Single-molecule charge transport through positively charged electrostatic anchors

Article abstract of DOI:10.1021/jacs.0c12664

The charge transport in single-molecule junctions depends critically on the chemical identity of the anchor groups that are used to connect the molecular wires to the electrodes. In this research, we report a new anchoring strategy, called the electrostat

Full text of DOI:10.1021/jacs.0c12664

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Single-Molecule Charge Transport through Positively Charged
Electrostatic Anchors
Hongliang Chen,$ Vitor Brasiliense,$ Jingshan Mo, Long Zhang, Yang Jiao, Zhu Chen, Leighton O. Jones,
Gen He, Qing-Hui Guo, Xiao-Yang Chen, Bo Song, George C. Schatz, and J. Fraser Stoddart*
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ABSTRACT: The charge transport in single-molecule junctions depends critically on the
chemical identity of the anchor groups that are used to connect the molecular wires to the
electrodes. In this research, we report a new anchoring strategy, called the electrostatic anchor,
formed through the efficient Coulombic interaction between the gold electrodes and the
positively charged pyridinium terminal groups. Our results show that these pyridinium groups
serve as efficient electrostatic anchors forming robust gold−molecule−gold junctions. We have
also observed binary switching in dicationic viologen molecular junctions, demonstrating an
electron injection-induced redox switching in single-molecule junctions. We attribute the
difference in low- and high-conductance states to a dicationic ground state and a radical
cationic metastable state, respectively. Overall, this anchoring strategy and redox-switching
mechanism could constitute the basis for a new class of redox-activated single-molecule
switches.
the strategy of employing electrostatic interactions (Figure 1d)
to form stable molecular junctions between the Au electrodes
and positively charged methylpyridinium-terminated mole-
cules. The conductance is measured using a home-built
scanning tunneling microscope-based break junction (STM-
BJ)13,45,46 technique. By comparing the charge transport
behavior of neutral pyridine-terminated and charged pyridi-
nium-terminated single-molecule junctions, we have estab-
lished that the electrostatic interaction can form stable
connections to the Au electrodes in a similar manner as the
lone electron pair does in dative anchors.
Such a binding strategy at the molecule/electrode interface,
which we refer to here as electrostatic anchoring, also applies
to many other backbones, showing that positively charged
pyridinium units can serve as efficient anchors to form robust
Au−molecule−Au junctions, resulting in conductive molecular
bridges. In addition, we observe binary conductance states,
namely, a dicationic ground state and a radical cationic
metastable state, in the dimethyl viologen molecular junction.
Combining the results from single-molecule experiments and
electrochemical measurements, we ascribe the binary con-
ductance signature to an electron injection/extraction-induced
redox reaction in the junction, an attribution which differs
INTRODUCTION
■
Advances in molecular electronics1 during the past half century
have enabled us to identify2−5 hundreds of single-molecules
that offer fundamental insight into the correlations6−11
between molecular electronic structures and charge transport
properties. Single-molecule junctions are generally constituted
of a central conducting bridge, terminated8,9 by anchors that
are used to attach the molecular bridges to electrodes. From
this perspective, an empirical rule-of-thumb for the design of
single-molecules is that researchers should consider not only
the electronic structure of the conducting bridges but also the
chemical identity of the anchors and their binding behavior at
the molecule/electrode interface.9,12 On the basis of the nature
of the binding interactions with the electrodes, anchors can be
divided into three categories, namely, (i) dative anchors13−22
(Figure 1a) which are bound to metal electrodes via
coordinative interactions, (ii) supramolecular anchors23−26
(Figure 1b) which couple to graphene electrodes through
[π···π] interactions, and (iii) covalent anchors27−34 (Figure 1c)
which provide direct electrode−molecule contacts. The nature
of these different binding interactions can be exploited in order
to (i) improve33,35−37 the stability and function of the
connected conducting bridges, (ii) install38−40 the current
rectification in a junction, and (iii) modify41 the dominant
transport mechanism.
Received: December 5, 2020
Published: February 12, 2021
The electrostatic interactions between a molecule and the
Au electrode have been observed to form stable Au−
molecule−Au junctions in protonated pyrazolyl42 and
pyridinyl43 anchors as well as in deprotonated imidazolate44
molecular junctions. In this work, we investigate systematically
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Figure 1. The development of an electrostatic anchor. Three widely used anchors employed in single-molecule junctions: (a) dative anchor, (b)
supramolecular anchor, and (c) covalent anchor. (d) Electrostatic anchor developed in this work. (e) X-ray photoelectron spectra (XPS) showing
different binding mechanisms for the monolayer of bipyridine (BIPY) (left) and the monolayer of dimethyl viologen (BIPY-Me2+) (right)
adsorbed on Au substrates.
X-ray Photoelectron Spectroscopy (XPS) Characterization.
XPS Analyses were carried out on thin films of 4,4′-bipyridine (BIPY)
and methyl viologen (BIPY-Me·2PF6) obtained by spin-coating. The
thickness and coverage can be tuned49 by changing the speed of
rotation from 3000 to 6000 rpm. XPS Depth analysis was performed
by sputtering Ar ions on to the BIPY-Me·2PF6 films.
Electrochemical Experiments. Cyclic voltammetry (CV) was
carried out at room temperature in N2-purged MeCN with a Gamry
Multipurpose instrument (Reference 600) interfaced to a computer.
CVs on all samples (1 mM in MeCN) were performed using a glassy
carbon working electrode (0.071 cm2), a Pt wire counter electrode,
and a Ag/AgCl (3 M NaCl) reference electrode. Tetrabutylammo-
nium hexafluorophosphate (TBAPF6) of 0.1 M concentration was
used as the supporting electrolyte.
UV−vis spectro-electrochemical experiments were performed using
a LAMBDA SYSTEM static experiment cuvette (1 mm optical path)
with a Pt 80 mesh working electrode, a Pt wire counter electrode, and
a Ag/AgCl (3 M NaCl) reference electrode. Compounds of 0.25 mM
in MeCN were degassed using N2 before being sealed in the cuvette.
TBAPF6 of 0.1 M was used as the supporting electrolyte. UV−vis
Absorption spectra of BIPY-Me·2PF6 at different potentials were
recorded by applying a potential to the solution of BIPY-Me·2PF6 for
1 min in a cuvette.
Electrochemical surface-enhanced Raman scattering (EC-SERS)
spectra were recorded in a home-built cell setup with the target
compounds (10 μM in H2O) adsorbed on AuFON SERS substrates.
Substrates of Au film over nanospheres (AuFONs) were prepared
according to a well-established procedure.50,51 First, we cleaned a 25-
mm silicon wafer (WaferNet Inc.) with a sequence including
incubation in Piranha solution (H2SO4/H2O2 = 3:1) and sonication
from the contact geometry-induced switching mechanism in
pyridine-terminated molecular junctions.
We explore systematically the length-dependence and the
role of counterions on the pyridinium-terminated molecular
junctions. Our results show that oligophenylenes terminated
with pyridinium anchors follow a quantum tunneling charge
transport mechanism, with a molecular decay constant similar
to that of pyridine-terminated analogues. We compare the
remnant conductance signature of the N−N electrostatic
anchor with that of the S−S dative anchor when they are
present in very similar molecules. A three-level conductance
feature has been demonstrated in these molecules with clear
attribution to different contact conditions. These results
establish our approach as a robust, yet flexible, strategy to
distinguish the conductance states in positively charged
molecular junctions43,47 and macrocyclic circuits.48 This
work expands the library of chemical anchor groups and
explores the intrinsic redox switching property in the viologen
family of molecules, thereby opening up new opportunities for
molecular electronics in the future.
EXPERIMENTAL METHODS
■
Synthesis. The structural formulas and descriptors for all
compounds used in the research reported in this paper are listed in
Scheme S1. The pyridinium derivatives were prepared by microwave-
assisted SN2 reactions at 90 °C. The full synthetic procedures and
characterizations are outlined in the Supporting Information. See SI
Sections A−C, Schemes S2−11, and Figures S1−20.
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in H2O/H2O2/NH4OH (5:1:1). We then cleaned the substrates
thoroughly with Milli-Q water, drop-cast a suspension of 300-nm
SiO2 microspheres (Bangs Laboratories, Inc.) onto the wafer, swirled
them, and left them to dry under ambient conditions. Finally, a
physical deposition system (PVD-75, Kurt J. Lesker) was used to
deposit a 150-nm Au film at a rate of 0.5 Å s−1 under low pressure
(10−7 Torr), forming the AuFONs substrates. The reference and
counter electrodes used were Ag/AgCl (3 M NaCl) and Pt wires
(99.99%, Alfa Aesar), respectively. The AuFONs substrate was
employed as the working electrode. A potentiostat (EC301, Stanford
Research Systems) was used to perform potential-step experiments in
aerated or N2-saturated 0.1 M HClO4.
STM-BJ Measurements. Single-molecule conductances were
measured using an STM-BJ,13,45,46 housed inside a plastic glovebox
filled with N2, with the target compounds dissolved (0.1 to 1 mM) in
MeCN (Sigma-Aldrich, 99% purity). In order to suppress the
background conductance of the solvent, an electrochemically etched
Au tip (Ø = 0.25 mm, 99.999% purity, Alfa Aesar) was coated with
Apiezon wax. The Au substrates were prepared by the evaporation of
Au (99.999% purity, Alfa Aesar) at ∼1 Å s−1 to give 100-nm-thick Au
on fresh silica (300 nm SiO2) substrates and were cleaned with
Piranha solution before making measurements. During the STM-BJ
experiments, 20 μL of the solution containing the test compounds was
added to the substrate. Thousands of traces were collected and
presented as one-dimensional (1D) and two-dimensional (2D)
conductance-displacement histograms without data selection in
order to obtain the most frequently observed conductance values.
Further details are provided in SI Section F.
powder spectrum, registered for BIPY-Me·2PF6, can be fitted
(Figure S21b) into a single peak at 401.4 eV, attributable to
the positively charged pyridinium−N atom. The fitted XPS
spectrum of the monolayer of BIPY-Me·2PF6 results (Figure
1e, bottom right and Figure S23) in an intense peak at 401.7
eV. The 0.3-eV shift in energy between the powder sample and
the monolayer confirms the existence of an electrostatic
interaction between the dicationic BIPY-Me·2PF6 and the Au
substrate. We performed (Figure S24) a depth analysis on the
BIPY-Me·2PF6 film by removing the material slowly between
each cycle of analysis without damaging the underlying BIPY-
Me·2PF6 film. Such depth-profiling XPS enables high-
resolution chemical analysis of the molecules close to the
electrodes. After sputtering with Ar ions for 1 s, the
pyridinium−N 1s core-level spectrum of the BIPY-Me·2PF6
film gave (Figure S24) two well-resolved peaks corresponding
to the viologen radical−N• at 399.6 eV and the positively
charged viologen−N+ at 401.7 eV. With an extension of the
sputtering time, no XPS peak was found for N+, suggesting an
efficient reduction (Figures S25−S27) of dicationic BIPY-
Me2+ to radical cationic BIPY-Me•+.
Single-Molecule Conductance Measurements. In
order to examine the electronic properties of pyridinium-
terminated compounds, we performed single-molecule con-
ductance measurements using the STM-BJ technique (SI
Section F). Figure 2 shows logarithmically binned 1D and 2D
single-molecule conductance-displacement histograms for
three compounds, namely, neutral BIPY (Figure 2a),
dicationic BIPY-Me2+ (Figure 2e), and radical cationic
BIPY-Me•+ (Figure 2i). These 1D and 2D conductance
histograms show peaks around integer multiples of G0, where
G0 = 2e2/h ≈ 77.5 μS is the conductance quantum,
corresponding to Au−Au point contacts and more peaks
below 1 G0 corresponding to molecular junctions. The 1D
conductance histogram (Figure 2b) shows two distinct
conductance peaks for single BIPY junctions, corresponding
to the most frequently observed conductance values, namely, a
low-conductance (LC) state centered at ∼10−3.9 G0 and a high-
conductance (HC) state centered at ∼10−3.3 G0. The LC/HC
bistable conductance signatures can be attributed54,55 to the
contact geometry change of the single-molecule junction
during elongation, i.e., the HC state of the molecule resides at
an angle between the Au electrodes, while in the LC state, the
molecule is fully stretched out in the junction. The 2D
conductance histogram also shows (Figure 2c) two well-
distinguished conductance features with almost four times the
difference in conductance between the HC and LC states.
Moreover, we analyzed (Figure 2d) the correlation between
these two kinds of conductance traces by compiling56,57 the 2D
covariance histogram. The significant correlation in the
intersection of HC and LC states indicates that these two
states occur sequentially in a single trace.
Theoretical Methods. The local electric field intensity and
distribution in the nanogap between the tip and substrate in an STM
were simulated using the AC/DC Module (steady state) of the
COMSOL Multiphysics finite-element-analysis software (COMSOL
Inc.). The molecular structures of BIPY-Me•+ and BIPY-Me2+ were
optimized on a slab of two layers of Au(111), in the Spanish Initiative
for Electronic Simulations with Thousands of Atoms (SIESTA,
version 4.1-b3) program (PBE/DZP for CHN atoms, PBE/SZP for
frozen Au atoms), using a k-grid size of 4 × 4 × 1 for the scattering
region, a density matrix tolerance of 1.0 × 10−4, and a force tolerance
of 0.05 eV Å−1. Another slab of Au was added to the other side of the
molecule, and together with the former, constitutes the scattering
−
region. One PF6 anion was added to one BIPY-Me•+ junction, and
two PF6− anions were added to BIPY-Me2+ junction in order to form
the radical cationic and dicationic molecular junctions, respectively.
These junctions were optimized subsequently at the same theoretical
level. The Green’s functions were calculated individually for the left
and right electrodes with six layers of Au(111) with each using the
same level (k-grid size of 4 × 4 × 50 for the electrodes). The
coordinates of these electrodes were added subsequently to each side
of the scattering region and form the device junction collectively. The
spin-polarized transport properties were computed with the Landau
formalism following previous methods.52,53 A spin of one unpaired
electron was fixed in the BIPY-Me·PF6 junction to simulate its
monoradical nature. Further details are provided in SI Section E.
RESULTS AND DISCUSSION
■
Film Characterization. In order to test the binding
mechanisms of the pyridinium anchors on Au surfaces (Figure
1e), XPS characterization of monolayer samples on Au films, as
well in powder samples, were recorded. BIPY was chosen as a
reference compound. The XPS powder spectrum of BIPY
exhibits (Figure S21a) a single peak at 397.9 eV, attributed to
the pyridine−N. The deconvolution of the XPS spectrum of
the BIPY monolayer also results (Figure 1e, bottom left and
Figure S22) in a single peak that shows a binding energy
displacement of 0.7 eV for the pyridine−N with respect to the
corresponding peaks in the powder spectrum. This result is
indicative of chemisorption of BIPY onto the Au surface by the
lone electron pair on the pyridine−N atom. Similarly, the XPS
The single BIPY-Me2+ junction (Figure 2e) also shows
(Figure 2f and Figure S31) two well-separated peaks in the 1D
conductance histograms, namely, a LC state centered at
∼10−3.8 G0 and a HC state centered at ∼10−2.0 G0, that differ
significantly from the conductance signature obtained for
BIPY. The HC state of the BIPY-Me2+ junction is much more
conductive (×20) than the HC state of the BIPY junction: this
observation indicates that the bistable conductance signatures
for BIPY-Me2+ cannot be attributed simply to the contact
geometry change of the single-molecule junction during tip
elongation. A 2D conductance histogram shows (Figure 2g and
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Figure 2. Electron transport of pyridine-terminated BIPY and pyridinium-terminated BIPY-Me2+ molecular junctions. (a) Schematic of a single
BIPY junction. (b) 1D and (c) 2D conductance−displacement histograms, and (d) 2D covariance histogram constructed from 3000 conductance−
displacement traces, showing a bistable conductance signature. (e) Schematic of a single BIPY-Me2+ junction. (f) 1D and (g) 2D conductance
histograms, and (h) 2D covariance histogram constructed from 4100 conductance−displacement traces, also showing a bistable conductance
signature. (i) Schematic of a single BIPY-Me•+ junction. (j) 1D and (k) 2D conductance histograms showing that the percentage of the HC state is
increased significantly. (l) Conductance summary of viologens terminated with different terminal substituent groups on pyridinium rings.
Figure S31) an HC state starting (displacement = ∼0.2 Å)
right after the break of the Au−Au point contact (displacement
= 0), followed by a LC state (displacement = 0.2 to 0.5 nm),
demonstrating two separate regions. Accounting for an Au−Au
‘snap-back’ distance28 of 0.5 0.1 nm, the molecular length
obtained from the relatively stretched distance distribution was
determined to be ∼0.7 nm for the HC state and ∼1.0 nm for
the LC state. We analyzed the correlation between these HC
and LC traces by compiling56,57 (Figure 2h) the 2D covariance
histogram. Similar to the case of the BIPY junction, the more
significant correlation in the intersection of the HC and LC
states indicates that the molecular junction is constructed by
these two states sequentially during each stretching cycle.
In order to confirm the original identity of the HC state, we
performed a control experiment on the BIPY-Me•+ junction
(prepared (Figure S27) by reducing the dicationic BIPY-Me2+
chemically with Zn dust in order to obtain its radical cationic
state) and measured its conductance in an N2-filled glovebox.
Radicals are believed to enhance58 charge transport in single-
molecule junctions on account of the increased transmission
close to the SOMO orbital when the radicals are formed. In
practice, the HC peak (Figure 2j) of the radical, centered on
∼10−2.8 G0, is similar in its magnitude to the HC value of the
BIPY-Me2+ junction. We have also obtained several differences
in the conductance signatures of the BIPY-Me2+ and BIPY-
Me•+ junctions. First, the relative percentage of the HC state is
larger in the BIPY-Me•+ junction for the simple reason that the
starting material is a radical. Second, the displacement of the
HC state for the BIPY-Me•+ junction is longer with a more
pronounced slope compared with that for BIPY-Me2+. These
differences can be explained by the formation59 of a BIPY-
Me•+ dimer in solution where one electrostatic anchor on each
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Figure 3. Electric-field-induced redox switching mechanism. (a) UV−vis Spectro-electrochemical measurements carried out on BIPY-Me·2PF6.
The UV−vis spectra recorded while applying a potential (vs Ag/AgCl) to the working electrode (Pt mesh) for 1 min. 0.1 M TBAPF6 in MeCN was
employed as the supporting electrolyte. (b) Electrochemical Surface-Enhanced Raman Scattering (EC-SERS) spectroscopy carried out on BIPY-
Me·2Br showing the formation of BIPY-Me•+ while applying forward bias (from +100 to −900 mV with steps of −100 mV) on an Au substrate.
COMSOL simulation showing the spatial distribution of the electric field across the break junction with a gap of (c) 0.5 nm and (d) 2 nm between
the STM tip and the substrate. (e) A plausible mechanism for the electron injection-induced redox switching mechanism in the single-molecule
junction during elongation.
molecule binds to the Au electrode and the conduction occurs
through the π−π conjugation associated with the two viologen
radical backbones. Qualitatively, a comparison of the 2D
histograms, illustrated in Figure 2c,g,k, suggests that in the
BIPY-Me2+ junction, the HC state with a short displacement
behaves like a radical species of BIPY-Me•+, while the LC state
with a fully stretched conformation corresponds to the
oxidized BIPY-Me2+. These results were supported by spin-
polarized non-equillibrium Green’s functions density func-
tional theory (NEGF-DFT) calculations (Figure S28 and
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Table S1), which reproduces the same trend, namely, the HC
state being the BIPY-Me•+ and the LC state being the BIPY-
Me2+. We hypothesize that an in situ redox reaction is taking
place in the junction by the electric-field-induced electron
injection/extraction to/from the single molecule.
rupture of the junction occurs. This process cannot happen in
BIPY because it is not a redox-active compound (Figure S25).
We note that the injection/extraction of electrons is essential
for the redox switching mechanism and is related to the barrier
provided by the substituents on the pyridinium−N. As a result,
we prepared (Figure 2l, inset) four more viologens with
different substituents of increasing size, namely, ethyl, n-propyl,
i-propyl, and adamantyl. Conductance studies show (Figure 2l
and Figure S32) that only BIPY-Et2+ with ethyl substituents
maintains the binary conductance feature characteristic of
BIPY-Me2+. Since larger substituents, namely, n-propyl, i-
propyl, and adamantyl, form higher barriers, electrons cannot
be injected effectively. It follows that we only obtain (Figure 2i,
blue line) direct tunneling currents in the case of BIPY-nPr2+,
BIPY-iPr2+, BIPY-Ada2+. This distinction supports the
electron injection-induced redox switching mechanism in the
case of BIPY-Me2+ and BIPY-Et2+. It also rules out the
possibility that the HC state only comes from the molecule-
electrode contact geometry when the molecules are lying on
the Au substrate.
System Expansion. Next, we compared (Figure 4a) the
length dependence of two series of molecules, namely, one
based on the ExnBIPY (n = 0−3) family with pyridine-
terminated dative anchors and the other one based on the
ExnBIPY-Me·2PF6 (n = 0−3) family with pyridinium-
terminated electrostatic anchors, by extracting (Figures S33−
S35) the conductance values from the 1D and 2D histogram
peak values and then plotting them against the length of the
molecules on a semilog scale. We find (Figure 4a) that the
conductance trends for both families are nearly linear on the
plot, indicating that the conductance depends66 exponentially
on length according to the relationship
Redox Switching Mechanism. We investigated the
hypothesis of electric-field induced redox reactions by relying
on spectro-electrochemical experiments to measure the
required driving force for carrying out the reduction from
BIPY-Me2+ to BIPY-Me•+. Figure 3a shows the in situ
formation of BIPY-Me•+ under potentiostatic control by
UV−vis spectro-electrochemistry. At low potentials, we
observe a band at 257 nm, characteristic of the HOMO−
LUMO transition of BIPY-Me2+. At about −400 mV, the
starting dication begins to be converted to the radical cationic
BIPY-Me•+, as supported by the presence of isosbestic points
at 250 and 300 nm as well as new peaks which arise at 398 and
605 nm for BIPY-Me•+. These conclusions are corroborated
(Figure 3b) by EC-SERS. As the potential is lowered beyond
−400 mV, characteristic bands appear at 1350, 1530, and 1665
cm−1 for BIPY-Me•+, accompanied by an increase in intensity
of these peaks on account of the resonance of the radical
species with the excitation wavelength, namely λ = 633 nm.
BIPY-Me2+ is usually employed60−62 as a probe molecule to
determine the quantum yield of electron transfer between
excited Au clusters and BIPY-Me2+ in aqueous solution. Break-
junctions with very small distances (<2 nm) between the metal
tip and substrate appear to facilitate the electron transfer which
can be attributed63−65 to the strong electric field (∼3 × 108 V
m−1) formed (Figure 3c,d) when a small bias (100 mV) is
imposed at small distances. In order to reveal the role of
electric fields on electron catalysis, we simulated (SI Section E)
the electric field distribution in the break-junction using an
AC/DC Module (steady state) of COMSOL. The simulation
was performed (Figure S29) using a 3D model with the
electrical field generated between a top and bottom electrode
in order to evaluate the focus of the localized electrical field in
STM-BJs. The results of the simulation (Figure S30) reveal
that the electrical field was focused on the end of the top
electrodes and decreased rapidly during tip elongation. The
intensity measured in the middle of the junction with a 0.5-
and 2-nm gap size differs by almost a factor of 10. We are led,
therefore, to propose that, only in the case when a molecule is
trapped in the junction with a small gap size and high electric
field (Figure 3c), does the electron injection happen63−65 and
reduction take place. Otherwise, direct tunneling (Figure 3d)
through the ground state of the molecule occurs.
G = Gcexp(−βL)
(1)
where G is the conductance, Gc is the contact conductance
arising from the molecule−electrode contact, L is the length of
the molecule, and β is the decay constant that depends on the
tunneling barrier associated with the single-molecule junction.
Using eq 1, we determined (Figure 4a, red dots and lines) a
decay constant for ExnBIPY of β = 2.91 nm−1, a value which is
comparable with those recorded14,22,46 in previous reports. By
contrast, the conductance of pyridinium-terminated ExnBIPY-
Me·2PF6 shows (Figure 4a, blue dots and lines) a slightly
smaller decay constant of 2.29 nm−1, indicating that electro-
static anchors form robust contacts with Au electrodes and
enable stable measurements to be made of conductances.
The counterion effect was also examined (Figure 4b) by
exchanging the PF6− with BF4− and SbF6 . BIPY-Me2+ with all
−
Combining all the results from theory and experiment, we
propose (Figure 3e) an electron injection-induced redox
switching mechanism in order to explain the single-molecule
bistable conductance signature of the positively charged
viologen during the elongation of the junction. The Au tip
picks up a BIPY-Me2+ molecule at its terminal group through
the electrostatic interaction between Au and the pyridinium-
N+. The gap, which is below 0.5 nm with an extremely large
electric field, will facilitate the electron injection from the Au
tip to BIPY-Me2+, creating a radical cationic BIPY-Me•+
species in its HC state. When the junction keeps increasing
and reaches 0.7 nm, the electric field decreases, leading to the
loss of one electron from the SOMO of the BIPY-Me•+ to the
bottom Au electrode, i.e., the BIPY-Me•+ is oxidized to its
dicationic ground state of BIPY-Me2+ in its LC state before the
three of these counterions dissolves easily in MeCN. Other
anions, for example, halide counterions, are not suitable
candidates since their salts are almost insoluble in MeCN. The
choice of solvent has to remain constant in all the experiments
since the influence of solvent effects67,68 on single-molecule
conductances cannot be ruled out. By extracting (Figure S36)
the conductance values from the 1D and 2D histogram peaks,
we obtained (Figure 4b) a binary conductance feature in the
case of all three compounds, namely, BIPY-Me·2BF4, BIPY-
Me·2PF6, BIPY-Me·2SbF6, with their LC states centered on
∼10−4.0 G0, while their HC states are distributed across a
broader range between 10−2.0 to 10−3.0 G0. This observation
indicates that the radical metastable state is more sensitive to
counterion exchange than the dicationic ground state.
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Figure 4. Applying pyridinium electrostatic anchors to other backbones. (a) Conductance determined from Gaussian fits to 1D histogram peaks for
the ExnBIPY (red) and ExnBIPY-Me·2PF6 (blue) (n = 0−3) families as a function of the molecular length. The dashed lines show an exponential
fit to the data points. (b) Conductance determined from Gaussian fits to 1D histogram peaks of BIPY-Me2+ with different counterions. (c)
Structural formulas of BIPY-Bn·2PF6, BIPY-SMeBn·2PF6, and BIPY-SPhBn·2PF6 terminated by benzyl, methyl sulfide benzyl, and phenyl sulfide
benzyl, respectively. (d) Typical individual conductance−displacement traces recorded in break junction measurements. 1D conductance
histograms for BIPY-Bn·2PF6 (e), BIPY-SMeBn·2PF6 (f), and BIPY-SPhBn·2PF6 (g), showing multilevel conductance signatures.
Finally, in order to compare the competitive relationship
between electrostatic anchors and other anchors when they are
present in the same molecule, we prepared (Figure 4c) three
benzyl derivatives, namely, BIPY-Bn·2PF6, BIPY-SMeBn·
2PF6, and BIPY-SPhBn·2PF6, and measured (Figure 4d)
their conductances. In the case of the BIPY-Bn2+ junction,
since the benzyl substituents stabilize the radical, we observed
(Figure 4e and Figure S37a) two-level conductance states,
namely, a HC radical state at 10−2.6 G0 and a LC dicationic
state at 10−4.2 G0, regardless of the steric bulk size of the end
group. In the case of the BIPY-SMeBn2+ junction, however, we
observed (Figure 4f and Figure S37d) three conductance
states, namely, an HC, a middle-conductance (MC), and an
LC state centered on ∼10−2.9, ∼10−4.2, and ∼10−5.1 G0,
respectively. The MC and HC states of BIPY-SMeBn2+ are
close to the values for the LC and HC states of BIPY-Bn2+.
Furthermore, considering the similar displacement of these two
pairs of states in the 2D histograms (Figure S37), the MC and
HC states of BIPY-SMeBn2+ can be ascribed, respectively, to
the dicationic ground state and a metastable radical state. The
LC state of BIPY-SMeBn2+, with its much lower conductance
and longer displacement, is the result of the tunneling through
S-to-S connections. These observations are confirmed in the
case of the BIPY-SPhBn2+ junction. A three-level conductance
feature similar to that for BIPY-SMeBn2+ was observed
(Figure 4g and Figure S37f). We note that in comparison
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J. Am. Chem. Soc. 2021, 143, 2886−2895

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
with methyl substituents on S, phenyl substituents on S help to
form a stronger binding between the S and Au atoms. Thus, in
the BIPY-SPhBn2+ junction, the percentage of the LC state
increases, while the HC state decreases simultaneously.
Yang Jiao − Department of Chemistry, Northwestern
University, Evanston, Illinois 60208, United States;
orcid.org/0000-0002-8437-0038
Zhu Chen − Department of Chemistry, Northwestern
University, Evanston, Illinois 60208, United States;
orcid.org/0000-0001-5889-5415
Leighton O. Jones − Department of Chemistry, Northwestern
University, Evanston, Illinois 60208, United States;
orcid.org/0000-0001-6657-2632
Gen He − State Key Laboratory of Optoelectronic Materials
and Technologies, School of Electronics and Information
Technology, Sun Yat-Sen University, Guangzhou 510006,
China; orcid.org/0000-0003-1394-0053
Qing-Hui Guo − Department of Chemistry, Northwestern
University, Evanston, Illinois 60208, United States;
orcid.org/0000-0002-9946-0810
Xiao-Yang Chen − Department of Chemistry, Northwestern
University, Evanston, Illinois 60208, United States;
orcid.org/0000-0003-3449-4136
Bo Song − Department of Chemistry, Northwestern University,
Evanston, Illinois 60208, United States; orcid.org/0000-
0002-4337-848X
George C. Schatz − Department of Chemistry, Northwestern
University, Evanston, Illinois 60208, United States;
orcid.org/0000-0001-5837-4740
CONCLUSION
■
In this research, we have investigated the charge transport
properties of a series of positively charged pyridinium-
anchored compounds with different substituents and back-
bones. Using a combination of ensemble experiments and
single-molecule measurements, we have demonstrated that the
pyridinium units serve as stable anchors facilitating con-
nections between the molecules and Au electrodes. Our results
show that the charge transport in pyridinium-anchored
oligophenylenes exhibits a quantum tunneling mechanism. In
addition, we observe binary switching states, namely, a cationic
ground state and a radical cationic metastable state, in the case
of the dimethyl viologen junction. These results reveal the role
of anchors in controlling redox reactions. Anchors facilitate an
increased understanding of conductance in junctions involving
molecules with complex structures. This research will aid in the
design and development of new chemistry as it relates to
molecular electronics.
ASSOCIATED CONTENT
■
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.0c12664
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.0c12664.
Author Contributions
$H.C. and V.B. contributed equally.
Detailed synthetic procedures and characterization data
(NMR spectroscopy) for all the compounds (PDF)
Notes
The authors declare no competing financial interest.
Cartesian coordinates (PDF)
ACKNOWLEDGMENTS
■
AUTHOR INFORMATION
■
We thank Northwestern University for its continued support of
this research. This work made use of the IMSERC NMR
facility at Northwestern University, which has received support
from the Soft and Hybrid Nanotechnology Experimental
(SHyNE) Resource (NSF ECCS-2025633) and Northwestern
University. This work made use of the Keck-II XPS facility of
Northwestern University’s NUANCE Center, which has
received support from the SHyNE Resource, the IIN, and
Northwestern’s MRSEC program (NSF DMR-1720139).
L.O.J. and G.C.S. acknowledge support from the Department
of Energy, Office of Basic Energy Science, under grant DE-
SC0004752. This research was also supported in part through
the computational resources and staff contributions provided
by the Quest high performance computing facility at
Northwestern University which is jointly supported by the
Office of the Provost, the Office for Research, and Northwest-
ern University Information Technology. We thank Prof. W.
Hong for help with the STM-BJ setup.
Corresponding Author
J. Fraser Stoddart − Department of Chemistry, Northwestern
University, Evanston, Illinois 60208, United States; School of
Chemistry, University of New South Wales, Sydney, NSW
2052, Australia; Stoddart Institute of Molecular Science,
Department of Chemistry, Zhejiang University, Hangzhou
310021, China; ZJU-Hangzhou Global Scientific and
Technological Innovation Center, Hangzhou 311215, China;
orcid.org/0000-0003-3161-3697; Email: stoddart@
northwestern.edu
Authors
Hongliang Chen − Department of Chemistry, Northwestern
University, Evanston, Illinois 60208, United States;
orcid.org/0000-0002-4442-7984
Vitor Brasiliense − Department of Chemistry, Northwestern
University, Evanston, Illinois 60208, United States;
Université Paris-Saclay, ENS Paris-Saclay, CNRS, PPSM,
91190 Gif/Yvette, France; orcid.org/0000-0002-5515-
2218
Jingshan Mo − State Key Laboratory of Optoelectronic
Materials and Technologies, School of Electronics and
Information Technology, Sun Yat-Sen University, Guangzhou
510006, China; orcid.org/0000-0003-2844-6314
Long Zhang − Department of Chemistry, Northwestern
University, Evanston, Illinois 60208, United States;
orcid.org/0000-0002-4631-158X
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