Dependence of Mesomorphism on Terminal End Group of a Molecule

Article abstract of DOI:10.1080/15421406.2015.1017286

A novel azoester homologous series of liquid crystals consisting of 12 members is majorly nematogenic without the exhibition of smectic mesomorphism, except for the hexadecyloxy derivative. The methoxy and ethoxy members are monotropically nematic and the rest of the homologs are enantiotropically nematogenic. Transition temperatures and mesophase morphology were determined on an optical polarizing microscope equipped with a heating stage. The textures of the nematic mesophase are threaded or Schlieren in type. Infra red, ¹H NMR, mass spectral data, and elemental analysis data confirm the molecular structures of the novel homologs. The mesophase length varies from 00 to 72°C and the average thermal stability for the nematic phase is 133.5°C. Liquid crystal properties vary from homolog to homolog in present novel series. The liquid crystal behavior of the presently investigated novel series is compared with structurally similar known series. The transition curves (Cr-N/Cr-I and N-I/I-N) behave in a normal manner. The N-I/I-N transition curve exhibit an odd-even effect. The series is majorly nematogenic without exhibition of smectogenic character. Only the C16 homolog is nonmesogenic.

Full text of DOI:10.1080/15421406.2015.1017286

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Dependence of Mesomorphism on Terminal End
Group of a Molecule
N. G. Makwana, H. R. Prajapati & A. V. Doshi
To cite this article: N. G. Makwana, H. R. Prajapati & A. V. Doshi (2015) Dependence of
Mesomorphism on Terminal End Group of a Molecule, Molecular Crystals and Liquid Crystals,
623:1, 119-128, DOI: 10.1080/15421406.2015.1017286
To link to this article: http://dx.doi.org/10.1080/15421406.2015.1017286
Published online: 21 Dec 2015.
Submit your article to this journal
Article views: 12
View related articles
View Crossmark data
Full Terms & Conditions of access and use can be found at
http://www.tandfonline.com/action/journalInformation?journalCode=gmcl20
Download by: [Orta Dogu Teknik Universitesi]
Date: 30 January 2016, At: 22:47

Mol. Cryst. Liq. Cryst., Vol. 623: pp. 119–128, 2015
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2015.1017286
Dependence of Mesomorphism on Terminal End
Group of a Molecule
N. G. MAKWANA,1 H. R. PRAJAPATI,2 AND A. V. DOSHI3,∗
1General Department (Chemistry), R.C. Technical Institute, Ahmedabad,
Gujarat, India
2C.U. Shah Science College, Ahmedabad, Gujarat, India
3Matushri Virbaima Mahila Science and Home Science College, Saurashtra
University, Rajkot, Gujarat, India
A novel azoester homologous series of liquid crystals consisting of 12 members is
majorly nematogenic without the exhibition of smectic mesomorphism, except for the
hexadecyloxy derivative. The methoxy and ethoxy members are monotropically nematic
and the rest of the homologs are enantiotropically nematogenic. Transition tempera-
tures and mesophase morphology were determined on an optical polarizing microscope
equipped with a heating stage. The textures of the nematic mesophase are threaded or
Schlieren in type. Infra red, 1H NMR, mass spectral data, and elemental analysis data
confirm the molecular structures of the novel homologs. The mesophase length varies
from 00 to 72◦C and the average thermal stability for the nematic phase is 133.5◦C.
Liquid crystal properties vary from homolog to homolog in present novel series. The
liquid crystal behavior of the presently investigated novel series is compared with struc-
turally similar known series. The transition curves (Cr-N/Cr-I and N-I/I-N) behave in
a normal manner. The N-I/I-N transition curve exhibit an odd-even effect. The series is
majorly nematogenic without exhibition of smectogenic character. Only the C16 homolog
is nonmesogenic.
Keywords Azoester; liquid crystal; monotropy; mesogen; nematic
Introduction
Liquid crystal (LC) properties of substances have been exploited for the benefit of a
mankind since its recognition [1] as a separate state between the crystalline solid and
the isotropic liquid. Chloro substituted LC materials are useful as an optical imaging,
LC displays, organic light-emitting diodes, semiconductor materials, anisotropic networks,
hole transporting materials in organic light emitting devices, [2,3] etc. Azoester LC dyes and
their benzoyloxy and cinnamoyloxy derivatives are also find equal importance as useful
as other LC materials with reference to polarization of light and related study. Number
of ester homologous series with –N = N- central groups have been reported [4–6]. The
present investigation is planned with a view to understanding and establishing the effects
of molecular structure on LC property [7,8] with the dichloro laterally substituted groups
causing steric hindrance in space at the middle phenyl ring as well as OCH3 and –OR left
and right terminal end groups on para positions of third and first phenyl rings. The study is
∗Address correspondence to A. V. Doshi, Matushri Virbaima Mahila Science and Home Science
College, Saurashtra University, Rajkot, Gujarat, India. E-mail: arvindbhaidoshi@yahoo.in
119

120
N. G. Makwana et al.
Table 1. Elemental analysis for methoxy, butoxy, hexyloxy, and decyloxy derivatives
Elements% found (calculated%)
Sr. no.
Molecular formula
C
H
N
1
2
3
4
C21H16N2O4Cl2
C24H22N2O4Cl2
C26H26N2O4Cl2
C30H34N2O4Cl2
58.39(58.47)
60.80(60.89)
62.20(62.27)
64.60(64.63)
3.78(3.71)
4.60(4.65)
5.11(5.19)
6.18(6.10)
6.58(6.50)
5.90(5.92)
5.57(5.59)
5.12(5.03)
aimed to understand the effect of terminal end groups on mesomorphic behaviors keeping
rest of the molecular part unchanged for the same homolog among the structurally similar
series, in terms of molecular rigidity and flexibility, dipole moment, dispersion forces, etc.
[9–12].
Experimental
Synthesis
4-n-Alkoxy benzoic acid was alkylated by suitable alkylating agents (R-X) by the modified
method of Dave and Vora [13]. Azodye 4-Hydroxy 3,5-dichloro phenyl azo-4’-methoxy
benzene was prepared by a usual established method (M.P is 102.0◦C and Yield is 69.5%)
[14]. 4-n-Alkoxy benzoic acids and azodye were individually condensed in ice-cooled
pyridine to get a series of final azoester products [15]. Final products were individually
decomposed, filtered, washed, dried, and purified till constant transition temperature, ob-
tained.
The chemicals, 4-hydroxy benzoic acid, alkyl halides [R-X], methanol, ethanol, KOH,
thionyl chloride, pyridine, 2,6 dichloro phenol, 4-methoxyaniline, HCl, NaNO2, etc. re-
quired for synthesis were used as received excepts solvents, which were dried and distilled
prior to use. The synthetic route to the series is shown below in Scheme 1.
Characterization
Some selected members of the novel series were characterized by elemental analysis, polar-
izing microscopy, mass spectra, infra red spectra, 1H NMR spectra, texture determination
by a miscibility method. Microanalysis of the compound was performed on 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.
Analytical Data
IR in cm−1 for n-heptyl derivative. 2933.0, 2867.0, 1455.0, 1386.7 (alkyl group), 1733.9,
1241.1 (ester group), 1601.0 ( N N ), 1582.5 ( C C aromatic stre.), 1142.7 and
1164.9 (ether group), 839.0 (p-sub. benzene ring), 724.2 (poly -CH2- rocking bending),
1037.6 (C Cl aromatic).

Dependence of Mesomorphism on Terminal End Group of a Molecule
121
Scheme 1. Synthetic route to the series.
IR in cm−1 for n-octyloxy derivative. 2922.0, 2852.5, 1473.0, 1386.7 (alkyl group), 1733.9,
1242.1 (ester group), 1599.0 ( N N ), 1581.5 ( C C aromatic stre.), 1142.7 and
1164.0 (ether group), 839.9 (p-sub. benzene ring), 723.3 (poly-CH2- rocking bending),
1033.8 (C Cl aromatic).
1H-NMR in CDCl3, δ ppm for n-propyloxy derivative. 1.07 (t, 3H, CH3), 1.88 (m, 2H,
OCH2-CH2-), 3.9 (s, 3H, OCH3), 4.02 (t, 2H, OCH2-), 6.98–7.04 (d, 4H, Ar-H), 7.9
(s, 2H, Ar-H), 8.19 and 8.23 (d, 4H, Ar-H).
1H-NMR in CDCl3, δ ppm for n-hexyloxy derivative. 0.92 (t, 3H, CH3), 1.35–1.47 (m,
6H, 3x CH2-), 1.83 (m, 2H, OCH2-CH2-), 3.9 (s, 3H, OCH3), 4.06 (t, 2H, OCH2-),
6.90–7.04 (d, 4H, Ar-H), 7.94 (s, 2H, Ar-H), 8.18 and 8.23 (d, 4H, Ar-H).

122
N. G. Makwana et al.
Table 2. Transition temperatures for 4-(4ꢀ-n-alkoxy benzoyloxy)-3,5-dichloro phenylazo-
Transition temperatures in ◦C
Compound
no.
n-Alkyl group
CnH2n+1 (n)
Sm
N
Isotropic
1
2
3
4
5
6
7
8
9
C1
C2
C3
C4
C5
C6
C7
C8
C10
C12
C14
C16
—
—
—
—
—
—
—
—
—
—
—
—
(208.0)
(205.0)
142.0
147.0
100.0
97.0
86.0
75.0
80.0
40.0
210.0
207.0
172.0
171.0
146.0
147.0
131.0
126.0
117.0
112.0
80.0
10
11
12
71.0
—
60.0
Mass Spectra (Molecular Weight) Pentyloxy Homolog
Theoritical – 487.
Experimental – 487.
Textures by Miscibility Method
Pentyloxy homolog – Threaded texture
Heptyloxy homolog – Threaded texture
Tetradecyloxy homolog – Schlieren texture
Results and Discussion
Azodye 4-hydroxy 3,5-dichlorophenyl azo-4ꢀ-methoxy benzene is nonmesomorphic (M.P.
is 102.0◦C). However on condensing it with dimerized 4-n-alkoxy benzoic acids through
their corresponding acid chlorides yielded azoester molecules with nematic type liquid
crystalline behaviors throughout the series. Transition temperatures (Table 2) as determined
by an optical polarizing microscopy equipped with a heating stage were plotted against the
number of carbon atoms present in n-alkyl chain of the left n-alkoxy terminal end group to
construct a phase diagram showing mesophase behaviors of a novel series. Transition curves
were drawn by linking like or related points of a phase diagram (Fig. 1). Solid-isotropic
(Cr–I) or Solid-Nematic (Cr–N) transition curve follows a zigzag path of rising and falling
in usual established manner and behaved with normal behavior. Nematic-Isotropic (N-I) or
Isotropic-Nematic (I-N) transition curve continuously descended as series is ascended up
to tetradecyloxy(C14) derivative. Hexadecyloxy (C16) homolog derivative being nonliquid
crystal, the N-I/I-N transition curve is extended [16] upto C16 homolog and it merges into
Cr-I point of it, indicating that Cr-I and N-I point of C16 is the same and its mesophase
length is zero, i.e., nonmorphicity of C16 is supported by the trend of N-I/I-N transition
curve. And I-N/N-I transition curve exhibited odd-even effect with odd member’s curve

Dependence of Mesomorphism on Terminal End Group of a Molecule
123
Figure 1. Phase behaviors of series.
lower than even member’s curve. Curves for odd and even members merge into each other
nearby nonyloxy (C9) homologue and then from and beyond merging of each other, it
follows into single N-I transition curve for higher homologs (C10–C16). Thus, odd-even
effect diminishes for higher homologues from and beyond merging (C9) of N-I curves for

124
N. G. Makwana et al.
odd and even homologues. Average Thermal Stability for nematic mesophase formation
(C3–C14) is 133.5 0C and mesomorphic phase length ranges from 0 to 72◦C. Mesomorphism
commences from very first member of a novel series. Mesomorphic properties of a novel
azoester series undergo variations from homologue to homologue with varying n-alkyl
chain length keeping right terminal and other molecular part unchanged. Thus, presently
investigated novel series is predominantly nematogenic (C1–C14), with partly high melting
and partly middle ordered melting type, without exhibition of any smectic property.
4ꢀꢀ-Methoxy Benzenes
Sm, Smectic; N, Nematic. Value in parenthesis indicate monotropy.
The molecular length increases on linking dimerized 4-n-alkoxy benzoic acids with
azodye. Hence, the molecular length, length to breadth ratio, molecular polarity and po-
larizability, dipole–dipole interactions, dispersion forces, etc. are enhanced and attains to
the suitable magnitudes of anisotropic forces of intermolecular end to end attractions as a
consequence of favorable molecular rigidity and flexibility to induce nematogenic character
in the members of a novel series from C1 to C14 homolog derivatives. The dimerization
of 4-n-alkoxy disappears on esterification through corresponding acid chlorides, due to
breaking of hydrogen bonding between two acid molecules. Azoester molecules from C3
to C14 homologs resist exposed thermal vibrations and disalign at an angle less than ninety
degree for some temperature range on floating surface with statistically parallel orien-
tational order. However, on continued further heating beyond certain temperature limit
intermolecular anisotropic forces are loosened and a sample substance exhibit isotropic
state of existence from and beyond isotropic temperature at which molecules are randomly
oriented in all possible directions with high order of disorder or high entropy(ꢀS = ꢀH/T).
However, on cooling the isotropic melt, the C1–C2 homologs undergo exhibition of statis-
tically parallel orientational molecular order and showed monotropic nematic mesophase
formation below isotropic temperature in irreversible manner. Rest of the enantiotropic
nematogenic azoester molecules from C3 to C14, on cooling the isotropic melt, showed
nematic mesophase formation from and below isotropic temperature in reversible manner
till solidification. Hexadecyloxy (C16) derivative of a series is unable to resist exposed
thermal vibrations and abruptly breaking its crystal lattices. Thus, C16 homolog sharply
and smoothly transform into isotropic liquid from solid state without passing through an
intermediate state of existence between crystalline solid and isotropic liquid, i.e., liquid
crystalline state of a substance is missing. The inadequate magnitudes of end-to-end in-
termolecular adhesion and closeness hinder the exhibition of mesophase by C16 homolog.
The absence of smectogenic character is attributed to the absence of lamellar packing
of molecules in the crystal lattices of the homologs C1–C16 and the unsuitable extent of
molecular noncoplanarity and steric hindrance caused by laterally substituted two chloro
groups bonded to middle phenyl ring towards smectogenic –COO- central group. The
exhibition of odd-even effect by N-I/I-N transition curve is attributed to the sequentially
added methylene unit to n-alkoxy terminal end group making n-alkyl chain with odd and
even numbered carbon atoms. The disappearance of odd-even effect from and beyond C9
homolog for higher homolog is attributed to the coiling or bending or flexing or coupling
of longer n-alkyl chain with principal axis of a core structure of molecules (C10–C14). The
variations of liquid crystalline properties from homologue to homologue in the present
series is attributed to the changing number of CH2- unit or units in n-alkyl chain which
alters the degree of intermolecular cohesive forces from homolog to homolog in the same
series. Thus, transition and melting temperatures undergo depression as series is ascended

Dependence of Mesomorphism on Terminal End Group of a Molecule
125
Figure 2. . Structurally Similar series.
from C1 to C14 and only nematogenic mesophase acquires stabilization. The mesomorphic
properties of presently investigated novel azoester homologous series 1 are compared with
other structurally similar known homologous series X [17] and Y [18] as under in Fig. 2.
Homologous series of present investigation series 1 and the homologous series X and
Y chosen for comparative study are identical and structurally similar with respect to three
phenyl rings, two central bridges COO and N N as well as laterally substituted
two flexible 3,5-dichloro groups. Moreover flexible n-alkoxy (-OR-) terminal end groups
are also identical for the same homologue from series to series. However series 1, X, and Y
differ with respect to right handed terminal end groups OCH3, CH3, and Cl, whose
individual group polarity and hence the molecular polarity as well as related degree of cohe-
sive intermolecular suitable magnitudes of anisotropic forces as a consequence of combine
effect of molecular rigidity and flexibility undergo variations. Thus, changing molecular
flexibility and rigidity for the same homolog from series to series vary accordingly. Thus,
changing trend of mesomorphic properties from series to series or homolog to homolog in
the same series depends upon the differing or changing part of structurally similar homolo-
gous series selected for comparative study. Thus, mesomorphic behaviors and the degree of
mesomorphism of any individual or structurally similar homologous series depends upon
the suitable or favorable magnitudes of molecular rigidity and flexibility as emerged from
its molecular structure. Following Table 3 represents some mesomorphic properties like

126
N. G. Makwana et al.
Table 3. Average thermal stabilizes in ◦C
Series →
1
X
Y
Smectic-Isotropic
or
Smectic-Nematic
Commencement
of Smectic
—
—
—
phase
Nematic-Isotropic
or
133.5
111.2
96.3
Isotropic-Nematic
Commencement
of Nematic
phase
(C3-C14)
C1
(C5-C10)
C4
(C4-C10)
C4
Mesophase length
00 to 72—–Nm
00 to 31
00 to 21
range in ◦c
thermal stabilities, commencement of mesophase, mesophase length range, etc. in compar-
ative manner for presently investigated novel series 1 and the structurally similar series X
and Y as chosen for comparison.
Table 3 indicates that
• Azoester series 1, X, and Y are only nematogenic without exhibition of any smec-
togenic character.
• Nematogenic mesophase formation commences from very first member of a series
1, while it commences late from Butyloxy (C4) homolog for series X and Y chosen
for comparison.
• Thermal stability gradually undergo depressed from series 1 to X to Y.
• Mesogenic phase length is the highest for a series 1 of present investigation and it
gradually reduced for series X and Y, respectively.
The nematogenic mesophase formation tendency of a substance depended upon the
extent of suitable magnitudes of anisotropic forces of intermolecular end to end attractions
as a consequence of favorable molecular rigidity and flexibility conducive to cohesive
forces emerged, which maintain the statistically parallel orientational order of molecules,
while floating on the surface.
Homologous series 1, X, and Y contain three phenyl rings, two central bridges viz.,
COO and N N as well as two (3,5 – dichloro) chloro groups on middle phenyl
ring perfectly equal, similar or identical. Moreover, n-alkoxy terminal (left) end group
is also identical for the same homolog from series to series. Therefore, intermolecular
cohesive forces playing their role due to identical molecular part which contribute almost
in equal manner. However the right-handed terminally situated OCH3, CH3, and Cl
intermolecular cohesion end group differ for series 1, X, and Y, respectively. Therefore,
molecular dipole moment across the long molecular axis, dipole–dipole interactions, and
the magnitudes of dispersion forces differ from series to series depending upon group
polarities, of OCH3, CH3, and Cl. All the three terminal end groups viz., OCH3,
CH3, and Cl bonded to third phenyl ring occupying identical para position. But a bond

Dependence of Mesomorphism on Terminal End Group of a Molecule
127
between carbon of phenyl ring and functional end groups, i.e.,
,
, and
bear
different values of bond polarity. Therefore, vector sum of polarities of all bonds of same
homolog of different series under comparison causes difference in dipole moment across the
long molecular axis, dipole–dipole interactions and magnitudes of dispersion forces or end
to end attractions. Thus, intermolecular closeness and suitable magnitudes of anisotropic
forces of end to end cohesion energy against the steric hindrance facilitates and stabilizes
only nematic mesophase formation in all the series 1, X, and Y, Thus, the magnitudes
of end to end attractions vary for the same homolog from series to series. Hence, the
average thermal stabilities, commencement of nematic mesophase and mesophase length
and missing of smectic phase for the same homologue vary from series to series OCH3
terminal end group is highly polar and bearing higher group efficiency order for nematic
than CH3 and Cl, which shows highest thermal stability, early commencement (from C1)
of nematic phase and the highest mesophase length CH3 and Cl groups are weakly polar
than OCH3 and mutually almost equally polar [19] ( CH3 ≈ Cl) have nearer values of
group efficiency order for nematic, thermal stability as well as equal commencement (from
C4) of nematic mesophase formation with nearer values towards mesophase lengths. Thus,
a phenomenon of mesomorphism and the degree of mesomorphism are very sensitive to
molecular structure and its molecular rigidity and flexibility as related to molecular polarity,
polarizability, intermolecular cohesive forces emerging from dipole–dipole interactions,
dipole moment across the long molecular axis and the magnitudes of dispersion forces.
Conclusions
• Presently investigated novel azoester series is majorly nematogenic with middle
ordered melting type whose mesogenic phase length is relatively high.
• The group efficiency order for nematic derived on the basis of (i) thermal stability
(ii) early commencement of nematic mesophase and (iii) mesophase length is as
under.
(i) Nematic: OCH3 > CH3 > Cl
(ii) Nematic: OCH3 > CH3 = Cl
(iii) Nematic: OCH3 > CH3 > Cl
• The mesomorphic properties vary from homolog to homolog in the same series due
to the sequentially added methylene unit or units.
• The mesomorphic properties for the same homolog of different series vary with the
changing polarity of right-handed terminal end group which remains unchanged for
same series.
• Mesomorphic properties of a substance are sensitive and susceptible to its molecular
structure.
Acknowledgments
The authors are thankful to Head and Teaching staff of Applied Chemistry of Faculty of
Technology and Engineering, The M.S. University of Baroda, Vadodara for their valuable
cooperation in the work. Authors are also thankful to CSMCRI, Bhavnagar, for the analysis
of samples.

128
N. G. Makwana et al.
References
[1] Reinitzer, F. (1888). Monatsch., 9, 421.
[2] Naemura, S. (2001). Displays, 22(1), p. 1.
[3] Kim, W. S., Elston, S. J., & Raynes, F. P. (2008). Displays., 29, 458–463.
[4] Demus, D. (1988). Mol. Cryst. Liq. Cryst., 165, 45–84.
[5] Demus, D. (1989). Liq. Cryst., 5, 75–110.
[6] (a)Doshiet al (i) Suthar, D. M., & Doshi, A. V. (2013). Mol. Cryst. Liq. Cryst., 575, 76–83 (ii)
Chauhan, H. N., & Doshi, A. V. (2013). Mol. Cryst. Liq. Cryst., 570, 92–100. (iii) Chaudhari,
R. P., Chauhan, M. L., & Doshi, A. V. (2013). 575, 88–95. (iv) Bhoya, U. C., Vyas, N. N., &
Doshi, A. V. (2012). Mol. Cryst, Liq. Cryst., 552, 104–110. (b). Shah, N. H., Vora, R. A., &
Jadav, N. D. (1991). Mol. Cryst. Liq. Cryst., 209, 291.
[7] Gray, G. W. (1974). In: Liquid Crystals and Plastic Crystals, Gray, G. W. & Winsor, P. A.
(Eds.), (Vol. 1. pp. 103–152) Chapter-4.1, Gray Ellishorwood Ltd. Chichester john wilay &
sons: London.
[8] Gray, G. W. (1962). Molecular Structure and Properties of Liquid Crystals, Academic Press:
London.
[9] Hird, M., Toyne, K. J., Gray, G. W., Dok, S. E., & Mc. Donnell, D. G. (1993). Liq. Cryst., 15,
123.
[10] Hird, M., Toyne, K. J., & Gray, G. W. (1993). Liq. Cryst., 14, 741.
[11] Marcos, M., Omenat, A, Serrano, J. L., & Ezcurra. (1992). Adv. Mater., 4, 285.
[12] Collings, P. J., & Hird, M. (1998). Introduction to Liquid Crystals Chemistry and Physics, Taylor
and Francis Ltd.: U.K.
[13] Dave, J. S, & Vora, R. A. (1970). In: Liquid Crystal and Ordered Fluids, Johnson, J. F. & Porter,
R. S. (Eds.), p. 477, Plenum Press: New York.
[14] Furniss, B. S., Hannford, A. J., Smith, P. G. W., & Tatchelll, A. R. (Revisor). (1989). Vogel’s
Text Book of Practical Organic Chemistry, 4th ed.. Longman publishers Pvt.Ltd.: Singapore,
pp. 563–649.
[15] Suthar, D. M., & Doshi, A. V. (2013). Mol. Cryst. Liq. Cryst., 577, 51–58.
[16] (a). Lohar, J. M., & Doshi, A. V. (1993). Proc Indian Acad of Sci– Bangalore., 105(3), 209–214.
(b). Ganatra, K. J., & Doshi, A. V. (2000). J. Indian Chem. Soc., 77, 322–325. (c). Doshi, A. V.,
Bhoya, U. C., & Travadi, J. J. (2012). Mol. Cryst. Liq. Cryst., 552, 10–15.
[17] Makwana, N. G., & Doshi, A. V. (2011). Der Pharma Chemica., 3(2), 433–439.
[18] Doshi, A. V., & Makwana, N. G. (2011). Der Pharma Chemica., 3(1), 580–587.
[19] Gray, G. W. & Winsor, P. A.(Eds.), In: Liquid Crystals and Plastic Crystals, chapter 4.2, Dave,
J. S. and Vora, R. A., pp. 161 (153–174). Gray Ellishorwood Ltd. Chichester John Wiley &
Sons: London Volume 1. pp. 103–152.