Supramolecular control of two-dimensional phase behavior

Article abstract of DOI:10.1002/chem.200390137

We have used directed two-component self-assembly to "pattern" organic monolayers on the nanometer scale at the liquid/solid interface. The ability of the scanning tunneling microscope to investigate structural details in these adlayers was used to gain insight into the two-component two-dimensional phase behavior. The components are symmetrically alkylated bisurea derivatives (R1-urea-spacer-urea-R2; R1, R2=alkyl, spacer=alkyl or bisthiophene). The bisthiophene unit acts as a marker and its bisurea derivative (T2) is a component in all the mixtures investigated. By varying the position of the hydrogen-bond forming urea groups along the molecule and the length of the alkyl chains of the other components, the effect of 1) hydrogen bonding, 2) molecule length, 3) odd-even effects, and 4) shape complementarity on the two-dimensional phase behavior was investigated. Insight into the effect of these parameters leads to the control of the two-dimensional patterning: from randomly intermixed systems to phase separation.

Full text of DOI:10.1002/chem.200390137

FULL PAPER
Supramolecular Control of Two-Dimensional Phase Behavior
Steven De Feyter,*[a] Mattias Larsson,[a, c] Norbert Schuurmans,[b] Bas Verkuijl,[b]
George Zoriniants,[a] Andre¬ Gesquie¡re,[a] Mohamed M. Abdel-Mottaleb,[a]
Jan van Esch,*[b] Ben L. Feringa,[b] Jan van Stam,[c] and Frans De Schryver[a]
Abstract: We have used directed two-
component self-assembly to ™pattern∫
organic monolayers on the nanometer
scale at the liquid/solid interface. The
ability of the scanning tunneling micro-
scope to investigate structural details in
these adlayers was used to gain insight
into the two-component two-dimension-
al phase behavior. The components are
symmetrically alkylated bisurea deriva-
tives (R1-urea-spacer-urea-R2; R1,
R2 ˆ alkyl, spacerˆ alkyl or bisthio-
phene). The bisthiophene unit acts as a
marker and its bisurea derivative (T2) is
a component in all the mixtures inves-
tigated. By varying the position of the
hydrogen-bond forming urea groups
along the molecule and the length of
the alkyl chains of the other compo-
nents, the effect of 1) hydrogen bonding,
2) molecule length, 3) odd even effects,
and 4) shape complementarity on the
two-dimensional phase behavior was
investigated. Insight into the effect of
these parameters leads to the control of
the two-dimensional patterning: from
randomly intermixed systems to phase
separation.
Keywords: monolayers ¥ physisorp-
tion ¥ scanning probe microscopy ¥
self-assembly
molecular assemblies formed by the adsorption of an active
surfactant on a solid surface.[11] Many SAMs have been
investigated, but monolayers of alkane thiolates on gold are
the most popular ones. Several groups have taken advantage
of the spontaneous formation of SAMs and the formation of
nanometer-sized domains by the coadsorption of two or more
adsorbates.[12, 13] If more than one adsorbate is involved in the
self-assembly process, one must consider the interactions
between the different components. STM and AFM proved to
be useful in identifying phase separation on the nanometer
scale.[13, 14] In studies involving mixtures of molecules differing
only in the nature of the end groups, it was shown that the
difference in their polarity drives the extent of phase
separation.[15] In addition to phase separation induced by
differences in the alkyl chain length,[14] it was demonstrated
that phase separation can be driven by intermolecular
interactions buried within the film.[16]
Introduction
The control of the lateral assembly and spatial arrangement of
micro- and nano-objects at interfaces is a prerequisite when it
comes to potential applications in the field of nanoscience and
technology. To create two-dimensional patterns, one can take
advantage of ™active∫ manipulation techniques, such as
photolithography, electron beam lithography,[1] and ™soft
lithography∫.[2] Scanning probe microscopy (SPM) techni-
ques, such as scanning tunneling microscopy (STM) and
atomic force microscopy (AFM), are another class of
techniques that can be implemented for the controlled
9]
manipulation of matter.[3
Self-assembly methods provide an alternative approach to
make defined structures with dimensions on the nanometer
scale. Self-assembly is a natural phenomenon that can be
observed in many biological, chemical, and physical proc-
esses.[10] Self-assembled monolayers (SAMs) are ordered
Less attention has been given to the self-assembly of
physisorbed layers at surfaces. In contrast to chemisorbed
structures, physisorption is not very suitable for making
™permanent∫ architectures. Nevertheless, these physisorbed
adlayers are model systems to investigate the interplay
between molecular structure and the formation of ordered
¡
[a] Dr. S. De Feyter, M. Larsson, Dr. G. Zoriniants, Dr. A. Gesquiere,
Drs. M. M. Abdel-Mottaleb, Prof. F. De Schryver
Katholieke Universiteit Leuven, Departement Scheikunde
Celestijnenlaan 200 F, 3001 Leuven (Belgium)
Fax : (‡32)16-327989
E-mail: steven.defeyter@chem.kuleuven.ac.be
assemblies in two dimensions and can be studied in great
21]
detail with STM.[17
A very convenient method for the
[b] Dr. J. van Esch, Drs. N. Schuurmans, B. Verkuijl, Prof. B. L. Feringa
University of Groningen, Material Science Center, Stratingh Institute
Laboratory of Organic and Inorganic Molecular Chemistry
Nijenborg 4, 9747 AG Groningen (The Netherlands)
E-mail: j.van.esch@chem.rug.nl
formation of extended 2D structures is physisorption at the
liquid/solid interface.[22, 23] The preparation is relatively sim-
ple, and STM allows a detailed investigation of the two-
27]
dimensional patterns.[24
However, it still remains a chal-
[c] M. Larsson, Prof. J. van Stam
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1198 1206
lenge to control the ordering in multicomponent mixtures at
the supramolecular level. Most binary mixtures investigated
33]
so far show phase separation on the nanometer scale[28 or
41]
the formation of randomly mixed monolayers.[34
Highly
ordered bimolecular two-dimensional adlayers are only
49]
formed in a few cases.[42
Recently, we found that simple alkyl and oligothiophene
bisurea compounds can form mixed bimolecular adlayers, and
we have exploited this feature to study the electronic proper-
ties of the oligothiophene moieties and its supramolecular
arrays by means of tunneling spectroscopy.[50] The different
contrast from the thiophene and alkyl moieties greatly
facilitates the observation of phase-separated or mixed
adlayers, and this feature was used to study pattern formation
in physisorbed adlayers of bimolecular mixtures of alkyl and
thiophene bisurea compounds on graphite in more detail.
Here we report on the results of these studies, in which we
address the interplay between molecular structure, supra-
molecular interactions, and occurrence of phase separation or
mixing in two-dimensional monolayers. The results clearly
show that it is possible to direct pattern formation in
monolayers via intermolecular interactions, which will be
exploited in future studies on the patterning of two-dimen-
sional monolayers at the supramolecular level.
Figure 1. STM images reflecting the ordering of the bisurea derivatives in
monocomponent systems. A) C12-12, B) C9-12, C) T2. The inset (top)
shows a molecular model based upon semiempirical calculations. The same
symbols as in Figure 4 are used, a few molecules are indicated. D) Hydro-
gen-bonding pattern of urea groups. The scale bar measures 2 nm. The
difference in contrast between the urea groups in C is attributed to a
scanning artefact.[45]
Results and Discussion
The compounds investigated are alkylated bisurea derivatives
determines the intermolecular distance of 0.46 nm. As a
result, it was shown that, in the case of T2 (Scheme 1), the
thiophene rings are tilted with respect to the graphite
substrate. This allows the possibility of p p interactions
between adjacent molecules in a stack.[50] The thiophene rings
can easily be recognized as the brightest spots in the images;
this is caused by the enhanced tunneling current associated
with them. The alkyl chains appear less bright. In CX-Y-type
molecules (Scheme 1), the contrast associated with the urea
groups is often quite different
(Scheme 1), which are known to assemble efficiently at the
53]
liquid/solid interface (Figure 1).[50
These molecules phys-
isorb on the graphite substrate with their long molecular axis
parallel to the substrate while forming extended tapes or
lamellae. In monocomponent monolayers, the dominating
intermolecular interaction is the formation of hydrogen bonds
between the urea groups (Figure 1D). Within a tape, each
molecule is stabilized by eight hydrogen bonds, which
from the alkyl chains so that
they can be easily located (Fig-
ure 1). In T2 molecules, this is
often more difficult because of
the adjacent ™bright∫ bisthio-
phene groups.
The number of carbon atoms
of the alkyl group linking both
urea groups (called ™spacer∫),
being odd or even, determines
the shape of the molecule. If the
number of carbon atoms is even
the molecule adopts an extend-
ed ™zigzag∫ shape, with the urea
groups pointing in opposite di-
rections. If the alkyl spacer
contains an odd number of
carbon atoms, the molecule is
™bow∫-shaped, and the urea
Scheme 1. Chemical structures of the bisthiophene derivative (T2) and the other derivatives containing bisurea
(CX-Y) used as coadsorbents. The two different chemical structures shown for CX-Y relate to the effect of the
number of methylene groups connecting both urea groups: upper one (odd), lower one (even).
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S. De Feyter et al.
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groups point in the same direction. This is illustrated in
Figure 1A, B.
Variation of alkyl spacer length
In a first series of experiments, the spacer length–the alkyl
chain connecting both urea groups–was varied systematically
and its effect on the mixing behavior was studied by STM. The
length of the outermost alkyl chains (dodecyl side chain) is
kept constant and is identical to those of the T2 molecules. T2
was mixed with Cx-12 (where x ˆ 6, 9, 12, 14, 15, or 16) in a
1:1 molar ratio and the composition of the mixed monolayers
was studied at the HOPG/1-octanol interface. In good agree-
ment with the studies of the pure compounds, the mixtures
form monolayers composed of tapes (Figures 2 and 3). Within
the monolayers, the T2 molecules are easily identified by the
appearance of the bright features located in the center of the
tapes, corresponding to the location of the bisthiophene
moieties. The molecules are lying ™flat∫ on the HOPG surface
and adopt a fully extended conformation. The intermolecular
distance measures 0.46 nm.
T2/C6-12: In T2/C6-12 mixtures (Figure 2A, B) exclusively
microphase separation occurs into T2 and C6-12 blocks. From
a practical point of view, phase separation is defined to reflect
those situations where (almost) all T2 blocks consist of more
than five T2 molecules. Pure T2 and C6-12 blocks are formed;
however, they are not ordered in a random way. They are in
line and at one side of the lamellae, the dodecylurea groups
are in registry. At the boundary between two blocks, this ™fit∫
allows hydrogen bonding between T2 and C6-12 molecules,
given that the orientation of the urea groups is the same
(model Figure 4A).
T2/C14-12: If T2 is mixed with C14-12, which is identical in
size including the spacer connecting the urea groups (model
Figure 4C), a totally different behavior is observed (Figure 3).
In addition to the formation of extended ™pure∫ tapes (five‡),
pentamers, tetramers, trimers, dimers, and even single T2
molecules are dispersed in the C14-12 matrix and mixed
lamellae are formed.
The two-dimensional phase behavior of the other mixtures
varies between both extremes (Figure 2).
The 1:1 ratio of T2/CX-12 in solution is not exactly reflected
in the monolayer composition, which ranges from 0.8 to 3.2
for the different systems measured.[54] However, this deviation
from the solution composition is small compared to other
mixtures investigated.[33, 37, 40] This indicates that T2 and CX-12
show very similar two-dimensional nucleation behavior,
growth, and interaction with the graphite support. The
minimum average lamella length ranges between 15 and
21 molecules.[54, 55]
Figure 2. STM images of the mixtures. Solid (dashed) lines reflect the
length, measured perpendicular to the ™lamella∫ axis, of T2 (CX-12)
molecules. A, B) T2/C6-12 mixture. C, D) T2/C9-12 mixture. E, F) T2/
C12-12 mixture. G, H) T2/C15-12 mixture. I, J) T2/C16-12 mixture. The
scale bar measures 4 nm.
Quantitative analysis: The images of the mixed systems were
analyzed in terms of the size of the T2 aggregates formed, and
the histogram representing this analysis is shown in Fig-
ure 5A.[56] The histogram shows the average percentage of
monomers, dimers, trimers, tetramers, pentamers, and five‡
observed. The histogram reveals a clear size-related tendency:
the monomer (five‡) content increases (decreases) from C6-
12 to C14-12 and decreases (increases) with increasing spacer
length (X > 14). If the difference in spacer length and 14 is ꢀ5
carbon atoms (i.e. T2/C6-12; T2/C9-12), the T2 and CX-
12 molecules do not mix, which is reflected by the (almost)
complete absence of T2 monomers and small aggregates. If
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Supramolecular Control of Two-Dimensional Phase Behavior
1198 1206
Figure 3. STM images of the T2/C14-12 mixture. Solid (dashed) lines
reflect the length, measured perpendicular to the ™lamella∫ axis, of T2
(C14-12) molecules. The molecules ™mix∫ very well. A, B) The molecules
are organized in lamellae. Different domains meet in A. C, D) Because of
1
the molecular symmetry, molecules can easily shift by ³3 of the molecule
length (model Figure 4D), creating defects indicated by arrows. Depending
on the size of these defects, they can contain immobilized molecules, such
as T2 in C, or diffusing ones, such as in D, indicated by white arrows. The
scale bar measures 4 nm.
the difference in spacer length and 14 is smaller (i.e. T2/C12-
12; T2/C15-12; T2/C16-12), mixtures of T2 with CX-12 show,
in addition to the formation of large aggregates (five‡), a
substantial contribution of monomers and small aggregates.
The mixture of T2 and C14-12 shows maximum mixing, as this
system contains the largest relative amount of monomers and
the smallest amount of five‡ aggregates.
Figure 4. Schematic representation of the two-dimensional ordering of T2
and CX-Y molecules within the same ™lamella∫. The lines reflect the alkyl
chains. The black box represents the bisthiophene unit. The ™V∫ or ™L∫-
shaped symbols reflect the position and the orientation of the urea groups.
The point of these symbols corresponds to the carbonyl groups. The slight
nonlinear shape (™zigzag∫-like or ™bow∫-like corresponds to an even or odd
number of carbon atoms in the spacer) of the molecules is neglected.
A) T2/C6-12 mixture. B) T2/C9-12 mixture. C, D) T2/C14-12 mixture.
E) T2/C15-12 mixture. F) T2/C14-6 mixture. G, H) How to accommodate
for different ™block∫ sizes: molecules from the adjacent lamella are shifted
to allow for an efficient packing efficiency (G); defects are created, filled by
molecules oriented perpendicular to the adjacent ones (H). I, J, K) Pos-
sible interactions in a T2/C15-8 mixture.
Note that a straightforward interpretation of the histogram
mentioned above in terms of the tendency of the different
mixtures toward phase separation is only valid if the average
lamella length and co-deposition ratio are identical for all the
mixtures, which is not the case.[54] However, the differences
are relatively small. In addition, for a randomly intermixing
system, the ratio of monomer versus large-aggregate popula-
tion will decrease for both increasing average lamella length
and increasing T2/CX-12 ratio, and vice versa. Because both
the co-deposition ratio and average lamella length are smaller
in T2/C15-12 compared to T2/C14-12, one would expect a
larger monomer versus large-aggregate ratio for T2/C15-12;
however, this is contradicted by the experimental results. This
shows that the trend for phase separation deduced from the
histogram is not an artefact caused by differences in average
lamella length or co-deposition ratio for the different
mixtures.
under pure statistical control. In this simulation, the lamella
length was kept fixed to 19 molecules, which equals the
average experimental lamella length for the T2/C14-12
mixtures.[54] During the simulations, the average T2 content
was also kept fixed according to the experimental value
(58%). The histogram with the simulated values and the
experimental data is given in Figure 5B. The experimental
and simulated data show good resemblance given the
approximations used in the simulation. However, there is a
higher preference to form five‡ aggregates than simulated.[55]
This is probably the result of interactions between the
thiophene rings, which slightly favor neighboring T2 mole-
cules. Nevertheless, the comparison of the data shows that
there is very good mixing between T2 and C14-12.
Simulation: To investigate whether the mixing of T2 and C14-
12 reflects ideal behavior, we compared the experimental
results of the T2/C14-12 mixture with a simulation giving
relative amounts of ™n-mers∫, assuming that the mixing is
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high degree of complementarity between T2 and C14-12,
leading to an almost ideal statistical two-dimensional mixing.
Apparently, the bisthiophene unit does not affect the two-
dimensional ordering significantly. Therefore, the T2 mole-
cules can safely be used as probes to study the two-dimen-
sional phase behavior.
For all other systems investigated, only one of the hydrogen
bonds between T2 and CX-12 can be maintained, as the
terminal alkyl chain length is identical although the alkyl
spacer length is different. Even increasing the length of the
alkyl spacer with one methylene group (C15-12) induces
phase-separation. If the difference is more than five methyl-
ene groups, the phase separation is maximal (no isolated CX-
12 molecules are observed). The difference in molecular
length, leading to less favorable van der Waals interactions, in
combination with the need for optimal hydrogen bonding,
drives the molecules in T2/CX-12 mixtures (X=14) to phase
separation.
Odd even effect: Both for T2 and C14-12, the alkyl spacer
contains an even number of atoms linking both urea groups.
Hence, in both molecules the urea groups point in opposite
directions. In Figure 3B for instance, the z-shape of C14-12,
indicating the orientation of the urea groups, can easily be
distinguished; the urea groups are well-resolved and the
transition between C14-12 and T2 molecules occurs seam-
lessly (model Figure 4C). As stated above, optimal hydrogen
bonding between C14-12 and T2 is expected to take place.
The spacer of C15-12 is not only longer, both urea groups
are now directed in the same direction (model Figure 4E),
making the formation of ideal hydrogen bonds between T2
and C15-12 difficult. Given the assumption that only one urea
group is involved in hydrogen bonding, it came as a surprise
that T2 and C15-12 mix rather well (histogram: Figure 5A)
which emphasizes that hydrogen bonding is an important
factor, although it is not the only factor driving the extent of
phase separation in these systems.
Figure 5. A) Histogram reflecting the relative content of T2 monomers,
dimers, trimers, tetramers, pentamers, and five‡ clusters in the mixtures
investigated. B) Histogram comparing the T2/C14-12 experimental data
with a simulation (see text).
Phase-separation versus statistical mixing: What drives these
molecules to ™mix∫ or ™phase-separate∫ in two dimensions?
In order to address these questions, we need to explore the
intermolecular interactions in detail. The first thing to
consider is why these molecules order into lamellae. The
formation of lamellae is the best way to realize an optimum
packing efficiency for this kind of compound that has a rather
extended shape with long alkyl chains. For instance, n-alkanes
(e.g. C24H50) are known to arrange in lamellae and form a
crystalline phase.[23] Only very long alkyl chains (e.g. C192H386
)
show a tendency to form a quasinematic phase.[57] The
compounds under investigation have two urea groups, allow-
ing strong intermolecular hydrogen bonding. In combination
with the ™molecular-shape complementarity∫, it should not
come as a surprise that lamella formation is favored.
Monolayer defects: As a result of the difference in width
between the T2 blocks and CX-12 blocks, resulting in phase
separation, monolayer defects are created that may give rise
to uncovered areas. This is energetically unfavorable, and the
physisorbed monolayers tend to avoid ™empty∫ space in these
mixtures in two different ways: 1) molecules from the
adjacent lamella can shift in order to fill up the gap (case 1,
model Figure 4G). This does not happen that often (see
below), a nice illustration can be found in Figure 6D. In the
upper part of that image, at the boundary between different
blocks, the molecules are arranged in such a way that a close
packing is achieved. 2) The gap is filled by molecules which
are oriented perpendicular to the molecules in the adjacent
lamellae (T2 or CX-12 or 1-octanol) (case 2, model Fig-
ure 4H). Examples can be found in Figure 2B, D (arrow).
Quite often, no molecular or submolecular resolution is
obtained in these defect areas, which is attributed to the
mobility of the molecules in these gaps. This increase in
mobility has several reasons: often neither the width nor the
length of the defect area allows the optimal tight packing of
the molecules. In addition, as the molecules are oriented
In the mixtures, the molecules will also order in such a way
as to optimize the intermolecular interactions, that is, to
optimize the packing efficiency, the van der Waals interac-
tions, and to allow, where possible, the formation of hydrogen
bonds. The properties of the molecules in the mixtures
discussed above are similar. All these molecules consist of two
dodecyl urea groups. They only differ in the length (CX-12) or
nature of the spacer (alkyl or bisthiophene). The dodecyl urea
groups allow optimal van der Waals and hydrogen-bonding
interactions between T2 and CX-12 molecules in a stack.
Therefore, the terminal methyl groups at one side of the
lamellae are found to be in line.
The T2/C14-12 mixture is the only system where the length
of the components is identical, and the spacer length of T2 and
C14-12 is also the same. As a result, this is the only mixture
where both dodecyl urea groups can be involved in optimal
intercomponent hydrogen bonding. Comparison of the ex-
perimental data with the simulation has demonstrated the
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Supramolecular Control of Two-Dimensional Phase Behavior
1198 1206
of this molecule allows, in principle, the optimum formation of
hydrogen bonds and this should favor mixing. The difference
in length between T2 and C14-6 (12 methylene groups) should
promote phase separation. Figure 6 shows some representa-
tive images of this mixture. The C14-6 molecules can easily be
identified by the short terminal hexyl chains and the long
spacer connecting both urea groups. As for the other systems,
T2 molecules are also codeposited. In Figure 6A, a few
isolated T2 molecules are visible. The bright structures,
corresponding to the bisthiophene unit, are centered and
located between the urea groups of adjacent C14-6 molecules.
This indicates that the isolated T2 molecules are anchored by
hydrogen bonding. As a result, the terminal methyl groups of
the dodecyl chains of the T2 molecules cannot be in line with
the terminal methyl groups of the hexyl chains of the C14-6
molecules. In addition to isolated T2 molecules, in Figure 6B,
some small clusters of T2 can be observed (model Figure 4F).
In the center of the image, a T2 pentamer is trapped in a C14-6
lamella. In this case, the dodecyl groups can easily be resolved
and they are fully absorbed on the graphite substrate.
Evidently, in terms of packing efficiency, this is not the best
™solution∫; however, it demonstrates the effect of hydrogen
bonding on the two-dimensional ordering. Another clear
example is given in Figure 6C. This image shows some pure
T2 and C14-6 blocks. Of special interest is the area indicated
with the arrow where a T2 block meets a C14-6 block. The
C14-6 block runs parallel to the adjacent T2 lamellae, but the
T2 block does not. At the boundary between the T2 and the
C14-6 block, the T2 molecule complements well with the
adjacent C14-6 molecule and forms hydrogen bonds. How-
ever, because of the adjacent T2 lamella at its lower side, there
is no space for the dodecyl groups. As a result, the molecules
in that T2 block gradually shift ™upwards∫ to accommodate
space for all the dodecyl groups, while maintaining hydrogen
bonding. The examples illustrate the importance of hydrogen-
bond formation, which is also confirmed by the monolayer
formation, as shown in Figure 6D. Instead of isolated T2
molecules, pure strands of T2 and C14-6 are formed. Never-
theless, the ordering of the molecules allows a seamlessly
perfect fit of the hydrogen bonds on account of the small
relative shift of the lamellae, which creates an optimum
packing efficiency.
Figure 6. STM images of the T2/C14-6 mixture. Solid (dashed) lines reflect
the length, measured perpendicular to the ™lamella∫ axis, of T2 (C14-6)
molecules. A) Isolated T2 molecules in the C14-6 matrix. B) Isolated
molecules and some small ™clusters∫: a pentamer is located in the center
(arrow). C) Hydrogen bonding domination at the boundary between a T2
and C14-6 block (arrow). D) Efficient monolayer packing at a domain
boundary (arrow). The scale bar measures 4 nm.
perpendicular to the adjacent molecules, the alkyl chains are
not aligned parallel to one of the major graphite axes of the
underlying graphite support. Therefore, the interactions
between those alkyl chains and the graphite support are
smaller. Case 2 occurs more often than case 1, which might
seem somewhat surprising. However, one should keep in
mind that along pure blocks, intermolecular hydrogen bond-
ing is very strong and the formation of ™straight∫ lamellae is
favored. Occasionally, similar defects are formed but for
another reason. Although the formation of ™straight∫ lamel-
lae is favored, it can happen that molecules laterally shift with
respect to each other by roughly 1³3 of the molecule length, as
is shown in Figure 3C, D (arrows) for the T2/C14-12 mixture.
At such a dislocation, hydrogen bonding is still possible
between urea groups (model Figure 4D). This type of
dislocations often induces monolayer defects with a defined
width, but not with a defined length. For instance, in
Figure 3C, five T2 molecules are trapped and oriented
perpendicular to the adjacent molecules. Because of the size
of the defect, the molecules are immobilized. In the same
image, the two long lines running from left to right are caused
by mobile molecules. In Figure 3D (center), the trapped
molecules are not immobilized and individual molecules are
not visible.
Based upon the large difference in size, T2 and C14-6
should exclusively show phase separation (T2/C6-12 shows
100% phase separation!). However, as shown in the images,
isolated T2 molecules and clusters are observed. If, in
molecules with identical spacer length, all the urea groups
can be involved in hydrogen bonding, the latter noncovalent
interaction dominates the two-dimensional phase behavior.
Phase separation in molecules with identical size
So far, except for the T2/C14-12 mixture, the components
differ in size. Although it was shown that hydrogen bonding is
definitely important (and dominant for systems with a
tetradecyl spacer: all urea groups are involved in hydrogen
bonding) in the two-dimensional phase behavior, it was
clearly demonstrated that increasing the difference between
the T2 and CX-12 spacer length promotes phase separation.
T2/C14-6 versus T2/C14-12
To evaluate the importance of hydrogen bonding versus
molecule length with respect to the two-dimensional phase
behavior, a mixture of T2 with C14-6, a bisurea derivative
containing hexylurea side chains, was investigated. The design
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In order to probe exclusively the effect of hydrogen bonding,
mixtures of T2 and bisurea derivatives were studied with
(almost) identical molecule length, but with differing posi-
tions of the urea groups. To realize this, symmetrical bisurea
derivatives were studied with pentadecyl alkyl chains and an
octyl or nonyl spacer group, with a total of 38 (C8-15) or 39
(C9-15) carbon atoms, respectively. In principle, three differ-
ent situations are possible for the mixtures (model Figure 4I
K). Situation 1) Phase separation (model Figure 4I): the
molecules stack in rows with alternating ™blocks∫ of T2 and
CX-15. Both types of molecules are perfectly aligned,
lamellae are ™straight∫ and hydrogen bonding at the boundary
between T2 and CX-15 blocks is not possible. Situation 2) -
Phase separation (model Figure 4J): the molecules align in
rows and phase separate, but in such a way that one hydrogen
bond can be formed at the boundary between T2 and CX-15
lamellae. Situation 3) Mixing with and without formation of
hydrogen bonds (model Figure 4 K). Figure 7 shows the STM
can be concluded that both situation 1 and situation 2 are
possible while situation 3 does not occur. For situation 1,
™straight∫ lamellae are formed, limiting the number of
monolayer defects formed, at the expense of hydrogen-bond
formation. For situation 2, one hydrogen bond can be formed
at the domain boundary between a T2 and C8-15 block,
inducing a small lateral offset, which could potentially lead to
an enhanced formation of monolayer defects. The absence of
situation 3 illustrates that both types of molecules do not ™fit∫
well together and that, in absence of hydrogen bonding
™glue∫, phase separation occurs.
Conclusion
This study aimed to investigate the relationship between
molecular structural differences–length, location of func-
tional groups, odd even effects, shape complementarity–
and the two-dimensional phase behavior.
As expected, phase separation is promoted by an increase
in the difference in molecule length while randomly intermix-
ing is optimal when the length of both components is identical.
In addition, the presence and the location of the hydrogen-
bonding units in the molecules plays an important role.
Hydrogen bonding can counteract the effect of the difference
in molecule length on the two-dimensional phase behavior.
Although it is not possible to control the size of the
aggregates, it is possible to influence to a large extent the
phase behavior leading to optimal intermixing or phase
separation by paying attention to the possible intermolecular
interactions.
Experimental Section
Materials and methods: All solvents were dried according to standard
procedures. Starting materials were purchased from Aldrich or Acros.
1H NMRspectra were recorded on a Varian VXR-300 spectrometer (at
300 MHz) in TFA ‡ CD3OD (10% v/v), chemical shifts are given in ppm
relative to methanol (d ˆ 3.35). 13C NMRspectra were recorded on a
Varian VXR300 spectrometer (at 75.48 MHz) in TFA ‡ D2O (10% v/v),
chemical shifts are given relative to TFA (d ˆ 154.3). The splitting patterns
in the 1H NMRspectra are designated as follows: s (singlet), d (doublet), t
(triplet), m (multiplet), br (broad). Melting points were measured on Stuart
scientific SMP1 apparatus. Infrared spectra were recorded on a Nexus
FTIRspectrometer. Elemental analyses were carried out in the Micro-
analytical department of the Stratingh Institute, University of Groningen
(The Netherlands).
Figure 7. STM images of the T2/C15-Y mixture. Solid (dashed) lines reflect
the length, measured perpendicular to the ™lamella∫ axis, of T2 (C15-Y)
molecules. A, B) T2/C15-8 mixture. C, D) T2/C15-9 mixture. The solid
arrow refers to ™situation 1∫: see text and model Figure 4I. The dashed
arrow refers to ™situation 2∫: see text and model Figure 4J. The scale bar
measures 4 nm.
images of T2/C8-15 and T2/C9-15 mixed monolayers. Without
any exceptions, complete phase separation is observed,
regardless of the C8 or C9 spacer length. As anticipated, the
molecules stack in rows with alternating ™blocks∫ of T2 and
CX-15. Close inspection of Figure 7 reveals that both
situation 1 (model Figure 4I) and situation 2 (model Fig-
ure 4J) occur. For instance, in the lamella in the center of
Figure 7A and B (solid arrow), it is clear that the urea groups
of the T2 molecules are not in line with the urea groups of the
C8-15 block. For the right lamella in Figure 7A (dashed
arrow), the left row of urea groups of the T2 and C8-15 block
appear to be in line. It is more difficult to distinguish between
both situations for the T2/C9-15 mixture; however, in the
images in Figure 7C and D, urea groups tend to be in line. It
The synthesis of compounds C12-12, C9-12, C6-12, and the bisthiophene
(T2) (Scheme 1) has been reported.[51, 58] The other linear bisurea com-
pounds were synthesized by reaction of a,w-diamines with the appropriate
isocyanates. Diaminooctane and diaminononane are commercially avail-
able. Diaminotetradecane, diaminopentadecane, and diaminohexadecane
were synthesized from the corresponding diols, in a sequence that converts
them to the dibromides,[59] then to the diazides,[60] which were then reduced
to the corresponding diamines.[61]
1-Hexyl-3-[14-(3-hexylureido)tetradecyl]urea (C14-6): Hexylisocyanate
(250 mg, 2 mmol) was slowly added to
a stirred solution of 1,14-
diaminotetradecane (180 mg, 0.8 mmol) in hot toluene (20 mL). An off-
white suspension formed immediately. After stirring for 2 h, the mixture
was poured into diethyl ether, and the product precipitated as a white solid.
After sonication for 1 h, the precipitate was collected by filtration and
washed with diethyl ether. The product could be purified by repeated
precipitation from p-xylene. Yield: 0.3 g (0.6 mmol, 75%); m.p. 163
1204
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Chem. Eur. J. 2003, 9, No. 5

Supramolecular Control of Two-Dimensional Phase Behavior
1198 1206
1658C (decomp); 1H NMR(300 MHz, TFA
‡
CD3OD): d ˆ 2.70 (t,
tunneling current. STM experiments were performed with a Discoverer
scanning tunneling microscope (Topometrix Inc., Santa Barbara, CA)
along with an external pulse/function generator (Model HP8111A), with
negative sample bias. Tips were electrochemically etched from Pt/Ir wire
(80%/20%, diameter 0.2 mm) in a 2n KOH/6n NaCN solution in water.
3J(H,H) ˆ 6.9 Hz, 8H), 1.01 (m, 8H), 0.67(m, 32H), 0.23 ppm (s, 6H);
13C NMR(75.48 MHz, TFA ‡ D2O): d ˆ 152.5, 35.6, 24.4, 22.8, 22.7, 22.6,
22.3, 21.9, 19.7, 19.4, 15.5, 6.0 ppm; IR(KBr): nÄ ˆ 3336, 1614, 1576 cmÀ1
;
C28H58N4O2 (482.79): m/z: 482; elemental analysis (%) calcd: C 69.66, H
12.11, N 11.60; found: C 69.53, H 12.02, N 11.52.
The experiments were repeated in several sessions with different tips to
check for reproducibility and to avoid artifacts. Different settings for the
tunneling current and the bias voltage were used, ranging from 0.3 nA to
1.0 nA and À10 mV to À1.5 V, respectively. All STM images contain raw
data and are not subjected to any manipulation or image processing.
1-Dodecyl-3-[14-(3-dodecylureido)tetradecyl]urea (C14-12): This was syn-
thesized as described for C14-6, starting from 1,14-diaminotetradecane
(200 mg, 0.8 mmol) and dodecylisocyanate (400 mg, 1.9 mmol). Yield:
0.34 g (65%); m.p. 159 1608C (decomp); 1H NMR(300 MHz, TFA
‡
CD3OD): d ˆ 2.71 (t, 3J(H,H) ˆ 6.6 Hz, 8H), d 1.02 (m, 8H), 0.67 (m, 56H),
0.23 ppm (s, 6H); 13C NMR(75.48 MHz, TFA ‡ D2O,): d ˆ 36.1, 25.9, 23.5,
23.3, 22.9, 22.6, 20.4, 16.5, 7.0 ppm; IR(KBr): nÄ ˆ 3336, 1611, 1574 cmÀ1
;
Acknowledgement
elemental analysis (%) calcd for C40H82N4O2 (651.12): C 73.79, H 12.69, N
8.60; found: C 73.60, H 12.64, N 8.37.
The authors thank the DWTC, through IUAP-V-03, the Institute for the
promotion of innovation by Sciences and Technology in Flanders (IWT).
ESF SMARTON made the Leuven Groningen collaboration possible.
S.D.F. is a postdoctoral fellow of the Fund for Scientific Research-Flanders.
J.v.E. gratefully acknowledges the Royal Academy of the Netherlands for a
fellowship. M.L. is an Erasmus student from Karlstad University (Sweden).
B.V. is an Erasmus student from University of Groningen (The Nether-
lands).
1-Dodecyl-3-[15-(3-dodecylureido)pentadecyl]urea (C15-12): This was
synthesized as described for C14-6, starting from 1,15-diaminopentadecane
(200 mg, 0.8 mmol) and dodecylisocyanate (400 mg, 1.9 mmol). Yield:
0.38 g (71%); m.p. 162 1648C (decomp); 1H NMR(300 MHz, TFA
‡CD3OD): d ˆ 2.71 (t, 3J(H,H) ˆ 6.6 Hz, 8H), 1.02 (m, 8H), 0.67 (m,
58H), 0.22 ppm (s, 6H); 13C NMR(75.48 MHz, TFA ‡ D2O): d ˆ 152.7,
35.7, 23.0, 22.9, 22.8, 22.4, 22.1, 19.9, 16.0, 6.4 ppm; IR(KBr): nÄ ˆ 3336, 1611,
1574 cmÀ1; C41H84N4O2 (665.14): m/z: 665.8; elemental analysis (%) calcd C
74.04, H 12.73, N 8.42; found: C 73.88, H 12.69, N 8.45.
1-Dodecyl-3-[16-(3-dodecylureido)hexadecyl]urea (C16-12): This was syn-
thesized as described for C14-6, starting from 1,16-diaminohexadecane
(200 mg, 0.8 mmol) and dodecylisocyanate (400 mg, 1.9 mmol). Yield:
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430 mg (78%); m.p. 164 1658C (decomp); 1H NMR(300 MHz, TFA
‡
CD3OD): d ˆ 2.71 (t, 3J(H,H) ˆ 6.6 Hz, 8H), 1.02 (m, 8H), 0.67 (m, 60H),
0.23 ppm (s, 6H); 13C NMR(75.48 MHz, TFA ‡D2O): d ˆ (152.7), 35.5,
25.2, 22.9, 22.8, 22.4, 22.3, 22.1, 19.9, 16.0, 6.3 ppm; IR(KBr): nÄ ˆ 3331,
1612, 1569 cmÀ1; elemental analysis (%) calcd for C41H84N4O2 (679.17): C
74.28, H 12.76, N 8.25; found: C 74.09, H 12.81, N 8.13.
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1-Isocyanatopentadecane: Palmitoyl chloride (5 g, 0.018 mol) was dis-
solved in p-xylene (40 mL). Sodium azide (1.5 g, 0.023 mol) was added. The
reaction mixture was heated at reflux for 2 h under a continuous flow of
nitrogen, after which conversion to the isocyanate was complete. The hot
reaction mixture was filtered to remove insoluble residue. The solvent was
removed in vacuo to yield the product as a semisolid material. Yield: 4.5 g
(0.018 mol, 99%); 1H NMR(300 MHz, CDCl 3): d ˆ 3.22 (t, 3J(H,H) ˆ
6.6 Hz, 2H), 1.55 (m, 2H), 1.21 (m, 24H), 0.82 ppm (t, 3H); 13C NMR
(75.48 MHz, CDCl3, TMS): d ˆ 134.0, 42.1, 31.2, 30.5, 29.9, 29.8, 29.4, 28.8,
26.4, 22.1, 20.2, 16.0, 13.5 ppm.
1-Pentadecyl-3-[8-(3-pentadecylureido)octyl]urea (C15-8): This was syn-
thesized as described for C14-6, starting from 1-isocyanato-pentadecane
(0.4 g, 1.6 mmol) and 1,8-diaminooctane (100 mg, 0.7 mmol) in hot toluene
(20 mL). Yield: 380 mg (85%); m.p. 167 1698C (decomp); 1H NMR
(300 MHz, TFA ‡ CD3OD): d ˆ 2.70 (t, 3J(H,H) ˆ 6.6 Hz, 8H), 1.02 (m,
8H), 0.66 (m, 56H), 0.22 ppm (s, 6H); 13C NMR(75.48 MHz, TFA ‡ D2O):
d ˆ (152.7), 36.3, 26.1, 23.7, 23.6, 23.5, 23.4, 23.1, 22.9, 22.8, 20.6, 20.5,16.7,
7.3 ppm; IR(KBr): nÄ ˆ 3336, 1611, 1576 cmÀ1; C40H82N4O2 (651.12): m/z:
651.6; elemental analysis (%) calcd: C 73.79, H 12.69, N 8.60; found: C
73.41, H 12.57, N 8.35.
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1-Pentadecyl-3-[9-(3-pentadecylureido)nonyl]urea (C15-9): This was syn-
thesized as described for C14-6, starting from 1-isocyanato-pentadecane
(0.35 g, 1.4 mmol) and 1,9-diaminononane (100 mg, 0.65 mmol) in hot
toluene (20 mL). Yield: 390 mg (90%); m.p. 161 1638C; 1H NMR
(300 MHz, TFA ‡ CD3OD): d ˆ 2.72 (t, 3J(H,H) ˆ 6.6 Hz, 8H), 1.03 (m,
8H), 0.67 (m, 58H), 0.24 ppm (s, 6H); 13C NMR(75.48 MHz, TFA ‡ D2O):
d ˆ (153.1), 35.9, 25.8, 23.5, 23.4, 23.3, 23.2, 23.1, 22.8, 22.7, 20.2,16.4,
7.0 ppm; IR(KBr): nÄ ˆ 3336, 1611, 1574 cmÀ1; C41H84N4O2 (665.14): m/z:
665.6; elemental analysis calcd: C 74.04, H 12.73, N 8.42; found: C 73.68, H
12.75, N 8.39.
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1206
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Chem. Eur. J. 2003, 9, No. 5