Electro-optical switching studies on 1,3-phenylene based banana shaped liquid crystals

Article abstract of DOI:10.1016/j.molstruc.2011.06.027

This work deals with the study of the thermotropic liquid crystalline properties of new asymmetric bent core liquid crystals. The thermotropic properties was analyzed by polarized optical microscopy, differential scanning calorimetric analysis and variable temperature X-ray diffraction measurements. Effect of equal terminal chain with asymmetric bent core system on mesomorphic behavior was examined. The shorter methyl chain compound 10a of exhibits B ₁ mesophase and higher homologs (10b and 10c) shows antiferroelectric B₂ mesophases, which was studied by electro-optical measurements.

Full text of DOI:10.1016/j.molstruc.2011.06.027

Journal of Molecular Structure 1001 (2011) 118–124
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Electro-optical switching studies on 1,3-phenylene based banana shaped
liquid crystals
b
b
S. Balamurugan a, P. Kannan a, , K. Yadupati , A. Roy
⇑
a Department of Chemistry, Anna University, Chennai 600 025, India
b Soft Condensed Matter, Raman Research Institute, C.V. Raman Avenue, Sadashivanagar, Bangalore 560 080, India
a r t i c l e i n f o
a b s t r a c t
Article history:
This work deals with the study of the thermotropic liquid crystalline properties of new asymmetric bent
core liquid crystals. The thermotropic properties was analyzed by polarized optical microscopy, differen-
tial scanning calorimetric analysis and variable temperature X-ray diffraction measurements. Effect of
equal terminal chain with asymmetric bent core system on mesomorphic behavior was examined. The
shorter methyl chain compound 10a of exhibits B1 mesophase and higher homologs (10b and 10c) shows
antiferroelectric B2 mesophases, which was studied by electro-optical measurements.
Ó 2011 Elsevier B.V. All rights reserved.
Received 27 April 2011
Received in revised form 23 June 2011
Accepted 23 June 2011
Available online 1 July 2011
Keywords:
Bent core liquid crystals
B1 mesophase
B2 mesophase
Antiferroelectric
Thermotropic liquid crystals
1. Introduction
complexes, hydrogen bonds) for the formation of mesophase
[10]. Niori et al. [11] initially demonstrated smectic phases of achi-
Banana shaped (bent-core) liquid crystals are usually architect-
ed by covalent linking of two mesogenic arms at 1, 3 position of
resorcinol or 2, 7 position of naphthol [1]. In lamellar smectic mes-
ophases the bent molecules are polar and packed in the bent direc-
tion. The polar packing generates macroscopic polarization in the
smectic layers with physical characteristics such as ferroelectric
or antiferroelectric properties [2]. These molecules can tilt in syn
or anti fashion in successive layers which results in four types of
arrangements in both ferro-(homochiral SmCSPF or recemic
SmCAPF) and antiferro-(recemic SmCSPA or homochiral SmCAPA)
electric properties. The antiferroelectric properties are prevalent
in majority of the bent-core compounds so far investigated [3].
Several symmetrical and unsymmetrical bent-core compounds
have been synthesized till date, and the liquid crystalline phases
exhibited have been classified as B1 to B8 [4,5]. It is apparent that
the formation of mesophase depends on structural variations of
the compounds in comparison with calamitic compounds. The
mesophase behavior is strongly influenced by number of rings
[6], shape of the molecules, positions and direction of the linking
groups [7], effect of lateral substituent [8] and length of the termi-
nal chains [9].
ral molecules with bent shapes can exhibit ferroelectric switching
property due to specific steric interaction in the bent shape mole-
cules. The molecules can preferably be packed in bent direction
giving rise to a long-range correlation of lateral dipole moments
[12]. Later, antiferroelectric liquid crystals were discovered and fi-
nally identified by tristable switching with a sharp threshold and a
double hysteresis [13]. This has paved development in very fast
electro-optic devices due to quick response of these materials used
for flat-panel displays [14–17]. The tristable switching is observed
by two methods, namely, electro-optic effect and switching current
measurements.
In this paper, we report the synthesis of three different com-
pounds consisting of bent-core unsymmetrical mesogen con-
structed by different linking groups and equal terminal chain
lengths. Herein, the effect of equal terminal chain length is studied
through thermal, electro-optical and X-ray diffraction studies.
2. Experimental section
2.1. Materials
The dependence on molecular structure, intermolecular interac-
tion play an important role (e.g. polar forces, charge-transfer
Solvents such as benzene, methanol, ethanol, dichloromethane,
ethyl acetate, tetrahydrofuran and acetone were purified by
the reported procedure. Potassium hydroxide, sodium hydroxide,
potassium carbonate, hydrochloric acid (Merck, India), 4-hydroxy-
benzoic acid, 4-hydroxybenzaldehyde, potassium iodide,
⇑
Corresponding author. Tel.: +91 44 2235 8666; fax: +91 44 2220 0660.
E-mail address: pakannan@annauniv.edu (P. Kannan).
0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2011.06.027

S. Balamurugan et al. / Journal of Molecular Structure 1001 (2011) 118–124
119
triethylamine (Spectrochem, India), N,N-dicyclohexylcarbodiimide
(DCC), palladium carbon (10%), potassium dichromate, resorcinol,
benzyl chloride and 4-(dimethylamino)pyridine (DMAP) (Aldrich)
were used as received.
cmꢀ1): 2918 (C–H), 2546 (–OH of COOH), 1685 (C@O), 1256
(C–O). 1H NMR (CDCl3, 400 MHz), d (ppm): 11.02 (s, 1H, –COOH),
8.12 (d, 2H, Ar–H), 6.96 (d, 2H, Ar–H), 4.01 (t, 2H, –OCH2), 1.24–
1.39 (m, 10H, –CH2), 0.93 (t, 3H, –CH3). 13C NMR (75 MHz, CDCl3):
d = 172.4 (–COOH), 163.8, 132.3, 121.4, 114.3 (aromatic), 68.2 (Ar–
O–CH2), 31.6, 29.0, 25.7, 22.4 (aliphatic).
2.2. Techniques
Elemental analysis was carried out on a Carlo-Erba 1106 sys-
tem. Infrared spectra were obtained on Thermo Electron Corpora-
tion Nicolet 380 FT-IR spectrometer. 1H NMR (400 MHz) and 13C
NMR (75 MHz) spectra were recorded on a Bruker AM-400 spec-
trometer with Me4Si as internal standard. Differential scanning
calorimetry (DSC) was conducted on a Perkin–Elmer model DSC
Pyris 1 system calibrated with indium and zinc standards. Polariz-
ing optical microscopy (POM) was performed with a polarizing
microscope (Leitz, model Ortholux Pol-BK) equipped with a heat-
ing stage (Mettler, model FP 80HT). Spontaneous polarization (Ps)
in the B2 phase was measured by a triangular wave method [18]
in a planner-aligned cell using a digital oscilloscope (Agilent, mod-
el MSO6012A) and an arbitrary waveform generator (Agilent,
model 33220A) coupled with a power amplifier (Trek, model 601).
X-ray diffraction (XRD) measurements of non-oriented (Linde-
mann capillary, diameter 0.7 mm) samples were performed using
2.5. Synthesis of 4(4-n-heptyloxybenzoyloxy)benzaldehyde (3a)
To a mixture of 4-heptyloxybenzoic acid (5.2 g, 22 mmol),
4-hydroxybenzoldehyde (1.8 g, 22 mmol), dicyclohexylcarbodiim-
ide (DCC) (3 g, 25 mmol), and 5% w/w of 4-(N,N0-dimethyl-
amino)pyridine (DMAP) were dissolved in methylene chloride
(200 mL) and the resulting solution stirred for 12 h at room tem-
perature under nitrogen atmosphere. Precipitated byproduct urea
was filtered from the reaction mixture and filtrate concentrated
by vacuum evaporation. Crude product was purified by silica gel
column chromatography using chloroform as eluent. The product
4-(4-n-heptyloxybenzoyloxy) benzaldehyde (5.5 g; 85 %) was ob-
tained as a white powder and a similar procedure adopted for
preparation of 3b and 3c.
Compound 3a: M.P. 120 °C. C21H24O4: Calcd C, 74.09; H, 7.11;
O, 18.8; found C, 74.44; H, 6.80; O, 18.76. FT-IR (KBr pellet,
cmꢀ1): 2912, 2865 (C–H), 1734 (C@O of ester), 1605 (C@O of
CHO), 1269 (C–O of ether). 1H NMR (CDCl3, 400 MHz), d (ppm):
10.02 (s, 1H, –CHO), 8.14 (d, 4H, Ar–H), 7.96 (d, 2H, Ar–H), 7.4 (d,
1H, Ar–H), 6.98 (d, 1H, Ar–H), 4.05 (t, 2H, –OCH2), 1.71 (dd, 2H,
–CH2), 1.24–1.39 (m, 8H, –CH2), 0.84 (t, 3H, –CH3). 13C NMR
(75 MHz, CDCl3): d = 191.7 (–CHO), 166.5 (ester), 163.7, 163.3,
154.5, 132.0, 130.6, 128.2, 122.0, 120.3, 114.6 (aromatic), 67.9
(Ar–O–CH2), 31.1, 28.6, 25.4, 22.02, 13.87 (aliphatic).
Cu Ka radiation from a rotating anode generator (Rigaku Ultrax-
18) having a graphite crystal as monochromator. The diffraction
pattern of each sample was recorded on a 2D image plate
(Marresearch).
2.3. Synthesis of resorcinolmonobenzyl ether (1)
Resorcinol (11 g, 100 mmol) was dissolved in acetone (100 mL).
To this solution, powdered potassium carbonate (13.8 g,
300 mmol) and potassium iodide (pinch) were added slowly. The
mixture was refluxed and benzyl chloride (11.6 mL, 100 mmol)
added drop wise to the refluxing mixture over a period of
30 min. The reaction was carried at for 48 h and cooled the reaction
to room temperature and potassium carbonate was filtered and
washed with acetone, the collected filtrates concentrated by vac-
uum distillation. Product thus obtained was purified by column
chromatography using chloroform and hexane (3:7) mixture as
eluent to yield pale brown color solid (12 g; 72%).
Compound 1: M.P. 52 °C. C13H10O2: Calcd C, 77.67; H, 6.47; O,
15.85; found C, 77.58; H, 7.29; O, 15.71; FT-IR (KBr pellet, cmꢀ1):
3290 (–OH), 3047 (C–H), 1257 (Ar–O–CH2). 1H NMR (CDCl3,
400 MHz), d (ppm): 8.27 (d, 2H, Ar–H), 7.65–7.71 (m, 3H, Ar–H),
7.32 (t, 1H, Ar–H), 6.91–6.97 (m, 3H, Ar–H), 5.38 (s, 1H, –OH),
5.19 (s, 2H, –OCH2). 13C NMR (75 MHz, CDCl3): d = 163.8, 157.2,
152.6, 132.2, 130.8, 128.3, 113.2, 112.6, 106.5 (aromatic) 70.4
(aliphatic).
2.6. Synthesis of 4-(4-n-heptyloxybenzoyloxy) benzoic acid (4a)
4-(4-n-Heptyloxybenzoyloxy) benzaldehyde (5.1 g, 15 mmol)
was dissolved in acetone (20 mL) and diluted with water to make
the final volume of 100 mL. Addition of Jones reagent [mixture
chromium oxide (26.72 g) with concentrated sulfuric acid
(23 mL)] was continued till red color persisted for at least 1 min.
Resultant mixture was stirred at room temperature for 30 min to
ensure the completion of oxidation. Excess oxidizing reagent was
quenched with 2-propanol. Reaction solution was then diluted
with water and repeated extraction with ether. Combined organic
extracts were dried over anhydrous sodium sulfate, filtered and
concentrated in vacuum. Crude product was purified by column
chromatography (4.7 g; 87%). A similar procedure was adopted
for the preparation of 4b and 4c.
Compound 4a: M.P. 97 °C. C21H22O5: Calcd C, 70.16; H, 6.48; O,
23.36; found C, 70.23; H, 6.65; O, 23.12. FT-IR (KBr pellet, cmꢀ1):
2964, 2866 (C–H), 1739 (C@O of ester), 1604 (C@O of –COOH),
2.4. Synthesis of 4-heptyloxybenzoic acid (2a)
1258 (–C–O). 1H NMR (CDCl3, 400 MHz),
d (ppm): 10.97
(s, 1H, –COOH), 8.20–8.06 (dd, 4H, Ar–H), 7.96 (d, 2H, Ar–H),
7.09 (d, 2H, Ar–H), 4.05, (t, 2H, –OCH2), 1.68–1.73 (m, –CH2),
1.23–1.39 (m, 8H, –CH2), 0.84 (t, 3H, –CH3). 13C NMR (75 MHz,
CDCl3): d = 166.6 (–COOH), 163.7 (ester), 163.4 (ether), 154.4,
132.4, 130.6, 128.2, 122.1, 120.1, 114.5 (aromatic), 67.9 (Ar–O–
CH2), 31.1, 28.6, 25.3, 22.0, 13.6 (aliphatic).
4-Hydroxybenzoic acid (6.4 g, 46 mmol) and potassium hydrox-
ide (19 g, 138 mmol) were dissolved in ethanol (30 mL) and stirred
for 1 h at room temperature. 1-Bromoheptane (8.2 g, 46 mmol)
was then added drop wise to this solution followed by addition
of potassium iodide (0.8 g, 4.6 mmol) in one portion. The solution
refluxed for 24 h with constant stirring. The reaction mixture
was concentrated by distillation of ethanol. Then the mixture
was poured in ice water (500 mL) and neutralized with 10% hydro-
chloric acid solution. Resultant precipitate was filtered and recrys-
tallized from absolute ethanol to get the desired product (Yield
67%). A similar procedure was adopted for preparation of 2b
and 2c.
2.7. Synthesis of 4-((3-(benzyloxy)phenoxy)carbonyl)phenyl
4-(heptyloxy)benzoate (5a)
To
a mixture of 4-(4-n-heptyloxybenzoyloxy)benzoic acid
(3.6 g, 10 mmol), resorcinol monobenzylether (2 g, 10 mmol),
DCC (2.5 g, 12 mmol) and a catalytic amount of DMAP in dry
dichloromethane (50 mL) were stirred for 12 h. The precipitated
N,N0-dicyclohexylurea was filtered, washed with excess of
Compound 2a: M.P. 146 °C. C14H20O3: Calcd C, 71.16; H, 8.53;
O, 20.31; found C, 71.02; H, 8.79; O, 20.19. FT-IR (KBr pellet,

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S. Balamurugan et al. / Journal of Molecular Structure 1001 (2011) 118–124
dichloromethane and filtrate concentrated in a rotary evaporator.
The residue was purified by silica gel column using chloroform
as eluent. The product obtained on removal of chloroform was fur-
ther purified by recrystallization using mixture of chloroform and
hexane (1:3) to get 4.8 g yield (84%). A similar procedure was
adopted for preparation of 5b and 5c.
(75 MHz, CDCl3): d = 191.5 (–CHO), 164.3, 164.2 (ester), 163.7,
155.3, 151.2, 151.2, 139.6, 134.1, 132.4, 131.8, 130.5, 130.0, 129.6,
126.6, 122.3, 120.9, 119.5, 119.0, 115.9, 114.4 (aromatic), 68.3
(Ar–O–CH2), 31.7, 29.0, 29.0, 25.6, 22.1, 14.0 (aliphatic).
2.10. Synthesis of 4-heptyloxyacetanilide (8a)
Compound 5a: M.P. 97 °C. C35H34O6: Calcd C, 75.82; H, 6.36; O,
17.82; found C, 75.76; H, 6.44; O, 17.8. FT-IR (KBr pellet, cmꢀ1):
2936, 2866 (C–H), 2367 (C–H of benzyl), 1740 (C@O), 1258
(C–O). 1H NMR (CDCl3, 400 MHz), d (ppm): 8.29 (d, 2H, Ar–H),
8.15 (d, 2H, Ar–H), 7.68 (d, 2H, Ar–H), 7.40–7.41 (m, 8H, Ar–H),
7.20–7.25 (m, 3H, Ar–H), 7.18 (s, 1H, Ar–H), 7.03 (d, 2H, Ar–H),
5.23 (t, 2H, –COCH2), 5.19 (s, 2H, –OCH2) 1.71 (dd, 2H, –CH2),
1.24–1.39 (m, 8H, –CH2), 0.84 (t, 3H, –CH3). 13C NMR (75 MHz,
CDCl3): d = 166.8, 163.8, 163.1 (ester), 154.3, 151.7, 136.1, 132.0,
130.8, 130.6, 129.5, 129.0, 128.2, 122.3, 120.3, 114.6, 113.9,
111.8, 108.2 (aromatic), 69.5, 67.9 (Ar–O–CH2), 31.2, 28.6, 25.7,
22.0, 13.8 (aliphatic).
A mixture of 4-hydroxyacetanilide (3 g, 20 mmol), anhydrous
K2CO3 (8.4 g, 60 mmol), 1-bromoheptane (3.4 g, 19 mmol) and
pinch of KI in 200 mL of acetone were stirred at 70 °C for 48 h. Then
the reaction mixture was cooled to room temperature, filtered,
washed with excess of acetone. The solvent was removed under
vacuum to give white solid. The solid obtained was dissolved in
diethyl ether and washed with water (3 ꢁ 300 mL) to remove the
unreacted 4-hydroxyacetanilide. The organic layer was dried over
anhydrous sodium sulfate, solvent removed under vacuum and
recrystallized from methylene chloride to give the 3.9 g of bright-
white crystals of 4-heptyloxyacetanilide (Yield 87%) (Henderson
et al. 2001). A similar procedure was adopted for the preparation
of 8b and 8c.
2.8. Synthesis of 4-((3-hydroxyphenoxy)carbonyl)phenyl
4-(heptyloxy)benzoate (6a)
Compound 8a: M.P. 51 °C. C14H12NO2: Calcd C, 71.46; H, 8.99;
N, 5.95; O, 13.60; found C, 71.41; H, 9.09; N, 5.92; O, 13.58. FT-IR
(KBr pellet, cmꢀ1): 3320 (N–H), 2924, 2850 (C–H), 1660 (C@O of
ester), 1240 (C–O). 1H NMR (CDCl3, 400 MHz), d (ppm): 8.01 (s,
1H, NH2), 7.28 (d, 2H, Ar–H), 6.78 (d, 2H, Ar–H), 3.90 (t, 2H,
–OCH2), 1.70 (m, 2H, –CH2), 1.27–1.43 (m, 8H, –CH2), 0.85 (t, 3H,
–CH3). 13C NMR (75 MHz, CDCl3): d = 168.7 (C@O), 155.8, 131.5,
121.0, 114.5 (aromatic), 68.5 (Ar–O–CH2), 31.4, 29.1, 29.3, 29.2,
29.2, 26.5, 22.0, 14.9 (aliphatic).
4-(4-n-heptyloxybenzoyloxy) phenylmonobenzylether-3-ben-
zoate (4.3 g, 8 mmol) was dissolved in ethyl acetate (150 mL) con-
taining a suspension of Pd/C catalyst (10% Pd/C). The mixture was
stirred for 24 h under H2 atmosphere, filtered and concentrated
under vacuum. Crude product thus obtained was purified by silica
gel column chromatography using ethyl acetate–hexane (1:3) as
eluent to get 3.2 g yield 88%. A similar procedure was adopted
for the preparation of 6b and 6c.
Compound 6a: M.P. 128 °C. C27H28O6: Calcd C, 72.30; H, 6.29;
O, 21.40; found C, 71.87; H, 6.31; O, 21.82. FT-IR (KBr pellet,
cmꢀ1): 3310 (–OH), 2937, 2864 (C–H), 1740 (C@O), 1258 (C–O).
1H NMR (CDCl3, 400 MHz), d (ppm): 8.29 (d, 2H, Ar–H), 8.15 (d,
2H, Ar–H), 7.68 (d, 2H, Ar–H), 7.42 (t, 1H, Ar–H), 7.23 (d, 2H, Ar–
H), 7.00–7.06 (m, 23H, Ar–H), 5.57 (s, 1H, OH), 5.23 (t, 2H, –
OCH2), 1.71 (dd, 2H, –CH2), 1.24–1.39 (m, 6H, –CH2), 0.84 (t, 3H,
–CH3). 13C NMR (75 MHz, CDCl3): d = 166.7, 163.5 (ester), 154.3,
151.9, 130.8, 129.1, 128.4, 122.1, 114.5, 111.8, 108.1 (aromatic),
69.4, 67.9 (Ar–O–CH2), 31.2, 28.5, 25.7, 22.0, 13.8 (aliphatic).
2.11. Synthesis of 4-heptyloxyaniline (9a)
4-Heptyloxyacetanilide (4.7 g, 20 mmol) was dissolved in etha-
nol (150 mL), 10 mL of concentrated HCl in ethanol (25 mL) added
drop wise to the mixture. The reaction mixture was heated to re-
flux for 12 h, cooled, poured into ice-water mixture. The white so-
lid thus obtained was extracted with ether, washed with water
(3 ꢁ 100 mL), brine solution (3 ꢁ 100 mL) and dried over anhy-
drous sodium sulfate. The solvent was removed under rotary evap-
orator to give 3.2 g of white crystal (Yield 82%). A similar procedure
was adopted for the preparation of 9b and 9c.
2.9. Synthesis of 3-(4-(4-(heptyloxy)benzoyloxy)benzoyloxy)phenyl
4-formylbenzoate (7a)
Compound 9a: M.P. 44–48 °C. C12H19NO: Calcd C, 74.54; H,
9.91; N, 7.25; O, 8.28; found C, 74.50; H, 9.89; N, 7.26; O, 8.35.
FT-IR (KBr pellet, cmꢀ1): 3409 (N–H), 2933, 2857 (C–H), 1121
(–C–N). 1H NMR (CDCl3, 400 MHz), d (ppm): 7.09 (d, 2H, Ar–H),
6.52 (d, 2H, Ar–H), 3.64 (t, 2H, –OCH2), 1.85 (s, 2H, –NH2), 1.56–
1.61 (m, 2H, –CH2), 1.22–1.27 (m, 8H, –CH2), 0.79 (t, 3H, –CH3).
13C NMR (75 MHz, CDCl3): d = 158.89, 124.48, 122.46, 115.20 (aro-
matic), 68.77 (Ar–O–CH2), 31.77, 29.25, 25.97, 22.61, 20.80, 14.02
(aliphatic).
A mixture of 4-((3-hydroxyphenoxy)carbonyl)phenyl 4-(hep-
tyloxy)benzoate (4.5 g, 10 mmol), and a 4-formylbenzoic acid
(1.5 g, 10 mmol) and a catalytic amount of 4-(N,N-dimethyl-
amino)pyridine (DMAP) in dry dichloromethane (25 mL) was stir-
red for 10 min. To this mixture, N,N0-dicyclohexylcarbodiimide
(DCC) (2.5 g, 12 mmol) was added and stirred for about 12 h at
room temperature. The precipitated N,N0-dicyclohexyl urea was fil-
tered off and washed with excess of dichloromethane. The comba-
ined organic solution was washed with ice-cold aqueous 5%
sodium hydroxide solution (3 ꢁ 75 mL), 5% HCl (3 ꢁ 75 mL) and fi-
nally with wáter (3 ꢁ 75 mL) and dried over anhydrous sodium
sulfate. The residue obtained on removal of solvent was chromato-
graphed on silica gel using chloroform as eluent to yield 4.9 g
(86.5%) A similar procedure was adopted for 7b and 7c.
Compound 7a: M.P. 137 °C. C35H32O8: Calcd C, 72.40; H; 5.56;
22.04; found C, 72.36; H, 5.51; O, 22.13. FT-IR (KBr pellet, cmꢀ1):
2926, 2853 (C–H), 1733 (C@O of ester), 1516 (C–O of CHO). 1H
NMR (CDCl3, 400 MHz), d (ppm): 10.05 (s, 1H, –CHO), 8.27 (d, 2H,
Ar–H), 8.20 (d, 2H, Ar–H), 8.05 (d, 2H, Ar–H), 7.93 (d, 2H, Ar–H),
7.43 (t, 1H, Ar–H), 7.28 (d, 2H, Ar–H), 7.18 (s, 1H, Ar–H), 7.10 (d,
2H, Ar–H), 6.89 (d, 2H, Ar–H), 3.97 (d, 2H, –OCH2), 1.70–1.77 (dd,
2H, –CH2), 1.24–1.39 (m, 8H, –CH2), 0.80 (t, 3H, –CH3). 13C NMR
2.12. Synthesis of 3-(4-(4-(heptyloxy)benzoyloxy)benzoyloxy)phenyl-
4-((4-(heptyloxy)phenylimino)methyl)benzoate(10a)
A mixture of compound 7a (0.62 g, 3.2 mmol) and compound 9a
(1.20 g, 3.2 mmol) dissolved in chloroform (25 mL) and refluxed for
4 h. The reaction mixture was concentrated and recrystallized from
ethanol to give 10a (Yield 71%) as pale yellow powdered crystals. A
similar procedure was adopted for the preparation of compounds
10b and 10c.
Compound 10a: M.P. 93–129 °C. C48H51NO8: Calcd C, 74.88; H,
6.68; N, 1.82; O, 16.62; found C, 74.84; H, 6.59. N, 1.79. O, 16.78.
FT-IR (KBr pellet, cmꢀ1): 2924, 2853 (C–H), 1732 (C@O of ester),
1605 (–C@N). 1H NMR (CDCl3, 400 MHz), d (ppm): 8.57 (s, 1H,
CHO), 8.28 (d, 2H, Ar–H), 8.26 (d, 2H, Ar–H), 8.14 (d, 2H, Ar–H),
8.01 (d, 2H, Ar–H), 7.48 (t, 1H, Ar–H), 7.36 (d, 2H, Ar–H), 7.28 (d,

S. Balamurugan et al. / Journal of Molecular Structure 1001 (2011) 118–124
121
2H, Ar–H), 7.25 (s, 1H, Ar–H), 7.22 (d, 2H, Ar–H), 7.18 (d, 2H, Ar–H),
6.93–6.99 (dd, 4H, Ar–H), 4.05 (t, 2H, OCH2), 3.98 (t, 2H, –OCH2),
1.76–1.86 (m, 4H, –CH2), 1.25–1.58 (m, 22H, –CH2), 0.89 (t, 6H, –
CH3). 13C NMR (75 MHz, CDCl3): d = 164.2 (ester), 158.6, 156.4,
155.6, 151.5, 144.7, 132.7, 132.0, 130.7, 130.7, 128.6, 122.6,
122.0, 121.8, 199.4, 155.9, 155.2, 144.8 (aromatic), 68.8 (Ar–O–
CH2), 32.0, 32.1, 29.4, 29.7, 29.6, 29.5, 29.0, 29.6, 29,3, 29.2, 26.0,
26.1, 22.2, 14.2 (aliphatic).
were prepared by following Williamson ether synthesis proce-
dure. The key intermediate 4(40-n-alkyloxybenzoyloxy)benzalde-
hyde (3a–3c shown in Scheme 1) was synthesized by the
reaction between n-alkyloxybenzoic acid and 4-hydroxybenzalde-
hyde which was oxidized by Jones reagent in the presence of ace-
tone. 4-(4-n-Alkyloxybenzoyloxy)benzoic acid (4a–4c shown in
Scheme 1) was treated with resorcinol monobenzyl ether (1) in
the presence of N,N0-dicyclohexylcarbodiimide as dehydrating
agent and 4-(N,N0 dimethylamino)pyridine as a catalyst to get
4-(4-n-alkyloxybenzoyloxy) phenylmonobenzylether-3-benzoate
(5a–5c). Further the compound 4-(4-n-hexyloxy benzoyloxy) phe-
nyl-3-hydroxybenzoate (6a–6c) was obtained using 5% Pd–C in
dried ethyl acetate in the presence of hydrogen atmosphere.
The phenolic compounds (6a–6c) obtained was condensed with
4-formylbenzoic acid to obtain 4-((3-(4-formyl benzoyloxy)phen-
3. Results and discussion
3.1. Synthesis
The bent core compounds were prepared by following syn-
thetic pathway as shown in Scheme 1. n-Alkoxybenzoic acids
Scheme 1. Synthetic pathway to prepare the final compounds 10a, 10b and 10c.

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S. Balamurugan et al. / Journal of Molecular Structure 1001 (2011) 118–124
oxy)carbonyl)phenyl 4-(hexyloxy)benzoate (7a–7c) by esterifica-
tion reaction, which was further treated with 4-alkyloxyaniline
to get the target compounds (10a–10c). The structures of the syn-
thesized compounds were confirmed by spectral techniques
(Fig. 1).Asymmetric compound is breaking the symmetrical struc-
ture by lateral substituent with different linking groups and dif-
ferent terminal chain lengths [18]. As a result the melting point
of compounds was reduced with wide temperature range of B
phases. In this paper we studied asymmetric compounds in which
the asymmetry is introduced in the bent core of the molecules
using different linking groups but with symmetrical terminal
chains. We have used the ester and azomethine linking groups
to achieve asymmetric core of the molecules. Hence, the dipole
moments of linking groups of the left and right arms are not
compensated to each other in the asymmetric mesogens. The
asymmetry in the mesogens is used to achieve lower melting
temperature with retention of B-phases.
3.2. Mesomorphic properties of the compounds
Fig. 2. DSC thermogram traces of compounds 10a, 10b and10c.
DSC thermogram of compounds 10a, 10b and 10c was shown in
Fig 2. The transition temperatures and associated enthalpy values
obtained for the compounds are summarized in Table 1. First heat-
ing and cooling thermograms of DSC were measured at the rate of
5 °C/min. The melting temperature (Tm) of the series of compounds
was observed in the range of 63.3–92.9 °C.
The homologous series of compounds exhibited mesophase at
different temperatures and phases are enantiotropic in nature.
The compound 10a exhibits B1 phase, whereas higher homologous
10b and 10c shows B2 mesophase. Mesophase B1 appears with for-
mation of small colored domains with mosaic texture. This texture
is usually associated with two-dimensional rectangular columnar
phase [19]. Typically, B1 phases are replaced by SmCPA phase on
increasing the length of terminal alkyl chains and this transition
depends on the ratio of core size to the chain length. These trends
well accompanied with the similar compounds containing even
number methyl chain variations [20]. The mesophase stability
range of B2 phase for these compounds is quite large. Thus higher
methylene chain compounds appear to favor the stabilization of B2
phase. This phase assignment was further confirmed by XRD stud-
ies. The XRD diffraction pattern obtained for the compound 10a at
110 °C is described Fig. 3. The powder XRD pattern shows two
reflections in a small angle region corresponding to d1 = 28.33 Å
(02) and d2 = 22. 11 Å (11) could be indexed onto a rectangular lat-
tice. Thus, this mesophase is
mesophase.
a rectangular columnar (Colr)
To investigate the texture of B2 phases as indicated in the POM
observations, XRD investigations suggested a smectic phase with-
out in-plane positional order. For example compound 10c at
120 °C (Fig. 4) showed two reflections at d1 = 40.43 Å and
d2 = 20.13 Å and the ratio of lattice spacing corresponding to the
two reflections is 1:1/2 indicating a lamellar or smectic type order-
ing. Moreover, a diffused peak in the wide-angle region with d-
spacing of 4.7 Å is due to liquid like in-plane order of molecules
Fig. 1. Representative 1H NMR spectra of compound 10b.

S. Balamurugan et al. / Journal of Molecular Structure 1001 (2011) 118–124
123
Table 1
Transition temperatures (°C) and associated enthalpies (kJꢀ1, in italics) for the compounds.
Sample No.
Methylene chain
7
Cr
Heating cooling
B2
–
Heating cooling
B1
Heating cooling
I
10a
d
93.9[14.4]
92.9[09.5]
d
129.9[31.3]
121.6[07.4]
d
d
d
10b
10c
9
d
d
82.6[25.3]
74.2[13.4]
d
d
138.2[15.4]
131.5[21.3]
–
–
11
82.9[27.0]
63.3[24.7]
139.0[12.8]
125.7[12.4]
Fig. 3. X-ray diffraction pattern and POM texture in the B1 phase of compound 10a
at 110 °C.
in the layers. The XRD pattern observed is typical of B2 mesophase.
The tilt angle calculated from the observed layer spacing from XRD
pattern and molecular length estimated from the molecular for-
mula is found to be about 50°.
Fig. 4. X-ray diffraction pattern of an un-oriented sample in the SmCPA (B2) phase
of compound 10c at 120 °C.
3.3. Electro-optical investigations
The polar character of these smectic phases could be detected
by electro-optical studies. The d.c. electric field was applied to a
rubbed polyimide coated cells of thickness 7 lm between crossed
polarizer. When a representative compound 10b exhibiting the
B2 phase was cooled slowly from isotropic phase, filamentary
growth patterns could be seen. Microphotograph of a typical grow-
ing filament is shown in Fig. 5a. Under the application of an AC
voltage (40 V, 10 Hz), the texture shown in Fig. 5a is transformed
to a clear fan texture. This fan texture (Fig. 5b) remains even after
removal of electric field. This clearly indicates the presence of
spontaneous polarization in the smectic layers and the polarization
tends to align along the applied electric field in the B2 phase. The
color and orientation of dark brushes in the B2 phase depend upon
the applied voltage. The presence of dark brushes parallel and per-
pendicular to the polarizer in the zero field ground state (Fig. 5b)
indicates an anticlinic organization of the molecules in successive
Fig. 5. Optical photomicrographs in the B2 phase of compound 10b at 115 °C (a) before applying electric field (b) in the absence of field but after the sample is treated with a
low frequency electric field.

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S. Balamurugan et al. / Journal of Molecular Structure 1001 (2011) 118–124
symmetric terminal chain lengths. The mesomorphic properties
and electro-optical investigations of the asymmetric BC mesogens
were analyzed. As expected the compounds seems to be promising
with regards to their melting temperatures being lowered. The
higher methylene chain containing compounds 10b and 10c show
B2 mesophase and higher mesophase range. Shorter chain com-
pound 10a exhibit apolar columnar (B1) phase. Mesomorphic
behavior and physical properties strongly depends on symmetry
of the molecules. The Compounds 10b and 10c exhibit an anticlinic
antiferroelectric B2 phase, these compounds show high spontane-
ous polarization values 586 nC/cm2 and 670 nC/cm2 respectively.
Acknowledgment
P.K. gratefully acknowledges the University Grants Commission,
New Delhi, Government of India [F. 31-110/2005 (SR)] for the
financial support. S.B. sincerely acknowledges the Council of Scien-
tific and Industrial Research (CSIR), New Delhi, India, for the award
of Senior Research Fellowship. Instrumentation facility provided
under FIST-DST and DRS-UGC to department of chemistry, Anna
University were sincerely acknowledged.
Fig. 6. The current response to a triangular voltage (55 V, 200 Hz) in the B2 phase of
compound 10b showing the antiferroelectric characteristics at 115 °C.
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7
l
m, which was treated for homogeneous alignment. The current
response was monitored by measuring the voltage drop across a
1 k resistance connected in series with the LC cell under the
X
application of a triangle wave voltage of 55 V and a frequency of
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4. Conclusion
A novel asymmetric bent core compounds have been synthe-
sized. These bent core compounds posses asymmetric core but