Mesogens based on cholesterol derivatives: Synthesis and characterization

Article abstract of DOI:10.1080/15421406.2012.687323

A new series of liquid crystals involving cholesterol based mesogenic units and Schiff base moiety interlinked through ester linkage were designed and synthesized. The target compounds were obtained by the reaction of 4-(3-cholesteryl oxy carbonyl)benzald

Full text of DOI:10.1080/15421406.2012.687323

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Mesogens Based on Cholesterol
Derivatives: Synthesis and
Characterization
Ashwini Pandey a & Bachcha Singh a
a Department of Chemistry, (Centre of Advanced Study), Faculty of
Science, Banaras Hindu University, Varanasi, India
Published online: 30 Jul 2012.
To cite this article: Ashwini Pandey & Bachcha Singh (2012) Mesogens Based on Cholesterol
Derivatives: Synthesis and Characterization, Molecular Crystals and Liquid Crystals, 562:1, 166-176,
DOI: 10.1080/15421406.2012.687323
To link to this article: http://dx.doi.org/10.1080/15421406.2012.687323
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Mol. Cryst. Liq. Cryst., Vol. 562: pp. 166–176, 2012
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2012.687323
Mesogens Based on Cholesterol Derivatives:
Synthesis and Characterization
ASHWINI PANDEY AND BACHCHA SINGH∗
Department of Chemistry, (Centre of Advanced Study), Faculty of Science,
Banaras Hindu University, Varanasi, India
A new series of liquid crystals involving cholesterol based mesogenic units and
Schiff base moiety interlinked through ester linkage were designed and synthesized.
The target compounds were obtained by the reaction of 4ꢀ-(3-cholesteryl oxy car-
bonyl)benzaldehyde with different substituted aromatic and heterocyclic amines. The
molecular structures of the compounds were confirmed by Fourier transform infrared
1
(FT-IR), H, and 13C nuclear magnetic resonance spectra. The mode of linkage has
been made via COO and
C
N
bonds. The linkages and different substituted
aromatic and heterocyclic groups were undertaken to study their influence on the
isotropization temperature (Ti) and on mesomorphic property. The thermal phase be-
havior of the synthesized compounds was investigated by polarizing optical microscopy
coupled with hot stage and differential scanning calorimetry (DSC). Enantiotropic
cholesteric phase were shown by 4ꢀ-[3-Cholesteryloxycarbonyl]benzylidene- 1, 2, 4-
triazine-3-yl and monotropic nematic mesophase were shown by 4ꢀ-[3-Cholesteryl
oxycarbonyl]benzylidene-(4- methyl)aniline whereas 4ꢀ-[3-Cholesteryloxycarbonyl]
benzylidene-(4-butoxycarbonyl)aniline defined oily streak phase in heating cycle. The
remaining members of the series exhibited monotropic droplet texture in cooling cycle
with exclusive suppression of cholesteric phase.
Keywords Cholesterol based Schiffꢀs base; differential scanning calorimetry; enan-
tiotropic mesophase; mesogenic units; monotropic mesophase; polarizing optical mi-
croscopy; synthesis
1. Introduction
Cholesterol-based liquid crystals have attracted attention of researchers, not only due to the
fact that cholesterol is wide-spread in nature and commercially available, but also because
of the helical supermolecular structure of cholesterol-based liquid crystals impart some
unique optical properties, such as selective reflection of circularly polarized light, high
optical rotatory power, circular dichroism, and electro-optic effect [1]. Additionally, these
properties are dependent on conditions such as temperature, pressure, and electric field and
thus they could be potential candidates for applications in optical storage, color display
techniques and full color rewritable recording devices [2–4].
The work has been presented in oral lecture session of “National Symposium on Advanced
Functional Materials (NSAFM 2012)” held in the Department of Chemistry of Banaras Hindu
University on 11th and 12th February 2012.
∗Address correspondence to Bachcha Singh, Department of Chemistry, Faculty of Science,
BHU, Varanasi-221005, India. Tel: +915426702475; Fax: 05422368127. E-mail: bsinghbhu@
rediffmail.com
166

Mesogens Based on Cholesterol Derivatives
167
The C3-hydroxyl group of cholesterol allows the synthesis of various cholesteryl esters
or ethers. Such compounds are widely utilized in toiletries and cosmetics, pharmaceuticals,
and chemical industry [5]. Both the rigid steroid skeleton and its hydrophobic nature create
a tendency of such molecules to aggregate into large three-dimentional structures in which
the position and orientation of the molecule is organized [6,7].
Cholesterol itself modulates the structural and dynamic properties of phospholipid
double-layer membranes [7].
Due to its rigid structure with eight chiral centers and the ease with which the structure
can be derivatized, cholesterol has been incorporated extensively in chiral liquid crystalline
materials. The ability of cholesterol in inducing a liquid crystalline property in its various
derivatives motivated many researchers to synthesize thousands of monomers, oligomers,
and polymers, based on cholesterol [8].
Imrie et al. have published some critical reviews [9–11], which systematically describe
the relationships between molecular structure and mesomorphic behavior. The dimers
[12–28] and trimers [29–32] consisting of cholesteryl ester unit as chiral segment joined to
different mesogenic moieties such as benzoate ester, Schiffꢀs base, azobenzene , biphenyl
or tolane, etc., through flexible spacers have been synthesized and exclusively studied.
Itahara et al. [33] reported the liquid crystalline materials by connecting adenine or
thymine to cholesteryl benzoate or related steroid groups throughpolymethylenespacer.
Yellamaggad et al. [34] also reported the liquid crystalline properties of dimesogenic
compounds in which one of the mesogenic units was the cholesterol moiety.
Here, we report Synthesis and characterization of new mesogenic compounds con-
taining aromatic and heterocyclic moieties connected to a cholesteryl ester moiety through
Schiff base linkage.
2. Experimental Details
2.1 Materials
Thionyl chloride and pyridine (Qualigens) were used as received. 4-Formylbenzoic acid,
cholesterol p-toludine, p-anisidine, methyl p-aminobenzoate, ethyl p-aminobenzoate, butyl
p-aminobenzoate, and 3-amino-1,2,4-triazine were procured from Sigma-Aldrich Chemi-
cals, USA and were used as received. All other solvents and reagents were AR grade and
used without further purification.
2.2 Techniques
Elemental analyses were performed on a CE-440 Exeter Analytical CHN analyzer. IR spec-
tra were performed on an FT-IR Varian 3100 Excalibur series spectrophotometer in KBr
pellet in the 4000–100 cm−1 region. 1H and 13C nuclear magnetic resonance spectra were
obtained on a JEOL FT-NMR AL 300 MHz spectrometer using tetramethylsilane as the in-
ternal standard. Differential scanning calorimetry (DSC) thermograms were recorded with a
Mettler Toledo TC15 TA differential scanning calorimeter at the rate of 10.0 K min−1 under
a nitrogen atmosphere using spec pure grade indium as standard by taking samples in close
lid aluminum pans. The transition temperatures from DSC have been determined with an
accuracy of 0.1 K. The mesophase type of the compounds were identified by visual com-
parison with known phase standards using an HT 30.01 NTT 268 Lomo polarizing optical
microscope (POM) fitted with a hot stage with temperature controlling accuracy of 0.1 K.

168
A. Pandey and B. Singh
2.3. Synthesis of Compounds
2.3.1 Synthesis of 4ꢀ-(3-Cholesteryloxycarbonyl)benzaldehyde (3). 4-Formylbenzoic acid
(1.5 g, 10 mmol) (1) was dissolved in dry THF (50 ml), and thionyl chloride (2.4 g, 20 mmol)
was added and refluxed for 7 h. The solvent and excess thionyl chloride was removed under
reduced pressure to give 4-formylbenzoyl chloride (2) as yellow oil. The acid chloride (2)
was treated with cholesterol (3.86 g, 10 mmol) in dry THF (50 ml) containing 2–3 drops of
pyridine. The reaction mixture was refluxed for 8 h resulting in a deep yellow color solution.
The solvent was removed under reduced pressure to give a cream color product, which was
washed four times with water (50 ml) dried and recrystallized from chloroform to afford 4ꢀ-
(3-cholesteryloxycarbonyl)benzaldehyde (3) as crystals. Yield: 75%. FT-IR (KBr, cm−1):
2936, 2899, 2866 (aliphatic C H stretching), 1737 (C O ester), 1705 (C O aldehyde),
1
1637, 1575(Ph), 1463 (aromatic C C), 1378, 1276. H NMR (CDCl3) δH (ppm): 10.10
(s, 1H, CHO), 8.25–8.18 (d, 2H, ArH), 7.95–7.92 (d, 2H, ArH), 5.35 (m, 1H, C CH ),
3.54 (t, 1H, OCH CH2), 2.27 (q, 2H, OCH CH2), 2.03–0.67 (m, 41H, CH, CH2
and CH3). 13C NMR (CDCl3) δc (ppm): 191.6 (CHO), 167.0 (COO), 140.7, 130.0, 129.4,
129.3, 122.9, 121.4, 77.4, 77.0, 76.5 (CDCl3) 75.2, 71.5, 56.6, 56.5, 56.0, 50.0, 42.2, 39.6,
39.4, 37.1, 36.5, 36.0, 35.6, 31.7, 28.1, 27.9, 24.1, 23.7, 22.7, 22.4, 20.9, 19.3, 18.6, 11.7.
Elemental analyses: calculated for C35H48O3 (%), C, 81.03; H, 9.71; Found, C, 80.92; H,
9.38.
2.3.2. Synthesis of 4ꢀ-(3-cholesteryloxycarbonyl)benzylidene- (4-methyl)aniline (4a). To a
solution of 4ꢀ-(3-cholesteryloxycarbonyl) benzaldehyde (3) (2.59 g, 5 mmol) in chloroform
(30 ml) was added (0.53 g, 5 mmol) of 4-methylaniline (0.53g, 5 mmol) and 2–3 drops of
glacial acetic acid. The reaction mixture was refluxed for 6 h and then cooled to give a deep
yellow color product. The compound was washed 2–3 times with distilled water (25 ml)
then with ethanol (50 ml) two times and dried. Yield: 75%. FT-IR (KBr, cm−1): 2935, 2853
(aliphatic C H stretching), 1711, 1643 (C N), 1462, 1375, 1274, 1191, 1112, 1051, 954,
1
810. H NMR (CDCl3) δH (ppm): 8.52 (s, 1H, CH N), 8.13–7.18 (m, 8H, ArH), 5.35
(m, 1H, C CH ), 3.56 (t, 1H, OCH CH2), 2.38 (q, 2H, OCH CH2), 2.35 (t, 3H,
CH3C6H4N CH ) 2.22–0.67 (m, 41H, CH, CH2 and CH3). 13C NMR (CDCl3) δc
(ppm): 167.0, 163.7, 140.7, 129.8, 128.4, 122.8, 121.7, 120.8, 77.4, 77.0, 76.5 (CDCl3)
74.9, 71.7, 56.7, 56.1, 50.0, 42.2, 39.7, 39.4, 37.2, 36.4, 36.1, 35.7, 31.8, 31.6, , 29.6,
28.2, 27.9, 24.2, 23.8, 22.8, 22.5, 21.0, 19.3, 18.6, 11.8. Elemental analyses: calculated for
C42H57NO2 (%) C, 82.98; H, 2.30; N, 2.30; Found, C, 82.63; H, 8.98; N, 2.16.
The procedure for the preparation of 4ꢀ-(3-cholesteryloxycarbonyl)benzylidene-(4-me
thoxy)aniline (4b), 4ꢀ-(3-cholesteryloxycarbonyl)benzylidene-(4-methoxycarbonyl)aniline
(4c), 4ꢀ-(3-cholesteryloxycarbonyl)benzylidene-(4-ethoxycarbonyl)aniline (4d), 4ꢀ-(3-cho
lesteryloxycarbonyl)benzylidene-(4-butoxycarbonyl)aniline (4e), and 4ꢀ-(3-cholesteryloxy
carbonyl)benzylidene-3-amino-1, 2, 4-triazine (4f) are similar to that described above,
2.3.3. 4ꢀ-(3-cholesteryloxycarbonyl)benzylidene-(4-methoxy)aniline (4b). Yellow powder
(Yield: 61%). FT-IR (KBr, cm−1) 2852, 2831 (aliphatic C H stretching), 1731, 1641
1
(C N), 1450, 1371, 1272, 1185, 1121, 1052, 972, 815. H NMR (CDCl3) δH (ppm):
8.53 (s, 1H, CH N ), 8.13–6.93 (m, 8H, ArH), 5.35 (m, 1H, C CH ), 3.56 (t, 1H,
OCH CH2), 2.50 (q, 2H, OCH CH2), 2.22–0.67 (m, 41H, CH, CH2 and CH3).
13C NMR (CDCl3) δc (ppm): 163.9, 160.1, 158.6, 140.7, 129.8, 122.3, 121.6, 114.4, 77.4,
77.0, 76.5 (CDCl3), 71.7, 56.7, 56.6, 56.0, 55.4, 50.0, 42.2, 39.7, 39.4, 37.2, 36.4, 36.1,
35.7, 31.9, 31.8, 31.7, 31.6, 29.6, 28.2, 27.9, 24.2, 23.8, 22.8, 22.5, 21.0, 19.3, 18.6, 11.8.

Mesogens Based on Cholesterol Derivatives
169
Elemental analyses: calculated for C42H57NO3 (%) C, 80.85; H, 9.20; Found, C, 80.62; H,
8.91; N, 1.84.
2.3.4 4ꢀ-(3-cholesteryloxycarbonyl)benzylidene-(4-methoxycarbonyl)aniline (4c). Yellow
powder (Yield: 60%). FT-IR (KBr, cm−1) 2934, 2824 (aliphatic C H stretching), 1715,
1642 (C N), 1458, 1372, 1219, 1171, 1109, 1051, 960, 844. 1H NMR (CDCl3) δH (ppm):
8.49 (s, 1H, CH N ), 8.15-6.91 (m, 8H, ArH), 5.43 (m, 1H, C CH ), 4.33 (t, 1H,
OCH CH2), 2.50 (q, 2H, OCH CH2), 2.20–0.67 (m, 41H, CH, CH2, and CH3).
13C NMR (CDCl3) δc (ppm): 166.0, 160.6, 139.5, 130.9, 129.9, 128.8, 122.9, 120.6, 77.4,
77.0, 76.5 (CDCl3), 75.0, 56.6, 56.1, 42.3, 39.5, 36.6, 36.1, 35.7, 31.8, 28.2, 28.0, 24.2,
23.8, 22.8, 22.5, 21.0, 19.3, 18.7, 11.8. Elemental analyses: calculated for C43H57NO4 (%)
C, 79.22; H, 8.81; N, 2.14; Found C, 78.90; H, 8.43; N, 2.13.
2.3.5 4ꢀ-(3-cholesteryloxycarbonyl)benzylidene-(4-ethoxycarbonyl)aniline (4d). Yellow
powder (Yield: 65%). FT-IR (KBr, cm−1) 2938, 2831 (aliphatic C H stretching), 1710,
1
1643 (C N), 1604 (Ph), 1517, 1464, 1372, 1219, 1171, 1109, 1051, 955, 844, 770. H
NMR (CDCl3) δH (ppm): 8.48 (s, 1H, CH N ), 8.15–6.62 (m, 8H, ArH), 5.35 (m, 1H,
C CH ), 4.40 (t, 1H, OCH CH2), 2.50 (q, 2H, OCH CH2), 2.22–0.67 (m, 41H,
CH, CH2, and CH3). 13C NMR (CDCl3) δc (ppm): 167.3, 165.4, 160.5, 155.6, 139.5,
139.3, 133.5, 130.8, 129.9, 128.7, 128.2, 122.9, 120.6, 77.4, 77.0, 76.5 (CDCl3), 75.0, 64.8,
56.5, 56.2, 50.0, 42.3, 39.7, 39.4, 38.1, 37.0, 36.5, 35.8, 30.8, 28.0, 27.8, 24.2, 23.8, 22.7,
22.5, 21.0, 19.3, 19.2, 18.7, 13.7, 11.8. Elemental analyses: calculated for C44H59NO4 (%)
C, 79.35; H, 8.92; N, 2.10; Found, C, 78.95; H, 8.65; N, 2.15.
2.3.6 4ꢀ-(3-cholesteryloxycarbonyl)benzylidene-(4-butoxycarbonyl)aniline (4e). Yellow
powder (Yield: 68%). FT-IR (KBr, cm−1) 2935, 2903, 2869 (aliphatic C H stretching),
1738, 1710, 1630 (C N), 1597 (Ph), 1464, 1379, 1274, 1193, 1166, 1106, 1057, 1014, 958,
889, 859, 774. 1H NMR (CDCl3) δH (ppm): 8.48 (s, 1H, CH N ), 8.15–7.95 (m, 8H,
ArH), 5.43 (m, 1H, C CH ), 4.33 (t, 1H, OCH CH2), 2.50 (q, 2H, OCH CH2),
2.20–0.67 (m, 41H, CH, CH2, and CH3). 13C NMR (CDCl3) δc (ppm): 166.3, 165.3,
160.5, 155.6, 139.5, 139.3, 133.5, 130.8, 129.9, 128.7, 128.2, 122.9, 120.6, 77.4, 77.0,
76.5 (CDCl3), 64.8, 56.6, 56.1, 50.0, 42.3, 39.7, 39.5, 38.1, 37.0, 36.1, 35.7, 31.8, 30.8,
28.2, 28.0, 27.8, 24.2, 23.8, 22.8, 22.5, 21.0, 19.2, 18.7, 13.7, 11.8. Elemental analyses:
calculated for C46H63NO4 (%) C, 79.61; H, 9.14; N, 2.01; Found, C, 79.18; H, 9.11; N,
1.81.
2.3.7. 4ꢀ-(3-Cholesteryloxycarbonyl)benzylidene- 3-amino-1,2,4-triazine (4f). Yellow
powder (Yield: 64%.) FT-IR (KBr, cm−1) 2950, 2903, 2869 (aliphatic C H stretching),
1738, 1710, 1630 (C N), 1597 (Ph), 1464, 1379, 1276, 1201, 1112, 1015, 980, 821, 793.1H
NMR (CDCl3) δH (ppm): 10.09 (s, 1H, CH N ), 8.20–7.92 (m, 6H, ArH), 5.42 (m, 1H,
C CH ), 4.30 (t, 1H, OCH CH2), 2.46 (q, 2H, OCH CH2), 2.00–0.69 (m, 41H,
CH, CH2 and CH3). 13C NMR (CDCl3) δc (ppm): 191.7, 166.0, 160.1, 149.0, 145.6,
130.1, 129.4, 77.4, 77.0, 76.5 (CDCl3), 75.3, 56.6, 56.1, 39.5, 36.4, 31.9, 28.2, 28.0, 27.8,
24.2, 23.8, 22.8, 22.5, 18.7, 11.8. Elemental analyses: calculated for C38H52N4O2 (%), C,
76.47; H, 8.78; N, 9.38; Found, C, 76.21; H, 8.27; N, 9.14.

170
A. Pandey and B. Singh
3. Results and Discussion
The synthetic route for the preparation of cholesterol based mesogens are outlined in
Scheme 1. The elemental analyses, FT-IR and NMR spectra are fully consistent with the
structure. The compound 4ꢀ-(3-cholesteryloxy carbonyl)benzaldehyde (3) exhibits major
bands at 2936, 2899, 2866 ν(C H aliphatic), 1737 ν(C O ester), 1705ν(C O alde-
hyde), 1637 ν(Ph), 1575 cm−1 ν(C C aromatic) stretch. The compound (3) after con-
densation with p-toludine displays major IR bands at 2935, 2853 ν(C H aliphatic), 1711
ν(C O ester), 1643 ν(C N). The disappearance of band at 1705 cm−1 due to ν(CHO)
and appearance of a new band at 1643 cm−1 due to ν(C N) indicates condensation of
aldehyde with amine forming Schiff base and confirming the structure of the compound
4ꢀ-(3cholesteryloxycarbonyl)benzylidene- (4-methyl)aniline (4a).
The proton NMR of 4ꢀ-(3-cholesteryloxycarbonyl)benzaldehyde (3) depicts signals
at δ10.10 (s), 8.25–8.18 (d), 7.95–7.92 (d), 5.35 (m) , 2.27 (q), 2.03-0.67 (m) ppm
that are attributed to CHO, ring, C CH
,
OCH CH2, methyne, methylene,
and methyl protons of cholesterol moiety, respectively. The proton NMR of 4ꢀ-(3-
cholesteryloxycarbonyl)benzylidene-(4-methyl)aniline (4a) displays signals at 8.52 (s, 1H,
CH N), 8.13–7.18 (m, 8H, ArH), 5.35 (m, 1H, C CH ), 3.56 (t, OCH CH2),
2.38 (q, 2H, OCH CH2), 2.22–0.67 (m, 41H, -CH, -CH2 and CH3) ppm respectively.
The characteristic signal due to azomethine group is observed at 8.52 ppm that con-
firms the formation of 4ꢀ-(3-cholesteryloxycarbonyl)benzylidene–(4-methyl)aniline (4a).
The 13C NMR spectrum of the compound 4ꢀ-(3-cholesteryloxy carbonyl)benzaldehyde (3)
exhibits signals at δ 191.6 for CHO bonded to an aromatic ring , 167.0 for COO ester,
140.7, 130.0, and 129.4 for aromatic ring, signals 77.4, 77.0, 76.5 for CDCl3. The signals
for the carbons of the cholesterol moiety are listed in the synthetic part. The compound
4ꢀ-(3-cholesteryloxycarbonyl)benzylidene-(4-methyl)aniline (4a) dispays major 13C NMR
signals at δ 167.0 (–COO ester), 163.7 (azomethine carbon CH N) and 140.7, 130.0,
129.8 (carbons of aromatic ring). Thus, the appearance of a new 13C NMR signal, character-
istic of azomethine carbon (CH N) (4a) is observed δ 163.7, which confirms the formation
of Schiff base (4a).
All other members of the series (4b–4f) exhibit similar IR, NMR (1H and 13C) bands
are listed in the synthesis.
4. Optical Properties
The phase transitions of the new mesogenic compounds 4a–4e were measured using DSC
at the rate of 10.0 K min−1. The transition temperatures and associated enthalpies obtained
are shown in Table 1. Textural analyses were performed with the help of a POM. The
compound 4a exhibited polymorphic changes in the heating cycle with the appearance
of several endothermic peaks while in cooling cycle it displayed nematic phase with the
appearance of exothermic peak centered at 226.7◦C (ꢀH = 0.56 KJ mol−1)(ꢀS = 1.12
JK−1 mol−1) (ꢀS = 1.48 JK−1 mol−1). Under optical microscope a monotropic nematic
phase observed for this compound was judged by the appearance of numerous globular
radial bubbles (nematic droplets) (Fig. 1). Then, this sharply transformed to a crystal phase
centered at 224.4◦C (ꢀH = 0.74 kJ mol−1, ꢀS = 1.48 JK−1 mol−1) followed by successive
crystal phases in cooling cycle. Similar patterns were observed for the compounds 4b, 4c,
and 4d also.
The compound 4b exhibited successive endothermic peaks in the heating cycle with
more polymorphic changes and a similar isotropization temperature (Ti), centered at

Mesogens Based on Cholesterol Derivatives
171
Table 1. Transition temperatures of cholesterol based mesogens
Compounds
Transition temperatures (◦C)
4a
Cr1 65.4 (0.22)∗(0.65)∗∗ Cr2 68.6 (0.97) (2.83) Cr3 107.2(0.86) (2.26)
Cr4 138.6 (37.82) (91.81) Cr5147.3 (1.09) (2.59) Cr6 172.3
(0.16)(0.35) Cr7 187.16 (1.02) (2.21) Cr8 227.19 (1.30) (2.59) I
226.75 (0.59)(1.12) N 224.4 (0.74) (1.48) Cr8 184.2 (0.60) (1.13)
Cr7 178.7 (0.60) (1.32) Cr6 144.0 (1.23) (2.94) Cr5 144.0 (28.76)
(68.95) Cr462.5 (1.25) (3.75) Cr.
4b
4c
4d
Cr1 68.6 (1.77)∗(5.18)∗∗ Cr2 107.6 (1.59) (4.17) Cr3 129.9 (1.27)
(3.15) Cr4 142.7 (32.11) (77.23) Cr5 147.3 (0.88) (2.09) Cr6
187.3(1.10) (2.38) Cr7 227.19 (1.26) (2.51) I 224.5(1.11) (2.23) N
214.5 (0.45) (0.92) Cr7 184.0 (0.77) (1.68) Cr6 144.25 (1.24) (2.97)
Cr5 104.8 (36.97) (97.83) Cr4 74.8 (0.54) (1.55) Cr3 61.92 (0.84)
(2.50) Cr.
Cr1 68.8 (2.92)∗(8.54)∗∗ Cr2 107.3 (0.89) (2.34) Cr3146.8 (1.75)
(4.16) Cr4 179.3(14.99) (33.14) Cr5186.6 (0.88) (0.87) (1.19) Cr6
227.4 (1.88) (0.37) I 224.5 (1.57) (3.15) N 184.0 (1.84) (4.02) Cr6
227.4 (1.88) (0.37) I 224.5 (1.57) (3.15) N 184.0 (1.84) (4.02) Cr6
144.0 (1.92) (4.61) Cr5 127.62 (1.56) (3.96) Cr4 104.5. (2.53) (6.70)
Cr3 62.0 (1.23) (3.67) Cr.
Cr1 65.6 (0.05)∗(0.15)∗∗ Cr2 68.6 (0.69) (2.01) Cr3 89.7 (0.07) (0.19)
Cr4 107.4 (0.92) (2.41) Cr5 147.3 (0.45) (1.07) Cr6 186.6 (0.35)
(0.77) Cr7 227.1 (0.42) (0.83) Cr8 268.1 (0.69) (1.27) Cr9 309.4
(0.76) (1.30) I 307.1 (0.51) (0.87) N 265.6 (0.40) (0.74) Cr9 224.5
(0.38) (0.76) Cr8 104.5 (1.02) (2.71) Cr7 59.42 (0.12) (0.38) Cr.
Cr1 68.6 (2.71)∗(7.93)∗∗ Cr2 107.4.6 (2.44) (6.41) Cr3 124.8 (18.60)
(46.75) Cr4 147.0 (2.81) (6.69) Cr5 187.0 (2.66) (5.78) N∗ 227.4
(2.07) (4.13) I 224.3 (2.29) (4.60) Cr5 184.05 (1.60) (3.50) Cr
Cr1 49.6 (5.78)∗(17.91)∗∗ Cr2 68.7 (0.82) (2.41) Cr3 119.2 (2.58)
(6.57) Cr4 147.1 (0.55) (1.32) Cr5 186.6 (0.29) (0.63) SmA 224.1
(0.06) (0.13) N∗ 227.1 (0.59) (1.17) I 184.2 (0.58) (1.26) N∗ 144.2
(0.75) (1.79) Cr5 104.5 (0.91) (2.42) Cr4 73.0 (0.24) (0.69) Cr3
62.44 (0.64) (1.90) Cr
4e
4f
∗The value in parentheses denotes enthalpy in kJ mol−1
.
∗∗The value in parentheses denotes entropy in JK−1 mol−1.
where Cr signifies crystal phase , Sm A signifies smectic A phase , N signifies nematic phase, N∗
signifies chiral nematic phase (cholesteric phase), and I signifies the isotropic liquid phase.
227.1◦C in the heating cycle compared to the isotropization temperature (Ti) of 4a. The
compounds 4c and 4d exhibited a successive increase in isotropization temperature (Ti),
which was noticed at 227.4◦C for 4c and 309.4◦C for 4d. A similarity in polymorphism
was noticed for the appearance of exothermic peaks in cooling cycle for compounds 4b
and 4c respectively. For the compound 4d, an incremental enhancement in the polymorphic
behavior was noticed with the appearance of successive endothermic and exothermic peaks
in both the heating and cooling cycles compared to that of 4b and 4c.

172
A. Pandey and B. Singh
Figure 1. Microphotograph of the nematic droplets of the compound 4ꢀ-[3-Cholesteryloxy
carbonyl]benzylidene- (4-methyl)aniline (4a) at 226.7◦C in cooling cycle.
Thus, the compounds 4b, 4c, and 4d exhibited a similar monotropic nematic droplets in
cooling cycle centered at 224.5◦C (ꢀH = 1.11 kJ mol−1 ꢀS = 2.23 JK−1 mol−1), 224.5◦C
(ꢀH = 1.57 kJ mol−1ꢀS = 3.15 JK−1 mol−1) and 307.1◦C (ꢀH = 0.51 kJ mol−1 ꢀS =
0.87 JK−1 mol−1), respectively. The monotropic nematic phase transition temperature de-
creases from 4a to 4f in the cooling cycle except 4e for which it occurs in heating cycle.
The compound 4d shows the phase transition at higher temperature due to increase in
chain length. The isotropization temperature is almost same for the compounds except for
the compound 4d, which changes to isotropic state at 309.4◦C due to chain length. The
monotropic and enantiotropic cholesteric (N∗) phases were observed for the compounds 4e
and 4f as 4e possesses an increased carbon chain and 4f possesses a hetrocyclic aromatic
moiety. The remaining compounds 4a–4d showed the cholesteric phase though they do
contain a cholesterol moiety. This shows that p-CH3C6H4, p-CH3OC6H4, p-MeOOCC6H4,
and p-EtOOCC6H4 groups suppress the formation of the N∗ phase. This is in agreement
with the observations reported by Kim and Park [35, 36] where the perfluoroalkoxy group
suppressed the formation of the N∗ phase due to variation in length in the cholesterol con-
taining compounds. The suppression of cholesteric phase has also been reported by other
workers [37–39].
The compound 4e exhibited at characteristic cholestric oily-streak texture with stripes
of fine battons arranged in layers (beads on string texture) (Fig. 2) at 187.0◦C under the
Figure 2. Microphotograph of the compound 4ꢀ-[3-Cholesteryloxycarbonyl]benzylidene- (4-
butoxycarbonyl)aniline (4e) showing oily-streak texture at 187.0◦C in heating cycle.

Mesogens Based on Cholesterol Derivatives
173
Figure 3. Microphotograph of the compound 4ꢀ-[3-Cholesteryloxycarbonyl]benzylidene-1, 2, 4-
triazine-3-yl (4f) showing smectic A phase at 186.6◦C in heating cycle.
POM in heating cycle. A green color in the cholesteric state is also observed. It is well
known that the pitch of chiral nematics is dependent on temperature, and that the selective
reflection of light occurs when its wavelength is equal to the pitch of the helical structure
in the chiral nematic phase. We did not observe colors other than green on increasing
the temperature, probably due to the temperature ranges of the chiral nematic phase being
narrow. The exothermic peak at 224.3◦C for this compound defines transition from isotropic
to crystal phase followed by the next crystal phase Thus, the compound 4e displayed an
oily-streak texture of the N∗ phase only in heating cycle. This is to be noted here that
only the virgin sample of compound 4e exhibited the appearance of green color with the
appearance of oily-streak texture in heating cycle. For the successive cycle run of the same
compound in heating and cooling cycle no such phenomenon was observed.
The compound 4f exhibited endothermic peaks at 49.6, 186.6, and 224.1◦C owing to
crystal to crystal, crystal to SmA and SmA to cholesteric (N∗) phase (Fig. 3) transitions
respectively . Under optical microscope, a focal-conic texture is seen with dark homeotropic
domains, which coalesce to form fan-like texture of cholesteric phase (Fig. 4) in accordance
with the reported texture for the cholesteric phase. The transition temperature corresponding
to 227.1◦C in heating cycle accounts for chiral nematic to isotopic phase transition. The
compound 4f exhibits exothermic peak at 184.2◦C because of isotropic to chiral nematic
phase (cholesteric phase) (Fig. 5) and the peaks at 144.2,144.3, 104.5, 73.0, and 62.4◦C
Figure 4. Microphotograph of the compound 4ꢀ-[3-Cholesteryloxycarbonyl]benzylidene-1, 2, 4-
triazine-3-yl (4f) showing cholesteric phase at 224.1◦C in heating cycle.

174
A. Pandey and B. Singh
Figure 5. Microphotograph of the compound 4ꢀ-[3-Cholesteryloxycarbonyl]benzylidene-1, 2, 4-
triazine-3-yl (4f) showing cholesteric phase at 184.2◦C in cooling cycle.
define successive transition to the crystal phases. The compound 4f displays smectic A
phase only in the heating cycle. This may be due to heterocyclic aromatic group linked to
cholesterol moiety. The position of the nitrogen may play important role in layer formation.
The lone pair of electrons on the nitrogen atoms act to broaden the molecule and also
introduce attractive forces, which leads to smectic phase formation [40].
SOCl2
OHC
C
O
Cl
OHC
COOH
CHCl3
reflux
7 h
2
1
Pyridine /
CHCl3
H
reflux
8 h
H
H
HO
H
H
H
OHC
C
O
3
O
Ethanol /
glacial
acetic
ArNH2
acid
(2-3 drops)
reflux
6 h
H
H
H
Ar
N
CH
C
O
O
4
where Ar = p-CH3C6H4, p-CH3OC6H4, p-MeOOCC6H4, p-EtOOCC6H4, p-BuOOCC6H4,
1,2,4-triazin-3-yl.
Scheme 1. Schematic route for the synthesis of cholesterol based mesogens.

Mesogens Based on Cholesterol Derivatives
175
Many cholesterol based compounds in the chiral nematic phase can reflect iridescent
colors under the visible light, because the pitch of the helical structure and the wavelength of
visible light are of the same order of magnitude, around hundreds of nanometers. However,
the compounds 4a, 4b, 4c, 4d, and 4f do not reflect brilliant colors owing to the fact that
the pitch of the compound is far greater than the wavelength of visible light.
5. Conclusions
A series of novel compounds containing cholesterol and Schiff base groups interlinked
through ester bonds were synthesized. The linking group increase the flexibility and mobility
of particular part of liquid crystal molecule and produce a higher nematic stability. The
linkages and different ring substituted aromatic and heterocyclic groups were undertaken to
understand their influence on isotropization temperature and mesomorphic properties. Some
of these compounds displayed cholesteric phase with high phase transition temperature over
a wide range during heating along with exhibition of green liquid crystalline state because
the cholesteric pith length was equal to the wavelength of visible light whereas the remaining
members of the homologous series exhibited monotropic nematic droplet in cooling cycle
with exclusive suppression of the cholesteric phase in both the cycles though they contain
a cholesterol moiety. The isotropization temperature, melting point and the mesomorphic
properties are dependent on substituted aromatic and heterocyclic cores.
Acknowledgment
The authors thank the Head of the Department of Chemistry, Banaras Hindu University
(BHU), Varanasi for providing laboratory facilities. One of the authors (Ashwini Pandey)
thanks the council of scientific and industrial research (CSIR-New Delhi) for financial
support as a senior research fellow at BHU, Varanasi. Thanks are also due to the staff of
1
analytical division for C, H, and N analyses and for recording of H, 13C NMR and IR
spectra.
References
[1] (a) Bobrovsky, A. Y., Boiko, N. I., & Shilaev, V. P. (1998). Polym. Sci. 40, 232; (b) Peter, P. M.
(1998). Nature, 391, 745 (c) Kurihara, S., Nomiyama, S., & Nonaka, T. (2001). Chem. Mater.,
13, 1992.
[2] Wang, L. Y., & Liao, S. S. (1985). Liquid Crystal Chemistry, Science Publication: Beijing,
p. 24.
[3] Tamaoki, N. (2001). Adv. Mater., 13, 1135.
[4] Mallia, V. A., & Tamaoki, N. (2004). Chem. Soc. Rev., 33, 76.
[5] Urata, K., & Takaishi, N. (1997). Cholesteryl ester compounds containing alkyl branched acyl
groups-their preparations, properties and applications. Fett/Lipid., 99, 327.
[6] Thiemann, T., & Vill, V. (1997). J. Phys. Chem. Ref. Data, 26, 291.
[7] Inoue, K., & Shinkai, S. (1998). Hyomen, 36, 421.
[8] (a) Davis, A. P. (1993). Chem. Soc. Rev., 22, 243 (b) (2007) Chem. Soc. Rev., 36, 2096;
(c) Yelamaggad, C. V., Hiremath, U. S., Shankar Rao, D. S., & Prasad, S. K. (2000). Chem,
Commun., 57.
[9] Imrie, C. T., & Henderson, P. A. (2007). Chem. Soc. Rev., 36, 2096.
[10] Imrie, C. T., Henderson, P. A., & Yeap, G. Y. (2009). Liq. Cryst., 36, 755.
[11] Imrie, C. T. (1999). Struct. Bonding, 95, 150.
[12] Yelamaggad, C. V., & Shankar, G. (2007). Liq. Cryst., 34, 799.

176
A. Pandey and B. Singh
[13] Yelamaggad, C. V., Shankar, A., Rao, D. S. S., & Prasad, Krishna S. (1999). Liq.Cryst., 26,
1547.
[14] Sharma, R. K., Gupta, V. K., Mathews, M., & Yelamaggad, C. V. (2008). Liq. Cryst., 35, 1161.
[15] Sharma, R. K., Gupta, V. K., Mathews, M., & Yellamaggad, C. V. (2009). Liq. Cryst., 36, 225.
[16] Gupta, V. K., Sharma, R. K., Mathews, M., & Yelammagad, C. V. (2009). Liq. Cryst., 36, 339.
[17] Pandey, A. S., Dhar, R., Pandey, M. B., Achalkumar, A. S., & Yelamaggad, C. V. (2009). Liq.
Cryst., 36, 13.
[18] Hardouin, F., Achard, M. F., Jin, J.-I., & Shin, J.-W. (1994). J. Phys. II Fr., 4, 627.
[19] Hardouin, F., Achard, M. F., Laguerre, M., Jin, J.-I., & Ko, D.-H. (1999). Liq. Cryst., 26, 589.
[20] Lee, D. W., Jin, J.-I., Laguerre, M., Achard, M. F., & Hardouin, F. (2000). Liq. Cryst., 27, 145.
[21] Mallia, V. A., & Tamaoki, N. (2003). J. Mater. Chem., 13, 219.
[22] Tamaoki, N., Aoki, Y., Moriyama, M., & Kidowaki, M. (2003). Chem. Mater., 15, 719.
[23] Lee, W.-K., Kim, K.-N., Achard, M. F., & Jin, J.-I. (2006). J. Mater. Chem., 2289.
[24] Wu, C. C. (2007). Mater. Lett., 61, 1380.
[25] Marcelis, A. T. M., Koudijs, A., Klop, E. A., & Sudho¨lter, E. J. R. (2001). Liq. Cryst., 28, 881.
[26] Zhang, D. W., Huang, W., Pan, G. H., He, W. L., Cao, Y. B., Guo, J. B., Cao, H., & Yang, H.
(2009). Chem. Lett., 20, 562.
[27] Yu, H. B., Huang, Z. G., Yin, B. Z., Jamil, M., & Jeon, Y. J. (2008). Mater.Lett., 62, 3284.
[28] Zhang, C. B., Jin, L. Y., Yin, B. Z., Jamil, M., & Jeon, Y. J. (2008). Liq. Cryst., 35, 39–44.
[29] Marcelis, A. T. M., Giesbers, M., & Koudijs, A. (2007). Liq. Cryst., 34, 811.
[30] Yelamaggad, C. V., Nagamani, S. A., Hiremath, U. S., Rao, D. S. S., & Prasad, S. K. (2001). J.
Chem. Res., 2001, 493.
[31] Yelamaggad, C. V., Nagamani, S. A., Hiremath, U. S., Rao, D. S. S., & Prasad, S. K. (2001).
Liq. Cryst., 2001, 493.
[32] Aldred, M. P., Hudson, R., Kitney, S. P., Vlachos, P., Lidetke, A., Woon, K. L., Neill, M. O., &
Kelly, S. M. (2008). Liq. Cryst., 35, 413.
[33] Itahara, T., Sunose, M., Kameda, T., & Ueda, T. (2002). Chem. Phys., 4, 378.
[34] (a) Majumdar, K. C., Chakravorty, S., Pal, N., & Rao, N. V. S. (2009).Tetrahedron, 65, 152;
(b) Majumdar, K. C., Chattopadhyay, B., Chakravorty, S., Pal, N., & Sinha, R. K. (2008).
Tetrahedron Lett., 49, 7149; (c) Majumdar, K. C., Pal, N., Debnath, P., & Rao, N. V. S. (2007).
Tetrahedron Lett., 48, 6330; (d) Majumdar, K. C., Pal, N., Nath, S., Choudhary, S., & Rao, N.
V. S. (2007). Mol. Cryst. Liq. Cryst., 37, 461; (e) Majumdar, K. C., Pal, N., & Rao, N. V. S.
(2006). Liq. Cryst., 33, 531.
[35] Kim, K. N., Do, E-D., Kwon, Y-W., & Jin, J. (2010). Liq. Cryst., 32, 299.
[36] Lee, J. W., Park, Y. S., Jin, J.-I., Achard, M. F., & Hardouin, F. (2003). J. Mater. Chem., 13,
1367.
[37] Liu, H., & Nohira, H. (1997). Liq. Cryst., 22, 217.
[38] Wen, J., Chen, H., & Shen, Y. (1999). Liq. Cryst., 26, 1833.
[39] Wang, K., Li, H., & Wen, J. (2001). J. Fluorine Chem., 109, 205.
[40] Coates, D. (1995). Chemical structure, Molecular Engineering and Mixture Formulation, vol. 1.
Chapter 3 In Behadur, B (Ed.). Liquid Crystals: Applications and uses. Singapore: Scientific
Publishing Co. Pte. Ltd.