Bent-shaped mesogens without an azomethine joint

Article abstract of DOI:10.1039/b205036f

A new series of banana-shaped mesogens with ethylene links in the branches and laterally substituted are presented. They form B₁, B₆ B₆ and B₇-like phases. The B₇-like and B₂ phases are not

Full text of DOI:10.1039/b205036f

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Bent-shaped mesogens without an azomethine joint
Jozef Mieczkowski,a Jadwiga Szydlowska,*a Joanna Matraszek,a Damian Pociecha,a
Ewa Gorecka,a,b Bertrand Donniob and Daniel Guillonb
aDepartment of Chemistry, Warsaw University, Al. Zwirki i Wigury 101, 02-089 Warsaw,
Poland. E-mail: jadszyd@chem.uw.edu.pl; Fax: 148 22 8221075; Tel: 148 22 8221075
bInstitut de Physique et Chimie des Mate´riaux de Strasbourg, Groupe des Mate´riaux
Organiques, 23 rue du Loess, 67037 Strasbourg Cedex, France
Received 24th May 2002, Accepted 19th August 2002
First published as an Advance Article on the web 12th September 2002
A new series of banana-shaped mesogens with ethylene links in the branches and laterally substituted are
presented. They form B1, B2, B6 and B7-like phases. The B7-like and B2 phases are not miscible although
identical crystallographic structures are deduced from X-ray studies. The switching in the B7-like phase is
without a threshold and occurs only in a small number of domains. Dielectric measurements do not confirm
ferroelectric properties for the B7-like phase, and only a weak non-collective relaxation process was observed.
In the B2 phase a collective and weak dielectric mode was detected. The B1 phase exhibits a columnar structure
2
˚
with a large rectangular cell 111.6 6 51 A .
Introduction
Experimental
In recent years a large number of substances made of V-shaped
(banana) molecules and revealing liquid crystalline properties
have been synthesised and studied. They form a new class of
thermotropic mesogens that exhibit polar mesophases.1–4 Most
of the reported compounds are resorcinol derivatives, symme-
trically substituted at the 1 and 3 positions. Biphenyl,5 terphenyl,3
naphthalene6 and other symmetric and non-symmetric bent
structures have also been presented.7 The central ring has been
additionally substituted at the available positions by a chlorine
or bromine atom, a nitro, methyl or even hexyl group.8–11 The
branches of the banana molecule have usually contained two or
three phenyl rings joined directly12 or connected through ester,
imino or vinylic groups. The phenyl rings of the branches have
been substituted at different positions by fluorine6,13,14 and
chlorine15,16 atoms and methyl groups. Then alkoxy or alkyl
chains have been attached at the terminal poitions of the
molecules.
Most of the synthesised banana-shaped compounds contain
in their structures the imino (-NLCH-) linking group, which
is sensitive to air and moisture and causes decomposition in
a short time. In this work, we present five new homologous
series of banana-shaped compounds that do not contain this
azomethine group. They are symmetric five rings resorcinol
derivatives, with the branches being attached to the central
phenyl ring via carboxy (-C(O)-O-) groups (Scheme 1, series
1–5). The branches contain two phenyl rings connected by
ethenyl linking groups and terminated at the para position by
alkoxy chains of different lengths. The external p-alkoxyphenyl
moiety of the branches was broadened by a lateral meta chloro-,
bromo-, nitro, methoxy- or other substituent. The main
resorcinol ring was also modified by the attachment of a methyl
group at the 2- (series 2) or 5-position (series 3). Compounds of
the series 1 have no additional substituents at the central ring.
The compounds of series 1–3 reveal mainly B7 or B2 liquid
crystalline phases. In the compounds of series 4, the substituent
attached at the 5-position of the central ring is a pentyl chain.
The central rings of the molecules of series 5 are asymmetrically
substituted at the 4-position by alkyl or ester chains.
Mesophase identification was based on microscopic routine
examination of the textures formed by samples between two
glass plates or in free suspended films. A Zeiss Jenapol-U
polarising microscope equipped with a Mettler FP82HT hot
stage was used. The structure of the detected LC phases was
supported by X-ray studies carried out on powder samples in
Lindemann capillaries. The patterns were registered with an
Inel CPS 120 curved positional sensitive counter and confirmed
with a Gunier set-up. The sample temperature was controlled
to within 0.1 K.
The phase transition temperatures were mainly determined
by calorimetric measurements performed with a Perkin-Elmer
DSC-7 apparatus. When a LC phase was monotropic or a
transition temperature was too high, the phase transition point
was taken from microscopic observations.
The birefringence was estimated in cells of 2–6 mm by
comparison with tabulated interference colours of a quartz
wedge between crossed polarisers in white light. The intensity
of light transmitted through the sample was taken by a
microphotometry system Nikon P102 attached to the polaris-
ing microscope Optiphot2-Pol. The changes in sample textures
under electric field, spontaneous polarisation and dielectric
dispersion measurements were studied in glass cells 2 and 4 mm
thick. They were prepared from glass plates coated with ITO
transparent electrodes and carrying a rubbed planar surfactant,
separated by Mylar spacers. The dielectric dispersion studies
were performed with a Wayne Kerr Precision Component
Analyser 6425 in the frequency range 20 Hz–300 kHz. The
relaxation frequency fr and the strength De of the dielectric
modes were obtained by fitting the complex permittivity e*(v)
~ e’ 2 ie@ to the Cole–Cole dispersion law:
De
1z(if =fr)(1{a)
s
2pe0f
eÃ{e?~
{i
where a, e‘, s and e0 are the distribution parameter of the
mode, the high frequency permittivity, the conductivity of
the sample and the permittivity of the vacuum, respectively.
Spontaneous polarisation was measured using the switching
current method under triangular wave voltage. Molecular
dimensions were estimated by molecular modelling
(HYPERCHEM).
In this paper, we report on the structural characterization
and dielectric measurements of the mesomorphic phases exhibited
by these new series of banana-shaped molecules.
3392
J. Mater. Chem., 2002, 12, 3392–3399
This journal is # The Royal Society of Chemistry 2002
DOI: 10.1039/b205036f

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Scheme 1
To confirm the molecular structure of the synthesised
compounds the following analytical methods were applied.
Infrared (IR) spectra were obtained using a Nicolet Magna IR
500 spectrophotometer. 13C NMR spectra were recorded on
a Varian Unity Plus spectrometer operating at 125 MHz,
whereas 1H NMR spectra were recorded at 200 MHz or at
500 MHz when high signal separation was necessary. Tetra-
methylsilane was used as an internal standard. Chemical shifts
are reported in ppm. TLC analyses were performed on Merck
60 silica gel glass plates and visualised using iodine vapour.
Column chromatography was carried out at atmospheric
pressure using silica gel (100–200 mesh, Merck). Compositions
of the synthesised compounds, determined by elemental
analysis, confirmed the expected molecular structures.
4-{(E)-2-[3-Chloro-4-(dodecyloxy)phenyl]ethenyl}benzoic acid
methyl ester (a) (n ~12). Anhydrous potassium carbonate
(13.8 g, 0.1 mol) and 18-crown-6 (100 mg) were added to
phosphonium salt (methyl 4-(triphenylphosphoniomethyl)-
benzoate bromide) (34 g, 0.0692 mol) dissolved in dry
methylene dichloride (300 ml) and mixed at room temperature
for about one hour. 3-Chloro-4-(dodecyloxy)benzaldehyde
(21 g, 0.0647 mol) dissolved in tetrahydrofuran (200 ml) was
added to the yellow reaction mixture which was heated under
reflux for 30 hours. After the inorganic salts were filtered, the
filtrate was evaporated to dryness and methanol was added
to the remaining oil, from which the product (a) crystallized
slowly (mp 69–71 uC; 12.4 g, 42.2%). According to the spectral
data, only (E)-stilbene was crystallized from the reaction
mixture.
Elemental analysis for C28H37O3Cl (M ~ 456.5): calc. C
73.60, H 8.10; found C 73.49, H 8.25%; IR nmax (KBr)/cm21
Synthesis
:
The synthetic route to the described compounds is sketched in
Scheme 1.
2940, 2910, 2850, 1720, 1510, 1500, 1455, 1420, 1290, 1100, 980;
1H NMR dH(CDCl3): 8.004 (d, 2H, J ~ 8.3 Hz), 7.556 (d, 1H,
J ~ 1.8 Hz), 7.514 (d, 2H, J ~ 8.5 Hz), 7.319 (dd, 1H, J1 ~
2.0 Hz, J2 ~ 8.4 Hz), 7.017 (d, 2H, J ~ 9.8 Hz), 6.886 (d, 1H,
J ~ 8.8 Hz), 4.029 (t, 3H), 3.991 (s, 3H), 1.839 (m, 2H), 1.476
(m, 2H), 1.264 (m, 18H), 0.880 (t, 3H); 13C NMR dC(CDCl3):
167.020, 154.753, 141.897, 130.359, 130.199, 129.711, 128.939,
128.283, 126.666, 126.593, 126.302, 123.527, 113.358, 69.421,
52.224, 32.091, 29.818, 29.760, 29.724, 29.512, 29.228, 26.103,
22.862, 14.296.
3-Chloro-4-(dodecyloxy)benzaldehyde (n ~ 12). Anhydrous
potassium carbonate (3 0 g, 0.22 mol) and dodecyl bromide
(60 g, 0.24 mol) were added to 3-chloro-4-hydroxybenzaldehyde
(32 g, 0.2 mol) dissolved in dimethylformamide (200 ml). The
reaction mixture was heated at 120 uC for 12 hours, then
poured into water and the crude product was extracted with
toluene. The toluene layer was dried with anhydrous magne-
sium sulfate and after evaporation of the solvent the product
was distilled under reduced pressure (bp 270 uC/10 Torr) and
slowly solidified in a refrigerator (mp 55–57 uC; 45.5 g, 68.5%).
Elemental analysis for C12H29O2Cl (M ~ 324.5): calc. C
4-{(E)-2-[3-Chloro-4-(dodecyloxy)phenyl]ethenyl}benzoic acid
chloride (b) n ~ 12. Potassium hydroxide (2 g, 0.0357 mol)
was added to methyl ester (a) (12 g, 0.0262 mol) dissolved in
hot ethanol, and the reaction mixture was heated under reflux
for 24 hours. After cooling, the potassium salt (10 g) was
crystallized, then dried under vacuum over phosphorus
pentaoxide suspended in toluene, then treated with excess of
oxalyl chloride (10 ml). The reaction mixture was heated for
8 hours under reflux. After filtration of precipitated potassium
chloride, the solvent was evaporated to dryness. The product
(b) slowly solidified at room temperature (8 g).
70.26, H 8.93; found C 70.01, H 9.01%; IR nmax (KBr)/cm21
:
2950, 2940, 2918, 2845, 1695, 1600, 1580, 1500, 1490, 1290,
1200, 1050, 820; 1H NMR dH(CDCl3): 8.941 (s, 1H), 7.899
(d, 1H, J ~ 2.0 Hz), 7.44 (dd, 1H, J1 ~ 2.0 Hz, J2 ~ 8.3 Hz),
7.013 (d, 1H, J ~ 8.4 Hz), 4.115 (t, 3H), 1.879 (m, 2H), 1.492
(m, 2H), 1.266 (m, 18H), 0.880 (t, 3H); 13C NMR dC(CDCl3):
189.869, 159.670, 131.394, 130.578, 130.133, 124.088, 112.594,
69.684, 32.069, 29.789, 29.716, 29.665, 29.498, 29.418, 29.010,
26.023, 22.840, 14.274.
J. Mater. Chem., 2002, 12, 3392–3399
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3-[4-{(E)-2-[3-Chloro-4-(dodecyloxy)phenyl]ethenyl}benzoyloxy]-
phenyl 4-{(E)-2-[3-chloro-4-(dodecyloxy)phenyl]ethenyl}benzoate
(1), X ~ Cl, n ~ 12. Triethylamine (10 ml), DMAP (50 mg)
and the chloride (b) (4.5 g, 0.00976 mol) were added to
resorcinol (0.5 g, 0.00454 mol) dissolved in dry methylene
dichloride (50 mol). The reaction mixture was heated under
reflux for 8 hours, then the excess of the solvents was
evaporated and the remaining oil was chromatographed on
silica gel eluted with methylene dichloride. The crude product
was recrystallized twice from ethyl acetate (2.1 g, 48.2%).
Elemental analysis for C60H72O6Cl2 (M ~ 959): calc. C
119.199, 115.845, 113.146, 69.230, 31.927, 29.675, 29.653,
29.599, 29.562, 29.366, 29.348, 29.053, 25.939, 22.702, 14.143.
Compounds of series 2–5 were prepared similarly except
of the use of the central unit (2-methylresorcinol, 5-methyl-
resorcinol, 5-pentylresorcinol (olivetol), 4-hexylresorcinol,
4-octanoyloxyresorcinol).
Results and discussion
The phase transition temperatures and the corresponding
enthalpies are collected in . The phase diagrams of the series
1–3 are shown in Fig. 1.
75.07, H 7.50; found C 75.18, H 7.65%; IR nmax (KBr)/cm21
:
2950, 2920, 2850, 1720, 1600, 1500, 1490, 1290, 1240, 1110,
1050, 1010; 1H NMR (500 MHz) dH(CDCl3): 8.148 (d, 4H, J ~
8.5 Hz), 7.568 (m, 6H), 7.465 (t, 1H), 7.333 (dd, 2H, J1 ~
Series 1—B7-like phase
2.0 Hz, J2 ~ 8.0 Hz), 7.199 (t, 1H), 7.164 (dd, 2H, J1 ~ 2.0 Hz,
J2 ~ 8.0 Hz), 7.111 (m, 2H), 6.988 (m, 2H), 6.895 (d, 2H, J ~
8.0 Hz), 4.029 (t, 4H), 1.837 (m, 4H), 1.486 (m, 4H), 1.40–1.20
(m, 36H), 0.881 (t, 6H); 13C NMR dC(CDCl3): 164.252,
154.680, 151.479, 142.552, 130.676, 130.064, 130.062, 130.032,
129.806, 128.713, 127.747, 126.551, 126.318, 126.289, 123.353,
Compounds of series 1 exhibit the enantiotropic liquid
crystalline phase whose microscopic textures are similar to
those of the B7 phase described previously.8,9,15 Upon cooling
from the isotropic phase a characteristic tiny ‘spiralling’ germ
texture appears. When the temperature is lowered rather slowly
a variety of domains are created that resemble B7 phase
Table 1 Phase sequence, phase transition temperatures (uC) and enthalpy changes (in parentheses, J g21) for compounds of series 1
n
X
Cry
LC phases
Iso
12
8
9
10
11
12
15
12
12
H
$
$
$
$
$
$
$
$
$
182.9 (11.3)
124.8 (24.6)
121.7 (20.4)
118.0 (20.8)
115.5 (19.1)
116.4 (18.9)
107.1 (16.6)
118.3 (18.5)
141.8 (28.0)
Unknown
B7-like
B7-like
B7-like
B7-like
B7-like
B7-like
B7-like
B1 – 149.8
(1.32) – B6
—
y200 dec
$
$
$
$
$
$
$
$
$
Cl
Cl
Cl
Cl
Cl
Cl
Br
NO2
139.7 (18.9)
137.4 (15.8)
140.4 (17.9)
136.6 (18.8)
144.4 (18.2)
137.6 (16.2)
131.3 (15.6)
150.6 (15.0)
8
12
1
12
8b
OCH3
OCH3
OC12H25
Br, OCH3
H
$
$
$
$
$
85.4 (55.5)
126.8 (75.3)
105.6 (115.7)
92.8 (41.3)
—
—
—
—
—
$
$
$
$
$
—
—
—
—
a
132.3 (52.1)
aBoth meta positions are substituted. bThe terminal chain is chiral -OCH2CH(C2H5)C4H9.
Table 2 Phase sequence, phase transition temperatures (uC) and enthalpy changes (in parentheses, J g21) for compounds of series 2
n
X
Cry
B2
Iso
8
9
Cl
Cl
Cl
Cl
Cl
Cl
Br
$
$
$
$
$
$
$
$
$
$
120.4 (48.0)
108.3 (29.6)
107.2 (34.5)
111.6 (41.6)
129.6 (70.1)
101.7 (52.8)
127.0 (50.5)
167.0 (58.2)
92.1 (19.4)
160.1 (36.4)
$
$
$
$
$
$
$
—
—
—
117.6 (12.5)
119.8 (14.1)
126.0 (12.6)
121.3 (3.47)
123.4 (11.2)
122.7 (12.9)
103.1 (5.6)
$
$
$
$
$
$
$
$
$
$
10
11
12
15
12
12
12
12b
NO2
Br, OCH3
Cl
a
aboth ortho positions are substituted. binstead of CH3 group in position 2 of central ring NO2 group is substituted.
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J. Mater. Chem., 2002, 12, 3392–3399

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Table 3 Phase sequence, phase transition temperatures (uC) and
enthalpy changes (in parentheses, J g21) for compounds of series 3
n
X
Cry
B2
Iso
8
9
Cl
Cl
Cl
Cl
Cl
Cl
Br
NO2
127.7 (83.1)
125.7 (75.7)
126.9 (39.7)
123.5 (36.9)
122.0 (40.6)
117.9 (32.0)
134.5 (46.1)
180.6 (51.1)
80.4 (17.8)
$
$
$
$
$
$
$
103a
99a
$
$
$
$
$
$
$
$
$
10
11
12
15
12
12
12
$
$
$
$
$
$
$
109a
104a
109a
106a
95a
—
—
b
Br, OCH3
aFrom microscopy. bBoth ortho positions are substituted.
Table 4 Phase sequence, phase transition temperatures (uC) and
enthalpy changes (in parentheses, J g21) for compounds of series 4
Fig. 1 Phase diagram for the series (a) 1, X ~ Cl, (b) 2, X ~ Cl, (c) 3,
X ~ Cl; full squares are melting points and open circles correspond to
the isotropization.
n
X
Cry
Iso
10
15
Cl
Cl
$
$
37.4 (27.3)
61.3 (30.3)
$
$
domains (Fig. 2). Among them double-twisted helices, fans or
myelinic textures can be observed. However, since there are
contradictory reports on the features of the B7 phase and as
there is no clear standard as to what constitutes the B7 phase,
Table 5 Phase sequence, phase transition temperatures (uC) and
enthalpy changes (in parentheses, J g21) for compounds of series 5
n
W
X
Cry
N
Iso
8
C6H13
OCH3
OCH3
Br
$
$
$
$
$
$
111.2 (52.0)
76.6 (56.0)
47.7 (40.3)
46.5 (46.9)
62.5 (35.3)
66.4 (35.6)
—
—
—
—
$
—
$
$
$
$
$
$
12 C6H13
12 C6H13
1
12 COOC8H17 Cl
12 COOC8H17 Br
C6H13
OC12H25
44.1 (2.3)
Fig. 2 Growth of B7-like phase domains on cooling the isotropic
phase—series 1, X ~ Cl, (a) n ~ 11, (b) n ~ 12.
J. Mater. Chem., 2002, 12, 3392–3399
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shorter,12,13,18 or much longer13,15 than those giving rise to
the main signal. Also splitting of the main layer thickness signal
has been reported.8,19 For all our compounds the measured
layer thickness d is significantly smaller than the longest
molecular dimension L, defined by the molecular diagonal,
estimated by molecular modelling. This result suggests a mole-
cular plane tilt in the smectic layer. The ratio obtained for all
compounds is d/L y 0.8 which gives a tilt angle y36u.
In the microscopic studies (for cells 6 and 2 mm thick), a pale
texture containing ribbon structures as well as fan-like circular
domains was observed. In addition, some bright circular focal
conic domains were created. The birefringence of pale and
bright domains was estimated to be Dn v 0.04 and Dn y0.09,
respectively. These are very low values in comparison to the
birefringence of the related compounds of series 3 (Dn ~ 0.16)
showing the synclinic B2 phase. The extinction brushes in both
types of the circular domains are formed along the directions of
the polarisers.
Application of a low frequency a.c. electric field influences
the texture. For some small bright circular domains a brush
rotation appears9 (Fig. 4). A linear reaction to the electric field
is observed. The light transmission measured in the bright part
of a circular domain, at the edge of the brush, is linearly
dependent on the applied triangular wave voltage (Fig. 5).
Removing the field pushes the brushes immediately back to
their initial orientation. This behaviour could reflect the mole-
cular switching from an anticlinic to a synclinic arrangement.
However, the rotation of the brushes is not accompanied by
changes of birefringence. Increasing the electric field strength
increases the number of circular domains that switch. Once
the domains have been switched, they become sensitive even to
a low field. The amplitude of motion is different for different
domains but is proportional to the applied voltage. The largest
recorded tilt was y28u. Switching exhibits neither the thresh-
old nor saturation in a broad voltage range, which is similar to
the linear electroclinic effect in the smectic A phase of chiral
compounds. When the field is sufficiently high (50 Vpp mm21),
the switchable bright circular domains become, for a short
time, yellowish in a sample 2 mm thick or bluish in a sample
4 mm thick, thus they are highly birefringent (Dn ~ 0.16) with
brushes tilted at about 31u. Then the whole texture, together
with the pale parts, is rebuilt into a grainy contrasting texture
that is no longer sensitive to the field. No polarisation reversal
current peak is observed for any applied voltage, which might
suggest that the electric polarisation of the sample is linearly
dependent on the electric field or that in the sample there is a
relatively small amount of switchable domains.
Fig. 3 X-Ray diffraction from a powder sample in the B2 phase (series
2, n ~ 10, X ~ Cl), B7-like phase (series 1, n ~ 10, X ~ Cl), and B1
phase (series 1, n ~ 12, X ~ NO2). For the B1 phase, the signal (200)
is partially covered by the beam stopper.
we would like to call this phase ‘B7-like’. In a contact
preparation between compounds showing the B7-like phase
(series 1, n ~ 11) and the B2 phase (series 2, n ~ 10), a sharp
border between the two phases was observed, indicating a lack
of miscibility between them.
In the DSC thermogram, the enthalpy change signal at this
point reveals substantial broadening on the side of the B7-like
phase for all compounds. The B7-like phase exhibits a simple
layered structure. The X-ray diffraction pattern (Fig. 3) shows
the first and second order signals derived from the layer
˚
thickness and a diffuse reflection at 4.5 A related to the liquid-
like order within the layers. In the low angle region no
additional incommensurate signals were found. Such addi-
tional signals have been reported previously for the B7 phase
and, by some authors, are considered to be the proof of the
existence of the B7 phase.8,12,13,17,18 However, it should be
noted that the positions of these signals in relation to the signal
associated with the layer thickness are not always the same.
Additional signals have been found for distances much
The dielectric investigation showed only a single, weak, high
frequency mode at about 105–106 Hz. The relaxation frequency
decreases with decreasing temperature. This mode is observed
Fig. 5 Light transmission measured for the circular domain in the
B7-like phase at the edge of an extinction brush, serrated curve; solid
lin, applied electric field (series 1, n ~ 10, X ~ Cl, temp. 120 uC, sample
2 mm thick).
Fig. 4 Changes in the extinction brush direction in a small circular
domain in the B7-like phase under an electric field (series 1, n ~ 10,
X ~ Cl; sample 2 mm thick, 0 V and y 10 V mm21).
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Fig. 8 X-Ray diffraction of a powder sample of the B1 phase, recorded
with a Guinier camera (series 1, n ~ 12, X ~ NO2). Arrows indicate
˚
signals at 23.02, 24.9, 27.6, 38.1, 46.4 and 55.8 A.
Series 1—B1 and B6 phases
The two phases B1 and B6 are exhibited by the compounds of
series 1 containing the nitro group at the X position on the
outer ring of the branches. The temperature range of the B6
phase is very narrow, less than 1 uC. The texture formed at the
transition from the isotropic liquid forms focal conic defects of
low viscosity which is similar to the texture of the smectic A or
C phases (Fig. 6a). Attempts to obtain a dark homeotropic
texture were unsuccessful which proves that the phase is not
the smectic A phase. In the microscopic observations, texture
changes are visible at the transition temperature to the B1 phase
only in thick cells, while the fan-shaped texture remains almost
unchanged (Fig. 6b) in thin cells. The extinction brushes—the
dark area around air bubbles—are parallel to the polarising
directions. This reflects anticlinic or non-tilted molecular align-
ment in the smectic layers. The viscosity of the B1 phase is
Fig. 6 Texture of (a) the B6 phase, (b) the B1 phase (series 1, n ~ 12,
X ~ NO2).
rather high. Applying a triangular voltage (1 Hz, 150 Vpp mm21
)
perpendicularly to the sample substrates does not influence the
texture, e.g. doesn’t cause any brush movement or birefrin-
gence change.
In the dielectric measurements, a single mode with the
relaxation frequency in the MHz region was found in the liquid
crystalline as well as in the isotropic liquid phases. This seems
to be due to the non-collective rotation of the molecules or
some of their parts. The mode strength is De y 3 and becomes
slightly higher in the region of the B6 phase (Fig. 7). The
molecular movement, thus the mode, is quenched in the
crystalline phase.
The X-ray studies of a powdered sample of the B1 phase
reveal few low angle reflections22 (Fig. 8). These signals match
a centred rectangular crystallographic lattice (Table 6). Among
the registered signals, there is the (210) reflection, revealing a
broken glide symmetry of the structure, which results from
the non-equivalent scattering centres in the rectangular
lattice—the molecule in the middle of the cell is a mirror
image of the molecules in the cell corners. The dimension
Fig. 7 Temperature dependence of the real part of the dielectric
permitivity measured on cooling at 10 kHz for the Iso, B6 and B1 phases
of compounds of series 1, n ~ 12, X ~ NO2.
in the B7-like phase as well as in the isotropic phase, but
disappears in the crystalline phase, and is probably due to
a non-collective molecular rotation. The mode strength is
De y 1–2 and it slightly increases near the transition tempera-
ture between the B7-like and the isotropic liquid phase.
Concluding, we observe the B7-like phase whose textures are
identical to the B7 phase textures. However, the B7-like phase
has a layered crystal structure similar to that of the B2 phase.
Simultaneously, the B7-like phase shows much different elec-
trooptic properties than the antiferroelectric B2 phase. In the
B7-like phase, switching without a threshold occurs only in
some domains.9 Moreover, low dielectric permittivity excludes
the ferroelectric properties from the B7-like phase, although
they have been reported for the B7 phase.20,21
˚
b ~ 51 A of the crystallographic unit cell is related to the full
molecular length, which is estimated by molecular modelling
(L ~ 52.8 A). The dimension a ~ 111.6 A is rather large.22 In
the B1 phase the wide angle diffraction signal related to the
liquid-like arrangement is obviously weaker and broader
than in the B7-like or B2 phases, which points to a shorter
correlation length of positional molecular ordering (Fig. 3).
For a most probable density of 1 g cm23, considering the
˚
˚
2
˚
dimensions of the crystallographic lattice (111.6 6 51 A ) and
the average packing distance between molecules along the
˚
columnar axis of about 5 A, the number of molecules in the
unit cell can be estimated to be around 18, which gives 9
molecules in each section of the column. This indicates an
Table 6 X-Ray diffraction signals for the B1 phase of compound of series 1, n ~ 12, X ~ NO2, and the corresponding crystallographic distances
(020)
(120)
˚
Calculated cell dimensions/A
Signal indexes
Registered signals/A
(200)
(110)
(210)
(400)
(220)
˚
55.8
55.8
46.4
46.4
38.1
37.6
27.6
27.6
24.9
25.5
24.9
23.02
23.02
˚
Calculated signals/A
a ~ 111.6
b ~ 51.0
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˚
average distance of about 6.2 A between the centres of the
molecules along the ‘a’ axis.
The compound of series 1 (n ~ 12) having a hydrogen
atom at the X position in the branches reveals a monotropic
liquid crystalline phase, and thus is difficult to investigate.
Its microscopic texture is not similar to any of the known LC
phases. The fully developed phase is soft but very viscous.
an electric field. In the thinner sample (2 mm), only the synclinic
domains were observed. The application of a low d.c. electric
field to the thin sample (of the order of 10 V mm21) orients
extinction brushes along the polarising directions and lowers
the birefringence without any destruction of the texture details.
Thus it shows switching to an anticlinic–ferroelectric state
under an applied electric field. However, switching off the
field does not restore the initial synclinic–antiferroelectric state;
the brushes remain in their positions and the birefringence is
not changed. This suggests that either the ferroelectricity is
preserved or the molecules in every second layer change their
dipole moment direction, rotating around their long axis,
giving the anticlinic–antiferroelectric state. The latter seems
more plausible as this structure is preferred in thick samples,
and thus is probably thermodynamically stable. Applying an
a.c. electric field smaller than 20 Vpp mm21 to the thus created
anticlinic–antiferroelectric state does not influence it—the
texture does not change. A higher alternating electric field
makes some small domains switchable, between an anticlinic–
antiferroelectric state at zero field, and a synclinic–ferroelectric
state under an applied field.20,24
Series 2 and 3—B2 phase
In the compounds of series 2 and 3, the B2 phase23 was found to
be enantiotropic for series 2 and monotropic for series 3.
Similar to the B7-like phase, a broad (a few degrees) two phase
region is observed in microscopic and DSC studies at the
transition to the isotropic phase. The X-ray studies (Fig. 2)
confirm the simple layered structure of the B2 phase. The
measured interlayer distance is shorter than the molecular
length (d/L y 0.84), in agreement with the assumed tilted
structure of the phase. In the microscopic studies of a freshly
prepared sample (4 mm thick) mainly orange circular domains
with low birefringence (Dn ~ 0.08) were visible. The extinction
brushes are directed along the polarising directions, showing
an anticlinic molecular organisation (Fig. 9a). Application of
an electric field moves the extinction brushes and raises the
birefringence, due to the rebuilding of the phase structure from
anticlinic–antiferroelectric to synclinic–ferroelectric. The brush
tilt saturates at an angle of 35.5u for a field of 4.9 V mm21
(Fig. 9b) when the domains reveal a blue–green colour that
corresponds to a birefringence of Dn ~ 0.17. Increasing
the applied field to 7 V mm21 still raises the birefringence to
Dn ~ 0.23. This suggests a strong Kerr effect i.e. enhancement
of the orientational order parameter S induced by the electric
field. Raising the electric field to almost 12 V mm21 does not
change the birefringence. Decreasing the electric field reverses
the switching process.
The spontaneous polarization of the compounds of series 2
was found to be 540–590 nC cm22, which is typical for banana
materials. In the dielectric measurements, a single relaxation
process was observed (Fig. 10). This mode is quenched in the
In the fresh sample, there are also some small blue colour
circular domains with extinction brushes inclined from the pola-
rising directions, showing the presence of a synclinic molecular
organisation. Their estimated birefringence is Dn ~ 0.16, which
is similar to that of the domains with saturated brush tilt under
Fig. 10 (a) Temperature–frequency plot of the imaginary parts of
dielectric permitivity and (b) the mode strength De and the fitted mode
frequency in logarithm scale as a function of inverse temperature for
the B2 phase measured on heating in planar, 2 mm thick cell (series 2,
n ~ 10, X ~ Cl).
Fig. 9 (a) Circular domains in the B2 phase under different electric fields,
(b) field dependence of brush tilt and birefringence. (series 3, n ~ 12, X ~
Cl, sample 4 mm thick). At 0 V the structure is anticlinic–antiferroelectric.
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J. Mater. Chem., 2002, 12, 3392–3399

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isotropic phase. It originates plausibly in a collective movement
of molecules—the antiphase rotation of the molecules from
subsequent layers.24 In a sample 2 mm thick, the relaxation
frequency varies with temperature, from y50 kHz near the
isotropisation temperature, to y100 Hz close to crystallisation,
within a 50 K range. The mode strength increases with
decreasing temperature, and reaches De y 15. The relaxation
frequency and the dielectric strength are of the same order of
magnitude as for the B2 phase in other compounds.24 How-
ever the observed dependence of the relaxation frequency on
temperature is much stronger than found previously,22 the
The thermodynamically stable arrangement seems to be an
anticlinic–antiferroelectric structure, however, in a thin cell, the
synclinic–ferroelectric domains are also formed. The X-ray
studies of the B2 and B7-like phases revealed identical spectra,
but nevertheless the two phases are not miscible and have quite
different properties; in particular for the B7-like phase the
electrooptic response is weak and no collective mode in the
dielectric studies has been found.
Acknowledgements
corresponding activation energy being 110.3 kJ mol21
.
The studies were supported by grant BW – 1562/13/02.
E. Go´recka wishes to thank the CNRS and the University
Louis Pasteur of Strasbourg for financial support.
Series 4 and 5
Lengthening the alkyl chains substituted at the central part
of the molecule (at position 5, series 4) up to 5 carbon atoms
changes the banana molecular shape and destroys the liquid
crystalline properties. Also the banana compounds of series 5,
when modified by addition of a lateral substituent at the central
resorcinol ring at position 4, are not mesogenic. Only for one
of the compounds (series 5 with COOC8H16 substituent in
the central ring and Cl atoms in the branches) is the uniaxial
nematic phase is formed.
References
1
2
3
T. Sekine, T. Niori, M. Sone, J. Watanabe, S. Choi, Y. Takanishi
and H. Takezoe, Jpn. J. Appl. Phys., 1997, 36, 6455.
J. Watanabe, T. Niori, T. Sekine and H. Takezoe, Jpn. J. Appl.
Phys., 1998, 37, L139.
D. Shen, S. Diele, G. Pelzl, I. Wirth and C. Tschierske, J. Mater.
Chem., 1999, 9, 661.
4
5
G. Pelzl, S. Diele and W. Weissflog, Adv. Mater., 1999, 11, 707.
D. Shen, A. Pegenau, S. Diele, I. Wirth and C. Tschierske, J. Am.
Chem. Soc, 2000, 122, 1593.
Conclusions
6
7
8
R. A. Reddy and B. K. Sadashiva, Liq. Cryst., 2000, 27, 1613.
V. Prasad, D. S. Rao and K. Prasad, Liq. Cryst., 2001, 28, 643.
G. Pelzl, S. Diele, A. Ja´kli, Ch. Lischka, I. Wirth and W. Weissflog,
Liq. Cryst., 1999, 26, 135.
A. Ja´kli, Ch. Lischka, W. Weissflog, G. Pelzl and A. Saupe, Liq.
Cryst., 2000, 27, 1405.
A new series of bent shaped compounds containing ethylene
links in the branches are presented. Similar to the frequently
used imino links, they are able to transmit p electrons through
the molecular branches, forming conjugated p bond systems.
Moreover, due to their high symmetry, the charge density
repartition between two phenyl rings should be more homo-
genous than was the case for imino links.25,26
9
10 H. Dehne, M. Po¨tter, S. Sokolowski, W. Weissflog, S. Diele,
G. Pelzl, I. Wirth, H. Kresse, H. Schmalfuss and S. Grande,
Liq. Cryst., 2001, 28, 1269.
The various lateral-substituents in the molecular branches
induce lower melting and isotropization temperatures in compa-
rison to the non-substituted compounds (series 1, n ~ 12 X ~ H).
The chlorine and bromine atoms strongly promote meso-
morphic phases of ‘‘banana’’ type. These atoms are known to
be suppliers of p electrons to the aroyl rings and give relatively
low dipole moments. The strong van der Waals interactions
accompanied by weak dipole–dipole interactions between
molecules usually promote mesophases. When a nitro moiety,
with a high dipole moment, is used, the LC phase is found
only for the compound of series 1 with n ~ 12, X ~ NO2. In
the series 2 and 3, the lowering of the melting points by
introduction of a meta-nitro group is insufficient for LC phases
to be formed.
Other substituents like methoxy and dodecanoxy or at the
two meta positions, bromine and methoxy groups, suppress
the liquid crystalline properties, their bulkiness disturbing the
molecular V-shape and the good lateral interactions necessary
for promoting mesomorphism.
Although the stability of the liquid crystalline phase is
predominately determined by lateral substituents in the banana
branches, the type of phase is determined by the substituents
at the central ring. The compounds studied, if they form LC
phases, have a rather simple polymorphism, only one banana
phase appears between the isotropic liquid and the solid crystal
phases. With the methyl group at the 2- or 5-position, the B2
phase is observed, while the B7-like phase is formed when the
central resorcinol ring is not substituted.
11 J. Matraszek, J. Mieczkowski, J. Szydłowska and E. Go´recka,
Liq. Cryst., 2000, 27, 429.
12 V. Prasad, Liq. Cryst., 2001, 28, 1115.
13 J. P. Bedel, J. C. Rouillon, J. P. Marcerou, M. Laguerre,
H. T. Nguyen and M. F. Achard, Liq. Cryst., 2000, 27, 1411.
14 Ch.-K. Lee, J.-H. Kim, E.-J. Choi, W.-Ch. Zin and L.-Ch. Chien,
Liq. Cryst., 2001, 28, 1749.
15 Ch.-K. Lee, A. Primak, A. Ja´kli, E.-J. Choi, W.-Ch. Zin and
L.-Ch. Chien, Liq. Cryst., 2001, 28, 1293.
16 B. K. Sadashiva, H. N. Sh. Murthy and S. Dhara, Liq. Cryst.,
2001, 28, 483.
17 W. Weissflog, H. Nadasi, U. Dunemann, G. Pelzl, S. Diele,
A. Eremin and H. Kresse, J. Mater. Chem., 2001, 11, 2748.
18 D. S. S. Rao, G. G. Nair, S. K. Prasad, S. A. Nagamani and
C. V. Yelamaggad, Liq. Cryst., 2001, 28, 1239.
19 N. A. Clark, D. R. Link, D. Coleman, W. G. Jang, J. Fernsler,
C. Boyer, J. Zasadzinski, D. M. Walba, E. Korblova and
W. Weissflog, presented at the 19th Iternational Liquid Crystals
Conference, Edinburgh, 2002.
20 D. M. Walba, E. Ko¨rblova, R. Shao, J. E. Maclennan, D. R. Link,
M. A. Glaser and N. A. Clark, Science, 2000, 288, 2181.
21 A. Ja´kli, D. Kru¨erke, H. Sawade and G. Heppke, Phys. Rev. Lett.,
2001, 86, 5715.
22 W. Weissflog, I. Wirth, S. Diele, G. Pelzl, H. Schmalfuss, T. Schoss
and A. Wu¨rflinger, Liq. Cryst., 2001, 28, 1603.
23 D. R. Link, G. Natale, R. Shao, J. E. Maclennan, N. A. Clark,
E. Ko¨rblova and D. M. Walba, Science, 1997, 278, 1924.
24 G. Heppke, A. Ja´kli, S. Rauch and H. Sawade, Phys. Rev. E, 1999,
60, 5575.
25 J. P. Bedel, J. C. Rouillon, J. P. Marcerou, M. Laguerre,
M. F. Achard and H. T. Nguyen, Liq. Cryst., 2000, 27, 103.
26 J. P. Bedel, J. C. Rouillon, J. P. Marcerou, M. Laguerre,
H. T. Nguyen and M. F. Achard, Liq. Cryst., 2001, 28, 128.
The B2 phase is strongly sensitive to surface interactions.
J. Mater. Chem., 2002, 12, 3392–3399
3399