Single-molecule conductance of a chemically modified, π-extended tetrathiafulvalene and its charge-transfer complex with F4TCNQ

Article abstract of DOI:10.3762/bjoc.11.120

We describe the synthesis and single-molecule electrical transport properties of a molecular wire containing a π-extended tetrathiafulvalene (exTTF) group and its charge-transfer complex with F₄TCNQ. We form single-molecule junctions using the

Full text of DOI:10.3762/bjoc.11.120

Single-molecule conductance of a chemically modified,
π-extended tetrathiafulvalene and its charge-transfer complex
with F4TCNQ
Raúl García1, M. Ángeles Herranz1, Edmund Leary*2, M. Teresa González2,
Gabino Rubio Bollinger3, Marius Bürkle4, Linda A. Zotti5, Yoshihiro Asai4, Fabian Pauly6,
Juan Carlos Cuevas5, Nicolás Agraït2,3 and Nazario Martín*1,2
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2015, 11, 1068–1078.
1Departamento de Química Orgánica, Facultad de Química,
Universidad Complutense, E-28040 Madrid, Spain, 2Fundación
IMDEA Nanoscience, Campus de Cantoblanco, Universidad
Autónoma, E-28048 Madrid, Spain, 3Departamento de Física de la
Materia Condensada, and Instituto “Nicolás Cabrera”, Universidad
Autonoma de Madrid, E-28049 Madrid, Spain, 4Nanosystem
Research Institute, National Institute of Advanced Industrial Science
and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan,
5Departamento de Física Teórica de la Materia Condensada,
Universidad Autónoma de Madrid, Spain and 6Department of Physics,
University of Konstanz, D-78457 Konstanz, Germany
doi:10.3762/bjoc.11.120
Received: 11 March 2015
Accepted: 22 May 2015
Published: 24 June 2015
This article is part of the Thematic Series "Tetrathiafulvalene chemistry".
Guest Editor: P. J. Skabara
© 2015 García et al; licensee Beilstein-Institut.
License and terms: see end of document.
Email:
Edmund Leary* - edmund.leary@imdea.org; Nazario Martín* -
nazmar@quim.ucm.es
* Corresponding author
Keywords:
break junction measurements; charge-transfer complex; DFT-based
transport; molecular electronics; tetrathiafulvalene
Abstract
We describe the synthesis and single-molecule electrical transport properties of a molecular wire containing a π-extended tetrathia-
fulvalene (exTTF) group and its charge-transfer complex with F4TCNQ. We form single-molecule junctions using the in situ break
junction technique using a homebuilt scanning tunneling microscope with a range of conductance between 10 G0 down to 10−7 G0.
Within this range we do not observe a clear conductance signature of the neutral parent molecule, suggesting either that its conduc-
tance is too low or that it does not form a stable junction. Conversely, we do find a clear conductance signature in the experiments
carried out on the charge-transfer complex. Due to the fact we expected this species to have a higher conductance than the neutral
molecule, we believe this supports the idea that the conductance of the neutral molecule is very low, below our measurement sensi-
tivity. This idea is further supported by theoretical calculations. To the best of our knowledge, these are the first reported single-
molecule conductance measurements on a molecular charge-transfer species.
1068

Beilstein J. Org. Chem. 2015, 11, 1068–1078.
Introduction
The development of molecular electronics is a current chal- dation peaks to form the radical cation and dication species,
lenge in nanoscience. The ultimate goal is to fabricate different shows only one oxidation peak involving a two electron process
electronic devices based on a variety of active elements such as to form the dication state. Furthermore, the geometrical prop-
wires, transistors, diodes or switches (to name just a few), erties of exTTF are quite different from pristine TTF. Thus,
where each is built from individual, suitably functionalized whereas TTF is mostly planar in the neutral form, exTTF is a
molecules. Although the controlled handling of molecules to highly distorted, out-of-plane molecule with a butterfly shape in
form reliable molecule-based circuits remains a demanding its neutral state. It undergoes a dramatic gain of planarity and
task, modern organic synthesis allows the design and prepar- aromaticity upon oxidation (Figure 1) [18,19]. This gain of
ation of nearly any challenging molecule [1].
stability upon oxidation has been skillfully used in a variety of
D–π–A systems, namely exTTF–π-bridge–C60 derivatives, for
Among the aforementioned electroactive elements, the study of determining the attenuation factor of the molecular wire
molecular wires has received great attention and, in this regard, (oligomer) acting as the π-bridge [20-22].
a great variety of molecules of different nature involving single,
double and triple C–C bonds (conjugated or not) have been
extensively studied [2-4]. Furthermore, most of these systems
have been covalently connected to a great variety of different
anchor groups in order to improve the connection to various
metal electrodes. However, despite the huge number of molec-
ular systems that find application as wires, the use of electroac-
tive molecules exhibiting different oxidation states that can
modify/control the conductance through the wire have been
considerably less studied.
In the realm of organic chemistry there are a great variety of
Figure 1: Depictions of 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydro-
organic compounds able to show different redox states that are
very appealing candidates to be used as wires and/or switches in
molecular electronics. In this regard, molecules having redox
centers such as viologen [5,6], aniline [7,8], thiophene [9],
anthracene (exTTF) in the (a) neutral form and (b) in the oxidized (2+)
state. In the oxidized state the two positive charges reside on the two
1,3-dithiole rings.
anthraquinone [10] and ferrocene [11] have been previously In this paper, we describe the synthesis of a new exTTF deriva-
studied. However, a particularly suitable redox-active molecule tive, suitably functionalized with two (p-acetylthio)phenyl-
for molecular electronics is the well-known electron donor ethynyl substituents at positions 2 and 6 of the anthracene
tetrathiafulvalene (TTF) molecule. Pristine TTF, as well as the central core. We then describe single-molecule conductance
tetraselenafulvalene analogue (TSF), have been previously measurements on this new derivative, along with measure-
reported. In this study, the authors hypothesized that in the ments of the charge-transfer complex formed with F4TCNQ.
Au–TTF–Au junctions, the molecule is connected to the elec- Finally, we present theoretical calculations to understand its
trodes in a face-to-face overlapping configuration [12]. In electrical transport properties.
contrast, since the first report on a suitably functionalized TTF
Results and Discussion
as a molecular wire using two thioacetate anchoring groups
Synthesis and characterization of molecular
wire 5
[13], most of the TTF derivatives synthesized for this purpose
have been functionalized with sulfur atoms as alligator clips.
These belong either to a fused ring on the TTF [14], or to a
chain covalently connected to the TTF core bearing a thiol
group at the end [15,16]. Furthermore, extended TTF cruciform
molecules, formed by two orthogonally placed, π-conjugated
moieties bearing the 1,3-dithiole rings at the ends, have also
been used for single-molecule measurements [17].
The synthesis of the target molecule is shown in Scheme 1 and
starts from the previously reported 2,6-diiodo-exTTF 1 [23].
Reaction of 1 under Sonogashira conditions (Pd(II), CuI,
DIPEA) with trimethylsilylacetylene affords the symmetrically
substituted exTTF 2 in good yield. Further removal of the
trimethylsilyl group is easily achieved by treatment with potas-
sium carbonate, yielding the free terminal alkyne 3 in quantitat-
ive yield. The introduction of the two anchor groups in 3 is
carried out through a second Sonogashira reaction. The
coupling of 2,6-diethynyl-exTTF 3 with 1-acetylthio-4-iodoben-
A singular TTF analogue is the so-called π-extended TTF
(exTTF, (9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthra-
cene) which, in contrast to pristine TTF that exhibits two oxi-
1069

Beilstein J. Org. Chem. 2015, 11, 1068–1078.
Scheme 1: Synthetic route to the target exTTF-based molecular wire 5.
zene (4), in the presence of Pd(II) and copper iodide and DIPEA to the oxidation peak found for pristine exTTF (Epa = 244 mV,
in THF, led to the target molecule 5 in moderate yield (35%). ΔE = 350 mV) (Figure 2).
Compound 4 was obtained in one step and with high yield from
1,4-diiodobenzene by reaction with n-BuLi in ether, followed
by reaction with S8, and eventually, acetyl chloride [24].
All new compounds were fully characterized by spectroscopic
and electrochemical means. Interestingly, compounds 2, 3 and 5
showed similar 1H NMR spectra due to their symmetry result-
ing in relatively simple spectra, which confirms the proposed
structures. In particular, compound 5 exhibits the methyl groups
as a singlet at 2.45 ppm, and the protons corresponding to the
1,3-dithiole rings appear as a singlet at 6.37 ppm. In the
13C NMR of the target molecule 5, the carbonyl groups appear
at 192.4 ppm and the carbons of the alkyne moieties at 88.0 and
90.4 ppm, with the terminal methyl groups at 28.7 ppm.
The redox properties of compound 5 (0.2 mM) were deter-
mined by cyclic voltammetry at room temperature in THF using
TBAPF6 (0.1 M) as a supporting electrolyte under argon atmos-
phere and at a scan rate of 0.1 Vs−1. The electrochemical cell
Figure 2: Cyclic voltammograms of compound 5 and pristine exTTF
(at concentrations of approximately 0.2 mM) using THF as a solvent
and TBAPF6 (0.1 M) as a supporting electrolyte. Scan rate: 0.1 Vs−1.
consisted of a glassy carbon working electrode, Ag/AgNO3 Break junction measurements
reference electrode and a Pt wire counter electrode. It is worth Neutral molecule
mentioning that ferrocene was not employed as the inner refer- We first tried to form neutral-molecule molecular junctions.
ence since its oxidation potential overlaps with that of the Simply, compound 5 can be seen as an analogue of an
exTTF unit. Similar to pristine exTTF, the new exTTF deriva- oligo(phenylene ethynylene), specifically an OPE3-dithiol com-
tive 5 exhibited only one quasi-reversible oxidation peak, pound (where 3 indicates the total number of phenyl rings), in
involving a two-electron process to form the dication state which the central phenyl ring has been substituted by an exTTF
[25,26]. This oxidation peak appears at Epa = 217 mV unit. We could, therefore, expect compound 5 to form molec-
(ΔE = 285 mV, peak-to-peak separation), which is quite similar ular junctions in a similar way to the OPE3-dithiol, which
1070

Beilstein J. Org. Chem. 2015, 11, 1068–1078.
readily does so either in solution, or under solvent-free condi-
tions as recently reported [27]. Hence, as a starting point, we
followed the same sample preparation conditions as previously
with the OPE3-dithiol [27]. We prepared 0.1–1 mM solutions of
compound 5 in both 1,2,4-trichlorobenzene (TCB) or mesity-
lene/tetrahydrofuran (Mes/THF 4:1), and exposed a clean gold
substrate to the solution for a period of approximately 30 min.
The sample was then dried under a flow of argon and mounted
inside the scanning tunneling microscope (STM). All experi-
ments were then performed under solvent-free, ambient condi-
tions. In order to form molecular junctions of compound 5, we
followed the break junction technique [28]. During the experi-
ment, the variation in conductance (G) is recorded while an
STM tip is moved vertically (z) in and out of contact with a
gold substrate, forming and breaking gold nanocontacts (G vs z
trace). When the two gold electrodes (the STM tip and the sub-
strate) are in contact, or in close enough proximity, one or more
molecules of the compound adsorbed on the surface can bridge
the two electrodes. This binding occurs through one of the
terminal groups (thiols in this case) to each of the electrodes,
hence forming a molecular junction. In this configuration, when
separating the electrodes (larger z values), we observe conduc-
tance plateaus while the gold nanocontact or a molecular junc-
tion remains intact during the pulling. The plateaus then end
abruptly when the junction is broken.
Figure 3: 2D histograms resulting from break junction experiments on
an unmodified gold sample (a), OPE3-dithiol drop-cast from TCB (26%
of traces displayed plateaus around 10−4 G0) (b), compound 5 drop-
cast from TCB (c) and, compound 5 dip-cast from dichloromethane (d).
All measurements were performed in dry ambient conditions.
Figure 3c and 3d show the 2D histograms consisting of several gories of traces: (1) those with only a smooth exponential decay
thousand individual G vs z traces recorded in a break junction (Figure 4a and Figure 4d, labeled “No Plat.”), (2) those with
experiment in the presence of compound 5. These are compared poorly-defined plateaus (Figure 4b and Figure 4e, labeled “Plat.
with those recorded on an unmodified gold substrate and in the 1”), and (3) those with well-defined plateaus (Figure 4c and
presence of OPE3-dithiol. These histograms include all the Figure 4f, labeled “Plat. 2”). We considered the conductance
measured traces without filtering out the tunneling-only traces range between log(G/G0) = −5.3 and −0.5 and established that a
from those with plateaus. One can see that the colored region of trace has a plateau whenever a z interval (Δz) longer than
the 2D histogram of OPE3-dithiol extends significantly to larger 0.15 nm is needed to observe a change of conductance
inter-electrode distances than the unmodified gold substrate. Δlog(G/G0) = 0.12 along the trace. Amongst these traces, we
Also, it does so within a narrow band of conductance values separate the well-defined plateaus as those traces for which a Δz
(roughly between log(G/G0) = −3 and −4.5). This 2D profile larger than 0.2 nm is needed to observe a Δlog(G/G0) = 0.1
exemplifies the presence of plateaus occurring in a narrow along the trace. These are empirical criteria which gave us the
region of conductance. In the 2D histograms of compound 5, we best separation results. We see that the histograms built from
can also see that the colored region extends to higher electrode the trace of the third group present protuberances, suggesting
interdistance values (z) than for the unmodified gold substrate. that molecular junctions are successfully formed from
However, as opposed to the case of OPE-dithiol, there is no compound 5.
clear protuberance in the histogram that would indicate the
concentration of plateaus at a given conductance value.
From a close inspection of Figure 4c and Figure 4f, we see that
the plateau conductance in these experiments varies by more
Therefore, in order to determine whether the signature of com- than 2 orders of magnitude (between log(G/G0) = −3 and −5),
pound 5 is simply very weak, we performed a trace separation significantly greater than OPE3-dithiol, and also that they
process to build histograms of only the traces displaying extend to between 1 and 2 nm. However, it is important to note
plateaus. Figure 4 shows the results of this separation for the here that the percentage of traces with well-defined plateaus
traces considered in Figure 3. In particular, for compound 5, (given in parentheses in Figure 4) is very low. Even with a total
two separation steps were carried out, resulting in three cate- number of 5000 measured traces, the final number of traces
1071

Beilstein J. Org. Chem. 2015, 11, 1068–1078.
Figure 4: 2D histograms corresponding to compound 5 after exposing a gold substrate to the solution of the compound in TCB. For each measure-
ment, the traces were separated into 3 groups: traces with no plateau (No Plat.), traces with poorly defined plateaus (Plat. 1), and traces with well-
defined plateaus (Plat. 2). See text for details on the separation process. The percentage of traces included in each group is indicated in the brackets.
with well-defined plateaus is less than 50 traces. We also note tained along the OPE3 backbone. For this series of compounds,
here that these results correspond to the most successful we could observe a clear signature for each, which was similar
measurement runs, and the percentage of traces with plateaus to the unmodified OPE3. We showed that the presence of the
was even lower in other cases. We stress that under the same DTF side groups does not influence the low-bias conductance
experimental conditions, we obtained percentages of around and secondly, and that their presence does not significantly
35% of well-defined plateaus for OPE3-dithiol.
hinder molecular junction formation. Different to the measure-
ments carried out thus far on compound 5, we carried out these
We have recently measured the conductance of a series of mole- measurements using dichloromethane as the deposition solvent,
cules based on a similar OPE3 backbone, also terminated with and since the results were clear, we decided to applied this also
thiols, but with differing numbers of dithiafulvalene (DTF) to compound 5. The sample was prepared by exposing the gold
substituents placed at various positions along the OPE back- to a solution of compound 5 in dichloromethane (DCM) for 1 h,
bone [17]. One of the main structural differences with these followed by drying the sample with a flow of N2. The results of
molecules compared to compound 5 is that conjugation is main- these measurements are shown in Figure 5.
Figure 5: 2D histograms corresponding to compound 5 after exposing a gold substrate to a solution of the compound in DCM. The traces were sep-
arated into three groups as described in Figure 4. The total number of traces recorded was 3830.
1072

Beilstein J. Org. Chem. 2015, 11, 1068–1078.
As with the measurements on compound 5 using the TCB drop- considered. Specifically, we switched from TCNQ to the
casting method, after dip-casting from DCM, we could observe stronger acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodi-
the presence of G(z) plateaus, in this case down to just above methane (F4TCNQ). This produced a clear change of color as
10−7 G0 facilitated by the use of larger gains. The percentage of soon as the first drops of the acceptor were added to the solu-
traces displaying these plateaus was again low, however, and tion of compound 5 in DCM, yielding a green solution. We
similar to that found before, with less than 2% fitting the well- monitored the formation of the CT complex by recording the
defined criteria. From the 2D histograms in Figure 4, we see UV–vis spectrum of the solution. As successive amounts of the
again that no favored region of conductance develops after acceptor are added to the solution of the donor, the peak at
separating the junctions displaying plateaus (labeled Plat. 1 and 448 nm of the neutral donor species decreases and new peaks
Plat. 2). This, we believe, indicates that whilst the formation of appear between 600–900 nm that grow with the addition of
molecular junctions is possible, it is unclear exactly how the more acceptor and also methanol (see Supporting Information
molecule binds in these junctions. The low percentage of mole- File 1, Figure S2). Specifically, we observe three peaks at
cular junction formation can be considered consistent with our 689 nm, 764 nm and 867 nm, which can be assigned to the
observations on the OPE3 series with DTF side groups, in that radical anion F4TCNQ species [30]. For the break junction
the presence of sulfur atoms in the center of the molecule measurements, it was necessary to avoid having an excess of
slightly reduces the probability of forming junctions. This is the acceptor because F4TCNQ forms molecular junctions itself
likely because the mobility of the molecule over the surface is due to its four terminal cyano groups (see Supporting Informa-
reduced, which in turn makes it difficult for molecules to tion File 1, Figure S7). Hence, we formed the charge-transfer
diffuse into the freshly created nanogaps.
(CT) complex by adding approximately 0.5 mL of a 10−4 M
solution of F4TCNQ in DCM to a 1 mL 10−4 M solution of
The lack of a clear molecular signature may ultimately be due to compound 5. In this way, we can avoid having significant
several reasons. Aside from reducing the probability of junc- amounts of free acceptor on the surface. This, however, means
tion formation, the sulfur atoms in the center of the molecule we do not exactly know the ratio of donor to acceptor in our
can also bind to the electrodes during the evolution of the junc- case. It is known that the CT complex of the parent exTTF with
tions, preventing the wire from becoming fully stretched. This TCNQ forms in a 1:4 ratio [31]. However, as we have evidence
is a distinct possibility for this molecule due to the well-known of the formation of a radical anion of F4TCNQ, we believe a
interaction of the parent extended-TTF and gold [29]. An alter- donor to acceptor ratio of 1:2 is more likely, for which there is a
native explanation for the lack of a clear signal for compound 5 precedent in a substituted exTTF complex with TCNQ [32]. We
is that its end-to-end conductance is too low for it to be then allowed 24 h for the molecules to adsorb onto the gold in
observed in our setup. If this is the case, this would mean the order to obtain as high a density of molecules as possible. This
conductance is lower than 10−7 G0. This is quite likely due to increases the possibility that a significant fraction of the solu-
the cross-conjugated nature of the exTTF unit, and the known tion is still the free donor species. Although there will still be
effect this has on conductance.
some of the neutral donor species present on the surface, as we
have shown that this molecule does not give a clear signal in
break junction experiments, there will be no problem of signal
overlap with the CT complex.
Conductance measurements on the exTTF-
F4TCNQ charge-transfer complex
Despite the lack of a clear signal for the neutral molecule, we
decided to proceed by trying to form the charge-transfer (CT) In contrast to the neutral form of compound 5, a significant
complex of compound 5 with 7,7,8,8-tetracyanoquinodi- percentage of conductance plateaus for the CT complex sample
methane (TCNQ). When combining two equimolar solutions of were observed. They were found fall into two main groups,
the two compounds in DCM, we observed no color change, labeled as high and low conductance. Figure 6a shows exam-
suggesting no complexation. The solvent was then changed to ples of individual G(z) traces displaying conductance plateaus.
acetonitrile, a more polar solvent better able to stabilize com- Firstly, we separate traces showing only tunneling (Figure 6b)
plex formation, but this too did not give the anticipated strong from those containing plateaus (Figure 6c) using the following
color change. Only when the solution was heated at reflux for criteria: z interval (Δz) longer than 0.12 nm needed to observe a
3 h under ambient conditions and a large excess of TCNQ was change of conductance Δlog(G/G0) = 0.16. We then further
added (5 equiv), a dark green color developed. UV–vis absorp- divided the traces into two more groups for those with plateaus
tion spectroscopy confirmed the formation of a CT complex above or below log(G/G0) = −3.8. Dividing the traces using a
(Supporting Information File 1, Figure S1). To ensure the quan- value slightly above or below this does not change the sep-
titative formation of the CT complex, a more straightforward aration significantly as the difference between the two types of
method compatible with the conductance measurements was plateaus is clear. By fitting a Gaussian function to the
1073

Beilstein J. Org. Chem. 2015, 11, 1068–1078.
Figure 6: a) Examples of individual G(z) traces showing clear conductance plateaus. b–e) 2D histograms corresponding to the CT complex between
compound 5 and F4TCNQ after exposing a gold substrate to the solution of the complex in DCM. The traces are separated into four groups: b) traces
without plateaus; c) traces with plateaus between log(G/G0) = −0.5 and −6.0. From these traces the traces were divided into high and low plateau
groups: d) traces with plateaus above log(G/G0) = −3.8; e) traces with plateaus below log(G/G0) = −3.8. The total number of traces recorded was
4868.
histograms we find peak positions of log(G/G0) = −3.0 for molecule of F4TCNQ. The length and conductance are similar
the high group, and log(G/G0) = −4.7 for the low group (see to the control test we carried out on only this molecule (see
Supporting Information File 1, Figure S6 for the 1D Supporting Information File 1, Figure S7). It may also be
histograms). We also measured the junction break-off distance possible that the high conductance state arises through contact
for the two groups (which we define as the separation required to the central part of the CT complex and one of the terminal
to move from log(G/G0) = −0.5 to either log(G/G0) = −4 for the thiol groups. We observed a similar feature for the neutral
high plateaus, or log(G/G0) = −6.1 for the low plateaus). We molecule (see Figure 4b and Figure 4c) although for the neutral
found mean values of 0.65 ± 0.25 nm and 1.14 ± 0.30 nm for molecule, this signal was never very reproducible. The low
the high and low groups, respectively (see Supporting Informa- group, on the other hand, fits well with conductance taking
tion File 1, Figure S3 for break-off histograms). We repeated place across the whole molecule (i.e., thiol to thiol). We calcu-
the measurements using a freshly prepared CT complex and late an S–S distance of 2.4 nm for the CT complex (substituting
gold electrodes, and obtained a very similar result for the low the dihydroanthracene core of compound 5 for anthracene). We
conductance group (see Supporting Information File 1, Figures estimate the amount of gold retraction to be 0.5 nm, which then
S4 and S5). In the repeated measurement, however, we did not gives a real Au–Au separation of 1.64 ± 0.30 nm for the mean
observe a signal in the high conductance region, above 10−3 G0. breaking distance of the low group, corresponding well with the
length of the molecule. The fact that the molecule does not
As can be seen from the separation of plateaus into high and seem to fully stretch inside the junctions (unlike the OPE3-
low groups, the two types generally occur independently. The dithiol) may be due to the bulky nature of the complex, which
origin of the high conductance state is difficult to be totally sure contains many groups with the potential to interact with gold.
about. The two groups may arise from independent chemical
species, in which case it would be natural to label the high We cannot be totally sure of the species which gives rise to the
group as junctions for which transport takes place through a low conductance state in so much as we do not know its exact
1074

Beilstein J. Org. Chem. 2015, 11, 1068–1078.
charge and ratio of donor to acceptor molecules. It is conceiv-
able that some charge is transferred to the electrodes upon
molecular junction formation, as is common for molecular junc-
tions [33]. However, it is well known that the cationic (1+) state
of exTTF is less thermodynamically stable than the dication
(2+) [34], resulting in inverted oxidation potentials of the cation
and dication. Additionally, it is known that the resonance
between the neutral and dication is impossible due to the strong
difference in geometries. Thus, it is more than plausible that the
molecule retains its cationic state in the junction. We further
point out that in several recent studies, a very similar conduc-
tance was found for a molecule that is structurally similar and
contains only the anthracene central group. In a study by Hong
et al., a conductance peak was found at log(G/G0) = −4.5, which
is close to the low conductance group we observe [10]. As it is
known that the oxidation of exTTF compounds results in the
planarization and aromatization of the central three rings to the
anthracene unit, the similarity in conductance is perhaps indica-
tive of these changes. We have also recently shown that adding
substituents to the central phenyl rings of OPE wires has no
noticeable influence on the molecular conductance (groups
were tested ranging from electron-withdrawing fluorine to elec-
tron-donating methoxy groups) [35]. It is also rational to
suppose that the, albeit charged, dithiole side groups of our CT
Figure 7: Frontier orbitals of compound 5 in the gas phase.
complex would not strongly modify the conductance of the cluster in a top and hollow position, respectively (Figure 8). We
otherwise neutral anthracene-containing wire. Finally, we point then computed the zero-bias electron transmission following the
out that recent conductive AFM measurements carried out on procedure explained in Supporting Information File 1. The
self-assembled monolayers of anthracene and anthraquinone corresponding transmission curves are shown in Figure 9.
OPE derivatives showed a difference of approximately two Notice that the molecular HOMO–LUMO gap was corrected,
orders of magnitude between their respective conductance following the procedure previously reported [37]. The
values [36]. This would fit with our hypothesis that the conduc- HOMO and all other occupied orbital energies were shifted by
tance of the neutral exTTF molecule, which has a similar Σocc=−IP − εH +Δocc, while the LUMO and all other unoccu-
bonding pattern to anthraquinone, is below our experimental pied orbital levels were shifted by Σvirt=−EA − εL +Δvirt. Here,
resolution, based on the value we measure for the CT complex. Δocc (Δvirt) is the image charge correction for the occupied
(unoccupied) states, εH (εL) is the Kohn–Sham energy of the
Ab initio calculations
gas phase HOMO (LUMO), and IP(EA) is the gas phase ioniza-
To gain insight into the conduction mechanism of compound 5, tion potential (electron affinity). All quantities are reported in
we performed theoretical calculations based on a combination Table 1 for both binding geometries.
of density functional theory (DFT) and Green’s functions tech-
niques within the framework of the Landauer theory for The alignment of the Breit–Wigner resonances related to both
coherent transport. The complete technical details are reported the HOMO and LUMO (at approximately −1 and 2.7 eV from
in Supporting Information File 1.
the Fermi level, respectively) do not show a strong dependence
on the binding geometry (Figure 9). The electron transport is
We first optimized the geometry of the molecule in the gas dominated by the HOMO, although interference features (reso-
phase. The HOMO was found to be localized on the exTTF nance–antiresonance pairs) appear in the energy region close to
unit, which presented the expected butterfly shape, while the Fermi level. This is not surprising, given the spatial exten-
the LUMO appeared delocalized over the whole molecule sion of the frontier orbitals, in particular of the HOMO, which
(Figure 7).
is localized on the ex-TTF unit only. In fact, Fano resonances
are known to arise when a “pendant” orbital, which is weakly
Subsequently, we constructed two kinds of metal–mole- coupled to the electrodes, is coupled to an orbital delocalized
cule–metal junctions, where the molecule is bound to a gold over the main axis of the molecular wire [38,39].
1075

Beilstein J. Org. Chem. 2015, 11, 1068–1078.
Figure 8: Top a) and hollow b) binding geometries of 5 to a gold cluster in metal–molecule–metal junctions.
idea that the real conductance of the molecule is too low to be
recorded in the experiments. Nevertheless, we cannot absolutely
rule out the possibility that the molecule does not form molec-
ular junctions in the experiments.
Conclusion
We have synthesized a molecular wire containing a π-extended
tetrathiafulvalene (exTTF) group and studied its single-mole-
cule electrical transport properties along with those of its
charge-transfer complex with F4TCNQ. Within the accessible
conductance range (10 to 10−7 G0) we did not observe a clear
conductance signature of the neutral parent molecule. This
alone could suggest either that its conductance is too low or that
it does not form stable junctions. We did, however, find a clear
conductance signature in the experiments carried out on the
charge-transfer complex. As complexation with the acceptor
oxidizes the molecule by removing two electrons from the
exTTF group, thus converting it from a buckled and cross-
conjugated group into a planar aromatic group, we predict the
CT species to have a higher conductance than the neutral mole-
cule. This, we believe, supports the idea that the conductance of
the neutral molecule is very low, below our measurement sensi-
tivity. This would make the conductance difference between the
neutral and CT species at least two orders of magnitude. This
can be considered as favorable for the use of single molecules
Figure 9: Transmission as a function of energy for the top and hollow
binding geometries.
Table 1: Kohn–Sham HOMO and LUMO, ionization potential (IP),
electron affinity (EA) and image charge correction for occupied Δocc
and Δvirt unoccupied orbitals. All quantities are in eV.
HOMO IP
Δocc
LUMO EA
Δvirt
top
−4.27 5.68
−0.35 −2.54 −1.16 −0.38
−0.32 −2.54 −1.16 −0.33
hollow −4.27 5.68
The computed conductance values, evaluated as the transmis- as chemical sensors, in which analyte molecules may bind to a
sion at the Fermi level, are in the range 10−6 to 10−7 G0. These backbone to alter its conductance. Further combinations of
low conductance values are at the limit of what we can observe donors and acceptors should be explored in order to evaluate
experimentally. This, therefore, would be consistent with the this potential.
1076

Beilstein J. Org. Chem. 2015, 11, 1068–1078.
8. Chen, F.; Nuckolls, C.; Lindsay, S. Chem. Phys. 2006, 324, 236–243.
doi:10.1016/j.chemphys.2005.08.052
Supporting Information
9. Xu, B. Q.; Li, X. L.; Xiao, X. Y.; Sakaguchi, H.; Tao, N. J. Nano Lett.
2005, 5, 1491–1495. doi:10.1021/nl050860j
Supporting Information File 1
Detailed experimental procedures for the synthesis and
characterization of 5, break junction experiments and
theoretical methods.
10.Hong, W.; Valkenier, H.; Mészáros, G.; Manrique, D. Z.;
Mishchenko, A.; Putz, A.; Moreno García, P.; Lambert, C.;
Hummelen, J. C.; Wandlowski, T. Beilstein J. Nanotechnol. 2011, 2,
699–713. doi:10.3762/bjnano.2.76
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-11-120-S1.pdf]
11.Xiao, X.; Brune, D.; He, J.; Lindsay, S.; Gorman, C. B.; Tao, N.
Chem. Phys. 2006, 326, 138–143.
doi:10.1016/j.chemphys.2006.02.022
12.Taniguchi, M.; Tsutsui, M.; Shoji, K.; Fijiwara, H.; Kawai, T.
J. Am. Chem. Soc. 2009, 131, 14146–14147. doi:10.1021/ja905248e
13.Giacalone, F.; Ángeles Herranz, M.; Grüter, L.; González, M. T.;
Calame, M.; Schönenberger, C.; Arroyo, C.; Rubio-Bollinger, G.;
Vélez, M.; Agraït, N.; Martín, N. Chem. Commun. 2007, 4854–4856.
doi:10.1039/b710739k
Acknowledgements
We thank Dr. Jose Manuel Santos Barahona for help with pre-
paring the CT complex with TCNQ, and recording and inter-
preting the UV–vis spectra. Financial support by the European
Commission (EC) FP7 ITN “MOLESCO” Project No. 606728,
the European Research Council (ERC-436 2012
ADG_20120216-Chirallcarbon), the CAM (PHOTOCARBON
project S2013/MIT-2841, NANOFRONTMAG-CM project
S2013/MIT-2850 and MAD2D project S2013/MIT-3007), FP7-
ENERGY-2012-1-2STAGE-number 309223 (PHOCS) and the
Spanish MICINN/MINECO through the programs MAT2011-
25046, MAT2014-57915-R and PRI-PIBUS-2011-1067 is
acknowledged. MB was partly supported by a FY2012
(P12501) Postdoctoral Fellowship for Foreign Researchers from
the Japan Society for Promotion of Science (JSPS) and by a
JSPS KAKENHI, “Grant-in-Aid for JSPS Fellows”, grant no.
24·02501. YA is also thankful to another KAKENHI, “Grant-
in-Aid for Scientific Research on Innovation Areas, Molecular
Architectonics: Orchestration of Single Molecules for Novel
Functions” (#25110009). LAZ was supported by the Spanish
MICINN under Grant MAT2011-23627. FP acknowledges
support by the Carl-Zeiss foundation and the Collaborative
Research Center 767 “Controlled Nanosystems: Interaction and
Interfacing to the Macroscale”.
14.Li, M.-J.; Long, M.-Q.; Chen, K.-Q.; Xu, H. Solid State Commun. 2013,
157, 62–67. doi:10.1016/j.ssc.2012.12.001
15.Leary, E.; Higgins, S. J.; van Zalinge, H.; Haiss, W.; Nichols, R. J.;
Nygaard, S.; Jeppesen, J. O.; Ulstrup, J. J. Am. Chem. Soc. 2008, 130,
12204–12205. doi:10.1021/ja8014605
16.Liao, J.; Agustsson, J. S.; Wu, S.; Schönenberger, C.; Calame, M.;
Leroux, Y.; Mayor, M.; Jeannin, O.; Ran, Y.-F.; Liu, S.-X.; Decurtins, S.
Nano Lett. 2010, 10, 759–764. doi:10.1021/nl902000e
17.Parker, C. R.; Leary, E.; Frisenda, R.; Wei, Z.; Jennum, K. S.;
Glibstrup, E.; Abrahamsen, P. B.; Santella, M.; Christensen, M. A.;
Della Pia, E. A.; Li, T.; González, M. T.; Jiang, X.; Morsing, T. J.;
Rubio-Bollinger, G.; Laursen, B. W.; Nørgaard, K.; van der Zant, H.;
Agraït, N.; Nielsen, M. B. J. Am. Chem. Soc. 2014, 136, 16497–16507.
doi:10.1021/ja509937k
18.Martín, N.; Sánchez, L.; Herranz, M. A.; Illescas, B.; Guldi, D. M.
Acc. Chem. Res. 2007, 40, 1015–1024. doi:10.1021/ar700026t
19.Brunetti, F. G.; López, J. L.; Atienza, C.; Martín, N. J. Mater. Chem.
2012, 22, 4188–4205. doi:10.1039/c2jm15710a
20.Giacalone, F.; Segura, J. L.; Martín, N.; Guldi, D. M. J. Am. Chem. Soc.
2004, 126, 5340–5341. doi:10.1021/ja0318333
21.Molina-Ontoria, A.; Wielopolski, M.; Gebhardt, J.; Gouloumis, A.;
Clark, T.; Guldi, D. M.; Martín, N. J. Am. Chem. Soc. 2011, 133,
2370–2373. doi:10.1021/ja109745a
22.Wielopolski, M.; Molina-Ontoria, A.; Schubert, C.; Margraf, J. T.;
Krokos, E.; Kirschner, J.; Gouloumis, A.; Clark, T.; Guldi, D. M.;
Martín, N. J. Am. Chem. Soc. 2013, 135, 10372–10381.
doi:10.1021/ja401239r
References
1. Leary, E.; La Rosa, A.; González, M. T.; Rubio-Bollinger, G.; Agraït, N.;
Martín, N. Chem. Soc. Rev. 2015, 44, 920–942.
23.Illescas, B. M.; Santos, J.; Martín, N.; Atienza, C. M.; Guldi, D. M.
Eur. J. Org. Chem. 2007, 5027–5037. doi:10.1002/ejoc.200700226
24.Pearson, D. L.; Tour, J. M. J. Org. Chem. 1997, 62, 1376–1387.
doi:10.1021/jo962335y
doi:10.1039/C4CS00264D
2. Guldi, D. M.; Nishihara, H.; Venkataraman, L., Eds. Molecular wires.
Chem. Soc. Rev. 2015, 4, 835–1030. doi:10.1039/C5CS90010G
3. Forrest, S. R.; Thompson, M. E., Eds. Organic Electronics and
Optoelectronics. Chem. Rev. 2007, 107, 923–1386.
doi:10.1021/cr0501590
25.Liu, S.-G.; Pérez, I.; Martín, N.; Echegoyen, L. J. Org. Chem. 2000, 65,
9092–9102. doi:10.1021/jo001149w
26.Herranz, M. A.; Yu, L.; Martín, N.; Echegoyen, L. J. Org. Chem. 2003,
68, 8379–8385. doi:10.1021/jo034894s
4. Guldi, D. M.; Illescas, B. M.; Atienza, C. M.; Wielopolski, M.; Martín, N.
Chem. Soc. Rev. 2009, 38, 1587–1597. doi:10.1039/b900402p
5. Haiss, W.; van Zalinge, H.; Higgins, S. J.; Bethell, D.; Höbenreich, D.;
Schiffrin, D. J.; Nichols, R. J. J. Am. Chem. Soc. 2003, 125,
15294–15295. doi:10.1021/ja038214e
27.González, M. T.; Leary, E.; García, R.; Verma, P.; Herranz, M. A.;
Rubio-Bollinger, G.; Martín, N.; Agraït, N. J. Phys. Chem. C 2011, 115,
17973–17978. doi:10.1021/jp204005v
28.Chen, F.; Tao, N. J. Acc. Chem. Res. 2009, 42, 429–438.
doi:10.1021/ar800199a
6. Gittins, D. I.; Bethell, D.; Schiffrin, D. J.; Nichols, R. J. Nature 2000,
408, 67–69. doi:10.1038/35040518
7. Chen, F.; He, J.; Nuckolls, C.; Roberts, T.; Klare, J. E.; Lindsay, S.
Nano Lett. 2005, 5, 503–506. doi:10.1021/nl0478474
1077

Beilstein J. Org. Chem. 2015, 11, 1068–1078.
29.Urban, C.; Écija, D.; Wang, Y.; Trelka, M.; Preda, I.; Vollmer, A.;
Lorente, N.; Arnau, A.; Alcamí, M.; Soriano, L.; Martín, N.; Martín, F.;
Otero, R.; Gallego, J. M.; Miranda, R. J. Phys. Chem. C 2010, 114,
6503–6510. doi:10.1021/jp911839b
30.Jain, A.; Rao, K. V.; Mogera, U.; Sagade, A. A.; George, S. J.
Chem. – Eur. J. 2011, 17, 12355–12361. doi:10.1002/chem.201101813
31.Bryce, M. R.; Moore, A. J.; Hasan, M.; Ashwell, G. J.; Fraser, A. T.;
Clegg, W.; Hursthouse, M. B.; Karaulov, A. I.
Angew. Chem., Int. Ed. Engl. 1990, 29, 1450–1452.
doi:10.1002/anie.199014501
32.Bryce, M. R.; Finn, T.; Batsanov, A. S.; Kataky, R.; Howard, J. A. K.;
Lyubchik, S. B. Eur. J. Org. Chem. 2000, 1199–1205.
doi:10.1002/1099-0690(200004)2000:7<1199::AID-EJOC1199>3.0.CO
;2-F
33.Peng, G.; Strange, M.; Thygesen, K. S.; Mavrikakis, M.
J. Phys. Chem. C 2009, 113, 20967–20973. doi:10.1021/jp9084603
34.Bendikov, M.; Wudl, F.; Perepichka, D. F. Chem. Rev. 2004, 104,
4891–4946. doi:10.1021/cr030666m
35.González, M. T.; Zhao, X.; Manrique, D. Z.; Miguel, D.; Leary, E.;
Gulcur, M.; Batsanov, A. S.; Rubio-Bollinger, G.; Lambert, C. J.;
Bryce, M. R.; Agraït, N. J. Phys. Chem. 2014, 118, 21655–21662.
doi:10.1021/jp506078a
36.Guédon, C. M.; Valkenier, H.; Markussen, T.; Thygesen, K. S.;
Hummelen, J. C.; van der Molen, S. J. Nat. Nanotechnol. 2012, 7,
305–309. doi:10.1038/nnano.2012.37
37.Zotti, L. A.; Bürkle, M.; Pauly, F.; Lee, W.; Kim, K.; Jeong, W.; Asai, Y.;
Reddy, P.; Cuevas, J. C. New J. Phys. 2014, 16, 015004.
doi:10.1088/1367-2630/16/1/015004
38.Cuevas, J. C.; Scheer, E. Molecular electronics: An Introduction to
Theory and Experiment; World Scientific Publishing Co Pte Ltd:
Singapore, 2010; Vol. 1. doi:10.1142/7434
39.Lambert, C. J. Chem. Soc. Rev. 2015, 44, 875–888.
doi:10.1039/C4CS00203B
License and Terms
This is an Open Access article under the terms of the
Creative Commons Attribution License
(http://creativecommons.org/licenses/by/2.0), which
permits unrestricted use, distribution, and reproduction in
any medium, provided the original work is properly cited.
The license is subject to the Beilstein Journal of Organic
Chemistry terms and conditions:
(http://www.beilstein-journals.org/bjoc)
The definitive version of this article is the electronic one
which can be found at:
doi:10.3762/bjoc.11.120
1078