Synthesis of compounds presenting three and four anthracene units as potential connectors to mediate infinite lateral growth at the air/water interface

Article abstract of DOI:10.1002/chem.200800478

We report the synthesis of molecular sheets based on the photochemically initiated dimerization of monomers with lateral anthracene units. The film thickness and composition were investigated by ellipsometry and X-ray photoelectron spectroscopy (XPS). The

Full text of DOI:10.1002/chem.200800478

FULL PAPER
DOI: 10.1002/chem.200800478
Synthesis of Compounds Presenting Three and Four Anthracene Units
as Potential Connectors To Mediate Infinite Lateral Growth
at the Air/Water Interface
Cindy Mꢀnzenberg,[a] Antonella Rossi,[a, b] Kirill Feldman,[a] Reto Fiolka,[c]
Andreas Stemmer,[c] Katarzyna Kita-Tokarczyk,[d] Wolfgang Meier,[d] Junji Sakamoto,[a]
Oleg Lukin,[a] and A. Dieter Schlꢀter*[a]
Abstract: We report the synthesis of
molecular sheets based on the photo-
chemically initiated dimerization of
monomers with lateral anthracene
units. The film thickness and composi-
tion were investigated by ellipsometry
and X-ray photoelectron spectroscopy
(XPS). The mechanical stability of the
film was sufficient to span it over 45ꢀ
45 mm-sized holes. Several model reac-
tions were performed to illustrate the
underlying chemistry and to assist in
analysis. The reported experiments are
considered first steps towards the ulti-
mate goal of the rational synthesis of
laterally “infinite”, one-monomer-unit-
thick molecular sheets with a long-
range positional order and a periodic
covalent-bonding pattern. Such sheets
are referred to as 2D polymers and are
considered a prime goal of chemical
synthesis with intriguing applications.
Keywords:
calixarenes
anthracenes
photodimerization
·
·
·
polymers · unimolecular films
Introduction
form and cease to be considered as single stranded. Because
the segments between the netpoints have a length distribu-
tion they cannot be regarded as ordered 2D or 3D structures
either. The creation of useable, “infinitely” extended, cova-
lently constructed and structurally defined, periodic 2D
polymers has been a dream of chemists for decades and nu-
Practically all bio- and synthetic polymers known today con-
sist of single-stranded, covalent backbones. The indispensa-
ble functions of many for both life in general and in the
daily life of every human being result from their chemical
structure and how these macromolecules arrange into
higher-order structures. For some applications, polymers
both as ultrathin film and in bulk are used in a networked
merous attempts towards this goal have been reported.[1]
A
non-comprehensive list of relevant articles can be found in
the literature.[2–7] Though enormous progress has been made,
even today no synthetic polymer is known that meets all the
above criteria.[8] The mastering of the very high level of
structure control inherently associated with a realization of
such a goal can be considered a major driving force for our
work in this area. Also, given the structural novelty of peri-
odic 2D polymers, the exploration of their property space is
another major motivation. We have initiated projects aiming
at achieving the above goal and wish to disclose some initial
steps here. These steps are based on amphiphilic monomers
capable of forming stable Langmuir monolayers at the air/
water interface and of undergoing subsequent polymeri-
zation in the compressed state. The monomers presented
here are based on three-armed branched molecules of kind
A and calixarenes of kind B (Figure 1) with three and four
pendent anthracenes, respectively. They are designed so as
to exploit the well-studied photochemically induced anthra-
cene dimerization[9] as the key reaction ideally resulting in
[a] Dipl.-Chem. C. Mꢁnzenberg, Prof. Dr. A. Rossi, Dr. K. Feldman,
Dr. J. Sakamoto, Dr. O. Lukin, Prof. Dr. A. D. Schlꢁter
Department of Materials, Institute of Polymers
HCI 541, ETH Zꢁrich, 8093 Zꢁrich (Switzerland)
Fax : (+41)44-633-1395
E-mail: oleg.lukin@mat.ethz.ch
dieter.schluter@mat.ethz.ch
[b] Prof. Dr. A. Rossi
Department of Inorganic and Analytical Chemistry
University of Cagliari, 09100 Cagliari (Italy)
[c] R. Fiolka, Prof. Dr. A. Stemmer
Professur fꢁr Nanotechnik, ETH Zꢁrich
8092 Zꢁrich (Switzerland)
[d] Dr. K. Kita-Tokarczyk, Prof. Dr. W. Meier
Department of Chemistry, University of Basel
Klingelbergstr. 80, 4056 Basel (Switzerland)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200800478.
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the desired lateral polymerizations. These monomers serve
to prove the principle, but may not yet have the optimum
molecular structures for the achievement of completely reg-
ular (periodic) 2D polymers and, perhaps even more impor-
tantly, the polymersꢃ unequivocal structure proof.[1]
Scheme 1. Benzotriazol-1-yl-oxytripyrrolidinophosphonium hexafluoro-
phosphate (PyBOP), N,N-diisopropylethylamine (DIPEA), CH2Cl2,
DMF.
prepare a small collection of tripods with different polar
groups so as to explore which specific structure shows the
best spreading behavior at the air/water interface.
Schemes 2–4 summarize the steps taken. The attachment of
Figure 1. Schematic representation of monomer structures discussed in
this work. Rectangles in both structures indicate hydrophilic groups that
should help render the monomers spreadable at the air/water interface.
We describe the synthesis of some representatives of the
compound families A and B and their spreading behavior at
the air/water interface. Of those forming stable and reversi-
bly compressible monolayers, one was selected for the next
step, the photochemical treatment of its compressed mono-
layer. This treatment led to a covalent connection between
the individual monomer units so as to render the layer a me-
chanically stable entity. The transfer of the untreated mono-
layer onto a solid substrate (oxidized silicon wafers) and the
subsequent analysis of its composition and orientation on
the substrate by the angle-resolved X-ray photoelectron
spectroscopy (ARXPS) is discussed, as well as the determi-
nation of its thickness by ellipsometry and XPS. We also de-
scribe how the monolayer, photochemically treated at the
interface, was analyzed in terms of thickness and mechanical
stability by using ellipsometry (after transfer) and scratching
experiments, respectively. The important aspect of stability
was addressed by transferring the treated monolayer onto
an electron microscopy grid with 2025 mm2-sized holes to see
whether they could be spanned by this film. Finally, we pres-
ent model studies on photodimerizations of some relevant
anthracene derivatives, mainly to help analytical aspects,
which will be important for future work.
Scheme 2. i) KOH, MeOH/THF, H2O; ii) KOH, MeOH/THF, H2O.
the polar units 6b, 7, 8, and 9b proceeded best by using
esterification with the 4-(dimethylamino)-pyridinium 4-tol-
uenesulfonate (DPTS)/1-ethyl-3-(3-dimethylaminopropyl)-
carbodiimide (EDC) system (see Supporting Information).
This afforded the corresponding monomers 10–12 and 13b,
respectively, in yields of 30–50%. In addition to these tri-
pods, calix[4]arene-based tetrapods were also synthesized.
Calixarenes 16a and 16b, representing group B monomers,
were prepared from starting materials 14a and 14b by react-
ing them with anthracene-9-carbaldehyde to furnish the
Schiff bases 15a and 15b, whose reduction with NaBH4 gave
the desired compounds 16a and 16b on the 100-mg scale.
The purification of the intermediates 15 turned out to be
somewhat problematic in that they quickly hydrolyzed on
column. This problem could be overcome, however, by
Results and Discussion
Monomer synthesis: The synthesis of potential group A
monomers started from the commercially available 9-anthra-
cene carboxylic acid 1, which was reacted with the trisamino
hydroxyl compound 2 to give alcohol 3 in 50% yield and on
the 3-g scale (Scheme 1). Product 3, referred to in the fol-
lowing as tripod, carries three anthracene units for dimeriza-
tion reactions and a free hydroxyl function and was used to
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Infinite Lateral Growth at the Air/Water Interface
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reochemical course, reversibility,[10] and accessible yields.
Comprehensive reviews on this matter are available.[11] Be-
sides simple derivatives, complex compounds containing an-
thracene units were also investigated. This involved the use
of anthracenes as monomer units in polymer synthesis,[12] as
cross-linking units in polymer network formation,[13] as well
as in dendrimers[14] and other topologically unusual com-
pounds.[15] By and large, dimerizations take place across the
9,10-positions and in the case of 9-substituted derivates,
head/tail dimers are the preferred products, though excep-
tions for both behaviors have been described.[16] Yields of
the photodimerization range typically from 40–70%.
Though this may be considered insufficient for the ambi-
tious goal described in the introduction, it should be pointed
out that these figures refer to products formed under homo-
genous solution conditions. If photodimerization is applied
to compressed monolayers in which the monomers are at
tight, ideally almost van der Waals distance, higher yields
for the coupling are to be expected. This is supported by re-
ports that yields may go up for dimerizations of anthracenes
in constrained geometries.[17] Scheme 5 shows the dimeriza-
tions that were studied in the frame of the present research.
Scheme 3. i) Pd/C, H2, EtOAc/MeOH.
Scheme 4. i) Anthracene-9-carbaldehyde, dry MeOH; ii) NaBH4, MeOH.
adding 5% triethylamine to the eluent. All new compounds
were fully characterized.
Model photodimerizations: The photodimerization of an-
thracene and its derivatives has been studied in great
detail,[9] including experimental conditions, regio- and ste-
Scheme 5. Test and model studies performed with tripod 3 and the litera-
ture-known anthracenes 18–20.
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A. D. Schlꢁter, O. Lukin et al.
The question of how many of the functional groups (an-
thracenes) of the several oligofunctional compounds synthe-
sized in the present study can actually be engaged in dimeri-
zations was addressed using tripod 3. This has the least com-
plicated substitution pattern of all compounds and it was,
therefore, expected that work-up and purification of the
model reactionsꢃ products would be easier than for those
representatives with flexible alkyl and oligoethyleneoxy
(OEO) chains. At first, compound 3 was irradiated with a fi-
vefold excess of parent anthracene per anthracene function-
ality in dichloromethane at 208C for 12 h. Product 17 was
isolated by column chromatography and obtained reproduci-
bly as colorless solid in yields of 45–50%. Other products
were formed, but these were neither isolated nor identi-
fied.[18]
In a future stage of the project it will be important to
prove that anthracene dimerization takes place at the air/
water interface when spread and compressed monolayers
are irradiated. Because under such conditions just a very
small quantity of molecules is involved, the analytical tools
applied need to account for this. Grazing incidence IR spec-
troscopy (GI-IR) is a powerful tool for monolayer character-
ization and it was thus a prime task to obtain IR spectra of
relevant compounds as a reference for these future studies.
The top part of Figure 3 compares sections of the IR spectra
The structure proof for 17 is based upon NMR spectros-
copy and mass spectrometry results, but also the correct
data from elemental analysis. Figure 2a and b compare its
solution 13C NMR spectrum with starting material 3. Espe-
cially indicative are the new signals at approximately d=
142, 66, and 55 ppm which correspond to the ortho-phenyl-
ene units and the bridgehead carbons, respectively. Thus, all
three anthracene moieties of 3 were individually engaged in
dimerization reactions and it was thus assumed that the
other tri- and tetrapods would behave similarly.
Figure 3. IR spectra in KBr of the four anthracenes 3, 18, 19, and 20 and
their dimerization product(s) 17 and C, dim-18, dim-19, as well as ht dim-
20 and hh dim-20, respectively. Despite the rather different substitution
patterns, all dimers show two signals at or near 1454 and 1474 cmÀ1
.
(in KBr) of starting compound 3 with 17. Evidently, the di-
merization is associated with the appearance of absorptions
at 1454 and 1474 cmÀ1, as was already reported for hydrocar-
bon cages closely related to the one contained in 17.[11,19] Be-
cause of the anticipated importance of this analytical aspect,
three other anthracene derivatives, 18–20, were also synthe-
sized (see Experimental Section) and photochemically di-
merized into their head/tail (ht) dimers [dim-18 (for an X-
ray structure, see Supporting Information), dim-19] and iso-
Figure 2. Solution 13C NMR spectra of starting tripod 3 (a) and its three-
fold anthracene adduct 17 (b), and solid-state cross polarization magic
angle spinning (CPMAS) 13C NMR spectrum of network C formed by
the tripodꢃs self-addition reaction (c).
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Infinite Lateral Growth at the Air/Water Interface
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meric head/head (hh) and ht dimers for dim-20.[20] This latter
mixture was separated into the individual components. The
respective IR spectra are also compiled in Figure 3. As can
be seen, all dimers exhibit these two signals at almost the
same wave numbers to the ones above. This applies also to
the two different hh and ht stereoisomers of dim-20, which
is somewhat unfortunate. It was hoped that GI-IR spectros-
copy would enable extraction of information not only about
whether or not dimerization has taken place in a monolayer
at the air/water interface, but also about the stereochemistry
of the product.[21,22] The UV spectrum of 17 confirmed the
complete disappearance of all anthracene-typical absorp-
tions (Figure 4a).
much like model compound 17. Network C also shows an-
thracene signals in the UV range (measured in reflection)
indicative of unreacted (end) groups (Figure 4b). The inten-
sity of these signals was not quantified. In addition, the IR
spectrum exhibits the characteristic signals at 1454 and
1474 cmÀ1. Thus, the expected network had formed at least
for the major part of the material.
The reactivity of compound 16a under irradiation was
also briefly investigated using UV spectroscopy. The out-
come of these reactions depended strongly on concentration.
For highly diluted dichloromethane solutions (0.1 mm) a
soluble product was obtained that gave the UV spectrum
depicted in Figure 4c. The anthracene-typical resonances
had disappeared, suggesting that under these highly diluted
conditions a twofold intramolecular dimerization took place.
Such reactions have been reported for simpler systems with
just two anthracene units.[23] At much higher concentrations
(125 mm) a precipitate formed, whose UV spectrum (Fig-
ure 4d) was taken in reflection. As for the self-addition of
compound 3, some residual anthracene signals were detect-
ed and tentatively ascribed to end groups.
Experiments and analyses at air/water and air/solid
interfaces
Non-polymerized monolayers: The ability of a compound to
form stable Langmuir monolayers depends on its solubility
and the amphiphilic (hydrophilic-to-hydrophobic) balance.
Film formation was tried with compounds 3, 10–12, 13b, and
16a using conventional techniques (see Experimental Sec-
tion). Compound 3 (hydrophobic) formed films, however,
these were not reversible and rather crystalline; compounds
10–12 (hydrophilic) did not spread but rather submerged
into the subphase. Compounds 13b and 16a formed stable,
compressible films with different reversibilities, with that of
16a being superior. The surface-pressure/-area isotherms of
both compounds are depicted in Figure 5. The full isotherm
Figure 4. UV spectra in chloroform comparing tripods 3 and 17 (a),
tripod 3 and network C (b), tetrapod 16a and the product of its irradia-
tion in highly diluted (0.1 mm) (c), and highly concentrated (125 mm)
CH2Cl2 solutions (d).
Next, the reaction of 3 with itself was studied. This was
necessary because this self-addition involves only monosub-
stituted anthracenes (at C-9) rather than a monosubstituted
one and the parent anthracene. This could result in a re-
duced propensity to undergo self-dimerization. It was ex-
pected that if dimerization worked, an insoluble network
should form, the formula C of which (Scheme 5) for simplic-
ity considers neither eventual hh dimers nor incomplete re-
actions. A 10-mm dichloromethane solution of 3 was irradi-
ated at 208C for 12 h under nitrogen during which time a
precipitate formed. This was filtered off (approx. 77% of
total mass) and the supernatant solution was concentrated
up to give a complex mixture of compounds (approx. 23%
of total mass). The precipitateꢃs solid-state 13C NMR spec-
trum (Figure 2c) shows signals at approximately d=144 ppm
as well as d=67 and 55 ppm that correspond to the ortho-
phenylene units and the bridgeheads, respectively, very
Figure 5. Surface-pressure/-area isotherms for monomers 13b (a) and 16a
(b) at the air/water interface. Arrows indicate compression and expansion
isotherms.
of 16a can be found in the Supporting Information (Fig-
ure S2). On the basis of these curves, monomer 16a was se-
lected for the initial polymerization experiments. First the
area per molecule was estimated. For this purpose a tangent
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A. D. Schlꢁter, O. Lukin et al.
was drawn near the surface pressure of 25 mNmÀ1, which in-
tersected the x-axis at approximately 2.2 nm2. An alternative
value was obtained by measuring the area at this pressure,
which was found to be 1.85 nm2. From the two extreme con-
formations of stick models shown in Figure 6, the expected
Figure 8. Brewster angle microscopy images of film formation during in-
crease in surface pressure. The rightmost image indicates a rather smooth
monolayer that represents the state at which transfers and irradiations
were performed. The size of the images is 430 mmꢀ498 mm.
Although the isotherm indicated that at 25 mNmÀ1 the film
of 16a should still be a monolayer, this was confirmed by el-
lipsometry measurements of Langmuir–Blodgett layers
transferred on oxidized silicon wafers; each time, a single
layer was transferred upon the dipper upstroke. These meas-
urements were carried out at three points and representative
results are compiled in Table 1 (entries 1–3). The average
Figure 6. Molecular models of the two extreme conformations of mono-
mer 16a with the four anthracene units in upright (left) and horizontal
(right) orientations, respectively. The areas covered by these two con-
formers differ significantly between approximately 1.0 and 4.8 nm2.
value should be somewhere between 1.0 and 4.8 nm2. An in-
termediate value is suggested by the X-ray structure of com-
pound 16b (Figure 7). This compound was chosen because it
Table 1. Thickness [ꢄ] of transferred monolayers of 16a.
Entry
Vertical transfer
Horizontal transfer
Monolayer
UV-treated
Monolayer
UV-treated
monolayer
monolayer
1
2
3
4
18
19
19
19Æ1
20
20
20
20Æ1
17
18
17
17Æ1
21
20
21
21Æ1
height of the film was found to be in the order of (19Æ1) ꢄ
(Table 1, entry 4, left column). This value corresponds well
with the height of the individual molecule 16a, assuming
that its flexible chains are somewhat contracted and not in
the all-trans conformation.[24] This assumption is reasonable
considering the fact that both the oligoethyleneoxy (OEO)
chains and the substrate are polar. The film height was also
checked by Langmuir–Schꢅfer transfer, which gave a slightly
smaller value of (17Æ1) ꢄ (Table 1, entry 4, third column).
Next, XPS measurements were performed aiming at com-
positional, structural, and orientation information of this
monolayer. Figure 9 shows the survey spectrum collected of
a monolayer of 16a transferred onto a silicon substrate.
Carbon, oxygen, and nitrogen signals are detected together
with the Si2s and Si2p that are due to the substrate. The ni-
trogen signal, N1s, indicates a successful transfer of the
monolayer as this element is the only one that is exclusively
present in the monomer. Because no other signals are de-
tected, it can also be concluded that the monolayer is free
of contamination. Detailed spectra of C1s, O1s, N1s, and
Si2p were also taken (Figure S3) and the peak areas used to
calculate atomic concentrations and monolayer thickness.
The Si2p spectrum exhibits two signals, one at 99.4 eV and
another one at 103.4 eV. The first signal is due to the parent
Figure 7. X-ray molecular structure of calix[4]arene 16b, which serves as
a model for 16a, showing that in the crystal this compound attains an
area between the two extreme models for 16a shown in Figure 6.
was easier to grow single crystals from it than from 16a and
the anthracene geometry should not be affected much by
the substitution at the lower rim. It is apparent that the rela-
tive orientation of the anthracene moieties in 16a at the in-
terface needs to be different from that of the same units in
the single crystal of 16b, if a high degree of intermolecular
[4+4] dimerizations is to be brought about.
The film formation upon increasing surface pressure was
monitored by Brewster angle microscopy (BAM). As can be
seen from the representative snapshots depicted in Figure 8,
for low pressures homogenous areas are initially separated
by less-ordered stripes. However, at higher pressure (typi-
cally 25 mNmÀ1), a fully homogenous film of 16a was ob-
served, which was used for all transfers and also irradiations.
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this calculation: a more appro-
priate model, such as the three-
layer model, would require
knowledge of the monolayer
density.[27]
Finally, angle-resolved XPS
measurements were carried out
on 16a to get an insight into
the monolayerꢃs orientation.
Figure 9 shows the plot of the
atomic concentration vs. the
emission angle, whereby the
latter is the one formed by the
direction of the emitted elec-
trons with the surface normal.
This plot suggests that the C-O-
Figure 9. Survey XPS spectrum of a non-polymerized monolayer of 16a transferred onto an oxidized silicon
wafer (left), and apparent atomic concentration vs. emission angle of non-polymerized monolayer of 16a trans-
ferred onto an oxidized silicon wafer (right). The apparent concentration is the concentration assuming a ho-
mogenous compound within the analyzed depth. If the concentration of an element increases as emission
angle increases (e.g., carbon) then it is mainly present at the outer part of the analyzed volume (data acquired C units of the monomers are in
with the Theta Probe Thermo, Fisher).
the inner part of the film
whereas the carbon species
seem to be located in the outer
substrate and the second can be assigned to silicon diox-
ide.[25]
part of the monolayer. Thus, the monolayer has an orienta-
tion as one would have intuitively suggested.
The O1s spectrum contains the contributions of both the
silicon dioxide at 533.1 eV and the monolayer of monomer
16a. In fact, the signal that is symmetric in the case of pure
silicon dioxide shows here a non-symmetry at the lower-
binding-energy side due the presence of oxygen signals of
the monolayer. The components at 532.6 and 533.3 eV were
identified by least-square peak-synthesis routine using the
fitting parameters obtained on the basis of reference sam-
ples: the untreated monolayer and the drop-cast compound
analyzed using the same spectrometer settings. These were
assigned to the C-O-C chains and to the Ar–O, respectively.
These values are in agreement with the literature.[26] The
C1s signal is also non-symmetric as it contains the signals
due to the aromatic rings (284.7 eV) of the calixarene, the
anthracenes (285.5 eV), and the C-O-C units at 286.7 eV[26]
The binding-energy value of the anthracene units of the
monolayer is lower than that of parent anthracene (not
shown here); this shift is attributed to the presence of the
donor substituents in the former.
Polymerized monolayer: With this information at hand, the
first irradiation experiments were performed. A monolayer
of 16a was compressed to 27 mNmÀ1 and irradiated for
30 minutes with unfiltered UV light of a medium-pressure
mercury lamp that was positioned above the film at a dis-
tance of approximately 20 cm. This distance was kept con-
stant throughout all irradiations described in this paper. The
whole trough was placed inside a box, which allowed appli-
cation of gentle nitrogen flow during the entire irradiation
time to exclude peroxide formation. First, it was tested
whether irradiation has any effect on the appearance of the
BAM images. Figure 10 compares images before and after
irradiation. As can be seen, there was no difference at all,
even though the image of the treated film was taken after
the barriers had been opened. To test whether cross-linking
had actually taken place, both the treated and non-treated
monolayers were scratched with a needle and the responses
compared.
The N1s signal is symmetric at 400.4 eV. This value is
3.4 eV higher than that measured on the pure drop-cast
compound, suggesting that the nitrogen atom might be
bonded to water molecules. The atomic composition of the
monolayer was based on the line intensities determined by
integration of the fitted Gaussian/Lorentzian model func-
tions after a Shirley background subtraction and corrected
for the photoionization cross-sections, the asymmetry func-
tion, the mean free path, and the transmission function of
the spectrometer. The values (%) found: C 82.5, N 2.7, O
14.8, are in good agreement with the calculated values (%):
C 83.8, N 2.7, O 13.5. The monolayer thickness was calculat-
ed to be (26Æ0.1) ꢄ, which is significantly larger than the
values obtained by ellipsometry, even if it is still in agree-
ment with the assumption of a monolayer. The difference
might be due to the fact that the density does not feature in
Both responses were monitored by BAM. Whereas with-
out irradiation the “wounds” healed instantaneously (not
shown), those in the treated film remained completely un-
changed, even if the barrier was opened. This proves the
non-treated film to behave like a fluid and the treated one
to be covalently cross-linked. It should be stressed that
these experiments do not enable conclusions to be drawn re-
garding how many of the anthracenes were actually involved
and whether a periodic structure was formed. These impor-
tant questions are presently being pursued.[19] Next, the irra-
diated film was transferred onto silica and the height mea-
surement repeated. As can be seen from Table 1, the values
are (20Æ1) ꢄ for the vertical and (21Æ1) ꢄ for the horizon-
tal transfers. Clearly, the irradiation process did not change
the monolayer nature of the film, though there may be
some thickening.
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A. D. Schlꢁter, O. Lukin et al.
Figure 10. BAM images of non-irradiated (top left) and UV-irradiated
monolayer of 16a at the air/water interface (top right) indicating that ir-
radiation does not cause morphological changes and crack formation visi-
ble by BAM. BAM images of UV-irradiated monolayers of 16a after
scratching with a needle (bottom left and right). The “wounds” stay un-
changed even if the film is decompressed. The size of the images is
430 mmꢀ498 mm.
Figure 11. Light-microscopy images of transferred UV-treated monolay-
ers of monomer 16a on a Cu grid with holes sized 45ꢀ45=2025 mm2.
The top image shows a case considered unsuccessful because film rupture
could not be prevented, the bottom image represents a successful case
because many holes are over spanned by non-ruptured films. The arrows
point towards cracks.
The transferred and cross-linked monolayers were also in-
vestigated by contact-angle measurements with water. From
the XPS measurements it was expected that this layer
should show the anthracene units at the top to be at least
partially dimerized. Contact angles for self-assembled mono-
layers (SAMs) with aromatic units at their surface are
known to be in the order of 80–858.[28] Measurements on our
films directly after transfer reproducibly gave contact angles
between 32–458. These values showed considerable scatter
and were clearly too low. However, after drying the samples
under high vacuum, the values were reproducibly between
82–888 and, thus, well within the expected range. This sub-
stantial change of contact angle upon drying is explained by
adsorbed water, which in the non-dried samples is abundant-
ly available (OEO chains!) and can reach the surface
through the many pores necessarily present in the cross-
linked film.
non-cross-linked film was transferred onto such a grid, how-
ever, in none of many attempts was it possible to cover
holes, not even for small parts. Cross-linking is thus a pre-
requisite for such experiments with films from 16a. The
transferred films are stable for at least several days under
ambient conditions. The differential interference contrast
(DIC) micrograph (Figure 12) shows a film five days after
transfer, with no visible change detected. Some of the grid
holes are covered (right-hand side) and some are not (left-
hand side). The holes in the center are (still) partially cov-
ered; due to rupture, the film in part folds back on itself,
wrinkles, and shows a tendency to roll up at the edge.
To test the mechanical stability of the cross-linked mono-
layer, transfers on electron microscopy grids with holes
sized 45ꢀ45 mm were performed to see whether they can be
covered with films without rupture. The literature describing
ultrathin layers over-spanning holes is scarce.[29] Attempts
with Langmuir–Blodgett transfer were unsuccessful, howev-
er, the horizontal transfer (Langmuir–Schꢅfer method) was
successful. However, it should be noted that not every trans-
fer was actually successful; only approximately every fifth
transfer gave usable results. Figure 11 compares an unsatis-
factory and a rather successful transfer. The images were
taken with a conventional optical microscope and the con-
trast is based on interference. In the first case, practically all
films over holes are ruptured. Note that the direction of the
cracks is more or less the same, which seems to indicate that
during coverage a stress was applied to the entire film while
near the grid. To say this crack formation is associated with
drying effects is merely speculation. In a blind test, also the
Figure 12. DIC image (ꢀ40) of a cross-linked monolayer of 16a taken
5 days after the transfer event. The film is on a Cu grid with 2025 mm2-
sized holes, some of which are spanned over by non-ruptured films (holes
on the right-hand side) and others are not at all (holes on the left-hand
side). The holes in the center are partially covered by films and overlay-
ing parts, wrinkles, and rolled-up edges are visible.
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Infinite Lateral Growth at the Air/Water Interface
FULL PAPER
Langmuir–Blodgett and Langmuir–Schꢅfer transfer were performed ac-
cording to the protocol.[30] For the Schꢅfer transfer a Cu grid purchased
from Plano with a pitch of 45 mm was used.
Conclusion
Monomer 16a, decorated with four anthracene units,
spreads reversibly at the air/water interface, and after com-
pression to 25 mNmÀ1 can be photochemically cross-linked
into a sheet-like object, whose mechanical stability is suffi-
cient to allow transfer onto both a solid substrate and a Cu
grid with 2025 mm2 sized holes. These holes can actually be
spanned over, whereby the films remain unchanged under
ambient conditions at least for several days. The sheets have
the thickness of a monolayer and two different faces, one
that consists of more or less dimerized anthracene units and
the other of oligoethyleneoxy chains. Thus, the sheets are
amphiphilic. Based on these results the next steps involve
1) proving the level of order in the sheet by grazing inci-
dence X-ray diffraction and IR spectroscopy, the near field
microscopies, as well as other techniques, 2) design and test-
ing of other monomers that may have an even higher
chance to lead to sheets with long-range positional order
and periodic bonding patterns,[1] 3) testing properties such as
rupture forces.
Brewster angle microscopy: Monolayer morphology was visualized by
using a BAM2plus Brewster angle microscope (Nanofilm, Germany),
with a 50-mW laser at the wavelength of 532 nm. With a 10ꢀ Nikon
long-distance objective, the microscope has a resolution of 2 mm; record-
ed images correspond to 430 mm in width.
Ellipsometry: The dry thicknesses of the films were determined by varia-
ble angle spectroscopic ellipsometry (VASE, M-2000F, L.O.T. Oriel
GmbH, Germany). Measurements were conducted under ambient condi-
tions at three angles of incidence (65, 70, and 758) in the spectral range
of 370–995 nm. Measurements were fitted with the WVASE32 analysis
software using a three-layer model for an organic layer on a silicon sub-
strate.
X-ray photoelectron spectroscopy: Small-area X-ray photoelectron spec-
tra (XPS) and angle-resolved XPS (ARXPS) were acquired using a
Theta Probe (Thermo Fisher Scientific, Waltham MA, USA). The residu-
al pressure during the analysis was 10À7 Pa. The measurements were car-
ried out using an AlKa (1486.6 eV) radiation source run at 70 W, 400-mm
beam diameter. The emitted electrons were collected by using a radian
lens with a conical angle of acceptance of ca. 38. The emission angle
ranges from 23 to 838. The spectra of O1s, C1s, N1s, Si2s, and Si2p were
obtained in constant analyzer transmission mode with a pass energy of
50 eV and a step size of 0.1 eV (full width at half maximum height,
FWHM, for Ag3d5/2 =0.7 eV). The angular information was summed into
16 channels.
The survey spectra were acquired with a pass energy of 100 eV. The in-
strument was calibrated using the inert-gas-ion-sputter cleaned reference
materials SCAA90 of Cu, Ag, and Au.10 following ISO 15472. Sample
charging was corrected by referring all binding energies to the carbon 1s
signal at 285.0 eV.
Experimental Section
General: The elemental analyses were performed using a Leco CHN-900
or Leco CHNS-932 instrument. The melting points were measured in
open capillaries by using a Bꢁchi B-540 and were uncorrected. MALDI-
TOF mass spectra were recorded by using an electron-ionization (EI)
MS spectrometer (Micromass AutoSpec-Ultima) or a FTMALDI MS
Contact-angle measurements were carried out by using a Ramꢆ-hart, Inc.
NRL C.A. Goniometer (Model No 100–00–230). All reported values are
the average contact angles at three different spots per sample determined
by sessile drop technique.
1
spectrometer (IonSpec Ultra Instrument). H- and 13C NMR spectra were
recorded by using Bruker Avance 300 and 500 MHz spectrometers at RT
in the indicated deuterated solvents, which were purchased from Merck
or Deutero GmbH. The resonance multiplicities in the 1H NMR spectra
are described as s (singlet), d (doublet), t (triplet), and m (multiplet).
Broad resonances are indicated by br. Reactions were monitored by TLC
using TLC silica-coated aluminium sheets Alugram by Macherey–Nagel
(SIL-G/UV254). The compounds were visualized by applying 254 or
366 nm UV light. Column chromatography purifications were performed
with silica gel, BIO-RAD Bio-Beads S-X1 [200–400 mesh], or prepara-
tive recycling GPC (Japan Analytical Industry Co. Ltd., LC 9101)
equipped with a pump (Hitachi l-7110, flow rate 3.5 mLminÀ1), a degas-
ser (GASTORR-702), a RI detector (Jai RI-7), a UV detector (Jai UV-
3702, l=254 nm), and two columns (Jaigel 2H and 2.5H, 20ꢀ600 mm for
each) using chloroform as eluent at RT. Optical microscopy experiments
were carried out by using a Leica DM-RP. FTIR measurements were per-
formed by using a Bruker Vector22 instrument in KBr pellets. UV/Vis
spectra were recorded by using a Perkin–Elmer UV/Vis Lambda20 spec-
trometer.
Synthesis
Compounds 1, 5, 7, 18, and the parent anthracene were purchased from
commercial sources and used without additional purification. Compounds
2,[31] 4,[32] 9a,[33] 14a,[34] 14b,[35] 19,[36] 20,[37] and dim-20[20] were prepared
according to the literature procedures. Preparation and characterization
of the remaining compounds is given in the Supporting Information.
General procedure for esterification: Alcohol (1.45 equiv) and DPTS
(1.45 equiv) were added to a solution of acid (1 equiv) in a mixture of
dry CH2Cl2 and dry DMF at RT. After 15 min EDC (1.45 equiv) was
added and the mixture was stirred at RT overnight. The reaction mixture
was diluted with CH2Cl2 and washed twice with brine. The organic layer
was separated and dried over MgSO4. The solvent was removed under re-
duced pressure. The crude product was purified by silica gel column chro-
matography or GPC.
General procedure for the dimerization of anthracene derivatives: The
compound was dissolved in dichloromethane and then the solution was
degassed. The degassed solution was irradiated with UV light for 12 h.
Then the solvent was removed under reduced pressure and subjected to
the preparative GPC purification.
Langmuir monolayers: Monolayers of 16a were prepared by spreading
an aliquot of a solution (1 mgmLÀ1 in CHCl3; spectroscopy-grade sol-
vents) on Millipore water of pH 6 on a mini Langmuir–Blodgett trough
(total area 242 cm2, from KSV, Finland), placed on an anti-vibration
table in a dust-reduced environment (no clean-room). After spreading,
the solvent was allowed to evaporate for 5 min, followed by compression
of the film at 10 mmminÀ1. The surface pressure of the monolayers was
measured at a precision of Æ0.01 mNmÀ1 with a Wilhelmy plate (chro-
matography paper, ashless Whatman Chr 1) on an electrobalance. Mono-
layers were compressed at 228C.
CCDC 680390 and 680391 contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
Acknowledgements
Irradiation and transfer of monolayers: An UV 250-Watt lamp fitted
with a Gallium bulb from UV Light Technology was placed over the
trough at a distance of 20 cm. The time of irradiation was 30 min. During
irradiation the whole set-up was purged with a gentle stream of nitrogen.
This research was supported by the Schweizer Nationalfonds (200021-
111739 and 200020-117572) and ETH grants (TH-05 07-1). We wish to
Chem. Eur. J. 2008, 14, 10797 – 10807
ꢂ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10805

A. D. Schlꢁter, O. Lukin et al.
thank cordially several people for their support of this research: Prof. G.
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Bçhmer, Mainz University; Dr. G. Brezesinski, MPI-KG, Golm; Dr. K.
Wagner, MPI-KG, Golm; Dr. W. B. Schweizer, B. Malisova, ETHZ; C.
Zink, ETHZ.
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Infinite Lateral Growth at the Air/Water Interface
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ꢂ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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