The effect of molecular rigidity and flexibility on the mesomorphism of azoesters

Article abstract of DOI:10.1080/15421406.2016.1190505

ASBTRACT: An Azoester novel homologous series RO-C₆H₄-COO-C₆H₄-N?N-C₆H₄-OC₄H_9(n) (para) of liquid crystalline (LC) materials are synthesized and studied with a view to understanding and establishing the effects of molecular structure on liquid crystal behavior with reference to lateral, terminal, or central group or groups on the basis of molecular rigidity and/or flexibility. The novel homologous series consists of thirteen (C₁ to C₁₈) homologues. C₁ to C₅ homologues are nonmesogenic and the rest of the homologues are enantiotropically mesogenic. C₈ to C₁₈ homologues are smectogenic plus nematogenic and the remaining two C₆ and C₇ homologues are only nematogenic without exhibition of smectic property. Analytical, thermal and spectral data confirms the molecular structures. Textures and transition temperatures of homologues were determined by an optical polarizing microscope (POM) equipped with a heating stage. Textures of a nematic phases are threaded or Schlieren and that of a smectic phase are of the type of A or C. The average thermal stabilities for smectic and nematic are 111.3°C and 124.7°C, respectively. The smectic and nematicmesophase lengths from 10.0°C to 28.0°C and 7.0°C to 16.0°C, respectively. The transition temperatures are compared with known series.

Full text of DOI:10.1080/15421406.2016.1190505

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
The effect of molecular rigidity and flexibility on
the mesomorphism of azoesters
U. H. Jadeja & R. B. Patel
To cite this article: U. H. Jadeja & R. B. Patel (2016) The effect of molecular rigidity and
flexibility on the mesomorphism of azoesters, Molecular Crystals and Liquid Crystals, 637:1,
28-36, DOI: 10.1080/15421406.2016.1190505
To link to this article: http://dx.doi.org/10.1080/15421406.2016.1190505
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Date: 11 November 2016, At: 00:25

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –

http://dx.doi.org/./..
The effect of molecular rigidity and flexibility on the
mesomorphism of azoesters
U. H. Jadeja and R. B. Patel
Chemistry Department, K. K Shah Jarodwala Maninagar Science College, Ahmedabad
ASBTRACT
KEYWORDS
An Azoester novel homologous series RO-C H -COO-C H -N=N-C H -
Azoester; liquid crystal;
smectic; nematic; monotropy
6
4
6
4
6
4
OC H (para) of liquid crystalline (LC) materials are synthesized and
4
9(n)
studied with a view to understanding and establishing the effects of
molecular structure on liquid crystal behavior with reference to lateral,
terminal, or central group or groups on the basis of molecular rigidity
and/or flexibility. The novel homologous series consists of thirteen (C to
1
C ) homologues. C to C homologues are nonmesogenic and the rest of
1
8
1
5
the homologues are enantiotropically mesogenic. C to C homologues
8
18
are smectogenic plus nematogenic and the remaining two C and C7
6
homologues are only nematogenic without exhibition of smectic prop-
erty. Analytical, thermal and spectral data confirms the molecular struc-
tures. Textures and transition temperatures of homologues were deter-
mined by an optical polarizing microscope (POM) equipped with a heat-
ing stage. Textures of a nematic phases are threaded or Schlieren and
that of a smectic phase are of the type of A or C. The average thermal
stabilities for smectic and nematic are 111.3°C and 124.7°C, respectively.
The smectic and nematicmesophase lengths from 10.0°C to 28.0°C and
7
.0°C to 16.0°C, respectively. The transition temperatures are compared
with known series.
Introduction
Azoester LC materials [1] are useful for thermographic and LC devices, optical imaging, light
emitting diods, semiconductor materials and equally important as dye materials [2–6]. More-
over the central ester group is biologically active, which is useful for mankind [7–10]. The aim
is to understand and establish the relation between mesomorphism and molecular structure
[
11–14] on the basis of molecular rigidity and flexibility [15–18] by changing single phenyl
or nepthyl ring as well as fixed tail end group and varying left –OR group for the same series
but differ tailed group from series to series for the same homologue. Thus, present investiga-
tion is planed with a view to synthesize and then to characterize, through analytical, spectral
and thermotropic data. The evaluated data will be discussed and interprited with reference to
molecular rigidity and flexibility. Number of ester or azoester series are reported till the date
[19–24].
CONTACT U. H. Jadeja
upenjadeja@gmail.com
Gujarat University, Ahmedabad, Gujarat, India.
©
 Taylor & Francis Group, LLC

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
29
Scheme . Synthetic route of Series.
Experimental
Synthesis
4
-Hydroxy benzoic acid was alkylated using the appropriate alkylating agent(R-X) to convert
it into 4-n-alkoxy benzoic acids(A) by a modified method of Dave and Vora [25]. Alkyla-
tion of paracetamol using alkylating agent n-C H Br was carried out to form 4-n-butyloxy
4
9
acetanilide, which on hydrolysis was converted to 4-n-butyloxy aniline by a usual establish
ꢀ
method. Azodye (B) 4-hydroxy phenyl azo 4 -butyloxy benzene (m.p. 126°C, yield 73%) was
prepared by well-known azotization method [26], Final azoester products were synthesized
by condensation of (A) and (B) [27]. Thus, the azo-ester homologue derivatives were filtered,
washed with sodium bicarbonate solution followed by distilled water, dried and purified until
constant transition temperatures obtained, using an optical polarizing microscope equipped
with a heating stage. 4-Hydroxy benzoic acid, alkyl halides, paracetamol, phenol, NaNO2,
dicyclohexylcarbodimide (DCC), dimethyl amino pyridine (DMAP), K CO , DCM, MeOH,
2
3
acetone required for synthesis were used as received except solvents which were dried and
distilled prior to use. The synthetic route to the series is shown in Scheme 1.

3
0
U. H. JADEJA AND R. B. PATEL
Table . Elemental analysis for propyloxy, hexyloxy, decyloxy, dodecyloxyderivatives.
%
Elements found
%Elements calculated
H
Sr. No.
Molecular formula
C H O N
C
H
N
C
N

.%
.%
.%
.%
.%
.%
.%
.%
.%
.%
.%
.%
.%
.%
.%
.%
.%
.%
.%
.%
.%
.%
.%
.%










C H O N

  
C H O N

  
C HON


Characterization
Representative homologues of the series were characterized by elemental analysis, infrared
1
1
spectroscopy, H NMR spectra. IR spectra were recorded ona Perkin-Elmer spectrum GX, H-
NMR spectra were recorded on a Bruker using CDCl as solvent. Microanalysis was per-
3
formed on a Perkin-Elmer PE 2400 CHN analyzer (Table 1). Transition temperatures and
LC properties were determined using an optical polarizing microscope equipped with a heat-
ing stage (POM). Texture images of nematic phase were determined by miscibility method
(Table 2).
Analytical data
−
IR spectra (KBr) in cm for hexyloxy, hexadecyloxy derivatives
Hexyloxy: 761 Poly methylene (–CH -)n of -OC H , 885(-C-H- def. di-substituted-Para),
2
6
13
6
90 Polymethylene (–CH -) of -OC H , 989 (-C-H- def. hydrocarbon), 1060 and 1105 (-C-
2 6 13
O-) Str, 1296 and 1321 and 1423, 1494 (-C-O str in –(CH )n chain), 1579 (-C-H- def. in CH ),
2
2
1
390 (-N=N-)str, 1641 (-C = O group), 1735 (-COO- ester group), 2870 and 3072 and (-C-H
str in CH ). IR confirms the molecular structure.
3
Hexadecyloxy: 759 and796 Polymethylene (–CH -)n of -OC H , 883(-C-H- def. m di-
2
16 33
substituted-Para), 989 (-C-H- def. hydrocarbon), 1008, 1056, (-C-O-) Str, of -OC H ,1276
5
11
and 1392 and 1373, 1246(-C-O str in –(CH )n chain, 1496(-C-H- def. in CH ),1512 and 1392
2
2
(
-N=N-)str, 1598 (-C = O group), 1737 (-COO- ester group), 2733 and 2916 and 3084 (-C-H
str in CH ).IR confirms the molecular structure.
3

HNMR spectra in CDCl in δ ppm for tetradecyloxy & octyloxy derivative

Tetradecyloxy: 0.89(t, –CH –CH , 4H), 1.26 (m, n-poly methylene groups of -OC H ),
2
3
14 29
1
,
.31–1.76(q, –CH –CH , 6H), 1.76(m, 8H of poly methylene group),3.98(t,-OCH –CH -
4H), 7.19 and 7.83(s, Ar-H, p-disubstituted),8.07–8.19(s, ArH, p-di substituted phenyl ring).
2
3
2
2
NMR confirms the molecular structure.
Octyloxy: 0.88–0.89(t, –CH –CH , 4H, of -C H ), 1.29(m, n-poly methylene groups
2
3
8
17
of -OC H ), 1.76(m, 8H of polymethylene), 4.06 (t, 4H, -OCH –CH -of -OC H and -
8
17
2
2
6
13
OC H ), 7.18–7.57 (s, Ar-H, substituted benzene), 8.07–8.11 (s, Ar-H, p-substituted phenyl
8
17
ring).NMR confirms the molecular structure.
Table . Texture of nematic and smectic phase of C , C , C , C by miscibility method.


 
Sr. No.
Homologue
Texture

C
C
C
C
Threaded Nematic
Smectic-A
Schlieren Nematic
Smectic-C




MOLECULAR CRYSTALS AND LIQUID CRYSTALS
31
Table . Transition temperatures in°C for series-.

-n-Alkoxy benzoyloxy phenylazo-"-butoxy benzene.
Transition Temperatures in(˚C)
Compound No
n-alkyl chain C H
Smectic
Nematic
Isotropic
n
n+

C
C
C
C
C
C
C
C
C
C
C
C
C
—
—
—
—
—
—
—
.
.
.
.
.
.
—
—
—
—
—
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
















Results and discussion
Novel azoester homologous series of LC substances is synthesized by extending molecular
length by linking dimeric 4-n-alkoxy benzoic acids and an azodye (m.p.: 126°C, yield 73%)
with its nonmesomorphicity which yielded C to C homologues as mesogenic and the rest
6
18
of the homologues as non-mesogenic. Mesogenic homologues are smectogenic plus nemato-
genic except C to C which are only nematogenic without exhibition of smectic property.
6
7
Thus, mesomorphism commences from C homologue and their transition temperatures are
6
lowered as compared to corresponding dimerisedn-alkoxy benzoic acids on esterification pro-
cess. Transition temperatures (Table 3) as determined from POM were plotted versus the
number of carbon atoms present in n-alkyl chain “R” of –OR group, and then on linking
like or related points, the Cr-M / I, Sm-N and N-I transition curves are obtained as shown in
a phase diagram (Fig. 1); Cr-M / I transition curve follows a zigzag path of rising and falling
with overall descending tendency and behaved in normal manner. Sm-N transition curve
rises, passes through maxima at C homologue of a series and then descended with absence
12
Table . Relative thermal stabilities in°C.
Series→
Series-
Series-X
.
Series-Y
.
Sm-N or Sm-I
Commencement of smectic phase
.
(C -C ).
(C -C )
(C -C )


 
 
C
C
C
N-I
.
(C -C )
.
(C -C )
.
(C -C )
Commencement of nematic phase






C
C
C
Total (Sm+N) mesophase lengths (from upper to lower)
. to .
C C
. to .
C C
. to .
C C
Above table- indicate that,
•
•
All the homologous series-, X and Y under comparative study are smectogenic plus nematogenic.
Thecommencementofsmecticphaseisindecreasingorderofearlycommencementofmesophase.i.e.,earliestforseries-
and the latest for series-Y.

•
•
Smectic, nematic thermal stabilities and lower mesophase lengths are in increasing order of their sequential order of
series under comparison.
Upper mesophase lengths is the highest for series-Y and the lowest for a series- among the series-, X and Y.

3
2
U. H. JADEJA AND R. B. PATEL
Figure . Phase behaviors of Series.
of exhibiting odd–even effect. Thus it behaved in normal established manner. N-I transition
curve initially ascended and then descended with negligible deviation at C homologue with
10
exhibition of odd–even effect from C to C members of a series. Thus, it behaved in normal
6
9
manner. N-I transition curves for odd and even homologues are merging into each other at
C and then odd–even effect disappears for higher homologues of longer n-alkyl chain “R”
9
of –OR group. N-I transition curve for odd members occupy lower position as compared
to even members of a series. Textures of a nematic phase are threaded or schlieren and that
of a smectic phase are of the type A or C. Thermal stabilities for smectic and nematic are
1
11.3°C and 124.7°C, whose total mesophase length (Sm+N) vary from 7.0 to 40.0 at the C6
and C14 homologues respectively. The mesogenic properties are varied from homologue to
homologue in the same series. Thus, it is predominantly nematogenic and partly smectogenic
series of middle ordered melting type series. Sm-N and N-I transition curves are extrapolated
to C and C as well as C homologues for predicting and determining [28–31] their latent
6
4
5
ability to exhibit smectic or nematicmesophase formation keeping in mind the trend of the
transition curves, respectively.
The disappearance of dimerization of n- alkoxy benzoic acids is attributed to the breaking
of the hydrogen bonding between the two molecules of aromatic acids by way of esterification
process. The exhibition of smectic and/or nematic phase are attributed to extended molecular
length which increases length to breath ratio, ratio of the polarity to polarizability, permanent
dipole moment across the long molecular axis, dipole–dipole interactions, intermolecular
dispersion forces and suitable magnitudes of anisotropic forces of cohesions and closeness

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
33
as a consequence of favorable molecular rigidity and / or flexibility, which causes the dis-
alignment of molecules at an angle perpendicular to the plane of a floating surface and then
at the less than ninety degree with the plane of the floating surface, resisting exposed thermal
vibrations containing lamellar packing of molecules in their crystal lattices (C -C ) to float
8
18
with sliding layered molecular arrangement in addition to maintaining statistically parallel
orientational order of molecules as facilitated by C to C homologues and maintaining only
8
18
statistically parallel orientational order of molecules like C and C homologues to induce
6
7
either only nematic phase orsmectic plus nematicmesophase or, only nematic phase for
definite ranges or range of temperatures. The exhibition of odd–even effect by N-I transition
curve is due to the expected status of n-alkyl chain ‘R’ of –OR in which –CH - group is
2
added in sequential manner. Absence of odd- even effect for Sm-N transition curve is due
to the presence of smectogenic character in only even members of homologues C , C , C ,
8
10
12
C , C , and C18 or due to the absence of odd members. The disappearance of odd-even
14
16
effect for higher homologues from and beyond C homologue, of longer n-alkyl chain for
9
N-I transition curve, which, may coil or bend or flex or couple to lie with major axis of
core structure and adopt unexpected status of n-alkyl chain and modify the magnitudes of
intermolecular cohesions, rigidity, flexibility or parameters related to exhibition of meso-
morphism. Thus, uncertainity in the status of n-alkyl chain ‘R ’, the intermolecular cohesive
forces for odd and even members of homologues beyond merging of transition curves do not
alternate. The variations in mesogenic properties from homologue to homologue in the same
novel series is attributed to the sequentially and progressively added methylene unit or units
to the n-alkyl chain ‘R’ which alters the molecular flexibility and the suitable magnitudes
of permanent dipole moment and intermolecular dispersion forces, responsible to induce
mesomorphism. The absence of mesomorphic property in case of C to C homologues
1
5
is attributed to their low magnitudes of dipole-dipole interactions and low magnitudes of
dispersion forces which induces low thermal resistivity towards exposed thermal vibrations.
Therefore, they possess inability to resist thermal break down of their crystal lattices and
sharply transform into isotropic liquid from crystalline solid state without passing through
LC state and adopt high crystallizing tendency on cooling the same. All the homologues of a
series further heating from and beyond isotropic temperature, their molecules are randomly
oriented in all possible directions with high order of disorder or randomness or high entropy
(
ꢀS = ꢀH/T) or with uncontrolled irregular motion. But on cooling the same with proper
rate of cooling, the mesogenic homologues reversibly exhibited (or mesophase reappeared)
their respective type of mesophase or mesophases at the same temperature at which the
mesophases were appear on heating (C to C ). Moreover under identical condition, the
6
18
none of the non-mesogenic (C to C ) homologues did exhibited any sort of mesomorphism
1
5
in irreversible manner in monotropic condition. Thus, all transition temperatures of meso-
genic homologues are enantiotropic transitions. Therefore N-I and Sm-N transition curves
extrapolated to C , C and C to predict their latent transition temperature (LTT) which
4
5
6
are hypothetical and not real or they are unrealizable due to their respective position, much
below to their respective isotropic temperature. Thus, no sooner homologues C , C and C
4
5
6
reach to their targeted mesophase formation; than prior to that, sample homologue either
undergo or acquire crystallization state or matches with isotropic state or acquire untargeted
mesophase formation (C ). The mesogenic behaviors of present novel series-1 are compared
6
with the structurally similar analogous series X [32] and Y[33] as mentioned below in
Fig. 2.

3
4
U. H. JADEJA AND R. B. PATEL
Figure . Structurally similar analogous series.
Homologous series-1 of present investigation and a series-Y chosen for comparison are
identical with respect to three phenyl rings and two central bridges -COO- and –N=N- con-
tributing to the total molecular rigidity. Moreover left n-alkoxy (-OR) terminal end groups
are identically same, contributing partly to the molecular flexibility for the same homologue
from series to series, but differs with respect to the tailed end groups -OC H and -OCH3,
4
9(n)
which contributes partly to the total molecular flexibility. Homologous series-1 and series-X
are identical with respect to two central bridges contributing partly to the molecular rigid-
ity but are identically same for the same terminal groups –OR and -OC H
for the same
4
9(n)
homologue from series to series, contributing total molecular flexibility. However, series-1
and X differ with respect to total molecular rigidity due to presence of middle phenyl ring
(
series-1) and a middle nepthyl ring (series-X) affecting molecular rigidity. Thus, a homolo-
gous series-X differs from series-1 with respect to identical molecular flexibility, but differing
with respect to total molecular rigidity for the same homologue and vice versa for the series-
1
and Y, i.e. series-1 and Y are identical with respect to molecular rigidity but, differing with
respect to molecular flexibility for the same homologue. Thus, difference in mesogenic behav-
iors, properties and the degree of mesomorphism etc. depended upon the differing features
among the analogous series-1, X and Y for the same homologue. Following table-4 represents
some thermometric properties of series-1, X and Y in comparative manner as under.
Homologous series-1, X and Y are bearing high polar n-alkoxy tailed ends -OC H or
4
9(n)
-
OCH and left –OR groups, whose combined effects of molecular rigidity and flexibility are
3
suitable to induce only nematic or smectic plus nematicmesophase or mesophases. Presence
of -OCH tailed end group being highly polar and polarizable as compared to tail ended -
3
OC H group which maintain end to end attractions strongly and intermolecular closeness
4
9(n)
which is more favorable of series-Y than series-1 and X. Moreover, the extent of molecular non
co-planarity is less favored by –N=N- central bridge in trans condition which causes varia-
tions in the magnitudes of intermolecular cohesions, molecular polarity and polarizability or
molecular rigidity and flexibility, resulting into early or late commencement of mesophase,
thermal resistivity towards exposed thermal vibrations and the magnitudes of lamellar pack-
ing of molecules in their crystal lattices. Thus, the variation and stabilization of a mesophase
or mesophases are facilitated. Now as regards to the increasing order observed for smectic and
nematic thermal stabilities which are related with the energy stored (ꢀH) in a molecule for
the same homologue from series to series which varied with the flexibility of shorter (-OCH3)
and longer (-OC H or -OCH –CH –CH –CH ) tailed ended groups; differing into ther-
4
9(n)
2
2
2
3
mal resistivity or acceptability of thermal vibrations as related to maintenance of magnitudes
of ordered molecular arrangement under floating condition on the surface. Thus, magnitudes
of stabilization of mesophase or mesophases or degree of mesomorphism, commencement of

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
35
nematic phase, and thermal stabilities of series-Y are highest and of series-1 is the lowest or
lower.
Conclusions
Novel azoester series of LC substances is predominantly nematogenic and partly smectogenic,
whose total mesophase lengths vary from 7.0 to 40°C and is of the middle ordered melting
type.
The group efficiency order derived for smectic and nematic on the basis of (a) thermal
stabilities (b) early commencement of mesophase and (c) upper mesophase lengths are as
under.
a) Smectic
Series − Y > Series − X > Series − 1
Nematic
Series − Y > Series − X > Series − 1
b) Smectic
Series − 1 > Series − X > Series − Y
Nematic
Series − Y > Series − X = Series − Y
c) Total mesophase lengths(Sm = +N)
Upper: Series − Y > Series − 1 > Series − X
Lower: Series − Y > Series − X > Sereis − 1
Phenomena of mesomorphism is very sensitive and susceptible to a molecular struc-
ture of a substance.
LC materials of present investigation may be useful for optical imaging and as useful
as other dying material.
Present investigation supports and raises credibility to the conclusions drawn earlier.
r
r
r
Acknowledgments
Authors acknowledge thanks to Dr. R.R. Shah, principal and management of K. K. Shah Jarodwala
Maninagar Science College, Ahmedabad. Authors are also thankful to Dr. A.V. Doshi, Ex-principal of
M.V.M. Science and Home Sci. College-Rajkot for his constant support, inspirations help, and valuable
comments and suggestions, as and when needed during the course of present investigation. Authors
also thank to NFDD Centre for the spectral services.
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[
[
[
[
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