Synthesis and characterization of cholesterol-based tetramers

Article abstract of DOI:10.1080/15421406.2010.504628

Novel symmetrical liquid-crystal tetramers were designed and synthesized. The length of the central spacer and the outer spacers has been varied. The molecular structures of the target compounds were confirmed by Fourier transform infrared and proton nucl

Full text of DOI:10.1080/15421406.2010.504628

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Synthesis and Characterization of
Cholesterol-Based Tetramers
K. C. Majumdar a , S. Ponra a & S. Chakravorty a
a Department of Chemistry, University of Kalyani, Kalyani, West
Bengal, India
Published online: 20 Oct 2010.
To cite this article: K. C. Majumdar , S. Ponra & S. Chakravorty (2010): Synthesis and
Characterization of Cholesterol-Based Tetramers, Molecular Crystals and Liquid Crystals, 528:1,
113-119
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Mol. Cryst. Liq. Cryst., Vol. 528: pp. 113–119, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421406.2010.504628
Synthesis and Characterization
of Cholesterol-Based Tetramers
K. C. MAJUMDAR, S. PONRA,
AND S. CHAKRAVORTY
Department of Chemistry, University of Kalyani, Kalyani,
West Bengal, India
Novel symmetrical liquid-crystal tetramers were designed and synthesized. The
length of the central spacer and the outer spacers has been varied. The molecular
structures of the target compounds were confirmed by Fourier transform infrared
and proton nuclear magnetic resonance spectroscopy. The thermal phase behavior
of the tetramers was investigated by polarizing optical microscopy and differential
scanning calorimetry. All of the liquid-crystal tetramers showed either a chiral
nematic mesophase or chiral nematic and smectic mesophases.
Keywords Chiral nematic phase; cholesterol-based tetramer; DSC; POM;
smectic phase
Introduction
Since the discovery of the liquid-crystal dimers [1] and subsequent interest in these
materials [2], many dimeric systems have been reported. Liquid-crystal dimers are
of interest because they can act as models for main group liquid-crystalline polymers
that allow the study of the flexibility and properties of different mesogenic com-
pounds. Thus, dimeric liquid crystals retain the crucial structural components of
many thermotropic main group polymers, namely, a flexible spacer linking two
mesogenic groups, yet they are easier to study and are amenable to a more straight-
forward interpretation. Furthermore, as a distinct class of compounds with unusual
properties and potential for application, dimeric liquid crystals are of interest in their
own right. There are remarkable differences in the meosphase behaviors of nonsym-
metric and symmetric dimers. Nonsymmetric liquid-crystal dimers usually exhibit an
intercalated smectic phase, in which specific molecular interactions between the two
different meosgenic units account for this specific behavior [3–5]. In contrast, sym-
metric liquid-crystal dimers exhibit monolayer smectic phases [6]. In general, meso-
phase behavior of liquid-crystal dimers depends on several factors, such as the
structure and size of the mesogenic units, the chemical structure, and the length of
the spacers and terminal groups [7–10]. Due to the fact that cholesterol is abundant
in nature and a commercially available chiral compound, liquid-crystal dimers
Address correspondence to K. C. Majumdar, Department of Chemistry, University of
Kalyani, Kalyani, West Bengal 741235, India. E-mail: kcm_ku@yahoo.co.in
113

114
K. C. Majumdar et al.
possessing a cholesteryl ester unit joined to different aromatic mesogens through an
alkyne spacer are currently receiving considerable interest as chiral dimers [11–15].
The main interest behind the design of new materials containing cholesteryl groups
results from its rigid, long shape and chiral structure. A rigid, long shape induces
large intramolecular interaction via Van der Waals forces stabilizing parallel molecu-
lar arrangements and the chiral structure induces chirality in molecular order; that is,
a helical superstructure. A liquid-crystal trimer consists of molecules containing
three mesogenic units interconnected via two flexible spacers. These structural com-
ponents may be assembled in a number of ways to give, for example, linear trimers
[16,17], trimers in which one or more mesogenic units are connected in a lateral
position [18,19], cyclic trimers [20–21], star-shaped trimers [22,23], and trimers con-
taining rod-like and disc-like mesogenic moieties [24,25]. On the other hand,
liquid-crystal tetramers consist of molecules containing four interconnected
mesogenic units linked via three flexible spacers.
In continuation of our effort to synthesize different cholesterol-based liquid-
crystalline compounds [26–29] we present a series of cholesterol-based liquid-crystal-
line tetramers that look like a combination of two nonsymmetric dimers separated
by a flexible central spacer. We have varied the length of the central mesogenic units
as well as the spacer length of the two unsymmetrical dimeric counterparts.
The reaction between cholesterol and n-bromohexanoyl chloride in tetrahydro-
furan (THF) at room temperature gave the corresponding ester derivatives 2a,b. The
ester derivatives were then subjected to alkylation with p-nitrophenol in refluxing
acetone in the presence of anhydrous potassium carbonate to afford the desired
nitro derivatives 3a,b. The nitro derivatives 3a,b on hydrogenation afforded the
Scheme 1. Reagents and reaction conditions: (i) THF, pyridine, bromo-alkanoylchloride rt,
12 h, (ii) p-nitrophenol, acetone, reflux, 16 h; (iii) Pd=C, H2, EtOAc (solvent); (iv) EtOH,
glacial AcOH, reflux.

Cholesterol-Based Tetramers
115
corresponding amine derivatives 4a,b. The aldehydes 5a,b and amines 4a,b deriva-
tives on condensation afforded a series of compounds 6a-d (Scheme 1).
Experimental
General Procedure for the Preparation of Compounds 5a,b
Compounds 5a,b were prepared according to the literature procedure [30].
General Procedure for the Preparation of Compounds 2, 3, and 4
All compounds were prepared according to the published procedure [26].
General Procedure for the Preparation of Compounds 6a-d
Compound 5a (0.1 g, 0.03 mmol) was heated under reflux with the amine derivative
4a (0.38 g, 0.06 mmol) in absolute ethanol (10 mL) in the presence of a catalytic
amount of glacial acetic acid to afford the desired product 6a. All other compounds
were prepared similarly.
Compound 6a. Yield 92%, white solid. IR (KBr) nmax: 2932, 1760, 1738, 1625,
1603 cmꢀ1
;
1H-NMR (CDCl3, 500 MHz): dH ¼ 8.46 (s, 2H), 7.90 (d, 4H,
J ¼ 8.6 Hz), 7.20–7.22 (m, 8H), 6.90 (d, 4H, J ¼ 8.8 Hz), 5.36 (d, 2H, J ¼ 4.0 Hz),
4.61–4.63 (m, 2H), 3.96 (t, 4H, J ¼ 6.5 Hz), 2.75 (t, 4H, J ¼ 7.2 Hz), 2.30 (t, 8H,
J ¼ 7.5 Hz), 0.67–2.24 (m, 96H). Molecular formula: C97H134N2O10, CHN
calculated, C, 78.29; H, 9.08; N, 1.88%; found, C, 78.50; H, 8.82; N, 1.97%.
Compound 6b. Yield 95%, white solid. IR (KBr) nmax: 2932, 1760, 1738, 1625,
1603 cmꢀ1
;
1H-NMR (CDCl3, 500 MHz): dH ¼ 8.46 (s, 2H), 7.90 (d, 4H,
J ¼ 8.6 Hz), 7.20–7.21 (m, 8H), 6.90 (d, 4H, J ¼ 8.6 Hz), 5.36 (d, 2H, J ¼ 4.0 Hz),
4.57–4.64 (m, 2H), 3.95 (t, 4H, J ¼ 7.2 Hz), 2.75 (t, 4H, J ¼ 7.2 Hz), 0.67–2.31 (m,
124H). 13C-NMR (CDCI3, 100 MHz): 173.3, 171.0, 158.0, 156.9, 152.6, 144.5,
139.7, 134.3, 129.7, 122.6, 122.2, 121.9, 115.0, 73.7, 68.3, 56.7, 56.1, 50.0, 42.3,
39.7, 39.5, 38.1, 37.0, 36.6, 36.2, 35.8, 34.7, 33.2, 31.9, 31.8, 29.5, 29.4, 29.3, 29.2,
29.1, 28.2, 28.0, 27.8, 26.0, 25.0, 24.3, 23.8, 22.8, 22.6, 21.0, 19.9, 19.3, 18.7, 11.9.
Molecular formula: C107H154N2O10, CHN calculated, C, 78.92; H, 9.53; N, 1.72%;
found, C, 79.10; H, 9.58; N, 1.94%.
Compound 6c. Yield 86%, white solid. IR (KBr) nmax: 2944, 1743, 1623, 1603,
1578 cmꢀ1
;
1H-NMR (CDCl3, 400 MHz): dH ¼ 8.46 (s, 2H), 7.89 (d, 4H,
J ¼ 8.6 Hz), 7.20–7.21 (m, 8H), 6.89–6.93 (m, 4H), 5.36 (d, 2H, J ¼ 4.0 Hz),
4.58–4.66 (m, 2H), 3.96 (t, 4H, J ¼ 6.4 Hz), 2.67 (t, 4H, J ¼ 7.2 Hz), 2.30 (t, 8H,
J ¼ 7.6 Hz), 0.67–1.98 (m, 98H). Molecular formula: C98H136N2O10, CHN
calculated, C, 78.36; H, 9.13; N, 1.86%; found, C, 78.10; H, 9.41; N, 1.97%.
Compound 6d. Yield 85%, white solid. IR (KBr) nmax: 2935, 1752, 1736, 1622,
1604 cmꢀ1
;
1H-NMR (CDCl3, 500 MHz): dH ¼ 8.45 (s, 2H), 7.89 (d, 4H,
J ¼ 8.6 Hz), 7.20 (d, 4H, J ¼ 8.8 Hz), 7.17 (d, 4H, J ¼ 8.6 Hz), 6.90 (d, 4H,
J ¼ 8.6 Hz), 5.37 (d, 2H, J ¼ 4.1 Hz), 4.61–4.66 (m, 2H), 3.95 (d, 4H, J ¼ 6.5 Hz),
2.59 (t, 4H, J ¼ 7.2 Hz), 0.67–2.35 (m, 126H). Molecular formula: C108H156N2O10,
CHN calculated, C, 78.98; H, 9.57; N, 1.71%; found, C, 79.19; H, 9.73; N, 1.78%.

116
K. C. Majumdar et al.
Results and Discussion
The phase transition temperatures were determined using differential scanning calor-
imetry. Table 1 shows the phase sequence, transition temperatures, and associated
enthalpies of the tetramers.
All the tetramers exhibit simple thermal behavior. All the tetramers 6a-d exhibit
enantiotropic phase behavior. The tetramers 6a, 6c, 6d exhibit only one mesophase in
addition to several solid–solid transitions, whereas the tetramer 6b exhibits two
mesophases in addition to one solid–solid phase transition. On cooling from the
isotropic phase the tetramers 6a, 6c, and 6d exhibit characteristic cholesteric fan tex-
tures of the Nꢁ phase [31]. The three compounds also exhibit oily streak textures of
the Nꢁ phase in heating cycles. On cooling from the isotropic phase, compound 6b
(compound was placed in a polyamide-coated thin cell of thickness d ¼ 5 ꢂ .02 mm)
exhibits a characteristic texture of the Nꢁ phase [31]; on further cooling a phase tran-
sition occurs at around 179ꢃC and after that a broken focal conic texture of the SmA
phase appears. On further cooling compound 6b solidifies.
Powder X-ray diffraction was carried out on a Philips powder diffractometer
(The Raman Research Institute, Bangalore, India), equipped with a temperature con-
troller permitting low- as well as high-temperature operation as needed (with CuKa
radiation of k ¼ 1.5418 nm) to confirm the mesophase structure of compound 6b. A
high-resolution X-ray diffraction (HRXRD) diagram of compound 6b at 160ꢃC is
represented in Fig. 1. The diffraction diagram exhibits a sharp peak in the small angle
´
˚
region with h ¼ 0.693 (d ¼ 63.68 A) and a broad, diffuse peak in the wide angle region.
The sharp Bragg’s reflection in the small angle region is due to one-dimensional
layering in the condensed SmA phase and a diffuse peak in the wide angle region is
due to liquid-like correlation of the molecules. The calculated molecular length from
˚
molecular modeling is about 56.77 A. So we expected a partial bilayer arrangement
within the SmA phase.
Tamaoki et al. [32] synthesized dicholesteryl ester of diacetylenedicarboxylic acid
with different lengths of methylene linkages. They observed that all the compounds
Table 1. Phase transition temperature (degree) and
associated enthalpies (KJ=molꢀ1 in parantheses) in
heating cycles of DSC experiment
Compound 6a:
161:5
178:6
258:4
Cr ꢀ! Cr1 ꢀ! Nꢁ ꢀ! I
ð1:9Þ
ð50:6Þ
ð12:5Þ
Compound 6b:
78:4
152:7
179:2
196:8
Cr ꢀ! Cr1 ꢀ! SmA ꢀ! Nꢁ ꢀ! Cr
ð3:7Þ
ð17:6Þ
ð2:6Þ
ð3:9Þ
Compound 6c:
136:6
174:6
178:6
278:9
Cr ꢀ! Cr1 ꢀ! Cr2 ꢀ! Nꢁ ꢀ! I
ð21:8Þ
ð2:0Þ
ð43:2Þ
ð29:4Þ
Compound 6d:
145:3
184:8
Cr ꢀ! Nꢁ ꢀ! I
ð86:0Þ
ð1:8Þ

Cholesterol-Based Tetramers
117
Figure 1. Compound 6b at 190ꢃC.
Figure 2. Compound 6b at 171.9ꢃC.
Figure 3. Compound 6c at 260ꢃC.

118
K. C. Majumdar et al.
Figure 4. HRXRD diagram of compound 6b at 160ꢃC.
exhibit a cholesteric phase. Henderson and coworkers [33–35] have synthesized six
closely related homologous series of tetramers by varying the length and parity of
the flexible spacers and by varying the molecular shape. The tetramers exhibit Nꢁ
and SmA and=or Nꢁ, SmA phase behavior. In this article we have mainly focused
on the synthesis of cholesterol-based tetramer. All the compounds exhibit meso-
morphism. Three of the four tetramers exhibit the Nꢁ phase and one exhibits the
Nꢁ phase in addition to the SmA phase.
Acknowledgment
Two of us (S.P. & S.C.) thank the Department of Science and Technology
(New Delhi) for financial assistance. We are grateful to the Council of Scientific &
Industrial Research (CSIR) (New Delhi) for their fellowships. We are also thankful
to Dr. Raghunathan of Raman Research Institute, Bangalore, for providing the
HRXRD data.
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