Helical nanofibers from aqueous self-assembly of an oligo(p-phenylene)- based molecular dumbbell

Article abstract of DOI:10.1021/ja051961m

We have synthesized an amphiphilic dumbbell-shaped molecule consisting of dodeca-p-phenylene and aliphatic polyether dendrons as flexible end groups. The molecular dumbbell in aqueous solution self-assembles into well-defined left-handed helical cylinders

Full text of DOI:10.1021/ja051961m

Published on Web 06/15/2005
Helical Nanofibers from Aqueous Self-Assembly of an
Oligo(p-phenylene)-Based Molecular Dumbbell
Jinyoung Bae, Jin-Ho Choi, Yong-Sik Yoo, Nam-Keun Oh, Byung-Sun Kim, and Myongsoo Lee*
Center for Supramolecular Nano-Assembly and Department of Chemistry, Yonsei UniVersity, Seoul 120-749, Korea
Received March 28, 2005; E-mail: mslee@yonsei.ac.kr
The self-assembly of incompatible molecular components leading
to microphase separation comprises a powerful approach toward
the fabrication of complex nanoarchitectures and plays an essential
role in living systems, for example, in protein folding and the
formation of biological membranes.1 Extensive efforts thus have
been directed toward bioinspired supramolecular systems for
exploration of novel properties and functions that are difficult
without specific assembly of molecular components.2 Self-as-
sembling molecules based on p-conjugated rods are receiving
increased attention as building blocks for electrooptically active
supramolecular structures, such as discrete bundles, ribbons, and
vesicles.3 One can envision that incorporation of a conjugated rod
into an amphiphilic dumbbell-shaped molecular architecture, by
grafting bulky flexible dendrons to its both ends, extends the
supramolecular organization capabilities of conjugated aromatic
rods. The attachment of bulky dendrons into both ends of a rod
would frustrate a parallel arrangement of the rod segments
commonly observed for linear rod-coil molecules,4 in order to
minimize a steric repulsion between bulky dendritic segments.
Instead, the rod segments should be strongly driven to aggregate
in one dimension with a regular helical arrangement through
microphase separation between incompatible molecular components
and π-π stacking interactions between the aromatic units.
We present here the formation of helical nanofibers from the
self-assembly of a dumbbell-shaped molecule based on a conjugated
rod segment in aqueous solution (Figure 1). The dumbbell-shaped
molecule that forms this aggregate consists of a dodeca-p-phenylene
rod and aliphatic polyether dendrons that are covalently linked at
both ends of the rod segment. The synthesis of the dumbbell-shaped
molecule started with the preparation of an aromatic scaffold
containing three oligoether dendrons according to the procedures
described previously.5 The final dumbbell-shaped molecule was
synthesized by palladium-catalyzed homocoupling reaction of the
precursor molecules. The resulting molecule was characterized by
1H and 13C NMR spectroscopy, elemental analysis, and MALDI-
TOF mass spectroscopy and shown to be in full agreement with
the structures presented.
The molecular dumbbell (1), when dissolved in a selective
solvent for one of the blocks, can self-assemble into an aggregate
structure because of its amphiphilic characteristics.6 Aggregation
behavior of the molecule was subsequently studied in aqueous
solution by using UV-vis, fluorescence, and circular dichroism
(CD) spectra (Figure 2). The absorption spectrum of 1 in aqueous
solution (0.5 wt %) exhibits a broad transition with a maximum at
351 nm, resulting from the conjugated rod block. The fluorescence
spectrum of 1 in chloroform solution (0.32 wt %) exhibits a strong
emission maximum at 419 nm. However, the emission maximum
in aqueous solution shows to be red-shifted with respect to that
observed in chloroform solution, and the fluorescence is signifi-
cantly quenched, indicative of aggregation of the conjugated rod
segments (Figure 2a).7 CD spectra of the aqueous solutions of 1
Figure 1. Molecular structure of 1 and schematic representation of a helical
nanofiber.
Figure 2. (a) Absorption (left) and emission (right) spectra of 1 in
chloroform solution (0.32 wt %, blue dashed line). Absorption (left) and
emission (right) spectra of 1 in aqueous solution (0.5 wt %, black solid
line). (b) CD spectra of 1 at 25 °C.
above certain concentrations (from 0.05 wt %) show first a negative
Cotton effect followed by a positive Cotton effect at higher
wavelength with the CD signal passing through zero near the
absorption maximum of the chromophore (Figure 2b), indicating
the formation of a helical superstructure with a preferred handed-
ness.8
Dynamic light scattering (DLS) experiments were performed with
1 in aqueous solution to further investigate the aggregation
behavior.9 The CONTIN analysis of the autocorrelation function
shows a broad peak corresponding to an average hydrodynamic
radius of approximately 270 nm. The angular dependence of the
apparent diffusion coefficient (Dapp) was measured because the slope
is related to the shape of the diffusing species. The slope was
observed to be 0.02, consistent with the value predicted for
cylindrical micelles (0.03).10 The formation of cylindrical micelles
was further confirmed by the Kratky plot that shows a linear angular
dependence over the scattering light intensity of the aggregates.11
The evidence for the formation of the cylindrical aggregates was
also provided by transmission electron microscopy (TEM) experi-
ments performed with the solutions of 1 (0.05 wt %) (Figure 3).
The micrographs of the unstained samples show cylindrical
aggregates with a left-handed helical arrangement with lengths up
to several micrometers and widths of 340 and 80 nm, respectively.
When the sample was negatively stained with a 2 wt % aqueous
solution of uranyl acetate, the images show predominantly the
smallest left-handed helical objects with a uniform diameter of about
8 nm and a pitch length of 5.6 nm. Considering the extended
9
9668
J. AM. CHEM. SOC. 2005, 127, 9668-9669
10.1021/ja051961m CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
In summary, the results described here demonstrate that the
molecular dumbbell based on an oligo-p-phenylene self-assembles
into well-defined left-handed helical cylinders with a diameter of
a molecular length scale and a pitch length of 5.6 nm. These
elementary fibrils are further assembled to superhelical fibers with
lengths up to several micrometers. The primary driving force
responsible for the helical arrangement of the conjugated rods is
believed to be the energy balance between repulsive interactions
among the adjacent bulky dendritic segments and π-π stacking
interactions. Such a well-defined helical arrangement of conjugated
rod building blocks may provide a new strategy for the design of
one-dimensional nanostructured materials with biomimetic, elec-
tronic, and photonic functions.
Acknowledgment. We gratefully acknowledge the National
Creative Research Initiative Program of the Korean Ministry of
Science and Technology for financial support of this work.
Supporting Information Available: Synthetic procedures, char-
acterization, and DLS data. This material is available free of charge
via the Internet at http://pubs.acs.org.
Figure 3. TEM images of 1 (a) without staining, (b) and (c) with negative
staining, with density profile inset.
molecular length (8.3 nm by CPK), the image indicates that the
diameter of the elementary cylindrical objects corresponds to one
molecular length. The density profiles taken parallel to the long
axis of a helical fiber confirmed a regular pitch to be 5.6 nm (Figure
3c).
On the basis of the results described thus far, it can be concluded
that molecular dumbbell 1 self-assembles into cylindrical micelles
in which the molecules are aligned perpendicularly to the cylinder
axis. However, the rod segments stack on top of each other with
mutual rotation in the same direction to avoid steric hindrance
between the bulky dendritic wedges. Consequently, this stacking
of the aromatic rod segments would lead to helical objects,
consisting of hydrophobic aromatic cores surrounded by hydrophilic
dendritic segments that are exposed to the aqueous environment.
The observed supramolecular handedness is believed to arise from
steric constraints imposed by the chiral centers in the dendritic
wedges.
The formation of a helical structure is also illustrated by a
computer model in which the COMPASS empirical force-field
calculation is used on a small cluster of 12 molecules stacked on
top of each other. Energy minimization of the cluster suggests that
a helical arrangement of the rod segments is energetically favorable.
The calculation revealed that the distance between two adjacent
rods is 0.46 nm and the angle of rotation is close to 15 degrees,
resulting in a pitch length of 5.4 nm. These single elementary fibrils
are further assembled via amphiphilic interactions to give left-
handed superhelical fibers. Although the origin of superhelicity is
not clear at present, winding of the individual helical strands around
each other seems to be responsible for the formation of a
superhelical structure.
References
(1) (a) Elemans, J. A. A. W.; Rowan, A. E.; Nolte, R. J. M. J. Mater. Chem.
2003, 13, 2661-2670. (b) Wong, G. C. L.; Tang, J. S.; Lin, A.; Li, Y.;
Janmey, P. A.; Safinya, C. R. Science 2000, 288, 2035-2039.
(2) (a) Moreau, L.; Barthelemy, P.; El Maataoui, M.; Grinstaff, M. W. J.
Am. Chem. Soc. 2004, 126, 7533-7539. (b) Lashuel, H. A.; LaBrenz, S.
R.; Woo, L.; Serpell, L. C.; Kelly, J. W. J. Am. Chem. Soc. 2000, 122,
5262-5277. (c) Ky Hirschberg, J. H. K.; Brunsveld, L.; Ramzi, A.;
Vekemans, J. A. J. M.; Sijbesma, R. P.; Meijer, E. W. Nature 2000, 407,
167-170. (d) Aggeli, A.; Bell, M.; Carrick, L. M.; Fishwick, C. W. G.;
Harding, R.; Mawer, P. J.; Radford, S. E.; Strong, A. E.; Boden, N. J.
Am. Chem. Soc. 2003, 125, 9619-9628. (e) Cuccia, L. A.; Ruiz, E.; Lehn,
J.-M.; Homo, J.-C.; Schmutz, M. Chem.-Eur. J. 2002, 8, 3448-3457.
(f) Enomoto, M.; Kishimura, A.; Aida, T. J. Am. Chem. Soc. 2001, 123,
5608-5609.
(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) Wang, H.; You, W.; Jiang, P.; Yu, L.; Wang, H. H. Chem.-
Eur. J. 2004, 10, 986-993. (c) 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. (d) Zahn, S.; Swager, T. M. Angew. Chem., Int. Ed. 2002,
41, 4225-4230. (e) Lee, M.; Kim, J.-W.; Hwang, I.-W.; Kim, Y.-R.; Oh,
N.-K.; Zin, W.-C. AdV. Mater. 2001, 13, 1363-1368. (f) Lee, M.; Jeong,
Y.-S.; Cho, B.-K.; Oh, N.-K.; Zin, W.-C. Chem.-Eur. J. 2002, 8, 876-
883.
(4) Lee, M.; Cho, B.-K.; Zin, W.-C. Chem. ReV. 2001, 101, 3869-3892.
(5) Yoo, Y.-S.; Choi, J.-H.; Song, J.-H.; Oh, N.-K.; Zin, W.-C.; Park, S.;
Chang, T.; Lee, M. J. Am. Chem. Soc. 2004, 126, 6294-6300.
(6) Fo¨rster, S.; Plantenberg, T. Angew. Chem., Int. Ed. 2002, 41, 688-714.
(7) (a) Messmore, B. W.; Hulvat, J. F.; Sone, E. D.; Stupp, S. I. J. Am. Chem.
Soc. 2004, 126, 14452-14458. (b) Varghese, R.; George, S. J.; Ajayag-
hosh, A. Chem. Commun. 2005, 593-595.
(8) Jonkheijm, P.; Hoeben, F. J. M.; Kleppinger, R.; van Herrikhuyzen, J.;
Schenning, A. P. H. J.; Meijer, E. W. J. Am. Chem. Soc. 2003, 125,
15941-15949.
(9) See Supporting Information.
(10) Gohy, J.-F.; Lohmeijer, B. G. G.; Alexeev, A.; Wang, X.-S.; Manners, I.;
Winnik, M. A.; Schubert, U. S. Chem.-Eur. J. 2004, 10, 4315-4323.
(11) Bockstaller, M.; Ko¨hler, W.; Wegner, G.; Vlassopoulos, D.; Fytas, G.
Macromolecules 2000, 33, 3951-3953.
JA051961M
9
J. AM. CHEM. SOC. VOL. 127, NO. 27, 2005 9669