The Dependence of Mesomorphism on the Terminal Group

Article abstract of DOI:10.1080/15421406.2015.1017307

A novel azoester homologous series of liquid crystals (LCs) was synthesized and studied with a view to understanding and establishing the effect of molecular structure on LC behavior. The series consists of twelve members. All the members of the series exhibit a nematogenic mesophase. The methoxy and propyloxy derivatives are monotropic and the rest of the homologs are enantiotropic nematogenic. Smectic mesophase formation is completely absent. Transition temperatures and the textures of the nematic phase were observed and determined through an optical polarizing microscope equipped with a heating stage. Transition curves (CrN and NI) of a phase diagram behave in a normal manner except for the hexadecyloxy homolog which deviates from normal behavior in the NI transition curve. The NI transition curve exhibits an odd-even effect. The textures of the nematic phase are threaded or Schlieren in type. The average thermal stability for the nematic phase is 158°C. The nematogenic mesophase length ranges from 27°C to 226°C. The analytical and spectral data confirms the molecular structures of the homologs. The LC properties of the presently investigated novel series are compared with a structurally similar known series. The novel azoester series is entirely nematogenic without exhibition of smectogenic character.

Full text of DOI:10.1080/15421406.2015.1017307

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
The Dependence of Mesomorphism on the
Terminal Group
N. G. Makwana, H. R. Prajapati, Y. K. Chahar & A. V. Doshi
To cite this article: N. G. Makwana, H. R. Prajapati, Y. K. Chahar & A. V. Doshi (2015) The
Dependence of Mesomorphism on the Terminal Group, Molecular Crystals and Liquid Crystals,
623:1, 148-156, DOI: 10.1080/15421406.2015.1017307
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Date: 09 February 2016, At: 03:36

Mol. Cryst. Liq. Cryst., Vol. 623: pp. 148–156, 2015
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2015.1017307
The Dependence of Mesomorphism on the Terminal
Group
1
2,∗
3
N. G. MAKWANA, H. R. PRAJAPATI, Y. K. CHAHAR,
AND A. V. DOSHI4
1General Department (Chemistry), R.C. Technical Institute, Ahmedabad,
Gujarat, India
2
C.U.Shah Science College, Ahmedabad, Gujarat, India
S.B.D. Govt. P.G. College, Sardarshahr (Chum), Rajasthan, India
M. V. M. Science and Home Science College, Rajkot, Gujarat, India
3
4
A novel azoester homologous series of liquid crystals (LCs) was synthesized and studied
with a view to understanding and establishing the effect of molecular structure on LC
behavior. The series consists of twelve members. All the members of the series exhibit
a nematogenic mesophase. The methoxy and propyloxy derivatives are monotropic and
the rest of the homologs are enantiotropic nematogenic. Smectic mesophase formation
is completely absent. Transition temperatures and the textures of the nematic phase
were observed and determined through an optical polarizing microscope equipped with
a heating stage. Transition curves (Cr N and N I) of a phase diagram behave in
a normal manner except for the hexadecyloxy homolog which deviates from normal
behavior in the N I transition curve. The N I transition curve exhibits an odd-even
effect. The textures of the nematic phase are threaded or Schlieren in type. The average
◦
thermal stability for the nematic phase is 158 C. The nematogenic mesophase length
◦
◦
ranges from 27 C to 226 C. The analytical and spectral data confirms the molecular
structures of the homologs. The LC properties of the presently investigated novel series
are compared with a structurally similar known series. The novel azoester series is
entirely nematogenic without exhibition of smectogenic character.
Keywords Azoester; enantiotropy; liquid crystal; nematic; smectic
Introduction
Liquid crystalline state (LC) of a matter in addition to well known three states of matter of
solid, isotropic liquid, and gas, and has been known since 1888 [1]. Thereafter, many LC
compounds [2–5] as simple monomers and dimers and polymers, etc. have been synthesized
and studied with different aims objectives and views by researchers of different disciplines.
We being chemists synthesize novel substances by varying molecular structure through
alteration of the number of phenyl rings, central bridge or bridges, lateral, or/and terminal
groups and then correlate the effect of molecular structure [6–8] on LC properties after their
characterization. The present investigation is planned to synthesize and characterize novel
azoester LC compounds consisting of three phenyl rings, two central bridges, two lateral
∗
Address correspondence to H. R. Prajapati, C.U. Shah Science College, Ashram Road, Income
Tax, Ahmedabad 380014, India. E-mail: drhrprajapati@yahoo.co.in
148

The Dependence of Mesomorphism on the Terminal Group
149
Table 1. Elemental analysis for ethoxy, pentyloxy, octyloxy, and dodecyloxy derivatives
Elements % found (calculated %)
Sr. no.
Molecular formula
C
H
N
1
2
3
4
C23H20N2O4Cl2
C26H26N2O4Cl2
C29H32N2O4Cl2
C33H40N2O4Cl2
60.05(60.13)
62.22(62.27)
64.15(64.09)
66.18(66.11)
4.30(4.36)
5.11(5.19)
5.80(5.89)
6.60(6.68)
6.18(6.10)
5.51(5.59)
5.11(5.16)
4.16(4.67)
chloro groups, and two terminal end groups, with a view to understanding and establishing
the relation between LC properties and the molecular structure as a consequence of molec-
ular rigidity and flexibility [9–12]. The novel proposed azoester substances may find use in
optical imaging display systems and for the study of cis-trans isomerism [13, 14].
Experimental
Synthesis
4
-n-Alkoxy benzoic acid was alkylated by suitable alkylating agents (R-X) by the modified
ꢀ
method of Dave and Vora [15]. Azodye 4-Hydroxy 3,5-dichloro phenyl azo-4 -ethoxy
benzene was prepared by a usual established method (M.P is 84.0 C and Yield is 67.40%)
◦
[16]. 4-n-Alkoxy benzoic acids and the azodye were condensed in ice cooled pyridine
to obtain a series of final azoester products [17]. The final products were individually
decomposed, filtered, washed, dried, and purified until constant transition temperature
were obtained.
The chemicals, 4-hydroxy benzoic acid, alkyl halides [R-X], methanol, ethanol, KOH,
thionyl chloride, pyridine, 2,6 dichloro phenol, 4-ethoxy aniline, HCl, NaNO2, etc. required
for synthesis were used as received excepts solvents, which were dried and distilled prior
to use. The synthetic route to the series is shown in scheme 1.
Characterization
Some selected members of the novel series were characterized by elemental analysis, polar-
1
izing microscopy, mass spectra, infra red spectra, H NMR spectra, texture determination
by 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
1
spectrometer, and H NMR spectra were performed on Bruker spectrometer using CDCl3
as solvent.
Analytical Data
−1
IR in cm for the pentyl derivative. 2924.8, 2849.6, 1451.1, 1387.7 (alkyl group), 1733.9,
1252.7 (ester group), 1596.0 ( N N ), 1559.3 ( C C aromatic stre.), 1140.8 and
1163.0 (ether group), 842.8 (p-sub. benzene ring), 724.2 (poly CH2- rocking bending),
1041.5 (C Cl aromatic).

150
N. G. Makwana et al.
Scheme 1. Synthetic route to the series.
−1
IR in cm for the hexyl derivative. 2934.0, 2856.0, 1442.7, 1387.7 (alkyl group), 1734.9,
1241.1 (ester group), 1588.0 ( N N ), 1560.3 ( C C aromatic stre.), 1141.8 and
1164.9 (ether group), 839.0 (p-sub. benzene ring), 724.2 (poly-CH2- rocking bending),
1043.4 (C Cl aromatic).
1H NMR in CDCl3, δ ppm for the butyloxy derivative. 1.00 (t, 3H, CH3), 1.57 (m, 2H,
CH2-) 1.82 (m, 2H, OCH2-CH2-), 4.07(t, 2H, OCH2-), 1.46(t, 3H, OCH2-CH3-),
4.15 (q, 3H, OCH2-CH3-), 6.92–8.23 (m, 10H, Ar-H)
1H NMR in CDCl3, δ ppm for the decyloxy derivative. 0.91 (t, 3H, CH3), 1.43–1.56
(m, 7H, 2x –CH2-), 1.83 (m, 2H, OCH2-CH2-), 4.06 (t, 2H, OCH2-), 1.47 (t, 3H,
OCH2-CH3-), 4.15 (q, 2H, OCH2-CH3-), 6.98–8.23 (m, 10H, Ar-H)

The Dependence of Mesomorphism on the Terminal Group
151
ꢀ
Table 2. Transition temperatures for 4-(4 -n-alkoxy benzoyloxy)-3,5-dichloro phenylazo-
4ꢀꢀ-ethoxy benzenes
◦
Transition temperatures in C
Compound
no.
n-Alkyl group
CnH2n+1(n)
Smectic
(Sm)
Nematic
(N)
Isotropic
(I)
1
2
3
4
5
6
7
8
9
C1
C2
C3
C4
C5
C6
C7
C8
C10
C12
C14
C16
—
—
—
—
—
—
—
—
—
—
—
—
(195.0)
66.0
196.0
292.0
160.0
224.0
142.0
169.0
112.0
138.0
113.0
103.0
105.0
182.0
(155.0)
152.0
113.0
103.0
71.0
98.0
86.0
57.0
76.0
1
1
1
0
1
2
109.0
() indicate monotropy.
Mass Spectra(Molecular Weight) Butyloxy Homolog
Theoritical 487.
Experimental 487.
Texture Determination by Miscibility Method
Propyloxy homolog: Threaded texture
Hexyloxy homolog: Threaded texture
Hexadecyloxy homolog: Schlieren texture
Results and Discussion
The homologous series of the present investigation consists of twelve homologs and is
entirely nematic in terms of mesomorphic behavior. The azodye 4-hydroxy 3,5 dichloro
ꢀ
◦
phenyl azo-4 -ethoxy benzene is a nonliquid crystal (NLC) (M.P. is 84 C and Yield is
7.40%) component. However on linking it with the 4-n-alkoxy benzoic acids through their
6
corresponding acid chlorides in dry cold pyridine twelve homologs resulted, all with liquid
crystal properties. Transition temperatures (Table 2) of the homologs are plotted versus the
number of carbon atoms present in the alkoxy terminal end group. Transition curves (Cr-I
or Cr N and N I or I N) are obtained on linking like or related points showing phase
behaviors of series in a phase diagram (Fig. 1).
The solid-isotropic or nematic (Cr N or Cr-I) transition curve adopts a zigzag path
of rising and falling values with an overall descending tendency and behaves in the usual
established manner. The N I or I N transition curve is descended as the series is ascended
up to the tetradecyloxy homolog and behaves in a normal manner. However, the hexadecy-
◦
loxy homolog deviates from the descending tendency by ascending by about 82 C higher

152
N. G. Makwana et al.
Figure 1. Phase behaviors of series.
than the expected normal behavior. The N I Transition curve for the odd members (C1–C7)
of the series lie below the transition curve for the even members and both transition curves
(for odd and even members) merge into each other at the dodecyloxy (C12) homolog and
then continue as a single N I curve. The mesomorphic (LC) properties vary from homolog

The Dependence of Mesomorphism on the Terminal Group
153
Figure 2. Structurally similar series.
to homolog in the present novel series. The average thermal stability for the nematic phase
◦
◦
is 158 C and their mesomorphic mesophase length ranges from a maximum of 226 C at
◦
the C2 homolog to a minimum of 27 C at the C10 homolog. Thus, homologous series under
discussion is entirely nematogenic without exhibition of smectic mesophase formation.
Linking of 4-n-alkoxy benzoic acids with a NLC component Azodye, increases the
length of molecules and hence the molecular rigidity and flexibilities are also enhanced as
a consequence of the resulted permanent dipole moment acting across the long molecular
axis, dipole–dipole interactions, dispersion forces, etc., which generate suitable magnitudes
of anisotropic forces of end to end intermolecular attractions caused by molecular polar-
izability and other cohesive forces. Hence, the molecules of all the homologs (C1–C16)
exhibit only a statistically parallel orientational order; either in reversible or irreversible
manner, resisting exposed thermal vibrations for definite range of temperature depending
upon the degree of resistivity toward heat and the angle of molecular disalignment. Thus,
only nematogenic mesophase formation is facilitated. The diminishing of the odd-even
effect from and beyond the dodecyloxy (C12) homolog is attributed to the coiling or flex-
ing or bending or coupling of n-alkyl chain with the major axis of the core structure of
a molecule. The variations in mesomorphic properties from homolog to homolog in the
present series is attributed to the sequentially added methylene unit in the n-alkyl chain,
keeping the right handed terminal end group OC2H5 unchanged. Thus, present novel
series is nematogenic, and of middle ordered melting type, whose mesogenic phase length
is wide, without exhibition of smectogenic property. The mesomorphic (LC) property of a
present novel series is compared with a structurally similar homologous series X [18] as
mentioned below in Fig. 2.
Novel homologous series 1 and a homologous series chosen for comparison X as
shown in Fig. 2 are identical with respect to three phenyl rings, central bridges COO
and N N , two laterally substituted chloro groups at the middle phenyl ring and left
n-alkoxy ( OR) terminal end group for the same homolog from series to series. However,
they differ with respect to right side terminal end groups OC2H5 (series 1) and NO2

154
N. G. Makwana et al.
◦
Table 3. Relative thermal stabilizes in C
Series →
1
X
Smectic-isotropic
or
—
—
Smectic-nematic
Commencement of smectic
phase
Nematic-isotropic
Commencement of N phase
158.0
(C1–C16)
C1
113.25
(C4–C12)
C2
◦
Mesophase length range in C
27–226
21–35
(series X). Therefore, the contribution of identical parts of the series under comparison
for the same homolog toward molecular rigidity, flexibility, polarity, and polarizability,
etc. related to suitable magnitudes of anisotropic forces of intermolecular end to end
attractions are equivalent. However, the flexibility contribution by the OC2H5 and NO2
terminal end groups differs in considerable magnitudes due to variation in permanent
dipole moment across the long molecular axis, dipole–dipole interactions, dispersion forces,
group polarities of individual terminal end groups OC2H5 and NO2 for phenyl ring
OC2H5 and phenyl ring NO2 covalent bond (C O and C N bond) polarity. Therefore,
total molecular rigidity and flexibility of the same homolog from series to series under
comparative study and the homolog to homolog in the same series acquires difference of
liquid crystal state inducing capacity or resistivity toward exposed thermal vibrations. Thus,
mesomorphism and the degree of mesomorphism for the same homolog from series to series
and from homolog to homolog in the same series depend upon changing suitable magnitudes
of anisotropic forces of intermolecular end to end attractions as a consequence of resultant
molecular rigidity and flexibility offered by individual molecular structure. Following
Table 3 represents some liquid crystal properties of series 1 and X in a comparative manner.
Careful observation of Table 3 indicates that
•
Azoester homologous series of liquid crystals of present investigation 1 and series
X are only nematogenic.
•
•
Smectogenic character is totally absent.
Thermal stability for nematic of Series 1 and X are 158.0 and 113.25 as calculated
on the basis of enantiotropic homologs, i.e., thermal stability of series 1 is higher
than a series X.
•
•
Nematogenic mesophase commences from very first member (C1) of a series 1 to last
C16 member and it commences from second member (C2) of series X to dodecyloxy
(
C12) member of a series.
Mesophase length of series 1 is greater than a series X chosen for comparison.
Nematogenic mesophase formation and the degree of mesomorphism being attributed
to the differing characteristics of right terminals OC2H5 and NO2; the group efficiency
order and mesophase stabilization are related to the group polarity of functional group
OC2H5 and NO2. A covalent bond between carbon of phenyl ring and oxygen atom of
OC2H5 is more polar than a covalent bond between a carbon of phenyl ring and Nitrogen
atom of NO2 due to the difference between electron affinities of oxygen and Nitrogen

The Dependence of Mesomorphism on the Terminal Group
155
atom. Thus, on thinking the bond polarities concept between O and C, C and H of OC2H5
as well as N and O of NO2, the OC2H5 group releases electron clouds toward phenyl
ring, whereas NO2 group instead of releasing toward phenyl ring it withdraws electron
clouds from phenyl ring. Therefore, molecular rigidity and the end-to-end attractions among
the molecules of series 1 for the same homolog are strengthened and therefore the relative
thermal stability of series 1 is more than the thermal stability of series X. Thus, enthalpy
(
ꢀH) value as related to thermal stability at a given temperature, i.e., energy stored in a
homolog substance for the same homolog from series to series is relatively more for a
series 1 than a series X, which facilitated the more resistivity against externally exposed
thermal vibrations from thermodynamic surroundings for the molecules of series 1, than
the corresponding molecules of series X. Therefore, comparatively mesophase length range
and early commencement of nematic mesophase in case of series 1 are relatively superior
to series X.
Conclusions
•
Presently investigated novel series is entirely nematogenic without exhibition of
smectic mesophase formation whose mesophase (nematic) commences from very
first member of a series to the last member (C1–C16) of a series.
Group efficiency order derived on the basis of (i) thermal stability, (ii) early com-
mencement of mesophase, and (iii) the mesophase length range for nematic are as
under.
•
(
i) Nematic : OC2H5 > NO2
(
ii) Nematic : OC2H5 > NO2
(
iii) Nematic : OC2H5 > NO2
•
Suitable magnitudes of molecular rigidity and flexibility are important to induce LC
state formation.
•
Molecular rigidity and flexibility are very sensitive and susceptible to molecular
structure.
•
Novel azoester substances of present investigation may be useful for photoinduced
study and optical imaging LC displays, etc.
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.
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