Increased Speed of Rotation for the Smallest Light-Driven Molecular Motor

In this paper we present the smallest artificial light-driven molecular motor consisting of only 28
carbon and 24 hydrogen atoms. The concept of controlling directionality of rotary movement at the molecular
level by introduction of a stereogenic center next to the central olefinic bond of a sterically overcrowded
alkene does not only hold for molecular motors with six-membered rings, but is also applicable to achieve
the unidirectional movement for molecular motors having five-membered rings. Although X-ray analyses
show that the five-membered rings in the cis- and trans-isomer of the new molecular motor are nearly flat,
the energy differences between the (pseudo-)diaxial and (pseudo-)diequatorial conformations of the methyl
substituents in both isomers are still large enough to direct the rotation of one-half of the molecule with
respect to the other half in a clockwise fashion. The full rotary cycle comprises four consecutive steps: 
two photochemical isomerizations each followed by a thermal helix inversion. Both photochemical cis−trans isomerizations proceed with a preference for the unstable diequatorial isomers over the stable diaxial
isomers. The thermal barriers for helix inversion of this motor molecule have decreased dramatically
compared to its six-membered ring analogue, the half-life of the fastest step being only 18 s at room
temperature.