Synthesis and comparative studies of phase transition behaviour of new dimeric liquid crystals consisting of dimethyluracil and biphenyl cores

Article abstract of DOI:10.1016/j.molliq.2016.04.025

A new homologous series of mesogens containing 6-amino-1,3-dimethyluracil moiety have been synthesized. The structures of the compounds were characterised by elemental analysis, FT-IR, ¹H and ¹³C NMR spectroscopic techniques. Their m

Full text of DOI:10.1016/j.molliq.2016.04.025

Journal of Molecular Liquids 219 (2016) 765–772
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Synthesis and comparative studies of phase transition behaviour of new
dimeric liquid crystals consisting of dimethyluracil and biphenyl cores
b
c
d
e
Mohammad AbdulKarim-Talaq a, , H.T. Srinivasa , Ha Sie-Tiong , S. Hariprasad , Yeap Guan-Yeow
⁎
a
Chemistry Department, College of Science, University of Anbar, Ramadi, Iraq
b
Raman Research Institute, Soft Condensed Matter Group, Sadashivanagara, Bengaluru, 560080 Karnataka, India
Department of Chemical Science, Faculty of Science, Universiti Tunku Abdul Rahman, Jln Universiti, Bandar Barat, 31900 Kampar, Perak, Malaysia
Department of Chemistry, Central College Campus, Bangalore University, Bengaluru, 560001 Karnataka, India
Liquid Crystal Research Laboratory, School of Chemical Sciences, Universiti Sains Malaysia, Minden, 11800 Penang, Malaysia
c
d
e
a r t i c l e i n f o
a b s t r a c t
Article history:
A new homologous series of mesogens containing 6-amino-1,3-dimethyluracil moiety have been synthesized.
The structures of the compounds were characterised by elemental analysis, FT-IR, 1H and 13C NMR spectroscopic
techniques. Their mesomorphic properties were studied by polarising optical microscopy attached to a heating
stage. Microscopy data were supported with transition temperatures and enthalpy change values obtained
from the differential scanning calorimetry analysis. The studies have shown that the mesomorphic properties
of the compounds are dependent on the lengths of alkoxy-spacers. Compounds 4a-f with a shorter alkoxy-
spacer chain (n = 6) exhibited smectic A phase, while compounds 4g-r with a alkoxy-spacer chain (n = 8 or
10) displayed nematic phase.
Received 19 January 2016
Received in revised form 4 April 2016
Accepted 9 April 2016
Available online 20 April 2016
Keyword:
Biphenyl
Dimethyluracil
Dimer
© 2016 Elsevier B.V. All rights reserved.
Mesomorphic
Smectic A
Nematic
1. Introduction
molecules, but also due to fact that the insertion of heteroatoms
strongly influences the formation of mesomorphic phases. The inclusion
Liquid crystals play an important role in today's world of science and
technology, due to their technological applications, particularly as opti-
cal imaging liquid crystal displays, organic light emitting diodes
(OLED's), anisotropic networks and semiconductor materials [1–3].
The mesomorphic behaviour of organic compounds depends on the
structure of the molecules and can be varied by modifying its molecular
structure including linking, terminal and core groups [4].
The simplest molecules consist of two liquid crystalline/non-liquid
crystalline units connected by a flexible spacer usually in the sense of
linearity, and are known as LC dimers [5,6]. A wide range of low molar
mass dimeric molecular architectures are known to exhibit interesting
LC properties when compared to the conventional monomer low
molar mass LCs [7]. Moreover, the studies directed toward the dimers
are enormous due to their LC favorable chemical structure, size/type
of the core, connecting groups and terminal/spacer alkyl chains [8,9].
In recent years, many mesomorphic compounds containing differing
heterocyclic units have been synthesized, with the interest in such
structures growing constantly [10–12]. This is not only because of the
greater possibilities with heterocyclics in the design of new mesogenic
effect of heteroatoms (S, O and N) alters the geometric shape of the mol-
ecule considerably, thereby influencing the polarity, polarisability and
sometime type of mesophase, phase transition temperatures,
dielectric- and other properties of the mesogens [13].
Nucleic acids are biologically very important for the sustenance of
life on earth, occurring in both RNA and DNA. The heterocyclic base moi-
eties present in nucleic acids are well-known. These bases present in the
nucleic acids are expected to be applicable to the design of molecular as-
semblies with diverse liquid-crystalline properties, since each
nucleobase specifically forms a base pair with its predetermined part-
ner. Some attempts to prepare thermotropic liquid crystals of
nucleobase derivatives were unsuccessful [14–16]; but there are several
reported examples of lyotropic liquid crystals derived from DNA and
nucleotides [17,18].
This article reports on our on-going research on the synthesis and
characterization of heterocyclic based liquid crystalline materials
[19–23]. The present work expands our understanding of the
structure-liquid crystalline property relationship of these materials.
Specifically we focus on a new series of mesogenic compounds contain-
ing 1.3-dimethyluracil core system having alkoxy-spacer [–O–(CH2)n-
O–] with an even number of carbon atoms ranging from C6H10 to
C
10H24. A pendant biphenyl moiety is connected to this core by an
⁎
Corresponding author.
E-mail address: drmohamadtalaq@gmail.com (M. AbdulKarim-Talaq).
alkyl chain varying in length from n = 6–16 carbons.
http://dx.doi.org/10.1016/j.molliq.2016.04.025
0167-7322/© 2016 Elsevier B.V. All rights reserved.

766
M. AbdulKarim-Talaq et al. / Journal of Molecular Liquids 219 (2016) 765–772
2. Experimental
2.1. Materials
Data sets of the compounds were entered as two-dimensional sketches
into the HyperChem program.
2.3. Synthesis and characterization of 4a–r
A series of α,ω-dibromoalkanes, 4-hydroxybenzaldehyde, 6-amino-
1,3-dimethyluracil, 4′-hydroxybiphenyl-4-carboxylic acid, hexanol,
octanol, decanol, dodecanol, tetradecanol, hexadecanol were purchased
from Sigma-Aldrich. The chemicals were used directly as received with-
out further purification. Thin-layer chromatography (TLC) was per-
formed on pre-coated silica-gel on aluminum plates using 2:8 ratio of
ethyl acetate and petroleum ether as an eluent.
The synthetic approach to the intermediates 1a–c, 2a–f, 3a–r and
title compounds 4a–r are illustrated in Scheme 1. Refluxing 4-
hydroxybenzaldehyde and anhydrous Na2CO3 with 1,6-
dibromohexane/1,8-dibromohexane/1,10-dibromohexanes in DMF sol-
vent gave the corresponding compounds 1a–c [25,26]. The esters 4′-
hydroxybiphenyl-4-carboxyalkanoates 2a–f were synthesized
employing the esterification reaction between 4-hydroxyphenyl-4-
benzoic acid and hexyl to hexadecyl alcohols at 120 °C in the presence
of 1 ml of sulphuric acid as dehydrating agent. Intermediates 3a–r
were synthesized via the Williamson etherification between 1a–c and
2a–f in DMF solvent using anhydrous Na2CO3 as a catalyst. The new di-
meric target compounds 4a–r were synthesized by the reaction of alkyl-
4′-(n-(4-formylphenoxy)alkyloxy)biphenyl-4-carboxylate 3a–r with
6-amino-1.3-dimethyluracil by refluxing in ethanol at 98 °C with cata-
lytic quantity of glacial acetic acid. Complete FT-IR, 1H and 13C NMR as-
signments of compounds 4a–r were obtained and substantiated with
the aid of DEPT and two-dimensional 1H–1H correlation spectroscopy
(COSY, NOESY), 1H-13C heteronuclear multiple quantum correlation
(HMQC) and 1H–13C heteronuclear multiple bond correlation (HMBC)
spectroscopies.
2.2. Measurements
Elemental (CHN) microanalyses were performed using a Perkin
Elmer 2400 LS Series CHNS/O analyzer. FT-IR spectra were obtained
using KBr pellets and the spectra were recorded in the range of 4000–
400 cm−1 using a Perkin Elmer 2000-FT-IR spectrophotometer. 1H
and 13C NMR spectra were recorded in dimethylsulphoxide (DMSO-
d6) at 25 °C on a Bruker 400 MHz Ultrashied™ FT-NMR spectrometer
equipped with a 5 mm BBI inverse gradient probe. Chemicals shift
was referenced to internal tetramethylsilane (TMS). The concentration
of solute molecules was 50 mg in 1.0 ml DMSO. Standard Bruker pulse
programs [24] were used throughout the entire experiment. The transi-
tion temperatures and associated enthalpy values were determined
using a differential scanning calorimeter (Elmer Pyris 1 DSC) operated
at a scanning rate of 5 °C/min−1 on heating and cooling respectively.
Texture observation was carried out using Carl Zeiss Axioskop 40 optical
microscope equipped with Linkam LTS350 hot stage and TMS94 tem-
perature controller.
In the FT-IR spectrum of compound 4a, the bands assignable to the
C\\H stretching at 2982–2875 cm−1 can be ascribed to the alkyl spacer
and the alkyl ester group attached to one of the terminals of the dimer.
The strong band observed at 1752 cm−1 is assigned to carbonyl group
C_O. On the other hand, the C_N gave a band at 1625 cm−1 with me-
dium intensity. The band at 1256 cm−1 is due to the ether C\\O stretching
in the fingerprint region. The FT-IR spectra of compounds 4b–r exhibited
similar diagnostic bands as those observed in compound 4a.
Molecular models were obtained using HyperChem 8.0.8 (Hyper-
cube Inc.) in the Liquid Crystal Institute of Kent State University, USA.
Scheme 1. Synthetic route and reagents for the formation of 4a–r. (a) DMF, anhydrous Na2CO3, reflux, 12 h. (b) Conc H2SO4, 120 °C, (c) ethanol, 98 °C, 6 h, catalytic acetic acid.

M. AbdulKarim-Talaq et al. / Journal of Molecular Liquids 219 (2016) 765–772
767
From the 1H NMR data for compound 4a, twelve aromatic protons of
compound 4a gave rise to six doublets with their resonances observed
at δ 7.01–8.56 ppm. At downfield region of 1H NMR spectrum, the reso-
nance of uracil ring proton and azomethine proton showed characteris-
tic singlets at 9.76 and 8.86 ppm, respectively. A triplet at 0.89 ppm is
attributed to the methyl protons of the terminal chain. Two triplets at-
tributed to the methoxy protons of the spacer were evident at a slightly
higher field at 4.01 and 3.99 ppm. The fact that two triplets were ob-
served instead of one triplet suggests that the molecule is non-
symmetric, while a triplet assigned to the ester\\OCH2 protons could
be observed at 4.18 ppm. Two singlets were observed at 3.37 and
3.19 ppm and assigned to two CH3 groups in the uracil ring.
−N_CH), 7.01–8.57 (6d, 12H, Ar\\H), 4.16 (t, 2H, J = 6.83 Hz, –COO-
CH2–), 4.07 (t, 2H, J = 6.33 Hz, –OCH2–), 3.93 (t, 2H, J = 6.79 Hz, –
OCH2–), 3.36 (s, 3H, −NCH3), 3.12 (s, 3H, −NCH3), 1.77–1.20 (m,
32H, Alkyl –CH2–), 0.85 (t, 3H, alkyl CH3). 13C NMR δ (ppm): 173.79,
164.00, 162.59 (C = O), 161.08 (C = N), 159.65 (Ar\\C\\O), 114.07–
139.87 (Ar\\C), 60.58 (C\\O\\C), 21.06 (CH2), 14.18 (CH3). Elemental
analysis found for C46H61N3O6 (%): C, 73.35; H, 8.27; N, 5.46. Calc (%),
C, 73.47; H, 8.18; N, 5.59.
4f. Yield 74%. IR: υmax (KBr, cm−1): 2988, 2869, 1767 1620, 1572,
1254. 1H NMR δ (ppm, DMSO-d6): 9.83 (s, 1H, Uracil-H), 8.82 (s, 1H,
−N_CH), 7.03–8.58 (6d, 12H, Ar\\H), 4.20 (t, 2H, J = 6.10 Hz, –COO-
CH2–), 4.09 (t, 2H, J = 6.38 Hz, –OCH2–), 3.90 (t, 2H, J = 6.89 Hz, –
OCH2– ), 3.30 (s, 3H, −NCH3), 3.10 (s, 3H, −NCH3), 1.79–1.26 (m,
36H, Alkyl –CH2–), 0.86 (t, 3H, alkyl CH3). 13C NMR δ (ppm): 176.06,
165.40, 163.64 (C_O), 162.29 (C_N), 161.75 (Ar\\C\\O), 114.88–
140.29 (Ar\\C), 60.44 (C\\O\\C), 22.30 (CH2), 14.69 (CH3). Elemental
analysis found for C48H65N3O6 (%): C, 73.35; H, 8.27; N, 5.46. Calc (%),
C, 73.91; H, 8.40; N, 5.39.
2.3.1. General methodology for the synthesis of 4a–r
The title compounds were synthesized according to a general
method described by Majumdar et al. [27]. As an example of synthetic
procedure for 4a is given below:
A mixture of 6-amino-1,3-dimethyluracil (0.155 g, 0.001 mol) and
hexyl-4′-(6-(4-formylphenoxy)hexyloxy)biphenyl-4-carboxylate, 3a
(0.502 g, 0.001 mol) was refluxed in absolute ethanol in the presence
of a catalytic amount of glacial acetic acid for 2 h. The Schiff base 4a
was obtained as a precipitate from the hot reaction mixture. The precip-
itate was filtered out and repeatedly washed with hot ethanol and dried
under vacuum.
4g. Yield 60%. IR: υmax (KBr, cm−1): 2990, 2872, 1752 1622, 1580,
1252. 1H NMR δ (ppm, DMSO-d6): 9.72 (s, 1H, Uracil-H), 8.83 (s, 1H,
−N_CH), 7.05–8.60 (6d, 12H, Ar\\H), 4.19 (t, 2H, J = 6.58 Hz, –COO-
CH2–), 4.06 (t, 2H, J = 6.67 Hz, –OCH2–), 3.96 (t, 2H, J = 6.47 Hz, –
OCH2–), 3.35 (s, 3H, −NCH3), 3.14 (s, 3H, −NCH3), 1.79–1.28 (m,
20H, Alkyl –CH2–), 0.86 (t, 3H, alkyl CH3). 13C NMR δ (ppm): 177.06,
168.20, 165.17 (C_O), 163.79 (C_N), 160.65 (Ar\\C\\O), 114.98–
141.08 (Ar\\C), 61.53 (C\\O\\C), 22.03 (CH2), 14.85 (CH3). Elemental
analysis found for C40H49N3O6 (%): C, 71.80; H, 7.51; N, 6.19. Calc (%),
C, 71.94; H, 7.40; N, 6.29.
4a. Yield 68%. IR: υmax (KBr, cm−1): 2982, 2875, 1752 1625, 1581,
1256. 1H NMR δ (ppm, DMSO-d6): 9.70 (s, 1H, Uracil-H), 8.90 (s, 1H,
−N_CH), 7.01–8.56 (6d, 12H, Ar\\H), 4.18 (t, 2H, J = 6.70 Hz, –COO-
CH2–), 4.01 (t, 2H, J = 6.30 Hz, –OCH2–), 3.99 (t, 2H, J = 6.67 Hz, –
OCH2–), 3.37 (s, 3H, −NCH3), 3.19 (s, 3H, −NCH3), 1.78–1.28 (m,
16H, Alkyl –CH2–), 0.89 (t, 3H, alkyl CH3). 13C NMR δ (ppm): 179.45,
168.33, 164.27 (C_O), 163.45 (C_N), 160.00 (Ar\\C\\O), 114.30–
142.16 (Ar\\C), 61.06 (C\\O\\C), 22.62 (CH2), 14.33 (CH3). Elemental
analysis found for C38H45N3O6 (%): C, 71.18; H, 7.23; N, 6.43. Calc (%),
C, 71.34; H, 7.09; N, 6.57.
4 h. Yield 66%. IR: υmax (KBr, cm−1): 2986, 2867, 1750 1620, 1582,
1254. 1H NMR δ (ppm, DMSO-d6): 9.81 (s, 1H, Uracil-H), 8.80 (s, 1H,
−N_CH), 7.03–8.57 (6d, 12H, Ar\\H), 4.18 (t, 2H, J = 6.77 Hz, –COO-
CH2–), 4.01 (t, 2H, J = 6.08 Hz, –OCH2–), 3.93 (t, 2H, J = 6.70 Hz, –
OCH2–), 3.38 (s, 3H, −NCH3), 3.15 (s, 3H, −NCH3), 1.73–1.20 (m,
24H, Alkyl –CH2–), 0.89 (t, 3H, alkyl CH3). 13C NMR δ (ppm): 176.60,
167.69, 165.50 (C_O), 162.68 (C_N), 161.08 (Ar\\C\\O), 114.40–
140.38 (Ar\\C), 61.08 (C-O-C), 22.62 (CH2), 14.74 (CH3). Elemental
analysis found for C42H53N3O6 (%): C, 72.53; H, 7.79; N, 6.20. Calc (%),
C, 72.49; H, 7.68; N, 6.04.
4b. Yield 60%. IR: υmax (KBr, cm−1): 2980, 2862, 1750 1621, 1577,
1250. 1H NMR δ (ppm, DMSO-d6): 9.72 (s, 1H, Uracil-H), 8.91 (s, 1H,
−N_CH), 7.00–8.40 (6d, 12H, Ar\\H), 4.19 (t, 2H, J = 6.80 Hz, –COO-
CH2–), 4.03 (t, 2H, J = 6.55 Hz, –OCH2–), 3.98 (t, 2H, J = 6.60 Hz, –
OCH2–), 3.35 (s, 3H, −NCH3), 3.12 (s, 3H, −NCH3), 1.77–1.27 (m,
20H, Alkyl –CH2–), 0.87 (t, 3H, alkyl CH3). 13C NMR δ (ppm): 177.03,
167.89, 162.44 (C_O), 161.89 (C_N), 159.64 (Ar\\C\\O), 115.00–
141.84 (Ar-C), 60.94 (C\\O\\C), 23.77 (CH2), 14.77 (CH3). Elemental
analysis found for C40H49N3O6 (%): C, 71.80; H, 7.53; N, 7.32. Calc (%),
C, 71.94; H, 7.40; N, 7.29;
4i. Yield 70%. IR: υmax (KBr, cm−1): 2980, 2860, 1755 1624, 1580,
1250. 1H NMR δ (ppm, DMSO-d6): 9.86 (s, 1H, Uracil-H), 8.77 (s, 1H,
−N_CH), 7.06–8.61 (6d, 12H, Ar\\H), 4.17 (t, 2H, J = 6.80 Hz, –COO-
CH2–), 4.07 (t, 2H, J = 6.77 Hz, –OCH2–), 3.96 (t, 2H, J = 6.19 Hz, –
OCH2–), 3.32 (s, 3H, −NCH3), 3.10 (s, 3H, −NCH3), 1.74–1.28 (m,
28H, Alkyl –CH2–), 0.90 (t, 3H, alkyl CH3). 13C NMR δ (ppm): 177.20,
167.19, 165.88 (C_O), 161.16 (C_N), 160.50 (Ar\\C\\O), 114.00–
140.06 (Ar\\C), 61.38 (C\\O\\C), 21.59 (CH2), 15.36 (CH3). Elemental
analysis found for C44H57N3O6 (%): C, 73.12; H, 7.83; N, 5.95. Calc (%),
C, 73.00; H, 7.94; N, 5.80.
4c. Yield 71%. IR: υmax (KBr, cm−1): 2988, 2873, 1757 1623, 1580,
1250. 1H NMR δ (ppm, DMSO-d6): 9.81 (s, 1H, Uracil-H), 8.87 (s, 1H,
−N_CH), 7.04–8.50 (6d, 12H, Ar\\H), 4.20 (t, 2H, J = 6.86 Hz, –COO-
CH2–), 4.09 (t, 2H, J = 6.67 Hz, –OCH2–), 3.93 (t, 2H, J = 6.26 Hz, –
OCH2–), 3.42 (s, 3H, −NCH3), 3.20 (s, 3H, −NCH3), 1.78–1.25 (m,
24H, Alkyl –CH2–), 0.90 (t, 3H, alkyl CH3). 13C NMR δ (ppm): 177.20,
167.14, 163.83 (C_O), 162.09 (C_N), 161.80 (Ar\\C\\O), 114.90–
141.00 (Ar\\C), 60.39 (C\\O\\C), 22.67 (CH2), 14.88 (CH3). Elemental
analysis found for C42H53N3O6 (%): C, 72.60; H, 7.52; N, 6.19. Calc (%),
C, 72.49; H, 7.68; N, 6.04.
4j. Yield 70%. IR: υmax (KBr, cm−1): 2988, 2874, 1760 1626, 1584,
1253. 1HNMR δ (ppm, DMSO-d6): 9.83 (s, 1H, Uracil-H), 8.80 (s, 1H,
−N_CH), 7.03–8.58 (6d, 12H, Ar\\H), 4.14 (t, 2H, J = 6.94 Hz, –COO-
CH2–), 4.02 (t, 2H, J = 6.18 Hz, –OCH2–), 3.94 (t, 2H, J = 6.70 Hz, –
OCH2–), 3.37 (s, 3H, −NCH3), 3.16 (s, 3H, −NCH3), 1.79–1.23 (m,
32H, Alkyl –CH2–), 0.88 (t, 3H, alkyl CH3). 13C NMR δ (ppm): 176.60,
168.05, 166.49 (C_O), 162.59 (C_N), 161.40 (Ar\\C\\O), 114.89–
140.76 (Ar\\C), 61.94 (C\\O\\C), 22.67 (CH2), 14.80 (CH3). Elemental
analysis found for C46H61N3O6 (%): C, 73.28; H, 8.29; N, 5.37. Calc (%),
C, 73.47; H, 8.18; N, 5.59.
4d. Yield68%. IR: υmax (KBr, cm−1): 2990, 2879, 1760 1625, 1586,
1254. 1H NMR δ (ppm, DMSO-d6): 9.83 (s, 1H, Uracil-H), 8.84 (s, 1H,
−N_CH), 7.02–8.47 (6d, 12H, Ar\\H), 4.18 (t, 2H, J = 6.38 Hz, –COO-
CH2–), 4.04 (t, 2H, J = 6.80 Hz, –OCH2–), 3.91 (t, 2H, J = 6.33 Hz, –
OCH2–), 3.38 (s, 3H, −NCH3), 3.17 (s, 3H, −NCH3), 1.79–1.24 (m,
28H, Alkyl –CH2–), 0.88 (t, 3H, alkyl CH3). 13C NMR δ (ppm): 175.14,
165.60, 163.78 (C_O), 162.80 (C_N), 160.30 (Ar\\C\\O), 115.09–
141.80 (Ar\\C), 61.07 (C\\O\\C), 22.72 (CH2), 14.70 (CH3). Elemental
analysis found for C44H57N3O6 (%): C, 73.18; H, 7.85; N, 5.90. Calc (%),
C, 73.00; H, 7.94; N, 5.80.
4 k. Yield 67%. IR: υmax (KBr, cm−1): 2979, 2870, 1756 1622, 1580,
1251. 1H NMR δ (ppm, DMSO-d6): 9.85 (s, 1H, Uracil-H), 8.72 (s, 1H,
−N_CH), 7.00–8.40 (6d, 12H, Ar\\H), 4.18 (t, 2H, J = 6.64 Hz, –COO-
CH2–), 4.08 (t, 2H, J = 6.59 Hz, –OCH2–), 3.90 (t, 2H, J = 6.19 Hz, –
OCH2–), 3.33 (s, 3H, −NCH3), 3.18 (s, 3H, −NCH3), 1.75–1.20 (m, 36H,
Alkyl –CH2–), 0.86 (t, 3H, alkyl CH3). 13C NMR δ (ppm): 177.20, 168.56,
165.03 (C_O), 162.48 (C_N), 161.00 (Ar\\C\\O), 114.12–140.07
(Ar\\C), 61.60 (C\\O\\C), 22.15 (CH2), 14.27 (CH3). Elemental analysis
4e. Yield 72%. IR: υmax (KBr, cm−1): 2984, 2860, 1764 1623, 1575,
1250. 1H NMR δ (ppm, DMSO-d6): 9.80 (s, 1H, Uracil-H), 8.82 (s, 1H,

768
M. AbdulKarim-Talaq et al. / Journal of Molecular Liquids 219 (2016) 765–772
found for C48H65N3O6 (%): C, 73.82; H, 8.56; N, 5.46. Calc (%), C, 73.91; H,
8.40; N, 5.39.
CH2–), 4.07 (t, 2H, J = 6.20 Hz, –OCH2–), 3.92 (t, 2H, J = 6.07 Hz, –
OCH2–), 3.32 (s, 3H, −NCH3), 3.11 (s, 3H, −NCH3), 1.82–1.24 (m,
36H, Alkyl –CH2–), 0.85 (t, 3H, alkyl CH3). 13C NMR δ (ppm): 176.70,
164.42, 163.17 (C_O), 162.06 (C_N), 160.11 (Ar\\C\\O), 114.27–
141.60 (Ar\\C), 61.59 (C\\O\\C), 22.30 (CH2), 14.89 (CH3). Elemental
analysis found for C48H65N3O6 (%): C, 73.76; H, 8.53; N, 5.22. Calc (%),
C, 73.91; H, 8.40; N, 5.39.
4 l. Yield 61%. IR: υmax (KBr, cm−1): 2983, 2868, 1752 1625, 1583, 1255.
1H NMR δ (ppm, DMSO-d6): 9.87 (s, 1H, Uracil-H), 8.78 (s, 1H, −N_CH),
7.03–8.54 (6d, 12H, Ar\\H), 4.12 (t, 2H, J = 6.09 Hz, –COO-CH2–), 4.07 (t,
2H, J = 6.48 Hz, –OCH2–), 3.96 (t, 2H, J = 6.53 Hz, –OCH2–), 3.32 (s, 3H,
−NCH3), 3.15 (s, 3H, −NCH3), 1.76–1.22 (m, 40H, Alkyl –CH2–), 0.89 (t,
3H, alkyl CH3). 13C NMR δ (ppm): 176.56, 167.49, 164.69 (C_O), 161.10
(C_N), 160.78 (Ar\\C\\O), 114.63–141.26 (Ar\\C), 61.47 (C\\O\\C),
22.00 (CH2), 15.08 (CH3). Elemental analysis found for C50H69N3O6 (%):
C, 74.23; H, 8.72; N, 5.11. Calc (%), C, 74.31; H, 8.61; N, 5.20.
4q. Yield 63%. IR: υmax (KBr, cm−1): 2983, 2873, 1753 1620, 1577,
1253. 1H NMR δ (ppm, DMSO-d6): 9.76 (s, 1H, Uracil-H), 8.86 (s, 1H,
−N_CH), 7.10–8.42 (6d, 12H, Ar\\H), 4.13 (t, 2H, J = 6.63 Hz, –COO-
CH2–), 4.09 (t, 2H, J = 6.86 Hz, –OCH2–), 3.90 (t, 2H, J = 6.69 Hz, –
OCH2–), 3.37 (s, 3H, −NCH3), 3.10 (s, 3H, −NCH3), 1.79–1.22 (m,
40H, Alkyl –CH2–), 0.89 (t, 3H, alkyl CH3). 13C NMR δ (ppm): 178.05,
166.10, 164.89 (C_O), 162.58 (C_N), 161.28 (Ar\\C\\O), 115.00–
140.29 (Ar\\C), 62.72 (C\\O\\C), 22.52 (CH2), 15.08 (CH3). Elemental
analysis found for C50H69N3O6 (%): C, 74.16; H, 8.77; N, 5.09. Calc (%),
C, 74.31; H, 8.61; N, 5.20.
4 m. Yield 74%. IR: υmax (KBr, cm−1): 2989, 2873, 1756 1624, 1580,
1253. 1H NMR δ (ppm, DMSO-d6): 9.86 (s, 1H, Uracil-H), 8.74 (s, 1H,
−N_CH), 7.00–8.60 (6d, 12H, Ar\\H), 4.19 (t, 2H, J = 6.41 Hz, –COO-
CH2–), 4.04 (t, 2H, J = 6.39 Hz, –OCH2–), 3.90 (t, 2H, J = 6.07 Hz, –
OCH2–), 3.39 (s, 3H, −NCH3), 3.16 (s, 3H, −NCH3), 1.77–1.26 (m,
24H, Alkyl –CH2–), 0.90 (t, 3H, alkyl CH3). 13C NMR δ (ppm): 177.07,
166.27, 164.06 (C_O), 162.97 (C_N), 161.43 (Ar\\C\\O), 115.39–
140.44 (Ar\\C), 62.04 (C\\O\\C), 22.60 (CH2), 14.76 (CH3). Elemental
analysis found for C42H53N3O6 (%): C, 72.30; H, 7.54; N, 6.21. Calc (%),
C, 72.49; H, 7.68; N, 6.04.
4r. Yield 69%. IR: υmax (KBr, cm−1): 2990, 2887, 1760 1623, 1582,
1255. 1H NMR δ (ppm, DMSO-d6): 9.84 (s, 1H, Uracil-H), 8.89 (s, 1H,
−N_CH), 7.14–8.64 (6d, 12H, Ar\\H), 4.20 (t, 2H, J = 6.06 Hz, –COO-
CH2–), 4.10 (t, 2H, J = 6.59 Hz, –OCH2–), 3.95 (t, 2H, J = 6.20 Hz, –
OCH2–), 3.32 (s, 3H, −NCH3), 3.19 (s, 3H, −NCH3), 1.80–1.20 (m,
44H, Alkyl –CH2–), 0.90 (t, 3H, alkyl CH3). 13C NMR δ (ppm): 176.30,
165.70, 163.04 (C_O), 162.12 (C_N), 160.86 (Ar\\C\\O), 114.30–
141.35 (Ar\\C), 61.00 (C\\O\\C), 22.73 (CH2), 14.29 (CH3). Elemental
analysis found for C52H73N3O6 (%): C, 74.80; H, 8.66; N, 5.20. Calc (%),
C, 74.69; H, 8.80; N, 5.03.
4n. Yield 70%. IR: υmax (KBr, cm−1): 2984, 2870, 1752 1622, 1586,
1250. 1H NMR δ (ppm, DMSO-d6): 9.89 (s, 1H, Uracil-H), 8.80 (s, 1H,
−N_CH), 7.08–8.66 (6d, 12H, A\\H), 4.11 (t, 2H, J = 6.73 Hz, –COO-
CH2–), 4.08 (t, 2H, J = 6.04 Hz, –OCH2–), 3.97 (t, 2H, J = 6.29 Hz, –
OCH2–), 3.32 (s, 3H, −NCH3), 3.11 (s, 3H, −NCH3), 1.79–1.28 (m,
28H, Alkyl –CH2–), 0.88 (t, 3H, alkyl CH3). 13C NMR δ (ppm): 176.83,
165.49, 163.67 (C_O), 162.18 (C_N), 161.20 (Ar\\C\\O), 114.89–
140.62 (Ar\\C), 61.78 (C\\O\\C), 22.37 (CH2), 14.08 (CH3). Elemental
analysis found for C44H57N3O6 (%): C, 73.18; H, 7.83; N, 5.61. Calc (%),
C, 73.00; H, 7.94; N, 5.80.
3. Results and discussion
3.1. Phase transition and mesomorphic behaviour
4o. Yield 63%. IR: υmax (KBr, cm−1): 2989, 2876, 1750 1624, 1580,
1253. 1H NMR δ (ppm, DMSO-d6): 9.85 (s, 1H, Uracil-H), 8.84 (s, 1H,
−N_CH), 7.01–8.60 (6d, 12H, Ar\\H), 4.20 (t, 2H, J = 6.95 Hz, –COO-
CH2–), 4.01 (t, 2H, J = 6.88 Hz, –OCH2–), 3.96 (t, 2H, J = 6.73 Hz, –
OCH2–), 3.39 (s, 3H, −NCH3), 3.14 (s, 3H, −NCH3), 1.81–1.29 (m,
32H, Alkyl –CH2–), 0.86 (t, 3H, alkyl CH3). 13C NMR δ (ppm): 178.30,
165.19, 164.69 (C_O), 162.68 (C_N), 161.20 (Ar\\C\\O), 114.30–
141.59 (Ar\\C), 62.20 (C\\O\\C), 22.06 (CH2), 15.11 (CH3). Elemental
analysis found for C46H61N3O6 (%): C, 73.58; H, 8.30; N, 5.70. Calc (%),
C, 73.47; H, 8.18; N, 5.59.
The second heating and cooling transition temperatures and en-
thalpies for the title compounds were determined by DSC. The
mesophases are identified according to the textures observed through
POM and the classification system reported by Dierking [28] and Gray/
Goodby et al. [29]. The phase transitions and associated enthalpy
value changes data of the title compounds 4a–r are summarized in
Table 1. All the compounds 4a–r formed liquid crystalline phases re-
gardless of the length of the alkoxy spacer or the terminal alkyl chain.
However, we find that the type of mesophase is dependent on the
length of the spacer chain. The melting points of the all compounds in-
creased as the length of the alkoxy spacer chains and terminal chains
increased.
4p. Yield 75%. IR: υmax (KBr, cm−1): 2980, 2884, 1760 1623, 1582,
1251. 1H NMR δ (ppm, DMSO-d6): 9.70 (s, 1H, Uracil-H), 8.81 (s, 1H,
−N_CH), 7.12–8.30 (6d, 12H, Ar\\H), 4.12 (t, 2H, J = 6.40 Hz, –COO-
Table 1
Transition temperature °C (corresponding enthalpy changes in KJ/mol−1) on second heating/cooling scans for the final compounds 4a–r.
No
n/R
Heating scan
Cooling scan
4a
4b
4c
4d
4e
4f
4g
4h
4i
4j
4k
4l
4m
4n
4o
4p
4q
4r
6/6
6/8
Cr 106.2 (6.45) SmA 124.7 (60.12) I
Cr 114.6 (3.78) SmA 132.4 (47.37) I
Cr 116 (6.23) SmA 135.6 (80.19) I
Cr 128.1 (4.30) SmA 150.1 (63.46) I
Cr 132.9 (2.11) SmA 158 (80.72) I
Cr 144.3 (3.70) SmA 166.5 (50.27) I
Cr 114.2 (9.03) N 130.9 (33.41) I
Cr 118.3 (1.07) N 142.7 (31.30) I
Cr 127 (1.15) N 149.7 (19.55) I
Cr 130.9 (2.20) N 155.3 (14.87) I
Cr 133.9 (3.60) N 164 (18.30) I
Cr 140.6 (1.30) N 170.3 (28.09) I
Cr 125.1 (3.37) N 150.4 (19.90) I
Cr 129.4 (6.70) N 158.8 (24.67) I
Cr 136.8 (1.56) N 171.4 (16.30) I
Cr 140 (1.39) N 187 (31.15) I
I 120.1 (−7.33) SmA 101.2 (−59.67) Cr
I 121.2 (−5.77) SmA 98.5 (−30.78) Cr
I 129.3 (−2.89) SmA 101.1 (−44.15) Cr
I 144.1 (−5.58) SmA110.3 (−65.20) Cr
I 154.1 (−7.20) SmA 119.5 (−50.29) Cr
I 160.2 (−4.09) SmA 128.1 (−20.57) Cr
I 123.8 (−1.69) N 102.7 (−24.48) Cr
I 136 (−2.31) N 111.3 (−18.06) Cr
I 140 (−1.60) N 117.3 (−30.21) Cr
I 148.1 (−1.39) N 123.6 (−21.00) Cr
I 158.1 (−2.66) N 130.2 (−29.11) Cr
I 164.4 (−1.06) N 131.2 (−14.20) Cr
I 143.1 (−4.39) N 122.1 (−19.47) Cr
I 146.6 (−3.59) N 124.3 (−27.03) Cr
I 166.2 (−7.23) N 142.1 (−20.55) Cr
I 172.3 (−4.30) N 137.1 (−17.93) Cr
I 177.6 (−1.76) N 142.6 (−23.18) Cr
I 190 (−2.04) N 158 (−14.67) Cr
6/10
6/12
6/14
6/16
8/6
8/8
8/10
8/12
8/14
8/16
10/6
10/8
10/10
10/12
10/14
10/16
Cr 154.3 (2.47) N 184.6 (22.50) I
Cr 161.3 (1.50) N 196.1 (15.60) I
Note: Cr = crystal; SmA = smectic A phase; N = nematic phase; I = isotropic phase.

M. AbdulKarim-Talaq et al. / Journal of Molecular Liquids 219 (2016) 765–772
769
Compounds 4a–f with spacer chain (n = 6) with varied terminal
alkyl chains (n = 6, 8, 10, 12, 14 and 16) show enantiotropic liquid crys-
talline SmA phase. The DSC thermograms of compounds 4a–f exhibited
Cr-to-SmA and SmA-to-I transitions typically in heating and cooling cy-
cles. The liquid crystalline texture was observed under crossed
polarizers and demonstrated the formation of batonnets initially. After
a few minutes, this enlarged and eventually merged to yield a classic
focal conic fan shaped texture of SmA phase on cooling for compounds
4a–f.
A typical example is illustrated for the compound 4a, which melts
from crystal to smectic A mesophase at 106.2 °C (ΔH = 6.45). With
followed heating, it went to isotropic state at 124.7 °C (ΔH = 60.12).
Again, when it was cooled down, it changed from the isotropic state
and started to appear with batonnets texture. It became completely
focal-conic texture at 120.1 °C (ΔH = −7.33), and finally transformed
back to crystalline state at 101.2 °C (ΔH = −59.67). Fig. 1(a) shows a
representative optical photomicrograph of compound 4a exhibiting
focal conic fan-shaped texture during heating cycle.
Similar transition behaviors with existence of fan-shaped textures
were observed for the remaining compounds 4b–f. All the observed liq-
uid crystalline textures are typical according to reports [25,26].
Compounds 4g–l with spacer (n = 8) with varied terminal alkyl
chains exhibit enantiotropic nematogenic properties. The DSC thermo-
gram of 4g–l reveals the Cr-N-I transitions on heating, and I-N-Cr tran-
sitions on cooling scan. The nematic to isotropic transitions are
increased with increase in the terminal chains. The temperature ranges
of mesophase were found to be 16.75, 24.39, 22.86, 24.3, 30.13 and
29.72 °C for 4g–l respectively. The clearing temperatures of the com-
pounds 4g–l, increased smoothly as the chain length increases. The
POM also reveals the N-I phase transition upon the heating run for com-
pounds 4g–l, whereby the POM shows only a Schlieren texture nematic
phase.
Compounds 4m–r possess a little longer spacer chain (n = 10) with
terminal chains of even parity numbers from 6 to 18. The representative
DSC thermogram of compound 4p is given in Fig. 2. It shows phase tran-
sitions of Cr-N-I and I-N-Cr on heating and cooling scans. The texture
observed for 4p compound upon the heating scan shown in Fig. 1
(b) with Schlieren texture for nematic phase. Similar phase transition
and texture observations were found for the whole set of compounds
4m–r suggesting that the mesophase is same for the homologous series
of compounds 4g–l and 4m–r. Moreover, compounds 4m–r exhibited a
nematic phase on cooling from the isotropic state, whereby nematic
droplets are seen to form and gradually fuse together to form the Schlie-
ren texture. The nematic phase appears to have high mobility with
flashes when subjected to mechanical stress [25].
spacer groups have high melting transition temperatures. This behav-
iour is most commonly attributed to the pronounced dependence on
the molecular shape, length and parity of the spacer. Such behaviour
has been accounted by theory, by means of the different shapes of the
conformers having odd or even membered spacers and their associated
conformational distributions [5–7].
In our work and the present series of dimmers, the odd-even effect is
eliminated and the values of the Cr-N and Iso-N entropies are irregular
for some compounds. This is presumably due to the rather bulky shape
of the dimethyluracil group and the increased molecular biaxiality,
which has been used to account for irregular entropies. The phase be-
haviour of compounds 4 g-r is unusual. The conventional behaviour in
liquid crystal dimers is smectic behaviour to be favoured on increasing
the spacer length. Since in the present series of compounds nematic
phase is favoured in higher methylene spacer containing members
this behaviour is unexpected [8–9].
A plot of the cooling transition temperatures as a function of the
number of carbon atoms in the connecting spacer and terminal n-
alkoxy chain for the compounds 4a–f, 4g–l and 4m–r is shown in Fig.
3(a), (b) and (c). It can be seen that the clearing temperature curve
gradually increases with increasing in the number of carbon atoms in
the terminal alkyl chain for the sets of 4a–f, 4g–l and 4m–r homologous
series of compounds.
From this we deduce that the type of phase depends on the length of
spacer alkoxy chains and not on the length of the terminal chains. In
shorter spacer chain members (n = 6) the SmA is observed regardless
the length of terminal alkyl chain. As the length of the spacer chain in-
creases to n = 8 and 10, the nematic (N) phase is observed as in the
compounds 4g–r. The nematogenic behaviour between compounds
4g–r, suggests that when the spacer carbon chains reach a certain
length (n = ≥8) the nematic (N) phase at the higher temperatures, is
induced [25]. On the other hand, as mentioned above, the melting
points increase when the number of carbons in terminal alkoxy chains
increases. This may be due to the excessive Van der Waals attractive
forces between the terminal alkyl chains or between the alkoxy chains
where the melting temperatures obviously increase from n = 6 to 18
[33].
3.2. Molecular modeling
In order to get a better understanding of the relationship between
the structure and type of phase, molecular modeling studies have
been carried out by HyperChem program. The variation of length at
the terminal alkyl chain did not change the type of phase. Surprisingly,
the type of phase is mainly depending on the length of spacer alkoxy
chain. Molecular models of compounds 4a, 4g and 4r are given in Fig.
4, in which the length of spacer alkoxy chain varied from n = 6, 8 and
It may be noted here that generally in dimeric compounds, transition
temperatures follows pronounced odd-even effect. Odd number spacer
groups have low melting transition temperatures and even number
Fig. 1. Optical photomicrographs of (a) SmA phase shown by 4a at 117 °C upon heating; (b) Nematic phase shown by 4p at 170 °C upon heating.

770
M. AbdulKarim-Talaq et al. / Journal of Molecular Liquids 219 (2016) 765–772
Fig. 2. DSC thermogram (Endothermic/Exothermic scans) of compound 4p.
Fig. 3. A plot of cooling transition temperature as a function of the number of carbon atoms in the methylene spacer and terminal chains for compounds (a) 4a–f, (b) 4g–l and (c) 4m–r.

M. AbdulKarim-Talaq et al. / Journal of Molecular Liquids 219 (2016) 765–772
771
Fig. 4. Molecular models of compound 4a, 4g and 4r using HyperChem program.

772
M. AbdulKarim-Talaq et al. / Journal of Molecular Liquids 219 (2016) 765–772
10, respectively. The data are also in agreement with the experimental
results of title compounds 4a–r.
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The main author would like to thank Professor John West and the
Liquid Crystal Institute at the Kent State University for performing the-
oretical studies and hospitality. The authors would also like to thank the
University of Anbar and Universiti Sains Malaysia for supporting this
project.