Nature of electron transport by pyridine-based tripodal anchors: Potential for robust and conductive single-molecule junctions with gold electrodes

Article abstract of DOI:10.1021/ja109577f

We have designed and synthesized a pyridine-based tripodal anchor unit to construct a single-molecule junction with a gold electrode. The advantage of tripodal anchoring to a gold surface was unambiguously demonstrated by cyclic voltammetry measurements. X-ray photoelectron spectroscopy measurements indicated that the π orbital of pyridine contributes to the physical adsorption of the tripodal anchor unit to the gold surface. The conductance of a single-molecule junction that consists of the tripodal anchor and diphenyl acetylene was measured by modified scanning tunneling microscope techniques and successfully determined to be 5 ± 1 ×10^-4G₀. Finally, by analyzing the transport mechanism based on ab initio calculations, the participation of the π orbital of the anchor moieties was predicted. The tripodal structure is expected to form a robust junction, and pyridine is predicted to achieve π-channel electric transport.

Full text of DOI:10.1021/ja109577f

ARTICLE
pubs.acs.org/JACS
Nature of Electron Transport by Pyridine-Based Tripodal Anchors:
Potential for Robust and Conductive Single-Molecule Junctions with
Gold Electrodes
Yutaka Ie,†,‡ Tomoya Hirose,† Hisao Nakamura,*,# Manabu Kiguchi,*,§ Noriaki Takagi,|| Maki Kawai,||,^ and
Yoshio Aso*,†
†The Institute of Scientific and Industrial Research (ISIR), Osaka University, 8-1, Mihogaoka, Ibaraki, Osaka 567-0047, Japan
‡PRESTO-JST, 4-1-8, Honcho, Kawaguchi, Saitama 333-0012, Japan
#Nanosystem Research Institute (NRI) “RICS”, National Institute of Advanced Industrial Science and Technology (AIST), Central 2,
Umezono 1-1-1, Tsukuba, Ibaraki 305-8568, Japan
§Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 W4-10, Ookayama,
Meguro-ku, Tokyo 152-8551, Japan
Department of Advanced Materials Science, The University of Tokyo, Kashiwa, Chiba 277-8561, Japan
^Surface Chemistry Laboratory, RIKEN, Wako, Saitama 351-0198, Japan
S Supporting Information
b
ABSTRACT: We have designed and synthesized a pyridine-based
tripodal anchor unit to construct a single-molecule junction with a
gold electrode. The advantage of tripodal anchoring to a gold surface
was unambiguously demonstrated by cyclic voltammetry measure-
ments. X-ray photoelectron spectroscopy measurements indicated
that the π orbital of pyridine contributes to the physical adsorption
of the tripodal anchor unit to the gold surface. The conductance of a
single-molecule junction that consists of the tripodal anchor and
diphenyl acetylene was measured by modified scanning tunneling
microscope techniques and successfully determined to be 5 ( 1 ꢀ 10-4 G0. Finally, by analyzing the transport mechanism based on
ab initio calculations, the participation of the π orbital of the anchor moieties was predicted. The tripodal structure is expected to
form a robust junction, and pyridine is predicted to achieve π-channel electric transport.
’ INTRODUCTION
issues for developing molecular electronics as well as fabricating
practical molecular devices.
Developing functional organic electronic devices is one of the
most active research fields in nanoscience.1 For instance, there
are several candidate devices in nanoelectronics, nanofabrication
(dense nanowire),2 and nanomaterial electronics (carbon nano-
tube, graphene).3 Single-molecule electronics are also one of the
promising candidates in terms of ultimate miniaturization. They
have a number of potential advantages for constructing func-
tional devices, primarily because a great variety of molecules and
their flexible conformations enable the design of various
functionalities.4 For instance, the first proposal of a molecular
device was introduced by Aviram and Ratner, who focused on the
function of molecular rectification by designing donor-acceptor
linked molecules.5 Given that the advantage of molecular elec-
tronics relies on varieties of molecules and their fine-tuned
properties, chemistry should play a key role in designing and
synthesizing functional electric materials using the encyclopedic
methodological knowledge base of organic synthetic chemistry.
In this sense, clarifying the relationships between “molecular
properties” and their “device characters” is one of the central
However, realization of a practical single-molecule device
requires reliable contact between the bridging molecule and
the electrodes, where sufficiently strong binding between two
terminal anchoring groups of the bridging molecule and the
metal electrodes is achieved.6 In this context, scanning tunneling
microscopy (STM), conducting atomic force microscopy
(AFM), and mechanically controllable break junctions (MCBJ)
are now established as standard techniques to form the contact;
they have been extensively investigated to measure the conduc-
tance of many metal-molecule-metal junctions.7 Hence, selec-
tion of the proper anchoring group is quite important in creating
the junctions. An anchoring group should satisfy the following
conditions. It should (i) form sufficiently strong connections
between a molecule and metal surfaces, and (ii) maintain
sufficient electron density of states (DOS) close to the Fermi
Received: October 25, 2010
Published: February 10, 2011
r
2011 American Chemical Society
3014
dx.doi.org/10.1021/ja109577f J. Am. Chem. Soc. 2011, 133, 3014–3022
|

Journal of the American Chemical Society
ARTICLE
level to pass an electron or hole through the molecule. Further-
more, in order to design tailor-made functional devices, it must
be able to (iii) control well-ordered molecular orientations.
As a simple strategy, a single point (region) can be designated
on each electrode as the “anchoring point,” where the connection
is formed by σ-bond formation (e.g., thiol (SH),8 selenol
(SeH)9,10), or physical adsorption (e.g., amine (NH2),8,11 pyri-
dine7,12-15) between functional groups and noble metal electro-
des. Recently, a different anchoring strategy utilizing direct
hybridization of the π orbital of a conjugated molecule (e.g.,
benzene,16 fullerene,17 etc.) and the metal has been examined,
with high conductance values of single-molecule junctions
reported by several groups. This hybridization behavior is quite
similar to the case of the interface between an organic semi-
conductor film such as 3,4,9,10-perylenetetracarboxylic dianhy-
dride (PTCDA) film and a metal surface such as silver.18
However, there are still limits to extending this strategy to design
new anchoring units, especially for application to single-molecule
junctions, that satisfy all three requirements above.
One reason for the difficulty of anchoring the molecule while
maintaining an ordered molecular orientation is that the anchor-
ing point is spatially limited and/or a single molecule can slide in
parallel to the electrode (tip) surface. Hence, the use of a tripodal
structure has been proposed by several groups.10,19-22 We have
demonstrated that the tripodal structure contributes not only to
maintaining the robustness of surface attachment but also to
controlling the molecular orientation.10,21 However, to the best
of our knowledge, there are no measurements of electrical con-
ductance for a single molecule bearing tripodal anchoring groups.
In the present study, we have examined the possibility of a
“conductive” molecular junction connected by tripodal anchors,
which can be expected to satisfy the conditions of robustness of
adsorption and well-ordered orientation. Recently, conductance
measurements of simple bipyridine junctions were reported,7,12a,12e
and a transport mechanism based on π-metal interactions
was proposed.13 Hence, we adopt pyridine as a candidate
anchoring group in the tripodal structure, as shown in Figure 1.
Here, the notations Py and PE represent the pyridine and
diphenylene-ethynylene groups, respectively. We labeled the
tripodal structure of the pyridine as 3Py. We report a compre-
hensive study toward a single-molecule device. Specifically, we
report the synthesis of 3Py-TIPS, 3Py-Ph4T, and 3Py-PE-3Py,
the formation of 3Py-TIPS and 3Py-Ph4T monolayers and their
evaluations by electrochemical measurements and X-ray photo-
electron spectroscopy (XPS), and subsequently the measure-
ments of single-molecule conductance for 3Py-PE-3Py. Further-
more, we analyze the potential of our newly developed pyridine
tripodal structure as an anchor for molecular devices using ab
initio transport calculations.
Figure 1. Chemical structures.
and elemental analysis. The synthetic details and compound-
identification data are summarized in the Experimental Section.
Evaluation of Monolayers. Monolayers of 3Py-Ph4T and
1Py-Ph4T were prepared by immersing Au-on-mica substrates
in their dichloromethane solution (5.0 ꢀ 10-4 M) for 12 h. The
electrochemical properties of the monolayers were investigated
by CV measurements, which were carried out at a scan rate of
100 mV s-1 in dichloromethane containing 0.1 M tetrabutylam-
monium hexafluorophosphate (TBAPF6) as a supporting elec-
trolyte using the monolayer-modified gold as a working elect-
rode, platinum wire as a counter electrode, and Ag/AgNO3 as a
reference electrode. As shown in Figure 2a, CVs with the 3Py-
Ph4T-modified gold electrodes displayed a reversible one-elec-
tron redox wave that corresponds to the oxidation process of the
Ph4T moiety. The surface coverage of the adsorbed molecules
was estimated to be 7.1 ꢀ 10-11 mol cm-2 by integrating the
anodic peak area. On the other hand, 1Py-Ph4T-modified
electrodes did not show any corresponding redox wave under
the same measurement conditions, indicating that 1Py-Ph4T has
a weak tendency to be adsorbed on gold in comparison with 3Py-
Ph4T. This difference can be explained by the multiplier effect of
the pyridyl tripodal structure. It is important to note that the
electrochemical response (surface coverage) of the monolayer
3Py-Ph4T remains at 30% of the original value after 10 scans
within the range of 0-0.55 V (Figure 2b and Figure S1). This
indicates that robust junctions, which are indispensible for the
fabrication of molecular devices, were formed.
’ RESULTS AND DISCUSSION
To obtain insight into the 3Py-TIPS-adsorbed state, XPS
measurements were performed at room temperature in an
ultrahigh vacuum chamber. As shown in Figure 3, the XPS
spectrum exhibits a peak at 399.4 eV, corresponding to N 1s.23
It has been reported that the perpendicular geometry of the
pyridine ring on the gold surface causes interactions to occur
between the gold and nitrogen atoms, leading to the detection of
a positively charged nitrogen peak at around 402 eV in addition
to the characteristic N 1s peak.24 However, such a peak is not
observed in our measurements. This suggests that parallel
Synthesis. The target compounds 3Py-TIPS, 3Py-Ph4T,
3Py-PE-3Py, and the reference compound 1Py-Ph4T, were
synthesized using the Suzuki, Sonogashira, and Stille coupling
reactions (Scheme 1). Phenyl-capped oligothiophene (Ph4T)
was introduced as an active component for cyclic voltammetry
(CV) measurements. The target compounds, being soluble in
common organic solvents such as chloroform, were thus success-
fully purified by gel-permeation liquid chromatography. These
compounds were unambiguously characterized by nuclear mag-
netic resonance (NMR) spectroscopy, mass spectroscopy (MS),
3015
dx.doi.org/10.1021/ja109577f |J. Am. Chem. Soc. 2011, 133, 3014–3022

Journal of the American Chemical Society
ARTICLE
Scheme 1a
a Reagents and conditions: (a) Br2, CH2Cl2, r.t.; (b) PdCl2(PPh3)2, CuI, (triisopropylsilyl)acetylene, THF/NEt3, r.t.; (c) Pd(PPh3)4, K2CO3,
4-pyridineboronic acid, THF/H2O, 80 ꢀC; (d) n-Bu4NF, THF, r.t.; (e) Pd(PPh3)4, Ph4T-I, toluene, reflux; (f) Pd(PPh3)4, CuI, 2, THF/i-Pr2NEt, r.t.;
(g) PdCl2(PPh3)2, CuI, Ph4T-I, THF/NEt3, r.t.
Figure 3. XPS spectrum of 3Py-TIPS on gold.
Figure 4 shows the typical conductance traces and histograms
of the Au point contacts in solutions with and without 3Py-PE-
3Py. In the solution containing 3Py-PE-3Py, the conductance
decreased in a stepwise fashion with each step occurring pre-
ferentially at an integer multiple of 5 ꢀ 10-4 G0 (arrow in
Figure 4(a)). Here, the G0 is the unit of the conductance, 2e2/h,
where e and h are the elementary electric charge and Planck’s
Figure 2. (a) CVs of 3Py-Ph4T/Au (red) and 1Py-Ph4T/Au (black).
constant, respectively. The corresponding conductance histo-
(b) Surface coverage as a function of the scan number.
gram showed a distinctive feature at 5 ꢀ 10-4 G0. Neither plateau
nor any significant features were observed below 2 ꢀ 10-4 G0 in
adsorption via the π orbitals of the pyridine ring is favored for
3Py-TIPS.
the conductance traces and conductance histograms, respec-
tively. In the solution without the molecules, neither plateaus nor
peaks were observed in the conductance traces and conductance
histograms. These experimental results indicate that the plateaus
in the conductance traces and the distinctive feature in the
conductance histogram, both of which occur at 5 ꢀ 10-4 G0,
could be ascribed to the bridging of a single 3Py-PE-3Py
molecule between the Au electrodes. The conductance of the
single-molecule junction was determined to be 5 ( 1 ꢀ 10-4 G0
by statistical analysis of repeated measurements (more than 3000
times on three independent samples). It has been referred that
the phenylene-ethynylene π-conjugated system having the pyr-
idine anchor unit 5 showed a single-molecule conductance of
3.5 ꢀ 10-6 G0 in the MCBJ setup (Chart 1).14 Compared to this
molecule, 3Py-PE-3Py showed a conductance larger by 2 orders
of magnitude despite its longer molecular length. Furthermore,
the conductance measurement of the contact created by a single
pyridine anchor (1Py) was carried out recently though the used
spacer (CH2 or phenyl) is shorter than the PE.12h The present
Conductance Measurements. Experiments for measuring
the single-molecule conductance of 3Py-PE-3Py were per-
formed using a modified STM (Pico-SPM, Molecular Imaging
Co.) with a Nano Scope IIIa controller (Digital Instruments Co.)
in an electrochemical cell. Details of the experimental design
used in this study have been previously reported by some of
present authors.25,26 The STM tip was made of an Au wire
(diameter ∼0.25 mm, purity >99%). The Au(111) substrate was
prepared by the flame-annealing and quenching method. The
solution of 3Py-PE-3Py was adjusted to 0.2 mM in mesitylene.
The STM tip was repeatedly moved into and out of contact with
the substrate at a rate of 50 nm s-1 in the solution. The
conductance was measured during the breaking process under
an applied bias of 20 mV between the tip and substrate. All
statistical data were obtained from a large number (over 1000) of
individual conductance traces. The experiments were performed
for three independent samples.
3016
dx.doi.org/10.1021/ja109577f |J. Am. Chem. Soc. 2011, 133, 3014–3022

Journal of the American Chemical Society
ARTICLE
Chart 1. Chemical Structures and Conductances
energetically optimized by the following procedure. First, we
searched the adsorbed structure of the surface system, which
consists of the 3Py-PE-3Py and only a left side electrode. Using
the obtained molecular conformation, we attached the electrode
on the opposite side as a right side electrode. Then we performed
the geometry optimization with relaxing 3Py-PE-3Py and Au
atoms of the top layers. We carried out this optimization with
changing the distance between the left and right electrodes. Then
we checked the total energy as the function of the distance. The
adopted model junctions for the transport calculation give the
minimum energy in the above procedures for (111) and (001)
electrodes. We denote the resulting two junction models as “3Py-
PE-3Py(111)” and “3Py-PE-3Py(001).”
Figure 4. (a) Conductance traces measured when breaking the Au
point contacts in solutions (red) with and (black) without 3Py-PE-3Py.
(b) Corresponding conductance histograms constructed without data
selection from 1000 traces. Each histogram is normalized by the number
of traces used to construct the histogram. The bin size is 10-5 G0.
The electronic structure calculations were performed by
density functional theory (DFT) using the SIESTA package.27
Transport calculations were then carried out by the nonequili-
brium Green function technique combined with DFT (NEGF-
DFT).28 The conductance can be represented as G0T(EF), where
EF is the Fermi level of the entire (semi-infinite) system and the
transmission coefficient T(E) is calculated by Green’s function
G(E) and relevant lead self-energy terms. All of the NEGF
calculations were performed by the HiRUNE module pro-
gram,29,30 developed by the one of the authors and implemented
in SIESTA. To analyze the transport properties, we estimated the
projected molecular orbitals (PMOs), where a PMO is defined as
the eigenstate of the Hamiltonian projected onto the molecular
conductance is also larger by one (or two) orders of magnitude
than the results given in ref 12h. This suggests effective hybridi-
zation of the metal electrode with the π orbital of the pyridine
ring caused by 3Py structure. In a later section, we check the
validity of our consideration that the obtained conductance is
provided by the contact of the 3Py anchor, not by 1Py, using ab
initio calculations
Theoretical Analysis of Electric Transport. In order to
analyze electron transport at the 3Py-PE-3Py molecular junc-
tion, we performed ab initio calculations. A theoretical model of
the junction was introduced according to the following proce-
dures. First, we adopt two types of Au electrodes: clean Au(111)
and Au(001) surfaces. This is because it is difficult to identify the
rigorous structure of the electrode surfaces in the present
experiments. An apex structure is often adopted in order to
model devices created by break junctions for a single pyridine
anchor.12h,15k However, we did not use the apex model in order
to avoid an arbitrary conformation of the tripodal leg, as the
conformation depends more strongly on the positions of apexes
than in the case of a single-point anchor. Instead, we analyze the
effects of varying the adsorption structures by adopting two clean
surface structures as the theoretical model. We started from two
0
region and the relevant eigenenergy is denoted as ER . The actual
0
PMO energy shifts by Δ from ER0 (i.e., ER = ER þ Δ) and has an
imaginary part γ because of coupling with the electrodes. The
term γ relates to the molecule-electrode coupling strength.
They can be strictly defined for each PMO ΨR using renorma-
lized Green functions.31 Hereafter, we label PMOs using stan-
dard terminologies: HOMO (highest occupied molecular
orbital), LUMO (lowest unoccupied molecular orbital), etc.,
where the terminology of “HOMO/LUMO” is defined by the
energy level ER relative to EF.
kinds of structures, where one is constructed by the p(6 ꢀ 6)
√
√
(111) and the other by 5 2 ꢀ 5 2 (001) unit cell elec-
trodes. The atomic positions in the 3Py-PE-3Py were then
The structures of the device region obtained by DFT optimi-
zation are shown in Figure 5 as well as a few distinctive angles.
3017
dx.doi.org/10.1021/ja109577f |J. Am. Chem. Soc. 2011, 133, 3014–3022

Journal of the American Chemical Society
ARTICLE
Figure 5. Structure of junctions adopted for the ab initio transport calculations. (a) 3Py-PE-3Py(111) model and (b) 3Py-PE-3Py(001) model,
respectively. The left panels show side views of each system, whereas the middle panels show views from the top. The right panels give the details of the
parameters to identify the conformations for 3Py, such as dihedral and bending angles.
Table 1. Summary of the Calculated Conductance and Re-
levant PMOs for the 3Py-PE-3Py(111) and (001) Systems
The three pyridine legs of 3Py-PE-3Py(111) are anchored by the
point contact between each N atom and metal. The tilt angle ϑ
between the pyridine molecular plane and the Au surface is about
18.9ꢀ and each N atom is placed close to the on-top site while it
slightly shifts in the hollow direction. The p orbital, which relates
to the lone pair of N, connects to the electrode to form the
contact. A little hybridization of the other p orbitals makes the π*
tilt slightly toward the molecular plane of the pyridine. In
contrast, the pyridine rings of each leg of 3Py-PE-3Py(001) lie
on the Au surface, and their tilt angles ϑ are smaller than 5.0ꢀ. In
particular, the pyridine plane for one of the three legs is almost
parallel to the surface. Therefore, sufficient hybridization may
occur between the metal and π state delocalized on the pyridine
ring, where such a π contact potentially leads to charge donation
and high conductance. When a reference plane (dotted line part
in Figure 5) is defined as the plane consisted of the C3-axis and a
pointed N atom of 3Py, the dihedral angles of the phenyl (jph)
and the pyridine (jpy) in 3Py are 64.9ꢀ and 86.3ꢀ, respectively.
We also defined the bending angles R and β (see Figure 5). The
angle β between the axis of the phenyl and pyridine is 162.7ꢀ, and
the loss of energy by bending is compensated by the adsorption
energy. Note that the legs of the tripodal anchor, particularly, of
3Py-PE-3Py(001), were not equivalently connected to the
electrodes in the present calculations. This is quite reasonable
model
(111)
conductance
PMOa
ER (eV)a
γ (eV)
PDOS b
1.16 ꢀ 10-6 G0
HOMO
LUMO
HOMO
LUMO
-1.97
0.78
-0.005
-0.046
-0.016
-0.071
—
0.36
—
(001)
2.27 ꢀ 10-4 G0
-1.87
0.64
3.40
a Definitions of the PMO energy ER and molecule-lead coupling γ are
given in the text. Here we set the Fermi level to 0. b The PDOS is the
value at the Fermi level. The value of HOMO is lower than 1.0 ꢀ 10-4
for both models.
because not all subparts of the (001) surface can satisfy C3
symmetry.
The calculated conductance for 3Py-PE-3Py(111) and (001)
are 1.16 ꢀ 10-6 G0 and 2.27 ꢀ 10-4 G0, respectively, as shown in
Table 1. The conductance depends on the surface Miller index of
the electrode, which in turn strongly depends on the orientations
of the attached pyridine as expected. The (001) electrode is far
more electrically conductive than the (111) electrode. As stated
above, there are potential differences in the electronic features of
the π contact between 3Py-PE-3Py(001) and (111). Therefore,
it is worthwhile to compare the transport mechanisms of the two
3018
dx.doi.org/10.1021/ja109577f |J. Am. Chem. Soc. 2011, 133, 3014–3022

Journal of the American Chemical Society
ARTICLE
indicating that the formed 3Py-PE-3Py could be sufficiently
conductive. Furthermore, the transport mechanism itself, i.e.,
electron transport through the LUMO, is independent of
electrode structure, although the conductance could be highly
sensitive to the formed contact structure because of different
types of pyridine-metal interactions.
Now, we examine the distinction between the tripodal anchor
and single pyridine contact to show the consistency between the
present experimental data and theoretical analysis. We per-
formed additional transport calculations for 1Py-PE-1Py(001),
i.e., the unit 5 with (001) electrode (see Chart 1). Although we
calculated several conformations of a single pyridine contact with
the same procedures, the resulting conductance values were
typically only ∼20% of 3Py-PE-3Py(001). This value, 20%, is
slightly large when the conductance by 1Py anchoring reported
previously is adopted.14 However, the calculated conductance of
1Py is sufficiently small compared with both the calculated (3Py)
and the measured result. Hence, we conclude that the tripodal
anchoring is essential to provide the present high conductance.
Finally, we briefly comment on the use of GGA level DFT in the
present NEGF framework. There are a few “beyond-DFT”
transport calculations for the pyridine type systems, and their
results improve the absolute values of conductance.12h,13 How-
ever most of the calculations are based on only the partial
correction scheme such as the GW correction for the bridge
molecular part. According to several calculations of similar
pyridine systems by NEGF-DFT,12f,12i,12j the use of GGA level
is sufficient qualitatively for the present purpose to discuss the
transport mechanism.
Figure 6. Plots of LUMO wave functions for 3Py-PE-3Py(111) (upper
left panel) and 3Py-PE-3Py(001) (lower left panel), respectively. Each
box of dashed lines represents the pyridine part of the wave function
relevant to anchoring. Right panels show enlarged views of each boxed
region.
’ CONCLUSION
We have designed a new tripodal pyridine anchor to realize
robust contact with metal electrodes and to achieve effective
hybridization of the pyridine π orbital with metal electrodes. We
successfully synthesized a series of tripodal pyridine-containing
molecules. CV measurements of their monolayers on gold
revealed that the tripodal structure has a great advantage in
adsorption tendency compared to a single pyridine contact and
in robustness even under biased conditions. These features can
be explained by the multiplier effect of the tripodal pyridine
anchor. As expected, XPS measurement indicated that the π
orbital of pyridine contributes to the physical adsorption of our
developed tripodal anchor on gold. Measurement of single-
molecule conductance was successfully carried out using mod-
ified STM techniques for the phenylene-ethynylene chain con-
nected with the present anchors. To our best knowledge, this is
the first example wherein the electrical conductance for single-
molecule bearing tripodal anchoring groups has been measured.
The obtained conductance of 5 ( 1 ꢀ 10-4 G0 is substantially
higher than that of previously reported shorter π-conjugated
chains with single-pyridine anchors. This result suggests that
effective hybridization occurred as expected. Theoretical analysis
based on ab initio calculations clearly supported the experimental
results and our hypothesis of π channel conductance. The
calculated results indicate an electron-transport mechanism for
the present conduction system in which the LUMO dominates
the transport. In pyridine-based tripodal anchoring, the π* orbital
of the pyridine part could directly interact with the electrode.
Again, we emphasize our theoretical prediction that the above
effective π contact enhances the conductance. The formation of
the effective channel is caused by the adsorption structure of the
model junctions. The calculated PMOs and relevant couplings
are listed in Table 1. For both 3Py-PE-3Py(111) and (001), the
difference between the LUMO energy and EF is much closer (1-
1.2 eV) than that between the HOMO energy and EF; thus,
LUMO should dominate the transport. Furthermore, the mole-
cule-lead coupling, γ, has a nonzero value for LUMO. As a result,
we concluded that the conductive orbital is LUMO, and the
transport carrier is an electron (electron transport) for each
system.
To proceed with the analysis, we plotted the LUMO wave
functions in Figure 6. The pyridine parts in LUMO are the π*
orbitals for both 3Py-PE-3Py(111) and (001). However, the
nodal planes of the p orbital of the N atom and the other
delocalized π* part of the pyridine are twisted around one
another for 3Py-PE-3Py(111). In contrast, the distribution of
the wave function for LUMO of 3Py-PE-3Py(001) is similar to
that of a standard delocalized benzene π conjugate orbital, and
the amplitude of the C-C π orbital overlaps with the metal.
Using the PMO and Green’s function, we calculated the pro-
jected density of states (PDOS) of (111) and (001) LUMO as a
function of energy and found that the PDOS of LUMO for 3Py-
PE-3Py(001) are sufficiently large below EF with a long tail: i.e.,
electron donation to π* in the tripodal anchor is confirmed.
We emphasize that the (111) and (001) electrodes are only
extremely idealized models: thus, the agreement of the conduc-
tances between the experimental and calculated values should
be treated with a measure of caution. However, at the very least,
the theoretical calculations support the experimental results,
3019
dx.doi.org/10.1021/ja109577f |J. Am. Chem. Soc. 2011, 133, 3014–3022

Journal of the American Chemical Society
ARTICLE
pointed pyridine, and anchoring by the tripodal moiety plays an
important role to realize higher conductance than by a single
atomic/molecular anchor. Therefore, we conclude that our
newly developed tripodal pyridine anchor has opened the door
to new strategies for achieving an ideal metal-molecular junc-
tion. We are now in the process of demonstrating this concept by
investigating the dependence of conductance on the molecular
length, the results of which will be reported in due course.
m/z 734.78 (Mþ, calcd 734.02). Anal. Calcd for C36H37Br3Si: C, 58.63;
H, 5.06; Found: C, 58.38; H, 4.86.
Synthesis of 3Py-TIPS. Compound 3 (800 mg, 1.08 mmol), 4-pyr-
idineboronic acid (797 mg, 6.48 mmol), K2CO3 (1.19 g, 8.64 mmol),
and Pd(PPh3)4 (125 mg, 0.108 mmol) were placed in a 200 mL round-
bottomed flask and dissolved with THF (50 mL) and water (10 mL).
The reaction mixture was stirred at 80 ꢀC for 12 h. After being stirred for
12 h, the reaction was quenched by addition of water, and the organic
layer was separated. The aqueous layer was extracted with EtOAc, and
the combined organic layer was washed with brine and dried over
Na2SO4. After removal of the solvent under reduced pressure, the
residue was purified by column chromatography on silica gel (EtOAc/
MeOH = 9/1) to give 3Py-TIPS (647 mg, 82%). White solid; mp 227-
229 ꢀC; 1H NMR (CDCl3) δ 1.12 (s, 21H), 7.27 (d, J = 8.4 Hz, 2H),
7.41 (d, J = 8.2 Hz, 6H), 7.45 (d, J = 8.4 Hz, 2H), 7.51 (d, J = 4.8 Hz, 6H),
7.59 (d, J = 8.4 Hz, 6H), 8.66 (d, J = 4.8 Hz, 6H); 13C NMR (CDCl3)
δ 11.3, 18.7, 64.6, 91.3, 106.6, 121.4, 121.7, 126.5, 130.7, 131.6, 131.6,
136.0, 146.1, 146.9, 147.4, 150.3; MS (MALDI-TOF, 1,8,9-trihydrox-
yanthracene matrix) m/z 732.14 (Mþ, calcd 731.37). Anal. Calcd for
C51H49N3Si: C, 83.68; H, 6.75; N, 5.74; Found: C, 83.31; H, 6.87; N,
5.75.
’ EXPERIMENTAL SECTION
General Information. Column chromatography was performed
on silica gel, KANTO Chemical silica gel 60N (40-50 μm). TLC plates
were visualized with UV light. Preparative gel-permeation chromatog-
raphy (GPC) was performed on Japan Analytical Industry LC-908
equipped with JAI-GEL 1H/2H columns. Melting points are uncor-
1
rected. H and 13C NMR spectra were recorded on a JEOL JMN-400
spectrometer in CDCl3 with tetramethylsilane as an internal standard.
Data are reported as follows: chemical shift in ppm (δ), multiplicity (s =
singlet, d = doublet, t = triplet, m = multiplet), coupling constant (Hz),
and integration. Mass spectra were obtained on Shimadzu AXIMA-
TOF. Cyclic voltammetry was carried out on a BAS ALC 620C
voltammetric analyzer. Elemental analyses were performed on Perkin-
Elmer LS-50B by the Elemental Analysis Section of Comprehensive
Analysis Center (CAC), the Institute of Scientific and Industrial
Research (ISIR), Osaka University. In the XPS measurement, Al KR
(1486.6 eV) and Mg KR (1253.6 eV) were used as X-ray source. The
calibration of binding energy was carried out with the peak of Au 4f7/2 at
84.0 eV as an energy reference.
Synthesis of Ph4T-I. Ph4T (451 mg, 0.784 mmol) was placed in a
100 mL three-necked round-bottomed flask and dissolved with THF
(20 mL). To the mixture was added n-BuLi (1.6 M hexane solution,
0.59 mL, 0.94 mmol) at -78 ꢀC. After the mixture was stirred for 30 min
at -78 ꢀC, I2 (299 mg, 1.18 mmol) was added. The mixture was
gradually warmed to room temperature. After being stirred for 30 min,
the reaction was quenched by addition of satd Na2S2O3 aq, and the
organic layer was separated. The aqueous layer was extracted with
hexane, and the combined organic layer was washed with brine and dried
over Na2SO4. After removal of the solvent under reduced pressure, the
residue was purified by column chromatography on silica gel (hexane) to
Materials. All reactions were carried out under a nitrogen atmo-
sphere. Solvents of the highest purity grade were used as received.
Unless stated otherwise, all reagents were purchased from commer-
cial sources and used without purification. 4-Trityliodobenzene (1)32
1
give Ph4T-I (520 mg, 95%). Yellow solid; mp 61-63 ꢀC; H NMR
1
and Ph4T33 were prepared by reported procedures, and H NMR
(CDCl3) δ 0.87-0.92 (m, 6H), 1.25-1.48 (m, 12H), 1.56-1.75 (m,
4H), 2.73 (t, J = 8.1 Hz, 2H), 2.80 (t, J = 8.1 Hz, 2H), 6.97 (d, J = 3.7 Hz,
1H), 7.06 (d, J = 3.7 Hz, 1H), 7.08 (s, 1H), 7.12 (d, J = 3.7 Hz, 1H), 7.14
(d, J = 3.7 Hz, 1H), 7.17 (s, 1H), 7.28-7.30 (m, 1H), 7.36-7.40 (m,
2H), 7.59-7.61 (m, 2H); 13C NMR (CDCl3) δ 14.1, 14.1, 22.6, 22.6,
29.0, 29.1, 29.3, 29.6, 30.6, 30.6, 31.6, 31.7, 71.8, 123.8, 124.1, 125.6,
126.2, 126.3, 127.0, 127.6, 128.9, 129.8, 133.8, 134.0, 135.6, 136.3, 136.4,
137.4, 139.8, 140.8, 141.7, 142.1; MS (MALDI-TOF, 1,8,9-trihydrox-
yanthracene matrix) m/z 700.92 (Mþ, calcd 700.08). Anal. Calcd for
C34H37IS4: C, 58.27; H, 5.32; Found: C, 58.16; H, 5.23.
data of these compounds were in agreement with those previously
reported.
Synthesis of 2. Compound 1 (7.89 g, 17.7 mmol) was placed in a
500 mL round-bottomed flask and dissolved with dichloromethane
(180 mL). To the mixture was added bromine (90.7 mL, 1.77 mmol) at
0 ꢀC. The mixture was gradually warmed up to room temperature. After
being stirred for 12 h, the mixture was added to satd Na2S2O3 aq, and the
organic layer was separated. The aqueous layer was extracted with
dichloromethane, and the combined organic layer was washed with brine
and dried over Na2SO4. After removal of the solvent under reduced
pressure, the residue was washed by hexane and methanol to give 3
(10.2 g, 84%). White solid; >300 ꢀC; 1H NMR (CDCl3) δ 6.88 (d, J =
8.7 Hz, 2H), 7.01 (d, J = 8.7 Hz, 6H), 7.39 (d, J = 8.7 Hz, 6H), 7.59 (d, J =
8.7 Hz, 2H); 13C NMR (CDCl3) δ 63.9, 92.5, 120.9, 131.2, 132.4, 132.7,
137.2, 144.5, 145.3; MS (MALDI-TOF, 1,8,9-trihydrioxyanthracene
matrix) m/z 678.85 (Mþ, calcd 679.78). Anal. Calcd for C25H16Br3I:
C, 43.96; H, 2.36; Found: C, 43.60; H, 2.00.
Synthesis of 3. Compound 2 (6.40 g, 9.37 mmol), PdCl2(PPh3)2
(329 mg, 0.469 mmol), and CuI (89 mg, 0.467 mmol) were placed in a 2
L round-bottomed flask and dissolved with THF (300 mL) and
triethylamine (300 mL). To the mixture was added (triisopropylsilyl)-
acetylene (4.15 mL, 18.7 mmol) at 0 ꢀC. The mixture was gradually
warmed up to room temperature. After being stirred for 12 h, the
reaction mixture was passed through Celite. After removal of the solvent
under reduced pressure, the residue was purified by column chroma-
tography on silica gel (hexane) to give 3 (6.32 g, 91%). White solid; mp
239-241 ꢀC; 1H NMR (CDCl3) δ 1.18 (s, 21H), 7.02 (d, J = 8.9 Hz,
6H), 7.07 (d, J = 8.7, 2H), 7.36-7.39 (m, 8H); 13C NMR (CDCl3) δ
11.3, 18.6, 64.0, 91.4, 106.4, 120.7, 121.8, 130.5, 131.0, 131.6, 132.4,
144.5, 145.5; MS (MALDI-TOF, 1,8,9-trihydroxyanthracene matrix)
Synthesis of 3Py-Ph4T. 3Py-TIPS (349 mg, 0.48 mmol) was placed
in a 100 mL round-bottomed flask and dissolved with THF (50 mL). To
the mixture was added nBu4NF (1.0 M THF solution, 0.95 mL, 0.95
mmol), and the mixture was stirred at room temperature for 30 min. The
reaction was quenched by addition of satd NaHCO3 aq, and the organic
layer was separated. The aqueous layer was extracted with EtOAc, and
the combined organic layer was washed with brine and dried over Na2-
SO4. After removal of the solvent under reduced pressure, the residue
was used for the next reaction without further purification.
The residue, Ph4T-I (401 mg, 0.57 mmol), PdCl2(PPh3)2 (33 mg,
0.048 mmol), and CuI (9 mg, 0.047 mmol) were placed in a 200 mL
round-bottom flask and dissolved with THF (50 mL) and triethylamine
(10 mL). The reaction mixture was stirred at room temperature. After
being stirred for 12 h, the reaction mixture was passed through Celite.
After removal of the solvent under reduced pressure, the residue was
purified by column chromatography on silica gel (EtOAc/MeOH = 9/1)
to give 3Py-Ph4T (286 mg, 52% (two steps)). Yellow solid; mp 209-
211 ꢀC; 1H NMR (CDCl3) δ 0.90 (t, J = 7.1 Hz, 6H), 1.29-1.50 (m,
12H), 1.56-1.68 (m, 4H), 2.74-2.82 (m, 4H), 7.06 (d, J = 4.1 Hz, 1H),
7.07 (d, J = 4.1 Hz, 1H), 7.11 (s, 1H), 7.14 (d, J = 4.1 Hz, 1H), 7.15 (d, J =
4.1 Hz, 1H), 7.16 (s, 1H), 7.27-7.32 (m, 3H), 7.35-7.43 (m, 8H), 7.46
3020
dx.doi.org/10.1021/ja109577f |J. Am. Chem. Soc. 2011, 133, 3014–3022

Journal of the American Chemical Society
ARTICLE
(d, J = 8.7 Hz, 2H), 7.54-7.63 (m, 14H), 8.66 (d, J = 6.4 Hz, 6H); 13
C
voltammogram of 3Py-Ph4T. Complete ref 1. This material is
available free of charge via the Internet at http://pubs.acs.org
NMR (CDCl3) δ 14.1, 22.6, 29.2, 29.3, 29.3, 29.6, 30.4, 30.5, 31.7, 31.7,
64.6, 83.3, 93.6, 120.8, 121.1, 123.9, 124.1, 125.5, 126.2, 126.3, 126.5,
126.9, 127.6, 128.9, 129.8, 130.9, 130.9, 131.6, 132.5, 133.9, 134.2, 135.1,
135.6, 136.0, 136.3, 137.3, 139.7, 40.8, 142.1, 142.6, 146.9, 147.4, 150.0;
MS (MALDI-TOF, 1,8,9-trihydroxyanthracene matrix) m/z 1147.84
(Mþ, calcd 1147.41); Anal. Calcd for C76H65N3S4: C, 79.47; H, 5.70; N,
3.66; Found: C, 79.31; H, 5.61; N, 3.43.
’ AUTHOR INFORMATION
Corresponding Author
hs-nakamura@aist.go.jp; kiguti@chem.titech.ac.jp; aso@sanken.
osaka-u.ac.jp
Synthesis of 3Py-PE-3Py. 3Py-TIPS (83 mg, 0.11 mmol) was
placed in a 100 mL round-bottomed flask and dissolved with THF
(20 mL). To the mixture was added nBu4NF (1.0 M THF solution,
0.22 mL, 0.23 mmol), and the mixture was stirred at room temperature
for 30 min. The reaction was quenched by addition of satd NaHCO3 aq,
and the organic layer was separated. The aqueous layer was extracted
with hexane, and the combined organic layer was washed with brine and
dried over Na2SO4. After removal of the solvent under reduced pressure,
the residue was used for the next reaction without further purification.
The residue, 2 (116 mg, 0.17 mmol), Pd(PPh3)4 (13 mg, 0.011
mmol), and CuI (2 mg, 0.011 mmol) were placed in a 30 mL round-
bottom flask and dissolved with THF (6 mL) and iPr2NEt (2 mL). The
reaction mixture was stirred at room temperature for 12 h. After removal
of the solvent under reduced pressure, the residue was purified by
column chromatography on silica gel (EtOAc/MeOH/Et3N = 18/1/1)
to give compound A (75 mg, 58% (two steps)). White solid; mp 132-
134 ꢀC; 1H NMR (CDCl3) δ 7.03 (d, J = 8.8 Hz, 6H), 7.12 (d, J = 8.3 Hz,
2H), 7.30 (d, J = 8.3 Hz, 2H), 7.37-7.45 (m, 14H), 7.48 (d, J = 8.8 Hz,
2H), 7.51 (d, J = 6.1 Hz, 6H), 7.61 (d, J = 8.8 Hz, 6H), 8.66 (d, J = 6.1 Hz,
6H); MS (MALDI-TOF, 1,8,9-trihydroxyanthracene matrix) m/z
1127.98 (Mþ, calcd 1127.11).
’ ACKNOWLEDGMENT
The authors are grateful to T. Ohto (the University of Tokyo)
for helpful discussions and support through calculations. This
work was supported in part by Grants-in-Aids for Scientific
Research on Priority Areas, “Electron Transport Through a
Linked Molecule in Nano-Scale” (Grant No. 17069006) and
Management Expenses Grants for National Universities Cor-
porations from the Ministry of Education, Culture, Sports,
Science, and Technology, Japan. Thanks are extended to the
Elemental Analysis Section of the Comprehensive Analysis
Center (CAC) of the Institute of Scientific and Industrial
Research (ISIR), Osaka University, for assistance in obtaining
elemental analyses.
’ REFERENCES
(1) Adams, D. M.; et al. J. Phys. Chem. B 2003, 107, 6668.
(2) Melosh, N.; Boukai, A.; Diana, F.; Gerardot, B.; Badolato, A.;
Petroff, P.; Heath, J. R. Science 2003, 300, 112.
Compound A (55 mg, 0.049 mmol), 4-pyridineboronic acid (36 mg,
0.29 mmol), K2CO3 (54 mg, 0.39 mmol), and Pd(PPh3)4 (17 mg, 0.015
mmol) were placed in a test tube and dissolved with THF (5 mL) and
water (1 mL). The reaction mixture was stirred at 80 ꢀC for 12 h. The
reaction mixture was extracted with hexane and the organic layer was
washed with brine and dried over Na2SO4. After removal of the solvent
under reduced pressure, the residue was purified by column chromatog-
raphy on alumina (CHCl3/MeOH = 9/1) to give 3Py-PE-3Py (49 mg,
90%). White solid; mp >300 ꢀC; 1H NMR (CDCl3) δ 7.28 (d, J = 8.8 Hz,
4H), 7.41 (d, J = 8.5 Hz, 12H), 7.46-7.53 (m, 16H), 7.60 (d, J = 8.5 Hz,
12H), 8.66 (d, J = 6.4 Hz, 12H); MS (MALDI-TOF, 1,8,9-trihydroxyan-
thracene matrix) m/z 1123.78 (Mþ, calcd 1124.46). Anal. Calcd for
C82H56N6: C, 87.52; H, 5.02; N, 7.47; Found: C, 87.80; H, 5.07; N, 7.19.
Synthesis of 1Py-Ph4T. 4-Ethynylpyridine (4) (55 mg, 0.53 mmol),
Ph4T-I (308 mg, 0.44 mmol), PdCl2(PPh3)2 (31 mg, 0.044 mmol), and
CuI (8 mg, 0.042 mmol) were placed in a 50 mL round-bottom flask and
dissolved with THF (21 mL) and Et3N (7 mL). The reaction mixture
was stirred at room temperature for 12 h. After removal of the solvent
under reduced pressure, the residue was purified by column chroma-
tography on silica gel (hexane/EtOAc = 3/1) to give 1Py-Ph4T (59 mg,
20%). Black oil; 1H NMR (CDCl3) δ 0.90 (t, J = 7.1 Hz, 6H), 1.22-1.50
(m, 12H), 1.56-1.68 (m, 4H), 2.75-2.82 (m, 4H), 7.07 (d, J = 4.1 Hz,
1H), 7.15-7.17 (m, 3H), 7.19 (s, 1H), 7.27-7.33 (m, 1H), 7.34-7.41
(m, 4H), 7.60 (d, J = 7.4 Hz, 2H), 8.60 (d, J = 6.4 Hz, 2H); 13C NMR
(CDCl3) δ 14.1, 22.6, 22.6, 29.1, 29.3, 29.3, 29.6, 30.4, 30.5, 31.6, 31.7,
87.6, 91.4, 119.4, 123.9, 124.2, 125.0, 125.5, 126.1, 126.3, 127.2, 127.6,
128.9, 129.7, 131.1, 133.9, 133.9, 135.7, 136.2, 136.3, 137.7, 139.8, 140.8,
142.1, 149.7; MS (MALDI-TOF, 1,8,9-trihydroxyanthracene matrix)
m/z 675.97 (Mþ, calcd 675.21) ; Anal. Calcd for C41H41NS4: C, 72.84;
H, 6.11; N, 2.07; Found: C, 73.10; H, 6.08; N, 1.94.
(3) Novoselov, K.; Geim, A. K.; Morozov, S. V.; Jiang., D.; Katsnelson,
M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A. Nature2005, 438, 197.
(4) (a) Tao, N. J. Nat. Nanotechnol. 2006, 1, 173.(b) Cuevas, J. C.;
Scheer, E. Molecular Electronics: An Introduction to Theory and Experi-
ment; World Scientific: Singapore, 2010.
(5) Aviram, A.; Ratner, M. Chem. Phys. Lett. 1974, 29, 277.
(6) Reed, M. A.; Zhou, C.; Muller, C. J.; Burgin, T. P.; Tour, J. M.
Science 1997, 278, 252.
(7) Xu, B.; Tao, N. J. Science 2003, 301, 1221.
(8) Chen, F.; Li, X.; Hihath, J.; Huang, Z.; Tao, N. J. Am. Chem. Soc.
2006, 128, 15874.
(9) (a) Patrone, L.; Palacin, S.; Bourgoin, J. P. Appl. Surf. Sci. 2003,
212-213, 446. (b) Patrone, L.; Palacin, S.; Charlier, J.; Armand, F.;
Bourgoin, J. P.; Tang, H.; Gauthier, S. Phys. Rev. Lett. 2003, 91, 096802/
1. (c) Yasuda, S.; Yoshida, S.; Sasaki, J.; Okutsu, Y.; Nakamura, T.;
Taninaka, A.; Takeuchi, O.; Shigekawa, H. J. Am. Chem. Soc. 2006, 128,
7746. (d) Yokota, K.; Taniguchi, M.; Kawai, T. J. Am. Chem. Soc. 2007,
129, 5818. (e) Yokota, K.; Taniguchi, M.; Tanaka, H.; Kawai, T. Phys.
Rev. B 2008, 77, 165416/1.
(10) Ie, Y.; Hirose, T.; Yao, A.; Yamada, T.; Takagi, N.; Kawai, M.;
Aso, Y. Phys. Chem. Chem. Phys. 2009, 11, 4949.
(11) (a) Venkataraman, L.; Klare, J. E.; Tam, I. W.; Nuckolls, C.;
Hybertsen, M. S.; Steigerwald, M. L. Nano Lett. 2006, 6, 458. (b) Quinn,
J. R.; Foss, F. W., Jr.; Venkataraman, L.; Hybertsen, M. S.; Breslow, R.
J. Am. Chem. Soc. 2007, 129, 6714. (c) Venkataraman, L.; Park, Y. S.;
Whalley, A. C.; Nuckolls, C.; Hybertsen, M. S.; Steigerwald, M. L. Nano
Lett. 2007, 7, 502.
(12) (a) Xu, B.; Xiao, X.; Tao, N. J. J. Am. Chem. Soc. 2003, 125,
16164. (b) Li, X.; Hihath, J.; Chen, F.; Masuda, T.; Zang, L.; Tao, N.
J. Am. Chem. Soc. 2007, 129, 11535. (c) Li, C.; Mishchenko, A.; Li, Z.;
Pobelov, I.; Wandlowski, T.; Li, X. Q.; W€urthner, F.; Bagrets, A.; Evers,
F. J. Phys., Condens. Matter. 2008, 20, 374122. (d) Zhou, X.-S.; Chen,
Z.-B.; Liu, S.-H.; Jin, S.; Liu, L.; Zhang, H.-M.; Xie, Z.-X.; Jiang, Y.-B.;
Mao, B.-W. J. Phys. Chem. C 2008, 112, 3935. (e) Horiguchi, K.;
Kurokawa, S.; Sakai, A. J. Chem. Phys. 2009, 131, 104703. (f) Wang,
C.; Batsanov, A. S.; Bryce, M. R.; Martin, S.; Nichols, R. J.; Higgins, S. J.;
Garcia-Suꢀarez, V. M.; Lambert, C. J. J. Am. Chem. Soc. 2009, 131, 15647.
’ ASSOCIATED CONTENT
Supporting Information. 1H NMR spectra of 3Py-TIPS,
S
b
3PY-Ph4T, 1Py-Ph4T, and 3Py-PE-3Py as well as the cyclic
3021
dx.doi.org/10.1021/ja109577f |J. Am. Chem. Soc. 2011, 133, 3014–3022

Journal of the American Chemical Society
ARTICLE
(g) Pan, S.; Zhao, A.; Wang, B.; Yang, J.; Hou, J. Adv. Mater. 2010, 22,
1967. (h) Kamenetska, M.; Quek, S. Y.; Whalley, A. C.; Steigerwald,
M. L.; Choi, H. J.; Louie, S. G.; Nuckolls, C.; Hybertsen, M. S.; Neaton,
J. B.; Venkataraman, L. J. Am. Chem. Soc. 2010, 132, 6817. (i) Stadler, R.
Phys. Rev. B 2010, 81, 165429. (j) Garcia-Suꢀarez, V. M.; Lambert, C. J.
Nanotechnology 2008, 19, 455203.
(13) Quek, S. Y.; Kamenetska, M.; Steigerwald, M. L.; Choi, H. J.;
Louie, S. G.; Hybertsen, M. S.; Neaton, J. B.; Venkataraman, L. Nat.
Nanotechnol. 2009, 4, 230.
(24) Behzadi, B.; Ferri, D.; Baiker, A.; Ernst, K.-H. Appl. Surf. Sci.
2007, 253, 3480.
(25) Kiguchi, M.; Miura, S.; Hara, K.; Sawamura, M.; Murakoshi, K.
Appl. Phys. Lett. 2007, 91, 053110.
(26) Kiguchi, M.; Miura, S.; Takahashi, T.; Hara, K.; Sawamura, M.;
Murakoshi, K. J. Phys. Chem. C 2008, 112, 13349.
(27) Soler, J. M.; Artacho, E.; Gale, J. D.; Garcia, A.; Junquera, J.;
Ordejon, P.; Sanchez-Portal, D. J. Phys.: Condens. Matter 2002, 14, 2745.
(28) Brandbyge, M.; Mozos, J. L.; Ordejon, P.; Taylor, J.; Stokbro, K.
Phys. Rev. B 2002, 65, 165401–1.
(29) Nakamura, H.; Yamashita, K. J. Chem. Phys. 2006, 125, 194106–1.
(30) Nakamura, H.; Yamashita, K.; Rocha, A. R.; Sanvito, S. Phys.
Rev. B 2008, 78, 235420–1.
(14) Grunder, S.; Huber, R.; Wu, S.; Sch€onenberger, C.; Calame, M.;
Mayor, M. Eur. J. Org. Chem. 2010, 833.
(15) (a) Biliꢀc, A.; Reimers, J. R.; Hush, N. S. J. Phys. Chem. B 2002,
106, 6740. (b) Tada, T.; Kondo, M.; Yoshizawa, K. J. Chem. Phys. 2004,
121, 8050. (c) Stadler, R.; Thygesen, K. S.; Jacobsen, K. W. Phys. Rev. B
2005, 72, 241401-1. (d) Li, Q.; Wu, X.; Huang, J.; Yang, J. Ultramicro-
scopy 2005, 105, 293. (e) Wu, X.; Li, Q.; Huang, J.; Yang, J. J. Chem. Phys.
(31) Nakamura, H. J. Phys. Chem. C 2010, 114, 12280.
(32) Li, Q.; Rukavishnikov, A. V.; Petukhov, P. A.; Zaikova, T. O.;
Keana, J. F. W. Org. Lett. 2002, 4, 3631.
(33) Negishi, N.; Ie, Y.; Taniguchi, M.; Kawai, T.; Tada, H.; Kaneda,
T.; Aso, Y. Org. Lett. 2007, 9, 829.
ꢀ
2005, 123, 184712. (f) Pꢀerꢀez-Jimꢀenez, A. J. J. Phys. Chem. B 2005, 109,
10052. (f) Stadler, R.; Jacobsen, K. W. Phys. Rev. B 2006, 74, 161405-1.
(g) Hou, S.; Zhang, J.; Li, R.; Ning, J.; Han, R.; Shen, Z.; Zhao, X.; Xue,
Z.; Wu, Q. Nanotechnology 2005, 16, 239. (h) Hou, S.; Ning, J.; Shen, Z.;
Zhao, X.; Xue, Z. Chem. Phys. 2006, 327, 1. (i) Li, R.; Hou, S.; Zhang, J.;
Qian, Z.; Shen, Z.; Zhao, X. J. Chem. Phys. 2006, 125, 194113. (j)
Strange, M.; Kristensen, I. S.; Thygesen, K. S.; Jacobsen, K. W. J. Chem.
Phys. 2008, 128, 114714. (k) Stadler, R. Phys. Rev. B 2010, 81, 165429-1.
(16) Kiguchi, M.; Tal, O.; Wohlthat, S.; Pauly, F.; Krieger, M.; Djukic,
D.; Cuevas, J. C.; van Ruitenbeek, J. M. Phys. Rev. Lett. 2008, 101, 046801.
(17) (a) Martin, C. A.; Ding, D.; Sørensen, J. K.; Bjørnholm, T.; van
Ruitenbeek, J. M.; van der Zant, H. S. J. J. Am. Chem. Soc. 2008, 130,
13198. (b) Kiguchi, M. Appl. Phys. Lett. 2009, 95, 073301.
(18) Pump, F.; Temirov, R.; Neucheva, O.; Soubatch, S.; Tautz, S.;
Rohlfing, M.; Cuniberti, G. Appl. Phys. A: Mater. Sci. Process. 2008, 98, 335.
(19) (a) Whitesell, J. K.; Chang, H. K. Science 1993, 261, 73. (b) Fox,
M. A.; Whitesell, J. K.; McKerrow, A. J. Langmuir 1998, 14, 816. (c) Yao,
Y.; Tour, J. M. J. Org. Chem. 1999, 64, 1968. (d) Guo, W.; Galoppini, E.;
Rydja, G.; Pardi, G. Tetrahedron Lett. 2000, 41, 7419. (e) Hu, J.; Mattern,
D. L. J. Org. Chem. 2000, 65, 2277. (f) Jian, H.; Tour, J. M. J. Org. Chem.
2003, 68, 5091. (g) Yam, C. M.; Cho, J.; Cai, C. Langmuir 2003, 19,
6862. (h) Park, J.-S.; Vo, A. N.; Barriet, D.; Shon, Y.-S.; Lee, T. R.
Langmuir 2005, 21, 2902. (i) Shirai, Y.; Cheng, L.; Chen, B.; Tour, J. M.
J. Am. Chem. Soc. 2006, 128, 13479. (j) Kitagawa, T.; Idomoto, Y.;
Matsubara, H.; Hobara, D.; Kakiuchi, T.; Okazaki, T.; Komatsu, K.
J. Org. Chem. 2006, 71, 1362. (k) T. Weidner, T.; Kr€amer, A.; Bruhn, C.;
Zharnikov, M.; Shaporenko, A.; Siemeling, U.; Tr€ager, F. Dalton Trans.
2006, 2767.
(20) (a) Galoppini, E.; Guo, W.; Qu, P.; Meyer, G. J. J. Am. Chem.
Soc. 2001, 123, 4342. (b) Galoppini, E.; Guo, W.; Zhang, W.; Hoertz,
P. G.; Qu, P.; Meyer, G. J. J. Am. Chem. Soc. 2002, 124, 7801. (c) Deng,
X.; Mayeux, A.; Cai, C. J. Org. Chem. 2002, 67, 5279. (d) Deng, X.; Cai,
C. Tetrahedron Lett. 2003, 44, 815. (e) Loewe, R. S.; Ambroise, A.;
Muthukumaran, K.; Padmaja, K.; Lysenko, A. B.; Mathur, G.; Li, Q.;
Bocian, D. F.; Misra, V.; Lindsey, J. S. J. Org. Chem. 2004, 69, 1453. (f)
Yam, C. M.; Cho, J.; Cai, C. Langmuir 2004, 20, 1228. (g) Deluge, M.;
Cai, C. Langmuir 2005, 21, 1917. (h) Thamyongkit, P.; Yu, L.; Padmaja,
K.; Jiao, J.; Bocian, D. F.; Lindsey, J. S. J. Org. Chem. 2006, 71, 1156. (i)
Quintiliani, M.; Kahnt, A.; Vꢀazquez, P.; Guldi, D. M.; Torres, T. J. Mater.
Chem. 2008, 18, 1542.
(21) (a) Zhu, L.; Tang, H.; Harima, Y.; Yamashita, K.; Hirayama, D.;
Aso, Y.; Otsubo, T. Chem. Commun. 2001, 1830. (b) Hirayama, D.;
Takimiya, K.; Aso, Y.; Otsubo, T.; Hasobe, T.; Yamada, H.; Imahori, H.;
Fukuzumi, S.; Sakata, Y. J. Am. Chem. Soc. 2002, 124, 532.
(22) (a) Wei, L.; Padmaja, K.; Youngblood, W. J.; Lysenko, A. B.;
Lindsey, J. S.; Bocian, D. F. J. Org. Chem. 2004, 69, 1461. (b) Wei, L.;
Tiznado, H.; Liu, G.; Padmaja, K.; Lindsey, J. S.; Zaera, F.; Roclan, D. F.
J. Phys. Chem. B 2005, 109, 23963. (c) Sakata, T.; Maruyama, S.; Ueda,
A.; Otsuka, H.; Miyahara, Y. Langmuir 2007, 23, 2269.
(23) Moulder, J. F.; Stickle, W. F.; Sobol, K. D.; Bomben, K. D.;
Chastain King, Jr., J.; Handbook of X-ray Photoelectron Spectroscopy, 2nd
ed.; Physical Electronics, Inc.: Eden Prairie, 1995.
3022
dx.doi.org/10.1021/ja109577f |J. Am. Chem. Soc. 2011, 133, 3014–3022