J. Am. Chem. Soc. https://doi.org/10.1038/s41467-018-04928-9 (2018)

Light-driven molecular motors, such as the Feringa motor, generally work through a combination of light-induced and thermally induced steps. Now, Gerwien et al. have demonstrated a different light-driven unidirectional molecular motor in which all the steps are driven by light.

The researchers use a hemithioindigo chromophore that bears a sulfoxide stereocentre attached to a chiral axis formed by a sterically crowded styrene fragment. As a result, it can exist as four different diastereoisomers, all of which are thermodynamically stable at room temperature. The photochemical steps consist in a single-bond rotation, a double-bond isomerization and a hula-twist motion (a concerted double-bond isomerization and an adjacent single-bond twist by 180°). In total, it takes three steps for the molecule to return to the initial configuration. Gerwien et al. demonstrate that their hemithioindigo can indeed undergo unidirectional motion under steady-state irradiation at 442 nm. At room temperature, about 80% of molecules rotate in the same direction with a cycling probability of about 10−9. Finally, because the motor has no thermally induced step, the efficiency and unidirectionality increase on lowering the temperature.