The effect of position of (S)-2-octyloxy tail on the formation of frustrated blue phase and antiferroelectric phase in Schiff base liquid crystals

Article abstract of DOI:10.1039/c4sm01829j

Two series of chiral salicylaldimine-based liquid crystals which differ from each other in the position of the (S)-2-octyloxy tail have been synthesized and characterized by polarizing optical microscopy, differential scanning calorimetry, and electrical

Full text of DOI:10.1039/c4sm01829j

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The effect of position of (S)-2-octyloxy tail on the
formation of frustrated blue phase and
Cite this: DOI: 10.1039/c4sm01829j
antiferroelectric phase in Schiff base liquid crystals†
*
Chiung-Cheng Huang, Ching-Chung Hsu, Li-Wen Chen and Yu-Lun Cheng
Two series of chiral salicylaldimine-based liquid crystals which differ from each other in the position of the
(S)-2-octyloxy tail have been synthesized and characterized by polarizing optical microscopy, differential
scanning calorimetry, and electrical switching. Compounds OH I (n ¼ 6–7) having (S)-2-octyloxy tail
close to the salicylaldimine core and compounds OH II (n ¼ 6–11) having (S)-2-octyloxy tail far from the
salicylaldimine core exhibit polymorphism of mesophases including frustrated blue phase and
antiferroelectric (SmC*A) phases. Notably, as compared with structurally similar Schiff base compounds H
I (n ¼ 7), intramolecular hydrogen bonding in antiferroelectric salicylaldimine-based compounds OH I (n
¼ 7) induces the frustrated blue phase. However, as compared with structurally similar Schiff base
compounds OH II (n ¼ 8), the lack of intramolecular hydrogen bonding in Schiff base compounds H II (n
¼ 8) suppresses antiferroelectric properties.
Received 18th August 2014
Accepted 3rd October 2014
DOI: 10.1039/c4sm01829j
www.rsc.org/softmatter
amount of chiral dopant into achiral LC hosts. For synthetic
chemists, it can be achieved by incorporating optically pure
1 Introduction
groups into the geometric structures of liquid crystal mole-
cules.14,19–25 Consequently, chirality plays an important role in
BPs.26,27
Chirality and liquid crystals are both widely manifested in
nature and biology. Chiral liquid crystal materials have been
attracting signicant attention because of the discoveries of the
*
The best known smectic phases are the ferroelectric chiral
smectic C (SmC*) phase and the antiferroelectric chiral smectic
ferroelectric (SmC*), antiferroelectric (SmCA), ferrielectric
*
(SmCr), twist grain boundary phase (TGB*) and blue phases
*
C (SmCA) phase. In the SmC* phase the chiral molecules tilt
(BPs), and the potential for the application in electro-optical
devices.1 Both blue phases and twist grain boundary phases are
two kinds of frustrated structures that can be induced by chiral
groups in liquid crystals. Blue phases are usually found between
the isotropic liquid and cholesteric phase of short pitches.
According to the characteristic of double twist helical structure,
they are classied as blue phase III (BPIII), blue phase II (BPII)
and blue phase I (BPI), which appear as the temperature
decreases from the isotropic liquid phase.2,3 The packing
structures are macroscopically amorphous, simple cubic, and
body-centered cubic. Blue phases are potentially useful for fast
light modulators or tunable photonic crystals in applications.
However, they are usually observed in a very narrow tempera-
ture range in a single molecule, which limits their practical
applicability.4 Therefore, both stabilization and broadening of
the temperature range of blue phase are the most important
issues and attract signicant attention.3,5–20 There are several
different methods to broaden blue phase range.5–18 The most
popular way to induce blue phase is the addition of a sufficient
with respect to the layers normal. In contrast, the tilting direc-
*
tion of the chiral molecules in the SmCA phase alternates in a
*
zig-zag mode between the layers. The antiferroelectric (SmCA)
phase in the chiral liquid crystals exhibiting a DC threshold,
double hysteresis, and tristable switching properties28–30 has
been demonstrated to have potential in the eld technology.
And the correlation between the molecular structure of the
materials and the appearance of the antiferroelectric liquid
crystals has been primarily successfully established.31–33 It has
been generally concluded that a rod-like molecule is made up of
a three-ring core, with at least two rings that are not directly
linked, and a chiral tail with a transversal disposition con-
cerning the long molecular axis and connected to the rigid core
through a carboxylate linkage. Dipole-associated functional
groups such as ester or amido groups are the main types of
*
connectors within the cores. And the formation of the SmCA
phase in chiral liquid crystals also depends strongly on the high
chirality of the molecules.
The possibility of getting both frustrated BP and antiferro-
*
electric SmCA phase in a rod-like molecule turned out to be
Department of Chemical Engineering, Tatung University, Taipei 104, Taiwan, Republic
of China. E-mail: chchhuang@ttu.edu.tw
difficult. It is probably due to both the frustrated BP and the
nematic phase possessing long-range order of the mean orien-
tations, but lacking any long-range translational order. In
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4sm01829j
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*
*
contrast, the antiferroelectric SmCA phase demands an excep- TGBA*, SmA* and SmCA phases.45 Nevertheless, no chiral
tionally long-range of translational order along the smectic layer Schiff-based liquid crystal with rod-like molecules possessing
*
normal. Consequently, only a few single chiral rod-like mate- both frustrated BP and antiferroelectric SmCA phase has been
*
rials possessing both frustrated BP and SmCA phases have been reported until now. In addition, the correlation of chiral Schiff
reported. In 1998, Li et al. synthesized a cyclohexane-based base molecular structure with the appearance of frustrated BP
*
trimesogen, the rst oligomeric antiferroelectric liquid crystal and antiferroelectric (SmCA) phases is still not established. In
that exhibits a fairly rich polymorphism of mesophases order to shed light on the molecular factors of the resulting
including frustrated blue phase.34 Lagerwall et al. have pre- mesophases in this type of Schiff base compounds, we report
sented a chiral twin dimer with a change of the molecular two series of chiral salicylaldimine-based liquid crystals with an
conformation from the rod to bent shape on cooling, possessing (S)-2-octyloxy tail to investigate the effect of the position of the
the smectic blue phase and antiferroelectric property.35 More- (S)-2-octyloxy tail and the length of achiral alkoxy tail on the
over, Wu and Hsieh have reported two antiferroelectric rod-like mesomorphic formation. To best of our knowledge, these
molecules with two asymmetric carbons, representing another simple rod-like and chiral salicylaldimine-based liquid crystals
two frustrated phases – BP and TGBA*.36,37
that possess frustrated blue phase and antiferroelectric behav-
Schiff base (azomethine) has been widely employed as a iors have not been reported before. In addition, in order to
linking group in the synthesis of many forms of liquid assess the effect of the intramolecular hydrogen bond on
crystals.1 Its analogues, salicylaldimine[N-(2-hydroxy-4-alkox- mesomorphic behavior, corresponding Schiff base compounds
ybenzylidene) aniline] compounds are more stable to heat and H I (n ¼ 7) and H II (n ¼ 8) without hydroxy group were addi-
moisture due to the presence of intramolecular hydrogen tionally prepared. We also examine the electro-optic behaviors
bonding between the hydroxy group and the imine group. In of the materials under the appropriate applied electric eld to
addition, with their synthetic versatility and ability to coordi- prove the antiferroelectric properties of the molecules.
nate metals, they have been also employed in the synthesis of
metallomesogens.38 In general, rod-like Schiff base liquid crys-
2 Results and discussion
tals exhibited nematic and/or smectic mesophases.1,38 With the
introduction of asymmetric carbons centers in the form of a
chiral alkyl tail or present within the molecule, frustrated
mesophases and new phase sequences have been discovered in
Schiff base compounds, such as a monosubstituted ferrocene-
based Schiff base derivative tethering a (S)-2-methylbutoxy tail,
the rst chiral metallomesogen39 exhibited two frustrated pha-
ses, BP and TGBA* phases. Moreover, a Schiff base pro-meso-
genic unit could also be an electron rich moiety that links a
cholesteryl ester unit through a exible polymethylene spacer to
obtain unsymmetrical dimesogen compounds possessing frus-
trated BP and TGBA* phases.5 On the other hand, a few chiral
Schiff base liquid crystals consisting of a three-ring mesogenic
core have been employed in the design of antiferroelectric
liquid crystals. Serrano and his coworkers have synthesized
three such Schiff base liquid crystals through a carboxylate
linkage with a terminal (R)-2-octyloxy tail exhibiting SmC* and
SmA* phases and antiferroelectric behavior.40 It can be seen
that the mesophase interval of the salicylaldimine-based
compound is wider than that of its analogue, the Schiff base
molecule, on account of the rigidity of the mesogenic nucleus
and its polarizability enhanced by intramolecular hydrogen
bonding between the hydroxy group and the imine group.41,42 In
addition, the intramolecular hydrogen bonding not only forces
the molecule to arrange itself in a coplanar manner, but also
enhances the intermolecular attraction to make it easy for
molecules to array parallel and form a layer structure. This
situation was also found in another series of Schiff base
2.1 Synthesis and molecular structure characterization
The target chiral Schiff base compounds and their intermedi-
ates were synthesized as outlined in Fig. 1. The key amines, 4-
alkoxyaniline and 4-[(1S)-1-methylheptyloxy]aniline were
obtained in quantitative yields by catalytic hydrogenation of 4-
alkoxynitrobenzene and 4-[1-(1S)-methylheptyloxy]nitroben-
zene, respectively. The 4-alkoxynitrobenzene was prepared by
known etherication of 4-nitrobenzene with bromoalkane in
the presence of K2CO3 (process A). 4-[1-(1S)-Methylheptyloxy]
nitrobenzene was prepared by the Mitsunobu reaction of 4-
nitrobenzene with (R)-2-octanol (process B). The other key
intermediates, 4-formyl-3-hydroxyphenyl 40-[(1S)-(1-methyl-
heptyl)oxy]benzoate, 4-formyl-3-hydroxyphenyl 4-(alkyloxy)
benzoate were synthesized by the selective O-alkylation of 2,4-
dihydroxybenzaldehyde with (S)-4-(1-methylheptyloxy)benzoic
acid and 4-(alkyloxy)benzoic acid, respectively. 4-Formylphenyl
40-[(1S)-(1-methylheptyl)oxy]benzoate and 4-formylphenyl 4-(al-
kyloxy)benzoate were prepared by a similar method starting
from 4-hydroxybenzaldehyde. In the nal step, the amines and
aldehydes were reuxed in alcohol to perform Schiff base
condensation to give the target molecules. The molecular
structures of the target products and the intermediates were
conrmed by spectroscopic analysis and microanalytical data,
as described in ESI.†
2.2 Mesophase characterizations and calorimetric studies
compounds, the rst ferroelectric SmC* liquid crystal The mesophases and transition temperatures and enthalpies
DOBAMBC, reported by Meyer43 and its derivative HDOBAMBC, for all Schiff base liquid crystals were determined by differential
prepared by Takao.44
scanning calorimetry (DSC) and polarizing optical microscopy
Recently, we also reported a novel homologous series of (POM). These results are presented in Table and 2.
1
chiral salicylaldimine-based liquid crystals in connection with a Compounds OH I (n ¼ 6, 7) in which the (S)-2-octyloxy tail far
linear alkynyl chain and (S)-2-octyloxy tail possessing N*, from the salicylaldimine core exhibits fairly rich polymorphic
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*
Iso.–BPI–N*–SmA*–SmCA–Cr. When the achiral alkoxy chain
length is extended, the higher homologues OH I (n ¼ 8–12)
show the absence of the frustrated blue phase. This suggests
that extending the terminal alkoxy chain length could increase
the intermolecular interaction such that it diminishes the
formation of the blue phase in this series of Schiff base liquid
crystals.
In order to investigate the correlation of the position of (S)-2-
octyloxy tail on the formation of frustrated phases, we prepared
compounds OH II (n ¼ 6–12) in which the position of (S)-2-
octyloxy tail is closer to the salicylaldimine core. These mate-
rials also exhibited more rich polymorphic sequences than
compounds OH II (n ¼ 6–12). When slow cooling from their
isotropic phase, the BPII appeared as an iridescent platelet
texture as shown in Fig. 2b and Fig. S2 in ESI† for compound
OH II (n ¼ 8) as a representative case. On further cooling, the
BPI was conrmed by the formation of the grazed plane texture.
The colors of both BPII and BPI textures changed with the
temperatures mainly because of the variation of the helical
pitches with temperatures. Compound OH II (n ¼ 8) exhibits
chiral nematic (N*) phase with fan-liked texture, indicating the
characteristic of strongly twisted material. On further cooling,
the homeotropic (dark) texture of smectic A* phase and a petal
texture similar to compounds OH I (n ¼ 7), indicating the
Fig. 1 Chemical structures of compounds OH I (n ¼ 6–12), OH II (n ¼
6–12), H I (n ¼ 7) and H II (n ¼ 8).
sequence including frustrated blue phase (BPI) and antiferro-
*
electric SmCA phase that were determined by observation of
characteristic textural patterns. Compound OH I (n ¼ 7), on
slow cooling (0.2 ꢀC minꢁ1) from their isotropic phase, displays
the grazed platelet textural pattern as shown in Fig. 2a, indi-
cating the presence of blue phase (BPI). On further cooling, the
paramorphotic defect texture also grows; in addition and
concomitantly, the characteristic of chiral nematic (N*) thread-
like texture in which dark lines connect two s ¼ ꢂ1/2 point
defects or from closed loops appears as shown in Fig. 2a and in
full texture shown in ESI.† As the temperature continued to cool
down from the nematic (N*) phase, the focal-conic texture
appeared, indicating the characteristic of SmA* phase. On
further cooling, the red petal texture started growing and other
color petal textures appeared as temperature declined, as shown
*
characteristic of antiferroelectric SmCA phase are shown in
*
Fig. 2b. Antiferroelectric SmCA phases for compounds OH I and
OH II were further characterized by the Schlieren texture. Two
brush and a few of four brush singularities can be observed in
the Schlieren texture for compound OH I (n ¼ 7) as shown in
Fig. 3a, and mostly two brush singularities for compound OH II
(n ¼ 9) as shown in Fig. 3b.36,48,49
*
in Fig. 2a. The antiferroelectric SmCA phase appeared as a petal
In addition, in order to study the effect of the intramolecular
hydrogen bonding between the hydroxyl group and the imine
texture and the colors of texture changed with temperature
caused by the variation of helical pitches with changing
temperature in the visible region of the spectrum.46,47
Compounds OH I (n ¼ 6, 7) show a phase transition sequence of
*
group on the formation of blue phases and SmCA phases, we
also prepared Schiff base compounds H I (n ¼ 7) and H II (n ¼ 8)
Table 1 Mesophases, transition temperatures (ꢀC)ꢁa1nd enthalpies DH (in brackets, kJ molꢁ1) of compounds OH I (n ¼ 6–12) recorded from DSC
thermograms at heating/cooling rate of 3 ꢀC min
*
Compound
Iso
BPI
c
N*
SmA*
SmCA
Cr.a
Mpb
OH I (n ¼ 6)
OH I (n ¼ 7)
OH I (n ¼ 8)
OH I (n ¼ 9)
OH I (n ¼ 10)
OH I (n ¼ 11)
OH I (n ¼ 12)
c
c
c
c
c
c
c
147.7
[1.1]
146.6c
c
c
c
c
c
c
c
90.1
[0.6]
95.1
c
c
c
c
c
c
c
70.0c
c
c
c
c
c
c
c
58.8
[19.6]
60.0
[28.9]
62.5
[19.6]
55.6
[11.4]
46.7
[20.3]
42.7
[18.4]
44.2
[19.8]
c
c
c
c
c
c
c
89.9
[22.6]
87.2
[26.5]
92.2
[22.6]
88.4
[19.7]
80.0
[25.5]
67.9
[21.8]
67.2
[23.4]
d
e
149.6
[1.6]
c
146.0c
87.0c
d
e
[0.6]
107.6
[0.8]
113.2
[1.0]
118.6
[1.3]
121.3
[1.5]
122.6
[1.7]
149.0c
[0.75]
144.7c
[0.7]
—
—
—
—
—
91.0
e
87.0c
e
143.4c
[1.0]
83.0c
e
140.4c
[0.8]
76.0c
e
138.1c
[1.1]
76.0c
e
a
c
Cr. refers to crystal. b Mp refers to melting point taken from DSC thermograms recorded at heating rates of 3 ꢀC minꢁ1
.
The phase transition
temperature was determined by optical microscopy at cooling rate of 0.1 ꢀC minꢁ1
.
The enthalpy of BPI-N* transition was added into that of Iso-
d
BPI transition. e The enthalpy of phase transition was too small to be determined at a cooling rate of 3 ꢀC minꢁ1
.
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Table 2 Mesophases, transition temperatures (ꢀC)ꢁa1nd enthalpies DH (in brackets, kJ molꢁ1) of compounds OH II (n ¼ 6–12) recorded from DSC
thermograms at heating/cooling rate of 3 ꢀC min
Cr.a
Mpb
*
Compound
Iso
BPII
BPI
N*
SmA*
SmCA
OH II (n ¼ 6)
OH II (n ¼ 7)
OH II (n ¼ 8)
OH II (n ¼ 9)
OH II (n ¼ 10)
OH II (n ¼ 11)
OH II (n ¼ 12)
c
c
c
c
c
c
c
146.4
[0.9]
146.1
[1.0]
147.6
[0.9]
143.5
[0.9]
142.0
[0.9]
139.0
[1.3]
136.9
[1.6]
c
c
c
c
c
c
c
145.6c
c
c
c
c
c
c
c
145.2
c
c
c
c
c
c
c
83.7
[1.2]
97.0
c
c
c
c
c
c
c
76.0c
c
45.1
[19.1]
39.3
[18.7]
56.8
[22.1]
57.1
[23.3]
55.6
[23.1]
47.4
[24.7]
41.5
[23.2]
c
c
c
c
c
c
c
93.1
[28.2]
80.7
[27.2]
80.8
[23.7]
86.0
[29.3]
90.0
[28.2]
79.3
[32.2]
65.6
[26.5]
d
d
e
145.7c
145.2
84.3c
c
d
d
e
[1.9]
105.6
[1.9]
111.4
[1.9]
114.4
[2.1]
118.1
[3.2]
118.5
[2.8]
147.2c
146.6
71.6c
c
d
d
e
143.0c
142.2
68.1c
c
d
d
e
141.4c
141.1
63.1c
c
d
d
e
137.7c
137.1
48.3c
c
d
d
e
136.3c
136.1
—
d
d
a
c
Cr. refers to crystal. b Mp refers to melting point taken from DSC thermograms recorded at heating rates of 3 ꢀC minꢁ1
.
The phase transition
temperature was determined by optical microscopy at cooling rate of 0.1 ꢀC minꢁ1
.
The enthalpy of BP-N* tꢁra1nsition was added to that of Iso-BPII
d
transition. e The enthalpy of phase transition was too small to be determined at a cooling rate of 3 ꢀC min
.
Fig. 2 Microphotographic textures of the mesomorphic phases obtained from (a): OH I (n ¼ 7), (b) OH II (n ¼ 8), (c) H I (n ¼ 7), (d) H II (n ¼ 8) at
various mesogenic phase temperatures (magnification ꢃ 400).
*
that are in the absence of intramolecular hydrogen bonding and The temperature range of antiferroelectric SmCA phase for
structurally similar to compound OH I (n ¼ 7) and compound compound I (n ¼ 7) exists from 87.6 ꢀC to the crystallization
OH II (n ¼ 8), respectively (Table 3). Schiff base compound H I (n point 59.2 ꢀC, and is similar to that of compound OH I (n ¼ 7).
¼ 7) exhibits mesomorphic behavior similar to compound OH I On the other hand, we repeated DSC measurements to examine
(n ¼ 7) except that the blue phase gets eliminated, as shown in the stability of Schiff base compound H I (n ¼ 7). The DSC
Fig. 2c. This SmCA phase was further characterized by Schlieren thermograms of the 2nd and 3rd heating-cooling cycles exhibited
*
texture with mostly two-brush singularities, as shown in Fig. 3c. traces similar to those of 1st heating-cooling cycles, as shown in
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Table 3 Mesophases, transition temperatures (ꢀC) and enthalpies DH (in brackets, kJ molꢁ1) of compounds I and II recorded from DSC ther-
mograms at heating/cooling rate of 3 ꢀC minꢁ1
*
Compound
I (n ¼ 7)
Iso
BPII
—
c
N*
SmA*
SmCA
Cr.a
Mpb
c
c
138.9
[1.8]
144.4
[1.1]
c
c
90.7
[2.1]
121.0
[0.6]
c
c
87.6c
c
59.2
[33.1]
65.1
c
c
77.0
[26.9]
86.6
e
II (n ¼ 8)
143.8c
—
d
[38.3]
[37.6]
a
c
Cr. refers to crystal. b Mp refers to melting point taken from DSC thermograms recorded at heating rates of 3 ꢀC minꢁ1
. The phase transition
temperature was determined by optical microscopy at cooling rate of 1 ꢀC minꢁ1
.
The enthalpy of BPII-N* tꢁra1nsition was added to that of Iso-BPII
d
transition. e The enthalpy of phase transition was too small to be determined at a cooling rate of 3 ꢀC min
.
attraction of mesogenic nucleus to make it easy for molecules to
array parallel and form a layer structure. These tendencies are
predictable and similar to mesogenic molecules derived from
salicylaldimines, such as the previously mentioned ferroelectric
SmC* liquid crystal HDOBAMBC 45 as well as antiferroelectric
salicylaldimine-based liquid crystals reported by Serrano.42
These salicylaldimine-based liquid crystals and each of their
analogues exhibit similar mesomorphic behaviors. In addition,
recently, Nandiraju has also reported that cholesterol-based
dimeric Schiff base liquid crystal KI-4 exhibited frustrated blue
phases, N*, TGBC* and SmC* phases.50 However, the ortho
hydroxyl group in the salicylaldimine moiety of the homologue
KI-5(OH) suppressed the formation of a blue phase and
widened the temperature range of the SmA* phase. Interest-
ingly, in our case, the presence of intramolecular hydrogen
bonding in salicylaldimine-based compounds OH I (n ¼ 6, 7)
with (S)-2-octyloxy tail far from the salicylaldimine core further
induces the formation of frustrated uid structure, BPI.
However, when the position of the (S)-2-octyloxy tail is closer to
the salicylaldimine core, the presence of intramolecular
hydrogen bonding in salicylaldimine-based compounds OH II
(n ¼ 6–12) further induces the formation of frustrated uid
Fig. 3 Microphotomorphic textures: (a) Schlieren texture displaying
both two- and four-brush singularities of the SmC*A phase obtained
from compound OH I (n ¼ 7) at 82.2 ꢀC. (b) Schlieren texture displaying
mostly two-brush singularities of the SmC*A phase obtained from
compound H I (n ¼ 7) at 66.4 ꢀC. (c) Schlieren texture displaying mostly
two-brush singularities of the SmC*A phase obtained from compound
OH II (n ¼ 9) at 60.5 ꢀC (magnification ꢃ 400).
*
structure, antiferroelectric SmCA phase. This means that the
two types of Schiff base molecular structures in conjunction
with the (S)-2-octyloxy tail generates high chirality. The plot of
temperature vs. the length of achiral alkoxy tail (n) (Fig. 4) shows
the increasing n decreases melting point and depresses the
formation of a frustrated uid blue phase for compounds OH I
with (S)-2-octyloxy tail far from the salicylaldimine core.
However, Fig. 5 shows increasing n destabilizes the formation of
ESI.† It can be seen that Schiff base compound H I (n ¼ 7) is
stable under repeated heating condition. However, Schiff base
compound H II (n ¼ 8) displays BPII, N* and SmA* without an
*
*
antiferroelectric SmCA phase. Comparison of mesomorphic
antiferroelectric SmCA phase for compounds OH II with (S)-2-
behaviors of two Schiff base compounds and their respective
structurally similar salicylaldimine-based compounds shows
that the mesomorphic interval increases when the rigidity and
polarity of mesogenic nucleus are enhanced by intramolecular
hydrogen bonding between the hydroxyl group and the imine
linkage. Thus salicylaldimine-based compound OH I (n ¼ 7)
displays a higher melting point and clearing point and a wider
mesophase interval (ca.10 ꢀC) as compared with compound H I
(n ¼ 7). In addition, a wider temperature range of smectic layers
can be observed for compound OH II (n ¼ 8) when compared
with compound H II (n ¼ 8). These results indicate that the
octyloxy tail close to the salicylaldimine core. Notably,
compound OH II (n ¼ 7) exhibits the widest temperature range
*
of an antiferroelectric SmCA phase (45 ꢀC) and compound OH II
*
(n ¼ 12) displays lack of the antiferroelectric SmCA phase.
Therefore, it can be seen that increasing the achiral alkoxy tail
(n) supports the formation of a layered structure to widen the
interval of smectic mesophases, especially the SmA phase, in
which molecules are parallel to the layer normal.
2.3 Electrical switching study
*
existence of intramolecular hydrogen bonding in the molecular The antiferroelectric SmCA phase was also further characterized
structure possibly enhances the rigidity and the intermolecular from the switching current and dielectric permittivity
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Fig. 4 Plot of transition temperatures vs. achiral alkoxy tail length (n) for compounds OH I (n).
Fig. 5 Plot of transition temperatures vs. achiral alkoxy tail length (n) for compounds OH II (n).
measurements. The triangular wave is the most effective among
DC, square and triangular waveforms for the alignment recovery
of antiferroelectric liquid crystal molecules.51 In addition, the
appearance of two switching current peaks means the instance
of the transmittance changes, indicating the switchings from
and to the third stable state occur at different voltages.28 We
take compound OH II (n ¼ 7) as representative material for
these measurements. Their switching current behaviors were
measured in a 5 mm homogeneously aligned cell, and the
switching proles are presented in Fig. 6. For compound OH II
(n ¼ 7), besides a broad current peak found close to the zero
eld point due to the ionic effect, it is seen that there are two
switching current peaks appearing simultaneously, located
approximately at opposite sides of the zero eld point on
Fig. 6 Switching current in a triangular electric field (50 Hz, 6 Vpp) in
the SmC*A phase at 82.0 ꢀC for compound OH II (n ¼ 7) in 5 mm
thickness of homogeneously aligned cell.
applying a triangular wave and representing the antiferro-
electric–ferroelectric and ferroelectric–ferroelectric switchings
*
in the SmCA phase, similar to that reported previously in
MHPOBC.30
In clarifying the characteristic and determining the origin of method. Fig. 7 shows texture change of compounds OH I (n ¼ 7)
*
the layer switching, microscopic observation of the texture upon by turning on and off an electric eld in the SmCA phase. Before
application of the triangular wave eld serves as a useful the application of an electric eld, ellipse–hyperbola focal
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Fig. 7 Electric-field-induced textural changes observed at 82.0 ꢀC in a 5.0 mm thickness of homogeneously aligned cell of compound OH I
(n ¼ 7).
ꢀ
conics are observed in a 5 mm homogeneously aligned cell. With transition is seen as a small peak at approximately 85 C to 87
the application of the triangular wave eld (50 Hz, 5 Vpp), the ꢀC. Since the macroscopic polarization from ferroelectric
texture changes in such a way that ellipse–hyperbola focal contribution does not exist in the antiferroelectric phase, the
conics gradually disappeared. When the eld is turned off, contribution of the Goldstone mode (the small winding and
microscopic striped pattern of the homogeneous texture unwinding motions of helical structure) cannot be detected by
appeared. This possibly suggests that dechiralization lines are means of the dielectric measurement; thus, the enhancement of
perpendicular to the layer normal and the ferroelectric domains the dielectric constant in the antiferroelectric phase is consid-
appear. This rst change of defects is irreversible. However, the ered to be the result of the antiferroelectric so mode53 and the
texture changes reversibly between Fig. 7b and c during the vibration of the azimuthal molecular motion.54 Moreover, a
subsequent switching.52 In addition, Fig. 8 and 9 also show the broad peak shown in the antiferroelectric phase possibly
electric-eld-induced textural changes of compound H II (n ¼ 7) resulted from the weaker vibration of the azimuthal molecular
and compound OH II (n ¼ 7), respectively, indicating the motion. Similar trends for other compounds such as OH I (n ¼
switching behaviors similar to compound OH I (n ¼ 7).
7) and H I (n ¼ 7) could also be observed, as shown in ESI.†
*
The temperature dependence of the dielectric constants 30 There was no existence of Goldstone relaxation in the SmCA
measured at 100 Hz in the 25 mm homogeneously aligned cell phase, further demonstrating the existence of antiferroelectric
was investigated. The represented results for compound OH II property. All of the results from switching behaviors and
(n ¼ 7) from the cooling measurements are shown in Fig. 10. dielectric permittivities are similar to our previously reported
During cooling, the dielectric constants 30 in the SmA* phase are antiferroelectric salicylaldimine-based compounds Cm-
*
small and slightly increase at the SmA* to SmCA transition. The YCBOC8* (m ¼ 6–10) having the (S)-2-octyloxy tail.
*
characteristic point corresponding to the SmA* to SmCA
Fig. 8 Electric-field-induced textural changes observed at 78.0 ꢀC in a 5.0 mm thickness of homogeneously aligned cell of compound H I (n ¼ 7).
Fig. 9 Electric-field-induced textural changes observed at 82.0 ꢀC in a 5.0 mm thickness of homogeneously aligned cell of compound OH II
(n ¼ 7).
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Fig. 10 Temperature dependence of the dielectric constant 30 for compound OH II (n ¼ 7) at 100 Hz in a cell with 25 mm thickness under 1 ꢀC
minꢁ1 cooling process.
layer structure. It can be concluded that this type of Schiff base
compounds in conjunction with an (S)-2-octyloxy tail generate
3 Conclusion
high chirality to easily exhibit a BP and an antiferroelectric
SmCA phase. More related compounds will be studied and
In this article, we have described the synthesis and thermo-
tropic properties of two series of chiral salicylaldimine-based
liquid crystals having an (S)-2-octyloxy tail with two different
positions. Two homologues have been investigated with the
correlation of the position of (S)-2-octyloxy tail on the frustrated
*
reported on the correlation of molecular structure to the
*
appearance of the blue phase and the SmCA phase in the nearest
future.
*
blue phase and antiferroelectric (SmCA) phase. In addition, the
length of achiral alkoxy tail has been varied from a hexyloxy to a
dodecyloxy tail with the intention of understanding the struc-
ture and property relationship. In general, the majority of chiral
salicylaldimine-based liquid crystals exhibit an antiferroelectric
Acknowledgements
We thank the nancial support from the National Science
Council of Taiwan (R.O.C.) through NSC99-2113-M-036-001-
MY2 and Tatung University through B103-C04-008. In addition,
we particularly thank Professor Shune-Long Wu for discussions
on antiferroelectric liquid crystals.
*
(SmCA) phase. For compound OH I (n ¼ 6–12) in which the (S)-2-
octyloxy tail is far from the salicylaldimine core, frustrated blue
*
phase destabilizes and the temperature range of SmCA phase
extends as the length of the achiral alkoxy tail (n) increases. For
compound OH II (n ¼ 6–12) with the (S)-2-octyloxy tail close to
the salicylaldimine core, all the compounds exhibit BPII and
BPI. However, increasing the length of the achiral alkoxy tail
References
1 D. Demus, J. Goodby, G. W. Gray, H.-W. Spiess and V. Vill,
Handbook of Liquid Crystals, Wiley-VCH, New York, USA.
1998.
2 P. Crooker, in Chirality in Liquid Crystals, ed. H.-S. Kitzerow
and C. Bahr, Springer, New York, 2001.
3 H. Kikuchi, Struct. Bonding, 2008, 128, 99–117.
4 H.-S. Kitzerow, ChemPhysChem, 2006, 7, 63–66.
5 C. V. Yelamaggad, I. S. Shashikala, G. Liao, D. S. Shankar
*
depresses the stability of the antiferroelectric SmCA phase. Only
chiral salicylaldimine-based compound OH II (n ¼ 12) exhibits
*
lack of the antiferroelectric SmCA phase. In addition, Schiff base
compounds H I (n ¼ 7) without a hydroxy group also exhibits an
*
antiferroelectric SmCA phase but no blue phase sequence.
However, Schiff base compounds H II (n ¼ 8) without hydroxy
group exhibits a BPII phase but no antiferroelectric property. It
can be seen that the existence of the intramolecular hydrogen
bonding in the chiral salicylaldimine-based compounds OH I
with an achiral alkoxy tail close to the salicylaldimine core could
enhance the rigidity and polarity of the mesogenic nucleus and
further induce the formation of the frustrated blue phase.
However, the antiferroelectric property of the chiral salicy-
laldimine-based compounds OH II with (S)-2-octyloxy tail close
to the salicylaldimine core could result from the intermolecular
attraction of mesogenic nucleus to array parallel and form a
´
Rao, S. K. Prasad, Q. Li and A. Jakli, Chem. Mater., 2006,
18, 6100–6102.
6 A. Yoshizawa, M. Sato and J. Rokunohe, J. Mater. Chem.,
2005, 15, 3285–3290.
7 M. Sato and A. Yoshizawa, Adv. Mater., 2007, 19, 4145–4148.
8 H. Iwamochi, T. Hirose, Y. Kogawa and A. Yoshizawa, Chem.
Lett., 2010, 39, 170–171.
9 H. Kikuchi, M. Yokota, Y. Hisakado, H. Yang and
T. Kajiyama, Nat. Mater., 2002, 1, 64–68.
Soft Matter
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
Soft Matter
10 E. Karatairi, B. Rozic, Z. Kutnjak, V. Tzitzios, G. Nounesis, 32 S. T. Lagerwall, Ferroelectric and Antiferroelectric Liquid
G. Cordoyiannis, J. Thoen, C. Glorieux and S. Kralj, Phys.
Rev. E: Stat., Nonlinear, So Matter Phys., 2010, 81, 041703.
Crystals, Wiley-VCH, Weinheim, New York, 1999.
33 M. Hird, Liq. Cryst., 2011, 38, 1467–1493.
11 W. He, G. Pan, Z. Yang, D. Zhao, G. Niu, W. Huang, X. Yuan, 34 M.-H. Li, L. Detre, P. Cluzeau, N. Isaert and H.-T. Nguyen,
J. Guo, H. Cao and H. Yang, Adv. Mater., 2009, 21, 2050–2053. Liq. Cryst., 1998, 24, 347–359.
12 H. J. Coles and M. N. Pivnenko, Nature, 2005, 436, 997–1000. 35 J. P. F. Lagerwall, F. Giesselmann, M. D. Wand and
13 M. Lee, S. T. Hur, H. Higuchi, K. Song, S. W. Choi and
D. M. Walba, Chem. Mater., 2004, 16, 3606–3615.
H. Kikuchi, J. Mater. Chem., 2010, 20, 5813–5816.
36 S.-L. Wu and W.-J. Hsieh, Chem. Mater., 1999, 11, 852–854.
14 Z. Zheng, D. Shen and P. Huang, New J. Phys., 2010, 12, 37 S.-L. Wu and W.-J. Hsieh, Chem. Mater., 2003, 15, 4515–4520.
113018.
38 J. L. Serrano and T. Sierra, Metallomesogens, Wiley-VCH,
15 S. Taushanoff, K. V. Le, J. Williams, R. J. Twieg,
Weinheim, 1996.
´
B. K. Sadashiva, H. Takezoe and A. Jakli, J. Mater. Chem., 39 T. Seshadri and H. Haupt, Chem. Commun., 1998, 735–736.
˜
2010, 20, 5893–5898.
40 D. L. de Murillas, R. Pinol, M. B. Ros, J. L. Serrano, T. Sierra
16 Y. Kogawa and A. Yoshizawa, Liq. Cryst., 2011, 38, 303–307.
and M. R. de la Fuente, J. Mater. Chem., 2004, 4, 1117–1127.
17 K. V. Le, S. Aya, Y. Sasaki, H. Choi, F. Araoka, K. Ema, 41 T. Tian, X. Xu, Y. Zhao and X. Tang, Liq. Cryst., 1995, 19, 295–
´
J. Mieczkowski, A. Jakli, K. Ishikawa and H. Takezoe, J.
300.
42 T. Sierra, E. Melendez, J. L. Serrano, A. Ezcurra and
´
Mater. Chem., 2011, 21, 2855–2857.
´
18 L. Wang, L. Yu, X. Xiao, Z. Wang, W. He, P. Yang and
H. Yang, Liq. Cryst., 2012, 39, 629–638.
19 A. Yoshizawa, J. Soc. Inf. Disp., 2008, 16, 1189–1194.
M. A. Perez-Jubindo, Chem. Mater., 1991, 3, 157–163.
43 R. B. Meyer, L. Liebert, L. Strzelecki and P. Keller, J. Phys.,
Lett., 1975, 36, L69–L71.
20 H. Iwamochi and A. Yoshizawa, Mol. Cryst. Liq. Cryst., 2009, 44 S. Takao, S. Kazutami, H. Masao, Y. Katsumi and
509, 223–232. O. Masanori, Ferroelectrics, 1984, 58, 21–32.
21 M. Nakata, Y. Takanishi, J. Watanabe and H. Takezoe, Phys. 45 C. C. Huang, C. C. Kao, N. C. Wang, M. C. Yu and S. L. Wu,
Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top.,
2003, 68, 41710–41716.
22 H. Iwamochi, T. Hirose, Y. Kogawa and A. Yoshizawa, Chem.
Lett., 2010, 39, 170–171.
Liq. Cryst., 2013, 40, 441–445.
46 S. Inui, N. Iimura, T. Suzuki, H. Iwane, K. Miyachi,
Y. Takanishi and A. Fukuda, J. Mater. Chem., 1996, 6, 671–
673.
23 H. Iwamochi, T. Hirose, Y. Kogawa and A. Yoshizawa, Chem. 47 I. Nishyama, E. Chin and J. W. Goodby, J. Mater. Chem., 1993,
Lett., 2010, 39, 170–171. 3, 161–168.
24 K.-W. Park, M.-J. Gim, S. Kim, S.-T. Hur and S.-W. Choi, ACS 48 P. Gisse, J. Pavel, H. T. Nguyen and V. L. Lorman,
Appl. Mater. Interfaces, 2013, 5, 8025–8029. Ferroelectrics, 1993, 147, 27–41.
25 I.-H. Chiang, C.-J. Long, H.-C. Lin, W.-T. Chuang, J.-J. Lee 49 S. L. Wu and C. T. Chiang, Liq. Cryst., 2002, 29, 39–45.
and H.-C. Lin, ACS Appl. Mater. Interfaces, 2014, 6, 228–235. 50 D. D. Sarkar, R. Deb, N. Chakraborty, G. Mohiuddin,
26 H. Grebel, R. M. Hornreich and S. Shtrikman, Phys. Rev. A,
1983, 28, 1114–1138.
R. K. Nath and V. S. Rao Nandiraju, Liq. Cryst., 2013, 40,
468–481.
27 V. A. Belyakov, E. I. Demikhov, V. E. Dmitrienko and 51 K. Itoh, M. Johno, Y. Takanishi, Y. Ouchi, H. Takezoe and
V. K. Dolganov, J. Exp. Theor. Phys., 1985, 62, 1173–1182. A. Fukuda, Jpn. J. Appl. Phys., 1991, 30, 735–740.
28 A. D. L. Chandani, T. Hagiwara, Y. Suzuki, Y. Ouchi, 52 Y. I. Suzuki, T. Isozaki, S. Hashimoto, T. Kusumoto,
H. Takezoe and A. Fukuda, Jpn. J. Appl. Phys., 1988, 27,
L729–L732.
T. Hiyama, Y. Takanishi, H. Takezoe and A. Fukuda, J.
Mater. Chem., 1996, 6, 753–760.
29 A. D. L. Chandani, Y. Ouchi, H. Takezoe, A. Fukuda, 53 S. Hiller, S. A. Pikin, W. Haase, J. W. Goodby and
K. Terahima, K. Furukawa and A. Kishi, Jpn. J. Appl. Phys.,
1989, 28, L1261–L1264.
I. Nishiyama, Jpn. J. Appl. Phys., 1994, 33, L1170–L1173.
54 M. Buivyads, F. Gouda, G. Andersson, S. T. Lagerwall,
B. Stebler, J. Bomelburg, G. Heppke and B. Gestblom, Liq.
Cryst., 1997, 23, 723–729.
30 A. Fukuda, Y. Takanishi, T. Iasozki, K. Ishikawa and
H. Takezoe, J. Mater. Chem., 1994, 4, 997–1016.
31 J. W. Goodby, A. J. Slany, C. J. Booth, I. Nishyama, J. D. Vuijk,
P. Syring and K. J. Toyne, Mol. Cryst. Liq. Cryst., 1994, 243,
231–298.
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