Octyl-decorated frechet-type dendrons: A general motif for visualisation of static and dynamic behaviour using scanning tunnelling microscopy?

Article abstract of DOI:10.1002/chem.200400984

A detailed STM study of monolayers of 3,5-bis[(3,5-bisoctyloxyphenyl) methyloxy]benzaldehyde and 3,5-bis[(3,5-bisoctyloxyphenyl)methyloxy]benzyl alcohol adsorbed on graphite is presented. Very highly resolved scanning tunnelling microscopy images are obse

Full text of DOI:10.1002/chem.200400984

FULL PAPER
Octyl-Decorated Frꢀchet-Type Dendrons: A General Motif for Visualisation
of Static and Dynamic Behaviour Using Scanning Tunnelling Microscopy?
Leo Merz,[a] H.-J. Gꢁntherodt,[a] Lukas J. Scherer,[b] Edwin C. Constable,*[b]
Catherine E. Housecroft,*[b] Markus Neuburger,[b] and B. A. Hermann*[c]
Abstract: A detailed STM study of
monolayers of 3,5-bis[(3,5-bisoctyloxy-
These long-chain alkyl-decorated Frꢀ-
chet-type dendrons are a powerful as-
sembly motif and initially form a pat-
tern based on trimeric units, assembled
into hexagonal host structures with a
pseudo-unit cell of seven molecules,
one of which remains highly mobile.
Over time, the supramolecular order-
ing changes from a trimeric into a di-
meric pattern. The chirality arising
from the adsorption onto a surface of
the dendrons is discussed.
phenyl)methyloxy]benzaldehyde
and
3,5-bis[(3,5-bisoctyloxyphenyl)methyl-
oxy]benzyl alcohol adsorbed on graph-
ite is presented. Very highly resolved
scanning tunnelling microscopy images
are observed at room temperature in
air allowing the analysis of the confor-
mation of the adsorbed molecules.
Keywords: adsorption · conforma-
tional analysis · dendrimers · scan-
ning probe microscopy · self-assem-
bly
Introduction
optimised to ever smaller scales. As one approaches the
nanoscale with nanometre and subnanometre-scaled devices,
the top-down approach reaches its practical and theoretical
limits, as it is based on bulk state theories. As sizes shrink,
the ratio of surface area to volume increases and surface
properties tend to dominate the system. For example, gold
clusters and nanoparticles have properties which differ sig-
nificantly from bulk, metallic gold.[4,5] A potential alterna-
tive is the bottom-up approach in which molecular compo-
nents are used for the assembly of nanoscaled devices. The
potential advantages in terms both of reaching the ultimate
nonsubatomic limits and of material economy are compel-
ling reasons for investigating the bottom-up methodology.[6]
Fundamental questions relating to the building, addressing,
and control of such molecular-based devices remain unan-
swered. However, self-assembly, using molecular recognition
interactions between molecules or between molecules and
substrates, is an attractive candidate for the construction of
nanoscaled devices.[7] If the interactions are sufficiently well
understood and the molecular components sufficiently well
designed, then the self-assembled structure can be spontane-
ously formed.[8] Applications require control at the molecu-
lar level, although the properties of surface-bound individu-
al molecules or of the ensemble may not parallel the proper-
ties of single molecules. In particular, properties are likely
to be highly dependent upon imperfections arising from sub-
strate structure, foreign molecules, or ambiguous self-assem-
bly motifs.
Self-assembly is a well-established phenomenon in chemis-
try[1] and is widely utilised for the construction of extended
two- or three-dimensional structures.[2] Phase interfaces
(solid–solid, solid–liquid, solid–gas, liquid–liquid, liquid–gas)
are particularly suitable for the assembly of two-dimensional
systems. Although originally little more than a chemical cu-
riosity, two-dimensional self-assembly is assuming a signifi-
cant technological importance.[3] The current drive for mini-
aturisation of devices commenced with so-called top-down
approaches, in which an existing macroscopic technology is
[a] Dr. L. Merz, Prof. Dr. H.-J. Gꢁntherodt
Institute of Physics, Klingelbergstrasse 82, 4056 Basel (Switzerland)
[b] Dipl.-Chem. L. J. Scherer, Prof. Dr. E. C. Constable,
Prof. Dr. C. E. Housecroft, M. Neuburger
Department of Chemistry, Spitalstrasse 51, 4056 Basel (Switzerland)
Fax : (+41)61-267-1005
E-mail: edwin.constable@unibas.ch
catherine.housecroft@unibas.ch
[c] Prof. Dr. B. A. Hermann
Walther-Meissner Institute and Faculty of Physics/Center for Nano-
science
LMU Munich, Walther-Meissner-Strasse 8, 85748 Garching
(Germany)
Fax : (+49)89-289-14206
E-mail: b.hermann@cens.de
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DOI: 10.1002/chem.200400984
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Scanning tunnelling microscopy (STM) is a powerful tool
for obtaining structural and electronic data of surfaces and
is particularly useful for characterising surfaces with non-
periodic properties, observing small numbers of (or even in-
dividual) molecules, and for visualising irregularities, dynam-
ic behaviour, or non-systematic defects.[9,10] It is well estab-
lished that STM may be used to monitor both the extent
and detailed structure of surface-bound monolayers. In a
few cases it has been possible not only to observe chemical
species by STM, but also to move atoms[11] or small mole-
cules[12] or to initiate and monitor chemical reactions of ad-
sorbed molecules,[13] although most studies of chemical reac-
tions have been made at low temperatures on isolated mole-
cules. Dynamic phase changes (two-dimensional Ostwald
ripening) have been observed in adsorbed monolayers,[14] as
have reorganisation processes.[15] The conformation of adsor-
bed molecules has a great effect on the properties of a mono-
layer[16] and with scanning probe microscopes, it is possible
to identify[17–19] and change the conformation of mole-
cules.[20]
Early studies of molecular systems with STM were in
ultra high vacuum[10] but measurements under ambient con-
ditions soon followed.[21] In some cases, molecular and sub-
molecular resolution STM images can be obtained at ambi-
ent temperature in air under conditions of chemical rele-
vance.[22–25]
A prerequisite for high resolution is a stable assembly
that is not perturbed during the scanning process by the
STM tip.[22] High-resolution imaging allows both a direct or
indirect analysis of the conformation of the adsorbed spe-
cies,[17,20,22,26–29] and the determination of the periodic proper-
ties of the ensemble.[24,30] The technique can also be used to
observe single events in real-space and -time.[31]
We and others have shown that high-resolution images
with submolecular resolution can be obtained of self-assem-
bled monolayers of (S,S)-1,4-bis(dimethylamino)-2,3-dime-
thoxybutanes or 2,2’-bipyridines functionalised with octyl-
decorated Frꢀchet dendrons and that in the latter case a de-
tailed conformational analysis is possible.[2,23] This leads us
to the idea of suggesting the general use of long-chain alkyl-
decorated Frꢀchet-type dendrons as visualisation markers
for STM. The dendron functionalisation serves two purpos-
es: firstly, the aromatic-rich structure and the alkyl chains
combine to give a powerful self-assembly motif on graphite,
and secondly, the high concentration of aromatic residues
makes visualisation facile, as they act as STM markers. As a
first step towards this proposition, we present a study of
higher-generation Frꢀchet dendrimers 3,5-bis[3,5-bis(octyl-
oxy)phenylmethyloxy]benzyl alcohol (3) and 3,5-bis[3,5-bis-
(octyloxy)phenylmethyloxy]benzaldehyde (4) (Scheme 1),
the former having a known 3D crystal structure.[32] Here we
present studies of these achiral molecules and show that
they form chiral domains upon adsorption on graphite. We
also describe the STM observation of a metastable pattern,
which rearranges to a stable one as a result of changing the
supramolecular arrangement.
Scheme 1. Synthesis of alcohol
3 and aldehyde 4. a) MesCl, NEt3,
CH2Cl2, À158C, 1 h; b) K2CO3, [18]crown-6, acetone, 608C, 48 h; c) pyri-
dinium chlorochromate, molecular sieves 3 ꢃ, CH2Cl2, room tempera-
ture, 4 h.
Results and Discussion
Synthesis and solid-state characterisation of compounds:
The octyl-decorated alcohol 3 has been reported by Seebach
and co-workers[32] and was the starting point for our investi-
gations. An attractive feature of this compound was that a
three-dimensional solid-state X-ray structure had been de-
termined (CCDC Refcode WIRXOK). We used a modifica-
tion of the literature method for the preparation of 3 using
the mesylate 2 rather than the corresponding bromo deriva-
tive (Scheme 1). The mesylate 2 was prepared in high yield
by the reaction of alcohol 1 with methanesulfonyl chloride
and NEt3 in dichloromethane at À158C. The crude mesylate
2 was then treated directly with 3,5-dihydroxybenzyl alcohol
to give alcohol 3 in 76% yield (Scheme 1). The spectroscop-
ic properties of 3 match those in the literature.[3]
The reported solid-state structure of 3 was collected at
295 K and was refined to R = 0.0872, wR2 = 0.229. Seebach
mentions that no hydrogen bonding to the alcohol was ob-
served. There was disorder present in the structure, with the
alcohol oxygen disordered over two sites with C O distan-
ces of 1.219 and 1.094 ꢃ. We have determined the structure
of 3 at 123 K (Figure 1).
À
All cell dimensions were within 2.5% of those of the pub-
lished structure and the structure was refined to final R and
wR factors of 0.0485 and 0.0565 (I>3s(I)). The structure at
123 K closely resembles that at room temperature, with the
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Dendrimer monolayers by STM
FULL PAPER
the height of the STM tip
above the surface. The height
of the tip above the surface is
a function of the conductivity
and of the physical topography
of the sample. The observed
ꢄflower patternꢅ, which is
shown in Figure 2, was seen for
both 4 and 3.
To maximise the two-dimen-
sional crystallisation energy,
the alkyl chains form interdigi-
tated patterns with the alkyl
chains of the neighbouring
molecules, as is often observed
for alkoxylated molecules ad-
sorbed on surfaces,[25,34,35] and
the molecular arrangement is
easily identified. The interdigi-
tation of alkyl chains is also
observed in the solid-state
Figure 1. a) ORTEP plot (50% probability ellipsoids) of the solid state structure of 3 at 123 K. b) Part of a
layer from the packing diagram of 3 showing the interdigitation of the alkyl chains.
structure of
3
(Figure 1b).
À
exception that the alcohol is ordered with a C O bond
length of 1.417(3) ꢃ. No intermolecular hydrogen bonding
between the alcohol groups is observed.
Both 3 and 4 initially form monolayers. In each case, these
comprise multiple domains with similar structures but differ-
ent orientation.[36] For each compound, three orientations of
the domains were observed, reflecting the threefold symme-
try resulting from the ABAB layer structure of a-graphite,
in which every second atom in the A layer has an atom
from the B layers directly underneath it. These domains
consist dominantly of trimeric substructures as previously
reported by the group of Bai for 3,5-bis[(3,5-bisdodecyloxy-
phenyl)methyloxy]benzoic acid.[37] However, we also com-
ment upon an additional feature. In some domains of trim-
ers, rows of embedded dimers were observed (Figure 2a and
c). As expected, the rows of dimers show three orientations
with respect to the surface. The STM images obtained were
of sufficiently high resolution to allow a detailed analysis of
the molecular conformation within the monolayers.
We have also prepared the aldehyde 4, which we initially
thought might be an oxidation product in the single-crystal
structure reported by Seebach et al. As the phenyl ether
groups of the Frꢀchet-type dendrimers do not withstand
treatment with MnO2, alcohol 3 was oxidised with pyridini-
um chlorochromate and aldehyde 4 was formed in 91%
yield. The aldehyde exhibited a characteristic C=O stretch-
ing mode at 1705 cmÀ1 in its IR spectrum and in the 1H
NMR spectrum, the aldehyde CH was observed at d =
9.89 ppm.
Initial crystallographic studies of poor quality crystals of
authentic 4 at 123 K revealed cell dimensions within 4% of
those for 3, and accordingly we can make no useful com-
À
ment about the short C O distances in the reported room-
temperature structure of 3.
Analysis of the molecular and supramolecular conformation
of the trimeric pattern: Images with very high resolution
were obtained at room temperature in air. A further reduc-
tion of (random) noise by using an averaging procedure was
helpful, but not mandatory to perform a conformational
analysis, as we have reported previously.[22] Figure 3 shows
an averaged enlargement, and the proposed molecular ar-
rangement of 4.
The alcohol 3 was expected to form a six-membered cen-
tral ring of hydrogen-bonded alcohol groups. The existence
of the hydrogen bonds could only be inferred indirectly. The
distances found between the aromatic moieties are consis-
tent with an arrangement with a central ring of alcohol
groups. Perhaps the existence of three hydrogen bonds is
the reason for the increased stability of the layers of 3. The
layers of 4 are destroyed by scanning with about 50–100 pA
tunnelling current, while the monolayers of 3 are stable up
to about 100 pA. A higher current setpoint in STM also
Self-organised monolayers of 4 and 3: Monolayers of 4 and
3 were prepared at room temperature in air by placing a
droplet of dilute solution (about 0.2mm, n-pentane, n-
hexane, n-decane, toluene, dichloromethane, acetone, or 1:1
dichloromethane/methanol) onto a freshly cleaved sample
of highly oriented pyrolytic graphite (HOPG). After evapo-
ration of the solvent, the samples were mounted in a com-
mercial STM apparatus. Immediately after the approach of
the STM tip, a periodic pattern was observed. STM images
of organic molecules are often compared to representations
of frontier orbitals.[25,33] In a simplified treatment, a high
conductivity results in a high intensity in the STM image
and aromatic rings give particularly high contrasts. On the
other hand, STM images of alkyl chains are usually of low
intensity and low contrast. The images were recorded in
constant-current mode in which the contrast is a measure of
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E. C. Constable, C. E. Housecroft, B. A. Hermann et al.
means a shorter distance between tip and sample, and there-
fore more interaction between tip and sample (even though
there is no contact). It can be concluded that the trimeric
structure formed by 3 is slightly more stable than the struc-
ture formed by 4 under STM scanning conditions. Addition-
ally, the monolayers of 3 are stable over larger periods of
time than layers of the aldehyde. For months of measure-
ments and reproducible data recording we thought that 3
does not undergo the transformation described below, be-
cause we only measured each sample for a couple of days;
then we discovered that 3 also undergoes the conversion de-
scribed below (see section about the conversion of the
supramolecular arrangement). Often, STM images are com-
pared to the frontier orbitals of the molecules, and a rela-
tionship between them can be found.[25,33] However, images
with the highest resolution should be compared to the fron-
tier orbitals of the adsorbed species, or the frontier orbital
interacting with the orbitals of the substrate, respectively.
Such calculations would be a major undertaking. For small
molecules, the STM image can be simulated by use of scat-
tering theories, as has been done for molecules like benzene
on graphite.[38] Fisher and Blçchl calculated that the STM
image of benzene strongly depends on the adsorption site
and the applied potential. They predicted a three-fold sym-
metry for benzene on graphite (at certain lattice spaces), re-
flecting the three atoms that are seen with STM of each
graphite hexagon.[38] Our most highly resolved images of 4
indeed show three protrusions per phenyl ring, even for
molecules much larger than benzene, as can be seen in Fig-
ure 3a. This high resolution allows a conformational analysis
of the adsorbed molecules. We showed earlier[22] that highly
resolved STM images of adsorbed Frꢀchet dendrimers can
be used to identify the conformation. Owing to the low con-
trast of the alkyl chains, only the conformation of the Frꢀ-
chet-type part of the dendrons can be determined with cer-
tainty. Molecules 3 and 4 adopted the asymmetric conforma-
tion shown in Figure 3b and were found adsorbed on either
face.
Chirality of the layers: Another reason for the interest in
self-assembled monolayers is the breaking of symmetry that
results from the interaction with the substrate on only one
face.[24,39] Both 4 and 3 are achiral molecules. They become
prochiral when they are constrained to a planar conforma-
Figure 2. Two 40ꢆ40 nm STM images and a cartoon of the observed pat-
tern. Bright features represent protrusions; dark features depressions.
The z scale (height) of all shown images is approximately 0.2 nm (from
black to white). a) A monolayer of 4 prepared from a hexane solution,
forming trimers and a row of dimers (parameters: It = 8 pA, Ubias
=
À700 mV). b) A monolayer of 3, prepared from a toluene solution (pa-
rameters: It = 20 pA, Ubias = À600 mV). c) A representation of the ob-
served pattern observed for 4, including a row of dimers (purple). The
inset explains the schematic representation; the diagram is simplified and
only four alkyl chains are shown for clarity. Aromatic moieties give a
high contrast in STM, and aliphatics only a low contrast, owing to their
low conductivity. See also Figure 4. The scanning parameters are listed in
the Experimental Section.
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Dendrimer monolayers by STM
FULL PAPER
Figure 4. Mirror images of the same trimer of 4. These prochiral trimers
become chiral upon adsorption on a surface. The sense of orientation de-
pends on the face onto which they adsorb to the surface.
chains to the neighbouring trimer. As a result, optimum in-
terdigitation is achieved and the two-dimensional crystallisa-
tion energy is maximised.
The green triangles formed by the aromatic moieties of
the molecules (Figure 4) do not point towards the centre of
the trimers, but either to the left or to the right of it. The tri-
angles formed by the trimers (Figure 5) do not point to-
wards the centre of the hexagons. A hypothetical substrate-
free monolayer of trimers would be correctly described as
prochiral—adsorption on the graphite would differentiate
the two prochiral faces and result in the formation of a
chiral monolayer. Figure 5 shows both mirror images of the
trimeric pattern. We call the orientation either clockwise
(Figure 5a) or counter-clockwise (Figure 5b), depending on
the arrangement of the trimers. Only homochiral domains of
this trimeric pattern were observed. Both chiralities were
found in equal proportions, but they were separated in dif-
ferent domains. This chiral, trimeric flower pattern was ob-
served for both 3 and 4.
The seventh molecule: At the centre of each hexagonal
array of trimers of 3 or 4, we observed an unresolved, noisy
centre. Using the averaging procedure described in the ex-
perimental section, the ordered molecules become better re-
solved, but the noisy centre is smoothed out. The height of
the centre in the raw data (Figure 2) is roughly the same as
for the ordered molecules. In the averaged image (Figure 3),
the height of the centre is less than the aromatic parts of the
fixed molecules. This indicates random noise, which is gener-
ated by mobile molecules. Further proof that the random
noise arises from the presence of one molecule confined to
an area of roughly 2 nm2 but remaining mobile is given
below.
Figure 3. a) A 10ꢆ10 nm image (It = 8 pA, Ubias = À700 mV) of a mono-
layer of 4 prepared from a hexane solution, averaged over ten positions
of a domain with counter-clockwise orientation with atomic resolution,
showing three protrusions per phenyl ring (example marked with a
circle). The conformation of the molecules is indicated with the over-
layed molecular structure (C: green; O: red). The alkyl chains are drawn
semi-transparent because they give little to no contrast in the STM
images. b) The proposed molecular arrangement for 4. Note that in
image 3a, it appears that poorly resolved alkyl chains can be seen.
On very rare occasions, a hexagonal motif without a noisy
centre could be observed for 3. Such a vacancy, where the
height of the bare graphite is seen,[40] is shown in Figure 6,
which further indicates that there is a mobile molecule in
most noisy centres.
Similar phenomena were observed in X-ray analyses of
trimesic acid (TMA) hydrogen-bonded structures that leave
cavities filled with solvent molecules, which may retain a
degree of mobility within a cavity.[41]
tion. Figure 4 shows trimers of 4 which are mirror images.
These trimers are prochiral. Upon adsorption onto a surface,
the symmetry is reduced and the supramolecular arrange-
ment becomes chiral, having a sense of direction as indicat-
ed with the circular arrows.
Because only the aromatic moieties give a high contrast
in STM, the molecules can be represented in cartoon form
as triangles, as shown in Figure 3b (green). These form trim-
ers that again can be represented as triangles (blue). The
trimers form hexagons and each presents a row of alkyl
Scanning tunnelling microscopy can resolve and distin-
guish static and dynamic molecules. The mobile molecule in
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E. C. Constable, C. E. Housecroft, B. A. Hermann et al.
Figure 5. Both mirror images, clockwise (a) and (c) and counter-clockwise (b) and (d) of the trimeric pattern formed by 4. STM images (c) and (d) are
each 10ꢆ10 nm; (c) is averaged over 21 positions and (d) is averaged over 16 positions.
the centre of the hexagonal array simply has no unique
“partner” with which to form an interdigitating pattern with
its alkyl chains. At room temperature, the motion of the
molecule is faster than the time scale of the STM measure-
ments. Neglecting the randomness of the orientation of the
central molecule, these ’hexagonal’ patterns have a unit cell
of seven molecules, comprising two trimers and one mobile
molecule in the centre.
matic representation of this pattern is shown in Figure 2c.
These single molecules have less space available than the
mobile molecules in the centre of a hexagon and, what
seems more important, they have a “partner” available to
allow interdigition of the alkyl chains. One molecule of the
row of dimers extends its alkyl chains towards this molecule,
and the molecule is fixed in its location and can therefore
be imaged with STM. These molecules present their aromat-
ic region towards the half-hexagon, which is a further indica-
tion that no alkyl chains are extended towards the centre of
the hexagons.
Rows of dimers: Embedded in the domains of trimers of ad-
sorbed 3 or 4, are single rows of dimers, as can be seen in
Figure 2a and 2c. The space between the neighbouring half-
hexagons (yellow) and a row of dimers (purple) is filled
with single molecules (red) that form an interdigitating pat-
tern of their alkane chains with those of the dimers. A sche-
Delayed conversion of the supramolecular arrangement:
The trimeric patterns observed for both 3 and 4 were not
stable over time. After minutes to hours, a conversion into a
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Dendrimer monolayers by STM
FULL PAPER
Figure 6. A rare 40ꢆ40 nm image (It = 8 pA, Ubias = À700 mV) of a va-
cancy in the trimeric pattern (marked with an arrow) for 3. The monolay-
er was prepared from a hexane solution. On the top right of the image,
the edge of the domain can be seen. The graphite there is covered with
highly mobile molecules, leading to streaky noise. The vacancy inside the
pattern shows the bare graphite.
different assembly started. This conversion could be fol-
lowed in real time by STM measurements. While measuring
the domains of trimers, a domain, consisting of dimers, ap-
peared from outside of the observed window in the meas-
urement and spread over all the observed area. The dimers
formed a lamellar phase. One of the observed conversions is
shown in Figure 7.
The newly formed pattern of dimers was stable over days,
and no further conversion could be observed. The structural
and conformational analysis of this new monolayer is given
in the next section. The dimers also form a chiral pattern,
because the two molecules do not face each other directly,
but are closer together with a lateral offset. Figure 8 shows
an example of the final dimeric pattern.
Both chiralities (offset to the right or offset to the left)
were observed, separated in homochiral domains. The do-
mains of dimers were of much larger size than those of the
trimers. Again, the pattern was observed in the three equiv-
alent orientations of the graphite surface. These lamellar re-
gions were very well ordered and images of domains up to
400ꢆ400 nm were commonly observed. The software resolu-
tion (512ꢆ512 pixels) precluded the observation of molecu-
lar structure within larger windows and we can merely state
Figure 7. A sequence of 100ꢆ100 nm STM images for compound 4, show-
ing a conversion of the supramolecular arrangement (parameters: It
=
9 pA, Ubias = À700 mV). a) A domain of dimers appears at the bottom
left of the scan-window. At the top right of the image, several domains of
trimers are seen. b) As the conversion continues, more and more of the
trimers rearrange into dimers. c) The conversion is almost complete. The
sample was prepared from hexane. Each image took about 6 min to
record.
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E. C. Constable, C. E. Housecroft, B. A. Hermann et al.
Figure 8. A 50ꢆ50 nm image of dimers of 4, prepared from a hexane so-
lution. Even the low contrast of the alkyl chains is recognisable, forming
aliphatic and aromatic lamellae. Compound 3 shows identical lamellar
phases after the conversion. (Parameters: It = 8.5 pA, Ubias = À700 mV.)
that individual domains were significantly greater than
160000 nm2 without defects or vacancies. The structure of
the dimeric domains differs from that of the rows of dimers
observed in the original trimeric monolayers and no isolated
dimeric rows were seen in the large dimeric domains. In
rare cases, two domains were found with either an angle of
1208 or a linear offset between them. By thermal annealing
(708C) of a freshly prepared trimeric sample, a structure
with smaller domains consisting of dimers could be pre-
pared. Under these thermal conditions, multiple domains of
dimers were observed.
Conformational analysis of the dimeric pattern: As a result
of the high resolution of dimeric domains, the conformation
of the individual molecules forming the dimers could be as-
signed, as shown in Figure 9. In the dimeric arrangement,
molecules of both 3 and 4 showed the asymmetric confor-
mation shown in Figures 9 and 3b. The compounds poten-
tially exhibit a high degree of conformational divergence, al-
though the possibilities in conformational space are consid-
erably reduced if we exclude the (unobserved) polymethyl-
ene chains from the discussion. The principal conformation-
al freedom is then associated with the C-O-CH2-C units.
Somewhat surprisingly, a survey of all aromatic ethers in the
Cambridge Crystallographic Data Base revealed a strong
preference for the OCH2C residue to lie in the plane of the
attached aromatic ring.[42] With an assumption of planarity,
each ether unit can adopt a syn or an anti conformation (de-
fined with respect to the C4 proton). A unique fit to the
STM images is obtained with a syn,anti conformation at the
central aromatic ring and syn,syn and syn,anti conforma-
tions at the outer rings. Detailed analysis of the conforma-
Figure 9. a) A 10ꢆ10 nm enlargement of the dimeric structure of 4 aver-
aged over 14 positions. Even the lower contrast of the alkyl chains can be
observed. The sample was prepared from a hexane solution. (Parameters:
It = 8.5 pA, Ubias = À700 mV.) b) A molecular model of the proposed
arrangement of molecules of 4. The chirality or handedness can be clear-
ly seen as an offset to the left or right with respect to the other molecule
of the dimer.
tion of Frꢀchet-type dendrimers and their metal complexes
will be given in a future publication.
Because of the observed spontaneous conversion of the
trimeric to dimeric arrangements, the question of the nature
of the mobile centre of the hexagons could be answered.
The surface coverage was determined for both arrange-
ments. Under the assumption that the number of molecules
per unit area for a given domain remains constant, we calcu-
lated that exactly one molecule must form the mobile centre
of the hexagons. The lamellar phase was observed to be
stable over several days, and no further change was ob-
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Dendrimer monolayers by STM
FULL PAPER
Figure 7. A few of the other obvious candidates for the ini-
tiation could be excluded as follows. The STM scanning
process always has a certain influence on the sample. Some-
times it is deliberately used to inflict an ordering process[43]
or to manipulate individual molecules.[14,44] By using a low-
current STM with currents below 10 pA, we hoped that this
influence could be minimised. The following two clues allow
us to exclude the STM as a possible trigger. Firstly, immedi-
ately after the observation of a conversion at a certain posi-
tion of the graphite sample, a location millimetres away was
measured (too far away to be influenced by the previous
measurements)—only dimers were observed. If the STM
triggered the conversion, one should always observe trimers
first, which would then be converted. It is worth emphasis-
ing that no actual start of the conversion was ever observed
inside the imaging window; it always started outside of the
measured range and spread to areas much larger than the
observed window. This was directly observed when the scan
range was enlarged after the observation of the conversion.
Secondly a sample of 4 that was annealed for half an hour
at 708C showed only dimers (Figure 10).
We believe that the trimer–dimer interconversion is ther-
mal in origin. Every sample is slightly cooled by the evapo-
ration of the solvent, and then heated to about 30–408C in
the STM by the measurement. The images showed very
little drift at the time of the trimer–dimer conversion, which
indicates that a constant temperature had been attained
over the sample and microscope. Usually, we observed only
one or two domains over a 500 nmꢆ500 nm area. In con-
trast, a thermally annealed sample showed dimeric struc-
tures in many small domains. These observations are consis-
tent with the conversion having a relatively high activation
energy that results in only a few initiation events at ambient
(30–408C) temperatures.
In contrast to other studies,[23] the observed patterns did
not change depending on the measurement technique. The
images at the solid–liquid interface (tunnelling while the tip
is immersed in a droplet of a solution near saturation lying
on a graphite sample) look very similar to those of monolay-
ers prepared by evaporation techniques. Which technique
delivers the best resolution in the end seems to depend
more on the skills and experience of the operator than the
technique. However, no conversion was observed at the
liquid-solid interface. An image of 3 at the solid–liquid inter-
face is shown in Figure 11.
In these measurements, only single domains of trimers
were observed, and these were error-free. No domain mis-
matches, no rows of dimers, no vacancies or other irregulari-
ties were seen. This indicates that the irregularities of the
monolayer, or even impurities, might trigger the conversion.
The suggestion that inverse micelles formed in solutions
of n-hexane are imprinted directly in two-dimensional ana-
logues on the graphite could be disproved. Measurements
from aprotic and protic, polar and apolar solutions show the
same metastable trimeric domains, which are converted into
a dimeric domain after a delay. All tested solvents (n-pen-
tane, n-hexane, n-decane, toluene, dichloromethane, ace-
Figure 10. a, b) Two 10ꢆ10 nm 3D illustrations of the trimeric and dimer-
ic patterns observed for compound 4. The smooth central part of lower
height in the centre of the averaged trimeric pattern can be seen (a). The
samples were prepared from a hexane solution. The images are rendered
in Pov-Ray 3.5 using the averaged three-dimensional data from the pro-
gram SXM-shell.
served in any case. The next question to be addressed is
what triggers the conversion of trimeric to dimeric monolay-
ers and why the trimeric arrangement often appears meta-
stable.
Trigger and driving forces for the trimer to dimer conver-
sion: There is no evidence that the rows of dimers found in
the trimer phase initiate the trimer to dimer conversion.
They have a different orientation on the graphite than the
dimers of the lamellar pattern, and the embedded rows of
dimers proved to be stable over observed times of minutes
to days; they were even on occasion converted into the la-
mellar domain with a different orientation, as can be seen in
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E. C. Constable, C. E. Housecroft, B. A. Hermann et al.
stacking interactions of the aromatic rings with the graphite
result in a very strong binding. Taken in combination with
previous studies,[22,23,42] we propose the use of these residues
as a general visualisation marker for the functionalisation of
molecular fragments for visualisation in STM.
The specificity of these binding interactions results in the
formation of monolayers, which allow us to directly observe
a range of phenomena (self-assembly, phase transformation,
molecular motion) that have not previously been observed
within one molecular system. Specifically, we have observed
the initial formation of a metastable monolayer based upon
trimeric units, which spontaneously and irreversibly trans-
forms into a monolayer composed of dimeric subunits.
The images obtained are so well resolved that it is possi-
ble to unambiguously assign the conformation within indi-
vidual molecules and describe the supramolecular ordering
of molecules into trimeric and dimeric structures and their
subsequent assembly into higher-order superstructures. We
have explicitly observed the formation of chiral monolayers
as a result of the graphite surface binding on only one face
of the prochiral monolayers.
Figure 11. A 40ꢆ40 nm image of compound 3 at the solid–liquid inter-
face, measured in a 1-phenyloctane solution. Scanning frequency 5 Hz, It
= 8 pA, Ubias = À700 mV.
We are currently synthesising and studying a variety of
other dendron-functionalised structures to establish the gen-
erality of this approach.
tone, and 1:1 dichloromethane/methanol) initially showed
only trimers with embedded rows of dimers. No influence of
the solvent on the formation of trimers or dimers was ob-
served.
In three-dimensional crystallography, it is well known that
metastable crystals can be formed, which are then converted
into a more stable form.[45]
Intuitively, a loss of entropy accompanies the observed
conversion of trimer to dimer owing to the localisation of
the seventh molecule. Clearly for the spontaneous conver-
sion of the trimers to the dimers, the global Gibbs energy
change must be negative. It follows that the decrease in en-
tropy must be compensated for by a negative enthalpy
change associated with the rearrangement—whether this
predominantly arises from the intermolecular or the mole-
cule–substrate interactions is unclear. In the spontaneous
conversion, the racemic conglomerate of small trimer do-
mains generates large dimer domains, each of a single chir-
ality.
Experimental Section
General: Commercially available chemicals were reagent grade and were
used without further purification. Alcohol 1 was synthesised as described
in the literature.[32] 1H and 13C NMR spectra were recorded on a Bruker
DRX500 spectrometer; d is relative to TMS, referenced to residual
CHCl3. Infrared spectra were recorded on a Shimadzu FTIR-8400S spec-
trophotometer on neat samples, using a Golden Gate ATR. MALDI-
TOF mass spectra were recorded on a Vestec Voyager Elite without
matrix. The microanalyses were performed with a Leco CHN-900 instru-
ment.
Compound 3: Methanesulfonyl chloride (1.04 mL, 13.5 mmol) was added
over 15 min to a mixture of 1 (1.23 g, 3.38 mmol) and NEt3 (2.08 mL,
16.9 mmol) in dry CH2Cl2 (20 mL) at À158C under N2. After stirring for
1 h at À158C, the reaction mixture was poured into a mixture of crushed
ice (100 g) and concentrated HCl (10 mL). The CH2Cl2 layer was separat-
ed, washed with saturated NaHCO3 solution, dried (Na2SO4), and evapo-
rated to give
2
(1.80 g, ca. 80% pure by 1H NMR spectroscopy,
3.20 mmol) as an oil. Crude 2 (1.33 g, ca. 80% pure, 75.0 mmol), 3,5-dihy-
droxybenzyl alcohol (210 mg, 1.50 mmol), K2CO3 (840 mg, 6.00 mmol),
and [18]crown-6 (15.8 mg, 60.0 mmol) were stirred vigorously in acetone
(30 mL) at 608C for 48 h. The solvent was evaporated, water (20 mL)
was added, and the mixture extracted three times with dichloromethane
(20 mL). The combined organic layers were dried (Na2SO4) and evapo-
rated. Chromatography on silica (ethyl acetate/hexane 1:7) yielded 3 as
an off-white powder (950 mg, 1.14 mmol, 76%). 1H NMR spectroscopic
and MS data were identical to those previously reported.[32]
Conclusion
Octyl-decorated Frꢀchet dendritic wedges are a powerful
recognition motif for the assembly of monolayers on graph-
ite. Herein, we have shown that first generation alcohol 3
and aldehyde 4 both spontaneously form highly structured
monolayers upon evaporation of dilute solutions. The aro-
matic portions of these monolayers may be directly ob-
served by using STM techniques and molecular and submo-
lecular resolution images obtained at room temperature in
air. The combination of strong interdigitation interactions
between octyl chains of adjacent molecules, of interactions
between methylene groups and the graphite surface, and
Compound 4: A solution of 3 (64 mg, 76.8 mmol) in dichloromethane
(2 mL) was added to a dry mixture of pyridinium chlorochromate (PCC)
(16.7 mg, 77.0 mmol) and molecular sieves (3) ꢃ, 0.4 g) in dichlorome-
thane (6 mL). The reaction mixture was then allowed to stir at room tem-
perature for 4 h, filtered through Celite, and the solvent evaporated.
Chromatography on silica (ethyl acetate/hexane 1:8) yielded 4 as a white
powder (58 mg, 69.8 mmol, 91%). 1H NMR (500 MHz, CDCl3, 238C): d
= 9.89 (s, 1H; CHO), 7.09 (d, 3J = 2.3 Hz, 2H; H2A), 6.85 (t, 3J =
2.3 Hz, 1H; H4A), 6.55 (d, 3J = 2.2 Hz, 4H; H2B), 6.41 (t, 3J = 2.2 Hz,
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Dendrimer monolayers by STM
FULL PAPER
2H; H4B), 5.00 (s, 4H; HOCH B), 3.94 (t, 3J = 6.6 Hz, 8H; HOCH CH ), 1.77
2
2
2
Acknowledgements
(tt, 3J
=
2
7.1, 6.7 Hz, 8H; HOCH CH ), 1.44 (tt, 3J
2
= 7.3 Hz, 8H;
), 0.88 ppm (t, 3J = 7.0 Hz, 12H;
2
2
(CH2)4
HOCH CH CH ), 1.25–1.37 (m, 32H; H
2
We thank the Swiss National Science Foundation for financial support
through the NRP47 program ’Supramolecular Functional Materials’ as
well as the Universities of Basel and Munich. B. A. H. acknowledges
fruitful discussions with Roland Netz.
HCH ); 13C NMR (125 MHz, CDCl3, 258C): d = 192.01, 160.72, 160.50,
3
138.51, 138.48, 108.84, 108.42, 105.86, 101.05, 70.54, 68.25, 31.97, 29.51,
29.39, 29.39, 26.20, 22.81, 14.26 ppm; IR (neat): n˜
= 2924 s, 2854 m,
1705 m, 1597 s, 1458 m, 1165 s, 1057 m, 833 cmÀ1 w; MS (MALDI-TOF):
m/z: 869.7 [M+K]+, 853.7 [M+Na]+; elemental analysis calcd (%) for
C53H82O7: C 76.58, H 9.94, N 0.0; found: C 76.23, H 9.86, N 0.0.
X-ray crystal structure analysis of C53H82O7 3: Determination of the cell
parameters and collection of the reflection intensities were performed on
an Enraf-Nonius Kappa CCD diffractometer (graphite monochromated
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Received: September 27, 2004
Published online: February 4, 2005
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Chem. Eur. J. 2005, 11, 2307 – 2318