Mesomorphic dependence on molecular rigidity by central bridge and homo-hetero ring systems

Article abstract of DOI:10.1080/15421406.2015.1069000

A novel homologous series of thermotropic derivatives containing a heterocyclic ring was synthesized and studied with a view to understanding and establishing a relation between liquid crystal (LC) properties and the molecular structure of chalconyl derivative in general. The series comprises 12 homologues, of which smectogenic LC property commences from the decyloxy (C₁₀) homologue and continues for C₁₂, C₁₄, and C₁₆ homologues. Homologues C₁ to C₈ are non-liquid crystals. Exhibition of nematogenic mesophase is totally absent. Transition and melting temperatures were determined by an optical polarizing microscope equipped with a heating stage. Spectral and analytical data confirmed the molecular structures of homologues. The textures of smectic mesophase are A or C, and are determined by a miscibility method. The average thermal stability of the smectic phase is 113.57°C. The mesogenic phase length ranges between 4.1°C and 11.1°C. Thus, it is a middle-ordered melting-type series of shorter range of liquid crystallinity with absence of nematogenic character. Some LC properties of the present series are compared with a structurally similar known homologous series.

Full text of DOI:10.1080/15421406.2015.1069000

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Mesomorphic dependence on molecular rigidity by
central bridge and homo–hetero ring systems
R. H. Maheta & U. C. Bhoya
To cite this article: R. H. Maheta & U. C. Bhoya (2016) Mesomorphic dependence on molecular
rigidity by central bridge and homo–hetero ring systems, Molecular Crystals and Liquid
Crystals, 625:1, 72-80, DOI: 10.1080/15421406.2015.1069000
To link to this article: http://dx.doi.org/10.1080/15421406.2015.1069000
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Date: 05 March 2016, At: 22:30

MOL. CRYST. LIQ. CRYST.
, VOL. , –

http://dx.doi.org/./..
Mesomorphic dependence on molecular rigidity by central
bridge and homo–hetero ring systems
R. H. Maheta and U. C. Bhoya
Department of Chemistry, Saurashtra University, Rajkot, Gujarat, India
ABSTRACT
KEYWORDS
Liquid crystal;
mesomorphism; nematic;
smectic, thermotropic
A novel homologous series of thermotropic derivatives containing a
heterocyclic ring was synthesized and studied with a view to under-
standing and establishing a relation between liquid crystal (LC) prop-
erties and the molecular structure of chalconyl derivative in general.
The series comprises 12 homologues, of which smectogenic LC prop-
erty commences from the decyloxy (C ) homologue and continues
1
0
for C , C , and C homologues. Homologues C to C are non-liquid
1
2
14
16
1
8
crystals. Exhibition of nematogenic mesophase is totally absent. Tran-
sition and melting temperatures were determined by an optical polar-
izing microscope equipped with a heating stage. Spectral and analyt-
ical data confirmed the molecular structures of homologues. The tex-
tures of smectic mesophase are A or C, and are determined by a mis-
cibility method. The average thermal stability of the smectic phase is
113.57°C. The mesogenic phase length ranges between 4.1°C and 11.1°C.
Thus, it is a middle-ordered melting-type series of shorter range of liquid
crystallinity with absence of nematogenic character. Some LC proper-
ties of the present series are compared with a structurally similar known
homologous series.
Introduction
The liquid crystalline (LC) state was discovered in 1888 [1], and the thermotropic type is
useful for liquid crystal devices (LCDs) and other purposes. Moreover, the bioactive lyotropic
type of LC is useful in pharmaceutical and medicinal fields to cure some diseases [2–8]. There-
fore, the present investigation is planned with a view to understanding and establishing the
effects of molecular structure on LC behavior by synthesizing a novel homologous series of
chalconyl derivatives [9–11]. Chalconyl derivatives being biologically active molecules can
be studied lyotropically to measure their pharmaceutical and medicinal potency. The present
investigation will be limited to the synthesis of novel LC substances and the characterization
of some LC properties, and a comparison with a structurally similar known series. Thus, the
present investigation will be useful to many research groups working on LC (thermotropic
and lyotropic) with various aims and objectives. Numerous homologous series are reported
[
12–15] to date. Results of our research reported here will be discussed and interpreted in
terms of molecular rigidity and flexibility [16–19].
CONTACT U. C. Bhoya
India.
drucbhoya@gmail.com
Department of Chemistry, Saurashtra University, Rajkot , Gujarat,
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.
73
Experimental
Synthesis
4
-Hydroxy benzoic acid was alkylated by suitable alkylating agents by the modified method
of Dave and Vora [20]. 3-(2-Thiophenyl)-1-(4-hydroxyphenyl) 2-propen-1-one (B) was pre-
pared by an established method [21]; m.p.: 170–172°C, yield: 64.8%. Coupling of compounds
A and B is done by the Steglich esterification to yield α-4(4’-n-alkoxy benzoyloxy) benzoyl-
β-(2”-thiophenyl) ethylene [22].
The synthetic route to the novel homologous series of ethylene derivatives is shown in
Scheme 1.
Scheme . Synthetic route to the novel series.
Characterization
Representative members of a novel series were characterized by elemental analysis (Table 1),
1
infrared (IR) spectroscopy, H Nuclear magnetic resonance (H NMR) spectroscopy, and mass
spectroscopy (MS). Microanalysis was performed on Perkin–Elmer PE 2400 CHN analyzer.
1
IR spectra were recorded on Shimadzu FT-IR-8400, H NMR spectra were recorded on Bruker

74
R. H. MAHETA AND U. C. BHOYA
Table . Elemental analysis for () ethyloxy, () pentyloxy, and () hexadecyloxy derivatives.
Elements found (%)
C, H, O
Elements calculated (%)
Sr. No.
Molecular formula
C H O S
C, H, O

.
.
.
., ., .
., ., .
., ., .
., ., .
., ., .
., ., .

  

C H O S
  

C HOS


spectrometer using CDCl as solvent, and mass spectra were recorded on Shimadzu GC-MS
3
Model No. QP-2010. The mesogenic behavior of homologue derivatives was observed through
polarizing microscopy. The textures of smectogenic mesophase were determined by miscibil-
ity method.
Analytical data
Spectral data

H NMR in ppm for hexyloxy derivative
1
H NMR (400 MHz, CDCl ) δ 0.89–0.93 (t, J = 7.8 Hz, 3H), 1.32 (m, 4H), 1.43–1.46 (m,
3
2
7
1
8
H), 1.71–1.78 (m, 2H), 4.10–4.13 (t, J = 7.5 Hz, 2H), 7.11–7.15 (m, 2H), 7.20–7.22 (t, J =
.5 Hz, 1H), 7.47–7.49 (m, 2H), 7.62–7.64 (d, J = 15.0 Hz, 1H), 7.73–7.75 (dd, J = 1.5, 7.5 Hz,
H), 7.81–7.83 (dd, J = 1.5, 7.5 Hz, 1H), 7.92–7.96 (d, J = 15.0 Hz, 1H), 8.10–8.12 (m, 2H),
.21–8.26 (m, 2H).

H NMR in ppm for heptyloxy derivative
1
H NMR (400 MHz, CDCl ) δ 0.86–0.88 (t, J = 7.6 Hz, 3H), 1.26–1.28 (m, 8H), 1.42–1.44 (m,
3
2
(
7
2
H), 1.74–1.76 (m, 2H), 4.10–4.14 (t, J = 7.4 Hz, 2H), 7.09–7.17 (d, J = 7.5 Hz, 2H), 7.20–7.22
m, 1H), 7.47–7.49 (d, J = 8.1 Hz, 2H), 7.60–7.64 (d, J = 15.2 Hz, 1H), 7.72–7.74 (m, 1H),
.82–7.84 (m, 1H), 7.93–7.97 (d, J = 15.1 Hz, 1H), 8.09–8.12 (s, 2H), 8.21–8.23 (d, J = 7.6 Hz,
H).

H NMR in ppm for tetradecyloxy derivative
1
H NMR (400 MHz, CDCl ) δ 0.85–0.94 (t, J = 7.4 Hz, 3H), 1.22–1.36 (m, 24H), 1.42–1.46
3
(
m, 2H), 1.74–1.78 (m, 2H), 4.08–4.12 (t, J = 7.5 Hz, 2H), 7.05–7.11 (m, 2H), 7.18–7.22 (t,
J = 7.5 Hz, 1H), 7.30–7.34 (m, 2H), 7.52–7.54 (d, J = 15.0 Hz, 1H), 7.78–7.80 (dd, J = 7.5,
1
.5 Hz, 1H), 7.96–7.99 (d, J = 15.0 Hz, 1H), 8.07–8.13 (m, 2H), 8.26–8.32 (m, 2H).
IR in cm  for heptyloxy derivative
−
IR (KBr): 3070 (Ar str.), 2946, 2852 (–CH str.), 1728 (–C=O str.), 1651, 1589, 1510 (–C=C–
3
−1
Ar str.), 1415, 1271, 1213, 1074 (–C–O str.), 844, 707 (–CH Ar bend) cm .
IR in cm  for butyloxy derivative
−
IR (KBr): 3038 (Ar str.), 2930 (–CH str.), 1750 (–C=O str.), 1656, 1544, 1508 (–C=C– Ar
3
−1
str.), 1056 (–C–O str.), 880, 742 (–CH Ar bend) cm .
IR in cm  for methyloxy derivative
−
IR (KBr): 3068 (Ar str.), 2955 (–CH str.), 1719, 1728 (–C=O str.), 1649, 1560, 1506 (–C=C–
3
−1
Ar str.), 1415, 1269, 1064 (–C–O str.), 885, 732 (–CH Ar bend) cm .

MOL. CRYST. LIQ. CRYST.
75
Table . Transition temperatures in °C.
n-alkyl chain
CnHn+ (n)
Compound No.
Sm
Nm
Isotropic



















–
–
–
–
–
–
–
–
.
.
.
.
–
–
–
–
–
–
–
–
–
–
–
–
.
.
.
.
.
.
.
.
.
.
.
.








Mass of hexyloxy derivative
+
m/z (rel. int%): 434 (M) , 149, 121, 93, 65, 44.
Mass of octyloxy derivative
+
m/z (rel. int%): 462 (M) , 205, 121, 93, 65, 44.
Mass of dodecyloxy derivative
+
m/z (rel. int%): 518 (M) , 219, 121, 93, 57, 43.
Result and discussion
The chalconyl derivative of the present homologous series was synthesized by the conden-
sation of dimeric 4-n-alkoxy benzoic acids and non-mesomorphic 3-(2-thiophenyl)-1-(4-
hydroxyphenyl) 2-propen-1-one as shown in Scheme 1. Dimerization disappears on esteri-
fication by the lowering of transition temperatures than the corresponding n-alkoxy acids.
Alternation of Cr-I temperatures are observed. Mesomorphism of smectogenic character
commences late from the C member of the series enantiotropically with absence of nematic
10
property. Transition and melting temperatures as determined using an optical polarizing
microscope equipped with a heating stage (Table 2) were plotted against the number of carbon
atoms present in the n-alkyl chain. A phase diagram (Fig. 1) is obtained, showing phase behav-
ior of novel series by linking like or related points. Cr-I or Cr-Sm transition curve follows a
zigzag path of rising and falling with overall descending tendency as series is ascended. Sm-I
transition curve initially rises and then descends after passing through maxima at C member
12
of a series without exhibition of odd-even effect. Thus, both transition curves behaved in nor-
mal established manner. The Sm-I transition curve is extrapolated to C homologue [23–26]
8
to determine and predict its latent transition temperature for smectic (109°C) and its latent
ability to induce mesomorphism, following the trend of Sm-I transition curve. However, it
is hypothetical and not realizable. Therefore, C to C homologues of a series are non-liquid
1
8
crystals (NLC). Hence, the present series is partly smectogenic with the absence of nematic
property, middle-ordered melting-type, and with low mesophase length range.
The dimerization of n-alkoxy benzoic acids disappears by breaking of hydrogen bond-
ing between two molecules of n-alkoxy benzoic acids, and subsequently esterification pro-
cess with 4-hydroxy chalcone derivative. Thus, the condensation process increases molec-
ular length, lateral to terminal attractions, and molecular polarity and polarizability due to

76
R. H. MAHETA AND U. C. BHOYA
Figure . Phase behavior of series.
increase in permanent dipole moment across the long molecular axis, dipole–dipole inter-
actions, electronic interaction, dispersion forces, etc. But suitable magnitudes of anisotropic
forces of intermolecular attraction commence from decyloxy homologues (C ) of preoc-
10
cupied lamellar packing of molecules in their crystal lattices as a consequence of favorable
molecular rigidity and flexibility required to cause smectic mesophase formation. The end-to-
end intermolecular attractions failed to maintain the statistically parallel orientational order of
molecules under the influence of exposed thermal vibrations in floating condition through-
out the entire present series. Thus, nematogenic mesophase formation is totally hindered.
Only sliding layered molecular arrangement occurs under thermal vibrations for C , C ,
10
12
C , and C homologues under floating condition of preoccupied lamellar molecular pack-
14
16
ing and layered structure, which induces smectic mesophase in enantiotropic manner. The
non-mesomorphic behaviors of C –C homologues is attributed to their high crystallizing
1
8
tendency, which arises due to unsuitable magnitudes of anisotropic forces of intermolecular
attractions. These intermolecular attractions immerge from low dipole–dipole interactions
and low magnitudes of dispersion forces by interactions between instantaneous dipoles pro-
duced by the spontaneous oscillations of electron clouds of molecules that hinder the thermal
environment of molecule arrangement and required to cause either smectic or/and nematic
mesophase formation under floating condition on surface. The absence of odd-even effect in
Sm-I transition curve is attributed to the absence of mesogenic phase formation either smectic
or nematic in the initial (C to C ) homologues of the series under discussion. However, alter-
1
8
nations in transition temperature are observed in isotropic temperatures due to odd and even
number of carbon atoms in n-alkyl chain of n-alkoxy group. The changing trend of mesomor-
phic property and the degree of mesomorphism from homologue to homologue in the present
series are attributed to the varying number of methylene unit or units present in left n-alkoxy
terminal end group keeping right-handed thiophene ring as an intact end of molecules. The
shorter phase length range or degree of mesomorphism is attributed to low resistivity (low
enthalpy value, ꢀH) of smectogenic homologues toward exposed thermal vibrations.
Variation in some mesomorphic properties of presently investigated series 1 is compared
with structurally similar other known homologous series X [27] and Y [28] as mentioned in
Fig. 2.

MOL. CRYST. LIQ. CRYST.
77
Figure . Structurally similar series.
Homologous series 1 of the present investigation and the homologous series X and Y cho-
sen for comparative study are identical to each other with respect to major structural sim-
ilarity, including minor changes in their molecular structures. Homologous series 1 and X
are identical with respect to two phenyl rings, heterocyclic thiophene ring, and a central
bridge –CO–CH=CH– linking middle phenyl ring and heterocyclic thiophene ring. More-
over, these are identical with respect to flexible n-alkoxy end group for the same homologue
from series 1 to series X. However, these differ with respect to central bridges –COO– and
–
CH=CH–COO– contributing to the molecular rigidity of the same homologue from series
1
to series X. Homologous series 1 and series Y are identical with respect to two phenyl (first
and middle) rings, two central bridges –COO– and –CO–CH=CH–, and the left n-alkoxy
flexible end group for the same homologue from series 1 to series Y. However, they differ in
the ring system, viz. phenyl ring and heterocyclic ring linked to –CO–CH=CH– at the right-
sided end of molecules in series Y and series 1 respectively, which contribute to the molecular
rigidity of the same homologue from series to series, and for homologue to homologue in the
same series. Thus, variations in mesomorphic properties and the degree of mesomorphism
for the same homologue from series to series and from homologue to homologue in the same
series will vary with the changing features of series 1, X, and Y. Table 3 represents some meso-
morphic properties of series 1, X, and Y under comparative study as mentioned below.
Table 3 indicates the following:
r
Homologues series 1 and Y are only smectogenic and only nematogenic respectively, but
series X is nematogenic in addition to smectogenic in character.
r
Smectic mesophase commences from the C10 homologue of series 1 and from the C4
homologue of series X, but it does not commence till the best member of series Y.
Table . Average thermal stability in °C.
Series→

X
Y
Smectic–isotropic
Or
Smectic–nematic
.
.
–
–
(C –C )
(C –C )


C



C
Commencement of smectic phase
Nematic–isotropic
Commencement of nematic phase
–
–
.
.
(C –C )
(C –C )




C
C
Total mesophase length range °C (Smectic +
. to .
C , C
. to .
. to .
nematic)
C , C


C, C

 

7
8
R. H. MAHETA AND U. C. BHOYA
r
Nematic mesophase commences from the very first member (C ) of series X and from
1
sixth (C ) member of series Y, but it does not commence till the last (C ) member of
6
16
series 1 of the present investigation.
Smectic thermal stability of the present series 1 (113.57) is lower than that of series X
r
r
r
(
136.56), whereas smectic mesophase is totally destabilized for series Y.
Nematic thermal stability (164.5) of series Y is greater than that of series X (152.78),
whereas the same does not stabilize through the present series 1.
The total mesophase length range (minimum to maximum) bears increasing order from
series 1 to X to Y.
The energy (ꢀH) stored in the molecules of a substance depends upon its quantity of mass
and characteristics during its formation as heat of formation per mole. Therefore, the ther-
modynamic quantity, ꢀH, varies for every substances from homologue to homologue in the
same series and for the same homologue from series to series depending upon molecular
rigidity and flexibility. The molecular rigidity in the present comparison varies with either one
of the central bridges (series 1 and X) or with the ring system (series 1 and Y) for the same
homologue from series to series. Therefore, the thermal resistivities toward exposed thermal
vibrations vary for the same homologue from series to series. Comparing the thermal stabil-
ity for smectic, which is higher for series X than for the present series 1, the vinyl carboxylate
–
CH=CH–COO– central bridge of series X and the carboxylate –COO– central bridge of
series 1, although closely similar to each other, differ in length and added rigidity because
of the presence of multiple bond of –CH=CH– unit. Therefore, the vinyl carboxylate group,
because of its longer length, causes more non-coplanarity by the twist obtained as the oxygen
atom of the vinyl carboxylate group bump into the non-bonded adjacent hydrogen atoms of
aromatic rings. On account this difference, the smectic–isotropic thermal stability of series 1
is lower than the smectic–nematic thermal stability of series X (or vice versa). Absence of pre-
occupied lamellar packing of molecular rigidity in the crystal lattices of series Y hinders the
stabilization of smectic phase formation till the last member of series Y. Absence of nematic
phase formation is attributed to the weaker end-to-end unsuitable magnitudes of anisotropic
forces of intermolecular attractions and closeness as compared with series X and Y chosen for
comparative study as a consequence of unfavorable molecular rigidity. The late commence-
ment of mesophase (C ) in the present series 1 and other series X and Y is attributed to
10
the corresponding difference in the extent of non-coplanarity of molecules related to the
molecular rigidity of each series. The observed difference in the total mesophase length range
after acquiring suitable magnitudes of anisotropic forces of intermolecular attractions induces
molecular resistivity toward exposed thermal vibrations, thus increasing from series 1 to
series X to series Y. Hence, the stabilization of total mesophase length range is progressively
enhanced from minimum to maximum for the series under comparison.
Conclusions
r
A novel homologous series of 12 chalconyl ester homologues containing heterocyclic
ring as end group has been synthesized, whose only smectogenic mesophase formation
commences late from C homologue without exhibition of nematic property through-
10
out the series.
r
Group efficiency order derived for smectic and nematic on the basis of (i) thermal sta-
bility, (ii) commencement of mesophase, and (iii) total mesophase length range is as
follows:

MOL. CRYST. LIQ. CRYST.
79
(
i) Smetic
–
CH=CH–COO– –COO– –COO–
with heterocyclic > with heterocyclic > with homocyclic
thiophene ring thiophene ring benzene ring
Nematic
–
COO– –CH=CH–COO– –COO–
with homocyclic > with heterocyclic > with heterocyclic
benzene ring thiophene ring thiophene ring
(
–
ii) Smectic
CH=CH–COO– –COO– –COO–
with heterocyclic > with heterocyclic > with homocyclic
thiophene ring thiophene ring benzene ring
Nematic
–
CH=CH–COO– –COO– –COO–
with heterocyclic > with homocyclic > with heterocyclic
thiophene ring benzene ring thiophene ring
(
iii) Sm + N
–
COO– –CH=CH–COO– –COO–
with homocyclic > with heterocyclic > with heterocyclic
benzene ring thiophene ring benzene ring
r
Molecular rigidity and flexibility are the controlling factors to induce mesomorphism.
Mesomorphism is very sensitive and susceptible to molecular structure.
Chalconyl derivatives are biologically active, and sulfur-containing heterocyclic ring is
helpful for dermatological treatment.
Study of binary systems themotropically may be useful for the manufacture of LC devices
at desired temperature.
r
r
r
Acknowledgments
We are thanks to the Department of Chemistry (DST-FIST-funded and UGC-SAP-sponsored),
Saurashtra University, Rajkot, for research work. Special thanks to the “National Facility for Drug Dis-
covery through New Chemical Entities (NCE’s) for sample analysis. We are also thankful to Dr. A. V.
Doshi, Ex. Principal M.V.M. Science and Home Science College, Rajkot, for his valuable cooperation
and suggestions during the present investigations as and when needed.
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