Mesomorphism dependence on central bridge (-COO-CH2-) and terminal end group (-Br)

Article abstract of DOI:10.1080/15421406.2015.1044639

A novel series of liquid crystalline (LC) ester materials has been synthesized and studied with a view to understanding the effects of molecular structure on LC behavior. The homologous series of eleven members is entirely enantiotropically smectogenic without exhibition of a nematic phase. Transition temperatures and the texture of smectic phase was determined by an optical polarizing microscope equipped with a heating stage (POM). Transition curves (Cr-Sm and Sm-I) showing phase behavior in a phase diagram behave in a normal manner. Textures of smectic phase are focal conic fan shaped of the type Smectic-A or Smectic-C. An odd-even effect is exhibited by the Sm-I transition curve. The average thermal stability for smectic is 91.2°C and the mesogenic phase length ranges between 3.6°C and 39.3°C. Thus, the novel smectogenic homologous series is a middle-ordered melting type whose LC phase is relatively middle ordered. The LC properties of the present series are compared with a structurally similar homologous series. Analytical and spectral data support the molecular structures.

Full text of DOI:10.1080/15421406.2015.1044639

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Mesomorphism dependence on central bridge (-
COO-CH -) and terminal end group (-Br)
2
Rajesh B. Marathe & A. V. Doshi
To cite this article: Rajesh B. Marathe & A. V. Doshi (2016) Mesomorphism dependence on
central bridge (-COO-CH -) and terminal end group (-Br), Molecular Crystals and Liquid Crystals,
2
624:1, 122-131, DOI: 10.1080/15421406.2015.1044639
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Date: 23 March 2016, At: 14:36

MOL. CRYST. LIQ. CRYST.
, VOL. , NO. , –

http://dx.doi.org/./..
Mesomorphism dependence on central bridge (-COO-CH -) and

terminal end group (-Br)
a
b
Rajesh B. Marathe and A. V. Doshi
a
b
Huntsman International India Pvt. Ltd, Umraya village, Taluka, Padra, Vadodara, Gujarat, India; Dr K. N.Modi
University, Newai, Dist. Tonk, Rajasthan, India
ABSTRACT
KEYWORDS
A novel series of liquid crystalline (LC) ester materials has been synthe-
sized and studied with a view to understanding the effects of molecular
structure on LC behavior. The homologous series of eleven members is
entirely enantiotropically smectogenic without exhibition of a nematic
phase. Transition temperatures and the texture of smectic phase was
determined by an optical polarizing microscope equipped with a heat-
ing stage (POM). Transition curves (Cr-Sm and Sm-I) showing phase
behavior in a phase diagram behave in a normal manner. Textures of
smectic phase are focal conic fan shaped of the type Smectic-A or
Smectic-C. An odd–even effect is exhibited by the Sm-I transition curve.
The average thermal stability for smectic is 91.2°C and the mesogenic
phase length ranges between 3.6°C and 39.3°C. Thus, the novel smec-
togenic homologous series is a middle-ordered melting type whose
LC phase is relatively middle ordered. The LC properties of the present
series are compared with a structurally similar homologous series. Ana-
lytical and spectral data support the molecular structures.
Enantiotropy; liquid crystals;
mesomorphism; nematic;
smectic
Introduction
Liquid crystalline state (LC) is popular among scientific and technological community since
888 [1] for the combination of liquidity and anisotropic properties. Thus, thermotropic and
1
lyotropic LCs are well known for their applicability in various fields of research and for appli-
cations such as display devices and for biological or microbiological activity [2–10]. A critical
survey of the literature about LC studies [11– 15] reveals much interest in the effect of molec-
ular structures on LC properties. Based on literature survey of synthesis of novel substances
through homologous series, [16–19], the present proposed study is planned to synthesize an
ester homologous series of three phenyl rings and two central bridges viz. –COO- and –COO-
CH - with –Br and –OR terminal end groups and their LC characteristics to be discussed and
2
interpreted in terms of molecular rigidity and flexibility [20–23], depending upon changing
novel molecular structures.
CONTACT Rajesh B. Marathe
Taluka, Padra, Vadodara  , Gujarat, India.
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
maratha_raj@rediffmail.com
Huntsman International India Pvt. Ltd., Umraya village,
©

MOL. CRYST. LIQ. CRYST.
123
Experimental
Synthesis
4
-Hydroxy benzoic acid was alkylated by suitable alkylating agent R-X to form n-alkoxy ben-
zoic acid by modified method of Dave and Vora [24] as main component of a series. 4-
ꢀ
Hydroxy-4 -bromobenzyl benzoate (m.p: 115–122°C) was prepared from 4-bromo benzyl
bromide or alcohol and 4-hydroxy benzoic acid by applying the method of European patent
and the modified method of Doshi, Patel, and Marathe [25]. n-Alkoxybenzoic acids through
ꢀ
their corresponding acid chlorides and 4-hydroxy-4 -bromobenzyl benzoate were condensed
in dry pyridine to form final products by a usual established method [26]. Final esterified
products were individually decomposed, filtered, washed, dried, and purified until their con-
stant transition temperatures obtained.
4
-Hydroxy benzoic acid, alkyl halides, methanol, KOH, 4-bromo benzyl bromide or alco-
hol, N,N-dimethyl formamide, NaHCO , HCl, dry pyridine, thionyl chloride, EtOH etc.
3
required for the synthesis were used as received, except solvents which were dried and purified
prior to synthesis. The synthetic route to the series is shown in Scheme 1.
Characterization
The selected members of novel homologues series were characterized and analyzed for
1
elemental analysis. The structure elucidated by Infra-red spectra, HNMR spectra, and
mass spectra. Textures and transition temperatures of homologues as well as of related
materials were determined by an optical polarizing microscope, equipped with a heating
stage. Elemental analysis was performed on Perkin–Elmer PE 2400 CHN analyzer. IR spec-
1
tra were recorded on Perkin–Elmer spectrum GX. HNMR spectra were determined on
Bruker spectrometer using CDCl solvent. Textures of smectic mesophase of some homo-
3
logues were recognized either directly from the microscopic observations or by miscibility
method.
Analytical data
Spectra data

HNMR in ppm for Dodecyloxy and Hexyloxy Derivatives
1
HNMR in ppm for the Dodecyloxy Derivative: 1.0 (-CH of C H ), 1.50 (-CH - of C H ),
3
12 25
2
12 25
2
6
.70(-O-CH -CH - of –C H ), 4.04 (-O-CH - of C H ), 4.14 (-COO-CH -Aromatic)
2
2
12 25
2
12 25
2
.25–6.29 (p-substituted phenyl ring), 7.82–7.84 (p-substituted benzene)
1
HNMR in ppm for the Hexyloxy Derivative: 0.93 (-CH of C H ), 1.35 (-CH - of –C H ),
3
6
13
2
6
13
1
.78 (-O-CH -CH - of –C H ), 4.02 (-O-CH - of C H ), 6.92–6.97 (p-substituted phenyl
2
2
6
13
2
6
13
ring), 7.51–7.57 (p-substituted benzene), 7.91–8.13 (p-substituted benzene).
NMR data confirm the structure.
−
IR in cm for Pentyloxy and Decyloxy Derivatives
−1
IR in cm for Pentyloxy Derivative: 844para-substituted phenyl ring, 1096, 1251, 1729 ester
linkage of (COO-C H ),759 Aromatic –Br linkage, 1070 ether linkage of (C H -O-C H ).
5
11
5
11
6
4
−1
IR in cm for Decyloxy Derivative: 844para substituted benzene, 1173, 1258, 1714 ester
linkage of (COO-C H ), 756 Aromatic –Br linkage, 1122ether linkage of (C H -O-C H ).
10
21
10 21
6
4
IR data confirm the structure

1
24
R. B. MARATHE AND A. V. DOSHI
Scheme . Synthetic route to the series.
Mass spectra
Mass spectra for Methoxy Derivative:
Molecular weight: Calculated: 442 Experimental: 445.
.
Texture of smectic phase by miscibility method
Butyloxy(C ) derivative → Smectic—A
4
Octyloxy (C ) derivative → Smectic—A
8
Tetradecyloxy (C ) derivative → Smectic—C
14
Results and discussion
ꢀ
4
-Hydroxy-4 -bromobenzyl benzoate is not a liquid crystal (M.P.115°–122°C, Yield- 70%).
Liquid crystallinity is induced on linking it with dimeric 4-n-alkoxy benzoic acids through
their corresponding acid chlorides. Transition temperatures of final novel ester derivatives

MOL. CRYST. LIQ. CRYST.
125
Table . Elemental analysis for ethoxy, propyloxy, butyloxy, and hexadecyloxy derivatives.
%
Elements calculated (Experimental%)
Molecular
formula
Sr.No.
C
H
Br

C H BrO
. (.)
. (.)
. (.)
. (.)
. (.)
. (.)
. (.)
. (.)
.(.)
. (.)
. (.)
. (.)








C H BrO
 
C H BrO





C H BrO

 
are relatively lower than the corresponding n-alkoxy benzoic acids. Only smectogenic meso-
morphism induced throughout a series i.e. enantiotropic mesomorphism is commences from
very first member of series to last C member of a series. Nematogenic mesophase formation
16
is totally absent for all the homologues. Transition temperatures (Table 2) of novel homo-
logues are plotted versus the number of carbon atoms present in n-alkyl chain of n-alkoxy
terminal end groups. Cr-Sm and Sm-I transition curves are obtained, which shows phase
behavior of the series as shown in phase diagram (Fig. 1). An odd–even effect is observed
for Sm-I transition curve. Cr-Sm transition curve follows a zigzag path of rising and falling
with overall descending tendency and behaved in normal manner. Sm-I transition curves for
even members initially falls upto hexyloxy (C ) homologue and it rises (from C ) and falls
6
1
to C homologue, then both curves are merging into each other at the hexyloxy (C ) deriva-
5
6
tive and then gradually rises through C , C , C homologues and finally maximum at C
8
10
12
14
homologue. Then a single Sm-I transition curve is descended to C homologue after passing
16
through maxima at C homologue derivative. Thus Sm-I transition curve behaved in normal
14
expected manner. The odd–even effect diminishes as series is ascended and transition curves
for odd and even members transformed into a single Sm-I transition curve after merging into
each other from and beyond hexyloxy homologue (C ) derivative. The mesomorphic proper-
6
ties and the degree of mesomorphism vary from homologue to homologue in present novel
series. Thus, present series is entirely smectogenic whose mesophaselength (smectic) ranges
minimum from 3.6°C at (C ) to maximum of 39.3°C at (C ). Smectic thermal stability is 91.2
16
2
and it is middle ordered melting type series with the absence of nematic mesophase forma-
tion.
Linking of dimeric 4-n-alkoxy benzoic acids through their corresponding acid chlorides
ꢀ
with 4-hydroxy-4 -bromobenzylbenzoate which increases the molecular length, length to
breadth ratio, ratio of the polarity to polarizability, permanent dipole moment across the
Table . Transition temperatures of series in °C.
Transition temperature in °C
Compound no.
R = C H
(n)
Sm
Nm
Isotropic
n
n+

C
C
C
C
C
C
C
C
C
C
C
.
.
.
—
—
—
—
—
—
—
—
—
—
—
.
.
.
.
.
.
.
.
.










.
.
.
.
.
.
.
.


.
.
Sm: Smectic
Nm:Nematic

1
26
R. B. MARATHE AND A. V. DOSHI
Figure . Phase behaviour of series.
long molecular axis, dipole-dipole interactions, electronic interactions and dispersion forces
of suitable magnitudes to build up lamellar packing of molecules in crystal lattices to form
layered arrangement in crystalline solid state and then, on exposure of thermal vibrations on
crystalline state, the C to C homologues under individual microscopic observations resisted
1
16
thermal vibrations for same range of temperatures, during which sample homologue acquire
sliding layered arrangement of molecular network in floating condition on the surface, to
show smectic mesophase formation due to end to end and lateral attractions as a consequence
of favorable molecular rigidity and flexibility. The smectogenic mesophase formation disap-
pears from and beyond isotropic temperatures on further heating. The molecules of present
homologue derivatives randomly orient in all possible directions with high order of disor-
der or high entropy (ꢀS = ꢀH/T). The same sample of homologues (C to C ) on cooling
1
16
with appropriate rate from and below isotropic temperature exhibit same textures of smec-
tic mesophase, i.e. smectic mesophase reappears in reversible manner for same temperatures
range which appeared on heating. The lowering of transition temperatures as compared to

MOL. CRYST. LIQ. CRYST.
127
4
-n-alkoxy acids are due to the breaking of hydrogen bonding between two molecules of aro-
matic n-alkoxy acids by the process of esterification through their corresponding acid chlo-
rides. Alternation of transition temperatures and odd-even effect observed upto C homo-
6
logue are due to the sequentially and progressively added methylene unit or units at the n-
alkyl chain bonded to phenyl ring through oxygen atom. Disappearance of odd–even effect
from and beyond C homologue derivatives of longer n-alkyl chain is attributed to the unusual
6
coiling or bending or flexing or coupling of n-alkyl chain with major axis of core structure.
Thus, uncertainity in the status of n-alkyl chain hinders the odd–even effect. The absence of
nematogenic mesophase formation throughout a present series is attributed to the absence
of residual suitable magnitudes of anisotropic forces of end to end intermolecular attractions
after the completion of smectic mesophase. The residual intermolecular end to end attrac-
tions are inadequate or insufficient to maintain statistically the parallel orientational order
of molecules to float on the surface which disallows or restricts nematic mesophase forma-
tion. Variations in mesomorphic behaviours or properties from homologue to homologue in
present series is attributed to the changing number of methylene unit or units at the n-alkyl
chain, keeping rest of the molecular part including right handed terminal or lateral group
intact or unchanged; throughout the same series. Low thermal stability (91.2) is associated
with enthalpy change (ꢀH) occurred by the difference of enthalpies between products and
reactants, for particular homologue derivatives which differ from homologue to homologue
with respect to mesophase stabilization energy and related to respective transition temper-
ature. Thus, suitable magnitudes of anisotropic forces of intermolecular end to end and/or
lateral attractions are sufficient to facilitate, induce, and stabilize smectic phase but hinders
to facilitate and stabilize nematic phase formation against exposed thermal vibrations. The
smectogenic mesophase length 3.6°C to 39.3°C depends upon the magnitudes of thermal
resistivity against exposed thermal vibrations as a consequence of molecular rigidity, flexi-
bility, intermolecular closeness, the extent of disalignment of molecules with the plane of a
floating surface or the extent of noncoplanarity of the molecules. The liquid crystal proper-
ties of present homologous series-1 are compared with the structurally similar known series-X
[
27] and series-Y [28] as shown below in Fig. 2.
Homologous series-1 of present investigation and a structurally similar homologous series-
X, chosen for comparison are identical with respect to three phenyl rings, central bridges –
COO- and –COO-CH and left n-alkoxy –OR, terminal end groups for the same homologue
2
from series-1 to series-X. But, they differ with respect to other terminal end group –Br and
–
H. Homologous series-1 and series-Y are identical with respect to three phenyl rings, two
terminal end groups –OR and –Br, but they (1 and Y) differ with respect to one of the cen-
tral bridges i.e. –COO-CH - (series-1) and –N=N– (series-Y), excluding a common –COO-
2
central bridge linking first and middle phenyl rings. Therefore, variations in mesomorphic
Figure . Structurally similar series.

1
28
R. B. MARATHE AND A. V. DOSHI
Table . Average thermal stability in °C.
Series→

X
Y
Smectic–Nematic
Or
.
.
Smectic– Isotropic
Commencement of
smectic phase
Nematic–Isotropic
Commencement of
Nematic phase
Mesophaselength
Range in °C
(C – C )
—
(C -C )




C
C
.
(C – C )
.
—
(C – C )




C
C
Sm- .
to .
 to 
 to 
properties and the degree of mesomorphism arising among the structurally similar series-1,
X and Y will depend upon varying molecular part either right ended end group or a central
bridge linking middle and third phenyl ring. Following Table 3 represents some LC prop-
erties of series-1, X, and Y which shows differences in thermal stability, commencement of
mesophase or mesophases, mesophase length ranges and odd–even effects etc. depending
upon molecular structures as a result of changing molecular rigidity and/or flexibility which
related to internal energy or enthalpy change values of different homologues of same series or
for the same homologue from series to series.
Homologous series-1 of present investigation and a structurally similar homologous series-
X chosen for comparative study are identical with respect to three phenyl rings and two cen-
tral bridges -COO- and –COO-CH - i.e. aromaticity of series-1 and series-X contributing to
2
the molecular rigidity are almost equal. The left n-alkoxy terminal end groups though, vary
from homologue to homologue for the same series, but remain unaltered for the same homo-
logue from series to series. Therefore, the molecular flexibility due to n-alkoxy terminal end
group for the same homologue from series to series remains unaltered but alters from homo-
logue to homologue in the same series. However, series-1 and series-X differ with respect
to terminal end group –Br and –H at the other end of the molecules contributing to total
molecular flexibility. Thus, magnitudes of combined effects of molecular rigidity and flexibil-
ity governing the tendency of a substance to induce or facilitate and stabilize liquid crystal
property which vary from homologue to homologue in the same series. It also vary for the
same homologue from series to series. Therefore, the mesomorphic (LC) property and the
degree of mesomorphism are depended upon the suitable magnitudes of anisotropic forces
of intermolecular attractions as a consequence of individual molecular rigidity and flexibility
as emerged due to dipole-dipole interactions, permanent dipole moment of molecule, dis-
persion forces, intermolecular closeness etc which vary with molecular structure and induce
variations in mesomorphic behaviour of every substance. Following Table 3; represents some
liquid crystal properties, like thermal stability, commencement of liquid crystal phase, type of
mesophase and the mesophase length range etc. for present novel series-1 and series-X chosen
for comparison purpose.
From the Table 3, it is clear that,
r
Homologous series-1 of present investigation is smectogenic only, without exhibition of
nematic property whereas series-X is nematogenic only without exhibition of smectic
property. Series-Y is smectogenic in addition to nematogenic character.
r
Smectogenic mesophase of series-1 commences from very first member of a series and
that of series-Y, it commences from hexyloxy (C ) homologue, but it does not commence
6
till the last member of a series-X.

MOL. CRYST. LIQ. CRYST.
129
r
Nematic mesophase commences from very first member of a series-Y and from hexyloxy
(
C ) member of a series-X, but it does not commence till the last member of a present
6
series-1.
r
r
r
Thermal stability for smectic of present series-1 is lower than a series-Y.
Nematic thermal stability of series-Y is more than a series-X.
Total mesophase length of present series is higher than a series-X, but it is lower than a
series-Y.
r
Odd–even effect is exhibited by the series-1, X, and Y, under comparative
study.
On comparing thermal stability for smectic in case of series-1 and Y, which is lower for
a series-1(91.2) than series-Y(149.7). Both these series are though structurally similar in all
other respects except the central bridges which are –COO-CH - in case of present series-
2
1
and azo –N=N– in case of series-Y under study are closely similar to each other. Both
these central bridges are comparable. The oxygen atom of the central –COO-CH - group
2
containing carboxy group in the molecule of series-1 will be bumping into the nonbounded
sides of the adjacent hydrogen atoms of the aromatic ring which causes considerable strain on
the molecule. Consequently, a twist around C-O bond occurs, forcing the phenyl ring out of
the plane of a molecule. Thus, the molecular co-planarity is reduced to some extent making
them thick. The central bridge –N=N– certainly endows the molecular coplanarity in Trans
position. Thus, on account of these differences, the smectic–isotropic thermal stabilities of
series-1 is in general way lower than corresponding smectic thermal stability of a homolo-
gous series-Y. As regards to the commencement of smectic phase, it depends upon the extent
of noncoplanarity of molecules. The molecules of series-Y are relatively shorter in length due
to shorter –N=N– central group than –COO-CH - longer central group of series-1. Thus,
2
molecular coplanarity for the same homologue from series-1; to series-Y, varies. Hence, smec-
tic mesophase commences from very first member of present series-1 while it commences
late from C homologue of series-Y. The absence of nematic property in present series-1 and
6
presence of nematic mesophase in series-X and Y is attributed to the difference of suitable
magnitudes of anisotropic forces of intermolecular end to end attractions needed for occur-
rence of statistically parallel orientational order of molecular arrangement, while floating on
the surface, under the influence of identically rated exposed thermal vibrations; for the same
homologue from series to series, which commences earliest from very first (C ) member of a
1
series-Y, from C member of a series-X and it does not commence till the last member (C )
6
16
of present series-1. Alteration of transition temperature in all the series-1, X, and Y under
comparative study which alternates for exhibition of odd-even effect. The mesophase length
ranges depends upon suitable magnitudes of the degree of molecular rigidity and flexibility
which causes mesophase stabilization energy related to differing values of enthalpies (ꢀH)
for the same homologue from series to series, which may resist more or less exposed ther-
mal vibrations depending upon characteristics of molecular structure to facilitate appropriate
molecular arrangement to cause smectic and/or nematic mesophase formation with different
degree of mesomorphism.
Conclusions
r
Presently investigated homologous series is entirely smectogenic without exhibition of
nematic property and of middle-ordered melting type.
r
Mesophase range varies between 3.6°C and 39.3°C and of low thermal stability.

1
30
R. B. MARATHE AND A. V. DOSHI
r
Group efficiency order derived on the basis of (i) Thermal stability (ii) Commencement
of mesophase (iii) Mesophase length range for smectic and/or nematic are as under
(
(
(
i) Smectic:
Nematic:
ii) Smectic
Nematic
iii) Smectic + Nematic
r
Suitable magnitudes of molecular rigidity and flexibility are operating parameter for
exhibition of LC properties.
Phenomena of mesomorphism and the degree of mesomorphism are very sensitive and
susceptible to molecular structure.
Present study is useful for study of mixed mesomorphism and biological activity for agri-
cultural productions and physiological therapy.
r
r
Acknowledgments
Authors acknowledge thanks to the Baldev Prajapati (Rajesh Pharma, Ankleshwar) for providing
research chemicals (4-Bromobenzyl alcohol/bromide) and Green Circle Inc. for providing research
facilities services as and when needed. Authors are also thankful to Dr. N.N. Vyas, Dr. Vipul Patel and Dr.
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 Analytical Instrumentation Facility, Punjab University,
Chandigarh for extending their help for analytical services.
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[
[
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