Isophthalic acid based mesogenic dimers: Synthesis and structural effects on mesophase properties

Article abstract of DOI:10.1016/j.molstruc.2013.01.047

Two series of mesogens based on isophthalic acid and side arms containing two and three phenyl rings linked through azomethine or ester groups were synthesized via divergent approach. Another type of structurally similar mesogen was synthesized via convergent approach in which the azomethine groups were replaced by ester groups. All the synthesized mesogens and their intermediates were structurally well characterized using FT-IR, ¹H NMR, ¹³C NMR and EI-MASS spectrometry. The mesophase characterization was carried out using hot stage-polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD) techniques. The structural changes made in the mesogens i.e. increase of terminal chain length and replacement of azomethine groups with ester groups decreased the transition temperatures whereas the increase of side arm length improves the phase stability to a wide temperature range. Nematic phase was observed for the mesogens with short terminal chain length whereas the smectic polymorphism was observed on increasing the terminal chain length. The structural modification (three ring) on the side arm core stabilizes the mesophases enantiotropically.

Full text of DOI:10.1016/j.molstruc.2013.01.047

Journal of Molecular Structure 1038 (2013) 126–133
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Isophthalic acid based mesogenic dimers: Synthesis and structural effects
on mesophase properties
A. Shanavas a, S. Sathiyaraj a, A. Chandramohan b, T. Narasimhaswamy c, A. Sultan Nasar a,
⇑
a Department of Polymer Science, University of Madras, Guindy Campus, Chennai 600 025, India
b Department of Chemical Engineering, A.C. Tech., Anna University, Chennai 600 025, India
c Polymer Division, Central Leather Research Institute, Adyar, Chennai 600 020, India
h i g h l i g h t s
"
"
"
"
Mesogenic dimers containing two and three phenyl ring side arm core were synthesized.
The structural changes made in the mesogens causes significant effect in the melting and clearing temperatures.
The increase of arm length stabilizes the enantiotropic fashion.
Smectic polymorphism was observed on cooling.
a r t i c l e i n f o
a b s t r a c t
Article history:
Two series of mesogens based on isophthalic acid and side arms containing two and three phenyl rings
linked through azomethine or ester groups were synthesized via divergent approach. Another type of
structurally similar mesogen was synthesized via convergent approach in which the azomethine groups
were replaced by ester groups. All the synthesized mesogens and their intermediates were structurally
well characterized using FT-IR, 1H NMR, 13C NMR and EI-MASS spectrometry. The mesophase character-
ization was carried out using hot stage-polarizing optical microscopy (POM), differential scanning calo-
rimetry (DSC) and X-ray diffraction (XRD) techniques. The structural changes made in the mesogens i.e.
increase of terminal chain length and replacement of azomethine groups with ester groups decreased the
transition temperatures whereas the increase of side arm length improves the phase stability to a wide
temperature range. Nematic phase was observed for the mesogens with short terminal chain length
whereas the smectic polymorphism was observed on increasing the terminal chain length. The structural
modification (three ring) on the side arm core stabilizes the mesophases enantiotropically.
Ó 2013 Elsevier B.V. All rights reserved.
Received 7 November 2012
Received in revised form 9 January 2013
Accepted 18 January 2013
Available online 26 January 2013
Keywords:
Dimers
Isophthalic acid
Azomethine
Nematic
Smectic polymorphism
1. Introduction
gens or twins have attracted much attention not only because of
exhibiting variety of mesophases but also they serve as useful
Thermotropic liquid crystals are important functional materials
[1]. The unique combination of order and mobility present in these
materials made them as stimuli responsive [2]. Apart from the con-
ventional liquid crystals of rod [3–6] and disc [7,8] shaped mole-
cules, a wide variety of molecular shapes includes V or bent-core
model compounds for liquid crystalline polymers [22–24].
Generally, a liquid crystal dimer is designed with two identical or
non-identical mesogenic groups linked, often, via a flexible spacer
or, more rarely, through a rigid unit. Literature search indicates
isophthalic acid [25], 1,3-diaminobenzene [26] and resorcinol
[27], etc. are used as rigid central connecting units in liquid crystal
dimers and the mesogens based on these molecules exhibiting
lamellar, banana, and columnar mesophases. The research works
on mesogenic dimers with oxymethylene spacer connecting the
central rigid unit and mesogenic unit are less reported [28].
In this context, here we report two series of isopthalic acid-
based mesogenic dimers with ester and azomethine linkages and
a structurally similar mesogenic dimer without azomethine link-
age. In both the series, hexamethylene spacer is used to connect
[9], T [10], H [11], k [12], —C [13],
w [14], dimers [15–17], stars
[18,19] and dendrimers [20,21] are continuously gaining impor-
tance due to the self assembly of these molecules forming the
new mesophase structures which are very much interesting for
investigation. In recent years, liquid crystalline dimers or dimeso-
⇑
Corresponding author. Tel.: +91 44 22202820.
E-mail addresses: drasultannasar@yahoo.com, drasultannasar@unom.ac.in
(A. Sultan Nasar).
0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2013.01.047

A. Shanavas et al. / Journal of Molecular Structure 1038 (2013) 126–133
127
the central and mesogenic units. These mesogens are designed in
2.3. Synthesis
such a manner to investigate the influence of side arm core length,
linking unit and terminal chain length on the mesomorphic behav-
ior and mesophase stability. Variation in the side arm core length
and in the terminal chain length is found to improve the liquid
crystalline property of mesogens.
2.3.1. 4-Dodecyloxybenzaldehyde (2c)
In a 500 mL two necked flask, 12.5 g (90 mmol) of K2CO3 and
11.8 g (85 mmol) of 4-hydroxybenzaldehyde were taken and dis-
solved in 200 mL of DMF. The mixture was stirred and heated to
90 °C followed by the drop wise addition of 100 mL of DMF solu-
tion containing 22.4 g (90 mmol) of n-bromododecane. The tem-
perature of the mixture was maintained at 90 °C for 5 h. Then
the reaction mixture was poured into 1 L of distilled water; the
product was extracted with 300 mL (3 ꢀ 100 mL) of diethyl ether.
The organic layer was washed with 5% aqueous KOH solution fol-
lowed by brine, dried over anhydrous sodium sulfate. The residue
obtained after removal of the solvent was washed with hexane.
The product was colorless liquid.
2. Experimental
2.1. Materials
Bis[(4-aminophenoxy)hexyl]benzene-1,3-dicarboxylate (1) was
synthesized using the reported procedure [29]. 4-Ethoxybenzalde-
hyde (2a), 4-hexyloxybenzaldehyde (2b), 4-hydroxybenzaldehyde,
4-dodecyloxybenzoicacid, 4-octyloxybenzoicacid, 4-ethoxy benzo-
icacid, 6-Chlorohexanol, n-bromododecane, 4-benzyloxyphenol,
palladium on charcoal (Pd/C, 10%), N,N0-dicyclohexylcarbodiimide
(DCC) and N,N-dimethylaminopyridine (DMAP) were purchased
from Aldrich and used without further purification. N,N0-Dimethyl-
formamide, tetrahydrofuran, isopropanal, ethanol and methanol
(all obtained from SD Fine chemicals) were purified by standard
procedures. Dichloromethane, ethyl acetate, diethyl ether, n-hex-
ane, 2-butanone, acetone, potassium hydroxide (pellets), Celite-
540, anhydrous potassium carbonate, anhydrous sodium sulfate,
silica gel (100–200 mesh) were obtained from Merck (India) and
used as such. Triethylamine (SD Fine chemicals) was distilled and
used.
Yield: 73%. 1H NMR ppm (CDCl3): d 9.80 (s, ACHO), 7.74
(d, J = 8.0, 2H), 6.92 (d, J = 8.0, 2H), 3.96 (t, J = 6.0, 2H), 1.74
(p, J = 5.5, 2H), 1.40 (p, J = 5.5, 2H), 1.27 (m, 16H), 0.82 (t, J = 5.5,
3H). 13C NMR ppm (CDCl3): d 190.72, 164.31, 131.99, 129.77,
114.76, 68.43, 31.98, 29.73, 29.68, 29.55, 29.41, 29.38, 29.12,
26.02, 22.75, 14.18. IR (KBr, cmꢂ1): 2924 and 2853 (ACH2str),
2732 (ACHO), 1695 (C@Ostr ester), 1601, 1577 and 1508 (C@Cstr
aromatic), 1467 (ACHben), 1159 (CAOACstr). EI-MS m/z: 290.1 (M+).
2.3.2. 4-(4-Alkoxybenzoyloxy)benzaldehydes (3a–c)
Eighteen millimole of 4-ethoxybenzoicacid or 4-octyloxybenzo-
icacid or 4-dodecyloxy benzoicacid and 18 mmol (2.2 g) of
4-hydroxybenzaldehyde were taken and were dissolved in 60 mL
of THF. To this stirred solution, dicyclohexylcarbodiimide
(18 mmol) dissolved in 20 mL of THF was added followed by the
addition of 4-dimethylaminopyridine (1.8 mmol) and stirring was
continued at room temperature for overnight. Filtration of solid
dicyclohexyl urea and subsequent evaporation of the solvent
yielded a solid that was purified by recrystallization in isopropanal.
The product was colorless crystalline solid.
2.2. Measurements
The FT-IR spectra of samples were recorded by the KBr pellet
method on a Thermo-Mattson FT-IR spectrometer. 1H NMR and
13C NMR spectra of the compounds were recorded using JEOL
GSX 500 NMR spectrometer; the samples were dissolved in CDCl3
and TMS was used as an internal standard. The EI mass spectra
were recorded using a JEOL DX-303 spectrometer. Polarizing opti-
cal microphotographs were taken using Olympus BX50 polarizing
microscope equipped with a Linkam THMS heating stage, and
TMS 94 temperature programmer with a C7070 digital camera.
The samples were placed between two 12 mm cover slips and were
heated with a programmed heating rate. Differential scanning cal-
orimetry calibrated for enthalpy and temperature was performed
using TA Instruments Q-10 series. The experiments were carried
out in nitrogen atmosphere at a heating rate of 10 °C/min. Each
sample was subjected to two heating and cooling cycles and the
data obtained from second heating and cooling was considered
for discussion. All the samples were crimpled in aluminum pans
to ensure good thermal contact between the sample and the pan.
Small-angle X-ray scattering spectra were measured with an evac-
uated high performance SAXS instrument ‘‘SAXSess’’ (Anton Paar
KG, Graz, Austria). The ‘‘SAXSess’’ was attached to a conventional
X-ray generator (Philips, Holland) equipped with a sealed X-ray
2.3.3. 4-(4-Ethoxybenzoyloxy)benzaldehyde (3a)
Yield: 75%. m.p.: 120–122 °C. 1H NMR (CDCl3):
d 10.00
(s, ACHO), 8.14 (d, J = 8.5, 2H), 7.96 (d, J = 8.5, 2H), 7.39 (d,
J = 8.0, 2H), 6.98 (d, J = 8.5, 2H), 4.13 (q, J = 6.0, 2H), 1.45 (t,
J = 5.5, 3H). 13C NMR (CDCl3): d 191.15, 164.34, 163.75, 155.99,
133.96, 132.55, 131.35, 122.72, 120.93, 114.49, 63.98, 14.75. IR
(KBr, cmꢂ1): 2937 and 2823 (ACH2str), 2736 (ACHO), 1731 and
1690 (C@Ostr ester), 1604 and 1506 (C@Cstr aromatic), 1460
(ACHben), 1272 (CAOAC). EI-MS m/z: 270.1 (M+).
2.3.4. 4-(4-Octyloxybenzoyloxy)benzaldehyde (3b)
Yield: 75%. m.p.: 54–55 °C. 1H NMR (CDCl3): d 9.99 (s, ACHO),
8.13 (d, J = 8.0, 2H), 7.94 (d, J = 8.0, 2H), 7.39 (d, J = 8.0, 2H), 6.97
(d, J = 8.0, 2H), 4.02 (t, J = 6.0, 2H), 1.81 (p, J = 5.5, 2H), 1.46 (p,
J = 5.5, 2H), 1.32 (m, 8H), 0.88 (t, J = 6.0, 3H). 13C NMR (CDCl3):
d 191.11, 164.32, 163.96, 155.99, 133.95, 132.52, 131.32, 122.71,
120.86, 114.51, 68.47, 31.89, 29.42, 29.33, 29.16, 26.07, 22.75,
14.22. IR (KBr, cmꢂ1): 2914 and 2846 (ACH2str), 2736 (ACHO),
1736 and 1692 (C@Ostr ester), 1602 and 1507 (C@Cstr aromatic),
1465 (ACHben), 1272 (CAOACstr of ester). EI-MS m/z: 354.3 (M+).
tube (Cu anode target type producing Cu K
a X-rays with a wave-
length of 0.154 nm) operating at 40 kV and 50 mA. The samples
were measured as past cell method. The scattered X-ray intensities
were detected with a 2D-imaging plate detection system Cyclone
(Packard, A Packard Bioscience Company) with the spatial resolu-
2.3.5. 4-(4-Dodecyloxybenzoyloxy)benzaldehyde (3c)
Yield: 75%. m.p.: 48–49 °C. 1H NMR ppm (CDCl3): d 10.00(s,
ACHO), 8.13 (d, J = 7.5, 2H), 7.94 (d, J = 7.5, 2H), 7.39 (d, J = 7.0,
2H), 6.97 (d, J = 7.5, 2H), 4.02 (t, J = 6.0, 2H), 1.81 (p, J = 5.5, 2H),
1.46 (p, J = 5.5, 2H), 1.30 (m, 16H), 0.87 (t, J = 5.5, 3H). 13C NMR
ppm (CDCl3): d 191.12, 164.34, 163.97, 156.00, 133.95, 132.52,
131.32, 122.71, 120.86, 114.51, 68.47, 32.02, 29.75, 29.74, 29.69,
29.66, 29.45, 29.16, 26.07, 22.79, 14.24. IR (KBr, cmꢂ1): 2916 and
2849 (ACH2str), 2733 (ACHO), 1734 and 1694 (C@Ostr ester),
tion of 50 ꢀ 50
lm per pixel at sample to detector distance of
265 mm. The measuring times of 30 s yielded sufficient measuring
statistics. Scattering data that were read off from the imaging plate
were first corrected for the absorption of the X-rays in the sample
and transformed to the q scale (q = 4
Quant; Anton Paar KG, Graz, Austria). The SAXS measurements
p
/kꢁsin h/2; program SAXS
were further corrected for the empty cell.

128
A. Shanavas et al. / Journal of Molecular Structure 1038 (2013) 126–133
1602 and 1509 (C@Cstr aromatic), 1463 (ACHben), 1272 (CAOAC str
of ester). EI-MS m/z: 410.1 (M+).
2.3.11. Bis-{[(4-ethoxybenzoyloxyphenyl)methylidene]amin
ophenoxyhexyl}benzene-1,3-di carboxylate (5a)
Yield: 60%. m.p.: 116.7 °C. 1H NMR ppm (CDCl3): d 8.69 (s, 1H),
8.47 (s, ACH@N), 8.23 (d, J = 6.5, 2H), 8.16 (d, J = 8.5, 4H), 7.95 (d,
J = 8.5, 4H), 7.53 (t, J = 7.0, 1H), 7.33 (d, J = 8.5, 4H), 7.23 (d,
J = 8.5, 4H), 6.99 (d, J = 8.0, 4H), 6.92 (d, J = 8.5, 4H), 4.37 (t,
J = 6.5, 4H), 4.07 (q, J = 6.5, 4H), 3.96 (t, J = 6.0, 4H), 1.83–1.54 (m,
16H), 1.44 (t, J = 6.0, 6H). 13C NMR ppm (CDCl3): d 165.96,
164.72, 163.76, 157.91, 157.20, 153.28, 144.70, 134.12, 133.80,
132.43, 130.97, 130.75, 129.81, 128.69, 122.30, 121.31, 115.05,
114.44, 68.11, 65.43, 63.70, 29.32, 28.74, 25.95, 25.91, 14.86. IR
(KBr, cmꢂ1): 2925 and 2854 (ACH2str), 1730 (C@Ostr ester), 1606
and 1509 (C@Cstr aromatic), 1468 (ACHben), 1248 and 1109
(CAOACstr of ester). EI-MS m/z: 1053.7 (M++1).
2.3.6. Bis-{[(4-alkoxyphenyl)methylidene]amino-
phenoxyhexyl}benzene-1,3-dicarboxylate (4a–c)
In a typical synthesis, 1:2 mole ratio of amine (1) and aldehyde
[2(a–c)] (total mass restricted to 0.9–1.2 g) were taken in a 100 mL
conical flask. 3 mL of ethanol was added to the conical flask, mixed
the reactants thoroughly and kept the flask inside the microwave
oven its power was fixed at 80 W and the time set was 10 min.
For every 1.5 min time intervals, the flask was taken out; the con-
tent was mixed thoroughly and again kept inside the oven. After
10 min, the flask was taken out from the oven and the product
was washed with methanol and recrystallised in 1:3 mixture of
MEK and methanol. All the products obtained were pale brown col-
ored solids.
2.3.12. Bis-{[(4-octyloxybenzoyloxyphenyl)methylidene]amin
ophenoxyhexyl}benzene-1,3-di carboxylate (5b)
Yield: 60%. m.p.: 117.2 °C. 1H NMR ppm (CDCl3): d 8.69 (s, 1H),
8.47 (s, ACH@N), 8.22 (d, J = 7.0, 2H), 8.14 (d, J = 8.5, 4H), 7.94 (d,
J = 8.5, 4H), 7.52 (t, J = 6.5, 1H), 7.31 (d, J = 8.5, 4H), 7.22 (d,
J = 8.5, 4H), 6.97 (d, J = 8.5, 4H), 6.91 (d, J = 8.5, 4H), 4.37 (t,
J = 6.5, 4H), 4.03 (t, J = 6.5, 4H), 3.98 (t, J = 6.5, 4H), 1.84–1.29 (m,
40H), 0.89 (t, J = 6.0, 6H). 13C NMR ppm (CDCl3): d 165.96,
164.72, 163.76, 157.91, 157.20, 153.28, 144.70, 134.12, 133.80,
132.43, 130.97, 130.75, 129.81, 128.69, 122.30, 121.31, 115.05,
114.44, 68.44, 68.12, 65.43, 31.90, 29.42, 29.32, 29.18, 28.73,
26.08, 25.96, 25.91, 22.75, 14.21. IR (KBr, cmꢂ1): 2924 and 2853
(ACH2str), 1730 (C@Ostr ester), 1605 and 1508 (C@Cstr aromatic),
1468 (ACHben), 1248 and 1109 (CAOACstr of ester). EI-MS m/z:
1221.9 (M++1).
2.3.7. Bis-{[(4-ethoxyphenyl)methylidene]amino-
phenoxyhexyl}benzene-1,3-dicarboxylate (4a)
Yield: 70%. m.p.: 131 °C. 1H NMR (CDCl3): d 8.69 (s, 1H), 8.38 (s,
ACH@N), 8.22 (d, J = 8.5, 2H), 7.80 (d, J = 8.5, 4H), 7.51 (t, J = 6.5,
1H), 7.16 (d, J = 8.0, 4H), 6.93 (d, J = 8.5, 4H), 6.88 (d, J = 8.5, 4H),
4.36 (t, J = 6.0, 4H), 4.08 (q, J = 6.5, 4H), 3.96 (t, J = 6.0, 4H), 1.85–
1.54 (m, 16H), 1.43 (t, J = 5.5, 6H). 13C NMR (CDCl3): d 165.95,
161.47, 157.93, 157.52, 145.24, 133.80, 130.98, 130.75, 130.32,
129.41, 128.69, 122.14, 115.02, 114.70, 68.11, 65.43, 63.70, 29.32,
28.74, 25.95, 25.91, 14.86. IR (KBr, cmꢂ1): 2936 and 2861 (ACH2str),
1718 (C@Ostr ester), 1611 and 1509 (C@Cstr aromatic), 1471
(ACHben), 1245 and 1103 (CAOACstr of ester). EI-MS m/z: 812.2
(M+).
2.3.13. Bis-{[(4-dodecyloxybenzoyloxyphenyl)methylidene]amin
ophenoxyhexyl}benzene-1,3-dicarboxylate (5c)
2.3.8. Bis-{[(4-
hexyloxyphenyl)methylidene]aminophenoxyhexyl}benzene-1,3-
dicarboxy late (4b)
Yield: 60%. m.p.: 117.5 °C. 1H NMR ppm (CDCl3): d 8.70 (s, 1H),
8.48 (s, ACH@N), 8.20 (d, J = 7.5, 2H), 8.13 (d, J = 8.5, 4H), 7.96
(d, J = 8.5, 4H), 7.53 (t, J = 6.0, 1H), 7.30 (d, J = 8.0, 4H), 7.21 (d,
J = 8.5, 4H), 6.96 (d, J = 8.5, 4H), 6.91 (d, J = 8.5, 4H), 4.37
(t, J = 6.0, 4H), 4.02 (t, J = 6.0, 4H), 3.98 (t, J = 6.0, 4H), 1.88–1.22
(m, 64H), 0.88 (t, J = 6.0, 6H). 13C NMR ppm (CDCl3): d 165.93,
164.71, 163.73, 157.89, 157.21, 153.26, 144.70, 134.13, 133.80,
132.41, 130.96, 130.75, 129.81, 128.67, 122.29, 121.31, 115.06,
114.44, 68.26, 68.11, 65.43, 32.02, 29.74, 29.70, 29.67, 29.49,
29.45, 29.33, 29.29, 28.74, 26.11, 25.95, 25.91, 22.79, 14.24. IR
(KBr, cmꢂ1): 2925 and 2855 (ACH2str), 1730 (C@Ostr ester), 1605
and 1508 (C@Cstr aromatic), 1468 (ACHben), 1246 and 1110
(CAOACstr of ester). EI-MS m/z: 1333.8 (M++1).
Yield: 70%. m.p.: 127 °C. 1H NMR ppm (CDCl3): d 8.69 (s, 1H),
8.38 (s, ACH@N), 8.22 (d, J = 8.5, 2H), 7.80 (d, J = 8.5, 4H), 7.51 (t,
J = 6.5, 1H), 7.16 (d, J = 8.0, 4H), 6.93 (d, J = 8.0, 4H), 6.88 (d,
J = 8.5, 4H), 4.36 (t, J = 6.0, 4H), 3.96 (m, 8H), 1.90–1.30 (m, 32H),
0.90 (t, J = 6.0, 6H). 13C NMR ppm (CDCl3):
d
165.95,
161.47, 157.93,157.52, 145.24, 133.80, 130.98, 130.75, 130.32,
129.41, 128.69, 122.14, 115.02, 114.70, 68.26, 68.11, 65.43,
31.67, 29.32, 29.25, 28.74, 25.95, 25.91, 25.79, 22.70, 14.14. IR
(KBr, cmꢂ1): 2933 and 2864 (ACH2str), 1718 (C@Ostr ester), 1611
and 1509 (C@Cstr aromatic), 1470 (ACHben), 1245 and 1103
(CAOACstr of ester). EI-MS m/z: 924.2 (M+).
2.3.9. Bis-{[(4-dodecyloxyphenyl)methylidene]amino-
phenoxyhexyl}benzene-1,3-dicarboxy late (4c)
2.3.14. 4-(Benzyloxy)phenyl 4-ethoxybenzoate (6)
2.9 g (18 mmol) of 4-ethoxybenzoicacid and 3.6 g (18 mmol) of
4-(benzyloxy)phenol were taken in a flask containing 60 mL of
THF. To this stirred solution, dicyclohexylcarbodiimide (18 mmol)
dissolved in 20 mL of THF was added followed by the addition of
4-dimethylaminopyridine (1.8 mmol) and stirring was continued
at room temperature for overnight. Filtration of solid dicyclohexyl
urea and subsequent evaporation of the solvent yielded a solid that
was purified by recrystallization in isopropanal. The product was
colorless crystalline solid.
Yield: 70%. m.p.: 121 °C. 1H NMR (CDCl3): d 8.69 (s, 1H), 8.38 (s,
ACH@N), 8.22 (d, J = 8.5, 2H), 7.80 (d, J = 8.5, 4H), 7.51 (t, J = 6.0,
1H), 7.16 (d, J = 8.0, 4H), 6.93 (d, J = 8.5, 4H), 6.88 (d, J = 8.5, 4H),
4.36 (t, J = 6.0, 4H), 3.96 (m, 8H), 1.85–1.26 (m, 56H), 0.87
(t, J = 6.0, 6H). 13C NMR (CDCl3): d 165.95, 161.47, 157.93,157.52,
145.24, 133.80, 130.98, 130.75, 130.32, 129.41, 128.69, 122.14,
115.02, 114.70, 68.26, 68.11, 65.43, 32.02, 29.74, 29.70, 29.67,
29.49, 29.45, 29.33, 29.29, 28.74, 26.11, 25.95, 25.91, 22.79,
14.24. IR (KBr, cmꢂ1): 2931 and 2866 (ACH2str),1718 (C@Ostr ester),
1611 and 1509 (C@Cstr aromatic), 1470 (ACHben), 1245 and 1103
(CAOACstr of ester). EI-MS m/z: 1092.3 (M+).
Yield: 73%. m.p.: 132–134. 1H NMR ppm (CDCl3): d 8.13 (d,
J = 7.5, 2H) 7.43 (t, J = 7.0, 2H) 7.40 (t, J = 7.0, 1H), 7.35 (d, J = 8.0,
2H), 7.09 (d, J = 8.0, 2H), 7.05 (d, J = 8.0, 2H), 6.19 (d, J = 8.0, 2H),
5.15 (s, 2H), 3.95 (q, J = 6.5, 2H), 0.97 (t, J = 6.0, 3H). 13C NMR
ppm (CDCl3): d 165.55, 163.20, 156.93, 144.30, 136.26, 132.35,
128.81, 128.37, 127.60, 122.54, 122.03, 115.15, 114.73, 70.25,
68.18, 13.96. IR (KBr, cmꢂ1): 2934 and 2883 (ACH2str), 1719
(C@Ostr ester), 1605 and 1506 (C@Cstr aromatic), 1470 (ACHben),
1239 (CAOACstr of ester). EI-MS m/z: 348.3 (M+).
2.3.10. Bis-{[(4-alkoxybenzoyloxyphenyl)methylidene]
aminophenoxyhexyl}benzene-1,3-dicarboxylate (5a–c)
The procedure adopted for these compounds was identical to
that adopted for compounds (4a–c). The compounds obtained
were brown colored solids.

A. Shanavas et al. / Journal of Molecular Structure 1038 (2013) 126–133
129
2.3.15. 4-Hydroxyphenyl 4-ethoxybenzoate (7)
Yield: 63%. m.p.: 82–84. 1H NMR ppm (CDCl3): d 8.13 (d, J = 7.5,
2H) 7.09 (d, J = 8.0, 2H) 6.95 (d, J = 7.5, 2H), 6.91 (d, J = 8.0, 2H), 4.11
(q, J = 6.5, 2H), 3.95 (t, J = 6.0, 2H), 3.65 (t, J = 6.0, 2H), 1.79 (p,
J = 6.0, 2H), 1.60 (p, J = 6.0, 2H), 1.50–1.40 (m, 4H), 1.45 (t, J = 6.0,
3H). 13C NMR ppm (CDCl3): d 165.46, 163.32, 156.80, 144.47,
132.32, 122.58, 121.80, 115.14, 114.32, 68.32, 63.87, 62.98, 32.76,
29.33, 25.97, 25.62, 14.77. IR (KBr, cmꢂ1): 3236 (AOHstr), 2942
and 2854 (ACH2str), 1724 and 1626 (C@Ostr ester), 1607 and
1513 (C@Cstr aromatic), 1464 (ACHben), 1232 and 1108 (CAOACstr
of ester). EI-MS m/z: 358.3 (M+).
2.5 g (9.92 mmol) of the compound (6) was reduced under
hydrogen atmosphere in the presence of 0.25 g of 10% Pd-charcoal
catalyst in 100 mL of tetrahydrofuran at room temperature for
12 h. Pd/C catalyst was removed by filtering the reaction mixture
through Celite-540 filtering aid and subsequent evaporation of
the solvents yielded the product. The product was recrystallised
in 1:3 mixture of ethyl acetate and hexane. The product was color-
less solid.
Yield: 93%. m.p.: 161–163. 1H NMR ppm (CDCl3): d 8.13 (d,
J = 7.5, 2H) 6.99 (d, J = 8.0, 2H) 6.95 (d, J = 8.0, 2H), 6.79 (d, J = 8.0,
2H), 5.55 (s, 1H, OH), 4.12 (q, J = 6.5, 2H), 1.45 (t, J = 6.5, 3H). 13C
NMR ppm (CDCl3): d 165.90, 163.44, 153.58, 144.30, 132.40,
122.69, 121.47, 116.24, 114.37, 63.92, 14.76. IR (KBr, cmꢂ1): 3438
(AOHstr), 2939 and 2891 (ACH2str), 1698 (C@Ostr ester), 1606 and
1513 (C@Cstr aromatic), 1450 (ACHben), 1260 and 1110 (CAOACstr).
EI-MS m/z: 258.2 (M+).
2.3.17. Bis[(4-ethoxybenzoyloxyphenoxy)hexyl]benzene-1,3-
dicarboxylate (9)
1.9 g (5.5 mmol) of (8) and 0.5 g (5.5 mmol) of triethylamine
were dissolved in 40 mL of MEK. To this solution, a 20 mL of
MEK solution containing 0.5 g (2.7 mmol) of isophthaloylchloride
was added drop wise at 0 °C and the reaction mixture was stirred
for 3 h at room temperature. Then, the triethylamine salt was fil-
tered off and the solvent was evaporated. The solid obtained was
recrystallized in 1:3 ratio of acetone–methanol mixed solvent.
The product obtained was colorless solid.
2.3.16. 4-(6-Hydroxyhexyloxy)phenyl 4-ethoxybenzoate (8)
1.2 g (9 mmol) of K2CO3 and 2.0 g (8 mmol) of (7) were taken in
a 100 mL two necked flask containing 40 mL of DMF. The solution
was stirred and heated to 90 °C followed by the drop wise addition
of 20 mL of DMF solution containing 1.2 g (9 mmol) of 6-chloro-
hexanol. The temperature of the mixture was maintained at
90 °C for 5 h. Then the reaction mixture was allowed to cool to
room temperature and poured into 1L of distilled water to get pre-
cipitate. The precipitate was filtered, washed with distilled water
and dried under vacuum. The product was recrystallised in 1:3
ratio of ethyl acetate–hexane mixed solvent. The product was col-
orless solid.
Yield: 55%. m.p.: 104–106 °C. 1H NMR ppm (CDCl3): d 8.71
(s, 1H) 8.21 (dd, J = 2.3, 2.1, 2H) 8.12 (d, J = 6.5, 4H), 7.52 (t,
J = 4.5, 1H), 7.08 (d, J = 6.5, 4H), 6.95 (d, J = 6.0, 4H), 6.91 (d,
J = 6.0, 4H), 4.36 (t, J = 4.5, 4H) 4.11 (q, J = 4.0, 4H), 3.96 (t, J = 4.5,
4H), 1.84–1.53 (m, 16H), 1.49 (t, J = 4.5, 6H). 13C NMR ppm (CDCl3):
d 165.96, 165.40, 163.32, 156.77, 144.51, 133.80, 132.31, 130.98,
130.76, 128.69, 122.58, 121.83, 115.13, 114.30, 68.26, 65.42,
63.87, 29.27, 28.74, 25.94, 25.89, 14.77. IR (KBr, cmꢂ1): 2939 and
2866 (ACH2str), 1727 (C@Ostr ester), 1610 and 1509 (C@Cstr
Scheme 1. Synthesis of mesogenic dimers with two and three phenyl ring side arm.

130
A. Shanavas et al. / Journal of Molecular Structure 1038 (2013) 126–133
aromatic), 1467 (ACHben), 1250 (CAOACstr of ester). EI-MS m/z:
EI-MASS spectral techniques. The data obtained from these tech-
niques are very much consistent with the structures of the com-
pounds and the assignments for individual group, proton and
carbon are given in Section 2.
846.1 (M+).
3. Results and discussions
3.1. Synthesis of mesogens
3.2. Liquid crystalline properties
The Scheme 1 describes the divergent synthesis of two series of
mesogenic dimers (4a–c) and (5a–c) with side arm core con-
structed with two and three phenyl rings respectively. It is note-
worthy to mention here that when compared to conventional
synthetic method, the condensation reaction between the diamine
(1) and the aldehydes (2a–c) and (3a–c) under microwave
condition afforded the mesogens (4a–c) and (5a–c) in good yield
for a short time.
The Scheme 2 describes the convergent synthesis of mesogenic
dimer containing only ester linkages (9). In this mesogen, the
azomethine linkages were replaced by ester groups. For this
structural modification, initially, ethoxybenzoic acid was reacted
with benzyloxyphenol in the presence of DCC as coupling reagent
and DMAP as catalyst to yield the benzylated intermediate (6)
which was then debenzylated to phenol (7) in the presence of
Pd/C as catalyst under hydrogen atmosphere. The phenolic OH of
(7) was alkylated with 6-chlorohexenol and the resultant O-alkyl-
ated product (8) was used as arm of the mesogen which was re-
acted with acid chloride groups of the core unit (isophthaloyl
chloride) to yield the target mesogen (9).
The liquid crystalline properties of mesogens were investigated
by hot stage-polarizing optical microscope (POM) and differential
scanning calorimetry (DSC). The POM was used to observe the
characteristic textures for identification of phases, whereas DSC
was employed to confirm the phases observed in the microscope
by determining the transition enthalpy values. In this work, the
mesogens were designed to have a 1,3-disubstituted benzene ring
as central unit and a core consisting of two or three phenyl rings
linked through azomethine or ester groups and decorated with
terminal alkoxy chains as side arms. The central unit and the side
arms are connected with hexamethylene spacer. Terminal alkoxy
chain length, side arm core length and linking unit have been var-
ied to study their role on mesophase and transition temperatures.
The phase transition temperature, transition enthalpy values and
type of the phases observed with these mesogens are given in Ta-
ble 1 and Table 2 and the representative POM photographs and
DSC traces are shown in Figs. 1–3. The structural changes made
in these mesogens were found to have significant effect on meso-
phase formation, stability and their transition temperatures. The
later parameter was found to be decreased when increasing the
terminal chain length and also when replacing the azomethine
linking groups with ester groups. The mesophase stability was in-
The structural characterization of all the intermediates and the
target mesogens was carried out using FT-IR, 1H NMR, 13C NMR and
Scheme 2. Synthesis of mesogenic dimer containing ester groups.

A. Shanavas et al. / Journal of Molecular Structure 1038 (2013) 126–133
131
Table 1
DSC and POM data of mesogenic dimers (4a–c) in cooling cycle.
Mesogen Transitions
Transition
D
H (kJ/
Phases
observed by DSC temperature (°C) mole)
observed by
POM
(4a)
(4b)
I ? N
123.1
1.4
Nematic
I ? N
N ? SA
115.3
108.5
99.7
2.5
3.1
0.2
2.4
Nematic
Smectic A
Smectic C
Smectic B
S
S
A ? SC
C ? SB
76.2
(4c)
(9)
I ? SA
102.7
94.3
86.0
83.5
9.1
0.1
3.6
–
Smectic A
Smectic C
Smectic B
Smectic F
S
S
S
A ? SC
C ? SB
B ? SF
I ? N
112.0
2.5
Nematic
Table 2
DSC and POM data of mesogenic dimers (5a–c) in heating cycle.
Mesogen Transitions
observed by DSC
Transition
temperature (°C) mole)
DH (kJ/ Phases observed
by POM
Fig. 1. DSC cooling scans of mesogens containing two phenyl ring side arm.
(5a)
Cr ? N
N ? I
116.7
265.3
63.8
1.3
Nematic
Isotropic
(5b)
Cr ? SmC
SmC ? SmA
SmA ? I
117.2
145.8
219.2
68.8
0.4
1.5
Smectic C
Smectic A
Isotropic
(5c)
Cr ? SmC
SmC ? SmA
SmA ? I
117.5
162.6
200.9
72.3
0.07
1.2
Smectic C
Smectic A
Isotropic
creased with increasing the side arm core length. The mesogens
with two phenyl rings side arm core (4a–c) and (9) showed meso-
morphism only on cooling the sample (monotropic) whereas the
mesogens with three phenyl rings side arm core (5a–c) showed
the mesomorphism both in heating and cooling the sample
(enantiotropic).
In the POM experiment, the C2 isomer (4a) which is the lower
homolog of the series showed the birefringent spherical droplets
which coalesced to characteristic threaded texture of nematic
phase (Fig. 3) on cooling from the isotropic melt. This observation
was further confirmed by DSC where a single transition at 123 °C
associated with low enthalpy value (DH = 1.4 kJ/mole) correspond-
ing to the nematic phase was noted (Table 1). The mesogen (9)
containing ester group in the place of azomethine also showed
the nematic phase (Fig. 3). In the cooling cycle of POM experiment,
threaded nematic texture was observed and this was further con-
firmed by an exothermic peak at 112 °C observed in DSC.
Fig. 2. DSC heating scans of mesogens containing three phenyl ring side arm.
Polymesomorphism was observed in the case of C6 isomer (4b).
In the POM cooling cycle, the formation of nematic phase was con-
firmed in this case too by observing spherical birefringent droplets
that coalesced to homeotropic texture with birefringent spot. The
sample upon further cooling exhibited homeotropy characteristic
of SA phase. DSC confirms this transition with characteristic
enthalpy of phase change for N–SA [30]. This further underwent
transition to birefringent smectic shlieren texture of SC phase.
The appearance of SC in this homolog was confirmed by noticing
a small peak at 100 °C with change in baseline and this pattern is
typical of a weak first order transition usually observed for SA–SC
phases [31]. On further cooling, the sample finally changed into
SB phase as identified by appropriate transition enthalpy value
associated with low temperature peak found in DSC (Table 1).
The POM observations of (4b) were very much consistent with
the transitions observed in DSC cooling scan where four peaks
were noticed with appropriate enthalpy change for Isotro-
pic ? nematic ? smectic A ? smectic C ? smectic
transitions in the temperature range of 115.3–76.2 °C (Fig. 1 and
Table 1).
Compound (4c) the highest homolog of the series, also showed
polymesomorphism. Here, the nematic phase was disappeared and
the observed mesophase sequence was smectic A, smectic C,
smectic B and smectic F. In the POM cooling cycle, formation of
battonets and their coalescence leading to focal conic fan texture
of SA was observed. Upon further cooling, broken fan texture of
B
phase
SC and the texture of concentric arcs on the fans (Fig. 3) of SB were
observed subsequently [32]. Finally, the concentric arcs on the fans
were disappeared and the phase was changed into smectic F. The
DSC cooling scan showed four peaks with different enthalpy values
corresponding for SA, SC, SB and SF phases (Table 1).

132
A. Shanavas et al. / Journal of Molecular Structure 1038 (2013) 126–133
Fig. 3. The POM photographs of (A) birefringent droplets of nematic phase of (4a) at 122 °C, (B) threaded nematic texture of (9) at 107 °C, (C) focal conic fan texture of smectic
A phase of (5b) at 182 °C and (D) texture of concentric arcs on the fans of smectic B phase of (4c) at 86.5 °C.
In the series of (5a–c) mesogens, the melting i.e.,
crystal ? mesophase temperatures of all the three compounds
were found to be almost same whereas the clearing i.e.,
mesophase ? isotropic temperatures were found to be decreased
with increasing the terminal chain length from C2 to C12. Such
trend is commonly observed in calamitic mesogens [33]. The
POM investigation of (5a) showed the threaded nematic texture
both in heating and cooling cycle. This was further confirmed by
DSC where two endothermic transitions with wide temperature
interval (116.7–265.3 °C) were noticed (Fig. 2). The enthalpy values
associated with each transition were found to be appropriate to the
crystal to nematic (63.8 kJ/mole) and nematic to isotropic (1.3
kJ/mole) phase (Table 1).
The diffractogram recorded at 121 °C and 150 °C were found to
be nearly identical (Fig. 4); a sharp reflection at around 2h = 0.82°
and a broad hump at around 2h = 13.3° (not shown in figure) were
noticed in the small angle and wide angle region respectively. The
sharp reflection at small angle region suggests the lamellar order-
ing [34] and confirming the presence of smectic phases whereas
the broad hump at wide angle region suggests that the terminal
chains are undergoing dynamics typical of liquid like nature. The
‘d’ values calculated by Bragg equation from the small angle reflec-
In the case of 5b and 5c, smectic C and smectic A phases were
observed enantiotropically. The DSC heating scan of these homo-
logs showed three peaks with varying temperature. The weak first
order transitions observed in these mesogens confirmed the
SmA ? SmC transition as observed in the POM. The transition tem-
peratures and the enthalpy values of these transitions are listed in
Table 1. In addition to the SmA and SmC, the homolog (5c) showed
SmB phase monotropically. In POM cooling cycle of (5c), battonets,
focal conic fans (Fig. 3), broken focal conic fans and concentric arcs
on the fans were observed which were similar to that observed in
the case of (4c). When comparing (4a–c) and (5a–c), it is very clear
that monotropic fashion of former analogs was changed into stabi-
lized enantiotropic fashion with retaining polymorphism in the la-
ter case.
3.3. Variable temperature X-ray diffraction (VT-XRD) studies
Powder X-ray diffraction technique was also used to confirm
the mesophases observed by POM and DSC for a representative
mesogen (5b). The diffraction patterns of (5b) obtained as a func-
tion of temperature in different mesophases is shown in Fig. 4.
Fig. 4. X-ray diffractogram of mesogen (5b).

A. Shanavas et al. / Journal of Molecular Structure 1038 (2013) 126–133
133
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of side arm core length from two to three phenyl rings exhibited
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Appendix A. Supplementary material
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Supplementary data associated with this article can be found, in the
online version, at http://dx.doi.org/10.1016/j.molstruc.2013.01.047.
These data include MOLfilesand InChiKeys of the most important com-
pounds described in this article.
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