Thiol groups, despite being proposed as rigid linker for many molecular electronics applications, anchor aromatic molecules to metal surfaces in a fashion that permits rapid rearrangement of the molecule on the substrate even at temperatures as low as 60K. The aromatic ring of individual benzenethiol molecules experiences a rotation barrier around the substrate linker of ca. 120 meV; even diffusion of the sulfur anchor on the substrate has a barrier of only ca. 150 meV.1 Placement of two thiol groups in a molecule (9,10-dithionanthracene, DTA) does not increase the diffusion barrier. Rather it renders the molecule\ufffds diffusive behavior on Cu(111) anisotropic, i.e. following a single line on the otherwise threefold symmetric substrate.2 Density functional theory calculations show, that this behavior originates from sequential occupation of optimal adsites by each of the two substrate linkers, strikingly resembling bipedal locomotion ("walking"). The resultant linear (uniaxial) motion on the surface can be used for transport of CO2 "cargo" molecules across a surface3. Asymmetric addition of methyl groups to DTA allows to explore the behavior of individual molecules in an asymmetric potential well, a Gedankenexperiment underlying Tolman's and Marcelin's prediction of Microscopic Reversibility and Detailed Balance. Extension of the molecule to its pentacene-based counterpart generates a quadrupedal species and I will show whether this molecule should be regarded (in equestrian terms) as a 'pacer (germ: Passgänger)' or a 'trotter (germ: Traber)'.
[1] K.L. Wong et al., Appl. Phys. Lett. 88, 183106 (2006)
[2] K.-Y. Kwon et al., Phys. Rev. Lett. 95, 166101 (2005)
[3] K.L. Wong et al., Science 315, 1391 (2007)
Thiol groups, despite being proposed as rigid linker for many molecular electronics applications, anchor aromatic molecules to metal surfaces in a fashion that permits rapid rearrangement of the molecule on the substrate even at temperatures as low as 60K. The aromatic ring of individual benzenethiol molecules experiences a rotation barrier around the substrate linker of ca. 120 meV; even diffusion of the sulfur anchor on the substrate has a barrier of only ca. 150 meV.1 Placement of two thiol groups in a molecule (9,10-dithionanthracene, DTA) does not increase the diffusion barrier. Rather it renders the molecule\ufffds diffusive behavior on Cu(111) anisotropic, i.e. following a single line on the otherwise threefold symmetric substrate.2 Density functional theory calculations show, that this behavior originates from sequential occupation of optimal adsites by each of the two substrate linkers, strikingly resembling bipedal locomotion ("walking"). The resultant linear (uniaxial) motion on the surface can be used for transport of CO2 "cargo" molecules across a surface3. Asymmetric addition of methyl groups to DTA allows to explore the behavior of individual molecules in an asymmetric potential well, a Gedankenexperiment underlying Tolman's and Marcelin's prediction of Microscopic Reversibility and Detailed Balance. Extension of the molecule to its pentacene-based counterpart generates a quadrupedal species and I will show whether this molecule should be regarded (in equestrian terms) as a 'pacer (germ: Passgänger)' or a 'trotter (germ: Traber)'.
[1] K.L. Wong et al., Appl. Phys. Lett. 88, 183106 (2006)
[2] K.-Y. Kwon et al., Phys. Rev. Lett. 95, 166101 (2005)
[3] K.L. Wong et al., Science 315, 1391 (2007)
