Dependence of molecular structure on mesomorphic behavior with special reference to central bridge

Article abstract of DOI:10.1080/15421406.2014.949594

A novel homologous series of liquid crystalline 4-(4′-n-alkoxy cinnamoyloxy)-4^″-methoxy benzyl cinnamates was synthesized and studied with a view to understand and establish the effects of molecular structure on liquid crystal behavior. The series consists of eleven members of the series. None of the homologue derivatives are either nonliquid crystal or smectogenic, i.e. all the eleven members of the novel homologous series are enantiotropcially nematogenic only. Transition and melting temperatures as well as textures of the nematic mesophase were determined on an optical polarizing microscope equipped with a heating stage. The transition curves of the phase diagram show phase behavior in a normal expected manner. An odd-even effect is observed for the nematic-isotropic transition curve. The textures of the nematic mesophase are threaded or Schlieren as determined a by miscibility method. Analytical and spectral data confirm the molecular structures of the homologues. The average thermal stability for nematic is 213.8°C. The isotropic temperatures vary between 190°C and 240°C. The nematogenic mesophase length varies from 17°C to 102°C. Thus, the novel homologous series is entirely nematogenic without the exhibition of smectogenic character. The liquid crystal (LC) properties of the present novel series are compared with structurally similar other known homologous series.

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Dependence of Molecular Structure
on Mesomorphic Behavior with Special
Reference to Central Bridge
Brijesh H. Patela, Ankita A. Doshib & A. V. Doshic
a Huntsman, Baroda Textile Effects Pvt. Ltd., Umraya village, Taluka
Padra, Vadodara, Gujarat, India
b Department of Biochemistry, The M.S. University of Baroda,
Vadodara, Gujarat, India
c Dr. K.N. Modi University, Plot No. INS-1, RIICO Industrial Area Ph-II,
Newai, Dist. Tonk, Rajasthan, India
Published online: 03 Mar 2015.
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To cite this article: Brijesh H. Patel, Ankita A. Doshi & A. V. Doshi (2015) Dependence of Molecular
Structure on Mesomorphic Behavior with Special Reference to Central Bridge, Molecular Crystals and
Liquid Crystals, 608:1, 38-46, DOI: 10.1080/15421406.2014.949594
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Mol. Cryst. Liq. Cryst., Vol. 608: pp. 38–46, 2015
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2014.949594
Dependence of Molecular Structure on Mesomorphic
Behavior with Special Reference to Central Bridge
BRIJESH H. PATEL,1,* ANKITA A. DOSHI,2 AND A. V. DOSHI3
1Huntsman, Baroda Textile Effects Pvt. Ltd., Umraya village, Taluka Padra,
Vadodara, Gujarat, India
2Department of Biochemistry, The M.S. University of Baroda, Vadodara,
Gujarat, India
3Dr. K.N. Modi University, Plot No. INS-1, RIICO Industrial Area Ph-II, Newai,
Dist. Tonk, Rajasthan, India
A novel homologous series of liquid crystalline 4-(4ꢀ-n-alkoxy cinnamoyloxy)-4ꢀꢀ-
methoxy benzyl cinnamates was synthesized and studied with a view to understand
and establish the effects of molecular structure on liquid crystal behavior. The series
consists of eleven members of the series. None of the homologue derivatives are either
nonliquid crystal or smectogenic, i.e. all the eleven members of the novel homologous
series are enantiotropcially nematogenic only. Transition and melting temperatures as
well as textures of the nematic mesophase were determined on an optical polarizing
microscope equipped with a heating stage. The transition curves of the phase diagram
show phase behavior in a normal expected manner. An odd-even effect is observed
for the nematic-isotropic transition curve. The textures of the nematic mesophase are
threaded or Schlieren as determined a by miscibility method. Analytical and spectral
data confirm the molecular structures of the homologues. The average thermal stability
for nematic is 213.8◦C. The isotropic temperatures vary between 190◦C and 240◦C. The
nematogenic mesophase length varies from 17◦C to 102◦C. Thus, the novel homologous
series is entirely nematogenic without the exhibition of smectogenic character. The liq-
uid crystal (LC) properties of the present novel series are compared with structurally
similar other known homologous series.
Keywords Enantiotropy; liquid crystal; nematic; smectic; thermotropic
Introduction
Thermotropic liquid crystals show their an intermediate state of existence between crys-
talline solid state and isotropic liquid state between two temperatures and are essentially free
from solvent [1, 2] have more scientific, biological and pharmaceutical, technological and
industrial importance [3–5]. Kazuki Iwahata et al. [6] studied applications of biomaterials
to chemically synthesized thermotropic liquid crystals. Such study covers the improvement
of the performance of liquid crystal displays using LC physical gels consisting of a liquid
crystal admixed with amino acid based gelator and some other functionalization like electro
optical measurements, electronic device industry and refining of industrial products. The
∗
Address correspondence to present address of Brijesh H. Patel, A-21 Saket Bunglows Opp Yash
Complex Gotri Baroda, 390021, Gujarat, India. E-mail: brijpatel79@gmail.com
Color versions of one or more of the figures in the article can be found online at
www.tandfonline.com/gmcl.
38

Molecular Structure on Mesomorphic Behavior with Special Reference
39
derivatives of benzoic acids and cinnamic acids are biologically active molecules giving
rise to exhibit liquid crystal property and are useful to the pharmaceutical, medicinal, elec-
tronic etc fields of applications which require LC materials workable at desired temperatures
[7, 8]. The LC materials workable at low or desired temperature requires imagination of con-
structing a molecule of conventional or unconventional LC substances with proper central,
terminal and lateral groups substituted at appropriate position or positions of phenyl rings
[9, 10]. Proposed investigation is planned with a view to construct a molecules consisting
of three phenyl rings bonded through CH CH COO and CH CH COO CH2−
central bridges, OCnH2n+1 and OCH3 terminal end groups without any lateral sub-
stitution as long linear molecule. Present study will include only construction of novel
1
LC molecules, their synthesis, their characterization by elemental analysis IR, HNMR,
mass spectra, microscopic examination texture determination and comparative study of LC
properties with structurally similar known substances. Thus, the present proposed investi-
gation is aimed with a view to understand and establish the effects of molecular structure
[11, 12] on liquid crystal behavior dependence on central group as a consequence of
molecular rigidity and flexibility [13–17].
Experimental
Synthesis
4-Hydroxy cinnamic acid was converted to 4-n-alkoxy cinnamic acids using suitable alky-
lating (R-X) agents by modified method of Dave and Vora [18]. 4-Hydroxy cinnamic acid
was converted to 4-hydroxy-4ꢀꢀ-methoxy benzyl cinnamate by the modified method of Eu-
ropean patent EP0117502B1 [19]. 4-n-alkoxy cinnamic acids were individually condensed
with 4-hydroxy-4ꢀꢀ-methoxy benzyl cinnamate (m.p. 204◦C, yield is 35%) in pyridine
through their acid chlorides in dry cold pyridine [20–22]. Final products were individually
decomposed, filtered, washed dried and purified in ethanol till constant transition temper-
atures obtained. Transition and melting temperatures were determined by an optical hot
stage polarizing microscopy.
The chemicals 4-hydroxy cinnamic acid, 4-methoxy benzyl alcohol, dibutyltin oxide,
pyridine, alkyl halides, MeOH, EtOH, thionyl chloride, HCl, KOH etc. were used as
received, except solvents, which were dried and distilled prior to use. The synthetic route
to the series is shown in Scheme 1.
Characterization
Some selected members of a present novel series were characterized by elemental analysis,
1
Infra red spectra, HNMR spectra and mass spectra. Micro analysis was performed by
1
Perkin-Elmer PE 2400 CHN analyser, HNMR spectra were recorded on a Bruker spec-
trometer using DMSO solvent. Infra red spectra were recorded on a Perkin Elmer spectrum
GX spectrometer. Transition temperatures and the textures of nematic mesophases are
determined by amiscibility method, which was characterized by an optical polarizing mi-
croscope equipped with a heating stage. Thermodynamic quantity enthalpy change (ꢀH)
and entropy change (ꢀS = ꢀH/T) are qualitatively discussed instead of DSC scane.
Analytical Data
1
Spectral Data. HNMR in ppm for the Decyloxy Derivative: 0.84–1.68 (H of C10H21), 2.52
( O CH2 CH2 of OC10H21), 3.76 ( OCH3), 5.06 ( CH CH ), 6.51 (p-substituted
benzene ring), 6.9–7.7 (indicates presence of more than one benzene).

40
B. H. Patel et al.
Table 1. Elemental analysis for methyloxy, ethyloxy, and propyloxy derivatives
Elements % found (% calculated)
Sr. no.
Molecular formula
C
H
1
2
3
C27H24O6
C28H26O6
C29H28O6
72.74 (72.97)
73.10 (73.36)
73.58 (73.72)
5.28 (5.40)
5.47 (5.67)
5.68 (5.93)
1HNMR in ppm for the Dodecyloxy Derivative: 0.84–1.69 (H of C12H25), 3.77
( OCH3), 2.53 ( OCH2 of OC12H25), 5.07 ( CH CH ), 6.31 (p-substituted benzene
ring), 6.9–8 (presence of more than one benzene rings).
IR in cm−1 for tetradecyloxy Derivative: 830 para-substituted phenyl ring, 1218 ether
linkage of ( C14H29 O C6H4), 1171, 1286, 1679 ( COO ester group), 982 (trans
CH CH )
IR in cm−1 for Hexadecyloxy Derivative: 830 para-substituted phenyl ring, 1217 ether
linkage of ( C16H33 O C6H4), 1171, 1286, 1680 ( COO ester group), 982 (trans
CH CH )
CH3OH
Reflux
R-X
CH=CHCOOH
CH=CHCOOH
HO
RO
RO
+
KOH
- HX
-SO2
- HCl
Reflux
SOCl2
+
CH=CHCOCl
A
DBTO
- H2O
+
O-CH3
CH=CHCOOH HOCH2
HO
O-CH3
CH=CHCOOCH2
HO
B
dry cold
O-CH3
+
CH=CHCOOCH2
CH=CHCOCl
HO
RO
Pyridine
- HCl
B
A
O-CH3
CH=CHCOOCH2
CH=CHCOO
RO
Final product
Where R = CnH2n+1; n = 1,2,3,4,5,6,8,10,12,14,16
Scheme 1. Synthetic route to the series. Series: 4-[4ꢀ-n-alkoxy cinnamoyloxy]-4ꢀꢀ-methoxy benzyl
cinnamates.

Molecular Structure on Mesomorphic Behavior with Special Reference
41
Mass Spectra. Mass spectra for butyloxy derivative:
Molecular formula C30H30O6 Calculated Molecular weight 486. Practical mass ob-
served by Mass spectroscopy 486.
Texture of Nematic by Miscibility Method
Pentyloxy derivative – –– –– –→Threaded nematic
Octyloxy derivative – –– –– –→Schlieren nematic
Results and Discussion
Cis-4-n-alkoxy cinnamic acid and 4-hydroxy-4ꢀꢀ-methoxy benzyl cinnamate (m.p. 204◦C)
are nonliquid crystal (NLC) components. But the liquid crystal property is induced from
the first to the last member of the novel series as an enantiotropic nematic phase without
exhibition of any smectogenic mesophase. The homologous series consists of eleven mem-
bers (C1 to C6, C8, C10, C12, C14, and C16), which are all liquid crystals of considerable
nematogenic phase length. Transition temperatures (Table 2) of the homologues were plot-
ted for the number of carbon atoms present in n-alkyl chain of the left terminal end group
bonded to the phenyl ring through oxygen atom. The solid-nematic and nematic-isotropic
transition curves drawn by linking like or related points constitute a phase diagram (Fig. 1)
showing the phase behavior of the presently investigated novel series of cinnamate diesters
of unchanged methoxy group as the right handed terminal end group. An odd-even effect
for nematic-isotropic transition curve and alternation of transition temperatures is observed
for the novel series. The solid-nematic transition curve follows a zigzag path of rising and
falling values and behaves in the usual established manner. The nematic-isotropic transition
curve continuously descends as series is ascended with an overall falling tendency from the
first (methoxy) to the last (hexadecyloxy) homologue with the exhibition of an odd-even
effect. The nematic-isotropic curves for odd and even homologues merge into each other
Table 2. Transition temperatures of series in ◦C
Transition temperature in ◦C
n-alkyl group
Compound no.
( CnH2n+1) (n)
Sm
Nm
Isotropic
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
8
10
12
14
16
—
—
—
—
—
—
—
—
—
—
—
155.0
160.0
119.0
153.0
173.0
127.0
161.0
167.0
150.0
152.0
173.0
240.0
238.0
221.0
227.0
214.0
219.0
210.0
203.0
198.0
192.0
190.0
10
11
Sm: Smectic; Nm: Nematic.

42
B. H. Patel et al.
280
260
240
220
200
180
160
140
120
100
SOLID - ISOTROPIC
OR
SOLID - NEMATIC
NEMATIC - ISOTROPIC
ISOTRPIC
NEMATIC
SOLID
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16
No.of carbon atoms in n-alkyl chain-R
X-axis
Figure 1. Phase behavior of series.
at the nonyloxy homologue and then they adopt a single curve for higher homologues.
Liquid crystal behavior varies from homologue to homologue in the present series of ester
derivatives. The entire novel series is nematogenic and a high melting type. The meso-
morphic phase length varies from a minimum of 17◦C at the hexadecyloxy homologue to
a maximum of 102◦C at the propyloxy homologue. The nematogenic mesophase length
alternates up to the hexy n-alkyl chain and then gradually diminishes up to the hexadecyl
n-alkyl chain with exceptional variation at the fifth and twelfth homologues respectively.
The introduction of mesomorphism to a molecule formed by linking two nonmesomor-
phic (NLC) components through acid chlorides of corresponding cis n-alkoxy cinnamic
acids, which increases molecular length, polarity and polarizability, length to breadth ratio,
dispersion forces including the presence of the highly polar methoxy end group and the con-
jugated double bonds. Such an environment raises the suitable magnitudes of anisotropic
forces of end to end intermolecular attractions as a result of favourable molecular rigidity

Molecular Structure on Mesomorphic Behavior with Special Reference
43
and flexibility for all the members of the present homologous series. The molecules of all
the members of the series individually disalign at an angle less than ninety degree with the
plane of a floating surface under the influence of externally exposed thermal vibrations.
Molecules of all novel homologues individually on their microscopic examination travel in
two dimensional ordered array with statistically parallel orientational order against exposed
thermal energy or by resisting exposed thermal vibrations for definite range of temperature,
termed the nematogenic mesophase length of the homologue. The absence of smectogenic
character of all the homologues of a present series is attributed to the absence of lamellar
packing of molecules in their crystal lattices due to the cis configuration of n-alkoxy cin-
namic acids from which final products are synthesized. Thus the suitable magnitudes of
anisotropic forces of intermolecular attractions are adequate to induce nematic mesophase
but inadequate to induce smectic mesophase formation. The exhibition of odd-even effect
for nematic-isotropic transition curve is attributed to the alternation of transition tempera-
tures for odd-even homologues. The odd-even effect for nematic-isotropic transition curve
diminished from and beyond the nonyloxy derivative of the present series because longer
n-alkyl chains may coil, bend, flex or couple to lie with major axis of a core structure. High
magnitudes of thermal resistivity by the molecular structure of homologues with absence of
lamellar packing, led to nematogenic mesophase formation at high transition temperature
and relatively high mesomorphic phaselength. “Cinnamoyloxy mesomorphic derivatives
are generally nematogenic” [23]. This early statement is very well supported by present
investigation. The liquid crystal behavior of the present novel series-1 is compared with
structurally similar known series X [24] as shown in Fig. 2.
The novel homologous series-1 and a known series-X are identical in all respects except
for the central bridge linking middle and third phenyl rings CH CH COO CH2−
and CH CH CO respectively. Thus molecular flexibility varies from homologue to
homologue in the same series but it remains unchanged for the same homologue from
series to series because of the common OCH3 terminal end group in both series under
comparison. The molecular rigidity remains unchanged from homologue to homologue in
the same series, but it varies for the same homologue from series to series due to changing
length and polarity of the differing central groups. The combined effects of molecular
rigidity and flexibility are operative to the mesophase formation as a consequence of
favourable molecular rigidity and flexibility, which cause suitable or unsuitable magnitudes
of anisotropic forces of intermolecular end to end and lateral attractions depending upon
intermolecular closeness for the same homologue from series to series under comparison.
Thus, liquid crystal behavior and the degree of mesomorphism are dependent upon the
differing features of two series under comparison. Table 3 shows the thermal stabilities of
the mesophase, commencement of mesophase etc. for series 1 and X.
Figure 2. Structurally similar homologous series.

44
B. H. Patel et al.
Table 3. Average thermal stability in ◦C
Series→
Smectic-Nematic
Or
Smectic-isotropic
Commencement of
Smectic phase
Nematic-Isotropic
Commencement of
Nematic phase
Series 1
Series X
—
—
235.2
(C1 C16)
C1
164.6
(C5 C14)
C5
Table 3 indicates the following.
• Homologous series 1 and X are nematogenic only without exhibition of any smec-
togenic character.
• The nematic thermal stability of series-1 is greater than a series-X under comparison
by 70.6◦C.
• Liquid crystal nematogenic mesophase commences from very first member of a
series-1, while it commences from a fifth member of a series-X.
• Isotropic temperatures of series-1 are varied between 190◦C and 240◦C by hex-
adecyloxy and methoxy derivatives of LC homologues, while, the same are varied
between 160◦C and 170◦C among LC homologues as well as it vary between 160◦C
LC and 205◦C NLC homologue of series-X.
• Mesogenic temperatures are varied from 119◦C to 173◦C for series-1, while, it vary
from 110◦C to150◦C.
• Nematogenic mesophase length vary from 17◦C to 102◦C in case of series-, while,
it varied from 10◦C to 58◦C in case of series-X.
Thus, all above points of thermometric data indicate that, presently investigated ho-
mologous series-1 possess richer degree of mesomorphism and relatively higher values of
thermometric data. Hence molecular rigidity induced by CH CH COO CH2- central
group (series-1) is more than a central group CH CH CO (series-X) for the same
homologue from series-1 to series-X. Hence, the molecules tend to pack more efficiently in
case of series 1 with higher thermal stability. The central bridge CH CH COO CH2-
of series-1 and CH CH CO in case of series-X under comparison are closely similar
to each other. Both these central bridges are comparable, though the vinyl carboxylate with
methylene unit CH CH COO CH2− has relatively greater length and causes more
non coplanarity due to the unequal twist obtained as the oxygen atoms of the vinyl carboxy
group bump into the nonbonded adjacent hydrogen atoms of the aromatic phenyl ring. On
account of this difference the nematogenic thermal stabilities and thermometric data of
series-1 are relatively greater than a series-X, under comparison.
Conclusions
• Homologous series 4-(4ꢀ-n-alkoxy cinnamoyloxy)-4ꢀꢀ-methoxy benzyl cinnamates
is entirely nematogenic without exhibition of smectogenic character with wide range
of liquid crystallinity and high melting type.

Molecular Structure on Mesomorphic Behavior with Special Reference
45
• Mesomorphic properties varies from homologue to homologue in the same series
and they vary from series to series for the same homologue due to only one differing
feature (one of the central bridges) keeping the rest of the molecular parts unchanged.
• The group efficiency order derived on the basis of (i) thermal stability and (ii) the
early commencement of mesophase for nematic are as under.
(i) Nematic: CH CH COO CH2 > CH CH CO
Smectic: CH CH COO CH2 CH CH CO = zero
(ii) Nematic: CH CH COO CH2− > CH CH CO
Smectic: CH CH COO CH2− CH CH CO = Zero
• Molecular rigidity and flexibility are very sensitive and susceptible to mesophase
formation, the degree of mesomorphism and the commencement of the smectic
or/and nematic mesophase.
• Molecular rigidity and flexibility are susceptible to molecular structure only.
• Early statement that “Vinyl carboxy group is nematogenic” is very well supported
by present investigation and raised the reliability and credibility to the conclusions
drawn earlier.
Acknowledgments
Authors acknowledge thanks to the Green Circle Inc. Lab for research facilities services as
and when needed. Authors also thank Dr. Vipul Patel and Dr. Umed Bhoya for their valuable
helping hand and microscopic facility. Thanks are due to the Sophisticated Analytical
Instrumentation Facility, Punjab University, Chandigarh for analytical services.
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