title: Increased speed of rotation for the smallest light-driven molecular motor.
abstract: 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.
authors.affiliation: Department of Organic Chemistry, Stratingh Institute, University of Groningen, Nijenborgh 4, 9747 AG, Groningen, The Netherlands.
authors.forename: Matthijs K J
authors.initials: MK
authors.lastname: ter Wiel
authors.forename: Richard A
authors.initials: RA
authors.lastname: van Delden
authors.forename: Auke
authors.initials: A
authors.lastname: Meetsma
authors.forename: Ben L
authors.initials: BL
authors.lastname: Feringa