First symmetrical banana compounds exhibiting SmAPR mesophase and unique transition between two orthogonal polar phases

Article abstract of DOI:10.1039/b912150a

Novel banana-shaped liquid crystals with stilbene bridges were synthesized. All presented materials exhibit the polarization-randomized non-tilted smectic phase (SmAP_R) in a broad temperature range and the antiferroelectric non-tilted smectic p

Full text of DOI:10.1039/b912150a

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First symmetrical banana compounds exhibiting SmAPR mesophase and
unique transition between two orthogonal polar phasesw
Kinga Gomola,a Lingfeng Guo,a Ewa Gorecka,b Damian Pociecha,b Jozef Mieczkowski,b
Ken Ishikawaa and Hideo Takezoea
Received (in Cambridge, UK) 19th June 2009, Accepted 22nd August 2009
First published as an Advance Article on the web 14th September 2009
DOI: 10.1039/b912150a
Novel banana-shaped liquid crystals with stilbene bridges were
synthesized. All presented materials exhibit the polarization-
randomized non-tilted smectic phase (SmAPR) in a broad
temperature range and the antiferroelectric non-tilted smectic
phase (SmAPA) at room temperature.
Recently we synthesized and reported some new asymmetric
compounds, which show uniaxial, orthogonal phase with
ferroelectric-like properties, SmAPR.13 Herein, we report the
first symmetric BC molecules showing the SmAPR mesophase.
In this work, we chose 1,3-dihydroxyacetophenone as a
central unit. Stilbene arms were used, similarly as in previous
compounds,13 bearing NO2 or Cl substituents. The presented
materials are interesting because: (1) they possess wide
temperature ranges of the expected mesophase, (2) a lower
temperature phase was found to be also non-tilted polar
phase. To the best of our knowledge, this is the first example
of compounds showing a direct transition between two
orthogonal phases; i.e., polar uniaxial smectic A phase to
polar biaxial smectic A phase. Only a few papers have
described the transitions between non-polar and polar smectic
A phases (SmA–SmAPA).6–8
Liquid crystalline materials possessing a bent molecular shape,
so called banana-shaped LC, are fascinating in view of their
academic research1,2 and their potential applications.3 Mainly
because of the shape, smectic and columnar phases formed by
these molecules exhibit polar order which lead to antiferro-
electric and ferroelectric switching.4 The B2 phase is the most
common and the most widely studied phase in bent-core (BC)
liquid crystals (BC-LCs). This tilted lamellar polar phase can
appear in four different supramolecular packing arrangements,
two of them form chiral conglomerates (SmCaPA, SmCsPF),
and two of them appear as racemates (SmCsPA, SmCaPF).5
Although much effort has been directed toward the synthesis
of novel banana compounds, orthogonal polar smectic-A
analogue phases have not been so often reported in the
literature.
The synthetic pathway leading to symmetrical banana-
compounds AP-n-NN-n14 with nitro groups and AP-n-ClCl-n
with chlorine substituents is presented in Scheme 1, where AP
The first example of non-tilted polar biaxial smectic A phase
(SmAPA) was reported by Eremin6 for bent-core compounds
possessing imine linkers in the arms and a cyano (CN) group
in the central core. Another example of the biaxial polar SmA
phase was reported by several groups; i.e., asymmetric
BC-LCs with stilbene bridges in the arms,7 asymmetric BC-LCs,
in which one of the terminal chains are replaced by a highly
polar cyano group,8 and BC-LCs with cyclic ureas as a central
unit.9 Usually, polar order within the layer leads to phase
biaxiality. However, Pociecha et al.10 reported an example of a
polar uniaxial SmA-like phase—SmAPR, i.e., polarization-
randomized nontilted smectic phase. The important technological
application of the SmAPR phase for a fast switching display
device has been described previously and was confirmed to be
a promising candidate for a new-generation liquid crystal
display.11,12 For such practical application, extensive syntheses
of new materials which exhibit SmAPR with wider and lower
temperature ranges is an urgent issue.
a Department of Organic and Polymeric Materials, Tokyo Institute of
Technology, 2-12-1-S8-42, O-okayama, Meguro-ku,
Tokyo 152-8552, Japan. E-mail: takezoe.h.aa@m.titech.ac.jp;
Fax: +81-3-5734-2876; Tel: +81-3-5734-2436
Scheme
1 Synthetic route for the preparation symmetric and
b Chemistry Department, Warsaw University, Al. Zwirki i Wigury
101, 02-089 Warsaw, Poland. E-mail: gorecka@chem.uw.edu.pl;
Fax: +48-22-8221075; Tel: +48-22-8221075
asymmetric bent-core molecules. Reagents and conditions: (i) TEA,
THF, DMAP, room temp. 4–5 h, (ii) 4-{(E)-2-[3-chloro-4-(tetra-
decyloxy)phenyl]ethenyl}benzoic acid chloride, TEA, THF, DMAP,
reflux 8 h; X = NO2, Cl; n = 14, 16, 18, 20.
w Electronic supplementary information (ESI) available: Experimental
and analytical data. See DOI: 10.1039/b912150a
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6592 | Chem. Commun., 2009, 6592–6594

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stands for central unit – acetophenone. Appropriate benzoic
acid chlorides 2, which were obtained previously,13 were
subjected to esterification reaction with 1,3-dihydroxy-
acetophenone (1) in dry THF in the presence of triethylamine
and DMAP as a catalyst. The usage of large excess of acid
chlorides allowed synthesis of symmetrical banana compounds
as the main products (B40% average yield) possible,
while monosubstituted central units were obtained as minor
products. Monosubstituted central unit 3 was used in the
synthesis of one asymmetrical analogue AP-14-NCl-14,
possessing both nitro group and chlorine atom in the arms.
After refluxing of compound 3 in THF for 8 h, in a presence of
triethylamine, AP-14-NCl-14 was obtained with 30% yield.
All final compounds were purified by column chromatography
using toluene or toluene–ethyl acetate as eluents and single or
double recrystallizations from ethyl acetate.
Fig.
1
Photomicrographs in the SmAPR phase of compound
AP-18-NN-18 at 100 1C: homeotropic texture of the SmAPR phase
(a) and homeotropic texture of the SmAPA phase (b). The sample
thickness was 5.2 mm.
characteristic of the SmA phase (Fig. 2(a)). During cooling the
texture was retained with only birefringence change (Fig. 2(c).
The application of an electric field also gave slight birefringence
increase both in the SmAPR and SmAPA phases (Fig. 2(b) and
(d)). A switching current measurement was also made. Under
the application of a triangular wave voltage to AP-18-NN-18
in the SmAPR phase (97 1C), one broad polarization switching
current peak for each half cycle was obtained as shown
in Fig. 3(a). The broad and single switching current peak
indicates field-induced alignment of dipoles from random
The mesomorphic behaviour of compounds AP-n-NN-n
and AP-14-NCl-14 was investigated by polarizing optical
microscopy (POM), differential scanning calorimetry (DSC-7
Perkin Elmer), X-ray diffraction, and electrooptical measure-
ments. The mesophases and transition temperatures of the
synthesized compounds are summarized in Table 1. All the
compounds exhibited LC phases, and the mesophase stability
increased with elongation of terminal chains. We should note
that the transition enthalpy at the SmAPR–SmAPA transition
was quite small and was even hard to be detected, particularly
in AP-20-NN-20 (see ESIw). The introduction of an
asymmetric substituent leads to broader mesophase ranges at
lower temperatures, as noticed by comparing AP-14-NN-14
with the asymmetric analogue AP-14-NCl-14. Fig. 1 and 2
reveal optical photomicrographs of homeotropic and homo-
geneous cells in SmAPR and SmAPA phases. In homeo-
tropically aligned samples of AP-18-NN-18, the high-temperature
phase exhibited a uniform dark texture with some minor
defects, indicating optical uniaxiality of the phase (Fig. 1(a)).
Then the transition to the low-temperature phase was
recognized as the appearance of grain-like textures, as shown
in Fig. 1(b).
Fig.
2 Photomicrographs in the SmAPR phase of compound
The occurrence of grain texture in the homeotropically-
aligned sample indicates a transition to a biaxial smectic
phase at 87 1C. When the compound was introduced in
homogeneous cells, the sample gave the fan-shaped texture
AP-18-NN-18 at 100 1C: homogeneous cell at (a) E = 0 V and
(b) E = 140 Vpp. Photomicrographs in the SmAPA phase at 84 1C:
homogeneous cell at (c) E = 0 V and (d) E = 140 Vpp. The sample
thickness was 5.2 mm.
Table 1 Phase transition temperatures (1C) and corresponding enthalpies (J gÀ1) for compounds of AP-n-NN-n; AP-14-NCl-14 and AP-14-ClCl-
14. Numbers at the both sides of the sample names stand for the numbers of carbons in both end chains. AP stands for the central
unit—acetophenone; N and Cl stand for substituents NO2 and Cl at the left and right wings, respectively
Code
X
n
Cry
SmAPA
SmAPR
Iso
DT (SmAPR)
AP-14-NN-14
NO2
14
ꢁ
105.23
[0.03]
94.10
[0.20]
88.97
[0.18]
84a
ꢁ
118.70
[5.70]
130.10
[4.73]
136.03
[3.67]
134.70
[2.85]
113.77
[3.78]
ꢁ
13.44
AP-16-NN-16
AP-18-NN-18
AP-20-NN-20
AP-14-NCl-14
AP-14-ClCl-14
NO2
NO2
NO2
NO2/Cl
Cl
16
18
20
14
14
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
36.00
44.30
50.70
16.77
—
97a
ꢁ
79.10
[1.33]
a
Determined by texture observation, since corresponding DSC peaks are so small (see ESIw).
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X-ray pattern did not change after cooling to room temperature.
The layer spacing continuously increases with decreasing
temperature in both phases (Fig. 4(c)), being almost equal to
the estimated molecular length (61 A in optimized conformation)
in the SmAPA phase. It is quite reasonable to conclude that
both phases are non-tilted phases.
In summary, we reported herein the synthesis of the first
symmetric bent-core liquid crystalline materials exhibiting the
SmAPR phase with a broader temperature range comparing to
previously reported unsymmetrical compounds. Additionally,
we observed for the first time a new transition from a uniaxial
non-tilted polar phase (SmAPR) to a biaxial non-tilted polar
phase (SmAPA).
Fig. 3 Switching current response obtained under a triangular wave
voltage (a) in the SmAPR phase 97 1C (140 Vpp, 6 Hz) and (b) in
the SmAPA phase at 87 1C (140 Vpp, 0.2 Hz). Sample thickness was
5.2 mm.
Notes and references
1 H. Takezoe and Y. Takanishi, Jpn. J. Appl. Phys., 2006, 45, 597.
2 R. Amaranatha Reddy and C. Tschierske, J. Mater. Chem., 2006,
16, 907.
3 J. Etxebarria and M. Blanca Ros, J. Mater. Chem., 2008, 18, 2919.
4 T. Niori, T. Sekine, J. Watanabe, T. Furukawa and H. Takezoe,
J. Mater. Chem., 1996, 6, 1231.
5 D. R. Link, G. Natale, R. Shao, J. E. Maclennan, N. A. Clark,
E. Korblova and D. M. Walba, Science, 1997, 278, 1924.
¨
6 A. Eremin, S. Diele, G. Pelzl, H. Nadasi, W. Weissflog,
J. Salfetnikolva and H. Kresse, Phys. Rev. E: Stat., Nonlinear,
Soft Matter Phys., 2001, 64, 051707.
7 D. Pociecha, E. Gorecka, M. Cepic, N. Vaupotic, K. Gomola and
J. Mieczkowski, Phys. Rev. E: Stat., Nonlinear, Soft Matter Phys.,
2005, 72, 060701.
8 R. Amaranatha Reddy and B. K. Sadashiva, J. Mater. Chem.,
2004, 14, 310.
Fig. 4 X-Ray patterns of compound AP-18-NN-18: (a) SmAPR at
140 1C, (b) SmAPA phase at 30 1C. Slight up/down asymmetry in the
pattern is due to shadowing by sample heating stage. (c) Temperature
dependence of the layer spacing d.
9 B. Glettner, S. Hein, R. Amaranatha Reddy, U. Baumeister and
C. Tschierske, Chem. Commun., 2007, 2596.
10 D. Pociecha, M. Cepic, E. Gorecka and J. Mieczkowski, Phys. Rev.
Lett., 2003, 91, 185501.
11 Y. Shimbo, E. Gorecka, D. Pociecha, F. Araoka, M. Goto,
Y. Takanishi, K. Ishikawa, J. Mieczkowski, K. Gomola and
H. Takezoe, Phys. Rev. Lett., 2006, 97, 113901.
12 Y. Shimbo, Y. Takanishi, K. Ishikawa, E. Gorecka, D. Pociecha,
J. Mieczkowski, K. Gomola and H. Takezoe, Jpn. J. Appl. Phys.,
2006, 45, L282.
13 K. Gomola, L. Guo, S. Dhara, Y. Shimbo, E. Gorecka,
D. Pociecha, J. Mieczkowski and H. Takezoe, J. Mater. Chem.,
2009, 19, 4240.
14 Typical procedure for the synthesis of AP-14-NN-14: To a solution
of 2,6-dihydroxyacetophenone 1 (200 mg, 1.31 mmol), triethyl-
amine (1 ml, 7.13 mmol) and DMAP (20 mg, 0.16 mmol) in
tetrahydrofuran (100 ml), 4-{(E)-2-[4-nitro-3-(tetradecyloxy)-
phenyl]ethenyl}benzoic acid chloride 2 (1.44 g, 2.89 mmol) was
added. The reaction mixture was kept under reflux for 8 h. The
final product AP-14-NN-14 was purified by column chromato-
graphy and double recrystallization from ethyl acetate. Compound
AP-14-NN-14: NMR: dH (300 MHz, CDCl3) 8.16 (d, J = 8.4 Hz,
4H, Ar), 8.03 (d, J = 2.1 Hz, 2H, Ar), 7.64 (dd, J1 = 2.1 Hz,
J2 = 8.7 Hz, 2H, Ar), 7.55–7.52 (m, 5H, Ar), 7.25–7.07 (m, 8H,
CHQCH, Ar), 4.14 (t, J = 6.6 Hz, 4H, OCH2CH3), 2.49 (s, 3H,
COCH3), 1.85–1.79 (m, 4H, OCH2CH2), 1.55–1.21 (m, 44H, CH2),
0.88 (t, J = 6.9 Hz, 6H, CH2CH3); dC (75 MHz, CDCl3) 198.41,
164.20, 152.29, 148.01, 142.31, 140.03, 132.11, 130.87, 129.13,
129.08, 128.39, 127.80, 127.58, 126.62, 123.48, 120.62, 114.66,
69.88, 31.91, 31.40, 29.64, 29.57, 29.50, 29.35, 29.27, 28.89, 25.80,
22.69, 14.12; IR (KBr): nmax/cmÀ1, 2921, 2851, 1738, 1603, 1529,
1457, 1352, 1261, 1223, 1177, 1077; Elemental analysis: calc. (%)
for C66H82N2O11 (1079.36): C, 73.44, H 7.66, N, 2.60, found: C,
73.13, H, 7.63, N, 2.60.
orientation through the Langevin process,11,12 and is
characteristic of the switching process in SmAPR.10 The
spontaneous polarization Ps obtained by current peak was
B170 nC cmÀ2 at 92 1C. The low-temperature phase of
AP-18-NN-18 showed antiferroelectric switching behaviour
(Ps B 300 nC cmÀ2 at T = 87 1C). In addition, no rotation
of the extinction crosses could be seen during the switching.
Hence we can conclude that the low-temperature phase is the
non-tilted SmAPA phase. No changes in the texture were
observed on cooling down to room temperatures, suggesting
that the phase remained or froze at room temperature.
In order to convince the layer structure of the phases, X-ray
analysis using the X-ray beam almost parallel to the substrate
surface was also made using homeotropically aligned samples
of AP-18-NN-18. As shown in Fig. 4(a), diffraction peaks were
observed along the meridian line in SmAPR, and were assigned
to the reflection due to smectic layers parallel to the substrate
surface. Diffuse scattering corresponding to the distance 4.5 A
appeared along the equatorial line, signifying the liquid-like
non-tilted arrangement of molecules within each layer. The
low-temperature SmAPA phase exhibited qualitatively the
same pattern except for the appearance of higher harmonics
of the signal coming from layer periodicity indicating an
increase of the smectic order parameter (Fig. 4(b)). The
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This journal is The Royal Society of Chemistry 2009
6594 | Chem. Commun., 2009, 6592–6594