Mesomorphism Dependence of Chalconyl Derivatives on Position of -CH=CH- Unit

Article abstract of DOI:10.1080/15421406.2015.1106280

A novel homologous series of heterocyclic chalconyl esters (C₁ to C₁₆) has been synthesized and studied with a view to understanding and establishing the effects of molecular structure on liquid crystal (LC) properties and the degree of liquid crystallinity. The C₁ to C₆ homologues are not liquid crystals, the C₇ homologue is enantiotropic nematic, the C₈ to C₁₆ homologues are smectogenic in addition to nematogenic (C₈ & C₁₀ monotropic smectic). The texture of nematic phases are threaded or Schlieren and that of the smectic phase are fan-shaped smectic-A or smectic-C type. Transition temperatures and the textures of mesophases were determined by an optical polarizing microscope equipped with a heating stage. Transition curves Cr-I/M, Sm-N/N-Sm, and N-I showing phase behaviors of series in a phase diagram behave in normal manner. Thermal stability for smectic and nematic are 115.66°C and 126.83°C and their mesophase lengths minimum to maximum are 6.3 to 12.9°C and 6.9°C to 17.4°C, respectively. Analytical and spectral data confirms the molecular structures of homologues. LC properties of present novel series are compared with structurally similar other reported series.

Full text of DOI:10.1080/15421406.2015.1106280

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Mesomorphism Dependence of Chalconyl
Derivatives on Position of -CH=CH- Unit
P. K. Rakhasia, G. N. Bhola & U. C. Bhoya
To cite this article: P. K. Rakhasia, G. N. Bhola & U. C. Bhoya (2016) Mesomorphism
Dependence of Chalconyl Derivatives on Position of -CH=CH- Unit, Molecular Crystals and
Liquid Crystals, 626:1, 81-89, DOI: 10.1080/15421406.2015.1106280
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Date: 19 May 2016, At: 20:12

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
http://dx.doi.org/./..
Mesomorphism Dependence of Chalconyl Derivatives on
=
Position of -CH CH- Unit
P. K. Rakhasia, G. N. Bhola, and U. C. Bhoya
Department of Chemistry, Saurashtra University, Rajkot, Gujarat, India
ABSTRACT
KEY WORDS
Liquid crystals;
mesomorphism; nematic;
smectic
A novel homologous series of heterocyclic chalconyl esters (C1 to C16) has
been synthesized and studied with a view to understanding and estab-
lishing the effects of molecular structure on liquid crystal (LC) proper-
ties and the degree of liquid crystallinity. The C1 to C6 homologues are
not liquid crystals, the C7 homologue is enantiotropic nematic, the C8
to C16 homologues are smectogenic in addition to nematogenic (C8 &
C10 monotropic smectic). The texture of nematic phases are threaded
or Schlieren and that of the smectic phase are fan-shaped smectic-A or
smectic-Ctype. Transitiontemperaturesandthetexturesofmesophases
were determined by an optical polarizing microscope equipped with a
heating stage. Transition curves Cr-I/M, Sm-N/N-Sm, and N-I showing
phase behaviors of series in a phase diagram behave in normal man-
ner. Thermal stability for smectic and nematic are 115.66°C and 126.83°C
and their mesophase lengths minimum to maximum are 6.3 to 12.9°C
and 6.9°C to 17.4°C, respectively. Analytical and spectral data confirms
the molecular structures of homologues. LC properties of present novel
series are compared with structurally similar other reported series.
Introduction
The liquid crystal (LC) is an intermediate state of matter between a crystalline solid and an
isotropic liquid The LC state has proved useful to mankind and has developed the interest
of scientists and technologists all over the world irrespective of their subject of specialization
[1]. This study of the LC state through novel chalconyl substances was planned to understand
and establish the effects of molecular structure on thermotropic LC properties [2–5] with a
view to providing novel substances to explore the thermometric properties and then biological
activities by lyotropic properties. Thus, the proposed investigation can be useful in providing
novel LC materials to other scientific and technological research communities [6–14]. Many
homologous series have been reported to date [15–19]. The novel homologous series consists
–
–
–
=
–
–
of two phenyl rings and one heterocyclic ring bonded through COO and CH CH CO
central bridges and para substituted -OR terminal end group whose flexibility changes with n-
alkoxy group; whose study of thermotropic LC properties are included in present investigation
but their lyotropic study is for pharmaceutical and medicinal research. Some thermotropic
LC properties of the novel homologous series will be compared, discussed and interpreted in
terms of molecular rigidity and flexibility [20–24] of structurally similar isomeric series.
CONTACT U. C. Bhoya
drucbhoya@sauuni.ernet.in
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

82
P. K. RAKHASIA ET AL.
Scheme . Synthetic route to the novel series.
Experimental
Synthesis
4-Hydroxy benzoic acid was alkylated by suitable alkylating agents by the modified method of
Dave and Vora [25]. 3-(4-Hydroxy phenyl) 1-(thiophene-2-yl) prop-2-en-1-one was prepared
by a well-known method [26]. 4-n-Alkoxy benzoic acids were converted to corresponding
acid chlorides using freshly distilled thionyl chloride and condensed with an ice cold solu-
tion of the chalcone in pyridine [27]. Final products were individually decomposed in 1:1
hydrochloric acid aqueous solution, filtered, washed, dried and purified until a homologue
gave constant transition temperatures.
4-Hydroxy benzoic acid, alkyl halides, KOH, thionyl chloride, methanol, ethanol, pyridine,
4-hydroxy benzaldehyde, 2-acetyl thiophene etc. were used as received except solvents which
were dried and purified prior to use. The synthetic route of the novel homologous series is
shown in Scheme 1.
Table . Elemental analysis for () Propyloxy () Heptyloxy () Decyloxy derivatives.
Elements%Found
H
Elements% Calculated
H
Sr.No.
Molecular formula
C
O
C
O



CHOS
CHOS
HOS
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
C

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
83
Characterization
Representative members of the novel series were characterized by elemental analysis (Table 1),
Infrared spectroscopy, 1HNMR spectroscopy, and mass spectroscopy. Microanalysis was per-
formed on Perkin-Elmer PE 2400 CHN analyzer. IR spectra were recorded on Shimadzu
FTIR-8400, 1H NMR spectra were recorded on Bruker spectrometer using CDCl3 as solvent
and mass spectra were recorded on Shimadzu GC-MS Model No.QP-2010. The mesogenic
behavior of the homologue derivatives were observed through polarizing microscopy. The
textures of nematogenic mesophase were determined by miscibility method.
Analytical data
Spectral data
H NMR in δ ppm for ethyloxy derivative
1.44–1.48 (t, 3H,-CH3 of -OC2H5 group), 4.00–4.15 (m, 2H, Ar-O-CH2), 7.18–7.20 & 8.13–
8.15 (4H of Phenyl ring with n-propoxy group), 7.38–7.42 & 7.84–7.88 (4H of middle Phenyl
ring), 7.26–7.29 & 8.13–8.15 (3H of thiophene ring with chloro group) 7.68–7.71 (d, 1H,
–
–
=
–
–
–
=
–
CO CH CH ), 6.96–6.98 (d, 1H, CO CH CH ). The NMR data are consistent with the
molecular structure.
H NMR in δ ppm for pentyloxy derivative
0.94–0.96 (t, 3H,-CH3 of –OC5H11 group), 1.34–1.36 (m, 2H, Ar-O-CH2-CH2-CH2-CH2),
1.38–1.49 (m, 2H, Ar-O-CH2-CH2-CH2), 1.77–1.84 (m, 2H, Ar-O-CH2-CH2), 4.00–4.03 (m,
2H, Ar-O-CH2), 7.15–7.17 & 8.14–8.16 (4H of Phenyl ring with n-propoxy group), 7.37–7.41
& 7.82–7.87 (4H of middle Phenyl ring), 7.25–7.87 (3H of thiophene ring with chloro group),
=
=
7.66–7.69 (d, 1H, -CO-CH CH-), 6.95–6.97 (d, 1H, -CO-CH CH-). The NMR data are
consistent with the molecular structure.
IR in cm− for methyloxy derivative
3078 (= C-H Str. of Phenyl nucleus), 2931 & 2846 (CH2 sym. & asym. stra.), 1720 (Aromatic
=
–
C = O Str.), 1643 (aromatic C C str.), 1512 (C-C aromatic str.), 1465 (C H bend alkanes),
1327 (aromatic& aliphatic C-O str.), 1157 (Ar-O str.), 840 (C-H oop. phenyl ring), 725 & 686
(C-H oop.bending of phenyl ring). The IR data are consistent with the molecular structure.
IR in cm− for hexyloxy derivative
= –
3070 ( C H Str. of Phenyl nucleus), 2954 & 2854 (CH2 sym. & asym. stra.), 1720 (Aromatic
=
=
–
–
C O Str.), 1643 (aromatic C C str.), 1512 (C C aromatic str.), 1465 (C H bend alkanes),
–
–
1327 (aromatic& aliphatic C O str.), 1165 (Ar-O str.), 848 (C H oop. phenyl ring), 709 & 694
=
(C H oop.bending of phenyl ring). The IR data are consistent with the molecular structure.
Mass of octyloxy derivative
m/z (rel.int%): 462 (M)+, 233, 201, 121, 111, 93, 57, 43
Mass of dodecyloxy derivative
m/z (rel.int%): 518 (M)+, 317, 289, 201, 191, 121, 111, 93, 57, 43

84
P. K. RAKHASIA ET AL.
Table . Transition temperatures in°C.
Transition temperatures in°C
Compound No.
R = n-alkyl chain CnHn+
Sm
N
Isotropic
























—
—
—
—
—
—
—
(.)
(.)
.
.
.
—
—
—
—
—
.
.
.
.
.
.
.
.
.
.
.
.
—
.
.
.
.
.
.
Sm, Smectic; N, Nematic; () indicate monotropy.
Results and discussion
3-(4-Hydroxy phenyl) 1-(thiophene-2-yl) prop-2-en-1-one (m.p-128–130°C; yield- 68.50%)
is a nonliquid crystalline component, which on condensing it with dimeric 4-n-alkoxy ben-
zoic acids yielded targeted molecules of chalconyl ester heterocyclic derivatives. Dimerization
of n-alkoxy benzoic acids disappears on esterification process through the corresponding acid
chloride. Transition temperatures (Table 2) were plotted versus the number of carbon atoms
present in n-alkyl chain bonded to first phenyl ring through oxygen atom. Smooth curves are
drawn through like or related points to show the phase behavior of the series in a phase dia-
gram (Fig. 1). Cr-I/M transition curve partly adopts a zigzag path of rising and falling with
overall descending tendency. Sm-N, or N-Sm transition curve initially rises from C8 homo-
logue, then it passed through a maxima at C14 homologue and finally descended to C16 in
usual expected manner. Hence Cr-I/M and Sm-N/N-Sm transition curves behaved in normal
Figure . Phase Behavior of Series.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
85
manner. Sm-N Curves behaved in normal manner. Sm-N curve is extrapolated to C6 homo-
logue to predict its latent transition temperature (L.T.T.) for smectic (83°C). N- I transition
curve initially rises from C7 to C14 and then descended, as series is ascended. Thus, N-I tran-
sition curve behaved in normal manner except negligible derivation at C14. Odd–even effect
in the transition curves of phase diagram is completely absent. The mesomorphic properties
and mesophase behaviors from homologue to homologue in the present novel series undergo
variations. The smectic and nematic thermal stabilities are 115.66°C and 126.83°C respec-
tively, whose total mesophase length (Sm+N) very between 6.9°C and 26.8°C at the C7 and
C
14 homologues, respectively. Smectic and nematic mesophases commences from C8 and C7
homologues respectively. Hence a novel chalconyl ester homologous series under discussion
is party nematogenic and party smectogenic of middle ordered melting type and of low degree
of mesomorphism.
Disappearance of dimerization of n-alkoxy benzoic acids is attributed to the breaking of
hydrogen bonding between acid molecules by the esterification process through their corre-
sponding acid chlorides. The nonmesomorphicity of C1 to C6 homologues is attributed to
their high crystallization tendency with arises from the low diploe–dipole interaction and
the low magnitudes of dispersion forces by the interaction between instantaneous dipole pro-
duced by the spontaneous oscillations of electron clouds of the molecules C1 to C6 which
induces unsuitable magnitudes of anisotropic forces of end to end and lateral attractions as
consequence of unfavorable molecular rigidity and flexibility including abrupt breaking of
crystal lattices and sharply transform into isotropic state without passing through LC state at
Cr-I transition temperature. As series is ascended by progressive addition of methylene unit
the molecular length, length to breadth ratio, polarity and polarizibility, permanent dipolemo-
ment across, the long molecular axis, dispersion forces, dipole-dipole and electronic interac-
tions, magnitudes of end to end and lateral attractions etc. alters and create the favorable envi-
ronment to include mesophase formation by disalignment of molecules at an angle ninety or
less than ninety degree and resists the exposed thermal vibration. Hence C8 to C16 homologue
molecules arrange themselves in sliding layered arrangement on reversible heating (C12 to
C16) or irreversibly (C8 and C10) on coiling due to preoccupied lamellar packing of molecules
in their crystal lattices of rigid crystals below N-I transition temperature under floating condi-
tion to exhibit smectic mesophase formation. C7 to C16 homologue molecules resist or extend
to resist above Cr-Sm or Cr-N transition temperatures; arrange themselves to occupy sta-
tistically parallel orientational order of molecules in floating condition to cause formation
of nematic phase. However, molecules of C7 to C16 and C1 to C6 are randomly oriented in
all possible directions with high order of disorder or high entropy (ꢀS = ꢀH/T) from and
beyond isotropic temperature. The isotropic mass on careful cooling, the C1 to C6 homo-
logues solidified to crystalline state, C7 homologues solidified to crystalline state after acquir-
ing nematogenic phase from and below isotropic temperature, C8 and C10 acquired reversible
nematic phase and then irreversibly or monotropically smectic phase and then solidified to
crystalline state below isotropic temperature. Homologues C12 to C16 molecules acquired on
coiling solid crystalline state in reversible manner below isotropic temperature i.e. the smectic
as well as nematic mesophases persisted exactly at the same transition temperatures at which
they were appeared on heating i.e. Sm-N and N-I transition temperatures are enantiotropic
and reversible for C12 to C16 homologues. Absence of odd–even effect in Sm-N and N-I tran-
sition curves is due to the late commencement of mesophase formation (C7 or C8) for longer
n-alkyl chain, whose status of n-alkyl chain possess uncertainty by coiling bending, flexing or
coupling with major axis of the core structure. The changing trend in mesomorphic behaviors
from homologue to homologue in the present series is attributed to the sequential addition of

86
P. K. RAKHASIA ET AL.
Figure . Structurally similar series.
methylene unit or units to the n-alkyl chain of the left n-alkoxy terminal end group, keeping
heterocyclic thiophene ring intact.
The LC behaviors, properties and the degree of mesomorphism which varies from homo-
logue to homologue in the same series and for the same homologue from series to series are
compared with an isomerism series-X [28] as under in Figure 2.
Homologous series-1 of present investigation and an isomeric series-X chosen for compar-
ative study are identical with respect to two phenyl rings bonded through same -COO- cen-
tral bridge heterocyclic thiophene tail end group but they differ from each other with respect
–
=
–
–
–
=
–
to other central bridges viz. CH CH CO and CO CH CH linking middle phenyl ring
and five-membered sulpher containing heterocyclic ring. However series-I and X can be com-
pared for the same homologue from series to series, depending upon the positional difference
–
–
–
=
–
or displacement of CO and/or CH CH of central bridge linking phenyl ring and hete-
rocyclic tail ring. Thus the LC properties and the degree of mesomorphism of present novel
series-1 and an isomeric series-X as chosen for comparison can be compared on the basis of
differing features of uncommon central bridge other than a common central bridge other than
–
–
a common central bridge COO for the same homologue. Some LC properties of series-1
and X are recorded in Table 3.
From Table 3 it is clear that,
• Present homologues series-1 is smectogenic in addition to nematogenic, whereas series-
X under comparative study is only smectogenic.
• Thermal stabilities for smectic of series-1 and X are nearly equivalent because it is higher
by 2.09 unit from series-X (115.66°C–113.57°C).
• Nematic thermal stability of series-1 is 126.83°C but nematic mesophase does not stabi-
lize till the lost homologue of series-X.
• Smectic and nematic mesophases commences from C8 and C7 homologues of a series-
1, where as nematic mesophase it totally absent and smectic mesophase commence late
from C10 member of a series-X under comparison.
Table . Average thermal stabilities in°C.
–
=
–
–
–
–
=
–
Series
 ( CH CH CO )
X ( CO CH CH )
Smectic-Isotropic or
Smectic-Nematic
Commencement of smectic phase
Nematic-Isotropic commencement of nematic phase
.
(C-C)
C
.
(C-C)
C
.
(C-C)
C
—
—
Total Mesophase length range in°C (Sm+N)
. – .
C C
. - .
C C

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
87
• Mesophase length (Sm+N) range of present series-1 is more than a series-X.
• Odd–even effect is absent in transition curves of both series-1 and series-X under com-
parative study.
Homologous series -1 of present investigation and an isomeric series -X of selected for com-
–
–
–
=
–
parative study resembles each other except positional replacement of CO or CH CH .
2
2
–
=
–
–
–
The sp carbon of CH CH bonded to sp carbon of phenyl ring or CO group of uncom-
mon central bridge is bonded to heterocyclic ring in the molecules of series-1 whereas
–
=
–
–
–
CH CH and CO groups are conversely bonded to heterocyclic ring and to the phenyl
–
–
ring respectively. The first alternative and second alternative of the position of CO and
–
=
–
CH CH are almost equally competent to induce and stabilize smectogenic property of
nearly equal magnitudes of thermal stability (115.66°C and 113.57°C) due to the suitable mag-
nitudes of inter molecular lateral attractions whose layers are shielded out from the lattices
of rigid crystals under the influence of exposed thermal vibrations. However, the molecules
=
–
–
of series-1 due to the –CH CH – bonded to middle phenyl ring (or CO group bonded to
hetero cyclic ring) strengthens the end to end attractions of such a suitable magnitudes that
layers under floating condition arranges or acquires statistically parallel orientational molecu-
lar ordered arrangement subsequent to smectic to induce nematic phase at Sm-N or Cr-N/I-N
temperature. The end to end intermolecular attractions of series-X of second alternative are
weaker as compared to series-1 of first alternative of chalconyl group which fails to main-
tain or emerge or stabilize nematic phase formation. Thus, magnitudes of molecular rigidity
differs (ꢀH) due to replacement of CO or CH CH plays the role of mesomorphism.
Thus, nematogenic mesophase stabilized beyond smectogenic phase in series-1 but nemato-
genic mesophase formation does not stabilize beyond smectogenic mesophase in isomeric
series-X. The commencement of mesophase early or late depends upon the extent of non-
–
–
–
=
–
–
=
–
planarity of a molecule. The bonding of CH CH unit directly at the middle phenyl ring
strengthens the molecular rigidity comparatively more in series-1 than, if it ( CH CH )
–
=
–
–
–
is bonded through CO group with middle phenyl ring, which reduces the extent of non-
coplanarity of molecules and favors layered arrangement of molecules and favorable end to
end attraction more efficiently in the preoccupied lamellar molecular packing in crystal lat-
tices of rigid crystal and in sliding layered arrangement under heating condition. Therefore
smectic mesophase formation take place earlier from C8 homologue in series-1 as compared
to isomeric series-X, in which smectic mesophase commences late from C10 homologue.
–
=
–
Moreover reduction in the extent of molecular non co-planarity by bonding CH CH
unit directly with middle phenyl ring favors nematic mesophase formation beyond smec-
–
=
–
–
–
tic phase; whereas bonding of CH CH unit through CO group with middle phenyl
ring and the resulted cohesive forces hinders or are unable to induce nematic phase beyond
smectic phase under exposed thermal vibrations while floating on the surface as observed
in series-X. The exhibition of total mesophase formation for the same homologue from
series to series is related with the intermolecular cohesive energy associated with individ-
ual molecules for the same homologue from series to series and from homologue to homo-
logue in the same series i.e. thermal resistivity towards exposed thermal vibration of different
suitable magnitudes vary from homologue to homologue or in general like or unlike sub-
stance to substance. Hence, variation in mesophase length range from minimum to maxi-
mum varies from series-1 to isomeric series-X under comparison. The absence of odd- even
effect for series-1 and X are attributed to the late commencement of mesophases or due to
the absence of mesophase form C1 to C6 or C1 to C8 homologue molecules in series-1 and X,
respectively.

88
P. K. RAKHASIA ET AL.
Conclusions
• Presently investigated novel chalconyl ester series-1 is partly smectogenic and partly
nematogenic with middle-ordered melting type.
• The group efficiency order derived on the basis of
r
(a)Thermal stability (b) early commencement of mesophase (c) mesophase length range
for smectic and nematic in isomeric series are as under.
–
=
–
–
–
–
=
–
(a) Smectic : CH CH CO > CO CH CH
–
=
–
–
–
–
=
–
Nematic: CH CH CO > CO CH CH
–
=
–
–
–
–
=
–
(b) Smectic : CH CH CO > CO CH CH
–
=
–
–
–
–
=
–
Nematic: CH CH CO > CO CH CH
–
=
–
–
–
–
=
–
(c) Sm+N: CH CH CO > CO CH CH
–
=
–
–
• CH CH CO central bridge is more efficient to from mesophase or mesogenic phase
–
–
=
–
than an isomeric chalconyl central group CO CH CH .
–
=
–
• The bonding of CH CH group directly to middle phenyl ring is more efficient to
induce mesophase or mesophase of higher phase length and earlier commencement of
–
=
–
–
–
mesophase as compared to the linking of CH CH group through CO with middle
phenyl ring.
• Phenomena of mesomorphism are very sensitive and susceptible to molecular structure.
• Chalconyl ester homologues of present investigation are useful for the study of binary
systems to prepare articles workable at required temperature. Chaloconyl derivative are
being biologically active molecules related to anticancer, antimalarial, antibacterial, anti-
dermitological etc. may be studied by the researchers working for pharmaceutical and
medicinal fields.
Acknowledgments
Our thanks to the Department of Chemistry (DST-FIST Funded & UGC-SAP Sponsored), Saurashtra
University, Rajkot, for research work. Special thanks to “National Facility for Drug Discovery 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 co-operation and suggestions during
present investigation as and when needed.
References
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[2] Imrie, C. T. (1999). Liq. Cryst. Dim., Struct., Bond, 95, 149.
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