Mesomorphism dependence on the combined effect of molecular rigidity and flexibility

Article abstract of DOI:10.1080/15421406.2016.1217704

A novel liquid crystalline (LC) homologous series of azoesters with a laterally substituted methoxy group RO-C₆H₄-COO-C₆H₃-(-OCH₃)-N═N-C₆H₄-COO-C₄H_9(n) has been synthesized and studied with a view to understanding the effect of molecular structure on thermotropic mesomorphism. The novel homologous series consists of thirteen homologues (C₁–C₁₈). The C₁–C₅ homologues are nonliquid crystals. The C₆ and C₇ homologues are only enantiotropically nematogenic and the rest of the mesomorphic homologues (C₈–C₁₈) are enantiotropically smectogenic and nematogenic. Transition temperatures and the textures of the mesophases were determined using an optical polarizing microscope (POM) equipped with a heating stage. The novel azoester homologues were characterised and confirmed using their analytical, spectral and thermometric data. Transition curves Cr-M/I, Sm-N and N-I behaved in normal manner without (Sm-N) and with (N-I) exhibition of odd-even effect respectively in a phase diagram. Thermal stabilities for smectic and nematic are 100.0°C and 127.7°C whose, mesomorphic phase length vary from 13.0°C to 24.0°C and 11.0°C to 33.0°C, respectively. The mesomorpism is compared with other known series.

Full text of DOI:10.1080/15421406.2016.1217704

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Mesomorphism dependence on the combined
effect of molecular rigidity and flexibility
U. H. Jadeja & R. B. Patel
To cite this article: U. H. Jadeja & R. B. Patel (2016) Mesomorphism dependence on the
combined effect of molecular rigidity and flexibility, Molecular Crystals and Liquid Crystals,
638:1, 17-26, DOI: 10.1080/15421406.2016.1217704
To link to this article: http://dx.doi.org/10.1080/15421406.2016.1217704
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Date: 16 November 2016, At: 00:11

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –

http://dx.doi.org/./..
Mesomorphism dependence on the combined effect
of molecular rigidity and flexibility
U. H. Jadeja and R. B. Patel
Chemistry Department, K. K Shah Jarodwala Maninagar Science College, Gujarat University, Ahmedabad,
Gujarat, India
ABSTRACT
KEYWORDS
A novel liquid crystalline (LC) homologous series of azoesters with a
laterally substituted methoxy group RO-C H -COO-C H -(-OCH )-N═N-
Azoester; enantiotropy;
liquid crystals; smectic;
nematic
6
4
6
3
3
C H -COO-C H has been synthesized and studied with a view to
6
4
4
9(n)
understanding the effect of molecular structure on thermotropic meso-
morphism. The novel homologous series consists of thirteen homo-
logues (C –C ). The C –C homologues are nonliquid crystals. The C
1
18
1
5
6
and C homologues are only enantiotropically nematogenic and the rest
7
of the mesomorphic homologues (C –C ) are enantiotropically smecto-
8
18
genic and nematogenic. Transition temperatures and the textures of the
mesophases were determined using an optical polarizing microscope
(
POM) equipped with a heating stage. The novel azoester homologues
were characterised and confirmed using their analytical, spectral and
thermometric data. Transition curves Cr-M/I, Sm-N and N-I behaved in
normal manner without (Sm-N) and with (N-I) exhibition of odd-even
effect respectively in a phase diagram. Thermal stabilities for smectic
and nematic are 100.0°C and 127.7°C whose, mesomorphic phase length
vary from 13.0°C to 24.0°C and 11.0°C to 33.0°C, respectively. The meso-
morpism is compared with other known series.
Introduction
The technological importance of the liquid crystalline (LC) [1] state and its utility have
attracted scientists from many disciplines [2–9]. The ester central group is biologically active
as antibacterial and antifungal and useful in the growth of agricultural plants like flowers and
fruit forming plants. Also they are used in light emitting and to manufacture display devices
to be operated at desired temperature. Therefore, the present investigation is planned with
a view to studying, understanding and establishing the relation between LC properties and
the molecular structure of substances by synthesizing novel LC azoesters [10–14]. The target
homologous series consist of three phenyl rings and two central bridges, viz., ─COO─ and
─
N═N─, left ─OR group and tailed ended ─COOC H group, whose LC behaviors will be
4 9(n)
interpreted and discussed on the basis of molecular rigidity and flexibility [15–18] after due
characterization through thermal analytical and spectral data. Group efficiency order will be
derived from the comparative study of present homologous series with structurally similar
analogous series. Several homologous series with ester group are reported to date [19–25].
CONTACT R. B. Patel
College, Ahmedabad-, Gujarat University, Ahmedabad, Gujarat, India.
 Taylor & Francis Group, LLC
roshanpatel@gmail.com
Chemistry Department, K. K. Shah Jarodwala Maninagar Science
©

1
8
U. H. JADEJA AND R. B. PATEL
Experimental
Synthesis
4
4
-Hydroxy benzoic acid was alkylated using suitable alkylating agent (R-X) to convert it into
-n-alkoxy benzoic acids (A) by a modified method of Dave and Vora [26]. 4- amino butyl
benzoate ester (B) was prepared by a usual establish method [27]. Azo dye (C) 4-hydroxy 3-
ꢀ
methoxy phenyl azo 4 -butyl benzoates (m.p. 121°C, yield 74%) was prepared by a well-known
azotization method [28], Final azoester [D] products were synthesized by condensation of (A)
and (C) [29]. 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 polarising microscope equipped with a heating stage.
4
-Hydroxy benzoic acid, Alkyl halides, O-Cresol, NaNO , SOCl , 1-Butanol, MeOH, required
2 2
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.
Characterization
Some selected representative homologues of the series were characterized by infrared [IR],
1
HNMR technique and elemental analysis. IR spectra were recorded on Perkin Elmer spec-
1
trum GX. HNMR spectra were recorded on brucker using CDCl as solvent. Elemental anal-
3
ysis was performed on Perkin Elmer PE 2400 CHN analyzer (Table 1). Liquid Crystal proper-
ties, i.e., transition and melting temperatures of homologues were investigated by an optical
polarizing microscopy equipped with heating stage. Textures of the novel homologues were
determined by miscibility method. Thermodynamic quantities enthalpy (ꢀH) and entropy
(ꢀS) are qualitatively discussed.
Analytical data
IR Spectra (KBr) in cm  for Heptyloxy, Hexadecyloxy Derivatives
−
Hexyloxy: 760 Poly methylene (─CH ─)n of ─OC H , 883(─C-H- def. di-substituted-
2
7
15
Para), 690 Polymethylene (─CH ─) of ─OC H , 989 (─C─H─ def. hydrocarbon), 1060
2
7
15
and 1105(─C─O─) Str, 1296 and 1321 and 1423, 1494 (─C─O str in ─ (CH )n chain), 1579
2
(
2
─C─H─ def. in CH ), 1390 (─N═N─)str, 1641 (─C═O group), 1735 (-COO- ester group),
2
872 and 3272 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.
2
16 33
m di-substituted-Para), 989 (─C─H─ def. hydrocarbon), 1008, 1056, (─C─O─) Str, of
C H ,1276 and 1392 and 1373, 1246(─C─O str in ─(CH )n chain, 1496(─C─H─ def. in
─
4
9
2
CH ),1512 and 1392 (─N═N─)str, 1660 (─C═O group), 1737 (─COO─ ester group), 2733
2
and 2930 and 3084 (-C-H str in CH ). IR confirms the molecular structure.
3

H NMR spectra in CDCl in δ ppm for Tetradecyloxy and Octyloxy Derivative

Tetradecyloxy: 0.88 (t, ─OC H , 3H, of ─C H ), 0.93 (t, 3H, ─C H ), 1.45 (p, of ─C H ),
14
29
14 29
4
9
4
9
1
─
.80 (p, of polymethylene ─C H ), 4.06 (t, 4H, ─OCH ─CH ─ of ─OC H ), 1.43 (p,
OC H ), 3.83 (s, 3H, ─OCH ), 8.25-8.11 (s, Ar-H, substituted first and third phenyl ring),
14 29 3
4 9 2 2 14 29
7
.63, 7.62–7.45 (s, Ar-H, p-substituted second phenyl ring), 7.15 (S, Ar-H, para substituted
phenyl ring), NMR confirms the molecular structure.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
19
Scheme . Synthetic route to the series.
Table . Elemental analysis for C , C , C , C , C .



 
%
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


C H O N
   

20
U. H. JADEJA AND R. B. PATEL
Table . Texture of nematic phase of C , C , C , C by miscibility method.

  
Sr. no.
Homologue
Texture

C
C
C
C
Threaded Nematic
Schlieren nematic
Schlieren Nematic
Smectic-C



Octyloxy: 0.88 (t, ─OC H , 3H, of ─C H ), 0.93 (t, 3H, ─C H ), 1.45 (p, of ─C H ), 1.80
8
17
8
17
4
9
4
9
(
p, of polymethylene ─C H ), 4.06 (t, 4H, ─OCH ─CH ─ of ─OC H ), 1.43 (p, ─OC H ),
4 9 2 2 8 17 8 17
3
7
.83 (s, 3H, ─OCH ), 8.25–8.11 (s, Ar-H, substituted first and third phenyl ring), 7.63, 7.61-
.46 (s, Ar-H, p-substituted second phenyl ring), 7.14 (S, Ar-H, para substituted phenyl ring),
3
NMR confirms the molecular structure.
ꢀ
ꢀꢀ
Homologous Series: 4-(4 -n-alkoxy benzoyloxy)-3-methoxyphenyl azo -4 -Butyl
benzoates
Homologous Series: 4-(4 -n-alkoxy benzoyloxy)-3-methoxyphenyl azo -4 -Butyl
benzoates.
ꢀ
ꢀꢀ
Results and discussion
A novel liquid crystalline azoester homologous series is formed by condensing dimeric n-
alkoxy benzoic acids and non-mesomorphic component azo dye 4-hydroxy 3-methoxy phenyl
ꢀ
azo-4 -n-butyl benzoate (M. P.: 121.0°C, yield: 74%). The transition temperatures of novel
homologues are relatively lower than the corresponding dimeric n-alkoxy acids. Mesomor-
phism commences from C homologue as nematic and from C homologue as smectic until
6
8
C member of a series in enantiotropic manner. The rest of the C –C homologues are non-
18
1
5
mesomorphic. Transition temperatures as determined from POM are plotted versus the num-
ber of carbon atoms present in n-alkyl chain ‘R’ of –OR group, and on linking the like or
related points, the Cr-N/I, Sm-N and N-I transition curves are obtained showing their phase
behaviours in a phase diagram (Fig.-1). A Cr-M/I transition curve follows a zigzag path of
rising and falling with overall descending tendency and behaved in normal manner. A Sm-N
transition curve commences from its maxima point and then descended until the last mem-
ber of a series in usual manner with absence of odd-even effect. N-I transition curve initially
Table . Transition temperatures in°C.
Compound no.
Homologue (n-alkyl chain)
Transition temperatures in°C
Smectic
—
—
—
—
—
—
—
.
.
.
.
.
.
Nematic
—
—
—
—
Isotropic
.
.
.
.
.
.
.
.
.
.
.
.

C
C
C
C
C
C
C
C
C
C
C
C
C






—
.
.
.
.
.
.
.
.










.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
21
descends from C to C and then ascended to C and then continuously descended in nor-
6
8
10
mal established manner with exhibition of short and narrow odd even effect. Thus, transition
curves of a phase diagram behaved in normal manner with negligible deviation at C and C
8
18
member of a novel series. N-I transition curves for odd and even homologues are extrapolated
30–33] to C and C homologue to intensify odd-even effect and to determine their latent
[
4
3
ability for nematic and the corresponding latent transition temperatures (LTT) which merges
into their (C and C ) isotropic temperatures, eliminating the possibilities of mesophase for-
4
3
mation. Similarly Sm-N transition curve is extrapolated to C homologue which predicts
6
its LTT for smectic as121.5°C, as monotropic transition temperature. Odd-even effect for
nematic merges into each other at C homologue and then prolonged up to C homologue as
8
18
a single transition curve for higher homologues of longer n-alkyl chain ‘R’ of –OR. Thermal
stability for smectic and nematic are 100.0 and 127.7 with their mesophase lengths ranging
from 13.0°C to 24.0°C and 11.0°C to 33.0°C, respectively. (Sm+N) mesophase length ranges
from 12.0°C to 48°C at the C and C homologues. Textures of nematic phase are threaded or
6
18
Schlieren and that of a focal conic fan shaped of type A or C. Spectral, analytical and thermal
data supported the molecular structures of homologues. The changing trend in mesomorphic
properties and the degrees of mesomorphism etc. from homologue to homologue in the same
present novel series undergoes variation with changing number of carbon atoms present in
n-alkyl chain ‘R’ of –OR group at left terminal end, keeping the rest of the molecular part
unchanged throughout a series.
Lowering of transition temperatures of present novel series as compared to corresponding
dimeric n-alkoxy benzoic acids are attributed to the breaking of hydrogen bonding between
aromatic acid molecules through esterification process. The lack of LC properties by the C1–
C homologues is due to the low magnitudes of dispersion forces and the low magnitudes
5
of dipole–dipole interactions which induces high crystallizing tendency in the molecules of
nonmesomorphic homologues. Thus, the molecules of C to C members of a series sharply
1
5
transform into isotropic state without passing through LC state and from isotropic state to
crystalline solid state excluding monotropic mesophase formation as a consequence of unfa-
vorable and unsuitable magnitudes of molecular rigidity and flexibility. The facilitation of
smectic and then nematogenic or only nematic mesophase formation from C or C homo-
8
6
logue members of a series is attributed to the disalignment of molecules perpendicular to the
plane of floating surface or/and at an angle less than ninety degree to the plane of a float-
ing surface, which arranges the floating of the molecules of mesogenic homologues, resisting
exposed thermal vibrations; either in sliding layered molecular ordered organization and then
with statistically parallel orientational ordered molecular organization (C –C ) or with only
8
18
statistically parallel orientational ordered molecular organization as depending upon magni-
tudes of intermolecular cohesion and closeness to facilitate either smectic plus nematic phase
(
C to C ) or only nematic phase formation for different ranges of liquid crystallinity. Thus,
8 18
suitable magnitudes of anisotropic forces of intermolecular attractions as a consequence favor-
able molecular rigidity and flexibility, through suitable magnitudes of dispersion forces and
dipole–dipole interactions facilitate or induce to cause mesophase formation. The exhibition
of odd-even effect by N-I transition curves by C , C , and C homologues is due to the C
6
7
8
7
odd and C plus C even numbered homologues, whose number of carbon atoms are sequen-
6
8
tially added in n-alkyl chain ‘R’ of –OR group. However, absence of odd-even effect in Sm-N
transition curve of a phase diagram is attributed to the presence of none of the odd meso-
genic member from the group of smectogenic homologues or alternatively one can say that, all
the smectogenic homologues are even numbered. Disappearance of odd-even effect of higher
homologues of longer n-alkyl chain from and beyond C is attributed to the possibilities of the
8

2
2
U. H. JADEJA AND R. B. PATEL
coiling or bending or flexing or coupling of n-alkyl chain ‘R’ with the major axis of core struc-
ture which may cause uncertainty in the status of longer n-alkyl chains and modifies magni-
tudes of molecular rigidity and flexibility as well as predominancy of intermolecular cohesions
operating oppositely at a time depending upon decreasing tendency of intermolecular dis-
tance and presence of highly polar –OCH lateral group substitution together, which increases
3
molecular polarizability and consequently increasing intermolecular attractions. Thus, the
net intermolecular cohesions, molecular rigidity and flexibility, exhibition of either smectic
or/and nematic mesophase and the degree of mesomorphism are the results of Net effects of
the status of n-alkyl chain ‘R’ of –OR under the influence of exposed thermal vibrations. The
extrapolations of N-I transition curves for C and C eliminate the possibilities of monotropic
4
3
nematic phase because, extrapolated curve matches with isotropic temperatures of C and C3
4
homologues but the extrapolation of Sm-N transition curve predicts monotropic transition
temperature as 121.5°C. But actually it is not realizable due to its high crystallising tendency.
The variations in mesogenic properties and their magnitudes from homologue to homologue
in the same series is attributed to the varying number of methylene unit or units which affects
the parameters, responsible and their suitable magnitudes for possibility to induce and facil-
itate mesophase or mesophases of their type. Following Fig. 2 represents some thermotropic
azoester series-1 and the structurally similar analogous series-X [34] and Y [35] which are
chosen for comparative study are as under.
Figure 2 of presently investigated series-1 and structurally similar analogous series X and
Y are thermotropically mesomorphic and identical with respect to first and third phenyl rings
which are liked through –COO- and –N═N- central bridges with middle phenyl ring substi-
tuted by lateral –OCH group (series-1) or another fused phenyl ring in naphthyl unit (series-
3
X) or –H in series-Y respectively, which constitute differing total molecular rigidity of each
individual series under comparative study. Flexible –OR group is commonly and identically
present as left n-alkoxy group for the same homologue which partly contributes to the total
molecular flexibility in equal manner for the same homologue from series to series; but, the
total molecular flexibility varied with changing tailed end groups -COOC H9(n) in which –
4
OC H
bonded through >C═O with third phenyl ring or directly bonded as –OC H
4
9(n)
4 9(n)
and through lateral –OCH , fused phenyl ring or –H for the same homologue from series to
3
series. Thus, facilitating mesophase and its type and magnitudes in terms of mesophase length
or degree of mesomorphism depended upon the magnitudes of differing features, including
combined effect of molecular rigidity and flexibility which varied for the same homologue
from series to series as well as homologue to homologue in the same series. Following Table 4
represents some LC properties of series-1, X and Y in comparative manner as mentioned
below.
Above comparative data of series-1, X and Y under comparison suggests that,
•
Series-1, X and Y are smectogenic plus nematogenic.
•
Thermal stabilities for smectic adopted increasing order from series-1 to series-Y to
series-X, i.e., smectic thermal stability of series-1 is the lowest and of series-X is the high-
est.
•
•
•
Thermal stabilities for nematic are in increasing order from series–Y to series-1 to series-
X, i.e., Nematic thermal stability of series-Y is the lowest and of series-X is the highest.
Smectogenic mesomorphism commences earliest from C homologue for series-1 and
8
Y but it commences late from C homologue from series-X.
12
Nematogenic mesomorphism commences from C homologue for Series-1, X and Y in
6
equal manner.
•
•
Lower mesophase lengths follow increasing order from series-Y to series-1 to series-X.
Upper mesophase lengths follow increasing order from series-X to series-Y to series-1.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
23
Figure . Phase behaviors of series.
Thermal stability of mesophase is a direct effect of the combined effect of molecular rigid-
ity and flexibility as a consequence of internal energy stored by a thermodynamic system due
to mass and characteristics at constant pressure (ꢀH), or the amount of energy released or
absorbed, i.e., exchanged with surroundings during its formation by breaking of old bonds
(
reactant) and forming of new chemical bonds (product), which constitutes the molecular
rigidity and flexibility, and the magnitudes of which alters from substance to substance and
from series to series depending upon respective molecular structure. Thus, suitable and favor-
able magnitudes of intermolecular cohesion and closeness as a consequence of combine effects
of molecular rigidity and flexibility is a thermally resisting internal power of a substance or
energy of mesophase stabilization under floating condition on the surface, when external heat
is exposed upon a sample substance. All the homologous series-1, X and Y under compara-
tive study are differing with respect to combined effect of molecular rigidity and flexibility
for the same homologue from series to series in which molecules of a homologous series-X
are comparatively more rigid in presence of common flexible end groups –OR and –OC H
4
9(n)
whose contribution to flexibility is almost identical, neglecting bonding of –OC H through
4
9(n)

24
U. H. JADEJA AND R. B. PATEL
Figure . Structurally similar analogous series.
Table . Relative thermal stability in°C.
Series→
Series-
Series-X
Series-Y
Sm-N or Sm-I Commencement of smectic phase
N-I Commencement of nematic phase
Total (Sm+N) mesophase length in°C
. (C –C ) C
. (C –C ) C
. (C –C ) C




  


. (C –C ) C
. to . C C
. (C –C ) C
. to . C C
. (C –C ) C






  
. to . C C
 




-
C═O in series-1. Thus, the most rigid series-X facilitated stabilization of smectic and nematic
mesophase to higher extent as compared to series -1 and Y. The lower nematic thermal sta-
bility and a little bit higher smectic thermal stability of series-Y as compared to series-1 is
attributed to the absence and presence of lateral substitution in series-Y as compared to series-
1
including effect due to its linking –OC H9(n) through >C═O part of tailed end, as well as
4
difference in intermolecular distance or intermolecular closeness, of almost equally rigid but,
of lower rigidity than series-X. The early or late commencement of mesophase depend upon
the extent of molecular noncoplanarity. All the series-1, X and Y are almost equally planer or
noncoplaner, but, fused phenyl ring at the middle part of molecules of series-X causes distur-
bance comparatively more than the molecules of series-1 and Y due to combined effects of
effective molecular polarity and polarizability. Therefore, smectogenic mesophase formation
commences late from C homologue of series-X, but it commences earlier from C homo-
12
8
logue for series-1 and Y. The commencement of nematic mesophase formation takes place
in equal manner from C homologue, irrespective of difference in molecular rigidity or the
6
extent of molecular noncoplanarity, because molecular structure is loosened more under the
influence of exposed thermal vibrations beyond ordered smectogenic molecular organization,
which reorganizes and arrange easily in less ordered texture of nematic type at relatively higher
temperature. Increasing or decreasing order of mesophase lengths depends upon the magni-
tudes of thermal resistivity induced during floating of molecules in LC state on the floating
surface.
Conclusions
•
An azoester novel homologous series of liquid crystals is predominantly nematogenic
and partly smectogenic whose degree of mesomorphism is 12°C to 48°C and it is a mid-
dle ordered melting type series.
•
The group efficiency order derived on the basis of (i) thermal stabilities (ii) early com-
mencement of mesophase and (iii) mesophase length foe smectic and nematic are as
under.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
25
(
i) Smectic Series-X > Series-Y > Series-1 Nematic Series-X > Series-1 > Series-Y
(
ii) Smectic Series-1 = Series-Y > Series-X Nematic Series-1 = Series-X = Series-Y
(
iii) (Sm± N) mesophase lengths: Lower: Series-X > Series-1 > Series-Y Upper: Series-1 >
Series-Y > Series-X
•
•
•
Mesomorphism is very sensitive and susceptible to molecular structure; as a result of
effective molecular rigidity and/or molecular flexibility.
Present novel LC dyes reported here are useful as equal as other dyes and other light
emitting devices or devices related to thermography.
Present investigation supports and raises the credibility to the early conclusions related
to the relation between molecular structure and LC properties.
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 and analytical services.
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