Dependence of Liquid-Crystal Behavior of a Substance on Molecular Structure

Article abstract of DOI:10.1080/15421406.2014.990277

A novel azoester homologous series of liquid crystals (LCs) consisting of 12 members has been synthesized and evaluated. All members of the series exhibit the nematic mesophase enantiotropically without the exhibition of a smectic mesophase. The textures of the nematic phase are threaded or Schlieren in type. The transition curves in the phase diagram behave in a normal manner. The N-I transition curves exhibit an odd-even effect. The analytical and spectral data support the molecular structures of the homologs. The transition temperatures and mesophase morphology were determined by an optical polarizing microscope equipped with a heating stage. The azoester compounds consist of three phenyl rings with a meta substituted -Cl group at the middle phenyl ring and -OR and -OCH₃ as terminal end groups. The average nematic thermal stability is 195°C and the nematogenic mesophase length ranges from 9°C to 186°C. The LC behaviors of the series are compared with a structurally similar series. The novel series is entirely nematogenic and of a high melting point type.

Full text of DOI:10.1080/15421406.2014.990277

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Dependence of Liquid-Crystal Behavior of a
Substance on Molecular Structure
N. G. Makwana & A. V. Doshi
To cite this article: N. G. Makwana & A. V. Doshi (2015) Dependence of Liquid-Crystal Behavior
of a Substance on Molecular Structure, Molecular Crystals and Liquid Crystals, 616:1, 93-101,
DOI: 10.1080/15421406.2014.990277
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Date: 15 March 2016, At: 13:05

Mol. Cryst. Liq. Cryst., Vol. 616: pp. 93–101, 2015
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2014.990277
Dependence of Liquid-Crystal Behavior
of a Substance on Molecular Structure
N. G. MAKWANA1,∗ AND A. V. DOSHI1,2
1General Department (Chemistry), R.C. Technical Institute, Ahmedabad,
Gujarat, India
2Saurashtra University, Rajkot, Gujarat, India
A novel azoester homologous series of liquid crystals (LCs) consisting of 12 members
has been synthesized and evaluated. All members of the series exhibit the nematic
mesophase enantiotropically without the exhibition of a smectic mesophase. The textures
of the nematic phase are threaded or Schlieren in type. The transition curves in the phase
diagram behave in a normal manner. The N-I transition curves exhibit an odd-even effect.
The analytical and spectral data support the molecular structures of the homologs. The
transition temperatures and mesophase morphology were determined by an optical
polarizing microscope equipped with a heating stage. The azoester compounds consist
of three phenyl rings with a meta substituted Cl group at the middle phenyl ring
and –OR and OCH3 as terminal end groups. The average nematic thermal stability
is 195◦C and the nematogenic mesophase length ranges from 9◦C to 186◦C. The LC
behaviors of the series are compared with a structurally similar series. The novel series
is entirely nematogenic and of a high melting point type.
Keywords Liquid crystal; mesomorphism; nematic; odd-even effect
Introduction
The recognition of a novel physical state of a substance between the crystalline solid state
and the isotropic liquid state was made in 1888 by Austrian Botanist [1] who termed this
phase as the liquid crystalline (LC) state. Such a novel physical state has dual character
of flow as a liquid and optical properties like a crystalline solid. Since 1888, scientists of
all disciplines have shared their research interest with different aims, objects, and views
according to their fields of research in specific liquid crystal area. Chemists synthesize
novel substances and characterize them with analytical, spectral, and thermal data and
study the effect of molecular structure on LC properties of the novel substances. Other
research groups of other disciplines study the physical properties and applications using
the novel substances reported by chemists [2–6]. The present proposed investigation is to
synthesize novel azoester molecules with three aromatic rings bonded through COO
and N N central groups, two flexible terminal end groups –OR and OCH3 and
one laterally substituted Cl group at meta position with respect to N N on middle
phenyl ring, and to study effect of the mono substituted Cl or OCH3 lateral group on
∗Address correspondence to N. G. Makwana, General Department (Chemistry), R.C. Technical
Institute, Ahmedabad, 380060, Gujarat, India. E-mail: ngmakwana@yahoo.com
93

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N. G. Makwana and A. V. Doshi
mesomorphic behaviors of a substance or the effect of molecular structure on LC properties
[7,8] in terms of molecular rigidity and/or flexibility [9–12]
Scheme 1. Synthetic route to series-1.
Experimental
Synthesis
2-Chlorophenol, 4-methoxyaniline, sodium nitrite, pyridine, thionyl chloride, 4-hydroxy
benzoic acid, and n-alkyl halides were used directly as received. The solvents were dried
and distilled before use. The synthetic route to the present series is illustrated in Scheme
1. The 4-n-alkoxy benzoic acid and corresponding 4-n-alkoxy benzoyl chlorides (A) were

Liquid Crystals and Molecular Structure
95
Table 1. Elemental analysis for propyloxy, pentyloxy, octyloxy, and decyloxy derivatives
Elements% found (% calculated)
Sr. no.
Molecular formula
C
H
N
1
2
3
4
C23H21N2O4Cl
C25H25N2O4Cl
C28H31N2O4Cl
C30H35N2O4Cl
65.08(65.01)
66.36(66.30)
67.92(67.95)
68.98(68.90)
4.99(4.95)
5.57(5.52)
6.24(6.27)
6.71(6.70)
6.53(6.59)
6.14(6.19)
5.61(6.27)
5.39(5.36)
synthesized by the modified method of Dave and Vora [13]. 4-Hydroxy-3-chlorophenylazo-
4ꢀ-methoxybenzene (B) was prepared by a known method [14a, 14b]. The 4-(4ꢀ-n-alkoxy
benzoyloxy)-3-chlorophenylazo-4ꢀꢀ-methoxybenzenes were synthesized by condensing the
4-n-alkoxy benzoyl chlorides (A) with 4-hydroxy-3-chlorophenylazo-4ꢀ-methoxybenzene
(B) in pyridine [15a–15d].
Characterization
Some selected members of the series were characterized by Elemental analysis, IR spectra,
1H-NMR spectra, mass spectra, and texture determination by a miscibility method. Micro-
analysis of the representative compounds was performed on a Perkin Elmer PE 2400 CHN
analyzer as shown in Table 1. IR spectra were performed on Perkin Elmer spectrometer
and 1H NMR spectra were performed on Bruker spectrometer using CDCl3 as solvent. LC
properties were investigated on a Leitz Laborlux 12 POL polarizing microscope equipped
with a heating stage.
Spectral Data
IR in cm−1 for n-Octyloxy Derivative
2920.0, 2851.6, 1467.0, 1350.1 (alkyl group), 1733.9, 1237.3 (ester group), 1602.0
( N N ), 1559.3 ( C C aromatic stre.), 1145.6 & 1166.9 (ether group), 840.9 (p-
sub. benzene ring), 755.1(tri sub. benzene ring), 720.4 (poly CH2- rocking bending),
1026.1(C Cl aromatic).
IR in cm−1 for n-Tetradecyloxy Derivative
2916.2, 2849.6, 1478.0, 1381.9 (alkyl group), 1734.9, 1237.3 (ester group), 1600.0
( N N ), 1559.3 ( C C aromatic stre.), 1141.8 & 1167.8 (ether group), 833.2 (p-sub.
benzene ring), 758.0 (tri sub. benzene ring), 716.5 (poly-CH2- rocking bending), 1033.8
(C Cl aromatic).
1H-NMR in CDCl3, δppm for n-Pentyloxy Derivative
0.95 (t, 3H, CH3), 1.41-1.46 (m, 4H, 2x CH2-), 1.83 (m, 2H, OCH2-CH2-), 4.05 (t,
2H, OCH2-), 3.9 (s, 3H, OCH3), 6.90-8.21 (m, 11H, Ar-H).

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N. G. Makwana and A. V. Doshi
1H-NMR in CDCl3, δppm for n-Hexyloxy Derivative
0.92 (t, 3H, CH3), 1.35-1.47 (m, 6H, 3× CH2-), 1.83 (m, 2H, OCH2-CH2-), 3.90 (s,
3H, OCH3), 4.05 (t, 2H, OCH2-), 6.90 8.21 (m, 11H, Ar-H).
Mass Spectra for Heptyloxy Homolog
Calculated value = 480.5
Experimental value = 480.5
Textures of Nematic Phase by Miscibility Method
Pentyloxy homolog = Threaded nematic
Heptyloxy homolog = Schlieren nematic
Decyloxy homolog = Schlieren nematic
Results and Discussion
4-Hydroxy-3-chlorophenylazo-4ꢀ-methoxy benzene (m.p. 111◦C, yield 68.5%) is a non-
LC component, but on linking it with dimeric 4-n-alkoxy benzoic acids through their
corresponding acid chlorides in pyridine an entirely nematogenic novel azoester homolo-
gous series is generated with a 9–186◦C LC mesophase length in anenantiotropic manner.
Smectogenic mesomorphism is absent throughout the series. Dimerization of 4-n-alkoxy
benzoic acids disappears on esterification. Transition temperatures [Table 2] as determined
by an optical polarizing microscopy equipped with a heating stage were plotted versus
the number of carbon atoms present in n-alkyl chain, bonded to phenyl ring through oxy-
gen atom. Like or related points are linked to form solid-nematic and nematic-isotropic
transition curves showing phase behaviors of a novel azoester series in a phase diagram
[Fig.1]. Solid-nematic transition curve adopt a zigzag path of rising and falling values as the
series is ascended with an overall descending tendency. The nematic-isotropic transition
curve descended as series is ascended with the exhibition of an odd-even effect. Thus,
both transition curves of a phase diagram behaved in a normal manner. The textures of the
nematic phase are threaded or Schlieren, and characterization data supports the molecular
structures of the novel homologs. The odd-even effect diminishes as the series is ascended.
Curves for odd and even members match with each other near the heptyloxy homolog and
then continue for higher homologs as a single curve. The average thermal stability for the
nematic phase is 195◦C. Thus, the entire series is nematogenic without the exhibition of a
smectic mesophase with a mesophase length from 9◦C to 186◦C.
Homologous Series: 4-(4ꢀ-n-Alkoxy Benzoyloxy) -3-Chloro phenylazo-4ꢀꢀ-Methoxy
A long molecular length in the final azodyes is generated through testerification of
4-n-alkoxy benzoic acids through their corresponding acid chlorides with the azodye phe-
nols. The long molecular length, lateral and terminal end to end attractions, molecular

Liquid Crystals and Molecular Structure
97
Figure 1. Phase behaviors of series.
polarity, and polarizability induce LC properties in all the twelve homologues of the novel
series. The molecules of each homolog under the influence of exposed thermal vibrations
are floated with a statistically parallel orientational order as a consequence of favorable
molecular rigidity and flexibility to cause nematic mesophase formation in an enantiotropic
manner with mesomorphism ranging from 9◦C to 186◦C without the exhibition of a smec-
tic mesophase. The absence of smectogenic character indicates the absence of lamellar
packing of molecules in their crystal lattices and unsuitable extent of molecular planarity,
which hinders the formation of three dimensional molecular networks to cause sliding
layered molecular arrangement in floating condition. An odd-even effect is observed due
to the progressively and sequentially added methylene group to the n-alkoxy terminal end
group. The disappearance of the odd-even effect beyond the heptyloxy (C7) homolog is
attributed to the coiling or bending or flexing or coupling of n-alkyl chain with the major
axis of the core structure of a molecule. The changing trend in LC behaviors from homolog
to homolog in the same series is also attributed to the changing number of methylene
units and hence the changing magnitudes of molecular flexibility responsible for mesomor-
phism. The LC behaviors of presently investigated homologous series-1 are compared with
structurally similar other known homologous series-X [16] and Y [17] as shown below in
Fig. 2.
The novel homologous series-1 [Fig. 2] and series-X selected for comparison is struc-
turally isomeric and similar, but their laterally and terminally substituted Cl and OCH3
groups are exchanged with each other. Therefore, their molecular rigidity and molecular
flexibility will differ. The novel homologous series-1 and series-Y only differ in respect of
one terminal group as seen in Fig. 2. Thus, variations in mesomorphic behaviors and the

98
N. G. Makwana and A. V. Doshi
Table 2. Transition temperatures in ◦C for 4-(4ꢀ-n-alkoxy benzoyloxy) -3-chloro phenylazo-
4ꢀꢀ-methoxybenzenes
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
7
8
10
12
14
16
—
—
—
—
—
—
—
—
—
—
—
—
158.0
183.0
102.0
130.0
89.0
71.0
63.0
79.0
72.0
65.0
58.0
92.0
284.0
330.0
288.0
257.0
187.0
192.0
167.0
152.0
138.0
142.0
102.0
101.0
9
10
11
12
Sm, smectic; N, nematic.
degree of mesomorphism will depend upon changing trend of molecular rigidity and/or
flexibility from homolog to homolog and series to series as a consequence of even minor
variations in a molecular structure. Table 3 shows the comparison of LC behavior, thermal
stability, commencement of mesophase, mesophase length for the novel series-1 and the
series-X and series-Y chosen for comparison.
Table 3 indicates that
Figure 2. Structurally similar homologous series.

Liquid Crystals and Molecular Structure
99
Table 3. Average thermal stability in ◦C
Series
1
X
Y
Smectic-nematic or
smectic-isotropic
commencement of
Smectic phase
—
87.55 (C4–C10) C4
—
Nematic-isotropic
commencement of
nematic phase
195.0 (C1–C16) C1 125.60 (C3--C16) C3 175.75 (C1–C16) C1
Mesophase length
9–186 = 177
28–64 = 36
66–129 = 63
range in ◦c
Homologous series-1 and series-Y are only nematogenic without exhibition of smectogenic
character, but a homologous series-X is nematogenic with exhibition of smectogenic
property.
Nematogenic property commences from very first (C1) member of series-1 and series-Y, but
it commences from third (C3) member of a series-X. Smectic mesophase commences
early from fourth (C4) member of a series-X.
Nematic thermal stability of present series-1 is the highest (195) and that of the series-X
is the lowest (125.60) among the homologous series-1, series-X, and series-Y under
comparative study.
Smectogenic mesophase is stabilized from C4 to C16 homologs of series-X while, it desta-
bilized in case of series-1 and series-Y.
Smectic and/or nematic phase length range is in the order of
Series-1 (177◦C) > Series-Y (63◦C) > Series-X (36◦C)
Molecules of homologous series-1 and series-Y under comparison bear commonly
lateral Cl group at meta position to N N central bridge and changing OCH3 or
CH3 group as end group while OCH3 group is substituted at the same identical meta
position in series-X, with Cl as end group. The Cl or OCH3 polar functional group
substituted at meta position to N N central bridge almost equally increases molecular
width and reduces intermolecular attractions, but at the same time the same laterally
substituted groups increases intermolecular attractions through molecular polarizability in
unequal manner due to their different group polarity. The changing molecular polarity and/or
polarizability increases intermolecular end to end attractions for the same homologue from
series to series more or less depending upon group polarity of OCH3, Cl and CH3.
Thus, by exchanging the positions of polar groups OCH3 and Cl for isomeric series-
1 and series-X. The suitable magnitudes of anisotropic forces of intermolecular end to
end attractions of series-1 exceeds to series-X, due to highly polar end group OCH3
in comparison with weakly polar Cl group for inducing and stabilizing nematic phase.
However, the lamellar packing of molecules in the crystal lattices of the homologs belonging
to series-1 are hindered, but, the same are not hindered in case of the molecules of series-X,
due to stronger interlocking of molecules by highly polar OCH3 lateral group as compared
to weaker interlocking of laterally substituted Cl group of series-1. Thus, smectogenic
character is favorably induced in the molecules of series-X, but it is hindered to stabilize
and facilitate smectic mesophase formation in case of the molecules of series-1. The early

100
N. G. Makwana and A. V. Doshi
or late mesophase formation depends upon magnitudes of suitability of anisotropic forces
of intermolecular closeness and end to end or/and lateral attractions as a consequence of
favorable molecular rigidity and/or flexibility as well as the extent of noncoplanarity among
the molecules under comparative study. The extent of molecular noncoplanarity, end to end
lateral attractions, molecular polarizability, dispersion forces, etc. changes with the laterally
substituted penta atomic OCH3 and mono atomic Cl groups. Therefore series-1 and
series-Y which are laterally substituted by Cl group on identical position to N N
showed commencement of LC state from very first member (C1) of the series-1 and Y,
but the commencement of LC state takes place a little bit late from third (C3) member
of a series to which Cl and OCH3 groups are exchanged on identical positions. The
mesophase length varies from series to series due to varying molecular resistivity against
exposed thermal vibrations as reflected through the values of average thermal stabilities
mentioned in Table 3. Thermal stabilities of the series under comparison are associated
with energy stored of every substance due to its mass and characteristics (ꢀH). Thus,
highest and the lowest resistivity against exposed thermal vibrations are associated with
the molecules of series-1 (177) and series-X (36), respectively. Thus, LC behaviors from
homolog to homolog in the same series are associated with changing number of methylene
unit or units keeping right terminal end group unchanged. However, changing trend of LC
behaviors for the same homolog from series to series is attributed to the changing functional
end groups OCH3, Cl, or CH3. The difference of LC behaviors between series-1 and
series-Y depends upon the difference of only one oxygen atom, i.e., between terminal
end group OCH3 and CH3 for the same homolog, keeping rest of the molecular part
unchanged.
Conclusions
Novel azoester series of twelve members with three phenyl rings, two central bridges, one
meta-substituted lateral group Cl and two para-substituted terminal end groups is entirely
nematogenic without exhibition of smectogenic character as a consequence of molecular
rigidity and flexibility of suitable magnitudes to induce nematic mesophase depending upon
molecular structure.
LC mesophase formation is very sensitive and susceptible to a molecular structure.
LC behaviors of isomeric substances with exchange of same polar functional groups
at the same position and phenyl rings can have unequal LC behaviors.
The group efficiency order derives for smectic and/or nematic mesophase formation on
the basis of (i) thermal stability, (ii) early commencement of mesophase, and (iii) mesophase
length range are as under.
(i) Nematic: OCH3 > CH3 > Cl—Smectic: Cl > OCH3
(ii) Nematic: OCH3 CH3 > Cl—-Smectic: Cl > OCH3
(iii) Smectic + nematic: OCH3 > CH3 > Cl
CH3
CH3
Variations in LC behaviors from homolog to homolog in the same series depend upon
changing sequentially or progressively molecular part, (methylene unit) keeping rest of the
molecular part unchanged.
Variations in LC behaviors from series to series for the same homolog depends upon
varying only lateral or only terminal end group keeping rest of the molecular part intact.
Molecular LC behaviors are the function of molecular rigidity and/or flexibility as a
consequence of molecular structure.

Liquid Crystals and Molecular Structure
101
Acknowledgments
The authors are thankful to Head and Teaching staff of Applied Chemistry of Faculty of
Technology and Engineering, M.S. University, Vadodara, for their valuable cooperation in
the work. Authors are also thankful to CSMCRI, Bhavnagar, for the analysis of samples.
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