Dependence of thermotropic mesomorphism on varying rigidity of central bridge in liquid crystals

Article abstract of DOI:10.1080/15421406.2016.1221983

A novel liquid crystalline(LC) homologous series of chalconyl esters: RO-C₆H₄-COO-C₆H₄-CH = CH-CO-C₆H₄F has been synthesized and studied with a view to understanding and establishing the relationship between thermotropic LC properties and molecular structure with reference to varying molecular rigidity due to linking groups. The novel homologous series consists of eleven members (C₁ to C₁₆). C₁ and C₂ homologues are nonliquid crystals (NLC) and the rest of the homologues are enantiotropically nematic (C₃ to C₁₆). The smectogenic character, either in enantiotropic or monotropic manner is totally absent for a series. Transition temperatures and textures of nematic phase were determined by an optical polarizing microscope (POM) equipped with a heating stage. Textures of the nematic phase are threaded or Schlieren. The analytical, thermal and spectral data support the molecular structures. Cr-N/I and N-I transition curves behaved in normal manner except for the C₁₄ homologue. N-I transition curve exhibits an odd- even effect with a negligible deviating effect from the C₁₀ homologue. Thermal stability for the nematic is 105.2°C and mesophase lengths range from 06.0°C to 46.0°C at C₈ and C₁₄ homologues respectively. The group efficiency order derived for nematic from the comparative study of present novel series with structurally similar analogous series on the basis of thermal stability as under.

Full text of DOI:10.1080/15421406.2016.1221983

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Dependence of thermotropic mesomorphism on
varying rigidity of central bridge in liquid crystals
S. H. Pandya & V. R. Patel
To cite this article: S. H. Pandya & V. R. Patel (2016) Dependence of thermotropic
mesomorphism on varying rigidity of central bridge in liquid crystals, Molecular Crystals and
Liquid Crystals, 638:1, 86-94, DOI: 10.1080/15421406.2016.1221983
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Date: 19 November 2016, At: 03:13

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
http://dx.doi.org/./..
Dependence of thermotropic mesomorphism on varying
rigidity of central bridge in liquid crystals
S. H. Pandya and V. R. Patel
Sheth P. T. Arts & Science College, Godhra, Gujarat University, Panchmahal, Gujarat, India
ABSTRACT
KEYWORDS
Chalconyl ester; Liquid
Crystal; Mesomorphism;
Smectic; Nematic;
Smectic: -CO-CH = CH- > -N
= N- > -CH = CH-CO-;
Nematic: -N = N > -CH =
CH-CO > -CO-CH = CH-
A novel liquid crystalline(LC) homologous series of chalconyl esters: RO-
C6H4-COO-C6H4-CH = CH-CO-C6H4F has been synthesized and studied
with a view to understanding and establishing the relationship between
thermotropic LC properties and molecular structure with reference to
varying molecular rigidity due to linking groups. The novel homologous
series consists of eleven members (C1 to C16). C1 and C2 homologues
are nonliquid crystals (NLC) and the rest of the homologues are enan-
tiotropically nematic (C3 to C16). The smectogenic character, either in
enantiotropic or monotropic manner is totally absent for a series. Tran-
sition temperatures and textures of nematic phase were determined by
an optical polarizing microscope (POM) equipped with a heating stage.
Textures of the nematic phase are threaded or Schlieren. The analyti-
cal, thermal and spectral data support the molecular structures. Cr-N/I
and N-I transition curves behaved in normal manner except for the C14
homologue. N-I transition curve exhibits an odd- even effect with a
negligible deviating effect from the C10 homologue. Thermal stability
for the nematic is 105.2°C and mesophase lengths range from 06.0°C
to 46.0°C at C8 and C14 homologues respectively. The group efficiency
order derived for nematic from the comparative study of present novel
series with structurally similar analogous series on the basis of thermal
stability as under.
Introduction
There is considerable scientific and technological interest in research of the liquid crys-
talline(LC) state [1] of both thermotropic or lyotropic type in the fields of thermographic
LC devices including TV screens. Moreover their bioactivity may be utilized as antibacterial,
antifungal, antimalarial, anticancer, antioxidant agents in the field of pharmaceutical prepa-
rations or in agricultural production [2–13]. The present investigation is planned with a view
to understanding and establishing the effects of molecular structure on LC properties of ther-
motropic LC substances with reference to a central bridge linking middle and tail ended third
phenyl ring of molecules consisting of three phenyl rings and two terminal units of –OR (vary-
ing) and –F (constant) polar groups [14–18]. Therefore a novel thermotropic homologous
series of chalconyl esters will be synthesized, characterized and their thermometric proper-
ties will be studied by an optical polarizing microscopy (POM) equipped with a heating stage.
Then the evaluated thermometric data will be compared with the structurally similar isomeric
CONTACT V. R. Patel
Panchmahal, Gujarat, India.
vrp@yahoo.co.in
Sheth P. T. Arts & Science College, Gujarat University, Godhra  ,
©  Taylor & Francis Group, LLC

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
87
or nonisomeric analogous series. The group efficiency order will be derived on the basis of
thermal stability etc. Thus, present study will include synthesis and thermotropic studies of
LC substances. The experimental results will be discussed on the basis of molecular rigidity
and flexibility [19–22]. Such studies are beneficial and provide scientific information to those
working in other areas of liquid crystals, pharmaceuticals or agricultural purposes. Several
ester homologous series of LCs have been reported to date [23–30].
Experimental
Synthesis
4-n-Alkoxy benzoic acids (A) were synthesized from 4-hydroxy benzoic acid using the corre-
sponding suitable alkylating agent (RX) by the modified method of Dave and Vora [31].α-4-
Flouro benzoyl –β-4ꢀ-Hexyloxy phenyl ethylene (B) (Chalcone) was prepared from 4-flouro
acetophenone and 4-hexyloxy benzaldehyde by a known condensation[32] method. 4-n-
Alkoxy benzoic acids (A) and α-4-hydroxy benzoyl –β-4ꢀ-Hexyloxy phenyl ethylene (B) were
condensed in 1,3-dicyclohexylcarbodiimide (DCC), 4-dimethylaminopyridine (DMAP) and
CH2Cl2 [33]. The synthetic route to the series is shown below in scheme-1. The final products
were individually filtered, washed, dried and purified until constant transition temperatures
were obtained.
Scheme . Synthetic route to the series-.
Characterization
Some selected members of the novel series were characterized by elemental analysis, 1HNMR
spectra and IR spectra, Textures of mesophases were characterized by a miscibility method.
Microanalysis for C,H,N, elements was performed on Perkin Elmer PE 2400 analyzer
(Table 1). 1HNMR spectra were obtained on a Bruker spectrometer using CDCl3 as solvent.
IR spectra were recorded on a Perkin- Elmer spectrum GX. Transition temperatures and

88
S. H. PANDYA AND V. R. PATEL
Table . Elemental analysis for the hexyloxy, octyloxy, and decyloxy derivatives.
Elemental % found (Compared with % calculated)
Molecular formula
C
H
CHOF
CHOF
CHOF
. (.)
. (.)
. (.)
. (.)
. (.)
. (.)
liquid crystal properties were investigated using an optical polarizing microscope with a heat-
ing stage [POM].
Analytical Data
H1NMR: in ppm. For Hexyloxy homologue.
Ethylenes: (200 MHZ) δ (CDCl3) (ppm) 0.98 (-CH3-CH2 of –C8H17), 1.40 (long-CH2-
chain), 3.8 -OCH2 of (−OCH2 of C8H17), 6.58& 6.45, 8.40 & 8.30 (p-sub. benzene rings)
H1NMR: in ppm. For octyloxy homologue.
Ethylenes: (200 MHZ) δ (CDCl3) (ppm) 1.02 (-CH3-CH2 of –C8H17), 1.52 (long-CH2-
chain), 3.25 -OCH2 of (−OCH2 of C8H17), 6.69 & 6.45, 8.35 & 8.28(p-sub. benzene rings)
IR in Cm−1,
IR Spectrum For Hexyloxy homologue
Ethylenes: (vmax/cm−1):2982, 2878, 1475, 1338 (-C-H, aliphatic), 1728, 1256(ester group),
1740 (>C=O group), 1558 (-C=C-, aromatic), 1031, 1235 (ether group), 855.0 (p- sub.
benzene ring), 1622, 1475, 1464 (Aromatic ring).
Spectrum For Octyloxy homologue
Ethylenes:(vmax/cm−1):2928, 2828, 1450, 1364 (-C-H, aliphatic), 1716, 1233(ester group),
1728 (>C=O group), 1556 (-C=C-, aromatic), 1026, 1265 (ether group), 844.2 (p- sub.
benzene ring), 1632, 1470, 1478 (Aromatic ring),
Result and discussion
Novel chalconyl esters of liquid crystal materials were synthesized by condensing 4-n-alkoxy
benzoic acids and α-4-hydroxy phenyl 8–4ꢀ-fluoro benzoyl ethylene (m.p.90°C, yield 85%).
The transition temperatures of novel homologues are lower than corresponding dimeric 4-n-
alkoxy benzoic acids. Mesomorphic property commences from C3 homologue. Totally eleven
homologue derivatives (C1 to C16) are synthesized. C1 and C2 homologues are nonliquid crys-
tals and the rest of the homologues (C3 to C16) homologues are enantiotropically nematogenic
without exhibition of smectic property. Transition temperatures as recorded in Table 2 were
plotted versus the number of carbon atoms present in n- alkyl chain -R of –OR left terminal
end group and then Cr-I/N and N-I transition curves are obtained on linking like or related
transition points; as depicted in a phase diagram (Fig. 1). Cr-I/N transition curve follows its
zigzag path and descended in normal manner. N-I transition curve behaved in normal man-
ner with negligible deviation from C10 to C12 and then maximum at C14 homologue derivative.
Odd-even effect is observed for N-I transition curve from C3 to C8 homologue and then neg-
ligible or considerable deviation from normal expected behavior observed between C10 to C14
homologue. Odd-even effect diminishes from C3 to C8 and both curves merge into each other
at the C8 homologue. N-I transition curve propagates as a single transition curve from and
beyond the C8 derivative. The textures of nematic phase are threaded or Schlieren. Thermal

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
89
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
—
—
—
—
—
—
—
—
—
—
—
—
—
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
stability in average for nematic is 105.2°C and total mesophase length is ranging from 6.0°C
to 46.0°C at C8 and C14 homologue respectively. Analytical, thermal and spectral data sup-
port molecular structures of novel homologues. The thermotropic LC properties vary from
homologue to homologue in the same novel series.
Lowering of transition temperatures of novel homologues as compared to corresponding
dimeric n-Alkoxy benzoic acids are attributed to the breaking of hydrogen bonding between
two molecules of aromatic acids by esterification process. The condensation of two unlike
molecules of 4-n-alkoxy benzoic acid and a 4-hydroxy derivative of chalcone increases molec-
ular length, length to breadth ratio, ratio of the molecular polarity to polarizability, perma-
nent dipole moment across the long molecular axis, favorable magnitudes of dispersion forces
and dipole-dipole interactions, favorable combined effect of molecular rigidity and flexibil-
ity etc. which emerges the suitable magnitudes of anisotropic forces of intermolecular end
Figure . Phase behavior of series.

90
S. H. PANDYA AND V. R. PATEL
Figure . Structurally similar homologous series.
to end attractions and closeness commencing from C3 homologue to disalign and arrange
their molecules at an angle less than ninety degree on the plane of floating surface with sta-
tistically parallel orientational order of molecules, resisting exposed thermal vibrations of
proper rate of heating and of proper intensity of radiation. Thus, molecules of novel chal-
conyl ester derivatives(C3 to C16) facilitate the formation of nematic phase for definite range
of temperature in enantiotropic manner with total absence of smectogenic character even in
the monotropic condition. The absence of either smectic and/or nematic mesophase forma-
tion in either reversible or irreversible manner in C1 and C2 homologues is attributed to low
magnitudes of intermolecular dispersion forces and the low magnitudes of dipole dipole inter-
actions, leading to high crystallizing tendency. The exhibition of odd-even effect observed for
C3 to C8 homologue is due to the sequentially added methylene unit at the n-alkyl chain ‘R’ of
–OR group. The diminishing of odd-even effect from and beyond C8 homologue for higher
homologues of longer n-alkyl chain ‘R’ is attributed to the n-alkyl chain ‘R’ which may coil or
bend or flex or couple to lie with major axis of core structure of a molecule. The variations in
LC properties from homologue to homologue in the same series is due to the varying number
of carbon atoms present in n-alkyl chain R; which vary molecular flexibility in combination
with molecular rigidity from homologue to homologue as depended upon either its usual
expected or unusual unpredicted status, affecting suitable magnitudes of anisotropic forces of
intermolecular cohesions and closeness. The LC properties of presently investigated homolo-
gous series-1 are compared with structurally isomeric or similar analogous series-X[34] and
Y[35] selected for comparative study as shown below in Fig. 2.
Figure 2 shows that, homologous series-1 and X are isomeric with respect to each other,
whereas series-1 and Y are structurally analogous or similar to each other. Thus, homologous
series-1, X and Y are identical with respect to three phenyl rings and a central bridge linking
first and middle phenyl rings –COO-, which partly contributes to the total molecular rigid-
ity. Left n-alkoxy group –OR and tail ended flexible group –F are same for the same homo-
logue from series to series, which contributes equally to the total molecular flexibility for same
homologue but differ with respect to molecular rigidity. However, the homologous series 1,
Table . Relative thermal stability in °C.
Series
Series-
—
Series-X
Series-Y
Sm-N or Sm-I
.
(C– C)
C
.
(C– C)
C
.
(C– C)
C
Commencement of
smectic phase
N-I
Commencement of
nematic phase
Total mesophase lengths
from t°C to t°C
.
(C- C)
C
. to .
CC
—
. to .
CC
. to .
CC

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
91
X and Y differ with respect to a second central bridges–CH=CH-CO-, -CO-CH=CH- and
–N=N- linking middle and third phenyl rings which contributes partly to the total molecular
rigidity. Thus, variations in Thermotropic LC properties, behaviors or degree of mesomor-
phism will depend upon the varying features of series 1, X and Y for the same homologue from
series to series and from homologue to homologue in the same series. The molecular flexibil-
ity for the same homologue from series to series are same but their molecular rigidity differs
for the same homologue from series to series. The phenomena of mesomorphism are oper-
ated by combined effect of molecular rigidity plus molecular flexibility. Therefore combined
effect of molecular rigidity plus molecular flexibility varies from homologue to homologue
in the same series and for the same homologue from series to series depending on individ-
ual molecular structure of each, homologue of each series. Table 3 shows some thermotropic
properties of the series under comparative study.
Table 3 indicates that,
• Presently investigated novel series-1 is only nematogenic, series-X is only smectogenic,
and series-Y is smectogenic plus nematogenic.
• Smectogenic thermal stabilities decreases from series-X to Y to 1 or it increases from
series-1 to Y to X.
• Smectic mesophase commences from very first(C1) member of a series-X and it com-
mences late from C8 member of a series-Y whereas it does not commence until the last
member(C16) of series-1.
• Nematic mesophase commences from very first(C1) member of a series-Y and it com-
mences from C3 member of a series-1, but it does not commence until the last C16 homo-
logue of a series-X.
• Upper total mesophase length increases from series-1 to X to Y whereas it decreases from
series-Y to 1 to X.
A novel homologous series-1 and a series-X are isomeric to each of each other with dif-
fering place of linking carbonyl group of a chalconyl central bridge attached directly to third
or tail ended last phenyl ring (-CH=CH-CO-C6H5F-Series-1) or attached to middle phenyl
ring as (-C6H4-CO-CH=CH-Series-X). Therefore, the positional placement of carbonyl unit
of chalconyl central group, linking directly either with middle or third phenyl ring causes
variations in molecular rigidity and the combined effect of molecular rigidity plus molecular
flexibility and of differing magnitudes of its induced effectivity on respective flexibility of
series-1 and X, in terms of differing thermodynamic quantity ꢀH or heat of formation for
the same homologue from series to series. Thus, suitable magnitudes of anisotropic forces
of intermolecular cohesions and closeness are strong enough to arrange the molecules of
presently investigated series-1 in only statically parallel orientational order without facili-
tating the formation of lamellar packing of molecules in their crystal lattices under floating
condition under differing degree of molecular noncoplanarity which maintain only forma-
tion of nematic phase commencing from C3 homologue to last C16 homologue with absence
of smectogenic character for definite range of temperature. However, the positional status of
carbonyl unit of central chalconyl group (series-X) attached directly with middle phenyl ring
induces suitable magnitudes of anisotropic forces of intermolecular attraction which is being
stronger than intermolecular attraction between two molecules of series-1, which facilitate
the formation lamellar packing of molecules in the crystal lattices of molecules of series-X
to disalign the molecules of series, perpendicular to the plane of a floating surface and
subsequently layered structure of crystal lattices facilitate the floating of ordered molecular
organization, with sliding layered molecular arrangement favoring the extent of molecular
coplanarity under exposed thermal vibrations for definite range of temperature; excluding

92
S. H. PANDYA AND V. R. PATEL
formation and facilitation of subsequent nematic phase. Thus, the direct attachment of
carbonyl unit of chalconyl central bridge with middle or third phenyl ring of a substance
containing three phenyl rings and two central bridge; leads the facilitation and formation
of only smectic or only nematic phase formation respectively. The geometrical shape, size,
the extent of molecular planarity, polarity and polarizability, combined effect of molecular
rigidity and flexibility, intermolecular dispersive forces and favorable or suitable magnitudes
of dipole-dipole interactions and resulting molecular rigidity and flexibility due to planner
–N=N- in trans configuration of the molecules of a series-Y are strong enough and of suitable
magnitudes which facilitated the formation of nematic phase from very first member C1
homologue of a series to C10 homologue and the smectic mesophase formation from C8 to
C16 homologue. Thus, -N=N- (series-Y) central bridge is smectogenic plus nematogenic,
-(-ph-)-CO-CH=CH-(series-X) central bridge is only smectogenic and –CH=CH-CO-(-ph)
(Series-1) central bridge is only nematogenic. Thermal resistivity and mesophase lengths and
the commencement of mesophase (smectic or nematic) depends upon only suitable magni-
tudes of anisotropic forces of intermolecular cohesions and closeness as well as the extent of
molecular noncoplanarity which may cause early or late mesophase formation for the definite
time period or for the temperature range which are related to intensity of exposed thermal
vibrations. Thermal stabilities in average of LC substances are depended upon the resistivity
offered by the individual molecules against exposed thermal vibrations through energy stored
by them, which vary from one molecular structure to another molecular structure for the
same homologue from series to series or from homologue to homologue in the same series.
Conclusion
• Novel homologous series of present investigation is enantiotropically nematogenic with
absence of smectic property, whose mesophase length ranges between 6.0 and 46.0°C
and it is of lower middle ordered melting type.
• Chalconyl central group containing three phenyl rings may be only smectogenic if car-
bonyl unit of a central chalconyl group is directly bonded to middle phenyl ring as
–ph-CO-CH=CH- and it may be only nematogenic if carbonyl unit of a central chal-
conyl group is directly bonded to third (tailed) phenyl ring as –CH=CH-CO-ph- in case
of isomeric series, but it may not be true completely for nonisomeric and nonchalconyl
series.
• The group efficiency order derived for smectic and nematic on the basis of (a) ther-
mal stability(b) early commencement of mesophase and (c) upper and lower mesophase
lengths with reference to uncommon central bridges between any two phenyl rings.
(a) Smectic
-CO-CH=CH- > -N=N- > -CH=CH-CO-
Nematic
-N=N- > -CH=CH-CO- > -CO-CH=CH-
(b) Smectic
-CO-CH=CH- > -N=N- > -CH=CH-CO-
Nematic
-N=N- > -CH=CH-CO- > -CO-CH=CH-
(c) Total mesophase lengths
Lower: -N=N- > -CH=CH-CO- > -CO-CH=CH-
Upper: -N=N- > -CO-CH=CH- > -CH=CH-CO-

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
93
• Mesomorphism is very sensitive and susceptible to molecular structures with reference
to rigidity due to uncommon central bridges.
• Present study supports and raises the credibility to the conclusions drawn earlier and
adds a conclusion for positional status of carbonyl unit of chalconyl central bridge as
–ph-CO-CH=CH- and –CH=CH-CO-ph-.
Acknowledgments
Authors acknowledge thanks to the Dr. N.N. Vyas, the head of chemistry department and the principal
Dr. M.B. Patel and Prof. M.L. Chauhan 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. Sci-
ence and Home. Sc. College, Rajkot for his valuable suggestions, comments, co operation and support as
and when needed throughout this present investigation. Authors acknowledge thanks to NFDD center,
dept. of chemistry, Rajkot, for analytical and spectral services.
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