Synthesis of novel azoester homologous series of liquid crystalline behavior and the study of mesomorphism dependence on lateral substitution of middle phenyl ring

Article abstract of DOI:10.1080/15421406.2015.1044162

The titled azoester liquid crystalline homologous series consists of eleven homologues. The pentyl to tetradecyl derivatives of the series are nematogenic without exhibition of smectogenic character. Rest of the members of the series are nonliquid crystalline in nature. Textures of the nematic mesophase are schlieren or threaded type. Transition curves in the phase diagram showed normal phase behavior. Transition temperatures and liquid crystal behavior observed under an optical polarizing microscope equipped with a heating stage. An odd even effect is observed for nematic-isotropic transition curve. Analytical and spectral data confirms the structures of the molecules. Present homologous series is predominant nematogenic and partly nonmesogenic. Average thermal stability for nematic is 125.33°C and nematogenic mesophase length varies between 12°C to 48°C at tetradecyl (C₁₄) and octyl (C₈) derivatives, respectively.

Full text of DOI:10.1080/15421406.2015.1044162

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Synthesis of novel azoester homologous series
of liquid crystalline behavior and the study
of mesomorphism dependence on lateral
substitution of middle phenyl ring
D. H. Patel, Ankita A. Doshi, H. R. Prajapati & A. V. Doshi
To cite this article: D. H. Patel, Ankita A. Doshi, H. R. Prajapati & A. V. Doshi (2016) Synthesis
of novel azoester homologous series of liquid crystalline behavior and the study of
mesomorphism dependence on lateral substitution of middle phenyl ring, Molecular Crystals
and Liquid Crystals, 624:1, 69-76, DOI: 10.1080/15421406.2015.1044162
To link to this article: http://dx.doi.org/10.1080/15421406.2015.1044162
Published online: 11 Feb 2016.
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Date: 01 April 2016, At: 16:38

MOL. CRYST. LIQ. CRYST.
, VOL. , NO. , –
http://dx.doi.org/./..
Synthesis of novel azoester homologous series
of liquid crystalline behavior and the study of mesomorphism
dependence on lateral substitution of middle phenyl ring
D. H. Patela, Ankita A. Doshib, H. R. Prajapatic, and A. V. Doshid
aSheth P.T. Arts & Science College, Godhra, Gujarat, India; bDepartment of Biochemistry, The M.S. University of
Baroda, Vadodara, Gujarat, India; cC. U. Shah Science College, Ahmedabad, Gujarat, India; dM. V. M. Science and
Home Science College, Rajkot, Gujarat, India
ABSTRACT
KEYWORDS
Azoester; enantiotropic;
liquid crystal; nematic;
smectic
The titled azoester liquid crystalline homologous series consists of
eleven homologues. The pentyl to tetradecyl derivatives of the series
are nematogenic without exhibition of smectogenic character. Rest of
the members of the series are nonliquid crystalline in nature. Textures of
thenematicmesophaseareschlierenorthreadedtype. Transitioncurves
in the phase diagram showed normal phase behavior. Transition tem-
peratures and liquid crystal behavior observed under an optical polar-
izing microscope equipped with a heating stage. An odd even effect
is observed for nematic-isotropic transition curve. Analytical and spec-
tral data confirms the structures of the molecules. Present homologous
series is predominant nematogenic and partly nonmesogenic. Average
thermal stability for nematic is 125.33°C and nematogenic mesophase
length varies between 12°C to 48°C at tetradecyl (C14) and octyl (C8)
derivatives, respectively.
Introduction
An Austrian botanist Friedrich Reinitzer first observed and recognized a novel state of matter
intermediate between crystalline solid and isotropic true Liquid in the year 1888 [1], which
has flow property like liquid but optical properties like crystals. Such physical distinct state of
a matter is called liquid crystalline (LC), mesomorphic or mesogenic state of a matter. Not all
but some of the substances with suitable magnitudes of molecular rigidity and flexibility [2–5]
based on molecular shape, size, and molecular structure do exhibit physically distinct state like
liquid is also termed as anisotropic liquid. Thus, the presently proposed work is planned to
–
–
synthesize novel liquid crystalline materials with three phenyl rings bridged through COO
– = –
–
–
and N N central bridges as rigid core and the rest of the molecular part viz., OR, Cl,
OCH3 (terminal end group or/and lateral group) as flexible core. Thus, effect of broadening
–
of molecular structure can be correlated with liquid crystalline behavior of the novel sub-
stances based on molecular rigidity and flexibility. Hence, present investigation is aimed to
understanding and establishing the effect of molecular structure on liquid crystal properties
[6–8].
CONTACT D. H. Patel
drhrprajapati@yahoo.co.in
Sheth P.T. Arts & Science College, Godhra , Gujarat, India.
©  Taylor & Francis Group, LLC

70
D. H. PATEL ET AL.
Experimental
Synthesis
4-Hydroxy benzoic acid was alkylated by suitable corresponding alkyl halides [R-X] and con-
verted to corresponding 4-n-alkoxy benzoyl chlorides (X) using thionyl chloride by the mod-
ified method of Dave and Vora [9]. 2-Methyl -4- hydroxyphenyl azo -2ꢀ- methoxy benzene (Y)
was prepared by a known method [10]. Component (X) acid chloride and (Y) azo dye were
condensed in dry cold pyridine to give the final azoester homologues [11]. 4-Hydroxy benzoic
acid, alkyl halides, thionyl chloride, pyridine, KOH, methanol, O-anisidine, m-methyl phe-
nol etc., required for synthesis were used as received except solvents which were dried and
distilled prior to use. Synthetic route to the series is shown below as Scheme 1.
Scheme . Synthetic route to the series.

MOL. CRYST. LIQ. CRYST.
71
Characterization
Some selected members of the series were characterized by elemental analysis, infra red spec-
tra (IR), and 1H NMR spectra. Microanalysis was performed on Perkin-Elmer PE C, H, N
1
analyzer (Table 1). IR spectra were recorded on Perkin–Elmer spectrum GX and H NMR
spectra were recorded on Bruker using CDCl3 solvent. Liquid crystal properties were deter-
mined using an optical polarizing microscopy equipped with a heating stage. Textures of the
nematic phase determined by miscibility method.
Analytical data
IR spectra in Cm−1 for C8 and C14 homologues
Octyl
2920.0, 2850.6, 1370.6 (alkyl group), 1685.5, 1126.4 (ester group), 1257.5, 1168.8 (ether
– = –
group), 1431.1( N N ), 844.8(p-sub benzene), 771.5 (o-sub. benzene).
Tetradecyloxy
2920.0, 2850.6, 1396.4(alkyl group), 1685.7, 1114.8, 1064.6(ester group), 1164.9, 1257.5(ether
– = –
group), 1431.1( N N ), 844.8(p-sub benzene), 767.6 (o-sub. benzene).
1H- NMR Spectra in CDCl3 in δ ppm for Octyloxy (C8) & Dodecyloxy (C10) homologues:
Octyl
0.889(-CH3 of -C8H17), 1.306(-CH2 of -C8H17),1.806(-CH2CH2O of -C8H17), 2.742(-OCH3,
-C8H17) 4.035(-OCH2CH2--), 6.905 & 6.934(p-sub Benzene), 7.253(o-sub Benzene).
Dodecyl
0.880(-CH3 of -C10H23), 4.035(-OCH2CH2 of -C12H23), 1.264 (-CH2CH2 of -C12H23), 2.742(-
OCH3)
Textures by miscibility method
Nematic
Octyloxy homologue—Threaded texture
Dodedecyloxy homologue—Schlieren texture
Results and discussion
n-Alkoxy benzoic acids are dimeric. Their dimerization disappears on esterification by link-
ing with azo dye (Y) due to breaking of hydrogen bonding. Methyl to butyl and hexadecyl
derivatives of the series are nonliquid crystalline. The rest of the derivatives of the series are
liquid crystalline with enantiotropically nematogenic character and absences of smectogenic
character. Plot of the transition temperatures (Table 2) versus the number of carbon atoms
in n-alkyl chain of the left n-alkoxy terminal end group shows phase behavior (Fig. 1) of the
series.
The solid–isotropic or nematic transition curve continuously falls from methyl (C1) to
hexyl (C6) derivative and then rises by 14°C at octyloxy homologue and again rises in minor
magnitudes and fall by very few degrees of temperature. Nematic–isotropic transition curve

72
D. H. PATEL ET AL.
Table . Elemental analysis for propyloxy, hexyloxy, decyloxy derivatives.
Elements% found (Calculated%)
Sr. No.
Molecular Formula
C
H
N



CHNO
CH NO
CH NO
. (.)
. (.)
. (.)
. (.)
. (.)
. (.)
. (.)
. (.)
. (.)
Table . Transition temperatures of the homologous series.
Transition Temperatures in °C
n-alkyl group
CnHn+ (n)
Smectic
(Sm)
Nematic
(N)
Isotropic
(I)
Compound No.











C
C
C
C
C
C
C
C
C
C
C
–
–
–
–
–
–
–
–
–
–
–
–
–
–
.
.
.
.
.
.
.
.
.
.
.
–
.
.
.
.
.
.
–
(ꢀ-n-alkoxy benzoyloxy) -- methyl phenyl azo-ꢀꢀ -methoxy benzenes.
Figure . Phase behaviors of series.

MOL. CRYST. LIQ. CRYST.
73
showed descending tendency as series is ascended in usual expected manner with exhibition
of odd-even effect. Nematic–isotropic transition curve is extrapolated [12–14] to the nonliq-
uid crystalline hexadecyloxy homologue to predict latent transition temperature [LTT] for
nematic. LTT merges or coincide into the solid-isotropic point of hexadecyl derivative of
the series i.e. 90°C. This indicates that nematogenic phase length of hexadecyloxy homo-
logue must be 90–90 = 0 i.e. nematic phase cannot occur and actually or practically does
not occur is very well proved from the phase diagram. Nematic–isotropic transition temper-
ature vary between 100°C to 145°C with nematogenic mesophase length varying minimum
of 12°C at the tetradecyloxy derivative to the maximum of 48°C at the octyloxy derivative
of the series. Thus, presently investigated homologous series is predominantly nematogenic
and partly nonmesogenic without exhibition of smectogenic character. Nonmesogenic prop-
erty showed by methyl to butyl derivatives of the series is attributed to their high crystallizing
tendency arising from stronger intermolecular attractions due to relatively shorter n-alkyl
chain length of left n-alkoxy terminal end group. The energy associated to stabilize and facil-
itate mesophase formation is insufficient to arrange statistically parallel orientations order of
molecule in floating condition by unsuitable and unfavorable magnitudes of molecular rigid-
ity and flexibility. Therefore, methyl to butyl derivatives sharply passes into isotropic state
directly, without showing liquid crystal state at their relatively higher melting point. The non-
mesogenic behavior of hexadecyl derivative of the series is attributed to its amorphous nature
due to longer n-alkyl chain of left n-alkoxy end group. Thus hexadecyl homologue molecules
are unable to resist thermal vibrations exposed upon them. Thus, unsuitable magnitudes of
anisotropic forces of intermolecular end to end attractions of C1 to C4 and C16 homologues
are unable to resist exposed thermal vibrations to exhibit mesomorphic state of either type
or the mesophase formation is missing. Hence, it melts sharply at its melting point relatively
at lower temperature without passing through liquid crystal state due to lower intermolecu-
lar attractions or heat content (ꢀH). Thus, reasons for inability, to resist thermal vibrations
for methyl to butyl group of homologues and hexadecyloxy homologue are different. Pentyl
to tetradecyl derivatives of the series showed ability to withstand or resist thermal vibrations
exposed upon them from thermodynamic surroundings. Therefore a thermodynamic system
of molecules under examination disalign at an angle less than ninety degree from the plane
of the surface within definite range of temperature and arrange themselves to maintain, sta-
tistically parallel orientational order of two dimensional array of restricted randomness or
entropy (ꢀS = ꢀH/T) in floating condition, which maintain suitable magnitudes of inter-
molecular anisotropic forces of end to end attractions. Thus, nematogenic mesophase for-
mation stabilizes within different range of temperature for different homologues of the same
series and or the same homologue from series to series. Absence of smectogenic mesophase
is attributed to absence of lamellar molecular packing in the crystal lattices of mesomorphic
homologues. Thus, absence of lamellar packing in the crystal lattices hinders the smectogenic
mesophase formation. Nematogenic mesophase disappears from and beyond isotropic tem-
perature because, the limit of withstanding power (ꢀH) against exposed thermal vibrations of
a sample substance ceases and its molecules randomly orient in all possible directions without
any sort of regularity of molecular order or one can say a high order of disorder of molecules
or entropy ((ꢀS = ꢀH/T) occur in destabilized state or floating condition. Such situation of a
physical state of a matter is called the isotropic state. The mesomorphic properties including
thermal stability of any homologue in same series and for the same homologue from series to
series vary due to terminal end group and/or lateral group or a flexible part of molecules. The
odd even effect and variations in mesomorphic properties from homologue to homologue in
the same series is attributed to the sequentially added methylene unit at left n-alkyl chain of

74
D. H. PATEL ET AL.
Figure . Structurally similar series.
n-alkoxy terminal end groups. The odd–even effect diminishes as series is ascended because
longer n-alkyl chain of left n-alkoxy group may coil, bend or flex or couple to lie with the line
of a major axis of the core. The mesomorphic properties of the presently investigated series-1
are compared with structurally similar homologous series A [15] and B [16] as mentioned
below in Fig. 2.
Homologous series 1, A and B consisted of identically three phenyl rings bridged through
–
–
– = –
COO and N N as rigid core. Left n-alkoxy terminal end group for the same homo-
logue and –OCH3 lateral groups are commonly present at the left phenyl ring and right sided
– = –
phenyl ring at ortho position to N N central group. The only uncommon features are at
the middle phenyl ring, which involves –CH3, fused phenyl ring (naphthyl) and –Cl group
as laterally substituted varying flexible core. Therefore, overall flexibility and rigidity vary for
the same homologue from series to series and homologue to homologue in the same series.
Table 3 represents the average thermal stability of all the three series under comparison and
commencement of nematogenic mesophase formation.
Table 3 indicates that average thermal stability of series 1 is less than the series A [contain-
ing fused phenyl ring] and more than series B [containing –Cl]. All the three series under com-
parison are equally lengthened but they differ with respect to the ortho or meta-substituted
lateral groups at the middle phenyl ring, whose, polarities are different. Moreover, all the
three series 1, A, and B are broadened by –CH3, fused phenyl ring and –Cl group, respec-
– = –
tively, in addition to the commonly present ortho-substituted –OCH3 group to N N at
the third phenyl ring. These two variations causes variations in length to breadth ratio, ratio
Table . Relative thermal stabilities in °C.
Average thermal stabilities in °C
Series
Series – ()
Series – (A)
Series – (B)
Smectic-Isotropic
Commencement of smectic phase
Nematic-Isotropic
Commencement of Nematic phase
–
–
–
–
. (C - C)
C
–
. (C - C)
C
–
. (C - C)
C

MOL. CRYST. LIQ. CRYST.
75
of the molecular polarity to polarizability, molecular aromaticity, electron–electron interac-
tions, inductive effect, magnitude of steric hindrance etc. Hence, the overall molecular rigidity
and flexibility [1–4] as a consequence of changing permanent dipole moment across the long
molecular axis and dipole–dipole interactions and dispersion forces, affects LC properties of
the substance depending upon molecular structure. The broadening or increased width of the
molecule increases intermolecular distance and hence decreases intermolecular attractions on
one hand and at the same time it increases molecular polarizability for which increases inter-
molecular attractions, on other hand. Thus, two opposing effects [7,8] are operating at a time
for the same reason i.e. due to broadening of a molecule for all the three series 1, A, and B. The
resultant net effect will be the predominating effect out of two opposing effects. All the three
series [1,A,B] are only nematogenic without exhibition of smectogenic character. This obser-
vation substantiates relatively lower magnitudes of intermolecular attractions due to which
absence of lamellar packing and hence absence of smectic mesophase formation of sliding
layered molecular arrangement fails to appear, and lowering of nematogenic thermal stabil-
ity resulted. Thus, polarizability factor does not predominate but declining of intermolecu-
lar attractions by increased molecular width factor predominates and raises its effectiveness
to maintain only statistically parallel orientational order of molecule in floating condition,
which enables only nematic mesophase formation. The highest value of nematic thermal sta-
bility of series A is attributed to the fused phenyl ring which raises molecular aromaticity or
molecular rigidity at the cost of molecular flexibility in comparison with series 1 and B. Lowest
value of nematic thermal stability of series B is attributed to the difference of steric hindrance
caused by monoatomic –Cl group and tetra atomic -CH3 group or fused phenyl ring. The
unfavorable extent of noncoplanarity restricted to commence smectic mesophase till the last
hexadecyloxy homologue of series-1, A, and B. Nematogenic mesophase commences from
fifth, sixth and sixth members of the series 1, A, and B, respectively, due to difference of mag-
nitudes of end to end attractions which causes present series 1 is as predominantly nemato-
genic and partly nonmesomorphic. Thus, variations in mesomorphic properties for the same
homologue from series to series is attributed to molecular structure and especially the later-
ally substituted –CH3, fused phenyl ring and –Cl group of series 1, A, and B of fixed polarity
respectively.
Conclusions
1) Broadening of molecular width may increase or decrease the intermolecular attrac-
tions.
2) Homologous series with naphthyl moiety are generally nematogenic.
3) A Homologous series without right terminal (-H) end group and with laterally substi-
tuted functional group or groups hinders lamellar packing of molecules and are miss-
ing smectogenic character with exhibition of only a nematogenic mesophase forma-
tion.
4) Laterally substituted groups considerably affects the molecular rigidity and flexibility
which are directly related to the magnitudes of anisotropic forces of intermolecular
end to end attractions, and the lamellar packing of molecules.
I) The group efficiency order derived for nematic on the basis of (i) thermal stability and
the (ii) early commencement of mesophase with reference to functions group attached
to middle phenyl ring at ortho or meta position to the central group is as below.

76
D. H. PATEL ET AL.
–
–
II) Group efficiency Fused Phenyl ring > CH3 > Cl
order for nematic :
III) Nematic: CH3 > Cl = Fused Phenyl ring
–
–
Acknowledgments
The authors are thankful to the Head, Applied Chemistry Department, Faculty of Technology and Engi-
neering, The M.S. University of Baroda, Vadodara for their valuable co-operation in the microscopic
work. Authors are also thankful to the authorities and scientists of Central Salts & Marine Chemicals
Research Institute, Bhavnagar for the analytical services of the samples.
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