16766 J. Phys. Chem. B, Vol. 108, No. 43, 2004
Dameron et al.
the SAM, any molecule that falls below the level of the SAM
will have an apparent height of zero resulting in narrower
distributions for the OFF states.
Supporting Information Available: A detailed description
of the synthesis of 2-thioacetyl-biphenyl. This material is
available free of charge via the Internet at http://pubs.acs.org.
The 2-thiophenanthrene also exhibits two conductance states,
with an apparent height difference between them of 5.1 ( 1.2
Å. The topographic height of these two molecules normal to
the surface is comparable, so the deviations in apparent height
are associated with differences in conductance. It is expected
that the 2-thiophenanthrene molecule is more conductive because
References and Notes
(1) Bumm, L. A.; Arnold, J. J.; Dunbar, T. D.; Allara, D. L.; Weiss,
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(4) Hwang, J. J.; Tour, J. M. Tetrahedron 2002, 58, 10387.
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of the increased conjugation of the middle ring extending the
overlap of the π orbitals.6
2-64
The existence of switching in
the 2-thiophenanthrene molecule eliminates internal ring rotation
as the sole mechanism responsible for the change in conductance
states. In addition, internal rotation of biphenyl at room
temperature would give states that are too short-lived to be
observed by these methods.8
Qualitatively, the 2-thiophenanthrene molecules switch states
less frequently and favor the off state. This is quantitatively
verified by the calculated ON/OFF ratios (calculated from the
areas under the Gaussian fits) of 2.45 for the 2-thiophenanthrene
and 4.61 for the 4-thiobiphenyl molecules. Considering that both
molecules are in similar environments, this difference can be
explained by the degrees of freedom available to each molecule.
In both cases, the alkanethiol SAM is surrounding the molecule
59, 2497.
(7) Tour, J. M.; Rawlett, A. M.; Kozaki, M.; Yao, Y. X.; Jagessar, R.
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(
(
(
2
see Figure 1). Due to size and rotation constraints, the
-thiophenanthrene molecule needs larger defect sites to be able
M. T.; Tour, J. M.; Reinerth, W. A.; Yao, Y.; Kozaki, M.; Jones, L. In
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to insert and to have the same degrees of freedom afforded to
the 4-thiobiphenyl. For the same defect site, one would expect
that the biphenyl is more likely to switch, but we do not know
if the distributions of sites at which the two molecules insert
are equivalent. Theory predicts that a tilted thiolate bond is the
energetically favorable and a more conductive (ON) molecular
conformation, although it is not possible to measure experi-
mentally only the thiolate-substrate bond angle with the
(15) Smith, R. K.; Lewis, P. A.; Weiss, P. S. Prog. Surf. Sci. 2004, 75,
1.
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9
,65,66
STM.
It is expected that a molecule that is increasingly
constrained by the host SAM is less likely to undergo motion
that will result in a dramatic change of conductivity.
(19) Lewis, P. A.; Donhauser, Z. J.; Mantooth, B. A.; Smith, R. K.;
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(
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(
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Conclusions
In order for the mechanism of molecular switching to be more
fully understood, 2-thiophenanthrene has been used to maximize
the conductance and to minimize the mechanical freedom
present in 4-thiobiphenyl. The mechanical hindrances are
apparent in the structural order of the SAMs made from each
molecule. The 2-thiophenanthrene SAMs exhibit no observable
ordered structures, regardless of insertion time. 4-thiobiphenyl
SAMs display two packing structures with a primarily striped
phase for insertion times less than 12 h and a hexagonal phase
for insertion times greater than 30 h. Conductively, the
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(
(
2-thiophenanthrene SAMs are slightly more symmetric around
(
zero sample bias than the 4-thiobiphenyl SAMs, although further
investigation of the system is needed. Upon insertion into
alkanethiolate SAMs, both molecules exhibit similar bistable
conductance switching. While internal rotation is therefore not
the sole mechanism responsible for conductance switching, it
does allow the molecules to relax conformationally within the
matrix, making switching more likely.
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Acknowledgment. The authors acknowledge Brent Man-
tooth and Penelope Lewis for their experimental and analytical
assistance and Kevin Kelly and Charlie Sykes for their helpful
discussions. The Army Research Office, Defense Advanced
Research Projects Agency, National Science Foundation, and
Office of Naval Research are gratefully acknowledged for their
support.
(
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