Ordered nanostructures from the self-assembly of reactive coil-rod-coil molecules

Article abstract of DOI:10.1002/anie.200502911

Keeping in shape: Polymerizable coil-rod-coil molecules self-organize into 2D columnar and 3D bicontinuous liquid crystals whose supramolecular architectures and lattice dimensions are preserved upon photopolymerization (see picture). Nanofibers resulting

Full text of DOI:10.1002/anie.200502911

Communications
[
1]
by the self-assembly of programmed molecules. Control of
the supramolecular structure of self-assembling molecules
and subsequent cross-linking of specific segments within the
organized structure are an efficient way to produce robust
organic nanomaterials while maintaining the precise size and
[
2]
shape of the self-assembled nanostructure. Rod-shaped
molecules containing flexible chains have been shown to self-
assemble into a large variety of supramolecular structures,
which include 3D bicontinuous cubic and 2D columnar
[
3]
structures. To enhance the structural robustness of the
self-assembled nanoarchitectures, we designed a coil–rod–coil
molecule that contains polymerizable vinyl groups in the
middle of the rod segment. Herein, we describe the self-
assembling behavior of the coil–rod–coil molecule,
1
(Figure 1), which bears reactive divinyl groups in the center
of the rod block, and the subsequent polymerization of 1 from
an ordered state.
Liquid Crystals
DOI: 10.1002/anie.200502911
Ordered Nanostructures from the Self-Assembly
of Reactive Coil–Rod–Coil Molecules**
Figure 1. Schematic representation of liquid crystalline phases of 1 and
subsequent polymerization to yield ordered nanostructures.
Long Yi Jin, Jinyoung Bae, Ja-Hyoung Ryu, and
Myongsoo Lee*
The triblock molecule, 1, consisting of five biphenyl units
linked through benzyl ether linkages and bis(pentaethylene
glycol) dendrons as coil segments at both ends of the rod, was
synthesized by etherification of penta(ethylene glycol) mono-
methyl ether and 4,4’-biphenol, and the resulting product was
subsequently treated with excess 4,4’-bis(bromomethyl)bi-
phenyl to obtain a white waxy solid, 1c (Scheme 1). Simple
etherification of 1c with 2,2’-divinyl-4,4’-biphenol in the
One of the most important challenges in the areas of
nanomaterials and biomimetic chemistry is to construct
supramolecular structures with well-defined shape and size
[
+]
[
*] Dr. L. Y. Jin, J. Bae, J.-H. Ryu, Prof. M. Lee
Center for Supramolecular Nano-Assembly and Department of
Chemistry
presence of K CO afforded 1. The final product was
2
3
1
13
characterized by H NMRand
C NMRspectroscopy and
Yonsei University
Shinchon 134, Seoul 120-749 (Korea)
Fax: (+82)2-393-6096
by elemental analysis. The results were in full agreement with
the structure presented in Scheme 1.
The mesophase structure of 1 was investigated by differ-
ential scanning calorimetry (DSC), thermal optical polarized
microscopy, and small-angle X-ray scattering (SAXS). On
heating, 1 melts into a liquid crystalline phase at 838C and
then converts into a second liquid crystalline phase which, in
turn, undergoes isotropization at 99.58C (Figure 2). On slow
cooling from the isotropic liquid phase, first a spherulitic
texture is observed and finally pseudofocal conic domains
develop, which is characteristic of a hexagonal columnar
mesophase. On further cooling from the hexagonal columnar
E-mail: mslee@yonsei.ac.kr
+
[
] Permanent address: Department of Chemistry
College of Science and Engineering
Yanbian University, Yanji, 133002 (China)
[
**] This work was supported by the Creative Research Initiative
Program of the Ministry of Science and Technology, Korea, and
Pohang Accelerator Laboratory, Korea (for using synchrotron
radiation). J.B. thanks the Seoul Science Fellowship Program
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
6
50
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 650 –653

Angewandte
Chemie
[
3]
7
.4 nm (Figure 3c). These
results, together with opti-
cal microscopic observa-
tions, indicate that 1 exhib-
its 3D bicontinuous cubic
and hexagonal columnar
liquid crystalline phases.
The polymerization of 1
in the liquid crystalline state
provides a strategy for con-
structing
well-defined
nano-objects if the polymer-
ization proceeds with reten-
tion of a liquid crystalline
structure. To demonstrate
this concept, the photopo-
lymerization of 1 was per-
formed at various liquid
crystalline states, with 2,2-
dimethoxy-2-phenylaceto-
phenone being used as a
photoinitiator. After photo-
polymerization in each
Scheme 1. Synthesis of 1.
Figure 2. DSC traces of 1 recorded during the heating and cooling
scans (K=crystalline, cub=cubic, col =hexagonal columnar, i=iso-
h
tropic phase.
phase, the birefringent texture disappears completely, thus
indicating the presence of a cubic mesophase (see Supporting
[
4]
Information). When 1 is in the optically isotropic liquid
crystalline phase, the corresponding small-angle X-ray dif-
fraction patterns show a number of sharp reflections (Fig-
ure 3a), which can be indexed as a bicontinuous cubic order
[
3b,5]
¯
(
Ia3d symmetry) with a lattice constant a = 16.5 nm.
At a
wide angle, only a diffuse halo remains as evidence of the lack
of any positional long-range order other than the 3D cubic
packing of supramolecular units (see Supporting Informa-
tion). In the birefringent liquid crystalline phase at higher
temperature, the SAXS patterns show three sharp reflections
that can be assigned as the (10), (11), and (20) reflections of a
hexagonal columnar structure with a lattice constant a =
Figure 3. Representative SAXS spectra: a) bicontinuous cubic structure
of 1; b) bicontinuous cubic structure after polymerization at 858C;
c) hexagonal columnar structure of 1; d) hexagonal columnar structure
after polymerization at 978C.
Angew. Chem. Int. Ed. 2006, 45, 650 –653
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
651

Communications
liquid crystalline phase, FTIRspectroscopy clearly showed
place with retention of the 2D liquid crystalline structure. The
that the peaks assigned to the out-of-plane bending band of
TEM images stained with RuO showed a hexagonal array of
4
À1
CÀH in the vinyl group at 919 and C=C band at 1640 cm
dark aromatic domains in a matrix of light polyethylene oxide
chains as well as views in the direction of the column. The
interdomain distance appears to be approximately 7 nm,
which is consistent with that obtained from the SAXS data
(Figure 4c). Interestingly, when the hexagonal columnar
structure was sonicated in water for several hours, the solid
dispersed into individual nanofibers (Figure 4d). Thus, poly-
merization takes place only within the cylindrical cores. Bulk
polymerization in the hexagonal columnar state gives rise to
well-defined nanofibers with a uniform diameter. It should be
noted that the wide-angle X-ray scatterings (WAXS) after
polymerization in each liquid crystalline state showed that the
reflection corresponding to 0.47 nm shifts to a slightly higher
angle (0.45 nm), thus indicating that polymerization occurs
with only a small degree of shrinkage. This shift is consistent
with the results of very small lattice constant reductions in
SAXS patterns (see Supporting Information).
were strongly suppressed, which is indicative of extensive
[
6]
polymerization (see Supporting Information). The SAXS
pattern of the sample after polymerization at 858C shows the
same reflections as those of the bicontinuous cubic liquid
crystalline structure of 1. This result is consistent with the
preservation of the 3D bicontinuous cubic structure upon
polymerization. To corroborate the detailed nanostructures,
TEM experiments were performed with the polymerized
samples. The TEM images of an ultra-microtomed thin film of
the samples (stained with RuO ) polymerized in the bicon-
4
tinuous cubic liquid crystalline phase showed tetragonal and
hexagonal arrays of dark rod domains in a light coil matrix
¯
¯
along the [011] and [111] directions, respectively, which is
[
7]
characteristic of a 3D bicontinuous cubic structure. The
lattice constant, calculated to be 16 nm, is in agreement with
the value estimated from the SAXS data (Figure 4a, b). Bulk
Recently, there have been many reports of Suzuki
[
8a]
couplings in water, and we have demonstrated that self-
assembled aromatic rod bundles encapsulated by hydrophilic
chains can be used as an efficient nanoreactor for Suzuki
[
8b]
coupling reactions. This result motivated us to investigate
whether our nanofibers are suitable to use as nanoreactors in
aqueous solution because the nanofibers consist of an
aromatic core surrounded by hydrophilic chains. To inves-
tigate the capability of the nanofibers as a reactor, we have
performed the Suzuki coupling reaction with phenyl boronic
acid and bromoanisole in the presence of the nanofibers in
aqueous solution at room temperature. Remarkably, the
reaction conversions were almost quantitative. This reactivity
demonstrates that the nanofibers are highly efficient nano-
reactors (Table 1). Owing to their covalently fixed character-
Table 1: Suzuki cross-coupling reaction of phenyl boronic acid and
[
a]
bromoanisole in a nanofiber nanoreactor.
Cycle
First
95
Second
94
Third
93
Fourth
93
[
b]
yield [%]
[a] The reaction was carried out in water at room temperature. Reaction
conditions: bromoanisole (0.1 mmol), phenyl boronic acid (0.12 mmol),
Pd(OAc) (0.5 mol%), P(Ph) (1.0 mol%), NaOH (0.2 mmol), nanofiber
2
3
Figure 4. a,b) TEM images of an ultra-microtomed thin film of the
sample polymerized in the optically isotropic mesophase stained with
(
0.025 mmol), H O (10 mL), stirring at room temperature for 12 h.
2
[b] Percent yields are calculated on the basis of GC analysis with xylene
RuO , which exhibit an ordered array of tetragonal and hexagonal dark
4
used as the internal standard.
¯
¯
rod domains in a light coil matrix along the [011] (a) and [111]
directions (b); c) an ultra-microtomed thin film of the sample polymer-
ized in the birefringent mesophase stained with RuO , which reveals
alternating dark rod layers and light coil layers; the inset shows an in-
plane hexagonally ordered array of dark rods in a coil matrix;
4
istics, nanofibers can be easily recycled for use in further
reactions by filtering the mixture and washing the nanofibers
with ethyl acetate. As shown in Table 1, the nanofibers can be
used up to four times and still maintain activity. The aqueous
Suzuki coupling reactions in the presence of the nanofibers
take place within the aromatic cores of the nanofibers at room
temperature as these cores are able to entrap solvophobic
aromatic reactants through intermolecular interactions,
including hydrophobic interactions and p–p interactions.
This entrapment leads to a highly concentrated reaction site
that lowers the energy barrier for the coupling reaction. The
confinement of the aromatic substrates within the aromatic
d) dispersed nanofibers in aqueous solution (stained with RuO ).
4
polymerization of 1 at 978C also proceeded with retention of
the hexagonal columnar liquid crystalline structure, as con-
firmed by SAXS and TEM (Figure 3b and d). The SAXS
pattern showed reflections similar to those of hexagonal
columnar mesophase with a small lattice contraction (from
7
.4 nm to 7.3 nm) as a consequence of the stitching together of
rod segments. This result indicates that polymerization takes
6
52
www.angewandte.org
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 650 –653

Angewandte
Chemie
cores of the nanofibers was confirmed by using fluorescence
spectroscopy on aryl halides, boronic acids, and triphenyl-
phosphine (see Supporting Information). Upon addition of
the aromatic substrates, the fluorescence intensities of the
nanofiber solutions were suppressed, thus demonstrating that
they are effectively entrapped within the aromatic bundle of
28, 8796 – 8806; b) B.-K. Cho, A. Jain, S. M. Gruner, U. Wiesner,
Science 2004, 305, 1598 – 1601.
6] Y. Li, Y. Fan, J. Ma, Polym. Degrad. Stab. 2001, 73, 163 – 167.
7] a) V. Z.-H. Chan, J. Hoffman, V. Y. Lee, H. Iatrou, A. Avger-
opoulos, N. Hadjichristidis, R. D. Miller, E. L. Thomas, Science
[
[
1
2
999, 286, 1716 – 1719; b) C. Yu, Y. Yu, D. Zhao, Chem. Commun.
000, 7, 575 – 576; c) R. C. Hayward, P. C. A. Alberius, E. J.
[
8]
the nanofibers.
Kramer, B. F. Chmelka, Langmuir 2004, 20, 5998 – 6004.
[8] a) N. E. Leadbeater, M. Marco, Angew. Chem. 2003, 115, 1445 –
447; Angew. Chem. Int. Ed. 2003, 42, 1407 – 1409; b) M. Lee, C.-J.
Jang, J.-H. Ryu, J. Am. Chem. Soc. 2004, 126, 8082 – 8083.
In summary, we have synthesized a polymerizable coil–
rod–coil molecule, which self-organizes into 2D columnar and
1
3
D bicontinuous cubic structures at the liquid crystalline
state. Photopolymerizations of 1 in liquid crystalline state
proceeds with preservation of the ordered supramolecular
architectures and maintenance of the lattice dimensions.
Photopolymerization of 1 in the bicontinuous cubic liquid
crystalline state gives rise to a 3D ordered nanostructure,
while in the hexagonal columnar liquid crystalline state it
produces a 2D ordered nanostructure that in aqueous solution
can be dispersed into individual nanofibers with a uniform
diameter. The remarkable feature of these nanofibers, which
have aromatic cores, is their ability to be used as recyclable
nanoreactors for room-temperature Suzuki coupling reac-
tions in aqueous solution. The covalent stitching of reactive
rod segments within the ordered state by photopolymeriza-
tion offers a strategy to construct shape-persistent organic
nanomaterials with well-defined size and shape, which
potentially have applications in macromolecular electronics,
nanoreactors, and hybrid nanomaterials.
Received: August 16, 2005
Revised: October 24, 2005
Published online: December 15, 2005
Keywords: liquid crystals · nanotechnology · polymers ·
.
supramolecular chemistry · Suzuki coupling
[
1] a) T. Shimizu, M. Masuda, H. Minamikawa, Chem. Rev. 2005, 105,
401 – 1444; b) F. J. M. Hoeben, P. Jonkheijm, E. W. Meijer,
1
A. P. H. J. Schenning, Chem. Rev. 2005, 105, 1491 – 1546; c) M.
Lee, B.-K. Cho, W.-C. Zin, Chem. Rev. 2001, 101, 3869 – 3892.
2] a) E. R. Zubarev, M. U. Pralle, L. Li, S. I. Stupp, Science 1999, 283,
[
5
9
23 – 526; b) Y.-Y. Won, H. T. Davis, F. S. Bates, Science 1999, 283,
60 – 963; c) W. Jin, T. Fukushima, A. Kosaka, M. Niki, N. Ishii, T.
Aida, J. Am. Chem. Soc. 2005, 127, 8284 – 8285; d) L. Y. Jin, J.-H.
Ahn, M. Lee, J. Am. Chem. Soc. 2004, 126, 12208 – 12209; e) B. A.
Pindzola, J. Jin, D. L. Gin, J. Am. Chem. Soc. 2003, 125, 2940 –
2
949; f) A. Müller, D. F. OꢀBrien, Chem. Rev. 2002, 102, 727 – 758.
3] a) M. Lee, B.-K. Cho, H. Kim, W.-C. Zin, Angew. Chem. 1998, 110,
61 – 663; Angew. Chem. Int. Ed. 1998, 37, 638 – 640; b) M. Lee,
B.-K. Cho, Y.-G. Jang, W.-C. Zin, J. Am. Chem. Soc. 2000, 122,
449 – 7455; c) B.-K. Cho, M. Lee, N.-K. Oh, W.-C. Zin, J. Am.
[
6
7
Chem. Soc. 2001, 123, 9677 – 9678; d) M. Lee, B.-K. Cho, H. Kim,
J.-Y. Yoon, W. C. Zin, J. Am. Chem. Soc. 1998, 120, 9168 – 9179;
e) L. Y. Jin, J. Bae, J.-H. Ahn, M. Lee, Chem. Commun. 2005, 9,
1197 – 1199.
[
4] a) D. Demus, L. Richter, Texture of Liquid Crystals, Verlag
Chemie, Weinheim, 1978; b) G. W. Gray, J. W. Goodby, Smectic
Liquid Crystals. Textures and Structures, Leonard Hill, Glasgow,
1984; c) K. Borisch, S. Diele, P. Gꢁring, H. Kresse, C. Tschierske,
J. Mater. Chem. 1998, 8, 529 – 543.
[
5] a) A. K. Khandpur, S. Fꢁrster, F. S. Bates, I. W. Hamley, A. J.
Ryan, W. Bras, K. Almdal, K. Mortensen, Macromolecules 1995,
Angew. Chem. Int. Ed. 2006, 45, 650 –653
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
653