Study of mesomorphism and its relation to molecular structure through molecular rigidity and flexibility

Article abstract of DOI:10.1080/15421406.2016.1221975

Novel liquid Crystalline (LC) substances of chalconyl ester derivatives from Vinyl aromatic carboxylic acids are synthesized and studied with a view to understanding and establishing the relation between molecular structure, RO-C₆H₄-CH=CH-COO-C₆H₄-CH=CH-CO-C₆H₄-F (para) and LC properties of the thermotropic variety. Homologous series consists of eleven members (C₁ to C₁₆). C₁ and C₂ members of novel series are nonliquid crystals (NLC) and the rest of the homologues (C₃ to C₁₆) are enantiotropic nematic with absence of smectic property throughout. Transition temperatures and textures of homologues were determined by an optical polarizing microscope (POM) equipped with a heating stage. Textures of nematic phase are threaded or Schlieren. The transition temperatures of novel homologues are relatively lower than the corresponding dimeric trans n-Alkoxy Cinnamic acids from which, novel homologues are synthesized. Thermal, analytical, spectral data confirmed molecular structures of novel homologues. Cr-N/I and N-I transition curves behaved in normal manner except C₈ to C₁₄ homologues which deviated from normal descending tendency in N-I transition curve. N-I transition curve exhibited odd-even effect up to C₆ homologue and odd member's N-I transition curve occupied lower position than even member's N-I transition curve. Thermal stability for nematic is 118.2°C; with its total mesophase length ranging from 12.0 to 50.0°C. Thus, it is a middle ordered melting type series with absence of smectic property. Group efficiency order derived on the basis of thermal stability using comparative study of present novel series with structurally similar analogous series as mentioned below. Smectic: Series-Y > Series- X = Series-1 and Nematic: Series-X > Series- 1 > Series-Y

Full text of DOI:10.1080/15421406.2016.1221975

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Study of mesomorphism and its relation to
molecular structure through molecular rigidity
and flexibility
N. R. Muniya & V. R. Patel
To cite this article: N. R. Muniya & V. R. Patel (2016) Study of mesomorphism and its relation
to molecular structure through molecular rigidity and flexibility, Molecular Crystals and Liquid
Crystals, 638:1, 77-85, DOI: 10.1080/15421406.2016.1221975
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Date: 19 November 2016, At: 02:56

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
http://dx.doi.org/./..
Study of mesomorphism and its relation to molecular structure
through molecular rigidity and flexibility
N. R. Muniya and V. R. Patel
Chemistry Department Sheth P. T. Arts & Science College, Gujarat University, Godhra, Gujarat, India
ABSTRACT
KEYWORDS
Liquid crystal;
Mesomorphism; Molecular
Flexibility; Nematic; Smectic
Novel liquid Crystalline (LC) substances of chalconyl ester derivatives
from Vinyl aromatic carboxylic acids are synthesized and studied with
a view to understanding and establishing the relation between molec-
=
=
ular structure, RO-C6H4-CH CH-COO-C6H4-CH CH-CO-C6H4-F (para)
and LC properties of the thermotropic variety. Homologous series con-
sists of eleven members (C1 to C16). C1 and C2 members of novel series
are nonliquid crystals (NLC) and the rest of the homologues (C3 to C16)
are enantiotropic nematic with absence of smectic property through-
out. Transition temperatures and textures of homologues were deter-
mined by an optical polarizing microscope (POM) equipped with a heat-
ing stage. Textures of nematic phase are threaded or Schlieren. The
transition temperatures of novel homologues are relatively lower than
the corresponding dimeric trans n-Alkoxy Cinnamic acids from which,
novel homologues are synthesized. Thermal, analytical, spectral data
confirmed molecular structures of novel homologues. Cr-N/I and N-I
transition curves behaved in normal manner except C8 to C14 homo-
logues which deviated from normal descending tendency in N-I transi-
tioncurve. N-Itransitioncurveexhibitedodd-eveneffectuptoC6 homo-
logue and odd member’s N-I transition curve occupied lower position
than even member’s N-I transition curve. Thermal stability for nematic
is 118.2°C; with its total mesophase length ranging from 12.0 to 50.0°C.
Thus, it is a middle ordered melting type series with absence of smectic
property. Group efficiency order derived on the basis of thermal stability
using comparative study of present novel series with structurally similar
analogous series as mentioned below.
Smectic: Series-Y > Series- X = Series-1 and
Nematic: Series-X > Series- 1 > Series-Y
Introduction
Liquid Crystal (LC) properties [1] and their behaviors are directly linked to the molecular
structure [2–6] through molecular rigidity and flexibility [7–10]. The study of structure-
property relationships generates a wealth of knowledge and aids the generation of liquid
crystals with specific properties for a range of applications [11–13]. Moreover, chalcones
are particularly useful in the pharmaceutical preparations such as antifungal, antibacterial,
anticancer agents and in agricultural products due to their bioactivity [14–20]. Thus, the
present investigation was planned to synthesize novel chalconyl ester derivatives through
CONTACT V. R. Patel
vrp@yahoo.co.in
(Affiliated with Gujarat University) PIN:, Gujarat, India.
Department of Chemistry, Sheth P.T. Arts and Science College, Godhra
©  Taylor & Francis Group, LLC

78
N. R. MUNIYA AND V. R. PATEL
=
homologous series containing three phenyl rings bonded through –CH CH-COO- and
=
–CH CH-CO- central bridges with varying –OR and fixed –F terminal end groups, with a
view to understanding and establishing the effect of molecular structure on LC properties.
The present investigation concerns the synthesis and characterization by analytical, thermal
and spectral data. The evaluated thermometric data will be interpreted and discussed on the
basis of molecular rigidity and flexibility. Several ester homologous series have been reported
to date [21–26].
Experimental
Synthesis
4-Hydroxy benzaldehydes were treated with malonic acid to convert them into trans 4-n-
alkoxy cinnamic acids (A) [27]. n-Alkoxy cinnamic acids (A) were condensed with α-4-flouro
benzoyl –β-4ꢀ-hexyloxy phenyl ethylene [28] (M.P.-112 °C)[B] by a usual established method
[29] to give final products. The synthetic route to series is shown below in Scheme 1. Final
products were individually, filtered, washed, dried, and purified until the constant transition
temperatures obtained.
Scheme . Synthetic route to the series-.
Characterization
Some selected members of the titled series were characterized by elemental analysis, 1HNMR
spectra and IR spectra, Textures of mesophases were characterized by miscibility method.
Microanalysis for C,H,N elements were performed on Perkin Elmer PE 2400 analyzer
(Table 1). 1HNMR spectra were obtained on Bruker spectrometer using CDCl3 as solvent. IR
spectra were recorded on a Perkin- Elmer spectrum GX. Transition temperatures and liquid
crystal properties were investigated using optical polarizing microscope (POM) with heating
stage.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
79
Table . Elemental analysis for the octyloxy, decyloxy, and dodecyloxy derivatives.
Elemental % found (Compared with % calculated)
Molecular formula
C
H
CHOF
CHOF
HOF
. (.)
. (.)
. (.)
. (.)
. (.)
. (.)
C
Analytical data
H1NMR: in ppm. For octyloxy homologue.
Ethylenes: (200 MHZ) δ (CDCl3) (ppm) 0.98 (-CH3-CH2 of –C8H17), 1.40 (long-CH2-
chain), 3.4 -OCH2 of (–OCH2 of C8H17), 6.35 & 6.45, 8.45, and 8.24 (p-sub. benzene rings)
H1NMR: in ppm. For decyloxy homologue.
Ethylenes: (200 MHZ) δ (CDCl3) (ppm) 1.05 (-CH3-CH2 of –C8H17), 1.52 (long-CH2-
chain), 3.25 -OCH2 of (–OCH2 of C8H17), 6.52 & 6.29, 8.41, and 8.25 (p-sub. benzene rings)
IR in Cm−1
,
IR Spectrum. For octyloxy homologue
Ethylenes: (vmax/cm-1): 2958, 2858, 1482, 1345 (-C-H, aliphatic), 1735, 1266 (ester group),
=
=
17 (>C O group), 1558 (-C C-, aromatic), 1031, 1235 (ether group), 845.0 (p- sub. benzene
ring), 1599, 1510, 1494 (aromatic ring).
Spectrum. For Decyloxy homologue
Ethylenes: (vmax/cm-1): 2998, 2825, 1466, 1382 (-C-H, aliphatic), 1713, 1245 (ester group),
=
=
1736 (>C O group), 1566 (-C C-, aromatic), 1036, 1266 (ether group), 845.2 (p- sub. ben-
zene ring), 1642, 1475, 1480 (Aromatic ring).
Result and discussion
A novel chalconyl ester homologous series of thermotropic LC derivatives were synthesized
by condensing trans-4-n-alkoxy cinnamic acids individually with α-4-hydroxy phenyl β-4ꢀ-
fluorobenzoyl ethylene. Lowering of transition temperature of novel compounds as compared
to corresponding dimeric trans-n-alkoxy cinnamic acids take place by esterification process.
Enantiotropic nematogenic mesomorphism commences from the C3 homologue and contin-
ued up to the C16 homologue. The C1 and C2 homologues are not liquid crystals. Transition
temperatures (Table 2) as determined by POM equipped with a heating stage were plotted
against the number of carbon atoms present in n-alkyl chain ‘R’ of –OR group and Cr-N/I
Table . Transition temperatures in °C.
Compound no.
n-Alkyl (CnHn+) group
Smectic
Nematic
Isotropic











C
C
C
C
C
C
C
C
C
C
C
—
—
—
—
—
—
—
—
—
—
—
—
—
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

80
N. R. MUNIYA AND V. R. PATEL
Figure . Homologous series: α - -[ꢀ-n-alkoxy cinnamoyloxy] phenyl- β-ꢀꢀ-floro benzoyl ethylenes.
as well as N-I transition curves are obtained on linking like or related points as depicted in
a phase diagram as shown in Fig 1. The Cr-N/I transition curve follows a zigzag path of ris-
ing and falling values with an overall descending tendency and behaves in normal manner.
The N-I transition curve initially descended and then ascended from C8 to C14 in an unex-
pected deviating manner and then finally descended in the C16 homologue in a normal man-
ner with exhibition of the odd-even effect from C3 to C6 homologue. The smectic phase is
totally absent either in a monotropic or an enantiotropic manner. The odd-even effect disap-
pears from the C6 member of the series and continues as a single transition curve for higher
homologues of longer n-alkyl chain ‘R’ of –OR group. Odd member’s N-I transition curve
occupied lower position as compared to even member’s N-I transition curve. Textures of the
nematic phase are threaded or Schlieren. Analytical, thermal and spectral data confirmed
the molecular structures of homologues. Thermal stability in average for nematic phase is

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
81
Figure . Structurally similar analogous series.
118.2 and mesophase lengths ranges between 12.0 and 50.0°C. Thus, novel chalconyl ester
homologous series is nematogenic without exhibition of smectic property. The LC properties
of series from homologue to homologue shows variations depending upon the suitable mag-
nitudes of anisotropic forces of intermolecular attractions and closeness as a consequence of
individual molecular structural rigidity and flexibility. Lowering of transition temperatures of
novel homologues as compared to corresponding to trans dimeric n-Alkoxy Cinnamic acids
is attributed to the breaking of hydrogen bonding between two same molecules of vinyl aro-
matic acid through esterification process. The nonmesomorphicity of methoxy and ethoxy
derivatives of present novel series is attributed to their high crystallizing tendency. Thus, the
molecules of C1 and C2 homologues sharply and directly transform into isotropic state. The
exhibition of a nematogenic mesophase commences from the C3 homologue and continues up
to last C16 homologue of the series in an enantiotropic manner. The molecular arrangement
required to facilitate smectic mesophase formation is hindered among the molecules of entire
series due to absence of lamellar packing of molecules in their respective crystal lattices. The
appearance of odd-even effect up to C6 homologue is attributed to the sequentially added
methylene unit in n-alkyl chain ‘R’ of –OR group which maintained expected status of n-
alkyl chain ‘R’ and the rest of the molecular part including tailed ended monoatomic fluoro
(F) group. The disappearance or diminishing of odd-even effect from and beyond merging
of odd and even N-I transition curves for higher homologues of longer n-alkyl chain ‘R’ is
attributed to coiling or bending or flexing or coupling of n-alkyl chain ‘R’ with core struc-
ture of a molecule which modifies the magnitudes of molecular rigidity and flexibility. The
changing trend in mesogenic (LC) behavior or properties with changing molecular structure
of present novel series-1 are compared with the structurally similar analogous series : X [30]
and Y [31] as mentioned below in Fig. 2.
Series 1 and X are identical with respect to three phenyl rings and two central bridges link-
ing them, which contributes to the total molecular rigidity. Their left flexible n-alkoxy group
–OR of series 1 and X are identical for the same homologues contributing partly to the total
flexibility, but their right handed tailed ended –F and –H groups differ, which partly differs
with respect to respective total molecular flexibility. Homologous series 1 and Y are identi-
cal with respect to three phenyl rings and one of the central groups linking middle and third
phenyl rings which contributes partly to the total molecular rigidity, but, differs with respect
=
to –CH CH- unit for a central bridges linking first and middle phenyl rings and causes dif-
=
ference in total molecular rigidity offered by –CH CH- conjugated unit through molecular
rigidity offered double bond and the negligible positional difference of –CO- in central group,
nearest and away from third phenyl ring respectively. The total molecular flexibility of a novel
series-1 of present investigation and a series-Y chosen for comparative study are the same for
the same homologue, because the left ended terminal group –OR and the right ended tailed
group-F are same for the same homologue from series-1 and Y. Therefore, the variations in
LC properties, behaviors and the degree of mesomorphism for the same homologue from
series to series will depend upon the magnitudes of differing features of series-1, X and Y

82
N. R. MUNIYA AND V. R. PATEL
Table . Relative thermal stabilities in °C.
Series →
Series-
Series-
Series-
Sm-N or Sm-I Commencement of smectic phase
N-I Commencement of nematic phase
Total mesophase lengths from t°C to t°C (Sm+N)
—
—
. (C–C) C
—
.–. C C
. (C–C) C
.–. C C
. (C–C) C
.–. C C
under comparative study. However variations in mesomorphic properties from homologue
to homologue in the (any) same series will depend upon the magnitudes of added methylene
unit and its related magnitudes of flexibility, keeping rest of the molecular part and its molecu-
lar rigidity plus flexibility due to tail end group remain unaltered, throughout the same series.
Following Table 3 represents comparative thermometric data for series-1, X and Y as under.
Thermometric data of Table 3 suggest that,
• Homologous series -1 and X are nematogenic, whereas series-Y is smectogenic.
• The smectogenic mesomorphism commences from very first member of a series-Y, but,
it does not commence till the last member of series 1 and X.
• The nematogenic mesomorphism commences earliest from C3 homologue of present
novel series-1 and late from C6 homologue of series-X, but, it does not commence till
the last C16 homologue of series-Y.
• Thermal stability for smectic is the highest for series-Y but it does not stabilize for series
X and 1.
• Thermal stability for nematic of present series-1 is lower than series-X, but it does not
stabilize for series-Y till the last homologue.
• Lower total mesophase length decreases from series-1 to X to Y and upper mesophase
lengths increases from series-X to 1 to Y.
Homologous series 1 and Y identical with respect to molecular flexibility for the same
homologue but differs with respect to molecular rigidity due to difference of presence and
=
absence of –CH CH- unit of first central bridge linking first and a middle phenyl ring includ-
ing difference in the place of Carbonyl group of direct attachment to middle (series-Y) or third
phenyl ring (series-1) longing to chalconyl central bridge. The same homologue from series to
series. Therefore, molecules of series-1 being more rigid but, facing unsuitable magnitudes of
anisotropic forces of intermolecular cohesive forces of attraction to build up layered molec-
ular arrangement in preoccupied crystal lattices and subsequently unable to maintain slid-
ing layered molecular organization under floating condition to facilitate smectic mesophase
formation. However intermolecular end to end forces of cohesions are suitable and sufficient
enough to arrange and maintain the molecules in statically parallel orientational order in float-
ing condition to facilitate nematic mesophase only with neglecting formation of smectic phase
as a result of unfavor magnitudes of dispersion forces, dipole–dipole interactions, length to
breadth ratio and the ratio of the molecular polarity to polarizability and absence of lamel-
lar packing of molecules of presently investigated series-1. On the other hand the molecules
=
bearing –COO- central bridge instead of –CH CH-COO- and -CO group of chalconyl cen-
tral bridge being directly attached to middle phenyl ring linking middle and third phenyl ring
=
through –CH CH- unit facilitate suitable magnitudes of anisotropic forces of intermolecular
attractions which builds up lamellar packing of molecules in the molecules of crystal lattices
with the positional status of -CO group attached to central phenyl ring; right from the very
first member of a series-Y and subsequently focal conic sliding layered molecular arrange-
ment is maintained under floating condition to facilitate only smectic phase, but, residual
end to end intermolecular dispersion forces are insufficient to induce nematic phase. Thus,

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
83
the role of carbonyl unit near or away from middle phenyl ring by chalconyl central bridge as
=
well as presence or absence of –CH CH- unit in ester central bridge are very important for a
molecule made up of three phenyl rings bonded through two central bridges and identically
similar terminal ends of similar or same polarity and polarizability for the same homologue.
The molecular polarity and polarizability of series-1 for the same homologue leading to inter-
molecular end to end attractions as compared to series-X arising out of combined effects of
molecular rigidity and flexibility as a consequence of favorable length to breadth ratio is rela-
tively lower than a series-X and Y which facilitate nematic mesophase in absence of smecto-
genic character of lowest thermal stability. The increasing order of smectic or nematic ther-
mal stabilities from series-1 to X to Y indicates the increasing proportions of internal thermal
resistivity or internal energy stored (ꢀH) due to mass and characteristics of a thermodynamic
system of structurally similar but of differing molecular structure to some extent against exter-
nally exposed thermal vibrations under identical condition or intensity of heat which causes
stabilization of mesophase to more or less extant of differing period of time or differing ranges
of temperature as mesophase lengths from t1 °C to t2 °C. The early or late commencement
of mesophase depends upon the extent of molecular noncoplanarity which decides suitable
(not more or not less) magnitudes of intermolecular attractions to which victim molecules
start to float with either statistically parallel orientational ordered molecular arrangement or
with sliding layered molecule arrangement under floating condition which is independent of
energy stored as ꢀH. Thus, mesophase commences earliest from C1 homologue in series-Y,
then from C3 homologue of series-1 and the latest from C6 homologue of series X.
Hence, the variations in mesomorphic properties and the degree of mesomorphism for the
same homologue from series to series or from homologue to homologue in the same series
depended upon the varying proportions of suitable magnitudes of intermolecular anisotropic
forces of effective cohesion and closeness as a consequence of effective dipole–dipole interac-
tions, electronic interactions, dispersion forces, effective length to breadth ratio, the ratio of
the molecular polarity to polarizability which results into favorable molecular rigidity and/or
flexibility, needed for phenomina of mesomorphism.
Conclusions
• Novel homologous series is nematogenic without exhibition of smectic property and of
middle ordered melting type.
• Carboxylate (-COO-) ester central group containing homologous series including, car-
=
bonyl group of chalcone directly linked to middle phenyl ring (-Ph-CO-CH CH-) are
=
smectogenic, but, vinyl carboxylate (-CH CH-COO-) ester central group containing
homologous series including, carbonyl group of chalcone linked to third phenyl ring
=
(-CH CH-CO-Ph) are only nematogenic or partly nematogenic and partly smecto-
genic.
• The group efficiency order derived for smectic and nematic on the basis of (1) thermal
stabilities (2) early commencement of mesophase and (3) total mesophase lengths are as
under.
(1) Smectic
Series-Y > Series-X = Series-1
Nematic
Series-X > Series-1 > Series-Y

84
N. R. MUNIYA AND V. R. PATEL
(2) Smectic
Series-Y > Series-1 = Series-X
Nematic
Series-1 > Series-X > Series-Y
(3) Total mesophase lengths (Sm N)
Lower: Series-1 > Series-X > Series-Y
Upper: Series-Y > Series-1 > Series-X
• Mesomorphism is very sensitive and susceptible to a molecular structure.
• Carboxylate or vinyl carboxylate group containing chalconyl derivatives are bioactive
which may be exploited and useful to the pharmaceutical, agricultural or medicinal
preparation and mesomorphic property may be exploited for thermographic LC devices
to be operated at desired temperature.
• Present investigation very well supports and raises the credibility to the conclusions
drawn earlier.
Acknowledgments
Authors acknowledge thanks to the Dr. N.N. Vyas, the head of chemistry department and the princi-
pal, Dr. M.B. Patel of the college for their supportive view and providing research facility of present
investigation. Authors are also thankful to Dr. A.V. Doshi, Ex. Principal, M.V.M. Science and Home. Sc.
College, Rajkot for his valuable suggestions, co-operation and support as and when needed throughout
present investigation. Authors acknowledge comment thanks to L. M. Pharmacy college, Ahmedabad
for analytical and spectral services.
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