title: Exploring the boundaries of a light-driven molecular motor design: new sterically overcrowded alkenes with preferred direction of rotation.
abstract: Insight in the steric and electronic parameters governing isomerization processes in artificial molecular motors is essential in order to design more advanced motor systems. A subtle balance of steric parameters and the combination of helical and central chirality are key features of light-driven unidirectional rotary molecular motors constructed so far. In an approach to decrease the steric hindrance around the central olefinic bond (rotary axis) and thereby lowering the energy barrier for helix inversion resulting in an increased rotation rate, the boundaries of our molecular motor design are explored. In a new design of a light-driven molecular motor based on a sterically overcrowded alkene the methyl substituent adjacent to the stereogenic center, which is responsible for the control of the direction of rotation, is shifted one position away from the fjord region of the molecule compared to the second-generation motor systems. In contrast to previously developed light-driven molecular motors, there is a preference for the methyl substituent to adopt a pseudo-equatorial orientation. Nevertheless, this new type of motor is capable of functioning as a rotary molecular motor, albeit not with full unidirectionality. Under the combined influence of light and heat, there is a preferred clockwise rotation of one half of the molecule. Surprisingly, the effect of shifting the methyl substituent on the energy barrier for helix inversion is small and even a slight increase in the barrier is observed.
authors.affiliation: Department of Organic and Molecular Inorganic Chemistry Stratingh Institute, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands.
authors.forename: Richard A
authors.initials: RA
authors.lastname: van Delden
authors.forename: Matthijs K J
authors.initials: MK
authors.lastname: ter Wiel
authors.forename: Harmen
authors.initials: H
authors.lastname: de Jong
authors.forename: Auke
authors.initials: A
authors.lastname: Meetsma
authors.forename: Ben L
authors.initials: BL
authors.lastname: Feringa
mesh_terms: D000475:Alkenes / Q000138:chemical synthesis* / Q000737:chemistry / Q000528:radiation effects; D002942:Circular Dichroism; D018360:Crystallography, X-Ray; D003500:Cyclization; D006575:Heterocyclic Compounds, 3-Ring / Q000138:chemical synthesis / Q000737:chemistry; D006358:Hot Temperature; D006835:Hydrazones / Q000737:chemistry; D007536:Isomerism; D008027:Light; D008958:Models, Molecular; D008968:Molecular Conformation; D009281:Naphthalenes / Q000737:chemistry; D009570:Nitriles / Q000737:chemistry; D010777:Photochemistry; D010880:Piperidines / Q000737:chemistry; D012399:Rotation; D013056:Spectrophotometry, Ultraviolet; D013237:Stereoisomerism; D013438:Sulfhydryl Compounds / Q000737:chemistry; D013457:Sulfur Compounds / Q000138:chemical synthesis / Q000737:chemistry; D013816:Thermodynamics
chemical_list: D000475:Alkenes; D006575:Heterocyclic Compounds, 3-Ring; D006835:Hydrazones; D009281:Naphthalenes; D009570:Nitriles; D010880:Piperidines; D013438:Sulfhydryl Compounds; D013457:Sulfur Compounds; C031103:crotononitrile; C032727:piperidine; C401958:2-naphthalenethiol