Three armed star mesogens based on 1,3,5-benzenetricarboxylic acid: Synthesis and mesophase characterization

Article abstract of DOI:10.1016/j.molstruc.2013.09.030

A series of star mesogens based on 1,3,5-benzenetricarboxylic acid central core, side arms containing two phenyl rings, linked through azomethine groups and hexamethylene spacers, which connect the central core and side arms were synthesised via divergent

Full text of DOI:10.1016/j.molstruc.2013.09.030

Accepted Manuscript
Three armed star mesogens based on 1,3,5-benzenetricarboxylic acid: Synthesis
and mesophase characterization
A. Shanavas, T. Narasimhaswamy, E. Rotimi Sadiku
PII:
S0022-2860(13)00805-3
DOI:
http://dx.doi.org/10.1016/j.molstruc.2013.09.030
MOLSTR 20027
Reference:
To appear in:
Journal of Molecular Structure
Received Date:
Revised Date:
Accepted Date:
31 July 2013
11 September 2013
18 September 2013
Please cite this article as: A. Shanavas, T. Narasimhaswamy, E. Rotimi Sadiku, Three armed star mesogens based
on 1,3,5-benzenetricarboxylic acid: Synthesis and mesophase characterization, Journal of Molecular Structure
(
2013), doi: http://dx.doi.org/10.1016/j.molstruc.2013.09.030
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Three armed star mesogens based on 1,3,5-benzenetricarboxylic acid:
Synthesis and mesophase characterization
1
*
2*
1
A. Shanavas , T. Narasimhaswamy and E. Rotimi Sadiku
1
Chemical, Metallurgical and Material Engineering, Polymer Technology Division, Tshwane
University of Technology, Pretoria, South Africa;
Polymer Laboratory, CSIR-Central Leather Research Institute, Adyar, Chennai-600020,
2
India.
ABSTRACT
A series of star mesogens based on 1,3,5-benzenetricarboxylic acid central core, side
arms containing two phenyl rings, linked through azomethine groups and hexamethylene
spacers, which connect the central core and side arms were synthesized via divergent
approach. The mesogens synthesized and their intermediates were structurally well
1
13
characterized, using FT-IR, HNMR, CNMR spectroscopy and elemental analysis
techniques. The mesophase characterization was carried out using a hot stage-polarizing
optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction
(
XRD) techniques. Enantiotropic mesophases were observed for all the mesogens
synthesized. An increase in the terminal chain length stabilized the smectic phase. Nematic
phase was observed for the mesogens with short terminal chain length, whereas smectic
polymorphism was observed on increasing the terminal chain length. XRD results suggested
a tripod-like assembly of molecules in the mesophase.
Key words: Thermotropic liquid crystals, 1,3,5-benzene tricarboxylic acid, Three armed
mesogens, Nematic, Smectic
Correspondence to: A. Shanavas, (Email: shanavaslc@gmail.com)
1

1
. Introduction
The contemporary concept of designing thermotropic liquid crystals emphasizes on;
shape anisometry, nanosegregation and non-linear core units [1]. Conventionally, liquid
crystals were designed with rod [2-4] or disc [5, 6] shaped cores. The challenge posed by
banana or bent-core mesogens has in turn motivated researchers to focus on non-conventional
mesogens, deviating from linear core, typical for calamitic mesogens [7]. Thus, topologically,
different molecular shapes, such as; bent-core [8], multiarm or stars [9], cones [10],
shuttlecocks [11], rings [12], dendrimers [13] etc, have made inroad in liquid crystal research.
These non-linear mesogens resulted in new mesophase morphologies which have paved the
way for novel applications. In view of the major developments in the area of dendrons and
dendrimers (multi-arm), star mesogens are gaining popularity. It is important to note that, the
open chain oligomers have been shown to exhibit interesting mesophase sequence and also
serve as model compounds for liquid crystalline polymers [14]. Thus, star mesogens based on
tri-functional groups, such as 1,3,5-benzenetricarboxylic acid, are classified as cyclic trimers
[
15]. Recently, Lehmann [16] named these cyclic trimers as ‘Hekates’.
From the literature [17, 18], it is found that studies on mesogens, based on 1,3,5-
benzenetricarboxylic acid are relatively less and they are suitable examples of star mesogens.
Aggregation behavior, charge carrier mobility and nanoscale properties of 1,3,5-
benzenetricarboxylic acid core, based star mesogens with C symmetry, have been reported
3
recently [19-21]. The flexible spacers used to connect the central core and side arms would
make the molecule to assemble in different ways in liquid crystalline phases, which causes
the appearance of the different mesophase sequences. Yet, the studies on the spacer-
containing arms are less reported. In this context, a series of spacer-containing star mesogens,
are hereby reported, following their synthesis with a view to investigate the structure-
property relationships.
2

2
. Experimental
2
.1 Materials
1
,3,5-benzenetricarbonyltrichloride, 4-hydroxybenzaldehyde, 4-butoxybenzaldehyde,
4
-hexyloxybenzaldehyde, 4-nitrophenol, 6-chlorohexanol, n-bromooctane, n-bromodecane,
n-bromododecane, n-bromotetradecane, n-bromohexadecane and palladium on charcoal
Pd/C, 10%) were purchased from Aldrich and used without further purification. N,N’-
(
Dimethylformamide, tetrahydrofuran, ethanol and methanol (all obtained from SD Fine
chemicals) were purified by standard procedures. Dichloromethane, ethyl acetate, diethyl
ether, n–hexane, 2-butanone, acetone, potassium hydroxide (pellets), Celite-540, anhydrous
potassium carbonate, anhydrous sodium sulphate were obtained from Merck (India) and used
as such. Triethylamine (SD Fine chemicals) was distilled before use.
2
.2 Measurements
The FT-IR spectra of samples were recorded on a Thermo–Mattson FT–IR
1
13
spectrometer by the KBr pellet method. H-NMR and C-NMR spectra of the compounds
were recorded using JEOL GSX 500 NMR spectrometer; the samples were dissolved in
CDCl and TMS was used as an internal standard. Elemental analyses of the samples were
3
recorded using Elemental Analyzer for CHNS, Model-Euro EA 3000, Euro Vector S.P.A.
Optical polarizing microphotographs were taken using Olympus BX50 polarizing
microscope equipped with a Linkam THMS heating stage and a TMS 94 temperature
programmer with a C7070 digital camera. The samples were placed between two 12 mm
diameter cover slips and were heated with a programmed heating rate.
Differential scanning calorimetry 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 (~5 mg) was subjected to two heating
nd
nd
and two cooling cylces and data obtained from the 2 heating and 2 cooling cycles were
3

considered for discussion. Samples were crimpled in aluminium pans in order to ensure good
thermal contact between the sample and the pan.
Small-angle X-ray scattering spectra were measured with an evacuated 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 tube (a Cu anode target type, producing CuKα X-ray radiation with a wavelength 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 a spatial resolution of 50×50 μm per pixel
and at sample-to-detector distance of 265 mm. A measuring time of 30 sec yielded sufficient
measuring statistics. Scattering data that were read off from the imaging plate and were first
corrected for the absorption of the X-rays in the sample and later transformed to the q-scale
(
q = 4π/λ ·sin θ/2; program SAXS Quant; Anton Paar KG, Graz, Austria). The SAXS
measurements were further corrected for the empty cell.
2
2
.3 Synthesis
.3.1. 6-(4-nitrophenoxy)hexan-1-ol (1).
2 3
In a 500 mL two necked flask, 12.5 g (90 mmol) of K CO and 11.8 g (85 mmol) of
4
-nitrophenol were measured and dissolved in 200 mL of DMF. The solution was stirred and
heated to 90°C, followed by the drop-wise addition of 100 mL DMF solution containing 12.3
g (90 mmol) of 6-chlorohexanol. The temperature of the mixture was maintained at 90°C for
4
h. Then, the reaction mixture was allowed to cool to room temperature and subsequently
poured into 1L of distilled water in order to obtain a precipitate. 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 solid product was pale yellow in color.
4

o
-1
Yield: 74%. Melting point: 66-67 C. IR (KBr, cm ): 3522 (-OHstr), 2950 and 2860 (-
CH2str), 1596 and 1504 (C=Cstr aromatic), 1463 (-CHben), 1336 (-NO2str), 1260 and 1108
1
(
C−O−Cstr). H-NMR ppm (CDCl
3
): δ 8.13 (d, 2H), 6.89 (d, 2H), 4.00 (t, 2H), 3.63 (t, 2H),
1
3
1
1
.90 (bs, -OH), 1.79 (m, 2H), 1.56 (m, 2H), 1.45 (m, 4H). C-NMR ppm (CDCl
3
): δ 164.30,
41.32, 125.95, 114.47, 68.82, 62.74, 32.63, 28.99, 25.81, 25.55. Elemental analysis: calcd.
(
%) for C12H17NO
4
: C 60.23, H 7.16, N 5.85; Found: C 60.01, H 6.97, N 5.63.
.3.2. Tris[(4-nitrophenoxy)hexyl]benzene-1,3,5-tricarboxylate (2).
5 mmol (1) and 5.5 g (55 mmol) of triethylamine were dissolved in 200 mL of 2-
butanone. To this solution, a 100 mL of 2-butanone solution containing 4.8 g (18.3 mmol) of
,3,5-benzenetricarbonyltrichloride was added drop wise at 0°C and the reaction mixture was
2
5
1
stirred for 3 h at room temperature. Then the triethylamine salt was filtered off and then the
solvent was evaporated. The solid product obtained was recrystallized in 1:3 ratio of acetone-
methanol mixed solvent. The solid product obtained was pale yellow in color.
o
-1
Yield: 55%. m.p.: 71-73 C. IR (KBr, cm ): 3064 (CHstr aromatic), 2954 and 2874 (-
CH2str), 1724 (C=Ostr ester), 1597 and 1504 (C=Cstr aromatic), 1448 (-CHben), 1340 (-NO2str),
1
1
260 and 1109 (C−O−C str of ester). H-NMR ppm (CDCl
3
): δ 8.81 (s, 3H), 8.16 (d, 6H), 6.90
1
3
(
(
d, 6H), 4.39 (t, 6H), 4.05 (t, 6H), 1.84 (m, 6H), 1.63 (m, 6H), 1.55 (m, 12H). C-NMR ppm
CDCl
3
): δ 165.15, 164.20, 141.39, 134.53, 131.48, 126.00, 114.46, 68.68, 65.76, 28.92,
2
8.63, 25.80, 25.73. Elemental analysis: calcd (%) for C45 15:C 61.84, H 5.88, N 4.80;
51 3
H N O
Found: C 61.61, H 5.72, N 4.58.
.3.3. Synthesis of Tris[(4-aminophenoxy)hexyl]benzene-1,3,5-tricarboxylate (3).
.92 mmol of the compound (2) was reduced under hydrogen atmosphere in the
2
9
presence of 10% Pd-charcoal catalyst in a 100 mL of 1:1 mixture of ethanol and
tetrahydrofuran at room temperature for 48 h. Pd/C catalyst was removed by filtering the
reaction mixture through Celite-540 filtering aid and evaporation of the solvents, yielded the
5

product. The product was recrystallised in 1:3 mixture of ethyl acetate and hexane. The solid
product was reddish brown in color.
o
-1
Yield: 92%. m.p.: 78-80 C. IR (KBr, cm ): 3467 and 3387 (-NH2str free amine), 2934
and 2860 (-CH2str), 1724 (C=Ostr ester), 1634 (-NHben), 1513 (C=Cstr aromatic), 1472 (-
1
CHben), 1239 (C−O−Cstr of ester). H-NMR ppm (CDCl
3
): δ 8.82 (s, 3H), 6.72 (d, 6H), 6.61
1
3
(
(
d, 6H), 4.37 (t, 6H), 3.88 (t, 6H), 1.83 (m, 6H), 1.76 (m, 6H), 1.51 (m, 12H). C-NMR ppm
CDCl
3
): δ 165.20, 152.30, 139.82, 134.02, 131.55, 116.51, 115.70, 68.50, 65.80, 29.40,
2
8.70, 25.88. Elemental analysis: calcd (%) for C45 : C 68.94, H 7.32, N 5.36; Found:
57 3 9
H N O
C 68.78, H 7.21, N 5.14.
.3.4. Synthesis of 4-Alkoxybenzaldehydes 4(c-g).
Synthetic procedures and analytical data of 4-Alkoxybenzaldehydes 4(c-g) are given
in supporting information. 4a and 4b were purchased from Aldrich, India.
2
2
5
.3.5. Tris-{[(4-alkoxyphenyl)methylidene]aminophenoxyalkyl}benzene-1,3,5-tricarboxylate
(a-g)).
In a typical experiment, 1.0 mmol of amine (3) and 3.0 mmol of 4-
alkoxybenzaldehydes (4a-4g) were taken in a 100 mL conical flask. 3 mL ethanol was added
to the conical flask and the reactants were thoroughly mixed and the flask kept in the
microwave oven (power 80W) for 8 minutes. For every 1.5 minutes, the flask was taken out
and the contents were mixed thoroughly and the process was repeated. The flask was
allowed to cool to room temperature and the solid product obtained was washed with
methanol and recrystallised in 2-butanone. All the products obtained were solid and all were
pale brown in color.
2
.3.5.1. Tris-{[(4-butoxyphenyl)methylidene]aminophenoxyhexyl}benzene-1,3,5-tricarboxy
late (5a).
6

o
-1
Yield: 60%. m.p.: 134 C. IR (KBr, cm ): 2936 and 2866 (-CH2str), 1729 (C=Ostr
ester), 1611 and 1509 (C=C str aromatic), 1472 (-CHben), 1245 (C−O−C str of ester), 1166
1
(
C−O−C str). H-NMR ppm (CDCl
3
): δ 8.84 (s, 3H), 8.35 (s, -CH=N), 7.78 (d, 6H), 7.13 (d,
6
1
1
2
H), 6.94 (d, 6H), 6.89 (d, 6H), 4.38 (t, 6H),4.00 (t, 6H),3.96 (t, 6H), 1.81 (m, 18H), 1.52 (m,
1
3
8H), 0.98 (t, 9H). C-NMR ppm (CDCl
3
): δ 165.18, 161.69, 157.98, 157.50, 145.25,
34.54, 131.55, 130.30, 129.35, 122.12, 115.01, 114.72, 68.07, 67.93, 65.79, 31.32, 29.32,
93 3
8.71, 25.90, 19.32, 13.96. Elemental analysis: calcd (%) for C78H N O12: C 74.08, H 7.41,
N 3.32; Found: C 74.34, H 7.36, N 3.37.
.3.5.2. Tris-{[(4-hexyloxyphenyl)methylidene]aminophenoxyhexyl}benzene-1,3,5-tricarbo
xylate (5b).
2
o
-1
Yield: 60%. m.p.: 135 C. IR (KBr, cm ): 2935 and 2860 (-CH2str), 1728 (C=Ostr
ester), 1611 and 1507 (C=Cstr aromatic), 1466 (-CHben), 1242 and 1109 (C−O−C str of ester).
1
H-NMR ppm (CDCl
3
): δ 8.84 (s, 3H), 8.37 (s, -CH=N), 7.78 (d, 6H), 7.13 (d, 6H), 6.93 (d,
6
H), 6.84 (d, 6H), 4.39 (t, 6H),3.99 (t, 6H),3.96 (t, 6H), 1.81 (m, 18H), 1.54 (m, 6H), 1.45
1
3
(
m, 6H), 1.31 (m, 18H), 0.90 (t, 9H). C-NMR ppm (CDCl
3
): δ 165.19, 161.46, 157.96,
1
3
C
2
57.50, 145.20, 134.55, 131.53, 130.30, 129.37, 122.13, 114.99, 114.72, 68.25, 68.07, 65.81,
1.67, 29.32, 29.25, 28.71, 25.90, 25.79, 22.70, 14.14. Elemental analysis: calcd (%) for
84
H
105
N
3
O12: C 74.79, H 7.84, N 3.11; Found: C 74. 45, H 7.76, N 3.07.
.3.5.3.
Tris-{[(4-octyloxyphenyl)methylidene]aminophenoxyhexyl}benzene-1,3,5-tricarbo
xylate (5c).
o
-1
Yield: 60%. m.p.: 133 C. IR (KBr, cm ): 2923 and 2856 (-CH2str), 1729 (C=Ostr
ester), 1610 and 1509 (C=Cstr aromatic), 1468 (-CHben), 1245 and 1109 (C−O−C str of ester).
1
H-NMR ppm (CDCl
3
): δ 8.84 (s, 3H), 8.35 (s, -CH=N), 7.78 (d, 6H), 7.13 (d, 6H), 6.94 (d,
6
H), 6.89 (d, 6H), 4.38 (t, 6H),4.00 (t, 6H),3.96 (t, 6H), 1.82 (m, 18H), 1.54 (m, 6H), 1.45
1
3
(
m, 6H), 1.31 (m, 30H), 0.88 (t, 9H). C-NMR ppm (CDCl
3
): δ 165.20, 161.46, 157.98,
7

1
3
57.50, 145.20, 134.59, 131.53, 130.35, 129.37, 122.18, 114.99, 114.69, 68.25, 68.07, 65.81,
2.00, 29.46, 29.34, 29.29, 28.71, 26.12, 25.92, 22.77, 14.23. Elemental analysis: calcd (%)
for C90
H
117
N
3
O
12: C 75.43, H 8.23, N 2.93; Found: C 75.12, H 8.13, N 2.90.
.3.5.4. Tris-{[(4-decyloxyphenyl)methylidene]aminophenoxyhexyl}benzene-1,3,5-tricarbo
xylate (5d).
2
o
-1
Yield: 60%. m.p.: 131 C. IR (KBr, cm ): 2928 and 2858 (-CH2str), 1730 (C=Ostr
ester), 1612 and 1509 (C=Cstr aromatic), 1468 (-CHben), 1244 (C−O−C str of ester), 1163
1
(
C−O−Cstr). H-NMR ppm (CDCl
3
): δ 8.84 (s, 3H), 8.35 (s, -CH=N), 7.78 (d, 6H), 7.13 (d,
6
6
1
6
H), 6.94 (d, 6H), 6.89 (d, 6H), 4.38 (t, 6H),4.00 (t, 6H),3.96 (t, 6H), 1.82 (m, 18H), 1.54 (m,
13
H), 1.45 (m, 6H), 1.29 (m, 42H), 0.88 (t, 9H). C-NMR ppm (CDCl
3
): δ 165.20, 161.46,
57.98, 157.50, 145.20, 134.59, 131.53, 130.35, 129.37, 122.18, 115.01, 114.73, 68.25,
8.07, 65.81, 32.00, 29.65, 29.54, 29.46, 29.34, 29.29, 28.71, 26.12, 25.92, 22.77, 14.23.
129 3
Elemental analysis: calcd (%) for C96H N O12: C 76.00, H 8.57, N 2.76; Found: C 75.61, H
8
.82, N 2.71.
.3.5.5. Tris-{[(4-dodecyloxyphenyl)methylidene]aminophenoxyhexyl}benzene-1,3,5-tricarbo
2
xylate (5e).
o
-1
Yield: 60%. m.p.: 127 C. IR (KBr, cm ): 2921 and 2852 (-CH2str), 1729 (C=Ostr
ester), 1612 and 1508 (C=Cstr aromatic), 1466 (-CHben), 1243 (C−O−C str of ester), 1163
1
(
C−O−Cstr). H-NMR ppm (CDCl
3
): δ 8.84 (s, 3H), 8.35 (s, -CH=N), 7.78 (d, 6H), 7.13 (d,
6
6
1
6
H), 6.94 (d, 6H), 6.89 (d, 6H), 4.38 (t, 6H),4.00 (t, 6H),3.96 (t, 6H), 1.82 (m, 18H), 1.54 (m,
13
H), 1.45 (m, 6H), 1.31 (m, 54H), 0.88 (t, 9H). C-NMR ppm (CDCl
3
): δ 165.20, 161.46,
57.98, 157.50, 145.20, 134.59, 131.53, 130.35, 129.37, 122.18, 115.01, 114.73, 68.25,
8.07, 65.81, 32.00, 29.73, 29.54, 29.46, 29.34, 29.32, 28.71, 26.12, 25.92, 22.77, 14.23.
Elemental analysis: calcd (%) for C102
.64, N 2.60.
H
141
3
N O12:C 76.50, H 8.87, N 2.62; Found: C 76.08, H
8
8

2
.3.5.6. Tris-{[(4-tetradecyloxyphenyl)methylidene]aminophenoxyhexyl}benzene-1,3,5-tricar
boxylate (5f).
o
-1
Yield: 60%. m.p.: 126 C. IR (KBr, cm ): 2920 and 2852 (-CH2str), 1729 (C=Ostr
ester), 1613 and 1510 (C=Cstr aromatic), 1467 (-CHben), 1244 (C−O−C str of ester), 1163
1
(
C−O−Cstr). H-NMR ppm (CDCl
3
): δ 8.84 (s, 3H), 8.35 (s, -CH=N), 7.78 (d, 6H), 7.13 (d,
6
6
1
6
2
H), 6.94 (d, 6H), 6.89 (d, 6H), 4.38 (t, 6H),4.00 (t, 6H),3.96 (t, 6H), 1.82 (m, 18H), 1.54 (m,
1
3
H), 1.45 (m, 6H), 1.25 (m, 66H), 0.87 (t, 9H). C-NMR ppm (CDCl
3
): δ 165.20, 161.46,
57.98, 157.50, 145.20, 134.59, 131.53, 130.35, 129.37, 122.18, 114.99, 114.73, 68.25,
8.07, 65.81, 32.00, 29.76, 29.54, 29.51,29.46, 29.42, 29.34, 29.29, 28.71, 26.12, 25.92,
153 3
2.77, 14.23. Elemental analysis: calcd (%) for C108H N O12: C 76.96, H 9.15, N 2.49;
Found: C 76.65, H 9.02, N 2.42.
.3.5.7. Tris-{[(4-hexadecyloxyphenyl)methylidene]aminophenoxyhexyl}benzene-1,3,5-tricar
boxylate (5g).
2
o
-1
Yield: 60%. m.p.: 128 C. IR (KBr, cm ): 2921 and 2853 (-CH2str), 1729 (C=Ostr
ester), 1612 and 1509 (C=Cstr aromatic), 1468 (-CHben), 1245 (C−O−C str of ester), 1164
1
(
C−O−C str). H-NMR ppm (CDCl
3
): δ 8.84 (s, 3H), 8.35 (s, -CH=N), 7.78 (d, 6H), 7.13 (d,
6
6
1
6
H), 6.94 (d, 6H), 6.89 (d, 6H), 4.38 (t, 6H),4.00 (t, 6H),3.96 (t, 6H), 1.82 (m, 18H), 1.54 (m,
13
H), 1.45 (m, 6H), 1.25 (m, 78H), 0.87 (t, 9H). C-NMR ppm (CDCl
3
): δ 165.20, 161.46,
57.98, 157.50, 145.20, 134.59, 131.53, 130.35, 129.37, 122.18, 114.99, 114.73, 68.25,
8.07, 65.81, 32.00, 29.81, 29.76, 29.61,29.46, 29.29, 28.71, 26.12, 25.92, 22.77, 14.23.
165 3
Elemental analysis: calcd (%) for C114H N O12:C 77.37, H 9.39, N 2.37; Found: C 77.06, H
9
3
3
.32, N 2.39.
. Results and discussions
.1 Synthesis of mesogens
9

Generally, trifunctional core derivatives can be obtained either by convergent or
divergent protocols. Each approach has its own advantages and disadvantages. In the present
work, the target mesogens were prepared by divergent approach, as shown in Scheme 1.
Accordingly, the compound (1) was prepared by reacting 4-nitrophenol with 6-chlorohexenol
in the basic condition. Thereafter, the compound (1) was treated with 1,3,5-
benzenetricarbonyltrichloride in the presence of triethylamine to get the compound (2). The
three nitro groups in compound (2) were reduced selectively to amines, under hydrogen
atmosphere by using Pd/C as catalyst. Finally, the reaction between the resulting amine and
the 4-alkoxybenzaldehydes (under microwave condition) yielded the target mesogens. It is
noteworthy to mention that when compared to thermal condition, microwave-assisted
reaction yielded the product with relatively good yield in a short time. In all, seven mesogens
were synthesized by varying the length of the terminal alkoxy chains. The intermediates
1
13
synthesized and target mesogens were well characterized by FT-IR, H NMR and C NMR
spectroscopy and elemental analysis techniques and the data for all the molecules were given
in the preceding section.
3
.2 HOPM and DSC studies
The mesogens synthesized were subjected to POM and DSC experiments for the
identification of mesophases and their sequences. The data obtained from these techniques
are in close agreement. The mesophases and thermal stability, as determined by POM and
DSC, are listed in Table 1. POM observation confirmed enantiotropic and monotropic
mesophases for all the mesogens. The mesogens (5a) and (5b) exhibited nematic phase
enantiotropically on both the heating and cooling of the samples under POM. In the case of
(
5a), the formation of nematic phase was confirmed by observing spherical birefringent
droplets that coalesced to homeotropic texture with birefringent spot. The sample on further
cooling, exhibited homeotropy characteristic of S phase. DSC experiment confirms this
A
1
0

transition by showing characteristic enthalpy of phase change for N-S
underwent transition to birefringent smectic shlieren texture, characteristic of S
sample, on further cooling, changed into more birefringent schlieren texture and is
subsequently assigned as S phase, due to the low transition enthalpy value found in the DSC
A
[22]. This further
C
phase. The
I
experiment. Finally, the sample underwent crystallization.
The next higher homolog C6 (5b) also showed birefringent nematic droplets (Figure
1
A). This texture changed into polygonal texture on cooling, which indicates a phase
transition to smectic A phase (Figure 1B). On further cooling, the texture changed into
shlieren texture of S phase (Figure 1C). This, on further cooling changed into the S phase
C
B
at a relatively lower temperature and finally changed into liquid crystal glass with frozen fan
texture. DSC confirms all these textural changes by showing five peaks of different enthalpy
values in the cooling scan (Figure 2). On the other hand, in the heating scan, only one peak
was observed even though POM showed nematic phase in the heating cycle. This is because
Cr-N and N-I temperatures were very close to each other, which leads to the occurrence of
overlap in the DSC thermogram.
On the heating cycle, the POM of next homolog (5c) showed enantiotropic S
This on cooling from isotropic liquid, led to the formation of battonets (Figure 1D) and their
coalescence, leading to the focal conic fan texture of S phase. On further cooling, it changed
to S phase where the transient stripes on the fan texture was noticed (Figure 1E). On
continuous cooling, no major changes were noticed. In the DSC cooling cycle, three peaks
were noticed and the peak at the lowest temperature was assigned to S -S (Figure 2). The
DSC heating cycle showed two peaks that were assigned to Cr-S and S -I transition (Figure
). The later peak was a shoulder signal to the former peak. The disappearance of the nematic
phase in the mesogen is the notable feature when compared to the earlier homologs.
A
phase.
A
B
B
F
A
A
3
1
1

The C10 homolog (5d) also showed polymorphism on cooling the sample from
isotropic phase. The appearance of fan texture on cooling the isotropic phase, suggested that
the phase under observation was S
conic fans usually noted in S phase. Upon cooling the sample, transition bars typical for S
was observed. The S phase changed into S phase at lower temperature. The DSC
cooling curve also confirms these observations. The DSC, on heating, showed only one peak
even though POM showed enantiotropic S phase. Due to the low mesophase stability, the
A
. The fans formed in this texture were different from focal
A
A
-
S
B
B
F
A
melting and clearing transitions were superimposed and hence one peak was noticed.
A
The C12 (5e) isomer also showed the fan texture and it was assigned as S phase.
Like the case of the C10 isomer (5d), the DSC heating curve of C12 isomer (5e) also showed
only one peak (Figure 4). The cooling curve, on the other hand, showed two peaks with
different enthalpy values (Figure 2). The high temperature peak was assigned to I-SA, while
the lower peak was attributed to S
F
phase. POM investigation of C14 (5f) also indicated the
formation of S and S when cooling the sample from isotropic phase. This was supported by
A
B
DSC curve, wherein two peaks matching the POM temperature were observed (Figure2). The
heating curve (DSC) showed one peak, similar to the C10 (5d) and C12 (5e) homologs, due
to the low mesophase stability (Figure 3).
The C16 (5g) isomer, on heating, showed S
further confirmed by noticing similar texture (broken fan) on cooling of the isotropic phase.
On further cooling, S phase changed to S phase and then underwent textural change
following the continuation of the cooling process. The mesophase texture, however, does not
match with known smectic textures [23] and has been assigned as S phase. These textural
C
phase as broken fan texture. This was
C
F
X
changes were confirmed by observing three transitions with different enthalpy values in the
DSC cooling scan (Figure 5). The DSC heating curve showed two peaks associated with Cr-
S
C
and S -I, in contrast to other homologs of the series (Figure 4).
C
1
2

POM and DSC studies of the series of mesogens indicated the presence of rich
polymesomorphism in these compounds; most of these phases are monotropic in nature. In
the DSC heating curves, majority of the mesogens of this series showed only one peak due to
the overlapping of crystals to mesophase and mesophase to isotropic transitions. However,
the cooling curves explicitly showed polymorphism and clearly matching with the POM
textures. Thus, the tendency towards smectic phases was clearly evident from (5a) to (5g)
homologs.
3
.3 Variable Temperature powder X-ray diffraction
Powder X-ray diffraction technique was also used to confirm the mesophase exhibited
by representative mesogen (5g) and to propose a model to explain the packing the molecules
in mesophase. The X-ray diffraction spectrum of mesogen (5g) was recorded at 100 °C, 133
°C and 147 °C. The spectrum recorded at 100°C (spectrum not given) showed sharp peaks at
low diffraction angle and high diffraction angle regions, which confirms the crystalline nature
of the sample at 100°C. At 133°C, the diffractogram (Figure 6) exhibited a very sharp
reflection at 2θ = 0.97° and a broad hump at 2θ = 13.5°. The disappearance of large number
of peaks and the presence of very sharp intense reflection at 2θ = 0.97° in the spectrum
obtained at 133°C, indicated the lamellar ordering [24], typical of smectic phase and confirms
the smectic C phase as observed by POM. The broad hump at high diffraction angle region
indicates the liquid-like nature of terminal chains. On the other hand, on increasing the
temperature to 147°C, the disappearance of low angle reflection and the presence of broad
hump at high diffraction angle region suggest the isotropic phase of the sample. These results
are highly consistent with the results obtained from POM and DSC experiments.
Reports on similar type of mesogens suggest that the possible conformation of the
mesogen can be λ, tuning fork or Є shape. Here, the d-spacing, calculated from X-ray
difractogram, gives some directions in the proposing possible structure of the mesogen. The
1
3

calculated d-spacing was 88 Å. But, the geometry optimized space-filling model of the same
mesogen indicated that the length of one arm of the mesogen is ~ 44 Å. Therefore, it is
clearly evident [25] from the d/l ratio, which is equal to 2.0 that the interlayer d-spacing had
contributions from two molecules. The λ or tuning fork conformation can support the d-
spacing obtained, if the length of the molecule is equal to two arms length. Therefore, we
propose the Є or tripod-like conformation, which explains the requirement of a bilayer
ordering (layer spacing (d) was contributed by two molecules) from the x-ray data.
Furthermore, the volume occupied by Є or tripod-like conformation, which is less than that
of λ and tuning fork conformations, also favor this conformation.
4
. CONCLUSIONS
A series of star mesogens, based on 1,3,5-benzenetricarboxylic acid was synthesized
via divergent approach and the mesophase characteristics of these mesogens were studied
using POM, DSC and XRD techniques. The stable nematic, smectic A and smectic C phases
and the meta-stable higher order smectic phases were observed in the series. The notable
feature of the investigation is the observation of rich polymesomorphism in all the mesogens
of the series. The disappearance of nematic phase at C8 homolog and the stabilization of the
smectic phases were observed when increasing the terminal chain length from C4 to C16.
The sharp reflection at the small angle region of the X-ray diffractogram confirms the smectic
layer ordering and the d/l ratio, calculated from X-ray data, supports the Є or tripod-like
assembly of molecules in the mesophase.
Acknowledgement
One of the authors A.S thanks Tshwane University of Technology, Pretoria, South
Africa and National Research Foundation (NRF), South Africa for the financial support.
1
4

References
[
[
1]
2]
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[
[
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[
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3
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(a). R. Amaranatha Reddy, B. K. Sadashiva, V. A. Raghunathan, Chem. Mater., 16,
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[
[
8]
9]
(
A. Sultan Nasar, J. Mol. Struct., 1038 (2013) 126.
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[
[
10] B. Xu, T. M. Swager, J. Am. Chem. Soc., 115, (1993) 1159.
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[
[
1
13] V. Percec, M. N. Holerca, S. Uchida, W. D. Cho, G. Ungar, Y. Lee, D. J. P. Yeardley,
Chem. Eur. J., 8, (2002) 1106.
[
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[
14] C. T. Imrie, Struct. Bonding, 95 (1999) 149.
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1
5

[
[
[
17] P. Goring, G. Pelzl, S. Diele, Liq. Cryst., 19, (1995) 629.
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Am. Chem. Soc., 127, (2005) 5490.
[
20] I. Paraschiv, M. Giesbers, B. Lagen, F. C. Grozema, R. D. Abellon, L. D. A.
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21] C. F. C. Fitie, I. Tomatsu, D. Byelow, W. H. de Jeu, R. P. Sijbesma, Chem.
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2
375.
[
[
[
23] I. Dierking, Textures of Liquid Crystals, Wiley-VCH, 2003.
24] A. De Vries, Mol. Cryst. Liq.Cryst., 131, (1985) 125.
25] D. Demus, J. W. Goodby, G. W. Gray, H. W. Spiess, V.Vill, Eds. Handbook of
Liquid Crystals, Wiley-VCH, Weinheim, 1998.
1
6

NO
2
CH3
Cl (CH
OH
)
2 6
(
CH
)
2 m
O
+
OH
DMF, 90
hrs.
0
C
K CO3
2
4
NH2
N
O
NO
2
CH3
NO
2
(
CH
2 m
)
O
O
6
O
(H
2
C)
(H
2
C)
6
(CH
2 n
)
O
6
O
O
(H2
C)
O
O
O
O
Cl
O
O
OH
Pd/C, H2
4 (a-g)
1
O
O
O
O
O
O
O
Microwave
EtOH, 8 mins.
O
O
O
O
O
O
TEA, MEK
THF : EtOH
0
0
C, 3 hrs
O
O
O
O
O
O
Cl
Cl
(
CH
2
)
6
(H
2
C)
6
(CH
2
)
6
(H
2
C)
6
(CH
2
)
n
(CH
)
2 n
O
NH2
NH2
N
N
NO
2
2
NO
2
3
Where,
m = 3, 5, 7, 9, 11, 13, 15.
O
O
(
CH
2
)
m
(CH
)
2 m
CH3
5 (a- g)
CH3
Scheme 1. Synthesis of star (three arm) mesogens
1
7

Fig. 1. POM textures of nematic droplets of (5b) at 137.3 °C (A), polygonal texture of
smectic A phase of (5b) at 130.7 °C (B), schlieren texture of smectic C phase of (5b) at 125.4
°C (C), battonets formation of smectic A phase of (5c) at 136.0 °C (D) and smectic B phase
of (5c) at 97.2 °C (E).
1
8

Fig. 2. DSC cooling curves of star mesogens
Fig. 3. DSC heating curves of star mesogens
1
9

Fig. 4. DSC heating curves of mesogens (5e) and (5g)
Fig. 5. DSC cooling curve of mesogen (5g)
2
0

Fig. 6. VT-XRD spectrum of star mesogen (5g) at 133 °C.
2
1

Table 1. POM and DSC results of star mesogens
Transitions Transition
observed temperature
Phases
observed by
HOPM
S.No
ΔH
(kJ/mole)
by DSC
(°C)
5
a
I-N
139.0
116.0
114.8
80.0
03.4
04.1
00.2
00.8
Nematic
Smectic A
Smectic C
Smectic I
[
[
[
N-S
-S
-S
A
]
S
A
C
]
I
]
S
C
5b
I-N
N-S
135.9
130.1
129.3
88.6
03.7
05.9
00.2
01.3
Nematic
Smectic A
Smectic C
Smectic B
[
[
[
A
]
S
S
A
-S
-S
C
]
]
C
B
5
c
I-S
-S
-S
A
133.9
99.8
95.3
20.2
02.4
01.6
Smectic A
Smectic B
Smectic F
[
[
S
S
A
B
]
]
B
F
5d
I-S
-S
-S
A
123.5
93.0
87.0
24.4
03.3
01.5
Smectic A
Smectic B
Smectic F
[
[
S
S
A
B
]
C
F
]
5
e
I-S
A
136.1
127.0
06.2
11.1
Smectic A
Smectic F
[
A
S -S
F
]
5
f
I-S
A
131.4
100.3
94.4
32.7
04.2
01.7
Smectic A
Smectic B
Smectic F
[
[
A
S -S
B
]
S
B
-S
F
]
5
g
I-S
-S
-S
C
132.6
99.2
73.5
10.6
05.7
07.1
Smectic C
Smectic F
Smectic X
[
[
S
C
F
]
S
F
X
]
[
] indicates monotropic phases.
2
2

Three armed star mesogens based on 1,3,5-benzenetricarboxylic acid:
Synthesis and mesophase characterization
1
,2*
3*
1
A. Shanavas , T. Narasimhaswamy and E. Rotimi Sadiku
1
Chemical, Metallurgical and Material Engineering, Polymer Technology Division, Tshwane
University of Technology, Pretoria, South Africa;
Department of Polymer Science, University of Madras, Guindy campus, Chennai-600025,
2
India.
3
Polymer Laboratory, CSIR-Central Leather Research Institute, Adyar, Chennai-600020,
India.
Highlights
¾
Star mesogens containing 1,3,5-benzenetricarboxylic acid central unit, two phenyl
ring side arm core, ester and azomethine linking groups and terminal alkoxy groups
were synthesized.
¾
The structural changes made in the mesogens such as increasing the terminal chain
length causes significant effect in the mesophase sequence.
The increase of arm length stabilizes the smectic phases.
¾
¾
Smectic polymorphism was observed on cooling.
2
3