Effect of a central group on mesomorphism

Article abstract of DOI:10.1080/15421406.2014.939601

A novel homologous series: 4-(4'-n-alkoxy benzoyloxy)-4^'-methoxy benzyl benzoates is studied with a view to understanding the effect of a central group on the phenomenon of mesomorphism. The novel homologous series consists of eleven compounds. Mesomorphism commences from the second member of a series. All the mesogenic homologues from ethoxy to hexadecyloxy exhibit nematogenic mesomorphism without the exhibition of smectogenic character. The nematic-isotropic transition curve of the phase diagram behaves in normal manner up to the dodecyloxy homologue and then deviated for the tetradecyloxy and hexadecyloxy homologues. The solid-isotropic or nematic transition curve adopts a normal behavior in a zigzag manner. The nematic-isotropic transition curve shows an odd-even effect and continues up to the octyloxy homologue. Transition and melting temperatures including the textures of mesophases are determined by an optical polarizing microscope, equipped with a heating stage. The textures of the nematic mesophase are threaded or Schlieren in type. Analytical data and spectral data support the molecular structures. The mesomorphic behavior of the present series is compared with structurally similar other known series of differing central group (viz. NN and COCHCH) keeping rest of a molecular part unchanged. The nematic thermal stability of the series is 236.8°C of a high melting type present series whose nematogenic mesophase length varies between 27°C and 98°C. Thus, series is only nematogenic without exhibition of a smectic mesophase.

Full text of DOI:10.1080/15421406.2014.939601

Mol. Cryst. Liq. Cryst., Vol. 607: pp. 114–122, 2015
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2014.939601
Effect of a Central Group on Mesomorphism
BRIJESH H. PATEL,1,∗ N. N. VYAS,2 AND A. V. DOSHI3
1Huntsman, Baroda Textile Effects Pvt. Ltd., Umraya village, Taluka Padra,
Vadodara, Gujarat, India
2Sheth P.T. Arts & Science College, Godhra, 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: 4-(4ꢀ-n-alkoxy benzoyloxy)-4ꢀꢀ-methoxy benzyl benzoates is
studied with a view to understanding the effect of a central group on the phenomenon of
mesomorphism. The novel homologous series consists of eleven compounds. Mesomor-
phism commences from the second member of a series. All the mesogenic homologues
from ethoxy to hexadecyloxy exhibit nematogenic mesomorphism without the exhibition
of smectogenic character. The nematic–isotropic transition curve of the phase dia-
gram behaves in normal manner up to the dodecyloxy homologue and then deviated
for the tetradecyloxy and hexadecyloxy homologues. The solid-isotropic or nematic
transition curve adopts a normal behavior in a zigzag manner. The nematic-isotropic
transition curve shows an odd-even effect and continues up to the octyloxy homologue.
Transition and melting temperatures including the textures of mesophases are deter-
mined by an optical polarizing microscope, equipped with a heating stage. The textures
of the nematic mesophase are threaded or Schlieren in type. Analytical data and spectral
data support the molecular structures. The mesomorphic behavior of the present series
is compared with structurally similar other known series of differing central group (viz.
N
N
and CO CH CH ) keeping rest of a molecular part unchanged. The
nematic thermal stability of the series is 236.8◦C of a high melting type present series
whose nematogenic mesophase length varies between 27◦C and 98◦C. Thus, series is
only nematogenic without exhibition of a smectic mesophase.
Keywords Central bridge; mesogen; mesomorphism; nematic; smectic
Introduction
Liquid crystal materials workable at room temperature or at a desired temperature enjoy
wide industrial applications in the present era [1, 2]. Liquid crystal properties can be induced
in a substance if favorable and suitable magnitudes of anisotropic forces of intermolecular
end to end and lateral attractions are generated by a molecular structure [3–5] depending
upon combined effects of molecular rigidity and flexibility [6–9]. The construction of a
molecular shape, size aromaticity, polarities of functional groups, their positions at a termi-
nal end group or at lateral positions [10, 11] or as a central bridge or molecular polarizability
contribute to the molecular rigidity and flexibility. Thus, the present investigation is planned
∗Address correspondence to Brijesh H. Patel, Huntsman, Baroda Textile Effects Pvt. Ltd.,
Umraya village, Taluka Padra, Vadodara, Gujarat, 391 440 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.
114

Effect of a Central Group on Mesomorphism
115
with a view to understanding and establishing the molecular structure [12, 13] dependence
of mesomorphism by varying the central group of a linear lath like molecule and keeping
rest of the molecular structure unchanged. The proposed molecules consist of three phenyl
rings bonded through two central bridges of COO and COO CH2 as rigid core and
n-alkoxy ( OR) and OCH3 groups as terminal end groups (R CnH2n + 1) without any
lateral substitution, which will act as flexible core of a novel molecule. The study will be
restricted to the synthesis of a novel homologous series, characterization of some selected
1
homologues by elemental analysis IR, HNMR, mass spectra, and the determination of
mesophase textures by a miscibility method, transition and melting temperatures by hot
stage polarizing microscope, and to correlate their liquid crystal properties by comparing
with azoester [14] and chalconyl [14] homologous series.
Experimental
Synthesis
4-Hydroxy benzoic acid was converted into dimeric 4-n-alkoxy benzoic acids using suitable
alkylating (R-X) agents by a modified method of Dave and Vora [15]. 4-Hydroxy benzoic
acid was converted to 4-hydroxy-4ꢀ-methoxy benzyl benzoate by the modified method
of a European patent EP0117502B1 [16]. 4-n-alkoxy benzoic acids were individually
condensed with 4-hydroxy-4ꢀ-methoxy benzyl benzoate (m.p. 180◦C, yield is 33%) in
pyridine through their acid chlorides in dry cold pyridine [17, 18]. Final products were
individually decomposed, filtered, washed dried and purified in ethanol until constant
transition temperatures obtained. Transition and melting temperatures were determined by
an optical hot stage polarizing microscopy.
The chemicals 4-hydroxy benzoic acid, 4-methoxy benzyl alcohol, dibutyltin oxide,
pyridine, alkyl halides, MeOH, EtOH, thionyl chloride, HCl, KOH were used as received,
except solvents, which were dried and distilled prior to use. The synthetic route to the series
is outlined in Scheme 1.
Characterization
Selected members of the novel series were characterized by elemental analysis, infra red
1
spectra, HNMR spectra and mass spectra. Micro analysis was performed on a Perkin-
1
Elmer PE 2400 CHN analyzer, HNMR spectra were recorded on Bruker spectrometer
using DMSO solvent. Infra red spectra were recorded on Perkin Elmer spectrum GX
spectrometer. Transition temperatures and the textures of the nematic mesophase were
determined by a miscibility method using an optical polarizing microscope 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
Spectral Data
1HNMR in ppm for the hexyloxy derivative: 0.88–1.72 (H of C6H13), 2.51 ( O CH2 CH2
of OC6H13), 3.76 ( OCH3), 3.99 ( OCH2 of OC6H13), 5.1 ( COO CH2 C6H4),

116
B. H. Patel et al.
CH3OH
Reflux
COOH
R-X
+
RO
COOH
HO
KOH
- HX
-SO2
SOCl2
+
- HCl
Reflux
COCl
RO
A
DBTO
- H2O
O-CH3
COOH +
O-CH3
HO
HOCH2
COOCH2
HO
B
dry cold
O-CH3
COOCH2
COCl HO
RO
+
Pyridine
- HCl
B
A
O-CH3
COOCH2
COO
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 nenzoyloxy]-4ꢀꢀ-methoxy benzyl
benzoates where R = CnH2n + 1; n = 1,2,3,4,5,6,8,10,12,14,16.
6.92 & 7.01 (p-substituted benzene ring), 7.34 (presence of another benzene ring), 7.89 &
8.03 (p-substituted benzene ring).
1HNMR in ppm for the octyloxy derivative: 0.83–1.71 (H of C8H17), 3.73 ( OCH3),
4.0 ( OCH2 of OC8H17), 5.0 ( COO CH2 C6H4), 6.9–7.01 (p-substituted benzene
ring), 7.32 (another benzene ring), 7.86–8.03 (p-substituted benzene rings).
IR in cm−1 for tetradecyloxy derivative: 827 para-substituted phenyl ring, 1250
ether linkage of ( C14H29O C6H4), 1681 & 1169 ( COO ester group).
IR in cm−1 for tetradecyloxy derivative: 827 para-substituted phenyl ring, 1250
ether linkage of ( C16H33O C6H4), 1681 & 1170 ( COO ester group).
Mass Spectra
Mass spectra for butyloxy derivative:
Molecular formula C26H26O6 calculated molecular weight 434.
Mass observed by mass spectroscopy 434.
Texture of nematic by miscibility method
Pentyloxy derivative → Threaded nematic
Decyloxy derivative → Schlieren nematic

Effect of a Central Group on Mesomorphism
117
Table 1. Elemental analysis for methyloxy, ethoxy and propyloxy derivatives.
Elements % found (% calculated)
Sr. no.
Molecular formula
C
H
1
2
3
C23H20O6
C24H22O6
C25H24O6
70.22 (70.40)
70.28 (70.93)
71.18 (71.42)
5.08 (5.10)
5.37 (5.41)
5.47 (5.71)
Results and Discussion
The novel homologous series 4-[4ꢀ-n-alkoxy benzoyloxy]-4ꢀꢀ-methoxy benzyl benzoates
consists of eleven members. The ethoxy to hexadecyloxy homologues are enantiotropi-
cally nematogenic and only first member of the series is non-mesogenic. The smectogenic
mesophase is totally absent. The transition temperatures (Table 2) of the homologues
are plotted versus the number of carbon atoms present in n-alkyl chain of left n-alkoxy
terminal end group. Like or related points are joined to draw the appropriate curves of solid-
mesomorphic/isotropic and nematic-isotropic to construct a phase diagram (Figure 1) show-
ing the phase behavior of the novel homologous series. The solid-mesomorphic/isotropic
transition curve follows a zigzag path of rising and falling values as series is ascended and
behaved in normal manner. The nematic-isotropic transition curve initially rises and then
descends as the series is ascended up to the dodecyloxy homologue, but it shows deviating
behavior for the tetradecyloxy and hexadecyloxy homologues. Thus, the nematic-isotropic
transition curve majorly behaved in a normal manner. An odd-even effect is observed for
the nematic-isotropic transition curve. The curves for the odd and even members of a series
merge into each other at the octyloxy homologue and then it propagates up to hexadecy-
loxy homologue in a single transition curve. Thus, the odd-even effect disappears from
Table 2. Transition temperatures of series in ◦C.
Transition temperature in ◦C
Compound no.
n-alkyl group ( CnH2n + 1) (n)
Sm
N
Isotropic
1
2
3
4
5
6
7
8
1
2
3
4
5
6
8
10
—
—
—
—
—
—
—
—
—
—
—
—
201.0
240.0
220.0
229.0
249.0
255.0
270.0
220.0
212.0
242.0
231.0
202.0
198.0
205.0
201.0
181.0
172.0
188.0
162.0
215.0
204.0
9
10
11
12
14
16
Sm: Smectic
N: Nematic

118
B. H. Patel et al.
300
280
260
240
220
200
180
160
140
120
SOLID - ISOTROPIC
OR
SOLID - NEMATIC
ISOTROPIC
NEMATIC - ISOTROPIC
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.
and beyond octyloxy homologue. The nematic-isotropic transition curve for the first non-
mesomorphic member of the series is extrapolated [19, 20] to predict the nematic-isotropic
transition temperature. Analytical and spectral data confirm the molecular structures of ho-
mologues. The mesomorphic properties vary from homologue to homologue in the present
novel series. The thermal stability for the nematogenic mesophase is 236.8◦C and meso-
morphic phase length varies between 27◦C (tetradecyloxy and hexadecyloxy homologues)
and 98◦C (octyloxy homologue). Thus, the present series under discussion is nematogenic
and high melting type with absence of smectogenic character.
Non-mesomorphicity of first member of the series is attributed to the high crystalising
tendency due to the presence of two identical polar OCH3 end groups of the first homo-
logue of the series. The intermolecular closeness and intermolecular attractions generated
as a consequence of unfavourable molecular rigidity and flexibility against exposed thermal
vibrations are relatively weaker or imbalanced to cause mesomorphism and hence sharply

Effect of a Central Group on Mesomorphism
119
O-CH3
COOCH2
COO
Series 1
Series X
Series Y
RO
O-CH3
N=N
COO
RO
O-CH3
CO-CH=CH
COO
RO
Figure 2. Structurally similar homologous series.
transforms into an isotropic liquid from the solid crystalline state without passing through
anisotropic state of mesomorphism. The nematogenic mesophase formation is attributed
to the disalignment of the molecules on the plane of a surface at an angle less than 90◦
and withstanding externally exposed thermal vibrations for definite temperature range with
statistically parallel orientational order of molecules in a floating condition. As a result of
suitable magnitudes of anisotropic forces of intermolecular favourable closeness and end
to end attractions for some temperature difference prior to isotropic temperature. Thus,
favorable quantum of molecular rigidity and flexibility facilitated nematogenic mesophase
formation for the ethoxy to hexadecyloxy homologues without exhibition of smectic charac-
ter. The absence of smectogenic character for all the homologues of presently investigated is
attributed to the absence of lamellar packing of molecules in their crystal lattices which did
not facilitate a sliding layered arrangement of molecules, prior to nematogenic mesophase
formation. The ordered arrangement of molecules gradually diminishes with rising of
temperature and the molecule of a sample substances under polarising microscopy are ran-
domly oriented freely in all possible directions with high order of disorder or randomness
or entropy (ꢀS = ꢀH/T). The reversal of mesophase reappears on cooling the isotropic
melt of the mesomorphic homologue at the same temperature at which it disappears on
heating if appropriate rate of heating and cooling is maintained for enantiotropic transitions.
The disappearance of odd-even effect from and beyond octyloxy homologue is attributed
to the effect of coiling or bending or flexing or coupling of longer n-alkyl chain, bonded
through oxygen atom with core structure of a molecule. The changing trend of mesogenic
properties from homologue to homologue in present series is due to the sequencial addition
of methylene unit or units in left n-alkoxy terminal end group keeping unaltered, polar
OCH3 group at the other end of a molecule. The nematic-isotropic transition curve for
odd members of a series is extrapolated [21–23] to the nonmesomorphic first member of
a present series to determine and predict nematic-isotropic transition temperature of first
homologue, keeping in view of the trend of the transition curve, it matches to the point at
201◦C, which is a solid-isotropic temperature of first odd member of a series i.e., 201=201=
0, or mesophase length zero degree centigrade indicates or means, there is no possibility of
mesophase formation beyond 201◦C. The mesomorphic behaviors of a present novel series
are compared with structurally similar known homologous series X [24] and Y [25] which

120
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
Series Y
137.0
(C14–C16)
C14
118.0
(C7–C14)
C7
236.8
(C2–C16)
C2
251.7
(C1–C14)
C1
157.0
(C5–C16)
C5
differs with respect to one of the central groups, keeping rest of the molecular part identical
for the same homologue from series to series , shown as under in Fig. 2.
Homologous series-1 investigated presently and series X and Y chosen for comparative
study are identical with respect to three phenyl rings and one of the central bridge linking
first and middle phenyl ring. But their second central bridges linking middle and third
phenyl rings are different. Therefore, series 1, X and Y differ with molecular rigidity.
Moreover all the three series under comparison have same terminal end groups for the
same homologue from series to series responsible to contribute identical molecular rigidity.
However, combine effects of suitable magnitudes of molecular rigidity and flexibility differ
form series to series and homologue to homologue in the same series. Thus, mesomorphism
and the degree of mesomorphism vary from series to series and homologue to homologue
in the same series. Following Table 3 represents some mesomorphic properties of the series
1, X and Y under comparison.
From Table 3, it is clear that,
• Homologous series 1 is only nematogenic, while series X and Y are nematogenic in
addition to smectogenic.
• Thermal stabilities for nematic mesophase decreases from series X to 1 to Y while
smectic thermal stability decreases from X to Y to 1.
• Smectogenic mesophase stabilizes for series X and Y but it destabilized in case of
series 1 until the last hexadecyloxy homologue.
• Smectic mesophase commences earliest from seventh member of a series Y, from
tetradecyloxy homologue in series X, but it did not commence until the last member
of series 1.
• Nematic mesophase commences from very first member of a series X, from second
member of a series 1and from fifth member of series Y.
The central group of COO linking first and middle phenyl ring is commonly
present in all the homologous series under comparison with changing central groups
( COO CH2
,
N N , CO CH CH ) linking middle and third phenyl rings, for
the same homologue from series to series. Central groups COO CH2 and N N in
case of present series and series X are comparable to each other. The oxygen atom of central
carboxy group bumps into the nonbonded sides of the adjacent hydrogen atoms of aromatic
phenyl ring, which causes considerable strain on the molecule. Consequently, a twist around
C-O bond occurs forcing the phenyl ring out of the plane of a molecule. Thus, molecular
coplanarity is reduced to some extent making them thick. The central group N N

Effect of a Central Group on Mesomorphism
121
certainly endows the molecular coplanarity in trans position. On account of these differ-
ences the smectic-Nematic and nematic-isotropic thermal stabilities of series-1 is lower
than a series X. Central bridge CO CH CH is relatively longer than COO CH2
central bridge with absence of conjugated double bond in series-1, which causes more
noncoplanarity due to the twist obtained by oxygen atom of vinyl C=O carbonyl group of
series Y. On account of this difference the nematic-isotropic thermal stability of series Y
is lower than a series 1. Smectic mesophase formation is hindered by absence of lamellar
packing of molecules in the crystal lattices of series 1. Early or late commencement of the
smectic or nematic mesophase also depends upon the extent of noncoplanarity caused by
the molecules. Thus, the degree of molecular noncoplanarity of series 1 does not permit
the lamellar packing of molecules in their crystal lattices to cause smectic mesophase for-
mation until the last member of a series 1. The suitable magnitudes of anisotropic forces of
inter molecular closeness and end to end as well as lateral attractions as a consequence of
varying molecular rigidity and flexibility is attributed to varying polarity and polarizability
of changing central bridges of series 1, X and Y. Thus, variations in mesophase forma-
tion and the varying degree of mesomorphism is susceptible to differing central groups of
COO CH2
,
N N , CO CH CH . Hence, observed variation in mesomorphic
behavior for the same homologue from series to series is attributed to changing central
group, keeping rest of the molecular parts unchanged.
Conclusions
• The group efficiency order derived on the basis of (i) thermal stability and (ii) the
early commencement of mesophase for smectic and nematic are as under.
(i) Smectic :
Nematic :
N N
N N
>
>
CO CH CH
COO CH2
>
>
COO CH2 −
CO CH CH −
COO CH2 −
CO CH CH
(1)
(2)
(3)
(4)
>
(ii) Smectic :
Nematic :
CO CH CH
>
N N
N N COO CH2
>
>
• The phenomena of mesomorphism and the degree of mesomorphism depend upon
the polarity and polarizability of central group of a mesomorphic molecule.
• Mesogenic tendency of a substance depends upon suitable magnitudes of molecular
rigidity and flexibility.
• Mesogenic properties of a substance is susceptible to suitable magnitudes of
anisotropic forces of intermolecular attractions as a consequence of favorable molec-
ular rigidity and flexibility.
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. M. L. Chauhan for
their valuable helping hand and microscopic facility. Thanks are due to the Sophisticated
Analytical Instrumentation Facility, Punjab University, Chandigarh for analytical services.

122
B. H. Patel et al.
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