Ferroelectric, ferrielectric and antiferroelectric mesophases in compounds with a polybenzyloxycarbonyl mesogenic core

Article abstract of DOI:10.1039/b417144f

It was found that the synclinic interlayer interactions for materials having mesogenic cores made of repeating benzyloxycarbonyl units are stronger than for the parent MHPOBC and analogous compounds having biphenyl-phenyl mesogenic cores, as evidenced by significantly broader range of the synclinic phase and less frequent appearance of anticlinic phases in the homologous series. The biaxiality of the three-layer structure of the SmC* _Fil* phase was directly observed at the temperature where the optical pitch is spontaneously unwound. The ferrielectric phase structure is nearly Ising-like. The Ising structure is most probably enforced by strong interlayer quadrupole order, in materials in which the ferri-phase appears nearly 20 K below the SmA phase. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b417144f

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Ferroelectric, ferrielectric and antiferroelectric mesophases in compounds
with a polybenzyloxycarbonyl mesogenic core
Ewa Dzik, Jo´zef Mieczkowski,* Ewa Gorecka and Damian Pociecha
Received 9th November 2004, Accepted 20th December 2004
First published as an Advance Article on the web 26th January 2005
DOI: 10.1039/b417144f
It was found that the synclinic interlayer interactions for materials having mesogenic cores made
of repeating benzyloxycarbonyl units are stronger than for the parent MHPOBC and analogous
compounds having biphenyl–phenyl mesogenic cores, as evidenced by significantly broader range
of the synclinic phase and less frequent appearance of anticlinic phases in the homologous series.
The biaxiality of the three-layer structure of the SmC*Fi1 phase was directly observed at the
temperature where the optical pitch is spontaneously unwound. The ferrielectric phase structure is
nearly Ising-like. The Ising structure is most probably enforced by strong interlayer quadrupole
order, in materials in which the ferri-phase appears nearly 20 K below the SmA phase.
Most of the research on the SmC* subphases has been done
using compounds of the MnPOBC series,2 that are homo-
Introduction
Although more than 10 years have passed since the discovery
of smectic C* subphases1 it is still not clear what kind of
MHPOBC, or other materials having a biphenyl–phenyl core
logues of the prototype antiferroelectric liquid crystal
structure.7 In this contribution we present the synthesis and
molecular interactions prompt the interlayer order. The vast
majority, thousands of materials, form a synclinic SmC*
characterization of a homologous series having a different
phase, while anticlinic order, leading to antiferroelectric
mesogenic core consisting of repeating benzyloxycarbonyl
properties for enantiomeric materials, is observed only in
units. In the studied materials either the chiral terminal chains
about 400 compounds. Most of them have branched terminal
were varied, or for fixed terminal chains different substituents,
chains and a three-ring mesogenic core.2 In some of these
i.e. halogen atoms or NO2 groups, were attached laterally to
systems other tilted subphases, SmC*a, SmC*Fi1 (described
the mesogenic core.
also as SmC*c or SmC*1/3) and SmC*Fi2 (described also as
SmC*AF or SmC*1/4), also appear. They are observed only in
Experimental
chiral systems, in the temperature range where the synclinic
and anticlinic interactions are almost equal.3 It took nearly
The mesophase identification was based on microscopic routine
10 years of intensive research to recognize the structure of
these phases. The SmC*a phase has a clock-like structure in
which the director rotates by a constant azimuthal angle in the
consecutive layers. The periodicity of this structure is from five
to several smectic layers. The SmC*Fi1 and SmC*Fi2 phases
have distorted clock-like structures with three and four layer
periodicity, respectively.4 It is believed that the structure of
these phases is driven by the competition between synclinic or
anticlinic nearest neighbour (NN) interactions and the anti-
clinic next nearest neighbour (NNN) interactions in systems
with relatively strong NN quadrupole interactions5 or thermal
fluctuations that undulate the smectic layers.6
examination of the textures formed by samples between two
glass plates. A Nikon Opthiphot-2Pol polarising microscope
equipped with a Mettler FP82HT hot stage was used. The
differential scanning calorimetry measurements were per-
formed using a Perkin Elmer DSC-7 calorimeter in the cooling
and heating runs at scanning rates of 0.2–5 K min21. For
dielectric and electro-optic measurements, glass cells of various
thicknesses (from 5 to 50 mm) with ITO or gold electrodes
coated with polyimide were used. Dielectric spectroscopy
studies were performed with an impedance analyzer (Solotron
SI 1260) in the frequency window 1 Hz–10 MHz. The dielectric
dispersion data were analyzed by fitting the temperature-
dependent complex dielectric constant, e*, to the Cole–Cole
equation:
In smectic SmC*Fi1 and SmC*Fi2 phases apart from the
chiral mesoscopic structure, defining the basic structural unit,
long wavelength helical modulation also exists. It is still
not clear how the chirality influences the unit structure of
subphases and if and how the chirality is transferred in these
phases between mesoscopic and macroscopic levels. For the
SmC*Fi2 phase it has been observed that the sign of the long
wavelength helix may be reversed at certain temperatures
although the handedness of the four layer structure is
conserved.5
De
s
ꢀ
e {e?~
1{a zi
2pe0f
1zðif =frÞ
where De, fr and a are the dielectric strength, relaxation
frequency, and distribution parameter of the mode, respec-
tively. For a single, Debye type relaxation the distribution
parameter a 5 1. Spontaneous polarization was measured by
integrating the switching current recorded during polarization
reversal under applied triangular voltage. Tilt angle was
measured in a planar cell placed between crossed polarizers,
*mieczkow@chem.uw.edu.pl
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by applying the opposite dc electric field and determining the
angle difference between the minima of the light transmission
positions. Optical rotatory power (ORP) was studied using a
system consisting of a He–Ne laser, a photodiode (FLC PIN
20) and a lock-in amplifier (EG&G 7265), with 50–100 mm
thick free-standing film samples. The light transmission (LT)
was recorded versus the angle between the polarizer and
analyzer, the LT minimum was found by fitting LT to a square
equation. The rotation angle, hence ORP, could be determined
with an accuracy better than 0.02u in SmC*A and SmC*
phases, and y0.1u in SmC*Fi1 phase, where considerable
light scattering was observed upon approaching the helix
inversion temperature. The selective reflection measurements
were performed in transmission mode at normal incidence
with a Shimadzu PC3101 spectrophotometer for free standing
film samples in the wavelength range 200–3200 nm. At
normal incidence the selective reflection from half pitch
band was observed, at l 5 np, the mean refractive index,
n 5 1.5, that is assumed to be temperature independent in the
temperature range of measurements, was used for calculating
the pitch length.
some analytical methods were applied. Infrared (IR) spectra
were obtained using a Nicolet Magna IR 500. The NMR
spectra (13C) were recorded on a Varian Unity Plus spectro-
1
meter operating at 200 MHz, whereas H NMR spectra were
recorded at 200 or 500 MHz when high signal separation was
necessary. Tetramethylsilane was used as an internal standard.
Chemical shifts are reported in ppm. TLC analyses were per-
formed on Merck 60 silica gel plasticfolien and visualised using
iodine vapour. Column chromatography was carried out at
atmospheric pressure using silica gel (100–200 mesh, Merck).
Composition of the synthesised compounds, determined by
elemental analysis, confirmed the expected molecular structures.
4-[(2,2,2-Trichloroethoxy)carbonyl]phenyl-4-dodecyloxy-
benzoate (c, X 5 H). To 2,2,2-trichloroethyl 4-hydroxybenzo-
ate b (2 g; 7.4 mmol) dissolved in dry THF (100 mL), TEA
(10 mL), appropriate acid chloride a (X 5 H) (2.4 g; 7.4 mmol)
in THF (100 mL) and 50 mg of DMAP were added. The
mixture was refluxed with stirring during 6 h. The solvents
were evaporated and the reaction mixture was chromato-
graphed on silica gel using toluene as eluent. Compound c was
obtained (2.56 g; 62% yield).
Synthesis
Elemental analysis for C28H35O5Cl3: Calc. C 60.27, H 6.28;
Found C 60.20, H 6.32%; nmax (KBr)/cm21 2920, 1730, 1610,
1290, 1030, 810, 690; dH (CDCl3) 8.11–8.22 (m, 4H), 7.32–7.36
(m, 2H), 6.95–6.99 (m, 2H), 4.98 (s, 2H), 4.04 (t, 2H, J 5
6.6 Hz), 1.75–1.89 (m, 2H), 1.27 (s, 54H), 0.85–0.91 (m, 9H);
dC (CDCl3) 164.20, 163.84, 155.56, 132.42, 131.71, 126.02,
122.17, 120.87, 114.41, 95.04, 74.46, 68.37, 31.92, 29.64, 29.60,
29.56, 29.35, 29.08, 25.97, 22.70, 14.14.
The general synthesis procedure of the studied compounds is
outlined in Schemes 1 and 2. The chiral substrates used in the
synthesis were supplied by Aldrich ((R)-(2)-2-butanol: 99%
ee/GLC, (S)-(+)-2-pentanol: 98% ee/GLC, (R)-(2)-2-heptanol:
95% ee/GLC, (R)-(2)-2-octanol: 98% ee/GLC, [(1S)-endo]-
(2)-borneol: 98% ee/GLC, (1R,2S,5R)-(2)-menthol: 99%
ee/GLC, (1R)-endo-(+)-fenchyl alcohol:97% ee/GLC). To con-
firm the molecular structures of the synthesised compounds
4-{[4-(2,2,2-Trichloroethoxycarbonyl)phenoxy]carbonyl}phenyl-
4-dodecyloxybenzoate (e, X 5 H). Ester c (2.0 g; 3.6 mmol),
THF (100 mL), zinc dust (2.0 g; 30.7 mmol) and acetic acid
(10 mL) were stirred at room temperature for 6 h. Zinc dust
was filtered off and the filtrate was evaporated with toluene
(100 mL). Remaining solid was washed with hexane and dried
over phosphorus anhydride and was subsequently suspended
in toluene (75 mL) to which oxalyl chloride (3.5 mL;
41.0 mmol) was added. The reaction mixture was heated for
Scheme 2 Reagents and conditions : i) TEA, THF, DMAP; ii) CrO3/
Scheme 1 Reagents and conditions : i) TEA, THF, DMAP; ii) Zn/
AcOH; iii) SOCl2, toluene, reflux.
AcOH, THF; iii) (COCl)2, toluene, reflux.
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6 h at reflux. The precipitate was filtered off and the solvent
was evaporated to dryness. The acid chloride d was obtained
(1.4 g; 87%).
Elemental analysis for C26H33ClO4: Calc. C 70.19, H 7.42;
Found C 70.11, H 7.47%; nmax (KBr)/cm21 3020, 2920, 2740,
1715, 1400, 1280, 1040, 820, 715; dH (CDCl3) 10.02 (s, 1H),
8.12–8.17 (m, 3H), 7.94–7.99 (m, 1H), 7.32–7.41 (m, 1H), 6.96–
7.01 (m, 2H), 4.05 (t, 2H, J 5 6.1 Hz), 1.76–1.89 (m, 2H), 1.27–
1.43 (m, 18H), 0.88 (t, 3H, J 5 6.2 Hz), dC (CDCl3) 191.00,
164.51, 163.34, 151.67, 136.29, 132.02, 131.21, 128.82, 127.59,
122.51, 118.45, 113.84, 68.40, 31.92, 29.64, 29.59, 29.55, 29.35,
29.08, 25.97, 22.69, 14.13.
The acid chloride d (1.4 g; 3.1 mmol) dissolved in THF
(25 mL) was added to the mixture of 2,2,2-trichloroethyl-4-
hydroxybenzoate b (0.85 g; 3.1 mmol) in TEA (5 mL) and
50 mg of DMAP. The mixture was refluxed with stirring
during 10 h. After evaporation of the solvents the product was
purified with column chromatography on silica gel using
toluene as eluent. Compound e was obtained (1.3 g; 62%).
Elemental analysis for C35H39Cl3O7: Calc. C 61.99, H 5.76;
Found C 61.27, H 5.88%; nmax (KBr)/cm21 3040, 2910, 1750,
1610, 1450, 1260, 1040, 820, 710; dH (CDCl3) 8.13–8.30 (m,
6H), 7.35–7.41 (m, 4H), 6.96–7.01 (m, 2H), 4.99 (s, 2H), 4.02–
4.08 (m, 2H), 1.76–1.86 (m, 2H), 1.27 (s, 18H), 0.85–0.88 (m,
3H), dC (CDCl3) 164.46, 164.34, 164.06, 155.87, 155.47, 132.62,
132.11, 131.99, 126.51, 126.45, 122.44, 122.30, 121.03, 114.63,
95.21, 74.69, 68.59, 32.11, 29.83, 29.78, 29.75, 29.55, 29.27,
26.17, 22.89, 14.33.
2-Chloro-4-(chlorocarbonyl)phenyl-4-dodecyloxybenzoate (i,
Y 5 Cl). To aldehyde h (2 g, 4.5 mmol), dissolved in con-
centrated acetic acid (100 mL), was added chromium(VI) oxide
(0.9 g, 9.0 mmol) in 70% aqueous acetic acid (30 mL) and the
mixture was stirred for 24 h at room temperature. Then the
mixture was poured into water. The precipitated product was
filtered and dried in a desiccator over phosphorus anhydride.
The acid was purified by single crystallisation from methanol
and had sufficient purity for the next step. To the acid in
toluene (80 mL) was added thionyl chloride (1.5 mL,
20.5 mmol) and the reaction mixture was heated for 8 h at
reflux. The solvents were evaporated to dryness. The acid
chloride i was obtained (1.4 g, 65%).
4-{[4-((R)-2-Octyloxycarbonyl)phenoxy]carbonyl}phenyl-4-
dodecyloxybenzoate (compound 4). To the ester e (1.3 g;
1.92 mmol) dissolved in THF (50 mL), zinc dust (1.3 g;
19.9 mmol) and acetic acid (10 mL) were added. The mixture
was stirred at room temperature during 6 h. Zinc dust was
filtered off, and the filtrate was evaporated with toluene
(50 mL). The crude product was washed with hexane and dried
over phosphorus anhydride. The obtained product was
suspended in toluene (100 mL) and oxalyl chloride (2.5 mL,
29.5 mmol) was added. Reaction mixture were stirred and
heated for 6 h at reflux. The precipitated solid was filtered and
the filtrate evaporated to dryness. The acid chloride f was
obtained (0.97 g; 90%). The acid chloride thus formed (0.97 g,
1.72 mmol), dissolved in THF (50 mL), was added to the
mixture of (R)-(2)-2-octanol (0.21 mL, 1.38 mmol) in TEA
(5 mL) and 50 mg of DMAP. The mixture was refluxed with
stirring during 10 h. Then the solvents were evaporated and the
product was purified via chromatography on silica gel eluting
with toluene. Compound 4 was obtained (0.47 g; 52%).
Elemental analysis for C41H54O7: Calc. C 74.77, H 8.21;
Found C 74.89, H 8.32%; nmax (KBr)/cm21 3040, 2920, 2880,
1720, 1610, 1260, 1020, 830; dH (CDCl3) 8.11–8.30 (m, 6H),
7.40–7.28 (m, 4H), 7.01–6.97 (m, 2H), 5.17 (sextet, 1H, J 5
6.2 Hz), 4.05 (t, 2H, J 5 6.6 Hz), 1.86–1.59 (m, 2H), 1.34 (d,
3H, J 5 6.2 Hz), 1.27 (s, 28H), 0.90–0.85 (m, 6H); dC (CDCl3)
165.65, 164.51, 164.07, 155.80, 154.58, 132.64, 131.38, 128.84,
126.67, 122.40, 121.87, 121.08, 114.64, 72.17, 68.61, 36.28,
32.13, 31.95, 29.85, 29.80, 29.56, 29.37, 29.29, 26.18, 25.62,
22.90, 22.80, 20.31, 14.33, 14.28.
4-{[(1-Methylheptyl)oxy]carbonyl}phenyl 3-chloro-4-{[4-
(dodecyloxy)benzoyl]oxy}benzoate (compound 7). To acid
chloride i (0.46 g, 0.96 mmol), dissolved in dry THF (25 mL),
was added (R)-2-octyl-4-hydroxybenzoate j (Z 5 H) (0.20 g;
0.8 mmol) in dry THF (25 mL), TEA (3 mL) and 50 mg of
DMAP. The reaction mixture was refluxed with stirring during
10 h. After evaporation of the solvents the reaction mixture
was chromatographed on silica gel using toluene as eluent.
Compound 7 was obtained (0.32 g; 57.7%).
Elemental analysis for C41H53ClO7: Calc. C 71.03, H 7.71;
Found C 70.96, H 8.01%; nmax (KBr)/cm21 3040, 2930, 1710,
1620, 1300, 1030, 830, 70; dH (CDCl3) 8.09–8.16 (m, 4H), 7.26–
7.30 (d, 2H, J 5 8.8 Hz), 6.95–7.00 (d, 2H, J 5 9.2 Hz),
5.15 (sextet, 1H, J 5 6.4 Hz), 4.05 (t, 2H, J 5 6.6 Hz), 1.22–
1.86 (m, 33H), 0.85–0.91 (m, 6H), dC (CDCl3) 165.54,
164.47, 163.75, 154.64, 132.39, 131.09, 128.36, 121.75, 121.08,
114.37, 71.90, 68.38, 36.08, 31.93, 31.75, 29.65, 29.60, 29.57,
29.36, 29.16, 29.09, 25.98, 25.41, 22.70, 22.60, 20.10, 14.13,
14.07.
Results
Variation of the chiral terminal group
Anticlinic/antiferroelectric order is readily obtained if 1-methyl-
alkyl is used as one of the terminal chains. Similarly as for
MnPOBC8 or MnOMBB9 (1-methylalkanyl 4-[(49-alkoxy-4-
oxymethyl)biphenyl]benzoate) analogous series having car-
boxy and methyloxy linkages between the biphenyl and
phenyl units in the mesogenic core, respectively, in the
studied homologous series the appearance of an antiferro-
electric phase depends on the number of carbon atoms in
the chiral alkyl chain, however it is not a simple odd–even
effect (Table 1). For the studied compounds the range of the
synclinic SmC* phase, that appears above the antiferro-
electric phase, is significantly broader (about 20 K) than for
2-Chloro-4-formylphenyl-4-(dodecyloxy)benzoate (h, Y 5 Cl).
To 3-chloro-4-hydroxybenzaldehyde (Y 5 Cl) (2 g; 13 mmol),
dissolved in dry tetrahydrofuran (50 mL), was added TEA
(10 mL), acid chloride a (5.1 g; 15.7 mmol) in THF (50 mL)
and finally 50 mg of DMAP. Reaction mixture was refluxed
with stirring during 10 h. The solvents were evaporated to
dryness and crude products were separated by column
chromatography with silica gel using toluene as eluent.
Compound h was obtained (3.7 g; 65%).
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Table 1 The phase transition temperatures (in uC) and enthalpy changes (in J g21) for the studied compounds
Compound
n
X
Y
Z
m.p.
SmC*A
SmC*Fi1
SmC*
SmA
Iso
1
2
3
4
5
6
7
8
9
2
3
5
6
6
6
6
6
6
H
H
H
H
F
Cl
H
H
H
H
H
H
H
H
H
Cl
H
NO2
H
H
H
H
H
H
H
74.0 (38.9)
69.6 (63.3)
68.2 (60.5)
69.4 (54.9)
77.5 (59.8)
70.7 (28.9)
below 40
N
N
N
N
N
N
118.2 (0.07)
116.5 (0.2)
113.4 (.3)
117.6 (0.5)
114.9 (0.75)
90.0 (0.3)
N
N
N
N
N
N
N
N
N
153.0 (9.4)
140.7 (9.5)
128.4 (8.0)
130.5 (8.4)
123.1 (7.5)
103.7 (5.6)
74. (7.6)
N
N
N
N
N
N
N
N
N
N
96.9 (0.04)
N
N
96.2 (0.01)
97.5 (0.04)
N
98.9 (0.02)
Cl 50.6 (28.8)
43.1 (29.6)
N
N
65.4 (0.1)
72.3 (0.1)
85.8 (6.6)
76.5 (5.9)
H
R
Phase sequence
10
11
12
–CH(CH3)COOC2H5
–PhCOOCH(CH3)C6H13
bornyl
Cry 70.6 (46.2) SmC* 107.4 (0.2) SmA 138.7 (6.0) Iso
Cry 120.4 (44.0) SmC* 200.6 (0.3) SmA 227.4 (6.3) Iso
Cry 133.8 (67.0) SmC* 125.1 (0.7) SmA 140.9 (0.06) TGBA*
141.7 (0.2) N* 146.1 BP (0.06) BP 146.7 (0.01) BP 147.2 (1.7) Iso
Cry 114.7 (70.4) [SmA 90.1 (3.7)] Iso
Cry 123.3 (66.5) Iso
13
14
menthyl
fenchyl
MnPOBC and MnOMBB compounds. The spontaneous
polarization increases with increasing chiral terminal chain
length, from below 20 to above 100 nC cm22 (Fig. 1) for the
shortest and longest homologues, respectively. This is a
rather typical dependence, extending the terminal chain
makes the zig-zag shape of the molecule10 more pronounced,
which results in strongly hindered rotation around the long
axis, thus stronger in-plane quadrupolar order.11 There is no
detectable anomaly in Ps value at the SmC*–SmC*A phase
transition. The temperature dependence of Ps follows the
power law, Ps y (T 2 Tc)b, with critical exponents varying
from 0.4 to 0.3, for the shortest and longest homologues,
respectively. The angle at which the tilt saturates is y30–35u
for all homologues. The helical pitch gives a selective
reflection in the SmC* phase in the IR range for the
shortest homologue and in the visible range for the longest
homologue, while for the SmC*A phase it always is in the
visible range (Fig. 2). In both phases the pitch is only
weakly temperature dependent. The sign of the pitch reverses
on going from the ferro to the antiferro phase.
Detailed studies were performed for longest homologue, 4,
for which the ferrielectric SmC*Fi1 phase was additionally
observed between the SmC* and SmC*A phases. This
particular compound was synthesized previously by Nguyen
et al. (designated by the acronym 12HHBBM7) and was
described as having two ferrielectric mesophases with identical
electrooptic properties.12 In our studies, however, we do not
see evidence for the additional phase transitions. In the
temperature range between the SmC* and SmC*A phases, in
DSC measurements, only SmC*–SmC*Fi1 and SmC*Fi1
–
SmC*A phase transitions could be resolved on slow cooling
and heating runs. The proposed phase sequence is also
consistent with the optical and dielectric studies described
below.
Fig. 1 Spontaneous polarization versus temperature for homologous
compounds 1 (open circles), 2 (solid circles), 3 (open squares) and 4
(solid squares). The lines are fits to power law, Ps y (T 2 Tc)b, the
critical exponents b are given on the curves.
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the vicinity of the unwinding, and reappears with the opposite
sign above Ti. We conclude that for the R-enantiomer of 4
there is a right-handed helix in the SmC*A phase, right-handed
reversing to left-handed in the SmC*Fi1 phase upon increasing
temperature, and a left-handed helix in the SmC* phase.
Studies of the free-standing film samples around the unwind-
ing temperature enable observation of the director field, since
around Ti the averaging of the optical dielectric tensor due to
the helical superstructure is absent. Around temperature Ti the
birefringent Schlieren-type texture with numerous disclination
defects was observed and the non-zero birefringence evidences
the distortion from the perfect clock structure of the crystal-
lographic unit cell (Fig. 4). The perfect clock structure, due to
the averaging effect in the three layer unit, should give zero
birefringence for the light propagating along the layer normal
and the maximum birefringence should be observed for the
perfect Ising i.e. all-in-one-plane structure. According to the de
Vries formula13 ORP is a function of helical pitch (p) and in-
plane dielectric anisotropy (De):
Fig. 2 Selective reflection wavelength obtained for normal light
incidence for thick (y100 mm) free standing film samples of
compounds 2 (circles) and 4 (triangles) and fluorinated compound 5
(squares). In the inset is shown the selective reflection wavelength
measured in SmC*Fi1 of compound 4.
ꢀ
ꢁ
2
À
À
ÁÁ
2p De
{1
ORP~
8l2 1{l2
ꢀ
p
e
In compound 4 the helical structure of the SmC* and
SmC*A phases gives rise to selective light reflection in the
visible light range nearly of the same wavelength, while the
sign of the helix is reversed as evidenced by the reversed sign of
the ORP (Fig. 3). The helical pitch in SmC*Fi1 is considerably
longer and strongly temperature dependent. However, in the
narrow, about 1 K, temperature range of the SmC*Fi1 phase
above the SmC*A phase, selective reflection could be still
detected by spectroscopic methods. As the temperature
increases, the helix unwinds and rewinds with opposite
handedness. The inversion temperature Ti is y2 K above the
transition from the SmC*A phase. The temperature depen-
dence of the ORP follows the pitch temperature dependence
(Fig. 3). In the SmC*Fi1 phase it increases in magnitude on
heating towards Ti, becomes out of the measurable range in
with l 5 lo/np, lo is the wavelength of the light, e¯ is the mean
dielectric tensor, and n the mean refractive index. Knowing the
change of ORP and helical pitch between the SmC*A and
SmC*Fi1 phases the change in the in-plane dielectric aniso-
tropy for the light propagating parallel to the layer normal
could be obtained.
ꢂ
À
Á
1
DeFi1 DeC
~
4 cos2ða=2Þ{1
A
3
This change is due to the distortion from the all-in-one-
plane (a 5 0) structure. For the studied system it was found
that DeFi1/DeCA y 1 ¡ 0.02. This gives an azimuthal angle a
that is not bigger than 14 degrees, and constant within the
temperature range of y1 K above the SmC*A–SmC*Fi1 phase
transition, where reliable data for helical pitch could be
obtained. Our preliminary studies show that for the studied
system the a angle is temperature independent but might
depend essentially on the enantiomeric purity of the material.
The change of dielectric anisotropy between SmC* and SmC*A
phases for the studied compound, calculated from ORP
measurements, is DeC/DeCA y 0.91 ¡ 0.03, that is slightly
different than 1 due to a small decrease of the tilt angle
magnitude, which decreases the in-plane dielectric anisotropy
in the SmC* phase.
The dielectric response in the SmC*A phase shows a single,
weak relaxation mode (Fig. 5) that has an Arrhenius type
temperature dependence fr 5 foexp(2Ea/T) with activation
Fig. 4 Three layer basic structural unit of the SmC*Fi1 phase (viewed
along the layer normal). Azimuthal angles a and b different than 120u
define the distortion from the perfect clock-like structure.
Fig. 3 Optical rotatory power vs. temperature for compound 4
measured for a free standing film sample of thickness y50 mm. The
arrow indicates the helix twist inversion temperature.
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which is consistent with optical measurements. The distortion
of the long wavelength helix is probably the main contribution
to the mode, but its polydispersed nature shows that other
relaxation mechanisms are also present in the same frequency
window, for example movements of the defect walls formed at
the boundaries of the domains with opposite directions of Ps
and 2Ps in crystallographic units.5
In all studied compounds having SmC* and SmC*A phases
strong hysteresis of the dielectric response is observed in the
SmC* phase (Fig. 6). In this phase, on cooling, the helix
distortion (so called goldstone) mode is nearly temperature
independent. However, on heating from the SmC*A phase
(directly or through the SmC*Fi1 phase) the dielectric response
is much stronger and has lower relaxation frequency that
observed in the cooling scan. On further heating, a few degrees
above the transition to the SmC* phase the dielectric mode
strength suddenly decreases and its relaxation frequency
increases. These changes are correlated with the sample texture
changes. In the temperature range of stronger dielectric
response the helical pitch, judged from the periodicity of
disclination lines observed in the planar cell, so-called
dechiralization lines,15 is in the micrometer range and suddenly
decreases at the temperature where the dielectric response
decreases. The effect was confirmed in samples of various
thickness (from 5 to 100 mm) and for different electrode layers
(ITO or gold). Since measurements of helical pitch by
spectroscopic methods in free suspended film samples do not
show any anomaly in this temperature range (see Fig. 2), the
effect is clearly related to surface interactions.
Fig. 5 Imaginary part of dielectric constant vs. temperature and
frequency for compound 4 in the temperature range of the SmC*Fi1
phase.
energy Ea 5 98 kJ mol21. This mode is probably related to the
distortion of the antiferroelectric, two layer structure of the
crystallographic unit cell.14 The crystallographic unit cell
distortion dielectric mode is also observed in the SmC*Fi1
phase. Additionally, in the SmC*Fi1 phase a low frequency
mode is present; however its frequency could not be
determined precisely. The attempts to fit the dielectric data
with a Cole–Cole dependence indicate that the mode relaxa-
tion frequency is in the range 1–100 Hz and the mode has a
large distribution parameter (0.2–0.3). The presence of this
mode, with strength significantly lower than in the SmC*
phase, shows that polarization is partially compensated in the
three layer crystallographic unit cell of the SmC*Fi1 phase,
Other compounds with cyclic chiral terminal groups:16
fenchyl (14) and menthyl (13) units, did not show liquid
crystalline properties or exhibited only a monotropic meso-
phase (Table 1). However, for compound 12 with a bornyl
unit, a broad range of liquid crystalline phases with complex
polymorphism Iso–BP–N*–TGBA*–SmA–SmC* is observed.
The helical pitch in the N* and SmC* phases is short, giving
Fig. 6 Dielectric dispersion vs. temperature for compound 4 taken in cooling (left) and heating (right) scans, in a 6 mm thick glass cell with ITO
electrodes. The sudden change in the relaxation process observed in the heating scan is correlated with texture changes. In the inset is shown the
texture with dechiralization lines observed on heating at 103 uC.
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selective reflection in the UV range; in the TGBA* phase it is
in the visible range. The spontaneous polarization in the SmC*
a y 40–45u,5,18 in 10OTBBB1M7 a y 45–50u5,18 and for
MHPBC a y 36u19 was found by optical methods. The
difference is probably due to the range of the SmC* phase that
precedes the SmC*Fi1 phase, in the studied system the SmC*Fi1
phase appears y20 K below the SmA–SmC* phase transition.
This indicates that quadrupole interlayer interactions, that
tend to keep molecules in the same tilt plane, might indeed play
an important role in the distortion of the clock-like structure.
It is reasonable to expect that these interactions grow with
lowering temperature and become considerable in materials
with a broad range of the SmC* phase.20
phase is below 30 nC cm22
.
For the material 10 with a lactic acid end group only SmA
and ferroelectric SmC* phases are observed showing that this
terminal chain has lower induction ability for anticlinic order
than a 2-methylalkyl chain.
Variation of the mesogenic core
Introducing the lateral substituent to the mesogenic core
suppresses the range of liquid crystalline phases, as the
substituent increases the width to length ratio. In all materials
only synclinic order is observed, except for the smallest
fluorine substituent (compound 5) for which also the anticlinic
phase was detected. It is interesting to note that the position of
the lateral group in the mesogenic core influences the ferro-
electric properties profoundly. If the chlorine atom is attached
to the phenyl ring most distant from the chiral centre
(compound 6), the polarization is only slightly decreased
In studied material 4 for the first time a helix twist inversion
within the temperature range of the SmC*Fi1 phase was
observed. The long wavelength helical modulation is a result of
the small incommensurability of the three layer structure.6 In
the SmC*Fi1 phase the azimuthal angle changes in consecutive
layers as a, b, b with a + 2b slightly different from 2p. The helix
inversion occurs at the temperature at which the structure
becomes exactly commensurate, with a + 2b 5 2p. It should be
stressed that the basic three layer structural unit does not
change at the optical helix reversing temperature. This shows
that there is no simple relation between the molecular chirality,
chirality of the crystallographic unit cell and long wavelength
helical modulations.
compared to the non-substituted compound (Ps 5 78 nC cm22
,
30 K below the SmA–SmC* phase transition). If the chlorine
atom is substituted at the phenyl ring closest to the chiral
centre (compound 8) the polarization strongly decreases (Ps 5
45 nC cm22, 30 K below the SmA–SmC* phase transition). In
this case, apparently the dipole moment of the carbonyl group
(the one close to the chiral centre), which is to a great extend
responsible for the spontaneous polarization,17 is diminished
in the presence of the nearby electron-accepting chlorine
substituent. If the chlorine atom is substituted at the inter-
mediate phenyl ring (compound 7) only an orthogonal SmA
phase is observed. However, if the chlorine group is exchanged
for an NO2 substituent (compound 9) the SmA phase is
strongly suppressed and a ferroelectric SmC* phase again is
observed, with spontaneous polarization even higher than for
material 4 (Ps 5 100 nC cm22, 30 K below the SmA–SmC*
phase transition).
For the broad range of the SmC* phase, above the SmC*A
phase strong hysteresis of the dielectric response for heating
and cooling scans was detected. The effect is clearly related to
surface interactions. It is reasonable to assume that upon
heating into the SmC* phase, from the SmC*A phase, the
surface layers remain anticlinic/antiferroelectric, while the bulk
is ferroelectric. It is observed that such geometry leads to the
helix unwinding, the helical pitch is much longer than its
equilibrium value, found in surface free samples. Above a
certain temperature apparently the surface layer undergoes a
transition into a synclinic/ferroelectric state and the change in
the surface layer allows the formation of a helix with shorter,
close to equilibrium pitch. Since the dielectric response is
related to the periodicity of helical modulations in the cell, the
change in the pitch corresponds to the sudden change in the
dielectric response within the SmC* phase temperature range.
Extending the mesogenic core by an additional benzyloxy-
carbonyl unit (compound 11) raises the clearing temperature
by about 100 K and only a synclinic tilted phase is observed.
The spontaneous polarization, up to 70 nC cm22, is lower than
for analogous three ring compound 4.
Acknowledgements
Discussion
This work was supported by the KBN grant 4T09A 00425.
Results of our studies show that attaching a 2-methylalkyl
chain to a mesogenic core made of repeating benzyloxy-
carbonyl units gives compounds able to form synclinic and
anticlinic phases. However, the interactions leading to
synclinic structure are for the studied materials apparently
stronger than for the parent MnPOBC or MnOMBB com-
pounds as evidenced by the much broader range of the
synclinic phase and less frequent appearance of anticlinic
phases in the homologous series. The ferrielectric SmC*Fi1
phase that is usually observed between the SmC* and SmC*A
phases for materials with a narrow temperature range of the
SmC* phase, in the studied series is observed only for
compound 4 with the longest chiral alkyl chain, below the
broad range of the SmC* phase. The ferrielectric phase has a
nearly Ising structure (a , 14u), for comparison in MHPOBC
Ewa Dzik, Jo´zef Mieczkowski,* Ewa Gorecka and Damian Pociecha
Warsaw University, Department of Chemistry, ul. Pasteura 1, 02-093,
Warsaw, Poland. E-mail: mieczkow@chem.uw.edu.pl;
Fax: +48 22 8221075; Tel: +48 22 8221075
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