Mesomorphism and flexibility of alkyl chains in chalcone esters

Article abstract of DOI:10.1080/15421406.2016.1221989

A novel homologous series of chalconyl ester liquid crystals (LCs): RO?C₆H₄–CH:CH–COO?C₆H₄–CO?CH:CH?C₆H₄–OC₁₀H₂₁ (n) (para) has been synthesized and studied with a view to understanding and establishing the relationship between molecular structure and LC properties with reference to flexibility of the terminal chain. The novel series consists of eleven homologs (C₁–C₁₆). The C₁ and C₂ homologs are nonliquid crystals and the rest of the homologs (C₃–C₁₆) are enantiotropic nematic without exhibition of smectic mesophase. Transition and melting temperatures were determined by an optical polarizing microscope equipped with a heating stage. Textures of nematic phases are threaded or Schlieren. Cr-N/I transition curve adopts a zigzag path in a normal manner with overall descending behaviors. The N-I transition curve exhibits a sharp and shorter odd-even effect from C₃ to C₆ and it diminishes from and beyond C₆ for higher homologs of longer n-alkyl chain (R) in more or less or negligible deviating manner from normal descending tendency. Thermal stability for the nematic is 116.4°C and mesophase length ranges from 19°C to 67°C at the C₃ and C₁₂ homolog respectively. The LC properties of present novel series are compared with structurally similar analogous series to derive group efficiency order.

Full text of DOI:10.1080/15421406.2016.1221989

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Mesomorphism and flexibility of alkyl chains in
chalcone esters
N. R. Muniya & V. R. Patel
To cite this article: N. R. Muniya & V. R. Patel (2016) Mesomorphism and flexibility of alkyl
chains in chalcone esters, Molecular Crystals and Liquid Crystals, 638:1, 95-102, DOI:
10.1080/15421406.2016.1221989
To link to this article: http://dx.doi.org/10.1080/15421406.2016.1221989
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Date: 19 November 2016, At: 03:17

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
http://dx.doi.org/./..
Mesomorphism and flexibility of alkyl chains in chalcone esters
N. R. Muniya and V. R. Patel
Chemistry Department, Sheth P. T. Arts & Science College, Godhra, Gujarat University, Gujarat, India
ABSTRACT
KEYWORDS
Degree of mesomor;
enantiotropy; phismnematic;
smectic
A novel homologous series of chalconyl ester liquid crystals (LCs):
–
–
–
–
RO C6H4–CH:CH–COO C6H4–CO CH:CH C6H4–OC10H21 (n) (para) has
been synthesized and studied with a view to understanding and estab-
lishing the relationship between molecular structure and LC properties
with reference to flexibility of the terminal chain. The novel series con-
sists of eleven homologs (C1–C16). The C1 and C2 homologs are nonliq-
uid crystals and the rest of the homologs (C3–C16) are enantiotropic
nematic without exhibition of smectic mesophase. Transition and melt-
ing temperatures were determined by an optical polarizing microscope
equipped with a heating stage. Textures of nematic phases are threaded
or Schlieren. Cr-N/I transition curve adopts a zigzag path in a normal
manner with overall descending behaviors. The N-I transition curve
exhibits a sharp and shorter odd-even effect from C3 to C6 and it dimin-
ishes from and beyond C6 for higher homologs of longer n-alkyl chain
(R) in more or less or negligible deviating manner from normal descend-
ingtendency. Thermalstabilityforthenematicis116.4°Candmesophase
length ranges from 19°C to 67°C at the C3 and C12 homolog respectively.
The LC properties of present novel series are compared with structurally
similar analogous series to derive group efficiency order.
Introduction
Chalconyl ester thermotropic liquid crystalline [1] substances are useful to mankind in LC
devices and the pharmaceutical preparations through their bioactive properties [2–7]. The
present investigation is aimed to study the relationship between molecular structure [8–12]
and liquid crystal properties with special reference to the molecular flexibility of n-alkoxy
groups of longer n-alkyl chain [13–16]. The proposed investigation involves synthesis,
characterization of novel substances, and study of thermometric behaviors using optical
polarizing microscope (OPM) to provide information on thermal stability, commencement
of mesophase, degree of mesomorphism, and derivation of group efficiency order. Several
homologous series of esters, azoesters, and chalconyl esters have been reported to date
[17–23].
Experimental
Synthesis
4-Hydroxy benzaldehyde was alkylated by a usual established method [24a]. n-Alkoxy ben-
zaldehydes were treated with malonic acid to convert them into 4-n-alkoxy cinnamic acids
CONTACT V. R. Patel
University, Gujarat, India.
vrp@yahoo.co.in
ChemistryDept.,ShethP.T.Arts&ScienceCollege,Godhra,Gujarat
©  Taylor & Francis Group, LLC

96
N. R. MUNIYA AND V. R. PATEL
(A) [24b]. n-Alkoxy cinnamic acids were condensed with α-4hydroxy benzoyl-4ꢀ decyloxy
phenyl ethylene [25] [B] (m.p. 130°C) by a usual established method. Components A and B
were condensed [26] to give final products. The synthetic route to the series is shown below in
scheme 1. Final products were individually decomposed, filtered, washed, dried, and purified
until the constant transition temperatures obtained.
Scheme . Synthetic route to the series-.
Final product
The chemicals 4-hydroxyl benzaldehyde, malonic acid, alkyl halides, 4- hydroxy acetophe-
none, 4-decyloxy benzaldehyde, potassium Hydroxide, hydrochloric acid, etc. required for
synthesis were used as received except solvents which were dried and distilled prior to use.
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 was performed on Perkin Elmer PE 2400 analyzer
1
(Table 1). HNMR spectra were obtained on Bruker spectrometer using CDCl3 as solvent.
IR spectra were recorded on a Perkin-Elmer spectrum GX. Transition temperatures and liq-
uid crystal properties were investigated using an OPM with a heating stage.
Analytical data
H1NMR: in ppm. For Octyloxy homolog.
–
–
–
Ethylenes: (200 MHZ) δ (CDCl3) (ppm) 0.75 (−CH3 CH2 of –C8H17), 1.20 (long CH2
–
chain), 3.2 OCH2 of (–OCH2 of C8H17), 6.81 & 6.80, 8.20 & 8.05 (p-sub. benzene rings)

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
97
Table . Elemental analysis for the hexyloxy, octyloxy, and decyloxy derivatives.
Elemental % found (compared with % calculated)
Molecular formula
C
H
CHO
CHO
CHO
. (.)
.(.)
. (.)
. (.)
. (.)
. (.)
H1NMR: in ppm. For Decyloxy homolog.
–
–
–
Ethylenes: (200 MHZ) δ (CDCl3) (ppm) 0.90 (−CH3 CH2 of –C8H17), 1.19 (long CH2
–
chain), 3.86 OCH2 of (–OCH2 of C8H17), 6.69 & 6.89, 8.16 & 8.21 (p-sub. benzene rings)
IR in Cm−1,
IR Spectrum. For Oct−yl1oxy homolog
– –
Ethylenes: (vmax/cm ): 2928, 2835, 1456, 1389 ( C H, aliphatic), 1739, 1260 (ester
=
– = –
group), 1720 (>C O group), 1561 ( C C , aromatic), 1059, 1253 (ether group), 846.2 (p-
sub. benzene ring), 1620, 1498, 1456 (Aromatic ring).
IR Spectrum. For Decyloxy homolog
Ethylenes: (vmax/cm ): 2920, 2839.1, 1432, 1382 ( C H, aliphatic), 1736, 1239 (ester
−1
– –
=
– = –
group), 1730 (>C O group), 1523 ( C C , aromatic), 1063, 1253 (ether group), 849 (p-
sub. benzene ring), 1622, 1483.2, 1463 (Aromatic ring).
Textures by miscibility method
C5 … … … …..Threaded nematic
C8 … … … … …Threaded nematic
C16 … … … …Schlieren nematic
Result and discussion
Transition temperatures of novel homologs are relatively lower than corresponding n-alkoxy
aromatic acids. Eleven homologs were synthesized by condensing dimeric 4-n-alkoxy cin-
namic acids and α-4-hydroxy benzoyl β-4ꢀ-n-decyloxy phenyl ethylene [m.p. 130 yield
76.0%]. Transition temperatures (Table 2) as determined by OPM are plotted against the num-
ber of carbon atoms present in n-alkyl chain ‘R’ of –OR of left terminal end group. A phase
diagram (Fig. 1) is obtained by linking like or related points to form Cr-N/I and N-I transition
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
—
—
—
—
—
—
—
—
—
—
—
—
—
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

98
N. R. MUNIYA AND V. R. PATEL
Figure . Homologous Series: α- -[ꢀ-n-Alkoxy Cinnamoyloxy] β-benzoyl - ꢀꢀ- Decyloky phenyl Ethylenes.
curves showing phase behaviors of novel series. Cr-N/I transition curve adopted zigzag path
of rising and falling tendency in a usual established normal manner. The N-I transition curve
exhibits an odd-even effect from C3 to C6 which merges into a single curve from and beyond
the C8 homolog. The N-I transition curves for odd and even members are extrapolated to
nonliquid crystal homologs C1 and C2 to predict their hypothetical latent transition temper-
atures. Transition curve for odd members occupies a higher position for the N-I curve than
even members. The variations in mesomorphic (LC) properties from homolog to homolog in
the same series are observed with changing number of carbon atoms present in n-alkyl chain
–
‘R’ of –OR keeping OC10H21 (n) flexible tail end unchanged throughout entire novel series.
Analytical and spectral data support the molecular structures of homologs with threaded or
Schlieren textures of nematic phase.
Lowering of transition temperatures of novel homologs as compared to corresponding
dimerized n-alkoxy cinnamic acids are attributed to the breaking of hydrogen bonding
by way of esterification process. The exhibition of only nematic property with absence of

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
99
Figure . Structurally similar series.
smectic property by C3–C16 homologs is attributed to the favorable and suitable magnitudes
of anisotropic forces of end to end attractions to cause a statistically parallel orientational
order of the molecules. The absence of lamellar packing of molecules due to insufficient
intermolecular adhesion precludes the generation of a smectic phase. The lack of any meso-
morphism in the C1 and C2 homologs is attributed to the low magnitudes of intermolecular
dispersion forces and low magnitudes of dipole–dipole interactions leading to their high
crystallizing tendency. The exhibition of odd-even effect in N-I transition curve is attributed
to the odd and even number of carbon atoms in n-alkyl chain ‘R’ of –OR terminal group
from C3 to C6 homolog. Then the deviating behavior of N-I transition curve from C8 to C16
homologs is attributed to the uncertainty and unusual status of longer n-alkyl chain ‘R’ of
–OR of higher homologs from and beyond C6 or C8 homologs, from normal descending
tendency. The disappearance of odd even effect from C6 homolog is attributed to the coiling
or bending of the longer n-alkyl chain. The extrapolated [27–30] N-I transition curve to
C1 from C3 homolog matches with the isotropic temperature of C1 which precludes the
possibility of the nematic phase even in the monotropic condition. The predicted L.T.T. for
C2 from extrapolation is 110°C, but it is actually not realizable because of its high crystal-
lizing tendency. The changing trend in mesomorphic or LC properties from homolog to
homolog in the same present series is attributed to the sequentially and progressively added
methylene unit or units at the n-alkyl chain ‘R’ or –OR, left terminal end group which alters
the intermolecular forces of cohesion by varying magnitudes of molecular length and other
related cohesive forces of end to end attraction as a result of altering molecular flexibility from
homolog to homolog in the same series. The thermotropic behavior of present series such as
thermal stability, commencement of mesophase, and upper and lower mesophase lengths are
compared with the structurally similar analogous series X[31] and Y[32] as shown in Fig. 2.
It is clear from Fig. 2, that a homologous series-1 of present investigation and analogous
series-X and Y chosen for comparative study are identical with respect to three phenyl rings
–
=
–
and two central bridges vinyl carboxylate and –CO CH CH contributing to total molecular
rigidity and n-alkoxy group for the same homolog from series to series, but they differ from
homolog to homolog in the same series and for the same homolog from series to series with
–
respect to tailed end groups –OC10H21(n), OC14H29(n), and –OC8H17(n) which partially
contributes to total molecular flexibilities. Therefore, thermotropic properties and the degree
of mesomorphism, etc. depend upon the changing features of homologous series-1, X, and
Y. Thus, the changing magnitudes of molecular flexibility from series to series for the same
homolog and from homolog to homolog in the same series can be linked with changing trend
of LC properties of series-1, X, and Y under comparison. Some LC properties of series-1, X,
and Y are recorded in following Table 3 as under.
It is clear from Table 3 that,
r
The homologous series-1, X, and Y are enantiotropic nematic without exhibition of
smectic property.

100
N. R. MUNIYA AND V. R. PATEL
Table . Thermal stability in °C.
Series
Series- (-OCH)
Series- (-OCH
)
Series- (-OCH)
Sm-I or Sm-N Commencement of smectic phase
N-ICommencement of nematic phase
Total upper and lower mesophase lengths in °C
—
—
—
.(C–C)C
.–. C C
.(C–C)C
.–. C C
.(C–C)C
.–. C C
r
Mesomorphic properties commences earliest from C3 homolog in case of series-1 and
Y, but it commences late from C6 homolog for series-X.
Thermal stabilities are in decreasing order from series-1 to series-Y to series-X.
The upper mesophase length follows the same decreasing order of thermal stabilities
but lower mesophase length follows increasing order from series-Y to series-X equal to
series-1.
More or less or negligible deviation from normal descending tendency exhibited by
homolog of longer n-alkyl chain ‘R’ of –OR group from and beyond the merging of N-I
transition curves for odd and even homologs.
r
r
r
The missing of smectogenic character by all the series-1, X, and Y are attributed to the
absence of lamellar packing of the molecules. The exhibition of nematic property by the
C3–C16 or C6–C18 homologs is attributed to the suitable magnitudes of anisotropic forces of
intermolecular end to end attractions and closeness as a consequence of favorable molecular
flexibility at constant molecular rigidity which arrange the molecules in statistically parallel
orientational order in floating condition under the influence of exposed thermal vibrations,
facilitating nematic mesophase formation and stabilizing for some temperature difference
called as mesophase length or the degree of mesomorphism. The absence of LC state in case
of nonmesogenic homologs is due to their high crystallizing tendency from low dispersion
forces and low magnitudes of dipole–dipole interactions. Increasing order of thermal stability
and simultaneously same increasing order of upper mesophase lengths of series-1 to Y to
X is attributed to the order of thermodynamic quantity enthalpy ꢀH1, ꢀHX, and ꢀHY for
series-1, X, and Y, respectively, where ꢀH1 > ꢀHY > ꢀHX, that is, suitable magnitudes of
energy required to commence and stabilize, or to maintain and then to resist against exposed
thermal vibrations which corresponds to varying magnitudes of ꢀH, the heat of formation;
which vary from homolog to homolog in the same series and series to series for the same
homolog as a result of changing molecular flexibility due to changing tailed end groups,
–
–
–
OC10H21, OC14H29, and OC8H17. The observed deviations in N-I transition curves are
attributed to unusual and unexpected status of n-alkyl chain “R” of –OR and tailed end
groups of longer n-alkyl chains. Early or late commencement of mesophase depends upon
the extent of molecular noncoplanarity. Therefore, the extent of molecular noncoplanarity is
almost equivalent for series-1 and series-Y, but it differs for series-X which restricts suitable
magnitudes of anisotropic dispersion forces and the molecular arrangement as required to
form nematic mesophase under floating condition.
Conclusions
r
Homologous series of present investigation is enantiotropically nematogenic with
absence of smectic property and of considerable degree of mesomorphism.
Homologous series bearing longer n-alkoxy tail ends are of nearly equivalent thermo-
metric properties.
r

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
101
r
Group efficiency order derived on the basis of (a) thermal stability, (b) commencement
of mesophase, and (c) mesophase lengths for nematic as are under.
–
–
–
(a) Nematic: OC10H21(n) > OC8H17(n) > OC14H29(n)
–
–
–
–
–
–
(b) Nematic: OC8H27(n) = OC10H21(n) > OC14H29(n)
–
(C) Nematic: OC10H21(n) > OC8H17(n) > OC14H29(n)
–
–
OC10H21(n) = OC14H29(n) > OC8H17(n)
r
r
r
r
The molecular flexibility contribution towards mesomorphic properties is less effective
as compared to molecular rigidity.
Mesomorphism phenomena are very sensitive and susceptible to molecular structure of
a substance.
Chalconyl ester LC derivatives can be further studied as binary systems in the manufac-
ture of LC devices and for pharmaceutical preparation as being bioactive molecules.
Present investigation supports and raises credibility to the conclusions drawn earlier.
Acknowledgments
The authors acknowledge thanks to the Dr. N.N. Vyas, the head of chemistry department and the prin-
cipal Dr. M.B. Patel of the college for their supportive view and providing research facility of present
investigation. The authors are also thankful to Dr. A.V. Doshi, Ex. Principal, M.V.M. Science and Home.
Sc. College, Rajkot for his valuable suggestions, cooperation, and support as and when needed through-
out this investigation. The authors acknowledge thanks to L. M. Pharmacy collage, Ahmedabad for
analytical and spectral services.
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