Mesomorphism dependence on terminal end groups

Article abstract of DOI:10.1080/15421406.2015.1044696

A novel homologous series of liquid crystals (LC) 4-(4′-n-alkoxy cinnamoyloxy)-4′′-bromobenzylbenzoate was synthesized and studied with a view to understanding and establishing the relation between LC properties and the molecular structure. The novel series comprises 11 members, and all the members exhibit mesomorphism as enantiotropic smectic. None of the homologues are nematogenic, even in the monotropic condition. Transition temperatures were determined by an optical polarizing microscope equipped with a heating stage. The textures of smectic phase are of the type A or C. The transition curves Sm-I and Cr-Sm behave in a normal manner in the phase diagram. An odd-even effect is exhibited by the Sm-I transition curve. Thermal stability for smectic is 102.6°C. Mesomorphism commences from the very first member of the series. The series is entirely smectogenic and of middle-order melting type whose mesomorphic phase length ranges from 7.4°C to 60.9°. Analytical and spectral data confirmed the molecular structures of homologues. The mesomorphic properties of the present series are compared with structurally similar homologous series.

Full text of DOI:10.1080/15421406.2015.1044696

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Mesomorphism dependence on terminal end
groups
Rajesh B. Marathe & A. V. Doshi
To cite this article: Rajesh B. Marathe & A. V. Doshi (2016) Mesomorphism dependence
on terminal end groups, Molecular Crystals and Liquid Crystals, 624:1, 144-151, DOI:
10.1080/15421406.2015.1044696
To link to this article: http://dx.doi.org/10.1080/15421406.2015.1044696
Published online: 11 Feb 2016.
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Date: 04 April 2016, At: 13:42

MOL. CRYST. LIQ. CRYST.
, VOL. , NO. , –
http://dx.doi.org/./..
Mesomorphism dependence on terminal end groups
Rajesh B. Marathea and A. V. Doshib
aHuntsman International India Pvt. Ltd., Umraya Village, Taluka, Padra, Vadodar, Gujarat, India, and Dr. K. N. Modi
University, Newai, Dist Tonk, Rajasthan, India; bDr. K. N. Modi University, RIICo Industrial Area Ph-II, Newai, Dist.
Tonk, Rajasthan, India
ABSTRACT
KEYWORDS
Mesomorphism;
enantiotropic; liquid crystals;
nematic; smectic
A novel homologous series of liquid crystals (LC) 4-(4ꢀ-n-alkoxy
cinnamoyloxy)-4ꢀꢀ-bromobenzylbenzoate was synthesized and studied
with a view to understanding and establishing the relation between
LC properties and the molecular structure. The novel series comprises
11 members, and all the members exhibit mesomorphism as enan-
tiotropicsmectic. Noneofthehomologuesarenematogenic, eveninthe
monotropic condition. Transition temperatures were determined by an
optical polarizing microscope equipped with a heating stage. The tex-
tures of smectic phase are of the type A or C. The transition curves Sm-I
and Cr-Sm behave in a normal manner in the phase diagram. An odd-
even effect is exhibited by the Sm-I transition curve. Thermal stability for
smectic is 102.6°C. Mesomorphism commences from the very first mem-
ber of the series. The series is entirely smectogenic and of middle-order
melting type whose mesomorphic phase length ranges from 7.4°C to
60.9°. Analytical and spectral data confirmed the molecular structures
of homologues. The mesomorphic properties of the present series are
compared with structurally similar homologous series.
Introduction
A common man is familiar with the word LCD in the present era, which is an abbrevia-
tion for liquid crystal display. Hence, the liquid crystalline state of matter has maintained
its importance in the various fields of applications since its discovery in 1888 [1]. The scien-
tific and technological communities are working extensively on liquid crystals (LCs) [2–12].
The present investigation is aimed to synthesize novel LC compounds through an homolo-
gous series of binary systems with a view to understanding and establishing the relationship
between mesomorphism and molecular structure [13–15]. Therefore, the novel LC materials
may be useful for researchers in branches of science and technology, other than chemistry,
particularly applications. A large number of homologous series and binary or ternary systems
have been reported [16–20]. The proposed investigation aims to synthesize a novel structure
containing three phenyl rings bonded through two central bridges, viz. –CH=CH-COO- and
–COO-, and two terminal end groups, viz. –OCnH2n+1 and –Br polar groups. The effect of
central and terminal groups on LC properties with reference to molecular rigidity and flexibil-
ity [21–24] will be discussed in a comparative manner with structurally similar LC materials.
CONTACT Rajesh B. Marathe
Padra, Vadodar, Gujarat, India.
maratha raj@gmail.com
Huntsman International India Pvt. Ltd., Umraya Village, Taluka,
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

MOL. CRYST. LIQ. CRYST.
145
Experimental
Synthesis
4-Hydroxy cinnamic acid was alkylated by suitable alkylating agent R-X to form n-alkoxy
cinnamic acid by the modified method of Dave and Vora [25] as a main component of series.
4-Hydroxy-4ꢀ-bromobenzyl benzoate (m.p.: 115–122°C) was prepared from 4-bromobenzyl
bromide and 4-hydroxy benzoic acid by applying the method of US patent No. 0053964 and
European patent and the modified method of Doshi et al. [26]. n-Alkoxy cinnamic acids,
through their corresponding acid chlorides, and 4-hydroxy-4ꢀ-bromobenzylbenzoate con-
densed in dry pyridine to form final products by an established method [27–29]. The final
esterified products were individually decomposed, filtered, washed, dried, and purified until
constant transition temperatures were obtained.
4-Hydroxy cinnamic acid, alkyl halides, methanol, KOH, 4-bromobenzyl bromide/alcohol,
N,N-dimethyl formamide, NaHCO3, HCl, dry pyridine, thionyl chloride, EtOH, etc. required
for the synthesis were used as received, except solvents which were dried and purified prior
to synthesis. Synthetic route to the series is mentioned below as Scheme 1.
Scheme . Synthetic route to the series: -[ꢀ-n-Alkoxy cinnamoyloxy]-ꢀꢀ-bromobenzyl benzoates.

146
R. B. MARATHE AND A. V. DOSHI
Table . Elemental analysis for methoxy, propyloxy, butyloxy, and pentyloxy derivatives.
% Elements calculated (Experimental%)
Sr. No.
Molecular formula
C
H
Br
.
CHBrO
CHBrO
CHBrO
CHBrO
. (.)
. (.)
. (.)
. (.)
. (.)
. (.)
. (.)
. (.)
.(.)
. (.)
. (.)
. (.)
.
.
.
Characterization
The selected members of a novel homologues series were characterized and analyzed by ele-
mental analysis and the structure elucidation by infra-red (IR) spectra, 1H NMR spectra, and
mass spectra. Textures and transition temperatures of homologues as well as related materials
were determined by an optical polarizing microscope, equipped with a heating stage. Elemen-
tal analysis was performed on Perkin-Elmer PE 2400 CHN analyzer (Table 1). The percentage
of halide content was determined by the usual analytical method. IR spectra were recorded
on Perkin-Elmer spectrum GX. 1H NMR spectra were determined on Bruker spectrometer
using CDCl3 solvent. Textures of smectic mesophase of some homologues were recognized
either directly from the microscopic observations or by miscibility method.
Analytical data
Spectral data
1H NMR in ppm for the octyloxy derivative: 1.19–1.23 (-CH3 of C8H17), 3.86-3.92 (-CH2-
of –C8H17), 5.14–5.21 (-CH=CH-), 6.19–6.82 (p-substituted phenyl ring), 7.80–7.82 (p-
substituted benzene).
1H NMR in ppm for the dodecyloxy derivative: 0.81 (-CH3 of C12H25), 1.17–1.22 (-CH2-
of –C12H25), 3.88–3.92 (-O-CH2-), 5.16–5.21 (-CH=CH-), 6.20–6.24 (p-substituted phenyl
ring), 6.76–6.83 (p-substituted benzene).
NMR data confirm the structure
IR in cm−1 for ethoxy derivative: 753–770 polymethylene of C2H5, 823 para-substituted phenyl
ring, 1715, 1118, 1675.9 ester linkage of (COO-C2H5), 982 trans of (-CH=CH-) 1121 ether
linkage of (C2H5-O- of phenyl), 1010 aromatic -Br linkage 512, 630 (C-Br stretch).
IR in cm−1 for hexadecyloxy derivative: 720–771 poly-methylene of C16H33, 807–829 para-
substituted benzene, 982 trans (-CH=CH-), 1011 aromatic –Br linkage, 1105, 1307, 1675,
1716 (-COO-), 1220-1269 ether linkage of (C16H33-O-C6H4), 3410 (H-bonding of OH), 2850,
-COOH.
IR data confirm the structure
Mass spectra
Mass spectra for hexyloxy derivative
Molecular weight, calculated: 537.
Molecular formula: C29H29BrO5
Experimental: 539.
Texture of smectic phase by miscibility method
Pentyloxy (C5) derivative
→ Smectic-A

MOL. CRYST. LIQ. CRYST.
147
Hexyloxy (C6) derivative
Dodecyloxy (C12) derivative
Tetradecyloxy (C14) derivative
→ Smectic-A
→ Smectic-C
→ Smectic-C
Results and discussion
Cis-4-n-alkoxy cinnamic acid and 4-hydroxy-4’-bromobenzylbenzoate (mp: 115–122°C,
yield = 70%) are not LCs. However, mesomorphism is induced in the present novel ester
homologous series by linking both non-mesomorphic components in dry cold pyridine. The
smectogenic mesomorphism is induced from first to last (C16) member of the series in an
enantiotropic manner. Nematogenic character is totally absent. Transition and melting tem-
peratures were determined by an optical polarizing microscopy equipped with a heating stage.
Transition temperatures (Table 2) of the homologues were plotted against the number of car-
bon atoms present in n-alkyl chain bonded to phenyl ring through oxygen atom. Like points
were linked to draw Cr-Sm and Sm-I transition curves indicating phase behavior of the series
(Fig. 1). The Cr-Sm transition curve follows a zigzag path of rising and falling values with
overall descending tendency in a typical manner. The Sm-I transition temperatures initially
rise for odd and even members of the series up to the octyloxy member (C8) of the series at a
maximum and then descends up to the last C16 member. An odd-even effect is observed for
Sm-I transition curve. Thus, Cr-Sm and Sm-I transition curves of a phase diagram behaves in
a normal established manner. Curves (Sm-I) for odd and even members matches at the hep-
tyl derivative (C7) of a series and then, Sm-I transition curve propagate as a single transition
curve until the C16 homologues derivative. Textures of the smectogenic homologues are focal
conic fan shape of smectic A or C. Analytical and spectral data support the molecular struc-
tures of the homologues. LC properties, such as thermal stability, molecular planarity, type
of mesophase behavior, mesophase length range etc., vary from homologue to homologue
in the same series as a consequence of changing molecular rigidity and flexibility depend-
ing upon the end to end and lateral intermolecular attractions of varying magnitudes. The
condensation to generate the ester homologues increases molecular length, ratio of the polar-
ity to polarizability, permanent dipole moment across the long molecular axis, dipole-dipole
and electronic interactions, dispersion forces, molecular rigidity and flexibility, anisotropic
forces of intermolecular end to end and lateral attractions. However, due to the suitable mag-
nitudes of anisotropic forces of lateral and terminal attractions, the molecules of all the homo-
logues generate mesomorphism as a result of differing molecular rigidity and flexibility. The
Table . Transition temperatures of series.
Transition temperature (°C)
Compound No.
R=CnHn+ (n)
Smectic
Nematic
Isotropic











C
C
C
C
C
C
C
C
C
C
C
.
.
—
—
—
—
—
—
—
—
—
—
—
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

148
R. B. MARATHE AND A. V. DOSHI
Figure . Phase behavior of series.
molecules of all the homologues show a lamellar packing arrangement and hence generate a
smectic phase, but the smectic to nematic transition does not appear for any of the homo-
logues because residual end to end intermolecular attractions are weak. The exhibition of an
odd-even effect is attributed to alteration of transition temperatures due to the sequentially
added methylene unit at the n-alkyl chain bonded to the phenyl ring through oxygen atom.
The disappearance of the odd-even effect from and beyond the heptyloxy (C7) homologue of
the series is attributed to the longer n-alkyl chain which may coil or bend or flex or couple
to occupy its position with principle axes of the core structure. The early commencement of
smectic phase (from C1) is attributed to the suitable and favorable extent of molecular non-
coplanarity, which causes suitability in lamellar packing of molecules. The changing trend in
LC properties and the degree of mesomorphism from homologue to homologue is attributed
to the changing the number of carbon atoms present in the n-alkoxy terminal end group. The
LC properties of the present series are compared with structurally similar homologous series
X [30] and Y [31] as shown below in Fig. 2.
A homologous series-1 of present investigation and the series-X and Y chosen for compari-
son are identical with respect to three phenyl rings bridged through vinyl carboxy and –COO-
CH2- groups constituting molecular rigidity and the flexible –OR terminal end group for the
same homologue from series to series, but all the three series under comparative study are

MOL. CRYST. LIQ. CRYST.
149
Figure . Structurally similar series.
differing with respect to flexible terminal end groups –Br, -OCH3 and –Cl. Therefore meso-
morphic properties or behaviors and the degree of mesomorphism as depended upon varying
features of homologous series for the same homologue from series to series. The mesomor-
phic properties and related data are recorded in following Table 3 for the series 1, X and Y as
under.
It is clear from Table 3 that
r
Series-1 and Y are only smectogenic, and series-X is smectogenic in addition to nemato-
genic in character.
Smectic and nematic thermal stabilities are highest in series X, whereas only smectogenic
thermal stabilities of series 1 and Y are relatively lower than in series X.
The smectic thermal stability of present series 1 is lowest among all three series under
comparison.
Smectogenic mesophase commences from the very first member of series 1 and Y, and
it commences from C10 member of series X.
Nematic mesophase commences from the C2 member of series X, but it does not com-
mence till the last member of series 1 and Y.
Mesophase length range is lowest for the present series 1 and highest for series X among
all the series under comparison.
r
r
r
r
r
Differing features among the series 1, X, and Y are the magnitudes of flexibility for the
same homologue from series to series caused by changing right-handed terminal end groups,
-Br, -OCH3, and -Cl, respectively. Terminal end groups —Br and -Cl of series 1 and Y are
mono-atomic with three lone pairs of electrons, and that of series X are penta-atomic with
two lone pairs of electrons that cause difference in the extent of molecular non-coplanarity,
Table . Average thermal stability (°C).
Series→

X
Y
Smectic—isotropic
or
Smectic–nematic
.
(C–C)
C
.
.
(C-C)
C
(C-
C)
C
Commencement of
smectic phase
Nematic–isotropic
Commencement of
nematic phase
Mesophase length
Range (Sm + N) in
°C
.
(C-C)
C
—
—
. to
.
 to

. to
.
Minimum to
maximum
C C
C C
C C
Cα Cβ

150
R. B. MARATHE AND A. V. DOSHI
which affects lamellar packing of molecules. The combined effect of molecular rigidity and
flexibility varies from homologue to homologue in the same series and for the same homo-
logue from series to series. Hence, stabilization of only smectic phase or smectic phase in
addition to nematic phase is facilitated or hindered. Thus, thermal stabilities and mesophase
length ranges vary from series to series as mentioned in Table 3.
Conclusions
r
Novel ester homologous series is smectogenic without exhibition of nematic property,
whose mesomorphic range is 7.4 to 62.2, and low thermal stability of 102.6.
-OCH3 group being nematogenic and halogen group being smectogenic are supported
by the present investigation.
r
r
The group efficiency order derived on the basis (i) thermal stability, (ii) commencement
of mesophase, and (iii) mesophase length range for smectic and nematic are as under:
(i) Smectic:
Nematic:
(ii) Smectic
Nematic
(iii) Smectic Nematic
r
As size of halogen atom end groups increases, the smectic thermal stability decreases.
Mesomorphic properties are sensitive and susceptible to molecular structure.
Present homologues are useful in the study of binary systems for the devices operated
between 40°C and 60°C. The ester derivatives may be useful in the agricultural field for
r
r
the growth of flower- and fruit-giving plants.
Acknowledgments
Authors acknowledge thanks to the Green Circle Inc. Laboratory for providing research facilities and
services as and when needed. Authors acknowledge thanks to the Baldev Prajapati (Rajesh Pharma,
Ankleshwar) for providing research chemicals (4-Bromobenzyl alcohol/bromide). Authors are also
thankful to Drs. N. N. Vyas, Vipul Patel, and M. L. Chauhan, P. T. Arts and Science College, Godhara,
for their valuable helping hand and microscopic facility. Also thanks are due to the Sophisticated Ana-
lytical Instrumentation Facility, Punjab University, Chandigarh, for extending their help for analytical
services.
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