation process with lead ions.
In another system (Feringa et al. 1997 (DOI)) free rotation in a biphenyl group is found to be more difficult for the cis-isomer than for the trans isomer which gives a potential for photochemical switching.
Both researchers, Kelly and Feringa, would a few years later also go head to head in the development of synthetic molecular motors.
Other systems of later date are based on a sulfide-sulfoxide redox cycle (Jog et al. 2003 DOI) and a solvent-mediated system interrupting hydrogen bonding (Alfonso et al. 2006 DOI)
In the novel Taiwanese system a three-bladed trypticene (Diels-Alder reaction product of anthracene and an aryne) is replaced by four-bladed pentiptycene molecule (same reaction but with a diaryne synthon).
To the pentiptycene scaffold is attached a stilbene unit at the central benzene ring. Free rotation around the aryne carbon and alkene bond is very easy with 109 revolutions per second (room temperature). Photochemical trans-cis isomerization generates the the cis-isomer, forcing the benzene ring in between the blades. This is effectively a break with the number of revolutions per second reduced to just 3 (based on NMR spectroscopy). The process is reversible by selecting a different wavelength.
A big difference with the molecular motors is that motion is not unidirectional , that is the propelling motion goes back and forth. In this sense an enthusiastic YouTube cartoon related to this particular research is misleading.