Hemilabile Ligands as Mechanosensitive Electrode Contacts for Molecular Electronics

Article abstract of DOI:10.1002/anie.201906400

Single-molecule junctions that are sensitive to compression or elongation are an emerging class of nanoelectromechanical systems (NEMS). Although the molecule–electrode interface can be engineered to impart such functionality, most studies to date rely on poorly defined interactions. We focused on this issue by synthesizing molecular wires designed to have chemically defined hemilabile contacts based on (methylthio)thiophene moieties. We measured their conductance as a function of junction size and observed conductance changes of up to two orders of magnitude as junctions were compressed and stretched. Localised interactions between weakly coordinating thienyl sulfurs and the electrodes are responsible for the observed effect and allow reversible monodentate?bidentate contact transitions as the junction is modulated in size. We observed an up to ≈100-fold sensitivity boost of the (methylthio)thiophene-terminated molecular wire compared with its non-hemilabile (methylthio)benzene counterpart and demonstrate a previously unexplored application of hemilabile ligands to molecular electronics.

Full text of DOI:10.1002/anie.201906400

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Hemilabile ligands as mechanosensitive electrode contacts for
molecular electronics
Authors: Nicoló Ferri, Norah Algethami, Andrea Vezzoli, Sara
Sangtarash, Maeve McLaughlin, Hatef Sadeghi, Colin J.
Lambert, Richard J. Nichols, and Simon J. Higgins
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10.1002/anie.201906400
Hemilabile ligands as mechanosensitive electrode contacts
†
for molecular electronics
#
#
#,*
#,*
Nicolò Ferri , Norah Algethami , Andrea Vezzoli , Sara Sangtarash , Maeve McLaughlin,
*
*
Hatef Sadeghi, Colin J. Lambert , Richard J. Nichols and Simon J. Higgins
†
Supporting Information (SI) available: Synthetic procedures and characterization data, further details on the STM-BJ
experiments, equipment used, measurement procedure and data analysis algorithms, and further theoretical calculations.
Dr. Nicoló Ferri, Dr. Andrea Vezzoli, Ms. Maeve McLaughlin, Prof. Dr. Richard J. Nichols, Prof. Dr. Simon J. Higgins
Department of Chemistry, University of Liverpool, Crown Street, Liverpool L69 7ZD, U.K.
E-mail for Prof. Dr. Simon J. Higgins: shiggins@liverpool.ac.uk
E-mail for Dr. Andrea Vezzoli: andrea.vezzoli@liverpool.ac.uk
Ms. Norah Algethami, Dr. Sara Sangtarash, Dr. Hatef Sadeghi, Prof. Dr. Colin J. Lambert
Department of Physics, Lancaster University, Lancaster LA1 4YB, U. K.
E-mail for Prof. Dr. Colin J. Lambert: c.lambert@lancaster.ac.uk
E-mail for Dr. Sara Sangtarash: s.sangtarash@lancaster.ac.uk
Dr. Sara Sangtarash and Dr. Hatef Sadeghi:
School of Engineering, University of Warwick, Conventry CV4 7AL, U.K.
*
corresponding authors
#
these authors contributed equally to this work
1
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Abstract
Single-molecule junctions that are sensitive to compression or elongation are an emerging class of
nanoelectromechanical systems (NEMS). Although the molecule-electrode interface can be
engineered to impart such functionality, most studies to date rely on poorly-defined interactions. We
focused on this issue by synthesizing molecular wires designed to have chemically-defined hemilabile
contacts, based on (methylthio)thiophene moieties. We measured their conductance as a function of
junction size, and we observed conductance changes of up to two orders of magnitude as junctions
were compressed and stretched. Localized interactions between weakly-coordinating thienyl sulfurs
and the electrodes are responsible for the observed effect, which allow reversible
monodentate bidentate contact transitions as the junction is modulated in size. We observed an up to
~
100-fold sensitivity boost of the (methylthio)thiophene-terminated molecular wire compared with its
non-hemilabile (methylthio)benzene counterpart, and demonstrate a previously unexplored
application of hemilabile ligands to molecular electronics.
Main Text
Hemilabile ligands are heterobifunctional ligands in which one donor coordinates strongly to a metal
and the other coordinates weakly, often reversibly, as a consequence of the chelate effect. They have
[
1]
found wide applications in homogeneous catalysis, where they are used, for instance, to reduce the
activation energy of steps such as oxidative addition / reductive elimination and increase reaction rate
and turnover, to stabilise reactive intermediates and boost yield values, and to obtain high
[2]
enantiomeric excesses in catalytic hydrovinylation reactions. More niche applications can be found
[3]
in metal-organic frameworks (MOF) to greatly increase their hydrolytic stability, and in their use as
[4]
[5]
chemosensors and redox-switchable ligands. The possibility of using external stimuli to change
[6–8]
the coordination motif (monodentate-chelate transitions)
triggered our interest in their
[
9]
development as contacts for molecular electronics, where metallophilic termini (e.g. thiols,
[
10]
[11]
[12]
[13]
amines,
thioethers,
phosphines,
selenols,
etc.) are regularly used to chemically and
electrically couple the molecular backbone to the nanoelectrodes and fabricate metal-molecule-metal
2
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devices. In such junctions, the interface between the molecule and the electrodes is a key parameter
for the control of its charge-transport properties. In addition to introducing an additional (contact)
[11]
resistance,
the metal-molecule interface controls the extent of coupling between the molecular
backbone and the two metallic leads, which can drastically affect the final conductance of the
[
14,15]
junction.
The zero-bias electrical conductance of a molecular electronic device is defined by the Landauer
[16]
formula, which takes the form
ꢀ
ꢁꢀꢃ(ꢄ)
(1)
ꢂ
ꢅ
ꢈ
at low temperature, where ꢀꢂ is the quantum of conductance (ꢆꢇ ⁄ꢉꢊꢋꢌꢌꢍꢎꢏꢊꢐꢑ) and the
transmission coefficient ꢃ(ꢄ)is the probability for an electron of energy ꢄto tunnel through the
molecule. The transmission coefficient for a one-level system with a molecular orbital of energy ꢒ
[17]
coupled to the electrodes can be described by the Breit-Wigner formula
ꢓꢔꢕ
ꢃ(ꢄ)ꢁꢊ
(2)
ꢅ
ꢕ
ꢕ
(
ꢖ ꢘꢙꢘꢚ) ꢛꢓꢔ
ꢗ
where ꢜis an energy shift of the orbital due to interactions with the electrodes, and ꢝdescribes the
strength of the coupling that causes a broadening of the orbital resonance, as shown in Figure 1a. It
can be inferred from the above equations that (i) the off-resonance transmission coefficient (and
therefore the conductance of the molecular junction) is strongly dependent on the strength of the
coupling to the electrodes and (ii) if ꢝcan be modulated reversibly and reproducibly in response to an
external stimulus, then this would yield a functional nanoelectronic device. We focussed our efforts
on the use of mechanical forces as the triggering stimuli, because these have been used to exert some
degree of control on ꢝ by exploiting weak interactions between the metallic electrodes and a
[
18]
neighbouring π-system in a hapto coordination, for instance by using 4-pyridyl contact groups, or
[19–21]
vicinal phenyl rings.
In what follows, we demonstrate that hemilabile ligands also satisfy the
above requirements, with further advantages over the existing strategies lying in chemical control on
ꢝmodulation, chemically-defined contact configurations and improved sensitivity and amplitude of
3
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conductance changes. The reason lies in an enhanced metal-molecule coupling in the bidentate
contact compared with the monodentate configuration (Figure 1c), with a fully reversible mono- to
bidentate transition and reproducible behaviour imparted by the hemilabile ligands.
F
Figure 1: a) Examples of Breit-Wigner resonances for transport through a LUMO of energy E +0.5 eV, over
a range of values of the coupling parameter ꢝ. No additional energy shift ꢜwas used in the calculations for
clarity. b) Schematics of the hemilabile ligand used in this study, (methylthio)thiophene and (c) examples of
its monodentate and bidentate contact configurations at a Au electrode. d) Structures and numbering of the
compounds used in this study.
We designed molecular wires with hemilabile contacts, where the strong coordination site is placed at
the two termini of the molecule, and compression of the junction would allow the interaction of the
metal electrode with a weaker coordinating group, promoting a transition to a bidentate configuration
and increasing the coupling parameter ꢝ. We synthesised molecules terminated with chemically-
defined hemilabile (methylthio)thiophene contacts (Figure 1b). The methyl thioether substituent acts
as primary contact and creates strong mechanical coupling, while the thienyl moiety interacts less
strongly with the metallic electrode (through the S-atom lone pair), thereby providing the additional
electronic coupling to enhance conductance in the compressed junction, with a DFT-calculated
4
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S
THIOETHER↔STHIOPHENE separation of 2.84 Å. Thiophenes are known to interact with Au electrodes,
[
22–28]
and have been successfully used as molecular termini in several single-entity electronics studies.
[
29]
The interaction is reported as being weaker than traditional contact groups,
consistent with the
and their established use in hemilabile ligand
thus making them an ideal “supporting” molecular contact to the stronger methyl
thioether. Furthermore, there are reports in the literature of unusual properties of oligothiophene-
[
30–32]
known coordination chemistry of thiophenes
[33–35]
design,
[
36–38]
based molecular wires, showing non-monotonic conductance attenuation with length,
stretching-
[38]
[39]
induced conductance decrease and large spread of conductance values (see SI for further details).
We reasoned these were strong enough evidence in support of our hemilabile contact design, worthy
[21]
of a more thorough investigation with an electromechanical approach.
We synthesized the compounds presented in Figure 1c (synthetic details and characterization in the
SI), focusing on the bridged bithiophenes 1 - 2 to reduce the conformational isomerism issues arising
[39]
with conventional oligothiophenes, which results in a large spread of conductance values.
We
[
18,20]
characterized the electromechanical behaviour of these compounds by employing a modified
[
9]
scanning tunnelling microscopy break junction (STM-BJ) technique. In a typical experiment, a
piezoelectric actuator drives a Au tip into a Au substrate under fixed bias, to form a contact with
conductance >5 G . The Au-Au junction is then abruptly stretched by 1 nm, and a modulation is
0
applied to the piezo signal for 100 ms (Figure 2a). The junction is then further stretched and
modulated several times, to ensure the formation of a single-molecule junction and its subsequent
clean rupture, with an overall stretching of 8 nm. The tip is then driven towards the substrate until the
contact once again has a conductance >5 G , and the process is then repeated thousands of times.
0
After data collection, we used an algorithm to slice the traces between abrupt stretches and to discard
the traces with no evidence of junction formation and those where the single-molecule junction did
not survive the whole modulation process. On average across the experiment presented here, 38% of
the traces are kept by the algorithm, and these are subsequently compiled into density maps. More
details about the measurements and data analysis can be found in the methods section and the SI. We
started our investigation on compound 1 by applying a 3 Å square-wave modulation to the piezo
5
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actuator, which abruptly changes the electrode separation and holds the position for 12.5 ms to allow
junction relaxation. The amplitude of 3 Å was chosen to be very close to the S↔S distance in the
(
methylthio)thiophene moiety, and therefore a mechanically-induced monodentateꢞbidentate
transition is expected.
Figure 2: Example of a typical 3 Å modulation measurement (a), with the imposed piezo signal (red) and the
corresponding conductance trace (blue), showing the correlation between increase in conductance and
junction compression. Idealized position of the electrodes during the modulation is depicted in (b).
Conductance vs time density maps for compound 1 (c) and 3 (d), under square-wave modulation of 3 Å.
Density maps are normalized to the number of scans used to compile them, which is 828 (c) and 980 (d). The
structure of the molecular wire is superimposed on the relevant density map.
As can be observed in Figure 2c, the square-wave modulation applied to the electrode separation
results in large and well-defined changes in the conductance of 1. Conductance modulation is reduced
when the amplitude of the square wave applied to the piezo is decreased from 3 Å to 2 Å but does not
change significantly when the amplitude is increased to 4 Å (See SI for density maps). For
comparison, the biphenyl-based compound 3 showed a very small conductance change upon
modulation (Figure 2d), which might be due to increased electrode-electrode direct tunnelling or the
already reported increased interactions of the electrodes with the phenyl π-system as these junctions
[19,21]
are compressed (lateral coupling).
The striking difference in behaviour between these two simple
biaryl compounds suggests that the presence of an additional, labile binding site lead to a larger
6
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overall increase of the coupling parameter ꢝ, as hypothesised earlier. To further characterise this
phenomenon, and the versatility of (methylthio)thiophene as a hemilabile contact moiety, we turned
our attention to compounds 2 and 4. In compound 2 the carbonyl substituent has an electron
withdrawing effect, and we reasoned this would result in reduced thienyl S-electrode coupling,
thereby decreasing the amplitude of mechanoresistive modulation. Compound 4 is a longer
oligothiophene, and its purpose is to test whether mechanosensitive properties are retained in longer
molecular wires. By using the same 3 Å square-wave modulation presented earlier we found, as
expected, that molecule 2 shows conductance changes of significantly reduced magnitude compared
with 1 (Figure 3a). Well-defined conductance variations are also found for 4 (Figure 3b), although this
longer molecule also shows a smaller modulation than 1.The overall results confirm that the labile
behaviour of the thienyl moiety is indeed responsible for the observed behaviour, with 1 being the
compound providing the largest conductance variation upon modulation of the electrode position as
evidenced by analysing the modulation profile (Figure 3c). The fact that both conformationally-locked
molecules 1 and 2, and conformationally-flexible 4 show significant modulation in conductance upon
junction compression supports the notion that an explanation for the conductance switching based
[
40]
[36]
upon possible different conformers of oligothiophenes like 4 is not tenable here.
Figure 3: Conductance vs. time density maps of compound 2 (a) and 4 (b) under 3 Å modulation. The
structure of the molecular wire is superimposed on the relevant density map. Density maps are normalized to
7
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the number of scans used to compile them, which is 1221 (a) and 1072 (b). 1D histograms (c) of the four
compounds presented in this study, obtained from the last modulation shown in the density maps, by
collapsing it on the y axis. The compressed junction is reproduced in darker colour, and the extended
junction is overlaid in lighter colour.
In order to better characterise the phenomena behind the metal-molecule monodentateꢞbidentate
contact transitions we applied more incremental compression/elongation ramps, which would also
allow us to assess the evolution of the coupling parameter ꢝas the junction is compressed and relaxed,
to gain insights into the kinetics of hemilabile coordination. Surprisingly, we did not observe a sudden
change in conductance as the junction is slowly compressed or stretched, marking a clear
monodentate bidentate configuration change. The overall results (see Figure 4a) suggest there is no
abrupt coordination transition for these compounds, but rather an increasing metal-thiophene
interaction as the two entities are brought in spatial vicinity, which accounts for the observed increase
in conductance. The junctions we fabricated behave therefore more like a nanoelectromechanical
rheostat than a simple, ON/OFF molecular switch. For example, by slowly compressing (stretching)
junctions fabricated with 1 with a triangular ramp at a rate of 24 nm/s, their conductance continuously
increases (decreases) at an average rate of ꢇꢟꢂꢍꢠꢡꢊ⁄ꢢ, and similar results were obtained under
sinusoidal modulation of the junction size. This absence of a clean coordination change may be of
interest for the possible mechanisms of metallorganic configuration transitions in the fluxional
[
41,42]
[43–45]
behaviour in solution of thienyl-based
and other weak-link hemilabile ligands,
although
other factors will be of importance in solution reactions, such as solvation energies and related
factors’, and have later emphasised the relevance to single molecule junctions.
8
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Figure 4: Conductance signal for compound 1 under different types of modulation (a) and under high-speed
10 kHz) trapezoidal modulation (b). The piezo signal is displayed along with the conductance curves in red.
(
Only a 2 ms portion of a single high-frequency modulation trace is displayed here for clarity. Additional data
can be found in the SI.
We also performed high-frequency modulation experiments (10 kHz, Figure 4b), and compound 1
showed regular changes in conductance correlated with the signal imposed to the piezo actuator
(additional details in the SI), with no apparent fatigue for up to 1000 modulations. It should be noted
that a 10 kHz frequency is at our instrument limit (200 kSa/s data acquisition system, resulting in 20
points per modulation cycle) and it is therefore possible that even higher frequencies can be attained.
To better understand the phenomena leading to the observed changes in conductance, and thereby to
quantify the effect of variations of ꢝ, we used density functional theory (DFT) to compute the
quantum transport properties for all the molecules (see SI for details). The quantum transport code
[
46]
Gollum was used to calculate the transmission coefficient ꢃ(ꢄ)for electrons of energy E passing
from the source to the drain electrode via the molecule. Using compound 4 as an example (Figure 5a),
we predict that at small tip-tip distances, the gold electrodes interact with both thienyl and thioether
sulfurs, resulting in high molecule-electrode coupling and a broadening of transport resonances
(Figure 5c). As the electrode separation is increased, the coupling to the thiophene moiety is reduced
and the resonance becomes sharper. No such effects are found in 3 (Figure 5b and 5d). It is worth
9
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noting that, when there is a change in the degree of molecule-electrode Au-S interaction, the position
of the energy levels relative to ꢄ changes as well. This is due to different degrees of charge transfer,
ꢅ
which lead to shifts of the whole transmission spectrum.
Figure 5: Relaxed structure of molecule 4 (a) and 3 (b) between two Au electrodes, at various tip-tip
distances (11.9 and 8.8 Å respective starting separation, increased at any step by approximately 2 Å). Key: H
=
white, C = grey, S = orange, Au = yellow. Corresponding transmission coefficient T(E) versus electron
energy of compound 4 (c) and 3 (d) at the four tip-tip distances. ꢄis plotted relative to the DFT-predicted
Fermi energy of gold (as ꢄꢣꢄ). The grey region shows the window of Fermi energy values where the
ꢅ
amplitude of conductance is in good agreement with measured values.
To predict the effect on the conductance ꢀꢁꢀꢃ(ꢄ), a value for ꢄ is needed. In Figure 5c-d, the
ꢂ
ꢅ
ꢅ
HOMO and LUMO levels correspond to the resonances in the transmission plots ꢃ(ꢄ)located
immediately below and above ꢄꢣꢄ ꢁꢤrespectively. The precise values of the HOMO and LUMO
ꢅ
levels relative to ꢄ (and therefore the exact zero of the horizontal axis) depend on environmental
ꢅ
conditions and on the unknown shape of the electrodes. However, since the molecules are neither
oxidized nor reduced, ꢄ lies within the energy gap between the HOMO and the LUMO, and the
ꢅ
qualitative change in the conductance with tip-tip distance is determined by the behaviour of ꢃ(ꢄ)
within the gap. In compound 4, the additional thiophene-Au interactions described earlier result in
10
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broadened transport resonances with an accompanying increase of the mid-gap value of ꢃ(ꢄ)as the
junction is compressed, and therefore the conductance is also expected to increase. In compound 3,
which lacks a thiophene moiety, the resonances stay sharp as the junction is compressed and value of
ꢃ(ꢄ)within the gap does not change significantly. Therefore, the conductance is predicted to be
almost independent of the electrode separation. The process was repeated for compounds 1 and 2
(details in the SI) and, in agreement with our experiments, compressing the electrodes leads to strong
interactions between the electrodes and the thiophene moieties, causing significant variations of the
transmission coefficient over a wide range of energies within the HOMO-LUMO gap. Furthermore,
for values of ꢄ lying in the window highlighted as a grey box in Figure 5c-d, there is good
ꢅ
agreement with the experimental values shown in Figure 3c, with compound 1 having the largest
variations in ꢃ(ꢄ)as the electrode separation is modulated, and compound 3 showing little or no
effect. By fitting the Breit-Wigner formula to the LUMO resonance of the ꢃ(ꢄ)curves for the four
compounds presented in this study, we found a pattern of increasing values of ꢝfor compounds 1, 2
and 4 as the junction is compressed, but ꢝfor compound 3 does not change significantly over a range
of several Å of junction size modulation (see SI for more details). As further confirmation of the
proposed mechanism, the coupling parameters for compound 3 were found to be consistently lower
than for compound 1 at similar junction size, in agreement with the reduced conductance modulation
observed during junction compression and elongation.
In conclusion, we have shown how hemilabile ligands, in this case based on the
(
methylthio)thiophene moiety, can be exploited to impart mechanosensitive behaviour to a single-
molecule junction. The presence of multiple binding sites and the monodentate bidentate contact
transition was exploited to modulate the coupling parameter ꢝas the junction is compressed and
relaxed. The junctions displayed chemically-controlled sensitivity, can operate at kHz speed and show
no sign of fatigue for up to 1000 cycles. In addition to expanding the molecular electronics toolbox
with new design ideas for molecular termini and contacts, our results shed light on the mechanism of
configuration transitions in thienyl-based hemilabile ligands, showing that the fabrication of single-
molecule junctions doubles also as a valuable characterisation tool to probe metal-ligand interactions.
11
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Furthermore, (methylthio)thiophenes and thiophenethiols are widely used as molecular wire termini in
[
36,39,47–53]
molecular electronics,
and our results unambiguously demonstrate that contact configuration
changes must be taken into account when interpreting their conductance vs electrode separation
trajectories during a single-entity break-junction experiment.
Experimental Section
Synthesis of compounds 1-4: Compound 1 was synthesized by methylation of commercial 4H-
cyclopenta[2,1-b:3,4-b']dithiophene with methyl iodide and potassium hydroxide, followed by
selective bromination with NBS, halogen-lithium exchange with n-butyllithium and quench with
dimethyl disulfide. Compound 2 was prepared from commercial 2,2’-bithiophene by tetrabromination
with elemental bromine, followed by halogen-lithium exchange with n-butyllithium and quench with
dimethyl disulfide to yield 3,3’-dibromo-5,5’-bis(thiomethyl)-2,2’-bithiophene. Halogen-lithium
exchange with n-butyllithium, followed by low-temperature quench with dimethylcarbamoyl chloride
gave 2. Compound 3 was prepared by halogen-lithium exchange of 4,4’-diiodo-1,1’-biphenyl with n-
butyllithium, followed by quench with dimethyl disulfide. Compound 4 was prepared by double direct
lithiation of 2,2':5',2''-terthiophene with n-butyllithium, followed by quench with dimethyl disulfide.
Detailed procedures and characterization data can be found in the SI.
Scanning Tunneling Microscope Break Junction: molecular junctions are formed by repeatedly
driving a Au tip into an Au substrate and then pulling it apart in a solution of the desired molecular
wire (mesitylene, 1 mM). A constant bias (200 mV) is applied between the tip and the substrate
during the measurements, which are performed at room temperature. The tip is retracted in a stepwise
manner, creating a nanogap which is held for 100 ms while its size is modulated (see Figure 2a for the
signal applied to the piezoelectric actuator). The tip is then retracted until junction rupture and driven
into the substrate again. The process is continuously repeated. 1000 cycles are performed on the bare
Au substrate prior to the measurements, to perform a mechanical annealing of the tip and ensure
absence of contaminants. After the introduction of the molecular wire, 2000-3000 scans are collected
and then analyzed using automated algorithms. Details on the instrumentation used and data analysis
can be found in the SI.
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DFT calculations: the optimized geometry and ground state Hamiltonian and overlap matrix
[
54]
elements of each structure was self-consistently obtained using the SIESTA implementation of
density functional theory (DFT). SIESTA employs norm-conserving pseudo-potentials to account for
the core electrons and linear combinations of atomic orbitals to construct the valence states. The
generalized gradient approximation (GGA) of the exchange and correlation functional is used with the
Perdew-Burke-Ernzerhof parameterization (PBE) a double-ζ polarized (DZP) basis set, a real-space
grid defined with an equivalent energy cut-off of 250 Ry. The geometry optimization for each
structure is performed to the forces smaller than 10 meV/Å.
Transport calculations: The mean-field Hamiltonian obtained from the converged DFT calculation
[
46]
was combined with Gollum implementation of the non-equilibrium Green’s function method to
calculate the phase-coherent, elastic scattering properties of each system consist of left gold (source)
and right gold (drain) leads and the scattering region.
Data Availability
NMR spectra (Bruker format) for compounds 1 – 4 and raw STM-BJ data can be found in the data
catalogue in Liverpool at https://datacat.liverpool.ac.uk/id/eprint/549 and at DOI:
10.17638/datacat.liverpool.ac.uk/549.
Acknowledgements
This work was supported by UK EPSRC (grants EP/M005046/1, EP/M029522/1, EP/M014169/1
EP/P027156/1, EP/N03337X/1, EP/N017188/1) and the ECH2020 FET Open project 767187
“
QuIET”. H.S. acknowledges the Leverhulme Trust for Early Career Fellowship ECF-2017-186 and
UKRI for the Future Leaders Fellowship no. MR/S015329/1. S.S. acknowledges the Leverhulme
Trust for Early Career Fellowship ECF-2018-375.
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References
[1]
[2]
[3]
A. Bader, E. Lindner, Coord. Chem. Rev. 1991, 108, 27–110.
T. V. RajanBabu, Chem. Rev. 2003, 103, 2845–2860.
L. N. McHugh, M. J. McPherson, L. J. McCormick, S. A. Morris, P. S. Wheatley, S. J. Teat,
D. McKay, D. M. Dawson, C. E. F. Sansome, S. E. Ashbrook, et al., Nat. Chem. 2018, 10,
1
096–1102.
S. E. Angell, C. W. Rogers, Y. Zhang, M. O. Wolf, W. E. Jones, Coord. Chem. Rev. 2006,
50, 1829–1841.
H. F. Cheng, A. I. D’Aquino, J. Barroso-Flores, C. A. Mirkin, J. Am. Chem. Soc. 2018, 140,
4590–14594.
[
4]
5]
2
[
1
[
6]
7]
A. J. M. Miller, Dalt. Trans. 2017, 46, 11987–12000.
[
A. M. Lifschitz, C. M. Shade, A. M. Spokoyny, J. Mendez-Arroyo, C. L. Stern, A. A. Sarjeant,
C. A. Mirkin, Inorg. Chem. 2013, 52, 5484–5492.
[8]
A. M. Allgeier, C. S. Slone, C. A. Mirkin, L. M. Liable-Sands, G. P. A. Yap, A. L. Rheingold,
J. Am. Chem. Soc. 1997, 119, 550–559.
[
[
9]
B. Xu, N. Tao, Science 2003, 301, 1221–1223.
10] Y. Zang, A. Pinkard, Z.-F. Liu, J. B. Neaton, M. L. Steigerwald, X. Roy, L. Venkataraman, J.
Am. Chem. Soc. 2017, 139, 14845–14848.
[
[
[
11] P. Moreno-García, M. Gulcur, D. Z. Manrique, T. Pope, W. Hong, V. Kaliginedi, C. Huang,
A. S. Batsanov, M. R. Bryce, C. Lambert, et al., J. Am. Chem. Soc. 2013, 135, 12228–12240.
12] Y. S. Park, A. C. Whalley, M. Kamenetska, M. L. Steigerwald, M. S. Hybertsen, C. Nuckolls,
L. Venkataraman, J. Am. Chem. Soc. 2007, 129, 15768–9.
13] S. Yasuda, S. Yoshida, J. Sasaki, Y. Okutsu, T. Nakamura, A. Taninaka, O. Takeuchi, H.
Shigekawa, J. Am. Chem. Soc. 2006, 128, 7746–7.
14
This article is protected by copyright. All rights reserved.

Angewandte Chemie International Edition
10.1002/anie.201906400
[
[
[
[
[
14] C. J. Lambert, Chem. Soc. Rev. 2015, 44, 875–888.
15] M. Koch, Z. Li, C. Nacci, T. Kumagai, I. Franco, L. Grill, Phys. Rev. Lett. 2018, 121, 47701.
16] R. Landauer, IBM J. Res. Dev. 1957, 1, 223–231.
17] G. Breit, E. Wigner, Phys. Rev. 1936, 49, 519–531.
18] S. Y. Quek, M. Kamenetska, M. L. Steigerwald, H. J. Choi, S. G. Louie, M. S. Hybertsen, J. B.
Neaton, L. Venkataraman, Nat. Nanotechnol. 2009, 4, 230–234.
[
[
[
[
[
[
[
19] I. Diez-Perez, J. Hihath, T. Hines, Z.-S. Wang, G. Zhou, K. Müllen, N. Tao, Nat. Nanotechnol.
2011, 6, 226–231.
20] A. K. Ismael, K. Wang, A. Vezzoli, M. K. Al-Khaykanee, H. E. Gallagher, I. M. Grace, C. J.
Lambert, B. Xu, R. J. Nichols, S. J. Higgins, Angew. Chemie Int. Ed. 2017, 56, 15378–15382.
21] R. Ramachandran, H. B. Li, W.-Y. Lo, A. Neshchadin, L. Yu, J. Hihath, Nano Lett. 2018, 18,
6638–6644.
22] M. Kiguchi, T. Ohto, S. Fujii, K. Sugiyasu, S. Nakajima, M. Takeuchi, H. Nakamura, J. Am.
Chem. Soc. 2014, 136, 7327–32.
23] R. Frisenda, R. Gaudenzi, C. Franco, M. Mas-Torrent, C. Rovira, J. Veciana, I. Alcon, S. T.
Bromley, E. Burzurí, H. S. J. van der Zant, Nano Lett. 2015, 15, 3109–3114.
24] J.-C. Mao, L.-L. Peng, W.-Q. Li, F. Chen, H.-G. Wang, Y. Shao, X.-S. Zhou, X. Zhao, H.-J.
Xie, Z. Niu, J. Phys. Chem. C 2017, 121, 1472–1476.
25] A. Etcheverry-Berríos, I. Olavarría, M. L. Perrin, R. Díaz-Torres, D. Jullian, I. Ponce, J. H.
Zagal, J. Pavez, S. O. Vásquez, H. S. J. van der Zant, et al., Chem. - A Eur. J. 2016, 22,
12808–12818.
[26] C. R. Arroyo, S. Tarkuc, R. Frisenda, J. S. Seldenthuis, C. H. M. Woerde, R. Eelkema, F. C.
Grozema, H. S. J. van der Zant, Angew. Chemie Int. Ed. 2013, 52, 3152–3155.
15
This article is protected by copyright. All rights reserved.

Angewandte Chemie International Edition
10.1002/anie.201906400
[
[
[
27] S. Bock, O. A. Al-Owaedi, S. G. Eaves, D. C. Milan, M. Lemmer, B. W. Skelton, H. M.
Osorio, R. J. Nichols, S. J. Higgins, P. Cea, et al., Chem. - A Eur. J. 2017, 23, 2133–2143.
28] Y. Ie, K. Tanaka, A. Tashiro, S. K. Lee, H. R. Testai, R. Yamada, H. Tada, Y. Aso, J. Phys.
Chem. Lett. 2015, 6, 3754–3759.
29] F. Bejarano, I. J. Olavarria-Contreras, A. Droghetti, I. Rungger, A. Rudnev, D. Gutiérrez, M.
Mas-Torrent, J. Veciana, H. S. J. van der Zant, C. Rovira, et al., J. Am. Chem. Soc. 2018, 140,
1691–1696.
[
[
[
30] J. Chen, R. J. Angelici, Coord. Chem. Rev. 2000, 206–207, 63–99.
31] R. J. Angelici, Organometallics 2001, 20, 1259–1275.
32] T. Jiang, W. Malone, Y. Tong, D. Dragoe, A. Bendounan, A. Kara, V. A. Esaulov, J. Phys.
Chem. C 2017, 121, 27923–27935.
[33] D. A. Weinberger, T. B. Higgins, C. A. Mirkin, C. L. Stern, L. M. Liable-Sands, A. L.
Rheingold, J. Am. Chem. Soc. 2001, 123, 2503–2516.
[34] O. Clot, M. O. Wolf, G. P. A. Yap, B. O. Patrick, J. Chem. Soc. Dalt. Trans. 2000, 2729–2737.
[
35] D. A. Weinberger, T. B. Higgins, C. A. Mirkin, L. M. Liable-Sands, A. L. Rheingold, Angew.
Chemie Int. Ed. 1999, 38, 2565–2568.
[
36] B. Capozzi, E. J. Dell, T. C. Berkelbach, D. R. Reichman, L. Venkataraman, L. M. Campos, J.
Am. Chem. Soc. 2014, 136, 10486–92.
[
37] L. Xiang, T. Hines, J. L. Palma, X. Lu, V. Mujica, M. A. Ratner, G. Zhou, N. Tao, J. Am.
Chem. Soc. 2016, 138, 679–687.
[38] B. Q. Xu, X. L. Li, X. Y. Xiao, H. Sakaguchi, N. J. Tao, Nano Lett. 2005, 5, 1491–5.
[39] E. J. Dell, B. Capozzi, K. H. DuBay, T. C. Berkelbach, J. R. Moreno, D. R. Reichman, L.
Venkataraman, L. M. Campos, J. Am. Chem. Soc. 2013, 135, 11724–7.
16
This article is protected by copyright. All rights reserved.

Angewandte Chemie International Edition
10.1002/anie.201906400
[
[
[
40] L. Mejía, N. Renaud, I. Franco, J. Phys. Chem. Lett. 2018, 9, 745–750.
41] S. Liu, C. R. Lucas, M. J. Newlands, J. P. Charland, Inorg. Chem. 1990, 29, 4380–4385.
42] G. R. Haire, N. E. Leadbeater, J. Lewis, P. R. Raithby, A. J. Edwards, E. C. Constable, J.
Chem. Soc. Dalt. Trans. 1997, 2997–3004.
[43] F. E. Kühn, W. A. Herrmann, M. Cokoja, A. Raba, A. Pöthig, K. Riener, M. J. Bitzer, Inorg.
Chem. 2014, 53, 12767–12777.
[44] G. M. Adams, A. S. Weller, Coord. Chem. Rev. 2018, 355, 150–172.
[45] E. Lindner, M. Haustein, H. A. Mayer, H. Kühbauch, K. Vrieze, B. de Klerk-Engels,
Inorganica Chim. Acta 1994, 215, 165–172.
[46] J. Ferrer, C. J. Lambert, V. M. García-Suárez, D. Z. Manrique, D. Visontai, L. Oroszlany, R.
Rodríguez-Ferradás, I. Grace, S. W. D. Bailey, K. Gillemot, et al., New J. Phys. 2014, 16,
093029.
[47] M. Tateno, M. Takase, T. Nishinaga, Chem. Mater. 2014, 26, 3804–3810.
[48] G. Gryn’ova, P. J. Ollitrault, C. Corminboeuf, Phys. Chem. Chem. Phys. 2017, 19, 23254–
23259.
[
[
[
[
49] W. B. Chang, C.-K. Mai, M. Kotiuga, J. B. Neaton, G. C. Bazan, R. A. Segalman, Chem.
Mater. 2014, 26, 7229–7235.
50] Z. Cai, W.-Y. Lo, T. Zheng, L. Li, N. Zhang, Y. Hu, L. Yu, J. Am. Chem. Soc. 2016, 138,
1
0630–10635.
51] D. Dulić, F. Pump, S. Campidelli, P. Lavie, G. Cuniberti, A. Filoramo, Angew. Chemie Int. Ed.
009, 48, 8273–8276.
2
52] C. Kergueris, J. P. Bourgoin, D. Esteve, C. Urbina, M. Magoga, C. Joachim, Phys. Rev. B -
Condens. Matter Mater. Phys. 1999, 59, 12505–12513.
17
This article is protected by copyright. All rights reserved.

Angewandte Chemie International Edition
10.1002/anie.201906400
[53] D. Dulić, S. van der Molen, T. Kudernac, H. Jonkman, J. de Jong, T. Bowden, J. van Esch, B.
Feringa, B. van Wees, Phys. Rev. Lett. 2003, 91, 207402.
[
54] J. M. Soler, E. Artacho, J. D. Gale, A. García, J. Junquera, P. Ordejón, D. Sánchez-Portal, J.
Phys. Condens. Matter 2002, 14, 2745–2779.
Table of Contents
Molecular junctions with hemilabile contact moieties show
enhanced mechanoresistive behaviour. The molecule-metal
contact is forced to transition from a monodentate to a bidentate
configuration as the junction is compressed and stretched, with a
resulting modulation in conductance of up to two orders of
magnitude.
Keywords





Density Functional Calculations
Hemilabile Ligands
Molecular Electronics
Molecular Devices
S Ligands
18
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