Probing π-coupling in molecular junctions

doi:10.1073/pnas.0500002102

Probing π-coupling in molecular junctions

^*Departments of Chemistry and Biochemistry and Materials, Institute for Polymers and Organic Solids, University of California, Santa Barbara, CA 93106; and ^†Center for Bio/Molecular Science and Engineering, Naval Research Laboratory, Washington, DC 20375

Edited by Mark A. Ratner, Northwestern University, Evanston, IL, and approved May 5, 2005 (received for review January 1, 2005)

Abstract

Charge transport characteristics for metal–molecule–metal junctions containing two structurally related π-conjugated systems were studied to probe π–π interactions in molecular junctions. The first molecule contains a typical π-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 π-conjugation. The observed high conductivity is consistent with density functional theory calculations, which demonstrate strong through-space π–π coupling across the pCp moiety.

Footnotes

↵ ‡ To whom correspondence may be addressed at: Department of Chemistry, University of California, Santa Barbara, CA 93106. E-mail: bazan{at}chem.ucsb.edu. §To whom correspondence may be addressed at: 4555 Overlook Avenue Southwest, Washington, DC 20375. E-mail: kushmerick{at}nrl.navy.mil.
Author contributions: G.C.B. and J.G.K. designed research; D.S.S., S.A.T., and J.G.K. performed research; D.S.S. and G.C.B. contributed new reagents/analytic tools; D.S.S., S.A.T., G.C.B., and J.G.K. analyzed data; and D.S.S., G.C.B., and J.G.K. wrote the paper.
This paper was submitted directly (Track II) to the PNAS office.
Abbreviations: HOMO, highest occupied molecular orbital; I–V, current–voltage; pCp, [2.2]paracyclophane.