Mesomorphism Dependence on Molecular Structure

Article abstract of DOI:10.1080/15421406.2015.1106222

A novel homologous series of liquid crystal (LC) derivatives of general structure: RO·C₆H₄·COO·C₆H₃·OCH₃(ortho)CH = CH·COO·(n)C₅H₁₁ was synthesized and studied with a view to understanding the effect of molecular structure on liquid crystal behavior with reference to lateral-OCH₃ and terminal end group. Homologous series consists of 12 derivatives (C₁-C₁₆), the first five (C₁-C₅), and the last (C₁₆) members are not liquid crystals and the rest of the homologs (C₆-C₁₄) are enantiotropically smectogenic without exhibition of the nematic phase. The textures of the smectic phases are focal conic fan shaped or batonets of smectic-A or smectic-C. Average thermal stability for smectic is 81.8°C and mesophase length ranges from 9°C to 31°C. Transition curves of a phase diagram (Sm-I and Cr-Sm/I) behave in a normal manner. The Sm-I transition curve exhibits an odd-even effect. Analytical and spectral data support the molecular structures. The series is smectogenic of a middle ordered melting type. LC properties of the present series are compared with structurally similar known homologous series. Transition temperatures were determined by an optical polarizing microscope equipped with a heating stage.

Full text of DOI:10.1080/15421406.2015.1106222

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Mesomorphism Dependence on Molecular
Structure
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 Structure, Molecular Crystals and Liquid Crystals,
626:1, 21-30, DOI: 10.1080/15421406.2015.1106222
To link to this article: http://dx.doi.org/10.1080/15421406.2015.1106222
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Date: 07 April 2016, At: 08:23

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –

http://dx.doi.org/./..
Mesomorphism Dependence on Molecular Structure
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
Liquid crystals;
mesomorphic; nematic;
smectic; thermotropic
A novel homologous series of liquid crystal (LC) derivatives of general
structure: RO·C H ·COO·C H ·OCH (ortho)CH = CH·COO·(n)C H was
6
4
6
3
3
5
11
synthesized and studied with a view to understanding the effect of
molecular structure on liquid crystal behavior with reference to lateral
–
OCH and terminal end group. Homologous series consists of 12 deriva-
3
tives (C –C ), the first five (C –C ), and the last (C ) members are not
1
16
1
5
16
liquid crystals and the rest of the homologs (C –C ) are enantiotrop-
6
14
ically smectogenic without exhibition of the nematic phase. The tex-
tures of the smectic phases are focal conic fan shaped or batonets of
smectic-A or smectic-C. Average thermal stability for smectic is 81.8°C
and mesophase length ranges from 9°C to 31°C. Transition curves of a
phase diagram (Sm-I and Cr-Sm/I) behave in a normal manner. The Sm-I
transition curve exhibits an odd-even effect. Analytical and spectral data
support the molecular structures. The series is smectogenic of a middle
ordered melting type. LC properties of the present series are compared
with structurally similar known homologous series. Transition temper-
atures were determined by an optical polarizing microscope equipped
with a heating stage.
Introduction
The liquid crystalline (LC) state [1] of matter is well known and has well established its util-
ity [1–8] in the benefit of mankind. The researchers of science and technology have shared
their interest to study the LC state in different fields, such as pharmaceutical preparations,
electronic devices, agriculture, biological, and/or microbiological activity, medicinal treat-
ment, and medical instruments in addition to the study of chemistry, physics, and biology.
We being chemists, synthesize novel compounds through homologous series with a view to
understanding the effects of molecular structure on liquid crystal behavior. The proposed
investigation of novel compounds can be useful to the scientific and technological research
fields as well as to derive novel conclusions about the effects of molecular structure [9–11] on
liquid crystal properties which may be useful to predict the type of moiety required to gen-
erate a required temperature range. Thus, the proposed investigation aims to provide novel
materials to research groups working in other areas of LCs. Several novel homologous series
have been reported [12–16] and the present proposed homologous series will consist of two
phenyl rings bridged through carboxy group with lateral –OCH group as well as –OR and
3
CONTACT J. J. Travadi
j.travadi@gmail.com
©
 Taylor & Francis Group, LLC

2
2
J. J. TRAVADI ET AL.
-
CH=CH-COOC H (n) terminal end groups. Results will be discussed in terms of molec-
5
11
ular rigidity and flexibility [17–20], including comparison of some LC properties of present
novel series with structurally similar other known series.
Experimental
Synthesis
4
4
3
-Hydroxy benzoic acid was alkylated by suitable alkylating agents (R-X) to provide dimeric
-n-alkoxy benzoic acids through the modified method of Dave and Vora [21]. 4-Hydroxy-
-methoxy cinnamic acid (m.p. 64°C) was esterified by n-pentanol by using conc. H SO by
2
4
a usual established method [22]. The ester was very viscous and dense brown liquid, and was
purified from distilled n-hexane. The yield was 61.5%. Dimeric 4-n-alkoxy benzoic acids were
individually condensed with 4-hydroxy-3-methoxy n-pentyl cinnamate in dry cold pyridine
through corresponding acid chlorides [23, 24]. Final products were individually decomposed,
filtered, washed, dried, and purified until constant transition 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), n-pentanol, Conc. H SO , pyri-
2
4
dine, 1:1 HCl, NaHCO , NaOH, anhydrous CaCl , silica gel, petroleum ether, methyl acetate,
3
2
and n-hexane were used as received, except solvents which were dried and purified prior to
use.
Scheme . Synthetic route to the series. R = –C H
where, n = , , , , , , , , , , , .
n+
n

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
23
Table . Elemental analysis for methyloxy, pentyloxy, octyloxy and dodecyloxy derivatives.
Mol.% of C
experimental
(theoretical)
Mol.% of H
experimental
(theoretical)
Compound no.
Molecular formula
C H O
C
. (.)
. (.)
. (.)
. (.)
. (.)
. (.)
. (.)
. (.)




C
C
C
C H O
 
CHO
CHO
Characterization
Representative homologs of the series were selected for characterization of structure by ele-
1
mental analysis Table 1, IR spectra, mass spectra, and H 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 qualitative without performance
1
on DSC scan. H NMR spectroscopy performed was recorded on Bruker using CDCl as
3
solvent.
Mesomorphic properties were investigated using hot stage polarizing microscope.
Analytical data
−1
IR spectra in cm for ethyloxy homolog derivatives. The IR spectrum of the compound
−
1
−1
showed 691.18 cm polymethylene of -C H group. 759.52 and 840.52 cm as p-sub. ben-
5
11
−1
−1
zene ring, 1146.45 cm as -CO- of alkoxy group, 1243.66, 1599.67, and 1711.11 cm con-
−1
firm -COO- and -CO- of ester group, 1502.55 cm confirms aromatic -C=C-, 1030.13 and
−1
−1
1
114.95 cm confirm trans -CH=CH- group, and 2961.73 cm confirms alkyl group.
IR data supports the molecular structure.
−1
IR spectra in cm for hexyloxy homolog derivatives. The IR spectrum of the compound
−
1
−1
showed, 848.78–769.28 cm confirm p-sub. benzene ring, 645.03 and 693.13 cm confirm
polymethylene of –C H group.
6
13
−
1
−1
2
932.76 and 2850.32 cm confirm alkyl group, 1697.17, 1607.41, and 1250.20 cm con-
−1
firm –COO- and -CO- group, 1508.35 and 1426.32 cm confirm aromatic -C=C-, 1067.55
−1
−1
and 1167.63 cm as -CO- of alkoxy group, and 996.13 cm confirms trans –CH=CH- group.
IR data supports the structure.
1
H NMR spectra in ppm for butyloxy and tetradecyloxy derivatives.
1
Butyloxy. H NMR data in δ ppm (n-butyloxy derivative, 400 MHz, CDCl , standard TMS):
3
−
0.9088 (-CH of –C H ), 1.4190 (-CH - of –C H ) 1.5176 (m, -CH - of –COOC H ),
3
5
11
2
5
11
2
5
11
1
.7208 (-CH - of -OC H ), 3.7613 (-O-CH ), 3.9421 (-O-CH - of -OC H ), 4.1653 (triplet, -
2
4
9
3
2
4
9
CH - of –COOC H ), 6.8906–7.0774 (-O-C H -CH=CH-CO-), 7.6029 (-CH = of ethylenic
2
5
11
6
3
-
CH=CH-), and 8.0440–8.0795 (p-sub. benzene ring).
The data supports the molecular structure.
1
Tetradecyloxy. H NMR data in δ ppm (n-tetradecyloxy derivative, 400 MHz CDCl , stan-
3
dard TMS): −0.9066 (-CH of –C H ), 1.1862 (-CH - of –C H ) 1.5239 (m, -CH - of –
3
5
11
2
5
11
2
COOC H ), 1.7392 (-CH - of –OC H ), 3.7653 (-O-CH ), 3.9419 (-O-CH - of -O C H ),
5
11
2
14 29
3
2
14 29
4
(
.1706 (triplet, -CH - of –COOC H ), 6.8789–7.0840 (-O-C H -CH=CH-CO-), 7.6078
2
5
11
6
3
-CH = of ethylenic -CH=CH-), and 8.0557–8.0778 (p-sub. benzene ring).

24
J. J. TRAVADI ET AL.
The data supports the molecular structure.
Mass spectra
Homolog:
Theoretical
Experimental
C
C
.
.
.
.
Texture Determination by Miscibility Method:
•
•
•
C ——→ Smectic – A
6
C ——→ Smectic – A
8
C ——→ Smectic – C
16
Results and Discussion
Transition temperatures (Table 2) determined by an optical polarizing microscope are plot-
ted versus the number of carbon atoms present in n-alkyl chain of left n-alkoxy terminal end
group. The methoxy to pentyloxy (C –C ) and hexadecyloxy (C ) derivatives of the series
1
5
16
are not liquid crystals. Mesomorphic properties commence from the hexyloxy (C ) deriva-
6
tive and continue up to the tetradecyloxy (C ) derivative of the series in an enantiotropic
14
manner solely as a smectic phase, without exhibition the nematic phase. Transition curves
(
Figure 1) are obtained on linking like or related points showing phase behaviors of a series.
Transition temperatures as determined by an optical polarizing microscopy were plotted
against the number of carbon atoms present in left n-alkoxy terminal end group and the like or
related points were linked to obtain Cr-I/Sm as well as Sm-I transition curves. Sm-I transition
curve initially rises up to octyloxy (C ) homolog and then descended up to C homolog after
8
14
passing through maxima at C homolog. Odd-even effect is observed for the curve. Transition
8
curve (Sm-I) is extrapolated for odd member C [25, 26] to intensify the effect keeping in view
5
of the trend of respective curve. Cr-I/Sm transition curve adopts a zigzag path of rising and
Table . Transition temperatures in °C.
Compound no.
n-Alkyl chain R = -C H
Sm
N
Isotropic
n
n+

C
C
C
C
C
C
C
C
C
C
C
C
–
–
–
–
–
.
.
.
.
.
.
–
–
–
–
–
–
–
–
–
–
–
–
–
.
.
.
.
.
.
.
.
.
.
.
.














Sm, smectic; N, nematic.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
25
Figure . Phase behaviors of series.
falling as shown in phase diagram. Thus, both the curves of a phase diagram behaved in nor-
mal expected manner. Curves for odd and even members merge into each other at the octyloxy
homolog, and then propagated as a single transition curve from and beyond C homolog.
8
Mesomorphic behaviors of series changes from homolog to homolog depending upon the
length of n-alkyl chain of left n-alkoxy terminal end group keeping the rest of the molecular
part unchanged. Disappearance of dimerization of n-alkoxy benzoic acids is attributed to the
breaking of hydrogen bonding between two molecules of n-alkoxy acid by esterification pro-
cess. Laterally substituted methoxy group and its polarity and polarizability increases length
to breadth ratio and the suitable magnitudes of intermolecular lateral attractions as a conse-
quence of resulted favorable molecular rigidity and flexibility; which induces mesophase for-
mation from C homolog to C homolog in enantiotropic manner. The absence of mesophase
6
14
formation by C –C and C members of the present novel series is attributed to their high
1
5
16
crystallizing tendency arising from unsuitable magnitudes of anisotropic forces of intermolec-
ular end to end and the lateral attractions; which neither allows statistically parallel orienta-
tional order of molecules, nor it allows sliding layered arrangement of molecules to float on the
surface, but crystal lattices of respective homologs are abruptly breaking and sharply trans-
form into isotropic liquid without passing through LC state, under exposed thermal vibra-
tions, at which molecules under observation randomly orient in all possible directions with
high order of disorder or entropy (ꢀS = ꢀH/T). However molecules of homologs C –C
6
14
support lamellar packing of molecules in their crystal lattices at a suitable temperature and
continues to higher temperature depending upon favorable magnitudes of molecular rigidity
and flexibility until the isotropic temperature. Thus, C –C homologs are smectogenic. The
6
14
absence of nematic property is attributed to the weaker end to end attractions which hinders
the formation of statistically parallel orientational order. The extrapolated transition temper-
ature [27] is much below the isotropic temperature for C , and hence it is unrealizable. The
5
exhibition of odd-even effect by Sm-I transition curve is due to the sequential addition of a

26
J. J. TRAVADI ET AL.
methylene unit at the left n-alkoxy terminal end group. The disappearance of odd-even effect
from and beyond C member of a present series is attributed to the flexing or bending or
8
coiling or coupling of the n-alkyl chain (C –C ) with the major axis of the core structure
10
14
of higher homologs with longer n-alkyl chains. Variations in mesomorphic behaviors from
homolog to homolog in the same series is due to the changing molecular length by added
methylene unit and hence the changing permanent dipole moment across its long molecu-
lar axis, dipole–dipole interactions, electronic interactions, dispersion forces, etc. which are
changing accordingly. Thus, changing trend varies with changing molecular rigidity and flex-
ibility, which reflected in mesomorphic behaviors and the degree of mesomorphism from
homolog to homolog in the same present series. The mesomorphic properties of presently
investigated novel series are compared with structurally similar homologous series X [28]
and Y [29] as shown below in Figure 2.
Homologous series of present investigation-1 and the structurally similar homologous
series-X chosen for comparison are identical with respect to two phenyl rings, central bridge
-
COO- linking two phenyl rings, the left n-alkoxy terminal end group for the same homolog
and the part of the right handed terminal end group -CH=CH-COO- linking to -C H group
5
11
of normal and iso-type structure for all homologs. However series 1 and X differ by later-
ally substituted highly polar and polarizable -OCH group as well as -C H (n) and -C H
3
5
11
5
11
(iso) linked terminal end groups respectively. Homologous series-1 and -Y are identical with
respect to two phenyl rings and the left n-alkoxy terminal end group for the same homolog,
but differs with respect to central bridges -COO- and -CH=CH-COO-, respectively, linking
two phenyl rings, and laterally substituted -OCH group keeping -CH=CH-COO-C H (n)
3
5
11
terminal (right sided) end group unchanged for all homologs of both series under discussion,
respectively. Thus, LC properties and the degree of mesomorphism will vary with the differing
features of each series under comparative study as a consequence of changing magnitudes of
anisotropic intermolecular end to end and lateral forces due to molecular polarity, changing
polarizability, rigidity, flexibility, permanent dipole moment across the long molecular axis,
Figure . Structurally similar series.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
27
dipole–dipole interactions, electronic interactions, changing dispersion forces, etc. Follow-
ing Table 3 represents the variation of some LC properties for the series-1, X and Y under
comparison.
Table 3 indicates that
•
•
Presently investigated novel series-1 is only smectogenic.
Homologous series X and Y selected for comparison are smectogenic in addition to
nematogenic.
•
•
•
•
•
•
Smectogenic mesophase commences from C member of the series-1 and Y, where as it
6
commences late from C homolog of the series-X.
8
Nematogenic mesophase commences from C and C homolog member of the series X
6
5
and Y respectively, where as it does not commence till the last C member of a series-1.
16
Smectic thermal stability is the lowest (81.8) for the present series-1 and the highest
122.2) for the series-X among the series under comparison.
(
Nematic thermal stability of a series-Y is (169.3) higher than series-X (139.0) whereas
series-1 is nonnematogenic.
Total mesophase length range (Sm + N) of series-Y (21–53) is greater than present series-
1
(09–31) and less than a series-X (11–59).
Odd-even effects of transition curve or curves are observed for the series under compar-
ative study.
The molecules of all the homologous series-1, X and Y under comparison are disaligned
at an angle 90° or less than ninety degree on the plane of a surface under the influence of
exposed thermal vibrations. The suitable magnitudes of anisotropic forces of intermolecu-
lar end to end or/and lateral attractions as a consequence of more or less molecular rigidity
and/or flexibility, depending upon individual molecular structure which may arrange them-
selves either in statistically parallel orientational order or/and as the sliding layers, while float-
ing on the surface. The molecules of homologous series-1, X and Y with originally present
lamellar packing of molecules in rigid crystals adopts sliding-layered arrangement in floating
condition for the range of temperature depending upon the magnitudes of thermal resistivity
against externally exposed thermal vibrations. The molecules of series-1 with their polariz-
ability induced by -OCH lateral group, maintains only sliding layered molecular arrangement
3
in floating condition (C –C ) but the residual intermolecular forces are inadequately unable
6
14
to maintain statistically parallel orientational molecular order under floating condition. Thus,
series-1 is only smectogenic without exhibition of nematic property. The magnitudes of inter-
molecular anisotropic attractive forces of series X and Y are sufficient to maintain required
molecular arrangement for smectic and nematic mesophases formation; one after another
in floating condition, for definite range of temperature, called as degree of mesomorphism
Table . Average thermal stability in °C.
Series:

X
Y
Smectic-isotropic or
smectic-nematic
Commencement of smectic C
phase
Nematic-isotropic
Commencement of nematic
phase
. (C –C )
. (C –C )
. (C –C )
 




C
. (C –C )
C
. (C –C )
–
–




C
C
. to . C C

Total mesophaselength
. to . C C
 
. to . C C

range in °C (Sm + N)

28
J. J. TRAVADI ET AL.
or mesophaselength range. On comparing intermolecular attraction due polarizability fac-
tor, between series-1 and X the polarizability is contributed by only –OCH lateral group in
3
series-1, but polarizability factor is strengthened by added molecular polarizability by “iso”
linked -C H (iso) group in case of series-X. Therefore, smectic mesophase stabilizes to the
5
11
highest extent among the series-1, X and Y, and more than series-1; with respect to the effect
due to polarizability factor induced by broadening of molecules through lateral substitution.
Therefore, residual intermolecular attractions after complete display of smectic mesophase is
insufficient to maintain statistically parallel orientational order of molecules in floating con-
dition to exhibit or stabilize nematic phase for series-1, but it is sufficient to induce nematic
phase for the molecules of series-X. Thus, smectic and nematic thermal stability for the series-
X are more than a series-1. Now on comparing thermal stability of series-1 and Y, the polar-
izability by lateral substitution, which is absent and the central bridge -COO- replaced by
-
CH=CH-COO- in series-Y. Therefore, inter molecular width between two molecules of
series-Y is closer than the molecules of series-1. Thus, anisotropic forces of inter molecular
end to end and lateral to terminal attractions for series-Y are relatively stronger than a series-1
due to longer and stronger -CH=CH-COO- central bridge. Therefore smectic and nematic
mesophases are facilitated to stabilize for series-Y, more efficiently than a series-1 in which
only smectic phase stabilizes with lower smectic thermal stability. Hence, the energy associ-
ated due to differing features of molecules in terms of enthalpy (ꢀH) vary from homolog to
homolog in same series due to changing length of n-alkyl chain and for the same homolog
from series to series due to changing right handed terminal end group –C H (n) and –
5
11
C H (iso), or/and central bridge, linking two phenyl rings. The commencement of smectic
5
11
phase or in general mesophase occurs from C or C homolog except it commences late from
5
6
C homolog for smectic (Series-X). Early or late commencement of smectic or mesophase is
8
related with the extent of noncoplanarity of a molecule. Early commencement of mesophase
is related with less noncoplanarity of a molecule and the late commencement is related to the
more extent of noncoplanarity of a molecule. The molecules of series-X are more noncopla-
nar than the corresponding same homolog of series-1 and Y. Therefore smectic mesophase
commences a little later from C homolog of series-X due to ‘iso’ linked –C H end group;
8
5
11
whereas it commences from C homolog of series-1 and Y because, both series [1 and X]
6
are equally coplanar or say equally noncoplanar. Odd-even effects observed for transition
curves are attributed to the alternation of transition temperatures due to sequential addition of
methylene unit or units at the left flexible n-alkoxy terminal end group for the same homolog
from series to series and the homolog to homolog in the same series. Thus, presently inves-
tigated homologous series is smectogenic and low melting type, whose mesomorphic phase-
length vary between 9°C and 31°C.
Conclusions
•
Presently investigated novel homologous ester series of twelve members consisted of two
phenyl rings bridged through carboxy group with laterally substituted -OCH group
3
and terminally substituted by -OR and -CH=CH-COOC H (n) groups is smectogenic
5
11
from C to C and low melting type.
6
14
•
Group efficiency order derived for smectic and nematic on the basis of, (i) thermal stabil-
ity (ii) early commencement of LC phase and (iii) mesophaselength range are as under.
(
i) Smectic:-
-
OC H (iso) > -OC H (n) > -OC H (n)
5
11
5
11
5
11

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
29
Without -OCH lateral group Without -OCH lateral group With -OCH lateral
3
3
3
group
Nematic:-
-
OC H (n) > -OC H (iso) > -OC H (n)
5
11
5
11
5
11
Without -OCH lateral group Without -OCH lateral group With -OCH lateral
3
3
3
group
(
ii) Smectic:-
-
OC H (n) = -OC H (n) > -OC H (iso)
5
11
5
11
5
11
With -OCH lateral group Without -OCH lateral group Without -OCH lateral
3
3
3
group
Nematic:-
-
OC H (n) > -OC H (iso) > -OC H (n)
5
11
5
11
5
11
Without -OCH lateral group Without -OCH lateral group With -OCH lateral
3
3
3
group
iii) Smectic + Nematic:-
OC H (iso) > -OC H (n) > -OC H (n)
(
-
5
11
5
11
5
11
Without -OCH lateral group Without -OCH lateral group With -OCH lateral
3
3
3
group
•
Suitable magnitudes of anisotropic forces of intermolecular attractions as a consequence
of molecular rigidity and/or flexibility can induce mesomorphic behaviors in a sub-
stance.
Molecular rigidity and flexibility are very sensitive and susceptible to molecular struc-
ture.
Sm-I transition temperatures varies between 36°C and 96°C for mesomorphic
homologs. Such mesomorphs are useful for the study of binary systems for the devices
workable at room temperatures.
Novel ester homolog derivatives may be useful for better growth of fruits and flowers in
the agricultural production and to the scientific community working in application field.
•
•
•
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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