Mesomorphism Dependence on the Position of Substitution of a Pair of Esters

Article abstract of DOI:10.1080/15421406.2015.1106279

A novel homologous series of ethylene derivatives of thermotropic liquid crystals has been synthesized. The methoxy to octyloxy derivatives are nematogenic, the decyloxy to tetradecyloxy derivatives are smectogenic, in addition to nematogenic, and the hexadecyloxy homologue is smectogenic only. All the members of the series are enantiotropically mesogenic. Thermotropic behavior was determined by an optical polarizing microscope equipped with a heating stage and Differential Scanning Calorimetry (DSC) study. Analytical and spectral data confirm the molecular structures of homologues (infrared, nuclear magnetic resonance, mass spectra, X-ray, and DSC data). Textures of the nematic phase are threaded or Schlieren and that of smectic phase are focal conic fan-shaped of smectic A or C. Transition curves of the phase diagram behave in a normal manner except one or two deviations from the normal trend. The mesophase range (Sm+N) varies from 3°C to 44°C. The average thermal stability for smectic is 93°C and that for nematic 117.4°C. The LC behavior of the novel series is compared with a structurally similar known series.

Full text of DOI:10.1080/15421406.2015.1106279

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Mesomorphism Dependence on the Position of
Substitution of a Pair of Esters
M. D. Khunt, V. C. Kotadiya & U. C. Bhoya
To cite this article: M. D. Khunt, V. C. Kotadiya & U. C. Bhoya (2016) Mesomorphism
Dependence on the Position of Substitution of a Pair of Esters, Molecular Crystals and Liquid
Crystals, 626:1, 68-80, DOI: 10.1080/15421406.2015.1106279
To link to this article: http://dx.doi.org/10.1080/15421406.2015.1106279
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Date: 09 April 2016, At: 10:27

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –

http://dx.doi.org/./..
Mesomorphism Dependence on the Position of Substitution of a
Pair of Esters
M. D. Khunt, V. C. Kotadiya, and U. C. Bhoya
Department of Chemistry, Saurashtra University, Rajkot, Gujarat, India
ABSTRACT
KEYWORDS
A novel homologous series of ethylene derivatives of thermotropic liq-
uid crystals has been synthesized. The methoxy to octyloxy derivatives
are nematogenic, the decyloxy to tetradecyloxy derivatives are smec-
togenic, in addition to nematogenic, and the hexadecyloxy homologue
is smectogenic only. All the members of the series are enantiotropically
mesogenic. Thermotropic behavior was determined by an optical polar-
izing microscope equipped with a heating stage and Differential Scan-
ning Calorimetry (DSC) study. Analytical and spectral data confirm the
molecular structures of homologues (infrared, nuclear magnetic reso-
nance, mass spectra, X-ray, and DSC data). Textures of the nematic phase
are threaded or Schlieren and that of smectic phase are focal conic fan-
shaped of smectic A or C. Transition curves of the phase diagram behave
in a normal manner except one or two deviations from the normal trend.
The mesophase range (Sm+N) varies from 3°C to 44°C. The average
thermal stability for smectic is 93°C and that for nematic 117.4°C. The
LC behavior of the novel series is compared with a structurally similar
known series.
Enantiotropy; liquid crystals;
nematic; smectic;
thermotropic
Introduction
An intermediate physical state of a substance between crystalline solid and a true isotropic
liquid is called an anisotropic liquid or a liquid crystalline (LC) state. The mesophase, known
since 1888 [1], has proved to be of value in applications such as electronic display devices,
pharmaceutical preparations from biologically active molecules, medical instruments, engi-
neering and technology, and space research [2–5]. Ester derivatives of benzoic acid and/or
cinnamic acid are biologically active molecules, and several materials with or without LC
properties have been reported to date [6–10]. The study of LC state is multidisciplinary
and studied with different aims and objectives depending on the research group. However,
every LC research group requires novel LC substances to continue their research. In order
to produce novel LC substances one must have adequate knowledge of molecular geome-
try, shape, size, polarity and polarizibility, dipole moment, and intermolecular forces con-
ducive to LC state. Therefore, the aim here was to synthesize novel LC substances with three
phenyl rings bridged through -COO and -CO-CH=CH- central groups with a view to under-
standing and establishing the effect of molecular structure on LC properties as a consequence
CONTACT U. C. Bhoya
India.
drucbhoya@gmail.com
Department of Chemistry, Saurashtra University, Rajkot , Gujarat,
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
©
 Taylor & Francis Group, LLC

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
69
Scheme . Synthetic route to novel series.
of molecular rigidity and flexibility [11–16]. The proposed novel substances may prove their
ability and usefulness as anti-inflammatory, anticancer, antioxidant, anti-angiogenis, antitu-
mor, antiradiation-induced oxidative stress and protect against radiation injury, antimalarial
and antibacterial agents [17–22].
Experimental
Synthesis
n-Alkoxy benzoic acids were prepared from 4-hydroxyl benzoic acid by suitable alkylat-
ꢀ
ꢀ
ing agents by a modified method of Dave and Vora [23]. α-4-Hydroxybenzoyl β-2 ,5 -
dimethoxy phenyl ethylene was prepared by a usual established method [24]. 4-n-Alkoxy
ꢀ
ꢀ
benzoic acids and α-4-hydroxybenzoyl-β-2 ,5 dimethoxy phenyl ethylene were condensed in
1
,3-dicyclohexylcarbodiimide (DCC), 4-dimethylaminopyridine (DMAP), and CH Cl [25].
2
2
The synthetic route to the series is shown in Scheme 1. Final products were individually
decomposed, filtered, washed, dried, and purified until constant transition temperatures were
obtained.
The following chemicals were used “as received” except solvents, which were
dried, purified, and distilled prior to use: 4-hydroxy benzoic acid, 4-hydroxy ace-
tophenone, 2,5 dimethoxybenzaldehyde, alkyl bromide (R-Br), methanol, KOH, 1,3-
dicyclohexylcarbodiimide, 4-dimethylaminopyridine, dichloromethane, and ethanol.

7
0
M. D. KHUNT ET AL.
Table . Elemental analysis for () ethyloxy, () butyloxy, and () hexyloxy derivatives
Elements found (%)
H
Elements calculated (%)
Sr. No.
MF
C
N
C
H
O



.
.
.
C H O
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.






C H O
 
C H O

 
Characterization
Some selected members of the series were characterized by elemental analysis (Table 1),
1
infrared (IR) spectra, H NMR spectra, mass spectra, X-ray diffraction (XRD; single crys-
tal). Transition temperatures and textures of nematic and smectic phases were determined
by an optical polarizing microscope equipped with a heating stage. Thermodynamic data
enthalpy (ꢀH) and entropy (ꢀS) were determined from differential scanning calorimetry
(
DSC).
Elemental analyses were performed on a Euro EA elemental analyzer. IR spectra were
1
recorded on a Shimadzu FTIR-8400, H NMR spectra were recorded on a BRUKER Avance-
III (400 MHz) spectrometer using CDCl as a solvent, and mass spectra were recorded on a
3
Shimadzu GC-MS Model No. QP-2010. Phase-transition temperatures and enthalpy changes
were measured using a differential scanning calorimeter (Shimadzu DSC-60) at a scanning
−1
rate of 10°C min . XRD measurements were performed at beamline BL17A. The liquid crys-
tal behavior and the type of textures were determined by a miscibility method on microscopic
observations.
Analytical data
Spectral data

HNMR in ppm for pentyloxy derivative
1
H NMR (CDCl , 400 MHz): 0.86–0.88 (t, 3H, -CH of –OC H group), 1.42–1.45 (m,
3
3
5
11
4
3
1
H, polymethylene group of –OC H group), 1.80–1.89(p, 2H, Ar-O-(CH )-CH ), 3.80–
5
11
2
2
.85 (6H, s, Ar-O-CH ), 4.00–4.03 (t, 2H, Ar-O-CH ), 6.84–8.15 (m,11H, Ar-H), 7.61 (d,
3
2
H, chalcone), 8.12 (d, 1H, chalcone). The NMR data are consistent with the molecular
structure.

HNMR in ppm for decyloxy derivative
1
H NMR (CDCl , 400 MHz): 0.88–0.90 (t, 3H, -CH3 of –OC H group), 1.27–
3
10 21
1
.45 (m, 12H, polymethylene group of –OC H group), 1.47–1.50 (p, 2H, Ar-O-
10
21
(
CH ) -CH ), 1.78–1.85 (p, 2H, Ar-O-(CH )-CH ), 3.86 (6H, s, Ar-OCH ), 4.02–
2
2
2
2
2
3
4
.05 (t, 2H, Ar-O-CH ), 6.86–8.11 (m, 11H, Ar-H), 7.35–7.57 (d, 1H, chalcone),
2
8
.08–8.10 (d, 1H, chalcone). The NMR data are consistent with the molecular
structure.
IR in cm  for tetradecyloxy derivative
−
3
066 (=C-H Str. of phenyl nucleus), 2929 and 2850 (CH sym. & asym. stra.), 1728 (aromatic
2
C=O Str.), 1604 (aromatic C=C str.), 1510 (C-C aromatic str.),1467 (C-H bend alkanes), 1315
(
aromatic and aliphatic C-O str.), 1166 (Ar-O str.),844 (C-H oop. phenyl ring), 758 (C-H oop.
bending of phenyl ring). The IR data are consistent with the molecular structure.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
71
Figure . Phase behaviors of series.
IR in cm  for hexadecyloxy derivative
−
3
074 (=C-H Str. of phenyl nucleus), 2916 and 2850 (CH sym. and asym. stra.), 1778 (aro-
2
matic C=O Str.), 1606 (aromatic C=C str.), 1508 (C-C aromatic str.), 1473 (C-H bend
alkanes), 1315 (aromatic and aliphatic C-O str.), 1170 (Ar-O str.), 844 (C-H oop. phenyl
ring), 759 (C-H oop bending of phenyl ring). The IR data are consistent with the molecular
structure.
Mass of propyloxy derivative
m/z (rel. int%): 445 (M)+, 398, 231, 147, 119.
Mass of octayloxy derivative
m/z (rel. int%): 516(M)+, 485, 233, 121, 104.
Texture determination by miscibility method
•
•
•
•
Butyloxy derivative (C ): Threaded nematic
4
Hexyloxy derivative (C ): Schlieren nematic
6
Decyloxy derivative (C ): Smectic-A
10
Hexadecyloxy derivative (C ): Smectic-C
16
Results and discussion
ꢀ
ꢀꢀ ꢀꢀ
The novel homologous series α-4-(4 -n-alkoxy benzoyloxy) benzoyl β-2 ;,5 -dimethoxy
phenyl ethylene consists of 12 homologues of mesogenic characteristics. The methoxy to octy-
loxy homologue derivatives are only nemetogenic; the decyloxy to tetradecyloxy homologues
are smectogenic, in addition to nematogenic, and the hexadecyloxy homologue is only smec-
togenic. All phases are generated enantiotropically. The transition temperatures (Table 2(a)),
as determined on an optical polarizing microscope equipped with a heating stage, are plotted
against the member of carbon atoms present in n-alkoxy chain of n-alkoxy (-OR) terminal
end group. Transition curves (N-I, Sm-N, Sm-I, solid-mesomorphic) are obtained on link-
ing with similar or related transition points, showing phase behaviors of a series in a phase
diagram (Fig. 1). The solid-mesomorphic transition curve adopts a zigzag path of rising and

7
2
M. D. KHUNT ET AL.
Table . Transition temperatures in °C.
Transition temperature
N
n-alkyl group
Compound No.
CnH
(n)
Sm
Isotropic
n+
(
a) Transition temperatures in °C of series-













-
-
-
-
-
-
-
-















-


























(
b) Transition temperatures in °C of series-X













-
-
-
-
-
-
.
.
.
.
.
.
.
.
.
.
.
.






.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
-








(
c) Transition temperatures in °C of series-Y













-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
.
.
.
.
.
.
.
.
.
.
.
.











.
.
.
.
.
.
.
.



Sm = Smectic, Nm = Nematic.
falling values and behaves in a normal manner. The Smectic–Nematic transition curve shows
a descending tendency and behaves in a usual established manner. A point of Sm-I transition
temperature does not match with the descending trend of Sm-N transition curve and deviates
from the normal behavior. The Nematic–isotropic (N-I) transition curve shows mainly nor-
mal descending behavior as the series is ascended with the exhibition of an odd-even effect
and a deviating trend at the pentyloxy homologue. Curves for odd and even members merge
at the heptyloxy (C ) homologue and progresses as a single curve up to the tetradecyloxy (C )
7
14
homologue.
Thus, the odd-even effect diminishes as the series is ascended. The Sm-N transition curve
is extrapolated [26,27] to the octyloxy (C ) homologue to predict its latent transition temper-
8
ature (LTT) for smectic keeping in view the trend of Sm-N transition curve, which merges
into the point of solid-nematic transition temperature (78°C). Initial members of a series are
nematogenic (C to C ), upper middle members are smectic and nematic (C to C ), and
1
8
10
14

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
73
Figure . Structurally similar series.
last (C ) member is only smectogenic. Thus, novel series follows normal trend of mesophase
16
sequence, with a little bit deviating tendency at pentyloxy (C ) and hexadecyloxy (C ) deriva-
5
16
tives. Sm-N and N-I thermal stabilities are 93°C and 117.4°C respectively, whose smectogenic
phase length varies between 9°C and 21°C and the nematogenic phase length varies between
3
°C and 44°C. Thus, novel series is predominately nematogenic and partly smectogenic of
middle-ordered melting-type, or a novel series of the present investigation is entirely meso-
morphic. Thus, mesomorphic properties of the present series vary from homologue to homo-
logue.
ꢀ
ꢀ
α-4-Hydroxy benzoyl β-2 ,5 -dimethoxy phenyl ethylene (m.p.: 205–210°C, yield 68–70%)
is a non-mesogenic. However, mesogenic property is induced on linking it with the 4-n-
alkoxy benzoic acids through their corresponding acid chlorides. The molecular length and
breadth of final ester derivatives are increased. Hence, the molecular polarity and polariz-
ibility are enhanced according to dipole moment acting across the long molecular dipole–
dipole interactions, dispersion forces, etc. related to suitable magnitudes of anisotropic forces
of end-to-end and lateral attractions because of resultant molecular rigidity and flexibility
required to induce mesogenic behaviors in a substance. All the members of a novel series
under the influence of heat resist individually the externally exposed thermal vibrations for
Table . Average thermal stability (°C).
Series

X
Y
Smectic–nematic
Commencement of
smectic phase
.
.
–
–
[C -C ]
[C -C ]




C
.
C
.
Nematic–isotropic
Commencement of
nematic phase
.
[C -C ]
[C -C ]
[C -C ]






C
 to  = 
C
 to  = 
C
 to  = 
Phase length range
in °C

74
M. D. KHUNT ET AL.
definite range of temperature. They misaligned at an angle of 90° or less than 90° with a float-
ing surface, depending upon suitable magnitudes of anisotropic intermolecular adhesion to
maintain mesophase formation with certain degree of entropy (ꢀS). The molecules of the
present series are under observation through heating stage polarizing microscopy but some
homologues are observed by DSC. Methoxy (C ) to octyloxy (C ) members maintained only
1
8
statistically parallel orientational order of molecules of definite entropy limit, while floating
on the surface prior to isotropic temperature with appearance of nematic phase. However, on
continued exposure to thermal vibrations beyond certain limit, the molecules under exami-
nation, from and beyond isotropic temperature, are randomly oriented in all possible direc-
tions with high order of disorder or high value entropy (ꢀS = ꢀH/T). However, on cooling
the same, with appropriate rate of cooling the same, the nematic mesophase appears from
and below isotropic temperature until solidification. The molecules from decyloxy (C ) to
10
hexadecyloxy (C ) homologues bear lamellar packing of molecules in their crystal lattices,
16
which allows the molecule to be set perpendicular to floating surface to build up two- or
three-dimensional network and acquires layered arrangement of crystal structure. However,
sliding layered arrangement of molecules is maintained under the exposed thermal vibrations
for some range of temperature with exhibition of smectic phase. C , C , and C homo-
10
12
14
logues exhibited smectic phase for the same temperature range and then adopted nematic
phase for higher range of temperature up to isotropic temperature in irreversible manner on
heating and cooling. C16 homologue adopts only smectogenic behavior due to high degree
of its intermolecular anisotropic forces of attractions. Disappearance of dimerization of n-
alkoxy benzoic acids through esterification is attributed to the breaking of hydrogen bonding
between two molecules of 4-n-alkoxybenzoic acids. Normal behavior of solid-mesomorphic
and Sm-N transition curves of a phase diagram is understandable, but the deviating trend
of N-I transition curve at the pentyloxy (C ) and unmatched tendency (Sm-I) of hexade-
5
cyloxy (C ) homologue with Sm-N transition curve requires some specific understandings.
16
The upper N-I transition curve may fall progressively, while the lower curve rises to the early
maxima before it falls and merges with the upper curve as alternation diminishes. Such behav-
ior has been reported earlier for including certain aldonitrones [28]. Therefore, exceptional
behavior of C homologue may occur for its exceptional magnitudes of molecular rigidity
5
and flexibility. The deviating trend of Sm-I transition temperature of C homologue may be
16
attributed to the under certainty of the status of longer n-alkyl chain of left n-alkoxy termi-
nal end group, and may cause exceptional variations of suitable magnitudes in anisotropic
forces of intermolecular attractions. The exhibition of odd-even effect by N-I transition curve
is due to the sequentially and progressively added methylene unit or units to n-alkyl chain
bonded through oxygen atom with a first phenyl ring. The diminishing of odd-even effect for
longer n-alkyl chain is attributed to its coiling or bending or flexing, or coupling of major axis
of a molecular core structure affecting suitable magnitudes of anisotropic forces of end-to-
end attractions because of varying molecular flexibility and rigidity operating mesomorphic
behavior of a substance. The changing mesomorphic property from homologue to homologue
in the present series is attributed to the changing molecular length, molecular polarity and
polarizibility, dipole moment across the molecular axis, etc. due to the changing number of
methylene units, i.e. changing molecular flexibility, keeping right terminal/lateral end groups
unchanged, which are also contributing to the total molecular flexibility, and hence causing
expected or unexpected variations in the type and the degree of mesomorphism. Smectic
and nematic average thermal stability of the present series are 93°C and 117.4°C respec-
tively. These values are relatively lower than the normal expected values, reflecting the lower
magnitudes of intermolecular anisotropic forces of intermolecular attractions due to two polar

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
75
-
OCH groups set at 2 and 5 positions of third phenyl ring. -OCH groups at 2 and 5 positions
3
3
are linked to two carbons of the same phenyl ring situated at an angle of 180° from each other.
Therefore, the C-OCH bond polarities and resulting dipole moment are equal in magnitudes
of C-OCH but opposite in directions, as bond polarity, dipole moment, dipole–dipole inter-
3
3
actions and polarizibility are vector quantities whose effectiveness depends upon the direction
in which they operate. Thus, such parameters governing LC behaviors being equal in magni-
tudes and opposite in directions cancel the effectiveness of each other caused by two -OCH
3
ꢀꢀ
ꢀꢀ
groups substituted at positions 2 , 5 on phenyl ring of targeted molecule. Thus, contribu-
tion of dipole moment across the long molecular axis is reduced, affecting transition tem-
peratures, which results into lowering of average thermal stabilities for smectic and nematic
phases.
The liquid crystal properties or behaviors of the present investigated novel homologous
series-1 are compared with the chosen structurally similar isomeric known series-X [28(ii)]
and series-Y [29] as shown in Fig. 2; transition temperatures of series-X and Y are shown in
Tables 2(b) and (c).
Homologous series-1 and series-X are identical with respect to three phenyl rings and two
central bridges -COO- and -CO-CH=CH- constituting molecular rigidity. In addition, these
are identical with respect to flexible end groups, -OR and two methoxy groups, for the same
homologue from series to series. However, both series under comparison differ with respect to
ꢀ
ꢀ
ꢀꢀ
the positional substitution of two -OCH groups bonded to third phenyl ring on 2 , 5 (series-
3
ꢀꢀ
ꢀꢀ
1
) and 3 ,4 (series-X) positions respectively. Therefore, magnitudes of molecular flexibility
cause variations of suitable magnitudes in the anisotropic forces of intermolecular end-to-end
and lateral attractions, responsible for possibility, type, and degree of mesophase formation.
Homologous series-1 and series-Y are identical but they are not isomeric to each other because
a central group -CH=CH-COO- linking the first and second phenyl ring of series-Y differ
from a central group -COO- of series-1 under identical condition. Moreover, in addition to
difference in central bridges, two methoxy groups set an angle of 60° of series-Y in compar-
ison with series-1, in which two -OCH groups set at an angle 180° affect the magnitudes of
3
vector quantity dipole moment across major molecular axis; dipole–dipole interaction and
dispersion forces also cause variations. The central bridge vinyl carboxylate -CH=CH-COO-
of series-Y and carboxylate -COO- of series-1 are comparable. However, vinyl carboxylate -
CH=CH-COO- has greater length than -COO-. Therefore, it causes more non-coplanarity
due to a twist obtained, as the oxygen atoms of vinyl carboxy group bump into the non-bonded
adjacent hydrogen atoms of aromatic rings. Thus, because of these effects, the mesomorphic-
isotropic thermal stabilities, i.e. liquid crystal properties and the degree of mesomorphism,
depend upon the varying parts and their varying magnitudes of differing molecular struc-
tures. Table 3 presents the comparative data of average thermal stabilities, commencement of
mesophase or mesophases, degree of mesomorphism, etc. for series-1, X, and Y under com-
parative study as mentioned below.
Table 3 clearly indicates the following:
•
Isomeric homologous series-1 and X have exhibited smectogenic and nematogenic
mesophase but a non-isomeric series-Y exhibited only nematogenic mesophase with
absence of smectogenic character.
•
Smectic thermal stability of presently investigated novel series-1 is lower (93.0) than an
isomeric series-X (115.6).
•
•
Nematic thermal stabilities are in increasing order from series-1 to series-X to series-Y.
Smectic mesophase commences late from decyloxy (C ) homologue in the present novel
10
series-1, whereas it commences earlier from hexyloxy (C ) homologue derivative of
6

76
M. D. KHUNT ET AL.
Table . DSC data for C, C, and C homologues at a heating rate of °C per minute
Transition Temp, t
(°C)
Absolute temp (°K) T
= t + 
ꢀH (J g 
−
)
ꢀS = ꢀH/T
Homologue
Transition
C
Cr ꢀS N
N ꢀS I
.
.
.
–
–
.
.
.
.
–
–
.
.
.
.
–
.
.
.
–

C
C
Cr ꢀS N

N ꢀS I
Cr ꢀS Sm
–
–

Sm ꢀS N
.
.
.
.

N ꢀSI
.
.
(
−): Missing transitions.
isomeric series-X and does not commence till the last (C ) member of
16
series-Y.
•
•
Nematic mesophase commences earliest from first member (C ) of the present novel
1
series-1, from second member (C ) of isomeric series-X, and late from the pentyloxy
2
(
C ) homologue member of non-isomeric series-Y.
5
Total mesomorphic phase length range is highest for series-X (60), lowest for series-Y
35), and intermittent (41) to series-X and Y for series-1 of the present investigation.
All three homologous series under comparison bear laterally substituted -OCH group or
(
3
groups. Therefore, their intermolecular distances parted somewhat more or less depending
on position and polarity of laterally substituted group or groups and magnitudes of inter-
molecular anisotropic end-to-end and lateral forces of attractions, which are affected because
of changing molecular rigidity and flexibility. Generally, enhancement in intermolecular dis-
tance reduces intermolecular attractions and depresses mesomorphic isotropic transition
temperatures as well as the possibility of facilitating tendency of LC phase formation. How-
ever, it also increases intermolecular forces of end-to-end and lateral attractions by increas-
ing molecular polarizibility through increasing molecular width. Thus, two opposing effects
of intermolecular forces (reducing and enhancing) are operated at a time for the same rea-
son (i.e. increasing molecular width). Hence, the resulting magnitudes of anisotropic forces
of intermolecular end-to-end or/and lateral attractions depend upon the predominance of
any one or other (second) effect. Thus, the possibility of facilitating smectic or/and nematic
mesophase formation depends upon resulting effects out of the two opposing effects operated
at a time. Now comparing the exhibition of mesophase formation in isomeric series-1 and X,
ꢀꢀ
ꢀꢀ
the two methoxy groups at positions 2 , 5 in series-1 are at 180° to each other; and those
ꢀꢀ
ꢀꢀ
at positions 3 , 4 in series-X are at 60° to each other. The bond polarity of C-OCH bonds
3
is equal in magnitudes for all the series under comparative study. However, bond polarity
being a vector quantity, the value of resultant bond polarities, i.e. vector sum of two identical
bonds, depends upon the angle between two identical bonds. The vector sum of two C-OCH3
bond polarities, although being equal in magnitudes, acting in opposite directions to each
other, is zero or cancels each other for an angle of 180° between two -OCH for the molecules
3
of the presently investigated novel series-1, thus affecting total molecular polarity, flexibil-
ity, dipole moment, dipole–dipole interactions, intermolecular dispersion, and anisotropic
forces of attraction. However, two C-OCH bonds in the molecules of series-X being at an
3
angle of 60°, the vector sum of two bond polarities are not equal to zero. Therefore, the mag-
nitudes of anisotropic forces of intermolecular end-to-end and lateral attractions of series-X
are greater than series-1. Thus, both isomeric series, although containing two C-OCH polar
3
bonds, have contributed differently to molecular polarities and polarizibility. However, both

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
77
Figure . (a) DSC thermogram for pentyloxy derivative; (b) DSC thermogram for octyloxy derivative; (c) DSC
thermogram for dodecyloxy derivative.

7
8
M. D. KHUNT ET AL.
Figure . X-ray diffractogram of C homologue.
isomeric series exhibited smectic mesophase in addition to nematic mesophase formation,
but their thermal stabilities for smectic and nematic, commencement of smectic and nematic
phases, degree of mesomorphism, etc. are different because of differing molecular rigidity
and flexibility depending on molecular structure. Cancellation of two -COCH bond polari-
3
ties (180°) lower Sm-N and N-I transition temperatures, and hence the corresponding ther-
mal stabilities for smectic and nematic mesophases of the present series-1 as compared with
isomeric series-X occur at the cost of the commencement of mesophase, which commences
late by one homologue, i.e. from the second member of series-X and from the first mem-
ber of series-1. The molecules of homologous series-Y have their two -OCH groups bonded
3
with two carbons at a bond angle of 60°, and equally affect the factors related to mesomor-
phism of series-X, but the effect due to the presence of longer vinyl carboxy -CH=CH-COO-
group in comparison with shorter carboxy -COO- group strengthens the end-to-end attrac-
tions. Therefore, lamellar packing of molecules in series-Y is hindered, and simultaneously
statistically parallel orientational order of molecules is favored in floating condition under
the influence of exposed thermal vibrations as a consequence of increased molecular rigidity
by -CH=CH-COO- more than -COO central bridge. Thus, the N-I transition temperature of
series-Y is higher than that of series-1 at the cost of smectic mesophase formation. Therefore,
smectic mesophase is missing for the molecules of series-Y, and nematic thermal stability is
highest among the series under comparative study. The late commencement of nematic phase
in series-Y (from C ) as compared with series-1 (from C ) is attributed to the suitable magni-
5
1
tudes of end-to-end intermolecular closeness required to resist exposed thermal vibrations to
suitable and facilitative mesophase for definite temperature range as a consequence of favor-
able molecular rigidity in combination with molecular flexibility. Thus, changing trend of LC
properties for the same homologue from series to series depends upon the changing position
of the same pair of polar groups with respect to the same central group linking middle and
third phenyl ring. The extrapolation [10(iv),30] of Sm-N transition curve to octyloxy (C8)
homologue keeping in view the trend of the curve, it matches with the solid-nematic transi-
tion point, indicating that there is no possibility of existence of smectic mesophase from and
prior to octyloxy (C ) homologue derivative.
8
−1
The transition temperature (in °C and °K) with associated enthalpy changes (in J g )
of C , C , and C homologues from DSC measurements are summarized in Table 4. The
5
8
12
homologues selected for the DSC study are enantiotropically mesogenic and undergo crystal-
mesomorphic small-isotropic transitions recorded during heating. DSC thermogram of
selected homologues is shown in Fig. 3.
An X-ray diffraction study of decyloxy (C ) homologue carried out for molecular arrange-
10
ment is depicted with related data in Fig. 4. A typical layer structure is observed for smectic-A
phase. A sharp diffraction peak implies the formation of layered structure.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
79
Conclusions
•
The LC properties of a substance from homologue to homologue in the same series and
for the same homologue from series to series depend upon the molecular structure of a
substance because of changing magnitudes of molecular rigidity and/or flexibility.
Molecular rigidity and flexibility are very sensitive and susceptible to molecular struc-
ture.
•
•
The group efficiency order derived on the basis of (i) thermal stabilities, and (ii) the early
commencement of smectic and nematic phases are as under:
ꢀ
ꢀ
ꢀꢀ
ꢀꢀ ꢀꢀ
(
i) Smectic 3 ,4 dimethoxy with -COO-(series-X) > 2 ,5 dimethoxy with -COO-
ꢀꢀ
ꢀꢀ
(
series-1) > 3 ,4 methoxy with-CH=CH-COO-(series-Y).
Nematic Series-Y > series-X > series-1.
(
ii) Smectic Series-X > series-1 > series-Y.
Nematic Series-1 > series-X > series-Y
•
•
•
Carboxy:-COO- central group is smectogenic and vinyl carboxy:-CH=CH-COO central
group is nematogenic.
Presently investigated novel series-1 is predominantly nematogenic and partly smecto-
genic with middle ordered melting type and middle ordered range of mesophase length.
Present study reveals that the mesophase range is greatly affected by the positional sub-
stitution of the same pair of functional group at the same phenyl ring and by changing
central group between first and middle phenyl rings.
Acknowledgments
Authors acknowledge thanks to the Department of Chemistry (DST-FIST funded & UGC-SAP spon-
sored), Saurashtra University, Rajkot, for research work. Authors are also thankful to Dr. A. V. Doshi,
Ex. Principal, M.V.M. Science and Home Science College, Rajkot, for his valuable cooperation during
the 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.
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