Amphiphilic hairy disks with branched hydrophilic tails and a hexa-peri-hexabenzocoronene core

Article abstract of DOI:10.1021/ja017553+

Amphiphilic discotic molecules with hydrophilic side branches consisting of hexaphenyl hexaperi-hexabenzocoronene and hexabiphenyl hexa-peri-hexabiphenylcoronene as the aromatic core and hexasubstituted oligoethers as the branched peripheral chains have been synthesized, and their microstructure has been characterized. The discotic molecules based on dibranched oligoether side chains have been observed to self-organize into a well-ordered hexagonal columnar structure within liquid crystalline phases, which possessed an exceptionally high thermal stability and an unusually wide temperature range over >300°C. We suggest that a combination of the large lateral dimensions of the rigid cores and disordered structure of the oxygen-containing branches tails is a driving force to the formation of a highly ordered columnar structure in the bulk state with enhanced molecular segregation. In contrast to the thermotropic phase behavior that favors the formation of highly ordered columnar aggregates through a strong stacking interaction, the hexabenzocoronene cores are packed in a face-on arrangement at the air/water interface and on solid surfaces with surface domains composed of an array of 7 × 7 molecules. We suggest a crablike molecular conformation and cluster-segregated monolayers with 6-fold symmetry and unusual face-on packing on a solid surface. Preliminary spectroscopic studies in the bulk state have shown that the molecules based on a hexaaromatic-substituted core may serve as functional supramolecular materials with high energy transfer characteristic within the columns due to near-perfect columnar ordering, which is unchanged over a wide temperature range. We believe that an absence of the crystallization phenomenon of side-branched oligoether chains is critical for the formation of long-range columnar ordering with strong intracolumnar correlation of conjugated disks important for high carrier mobility.

Full text of DOI:10.1021/ja017553+

Published on Web 07/13/2002
Amphiphilic Hairy Disks with Branched Hydrophilic Tails and
a Hexa-peri-hexabenzocoronene Core
Myongsoo Lee,*,† Jung-Woo Kim,† Sergiy Peleshanko,‡ Kirsten Larson,‡
Yong-Sik Yoo,† David Vaknin,§ Sergei Markutsya,‡ and Vladimir V. Tsukruk*,‡
Contribution from the Department of Chemistry, Yonsei UniVersity, Seoul 120-749, Korea,
Department of Materials Science & Engineering, Iowa State UniVersity, Ames, Iowa 50011,
and Ames Laboratory and Department of Physics and Astronomy, Iowa State UniVersity,
Ames, Iowa 50011
Received November 15, 2001
Abstract: Amphiphilic discotic molecules with hydrophilic side branches consisting of hexaphenyl hexa-
peri-hexabenzocoronene and hexabiphenyl hexa-peri-hexabiphenylcoronene as the aromatic core and hexa-
substituted oligoethers as the branched peripheral chains have been synthesized, and their microstructure
has been characterized. The discotic molecules based on dibranched oligoether side chains have been
observed to self-organize into a well-ordered hexagonal columnar structure within liquid crystalline phases,
which possessed an exceptionally high thermal stability and an unusually wide temperature range over
>300 °C. We suggest that a combination of the large lateral dimensions of the rigid cores and disordered
structure of the oxygen-containing branches tails is a driving force to the formation of a highly ordered
columnar structure in the bulk state with enhanced molecular segregation. In contrast to the thermotropic
phase behavior that favors the formation of highly ordered columnar aggregates through a strong stacking
interaction, the hexabenzocoronene cores are packed in a face-on arrangement at the air/water interface
and on solid surfaces with surface domains composed of an array of 7 × 7 molecules. We suggest a
crablike molecular conformation and cluster-segregated monolayers with 6-fold symmetry and unusual
face-on packing on a solid surface. Preliminary spectroscopic studies in the bulk state have shown that
the molecules based on a hexaaromatic-substituted core may serve as functional supramolecular materials
with high energy transfer characteristic within the columns due to near-perfect columnar ordering, which is
unchanged over a wide temperature range. We believe that an absence of the crystallization phenomenon
of side-branched oligoether chains is critical for the formation of long-range columnar ordering with strong
intracolumnar correlation of conjugated disks important for high carrier mobility.
Introduction
peri-hexabenzocoronene derivatives. They have a propensity to
form either hexagonal or rectangular columnar mesophases, with
The construction of disk-shaped molecules with rigid aromatic
cores opens a way to novel supramolecular aggregates, which
have attracted great interest in molecular and supramolecular
materials. Rigid disk-shaped molecular architectures containing
flexible chains on their periphery (“hairy disks”) have proven
to self-assemble into a large variety of organized supramolecular
columns.1-3 The individual columns can serve as nanowires in
molecular electronic devices for one-dimensional transport
processes such as energy migration, electric conductivity, and
photoconductivity with exceptionally high transport properties
along the columnar axes.4-6 A typical example of disk-shaped
molecules is provided by triphenylene and hexasubstituted hexa-
the flat aromatic core assisting in their formation.7-9 The
remarkable properties of the columnar mesophases of hexa-peri-
hexabenzocoronene derivatives include a charge carrier mobility
that is exhibited to be much higher than that of small discotic
molecules such as triphenylene derivatives due to the formation
of highly ordered columnar structures.10 Increasing the size of
the aromatic core can lead to highly ordered supramolecular
columns because the molecular architecture imparts microphase
separation of the rigid and flexible blocks into ordered periodic
(4) Adam, D.; Schuhmacher, P. S.; Simmerer, J.; Haussling, L.; Siemensmeyer,
K.; Etzbach, K. H.; Ringsdorf, H.; Haarer, D. Nature 1994, 371, 141.
(5) Adam, D.; Closs, F.; Frey, T.; Funhoff, D.; Haarer, D.; Ringsdorf, H.;
Schuhmacher, P.; Siemensmeyer, K. Phys. ReV. Lett. 1993, 70, 457.
(6) Boden, N.; Bushby, J.; Clements, J.; Jesudason, M. V.; Knowles, P. F.;
Williams, G. Chem. Phys. Lett. 1998, 152, 94
(7) (a) Herwig, P.; Kayser, C. W.; Muellen, K.; Spiess, H. W. AdV. Mater.
1996, 8, 510. (b) Lee, M.; Jung, Y.-S.; Cho, B.-K.; Oh, N.-K.; Zin, W.-C.
Chem.-Eur. J., in press.
(8) Fechtenkoetter, A.; Saalwaechter, K.; Harbison, M. A.; Muellen, K.; Spiess,
H. W. Angew. Chem., Int. Ed. 1999, 38, 3039.
(9) Ito, S.; Wehmeier, M.; Brand, J. D.; Kuebel, C.; Epsch, R.; Rabe, J. P.;
Muellen, K. Chem.-Eur. J. 2000, 6, 4327.
(10) Van de Craats, A. M.; Warman, J. M.; Muellen, K.; Geerts, Y.; Brand, J.
D. AdV. Mater. 1998, 10, 36.
* To whom correspondence should be addressed. E-mail: (M.L.)
mslee@yonsei.ac.kr; (V.V.T.) vladimir@iastate.edu.
† Yonsei University.
‡ Department of Materials Science & Engineering, Iowa State University.
§ Ames Laboratory and Department of Physics and Astronomy, Iowa
State University.
(1) Chandrasekhar, S. Liq. Cryst. 1993, 14, 3.
(2) Allen, M. T.; Diele, S.; Harris, K. M. D.; Hegmann, T.; Kariuki, M.; Lose,
D.; Preece, J. A.; Tschierske, C. J. Mater. Chem. 2001, 11, 302.
(3) Raja, K. S.; Ramakrishnan, S.; Raghunathan, V. A. Chem. Mater. 1997, 9,
1630.
9
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J. AM. CHEM. SOC. 2002, 124, 9121-9128
9121

A R T I C L E S
Lee et al.
Scheme 1. Synthesis of the Discotic Molecules Based on
Hexaphenyl Hexa-peri-hexabenzocoronene
Scheme 2. Synthesis of the Discotic Molecules Based on
Hexabiphenyl Hexa-peri-hexabenzocoronene
structures in combination with the parallel arrangement provided
by rigid cores.11 On the other hand, incorporation of hydrophilic
chains into the periphery of an aromatic core can significantly
enhance amphiphilic balance and result in reorganization of
edge-on orientation of the columnar structures at the air/water
interface and on solid substrates. Natural face-on orientation of
discotic molecules is important for formation of molecular films
with fast charge transfer across the film. Other complications
in chemical architecture, such as branching of the peripheral
tails, may induce amphiphilic behavior analogous to dendrimer
compounds rather than conventional discotic phases.12
In this article, we report the self-assembling behavior of am-
phiphilic discotic molecules 7a, 7b, and 16, consisting of a rigid
coronene core and hydrophilic branched oligoethers (Schemes
1, 2). We focus on the bulk microstructure and UV adsorption
properties of these compounds and their potentials to form
organized molecular films on solid surfaces as determined by
the length and degree of branching of the peripheral tails.
chemical structure of all products is characterized by MALDI-TOF-
MS, gel permeation chromatography (GPC), and 1H NMR and 13C
NMR. Bulk properties are studied using differential scanning calorim-
etry and X-ray scattering measurements performed in transmission mode
with synchrotron radiation at the 3C2 X-ray beam line at Pohang
Accelerator Laboratory, Korea. Monomolecular films of the dendritic
compounds are prepared by the Langmuir technique and are deposited
on solid substrates following the usual Langmuir-Blodgett (LB)
procedure.13 Monolayer thickness is measured by ellipsometry, and
imaging of monolayers is performed with scanning probe microscopes
in the tapping mode according to an experimental procedure described
in detail earlier.14-16 X-ray reflectivity measurements from Langmuir
monolayers at the air/water interface are conducted on an X-ray
spectrometer at the 6ID beam line at the Advanced Photon Source
synchrotron at Argonne National Laboratory.17-19 Molecular models
are created using Cerius2 software. For technical detail, see the
Supporting Information.
(13) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press:
San Diego, CA, 1991.
Experimental Section
(14) Tsukruk, V. V. Rubber Chem. Technol. 1997, 70, 430.
(15) Tsukruk, V. V.; Reneker, D. H. Polymer 1995, 36, 1791.
(16) Ratner, B.; Tsukruk, V. V. Scanning Probe Microscopy of Polymers; ACS
Symposium Series, 1998; Vol. 694.
(17) Vaknin, D. In Methods of Materials Research; Kaufmann, E. N., Abb-
aschian, R., Baines, P. A., Bocarsly, A. B., Chien, C. L., Doyle, B. L.,
Fultz, B., Leibowitz, L., Mason, T., Sanches, J. M., Eds.; John Wiley &
Sons: New York, 2001; p 10d.2.1.
Compounds 1a, 1b, 8, and 4-trimethylsilylphenylboronic acid are
synthesized using the procedures reported previously.7b,12 Thin-layer
chromatography is used to check the purity of the products. The
(11) (a) Tschierske, C. J. Mater. Chem. 1998, 8, 1485. (b) Lee, M.; Cho, B.-K.;
Zin, W.-C. Chem. ReV. 2001, 101, 3869.
(12) Tully, D. C.; Frechet, J. M. J. Chem. Commun. 2001, 1229. Kampf, J. P.;
Frank, C. W. Langmuir 1999, 15, 227. Pao, W. J.; Stetzer, M.; Heiney, P.;
Cho, W. D.; Percec, V. J. Phys. Chem. B 2001, 105, 2170.
(18) Weissbuch, I.; Leveiller, F.; Jacquemain, D.; Kjaer, K.; Als-Nielsen, J.;
Leiserowitz, L. J. Phys. Chem. 1993, 97, 12858.
(19) Vaknin, D.; Kelley, M. S. Biophys. J. 2000, 79, 2616.
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Amphiphilic Hairy Disks with Hydrophilic Tails
A R T I C L E S
Table 1. Characterization of Discotic Molecules 7a, 7b, and 16
intermolecular
distance (d001) nm
molecule
M /M a
state
columnar
d
100 nm
w
n
7a
1.03
3.29
0.349
hexagonal phaseb
isotropic liquid
columnar
7b
16
1.03
1.06
3.84
0.346
hexagonal phaseb
a Determined from GPC. b Limited by decomposition for T > 350 °C.
LC state exists at room temperature. Gellike state is characteristic of all
compounds.
Results and Discussion
Chemical Microstructure. The synthesis of these molecules
was performed in a stepwise fashion starting with a convergent
route of oligoether dendron using etherification chemistry7b,20
and continuing with Sonogashira reactions to generate tolane
derivatives 5a, 5b, 14.7a,8-10 Subsequent cyclotrimerization of
these molecules, catalyzed by [Co2(CO)8], in dioxane afforded
the compounds 6a, 6b, 15. The amphiphilic discotic molecules
(7a, 7b, 16) were obtained by oxidative cyclodehydrogenation
with a solution of FeCl3 in nitromethane. The resulting
compounds were purified by column chromatography (silica gel)
using ethyl acetate as an eluent.
Figure 1. Representative X-ray diffraction data of molecule 7a at room
temperature.
relatively high and can be attributed to high level of ordering.
All small-angle peaks are extremely sharp (close to the spatial
resolution of the instrument), which indicates very well-defined
long-range positional ordering in hexagonal lattice.
1
The final products were characterized by H and 13C NMR
spectroscopies elemental analysis, and were shown to be in a
full agreement with the chemical structures presented in
Schemes 1, 2 (see Supporting Information). In addition, GPC
data showed a very narrow molecular weight distribution with
the polydispersity index within 1.03-1.06, which indicated very
uniform chemical composition and a minute presence of other
fractions (Table 1).
In addition to these small-angle reflections, a broad halo
centered at 0.47 nm was observed, which is associated with
the liquidlike packing of flexible oligoether chains (Figure 1).21
The sharp (001) reflections at 0.349 and 0.346 nm for 7a and
16, respectively, indicate the well-defined periodic stacking of
the aromatic cores within the columns, usual for columnar
phases of discotic compounds.22,23 Compared to other similar
molecules,8,9 these intermolecular distances indicate an improved
long-range ordering of the molecules with spatial correlations
extending well beyond the short-range order and involving more
than 20 molecules in the column as evaluated from the width
of the (001) peaks according to the Sherrer equation.22-26 These
results indicate that the molecules 7a and 16 exhibit an ordered
hexagonal columnar mesophase. It should be noted that the
incorporation of dibranched oligoethers on the periphery of the
aromatic core suppresses a crystalline phase giving rise to a
room-temperature columnar mesophase with high thermal
stability, in contrast to the similar discotic molecules containing
peripheral n-dodecyl chains.8,9 Moreover, the discotic molecules
based on tetrabranched oligoether (7b) show only an isotropic
phase at room temperature. This is most likely because of the
higher conformation entropy of bulkier peripheral chains and
because of the fact that the volume fraction of rigid cores
became small, preventing their effective segregation into ordered
columns. We suggest that extra branching of the peripheral
chains (four short tails per one site) plays a significant role,
providing additional space constraints for the dense packing of
the rigid cores forcing the chains out of the disk plane. Indeed,
molecular modeling (Figure 2) shows that the effective thickness
Bulk Microstructure and Properties. The phase behavior
of the discotic molecules in the bulk state was investigated by
means of optical polarized microscopy and differential scanning
calorimetry (DSC) in combination with powder X-ray scatterings
(Table 1). The molecules based on dibranched oligoethers (7a
and 16) show a birefringent liquid crystalline (LC) phase at room
temperature without any signs of crystallization. Both com-
pounds are viscous gels at room temperature. This LC state is
retained at elevated temperature up to decomposition temper-
atures at approximately 350 °C without significant changes of
optical texture, as confirmed by optical microscopy and DSC.
The X-ray scattering study of 7a in the bulk state using
synchrotron radiation revealed a series of five well-resolved
reflections, which could be indexed as two-dimensional hex-
agonal columnar order in a usual manner (Figure 1).21-23 The
center-to-center distances between neighboring columnar struc-
tures, a, determined from the (100) reflections according to a
relationship for hexagonal lattice a ) 2d100/x3 are 3.81 and
4.43 nm for 7a and 16, respectively. These are lower than the
lateral dimensions of the disks as estimated from molecular
models for extended chains (5.0 and 6.1 nm, Figure 2), which
indicates a certain level of interdigitation of the peripheral groups
from neighboring columns. The number of reflections is
(20) Jayaraman, M.; Frechet, J. M. J. J. Am. Chem. Soc. 1998, 120, 12996.
(21) Janietz, D.; Festag, R.; Schmidt, C.; Wendorff, J. H.; Tsukruk, V. V. Thin
Solid Films 1996, 284/285, 289.
(24) Karthaus, O.; Ringsdorf, H.; Tsukruk, V. V.; Wendorff, J. H. Langmuir
1992, 8, 2279.
(22) Tsukruk, V. V.; Shilov, V. V. Structure of Polymeric Liquid Crystals;
Naukova Dumka: Kiev, 1990.
(25) Tsukruk, V. V.; Wendorff, J. H.; Karthaus, O.; Ringsdorf, H. Langmuir
1993, 9, 614.
(23) Janietz, D. J. Mater. Chem. 1998, 8, 285. Mo¨ller, M.; Tsukruk, V. V.;
Wendorff, J. H.; Bengs, H.; Ringsdorf, H. Liq. Cryst. 1992, 12, 17.
(26) Tsukruk, V. V.; Reneker, D. H.; Bengs, H.; Ringsdorf, H. Langmuir 1993,
9, 2141.
9
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A R T I C L E S
Lee et al.
Figure 3. Absorption and emission spectra of 16 in bulk state. The thick
solid line represents the UV absorption spectrum, the thin solid line presents
the 286 nm excitation spectrum, and the dashed line represents the 382 nm
excitation.
even at room temperature that, additionally, results in an
exceptionally wide range of the LC columnar state in these
compounds exceeding 300 °C.
We tested preliminary optical properties of the molecules in
LC state. All molecules possess similar UV spectra with strong
adsorption in the ranges of 260-285 nm and 360-385 nm in
the bulk state similar to that observed in solution. Figure 3 shows
an example of absorption spectra for 16 in the bulk state. The
absorption spectrum of this molecule exhibits two distinct bands
at 286 and 382 nm, corresponding to the biphenyl unit and the
hexa-peri-hexabenzocoronene core, respectively.27 We observed
that when the molecule was excited at 286 nm where most of
the radiation is absorbed by the biphenyl unit, the fluorescence
spectrum exhibits a strong increase with a maximum at 558
nm, corresponding to the hexa-peri-hexabenzocoronene core.
Because the emission attributable to the biphenyl unit at 360
nm disappears almost completely, this change indicates that
energy transfer takes place between the biphenyl and the
hexabenzocoronene units of the molecule within the supramo-
lecular columns.27 Its appearance and peculiar properties are
the subject of a future investigation.
Figure 2. Molecular models of compounds studied in top-view (left) and
side-view (right) projections. The dashed circles represent the surface area
calculated to include the hexabenzocoronene and terminal phenyl rings.
Surface Microstructure. As a next step, we considered the
molecular organization of the discotic molecules at solid surfaces
transferred with the Langmuir-Blodgett (LB) technique as a
first step in the formation of organized thin films with controlled
molecular packing and transport properties. Initially, pressure-
area isotherms were recorded for all compounds on a water
subphase after residual solvent evaporation (Figure 4). The
pressure-area isotherms for 7a and 16 possess shapes common
for amphiphilic molecules with large surface areas because of
considerable cross-sectional areas of discotic cores.13 They are
characterized by two distinct phase regions as the surface
pressure is increased. The areas per molecule at the two
monolayer transformations of 7a, obtained by extrapolation to
a zero pressure, are 5.80 and 1.40 nm2, respectively (Figure 4).
Molecular models show the theoretical surface area of the
aromatic core itself equals 1.02 nm2. The surface area occupied
by the molecule increases to 2.70 nm2 if terminal phenyl rings
are included (Figure 2). The dashed circles in Figure 2 represent
the area of the total circle calculated with a diameter of phenyl
ring to ring. This area would represent the cores packing with
of the shell area is higher for the tetrabranched compound as
compared with the dibranched molecules, thus preventing dense
packing of the thin (about 0.35 nm) central disks. Very short
branches cannot be arranged within the disk plane and inevitably
form loosely packed randomly oriented branches.
The microphase segregation between the hydrophilic flexible
and hydrophobic rigid blocks of these molecules is likely to be
the main driving force for the bulk microstructure and phase
behavior. As known, this is a major mechanism of the LC phase
formation in organic compounds with rigid rod-flexible tail
architecture.22 For these compounds, the effective dimension
of the rigid disks and the flexible tails is crucial in defining the
degree of segregation and molecular ordering. Obviously, the
large lateral dimensions of the hexa-peri-hexabiphenylcoronene-
based cores provide a much stronger trend for segregation and
formation of strongly coupled columnar structures as compared
with traditional LC discotics. On the other hand, branched
oligoether peripheral tails are much less ordered than traditional
alkyl tails that provide additional driving forces and promote
segregated packing of the rigid cores. These two factors are
important reasons for the formation of exceptionally ordered
columnar phases in the bulk state with outstanding thermal
stability. Moreover, a suppressed tendency to the crystallization
of the peripheral groups prevents crystallization of the LC phase
(27) Rau, H. In Photoisomerization of Azobenzenes in Photochemistry and
Photophysics; Rabek, J. F., Ed.; CRC Press: Boca Ranton, FL, 1990; Vol.
II, p 119. Sidorenko, A.; Houphouet-Boigny, C.; Villavicencio, O.;
McGrath, D. V.; Tsukruk, V. V. Thin Solid Films 2002, 410, 147. Devadoss,
C.; Bharathi, P.; Moore, J. S. J. Am. Chem. Soc. 1996, 118, 9635.
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Amphiphilic Hairy Disks with Hydrophilic Tails
A R T I C L E S
Table 2. Structural Parameters of Monolayers of 7a and 16
Deduced from Fitting Reflectivity Data with the Two-Box Models
for Different Pressures
7a
16
pressure (mN/m)
7
10
1.40
0.40
0.59
0.11
0.33
7
10
2.11
0.39
0.62
0.86
0.60
tail length (nm)
1.04
0.44
0.69
0.08
0.27
1.09
0.40
0.36
0.12
0.26
tail density (10-3 e/nm3)
core length (nm)
core density (10-3 e/nm3)
roughness (nm)
molecules at two pressures analyzed here (Table 2). On the other
hand, the low electron density of aromatic core areas reflects
their loose packing within the monolayer at very low surface
pressures. The length of the terminal tails projected on the
surface normal is equal to 1.0 nm for molecule 7a, slightly lower
than the contour length of 1.5 nm. Increasing surface pressure
slightly above the first transformation resulted in thickening of
the “tail layer” to 1.4 nm and a modest increase of the packing
density of aromatic cores (Table 2). These parameters cor-
respond to a crablike conformation of the discotic molecules
with the oligoether terminal chains being completely submerged
in water (Figure 6).
The thickness of the monolayers deposited on a solid substrate
was measured by ellipsometry and represents “effective” thick-
ness calculated assuming a uniform coverage and a value of
refractive index close to known bulk values. The uniform
coverage was confirmed by SPM, and possible variations of
the refractive index could give an error margin of 20% at most.
For monolayers deposited at pressures below the first transition,
the effective thickness equals 0.6 nm, which confirms virtually
flat orientation of the molecules. This demonstrates that the
presence of branched, water-soluble peripheral chains instead
of traditional hydrophobic single alkyl chains causes restructur-
ing from traditional, edge-on orientation of discotic molecules23-26
to flat, face-on orientation on both water and solid surfaces.
Increasing the pressure to slightly above the transition to the
condensed state results in a monolayer effective thickness of
1.2 nm corresponding to the molecular dimension of molecules
in a crablike conformation (Figure 6).
Figure 4. Pressure-area isotherms of Langmuir monolayers of compounds
studied.
no interdigitation of the terminal phenyl rings. The theoretical
cross-sectional area per molecule increases to 10.50 nm2 if
extended peripheral tails are located in the plane of the discotic
core and no interdigitation occurs. This is only slightly lower
than the cross-sectional area per column in the bulk LC state
calculated from the center-to-center distance (11.4 nm2).
Therefore, the experimentally observed cross-sectional areas for
first transformation (5.80 nm2) immediately point to interdigi-
tation of side chains in a condensed monolayer state. Moreover,
in a solid monolayer state (region with a sharp rise of the surface
pressure, Figure 4), significant bending of the hydrophilic tails
should occur to accommodate a dense packing of hard disks
with the cross-sectional area below 2 nm2 per molecule.
Therefore, we suggest that, at the first transformation, the
molecules adopt an orientation in which the hydrophobic
aromatic core lies flat on the water surface and the hydrophilic
peripheral chains are partially interdigitated and bent toward
the water surface, while at the second transformation they are
completely bent and submerged in the water subphase. Indeed,
X-ray reflectivity data confirm this suggestion and provide
quantitative parameters of structural organization.
In fact, X-ray reflectivity data confirm flat-on orientation of
the rigid cores at the air/water interface at low surface pressures.
It shows one broad maximum with fast decreasing intensity for
Q > 3.0 nm-1 (Figure 5). The best fit of the experimental data
can be obtained with a two-box model with higher density of
the flexible tails submerged in the water phase and lower density
of the loosely packed aromatic cores. The higher electron density
of terminal chains indicates significant presence of the water
Compression of the monolayer above 6 mN/m at the air/water
interface resulted in a steady increase of the surface pressure
over a wide range of areas followed by a sharp rise at very
small cross-sectional areas as was discussed above (Figure 4).
Considering that this increase happened at 1.40 nm2, which is
Figure 5. (a) X-ray reflectivity data at air/water interface for 7a and 16 with the best fit; the symbols represent the data, while the solid lines represent the
fits. (b) The two-box model with sharp interfaces and corresponding smeared electron density distribution along the normal to the surface plane for the
highest pressure for 7a.
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A R T I C L E S
Lee et al.
Figure 6. Schematic representations of 7a at the air/water interface and
deposited on solid substrate: (a) molecules are at low surface pressure, (b)
oligoether chains submerged in subphase as surface pressure is increased,
(c) formation of bilayer at the air/water interface, and (d) the formation of
LB film on the solid surface.
smaller than the area required for flat-on orientation but larger
than the area required for edge-on orientation of a single
molecule, we can suggest that this transition corresponds to a
collapse of a single monolayer and the gradual formation of
the bilayer structure (Figure 6). Edge-on orientation is dis-
counted also by the much smaller than expected thickness of
the high surface pressure monolayer films. The molecular model
estimates the height of edge-on oriented molecules to be in the
range from 3.8 to 5.0 nm. The total effective layer thickness is
much lower (2.7 nm), which confirms the bilayer film structure
of crablike molecules (Figure 6).
Direct confirmation of the formation of the microstructure
with 6-fold symmetry came from AFM imaging of the mono-
layers (Figure 7). At surface pressures below the first transfor-
mation, only uniform, featureless morphology was observed.
Similarly, at pressures greater than 20 mN/m, we observed
uniform surface morphology. However, at pressures in the
beginning of the sharp pressure rise, characteristic domain
texture was recorded with sizes of uniform domains of several
micrometers across (Figure 7). Six-fold orientational symmetry
clearly shows up. It manifests itself in needlelike islands
preferably oriented along one of three axes, which are frequently
intersected at ∼60 and ∼120° angles. Examples of the preferred
orientation of the islands are demonstrated in Figure 8 with the
desired axes labeled. The height of these islands determined
by AFM is 1.5-1.7 nm, close to the estimated thickness of the
molecules in the crablike conformation calculated by molecular
models, and is about 50% of the total effective thickness of the
films derived from ellipsometry.
Therefore, at intermediate surface pressures (6-10 mN/m),
we observed the initial stages of the formation of the second
layer through the initial growth of anisotropic islands. Symmetry
of the island orientations allows one to conclude that, first, the
underlying layer possesses 6-fold symmetry that is com-
mensurate with face-on orientation of the molecules at the
interface. Indeed, high-resolution imaging of the LB films
displayed additional details of surface structure and confirmed
its 6-fold character (Figure 8). First, all islands have a very
uniform width of 30-40 nm and a length in the range from
100 to 200 nm as estimated after correction on the SPM tip
dilation. They show clear grainy structures with, on average,
Figure 7. SPM images obtained in tapping mode: (a) 10 × 10 µm scan
of 7a at 12 mN/m surface pressure, (b) 2 × 2 µm scan of monolayer area,
(c) 2 × 2 µm scan of 16 at 10 mN/m surface pressure. The left images are
topography, and the right images are phase shift.
the islands composed of two grains with a width of 20 nm across
each (Figure 8a).
Scanning of the monolayer in the surface areas with scarce
islands resolved an ordered lattice with periodicity of about 24
nm (Figure 8b). This lattice generates the 2-D Fourier pattern
that confirms its 6-fold symmetry (inset of Figure 8b). Clusters,
which compose this lattice, have lateral sizes close to 20 nm.
Considering the average diameter of the 7a molecules in the
crablike conformation of about 3 nm, we can suggest that a
single cluster is composed of an array of 7 × 7 molecules. The
nature of such aggregation is not clear but, we suggest, is related
to in-plane frustration during structural relaxation caused by
transfer from water to solid surface. Indeed, as we suggested
for the monolayer at the water surface, the oligoether tails are
submerged in the water (Figure 6). In contact with a solid silicon
oxide surface, they are in more confined conditions with a minor
presence of water and must rearrange themselves to adapt a
monolayer of 1-1.2 nm thickness by partially rearranging
themselves beneath the aromatic cores. It can lead to contraction
of the lateral packing and the formation of in-plane aggregates
separated by regular defects.
Molecule 16, based on a larger aromatic core, also shows a
very similar phase behavior and monolayer microstructure to
that of 7a. Indeed, the pressure-area isotherm has the same
shape except slightly larger cross-sectional areas per molecule
at both transitions caused by the presence of additional phenyl
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9126 J. AM. CHEM. SOC. VOL. 124, NO. 31, 2002

Amphiphilic Hairy Disks with Hydrophilic Tails
A R T I C L E S
ity to form well-ordered columnar phases with exceptional
thermal stability in the bulk state and nontrivial face-on
molecular organization on solid surfaces. We have synthesized
amiphiphilic discotic molecules based on a hexa-peri-hexaben-
zocoronene core and oligoether peripheral chains with different
lengths and degrees of branching. The molecules based on
dibranched oligoether chains have been observed to organize
into hexagonal columnar LC structure at room temperature with
suppressed crystallization of the side chains usually observed
for alkyl tails. These mesophases are extremely thermally stable
and possess an exceptionally wide temperature interval of
columnar order extending over >300 °C. We suggest that a
combination of the large lateral dimensions of the rigid cores
and disordered structure of the oxygen-containing side branches
is a driving force to the formation of the highly ordered
columnar structure in the bulk state. Enhanced molecular
segregation results in the exceptional thermal stability of the
columnar ordering surpassing anything previously known for
the discotic compounds.
We speculate that these molecules may provide access to
prospective functional organic materials with characteristic high
energy transfer due to near-perfect columnar ordering, which
is unchanged over a wide temperature range. We believe that,
for this purpose, an absence of the crystallization phenomenon
of side-branched oligoether chains (unlike that usually observed
for alkyl-based discotics) is critical for the formation of long-
range columnar ordering with strong intracolumnar correlation
of conjugated disks important for high carrier mobility. As
known, cocrystallization of side alkyl chains is a dominant
reason for disturbance of columnar ordering and disruption of
the charge transfer along the columnar structures.31 In fact, the
hexa-peri-hexabenzocoronene cores with mainly alkyl side
chains have been used before to synthesize hairy disks and
fabricate organized films with promising high charge-transfer
mobility.10,32,33 However, cocrystallization phenomenon and,
thus, coexistence of two crystalline phases resulted in distorted
columnar ordering and highly skewed π-stacking reducing
dramatically the transfer properties of these compounds.
The introduction of side chains with hydrophilic terminal
groups has been used to induce sufficient amphiphilic balance
required for the fabrication of organized molecular films with
the LB technique.32 As a result of such an asymmetric balance,
a highly tilted packing of rigid cores and their edge-on
orientation has been observed for molecular films deposited on
solid surfaces. This ordering, along with additional crystalliza-
tion of alkyl chains, is unfavorable for the high charge transport
within surface films. Moreover, for such prospective applications
as light-emitting devices, the necessary direction of charge
transfer is across the film-substrate interface. This requires
orientation of the columnar structures perpendicular to the solid
surface.21-26 The approach proposed in this work, in fact,
promotes well-organized face-on orientation of the hairy disk
molecules, which is critical for perpendicular orientation of the
Figure 8. SPM images obtained in tapping mode: (a) 900 × 900 nm
topography scan of 7a at 12 mN/m surface pressure demonstrating
selectively oriented anisodiametric islands of second layer formed on top
of a monolayer, (b) 250 × 250 nm topography scan of a monolayer surface
with corresponding 2-D Fourier transformation of the image (insert).
rings in the aromatic core and an additional monomeric unit in
the oligoether tails (Figure 2). The first transition at 7.10 nm2
corresponds to the calculated area of an aromatic core (circle
with a diameter of 3.1 nm), indicative of face-on orientation to
the water surface with bent side chains similar to the 7a
molecule (Figure 6). The thickness of the monolayer at the air/
water interface, obtained from X-ray reflectivity data, is virtually
indistinguishable from the effective thickness of the monolayer
7a (Table 2). Also, very similar to that of 7a, the second
transition at 1.80 nm2 corresponded to the formation of the
second layer via needlelike island growth (Figure 7). The
effective thickness of the film (0.56 nm at 5 N/m) and the
parameters of the microstructural ordering of 16 are very similar
to that observed for 7a (Table 2). In contrast, molecule 7b based
on tetrabranched oligoethers shows steady increase of the surface
pressure during gradual collapse of the monolayer and the
formation of a disordered film.
(28) Mindyuk, O. Y.; Heiney, P. A. AdV. Mater. 1999, 11, 341.
(29) Janiets, D.; Katholy, S.; Dietzel, B.; Brehmer, L. Langmuir 1997, 13, 7217.
(30) Smolenyak, P.; Peterson, R.; Nebesny, K.; Toerker, M.; O’Brien, D. F.;
Armstrong, N. R. J. Am. Chem. Soc. 1999, 121, 8628.
(31) Bjornholm, T.; Hassenkam, T.; Reitzel, N. J. Mater. Chem. 1999, 9, 1974.
(32) Reitzel, N.; Hassenkam, T.; Balashev, K.; Jensen, T. R.; Howes, P. B.;
Kjaer, K.; Fechtenotter, A.; Tchebotareva, N.; Ito, S.; Mullen, K.;
Bjornholm, T. Chem.-Eur. J. 2001, 7, 4894.
Concluding Discussion
In summary, we introduced a novel type of “hairy” disk
molecule with branched peripheral chains and enhanced capabil-
(33) Christ, T.; Glussen, B.; Greiner, A.; Kettner, A.; Sander, R.; Stumpflen,
V.; Tsukruk, V. V.; Wendorff, J. H. AdV. Mater. 1997, 9, 48.
9
J. AM. CHEM. SOC. VOL. 124, NO. 31, 2002 9127

A R T I C L E S
Lee et al.
columnar structure and charge transfer across the film thickness
(i.e., between two electrodes).33,34 This dramatically distin-
guishes this class of molecules from all well-known hairy disks
with orientation of columnar structures which overwhelmingly
occurs in-plane of solid surfaces (edge-on orientation).
It is worth noting that this interfacial behavior is in contrast
to that in other symmetrically substituted rigid discotic molecules
with single side alkyl tails which favor an edge-on orientation
at the air/water interface.23-26,28-30 This interesting behavior is
believed to originate from a strong hydrophilic character of the
symmetrically substituted peripheral oligoether chains that have
a tendency to dip into the water subphase. On the other hand,
branching creates more challenging conditions for local ordering
of the side chains and prevents edge-on orientation. These results
demonstrate that the symmetrical incorporation of hydrophilic
chains at the periphery of an extended aromatic core provides
a powerful strategy to combine thermotropic liquid crystalline
behavior and amphiphilic self-organization within one molecule.
This approach is much more efficient than a common strategy
adapted widely of fractional substitution of a selected terminal
group with a hydrophilic group, which produces crystallizable
films with edge-on orientation of poorly ordered columnar
structures.
For dibranched molecules in the Langmuir monolayers, we
proposed the crablike molecular conformation with the branched
side chains submerged in water. This organization differs
drastically from columnar packing in the bulk state where side
chains are close to extended conformation and are, indeed,
frequently crystallized as a separate phase. This also demon-
strates that these branched compounds are very different from
conventional discotic molecules with hydrophobic, single-tail
side chains. After transfer to a solid substrate, the molecules
with dibranched short peripheral chains formed an organized
monolayer on solid supports with 6-fold symmetry of face-on
molecular packing and cluster aggregation. We attributed
aggregate formation to monolayer relaxation caused by periph-
eral chain restructuring during transfer from the water surface
to the solid substrate. Formation of the second molecular layer
on top of the surface layer showed well-defined 6-fold symmetry
that indicates very strong stacking correlations among rigid
cores, which overcomes usual poor ordering for the first surface
layer due to the disturbing influence of the solid substrate. This
is promising for further formation of perpendicularly oriented
columnar structures with high charge transfer across the film
thickness. On the other hand, a high level of flexibility of
peripheral chains and their readiness to change conformation
at different surface pressures may be a promising sign for
controlled manipulation of columnar ordering and, thus, transfer
properties.
Acknowledgment. This paper is dedicated to Prof. Yu.
Lipatov on the occasion of his 75th birthday. The authors thank
Dr. A. Sidorenko for technical assistance and useful discussion.
Funding from the National Science Foundation, DMR-0074241,
as well as support from CRM-KOSEF(2001) and KRF(BSRI
DP0228), is gratefully acknowledged. The Midwest Universities
Collaborative Access Team (MUCAT) sector at the APS is
supported by the U.S. Department of Energy, Basic Energy
Sciences, Office of Science, through the Ames Laboratory under
Contract No. W-7405-Eng-82. Use of the Advanced Photon
Source was supported by the U.S. Department of Energy, Basic
Energy Services, Office of Science, under Contract No. W-31-
109-Eng-38.
Supporting Information Available: Description of experi-
mental instrumentation and synthetic details and characterization
for 2-7 and 9-16 (PDF). This material is available free of
charge via the Internet at http://pubs.acs.org.
(34) Kevenhorster, B.; Kopitzke, J.; Seifert, A. M.; Tsukruk, V. V.; Wendorff,
J. H. AdV. Mater. 1999, 11, 246.
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