The effect of a thiophene ring in the outer position on mesomorphic properties of the bent-shaped liquid crystals

Article abstract of DOI:10.1039/c0jm00375a

Novel bent-shaped compounds with the naphthalene central core possessing an outer thiophene unit in the lengthening arm were synthesized. Mesomorphic behaviour of new materials was studied using optical polarising microscopy, DSC, and electrooptic methods. The structures of mesophases were identified by X-ray diffraction and the structural parameters established. In the case of the thiophene unit in one molecular arm (the phenyl in the opposite arm) we succeeded in obtaining mesogenic properties for all types of lateral substitutions on the central naphthalene core. All non-substituted compounds and materials with the central core substituted by CH₃ or Cl exhibited the B₂ phase. The B₇ phase has been observed for all compounds with CN-substituted central core. Materials with the thiophene unit in both lengthening arms were not liquid crystalline except for the CN-substituted compound exhibiting the B₇ phase. Mesomorphic properties of compounds with the thiophene are compared with those detected on analogous materials with the phenyl ring.

Full text of DOI:10.1039/c0jm00375a

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
The effect of a thiophene ring in the outer position on mesomorphic properties
of the bent-shaped liquid crystals†
a
a
a
a
Vaclav Kozmık, Petr Polasek, Arnost Seidler, Michal Kohout, Jirı Svoboda, Vladimıra Novotna,
a
b
*
ꢀ
ꢀ
Milada Glogarova and Damian Pociecha
ꢀ
ꢀꢁ
ꢁ
ˇꢀ
ꢀ
c
ꢀb
Received 10th February 2010, Accepted 7th June 2010
DOI: 10.1039/c0jm00375a
Novel bent-shaped compounds with the naphthalene central core possessing an outer thiophene unit in
the lengthening arm were synthesized. Mesomorphic behaviour of new materials was studied using
optical polarising microscopy, DSC, and electrooptic methods. The structures of mesophases were
identified by X-ray diffraction and the structural parameters established. In the case of the thiophene
unit in one molecular arm (the phenyl in the opposite arm) we succeeded in obtaining mesogenic
properties for all types of lateral substitutions on the central naphthalene core. All non-substituted
compounds and materials with the central core substituted by CH3 or Cl exhibited the B2 phase. The B7
phase has been observed for all compounds with CN-substituted central core. Materials with the
thiophene unit in both lengthening arms were not liquid crystalline except for the CN-substituted
compound exhibiting the B7 phase. Mesomorphic properties of compounds with the thiophene are
compared with those detected on analogous materials with the phenyl ring.
was joined to the thiophene, uniformly the C12H25 alkyl group.
In order to distinguish the influence of both the thiophene unit
and the character of the terminal chain (alkyl vs. alkoxy) on the
mesomorphic properties of the studied materials, both symmet-
rical dialkyl- and non-symmetrical alkyl-alkoxy-terminated
materials were also studied.
Introduction
Bent-shape liquid crystals have attracted a broad interest as they
exhibit polar smectic phases.1–4 Among others, only few mate-
rials possessing a heterocycle in the central core have been
investigated. While the bonding in the six-membered heterocy-
cles5–9 does not disturb the typical bend angle around 120ꢀ,
materials based on 2,5-disubstituted 1,3,4-oxadiazole,10–14 1,2,4-
oxadiazole,15 1,3-oxazole,16 1,3,4-thiadiazole12 obviously repre-
sent the borderline between the calamitic and bent-shaped
materials due to their increased bend angle. Recently in
a conference contribution bent-shaped liquid crystals with
a thiophene ring in the lengthening arms were shown,17 however
the five ring materials with a resorcinol and naphthalene-2,7-diol
central core lost their mesogenic properties.
Experimental
Synthesis
Synthesis of materials I–V was performed by a stepwise acylation
of the X-substituted naphthalene-2,7-diol by the methodology
described in the preceding papers.18,19 Their molecular structure
is shown in Scheme 1. Details of the synthesis and NMR spectra
of all synthesized compounds are described in ESI.†
Herein we report for the first time a complete study of the bent-
shaped liquid crystals with a thiophene unit in the outer position
based on laterally 1-substited naphthalene-2,7-diol. Unlike the
commonly exploited materials the intended introduction of a 2,5-
disubstitued thiophene unit leads to a further bend (148ꢀ) into the
molecular structure, which can substantially affect the supra-
molecular assembly, and thus, the mesomorphic properties of the
studied materials. In addition, for synthetic reasons instead of
the routinely used alkoxy terminal chain a simple alkyl group
Characterization
All materials were studied using DSC (Perkin–Elmer Pyris Dia-
mond). The samples, 2–4 mg, were hermetically closed in
aluminium pans and placed into a nitrogen atmosphere. Cooling
and heating rates of 5 K minꢁ1 were applied. The texture
observations were carried out on the planar samples (3 or 6 mm)
using the polarizing microscope Nikon Eclipse. We prepared the
cells from the glass plates provided with transparent ITO elec-
trodes with the area of 5 ꢂ 5 mm2. The glasses were glued
together and mylar sheets were used as (a) spacers. The cells were
filled in the isotropic phase. The temperature was stabilized with
accuracy ꢃ0.1 ꢀC in the hot stage (Linkam) placed on the table of
polarizing microscope. For some measurements the commercial
cells provided with aligning layers were used. The free standing
films were prepared by spreading melted material over a circular
hole in a metal plate.
aDepartment of Organic Chemistry, Institute of Chemical Technology,
CZ-166 28 Prague 6, Czech Republic. E-mail: Jiri.Svoboda@vscht.cz;
Fax: +420220444182; Tel: +420220444288
bInstitute of Physics, Academy of Science of the Czech Republic, Na
Slovance 2, CZ-182 21 Prague 9, Czech Republic. E-mail: novotna@fzu.
cz; Fax: +420286890527; Tel: +420266053111
cLaboratory of Dielectrics and Magnetics, Chemistry Department, Warsaw
University, Al. Zwirki i Wigury 101, 02-089 Warsaw, Poland. E-mail:
pociu@chem.uw.edu.pl; Fax: +48228221075; Tel: +48228221075
† Electronic supplementary information (ESI) available: Reaction
schemes, experimental procedures and characterization of new
compounds, DSC thermographs and X-ray diffraction patterns. See
DOI: 10.1039/c0jm00375a
For dielectric measurements, we prepared the cells from glass
plates evaporated by a non-transparent gold electrode. Due to
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Table 1 Temperature of melting point, m.p., was indicated on the
second heating. TM1-iso is the phase transition temperature from the
isotropic to the mesophase and Tcr, is the temperature of crystallization,
both temperatures were detected on the second cooling. TM1-M2 is the
temperature of the phase transition from mesophase M1 to M2 estab-
lished from X-ray measurements, round brackets symbolize that the
phase is monotropic. The temperatures are in ꢀC and the corresponding
enthalpies, DH, in square brackets behind the corresponding temperature
value, are in kJ molꢁ1. All thermographs were taken at a rate of 5 K minꢁ1
Tcr
[DH]
TM1-iso
M.p. [DH]
M2 TM1-M2 M1 [DH]
I/H
I/CH3
I/Cl
I/CN
II/H
156 [+ 42.3] 155 [ꢁ9.3]
135 [+ 47.4] 114 [ꢁ0.5]
113 [+ 29.1] 105 [ꢁ9.2]
110 [+ 30.0] 91 [ꢁ22.0]
152 [+ 34.3] 145 [ꢁ3.2]
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
B7
136 [ꢁ25.1]
(B2) 151 [ꢁ19.6]
(B2) 133 [ꢁ20.7]
(B2) 118 [ꢁ17.4]
II/CH3 136 [+ 42.4] 115 [ꢁ7.8]
II/Cl
II/CN
III/H
125 [+ 31.0] 115 [ꢁ6.4]
96 [+ 10.1] 62 [ꢁ12.0]
133 [+ 12.8] 117 [ꢁ11.9]
B7
B2
B2
B2
B7
B2
B2
B2
B7
B2
B2
B2
B7
160 [ꢁ25.4]
154 [ꢁ20.5]
136 [ꢁ22.5]
123 [ꢁ19.2]
151 [ꢁ23.4]
168 [ꢁ20.1]
149 [ꢁ16.8]
137 [ꢁ14.8]
170 [ꢁ21.4]
169 [ꢁ20.4]
150 [ꢁ20.3]
138 [ꢁ17.7]
165 [ꢁ20.7]
III/CH3 132 [+ 19.9] 112 [ꢁ9.3]
III/Cl
III/CN 75 [+ 10.3] 73 [ꢁ8.3]
IV/H
163 [+ 25.0] 142 [ꢁ27.7]
IV/CH3 136 [+ 24.1] 117 [ꢁ19.8]
IV/Cl
129 [+ 22.4] 98 [ꢁ15.2]
IV/CN 94 [+ 14.8] 81 [ꢁ8.2]
V/H
140 [+ 27.1] 125 [ꢁ26.5]
95 [+ 17.1] 86 [ꢁ16.0]
0
(B7 ) 115
—
—
—
V/CH3 138 [+ 20.9] 117 [ꢁ17.1]
V/Cl
V/CN
121 [+ 24.1] 91 [ꢁ4.4]
68 [+ 16.7] 58 [ꢁ7.9]
0
(B7 ) 82
Scheme 1 Structure of materials I–V.
their high conductivity the parasitic high frequency mode (which
causes cut-off frequency for a measurement) that is present at
about 1 MHz if the ITO electrodes are used, was shifted to the
frequency of one order higher. With the cells provided with the
gold electrodes weak high frequency modes could be studied.
Dielectric properties were studied using Schlumberger 1260
impedance analyzer. The frequency dispersions were measured
on cooling at a rate of about 0.2 K minꢁ1, keeping the temper-
ature stabilized during the frequency sweeps in the range of
10 Hz–1 MHz.
X stands for a type of substitution (X ¼ H, CH3, Cl and CN). II/
X or III/X are compounds with a thiophene unit in one length-
ening arm and alkyl or alkoxy chain in the terminal chain of the
other arm, respectively. To study the effect of thiophene in
comparison with the phenyl ring we synthesized series IV/X and
V/X with Ar ¼ phenyl and alkyl or alkoxy chain in the opposite
arm, respectively.
The typical DSC plots are presented for compounds I/CN, II/
CH3 and V/H in Fig. 1a, 1b and 1c, respectively. Other DSC
thermographs are in the ESI.† The mesomorphic behavior of all
the new materials has been studied using optical polarizing
microscopy, DSC and electro-optic measurements. The results of
the DSC analysis and the phase assignment are summarized in
Table 1. In several cases there is an incomplete crystallization
and/or additional phase transition in the solid state and
a discrepancy when comparing melting and crystallization
enthalpy value. It can be seen that the introduction of the thio-
phene ring in both lengthening arms (materials I/X) leads to
a destabilization of the mesomorphic behavior for some types of
the lateral substitutions. Namely materials I/H, I/CH3 and I/Cl
exhibited the crystalline phase only and the presence of the lateral
cyano substituent I/CN resulted in the occurrence of the B7 phase
in a broad temperature range. The materials of series II, III, IV
and V possess the B2 phase for non-substituted compounds and
with the substitution by CH3 or Cl. For CN substitution the B7
phase has been found. For compound IV/CN and V/CN an
additional columnar phase has been observed below the B7 phase
on cooling by X-ray measurements. This phase was designated as
Small angle X-ray diffraction studies were performed using
Bruker Nanostar setup (Cu Ka line, cross-coupled Goebel
mirrors, Vantec 2000 area detector) equipped with a heating
stage allowing a temperature control with a precision of 0.1 K.
The experiments were done in the transmission mode for the
samples in Lindemann capillaries. For broad angle XRD studies
a Bruker D8 GADDS was used (Cu Ka line, Goebel mirrors,
point beam collimator, HiStar area detector). The samples were
prepared as droplets on the heated plate.
Results
Mesomorphic properties
DSC studies have been performed for all compounds, the phase
transition temperatures and associated enthalpy changes are
collected in Table 1. According to the type of aromatic core in the
lengthening arm and alkyl or alkoxy chain, the compounds are
arranged into five groups. Compounds with the thiophene ring
and the alkyl chain in both lengthening arms are designated I/X,
0
the B7 phase and will be described in the next paragraph.
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Fig. 1 Typical DSC plots for (a) I/CN, (b) II/CH3 and (c) V/H. The
upper and lower curves show the second heating and cooling runs,
respectively. The slopes are adjusted for convenience, phases are indi-
cated.
Unfortunately, the phase transition between the B7 and B70 phase
in not recognizable by DSC and/or textural observation in the
polarizing microscope.
Fig. 3 The planar texture of the B2 phase after the electric field treatment
for IV/CH3 (a) without electric field and at electric field of about (b)
20 V mmꢁ1 and (c) 40 V mmꢁ1. The width of the figures corresponds to
about 250 mm.
The planar texture of the B2 phase appearing below the
isotropic phase exhibits different textural features. The most
frequently observed is a tiny texture. Besides, the dark low-
birefringent domains that appear can be distinguished by
decrossing the polarizers for an angle of about 6 degrees, the
contrast being interchanged in the clockwise and the anticlock-
wise rotation. These two types of textures can coexist (see Fig. 2
for II/Cl). Under an electric field of about 10 V mmꢁ1 applied for
several minutes both textures transform into a fan shaped texture
(Fig. 3). After switching off the electric field the fans are dark,
which indicates a low birefringence (see Fig. 3a), with the
extinction parallel to the crossed polarizers direction. It corre-
sponds to the SmCAPA state (chiral anticlinic antiferroelectric).
Under a d.c. electric field the birefringence significantly grows
(see Fig. 3b, c) and the extinction brushes rotate by an angle of
about 45 degrees clockwise or anticlockwise, respective to the
polarity. After switching off the field the dark texture with the
extinction parallel to the direction of the crossed polarizers is
restored (Fig. 3a). The virgin textures (Fig. 2) could be restored
by heating the isotropic phase and subsequent cooling.
The profile of switching current exhibits two peaks per half
cycle of applied voltage in the B2 phase for a low frequency a.c.
field, which prove the antiferroelectric character of this phase
(see Fig. 4 for methyl substituted III/CH3 and Cl-substituted III/
Cl, more profiles are in ESI†).
Fig. 2 The planar texture in the B2 phase of II/Cl (a) at crossed polar-
izers, with the analyzer rotated by an angle of about 6 degrees (b) anti-
clockwise and (c) clockwise from the crossed position. The width of the
figures corresponds to about 200 mm.
Fig. 4 The switching current profile under a triangular a.c. electric field
in the B2 phase for (a) III/CH3 at T ¼ 120 ꢀC and a frequency of 12 Hz
and (b) III/Cl at T ¼ 115 ꢀC and a frequency of 11 Hz.
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phase transition from the isotropic to the B7 phase on cooling
(see Fig. 6 for I/CN). The free standing films form a specific
texture (see Fig. 7 for IV/CN) similar to those observed in the B7
phase for other compounds.20
X-Ray studies
The structures of mesophases were identified using X-ray
diffraction. The X-ray patterns recorded for I/H, I/CH3 and I/Cl
confirmed a crystalline character and the absence of liquid
crystalline phases. On the contrary, for all CN substituted
compounds the X-ray pattern typical for a 2D lattice with an
oblique cell21 has been detected (Fig. 8). The strongest signal
indexed as (01) corresponds to the molecular length. It suggests
a high degree of lamellarization of the structure, which is char-
acteristic for the B7 phase.21 The cell parameters for all
compounds with the B7 phase measured by X-ray are summa-
rized in Table 2. For I/CN the c parameter as well as a parameter,
which reflects the block length within its cross-section, decreases
on cooling. The cell parameters of IV/CN differ only negligibly
from those established for V/CN because the only change in the
Fig. 5 The three-dimensional graph of the imaginary part of permit-
tivity versus the temperature and frequency for III/Cl.
In dielectric spectroscopy studies a single weak mode is estab-
lished in the B2 phase with the relaxation frequency of hundreds
kHz, which decreases with decreasing the temperature (see Fig. 5).
Using the gold electrodes the cut-off frequency was shifted to
a megahertz region and thus the high frequency mode could be
detected. The studied mode completely disappears in the isotropic
as well as in the crystalline phases (Fig. 5) so that the observed
mode can be ascribed as an antiferroelectric collective mode.
In the B7 phase existing for all cyano substituted compounds
typical spiral nuclei have been observed in the vicinity of the
Fig. 8 X-Ray intensity versus the scattering angle for I/CN at T ¼ 130 ꢀC
with corresponding Miller indices.
Fig. 6 The microphotograph of a planar texture shows the phase tran-
sition from the isotropic to the B7 phase for I/CN. The width of the photo
corresponds to about 150 mm.
Table 2 Parameters of the crystallographic unit cell measured by X-ray
in the B7 phase for selected compounds at selected temperatures, T. For
IV/CN and V/CN last two temperatures are in the B7 phase
0
T/ꢀC
b (ꢀ)r.
ꢂ
a/A
ꢂ
c/A
Comp.
I/CN
130
120
110
140
120
100
160
140
120
100
80
160
140
120
100
80
114.3
108.1
102.0
153.7
159.3
163.8
195.2
192.2
190.9
123.7
119.6
192.4
192.6
194.5
198.9
122
39.46
38.50
37.30
40.5
40.3
40.2
39.4
39.5
39.5
42.6
42.6
38.3
38.4
38.2
37.7
43.6
43.9
115.5
112.8
108.0
114.3
113.6
114.6
93.5
93.9
93.7
118.3
120
93.2
92.9
91.9
92.7
125.2
126.9
III/CN
IV/CN
V/CN
Fig. 7 The free-standing films of the B7 phase for IV/CN. The width of
the photo corresponds to about 100 mm.
70
119
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molecule between IV/CN and V/CN is one oxygen atom (dodecyl
and dodecyloxy chains, respectively). On the contrary, the
structural parameters significantly differ (when) comparing I/CN
with the thiophene unit in both lengthening arms and IV/CN
with the phenyl rings in the corresponding position, both having
the same type of dodecyl chains. For I/CN the length of the block
tilt angle, Q, can be roughly estimated to be approximately
45 degrees. This value fits well to the angle of rotation of the
extinction brushes from the crossed position when we observe the
fan-shaped textures under the polarizing microscope (see above).
The temperature dependence of the layer spacing reveals only
small increase on cooling, which can be connected with
a stretching of dodecyl or dodecyloxy chains (see Fig. 10 for III/
Cl). The identification of phases obtained from the X-ray study is
in agreement with the results acquired from the texture studies.
ꢂ
(a parameter) is about 110 A, which corresponds to a half of the
value established for IV/CN or V/CN (a changes from 190 to 198
ꢂ
A, see Table 2). Compound III/CN with one thiophene on one
arm and the phenyl on the opposite arm exhibits a block length
ꢂ
of a ꢄ 155 A on average between values established for I/CN and
IV/CN. For compound IV/CN and V/CN another columnar
0
Discussion and conclusions
phase designated as the B7 phas0e has been observed on further
cooling. The X-ray pattern of B7 can also be fitted with oblique
Influence of thiophene unit on mesomorphic properties of
banana compounds with the central naphthalene core
substituted with CH3, Cl or CN is studied. Properties of
synthesized bent-shaped molecules are compared with those of
similar compounds with phenyl instead of thiophene.
0
2D unit cell, however, in B7 phase a parameter is substantially
smaller and c parameter only slightly higher in comparison with
the B7 phase (Fig. 9).
For all compounds exhibiting the B2 phase the X-ray patterns
exhibit sharp, commensurate small angle reflections and a diffuse
scattering maximum in a wide angle region. It shows a lamellar
structure with the liquid-like molecular packing within the layers
and confirms the presence of the lamellar smectic B2 phase. From
the small angle reflections the layer spacing, d, was evaluated.
Temperature dependence of d is presented in Fig. 10 for III/H,
III/CH3, III/Cl, V/CH3. For II/CH3 d was found practically
Introduction of the thiophene unit in both arms of the studied
compounds (series I/X) destroys mesomorphic properties for
non-substituted as well as CH3 and Cl substituted cores (X ¼ H,
CH3, Cl). It is probably due to the presence of an additional bend
of molecule caused by thiophene units, which disturbs a close
packing of molecules. Only CN substitution enforces the meso-
genic behavior and the B7 phase is stabilized in a temperature
range 25 K above the melting point. It is the first bent-shaped
mesogen with a thiophene in both arms. One can speculate why
only bulky CN groups stabilize the mesomorphic properties for
compounds with the thiophene unit in both arms.
ꢂ
temperature independent with value of about 38.1 A. The layer
spacing is substantially smaller than the calculated molecular
ꢂ
length, l, which is about 57 A for example for II/CH3. Thus the
In all compounds with the thiophene ring in one arm,
regardless of substitution, the mesomorphic properties are
preserved. For series II/X the monotropic B2 phase occurs for
X ¼ H, CH3, Cl and a B7 phase is stabilized in a wide temperature
interval for CN substitution. The situation is changed when the
terminal alkyl chain is exchanged for alkoxy type (series III/X).
Then the melting point lowers and thus the B2 phases for X ¼ H,
CH3, Cl become enantiotropic.
When the thiophene is replaced by phenyl in both molecular
arms (series IV/X and V/X differing in the alkyl/alkoxy chain) the
phase transition temperatures become higher by 10 to 29 K, only
the crystallization temperatures are not affected. Besides, the
temperature range of the B7 phase, occurring in the CN
compounds, is in fact composed of two columnar B7-like struc-
tures not-distinguishable in DSC study and in the texture
observation. The first order phase transition between them is
established from temperature dependence of lattice parameters
(Fig. 9). The role of alkyl/alkoxy groups in the terminal chain can
be clarified for series IV/X and V/X. One can see the mesomor-
phic behavior of the related materials is uniform and presence of
the alkoxy chains causes only a slight lowering of the melting
point, changes in the clearing point and in the width of meso-
phases being nearly negligible. On the contrary, for series II/X
and III/X with the thiophene ring in one arm the role of the type
of the terminal chain (alkyl and/or alkoxy) is significant.
The presence of thiophene also influences structural parame-
ters of the B7 phase, namely the length of blocks a (periodicity of
smectic layer undulation). Without the thiophene (IV/CN)
parameter a is nearly as twice as long than that of I/CN with
thiophene in both molecular arms (see Table 2). Smectic layer
Fig. 9 Fitted parameters of the columnar lattice are shown for (a) IV/
CN and (b) V/CN.
Fig. 10 Temperature dependence of the layer spacing, d, in B2 phase for
indicated compounds.
7434 | J. Mater. Chem., 2010, 20, 7430–7435
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€
7 M. W. Schroder, S. Diele, G. Pelzl, U. Dunemann, H. Kresse and
W. Weissflog, J. Mater. Chem., 2003, 13, 1877–1882.
undulation for compound III/CN with one thiophene is averaged
between a values for I/C and IV/CN.
€
8 M. W. Schroder, G. Pelzl, U. Dunemann and W. Weissflog, Liq.
Cryst., 2004, 31, 633–637.
To sum up, for the first time we have successfully applied
thiophene ring for design of new bent-shaped mesogens. Thio-
phene bent-shaped mesogens are a promising class of compounds
for resonant X-ray scattering studies.22,23 Due to the presence of
the thiophene unit one can expect a better resolution of this
experimental technique, which can determine a layer organiza-
tion in the B2 and B7 phases in details.
€
9 U. Dunemann, M. W. Schroder, G. Pelzl, S. Diele and W. Weissflog,
Liq. Cryst., 2005, 32, 151–161.
10 T. J. Dingemans and E. T. Samulski, Liq. Cryst., 2000, 27, 131–
136.
11 L. A. Madsen, T. J. Dingemans, M. Nakata and E. T. Samulski, Phys.
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H. Takezoe and J. Watanabe, J. Phys. Chem. B, 2006, 110, 5205–5214.
€
13 V. Gortz, C. Southern, N. W. Roberts, H. F. Gleeson and
J. W. Goodby, Soft Matter, 2009, 5, 463–471.
14 W. Weissflog, U. Dunemann, S. Findeisen-Tandel, M. G. Tamba,
H. Kresse, G. Pelzl, S. Diele, U. Baumeister, A. Eremin, S. Stern
and R. Stannarius, Soft Matter, 2009, 5, 1840–1847.
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17 K. Geesse, M. Prehm, C. Tschierske, 12th International Conference
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Acknowledgements
This project was financially supported by the Ministry of
Education, Youth and Sports of the Czech Republic (projects
OC175 and OC176), Czech Science Foundation (project No. 202/
09/0047), and by the Grant Agency of AS CR (project No.
IAA100100911). The X-ray diffraction measurements were per-
formed at the Structural Research Laboratory, Chemistry
Department, University of Warsaw, Poland, established with the
financial support from European Regional Development Fund,
project No: WKP 1/1.4.3./1/2004/72/72/165/2005/U.
ꢀ
ˇ
ꢀ
ꢀ
18 V. Kozmık, M. Kuchar, J. Svoboda, V. Novotna, M. Glogarova,
U. Baumeister and S. Diele, Liq. Cryst., 2005, 32, 1151–1160.
ˇ
19 V. Novotna, J. Zurek, V. Kozmık, J. Svoboda and M. Glogarova,
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ꢀ
ꢀ
€
20 G. Pelzl, M. W. Schroder, U. Dunemann, S. Diele, W. Weissflog,
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