Study of Mesomorphism Through a Novel Homologous Series and Its Relation to Molecular Structure

Article abstract of DOI:10.1080/15421406.2014.915630

A novel homologous series 4-(4'-n-alkoxy benzoyloxy)-3-chloro phenyl azo,3^',4^'-dichlorobenzenes was synthesized with a view to understand and establish the relation between mesomorphism and the molecular structure. The novel series consists of 12 members, which showed commencement of monotropic nematic mesophase formation from the butoxy homologue and continued up to the hexadecyloxy homologue without exhibition of a smectic mesophase. The textures of the nematic mesophase are threaded or Schlieren in type. Transition temperatures were determined by an optical polarizing microscope equipped with a heating stage. Isotropic-nematic transition curve of the phase diagram behaved in normal manner with exhibition of odd-even effect. Solid-isotropic transition curve behaved descending tendency by adopting zigzag path up to the pentyloxy homologue; which include methoxy to propoxy homologues as nonmesomorphic derivatives. Thermal stability for the nematic is just a few degrees (1°C to 2°C). Analytical and spectral data support the molecular structures. The mesomorphic properties of present novel series are compared with structurally similar other known homologous series.

Full text of DOI:10.1080/15421406.2014.915630

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Study of Mesomorphism Through a Novel
Homologous Series and Its Relation to
Molecular Structure
A. A. Doshia & B. C. Chauhanb
a Department of Biochemistry, The M.S. University of Baroda-
Vadodara, Gujarat, (India)
b Shri P.H.G. Municipal Arts and Science College-Kalol, Gujarat,
(India)
Published online: 15 Jan 2015.
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To cite this article: A. A. Doshi & B. C. Chauhan (2015) Study of Mesomorphism Through a Novel
Homologous Series and Its Relation to Molecular Structure, Molecular Crystals and Liquid Crystals,
606:1, 66-74, DOI: 10.1080/15421406.2014.915630
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Mol. Cryst. Liq. Cryst., Vol. 606: pp. 66–74, 2015
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2014.915630
Study of Mesomorphism Through a Novel
Homologous Series and Its Relation
to Molecular Structure
A. A. DOSHI1 AND B. C. CHAUHAN2,∗
1Department of Biochemistry, The M.S. University of Baroda-Vadodara,
Gujarat, (India)
2Shri P.H.G. Municipal Arts and Science College-Kalol, Gujarat, (India)
A novel homologous series 4-(4ꢀ-n-alkoxy benzoyloxy)-3-chloro phenyl azo,3ꢀꢀ,4ꢀꢀ-
dichlorobenzenes was synthesized with a view to understand and establish the relation
between mesomorphism and the molecular structure. The novel series consists of 12
members, which showed commencement of monotropic nematic mesophase formation
from the butoxy homologue and continued up to the hexadecyloxy homologue without
exhibition of a smectic mesophase. The textures of the nematic mesophase are threaded
or Schlieren in type. Transition temperatures were determined by an optical polarizing
microscope equipped with a heating stage. Isotropic-nematic transition curve of the
phase diagram behaved in normal manner with exhibition of odd-even effect. Solid-
isotropic transition curve behaved descending tendency by adopting zigzag path up to
the pentyloxy homologue; which include methoxy to propoxy homologues as nonmeso-
morphic derivatives. Thermal stability for the nematic is just a few degrees (1◦C to
2◦C). Analytical and spectral data support the molecular structures. The mesomorphic
properties of present novel series are compared with structurally similar other known
homologous series.
Keywords Liquid crystals; monotropy; mesomorph; mesomorphism; nematic
Introduction
Thermotropic mesomorphic substances exhibit mesomorphism peculiarities based on the
variations of temperature and the range of temperature depending upon their individual
molecular rigidity and flexibility [1–3] generated from respective molecular structure.
Molecular structure causing suitable magnitudes of anisotropic forces of intermolecular
end to end attractions [4] as a consequence of favorable molecular rigidity and flexibility
can yield statistically parallel orientational order of molecules in floating condition giving
rise to formation of nematic mesophase. Present investigation is planned to synthesize an
azoester molecular consisting of three phenyl rings bridged through COO and N N
central groups [5] left n-alkoxy (–OR) terminal end group as well as three chloro [6–8]
group substituted at middle (meta) and third phenyl ring (meta and para) as lateral and
terminal positions.
∗Address correspondence to Bakul Chauhan, Shri P.H.G. Muni. Arts & Science College,
Near, Ambica Highway, Kalol (North Gujarat) India, Pin. 382721. Tel.: +(919428224104). E-mail:
principalphg1966@gmail.com
66

Study of Mesomorphism Through a Novel Homologous Series
Experimental
Synthesis
67
4-n-Alkoxy benzoic acids were alkylated by suitable alkylating agents (R-X) by mod-
ified method of Dave and Vora [9]. An azo dye 4-hydroxy-3-chloro phenyl azo-3ꢀ-4ꢀ-
dichlorobenzene was prepared by usual established method of diazotization [10, 11]. The
final, azo ester homologues were prepared by condensing the various 4-n-alkoxy benzoic
acids through their corresponding acid chlorides with an azo dye common component in dry
cold pyridine [12]. The azo-ester novel homologues were individually decomposed, filtered,
washed, dried, and purified until constant transition temperatures were obtained. 4-Hydroxy
benzoic acid, alkyl halides, MeOH, EtOH, KOH, HCl, 2-chlorophenol, 3,4-dichloro ani-
line, thionyl chloride, pyridine, etc. required for synthesis were used as received, except
for solvents, which were purified and dried prior to sue. The synthetic route to the series is
shown in Scheme 1.
SOCl2
RBr,KOH
MeOH
HO-
-COOH
-COOH
RO-
-COCl
RO-
(1)
(Excess)
-SO2,-HCl
(A)
1) 2-chlorophenol in
NaOH
2) HCl, 0-5ºC
Cl
Cl
-
Cl
-
-
NaNO2. HCL
Cl-N+ -
(2)
H2N-
-Cl
-Cl
HO-
Cl
-N=N-
(B)
-Cl
2
-
Cl
-
Dry Pyridine
Cold HCl 1:1
(3)
(A) + (B)
RO-
-COO-
-N=N-
-Cl
-
Cl
Where R = CnH2n+1; n = 1,2,3,4,5,6,7,8,10,12,14,16
Scheme 1. Synthetic route to the series. Series: 4-[4ꢀ-n-Alkoxy benzoyloxy]-3-chlorophenyl azo-
3ꢀꢀ,4ꢀꢀ-dichlorobenzenes.
Characterization
Representative members of the novel homologous series were characterized by elemental
analysis (Table 1), infra red spectra, 1HNMR spectra, mass spectra. Microanalysis for C,H,N
analysis was carried out by Perkin-Elmer PE 2400 CHN analyser, IR spectra were performed

68
A. A. Doshi and B. C. Chauhan
Table 1. Elemental analysis for methoxy, ethoxy, and propoxy derivatives
Elements% found (% calculated)
Sr. no.
Molecular formula
C
H
N
1
2
3
C20H13N2O3Cl3
C21H15N2O3Cl3
C22H17N2O3Cl3
54.26 (55.10)
56.21 (56.06)
57.08 (56.95)
3.09 (2.98)
3.46 (3.33)
3.71 (3.66)
6.49 (6.42)
6.55 (6.22)
5.98 (6.04)
on Perkin Elmer spectrum GX and 1HNMR spectra were recorded on a Bruker instrument
using CDCl3 as solvent. The texture of the nematic phase and transition temperatures were
determined by optical polarizing microscopy equipped with a heating stage.
Analytical Data
Spectral Data
1HNMR in ppm for the octyloxy derivative: 1.315 [s, (CH2)n of OC8H17], 4,055 (t,
O CH2 CH2 of OC8H17), 6.973 and 8.204 (dd, p.sub phenyl ring),
1HNMR in ppm for the dodecyloxy derivative: 1.27 [s, (CH2)n of OC12H25], 4,05
(t, O CH2 CH2 of OC12H25), 6.903 to 7.018 and 7.257 to 7.951 (m,tri.substituted
phenyl ring), 8.025, 8.067, 8.308, 8.319, (dd, p. substituted phenyl ring).
IR in cm−1 for hexyloxy derivative: 3421.5 H-bonded OH, 2929.7, 2858.3 confirms
alkyl group, 1722.3, 1249.8, 1170.7 confirms COO , 1078.1 C O of ether, 1423.4
confirms N N group, 1043,1024 Ar-Cl linkage, 893 and 819.7 confirms 1,2,4 tri. Sub
aromatic ring, 840.9 confirms p-sub benzene ring.
IR in cm−1 for decyloxy derivative: 3427.3 H-bonded OH, 2922, 2852.5 confirms
alkyl group, 1722.3, 1232.4, 1170.7 confirms COO , 1078.1 C O of ether, 1423.4
confirms N N group, 1045.3,1024.1 Ar-Cl linkage, 893 and 819.7 confirms 1,2,4 tri.
Sub aromatic ring, 840.9 confirms p-sub benzene ring.
Mass Spectra
Mass spectra for butoxy derivative: Molecular formula C23H19N2O3Cl3 Calculated Molec-
ular weight 477.5. Practical mass observed by Mass spectroscopy 477.
Texture of Nematic Phase by Miscibility Method
Pentyloxy derivative —————–→Threaded nematic
Heptyloxy derivative ———–→Schlieren nematic
Results and Discussion
Azodye, 4-hydroxy-3-chlorophenylazo-3ꢀ-4ꢀ-dichlorobenzene (M.P. 122◦C) is not liquid
crystalline. However, liquid crystal property is induced on linking 4-n-aloxy benzoic acids
from butoxy homologue of a novel series in monotropic manner, as nematogenic phase.
Azoester, novel homologues showed their transition temperatures lower than the corre-
sponding n-alkoxy benzoic acids. Methoxy to propoxy homologues are nonmesomorphic,

Study of Mesomorphism Through a Novel Homologous Series
69
Table 2. Transition temperatures of series in ◦C
Transition temperature in ◦C
Compound no.
n-alkyl group (—CnH2n+1) (n)
Sm
Nm
Isotropic
1
2
3
4
5
6
7
8
9
10
11
12
1
2
3
4
5
6
7
8
10
12
14
16
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
178.0
169.0
116.0
110.0
114.0
108.0
102.0
99.0
98.0
94,0
91.0
87.0
107.0
97.0
99.0
87.0
92.0
83.0
75.0
72.0
78.0
Sm: Smectic
Nm: Nematic
Where R = CnH2n+1; n = 1,2,3,4,5,6,7,8,10,12,14,16.
butyloxy to hexadecyloxy homologue derivatives are mesomorphic with exhibition of
nematic mesophase in monotropic manner and, without exhibition of any smectogenic
character. Transition temperatures (Table 2) as determined by an optical polarizing mi-
croscopy with heating stage, plotted versus the number of carbon atoms present in n-alkyl
chain of the left n-alkoxy terminal end group gives rise to a phase diagram (Figure 1);
showing phase behavior through solid-isotropic and isotropic-nematic transition curves.
Solid-isotropic transition curve partly adopt a zigzag path of rising and falling (up to penty-
loxy homologue) and then possesses descending tendency as series is ascended without
adopting zigzag path. Isotropic-nematic transition curve follows descending tendency as
series is ascended with exhibition of odd-even effect. Isotropic-nematic transition curve is
extrapolated [13, 14] to third member (Odd homologue) of a novel series following trend of
a curve to determine and predict its latent transition temperature (LTT) for nematic (116◦C).
Thus, both transition curves, (viz. solid-isotropic and isotropic-nematic) behaved in normal
manner. Transition curves for odd and even homologues in isotropic-nematic curve merges
into each other at decyloxy homologue and then, odd–even effect disappears. Analytical,
spectral data agreed with the molecular structures of the respective homologues. Thermal
stability for nematic is one or two degrees (or few degree) with uncertainty of its exactness,
to determine the degree of mesomorphism or phase length. However, the transition tem-
peratures for the nematogenic mesophase persistence is exactly detectable below isotropic
temperature as shown in Table 2 in bracket under column, headed by Nm. Mesomorphic
behaviors of series, gradually varies from homologue to homologue as series is ascended.
Thus, present homologous series is relatively low melting type with predominancy of
nematogenic character in monotropic condition and absence of smectogenic character.
Linking of a phenyl ring with n-alkoxy terminal end group with a nonmesomorphic azo
dye has increased molecular aromaticity, length, molecular rigidity and flexibility, molecu-
lar polarity, and polarizability resulted to induce mesomorphism in irreversible manner from
and beyond fourth homologue of a novel series. Lowering of transition temperatures of the
homologues, than the corresponding n-alkoxy acids and their disappearance of dimerization

70
A. A. Doshi and B. C. Chauhan
Series: 4-[4'-n-alkoxy benzoyloxy]-3-chlorophenyl azo-3",4"-dichlorobenzenes
Cl
COO
RO
N=N
Cl
Cl
220
200
180
160
140
120
100
80
SOLID - ISOTROPIC
ISOTROPIC-NEMATIC
ISOTROPIC
/ NEMATIC
SOLID
60
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
Figure 1. Phase behavior of series.
is attributed to the breaking of hydrogen bonding of n-alkoxy acids through esterification
process. Twelve homologues of a thermotropic homologous series, directly transform into
the isotropic liquid state, on gradual heating of a sample substance under a polarizing
microscope. Thus, sample substance observed under a polarizing microscope is considered
thermodynamically as an open system and the rest of the universe is then considered as
the thermodynamic surroundings, that is, the exchange of heat energy takes place between
a system and the surroundings. As temperature rises of a system, the supplied heat from

Study of Mesomorphism Through a Novel Homologous Series
71
Cl
Series 1
N=N
N=N
COO
COO
RO
RO
Cl
Cl
Cl
Cl
Cl
Series X
O
CH3
Series Y
N=N
COO
RO
Cl
Figure 2. Structurally similar homologous series.
surroundings weakens the intermolecular end to end and other attractive forces, and finally,
smooth breaking of crystal lattices sharply transform into isotropic melt at corresponding
melting temperature. Then, isotropically melted mass is allowed to cool without any dis-
turbing event, the homologues from and beyond butoxy derivative releases heat energy from
a system to surroundings. As a consequence of cooling the isotropic melt, the molecules
of homologue system set themselves and acquire statistically parallel orientational order in
floating condition irreversibly at monotropic temperature, below isotropic temperature, un-
der the influence of suitable magnitudes of anisotropic forces of end to end intermolecular
attractions. But, the nematogenic mesophase stabilization energy is insufficient to prolong
the mesophase stabilization, and immediately transforms the sample substance into solid
crystalline state from mesomorphic state. Thus, entropy (ꢀS = ꢀH/T) of a system increase
on heating a system and then decreases on cooling a sample, which observed under polar-
izing microscopy. Absence of smectogenic character is attributed to the high crystallizing
tendency of nematogenic mesophase without passing through smectogenic mesophase, due
to inadequate suitable magnitudes of anisotropic forces of intermolecular attractions which
hinders lamellar packing of molecules in the crystal lattices of homologues concerned. Two
opposing forces [4, 5, 6] viz (i) reducing intermolecular attractions by broadening molecules
through laterally substituted two chloro groups and (ii) the enhancing intermolecular at-
tractions by increasing molecular polarizability operating at a time causes predominancy
of the force (i), that is, the reducing intermolecular anisotropic forces of intermolecular
attractions, which hinders the enantiotropically nematogenic mesophase formation as well
as smectogenic mesophase formation in the present series. The diminishing tendency for
odd–even effect observed for isotropic-nematic transition curve, from and beyond decyloxy
homologue is, due to the coiling or bending or flexing or coupling of longer n-alkyl chain
of left n-alkoxy terminal end group to the major axis of the core molecule for higher homo-
logues. Exhibition of odd–even effect, alternation of transition temperatures and variations
of liquid crystal properties from homologue to homologue in the same present series are
attributed to the progressive and sequential addition of methylene unit or units in left n-
alkoxy terminal end group affecting suitable or unsuitable anisotropic forces of end to end
intermolecular attractions. The extrapolation of isotropic-nematic transition curve to the
third odd member of a series merges into solid-isotropic transition temperature indicates
that, there is no possibility of the formation of nematic or smectic phase, either enantiotrop-
ically or monotropically. Relatively low melting behavior of a series and shorter mesophase
length are attributed to the predominancy of the weaker intermolecular adhesion forces

72
A. A. Doshi and B. C. Chauhan
(i) operated by broadening of the molecules. The mesomorphic properties of a presently
investigated series 1 are compared with other known structurally similar homologous series
X [15] and Y [16] as shown in Fig. 2.
Homologous series-1 and series X, Y chosen for comparing mesomorphic properties as
shown in Fig. 2 are structurally identical with respect to three phenyl rings bonded through
COO and N N central bridges, left and right terminally situated end groups –OR
and Cl, another Cl group substituted at meta position to N N group on third
phenyl ring as lateral group. However, series 1,X and Y differ only with respect to a group
Cl, OCH3 and H substituted at ortho position to COO meta position to N N
on middle phenyl ring respectively. Therefore, variations in mesomorphic properties and
their degree of mesomorphism vary, due to the varying laterally substituted Cl, OCH3,
and H group at the middle phenyl ring at the identical position from N N central
bridge. Thus, variations can be co-related with differing molecular polarity, polarizabi-
ity, molecular rigidity and flexibility, etc. related to combine effect of anisotropic forces
of intermolecular lateral and terminal attractions. Following Table 3 represents thermal
stabilities, commencement of mesophases, type of mesophase etc for series 1, X and Y.
Homologous series 1 and X are monotropically either nematic or smectic respectively.
However, series Y is predominantly nematic and partly smectic. Series 1 and X structurally
differ by only lateral substitution at identical position on middle phenyl ring by Cl
and OCH3 group. The OCH3 group relatively being more polar [4, 6] than Cl, can
contribute more, toward molecular polarity and polarizability. Thus, overall molecular
rigidity, flexibility, polarity, and polarizability including other intermolecular adhering,
anisotropic forces facilitated, lamellar packing of molecules in crystal lattices to form
interlayered linking in floating condition for series-X below isotropic temperature; while
chloro group under the same condition in series 1 generate, such a little bit less magnitudes
of anisotropic forces by combine effect of lateral and terminal end to end attractions, that,
molecules of series 1 set an angle less than 90 degree to arrange themselves, statistically
parallel orientational order below isotropic temperature under floated movement, without
formation of lamellar packing of molecules in their crystal lattices. Thus, homologous series
1 exhibit only nematic phase while, series X, exhibit only smectic phase in irreversible
or monotropic manner. Now on comparing series Y with series 1, the –chloro group
bonded at middle phenyl ring is eliminated or to say replaced by H, the mesomorphic
behavior of series Y is drastically uplifted. Only elimination of Cl group from middle
Table 3. Average thermal stability in ◦C
Series→
Series 1
Series X
Series Y
Smectic-isotropic or
smectic-nematic or
nematic-smectic or
isotropic-smectic
commencement of
smectic phase
—
1 or 2 degree or few 106.6 (C6 C16) C6
degree (C5 C16) C5
Nematic-isotropic or 1 or 2 degree or few
156.25 (C1 C16)
C1
isotropic-nematic
commencement of
nematic phase
degree (C4 C16) C4

Study of Mesomorphism Through a Novel Homologous Series
73
phenyl ring modifies molecule of series Y as long linear path like without any steric
hindrances keeping molecular polarizability as equal as of series 1. Thus, intermolecular
closeness strengthened to raise suitable magnitudes of anisotropic forces of intermolecular
attraction drastically high enough to resist, externally exposed thermal vibrations which
permitted the molecules of series Y to build their lamellar packing and interlinked layers
in their crystal lattices as well as subsequently it allowed to maintain statistically parallel
orientational molecular order due to adequate end to end attractions. Thus, smectic and
nematic mesophase exhibited for definite range of temperature persisted enantiotropically
in case of series Y. Thus, even a smallest structural change can yield the drastic change in the
mesomorphic behavior of a substance. Now as regard to the comparison of commencement
of mesophase formation, the nematic mesophase commences from butyloxy homologue
of a series-1, from very first member of a series Y and it does not commence till the last
homologue of a series X. Thus, type of nematogenic behavior is attributed to the suitable
or unsuitable magnitudes of end to end intermolecular anisotropic forces of attractions
occurred as a consequence of varying molecular rigidity and/or flexibility which varies by
only laterally substituted groups Cl, OCH3, and H, keeping both terminal end groups
(–OR and Cl) unchanged. Thus, intermolecular lateral attractions plays the major role to
the commencement of nematic phase. The smectic mesophase commences from hexyloxy
homologue for series Y, pentyloxy homologue for series X and it does not commence till
the last homologue for the novel series 1 under present investigation. The early or late
commencement of smectic phase depend upon early or late commencement of lamellar
packing of molecules in their crystal lattices, which in turn depends upon the extent of
noncoplanarity of the respective molecules of different molecular structures. Thus, extent
of noncoplanarity played by differing individual group polarities of Cl, OCH3 and
H
substituted at middle phenyl ring at identical position. Thus, differing degree of molecular
coplanarity results, commencement of smectic phase from respective order of coplanarity.
The variations in mesomorphic properties from series to series for the same homologue
arise from uncommon molecular part or the differing entity of the corresponding molecular
structure. Differing entity may be terminal end group, lateral group, position of same group
or groups on same or different phenyl ring or rings, central group or groups, etc.
Conclusions
• The group efficiency order derived on the basis of thermal stability and early com-
mencement of mesophases with reference uncommon to lateral substitution is as
under.
• Thermal stabilities
Smectic : −H > −OCH3 > −Cl
Nematic : −H > −Cl > −OCH3
• Commencement of mesophase
Smectic : −OCH3 > −H > −Cl
Nematic : −H > −Cl > −OCH3

74
A. A. Doshi and B. C. Chauhan
• Present investigated novel series is predominantly monotropically nematogenic with-
out exhibition of smectogenic character and low melting type.
• Elimination or substitution of a same functional group at same position can create a
drastic change in liquid crystal behavior.
• Mesophase behavior is very sensitive and susceptible to molecular structure.
• Molecular rigidity and flexibility of favorable magnitudes can cause suitable mag-
nitudes of anisotropic forces of intermolecular attractions which can induce liquid
crystalline property in a substance.
• Variations in liquid crystal property and the degree of liquid crystallinity between two
different homologous series depend upon uncommon part of a molecular structure.
Acknowledgment
Authors acknowledge thanks to the head, applied chemistry faculty of technology, The
M.S. University of Baroda, Vadodara. Thanks are due to the CSMCRI-Bhavnagar for
their analytical and spectral services. Authors also acknowledge thanks to Brijesh H Patel,
Huntsman Ltd. Padra, Vadodara for his cooperation.
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