Study of Novel Symmetrical Liquid Crystalline Ester Dimers

Article abstract of DOI:10.1080/15421406.2014.990186

A novel homologous series of 12 symmetrical dimers, viz. 1,1'-Bis[4-(4'-n-alkoxy benzoyloxy)-3-methyl phenyl] cyclohexanes is reported. Nematogenic mesophase commences from the heptyloxy homologue dimer and continues up to the hexadecyloxy dimer in an enantiotropic manner without the exhibition of any smectogenic behavior. The rest of the dimers from methoxy to hexyloxy are nonmesogenic. The textures of the nematic phase are threaded or Schlieren. The transition temperatures and textures of the nematic phase were determined by an optical polarizing microscope equipped with a heating stage. Curves (solid-isotropic/nematic and N-I) of a phase diagram showing mesomorphism behave in normal manner except for a minor deviation at the dodecyloxy derivative. The analytical and spectral data confirm the molecular structure of dimers. The nematogenic mesophase lengths vary between 3.3°C and 26.6°C. The average thermal stability for nematic is 108.26°C; series is partly nematogenic with middle ordered melting type and low degree of mesomorphism. Liquid crystal properties of novel dimer series are compared with the structurally similar other known series.

Full text of DOI:10.1080/15421406.2014.990186

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Study of Novel Symmetrical Liquid Crystalline
Ester Dimers
V. C. Kotadiya & U. C. Bhoya
To cite this article: V. C. Kotadiya & U. C. Bhoya (2015) Study of Novel Symmetrical
Liquid Crystalline Ester Dimers, Molecular Crystals and Liquid Crystals, 616:1, 19-27, DOI:
10.1080/15421406.2014.990186
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Date: 09 April 2016, At: 16:07

Mol. Cryst. Liq. Cryst., Vol. 616: pp. 19–27, 2015
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2014.990186
Study of Novel Symmetrical Liquid Crystalline
Ester Dimers
V. C. KOTADIYA AND U. C. BHOYA∗
Department of Chemistry, Saurashtra University, Rajkot, Gujarat, India
A novel homologous series of 12 symmetrical dimers, viz. 1,1’-Bis[4-(4’-n-alkoxy
benzoyloxy)-3-methyl phenyl] cyclohexanes is reported. Nematogenic mesophase com-
mences from the heptyloxy homologue dimer and continues up to the hexadecyloxy
dimer in an enantiotropic manner without the exhibition of any smectogenic behavior.
The rest of the dimers from methoxy to hexyloxy are nonmesogenic. The textures of the
nematic phase are threaded or Schlieren. The transition temperatures and textures of
the nematic phase were determined by an optical polarizing microscope equipped with
a heating stage. Curves (solid-isotropic/nematic and N-I) of a phase diagram showing
mesomorphism behave in normal manner except for a minor deviation at the dodecyloxy
derivative. The analytical and spectral data confirm the molecular structure of dimers.
The nematogenic mesophase lengths vary between 3.3◦C and 26.6◦C. The average ther-
mal stability for nematic is 108.26◦C; series is partly nematogenic with middle ordered
melting type and low degree of mesomorphism. Liquid crystal properties of novel dimer
series are compared with the structurally similar other known series.
Keywords Dimers; enantiotropy; liquid crystal; nematic; smectic
Introduction
Liquid crystalline (LC) substances are useful in a wide range of applications. Therefore
physicists, chemist, biologists, pharmacists, microbiologists, and engineers have exploited
the special properties of liquid crystals. LC substances are flowing crystals discovered in
1888 [1–4] and since that time lots of novel conventional and unconventional LC substances
have been synthesized through homologous series and/or binary systems and characterized
by spectroscopic, microscopic, and other related analysis to derive the relations between
molecular structure and LC behavior [5–7]. Oligomers with LC properties are useful
materials. The simplest oligomer is a dimer [8,9]. Therefore, the work reported here is
aimed to synthesize novel symmetrical dimers with two same mesogenic units bonded to
a common spacer. The thermotropic symmetrical LC dimers will be studied through an
optical polarizing microscope equipped with a heating stage. The results will be interpreted
in terms of molecular rigidity and flexibility depending on molecular structure [10–13].
∗Address correspondence to U.C. Bhoya, Department of Chemistry, Saurashtra University,
Rajkot 360005, Gujarat, India. E-mail:drucbhoya@gmail.com
Color versions of one or more of the figures in the article can be found online at
www.tandfonline.com/gmcl.
19

20
V. C. Kotadiya and U. C. Bhoya
Experimental
Synthesis
As shown in Scheme 1, we synthesized 4-n-alkoxybenzoic acids by using 4-hydroxybenzoic
acid and suitable alkyl bromides (R-Br) in methanol in the presence of KOH [14]. Differ-
ent substituted 4-n-alkoxybenzoyl chlorides were prepared through the reaction between
4-n-alkoxybenzoic acids and an excess of thionyl chloride under reflux until the starting
material was totally consumed [14]. 1,1-bis (3-methyl-4-hydroxyphenyl) cyclohexane ma-
terial was synthesized by a reported method (m.p. 185–187◦C) (yield 75–80%) [15]. The
4-n-alkoxybenzoyl chlorides and 1,1’-bis (3-methyl-4-hydroxyphenyl) cyclohexane in dry
cooled pyridine were stirred overnight at room temperature [16]. Then reaction mixture
was poured into cold 1:1 HCl in ice then filtered, washed, dried, and purified until constant
transition temperatures were obtained. Purifications of all intermediate products and objec-
tive compounds were repeated many times and judged to be pure by TLC which showed
only one spot.
Scheme 1. Synthetic route to the novel series.
4-Hydroxybenzoic acid, methanol, KOH, thionyle chloride, cyclohexanone, 2-
methylphenol, pyridine, ethanol, acetic acid, hydrochloric acid, etc. were used as received,
except for solvents, which were dried, purified, and distilled prior to use.

Study of Novel Symmetrical Liquid Crystalline Ester Dimers
21
Table 1. Elemental analysis for (1) methyloxy, (2) propyloxy, and (3) tetradecyloxy
derivatives
Elements%found
H
Elements%calculated
Sr. No.
Molecular formula
C
O
C
H
O
1
2
3
C36H36O6
C40H44O6
C62H88O6
76.50
77.49
80.02
6.41
7.09
9.61
17.09
15.41
10.37
76.57
77.39
80.13
6.43
7.14
9.54
17.00
15.46
10.33
Characterization
The constitutions of all the synthesized compounds were characterized by various analyt-
ical techniques. Elemental analysis was performed using a Perkin Elmer PE 2400 CHN
Analyzer-Waltham, Massachusetts, USA system (Table 1). The structures of the materials
were elucidated by infrared (IR) spectroscopy (SHIMATZU FTIR-8400 Spectrophotometer
Nishinokyou Kuwabara-cho, Nakagyo-Ku Kyoto, Japan. infrared spectrophotometer), pro-
ton nuclear magnetic resonance (1H NMR) spectrometry (BRUKER Avance-III (400 MHz)
Spectrometer-Billerica, Massachusetts, USA using CDCl3 as a solvent) and Mass spectrom-
etry (Shimadzu GC-MS Model No.QP-2010, Nishinokyou Kuwabara-cho, Nakagyo-Ku,
Kyoto, Japan). The analyses of the structures of the products and intermediates by spectro-
scopic methods were found to be consistent with the expected structures.
Analytical Data
Elemental analysis (Table 1)
Spectral Data
1H NMR in ppm for Butyloxy Derivative
8.072–8.050 (d,4H, Ar–H), 7.313–7.305 (d, 2H, Ar–H), 7.221–7.200 (d, 2H, Ar–H),
7.126–7.104 (d, 2H, Ar–H), 7.094–7.073 (d, 4H, Ar–H), 4.104–4.072 (t, 4H, 2 ×
Ar–OCH2–), 2.116 (s, 6H, 2 × Ar-CH3), 1.901–1.857 (t, 4H, cyclohexane–H), 1.751–1.714
(t, 4H, 2×Ar-OCH2-CH2), 1.421–1.495 (m, 10H, cyclohexane–6H&aliphatic–4H),
1
0.961–0.924 (t, 6H, 2 × Ar-O-(CH2)3-CH3). The H NMR data are consistent with the
molecular structure.
1H NMR in ppm for Pentyloxy Derivative
8.094–8.048 (d,4H, Ar–H), 7.311–7.302 (d, 2H, Ar–H), 7.222–7.218 (d, 2H, Ar–H),
7.201–7.148 (d, 2H, Ar–H), 7.122–7.072 (d, 4H, Ar–H), 4.096–4.064 (t, 4H, 2 ×
Ar–OCH2–), 2.115 (s, 6H, 2 × Ar-CH3), 1.920–1.844 (t, 4H, cyclohexane–H), 1.761–1.724
(t, 4H, 2 × Ar-OCH2-CH2), 1.490–1.406 (m, 10H, cyclohexane–6H&aliphatic–4H),
1.335–1.300 (m, 4H, aliphatic–4H), 0.900–0.865 (t, 6H, 2 × Ar-O-(CH2)4-CH3). The
1H NMR data are consistent with the molecular structure.

22
V. C. Kotadiya and U. C. Bhoya
IR in cm−1 for Ethyloxy Derivative
3061.13 (w, C H Str. of aromatic ring), 2928.04, 2860.53 (s, C H Str. of alkane, Sp3
carbon), 1730.21(s, C
O Str. ester), 1604.83, 1504.53 (C
C Str. in aromatic ring),
1452.45 (m, C H bend alkane), 1257.63 (s, aromatic C O Str. & C C Str.)1163.11,
1114.89, 1057.03 (s, aliphatic C O Str. & second overtone C -H Str.), 860.28, 763.84
(meta-, ortho-, and para-substituted phenyl ring). The IR data are consistent with the
molecular structure.
IR in cm−1 for Octyloxy Derivative
3068.85 (w, C H Str. of aromatic ring), 2941.54, 2858.60 (s, C H Str. of alkane, Sp3
carbon), 2505.52, 2422.67, 2333.94 (combination of C H Str.), 1728.28 (s, C O Str.
ester), 1604.83,1504.53 (C C Str. in aromatic ring), 1450.50 (m, C H bend alkane),
1267.27 (s, aromatic C O Str. and C C Str.) 1192.11, 1161.19.89, 1072.46(s, aliphatic
C O Str. and second overtone C-H Str.), 858.35, 767.69 (meta-, ortho-, and para-substituted
phenyl ring) The IR data are consistent with the molecular structure.Where, s = strong,
m = medium, w = weak.
Mass Spectra of Octyloxy Derivative
m/z (rel. int%): 760 (M)+, 599,482,233,121
Mass spectra of Decyloxy Derivative
m/z (rel. int%): 817(M)+,534, 414, 294,121
Results and Discussion
The novel homologous series of symmetrical dimers of 1,1ꢀ-bis[4-(4ꢀ-n-alkoxy
benzoyloxy)-3-methyl phenyl] cyclohexanes consist of two identical mesogenic units in-
volving COO central groups bonded to a cyclohexyl ring at the 1 and 4 positions. The
mesogenic units are derived from n-alkoxybenzoic acids, whose dimerisation disappears by
the esterification process. The methoxy to hexyloxy dimers are nonliquid crystals (NLC),
while the heptyloxy, octyloxy, decyloxy, dodecyloxy, tetradecyloxy, and hexadecyloxy ho-
mologues are LC as enantiotropic nematogenic character. Smectogenic property is totally
absent for all the LC members of the series. Transition and melting temperatures of homo-
logues were determined by an optical polarizing microscope equipped with a heating stage.
Transition temperatures of the homologues (Table 2) as well as the textures of homologues
were plotted versus the number of carbon atoms present in n-alkyl chain of the left n-alkoxy
terminal end group. Transition curves, viz., solid-nematic and nematic-isotropic were ob-
tained by linking like or related transition points forming a phase diagram showing phase
behaviors of the novel series of dimers (Fig. 1). The solid-nematic transition curve follows
a zigzag path of rising and falling values with an overall descending tendency and behaves
in a normal manner. The nematic-isotropic transition curve initially rises and then descends
to the dodecyloxy homologue and then ascends to the tetradecyloxy dimer and again de-
scends to the hexadecyloxy derivative. Thus, the N-I transition curve adopts a serpentine
shape. The textures of the nematic mesophases are threaded or Schlieren as determined by
a miscibility method. The N-I transition curve exhibits an odd-even effect. The curves for

Study of Novel Symmetrical Liquid Crystalline Ester Dimers
23
Table 2. Transition temperatures in ◦C of series-1
Transition temperature in oC
Compound no.
n-alkyl group CnH2n+1 group (n)
Sm
N
Isotropic
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
10
12
14
16
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
117.9
115.8
120.4
110.2
130.8
90.1
101.7
98.0
122.2
111.1
115.1
101.5
—
96.1
93.0
95.6
100.2
90.8
98.2
10
11
12
Abbreviations: Sm, smectic; N, nematic.
odd and even homologues merge at the decyloxy dimer and then propagate as a single curve
for the higher homologues. The nematogenic mesophase behavior of the series varies from
homologue to homologue with the variation of number of carbon atom in n-alkyl chain.
The average thermal stability for the nematic phase is 108.26◦C and the mesophase length
varies from a minimum of 3.3◦C at the hexadecyloxy homologue to a maximum of 26.6◦C
at the decyloxy homologue. Thus, the present dimer series is partly nematogenic and partly
nonmesomorphic with a poor degree of mesomorphism and a middle ordered melting type.
The N-I transition curve is extrapolated to the pentyloxy homologue to determine its latent
Figure 1. Phase behavior of series.

24
V. C. Kotadiya and U. C. Bhoya
Figure 2. structurally similar dimer series.
transition temperature(LTT). The 1,1ꢀ-bis(3-methyl-4-hydroxyphenyl)cyclohexane (m.p. -
185 to 187◦C) spacer to which two identical mesogenic units are linked is nonmesogenic.
Though mesogenic temperatures of the dimer homologue are lower than the corresponding
n-alkoxy benzoic acids, but the degree of mesomorphism of the novel dimers is reduced
by esterification process. The presence of two methyl groups symmetrically situated under
identical lateral positions increases intermolecular distance and molecular width. Therefore,
increased intermolecular distance decreases the intermolecular attractions on one hand and
at the same time increase the intermolecular distance by lateral substitution, and the methyl
groups increase molecular polarizability and increase intermolecular attractions on the other
hand. In the present investigation of the novel dimer series, the first effect predominates
against second effect emerged by the polarizability effect. Therefore, suitable magnitudes
of anisotropic forces of intermolecular end to end attractions are sufficient to resist exposed
thermal vibrations for some temperature differences to maintain parallel orientational order
of molecules in a floating condition on the surfaces, but the same magnitudes of anisotropic
forces of intermolecular attractions is insufficient to cause lamellar packing of molecules
in the crystal lattices of mesogenic dimers. Thus, only nematogenic mesophase of short
range of liquid crystallinity occur very late from the heptyloxy homologue dimer without
exhibition of any smectic property. The disappearance of dimerisation of n-alkoxy benzoic
acids by esterification process is attributed to the breaking of hydrogen bonding between
two molecules of n-alkoxy benzoic acids. The NLC behavior of the methoxy to hexyloxy
homologues is attributed to their high crystallizing tendency arising from unsuitable mag-
nitudes of anisotropic forces of intermolecular attractions as a consequence of unfavorable
molecular rigidity and flexibility, which induces an inability to resist exposed thermal vibra-
tions. The merging of curves for the odd and even members of the series from and beyond
the decyloxy homologue into a single curve for higher homologues is because of the coiling
or bending or flexing or coupling of the longer n-alkyl chains of the mesogenic units to
the principle axis of the core structure of molecules. The changing mesomorphic behavior
from homologue to homologue in the series is due to the sequentially added methylene unit,
which contributes to the molecular flexibility in varying proportions for the same series.
The laterally substituted methyl groups reduce intermolecular end to end attractions. The
isotropic temperatures of mesomorphic dimers vary between 98◦C and 122.2◦C. The LTT
for nematic of pentyloxy homologue derivative is predicted 71◦C by extrapolation [17,18]
method. However, its realization is difficult due to its high crystallizing tendency and the
temperature of isotropic-nematic transition being far below by 59◦C from its isotropic tem-
perature. The liquid crystal behaviors of the present series-1 are compared with another
structurally similar series-X [19] as shown in Fig. 2.

Study of Novel Symmetrical Liquid Crystalline Ester Dimers
25
Table 3. Average thermal stability in ◦C
Series
[1]
[X]
Smectic-nematic or isotropic
Commencement of smectic phase
—
—
129.0
[C6-C10]
C6
Nematic –isotropic
Commencement of nematic phase
108.26
[C7-C16]
C7
147.7
[C6-C16]
C6
Homologous series-1 of present investigation and series-X chosen for comparison are
identical with respect to mesogenic units, but they differ with respect to rest of the molecular
part between the two mesogenic parts. Therefore the variations of mesomorphic properties
and the degree of mesomorphism observes from series to series for the same homologue
dimer and homologue to homologue for the same series can be attributed to the varying
molecular part of the respective dimer series under comparison. Table 3 shows the liquid
crystal properties of series-1 and X. The comparative points are mentioned below.
• Present series-1 is only nematogenic with the absence of smectogenic property,
whereas series-X is smectogenic in addition to nematogenic character.
• Nematogenic mesophase commences from the heptyloxy dimer of the present series-
1 while, it commences from hexyloxy dimer of a series-X.
• Smectogenic mesophase does not commence until the last member of the present
series-1 while it commences from the hexyloxy dimer of a series-X.
• Thermal stability for nematic in case of series-1 is 108.26◦C while thermal stabilities
for nematic and smectic are 147.7◦C and 129.0◦C.
• Mesophase length varies between 3.3◦C and 26.6◦C for series-1 and it varies between
18◦C and 56◦C for series-X.
The presence of the cyclohexyl ring in the molecules of series-1 and two methyl
groups substituted in the neighboring phenyl ring, comparatively increase the intermolecular
distance more than for series-X. Thus, the intermolecular distance factor in the case of series-
1 and X maintain suitable magnitudes of anisotropic forces of intermolecular end to end
attractions to facilitate nematic mesophase formation for both series (1 and X). However,
it is adequately sufficient for series X, but inadequately insufficient for series-1 to induce
smectic mesophase formation due to unfavorable molecular polarizability factor. Thus,
mesophase length and thermal stability of series-1 are relatively lower than for series-X.
The absence of a smectic mesophase in series-1 is also attributed to the steric hindrance
induced by the cyclohexyl ring and the presence of two methyl groups on the neighboring
phenyl rings, and the relative extent of molecular noncoplanarity. Hence, variations of
mesomorphic behaviors from homologue to homologue in same series and for the same
homologue from series to series are due to the sequentially added methylene unit and
changing molecular part at the middle of a molecule linking two phenyl rings or linking
two same mesogenic units of each molecule of dimer series-1 and X.

26
V. C. Kotadiya and U. C. Bhoya
Conclusions
The presently investigated novel ester homologous series of symmetrical dimers is nemato-
genic without exhibition of smectogenic character, whose, mesophase length is shorter and
is of middle ordered melting type.
Smectic and nematic group efficiency order derived on the basis of (i) thermal stabilities
and (ii) the early commencement of mesophase are as under
Solid-nematic transition temperature vary between 90◦C and 100◦C
Mesomorphism phenomenon is sensitive, susceptible, and depends upon a combine
effect of molecular rigidity and flexibility.
Molecular rigidity and flexibility are totally depending upon only molecular structure
of a substance.
Molecular flexibility varies with the status of n-alkyl chain bonded through oxygen
atom to a phenyl ring.
Acknowledgments
The Authors acknowledge thanks to the Department of Chemistry (DST-FIST Funded &
UGC-SAP Sponsored), Saurashtra University, Rajkot, for research work. Authors are also
especially thankful to Dr. A.V. Doshi, Ex. Principal M.V.M. Science and Home Science
College, Rajkot, for his valuable co-operation during present investigation as and when
needed. Also thanks are due to the National Facility for Drug Discovery through New
Chemical Entities (NCEs), for analysis of samples.

Study of Novel Symmetrical Liquid Crystalline Ester Dimers
27
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