A new class of extended tetrathiafulvalene cruciform molecules for molecular electronics with dithiafulvene-4,5-dithiolate anchoring groups

Article abstract of DOI:10.1002/adma.201201583

Cruciform motifs with two orthogonally oriented π-extended tetrathiafulvalenes and with differently protected thiolate end-groups are synthesized by stepwise coupling reactions. The molecules are subjected to single-molecule conductivity studies in a break-junction and to conducting probe atomic force microscopy studies in a self-assembled monolayer on gold. Copyright

Full text of DOI:10.1002/adma.201201583

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A New Class of Extended Tetrathiafulvalene Cruciform
Molecules for Molecular Electronics with
Dithiafulvene-4,5-Dithiolate Anchoring Groups
Christian Richard Parker, Zhongming Wei, Carlos R. Arroyo, Karsten Jennum, Tao Li,
Marco Santella, Nicolas Bovet, Guangyao Zhao, Wenping Hu, Herre S. J. van der Zant,
Marco Vanin, Gemma C. Solomon, Bo W. Laursen, Kasper Nørgaard,
and Mogens Brøndsted Nielsen*
Development of functional organic molecules as components
for molecular electronics has attracted wide interest in recent
years.[1] In particular, oligo(phenyleneethynylene)s (OPEs,
Figure 1) constitute a class of π-conjugated molecules that has
been used extensively as molecular wires for transmitting a
current between two electrodes.[2] The electron donor tetrathia-
fulvalene (TTF, Figure 1) is another attractive molecule, under-
going two reversible one-electron oxidations at potentials that
can be tuned by either peripheral substitution or by insertion of
a π-conjugated spacer between the two rings.[3] In fact, the sug-
gestion by Aviram and Ratner[4] in 1974 that a donor-σ-acceptor
dyad consisting of TTF and tetracyanoquinodimethane could be
employed as a molecular rectifier can be considered to be the
birth of molecular electronics. Several intramolecular charge-
transfer compounds have been synthesized with this objective[5]
in mind and some have been studied in junction devices.[5c]
In addition, TTF derivatives with thiol end-groups have been
inserted between electrodes and the switching of molecular
junction conductance by successive oxidation/reduction cycles
has been demonstrated.[6] With the objective of combining
the properties of OPEs and TTF, we have, in recent years,
developed synthetic protocols for so-called OPE-TTF cruciform
molecules in which a π-extended TTF is placed orthogonally to
an OPE wire.[7] Conducting-probe (CP) AFM measurements on
self-assembled monolayers (SAMs) of the OPE5-TTF, shown in
Figure 1, revealed a 9-fold increase in conductivity relative to a
simple OPE5.[8] Moreover, this molecule has been contacted in
a three-terminal geometry in which it was reversibly switched
between three redox states by a gate electrode.[9] In the quest for
more advanced molecules, we decided to include redox-active
dithiafulvene (DTF) units containing protected thiol groups
in each end of the wire as in molecules 1-4 (Figure 1). These
molecules can be seen as a new class of TTF-based cruciform
molecules where both the horizontal and vertical backbones
are extended TTFs. Alternatively, the molecules can be seen
as OPE3-TTF cruciforms end-capped with protected DTF-4,5-
dithiolate anchoring groups. As thiolate protection groups, the
molecules contain different combinations of cyanoethyl, acetyl,
and methyl. Here we describe synthetic protocols for obtaining
these molecular wires and their electronic properties, including
CP-AFM studies of SAMs and mechanically controllable break-
junction (MCBJ) experiments.
The cruciform molecules were prepared according to
Scheme 1. The known extended TTF 5[7a] with two ethynyl
groups was subjected to Pd-catalyzed cross-coupling reac-
tions with the aryliodides 6 and 7, respectively, to furnish the
OPE3-TTFs 8 and 9. Next, the dialdehyde 9 was subjected to a
phosphite-mediated coupling with the readily obtainable
1,3-dithiol-2-thione 10[10] to provide cruciform 1. One cyanoe-
thyl protecting group at each end of the molecule was selectively
removed by treatment with 2.1 molar equivalents of cesium
hydroxide in 1-propanol; this alcohol was used as solvent to pre-
vent transesterification reactions of the ester groups on the cen-
tral DTF units. The resulting dithiolate was treated with acetyl
chloride to give cruciform 2, while treatment with methyliodide
gave cruciform 3. Both these compounds were isolated as Z/E
Dr. C. R. Parker, Dr. Z. Wei, M.Sc. K. Jennum, Dr. T. Li,
M.Sc. M. Santella, Prof. N. Bovet, Dr. M. Vanin,
Prof. G. C. Solomon, Prof. B. W. Laursen,
Prof. K. Nørgaard, Prof. M. B. Nielsen
Department of Chemistry
Nano-Science Center
University of Copenhagen & Sino-Danish Centre for
Education and Research (SDC)
Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark
E-mail: mbn@kiku.dk
M.Sc. G. Zhao, Prof. W. Hu
Beijing National Laboratory for Molecular Sciences
Key Laboratory of Organic Solids
Institute of Chemistry
Chinese Academy of Sciences
Beijing 100190, China
1
mixtures (at least two isomers according to H-NMR spectros-
copy). Removal of the two remaining cyanoethyl groups of 3
followed by treatment with acetyl chloride gave cruciform 4.
We also attempted to remove all four cyanoethyl groups by dif-
ferent bases and subjected the suspected tetrathiolate to acetyla-
tion, but could not isolate the compound with four SAc groups
as it seemed to decompose quite readily.
Dr. C. R. Arroyo, Prof. H. S. J. van der Zant
Kavli Institute of Nanoscience
Delft University of Technology
2600 GA Delft, The Netherlands
DOI: 10.1002/adma.201201583
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DOI: 10.1002/adma.201201583
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CO2Pr
S
PrO2C
OPE
n
S
S
S
S
S
S
S
AcS
2
SAc
2
TTF
DTF
CO2Pr
PrO2C
S
OPE5-TTF
CO2Pr
R1S
S
S
S
SR2
S
PrO2C
S
S
1 (R1= R2 = CH2CH2CN)
S
R2S
2 (R1 = CH2CH2CN, R2 = Ac; Z/Z, Z/E, E/E)
3 (R1 = CH2CH2CN, R2 = Me; Z/Z, Z/E, E/E)
4 (R1= Ac, R2 = Me; Z/Z, Z/E, E/E)
S
SR1
S
CO2Pr
PrO2C
Figure 1. Molecular structures.
The longest wavelength absorption maxima of 1 and 8 (spectra
are shown in SI, together with electrochemical data) were sim-
ilar but the molar absorptivities were significantly stronger for
1 because it has twice as many DTF units, λmax (1): 402 (1.1 ×
CO2Pr
S
S
PrO2C
CO2Pr
S
S
PrO2C
−1
−1
105
M
cm−1 ), 437 (sh); λmax (8): 417 (4.9 × 104
M
cm−1 ),
5
PhI (6)
p-IC6H4CHO (7)
Pd(PPh3)4
CuI
425 (sh).
Pd(PPh3)2Cl2
Formation of SAMs from cyanoethyl-protected TTF thiolates
has previously been reported using hydroxide as base[11] and
includes studies on a bis(dodecylthio)-TTF tethered through
two thiolate groups.[11a] We were, however, unable to form
stable and homogenous monolayers from either 1, using
KOH/1-propanol, or 2, using either NEt3 (for removal of acetyl
group only) or KOH/1-propanol. Further, the simple OPE3-TTF
8 could not form a SAM on a gold substrate. Gratifyingly, how-
ever, high quality SAMs of 4 on gold surfaces were achieved
using 15% NEt3 in THF (conditions developed by Valkenier
et al.[12]). A cyclic voltammogram (CV) of an aqueous solution
of K3Fe(CN)6/K4Fe(CN)6 recorded with SAM-covered Au elec-
trodes shows that the waves for the Fe2+/Fe3+ redox couple are
decreased and shifted relative to a CV using bare Au electrodes
(see SI), which implies that the SAM partly blocks access to the
Au surface. As judged from the CV, it seems that the SAM is
somewhat less dense (contains pinholes) than that obtained
previously for OPE5-TTF,[8] and the methyl group might hinder
the S from reaching the Au surface as efficiently. The transport
properties of the SAM were measured by CP-AFM using a Au
coated conductive tip as the top electrode. Figure 2 shows a
typical I–V curve for the molecular junction and the resistance
histogram generated from about 160 measurements. All the
resistances were determined over a small voltage range ( 0.3 V).
The main peak of the Gauss fitting curve was on the order of
108 Ω, and the average resistance value was calculated to be
1.18 × 109 Ω.
CO2Pr
S
CuI
PrO2C
NEt3, THF
NH(i-Pr)2, THF
S
R
R
S
8 (R = H; 52%)
S
9 (R = CHO; 90%)
CO2Pr
PrO2C
CN
S
S
S
R = CHO
S
P(OEt)3, Δ
CN
S
10
1 (63%)
1) 2.1 equiv. CsOH H2O
DMF / PrOH
2) AcCl or MeI
CO2Pr
CN
SR2
PrO2C
S
S
S
S
S
S
S
R2S
NC
S
2 (R2 = Ac; 53%)
3 (R2 = Me; 87%)
S
S
CO2Pr
Z/E
In order to further investigate the molecular conductance of
the new cruciform motif, we turned to MCBJ experiments. Cru-
ciform 2 was found to form stable molecular junctions. Meas-
urements were performed at room temperature using gold
electrodes, and the molecules were deposited onto the sample
in a freshly prepared solution (0.1 × 10−3 M in CH2Cl2). The
PrO2C
1) 2.2 equiv. CsOH H2O
DMF / PrOH
2) AcCl
R2 = Me
4 (19%)
Scheme 1. Synthesis of cruciforms composed of two extended TTFs.
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(a)
0
−1
−2
−3
−4
−5
0
1
2
3
4
5
electrode displacement /nm
(b)
0
−1
−2
−3
−4
−5
−0.5
0
0.5
1
1.5
2
2.5
3
electrode displacement /nm
Figure 2. Top: A typical I−V curve of the SAM molecular junction based
on cruciform 4. Bottom: Histogram of the SAM molecular junction
resistance.
Figure 3. MCBJ experiments on cruciform 2 (0.1 × 10−3 M in CH2Cl2). Top:
Individual breaking traces of gold. Bottom: Trace histogram built from
500 breaking traces of gold.
solvent did not show any additional features compared with
empty gold junctions (the breaking traces of these are shown
in SI). Cruciform 2 showed a clear conductance plateau around
10−3 G0 (corresponding to a resistance of 1.3 × 107 Ω) with a
length of roughly 2 nm (Figure 3). It is possible that the gradual
drop in the conductivity between 2–2.5 nm is due to the pres-
ence of two or more π−π-stacked molecules in the junction,
which may slide relative to each other as the electrodes are dis-
placed. As noted above, the molecule exists as different E/Z iso-
mers, but it has previously been shown that cis/trans isomeriza-
tion about a double bond does not effect transport.[13] To ascer-
tain whether this is also the case here, we performed transport
calculations on different E/Z isomers and conformations,
anchoring the molecule to a gold atom at an external sulfur in
each end (see SI). The calculations revealed that the transmis-
sion at the Fermi level was almost the same for different E/Z
isomers, although the through-space length differed by 3.4 Å.
Yet, the through-bond length of the tunneling path is the same
for the two configurations and the two tunneling paths have
the same electronic coupling through the π-system, resulting
in the same transport properties. Different conformers, i.e.,
rotations about single bonds, also had similar transmission
around the Fermi level, although it may be that one confor-
mation is more accessible for binding with the electrodes and
dominates for purely steric reasons.
The much higher conductivity of 2 measured in the break
junction in comparison with that measured of 4 by CP-AFM
could possibly be explained by a more efficient anchoring con-
figuration in the break junction, which may involve not only
one of the end-sulfur atoms, but also direct binding to a DTF
ring-sulfur atom in each end. The accessibility of all of the
sulfur atoms in the end groups will depend on the degree of
deprotection and the orientation of the molecules in the junc-
tion. Transport calculations (SI) suggest that for most binding
configurations, the additional DTF units are likely to result in
high levels of transport, as was seen in the comparison of OPE5
with OPE5-TTF by CP-AFM,[8] as they introduce transport reso-
nances close to the Fermi energy. In our calculations, the trans-
port is sensitive to the degree of deprotection. Where end-sulfur
atoms are protected, we see narrower transmission resonances
and decreased transmission. Thus, the state of the end-sulfur
atoms in an experiment may have both a steric impact on the
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accessibility of these atoms for binding and also an electronic
impact on the interaction with the electrodes.
of these showed any field effect transistor performance (for
details, see SI).
An anchoring configuration in the break junction involving
one of the central DTF rings could also be imagined, as long
as an inefficient cross-conjugated current pathway[14] via the
meta-configuration is avoided, as this would contradict the high
conductance obtained. However, when subjecting OPE3 8 to
MCBJ studies (see SI), no binding was observed, in agreement
with the failure to grow SAM of this molecule, suggesting that
anchoring in 2 does indeed occur with via the DTF end-groups
and not the central ones. Cruciform 1 was also measured in
two different MCBJ samples, but it was difficult to obtain stable
molecular junctions and only few (5–10%) breaking traces
showed conductance plateaus (see SI) characterized by a high
resistance of 1.3 × 109 Ω. Thus, presence of the cleavable acetyl
group[15] seems important to enforce the anchoring of 2. It is
worth noting that the resistance of 1, containing four protected
thiolates, in the break junction is of the same order of magni-
tude as that of 4 in the monolayer, which supports inefficient
anchoring of 4 in the SAM to the AFM tip electrode.
In conclusion, a series of new OPE-TTF cruciforms, con-
taining two orthogonally oriented π-extended TTFs, has been
developed with different protection groups for the four thiolate
end-groups. The synthesis takes advantage of selective cya-
noethyl deprotection–alkylation/acetylation reactions. While
attempts at forming homogenous SAMs of cruciform 2 using
different bases to remove the cyanoethyl and/or acetyl pro-
tecting groups failed, this molecule did form stable molecular
junctions in break-junction experiments. A cleavable acetyl pro-
tecting group was required to obtain a stable junction, signal-
ling that anchoring via an end-sulfur atom (thiolate) is in play.
A remarkably low spread in bias conductance shows the great
potential of the DTF-thiolate anchoring group for such studies.
CP-AFM measurements on a SAM of the related cruciform 4
showed a resistance that was two orders of magnitude higher,
suggesting that the conductivity depends on subtle changes
in the ability to contact the molecule, i.e., less efficient con-
tact seems to be achieved between the AFM tip and the DTF
anchoring group. Anchoring via the DTF group presented in
this paper is particularly important for the future development
of gate-controlled devices based on the new cruciform motif,
in which its four redox-active DTF groups are used for control-
ling conductivity changes, or for development of donor-acceptor
dyads as molecular rectifiers.
The OPE3-TTF 8 is structurally identical to the central part
of cruciforms 1-4. We managed to obtain single crystals of 8
(grown from CH2Cl2/ heptane), which were subjected to X-ray
crystallography (Figure 4). Importantly, the central extended
TTF part has an almost planar structure, while the two phenyl
end-groups are slightly twisted out of this plane. Both single-
crystal and thin-film devices were fabricated from 8, but none
[CCDC 875277 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.]
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
This work was supported by The European Union seventh Framework
Programme (FP7/2007-2013) under the grant agreement no
270369 (“ELFOS”) and The Danish-Chinese Center for Molecular
Nanoelectronics funded by the Danish National Research Foundation.
MV and GCS received funding from the European Research Council
under the European Union’s Seventh Framework Programme (FP7/2007-
2013)/ERC Grant agreement no. 258806.
Received: April 20, 2012
Revised: July 4, 2012
Published online:
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DOI: 10.1002/adma.201201583
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