Mesomorphism dependence on molecular rigidity and flexibility

Article abstract of DOI:10.1080/15421406.2018.1499631

A novel homologous series of liquid crystalline properties is synthesized and studied with a view to understand the effect of molecular structure on its thermotropic properties. Novel homologous series consisted of thirteen homologs. All the homologs are enantiotropically smectogenic with absence of nematic property even in the monotropic condition. Textures of the homologs as observed through an optical hot stage polarizing microscopy. The mesophase temperature range vary minimum from 6.0°C to a maximum 37.0°C at the methyloxy and propyloxy derivatives of a series respectively with its thermal stability (Sm–I) 127.0°C. Transition curves of a phase diagram behaved in normal manner. Odd–even effect is observed for Sm–I transition curve.

Full text of DOI:10.1080/15421406.2018.1499631

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: https://www.tandfonline.com/loi/gmcl20
Mesomorphism dependence on molecular rigidity
and flexibility
Avani R. Ranchchh & U. C. Bhoya
To cite this article: Avani R. Ranchchh & U. C. Bhoya (2018) Mesomorphism dependence
on molecular rigidity and flexibility, Molecular Crystals and Liquid Crystals, 665:1, 43-51, DOI:
10.1080/15421406.2018.1499631
To link to this article: https://doi.org/10.1080/15421406.2018.1499631
Published online: 13 Feb 2019.
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MOLECULAR CRYSTALS AND LIQUID CRYSTALS
2018, VOL. 665, NO. 1, 43–51
https://doi.org/10.1080/15421406.2018.1499631
Mesomorphism dependence on molecular rigidity and
flexibility
Avani R. Ranchchh and U. C. Bhoya
Department of Chemistry, Saurashtra University, Rajkot, Gujarat, India
KEYWORDS
ABSTRACT
Enantiotropy; mesomor-
phism; nematic; smectic
A novel homologous series of liquid crystalline properties is synthe-
sized and studied with a view to understand the effect of molecular
structure on its thermotropic properties. Novel homologous series
consisted of thirteen homologs. All the homologs are enantiotropi-
cally smectogenic with absence of nematic property even in the
monotropic condition. Textures of the homologs as observed
through an optical hot stage polarizing microscopy. The mesophase
temperature range vary minimum from 6.0ꢀC to a maximum 37.0ꢀC
at the methyloxy and propyloxy derivatives of a series respectively
with its thermal stability (Sm–I) 127.0ꢀC. Transition curves of a phase
diagram behaved in normal manner. Odd–even effect is observed
for Sm–I transition curve.
Introduction
Liquid crystalline properties of thermotropic LC [1] substances is studied by chemists,
physicists, biologists, engineers, technologists, pharmacist, etc. with different aims,
objects, and views at different angles [2,3].The present investigation is planned with a
view to understand the effects of molecular structure [4–8] on thermotropic LC proper-
ties as a consequence of molecular structural geometry which results into more or less
suitable magnitudes of anisotropic forces of intermolecular end to end or/and lateral
attractions to cause or induce LC property for definite range of temperature. The
molecular rigidity and/or molecular flexibility change with molecular structure [9–12].
Therefore proposed research work is planned to synthesize novel LC materials through
homologous series with different geometrical shape, size, polarity, polarizability, disper-
sion forces, dipole moment, etc. with proper imagination. Numbers of homologous ser-
ies with different molecular rigidity and/or flexibility are reported till the date [13–19].
LC materials including dyes are useful as optical imaging, display devices, light emitting
diodes, semi-conductor material, anisotropic network transporting materials, light
emitting devices etc. [20,21]. Azoester LC material is used and applied equal as
dyes [22–25].
CONTACT Avani R. Ranchchh
avaniranchchh@gmail.com; U. C. Bhoya
drucbhoya@gmail.com
Department of
Chemistry, Saurashtra University, Rajkot 360005, Gujarat, India.
ß 2018 Informa UK Limited, trading as Taylor & Francis Group

44
A. R. RANCHCHH AND U. C. BHOYA
Experimental
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 [26]. The resulting 4-n-alkoxybenzalde-
hydes were reacted with malonic acid (1.2 equiv.) in the presence of 1–2 drops piperi-
dine as catalyst and pyridine as solvent to yield corresponding trans-4-n-alkoxy
cinnamic acids (A) [27]. 4-Hydroxy phenylazo-20,60-dimethyl benzene (B) was prepared
by the method of Vogel’s textbook of practical organic chemistry [28,29]. Coupling of
compound (A) and (B) is done by steglich esterification to yield 4(40-n- Alkoxy
Cinnamayloxy) phenyl azo - 2",6" dimethyl benzenes [30].
The synthetic route to the novel homologous series of azo cinnmoyl ester derivatives
is under 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, HNMR spectra and
mass spectroscopy. Microanalysis was performed on EuroEA Elemental Analyzer. IR
1
spectra were recorded on Shimadzu FTIR Model -IRAffinity-1S (MIRacle 10), HNMR
spectra were recorded on Bruker spectrometer using CDCl3 as a solvent and mass spec-
tra were recorded on Shimadzu GC–MS Model No.QP-2010. The liquid crystal behavior
and the type of textures were determined by miscibility method on microscopic
observations.
Analytical data
Spectral data
1H NMR in ppm for methyloxy derivative
1.279 (s, 6H, –CH3–Ph), 2.373(S, 3H –OCH3), 6.535–6357(d, 1H, CH¼CH–COO–),
6.970–6.992(dd, 2H nearer to alkoxy group of first phenyl ring), 7.167–7.160 (m, 2H of
third phenyl ring nearer to the azo group), 7.584–7.606(dd, 2H of second phenyl ring
nearer to ester group), 7.880–7.919(d, 1H, –CH¼CH–COO–), 7.892–8.004(2H of second
phenyl ring nearer to azo group). The NMR data are reliable with the molecu-
lar structure.
1H NMR in ppm for ethyloxy derivative
1.459–1.494(t, 3H –CH3 of –OC2H5 group), 2.374(s, 6H, –CH3–Ph), 4.092–4.144 (t, 2H
of CH3–CH2–O–Ph), 6.527–6.567(d, 1H, CH¼CH–COO–), 6.952–6.974(dd, 2H nearer
to alkoxy group of first phenyl ring), 7.149–7.179(m, 2H of third phenyl ring and 1H of
–CH¼CH–COO–), 7.352–7.374(dd, 2H of third phenyl ring nearer to azo
group),7.569–7.591(dd, 2H of second phenyl ring nearer to ester grop), 7.875(1H, d of

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
45
Scheme 1 Synthetic route to the novel series.
third phenyl ring),7.982–8.004 (dd, 2H of second phenyl ring nearer to azo group). The
NMR data are reliable with the molecular structure.

46
A. R. RANCHCHH AND U. C. BHOYA
Table 1. Elemental analysis for (1) Propyloxy (2) Octyloxy (3) Dodecyloxy
derivatives.
Elements %Found
Elements %Calculated
Sr. No.
Molecular formula
26H26N2O3
C31H36 N2O3
35H44N2O3
C
H
O
C
H
O
1
2
3
C
75.36
76.82
77.76
6.31
7.45
8.22
11.56
9.91
8.86
75.34
76.83
77.74
6.32
7.49
8.20
11.58
9.90
8.88
C
IR in cmꢁ1 for tetradecyloxy derivative
3635 (polymethylene (–CH2–)n group of OC14H29), 745, 771 (para & ortho substituted
phenyl ring), 835 (1,6 disubstituted phenyl ring), 1180 (>C¼O group), 1250 (–COO–
ester group), 1610 (–N¼N– Str.), 1753 (>CO, Str. of –COO group), 2840, 2926 (aro-
matic phenyl ring). The IR data are consistent with the molecular structure.
IR in cmꢁ1 for hexadecyloxy derivative
3645 (polymethylene (CH2–)n group of OC16H33), 740, 778 (para & ortho substituted
phenyl ring), 835 (1,6 disubstituted phenyl ring), 1186 (>C¼O group), 1255 (–COO–
ester group), 1610 (–N¼N– Str.), 1740 (>CO, Str. of –COO group), 2830, 2920
(Aromatic phenyl ring). The IR data are consistent with the molecular structure.
Mass spectra of heptyloxy derivative
m/z (rel.int%): 470(M)þ, 261, 209, 337, 133, 115, 356.
Mass spectra of butyloxy derivative
m/z (rel.int%): 428(M)þ, 219, 209, 73, 356, 295, 133.
Result and discussion
Azo dye 4-hydroxy phenyl azo 20,60 dimethyl benzene (B) is nonmesomorphic; the link-
ing of 4-n-alkoxy cinnamic acid (A) yielded smectogenic mesomorphs throughout a
novel series. Series consists of thirteen enantiotropically smectogenic mesomorphs with
none of the homologs exhibiting either nematic or nonmesogenic character. The transi-
tion temperatures (Table 2) as determined by an optical polarizing microscopy (POM)
equipped with a heating stage are plotted versus the number of carbon atoms present in
n-alkyl chain of left n-alkoxy group, and a phase diagram obtained by linking like or
related points is represented in Fig. 1. A Cr–Sm transition curve adopted zigzag path of
rising and falling with overall descending tendency in normal manner. A Sm–I transi-
tion curve continuously descended as series is ascended with exhibition of odd–even
effect and alternation of transition temperatures. A transition curve (Sm–I) for odd
members, occupied lower position than even members Sm–I transition curve.
Mesomorphic properties, undergo variation from homolog to homolog in the thermo-
tropic novel series, whose, mesogenic temperature range between 6.0 ꢀC and 37.0 ꢀC.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
47
Table 2. Transition temperatures in oC of Series 1, Series X and Series Y.
Series 1
Series X
Series Y
N
Compound No. n-alkyl chain CnH2nþ1 (n) Sm
N
Isotropic Sm
N
Isotropic Sm
Isotropic
1
2
3
4
5
6
7
8
C1
C2
C3
C4
C5
C6
C7
C8
C10
C12
C14
C16
C18
139
137
101
118
120
123
102
100
87
95
89
86
84
–
–
–
–
–
–
–
–
–
–
–
–
–
145
148
138
144
140
139
138
132
119
110
104
100
95
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
104.0
111.2
102.0
100.0
82.3
83.2
78.3
79.7
76.6
74.0
71.3
130.0
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
149.0
168.0
117.0
110.0
144.0
184.0
109.0
94.0
(147.0)
–
(54.0)
(122.0)
(70.0)
62.0
(85.0)
88.0
88.0
93.0
104.0
–
(71.9)
80.6
51.5
60.3
49.8
57.9
59.7
86.9
–
9
96.0
10
11
12
13
107.0
119.0
130.0
–
Sm¼Smectic, N¼Nematic, () indicate Monotropy.
Figure 1. Phase behavior of series.
Hence, a novel series is entirely enantiotropically smectogenic with absence of nematic
property and middle ordered melting type.
The exhibition of mesomorphic property C1–C18 homologs are attributed to the suit-
able magnitudes of anisotropic forces of intermolecular attractions caused by favorable
dipole–dipole interactions, molecular polarity and polarizability, molecular length, dis-
persion forces etc. as a consequence of molecular rigidity and flexibility, depending
upon the molecular geometry i.e., length, width, size, type of functional group or groups
and their ortho, meta and/or para substitutional position or position or positions, the
linearity and strength of central bridge or bridges, the number of phenyl ring or rings
their bond polarity etc. which govern the suitable magnitudes of anisotropic forces of
intermolecular attractions. All the molecules of all the members of a series (C1–C18) are
offering lamellar packing of molecules and disaligned at an angle 90ꢀ or perpendicular
to the plane of a floating surface to show focal conic fan shaped or batonate type

48
A. R. RANCHCHH AND U. C. BHOYA
Figure 2. Structurally similar series.
smectic-A or -C type mesophase. However, on cooling the same below isotropic tem-
perature, the smectogenic mesophase reappears excluding possibility of monotropic
nematic mesophase appearance or formation. The exhibition of odd–even effect in
Sm–I transition curve is attributed to the presence of even and odd number of methy-
lene (–CH2–) unit or units in n-alkyl chain of left n-alkoxy terminal end group.
Alternations of transition temperature of homologs are due to odd an even number of
carbon atoms, which are present in n-alkoxy group. Disappearance of odd–even effect
beyond C5 homolog for higher homologs of longer n-alkyl chain is attributed to the
coiling or bending or flexing or coupling of n-alkyl chain with a major axis of a core
structure of molecules. Thus, uncertainty in the status of a molecule causes absence of
odd–even effect for higher homologs of longer n-alkyl chains. The normal behaviors of
Sm–I transition curve is attributed to the depression of isotropic transition temperatures
from C1 to C18 homolog to homolog in present novel azoester series.
The mesogenic properties of presently investigated novel series 1 is compared with
structurally similar known analogs series X [31] and Y [32] as represented in following
Fig. 2.
The present novel non isomeric homologous Series 1 is identical with isomeric Series
X and Y with respect to three phenyl rings, left n-alkoxy terminal and substituted two
methyl groups and a central group –N¼N– linking middle and third phenyl rings.
However, it differs from Series X and Series Y commonly with respect to a first central
group –CH¼CH– COO– or –COO– linking first and middle phenyl rings. Moreover,
two methyl groups of Series 1 and Series X are identically substituted on third phenyl
rings at the same position (2", 6"). But in case of series Y the two methyl groups are
nonidentically substituted on middle and third phenyl rings, including with differing
first central bridge linking first and middle phenyl rings. Thus, the differences of varia-
tions in thermotropic mesogenic properties among the above Series 1, X, and Y arises

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
49
or depend upon the above mentions differing features of each series under comparative
study for the same homolog from series to series and homolog to homolog in the same
series. Therefore, the differing features among the structurally similar series causes dif-
ference in molecular rigidity and/or flexibility and reflect in mesogenic properties like
thermal stability, commencement of mesophase or mesophases etc. Following Table 2
represents transition temperatures of Series 1, Series X, and Series Y.
It is clear from Table 2 that,
ꢂ
Azoester novel homologous Series 1 of present investigation is entirely smecto-
genic without exhibition of nematic property, whereas the homologous Series X
and Y chosen for comparative study are only nematogenic (partly) without
exhibition of smectic property.
ꢂ
ꢂ
ꢂ
Smectic property commences from very first member C1 and ended to last mem-
ber C18 in case of present Series 1, but a nematogenic mesophase does not com-
mences till the last member of a Series 1.
Nematic mesophase commences from C5 to C3 for Series X and Y, respectively,
with absence of smectic mesophase till the last member of series under pre-
sent comparison.
Thermal stability for the exhibition of mesophase (irrespective of its type)
decreases from Series 1 to series Y to Series X.
The exhibition of only smectic property without exhibition of nematic property of
present Series 1 as compared to series X and Y which exhibited only nematic property
without exhibition of smectic property is attributed to the stronger molecular rigidity in
combination with suitable or unsuitable magnitudes of molecular flexibility occurred by
presence of two methyl group and their different positions and different central bridges
–CH¼CH–COO– or –COO– linking first and middle phenyl rings. The commonly pre-
sent –N¼N– linear central bridge between middle and third phenyl rings for Series 1,
X, and Y are almost equi effective, but a vinyl carboxy –CH¼CH–COO– central group
in case of a present homologous Series 1 is longer and stronger than –COO– carboxy
central bridge in case of Series X and Y present on an identical position under compari-
son causes more noncoplanarity due to a twist obtained, as the oxygen atoms of vinyl
carboxy group bump into non bonded adjacent hydrogen atoms of the aromatic rings.
On account of this, a lamellar arrangement and disalignment of molecules at an angle
perpendicular to the floating surface comparatively more favored than Series X and Y.
Thus, molecules of Series 1 are more rigid than Series X and Y. Therefore, the differ-
ence in rigidity of molecules simultaneously in combination with their individual flexi-
bility causes variations in the suitable or unsuitable magnitudes of anisotropic forces of
intermolecular end to end or lateral attraction and varying type or types of only smectic
or only nematic or smectic plus nematic mesophases, different values (higher to lower)
of thermal stability, higher to lower range of mesophase temperature range, early or late
commencement of smectic or/and nematic mesophase from series to series for the same
homolog and from homolog to homolog in the same series are observed. Thus, the
more or less molecular rigidity or planarity or the extent of noncoplanarity of molecules
causes difference in degrees of mesomorphism etc. LC properties.

50
A. R. RANCHCHH AND U. C. BHOYA
Conclusions
ꢂ
ꢂ
Novel homologous Series 1 of present investigation is enantiotropically entirely
smectogenic without exhibition of nematic property, whose degree of mesomor-
phism vary between 6.0 ꢀC and 37.0 ꢀC and it is of middle ordered melting type.
The group efficiency order derived on the basis of thermal stability and early
commencement of mesophase are as under.
Smectic
Series 1 > Series X¼Series Y
Nematic
Series Y > Series X > Series 1
ꢂ
ꢂ
Odd–even effect is observed from Sm–I transition curve.
Mesomorphism and the degree of mesomorphism are very sensitive and suscep-
tible to corresponding molecular structure, as consequence of molecular rigidity
and/or flexibility.
ꢂ
Present investigation supports the early views about mesomorphism and raises
the credibility to the conclusions drawn earlier.
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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