Liquid-crystalline Kagome

Article abstract of DOI:10.1002/anie.200802957

No place like kagome: A quaternary amphiphile displays a novel complex liquid-crystalline phase formed by a periodic array combining triangular and hexagonal cylinders, filled with hydrocarbon and fluorinated chains, respectively. This leads to a fluid nanoscale multicompartment structure with kagome geometry. The thickness of the intercompartment walls is equal to the width of the π-conjugated rods (shown as black lines). (Figure Presented)

Full text of DOI:10.1002/anie.200802957

Angewandte
Chemie
DOI: 10.1002/anie.200802957
Liquid Crystals
Liquid-Crystalline Kagome**
Benjamin Glettner, Feng Liu, Xiangbing Zeng, Marko Prehm, Ute Baumeister, Martin Walker,
Martin A. Bates, Peter Boesecke, Goran Ungar,* andCarsten Tschierske*
The kagome (from Japanese; kago = basket, me = hole, eye:
meaning a basket with holes) represents a periodic 2D tiling
of regular hexagons and triangles in a 1:2 ratio (see
Figure 1c). This pattern, also known as trihexagonal tiling,
is of outstanding interest not only for its aesthetic appeal, but
especially as it is of relevance for spin-frustrated magnetic
materials.[1] However, the kagome pattern is rare on a
molecular scale, and is mainly restricted to Jarosites[2] and a
few metal–organic frameworks (MOFs), that is, to solid-state
materials.[3] Kagome nets can also be found as atomic planes
in a number of metal alloys[4] and recently 2D kagome nets
were obtained by the organization of DNA strands[5] or
organic molecules on solid surfaces.[6]
Ternary amphiphiles (molecules containing three incom-
patible types of segments) with a T shape have been shown to
spontaneously assemble into liquid-crystalline (LC) phases
representing periodic arrays of tessellated infinitely long
triangular, square, pentagonal, or hexagonal prisms.[7–9] In
these new types of LC phases, the prism walls are formed by
rodlike aromatic segments (Figure 1a). The prismatic cells
are filled with fluid chains attached laterally to the rodlike
cores. Herein, this concept is extended and used to design a
new polyphilic block molecule capable of self-assembly into
the LC kagome, a fluid superstructure representing a periodic
array combining triangular and hexagonal cells.
Figure 1. Three polygonal 2D nets with identical vertices and side-
lengths showing hexagonal p6mm symmetry formed by a) hexagons,
b) triangles, and c) the kagome and the relations between side length l
and hexagonal lattice parameter ahex. In the polygonal cylinder phases
these nets are extended to infinity in the third dimension. The
hexagonal beehive honeycomb organization of some previously
reported T-shaped ternary amphiphilic molecules are shown
schematically in (a).[7,8]
Compound 1 (Scheme 1), specifically designed for this
purpose, was synthesized by a sequence of preparative steps
with a Sonogashira cross-coupling reaction[10] as the key step
[*] F. Liu, Dr. X. B. Zeng, Prof. Dr. G. Ungar
Department of Engineering Materials, University of Sheffield
Robert Hadfield Building, Mappin Street, Sheffield S1 3JD (UK)
E-mail: g.ungar@sheffield.ac.uk
Dipl.-Chem. B. Glettner, Prof. Dr. C. Tschierske
Institute of Chemistry, Organic Chemistry
Martin-Luther-University Halle-Wittenberg
Kurt-Mothes-Strasse 2, 06120 Halle (Germany)
E-mail: carsten.tschierske@chemie.uni-halle.de
Homepage: http://www.chemie.uni-halle.de/bereiche_der_che-
mie/organische_chemie/33685_47302
Scheme 1. Molecular structure and phase-transition temperatures of 1;
values in square brackets indicate the corresponding transition
enthalpy values (DH/kJmolÀ1, determined by DSC, first heating scan,
10 KminÀ1, see Figure S1 in the Supporting Information); Cr=crystal-
line solid, M=unknown mesophase, Colhex/p6mm=hexagonal colum-
nar LC kagome phase with plane group p6mm, Iso=isotropic liquid.
Dr. M. Prehm, Dr. U. Baumeister
Institute of Chemistry, Physical Chemistry, University Halle
Mühlpforte 1, 06108 Halle (Germany)
Dr. M. Walker, Dr. M. A. Bates
Department of Chemistry, University of York, York YO10 5DD (UK)
Dr. P. Boesecke
European Synchrotron Radiation Facility, BP220
38043 Grenoble Cedex (France)
(see Scheme 2). Two chemically different chains, an aliphatic
hydrocarbon chain and a semiperfluorinated chain, were
attached laterally at opposite sides of a p-conjugated 1,4-
bis(phenylethynyl)benzene-based aromatic rod to which
glycerol groups, capable of forming cooperative hydrogen
bonds, were fixed at each end. Hence, this compound
represents a Janus-type,[11] X-shaped quaternary amphiphile.
Two distinct lateral chains were combined to enable the
formation of two different compartments by microsegrega-
tion; a different volume of the chains[12] was required to fill
[**] This work, as part of the European Science Foundation EURO-
CORES Programme SONS project SCALES, was supported by the
DFG, the EPSRC, the EC Sixth Framework Programme (contract No.
ERAS-CT-2003-989409), and by the Government of Sachsen-Anhalt
in the framework of the Cluster of Excellence “Nanostructured
Materials”. We thank the European Synchrotron Radiation Facility
for beamtime on beamline ID02.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200802957.
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Scheme 2. Synthesis of compound 1. Reagents and conditions:
ꢀ
a) H-C C-SiMe3, [Pd(PPh3)4], CuI, Et3N, reflux, 5 h; b) KOH, MeOH,
208C, 3 h; c) BnCl, K2CO3, butanone, reflux, 2 h; d) C10H21Br, K2CO3,
CH3CN, reflux, 10 h; e) cyclohexene, Pd(OH)2/C, EtOAc, reflux, 48 h;
f) C12F25(CH2)4Br , K2CO3, CH3CN, reflux, 16 h; g) [Pd(PPh3)4], CuI,
Et3N, reflux, 6 h; h) pyridinium 4-toluenesulfonate, MeOH/THF, reflux,
6 h. Bn=benzyl.
the very different spaces inside the hexagonal and triangular
cylinders in the envisaged kagome structure.
This compound (1) forms two LC phases in the temper-
ature range between 758C and 1168C with a phase transition
at 868C (see also Figures S1and S2 in the Supporting
Information). Our discussion is focused on the high-temper-
ature phase occurring between 86 and 1168C. The texture of
this phase, as seen under a microscope between crossed
polarizers, is characterized by dark, homeotropically aligned
regions and birefringent filaments (Figure 2a), which indi-
cates an optically uniaxial columnar LC phase. It is easy to
shear the sample; this destroys the homeotropic texture,
converting it into a nonspecific birefringent texture, which
indicates the fluidity of the material. The lack of atomic-scale
order is additionally confirmed by the diffuseness of the wide-
angle X-ray scattering (Figure S3 in the Supporting Informa-
tion). The small-angle Bragg reflections (Figure 2b) have the
ratio of corresponding reciprocal spacing of 1:31/2:2:71/2:3…
These can be indexed on a hexagonal 2D lattice (Colhex/p6mm
phase) with a lattice parameter ahex = 5.76 nm (see Table S1in
the Supporting Information). This value corresponds to
approximately twice the length of the rigid bolaamphiphilic[13]
moiety of 1 as measured between the ends of the terminal
glycerol groups (2.80 nm to 3.05 nm depending on the
conformation of the glycerol groups, see Figure S4a,b in the
Supporting Information). As shown in Figure 1c, the side
length of the polygons (l) is half the hexagonal lattice
parameter ahex for the kagome lattice. This result suggests
that in the observed hexagonal columnar LC phase the
aromatic moieties form a kagome-type net with the hydrogen-
bonding glycerol groups at the vertices.
Figure 2. Investigation of compound 1: a) Texture as seen between
crossed polarizers at T=1008C (dark areas represent homeotropically
aligned regions); b) small-angle region of the X-ray diffraction pattern
at T=1058C; c) electron-density map calculated from this diffraction
pattern; the organization of a few molecules is shown on the bottom
right; d) snapshot of a molecular dynamics simulation. Dissipative
particle dynamics simulation: e) simulation at a temperature corre-
sponding to the isotropic phase, in which some segregation occurs;
the inset shows the bead model for the molecule, where similar
chemical functional groups are combined into a single bead; f) on
cooling, the kagome is formed. For easy comparison, color coding in
(c)–(f) is: red: alkyl chains, purple: RF chains, green: aromatic cores
and glycerol units; in (c), (d), and (f) the view is down the column
axis; further details are described in the Supporting Information.
To confirm the proposed structure, we have used two
approaches. We performed high-resolution X-ray diffraction
experiments using synchrotron radiation[14] and from the data
thus obtained reconstructed electron density maps.[9a] Fur-
thermore, we carried out computer simulations of relevant
models. The electron density map (Figure 2c; for the recon-
struction procedure see the Supporting Information) indi-
cates the 3.6.3.6 trihexagonal tiling of hexagons and triangles,
which corresponds to the kagome structure, in which triangles
and hexagons have very distinct electron densities. Accord-
ingly, the red triangles with the lowest density should be filled
with the nonfluorinated alkyl chains, whereas the semiper-
fluorinated chains, which provide a much higher electron
density, are organized in the blue/purple high-electron-
density hexagons. This distribution of the lateral chains is
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also in agreement with their different sizes; the shorter alkyl
chains fill the small triangles whereas the much larger
semiperfluorinated chains fill the hexagons.[15] Within the
hexagons, there is an electron density gradient that changes
from highest in the center (purple), where exclusively
fluorinated segments are located, to lower electron density
near the periphery (blue), where perfluorinated end groups
are mixed with the aliphatic spacers (see also the surface plot
in Figure S5a in the Supporting Information). The aromatic
rodlike moieties are located in the medium-electron-density
areas (green), which represent the thin walls separating the
hexagonal fluorous from the triangular aliphatic cells.[16]
Calculation of the number of molecules per unit cell,
assuming a height of h = 0.45 nm (see Table S2 in the
Supporting Information), gives a value of approximately
eight molecules. This corresponds to an average thickness of
the walls separating the distinct compartments of approx-
imately 1.4 aromatic cores. Molecular dynamics simulations
(Figure 2d) performed with six molecules per unit cell
confirm that phase separation is achieved by the proposed
structure and that it is stable under the given boundary
conditions, but suggests some overcrowding in the triangular
and some deficit in the hexagonal cylinders. It is possible that
the stacking of aromatic rods is staggered (see Figure S5b in
the Supporting Information), so that the average elevation
along the cylinder axis (the “unit cell height”) is reduced,
giving the required six molecules per cell. Such staggering
could be related to the need for some molecules to contribute
both side chains to the hexagonal cylinders to redress their
material deficit (see Figure S4 in the Supporting Informa-
tion).
Figure 2c). Only the aliphatic spacers (blue) are mixed with
parts of the fluorinated segments in the shells around the
fluorinated column cores. This complexity is typical of
biological systems, wherein a single cell incorporates many
different units to perform distinct biological functions, but, to
the best of our knowledge, has never been achieved in any
artificial soft-matter structure.[19] Related polyphilic mole-
cules should lead to numerous other LC phases with even
more complex tiling patterns, establishing a new level of
complexity in LC design. Likewise, the DPD simulation
model developed here should prove to be an efficient tool for
the prediction of new morphologies.
Received: June 20, 2008
Published online: October 16, 2008
Keywords: kagome structures · liquid crystals · nanostructures ·
.
self-assembly · soft matter
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