Supramolecular staircase via self-assembly of disklike molecules at the solid-liquid interface

Article abstract of DOI:10.1021/ja0111380

A series of soluble hexabenzocoronene (HBC) derivatives with pendant optically active (S)-3,7-dimethyloctanyl and (R,S)-3,7-dimethyloctanyl (mixture of stereoisomers) hydrocarbon side chains with and without a phenylene spacer were assembled into differen

Full text of DOI:10.1021/ja0111380

11462
J. Am. Chem. Soc. 2001, 123, 11462-11467
Supramolecular Staircase via Self-Assembly of Disklike Molecules at
the Solid-Liquid Interface
Paolo Samor´ı,† Andreas Fechtenko1tter,‡ Frank Ja1ckel,† Thilo Bo1hme,† Klaus Mu1llen,*,‡ and
Ju1rgen P. Rabe*,†
Contribution from the Department of Physics, Humboldt UniVersity Berlin, InValidenstrasse 110,
10115 Berlin, Germany, and MPI for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
ReceiVed May 7, 2001
Abstract: A series of soluble hexabenzocoronene (HBC) derivatives with pendant optically active (S)-3,7-
dimethyloctanyl and (R,S)-3,7-dimethyloctanyl (mixture of stereoisomers) hydrocarbon side chains with and
without a phenylene spacer were assembled into differently ordered arrays at the interface between a solution
and the basal plane of highly oriented pyrolytic graphite (HOPG). Molecularly resolved scanning tunneling
microscopy (STM) images revealed that all derivatives self-assemble into oriented crystals in quasi-two
dimensions. However, while for the alkyl-substituted HBCs (1,4) all of the single aromatic cores within a
monolayer exhibit the same contrast in the STM, the single aromatic cores with a phenylene group between
the alkyl side chains and the aromatic core (2a,2b,3) exhibit different contrasts within a monolayer. For the
disks carrying racemic branched or n-alkyl side chains (2b,3) a random distribution of the two different contrasts
within the 2D-crystal is observed, while the optically active phenylene-alkyl-substituted HBC (2a) exhibits a
periodical distribution of three contrasts within the monolayer. We attribute the different contrasts of the aromatic
cores in the presence of the phenylene groups to a loss of the planarity of the whole molecule and different
conformations, which allow the conjugated disks to attain different equilibrium positions above the surface of
HOPG. In the case of the optically active side chains a regular superstructure with three distinctly different
positions such as in a staircase is attained. The self-assembly processes are governed by the interplay of
intramolecular as well as intermolecular and interfacial interactions. In the present case, the interactions may
induce both the molecules to acquire well distinct positions along the z axis and to adopt different conformations.
The reported results open new avenues of exploration. For instance, the different couplings of conjugated
molecules with the substrate at different separations can be investigated by means of scanning tunneling
spectroscopy (STS). Furthermore, experiments on the STM tip-induced switching of single molecules embedded
in a monolayer appear feasible.
Introduction
Polycyclic aromatic hydrocarbons (PAHs), which can be
regarded as two-dimensional subsections of graphite, are well-
defined nanoobjects6-18 with interesting electronic proper-
ties.7,8,10,14,16 During the last few years, soluble derivatives,
namely, hexa-peri-hexabenzocoronenes (HBCs), and larger
analogues have been synthesized. From a synthetic point of
view, they are extremely versatile compounds that are able to
bear different chemical functionalities in their periphery.10-18
Scanning tunneling microscopy (STM) is a technique which
allows the investigation of physisorbed layers of PAHs both at
The self-assembly of nanometer-sized building blocks into
targeted molecular architectures at surfaces represents one of
the major goals of supramolecular chemistry and material
science, given the perspective of the potential applications of
these systems in nanotechnology, for example, for molecular
information storage devices or functional surfaces.1 This requires
an accurate control of the molecular arrangements over a wide
range of length scales, spanning from micrometers down to the
molecular size. Noncovalent intermolecular forces have been
used to engineer highly ordered three-dimensional (macro)-
molecular architectures where the single building blocks are held
together by specific interactions, such as metal-ligand bond-
ing,2 hydrogen bonding3 or π-π stacking.4 On the other hand,
the self-assembly at the solid-liquid interface has been suc-
cessfully used to control the molecular arrangement in two
dimensions.3b,5
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Schubert, D.; Weidl, C. H.; Schubert, U. S. Angew. Chem. 1999, 111, 2701-
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Lahav, M. J. Am. Chem. Soc. 1999, 121, 9589-9598. (c) Kurth, D. G.;
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Chem., Int. Ed. Engl. 1995, 34, 2311-2327. (b) Gottarelli, G.; Masiero,
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Eur. J. 2000, 6, 3242-3248. (c) Hirschberg, J. H. K. K.; Brunsveld, L.;
Ramzi, A.; Vekemans, J. A. J. M.; Sijbesma, R. P.; Meijer, E. W. Nature
2000, 407, 167-170. (d) Bisson, A. P.; Carver, F. J.; Eggleston D. S.;
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* Corresponding authors. Prof. Dr. K. Mu¨llen: e-mail: muellen@
mpip-mainz.mpg.de; fax: +49-6131-379350. Prof. Dr. J. P. Rabe: e-mail
rabe@physik.hu-berlin.de; fax: +49-30-20937632.
† Humboldt University Berlin.
‡ MPI for Polymer Research.
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1996, 108, 1242-1286; Angew. Chem., Int. Ed. Engl. 1996, 35, 1155-
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10.1021/ja0111380 CCC: $20.00 © 2001 American Chemical Society
Published on Web 10/26/2001

Self-Assembly into a Staircase Architecture
J. Am. Chem. Soc., Vol. 123, No. 46, 2001 11463
the graphite-solution interface8-10,14,17 and in dry thin films
on conductive substrates18 with molecular resolution. The first
type of PAH which has been explored were triphenylene
derivatives,8,9 which possess 18 carbon atoms in the aromatic
core. Following the recent achievements in the synthesis of
larger and larger PAHs, the packing at surfaces of compounds
with 4210,17,18 and 60 carbons14 in the conjugated core has been
also studied. These systems have been found to form two-
dimensional crystals where the disklike molecules lie flat on
the basal plane of the conductive substrate. Recently, an STM
investigation of self-assembled monolayers of a series of
triphenylene derivatives with increasing lengths of the alkyl side
substituents has been reported which revealed different periodi-
cally distributed contrasts of the conjugated disks: the observed
structures have been explained with a frustrated Ising net
model.9
In a previous report it has been shown that HBC-C12 (4)
(Figure 1) can crystallize in monolayers at the solution-graphite
interface with all conjugated cores lying equally flat on the basal
plane of the HOPG.10 STM and STS have been used to study
the molecular structure and electronic properties of the mono-
layer on the molecular scale. In the latter type of analysis, a
diode like electrical behavior of the conjugated HBC core in
the gap tip-substrate has been observed.10
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Figure 1. Chemical formulas.
With the aim of extending the molecular order of the
physisorbed layers to the third dimension (z axis) in the region
near the substrate surface, we have investigated HBC derivatives
symmetrically functionalized in the peripheral positions with
more bulky side chains, which, due to steric hindrances could
force neighboring HBCs to acquire different positions along the
z axis.
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We report here a submolecularly resolved STM investigation
of the self-assembly of different enantiomerically pure (S) or
(R,S) hydrocarbon-substituted HBCs with and without a phen-
ylene spacer (Figure 1) into monolayers highly ordered at the
graphite-solution interface.
Compound 1 is bearing alkyl side chains exhibiting a chiral
methyl function in the 3′-position and a racemic one in the 7′-
position. In compound 2a a phenylene has been introduced
between the HBC core and the enantiomerically pure aliphatic
chain in each of the six peripheral positions. Because of the
repulsive interactions between ortho protons in the phenylenes
and the ones of the neighboring aromatic rings, the phenylenes
are likely to be twisted with respect to the HBC core. This loss
of planarity of the HBC could play a role in the organization
of the molecules when physisorbed into layers at the HOPG-
solution interface. In addition, to gain insight into the role of
the chirality of the group in the 3′-position, the (R,S) mixture
of 2a, namely, compound 2b, and compound 3 have been also
analyzed.
The syntheses of alkyl-, and phenylene-alkyl-substituted
hexabenzocoronenes, HBC-C8* (1) and HBC-PhC12 (3)
respectively, have been previously described in the literature.15
Here, in the Experimental Section, we will just briefly discuss
the synthesis of the optically active HBC-PhC8* (2a) and the
mixture of stereoisomers HBC-PhC8 (2b), which have been
synthesized according to similar procedures.
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Ku¨bel, Ch.; Mu¨llen, K.; Leo, K. Surf. Sci. 2000, 445, 358-367.

11464 J. Am. Chem. Soc., Vol. 123, No. 46, 2001
Samor´ı et al.
Figure 3. Scanning tunneling microscopy current image of 3 at the
solution-HOPG interface. An adsorbed molecule with brighter contrast
(A) is marked with light gray arrows, while a molecule with darker
contrast (B) is labeled in dark gray. Unit cell: a ) 3.10 ( 0.10 nm; b
) 2.00 ( 0.10 nm; R ) 78 ( 3°. Ut ) 1.17 V; average It ) 251 pA.
Figure 2. Scanning tunneling microscopy current image of 1 at the
solution-HOPG interface. The brightness is proportional to the current.
The arrows indicate domain boundaries. Unit cell: a ) 1.86 ( 0.10
nm; b ) 2.03 ( 0.10 nm; R ) 58 ( 2°. Ut ) 1.00 V and average It )
40 pA.
of a monolayer of the (R,S)-phenylene-alkyl-substituted HBC
(3) physisorbed on HOPG. Similar to the case of 4, the 2D unit
cell of the monolayer is oblique. In contrast to 4, however, two
different types of contrast of the aromatic core have been
recorded here, as marked in the image by A (brighter) and B
(darker). It is noteworthy that their spatial distribution does not
follow a periodical motif. A similar contrast behavior has been
observed for the other (R,S)-phenylene-alkyl functionalized
HBC, that is, compound 2b (image not shown). However, in
the latter case, the 2D unit cell is nearly hexagonal (a ) (2.41
( 0.12) nm; b ) (2.21 ( 0.12) nm; R ) (62 ( 4)°).
The STM image of the (S)-phenylene-alkyl-substituted HBC
derivative, 2a, self-assembled at the solution-HOPG interface
is displayed in Figure 4. Although the 2D packing in the xy
plane is hexagonal within the experimental error, similar to that
of compounds 1 and 2b, the conjugated cores exhibit three
different contrasts distributed in a periodical motif across the
plane. In Figure 4, the different cores are marked with A, B,
and C, where A exhibits the brightest contrast, and C the lowest.
By assuming the unit cell of the aromatic cores marked with I
to be exactly hexagonal, molecules exhibiting the same contrast
Results and Discussion
Figure 2 displays the STM current image of a monolayer of
1 physisorbed at the interface between an organic solution and
the basal plane of highly oriented pyrolytic graphite (HOPG).
It shows a polycrystalline structure with domain boundaries
(indicated by the arrows in Figure 2) that are characterized by
a translation of the crystal lattice along one of the three
crystallographic axes. Inside each domain, bright spots are
arranged (within the experimental error) in a hexagonal motif.
These bright spots are attributed to the π-conjugated cores of
the molecules, because in STM current images of alkylated
aromatic molecules lying equally flat on HOPG, the aromatic
moieties appear brighter (corresponding to higher currents) than
the alkyl chains, due to the smaller energy difference between
their frontier orbitals and the Fermi level of the substrate.19 This
assignment is consistent with the fact that the area of the unit
cell corresponds to the area within the van der Waals contour
of a single molecule lying flat on the surface. Since the contrast
in STM depends to a great extent also on the spatial overlap of
the electronic states of the adsorbate and the substrate, molecules
which are placed at variable distances from the substrate can
be expected to exhibit different contrasts in the STM current
images. Consequently, the distribution of contrasts attained in
the STM image in Figure 2 suggests that the aromatic cores
are located at equal positions relative to the substrate. The high
conformational mobility of the side chains at the surface, which
occurs on a time scale faster than the scanning frequency, did
not allow us to resolve their structures.
x
x
form a 3 × 3 R30° superstructure indicated with II.
To quantify the contrasts in the STM current images, the
images have been processed. In Figure 4 the average brightness
(which corresponds to the mean value of the current determined
inside a circle with a diameter of 1.1 nm that has been centered
on the middle of the spot) and its standard deviation have been
measured for each bright spot. The evolutions of these values
along the three lattice axes are plotted versus the position
number in Figure 5. In all three profiles in Figure 5, there is a
periodicity corresponding to the three neighboring contrasts (A,
B, C), although, due to a nonperfect flatness of the image, the
absolute values vary across the image. While the long range
variations across the whole image are on the order of what is
generally observed for more simple crystalline monolayers, for
example of 4, the periodic structure is unique.
The introduction of a phenylene ring between the aromatic
core and the aliphatic side chains in all of the six peripheral
positions leads to a dramatic change in the supramolecular
structure on the solid surface. Figure 3 displays the STM image
The different contrasts, observed in the STM images of the
molecules with the phenylene-containing side chains 2a, 2b,
(19) Lazzaroni, R.; Calderone, A.; Bre´das, J. L.; Rabe, J. P. J. Chem.
Phys. 1997, 107, 99-105.

Self-Assembly into a Staircase Architecture
J. Am. Chem. Soc., Vol. 123, No. 46, 2001 11465
Table 1: Unit Cell and Contrasts of the Conjugated Cores in the
STM Images of Functionalized HBCs
unit cell
in xy
contrasts of the conjugated
cores in the STM image
molecule
1
hexagonal 1 contrast
hexagonal 3 contrasts periodically distributed: “staircase”
hexagonal 2 contrast randomly distributed
2a
2b
3
oblique
oblique
2 contrasts randomly distributed
1 contrast
4
Figure 6. Schematic cartoon of STM constant height investigation.
The molecule can be localized in different z locations within the gap
tip-substrate. In all three cases (A,B,C) the size of the not-filled gap,
which can be considered as vacuum, is equal: GA ) GB1 + GB2 ) GC1
+ GC2. In the extreme case (D) the gap consists of vacuum where GA
< GD.
Figure 4. Scanning tunneling microscopy current image of 2a at the
solution-HOPG interface. A-, B-, C-types of molecules are character-
ized by a different contrast. The two white arrows indicate vacancies
in the crystal structure. Around the vacancy 1 it is possible to notice
the presence of three neighboring (labeled dashed black arrow) + one
(filled black arrow) A-type of molecules. Unit cell of the 2D structure
I: a ) 2.55 ( 0.10 nm; b ) 2.44 ( 0.10 nm; R ) 58 ( 3°. Unit cell
of the superstructure II: a′ ) 4.28 ( 0.17 nm; b′ ) 4.28 ( 0.17 nm;
R′ ) 58 ( 3°. Ut ) 1 V; average It ) 50 pA.
Assuming II, the contrast A in Figure 4 could correspond to
three stacked disks, B to two, and C to one disk filling the gap
tip-substrate, since a stack of aromatic cores should exhibit a
better electron transfer than the solvent molecules. However,
observing carefully the molecular packing in the proximity of
lattice defects such as a missing molecule (indicated with white
arrows 1 and 2 in Figure 4), one can note that these defects
are located where one of the darker disks should be placed,
albeit it is desorbed. The desorption of a C-type molecule is
very unlikely, since among the three types it is the one which
is better coupled with the substrate, hence more strongly
physisorbed on it. This observation does not support the effect
II.
Phenylene groups which are directly attached to the molecular
core are common to derivatives 2a, 2b, and 3. To minimize the
steric interactions of the ortho protons with those of the
neighboring aromatic core, they are likely to arrange in a
nonplanar conformation giving rise to a nonplanarity of the
whole system.15a,16 The variation of the conformation (effect
III), from planar to nonplanar, can induce a change in the
electronic properties of the HBCs, causing different contrasts
in the STM current image.
According to interpretation IV, the aromatic cores may be
”frozen” at different positions (“staircase”) in the tunneling gap
tip-substrate, as sketched in the cartoon in Figure 6 In this
model, the asymmetric position of the molecule within the gap
tip-substrate has an influence on the contrast in STM current
images, according to a resonant tunneling contribution.20 In the
case of the STM images of 2a, which are characterized by three
different contrasts, the brighter disks (A) would be better coupled
to the substrate, while B and C would appear more and more
dark because of their increasing distance from the substrate,
and consequently lower coupling (Figure 6). Already a gap shift
on the 0.1 nm scale would cause a significant current change.21
The extreme case would be characterized by the absence of the
molecule in the gap (case D), which leads to the smallest current
in the STM image, as at vacancies in the 2D lattice (indicated
by the white arrows 1 and 2 in Figure 4). This model is also
Figure 5. Line profiles for the STM image shown in Figure 4. (a)
Profile from 9 to 0; (b) from b to O; (c) from [ to ]. The average
spot brightness and its standard deviation are plotted vs the position
number, i.e., each unit on the x-axis corresponds to one disk. The
presented data were obtained by analyzing all pixels in a circular area
with a diameter of 1.1 nm centered on the middle of the particular
spot.
and 3 (Table 1), could be due to four main effects: (I) defects
of the molecular packing on the surface; (II) different number
of HBCs in a stack filling the tunneling gap between tip and
substrate; (III) different conformation of the molecule; (IV)
different position of the molecule in the gap tip-substrate.
The spatially random distribution of two different contrasts
for compounds 3 (Figure 3) and 2b in single crystalline domains
physisorbed on HOPG could possibly be explained with effect
I, which could be due to: (a) defects in the HOPG lattice or
(b) impurities of the molecular material. While (a) can be
neglected since the defect density on the freshly cleaved surface
of HOPG has been proven with STM to be orders of magnitude
smaller, (b) can be ruled out since all of the compounds which
have been used are analytically pure (as determined by NMR
and mass spectrometry).
(20) Mizutani, W.; Shigeno, M.; Kajimura, K.; Ono, M. Ultramicrosopy
1992, 42-44, 236-241.
(21) Eigler, D.; Lutz, C. P.; Rudge, W. E. Nature 1991, 352, 600-603.

11466 J. Am. Chem. Soc., Vol. 123, No. 46, 2001
Samor´ı et al.
in accordance with the observation of a missing molecule (white
arrows in Figure 4) of type C, which can desorb more easily
since it is located further away from the substrate.
Scheme 1
We conclude, therefore, that either III or IV, or both of them,
cause the different contrast observed in Figures 3 and 4.
The observation that 2a self-assembles at the solid-liquid
interface not only in two dimensions but also with respect to
the third dimension is particularly interesting. In addition to the
role played by the phenylene groups attached to the molecular
core (effect III) inducing a loss of planarity of the whole system,
a contribution of the aliphatic side chains for the supramolecular
arrangement is likely to be relevant. The branched substitutions
themselves, being (S), also induce well-defined and periodical
losses of planarity. We believe that both of these two chemical
functionalities are required to attain the self-assembly into a
staircase architecture of 2a.
On the other hand, the mere existence of the first of the two
forces in the case of compound 3, namely the presence of a
phenylene function directly attached to the HBC core, induces
a loss of planarity of the system in the nonadsorbed state, that
upon physisorption at a surface leads to a molecular pattern
with a degree of order only in the xy plane, while the molecules
are located randomly, that is, in a nonperiodical fashion, in the
third dimension in any of two distinct positions. In the absence
of the phenylene attached directly to the aromatic core, the
molecules tend to physisorb equally flat on the basal plane of
graphite, as has also been observed for several other sym-
metrically functionalized PAHs.10,14
of the nonconjugated bulky peripheral groups, and consequently
a smaller flexibility of the system, the HBCs cannot arrange in
a two-fold symmetry, thus adopting an almost hexagonal
structure, that is reminiscent of columnar mesophases. Indeed,
this class of compounds exhibits liquid crystalline phases which
exist over a wide range of temperatures.12,16
Conclusions and Outlooks
Focusing again on the missing molecules in Figure 4, one
can notice that the superstructure is lost in their vicinity. For
example, a total of four A-type molecules in the vicinity of the
vacancy 1 in Figure 4 can be recognized: Three neighboring
ones are marked with dashed black arrows, and another one is
labeled with a filled black arrow. The free space provided by
the missing molecule enables the neighboring moieties to attain
a position which is more near the substrate if compared to the
standard location it would usually assume. This indicates once
again that the steric hindrances (intermolecular interactions) are
essential to achieve this type of 3D arrangement. Remarkably,
these vacancies are rather stable features with a lifetime of
several minutes.
A similar supramolecular nanostructure like that of 2a at the
solution-HOPG interface has been detected on triphenylenes
bearing long aliphatic side-chains by Charra and Cousty.9 These
authors have observed the appearance of a superlattice in
hexagonally packed monolayers with increasing lengths of the
side chains, where every third molecule appears brighter,
In summary, the chemical functionalization of HBCs with
phenylene-alkyl (S) side chains allowed us to extend the
established two-dimensional molecular self-assembly at the
solid-liquid interface to the third dimension in the region near
the substrate. The formation of the staircase architecture finds
its origin in the interplay between intramolecular as well as
intermolecular and interfacial interactions; a key role is played
by the steric hindrance suffered by the side chains. This latter
effect can also play a role inducing the molecules to adopt
different conformations, allowing the minimization of the
intermolecular repulsive interactions. The staircase architecture
appears to be an ideal candidate for scanning tunneling
spectroscopy studies of equally conjugated molecules possessing
different electronic properties due to their variable overlap with
the substrate. Furthermore, it is possible to envisage new
interesting experiments manipulating a single HBC in the xy
plane from near-vacancy to vacancy: Depending on the final
location that the manipulated molecule spontaneously adopts
along the z axis, the HBC layer might exhibit properties of a
molecular-level machine23 where the stimulated switch between
two different positions with respect to the substrate is controlled
by intermolecular interactions.
x
x
according to a 3 × 3 R30° superstructure. They explained
this phenomenon in the light of an antiferromagnetic triangular
Ising net. However, this model cannot be applied to our data
because it cannot be used to describe a structure consisting of
more than two contrasts, for example, the staircase built up with
2a.
Experimental Section
Synthesis. The synthesis of compounds 2a and 2b is depicted in
Schemes 1 and 2. In a first step the commercially available 1-bromo-
4-trimethylsilylbenzene (4) was alkylated under common Kumada-
coupling conditions with an (S)- and an (R,S)-side chain to afford 5a
and 5b, respectively.15 The trimethylsilyl-substituted alkyl-benzenes 5a
and 5b were converted to the iodine-functionalized derivatives 6a and
6b via electrophilic substitution24 using iodine monochloride in carbon
Drawing the attention to the symmetry of the molecular
arrangement in 2D (Table 1), compounds 1 and 2a exhibit a
2D hexagonal unit cell, while for 3 it is oblique. In the latter
case, the steric hindrance of the nonbranched, that is, linear,
aliphatic side chains is smaller, giving rise to a larger flexibility
of the molecular system. This may be the reason for 3 to pack
according to a two-fold symmetry, similarly to that of HBC-
(22) Samor´ı, P.; Severin, N.; Mu¨llen, K.; Rabe, J. P. AdV. Mater. 2000,
12, 579-582.
(23) Balzani, V.; Credi, A.; Raymo, F. M.; Stoddart, J. F. Angew. Chem.
2000, 111, 3484-3530; Angew. Chem., Int Ed. 2000, 39, 3348-3391.
(24) Wu, R.; Schumm, J. S.; Pearson, D. L.; Tour, J. M. J. Org. Chem.
1996, 61, 6906-6921.
C
12 (4).10 This packing is favored since the area per unit cell is
minimized; hence, the enthalpic gain upon adsorption at the
surface (interfacial interaction) is maximized.22 Instead, in the
first three cases 1, 2a, and 2b, due to a larger steric hindrance

Self-Assembly into a Staircase Architecture
J. Am. Chem. Soc., Vol. 123, No. 46, 2001 11467
Scheme 2
tetrachloride as outlined in Scheme 1. The aryl-aryl coupling of two
eqivalents of 6a or 6b with 4,4’-dibromodiphenylacetylene (7) was
carried out under the same conditions described earlier by our group,
to yield the (S)-bis-biphenyl-4-yl-acetylene 8a and its (R,S) derivative
8b (Scheme 2).15 In a cyclotrimerization reaction under catalytic the
action of Co2(CO)8 8a and 8b were both transferred to the hexabi-
phenylylbenzene derivatives 9a and 9b, respectively. The cyclode-
hydrogenation (Scheme 2) was carried out by adding a solution of FeCl3
in nitromethane to the precursors 9a and 9b to afford the hexabenzo-
coronenes 2a and 2b. Isolated yields after purification using column
chromatography and slow reprecipitation are of the order of 80%.15
STM investigation. The STM study has been carried out with a
home-built beetle-type STM25 operating at the solid-liquid interface.
STM tips have been prepared from a 0.25 mm thick Pt/Ir (80%,20%)
wire either by mechanical cutting or by electrochemical etching using
a solution of NaCN (6 N) + KOH (2 N). Almost saturated solutions
of 1-3 (Figure 1) in 1,2,4-trichlorobenzene have been applied to the
basal plane of the freshly cleaved highly oriented pyrolitic graphite
(HOPG) substrate. The lattice of the underlying HOPG has been
visualized during the measurements by simply changing the tunneling
parameters; this allowed the calibration of the piezo in the xy plane in
situ. Unit cells were averaged on several images after their correction
for the piezo drift (using SPIP Scanning Probe Image Processor, Version
1.720, Image Metrology ApS, Lyngby, Denmark). STM current images
with a submolecular resolution have been recorded using scan rates of
∼20-100 lines/sec. The different contrasts in the STM image of the
staircase nanostructure (Figure 4) have been quantified using NIH-
Image software (National Institutes of Health, Bethesda, Maryland),
selecting manually in the image each bright spot that has been assigned
to the conjugated core, and detecting the average gray-scale value and
its standard deviation inside a circle with a diameter of 1.1 nm which
has been centered on the spot. Profiles of the obtained results are plotted
in Figure 5.
Acknowledgment. This work was supported by the EU-
TMR project SISITOMAS, the Volkswagen-Stiftung (Elektro-
nentransport durch konjugierte molekulare Scheiben und Ketten)
and the European Science Foundation through SMARTON.
Supporting Information Available: Experimental proce-
dures (PDF). This material is available free of charge via the
Internet at http://pubs.acs.org.
(25) Hillner, P. E.; Wolf, J. F.; Rabe, J. P. Humboldt University: Berlin,
unpublished results.
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