Synthesis and evaluation of a novel homologous series of carbonylethylene-linked liquid crystals

Article abstract of DOI:10.1080/15421406.2012.762498

A novel homologous series of α-4-[4′-n-Alkoxy benzoyloxy] benzoyl β-phenyl ethylenes was synthesized with a view to understand the effect of molecular structure on liquid crystal (LC) properties. The novel series consists of 11 homolog derivatives. The LC properties commence from the hexyl derivative of the series. The methyl to pentyl derivatives of the series are not liquid crystalline; however, the hexyl to hexadecyl homolog derivatives are enantiotropically nematogenic with the absence of smectogenic phases. Thus the novel series is entirely nematogenic. The melting points, transition temperatures, and mesophase morphologies were determined by optical polarizing microscopy in conjunction with a heating stage. The microscopic textures show the nematic mesophase of the threaded or Schlieren type. Analytical and spectral data support the molecular structure. The nematic to isotropic liquid transition curve behaves in a normal manner without the exhibition of an odd-even effect. Typical melting points are around 140°C and the nematic-phase stability varies from around 150°C to 180°C. The LC properties of the novel series are compared with other structurally similar series.

Full text of DOI:10.1080/15421406.2012.762498

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Synthesis and Evaluation of a Novel
Homologous series of Carbonylethylene-
Linked Liquid Crystals
H. N. Chauhan a , V. R. Patel b & A. V. Doshi c
a Department of Chemistry , Rajasthan/K. K. Shah Jarodwala
Maninagar Science College , Ahmedabad , Gujarat , India
b Department of Chemistry , Sheth P. T. Arts and Science College ,
Godhara , Gujarat , India
c Department of Chemistry , Shri Jagdishprasad Jhabarmal Tibrewala
University , Jhunjhunu , Rajasthan , India
Published online: 02 Apr 2013.
To cite this article: H. N. Chauhan , V. R. Patel & A. V. Doshi (2013) Synthesis and Evaluation of a
Novel Homologous series of Carbonylethylene-Linked Liquid Crystals, Molecular Crystals and Liquid
Crystals, 574:1, 67-74, DOI: 10.1080/15421406.2012.762498
To link to this article: http://dx.doi.org/10.1080/15421406.2012.762498
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Mol. Cryst. Liq. Cryst., Vol. 574: pp. 67–74, 2013
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2012.762498
Synthesis and Evaluation of a Novel Homologous
series of Carbonylethylene-Linked Liquid Crystals
H. N. CHAUHAN,1 V. R. PATEL,2 AND A. V. DOSHI3,∗
1Department of Chemistry, Rajasthan/K. K. Shah Jarodwala Maninagar Science
College, Ahmedabad, Gujarat, India
2Department of Chemistry, Sheth P. T. Arts and Science College, Godhara,
Gujarat, India
3Department of Chemistry, Shri Jagdishprasad Jhabarmal Tibrewala University,
Jhunjhunu, Rajasthan, India
A novel homologous series of α-4-[4ꢀ-n-Alkoxy benzoyloxy] benzoyl β-phenyl ethylenes
was synthesized with a view to understand the effect of molecular structure on liquid
crystal (LC) properties. The novel series consists of 11 homolog derivatives. The LC
properties commence from the hexyl derivative of the series. The methyl to pentyl
derivatives of the series are not liquid crystalline; however, the hexyl to hexadecyl
homolog derivatives are enantiotropically nematogenic with the absence of smectogenic
phases. Thus the novel series is entirely nematogenic. The melting points, transition
temperatures, and mesophase morphologies were determined by optical polarizing
microscopy in conjunction with a heating stage. The microscopic textures show the
nematic mesophase of the threaded or Schlieren type. Analytical and spectral data
support the molecular structure. The nematic to isotropic liquid transition curve behaves
in a normal manner without the exhibition of an odd–even effect. Typical melting points
are around 140◦C and the nematic-phase stability varies from around 150◦C to 180◦C.
The LC properties of the novel series are compared with other structurally similar
series.
Keywords Enantiotropy; liquid crystal; nematic; odd-even effect; smectic
Introduction
Some substances exhibit an intermediate state of matter with a molecular ordering in
between that of the conventional crystalline solid and isotropic liquid phases, and this state
of matter is called a liquid crystal (LC) state or mesomorphic state, which is an anisotropic
liquid. Of course, the generation of liquid crystalline phases depends on molecular structure
of a substance. Molecular rigidity and flexibility combine to facilitate suitable magnitudes
of intermolecular forces of attractions [1–4] required to induce an LC phase. Favorable
molecular rigidity means suitable molecular aromaticity; molecular shape; size; polarity
of lateral, terminal, and central groups; molecular polarizability; electronic interaction etc.
that contribute to the intermolecular forces of attraction [4]. The present series involves
n-alkoxy (−OR) and –H terminal end groups; three phenyl rings bonded through –COO−
∗Address correspondence to A. V. Doshi, Department of Chemistry, Ph.D. Guide, Shri J.
J. T. University, Rajasthan, Jhunjhunu 333 001, Rajasthan, India. Tel: 0919825474811. E-mail:
shreeyashailee@yahoo.in
67

68
H. N. Chauhan et al.
and –CO−CH=CH− central bridges with absence of any lateral group. Thus, a high length
to breadth ratio of a linear lath-like molecule causing suitable magnitudes of molecular
rigidity and flexibility induces LC mesophase at moderate to long terminal chain lengths.
Experimental
Synthesis
4-hydroxybenzoic acid was alkylated with suitable alkylating agents (R-X) and then con-
verted into the corresponding n-alkoxybenzoyl chloride [A] using freshly distilled thionyl
chloride by a modified method of Dave and Vora [5]. α-4-Hydroxybenzoyl β-phenyl
ethylene [B] was prepared by a well-established method [6]. Components A and B were
condensed in dry, cold pyridine [7]. The final products were individually synthesized, fil-
tered, washed, dried, and purified until constant transition temperatures were obtained.
4-hydroxybenzoic acid, alkyl halides, MeOH, KOH, HCl, SOCl2, ethanol, pyridine, 4-
hydroxyacetophenone, benzaldehyde etc. were used as received. All solvents were dried
and distilled prior to use. The synthetic route to the series is shown in Scheme 1.
Scheme 1. Synthetic route to the series.

Synthesis and Evaluation of Novel L.C. Molecules
69
Characterization
Some of the selected homologs of the novel series were characterized for the determination
of molecular structures by elemental analysis, infrared (IR) and 1HNMR (nuclear magnetic
resonance) techniques. Microanalysis was performed using a Perkin Elmer PE 2400 CHN
analyzer. IR spectra were recorded on a Perkin-Elmer spectrum GX, and 1HNMR spectra
were recorded on a Bruker instrument using CDCl3 as solvent. The mesogenic properties
and transition temperatures of the series were investigated using an optical polarizing
microscope equipped with a heating stage. Thermodynamic quantities, enthalpy (ꢀH), and
entropy (ꢀS) are qualitatively discussed through differential scanning calorimeter (DSC)
scan.
Analytical Data:
Table 1 Spectral data:
NMR in ppm for decyloxy derivative:
0.885 (−CH3 of −OC10H21 group), 1.251 (−CH2− of −OC10H21), 3.998 (−OCH2CH2−
of −OC10H21), 7.488 (−CO−CH=CH− group), 6.88 to 8.045 (p-substituted phenyl ring).
NMR supports the structure.
NMR in ppm for tetradecyloxy derivative:
0.8822 (−CH3 of −OC14H29 group), 1.2 (−CH2− of −OC14H29), 3.184 (−OCH2CH2−
of −OC14H29), 6.856 (−CO−CH=CH− group), 7.3 3to 7.906 (p-substituted phenyl ring).
NMR supports the structure.
IR in cm−1 for butoxy derivative:
700 −CH2− group of −OC4H9, 840 phenyl ring, 1160 −CO− of −OC4H9 group, 1210,
1255, & 1600 −COO− ester group, 975 −CH=CH− group. IR supports the structure.
IR in cm−1 for ethoxy derivative:
760 −CH2− group of −OC2H5), 845 phenyl ring, 1160 −CO− of −OC2H5 group), 1250
&1600 −COO− ester group. IR supports the structure.
Results and Discussion
Dimeric 4-n-alkoxybenzoic acid on esterification gives rise to a product with relatively lower
transitions. But, on linking it to a third phenyl ring through a −CO−CH=CH− central
bridge, the transition temperatures of a final product rise. The presently investigated series
consists of the methyl to pentyl homolog derivatives, which are not liquid crystalline and
the hexyl to hexadecyl homolog derivatives as enantiotropically liquid crystals (LC) with
a nematogenic type of mesophase. None of the homolog derivatives exhibit smectogenic
character. The nematogenic mesophase is seen as a threaded or Schileren type of texture. The
transition temperatures (Table 2) were observed through an optical polarizing microscopy
with heating stage. The transition temperatures were plotted versus the number of carbon
atoms in n-alkyl chain [−R−] of left n-alkoxy terminal end group. The phase diagram
Table 1. Elemental analysis for methoxy, hexyloxy, and octyloxy derivatives
Elements % found (% Calculated)
Serial numbers
Molecular formula
C
H
1
2
3
C23H18O4
C28H28O4
C30H32O4
77.10 (76.92)
78.50 (78.12)
78.95 (79.18)
5.03 (5.14)
6.54 (6.44)
7.02 (6.98)

70
H. N. Chauhan et al.
Table 2. Transition temperatures of the novel series in ◦C
Transition temperatures in ◦C
Compound numbers
numbers
n-alkyl group
−CnH2n+1 (n)
Sm
N
Isotropic
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
8
10
12
14
16
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
148.0
150.0
135.0
140.0
132.0
138.0
210.0
192.0
202.0
182.0
162.0
167.0
178.0
180.0
165.0
152.0
145.0
10
11
Sm = smectic; N = nematic.
obtained is represented in Fig. 1. Solid to isotropic or nematic transition curve adopts a
zigzag path of rising and falling tendency with an overall descending manner. The nematic
to isotropic transition curve initially rises and then falls as series is ascended without the
exhibition of an odd–even effect. Thus, both transition curves behave in a normal manner
with absence of smectogenic behavior. The nematic–isotropic transition temperatures vary
between 180◦C and 145◦C. Mesomorphic phase temperature range varies from 7◦C to
45◦C.
Dimerization of 4-n-alkoxybenzoic acids disappears because the hydrogen bonding
between two molecules is removed by the esterification process. The nonmesomorphic
behavior of the methyl to pentyl homolog derivatives is due to their high crystallizing
tendency that arises from the relatively stronger intermolecular attractions from shorter n-
alkyl chain that induces more crystalline character, but unsuitable magnitudes of anisotropic
intermolecular forces of attraction mean that the molecules of a sample substance are
randomly oriented in all possible directions in an irregular manner under floating condition
with high entropy. Hence they sharply melt without showing any mesophase (LC) formation.
On extending the n-alkyl chain from and beyond the hexyl homolog to the hexadecyl
homolog, the molecules of a sample under the influence of heat gradually pass through
translational, rotational, and vibrational motion and resist the exposed thermal vibrations.
Consequently, molecules under examination disalign at an angle less than 90 degrees with
a plane of a surface at a particular temperature t1 and continue to resist thermal vibrations
up to a higher temperature t2. Thus, molecules are floating with a statistically parallel
orientational order between a t1 and t2 temperature range, which shows a nematogenic
mesophase formation. However from and beyond t2, the molecules are unable to resist
the thermal vibrations and disalign on the plane of a surface. At this point, the molecules
lose the LC state and are converted into the isotropic state. On gradual heating of a sample
homolog from room temperature, the intermolecular binding forces are loosened, depending
upon the molecular rigidity and flexibility [1–4] as a consequence of molecular polarity,
polarizability, length to breadth ratio etc., which governs the magnitudes of anisotropic
intermolecular forces of attraction. Thus, the anisotropic intermolecular forces of adhesion

Synthesis and Evaluation of Novel L.C. Molecules
71
Figure 1. Phase behavior of series.
for the n-hexyl to n-hexadecyl chains of left n-alkoxy terminal end group in addition to
the other molecular units favorably induce only nematogenic mesophase. However the
molecules show a lack of lamellar packing and so do not allow the smectogenic mesophase
even in the monotropic condition. The variation in mesomorphic properties from homolog
Figure 2. Structurally similar series.

72
H. N. Chauhan et al.
to homolog [4] in the same series is attributed to the sequentially added methylene unit
in n-alkyl chain of left n-alkoxy terminal end group. The absence of an odd-even effect is
attributed to the absence of mesomorphic behavior for the shorter chain homologs because
odd-even effects diminish with chain length. The mesomorphic properties of the presently
investigated novel homologous series 1 are compared with the other structurally similar
homologous series X [8] and Y [9].
Homologous series 1 and X are identical in all respects except for the position of the
−CO− group of a central bridge linking middle and last (third) phenyl rings i.e. both series
consist of three phenyl rings with left n-alkoxy terminal end group and right-sided unsub-
stituted phenyl ring. The first and middle phenyl rings bridged through −COO− common
central group. However the middle and third phenyl rings are bridged through isomeric cen-
tral groups −CO−CH=CH− and –CH=CH−CO− of series 1 and X, respectively. Hence,
the difference of mesomorphic properties and the degree of mesomorphism is attributed to
the positional difference of −CO− group between middle and third phenyl ring i.e. −CO−
group directly attached to middle phenyl ring and directly attached to third phenyl ring (or
away from middle phenyl ring) in case of series 1 and X, respectively. Homologous series 1
and Y are identical in all respects, but they differ in the case of one central bridge linking first
(left) and middle phenyl rings i.e. −COO− and −CH=CH−COO− central bridges in case
of series 1 and Y, respectively. Therefore, the observed difference in mesomorphic prop-
erties and the degree of mesomorphism are attributed [10] to the −CH=CH− unit present
in the −CH=CH−COO− central group of series Y, which is absent in case of −COO−
central group of series 1. Table 3 represents thermal stability and the commencement of the
mesophase for series 1 and the series (X and Y) selected for the comparative study.
On comparing the type of mesophase or mesophases formed by the molecules of series
1 and X, Table 3, indicates that series 1 is only nematogenic while series X exhibits smectic
and nematic characters. The smectic and nematic thermal stabilities of series X are 106.66
and 119.8, respectively while that of series 1, the only nematic thermal stability is 164.5,
which is much higher than series X. The mesophase formation (nematic) commences from
hexyl homolog in series 1, while it occurs from the octyl homolog in case of series X.
Thus, only a positional difference of the −CO− group in the isomeric homologous series 1
and X causes such drastic difference. Thus, only shifting of −CO− group in the molecular
structure drastically affects the magnitudes of molecular rigidity and flexibility [1–3] and
hence the suitable varying magnitudes of the anisotropic intermolecular forces of attraction
Table 3. Average thermal stabilities in ◦C
Average transition temperatures in ◦C
Series
Series (1)
Series (X)
Series (Y)
Smectic-nematic or
smectic-isotropic
Commencement of
smectic mesophase
Nematic-isotropic
—
106.66
(C10-C14)
C10
—
164.5
(C5–C16)
C5
119.8
(C8–C16)
C8
155.4
(C5–C16)
C5
Commencement of
nematic mesophase

Synthesis and Evaluation of Novel L.C. Molecules
73
as a consequence of their net molecular polarity, polarizability, length to breadth ratio,
molecular geometry, electronic interactions, etc. [4]. Thus in spite of very close molec-
ular similarities, the mesomorphic properties of a substance can be drastically different.
Homologous series 1 and Y differ only with respect to central group −CH=CH−COO−
and −COO− by −CH=CH− unit between first (left) and middle phenyl rings. Both these
central bridges are comparable, though the vinyl carboxylate has a greater length and causes
more noncoplanarity due to a twist obtained as the oxygen atoms of the vinyl carboxy group
bump into the nonbonded adjacent hydrogen atoms of the aromatic rings. The enhanced
length of the vinyl −CH=CH−COO− central group, increases the conjugation through
the −CH=CH− linking group, which should increase the rigidity and lateral attractions,
and hence intermolecular attractions should increase by more than the −COO− central
bridge. However, cis alkoxycinnamic acids from which final products of series Y are syn-
thesized are nonmesomorphic. The hydrogen bonded n-alkoxybenzoic acids from which
molecules of series 1 are formed, whose intermolecular attractions are almost equivalent
to the cis isomer of cinnamic derivatives. Therefore the thermal stability of series 1 and Y
is 164.5 and 155.4, respectively. Thus, the cis configured isomers of cinnamic acids yield
lower magnitudes of intermolecular attractions as compared to the hydrogen bonded ring
system of p-n-alkoxybenzoic acids derivatives (−COO− of series 1). Thus, the lower value
of thermal stability of series Y than the series 1 under present investigation is justified.
The commencement of nematogenic mesophase from pentyl homolog derivative and only
nematic mesophase from the hexyl to hexadecyl derivatives with absence of smectogenic
character in series 1 and Y, respectively, very well matches from the point of view of
nearly equivalent magnitudes of anisotropic forces of intermolecular attractions. Absence
of smectogenic property in series 1 in comparison with series X is attributed to the absence
of lamellar packing of molecules of series 1 in their crystal lattices due to direct bonding of
−CO− part of the group at the middle phenyl ring that inhibits to form lamellar packing of
molecule. Hence, smectogenic mesophase formation does not occur until the last homolog
of series 1, while presence of lamellar molecular packing in the crystal lattices of the
molecules of series X allows smectogenic character from the decyl homolog due to favor-
able extent of noncoplanarity of the molecules in which −CO− group of −CH=CH−CO−
central group linked directly to third phenyl ring of series X, instead of middle phenyl ring
of series 1. Thus suitable magnitudes of intermolecular anisotropic forces of attractions are
related to proper positioning of a functional group in a molecule or to a part of a functional
group. Thus, a variation in mesomorphic properties from series to series is attributed to the
variation in central group or groups keeping rest of the molecular part unchanged.
Conclusions
(1) The group efficiency order derived for the nematic mesophase formation is as
below.
−CO−CH CH−
>
−CO−CH CH−
>
−CH CH−CO−
with
with
with
−COO−
−CH CH−COO−
central group
−COO−
central group
central group
(2) Group efficiency order derived on the basis of early commencement of smectic
and nematic phase is as below.
Smectic:
−CO−CH CH−
−CH CH−CO−
with
<
with
−COO− and –CH CH−COO−
−COO−
central group
central group

74
H. N. Chauhan et al.
Nematic:
−CO−CH CH−
With
−CH CH−CO−
with
>
−COO− and −CH CH−COO−
−COO−
central group
central group
(3) Variations of mesomorphic properties depend upon the varying molecular rigidity
and flexibility.
(4) Mesomorphic properties and the degree of mesomorphism depend upon the posi-
tional difference of a functional group or part of the functional group of a central
bridge linking phenyl rings.
(5) Variation in mesomorphic properties from homolog to homolog in the same series
depends upon the sequentially added methylene unit.
(6) The cis isomer of the −CH=CH−COO− central group yields a poorer degree of
mesomorphism than the same homolog containing a −COO− central group at the
identical position.
(7) The presence of a −CO− group in any central region directly attached to a phenyl
ring does have relatively strong end-to-end intermolecular anisotropic forces of
attractions.
Acknowledgement
Authors acknowledge thanks to the principal, Dr. R. R. Shah and the head, Dr. R. B. Patel,
of chemistry of K. K. Shah Jarodwala Maninagar Science College, Ahmedabad, for their
valuable co-operation and also to the Vaibhav Laboratory for analytical services. Authors
are thankful to head and staff of chemistry, S.P.T. Science College Godhara for providing
their microscopic facility.
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