Effect of metal complexation on the conductance of single-molecular wires measured at room temperature

Article abstract of DOI:10.1021/ja5012417

The present work aims to give insight into the effect that metal coordination has on the room-temperature conductance of molecular wires. For that purpose, we have designed a family of rigid, highly conductive ligands functionalized with different termina

Full text of DOI:10.1021/ja5012417

Article
pubs.acs.org/JACS
Effect of Metal Complexation on the Conductance of Single-
Molecular Wires Measured at Room Temperature
Julia Ponce,†,⊥ Carlos R. Arroyo,‡,⊥ Sergio Tatay,† Riccardo Frisenda,‡ Pablo Gavina,∥ Daniel Aravena,§
̃
Eliseo Ruiz,§ Herre S. J. van der Zant,*,‡ and Eugenio Coronado*,†
†Instituto de Ciencia Molecular (ICMol), Universidad de Valencia, Catedrat
‡Kavli Institute of Nanoscience, Delft University of Technology, Lorentzweg 1, 2628 CJ Delft, The Netherlands
§Departament de Química Inorgan
ica and Institut de Recerca de Química Teorica i Computacional, Universitat de Barcelona,
Diagonal 645, 08028 Barcelona, Spain
∥Centro de Reconocimiento Molecular y Desarrollo Tecnolog
́ ́ ́
ico Jose Beltran 2, 46980 Paterna, Spain
̀
̀
́
́
ico (IDM), Unidad mixta Universidad Politecnica de
Valencia-Universidad de Valencia, Dr. Moliner 50, 46100 Burjassot, Spain
S
* Supporting Information
ABSTRACT: The present work aims to give insight into the effect that metal coordination has on the room-temperature
conductance of molecular wires. For that purpose, we have designed a family of rigid, highly conductive ligands functionalized
with different terminations (acetylthiols, pyridines, and ethynyl groups), in which the conformational changes induced by metal
coordination are negligible. The single-molecule conductance features of this series of molecular wires and their corresponding
Cu(I) complexes have been measured in break-junction setups at room temperature. Experimental and theoretical data show that
no matter the anchoring group, in all cases metal coordination leads to a shift toward lower energies of the ligand energy levels
and a reduction of the HOMO−LUMO gap. However, electron-transport measurements carried out at room temperature
revealed a variable metal coordination effect depending on the anchoring group: upon metal coordination, the molecular
conductance of thiol and ethynyl derivatives decreased, whereas that of pyridine derivatives increased. These differences reside on
the molecular levels implied in the conduction. According to quantum-mechanical calculations based on density functional theory
methods, the ligand frontier orbital lying closer to the Fermi energy of the leads differs depending on the anchoring group.
Thereby, the effect of metal coordination on molecular conductance observed for each anchoring could be explained in terms of
the different energy alignments of the molecular orbitals within the gold Fermi level.
complex embedded in an electromigrated break junction.9
Recent studies have also stated that the existence of high
permanent dipole moments in single transition-metal com-
plexes can lead to current-rectification behavior.5,6
In single-molecule electronics it is customary to functionalize
the target molecules with suitable anchoring groups in order to
improve the electron transport and mechanical stability. In the
particular case of metal complexes, on the basis of the location
of the anchoring groups, the molecular structures can fall into
two categories. In the first one, represented in Figure 1 A, the
anchoring groups are placed in two separate ligands. Hence,
when contacted, the charge current is forced to pass through
INTRODUCTION
■
The research field of single-molecule electronics emerged with
the aim of using tailored individual molecules as electronic
building blocks.1 Among other molecular systems, the
integration of functional metal complexes in electronic circuits
has attracted particular interest because of the additional
exciting prospects offered by the possibility of controlling and
manipulating metal redox and spin states at the molecular
level.2−4 On that basis, metal-containing molecular devices
capable of acting as molecular-based switches or memories have
been proposed.2−7 Moreover, several studies of charge
transport through single-molecule junctions4,8−11 have reported
a broad variety of quantum phenomena triggered by the
presence of a metal ion in the conduction pathway. Of special
relevance is the Kondo resonance observed for a single Co(II)
Received: February 13, 2014
© XXXX American Chemical Society
A
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Figure 1. Schematic representations of metallic complexes in single-
molecule junction configurations: (A) metal-complex junction with the
two anchoring groups located in different ligands; (B) metal-complex
junction in which the two anchoring groups are located in the same
ligand.
the metal ion, and in the absence of the metal center the
molecule breaks down.2,4,8−11 Figure 1 B illustrates the second
category, in which the anchoring groups are placed at both ends
of the same ligand. In this case, current flows through the ligand
even in the absence of the metal ion. This approach is
particularly interesting since it offers the possibility to
investigate electron transport in either the presence or the
absence of a metal center, enabling the study of the metal
coordination effect.
Figure 2. Chemical structures of molecular wires 1−3 and complexes
1c−3c. The molecular lengths were calculated from DFT-optimized
structures.
As far as we know, this kind of study has been performed
only on peptides12 and ligands of the 2,2′-bipyridine
family.5,6,13 All of these compounds showed an increase in
molecular conductance upon coordination, which in the case of
bipyridines was attributed to the increased conjugation derived
from the planar configuration adopted by the bipyridine unit in
the metal chelate. This explanation is supported by several
detailed studies of functionalized biphenyls, in which the
conductance showed a cos2 Φ dependence on the torsion angle
between the phenyl rings (Φ).14−16 Therefore, in this scenario
the effect of metal coordination is obscured by changes in the
geometry. Because of this, with the aim of isolating the effect of
metal coordination on the conductance of molecular wires, in
the present work we investigated the single-molecule electron
transport properties of a series of rigid ligands and their
corresponding Cu(I) complexes. For that purpose, we designed
a series of molecular wires bearing a 1,10-phenanthroline
(phen) metal-binding site and end-substituted with different
anchoring groups (see Figure 2). Both phens and 2,2′-
bipyridines are considered excellent coordination chelates
capable of forming stable complexes with most transition-
metal ions. However, in phens the torsional degree of freedom
is suppressed, and the imine nitrogens are fixed in a cis
conformation even before interaction with the metal. On that
basis, any change in the conductance of our phen-based
molecular wires upon metal chelation must be directly derived
from the effect of the metal, with minor contributions coming
from changes in molecular conformation.
In a first set of experiments, with the aim of systematically
studying the effect of the anchoring groups on the electron
transport properties, we synthesized a series of phenanthroline-
based molecular backbones functionalized with thiol (1),
pyridine (2), and acetylene (3) anchoring groups (Figure 2).
It is well-known that the anchoring groups affect the electron
transport through molecular wires.17−19 To date, thiols have
been the favorite choice in electron transport studies because of
the strong covalent bond formed between sulfur and gold
atoms.20 In these experiments, the use of protected thiols was
mandatory since in solution they suffer from spontaneous
oxidative dimerization to disulfides.20 The acetylthio (AcS)
group has been demonstrated to be a very appropriate
protecting group because it is resistant to Pd-catalyzed coupling
conditions and can be easily cleaved in the presence of gold
electrodes even without the assistance of any deprotecting
agent.21 In addition to thiols, although their use is not so
extended, terminal para-substituted pyridines have proved to be
promising linkers in molecular electronics.19 Several studies
have revealed that pyridyls form very stable contacts in which
molecules adopt particularly well-defined binding geometries
between the leads, thus affording more reproducible con-
ductance values than the aforementioned thiols.19,22,23 From
the chemical point of view, the use of pyridine derivatives is
very advantageous since they do not need to be protected and
are compatible with a wide variety of reaction conditions. More
recently, the formation of direct Au−C covalent bonds, starting
from either trimethyltin (SnMe3, TMT) methylenes or
trimethylsilyl (SiMe3, TMS) acetylenes, has been explored in
the study of molecular conductance.24−26 When TMS or TMT
groups are cleaved off on gold surfaces with (TMS) or without
(TMT) the assistance of a chemical agent, a σ bond between
gold and a carbon atom is formed. In the case of TMT
methylenes, these anchorings have given rise to conductance
values up to 100 times larger than those for the analogous
oligophenyl molecules with amine terminations.25
In a second phase, we studied the effect of Cu(I)
coordination on the conductance of wires 1, 2, and 3. Copper
complexes have been widely studied in coordination chemistry,
and they attract special interest since they possess two available
oxidation states, I and II, with very different magnetic and
spectroscopic properties.27
The single-molecule conductance features at room temper-
ature reported herein were investigated using the mechanically
controlled break junction (MCBJ) technique in the low-bias
regime. Furthermore, we employed a new junction-formation
method, the so-called self-breaking (SB) technique.28 This
method turned out to be well-suited for the evaluation of the
transport properties of bulky copper complexes 1c−3c.
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broken for the first time, atomic-sized contacts can be repeatedly
formed and broken by moving the electrodes toward and away from
each other. During the whole process, the conductance G = I/V is
simultaneously measured by applying a bias voltage of 0.1 V across the
electrodes. During the breaking process, the conductance decreases in
a stepwise fashion. Just before the complete rupture of the gold wire,
the conductance usually exhibits a plateau that is nearly flat and very
close to G0, where G0 is the conductance quantum (G0 = 2e2/h, where
e is the elementary charge and h is Planck’s constant). This plateau
characterizes the formation of a contact consisting of a single gold
atom bridging the gap between the electrodes. Upon further
stretching, the contact breaks and the conductance drops abruptly to
a value of 10−3−10−4G0. Beyond this point, electron tunneling
between the electrodes leads to an exponential conductance decay as a
function of the electrode displacement. The abrupt drop in
conductance after the breaking of the one-atom contact is associated
with mechanical relaxation and atomic rearrangements at the electrode
apexes.32 When a molecule is trapped between the electrodes,
conductance plateaus can be observed for long electrode displace-
ments. After the contact rupture, the electrodes are fused together to
give a conductance value around 80G0 to ensure the formation of clean
gold contacts.
Alternatively, a variation of the traditional MCBJ experiment, the
self-breaking method, was used to form molecular junctions.28 This
method has been previously used to investigate the stability of atomic
gold contacts. In these experiments, the junction is first stretched by
bending the substrate with a bias voltage of 0.1 V applied across the
electrodes. Once the conductance of the junction reaches a value
below 10G0 (typically 6G0), the bending process is stopped and the
conductance is continuously monitored as a function of time. As a
result of the high mobility of gold atoms at room temperature, the
junction breaks by itself. When the conductance drops below a
minimum conductance value (G < 10−7G0), the electrodes are fused
together again to give a conductance of 80G0, and a new SB trace is
started. The key point of this technique is that when no molecules are
present, the time interval over which the conductance falls from G0 to
10−7G0 is short (typically less than 100 s). However, when sample
molecules are present, conductance plateaus can be observed for long
periods of time, even several hours. These plateaus are indicative of the
presence of the target molecules trapped between the electrodes. We
define the junction lifetime as the time required for the conductance to
drop from G = G0 to G < 10−7G0. Considering that the lifetimes
measured for pure solvent molecules are typically below 100 s (see the
S.I.), we can assume that molecular junctions are formed when the
junction lifetime exceeds 100 s, and hence, a trapping probability can
be calculated.
Coordination to Cu(I) ion led to different changes in the
conductance depending on the anchoring group used, which
will be rationalized in terms of electronic and density functional
theory (DFT) considerations.
EXPERIMENTAL SECTION
■
Chemical Synthesis. The chemicals and solvents used were
purchased from commercial sources and used without further
purification, unless specially mentioned. Detailed synthetic procedures
and characterization data for the compounds described herein are
included in the Supporting Information (S.I.).
Spectrochemical Methods. 1H NMR spectra were acquired on a
Bruker AVANCE DRX 300 spectrometer. The spectra were referred to
residual solvent proton references.29 Electrospray mass spectrometry
[MS(ES)] was performed using a Waters Micromass ZQ spectrometer
in the positive ion mode. Absorption spectra were recorded in
dichloromethane solution at a concentration of 1 × 10−5 M on a
Shimadzu UV-2501PC spectrophotometer using 1 cm path length
quartz cuvettes.
Electrochemistry. Electrochemical measurements were performed
in a nitrogen glovebox using an Autolab PGSTAT 128N potentiostat
and a three-electrode electrochemical cell consisting of a glassy carbon
working electrode, a platinum wire counter electrode, and a silver wire
pseudoreference electrode. [Cu(dmp)2](PF6) (dmp = 2,9-dimethyl-
phenanthroline) was used as reference (0.64 V vs SCE in
dichloromethane).30 Supporting electrolyte solutions were prepared
from anhydrous dichloromethane and tetrabutylammonium hexafluor-
ophosphate (TBAPF6).
Deposition of the Conjugated Molecular Wires onto the
Electrodes. In the experiments reported here, the molecules were
deposited onto the MCBJ devices by pipetting 2 μL of a freshly
prepared 0.5 mM solution. In the case of 1 and 3, 1 equiv of
tetrabutylammonium hydroxide (TBAOH) or tetrabutylammonium
fluoride (TBAF) was added to the solution just before the deposition
to help with the deprotection of the AcS and TMS groups,
respectively. The solvents used for the different backbones were
dichlorobenzene for 1, dichloromethane for 2, and a 1:4 mixture of
tetrahydrofuran and octane in the case of 3. For the Cu(I) complexes
1c−3c, dichloromethane was used as the solvent. Moreover, in the
case of the complexes 1c−3c, 10 equiv of hydrazine in THF was added
to the solution just before the deposition to prevent oxidation to
Cu(II). Finally, after the solvent had dried over the electrode, a
vacuum can was placed, and the sample was pumped down to around
10−6 mbar for approximately 1 h before the start of the experiment.
Conductance Measurements. Single-molecule conductance
measurements were carried out at room temperature using the
MCBJ technique.31 Briefly, the device consists of a narrow gold bridge
suspended above a flexible substrate. This device is clamped in a three-
point bending configuration, as schematically shown in Figure 3. After
the deposition of the molecules from solution under ambient
conditions, the suspended gold bridge is stretched by driving the
pushing rod against the bottom part of the MCBJ device until it
breaks, leaving a pair of atomically sharp electrodes. Once the bridge is
Computational Studies. DFT calculations on the isolated
molecules were performed employing the B3LYP functional33 and
the TZV basis set, as implemented in the Gaussian09 software
package.34 Nonequilibrium Green’s function plus DFT (NEGF+DFT)
calculations were carried out by means of the TranSIESTA34 routine
included in SIESTA 3.135 using the PBE functional36 and the DZP
basis set for the molecule and the SZ basis set for the gold atoms.
Device models consisted of two five-layer Au(111) electrodes bridged
with gas-phase-optimized molecules. The adsorption sites and
distances of the anchoring groups to the gold surface were chosen
according to previous studies37−39 to be the following: bridge site, 2.5
Å for 1; atop site, 2.5 Å for 2; atop site, 2.0 Å for 3. Copper-
coordinated models preserved the adsorption geometry of the
corresponding backbones.
RESULTS AND DISCUSSION
■
Design and Synthesis. As previously mentioned, we were
interested in evaluating the effect of metal coordination on the
conductance of molecular wires while avoiding as much as
possible the contribution derived from structural or geometrical
distortions. For that purpose, we chose rigid phen as the
coordination unit. In order to symmetrically bind the phen
moiety to the gold leads, we designed molecular wires 1−3.
Figure 3. Layout of the experimental setup. Inset: scanning electron
micrograph of a device.
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The wires are formed by a central phen moiety substituted at
the 3 and 8 positions with suitable conjugated anchoring groups
(Figure 2). As anchoring groups we used p-arylethynyl spacers
functionalized with −SAc (1), pyridine (2) and ethynyl-TMS
(3) terminations.
The syntheses of molecular wires 1−3 are summarized in
Scheme 1. The preparation of 1 and 2 has been already
Despite this, the 2,9-aryl substituents in macrocycle M46
provide a very stable coordination environment for Cu(I) ions
and minimize metal exchange,47 and the closed nature of M
avoids the formation of the homoleptic Cu(M)2 complex. For
these reasons, the combination of Cu(I) ions with our 3,8-
substituted molecular backbones 1−3 and macrocycle M in a
1:1:1 stoichiometry assures the selective formation of the
heteroleptic Cu(M)(1−3) complexes.48
The formation of the interlocked species was evidenced by
the rapid appearance of a red-brown color characteristic of
Cu(I) complexes with two aromatic diimine ligands49,50 and
was further verified by 1H NMR and MS(ES) analyses. The C2v
symmetry of the complexes was evidenced by 1H NMR
analyses, since all of the signals in the spectra integrated to at
least 2H.
a
Scheme 1. Synthesis of Molecular Wires 1−3
Photo- and Electrochemical Characterization. Organic
backbones 1−3 and their respective Cu(I) complexes 1c−3c
were further characterized by means of UV−vis spectroscopy
and cyclic voltammetry. As can be observed in the UV−vis
spectra (see S.I. Figure 1), the organic backbones 1−3 present
very intense absorption bands (ε > 50 000 M−1 cm−1) around
285 and 345 nm arising from π−π transitions.51 Complexes
1c−3c show additional weak broad bands (ε < 4000 M−1 cm−1)
in the region between 400 and 550 nm associated with metal-
to-ligand charge transfer (MLCT) processes arising from the
presence of the metal atom.50,52
a
Reagents and conditions: (a) Pd(PPh3)Cl2, CuI, THF, i-Pr2NH (74%
for 6, 50% for 3); (b) K2CO3, MeOH (97%); (c) S-(4-iodophenyl)-
ethanethiolate (2 equiv), Pd(dba)2, PPh3, CuI, THF, DIEA (40%); (d)
4-iodopyridine (2 equiv), Pd(PPh3)4, CuI, THF, DIEA (84%).
The cyclic voltammograms of molecular backbones 1−3
present a single irreversible reduction process below −1.0 V
(see S.I. Figure 2). In the corresponding Cu(I) complexes these
irreversible reduction processes become somewhat more
defined, and reduction peaks can be observed. Additionally, a
quasi-reversible peak associated with the Cu(I/II) couple
appears at around 0.5 V.49
reported40,41 and their syntheses were accomplished following a
slight modification of the described synthetic procedures. In
short, the phen core was symmetrically grown by the coupling
of 3,8-dibromo-1,10-phenanthroline (4)42 with commercial
ethynyltrimethylsilane to give 6 followed by desilylation under
basic conditions (Na2CO3 in methanol) to afford diethynyl
intermediate 7 in high yield.43 Final coupling with 2 equiv of 1-
(S-acetylthio)-4-iodobenzene or 4-iodopyridine under Sonoga-
shira conditions afforded the desired molecular rods 1 and 2,
respectively. The yield of 1 was enhanced by the use of the
Pd(dba)2 catalyst in the last step of the synthesis, as described
by Flatt et al.44 The synthesis of compound 3, which is reported
here for the first time, was carried out by the coupling of 4 with
2 equiv of ((4-ethynylphenyl)ethynyl)trimethylsilane (5) under
the same coupling conditions as used in the case of 6.
Compound 5 had been previously prepared by the reaction of
1,4-diethynylbenzene with n-BuLi and subsequent quenching of
the lithium intermediate by the addition of chlorotrimethylsi-
lane.45
We estimated the optical HOMO−LUMO gaps (Eogpt) from
the onsets of the lowest-energy bands in the UV−vis spectra.
Moreover, according to the empirical relationship proposed by
Forrest and co-workers,53,54 we estimated the LUMO energies
of 1−3 and 1c−3c and also the HOMO energies and the
HOMO−LUMO gaps (Eeglec) of 1c−3c from the onset of the
reduction and oxidation waves. The results of these calculations
and some other relevant data are summarized in Table 1. The
optical data show a reduction of the HOMO−LUMO gap by
more than 1.0 eV upon metal coordination. The Eg values show
a good correlation between the experimental methods and DFT
calculations on the isolated molecules, although the latter values
are slightly overestimated.55
DFT Calculations. As a first approximation, we performed
DFT calculations for the isolated backbone molecules 1−3 and
those after the formation of the Cu(I) complexes 1c−3c. The
reduction of the gap in the copper complexes with respect to
the backbones is well-reproduced in the calculations, with the
calculated values being slightly higher, as shown in Table 1.
Besides, the coordination of Cu(I) results in a general shift of
the frontier orbitals of the three backbones to lower energies
(Figure 4). This trend is consistent with the inclusion of a
positively charged fragment that withdraws some electron
density from the backbone, and similar trends have been
observed before upon metal coordination of conjugated wires.55
Electrical Characterization. Conductance Histograms of
Backbone Molecules 1−3 Obtained by the MCBJ Method. As
described in the Experimental Section, we collected thousands
of breaking traces by measuring the conductance as a function
of electrode displacement (z) with a bias voltage of 0.1 V
The synthesis of the threaded copper complexes 1c−3c
(depicted in Scheme 2) was carried out by mixing equimolar
amounts of macrocycle M and [Cu(CH3CN)4]PF6 with the
corresponding molecular backbones 1−3. Cu(I) and phen
ligands form tetrahedral complexes that are kinetically labile.
Scheme 2. Copper-Templated Self-Assembly of Complexes
1c−3c
D
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Table 1. Electrochemical and Optical Properties of 1−3 and 1c−3c
a
a
b
b
c
d
e
red
1/2
ox
1/2
E
(V)
E
(V)
EHOMO(eV)
ELUMO (eV)
Eeglec (eV)
λmax (nm)
Eogpt (eV)
EDg FT (eV)
1
−1.67
−1.60
−1.10
−1.67
−1.52
−1.28
−
−
−
−
−
−
−4.35
−4.25
−4.84
−2.16
−2.21
−3.64
−2.38
−2.38
−3.35
−
−
−
1.97
1.88
1.40
384
371
393
596
612
510
3.23
3.34
3.15
1.80
1.91
1.70
3.43
3.65
3.45
2.64
2.50
2.59
2
3
1c
2c
3c
0.54
0.52
0.181
a
E1/2 values in V vs SCE recorded in CH2Cl2 with TBAPF6 as the supporting electrolyte. The concentration was 1 mM, and the scan rate 100 mV/s.
The HOMO and LUMO energies were determined from the onsets of the first oxidation wave and the first reduction wave in cyclic
b
c
d
voltammograms. The electrochemical HOMO−LUMO gap is defined as Eeglec = −(EHOMO − ELUMO). Optical HOMO−LUMO gaps were
determined from the onsets of the lowest-energy visible absorption bands. Calculations were performed using DFT at the B3LYP/TZV level. In
e
each case, the onset was calculated as the intersection between the baseline and a tangent line that touches the point of inflection.
selection were shifted along the electrode displacement
(horizontal) axis to fix the rupture of the one-atom gold
contact at zero. The areas with high counts denote the most
probable evolution during the formation of the molecular
junctions. The histograms for 1 and 2 show the occurrence of
conductance plateaus with distinguishable conductance values,
but traces measured in the presence of 3 appear noisier.
According to their length, the plateaus observed just after the
breaking of the metallic contact correspond to the formation of
single-molecule junctions, where the anchoring groups provide
the mechanical and electrical connection between the gold
electrodes and the individual molecule.56 However, at shorter
electrode displacements the phen unit may compete with the
terminal anchoring groups and contribute to the conduc-
tance.57−59
Figure 4. Orbital energy level diagram showing the stabilization of the
backbones (1−3) upon Cu(I) coordination (1c−3c). Colored
rectangles correspond to backbone levels, empty rectangles to copper
centers and macrocycle orbitals, and weakly colored rectangles to
mixed copper/macrocycle-backbone levels. The Fermi level corre-
sponding to the bulk gold phase (−5.53 eV) is presented for reference.
Figure 6 shows the one-dimensional (1D) conductance
histograms obtained for 1−3 from the data in Figure 5. The 1D
applied across the electrodes; for these settings, the noise level
in our setup was close to 1 × 10−6G0. In comparison with the
empty gold junctions, individual breaking traces measured with
electrodes exposed to pure solvents did not introduce
additional features. By contrast, when the MCBJ devices were
exposed to solutions containing the target molecules, additional
conductance plateaus were observed for sub-G0 conductance
values (Figure 5 insets).
Typical trace histograms obtained from 1000 consecutive
breaking traces measured in the presence of backbones 1−3 are
shown in Figure 5. To build these two-dimensional trace
histograms, the individual breaking traces without any data
Figure 6. One-dimensional conductance histograms obtained from the
trace histograms displayed in Figure 5. The most probable
conductance values were obtained from Gaussian fits of the
conductance histograms (black lines).
conductance histograms reveal well-defined peaks only in the
cases of 1 and 2. In the case of 1, a broad peak is centered
around 2.1 × 10−5G0, while for molecule 2 the peak is centered
around 5.8 × 10−6G0. Although single-molecule transport
through thiolated bipyridyl oligo(phenylene ethynyl) (BPE)
molecules has been studied before,13,60 the authors did not
report any low-bias conductance value for comparison.
Nevertheless, backbone 1 exhibits a conductance value close
to the value of 2.4 × 10−5G0 reported for the dithiolated
biphenyl oligo(phenylene ethynyl) derivative by Zhang and co-
workers.61 As in the case of 1, in this molecule the central
aromatic rings are forced to remain planar through proper
chemical substitution. Thus, the phen moiety affords a
molecular resistance value comparable to that reported for a
Figure 5. Trace histograms obtained from 1000 breaking traces such
as those shown in the insets for junctions exposed to 0.5 mM solutions
of 1, 2, and 3. The bias voltage was 0.1 V, and the electrode
displacement speed was 12 nm/s. The color scale indicates the
number of counts found at each conductance value for a certain
electrode displacement.
E
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planar biphenyl unit. In agreement with its molecular length,
molecular backbone 2 presents a molecular conductance value
intermediate between those reported for OAE3 and OAE4
[OAE = oligo(arylene ethylene)] with terminal pyridyl groups
measured in MCBJ.62 Previous works on tolane molecules
using MCBJ reported by Hong and co-workers19,26 observed an
increase in the resistance in passing from acetylenes to thiols
and then to pyridines [G(acetylenes) > G(thiols) >
G(pyridines)] and described the following sequence for
junction formation probability and stability: pyridines,
acetylenes > thiols. In the present case, we observed a similar
trend in conductance in passing from thiol to pyridine
anchoring groups. However, under our experimental con-
ditions, acetylenes did not afford well-defined conductance
plateaus in the measured breaking traces (Figure 5).
Conductance Histograms of 1c−3c Obtained by the
MCBJ Method. The trace histograms obtained for junctions
exposed to copper complexes 1c−3c did not show horizontal
regions of high counts at the same electrode displacement
speed used in the backbone measurements (12 nm/s). This fact
indicates a low probability of trapping a molecule between the
electrodes. However, we observed that the occurrence of
conductance plateaus increased considerably when the speed of
electrode displacement was reduced. A possible explanation for
the differences in the behaviors of the backbone molecules and
the copper complexes could rely on the fact that the central
ring, being so bulky, prevents the formation of molecular
junctions when the electrodes are moved apart at high speed
(Figure 7).
Figure 8. Self-breaking trace histograms obtained from 61, 336, and
139 breaking traces such as those shown in the insets for junctions
exposed to 0.5 mM solutions of 1, 2, and 3, respectively. The bias
voltage was 0.1 V.
decreases in the order thiols (86%) > pyridines (59%) >
acetylenes (39%). Moreover, the average lifetime for each
molecule was obtained from the Gaussian fit of the junction
lifetime histogram (see S.I. Figure 4). The thiol and pyridine
backbones display well-defined peaks at 97 and 3 min,
respectively. In the case of acetylenes, the trapping probability
is significantly lower, and thus, about 61% of the signal in the
lifetime histogram comes from nontrapped molecules. Never-
theless, a peak at a lifetime above 104 s (167 min), which
corresponds to the lifetime of the plateau visible in the self-
breaking trace histogram, can be distinguished.
The 1D conductance histograms constructed from the SB
traces in Figure 8 reveal well-defined peaks for all three
backbones (Figure 9). Gaussian fits to these peaks were used to
Figure 9. One-dimensional conductance histograms obtained from the
measured SB trace histograms of molecular backbones 1, 2, and 3
shown in Figure 8. The most probable conductance values were
obtained from Gaussian fits of the conductance histograms (black
lines).
Figure 7. Sketches representing a possible explanation for the different
trapping probabilities shown by the backbones and the copper
complexes.
Conductance Histograms of Backbone Molecules 1−3
Obtained by the SB Method. By using the self-breaking
method, we measured a total of 61, 336, and 139 self-breaking
traces for 1, 2, and 3, respectively. In contrast to the traditional
MCBJ method, the SB approach consists of following the
conductance evolution of atomic-sized contacts as a function of
time rather than electrode displacement. Figure 8 shows the
histograms recorded by measuring consecutive SB traces in the
presence of backbones 1−3 without any data selection. To
build these histograms, the individual SB traces were shifted
along the time axis to fix the rupture of the one-atom gold
contact at zero. In the case of pure solvent, the time that the
junction has a conductance between 0.3G0 and 10−6G0 is below
100 s (see S.I. Figure 3). However, in the presence of
backbones 1, 2, and 3, the lifetime of the junction increases
significantly (Figure 8 insets), which is a clear signature that a
molecule has been trapped between the electrodes. The
junction formation probability, calculated as described in the
Experimental Section, is different for each anchoring group and
identify the preferred conductance values of 1 (1.6 × 10−4G0),
2 (7.4 × 10−6G0), and 3 (8.3 × 10−5G0). Comparing these
results with those obtained in the previous section using the
traditional MCBJ method (Figure 8), we find that the SB
method provides narrower single-conductance peaks, even for
3, for which a particularly well-defined peak was found. In the
case of 1, we passed from a broad peak centered around 2.1 ×
10−5G0 to a peak at 1.6 × 10−4G0 with a variance comparable to
that of 2. For backbone 2, the values obtained with the two
methods compare quite well in terms of conductance value and
peak width.
Conductance Histograms of 1c−3c Obtained by the SB
Method. In contrast to the MCBJ experiments, well-defined
conductance plateaus could be obtained for compounds 1c−3c
using the SB method. Figure 10 shows the conductance
histograms obtained from 35, 77, and 52 self-breaking traces for
1c, 2c, and 3c, respectively. In this case, 88, 89, and 100% of the
curves displayed lifetimes greater than 100 s with average
lifetimes of 31, 7, and 163 min, respectively (see S.I. Figure 4).
F
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Article
coordination (see Table 1). Therefore, we resorted to
theoretical calculations to get a closer look into the energy
level alteration induced by copper coordination in the different
wires.
NEGF+DFT Calculations. We also performed more
realistic NEGF+DFT calculations for the gold devices with
backbones 1−3 and the Cu(I) complexes 1c−3c. As previously
observed in the calculations on the isolated molecules,
coordination of Cu(I) results in a general shift of the density
of states toward lower energies. Besides, the transmission
spectra and molecule-projected densities of states of 1, 2, and 3
(see S.I. Figure 5) are in concordance with previous studies on
similar molecules66,67 and indicate that transport in thiol
derivative 1 and acetylene derivative 3 is HOMO-dominated,
whereas in the pyridine-anchored system 2 the LUMO is the
orbital closest to the Fermi energy. The calculated zero-bias
conductance indicates that 2c (5.7 × 10−4G0) is significantly
more conductive than 2 (2.5 × 10−6G0), whereas 1c (3.8 ×
10−3G0) and 3c (4.0 × 10−4G0) are less conductive than their
parent wires 1 (1.4 × 10−2G0) and 3 (1.0 × 10−2G0),
respectively. These trends are in qualitatively good agreement
with the experimentally measured conductances, although a
quantitative comparison fails because of the well-known
drawback of the generalized gradient approximation functionals
in the estimation of the HOMO−LUMO gap due to the
presence of the self-interaction error, which leads to an
overestimation of the calculated conductance.68,69
In summary, experiment and theory agree that for pyridine-
ended systems there is a considerable increase of the
conductance upon coordination, whereas in the case of thiol-
and acetylene-functionalized molecules the molecular con-
ductance decreases upon coordination. This picture can be
explained in terms of the alignment between the molecular
orbitals and the gold Fermi level. Since the Fermi level of the
gold electrodes lies in the HOMO−LUMO gap, the orbital
stabilization resulting from the coordination of Cu(I) should
bring the LUMO level closer to it, enhancing its contribution to
the current. That is the reason why the conductance in
pyridines, where the transport is LUMO-mediated, increases
upon coordination. By contrast, the shift of the HOMO toward
lower energies moves this orbital away from the Fermi level,
thereby decreasing the conductivity of the molecular systems in
which the transport is HOMO-dominated, that is, the thiol and
acetylene derivatives.
Figure 10. SB trace histograms obtained from 35, 77, and 52 breaking
traces such as those shown in the insets for junctions exposed to 0.5
mM solutions of 1c, 2c, and 3c, respectively. The bias voltage was 0.1
V.
In the case of the copper complexes, we found the
probability of junction formation obtained by the MCBJ
method to be smaller than that obtained by the SB method. We
attribute this higher probability for the SB technique to a
difference in trapping probability when the electrode displace-
ment speed is varied.63 Specifically, in 1c−3c the presence of
the ring makes the molecule bulkier, and this may hamper the
junction formation when the electrodes are quickly displaced.
In the SB method, however, the time for junction formation is
longer, and thus, the chance of trapping the molecules in
between increases. Furthermore, we found that once the
complexes 1c, 2c, and 3c were trapped, they remain bonded to
the leads for times similar to those of backbones 1, 2, and 3,
respectively. These facts suggest that a similar configuration is
adopted in the junction and that the complexes are not
attached by the ring component. Moreover, the junction
average lifetimes are roughly in agreement with the anchor−Au
bond strengths according to the sequence Au−C > Au−S ≫
Au−N.64,65
The most probable conductance values of copper complexes
1c−3c were extracted from the Gaussian fits of the 1D
molecular histograms shown in Figure 11. Comparison with the
CONCLUSIONS
■
Figure 11. One-dimensional conductance histograms obtained from
the measured SB trace histograms of copper complexes 1c, 2c, and 3c
shown in Figure 10. The most probable conductance values and their
variances were obtained from Gaussian fits of the conductance
histograms (black lines).
We have used the mechanically controlled and SB junction
formation techniques to measure the room-temperature low-
bias conductance of a family of conjugated ligands and their
corresponding Cu(I) complexes. Specifically, we have studied
the single-molecule conductance of a family of oligo(arylene
phenylene) phen molecular wires functionalized with thiol,
pyridine, or acetylene anchoring groups before and after Cu(I)
coordination.
The prepared conjugated molecular backbones presented
good conductance features, in accordance with those of similar
reported systems. By the MCBJ method, the most probable
molecular conductance values could be extracted only from the
conductance histograms of the thiol and pyridine derivatives.
By contrast, acetylene derivatives did not show clearly defined
plateaus, and no precise conductance values could be extracted.
In all cases, pyridine-ended derivatives afforded very stable and
reproducible junctions, confirming that they are an excellent
conductance values obtained for the bare backbones showed
the effects of Cu(I) coordination: the conductance of 1c (1.3 ×
10−4G0) exhibited a subtle decrease with respect to that of 1
(1.6 × 10−4G0), whereas 2c (4.7 × 10−5G0) displayed an
increase in conductance by about 1 order of magnitude with
respect to backbone 2 (7.4 × 10−6G0) and 3c (9.2 × 10−6G0)
displayed a decrease by about 1 order of magnitude with
respect to 3 (8.3 × 10−5G0). These different trends observed
for the three types of termination cannot be attributed simply
to changes in the HOMO−LUMO gap, since 1, 2, and 3
present similar drops in the gap of about 1 eV upon Cu(I)
G
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Journal of the American Chemical Society
Article
option for charge transport studies. In contrast to the MCBJ
method, the SB technique afforded a high junction formation
probability for our bulky Cu(I) complexes and turned out to be
a very suitable alternative independent of the anchoring group
used. These differences between the two techniques are
probably related to the different time scales involved in the
junction formation.
ACKNOWLEDGMENTS
■
The present work was funded by the EU (Project ELFOS,
SAMSFERE-321739, and ERC Advanced Grants SPINMOL
and Mols@Mols), the Spanish MINECO (Grants MAT2011-
22785, MAT2007-61584, and CTQ2011-23862-C02-01 and
the CONSOLIDER-INGENIO Project on Molecular Nano-
science), and the Generalidad Valenciana (Prometeo and ISIC
Programmes of Excellence). The research reported here was
also supported by the Regional Generalitat de Catalunya
Authority (2009SGR-1459). J.P and S.T. thank the Spanish
MICINN for a predoctoral fellowship and a JdlC postdoctoral
contract, respectively. D.A. thanks Conicyt-Chile for a
predoctoral fellowship. The authors gratefully acknowledge
the computer resources, technical expertise, and assistance
provided by the BSC (Siesta calculations) and CESCA
(Gaussian calculations).
In regard to the effect of metal coordination, the design of
our molecular backbones enabled for the first time an
evaluation, at the single-molecule level, of the effect of Cu(I)
coordination on the conductance of molecular wires while
excluding any contribution from metal-induced conformational
changes. As expected, the spectral and electrochemical features
of the Cu(I) oligo(arylene phenylene)phenanthroline com-
plexes reflected significantly lower HOMO−LUMO energy
gaps than their parent backbones. However, we observed that
in contrast to what has been previously reported,5,6,12,13 metal
coordination was not always followed by a molecular
conductance enhancement. We studied the energy density of
states diagrams for both the ligands and the complex-contacted
molecules by NEGF+DFT calculations. In accordance with
previous works, in the thiol and acetylene backbones (1 and 3)
the electron transmission takes place through the HOMO level,
whereas in the pyridine system (2) the orbital mainly involved
in the conduction is the LUMO. The shift of the ligand levels to
lower energies induced by the coordination of Cu(I) increases
the energy difference between the HOMO and the gold Fermi
level, reducing the conductance in the cases of 1c and
particularly 3c. By contrast, in the case of 2c, the stabilization
of the frontier orbitals reduces the energy difference between
the LUMO and the Fermi level, leading to a better level
alignment that is reflected in a considerable conductance
increase. Consequently, the observed differences in conduc-
tance after metal coordination cannot be correlated with the
reduction of the HOMO−LUMO gap but instead are
correlated with the particular displacement of the molecular
levels involved in electron transport with respect to the Fermi
level of the electrodes. This shift of the ligand levels toward
lower energies is expected to have different magnitudes for
various transition-metal ions,55 and thus, similar trends in the
molecular conductance of thiol-, pyridine-, and acetylene-ended
conjugated wires are expected for other metals. Furthermore,
these results open the door to the study of the conductance
response to the redox switching behavior of copper-based
interlocked molecules embedded in molecular junctions.70
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