Aqueous nanofibers with switchable chirality formed of self-assembled dumbbell-shaped rod amphiphiles

Article abstract of DOI:10.1039/b913286d

We have demonstrated that fibrillar aggregates formed from the self-assembly of a dumbbell-shaped rod amphiphile undergo a reversible chiral-non-chiral transition triggered by temperature.

Full text of DOI:10.1039/b913286d

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COMMUNICATION
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Aqueous nanofibers with switchable chirality formed of self-assembled
dumbbell-shaped rod amphiphilesw
Zhegang Huang,ab Eunji Lee,a Ho-Joong Kima and Myongsoo Lee*a
Received (in Cambridge, UK) 3rd July 2009, Accepted 17th September 2009
First published as an Advance Article on the web 5th October 2009
DOI: 10.1039/b913286d
We have demonstrated that fibrillar aggregates formed from the
self-assembly of a dumbbell-shaped rod amphiphile undergo a
reversible chiral–non-chiral transition triggered by temperature.
The construction of helical nanostructures is of growing
interest in interdisciplinary areas combining chemistry,
biology and materials science, because they are ideally suited
for the design of intelligent materials.1,2 Supramolecular
Fig. 1 Schematic representation of the reversible transformation
from achiral extended to chiral compressed states of the cylindrical
helices can be generated through the intermolecular assembly
of chiral building blocks.3 Transfer of chiral information
stack.
through the molecular assembly provides another strategy to
construct elongated helical aggregates.4 One of the most
image indicates that the diameter of the elementary cylindrical
objects corresponds to one molecular length. This result
suggests that molecule 1 self-assembles into fibrillar aggregates
consisting of hydrophobic aromatic segments surrounded by
hydrophilic oligoether segments that are exposed to the
aqueous environment (Fig. 1). Within the core, the rod
segments stack on top of each other with mutual rotations
to reduce steric repulsions between the bulky dibranched
oligoether chains. Interestingly, when the aqueous solution
was subjected to circular dichroism (CD) measurements at
room temperature, no CD signals could be detected even
though 1 contains chiral side groups. This indicates that the
fibrillar objects are non-chiral, even though the rods contain
chiral side chains.
important strategies available to form these aggregates arises
from twist stacking of rigid aromatic segments containing
chiral side chains.5 Although the introduction of helicity into
one-dimensional fibrillar aggregates has been extensively
studied through grafting chiral side groups, their switching
behavior in response to external stimuli remains a challenge.6
In this Communication, we present the formation of
aqueous nanofibers with switchable supramolecular chirality
from the self-assembly of dumbbell-shaped molecules (Fig. 1).
Notably, the helicity of the non-chiral nanofibers is induced
in
a reversible way upon heating. The self-assembling
dumbbell-shaped molecules consist of a carbazole end-capped
phenanthrene as a rigid stem and chiral oligoether segments
laterally attached to the carbazole units as flexible chains, and
were synthesized according to previously reported similar
methods (Scheme 1).7
The formation of long nanofibers with a hydrophilic
oligoether exterior suggests that they may lead to temperature-
dependent solution behavior due to the lower critical solution
The dumbbell-shaped molecules can self-assemble into an
aggregate structure in an aqueous solution because of their
amphiphilic characteristics. The aggregation behavior of the
molecules was subsequently studied in aqueous solution by
using dynamic light scattering (DLS), transmission electron
microscopy (TEM), and absorption and emission spectro-
scopies. The CONTIN analysis of the autocorrelation function
of 1 shows a broad peak that corresponds to a hydrodynamic
radius (RH) of B250 nm (ESI, Fig. S2w). To confirm the
aggregation structures, TEM experiments were performed. As
shown in Fig. 2a, the image of 1 with a negatively stained
sample shows elongated fibers with a uniform diameter of
about 5 nm and lengths of several micrometres. Considering
the extended molecular length (5.3 nm by CPK modeling), this
a Center for Supramolecular Nano-Assembly, Department of
Chemistry, Seoul National University, Seoul 151-747, Korea.
E-mail: myongslee@snu.ac.kr; Fax: +82 2 393 6096;
Tel: +82 2 880 4340
b Department of Chemistry, Yonsei University, Seoul 120-749, Korea
w Electronic supplementary information (ESI) available: Experimental
details and supplementary figures. See DOI: 10.1039/b913286d
Scheme 1 Molecular structure of the dumbbell-shaped aromatic
amphiphiles
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Fig. 2 TEM images (negatively stained with uranyl acetate) of
aqueous solutions of 1 prepared (a) at 30 1C and (b) at 60 1C. (c) A
cryo-TEM image of an aqueous solution of 1 prepared at 60 1C.
temperature (LCST) behavior of the oligoether chains.8 The
transition temperature was subsequently determined to be
55 1C by turbidity measurements using UV-vis transmittance
(ESI, Fig. S1w). TEM images at higher temperatures than the
LCST also revealed elementary fibrillar objects with a
diameter of B5 nm (Fig. 2b), indicating that the elongated
nanofibers remained unchanged in shape upon heating. The
existence of the fibrillar aggregates at higher temperatures was
further confirmed by cryo-TEM and DLS measurements at
60 1C (Fig. 2c and ESI, Fig. S2w). However, the CD spectra
showed a strong Cotton effect above the transition tempera-
ture accompanied by notable changes in the absorption and
emission spectra (Fig. 3a), indicating the formation of helical
stacks of the rod segments with a preferred handedness.5b To
prove that the CD signal was not from artifacts, linear
dichroism (LD) experiments were performed at different
temperatures (ESI, Fig. S4w).9 The LD values appeared to
be negligibly smaller than those arising from CD, demonstrating
that the CD signal was from molecular assemblies.10 Upon
heating, the intensity of the absorption spectra is significantly
reduced and the fluorescence is quenched (Fig. 3b and c),
indicating that the p–p stacking interactions between the
aromatic rods are enhanced.11 These results suggest that the
helicity induction of the nanofibers upon heating arises from
closer packing between the adjacent aromatic units within the
core. The strengthened p-stacking interactions would lead to a
more planar conformation of the rod segment. This is reflected
in the bathochromic shift of an absorption maximum in UV,
which is attributed to the transformation into longer effective
conjugation length.
Fig. 3 (a) CD spectra and reversible switching cycles of the intensity
at 370 nm in an aqueous solution of 1 (0.01 wt%); (b) UV-vis, and
(c) fluorescence spectra of 1 (0.01 wt%) with temperature variation
(inset: 425 nm intensity of 1 upon cooling from 80 to 30 1C);
(d) fluorescence spectra of 1 (0.01 wt% aqueous solution) in the
absence and in the presence of 1 equiv. of Nile Red at 30 1C
(excited at 355 nm, inset: magnification between 550 and 750 nm).
be loosely stacked with each other to reduce steric repulsions
between the bulky oligoether chains. This packing frustration
would allow the rod segments to rotate in a random way
within the cores to block the chiral transfer from the side
groups to the aromatic cores, leading to non-chiral fibers.
Above the LCST, however, the ethylene oxide chains would be
dehydrated and collapse into molecular globules with reduced
effective volume, leading to a decrease in steric repulsions
between the oligoether chains.8 This shrinkage, together with
enhanced hydrophobic environment, leads to the increased
strength of the p-stacking interactions with restricted
rotational freedom, and helical stacks are formed with a
reduced angle between the rod axis. Consequently, the
non-chiral fibers, upon heating above the LCST, are reversibly
transformed into one-handed helical fibers through enhanced
p–p stacking interactions (Fig. 1).
To corroborate the role of the cross-sectional area of the
oligoether chains in the helicity induction, molecule 2,
based on a tetrabranched oligo(ethylene oxide) chain, was
investigated. Similar to 1, molecule 2 is also self-assembled
into fibrillar aggregates, as confirmed by TEM and DLS
experiments (see the ESIw). However, 2 shows the absence of
the Cotton effect at room temperature, and remains unchanged
even above the LCST transition (ESI, Fig. S6w), indicating
that the disordered packing of 2 is retained even above the
LCST due to steric crowding between bulky tetrabranched
chains. These results strongly support the idea that the cross-
sectional area of the oligoether chain plays a crucial role in the
reversible chiral switching behavior of the fibrillar aggregates.
In summary, the results described herein demonstrate that
non-chiral nanofibers can be reversibly switched to a chiral
state, triggered by external stimuli. The helicity induction of
the nanofibers takes place through molecular reorganization
within the aromatic core on heating. It is worth noting that,
upon heating, the helicity induction in the non-chiral
To gain more insight into the p–p stacking interactions with
temperature variation, fluorescence resonance energy transfer
(FRET) experiments were performed with a hydrophobic dye,
Nile Red. When the aqueous solution of 1 containing Nile Red
is excited at 355 nm at higher temperatures, the fluorescence
shows only a strong emission maximum at 425 nm corres-
ponding to aggregated aromatic segments of 1 (ESI, Fig. S5w).
However, on cooling to room temperature, the emission
intensity at 425 nm sharply decreases with the concomitant
formation of an emission at 620 nm, demonstrating that
energy transfer occurs only below the LCST, indicative of
separation between the adjacent rods within the fibers
(Fig. 3d).12 This result suggests that the nanofibers are some-
what lengthened through loose packing of the aromatic
segments on cooling.
Based on these results, the rod segments within the
cylindrical fibers at lower temperatures can be considered to
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nanofibers represents a remarkable contrast to other supra-
molecular nanofibers that dissolve into molecular components
and/or smaller non-chiral aggregates.5a,6b,13 Such fibrillar
aggregates with dynamic structural changes may provide a
new strategy for the construction of supramolecular device
with chiroptical switching behavior.
H. Zhang, M. Glasbeek, J. A. J. M. Vekemans and E. W. Meijer,
J. Am. Chem. Soc., 2000, 122, 6175.
5 (a) O. Henze, W. J. Feast, F. Gardebien, P. Jonkheijm,
R. Lazzaroni, P. Philippe Leclere, E. W. Meijer and
A. P. H. J. Schenning, J. Am. Chem. Soc., 2006, 128, 5923;
(b) P. Jonkheijm, F. J. M. Hoeben, R. Kleppinger,
J. V. Herrikhuyzen, A. P. H. J. Schenning and E. W. Meijer,
J. Am. Chem. Soc., 2003, 125, 15941; (c) A. Ajayaghosh,
R. Varghese, S. J. George and C. Vijayakumar, Angew. Chem.,
Int. Ed., 2006, 45, 1141; (d) J. Bae, J.-H. Choi, Y.-S. Yoo,
N.-K. Oh, B.-S. Kim and M. Lee, J. Am. Chem. Soc., 2005, 127,
9668; (e) J.-H. Ryu, H.-J. Kim, Z. Huang, E. Lee and M. Lee,
Angew. Chem., Int. Ed., 2006, 45, 5304; (f) B.-K. Cho, H.-J. Kim,
Y.-W. Chung, B.-I. Lee and M. Lee, Adv. Polym. Sci., 2008, 219, 69.
6 (a) H. Engelkamp, S. Middelbeek and R. J. M. Nolte, Science,
1999, 284, 785; (b) P. Jonkheijm, P. V. D. Schoot, A. P. H. J.
Schenning and E. W. Meijer, Science, 2006, 313, 80;
(c) W.-Y. Yang, E. Lee and M. Lee, J. Am. Chem. Soc., 2006,
128, 3484.
We gratefully acknowledge the National Creative Research
Initiative Program of the Ministry of Education, Science and
Technology (MEST). E.L. thanks the BK21 program of MEST.
Notes and references
1 (a) J. J. L. M. Cornelissen, A. E. Rowan, R. J. M. Nolte and
N. A. J. M. Sommerdijk, Chem. Rev., 2001, 101, 4039;
(b) L. Brunsveld, B. J. B. Folmer, E. W. Meijer and
R. P. Sijbesma, Chem. Rev., 2001, 101, 4071; (c) H.-J. Kim,
Y.-b. Lim and M. Lee, J. Polym. Sci., Part A: Polym. Chem.,
2008, 46, 1925; (d) V. Percec, J. G. Rudick, M. Peterca and
P. A. Heiney, J. Am. Chem. Soc., 2008, 130, 7503.
2 (a) A. Khan, C. Kaiser and S. Hecht, Angew. Chem., Int. Ed., 2006,
45, 1878; (b) H. Sugiura, Y. Nigorikawa, Y. Saiki, K. Nakamura
and M. Yamaguchi, J. Am. Chem. Soc., 2004, 126, 14858;
(c) T. Nishimura, K. Tsuchiya, S. Ohsawa, K. Maeda,
E. Yashima, Y. Nakamura and J. Nishimura, J. Am. Chem.
Soc., 2004, 126, 11711; (d) M. T. Stone and J. S. Moore, J. Am.
Chem. Soc., 2005, 127, 5928; (e) J. P. Hill, W. Jin, A. Kosaka,
T. Fukushima, H. Ichihara, T. Shimomura, K. Ito, T. Hashizume,
N. Ishii and T. Aida, Science, 2004, 304, 1481; (f) C. Tan,
M. R. Pinto, M. E. Kose, I. Ghiviriga and K. S. Schanze, Adv.
Mater., 2004, 16, 1208; (g) M. Barboiu, G. Vaughan,
N. Kyritsakas and J.-M. Lehn, Chem.–Eur. J., 2003, 9, 763;
(h) H. N. Bordallo, B. A. Kolesov, E. V. Boldyreva and
F. Juranyi, J. Am. Chem. Soc., 2007, 129, 10984; (i) H.-J. Kim,
J.-H. Lee and M. Lee, Angew. Chem., Int. Ed., 2005, 44, 5810.
3 (a) H. G. Borner and H. Schlaad, Soft Matter, 2007, 3, 394;
(b) Y.-b. Lim and M. Lee, J. Mater. Chem., 2008, 18, 723.
7 Z. Huang, J.-H. Ryu, E. Lee and M. Lee, Chem. Mater., 2007, 19,
6569.
8 (a) E. E. Dormidontova, Macromolecules, 2002, 35, 987;
(b) G. D. Smith and D. Bedrov, J. Phys. Chem. B, 2003, 107, 3095.
9 (a) C. C. Lee, C. Grenier, E. W. Meijer and A. P. H. J. Schenning,
Chem. Soc. Rev., 2009, 38, 671; (b) M. Wolffs, S. J. George,
Z. Tomoviæ, S. C. J. Meskers, A. P. H. J. Schenning and
E. W. Meijer, Angew. Chem., Int. Ed., 2007, 46, 8203;
(c) A. Tsuda, M. A. Alam, T. Harada, T. Yamaguchi, N. Ishii
and T. Aida, Angew. Chem., Int. Ed., 2007, 46, 8198.
10 K. Murata, M. Aoki, T. Suzuki, T. Harada, H. Kawabata,
T. Komri, F. Ohseto, K. Ueda and S. Shinkai, J. Am. Chem.
Soc., 1994, 116, 6664.
11 (a) R. W. Sinkeldam, F. J. M. Hoeben, M. J. Pouderoijen, I. De
Cat, J. Zhang, S. Furukawa, S. De Feyter, J. A. J. M. Vekemans
and E. W. Meijer, J. Am. Chem. Soc., 2006, 128, 16113;
(b) T. Nakano and T. Yade, J. Am. Chem. Soc., 2003, 125, 15474.
12 (a) E. Lee, J.-K. Kim and M. Lee, Angew. Chem., Int. Ed., 2008, 47,
6375; (b) V. K. Praveen, S. J. George, R. Varghese,
C. Vijayakumar and A. Ajayaghosh, J. Am. Chem. Soc., 2006,
128, 7542.
4 (a) F. J. M. Hoeben, P. Jonkheijm, E. W. Meijer and A. P. H. J.
Schenning, Chem. Rev., 2005, 105, 1491; (b) A. Ajayaghosh and
V. K. Praveen, Acc. Chem. Res., 2007, 40, 644; (c) L. Brunsveld,
13 P. Van der Schoot, M. A. J. Michels, L. Brunsveld, R. P. Sijbesma
and A. Ramzi, Langmuir, 2000, 16, 10076.
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