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ScienceWeek
2004 4 June A5
5. MATERIALS SCIENCE: ON SUPRAMOLECULAR DENDRITIC LIQUID
CRYSTALS
The following points are made by X. Zeng et al (Nature 2004
428:157):
1) A large number of synthetic and natural compounds
self-organize into bulk phases exhibiting periodicities on the
10^(-8) to 10^(-6) meter scale(1) as a consequence of their
molecular shape, degree of amphiphilic character, and often the
presence of additional non-covalent interactions. Such phases
are found in lyotropic systems(2) (for example, lipid-water,
soap-water), in a range of block copolymers(3) and in
thermotropic (solvent-free) liquid crystals(4). The resulting
periodicity can be one-dimensional (lamellar phases),
two-dimensional (columnar phases) or three dimensional ("micellar"
or "bicontinuous" phases).
2) All such two- and three-dimensional structures identified to
date obey the rules of crystallography and their symmetry can be
described, respectively, by one of the 17 plane groups or 230
space groups. The "micellar" phases have
crystallographic counterparts in transition-metal alloys, where
just one metal atom is equivalent to a 10^(3) to 10^(4)-atom
micelle. However, some metal alloys are known to defy the rules
of crystallography and form so-called quasicrystals, which have
rotational symmetry other than the allowed two-, three-, four-
or six-fold symmetry(5).
3) Research on bulk nanoscale self-assembly of organic matter is
partly motivated by the fact that such complex structures may
serve as scaffolds for photonic materials and other nanoarrays,
or as precursors for mesoporous ceramics or elements for
molecular electronics. Larger biological objects, such as
cylinder-like or sphere-like viruses, also pack on similar
macrolattices.
4) Dendrons and dendrimers (tree-like molecules) are proving
particularly versatile in generating periodic nanostructures.
Two micellar lattices, with space groups Imm (body-centered
cubic, b.c.c.), and Pmn, have been established. An analogue of
the Imm phase has also been observed in block copolymers, and
that of the Pmn phase in lyotropic liquid crystals. Recently, a
complex three-dimensional (3D) tetragonal lattice (space group
P42/mnm) was found, having 30 self-assembled micelles in the
unit cell.
5) In summary: The authors demonstrate that quasiperiodic
structures can also exist in scaled-up micellar phases,
representing a new mode of organization in soft matter.
References (abridged):
1. Supramolecular chemistry and self-assembly . Science 295,
2395-2421 (2002)
2. Seddon, J. M. Lyotropic phase behavior of biological
amphiphiles. Ber. Bunsenges. Phys. Chem. 100, 380-393 (1996)
3. Thomas, E. L., Anderson, D. M., Henkee, C. S. & Hoffman,
D. Periodic area-minimizing surfaces in block copolymers. Nature
334, 598-601 (1988)
4. Tschierske, C. Micro-segregation, molecular shape and
molecular topology partners for the design of liquid crystalline
materials with complex mesophase morphologies. J. Mater. Chem.
11, 2647-2671 (2001)
5. Janot, C. Quasicrystals: A Primer (Oxford Univ. Press,
Oxford, 1992)
Nature http://www.nature.com/nature
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GIANT SUPRAMOLECULAR LIQUID CRYSTAL LATTICE
The following points are made by G. Ungar et al (Science 2003
299:1208):
1) Molecular self-assembly into a variety of bulk phases with
two-dimensional (2D) and 3D nanoscale periodicity, such as
cubic, cylindrical, or mesh phases, has been researched
intensely in lyotropic (e.g., surfactant-water) liquid crystals
(LCs) (1), block copolymers (2-4), and thermotropic
(solvent-free) LCs (5). Lyotropics can provide templates for
porous inorganic materials with well-defined structures, and LC
complexes of DNA with cationic and neutral lipids are potential
carriers for gene delivery. Complex organic nanostructures may
serve as scaffolds for photonic materials and other nanoarrays
and for surface nanopatterning.
2) In the case of bicontinuous cubic phases (1,2,5), the same
structures have been observed in lyotropics, thermotropics, and
block copolymers. The recent inclusion of self-assembling
dendrons into the category of nanostructured soft matter has
shown their tapered shape to be responsible for creating similar
bulk phases and enabling "self-processing" of
electronics components. It has also become apparent that
manipulating the size and distribution of "micelles"
aggregated from self-assembling dendrons can be used in
controlling polymerization. Regarding creation of structural
diversity, dendron shape can be fine-tuned in ways unavailable
in lyotropic LCs or block copolymers, and hence have the
potential of forming hitherto unobserved phases.
3) In summary: Self-organized supramolecular organic
nanostructures have potential applications that include
molecular electronics, photonics, and precursors for nanoporous
catalysts. Accordingly, understanding how self-assembly is
controlled by molecular architecture will enable the design of
increasingly complex structures. The authors report a liquid
crystal (LC) phase with a tetragonal three-dimensional unit cell
containing 30 globular supramolecular dendrimers, each of which
is self-assembled from 12 dendron (tree-like) molecules. The
present structure is one of the most complex LC phases yet
discovered. A model explaining how spatial arrangement of
self-assembled dendritic aggregates depends on molecular
architecture and temperature is proposed.
References (abridged):
1. J. M. Seddon, Ber. Bunsenges. Phys. Chem. 100, 380 (1996)
2. E. L. Thomas, D. M. Anderson, C. S. Henkee, D. Hoffman,
Nature 334, 598 (1988)
3. I. W. Hamley, The Physics of Block Copolymers (Oxford Univ.
Press, Oxford, 1998)
4. M. A. Hillmyer, et al., Science 271, 976 (1996)
5. A. M. Levelut and M. Clerc, Liq. Cryst. 24, 105 (1998)
Science http://www.sciencemag.org
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SELF-ASSEMBLY OF PHASE-SEGREGATED LIQUID CRYSTAL STRUCTURES
The following points are made by Takashi Kato (Science 2002
295:2414):
1) Molecules in the liquid crystalline (LC) state, although
still mobile, have an orientational ordering that depends on
external conditions, such as temperature or electric fields
(1,2). The best known example would be the thermotropic LC
materials, which exhibit different phases as a function of
temperature, that are used in displays (3). In contrast to these
high-tech materials, biological self-assembled systems consist
of a variety of discrete molecules forming heterogeneous and
hierarchical structures (structures within structures) on
various scales of length ranging from the nanometer to the
micrometer scale. For example, lipids exhibit lyotropic LC
states in which phase formation depends on solution
concentration (4). Biological cell membranes are formed through
self-assembly of lipids with other biomolecules such as proteins
and steroids.
2) Although the intrinsic ordering within a LC material can be
controlled by external parameters, the ordering can also be
controlled internally through weak interactions to create more
complex structures that add functionality to the material. One
approach is to obtain self-assembled structures from a variety
of different molecules (2,5). Another is to form
phase-segregated structures of "block" molecules of
longer polymers and smaller molecules on the nanometer and
micrometer scale (2). The control of intermolecular interactions
(noncovalent interactions) within these structures is critical
because several competing effects are at work.
3) In the formation of the self-assembled LC structures,
specific intermolecular interactions such as hydrogen bonding
and ionic interactions play key roles. A notable example is the
formation of supramolecular liquid crystals (5). In the case of
these materials, well-defined structures such as rods and disks
are made through self-assembly of two or more complementary
molecular components by hydrogen bonding. For example, benzoic
acids and pyridines form supramolecular rod-like complexes
exhibiting thermally stable nematic and smectic (layered) LC
phases (5). Moreover, identical molecules self-assemble into
unconventional LC complexes. Studies of these materials have
initiated and stimulated the development of supramolecular
polymers.
4) In summary: Additional functionality can be incorporated into
liquid crystalline materials by using phase segregation and
self-assembly. Intermolecular interactions such as hydrogen
bonding and ionic interactions play key roles in the formation
of these complex structures. One-, two-, and three-dimensional
phase-segregated structures on various scales of length are
formed by self-assembly of a variety of partially incompatible
molecules. Such structures can enhance anisotropic properties
such as ionic conductivity.
References (abridged):
1. D. Demus, J. W. Goodby, G. W. Gray, H.-W. Spiess, V. Vill,
Eds., Handbook of Liquid Crystals (Wiley-VCH, Weinheim, Germany,
1998)
2. J. W. Goodby, Curr. Opin. Solid State Mater. Sci. 4, 361
(1999)
3. J. H. Haaren and D. Broer, Chem. Ind. 1998, 1017 (1998)
4. C. E. Fairhurst, S. Fuller, J. Gray, M. C. Holmes, G. J. T.
Tiddy, in (1), vol. 3, chap. 7.
5. T. Kato, Struct. Bonding (Berlin) 96, 95 (2000)
Science http://www.sciencemag.org
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