title: Ullmann coupling mediated assembly of an electrically driven altitudinal molecular rotor.
abstract: Surface-bound molecular rotation can occur with the rotational axis either perpendicular (azimuthal) or parallel (altitudinal) to the surface. The majority of molecular rotor studies involve azimuthal rotors, whereas very few altitudinal rotors have been reported. In this work, altitudinal rotors are formed by means of coupling aryl halides through a surface-mediated Ullmann coupling reaction, producing a reaction state-dependent altitudinal molecular rotor/stator. All steps in the reaction on a Cu(111) surface are visualized by low-temperature scanning tunneling microscopy. The intermediate stage of the coupling reaction is a metal-organic complex consisting of two aryl groups attached to a single copper atom with the aryl rings angled away from the surface. This conformation leads to nearly unhindered rotational motion of ethyl groups at the para positions of the aryl rings. Rotational events of the ethyl group are both induced and quantified by electron tunneling current versus time measurements and are only observed for the intermediate structure of the Ullmann coupling reaction, not the starting material or finished product in which the ethyl groups are static. We perform an extensive set of inelastic electron tunneling driven rotation experiments that reveal that torsional motion around the ethyl group is stimulated by tunneling electrons in a one-electron process with an excitation energy threshold of 45 meV. This chemically tunable system offers an ideal platform for examining many fundamental aspects of the dynamics of chemically tunable molecular rotor and motors.
authors.affiliation: Department of Chemistry, Tufts University, Medford, MA 02155, USA. Charles.sykes@tufts.edu.
authors.forename: Colin J
authors.initials: CJ
authors.lastname: Murphy
authors.affiliation: Department of Chemistry, Tufts University, Medford, MA 02155, USA. Charles.sykes@tufts.edu.
authors.forename: Zachary C
authors.initials: ZC
authors.lastname: Smith
authors.affiliation: Department of Chemistry, Tufts University, Medford, MA 02155, USA. Charles.sykes@tufts.edu.
authors.forename: Alex
authors.initials: A
authors.lastname: Pronschinski
authors.affiliation: Department of Chemistry, Tufts University, Medford, MA 02155, USA. Charles.sykes@tufts.edu.
authors.forename: Emily A
authors.initials: EA
authors.lastname: Lewis
authors.affiliation: Department of Chemistry, Tufts University, Medford, MA 02155, USA. Charles.sykes@tufts.edu.
authors.forename: Melissa L
authors.initials: ML
authors.lastname: Liriano
authors.affiliation: Department of Chemistry, Tufts University, Medford, MA 02155, USA. Charles.sykes@tufts.edu.
authors.forename: Chloe
authors.initials: C
authors.lastname: Wong
authors.affiliation: Department of Chemistry, Tufts University, Medford, MA 02155, USA. Charles.sykes@tufts.edu.
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Ivimey
authors.affiliation: Department of Chemistry, Tufts University, Medford, MA 02155, USA. Charles.sykes@tufts.edu.
authors.forename: Mitchell
authors.initials: M
authors.lastname: Duffy
authors.affiliation: Department of Chemistry, Tufts University, Medford, MA 02155, USA. Charles.sykes@tufts.edu.
authors.forename: Wojciech
authors.initials: W
authors.lastname: Musial
authors.affiliation: Department of Chemistry, Tufts University, Medford, MA 02155, USA. Charles.sykes@tufts.edu.
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Therrien
authors.affiliation: Department of Chemistry, Tufts University, Medford, MA 02155, USA. Charles.sykes@tufts.edu.
authors.forename: Samuel W
authors.initials: SW
authors.lastname: Thomas
authors.affiliation: Department of Chemistry, Tufts University, Medford, MA 02155, USA. Charles.sykes@tufts.edu.
authors.forename: E Charles H
authors.initials: EC
authors.lastname: Sykes