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Abstract We report the first conductance measurements of [n]staffane (bicyclopentane) oligomers in single‐molecule junctions. Our studies reveal two quantum transport characteristics unique to staffanes that emerge from their strained bicyclic structure. First, though staffanes are composed of weakly conjugated C−Cσ‐bonds, staffanes carry a shallower conductance decay value (β=0.84±0.02 n−1) than alkane chain analogs (β=0.96±0.03 n−1) when measured with the scanning tunneling microscopy break junction (STM‐BJ) technique. Staffanes are thus more conductive than otherσ‐bonded organic backbones reported in the literature on a per atom basis. Density functional theory (DFT) calculations suggest staffane backbones are more effective conduits for charge transport because their significant bicyclic ring strain destabilizes the HOMO‐2 energy, aligning it more closely with the Fermi energy of gold electrodes as oligomer order increases. Second, the monostaffane is significantly lower conducting than expected. DFT calculations suggest that short monostaffanes sterically enforce insulating gauche interelectrode orientations over syn orientations; these steric effects are alleviated in longer staffanes. Moreover, we find that [2‐5]staffane wires may accommodate axial mechanical strain by “rod‐bending”. These findings show for the first time how bicyclic ring strain can enhance charge transmission in saturated molecular wires. These studies showcase the STM‐BJ technique as a valuable tool for uncovering the stereoelectronic proclivities of molecules at material interfaces.
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Charge Transport through Self‐Assembled Monolayers of Monoterpenoids
Abstract The nature of the processes at the origin of life that selected specific classes of molecules for broad incorporation into cells is controversial. Among those classes selected were polyisoprenoids and their derivatives. This paper tests the hypothesis that polyisoprenoids were early contributors to membranes in part because they (or their derivatives) could facilitate charge transport by quantum tunneling. It measures charge transport across self‐assembled monolayers (SAMs) of carboxyl‐terminated monoterpenoids (O2C(C9HX)) and alkanoates (O2C(C7HX)) with different degrees of unsaturation, supported on silver (AgTS) bottom electrodes, with Ga2O3/EGaIn top electrodes. Measurements of current density of SAMs of linear length‐matched hydrocarbons—both saturated and unsaturated—show that completely unsaturated molecules transport charge faster than those that are completely saturated by approximately a factor of ten. This increase in relative rates of charge transport correlates with the number of carbon–carbon double bonds, but not with the extent of conjugation. These results suggest that polyisoprenoids—even fully unsaturated—are not sufficiently good tunneling conductors for their conductivity to have favored them as building blocks in the prebiotic world.
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- Award ID(s):
- 1808361
- PAR ID:
- 10102914
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Angewandte Chemie International Edition
- Volume:
- 58
- Issue:
- 24
- ISSN:
- 1433-7851
- Page Range / eLocation ID:
- p. 8097-8102
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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