Carbazole-Based Tetrapodal Anchor Groups for Gold Surfaces: Synthesis and Conductance Properties

Article abstract of DOI:10.1002/anie.201911652

As the field of molecular-scale electronics matures and the prospect of devices incorporating molecular wires becomes more feasible, it is necessary to progress from the simple anchor groups used in fundamental conductance studies to more elaborate anchor

Full text of DOI:10.1002/anie.201911652

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
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Accepted Article
Title: Carbazole-Based Tetrapodal Anchor Groups for Gold Surfaces:
Synthesis and Conductance Properties
Authors: Martin Robert Bryce, Luke O'Driscoll, Xintai Wang, Michael
Jay, Andrei Batsanov, Hatef Sadeghi, Colin Lambert, and
Benjamin Robinson
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201911652
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10.1002/anie.201911652
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RESEARCH ARTICLE
Carbazole-Based Tetrapodal Anchor Groups for Gold Surfaces:
Synthesis and Conductance Properties
Luke J. O’Driscoll,[a] Xintai Wang,[b] Michael Jay,[b] Andrei S. Batsanov,[a] Hatef Sadeghi,[b,c] Colin J.
Lambert,[b] Benjamin J. Robinson[b] and Martin R. Bryce*[a]
Abstract: As the field of molecular-scale electronics matures and
the prospect of devices incorporating molecular wires becomes more
feasible, it is necessary to progress from the simple anchor groups
used in fundamental conductance studies to more elaborate anchors
designed with device stability in mind. This study presents a series
of oligo(phenylene-ethynylene) wires with one tetrapodal anchor and
a phenyl or pyridyl head group. The new anchors are designed to
bind strongly to gold surfaces without disrupting the conductance
pathway of the wires. Conductive probe atomic force microscopy
(cAFM) was used to determine the conductance of self-assembled
monolayers (SAMs) of the wires in Au−SAM−Pt and
Au−SAM−graphene junctions, from which the conductance per
molecule was derived. For tolane-type wires, mean conductances
per molecule of up to 10−4.37 G0 (Pt) and 10−3.78 G0 (graphene) were
measured, despite limited electronic coupling to the Au electrode,
demonstrating the potential of this approach. Computational studies
of the surface binding geometry and transport properties rationalise
and support the experimental results.
Thiols, thioethers, and pyridines, amongst others, are
common anchor groups in fundamental molecular conductance
studies using gold surfaces.[2] Recently, several new anchor
groups have been designed to exhibit increased affinity for gold
5]
surfaces, with various applications.[1a-c,
Some examples
pertinent to molecular electronics are shown in Figure S1.29 in
the Supporting Information (SI). In many cases, these anchors
are tripodal with three conventional anchor groups working
together to achieve enhanced surface binding, akin to the
chelate effect. Surface-π interactions can contribute additionally
to this binding. Tripodal anchors often incorporate an
sp3-hybridised carbon centre to induce the desired geometry.
High molecular conductance is associated with conjugation,[2a]
meaning that sp3-hybridisation is considered undesirable in
conductive materials, although a study of a pyridine-anchored
tripod describes contrasting results.[5a] Positioning the sp3-centre
outside the conductance pathway of the molecule can avoid any
impact on conductance.[5b] An additional feature of larger
anchors of this type is that they increase the spacing between
conductive molecular backbones and thereby prevent
intermolecular interactions.[5c]
Introduction
In this work, we present a new tetrapodal anchoring motif
designed to bind strongly to gold surfaces and form ordered self-
Anchor groups fulfil a critical role in materials designed to
assemble on surfaces.[1] In the field of molecular-scale
electronics, they serve as contact points between conductive
molecules and electrode surfaces. The strength of binding
interactions and extent of electronic coupling between an anchor
group and an electrode are important factors when designing
conductive organic materials.[2] For fundamental studies, an
anchor able to form a conductive junction with a lifetime longer
than a molecular conductance experiment is adequate. However,
when working towards the goal of functional devices based on
conductive molecules,[2a] such as thermoelectric generators[3]
and Peltier coolers,[4] strengthening the interactions between
these molecules and electrode surfaces is necessary to achieve
effective device lifetime and performance. Improved anchor
groups can also result in more ordered surface assemblies.
assembled monolayers (SAMs) with well-spaced conductive
5b, 5d-i]
backbones. Inspired by tripodal systems,[5a,
multiple
anchoring points are incorporated into our design to ensure
efficient surface binding. In contrast to many current approaches,
we aimed to separate these additional anchoring points from the
conductive
backbone
of
the
molecule
and
avoid
sp3-hybridisation in the conductive pathway. We note that while
this study focuses on the molecular electronics applications of
our new anchoring motif, it may also prove adaptable as a useful
scaffold for other applications, e.g. gold surface-assemblies of
7]
optoelectronic materials,[6] switches[5d,
initiators.[8]
or polymerisation
Results and Discussion
[a]
[b]
[c]
Dr L. J. O’Driscoll, Dr A. S. Batsanov, Prof. M. R. Bryce
Department of Chemistry
Durham University
Lower Mountjoy, Stockton Road, Durham, UK, DH1 3LE
E-mail: m.r.bryce@durham.ac.uk
Dr X. Wang, M. Jay, Dr H. Sadeghi, Prof. C. J. Lambert, Dr
B. J. Robinson
Physics Department
Lancaster University
Lancaster, UK, LA1 4YB
Dr H. Sadeghi,
Taking the ubiquitous oligo(phenylene-ethynylene) (OPE)
backbone as a starting point, we selected the meta-positions
with regard to the conductive backbone as an ideal point to
incorporate additional anchoring functionality. Conductance
through a meta-conjugated pathway is considerably lower than
the favoured para-conjugated pathway due to destructive
quantum interference (QI) effects.[2b, 9] Therefore, any additional
meta-functionalisation should contribute minimally to the
conductance of the molecule. Inspired in part by the
5e]
spirobifluorene tripods reported by the Mayor group,[5b,
School of Engineering
University of Warwick
carbazole was selected as the basis of the additional anchors.
This choice further disfavours any conductance through the
poorly conducting meta-positions as steric factors should induce
a significant twist between the backbone and carbazole π-
systems, reducing conjugation between them. Furthermore, this
Coventry, UK, CV4 7AL
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201911652
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RESEARCH ARTICLE
twist should help to direct the conjugated backbone away from
the surface. In addition to any interactions between its π-
electrons and the surface, carbazole can be readily
functionalised in the 3- and 6-positions, allowing convenient
incorporation of additional binding functionality.
To test this design the model compound 1 was initially
prepared (see SI, Section 1.6). The single crystal X-ray structure
(Figure 1a) confirmed the expected twist between the backbone
and the carbazole units. To investigate the anchoring properties
of the unit a series of OPE2 type molecules (where 2 refers to
should bind to metallic electrodes through the nitrogen lone pair,
with the para derivative expected to show a higher molecular
conductance than the meta derivative due to QI effects. The
structures of these species are shown in Scheme 1. Our naming
convention is to indicate the anchor unit (B or P) followed by the
head unit (B, pP or mP), e.g. BpP is the compound with a
benzene-based anchor and a para-pyridyl head.
A key building block is the thiomethyl-substituted carbazole
derivative 2. Although it has been previously reported, limited
experimental details and characterisation data are available.[11]
the number of aryl rings in the backbone) were then synthesised, We
developed
an
alternative
synthesis
using
each with one tetrapodal unit (anchor) and one simple terminal
aryl group (head). Two anchors were proposed, with either a
central benzene (X = CH) or pyridine (X = N) ring, in each case
functionalised with two 3,6-bis(methylthio)carbazole units, i.e.
four thiomethyl groups per anchoring unit. The thiomethyl anchor
groups were preferred to acetyl-protected thiols to ensure
compatibility with the synthetic route and to reduce synthetic
complexity. The use of multiple thioethers has been reported to
result in strong anchoring to a gold surface, where each
thioether makes a contribution comparable to that expected from
a thiol.[10] For the benzene systems, anchoring is expected from
only the functionalised carbazole subunits, whereas the pyridyl
nitrogen may be able to interact additionally with the gold
surface. Three head groups were investigated, namely benzene,
p-pyridine and m-pyridine. The benzene head group has no
defined anchor point and should therefore interact poorly with
metallic electrodes. Benzene should, however, be able to
interact with graphitic surfaces through π-π overlap at
appropriate contact angles. Both p- and m-pyridyl head groups
3,6-diiodocarbazole[12] as the precursor. After TBDMS-protection
of the N-position, thiomethyl substituents were introduced via
lithiation followed by treatment with dimethyl disulfide, although
optimisation was required to minimise by-product formation and
achieve practical yields (see SI, Section 1.7). Straightforward
deprotection of the TBDMS group using TBAF afforded 2. By
using nucleophilic aromatic substitution (SNAr) reactions it was
then possible to prepare the benzene- and pyridine-based
scaffolds 3 and 4 (Scheme 1). Both are functionalised with an
iodide group as a convenient synthetic handle for subsequent
synthetic transformations. This is an advantage of the SNAr
route; alternatives based on metal-catalysed coupling reactions
would have a higher potential for by-product formation and
would either rely on statistical reactions or utilise a less-reactive
bromide group as the resulting synthetic handle. A disadvantage
of the SNAr approach is that it is poorly compatible with central
rings bearing electron-donating substituents, as these disfavour
SNAr reactions.
Scheme 1. Synthesis of tetrapodal molecular wires. Reagents and conditions: a) Cs2CO3, DMF, 100 °C, 17 h; b) K2CO3, DMSO, 70 °C, 2 h; c) CuI, Pd(PPh3)2Cl2,
THF, DIPEA, RT, 80–120 min. For the tetrapodal molecular wires, the anchoring unit is coloured red, and the head unit blue (note that for B(OPE1)pP the head
unit pyridine ring also forms part of the anchor group).
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With the iodide species in hand, the targeted library of
asymmetric molecular wires was prepared using Sonogashira
protocols (Scheme 1). For comparison in the conductance
studies, a series of analogous OPE2 derivatives were also
prepared with a simple (protected) thiol anchor (denoted with S
in our naming convention) and the three head groups used in
the tetrapodal series (See SI, Section 1.8). To probe the effect of
changes to the length of the conductive backbone, the shorter
(B(OPE1)pP) and longer (B(OPE3)pP) analogues of BpP
(Scheme 1) were also prepared. They differ by the effective
anchors rather than thiols. Although it binds strongly, the latter
triptycene design includes sp3-carbons so is likely poorly suited
to molecular electronics applications.
subtraction or addition of
conductive OPE2 backbone.
a phenylethynyl unit into the
Single crystal X-ray structures were obtained for model
compound 1 and for BmP. For 1, the torsion angles between the
central benzene ring and the two carbazole units are 56° and
60° (Figure 1a), whereas for BmP, which showed noticeable
structural disorder in the positions of the pyridyl-nitrogen and the
thiomethyl groups (see SI, Section 2.1), these angles are 55°
and 39° (Figure 1b). Using the SIESTA package,[13] density
functional theory (DFT) simulations (see SI, Section 3.1) of
model, alkyne-terminated systems in the gas phase gave
corresponding angles of 38° and 53° for a benzene (X = CH)
base (Figure 1c) and 24° and 29° for a pyridine (X = N) base
(Figure 1d). The reduced torsion in the latter can be attributed to
reduced steric hindrance. H-bonding interactions may also
contribute (see SI, Section 2.2). In all cases, to differing extents,
the designed twist is observed between the carbazole and OPE
π-systems, which should reduce the influence of the carbazole
units on the conductive backbone. DFT was also used to
simulate the assembly of these model systems on Au(111)
surfaces. As shown in Figures 1e and 1f, the two carbazole units
splay out to lie essentially flat on the gold surface (indicative of
favourable surface-π interactions), with the OPE backbone
protruding at an angle. The angle between the gold surface and
the conductive backbone is 48° for a benzene base and 37° for
Figure 1. a) X-ray structure of model compound 1; b) X-ray structure of BmP;
c) Gas phase DFT relaxed model system with benzene base; d) Gas phase
DFT relaxed model system with pyridine base; e) model system with benzene
base relaxed on Au(111) surface; f) model system with pyridine base relaxed
on Au(111) surface.
a
pyridine base. Therefore, little conjugation is expected
between the conductive backbone and the carbazole units in this
conformation.
The properties of SAMs of the tetrapodal molecules on
Au(111) surfaces were assessed using a range of techniques.
Details of the preparation of SAMs, characterisation methods
and associated images can be found in the SI (Section 2.3).
Atomic force microscope (AFM) imaging showed that the SAMs
were densely packed films with uniform structures in the range
of 0.5 – 1 nm. The thickness of the SAMs was determined using
a nano-scratching method.[16] The averaged film thickness for
the OPE2 molecules was in the range of 0.6-0.65 nm, and for
OPE3 derivative B(OPE3)pP it was 0.9 ± 0.04 nm. These values
correspond to tilting angles of ca. 35° between the OPE
backbones and the substrate surface, in reasonable agreement
with the DFT calculations (Figures 1e and 1f). SAMs of the
model compound 1 could not be prepared on gold, showing that
the thiomethyl groups play a critical role in surface assembly.
DFT simulations confirm this, showing stepwise increases in
binding energy as thiomethyl groups are sequentially added to
the carbazole-based anchoring motif (see SI, Section 3.1 for
further discussion).
The binding energies of these conformers to a Au(111)
surface, calculated using the counterpoise correction method,[14]
are −1.86 eV (X = CH) and −1.68 eV (X = N). These values
suggest that the pyridine nitrogen atom does not enhance
surface binding (at least in this conformation), and in fact results
in slightly weaker surface interactions than the benzene
analogue. The binding energies of some simple, conventional
anchors were reported previously using the counterpoise
method.[15] Comparison with these values shows that both
tetrapodal anchors have considerably higher binding energies
than
amine
(−0.30
eV),
nitrile
(−0.41
eV),
dihydrobenzothiophene (−0.41 eV) and pyridine (−0.50 eV)
anchors, and have enhanced binding compared to thiols (−1.51
eV). A significant increase in binding energy is also observed in
comparison to phosphine-based tripodal anchors with three
thiomethyl anchoring groups (ca. −1 eV).[5f] The adsorption
energies of triptycene tripods with three thiol anchors, which
account for loss of hydrogen upon binding to a gold surface,
were found to be −1.62 eV (for less flexible aryl thiols) and -2.67
eV (for more flexible benzylic thiols).[5g] The tetrapodal anchors
perform comparably to the former case despite using thiomethyl
The density of molecules on the gold substrate was
determined using reductive desorption and quartz crystal
microbalance (QCM) measurements. Reductive desorption
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allowed the molecular area to be calculated based on the charge
density of the desorption peak, under the assumption that four
electrons correspond to desorption of a single molecule (i.e. one
electron per thiomethyl anchor group). A second cycle of
desorption confirmed that all molecules were desorbed from the
surface in the first sweep (SI, Figure S2.07). BB and PB were
investigated as representative molecules and had estimated
molecular occupation areas of 265 Å2 and 300 Å2, respectively.
This agrees well with a DFT-estimated molecular footprint of 265
Å2 for either benzene- or pyridine-based tetrapods (see SI,
Section 3.1). The desorption potential of benzene-based
material BB was −0.57 V vs. SCE, whereas for PB the
desorption potential was −0.67 V vs. SCE. In this case, the
pyridine-based anchoring unit appears to provide a small
additional anchoring effect compared to the benzene-based
analogue. Although this appears to disagree with the DFT-
calculated binding energies stated above, we note that the
methods may not be directly comparable as reductive desorption
relates to electrochemical stability, whereas the binding energies
relate to adsorption. QCM measurements (see SI, Section 2.5)
gave estimated molecular areas around 30% lower than those
derived from reductive desorption. AFM imaging of the QCM
substrate (SI, Figure S2.09) showed that the surface was much
rougher than those used in the other measurements and was
therefore likely to result in underestimation of molecular areas.
As the substrate used for reductive desorption is comparable to
those used in conductance studies, we believe this is the more
reliable method for determining molecular area in this case.
analogues prepared under the same conditions: there are clearly
some uncovered regions in the electrical map in the pyridine
case (c.f. PpP and BB, SI, Figure S2.11). The electrical
conductance of the SAMs was determined through cAFM IV
measurements using both Pt- and graphene-coated probes (See
SI, Section 2.6). After gently approaching the surface with a new
cAFM probe, the contact force was set to 2 nN and the bias
voltage was swept (typically from −1 V to +1 V) at at least 20
randomly selected locations on the sample surface (Figures 2c,
S2.12, S2.13 and S2.15). At least 3 IV sweeps were conducted
at each location and used to calculate the differential
conductance of the junction (GJ). The number of molecular
junctions formed between the probe and the substrate was
estimated using the Hertz model[17] (see SI, Section 2.6). This
allowed the conductance contribution of a single molecule (GM)
to be calculated for each IV curve. At least 60 IV curves were
measured for each molecule, giving a distribution of GM values
(Figures 2a, 2b, S2.14 and S2.16). The mean GM values for
each molecule are listed in Table 2.
Table 2. Summary of mean single-molecule conductance (GM) values
derived from cAFM measurements (uncertainties are standard deviations).
Structures of thiol-anchored reference compounds SB, SpP and SmP can
be found in Scheme S3 in the SI.
Pt probe
Graphene Probe
Molecule
GM / nS
log(GM/G0)
GM / nS
log(GM/G0)
BB
0.89 (±0.07)
1.36 (±0.14)
2.8 (±0.15)
3.3 (±0.36)
0.49 (±0.03)
0.97 (±0.10)
29.5 (±2.16)
0.047 (±0.01)
6.4 (±0.58)
12.8 (±1.25)
5.2 (±0.47)
−4.94
−4.76
−4.44
−4.37
−5.20
−4.90
−3.42
−6.22
−4.08
−3.78
−4.17
4.5 (±0.38)
−4.24
Table 1. Summary of values derived from reductive desorption and
quartz crystal microbalance studies
PB
6.5 (±0.89)
−4.08
BpP
8.6 (±0.94)
−3.95
Molecule
Desorption
Potential
vs. SCE
Molecular area
/
Molecular
/
V
Å2
(Reductive
area
/
Å2
PpP
12.9 (±1.02)
−3.78
Desorption)
265
(QCM)
BmP
PmP
2.5 (±0.22)
−4.49
BB
PB
−0.57
−0.67
187
220
4.1 (±0.41)
−4.28
301
B(OPE1)pP
B(OPE3)pP
SB
-
-
-
-
-
-
-
-
-
-
Investigations of the conductance of molecules with
multiple binding sites using single-molecule junction techniques
can be challenging, as various junction configurations are
possible.[5h] This generally results in complex conductance data
from which it can be difficult to determine the contributions of
different configurations. To avoid this complication, the
conductance properties of the tetrapodal wires were investigated
using conductive probe AFM (cAFM). The analysis of SAMs of
the tetrapods indicated that the molecules were binding to the
gold surface with all four thiomethyl groups in the designed
manner. Therefore, it was expected that the protruding head
groups would be contacted with the cAFM probe and afford the
desired Au−anchor−head−probe junction configuration. A further
advantage of cAFM is that measuring the conductance of SAMs
should be more representative of potential large-area devices
than single-molecule methods.
SpP
SmP
Using either Pt- or graphene-coated probes, for a given
head group, the average molecular conductance, GM, of a
pyridine-based wire is higher than that of the equivalent
benzene-based wire. However, the conductance distribution of
pyridine-based wires tends to be broader than that of the
benzene-based analogues (Figures 2a and S2.14 in the SI).
This agrees with the observation above that SAMs of pyridine-
based species have lower electrical uniformity than SAMs of
their benzene analogues. Given the close proximity to the Au
surface, the slightly reduced steric bulk of a pyridine lone pair
versus a benzene hydrogen atom could mean that more
molecular conformations are possible within the Au−SAM−probe
Electrical maps obtained via cAFM revealed that, in
general, the SAMs showed very good electrical uniformity.
However, the electrical uniformity of SAMs of tetrapods with
pyridine bases was poorer than that of SAMs of benzene-based
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ensemble in the pyridine case. The extent of electronic coupling
between the Au surface and the conductive backbone through
the pyridine nitrogen, a known anchor group,[18] could vary
graphene, which could enhance electronic coupling and
therefore increase conductance. Alternatively, π-π overlap could
move the electronic contact point further down the backbone,
effectively shortening the conductive pathway, thereby affording
a higher conductance.
significantly with conformation, thus affording
conductance distribution.
a
broad
For molecules with a given anchor group, GM varies with
head group according to the trend pP > B > mP in all cases. The
pP head group was expected to perform best with a metallic
probe, due to interactions between the probe and the pyridine
lone pair, which were anticipated to enhance electronic coupling,
and constructive QI effects. Interestingly, this head group also
showed the highest GM when using a graphene-coated probe.
These results are consistent with constructive (pP) and
destructive (mP) QI effects occurring for both probes, even
though in short single molecules, such effects are sensitive to
the nature of the molecule-electrode contact and can be masked,
due to parallel transport through the σ-channel and π-σ
mixing.[19] The higher conductance of the B head group relative
to mP suggests that π-probe interactions contribute significantly
to electronic coupling for both probes, as the former has no
other binding functionality.
The effect of molecular length on GM was studied using
BpP and its two analogues B(OPE1)pP and B(OPE3)pP. As
seen in Figures 2b-d, conductance decreases with molecular
length, as expected for OPE-type molecular wires. Indeed, a plot
of ln(GM) against DFT-relaxed molecular length gives the
expected linear trend (Figure 2d), allowing for estimation of the
tunnelling decay factor, β. For this series, β = 0.42 ± 0.03 Å-1,
which is slightly larger than the reported range for OPEs of 0.2-
0.34 Å-1.[2b, 18] This discrepancy could relate to the incorporation
of the pyridine head group into the tetrapodal base of
B(OPE1)pP in order to create a shorter analogue of BP. This
structural change may have an additional effect on conductance
and therefore distort the β-value. From the plot in Figure 2d the
contact resistance of the benzene-based anchoring unit is
estimated as 9.8 MΩ.
We further investigated these three molecules and two
additional, longer analogues using DFT-based charge transport
calculations with the Gollum package[20] (see SI, Section 3.2).
The logarithms of the calculated conductances of the OPE2 to
OPE5 species follow the expected linear trend with molecular
length, with β = 0.21 Å-1 (SI, Figure S3.02). The calculated
conductance of B(OPE1)pP is much higher than would be
expected by extrapolating this trend, indicating that it may not be
a representative member of the OPE series. In the simulations,
the short conductive backbone of B(OPE1)pP results in
additional electronic coupling between the top electrode and the
carbazole units, which affords additional conductance pathways
and a higher molecular conductance. When this additional
coupling is artificially removed, the calculated conductance is
lower and closer to the expected trend (SI, Figure S3.02). The
linear trend observed in the cAFM experiments suggests that
electronic coupling through the carbazole units is hindered in
this case. This could be due to roughness in the top contact or
the presence of solvent molecules.
Figure 2. a) Histograms of conductance per molecule (GM) for Au−SAM−Pt
junctions containing tetrapodal OPE2 molecular wires at low bias voltage
(−0.1 V to 0.1 V), fit curves are a guide for the eye; b) Histograms of GM values
for Au−SAM−Pt junctions containing tetrapodal molecular wires of different
lengths at low bias voltage (−0.1 V to 0.1 V), fit curves are a guide for the eye;
c) Differential conductance (per molecule) versus voltage for Au−SAM−Pt
junctions containing tetrapodal molecular wires of different lengths; d) Plot of
ln (GM) vs. molecular length which allows the tunnelling decay factor, β, to be
calculated.
GM for the tetrapodal wires is 3-5 times higher in
Au−SAM−graphene junctions than in Au−SAM−Pt junctions.
Stronger molecule-probe interactions are possible in the former
case as π-π overlap may occur between the head groups and
Compared to thiol-anchored analogues SB, SpP and SmP,
results from the Pt probe show that GM for the benzene- and
pyridine-based tetrapodal wires is 5-10 times and 4-5 times
lower, respectively. This is reasonable as a thiol anchor provides
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strong electronic coupling to a gold surface through Au−S bond
formation, whereas the conductive backbones of the tetrapodal
species are unable to interact directly with the surface to the
same extent. To probe the nature of the electronic coupling
between the tetrapods and the gold surface, we conducted
charge transport simulations in which BB was compared to a
simple, unfunctionalised OPE2, which was held in the same
geometry as the OPE2 backbone of BB when relaxed in a
junction configuration (Figures 3a and 3b). The resulting
transmission functions (Figure 3c) are remarkably similar,
suggesting that the carbazole anchoring units have negligible
influence on electronic coupling and therefore on GM, despite
providing an efficient means of surface binding. In effect, our
molecular design decouples surface binding from electronic
coupling. This important observation potentially allows for
investigations of unconventional or weakly binding anchor
groups, which could be held in place near the surface by a
tetrapodal unit, allowing their electronic coupling to be probed
regardless of their surface binding properties.
using scanning tunnelling microsocopy or mechanically
controlled break-junction methods (STM-BJ and MCBJ,
respectively). Any comparisons with our GM values obtained
using cAFM therefore require benchmarking. The molecular
conductances of thiol-anchored species SpP and SmP have
been reported using the MCBJ method.[21] Our cAFM method
results in slightly lower conductances for both SpP
(log GM/G0 = −3.78
vs
log G/G0 = −3.2[21]
)
and
comparable
SmP
(log GM/G0 = −4.17 vs log G/G0 = −3.9[21]);
a
deviation between cAFM and MCBJ has been observed
previously.[22] Such discrepancies may result from differences in
the nature of the junctions or assumptions made when
calculating GM from GJ. It can be concluded, however, that it is
reasonable to compare our GM values with conductances
determined using MCBJ, with the caveat that the GM values may
be slight underestimates. As MCBJ conductances have been
shown to be comparable with those obtained using STM-BJ,[15,
23] it follows that comparisons with STM-BJ data are also valid.
Spirobifluorenes are amongst the most conductive tripodal
molecular wires, with reported[5b] log G/G0 of −3.2 (STM-BJ) and
−3.0 (MCBJ) for a wire of similar length to our OPE2 derivatives
(log GM/G0 ≤ −4.37), but with much stronger direct thiol
anchoring of the conductive backbone to the gold surface.
Although they are more conductive, the spirobifluorenes require
a much lengthier synthesis than the tetrapods. The OPE2
tetrapods have conductances comparable to molecular wires
based on tetraphenylmethane tripods, with three thiol anchors
(log G/G0 = −3.4 for a slightly shorter wire and log G/G0 = −4.8
for a slightly longer wire, both STM-BJ[5h]), pyridine anchors
(log G/G0 = −4.5
(STM-BJ)[5i]
)
or
thiophene
anchors
(log G/G0 = −4.7 (STM-BJ)[5i]).
Conclusion
Tetrapodal anchor units for gold surfaces based on thiomethyl-
substituted carbazole have been developed. The synthetic route
is convenient and adaptable for other possible applications. The
anchor units were incorporated into OPE-type molecular wires
bearing benzene, para-pyridine or meta-pyridine head groups,
and their conductance was investigated using cAFM and charge
transport calculations. The molecules form uniform SAMs on
Au(111) in which the carbazole π-systems and sulfur atoms bind
to the surface with the conductive OPE backbones protruding at
an angle of 35-50°. The stabilising effect of multiple thiomethyl
groups was demonstrated using DFT-calculated binding
energies, which were higher than those of conventional
anchoring groups and analogues where thiomethyl groups were
sequentially removed. Using cAFM, the conductance of
Au−SAM−Pt and Au−SAM−graphene junctions was measured
and used to determine the conductance contribution per
molecule, GM, based on the contact area of the AFM probe and
the area occupied by a molecule as determined by reductive
desorption studies. For OPE2 tetrapods in Au−SAM−Pt junctions,
GM varied from 10-5.20 G0 (BmP) to 10-4.37 G0 (PpP). In all cases,
tetrapods with pyridine in the base unit gave higher GM than their
benzene-based analogues, but tended to show broader
conductance distributions. For a given base, GM varied with
Figure 3. a) DFT-relaxed geometry of a BB tetrapod in a molecular junction;
b) a junction containing only the conductive OPE backbone of BB, held in the
same position as the backbone in a); c) Red curve: transmission function for a
BB tetrapod in a molecular junction as shown in a), Blue curve: transmission
function for the OPE2 backbone of BB, with the carbazole-based anchoring
groups removed, held in a molecular junction as shown in b).
Section 3.2 of the SI discusses additional charge-transport
calculations investigating the effects of factors such as the
spacing between the molecules and the top electrode, the tilt
angle of the conductive backbone in the junction, and binding
geometries between the head group and top contact.
The single-molecule conductance of several tripodal
molecular wires has been reported in the literature, typically
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10.1002/anie.201911652
Angewandte Chemie International Edition
RESEARCH ARTICLE
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Acknowledgements
We thank EPSRC for support (grants EP/P027520/1,
EP/P027156/1, EP/N017188/1 and EP/N03337X/1). This work
was additionally supported by the European Commission
through the FET Open projects 767187 (QuIET) and 766853
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assembled monolayers of molecular
wires with new anchoring units
containing four thiomethyl groups
were formed on gold and their
conductance was measured. The
tetrapodal anchors provide effective
binding but contribute minimally to
the conductance pathway of the
wires. Surface binding and electronic
coupling can therefore be considered
independently.
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