Web Release Date: December 12,
Odd-Even Effects in Self-Assembled Monolayers of
-(Biphenyl-4-yl)alkanethiols: A First-Principles Study
and
School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, Department of Material Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139-4307, and Institute of Solid State Physics, Graz University of Technology, Petersgasse 16, A-8010 Graz, Austria
Received August 2, 2007
In Final Form: October 12, 2007
Abstract:
Conjugated molecules with a saturated alkyl linker between a thiol docking group and the 
-conjugated core have
been shown to form self-assembled monolayers (SAMs) with a high degree of long-range order and uniformity.
Additionally, pronounced odd-even effects have been observed in a number of properties characterizing these SAMs.
We focus on -(biphenyl-4-yl)alkanethiols with n = 0-6 -(CH2)n- units deposited on Au(111) and investigate the
microscopic origin of these odd-even effects in terms of the local sulfur-gold bonding geometry by employing
first-principles calculations. An additional structural parameter, the torsion angle between the two phenyl rings in the
biphenyl moiety, is identified and its relation to the experimentally observed odd-even effects is discussed. More
importantly, we address relevant quantities for the application of these SAMs in molecular electronic devices, in
particular, the modification of the work function of the underlying metal substrate and the energetic alignment of the
molecular orbitals in the SAM with the Fermi level. While no clear trend emerges for the former, we find pronounced
odd-even effects for the latter. Furthermore, the insertion of a single methylene unit between the biphenyl core and
the thiol appears to largely decouple the valence electronic systems of the -conjugated segment and the gold substrate.
Our results thus provide a solid theoretical basis for the interface energetics in this important class of systems.
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