Oligofunctional amphiphiles featuring geometric core group preorganization: Synthesis and study of Langmuir and Langmuir-Blodgett films

Article abstract of DOI:10.1039/b509917j

Based on the principle of supramolecular preorganization, a new type of oligofunctional amphiphile, of which compounds 1-4 are representative structures, has been designed and synthesized. The typical feature of their structure is a highly rigid and geometrically well-defined central unit composed of ethynylene substituted aromatic spacers with different numbers of amphiphilic segment groups (also of a rigid geometric design) attached to it. The molecules form well-defined Langmuir films when spread from a solution at the air/water interface or when a 10^-4 M aqueous CaCl₂ solution was used as the subphase. By analysis of the surface pressure-surface area (π-A) isotherms, information on the packing behavior and orientation of the amphiphilic molecules depending on the molecular structure could be obtained. Morphological characterization of the dynamic process of monolayer compression at the air/water interface was carried out by Brewster angle microscopy, illustrating several phase states visualized as snap shots. Thin monolayer films produced on a 10^-4 M aqueous CaCl₂ subphase can be transferred to a mica solid support by the Langmuir-Blodgett technique. Tapping mode atomic force microscopy reveals a surface topography of the monofilms composed of 1 and 3 that differ in roughness and also in the properties of elasticity, hardness and adhesive strength. X-Ray crystal structure analysis of three relevant intermediate compounds of the synthesis were successfully determined giving an indication of the potential structural features inherent in the new amphiphiles. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b509917j

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A R T I C L E
Oligofunctional amphiphiles featuring geometric core group
preorganization: synthesis and study of Langmuir and
Langmuir–Blodgett films†
Petra U. Mu¨ller,a Edwin Weber,*a Gerd Rheinwaldb and Wilhelm Seichtera
a Institut fu¨r Organische Chemie, Technische Universita¨t Bergakademie Freiberg, Leipziger Str.
29, D-09596, Freiberg/Sachsen, Germany. E-mail: edwin.weber@chemie.tu-freiberg.de;
Fax: (+49) 37 31-39 31 70
b Institut fu¨r Chemie, Lehrstuhl fu¨r Anorganische Chemie, TU Chemnitz-Zwickau, Straße der
Nationen 62, D-09107, Chemnitz/Sachsen, Germany
Received 12th July 2005, Accepted 11th August 2005
First published as an Advance Article on the web 12th September 2005
Based on the principle of supramolecular preorganization, a new type of oligofunctional amphiphile, of which
compounds 1–4 are representative structures, has been designed and synthesized. The typical feature of their
structure is a highly rigid and geometrically well-defined central unit composed of ethynylene substituted aromatic
spacers with different numbers of amphiphilic segment groups (also of a rigid geometric design) attached to it. The
molecules form well-defined Langmuir films when spread from a solution at the air/water interface or when a 10−4
aqueous CaCl2 solution was used as the subphase. By analysis of the surface pressure–surface area (p–A) isotherms,
information on the packing behavior and orientation of the amphiphilic molecules depending on the molecular
structure could be obtained. Morphological characterization of the dynamic process of monolayer compression at
the air/water interface was carried out by Brewster angle microscopy, illustrating several phase states visualized as
snap shots. Thin monolayer films produced on a 10−4 M aqueous CaCl2 subphase can be transferred to a mica solid
support by the Langmuir–Blodgett technique. Tapping mode atomic force microscopy reveals a surface topography
of the monofilms composed of 1 and 3 that differ in roughness and also in the properties of elasticity, hardness and
adhesive strength. X-Ray crystal structure analysis of three relevant intermediate compounds of the synthesis were
successfully determined giving an indication of the potential structural features inherent in the new amphiphiles.
M
Introduction
The design and construction of molecular materials assembled
in organized structures with desirable functions and properties is
an area of great interest both from theoretical and applicational
aspects.1,2 Within this frame, thin-film materials have recently
attracted considerable attention due to the exciting possibilities
for use in optoelectronic, data storage and sensor devices.3
Materials of this type require the rational design of molec-
ular components that are programmed to assemble through
noncovalent intermolecular forces classified as supramolecular
interactions.4 Amphiphiles or surfactants are among the most
versatile examples of two-dimensional ordered systems.5 When
Scheme 1 Structural design concept.
spread from a solution at the air/water interface, molecules of
this type may form well-defined Langmuir films6 which can be
transferred to solid supports by the Langmuir–Blodgett (LB)
technique.7 This allows an elegant means of building up ultra-
thin films of amphiphilic molecules of lamellar order engineered
at the molecular level, thereby offering promising access to novel
supramolecular architectures and device structures.8
Rational engineering of new amphiphiles may arise from a
strict geometric instruction aiming at a particular molecular
shape and with specific functionality in mind.9 This way of
handling is an equivalent to the principle of preorganization
widely used in supramolecular receptors and complexants4,10
which is also followed here, thus producing a new type of largely
shape-persistent and geometrically controlled oligofunctional
amphiphiles as sketched in Scheme 1 and of which compounds
1–4 (Schemes 1 and 2) show representative structures.
We describe the synthesis of these compounds, report on
their ability to form Langmuir and Langmuir–Blodgett films on
different subphases, including packing behavior, film stability
and morphological properties. We also deal with a discussion of
crystal structures that have been obtained from three relevant
intermediates belonging to the synthetic pathway of the target
compounds.
Results and discussion
Developmental strategy of compound structures
However great in number the known amphiphilic compounds
are (including previous oligofunctional species9,11) that have
been used in the formation of Langmuir films5,8, in most cases
they feature a rather flexible structure arising from particular
building elements.12 On the other hand, highly rigid molecules
and shape-persistent architectures have recently proven very
productive in supramolecular chemistry and nanomaterials
research.1–4 Thus, structural preorganization has become an
† Electronic supplementary information (ESI) available: spectroscopic
and elemental analysis data, AFM surface topography and relief images.
See http://dx.doi.org/10.1039/b509917j
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 3 7 5 7 – 3 7 6 6
3 7 5 7
©

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Scheme 3 Synthesis of the lateral building block; (a) EtOH, H2SO4;
(b) TMS–acetylene, Pd(OAc)2, Ph3P, CuI, Et3N; (c) KF·H2O, DMF.
Synthesis
The synthesis of the amphiphilic preorganized compounds 1–
4 was performed by Pd-assisted coupling14 as a key reaction
step. In principle, the coupling may be achieved in two opposite
ways differing in the distribution of the ethynyl and bromo
functions among the educt species, i.e. the lateral or the aro-
matic unit. However, since the palladium cross-coupling between
the ethynylated aromatic core and the halogen substituted
lateral building block failed, the alternative route of ethynyl and
halogen substituent distribution was employed.
The reaction path for the preparation of the amphiphilic target
compounds is a multi step procedure, examples are outlined
in Schemes 3 and 4 for compound 1. In order to realize
the particular topological structures of 1–4, shape-persistent
central units with linear, extended linear, trigonal and tetragonal
geometry were applied, allowing defined positioning of the am-
phiphilic side groups. Along these lines, commercially available
1,4-diiodobenzene was used as short linear construction element
while the extended linear building block is given by bis(4-
bromophenyl)acetylene. This latter compound was synthesized
(48%) in two steps by direct bromination of diphenylethane in
water–acetic acid followed by twofold dehydrohalogenation of
the dibromo adduct with sodium ethoxide in ethanol according
to the literature procedures.15,16 In the case of the trigonal
core unit, 1,3,5-triiodobenzene was used as the preferred reac-
tion component because of the higher reactivity compared to the
bromo derivative. Transformation of the 1,3,5-tribromobenzene
into the analogous iodo compound (72%) was achieved by
halogen exchange reaction using nickel powder, potassium
iodide and iodine in anhydrous DMF.17 For the same reason,
1,2,4,5-tetraiodobenzene providing the tetragonal building unit
was prepared from benzene (67%) by application of the direct
iodination procedure using periodic acid and potassium iodide
in concentrated sulfuric acid.18
The preparation of the lateral building block, properly
functionalized for attachment to the central unit and acting as
mediator element between hydrophilic and hydrophobic func-
tionalities, is also a multistep reaction (Scheme 3). The starting
material, 3-nitrobenzoic acid, was first converted into 3-bromo-
5-nitrobenzoic acid (83%) by direct bromination under strongly
acidic conditions using silver sulfate in concentrated sulfuric
acid.19 Subsequent reduction of the nitro group with Sn powder
and hydrochloric acid gave the corresponding amino compound
in 88% yield, which was transferred via the diazonium salt to
3-bromo-5-hydroxybenzoic acid (9) in 86% yield.19
Protective esterification of 9 was carried out with ethanol
and concentrated sulfuric acid20 to obtain 10 in 90% yield.
For affixing of the terminal ethynyl unit to 10, monoprotected
trimethylsilylacetylene was used and reacted under palladium
catalyzed coupling conditions21 followed by cleavage of the
protecting group.22 The coupling reaction was carried out
under argon in boiling triethylamine with a mixture of cat-
alyst, composed of triphenylphosphane, Pd(II) acetate and
Scheme 2 Structures of amphiphilic compounds in this study.
essential cornerstone in this field of development, here meaning
the use of structurally defined building blocks to create a
molecular backbone that is prevented from collapse, but is highly
organized for a particular target.13 This principle is applied to
the present structural design which is based on the attachment
of different numbers of amphiphilic segment groups to a rigid
and geometrically well-defined central unit, giving rise to a new
type of preorganized oligofunctional amphiphiles (Scheme 1).
Here, the high degree of shape-persistency is obtained by
the use of particularly rigid and strictly geometrical defined
aromatic and ethynylene spacer units, including also aromatic
carboxylic acid groups that contain the lateral alkyl chains in
a fixed position as specified with compounds 1–4 (Schemes 2
and 3).
3 7 5 8
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 3 7 5 7 – 3 7 6 6

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Scheme 4 Example synthesis of compound 1; (a) 12, Pd(CH3COO)2, PPh3, CuI, diethyl amine, Ar; (b) C12H25Br, K2CO3, 18-crown-6, butanone;
(c) NaOH, EtOH–H2O.
Cu(I) iodide to yield 85% of compound 11. Cleavage of the
trimethylsilyl protecting group was performed using KF·H2O
in dimethylformamide to give the lateral building block 12 in
almost quantitative yield.
This latter building unit (12) was then coupled with the
respective di-, tri- and tetrahalogenated central construc-
tion elements according to the palladium catalyzed standard
procedures,14,21 yielding 64–74% of the corresponding esters 5a–
8a. The lipophilic alkyl chains were introduced to 5a–8a by
ether linkage which was carried out with 1-bromododecane and
anhydrous potassium carbonate in 2-butanone catalyzed by 18-
crown-623 to yield 5b–8b, respectively. In the final reaction step,
the esters 5b–8b were hydrolyzed with aqueous 1 N sodium
hydroxide in boiling ethanol–water24 to give the amphiphilic
target compounds 1–4 in 80–82% yield. They are rather high
melting solids that show the expected spectroscopic data. All
synthesized compounds were characterized by 1H and 13C NMR,
IR, MS and elemental analysis.
Langmuir films of the oligofunctional amphiphiles
Fig. 1 Isotherms (p–A) of the oligofunctional compounds 1–4 on a
pure water subphase at 25 ◦C.
The new compounds have been designed in the light of their
potential use for monolayer formation on the water surface. Ex-
amination of the adsorptive behavior at the air/water interface
is regarded as one of the effective methods for understanding the
relation between the surface active properties and the molecular
structure.5,25 Moreover, analysis of the surface pressure (p–
A) isotherms of the Langmuir films makes available much
information on the orientation of the amphiphilic molecules
at the interface as well as their packing behavior in the film.9,26
The compression isotherms of the new preorganized am-
In the case of compound 1, the first interaction between the
molecules of the linear bifunctional amphiphile, lying flat on
the surface of the water subphase, was detected at an area per
molecule of 0.75 nm2, referred to as the lift-off area (Table 1).
Further compression of the monolayer leads to the liquid
condensed state at an area per molecule of 0.7–0.6 nm2 to pass
into the solid–condensed state at an area of 0.6 nm2 (8 mN m−1),
which is characterized by a steep rise of the monolayer pressure.
Near the collapse point (K0) the molecules are arranged in a most
dense packing and the monolayer exhibits solid-state analogues
properties. Extrapolating the linear part of the isotherm to
the film pressure (p = 0 mN m−1), the average required space
per molecule (A0) can be estimated. At the collapse point K0,
the existence of the monolayer ends and further compression
leads to formation of disordered polylayers. The isotherms of
the extended linear compound 2 and the trigonal preorganized
amphiphile 3 exhibit comparable monolayer polymorphism.
phiphiles 1–4 which were measured on a water and a 10−4
M
aqueous CaCl2 subphase, recorded on a Langmuir film balance,
are shown in Fig. 1 and 2, respectively. Although the collective
feature derived from the traces of the respective isotherms is the
formation of monomolecular films of relatively high stability,
the shapes of the isotherms are distinctly different. This suggests
that the monolayers and monolayer polymorphism are different
for compounds 1–4, dependent on the molecular structure and
the used subphase.27
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 3 7 5 7 – 3 7 6 6
3 7 5 9

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Fig. 3 BAM images during the monolayer compression of the bifunc-
tional compound 1: (a) transition from the gas analogous to the liquid
expanded state, p = 3 mN m−1; (b) liquid condensed state, p = 8 mN
m−1; (c) solid-condensed state of the monolayer, p = 35 mN m−1; collapse
Fig. 2 Isotherms (p–A) of the compounds 1–4 on a 10−4 M aqueous
point of the Langmuir film, p = 45 mN m−1
.
CaCl2 subphase at 25 ◦C.
monolayers at the air/water interface can be visualized without
affecting the monolayer with probes. Fig. 3 illustrates the BAM
images during the compression procedure of the Langmuir
film composed of compound 1. Condensed phase domains are
formed directly after the spreading procedure, so that only
irregularly shaped condensed phase domains surrounded by
the fluid subphase are formed (Fig. 3a). The transition of the
monolayer from the gas analogous to the liquid expanded state
takes place at a pressure of approximately 3 mN m−1. Up to a
pressure of 8 mN m−1, two different morphological states coexist
which can be described under thermodynamic considerations as
the transition to the liquid condensed state of the monolayer
(Fig. 3b). In spite of the nearly closed coverage of the water
surface, loose clods and small regions of uncovered subphase
can be detected. Further compression of the monolayer leads to
the solid–condensed phase where a high degree of homogeneity
without inner texture could be detected, and a smooth surface
morphology was observed (Fig. 3c). This process is associated
with a strong increase in the surface pressure up to 45 mN
m−1. Continued compression of the thin film finally leads to
the evolution of non-regular morphological patterns due to
the increasing lateral pressure. At the collapse point, K0, the
existence of the dense packed monomolecular Langmuir film is
terminated and the molecules draw aside in the third dimension,
recognizable at the cracks and slips in Fig. 3d.
Under the same operating conditions with the tetrafunctional
compound 4, a liquid expanded state (1.5–1 nm2) can be
observed.
The experimental value of the lift-off area and the average
required space per molecule (A0) of the oligofunctional am-
phiphiles 1–4, compared with calculated data of A0, based on
molecular modelling and assuming a perpendicular arrange-
ment of the alkyl chains for each compound, are summarized in
Table 1. A good correlation between the measured and calcu-
lated values can be determined for the bifunctional compounds
1 and 2, whereas for compounds 3 and 4 a moderate and
bigger deviation can be observed, respectively. This suggests
that the molecules of 3 and 4 in the solid-state monolayer are
probably no longer laying flat on the water surface but tend
to tilt up in a quasi “edge-on” orientation, which is a known
property of aromatic disc-shaped compounds,28 giving rise to
the relatively low average spaces required per molecule which
are rather similar to the values for compounds 1 and 2.
Fig. 2 shows the p–A isotherms of the amphiphilic compounds
1–4 on a 10−4 molar aqueous CaCl2 subphase. The formation
of stable monolayers are also observed under these conditions.
Moreover, as follows from Table 1, the presence of the divalent
Ca2+ ions results in more space required per molecule for
all compounds. Obviously, this is due to an increase of the
hydrophilic interaction which is supported by the Ca2+ ions,
leading to enhanced conformational rigidity and favoring a
plane or “face-on” arrangement of the amphiphiles at the water
surface. In fact, here the data for A0 given in Table 1 are very
close to the calculated areas of the amphiphiles.
Langmuir–Blodgett films and AFM characterization
Since the amphiphiles 1–4 proved efficient in the formation
of stable Langmuir films on a water or on a 10−4 M CaCl2
subphase, transfer of the monolayers to a solid support (in
The application of the non-resonant Brewster angle mi-
croscopy (BAM)29 is a useful tool for investigation of the
morphological texture of the Langmuir film and of the dynamic
process of monolayer compression at the air/water interface.
The main advantage of BAM is that the morphology of
order to generate Langmuir–Blodgett type coating films7,27
)
is promising. This was performed by using the 10−4M CaCl2
subphase and freshly cleaved mica plates at a surface pressure
of 25 mN m−1. As expected, the transfer of the monolayers
1–4 to the hydrophilic mica plate occurs with a dipper speed of
Table 1 Results of the lift-off area (A0) and calculated minimum area of the compounds 1–4 on the water subphase and 10−4 molar aqueous CaCl2
subphase
Water subphase
Subphase 10−4 M CaCl2
Lift-off area/nm2
Compound
Lift-off area/nm2
A0/nm2
Calculated area/nm2
A0/nm2
1
2
3
4
0.75
0.98
1.04
1.56
0.61
0.73
0.70
0.79
0.62
0.74
0.88
1.12
0.83
1.00
1.46
1.65
0.70
0.79
0.84
1.07
3 7 6 0
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 3 7 5 7 – 3 7 6 6

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0.5 mm min−1 only by horizontal lifting from the Langmuir
films, producing a hydrophobic coating of the support. The
transfer ratio was found between 0.99 and 1.19 indicating the
effectiveness of the transfer.
type interactions involved in these structures are summarized in
Table 2.
Crystallization of the diphenolic diester 5a from dimethyl-
sulfoxide yielded a corresponding 1 : 2 solvent complex of the
triclinic space group P-1 with one half of the host molecule in
the asymmetric unit of the cell. The molecular structure which
is illustrated in Fig. 4 shows a deviation of 7.3◦ of the ethynylene
spacers from linearity to give the basic molecule a slightly
undulated geometry. The crystal packing is characterized by
supramolecular sheets of 5a and DMSO held together by a
close network of hydrogen bonds including different modes of
interaction (Fig. 5). The solvent molecules are associated with
the hydroxyl groups of 5a by conventional O–H · · · O hydrogen
bonds31 while one of the less acidic hydrogens of each methyl
group of DMSO is connected with the carbonyl oxygen of two
adjacent molecules of 5a. Moreover, weak hydrogen bonds of
the C–H · · · O type32 connect inversion related molecules of
5a. Due to the unsaturated nature of the backbone of 5a, p–
p stacking interactions33 dominate between the molecular layers
For characterization of the surface topography of the trans-
ferred monolayers, the tapping mode atomic force microscopy30
was used. The surface topography of the monofilms composed
of the amphiphiles 1 and 3 shows marked differences in the
roughness. While the linear amphiphile 1 gives rise to distinct
smoothness of the surface, the trigonal amphiphile 3 reveals
a rather rough structure, which is in agreement with the
property of Langmuir film formation where 1 also yielded a
more dense packing in the solid-condensed state compared
to 3. Nevertheless, coating of the mica surface is complete in
both cases. Accordingly, the LB-films of compounds 1 and 3
differ in their properties of elasticity, hardness and adhesive
strength registered through the phase shift of the oscillating
AFM cantilever.
˚
with mean distance between consecutive layers of 3.5 A involving
Crystal structure analysis
the interacting aromatic rings.
Although no crystal structures of the amphiphilic target com-
pounds could be obtained because of lacking single crystals, X-
ray crystal structures including the functionalized terminal unit
12 and two relevant intermediate compounds (5a and 5b) were
successfully determined.‡ The supramolecular hydrogen bond
In the crystal structure of compound 5b (space group P-1,
Z = 1), which is equated with the diether derivative of 5a or the
ester analogue of the amphiphile 1, the unsaturated part of the
molecule together with the non-polar dodecyl residues exhibit a
nearly planar arrangement showing anti orientation of both the
alkyl groups and ester functions relative to each other. Also in
the present structure, the molecules establish two-dimensional
supramolecular aggregates which are stabilized by van der Waals
forces and hydrogen bonding. The particular constitution of
‡ CCDC reference numbers 278011, 278012 and 278014. See http://dx.
doi.org/10.1039/b509917j for crystallographic data in CIF or other
electronic format.
Fig. 4 Molecular structure of 5a·DMF (1 : 2). H bond type interactions are specified as broken lines.
◦
˚
Table 2 Distances (A) and angles ( ) of hydrogen bond type interactions
D
H
A
D–H
H · · · A
D · · · A
D–H · · · A
Symmetry
5a
O1
C6
C15
C16
5a
H1
H6
H15C
H16B
O4
O1
O2
O2
0.84
0.95
0.98
0.98
1.83
2.52
2.35
2.57
2.662(3)
3.446(3)
3.251(4)
3.426(4)
172
164
153
146
x, y, z
1 − x, 2 − y, 1 − z
2 − x, 1 − y, 1 − z
2 − x, 1 − y, 1 − z
C18
C25
12
O1
C11
H18B
H25
O2
O3
0.98
0.95
2.66
2.46
3.570(2)
3.370(2)
155
160
2 − x, 2 − y, −z
−1 + x, 1 + y, z
H1
H11
O2
O1
0.82
0.93
1.91
2.35
2.727(3)
3.271(4)
175
172
1/2 − x, 1/2 + y, 1 − z
3/2 − x, −1/2 + y, 2 − z
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 3 7 5 7 – 3 7 6 6
3 7 6 1

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Fig. 5 Packing excerpt of 5a·DMF (1 : 2) showing the supramolecular layer structure with H bond type interactions given as broken lines.
Fig. 6 Packing illustration of 5b with H bond type interactions specified as broken lines.
5b induces a crystal packing in which hydrophobic domains
to the above structures, no aromatic stacking interactions33 are
formed by dodecyl residues alternate with regions established
by the tricyclic building blocks of the molecules (Fig. 6). A
weak C–H · · · O contact32 exists between the carbonyl oxygen
and a hydrogen atom of the central arene ring of a symmetry
related molecule. Moreover, the distance between the ester
alkoxy oxygen and the terminal dodecyl residue of a neighboring
molecule indicates an attractive contact to stabilize the packing
structure of 5b.
The phenolic ester 12, which can be considered as a molecular
fragment of 5a, generates a specific two-dimensional hydrogen
bonded aggregate in the crystalline state (Fig. 7). This involves
strong intermolecular O–H · · · O hydrogen bonds31 between
hydroxyl and carbonyl oxygens as well as weaker C–H · · · O
contacts32 including the ethynyl hydrogen and the phenolic
oxygen thus being in H-donor and acceptorship. In contrast
observed in the packing of 12.
Conclusions
Langmuir and Langmuir–Blodgett (LB) films composed of
molecules more complex than the conventional rod-like am-
phiphiles are clearly expected to display encouraging new
properties and structures, having prompted our effort in the field.
Thus, stimulated by the so-called “Gemini surfactants”11 con-
sisting of two or more hydrophilic head groups and hydrophobic
aliphatic chains, which have previously been modified by teth-
ering to an aromatic core,12 and are based on the well-known
principle of supramolecular preorganization,4,10 a new type of
oligofunctional amphiphile has been developed and synthesized,
represented by compounds 1–4. They feature a highly rigid and
geometrically well-defined central unit composed of ethynylene
and attached amphiphilic segment groups, also being of rigid
geometry.
This particular design of amphiphilic compounds proved
useful in the formation of well-defined Langmuir monolayers
on different aqueous subphases that can be transferred to a
mica solid support by the LB technique.6,7 Both the properties
of the Langmuir and LB films, including packing behavior
and surface topography, depend on the particular molecular
structure, as determined by analysis of the p–A isotherms,26,27
Brewster angle29 and atomic force microscopy.30 Thus, our strat-
egy has the potential to provide a controlled two-dimensional
surface arrangement of hydrophobic tails and polar heads with
desirable geometries and distances owing to the supramolecular
preorganization.
Future research will be directed to obtaining even more
control over the organization of the self-assembling components
by supplying the preorganized amphiphiles with additional
Fig. 7 H-bonded network structure in the packing of 12. H bonds are
given as broken lines.
3 7 6 2
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 3 7 5 7 – 3 7 6 6

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interacting groups according to the supramolecular synthon
approach34 which is a promising aspect for new applications.
temperature, the catalyst mixture (composed of palladium(II)
acetate (60 mg), triphenylphosphane (117 mg) and copper (I)
iodide (50 mg)) was added and the solution was heated under
reflux for 7 h. During this time, trimethylsilylacetylene (7.5 mL,
53.0 mmol) was slowly added to the boiling reaction mixture.
After cooling down to room temperature the precipitated tri-
ethylamine hydrobromide was removed by filtration and washed
with diethyl ether. The solvent was removed under reduced
pressure and the obtained oily residue dissolved in diethyl ether.
The mixture was washed with diluted HCl followed by water,
dried with Na2SO4 (with the aid of charcoal), filtered over celite
and evaporated in vacuum. Purification of the crude product
was carried out by stirring with petroleum ether (40–60) to yield
9.1 g (85%) of a light yellow solid, mp 89–90 ◦C.
Experimental
General methods
Melting points (uncorrected) were determined with a Kofler
melting point microscope (VEB Dresden Analytik). NMR
spectra (internal standard TMS, d in ppm, J in Hz) were
recorded on a Bruker Avance DPX 400 at 400.1 (1H) and
100.6 MHz (13C) at room temperature. IR spectra (m in cm−1)
were measured on a Perkin Elmer FT-IR 1600. GC-MS spectra
were recorded on a Hewlett-Packard (Palo Alto, USA) GC-MS-
coupling 5890 Series II/MS 5989A. EI-MS spectra were mea-
sured on a Micromass Autospec Q, MADLD-TOF spectra on a
Hewlett-Packard G2025A MALDI-TOF-MS system, ESI-TOF
spectra on a Mariner ESI-TOF-MS from Applied Biosystems
(Weiterstadt, Germany), and the FAB(+)-MS (LSIMS) spectra
were accomplished with a Micromass Autospec Q (Switzerland).
Elemental analyses were determined with a Heraeus CHN rapid
analyzer.
Ethyl 3-ethynyl-5-hydroxybenzoate (12). A solution of 11
(9.0 g, 34.0 mmol) and KF·H2O (7.6 g, 100 mmol) in dimethyl
sulfoxide (200 mL) and water (2 mL) was stirred at room
temperature for 5 h. The reaction mixture was poured into ice
cold 6 N HCl (300 mL) and extracted thrice with diethyl ether.
The combined organic layers were washed with diluted NaHCO3
solution and water, and dried over Na2SO4. After evaporation of
the solvent the crude oily product was purified by crystallization
from n-heptane to yield 6.1 g (95%) of a light yellow crystalline
solid, mp 129 ◦C.
Compounds and materials
The following reagents and compounds were purchased: bi-
benzyl (Fluka), 1-bromododecane (Fluka), 18-crown-6 (Merck),
1,4-diiodobenzene (Fluka), 3-nitrobenzoic acid (Fluka), ortho-
periodic acid (Riedel-de Hae¨n), trimethylsilylacetylene (ABCR),
and 1,3,5-tribromobenzene (Fluka). Organic solvents were
purified by standard procedures. For column chromatography,
silica gel (Merck, 63–100 lm) was used.
Preparation of compounds 5a–8a (General procedure)14,21
The corresponding aryl halide was dissolved in diethylamine
(iodide) or triethylamine–toluene (bromide). A slight stream
of argon is passed through the solution while heating under
reflux for 20 min to remove the oxygen. After cooling down to
room temperature the mixture of catalyst, being composed of
palladium(II) acetate (50.0 mg), triphenylphosphane (117.0 mg)
and copper(I) iodide (60.0 mg) was added and the solu-
tion heated under reflux for 20 min. The ethynyl-substituted
compound 12 dissolved in the respective amine was added
dropwise to the reaction mixture, and refluxing was continued
for further 7 h until completion of the reaction (monitored by
thin layer chromatography). The mixture was allowed to cool
down to room temperature and the precipitate (catalyst, di-
or triethylamine hydrobromide respectively hydroiodide) was
removed by filtration and washed with diethyl ether. The solvent
was removed under reduced pressure. Further details and data
of the individual compounds are given below.
1,3,5-Triiodobenzene was prepared from 1,3,5-tribromo-
benzene with potassium iodide, nickel powder and iodine in
DMSO using the literature procedure.17 Sublimation of the crude
◦
product yielded 72% of colorless fine crystals; mp 159–162 C
(lit.17 mp 163 ◦C).
1,2,4,5-Tetraiodobenzene was obtained from benzene and
periodic acid–potassium iodide in concentrated sulfuric acid
according to the literature.18 Crystallization from pyridine–
methanol (1 : 1) yielded 67% of a yellow solid; mp 247–149 ◦C
(lit.18 mp 249–252 ◦C).
Bis(4-bromophenyl)acetylene was synthesized by bromina-
tion of diphenylethane in water–acetic acid followed by elimina-
tion of the dibromo adduct with sodium ethoxide in ethanol.15,16
Crystallization from chlorofor◦m yielded 48% of a colorless solid;
mp 184 ◦C (lit.16 mp 182–184 C).
3-Amino-5-bromobenzoic acid was prepared from 3-
nitrobenzoic acid by direct bromination and subsequent reduc-
tion with Sn–HCl.19 Crystallization◦from ethanol yielded 73%
solid; mp 222 ◦C (lit.19 mp 220–222 C).
3-Bromo-5-hydroxybenzoic acid (9) was obtained from 3-
amino-5-bromobenzoic acid via diazotation and hydrolysis.19
Crystallized from water in the presence of charcoal yielded 86%
of a light yellow solid; mp 238 ◦C (lit.19 mp 238–239 ◦C).
Diethyl 5,5ꢀ -[benzene-1,4-diyl-bis(ethyne-2,1-diyl)]bis(3-
hydroxybenzoate) (5a). 1,4-Diiodobenzene (3.30 g, 10.0 mmol),
12 (3.80 g, 20.0 mmol) and the catalyst in diethylamine (100 mL)
were used. The crude product was dissolved in THF and treated
with charcoal. The solvent was removed in vacuum and the
residue taken up with a small amount of acetone. By addition of
n-heptane, a light-brown solid precipitated which was purified by
column chromatography (SiO2, petroleum ether (40–60)–THF
2 : 1) to obtain 3.7 g (68%) of a white solid, mp 263–265 ◦C.
Diethyl 5,5ꢀ-[tolan-4,4ꢀ-diyl-bis(ethyne-2,1-diyl)]bis(3-hydroxy-
benzoate) (6a). Bis-(4-bromophenyl)acetylene (3.51 g,
10.5 mmol), compound 12 (4.0 g, 21.0 mmol) and the catalyst in
triethylamine and toluene (140 mL, 1 : 1) were used. The crude
viscous product was stirred with n-heptane to obtain a light-
brown solid, which was purified using column chromatography
(SiO2, petroleum ether (40–60)–THF, 2 : 1) to yield 4.05 g (70%)
of a crystalline solid, mp 197–200 ◦C.
Ethyl 3-bromo-5-hydroxybenzoate (10). To a solution of 9
(12.5 g, 57.0 mmol) in anhydrous ethanol (60 mL) conc. H2SO4
(1.5 mL) was added. The reaction mixture was refluxed for
5 h, the solvent evaporated under reduced pressure, and the
resulting oily residue poured into water (400 mL). The organic
phase was separated and the aqueous phase extracted thrice
with diethyl ether (100 mL). The combined organic layers were
washed with diluted Na2CO3 (100 mL) and water (100 mL),
dried with Na2SO4, and the solvent was removed under reduced
pressure. The residue was crystallized from n-heptane to yield
12.5 g (90%) of a white crystalline solid, mp 96 ◦C.
Triethyl 5,5ꢀ,5ꢀꢀ -[benzene-1,3,5-triyl-tris(ethyne-2,1-diyl)]-
tris(3-hydroxybenzoate) (7a). 1,3,5-Triiodobenzene (2.4 g,
5.25 mmol), compound 12 (3.0 g, 15.75 mmol) and the catalyst
in diethylamine (100 mL) were used. Purification was carried
out by column chromatography (SiO2, petroleum ether (40–60)–
THF, 2 : 1 ) to yield 2.5 g (74%) of a light yellow solid, mp
153–155 ◦C.
Ethyl 3-hydroxy-5-[2-(trimethylsilyl)ethynyl]benzoate (11).
A solution of 10 (10.0 g, 41.0 mmol) in triethylamine (250 mL)
was heated under reflux for 15 min while argon is passed through
the solution to keep it free from air. After cooling to room
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 3 7 5 7 – 3 7 6 6
3 7 6 3

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Tetraethyl 5,5ꢀ,5ꢀꢀ,5ꢀꢀꢀ-[benzene-1,2,4,5-tetrayl-tetrakis(ethyne-
2,1-diyl)]tetrakis(3-hydroxybenzoate) (8a). 1,2,4,5-Tetraiodo-
benzene (3.05 g, 5.25 mmol), compound 12 (4.0 g, 21.0 mmol)
and the catalyst in diethylamine (150 mL) were used. The crude
viscous product was dissolved in THF, treated with charcoal
and filtered. The resulting yellow solution was slowly added
under stirring to the tenfold amount of petroleum ether (40–
60) giving rise to precipitate, which was purified by column
chromatography (SiO2, petroleum ether (40–60)–THF, 2 : 1 )
to yield 2.9 g (67%) of a light yellow solid, mp > 260 ◦C.
5,5ꢀ -[Benzene-1,4-diyl-bis(ethyne-2,1-diyl)]bis(3-dodecyloxy-
benzoic acid) (1). Compound 5b (2.70 g, 3.25 mmol) and
NaOH (5.0 g) in water and ethanol (50 mL, 1 : 1) were used to
yield 1.1 g (80%) of a light yellow solid, mp 213–215 ◦C.
5,5ꢀ -[Tolan-4,4ꢀ -diyl-bis(ethyne-2,1-diyl)]bis(3-dodecyloxy-
benzoic acid) (2). Compound 6b (1.70 g, 1.9 mmol) and NaOH
(5.0 g) in water and ethanol (150 mL, 2 : 1) were used to yield
1.27 g (80%) of a light yellow solid, mp 186–190 ◦C.
5,5ꢀ,5ꢀꢀ -[Benzene-1,3,5-triyl-tris(ethyne-2,1-diyl)]tris(3-dode-
cyloxybenzoic acid) (3). Compound 7b (1.80 g, 1.6 mmol) and
NaOH (5.0 g) in water and ethanol (60 mL, 1 : 5) were used to
obtain 1.4 g (82%) of a light yellow solid, mp 186–189 ◦C.
Preparation of compounds 5b–8b (General procedure)23
The corresponding phenol compound (5a–8a), anhydrous
K2CO3, 18-crown-6 and 1-bromododecane were dissolved in
anhydrous butanone and heated under reflux and under argon
for 72 h. After cooling down to room temperature, the sol-
vent was evaporated and the residue extracted into methylene
chloride. The solid was filtered off and the filtrate washed with
diluted HCl followed by aqueous 1 N NaHCO3 and water, then
dried (Na2SO4) and evaporated. Further details and data of the
individual compounds are given below.
5,5ꢀ,5ꢀꢀ,5ꢀꢀꢀ -[Benzene-1,2,4,5-tetrayl-tetrakis(ethyne-2,1-diyl)]-
tetrakis(3-dodecyloxy-benzoic acid) (4). Compound 8b (2.0 g,
1.33 mmol) and NaOH (5 g) in water and ethanol (60 mL, 1 :
5) were used to obtain 1.61 g (87%) of a light yellow solid, mp
250–255 ◦C.
Langmuir and Langmuir–Blodgett films
A computer-controlled Langmuir film balance system Lauda
FW2, equipped with a Wilhelmy platinum plate and a Teflon
coated trough was applied. As subphases, pure water, obtained
with a Milli-Q-Labo apparatus (18 MX cm), as well as a
10−4 M aqueous CaCl2 solution were used. The oligofunctional
amphiphiles, all prepared in high purity, were at first dissolved in
a few drops of DMSO followed by addition of CHCl3 to prepare
solutions of concentration of 1 mg ml−1. These solutions were
spread on the water surface with a 100 ll micro syringe and by
waiting 15 min for solvent evaporation. The monolayers were
compressed with a barrier speed of 10 mm min−1. To confirm
their reproducibility all isotherms were run at least three times in
the direction of increasing pressure with freshly prepared films.
The measurements were performed at the constant temperature
of 25 ◦C under clean room conditions.
The Langmuir–Blodgett films on mica plates were performed
by transfer of compressed Langmuir monolayers generated on
a 10−4 M aqueous CaCl2 subphase at a surface pressure of 25
mN m−1. The transfer to the mica substrate was carried out
with a dipper-speed of 0.5 mm min−1. The mica plates were
freshly cleaved before use in order to ensure a smooth, pure and
hydrophilic surface.
Diethyl 5,5ꢀ -[benzene-1,4-diyl-bis(ethyne-2,1-diyl)]bis(3-
dodecyloxybenzoate) (5b). Compound 5a (2.36 g, 5.2 mmol), 1-
bromododecane (3.82 g, 15.0 mmol), anhydrous K2CO3 (2.07 g,
15.0 mmol) and 18-crown-6 (10.0 mg) in anhydrous butanone
(50 mL) were used. Purification was achieved by column
chromatography (SiO2, methylene chloride–n-hexane, 2 : 1) to
yield 3.37 g (82%) of a light yellow solid, mp 83–84 ◦C.
Diethyl 5,5ꢀ -[tolan-4,4ꢀ -diyl-bis(ethyne-2,1-diyl)]bis(3-
dodecyloxybenzoate) (6b). Compound 6a (2.60 g, 4.7 mmol),
1-bromododecane (3.49 g, 14.0 mmol), anhydrous K2CO3
(1.93 g, 14 mmol) and 18-crown-6 (10 mg) in anhydrous
butanone (100 mL) were used. Purification was achieved by
column chromatography (SiO2, methylene chloride–n-hexane,
2 : 1). The product was taken up with n-heptane and mixed with
ethanol–water (3 : 1) to separate 3.5 g (84%) of a light yellow
solid, mp 75–79 ◦C.
Triethyl 5,5ꢀ,5ꢀꢀ-[benzene-1,3,5-triyl-tris(ethyne-2,1-diyl)]tris(3-
dodecyloxybenzoate) (7b). Compound 7a (1.50 g, 2.33 mmol),
1-bromododecane (1.78 g, 7.15 mmol), anhydrous K2CO3
(0.99 g, 7.15 mmol) and 18-crown-6 (5 mg) in anhydrous
butanone (40 mL) were used. The raw product was purified by
column chromatography (SiO2, methylene chloride–n-hexane,
2:1) to yield 2.03 g (76%) of a light yellow solid, mp 61–63 ◦C.
Brewster angle microscopy (BAM)
These investigations were carried out using a BAM1 setup from
Nanofilm Technologies GmbH with an argon laser illumination
source. The images were recorded on a CCD camera and are
snapshots of different states of the monolayer recorded upon
compression. The lateral resolution was about 4 lm.
Tetraethyl 5,5ꢀ,5ꢀꢀ,5ꢀꢀꢀ-[benzene-1,2,4,5-tetrayl-tetrakis(ethyne-
2,1-diyl)]tetrakis(3-dodecyloxybenzoate) (8b). Compound 8a
(2.70 g, 3.25 mmol), 1-bromododecane (2.50 g, 10.0 mmol),
anhydrous K2CO3 (1.38 g, 10 mmol) and 18-crown-6 (5 mg)
in anhydrous butanone (70 mL) were used. The raw product
was dissolved in petroleum ether (40–60) and precipitated
by addition of ethanol. Further purification was achieved by
column chromatography (SiO2, methylene chloride–n-hexane,
2 : 1) to yield 3.52 g (72%) of a light yellow solid, mp 62–63 ◦C.
AFM measurements
They were accomplished with an Extended Multimode
NanoScope IIIa (Digital Instruments, Cambridge, UK)
equipped with a silicon cantilever of 125 lm length using the
tapping-mode. For topographic and phase imaging determina-
tions the software of Digital Instruments, Version 4.23r3 was
applied.
Preparation of compounds 1–4 (General procedure)24
A mixture of the corresponding ethyl ester (5b–8b) in ethanol–
20% aqueous NaOH (1 : 1) was heated under reflux for 4 h.
After cooling down to room temperature, the reaction mixture
was filtered and acidified with diluted hydrochloric acid. The
resulting white precipitate was collected and washed with water
to free from acid. The raw product was dissolved in THF
and treated with charcoal which was filtered off. Addition of
ethanol–water (1 : 1) yielded a precipitate of the product which
was filtered and dried in vacuum. Further details and data of
the individual compounds are given below.
X-Ray diffraction‡
Crystals of 5a and 5b suitable for structure analysis were grown
by slow isothermal evaporation of solvent from dimethylsulfox-
ide solutions; crystals of 12 were obtained from chloroform.
Compound 5b crystallizes as trigonal prisms which turned out
to be twins.
The intensity data for 5a and 5b, collected on a SMART
diffractometer (graphite monochromated Mo Ka radiation,
˚
k = 0.71073 A) were measured in the x-scan mode, and for
3 7 6 4
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 3 7 5 7 – 3 7 6 6

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compound 12 in the x–2h scan mode, recorded on a CAD4
diffractometer (graphite monochromated Cu Ka-radiation, k =
8 (a) J.-H. Fuhrhop and J. Ko¨ning, Membranes and Molecular As-
semblies: The Synkinetic Approach, (Monographs in Supramolecular
Chemistry, vol. 5), Royal Society of Chemistry, Cambridge, UK,
1994; (b) I. K. Lednev and M. C. Petty, Adv. Mater., 1996, 8, 615.
9 (a) K. Ariga, T. Urakawa, A. Michiue, Y. Sasaki and J. Kikuchi,
Langmuir, 2000, 16, 9147; (b) N. Reitzel, T. Hassenkam, K.
Balashev, T. R. Jensen, P. B. Howes, K. Kjaer, A. Fechtenko¨tter,
N. Tchebotareva, S. Ito, K. Mu¨llen and T. Bjørnholm, Chem. Eur. J.,
2001, 7, 4894.
10 (a) D. J. Cram, Angew. Chem., 1986, 98, 1041 (Angew. Chem., Int.
Ed., 1986, 25, 1039); (b) E. Weber, in Encyclopedia of Supramolecular
Chemistry, ed. J. L. Atwood and J. Steed, Marcel Dekker, New York,
2004, p. 261.
11 (a) F. M. Menger and J. S. Keiper, Angew. Chem., 2000, 112, 1980
(Angew. Chem., Int. Ed., 2000, 39, 1906); (b) J. Haldar, V. K. Aswal,
P. S. Goyal and S. Bhattacharya, Angew. Chem., 2001, 113, 1278
(Angew. Chem., Int. Ed., 2001, 40, 1228); (c) A. J. Kirby, P. Camilli,
J. B. F. N. Engberts, M. C. Feiters, R. J. M. Nolte, O. So¨derman,
M. Bergsma, P. C. Bell, M. L. Fielden, C. L. Garcia Rodriguez, P.
Guedat, A. Kremer, C. McGregor, C. Perrin, G. Ronsin and M. C. P.
van Eijk, Angew. Chem., 2003, 115, 1486 (Angew. Chem., Int. Ed.,
2003, 42, 1448).
˚
1.5418 A). Reflections were corrected for background, Lorentz
and polarisation effects. The crystal structures were solved by
using direct methods35,36 and difference Fourier synthesis and
refined by full-matrix least-squares.37 Absorption correction
for compounds 5a and 5b were performed by using empirical
methods, and for 12 the WinGX program38 was used. All
non-hydrogen atoms were refined anisotropically. The hydrogen
atoms of the hydroxyl groups were partly obtained from the
difference Fourier map, in other cases included in the models
in calculated positions and refined as constrained to bonding
atoms.
Acknowledgements
We are grateful for generous financial support to this work
from the Deutsche Forschungsgemeinschaft (SFB 285) and the
Fonds der Chemischen Industrie. Mrs E. Delahaye (RWTH
Aachen) is gratefully acknowledged for execution of the AFM
measurements. We also thank Dr U. Oertel (IPF Dresden) and
Prof. Dr A. Bereck (Universita¨t-GH Wuppertal) for helpful
discussions.
12 (a) C. Helbig, H. Baldauf, T. Lange, R. Neumann, R. Pollex and E.
Weber, Tenside Surf. Det., 1999, 36, 58; (b) P. Mu¨ller, E. Weber, C.
Helbig and H. Baldauf, J. Surf. Det., 2001, 4, 407; (c) P. Mu¨ller, C. C.
Akpo, K. W. Sto¨ckelhuber and E. Weber, Adv. Colloid Interface Sci.,
2005, 291, 114–115.
13 S. Ho¨ger, Chem. Eur. J., 2004, 10, 1320.
14 S. Takahashi, Y. Kuroyama and K. Sonogashira, Synthesis, 1980,
627.
15 J. I. G. Cadogan and P. W. Inward, J. Chem. Soc., 1962, 4170.
16 (a) H. J. Barber and R. Slack, J. Chem. Soc., 1944, 612; (b) S.
Misumi, Bull. Chem. Soc. Jpn., 1962, 35, 135; (c) A. L. Rusanov,
M. L. Keshtov, M. M. Begretov, I. A. Khotina and A. K. Mikitaev,
Russ. Chem. Bull., 1996, 45, 169.
17 (a) U. Scho¨berl, T. F. Magnera, R. M. Harrison, F. Fleischer, J. L.
Pflug, P. F. H. Schwab, X. Meng, D. Lipiak, B. C. Noll, V. S. Allured,
T. Rudalevige, S. Lee and J. Michl, J. Am. Chem. Soc., 1997, 119,
3909; (b) S. H. Yang, C. S. Li and C. H. Cheng, J. Org. Chem., 1987,
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