An optically uniaxial antiferroelectric smectic phase in asymmetrical bent-core compounds containing a 3-aminophenol central unit

Article abstract of DOI:10.1039/c0jm01241f

Two new series of asymmetrical bent-core compounds possessing 3-aminophenol and its N-methyl derivative were synthesized. In the obtained compounds, the asymmetry comes from the different linkages between the central unit and its arms and in two cases also from different lateral substituents. The mesomorphic properties were investigated by differential scanning calorimetry, polarizing optical microscopy, switching current and electro-optical measurements, X-ray analysis, and second-harmonic generation. All materials possessing 3-aminophenol as a central unit showed liquid crystalline properties, and two of them exhibited the orthogonal smectic phase (SmA family phase) and very stable columnar phases, while the N-methyl analogues exhibited no mesophase. Because of the optically uniaxial nature, double switching current peaks, and SHG activity associated with a distinct threshold electric field, this orthogonal smectic phase is assigned to a novel type, which has a short-range antiferroelectric order with a long-range randomized polar plane (SmAP_AR). This is very unique in the sense that distinct threshold behavior emerges at the field-induced phase transition from the short-range antiferroelectric phase to a long-range ferroelectric phase.

Full text of DOI:10.1039/c0jm01241f

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
An optically uniaxial antiferroelectric smectic phase in asymmetrical
bent-core compounds containing a 3-aminophenol central unit
a
Kinga Gomola, Lingfeng Guo, Damian Pociecha, Fumito Araoka, Ken Ishikawa and Hideo Takezoe
a
b
a
a
a
*
Received 28th April 2010, Accepted 12th June 2010
DOI: 10.1039/c0jm01241f
Two new series of asymmetrical bent-core compounds possessing 3-aminophenol and its N-methyl
derivative were synthesized. In the obtained compounds, the asymmetry comes from the different
linkages between the central unit and its arms and in two cases also from different lateral substituents.
The mesomorphic properties were investigated by differential scanning calorimetry, polarizing optical
microscopy, switching current and electro-optical measurements, X-ray analysis, and second-harmonic
generation. All materials possessing 3-aminophenol as a central unit showed liquid crystalline
properties, and two of them exhibited the orthogonal smectic phase (SmA family phase) and very stable
columnar phases, while the N-methyl analogues exhibited no mesophase. Because of the optically
uniaxial nature, double switching current peaks, and SHG activity associated with a distinct threshold
electric field, this orthogonal smectic phase is assigned to a novel type, which has a short-range
antiferroelectric order with a long-range randomized polar plane (SmAPAR). This is very unique in the
sense that distinct threshold behavior emerges at the field-induced phase transition from the short-
range antiferroelectric phase to a long-range ferroelectric phase.
Since the first discovery of the SmAPR phase,11 several
compounds have been synthesized. The original molecules had
two ester groups attached to the central unit – a 1,3-substituted
phenyl ring with a methyl substitution as a central unit and
a stilbene bridge as a main element in the structure. The mole-
cules were asymmetric, due to different lateral substituents and/
or terminal alkoxy chain lengths. Some of them have one
unsaturated chain or both chains may be unsaturated. Recently
we showed that even symmetric molecules, which have 1,3-
dihydroxyacetophenone as a central unit and stilbene arms, can
exhibit the SmAPR phase.9 However, no compounds, which are
expected to have hydrogen bonding (H-bonding), have been
examined for stabilizing the SmAPR phase. It is known that
molecular interaction plays an important role not only in the
appearance of mesophases but also in their thermal stability.
Hydrogen bonding is one of the typical molecular interactions
responsible for the organization of molecules. Hydrogen bonding
interactions in liquid crystals could be introduced in two general
ways: by the introduction of appropriate functional groups, e.g.
amide groups or a hydroxy group in the ortho position to an
azomethine group or through the complexation of two different
components, in which one of them plays the role of a H-donor
(e.g., a carboxylic acid) and the second one plays the role of a H-
acceptor (e.g., pyridine derivatives).
1. Introduction
The bent-core (banana-shaped) liquid crystals, due to strong
bending of mesogenic cores, offer a new route to macroscopic
polar order and chiral superstructures even in achiral systems. In
the last decade, a number of bent-core mesogens exhibiting
various phase structures and phase transitions have been
synthesized and investigated.1,2 In addition to conventional
banana phases B1–B8 characteristic to bent-core mesogens,
another important type of banana phases is the orthogonal
smectic phase, in which molecules align perpendicular to the
smectic layer. Besides the conventional smectic A (SmA) phase,
in which molecules freely rotate about their long molecular axes,
there are also phases in which hindered rotation and the resultant
closed packing of molecules bring about polar layers or
domains:3 i.e., SmAdPA
4,5
SmAPA,6 and SmAPR.7–11 The
subscript A stands for antiferroelectric. Actually the SmAdPA
and SmAPA phases are birefringent even in homeotropic cells
and so biaxial, and exhibit the antiferroelectric–ferroelectric
phase transition by application of an electric field parallel to the
layer. By contrast, the SmAPR phase appears like a uniaxial
phase and exhibits ferroelectric switching.11 This is because the
phase consists of microscopic polar domains with the random-
ized polarization direction of the polar domains, so that the
phase is macroscopically nonpolar but becomes easily polar
upon field application. The potential application of the SmAPR
phase for a fast switching display device has been proposed and
demonstrated.8,10
Hydrogen bonding, intermolecular as well as intramolecular,
plays an important role in the design and engineering of the
architecture of many rod-like, disc-like and polymeric mate-
rials.12,13 However in the bent-shaped liquid crystals most of the
mesogenic molecules possess only covalent bonds. The first bent-
core liquid crystals, containing non-covalent bonds, were
described by Kato et al. in 1992.14 In those molecules, the bent
shape originates from interactions of phthalic or isophthalic
acids (H-donors) with stilbazole derivatives (H-acceptors).
Recently, a few reports have been published, which described
aDepartment of Organic and Polymeric Materials, Tokyo Institute of
Technology, O-okayama, Meguro-ku, Tokyo, 152-8552, Japan. E-mail:
takezoe.h.aa@m.titech.ac.jp
bChemistry Department, Warsaw University, Al. Zwirki i Wigury 101, 02-
089 Warsaw, Poland
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banana shape compounds possessing intra- or intermolecular
hydrogen bonding. In these examples, intermolecular H-bonding
is formed between the H-atom of the hydroxyl group and the N-
atom of the imine linkage.15 Intermolecular H-bonding appeared
in different combinations of stilbazole derivatives with carboxy-
lic acids, which acted as H-acceptors and H-donors, respec-
tively,16–19 resulting in the formation of bent-shaped molecules.
By contrast, intermolecular hydrogen bonding originating from
the amide linkage has not been explored in the field of banana-
shaped liquid crystals, although some achiral bent-rod-shaped
and columnar liquid crystalline compounds with amide linkages
have been reported.20,21 Amide linkage could promote the
intermolecular formation of hydrogen bonds between neigh-
boring molecules, which differs this kind of compounds from
what is described above.
In this work we synthesized bent-core compounds with 3-
aminophenol and its N-methyl derivative as a central unit, lateral
stilbene bridges and terminal dodecyloxy chains and found
a novel polar orthogonal smectic phase, which is optically
uniaxial but clearly shows antiferroelectric switching behavior. In
this sense, this phase is definitely different from the SmAPR
phase, so we call this phase SmAPAR phase. Actually, this phase
exhibits electric and optical properties characteristic to an anti-
ferroelectric phase; i.e., two switching current peaks upon
application of triangular voltage wave and second-harmonic
generation (SHG) upon application of an electric field with
a distinct threshold. From a chemical structure viewpoint, we
particularly focus on H-bonding for stabilizing the SmAPR phase
in addition to asymmetric molecular architectures.
Scheme 1 Synthetic pathway for the compounds: N–H NO2,NO2, N–H
Cl,Cl, N–H NO2,Cl. Reagents and conditions: (i) 4-{(E)2-[4-nitro-3-
(dodecyloxy)phenyl]ethenyl}benzoic acid chloride1 or 4-{(E)2-[3-chloro-
4-(dodecyloxy)phenyl]ethenyl}benzoic acid chloride,1 TEA, THF,
DMAP, room temp., 5 h; (ii) 4-{(E)2-[3-chloro-4-(dodecyloxy) phenyl]-
ethenyl}benzoic acid chloride,1 TEA, THF, DMAP, room temp., 4–5 h.
2. Results and discussion
2.1 Hydrogen bonding and mesomorphic properties
liquid crystalline behavior of both series was investigated by
polarizing optical microscopy (POM), differential scanning
calorimetric (DSC), electric and electro-optical studies, X-ray
We synthesized two series of asymmetric bent-core compounds
possessing 3-aminophenol and its N-methyl derivative as
a central unit as shown in Table 1 (see Schemes 1 and 2). The
Table 1 Transition temperatures (ꢀC) and associated enthalpy changes (in parentheses, kJ molꢁ1) as determined by DSC at a scanning rate of
10 ꢀC minꢁ1
Code
X
Y
L
Mesomorphic properties
N–H NO2,NO2
N–H NO2,Cl
N–H Cl,Cl
NO2
NO2
Cl
NO2
Cl
Col2 92.9[1.4] Col1 185.8[1.5]
SmAPAR 200.8[12.2] Iso
Col2 119.0[4.9] Col1 185.9[1.2]
SmAPAR 204.8[7.6] Iso
Cry 152.9[27.4] SmX 197.3 [19.5]
Iso
Cl
Cry 74.9 [10.6] Iso
N–CH3 NO2,NO2
N–CH3 NO2,Cl
NO2
NO2
NO2
Cl
Cry 70.8 [9.1] Iso
Cry 141.8[28.0] B1 149.8[1.3] B6
150.6[15.0] Iso
20
COO NO2,NO2
COO NO2,Cl21
NO2
NO2
NO2
Cl
Cry 113.1[13.2] B1RT 146.9[17.5] Iso
Cry 116.4[18.9] B7 144.4[18.2] Iso
COO Cl,Cl20
Cl
Cl
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were replaced with chlorine atoms, gave an unknown enantio-
tropic smectic phase. The reference compounds, COO NO2,NO2,
COO NO2,Cl, and COO Cl,Cl, showed the conventional so-
called banana phases but not SmAPAR, as shown in Table 1.
The introduction of the amide linkage led to the formation of
hydrogen bonding between molecules, which would stabilize
columnar liquid-crystalline mesophases. In the case of
compounds in the second series N-CH3 X,Y, where the hydrogen
of amide was replaced by a methyl group, both materials lost
their liquid crystalline properties. These facts suggest the
importance of intermolecular hydrogen bonding. Hence we
measured FT-IR spectra of N–H NO2,Cl as a function of
temperature. All materials of series N–H X,Y showed a broad
band in a range 3370–3337 cmꢁ1 at room temperature, which
corresponds to hydrogen bond of the amide group. The
absorption of the amide I band (C]O) emerges in the region of
1654 cmꢁ1. The lower frequency of C]O stretching in IR spectra
is explained by the existence of H-bonding. The FT-IꢀR absorp-
tion spectra for N–H NO2,Cl at 100 ꢀC (Col2), 150 C (Col1),
200 ꢀC (SmAPAR) and 225 ꢀC (isotropic phase) are shown in the
inset of Fig. 1. In the Col2 phase (100 ꢀC) the N–H peak was
sharp and its intensity was also higher, suggesting stronger
H-bonding compared to other phases. The absorption peak
wavelength of the N–H vibration shifted stepwise to higher
wavenumbers at the phase transition temperature (Fig. 1), indi-
cating considerable decrease of H-bond interactions with
increasing temperature. Decreased absorbance was also associ-
ated with this change. At 225 ꢀC, in the isotropic phase, the N–H
vibration shows very low intensity and high wavenumber
frequency around 3400 cmꢁ1. Moreover the wavenumber of the
C]O vibration increased up to 1680 cmꢁ1. These facts suggest
that the N–H group is free from hydrogen bonding in the
isotropic phase. We also excluded the formation of H-bonding
between amide and ester groups, as the C]O vibration peak
position of the ester group did not depend on temperature.
Scheme 2 Synthetic pathway for the compounds: N-CH3 NO2,NO2; N-
CH3 NO2,Cl. Reagents and conditions: (i) CH3I, K2CO3, DMF, 100 ꢀC,
2
h;2 (ii) 4-{(E)2-[4-nitro-3-(dodecyloxy)phenyl]ethenyl}benzoic acid
chloride,1 TEA, THF, DMAP, room temp. 4–5 h; (iii) 4-{(E)2-[3-chloro-
4-(dodecyloxy)phenyl]ethenyl}benzoic acid chloride,1 TEA, THF,
DMAP, room temp., 5 h.
2.2 SmAPAR phase
In order to prove the emergence of the SmAPAR phase,
compound N–H NO2,Cl was selected for various experiments.
The textures at various temperatures in cells with distinct surface
treatments are shown in Fig. 2. On cooling from the isotropic
phase, typical fan-shaped textures in planar cells appeared
(Fig. 2a). By applying an electric field, only a slight birefringence
change could be seen without rotation of extinction direction,
indicating the non-tilted (SmA) character of this phase. When the
phase transition occurred to Col1, the focal conic texture was
broken into small fragments, indicating the breaking of layer
structure (Fig. 2b). When the temperature was reduced further,
the transition to another columnar phase could be observed
under a microscope (Fig. 2c). In a homeotropically aligned
sample, a uniform dark image (Fig. 2d) was observed in the
highest mesophase, indicating unaxial phase, being consistent to
the SmAPAR phase. By contrast, typical mosaic textures with
lower and higher birefringence were observed in the Col1 and
Col2 phases (Fig. 2e and f), respectively.
diffraction and SHG measurements. Transition temperatures
and associated enthalpy changes obtained from DSC thermo-
grams are shown in Table 1, where SmAPAR stands for a new
phase which has polar (P) antiferroelectric (sub A) SmA random
(sub R) characteristics (see other experimental results later). Here
random means that the paired bent directions (polar plane of
antiferroelectric domain) have only a short-range order, but are
random in long range. For reference, Table 1 also includes
previously obtained compounds possessing two ester groups
attached to the central unit, 1,3-substituted phenyl ring.22,23 The
DSC thermograms of N–H NO2,NO2 show three mesophases.
Based on the microscope observations and X-ray measurements
the high temperature phase was identified as a non-tilted smectic
phase with polar order (SmAPAR), while both low-temperature
phases have a columnar (2D) structure. The low temperature
columnar phase remains even at room temperature. For asym-
metric analogue N–H NO2,Cl, when one nitro group was
replaced by a chlorine atom, the sequence of mesophases was
retained. The compound N–H Cl,Cl, in which both nitro groups
We also carried out X-ray diffraction measurements in both
unoriented and oriented samples, where the X-ray beam was
incident almost parallel to a homeotropically aligned surface. In
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frustrated layer structure should be considered. All signals can be
indexed assuming a 2D oblique lattice. At 150 ꢀC, in the Col1
ꢀ
phase the lattice parameters are as follows: a ¼ 118.1 A, c ¼ 49.96
ꢀ
ꢀ
ꢀ
ꢀ
A, b ¼ 113.4 and at 80 C in the Col2 phase a ¼ 112.8 A, c ¼
ꢀ
ꢀ
57.96 A, b ¼ 124.2 . The X-ray pattern also shows less diffuse
outer scattering in the Col2 phase than in the Col1 phase (Fig. 3c).
The pattern of the Col2 phase does not change even when cooled
down to room temperature.
Polarization reversal current was measured in a 5.2 mm thick
homogeneous cell using a triangular wave of 16 Hz to identify the
polar structure of the SmAPAR phase. Interestingly, two split
switching current peaks per half cycle ofꢀapplied voltage were
recorded in compound N–H NO2,Cl at 4 C above the Sm–Col
phase transition, as shown in Fig. 4(a), indicating antiferro-
electric behavior. With gradually decreasing temperature, the
first peak moved forward to the second peak, ꢀand the two peaks
became an overlapped single wide peak at 2 C above the Sm–
Col transition (Fig. 4b), being reminiscent of the behavior in the
SmAPR phase.9
Fig. 1 N–H stretching vibration of N–H NO2,Cl in variable temperature
FT-IR spectroscopy. Col2, Col1, SmAPAR, Iso indicate the columnar 2,
columnar 1, polar smectic A and isotropic phase, respectively. Stepwise
changes in the peak wavenumber are clearly displayed between these
phases. FT-IR spectra near the N–H stretching mode in each phase are
also shown in the inset.
To understand the difference between SmAPAR and SmAPR,
SHG measurements were conducted. Without applying an elec-
tric field, no SH signal was observed in the three phases, implying
that none of those phases has spontaneous macroscopic polari-
zation. By contrast, SH signal was induced by applying an
electric field to planar cells in the SmAPAR and in the vicinity of
the Sm–Col transition both on heating and cooling runs.
Fig. 5(a) shows the temperature dependence of the SH signal
recorded when a 5.2 mm thick cell was gradually cooled down
from the isotropic phase under applying a rectangular wave
voltage 75 Vpp at 10 Hz. Under a constant field strength, the SH
signal increases with decreasing temperature; electric-field-
induced polar order develops with decreasing temperature
because of the gradual increase of coherent lengths of polar
domains.10 When the transition takes place to the Col1 phase, the
threshold voltage for the polar order increased and as a result the
SH signal decreased with lowering temperature.
the SmAPAR phase, diffraction peaks due to the smectic layer
parallel to the substrate surface were observed along the
meridian line (not shown). Diffuse scattering appeared along the
equatorial line, signifying the liquid-like non-tilted arrangement
of molecules within each layer. The layer spacing continuously
ꢀ
ꢀ
increased from 43.9 A to 45.4 A in the SmAPAR phase (Fig. 3a),
which is characteristic of SmAPR.8 The significant drop of the
d spacing at 185 ꢀC shows the first order transition from smectic
to columnar phase. Fig. 3(b) and (c) shows wide angle two-
dimensional X-ray diffraction patterns in the two distinct
columnar phases. In a small angle region several sharp peaks
were observed in the powder X-ray diffraction. Because of the
incommensurability of the signals a two-dimensional columnar
Fig. 2 Optical photomicrographs of N–H NO2,Cl under crossed polarizers. (a–c) Homogeneous and (d–f) homeotropic textures at various temper-
atures. (a, d) SmAPAR (185 ꢀC), (b, e) Col1 (150 ꢀC) and (c, f) Col2 (80 ꢀC) phases.
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Fig. 4 Switching current in the SmAPAR phase recorded at relative
temperatures (a) 4 ꢀC and (b) 2 ꢀC above the Sm–Col phase transition.
The applied electric field was a 95 Vpp triangular wave and the cell was 5.2
mm thick.
Fig. 3 (a) Variation of d spacing obtained from the main X-ray signal as
a function of temperature. 2D X-ray diffraction pattern of (b) Col1
(160 ꢀC) and (c) Col2 (80 ꢀC) phases.
To examine the field-induced polarization, the field depen-
dence of the SH signal in the SmAPAR phase was also
The tristable switching has been reported even in the ferro-
electric liquid crystals by Itoh et al.24 in addition to conventional
antiferroelectric liquid crystals.25 In this case, the helical state was
strongly stabilized because of its ultrashort pitch, so that the
tristable switching was observed. To consider the molecular
orientational structure in the SmAPAR phase, we should take
into account five experimental facts; (1) orthogonal smectic
phase, (2) antiferroelectric (tristable) switching behavior, (3)
existence of threshold for the field-induced transition, (4) optical
uniaxiality, and (5) no helical structure. The most plausible
model satisfying these conditions is the SmAPAR phase; i.e., local
SmAPA structure for (1)–(3) and randomized polar plane for (4)
and (5).
ꢀ
measured. The measurement was carried out at 4 C above the
Sm–Col transition in a cooling process by applying a rectan-
gular wave voltage to a 4.7 mm thick cell. The result shown in
Fig. 5(b) is quite different from that in SmAPR; i.e., the SH
signal started to increase with a distinct threshold around 40
Vpp and saturated above 70 Vpp, whereas thresholdless increase
describable by the Langevin process was observed in SmAPR.10
This is the prime difference between the SmAPR and SmAPAR
phases. We confirmed that the threshold behavior was
observed over the whole the SmAPAR temperature range even
ꢀ
at 1 C above the Sm–Col transition, where a single switching
current peak was observed. In addition, the temporal change of
the SH signal intensity under a triangular wave field (90 Vpp
)
Finally we summarize the difference of SmAPR and SmAPAR
and the electric-field-induced transition to the ferroelectric
SmAPF phase as a cartoon in Fig. 6, by which we want to
emphasize the novelty of the SmAPAR phase. It is physically
natural that a short-range order develops to a larger scale
without a threshold, as in the case of the SmAPR phase, where
randomly ordered short-range polar domains gradually change
shows a hysteresis behavior; i.e., the onset and termination of
the SH signal occurred at different voltages (or phases), as
shown in Fig. 5(c). The hysteresis and associated tristable
switching behavior obtained are consistent with the two
switching current peaks and also strongly indicate the anti-
ferroelectric behavior.
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shown in Fig. 6(b) (left to middle). However, the detection of this
process is not easy because this process appears as only a slight
birefringence change. Then, the field-induced antiferroelectric–
ferroelectric transition occurs, and the system becomes polar, as
Fig. 5 (a) Temperature dependence of SHG obtained under a 75 Vpp
rectangular wave field at 10 Hz in the SmAPAR and Col1 phases. The cell
was 5.2 mm thick homogeneously aligned. (b) Electric field (E) depen-
dence as a function of an electric field (rectangular wave field at 10 Hz) at
4 ꢀC above the Sm–Col transition. (c) SHG intensity under application of
triangular wave voltage in the SmAPAR phase recorded at 4 ꢀC above the
Sm–Col transition. A planar cell with the thickness of 4.7 mm was used in
the measurements (b) and (c).
Fig. 6 Cartoons showing (a) SmAPR and (b) SmAPAR and the transi-
tions from these phases to the ferroelectric SmAPF phase. In the absence
of a field, (a) ferroelectric and (b) antiferroelectric orders are only in
a short range. By applying a field to SmAPR ((a) left), gradual formation
of macroscopic SmAPF ((a) middle) proceeds below the saturate field Esa,
and well saturated macroscopic SmAPF ((a) right) results. By contrast, by
applying a field to SmAPAR ((b) left) gradual formation of macroscopic
SmAPA ((b) middle) proceeds below the threshold field Eth, and field-
induced transition occurs to macroscopic SmAPF at Eth ((b) right).
their polar directions to the electric field without a threshold due
to the Langevin process, as shown in Fig. 6(a). By contrast, in
SmAPAR
,
randomly ordered short-range antiferroelectric
domains first gradually change their polar planes to the electric
field without a threshold, establishing macroscopic antiferro-
electric order without showing macroscopic polarization, as
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shown in Fig. 6(b) (middle to right). This type of field-induced
transition usually occurs with a distinct threshold Eth. This is true
in chiral smectic25 and bent-core liquid crystals,26 and also in the
present case. The SmAPAR phase is unique since this is not only
the new phase with a short-range order but also the phase
showing a field-induced transition with a distinct threshold, as
mentioned above. We are continuing further experiments to
definitely confirm the molecule level arrangement and the
dynamic switching process.
J ¼ 6.3 Hz, 2H, OCH2CH2), 1.76–1.65 (m, 2H, OCH2CH2),
1.38–1.18 (m, 18H, CH2), 0.85–0.80 (m, 3H, CH2CH3); 13C NMR
(DMSO, 75 MHz) d 165.0, 157.5, 150.8, 140.2, 139.9, 139.8,
133.8, 132.1, 129.5, 129.2, 128.2, 127.95, 127.88, 126.2, 122.6,
115.4, 111.1, 110.8, 107.5, 69.3, 40.3, 40.1, 31.3, 29.0, 28.9, 28.7,
28.6, 28.3, 25.2, 22.1, 14.0; Elemental analysis calc. (%) for
C33H40N2O5 (544.68) C, 72.77, H, 7.40, N, 5.14 found C, 72.71,
H, 7.55, N, 5.12.
N–H NO2,NO2: To the solution of 3-aminophenol (0.053 g,
0.48 mmol), triethylamine (1 ml, 7.13 mmol) and DMAP (20 mg,
0.16 mmol) in tetrahydrofuran (50 ml), 4-{(E)-2-[4-dodecyloxy-
3-nitro-phenyl]ethenyl}benzoic acid chloride 2 (0.50 g, 1.06
mmol) was added. The reaction mixture was stirred at 40 ꢀC for 5
h, then the solvent was evaporated to dryness under reduced
pressure. The obtained product H–N NO2NO2 was purified by
column chromatography and recrystallized twice from ethyl
acetate. Yield of 45%.
3. Experimental
3.1 Synthesis
The synthesis of novel asymmetrical bent-core compounds was
achieved by the following routes shown in Scheme 1 and Scheme
2. In Scheme 1, the synthesis and chemical structures of
compounds N–H X,Y, having 3-aminophenol as a central unit
are presented. The synthesis routes used in these schemes are
classical. 3-Aminophenol (1) was treated with 4-{(E)-2-[3-chloro-
4-(dodecyloxy)phenyl]ethenyl}benzoic acid chloride (2a) or
N–H Cl,Cl was synthesized using the same procedure.
N–H NO2,NO2: 1H NMR (CDCl3, 300 MHz) d 8.17–8.10 (m,
3H, N–H, Ar–H), 7.94 (dd, J1 ¼ 2.1 Hz, J2 ¼ 9.3 Hz, 2H, Ar–H),
7.82 (d, J ¼ 8.4 Hz, 2H, Ar–H), 7.72 (t, J ¼ 2.1 Hz, 1H, Ar–H),
7.63–7.55 (m, 4H, Ar–H), 7.46–7.14 (m, 3H, Ar–H), 7.33 (t, J ¼
8.1 Hz, 1H, Ar–H), 7.08–6.93 (m, 7H, Ar–H, CH]CH), 4.13–
4.04 (m, 4H, OCH2CH2), 1.91–1.76 (m, 4H, OCH2CH2), 1.54–
1.08 (m, 36H, CH2), 0.90–0.86 (m, 6H, CH2CH3); 13C NMR
(CDCl3, 75 MHz) d 165.1, 164.8, 152.2, 152.1, 151.3, 141.8,
140.3, 139.9, 139.0, 133.5, 132.1, 132.0, 130.7, 129.8, 129.2, 129.1,
128.8, 128.1, 127.9, 127.8, 127.6, 126.7, 126.5, 123.4, 123.3, 117.7,
114.6, 113.8, 69.8, 31.9, 29.62, 29.57, 29.5, 29.35, 29.26, 28.9,
25.8, 22.7, 14.1; IR (cmꢁ1), 3350 (br), 2923 (vs), 2853 (s), 1725 9
(m), 1654 (m), 1603 (s), 1531 (vs), 1350 (w), 1275 (s), 1252 (s),
1178 (m), 1152 (m), 1080 (w), 758 (w); Elemental analysis calc.
(%) for C60H73N3O9 (544.68) C, 73.52, H, 7.51, N, 4.29 found C,
73,74, H, 7.36, N, 4.13.
4-{(E)-2-[4-dodecyloxy-3-nitrophenyl]ethenyl}benzoic
acid
chloride (2b)23 in a presence of TEA and catalytic amounts of
DMAP in THF, which led to the formation of disubstituted
products N–H Cl,Cl and N–H NO2,NO2. In case of reaction with
2b, the excess of the 3-aminophenol (1) was used, which resulted
in a mixture of mono- and disubstituted products, separated by
column chromatography. Based on the NMR analysis, we
assigned the monosubstituted derivative as an amidophenole 3,
which was converted in the following step by the treatment with
2a into N–H NO2,Cl.
The synthesis and structures of compounds N-CH3 X,Y,
having N-methyl-3-aminophenol (4) as a central unit, are given in
Scheme 2. N-Methyl-3-aminophenol (4) was obtained by alkyl-
ation of 3-aminophenol (1) with methyl iodide, using a known
procedure.27 N-Methyl-3-aminophenol (4) was treated with 4-
{(E)-2-[4-dodecyloxy-3-nitrophenyl]ethenyl}benzoic acid chlo-
ride (2b)23 in a presence of TEA and catalytic amounts of DMAP
in THF, which resulted in a mixture of mono and disubstituted
products, separated by column chromatography. Based on the
NMR analysis, we assigned the monosubstituted derivative as an
aminoester 5, which was converted in the following step by the
treatment with (2a) into the N-CH3 NO2,Cl.
N–H Cl,Cl: 1H NMR (CDCl3, 300 MHz) d 8.16 (d, J ¼ 8.1 Hz,
2H, Ar–H), 7.93 (s, 1H, N–H), 7.84 (d, J ¼ 8.4 Hz, 2H, Ar–H),
7.17–7.67 (m, 1H, Ar–H), 7.64–7.50 (m, 6H, Ar–H), 7.49–7.13
(m, 4H, Ar–H), 7.18–6.87 (m, 7H, Ar–H, CH]CH), 4.12–4.02
(m, 4H, OCH2CH2), 1.93–1.80 (m, 4H, OCH2CH2), 1.52–1.10 (m,
36H, CH2), 0.91–0.80 (m, 6H, CH2CH3); 13C NMR (CDCl3,
75 MHz) d 165.3, 165.0, 154.6, 154.5, 151.2, 142.5, 140.8, 139.1,
133.0, 130.7, 130.1, 130.0, 129.7, 129.2, 128.1, 128.0, 127.8, 127.6,
126.6, 126.4, 126.3, 123.2, 117.6, 117.5, 113.8, 113.1, 69.2, 31.9,
29.7, 29.60, 29.56, 29.4, 29.0, 26.0, 22.7, 14.2; IR (cmꢁ1), 3370 (br),
2922 (vs), 2853 (s), 1716 (m), 1665 (m), 1594 (s), 1535 (m), 1507 (s),
1282 (vs), 1250 (vs), 1179 (m), 1152 (m), 1058 (w), 964 (w).
N–H NO2,Cl: 1H NMR (CDCl3, 300 MHz) d 8.14 (d, J ¼ 8.1
Hz, 2H, Ar–H), 8.01 (s, 1H, N–H), 7.97 (d, J ¼ 2.4 Hz, 1H,
Ar–H), 7.85 (d, J ¼ 8.4 Hz, 2H, Ar–H), 7.70 (t, J ¼ 1.8 Hz, 1H,
Ar–H), 7.63–7.52 (m, 6H, Ar–H), 7.48–7.32 (m, 3H, Ar–H),
7.14–6.97 (m, 6H, Ar–H), 6.91 (d, J ¼ 8.7 Hz, 1H, Ar–H), 4.12–
4.03 (m, 4H, OCH2CH2), 1.87–1.80 (m, 4H, OCH2CH2), 1.53–
1.20 (m, 36H, CH2), 0.91–0.86 (m, 6H, CH2CH3); 13C NMR
(CDCl3, 75 MHz) d 165.2, 165.0, 154.6, 152.1, 151.2, 142.5,
140.2, 139.8, 139.1, 133.5, 132.0, 130.7, 130.0, 129.7, 129.2, 128.1,
128.0, 127.8, 127.6, 126.6, 126.5, 126.3, 123.3, 117.6, 117.4, 114.5,
113.8, 113.1, 69.8, 69.2, 31.9, 29.65, 29.56, 29.54, 29.51, 29.35,
29.28, 29.0, 28.9, 25.9, 25.8, 22.7, 14.1; IR (cmꢁ1), 3337 (br), 2922
(vs), 2853 (s), 1725 (m), 1653 (m), 1603 (s), 1531 (vs), 1498 (s),
The synthesis and analytical data for 2a and 2b have been
described previously.23
3: To the solution of 3-aminophenol (1.16 g, 10.63 mmol),
triethylamine (1 ml, 7.13 mmol) and DMAP (20 mg, 0.16 mmol)
in tetrahydrofuran (100 ml), 4-{(E)-2-[4-dodecyloxy-3-nitro-
phenyl]ethenyl}benzoic acid chloride 2 (1 g, 2.12 mmol) in
dichloromethane (30 ml) was added dropwise. The reaction
mixture was stirred at room temperature for 4–5 h, then the
solvent was evaporated to dryness under reduced pressure. The
obtained product was purified by column chromatography and
recrystallized from a mixture of EtOH and THF (95 : 5 v/v, 15
ml). Yield 41%. Iso 122.8 SmA 12.2 Cry; 1H NMR (DMSO, 300
MHz) d 10.08 (s, 1H, N–H), 9.40 (s, 1H, OH), 8.13 (d, J ¼ 2.1 Hz,
1H, Ar–H), 7.95 (d, J ¼ 8.4 Hz, 2H, Ar–H), 7.88 (dd, J1 ¼ 2.1 Hz,
J2 ¼ 9.0 Hz, 1H, Ar–H), 7.70 (d, J ¼ 8.4 Hz, 2H, Ar–H), 7.44–
7.07 (m, 6H, Ar–H, CH]CH), 6.54–6.46 (m, 1H, Ar–H), 4.15 (t,
7950 | J. Mater. Chem., 2010, 20, 7944–7952
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1276 (vs), 1250 (vs), 1177 (m), 1151 (m), 1076 (m); Elemental
analysis calc. (%) for C60H73ClN2O7 (969.68) C, 74.32, H, 7.59,
N, 2.89, Cl, 3.66 found C, 74.32, H, 7.27, N, 2.92, Cl, 3.90.
The synthetic pathway leading to asymmetrical banana-shape
compounds: N-CH3 NO2,NO2; N-CH3 NO2,Cl is presented in
Scheme 2, where N-CH3 stands for N-methyl aminophenol as
a central unit.
Varian Unity Plus spectrometers. Chemical shift values for
protons are reported in parts per million (ppm, d scale) downfield
from tetramethylsilane (d ¼ 0) and are referenced to the carbon
resonance of CDCl3 (d 77.0). Data are presented as follows:
chemical shift, spin multiplicity (s ¼ singlet, d ¼ doublet, dd ¼
double doublet, t ¼ triplet, m ¼ multiplet), coupling constant in
Hz, and signal area integration in natural number. Thin layer
chromatography (TLC) analyses were performed on Merck 60
F254 silica gel aluminium plates. Column chromatography was
carried out under atmospheric pressure using silica gel (100–200
mesh, Merck). Infrared (IR) spectra were obtained using
a JASCO FTIR-460 PLUS spectrometer for the room tempera-
ture tests and are reported as the wavenumber in cmꢁ1 with band
intensities indicated as br (broad), vs (very strong), s (strong), m
(medium) and w (weak). Infrared spectra for high temperature
measurements were obtained using a JEOL IR-MAU200 spec-
trometer. In the IR spectroscopy, SrF2 substrates were used to
fabricate cells with 5 mm spacers and UV curable adhesive
between the top and bottom substrate. Compositions of the
synthesized compounds were determined by elemental analysis.
Phase transition temperatures and associated enthalpies were
determined by differential scanning calorimetry (Pyris Diamond
Perkin-Elmer) at a rate of 10 ꢀC minꢁ1 under cooling and heating
runs. Texture observationsand the identificationofthe mesophases
were done using a polarizing optical microscope (Nikon, Optiphot-
pol) equipped with a hot stage and temperature controller (Mettler
Toledo FP 82). A function generator connected with a high voltage
amplifier was utilized to study the electro-optical behavior. The
polarization reversal current was measured by the standard trian-
gular wave technique.28 The electric field applied to the sample was
a triangular wave voltage of 95 Vpp and frequency of 16 Hz. The
current was measured across a 1 MU resister.
N-Methyl-3-aminophenol (4) was prepared according to
a known procedure.27
1
5: Iso 237 SmA 190 Cry; H NMR (CDCl3, 300 MHz) d 8.17
(d, J ¼ 8.4 Hz, 2H, Ar–H), 8.00 (d, J ¼ 1.8 Hz, 1H, Ar–H), 7.64
(dd, J1 ¼ 2.1 Hz, J2 ¼ 8.7Hz, 1H, Ar–H), 7.59 (d, J ¼ 8.4 Hz, 2H,
Ar–H), 7.22–7.04 (m, 4H, Ar–H, CH]CH), 6.55–6.43 (m, 3H,
Ar–H), 4.11 (t, J ¼ 6.3 Hz, 2H, OCH2CH2), 3.85 (s, 1H, N–H),
2.82 (s, 3H, N–CH3), 1.88–1.79 (m, 2H, OCH2CH2), 1.48–1.19
(m, 18H, CH2), 0.90–0.85 (m, 3H, CH2CH3); 13C NMR (CDCl3,
75 MHz) d 164.9, 152.2, 152.1, 150.6, 141.6, 140.0, 132.0, 130.6,
129.8, 129.2, 128.8, 128.6, 128.0, 126.4, 123.4, 114.7, 110.2, 110.0,
105.2, 69.8, 31.9, 30.6, 29.6, 29.54, 29.47, 29.3, 29.2, 28.9, 25.8,
22.6, 14.1; Elemental analysis calc. (%) for C34H42N2O5 (558.71)
C, 73.09, H, 7.58, N, 5.01 found C, 72.96, H, 7.55, N, 5.10.
N–CH3 NO2, NO2: 1H NMR (CDCl3, 300 MHz) d 8.14 (d, J ¼
8.1 Hz, 2H, Ar–H), 8.00 (d, J ¼ 1.8 Hz, 1H, Ar–H), 7.92 (d, J ¼ 2.1
Hz, 1H, Ar–H), 7.65 (dd, J1 ¼ 1.8 Hz, J2 ¼ 8.4 Hz, 1H, Ar–H),
7.61–7.56 (m, 3H, Ar–H), 7.38–7.28 (m, 4H, Ar–H), 7.23–6.89 (m,
10H, Ar–H, CH]CH), 4.15–4.08 (m, 4H, OCH2CH2), 3.53 (s,
3H, N–CH3), 1.89–1.78 (m, 4H, OCH2CH2), 1.55–1.15 (m, 36H,
CH2), 0.88–0.86 (m, 6H, CH2CH3); 13C NMR (CDCl3, 75 MHz)
d 170.0, 164.4, 152.2, 151.9, 151.3, 145.9, 142.0, 140.0, 138.1,
134.8, 132.0, 131.7, 130.7, 129.8, 129.5, 129.4, 129.1, 129.0, 128.3,
128.0, 127.8, 127.2, 126.5, 125.9, 123.4, 123.1, 120.2, 119.9, 114.7,
114.6, 69.9, 69.8, 38.4, 31.8, 29.60, 29.59, 29.53, 29.46, 29.3, 29.2,
28.9, 25.8, 22.6, 14.1; IR (cmꢁ1), 2923 (vs), 2853 (s), 1732 (s), 1644
(m), 1602 (s), 1531 (vs), 1353 (s), 1272 (vs), 1259 (vs), 1177 (m),
Elemental analysis calc. (%) for C61H75N3O9 (994.26) C, 73.69, H,
7.60, N, 4.23 found C, 73.77, H, 7.48, N, 4.23.
N–CH3 NO2,Cl: 1H NMR (CDCl3, 300 MHz) d 8.14 (d, J ¼ 8.4
Hz, 2H, Ar–H), 8.01 (d, J ¼ 2.4 Hz, 1H, Ar–H), 7.65 (dd, J1 ¼ 2.1
Hz, J2 ¼ 8.7 Hz, 1H, Ar–H), 7.60 (d, J ¼ 8.4 Hz, 2H, Ar–H), 7.50
(d, J ¼1.8 Hz, 1H, Ar–H), 7.36–7.28 (m, 5H, Ar–H), 7.24–6.83 (m,
9H, Ar–H), 4.12 (t, J ¼ 6.6 Hz, 2H, OCH2CH2), 4.03 (t, J ¼ 6.6 Hz,
2H, OCH2CH2), 3.53 (s, 3H, N–CH3), 1.89–1.76 (m, 4H,
OCH2CH2), 1.53–1.15 (m, 36H, CH2), 0.89–0.85 (m, 6H,
CH2CH3); 13C NMR (CDCl3, 75 MHz) d 170.2, 154.3, 152.2,
151.3, 146.0, 142.0, 138.7, 134.2, 132.0, 130.7, 130.4, 129.8, 129.3,
129.1, 128.9, 128.4, 128.1, 127.9, 127.8, 126.7, 126.5, 126.2, 125.7,
123.4, 123.2, 120.1, 119.9, 114.7, 113.2, 69.9, 69.2, 38.5, 31.9, 29.6,
29.54, 29.48, 29.3, 29.2, 29.0, 28.9, 25.9, 25.8, 22.7, 14.1; IR (cmꢁ1):
2923 (vs), 2853 (s), 1732 (s), 1645 (m), 1636 (m), 1601 (s), 1532 (s),
1507 (s), 1498 (m), 1355 (m), 1260 (vs), 1178 (m), 1067 (w), 697 (w);
Elemental analysis calc. (%) for C61H75ClN2O7 (983.71) C, 74.48,
H, 7.68, N, 2.85, Cl, 3.60 found C, 74.55, H, 7.67, N, 2.84, Cl, 3.88.
In the measurements of second-harmonic generation (SHG),
Nd:YAG laser (1064 nm, 8 ns duration, and 10 Hz repetition)
was used as a fundamental light source. The intensity of the
illuminated laser was reduced to 6 mJ mmꢁ2 and the incidence
angle was 45ꢀ. The SH signal was detected in the transmission
direction by a photomultiplier tube (Hamamatsu, R955) after
removing the fundamental light by IR cut and interference filters.
The signal was accumulated for 30 s with a boxcar integrator
(Stanford Research Systems, SR-250). Cells used in SHG and
electro-optical measurements were prepared from indium tin
oxide (ITO) coated glass plates. The polyimides AL1254 (JSR)
and SE-1211 (Nissan Chemical) were spin coated as alignment
layers for planar and homeotropic cells, respectively. The plates
were rubbed antiparallel to achieve homogenous alignment of
the molecules. Liquid crystal materials were filled into cells by
capillary action in the isotropic phase.
X-Ray studies were performed with a Bruker D8-GADDS
system, parallel point beam of CuKa radiation was formed by
Goebel mirror and collimator. The diffraction patterns were
registered with a HiStar area detector. Oriented samples were
prepared as thin droplets of compound on a heated plate, having
one surface free.
3.2 Experimental methods
The structures of the synthesized compounds were confirmed by
the following analytical methods. Proton nuclear magnetic
resonance (1H NMR, 300 MHz) and carbon nuclear magnetic
resonance (13C NMR, 75 MHz) spectra were recorded with
4. Conclusions
We have reported the synthesis and mesomorphic properties of
asymmetric bent-core compounds possessing 3-aminophenol and
This journal is ª The Royal Society of Chemistry 2010
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N-methyl-3-aminophenol as a central unit. IR spectroscopy
confirmed the existence of hydrogen bonds between amide groups
of neighboring molecules. The extensive hydrogen bonding
induced liquid crystallinity and enhanced the phase stability.
Hydrogen bonding also induced a novel smectic phase SmAPAR
in two compounds with 3-aminophenol, N–H NO2,NO2 and N–H
NO2,Cl. Based on various experiments such as SHG and switch-
ing current measurements, we ascribed the SmAPAR phase to
a polar-plane-randomized SmAPA structure. We emphasized the
novelty of this phase as a phase showing both a short-range order
and a field-induced transition with a distinct threshold.
Acknowledgements
We acknowledge Prof. Y. Tezuka and Dr T. Yamamoto for
kindly allowing KG to use their chemistry lab and for useful
advice.
20 T. Kajitani, S. Kohmoto, M. Yamamoto and K. Kishikawa, Chem.
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This journal is ª The Royal Society of Chemistry 2010