Dependence of Mesomorphism on Molecular Rigidity with Reference to Lateral Substitution and Central Bridge

Article abstract of DOI:10.1080/15421406.2015.1106273

A novel mesogenic ester homologous series is synthesized and studied with a view to understanding and establishing the effect of laterally substituted-OCH₃ on mesomorphic behavior. The series consists of twelve members. C₁ to C₄ members are nonmesogenic, C₆ to C₁₂ are smectogenic in addition to nematogenic, and C₁₄ to C₁₆ are only smectogenic. The textures of smectic and nematic phases are A or C type and threaded or Schlieren, respectively. The transition curves in a phase diagram exhibit an odd-even effect and behave in normal manner except for the C₁₄ and C₁₆ derivatives in Sm-I transitions. Thermometric data were determined by an optical polarizing microscope equippedwith a heating stage. Average thermal stability for smectic and nematic are 107.7°C and 121.0°C, respectively. Smectogenic and nematogenicmesophase length ranges from 11.0 to 44.0°C and 12.0°C to 39.0°C, respectively. The mesomorphic properties of present series are comparedwith structurally similar other known series. Thus, present series is predominantly smectogenic and partly nematogenic ofmiddle ordered melting type.

Full text of DOI:10.1080/15421406.2015.1106273

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Dependence of Mesomorphism on Molecular
Rigidity with Reference to Lateral Substitution and
Central Bridge
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)
Dependence of Mesomorphism on Molecular Rigidity with Reference to Lateral
Substitution and Central Bridge, Molecular Crystals and Liquid Crystals, 626:1, 58-67, DOI:
10.1080/15421406.2015.1106273
To link to this article: http://dx.doi.org/10.1080/15421406.2015.1106273
Published online: 22 Mar 2016.
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Date: 01 May 2016, At: 14:13

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –

http://dx.doi.org/./..
Dependence of Mesomorphism on Molecular Rigidity with
Reference to Lateral Substitution and Central Bridge
a
b
b
c
J. J. Travadi , M. S. Vadodaria , K. D. Ladva , and A. V. Doshi
a
b
Chemistry Department, Kamani Sc. College and Prataprai Arts College, Amreli, Gujarat, India; Chemistry
c
Department, Shree M. & N. Virani Sc. College, Rajkot, Gujarat, India; Ex. Principal, Matushri Virbaima Mahila
Science and Home Science College, Rajkot, Gujarat, India
ABSTRACT
KEYWORDS
A novel mesogenic ester homologous series is synthesized and stud-
ied with a view to understanding and establishing the effect of later-
Liquid crystal; mesogenic;
mesomorphism; nematic;
smectic
ally substituted -OCH on mesomorphic behavior. The series consists
3
of twelve members. C to C members are nonmesogenic, C to C
1
4
6
12
are smectogenic in addition to nematogenic, and C14 to C16 are only
smectogenic. The textures of smectic and nematic phases are A or C
type and threaded or Schlieren, respectively. The transition curves in
a phase diagram exhibit an odd–even effect and behave in normal
manner except for the C14 and C16 derivatives in Sm-I transitions. Ther-
mometric data were determined by an optical polarizing microscope
equippedwithaheatingstage. Averagethermalstabilityforsmecticand
nematic are 107.7°C and 121.0°C, respectively. Smectogenic and nemato-
genic mesophase length ranges from 11.0 to 44.0°C and 12.0°C to 39.0°C,
respectively. The mesomorphic properties of present series are com-
pared with structurally similar other known series. Thus, present series is
predominantly smectogenic and partly nematogenic of middle ordered
melting type.
Introduction
The study of liquid crystalline (LC) or mesomorphic state is important from its ability to act
in the interest of mankind since its discovery in 1888 [1]. Researchers of different groups are
working with LC compounds with different aims and objectives. The present investigation is
planned with a view to synthesize novel LC substances through a novel homologous series to
understand and establish the effects of molecular structure on liquid crystal properties [2–5].
Several homologous series have been reported to date [6–10]. The scientific and technolog-
ical research groups have developed and reported number of LC devices and their applica-
tions [11–17]. The proposed molecular structure of novel homologous series consists of two
phenyl rings bonded through -COO- central bridge, two terminal end groups -OR and -CH =
CH-COOCH -CH -CH (n) and laterally substituted highly polar -OCH group. The novel
2
2
3
3
homologues are thermotropically and structurally characterized by evaluated data and then
the mesomorphic behaviors will be discussed and interpreted in terms of molecular rigidity
and flexibility [18–21]. The LC properties of present novel substances will be compared with
CONTACT J. J. Travadi
College, Amreli, , Gujarat, India.
 Taylor & Francis Group, LLC
jjt.ksc@gmail.com
Chemistry Department, Kamani Sc. College and Prataprai Arts
©

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
59
Scheme . Synthetic route to the series. R = –C Hn+ where, n = ,,,,,,,,,,,.
n
structurally similar series. The probable utility of novel substances will be suggested; to extend
and continue the research by researchers working in application part.
Experimental
Synthesis
4
-Hydroxy benzoic acid was alkylated by suitable alkylating agents (R-X) to convert it into
dimeric 4-n-alkoxy benzoic acids by the modified method of Dave and Vora [22]. 4-Hydroxy-
-methoxy cinnamic acid (m.p. 64°C) was esterified by n-propenol by using conc. H SO4
3
2
by usual established method [23]. The ester was very dense brown colored semisolid (yield
5.5%), which purified from n-hexane. Dimeric 4-n-alkoxy benzoic acids were individu-
6
ally condensed with 4-hydroxy-3-methoxy n-propyl cinnamate in dry cold pyridine through
corresponding acid chlorides [24]. Final products were individually decomposed, filtered,
washed, dried and purified until constant transition temperatures were obtained.
The synthetic route to the series is shown below in Scheme 1.
The chemicals required, 4-hydroxy benzoic acid, MeOH, KOH, alkyl halides (R-X), thionyl
chloride, 4-hydroxy-3-methoxy cinnamic acid (Ferulic acid), n-propenol, Conc. H SO , pyri-
2
4
dine, 1:1 HCl, NaHCO , NaOH, anhydrous CaCl , silicagel, petroleum ether, methyl acetate
3
2

6
0
J. J. TRAVADI ET AL.
and n-hexane were used as received, except solvents which were dried and purified prior to
use.
Characterization
Representative homologues of the series were selected for characterization of structure by
1
elemental analysis, IR spectra, mass spectra and H NMR spectroscopy. Microanalysis was
performed on Perkin Element PE 2400 CHN analyzer. IR spectra were recorded on a Perkin
1
Elmer spectra GX. H NMR spectroscopy performed was recorded on a Bruker spectrometer
using CDCl as solvent.
3
Mesomorphic properties were investigated using hot stage polarizing microscopy.
Analytical data
IR spectra in cm− for octyloxy homologoue derivative
The IR spectrum of the compound showed
−
1
6
7
1
1
1
1
2
82.35 cm polymethylene of -C H group.
8
17
−1
21.45 and 822.57 cm as p-sub. benzene ring,
−
1
154.33 cm as -CO- of alkoxy group,
−1
−1
256.51, 1606.64 cm and 1731.80 cm Conforms -COO- & -CO- of ester group,
−
−
−
1
456.41 cm Confirms aromatic -C = C-,
067.05 cm Confirms trans -CH = CH- group,
1
1
926.46 cm Confirms alkyl group.
IR data support the molecular structures.
IR spectra in cm− for decyloxy homologoue derivative
The IR spectrum of the compound showed,
−
1
−1
−1
6
8
2
1
1
1
1
9
48.53 cm & 691.22 cm Confirms polymethylene of -C H group,
10
21
−1
43.85 cm to 762.03 cm Confirms p-sub. benzene ring,
−1
852.88 & 2923.04 cm Confirms alkyl group,
−
1
−1
−1
732.05 cm , 1606.30 cm , 1256.40 cm , Confirms -COO- & -CO- group,
−1
508.58, 1418.53, 1435.24 & 1470.73 cm Confirms aromatic -C = C-,
636.13 cm confirms aliphatic -C = C-,
067.88 cm , 1119.59 cm and 1167.18 cm as -CO- of alkoxy group,
−
−
1
1
−1
−1
−1
86.33 cm Confirms trans -CH = CH- group,
IR data support the molecular structures.

H NMR spectra in ppm for hexyloxy and dodecyloxy derivatives
1
Hexyloxy. H NMR data in δ ppm (n-hexyloxy derivative, 400 MHz, CDCl , standard TMS):-
3
0
.9159 (-CH of -C H ), 1.2670 & 1.3870 (-CH of -C H ), 1.7251 to 1.6427 (-OCH -
3
6
13
2
6
13
2
CH of -C H ), 3.7480 (-O-CH ), 4.0877 & 3.9422 (-O-CH - of -C H ), 6.3153 to 7.1810
2
6
13
3
2
6
13
(
-O-C H -CH = CH-CO-), 7.6066 to 8.0675 (p-sub. Benzene ring).
6
3
The data supports the molecular structure.
1
Dodecyloxy. H NMR data in δ ppm (n-dodecyloxy derivative, 400 MHz, CDCl , standard
3
TMS):-

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
61
Table . Elemental analysis for ethyloxy, butyloxy, octyloxy and decyloxy derivatives.
%
of C Experimental
(Theoretical)
% of H Experimental
(Theoretical)
Compound no.
Molecular formula
C H O
C
. (.)
. (.)
. (.)
. (.)
. (.)
. (.)
. (.)
. (.)




C
C
C
C H O
 
C H O

 
CHO
0
2
.9159 (-CH of -C H ), 1.0266 & 1.3156 (-CH of -C H ), 1.7537 to 1.8377 (-OCH -
3
12 25
2
12 25
2
CH of -C H ), 3.8623 (-O-CH ), 4.0543 & 4.1983 (-O-CH - of -C H ), 6.4232 to 7.2859
12 25
3
2
12 25
(
-O-C H -CH = CH-CO-), 7.6763 to 8.1773 (p-sub. Benzene ring).
6
3
The data supports the molecular structure.
Mass Spectra
Homologue:
Theoretical
Experimental
C
C
.
.
.
.
Texture Determination by Miscibility Method
•
•
•
Hexyloxy homologue → Threaded nematic
Octyloxy homologue → Schlieren nematic
Tetradecyloxy homologue → Smectic—A
Results and discussion
The methoxy (C ) to pentyloxy (C ) homologues are nonmesomorphic, the hexyloxy (C ) to
1
5
6
dodecyloxy (C ) homologues are smectogenic in addition to nematogenic, and the C to
12
14
C16 homologues are only smectogenic. Transition temperatures (Table 2) and melting tem-
peratures as determined from an optical microscopy equipped with a heating stage are plot-
ted against the number of carbon atoms present in n-alkyl chain of left n-alkoxy end group.
Table . “Transition temperatures in °C”.
Compound No.
R = -C H
Smectic
Nematic
Isotropic
n
n+

C
C
C
C
C
C
C
C
C
C
C
C
–
–
–
–
–
.
.
.
.
.
.
.
–
–
–
–
–
.
.
.
.
.
–
.
.
.
.
.
.
.
.
.
.
.
.














–

6
2
J. J. TRAVADI ET AL.
Figure . “Phase Behaviors of Series”.
Smooth curves are drawn through like or related points; in a phase diagram (Fig. 1) show-
ing phase behaviors of series. Cr-I/Sm follows a zigzag path of rising and falling manner with
over all descending tendency as series is ascended. N-I transition curve is descended up to
C12 homologue in usual established manner as series is ascended. Sm-N/I transition curve
rises from C homologue, passes through maxima at C and then descended for C and C
6
8
10
12
homologues, but, then it again rises and reaches to second maxima through C to C homo-
14
16
logue derivative. Thus, it behaves in normal manner from C to C members of the series and
6
12
abnormally behaves for C14 and C16 member of a series. Sm-N/I transition curve is extrap-
olated [25–28] for odd and even-membered transition curves to C and C homologues to
5
4
determine latent transition temperature for nematic and smectic, respectively, and to highlight
or intensify odd–even effect of Sm-N/I transition curve. N-I and Sm-N/I transition curves
exhibited odd–even effect. The N-I and Sm-N/I transition curves for odd and even members
merges into each other at the C and C members of a series respectively and then propagate
10
8
as a single transition curve from and beyond C and C homologue, respectively, for higher
10
8
homologues. The N-I transition curve for odd members occupy lower position then even-
membered transition curve, but odd-membered Sm-N transition curve occupy higher posi-
tion then even-membered transition curve. The mesomorphic properties from homologue to
homologue in a same novel series undergo variations. The present series of ester homologues
is predominantly smectogenic and partly nematogenic with middle ordered degree of meso-
morphism and 107.7 as well as 121.0 thermal stability for smectic and nematic mesophases,
respectively xref.
The disappearance of dimerization of n-alkoxy benzoic acid is attributed to the breaking of
hydrogen bonding between the acid molecules by esterification process. The nonmesomor-
phicity of C to C homologues is attributed to their high crystallizing tendency arising from
1
5
unsuitable magnitudes of anisotropic forces of intermolecular end to end and lateral attrac-
tions due to the law dipole–dipole interactions and the law magnitudes of dispersion forces by
the interactions between instantaneous dipoles produced by the spontaneous oscillations of
electron clouds of the molecules concerned, inducing high crystallizing tendency. The exhi-
bition of smectogenic mesophase indicates the presence of preoccupied lamellar packing of
molecules in their rigid crystal lattices, which forms layered structure. The intermolecular
forces and closeness arising from various factors like molecular polarity and polarizability,
size, aromaticity, molecular rigidity and flexibility, lateral and/or terminal end groups etc.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
63
which stabilizes an existence of a molecule in varying physical state, to which the forces hold-
ing the planes of the layers together in the rigid crystal are weakened relatively to the lateral
attractions among the molecules within a given layer at Cr-M transition temperature and the
planes of the layers may become free to slide and rotate over one another giving rise to for-
mation and appearance of smectic phase under the influence of exposed thermal vibrations
in floating condition for definite range of temperature, which is termed as mesophaselength
range or the degree of smectogenic mesophaselength depending upon magnitudes of molec-
ular thermal resistivity (ꢀH). Now on further heating to the smectic to a higher temperature,
the intermolecular forces between the sides of the molecules are weakened to such an extent
that resultant intermolecular end to end attractions are sufficient and of suitable magnitudes,
which maintains, the statically parallel orientational order of molecules for C to C homo-
6
12
logues in floating condition causing hindrance to slide out the layers and induces nemato-
genic phase formation at Sm-N transition temperature. Thus, C to C homologues exhib-
6
12
ited smectic in addition to nematic phase for definite range of temperature. However, C and
14
C homologue molecules are unable to resist exposed thermal vibrations to such an extent
16
that, resultant end to end and lateral intermolecular forces allow the molecules to maintain
only sliding layered molecular arrangement for some temperature difference, but do not allow
a statistically parallel orientational order of molecules in floating condition. Thus, molecules
are randomly oriented in all possible directions, beyond exhibition of smectic phase with high
order of disorder or high entropy (ꢀS = ꢀH/T) at Sm-I transition temperature. The isotropic
mass of C to C homologues and C to C homologues, on carefully cooling, causes reap-
6
12
14
16
pearance of nematic and smectic or only smectic phase (C and C reversibly at the same
14
16)
transition temperatures at which they were appeared on heating. Odd–even effects observed
for Sm-N/I and N-I transition curves are attributed to the sequentially added methylene unit
or units at the left terminal end group. The disappearance of odd–even effect from and beyond
merging of curves for odd and even members is due to the coiling or bending or fluxing
or coupling of n-alkyl chain with the principle axis of core structure. Laterally substituted
methoxy group increases intermolecular distance and molecular polarizability operatable at
a time for the same reason, and acting oppositely with respect to magnitudes of intermolecu-
lar attractions to each other. The extent of molecular noncoplanarity delays commencement
of mesophase from C homologue. The absence of mesophase formation from C to C is
6
1
5
attributed to the unsuitable magnitudes of anisotropic forces of intermolecular cohesion as a
consequence of unfavorable molecular rigidity and flexibility possessed by the molecule. The
abnormal or unexpected behavior of C and C members in a transition temperature curve
14
16
is attributed to unusual status of longer n-alkyl chain of the left n-alkoxy terminal end group
29] in the presence of highly polar -OCH lateral group on second phenyl ring. Thus, varia-
[
3
tions in mesomorphic properties from homologue to homologue in the same present series is
governed by number of carbon atoms present in n-alkyl chain keeping tail group and a lateral
-
OCH group unchanged. The mesomorphic properties of present series are compared with
3
the structurally similar other known series as under in Figure 2.
The homologous series-1 of present investigation and series-X [30] and Y [31] chosen for
comparison are structurally resembled each other with respect to two phenyl rings and flexible
terminal end group -CH = CH-COO-CH -CH -CH as well as -OR for the same homologue
2
2
3
from series to series under comparative study. However, they differ with central bridge or
laterally substituted functional group. Thus, the minor dissimilarity in molecular structure of
substance or substances in their mesomorphic properties and the degree of mesomorphism
as a result of changing molecular rigidity and flexibility associated with individual molecules
for the same homologue from series to series and from homologue to homologue in the same

6
4
J. J. TRAVADI ET AL.
Figure . Structurally Similar Series.
series. Some mesomorphic properties of series 1, X and Y are mentioned below in Table 3 as
under, which varies with the changing molecular structure.
Above Table 3 represents the thermal stabilities for smectic and nematic, commence-
ment of smectic and/or nematic, mesophaselength etc for the series 1, X and Y respectively,
described as under.
•
•
•
•
•
Homologous series 1 and X are smectogenic in addition to nematogenic, whereas series-
Y is only smectogenic.
Smectogenic mesophase commences from C and C homologues for series 1 and X, but
6
8
it does not commence till the last member (C ) of a series-Y.
16
Nematic mesophase commences from C member of the series 1, X and Y, but it continue
6
till different homologue i.e. C , C and C member of series 1, X and Y, respectively.
12
16
14
Total mesophaselength range bears decreasing tendency for the degree of mesomor-
phism, from series-1 to X to Y.
Thermal stabilities for smectic and nematic are equal for the series 1 and X, but, the
smectogenic commencement differs. Moreover nematic thermal stability, greatly differs
of series-Y as compared to series-1 and X with complete destabilization of smectic phase.
Homologous series-1 of present investigation is laterally substituted by highly polar—
OCH functional group. Therefore two opposing forces are operated at a time. Intermolec-
3
ular forces (i) which undergo reduction due to molecular branching by -OCH lateral sub-
3
stitution due to increase in the intermolecular distance or by molecular widening, whereas
(
ii) intermolecular forces enhanced due to increase in the molecular polarizability by lat-
erally substituted -OCH group. Thus, two opposing intermolecular forces are operated at
3
a time for the same reason. Therefore, the net intermolecular force operated between two
molecules of same homologue will depend upon the predominancy of the factor, either (i) or
(ii) or the resultant magnitudes of net anisotropic forces of intermolecular attractions which
Table . Average thermal stability in °C.
Series:

X
Y
Smectic–nematic or Smectic–isotropic
Commencement of Smectic phase
Nematic–isotropic
. (C -C )
. (C -C )
–
–




C
C
. (C -C )
. (C -C )
. (C -C )






Commencement of nematic phase
Total mesophaselength range in °C (Sm + N)
C C
C
C
C
. to .
C C
. to .
C C
. to .
C C
i
p


MOLECULAR CRYSTALS AND LIQUID CRYSTALS
65
is suitable to cause mesomorphism from C to C homologues as a consequence of favorable
6
16
molecular rigidity and flexibility; depending upon permanent dipole moment across the long
molecular axis, dipole–dipole interactions, electron–electron interactions, dispersion forces
etc. However, in case of homologous series-X, the branching of a molecule is absent for all the
homologues, which do not affect to the resultant intermolecular cohesive forces and close-
ness. But, coincidently the suitable magnitudes of anisotropic forces of intermolecular end
to end and lateral attractions together approaches nearest or equivalent to each other for the
series-1 and X, though, their reasons for occurrence differ from each other; the part of which
is related to molecular resistivity against exposed thermal vibrations to facilitate and stabilize
smectic and then nematic phase. Their commencement of smectic phase differs (C and C )
6
8
but the commencement of nematic phase and Sm-N/I and N-I thermal stabilities are iden-
tical but their ending mesophase homologue differs; with differing intermolecular cohesive
energy associated with individual molecules. The molecular constitution being different for
the same homologue from series-1 to series-X, the individual thermometric behaviors are
differing except their thermal stabilities for smectic and nematic as well as commencement
of nematic mesophase. The absence of smectogenic character in series-Y may be attributed
to the absence of lamellar packing due to the replacement of -COO- by -CH = CH-COO-
central bridge joining first and middle phenyl ring which increases molecular rigidity due
to the added -CH = CH- unit and hinders to facilitate smectic mesophase formation and
favors the less-ordered nematic phase formation at the cost of smectic phase. The suitable
magnitudes of end to end intermolecular forces are strengthened by the presence of multi-
ple bond in vinyl carboxy group; and the reduction of the extent of noncoplanarity between
two molecules of the same homologues. Thus, the change in mesomorphic properties from
homologue to homologue and for the same homologue from series to series depends upon
molecular structure and the extent of dissimilarity of molecular structure among the compar-
ing molecules.
Conclusions
•
•
Present novel ester series is predominantly smectogenic and partially nematogenic with
sufficient mesophaselength range.
Group efficiency order derived for smectic and nematic on the basis of (I) Thermal sta-
bilities (ii) early commencement of mesophase (iii) total mesophaselength (Sm + N) as
under.
(
i) Smectic:-
Series-1 = Series-2 > Series-3
1
(Presence of lateral -OCH group) (Absence of lateral group) (Replacement of -
3
COO- by -CH = CH-COO- central bridge)
Nematic:-
(
-CH = CH-COO-) > -COO- = -COO- (with lateral substitution)
(ii) Smectic :-
-
(
COO- > -COO- > -CH = CH-COO-
with lateral substitution) (without lateral substitution)
Nematic:-
COO- = -CH = CH-COO- = -COO-
-
(
without lateral substitution) (without lateral substitution) (with lateral substitution)
(
iii) (Sm + N) :-

6
6
J. J. TRAVADI ET AL.
-
(
COO- > -COO- > -CH = CH-COO-
with lateral substitution) (without lateral substitution) (without lateral substitution)
•
Suitable magnitudes of favorable molecular rigidity and flexibility can induce mesomor-
phism depending on molecular structure.
•
•
A phenomena of mesomorphism is very sensitive and susceptible to molecular structure.
Ester derivatives containing -CH = CH- unit may be useful in agricultural production.
Acknowledgments
The authors are especially thankful to management, principal and HOD (chemistry Dept.) of Shree
M. & N. Virani Sc. College, Rajkot and Kamani Sc. College & Prataprai Arts College, Amreli for their
valuable cooperation to facilitate present research work, and IR spectra. The authors are also thankful
to the authorities of Saurashtra University for analytical services.
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