Synthesis and mesomorphic behavior of novel calamitic liquid crystalline dimesogens possessing a cholesteryl moiety connected to a pyrimidine core

Article abstract of DOI:10.1080/15421406.2013.779180

Novel non-symmetric dimesogens consisting of a cholesteryl moiety and a pyrimidine unit interconnected through a pentamethylene or decamethylene spacer have been prepared through palladium-mediated Sonogashira cross-coupling. The novel mesogens show only

Full text of DOI:10.1080/15421406.2013.779180

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Synthesis and Mesomorphic Behavior
of Novel Calamitic Liquid Crystalline
Dimesogens Possessing a Cholesteryl
Moiety Connected to a Pyrimidine Core
K. C. Majumdar a & Tapas Ghosh a
a Department of Chemistry , University of Kalyani , Kalyani , West
Bengal , India
Published online: 09 Jul 2013.
To cite this article: K. C. Majumdar & Tapas Ghosh (2013) Synthesis and Mesomorphic
Behavior of Novel Calamitic Liquid Crystalline Dimesogens Possessing a Cholesteryl Moiety
Connected to a Pyrimidine Core, Molecular Crystals and Liquid Crystals, 577:1, 15-24, DOI:
10.1080/15421406.2013.779180
To link to this article: http://dx.doi.org/10.1080/15421406.2013.779180
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Mol. Cryst. Liq. Cryst., Vol. 577: pp. 15–24, 2013
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2013.779180
Synthesis and Mesomorphic Behavior of Novel
Calamitic Liquid Crystalline Dimesogens Possessing
a Cholesteryl Moiety Connected to a Pyrimidine
Core
K. C. MAJUMDAR∗ AND TAPAS GHOSH
Department of Chemistry, University of Kalyani, Kalyani, West Bengal, India
Novel non-symmetric dimesogens consisting of a cholesteryl moiety and a pyrimidine
unit interconnected through a pentamethylene or decamethylene spacer have been
prepared through palladium-mediated Sonogashira cross-coupling. The novel mesogens
show only a chiral nematic phase. The liquid crystalline phase was investigated through
polarizing optical microscopy and differential scanning calorimetry. All the compounds
were characterized through elemental analyses and spectral data.
Keywords Calamitic liquid crystal; chiral nematic phase; cholesterol; dimesogens;
pyrimidine
Dimesogenic compounds (twins), consisting of two different mesogenic units interlinked
through a central spacer, are a relatively new class of liquid crystalline (LC) compounds
that still require more intensive research in order to establish the relationship between their
structure and LC properties [1–23]. The non-symmetrical dimers are markedly different
from symmetrical ones as they exhibit interesting polymorphic sequence [24, 25] and sta-
bilize wide-range chiral nematic (N∗) mesophases [26]. Due to commercial availability of
cholesterol as an inexpensive natural product, its rigid structure with eight chiral centers and
the ease with which the structure can be derivatized, it has been incorporated extensively
in chiral LC materials. The ability of cholesterol in inducing an LC property in its various
derivatives motivated many researchers to synthesize thousands of monomers, oligomers,
and polymers derived from cholesterol[27, 28]. Non-symmetric dimers derived from nat-
urally occurring cholesterol represent an exemplary and emerging class of chiral liquid
crystals. In general, cholesterol dimers contain cholesterol moiety and another mesogen,
such as a Schiff’s base [29] and an azobenzene [30, 31], stilbene [32], or tolane [33] unit.
Uracil is found in various natural products and has numerous applications in organic
synthesis [34]. However, some unsuccessful attempts [35–37] have been made to prepare
thermotropic liquid crystals of nucleobase. Itahara et al. [38] reported on the LC materials
formed by connecting adenine or thymine to cholesteryl benzoate or related steroid groups
through a polymethylene spacer. Their results suggested that base pairing plays an important
role in the control of LC properties. There are also few reports regarding the formation of
∗Address correspondence to K. C. Majumdar, Department of Chemistry, University of Kalyani,
Kalyani-741235, West Bengal, India. Tel: (O) +91-3712-267008; Fax: (O) +91-3712-267005.
E-mail: kcm@klyuniv.ac.in
15

16
K. C. Majumdar and T. Ghosh
lyotropic liquid crystals by DNA and nucleotides [39, 40]. In continuation to our ongoing
research on the synthesis of LC materials containing heterocyclic cores [41–55] we became
interested to synthesize uracil containing non-symmetrical dimesogens and in our recent
investigations [46, 47] we had demonstrated that when uracil is connected to a cholesteryl
moiety through C=N or C=C linkage, it exhibits a twist grain boundary (TGB) phase
over a wide temperature range along with a chiral nematic and smectic phases. Being
inspired by our results and for understanding the structure-LC property relationship, we have
undertaken a study on the synthesis and characterization of new dimesogenic compounds
containing uracil moiety connected to a cholesteryl moiety through a tolane linkage. Herein
we report our results.
2. Results and Discussions
The synthetic route for A new class of calamitic liquid crystals (5a–d) is depicted
in Scheme 1. The required precursors 3a,b were synthesized from naturally occur-
ring cholesterol. Esterification of cholesterol (1) with the bromoalkanoylchlorides was
carried out in tetrahydrofuran (THF) in the presence of pyridine to afford 2a,b com-
pounds. The cholesteryl 4-iodo alkyl esters (3a,b) were prepared [56] by the reaction
of compounds 2a,b with 4-iodophenol in refluxing acetone in the presence of anhy-
drous K2CO3 (Scheme 1). The other precursors 4a–c were synthesized following a stan-
dard literature procedure [57, 58]. Finally, the target compounds (5a–d) were success-
fully obtained by Sonogashira coupling between compounds 3a,b and 4a–c by using
Pd(PPh3)2Cl2 (10 mol%) as catalyst, CuI (10 mol%) as co-catalyst, and triethyl amine
as base in THF. The compounds were characterized from their elemental analysis and
spectral data.
2.1. Mesomorphic Properties
The mesomorphism of compounds 5a–d is characterized and studied by differential
scanning calorimetry (DSC) and polarizing optical microscopy (POM). With a view to
O
R
O
H
H
N
H
(i)
Br
(ii)
O
H
H
O
H
H
N
H
H
O
HO
(
)
n
R'
(
)
O
O
n
4a
, R = R' = Me
2a. n = 5
2b
3a. n = 5
. n = 10
1
I
4b, R = Me, R' = Et
. n = 10
3b
4c
, R = R' = Et
H
O
H
H
O
(
)
n
H
O
O
O
O
H
H
O
R
O
R
O
(
)
n
O
N
N
(iii)
+
3a. n = 5
3b
I
N
N
. n = 10
R'
R'
5a
, R = R' = Me, n = 10
4a
, R = R' = Me
5b, R = Me, R' = Et, n = 5
5c
4b, R = Me, R' = Et
4c, R = R' = Et
, R = Me, R' = Et, n = 10
5d, R = R' = Et, n = 5
Scheme 1. Reagents and conditions: (i) Bromoalkanoyl chloride, THF, pyridine, rt, 12 h; (ii) p-iodo
phenol, acetone, K2CO3, reflux, 12 h; (iii) Pd(PPh3)2Cl2, CuI, Et3N, THF, rt, 12 h.

Synthesis and Mesomorphic Behavior of LC Dimesogens
17
Figure 1. Compound 5b at 112◦C.
understanding the relationship between the structure and mesomorphic properties in
calamitic pyrimidine derivatives, we have synthesized a series of compounds 5a–d in
which we systematically changed the spacing of the carbon chain on the cholesteryl moiety
(n = 5, 10) and the alkyl group in the pyrimidine moiety.
All the mesogenic compounds showed phase sequence of crystal-N∗-isotropic. For the
compounds having the same N-alkyl groups, the melting as well as clearing temperatures
decreases with increase in the spacer length of the cholesteryl moiety. When undecamethy-
lene spacer of 5a was replaced by a hexamethylene spacer, compound 5b also displayed
the same phase sequence (Fig. 1) only with increase in the isotropic temperature. All the
other compounds exhibit similar type of phase behavior under POM observation (Fig. 2).
Figure 2. Compound 5c at 88◦C.

18
K. C. Majumdar and T. Ghosh
Table 1. Transition temperatures (◦C)a and associated enthalpies (KJ/mol) of unsymmetri-
cal LC dimesogens
Dimers
Heating cycle
Cooling cycle
5a
Cr 104.5 (0.1) N∗
115.4 (22.7) I
I105.6 (2.8) N∗
90.8 (0.01) Cr
I 121.4 (3.6) N∗
100.5 (15.9) Cr
5b
Cr 128.4 (0.03) Cr1
135.0 (0.03) N∗
155.0 (40.9) I
5c
Cr 108.1 (20.0) N∗
128.6 (0.1) I
I 91.0 (0.01) N∗
80.0 (2.7) Cr
5d
Cr 90.4 (0.1) N∗
151.0 (39.4) I
I 137.7 (0.01) N∗
88.1 (0.9) Cr
aPeark temperatures in the DSC thermograms obtained during the first heating and cooling cycles
at 5◦C min−1. I: isotropic liquid state; N∗: chiral nematic; Cr and Cr1: crystals.
The phase transition temperatures and associated enthalpies are summarized in Table 1.
All the compounds exhibit only two transitions in both heating and cooling cycles except
5b, which exhibits an additional transition in the heating cycle, probably due to a solid–solid
transformation. For all the compounds of the series, the enthalpy change across the crystal
to mesophase transition is much smaller than that for the mesophase to isotropic phase
transition in the heating mode, and while considering the cooling cycle, except compound
5a, for all the other dimers the enthalpy change across the isotropic phase to mesophase
transition is much smaller than that for the mesophase to crystal transition.
2.2. Optical Properties
The UV-Vis absorption and fluorescence spectra of the compounds 5a–d in CHCl3 solution
are shown in Figs. 3 and 4(a), (b) respectively. The absorption and emission spectra of
1.25
(---) 5a
(---) 5c
(---) 5b
1.00
(---) 5d
0.75
0.50
0.25
0.00
250
275
300
325
350
375
400
Wavelength (nm)
Figure 3. UV-Vis spectra of compounds 5a–d in CHCl3.

Synthesis and Mesomorphic Behavior of LC Dimesogens
19
600
500
400
300
200
100
0
(--)1: 5a
(--)1: 5b
(--)1: 5c
(--)1: 5d
1000
800
600
400
200
0
(--)1: 5d
(--)1: 5c
(--)1: 5b
(--)1: 5a
300
350
400
450
500
350
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
(a)
(b)
Figure 4. (a) Fluorescence spectra of compounds 5a-d in CHCl3 (excitation at 268 nm).
(b) Fluorescence spectra of compounds 5a-d in CHCl3 (excitation at 321 nm).
all the compounds 5a–d are very similar in shape due to their structural similarity. The
absorption peaks of all the compounds are found to be near about 268 nm and 321 nm. The
absorbance is maximum for compounds 5a, and for compound 5d it is at a minimum. For
compounds having similar N-alkyl groups, absorbance decreases with decrease in spacer
length. In order to record the fluorescence spectra, we excited the compounds at 268 nm
and 321 nm. In case of fluorescence spectra, the emission peaks occurred at around 407 nm
and 409 nm for all the compounds irrespective of the spacer length. Fluorescence intensity
was found to be at a maximum for compound 5b on excitation at 268 nm. Compound 5d
exhibited maximum intensity on excitation at 321 nm.
Itahara et al. [38] explained the thermotropic behavior of their mesogenic compounds
containing adenine and thymine moiety by considering Watson–Crick base pairing (in-
termolecular H-bonding). However, in our present instance, the compounds cannot form
Watson–Crick base pairing or similar hydrogen bonds.
3. Conclusion
In summary, we have reported the synthesis and characterization of novel calamitic dime-
sogens, in which a pyrimidine unit is covalently tethered to a bulky chiral rod-like mesogen
through a tolane linkage. The dimesogenic compounds show only a chiral nematic phase
irrespective of the spacer length or the N-alkyl groups. Such a molecular design favors
formation of nematic phase over a reasonable thermal range.
4. Experimental
All the chemicals were procured from either Sigma Aldrich Chemicals Pvt. Ltd. or Spec-
trochem, India. Silica gel (60–120 mesh) was used for chromatographic separation. Silica
gel G (E-Merck, India) was used for thin layer chromatography (TLC). Petroleum ether
refers to the fraction boiling between 60◦C and 80◦C. IR spectra were recorded on a
Perkin–Elmer L 120-000A spectrometer (νmax in cm−1) on KBr disks. UV and fluores-
cence spectra were recorded in CHCl3 on a Shimadzu UV-2401PC spectrophotometer
1
(λmax in nm). H NMR and 13C NMR (400 MHz, 500 MHz) spectra were recorded on a

20
K. C. Majumdar and T. Ghosh
Bruker DPX-400, DPX-500 spectrometer in CDCl3 (chemical shift in δ) with tetramethyl-
silane (TMS) as internal standard. The LC properties were established through thermal
microscopy (Nikon polarizing microscope LV100POL attached to Instec hot and cold stage
HCS302, with STC200 temperature controller configured for HCS302), and the phase tran-
sitions were confirmed (both heating and cooling rate is 5◦ min−1) by differential scanning
calorimetry (Perkin–Elmer DSC Pyris1 system). The HRXRD was carried out at Centre
for Soft Matter Research (CSMR), Bangalore, India by employing PAN analytical X’Pert
PRO diffractometer equipped with a high-resolution, fast detector PIXCEL.
General procedure for the synthesis of compounds 3a,b:
Compounds 3a,b were prepared according to the previously published procedures [56].
General procedure for the synthesis of compounds 4a–c:
Compounds 4a–c were prepared according to the previously published procedures [57,
58].
General procedure for the synthesis of compounds 5a–d:
Nitrogen gas was purged through a solution of compound 4a (100 mg, 0.61 mmol),
3b (566 mg, 0.732 mmol), and Et3N (4 mL) in dry THF (10 mL) for 30 min. Then catalyst
Pd(PPh3)2Cl2 (42.8 mg, 0.061 mmol) and co-catalyst CuI (11.6 mg, 0.061 mmol) were
added and stirred for 12 h at room temperature. THF was removed followed by extraction
with CHCl3 (3 × 30 mL), and the extract was washed with H2O (2 × 20 mL) followed by
brine (10 mL) and dried Na2SO4, and the solvent was evaporated to give crude product,
which was purified by column chromatography over silica gel by EA:PE (1:19) as eluant
to afford compound 5a. Compounds 5b–d were prepared with similar procedure.
Compound 5a: White solid, yield 92%, IR (KBr): 2933, 1735, 1711, 1695, 1656,
1602 cm−1, 1H NMR (400 MHz, CDCl3): δH = 7.49 (s, 1H), 7.42 (d, 2H, J = 8.4 Hz), 6.84
(d, 2H, J = 8.8 Hz), 5.38 (s, 1H), 4.60–4.62 (m, 1H), 3.95 (t, 2H, J = 6.8 Hz), 3.45 (s, 3H),
3.40 (s, 3H), 2.25–2.32 (m, 4H), 0.86–2.02 (m, 54H), 0.67 (s, 3H). 13C NMR (100 MHz,
CDCl3): 173.3, 161.8, 159.4, 150.9, 144.7, 139.7, 133.1, 122.6, 114.5, 99.6, 93.5, 79.0,
73.7, 68.1, 56.7, 56.1, 50.0, 42.3, 39.7, 39.5, 38.2, 37.3, 37.0, 36.6, 36.2, 35.8, 34.7, 31.90,
31.86, 29.5, 29.3, 29.23, 29.17, 29.1, 28.4, 28.2, 28.0, 27.8, 26.0, 25.1, 24.3, 23.8, 22.8,
22.6, 21.0, 19.3, 18.7, 11.9; Anal. Calcd. for C52H76N2O5: C, 77.18; H, 9.47; N, 3.46%.
Found: C, 77.16; H, 9.46; N, 3.49%.
Compound 5b: White solid, yield 91%, IR (KBr): 2947, 1737, 1721, 1698, 1655,
1
1602 cm−1, H NMR (500 MHz, CDCl3): δH = 7.49 (s, 1H), 7.42 (d, 2H, J = 8.4 Hz),
6.83 (d, 2H, J = 8.8 Hz), 5.37 (d, 1H, J = 4.4 Hz), 4.61–4.63 (m, 1H), 3.96 (t, 2H, J =
6.4 Hz), 3.86 (q, 2H, J = 7.2 Hz), 3.39 (s, 3H), 2.31 (t, 4H, J = 7.5 Hz), 0.86–2.02 (m,
47H), 0.68 (s, 3H). 13C NMR (100 MHz, CDCl3): 173.0, 161.8, 159.3, 150.5, 143.7, 139.7,
133.1, 122.6, 114.6, 114.4, 99.7, 93.4, 79.2, 73.8, 67.7, 56.7, 56.1, 50.0, 45.4, 42.3, 39.7,
39.5, 38.2, 37.0, 36.6, 36.2, 35.8, 34.5, 31.90, 31.86, 29.7, 28.9, 28.4, 28.2, 28.0, 27.8, 25.6,
24.8, 24.3, 23.8, 22.8, 22.6, 21.0, 19.3, 18.7, 14.5, 11.9; Anal. Calcd. for C48H68N2O5: C,
76.56; H, 9.10; N, 3.72%. Found: C, 76.50; H, 9.16; N, 3.79%.
Compound 5c: White solid, yield 90%, IR (KBr): 2932, 1732, 1713, 1660, 1647,
1604 cm−1, 1H NMR (500 MHz, CDCl3): δH = 7.49 (s, 1H), 7.42 (d, 2H, J = 8.8 Hz), 6.84
(d, 2H, J = 8.8 Hz), 5.37 (d, 1H, J = 4.7 Hz), 4.60–4.62 (m, 1H), 3.95 (t, 2H, J = 6.6 Hz),
3.86 (q, 2H, J = 7.2 Hz), 3.39 (s, 3H), 2.25–2.32 (m, 4H), 0.86–2.02 (m, 57H), 0.68 (s,
3H). 13C NMR (100 MHz, CDCl3): 173.3, 161.8, 159.4, 150.5, 143.7, 139.7, 133.1, 122.6,
114.5, 99.7, 93.4, 79.2, 73.7, 68.0, 56.7, 56.1, 50.0, 45.4, 42.3, 39.7, 39.5, 38.2, 37.0, 36.6,
36.2, 35.8, 34.7, 31.90, 31.85, 29.7, 29.5, 29.3, 29.2, 29.1, 28.4, 28.2, 28.0, 27.8, 26.0, 25.1,
24.3, 23.8, 22.8, 22.6, 21.0, 19.3, 18.7, 14.5, 11.9; Anal. Calcd. for C53H78N2O5: C, 77.33;
H, 9.55; N, 3.40%. Found: C, 77.46; H, 9.56; N, 3.49%.

Synthesis and Mesomorphic Behavior of LC Dimesogens
21
Compound 5d: White solid, yield 84%, IR (KBr): 2951, 1730, 1707, 1655, 1645,
1602 cm−1, 1H NMR (500 MHz, CDCl3): δH = 7.48 (s, 1H), 7.43 (d, 2H, J = 8.8 Hz), 6.83
(d, 2H, J = 8.8 Hz), 5.38 (s, 1H), 4.61–4.63 (m, 1H), 4.06 (q, 2H, J = 7.2 Hz), 3.96 (t, 2H,
J = 6.4 Hz), 3.85 (q, 2H, J = 7.2 Hz), 2.32 (t, 4H, J = 7.2 Hz), 0.85–2.02 (m, 50H), 0.68 (s,
3H). 13C NMR (100 MHz, CDCl3): 173.0, 161.8, 159.4, 150.5, 143.7, 139.7, 133.1, 122.5,
114.6, 99.7, 93.4, 79.2, 73.7, 68.0, 56.7, 56.1, 50.0, 45.4, 42.3, 39.7, 39.4, 38.2, 37.0, 36.6,
36.2, 35.8, 34.7, 31.90, 31.85, 29.7, 29.5, 29.4, 29.2, 29.1, 28.4, 28.2, 28.0, 27.8, 26.0, 25.1,
24.3, 23.8, 22.8, 22.6, 21.0, 19.3, 18.7, 14.5, 13.8, 11.9; Anal. Calcd. for C49H70N2O5: C,
76.72; H, 9.20; N, 3.65%. Found: C, 76.76; H, 9.46; N, 3.71%.
Acknowledgments
We thank DST (New Delhi) for financial assistance. We are also thankful to UGC and
CSIR (K. C. Majumdar to UGC, New Delhi for a UGC-Emeritus fellowship, and T. Ghosh
to CSIR, New Delhi for his senior research fellowship). We thank DST (New Delhi) for
providing Bruker NMR (400 MHz), Perkin-Elmer CHN Analyser, FT-IR, and UV-Vis
spectrometer.
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