Mesomorphism in relation to molecular structure with reference to a laterally substituted homologous series

Article abstract of DOI:10.1080/15421406.2014.927551

A novel homologous series 4-(4′-n-alkoxy benzoyloxy) 3-methyl phenyl azo-2^″, 6^″-dimethyl benzenes consists of 12 homologues. The methyl to hexyl derivatives are non-mesomorphic and the rest of the homologues are mesomorphic. Mesomorphism commences from heptyl homologue in a monotropic manner and continues as an enantiotropic nematic for octyl to dodecyl derivatives, and then monotropic mesomorphism as a nematic mesophase reappears for tetradecyl and hexadecyl derivatives without the exhibition of smectogenic character. The textures of the nematic mesophase are threaded, or are schlieren in type. The nematic-isotropic (or vice versa) transition curve of the phase diagram behaves in a normal manner without the exhibition of an odd-even effect. The average thermal stability for nematic to isotropic transition is 126.5°C. Analytical and spectral data confirm the structure of molecules. The mesomorphic to isotropic transition temperatures vary between 56°C and 130°C. The nematogenic mesophase appeared at the lowest temperature of 41.3°C. Transition and melting temperatures are determined using an optical polarizing microscopy equipped with a heating stage. The present novel series is of a low ordered melting type and predominantly nematogenic, without the exhibition of smectogenic character.

Full text of DOI:10.1080/15421406.2014.927551

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Mesomorphism in Relation to Molecular
Structure with Reference to a Laterally
Substituted Homologous Series
M. D. Khunta, V. C. Kotadiyaa & U. C. Bhoyaa
a Department of Chemistry, Saurashtra University, Rajkot, Gujarat,
India
Published online: 26 Feb 2015.
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To cite this article: M. D. Khunt, V. C. Kotadiya & U. C. Bhoya (2015) Mesomorphism in Relation to
Molecular Structure with Reference to a Laterally Substituted Homologous Series, Molecular Crystals
and Liquid Crystals, 607:1, 32-40, DOI: 10.1080/15421406.2014.927551
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Mol. Cryst. Liq. Cryst., Vol. 607: pp. 32–40, 2015
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2014.927551
Mesomorphism in Relation to Molecular Structure
with Reference to a Laterally Substituted
Homologous Series
M. D. KHUNT, V. C. KOTADIYA,
AND U. C. BHOYA∗
Department of Chemistry, Saurashtra University, Rajkot, Gujarat, India
A novel homologous series 4-(4ꢀ-n-alkoxy benzoyloxy) 3-methyl phenyl azo-2ꢀꢀ, 6ꢀꢀ-
dimethyl benzenes consists of 12 homologues. The methyl to hexyl derivatives are
non-mesomorphic and the rest of the homologues are mesomorphic. Mesomorphism
commences from heptyl homologue in a monotropic manner and continues as an enan-
tiotropic nematic for octyl to dodecyl derivatives, and then monotropic mesomorphism
as a nematic mesophase reappears for tetradecyl and hexadecyl derivatives without the
exhibition of smectogenic character. The textures of the nematic mesophase are threaded,
or are schlieren in type. The nematic–isotropic (or vice versa) transition curve of the
phase diagram behaves in a normal manner without the exhibition of an odd-even effect.
The average thermal stability for nematic to isotropic transition is 126.5◦C. Analytical
and spectral data confirm the structure of molecules. The mesomorphic to isotropic
transition temperatures vary between 56◦C and 130◦C. The nematogenic mesophase
appeared at the lowest temperature of 41.3◦C. Transition and melting temperatures are
determined using an optical polarizing microscopy equipped with a heating stage. The
present novel series is of a low ordered melting type and predominantly nematogenic,
without the exhibition of smectogenic character.
Keywords Liquid crystal; mesogen; monotropy; nematic; smectic
Introduction
Our continuous search in the field of liquid crystalline (LC) state has been used in the man-
ufacture of various articles such as electronic display devices, electronic screens, medical
instruments [1, 2], pharmaceutical and medicinal preparations, textiles and color indus-
try [3–5], and the study of LCs is important from the point of view novel liquid crystal
materials. There are various parts or branches of this field in which engineers, physicists,
biologists, and chemists are working in the present era. Our study is restricted or limited to
the following.
1. To synthesize novel thermotropic LC substances through low temperature homolo-
gous series.
∗Address correspondence to U. C. Bhoya, Department of Chemistry, Saurashtra University,
Rajkot 360005, Gujarat, India. Tel:+91-9824203183; Fax:+0281-2583984. E-mail: drucbhoya@
gmail.com
Color versions of one or more of the figures in the article can be found online at
www.tandfonline.com/gmcl.
32

Mesomorphism in Molecular Structure with Substituted Homologous Series
33
2. To characterize by elemental analysis and spectral data.
3. To determine thermal data and liquid crystal behavior with a range of liquid crys-
tallinity by an optical polarizing microscopy equipped with a heating stage etc. to
understand LC behavior dependence on the molecular structure of a substance.
The proposed homologous series of azo ester molecules consists of three phenyl
rings bridged through –COO and N N- central groups playing the role of a rigid
core and an n-alkoxy terminal end group playing the role of a flexible core [6–8]. Three
methyl groups substituted at lateral positions provide steric hindrance [9] and may reduce
transition temperatures, which may be useful for the devices to be operated at the desired
temperature. Moreover, the present study is mainly aimed to understand and establish the
relation between liquid crystal properties and the molecular structure of a substance.
Experimental
Synthesis
4-Hydroxy benzoic acid was alkylated by the suitable alkylating agent (R-X) by the modified
method of Dave and Vora [10]. 2, 6-Dimethyl aniline was diazotized by the usual established
method [11, 12] to form an azo dye, 4-hydroxy-3-methyl phenyl azo 2ꢀ, 6ꢀ-dimethyl benzene
(mp 95–97◦C). Dimerized 4-n-alkoxy benzoic acid was condensed with the azo dye in dry
cold pyridine through corresponding 4-n-alkoxy acid chloride using freshly distilled dry
thionyl chloride [13, 14]. The final azo ester derivatives were decomposed, filtered, washed,
dried, and crystallized until constant transition temperatures were obtained. 4-Hydroxy
benzoic acid, alkyl halides (R-X), thionyle chloride, MeOH, EtOH, KOH, ortho cresol,
HCl, 2,6-dimethyl aniline, pyridine were used as received except solvents, which were
dried and distilled prior to use.
The synthetic route to the series is shown below in Scheme 1.
Characterization
The molecular structures of the novel series were identified using various analytical tech-
1
niques such as H NMR, mass spectroscopy, IR spectroscopy, and elemental analysis.
Elemental analysis data obtained from the experimental Euro EA Elemental Analyser
(Table 1) and IR data were recorded on a SIMADZU-FTIR-8400 spectrometer. 1H NMR
spectra were recorded using BRUKER Avance-III (400 MHz) spectrometer instrument
using DMSO-d6 as a solvent. The molecular ion peak obtained from the experimental
Table 1. Elemental analysis for (1) propyloxy, (2) butyloxy, (3) heptyloxy, and (4) octyloxy
derivatives
Elements found (%)
Elements calculated (%)
Sr. no.
Molecular formula
C
H
N
C
H
N
1.
2.
3.
4.
C25H26N2O3
C26H26N2O3
C29H34N2O3
C30H36N2O3
74.63
74.92
75.89
76.32
6.53
6.81
7.51
7.64
6.91
6.75
6.13
5.89
74.60
74.97
75.95
76.24
6.51
6.78
7.47
7.68
6.96
6.73
6.11
5.93

34
M. D. Khunt et al.
CH3
NH2
CH3
O
HO
OH
Reflux
MeOH
KOH
l ,
HC
R-X
-HX
NO
Na
2
CH3
O
RO
N
N Cl
OH
CH3
Reflux
fresly distilled
SOCl2
OH
NaOH
0-5 oC
pH 8-8.5
-HCl
-SO2
CH3
H3C
N N
H3C
O
RO
HO
H3C
Cl
[A]
[B]
[B]
[A]+
Dry cold
Py
rid
ine
O
O
H3C
N N
H3C
RO
H3C
4-(4'-n-Alkoxy benzoyloxy)-3-methyl phenylazo-2"-6"-dimethyl benzenes
Where, R= CnH2n+1 and n= 1to8,10,12,14and 16.
Scheme 1. Synthetic route to the series.
EI-MS data was matched with synthesized molecular mass. It was analyzed with the help
of SHIMADZU-GC-MS, Model No. QP-2010. All analytical data were matched with the
synthesized molecule and found to be satisfied. The liquid crystal behavior and texture
types were determined by the miscibility method on microscopic observations.

Mesomorphism in Molecular Structure with Substituted Homologous Series
Spectral Data
35
NMR in ppm for Decyloxy Derivative. 0.86–0.87(t, 3H, -CH3 of –OC10H21 group),
1.26–1.40 (m, 14H ( CH2 )n group), 1.72–1.77 (t, 2H, CH2 CH2 O ), 4.08–4.12
(t, 2H, CH2 O ), 2.306 (s, 9H, CH3-Ph), 7.13–7.15, 8.11–8.12 (dd, 4H, p-substituted
phenyl ring), 7.43–7.45, 7.79–7.88 (m, 3H, o-substituted phenyl ring), 7.77 (m, 3H, 1,6 di-
substituted phenyl ring), 7.66–7.67 (d, 1H, in phenyl ring ortho to –N N-group), 8.02–8.12
(m, 4H, ortho to –COO and N N- group in phenyl ring). The NMR data confirm the
molecular structure.
NMR in ppm for Tetradecyloxy Derivative. 0.83–0.85 (t, 3H, -CH3 of –OC14H29 group),
1.24 (m, 22H ( CH2 )n group), 1.72–1.77 (q, 2H, CH2 CH2 O ), 4.08–4.12 (t, 2H,
CH2 O ), 2.305 (s, 9H, CH3-Ph), 7.13–7.15, 8.11–8.1 (dd, 4H, p-substituted phenyl
ring), 7.43–7.45, 7.79–7.88 (m, 3H, o-substituted phenyl ring), 7.20–7.22, 7.77 (m, 3H, 1,6
disubstituted phenyl ring). The NMR data confirm the molecular structure.
IR in cm−1 for Pentyloxy Derivative. 2868, 2949 (C-H Str. of (-CH2-) n group of
–OC5H11), 1470, 1593 (aromatic C C Str.), 3077 (C-H Str. of aromatic ring), 1008, 1166,
1253 (C O Str. of –COO group), 1727 (C O, Str.of –COO- group), 773, 842, 653 (para-,
ortho-, and meta-substituted phenyl ring), 1604 (-N N- Str.). The IR data are consistent
with the molecular structure.
IR in cm−1 for Dodecyloxy Derivative. 2854, 2924 (C-H Str. of (-CH2-) n group of
–OC12H25), 1465, 1510 (aromatic C C Str.), 3077 (C-H Str. of aromatic ring), 1012, 1168,
1255 (C-O Str. of –COO-group), 1730 (C O, Str.of –COO- group), 769, 844, 648 (para-,
ortho-, and meta-substituted phenyl ring) 1604 (-N N- Str.). The IR data are consistent
with the molecular structure.
Mass Spectra of Ethoxy Derivative. m/z (rel. int%): 388 (M)+, 245, 205, 121, 93.
Mass spectra of Hexyloxy Derivative. m/z (rel. int%): 444 (M)+, 345, 425, 317, 121.
Result and Discussion
4-Hydroxy 3-methyl phenyl azo 2ꢀ, 6ꢀ-dimethyl benzene is a non-mesomorphic component
of an azoester novel series reported herewith. However, on linking it with dimerized alkoxy
acids through their corresponding acid chlorides, azoester homologue derivatives result with
lower transition temperature as compared with corresponding n-alkoxy acid. The presently
investigated novel homologous series 4-(4ꢀ-n-alkoxy benzoyloxy) 3-methyl phenyl azo-2ꢀꢀ,
6ꢀꢀ-dimethyl benzenes consists of 12 homologues. The methyl to hexyl homologues are non-
liquid crystals (NLC) and heptyl to hexadecyl homologue derivatives are liquid crystals
(LC). Mesomorphism commences from the heptyl homologue as a monotropic nematic,
and mesomorphism from and beyond the octyl homologue to the dodecyl homologue is
enantiotropic nematic and finally the tetradecyl and hexadecyl homologues are monotropic
nematic. Smectogenic character is totally absent. The transition and melting temperatures
(Table 2) as determined by an optical polarizing microscope equipped with a heating
stage and plotted against the number of carbon atoms present in an alkyl chain of a
left n-alkoxy terminal end group. A phase diagram showing phase behavior of novel
series is represented in Fig. 1. Solid mesomorphic/isotropic transition curve adopt a zigzag
path of rising and falling values as the series is ascended and finally descended at the
hexadecyloxy homologue. The nematic–isotropic transition curve drastically rises from
the heptyl to the octyl homologue; descends slowly up to the dodecyl homologue; and
then drastically descends to tetradecyl and hexadecyl homologues as series is ascended.
Thus, both curves of the phase diagram behave in normal manner. An odd-even effect is

36
M. D. Khunt et al.
Table 2. Transition temperatures in ◦C
Transition temperature in ◦C
Compound no.
n-alkyl CnH2n+1 group (n)
Sm
N
Isotropic
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
10
12
14
16
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
93.0
75.4
114.6
102.1
109.0
83.8
(41.3)a
97.2
70.8
130.8
125.7
123.0
56.0
93.2
10
11
12
96.0
(46.3)a
(48.5)a
61.1
Note. aMonotropy. Sm = smectic; N = nematic.
absent for the nematic–isotropic (or vice versa) transition curve. Mesomorphic behavior
of the novel series from homologue to homologue varies with the addition of methylene
units. The mesomorphic range varies from 27.0◦C to 33.6◦C. Thermal stability for nematic
is 126.5◦C. Thus, novel series is predominantly nematogenic and partly non-mesogenic
whose isotropic temperatures vary between 56◦C and 130.8◦C. Therefore, the homologous
series under discussion is a low melting series.
Mesomorphic properties are induced in a non-mesomorphic azodye molecule by ex-
tending molecular length, linking the n-alkoxy acid through acid chloride. An extended
molecular length increases the length to breath ratio and lateral to terminal or end-to-end
Figure 1. Phase behavior of series.

Mesomorphism in Molecular Structure with Substituted Homologous Series
37
Figure 2. Structurally similar homologous series.
intermolecular anisotropic forces of attraction in suitable magnitudes required to exhibit
mesomorphic behavior by heptyl to hexadecyl homologue derivatives as a consequence
of favorable molecular rigidity and flexibility [6–8]. Disappearance of dimerisation of n-
alkoxy acids and the lowering of transition temperatures of azoester molecules are attributed
to the breaking of hydrogen bonding of n-alkoxy acids through the esterification process.
The absence of an odd-even effect in the nematic–isotropic transition curve is attributed
to the late commencement of mesomorphism from and beyond heptyl homologue. Ab-
sence of mesomorphism from methyl to hexyl homologues is due to the high crystallizing
tendency, which arises from relatively strong intermolecular forces due to shorter n-alkyl
chain. Hence, such molecules under exposed thermal vibrations sharply transform into
the isotropic state from the crystalline solid state without passing through a mesomorphic
(LC) state. The molecules of the rest of the homologues of the novel series are capable of
resisting exposed thermal vibrations reversibly or irreversibly and align with a statistically
parallel orientational order of molecules within definite range of temperature. The absence
of smectogenic character of the novel series is attributed to the absence of lamellar packing
of molecules in their crystal lattices, which is due to the presence of three laterally sub-
stituted methyl groups that hinder suitable molecular fitting arrangement to form lamellar
molecular packing in their crystal lattices. Vibration in mesomorphic behavior from ho-
mologue to homologue in the present novel series is attributed to the sequentially added
methylene units, which increase molecular length and lateral to terminal magnitudes of
anisotropic forces, including molecular polarity and polarizability affecting intermolecular

38
M. D. Khunt et al.
Table 3. Average thermal stability in ◦C
Series →
1
X
Y
Smectic–nematic or smectic–isotropic
Commencement of smectic phase
Nematic–isotropic
—
—
—
—
106.6 (C12-C16)
C10
126.5 [C8-C12] 125.57 [C6-C16] 161.58 [C1-C16]
C7 C6 C1
Commencement of nematic phase
adhesion. Thus, suitable magnitudes of intermolecular attractions result in the phenomena
of mesomorphism and varying degree and type of mesomorphism. The mesomorphic (LC)
behavior of the presently investigated novel series-1 is compared with structurally similar
known series X [15] and Y [16] as shown in Fig. 2.
It is clearly seen that homologue series 1, X, and Y are very similar with respect to three
phenyl rings bonded through –COO– and –N = N– central bridges and left n-alkoxy (–OR)
terminal end group. Therefore, the combined effect of molecular rigidity and flexibility
is due to similar molecular parts that operate and contribute equally to mesomorphic
behavior. However, variations in mesomorphic behavior in the presently investigated novel
series 1 and the homologous series X and Y chosen for comparison have arisen due to
the substituted flexible part or functional group or groups of different polarities and the
position of substitution which has contributed to the differing magnitudes of anisotropic
forces of intermolecular attractions as a consequence of combined and overall effects of
molecular rigidity and flexibility. Thus, variations in mesomorphic properties and the degree
of mesomorphism from series to series under discussion will vary with the changing part
of each series. Table 3 shows the mesomorphic (LC) properties of each series.
Table 3 shows comparative data for series 1, X, and Y as under.
• Homologous series 1 and X are non-smectogenic, while homologous series Y is
smectogenic.
• The nematogenic mesophase is exhibited by all homologous series 1, X, and Y under
comparison.
• Smectogenic property of series Y commences from the decyloxy homologue, while
it does not commence until the last member of series 1 and X.
• Nematogenic mesophase formation commences from the seventh member of series
1, from the sixth member of series X, and from the very first member of series Y.
• The lowest temperatures for the commencement of mesophase formation are 41.3◦C,
75.0◦C, and 63.0◦C for series 1, X, and Y respectively.
• Thermal stability for the nematic of series 1 and X differs by only 1.0 (more) unit
or say almost equivalent while it differs (less) by 35.08 units than series Y.
The absence of smectogenic character in series 1 and X and the presence of smectogenic
character in series Y are attributed to the absence and presence of lamellar packing of
molecules in their respective crystal lattices due to differing extent of non-coplanarity as a
consequence of lateral substitutions at two positions of the third phenyl rings of series 1 and
X, while homologous series Y possesses only one terminal end group –Cl, which reduces
the extent of non-coplanarity as compared with series 1 and X favoring lamellar packing
of molecule for series Y to facilitate smectogenic mesophase formation. The resistivity of
externally exposed thermal vibrations of series 1, X, and Y is strong enough to maintain
statistically parallel orientational order of molecules. However, the suitable magnitudes of

Mesomorphism in Molecular Structure with Substituted Homologous Series
39
anisotropic forces of intermolecular end-to-end attractions differ between the series due to
the methyl and chloro group or groups at third phenyl ring of all the series under discussion
of comparison, affecting intermolecular closeness, molecular steric hindrance, molecular
polarity, and polarizability. Thus, the nematic mesophase commences from the very first
member of series Y and from the seventh and sixth member of series 1 and X respectively.
The lowest temperatures for the commencement of mesophase either smectic or nematic
corresponds for presently investigated series 1. Such behavior may be attributed to the steric
hindrance caused by two methyl groups around –N N– group in 2ꢀꢀ and 6ꢀꢀ positions (ortho
to –N N–). The highest value of thermal stability for the member of a series Y as compared
with series 1 and X can be attributed to the differing molecular rigidity and flexibility. Thus,
variation in mesomorphic properties from series to series for same homologue is attributed
to the changing molecular part of series [17].
Conclusions
1. The group efficiency order derived on the basis of (a) thermal stabilities and (b) the
early commencement of mesophase for smectic and nematic are as under:
(a) Smectic: pera –Cl > 2,5-dichloro = 2,6-dimethyl
Nematic: pera –Cl > 2, 6-dimethyl > 2, 5-dichloro
(b) Smectic: pera –Cl > 2, 5-dichloro = 2, 6-dimethyl
Nematic: pera –Cl > 2, 5-dichloro > 2, 6-dimethyl
2. Presently investigated novel series is nematogenic only of low melting type and a
useful range of liquid crystallinity at low temperature.
3. Molecular rigidity and flexibility are very sensitive and susceptible to form liquid
crystal mesophase.
4. The variations in mesomorphic properties from homologue to homologue in the
same series and for the same homologue from series to series are due to the
changing sequentially added methylene unit or units and the changing molecular
part of a whole molecule keeping rest of the molecular part intact respectively.
5. The phenomenon of mesomorphism is a direct result of a molecular structure as a
consequence of its molecular rigidity and flexibility.
Acknowledgments
Authors are thankful to the Department of Chemistry (DST-FIST funded and UGC-SAP
sponsored), Saurashtra University, Rajkot for providing research facilities. Authors are
also thankful to the National Facility for Drug Discovery (NFDD) Centre, Saurashtra
University, Rajkot, for sample analysis. Authors are particularly thankful to Dr. A. V.
Doshi, ex-principal M.V.M. Science and Home Science College Rajkot, for his valuable
cooperation during present investigations as and when needed.
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