Observation of electrochemically controlled quantum interference in a single anthraquinone-based norbornylogous bridge molecule

Article abstract of DOI:10.1002/anie.201107765

A single-molecular switch based on the anthraquinone/hydroanthraquinone redox reaction is reported (see picture). A single norbornyl anthraquinone unit can be switched between a low-conducting and a high-conducting form using electrochemical gating. The potential range, upon which the conductance enhancement is observed, can be varied using different pH values of the electrolyte. Copyright

Full text of DOI:10.1002/anie.201107765

Angewandte
Chemie
DOI: 10.1002/anie.201107765
Single-Molecule Switches
Observation of Electrochemically Controlled Quantum Interference in
a Single Anthraquinone-Based Norbornylogous Bridge Molecule**
Nadim Darwish, Ismael Dꢀez-Pꢁrez, Paulo Da Silva, Nongjian Tao,* J. Justin Gooding,* and
Michael N. Paddon-Row*
There is considerable ongoing interest in understanding the
electrical properties of single molecules both from a funda-
mental point of view and for potential applications in single-
molecule technologies.[1–4] An important goal in molecular
electronics is the ability to switch, by means of electro-
chemical gating, the conductance through a single molecule
and, in this context, the anthraquinone/hydroanthraquinone,
AQ/H2AQ, redox couple has been proposed as a suitable
candidate for study.[5] Indeed, calculations[6] predict that
electrochemical gating of conductance in AQ-based molec-
ular switches should be strong, with conductance on(H2AQ)/
off(AQ) ratios of several orders of magnitude. The switching
mechanism is due to the presence of destructive quantum
interference (QI) between various conductance channels in
the cross-conjugated AQ, which is absent in the linear-
conjugated H2AQ, thereby resulting in lower conductance in
AQ, compared to H2AQ. Recently, Fracasso et al.[7] have
experimentally confirmed the operation of QI in bulk
conductance studies of self-assembled monolayers (SAMs)
of arylethynylene thiolates (aryl = anthracene, AQ, 9,10-
dihydroanthracene).[7]
We now report the first experimental evidence for the
operation of electrochemically controlled QI in a novel AQ-
based norbornylogous bridge tetrathiol, 5AQ5 (Scheme 1),
from single-molecule conductance measurements using the
scanning tunneling microscopy (STM) break junction tech-
nique.[8] We show that the AQ moiety in 5AQ5 can be
electrochemically and reversibly switched in situ between the
Scheme 1. Molecular structure of the compounds used in this study.
The 5AQ5 molecule possesses five bonds on each side and an AQ
moiety in the center. 8AQ8 possesses eight bonds on each side and an
AQ moiety in the center. The detailed experimental procedures for the
synthesis of compounds 5AQ5 and 8AQ8 along with analytical and
spectral information can be found in the Supporting Information.
high-conducting H2AQ form and the low-conducting AQ
system. Further, we demonstrate that the potential range of
the conductance enhancement can be shifted using different
pH values. This pH dependency of the AQ/H2AQ redox
reaction constitutes an extra degree of freedom that can
control single-molecule conductivity.
A key design feature of 5AQ5 is the cementing of the AQ
group into a rigid, structurally well-defined norbornylogous
(NB) unit bearing two pairs of thiol groups at each end,
thereby conferring additional stability to SAMs derived
therefrom. The 19.8 ꢀ length of 5AQ5 is much greater than
the gate thickness, that is, the electrochemical double layer
that relates to the diameter of the ions used in the electrolyte,
thereby ensuring that the field screening effect due to the
proximity of the source and drain electrodes is negligible.[9]
Norbornylogous bridges have played pivotal roles in
investigating many fundamental aspects of electron-transfer
(ET) processes,[10,11] including those involving SAMs derived
therefrom.[12–16] In particular, NB bridges are very efficient
mediators of ET by the superexchange mechanism and it was
hoped that the NB bridge would likewise facilitate coherent
charge transport in 5AQ5, which is a prerequisite for QI to be
operative. This issue was first investigated by determining the
magnitude and distance dependence of the single-molecule
conductivity in 5AQ5 and its longer cognate, 8AQ8. X-ray
photoelectron spectroscopy (XPS) and STM studies on SAMs
formed from 5AQ5 and 8AQ8 on gold surfaces confirmed
that the 5AQ5 and 8AQ8 molecules stand upright on the gold
surface, anchored by a pair of thiolates at one end and a pair
[*] Dr. N. Darwish, Dr. P. Da Silva, Prof. Dr. J. J. Gooding,
Prof. Dr. M. N. Paddon-Row
School of Chemistry, The University of New South Wales
Sydney, NSW, 2052 (Australia)
E-mail: justin.gooding@unsw.edu.au
m.paddonrow@unsw.edu.au
Dr. I. Dꢀez-Pꢁrez, Prof. Dr. N. Tao
Center for Bioelectronics and Biosensors
Biodesign Institute, Arizona State University
Tempe, AZ 6206 (USA)
E-mail: njtao@asu.edu
Dr. I. Dꢀez-Pꢁrez
Department of Physical Chemistry, University of Barcelona
Barcelona 08028 (Spain)
[**] J.J.G. and M.P.-R. thank the Australian research council for support.
Dr. Erwann Luais is acknowledged for the X-ray photoelectron
spectroscopy experiments. Dr. Mohan Bhadbhade and Australian
Synchrotron are acknowledged for their technical support. N.T.
thanks the National Science Foundation (CHE-1105588) for
support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201107765.
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of free thiols at the distal end that is easily accessible
to the gold STM tip (see the Supporting Informa-
tion). Single-molecule conductance measurements
were determined using the STM break junction
method with a two-electrode setup.[8] Conductance
histograms were built using several hundred cur-
rent–transient curves for 5AQ5 and 8AQ8 (see the
Experimental Section for details). The conductance
values (Go) for 5AQ5 and 8AQ8 are (2.7 Æ 1.1) ꢁ
10À4 Go and (1.7 Æ 1.0) ꢁ 10À5 Go, respectively
(Figure 1). These values are significantly larger
than those obtained previously for completely
saturated NB tetrathiolates of comparable
length.[14] For example, the conductance of 5AQ5,
with a bridge length of 16 bonds, is more than two
orders of magnitude greater than that measured for
a completely saturated 15-bond NB bridge molecule
(1.6 ꢁ 10À6 Go).[14] The distance dependence attenu-
[
]
*
ation factor, b, for the conductance of 5AQ5 and
8AQ8 is (0.46 Æ 0.17) bondÀ1. This value is smaller
than that obtained for saturated NB bridge systems
(around 1 bondÀ1).[2] The enhanced conductance and
smaller b values for 5AQ5 and 8AQ8, compared to
saturated NB bridges, signifies that the charge
transport in these molecules is occurring through
superexchange-mediated coherent (i.e. tunneling)
charge transport involving virtual ionic states of the
AQ group. An incoherent, ohmic scattering mech-
Figure 1. a) Conductance histogram of 5AQ5. b) Typical individual current–transi-
ent curves of 5AQ5. c) Conductance histogram of 8AQ8. d) Typical individual
current–transient curves of 8AQ8. The histograms were built from around
750 individual transient curves by counting the number of times each step occurs
and weighting that number by the time duration of the step.
anism is ruled out on the grounds that the conductance would
show a linear dependence on the bridge length resulting in
a conductance ratio for 5AQ5:8AQ8 of around 1.4, instead of
the observed value of 15.9.
0.5m phosphate buffer at two different pH values (pH 3 and
8). The CVs show reduction/reoxidation peaks corresponding
to a two-electron redox switching of the AQ redox center
(Figure 2a). The half-wave potential (E1/2) of the redox
Electrochemical gating
of 5AQ5 was performed
using a four-electrode setup
in which a counter elec-
trode, controlled through
a reference electrode, acts
as the “gate” for the tunnel-
ing process. The other two
electrodes (the STM tip and
the gold surface) act as con-
tacts to the molecules and
can be thought of as the
source or the drain of
a single molecular device.
The AQ moiety in 5AQ5
undergoes a proton-coupled
Figure 2. a) Cyclic voltammetry at 50 mVsÀ1 vs. Ag of an Au(111) surface modified with a SAM of 5AQ5 in
0.5m phosphate buffer, pH 3 (black line) and pH 8 (grey line). The inset is the redox reaction of the AQ
moiety to an H2AQ moiety. b) ACVs at pH 3 (black line) and pH 8 (grey line). Data was obtained at
a frequency of 1 Hz and an AC amplitude of 15 mV.
redox reaction in aqueous
solution and the redox
couple can be switched
between the oxidized AQ
form and the reduced
H2AQ form.[17] This is confirmed by cyclic voltammograms
(CVs) of a SAM formed from 5AQ5 on an Au(111) surface in
reaction was found to shift more cathodic by around
315 mV when the buffer used is changed from pH 3 to 8.
The values of E1/2 were obtained from the peak maximum in
alternating current voltammograms (ACVs) at low frequency
of 1 Hz (Figure 2b). The shift of E1/2 with pH is consistent
with a 2eÀ/2H+ redox reaction which is widely reported on
[*] Conductance is given by G=Gc eÀbL where Gc is the contact
conductance, b is the tunneling decay constant, and L is the length of
the molecule.
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AQ SAMs in this pH range.[17–19] The anodic and the cathodic
waves in the CVs scaled linearly with the scan rate indicating
a surface-related redox process. Plots showing the depend-
ence of the peak current and the peak potential on the scan
rate are presented in the Supporting Information.
Figure 3 shows the corresponding dependence of the
single-molecule conductance on the electrochemical potential
of 5AQ5 at pH 3 and 8. The conductance measurements were
performed at a constant tip–surface bias of + 100 mV. At
a surface potential of + 300 mV (vs. Ag), where the AQ is in
its oxidized form, the single-molecular conductance of 5AQ5
is (2.4 Æ 1.2) ꢁ 10À4 Go which is close to that obtained with the
two electrode systems, (2.7 ꢁÆ 1.1) ꢁ 10À4 Go.
Figure 4. Evolution of the conductance of 11-NB that lacks the AQ
moiety, with a gate potential at pH 3. The inset is the structure of 11-
NB. Typical individual curves along with histograms at different gate
potentials are presented in the Supporting Information.
5AQ5 at the E1/2 value is due to the redox switch from the AQ
to the more conducting H2AQ moiety.
The conductance of 11-NB is (3.5 Æ 1.2) ꢁ 10À5 Go. This
value is significantly lower than the conductance of 5AQ5 in
the oxidized form, (2.7 Æ 1.1) ꢁ 10À4 Go, despite the 5AQ5
being five bonds longer than 11-NB. The high conductance of
5AQ5 and 8AQ8 opens up the possibility to design partially
conjugated NB bridges that incorporate two or more AQ
moieties thus achieving very long molecules that are chemi-
cally stable, rigid, and can be electrochemically switched to
a higher conductance state by reducing the AQ moieties.
In summary, we have shown the successful operation of
a single-molecule switch in an AQ-NB system with a con-
ductance on/off ratio of an order of magnitude. This
magnitude, which is attributed to destructive QI effects
Figure 3. Evolution of the conductance of 5AQ5 with a gate potential
at pH 3 (black line) and pH 8 (grey line). Each data point is the peak
maximum in the histograms. Error bars are calculated from the full
width at half maximum (fwhm) of the histogram peaks. Each data
point is obtained at a fixed gate potential vs. Ag. The conductance
value plateaus at around À300 mV for pH 3 and around À700 mV for
pH 8. Typical individual curves along with histograms at different gate
potentials are presented in the Supporting Infomation.
[
]
*
operating in the AQ form, is smaller than that predicted
from simple theoretical calculations,[6] but is similar to the
experimentally found magnitude from bulk conductance
studies across SAMs.[7] The AQ moiety can be electrochemi-
cally switched in situ between the high-conducting H2AQ
system and the low-conducting AQ system. Further, it is
shown that the potential range of the conductance enhance-
ment can be shifted using different pH values. Therefore, such
systems could potentially be used as single-molecule pH-
gated transistors.
As the potential of the surface is shifted more cathodic,
the conductance histogram shifts to higher values and reach
a maximum value of (3 Æ 1.4) ꢁ 10À3 Go. Thus, the conduc-
tance is increased by more than an order of magnitude at
potentials more cathodic than the E1/2 of the redox reaction.
When the pH of the electrolyte was changed from 3 to 8, the
increase in the conductance is shifted to more cathodic values.
The conductance value reaches a maximum value at around
À300 mV for pH 3 and around À700 mV for pH 8. These
values are close to the E1/2 values obtained at pH 3 (À305 mV)
and pH 8 (À620 mV) in the CVs and ACVs. Once the
potential is shifted back to + 300 mV the conductance value
was found to restore its original value of (2.5 Æ 1.0) ꢁ 10À4 Go,
which indicates that the switching system is reversible.
As a control experiment, we found that a SAM con-
structed using a NB bridge (11-NB) that lacked the AQ
moiety (Figure 4, inset) displayed no dependence of the
conductivity on the electrochemical potential over the same
potential window that was used for 5AQ5 at pH 3 (Figure 4).
This finding confirms that the increase in the conductance of
[*] Evidence that the observed electrochemical switching in 5AQ5/
5H2AQ5, is due to QI, rather than to incoherent processes is: 1) For
relatively short molecules, such as 5AQ5, the time-scale for the
electrons is expected to be fast compared to the time-scale of
vibrations or solvent polarization, so the transport should be
coherent. 2) Theoretical work in ref. [7] confirms the presence of QI
in a related system. 3) The conductance vs. potential plot (Figure 3)
shows a plateau at negative potentials, rather than a drop as would
be predicted for an incoherent process, which further confirms that
the molecule is either in AQ or H2AQ form.
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Keywords: molecular switches · quantum interference ·
quinones · redox chemistry · single-molecule studies
Experimental Section
.
Sample preparation: Gold substrates were prepared by thermally
evaporating around 100 nm of gold (99.999% Alfa Aesar) on freshly
cleaved mica slides (Ted Pella, Inc.) in an ultrahigh-vacuum chamber
(around 5 ꢁ 10À8 torr). Prior to each experiment, the substrate was
briefly annealed in a hydrogen flame to remove possible contami-
nation and to form an atomically flat surface and then it was
immediately immersed into a 10 mm NB bridge solution in dichloro-
methane (DCM). The substrate was left in the modification solution
for three hours after which it was removed, washed thoroughly with
DCM and used for the measurements.
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Electrochemistry: The redox electrochemistry of SAMs formed
on freshly annealed Au(111) substrates of compound 5AQ5 were
studied by cyclic voltammetry using a BAS 100B electrochemical
analyzer. The counter electrode was a platinum mesh and the
reference electrode was a silver wire. The electrolyte used was 0.5m
phosphate buffer using Na2HPO4/NaH2PO4 for pH 8 and NaH2PO4/
H3PO4 for pH 3. The same setup was used to record the ACVs with
a Solartron Impedance/Gain-Phase Analyzer. The alternating current
(AC) amplitude was 15 mV. Data analysis was carried out using the
program Z view by Scribner Associates Inc.
Conductance measurements: The STM break junction setup was
a modified Pico-STM (Molecular Imaging) using a Nanoscope IIIa
controller. The setup and method have been described in details
elsewhere.[8] The SAM modified Au (111) substrate was placed in
a Teflon STM cell and the surface was covered with toluene. The
molecular conductance was measured by repeatedly forming and
breaking Au point contacts using an STM gold tip (99.998% Alfa
Aesar). The first step was to image the substrate in the regular STM
mode. Images showing clear and sharp atomic steps are good
indication of a clean substrate and a sharp tip. After surveying the
substrate and confirming the tip condition, the tip was fixed at the
center of an atomically flat terrace and the STM feedback loop was
turned off. Consequently, a Lab View program was used to move the
tip into and out of contact with the substrate at a typical rate of
40 nmsÀ1. During the contact process, molecules can bridge between
the tip and the molecules on the surface through the thiol linkers at
the distal end of the molecules. After reaching a preset current value,
the tip was pulled back until the current drops to zero. This process
was repeated automatically thousands of times. Typically 3000 curves
were collected for each experiment. Transient curves that are either
noisy or that showed smooth exponential decay because of the
absence of a bridging molecule were all rejected when building the
histograms. The percentage decay curves that showed clear molecular
steps were typically between (20–40)% and were all selected for
building the histograms.
Received: November 4, 2011
Revised: February 2, 2012
Published online: February 14, 2012
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