Single-molecule charge-transport measurements that reveal technique-dependent perturbations

Article abstract of DOI:10.1021/ja062898j

We compare scanning tunneling microscopy (STM) imaging with single-molecule conductive atomic force microscopy (C-AFM) measurements by probing a series of structurally related thiol-terminated oligo(phenylenevinylene)s (OPVs) designed to have unique charge-transport signatures. When one or two methylene spacers are inserted between the thiol points of attachment and the OPV core, a systematic reduction in the imaged molecular transconductance and the current transmitted through a metal-molecule-metal junction containing the molecule is observed, indicating good agreement between STM and C-AFM measurements. However, a structure where the OPV backbone is interrupted by a [2.2]paracyclophane core has a low molecular transconductance, as determined from STM images, and a high measured single-molecule conductance. This apparent disconnect can be understood by comparing the calculated molecular orbital topology of the OPV with one thiol bound to a gold surface (the geometry in the STM experiment) with the topology of the molecule with both thiol termini bound to gold (relevant to C-AFM). In the former case, a single contact splits low-lying molecular orbitals into two discrete fragments, and in the latter case, molecular orbitals that span the entire molecule are observed. Although the difference in observed conductance between the two different measurements is resolved, the overall set of observations highlights the importance of using combined techniques to better characterize charge-transport properties relevant to molecular electronics.

Full text of DOI:10.1021/ja062898j

Published on Web 08/08/2006
Single-Molecule Charge-Transport Measurements that Reveal
Technique-Dependent Perturbations
Dwight S. Seferos,† Amy Szuchmacher Blum,‡ James G. Kushmerick,*,§ and
Guillermo C. Bazan*,†
Departments of Chemistry & Biochemistry and Materials, Institute for Polymers and Organic
Solids, UniVersity of California, Santa Barbara, California 93106, NaVal Research Laboratory,
Washington, D.C. 20375, and National Institute of Standards and Technology,
Gaithersburg, Maryland 20899
Received May 3, 2006; E-mail: james.kushmerick@nist.gov; bazan@chem.ucsb.edu
Abstract: We compare scanning tunneling microscopy (STM) imaging with single-molecule conductive
atomic force microscopy (C-AFM) measurements by probing a series of structurally related thiol-terminated
oligo(phenylenevinylene)s (OPVs) designed to have unique charge-transport signatures. When one or two
methylene spacers are inserted between the thiol points of attachment and the OPV core, a systematic
reduction in the imaged molecular transconductance and the current transmitted through a metal-molecule-
metal junction containing the molecule is observed, indicating good agreement between STM and C-AFM
measurements. However, a structure where the OPV backbone is interrupted by a [2.2]paracyclophane
core has a low molecular transconductance, as determined from STM images, and a high measured single-
molecule conductance. This apparent disconnect can be understood by comparing the calculated molecular
orbital topology of the OPV with one thiol bound to a gold surface (the geometry in the STM experiment)
with the topology of the molecule with both thiol termini bound to gold (relevant to C-AFM). In the former
case, a single contact splits low-lying molecular orbitals into two discrete fragments, and in the latter case,
molecular orbitals that span the entire molecule are observed. Although the difference in observed
conductance between the two different measurements is resolved, the overall set of observations highlights
the importance of using combined techniques to better characterize charge-transport properties relevant
to molecular electronics.
bead.8 In addition to these ensemble measurements, a number
of methods have been developed that contact and measure the
Introduction
The possibility of using individual organic molecules for
molecular-scale electronic applications has prompted the de-
velopment of numerous techniques to measure the charge-
transport properties of a few or single molecules.1 Transport
measurements on a small collection of molecules can be
accomplished by probing a self-assembled monolayer2 (SAM)
of the molecule of interest with a top contact such as a
conducting atomic force microscope tip,3 a mercury drop
electrode,4 or an evaporated metal.5 The charge-transport
properties of SAMs can also be measured in crossed-wire6 or
inline metallic nanowire7 geometries or by bridging two closely
spaced SAM-functionalized electrode surfaces with a metallic
charge transport of individual molecules. These techniques
include mechanical break junctions9 and scanning probe mea-
surements, such as scanning tunneling microscopy10 (STM) and
conductive atomic force microscopy11 (C-AFM).
The growing number of measurements, and differences
between them, may prompt some critics to question how much
information can be obtained by using a single method. Cross-
correlation of techniques is important, not only to provide
confidence in the measurements but also because it yields insight
(7) Cai, L. T.; Skulason, H.; Kushmerick, J. G.; Pollack, S. K.; Naciri, J.;
Shashidhar, R.; Allara, D. L.; Mallouk, T. E.; Mayer, T. S. J. Phys. Chem.
B 2004, 108, 2827-2832.
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2002, 80, 2761-2763. (b) Long, D. P.; Patterson, C. H.; Moore, M. H.;
Seferos, D. S.; Bazan, G. C.; Kushmerick, J. G. Appl. Phys. Lett. 2005,
86, 153105.
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P.; Jones, L., II; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 271,
1705-1707. (b) Blum, A. S.; Kushmerick, J. G.; Pollack, S. K.; Yang, J.
C.; Moore, M.; Naciri, J.; Shashidhar, R.; Ratna, B. R. J. Phys. Chem. B
2004, 108, 18124-18128.
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† University of California.
‡ Naval Research Laboratory.
§ National Institute of Standards and Technology.
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Chem. ReV. 2005, 105, 1103-1169.
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E.; Terfort, A.; Rampi, M. N.; Whitesides, G. M. J. Am. Chem. Soc. 2001,
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Single-Molecule Charge-Transport Measurements
A R T I C L E S
reduced conductivity in molecular junction measurements15
when compared to their analogous fully delocalized structures.
Conversely, results on a molecule that contains a [2.2]para-
cyclophane core demonstrate that this structural perturbation
does not significantly reduce transport through the molecule
when compared to its fully through-bond delocalized analogue.16
Examination of these molecular structures provides a basis for
cross-comparison of STM and C-AFM measurements and can
be used to evaluate whether such a combined technique approach
yields a better understanding of molecular charge-transport
properties.
Results and Discussion
Molecular Design and Synthesis. As shown in Figure 1,
the molecular fragments in this study are thioacetate-terminated
oligomers of the phenylenevinylene (OPV) structural motif. The
thioacetate group hydrolyzes during the assembly protocol to
provide thiol functionalities for chemical attachment onto the
gold surfaces. The first molecular structure is the fully π-con-
jugated OPV, 1,4-bis[4′-(acetylthio)styryl]benzene (1), which
has thioacetate groups positioned directly on the OPV π-system.
The second compound, 4,12-bis[4′-(acetylthio)styryl][2.2]-
paracyclophane (2), is similar to 1 but has the phenylenevinylene
structure interrupted by the [2.2]paracyclophane core. This
arrangement represents a well-defined transannular extension
of conjugation17 and has been demonstrated to have a charge-
transport efficiency similar to that for 1.16 The last two
structures, 1,4-bis[4′-(thioacetylmethyl)styryl]benzene (3) and
1,4-bis[4′-(thioacetylethyl)styryl]benzene (4) are a progression
of structure 1 through the systematic incorporation of one or
two methylene spacer units between the π-conjugated OPV core
and the thioacetate points of attachment.
Compounds 1 and 2 were synthesized by using our previously
reported thioanisol precursor route.18 The synthesis of these
structures, which have their thiol anchor groups attached directly
onto their aromatic core, involves masking the reactive arylthiol
as an arylthiomethyl precursor. These precursors are tolerant
of Wittig19 and Heck-type20 reactions for forming the C-C
double bonds contained within the structures and can be
converted to their thioacetate analogues by following a simple
and high-yielding dealkylation step.21 Additionally, this step
isomerizes the π-backbone to the all-E isomers, regardless of
the starting material stereochemistry.18
Figure 1. Structures of the molecules examined in this study and a
representation of their films on a gold surface. In charge-transport
measurements, the top of the film is contacted by a metallic probe (not
shown).
into which effects arise from differences in contacts and/or
geometries (dependent on the measurement test structure) and
from inherent molecular properties.12 Although it is conceptually
simple to compare properties such as resistances obtained from
current-voltage measurements by simply scaling for the number
of molecules contacted in the measurement, the uncertainty in
that number presents a problem. Additionally, comparing
techniques that indirectly measure conductance, such as STM
imaging, with direct molecular conductivity measurements is
even less straightforward. STM offers the advantages of
simultaneous imaging and measurement and also gives ad-
ditional information about the local density of states of adsorbed
molecules. STM measurements have been used to examine
molecular switching13 and to provide a measure of molecular
transconductance14 by giving an estimation of the tunneling
decay constant. However, these data are at best an indirect
measure of molecular transconductance and their correlation to
molecular conductivity has not been directly investigated.
In this contribution, we compare the tunneling decay constant,
determined from STM imaging, with metal-molecule-metal
conductivity measurements, obtained from C-AFM measure-
ments, for single molecules isolated in an alkane thiol matrix.
We provide for these studies a series of structurally related
distyrylbenzene derivatives that are expected to display unique
charge-transport properties (Figure 1). For example, recent
independent reports have shown that the incorporation of a
methylene spacer into a conjugated molecule leads to both a
reduced transconductance in STM measurements14b and a
The synthesis of 3, a distyrylbenzene with methylthioacetate
end groups, begins with the conversion of 4-bromomethyl-
benzonitrile to 4-bromomethylbenzaldehyde in 71% yield by
DIBAL reduction (Scheme 1).22 Next, the 2-fold Wittig coupling
of 4-bromomethylbenzaldehyde and 1,4-bis(methyltriphenyl-
(15) Xiaoyin, X.; Xu, B.; Tao, N. J. Nano Lett. 2004, 4, 267-271.
(16) Seferos, D. S.; Trammell, S. A.; Bazan, G. C.; Kushmerick, J. G. Proc.
Natl. Acad. Sci. U.S.A. 2005, 102, 8821-8825.
(12) (a) Selzer, Y.; Cai, L.; Cabassi, M. A.; Yao, Y.; Tour, J. M.; Mayer, T. S.;
Allara, D. L. Nano Lett. 2005, 5, 61-65. (b) Blum, A. S.; Kushmerick, J.
G.; Long, D. P.; Patterson, C. H.; Yang, J. C.; Henderson, J. C.; Yao, Y.;
Tour, J. M.; Shashidhar, R.; Ratna, B. R. Nat. Mater. 2005, 4, 167-172.
(13) (a) Donhauser, Z. J.; Mantooth, B. A.; Kelly, K. F.; Bumm, L. A.; Monnell,
J. D.; Stapleton, J. J.; Price, D. W., Jr.; Rawlett, A. M.; Allara, D. L.;
Tour, J. M.; Weiss, P. S. Science 2001, 292, 2303-2007. (b) Lewis, P. A.;
Inman, C. E.; Maya, F.; Tour, J. M.; Hutchison, J. E.; Weiss, P. S. J. Am.
Chem. Soc. 2005, 127, 17421-17426.
(14) (a) Bumm, L. A.; Arnold, J. J.; Dunbar, T. D.; Allara, D. L.; Weiss, P. S.
J. Phys. Chem. B 1999, 103, 8122. (b) Moth-Poulsen, K.; Patrone, L.; Stuhr-
Hansen, N.; Christensen, J. B.; Bourgoin, J. P.; Bjornholm, T. Nano Lett.
2005, 5, 783-785. (c) Blum, A. S.; Ren, T.; Parrish, D. A.; Trammell, S.
A.; Moore, M. H.; Kushmerick, J. G.; Xu, G.-L.; Deschamps, J. R.; Pollack,
S. K.; Shashidar, R. J. Am. Chem. Soc. 2005, 127, 10010-10011. (d)
Monnell, J. D.; Stapleton, J. J.; Dirk, S. M.; Reinerth, W. A.; Tour, J. M.;
Allara, D. L.; Weiss, P. S. J. Phys. Chem. B 2005, 109, 20343-20349.
(17) (a) Bazan, G. C.; Oldham, W. J.; Lachicotte, R. J.; Tretiak, S.; Chernyak,
V.; Mukamel, S. J. Am. Chem. Soc. 1998, 120, 9188-9204. (b) Zyss, J.;
Ledoux, I.; Volkov, S.; Chernyak, V.; Mukamel, S.; Bartholomew, G. P.;
Bazan, G. C. J. Am. Chem. Soc. 2000, 122, 11956-11962. (c) Wang, S.;
Bazan, G. C.; Tretiak, S.; Mukamel, S. J. Am. Chem. Soc. 2000, 122, 1289-
1297. (d) Bartholomew, G. P.; Bazan, G. C. Acc. Chem. Res. 2001, 34,
30-39.
(18) Seferos, D. S.; Banach, D. A.; Alcantar, N. A.; Israelachvili, J. N.; Bazan,
G. C. J. Org. Chem. 2004, 69, 1110-1119.
(19) Maercker, A. Org. React. 1965, 14, 270-490.
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478-480. (b) Testaferri, L.; Tiecco, M.; Tingoli, M.; Chianelli, D.;
Montanucci, M. Synthesis 1983, 751-755.
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9
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A R T I C L E S
Seferos et al.
Scheme 1. Synthetic Route to
1,4-Bis[4′-(thioacetylmethyl)styryl]benzene (3)
films were examined by cyclic voltammetry (C-V) measure-
ments in ferricyanide solution24 (see Figure S1 in the Supporting
Information). Comparison of measurements using electrodes
modified with 1, 2, and 3 with an unfunctionalized electrode
indicates that these compounds significantly slow the rate of
heterogeneous electron transfer by forming a blocking film.
However, the electrode that was functionalized with 4 showed
a response similar to that of the unfunctionalized electrode. The
added flexibility induced by the incorporation of two ethylene
units into the rigid π-backbone of 4 could allow the molecule
to adopt a geometry where both thiols are bound to the gold
surface with the molecule lying flat on the surface. Although
the exact orientations of the molecules cannot be determined
by electrochemical experiments, the poor blocking quality
suggests that vertical, closely packed monolayers are not
obtained with 4. We conclude that in the case of 4 a poor quality
film significantly limits the number of techniques that can be
used to measure the conductivity across this type of structure
because most measurements require a well-formed film.1
STM Apparent Height Measurements. To circumvent the
difficulties presented by the poorly defined monolayers obtained
with 4, two-component SAMs were formed by inserting OPVs
1-4 into an undecanethiol on a Au(111) matrix monolayer.
Under these conditions, the conjugated molecules are inserted
into the matrix film at step edges and domain boundaries where
it has been demonstrated that they are electronically isolated
and exist in an upright fashion due to the rigidity of the
supporting monolayer matrix.25 The surfaces were imaged by
STM measurements with the tip biased under constant current
feedback control. Inserted molecules appear as bright spots in
the image due to differences in both their physical height and
their electronic properties relative to the alkane monolayer
(Figure 2). Although the apparent height measured from these
STM images represents a convolution of both physical and
electronic properties, this parameter can be used to calculate
the tunneling decay constant relative to the known physical
height differences of the two components and the well-studied
electronic properties of the matrix film.
a Reagents and conditions: (i) i-Bu2AlH, dichloromethane; (ii) LiOEt,
EtOH; (iii) KSAc, DMF.
Scheme 2. Synthetic Route to
1,4-Bis[4′-ethyl(thioacetyl)styryl]benzene (4)
a Reagent and conditions: (i) Pd(PPh3)4, toluene; (ii) LiOEt, EtOH; (iii)
AcSH, AIBN, toluene.
phosphonium chloride)benzene is used to form 1,4-bis[4′-
(bromomethyl)styryl]benzene (3a) in 50% yield. Finally, nu-
cleophilic displacement of the benzylic bromides with potassium
thioacetate affords 3 in 61% yield. A key advantage of this route
is that the surface-active thioacetate moieties are installed intact
in the last step, which eliminates the need for a transprotection
step and shortens the overall synthetic pathway. In this case,
however, a thermal isomerization step was required to produce
the desired E,E isomer.
To enable quantitative analysis of the STM images, a model
is needed that relates the apparent height measured by STM to
the molecular conductance. For an STM tip probing a surface-
bound molecule, the tunneling electrons transit two distinct
regions: the gap between the tip and the molecule and the
molecular backbone. Following the formalism of Weiss and co-
workers,14a,d this process can be represented by a two-layer
tunnel junction model (Figure 3) where the transconductances
(G) across each layer are
Synthetic entry into 4 begins with the conversion of 4-iodo-
benzaldehyde to 4-vinylbenzaldehyde by treatment with vinyl-
tributylstannane under Stille-type conditions (Scheme 2).23 Next,
a 2-fold Wittig-type reaction of 4-vinylbenzaldehyde and 1,4-
bis(methyltriphenylphosphonium chloride)benzene is used to
provide 1,4-bis(4′-vinylstyryl)benzene (4a) in 63% yield. Fi-
nally, treatment of 4a with thioacetic acid in the presence of
the radical initiator AIBN affords the desired product 4 in 47%
yield. In this example, the thermal conditions of the final reaction
concurrently isomerize the product and only the desired E,E
isomer was recovered. Olefin stereochemistry was confirmed
Ggap ) A exp(-Rdgap
)
(1)
and
Gmol ) B exp(-âhmol
)
(2)
1
by H and 13C NMR spectroscopy.
Monolayer Characterization. Self-assembled monolayers of
1-4 were prepared by immersing cleaned gold electrodes into
0.3 mM solutions of the compounds in ethanol and dichloro-
methane (1:2) for 14 h. After thorough rinsing in dichloro-
methane, ethanol, and water, the blocking characteristics of the
where R and â are the tunneling decay constants, A and B are
the contact conductances, and dgap and hmol are the thicknesses
of the gap and the molecule, respectively. The total trans-
(24) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem.
Soc. 1987, 109, 3559-3568.
(25) Cygan, M. T.; Dunbar, T. D.; Arnold, J. J.; Bumm, L. A.; Shedlock, N. F.;
Burgin, T. P.; Jones, L., II; Allara, D. L.; Tour, J. M.; Weiss, P. S. J. Am.
Chem. Soc. 1998, 120, 2721-2732.
(23) McKean, D. R.; Parrinello, G.; Renaldo, A. F.; Stille, J. K. J. Org. Chem.
1987, 52, 422-424.
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Single-Molecule Charge-Transport Measurements
A R T I C L E S
Figure 3. Schematic illustration of the two-layer tunnel junction model
used to represent the two-component SAMs. Here, h and d are the layer
thicknesses, R and â are the tunneling decay constants, and Ggap and Gmol
are the conductances of the gap and molecule, respectively. ∆STM is the
measured apparent height difference, and ∆h is the geometric height
difference of the different components of the film.
Figure 2. Constant current STM images (I ) 2.5 pA, V ) 1 V) of
molecules 1-4 on Au(111) inserted into an undecanethiol SAM. Inserted
molecules 1 (A), 3 (B), 4 (C), and 2 (D) appear as bright spots in the images.
Table 1. STM Apparent Height and Conductivity Data
conductivity
G (nS)e
relative conductivity
C-AFMf x-wireg
length
compounda (Å)b
apparent
height (Å)c
1
d
â )
(Å-
conductance of either the inserted molecules or the matrix is
1
2
3
4
19.3 10.0 ( 1.0 0.40 ( 0.12 15.8 ( 6.9 29 ( 13 29 ( 11
19.9 3.9 ( 2.2 1.16 ( 0.25 13.6 ( 5.7 25 ( 10 26 ( 13
20.8 8.2 ( 1.2 0.73 ( 0.13 5.4 ( 2.5 10 ( 4
the product of the conductances of these two layers, Gtotal
)
GgapGmol. Images are acquired in constant current mode;
24.0 5.1 ( 1.5 1.24 ( 0.14 0.55 ( 0.40 1 ( 0.7
therefore, Gtotal is constant over the entire image regardless of
which feature is probed so that
a Structures of the compounds are shown in Figure 1. b The sulfur-sulfur
distance of the compounds. c Measured above the height of the undecane
thiol film. d Calculated using eq 4. e Determined from the slope of the linear
low-bias region of the I-V characteristics measured by C-AFM. f Conduc-
tivity values normalized to molecule 4. g Determined from cross-wire
measurements and packing density calculations (ref 16), normalized to the
relative conductance of 1 as measured by C-AFM.
Ggap1Gmol1 ) Ggap2Gmol2
(3)
for a given image. By making the simplifying assumptions that
the tunneling properties of the gap are independent of the
molecular fragment (R1 ≈ R2, A1 ≈ A2) and that the contact
conductance of the Au-S attachment chemistry is the same for
all molecules (B1 ≈ B2), eq 3 can be simplified and solved for
the tunneling decay constant of the inserted molecule:
occurs as aliphatic spacer units are inserted between the metal
surface and the conjugated molecular fragment, in good agree-
ment with a recent report.14b It is significant that the â values
calculated for the OPV molecules in this study generally gave
much better electronic transparency than those previously
observed for other conjugated structures based on the oligo-
(phenyleneethynylene) structure, which (measured in the same
manner) provided a â value of approximately 1 Å-1 14b,c and
highlights the efficient charge transport that characterizes OPV-
type structures.
A surprising consequence of the STM imaging is the
inconsistency between the calculated tunneling decay constant
for 1 and 2 and our previous molecular conductivity measure-
ments using the cross-wire technique that demonstrate nearly
identical conductivities for these two molecules.16 Although
compound 1 showed the highest electron transparency, the
inserted compound 2 gave an apparent height of only 3.9 (
2.2 Å, corresponding to â ) 1.16 ( 0.25 Å-1. On the basis of
our previous conductivity measurements for monolayer samples,
we anticipated that 2 would have a tunneling decay constant
similar to that of 1. Instead, compound 2 has a tunneling decay
constant that most closely resembles that of 4, a compound with
a total of four methylene spacer moieties.
âOPV ) [âC11hC11 - R(∆STM - ∆h)]/hOPV
(4)
where ∆STM is the measured apparent height difference, ∆h
is the calculated physical height difference, and OPV and C11
denote the inserted and matrix molecules, respectively.26 The
decay constant for alkane thiols on gold such as C11 has been
determined both electrochemically27 and by STM measurements14a
and serves as a reference for evaluating the conductivity of the
inserted OPV molecules.
The apparent height differences in the STM images therefore
provide a measure of â, the tunneling decay constant of the
inserted molecules (eq 4). The apparent heights measured and
the standard deviation from 100 insertion events for each
molecule are 10.0 ( 1.0, 8.2 ( 1.2, and 5.1 ( 1.5 Å,
corresponding to â values of 0.40 ( 0.12, 0.73 ( 0.13, and
1.24 ( 0.14 Å-1, for 1, 3, and 4, respectively (Table 1). These
data indicate the expected trend for molecular conductivity, 1
> 3 > 4, and that a significant drop in the tunneling efficiency
(26) Literature values of R ) 2.3 and âC11 ) 1.2 Å-1 were used for the
calculation (ref 14). The thickness of the C11 monolayer is assumed to be
12.5 Å, corresponding to a tilt angle of 30°.
Molecular Conductivity Measurements. Although several
techniques are available to measure the charge-transport proper-
ties of molecular junctions, most require the molecules to be
(27) Weber, K.; Hockett, L.; Creager, S. J. Phys. Chem. B 1997, 101, 8286-
8291.
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A R T I C L E S
Seferos et al.
Figure 4. AFM images (contact mode) of inserted molecules 2 (A) and 4
(B) with attached gold colloids. Inset (C) shows a cartoon of the proposed
structure. Topographic images of inserted molecules 1 and 3 with attached
colloids were similar.
structurally robust and in an upright orientation relative to the
surface.1 Because of the uncertainties in monolayers prepared
from 4, a technique was chosen that directly measures the
conductivity of inserted molecules in a matrix film through the
attachment of gold colloids that are subsequently contacted and
measured with a C-AFM tip. There are two primary advantages
to this method. First, the molecules are all forced into a similar
upright orientation due to the nature of the insertion process.
Second, an analogous molecular geometry to the STM imaging
experiments is achieved, which should allow for the best
comparison of the two techniques. Lindsay and co-workers have
previously demonstrated that this technique provides a repro-
ducible measure of single-molecule conductivity for saturated
alkanedithiols28 and for similar conjugated oligo(phenylene-
ethynylene) dithiols.29
Inserted molecules 1-4 were functionalized with gold
colloids through attachment to their unbound protruding thiol
functionality by soaking the two-component SAMs in a solution
of 5 nm diameter colloids for 1 h, followed by rinsing with
water. After the attachment step, the inserted molecules were
visualized from AFM imaging by the appearance of their
attached colloids, as shown in Figure 4. Images could be
obtained in either noncontact (tapping mode) or contact mode
with a metal-coated tip. The colloids were stable toward multiple
scanning passes, indicating that they are mechanically robust
and that the scanning probe tip does not significantly perturb
the overall structure.
A closed-loop x-y piezo scanner was used to position the
conductive tip directly over the gold colloids, and the current-
voltage (I-V) characteristics were directly measured. I-V traces
recorded on 10-20 individually addressed colloids per sample
generally showed a symmetric I-V behavior that was stable to
multiple scans in the (1 V potential range (Figure 5). These
I-V measurements demonstrate the same trend in conductivity
that was observed in the STM images, 1 > 3 > 4. The
conductivities of the molecules, calculated from the slope of
the linear low-bias regime, are 15.8 ( 6.9, 13.6 ( 5.7, 5.4 (
2.5, and 0.55 ( 0.40 nS for 1, 2, 3, and 4, respectively (Table
1). The consequence of including a methylene spacer at each
end of the OPV structure results in a 3-fold decrease in
conductivity. Good agreement is found with a previous report
that compares the conductivity of 1,4-benzenedithiol with 1,4-
Figure 5. Single-molecule conductivity (current vs voltage) measured on
Au-molecule-Au colloid junctions formed from inserted molecules 1 (red),
2 (blue), 3 (green), and 4 (orange), averaged from 10 to 20 individually
contacted colloids per sample. Upper inset shows the same data plotted on
a semilog scale, and the lower inset shows a cartoon illustration of the
measurement.
benzenedimethanethiol and reveals a similar reduction in
conductivity that is imposed by the methylene spacer units.15
Additionally, when extending the study to 4, which has two
methylene units per thiol attachment, a nearly 30-fold drop in
conductivity relative to the parent compound 1 is observed
(Table 1). This last result highlights how further spacing of the
thiol functionality from the OPV core disrupts efficient charge
transport.
A notable observation is the high conductivity demonstrated
in the I-V characteristics of 2. Although STM imaging
suggested that this structure would have charge transport
properties similar to 4, molecular conductivity measurements
by using C-AFM demonstrate that this molecular fragment is
more similar to 1, which is in good agreement with our previous
report that demonstrates that these two structures have similar
charge-transport characteristics (Table 1). One possible explana-
tion for the discrepancy could be related to the difference in
the contacts that the molecules experience in the two measure-
ments. Specifically, in the AFM experiment, both thiol func-
tionalities are chemically bound to a gold surface, whereas in
the STM measurements, only one of the thiol end groups is
attached.
Electronic Structure Calculations. To gain insight into how
the different measurement techniques might influence charge-
transport properties, electronic structure calculations were
performed on 1 and 2 bound to one and two Au contacts within
the density functional theory approximation.30 In these calcula-
tions, one Au contact is representative of the electronic structure
probed in the STM measurement, and two Au contacts are used
to simulate the electronic structure probed in the C-AFM
measurements. The density functional theory calculations were
performed using the B3LYP functional31 coupled with the
(28) Cui, X. D.; Primak, A.; Zarate, X.; Tomfohr, J.; Sankey, O. F.; Moore, A.
L.; Moore, T. A.; Gust, D.; Nagahara, L. A.; Lindsay, S. M. J. Phys. Chem.
B 2002, 106, 8609-8614.
(29) Rawlett, A. M.; Hobson, T. J.; Nagahara, L. A.; Tsui, R. K.; Ramachandran,
G. K.; Lindsay, S. M. Appl. Phys. Lett. 2002, 81, 3043-3045.
(30) Frisch, M. J. et al. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford,
CT, 2004.
(31) Becke, A. D. J. Chem. Phys. 1998, 98, 5648-5652.
9
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Single-Molecule Charge-Transport Measurements
A R T I C L E S
Figure 6. Charge density plots of the highest-occupied molecular orbitals for 1 and 2 attached to one and two gold atoms (Au/1, Au/1/Au, Au/2, and
Au/2/Au, respectively).
LANL2DZ basis set32 for all atoms. Single Au atoms attached
to the thiol groups were used to represent the molecule electrode
interaction. Although single Au atoms are clearly not equivalent
to an extended gold surface, previous studies have demonstrated
the utility of such calculations to predict likely conductance
channels.33 Specifically, molecular orbitals that span the entire
length of the molecule and are energetically similar to the
electrode Fermi level are considered good conductance channels.
On the basis of these criteria, we identify the highest occupied
molecular orbital (HOMO) of 1 and the HOMO and HOMO-1
orbitals of 2 to be the most likely conduction channels for the
two systems, respectively (Figure 6).16
charge carrier has a farther distance to travel (experiences a
wider tunnel barrier) before reaching the delocalized molecular
orbital.
Summary Discussion and Conclusion
We have provided a comparison of two commonly employed
molecular conductance measurement techniques. Both agree-
ment and disconnect were found between the tunneling decay
constants obtained by barrier height calculations using STM
imaging and molecular conductivity measured by C-AFM on
inserted molecules (Table 1). From the molecular perspective,
the two techniques differ by the number of chemical (gold)
contacts that are made to molecules under investigation. A single
gold surface contacts the inserted molecules in STM measure-
ments, and in the case of C-AFM, a top gold colloid is
chemically attached to create a more symmetric metal-
molecule-colloid junction. Whether the different types of
contacts inherent to the measurements result in different
measured charge-transport characteristics, and thereby correlate
with each other, thus depends on the electronic structure of the
bridging molecule.
In the case of 1, calculations reveal similar topologies for
the HOMO, whether the molecule is bound to one (Au/1) or
two (Au/1/Au) gold atoms. The HOMO is delocalized across
the entire molecular framework for both Au/1 and Au/1/Au,
and the orbitals lie close in energy to the Au Fermi level (Ef )
-5.3 eV)34 (Figure 6). Close inspection reveals that a minor
asymmetry is apparent for Au/1. Different results are observed
when the analogous calculation was performed on 2. Although
the HOMO and HOMO-1 structure with two Au atoms (Au/2/
Au, Figure 6) has a fully delocalized electronic structure that
extends across the length of the molecule and demonstrates the
through-space mixing of states across the central [2.2]para-
cyclophane core, the HOMO and HOMO-1 of the singly bound
Au/2 molecule lacks these features. Specifically, the spatial
degeneracy of the HOMO and HOMO-1 orbitals is lifted by
the single Au contact and the charge density is localized on the
upper and lower π-conjugated fragment, respectively. Such
localization of the formerly continuous conduction channel
highlights why the singly bound molecule has such drastically
different charge-transport characteristics when compared to the
doubly bound species. In the STM measurements, the tunneling
In the series of related OPV molecules that incorporates
increasing methylene functionalities (1, 3, and 4), STM imaging
and conductivity measurements are well matched (Table 1). For
these compounds, the HOMO distributions are nearly identical
when one or two of the terminal thiols are bound to gold, as
determined by DFT modeling. However, the more topologically
complex structure 2 is more greatly perturbed under the two
different contact geometries. As demonstrated by DFT calcula-
tions, the Au/2 geometry results in localization of HOMO and
HOMO-1, when compared to the symmetric Au/2/Au situation.
In this case, the general electronic distribution of the molecular
bridge is considerably different for the two measurement
techniques. With this consideration, the apparent disconnect
between the measured barrier height and the measured conduc-
tance is understood.
(32) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299-310.
(33) (a) Derosa, P. A.; Seminario, J. M. J. Phys. Chem. B 2001, 105, 471-481.
(b) Heurich, J.; Cuevas, J. C.; Wenzel, W.; Schon, G. Phys. ReV. Lett.
2002, 88, 256803.
(34) Lide, D. R. CRC Handbook of Chemistry and Physics; CRC: Boca Raton,
FL, 1998.
The contact geometry in C-AFM measurements is similar to
the geometry contacted in previous cross-wire junction measure-
9
J. AM. CHEM. SOC. VOL. 128, NO. 34, 2006 11265

A R T I C L E S
Seferos et al.
of the title compound,36 which were used in the next step, without
isomerization. HRMS-EI: 465.9922 ∆ ) 2.0 ppm.
ments. This point is highlighted by the excellent experimental
agreement that we have observed between the relative conduc-
tance of 1 and 2 in the C-AFM-based measurements presented
here and previous cross-wire conductivity measurements (Table
1). The results obtained in these independent examinations
demonstrate that conductivity of 1 and 2 are the same, within
experimental error, when contacted via a symmetric metal-
molecule-metal junction geometry. This similar charge-
transport is a consequence of the strong through-space π-
coupling that is experienced when π-conjugated molecular
structures are brought in contact via the [2.2]paracyclophane
arrangement.17
As a supplement to these metal-molecule-metal junction
measurements, probing molecule 2 in the STM geometry yields
further information about the electronic properties of this
molecule. We speculate that the decreased conductance of 2
that results from a single contact is related to another feature
of the unique through-space delocalization across the central
[2.2]paracyclophane core. Namely, molecules closely related
to 2 have been shown to be more polarizable when compared
to structures similar to 1 which contain only through-bond
delocalization.35
The observed disconnect between the STM and C-AFM
measurements on 2 provides some evidence that STM imaging,
taken alone, may not always accurately predict the conductivity
of molecules assembled in the metal-molecule-metal junction
arrangement. Although this clearly is not true in all cases, it
represents one cautionary tale where a difference in the measured
molecular charge transport arises solely as a consequence of
the test structure. Employing multiple measurement techniques
not only serves to test for these types of phenomena but also,
as demonstrated in the case of 2, relates additional information
about molecular sensitivity to the local environment. We believe
that these observations should provide motivation for using a
combined technique approach to evaluate molecular charge
transport, especially in molecules that contain complex electronic
structures.
1,4-Bis[4′-methyl(thioacetyl)styryl]benzene (3). In a flame-dried,
three-neck flask fitted with a Teflon-coated magnetic stir bar and needle
valve, 3a (100 mg, 0.21 mmol) was partially dissolved in anhydrous
DMF (10 mL) under an argon atmosphere. Potassium thioacetate (57
mg, 0.50 mmol) was added at once, and the reaction was stirred at
room temperature for 75 min and was then heated to 80 °C for 90
min. Upon cooling to room temperature, the solution was diluted with
50 mL of dichloromethane (DCM), washed with water, dried, and
concentrated. Purification by chromatography using a 10 cm silica gel
column eluting with DCM afforded a mixture of three isomers of the
desired product. Thermal isomerization by refluxing in anhydrous
toluene with a few crystals of iodine, followed by chromatography,
1
yielded 59 mg (61%) of the all-E isomer of the title compound. H
NMR (CDCl3): 7.51 (s, 4H), 7.46 (d, 3J ) 8.37 Hz, 4H), 7.29 (d, 4H),
7.09 (s, 4H), 4.14 (s, 4H), 2.37 (s, 6H). 13C NMR (CDCl3): 195.4,
137.2, 136.9, 136.6, 129.4, 128.5, 128.2, 127.1, 126.9, 33.5, 30.6.
HRMS-EI: 458.1391 ∆ ) 3.7 ppm.
1,4-Bis[4′-vinylstyryl]benzene (4a). 4-Vinylbenzaldehyde (170 mg,
1.29 mmol) and 1,4-bis[(methyl)triphenylphosphine chloride]benzene
(401 mg, 0.58 mmol) were dissolved in anhydrous ethanol, treated with
1.0 M lithium ethoxide in ethanol (1.5 mL), and worked up in the same
manner as that described for 3a to yield 123 mg (63%) of a yellow
solid. The NMR spectra were consistent with the three isomers of the
title compound,36 which were used in the next step, without isomer-
ization. HRMS-EI: 334.1716 ∆ ) 1.8 ppm.
1,4-Bis[4′-ethyl(thioacetyl)styryl]benzene (4). In a flame-dried,
three-neck flask fitted with a Teflon-coated magnetic stir bar and a
needle valve, 4a (50 mg, 0.15 mmol), thioacetic acid (32 µL, 0.45
mmol), and AIBN (10 mg) were dissolved in anhydrous toluene. The
solution was degassed by three freeze-pump-thaw cycles and was
then heated to 90 °C for 24 h. Subsequently, additional AIBN (5 mg)
was added and the reaction was continued for an additional 24 h. Upon
cooling, the mixture was partitioned between DCM (50 mL) and
saturated NaHCO3 (50 mL), extracted with DCM, dried, and concen-
trated. Purification by chromatography using a 20 cm silica gel column
eluting with DCM afforded 34 mg (47%) of the desired product; only
the all-E isomer was present. 1H NMR (CDCl3): 7.51 (s, 4H), 7.47 (d,
3
3J ) 8.37 Hz, 4H), 7.23 (d, 4H), 7.10 (s, 4H), 3.14 (t, J ) 8.10 Hz,
4H), 2.89 (t, 4H), 2.36 (s, 6H). 13C NMR (CDCl3): 196.0, 139.7, 136.9,
135.9, 129.2, 128.9, 128.4, 128.1, 127.0, 126.9, 35.8, 31.0, 30.6. HRMS-
EI: 486.1693 ∆ ) 1.1 ppm.
Experimental Details
General Considerations. The preparation of 4-vinylbenzaldehyde23
and 4-(bromomethyl)benzaldehyde22 has been described in the literature,
and structures were verified by comparison to those references. Reagents
were purchased from Aldrich and used as received. Solvents were the
highest grade available, purified by a solvent purification system.
Synthetic manipulations were performed on a vacuum/argon manifold
under an inert atmosphere using Schlenk techniques. Gold-coated mica
substrates (150 nm Au, flame annealed) and 5 nm gold colloids (optical
density ) 1.1 ( 0.1 at 520 nm, approximately 0.1 g/L) were purchased
from SPI Supplies.
1,4-Bis[4′-(bromomethyl)styryl]benzene (3a). In a flame-dried,
three-neck flask fitted with a Teflon-coated magnetic stir bar and a
needle valve, a rapidly stirring suspension of 1,4-bis[(methyl)triphenyl-
phosphine chloride]benzene (698 mg, 1.00 mmol) and 4-(bromomethyl)-
benzaldehyde (408 mg, 2.05 mmol) in anhydrous ethanol (10 mL) was
treated dropwise with a 1.0 M solution of lithium ethoxide in ethanol
(2.5 mL) at 0 °C. The mixture was allowed to warm to room
temperature and was stirred overnight (∼14 h). Dilute hydrochloric
acid (5 mL) was added, and the precipitates were collected on a glass
frit, washed with hexanes, and dried in vacuo to yield 236 mg (50%)
of yellow solid. The NMR spectra were consistent with the three isomers
C11 Films. A fresh Au/mica Au(111) substrate was cleaned by 10
min exposure to UV-ozone, followed by rinsing with deionized water
and ethanol.37 The cleaned substrate was immersed into 10 mL of an
ethanolic solution containing 50 µL of undecanthiol (Aldrich) for 40
min, rinsed with EtOH, and dried with a stream of CO2.
OPV Insertion. Working in an inert atmosphere glovebox, 1 mg of
OPV compound was dissolved in 4 mL of anhydrous THF. Once
dissolved, 10 µL of NH4OH was added, and the solution was agitated
for about a minute and passed through a 0.1 µm membrane into a vial
containing the C11/Au/mica substrate. After 45 min, the vial was
removed, and the substrate was rinsed with THF and EtOH and dried
with a stream of CO2.
Nanoparticle Attachment. The substrate described above was
treated with a commercially available solution of 5 nm diameter Au
colloids without dilution for 1 h, rinsed with Milli-Q water, and dried
with a stream of CO2.
STM Measurements. Images were obtained in constant current
mode (I ) 2.5 pA, V ) 1.0 V) using a Digital Instruments Multimode
scanning probe microscope equipped with a low-current amplifier
attachment operated by a Nanoscope IIIa controller. A Pt/Ir tip was
used.
(35) Hong, J. W.; Woo, H. Y.; Liu, B.; Bazan, G. C. J. Am. Chem. Soc. 2005,
127, 7435-7443.
(36) Drefahl, G.; Plo¨tner, G.; Winnefeld, K. Chem. Ber. 1961, 94, 2002-2010.
(37) Ron, H.; Matlis, S.; Rubinstein, I. Langmuir 1998, 14, 1116-1121.
9
11266 J. AM. CHEM. SOC. VOL. 128, NO. 34, 2006

Single-Molecule Charge-Transport Measurements
A R T I C L E S
AFM Measurements. Images and spectroscopy data were obtained
using a PSIA XE-100 scanning probe microscope equipped with an
external preamplifier. Contact mode images and current voltage
measurements were obtained using a Ti-Pt-coated tip operating at a 2
nN setpoint.
Electronic Structure Calculations. The electronic structure of
molecules 1 and 2 coupled to either one or two Au atoms at their thiol
end groups was calculated within the density functional theory
approximation.30 The density functional theory calculations were
performed by using the B3LYP functional31 coupled with the LANL2DZ
basis set32 for all atoms. The entire “extended molecule” (molecule
plus Au atom(s)) was relaxed to find the final optimized structure before
performing the electronic structure calculation.
Acknowledgment. Financial support from the Institute for
Collaborative Biotechnology (D.S.S. and G.C.B) and DARPA
(J.G.K.) is gratefully acknowledged.
Supporting Information Available: Complete ref 29, cyclic
voltammetry data for electrodes modified with molecules 1-4,
Cartesian coordinates, and energy of optimized structures. This
material is available free of charge via the Internet at
http://pubs.acs.org.
JA062898J
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J. AM. CHEM. SOC. VOL. 128, NO. 34, 2006 11267