Mesomorphism dependence on molecular flexibility

Article abstract of DOI:10.1080/15421406.2016.1146867

A novel ester homologous series of liquid crystals with a highly polar laterally substituted –OCH₃ group is synthesized and studied with a view to understanding and establishing the effects of molecular structure involving lateral substitution on liquid crystal behavior. The novel series RO·C₆H₄·COO·C₆H₃·OCH₃(ortho)CH=CH·COO·C₂H₅ consists of 12 homologs of which C₇, C₈, C₁₀ are nematogenic and C₁₂, C₁₄, C₁₆ homologs are smectogenic. The rest of the homologs are nonliquid crystals. The textures of nematic phase are threaded and the smectic phase is of the type A or C. Transition temperatures and the textures of mesophases were determined by an optical polarizing microscope equipped with a heating stage. Analytical and spectral data support the molecular structures. The N-I and Sm-I transition curves behave in normal manner without exhibition of an odd-even effect. Average thermal stability for smectic and nematic are 127.3°C and 129°C, respectively. The total mesophase length ranges from 17°C to 57°C and series is of a middle ordered melting type with transition temperatures varying between 82°C and 148°C. The liquid crystal properties of present novel series are compared with, structurally similar known homologous series.

Full text of DOI:10.1080/15421406.2016.1146867

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Mesomorphism dependence on molecular
flexibility
J. J. Travadi, M. S. Vadodaria, K. D. Ladva & A. V. Doshi
To cite this article: J. J. Travadi, M. S. Vadodaria, K. D. Ladva & A. V. Doshi (2016)
Mesomorphism dependence on molecular flexibility, Molecular Crystals and Liquid Crystals,
630:1, 69-78, DOI: 10.1080/15421406.2016.1146867
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Date: 21 July 2016, At: 23:18

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
http://dx.doi.org/./..
Mesomorphism dependence on molecular flexibility
J. J. Travadia, M. S. Vadodariab, K. D. Ladvab, and A. V. Doshic
aChemistry Department, Kamani Sc. College and Prataprai Arts College, Amreli, Gujarat, India; bChemistry
Department, Shree M. & N. Virani Sc. College, Rajkot, Gujarat, India; cEx. Principal, Matushri Virbaima Mahila
Science and Home Science College, Rajkot, Gujarat, India
ABSTRACT
KEYWORDS
Azoester; liquid crystals;
nematic; mesomorphism;
smectic
A novel ester homologous series of liquid crystals with a highly polar
laterally substituted –OCH3 group is synthesized and studied with a
view to understanding and establishing the effects of molecular struc-
ture involving lateral substitution on liquid crystal behavior. The novel
series RO·C6H4·COO·C6H3·OCH3(ortho)CH=CH·COO·C2H5 consists of 12
homologsofwhichC7, C8, C10 arenematogenicandC12, C14, C16 homologs
are smectogenic. The rest of the homologs are nonliquid crystals. The
textures of nematic phase are threaded and the smectic phase is of the
type A or C. Transition temperatures and the textures of mesophases
were determined by an optical polarizing microscope equipped with a
heating stage. Analytical and spectral data support the molecular struc-
tures. The N-I and Sm-I transition curves behave in normal manner with-
out exhibition of an odd-even effect. Average thermal stability for smec-
tic and nematic are 127.3°C and 129°C, respectively. The total mesophase
length ranges from 17°C to 57°C and series is of a middle ordered melt-
ing type with transition temperatures varying between 82°C and 148°C.
The liquid crystal properties of present novel series are compared with,
structurally similar known homologous series.
Introduction
Liquid crystalline (LC) state of substance [1] is universally accepted as an useful physical state
of a substance to human beings [2–6]. The present investigation was planned with a view to
understanding and establishing the effects of molecular structure on LC properties by synthe-
sis of novel LC materials through homologous series by changing molecular structural geom-
etry with reference to laterally substituted group of esters. [7–10] Many homologous series
have been reported to date [11–20]. The present series consists of two phenyl rings, bonded
through –COO– central bridge, changing –OR and fixed –CH=CH-COO-CH2-CH3 flexible
end groups as well as –OCH3 laterally substituted functional group. The mesomorphicity, tex-
tures of mesophase and characterization of novel homologs will be performed and the results
will be evaluated and interpreted in terms of molecular rigidity and flexibility [21–25]. Group
efficiency order will be derived for smectic and nematic. Then, the data of the novel investi-
gation will be compared with the structurally similar known series.
CONTACT J. J. Travadi
Amreli, Gujarat, India.
j.travadi@gmail.com
Chemistry Department, Kamani Sc. College and Prataprai Arts College,
©  Taylor & Francis Group, LLC

70
J. J. TRAVADI ET AL.
RBr / KOH
RO
COOH
HO
COOH
MeOH / Reflux
+ SOCl2
(-SO2, -HCl)
CH=CH-COOC2H5
HO
RO
COCl
+
( In Pyridine )
H3CO
CH=CH-COOC2H5
RO
COO
H3CO
Ethyl-4-(4'-n-alkoxy benzoyloxy)-3-methoxy-cinnamate
R= –CnH2n+1 where, n = 1,2,3,4,5,6,7,8,10,12,14,16
Scheme . Synthetic route to the series.
Experimental
Synthesis
4-hydroxy benzoic acid was alkylated by suitable alkylating agent (R-X) to convert it into
dimeric 4-n-alkoxy benzoic acids by the modified method of Dave and Vora [26]. 4-Hydroxy
3-methoxy cinnamic acid (m.p. 64°C yield 67 %;) was esterified by ethanol by using conc.
H2SO4 by usual established method [27]. The ester was cream-colored soft crystalline sub-
stance (M. Pt 63°C–65°C), which purified from distilled n-hexane. The yield was 67.5%;.
Dimeric 4-n-alkoxy benzoic acids were individually condensed with 4-hydroxy, 3-methoxy,
ethyl cinnamate in dry cold pyridine through corresponding acid chlorides [28]. Final prod-
ucts were individually decomposed, filtered, washed, dried, and purified till constant transi-
tion temperatures obtained.
The synthetic route to the series is shown below in scheme 1.
The chemicals required, 4-Hydroxy benzoic acid, MeOH, KOH, Alkyl halide (R-X),
Thionyl chloride, 4-hydroxy,3-methoxy, cinnamic acid (Ferulic acid), Ethanol, Conc. H2SO4,

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
71
Pyridine, 1:1 HCl, NaHCO3, NaOH, anhydrous CaCl2, Silicagel, Petrolium ether, Methyl
acetate, and n-hexane were used as received, except solvents which were dried and purified
prior to use.
Characterization
Representative homologs of the series were selected for characterization of structure by ele-
mental analysis, IR spectra and 1H NMR spectroscopy.
Microanalysis was performed on Perkin Element PE 2400 CHN analyzer. IR spectra were
recorded on Perkin Elmer spectra GX. Enthalpy-entropy concept discussed qualitatively with-
out performance on DSC. 1H NMR spectroscopy performed were recorded on Bruker using
CDCl3 as solvent.
Mesomorphic properties were investigated using hot stage polarizing microscope.
Analytical data
IR spectra in cm−1 for propyloxy and hexyloxy homologoue derivatives
Propyloxy
The IR spectrum of the compound showed
693.95 cm−1 polymethylene of -C3H7 group.
847.42 and 771.69 cm−1 conforms para and ortho sub. benzene ring,
1165.74 and 1044.09 cm−1 as -CO- of alkoxy group,
1248.09, 1600.78, and 1708.40 cm−1 conforms -COO- of ester group,
966.59 cm−1 conforms trans -CH=CH- group,
2874.32 and 2964.13 cm−1 conforms alkyl group.
IR supports the structure.
Hexyloxy
The IR spectrum of the compound showed
691.50 and 644.09 cm−1 polymethylene of -C6H13 group.
844.67 and 769.44 cm−1 as p-sub. and ortho-substituted benzene ring,
1155.41 and 1024.09 cm−1 as -CO- of alkoxy group,
1730.82, 1679.84, 1601.77, and 1244.13 cm−1 as -COO- & -CO- of ester group,
984.12 cm−1 conforms trans -CH=CH- group,
2928.99 and 2868.20 cm−1 conforms alkyl group.
IR supports the structure.
1H NMR spectra in ppm for pentyloxy and dodecyloxy derivatives
Pentyloxy
H1 NMR data in δ ppm (n-pentyloxy derivative, 400 MHz, CDCl3, standard TMS)
0.8863 (-CH3 of -C5H11), 1.7454, 1.2628, and 1.1792 (-CH2- of -C5H11), 1.3475 (-OCH2-CH2
of -C5H11), 3.7744 (-O-CH3), 3.9511 (-O-CH2- of -C5H11), 6.8715, and 7.1917 (-O-C6H3-
CH=CH-CO-), 7.9651, and 7.9938 (p-sub. Benzene ring).
The data supports the molecular structure.

72
J. J. TRAVADI ET AL.
Table . Elemental analysis for propyloxy, hexyloxy, decyloxy, and dodecyloxy derivatives.
%; of C
Experimental
%; of H
Theoretical
Compound no.
Molecular formula
Experimental
Theoretical
C
C
C
C
CHO
CHO
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
C 
H O
CHO
Dodecyloxy
H1 NMR data in δ ppm (n-dodecyloxy derivative, 400MHz CDCl3, standard TMS):
0.8182 (-CH3 of -C12H25), 1.2425 to 1.7711 (-CH2- of -C12H25), 1.3928 (-OCH2-CH2 of -
C12H25), 3.8128 (-O-CH3), 3.9595 (-O-CH2- of -C12H25), 6.8535 & 6.8668 & 7.1923 ( -O-
C6H3-CH=CH-CO-), 7.9644, and 7.9930 (p-sub. Benzene ring).
The data support the molecular structure.
Mass spectra
Homolog
Theoretical
Experimental
C
C
.
.
.
.
Textures of mesophase by miscibility method
Octyloxy homolog→Threaded Nematic
Tetradecyloxy homolog → Smectic–C
Results and discussion
4-Hydroxy-3-methoxy ethyl cinnamate (M. Pt 64°C) is a nonmesomorphic in character, but
on linking it with the dimeric 4-n-alkoxy benzoic acids yielded LC homologs, whose meso-
morphism commences from C7 homolog and continued up to C16 homolog. The transi-
tion temperatures of homologs are lower than corresponding n-alkoxy acids and alternates
throughout the series. C7–C10 homologs are nematogenic and C12–C16 homologs are smecto-
genic. Rest of the C1–C6 homologs are nonliquid crystals. Transition temperatures as deter-
mined from an optical polarizing microscopy equipped with a heating stage are recorded in
Table 2. Smooth transition curves are drawn by linking like or related points for Cr-M, N-I
or Sm-I transitions; as depicted in a phase diagram as shown in Figure 1. A Cr-M/I transition
curve follows a zigzag path of rising and falling with overall descending tendency as series is
ascended and behaved in normal manner. N-I transition curve descends from C7 to C10 with-
out exhibition of odd-even effect. Sm-I transition curve ascended from C12 to C16 homolog
without exhibition of odd-even effect. Thus, N-I and Sm-I transition curves drawn for number
of carbon atoms present in n-alkyl chain of a left n-alkoxy terminal end group versus transition
temperatures behaved in normal manner. Sm-I transition curve and N-I transition curve are
extrapolated [29–31] to C10 and C12, respectively; to determine latent transition temperature
for smectic of C10 homolog and to examine absence of nematic property for C12 homolog
beyond smectic property. Mesomorphic property and thermal behaviors from homolog

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
73
Table . Transition temperatures in °C.
Compound no.
R = -CnHn+
n-Alkyl group
Sm
N
Isotropic












C
C
C
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—



—
—
—
.
.
.
.
.
.
.
.
.
.
.
.
C
C
C
C
C
C
C
C
C
.
.
.
to homolog in a same novel series which undergo variations keeping flexible -CH=CH-
COO-CH2-CH3 group and the molecular rigidity constant. Thus, homologous series under
discussion is partly smectogenic and partly nematogenic of middle ordered melting type
whose smectogenic and nematogenic average thermal stabilities are 127.3°C and 129.0°C,
respectively.
The disappearance of dimerization of alkoxy acids and the lowering of transition tem-
peratures of homolog ester derivatives (C1–C16) as compared to corresponding dimerized
n-alkoxy acids are attributed to the breaking of hydrogen bonding between two molecules of
aromatic acids, by esterification process. The exhibition or nonexhibition of mesophases is
attributed to the favorable or unfavor disalignment of molecules at an angle either perpendic-
ular to the plane of floating surface or at an angle less than 90° depending upon the thermal
Figure . Phase behaviors of series.

74
J. J. TRAVADI ET AL.
resistivity of molecules by suitable or unsuitable magnitudes of anisotropic forces of inter-
molecular anisotropic end-to-end or/and lateral attractions and closeness; as a consequence
of resultant favorable or unfavorable molecular rigidity and flexibility. The nonexhibition of
mesophase formation of C1–C6 is attributed to the inability to resist exposed thermal vibra-
tions by nonmesomorphs whose crystal lattices are abruptly breaking. Consequently, they
sharply transform into isotropic liquid without exhibition of an intermediate state of existence,
called as LC state. Such nonmesomorphic tendency arises due to their (C1–C6) high crystalliz-
ing tendency which arises from low dipole–dipole interactions and low magnitudes of disper-
sion forces by the interactions between instantaneous dipoles produced by the spontaneous
oscillations of electron clouds of the molecules. The exhibition of nematic mesophase by C7,
C8, and C10 is attributed to disalignment of molecules resisting exposed thermal vibrations by
the suitable magnitudes of anisotropic end-to-end cohesive forces to arrange the molecules in
statistically parallel orientational order under floating condition; due to favorable molecular
rigidity and flexibility. However, on increasing n-alkyl chain length of left n-alkoxy group, the
longer n-alkyl chain bearing C12, C14, and C16 homologs in which laterally substituted -OCH3
group due to molecular polarizability factor activate formation of lamellar packing in crystal
lattices and forms layered structure in rigid crystal; which forms sliding layered arrangement
under the influence of externally exposed thermal vibrations between two temperatures show-
ing smectogenic mesophase formation. The appearance of all mesomorphic or nonmesomor-
phic homologs (C1–C16) from and beyond isotropic temperature become identical at which
molecules under investigation are randomly oriented in all possible directions with high order
of disorder or high entropy (ꢀS = ꢀH/T); but on cooling the same with proper rate of cooling
the isotropic melt; the mesogenic homologs reversibly exhibited reappearance of respective
smectic or nematic phase which was appeared on heating the sample. None of the mesogenic
homolog showed monotropic transition or mesophase (monotropically) irreversible manner
on cooling sample.
The absence of odd-even effect is attributed to the late commencement of mesophase
from C7 homolog and the subsequent mesogenic homologs are of even number, i.e., C8,
C10, C12, C14, and C16. The LC properties of varying left flexible n-alkoxy terminal end
group and fixed right handed flexible tail end group, including fixed flexible lateral -OCH3
group with unchanging rigidity of presently investigated series, in which mesogenic ther-
mometric behaviors vary from homolog to homolog as a consequence of changing molec-
ular flexibility by sequential addition of methylene unit or units. The extrapolated LTT for
smectic of C10 homolog is predicted at 100°C, but it is not realizable due to its high crys-
tallization tendency. The extrapolation of N-I transition curve to C12 homolog matches with
Sm-I transition curve of C12; indicating the impossibility of nematic phase formation, because
N-I and Sm-I transition temperatures coincide at the same point in the phase diagram
(Figure 1).
Some LC properties of presently investigated homologous series-1 are compared with a
structurally similar other known series-X [32] and Y [33] as mentioned below in Figure 2.
Homologous series-1 of present investigation-1 and series-X selected for comparative
study, resemble to each other in all respect, i.e., two phenyl rings, central bridge -COO- right
handed tail –CH=CH-COOCH2-CH3, left n-alkoxy end group for the same homolog, but
differs with respect to laterally substituted -OCH3 group. Homologous series-Y differs from
series-1 with respect to laterally substituted -OCH3 group and a part of tail end group -CH3 or
-C6H5 of series-1 and -Y, respectively. Thus, series-1, X, and Y differs with respect to molec-
ular polarity, polarizability and flexibility or rigidity due to, lateral -OCH3 group (series-1),
the part of tail end groups -CH3 (series-X) and -C6H5 (series-Y) for the same homolog from

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
75
RO
CH=CH-COO-CH2-CH3
COO
Series-1
H3CO
RO
CH=CH-COO-CH2-CH3
COO
Series-X
RO
CH=CH-COO-CH2
COO
Series-Y
Figure . Structurally similar series.
series to series. Hence, the changing magnitudes of either molecular rigidity and/or flexibility
will be reflected to variations in mesomorphic properties and the degree of mesomorphism
from homolog to homolog in the same series or for the same homolog from series to series.
Following Table 3 represents some mesomorphic properties, commencement of mesophase,
thermal stability, total mesophaselength range, etc., for the series under comparative
study.
Table . Relative thermal stability in °C.
Series

X
Y
Smectic-isotropic or Smectic-nematic
Commencement of smectic phase
. (C–C)
C
. (C–C)
. (C–C)
C
C
Nematic-isotropic
.
.
.
(C–C)
(C–C)
(C–C)
Commencement of nematic phase
C
C
C
Total mesophaselength (Sm +N) in °C
Ci–Cp
–
–
–
C
 C
C C
C C

76
J. J. TRAVADI ET AL.
It is clear from Table 3 that,
r
Homologous series-1, X, and Y under comparative study are smectogenic in addition to
nematogenic.
Smectic and nematic thermal stability of series-1 is more than a series-X and less than
series-Y.
Smectic mesophase commences from C12, C6, and C6 homologs respectively for series-1,
X and Y.
Nematogenic mesophase commences from C7, C4, and C5 homologs for series-1, -X, and
-Y, respectively.
r
r
r
r
Total mesophaselength range adopts decreasing order from series-1 to -X to -Y.
All the three series under comparative study have the potential to exhibit and stabilize
r
smectic mesophase in addition to nematic mesophase in usual establish manner. How-
ever, their thermal stabilities, commencement of mesophase formation and the range of
liquid crystalanity differ for the same homolog from series to series or from homolog
to homolog in the same series due to varying magnitudes of molecular rigidity and/or
flexibility as a consequence of variable factors like length to breadth ratio, aromaticity,
permanent dipole moment across the long molecular axis, dispersion forces, dipole-
dipole interactions, electronic interactions, length-to-breadth ratio, molecular polarity,
and polarizability, inter molecular closeness as well as suitable magnitudes of anisotropic
end-to-end and lateral attractions, the bipolarity of -CH2-CH3 or -CH2-C6H5 bond or
C-OCH3 bond etc. Homologous series-1 is laterally substituted by highly polar -OCH3
group which induces molecular polarizability relatively more than the series-X and -Y;
whereas; series-X and -Y are linear and lath like with stratified structure bears, difference
of magnitudes of intermolecular closeness causing difference of suitable magnitudes of
anisotropic end-to-end and lateral attractions. Thus, the resistivity toward exposed ther-
mal vibrations adopts different total mesophaselength range, which follows the decreas-
ing order. Thus, highest order of series-1 is attributed to laterally substituted -OCH3
group, which predominantly induces and operates polarizability factor. The sp3 carbon
of a part of tail terminal end group is bonded with sp3 carbon of -CH3 in series-1 and
-X, but sp3 carbon of the identical part of series-Y is bonded with sp2 carbon of a phenyl
ring -C6H5 that causes difference of molecular rigidity and flexibility and intermolecu-
lar cohesive energy. Moreover, the extent of molecular noncoplanarity which results into
the early or late commencement of mesophases, viz., smectic and nematic. The values
of thermal stabilization and facilitations of mesophases are dependent upon the values
of heat of formation (ꢀH) which varies from homolog to homolog in the same series
and series to series for the same homolog. Thus, difference of transition temperatures
of mesogenic homologs stabilize smectic and nematic phase based on molecular struc-
ture, which alters from series to series for the same homolog by changing fixed tail group
or/and a lateral group, including changing number of methylene unit or units.
Conclusions
r
A novel homologous series of esters with two phenyl rings and lateral substitution is
partly smectogenic and partly nematogenic with middle ordered melting type and con-
siderable mesophaselength range.
The group efficiency order derived on the basis of (a) thermal stability (b) early com-
mencement of mesophase, and (c) total mesophaselength range for smectic and nematic
are as under.
r

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
77
(a) Smectic
−CH=CH−COOCH2·C6H5> −CH=CH−COOCH2·CH3
with lateral group
>CH=CH−COOCH2·CH3
Nematic
−CH=CH−COOCH2 · C6H5 >−CH=CH−COOCH2 · CH3
with lateral group
>−CH=CH−COOCH2 · CH3
(b) Smectic
CH=CH−COOCH2·C6H5= −CH=CH−COOCH2·CH3
> CH=CH−COOCH2·CH3
with lateral group
Nematic
−CH=CH−COOCH2·CH3>−CH=CH−COOCH2·C6H5
> −CH=CH−COOCH2·CH3
with lateral group
(c) Total mesophaselength range (Sm + N)
−CH=CH−COOCH2·CH3 >−CH=CH−COOCH2·CH3
with lateral group
>−CH=CH−COOCH2·C6H5
Favorable molecular rigidity and flexibility are governing mesomorphism depending on
molecular structure.
A phenomenon of mesomorphism is very sensitive and susceptible to molecular struc-
ture.
Present investigation may be useful for the study of binary systems to operate LC devices
at desired temperature between 60°C and 100°C and agricultural production.
Present investigation supports the early views about the effects of molecular structure on
mesomorphism and raises credibility to the established concepts.
r
r
r
r
Acknowledgments
The authors are especially thankful to management, principal and HOD (chemistry Dept.) of Shree
M. and N. Virani Sc. College, Rajkot and Kamani Sc. College and Prataprai Arts College, Amreli for
their valuable cooperation to facilitate for present research work, and IR spectra. The authors are also
thankful to the authorities of Saurashtra University for analytical services.
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