Polar switching in a crystalline modification of a five-ring bent-core compound

Article abstract of DOI:10.1039/b601454b

A new five-ring banana-shaped compound is presented which does not exhibit a mesophase but a high-temperature crystalline phase with unexpected electro-optical properties. By application of a sufficiently high electric field (15 V m^-1) this sol

Full text of DOI:10.1039/b601454b

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Polar switching in a crystalline modification of a five-ring bent-core
compound{
Gerhard Pelzl, H. N. Shreenivasa Murthy, Martin W. Schro¨der, Siegmar Diele, Zinaida Vakhovskaya,
Horst Kresse and Wolfgang Weissflog*
Received 30th January 2006, Accepted 2nd March 2006
First published as an Advance Article on the web 22nd March 2006
DOI: 10.1039/b601454b
A new five-ring banana-shaped compound is presented which does not exhibit a mesophase but a
high-temperature crystalline phase with unexpected electro-optical properties. By application of a
sufficiently high electric field (15 V mm21) this solid modification can be clearly switched where
the texture of the switched state is different for an opposite sign of the applied electric field. This
switching is accompanied by a current response which indicates an antiferroelectric ground state.
The switching polarisation was found to be 700 nC cm22. It is interesting that the polar switching
takes place only below a frequency of 5 Hz. The crystalline nature of the solid modification was
proved by X-ray diffraction measurements on partially oriented samples. The mechanism of the
polar switching is not yet clear. It can be assumed that the switchable modification has a structure
with sufficient free volume which enables the collective rotation of at least parts of the molecules.
Herein we present a novel bent-core compound which forms
Introduction
a crystalline phase with polar switching. This polar switching
Polar crystals have at least one polar axis along which a
in the crystalline modification of a bent-core compound is
macroscopic spontaneous polarization occurs. Ferroelectric
being reported for the first time.
crystals are the part of a variety of polar crystals in which the
direction of the spontaneous polarization can be reversed by
Results
applying an electric field. Ferroelectric (or antiferroelectric)
crystals are of great interest for applications in electronics and
optoelectronics (materials for capacitors, light modulators,
light frequency converters, etc.).1 The majority of ferroelectric
crystals are inorganic compounds. But among ferroelectrics
there are also organic crystals (e.g. triglycine sulfate,2
thiourea,2 tanane,3 co-crystals of phenazine and chloraniline
acid4) as well as crystalline polymers5 such as copolymers of
vinylidene fluoride–trifluoroethylene. Ferroelectric and anti-
ferroelectric properties can be also realized in liquid crystals,
for example in smectic phases with a tilted arrangement of
chiral calamitic molecules.6–8 In 1996, it was found that achiral
banana-shaped molecules can be packed in a polar fashion due
to restricted rotation of the molecules about their long axes
within the smectic layers. This leads to a long-range correlation
of the lateral dipoles and to a spontaneous polarization, which
is usually parallel to the layers.9–12 To the best of our
knowledge only two examples were reported in the literature
of a mesogenic compound forming a crystalline modification
which not only has a polar structure (with pyro- and
piezoelectric properties) but also shows a ferroelectric switch-
ing.13 The spontaneous polarization values were found to be
about 25 nC cm22 and 45 nC cm22, respectively.
The five-ring compound under study is derived from
2-methylresorcinol having ester linkage groups between the
aromatic rings and n-hexadecyloxy chains as terminal moieties
(Fig. 1).
It is seen from the DSC curves in Fig. 2 that two phases can
be distinguished and that there are no significant differences
for different heating or cooling rates. X-Ray investigations
give clear evidence for the crystalline nature of the low- and
high-temperature phases. The X-ray powder pattern shows
several sharp reflections not only in the small angle region but
also in the middle and wide-angle regions. To check the
crystalline nature of the CrII phase in more detail we
investigated samples which were aligned by cooling a drop of
the isotropic liquid on a glass plate at different cooling rates.
The different cooling rates (10 K min21 and 0.5 K min21
)
were applied to approach the experimental conditions in the
X-ray experiment to those of the optical studies (10 K min21
Martin-Luther-Universita¨t Halle-Wittenberg, Institut fu¨r Physikalische
Chemie, Mu¨hlpforte 1, 06108 Halle (Saale), Germany.
E-mail: wolfgang.weissflog@chemie.uni-halle.de
{ Financial support by the Deutsche Forschungsgemeinschaft
(GRK 894) and the Fonds der Chemischen Industrie are greatly
acknowledged.
Fig. 1 Chemical structure and transition temperatures (uC) of the
bent-core compound. The values in parentheses refer to the related
transition enthalpies in kJ mol21
.
#This phase can be supercooled by
about 15 K.
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do not exceed 25 and 30 kJ mol21, respectively. We found that
the sample of the high-temperature phase (CrII) could be
slightly and irreversibly deformed by a strong shearing of the
bounding glass plates.
On fast cooling the isotropic liquid, the CrII phase appears
preferably as a weakly birefringent modification designated
as CrIIa (Fig. 4). If the isotropic liquid is cooled very
slowly (0.5 K min21) (Fig. 5a) or in the presence of a dc field
(20–25 V mm21) preferably a different solid modification arises
designated as CrIIb. Particularly by application of an electric
field this modification adopts an unusual and exciting optical
texture with colourful ribbons, circular and oval domains
(see Fig. 5b).
It is remarkable that above the transition temperature to the
isotropic phase (165 uC) the nucleation of this modification
within the isotropic liquid can be induced by the application of
a sufficiently high electric field. We found that in this way the
transition temperature to the isotropic liquid can be enhanced
by #2 K with a field of 30 V mm21. If the applied field is
removed the field-induced phase disappears. The field-induced
phase is retained below 165 uC on terminating the field. Since
the textures of the domains on slow cooling and under the field
are comparable we argue that the crystalline modification
under the electric field is indeed the same as the modification
obtained without field. The field-induced formation of the
CrIIb modification above the melting temperature is similar to
the field-induced formation of the ferroelectric SmC phase
(SmCPF) above the clearing temperature in some bent-core
mesogens.14,15 This effect can be interpreted by the field-
induced alignment of ferroelectric clusters which already exist
in the short-range order region of the isotropic liquid.15,16
In contrast to the weekly birefringent modification CrIIa,
the modification CrIIb shows a clear electro-optical response.
Above a critical field (15 V mm21) and at frequencies ,5 Hz a
polar switching is visible where the texture of the switched
state depends on the polarity of the field. As seen in Fig. 6a
and b, the colour of the circular domains changes immediately
from yellow to blue and vice versa by the reversal of the
polarity of the applied field. The texture of the switched
state relaxes to a pink texture after a sufficiently long time
(5–10 min), i.e. the colour of the circular domain in the
switched state is different from that of the zero field (Fig. 6c).
The polar character of the switching is confirmed by the
current response using the triangular-wave voltage method.17
We found only one polarization current peak per half period
of the triangular voltage (Fig. 7) which points to a ferroelectric
switching of the CrIIb phase. The single polarization current
peak was observed even at low frequencies (,0.5 Hz) and also
in the modified triangular-wave method in which there will be
ample time for the molecules to relax to the ground state
structure. But because the texture of the switched state is very
slowly changed after removing the field we believe that the
ground state structure is antiferroelectric. In this context it is
important to note that above 165 uC (until about 167 uC) the
application of a triangular voltage (,5 Hz) gives the same
current response, which indicates that the field-induced
modifications above and below 165 uC are identical.
Fig. 2 DSC scans at different heating and cooling rates: (a) 1 K min21
and (b) 10 K min21
.
and 1 K min21) and DSC measurements (10 K min21 and
1 K min21). In one case, on fast cooling from the isotropic
phase a pattern of a well-developed mono-domain could be
obtained which clearly demonstrates the high crystallinity of
the sample (Fig. 3a).
In all other cases patterns similar to the pattern shown in
Fig. 3b were obtained which were not essentially changed,
neither by annealing nor by cooling the sample. The pattern
again displays a lot of strong reflections in the small- as well in
the wide-angle region, which characterize the phase under
discussion as crystalline in nature. In the wide-angle region—in
addition to the strong reflections—some diffuse scattering
maxima can bee seen which point to a certain amount of
disorder in the structure. The disorder is limited to short
distances and does not influence the whole structure. The
positions of the reflections suggest the assumption of a layer
˚
structure with a periodicity of 58 A.
It should be noted that the transition from the high-
temperature crystalline phase CrII into the low-temperature
phase CrI is not only indicated by a change of the optical
texture but also by the change of the X-ray scattering diagram.
On the other hand, the crystalline nature is also supported
by the high enthalpy value at the transition to the isotropic
phase (58 kJ mol21) whereas the clearing temperatures of the
B7 phase or columnar phases formed by bent-core compounds
The polar switching was observed up to 15 K below the
Iso–CrIIb transition temperature, at lower temperature the
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Fig. 3 (a) X-Ray pattern in the CrII phase of a sample which was cooled down from the isotropic state by 10 K min21 (154 uC). (b) The pattern
obtained by slow cooling, 0.5 K min21 (159 uC); (c) followed by an annealing process of 20 h (d) and the related intensity versus 2h profile.
As mentioned before the switching process in the solid
modification CrIIb can be only observed when the frequency
of the applied field is below 5 Hz. It is remarkable that by
application of a sufficiently high electric field (30 V mm21
)
below a frequency of 5 Hz the non-switchable CrIIa modifi-
cation can be completely transformed into the colourful
switchable modification CrIIb. If the frequency is increased
to >5 Hz under the same conditions the reverse transition
CrIIb A CrIIa can be realized (see Fig. 8b A c).
In order to get information about the dynamics of polar
units (molecules, parts of molecules or even aggregates)
dielectric measurements were performed on the crystalline
modification CrII. The measurements were carried out with an
alternating voltage of 1 V in a cell of 50 mm thickness (for
experimental details see ref. 18). Different experimental runs
were: the first during fast cooling from the isotropic liquid, the
second in the same way by slow cooling and the third directly
after the second on heating. In the first run no relaxation and
only a decrease of the dielectric constant is observed at the
Isotropic–CrII phase transition (Fig. 9a). Such behaviour is
typical for a true solid. But in the second and third runs a
broad and less intensive dielectric relaxation range with a
Fig. 4 Optical photomicrograph of the texture of the CrIIa modifica-
tion growing on fast cooling from the isotropic phase (10 K min21).
switching of the domains as well as the polarization current
peak disappear. The switching polarization PS obtained by
integrating the area under the current peak was found to be
700 nC cm22
.
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Fig. 5 Textures of the switchable solid modification CrIIb growing
on slow cooling of the isotropic liquid: (a) without field and (b) in the
presence of a dc field of 15 V mm21
.
relaxation frequency of about 1.5 kHz was detected (Fig. 9b).
The high frequency limit of the dielectric constant agrees well
with that seen in the first run. Assuming that the observed
dynamics is related to a slow crystallization process (two phase
range, Maxwell–Wagner relaxation), then the low frequency
limit of the dielectric function e0 should further decrease
during the heating run due to faster crystallization. This is not
observed. It should be emphasized that the relaxation
frequency and the applied voltage (1 V in a 50 mm cell) do
not agree with the frequency scale of the electro-optical
experiments. Probably the linear method is sensitive to the
dynamics of small polar groups whereas for the collective
reorientation of the ferroelectric units a considerably higher
electric field is necessary. Anyway, from the results of dielectric
measurements we can argue that the sample shows two high-
temperature modifications, one without dynamics and a
second one in which the free volume does allow a linear
response of polar units.
Fig. 6 Switching behaviour of the CrIIb phase. Texture in a circular
domain: (a) at 216 V mm21, (b) +16 V mm21, (c) at 0 V mm21 after
10 min (temperature: 160 uC; cell gap: 6 mm).
detailed structure of the CrIIb phase but it is reasonable to
assume that the polar structure and the polar switching of the
crystalline layered structure are in any way related to the polar
packing of the bent molecules as known for switchable
Now the question arises: what is the origin of the
ferroelectricity (polar properties) in the CrIIb phase? We can
exclude the displacive type as found for the inorganic1 and a
few organic crystals.2–4 Unfortunately, we do not know the
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effect can be understood if already in the isotropic liquid
ferroelectric clusters in the short range exist.15,16
2. Furthermore, the switching polarization of the CrIIb
phase was found to be of the same order of magnitude as
found for ‘‘banana mesophases’’ (SmAP, SmCP, ColP).12,19
3. The nucleation on cooling the isotropic liquid and the
optical appearance of the field-induced switching (also
its dependence on the polarity of the field) is in a sense
reminiscent of ‘‘banana phases’’.
At the present stage of research we are not able to present
a convincing model of the polar switching in the CrIIb
modification. But in this context it should be mentioned that
in some solid phases flips of the aromatic rings as well as
reorientation of elongated molecules around their para axis
are–in principle–possible.20–22 On the basis of the results of
our dielectric and electro-optical measurements we can assume
that in the CrIIb modification the collective motion of at least
bigger parts of the molecules is possible because the field-
induced change of the birefringence is relatively large. On the
other hand, the optical response of the CrIIb modification is
clearly different compared with polar ‘‘banana phases’’. We
can only speculate that the arrangement of the molecules in the
Fig. 7 Switching current response obtained in the CrIIb phase under
an applied triangular wave field of 30 V mm21 at 160 uC; frequency:
3 Hz.
‘‘banana mesophases’’.9–12 This assumption is based on several
experimental observations.
1. In resemblance to some SmCPA phases the transition of
the CrIIb phase into the isotropic liquid can be enhanced by a
few degrees by applying a sufficiently high electric field. This
Fig. 8 Frequency dependence of the transition of the switchable high-birefringent CrIIb modification into the non-switchable modification CrIIa
at a field of 30 V mm21 (temperature: 160 uC): (a) 3 Hz (CrIIb); (b) 7 Hz (after 15 s; CrIIb + CrIIa); (c) 7 Hz (after 20 s; CrIIa), and (d) again 3 Hz
(after 15 s; CrIIb).
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Linkam hotstage (THM 600/S). X-Ray diffraction measure-
ments on powder-like samples were performed with a Guinier
film camera. X-Ray studies on partially oriented samples were
carried out by means of a 2D detector (HI-Star, Siemens AG).
Partially oriented samples were obtained by cooling a drop of
the isotropic liquid on a glass plate. Electro-optical measure-
ments were carried out in commercial ITO cells (E. H. C.
Corp.; spacing: 5 mm or 6 mm). The switching polarization was
measured by means of the triangular wave voltage method.17
Dielectric measurements were performed with a Solartron
Schlumberger Impedance Analyzer SI1260 in combination
with a Chelsea interface.
The synthetic path to obtain the target compound is shown
in Scheme 1. The spectral and the analytical data for the
compound are given below.
2-Methyl-1,3-phenylene bis[4-(4-n-hexadecyloxyphenyloxy-
carbonyl)benzoate]
4-(4-n-Hexadecyloxyphenyloxycarbonyl)benzoic acid (0.5 g;
0.1 mmol) was added to a solution of 2-methylresorcinol
(0.064 g; 0.05mmol), DCC (0.23 g; 0.11 mmol) and DMAP
(20 mg) in about 20 ml of dry dichloromethane. The reaction
mixture was stirred for 24 h at room temperature. The solvent
was evaporated, and the crude product was repeatedly
recrystallised from DMF–ethanol. Yield: 0.3 g (54.0%).
1H NMR (400 MHz, CDCl3) d (ppm) = 8.32 (s, 8H, ArH),
7.33 (t, 3J = 8.0 Hz, 1H, Ar–H), 7.17 (d, 3J = 8.0 Hz, 2H,
3
4
Ar–H), 7.11 (dd, J = 9.0 Hz, J = 2.4 Hz, 4H, Ar–H), 6.93
(dd, 3J = 9.0 Hz, 4J = 2.4 Hz, 4H, Ar–H), 3.95 (t, 3J = 6.4 Hz,
4H, –OCH2), 2.12 (s, 3H, Ar–CH3), 1.80–1.74 (m, 4H, –CH2),
1.52–1.41 (m, 4H, –CH2), 1.33–1.23 (m, 48H, CH2), 0.87 (t,
Fig. 9 (a) Limits of the dielectric function obtained from the fit to a
Cole–Cole mechanism.18 The curve of the first run is related to the
CrIIa modification and the other curves correspond to the CrIIb
modification. (b) The complex dielectric function curve measured for
the CrIIb modification during heating (third run) and the fitted curve.
switchable CrIIb modification is more complicated than in
simple ‘‘banana phases’’ (possibly there is not only one polar
axis) which allows, nevertheless, a partial rotation of the
molecules or bigger parts of them on applying an electric field.
Conclusions
In summary, the search for new ferroelectric or antiferro-
electric materials is not only interesting from the basic
scientific point of view but also with respect to possible
practical application for example as materials with high NLO
coefficients. Here we presented for the first time a new variant
of crystalline organic ferroelectrics formed by bent-core mole-
cules whereby the switching mechanism seems to be in a sense
similar to that of polar ‘‘banana’’ mesophases.
Experimental
The compound was characterized by a combination of
polarized light microscopy, X-ray diffraction, dielectric and
electro-optical measurements. The transition temperatures and
transition enthalpies were determined using a Perkin-Elmer
Pyris 1 differential scanning calorimeter. The optical textures
and the field-induced texture changes were examined using a
polarizing microscope (Leitz Laborlux) equipped with a
Scheme 1 Synthetic route to prepare the bent-core compound.
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3J = 6.8 Hz, 6H, CH3); m/z: 1050 (M + 1 peak); elemental
analysis: calculated for C67H88O10 (%): C 76.39, H 8.42; found
C 76.67, H 8.24.
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J. Mater. Chem., 1996, 6, 1231.
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