Self-assembly of T-shaped aromatic amphiphiles into stimulus-responsive nanofibers

Article abstract of DOI:10.1002/anie.200702136

(Figure Presented) Stimulating fibers: Self-assembled nanofibers coated with hydrophilic oligo(ethylene oxide) dendrons transform reversibly, upon heating, into hydrophobic nanofiber bundles as a result of dehydration of the dendritic chains (see scheme). A thermoresponsive sol-gel phase transition is observed.

Full text of DOI:10.1002/anie.200702136

Angewandte
Chemie
DOI: 10.1002/anie.200702136
Amphiphilic Nanofibers
Self-Assembly of T-Shaped Aromatic Amphiphiles into Stimulus-
Responsive Nanofibers**
Kyung-Soo Moon, Ho-Joong Kim, Eunji Lee, and Myongsoo Lee*
Supramolecular assemblies formed by self-associating block
molecules have attracted considerable attention as a means of
creating well-defined nanostructured materials with tunable
properties.[1–6] Among the nanostructures formed by self-
assembly of designed molecules,a 1D fibrillar assembly has
proved to be particularly interesting for applications such as
nanowires and biomimetic macromolecules.[3] The 1D struc-
tures with stimulus-responsive features are likely to further
enhance their scope as intelligent materials. Although stim-
ulus-responsive nanostructures have been extensively studied
with spherical objects,[4] switching of the properties triggered
by external stimuli with 1D fibrillar objects remains challeng-
ing.[5,6]
The hierarchical assembly of such 1D structures could
result in the formation of 3D networks through interconnect-
ing the extended micelles.[7,8] For example,conjugated-rod
segments containing flexible alkyl chains self-assemble into
1D fibrils that are entangled with each other to form
reversible gels.[9] We have also shown that discrete cylindrical
micelles can be interconnected by addition of a bridging agent
to form reversible gels in aqueous solution.[10] Recently,we
demonstrated that facial amphiphiles consisting of a laterally
extended aromatic segment self-assemble into elongated
Figure 1. Stimulus-responsive sol–gel phase transition of self-assem-
bled supramolecular nanofibers.
nanofibers coated by oligo(ethylene oxide) dendrons.[11]
One can envision that aromatic amphiphiles based on oligo(-
ethylene oxide) dendrons lead to the formation of thermor-
esponsive nanofibers,because ethylene oxide chains become
dehydrated with increasing temperature.[12] With this idea in
mind,we prepared T-shaped aromatic amphiphiles containing
oligo(ethylene oxide) dendrons.
Herein,we present the formation of thermoresponsive
nanoscale fibrils from the aqueous self-assembly of T-shaped
aromatic amphiphiles based on oligo(ethylene oxide) den-
drons (Figure 1). The amphiphiles contain dibranched oligo-
(ethylene oxide) (1a) and tetrabranched oligo(ethylene
oxide) dendrons (1b) with similar volume fraction
(fdendron = 0.67 ꢀ 0.68) as hydrophilic side chains (Scheme 1).
For comparison,we also prepared an anisole-branched T-
shaped aromatic amphiphile (2b).
a
The aggregation behavior of 1a and 1b was studied in
aqueous solution by using fluorescence spectroscopy. The
emission maxima of both amphiphiles in aqueous solution
(0.001 wt%) were red-shifted with respect to those observed
in dichloromethane solution,and the intensities were signifi-
cantly reduced,indicative of aggregation of the conjugated
aromatic segments (Figure 2a).[13] Dynamic light scattering
(DLS) experiments were performed with the amphiphiles in
aqueous solution (0.1 wt%) to further investigate the aggre-
gation behavior.[14–16] The CONTIN analysis of the autocor-
relation function for both solutions showed a broad peak
corresponding to average hydrodynamic radii (RH) of approx-
imately 80 and 110 nm for 1a[14] and 1b (Figure 2b),
respectively.
[*] K.-S. Moon, H.-J. Kim, E. Lee, Prof. M. Lee
Center for Supramolecular Nano-Assembly and
Department of Chemistry
Yonsei University
Shinchon 134, Seoul 120-749 (Korea)
Fax: (+82)2-393-6096
E-mail: mslee@yonsei.ac.kr
[**] This work was supported by the Creative Research Initiative
Program of the Korean Ministry of Science and Technology. We
acknowledge the Pohang Accelerator Laboratory, Korea, for use of
the synchrotron radiation source. E.L. thanks the Seoul Science
Fellowship Program, and K.-S.M., H.-J.K., and E.L. acknowledge a
fellowship of the BK21 program from the Ministry of Education and
Human Resources Development.
Evidence for the formation of the fibrillar aggregates was
also provided by transmission electron microscopy (TEM)
experiments. The micrographs with negatively stained sam-
ples showed fibrillar aggregates with lengths up to several
hundred nanometers for 1a (Figure 2c) and several micro-
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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matic cores surrounded by hydrophilic dendritic segments
that are exposed to the aqueous environment (Figure 1).
Within the core,the T-shaped aromatic segments are stacked
with dimeric association to maximize p–p interactions. Note
that 1b,in the solid state,self-assembles into a 2D oblique
columnar structure with lattice constants a = 5.7 and b =
2.5 nm and a characteristic angle of 1008,thus implying that
the cross section of a cylinder consists of two molecules.
To gain insight into the role of the p–p stacking
interactions between T-shaped aromatic segments in the
formation of a 1D fibrillar structure,compound 2b,based on
a branched T-shaped aromatic unit,was prepared with the
aim of frustration of p–p stacking interactions through steric
hindrance. As expected, 2b showed no apparent aggregation
behavior,which was confirmed by DLS measurements.
Furthermore, 2b was an amorphous solid. This result suggests
that the primary driving force for the formation of stable
fibrillar aggregates in 1b in aqueous solution is p–p stacking
interaction of the T-shaped aromatic segments.
Self-assembly of T-shaped amphiphiles into a long fibrillar
structure with a hydrophilic oligoether dendritic exterior
suggests that they may lead to temperature-dependent
solution behavior,because the degree of hydration of the
ethylene oxide chains decreases with increasing temper-
ature.[12] To investigate the thermoresponsive solution behav-
ior,viscosities were measured with a capillary viscometer as a
function of temperature (Figure 3a). The aqueous solution of
1a (0.5 wt%) showed essentially no change in viscosity with
heating. This result suggests that the hydrophobicity of the
surfaces and the length of the micelles in 1a are not sufficient
to interconnect adjacent cylindrical micelles. Indeed, 1a was
highly soluble and no gels were produced under standard
conditions. In great contrast,the solution viscosity of 1b
(0.5 wt%) increases abruptly above 308C and results in
Scheme 1. Molecular structure of the T-shaped aromatic
amphiphiles.
meters for 1b (Figure 2d). The formation of the
much longer fibrillar aggregates in 1b is consis-
tent with the results obtained from DLS meas-
urements described above,thus indicating that
the tetrabranched oligoether dendrons,com-
pared to the dibranched ones,endow the T-
shaped aromatic segments with better aggrega-
tion ability. A dendritic architecture with a
higher generation is known to exhibit a more
hydrophobic nature because of the dendritic
effect,[17] which results in a higher ability to form
aggregates in aqueous solution. By considering
the extended molecular lengths (about 7 nm by
the Corey–Pauling–Koltun (CPK) model),the
images indicate that the diameter of the elemen-
tary cylindrical objects corresponds to one
molecular length. All of these data suggest that
the T-shaped aromatic amphiphiles self-assem-
ble into a fibrillar structure consisting of aro-
Figure 2. a) Fluorescence emission spectra of 1b in CH2Cl2 (0.001 wt%, curve A) and
aqueous solution (0.001 wt%, curve B). lex =350 nm. b) Size distribution of 1b at a
scattering angle of 908 (from CONTIN analysis of the autocorrelation function;
0.1 wt% in aqueous solution). c,d) TEM images of 1a and 1b, respectively, with
negative staining (0.01 wt%); scale bars: 100 nm.
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copy. When Disperse Orange 3 was added to a
solution of 1b (0.001 wt%),the characteristic
absorption band of the dye molecule increased
gradually (Figure 3d),which indicates that the
more hydrophobic nature of the fibrillar sur-
faces upon heating allows the more hydro-
phobic dye molecules to translocate into the
aromatic core.
Interestingly,the gel reversibly transforms
into a fluid solution with addition of a dye
molecule,such as 0.2 equivalents of Nile Red.
CONTIN analysis of the correlation functions
in DLS experiments showed RH to be 84 nm,
which demonstrates a decrease in the fibrillar
length upon addition of the dye (Figure 4a).
This decrease in the length was also confirmed
by TEM,which revealed discrete fibrillar
objects with an average length of about
150 nm,consistent with the result obtained
from DLS experiments (Figure 4b). Conse-
quently,the structural change of 1b to discrete
micelles upon addition of guest molecules is
responsible for the phase change from gel to a
fluid solution. The breakup of the long fibrils
of 1b into discrete micelles seems to arise from
the packing frustration of the T-shaped aro-
matic segments by intercalation of the guest
Figure 3. a) Relative viscosities of 1a and 1b in aqueous solution (0.5 wt%) with
increasing temperature. b) TEM image of gel with 0.5 wt% 1b at 458C; scale bar:
100 nm. c) Changes in the emission (lex =350 nm) spectrum of 1b in aqueous solution
(0.05 wt%) with increasing temperature. d) Changes in the absorption spectrum of 1b
in aqueous solution (0.001 wt%) containing five equivalents of Disperse Orange 3 with
increasing temperature.
gelation at 458C. This transition temperature decreases to
08C with increasing concentration up to 4.0 wt% (see the
Supporting Information).
molecules.[19]
In summary,we have demonstrated that T-shaped aro-
matic amphiphiles self-assemble into stimulus-responsive
fibrils consisting of aromatic cores surrounded by hydrophilic
oligo(ethylene oxide) dendritic segments. Upon heating,the
fibrils have more hydrophobic surfaces,which results in
The TEM image of the gel dried on a carbon-coated
copper grid revealed the formation of bundles of the fibrils at
higher temperatures (Figure 3b),which indicates that an
increase in the solution viscosity with increasing temperature
and subsequent formation of gel can be attributed to the
interconnection of fibrillar aggregates. These results show
that heating the solution of long fibrils drives the fluid state to
a gel in which the elementary fibrils are connected to each
other to immobilize water molecules (Figure 1). This ther-
moresponsive solution behavior can be explained by consid-
ering hydrophobic interactions between oligo(ethylene
oxide) dendron-coated nanofibers.[12,17,18] With increasing
temperature,the oligoether dendritic chains will be dehy-
drated because of the breakup of hydrogen bonding. As a
result,the fibrillar surfaces would become more hydrophobic,
thus resulting in enhanced hydrophobic interactions between
adjacent fibrils to form 3D networks.
This finding was confirmed by fluorescence spectroscopy
experiments. The fluorescence of an aqueous solution of 1a
(0.05 wt%) remained nearly unaltered with increasing tem-
perature up to 608C.[14] In contrast,the fluorescence of 1b
(0.05 wt%) was significantly reduced at the sol–gel transition
temperature (Figure 3c). This result indicates that,upon
heating,the aromatic segments are packed more closely
within the core because of enhanced hydrophobic environ-
ments caused by dehydration of the oligoether dendritic
surfaces. Evidence for the enhanced hydrophobicity of
fibrillar surfaces was also provided by hydrophobic dye
molecular-encapsulation experiments with UV/Vis spectros-
Figure 4. a) Size distribution at a scattering angle of 908 from
CONTIN analysis of the autocorrelation function of a solution of 1b
(0.1 wt%) containing 0.2 equivalents of Nile Red. b) TEM image of 1b
with 0.2 equivalents of Nile Red (0.01 wt%); scale bar: 100 nm.
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Communications
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enhanced hydrophobic interactions between the ethylene
oxide-coated nanofibrils. Consequently,this dynamic fibrillar
association leads to a reversible phase transition from a fluid
state to a gel with increasing temperature. In addition,the gel
was transformed into a fluid solution by addition of hydro-
phobic guest molecules,caused by the breakup of long fibrils
into discrete micelles. These stimulus-responsive features of
the fibrils show a remarkable contrast to conventional fibrillar
gels that dissolve upon heating.[20] The results described
herein represent a significant example of nanofibrils with
stimulus-responsive surfaces,which can provide a strategy for
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Keywords: amphiphiles · hydrophobic effect · nanofibers ·
.
self-assembly · sol–gel processes
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