Trimethylsilyl-terminated oligo(phenylene ethynylene)s: An approach to single-molecule junctions with covalent Au-C σ-bonds

Article abstract of DOI:10.1021/ja307544w

A new and efficient approach using cleaving of trimethylsilyl groups to create covalent Au-C anchoring sites has been developed for single-molecule junction conductance measurements. Employing the mechanically controllable break junction (MCBJ) technique in liquid, we demonstrate the formation of highly conducting single molecular junctions of several OPE derivatives. The created junctions are mechanically stable and exhibit conductances around one order of magnitude higher than those of their dithiol analogues. Extended assembly and reaction times lead to oligomerization. Combined STM imaging and gap-mode Raman experiments provide structure evidence to support the formation of covalent Au-C contacts and further oligomerization.

Full text of DOI:10.1021/ja307544w

Article
pubs.acs.org/JACS
Trimethylsilyl-Terminated Oligo(phenylene ethynylene)s: An
Approach to Single-Molecule Junctions with Covalent Au−C σ‑Bonds
Wenjing Hong, Hui Li, Shi-Xia Liu,* Yongchun Fu, Jianfeng Li, Veerabhadrarao Kaliginedi,
Silvio Decurtins, and Thomas Wandlowski*
Department of Chemistry and Biochemistry, University of Bern, CH-3012 Bern, Switzerland
S
* Supporting Information
ABSTRACT: A new and efficient approach using cleaving of
trimethylsilyl groups to create covalent Au−C anchoring sites
has been developed for single-molecule junction conductance
measurements. Employing the mechanically controllable break
junction (MCBJ) technique in liquid, we demonstrate the
formation of highly conducting single molecular junctions of
several OPE derivatives. The created junctions are mechan-
ically stable and exhibit conductances around one order of
magnitude higher than those of their dithiol analogues.
Extended assembly and reaction times lead to oligomerization.
Combined STM imaging and gap-mode Raman experiments provide structure evidence to support the formation of covalent
Au−C contacts and further oligomerization.
Alternative strategies to create covalent, highly directional
single metal (e.g., Au, Pt)−C bonds may involve aryldiazonium
salts29−33 or alkynyl compounds.34−42 Transition metal alkynyl
σ-complexes are well-known in the context of organometallic
coordination chemistry.34,35 However, anchoring a molecule via
an alkynyl group through a covalent carbon σ-bond to metal
surfaces such as gold and other coinage metals,37,38 remains
challenging.
Indeed, several groups reported the grafting of R−CCH
derivatives on rough36 and single crystalline Au(111)
surfaces,39,40 as well as on gold nanoparticles.37,38,41 DFT
calculations on the adsorption of an ethynylbenzene radical on
Au(111) showed that a strong covalent bond is formed with the
surface upon removal of the terminal hydrogen of the ethynyl
group.39 The fcc hollow sites are the most energetically
favorable with an interaction energy of ∼2.99 eV per bond. The
molecule is proposed to be adsorbed perpendicularly to the
surface through the terminal carbon. The authors in ref 39
discuss also an alternative pathway involving the heterolytic
cleaving of the C−H bond in R−CCH, followed by the
surface binding of an anion. This route is thought to be
favorable in solution. Transport calculations based on the
INTRODUCTION
■
The formation of well-defined, stable, and highly conducting
contacts between (single) molecules and electrodes represents
a major challenge for charge transport in nanoscale
assemblies.1−5 The most frequently used chemical anchoring
groups to bind organic molecules to metal electrodes are thiol
(-SH),1,6,7 amino (−NH2)8 and pyridyl.9−11 Other anchoring
groups explored are isocyano (-NC),12,13 cyano (-CN),14,15
isothiocyanato (-NCS),16 methylselenide (-SeCH3),17 methyl-
thiol (-SCH3),17 fused thiophene,17 dimethylphosphine,17
carboxylic acid (-COOH),6 dithiocarboxylic acid (-CSSH),18
nitro (-NO2),15 and even fullerene.19−22 However, most of
these experiments suffer from detriments as non-uniform
binding geometries and structural rearrangements of the leads,
strong metal−molecule coupling disturbing the molecular
orbitals, or decoupled electron systems with limited current
flow through the molecular junction.1,4,7,23,24
Rather high single-molecule junction conductances were
reported for metal−carbon (C) coupling, such as C60,19
benzene,25 and π-stacked benzene26 on gold (Au) and platinum
(Pt) electrodes. As an important new development, Venkatara-
man et al. demonstrated recently the formation of direct Au−
C-bonded single molecular junctions for alkanes and π-
conjugated aromatic molecules upon the spontaneous cleavage
of a trimethyl tin end group (-Sn(CH3)3).27,28 These covalent
σ-bonded junctions led to conductances up to ∼100 times
larger compared to analogous alkanes or aromatic molecules
with most other terminations. However, the widespread
application of this unique approach is currently limited by
the need of rather toxic precursors and the immediate
formation of dimers and oligomers.27,28
́
nonequilibrium Greens function (NEG) technique suggests
rather high conductances for Au−CC−Au single molecular
junctions.42
Inspired by the above experimental results and theoretical
predictions, we applied well-established protecting group
chemistry as a novel concept to create single-molecule
junctions with covalent Au−C σ-bonds. In this contribution
Received: July 31, 2012
© XXXX American Chemical Society
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commercial sources and used without further purification, unless
specifically mentioned.
Scheme 1. Schematics of the Au−C Junction Formation
Conductance Measurements. The transport characteristics were
studied in a mechanically controlled break junction setup (MCBJ).
Single molecular junctions were created by the repeated opening and
closing of a nanoscale gap between two gold electrodes, typically 500
(t ≤ 1000 s) or 2000 (t ≥ 1000 s) cycles with a stretching rate of 5 nm
s−1 in a solution containing tetrahydrofuran (THF) | decane (v:v =
1:4), 0.5 mM of the TMS-protected target molecules and 1 mM
TBAF. The conductance G (current/bias voltage) was measured as a
function of the relative displacement Δz of the two electrodes. The
individual traces were used to construct conductance histograms.
Details of measurement technique and data analysis were reported in
two previous studies of our group.11,47 Specifics of the sample
preparation with the TMS-protected OPE derivatives are described in
SI.
Scheme 2. Molecules Studied in This Work
Scanning Probe Experiments. The STM and AFM experiments
were carried out on (111) facets of bead gold single crystals51
mounted on a sheet of polycrystalline gold. Prior to each experiment,
the electrode was subjected to electrochemical polishing and annealing
in a hydrogen flame followed by cooling under Ar. Surface
modification in the target molecule-containing solution was carried
out in a sealed, stainless steel chamber in an argon atmosphere to
avoid oxygen exposure. After a certain reaction time, the sample was
removed from the assembly container, rinsed with THF, dried in
argon, and subsequently inspected by STM or AFM.
The STM measurements were performed in decane using a
Nanoscope E (Digital Instruments), which was equipped with a
1840AI scanner, in constant-current mode. The STM tips were
prepared by mechanical cutting of Pt−Ir wires (80/20%, Pt/Ir, 0.25
mm in diameter, Goodfellow).
The AFM measurements were typically carried out in noncontact
mode with a Nanosurf FlexAFM (Nanosurf AG, Switzerland) under
ambient conditions and in air. We employed PPP-NCH-W cantilevers
with a tip radius of ∼10 nm (Nanosensors).
Raman Experiments. We employed gap-mode Raman spectros-
copy52−54 in Au(111) | target molecule | Au nanoparticle (NP)
sandwich structures. The ∼55 nm gold NPs were obtained using a
modified citrate reduction method.55 Details are summarized in the SI.
The Au(111) substrates were prepared from 200-nm-thick gold films
with a 5 nm chromium adhesion layer deposited on a glass wafer of 2
× 2 cm2 size (Berliner Glas AG). The samples were cleaned in 98%
concentrated sulfuric acid with extended rinsing with ultrapure water
(Millipore, 3 ppb TOC), followed by flame annealing. Subsequently,
the gold samples were immersed in the target solution, such as 0.5 mM
OPE1 with 1 mM TBAF in THF. After a certain reaction time t, the
gold films were taken out, rinsed with absolute ethanol (Aldrich, p.a.),
dried in Ar (Carbagas, Alphagaz), and subsequently covered by drop-
casting with a submonolayer (4−6 μL) of gold NPs.
Raman spectra of the dried samples were recorded with a HR 800
confocal Raman spectrometer (Horiba JY, France). The excitation
wavelength was 632.8 nm from a He−Ne laser. A 50× magnification
long-working-distance (8 mm) objective was used to focus the laser
onto the sample and to collect the scattered light in a backscattering
geometry. The power on the sample was about 1 mW.
we demonstrate that a trimethylsilyl (TMS) group attached to
an alkynyl moiety is cleaved in situ (i.e., in solution) in the
presence of a base, such as tetrabutylammonium fluoride
(TBAF), to create Au−C σ-bonds. As shown in Figure S2-1
(Supporting Information) [SI], deprotection with fluoride
proceeds via formation of a pentavalent fluorosiliconate
intermediate with loss of fluorotrimethylsilane to liberate an
alkynylide as its tetrabutylammonium salt.43−45 Clearly, the
cleavage of the Si−C bond is through a heterolytic mechanism,
similar to the cleavage of silyl ethers.46 The generated
alkynylide binds selectively to the adjacent gold surface, leading
to the formation of a highly directional covalent Au−C σ−bond
(Scheme 1).
We have synthesized a series of TMS-terminated oligo-
(phenylene ethynylene)s (OPEs) (Scheme 2, R = OCH3) and
measured single molecular conductances of these wires
attached covalently to gold electrodes in solution by employing
the mechanically controllable break junction (MCBJ) techni-
que.47 The transport measurements were combined with in situ
STM imaging and gap-mode Raman spectroscopy to unravel
the nature of surface binding. Importantly, this new concept to
create Au−C σ-bonds paves the way toward further integration
of functional molecules in molecular junctions employing easily
accessible and versatile chemical strategies, thus providing a
promising platform in nanoscale assembly.
RESULTS AND DISCUSSION
■
Figure 1A displays individual conductance traces from experi-
ments with OPE1, OPE2, and OPE3 terminated at both ends
with TMS groups. The data were recorded within the first 1000
s after adding TBAF to the sample-containing solution. We
observe quantized charging conductance steps at integer
multiples of the quantum conductance 1 G0 upon elongation
of the Au−Au junction until a single-atom contact between the
two leads breaks at ∼1 G0. Subsequently, the conductance
decreases abruptly to G ≤ 0.1 G0, depending on the molecular
wire trapped, and well-defined single plateaus at molecule-
dependent conductances appear. These features are attributed
EXPERIMENTAL SECTION
■
Chemicals and Organic Synthesis. Compounds OPE1,48
OPE1_1H,49 and OPE1_D50 were synthesized according to literature
procedures. Details of synthesis and characterization of OPE2, OPE3,
and of the trimer OPE1_T are summarized in the Supporting
Information (SI). All chemicals and solvents were purchased from
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stretching cycle. The latter lasts about 100 ms. This observation
is also supported by a high C−H bond dissociation energy of
acetylene56,57 as well as by the estimated pKa(≡C−H) value of
∼30.
Finally, we want to mention that no indications of molecular
junction formation were observed in experiments with the
blank solution containing only THF, decane, and 1 mM TBAF
(see SI).
Typically, we recorded 2000 individual traces, which were
subsequently analyzed by constructing all-data point histograms
without any data selection to extract statistically significant
results. Figure 1B displays the corresponding one-dimensional
(1D) histogram in a logarithmic scale for three TMS-protected
OPEs, together with data from the above control experiments.
In addition to the Au−Au contact around 1 G0, we found clear
single peaks at 10−1.8 G0, 10−2.7 G0, and 10−3.6 G0, which are
assigned to the most probable molecular junction conductances
before rupture of OPE1, OPE2, and OPE3, respectively. These
values depend exponentially on the molecular length Lm
according to G ≈ G0 exp(−β·Lm) with a decay constant β =
(3.3 0.1) nm−1 (Figure 1C). This value is comparable with
that of dithiol-terminated OPEs β = (3.4
0.1) nm−1, as
obtained in our recent study under nearly identical
experimental conditions.58 We may conclude that the electron
transfer across molecular junctions formed by deprotection of
TMS-terminated OPEs proceeds by through-bond nonresonant
tunneling. We also point out two additional distinct
observations: (1) Extrapolation of the conductance data plotted
in Figure 1C toward Lm → 0 gives a value of approximately 1
G0, indicating that the apparent contact resistance of the
covalent bond formed is of the order of a metallic Au−Au
nanocontact, thus indicating an excellent electronic coupling of
the Au−C linker unit. (2) The comparison of these wires with
their dithiol-terminated analogues reveals that the single
molecular conductance values of the former are around one
order of magnitude higher. The contact resistance per Au−S
bond, ∼40 kΩ, as obtained under experimental conditions the
same as that for the Au−C data, is significantly larger.58 Similar
trends were reported in two recent studies by Venkataraman et
al.,27,28 which clearly indicates that our new approach based on
TMS-deprotection chemistry represents a promising alternative
to the Sn(CH3)3 route to form covalent, highly conductive Au−
C contacts.
Figure 1. (A) Individual conductance-distance traces of OPE1 (blue),
OPE2 (red), OPE3 (green), OPE1_1H (black) recorded in THF/
decane (v:v = 1:4) + 1.0 mM TBAF and 0.5 mM target molecules,
stretching rate ∼5 nms−1. (B) All-data point one-dimensional
conductance histograms of OPE1, OPE2, OPE3, and OPE1_1H
(Scheme 2) as constructed of 500 individual traces during the first
1000 s. (C) Most probable conductance of OPEs versus the molecular
length Lm (filled blue circles, cf. Table S4−2 in SI). The black circles
represent conductance data of the dithiol analogues taken from ref 58.
Open blue circles indicate the most probable conductances of
OPE1_D and OPE1_T.
The above analysis was extended by constructing two-
dimensional (2D) conductance vs displacement histograms.19
This strategy provides direct access to the evolution of
molecular junctions during the formation, elongation and
breakdown steps (Figure 2A). We have introduced a relative
displacement Δz with Δz = 0 at G = 0.7 G0 to assign a common
scale to each individual conductance vs distance trace. This
procedure allows the accurate alignment of the conductance vs
distance traces because of the sharp drop in conductance upon
breaking the monatomic Au−Au contact at 1 G0. Figure 2A
displays the corresponding plot of OPE1 (for other data see
SI). This graph shows a well-defined high-density data cloud
with a clear conductance decay from 10−1 G0 to 10−2.5 G0 upon
stretching up to Δz ≈ 0.35 nm (region H, see SI for further
details). This feature represents the stability range of a
molecular junction. Extending the displacement to Δz > 0.40
nm leads to the breakdown of the molecular plateau region.
The noise level is approached at G < 10−5.5 G0 (region T). The
inset in Figure 2A displays the relative-displacement histo-
grams11 with a maximum at (0.35 0.1) nm in the Δz scale as
to the conductance of (single) junctions formed between the
deprotected targeted molecules covalently bound between the
two gold point-contacts. The sample traces also show that the
plateau length increases with molecular length and decreases
upon stretching until junction rupture occurs with conductance
values reaching the noise level. We note that the presence of
electrolyte ions (TBA+, F−) determines the respective noise
background level in our experiments at ∼10−6 G0.
Control experiments with asymmetric OPE1_1H wires
having only one TMS-protecting group did not provide
evidence for creation of molecular junctions. As an example,
the black trace in Figure 1A displays an abrupt decrease of the
conductance after breaking a single-atom gold contact at ∼1 G0
to approximately 10−3 G0 followed by an exponential decay
until the noise level is reached. This signature represents clearly
a through-space tunneling behavior. The result further implies
that TBAF as a nucleophilic catalyst and the applied electric
field are not sufficient for cleaving the terminal C−H bond of
the unprotected ethynyl group in OPE1_1H within a typical
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of the ethynyl unit, the terminal carbon atom binds to the
energetically most favorable three-fold hollow sites of two
adjacent freshly broken gold leads. Other contact sites (bridge,
atop) are also possible due to marginally small differences in
the corresponding binding energies. At the immediate moment
of breaking a monatomic Au−Au contact (Δzcorr = 0.5 nm) and
cleaving the TMS protecting groups, the generated surface-
bound intermediate is attached to the gold leads in a side11 or
tilted42 geometry due to spatial restrictions. We hypothesize
that a freshly broken Au−Au contact may bear a catalytic
activity similar to that of a ‘naked’ Haruta-type Au nano-
cluster,60,61 thus facilitating the cleavage of the TMS leaving
group upon contact. Pulling the gold leads further apart causes
an orientation change of the molecular wire in the gap from
tilted toward more upright. The surface-bound species is quite
mobile (barrier to surface diffusion on Au(111) ≈ 0.22 eV39),
despite the strength of interaction with gold, and migrates from
an fcc hollow toward an atop site, where the carbon atom is
attached to a single gold atom.39,42 The Au−C bond is stronger
than the Au−Au bond and, as a consequence, the breaking of
the molecular junction upon further stretching is accompanied
by the detachment of a single gold atom from the surface, and
not by the breaking of an Au−C bond. The latter represents a
hybrid created by mixing the 2sp carbon orbital with the gold
5d orbitals, and thus provides a covalent carbon σ-bond to the
gold surface.39
Hoft et al.42 calculated transmission curves of relaxed Au−
OPE1−Au molecular junctions at various stages of the pulling
process, using a nonequilibrium Green’s function technique.
Their results are directly applicable to our experimental
situation. These authors found a resonance feature at ∼−1.0
eV, which tails into the Fermi level EF at relatively high
conductances (see also SI). The resonance is attributed to a
2sp(C)−5d(Au) σ-bonded hybrid, which mediates the
electronic coupling at the interface.39,42 The theoretically
predicted high transmission through alkynyl−Au molecular
junctions is also comparable with recent data reported for Au−
C σ-bonded alkanes.27 The transport calculations of Hoft et
al.42 predicted further that the broad transmission feature
around EF lowers upon stretching of a single molecular
junction, which is attributed to a reduced coupling between the
carbon and gold atoms as well as between the detached gold
atoms and the rest of the surface.
Indeed, our experimental results support the theoretically
predicted trends. Figure 2A displays, as an example, the
comparison of the calculated conductances vs displacement
data at various stages of the pulling process, as extracted from
ref 42 (blue circles; for further details see transmission curves
plotted in SI), with a set of real experimental data. The decrease
in conductance with increasing displacement Δz is clearly
resolved.
Despite the good agreement, we like to emphasize that the
above comparison should be considered with caution and only
treated as qualitative in nature. The DFT-based transport
calculations in ref 42 did not account for self-energy errors and
image charge corrections. However, as demonstrated previously
by Cheng et al. in a study of Au−C σ-bonded alkanes, which
are structurally related to our current system, these corrections
downshift the σ-resonance toward slightly more negative
energies (−0.90 eV) but do not change the qualitative trends.27
This comparison supports and strengthens our interpretation.
The experimental data shown in Figure 1 and Figure 2A
represent conductance vs displacement traces as recorded
Figure 2. All-data point 2D conductance vs relative distance (Δz)
histograms constructed from (A) 500 traces recorded during the first
1000 s and (B) long-term (3600 s) experiment with 2000 traces
analyzed. The characteristic molecular conductance regions are labeled
H, L1 and L2. The inset of (A) and (B) display relative-displacement
histograms as obtained from an stretched distance analysis47 in 0.7 G0
to 10−2.5 G0 (black) and up to 10−6 G0 (gray). Panel (A) also shows
the (scaled) conductance data (blue circles) from calculations reported
in ref 42. (C) 1D conductance histograms of OPE1 recorded during
the first 1000 s (green) and 3600 s (black) after the start of the
experiment as well as of the dimer OPE1_D (blue) and of the trimer
OPE1_T (red), both measured during the first 3600 s.
the most probably characteristic distance Δz* over which a
single OPE1 junction can be stretched. The narrow distribution
of a single peak and the estimated junction formation
probability of ∼90% (calculated from the two peak area ratio
in the inset of Figure 2A) demonstrate the uniformity and the
high stability of the Au−C anchoring site. Considering the
“snap-back” distance Δzcorr ≈ (0.50 0.1) nm10,11,59 (see also
SI), i.e. the fast relaxation of the electrode upon breaking a
monatomic Au−Au contact, we obtain z = Δz* + Δzcorr = (0.85
0.20) nm as the most probable absolute electrode separation
at which a single Au−OPE1−Au junction breaks. This value is
comparable with the C−C distance 0.83 nm42 of the two
carbon atoms attached to the gold leads in a fully extended
upright geometry.
In an attempt to qualitatively rationalize our experimental
observations, we refer to recent electronic structure and
transport calculations of ethynyl and diethynyl benzene
attached to Au(111) leads.39,42 By removal of the hydrogen
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during the first 1000 s after addition of THF/decane solution
(v:v = 1:4) containing 0.5 mM TMS protected OPE1 and 1
mM TBAF into the liquid cell of the MCBJ setup. Extending
the recording time to 3600 s and longer results in individual
conductance traces with more than one plateau. The
corresponding all-data point histograms either in a 1D (Figure
2C) or in a 2D (Figure 2B) representation show additional
features. Figure 2B illustrates, as an example, the conductance
vs relative displacement Δz for a series of 2000 OPE1 traces. In
addition to the data cloud already assigned to the Au−C σ-
bonded single molecular junctions of deprotected OPE1
monomers (region H), we observed two distinctly new
conductance clouds around 10−3 G0 and 10−4 G0, which are
labeled L1 and L2, respectively. Both features evolve more
pronounced with progressing experimental time (see SI for
details). Their relative-displacement (Δz) distribution, plotted
as inset in Figure 2B, illustrates a representative distribution of
the three characteristic conductance regions as obtained during
3600 s reaction time.
Next we present results of structure-sensitive experiments,
specifically STM imaging and gap-mode Raman spectrosco-
py,52−54 which provide evidence for the formation of surface-
confined covalent Au−CC−R bonds under conditions
similar to those applied in the MCBJ measurements. STM
experiments revealed a major restructuring of a freshly prepared
Au(111) (1 × 1) surface in THF containing 1 mM TBAF upon
addition of 0.5 mM OPE1 (Figure 3). Within less than 1000 s
In an attempt to rationalize the nature of the evolving
additional conductance features L1 and L2 we refer to recent
reports on copper (Cu+)- and gold (Au+)-catalyzed oxidative
coupling (dimerization) reactions of terminal alkynes.62−66
These experiments lead to the corresponding symmetric 1,3-
diynes in moderate to good yields and demonstrate the active
role of a charged coinage metal in triggering the coupling
reaction. On the basis of these results and considering that the
application of a bias voltage between the two gold electrodes in
the MCBJ configuration results in localized charges, we propose
that the gold surface mediates the oxidative coupling to afford
alkynylides. The mechanism is illustrated in SI, Figure S2-1.
This interpretation is supported by NMR and Raman
experiments of 0.5 mM OPE1 and 1 mM TBAF in THF
solution, e.g. in the absence of a gold source. We observed no
evidence of dimerization, even after 24 h of stirring.
On the basis of the above arguments and observations, we
assign the new conductance features L1 and L2 to products of a
slow oligomerization process. In an attempt to strengthen this
hypothesis further, we synthesized the TMS-protected dimer
and trimer of the OPE1 analogue, and measured these
conductance characteristics within a time interval of 1000−
3600 s (cf. Figure 2C and SI for details). The results of these
experiments confirmed unambiguously that the additional
features in the long-term reaction experiments of OPE1
represent conductances of the corresponding dimer OPE1_D
(L1 region) and trimer OPE1_T (L2 region), respectively. This
interpretation is also supported by the alignment of the latter
conductance data (open blue circles in Figure 1C) with the
linear relation log (G/G0) vs molecular length Lm (filled circles
in Figure 1C).
Figure 3. (A) STM image recorded in decane, size 200 nm × 200 nm
and (B) typical cross section of an Au(111) surface after 1000 s
exposure to a 0.5 mM OPE1/1.0 mM TBAF solution in THF (for
further details see SI) and cross-section analysis of the blue line.
monatomically deep vacancy islands and channels as well as
terraces develop as a result of the restructuring of the gold
surface over up to five atomic layers. Control experiments with
THF/decane + 1 mM TBAF and THF/decane + 0.5 mM
OPE1 showed clearly that the observed structure changes are
not related to the known gentle etching of a Au(111) surface in
THF (see also SI for details)67 but rather represent the result of
a surface-confined chemical reaction. We speculate that this
reaction comprises the formation of a covalent Au−C bond
through the alkynylide, which is generated by the fluoride-
induced cleavage of the TMS-leaving group, most probably in
direct contact with the gold surface. The comparison of our
results with a recent STM report on grafting ethynylbenzene on
Au(111)40 demonstrates convincingly that the formation of
covalent Au−C σ-bonds using TMS-protected precursors
proceeds more efficiently, and even at room temperature,
indicating that H-terminated ethynyl groups require additional
energy due to the endothermic breaking of the C−H σ-
bond.39,40
Additional evidence for the formation of covalent Au−C σ-
bonds is obtained from gap-mode Raman spectroscopic
experiments. We created Au molecule | (OPE1, OPE2, or
OPE3) | Au nanoparticle (NP) sandwich structures (Figure 4).
The adlayer was obtained by surface grafting from a THF/
decane solution containing 1 mM TBAF and 0.5 mM of the
target molecule, under argon to avoid the presence of oxygen.
The reaction was stopped at various time intervals. After
extended rinsing of the modified gold surface with ethanol to
remove unbound material, a submonolayer of citrate-stabilized
gold NPs was deposited to create nanoplasmonic gaps. Figure 4
displays Raman spectra of OPE1 as observed for 1000 and 3600
s reaction time during the immobilization process. The graphs
in Figure 4 show clear Raman signals at 1297 cm−1 (δCH), 1593
cm−1 (νRing), 1900−2200 cm−1 (νCC), and between 2800 to
3100 (νC−H), which confirm the surface immobilization of
OPE1. Details of the band assignment, as well as additional
Raman and SERS spectra of the various compounds, which
Similar results are also predicted for the other two OPE
derivatives. However, the expected single-junction conductance
data of the corresponding dimers and trimers were not
observed. This may be related to the reduced reactivity of the
longer wires or to the fact that the conductances of the
corresponding dimers and trimers are below the detection limit
of our current setup. In concluding this paragraph we
emphasize that a careful choice of experimental conditions
provides a sufficiently large time interval, within which single
molecular junctions of the Au−C σ-bonded monomers could
be identified and characterized without interference of
coexisting oligomers.
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protecting group chemistry. Employing the MCBJ technique in
liquid, we demonstrated the formation of highly conducting
single molecular junctions of several OPE-derivatives attached
to Au leads. However, we also demonstrated that extended
assembly and reaction times lead to oligomerization. The
created junctions are mechanically stable and exhibit con-
ductances around one order of magnitude higher as compared
to those of traditional anchoring groups. Binding geometry and
junction evolution were discussed by comparing our exper-
imental observations with theoretical predictions. The latter
could be confirmed convincingly. Structure evidence to support
the formation of covalent Au−C σ-bonds is derived from STM
imaging and gap-mode Raman experiments. On the basis of the
combination of structure and transport measurements we could
identify experimental conditions under which selectively only
monomeric Au−OPE−Au junctions were created. Anchoring
(functional) molecular wires through alkynyl groups covalently
attached to contact leads could benefit from the rich chemistry
available and offers considerable promise in the further
development of fundamental and applied single-molecule
charge transport studies. Use-inspired basic research will lead
us to further apply this new strategy to a variety of π-conjugated
systems with specific functions in a straightforward manner
thanks to the broad variability of the TMS-protected molecules.
Figure 4. Gap-mode Raman spectra of a gold surface immersed in
THF solution containing 0.5 mM OPE1 with (red line) and without
(black line) TBAF for 1000 s. The left panel illustrates the schematics
of the gap-mode configuration. The right panel represents a
magnification of the spectra in the CC stretching region recorded
after 1000 s (red) and after 3600 s (blue) reaction time.
were investigated under a wide range of different experimental
conditions, are summarized in the SI.
In this paragraph we discuss data of OPE1 as recorded during
the first 1000 s of surface modification. The νCC stretching
region in the SERS spectra is particularly informative. The
single stretching mode as monitored in the normal Raman
spectra at ∼2150 cm−1, due to the free CC group, is found to
broaden and to downshift to 1950 cm−1. The red-shift of ∼200
cm−1 indicates that the OPE1 molecules are bound to the gold
substrate. Both observations indicate a strong interaction with
the substrate. This result is in agreement with previous reports
on the formation of covalent Au−C bonds involving −CC−
linkers.37,38,41 The appearance of a small band at ∼2100 cm−1,
which represents the spectral position of a free CC group,
provides evidence that the deprotected OPE is adsorbed on a
planar Au(111) surface mainly through one of the two
acetylene groups with the other being pendent and/or only
weakly (electrostatically) attached to adjacent Au-NPs in the
top layer.37,38 A remarkable change in the Raman spectra is
found for samples exposed 3600 s or longer to the assembly
solution (inset in Figure 4). In addition to the two ν(CC)
bands already described, we detected a third band around 2200
cm−1. Comparative Raman experiments with OPE2 and OPE3
in normal Raman and upon immobilization on Au(111)
demonstrated that this spectral feature represents (CC)
groups interspaced between adjacent phenyl rings of the
molecular rod (see SI for details). These results demonstrate
that the appearance of these bands provides direct evidence of
forming surface-confined oligomers in long-time assembly/
reaction experiments with OPE1. Thus, the interpretation of
our transport experiments is nicely supported. We also notice
that the application of a negative charge to the gold leads
quenches dimerization, whereas the Au−C covalent bond
formation is selectively preserved. Paragraph 6.6 in SI
summarizes SERS experiments at the electrochemical gold/
electrolyte interface, which support this important discovery on
selectivity.
ASSOCIATED CONTENT
* Supporting Information
Details on the molecular synthesis, the experimental setup, data
analysis, and the Raman and SPM measurements. This material
is available free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
liu@iac.unibe.ch (S.-X.L.); thomas.wandlowski@dcb.unibe.ch
(T.W.)
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge particularly the enlightening discussions with
P. Renaud on the mechanism of the TMS-cleaving reaction.
This work was also supported by the Swiss National Science
Foundation (Grant Nos. 200020-116003, 200020-144471, and
200021-124643) and by EC FP7 ITN “FUNMOLS”, Project
212942.
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F
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