The locally polar smectic A phase in V-shaped molecules

Article abstract of DOI:10.1039/c2jm35530b

For focusing on the polar smectic A phase derived from the V-shaped molecules, bent-core molecules with an acute-angled central core (Ar = 1,2-phenylene or 2,3-naphthylene) and a lateral halogen substituent (X = F or Cl) were synthesized. Their mesomorphic properties were investigated by DSC, polarizing microscopy, X-ray and electro-optical measurements. We found that a compound with Ar/X = 2,3-naphthylene/Cl forms the conventional smectic A phase, whereas three compounds with Ar/X = 1,2-phenylene/F, 1,2-phenylene/Cl and 2,3-naphthylene/F form the locally polar smectic A phase (SmAP_R). According to our knowledge this is the first obvious experimental evidence that V-shaped molecules with an acute-subtended angled central core, in cooperation with a lateral halogen substituent, can form the SmAP_R. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2jm35530b

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PAPER
The locally polar smectic A phase in V-shaped molecules
a
E-Joon Choi, Eun-Chol Kim,a Sang-Byung Park,b Wang-Cheol Zin,b You-Jin Leec and Jae-Hoon Kimc
*
Received 15th August 2012, Accepted 1st October 2012
DOI: 10.1039/c2jm35530b
For focusing on the polar smectic A phase derived from the V-shaped molecules, bent-core molecules
with an acute-angled central core (Ar ¼ 1,2-phenylene or 2,3-naphthylene) and a lateral halogen
substituent (X ¼ F or Cl) were synthesized. Their mesomorphic properties were investigated by
DSC, polarizing microscopy, X-ray and electro-optical measurements. We found that a compound
with Ar/X ¼ 2,3-naphthylene/Cl forms the conventional smectic A phase, whereas three compounds
with Ar/X ¼ 1,2-phenylene/F, 1,2-phenylene/Cl and 2,3-naphthylene/F form the locally polar smectic A
phase (SmAPR). According to our knowledge this is the first obvious experimental evidence that
V-shaped molecules with an acute-subtended angled central core, in cooperation with a lateral halogen
substituent, can form the SmAPR.
helicoidally modulated polarization with the short pitch; R
1. Introduction
stands for a randomly oriented polarization. Moreover, Shimbo
Nowadays the polar property of the mesophase is a hot issue in
et al. reported that such polar SmAPF shows the fast polarization
the research field of liquid crystals because of its potential
orientation and the associated birefringence as an in-plane elec-
application in advanced information-display areas of tech-
tronic field was applied to vertically aligned cells, which originate
nology. In 1974, it was discovered that if chiral molecules with a
from the cooperative motion of the bent molecules with a quasi-
rod-like mesogen form the tilted smectic phase their mesophases
long-range order of dipoles based on a two-dimensional Lange-
can possess the spontaneous polarization: these mesophases are
vin process.12
the polar smectic C phase (SmCP).1 In 1996, Takezoe and
According to the bent angle of the central core, the bent-core
Watanabe’s group reported the first obvious example of a SmCP
molecules can be classified into two categories: the V-shaped
derived from an achiral molecule with a bent-core mesogen.2 In
molecules that have an acute angled central core and the banana-
1997, Clark and Walba’s group depicted and explained the layer
shaped molecules that have an obtuse angled central core. To
structure of the SmCP using three stereogenic elements such as
date, the polar mesophase of the banana-shaped molecules has
chirality (+/ꢀ), clinacity (S/A) and polarity (F/A) of the adjacent
been an object of study for a long time. However, there is little
layer: there are four states such as SmCSPA, SmCAPF, SmCAPA,
and SmCSPF.3 Since then, a large number of studies have been
agreement about the polar mesophase of the V-shaped mole-
cules. At the beginning of 1990, Japanese groups first reported
made on the SmCP.
that the V-shaped molecules with a 1,2-phenylene central core
On the other hand, so far a limited number of studies on SmAP
form conventional nematic, smectic A and smectic B phases
(the polar smectic A phase) have been reported.4–8 For an
depending on the length of flexible terminals.13,14 Since then,
example, asymmetric banana-shaped molecules can form the
more studies on this topic have been reported by several research
SmAPA.9,10 Recently, Pociecha et al. have reported that SmAP in
groups. Prasad reported that a series of 1,2-phenylene bis[4-(4-
asymmetric banana-shaped molecules has a layered, nontilted,
alkoxyphenylazo)benzoates] form nematic, smectic A and crystal
E phases.15 Yelamaggad et al. reported that V-shaped molecules
optically uniaxial and polarly ordered structure with a random
direction of the layer polarization.11 To account for the structure
with salicylaldimine segments form nematic and smectic A pha-
of such SmAP, five possible arrangements have been proposed,
ses due to the intramolecular hydrogen bonding.16 Watanabe
SmAPF, SmAPA, SmAPa, SmAP2, and SmAPR: a stands for a
et al. reported that V-shaped molecules with 1,2-, 1,3-, 1,7- and
2,3-dihydroxynaphthalene central cores form smectic A and B4
phases.17 Recently, we reported that a main-chain polymer with a
18
aDepartment of Polymer Science and Engineering, Kumoh National
Institute of Technology, Gumi, Gyungbuk 730-701, Korea. E-mail:
V-shaped mesogen based on 2,3-naphthalene forms SmCPF
and a V-shaped molecule with a 2,3-dihydroxynaphthalene
central core forms an electric-field-induced SmAP.19
ejchoi@kumoh.ac.kr; Fax: +82 544787710; Tel: +82 544787684
bDepartment of Material Science and Engineering, Pohang University of
Science and Technology, Pohang, Gyungbuk 790-784, Korea
cDepartment of Information Display Engineering, Department of
Electronics and Communications Engineering, Hanyang University,
Seoul, 133-701, Korea
In this paper, four new V-shaped molecules with an acute-
angled central core (Ar) and a lateral halogen substituent (X)
have been synthesized as shown in Scheme 1. Their mesomorphic
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Compound 4c. IR (KBr pellet, cmꢀ1): 3059 (aromatic ¼C–H,
st), 2917, 2848 (aliphatic C–H, st), 1741 (C]O, st), 1519, 1462
(aromatic C]C, st), 1628 (C]N, st), 1272, 1102, 1013 (C–O, st).
1H NMR (CF3COOD, d in ppm): 8.44 (2H, s, N]CH), 8.26–
6.89 (18H, m, Ar–H), 4.06–3.99 (4H, t, J ¼ 6.57 Hz, OCH2),
1.85–1.74 (4H, m, OCH2CH2), 1.44–1.08 (36H, m,
OCH2CH2CH2).
Elemental
analysis
calcd
(%)
for
C62H72F2N2O6: C 76.04, H 7.41, N 2.86; found C 76.13, H 7.60,
N 2.90.
Compound 4d. IR (KBr pellet, cmꢀ1): 3062 (aromatic ¼C–H,
st), 2922, 2851 (aliphatic C–H, st), 1734 (C]O, st), 1495, 1465
(aromatic C]C, st), 1617 (C]N, st), 1265, 1162, 1060 (C–O, st).
1H NMR (CF3COOD, d in ppm): 8.43 (2H, s, N]CH), 8.25–
6.72 (18H, m, Ar–H), 4.04–3.98 (4H, t, J ¼ 6.39 Hz, OCH2),
1.86–1.76 (4H, m, OCH2CH2), 1.44–1.25 (36H, m,
OCH2CH2CH2).
Elemental
analysis
calcd
(%)
for
C62H72Cl2N2O6: C 73.57, H 7.17, N 2.77; found C 73.42, H 7.35,
N 2.93.
Scheme 1 Synthetic route towards compounds: (a) DCC, DMAP,
CHCl3, rt, 24 h; (b) Na2CO3, DMF, reflux, 4 h; (c) 100 psi H2, 10% Pd–C,
EtOH, 70 ꢁC, 4 h; (d) EtOH, 80 ꢁC, reflux, 8 h.
2.2. Characterization
Instruments. IR and NMR spectra were obtained by a Jasco
300E FT/IR and a Bruker Avance 400 MHz NMR spectrometer,
respectively. Elemental analysis was performed with a Thermo
Scientific Flash 2000 Elemental Analyzer. The transition
behaviors were characterized by a differential scanning calo-
rimeter (Netzsch DSC 200 F3 Maia). DSC measurements were
performed in a N2 atmosphere with heating and cooling rates of
10 and 20 ꢁC minꢀ1. Optical texture observation was carried out
using a polarizing microscope (Carl Zeiss Axioskop 40 Pol) with
a heating stage (Mettler FP82HT).
properties were investigated by DSC, polarizing microscopy,
X-ray diffraction, and electro-optical measurements. We have
described the obvious experimental evidence that our V-shaped
molecules can form the SmAPR.
2. Experimental
2.1. Synthesis
The V-shaped molecules (4a–4d) were synthesized by a modifi-
cation of the procedure previously reported by our group:
compound 1 was prepared by the direct esterification,20 and then
compound 4 was prepared by nucleophilic addition followed by
dehydration.18 The purity and the structure of the final
compounds were confirmed by thin-layer chromatography,
X-ray investigation. XRD measurements were performed in
ꢀ
transmission mode with synchrotron radiation (l ¼ 1.54 A) at
the Pohang Accelerator Laboratory. When heating and
cooling, the sample, which was sealed on both sides with a 7 mm
thick Kapton film, was held in an aluminium sample holder.
The data are presented as a function of q ¼ 4psin q/l (q: the
scattering angle). The sample was heated with two cartridge
heaters, and its temperature was monitored by a thermocouple
placed close to the sample. A background scattering correction
was performed by subtracting the scattering from the Kapton
film.
1
elemental analysis, and FT/IR and H NMR spectrometry. The
resultant data were in accordance with the expected formula.
Compound 4a. IR (KBr pellet, cmꢀ1): 3074 (aromatic ¼C–H,
st), 2922, 2854 (aliphatic C–H, st), 1737 (C]O, st), 1512, 1488
(aromatic C]C, st), 1628 (C]N, st), 1242, 1106, 1070 (C–O, st).
1H NMR (CF3COOD, d in ppm): 8.70 (2H, s, N]CH), 8.23–
6.77 (16H, m, Ar–H), 4.02 (4H, t, J ¼ 6.55 Hz, OCH2), 1.84–1.66
(4H, m, OCH2CH2), 1.40–0.99 (36H, m, OCH2CH2CH2).
Elemental analysis calcd (%) for C58H70F2N2O6: C 74.97, H 7.59,
N 3.01; found C 75.13, H 7.89, N 3.21.
Electro-optical measurement. The LC was injected between
two ITO glass substrates by capillary action at 180 ꢁC. Each
non-patterned ITO glass substrate was spin-coated with a
commercial planar alignment regent (AL22620, JSR). The spin-
ꢁ
ꢁ
coated alignment layer was pre-baked at 100 C for 10 min to
Compound 4b. IR (KBr pellet, cmꢀ1): 3063 (aromatic ¼C–H,
st), 2923, 2850 (aliphatic C–H, st), 1738 (C]O, st), 1493, 1469
(aromatic C]C, st), 1617 (C]N, st), 1259, 1231, 1055 (C–O, st).
1H NMR (CF3COOD, d in ppm): 8.44 (2H, s, N]CH), 8.23–
6.74 (16H, m, Ar–H), 3.95–3.88 (4H, t, J ¼ 6.54 Hz, OCH2),
1.86–1.76 (4H, m, OCH2CH2), 1.43–1.25 (36H, m,
evaporate the solvent, and then cured at 210 C for 1 hour to
complete imidization reaction. The rubbing directions of the
upper and the lower substrates were antiparallel to each other.
The cell thickness was maintained using glass spacers of
4.5 mm. The temperature of the cell was maintained with an
accuracy of within 0.1 ꢁC using a heating stage (FP90, Met-
tler). The switching current response characteristics were
OCH2CH2CH2).
Elemental
analysis
calcd
(%)
for
C58H70Cl2N2O6: C 72.41, H 7.33, N 2.91; found C 72.60, H 7.53,
N 3.03.
measured by applying a triangle wave voltage (E ¼ 22 V mmꢀ1
f ¼ 3 Hz).
,
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plays an important role in enhancing directional ordering of
mesogens instead of steric hindrance in their mesophase.
In Fig. 1, the melting behavior of compound 4b with X ¼ Cl
depends on the thermal history whereas compound 4a with X ¼
F did not show that. The fluorine atom can bestow better
packing property due to higher electron negativity, lower
molecular cohesive energy and smaller van der Waals radius
compared with the chlorine atom. In the case of compounds 4c
and 4d, the similar result was observed although their Tis are high
enough to suffer from thermal decomposition (Fig. 1j and n).
Fig. 2 shows the X-ray diffraction patterns obtained during
heating and cooling processes. The powder samples were
mounted between two amorphous Kapton films. At tempera-
tures below Tm defined by DSC, all compounds exhibited many
diffraction peaks, indicating a crystalline state. When heated
above the Tm, a strong peak was found in the small angle region,
while all peaks in the wide-angle region disappeared. This is
indicative of the smectic phase. When further heated above Ti
defined by DSC, only broad scattering patterns were observed,
indicating an isotropic liquid phase. Subsequently, when the
isotropic liquid was cooled below the Ti, the smectic phase
appeared reversibly in all compounds. When further cooled to
room temperature, compound 4a exhibited most distinctive
recrystallization, according to the DSC result.
Fig. 1 DSC thermograms obtained with heating and cooling rates of 10
(a–d, f–i, k, m) or 20 ꢁC minꢀ1 (e, j, l, n): dashed line: 1st cooling; solid-
line: 1st heating; dash-dotted line: 2nd heating. (a–c) 4a; (d–g) 4b; (h–j) 4c;
(k–n) 4d.
In Table 2, the layer spacing (d) values of the smectic meso-
phases are in the range of 3.77–4.00 nm. In general the d-spacing
values show subtle differences not only depending on the struc-
ture but also on the temperature. This indicates that bent angles
of molecules can be altered by changing the temperature in the
melt state.
3. Results and discussion
3.1. Mesogenic properties
Mesogenic properties have been examined by thermal, optical,
and X-ray measurements. In Fig. 1, DSC thermograms of
compounds show two distinctive transition temperatures that
have been defined as melting (Tm) and isotropization (Ti)
temperature, respectively. The transition temperatures and
enthalpy changes defined by DSC are summarized in Table 1.
Among four compounds, compound 4c with Ar/X ¼ 2,3-Naph/F
shows the highest Tm value and compound 4d with Ar/X ¼ 2,3-
Naph/Cl exhibits the lowest one. This implies that a highly bent
2,3-phenylene central core with a second ring extruded can play
either a constructive or destructive role in molecular packing in
the solid state in cooperation with the lateral halogen substituent.
On the other hand, the DHi values of compounds 4c and 4d are
2.5 and 3.6 times greater than those of compounds 4a and 4b,
respectively. This means that the 2,3-naphthylene central core
To account for a stable conformation, in a similar way with
a 1,3-phenylene central unit,21,22 we have postulated that phenyl
rings are coplanar when carbonyl groups are attached to them,
and alkyloxy groups in all-trans conformation are equiplanar
to the benzene rings. Moreover, we assumed that the bent angle
is 60 degrees and there is no intercalation between chains.
Then, we can estimate the length of the molecule with the all-
trans-conformation to be 3.53 nm by using the PCFF force-
field model of martial studio. However, this value is not
matched with d values determined by X-ray measurements.
Nevertheless, the postulation that the molecules would form a
tilted smectic mesophase can be excluded because the d-spacing
value would be smaller than 3.53 nm. In the mean time, if we
assume that bent angles are larger than 60ꢁ, the fact that
d-values are larger than 3.53 nm can be explained by the
formation of the smectic A phase. Then, we can calculate the
opening angle of arms to be 76.9–82.6 degrees corresponding to
d ¼ 3.77–4.00 nm.
Table 1 Transition temperatures (ꢁC) and enthalpy changes (kJ molꢀ1
)
Compound
Ar/X
DSC scana
Tm
DHm
Ti
DHi
Fig. 3 shows cross-polarizing optical micrographs obtained
by using a normal slide glass. During the heating process,
compounds 4a and 4d showed the optical texture shown in
Fig. 3a and b, respectively, while compounds 4b and 4c showed
a homeotropic-like optical texture. When the isotropic liquid
was cooled, no birefringence occurred at the isotropic-to-
smectic transition temperatures and then the smectic phases
held a homeotropic optical texture until solidification. Note
that compounds 4a and 4c showed the Maltese-cross spheru-
lites with a homeotropic region as shown in Fig. 3c and d,
respectively.
4a
1,2-Phen/F
1-H
1-C
2-H
1-H
1-C
2-H
1-H
1-C
1-H
1-C
95
61
94
83
75
88
118
42
75
41
59.0
7.3
58.6
54.9
3.3
3.0
47.1
6.4
150
148
150
133
130
133
242
234
231
224
3.1
3.1
3.0
2.6
2.5
2.5
7.6
6.1
9.3
7.1
4b
1,2-Phen/Cl
4c
4d
2,3-Naph/F
2,3-Naph/Cl
3.3
2.5
a
Abbreviations: 1-H ¼ 1st heating; 2-H ¼ 2nd heating; 1-C ¼ 1st cooling.
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Table 2 The q-value (q) and layer spacing (d) for smectic phases
Process temp.a(ꢁC) q (nmꢀ1
Compound Ar/X
)
d (nm)
4a
4b
4c
1,2-Phen/F
H(120)
C(140)
H(110)
C(100)
H(150)
H(220)
C(200)
C(60)
1.64
1.64
1.65
1.64
1.67
1.61
1.58
1.57
1.62
1.61
1.64
3.83
3.83
3.80
3.83
3.77
3.90
3.97
4.00
3.87
3.90
3.83
1,2-Phen/Cl
2,3-Naph/F
4d
2,3-Naph/Cl H(200)
C(200)
C(60)
a
Heating (H) and cooling (C) process temps. are given in parentheses.
Fig. 3 Cross-polarizing optical micrographs obtained at given temper-
ature (ꢂ200). On heating: (a) 4a, at 127 ꢁC; (b) 4d, at 225 ꢁC. On cooling:
(c) 4a, at 37 ꢁC; (d) 4c, at 65 ꢁC.
Fig. 2 Synchrotron radiation X-ray diffraction profiles of compounds
obtained at given temperatures (ꢁC, in parentheses). Abbreviations:
RT ¼ room temperature; H ¼ heating process; C ¼ cooling process.
Fig. 4 Cross-polarized optical micrographs obtained using planarly
aligned cells at given temperatures.
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Fig. 6 (a) Peak separations of reverse current curves taken at the smectic
phase. (b) Temperature dependence of the polarization estimated from
the 1st peak ( ), 2nd peak ( ), and total peak (C) of reverse current.
3.2. Electro-optical property
For electro-optical investigation, planarly aligned cells were
used. The rubbing directions of the upper and the lower
substrates were antiparallel to each other. In Fig. 4, the
directions of a polarizer (P) and an analyzer (A) crossing each
other make an angle of 45ꢁ with the rubbing direction (R). On
the planar alignment layers, the optical textures of compounds
4a–4c show the incomplete unidirectional alignment similar to
micro-domains. The transmittance differences among these
compounds are attributed to the different effective retardation:
although the cell gaps are almost the same, the incomplete
uniformity in alignment and the different magnitude of bire-
fringence can lead each texture to different brightness and
colors. Nonetheless, compound 4d has a totally different
texture compared with compounds 4a–4c. This indicates that
the former forms a different type of smectic phase from the
latter.
Moreover, we have investigated the polar property of these
smectic mesophases using current peak evaluation. Fig. 5 shows
the switching current response on applying a triangular wave
voltage. The reverse current peaks appeared at every half
period of applied voltage, indicating a polar switching. In
Fig. 6a, all the reverse current peaks are clearly separated by
two individual peak functions. In Fig. 6b, the spontaneous
polarizations gradually increase as the temperature increases.
As a result, we have suggested that the smectic phases of 4a–4c
are locally polar, SmAPR. But the smectic phase of 4d is
conventional.
Fig. 5 Switching current response for compounds by applying a trian-
gular voltage (E ¼ 22 V mmꢀ1, f ¼ 3 Hz) at given temperatures.
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4. Conclusions
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This work was supported by the National Research Foundation
of Korea (KOSEF) grant funded by Korea government (MEST)
(no. R01-2008-000-11521-0). This work was supported in part by
KRCF (Korea Research Council of Fundamental Science &
Technology) and KIST (Korea Institute of Science and Tech-
nology) for NAP (National Agenda Project) program, and sup-
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This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 24930–24935 | 24935