The Nature of Electronic Contact in Self-assembled Monolayers for Molecular Electronics: Evidence for Strong Coupling

Vondrak, T.; Cramer, C. J.; Zhu, X.-Y.
J. Phys. Chem. B 1999, 103, 8915.

Self-assembled-monolayers (SAMs) of aromatic and conjugated thiolates on metal surfaces are candidates for molecular "quantum dots" or "wires". We probe the interfacial electronic structure using two-photon photoemission in SAM of pentafluorothiophenolate (C₆F₅S^-) on Cu(111). Compared to gas phase C₆F₅SH, the LUMO in the assembled molecule is stabilized by 3.1 eV and the HOMO-LUMO gap is reduced by 5.7 eV. Ab initio calculations reveal that stabilization of molecular states is, to a great extent, a consequence of the strong electronic coupling between metal and molecular orbitals via the -S- bridge. The -S-metal linker is a conducting contact in SAMs of molecular electronics.

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