In situ stepwise synthesis of functional multijunction molecular wires on gold electrodes and gold nanoparticles

Article abstract of DOI:10.1002/anie.200906607

Figure and Chemical Equation Present. Tunneling through: The sequencing of electron-donating and electron-accepting components that are separated by a σ-electron bridge has resulted in the highest rectification ratio to date from a molecular diode.

Full text of DOI:10.1002/anie.200906607

Communications
DOI: 10.1002/anie.200906607
Molecular Electronics
In Situ Stepwise Synthesis of Functional Multijunction Molecular Wires
on Gold Electrodes and Gold Nanoparticles**
Geoffrey J. Ashwell,* Barbara Urasinska-Wojcik, and Laurie J. Phillips
Interest in molecular electronics has focused on materials that
act as diodes[1,2] or wires,[3–5] and on the development of
innovative techniques for contacting to single molecules and
ultrathin films.[6–14] They include, for example, the in situ
synthesis of molecular wires on solid supports,[10–14] in which
recent advances have resulted in the bridging of nanometer-
sized gaps between electrodes by the coupling of amino-
terminated molecules between opposing surface-based alde-
hyde groups on the electrode coatings.[12–14] For photovoltaic
applications, molecules may be elongated in situ on nano-
particles with differently colored linked units to provide
charge-transport pathways along the conjugated wirelike
backbones.[15] This elongation method is highly versatile: it
Figure 1. Structure of the self-assembled monolayer (SAM), which was
obtained from 4-[(3-mercaptophenylimino)methyl]benzaldehyde
precursor 1, and aldehyde and amine substrates that were used for
in situ elongation of the wire.
combines a facile synthesis, user-defined functionality during
the initial self-assembly of the precursors to provide reactive
surface-based groups, and the sequencing of chemical build-
ing blocks (donors, acceptors, and bridges) to match the
energy levels with those of the electrodes. The current–
voltage (I–V) characteristics, which are usually symmetrical
for conventional wires, may be tuned by controlling the
molecular sequence. Donors and acceptors give rise to
rectifying characteristics when isolated at opposite ends and
the direction of electron flow at forward bias is from the
cathode to acceptor on one side and from the donor to anode
on the other. Improved rectification has been achieved by
incorporating a cyclohexane s bridge that inhibits intramo-
lecular charge transfer and maintains the integrity of the
electron-donating and electron-accepting moieties. This addi-
tional bridge has resulted in the highest rectification ratio to
date from a molecular diode.
user-defined and the wires may be elongated to any appro-
priate length by repeating the process before terminating with
a mono-substituted component. In this study, plasma-cleaned
substrates were immersed in a solution of 4-[(3-mercapto-
phenylimino)methyl]benzaldehyde in acetone (0.1 mgcmÀ3,
step 1) for 4 h and then rinsed with acetone to remove
physisorbed material. When assembled on the electrodes of
10 MHz quartz crystals, a Sauerbray analysis[16] of the limiting
frequency change gave an area of 0.3 nm2 moleculeÀ1, which
approximates to the molecular cross-section. Chemisorption
was verified by X-ray photoelectron spectroscopy (XPS): a
doublet at 162.1 eV (S 2p3/2) and 163.3 eV (S 2p1/2) is charac-
À
Molecular wires were synthesized in situ on gold elec-
trodes from a self-assembly reaction of a meta-substituted
thiophenol with a terminal aldehyde group to act as a
template for growth that is perpendicular to the substrate
(Figure 1). These embryonic wires were elongated by reacting
first with H2N-wire-NH2 and then OHC-wire-CHO to couple
the wire-like units through imino groups. The sequence is
teristic of the Au S link, and a peak at 399.1 eV (N 1s)
corresponds to the imino nitrogen atom. Quantitative analysis
of the areas under the S 2p and N 1s peaks, which was fitted
using a Gaussian-Lorentzian function and corrected for
atomic sensitivity factors, confirmed an N:S ratio of about
1:1, consistent with the atomic ratio within the self-assembled
molecule.
[17]
À
À
Mercaptoaniline (HS C6H4 NH2)
surface through both the nitrogen and sulfur substituents, as
shown by the characteristic XPS data of each bond: 162.0 and
binds to the gold
[*] Prof. G. J. Ashwell,[+] Dr. B. Urasinska-Wojcik, L. J. Phillips
College of Physical and Applied Sciences, Bangor University
Bangor, Gwynedd LL57 2UW (UK)
À
163.2 eV (S 2p, Au S); 163.6 and 164.8 eV (S 2p, SH);
À
398.3 eV (N 1s, Au N); 400.4 eV (N 1s, NH2). As such, 4-[(3-
[+] Current address: Physics Department, Lancaster University
mercaptophenylimino)methyl]benzaldehyde 1 was chosen as
the starting building block for the molecular wires (Figure 1).
However, mercaptoaniline was used in a preliminary study to
demonstrate the in situ growth of molecular wires on gold
nanoparticles. When functionalized by 3-mercaptoaniline,
these nanoparticles exhibited NH2 IR absorption stretching
modes at 3200 and 3300 cmÀ1, but subsequent reaction with
terephthalaldehyde resulted in the loss of them and the
Lancaster LA1 4YB (UK)
E-mail: g.j.ashwell@lancaster.ac.uk
[**] We thank the EPSRC, Technology Strategy Board, and the EC FP7
ITN “FUNMOLS” project no. 212942 for financial support, Prof. Ian
Gentle for access to the XPS facility at the University of Queensland,
and Piotr Wierzchowiec and Kym Ford for technical assistance.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200906607.
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Chemie
formation of stretching modes at 1609 cmÀ1 (C N) and
=
Table 1: XPS data: sulfur (S 2p3/2 and S 2p1/2) and nitrogen (N 1s)
binding energies (B.E.s) for each stage of the seven step process.
1703 cmÀ1 (C O; Figure 2). The latter stretches were
=
quenched by reaction with amino-substituted units, and
their appearance and disappearance is consistent with the
Step
B.E. [eV]
À
(Au S)
B.E. [eV]
B.E. [eV]
(NH2)
B.E. [eV]
(NO2)
=
(CH N)
1
2
3
4
5
6
7
162.1, 163.3
162.0, 163.2
162.2, 163.4
162.3, 163.5
162.0, 163.2
162.0, 163.2
162.1, 163.3
399.1
399.3
399.2
399.3
399.3
399.2
399.1
–
–
–
–
–
–
400.6
400.7
400.8
400.9
400.7
400.9
–
405.0
Figure 2. IR spectra: a) self-assembled 3-mercaptoaniline monolayers
on gold nanoparticles and b) evolution of peaks at 1609 cmÀ1 (C N)
=
and 1703 cmÀ1 (CO) following reaction with 5.
formation of imino links between the reacted units. As part of
an extended study, the analogous in situ coupling reaction has
been demonstrated for platinum and silver nanoparticles that
were functionalized by 3-mercaptoaniline and TiO2 nano-
particles treated with 4-aminobenzoic acid.
The in situ growth of embryonic molecular wires on planar
substrates was performed at ambient temperature by the
initial self-assembly of 4-[(3-mercaptophenylimino)methyl]-
benzaldehyde on, for example, the gold electrodes of 10 MHz
quartz crystals, and thereafter, by immersing the SAM-coated
(SAM = self assembled monolayer) substrates in solutions of
each of the following reactants (1 mg in 10 cm3; 0.05 cm3
glacial acetic acid was added in each case): 1,4-phenylenedi-
Figure 3. Molecular structure of the embryonic wire following step 2
and its deconvoluted N 1s core-level spectrum. The 2:1 area ratio of
the peaks at 399.3 and 400.6 eV from the imine and amine, respec-
tively, is consistent with the ratio in the molecule.
amine
2
in acetone (step 2); 2,5-bis(hexyloxy)-
Analytical data for each stage of the stepwise elongation
are included in the Supporting Information; quartz crystal
and XPS data for the rectifying wire, which was formed in
seven steps, are depicted in Figure 4. These data suggest a
quantitative reaction except in the third step where the bulky
C6H13O substituents of 2,5-bis(hexyloxy)terephthalaldehyde
restricted the coupling yield to 50%. This conclusion is
confirmed from the plot of frequency change versus relative
molecular mass of the stepwise elongated wire in which the
initial slope, steps 1 and 2, is double that of steps 3 to 7
(Figure 4b). XPS studies also showed that the N:S ratios
obtained for steps 4 to 7 are consistent with (50 Æ 10)%
coupling at step 3 (Figure 4c). Discrepancies may arise from
using different depth profiles; therefore, XPS studies were
performed at a takeoff angle of 458 to minimize the influence
of sulfur being bonded to the substrate when the nitrogen
atoms are located throughout the film. However, both XPS
and the quartz crystal analyses indicate that the steric bulk of
terephthalaldehyde 3 in acetone (step 3); 1,4-diaminocyclo-
hexane 4 in methanol (step 4); terephthalaldehyde 5 in
tetrahydrofuran (step 5); 2,6-diaminoanthra-9,10-quinone 6
in tetrahydrofuran (step 6); and 4-nitrobenzaldehyde 7 in
acetone (step 7). The monolayer was immersed in each
solution for 2–3 hours and rinsed with the appropriate solvent
to remove any physisorbed material. Coupling was indicated
by a stepwise decrease in the frequency of the quartz crystals,
and the condensation was verified by XPS data that are
consistent with the formation of imino links (Table 1). For
example, the N 1s core-level spectrum of the initially formed
SAM was altered following reaction with 1,4-phenylenedi-
amine, and exhibited peaks at 399.3 and 400.6 eV with an area
ratio of about 2:1 (Figure 3). Those peaks corresponded to
imino and amino groups, respectively, and, when corrected for
atomic sensitivity factors, their combined area relative to
sulfur was about 3:1 (cf. 1:1 for the SAM).
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Figure 4. Multijunction molecular wire: a) Molecular structure of the wire that was synthesized by the stepwise in situ coupling of seven
components on a gold electrode. b) Frequency change versus relative molecular mass (Mr) for the self-assembly of embryonic wires at the gold
electrodes of 10 MHz quartz crystals and its subsequent elongation in six steps. c) N:S ratios that were calculated from the peak areas of the N
1s and S 2p core-level spectra, corrected for atomic sensitivity factors; steps 3 and 5 showed no increase in the ratio as the terephthalaldehyde
linking units do not contain nitrogen. d) I–V characteristics obtained using a set point current of 600 pA and a voltage of 80 mV. The sign
corresponds to the substrate electrode; at a forward bias (positive quadrant), electrons tunnel from the gold probe to the gold-coated substrate.
the alkyloxy side chains hindered the third step of the reaction
and that, thereafter, the coupling reactions proceed stoichio-
metrically in solution with exposed surface-active groups.
The deconvoluted sulfur spectrum for the final step of the
chain elongation process had characteristic gold thiolate XPS
peaks at 162.1 eV (S 2p3/2) and 163.3 eV (S 2p1/2) with an
expected peak separation of 1.2 eV and area ratio of 2:1. The
deconvoluted nitrogen spectrum revealed peaks at 399.1
(strong), 400.9 (weak), and 405.0 eV (weak), which corre-
spond to the imino links, and the amino and nitro groups
respectively. No amino groups were present in the nitro-
terminated wire (Figure 4a); instead, the weak peak at
400.9 eV related to the amino substituent on the embryonic
wire, shown in Figure 2, whose growth was inhibited by the
bulky hexyloxy ring-substituted reactant for the next step.
Following the final elongation step, the wire had a 50%
occupancy and an experimental N:S ratio of (5.7 Æ 0.3), which
was consistent with a mean of 5.5 for the short (3:1) and long
(8:1) wires formed in steps 2 and 7, respectively.
The I–V characteristics of the nitro-group-terminated wire
were studied by STM (scanning tunneling microscopy) at
different locations across the films and, using contacting gold
probes, the data were averaged from multiple scans on the
same spot. The profile was found to be independent of the set
point current and the voltage, which affect the magnitude of
the current by altering the distance between the probe and the
surface but have minimal effect on the shape of the I–V curves
of the rectifying molecule. A slight asymmetry could be
induced in the shape of the I–V curve by varying the set point
conditions but, for example, the maximum current ratio from
I–V curves of symmetrical molecules is typically less than
about 5 at Æ 1 V. This value is trivial compared with data
obtained for the donor–acceptor structure. The forward bias
direction of the electron flow is consistent with the model
reported by Aviram and Ratner (Figure 4d).[1] It is from the
gold probe (cathode) to the lowest unoccupied molecular
orbital (LUMO) of the electron-accepting end of the wire on
one side, and from the highest occupied molecular orbital
(HOMO) of the electron-donating end to the gold substrate
(anode) on the opposite side. At reverse bias, a mismatch
between the Fermi energies of the electrodes and the energy
levels of the conjugated units that are isolated by the s bridge
is responsible for the I–V characteristics.
The rectification ratio of approximately 160 at Æ 1 V is the
highest to date for a molecular diode, although we note that
higher values have been obtained from a bilayer structure (an
organic rectifying junction).[18] The magnitude of this ratio is
influenced, in part, by the acceptor arrangement (nitro-
benzene and anthraquinone) on one side and electron-
donating (hexyloxy) substituents on the other, and also , in
part, by the s bridge. This cyclohexane tunneling barrier plays
a pivotal role in improving rectification by, for example,
suppressing zero-bias charge transfer and maintaining the
integrity of the electron-donating and electron-accepting
ends. The ratio may be contrasted with about 20 at Æ 1 V
for a p-bridged sequence with the same series of donors and
acceptors but no s-bridged tunneling barrier at step 4.
Moreover, the majority of molecular diodes[19–25] exhibit
slight rectification with ratios in the range 5–30 at Æ 1 V
when contacted by non-oxidizable electrodes. There are
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Angewandte
Chemie
exceptions,[2a] and high ratios have been reported for bilayer
arrangements in which cationic acceptor-(p-bridge)-donor
structures are ionically coupled to anionic phthalocyanine
donors.[18] However, the rectification ratio of 160 at Æ 1 V,
obtained herein, is by far the highest for a monolayer, and has
been achieved by the in situ coupling of the component parts.
In conclusion, we have sequenced electroactive units
along the length of a molecular wire and shown that the
energy levels on opposite sides may be manipulated to
achieve improved rectification by locating a tunneling barrier
between the electron-donating and electron-accepting moi-
eties. Difficulties in alignment have inhibited previous studies
of donor-(s-bridge)-acceptor diodes,[26,27] but they may be
readily designed, synthesized, and aligned by in situ growth.
This study has resulted in the highest rectification ratio to
date for a molecular diode and, as part of our continuing
work, the templated sequencing of electroactive moieties on
solid supports is being applied to other types of substrates,
including silicon. This technique is highly versatile and the
sequencing of donors and acceptors as well as electron bridges
permits the band structure and absorption characteristics to
be finely tuned for electronic and photonic applications.
Experimental Section
4-[(3-Mercaptophenylimino)methyl]benzaldehyde (1), was synthe-
sized from the reaction of 3-mercaptoaniline (0.75 g, 6 mmol) and
excess terephthalaldehyde (1.6 g, 12 mmol) in ethanol (50 cm3) at
ambient temperature. The solvent was removed in vacuo and the pale
yellow product was recrystallized from chloroform and acetone and
washed with ethanol: 90% yield; m.p. 2278C; UV (C2H5OH): lmax
=
À1
À1
À1
=
=
~
340 nm; IR (KBr): n = 1620 cm (C N), 1700 cm (C O), 2550 cm
À
(S H); elemental analysis calcd for C14H11NOS: C 69.69, H 4.60,
N 5.81%; found: C 69.4, H 4.4, N 5.5%; XPS: S 2p3/2 164.1 eV, S 2p1/2
=
À
165.3 eV (SH), C 1s 284.7 eV (CAr), 285.4 eV (C N), 286.1 eV (C S),
Figure 5. Core-level spectra of an elongated wire that was formed in
=
=
=
287.4 eV (C O), N 1s 399.3 eV (CH N), O 1s 531.9 eV (C O); MS
m/z (%): 241 (100); HRMS, m/z: 241.0552 [M]+, calculated 241.0556.
Functionalized gold nanoparticles were prepared by adapting the
method reported by Brust et al.[28] Tetraoctylammonium bromide
(2.2 g, 4 mmol) in toluene (100 cm3) was added to an aqueous solution
(100 cm3) of gold(III) chloride trihydrate (0.32 g, 0.8 mmol, 50 cm3).
The mixture was stirred to transfer the auric chloride into the organic
phase, to which 3-mercaptoaniline (0.25 g, 2 mmol) and then an
aqueous solution of sodium borohydride (0.76 g, 20 mmol, 50 cm3)
were added with stirring. The organic phase was separated after 16 h,
evaporated in vacuo, and the functionalized nanoparticles were
washed with water (2 ꢀ 30 cm3) and ethanol (2 ꢀ 30 cm3). The nano-
particles were then treated with a chloroform solution of terephtha-
laldehyde (0.40 g, 3 mmol, 50 cm3), for 48 h at ambient temperature,
filtered, and washed with ethanol. Reaction with the dialdehyde
=
seven steps: a) N 1s, 399.1 eV (C N), 400.9 eV (NH2) and 405.0 eV
À
(NO2); b) S 2p, 162.1 and 163.3 eV (Au S). The weak amino shoulder
arises from the embryonic wire formed in step 2 (see text).
1s peak at 284.7 eV respectively and analyzed using Casa XPS
software. Spectra for the wire formed from the seventh stage of the in
situ process are included in the Supporting Information, and shown in
Figure 5.
Received: November 23, 2009
Published online: March 31, 2010
Keywords: molecular diodes · molecular electronics ·
.
yielded a CO stretch at 1703 cmÀ1 and a C N stretch at 1609 cm
,
À1
nanotechnology · organic photovoltaics · self-assembly
=
with loss of the NH2 stretch at 3200–3300 cmÀ1. The resultant
spectrum is consistent with formation of the self-assembled form of
4-[(3-mercaptophenylimino)methyl]benzaldehyde; we note that sub-
sequent reaction with aniline resulted in the loss of the CO stretch,
the sequential appearance and disappearance of which is indicative of
coupling through imino linkages.
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