Liquid-crystalline assembly from rigid wedge-flexible coil diblock molecules

Article abstract of DOI:10.1002/anie.200461623

Shape sorting: The incorporation of a flexible coil of variable length into a rigid wedge-shaped building block leads to the spontaneous formation of a rich variety of supramolecular structures. These include a bilayer lamellar structure with in-plane hex

Full text of DOI:10.1002/anie.200461623

Communications
Supramolecular Chemistry
Liquid-Crystalline Assembly from Rigid Wedge–
Flexible Coil Diblock Molecules**
Jung-Keun Kim, Min-Ki Hong, Jong-Hyun Ahn, and
Myongsoo Lee*
There is a growing interest in the rational design of new self-
assembling molecules that possess various chemical or
biological functionalities and self-assemble into tunable and
predictable supramolecular structures.[1] Among self-assem-
bling molecular systems, a dendritic molecular architecture
has proved to be a promising scaffold for nanometer-sized
aggregation structures including spheres and cylinders.[2–6]
Rod–coil block molecules represent another class of self-
assembling systems that are increasingly used for the con-
struction of supramolecular architectures with well-defined
shape.[7] The supramolecular structures can be precisely
controlled by systematic variation of the type and relative
length of the respective blocks.[8,9]
Incorporation of a rigid wedge-shaped building block into
a diblock molecular architecture gives rise to a novel class of
self-assembling systems consisting of a rigid wedge and a
flexible coil because the molecule shares certain general
characteristics of both dendrons and block copolymers.
[*] J.-K. Kim, M.-K. Hong, J.-H. Ahn, Prof. M. Lee
Center for Supramolecular Nanoassembly 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 Ministry of Science and Technology, Korea, and
Pohang Accelerator Laboratory, Korea (use of synchrotron radia-
tion).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
328
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200461623
Angew. Chem. Int. Ed. 2005, 44, 328 –332

Angewandte
Chemie
Wedge-shaped molecules based on a short flexible coil should
self-assemble into discrete supramolecular structures with a
highly curved surface, similar to self-assembling dendron
molecules. As the length of the flexible coil increases,
however, supramolecular structures with a flatter interface
might be formed as a result of the increased effective area of
the flexible coil and consequent space-filling requirement.
Therefore, a combination of the organizing principles of
dendritic wedges and block copolymers may open up new
possibilities for the design of unique supramolecular arrange-
ments from wedge-shaped dendritic building blocks.
terminated PEO chains. The resulting block molecules were
purified by column chromatography on silica gel with ethyl
acetate as the eluent and then further purified by preparative
gel-permeation chromatography (prep. GPC, see the Exper-
imental Section). The resulting molecules were characterized
by NMR spectroscopy, elemental analysis, GPC, and
MALDI-TOF mass spectroscopy, and the data were shown
to be in full agreement with the structures presented.
As shown in Table 1, all of the molecules had a thermo-
tropic liquid-crystalline structure after melting. The wide-
angle X-ray diffraction patterns of all the molecules in the
melt state are characterized by a diffuse scattering, which
confirms their liquid-crystalline nature. However, a signifi-
cant structural variation in the melt state was observed as the
length of the PEO segment was varied, as evidenced by
optical microscopic textures and small-angle X-ray diffraction
scattering (SAXS, Figure 1). Compounds 1a and 1b, which
are based on a short PEO chain, exhibited a lack of
birefringence between crossed polarizers, thus indicating the
presence of a 3D cubic liquid-crystalline phase. The SAXS
study of 1a revealed a number of well-resolved reflections
corresponding to a body-centered cubic structure with Im3m
symmetry and a lattice parameter of 11.6 nm. However, the
SAXS pattern of 1b appeared to correspond to a 3D cubic
structure with a Pm3n space group symmetry with a lattice
constant of 20.3 nm. These results indicate that the symmetry
change in the 3D cubic phase, from an Im3m to a Pm3n
symmetry, occurs with increasing chain length. This symmetry
change was also observed for the 3D micellar cubic phase of
other dendritic molecules on changing the temperature.[11]
The number of molecules per sphere in spherical supramolec-
ular structures is estimated to be 200 and 237 for 1a and 1b,
respectively.
In contrast, compound 1c displayed a birefringent meso-
phase upon melting of the crystalline phase. SAXS studies
confirmed the formation of a 2D hexagonal columnar
structure with a lattice constant a = 10.3 nm, which is in
accordance with the phase assignment made by optical
microscopy (Figure 2a). Remarkably, further cooling from
the hexagonal structure gave rise to a metastable perforated
lamellar structure. On cooling from the hexagonal columnar
mesophase, arc-shaped striations appeared on the pseudo-
focal conic domains at 458C (see Supporting Information),
which was indicative of transformation into a 3D hexagonal
structure. This finding was confirmed by SAXS experiments
that showed a number of reflections with low intensity in
addition to a strong reflection at a lower angle, which can be
This work focuses on a novel combination of the above
perspectives, in which wedge–coil diblock molecules consist-
ing of a rigid wedge and a flexible poly(ethylene oxide)
(PEO) coil were investigated. The self-assembling behavior of
these diblock molecules in the melt state was studied by
optical polarized microscopy, differential scanning calorim-
etry (DSC), and X-ray scattering measurements. The mole-
cules self-assembled successively, as the length of the PEO
chain increased, into 3D micellar cubic (with Im3m and Pm3n
lattices), 2D hexagonal columnar, 3D perforated lamellar,
and smectic-like lamellar structures in the melt state.
The synthesis of rigid wedge–flexible coil diblock mole-
cules, which consisted of the wedge-shaped rigid aromatic
segment containing peripheral tetradecyloxy groups and a
flexible PEO chain, started with the preparation of an
aromatic scaffold with a tetradecyloxy periphery according
to the procedures described previously.[10] The final wedge–
coil diblock molecules were synthesized by an etherification
reaction of a phenolic precursor with the appropriate tosyl-
Table 1: Thermal transitions of 1a–e (data from heating and cooling scans).
Phase transition[a] [8C] and corresponding enthalpy changes [kJmolÀ1
]
[b]
Mw/Mn
Heating
Cooling
1a
1b
1c
1d
1e
1.08
1.04
1.04
1.05
1.05
cr 60.8 (31.5) cub(Im3m) 78.4 (0.14) i
cr 60.2 (32.0) cub(Pm3n) 88.4 (0.80) i
cr 56.5 (18.1) col 93.2 (0.99) i
cr 57.5 (29.6) Lhex 115.6 (1.90) i
cr 48.0 (49.6) sA 143.7 (0.36) i
i 74.5 (0.08) cub(Im3m) 18.5 (12.5) cr
i 83.9 (0.09) cub(Pm3n) 22.4 (14.9) cr
i 91.5 (0.34) col 50.7 (0.23) Lhex 21.0 (36.3) cr
i 112.9 (0.26) Lhex 36.1 (3.65) cr 28.8 (13.5) cr
i 142.1 (0.38) sA 32.4 (4.07) cr 24.0 (1.75) cr 10.3 (49.0) k
[a] cr: crystalline phase, cub: cubic phase, col: hexagonal columnar phase, Lhex: hexagonal perforated lamellar phase, sA: smectic A phase, i: isotropic
phase. [b] Determined by GPC.
Angew. Chem. Int. Ed. 2005, 44, 328 –332
www.angewandte.org
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
329

Communications
aromatic layers with in-plane hexagonal packing of coil
perforations. The consequent layers are stacked in a bilayer
AB–AB arrangement to generate 3D order. The tendency of
a hexagonal columnar phase to transform into a perforated
lamellar phase on cooling is consistent with the results from
rod–coil systems described previously.[9c,13]
Compound 1d displayed a stable perforated lamellar
mesophase. Small-angle X-ray scattering revealed a similar
diffraction pattern to that of the lower-temperature meso-
phase of 1c, which indicates the presence of a bilayer lamellar
structure with in-plane hexagonally ordered coil perforations
(a = 14.5 nm and c = 11.0 nm). This phase identification is
further supported by transmission electron microscopy
(TEM) and optical microscopic observations. The TEM
images stained with RuO4 showed a layered structure with
in-plane light-colored coil domains in a matrix of dark rod
segments (Figure 3a). An arc-shaped pseudofocal conic
Figure 3. Representative TEM images of a) an ultramicrotomed film of
1d stained with RuO4 revealing an ordered array of alternating light-
colored PEO layers and dark aromatic layers. The inset shows an in-
plane hexagonally ordered array of PEO perforations. b) An ultra-
microtomed film of 1e stained with RuO4 exhibiting alternating dark
aromatic layers and light PEO layers.
Figure 1. Small-angle X-ray diffraction patterns of 1a–e plotted against
q. a) The Im3m cubic lattice exhibited by 1a at 658C; b) the Pm3n
cubic lattice exhibited by 1b at 658C; c) the hexagonal columnar lattice
exhibited by 1c at 708C; d) the hexagonal perforated layer lattice exhib-
ited by 1c at 408C; e) the hexagonal perforated layer lattice exhibited
by 1d at 708C; and f) the lamellar lattice exhibited by 1e at 708C.
texture was observed on an optical polarized microscope
(Figure 2b) on cooling from the isotropic liquid, which
supports the formation of a 3D hexagonal structure.[9b] In
contrast, compound 1e exhibited a smectic A phase, as
confirmed by optical microscopic texture and X-ray diffrac-
tion studies. The layer thickness obtained from the X-ray
diffraction pattern appeared to be 11.5 nm, that is, much
smaller than the estimated molecular length (17.1 nm by CPK
models), and suggests that the molecules are packed with a
monolayer lamellar structure in which the rigid wedge
segments are interdigitated. This result is further supported
by TEM experiments that showed much thinner dark
aromatic layers compared to those of 1d (Figure 3b). These
findings indicate that increasing coil length leads to trans-
formation of a bilayer to an interdigitated monolayer
structure.
The results described above demonstrate the capability of
manipulating the supramolecular structure by grafting flexi-
ble PEO chains of different lengths to the apex of the same
wedge-shaped rigid building block (Figure 4). The variation in
the supramolecular structure can be rationalized by consid-
ering the microphase separation between the dissimilar parts
of the molecule and the space-filling requirement of the
flexible PEO chains. The molecules based on a short PEO
chain can be packed with a radial arrangement to fill the space
efficiently, which results in a spherical supramolecular
Figure 2. Representative optical micrographs (100ꢀ) of: a) the texture
exhibited by the hexagonal columnar mesophase of 1c at 808C in the
cooling scan; and b) the texture exhibited by the hexagonal perforated
lamellar mesophase of 1d at 908C in the cooling scan.
indexed as a 3D hexagonal close-packed structure with lattice
constants of a = 12.3 nm and c = 9.2 nm (Figure 1d). The peak
intensity associated with the (001) reflection appeared to be
the most intense, which suggests that the fundamental
structure is lamellar, similar to that of the perforated lamellar
structure reported previously.[9,12] Thus, this 3D hexagonal
structure can be considered as a system of perforated
330
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
Angew. Chem. Int. Ed. 2005, 44, 328 –332

Angewandte
Chemie
Figure 4. Schematic representation of supramolecular structures formed by wedge-shaped molecules.
structure. Increasing the length of the PEO chain results in
more space for the chains being required while maintaining
the radial arrangement of the rigid segments. Consequently,
the discrete PEO domains will be extended to an infinitely
long cylindrical domain in which the coils are less confined,
which results in the formation of a 2D hexagonal columnar
structure. Further increasing the length results in the rigid
wedge-shaped segments assembling into a perforated lamel-
lar structure with a radial arrangement of rigid segments,
which allows a greater volume for the PEO chains to explore
compared to that of the columnar structure. The radial
arrangement of the wedge-shaped rigid segments eventually
transforms into a parallel arrangement with interdigitation, to
produce a flat interface while maintaining a constant density
of the rigid hydrophobic domain. This transformation results
in a monolayer lamellar structure as in the case of 1e.
The self-organization phenomena of the series of rigid
wedge–flexible coil diblock molecules demonstrate that
systematic variation in the coil length can regulate the
supramolecular structure, from 3D micellar cubic with differ-
ent lattices, 2D hexagonal columnar, and perforated lamellar
to smectic-like liquid crystalline assemblies. The primary
force responsible for this structural change is believed to be
the combination of shape complementarity and microphase
separation between rigid and flexible segments. Compared to
other self-assembling systems based on dendritic mole-
cules,[2–6,11,14] the remarkable feature of the wedge-shaped
building blocks investigated here is their ability to self-
assemble not only into spherical cubic and columnar struc-
tures, but also into an unusual bilayer lamellar structure with
in-plane hexagonally ordered coil perforations. These results
demonstrate that this approach of controlling supramolecular
structures using wedge-shaped building blocks and only a
small variation in the length of grafted coils allows unexpect-
edly complex superstructures to be produced.
Received: August 12, 2004
Keywords: liquid crystals · nanostructures · polymers ·
.
self-assembly · supramolecular chemistry
[1] a) J.-M. Lehn, Supramolecular Chemistry: Concepts and Per-
spectives, VCH, Weinheim, 1995; b) J. A. A. W. Elemans, A. E.
Rowan, R. J. M. Nolte, J. Mater. Chem. 2003, 13, 2661; c) T. Kato,
Science 2002, 295, 2414.
[2] a) G. Ungar, Y. Liu, X. Zeng, V. Percec, W.-D. Cho, Science 2003,
299, 1208; b) V. Percec, C. M. Mitchell, W.-D. Cho, S. Uchida, M.
Glodde, G. Ungar, X. Zeng, Y. Liu, V. S. K. Balagurusamy, P. A.
Heiney, J. Am. Chem. Soc. 2004, 126, 6078.
[3] a) T. Kato, T. Matsuoka, M. Nishii, Y. Kamikawa, K. Kanie, T.
Nishimura, E. Yashima, S. Ujiie, Angew. Chem. 2004, 116, 2003;
Angew. Chem. Int. Ed. 2004, 43, 1969; b) M. Enomoto, A.
Kishimura, T. Aida, J. Am. Chem. Soc. 2001, 123, 5608; c) M.
Suarez, J.-M. Lehn, S. C. Zimmerman, A. Skoulios, B. Heinrich,
J. Am. Chem. Soc. 1998, 120, 9526.
[4] J. C. M. van Hest, D. A. P. Delnoye, M. W. P. L. Baars, M. H. P.
van Genderen, E. W. Meijer, Science 1995, 268, 1592.
[5] a) M. R. Leduc, W. Hayes, J. M. J. Frꢀchet, J. Polym. Sci. Polym.
Chem. 1998, 36, 1; b) J. Iyer, K. Fleming, P. T. Hammond,
Macromolecules 2004, 37, 2490; c) A. R. Hirst, D. K. Smith,
M. C. Feiters, H. P. M. Geurts, A. C. Wright, J. Am. Chem. Soc.
2003, 125, 9010.
[6] a) S. Lecommandoux, H.-A. Klok, M. Sayar, S. I. Stupp, J.
Polym. Sci. Polym. Chem. 2003, 41, 3501; b) E. R. Zubarev,
Angew. Chem. Int. Ed. 2005, 44, 328 –332
www.angewandte.org
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
331

Communications
M. U. Pralle, E. D. Sone, S. I. Stupp, J. Am. Chem. Soc. 2001, 123,
4105.
[7] Recent reviews: a) M. Lee, Y.-S. Yoo, J. Mater. Chem. 2002, 12,
2161; b) M. Lee, B.-K. Cho, W.-C. Zin, Chem. Rev. 2001, 101,
3869; c) H.-A. Klok, S. Lecommandoux, Adv. Mater. 2001, 13,
1217; d) S. I. Stupp, Curr. Opin. Colloid Interface Sci. 1998, 3, 20.
[8] a) M. Lee, B.-K. Cho, H. Kim, J.-Y. Yoon, W.-C. Zin, J. Am.
Chem. Soc. 1998, 120, 9168; b) M. Lee, D.-W. Lee, B.-K. Cho, J.-
Y. Yoon, W.-C. Zin, J. Am. Chem. Soc. 1998, 120, 13258; c) M.
Lee, B.-K. Cho, N.-K. Oh, W.-C. Zin, Macromolecules 2001, 34,
1987; d) M. Lee, J.-W. Kim, I.-W. Hwang, Y.-R. Kim, N.-K. Oh,
W.-C. Zin, Adv. Mater. 2001, 13, 1363; e) M. Lee, B.-K. Cho, Y.-
G. Jang, W.-C. Zin, J. Am. Chem. Soc. 2000, 122, 7449.
[9] a) M. Lee, B.-K. Cho, K. J. Ihn, W.-K. Lee, N.-K. Oh, W.-C. Zin,
J. Am. Chem. Soc. 2001, 123, 4647; b) B.-K. Cho, M. Lee, N.-K.
Oh, W.-C. Zin, J. Am. Chem. Soc. 2001, 123, 9677; c) J.-H. Ryu,
N.-K. Oh, W.-C. Zin, M. Lee, J. Am. Chem. Soc. 2004, 126, 3551.
[10] a) M. Lee, Y.-S. Yoo, M.-G. Choi, Bull. Korean Chem. Soc. 1997,
18, 1067; b) Y.-S. Yoo, J.-H. Choi, J.-H. Song, N.-K. Oh, W.-C.
Zin, S. Park, T. Chang, M. Lee, J. Am. Chem. Soc. 2004, 126,
6294.
[11] a) V. Percec, M. N. Holerca, S. Uchida, W.-D. Cho, G. Ungar, Y.
Lee, D. J. P. Yeardley, Chem. Eur. J. 2002, 8, 1106; b) Y. Li, S.-T.
Lin, W. A. Goddard III, J. Am. Chem. Soc. 2004, 126, 1872.
[12] It should be noted that this is in contrast to that of the 3D
hexagonal lattice based on discrete particles; see T. Kenichi,
Phys. Rev. Lett. 1995, 75, 1807.
[13] N.-K. Oh, W.-C. Zin, J.-H. Im, J.-H. Ryu, M. Lee, Chem.
Commun. 2004, 1092.
[14] a) D. J. Pesak, J. S. Moore, Angew. Chem. 1997, 109, 1709;
Angew. Chem. Int. Ed. Engl. 1997, 36, 1636; b) I. M. Saez, J. W.
Goodby, Chem. Commun. 2003, 1726; c) A. R. A. Palmans,
J. A. J. M. Vekemans, E. E. Havinga, E. W. Meijer, Angew.
Chem. 1997, 109, 2763; Angew. Chem. Int. Ed. Engl. 1997, 36,
2648; d) J. M. J. Frꢀchet, Proc. Natl. Acad. Sci. USA 2002, 99,
4782; e) C. Tschierske, J. Mater. Chem. 2001, 11, 2647.
332
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
Angew. Chem. Int. Ed. 2005, 44, 328 –332