Effect of bond-length alternation in molecular wires

Article abstract of DOI:10.1021/ja027090n

Current-voltage (I-V) characteristics for metal-molecule-metal junctions formed from three classes of molecules measured with a simple crossed-wire molecular electronics test-bed are reported. Junction conductance as a function of molecular structure is consistent with I-V characteristics calculated from extended Hueckel theory coupled with a Green's function approach, and can be understood on the basis of bond-length alternation. Copyright

Full text of DOI:10.1021/ja027090n

Published on Web 08/17/2002
Effect of Bond-Length Alternation in Molecular Wires
James G. Kushmerick,*,†,§ David B. Holt,† Steven K. Pollack,† Mark A. Ratner,‡ John C. Yang,†
Terence L. Schull,† Jawad Naciri,† Martin H. Moore,† and Ranganathan Shashidhar*,†
Center for Bio/Molecular Science and Engineering, NaVal Research Laboratory, Washington, D.C. 20375, and
Department of Chemistry, Northwestern UniVersity, EVanston, Illinois 60208
Received May 29, 2002
Table 1. Molecular Structures, Calculated HOMO-LUMO Gap
(Eg), Calculated (Gcalc) and Measured (Gmeas) Relative Junction
Conductance at 0.5 V
In this communication, we report experimental and theoretical
current-voltage (I-V) characteristics which demonstrate that bond-
length alternation plays an important role in determining the
conductance of molecular wires. Charge transport across organic
molecules as a function of molecular structure has previously been
investigated with a variety of experimental measurements including
electrochemical,1 scanning probe,2 donor-bridge-acceptor,3 and
mercury drop electrode,4 as well as theoretical calculations.5 Two
particular classes of molecular wires oligo(phenylene ethynylene)
(OPE) and oligo(phenylene vinylene) (OPV) have been the focus
of much study.1b,d,e,2a,b,3d While it has previously been argued that
the higher planarity of OPV makes it a better molecular wire than
OPE, the measurements and theoretical calculations presented here
highlight a second important contribution to molecular wire
conductance: the extent of bond-length alternation along the
π-conjugated molecular backbone.
a Eg is the HOMO-LUMO gap calculated from density-functional theory
at the B3LYP/6-31G* level. b Junction conductance (I/V) at 0.5 V normal-
ized to the conductance of the C12 junction.
The I-V characteristics of self-assembled monolayers (SAMs)
of the three molecules investigated (Table 1) are measured with a
crossed-wire tunnel junction (Figure 1). The crossed-wire tunnel
junction is formed from two 10 µm Au wires, one modified with
a SAM of the molecule of interest. The wires are mounted to a
custom built test-stage so that they are crossed, and the wire spacing
is controlled by the Lorentz force: dc current in one wire deflects
it in a magnetic field (B). This deflection current is slowly increased
to bring the wires gently together, forming a junction at the contact
point. By comparison to other charge transport measurements, we
calculate that the junction contains ∼103 molecules.2c,4 We have
previously used this experimental configuration to investigate the
role that metal-molecule contacts play in charge transport across
monolayers.6
Figure 1. Schematic representation of the crossed-wire tunnel junction
(not to scale). All measurements were made in a nitrogen purged Faraday
cage at room temperature.
Figure 2A shows the experimental I-V characteristics for
junctions formed from monolayers of the three classes of molecules
studied. The error bars associated with the measurements are
attributed to variations in junction area between independent
junctions. Multiple measurements on the same junction show a
much smaller deviation. As would be expected, there is a clear
difference in the magnitude of charge transport across monolayers
of the electrically insulating σ-bonded alkane (C12) and that of
the π-conjugated oligo(phenylene ethynylene) (OPE) and oligo-
(phenylene vinylene) (OPV) molecular wires. The difference in
conductance for OPE and OPV is discussed below.
techniques under the approximation that the entire potential drop
occurs at the metal-molecule interfaces. The details of the
theoretical methods used here have been reported previously.6,7
Briefly, the EHT/GF treatment allows the calculation of the
transmission function, which is then implemented within the
Landauer-Buttiker formalism to calculate the I-V characteristics.8
While we have previously shown that junctions with asymmetric
metal-molecule contacts can have unequal voltage drops at the
two metal-molecule interfaces, resulting in asymmetric I-V
characteristics, all of the junctions in this study have symmetric
metal-molecule attachments, and thus the voltage drop is expected
to be split equally at both interfaces.6
The calculated I-V characteristics are in excellent qualitative
agreement with the experimental measurements (Figure 2B). Both
experiment and theory show the same trend in molecular conduc-
tance, C12 < OPE < OPV (Table 1).9 Now we turn our attention
to the differences in charge transport for OPE and OPV. It has
previously been argued that the higher coplanarity and thus better
To better understand the physical basis for the measurements,
we calculated the I-V characteristics for the junctions studied using
extended Hu¨ckel theory (EHT) and Green’s functions (GF)
* To whom correspondence should be addressed. E-mail: J.G.K., kushmerick@
nrl.navy.mil; R.S., rshashidhar@cbmse.nrl.navy.mil.
† Naval Research Laboratory.
‡ Northwestern University.
§ Also at Geo-Centers Inc., Fort Washington, MD 20749.
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J. AM. CHEM. SOC. 2002, 124, 10654-10655
10.1021/ja027090n CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
differences in charge transport across π-conjugated molecular wires.
We are currently adding low-temperature capabilities to our crossed-
wire molecular electronics test-bed to further unravel the relative
importance of internal rotations and bond-length alternation in
molecular wires.
Acknowledgment. Financial support from the Defense Ad-
vanced Research Project Agency (DARPA) is gratefully acknowl-
edged. J.C.Y. is thankful to the National Research Council for a
fellowship.
Supporting Information Available: Synthesis of compounds, SAM
preparation, and bond lengths for OPE and OPV (PDF). This material
is available free of charge via the Internet at http://pubs.acs.org.
Figure 2. Experimental (A) and theoretical (B) plots of current (logarithmic
scale) as a function of the applied bias voltage for junctions formed from
the three compounds studied. The theoretical traces are given in units of
the quantum of current (2e2/h V).
References
(1) (a) Chidsey, C. E. D. Science 1991, 251, 919-922. (b) Sachs, S. B.; Dudek,
S. P.; Hsung, R. P.; Sita, L. R.; Smalley, J. F.; Newton, M. D.; Feldberg,
S. W.; Chidsey, C. E. D. J. Am. Chem. Soc. 1997, 119, 10563-10564.
(c) Weber, K.; Hockett, L.; Creager, S. J. Phys. Chem. B 1997, 101, 8286-
8291. (d) Creager, S.; Yu, C. J.; Bamdad, C.; O’Sonnor, S.; MacLean,
T.; Lam, E.; Chong, Y.; Olsen, G. T.; Luo, J.; Gozin, M.; Kayyem, J. F.
J. Am. Chem. Soc. 1999, 121, 1059-1064. (e) Sikes, H. D.; Smalley, J.
F.; Dudek, S. P.; Cook, A. R.; Newton, M. D.; Chidsey, C. E. D.; Feldberg,
S. W. Science 2001, 291, 1519-1523.
(2) (a) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T.
P.; Jones, L.; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 271,
1705-1707. (b) Cygan, M. T.; Dunbar, T. D.; Arnold, J. J.; Bumm, L.
A.; Shedlock, N. F.; Burgin, T. P.; Jones, L.; Allara, D. L.; Tour, J. M.;
Weiss, P. S. J. Am. Chem. Soc. 1998, 120, 2721-2732. (c) Wold, D. J.;
Frisbie, C. D. J. Am. Chem. Soc. 2001, 123, 5549-5556. (d) Wold, D. J.;
Haag, R.; Rampi, M. A.; Frisbie, C. D. J. Phys. Chem. B 2002, 106, 2813-
2816.
(3) (a) Closs, G. L.; Miller, J. R. Science 1988, 240, 440-447. (b) Fox, L.
S.; Kozik, M.; Winkler, J. R.; Gray, H. B. Science 1990, 247, 1069-
1071. (c) Helms, A.; Heiler, D.; McLendon, G. J. Am. Chem. Soc. 1992,
114, 6227-6238. (d) Davis, W. B.; Svec, W. A.; Ratner, M. A.;
Wasielewski, M. R. Nature 1998, 396, 60-63.
(4) (a) Holmlin, R. E.; Haag, R.; Chabinyc, M. L.; Ismagilov, R. F.; Cohen,
A. E.; Terfort, A.; Rampi, M. A.; Whitesides, G. M. J. Am. Chem. Soc.
2001, 123, 5075-5085. (b) Holmlin, R. E.; Ismagilov, R. F.; Haag, R.;
Mujica, V.; Ratner, M. A.; Rampi, M. A.; Whitesides, G. M. Angew.
Chem., Int. Ed. 2001, 40, 2316-2320.
π-conjugation of OPV systems as compared to that of OPE (in
which the phenylene rings are freely rotating at room temperature)
explain the more facile charge transport in OPV.1b,d,e,10 Charge
transport calculations as a function of coplanarity demonstrate that
conductance is reduced when the rings of an OPE system are
perpendicular to each other.11 While we do not dispute that the
increased coplanarity of OPV should enhance its conductivity
relative to OPE (at least in isolated molecules), our calculations
indicate a second important contribution. Because the transport
calculations reported in Figure 2B were performed for individual
molecules rigidly fixed at their energy minimized structures
(approximately planar for both molecules), the differences in
conductance cannot be attributed to disruption of π-conjugation
from phenylene ring rotation. Instead, the higher conductivity is
due to the smaller bond-length alternation in OPV as compared to
that in OPE.
If we consider the molecules as one-dimensional crystals, we
see that the short (1.218 Å) ethynylene linkage in OPE disrupts
the periodicity of the π-conjugated molecular backbone (1.41 (
0.01 Å) more drastically than the vinylene linkage in OPV (1.352
Å, backbone ) 1.41 ( 0.03 Å) (see Supporting Information). We
also know, from studies of conducting polymers, the size of the
energy gap between the highest occupied and lowest unoccupied
molecular orbitals (HOMO-LUMO gap) is directly related to the
extent of bond-length alternation,12 and thus the greater bond-length
alternation in OPE causes a larger HOMO-LUMO gap relative to
the OPV system (Table 1). Because, at low applied bias, transport
is dominated by charge carrier tunneling inside the HOMO-LUMO
gap, the smaller gap of OPV leads to higher conductance. In the
language of solid-state physics, OPV can be thought of as having
a smaller Peierls distortion than OPE and thus acts more like a
one-dimensional metal.13
(5) (a) Samanta, M. P.; Tian, W.; Datta, S.; Henderson, J. I.; Kubiak, C. P.
Phys. ReV. B 1996, 53, R7626-R7629. (b) Magoga, M.; Joachim, C. Phys.
ReV. B 1997, 56, 4722-4729. (c) Joachim, C. Superlattices Microstruct.
2000, 28, 305-315.
(6) Kushmerick, J. G.; Holt, D. B.; Yang, J. C.; Naciri, J.; Moore, M. H.;
Shashidhar, R. Phys. ReV. Lett. 2002, 086802.
(7) (a) Mujica, V.; Kemp, M.; Ratner, M. A. J. Chem. Phys. 1994, 101, 6849-
6855. (b) Datta, S.; Tian, W.; Hong, S.; Reifenberger, R.; Henderson, J.;
Kubiak, C. P. Phys. ReV. Lett. 1997, 79, 2530-2533. (c) Emberly, E. G.;
Kirczenow, G. Phys. ReV. B 1998, 55, 10911-10920. (d) Hong, S.;
Reifenberger, R.; Tian, W.; Datta, S.; Henderson, J.; Kubiak, C. P.
Superlattices Microstruct. 2000, 28, 289-303.
(8) Datta, S. Electronic Transport in Mesoscopic Systems; Cambridge
University Press: Cambridge, 1995.
(9) It must be noted that the transport calculations reported here are performed
on a single molecule, while the experiments are on an ensemble of
molecules in a SAM. Taking this into account along with the likely
difference in packing densities of SAMs formed from OPE and OPV
would bring the measurement and calculation into even better relative
agreement (specifically increasing the measured conductance of OPV with
respect to OPE).
(10) Dudek, S. P.; Sikes, H. D.; Chidsey, C. E. D. J. Am. Chem. Soc. 2001,
123, 8033-8038.
(11) Seminario, J. M.; Derosa, P. A. J. Am. Chem. Soc. 2001, 123, 12418-
12419.
In summary, we have measured the I-V characteristics of three
classes of molecules with a crossed-wire tunnel junction. The
measured I-V characteristics are in good agreement with I-V
characteristics calculated from extended Hu¨ckel theory coupled with
a Green’s function approach and point out that the degree of bond-
length alternation needs to be considered to fully understand
(12) (a) Heeger, A. J. J. Phys. Chem. B 2001, 105, 8475-8491. (b) Farchioni,
R., Grosso, G., Eds. Organic Electronic Materials: Conjugated Polymers
and Low Molecular Weight Organic Solids; Springer: New York, 2001;
Vol. 41.
(13) Peierls, R. E. Quantum Theory of Solids; Oxford University Press: London,
1955.
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