Side-Group-Mediated Mechanical Conductance Switching in Molecular Junctions

Article abstract of DOI:10.1002/anie.201709419

A key target in molecular electronics has been molecules having switchable electrical properties. Switching between two electrical states has been demonstrated using such stimuli as light, electrochemical voltage, complexation and mechanical modulation. A

Full text of DOI:10.1002/anie.201709419

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Side Group-Mediated Mechanical Conductance Switching in
Molecular Junctions
Authors: Simon J. Higgins, Ali Khalid Ismael, Kun Wang, Andrea
Vezzoli, Mohsin K. Al-Khaykanee, Harry E. Gallagher, Iain M.
Grace, Colin J. Lambert, Bingqian Xu, and Richard J. Nichols
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10.1002/anie.201709419
Angewandte Chemie International Edition
Side Group-Mediated Mechanical Conductance Switching in Molecular
Junctions
Ali Khalid Ismael#, Kun Wang#, Andrea Vezzoli#, Mohsin K. Al-Khaykanee, Harry E.
Gallagher, Iain M. Grace, Colin J. Lambert*, Bingqian Xu*, Richard J. Nichols, and
Simon J. Higgins*
#: these Authors contributed equally to this work.
*: Corresponding Authors
Dr. A. Vezzoli, Mr. H. E. Gallagher, Prof. Dr. R. J. Nichols, Prof. Dr. S. J. Higgins
Department of Chemistry, University of Liverpool, Crown Street, Liverpool L69 7ZD, U.K.
E-mail for Prof. Dr. S. J. Higgins: shiggins@liverpool.ac.uk
http://pcwww.liv.ac.uk/~nichols
Dr. K. Wang, Prof. Dr. B. Xu
College of Engineering & Department of Physics and Astronomy, University of Georgia, 220 Riverbend Road, Athens,
GA 30602, U.S.A.
E-mail for Prof. Dr. B. Xu: bxu@engr.uga.edu
http://xulab.uga.edu
Dr. A. K. Ismael, Mr. M. K. Al-Khaykanee, Dr. I. M. Grace, Prof. Dr. C. J. Lambert
Department of Physics, Lancaster University, Lancaster LA1 4YB, U. K.
E-mail for Prof. Dr. C. J. Lambert: c.lambert@lancaster.ac.uk
Dr. A. K. Ismael
Department of Physics, College of Education for Pure Science, Tikrit University, Tikrit, Iraq
Mr. Mohsin K. Al-Khaykanee
Department of Physics, College of Science, University of Babylon, Iraq.
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10.1002/anie.201709419
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Abstract
A key target in molecular electronics has been molecules having switchable electrical properties.
Switching between two electrical states has been demonstrated using such stimuli as light,
electrochemical voltage, complexation and mechanical modulation. A classic example of the latter is the
switching of 4,4’-bipyridine, leading to conductance modulation of ~1 order of magnitude. Here, we
describe the use of side-group chemistry to control the properties of a single-molecule
electromechanical switch, which can be cycled between two conductance states by repeated
compression and elongation. While bulky alkyl substituents inhibit the switching behaviour, π-
conjugated side-groups reinstate it. DFT calculations show that weak interactions between aryl moieties
and the metallic electrodes are responsible for the observed phenomenon. This represents a significant
expansion of the single-molecule electronics “tool-box” for the design of junctions with
electromechanical properties.
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Main Text
The ability to reliably cycle a metal-molecule-metal junction between two distinct conductive states is
technologically important for the development of devices with transistor or switch properties, and could
at the same time give valuable insights into the chemical and physical behaviour of isolated molecules
at the nanoscale. So far, a variety of stimuli has been demonstrated to trigger a reversible change in the
conductance state, the most studied being light and electrochemical potential. In these cases, the
stimulus acts on the molecular backbone through which charge is transported, for instance by
photoexcitation,[1] photoinduced E-Z isomerization[2] or cyclization[3–5] and electrochemical reduction or
oxidation.[6–10] A conductance switch can be induced in a molecular device also through mechanical
stimuli such as junction compression or elongation. In this case, the molecular component of the
junction is left chemically unchanged, and the switching behaviour depends on the properties of the
contacts and the molecule-metal interface. A classic example of this phenomenon is the switching of
pyridyl-terminated molecular wires (such as 4,4’-bipyridine 1) sandwiched between Au contacts, with a
conductance modulation of ~1 order of magnitude upon junction compression-elongation.[11] In this
case, the switching properties are due to the nature of the pyridyl-Au contact, which allows multiple
coordination geometries. It is general consensus that junctions are in a “low conductance” state when
the Au-N bond is perpendicular to the conducting π-system, and in a high conductance state when the
junction is tilted, with the pyridyl ring lying co-facially on the metallic contact. Conductance changes
upon mechanical modulation have also been observed in methyl-thioether terminated oligosilanes,
where the behaviour is due to the stereoelectronic properties of the contacts and the orientation of the
metallic lead with respect to the molecular backbone.[12] However, it has not been established how
chemical design and substitution can be used to control such mechanically actuated molecular switches.
In this submission, we studied the structural and chemical features necessary to control the reversible
switching properties of a single molecule junction, based on functionalised biaryl compounds.
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Figure 1: Structure of 4,4’-bipyridine 1 and synthetic pathway to compounds 2-6. i) LDA (1 h, -94 ºC, THF), CuCl2 (16
h, RT). ii) nBuLi, (1 h, -78 ºC, THF), tBuPCl2 (15 min, 40 ºC), followed by H2O2 (4 h, RT, CH2Cl2) iii) nBuLi, (1 h, -78
ºC, THF), CyPCl2 (15 min, 40 ºC), followed by H2O2 (4 h, RT, CH2Cl2). iv) nBuLi, (1 h, -78 ºC, THF), Ph2SiCl2 (16 h,
RT). v) nBuLi, (1 h, -78 ºC, THF), PhPCl2 (15 min, 40 ºC), followed by H2O2 (4 h, RT, CH2Cl2).
The substituted dipyridyls 2-6 were synthesised as described in Figure 1,[13,14] and the Scanning
Tunnelling Microscopy - Break Junction (STM-BJ) technique[15] was used in this study to fabricate
single-molecule junctions and to characterise their electrical behaviour. In brief, junctions are formed by
repeatedly driving a gold tip into and out of contact with a gold substrate, in the presence of molecules
with appropriate gold-binding end-group (adsorbed on the substrate). The current I is recorded during
the withdrawal process, under a constant bias V of 300 mV, and transport through molecular bridges
results in plateaux in the traces (examples in inset of Figure 2b). The process is repeated thousands of
times and the results are compiled in histograms, where the plateaux in individual current-displacement
traces result in peaks in the histograms that represent the distribution of conductance G values.
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Figure 2: Structure of 3 (a) and 4 (c) in the Au-Au junction. Conductance histogram of 3 (b) and 4 (d), with example
current-displacement traces as inset. Histograms compiled from >2000 current traces, at 20 nm s-1 withdrawal speed,
using a tip-substrate bias of 300 mV. Conductance G is calculated using Ohm’s law (G = I / V) and is given in units of
the quantum of conductance G0 ≈ 77.48 µS.
The phosphoryl-bridged compounds 3 and 4 both produced quite broad conductance histograms
although with no obvious signature of bistable conductance (Figure 2), in contrast with the clear double
peak observed for 1[16] and other pyridyl-terminated molecular wires.[17] The peak conductance values
for 3 and 4 are lower than for unsubstituted 1, with a most probable value of 2 × 10-4 G0 for 3 and 3.5 ×
10-4 G0 for 4. Although it may be tempting to suggest that there is non-switching behaviour, arising
from rotational constraint of the two pyridyl rings with the molecule being unable to adopt one of the
two possible geometries at the electrode contacts, the one-dimensional histograms are clearly very broad.
Interestingly, although 3,3’-dibromo-4,4’-bipyridine 2 is also rotationally constrained (in this case, the
dibromo substituents enforce orthogonality and make the coplanar geometry too high in energy), it
nevertheless does show two clear peaks in the conductance histogram (details in the SI, Figure S1), and
the values are consistent with the mechanism suggested by Quek et al.[11] To better understand the
observed phenomena, we performed piezo-modulation experiments.[18–20] Here, the junction was first
closed to achieve a conductance of several G0, the tip was abruptly stretched ~1 (± 0.1) nm (ds) to create
a nanogap that could accommodate the dipyridyl molecule, and the size of this nanogap was then
modulated by 0.2 nm (dm) 4 times in 30 ms (modulation frequency of 135 Hz). The modulation value
of 0.2 nm was chosen because it proved optimal to promote the switching between the high and low
conductance states of 4,4’-bipyridine 1 (more details in the SI).[11] After this modulation, the junction
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was stretched a further 1 nm, and hence broken. Example conductance traces along with their
corresponding piezo-transducer (PZT) movement signal are shown in Figure 3.
Figure 3: Example conductance vs time trace under piezo modulation for compounds 2 (a), 3 (c) and 4 (e), with piezo
signal superimposed as grey line. Modulation 2d density maps for compounds 2 (b), 3 (d) and 4 (f). Structure and piezo
signal are superimposed on the 2d maps for clarity.
Compound 2 behaved similarly to 4,4’-bipyridine 1, with a switching of about 1 order of magnitude
upon piezo modulation (Figure 3b). This phenomenon has been ascribed to different geometries at the
Au-N interface adopted by the molecule upon junction compression and stretching.[11] In contrast, 3 and
4 showed no conductance switching at all, and the modulation only resulted in increased noise (Figure
3d and 3f, respectively). We reasoned that the presence of bulky alkyl side-groups effectively prevent
the molecule from adopting the “high” conductance geometry suggested by Quek et al[11] by simple
steric hindrance. It is, therefore, an effective way to shut down the mechanically triggered conductance
switching of such molecular junctions.
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Figure 4: Conductance histogram (a) with example traces as inset, and modulation 2d density map (b) for compound 5.
Conductance histogram (c) with example traces as inset, and modulation 2d density map (d) for compound 6.
Experiments performed at 300 mV tip-substrate bias. Piezo signal as grey line and the molecular are superimposed on
the 2d maps for clarity.
To further investigate this phenomenon, we turned our attention to the Si-bridged dipyridyl 5, with two
orthogonal phenyl rings on the heteroatom. STM-BJ measurements on 5 resulted in a broad peak in its
conductance histogram, with a maximum at 2 × 10-4 G0 and a pronounced shoulder at 10-3 G0 (Figure
4a). Interestingly, piezo-modulation experiments on 5 showed the signature of cycling through the two
conductance states instead of shutting down the conductance switching as in the case of compound 4
(example conductance traces in Figure S2 of the SI). Metal/molecule/metal junctions with phenyl rings
acting as contacts (e.g. through η2 coupling) have been reported in the literature, for instance in
cyclophanes[21], C60,[22] and organometallic molecular wires.[23] We therefore hypothesised that the
switching mechanism proposed by Quek et al.[11] is disabled in 3 and 4 due to the steric bulk of the alkyl
substituents, but interactions of a side π-system with the metallic lead reinstate the binary conductance
behaviour. To further test this hypothesis and determine whether it is the Si bridge or the side-group
responsible for the switching properties, we prepared 6, which has a phenyl moiety on a phosphoryl
bridge. In Figure 4d, the piezo-modulation measurements on 6 revealed clear conductance switching
(example conductance traces in Figure S2 of the SI). In the case of 6 (phosphoryl bridge), the average
ratio between the two conductance states is only ≈ 2, increasing to ≈ 5 in 5 (Si bridge). Therefore, STM-
BJ measurements confirmed that it is the aryl substituent that reinstates conductance switching,
although the bridging heteroatom identity modulates its magnitude. As further evidence for the
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proposed mechanism, we noted that the piezo-modulation density maps for compounds 5 (Figure 4b)
and 6 (Figure 4d) show noisier switching than 2 (Figure 3b), consistent with the high mobility of the
phenyl side-substituent responsible for the increase in conductance. It was also found that the switching
behaviour is retained at a higher modulation frequency (400 Hz, see Supporting Information, Figure S4).
To validate the proposed switching mechanism, we performed DFT quantum transport calculations,
exploring both the Au-pyridyl contact geometry and the binding characteristics of the side group on the
metalloid bridge. As described in the SI, a variety of different junction geometries were explored (for
more details, see figures S6 to S14), to assess the most likely configuration of the molecular bridge and
the molecule-electrode coupling strength. The geometry which showed the higher binding energy (see
figure S6) in the compressed junction and better explained the experimental results is the one where the
dipyridyl unit lies tilted between the two leads, with a N terminus contacted to an under-coordinated Au
atom and the other end in a co-facial arrangement with the Au electrode. Results of DFT calculations on
4 and 5 are presented in Figure 5. In the relaxed junction, the molecular bridge lies parallel to the
electrode axis, and transport is LUMO-dominated, with a sharp resonance near EF. Upon junction
compression, the molecular bridge tilts in the junctions to reach the described geometry where the
pyridyl ring lies co-facially to the electrode. While it has previously been shown that such arrangement
leads to an increase in conductance, in the case of 4 there is little difference in the transmission curves
T(E) for the relaxed geometry A and the compressed B. The bulky side-group of 4 causes the Au-N
bond to stretch to ~ 0.3 nm, reducing the binding energy and the coupling to the electrode. The shorter
charge transport pathway is therefore countered by the weak coupling, resulting in no broadening of the
resonances in the T(E) curves and negligible change in the conductance value.
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Figure 5: DFT-optimised geometries for compound 4 (a) and 5 (b), in the relaxed (A) and compressed (B) geometry.
Relative transmission curves in each geometry for 4 (c) and 5 (d).
T(E) for compound 5 (Figure 5d) shows a different behaviour, with a broadening of molecular
resonances resulting a clear increase in the off-resonant transmission values upon junction compression,
that can be ascribed to an increase in the molecule-lead coupling strength. The binding geometry at the
co-facial end does not change from 4 (Figure 5a and 5b), and consequently the resonance broadening
arises from interactions between the π-system of the phenyl side-groups and the Au electrode, which
increase the overall coupling. Performing the same calculations on 3 and 6 resulted in a similar
predicted behaviour (details in the SI). We note that the introduced transport mechanism represents the
first example of conductance switching mediated by apparently innocuous substituents orthogonal to the
conductance path, which have no continuous σ-bond with the junction leads. Furthermore, our study
highlights the need for a more thorough characterisation of single-molecule junctions beyond the
traditional crash-withdraw STM-BJ technique, as many subtle but important effect can be hidden in the
broad distribution of conductance values routinely observed in one-dimensional histograms.
In conclusion, we presented the synthesis and single-molecule conductance measurements of a family
of planarized 4,4’-dipyridyls, with the two rings bridged in 3 and 3’ position by a heteroatom. The
resulting planarization (with formation of a 5-membered ring) and the presence of bulky orthogonal
substituents effectively switch off the mechanically controlled binary conductance switching of 4,4’-
dipyridyl 1,[11] by reducing the molecule-lead coupling strength in the high-conductance geometry. The
phenomenon can be reinstated by introducing aryl orthogonal substituents, which cause a broadening of
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10.1002/anie.201709419
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the transport resonance by interacting with the Au leads. The extent of switching depends on the
bridging heteroatom (conductance change of P-bridged 6 = 1 × 10-4 G0 and Si-bridged 5 = 8 × 10-4 G0),
showing the subtle effect of entities outside of the main conjugation path on the behaviour of single-
molecule junctions. We envisage that the mechanism of conductance switching demonstrated here may
stimulate the synthesis and testing of other related molecules.
Experimental Section
Synthetic procedures for the compounds used in this study, technical details on the STM measurements,
further information and parameters for the theoretical calculations, and additional experimental and
computational results can be found in the Supporting Information.
Data Availability
NMR spectra for compounds 2 - 6 are available in the data catalogue in Liverpool at:
https://datacat.liverpool.ac.uk/id/eprint/405
Acknowledgements
This work was supported by UK EPSRC (grants EP/H035184/1, EP/M005046/1, EP/N017188/1 and
EP/M014452/1), US NSF (grants ECCS 1231967 and ECCS 1609788), European Commission (project
606728 of FP7 ITN “MOLESCO”) and the Iraqi Ministry of Higher Education (SL-20). A.K.I.
acknowledges financial support from Tikrit University (Iraq), and M. K. A. acknowledges financial
support from Babylon University (Iraq).
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Table of Content
Designing molecular switches: The criteria for the development of single-molecule electromechanical
switches are presented in the manuscript. Conjugate moieties that provide additional coupling to the
metallic electrodes of a single-molecule junction increase the electron transmission efficient and
promote charge transport. The junction can therefore be cycled between two conductance states by
acting on the separation between the two electrodes, and chemical design can be used to tune the extent
of switching.
Keywords:
• Molecular Electronics
• Electron Transport
• Nanotechnology
• Scanning Probe Microscopy
• Density Functional Calculations
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