Liquid crystals with complex superstructures

Article abstract of DOI:10.1002/anie.200460762

The hexagonal-channeled layer phase (ChL_hex; see picture, right) is one of the new liquid-crystalline phases formed by the self-organization of the rigid rodlike alkali-metal carboxylates shown.

Full text of DOI:10.1002/anie.200460762

Angewandte
Chemie
are hard to modify, fluid self-organized structures can easily
respond to external stimuli and change as a result of changed
external conditions. This fundamental principle is used by all
biological systems.[2] Liquid-crystalline (LC) systems combine
order and mobility and therefore can be regarded as simple
model systems in which the driving forces of self assembly can
be studied in a systematic and controlled manner. However,
the known self-organized structures of LC materials, though
of great technological importance,[3] are relatively simple, and
are restricted mostly to nematic layered (smectic = Sm) and
columnar morphologies (Col).[4] More recently the formation
of bicontinuous networks and spherelike aggregates was
found to lead to new cubic and noncubic mesophase
structures.[5–7] Based on the concept of competitive polyphi-
licity, elaborated in our laboratories,[8,9] it now seems possible
to further increase the complexity of these self-organized
fluid systems. This concept was successfully applied first to
rodlike bolaamphiphiles with nonpolar lateral chains (see
Figure 1), which form honeycomb-like arrays of cylinders and
Liquid Crystals
Liquid Crystals with Complex Superstructures**
Bin Chen, Xiang Bing Zeng, Ute Baumeister,
Siegmar Diele, Goran Ungar, and Carsten Tschierske*
Figure 1. Schematic structures of ternary bolaamphiphiles and facial
amphiphiles.
The investigation of molecular self-organization is one of the
most exciting areas of contemporary chemical research.
Significant progress was recently achieved in the field of
crystal engineering, especially with coordination polymers.[1]
In contrast to these crystalline systems, which once formed
a series of novel layer structures.[8] However, facial amphi-
philes, in which the molecular topology is reversed, that is, a
polar group is attached to a lateral position on a rigid
aromatic core and lipophilic chains are tethered at the termini
(see Figure 1), have so far failed to give such complex
structures.[10]
[*] B. Chen, Prof. Dr. C. Tschierske
Institute of Organic Chemistry
Martin-Luther-University Halle-Wittenberg
Kurt-Mothes-Str. 2, 06120 Halle (Germany)
Fax: (+49)345-5525-346
Herein we report successful generation of novel soft
matter morphologies with facial polyphilic LC molecules of a
new type, composed of three incompatible segments. The new
LC phases comprise three distinct subspaces, each containing
one of the three molecular parts. Two of the incompatible
segments, aromatic and aliphatic, form alternating layers. The
third segment, which incorporates an ionic group, forms
closed spheroidic aggregates, which are distributed either
randomly within the aromatic sublayers (filled random-mesh
phase) or on an hexagonal net (rhombohedral 3D phase,
Rho). These polar ionic groups can also coalesce into infinite
columns, which penetrate the layer stacks leading to a
completely new type of liquid-crystal organization, charac-
terized by an orthogonal set of layers and columns in one
structure, designated as channeled-layer phases (ChL). The
first representative of such mesophases, the hexagonal
channeled-layer phase (ChLhex) is unambiguously proven by
E-mail: tschierske@chemie.uni-halle.de
Dr. U. Baumeister, Dr. S. Diele
Institute of Physical Chemistry
Martin-Luther-University Halle-Wittenberg
Dr. X. B. Zeng, Prof. Dr. G. Ungar
Department of Engineering Materials and
Centre for Molecular Materials
University of Sheffield
Robert Hadfield Building, Mappin Street, Sheffield S1 3JD (UK)
[**] This work was supported by the Deutsche Forschungsgemeinschaft,
the European Union within the framework of the RTN network
LCDD under contract: HPRN-CT-2000-00016 and the Fonds der
Chemischen Industrie. We thank A. Gleeson for helping to set up
the experiment at Daresbury Synchrotron and CCLRC for granting
the beamtime.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200460762
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Table 1: Transition temperatures, corresponding enthalpy values, and lattice parameters of the LC
phases of the compounds n-M.[a]
electron-density calculation based
on high-resolution X-ray diffrac-
tion. It is also shown that the
transitions between these three
new types of LC organization can
be tuned by molecular design and
triggered by external stimuli.
The compounds denoted 16-
Na, 16-Cs and 10-M (M = Li, Na,
K, Cs) were synthesized according
to Scheme 1 and the mesophase
properties are summarized in
Table 1. The sodium salt 16-Na
forms two mesophases. Between
608C and the isotropization tem-
perature at 1078C the X-ray dif-
fraction pattern is characterized by
two orders of a meridional-layer
reflection, which corresponds to a
periodicity of d1 = 4.0 nm (see Fig-
ure 2a). The diffuse wide-angle
scattering is circular and has two
clear maxima centered on the
equator (see Figure 2b). This fea-
ture indicates a smectic A (SmA)
liquid-crystal phase, with the mol-
ecules on average perpendicular to
the layer plane, but without long-
Complex
n
M
T/8C[b][DH/kJmolꢁ1 [b]
]
Lattice parameter/nmꢁ1[c]
16-Na
16 Na Cr1 37 [17.4] Cr2 52[d] [42.7] Rho 60 [0.1] SmAfrm 108 d1 =4.0 (908C), a=3.4,
[2.1] Iso
c=12.8 (508C)
16-Cs
10-Li
10-Na
10-K
16 Cs Cr1 37 [35.4] Cr2 90[d] [14.9] ChLhex 114 [2.0] Iso
10 Li G 7[e] ChLhex 99 [1.8] Iso
10 Na G 17[e] ChLhex 105 [1.7] Iso
a=3.9, c=4.1 (1038C)
a=3.8, c=3.6 (758C)
a=3.9, c=3.7 (708C)
a=3.8, c=3.6 (1158C)
10
K
G 4[e] ChLhex 123 [2.1] Iso
10-Cs
10 Cs Cr1 51[d] [20.3]Cr2 71[d] [3.3] (G-8) M1 120[f] [–] ChLhex a=4.08, c=3.72 (808C)
127 [2.3] Iso
[a] The analytical data and tables with X-ray data are provided in the Supporting Information;
abbreviations: Cr=crystalline solid state; G=glassy state; Iso=isotropic liquid state; Rho=meso-
phase with a rhombohedral 3D lattice (R3m), see Figure 3b; SmAfrm =filled random-mesh phase, see
Figure 3a; ChLhex =channeled-layer phase (P6/mmm), see Figure 3c; M1=low-temperature meso-
phase. [b] Obtained by differential scanning calorimetry (DSC-7, Perkin Elmer, 10 Kminꢁ1) during the
second heating scan and confirmed by polarizing microscopy. [c] Obtained from the Guinier pattern,
except the values of 10-Cs, which were taken from the synchrotron small-angle diffraction experiments.
[d] Observed only in the first heating scan. [e] Even after storage at 258C for about six months no
crystallization could be observed. [f] Transition can only be detected in the X-ray diffraction pattern.
¯
range positional order within the layer. As expected, d1 is
shorter than the length of the extended molecule (L =
5.6 nm), hence the alkyl chains from neighboring layers
show some interdigitation. However, in contrast to the
conventional SmA phase, even for a highly oriented sample,
the diffuse wide angle scattering forms a closed ring (see
Figure 2b), and the pattern contains an additional diffuse
small angle equatorial maximum, which corresponds to an
average distance of d2 = 3.2 nm (see Figure 2a). This diffuse
maximum indicates an additional short range periodicity in
the layer plane. An explanation of this unique feature can be
sought in the T-shaped triblock structure of the molecule. As
in other SmA phases the aromatic cores and the alkyl chains
are segregated into distinct sublayers. However, the polar
groups, fixed to the centers of the terphenyl cores and yet
strongly incompatible with them, are forced to cluster mainly
within the terphenyl sublayers (see Figure 3a). These clusters
are distributed randomly within the aromatic sublayers, with
an average distance of d2 ꢀ 3.2 nm. The polar groups are quite
flexible and likely to be orientationally disordered within the
aggregates, hence the circular shape of the diffuse wide-angle
scattering.
The layered LC phase can thus be described as a random-
mesh phase, in which the holes are filled by the polar lateral
groups, that is, by the third incompatible component (filled
random-mesh phase, SmAfrm). In this respect the mesophase
is new and different from conventional smectic phases and
from the mesh phases known in lyotropic systems,[12] block
copolymers,[13] and rod–coil molecules,[14] in which the holes
or dimples within a sublayer are filled not by a third
component but by excess material from the two adjacent
sublayers.
Scheme 1. Synthesis of compounds n-M. a) toluenesulfonyl chloride,
pyridine; b) 2,5-dichlorophenol, K2CO3; c) Pd(OAc)2, KF, THF;[11]
The rhombohedral phase—a correlated filled hexagonal
mesh phase—is described next. Upon cooling the SmAfrm
d) NaOH, H2O; e) HCl, H2O , EtO ; f) MO H or C2sCO3 (M is Na–Cs);
g) Pd(OH)2, cyclohexene, MeOH; h) C16H33Br, K2CO3, Bu4NI.
2
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Figure 3. Organization of the facial amphiphiles n-M depending on the
molecular structure and conditions. For clarity, the alkyl chains are not
shown in the models and the centers of the unit cells of the 3D lattices
are offset vertically from the inversion center by half a sublayer thick-
ness.
cally aligned) regions do not change.[15] Only a significant
increase in viscosity is observed. This observation confirms
that the mesophase is optically uniaxial, which is consistent
with the proposed rhombohedral lattice. To understand the
structure of the Rho phase it is reasonable to assume that at
the SmAfrm!Rho transition, the polar domains pack on a
hexagonal 2D lattice within the aromatic sublayers (filled-
hexagonal-mesh layers). The occurrence of this order within
the layers is accompanied by long-range positional correlation
between layers in an ABC fashion, thus leading to the 3D
Figure 2. X-ray diffraction pattern of aligned samples of the meso-
phases. a) Filled random mesh phase (SmAfrm) of 16-Na at 1028C
(small-angle region). b) Same as (a), but wide-angle region. c) Rhom-
¯
bohedral (R3m) phase of 16-Na at 508C (small-angle region). d) Wide-
¯
rhombohedral symmetry R3m, as shown in Figure 3b. In
angle region of the ChLhex (P6/mmm) phase of 10-K at 858C. e) Same
as (d), but small-angle region. In all diffraction patterns the c-axis is
vertical, perpendicular to the substrate and the X-ray beam. The sam-
ples are disordered around an ¥-fold axis perpendicular to the sub-
strate (fiber geometry).
contrast to the small-angle scattering, the overall appearance
of the wide-angle region of the X-ray diffraction pattern does
not change at the phase transition, that is, the diffuse ring with
distinct equatorial maxima remains. This is in line with the
proposed model and indicates that the phase is indeed liquid
crystalline with a 3D mesoscale lattice and that it belongs to
phase in 16-Na below 608C, the X-ray diffraction pattern
changes significantly. The diffuse small-angle scattering on
the equator splits and condenses into a series of row-line
Bragg reflections (see Figure 2c). This diffraction pattern can
correlated layer structures.[16] The structure is related to that
[12–14]
¯
of the R3m mesh phase in some other organized fluids,
but again unlike in those systems, the holes in the layers are
filled with a distinct third component.
¯
be indexed on a rhombohedral 3D lattice, space group R3m,
with the parameters a = 3.4 nm and c = 12.8 nm. No signifi-
cant change can be observed at this phase transition under the
polarizing microscope. The fanlike texture of the SmAfrm
phase remains and also the optically isotropic (homeotropi-
The third phase, described below, has a 3D hexagonal
structure consisting of layers penetrated by continuous polar
channels (channeled-layer phase, ChLhex). It is found in
compound 16-Cs, in which the Na+ ion of 16-Na is replaced by
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pattern of compound 10-K, obtained with an aligned sample
(see Figure 2d and e) shows diffuse wide angle scattering with
distinct equatorial maxima. This is very similar to the
appearance of the Rho phase, but the positions of the Bragg
reflections in the small-angle region are completely different.
The reflections can be indexed on a 3D hexagonal lattice,
space group P6/mmm, with the parameters a = 3.8 nm and c =
3.6 nm. Similar diffraction patterns were observed for the
mesophases of all other 10-M compounds as well as for 16-Cs;
the lattice parameters are collated in Table 1.
More detailed X-ray investigations were carried out for
compound 10-Cs by using a synchrotron source. Small-angle
diffraction intensities were measured from high-resolution
powder patterns. The corrected intensities and a description
of the electron-density-reconstruction procedure are given in
the Supporting Information. The electron-density map of the
ChLhex phase of compound 10-Cs is shown in Figure 4c. The
blue surfaces enclose the polar domains (high electron
density) and the pink surface encloses the low-density, that
is, aliphatic volume. According to the map, the terphenyls are
positioned perpendicular to the layer planes (as also indicated
by the diffuse wide angle maxima on the equator), thus
forming a perforated layer between the perforated alkyl
layers. The polar domains form cylinders with undulated
profile, arranged on a hexagonal 2D lattice and penetrating
the layer structure. At lower temperature additional meso-
phases were found for the Cs-salt 10-Cs, which require
additional investigations.
From the comparison of compounds 10-Na and 16-Na the
effect of the length of the terminal alkyl chains is evident. For
long chains the aliphatic sublayers are too thick to allow
fusion of the polar regions. For such compounds the ABC
packing of the layers (Rho phase) allows optimal space filling.
In the short-chain compounds the aliphatic sublayers are
thinner and fusion of the polar regions is possible. The
resulting reduction in interfacial area stabilizes the AAA
stacking (ChLhex structure), which allows the formation of
continuous polar channels (see Figure 3c). The same change
from Rho to ChLhex is observed when the Na+ ions of 16-Na
are replaced by Cs+ ions in 16-Cs. This change could be
interpreted in terms of the larger Cs+ ions increasing the
overall volume of polar domains and allowing them to fuse,
even across the thick hexadecyl sublayer of 16-Cs.
The mesophase type can also be influenced by solvents.
For example, the addition of a 5.0m sodium chloride solution
to 16-Na induces a ChLhex phase as indicated by the very
typical texture of this mesophase (see Figure 4d). Similar to
the effect of large cations, the added aqueous solution is
thought to induce fusion of the polar domains by increasing
their volume through the coordination of water molecules
and/or additional Na+ ions.
Figure 4. The channeled layer mesophase (ChLhex). a) Growth of an
optically isotropic hexagon of the ChLhex phase from the isotropic
liquid state at 1278C. b) Texture of the ChLhex phase of compound 10-
Cs as observed between crossed polarizers at 1258C (the optically iso-
tropic regions are homeotropically aligned regions). c) Reconstructed
electron-density map of the ChLhex mesophase of compound 10-Cs; the
blue isoelectron surfaces enclose the polar channellike domains (high
electron density) and the pink surface encloses the low density, that is,
aliphatic volume. Texture of the ChLhex phase induced in the contact
region between 16-Na and 5.0m aqueous NaCl solution at 958C. The
dark area at the right is the NaCl solution, the dark areas at the left
represent homeotropically aligned regions of the ChLhex phase (layers
parallel to the substrate).
the larger Cs+, and also in the series of salts 10-M (M = Li, Na,
K, Cs), in which the length of the two terminal alkyl chains is
reduced from hexadecyl to decyl. The common feature of all
five compounds is the appearance of a mosaic-like texture
with large optically isotropic regions (see Figure 4b) at the
transition from the isotropic liquid to the mesophase. The
optically isotropic regions grow as rounded hexagons (see
Figure 4a). This growth pattern, the optical uniaxiality and
the rather high viscosity of the mesophase, are preliminary
indications of a 3D hexagonal structure, and such a structure
was indeed confirmed by X-ray diffraction. The diffraction
The above results show that competition of multiple-level
microsegregation and anisotropic interaction of rodlike
molecular segments presents a successful strategy for the
design of new and exciting mesophase morphologies. The self-
organized structures obtained with the molecules presented
here are all quite distinct from conventional thermotropic and
lyotropic liquid-crystalline phases of other compounds with
low molecular weights and from the mesophases of some LC
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dendrimers[6,17] and main chain polymers.[18] The mesophases
we report here represent combinations of spheroids or
columns with layers. Hence, a higher-than-usual level of
structural complexity is achieved through the presence of
three incompatible molecular moieties, instead of two as in
more conventional amphiphiles. Though there have been
reports of several complex polymer morphologies in triblock
copolymers[19] the present morphologies are different with
regards to their structure and size. For example, “spheres-on-
layer” morphologies were reported for linear ABC block
copolymers, whereas the Rho phase represents a “spheres-in-
layer” morphology. Similarly, in the ChLhex phase the
cylinders are arranged perpendicular to the layer planes,
whereas in the “cylinder-on-layer” polymer morphology the
columns run parallel to the layers. Additionally, these ordered
structures occur at a significantly smaller length scale (3–
10 nm) than those of the block copolymers (10 to > 100 nm).
In the ChLhex phase reported here, the polar regions
represent well defined ion-carrying channels, which can be
modified by molecular design and influenced by external
stimuli. Hence, these or similar materials might be useful, for
example, as components in ion-conducting nanodevices. One
can also envisage their use as ion-triggered gate valves,
opened (ChLhex phase) by a solution carrying large ions and
closed (Rho phase) when small or no ions are present. The
study of the current structures may also shed new light on
ionic channels in biological systems and on transport across
lipid membranes.[20] Being mechanically more robust than
columnar liquid crystals and readily surface-aligned,[21] the 3D
mesophases may also be used for encapsulating low-molec-
ular and polymeric guests, such as drugs, and electrolumines-
cent[22] or electronically conducting polymers[23] and biopol-
ymers.[24]
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[16] M. Prehm, S. Diele, M. K. Das, C. Tschierske, J.Am.Chem.Soc.
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way. The homeotropic alignment is found over large areas
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bubbles a birefringent texture (defects) can be found.
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Published Online: August 19, 2004
Keywords: alkali metals · liquid crystals · mesophases ·
.
self-assembly · supramolecular chemistry
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