Molecular junctions composed of oligothiophene dithiol-bridged gold nanoparticles exhibiting photoresponsive properties.

Abstract

Three oligothiophene dithiols with different numbers of thiophene rings (3, 6 or 9) have been synthesized and characterized. The X-ray single crystal structures of terthiophene 2 and sexithiophene 5 are reported herein to show the exact molecular lengths, and to explain the difference between their UV-visible spectra arising from the different packing modes. These dithiols with different chain lengths were then treated with 2-dodecanethiol-protected active gold nanoparticles (Au-NPs) by means of in situ thiol-to-thiol ligand exchange in the presence of 1 microm gap Au electrodes. Thus the molecular junctions composed of self-assembled films were prepared, in which oligothiophene dithiol-bridged Au-NPs were attached to two electrodes by means of Au-S bonded contacts. The morphologies and current-voltage (I-V) characteristics of these films were studied by SEM and AFM approaches, which suggested that the thickness of the films (3-4 layers) varied within the size of one isolated Au-NP and typical distance-dependent semiconductor properties could be observed. Temperature dependent I-V measurements for these molecular junctions were performed in which the films served as active elements in the temperature range 6-300 K; classical Arrhenius plots and subsequent linear fits were carried out to give the activation energies (deltaE) of devices. Furthermore, preliminary studies on the photoresponsive properties of these devices were explored at 80, 160, and 300 K, respectively. Physical and photochemical mechanisms were used to explain the possible photocurrent generation processes. To the best of our knowledge, this is the first report in which oligothiophene dithiols act as bridging units to link Au-NPs, and also the first report about functionalized Au-NPs exhibiting photoresponse properties in the solid state.

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DOI: 10.1002/chem.200500822
Molecular Junctions Composed of Oligothiophene Dithiol-Bridged Gold
Nanoparticles Exhibiting Photoresponsive Properties
Wei Huang,*[a, b]Gou Masuda,[c]Seisuke Maeda,[c]Hirofumi Tanaka,[a, d]and
Takuji Ogawa*[a, d]
Introduction
Molecular electronics is a fairly new and fascinating area of
research that is firing the imagination of scientists as few re-
search topics have in the past, and significant advances have
been made during the last a few years in various applica-
tions of nanotechnology.[1–3]In general, it involves the search
for single molecules or small groups of molecules that can
be used as the fundamental units or self-contained electronic
devices, that is, wires, switches, and memory and gain ele-
ments. The goal is to use these molecules, designed from the
bottom-up to have specific properties and behaviors, instead
of present solid-state electronic devices that are constructed
by lithographic technologies from the top-down.[4]In recent
Abstract: Three oligothiophene dithiols
with different numbers of thiophene
rings (3, 6 or 9) have been synthesized
and characterized. The X-ray single
crystal structures of terthiophene 2 and
sexithiophene 5 are reported herein to
show the exact molecular lengths, and
to explain the difference between their
UV-visible spectra arising from the dif-
ferent packing modes. These dithiols
with different chain lengths were then
treated with 2-dodecanethiol-protected
active gold nanoparticles (Au-NPs) by
means of in situ thiol-to-thiol ligand ex-
change in the presence of 1mm gap Au
electrodes. Thus the molecular junc-
tions composed of self-assembled films
were prepared, in which oligothio-
phene dithiol-bridged Au-NPs were at-
tached to two electrodes by means of
Au?S bonded contacts. The morpholo-
gies and current–voltage (I–V) charac-
teristics of these films were studied by
SEM and AFM approaches, which sug-
gested that the thickness of the films
(3–4 layers) varied within the size of
one isolated Au-NP and typical dis-
tance-dependent semiconductor prop-
erties could be observed. Temperature
dependent I–V measurements for these
molecular junctions were performed in
which the films served as active ele-
ments in the temperature range 6–
300 K; classical Arrhenius plots and
subsequent linear fits were carried out
to give the activation energies (DE) of
devices. Furthermore, preliminary stud-
ies on the photoresponsive properties
of these devices were explored at 80,
160, and 300 K, respectively. Physical
and photochemical mechanisms were
used to explain the possible photocur-
rent generation processes. To the best
of our knowledge, this is the first
report in which oligothiophene dithiols
act as bridging units to link Au-NPs,
and also the first report about function-
alized Au-NPsexhibiting
ponse properties in the solid state.
photores-
Keywords: gold · nanostructures ·
oligothiophenes · photoresponse ·
semiconductors · thin films
[a] Dr. W. Huang, Dr. H. Tanaka, Prof. Dr. T. Ogawa
Research Center for Molecular Nanoscience
Institute for Molecular Science
National Institutes of Natural Sciences
5-1 Higashiyama, Myodaiji-cho, Okazaki
Aichi, 444-8787 (Japan)
Fax: (+ +81)564-59-5635
E-mail: whuang@nju.edu.cn
ogawat@ims.ac.jp
[b] Dr. W. Huang
State Key Laboratory of Coordination Chemistry
Coordination Chemistry Institute
Nanjing University, Nanjing 210093 (China)
[c] G. Masuda, S. Maeda
Department of Chemistry, Faculty of Science
Ehime University, Bunkyo-cho 2-5, Matsuyama
Ehime, 790-8577 (Japan)
[d] Dr. H. Tanaka, Prof. Dr. T. Ogawa
Core Research for Evolutional Science and Technology (CREST)
Japan Science and Technology Agency (JST)
Hon-machi 4-1-8, Kawaguchi, Saitama, 332-0012 (Japan)
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author. UV/Vis spectra
of 2, 5 and 8–11 in CHCl3and several optical microscopy, SEM and
AFM images of self-assembled films are also attached herein.
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years, several research groups have reported the measure-
ments of the current–voltage (I–V) characteristics of self-as-
sembled monolayers (SAMs) of conjugated polymers,[5–7]as
they have potential applications in semiconductor nanotech-
nology. In view of the intrinsic difficulties and fragility dem-
onstrated by single-molecular or SAM junctions, well-de-
fined self-assembled thin films formed between micro-gap
Au electrodes, in the presence of Au-NPs, are reported
herein. The resultant films exhibit good and reproducible I–
V curves; hence, it is possible for us to discuss the possible
conduction mechanism on the nanometer scale, which is still
a poorly understood field.[8]
It is known that thiol groups can be chemisorbed onto
gold surfaces strongly and that they can be used as a pro-
tected species to synthesize functionalized, ligand-stabilized
Au-NPs. Brust et al[9]developed a general approach for the
preparation of thiol-protected Au-NPs with a narrow size
distribution; this method is based on the reduction of
AuCl4
presence of certain phase-transfer catalysts. More important-
ly, it is also possible for terminal dithiols to replace the pro-
tecting thiol groups through thiol-to-thiol ligand-exchange
reactions. If gold electrodes are used, thiol groups can be
further absorbed chemically to form molecular junctions. As
a result, a variety of electro-optical devices can be fabricat-
ed successfully in this way. In previous work, we showed
that stable arrays of 1,10-decanedithiol-bridged Au-NPs
could be easily obtained between a pair of Au-electrodes.[10]
This material demonstrated semiconductor-like properties
with characteristic sigmoidal-shaped I–V curves and a linear
log(I) versus 1/T relationship when attached to gold micro-
electrodes, in contrast with the bulk material which showed
ohmic behavior.
For a given metal/dielectric-film/metal system, certain
conduction mechanisms may dominate in certain voltage
and temperature regimes. So it is interesting to study their
electronic correlation of voltage and temperature. We have
recently reported the I–V characteristics of molecular nano-
devices in which a polymer of an RuIIcomplex bearing the
3,8-bis[terthiophenyl-(1,10-phenanthroline)] ligand was elec-
trodeposited as functionalized molecular wires between
nanogap (~15 nm) electrodes.[11]Our strategy here is first to
prepare monothiol-potected active Au-NPs, in which the
monothiol (2-dodecanethiol) serves as a surfactant capping
molecule (the average size of them in our case is 3.3?
1.0 nm), and then use a thiol-to-thiol exchange reaction to
replace the monothiol with different dithiols, and finally
bridge a micro-gap source and drain electrodes by means of
a self-assembly method. Accordingly, metal–film–metal
junctions can be produced on the molecular level composed
of dithiol-bridged Au-NPs. Polythiophenes and oligothio-
phenes have been widely studied as conducting polymers
when doped, because they contain extensive conjugated p-
electron systems and they are much more stable than poly-
acetylene films toward oxidative degradation.[12]However,
they have not been involved in the research of self-assembly
with Au-NPs to fabricate nanodevices. In this paper, we
?in toluene with an aqueous NaBH4solution in the
report the syntheses and characterization of three thiocya-
no-terminated oligothiophene precursors and their respec-
tive dithiols; the subsequent self-assembly with Au-NPs for
fabricating molecular junctions (Figure 1), with characteriza-
tion of the formed films by SEM and AFM methods; and
the I–V characteristics and photoresponse properties of the
fillms.
Results and Discussion
Synthesis: Oligothiophenes with different lengths capped by
thiocyano groups were obtained after a multistep synthesis
(Scheme 1). Different molar ratios of N-bromosuccinimide
(NBS) and 3’,4’-dibutyl-2,2’:5’,2’’-terthiophene were used to
synthesize the mono- and dibromination products 3 and 6,
respectively. Compound 2 was obtained by a Ni-catalyzed
cross-coupling to 2,5-dibromo-3,4-dibutylthiophene and sub-
sequent reaction with Br2and KSCN to give the thiocyano
derivative, while compound 5 was yielded by the similar way
except that terthiophene 3 was used in the coupling reac-
tion. Compound 8 was prepared by Ni-catalyzed cross-cou-
pling of terthiophene 6 with the appropriate Grignard re-
agent and subsequent thiocyanation. Three oligothiophenes
end-capped by thiocyanate (9, 10, and 11) were used to fab-
ricate devices immediately after they were reduced by
LiAlH4in dry THF from their respective thiocyano precur-
sors (2, 5, and 8), because they were readily oxidized when
exposed to air. All steps gave moderate to high yields
making the strategy convenient for the preparation of these
functionalized oligothiophene derivatives. Except for the el-
emental analyses of unstable dithiols, NMR spectroscopy,
mass spectrometry, and elemental analyses of the com-
pounds were performed and are consistent with the struc-
tures shown in Scheme 1. In addition, 2 and 5 were further
characterized by X-ray single-crystal diffraction.
To obtain good films, all the dithiols, the Au-NPs with
narrow size distribution, and the gold electrodes were fresh-
ly prepared before use. Successful preparation of black self-
assembled films (which were insoluble in chloroform and
Figure 1. Schematic diagram of a self-assembled film consisting of dithiol-
bridged active Au-NPs between 1 mm gap Au electrodes with photores-
ponse properties.
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could be easily checked by the optical microscope and
SEM) by means of thiol-to-thiol exchange reactions was
verified according to the conductance between the Au-elec-
trodes, since the contrast experiments showed that neither
2-dodecanethiol-protected Au-NPs nor dithiols only can
generate current between the micro-gap electrodes. Further-
more, the presence, thickness, and I–V characteristics of the
films were also characterized by AFM topographs.
Structural description of terthiophene 2 and sexithiophene
5: Drawings of the molecular structures with the atom-num-
bering schemes for 2 and 5 are illustrated in Figure 2. From
X-ray single-crystal diffraction study, compound 2 crystal-
lizes in the monoclinic chiral space group Cc and has two
crystallographically unique molecules in each unit cell.
Three thiophene rings adopt a trans conformation, but they
are not coplanar. The bite angles between the side rings and
Scheme 1. Synthetic route for compounds 2, 5 and 8.
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the middle ring for each molecule are 14.7 and 21.98 8 and
16.0 and 20.18 8, respectively. n Butyl aliphatic chains bonded
to the middle thiophene rings are fully extended, also adopt-
ing the trans conformation to minimize the steric crowding.
However, two terminal linear thiocyanate groups are found
to be in a cis conformation with respect to the molecular
plane. The crystal packing of compound 2 is shown in
Figure 3.
Analysis of the single-crystal structure of 5 indicates that
it crystallizes in a monoclinic system of centrosymmetric
space group P21/c and each asymmetric unit contains half of
the crystallographically unique molecule. Six thiophene
rings adopt a zigzag chain so that two groups of n-butyl side
chains bound to their respective thiophene rings can be fully
extended, adopting a trans conformation to minimize the
spatial repulsion. In contrast to compound 2, two essentially
linear thiocyanate radicals (S1-C1-N1=177.9(7)8 8) adopt a
tran conformation with respect to the molecular plane. The
middle two thiophene rings are parallel with each other, but
the other thiophene rings are a little twisted because of the
presence of alkyl groups. The dihedral angles between the
n-butyl-substituted thiophene rings and the adjacent rings
are 21.78 8 (terminal) and 29.98 8 (middle). It should be men-
tioned that in each moiety all three sulfur atoms point in the
same directionwith short
3.106(2) ? and S3···S4, 3.167(2) ?) in contrast to a conven-
tional all-anti conformation, such as that in compound 2
with a shorter chain. The presence of the long-chain struc-
ture and n-butyl groups is believed to contribute significant-
ly to this array.
An offset layer packing structure is constructed in the
crystal packing of 5 in which strong p–p stacking interac-
tions between adjacent thiophene rings are observed. As
shown in Figure 4 (top), a shift of about one thiophene ring
is found among vicinal molecules forming the offset layer
S···S separations(S2···S3,
Figure 2. Ball-and-stick views of the molecular structures of 2 (top) and 5
(bottom) with the atom-numbering scheme.
Figure 3. Perspective view of the packing structure of 2 with the unit cell;
hydrogen atoms are omitted for clarity.
Figure 4. Perspective views of the layer packing structure of 5 parallel
(top) and perpendicular (bottom) to the molecular plane together with
the unit cell; hydrogen atoms are left out for clarity.
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packing. Adjacent middle thiophene rings are parallel with
the separation of 3.48 ?, indicative of strong p stacking.
Moreover, each terminal thiophene ring bound to the thio-
cyano groups is found to interact with one neighboring ter-
minal thiophene ring through the offset p–p stacking with
the shortest contact of 3.64 ?. In addition, a weak S···S con-
tact between SCN and middle thiophene sulfur atoms (S1?
S4A (1?x, 1?y, ?z), 3.418(2) ?) is observed between adja-
cent molecules, further stabilizing the structure (Figure 4
bottom). In contrast to 5, no p–p stacking interaction can be
observed in 2 because of the existence of strong spatial hin-
drance of n-butyl and thiocyano groups for short chain, as
shown in Figure 3. Moreover, neither inter- nor intramolecu-
lar hydrogen-bonding interactions are observed in either
structure.
To date, there have been several structural reports on oli-
gothiophenes with different numbers of thiophene rings and
different substituted groups such as 3’,3’’’’,4’,4’’’’-tetrabutyl-
sexithione, 3’,4’-dibutyl-5’5’’-diphenyl-2,2’:5’,2’’-terthiophene,
4’’,3’’’-dimethyl-
2,2’:5’,2’’:5’’,2’’’:5’’’,2’’’’:5’’’’,2’’’’’-
sexithiophene, and a,w-dicya-
nooligothiophenesNC-
(C4H2S)nCN(n=3–6).[13]
Our
motivation is to prepare oligo-
thiophenes capped at the ends
by dithiol groups that can then
bridge Au-NPs. Here thiocyano
groups are very stable and can
be easily transformed to thiol
groups through reduction by
LiAlH4. Since it is also the first
time for us to introduce termi-
nal thiocyano groups to oligo-
thiophenes, we determined the
crystal structures of the repre-
sentative compounds 2 and 5.
From their single-crystal struc-
tures, we know that the distance
betweenthe terminalsulfur
atoms is 1.36 nm in 2, and this
separation for 5 is 2.16 nm. Furthermore, we can predict the
length of 8 is approximately 3.16 nm, which can be estimat-
ed by its optimal configuration with the help of quantum
chemical calculations. These data will help us to understand
their electronic behaviors better and possible mechanism of
electronic conduction.
Electronic spectra: The UV-visible spectra of 2, 5, and 8 in
chloroform and in the solid state were measured and the
maximum absorptions were analogous to those reported oli-
gothiophenes.[14]Bathochromic shifts were observed for
both of them because this p–p* transition of thiophene rings
is known to be shifted to lower energies when the number
of thiophene rings is increased.[15,12a]In comparison with the
absorptions in solution, lower energy absorptions and larger
red shifts were observed for the crystalline solids, which is a
reflection of their solid structures. As mentioned above,
compound 5 has much better conjugated p system than 2
and stronger p–p stacking interactions are present between
molecules in its packing structure. Consequently, from 2 to
5, red shifts of 86 and 58 nm in solid and in solution were re-
corded, respectively. In comparison with 5, compound 8 has
only a slightly better delocalized p system; hence red shifts
of only 7 and 14 nm in solid and in solution were obtained
on going from 5 to 8, respectively. In this case, the influence
of changing the terminal group from SCN to SH is not so
evident compared with the main structure, thus similar elec-
tronic spectra were recorded with red shifts of 49 and 5 nm
on going from 9 to 10 and from 10 to 11, respectively (see
Supporting Information).
Characterization of the self-assembled films: SEM and
AFM studies provide further insight into the morphologies
of these self-assembled films between the 1 mm gap Au-elec-
trodes. Figure 5 shows SEM images of the molecular junc-
tion 14 on the electrodes, in which the self-assembled film
covers a wide region around the Au electrode pair. These
images clearly manifest the formation of the self-assembled
film between the micro-gap Au-electrodes. AFM determina-
tions on the same film uncover the detailed information on
the aggregation of the particles, thickness of the film, and
distance-dependent I–V behavior between the micro-gap
Au-electrodes. Figure 6 (top left) is the tapping mode AFM
(TM-AFM) morphology of the area A (1?1 mm2) marked in
Figure 5 (left), over which the height of film varies only
within 10 nm. In view of the average size of Au-NPs (3.3?
1.0 nm) and the molecular length of dithiol 11 (3.16 nm) in
this case, the size of one isolated gold nanoparticle can
reach 10 nm; this value is in agreement with the height var-
iation observed in the AFM images. Similar morphologies
can be observed from the AFM images for the films of 12
Figure 5. SEM images of the self-assembled film of 14 on the 1?1 mm2micro-gap gold electrodes with different
magnifications.
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and 13, which mean all the self-assembled films prepared in
this paper can cover a large zone including the micro-gap
Au-electrodes with uniform morphologies.
The presence of the cracks in the film makes the thickness
determination possible. TM-AFM images corresponding to
the areas C, D, and E (marked in Figure 5 left) in the 1–
3 mm2range exhibit similar thickness distributions of the
films at 18–24 nm, at which 3–4 layers dithiol bridged Au-
NPs are supposed to be present (images of areas B, D, and
E are included in Supporting Information). Conductive
mode AFM (CM-AFM) scanning gives the distance-depen-
dent I–V characteristics of the film, for which one Au-elec-
trode (the left one in Figure 6 bottom) is connected to the
brass substrate by means of a gold wire and a conductive
cantilever serves as the other electrode. The plots of average
current at different regions marked as A, B, and C within
the pair of micro-gap electrodes versus applied bias voltage
give three symmetric I–V curves in the voltage range ?3–
3 V. As shown in Figure 6 (bottom), the current is of the
order of nanoamperes, but decreases when the distances be-
tween two working electrodes increases; these results pro-
vide evidence for the semiconductor properties for this film.
All the above-mentioned results prompted us to explore fur-
ther the electronic characteristics of these devices.
Current–voltage characteristics
of self-assembled films for 12,
13, and 14: The devices pre-
pared in this paper are very
stable in air, although oligothio-
phenes end-capped by thiols
are easily oxidized, and they
remain unchanged over a large
range of voltage under repeated
cycling (?16 to + +16 V). In our
experiment, the
(measured current versus volt-
age) were recorded from ?4 to
4 V with 0.05 V intervals by
using a cyclic scanning method.
All the I–V curves are almost
linear in this range. If a higher
concentration is employed (pre-
pared from 0.5 molL?1
tions), a thicker film containing
more gold atoms and bridging
dithiols is formed. As a result,
higher values of current could
be recorded. However, the tun-
neling current could also be ob-
served at low temperature in
this case.
The I–V curves of one typical
junction of 12 are shown in
Figure 7 (top left); they are
linear over a wide range of tem-
perature (6–300 K). The maxi-
I–V curves
solu-
mal current at ?4 V and 300 K reaches 475 nA. As seen in
Figure 7 (top right), the Arrhenius plots (ln(T) versus 1/T)
can be used to characterize the I–V behaviors of this device,
from which two types of mechanisms can be found. At low
temperature (6–40 K), the currents are temperature inde-
pendent, since in this range the conductance is governed by
only the tunneling between Au-NPs in 1?1 mm2region.
From 40 to 160 K, the tunneling current and the thermal ex-
citation current (thermally excited electrons hop from one
isolated state to the next, the conductance of which depends
strongly on the temperature) co-dominate the I–V behavior,
since they are comparable during this stage (Figure 7
bottom left). However, when the temperature increases
from 160 to 300 K, thermal excitation dominates the con-
duction behavior. Linear fits for linear parts of eight curves
(160–300 K) at different voltages (0.5–4.0 V) give mean
value of the activation energy DE=0.0073 eV for 12
(Figure 7 bottom right). Similar I–V characteristics were ob-
served for the thick molecular junctions of 13 and 14, and
mean values of the activation barriers for hopping DE=
0.016 eV for 13 and DE=0.041 eV for 14, respectively, were
obtained by using the same method.
For the thin films (prepared from 0.1 molL?1solutions),
almost no current (less than 1?10?13A) was recorded at low
temperature (<40 K). Nevertheless, stable I–V curves are
Figure 6. Top: TM-AFM topographies of the self-assembled film of 14 on the micro-gap gold electrodes; A
and B correspond to the areas marked in Figure 5 (left). Bottom: C) topographic image of the electrode, and
D) I–V curves with different separations between cantilever and brass substrate by using CM-AFM.
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obtained when the temperature exceeds 60 K. As seen in
Figure 8 (top), the I–V curves of thin film 13 in the tempera-
ture range 60–300 K with 208 8C intervals show smooth and
reproducible, linear temperature-dependent characteristics
for this device. Figure 8 (middle) shows the experimental
points of currents at different voltages ranging from 0.5 to
4 Vat 0.5 V intervals, from which one can see the current in-
creases significantly to approximately 20 nA when the tem-
perature increases to 300 K. As illustrated in Figure 8
(bottom), the Arrhenius plots at different voltages (0.5–
4.0 V) exhibit good linearity in the temperature range 60–
300 K, which mean the semiconductor characteristics of this
molecular junction are governed by thermal conduction
mechanism. Subsequent linear fits for eight curves give the
mean value of activation energy DE=0.050 eV.
Different from bulk materials in which the electronic con-
duction is dominated by intermolecular or interdomain in-
teractions, the predominant mechanism of electronic con-
duction for the individual molecules is tunneling when the
Fermi level of the electrode does not match with the molec-
ular orbital levels at which a contact barrier is formed.[16]
The most prevalent theories for metal–molecule systems are
thermionic emission, direct tunneling, and defect-mediated
transport, such as hopping.[3]Selzer et al. have newly report-
ed the quantitative comparison of two types of junctions
with the same molecule; one based on an isolated individual
molecule and the other on a SAM.[17]In their SAM junction
with ~100 meV activation energy at 0.1 V, two possible elec-
tron-transport mechanisms of heat dissipation were pro-
posed: 1) local heating is attributed to electrons or holes dis-
sipating energy as they fall down the potential slope when
traversing the molecule, and 2) an inelastic electron-tunnel-
ing processes in the off-resonance regime can also store heat
in a molecule by exciting various vibration modes.[18]
In this paper, the sizes of three dithiols are far smaller
than the electrode gap; thereby two large-area Au-electro-
des can only be covalently connected by organic molecules
in large quantity by the assistance of the Au-NPs. These
three-dimensional nanostructures show similar I–V charac-
teristics and barriers to injection (DE) to the SAM junction.
Hence similar theories can be used to explain the conduc-
tion mechanisms. Since both Fower–Nordheim tunneling
and direct-tunneling processes do not depend on the tem-
perature (to first order), but strongly depend on the film
thickness and voltage,[3]the thickness of the film is relatively
important especially for the conduction mechanisms at low
Figure 7. I–V characteristics of the molecular junction 12 with thick film. Top left: full range of the I–V curves (6–300 K) for a typical device. Top right
and bottom left and right: the Arrhenius plots at 0.5–4 V in different temperature regions.
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temperature. That is why the contribution of tunneling cur-
rent can only be observed for thick films at low temperature
in our case. However, at high temperature, thermal effects
will dominate the conduction because the activation barriers
for hopping are relatively small (<50 meV), and more and
more resonance transmission channels will open for elec-
trons. Moreover, the current excited in this way is much
stronger than the temperature-independent tunneling cur-
rent and it will cover the contribution of the latter. As a
consequence, we can explain the temperature-dependent I–
V behavior at higher temperature for all the devices. In fact,
all the devices have similar I–V characteristics at high tem-
perature. However, thicker films, which contain more Au-
NPs and dithiols, may decrease the energy barriers of our
molecular junctions a little. It should be noted that conduc-
tion mechanism is also affected by the interactions of the
electrodes with the individual molecules; this is the basic
conception in molecular electronics. Thus similar but differ-
ent I–V behaviors and DE values for these above-mentioned
films can be obtained corresponding to the molecules with
different lengths.
Preliminary research for photoresponse properties of self-as-
sembled films for 12, 13, and 14: The I–V curves of the pho-
toresponse for 12, 13, and 14 were measured at 80, 160,
300 K (10 minutes after the metal halide lamp gave the max-
imal power), and those that were not exposed to the lamp
also collected for comparison. A time–current scan mode
with 0.3 s intervals was used at ?3 V potential to record the
process of the photoresponse, and the light source was
switched periodically. When the strong light was irradiated
to the devices, the surface temperature of the devices in-
creased slightly at first and then decreased with control of
the thermostat, but the range only varied within 0.5 K. All
the experiments were carried out at midnight in order to
prevent the influence of the surrounding light.
As clearly seen in Figure 9 (top), the linear I–V curve of
12 under irradiation (red) is increased compared with that
measured in the dark (black); this means that the photores-
ponse in this case is voltage independent. The linear fits of
two I–V graphs give the ratio of 2.00 (Klight/Kdark), which sug-
gests that the photoresponse can double the conductance of
this device. However, the rates of conductance are strongly
influenced by the temperature. In our experiments, this
value decreases to 1.23 at 160 K and 1.02 at 300 K for 12
(Figure 9 middle and bottom). Further study hints that the
degree of photoresponse will decrease when the molecular
length of the oligothiophene-based dithiols increase. For in-
stance, the ratios of conductance for 12, 13 and 14 at 80 K
decrease from 2.00 to 1.41 (Figure 10 top) and 1.32
(Figure 10 bottom). From the plots of time versus current, il-
lustrated in the insets of Figure 9, we can see the photores-
ponse processes for these devices are reversible and repro-
ducible. However the speed of photoresponse is a function
of time that can be divided into two parts. Repeated on–off
cycles of illumination show that a rapid process takes place
first and then a relatively slow process (more than 50 s) in-
creasing the current to the maximum when the light is
turned on or decreasing it to the minimum when the light is
turned off. This behavior is different from the self-assembled
porphyrin nanorods for which the photoconductivity grows
over hundreds of seconds with light exposure and decays
slowly when the light is off.[19]It is also different from the
self-assembled Au-S-C60 system in toluene on optically
Figure 8. I–V characteristics of molecular junction 13 with thin film. Top:
Full range of the I–V curves (60–300 K) for a typical device. Middle: I–T
curves in the bias voltage range of 0.5–4 V. Bottom: Classical Arrhenius
plots at 0.5–4 V with 0.5 V intervals in different temperature regions.
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W. Huang, T. Ogawa et al.

Page 9

transparent electrodes for which prompt photoresponsive
behavior was observed.[20]
Before discussing the possible mechanism of photocurrent
generation in the on–off cycles of illumination, we should
exclude the possibility of thermal effects on the generation
of current, because the light source we used has a wide
range of wavelength. From the experimental results we have
obtained, we can ignore these effects by using the effective
thermostat (?0.58 8) and the reference experiments at every
temperature. For the reference experiments, we prepared
films composed of substituted phenylethynylbenzene-based
dithiols with different chain lengths and the same active Au-
NPs under similar conditions, but no photoresponse was ob-
served at either low or room temperature.
Two distinctively different mechanisms, namely photogal-
vanic and photosensitization processes, can be operative in
the anodic photocurrent generation.[21]In our mode, Au-NPs
have the unique ability to store electrons[22]and promote
the charge separation and facilitate electron transporta-
tion[23]within the film during the photocurrent process. Fur-
thermore, the robust coverage and high surface area of the
nanostructured film play a key role in this process. On the
other hand, the selection of thiol molecules to combine with
Au-NPs is very important, because it will have a significant
Figure 9. Reversible photoresponsive I–V and I–T curves of device 12.
Top: I–V curves under irradiation (red one) and in dark conditions
(black one) in the range ?4 to + +4 V at 80 K. The inset is I–T curve
when the light is turned on or off periodically at ?3 V, 80 K. Middle and
bottom: I–V curves under irradiation (red) and in the dark (black) in the
range ?4 to + +4 V at 160 and 300 K, respectively. The insets are I–T
curves when the light is turned on or off periodically at ?3 V, 160 K and
300 K, respectively.
Figure 10. Photoresponsive I–V curves (red ones) of 13 (top) and 14
(bottom) at 80 K compared those in dark condition (black ones).
Chem. Eur. J. 2006, 12, 607–619 ? 2006 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim
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Page 10

influence on the electron energy levels of the combined
metal–molecule system. Molecules with a good delocalized
conjugated p system, such as porphyrin and oligothiophene,
and suitable functionized groups, such as thiol and thiol
ether, will be good candidates so that small activation barri-
ers for hopping can be obtained. When electrons are photo-
excited from the HOMOs to the LUMOs, if the LUMOs
are closely coupled (which can be achieved by using suitable
molecules), transport through the LUMOs and HOMOs
other than the relaxation from LUMOs back to the
HOMOs will be favored. Here temperature is also a factor
that should be taken into account, since it can influence the
HOMO–LUMO gap energy for a given device. Besides the
aforementioned physical factors, some photochemical proc-
esses are believed to occur in this process, because the I–T
curves of our devices show relatively slow photoresponses
(the second part). The irradiation appears to give rise to
some electron carriers and these photochemically generated
carriers can further increase the current between the junc-
tions, and vice versa. As the photochemical process is much
slower than the physical process, time-dependent increase
and decrease of the current can be observed for all junctions
when the light is switched on and off periodically.
There have been several reports about the photoresponse
of some silicon-based semiconductor devices and organic
compounds.[24–25]However, as far as we are aware, this is the
first report about the photoresponsive compounds in which
an Au-NP-containing self-assembled film is involved in the
solid state. In fact, photoresponse is a very interesting phe-
nomenon and it has many future opportunities for designing
photovoltaic cells, photoswitchable molecular-based devices,
and photomodulated sensors. As very stable and easily pre-
pared devices can be obtained by self-assembly method in
our case, this sort of research deserves more attention and it
is expected to have utility in molecular electronics.
Conclusion
In this paper, we described the synthesis and characteriza-
tion of three oligothiophene-based terminal dithiols with dif-
ferent molecular lengths (1.36, 2.16, and 3.16 nm) and their
self-assembled films with active Au-NPs (3.3?1.0 nm) by
using ligand-exchange and binding between microgap gold
electrodes. UV-visible spectra of these compounds in solu-
tion and in the solid state show different red shifts, which
are related to the different packing modes in the single-crys-
tal structures. SEM and AFM morphologies for one typical
device reveal further information on the morphologies of
these self-assembled films; the height variation of the well-
oriented film is less than 10 nm (corresponding to the size of
isolated Au-NPs) and the thickness of 18–24 nm suggests the
formation of 3–4 layers dithiol-bridged Au-NPs. Distance-
dependent I–V characteristics of the film were also probed
by using conductive AFM technologies. Temperature-depen-
dent I–V curves of these devices with different thickness
and molecular lengths in the range 6–300 K were discussed,
and an explanation for the possible conduction mechanism
was also included. Classical Arrhenius plots were further ap-
plied to calculate the activation energies for different
metal–film–metal junctions. More importantly, preliminary
reversible photoresponsive properties for the aforemen-
tioned devices at different temperatures are discussed and
the magnitude of the current can reach twice that recorded
in dark at 80 K for the experimental bias voltage range ?4
to + +4 V. Some physical and photochemical mechanisms
dominate the photoresponsive processes. Further studies on
this type of molecular junctions such as wavelength depen-
dence for the photoresponse properties and some RuIIcoor-
dination complexes bearing 3,8-disubstituted 1,10-phenan-
throline ligand with high photoresponse are now being un-
dertaken.
Experimental Section
Materials and measurements: All reagents and solvents were of analyti-
cal grade and used without further purification. The anhydrous solvents
were drawn up into a syringe under a flow of dry N2gas and were direct-
ly transferred into the reaction flask to avoid contamination. The nomen-
clature used in the drawings is related to molecular symmetry and does
not correspond to the IUPAC nomenclature used in the Experimental
Section. The intermediates and products were characterized by elemental
analysis and spectroscopic methods. 2-Dodecanethiol-protected active
Au-NPs were prepared by the modification of the Brust method.[26]
The UV-visible spectra were recorded with a Shimadzu UV-3150 double-
beam spectrophotometer. The IR spectra were recorded in a Horiba FT-
700 spectrophotometer.1H NMR spectra were collected on a Varian Unit
500 MHz spectrometer and a JEOL GSX 270 MHz spectrometer.
13C NMR data were obtained at 67.8 MHz with a JEOL GSX spectrome-
ter. The DI-EI (70 eV) mass spectra were given by a Hitachi M80-B
spectrometer. FAB-MS and TOF-MS spectra were measured with a
JEOL JMS-777 V spectrometer. Scanning electron microscope (SEM)
images were collected with a JEOL JSM-6700F microscope with the ac-
celeration voltage at 3 kV. An OLYMPUS BX60m optical microscope
was used to check all the electrodes before the determination of the I–V
curves. A Yanaco PLASMA ASHER LTA-102 instrument was used to
clean all the electrodes. The light source used for irradiating the samples
with the maximal intensity is a NPI PCS-UMX250 high-power metal
halide lamp. Atomic force microscope (AFM) images were recorded on
a JEOL-JSPM4210 instrument. The measurement was carried out in
vacuum to eliminate the influence of water adsorbed on the sample sur-
face and increase the sensitivity and reproducibility of the experiment.
The resonant frequency of the cantilevers was 250 kHz for tapping mode.
Contact-mode AFM was used to measure the I–V curve in which Pt-
coated conductive cantilevers were used to measure current.
The gold nanoelectrodes with 1 mm gaps were prepared as follows. Suc-
cessive layers of Ti, Au, and electron beam resist polymer were deposited
on a four inch n-doped silicon wafer, covered by a 200 nm layer of ther-
mally grown SiO2. It was patterned by conventional electron beam lithog-
raphy and the exposed metal was removed with Ar+ +beams to define the
nanogap electrodes. It was then covered with a 100 nm thick Si3N4layer
after stripping out the resist polymer and the pads and nanogaps were
open by conventional electron beam lithography as mentioned above.
Each 3?3 mm silicon chip was thoroughly washed with toluene, acetone,
and methanol and cleaned in an oxygen plasma asher before SEM analy-
sis. As for the preparation for the self-assembled films with 2-dodecane-
thiol-protected active Au-NPs in CHCl3, the electrodes with 1 mm gaps
were cleaned carefully by CF4plasma and checked by the microscopy
before use.
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? 2006 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim Chem. Eur. J. 2006, 12, 607–619
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W. Huang, T. Ogawa et al.

Page 11

The I–V curves were collected by an Advantest R6245 2Channels volt-
age–current source monitor interfaced to a microcomputer through a
GPIB-SCSI board and NI-488.2 protocol. The data were acquired with a
homemade procedure and IgorPro 4.0 (Wavemetrics) software. The sam-
ples were mounted on the top of an anti-vibration table with a tempera-
ture-controlled cryogenic chamber (? 0.0058 8C). All measurements were
carried out in high vacuum (P < 2.0?10?4Pascal) by means of the turbo
pump, and the samples were cooled by using liquid helium as coolant (6
to 300 K). Triaxial cables were used to connect the molecular devices and
the I–V monitor in order to minimize the external noise.
X-ray data collection and refinement: Single-crystal samples of 2 and 5
were covered in glue and mounted on a glass fiber and used for data col-
lection on a Rigaku Mercury CCD area-detector at 100(2) K using graph-
ite monochromated MoKaradiation (l=0.71073 ?). The collected diffrac-
tion data were reduced by using the program Crystalclear[27]and empiri-
cal absorption corrections were done. The original data files generated
by Crystalclear were transformed to SHELXTL97 format by the Texsan
program,[28]and then the structures were solved by direct methods and
refined by least-squares method on F2
ware package.[29]All non-hydrogen atoms were anisotropically refined
and all hydrogen atoms were inserted in the calculated positions assigned
fixed isotropic thermal parameters and allowed to ride on their respec-
tive parent atoms. All calculations and molecular graphics were carried
out with the SHELXTL PC program package on a PC computer. The
summary of the crystal data, experimental details and refinement results
for 2 and 5 is listed in Table 1, while selected bond lengths and bond
angles for 5 are given in Table 2.
oby using the SHELXTL-PC soft-
CCDC-272762 and CCDC-272763 contain the supplementary crystallo-
graphic 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.
Terthiophenes 1 and 3: Compounds 1 and 3 were prepared by means of
the method we have described previously.[11]
Terthiophene 2: A solution of compound 1 (3.6 g, 10.0 mmol) in CHCl3
(20 mL) was added dropwise to a mixture of KSCN (40.1 g, 412.6 mmol)
in methanol (100 mL) and Br2(10.3 mL, 1=3.12 gcm?3, 201.1 mmol) in
CHCl3(50 mL) at ?788 8C. This mixture was stirred for 4 h at room tem-
perature, quenched with water, extracted with CHCl3, and dried over an-
hydrous Na2SO4. The solvent was removed by a rotatory evaporator, and
the residue was purified by silica-gel column chromatography (n-hexane).
A yellow crystalline product was obtained (yield: 89.1%) after recrystalli-
zation from CHCl3/n-hexane. M.p. 87–888 8C;1H NMR (270 MHz, CDCl3,
258 8C, TMS): d=7.39 (d, J=3.9 Hz, 2H), 7.10 (d, J=3.9 Hz, 2H), 2.70 (t,
J=8.1 Hz, 4H), 1.58–1.44 (m, 8H), 0.97 ppm (t, 7.1 Hz, 6H);
(68 MHz, CDCl3, 258 8C, TMS): d=143.94, 141.85, 138.02, 129.59, 126.83,
117.52, 110.09, 32.77, 27.82, 22.90, 13.77 ppm; FT-IR (KBr pellets): n ˜ =
2952 (s), 2868 (s), 2154 (s), 1464 (s), 1417 (m), 793 cm?1(s); elemental
analysis calcd (%) for C22H22N2S5: C 55.66, H 4.67, N 5.90; found: C
55.38, H 4.66, N 6.00; UV/Vis (CHCl3): lmax=359 nm (387 nm for solid);
MS (DI-EI): m/z: 474 [C22H22N2S5]+ +. Yellow needlelike single crystals of
2 suitable for X-ray diffraction determination were grown from a mixed
solution of acetone and ethanol (2:1) by slow evaporation in air at room
temperature.
Sexithiophene 4: A solution of terthiophene 3 (6.17 g, 14.04 mmol) in dry
THF (20 mL) was added dropwise into a suspension of zinc (2.76 g,
42.26 mmol, NiCl2(0.20 g, 1.54 mmol), and PPh3(2.96 g, 11.29 mmol) in
dry THF (30 mL) under an N2atmosphere at 608 8C. The mixture was
then stirred overnight at room temperature, and was then filtered and ex-
tracted with CHCl3. The organic layer was washed with water and satu-
rated brine, and dried with anhydrous Na2SO4. The solvent was removed
by a rotatory evaporator and the residue was purified by silica-gel
column chromatography (n-hexane). An orange crystalline product was
obtained (yield: 78.0%) after recrystallization from CHCl3/n-hexane.
M.p. 117–1188 8C;
J=1.2 Hz, 2H), 7.30 (d, J=1.2 Hz, 2H), 7.15–7.04 (m, 6H), 2.74–2.68 (m,
8H), 1.54–1.43 (m, 16H), 1.00–0.93 ppm (m, 16H);
CDCl3, 258 8C, TMS): d=140.19, 136.68, 136.11, 135.25, 130.00, 129.99,
129.61, 127.36, 126.33, 125.89, 125.35, 123.81, 32.90, 32.84, 27.94, 27.80,
23.03, 22.99 ppm; FT-IR (KBr pellets): n ˜ =2954 (s), 2854 (s), 1496 (m),
1464 cm?1(s); elemental analysis calcd (%) for C40H46S6: C 66.81, H 6.45;
found: C 66.62, H 6.39; UV/Vis (CH2Cl2): lmax=412 nm; MS (DI-EI):
m/z: 718 [C40H46S6]+ +.
Sexithiophene 5: A solution of compound 4 (1.33 g, 1.85 mmol) in CHCl3
(15 mL) was added dropwise to a mixture of KSCN (10.9 g, 112.15 mmol)
in methanol (30 mL) and Br2(2.9 mL, 1=3.12 gcm?3, 56.62 mmol) in
CHCl3(10 mL) at ?788 8C. This mixture was stirred for 4 h at room tem-
perature, quenched with water, extracted with CHCl3, and dried over an-
hydrous Na2SO4. The solvent was removed by a rotatory evaporator and
the residue was purified by silica-gel column chromatography (n-hexane).
A yellow crystalline product was obtained (yield: 85.6%) after recrystalli-
zation from CHCl3/n-hexane. M.p. 174–1758 8C;
CDCl3, 258 8C, TMS): d=7.39 (d, J=3.9 Hz, 2H), 7.10 (d, J=3.9 Hz, 2H),
7.09 (d, J=3.9 Hz, 2H), 7.08 (d, J=3.9 Hz, 2H), 2.72–2.68 (m, 8H), 1.56–
1.45 (br, 16H), 1.01–0.96 ppm (br, 12H);
258 8C, TMS): d=144.62, 141.82, 140.51, 138.10, 137.01, 134.75, 131.26,
128.23, 126.88, 126.38, 124.07, 110.26, 32.86, 27.92, 22.99, 13.84 ppm; FT-
IR (KBr pellets): n ˜ =2954 (s), 2929 (s), 2860 (s), 2156 (m), 1518 (m),
1456 (s), 1435 cm?1(m); elemental analysis calcd (%) for C42H44N2S8: C
60.53, H 5.32, N 3.36; found: C 60.29, H 5.28, N 3.35; UV/Vis (CHCl3):
lmax=417 nm (473 nm for solid); MS (FAB): m/z: 833 [C42H44N2S8]+ +.
Orange needlelike single crystals of 5 suitable for X-ray diffraction deter-
mination were grown from a mixed solution of n-hexane and chloroform
(1:3) by slow evaporation in air at room temperature.
13C NMR
1H NMR (270 MHz, CDCl3, 258 8C, TMS): d=7.31 (d,
13C NMR (68 MHz,
1H NMR (270 MHz,
13C NMR (68 MHz, CDCl3,
Table 1. Crystal and refinement data for 2 and 5.
25
formula
Mr
crystal size [mm]
crystal system
space group
a [?]
b [?]
c [?]
b [8 8]
V [?3]
Z
1calcd[Mgm?3]
m (MoKa) [mm?1]
F(000)
max/min transmission
Flack parameter
parameters
R1/wR2 [I>2s(I)][a]
R1, wR2 (all data)[a]
goodness of fit (S)
D [e??3] (max/min)
C22H22N2S5
474.77
0.10?0.10?0.20
monoclinic
Cc (No. 9)
8.5419(17)
20.333(4)
26.445(5)
95.28(3)
4573.5(16)
8
1.379
0.519
1984
0.903/0.950
0.07(19)
491
0.0952/0.2416
0.0974/0.2427
1.151
0.918/?0.848
C42H44N2S8
833.27
0.10?0.10?0.40
monoclinic
P21/c (No. 14)
14.151(3)
5.4799(11)
26.475(5)
91.04(3)
2052.7(7)
2
1.348
0.468
876
0.835/0.955
–
237
0.0951/0.1645
0.1135/0.1717
1.106
0.352/?0.336
o)2]1/2. [a] R1=?jjFoj?jFcjj/?jFoj, wR2=[?[w(F2
o?F2
c)2]/?w(F2
Table 2. Selected bond lengths [?] and angles [8 8] for 5.
Bond lenghts Bond angles
S1?C1
S1?C2
S2?C2
S2?C5
S3?C6
S3?C9
S4?C10
S4?C13
N1?C1
1.708(8)
1.768(6)
1.716(6)
1.737(6)
1.721(6)
1.728(6)
1.725(6)
1.719(6)
1.142(11)
C1-S1-C2
C2-S2-C5
C6-S3-C9
C10-S4-C13
S1-C1-N1
S1-C2-S2
S1-C2-C3
S2-C2-C3
98.8(3)
91.3(3)
92.3(3)
92.6(3)
177.9(7)
120.6(3)
126.9(5)
112.5(4)
Chem. Eur. J. 2006, 12, 607–619? 2006 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim
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Terthiophene 6: A solution of NBS (7.88 g, 6.96 mmol) in 50 mL DMF
was added dropwise to a solution of terthiophene 1 (7.98 g, 22.1 mmol) in
DMF (150 mL) at room temperature. The mixture was stirred overnight,
poured into a saturated brine solution (400 mL), and extracted with
CHCl3. The organic phase was washed thoroughly with distilled water, sa-
turated brine and distilled water, and then dried over anhydrous Na2SO4.
The solvent was removed by a rotatory evaporator and the residue was
purified by silica-gel column chromatography (n-hexane). A yellow liquid
was obtained (yield 56.2%).
d=7.00 (d, J=1.5 Hz, 2H), 6.86 (d, J=1.5 Hz, 2H), 2.65 (t, J=2.4 Hz,
4H), 1.54–1.27 (m, 8H), 0.94 ppm (t, 3.6 Hz, 6H);
CDCl3, 258 8C, TMS): d=140.56, 137.45, 130.19, 129.35, 126.23, 111.97,
32.93, 27.82, 27.79, 13.82 ppm; FT-IR (KBr pellets): n ˜ =2950 (s), 1461 (s),
1060 (m), 778 cm?1(s); elemental analysis calcd (%) for C20H22Br2S3: C
46.34, H 4.28; found: C 46.38, H 4.25; MS (DI-EI): m/z: 518
[C20H22Br2S3]+ +.
Nonathiophene 7: Magnesium (0.96 g, 40 mmol) was transferred to a
three-necked round-bottomed flask and heated to 1008 8C in the N2atmos-
phere. After the mixture was cooled down to room temperature, dry
THF (10 mL) and some iodine were added. The mixture was left to react
for 10 min under magnetic stirring, and then terthiophene 3 (7.2 mL,
16.2 mmol) in THF (10 mL) was carefully added. The mixture was re-
fluxed for 5 h to obtain the corresponding Grignard reagent. This was
slowly added to a solution obtained by dissolving 6 (3.50 g, 8.0 mmol)
and [NiCl2(dppp)] (0.15 g, 0.16 mmol) in dry THF (40 mL). The mixture
was refluxed for 4 h in the N2atmosphere. The reaction mixture was stir-
red for additional 12 h at room temperature, quenched with 1 molL?1hy-
drochloric acid, and extracted with CHCl3. The organic phase was
washed thoroughly with saturated brine and distilled water, and then
dried over anhydrous Na2SO4. The solvent was re-
moved by a rotatory evaporator and the residue
was purified by silica-gel column chromatography
(30% CHCl3/n-hexane). A red powder was ob-
tained in 74.4% yield. M.p. 160–1618 8C;
(270 MHz, CDCl3, 258 8C, TMS): d=7.31 (d, J=
1.4 Hz, 2H), 7.14–7.02 (m, 12H), 2.73–2.69 (m,
12H), 1.54–1.46 (m, 24H), 0.98–0.92 ppm (m,
18H);
d=140.35, 140.19, 140.17, 136.78, 136.11, 135.31,
135.14, 130.03, 129.80, 127.36, 126.40, 126.35,
125.89, 125.36, 123.84, 32.92, 32.88, 32.80, 27.98,
27.92, 27.79, 23.05, 23.02, 22.98, 13.88, 13.84 ppm;
FT-IR (KBr pellets): n ˜ =2954 (s), 1457 (m), 786
(s), 686 cm?1(m) cm?1; elemental analysis calcd
(%) for C60H68S9: C 66.86, H 6.36; found: C 66.80,
H 6.33; MS (TOF-MS): m/z: 1079 [C60H69S9]+ +.
Nonathiophene 8: The synthetic procedure for the
preparation of 8 is analogous to those of 2 and 5.
A solution of compound 7 (0.30 g, 0.70 mmol) in
CHCl3(10 mL) was added dropwise to a mixture
of KSCN (0.068 g, 112.15 mmol) in methanol
(3 mL) andBr2
(0.4 mL,
1=3.12 gcm?3,
7.81 mmol) in CHCl3(10 mL) at ?788 8C. This mix-
ture was stirred for 4 h at room temperature,
quenched with water, extracted with CHCl3, and
dried over anhydrous Na2SO4. The solvent was re-
moved by a rotatory evaporator and the residue
was purified by silica-gel column chromatography
(60% CHCl3/n-hexane). A red crystalline product
was obtained (yield: 72.3%) after recrystallization
from CHCl3/n-hexane. M.p. 152–1538 8C;
(270 MHz, CDCl3, 258 8C, TMS): d=7.38 (d, J=
1.4 Hz, 2H), 7.15 (m, 4H), 7.08 (m, 6H), 2.74–2.72
(m, 12H), 1.54–1.49 (m, 24H), 1.01–0.96 ppm (m,
18H);
d=144.63, 141.77, 140.41, 138.09, 136.51, 135.51,
135.25, 134.51, 131.33, 129.97, 128.14, 126.83,
126.40, 126.31, 124.03, 123.87, 116.63, 110.27, 32.82,
27.85, 23.04, 13.87 ppm; FT-IR (KBr pellets): n ˜ =
1H NMR (270 MHz, CDCl3, 258 8C, TMS):
13C NMR (68 MHz,
1H NMR
13C NMR (68 MHz, CDCl3, 258 8C, TMS):
1H NMR
13C NMR (68 MHz, CDCl3, 258 8C, TMS):
2927 (s), 2156 (m), 1457 (s), 1072 (m), 786 cm?1(s); elemental analysis
calcd (%) for C62H66N2S11: C 62.48, H 5.58, N 2.35; found: C 62.39, H
5.56, N 2.35; UV/Vis (CHCl3): lmax=431 nm (480 nm for solid); MS
(TOF-MS): m/z: 1192 [C62H66N2S11]+ +.
Dithiols 9, 10, 11: Compounds 9, 10, and 11 were all prepared by a simi-
lar method. A suspension of LiAlH4(0.05 g, 1.3 mmol) in dry THF
(10 mL) in the N2atmosphere in a three-necked round-bottomed flask
equipped with a condenser was added carefully the solutions of 2 (0.21 g,
0.50 mmol), 5 (0.10 g, 0.12 mmol), or 8 (0.01 g, 0.0084 mmol), respective-
ly, in dry THF (10 mL) at room temperature. The mixture was then stir-
red for 2 h and quenched with hydrochloric acid (1 molL?1). It was then
poured into chloroform (50 mL), and washed thoroughly with distilled
water and dried with anhydrous Na2SO4. The solvent was removed by a
rotatory evaporator and the residues were dried in a vacuum.
Compound 9: Yellow powder (yield: 96.7%);
CDCl3, 258 8C, TMS): d=7.04 (d, J=1.5 Hz, 2H), 6.94 (d, J=1.5 Hz, 2H),
3.59 (s, 2H), 2.67 (t, J=3.0 Hz, 4H), 1.55–1.38 (m, 8H), 0.95 ppm (t, J=
2.5 Hz, 6H);13C NMR (68 MHz, CDCl3, 258 8C, TMS): d=140.37, 139.38,
134.33, 129.61, 126.16, 124.41, 32.87, 27.80, 22.97, 13.83 ppm; FT-IR (KBr
pellets): n ˜ =2931 (s), 2514 (m), 1456 cm?1(m); UV/Vis (CHCl3): lmax=
366 nm; MS (TOF-MS): m/z: 424 [C20H24S5]+ +.
Compound 10: Orange powder (yield: 94.3%);
CDCl3, 258 8C, TMS): d=7.13 (d, J=1.5 Hz, 2H), 7.05 (d, J=1.5 Hz, 2H),
7.04 (d, J=1.5 Hz, 2H), 6.97 (d, J=1.5 Hz, 2H), 3.59 (s, 2H), 2.75–2.65
(m, 8H), 1.56–1.45 (m, 16H), 1.00–0.93 ppm (m, 12H);
(68 MHz, CDCl3, 258 8C, TMS): d=140.48, 140.25, 135.08, 134.37, 129.93,
129.47, 126.47, 126.01, 123.88, 32.87, 27.83, 22.97, 13.87 ppm; FT-IR (KBr
1H NMR (500 MHz,
1H NMR (500 MHz,
13C NMR
Scheme 2. Schematic illustration of the preparation for the self-assembled films of 12, 13 and 14.
www.chemeurj.org
? 2006 Wiley-VCH Verlag GmbH&Co. KGaA, WeinheimChem. Eur. J. 2006, 12, 607–619
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W. Huang, T. Ogawa et al.

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pellets): n ˜ =2950 (s), 2510 (m), 1455 cm?1(m); UV/Vis (CHCl3): lmax=
415 nm; MS (TOF-MS): m/z: 782 [C40H46S8]+ +.
Compound 11: Red powder (yield: 93.8%);1H NMR (500 MHz, CDCl3,
258 8C, TMS): d=7.15 (d, J=1.5 Hz, 2H), 7.06 (m, 4H), 7.01 (m, 6H),
3.58 (s, 2H), 2.74–2.67 (m, 12H), 1.55–1.49 (m, 24H), 1.00–0.95 ppm (m,
18H); FT-IR (KBr pellets): n ˜ =2945 (s), 2510 (m), 1454 cm?1(m); UV/
Vis (CHCl3): lmax=420 nm; MS (TOF-MS): m/z: 1141 [C60H68S11]+ +.
Preparation of self-assembled films of 12, 13, and 14 in 1?1 mm2area:
Two concentrations were used to prepare films with different thickness.
The junctions were fabricated by the self-assembly method as illustrated
in Scheme 2. The freshly cleaned gold electrodes with 1 mm gap 1 mm
width were soaked into solutions of 9, 10, and 11 (0.1 or 0.5 mmolL?1) in
chloroform, respectively, for 30 minutes and then a solution of 2-dodeca-
nethiol-protected active Au-NPs (0.1 or 0.5 mmolL?1) in chloroform was
added. The mixtures were kept standing for 30 h at room temperature in
a glove box, then the electrodes were taken out, washed thoroughly with
chloroform in order to remove excess unreacted Au-NPs and dithiols,
and dried in vacuum.
Acknowledgements
This work was supported by a Grant-in-Aid for Scientific Research (No.
15201028 and No. 14654135) from the Ministry of Culture, Education,
Science Sports, and Technology of Japan.
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Received: June 7, 2005
Published online: September 29, 2005
Chem. Eur. J. 2006, 12, 607–619 ? 2006 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim
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FULL PAPER
Molecular Junctions