Self-assembly of small gold nanoparticles through interligand interaction

Article abstract of DOI:10.1021/ja064510q

Stable and monodisperse Au nanoparticles smaller than 2 nm are easily prepared by the reduction of HAuCl₄-4H₂O in DMF/H ₂O in the presence of a series of bidentate ligands, 2,6-bis(1′-(n-thioalkyl)benzimidazol-2-yl)pyridine (TC_nBIP, n = 3, 6, 8, 10, 12). The TC_nBIP ligands afford stronger coordination ability than alkanethiols due to their bidentate nature. These small nanoparticles form hexagonal close-packed (hcp) two-dimensional (2D) superlattices with tunable interparticle spacings (from 1.2 to 2.5 nm), produced by changing the length of the ligand at both the hydrophobic amorphous carbon and the air-water interface. Long-range-ordered hep 2D superlattices were fabricated through the cleavage and construction of interligand π-π interactions formed via an annealing process at the air-water interface.

Full text of DOI:10.1021/ja064510q

Published on Web 09/14/2006
Self-Assembly of Small Gold Nanoparticles through
Interligand Interaction
Masayuki Kanehara, Etsushi Kodzuka, and Toshiharu Teranishi*
Contribution from the Graduate School of Pure and Applied Sciences, UniVersity of Tsukuba,
1-1-1 Tennodai, Tsukuba, Ibaraki 305-8571, Japan
Received June 26, 2006; E-mail: teranisi@chem.tsukuba.ac.jp
Abstract: Stable and monodisperse Au nanoparticles smaller than 2 nm are easily prepared by the reduction
of HAuCl4‚4H2O in DMF/H2O in the presence of a series of bidentate ligands, 2,6-bis(1′-(n-thioalkyl)-
benzimidazol-2-yl)pyridine (TCnBIP, n ) 3, 6, 8, 10, 12). The TCnBIP ligands afford stronger coordination
ability than alkanethiols due to their bidentate nature. These small nanoparticles form hexagonal close-
packed (hcp) two-dimensional (2D) superlattices with tunable interparticle spacings (from 1.2 to 2.5 nm),
produced by changing the length of the ligand at both the hydrophobic amorphous carbon and the air-
water interface. Long-range-ordered hcp 2D superlattices were fabricated through the cleavage and
construction of interligand π-π interactions formed via an annealing process at the air-water interface.
Introduction
particle 2D superlattices are strongly affected by the following
three types of disorder: global structural disorder in the array
Chemical techniques for the preparation and size separation
of organically protected metal and semiconductor nanoparticles
have advanced rapidly over the past several years.1-10 From
the discovery of a Coulomb staircase through the double-tunnel
junction,11 metal nanoparticles have also attracted much attention
for their potential in single-electron-tunneling devices.12 Recent
synthetic developments in the fabrication of highly monodisperse
metal nanoparticles allow us to study not only the properties of
the individual nanoparticles, but also the collective effects of
the two-dimensional (2D) and three-dimensional (3D) super-
lattices of metal nanoparticles on their physical properties.13-23
The in-plane electron transport characteristics of metal nano-
topology, local structural disorder in the interparticle couplings,
and local charge disorder, due to random immobile charges in
the underlying substrate.16 To minimize an unpredictable
disorder effect, the fabrication of well-ordered metal nanoparticle
2D superlattices is of great importance. For the development
of practical nanoelectronic devices based on the single-electron-
tunneling effect at room temperature, it is indispensable to reveal
the electron transport properties of well-ordered 2D superlattices
of small (<2 nm) nanoparticles, which are expected to show
the Coulomb-blockade phenomenon at room temperature.
Although the fabrication techniques of relatively large metal
and semiconductor nanoparticle superlattices have been vigor-
ously investigated so far, well-ordered superlattices made from
quite small (<2 nm) particles are still limited. Whetten et al.
reported self-assembled, well-ordered hexagonal close-packed
(hcp) 2D superlattices of water-soluble Greengold, where the
1.3 nm Au cores are protected by amide-terminated PR3
compounds (R ) p-C6H4CONHCH3).24 An alternative technique
for the fabrication of ordered monolayers of Au cluster
compounds has been reported by Schmid’s group.25 Here, the
authors utilized the strong attractive interactions between
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10.1021/ja064510q CCC: $33.50 © 2006 American Chemical Society

Self-Assembly of Small Gold Nanoparticles
A R T I C L E S
polyelectrolytes, in which the imino groups on a substrate and
sulfonic groups on the cluster surfaces were used to promote
the self-assembly of the acidic [Au55{PPh2(m-C6H4SO3Na)12}-
Cl6] on a mica surface coated with poly(ethylenimine). In both
cases, however, the phosphine-capped nanoparticles are so
unstable that it is difficult to measure the physical properties
of the superlattices.
After synthesis of a series of 2,6-bis(1′-(n-thioalkyl)benzimid-
azol-2-yl)pyridines (TCnBIPs, n ) 3, 5, 8, 10, 12), TCnBIP-
protected small Au nanoparticles were self-assembled into well-
ordered hcp 2D superlattices, both on an amorphous-carbon-
coated Cu grid and on an air-water interface, where the
interparticle spacings were tuned according to the length of the
TCnBIP ligands. Well-ordered hcp superlattices of TCnBIP-
protected Au nanoparticles on water were subject to an annealing
process to achieve the long-range ordering of small Au
nanoparticles.
In addition to these disorder effects, the control of the
interparticle spacings, and the chemical structure of the ligands
greatly affecting the tunneling resistance, are key determinants
of the electronic property of metal nanoparticle superlattices.21,26-29
As for the interparticle spacing, we have recently demonstrated
that, for ∼2 nm alkanethiol-protected Au nanoparticles, the
tunneling resistance of the alkanethiol layer changes from 8.5
GΩ (1-octanethiol) to 500 MΩ (1-hexanethiol), implying that
the shorter ligand is desirable for a drastic reduction of the
tunneling resistance. Furthermore, the introduction of a π-con-
jugated unit to the ligand would also reduce the tunneling
resistance. On the other hand, it has been proposed that the
electrical properties of organic molecules can be controlled by
the occupancy of energy levels within these molecules, as
illustrated by the use of oligophenylene-ethynylene30 and
viologen bridges.31 In both cases, it was found that charge
trapping in molecular states leads to changes in electrical
behavior. Well-ordered metal nanoparticle superlattices having
redox-active organic units between the particles have a great
potential for numerous device applications, including high-
frequency oscillators, mixers, multipliers, analog-to-digital
converters, etc.31 Benzimidazoles and substituted benzimidazoles
show excellent electron-conductive properties, and have been
used as redox-active sites in electron-conductive polymers.32
We previously reported the preparation of 1.9 nm monodisperse
Au nanoparticles protected by the benzimidazole derivative 2,6-
bis(1′-(8-thiooctyl)benzimidazol-2-yl)pyridine (TC8BIP). These
protected nanoparticles have quite narrow size distribution and
have been shown to remain stably dispersed in DMF for more
than one year.33 Well-ordered 2D superlattices of these Au
nanoparticles were obtained through the self-assembly process
on an amorphous-carbon-coated copper grid, probably due to
π-π interactions between the neighboring 2,6-bis(benzimidazol-
2-yl)pyridine (BIP) groups of the TC8BIP layers.
Results and Discussion
General Remarks on Ligand Syntheses. To demonstrate
our concept for the formation of well-ordered hcp 2D super-
lattices of small nanoparticles based on the interligand π-π
interactions, we focused on the syntheses of a series of TCn-
BIP ligands (n ) 3, 6, 8, 10, 12). These ligands were designed
to have three functional groups, that is, disulfide groups to
coordinate with the surface Au atoms of the nanoparticles, BIP
groups to promote interligand π-π interactions and endow
redox-active sites to particles, and alkyl chains to control
interparticle spacings.
A previously reported synthetic route to TC8BIP ligands
involved the substitution of dibromoalkane with BIP in the first
step.33 This procedure principally affords unsatisfactory yields
of bissubstituted bromoalkyl-BIP due to an intramolecular ring
formation. To avoid this reaction, bromoalkene was used instead
of dibromoalkane. Scheme 1 shows the new synthetic routes
for a series of TCnBIP molecules. In the synthesis of TC10BIP,
the olefin 1 was synthesized through the mild bromination of
9-decen-1-ol with triphenylphosphine and tetrabromomethane.
The substitution of bromoalkenes with BIP gives the terminal
olefins 2a-d in satisfactory yields. The terminal olefin reacts
with thioacetic acid under UV irradiation to give the anti-
Markovnikov product. These reactions afford almost equivalent
yields of the terminal thioacetoxy derivatives 3a-d without any
radical initiators. For the synthesis of terminal bromododecyl-
BIP 2e, 10 equiv of 1,12-dibromododecane was used in quite
high concentration (dibromododecane:solvent ) 1:1 (v/v)) to
prevent an undesirable intramolecular ring formation, the yield
of 2e being 75%. The thioacetate 3e was equivalently obtained
from 2e and potassium thioacetate. The reduction of 3a using
NaBH4 gives the reduced form of TC3BIP in excellent yield
(93%), while alcoholysis and mild oxidation of 3b-e afford
the final products 4b-e in satisfactory yields (71-93%).
Formation of TCnBIP-Au Nanoparticles. The TCnBIP-
protected Au (TCnBIP-Au) nanoparticles were synthesized by
the NaBH4 reduction of HAuCl4‚4H2O in the presence of a
series of TCnBIP ligands in a DMF/H2O mixed solution. The
solution turned from yellow to brown-red immediately after the
addition of NaBH4 aqueous solution. Figure 1 shows the
representative UV-vis spectra of the solutions containing
HAuCl4‚4H2O and TC3BIP, TC8BIP, or TC12BIP (TCnBIP:Au
) 1:1 (mol/mol)) before and after addition of NaBH4 aqueous
solution. In the UV-vis spectra of the resulting solutions, the
peak at 320 nm assigned to the charge-transfer transition of
[AuCl4]- ions and the π-π* transition of TCnBIP (BIP groups)
decreased after addition of NaBH4, and broad tailing absorptions
were observed between the ultraviolet and visible regions due
to development of the band structure, indicating the formation
To reveal the electron-transport properties of 2D superlattices
of redox-active organic unit surrounded small nanoparticles,
establishing a general fabrication technique of a well-ordered
small metal nanoparticle superlattice is indispensable. In this
paper, we report the fabrication of hcp 2D superlattices of small
(1.5-2.2 nm) Au nanoparticles, whose protective ligands are
terminated with redox-active π-conjugated aromatic groups.
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183.
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A R T I C L E S
Kanehara et al.
Scheme 1. Synthetic Procedures of TCnBIP Ligands
of Au nanoparticles. Small surface plasmon resonance bands
around 520 nm were observed for all samples, indicating that a
small amount of larger nanoparticles is contained in the solution.
The other TCnBIP-Au nanoparticles showed similar spectra
depending on their sizes. These Au nanoparticles have been
shown to remain stably dispersed in a DMF/H2O mixed solution
under air for at least one year, probably due to their bidentate
nature.
Table 1 presents the mean diameters and standard deviations
of Au nanoparticles prepared at various TCnBIP:Au molar ratios.
Both the mean diameter and standard deviation decreased with
an increase in the amount of the protective agent, TCnBIP, which
was also observed in the cases involving polymer-protected Pd,34
Pt,35,36 and Au2 nanoparticles. For TCnBIP (n ) 8, 10, 12), Au
nanoparticles smaller than 2 nm could be obtained for TCnBIP:
Au molar ratios larger than 1.0. For TC6BIP-Au, 1.8 nm
particles were obtained when the TC6BIP:Au molar ratio was
2.0. On the other hand, when using TC3BIP as a protective
ligand, only Au nanoparticles larger than 2 nm were produced
even at large TC3BIP:Au molar ratios. Generally, to prepare
kinetically stabilized small Au nanoparticles, sterically bulky
ligands37-40 and a low reaction temperature41 have been
employed, because the growth rate of the metal core may
decrease with increasing ligand bulkiness and decreasing
reaction temperature. Our TCnBIP-Au nanoparticles follow this
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Soc. 1998, 120, 12696-12697.
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Self-Assembly of Small Gold Nanoparticles
A R T I C L E S
Table 1. Mean Diameters and Standard Deviations of a Series of
TCnBIP-Au Nanoparticles Synthesized at Various TCnBIP:Au
Molar Ratios
TCnBIP:Au
(mol/mol)
mean diam
(nm)
std dev
(nm)
ligand
TC3BIP
1.0
2.0
1.0
2.0
0.05
0.1
0.5
1.0
2.0
1.0
2.0
0.4
0.5
1.0
2.0
2.7
2.2
2.6
1.6
3.2
2.7
2.3
1.9
1.5
1.6
1.5
1.6
1.6
1.5
1.5
1.0
0.5
0.9
0.5
0.8
0.5
0.4
0.4
0.2
0.3
0.2
0.5
0.4
0.3
0.3
TC6BIP
TC8BIP
TC10BIP
TC12BIP
Figure 2. High-resolution TEM image of 1.5 nm TC8BIP-Au nanopar-
ticles.
Figure 1. UV-vis spectra of the solutions containing HAuCl4‚4H2O before
and after addition of a NaBH4 aqueous solution in the presence of (a) TC3-
BIP, (b) TC8BIP, and (c) TC12BIP.
tendency; that is, TCnBIP ligands with a longer alkyl chain tend
to produce smaller Au nanoparticles at a similar TCnBIP:Au
ratio. In addition, it is well-known that small gold clusters exhibit
a strong fluorescence; however, the TCnBIP-Au nanoparticles
described here are larger than the fluorescent gold clusters and
do not exhibit fluorescence.
Characterization of TCnBIP-Au Nanoparticles. High-
resolution transmission electron microscopy (HRTEM) provides
extensive information about the size and crystal structure of
inorganic nanoparticles. As shown in Figure 2, an HRTEM
image of 1.5 nm TC12BIP-Au shows lattice fringes originating
from a single crystal, and the lattice spacing of 0.24 nm is in
good agreement with that of the (111) plane (0.2355 nm) for
fcc bulk Au. The crystal structure of the TCnBIP-Au nano-
particles was also confirmed by X-ray diffraction (XRD)
measurements. An XRD pattern of the 1.9 nm TC8BIP-Au
Figure 3. XRD pattern of 1.9 nm TC8BIP-Au nanoparticles.
nanoparticles is representatively shown in Figure 3. Several
diffraction peaks were observed at 38.1°, 44.4°, 64.8°, 77.8°,
and 82.2°, which correspond to the (111), (200), (220), (311),
and (222) planes of the fcc bulk Au, respectively. Other TCn-
BIP-Au nanoparticles also show similar XRD patterns with
slight differences in the fwhm values. The chemical state of
gold in the TCnBIP-Au nanoparticles was confirmed by X-ray
photoelectron spectroscopy (XPS). The Au4f7/2 binding energies
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A R T I C L E S
Kanehara et al.
Table 2. Au Contents and the Number of Surface Au Atoms
Occupied by a Single Thiolate for TCnBIP-Au Nanoparticles
Synthesized at a Molar Ratio of TCnBIP:Au ) 1
mean diam
(nm)
Au contenta
(wt %)
no. of surface Au atoms
occupied by a single thiolateb
ligand
TC8BIP
TC10BIP
TC12BIP
1.9
1.6
1.5
72.2
69.0
67.4
2.5
2.2
2.4
a Obtained from TGA measurement. b Calculated on the assumption that
the particles have truncated octahedral shell structures.
conformations), respectively. The methylene peaks correspond-
ing to the d+ and d- bands appeared at 2920 and 2850 cm-1
for all TCnBIP-Au, demonstrating that the alkyl chains of the
TCnBIP ligands on the Au surface predominantly have an all-
trans conformation in the dry state.
The progressive heating of thiol-stabilized Au nanoparticles
leads to volatilization of the organic fraction as disulfides,
leaving a residue of gold.44 Thus, TGA can provide information
on the total organic content of the nanoparticles. Table 2 shows
the representative Au contents of TCnBIP-Au nanoparticles
estimated by TGA, the Au contents of TC8BIP-, TC10BIP-,
and TC12BIP-Au being 72.2, 69.0, and 67.4 wt %, respectively.
Table 2 also presents the number of surface Au atoms occupied
by a single thiolate ligand, which was calculated on the
assumption that the particle has a truncated octahedral shell
structure, in which the bithiolate (two sulfur ends) of one TCn-
BIP ligand protects a single Au nanoparticle. It was found that
two or three surface Au atoms were occupied by a single thiolate
group of the TCnBIP ligand in all cases. These surface coverages
are almost comparable to those of alkanethiol-protected Au
nanoparticles.43
Figure 4. S2p XPS spectra of (a) as-synthesized and (b) purified TC8-
BIP-Au nanoparticles.
of TCnBIP-Au nanoparticles have peaks centered at 84.0 eV.
The Au substrate covered with a 2D self-assembled monolayer
of alkanethiol (where the alkanethiol coverage is 33%) showed
a Au4f7/2 binding energy (84.0 ( 0.1 eV) similar to that of bulk
Au (84.0 eV).42 On dodecanethiol-protected Au nanoparticles
(diameter 2.0-2.5 nm, ∼60% coverage based on a ∼309 atom
core), Brust et al. observed a similar Au4f7/2 peak value.1 It
was thus a question of whether smaller cluster cores would
continue to exhibit bulk metal-like photoionization.
Then, the TCnBIP ligands protecting Au nanoparticles were
characterized by XPS, Fourier transform infrared (FT-IR)
measurements, and thermogravimetric analysis (TGA). Figure
4 shows the high-resolution S2p core level XPS spectra of as-
synthesized and purified 1.9 nm TC8BIP-Au nanoparticles.
After purification, the S2p peak at 169 eV assigned to sulfur
oxide (SOx) disappeared without a significant decrease in the
S2p peak at 163.5 eV assigned to thiolates, meaning that the
pure Au nanoparticles were obtained by the repeated-precipita-
tion technique, which is indispensable for the formation of a
long-range-ordered 2D superlattice of nanoparticles. The S2p
binding energy for TC8BIP-Au (163.5 eV) is comparable to
that of alkanethiol-protected Au nanoparticles,43 indicating that
the thiolate coordination site of the TC8BIP ligand possesses a
binding ability similar to that of alkanethiol.
Self-Assembled 2D Monolayers of TCnBIP-Au Nanopar-
ticles. Figure 5 shows the TEM images of a self-assembled 2D
monolayer of 2.2 nm TC3BIP-, 1.8 nm TC6BIP-, 1.9 nm TC8-
BIP-, 1.6 nm TC10BIP-, and 1.5 nm TC12BIP-Au nanopar-
ticles. Locally ordered hcp 2D domains were formed in all
samples, among which the TCnBIP-Au nanoparticles with
relatively short chain lengths (n ) 3, 6, 8) formed rather larger
hcp domains. The ligand density around the Au core decreases
with elongating alkyl chain length of the TCnBIP ligand and
may play an important role in the formation of well-ordered
superlattices. To obtain well-ordered superlattices, an optimum
outer density of effectively interactive aromatic BIP groups is
needed. For 1.5 nm TC8BIP-Au nanoparticles, the volume of
the aromatic BIP groups is ca. 0.3 nm3, considering that the
π-orbital expansion centering on the BIP plane is 0.35 nm.
Assuming the Au nanoparticles have a shell structure, the
volume ratio of total BIP groups with respect to the BIP shell
spread around the particle is ca. 20%. Since the well-ordered
hcp 2D superlattices were obtained from TC3BIP-, TC6BIP-,
and TC8BIP-Au nanoparticles, an outer density of the ef-
fectively interactive BIP groups of >20% was needed in our
samples. An introduction to the interactive aromatic groups of
the ligand is feasible to form well-ordered hcp 2D superlattices
of small nanoparticles. Since the BIP molecule has a highly
crystallized nature and shows very low solubility in most organic
solvents, the relatively strong interparticle interaction of TCn-
BIP-Au nanoparticles may stabilize their 2D superlattices.
FT-IR spectroscopy was adopted to investigate the conforma-
tion of alkyl chains of TCnBIP ligands on the surface of Au
nanoparticles. The peak positions for the symmetric (d+) and
asymmetric (d-) stretching bands of the methylene C-H bonds
can be used as a sensitive indicator to estimate the degree of
ordering of alkyl chains.4 For crystalline polyethylene (all-trans
zigzag confirmation), the d+ and d- bands appear at 2920 and
2850 cm-1, respectively, and these shift to 2928 and 2856 cm-1
for amorphous polyethylene (coexistence of trans and gauche
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Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.;
Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998,
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A R T I C L E S
Figure 6. (a) Relationship between the interparticle spacings of 2D
monolayers and the ligand lengths. The solid line stands for the length of
two TCnBIP ligands calculated on the assumption that the alkyl chains of
TCnBIP ligands have an all-trans conformation. (b) Schematic illustration
of the interligand π-π interaction between the neighboring TC8BIP-Au
nanoparticles.
Figure 5. TEM images of a self-assembled 2D monolayer of (a) 2.2 nm
TC3BIP-, (b) 1.6 nm TC6BIP-, (c) 1.9 nm TC8BIP-, (d) 1.6 nm TC10-
BIP-, and (e) 1.5 nm TC12BIP-Au nanoparticles on carbon-coated Cu
grids.
Heath et al. also reported the volume effect of linear alkyl
ligand-protected Au and Ag nanoparticles in LB monolayers.
The authors classified the nanoparticles into three categories
depending on their outer ligand densities. The nanoparticles with
an optimum ligand density gave well-ordered hcp 2D super-
lattices, while the nanoparticles with a low ligand density
afforded only foamlike 2D structures.45 It should be noted that
the 2D superlattices of TCnBIP-Au nanoparticles showed
excellent stability against heat and electron beam irradiation
during TEM observation due to a bidentate ligand structure,
and as such, stability is quite important for measurements of
both their physical properties and their applications.
The mean interparticle spacings from particle edge to edge
for TC3BIP-, TC6BIP-, TC8BIP-, TC10BIP-, and TC12BIP-
Au superlattices were estimated from TEM images to be 1.2,
1.6, 1.9, 2.3, and 2.5 nm, respectively, as shown in Figure 6a.
In any case, the interparticle spacing is shorter by ∼1 nm than
the sum of two ligand lengths (Figure 6a, solid line). These
shorter interparticle spacings are quite reasonable when one
considers the interligand overlap between the neighboring BIP
groups (ca. 0.8 nm), as illustrated in Figure 6b. The MM2
calculation of the TC8BIP ligand revealed that the two benz-
Figure 7. TEM image of self-assembled hcp domains of 1.5 nm TC8-
BIP-Au nanoparticles formed at the air-water interface.
imidazole groups of the TC8BIP ligand were distorted to the
same direction, and the distortion angle between the benzimi-
dazole and pyridine rings was smaller than 20°, indicating that
the π-π interaction between the neighboring highly crystalline
BIP groups may stabilize the hcp superlattice.
The formation of hcp superlattices of TC8BIP-Au nanopar-
ticles is clearly observed at the air-water interface, where the
particles easily combine to form a stable superstructure, as
compared with the solid carbon surface. When a few microliters
of a 5 mM chloroform solution of 1.5 nm TC8BIP-Au
nanoparticles was spread on water, the nanoparticles self-
(45) Heath, J. R.; Knobler, C. M.; Leff, D. V. J. Phys. Chem. B 1997, 101,
189-197.
9
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A R T I C L E S
Kanehara et al.
interaction also induces the formation of hcp superlattices at
the air-water interface. The interligand π-π interaction was
also implied by the small-angle X-ray diffraction of 1.5 nm TC8-
BIP-Au nanoparticle powder (Figure S1 in the Supporting
Information), which was obtained by the precipitation from a
TC8BIP-Au DMF solution with methanol. The mean interpar-
ticle spacing was calculated as 2.2 nm from a diffraction peak
at 2.06° assigned to the (111) plane for the 3D superlattice of
1.5 nm TC8BIP-Au nanoparticles. This value is roughly
consistent with that for a 2D superlattice within error, indicating
that there is an interdigitation or overlap of BIP groups (0.8
nm).
The UV-vis spectra of different amounts of TC8BIP-Au
nanoparticles on quartz substrates were measured to spectro-
scopically confirm the existence of a π-π interaction (Figure
8). TC8BIP-Au nanoparticle chloroform solutions with two
concentrations (3 and 30 mM) were deposited onto quartz
substrates to form self-assembled domains of nanoparticles. The
absorption peak of the BIP groups for 3 mM TC8BIP-Au
nanoparticles was centered at 324 nm, whereas that of the 30
mM nanoparticles occurs at 321 nm. Since highly concentrated
nanoparticles form multilayer topology, the extent of π-π
stacking between the BIP rings is much higher in the vertical
Figure 8. UV-vis spectra of TC8BIP-Au nanoparticles on quartz
substrates. TC8BIP-Au nanoparticle chloroform solutions of (a) 3 mM
(dotted line) and (b) 30 mM (solid line) concentration were deposited. The
spectra were normalized with BIP π-π* absorbance.
assembled into a number of isolated hcp domains (∼100 nm ×
∼100 nm, Figure 7). The mean interparticle spacing between
the neighboring particle edges is estimated from the TEM image
to be 1.9 ( 0.3 nm, which is compatible with that for the self-
assembled superlattices directly formed on an amorphous-
carbon-coated copper grid, indicating that the interligand π-π
Figure 9. (a) Low-magnification TEM image of long-range-ordered hcp 2D superlattices of 1.5 nm TC8BIP-Au nanoparticles after annealing at 50 °C for
12 h. The dark domains indicated by arrows are bilayers. (b) Enlarged TEM image of (a). (c) TEM image of a nearly perfectly ordered hcp 2D superlattice
of 1.5 nm TC8BIP-Au nanoparticles. The insets show FFT spots of each superlattice.
9
13090 J. AM. CHEM. SOC. VOL. 128, NO. 40, 2006

Self-Assembly of Small Gold Nanoparticles
A R T I C L E S
Figure 10. TEM images of long-range-ordered hcp 2D superlattices of 1.6 nm TC6BIP-Au nanoparticles after annealing at 50 °C for 12 h. The insets show
FFT spots of each superlattice.
direction to the substrate. This 3 nm blue shift of the absorption
peak clearly demonstrates the interligand π-π interactions. A
similar blue shift of the Soret band is observed for the porphyrin
H-aggregate, in which porphyrin molecules are in a face-to-
face π-stacking fashion.46 Consequently, we concluded that the
face-to-face interligand π-π stacking shown in Figure 6b is
responsible for the self-assembly of small TCnBIP-Au nano-
particles, and the interparticle spacing can be tuned by the ligand
length even for small nanoparticle superlattices without any
mechanical compression.
cm-1) phase in a 100 mL beaker, where the air-water interface
area was 15 cm2. Considering the total diameter of a single TC8-
BIP-Au nanoparticle (3.4 nm) and the fcc packing fashion,
the total area of TC8BIP-Au nanoparticles on the water
subphase is ca. 4 cm2, which is small enough to both obtain a
monolayer hcp 2D superlattice and avoid an unfavorable
interaction between the glass wall and the nanoparticles. To
avoid water evaporation, either the humidity inside the oven
was adjusted to 100% or the beaker was covered with a
polyethylene film. Then, the beaker was placed inside a sealed
oven and annealed at 50 °C for 12 h to weaken the π-π
interaction between the TC8BIP ligands and rearrange the Au
nanoparticles. The obtained film was transferred onto an
amorphous-carbon-coated copper grid by means of a horizontal
lifting method. Figure 9 shows the TEM images of the film
produced at the air-water interface after an annealing process.
It was found from Figure 9a that this film predominantly
consisted of a TC8BIP-Au nanoparticle monolayer with minor
fragments of a bilayer (indicated by arrows in Figure 9a; the
other dark spots mainly come from aerial dust). The enlarged
image of the monolayer in Figure 9b demonstrates that the long-
range-ordered hcp monolayer of 1.5 nm TC8BIP-Au nanopar-
ticles was generated without large vacancies. It should be noted
that such long-range-ordered superlattices were never observed
before annealing. Fast Fourier transform (FFT) spots in the
enlarged TEM image (see the inset in Figure 9b) are seen with
a spacing corresponding to an interparticle spacing of the
superlattice. With respect to the periodicity of the nanoparticles,
the as-formed monolayer is not uniform, but rather an aggregate
of many hcp domains, which is also suggested by the slight
expanse of the FFT spots. Nearly perfect ordering was often
observed (Figure 9c), and FFT spots proved the quite high
ordering degree (see the inset in Figure 9c). The multicrystalline
topology (Figure 9b) seems to be half the crystalline rearrange-
ment formed during the annealing process. This interligand-
interaction-based self-assembly process at the air-water inter-
face is fundamentally different from previous methods, such as
those relying on surface tension applied during annealing at
room temperature,47 self-assembly at the water/oil interface,48,49
Heat-Induced Long-Range Ordering of TC8BIP-Au Nano-
particles at the Air-Water Interface. The long-range ordering
of small nanoparticles is indispensable for measurement of the
electron-transport property of nanoparticle superlattices. The
Langmuir-Blodgett technique was first adopted for the forma-
tion of long-range-ordered 2D superlattices of TC8BIP-Au
nanoparticles. The monodisperse 1.5 nm TC8BIP-Au nano-
particles were employed as these are small and easily self-
assemble into hcp superlattices. In the surface pressure-
occupied area (π-A) curve (Figure S2 in the Supporting
Information), the surface pressure showed a gradual increase,
although no sharp phase transition was observed. Due to the
relatively strong interparticle interaction between the TC8BIP-
Au nanoparticles, and in contrast to alkanethiol-protected Au
nanoparticles, long-range-ordered superlattices were hardly
obtained, and the resulting LB film consisted of a mixture of
monolayer and multilayer domains with several vacant areas.
It appears that the stiff small hcp domains of TC8BIP-Au
nanoparticles remain unchanged even at high surface pressures,
and these eventually overlap to form multilayer domains (Figure
S3 in the Supporting Information). To convert the stiff hcp
domains of TC8BIP-Au nanoparticles into a long-range-ordered
hcp monolayer, it would be necessary to both weaken the
interligand interaction and rearrange the nanoparticles. Our
strategy was conceived from previous research on the thermal
stability of LB monolayers of alkanethiol-protected Au and Ag
nanoparticles reported by Heath and co-workers.21,45 A solution
(10 µL) of 1.5 nm TC8BIP-Au nanoparticles in chloroform
(0.5 mM as Au atoms) was spread onto the water (18.2 MΩ
(47) Brown, J. J.; Porter, J. A.; Daghlian, C. P.; Gibson, U. J. Langmuir 2001,
17, 7966-7969.
(48) Lin, Y.; Skaff, H.; Emrick, T.; Dinsmore, A. D.; Russell, T. P. Science
2003, 299, 226-229.
(46) Imahori, H.; Norieda, H.; Nishimura, Y.; Yamazaki, I.; Higuchi, K.; Kato,
N.; Motohiro, T.; Yamada, H.; Tamaki, K.; Arimura, M.; Sakata, Y. J.
Phys. Chem. B 2000, 104, 1253-1260.
9
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A R T I C L E S
Kanehara et al.
or solvent evaporation rate controlled self-assembly,50,51 because
no liquid/liquid interface, surface tension, or other additives
known to change the contact angle at the subphase have been
used in our approach. Our approach exploits only the affinity
between the aromatic groups of TCnBIP ligands, which is similar
to the crystalline growth process. To further extend this
approach, 1.6 nm TC6BIP-Au nanoparticles were also em-
ployed. After annealing at 50 °C for 12 h, an ordered hcp 2D
superlattice was obtained (Figure 10), indicating that annealing-
assisted self-assembly at the air-water interface can be applied
to other nanoparticles capped by ligands with terminal highly
crystalline groups. For the generation of perfectly ordered hcp
superlattices, optimum temperatures and heating times should
be applied, and further investigation is in progress.
oxide was filtered off, and the filtrate was concentrated under reduced
pressure. The product was purified by column chromatography (silica
gel, hexane) to give 1 (3.40 g, equivalent yield) as a colorless oil. H
NMR (400 MHz, CDCl3): δ 1.85 (q, 6H), 2.04 (q, 2H), 3.40 (t, 2H),
4.96 (m, 2H), 5.81 (m, 1H). GC-MS (EI): m/z 218, calcd exact mass
218.067.
1
2,6-Bis(1′-(prop-2-enyl)benzimidazol-2-yl)pyridine (2a). Sodium
hydride (NaH, 60%) in oil (644 mg, 16.1 mmol) was added to a three-
necked flask under an argon atmosphere, and then dry hexane was
added. After the resulting solution was stirred for 5 min, the hexane
was removed with a syringe, and dry N,N-dimethylacetamide (DMAc;
30 mL) was then added. BIP (1.00 g, 3.20 mmol) was added and the
mixture heated to 50 °C for 2 h. To this solution was added 3-bromo-
1-propene (7.16 mL, 82.8 mmol), and the mixture was continuously
heated for 2 h. The solution was then cooled to room temperature, and
dichloromethane was added. The resulting pale yellow solution was
washed with water, dried over Na2SO4, and concentrated under reduced
pressure. The product was purified by column chromatography (silica
gel, ethyl acetate:methanol ) 99:1) to give 2a (2.60 g, equivalent yield)
Conclusions
We synthesized a series of TCnBIP ligands (n ) 3, 6, 8, 10,
12) bearing terminal π-conjugated bis(benzimidazol-2-yl)-
pyridine groups and disulfides (or mercapto groups) at both ends.
Small and monodisperse TCnBIP-Au nanoparticles were suc-
cessfully synthesized, and well-ordered hcp 2D superlattices
were formed on amorphous-carbon membranes. Several small
domains of well-ordered hcp 2D superlattices of TC8BIP-Au
nanoparticles were also formed at the air-water interface, and
through an annealing process, these domains were converted
into long-range-ordered superlattices through a rearrangement
of the nanoparticles. The heat-induced rearrangement of the
metal nanoparticle superlattice is potentially applied to other
nanoparticles possessing relatively strong interparticle interac-
tions. To the best of our knowledge, the self-assembly of stable,
small nanoparticles into long-range well-ordered hcp superlat-
tices has been accomplished only by using TCnBIP-Au
nanoparticles. The monolayer produced at the air-water
interface can be advantageously transferred onto any substrate.
The interparticle spacings of TCnBIP-Au nanoparticles are
simply controlled by changing the ligand length from 1.2 nm
for TC3BIP-Au to 2.5 nm for TC12BIP-Au nanoparticles.
Especially, the spacing of 1.2 nm for TC3BIP-Au nanoparticles
is narrow enough to realize interparticle electron tunneling, and
easy handling, due to their excellent stability, may open new
doors to producing high-quality single-electron-tunneling de-
vices. Investigations into the electron-transport property of the
hcp superlattice of TCnBIP-Au nanoparticles using double-
probe STM or nanogap gold electrodes is currently in progress
and is expected to reveal the effect of π-conjugated terminal
aromatic groups.
1
as a colorless powder. H NMR (300 MHz, CDCl3): δ 4.70 (t, 4H),
4.84 (t, 1H), 5.12 (t, 1H), 5.41 (m, 4H), 7.37 (m, 4H), 7.48 (m, 2H),
7.91 (q, 2H), 8.03 (t, 1H), 8.38 (d, 2H). DI-MS (EI): m/z 391, calcd
exact mass 391.18.
2,6-Bis(1′-(hex-5-enyl)benzimidazol-2-yl)pyridine (2b). NaH (60%)
in oil (1.28 g, 32.1 mmol) was added to a three-necked flask under an
argon atmosphere, and then dry hexane was added. After the resulting
solution was stirred for 5 min, the hexane was removed with a syringe,
and dry DMAc (30 mL) was then added. BIP (2.00 g, 6.40 mmol) was
added, and the mixture was heated to 80 °C for 2 h. To this solution
was added 6-bromo-1-hexene (2.27 mL, 16.9 mmol), and the mixture
was continuously heated for 2 h. The solution was then cooled to room
temperature, and dichloromethane was added. The resulting pale yellow
solution was washed with water, dried over Na2SO4, and concentrated
under reduced pressure. The product was purified by column chroma-
tography (silica gel, ethyl acetate:methanol ) 99:1) to give 2b (3.05
1
g, equivalent yield). H NMR (300 MHz, CDCl3): δ 1.21 (m, 8H),
1.83 (m, 8H), 4.78 (m, 8H), 7.37 (m, 4H), 7.48 (m, 2H), 7.86 (m, 2H),
8.07 (t, 1H), 8.33 (d, 2H). DI-MS (EI): m/z 475, calcd exact mass
475.27.
2,6-Bis(1′-(oct-7-enyl)benzimidazol-2-yl)pyridine (2c). NaH (60%)
in oil (552 mg, 13.8 mmol) was added to a three-necked flask under
an argon atmosphere, and then dry hexane was added. After the resulting
solution was stirred for 5 min, the hexane was removed with a syringe,
and dry DMAc (30 mL) was then added. BIP (1.79 g, 5.75 mmol) was
added and the resulting solution stirred at 80 °C for 2 h. To this solution
was added 8-bromo-1-octene (3.47 mL, 20.7 mmol), and the resulting
mixture was continuously heated at 80 °C for 2 h. The solution was
then cooled to room temperature, and diethyl ether was added. The
resulting pale yellow solution was washed with water, dried over Na2-
SO4, and concentrated under reduced pressure. The product was purified
by column chromatography (silica gel, hexane:ethyl acetate:methanol
1
Experimental Section
) 4:1:1) to give 2c (3.25 g, equivalent yield). H NMR (300 MHz,
CDCl3): δ 1.08 (m, 12H), 1.76 (m, 8H), 4.70 (t, 4H), 4.84 (m, 4H),
5.62 (m, 2H), 7.37 (m, 4H), 7.47 (m, 2H), 7.88 (m, 2H), 8.06 (t, 1H),
8.32 (d, 2H). DI-MS (EI): m/z 531, calcd exact mass 531.336.
2,6-Bis(1′-(dec-9-enyl)benzimidazol-2-yl)pyridine (2d). NaH (60%)
in oil (432 mg, 10.8 mmol) was added to a three-necked flask under
an argon atmosphere, and then dry hexane was added. After the resulting
solution was stirred for 5 min, the hexane was removed with a syringe,
and dry DMAc (30 mL) was then added. BIP (1.12 g, 3.60 mmol) was
added, and the mixture was heated to 80 °C for 1 h. To this solution
was added 1 (2.37 g, 10.8 mmol), and the mixture was continuously
stirred at 70 °C for 17 h. The solution was then cooled to room
temperature, and diethyl ether was added. The resulting pale yellow
solution was washed with water, dried over Na2SO4, and concentrated
under reduced pressure. The product was purified by column chroma-
Materials. All chemicals were purchased from Sigma-Aldrich, Wako
Chemical, Kanto Chemical, and Tokyo Kasei Kogyo and used as is
without further purification unless otherwise noted.
Ligand Syntheses. 10-Bromo-1-decene (1). To a solution of
9-decen-1-ol (2.42 g, 15.5 mmol) in dry THF (50 mL) were added
triphenylphosphine (4.09 g, 15.6 mmol) and tetrabromomethane (5.10
g, 15.5 mmol) under cooling with a water bath at room temperature,
and the solution was stirred for 2.5 h. The resulting triphenylphosphine
(49) Reincke, F.; Hickey, S. G.; Kegel, W. K.; Vanmaekelbergh, D. Angew.
Chem., Int. Ed. 2004, 43, 458-462.
(50) Lin, X. M.; Jaeger, H. M.; Sorensen, C. M.; Klabunde, K. J. J. Phys. Chem.
B 2001, 105, 3353-3357.
(51) Narayanan, S.; Wang, J.; Lin, X.-M. Phys. ReV. Lett. 2004, 93, 135503-
(1-4).
9
13092 J. AM. CHEM. SOC. VOL. 128, NO. 40, 2006

Self-Assembly of Small Gold Nanoparticles
A R T I C L E S
tography (silica gel, hexane:ethyl acetate ) 1:1) to give 2d (1.73 g,
82%). H NMR (300 MHz, CDCl3): δ 1.08 (m, 12H), 1.76 (m, 8H),
4.70 (t, 4H), 4.84 (m, 4H), 5.62 (m, 2H), 7.35 (m, 4H), 7.45 (m, 2H),
7.87 (m, 2H), 8.05 (t, 1H), 8.31 (d, 2H). DI-MS (EI): m/z 587, calcd
exact mass 587.399.
6H), 2.84 (t, 4H), 4.71 (t, 4H), 7.36 (m, 4H), 7.47 (m, 2H), 7.87 (m,
2H), 8.06 (t, 1H), 8.32 (d, 2H). DI-MS (EI): m/z 739, calcd exact mass
739.395.
1
2,6-Bis(1′-(12-thioacetoxydodecyl)benzimidazol-2-yl)pyridine (3e).
An acetonitrile solution (100 mL) of 2e (3.57 g, 4.4 mmol) was bubbled
with Ar for 5 min. To this solution was added potassium thioacetate
(1.26 g, 11.1 mmol), and the resulting mixture was stirred under argon
at room temperature for 2 h. The solvent was removed under reduced
pressure, diethyl ether was added, and the solution was washed with
saturated aqueous NaHCO3 solution, dried over Na2SO4, and concen-
trated under reduced pressure. The product was purified by column
chromatography (silica gel, dichloromethane:ethyl acetate ) 3:1) to
2,6-Bis(1′-(12-bromododecyl)benzimidazol-2-yl)pyridine (2e). NaH
(60%) in oil (770 mg, 19.3 mmol) was added to a three-necked flask
under an argon atmosphere, and then dry hexane was added. After the
resulting solution was stirred for 5 min, the hexane was removed with
a syringe, and dry DMAc (10 mL) was then added. BIP (2.0 g, 6.4
mmol) was added, and the resulting mixture was heated to 75 °C for
1 h. This pale yellow solution was added to the preheated dry DMAc
solution (10 mL) of 1,12-dibromododecane (3.35 mL, 28.3 mmol)
through a needle at 75 °C, and the mixture was continuously heated
for 1 h. The solution was then cooled to room temperature, and diethyl
ether was added. The resulting pale yellow solution was washed with
water, dried over Na2SO4, and concentrated under reduced pressure.
The product was purified by column chromatography (silica gel, hexane,
then ethyl acetate:methanol ) 100:1) to give 2e (3.88 g, 75%). 1H NMR
(300 MHz, CDCl3): δ 1.08 (m, 12H), 1.76 (m, 8H), 4.70 (t, 4H), 4.84
(m, 4H), 5.62 (m, 2H), 7.35 (m, 4H), 7.45 (m, 2H), 7.87 (m, 2H), 8.05
(t, 1H), 8.31 (d, 2H). DI-MS (EI): m/z 805, calcd exact mass 805.312
2,6-Bis(1′-(3-thioacetoxylpropyl)benzimidazol-2-yl)pyridine (3a).
A dichloromethane solution (50 mL) of 2a (1.26 g, 320 mmol) and
thioacetic acid (0.69 mL, 9.7 mmol) was irradiated with a high-pressure
mercury lamp (400 W) under argon for 2 h. The solution was washed
with saturated aqueous NaHCO3 solution, dried over Na2SO4, and
concentrated under reduced pressure. The product was purified by
column chromatography (silica gel, dichloromethane:ethyl acetate )
3:1) to give 3a (1.31 g, 75%) as a pale yellow oil. 1H NMR (300 MHz,
CDCl3): δ 1.97 (s, 4H), 2.03 (m, 4H), 2.70 (t, 4H), 4.76 (t, 4H), 7.37
(m, 4H), 7.48 (m, 2H), 7.91 (q, 2H), 8.03 (t, 1H), 8.38 (d, 2H). DI-MS
(EI): m/z 543, calcd exact mass 543.18.
2,6-Bis(1′-(6-thioacetoxyhexyl)benzimidazol-2-yl)pyridine (3b). A
dichloromethane solution (50 mL) of 2b (3.05 g, 6.40 mmol) and
thioacetic acid (1.37 mL, 19.2 mmol) was irradiated with a high-pressure
mercury lamp (400 W) under argon for 2 h. The solution was washed
with saturated aqueous NaHCO3 solution, dried over Na2SO4, and
concentrated under reduced pressure. The product was purified by
column chromatography (silica gel, dichloromethane:ethyl acetate )
3:1) to give 3b (4.02 g, equivalent yield) as a pale yellow oil. 1H NMR
(300 MHz, CDCl3): δ 1.10 (m, 8H), 1.27 (m, 4H), 1.86 (m, 4H), 2.20
(s, 6H), 2.61 (t, 4H), 4.69 (t, 4H), 7.33 (m, 4H), 7.45 (m, 2H), 7.86
(m, 2H), 8.06 (t, 1H), 8.32 (d, 2H). DI-MS (EI): m/z 627, calcd exact
mass 627.27.
1
give 3e (3.56 g, equivalent yield) as a pale yellow oil. H NMR (300
MHz, CDCl3): δ 1.00-1.25 (m, 32H), 1.54 (m, 4H), 1.71 (m, 4H),
2.32 (s, 6H), 2.85 (t, 4H), 4.71 (t, 4H), 7.33 (m, 4H), 7.45 (m, 2H),
7.87 (m, 2H), 8.06 (t, 1H), 8.32 (d, 2H). DI-MS (EI): m/z 795, calcd
exact mass 795.458.
2,6-Bis(1′-(3-mercaptopropyl)benzimidazol-2-yl)pyridine (4a). So-
dium borohydride (1.31 g, 2.4 mmol) was added to a DMAc solution
(50 mL) of 3a (1.31 g, 2.4 mmol). The mixture was stirred at room
temperature for 1 h. After addition of water to the solution, the product
was extracted with dichloromethane, dried over Na2SO4, and concen-
trated under reduced pressure. The product was then purified by column
chromatography (silica gel, dichloromethane:ethyl acetate ) 3:1) to
give 4a (0.5 g, 93%) as a colorless solid. 1H NMR (300 MHz, CDCl3):
δ 2.32 (m, 4H), 2.94 (q, 4H), 4.79 (t, 4H), 7.33 (m, 4H), 7.45 (m, 2H),
7.86 (q, 2H), 8.06 (t, 1H), 8.31 (d, 2H). DI-MS (EI): m/z 459, calcd
exact mass 459.16.
2,6-Bis(1′-(6-thiohexyl)benzimidazol-2-yl)pyridine (4b). Sodium
methoxide (2.04 g, 38 mmol) was added to a 2-propanol solution (500
mL) of 3b (1.00 g, 1.6 mmol), and the mixture was vigorously stirred
under ambient conditions at room temperature for 48 h. The solvent
was removed under reduced pressure. After addition of water to the
solution, the product was extracted with chloroform, dried over Na2-
SO4, and concentrated under reduced pressure. The product was then
purified by column chromatography (silica gel, dichloromethane:ethyl
1
acetate ) 3:1) to give 4b (0.77 g, 89%) as a colorless solid. H NMR
(300 MHz, CDCl3): δ 1.11 (m, 8H), 1.76 (m, 4H), 4.70 (t, 4H), 4.85
(t, 4H), 7.35 (m, 4H), 7.46 (m, 2H), 7.88 (m, 2H), 8.07 (t, 1H), 8.32
(d, 2H). DI-MS (EI): m/z 541, calcd exact mass 541.23.
2,6-Bis(1′-(8-thiooctyl)benzimidazol-2-yl)pyridine (4c). Sodium
methoxide (2.04 g, 38 mmol) was added to a 2-propanol solution (500
mL) of 3c (1.02 g, 1.5 mmol), and the mixture was vigorously stirred
under ambient conditions at room temperature for 48 h. The solvent
was removed under reduced pressure. After addition of water to the
solution, the product was extracted with chloroform, dried over Na2-
SO4, and concentrated under reduced pressure. The product was then
purified by column chromatography (activated alumina, chloroform)
2,6-Bis(1′-(8-thioacetoxyoctyl)benzimidazol-2-yl)pyridine (3c). A
dichloromethane solution (50 mL) of 2c (2.41 g, 4.53 mmol) and
thioacetic acid (3.2 mL, 45.3 mmol) was irradiated with a high-pressure
mercury lamp (400 W) under argon for 3 h. The solution was washed
with saturated aqueous NaHCO3 solution, dried over Na2SO4, and
concentrated under reduced pressure. The product was purified by
column chromatography (silica gel, hexane:dichloromethane:methanol
1
to give 4c (0.83 g, 93%) as a colorless solid. H NMR (300 MHz,
CDCl3): δ 1.28 (m, 16H), 1.65 (m, 4H), 1.84 (m, 4H), 2.66 (t, 4H),
4.73 (t, 4H), 7.36 (m, 4H), 7.48 (m, 2H), 7.87 (m, 2H), 8.05 (t, 1H),
8.34 (d, 2H). ESI-FT-ICR-MS: m/z 598.303 (M + H+), calcd exact
mass 598.304 (M + H).
1
) 5:5:1) to give 3c (3.09 g, equivalent yield) as a pale yellow oil. H
NMR (300 MHz, CDCl3): δ 1.02-1.25 (m, 16H), 1.33 (m, 4H), 1.75
(m, 4H), 2.30 (s, 6H), 2.71 (t, 4H), 4.69 (t, 4H), 7.36 (m, 4H), 7.47
(m, 2H), 7.87 (m, 2H), 8.07 (t, 1H), 8.31 (d, 2H). DI-MS (EI): m/z
683, calcd exact mass 683.333.
2,6-Bis(1′-(10-thioacetoxydecyl)benzimidazol-2-yl)pyridine (3d).
A dichloromethane solution (50 mL) of 2d (1.68 g, 2.86 mmol) and
thioacetic acid (2.0 mL, 28.6 mmol) was irradiated with a high-pressure
mercury lamp (400 W) under argon for 3 h. The solution was washed
with saturated aqueous NaHCO3 solution, dried over Na2SO4, and
concentrated under reduced pressure. The product was purified
by column chromatography (silica gel, hexane:ethyl acetate ) 1:1) to
give 3d (2.09 g, 99%) as a pale yellow oil. 1H NMR (300 MHz,
CDCl3): δ 1.00-1.25 (m, 24H), 1.50 (m, 4H), 1.73 (m, 4H), 2.31 (s,
2,6-Bis(1′-(10-thiodecyl)benzimidazol-2-yl)pyridine (4d). An aque-
ous solution (5 mL) of potassium carbonate (0.30 g, 2.2 mmol) was
added to an ethanol solution (300 mL) of 3d (0.35 g, 0.47 mmol). The
mixture was vigorously stirred under ambient conditions at room
temperature for 48 h. The solvent was removed under reduced pressure.
After addition of water to the solution, the product was extracted with
chloroform, dried over Na2SO4, and concentrated under reduced
pressure. The product was then purified by column chromatography
(activated alumina, chloroform) to give 4d (0.25 g, 80%) as a colorless
1
solid. H NMR (300 MHz, CDCl3): δ 1.23 (m, 20H), 1.37 (m, 4H),
1.68 (m, 4H), 1.83 (m, 4H), 2.69 (t, 4H), 4.71 (t, 4H), 7.38 (m, 4H),
7.47 (m, 2H), 7.88 (m, 2H), 8.05 (t, 1H), 8.34 (d, 2H). ESI-FT-ICR-
MS: m/z 654.366 (M + H+), calcd exact mass 654.366 (M + H).
9
J. AM. CHEM. SOC. VOL. 128, NO. 40, 2006 13093

A R T I C L E S
Kanehara et al.
2,6-Bis(1′-(12-thiododecyl)benzimidazol-2-yl)pyridine (4e). So-
dium methoxide (1.02 g, 18.8 mmol) was added to a 2-propanol solution
(500 mL) of 3e (0.50 g, 0.63 mmol). The mixture was vigorously stirred
under ambient conditions at room temperature for 72 h. The solvent
was removed under reduced pressure. After addition of water to the
solution, the product was extracted with chloroform, dried over Na2-
SO4, and concentrated under reduced pressure. The product was then
purified by column chromatography (activated alumina, chloroform)
spectroscopy (HORIBA FT-720) was adopted to investigate the
conformation of the TCnBIP ligands on Au nanoparticles in the solid
state by a KBr method. The Au nanoparticles were observed by TEM
at 100 kV on a Hitachi H-7100. The crystal structure of the Au
nanoparticles was determined by both HRTEM at 300 kV on a Hitachi
H-9000NAR and XRD on a MAC Science M18XHF-SRA with Cu
KR radiation. XPS was used to examine the chemical state of the sulfur
in the TCnBIP nanoparticles and gold core. The S2p and Au4f peaks
of the TCnBIP-Au nanoparticles were measured on an Ulvac Phi
5600ci with monochromated Al X-ray radiation at 350 W.
1
to give 4e (0.32 g, 71%) as a colorless solid. H NMR (300 MHz,
CDCl3): δ 1.13-1.40 (m, 32H), 1.67 (m, 4H), 1.82 (m, 4H), 2.70 (t,
4H), 4.69 (t, 4H), 7.37 (m, 4H), 7.48 (m, 2H), 7.89 (m, 2H), 8.07 (t,
1H), 8.33 (d, 2H). ESI-FT-ICR-MS: m/z 710.428 (M + H+), calcd
exact mass 710.429 (M + H).
Particle Size and Interparticle Spacing Distribution Measure-
ments. Particle size distributions were measured by counting 200 of
nanoparticles for each TCnBIP-Au monolayer on an amorphous-
carbon-coated copper grid. The mean interparticle spacing was obtained
by direct measurement of the individual spacings in a well-ordered
hcp domain.
Preparation of TCnBIP-Au Nanoparticles. In a typical prepara-
tion, 2.5 mL of a 1 mM aqueous solution of HAuCl4‚4H2O (2.5 µmol)
was added to a mixture of 1 mL of a 2.5 mM DMF solution of TCn-
BIP (2.5 µmol) and 44 mL of DMF. After the solution was heated at
60 °C or cooled in an ice-water bath, 2.5 mL of a 10 mM aqueous
solution of NaBH4 (25 µmol) was swiftly added to the solution under
vigorous stirring, and the solution was continuously stirred for 1 h to
obtain TCnBIP-Au nanoparticles. The solvent was evaporated to ca.
3 mL at 40 °C under reduced pressure. Distilled methanol was then
added, and the mixture was centrifuged to give a crude precipitate of
Au nanoparticles. The precipitate was dissolved in 0.5 mL of distilled
chloroform, reprecipitated with distilled methanol, and centrifuged. The
reprecipitation processes were repeated three times to give pure TCn-
BIP-protected Au nanoparticles.
Measurements. The consumption of AuCl4- ions and the formation
of TCnBIP-Au nanoparticles were confirmed by UV-vis spectroscopy
with a Hitachi U-3310 UV-vis spectrophotometer. The ligand contents
of TCnBIP-Au nanoparticles were estimated by TGA on an ULVAC-
RIKO TGD9700 by heating from room temperature to 600 °C at a
heating rate of 2 °C min-1 and holding at 600 °C for 30 min. FT-IR
Acknowledgment. The present work was supported by the
Industrial Technology Research Grant Program in ’04 from
NEDO of Japan, a Grant-in-Aid for Scientific Research on
Priority Area “Chemistry of Coordination Space” from MEXT
(T.T.), and a Grant-in-Aid for Young Scientists (B) (No.
17710090) from MEXT, Japan (M.K.).
Supporting Information Available: Small-angle X-ray dif-
fraction pattern of 1.5 nm TC8BIP-Au nanoparticle powder,
π-A curve of a TC8BIP-Au nanoparticle LB film, and TEM
image of a 1.5 nm TC8BIP-Au nanoparticle LB film obtained
at 15 mN/m. This material is available free of charge via the
Internet at http://pubs.acs.org.
JA064510Q
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13094 J. AM. CHEM. SOC. VOL. 128, NO. 40, 2006