Oligo(aryleneethynylene)s with terminal pyridyl groups: Synthesis and length dependence of the tunneling-to-hopping transition of single-molecule conductances

Article abstract of DOI:10.1021/cm4029484

The synthesis is reported of a new series of oligo(aryleneethynylene) (OAE) derivatives of up to ca. 6 nm in molecular length (OAE9) using iterative Pd-mediated Sonogashira cross-coupling methodology. The oligo-p- phenyleneethynylene cores of the molecula

Full text of DOI:10.1021/cm4029484

Article
pubs.acs.org/cm
Oligo(aryleneethynylene)s with Terminal Pyridyl Groups: Synthesis
and Length Dependence of the Tunneling-to-Hopping Transition of
Single-Molecule Conductances
Xiaotao Zhao,†,⊥ Cancan Huang,‡,⊥ Murat Gulcur,† Andrei S. Batsanov,† Masoud Baghernejad,‡
Wenjing Hong,‡ Martin R. Bryce,*,† and Thomas Wandlowski*,‡
†Department of Chemistry, Durham University, Durham DH1 3LE, U.K.
‡Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012, Bern, Switzerland
S
* Supporting Information
ABSTRACT: The synthesis is reported of a new series of
oligo(aryleneethynylene) (OAE) derivatives of up to ca. 6 nm
in molecular length (OAE9) using iterative Pd-mediated
Sonogashira cross-coupling methodology. The oligo-p-phenyl-
eneethynylene cores of the molecular wires are functionalized
at both termini with pyridyl units for attachment to gold leads.
The molecular structures determined by single-crystal X-ray
analysis are reported for OAE4, OAE5, OAE7, and OAE8a.
The charge transport characteristics of derivatives OAE3−
OAE9 in single-molecular junctions have been studied using
the mechanically controlled break junction technique. The data demonstrate that the junction conductance decreases with
increasing molecular length. A transition from coherent transport via tunneling to a hopping mechanism is found for OAE wires
longer than ca. 3 nm.
KEYWORDS: molecular electronics, synthesis, oligomer, oligo(aryleneethynylene), single-molecule conductance, tunneling, hopping
tip revealed a transition from tunneling to hopping at a molecular
length of ca. 4 nm in junctions made of hundreds of molecules.25
Single-molecule measurements were not reported in these
studies. Hines et al. reported a transition from tunneling to
hopping at a molecular length between 5.2 and 7.3 nm by means
of STM-BJ measurements (at the single-molecule level) on four
oligofluorene-based molecular wires terminated with thiols.26
Oligo(phenyleneethynylene) (OPE) derivatives are ideal
candidates for studies on the correlation of conductance with
molecular length. They are highly conjugated, rigid, rodlike
molecules whose functional properties can be systematically
tuned by chemical synthesis, which makes them a very important
class of molecules in materials chemistry and molecular
electronics.27,28 Short derivatives, typically OPE3 systems, have
been widely studied in metal−molecule−metal junctions,29−31
but there are very few reports of transport studies through longer
derivatives.32 Lu et al. studied a series of OPEs of lengths ca. 1−5
nm with amino groups at both termini and probed the charge
transport mechanism in monolayers using STM-BJ and CP-AFM
methods. A transition from tunneling to hopping was observed at
a molecular length of ca. 2.75 nm.33 Studies on related derivatives
incorporating a ferrocene unit in the backbone, using the same
contacting techniques, established that ferrocene enhanced the
INTRODUCTION
■
Molecular electronics1−5 is receiving renewed interest due to the
intrinsic limitations of silicon-based electronics.6 Synthetic
chemistry has tailored organic molecules to achieve specific
electronic functions, even at the single-molecule level.7,8
Terminal anchor groups that have an affinity for gold are
attached to the molecules to achieve the in situ assembly of
metal−molecule−metal junctions.9−11 The electrical conduc-
tance of these nanoscale junctions is characterized using a range
of techniques, notably, scanning tunneling microscopy
(STM)12,13 conductive-probe atomic force microscopy (CP-
AFM),14,15 scanning tunneling microscopy break junctions
(STM-BJs),16−18 and mechanically controlled break junctions
(MCBJs).19−21
Studies that aim to understand how charge transport
mechanisms vary with changing the length of the molecular
wire22,23 have been very limited due to the challenges posed by
the synthesis, rigorous purification, and measurement of long
molecules anchored between two electrodes. Two distinct charge
transport mechanisms have been predicted theoretically and
identified experimentally: coherent transport via tunneling or
superexchange that dominates in short molecules, and
incoherent thermally activated hopping in long molecules.24
Choi et al. reported a series of oligophenyleneimine (OPI)
molecules of length ca. 1.5−7 nm with an anchored thiol unit at
one terminus and a phenyl unit at the other terminus. Contacting
a monolayer of the molecules assembled on gold with a CP-AFM
Received: July 24, 2013
Revised: October 7, 2013
Published: October 25, 2013
© 2013 American Chemical Society
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Chart 1. Structures of the OAE Derivatives Studied in This Work
a
Scheme 1. Synthesis of OAE4 and OAE9
a
Reagents and conditions: (i) 2-methyl-3-butyn-2-ol, [PdCl2(PPh3)2], CuI, (i-Pr)2NH, 20 h, 20 °C, 95% yield; (ii) NaOH, toluene, 6 h, 70 °C, 65%
yield; (iii) 4, [Pd(PPh3)4], CuI, THF, Et3N, 18 h, 50 °C, 95% yield; (iv) NaOH, toluene, 22 h, 100 °C, 36% yield; (v) 7, [Pd(PPh3)4], CuI, THF,
Et3N, 23 h, 20 °C, 88% yield; (vi) 8, [Pd(PPh3)4], CuI, THF/Et3N, 18 h, 20 °C, 60% yield; (vii) 1, [Pd(PPh3)4], CuI, THF, Et3N, 15 h, 20 °C, 21%
yield.
conductance in both the tunneling and the hopping regimes.34
Single-molecule measurements were not reported in these
studies.33,34
Terminal 4-pyridyl (PY) units anchor molecules effectively to
gold substrates, and unlike thiols,35,36 they are stable under
ambient conditions and no protecting group is needed.37−40 We
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a
Scheme 2. Synthesis of OAE5
a
Reagents and conditions: (i) NaOH, toluene, 12 h, 100 °C, 82% yield; (ii) 3, [Pd(PPh3)4], CuI, THF, Et3N, 18 h, 20 °C, 47% yield; (iii) NaOH,
toluene, 23 h, 100 °C, 70% yield; (iv) 7, [Pd(PPh3)4], CuI, THF, Et3N, 20 h, 20 °C, 68% yield.
Table 1. Characteristic Parameters of the UV/vis Absorption and Transport Measurements
a
b
c
d
e
f
g
molecule
λmax/nm
L/nm
G*/G0 1D
G/G0 2D
Δz*
z*
JFP/%
OAE3a
OAE3b
OAE4
OAE5
OAE6
OAE7
OAE8b
OAE9
319
1.6
1.6
2.3
3.0
3.7
4.4
5.1
5.8
10−4.5
10−4.5
10−5.5
10−6.7
10−6.8
10−6.9
10−6.8
10−6.9
10−5.0−10−4.0
10−5.0−10−4.0
10−6.0−10−5.0
10−7.0−10−6.2
10−7.3−10−6
10−7.0−10−6.5
10−7.2−10−6
10−7.5−10−6
1.08
1.00
1.88
1.92
2.22
2.31
1.78
1.36
1.58
1.50
2.38
2.42
2.72
2.81
2.28
1.86
100
100
100
71
306, 372
320, 401
324, 417
331, 409
339, 428
335, 435
335, 439
18
18
20
17
a
b
Maxima of the characteristic UV/vis absorption in CH2Cl2 solution; Molecular length L, which is defined as the distance between the center of the
nitrogen anchor atom at one end of a fully extended isolated molecule to the center of the anchor atom at the other end. The lengths of the
molecules were calculated by ACD/ChemSketch and are in very close agreement with the N···N distances obtained by single-crystal X-ray diffraction
c
for OAE4, OAE5, OAE7 and OAE8a (see the Supporting Information). Most probable molecular junction conductance as estimates from Gaussian
d
fits to the experimentally obtained 1D conductance histograms. Conductance range of molecular junctions extracted from the 2D conductance vs
e
f
relative displacement histograms. Δz* as most probable relative junction elongation (displacement). z* = Δz* + 0.5 as absolute junction
g
elongation (displacement). JFP is the junction formation probability.
have shown that, for tolane derivatives, pyridyl exhibits excellent
anchoring performance with high conductance and 100% of
molecular junctions formed.41 Therefore, pyridyl units are used
as anchors in the present series of molecules, in preference to
amine or thiol analogues.
could be easily purified, thereby facilitating the final step to the
target OAE derivatives. For the derivatives OAE4−OAE9,
hexyloxy substituents are attached to the inner phenyl rings to
give the molecules good solubility in organic solvents. Two
OAE3 derivatives were studied (OAE3a and OAE3b) to confirm
that alkoxy substituents do not affect the conductance behavior,
as was reported previously for thiol-terminated OPE3 ana-
logues.30 Two OAE8 derivatives were synthesized: OAE8a has
only very limited solubility, and although an X-ray crystal
structure was obtained, it could not be purified to the required
level for conductance measurements. The analog OAE8b with
additional hexyloxy side chains in the backbone has improved
solubility and was fully characterized. A key aspect of our strategy
is the use of 2-hydroxypropyl protecting groups for the
intermediate terminal alkynes, e.g., compounds 2 and 5 (Scheme
1). A benefit of this protecting group (compared to trialkylsilyl,
which is more commonly used)28 is that its high polarity
facilitates the separation of any unreacted starting material or
byproducts that do not contain this unit. Deprotection with loss
of acetone is easily accomplished using sodium hydroxide in
toluene.43 A notable feature of the deprotection reactions is that,
for bis-protected derivatives 2 and 5, the major product can be
exclusively monodeprotected (2 → 3) or bis-deprotected (5 →
6) depending on the temperature and the length of time of the
reaction. As representatives of the series, the syntheses of OAE4
and OAE9 are shown in Scheme 1, and OAE5 in Scheme 2. The
syntheses of the other OAEs are described in the Supporting
The motivation for this work is to study the length dependence
of the single-molecule conductance of oligoaryleneethynylene
(OAE) derivatives comprising OPE cores terminated at both
ends with pyridyl groups. This study is clearly distinct from
previous work on OPI25 and OAE33,34 derivatives that concerned
monolayers comprising hundreds of molecules in closely packed
arrangements. The benefit of single-molecule studies is that
artifacts arising from intermolecular interactions (e.g., π−π
stacking) that are well-known in OAE derivatives27 are absent.
We describe the synthesis and characterization of derivatives
OAE3−OAE9 of molecular length (i.e., intramolecular N···N
distance) ca. 2−6 nm. The charge transport characteristics of
these OAE derivatives in single-molecular junctions have been
studied using the MCBJ technique in solution and under
environmentally controlled conditions.
RESULTS AND DISCUSSION
■
Synthesis. The OAE derivatives studied in this work are
shown in Chart 1. The synthetic strategy uses iterative Pd-
catalyzed Sonogashira cross-coupling reactions42 of new, linearly
conjugated phenyleneethynylene building blocks. The retro-
synthetic disconnections of the molecules are based on (i)
commercial availability of starting materials, (ii) high yielding
reactions, (iii) and the synthesis of intermediate reagents that
1
Information. All the OAE derivatives were characterized by H
NMR, 13C NMR, and mass spectra that confirmed their expected
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structures and established their high level of purity. Single-crystal
X-ray structures obtained for OAE4, OAE5, OAE7, and OAE8a
give the molecular lengths and solid-state conformations of the
molecules. UV−visible absorption spectra in solution (Table 1)
show that, for OAE3-OAE9, there is extended conjugation
throughout the backbone. The saturation length has not been
reached in the present series. These crystallographic and
spectroscopic characterization data are discussed in detail in
the Supporting Information.
Conductance Measurements. The charge transport
characteristics of the derivatives OAE3−OAE9 in single-
molecular junctions were investigated using the mechanically
controlled break junction (MCBJ) technique. Key parameters
are listed in Table 1. Molecular junctions were formed by
opening and closing a nanogap between two gold electrodes in a
solution of 1,3,5-trimethylbenzene (TMB) and tetrahydrofuran
(THF), 4:1 (v/v), containing typically 0.1 mM of the target
molecules. Details of the experimental setup were described in
our previous publications,41,44 and are briefly summarized in the
Experimental Section.
process is reflected in additional features in the log(G/G0) vs Δz
stretching traces. The examples plotted in Figure 1 show well-
developed plateaus at G < 10−5 G0. These plateaus represent
stable molecular junctions “electrode−OAE−electrode”. We
notice that these traces show a distinct monotonic decrease in the
conductance with stretching distance. Upon further pulling, a
second decrease in conductance occurs, which is assigned to the
rupture of the contact between the molecular wire and the gold
leads.31,46 The conductance finally reaches the noise level of G ≤
10−8 G0 upon the complete breaking of the molecular junction.
We note that the plateau length increases with molecular length
L, as illustrated in Figure 1 for OAE4 and OAE7 as typical
examples. We also observed that rupture of the molecular
junction is less steep for junctions formed with the longer OAE
molecules. The overall shape of the stretching traces as recorded
for each molecule is qualitatively similar, but varies in detail.
Therefore, a careful statistical analysis of a large number of
individual traces is required to extract representative results.
Several thousands of individual conductance versus relative
displacement traces (G vs Δz) were recorded and analyzed
further by constructing all-data-points histograms without any
data selection to extract statistically significant results from the
different junction configurations. Figure 2 displays the
Figure 1 displays typical conductance (G) versus distance
(Δz) stretching traces, as plotted in a semilogarithmic scale, and
Figure 2. All-data-points 1D conductance histograms of OAE
derivatives as constructed from 2000 individual conductance vs relative
distance traces as plotted in Figure 3.
Figure 1. Typical conductance vs distances traces recorded in TMB/
THF in the absence (black lines) and in the presence of 0.1 mM OAE4
(blue lines) or OAE7 (red lines) in an MCBJ setup at 0.10 V bias
voltage, stretching rate of 2 nm s−1.
corresponding one-dimensional (1D) histograms of eight
OAEs in a semilogarithmic scale as constructed from 2000
experimental conductance versus distance traces for each
compound. The sharp peaks around 1 G0 represent the
conductance of a single atom gold−gold contact. The prominent
peaks between 10−7 G0 < G/G0 < 10−4 G0 represent molecular
features. The 1D histograms reveal one pronounced molecular
feature for each OAE derivative. The conductance peaks at G <
10−7.5 G0, which were also observed in OAE-free solutions, are
caused by instrumental noise.41,44 The statistically most probable
conductances of molecular junctions formed by the respective
OAEs as trapped between two gold contacts were obtained by
fitting Gaussians to the characteristic maxima in the conductance
histograms. The results are compiled in Table 1. The data
presented in Figures 2 and 3 are summarized in Figure 4. We note
that rather similar conductance values were found for molecular
concentrations ranging between 10−6 M up to 10−4 M, which
supports their assignment to single-molecular junction con-
ductances. The single junction conductances decrease with
increasing molecular length L from 10−4.5 G0 for OAE3 to 10−6.7
G0 for OAE5, by 3 orders of magnitude. However, this trend
levels off for longer molecular wires. The junction conductances
of OAE5 to OAE9 reveal rather small changes spanning from
10−6.7 to 10−6.9 G0, respectively. We also note that the width of
the molecular conductance features increases with molecular
recorded for 0.1 mM OAE4 or OAE7 in TMB/THF using the
MCBJ technique. For reference, we also plotted two traces (black
curves) representing the OAE-free solution, which reveal
classical tunneling characteristics, i.e., an exponential decrease
of the conductance upon junction elongation. The traces
recorded for OAE4 and OAE7, which are characteristic
representatives of the studied pyridyl-terminated OAE family,
are more complex. After the formation of the contact between
the two gold leads, the nanoscale constriction was stretched with
a typical rate of 2 nm s−1. All curves show initially a steplike
decrease of the conductance from 10 G0 up to 1 G0, with G0 =
2e2/h being the fundamental quantum conductance. Opening the
gap results in an elongation of the gold−gold junction and
decreases the number of gold atoms in the constriction, which
causes the conductance to change up to 1 G0, where the contact
between the two gold leads in the MCBJ consists of only one gold
atom. Subsequently, an abrupt decrease of the conductance over
several orders of magnitude up to 10−5 G0 occurs, which is
assigned to the “jump out of atomic contact”.45 The gold−gold
contact breaks, and OAE molecules from the solution or already
adsorbed at one of the two metal leads bridge the gap. This
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length, which most probably reflects the increasing number of
molecular conformations in the nanoscale junctions.
The above analysis was extended by constructing two-
dimensional (2D) conductance vs relative displacement Δz
histograms. Figure 3A,B shows typical results for OAE4 and
normal distribution with a well-defined maximum. No stretching
traces shorter than 1.2 nm were observed, indicating that no
significant contribution due to direct tunneling (T) through the
solution exists. Therefore, we conclude that the molecular
junction formation probability of OAE4 approaches 100%. The
single maximum in Figure 3C represents the most probable
relative characteristic stretching distance Δz* = 1.88 nm, which
may be considered as a measure of the most probable
characteristic plateau length of an OAE4-type molecular
junction.41 The most probable absolute displacement z* in an
experimental molecular junction formed between two gold tips
and OAE4 is obtained by adding the snap-back distance Δzcorr to
the relative displacement: z* = Δz* + Δzcorr. Referring to our
previous work with pyridyl-terminated rigid rodlike tolanes, we
used Δzcorr = (0.5 0.1) nm,41 which leads to z* = 2.38 nm for
OAE4. (This value is close to the molecular length L of OAE4.)
Similar trends were found with OAE3 (c.f. data in Table 1).
The 1D displacement histograms of the longer OAEs are more
complex. Figure 3D illustrates data of OAE7 as an example. In
addition to a well-developed molecular feature with a maximum
at Δz* = 2.31 nm, which gives z* = 2.81 nm as the most probable
absolute displacement, we observed a second peak with a
maximum around 1.2 nm. This peak originates from an
increasing number of individual conductances versus distance
traces without the formation of a molecular junction. In other
words, it represents contributions from direct tunneling. The
area ratio between the molecular contribution and the total data
density in the histogram plotted in Figure 3D leads to a junction
formation probability of 18% for OAE7. A similar analysis of our
experiments with OAE6 to OAE9 reveals the following trends:
(i) The molecular junctions break prior to a fully extended
configuration; (ii) the molecular features in the 1D displacement
histograms broaden; and (iii) the junction formation probability
decreases considerably with increasing molecular length L. These
observations demonstrate that the larger number of molecular
conformations and/or the increasing number of hexyloxy side-
chains of the longer OAEs appear to hamper the formation of
stable (single) molecular junctions. Changes in the molecular
structure near the pyridyl binding site may play a role. For
OAE3−OAE5, the phenyl rings adjacent to the pyridyls are
disubstituted with alkoxy groups, whereas for OAE6−OAE9
these rings are unsubstituted. The latter might strengthen the π-
interaction of the molecule with the gold substrate, and therefore,
an additional energy might be needed to lift the respective OAEs
from the substrate surface. This extra energy threshold might not
always be reached so that the percentage of stretching events that
are unsuccessful (no molecular junctions being formed) is larger,
which leads to a lower junction formation probability. This
process is currently being studied by molecular dynamics
simulations in combination with electromechanical force−
distance and conductance−distance measurements. However,
this topic is beyond the scope of this article.
Figure 3. (A, B) 2D conductance−relative displacement histograms
from MCBJ-BJ experiments for OAE4 (A) and OAE7 (B). (C, D)
Relative displacement (Δz) distribution from MCBJ-BJ experiments for
OAE4 (C) and OAE7 (D). The relative displacement histograms are
obtained from conductance traces between 0.1 G0 and 10−7 G0. The
envelope traces represent Gaussian fits to the molecular junction
contribution and to direct tunneling, labeled as M and T, respectively.
OAE7. The corresponding graphs of the other molecules are
summarized in the Supporting Information. The 2D con-
ductance histograms were obtained as follows.41,47,48 First, we
normalized all individual conductance traces to a common
distance scale by assigning the relative displacement Δz = 0 at G
= 0.7 G0. This procedure is justified by the sharp drop in
conductance around 1 G0. The conductance versus relative
displacement histogram was then constructed by counting the
occurrences of [log(G/G0), Δz] pairs in a 2D field. The 2D
histograms of OAE4 and OAE7 show features of gold quantum
contacts around G ≥ 1 G0 and a second cloudlike pattern in [0.5
nm < Δz < 1.8 nm, 10−6.2 G0 < G < 10 −4.8 G0] centered at G =
10−5.5 G0 (OAE4) and in [0.5 nm < Δz < 2.2 nm, 10−7.0 G0 < G <
10 −6.5 G0] centered at G = 10−6.9 G0 (OAE7). We attribute the
latter to the formation of single-molecule junctions. The
corresponding characteristic conductance data are rather close
to the positions of conductance peaks in the 1D histograms.
Similar conclusions were also obtained for the other OAE
derivatives (see the Supporting Information). We also note that
all 2D conductance histograms reveal a small, but distinct,
decrease of the single junction conductance G with increasing
displacement Δz. The corresponding intervals are given in Table
1 for all the compounds investigated. Finally, we comment that
the data cloud below 10−7.5 G0 represents the noise level.
Figure 4 displays the dependence of the most probable single
junction conductances of the pyridyl-terminated OAE family on
molecular length L. We found two distinctly different regions.
For the shorter molecules, from OAE3 to OAE5, the
conductance decreases exponentially according to eq 123,31
Analyzing the evolution of a molecular junction upon
stretching provides additional information about the stability
and junction formation probability. We constructed relative
displacement (Δz) histograms41 by calculating the displacement
from the relative zero position at 0.7 G0 to the end of the
molecular conductance region, just before the onset of the noise
level. Figure 3C,D displays characteristic displacement histo-
grams of OAE4 and OAE7. The plot of OAE4 reveals a uniform
G = Gce−βL
(1)
with an attenuation constant β = (3.3 0.1) nm−1 and a contact
conductance per pyridyl terminating group, Gc = (0.54 0.07)
μS at L = 0, which leads to a contact resistance Rc = 1/Gc = (1850
232) kΩ. Similar values of β were reported for other rigid-
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which involve contacting hundreds of molecules within self-
assembled monolayers, revealed the transition from tunneling to
hopping at a molecular length of ca. 2.75 nm for a series of amine-
terminated OPEs33 and ca. 4 nm for thiol-anchored oligophe-
nyleneimines.25 Single-molecule conductance experiments
employing the STM-BJ techniques reported characteristic
transition lengths of 5.6−6.8 nm for polythiophenes55 and
5.2−7.3 nm for oligofluorene-based molecular wires,26 both
terminated with thiol anchors. We note that, for the
oligofluorene series, the experiments are very limited within
the length range investigated.26 This comparison demonstrates
convincingly that the transition length from tunneling to
transport controlled by hopping depends critically on the
structure of the molecular backbone. However, the existing
limited database of single-molecular junction studies does not yet
allow general trends to be extracted. Comparative studies with
well-defined molecular wires, not only of known and tunable
length but, in particular, of molecular conformation, are essential
to achieve this goal.
Figure 4. Semilogarithmic plot of the most probable single-molecule
junction conductances (individual data points as obtained from the
analysis of the 1D conductance histograms) and of the conductance
range (bare symbols as extracted from the analysis of the 2D
conductance histograms) in units of log(G/G0) versus the molecular
length for a family of pyridyl-terminated OAE derivatives. The inset
represents the junction formation probability (JFP) dependent on the
molecular length L. All numerical data are compiled in Table 1
CONCLUSIONS
rodlike single molecular wires or assembled monolayers, such as
dithiol-terminated OPEs (3.4 0.1 nm−1),31 oligophenylenei-
mines (OPI, β = 3.0 nm−1),25 oligophenylenedithiols (PP, β =
(3.5−5.0) nm−1),49,50 or pyridyl-terminated oligoynes (OY, β =
(3.1 0.4) nm−1).51 Smaller values of β were reported for OPEs
with amine anchoring groups (OPE, β = 2.0 nm−1),33
■
We have presented the synthesis of a family of new linear OAE
derivatives with terminal pyridyl anchor groups of molecular
length ca. 2−6 nm. The charge transport properties of these
molecular wires were characterized by MCBJ experiments.
Single-molecule junction conductance measurements reveal a
transition from coherent transport via tunneling, which
dominates in shorter molecules, to incoherent hopping in longer
molecules. The transition occurs at a molecular length L ∼ 3.0
nm and for conductances below 10−6.5 G0. Increasing the
molecular length L leads also to a reduction of the probability for
molecular junction formation from 100% to values below 20%.
This trend is attributed to the larger number of molecular
conformations and the increasing number of hexyloxy side-
chains, which appear to hamper the formation of stable OAE-
type (single) molecular junctions. A comparison of our data with
literature data for other oligomer systems establishes that the
details of the tunneling-to-hopping transition depend on the
nature of the molecular wire backbone as well as its coupling to
the respective anchoring sites. This work represents the first
studies on length dependence of conductance of OAE derivatives
at the single-molecule level. However, unambiguous and more
general correlations between structure and transport character-
istics cannot yet be established due to the limited number of
reliable single junction conductance studies with long molecular
wires of well-defined structure, in particular, molecular
conformation. This topic comprises a major challenge for future
research in synthetic and materials chemistry.
oligoflourenes (OF, β = (2.06
0.09 nm−1)),26 oligopheny-
lene-vinylenes (OPV, β = (1.7−1.8) nm−1),46,52 oligothiophenes
(OT, β = ∼1 nm−1),53 and for metal-porphyrin containing
molecular wires (P, β ≪ 0.1 nm−1).54
The contact resistance Rc = 1850 kΩ per pyridyl anchoring
group for the family of OAE wires studied in this work is larger
than Rc = 205 kΩ, which is the corresponding value for pyridyl-
terminated oligoynes.51 A similar trend is observed with the
dithiol-terminated OPEs (40 kΩ)31 and oligoynes (∼3.2 kΩ).51
These data demonstrate that the values are not only determined
by the molecular anchoring site but also by the coupling of the
anchor group to the wire backbone.
Comparing the results of the single-molecule conductance
measurements for the short OAEs OAE3−OAE5 with the
literature data above, and, in particular, with results reported for
pyridyl-terminated oligoynes51 and thiol-terminated OPEs,31 as
well as the excellent fit of eq 1 to the experimental data,
demonstrates that electron transport across the OAE3−OAE5
wires is controlled by coherent transport via tunneling or
superexchange.
A distinctly different behavior is found for the longer
derivatives OAE6−OAE9. The most probable single junction
conductances are still exponentially dependent on molecular
length L. However, the fit of eq 1 to the experimental data gives
EXPERIMENTAL SECTION
Characterization Data for the OAE Derivatives. OAE3a. This
compound was synthesized following the literature route, and the
characterization data are in agreement with those reported.40
■
an attenuation constant β = (0.16
0.08) nm−1, which is
significantly smaller as compared to the β = (3.3 0.01) nm−1,
the values extracted from the analysis of the shorter OAEs.
Similar small β values were reported for longer amine-terminated
OPEs (β ∼ 0.3 nm−1),33 thiol-terminated oligofluorenes (β ≪
0.1 nm−1),26 and longer oligophenylenimines (β ∼ 0.9 nm−1).23
Comparing these results suggests that transport through the
longer pyridyl-terminated OAEs is characterized by a thermally
activated hopping mechanism.24 Transition from tunneling to
hopping was found at a molecular length of ∼3 nm for the
present system.
1
OAE3b. mp 189.1−190.2 °C. H NMR (400 MHz, CDCl3) δ 8.63
(dd, J = 4.5, 1.6 Hz, 4H), 7.44 (dd, J = 4.5, 1.6 Hz, 4H), 7.07 (s, 2H), 3.94
(s, 6H); 13C NMR (CDCl3, 101 MHz) 154.57, 150.04, 131.49, 125.72,
116.19, 113.63, 92.71, 90.21, 56.78; HR-MS (ASAP+) m/z Calcd for
C22H16N2O2 ([M]+ 340.1212, found m/z: [M]+ 340.1229.
OAE4. mp 107.0−107.5 °C. 1H NMR (700 MHz, CDCl3) δ 8.60 (d, J
= 5.6 Hz, 4H), 7.37 (d, J = 5.6 Hz, 4H), 7.02 (s, 2H), 7.01 (s, 2H), 4.03
(m, 8H), 1.84 (m, 8H), 1.52 (m, 8H), 1.34 (m, 16H), 0.89 (m, 12H);
13C NMR (151 MHz, CDCl3) δ 153.98, 153.45, 149.61, 131.69, 125.41,
117.14, 116.96, 115.33, 112.66, 91.94, 91.75, 90.80, 69.74, 69.51, 31.58,
31.54, 29.25, 29.23, 25.71, 25.64, 22.62, 14.01, 13.99; HR-MS (ASAP+)
Single-molecule studies of length dependence have not been
reported previously for OAE systems. CP-AFM experiments,
4345
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Chemistry of Materials
Article
m/z Calcd for C52H65N2O4 [M + H]+ 781.4944, found m/z: [M + H]+
781.4937.
repeated more than 2000 times to obtain statistically relevant data. In the
MCBJ setup, the current could be recorded as a feedback signal at a
given bias voltage (typically between 0.020 and 0.200 V). The two ends
of the “broken wire” were taken as working electrodes WE1 and WE2.
The MCBJ unit is controlled by a lab-built bipotentiostat with two
bipolar tunable logarithmic I − V converters as current measuring units,
which are operated by a custom-designed microcontroller. The system
provides three analog signals: the potential of WE1, the voltage
difference between the two working electrodes WE1 and WE2 (bias
voltage Vbias), driving the current through the two gold electrodes for the
conductance measurements, and the voltage output of the piezo stack in
the range of 0−50 V, allowing the displacement of the piezo stack up to 8
μm with rates ranging from 3 to 3000 nm s−1. The latter translates into
lateral pulling (pushing) rates between the two gold leads of 0.1−100
nm s−1. The distance between the two gold electrodes in the MCBJ
setup was calibrated with complementary STM-BJ experiments
assuming that the tunneling decay is identical under the same
experimental conditions. Further technical details of the MCBJ setup
are reported by Hong et al. in refs 41 and 44.
OAE5. mp 127.2−128.2 °C. 1H NMR (700 MHz, CDCl3) δ 8.59 (d, J
= 5.7 Hz, 4H), 7.36 (d, J = 5.7 Hz, 4H), (7.01 (m, 6H), 4.02 (m, 12H),
1.84 (m, 12H), 1.51 (m, 12H), 1.33 (m, 24H), 0.88 (m, 18H); 13C NMR
(176 MHz, CDCl3) δ 153.99, 153.53, 153.41, 149.63, 131.67, 125.40,
117.24, 117.18, 116.95, 115.54, 114.27, 112.49, 92.05, 91.87, 91.31,
90.81, 69.76, 69.66, 69.50, 31.59, 31.58, 31.54, 29.27, 29.24, 29.23, 25.70,
25.66, 25.64, 22.61, 14.00, 13.98; HR-MS (ASAP+) m/z Calcd for
C72H93N2O6 [M + H]+ 1081.7034, found m/z: [M + H]+ 1081.7070.
OAE6. mp 214.2−215.2 °C. 1H NMR (600 MHz, CDCl3) δ 8.62 (bs,
4H), 7.52 (bs, 8H), 7.39 (d, J = 4.2 Hz, 4H), 7.01 (s, 2H), 7.00 (s, 2H),
4.03 (m, 8H), 1.85 (m, 8H), 1.53 (m, 8H), 1.34 (m, 16H), 0.89 (m,
12H); 13C NMR (151 MHz, CDCl3) δ 153.72, 153.49, 149.53, 131.78,
131.54, 131.43, 125.58, 124.44, 121.64, 117.02 (2 overlapping peaks),
114.68, 113.51, 94.30, 93.85, 91.64, 88.61, 88.30, 69.72, 69.55, 31.60,
31.59, 29.30, 29.26, 25.74, 25.65, 22.62, 14.02; HR-MS (ASAP+) m/z
Calcd for C68H73N2O4 [M + H]+ 981.5570, found m/z: [M + H]+
981.5566.
OAE7. mp 203.5−204.2 °C. 1H NMR (600 MHz, CDCl3) δ 8.61 (d, J
= 6.0 Hz, 4H), 7.52 (s, 8H), 7.39 (d, J = 6.0 Hz, 4H), 7.01 (m, 3H), 4.03
(m, 12H), 1.84 (m, 12H), 1.52 (m, 12H), 1.34 (m, 24H), 0.89 (m,
18H); 13C NMR (151 MHz, CDCl3) δ 153.73, 153.50, 153.47, 149.49,
131.79, 131.54, 131.50, 125.55, 124.47, 121.61, 117.23, 117.04, 117.01,
114.78, 114.28, 113.42, 94.26, 93.92, 91.78, 91.45, 88.64, 88.27, 69.73,
69.67, 69.54, 31.61, 31.59, 29.30, 29.29, 29.26, 25.75, 25.67, 25.65, 22.62,
14.02; HR-MS (ASAP+) m/z Calcd for C88H101N2O6 [M + H]+
1281.7700, found m/z: [M]+ 1281.7615.
ASSOCIATED CONTENT
* Supporting Information
Details of molecular synthesis and characterization, X-ray
crystallographic data (CCDC 951746−951749), UV−vis
absorption spectra, and single-junction conductance measure-
ments (PDF). This material is available free of charge via the
Internet at http://pubs.acs.org.
■
S
OAE8b. mp 199.5−200.6 °C. 1H NMR (600 MHz, CDCl3) δ 8.61 (d,
J = 5.2 Hz, 4H), 7.53 (s, 8H), 7.38 (d, J = 5.2 Hz, 4H), 7.02 (m, 8H), 4.04
(m, 16H), 1.86 (m, 16H), 1.53 (m, 16H), 1.35 (m, 32H), 0.89 (m,
24H); 13C NMR (151 MHz, CDCl3) δ 153.74, 153.52, 153.50, 153.48,
149.72, 131.78, 131.54, 131.26, 125.48, 124.44, 121.67, 117.26, 117.24,
117.06, 117.02, 114.81, 114.40, 114.21, 113.42, 94.28, 93.68, 91.83,
91.61, 91.44, 88.65, 88.34, 69.74, 69.68, 69.66, 69.54, 31.62, 31.61, 31.60,
29.32, 29.30, 29.27, 25.76, 25.68, 25.57, 22.64,14.03; HR-MS (ASAP+)
m/z calcd for C108H128N2O8 [M]+ 1580.9671, found m/z: [M]+
1580.9725.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: m.r.bryce@durham.ac.uk.
*E-mail: Thomas.wandlowski@dcb.unibe.ch.
■
Author Contributions
⊥These authors contributed equally.
Notes
The authors declare no competing financial interest.
OAE9. mp 202.1−203.5 °C. 1H NMR (400 MHz, CDCl3) δ 8.62 (d, J
= 6.0 Hz, 4H), 7.54 (bs, 8H), 7.40 (d, J = 6.0 Hz, 4H), 7.02 (m, 10H),
4.03 (m, 20H), 1.86 (m, 20H), 1.53 (m, 20H), 1.34 (m, 40H), 0.89 (m,
30H); 13C NMR (101 MHz, CDCl3) δ 153.95, 153.74, 153.71 (two
overlapping peaks), 153.69, 149.91, 132.04, 131.80, 131.59, 125.77,
124.68, 121.89, 117.98, 117.44, 117.22, 115.01, 114.79, 114.61, 114.50,
114.38, 113.61, 112.82, 94.52, 94.01, 92.07, 91.87, 91.80, 91.66, 88.88,
88.56, 69.95, 69.88, 69.75, 31.87, 31.85, 31.78, 29.53, 29.50, 26.00, 25.92,
25.86, 22.89, 14.29; MS (ASAP+) m/z: 1883.1 ([M + H]+, 100%).
Mechanically Controlled Break Junction (MCBJ) Measure-
ments. The transport characteristics in single-molecule junctions were
studied by MCBJ measurements in solution at room temperature. The
latter contained typically 0.1 mM of the OAE-type molecules in a
mixture of 1,3,5-trimethylbenzene (TMB; Aldrich, p.a.) and tetrahy-
drofuran (THF; Aldrich, p.a), 4:1 (v/v).
ACKNOWLEDGMENTS
■
X.Z. thanks the China Scholarship Council for a studentship
award. The work was funded by The European Commission
(EC) FP7 ITN “FUNMOLS” Project No. 212942 and by the
Swiss National Science Foundation under contract
200020_144471/1.
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