Mesomorphic properties of (S)-MHPSBOn series

Article abstract of DOI:10.1016/j.tca.2010.02.006

This article reports new results on synthesis and mesomorphic properties of the homologous series of (S)-1-methylheptyl-4-(4′-alkyloxybiphenylthiocarboxy)-benzoates, referred to as (S)-MHPSBOn, where n varies from 7 to 10 and denotes the number of carbon

Full text of DOI:10.1016/j.tca.2010.02.006

Thermochimica Acta 502 (2010) 51–59
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Mesomorphic properties of (S)-MHPSBOn series
Mirosława D. Ossowska-Chrus´ciel∗, J. Chrus´ciel
Institute of Chemistry, University of Podlasie, 3-go Maja 54, 08-110 Siedlce, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
This article reports new results on synthesis and mesomorphic properties of the homologous series of
(S)-1-methylheptyl-4-(4ꢀ-alkyloxybiphenylthiocarboxy)-benzoates, referred to as (S)-MHPSBOn, where
n varies from 7 to 10 and denotes the number of carbon atoms in the alkyl chain. These compounds are
sulphur analogous to a liquid crystal known as MHPOBC, where the central ester bridge was replaced with
the –COS– connecting group. The (S)-MHPSBOn homologous series possesses enantiotropic SmA*, SmC*,
SmC∗A and monotropic SmI* phases for n = 7–10. The (S)-MHPSBO7 compound has also smectic chiral
subphases: SmC∗␣ and SmC∗␥. Their mesomorphic properties were investigated by means of polarizing
optical microscopy (POM), differential scanning calorimetry (DSC), transmittance light intensity (TLI),
X-ray diffraction and electro-optic (EO) measurements.
Received 3 August 2009
Received in revised form
29 December 2009
Accepted 3 February 2010
Available online 10 February 2010
Keywords:
Chiral liquid crystals
Smectic phases
Ferroelectric and antiferroelectric phases
Spontaneous polarization
Mesomorphic and electro-optic properties
DSC calorimetry
© 2010 Elsevier B.V. All rights reserved.
POM
TLI and X-ray measurements
1. Introduction
Among the constituents of liquid crystals core, ester and
thioester groups are listed among the most important links.
The antiferroelectric order in liquid crystals was first observed
in 4-(1-methylheptyloxycarbonyl)phenyl-4ꢀ-octyloxybiphenyl-4-
carboxylate in compound well known as MHPOBC in 1989 [1] and
since then is the subject of intensive investigations (see e.g. [2–9]).
The high optical purity MHPOBC is characterized by antifer-
roelectric SmC∗A, paraelectric SmA* and three chiral subphases:
A
thioester group often improves thermal stability of the
mesophases. It has been well known that many achiral liquid crys-
tals having a thioester linkage tend to exhibit smectic phases,
namely SmC and SmA. Chiral thioester materials described in
this article, particularly homologue n = 7, seem to be interest-
ing as mesogens forming intermediate ferro- and antiferroelectric
SmC* phases and having a sulphur atom in a rigid core are
rare, but, considering their practical applications, very much
desired.
In what follows, a convenient and efficient method of synthesis
will be given and the properties of four chiral liquid crystal com-
pounds belonging to a new homologous series of chiral thioben-
zoates, CnH2n+1O–Ph–Ph–COS–Ph–COO–C*H(CH3)C6H13, abbrevi-
ated to as (S)-MHPSBOn (n varies from 7 to 10 and denotes
the number of carbon atoms in the alkoxy chain) will be pre-
sented. In these compounds the 4-alkoxybiphenyl is connected
with –COS– central group and the chiral branched chain in (S)-
4-(1-methylheptyloxycarbonyl)phenyl is attached to the thioester
central group in the p-position. The phase polymorphism com-
pounds from (S)-MHPSBOn homologous series were investigated
by means of three complementary methods: POM, DSC and TLI. X-
ray diffraction and electro-optic (EO) studies were also carried out.
The phase situation was characterized by means dielectric spec-
troscopy as well [21]. A strong influence of alkoxy group on the
sequence of phases was observed.
SmC∗␣, SmC␥∗ and SmC∗ between SmA* and SmC∗A phases. The chiral
␤
subphases are observed for high optical purity (S)-MHPOBC or (R)-
MHPOBC isomers only. The SmC∗␤ subphase is not observed when
one enantiomer contains 1.5% second. In this case, the sequence
of SmC∗␣ ↔ SmC∗ ↔ SmC␥∗ ↔ SmC∗ phases is observed. The SmC␥∗
A
subphase disappears when an admixture of one enantiomer is
higher than 5.6%. The SmC∗␣ is not present in a racemic mixture
[8].
After antiferroelectric properties of MHPOBC were discovered,
many chiral compounds exhibiting the antiferroelectric phases
have been synthesized in the past several years [9–12] and the
influence of different chemical structures on the phase polymor-
phism has been studied. In particular, the influence of thioester
linking group on the stabilization of SmC phase in chiral and achiral
compounds is a well-known phenomenon [13–20].
∗
Corresponding author. Tel.: +48 25 6431010.
E-mail address: dch@ap.siedlce.pl (M.D. Ossowska-Chrus´ciel).
0040-6031/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2010.02.006

52
M.D. Ossowska-Chru´sciel, J. Chru´sciel / Thermochimica Acta 502 (2010) 51–59
2. Experimental
2.1. Characterization
The structures of the intermediate and final compounds were
confirmed by elemental analysis (EA), IR, 1H NMR, 13C NMR and MS
spectroscopic methods. IR spectra were recorded on a FTIR Nicolet
Magna 760 spectrometer using a minimum of 64 co-added scans
at a resolution of 1 cm−1. The NMR spectra were obtained with a
Varian Unity Plus spectrometer operating at 500 MHz (CDCl3, TMS
as internal standard). MS spectra were obtained by MS TOF ES+. The
chemical purity was checked by thin layer chromatography (TLC)
and further confirmed by elemental analysis using a Perkin-Elmer
2400 spectrometer. The optical purity of the compounds was deter-
mined by chiral HPLC and 13C NMR. Transitions temperatures were
examined by polarized optical microscopy (POM), transmitted light
intensity (TLI) and differential scanning calorimetry (DSC). Heating
and cooling rates for all POM, TLI and DSC measurements were the
same ( 0.5 and 2.0 ◦C min−1). The temperature was controlled
with a Linkam controller with an accuracy of 0.1 ◦C (Linkam pro-
grammable heating stage THMSE 600 was used). Our TLI measuring
set was further described in [22]. DSC measurements were per-
formed using a DSC 822e Mettler Toledo Star System differential
scanning calorimetry. The examinations of POM and TLI were done
on planar aligned samples, using AWAT electro-optic cells, type
of HG cell, 6.4 ␮m thick (a trade mark of electro-optic cells made
by AWAT Company, Poland) with ITO electrodes. Measurements of
the polarization reversal current were performed according to the
well-known triangular-wave technique. X-ray investigations were
performed on a Philips X’Pert diffractometer as well as on a Guinier
symmetrical focusing transmission photographic camera. The anal-
ysis of TG, DTG and DTA curves obtained for compounds from the
(S)-MHPSBOn homologous series (by repeatedly heating and cool-
ing the sample from crystalline to isotropic phases) confirmed the
high thermal stability of investigated liquid crystals.
Fig. 1. Chemical structure of mesogens from the (S)-MHPSBOn homologous series.
The method applied earlier was based on the reaction of chlo-
rides of 4-alkoxybiphenyl-carboxylic acid with 4-mercaptobenzoic
acid. The similar method of (S)-MHPSBO8 synthesis was previously
described in paper [9]. This method leads up to obtaining low yields
of higher homologous.
Taking into consideration the high vulnerability of
4-mercaptobenzoic acid to create sulfide and disulfide
bonds and therefore small chemical yields of 4-[4-(4ꢀ-
alkoxybiphenyl)carbonylthio]benzoic acids,
a new method of
synthesis was developed. Compounds from the (S)-MHPSBOn
series were prepared according to following the procedures pre-
sented in detail in Fig. 2. The synthesis of the homologous series of
mesogens described by formula 6 was carried out with the use of
4-mercaptobenzoic acid 1.
Our new method (Fig. 2) is an innovation and a considerable
improvement with respect to what has been presented previously
[9]. 4ꢀ-(4-Alkoxybiphenyl)carboxylic acid (nOBB with n from 7 to
10) and (S)-1-methylheptyl 4-mercaptobenzoate (4, (S)-MHCBSH)
were used as intermediate products. The nOBB acids were obtained
using the modified Williamson method [23]. Under slow oxidation
4-mercaptobenzoic acid 1 was transformed into dithiodibenzoic
acid. The acid chloride (2 and 5, Fig. 2) was obtained by means of
the usual method. The (S)-1-methylheptyl 4,4ꢀ-dithiodibenzoates 3
(Fig. 2) was obtained by the esterification of 2 with the (S)-2-octanol
in the presence of pyridine and CH2Cl2. The (S)-2-octanol was pur-
chased from Fluka Co. Chem., chiral select 99.5%. The compound 3
was reduced with sodium borohydride in absolute ethanol under
nitrogen. The final compounds (S)-MHPSBOn 6 were obtained by
the esterification of 4 and 4-alkyloxybiphenyl-carboxylic acid chlo-
ride 5 with 72–76% yield. All compounds, intermediate and final,
2.2. Preparation of materials
The chemical structure of chiral mesogens from the (S)-
MHPSBOn homologous series is presented in Fig. 1.
Fig. 2. A convenient preparation of new chiral of thiobenzoate from the (S)-MHPSBOn homologous series.

M.D. Ossowska-Chru´sciel, J. Chru´sciel / Thermochimica Acta 502 (2010) 51–59
53
Fig. 3. Molecular configuration of the (S)-MHPSBOn: (a) molecular model of the (S)-MHPSBO7 compound and (b) postulated molecular configuration.
were purified by column chromatography and crystallization until
their melting point was constant. The optical purity of all homol-
ogous was checked by chiral HPLC and also 13C NMR method of
comparative measurements. 13C NMR method is one of methods
applied to determine for optical purity (enantiomer contains). The
samples of (S)-MHPSBOn showed the formation of desired (S)-6
with 99.5% 0.1 ee with respect to the starting compound (S)-2-
octanol.
eluted with CH2Cl2 under dry argon. The product 4 was obtained
as hell oil with 68% yield. The optical purity was 99.5 0.1% (HPLC).
1H NMR (500 MHz, CDCl3), ı (ppm): 0.98 (t, 3H, –CH3); 1.39 (m, 11H,
alkyl); 1.75 (m, 2H, –C*–CH2); 3.72 (s, 1H, –SH); 5.18 (s, 1H, –C*H–);
7.10 (d, 2H–Ar ortho to –SH), 7.95 (d, 2H–Ar ortho to –COOR). IR
(KBr, cm−1): 2570 (SH), 1680 (COO), 1600 (phenyl ring stretching).
EA (%) calc. for C15H22O2S: C 67.67, H 8.27, S 12.03; found C 67.70,
H 8.23, S 12.00.
2.2.2. General procedure for the preparation of compound
(S)-MHPSBOn
2.2.1. Preparation of compound (S)-MHCBSH-4
40 ml thionyl chloride was added to the suspension of 4,4ꢀ-
dithiodibenzoic acid (4.77 g, 15 mmol) in anhydrous toluene
(40 ml). The mixture was refluxed until the dissolution was com-
plete. The mixture was extracted with hot hexane and then
concentrated. The crude product was then crystallized from
the hexane. 4,4ꢀ-Dithiodibenzoyl chloride 2 (m.p. 80–82 ◦C) was
obtained with 73% yield. 1H NMR (500 MHz, CDCl3); ı (ppm); 8.05
(m, 4H, ortho to –COCl); 7.59 (m, 4H, ortho to –S–S–).
Details for (S)-MHPSBO7 are given as typical. 11 mmol of dry
pyridine was added to the solution (10 mmol) of compound 5 in
50 ml of dry toluene and then stirred for 30 min. Next, the solu-
tion of 10 mmol compound 4 in 50 ml of dry toluene was added
to the mixture. The reaction mixture was stirred for 12–15 h at
35–45 ◦C. When the analysis by TLC revealed a complete reaction,
the reaction mixture was cooled to 10 ◦C and added to water con-
taining ice and hydrochloric acid (10 ml, 15%). The organic layer
was washed several times with water and an aqueous solution
of sodium carbonate. The organic phases were dried over anhy-
drous magnesium sulphate and then filtrated. The filtrated solvent
was evaporated. The residue was purified by column chromatog-
raphy on silica gel using CH2Cl2 to give a white solid, which was
then recrystallized from absolute ethanol to white crystals; m.p.
119.7–120.5 ◦C. The yield of final compound was 75.6%. 1H NMR
(500 MHz, CDCl3, ı, ppm): 8.08 (2d, J = 8.6, 4H; 2H ortho to –COS–,
2H ortho to –COO–), 7.60 (m, 6H, 2H ortho to –OCH2–, 4H ortho to
–C6H4–), 7.00 (d, J = 8.0, 2H, ortho to –SCO–), 5.17 (s, J = 6.4, 1H,
–C*H–), 4.00 (t, J = 6.6, 2H, –CH2–, ␣ to –OC6H4–), 1.71 (m, 4H,
–CH2–, ␤ to –OC6H4– and ␣ to –C*H–), 1.35 (m, 19H, –CH2–), 0.90
(m, 6H, –CH3). 13C NMR (500 MHz, CDCl3, ı, ppm): 188.61 (1C,
–COS–), 165.57 (1C, –COO–), 159.88 (1C, C5H4C–OR), 146.51 and
133.20 (2C, –C5H4C–CC5H4–), 134.89 (1C, Ar, ortho to –OR), 134.51
(1C, C5H4C–SCO–), 131.85 (2C, C5H4C–CO–), 130.27 (2C, Ar, meta
to –OR), 128.52 (2C, meta to –COS–), 128.30 (2C, ortho to –SCO–),
126.88 (2C, ortho to –COOR), 115.19 (2C, ortho to –COS–), 36.23
(1C*), 31.11–14.56 (13C, alkyl). IR (KBr, cm−1): 2931, 2858, 1714,
1674, 1528 and 1190.
The compound 2 (1.72 g, 5 mmol) was dissolved in dry toluene
(30 ml), to which 0.45 g (6 mmol) of dry pyridine was added. The
solution was stirred for 0.5 h and then 1.54 g (5 mmol) of (S)-2-
octanol in 30 ml dry toluene was added dropwise. The mixture was
stirred at 35–40 ◦C for 18 h (TLC control, Rf = 0.48 in CHCl3). Then
it was filtrated, and the filtrate was washed with cold water with
hydrochloric acid and again washed with cold water. The toluene
layer was dried over anhydrous magnesium sulfate and concen-
trated under vacuum to the yellow oil, then purified by column
chromatography on silica gel using CH2Cl2 as eluent. The product
3 was obtained as light yellow oil (72% yield). 1H NMR (500 MHz,
CDCl3); ı (ppm): 0.86 (t, 6H, –CH3); 1.29 (m, 22H, al); 1.62 (m, 4H,
–CH2–); 5.13 (s, 2H to C*); 7.50 (m, 4H, ortho to –S–S–); 7.95 (m,
4H, ortho to –COO). IR (KBr, cm−1): 1720, 1611, 1509, 445 cm−1
.
Portions of sodium borohydride (0.36 g, 10 mmol) were added to
the solution of 3 (2.7 g, 5 mmol) in absolute ethanol (70 ml), which
was then stirred under nitrogen at RT for 1 h (TLC, Rf of product
0.62 CH2Cl2). The reaction mixture was added to water containing
ice and hydrochloric acid and extracted with dichloromethane (3×
50 ml). The combined dichloromethane layers were washed with
water, next they were dried (MgSO4), filtrated, then dry silica gel
(2 g) was added to them and they were evaporated to dryness. The
silica gel plug was placed on top of chromatography column and
MS (m/z, %): 560.7 (M+, 100.00). EA (%) calc. for C35H44O4S: C
75.00, H 7.86, S 5.71; found C 75.07, H 7.80, S 5.70.

54
M.D. Ossowska-Chru´sciel, J. Chru´sciel / Thermochimica Acta 502 (2010) 51–59
Table 1
Phase behaviour of the (S)-MHPSBOn series obtained during heating and cooling.
(S)-MHPSBOn
(S)-MHPSBO7
CnH2n+1O–Ph–Ph–COS–Ph–COO–C*H(CH3)C6H13
Heating
Cooling
Cr2 82.1 (19.0), Cr1 117.3 (52.0), SmC∗A 140.9 (0.02), SmC∗␥
141.6 (0.04), SmC* 144.8 (0.02), SmC∗␣ 145.8 (0.05), SmA*
184.2 (12.3) I
I 184.0 (14.3), SmA* 145.8 (0.05), SmC∗␣ 144.8 (0.02), SmC*
141.4 (0.04), SmC∗␥ 140.6 (0.02), SmC∗ 116.9 (3.3), SmI*
A
78.1 (32.3), Cr1 71.4 (10.5) Cr2
(S)-MHPSBO8
(S)-MHPSBO9
(S)-MHPSBO10
Cr 111.6 (54.6), SmC∗A 146.1 (0.14), SmC* 151.1 (0.59),
SmA* 177.8 (11.5) I
I 177.8 (11.2), SmA* 151.6 (0.53), SmC* 144.4 (0.09), SmCA∗
107.6 (3.1), SmI* 72.9 (36.5) Cr
Cr 96.1 (5.4), SmC∗A 146.3 (0.06), SmC* 152.2 (0.13), SmA*
168.4 (8.4) I
I 167.9 (7.9), SmA* 151.3 (0.08), SmC* 145.2 (0.05), SmCA∗
92.9 (0.2), SmI* 49.2 (3.9) Cr
Cr 83.1 (56.1), SmC∗A 144.9 (0.14), SmC* 153.8 (1.03), SmA*
166.1 (9.1) I
I 165.9 (8.7), SmA* 153.4 (1.0), SmC* 142.9 (0.11), SmCA∗
85.9 (1.4), SmI* 29.6 (26.0) Cr
Transition temperatures (◦C) and enthalpies (in parentheses, kJ g−1) were determined from DSC scans at 2 and 0.5 K min−1; Cr denotes crystalline phase, SmC␣∗ and SmC∗␥
are chiral subphases, SmC* is chiral ferroelectric phase, SmC∗A is chiral antiferroelectric phase, SmI* is chiral smectic phase, SmA* and SmB* are orthogonal phases and I is
isotropic phase.
Fig. 4. DSC curve obtained during heating (a) and cooling (b) of (S)-MHPSBO7 ( 0.5 ◦C min−1).
3. Results and discussion
The net charge of the (S)-MHPSBOn molecule shows three
dipole groups: –COS– in the central part and two groups,
–COOC*H(CH3)C6H13 and –OR in the terminal chains. The largest
negative charges can be found at oxygen atoms O1, O2, O3 and
O4, whereas the sulphur atom S1 has a slight positive charge. The
chiral end chain is bent by about ı ca. 42◦ with respect to the
core direction. Crystallographic investigations in the crystalline
phase of MHPOBC prove that the chiral branched terminal chain
and the nearest phenyl plane are tilted with respect to each other
A molecular model of (S)-MHPSBOn (for n = 7) is shown in
Fig. 3. The all-trans conformation for alkyl and alkoxy chains
were found for the molecules of thioester series in the solid
state [24–26], however in the liquid and liquid crystalline state
many different conformations occur. Theoretical calculations for
the investigated molecules were carried out by means of the semi-
empirical MINDO3 method. The result is shown in Fig. 3.
Fig. 5. TLI curve obtained during cooling (−2 ◦C min−1) of (S)-MHPSBO7.

M.D. Ossowska-Chru´sciel, J. Chru´sciel / Thermochimica Acta 502 (2010) 51–59
55
Fig. 6. Textures of the SmA*, SmC∗˛, SmC*, SmC␥∗ , SmCA∗ , SmI* and Cr phases observed during cooling of the (S)-MHPSBO7.
by ca. 90◦ [27,28]. It was also confirmed that the molecules are
also bent even in the liquid crystalline phase. Polarized FT-IR [29]
and NMR [30,31] measurements of MHPOBC suggest that the chi-
The enthalpy changes for (S)-MHPSBOn have been established
by DSC investigations and are presented in Table 1. They show
that the (S)-MHPSBO7 is characterized by rich phase polymor-
phism: antiferroelectric SmC∗A, ferroelectric SmC* and paraelectric
SmA*, as well as two subphases: SmC∗˛ between phases SmA* and
SmC* and SmC∗␥ between SmC∗A and SmC* and highly ordered tilted
SmI* phase. The optical purity was comparable between all com-
pounds (99.5 0.1%), suggests that the presence of smectic chiral
subphases is the property of (S)-MHPSBO7 compound. It can be thus
concluded that alkoxy terminal chain length played the crucial role
in the phase polymorphism of (S)-MHPSBOn compounds. From this
series only the (S)-MHPSBO7 possesses rich phase polymorphism
and in this subphases. It is probably caused by structural differences
ral chain is bent at the magic angle (54.7◦) and ca. 43◦ 13C NMR)
(
or 38–76◦ (2H NMR) respectively. The results obtained from semi-
empirical methods indicate also that the angle between the chiral
terminal group (S)-1-methylheptyloxy and the nearest phenyl
plane is at ca. 90◦ in 5BoOSMH and 5PhOSMH compounds [16].
The postulated molecular configuration compounds from homol-
ogous series of (S)-MHPSOBn, of MHPOBC analogues are shown in
Fig. 3.
Transition temperatures for all compounds obtained during
heating and cooling are presented in Table 1.

56
M.D. Ossowska-Chru´sciel, J. Chru´sciel / Thermochimica Acta 502 (2010) 51–59
Fig. 7. Textures observed during cooling of the (S)-MHPSBO7 sample right behind I-SmA* phase transition (184.0 ◦C) for different cooling rates (from 5.0◦ min−1, through
2.0◦ min−1, 1.0◦ min−1, 0.5◦ min−1, 0.2◦ min−1, 0.1◦ min−1, 0.05◦ min−1 to 0.02◦ min−1).
between these compounds in steric packing and the dipole inter-
action resulting from different lengths of terminal chain.
The DSC scan for (S)-MHPSBO7 that illustrates an especially
important temperature region from 135 to 150 ◦C is presented in
Fig. 4. It refers to heating and cooling (Fig. 4a and b respectively).
Phase transitions SmC∗A ↔ SmC␥∗ , SmC␥∗ ↔ SmC∗, SmC∗ ↔ SmC␣∗
and SmC∗␣ ↔ SmA∗ have very low transition enthalpy (from 0.02 to
0.05 J g−1, Table 1). These phase transitions are also well identified
on the TLI curve (Fig. 5). On cooling of (S)-MHPSBO7, this phase
sequence was deduced from the course of the TLI curve: I 187.5 ◦C

M.D. Ossowska-Chru´sciel, J. Chru´sciel / Thermochimica Acta 502 (2010) 51–59
57
Fig. 8. Phase diagram for the (S)-MHPSBOn compounds obtained during cooling.
Fig. 9. Temperature dependence of layer spacing d for the (S)-MHPSBO7 compound
obtained during cooling.
SmA*, 146.5 ◦C SmC␣∗ , 145.2 ◦C SmC*, 141.3 ◦C SmC␥∗ , 140.6 ◦C SmC∗A,
116.8 ◦C SmI*, 80.0 ◦C Cr. The transition temperatures between sub-
phases and ferroelectric SmC* and antiferroelectric SmC∗A phases
were precisely determined on the basis of the first derivative of
transmittance with respect to the temperature, calculated within
the range of 139.0–148.0 ◦C. One can conclude that both DSC and TLI
methods appear to be fruitful and complementary in establishing
the phase sequence in liquid crystalline compounds.
layer spacing d clearly decreases in proportion to the decreasing
temperature in subphase SmC␥∗ as well as antiferroelectric SmC∗A.
The biggest change is noticeable in SmC∗A phase (from 36.44 Å at
141.0 ◦C to 35.75 Å at 136.0 ◦C). Representative results for temper-
ature dependence of the layer spacing d for ((S)-MHPSBO7 are given
in Fig. 9.
The layer spacing d for the orthogonal SmA* phase for (S)-
MHPSBO7 is equal to 36.57 Å in the vicinity of transition to SmC∗␣,
at 145.8 ◦C. In the SmA* phase the layer spacing d is approximately
0.97 of the actual molecular length l (37.70 Å calculated by means
of the semi-empirical MINDO3 method, it is not crystallographic
data) in its fully extended conformations, including the covalent
radii of the H atoms. The difference between d and l is probably
caused by (S)-MHPSBO7 molecules being slightly tilted in SmA*
phase which suggests that the SmA* phase can be of the de Vries
type. Further other measurements are needed to study SmA*–SmC␣∗
and SmC∗␣–SmC* phase transition.
The homogeneous textures were obtained during slow cool-
ing (0.02 ◦C min−1) from isotropic state to the SmA* phase. The
optical observations and electro-optical studies have been car-
ried out in the same conditions. Spontaneous polarization studies
have been conducted using triangular field for all compounds from
(S)-MHPSBOn homologous series. Temperature dependence of the
spontaneous polarization value PS for (S)-MHPSBO7 is presented in
Fig. 10. The transition temperatures put in Fig. 10 were determined
from DSC method.
Good experimental evidence has been obtained from the POM
texture images of the (S)-MHPSBO7 sample for phase transitions
from SmA* through subphase SmC∗␣ ferroelectric SmC*, subphase
SmC∗␥ and antiferroelectric SmC∗A to Cr. The substance was placed in
the cell identical to that used in TLI method (planar orientation). The
microscope setting was kept unchanged while taking micrographs
at different temperatures. However, the texture changes between
phases are visible. Fig. 6 (Photo a) presents the I-SmA* phase transi-
tion. During cooling of the SmA* phase a fan shaped texture (Photo
b) was found. This texture was changing upon further tempera-
ture decreasing and one could recognize characteristic textures of
SmC∗␣, SmC*, SmC∗␥, SmC∗ and SmI* phases seen in Photos c, d, e, f,
A
g (Fig. 6) respectively. Photo h presents the Cr phase.
The homogenization of the texture of the samples is the biggest
problem in optical investigation. Fig. 7 presents the textures
obtained during cooling of the (S)-MHPSBO7 sample right behind
I-SmA* phase transition (184.0 ◦C) for different cooling rates. The
last cooling rate was selected as an optimal rate and was applied to
other substances.
From XRD and miscibility data the compounds (S)-
MHPSBO7–(S)-MHPSBO10 have the enantiotropic SmC∗A, SmC*,
SmA* phases during heating and cooling and monotropic SmI*
during cooling. Temperature ranges of the above-mentioned
phases for all investigated compounds are presented in Fig. 8.
We can see that ꢀT(SmC∗A) for (S)-MHPSBO7, (S)-MHPSBO8,
(S)-MHPSBO9 and (S)-MHPSBO10 possess a different range of the
antiferroelectric SmC∗A phase (phase diagram, Fig. 8). The increas-
ing length of terminal alkoxy chain (from n = 7 to 10) extends the
mesophase range considerably during cooling (ꢀT(SmC∗A) = 23.6
and 61.8 ◦C for n = 7 and 10 respectively).
A small value of spontaneous polarization is visible already
at a temperature of over 155 ◦C degrees (SmA* phase). It is in
The interlayer distances of chiral mesogens from the (S)-
MHPSBOn homologous series were measured by means of X-ray
diffraction as a function of temperature. The layer spacing d clearly
decreases with lowering temperature in SmC* and subphase SmC␥∗
and antiferroelectric SmC∗A. The biggest change is in SmC∗A in
the region of SmC∗␥–SmC∗ phase transition ((S)-MHPSBO7, from
A
36.28 Å at 141 ◦C to 35.84 Å at 138 ◦C). The minimal change was
observed in subphase SmC␣∗ and in SmC∗␣–SmC* phase transition.
The change in the ferroelectric SmC* phase is similar to that in para-
electric SmA* (from 36.60 Å at 144.8 ◦C to 36.52 Å at 142.0 ◦C). The
Fig. 10. Spontaneous polarization PS for the (S)-MHPSBO7 compound vs. tempera-
ture obtained during cooling obtained at 10 Hz.

58
M.D. Ossowska-Chru´sciel, J. Chru´sciel / Thermochimica Acta 502 (2010) 51–59
Table 2
Comparison of values of spontaneous polarization for compounds from (S)-MHPSBOn homologous series.
Phase transitions
(S)-MHPSBO7
(S)-MHPSBO8
(S)-MHPSBO9
(S)-MHPSBO10
SmA*–SmC∗␣
SmC∗␣–SmC*
SmC*–SmC∗␥
39.23
45.35
61.58
66.60
105.33
SmA*–SmC*
23.49
SmA*–SmC*
28.90
SmA*–SmC*
34.59
P
SmC*–SmC∗A
69.86
SmC*–SmC∗A
107.53
SmC*–SmC∗A
110.90
SmC∗␥–SmC∗
A
SmC∗A–SmI*
136.11
162.16
182.02
Fig. 11. Transmitted light curves vs. applied triangular-wave electric field of frequency 1 Hz and∗amplitude 40 V/6.4 ␮m for the (S)-MHPSBO7 compound obtained during
cooling: (a) at 145.3 ◦C (SmC˛∗ ), (b) at 143.0 ◦C (SmC*), (c) at 141.0 ◦C (SmCꢁ∗ ), (d) at 130.0 ◦C (SmCA), (e) at 117.0 ◦C (SmI*) and (f) at 85.0 ◦C (SmI*).
accordance with tilt angle of molecules within the tempera-
ture range of SmA*–SmC∗␣ phase transition (from ca. 10.0◦ at
SmA*–SmC∗␣ transition point to 0.0◦ at 155 ◦C in SmA* phase).
The PS values are 39.23, 45.35, 61.58, 66.60 and 105.33 nC cm−2
Optical transmittance vs. electric field ( 60 V) was measured
for (S)-MHPSBOn sample in 6.4 ␮m homogeneous cells at the fre-
quency of 1.0 Hz of applied triangular-wave electric field. Some
representative results obtained during cooling of the (S)-MHPSBO7
were presented in Fig. 11.
at the SmA*–SmC␣∗ , SmC␣∗ –SmC*, SmC*–SmC␥∗ , SmC∗␥–SmC∗ and
A
SmC∗A–SmI* phase transitions respectively. Spontaneous polariza-
tion behaviour above 148.5 ◦C seems to be small, with a relatively
high value of the spontaneous polarization close to 116.9 ◦C to
SmC∗A–SmI* phase transition. These thermal characteristics include
a low temperature region which corresponds to the more orderly
smectic phase (SmI* ≤ phase). Temperature dependence of the
spontaneous polarization with theoretical fit is presented in Fig. 10.
The equation describes changes of the spontaneous polarization
between SmA*–SmC∗␣ and SmCA∗ –SmI* phase transitions. The crit-
ical exponent obtained by fitting the square root law is equal to
0.37 and implies that changes of PS values are increasing mono-
tonic with decreasing temperatures. As there is a drop of the PS
values at the SmC∗A–SmI* transition one can conclude that in this
case a first order transition takes place. Similar results were earlier
obtained for other compounds [32–34].
Small transmittance changes from 145.8 to 144.8 ◦C (SmC␣∗ ,
Fig. 11a) are visible. At a low frequency (1.0 Hz) the V-shaped
switching was observed. Similar loops were observed in SmC␣∗
for another compound [35]. The SmC∗␣–SmC* phase transition
at 144.8 ◦C was not observed as a considerable change in the
characteristic shape of the switching pattern (Fig. 11b). A slight
modulation of hysteresis loop was noticeable in the whole range
of the SmC* phase. Similar changes were observed in SmC* for
10OPOSMH [17]. In the SmC∗␥ a slight modulation of V-shaped
switching was observed (Fig. 11c). In the antiferroelectric SmCA∗
phase distinct changes in hysteresis loop were visible (Fig. 11d).
The characteristic tristable switching pattern was observed in the
whole temperature range of the antiferroelectric SmC∗A phase. The
SmC∗A–SmI* phase transition at 117 ◦C, i.e. exactly at the tem-
perature of SmC∗A–SmI* phase transition, was not observed as a
considerable change in the tristable switching pattern (Fig. 11e).
At a lower temperature the planar segment of the tristable switch-
ing becomes shorter during subsidence (Fig. 11f). This means that
the molecule remained shorter in the anticlinic order. Other com-
pounds from the (S)-MHPSBOn homologous series: (S)-MHPSBO8,
(S)-MHPSBO9 and (S)-MHPSBO10 displayed identical behaviour.
Between SmC∗␣ subphase and ferroelectric SmC* phase, fer-
roelectric SmC* and SmC␥∗ subphase as well as SmC∗␥ subphase
and antiferroelectric SmCA∗ no distinct change of PS value was
observed. A comparison of PS values between the homologous of
(S)-MHPSBOn series is presented in Table 2 (PS values were extrap-
olated at phase transition temperatures).
A similar characteristic of PS values was observed for (S)-
MHPSBO8, (S)-MHPSBO9 and (S)-MHPSBO10 in the region from
4. Conclusions
SmC* to SmC∗ . It seem to indicate that maximum of PS val-
A
ues increasing with elongated of alkoxy terminal chain, from
105.33 nC cm−2 for (S)-MHPSBO7 to 182.02 nC cm−2 for (S)-
MHPSBO10 at SmC∗A–SmI* phase transition.
A
convenient preparation of new chiral of thiobenzoate
ferroelectric–antiferroelectric liquid crystals, the (S)-MHPSBOn
homologous series, is described. The (S)-MHPSBO7 possesses rich

M.D. Ossowska-Chru´sciel, J. Chru´sciel / Thermochimica Acta 502 (2010) 51–59
59
phase polymorphism, including paraelectric SmA*, chiral ferroelec-
tric SmC*, antiferroelectric SmC∗A, monotropic SmI* phases and also
smectic chiral subphases: SmC∗␣ and SmC␥∗ . The higher homolo-
goues have enantiotropic SmA*, SmC*, SmC∗A and monotropic SmI*
phases (for n = 8–10). The temperature range of the antiferroelectric
SmC∗A phase increases proportionally to the extending length of ter-
minal alkoxy chain (for (S)-MHPSBO7 is equal to 23.6 ◦C and rises
considerably and for (S)-MHPSBO10 is equal to 61.8 ◦C). The val-
ues of spontaneous polarization PS in the ferroelectric SmC* phase
changed in the range of 66.6–106.7 nC cm−2 and from 105.3 to
182.0 nC cm−2 in antiferroelectric SmC∗A phase. These are relatively
high values which may reveal possible application potential. Analy-
sis of electro-optical switching properties of the SmI* phase leads to
the conclusion that SmI* phase of (S)-MHPSBOn homologous series
displays the behaviour typical of antiferroelectric phase.
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The authors would like to thank Prof. Jan Przedmojski (Warsaw
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