Mesomorphism dependence on central bridge and tail ended polar group

Article abstract of DOI:10.1080/15421406.2018.1499595

A novel azoester homologous series of laterally and terminally substituted viz, RO–C₆H₄–CH=CH–COO–C₆H₃–(CH₃) –N=N–C₆H₄–Cl is synthesized, characterized and studied with a view to understand and establish the relation between molecular structure and thermotropic liquid crystalline property of its homologs. Series consists of twelve homologs. LC properties commence from very first member of a series and continue up to last homolog as enantiotropic nematic with absence of smectic property even in the monotropic condition. Transition temperatures of homologs were determined by polarizing optical microscopy equipped with a heating stage. The transition curves Cr?N and N?I behaved in normal manner. Thermal stability for nematic is 196.7?C whose mesophaselength ranges from 35°C to 131°C.

Full text of DOI:10.1080/15421406.2018.1499595

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: https://www.tandfonline.com/loi/gmcl20
Mesomorphism dependence on central bridge and
tail ended polar group
Avani R. Ranchchh, Kavita Chavda, Roshani Bhanderi, Suman Patel & U. C.
Bhoya
To cite this article: Avani R. Ranchchh, Kavita Chavda, Roshani Bhanderi, Suman Patel & U. C.
Bhoya (2018) Mesomorphism dependence on central bridge and tail ended polar group, Molecular
Crystals and Liquid Crystals, 665:1, 33-42, DOI: 10.1080/15421406.2018.1499595
To link to this article: https://doi.org/10.1080/15421406.2018.1499595
Published online: 13 Feb 2019.
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MOLECULAR CRYSTALS AND LIQUID CRYSTALS
2018, VOL. 665, NO. 1, 33–42
https://doi.org/10.1080/15421406.2018.1499595
Mesomorphism dependence on central bridge and tail
ended polar group
Avani R. Ranchchh, Kavita Chavda, Roshani Bhanderi, Suman Patel, and
U. C. Bhoya
Department of Chemistry, Saurashtra University, Rajkot, Gujarat, India
KEYWORDS
ABSTRACT
Azoester; liquid crystal;
nematic; smectic;
thermotropic
A novel azoester homologous series of laterally and terminally substi-
tuted viz, RO–C6H4–CH¼CH–COO–C6H3–(CH3) –N¼N–C6H4–Cl is syn-
thesized, characterized and studied with a view to understand and
establish the relation between molecular structure and thermotropic
liquid crystalline property of its homologs. Series consists of twelve
homologs. LC properties commence from very first member of a ser-
ies and continue up to last homolog as enantiotropic nematic with
absence of smectic property even in the monotropic condition.
Transition temperatures of homologs were determined by polarizing
optical microscopy equipped with a heating stage. The transition
curves Cr–N and N–I behaved in normal manner. Thermal stability
for nematic is 196.7 ꢀC whose mesophaselength ranges from 35 ꢀC
to 131ꢀC.
Introduction
Azoester compound of LC variety [1] are applicable to electronic display devices, light
emitting materials and useful as equal as normal dyes including changing their color
with changing frequency of exposed radiation. Azoester are useful ingredients to
pharmaceutical industries [2–8]. However, present investigation is planned to study the
relation between molecular structure and mesomorphism with reference to central
bridge and tail ended variations polar groups [9–13]. Proposed investigation involves
synthesis and characterization of novel substances and the study of their relation of
mesogenic property with molecular structure, excluding their applications through
molecular rigidity and flexibility. Numbers of homologous series are reported till the
date [14–29].
Synthesis
4-n-Alkoxybenzaldehydes were synthesized by refluxing 4-hydroxybenzaldehyde (1 equiv.)
with corresponding n-alkyl bromides (1 equiv.) in the presence of potassium carbonate
(1 equiv.) and acetone as a solvent [30]. The resulting 4n-alkoxybenzaldehydes were
reacted with malonic acid (1.2 equiv.) in the presence of 1–2 drops piperidine as catalyst
CONTACT Avani R. Ranchchh
avaniranchchh@gmail.com
Department of Chemistry, Saurashtra University, Rajkot
360005, Gujarat, India.
ß 2018 Taylor & Francis Group, LLC

34
A. R. RANCHCHH AL.
Scheme 1 Synthetic route to the novel series.
and pyridine as solvent to yield corresponding trans 4-n-alkoxy cinnamic acids (A) [31].
Azo 4-hydroxy-3-methyl phenylazo-40, chlorobenzene[B] was prepared by the usual estab-
lished method of diazotization [32,33] Yield – 70.5%. Coupling of compound A and com-
pound B is done by steglich esterification to yield [34].

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
35
Table 1. Elemental analysis for (1) Pentyloxy (2) Heptyloxy (3) Dodecyloxy
derivatives.
Elements %Found
Elements %Calculated
Sr. No.
Molecular formula
27H27ClN2O3
C29H31ClN2O3
34H41ClN2O3
C
H
O
C
H
O
1
2
3
C
69.98
70.90
72.15
5.83
6.31
7.30
10.36
9.77
8.55
70.05
70.94
72.10
5.88
6.36
7.00
10.37
9.78
9.00
C
The synthetic route to the novel homologous series is mentioned in Scheme 1.
Characterization
Some of members of a novel series as the representative member of a series were char-
1
acterized by elemental analysis (Table 1), Infrared spectroscopy, H NMR spectra and
mass spectroscopy. Microanalysis was performed on EuroEA Elemental Analyzer. IR
1
spectra were recorded on Shimadzu FTIR Model-IR Affinity-1S (MIRacle 10), HNMR
spectra were recorded on Bruker spectrometer using CDCl3as solvent and mass spectra
were recorded on Shimadzu GC–MS Model No.QP-2010.
Spectral data:
1HNMR in ppm for ethyloxy derivative
1.32–1.40(t, 3H CH3 of –OC2H5), 2.15(s, 1H, –CH3–Ph), 3.90–4.0(q, 2Hof –OC2H5),
6.29–6.31(d,1H of –CH¼CH–COO–), 6.90–6.94(m, 2H of first phenyl ring), 7.37(s, 1H
of second phenyl ring meta to –CH3 group), 7.48–7.50(d, 1H of –CH¼CH–COO–),
7.62–7.69(m, 4H, 2H of first phenyl ring nearer to –CH¼CH–COO– and 2H of third
phenyl ring nearer to –Cl), 7.78–7.83(m, 2H of second phenyl ring nearer to –N¼N–
group), 7.90–7.94(m, 2H of third phenyl ring nearer to –N¼N– group).The NMR data
are consistent with the molecular structure.
1H NMR in ppm for propyloxy derivative
1.35–1.40(t, 3H CH3 of –OC3H7), 1.74 (M, 2H of –CH3–CH2–CH2–O–),2.17(s, 1H,
–CH3–Ph), 3.91–4.1(t, 2Hof –OC3H7), 6.24–6.31(d,1H of –CH¼CH–COO–),
6.80–6.84(m, 2H of first phenyl ring), 7.33(s, 1H of second phenyl ring meta to –CH3
group), 7.50–7.56(d, 1H of –CH¼CH–COO–), 7.60–7.65(m, 4H, 2H of first phenyl ring
nearer to –CH¼CH–COO– and 2H of third phenyl ring nearer to –Cl), 7.79–7.83(m,
2H of second phenyl ring nearer to –N¼N– group), 7.95–7.98(m, 2H of third phenyl
ring nearer to –N¼N– group). The NMR data are consistent with the molecu-
lar structure.
IR in cm21 for hexyloxy derivative
3037 (¼C–H str. of phenyl nucleus), 2922 & 2850 (C–H str. of (–CH2–)n group of
–OC6H13), 1724 (C¼O str. of carbonyl carbon of ester group), 1625 (C¼O str. of a,b
unsaturated ketone), 1589 (C¼C str. of alkene), 1510 & 1477 (C¼C str. of aromatic

36
A. R. RANCHCHH AL.
Table 2. Transition temperatures in ꢀC.
Compound No.
n-alkyl chain CnH2nþ1 (n)
Sm
N
Isotropic
1
2
3
4
5
6
7
8
C1
C2
C3
C4
C5
C6
C7
C8
C10
C12
C14
C16
–
–
–
–
–
–
–
–
–
–
–
–
172
160
131
91
115
109
129
111
102
104
108
114
274
263
205
222
185
201
180
183
175
167
157
149
9
10
11
12
Sm¼Smectic, N¼Nematic.
ring), 1423 (Ar–N¼N–Ar str.), 1394 (C–H bending of alkene disubstituted), 1307 (C–O
str. of ether linkage), 1128 (C–O str. of ester group), 972 & 891 (C–H bending of
alkene). The IR data are consistent with the molecular structure.
IR in cm21 for hepyloxy derivative
2964 (¼C–H str. of phenyl nucleus), 2922 & 2850 (C–H str. of (–CH2–)n group of
–OC6H13), 1722 (C¼O str. of carbonyl carbon of ester group), 1633 (C¼O str. of a,b
unsaturated ketone), 1598 (C¼C str. of alkene), 1510 & 1477 (C¼C str. of aromatic
ring), 1421 (Ar–N¼N–Ar str.), 1394 (C–H bending of alkene disubstituted), 1307 (C–O
str. of ether linkage), 1132 (C–O str. of ester group), 974 & 891 (C–H bending of
alkene). The IR data are consistent with the molecular structure.
Mass spectra of methyloxy derivative
m/z (rel.int%): 406 (3%, Mþ), 246 (21%, ester linkage break), 161 (100%), 107 (35%).
Results and discussion
Present azoester novel series is derived from trans n-alkoxy cinnamic acids and an azodye
4-hydroxy 3-methyl phenyl azo-40-chlorobenzene. Azodye is nonliquid crystal, but on
condensation with dimeric trans vinyl carboxylic acid yielded LC homologs by esterifica-
tion process with alternations of transition temperatures up to C8 derivative and then
alternation disappears. Esterification process breaks the hydrogen bonding between two
molecules of n-alkoxy aromatic cinnamic acid and lowers the transition temperatures of
final azoester homologs, including showing up to odd even effect. Transition temperatures
as determined by an optical polarizing optical microscopy (POM) (Table 2) were plotted
versus the number of carbon atom present in n-alkyl chain ‘R’ of –OR terminal (Fig. 1).
Transition curves Cr–N and N–I are obtained by linking like or related transition points
as depicted in Fig. 1. Cr–N transition curve follows zigzag path of rising and falling with
overall descending tendency and behaved in normal manner. N–I transition curve adopted
continuous descending tendency as series is ascended with exhibition of odd–even effect

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
37
Figure 1 Phase behaviors of homologous series.
up to C8 homolog, beyond which odd–even effect disappears for higher homologs of lon-
ger n-alkyl chain ‘R’ of –OR groups. Thus, N–I transition curve behaved in normal
expected manner. Thermal, analytical and spectral data confirmed molecular structures of
novel homologs. Thermal stability for nematic is 196.7 ꢀC and mesophaselength ranges
maximum 131.0 ꢀC to minimum 35.0 ꢀC at the C4 and C16 homolog respectively. Thus,
novel series is high melting series, with sufficient degree of mesomorphism.
Esterification and diazotization process introduces conjugated double bond between
two carbon atoms of vinyl group as well as double bond between two nitrogen atom
which strengthens the molecular rigidity and the presence of ortho substituted lateral
methyl group and para substituted polar/polarizable –OR, –Cl terminal groups raises
flexibility of the molecules of each homolog to such an extent that, suitable magnitudes
of anisotropic forces of intermolecular end to end attractions only, which maintains the
statically parallel orientational order of molecules under floating condition on the float-
ing surface. Thus, LC state of all the homologs of series facilitated the exhibition of
nematogenic mesophase formation as observed from the heating top of POM with con-
siderable ranges of temperature. However, smectogenic mesophase facilitation is hin-
dered due to insufficient or inadequate suitable magnitudes of anisotropic forces of
lateral to terminal intermolecular cohesion to adopt layered structure of molecules even
in the monotropic condition. Thus, smectogenic mesophase formation is missing to
appear. The exhibition of odd–even effect observed up to C8 homolog from very first
member of present novel series is attributed to the gradual change in molecular rigidity
and flexibility of the homolog molecules as depended upon their size, permanent dipole
moment across the long molecular axis, aromaticity, molecular polarity and polarizibil-
ity, length to breadth attraction etc. Thus, sequential addition of methylene unit play an
important role in alternation of transition temperatures causing odd–even effect in a

38
A. R. RANCHCHH AL.
Figure 2 Homologous series for comparative study.
transition curve (N–I) of a series. However, disappearance of odd–even effect from and
beyond C8 homolog for higher homologs of longer n-alkyl chain ‘R’ is attributed to the
coiling or bending or flexing or coupling of n-alkyl chain with the core structure of a
molecule. Thus unusual status of n-alkyl chain ‘R’ of –OR group causes such effects.
The relatively wider range of mesophaselength of nematic mesophase stabilization is
attributed to the favored magnitude of molecular rigidity and flexibility from homolog
to homolog in the same series, related to add –CH2– unit or –CH2–CH2– unit. Some
LC properties of a present novel Series 1 are compound with following structurally
analogous and similar homologous Series X [35] and Y [18] as under (Fig. 2).
In above homologous series shown in Fig. 2 for comparative study,
All the series, X and Y are identically same with respect to three phenyl rings,
–N¼N– central bridge linking middle and third phenyl ring as a one of the central
group partly contributing to the total molecular rigidity. Ortho substituted lateral –CH3
group which substituted identically on same position and same middle phenyl ring con-
tributing partly to the total molecular flexibility. However, they differ with respect to
first central bridge linking first and middle phenyl ring with present (Series 1) and
absence (Series X and Y) of –CH¼CH– unit contributing partly to the molecular rigid-
ity and changing homolog to homolog in same series at the left terminal –OR and fixed
tail ended terminal –Cl (Series 1 and X) and –OCH3 which contributes partly to the
total molecular flexibility from series to series for the same homolog. Thus, molecular
rigidity and flexibilities of homologs in the same series or from series to series for the
same homolog vary with more or less extent, which reflects in the variations in the
thermal resistivity toward exposed thermal vibrations causing difference in the meso-
genic properties or LC behaviors from homolog to homolog in the same series or for
the same homolog from series to series. Following Table 3 represent some variations of
LC properties for the same homolog from series to Series 1,
under comparison.
X and Y
Above data indicate that,

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
39
Table 3. Relative average thermal stabilities in ꢀC.
Series !
Series-1
–CH¼CH–COO– and –Cl
Series-X
–COO– and –Cl
Series-Y
–COO– and –OCH3
Smectic–Nematic
Or
106.6
(C12–C16
118.6
(C12–C16)
–
)
Smectic–Isotropic
Commencement of Smectic phase
Nematic–Isotropic
–
196.7
(C1–C16
C1
35.0–131.0
C16 C4
C10
161.5
(C1–C16
C1
31.0–74.0
C1 C4
C12
183.0
(C1–C12)
C1
23.0–137.0
C14 C2
)
)
Commencement of nematic phase
Upper and lower total mesophase lengths
In ꢀC
ꢁ
Present novel Series 1, is only nematogenic without exhibition of smectogenic
property whereas, Series X and Y chosen for comparative study are predomin-
ately nematogenic and partly smectogenic.
ꢁ
ꢁ
Mesomorphism commence from very first member of the Series 1, X and Y
under comparison.
Thermal stability for nematic is the lowest for Series X (161.5) lower for Series
Y(183.0) and the highest for Series 1(196.7), whereas, thermal stability for smec-
tic follows decreasing order from Series Y to X to 1.
ꢁ
ꢁ
Smectogenic character commences late from C10 homolog of Series X and from
C12 homolog of Series Y whereas, it does not commence even from last homolog
of a present novel Series 1.
Lower total mesophaselengths are in decreasing order from Series 1 to Series X
to Series Y whereas; upper total mesophaselengths are in increasing order from
Series X to Series 1 to Series Y under comparative manner.
Homologous Series 1 of present investigation and the analogous homologous Series X
and Y are almost identical in all respect except the first ester central bridge which is
–CH¼CH–COO– in case of Series 1 and carboxylate –COO– in case of Series X and Y
under comparative study. These are closely similar to each other except differing by
–CH¼CH– unit. Therefore, vinyl carboxylate –CH¼CH–COO– has greater or longer
molecular length and the permanent dipole moment across the long molecular axis, as
compared to –COO– central bridge of series X and Y. Therefore, Series1 causes more
non-coplanarity due to a twist obtained as the oxygen atoms of the vinyl carboxyl group
bump into the non-bonded adjacent hydrogen atom of the aromatic ring. On account
of these differences the thermal stability of Series1 is missing or the smectic–isotropic
thermal stabilities are in a general way lower than the corresponding smectic thermal
stabilities of Series X and Y. Thus, exhibition of only nematic character of Series 1 is
facilitated or smectic mesophase formation is hindered as compared to Series X and Y.
Early or late commencement of smectic phase is related with the extent of molecular
non-coplanarity of a molecule. However, though there is no much difference except tail
ended except tail ended terminal groups between the Series X and Y but much differs
in case of Series 1. The commencement of nematic phase are equally facilitated from
very first member of the each series, because, the suitable magnitude of anisotropic
forces of intermolecular end to end attractions as a consequence of favorable molecular
rigidity and flexibility which causes required environmental condition to facilitate

40
A. R. RANCHCHH AL.
nematogenic mesophase formation. Thermal stabilities and the degree of mesomor-
phism for smectic or/and nematic mesophases depended upon the internal energy (DH)
stored by individual molecules of each homolog of each series or substance, as a conse-
quence of combined effect of molecular rigidity and/or flexibility and their thermal
resistivity toward exposed thermal vibrations balance the suitable or unsuitable magni-
tudes of anisotropic or isotropic forces of intermolecular cohesion with or without
adoption of LC state or none LC state with different temperature range or at isotropic
temperature. Thus, variations in the thermal stabilities for mesophase or the sequential
status of thermal stabilities and the extent of total mesophaselengths for smectic and/or
nematic depended upon changing molecular rigidity and/or flexibilities varied through
numerical values of their DH for the same homolog from series to series or from homo-
log to homolog in the same series.
Thus, present novel series is entirely nematogenic whose mesophaselength is consist-
ently wide and considered as high melting azoester series.
Conclusions
ꢁ
ꢁ
Novel azoester series is entirely nematogenic with absence of smecto-
genic character.
Group efficiency order derived for smectic and nematic on the basis of [a] ther-
mal stabilities [b] early commencement of mesophase and [c] total mesophase-
lengths are as under.
(a) Smectic
SeriesY > Series X > Series 1
Nematic
Series 1 > Series Y > Series X
(b) Smectic
Series X > Series Y > Series 1
Nematic
Series 1¼Series X¼Series Y
(c) Mesophaselength
Upper
Series Y > Series 1 > Series X
Lower
Series 1 > Series X > Series Y
ꢁ
ꢁ
ꢁ
ꢁ
Molecular structure is very sensitive and susceptible to phenomena of mesomor-
phism and the degree of mesomorphism.
Present study may be useful for the sensing deviation to be operated to control
thermo processes 91.0 ꢀC–274.0 ꢀC.
Favorable magnitude of molecular rigidity and flexibility operates mesomorphic
tendancy of a substance and its magnitude of stabilization.
Present investigation supports and raises the credibility to the conclusions
drawn earlier.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
41
Acknowledgments
Authors acknowledge thanks to the Department of Chemistry (DST-FIST Funded & UGC-SAP
Sponsored), Saurashtra University, Rajkot, for research work. Authors are also thankful to
Dr. A.V. Doshi, Ex. Principal M.V.M. Science and Home Science College Rajkot, for his valuable
co-operation during present investigation as and when needed. Also thanks to the National
Facility for Drug Discovery through New Chemical Entities (NCE’s) for analysis of samples.
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