Turning the Tap: Conformational Control of Quantum Interference to Modulate Single-Molecule Conductance

Article abstract of DOI:10.1002/anie.201909461

Together with the more intuitive and commonly recognized conductance mechanisms of charge-hopping and tunneling, quantum-interference (QI) phenomena have been identified as important factors affecting charge transport through molecules. Consequently, esta

Full text of DOI:10.1002/anie.201909461

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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Accepted Article
Title: Turning the Tap: Conformational Control of Quantum Interference
to Modulate Single Molecule Conductance
Authors: Feng Jiang, Douglas Trupp, Norah Algethami, Haining
Zheng, Wenxiang He, Afaf Alqorashi, Chenxu Zhu, Chun
Tang, Ruihao Li, Junyang Liu, Hatef Sadeghi, Jia Shi, Ross
Davidson, Marcus Korb, Masnun Naher, Alexandre N.
Sobolev, Sara Sangtarash, Paul J. Low, Wenjing Hong, and
Colin Lambert
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201909461
Angew. Chem. 10.1002/ange.201909461
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10.1002/anie.201909461
Angewandte Chemie International Edition
RESEARCH ARTICLE
Turning the Tap: Conformational Control of Quantum Interference
to Modulate Single Molecule Conductance
Feng Jiang, Douglas I. Trupp, Norah Algethami, Haining Zheng, Wenxiang He, Afaf Alqorashi, Chenxu
Zhu, Chun Tang, Ruihao Li, Junyang Liu, Hatef Sadeghi, Jia Shi, Ross Davidson, Marcus Korb,
Alexandre N. Sobolev, Masnun Naher, Sara Sangtarash,* Paul J. Low,* Wenjing Hong,* Colin J.
Lambert*
Abstract: Together with the more intuitive and commonly
recognized conductance mechanisms of charge-hopping and
tunneling, quantum interference (QI) phenomena have been
identified as important factors affecting charge transport through
molecules. Consequently, establishing simple, flexible molecular
design strategies to understand, control and exploit QI in molecular
junctions poses an exciting challenge. Here we demonstrate that
destructive quantum interference (DQI) in meta-substituted
phenylene ethylene-type oligomers (m-OPE) can be tuned by
changing the position and conformation of pendant methoxy (OMe)
substituents around the central phenylene ring. These substituents
play the role of molecular-scale ‘taps’, which can be switched on or
off to control the current flow through a molecule. Our experimental
results conclusively verify recently postulated magic ratio and orbital
product rules, and highlight a novel chemical design strategy for
tuning and gating DQI features, to create single-molecule devices
with desirable electronic functions.
This concept was first introduced in 1909 to prove the wave
characteristics of photons in the double-slit experiments.[5] Now
it is widely investigated in diverse research areas, such as
nanophotonics,[6] superconductivity[7] and quantum metrology.[8]
In the studies of molecular junctions, QI arises when the de
Broglie waves of electrons progressing from one electrode to the
other pass through different energetically accessible pathways
across the molecule junction, causing interference patterns
within the molecule.[9] Constructive QI (CQI) occurs when the
interference pattern has a large amplitude at the point of
molecular contact to both the source and drain electrodes,
causing high through-molecule conductance. Conversely,
destructive QI (DQI) results in low amplitude of the propagating
electron wave at one or both of the electrode-molecule contacts,
causing extremely low through-molecule conductance.[10] The
ability to convert between these two scenarios by tuning the
molecular pathways can offer an exciting range of opportunities
to regulate charge transport through molecules, without
changing molecular backbone structure, length or redox state.
Recently
a number of studies have contributed to the
Introduction
identification of mechanisms for conductance tuning of single-
molecule junctions using QI phenomena, including the impact of
anchor groups,[11] the position of heteroatoms within the
molecular backbone[12] and bridge modification.[13] Following
theoretical predictions of the introduction of DQI effects due to
interactions of pendant oxygen and bipyridene groups with
conjugated molecular cores,[14] and subsequent experimental
confirmation,[15] a study of QI effects in molecular junctions of π-
conjugated systems was carried out by Guédon et al. using the
conducting atomic force microscopy technique.[16] Arroyo et al.
studied QI effects in a central phenyl ring by varying the coupling
to a variety of anchor groups.[17] Manrique et al. have also
demonstrated tuning of QI effects through heteroatom
substitution within a molecular core, and proposed a quantum
circuit rule for designing molecular devices and materials.[18]
Furthermore, the experimental and theoretical breakthroughs
realized by Garner et al. have demonstrated that destructive QI
effects are also achieved in silicon-based σ-orbital systems.[4a]
More recently, Li et al. and Bails et al. have demonstrated
control over through-molecule conductance by electrochemical
gating of compounds displaying pronounced DQI anti-resonance
features in their transmission profiles.[19] Although state-of-art
theoretical studies have been devoted to the investigation of
intramolecular QI patterns,[20] combined experimental and
theoretical studies of exploring the influence of both location and
conformation of additional pendant groups on QI effects is
lacking. This study set out to explore both theoretically and
experimentally whether varying the locations and conformations
of pendant groups around a molecular backbone could provide a
new strategy for tuning QI effects, and hence molecular
conductance.
Measurements of the conductance of electrode | molecule |
electrode junctions, interpreted with the aid of theoretical
treatments and computational modelling, have given insight into
the fundamentals of through-molecule electron transport, leading
to the design of molecular wires,[1] molecular switches[2] and
molecular diodes.[3] One of the most interesting aspects of
single-molecule electronics to emerge from these studies is the
phenomena of room-temperature quantum interference (QI).[4]
[*] Feng Jiang, Haining Zheng, Wenxiang He, Chenxu Zhu, Chun Tang,
Ruihao Li, Dr. Junyang Liu, Prof. Jia Shi, Prof. Wenjing Hong
State Key Laboratory of Physical Chemistry of Solid Surfaces
College of Chemistry and Chemical Engineering, iChEM
Xiamen University, Xiamen 361005 (China)
E-mail: whong@xmu.edu.cn
Douglas I. Trupp, Marcus Korb, Alexandre N. Sobolev, Masnun Naher,
Prof. Paul J. Low
School of Molecular Sciences, University of Western Australia, 35
Stirling Highway, Crawley, Western Australia, 6009, Australia
E-mail: paul.low@uwa.edu.au
Norah Algethami, Dr. Afaf Alqorashi, Dr. Hatef Sadeghi, Dr. Sara
Sangtarash, Prof. Colin J. Lambert
Department of Physics, Lancaster University
Lancaster LA1 4YB (UK)
E-mail: s.sangtarash@lancaster.ac.uk
c.lambert@lancaster.ac.uk
Dr. Ross Davidson
Department of Chemistry, Durham University
Durham, DH1 3LE (UK)
CCDC 1876564 and 1876565 contain the supplementary
crystallographic data for this paper. These data are provided free of
charge by The Cambridge Crystallographic Data Centre. Further
supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/anie.201909461
Angewandte Chemie International Edition
RESEARCH ARTICLE
where electrons tunnel through the HOMO-LUMO gap (i.e. the
Fermi energy of the electrodes lies within the molecular HOMO-
LUMO gap; SI section S3). For M1-3 and N1-3, injection and
collection points i and j (Figure 2b) are in meta positions relative
to each other, and as such the bare ‘parent’ molecule (with no
OMe pendant group) exhibits a DQI feature near the middle of
the HOMO-LUMO gap, and possesses a low conductance.[9a] To
a first approximation, if a perturbation to the parental structure is
imposed on the central ring at a position k, for example through
the introduction of a pendant or substituent group, to yield a
‘daughter’ molecule, then magic ratio theory predicts the
following ‘rules’:[12]
M1
M2
N1
N2
N3
M3
Figure 1. Schematic of STM-BJ setup and the chemical structures of the
molecules investigated in this work.
In this study, density functional theory (DFT) analysis (SI
sections S1 – S4) supported by experimental studies using the
scanning tunneling microscope break junction (STM-BJ)
technique[21] (Figure 1, SI sections S5 – S7) have been used to
investigate single-molecule charge transport through a series of
m-OPE molecules with thiolate (protected in the precursor as the
thioacetyl, SAc) (M1-3) or thiomethyl (SMe) (N1-3) anchor
groups (SI sections S8 and S9). The two anchor groups are
used and results compared to ensure that the phenomena
observed are due to the backbone and not some molecule-
contact artifacts. The molecular m-OPE backbones are
systematically modified by introduction of a pendant OMe
substituent at different positions around the central ring (Figure
1). The only difference between these molecules is the position
and lowest energy conformation of the pendant OMe group
relative to the planar meta-diethynylphenylene core. For these
molecular systems, DFT studies indicate that the OMe pendant
groups act as molecular ‘taps’, which can be used to switch on
or off the current flowing through the molecules by modulating
the QI signature. Figure 2a shows two conformations of the OMe
‘tap’ of M2 (modeled as the free thiol). The ‘off’ conformation (i)
features the OMe group perpendicular to the plane of the
molecule, and corresponds to a low conductance state for the
m-OPE fragment that is similar to the parent 1,3-
diethynylbenzene derived system. Rotating the OMe group into
the molecular plane gives the ‘on’ conformation (ii) and
increases molecular conductance through the m-OPE moiety as
the pendant oxygen becomes oriented to better serve as a π-
donor and more effectively interfere with QI effects in the
molecular π-system. These molecules have been designed such
that the most energetically favorable conformation of the OMe
tap belonging to M1 and N1 is ‘on’, whereas that of the more
sterically restricted M2 and N2 is ‘off’. As disussed below, these
molecules also demonstrate that the effectiveness of such
molecular taps in controlling conductance is critically dependent
on their connectivity to the central ring. Therefore the location of
the OMe taps of M3 and N3 has been chosen to render them
completely ineffective, such that the electrical conductances of
M3 and N3 are almost independent of the conformation of their
pendant OMe groups.
1. If, as in M1 and N1, k is ortho to i and para to j, then
the pendant group will increase the conductance by shifting
the DQI feature to a higher or lower energy.
2. If, as in M2 and N2, k is ortho to both i and j, then the
pendant group will increase the conductance by shifting the
DQI feature in the opposite direction to case 1.
3. If, as in M3 and N3, k is meta to either i or j, then the
pendant group will be ineffective and have only a small
effect on conductance. Consequently, the conductance of
the daughter remains low, as in the parent system.
(i)
(ii)
a
Off
On
Perturbed point k
k
b
Collection point j
Injection point i
i
j
Figure 2. a) Two conformations of the molecular tap M2 (drawn as the free
thiol). Rotating the OMe pendant group from the ‘off’ conformation (i) to the
‘on’ conformation (ii) switches the m-OPE molecule from low to high
conductance as the position of the DQI feature shifts. b) A conceptual view of
the magic ratio model of a molecular junction. The yellow regions represent
compound electrodes, which inject and collect electrons via triple bonds into π-
orbitals of the central ring at points i and j respectively. A perturbation
associated with
a pendant group is introduced at site k, illustrated
schematically here for the substitution pattern of M1 / N1.
The above changes in conductance are predicted to occur
only if the pendant group perturbs the central ring by coupling to
its π-system (SI section S2). When this happens, as for the
lowest energy conformation of M1 / N1 (which also conforms to
the crystallographically determined structure of N1, SI section
S9) or the (higher energy) ‘planar’ conformation of M2 / N2 (c.f.
Figure 2a (ii)), the QI tap is ‘on’ and the conductances of M1,
M2, N1 and N2 will be high. On the other hand, for the most
energetically-favourable conformation of M2 / N2, where for
steric reasons the OMe group rotates out of the molecular plane
The design of M1-3 and N1-3 is further informed by the
recently described ‘magic ratio’ theory (SI section S2).[22] This
theory describes the effect of connectivity on QI in the central
aryl ring, and views the moieties to the left and right as
“compound electrodes”, which inject and collect electrons via
triple bounds into π-orbitals at points i and j respectively (Figure
2b). The theory applies to the commonly encountered case,
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10.1002/anie.201909461
Angewandte Chemie International Edition
RESEARCH ARTICLE
((c.f. Figure 2a (i)) and the single crystal X-ray structure of N2, SI
section S9), the lone pair of electrons on the OMe oxygen atom
is orthogonal to the π-system of the central ring. In this lower
energy ‘orthogonal’ conformer, the OMe does not significantly
perturb the π system of the central ring and the tap is ‘off’.
Consequently, the conductance of the m-OPE daughters M2 /
N2 is expected to remain low. Therefore, by computing and
measuring the electrical conductances of M1-3 and N1-3, the
effect of both connectivity and conformation of the pendant
group on QI effects and hence the flow of electricity through the
m-OPE derived molecules can be explored.
HOMO-LUMO gap and that the function is almost coincident with
that of M3 over a wide range of energies, which demonstrates
that the OMe tap in this location is ineffective. In contrast, the
DQI dip of M1 is shifted to higher energies and therefore if the
Fermi energy ꢀ! in the experiment lies in the vicinity of ꢀ!!"#, the
electrical conductance of M1 is higher than that of M3 (Figure
3a). These features are also observed in N1 and N3 (Figure 3b),
and they are in agreement with rules 1 and 3 of the magic ratio
theory described above, and the trends observed in related
studies of the effect of substituent groups on QI.[12, 20b] On the
other hand, the DQI dip of M2 in the fully relaxed (non-planar)
geometry is not shifted and therefore the electrical conductance
of M2 remains low. This result for M2 is entirely consistent with
the experimental data presented below (Table 1), but at first
inspection would seem to be contrary to the effect of a
perturbation described by rule 2, which has been demonstrated
Results and Discussion
To probe the role of OMe pendant groups, DFT calculations
combined with the quantum transport code Gollum[23] were used
to compute the transmission coefficient of the systems. Plots of
the transmission coefficients T(E) of m-OPE M1-2 and N1-3 in
their fully relaxed geometries are shown in Figure 3a and 3b.
The transmission function T(EF) evaluated at the EF of the
electrodes reflects the magnitude and trends of electrical
conductance G of the molecules. The experimental location of
molecular orbital energies relative to EF relative may differ from
the DFT-predicted values. Typically EF is expected to lie in the
vicinity of the middle of the HOMO-LUMO gap (SI section S3),
and based on fitting to the experimentally determined
conductance values (vide infra), the present results suggest that
EF falls within the shaded regions in the figure (Table 1).[24]
to correctly predict the effect of substituents on QI in aryl
However in these earlier studies, molecular
conformation plays no role. Here, the transmission coefficients of
20b]
rings.[12,
the ‘off’ and ‘on’ conformations of molecules M2 (Figure 3c) and
N2 (Figure 3d) demonstrate
a
clear and significant
conformational effect. Whilst rotation of the pendant group of M2
or N2 into the planar conformation removes the DQI dip from the
middle of the HOMO-LUMO gap (Figure 3c, Figure 3d; Figure
S1), this local minimum described by the planar geometry of the
‘on’ state is higher than that of the orthogonal conformation
(+11.0 (M2); +12.7 (N2) kJ/mol). Therefore the lower energy ‘off’
conformations are favoured and determine the conductance
properties of the junctions.
0
-2
-4
-6
-8
0
-2
-4
-6
-8
Table 1. The calculated and experimentally determined single-molecule
b
a
Molecule
Calculated
conductance
(log(G/G0))
Experimental
conductance
(log(G/G0))
M1
-4.25
-4.18
-5.24
-5.05
-4.88
M1
M2
M3
m-OPE
N1
N2
N3
m-OPE
M2 (OMe ‘on’)
M2 (OMe ‘off’)
M3
-5.87
-5.55
-5.03
-1
0
1
-2
-1
0
1
E-EFDFT / eV
E-EFDFT / eV
N1
N2 (OMe ‘on’)
-5.35
-5.56
-6.26
-6.05
0
-2
-4
-6
-8
0
-2
-4
-6
-8
c
d
N2 (OMe ‘off’)
-5.98
-5.74
N3
conductance of M1-3 and N1-3.
The presence or absence of DQI transmission dips and the
effect on mid-gap conductance can also be linked to the
structure of the HOMO and LUMO through a recently highlighted
‘orbital product rule’ (Figure 4).[25] Even though these molecules
do not possess particle-hole symmetry and therefore the orbital-
M2-off
M2-on
M3
N2-off
N2-on
N3
-1
0
1
-2
-1
0
1
E-EFDFT / eV
E-EFDFT / eV
Figure 3. a-b) Transmission coefficients T(E) describing electrons of energy E
passing through the m-OPEs from one electrode to the other. In all cases, the
dashed lines show results for the parent m-OPE. c-d) Transmission
coefficients of the ’off’ and ‘on’ M2 (N2) molecules of Figure 2a (i) and (ii)
respectively. For comparison, the transmission of the relaxed M3 (N3) is also
shown.
symmetry rule[26]
and the Coulson-Rushbrooke pairing
theorem[27] do not apply, a qualitative indication of the presence
or absence of DQI can be obtained by examining interference
between the HOMO and LUMO. The rule is applied by noting
that if the HOMO (LUMO) amplitudes of a molecule at the
electrode contacts have the same sign, then the HOMO (LUMO)
is assigned an ‘orbital product’ aH (aL), which is positive.
Conversely, if the HOMO (LUMO) amplitudes at the contacts
have opposite sign the orbital product aH (aL) will be negative.
The conductance of the thiolate contacted parent m-OPE (i.e.
from the SAc protected derivative analogous to the M-series of
compounds) has been previously determined to be 10-5.5 G0.[12b]
Figure 3a shows that the transmission function of this parent m-
OPE (dashed line) possesses a DQI dip near the middle of the
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10.1002/anie.201909461
Angewandte Chemie International Edition
RESEARCH ARTICLE
The ‘orbital product rule’ states that if aH and aL possess the
same sign (i.e. if the product aH · aL is positive) then the HOMO
and LUMO interfere destructively and the transmission function
is likely to possess a DQI dip, otherwise CQI occurs and there
will be no dip. As indicated in Figure 4, the nodal properties and
distribution of the HOMO and LUMO of N2 in the lower energy
‘off’ conformation are identical to those of the parent m-OPE,
because the π system of the pendant OMe group is orthogonal
to, and therefore decoupled from, the π-system of the central
ring. Similar patterns are also found for N3, where the OMe
pendent is located at a node in both the HOMO and LUMO. In
each case, aH and aL are both negative, and the orbital product
rule predicts that N2, N3 and the parent m-OPE will all exhibit
DQI and possess low molecular conductances. In contrast, the
opposing signs of aH and aL indicate that N1 should exhibit CQI
and possess a high conductance, in agreement with the
observed conductance trends (the analogous trends and results
from the M-series are shown in Table S6). In addition, if the
OMe tap of N2 is artificially rotated to the ‘on’ position, as shown
in the bottom row of Figure 4, the orbital product switches sign
and CQI occurs.
conductance-distance traces of the solvent, M1-3 and N1-3 are
shown on a semi-logarithmic scale in Figure 5a and 5b.[28] The
curves for pure solvent (purple traces) exhibited the expected
exponential decay after the cleavage of the last gold-gold atomic
contact at the quantum conductance G0 (G0 = 2e2/h),[29] while the
traces for M1-3 and N1-3 showed distinct molecular plateaus,
indicating the formation of single-molecule junctions. Over 2,000
conductance-distance traces are binned to produce the
corresponding 1D histograms featuring distributions with clear
peaks (Figure 5c, Figure 5d). These 1D histograms were fitted
by Gaussian functions, and the peaks of these distributions are
attributed to the conductance of the most probable molecular
configurations in the junction i.e. 10-4.88 G0, 10-5.87 G0 and 10-5.55
G0 for M1-3, and 10-5.03 G0, 10-5.98 G0 and 10-5.74 G0 for N1-3,
respectively.
Solvent
Solvent
M1
b
a
0
-1
-2
-3
-4
-5
-6
0
-1
-2
-3
-4
-5
-6
N1
N2
N3
M2
M3
Δz / nm
Δz / nm
c
d
M1
M2
M3
N1
N2
N3
-5.87
-5.98
-5.74
-5.03
-5.55
-4.88
-6 -5 -4 -3 -2 -1
Conductance / log (G/G0)
0
-6 -5 -4 -3 -2 -1
0
Conductance / log(G/G0)
1
0
1
0
e
1.27!0.39
1.10!0.36
f
M1
N1
-1
-2
-3
-4
-5
-6
-1
-2
-3
-4
-5
-6
0
1
2
3
0
1
2
3
Δz / nm
Δz / nm
1000
0
1150
0
-0.5
0.0
0.5
1.0
1.5
2.0
-0.5
0.0
0.5
1.0
1.5
2.0
Δz / nm
Δz / nm
Figure 5. a) Typical individual conductance-distance traces of pure solvent
(TMB), M1, M2 and M3. b) Typical individual conductance-distance traces of
pure solvent (TMB), N1, N2 and N3. c) All-data-point 1D conductance
histograms of M1, M2 and M3 constructed from over 2000 traces for each
histogram without data selection. d) All-data-point 1D conductance histograms
of N1, N2 and N3 constructed from over 2000 traces for each histogram
without data selection. Representative 2D conductance-displacement
histogram and insert showing the displacement distribution of M1. f)
Representative 2D conductance-displacement histogram and insert showing
the displacement distribution of N1.
Figure 4. HOMOs and LUMOs of molecules N1-N3 and the parent m-OPE.
Blue regions are positive and red regions are negative. As an example, for N1
the HOMO at the left end of the molecule is positive (+) and at the right end it
is negative (-). Therefore, since these have opposite signs, aH is negative (-).
Similarly for N1, aL is negative. Since the product of aH aL is positive (+), N1
will exhibit DQI.
The displacement distribution histograms and two-dimensional
(2D) conductance-displacement histograms of M1 and N1 are
shown in Figure 5e and 5f by way of example (for the analogous
data from the other molecules see Figure S6).[30] Taking the data
from M1 as illustrative of the general analysis, the most probable
length of the molecular junction is determined to be 1.77±0.39
nm after correcting for the snap-back effect of the gold-gold
To provide experimental support for the above predictions, the
STM-BJ technique was employed to investigate the single-
molecule conductance of all molecules in trimethylbenzene
(TMB) at room temperature (SI section S5).[15a] Typical individual
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10.1002/anie.201909461
Angewandte Chemie International Edition
RESEARCH ARTICLE
atomic contact (0.5 nm) (Figure 5e, SI section S6).[15a] These
measurements and computational estimates of the junction
length are remarkably consistent with all the members of both
series (Figure S6, Table S3, Table S4), suggesting that the
molecules are contacted through the sulfur atoms in the
molecular junctions. The histograms composed from M2/N2 and
M3/N3 appear to capture a broader range of conductance
features than those of M1/N1 (Figure 5c, Figure 5d; Table S7).
This arises because the conductances of M2/N2 and M3/N3 are
very close to the lower detection limit of the instrument used for
the STM-BJ measurements (-7 ~ -6.5 log (G / G0)) and external
electromagnetic interference and mechanical vibration will have
more obvious influence on these conductance data. In order to
avoid the influence of the unstable molecular junctions on the
measurements, 1D conductance histograms have also been
reconstructed from data points only within the limited
displacement range (0.9 ~ 1.1 nm) where the conductance
plateaus are relatively flat and close to full extension (Figure
S7). The conductance peaks and corresponding most probable
conductance values from this limited data set are very similar to
the results of the original full-range 1D conductance histograms,
suggesting that the unstable junctions or lower yields of
junctions might affect the shapes and distributions of 1D
conductance histograms but not distinctly change the
conductance values.
In conclusion, we have investigated the possibility of tuning
DQI within m-OPE derived molecules by placing OMe pendant
‘taps’ at different positions around the central ring. Our
combined DFT predictions and MJBJ measurements
demonstrate that the position and orientation of the OMe taps
have a significant impact on the energy of the DQI induced dips
in the transmission function. As a consequence of this DQI
tuning, the conductance of M1/N1 is almost one order of
magnitude higher than the parent m-OPE system, whereas the
conductances of M3/N3 remain low. On the other hand, the
influence of the OMe group on the mid-gap DQI feature also
strongly depends on the on-off conformation of the OMe tap.
This is illustrated by M2/N2, whose most energetically
favourable conformation is ‘off’ and corresponds to weak
coupling between the pendant group and the molecular core.
This work presents a simple and convenient approach for
structurally tuning room-temperature DQI at a single-molecule
level and demonstrates a new strategy for designing molecular
devices with enhanced switching functionality.
Experimental Section
Synthesis: The target compounds M1-3 were prepared by
Sonogashira cross-coupling of an appropriate isomer of 1,3-
diethynyl anisole with S-4-iodophenyl ethanethioate. Compound
N1 was prepared in an analogous fashion from 1,3-diethynyl-4-
methoxybenzene and (4-iodophenyl)(methyl)sulfane, whilst it
proved more convenient to prepare N2 and N3 from (4-
ethynylphenyl)(methyl)sulfane and the appropriate 1,3-diiodo or
1,3-dibromo anisole as detailed in the supporting information (SI
section S8).
To exclude the influence of physisorbed sulphur contacts and
formation of junctions retaining the acetyl-protecting groups,[31]
single-molecule measurements of the M-series of molecules
were also recorded after adding 3 equiv. of Bu4NOH to the
sample solution to ensure cleavage of the acetyl moiety. The
resulting conductance-distance traces and conductance
histograms were consistent with those of junctions formed from
the same molecules by spontaneous cleavage of the protecting
group in pristine solvent (Figure S8-S10).[31-32] To rule out the
further possibilities of intramolecular π-π stacking effects within
the junction[33] or adventitious electrode contacts to the methoxy
Conductance measurements: Conductance measurements
were made using the STM-BJ technique to form gold-molecule-
gold junctions from 0.1 mM solutions of the target compound in
1,3,5-trimethylbenzene (TMB, 99%, Sigma-Aldrich used as
received). Gold wire (99.99%, 0.25mm diameter) was purchased
from Beijing Jiaming Platinum Nonferrous Metal Co, Ltd. for the
moiety[34]
,
two
model
tolane
compounds
(4-((4-
fabrication of STM tip.
A
polytef liquid cell and
a
methoxyphenyl)ethynyl)phenyl) (methyl)sulfane (O1) and (4-((3-
methoxyphenyl)ethynyl)phenyl) (methyl)sulfane (O2) were also
measured (Figure S11). Under conditions identical to those used
in the studies of M1-3 and N1-3, no clear molecular plateaus
were observed in the conductance histograms of O1 and O2.
These control experiments, together with the excellent
agreement of the break-off distance and the molecular length of
M1-3 and N1-3 give us confidence that the single-molecule
conductance features illustrated in Figure 5 can be attributed to
single-molecule junctions formed by contact of the gold
electrodes to the two sulfur atom anchors of M1-3 and N1-3.
The conductance values (G) within each series follow a
pronounced pattern of variation, with G (M1 / N1) > G (M3 / N3)
> G (M2 / N2). This pattern of behaviour is entirely consistent
with the broad predictions of the magic ratio theory described
above, combined with the DFT predictions for the conformation
of the OMe pendant relative to the central phenylene ring, which
determines the on-off nature of the OMe ‘tap’.
perfluoroelastomer O-ring (FFKM 6.07 × 1.78) were purchased
from Wuxi Bo Yate Sealing Technology Development Co. Ltd.
All measurements were carried out at room temperature.
Substrates were prepared by depositing gold film on the N<111>
monocrystalline face of
a silicon wafer. The gold-coated
substrate were cleaned by immersion in piranha solution (V
(H2SO4): V(H2O2) = 3:1 CAUTION piranha solution is extremely
corrosive) for ca. 2 hours, before being rinsed in fresh, boiling
deionized water for 5-15 minutes. After this time the substrate
was moved into fresh deionised water and the boiling procedure
repeated for a total of three cycles. The polytef liquid cell and a
perfluoroelastomer O-ring were also cleaned in a similar fashion
before being clamped to the substrate to create the liquid cell.
The gold wire was carefully cleaned and annealed in a butane
flame before use. After measurement, 1D conductance
histograms were used to determine the most likely conductance
values, whilst 2D conductance-displacement cloud maps
provided additional information of the stretching distance for the
molecular plateaus (SI section S5, section S6).
Theoretical methods: The optimized geometry and ground
state Hamiltonian and overlap matrix elements of each structure
were
self-consistently
obtained
using
the
SIESTA
Conclusion
implementation of density functional theory (DFT). SIESTA
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10.1002/anie.201909461
Angewandte Chemie International Edition
RESEARCH ARTICLE
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This work was supported by the National Key R&D Program of
China (2017YFA0204902), National Natural Science Foundation
of China (Nos. 21673195, 21703188, 21503179). It was also
supported by EPSRC grants EP/P027156/1, EP/N03337X/1,
EP/N017188/1, the EC H2020 FET Open project 767187
“QuIET”, the EU project Bac-to-Fuel and the Australian
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10.1002/anie.201909461
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RESEARCH ARTICLE
RESEARCH ARTICLE
Feng Jiang, Douglas I. Trupp,
Norah Algethami, Haining Zheng,
Wenxiang He, Afaf Alqorashi,
Chenxu Zhu, Chun Tang, Ruihao
Li, Junyang Liu, Hatef Sadeghi, Jia
Shi, Ross Davidson, Marcus Korb,
Alexandre N. Sobolev, Masnun
Naher, Sara Sangtarash,* Paul J.
Low,* Wenjing Hong,* Colin J.
Lambert*
The change of the conformation and
location of an –OMe group at
different positions can serve as a
‘molecular tap’, which will precisely
control the ‘on-off’ state of the m-
OPE in their electrical conductance
and offer a convenient and simple
way to control room-temperature
charge transport at the single-
molecule scale.
Page No. – Page No.
Turning the Tap: Conformational
Control of Quantum Interference
to Modulate Single Molecule
Conductance
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