Nanofibers from self-assembly of an aromatic facial amphiphile with oligo(ethylene oxide) dendrons

Article abstract of DOI:10.1039/b617404c

Novel block facial amphiphiles consisting of a laterally extended aromatic segment and oligo(ethylene oxide) dendrons as a flexible segment were synthesized in a stepwise fashion and their aggregation behavior was investigated in aqueous solution; self-as

Full text of DOI:10.1039/b617404c

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COMMUNICATION
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Nanofibers from self-assembly of an aromatic facial amphiphile with
oligo(ethylene oxide) dendrons{
Dong-Je Hong, Eunji Lee and Myongsoo Lee*
Received (in Cambridge, UK) 29th November 2006, Accepted 22nd January 2007
First published as an Advance Article on the web 12th February 2007
DOI: 10.1039/b617404c
Novel block facial amphiphiles consisting of a laterally
extended aromatic segment and oligo(ethylene oxide) dendrons
as a flexible segment were synthesized in a stepwise fashion and
their aggregation behavior was investigated in aqueous solu-
tion; self-assembly into elongated nanofibers with a uniform
diameter of 7 ¡ 0.5 nm and lengths up to several hundred
nanometers was observed.
Supramolecular assemblies formed by self-associating block
molecules have attracted considerable attention as a means of
creating well-defined nanostructured materials with tunable
properties.1 In most cases, amphiphilic block molecules consist
of different immiscible coil segments.2 These molecules are
dissolved in a selective solvent for one constituent, which results
in aggregates that consist of a core formed by the insoluble blocks
surrounded by a shell of the solvated block. Block molecules based
on rigid aromatic segments represent a unique class of self-
assembling systems due to their capability to render desired
functions.3,4 Recently, we have been devoted to developing novel
amphiphilic block molecules based on rigid aromatic segments that
self-assemble into various supramolecular structures in aqueous
Scheme 1 Synthesis of block facial amphiphiles. (i) Pd(0), 2M Na2CO3,
THF, 80 uC, 12 h (ii) ICl, CH2Cl2, 278 uC, 0.5 h (iii) BBr3, CH2Cl2, 0 uC,
6 h (iv) CH3I, acetone–ethanol, 70 uC, 2 h (v) excess K2CO3, CH3CN,
70 uC, 12 h (iv) Pd(0), 2M Na2CO3, THF, 80 uC, 12 h.
solution. For example, we have shown that incorporation of a
conjugated rod into an amphiphilic dumbbell-shaped molecular
architecture gives rise to the formation of a helical nanostructure,
consisting of hydrophobic aromatic cores surrounded by hydro-
philic flexible chains.5 We have also shown that rod building
blocks in rigid–flexible macrocycles self-assemble into discrete
barrel or long tubular structures depending on the relative rod
volume fraction.6 In a recent publication, we have demonstrated
that tree-shaped amphiphilic molecules based on octa-p-phenylene
self-assemble into a stable cage-like structure in aqueous solution.7
Although rod-like aromatic amphiphiles have been extensively
studied, the design of facial amphiphiles based on laterally
extended aromatic segments remains a challenge.8
described previously.7,9 The resulting silyl-substituted compound 1
was coupled with 2 equiv. of 49-methoxybiphenyl-4-boronic acid
by a Suzuki cross-coupling reaction to yield 2. The silyl group of 2
was subsequently substituted into iodine which is the most active
group for the next aromatic coupling. To increase the solubility,
aliphatic chains should be introduced to the aromatic building
block prior to further extension of the aromatic block of 3. Along
this line, dihydroxy compound 4 obtained from demethylation in
the presence of boron tribromide was coupled with oligo(ethylene
oxide) dendron in the presence of potassium carbonate to yield 6.
As expected, 6 was readily soluble in common organic solvents
methylene dichloride, acetonitrile, and methanol. The final
amphiphilic molecules 8 and 9 based on a laterally extended
aromatic segment were obtained from a Suzuki coupling reaction
of excess amounts of 6 and 7 with benzene-1,4-diboronic acid in
the presence of Pd(0) catalysis, respectively. The resulting facial
amphiphile was purified by column chromatography (silica gel)
and subsequent preparative HPLC to yield a waxy solid, and
characterized by 1H and 13C-NMR spectroscopy, elemental
analysis and MALDI-TOF mass spectroscopy, and showed to
be in full agreement with the structure presented.
In this communication, we describe the synthesis of facial
amphiphiles (8 and 9) consisting of a laterally extended aromatic
segment and oligo(ethylene oxide) dendrons as a flexible segment,
and their self-assembling behavior in aqueous solution. The
synthesis of the amphiphilic molecule is outlined in Scheme 1
and starts with the preparation of 1-trimethylsilyl-3,5-dibromo-
benzene and oligoether dendrons according to the procedures
Center for Supramolecular Nano-Assembly and Department of
Chemistry Yonsei University, Seoul 120-749, Korea.
E-mail: mslee@yonsei.ac.kr; Fax: +82 2 393 6096; Tel: +82 2 2123 2647
{ Electronic supplementary information (ESI) available: details of
synthetic procedures, characterization, DLS, TEM, UV/Vis specta of
encapsulation. See DOI: 10.1039/b617404c
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Chem. Commun., 2007, 1801–1803 | 1801

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Block molecule 8 can be considered as a novel class of
amphiphile because it consists of hydrophilic flexible dendrons and
a hydrophobic rigid aromatic segment. Aggregation behavior of
both block molecules 8 and 9 was subsequently studied in aqueous
solution using UV/vis and fluorescence spectra (Fig. 1a). The
absorption spectrum of 8 in aqueous solution (0.002 wt%) exhibits
a broad transition with a maximum at 296 nm, resulting from the
conjugated rod block. The fluorescence spectrum of 8 in methylene
dichloride solution (0.002 wt%) exhibits a strong emission
maximum at 372 nm. However, the emission maximum in
aqueous solution is red-shifted with respect to that observed in
methylene dichloride solution, and the fluorescence is significantly
quenched, indicative of aggregation of the conjugated rod
segments (Fig. 1b).10
Fig. 2 TEM images of nanofibers formed by self-assembly of 8 in
aqueous solution (a) with low magnification and (b) with high
magnification.
Dynamic light scattering (DLS) experiments were performed
with 8 in aqueous solution to further investigate the aggregation
behavior with a concentration of 0.01 wt% over a scattering
angular range of 30–150u (Fig. 1b). The CONTIN analysis of the
autocorrelation function at a scattering angle of 90u showed a
broad peak corresponding to an average hydrodynamic radius
(Rh) of approximately 117 nm (Fig. 1c). The angular dependence
of the apparent diffusion coefficient (Dapp) was measured because
the slope is related to the shape of the diffusing species. As shown
in Fig. 1d, the slope is 0.02, consistent with the value predicted for
elongated non-spherical micelles (0.03).11Compound 9 based on
dimethoxy groups showed similar aggregation behavior to that of
8 and its Rh value showed to be 125 nm.12
On the basis of the results described above, the aromatic facial
amphiphile can be considered to self-assemble into elongated
nanofibers with a uniform diameter of 7 ¡ 0.5 nm and several
micrometers long. The nanofibers consist of hydrophobic aromatic
inner cores surrounded by hydrophilic flexible dendritic segments
that are exposed to the aqueous environment. Within the core, the
laterally extended aromatic segments are stacked with a bilayer
packing to release steric crowding between bulky dendritic chains
(Fig. 3).13 This estimation is supported by molecular dimensions
and the diameter of the single strand in TEM.
To further confirm the aggregation structure, transmission
electron microscopy (TEM) experiments have been performed
with an aqueous solution of 8. Fig. 2 shows micrographs obtained
from a 0.01 wt% aqueous solution of 8 cast onto a TEM grid. The
negatively stained sample with uranyl acetate clearly shows
1-dimensional nanofibers with a uniform diameter of 7 nm.
Most of the strands have lengths up to several hundred
nanometers. Considering that from molecular modeling the fully
extended molecular length of the amphiphilic molecule is estimated
to be y4 nm, the 7 nm width is consistent with a bilayer packing
of aromatic segments.
In conclusion, we have successfully synthesized a novel block
facial amphiphile based on a laterally extended rigid aromatic
segment that shows a unique aggregation behavior in aqueous
solution. TEM along with dynamic light scattering studies
demonstrated that the amphiphilic molecule self-assembles into
elongated nanofibers with a uniform diameter of molecular length
scale, and lengths up to several hundred nanometers. The synthetic
approach described here can be extended to the preparation of a
series of amphiphilic molecules with control over the rigid-to-
flexible volume ratio to create tunable nanostructures. Such
Fig. 1 (a) Absorption and emission spectra of 8 in CH2Cl2 (0.002 wt%,
blue dot line) and aqueous solution (0.002 wt%, black line). (b)
Autocorrelation functions of 8 in aqueous solution (0.01 wt%). (c)
Hydrodynamic radius distribution of 8. (d) Angular dependence of
diffusion coefficient for 8.
Fig. 3 Schematic representation of the proposed self-assembly of facial
amphiphile 8.
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amphiphilic molecules are believed to be of interest for biological
applications due to the formation of well-defined supramolecular
nanostructures based on a highly hydrophobic aromatic core in
aqueous solution.
We gratefully acknowledge the National Creative Research
Initiative Program of the Korean Ministry of Science and
Technology for financial support of this work. E.L. thanks the
Seoul Science Fellowship program.
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12 See supporting Information{.
13 The hydrophobic nature of the core was also confirmed by dye
encapsulation experiments. See Supporting Information{.
This journal is ß The Royal Society of Chemistry 2007
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