Dielectric spectroscopy of bent-core thioesters' B phases

Article abstract of DOI:10.1080/15421406.2011.569249

Banana-shaped homologoues 1,3-phenylene bis{4-[(4-alkoxybenzoyl) sulfanyl]benzoate}, in short nOSOR, where n denotes the number of carbon atoms in the end chains, have been studied using dielectric and electrooptic methods. 9OSOR shows a ferroelectric B1

Full text of DOI:10.1080/15421406.2011.569249

Mol. Cryst. Liq. Cryst., Vol. 541: pp. 252=[490]–261=[499], 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421406.2011.569249
Dielectric Spectroscopy of Bent-Core
Thioesters’ B Phases
1
J. CHRUSCIEL, M. WIERZEJSKA-ADAMOWICZ,
2
´
1
D. M. OSSOWSKA-CHRUSCIEL, M. MARZEC,
2
´
A. WAWRZYNIAK,2 R. DOUALI,3 CH. LEGRAND,3
2
´
AND S. WROBEL
1Institute of Chemistry, Siedlce University of Natural Sciences and
Humanities, Siedlce, Poland
2
´
Jagiellonian University, Institute of Physics, Reymonta, Krakow, Poland
Universite du Littoral Cote d’Opale, LEMCEL, Calais, France
3
´
ˆ
Banana-shaped
homologoues
1,3-phenylene
bisf4-[(4-alkoxybenzoyl)
sulfanyl]benzoateg, in short nOSOR, where n denotes the number of carbon atoms
in the end chains, have been studied using dielectric and electrooptic methods.
9OSOR shows a ferroelectric B1 phase, whereas 12OSOR, 14OSOR exhibit antifer-
roelectric B2 phase. Dielectric spectra have been measured for two alignments of B2
phase. In the low frequency range a broad collective relaxation process is observed
under bias field. This process is connected with fluctuations of ferroelectric domains.
The high frequency relaxation originates from molecular reorientation around the
long axis.
Keywords Banana-shaped mesogenes; collective modes; dielectric relaxation;
molecular processes
1. Introduction
Liquid crystals composed of banana-shaped molecules belong to a new interesting
class of liquid crystals [1,2]. Smectic B phases of banana-shaped systems differ from
typical ferroelectric and antiferroelectric smectic phases studied so far. These new
materials attract our attention because of unexpected electro-optic properties, i.e.,
fast switching from planar to homeotropic state for the secondary optical axis [3,4].
The B2 phase can have four possible structures: synclinic – antiferroelectric,
antyclinic – antiferroelectric, synclinic – ferroelectric and antyclinic – ferroelectric
[5,6]. Two antiferroelectric states usually appear as ground state and two ferroelec-
tric states can be obtained by applying an external electric field [6,7]. Different types
of B2 phases can be distinguished by using complementary methods: texture
observation, electro-optic response and X-ray diffraction.
´
Address correspondence to S. Wrobel, Jagiellonian University, Institute of Physics,
´
Reymonta 4, Pl-30-059 Krakow, Poland. Tel.: þ48(12)6635684; E-mail: stanislaw.wrobel@uj.
edu.pl
252=[490]

Dielectric Spectroscopy of Bent-Core Thioesters’
253=[491]
Figure 1. Molecular structure of bent-core mesogenes studied and their phase diagrams
obtained during cooling.
In this work dielectric and electrooptic methods were used to investigate B
phases of three symmetric bent-core thioestr mesogenes nOSOR (n ¼ 9, 12, 14)
(Fig. 1). 9OSOR shows ferroelectric B1 phase, whereas 12OSOR, and 14OSOR
exhibit antiferroelectric B2 phase.
All these materials have been also studied by texture observations and reversal
current method. For B2 phases of 12OSOR and for 14OSOR quasi-planar and
quasi-homeotropic textures were observed. Polarization for B1 phase of 9OSOR is
rather small and it increases with temperature. For B2 phases of 12OSOR, and
14OSOR the spontaneous polarization is very high (about 600 nC=cm2). Shorter
homologue 8OSOR has been investigated by us and it shows B1 phase which displays
some ferroelectric properties [8]. Bent-core compounds with different configurations
of thioester’s group have been studied in [9].
2. Experimental
2.1. Synthesis
The synthesis of banana-shaped homologoues 1,3-phenylene bisf4-[(4-alkoxyben-
zoyl) sulfanyl]benzoateg, referred to as nOSOR, where n denotes the number of
carbon atoms in the terminal alkoxy chain, is outlined schematically in Figure 2.
These compounds have ester and thioester connecting groups and are derived from
resorcinol as the central core. These banana-shaped molecules correspond to the
formula shown in Figure 1.
The 4-[(40-alkoxybenzoyl)sulfanyl]benzoic acids (nOSB, IV) were obtained by
reaction of chlorides (II), of the appropriate 4-n-alkoxybenzoic acids (nOB, I) with
4-mercaptobenzoic acid (III). This process was carried out at room temperature, in
the presence of pyridine in toluene. Crude acids (nOSB) were purified by repetitive
crystallization from ethyl acetate until constant melting points were achieved.
The final products (VII) were synthesized by two methods: first the acids nOSB
(IV) were esterified with resorcinol in the presence of 1,3-dicyclohexylcarbodiimide
(DCC) and dimethylaminopyridine (DMAP) in dichloromethane (as solvent) at
the room temperature and second along the path a traditional synthetic route by
reaction of 4-(40-n-alkoxybenzoylthio)benzoic acid chloride (V) and resorcinol (VI)
in toluene in the presence of pyridine at 40–50ꢀC (Fig. 1). The progress of the reac-
tion was monitored by thin layer chromatography (TLC). The first method is much
quicker, however its yield is quite poor (24%). This is why the second method, of 47%
yield, is preferred.
All the final liquid crystal compounds were purified by means of column chro-
matography with silica gel filling (Merck, 60, 0.063 ꢁ 0.2 mm) and methyl dichloride

´
J. Chrusciel et al.
254=[492]
Figure 2. General synthetic pathway used for the preparation of banana-shaped nOSOR
mesogens.
as eluent. Crude nOSORs were crystallized from mixture: ethyl alcohol and acetate–
ethyl until constant melting points were achieved. At last, their structure was
1
confirmed by spectroscopic methods: H NMR,13C NMR, MS and FT-IR.
2.2. Experimental Methods
Electrooptic measurements and texture observations were done using Nikon Eclipse
Polarizing Microscope LV100POL and INSTEC temperature controller. Texture
observations and studies of electro-optic switching between the planar and homeo-
tropic textures for 12OSOR were done at LEMCEL using Olympus Polarizing
Microscope BX60 and LINKAM temperature controller. To observe textures in
electro-optic cell under high fields a wave form generator Agilent 33120A and
F20A amplifier were employed.
Texture observations allowed us to identify phases. 9OSOR compound shows
ferroelectric B1 phase, however, 12OSOR, and 14OSOR show antiferroelectric B2
phase. Using thin AWAT cells with ITO electrodes it was possible to observe
electro-optic switching between quasi-planar and quasi-homeotropic textures for
B2 phase of 12OSOR compounds. In Figure 3 one can see a transition between these
two states.
In Figure 3(a) one can see a characteristic texture of extinction crosses observed
under triangular electric field. A transition from quasi-planar (bright state,
Figure 3(a)), in which bent-core molecules ‘‘are parallel’’ to electrode’s surface and
their smectic planes are rolled up in onion-like circular cylinders [5,6], to the dark

Dielectric Spectroscopy of Bent-Core Thioesters’
255=[493]
Figure 3. Switching between planar and homeotropic texture of 12OSOR’s B2 phase, (a)
Quasi-planar texture of the B2 phase in thin 1.7 mm AWAT cell (T ¼ 112ꢀC), (b)
Quasi-homeotropic texture of B2 phase under electric field of 13 Vp-p=mm (T ¼ 112ꢀC). (Figure
appears in color online.)
state (quasi-homeotropic, Figure 3(b)). One should add that at a certain voltages of
triangular wave a completely dark state shows up in which the secondary optical axis
is normal to the electrodes. The character of the texture changes under triangular
electric field suggests that 12OSOR exhibits an anticlinic, antiferroelectric phase
(SmCaPA).
The measurements of spontaneous polarization were done by reversal current
method using Agilent 33120A wave form generator and F20A amplifier. Both the
driving voltage and the response current spectra of the samples were recorded on
the Agilent DSO6102A digital scope and transferred by LAN connection to the
computer. Temperature of the sample was controlled by means of INSTEC hot stage
driven by the computer.
Dielectric studies have been performed by using a dielectric spectrometer based
on Agilent 4294A impedance analyzer controlled by PC. The sample was put into the
capacitor consisting of two glass plates covered with gold layers separated by 5 mm
spacers (AWAT HG cells). The cell was calibrated by using standard liquids. The
substances being in isotropic phase were introduced into cells by means of
capillary effect.

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J. Chrusciel et al.
256=[494]
3. Results and Discussion
Spontaneous polarization measurements confirmed, that the B1 phase of 9OSOR is a
ferroelectric one – a single peak was observed (Fig. 4(a)) – and B2 phase of 12OSOR,
and 14OSOR exhibits antiferroelectric phase – two well separated peaks were
detected as a response current (Fig. 4(b)). Polarization for phase B1 of 9OSOR is
rather small and it’s temperature dependence is unusual for ferroelectric liquid crys-
tals – it increases with temperature (Fig. 5(a)). For B2 phase of 14OSOR spon-
taneous polarization is very high (Fig. 5(b)) – it is close to 600 nC=cm2 and
slightly smaller than for 12OSOR [4].
Dielectric measurements were done using dielectric spectrometer based on
Agilent 4294A impedance analyzer. The dielectric spectra measured for B1 phase
of 9OSOR without and with bias voltage show only one Debye-type dielectric relax-
ation process (Fig. 6(a)) connected with molecule reorientation around the short
axis. For B2 phase of 12OSOR, and 14OSOR dielectric spectra measured with bias
voltage show two well separated relaxation processes (Fig. 6(b)).
Figure 4. Reversal current spectra obtained for B1 (a) and B2 (b) phases. (Figure appears in
color online.)

Dielectric Spectroscopy of Bent-Core Thioesters’
257=[495]
Figure 5. Spontaneous polarization vs. temperature obtained for (a) ferroelectric B1 phase of
9OSOR – E ¼ 42.4 Vp-p=mm, f ¼ 5 Hz, d ¼ 1.7 mm and for (b) antiferroelectric B2 phase of
14OSOR compound – E ¼ 25 Vp-p=mm, f ¼ 15 Hz, d ¼ 3.2 mm.
The following complex function [10,11] was fit to the experimental points
measured without bias field:
eð0Þ ꢁ eð1Þ
1ꢁa ꢁ
iA
B
eꢂðnÞ ¼ eð1Þ þ
þ
;
ð1Þ
e0nM nN
1 þ ði2pnsÞ
where e(1) is a high frequency electric permittivity, e0 – electric permittivity of the
free space, e(0) – static electric permittivity, s – relaxation time, a – distribution para-
meter of relaxation times (0 ꢃ a ꢃ 1), r(n) – ionic conductivity, n is a frequency and
rðnÞ
A, B, M, N – phenomenological fitting parameters, where A ¼
M : Solid lines in
ð2pÞ
Figure 6(a) and (b) are theoretical fit curves of Eq. (1) to the experimental points.
To the experimental points obtained under the bias field (Figure 6(b)) the fol-
lowing complex fitting function was used:
e1ð0Þ ꢁ eð1Þ
e1ð0Þ ꢁ e1ð0Þ
;
eꢂðnÞ ¼ eð1Þ þ
ꢁ
ð2Þ
1ꢁa2
1ꢁa1
1 þ ði2pns2Þ
1 þ ði2pns1Þ

´
J. Chrusciel et al.
258=[496]
Figure 6. Dispersion and absorption curves for 9OSOR (a) and Cole-Cole plot (b) of the
dielectric spectra acquired for 12OSOR.
where e(1) is a high frequency electric permittivity, e0 – electric permittivity of the
free space, e(0) – static electric permittivity, e1(0) – quasi-static electric permittivity,
s1 and s2 – are relaxation times, a1 and a2 – distribution parameters of relaxation
times (0 ꢃ ai ꢃ 1) and n is a frequency.
The dielectric spectra were measured in the frequency range from 40 Hz to
10 MHz. The dielectric spectra acquired incorporate about 80 points pear decade.
In low frequency range the relaxation process is connected with fluctuations of
domains. In high frequency range – it is connected with reorientation around the
long axis and it is also observed without bias field. The value of activation energy
obtained for this process (Fig. 7(b)) substantiates such interpretation. For 12OSOR
the dielectric relaxation process observed without bias field is at low frequencies
influenced by ionic conductivity. The fitting parameters are gathered in Table 1.
The low frequency range the relaxation process (1) is connected with fluctua-
tions of ferroelectric domains. It is a low frequency process which is merged with
conductivity contribution in the absence of the bias field. After application of the
bias field process (1) is clearly seen (Fig. 6(b)). However, it displays a large value
(0.42) of the distribution parameter (Table 1) which makes the Cole-Cole plot far
from being a semi-circle. Similar results have been obtained for 14OSOR (Table 2).
It was found that all materials studied exhibit unusual frequency dependence of
e0(n) and e00(n) at low frequencies. Dielectric measurements show also that at low fre-
quencies there is an electrode polarization contribution to the dielectric permittivity
and the electrical ionic conductivity contribution to the dielectric loss for all materi-
als studied. However, the electric conductivity of these banana-shaped thioesters is

Dielectric Spectroscopy of Bent-Core Thioesters’
259=[497]
Figure 7. (a) Arrhenius plot for the dielectric relaxation process for 9OSOR B1 phase observed
with bias voltage equal to 5 V, (b) Arrhenius plots for the dielectric relaxation processes for
12OSOR B2 phase observed with bias voltage equal to 15 V.
distinctly lower than for other bent-core compounds studied before [12]. The M
parameter for all materials studied shows the difference between calamitic LCs
and B phases. One should notice that under bias field M ¼ 1 for B2 phases of both
Table 1. Fitting parameters for three relaxation processes shown in Figure 6(b) for
B2 phase of 12OSOR
T ¼ 101.25ꢀC
12OSOR
UB [V]
Proces
e (0)
s [s]
a
e (1)
18
4.1
3.7
r [S=m]
M
UB ¼ 15
–––1
–––2
–––
23.5
18
13.4
1 ꢄ E-4
4 ꢄ E-7
4 ꢄ E-7
0.42
0.01
0.12
0
1
1
0.6
6 ꢄ E-11
UB ¼ 0
3.9 ꢄ E-9

´
J. Chrusciel et al.
260=[498]
Table 2. Fitting parameters for three relaxation processes for B2 phase of 14OSOR
T ¼ 100ꢀC
14OSOR
UB [V]
Proces
e (0)
s [s]
a
e (1)
r [S=m]
M
UB ¼ 10
–––1
–––2
–––
29.3
19.4
15.7
1.3 ꢄ E-4
6 ꢄ E-7
0.48
0.06
0.07
19.4
3.9
3.9
0
1
1
0.6
3 ꢄ E-8
UB ¼ 0
5.9 ꢄ E-7
3.8 ꢄ E-9
12OSOR (Table 1) and 14OSOR (Table 2) as it has been found for nematic and
orthogonal smectic phases [13] without applying any bias field.
4. Conclusions
The dielectric spectra measured for B1 phase of 9OSOR with and without bias volt-
age showed only one dielectric relaxation process connected with molecular reorien-
tation around the short axis. However, for B2 phase of 12OSOR, and 14OSOR the
dielectric spectra measured with bias voltage show two well separated relaxation pro-
cesses. In low frequency range the relaxation process is connected with fluctuations
of domains. In high frequency range – it is connected with reorientation around the
long axis and it is also observed without bias field. For 12OSOR the dielectric relax-
ation process observed without bias field is complex: in the high temperature range
of B2 phase it is connected with molecular reorientation around the short molecular
axis, whereas in the low temperature range – around the long axis.
Dielectric measurements showed also that at low frequencies there is an elec-
trode polarization contribution to the dielectric permittivity and the electrical ionic
conductivity contribution to the dielectric loss for all materials studied. The electric
conductivity of these banana-shaped thioesters is distinctly lower than for other simi-
lar compounds. The M parameter for all materials studied shows the difference
between calamitic LCs and B phases.
Acknowledgment
Financial support of Polish Ministry of Science and Higher Education in scope of
Grant NN202 076435 is gratefully acknowledged.
The research was carried out with the equipment purchased thanks to the
financial support of the European Regional Development Fund in the framework
of the Polish Innovation Economy Operational Program (contract no. POIG.02.01.
00-12-023=08).
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