Tetrathiafulvalene-based molecular nanowires

Article abstract of DOI:10.1039/b710739k

A new molecular wire suitably functionalized with sulfur atoms at terminal positions and endowed with a central redox active TTF unit has been synthesized and inserted within two atomic-sized Au electrodes; electrical transport measurements have been perf

Full text of DOI:10.1039/b710739k

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COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Tetrathiafulvalene-based molecular nanowires{
a
Francesco Giacalone, M Angeles Herranz, Lucia Gru¨ter, M Teresa Gonza´lez, Michel Calame,
a
a
b
a
b
b
´
Christian Scho¨nenberger,b Carlos R. Arroyo,c Gabino Rubio-Bollinger,c Marisela Ve´lez,c Nicolas Agra¨ıt*c
and Nazario Mart´ın*a
Received (in Cambridge, UK) 13th July 2007, Accepted 11th October 2007
First published as an Advance Article on the web 5th November 2007
DOI: 10.1039/b710739k
Target TTF-based wire 4 was synthesized as shown in Scheme 1.
By following modified previously reported methods, intermediates
2,10 3,11 5,12 6,12 and 711,13 were prepared. Further cross-coupling
Sonogashira protocol for a mixture of 2 and 3 or 6 and 7 afforded
the target molecule 4{ which was obtained in 23% or 47% yield,
respectively.
A new molecular wire suitably functionalized with sulfur atoms
at terminal positions and endowed with a central redox active
TTF unit has been synthesized and inserted within two atomic-
sized Au electrodes; electrical transport measurements have
been performed in STM and MCBJ set-ups in a liquid
environment and reveal conductance values around 1022 G0
for a single molecule.
In order to investigate the different variables affecting the
electrical transport in a liquid environment, the solution redox
properties of 4 were studied by cyclic voltammetry (CV) in
different solvents (CH3CN, CH2Cl2 and THF) and using
tetrabutylammonium hexafluorophosphate (TBAPF6) or perchlo-
rate (TBAClO4) as supporting electrolytes. Derivative 4 gives two
pairs of well-defined and reversible redox couples typical of TTF,
although positively shifted due to the presence of electron-
withdrawing substituents (see Table S1 for detailed electrochemical
data{).7 It is evident from the results that solvent and supporting
electrolyte have a notable effect on the stability of the radical
species generated electrochemically. Furthermore, the redox
potentials of the cation-radical or dication could be tuned as a
function of the above factors (see ESI{).
To get a better understanding of the geometrical, electrical and
charge-transport properties of 4, semiempirical theoretical calcula-
tions were performed using the PM3 Hamiltonian. The optimized
geometry of derivative 4, with the TTF molecule in the neutral
state, resulted in a planar D2h symmetry in agreement with
previous related theoretical calculations.14 The distance for 4
measured between the two sulfur atoms was found to be 2.10 and
2.34 nm for the cis and trans isomers, respectively. In addition, the
electronic distribution structure of 4 shows, in the neutral state,
Electron transport measurements through single molecules sand-
wiched between metal electrodes are at the forefront of research in
molecular electronics.1 Several strategies including mechanically
controllable break junctions (MCBJ)2 and scanning tunneling
microscopy (STM) techniques,3 among others, have been
employed to measure the conductance of various molecular
devices based on carbon nanotubes,4 C60,5 and other organic
molecules.6
Currently, a major challenge is to make use of single molecules
that function as switches on metal surfaces in response to an
external trigger. In this sense, tetrathiafulvalene (TTF) is very
attractive because of its three stable redox states (TTF0, TTF+, and
TTF2+).7 Although TTF itself has been used as a bridge in
different systems,8,9 to the best of our knowledge, electrical
conductance on a single molecule containing the TTF unit has not
been previously studied.
In this communication we present the synthesis, solution
electrochemistry and preliminary electrical transport measure-
ments of a TTF-based molecular switch (4) that has thioacetyl end
groups for attachment to the metal electrodes. Conductance
measurements of single molecules in solution of 4 were performed
using an STM, showing high conductance at low bias voltages.
Additionally, stable current–voltage (I–V) characteristics recorded
in an MCBJ immersed in a solution of 4 show indications of a
gating effect.
aDepartamento de Qu´ımica Orga´nica, Facultad de Qu´ımica, Universidad
Complutense de Madrid, E-28040, Madrid, Spain.
E-mail: nazmar@quim.ucm.es; Fax: +34 91 394 4103;
Tel: +34 91 394 4227
bInstitut fu¨r Physik, Universita¨t Basel, Klingelbergstr. 82, 4056 Basel,
Switzerland
cLaboratorio de Bajas Temperaturas, Departamento de F´ısica de la
Materia Condensada C-III, Universidad Auto´noma de Madrid, E-28049,
Madrid, Spain. E-mail: nicolas.agrait@uam.es; Fax: +34 91 497 3961;
Tel: +34 91 497 4756
{ Electronic supplementary information (ESI) available: Synthesis and
structural characterization of intermediates 2, 3, 5, 6, 7 and molecular wire
4. Electrochemical data for 4 in different solvents. See DOI: 10.1039/
b710739k
Scheme 1 Synthesis of the TTF-based redox switch 4.
4854 | Chem. Commun., 2007, 4854–4856
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Fig. 1 Calculated HOMO and LUMO orbitals (PM3) for compound 4.
that the HOMO is predominantly located on the TTF donor
fragment (Fig. 1) and the LUMO is mainly spread on the
phenylalkynyl groups. In agreement with recent results,15 con-
ductance is expected to change with the relative twist angle
between the two dithiole rings, with the planar conformation of the
neutral TTF moiety having the highest conductance.
Fig. 3 I–V characteristics, at different gate voltages, for a wide open (A)
and almost closed (B) break junction immersed in a solution of 4. The
inset shows a schematic of the break junction setup working in liquid.
Charge transport measurements were carried out by repeatedly
forming and breaking Au point contacts in the presence of 4 using
an STM at room temperature.3c,15 The STM tip was made out of a
99.99% purity 250 mm gold wire, and the sample consisted of gold
evaporated on flat quartz. Au point contacts were formed in a
droplet of a dilute 1 mM solution of 4 in 1,2,4-trichlorobenzene
placed on the sample. As an Au point contact is broken by pulling
apart the electrodes, there is a certain probability that one or
several molecules will bridge the junction giving rise to character-
istic features in the conductance traces from which the average
conductance of the molecules can be obtained. The observation of
a conductance step at G0 = 2e2/h, the quantum of conductance, is
the signature of a one-atom gold contact16 and an indication that
only a few molecules are present in the junction. We have
measured 7000 conductance traces at a fixed bias of 25 mV. Most
of these traces show an exponential decrease of the conductance
with increasing tip–sample separation (trace a in Fig. 2A) as
expected for tunneling through the solvent. Interestingly, about 1%
of these traces show conductances larger than 1023 G0 for tip–
sample separations in excess of 1 nm followed by an abrupt drop
of conductance (traces b and c in Fig. 2A). The prominent peak in
the histogram constructed with these traces suggests a conductance
of about 1022 G0 for a single molecule.
its surface is elongated and finally broken. Two atomic-sized Au
electrodes are then formed. The gap between the electrodes can be
opened and closed repeatedly by moving the pushing rod up and
down. The resolution in the gap size variation is sub-Angstrom.17
A PtIr wire was added to the cell and operated as a gate electrode.
We prepared a 0.3 mM solution of 4 in a (4 : 1) mixture of
toluene–CH3CN. The thioacetyl end groups were deprotected with
tetrabutylammonium hydroxide (TBAOH) and 0.1 M Bu4NPF6
was added as an electrolyte. The solution with 4 was injected in the
liquid cell while the junction was kept closed. The junction was
then opened widely (¢ 3 nm) for a few minutes to let the
molecules assemble. Then, we performed several open/close cycles,
without allowing the Au electrodes to get into contact. In this
process, I–V characteristics were recorded at several gap openings
and for different gate voltages applied. Two reproducible kinds of
I–V characteristics were observed. These are depicted in Fig. 3.
(A)-type curves were found at large gap sizes. They present a
Simmons-like shape typical of tunneling, and show little sensitivity
to the gate voltage. In contrast, a gate effect is observed in (B)-type
curves, which present a linear profile in a larger voltage range.
(B)-type curves were observed when the electrodes were brought
very close together. Note that the precise gap size cannot be
determined here. After the gap is opened, the gold atoms on the
electrode tips will rearrange and retract.17 The resistance in the
linear regime of the (B)-type curves is approximately 2 MV. This
value is two orders of magnitude lower than the single-molecule
resistance reported for oligo(phenylene ethynylene),18 and four
orders of magnitude lower than that estimated from studies in self-
assembled monolayers of a similar TTF compound.8 These values
suggest that our results do not correspond to a single molecule, but
rather to a collection of confined (and not necessarily chemically
bonded) molecules between the electrodes getting together. These
encouraging observations emphasize the potential of TTF
derivatives as molecular wires with an electrostatically adjustable
conductance. Additional experiments are still needed to confirm
the single-molecule response.
Gating experiments were carried out using a MCBJ equipped
with a liquid cell.17 A schematic of the setup is shown in Fig. 3. By
bending the substrate with a central pushing rod, the Au wire on
In summary, we have prepared a new type of molecular wire
endowed with a redox active TTF unit. First liquid conductance
measurements by STM and MCBJ techniques have been carried
out and showed high conductance at low bias voltages. Work is
Fig. 2 (A) Conductance traces for an Au–Au junction (trace a) and two
molecular junctions comprising one or a few molecules (traces b and c).
(B) Conductance histogram constructed from the conductance traces that
show evidence of trapped molecules.
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5 (a) H. Park, J. Park, A. K. L. Lim, E. H. Anderson, A. P. Alivisatos and
currently in progress to determine how the different oxidation
states of TTF influence the conductance on this molecular wire.
This work has been supported by the Spanish MEC (project
MAT2002-11982-E) through the SONS program of the ESF
(Projects 02-PE-SONS-051-NANOSYN and 02-PE-SONS-055-
SASMEC) which is also supported by the EC, Sixth Framework
Program, and the MEC and Comunidad de Madrid (Projects
CTQ2005-02609/BQU and P-PPQ-000225-0505). F. G. is indebted
to the SONS program for a postdoctoral fellowship and M. A. H.
to the MEC of Spain for a ‘Ramo´n y Cajal’ contract.
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4856 | Chem. Commun., 2007, 4854–4856
This journal is ß The Royal Society of Chemistry 2007