Probing pi-coupling in molecular junctions.

Charge transport characteristics for metal-molecule-metal junctions containing two structurally related pi-conjugated systems were studied to probe pi-pi interactions in molecular junctions. The first molecule contains a typical pi-conjugated framework derived from phenylene vinylene units, whereas the second has the phenylene vinylene structure interrupted by a [2.2]paracyclophane (pCp) core. Electrochemical investigations were used to characterize the defects and packing density of self-assembled monolayers of the two molecules on gold surfaces and to enable quantitative comparison of their transport characteristics. Current-voltage measurements across molecular junctions containing the two species demonstrate that the pCp moiety yields a highly conductive break in through-bond pi-conjugation. The observed high conductivity is consistent with density functional theory calculations, which demonstrate strong through-space pi-pi coupling across the pCp moiety.