Self-assembling molecular trees containing Octa-p-phenylene: From nanocrystals to nanocapsules

Article abstract of DOI:10.1021/ja048856h

Tree-shaped molecules consisting of octa-p-phenylene as a stem segment and oligoether dendrons as a flexible head were synthesized and characterized. The molecular tree based on a small flexible head self-assembles into a lamellar structure, whereas the m

Full text of DOI:10.1021/ja048856h

Published on Web 04/30/2004
Self-Assembling Molecular Trees Containing
Octa-p-phenylene: From Nanocrystals to Nanocapsules
Yong-Sik Yoo,† Jin-Ho Choi,† Ji-Ho Song,† Nam-Keun Oh,‡ Wang-Cheol Zin,‡
Soojin Park,§ Taihyun Chang,§ and Myongsoo Lee*,†
Contribution from the Center for Supramolecular Nano-Assembly and Department of Chemistry,
Yonsei UniVersity, Shinchon 134, Seoul 120-749, Korea, and the Departments of Materials
Science and Engineering and Chemistry, Pohang UniVersity of Science and Technology,
Pohang 790-784, Korea
Received March 1, 2004; E-mail: mslee@yonsei.ac.kr
Abstract: Tree-shaped molecules consisting of octa-p-phenylene as a stem segment and oligoether
dendrons as a flexible head were synthesized and characterized. The molecular tree based on a small
flexible head self-assembles into a lamellar structure, whereas the molecule based on a larger headgroup
self-assembles into a discrete heptameric bundle that organizes into a 3-D primitive orthorhombic
supercrystals, as confirmed by X-ray scatterings and transmission electron microscopic (TEM) observations.
Optical studies revealed that the absorption and emission maxima and absorption edge of the 3-D structure
shift to higher energy compared to those of the lamellar structure. The molecules in dilute solution (THF/
water ) 1:10 v/v) were observed to self-assemble into capsule-like hollow aggregates, as confirmed by
dynamic and static light scatterings, scanning electron microscopy (SEM), and TEM investigations. These
results demonstrate that tree-shaped molecules are capable of packing into organized discrete nanocrystals
with parallel arrangement as well as hollow nanocapsules with radial arrangement, depending on the
presence of selective solvents for flexible headgroup.
Introduction
and the properties can be tuned by careful selection of the type
and relative length of the respective blocks.
An emerging field of chemistry involves the design and
synthesis of self-assembling molecules that organize into regular
organic nanostructures.1 Self-assembling molecules based on
conjugated rod building blocks promise the opportunity to
explore desired functions and properties as a result of aggrega-
tion into well-defined supramolecular architectures.2 Thus,
diverse molecular structures are being created as a means of
manipulating aggregation structure which has dramatic effects
on the physical properties of materials. Incorporation of a
conjugated rod into a block molecular architecture leads to a
novel class of self-assembling molecules, where the anisotropic
orientation of the rod segments and repulsion between the
covalently connected segments lead to self-organization into
unique aggregation structures.3 The aggregation architectures
Previous publications from our laboratory reported synthesis
and structural analysis of rod-coil block systems that self-
assemble into lamellar, cylindrical, and discrete nanostructures
depending on the relative volume fraction of the rod segments.4
In addition, we have demonstrated that rod-coil systems with
an elongated rod block self-assemble into discrete bundles or
perforated layers that organize into 3-D tetragonal or 3-D
hexagonal superlattices, respectively.5 The shape and size of
the aggregation structure have also been reported to have a
strong influence on the photophysical properties of conjugated
molecular materials.6 Thus, manipulation of aggregation struc-
ture in conjugated systems is of paramount importance in
achieving desirable properties in supramolecular materials.
(3) (a) Leclere, P.; Calderone, A.; Marsitzky, D.; Francke, V.; Geertz, Y.;
Mu¨llen, K.; Bredas, J. L.; Lazzaroni, R. AdV. Mater. 2000, 12, 1042-
1046. (b) Hempenius, M. A.; Langeveld-Voss, B. M. W.; van Haar, J. A.
E. H.; Janssen, R. A. J.; Sheiko, S. S.; Spatz, J. P.; Mo¨ller, M.; Meijer, E.
W. J. Am. Chem. Soc. 1998, 120, 2798-2804. (c) Wang, H.; Wang, H.
H.; Urban, V. S.; Littrel, K. C.; Thiyagarajan, P.; Yu, L. J. Am. Chem.
Soc. 2000, 122, 6855-6861. (d) Jenekhe, S. A.; Chen, X. L. Science 1999,
283, 372-375. (e) Jenekhe, S. A.; Chen, X. L. Science 1998, 279, 1903-
1907. (f) Kilbinger, A. F. M.; Schenning, A. P. H. J.; Goldoni, F.; Feast,
W. J.; Meijer, E. W. J. Am. Chem. Soc. 2000, 122, 1820-1821.
(4) (a) Lee, M.; Cho, B.-K.; Kim, H.; Yoon, J.-Y.; Zin, W.-C. J. Am. Chem.
Soc. 1998, 120, 9168-9179. (b) Lee, M.; Lee, D.-W.; Cho, B.-K.; Yoon,
J.-Y.; Zin, W.-C. J. Am. Chem. Soc. 1998, 120, 13258-13259.
(5) (a) Lee, M.; Cho, B.-K.; Jang, Y.-G.; Zin, W.-C. J. Am. Chem. Soc. 2000,
122, 7449-7455. (b) Lee, M.; Cho, B.-K.; Ihn, K. J.; Lee, W.-K.; Oh,
N.-K.; Zin, W.-C. J. Am. Chem. Soc. 2001, 123, 4647-4648.
† Yonsei University.
‡ Department of Materials Science and Engineering, Pohang University
of Science and Technology.
§ Department of Chemistry, Pohang University of Science and Technol-
ogy.
(1) (a) Lehn, J. M. Supramolecular Chemistry, Concepts and PerspectiVes;
VCH: Weinheim, Germany, 1995. (b) Muthukumar, M.; Ober, C. K.;
Thomas, E. L. Science 1997, 277, 1225-1232. (c) Stupp, S. I.; LeBonheur,
V.; Walker, K.; Li, L. S.; Huggins, K. E.; Kesser, M.; Amstutz, A. Science
1997, 276, 384-389. (d) Lee, M.; Cho, B.-K.; Zin, W.-C. Chem. ReV. 2001,
101, 3869-3892.
(2) (a) Segura, J. L.; Martin, N. J. Mater. Chem. 2000, 10, 2403-2435. (b)
Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int. Ed. 1998,
37, 402-428. (c) Bunz, U. H. F. Acc. Chem. Res. 2001, 34, 998-1010.
(d) Breen, C. A.; Deng, T.; Breiner, T.; Thomas, E. L.; Swager, T. J. Am.
Chem. Soc. 2003, 125, 9942-9943. (e) Berresheim, A. J.; Mu¨ller, M.;
Mu¨llen, K. Chem. ReV. 1999, 99, 9. 1747-1786.
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9
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J. AM. CHEM. SOC. 2004, 126, 6294-6300
10.1021/ja048856h CCC: $27.50 © 2004 American Chemical Society

Molecular Trees Containing Octa-p-phenylene
A R T I C L E S
Experimental Section
4-Trimethylsilyl-biphenyl-4′-boronic acid, 4-diphenyl boronic acid,
1,4-dibromo-2,5-dimethoxybenzene (3a), and dendritic oligoether coils
(R1 and R2) were prepared according to the similar procedures described
previously.9,10 For synthetic detail, see the Supporting Information. 1H
NMR and 13C NMR spectra were recorded from CDCl3 solutions on a
Bruker AM 250 or 500 spectrometer. The purity of the products was
checked by thin-layer chromatography (TLC; Merck, silica gel 60). A
Perkin-Elmer DSC-7 differential scanning calorimeter equipped with
1020 thermal analysis controller was used to determine the thermal
transitions, which were reported as the maxima and minima of their
endothermic or exothermic peaks. In all cases, the heating and cooling
rates were 10 °C/min. A Nikon Optiphot 2-pol optical polarized
microscopy (magnification: 100 ×) equipped with a Mettler FP 82
hot-stage and a Mettler FP 90 central processor was used to observe
the thermal transitions and to analyze the anisotropic texture. Mi-
croanalyses were performed with a Perkin-Elmer 240 elemental
analyzer. X-ray scattering measurements were performed in transmission
mode with synchrotron radiation at the 3C2 X-ray beam line at Pohang
Accelerator Laboratory, Korea. Molecular weight distribution (Mh w/Mh n)
was determined by gel permeation chromatography (GPC) with a
Waters R401 instrument equipped with Stragel HR 3, 4, and 4E
columns, M7725i manual injector, column heating chamber, and 2010
Millennium data station. MALDI-TOF mass was performed on a
Perseptive Biosystems Voyager-DE STR using a 2,5-dihydroxy benzoic
acid matrix. The molecular density (F) measurements were performed
in aqueous sodium chloride solution at 25 °C. Optical absorption spectra
were obtained from a Shimadzu 1601 UV spectrophotometer. The
steady-state fluorescence spectra were obtained from a Hitachi F-4500
fluorescence spectrophotometer. The SEM was recorded on a Hitachi
S-4200 at an acceleration voltage of 18 kV. A drop of the solution (1
× 10-5 g/mL in THF/water 1:10 v/v) was put on Al foil and dried
under reduced pressure. The sample was then coated with gold in the
ion coater for 30 s. The TEM was performed at 120 kV using JEOL
JEM 2010. The sample was embedded in epoxy resin and microtomed
at -170 °C using a cyro-ultramicrotome without staining. Light
scattering experiments were performed using a light scattering goni-
ometer (BI200-SM, Brookhaven Instruments) equipped with a digital
autocorrelator (BI-9000 AT, Brookhaven Instruments) and an Ar ion
laser (Model 95, Lexel Laser Inc.). The incident light was vertically
polarized at λ ) 488 nm, and the typical laser power was 75 mW. All
the measurements were carried out at the temperature of 25.0 ( 0.1
°C. Dynamic (DLS) and static light scattering (SLS) experiments were
performed over a scattering angular range of 30° to 120° at the sample
concentrations range of 5.0 × 10-6 to 1.0 × 10-5 g/mL. The
hydrodynamic radius of the self-assembly was determined from the
analysis of the DLS autocorrelation functions by the cumulants and
the CONTIN methods using the software provided by the manufacturer.
The radius of gyration and the molecular weight of the self-assembly
of the molecular tree were obtained by Zimm plot of the SLS intensity.
The specific refractive index increments, dn/dc, in water/THF mixture
was determined as 0.201 mL/g by differential refractometry.
Figure 1. Schematic representation of the molecular arrangement of
molecular trees. (a) Interdigitated parallel arrangement of molecules and
(b) orientationally ordered radial arrangement of molecules.
A strategy to manipulate the aggregation structure assembled
from a conjugated rod building block may be accessible by
connecting hydrophilic, flexible dendritic branches to its one
side, leading to a tree-shaped molecule. The molecular tree based
on small branches may self-assemble into a monolayer lamellar
structure in which the rod segments are fully interdigitated,
similar to simple rod-coil molecules. As the volume fraction
of the flexible part of the molecule increases, greater steric
repulsion between adjacent flexible parts could possibly cause
an increment in the interfacial area to relieve repulsive forces,
giving rise to the formation of a discrete nanostructure maintain-
ing parallel arrangement of rod segments (Figure 1a).1d,7,8
Furthermore, tree-shaped molecules can also be considered as
an amphiphilic molecule because they consist of hydrophilic
dendritic chains as a head and a hydrophobic rod as a stem.
When such molecules with an amphiphilic character are
dissolved in hydrophilic solvents, the molecules may self-
assemble into aggregates with interfacial curvature surrounded
by the hydrophilic flexible chains due to hydrophobic effect.
In contrast to bulk nanocrystals with parallel arrangement of
rod segments, rod segments in a selective solvent would be
packed with orientationally ordered radial arrangement to
maximize the contact of the hydrophilic chains with water,
giving rise to a unique supramolecular structure with interfacial
curvature (Figure 1b). Therefore, amphiphilic tree-shaped
molecules can provide the unique possibility to change the
nanostructure from discrete nanocrystals to spherical aggregates
by introduction of selective solvent to flexible chains.
In this article, we describe the synthesis of tree-shaped
molecules consisting of octa-p-phenylene as a stem segment
and oligo(ethylene oxide) dendrons as flexible branches and the
self-assembling behavior in the bulk and dilute THF/water (1:
10 v/v) solution characterized by optical polarized microscopy,
differential scanning calorimetry (DSC), powder X-ray diffrac-
tion, transmission electron microscopy (TEM), scanning electron
microscopy (SEM), and light scattering measurements. The
molecular tree based on a small hydrophilic head self-assembles
into a lamellar structure, whereas the molecule containing a large
head crystallizes into a discrete nanostructure that organizes into
a 3-D primitive orthorhombic supercrystal. In dilute solution,
however, the molecules self-assemble into a hollow nanocapsule,
as confirmed by a combination of light scattering, TEM, and
SEM measurements.
Results and Discussion
Synthesis. The synthesis of treelike molecules consisting of
octa-p-phenylene as a stem segment and oligoether dendrons
as flexible branches is outlined in Scheme 1 and start with the
preparation of 1,4-dibromo-2,5-dimethoxybenzene and oligo-
ether dendrons according to the procedures described previ-
ously.9,10 Trimethylsilyl-substituted biphenyl derivative 4a was
prepared from the Suzuki coupling reaction of 3b in the presence
(9) Vahlenkamp, T.; Wegner, G. Macromol. Chem. Phys. 1994, 195, 1933-
(7) Semenov, A. N. Mol. Cryst. Liq. Cryst. 1991, 209, 191.
(8) Williams, D. R. M.; Fredrickson, G. H. Macromolecules 1992, 25, 3561-
3568.
1952.
(10) Jayaraman, M.; Frechet, J. M. J. J. Am. Chem. Soc. 1998, 120, 12996-
12997.
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A R T I C L E S
Yoo et al.
Scheme 1. Synthesis of Molecular Tree 1 and 2
of Pd(0) catalysis.11 For the next Suzuki coupling reaction, the
trimethylsilyl group of 4a was substituted to aryl iodide, which
is the most active in Suzuki-type aromatic couplings, and then
subsequent aromatic coupling reaction produced 5a. After
iodination of 5a with ICl, the methoxy groups in the molecule
were deprotected, and then the subsequent etherification with
tosylated oligoether dendrons yielded 5d and 6d, showing
sufficient solubility for further couplings of rigid conjugated
building blocks in the reaction medium. The final tree-shaped
molecules were synthesized by following the same sequence
of reactions, i.e., by Suzuki coupling reaction of 5d and 6d with
4-bromobiphenyl-4′-boronic acid, subsequent iodination with
ICl, and finally, reaction with 4-biphenyl boronic acid. All of
the resulting tree-shaped molecules were purified by silica gel
column chromatography and subsequent reprecipitation of
diethyl ether solution to n-hexane as described in the Experi-
mental Section. The resulting tree-shaped molecules were
characterized by NMR spectroscopy, elemental analysis, GPC,
and MALDI-TOF mass spectroscopy. As confirmed by 1H NMR
spectroscopy, the ratio of the aromatic protons of the rod block
to the ethylene oxide protons is consistent with the ratio
calculated according to the number of dendritic units and octa-
p-phenylene. As shown in Figure 2, the MALDI-TOF mass
spectra of the molecular trees exhibit three signals that can be
assigned as the molecular ions, together with the Na+ and K+
labeled molecular ions. The mass corresponding to a representa-
tive peak in the spectrum is matched with the calculated
molecular weight of each of the molecular trees.
Figure 2. MALDI-TOF mass spectrum of 1.
transition temperatures and the corresponding enthalpy changes
are determined from DSC scans. The molecular trees appear to
be opaque waxy solid that melts into isotropic liquid at 135.3
°C and 76.6 °C for 1 and 2, respectively (Figure 3a). Between
cross polarizers, these waxy solids showed strong birefringence.
Although no discernible texture could be identified from 1, 2
showed a characteristic texture associated with supramolecular
ordering. On cooling from the optically isotropic phase of 2,
rectangular areas growing in four directions can be observed
with a final development of mosaic texture, characteristics of a
tetragonal structure exhibited by rod-coil systems (Figure 3b).5,6
To corroborate the solid-state structure of the molecular trees,
small- and wide-angle X-ray scattering experiments were
performed. The small-angle X-ray diffraction pattern of 1 based
on a small head displays sharp reflections that correspond to
equidistant q-spacings and thus index to a lamellar lattice. The
layer spacing of 4.2 nm is very close to the estimated molecular
Solid State Structure. The self-assembling behavior of the
tree-shaped molecules in the bulk was investigated by means
of DSC, thermal optical polarized microscopy, TEM, and X-ray
scatterings. The molecules show an ordered structure, and the
(11) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457-2483.
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Molecular Trees Containing Octa-p-phenylene
A R T I C L E S
Figure 3. (a) DSC curves recorded during the first heating scan of (A) 1 and (B) 2. (b) Representative optical polarized micrograph (100×) of the texture
exhibited by 2 at the transition from the isotropic state at 76 °C.
Figure 5. Transmission electron micrograph of 2 without staining.
Figure 4. (a) Small-angle X-ray diffraction pattern of 2 measured at 25
°C. (b) Wide-angle X-ray diffraction pattern of 2 measured at 25 °C.
on the lattice parameters of the 3-D unit cell and measured
densities indicates that the number of molecules in each bundle
length, indicating that the rods are packed into a fully inter-
digitated monolayer lamellar structure (see Supporting Informa-
tion). The wide-angle X-ray diffraction pattern shows a strong
peak which is due to crystal packing of the rod segments. The
small-angle X-ray diffraction pattern of 2 based on a large
flexible head shows three strong reflections together with a
number of low intensity reflections at higher angles (Figure 4a),
indicative of the existence of a highly ordered nanoscopic
structure with three distinct lattice parameters. These reflections
indeed can be indexed as a 3-D primitive orthorhombic structure
with lattice parameters a ) 2.5 nm, b ) 1.9 nm, and c ) 4.9
nm.12 The wide-angle X-ray diffraction pattern shows several
sharp reflections corresponding to a rectangular lattice with unit
cell dimensions of x ) 8.5 Å and y ) 5.2 Å, indicating that the
rod building blocks are arranged in a herringbone fashion within
the domains (Figure 4b).
When microtomed films of 2 without the use of stains were
characterized by TEM, dark domains in a lighter matrix could
be observed as shown in Figure 5. The dark regions are believed
to consist of crystalline rod blocks, similar to other rod-coil
systems reported previously.13 The image reveals that the
average diameter of the aggregates is roughly 2 nm, which is
comparable to the lattice parameters determined from small-
angle X-ray diffraction pattern.
is about seven.14 The tendency of the rod building blocks to be
arranged into anisotropic crystalline order along their axes is
likely to generate nonspherical aggregates, which are responsible
for the formation of the 3-D orthorhombic superlattice. Fur-
thermore, this orthorhombic nature with three characteristic
dimensions, together with a rectangular crystalline lattice,
suggests that the aggregation of seven molecules in each
aggregate generates the heptameric bundle with cross sections
that are more rectangular than circular in shape (Figure 6).
Both steric forces and crystallization are believed to play an
important role in the formation of discrete heptameric bundle.
Treelike molecules based on a small flexible head would be
packed into a continuous 2-D sheetlike structure as in the case
of 1. Increasing the volume fraction of the head causes the steric
forces among head parts to be greater. This increase in steric
repulsion could drive the sheetlike domain to break up into
discrete nanostructures that allow more space for the flexible
chains to adopt a less strained conformation.7,8,15 Accordingly,
2 based on a large flexible head is likely to self-organize into
a discrete 3-D structure with prolate shape.
Absorption and Emission Studies. The effects of supramo-
lecular structure on the bulk state optical properties were
investigated by using the UV/vis absorption and fluorescence
spectroscopies. Figure 7 shows the absorption and fluorescence
spectra of the treelike molecules and of the CH2Cl2 solution of
On the basis of the optical and transmission electron
microscopic observations and X-ray diffraction results, tree-
shaped molecule 2 can be considered to self-assemble into
discrete bundles encapsulated by ether chains that organize into
a 3-D primitive orthorhombic supercrystal. Calculation based
(14) From the three characteristic lattice parameters (a, b, and c) determined
from SAXS and the molecular density (r), the average number of molecule
per bundle (n) can be calculated according to eq 1, where M is the molecular
mass and NA is Avogadro’s number:
abc
n )
(12) Kitamura, M.; Nakano, K. Cryst. Growth Des. 2003, 3, 25-34.
(13) Tew, G. N.; Pralle, M. U.; Stupp, S. I. J. Am. Chem. Soc. 1999, 121, 9852-
9866.
{M}/{NAF}
(15) Mu¨ller, M.; Schick, M. Macromolecules 1996, 29, 8900-8903.
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A R T I C L E S
Yoo et al.
Figure 6. Schematic representation of (a) the self-assembly of 2 into a discrete bundle with the herringbone packing of the octa-p-phenylene rods and (b)
the subsequent crystallization into a primitive orthorhombic superlattice.
encapsulated by flexible segments, in contrast to that of 1, where
the rods are packed into sheetlike continuous domains. There-
fore, this 3-D confinement of optically active domains in
dimensions less than 10 nm is presumably responsible for this
interesting optical behavior. The absorption and emission
maxima and absorption edge in semiconducting nanoparticles
have also been observed to shift to higher energy with decreasing
particle size in the range less than 10 nm.16,17 This trend may
also arise from the slightly different intermolecular interactions
between adjacent conjugated rods that occur with variation in
the headgroup size of the molecular tree.18-20 However, the size
effect and intermolecular interactions seem to be cooperative,
and their contribution could be equally important in the optical
properties of the supramolecular materials.
Self-Assembly in Aqueous Solution. Amphiphilic block
molecules, when dissolved in a selective solvent for one of the
blocks, can self-assemble into a variety of aggregate structures,
including vesicular structures.21 The molecular trees can be
Figure 7. Absorption and emission spectra of 1 and 2. (Solid line)
absorption (left) of 2 in CH2Cl2 solution. (Dashed line) absorption of solid
considered as a new class of amphiphiles because they consist
of a hydrophobic rod and a hydrophilic flexible head. Aggrega-
thin film of 2 prepared from the THF/water (1:10 v/v) solution with
concentration of 1 × 10-5 g/mol. (Dotted line) absorption (left) and emission
(right) of solid film of 2. (Dash-dotted line) absorption (left) and emission
(right) of solid film of 1.
tion behavior of the molecular trees was subsequently studied
in water by dissolving a molecular tree solution in THF (1.1 ×
10-4 g/mL) into ultrapure water, which gave a final THF/water
ratio of 1:10 v/v. After the solutions were allowed to equilibrate
over 24 h, the aggregation structure was investigated by using
both DLS and SLS, TEM, and field emission scanning electron
microscopy (FE-SEM). Dynamic light scattering experiments
2 for comparison. Both the absorption and emission spectra of
thin solid films of the molecules appear to be red-shifted with
respect to those of the corresponding solutions and thus
demonstrate existence of intermolecular interactions or increase
in the planarization of conjugated rod segments due to crystal
packing. In the bulk state, both the absorption maximum and
absorption edge of 2 shift to higher energy relative to those of
1. Furthermore, the intensity of the emission band centered at
about 415 nm of 2 increases at the expense of the intensities
corresponding to 460 and 500 nm, in comparison with the
emission profile of 1.
(16) Henglein, A. Chem. ReV. 1989, 89, 1861-1873.
(17) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993,
115, 8706-8715.
(18) Cacialli, F.; Wilson, J. S.; Michels, J. J.; Daniel, C.; Silva, C.; Friend, R.
H.; Severin, N.; Samori, P.; Rabe, J. P.; O’Connell, M. J.; Tayor, P. N.;
Anderson, H. L. Nat. Mater. 2002, 1, 160-164.
(19) Fu, H.-B.; Yao, J.-N. J. Am. Chem. Soc. 2001, 123, 1434-1439.
(20) Lee, J.-K.; Koh, W.-K.; Chae, W.-S.; Kim, Y.-R. Chem. Commun. 2002,
138-139.
These results could be attributed to nanosize effect of the
optically active discrete nanostructure. As evidenced by TEM
and small-angle X-ray scatterings, the supramolecular structure
of 2 consists of three-dimensionally confined rod domains
(21) (a) Alexandridis, P.; Lindman, B. Amphiphilic Block Copolymer, Self-
Assembly and Applications; Elservier: New York, 2000. (b) Antonietti,
M.; Fo¨rster, S. AdV. Mater. 2003, 15, 1323-1333. (c) Discher, D. E.;
Eisenberg, A. Science 2002, 297, 967-973. (d) Fo¨rster, S.; Plantenberg,
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Molecular Trees Containing Octa-p-phenylene
A R T I C L E S
Figure 8. Autocorrelation function (left) and hydrodynamic radius distribution (right) of 2 in THF/water (1:10 v/v) solution with concentration of 1 × 10-5
g/mL.
Figure 9. Zimm plot of 2 in THF/water (1:10 v/v) solution, obtained at
25 °C over a concentration range of 1 × 10-5 to 5 × 10-6 g/mL.
were performed with the solutions of 1 and 2 with concentration
of 1 × 10-5 g/mL over a scattering angular range of 30° to
120° and a concentration range of 5 × 10-6 g/mL to 1 × 10-5
g/mL at 25 °C. The aggregate size was estimated using the
CONTIN analysis for evaluation of the autocorrelation func-
tion.22-24 In both cases, the analysis of the autocorrelation
functions showed a monomodal size distribution, thus confirm-
ing the formation of only one family of aggregate sizes (Figure
8), indicating well-equilibrated structures without unimers and
impurities. The average hydrodynamic radii (RH) of the corre-
sponding aggregates were observed to be approximately 100
nm for 1 and 46 nm for 2, indicating that the molecular tree
with a larger flexible headgroup assembles into a smaller size
of aggregate.
Figure 10. (a) TEM and (b) SEM images of capsules of 2.
To get more information about the size and the molecular
weight of the aggregates, SLS measurements were performed
with 2 under the same conditions. Figure 9 shows an example
of the Zimm plot of the results of 2 solutions. From the Zimm
plots, the weight-averaged molecular weight (Mw) was found
to be 2.18 × 107 g/mol. In addition, the radius of gyration (Rg)
was found to be 48.8 nm. Thus, the radius of gyration of 2 is
nearly identical to the hydrodynamic radius determined from
DLS, suggesting the existence of hollow spheres.24,25 The
measured diameter exceeds the extended molecular lengths
(approximately 4 nm) by a factor of about 20, also strongly
suggesting that these aggregates are rather capsule-like entities
than simple micelles. TEM studies on this sample revealed that
there is the obvious contrast between the periphery and center
(22) Koppel, D. E. J. Chem. Phys. 1972, 57, 4814-4820.
(23) Provencher, S. W. Makromol. Chem. 1979, 180, 201-209.
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M.; Hackler, U. E.; Isobe, H.; Nakamura, E. Science 2001, 291, 1944-
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A R T I C L E S
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Compared to other self-assembling systems, including block
copolymers,21,26,27-30 surfactant molecules,31 and liquid crys-
tals,32 the molecular tree possesses the unique feature that the
rigid building block is capable of packing into organized discrete
nanocrystals with parallel arrangement as well as hollow
nanocapsules with radial arrangement in one system, depending
on the presence of selective solvents. These results demonstrate
that rational design of a self-assembling molecule based on a
conjugated rod building block allows stable nanostructures to
be produced. These nanostructures potentially have a number
of applications, including the encapsulation and controlled
release of active species and nanoreactors.
Figure 11. Schematic representation of formation of the nanocapsule in
solution state.
Acknowledgment. This work was supported by National
Creative Research Initiative Program of the Korean Ministry
of Science and Technology. We thank Pohang Accelerator
Laboratory, Korea, for allowing us to use the Synchrotron
Radiation Source. T.C. acknowledges a financial support from
KOSEF (Center for Intergrated Molecular Systems).
in the sphere, characteristic of the projection images of hollow
spheres (Figure 10a).21c The formation of nanocapsule of 2 was
also confirmed by FE-SEM experiments. As shown in Figure
10b, the SEM micrograph shows spherical aggregates that are
approximately 80 nm in diameter and are thus consistent with
the results obtained from light scattering experiments. On the
basis of results described above, a schematic representation can
be illustrated as shown in Figure 11.
Supporting Information Available: Experimental details, 13
C
NMR and MALDI-TOF of 2, and SAXS of 1 (PDF). This
material is available free of charge via the Internet at
http://pubs.acs.org.
Conclusions
New treelike amphiphilic molecules consisting of octa-p-
phenylene as a stem and oligo(ethylene oxide) dendrons as
branches were synthesized, and their self-assembling behavior
was investigated in the solid and dilute THF/water solution (1:
10 v/v). The molecular tree based on a small flexible head was
observed to self-assemble into a monolayer lamellar structure,
whereas the molecule based on a larger headgroup self-
assembled into a heptameric bundle that self-organizes into a
3-D primitive orthorhombic supercrystal. Spectroscopic studies
of the molecular trees demonstrated the supramolecular structure
has a strong influence on the photophysical properties of the
optically active materials. In dilute solutions, the molecules were
observed to self-assemble into capsule-like aggregates in which
the rod building blocks are likely to be assembled with radial
arrangement.
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