Regulating a benzodifuran single molecule redox switch via electrochemical gating and optimization of molecule/electrode coupling

Article abstract of DOI:10.1021/ja5034606

We report a novel strategy for the regulation of charge transport through single molecule junctions via the combination of external stimuli of electrode potential, internal modulation of molecular structures, and optimization of anchoring groups. We have designed redox-active benzodifuran (BDF) compounds as functional electronic units to fabricate metal-molecule-metal (m-M-m) junction devices by scanning tunneling microscopy (STM) and mechanically controllable break junctions (MCBJ). The conductance of thiol-terminated BDF can be tuned by changing the electrode potentials showing clearly an off/on/off single molecule redox switching effect. To optimize the response, a BDF molecule tailored with carbodithioate (-CS₂^-) anchoring groups was synthesized. Our studies show that replacement of thiol by carbodithioate not only enhances the junction conductance but also substantially improves the switching effect by enhancing the on/off ratio from 2.5 to 8.

Full text of DOI:10.1021/ja5034606

Communication
pubs.acs.org/JACS
Regulating a Benzodifuran Single Molecule Redox Switch via
Electrochemical Gating and Optimization of Molecule/Electrode
Coupling
Zhihai Li,†,∥ Hui Li,‡,∥ Songjie Chen,‡ Toni Froehlich,§ Chenyi Yi,‡ Christian Schonenberger,§
̈
Michel Calame,*,§ Silvio Decurtins,‡ Shi-Xia Liu,*,‡ and Eric Borguet*,†
†Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122, United States
‡Department of Chemistry and Biochemistry, University of Bern, Bern, Switzerland
§Department of Physics and Swiss Nanoscience Institute, University of Basel, Basel, Switzerland
S
* Supporting Information
Single molecule gating experiments can be experimentally
ABSTRACT: We report a novel strategy for the
regulation of charge transport through single molecule
junctions via the combination of external stimuli of
electrode potential, internal modulation of molecular
structures, and optimization of anchoring groups. We
have designed redox-active benzodifuran (BDF) com-
pounds as functional electronic units to fabricate metal−
molecule−metal (m−M−m) junction devices by scanning
tunneling microscopy (STM) and mechanically control-
lable break junctions (MCBJ). The conductance of thiol-
terminated BDF can be tuned by changing the electrode
potentials showing clearly an off/on/off single molecule
redox switching effect. To optimize the response, a BDF
achieved using mechanical controllable break junction (MCBJ)
or scanning tunneling microscopy break junctions (STM-BJ).
In a break junction experiment, individual molecules are wired
between two electrodes, the source and drain electrodes, via
proper anchoring groups. A reference electrode is used as the
gating electrode to adjust the Fermi energy of the source and
drain electrodes so that the source−drain (S−D) current
through molecular junctions can be regulated/electrochemically
gated. Electrochemically induced molecular conductance
switching and the resulting gating effect have been detected
for redox active molecules such as viologens15,18,19 and
perylenes16,20 as well as recently for graphene fragments.21
However, the electrochemical gating effect of benzodifurans
(BDFs), an important class of redox-active model mole-
cules,22−25 has not been explored at the single molecule level.
Even the single molecule conductance of BDFs has not been
reported yet. Our keen interest in BDFs stems from their
unique p-type semiconductor characteristics and high hole
mobility which render them excellent in organic field effect
transistors (OFETs) and organic light emitting diodes
(OLEDs).26,27 Notably, they can undergo a reversible oxidation
forming stable cation-radical salts.22 Furthermore, although it is
clear that the molecular/junction conductance can be gated
electrochemically, no study has focused on the important
question of how the anchoring groups of molecules, or the
contact resistance between a molecule and the electrodes, can
influence the gating effect.
Herein, we report the design of redox-active benzodifurans
terminated by thiols (acetyl protected) and carbodithioate
(TMSE protected) anchoring groups, as shown in Chart 1, for
the formation of molecular junctions. The synthesis of BDF
derivatives involves a palladium-catalyzed Sonogashira cross-
coupling of 2,6-bis(dihexylamino)-4,8-diiodo-benzo[1,2-b:4,5-
b′]difuran-3,7-dicarbonitrile with 4-ethynyl-1-thioacetylbenzene
and 4-ethynyldithiobenzoic acid 2-(trimethylsilyl)ethyl ester,
respectively. The details of the preparation procedure and
characterization can be found in the Supporting Information.
While previous studies only investigated how different core
−
molecule tailored with carbodithioate (−CS2 ) anchoring
groups was synthesized. Our studies show that replace-
ment of thiol by carbodithioate not only enhances the
junction conductance but also substantially improves the
switching effect by enhancing the on/off ratio from 2.5 to
8.
key step in molecular electronics is to design and
Aconstruct sophisticated nanoscale devices which can
perform functions, such as switching, gating, and regulating,
thereby mimicking traditional electronic units.1−3 In memory
and logic, switching represents one of the most basic functions
and there is interest to study the switching behavior at the
single molecule level.4 In particular, it is highly desirable to
fabricate molecular devices through which the current flows can
be regulated by external “stimuli” or parameters. These
parameters can be environmental solution pH,5,6 light
irradiation,7−9 electrode potentials,10−14 etc. Modulation of
current flow through external electrode potential control, so-
called electrochemical gating, is one of the most exciting
approaches to facilitate conductance switching. In an electro-
chemical gating experiment, charge transport through mole-
cule-mediated junctions can be regulated by tuning the Fermi
level of the junction electrodes with respect to an electro-
chemical reference electrode.15 More dramatic changes in the
molecular junction conductance can occur when the molecules
are switched between different redox states.16,17
Received: April 7, 2014
Published: June 16, 2014
© 2014 American Chemical Society
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Communication
Chart 1. Chemical Structures of the Functionalized BDF
Derivatives with Thiolacetyl and Carbodithioic Acid TMSE
Ester Groups
structures affected the gating behavior, our experiments show
that the anchoring groups (contact resistance) also play an
essential role in the single molecule electrochemical con-
ductance gating.
STM-BJ and MCBJ techniques employed to investigate the
charge transport through molecular junctions in the present
study have been described in our previous reports5,28−30 and
Supporting Information. Basically, in both techniques, metal
junctions are formed and subsequently broken in the presence
of target molecules to form metal−molecule−metal (m−M−
m) junctions (Figure 1A) while current versus distance traces
are recorded. Then these current−distance traces are used to
construct current/conductance histograms to obtain the
statistical single molecule conductance values.30−34
Figure 1. (A) Schematic of a break junction measurement with a
molecule wiring between two electrodes. (B, C) Current histogram
constructed from current−distance traces measured at 0.1 V in an
−
STM break junction for BDF-S− (B) and BDF-CS2 (C). Inset in B
and C: sample current−distance traces.
Single molecule conductance experiments were first
performed on BDF-S− molecule, with mesitylene as the
solvent, using STM break junction techniques. Junctions were
formed and broken repeatedly allowing thousands of current−
distance traces to be recorded for statistical analysis. The
detailed data analysis and trace selection criteria can be found in
the Supporting Information. The current histogram in Figure
1B was constructed using 450 stepped traces from 2000 total
traces, showing a current maximum at about 2.3 nA at a bias
voltage of 0.1 V, corresponding to a single molecule
conductance of 23 nS (2.9 × 10−4 Go, where Go is the
quantum of conductance and equals 77 500 nS). This is
consistent with the conductance value, revealed by current
steps, from individual traces (Figure 1B, inset).
For comparison, we performed single molecule conductance
experiments in 0.25 mM solution of BDF-SAc in tetrahy-
drofuran/mesitylene (1:4 v/v) using the MCBJ technique. The
BDF-SAc molecules were deprotected in situ by adding tetra-n-
butylammonium hydroxide (TBAOH) to the solution (∼0.1
mM). The two-dimensional conductance versus relative
distance histogram shown in Figure 2A was constructed from
514 successive traces recorded at a bias voltage of 0.2 V without
data filtering. To generate the histogram, all conductance traces
were aligned to the sharp conductance drop below 1 G0. This
representation provides a useful overview of the breaking
process for all breaking cycles performed. A well-defined
accumulation of counts can be observed in the conductance
range from 1 × 10−4 Go to 2.5 × 10−3 Go. Note that we observe
only one clear accumulation region, supporting the idea that
the junction formation and breaking is statistically dominated
by a well-defined geometry of the molecular junction. Most
conductance traces break at a push-rod extension lower than 25
μm, which corresponds to a breaking distance of ∼6.9 Å using
our typical average attenuation factor of 2.75 × 10−5. This
corresponds well to the observed breaking distances in the
STM break junction traces shown in Figure 1B and 1C. The
accumulation of data points at conductances close to and lower
than 10−6 Go arises due to the presence of the deprotection
agent in the liquid cell. The one-dimensional histogram in
Figure 2B clearly shows the corresponding conductance peak
with a maximum at 28 nS (3.7 × 10−4 Go), consistent with the
conductance obtained in STM break junction experiments
(Figure 1B).
Replacement of thiol anchoring groups by carbodithioate
groups results in an enhanced single molecule conductance of
210 nS (Figure 1C). The increase of conductance by the
introduction of carbodithioate groups has been reported for
biphenyl,35 and OPE,36 systems and supported by theoretical
calculations, which indicates that the carbodithioate group can
increase the molecule−electrode coupling and thus reduce the
electrode−molecule contact resistance.
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Figure 2. (A) Two-dimensional conductance versus push-rod
displacement histogram constructed from 514 traces recorded at a
bias voltage of 0.2 V for BDF-S−. (B) One-dimensional conductance
histogram for BDF-S− showing a conductance peak at 28 nS (3.7 ×
10−4 Go).
−
Figure 3. (A) Cyclic voltammograms of BDF-S− (red) and BDF-CS2
(blue) on Au(111) electrodes in 0.1 M HClO4. Scan rate: 0.5 V/s. (B)
To explore the device functions of these molecule-mediated
junctions, we examined the potential-induced single molecule
conductance switching effect of BDF derivatives in electro-
chemical electrolyte. Au(111) electrodes covered with immo-
bilized thiol or carbodithioate terminated BDF derivatives show
apparent redox peaks around 0.50 V (vs SCE). Cyclic
voltammograms (CV) were measured in 0.1 M HClO4 solution
in an STM electrochemical cell with a sweep rate of 0.50 V/s
(Figure 3A). Multiple CVs at different scan rates (0.05 to 1 V/
s) are illustrated in the Supporting Information (Figure S4).
Cyclic voltammetry experiments carried out in ionic liquid did
not show such well-defined peaks. Therefore, we performed
single molecule break junction measurements in HClO4 instead
of in ionic liquid.
−
Single molecule conductance of BDF-S− (red circles) and BDF-CS2
(blue squares) as a function of gating (substrate electrode) potential
determined from STM-BJ experiments.
from the initial 220 nS to 1760 nS at the maximum for BDF-
−
CS2 . The increased on/off ratio is a consequence of the use of
the CS2 anchoring group as rationalized by a simple model and
calculation presented in the Supporting Information. Con-
ductance of BDF-S− (20 nS) at a sample potential of −0.05 V,
away from the redox formal potential, has a similar conductance
with the one measured in the mesitylene (23 nS), indicating
that the solvent effect on single molecule conductance is
negligible for the studied system.
The single molecule conductance of BDF-S− and BDF-CS2
In summary, we have demonstrated the first study of single
molecule charge transport through a benzodifuran corea
novel category of redox-active molecules for functional
molecular devicesusing both STM-BJ and MCBJ techniques.
Single molecule conductance values were determined by STM
break junction to be 23 nS (consistent with 28 nS by MCBJ
measurement). Replacing the thiol by carbodithioate anchoring
groups led to an increase of conductance to 220 nS, further
−
were measured by the STM break junction in 0.1 M HClO4
electrolyte solution at different substrate potentials across the
range of the electrochemical redox peaks. The general trend in
both cases is that the molecule conductance increases gradually
when the substrate potential steps positively from −0.05 V
toward the formal electrochemical potentials (Figure 3A). The
single molecule conductance reaches a maximum around 0.50
V, consistent with the electrochemical redox formal potentials
(about 0.48 V) observed in the cyclic voltammograms (Figure
3A).
Single molecule gating experiments show that the con-
ductance of BDF molecules changes as a function of the gating
(substrate electrode) potential, exhibiting an off/on/off switch-
ing behavior (Figure 3B). However, an enhancement of the on/
off ratio was observed when thiols were replaced by
carbodithioate anchoring groups. For example, for BDF-S−
the initial conductance is about 20 nS, and the maximum gated
conductance is 48 nS (Figure 3B, red and Supporting
Information) resulting in an on/off ratio of ∼2.5. In contrast,
the on/off ratio is enhanced to be 8 as the conductance changes
−
supporting our hypothesis that CS2 anchoring groups have a
small contact resistance and can enhance the junction
conductance. Furthermore, electrochemical gating experiments
in electrolyte show that both BDF derivatives terminated with
thiol or carbodithioate anchoring groups exhibit an off/on/off
gating effect, with an on/off ratio of 2.5 for the former and 8 for
the latter. This result indicates that reducing the contact
resistance between molecules and electrodes can improve the
gating effect. These experiments represent the first attempt to
correlate the molecule/electrode contact resistance to the
electrochemical gating, an important type of molecular
switching. Therefore, the present study may have a fundamental
impact on molecular devices and nanoelectronics, for example,
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Communication
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ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental section, details about synthesis of com-
pounds BDF-SAc and BDF-CS2TMSE, intuitive model
and calculations. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Authors
eborguet@temple.edu
shi-xia.liu@iac.unibe.ch
michel.calame@unibas.ch
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∥Z.L.and H.L. contributed equally to this work.
Neels, A.; Hauser, J.; Schurch, S.; Decurtins, S. J. Org. Chem. 2010, 75,
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3350.
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Dr. Yangjun Xing for help with data analysis.
Financial support from the National Science Foundation (CHE
0809838), the Swiss National Science Foundation (No.
200021-147143), and the Sino Swiss Science and Technology
Cooperation as well as the European Commission (EC) FP7
ITN “MOLESCO” Project No. 606728, EC FP7 STREP
“SYMONE” Project No. 318597 is gratefully acknowledged.
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