Molecular flexibility dependence on mesogenic behaviors of isomeric and nonisomeric series

Article abstract of DOI:10.1080/15421406.2016.1263119

A novel homologous series containing vinyl ester and azomethane central bridges and n-alkoxy as well as 3″,4″-dimethyl groups as flexible terminal/lateral groups viz. RO?C₆H₄?CH = CH?COO?C₆H₄?N?CH?C₆H₃-(CH₃)₂ have been synthesized and studied with a view to establishing the relation between molecular structure and thermotropic liquid crystal (LC) properties with reference to molecular flexibility within the series. The series consists of twelve homologues (C₁ to C₁₆). C₆ and C₇ homologues are smectogenic plus nematogenic and C₈ to C₁₆ homologues are only smectogenic, and the rest of the homologues (C₁ to C₅) are nonmesomorphic. Transition temperatures and the textures of the homologues were determined using an optical polarizing microscope equipped with a heating stage (POM). The textures of a nematic phase are threaded or Schlieren and that of a smectic phase are of the A or C type. Analytical, thermal and spectral data support the molecular structures. Smectic and nematic thermal stabilities are 116.85°C and 147.5°C, respectively. Whose Sm?N/I and N-I mesophase lengths are varied between 15°C to 21°C and 25°C to 62°C, respectively. The novel compounds are compared with structurally-similar series.

Full text of DOI:10.1080/15421406.2016.1263119

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Molecular flexibility dependence on mesogenic
behaviors of isomeric and nonisomeric series
P. K. Rakhasia, Avani R. Ranchchh & U. C. Bhoya
To cite this article: P. K. Rakhasia, Avani R. Ranchchh & U. C. Bhoya (2017) Molecular flexibility
dependence on mesogenic behaviors of isomeric and nonisomeric series, Molecular Crystals and
Liquid Crystals, 643:1, 159-167, DOI: 10.1080/15421406.2016.1263119
To link to this article: http://dx.doi.org/10.1080/15421406.2016.1263119
Published online: 23 Feb 2017.
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Date: 23 February 2017, At: 21:39

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
http://dx.doi.org/./..
Molecular flexibility dependence on mesogenic behaviors of
isomeric and nonisomeric series
P. K. Rakhasia, Avani R. Ranchchh, and U. C. Bhoya
Department of Chemistry, Saurashtra University, Rajkot, Gujarat, India
ABSTRACT
KEYWORDS
Enantiotropy;
mesomorphism; nematic;
smectic
A novel homologous series containing vinyl ester and azomethane cen-
tral bridges and n-alkoxy as well as 3ꢀꢀ,4ꢀꢀ-dimethyl groups as flexible ter-
–
–
–
–
– =
–
minal/lateral groups viz. RO C6H4 CH = CH COO C6H4 N CH C6H3-
(CH3)2 have been synthesized and studied with a view to establish-
ing the relation between molecular structure and thermotropic liquid
crystal (LC) properties with reference to molecular flexibility within the
series. The series consists of twelve homologues (C1 to C16). C6 and C7
homologues are smectogenic plus nematogenic and C8 to C16 homo-
logues are only smectogenic, and the rest of the homologues (C1 to
C5) are nonmesomorphic. Transition temperatures and the textures of
the homologues were determined using an optical polarizing micro-
scope equipped with a heating stage (POM). The textures of a nematic
phase are threaded or Schlieren and that of a smectic phase are of the
A or C type. Analytical, thermal and spectral data support the molecu-
lar structures. Smectic and nematic thermal stabilities are 116.85°C and
–
147.5°C, respectively. Whose Sm N/I and N-I mesophase lengths are var-
ied between 15°C to 21°C and 25°C to 62°C, respectively. The novel com-
pounds are compared with structurally-similar series.
Introduction
A physically distinct state of a matter between the two well-known states of crystalline solid
and isotropic liquid is called the liquid crystalline (LC) state [1] and it has proved very impor-
tant [2–5] in thermotropic and lyotropic environments. A wide range of scientific, technolog-
ical and industrial researchers are working on the unique state of matter of liquid crystals with
different, but complementary aims and objectives. One of the very important and basic aims
and objectives is the study of the effects of molecular structure on LC properties, which is the
focus of the present investigation [6–10]. The aim of study was to synthesis novel LC sub-
–
stances through a homologous series containing three phenyl rings, two central bridges, CH
–
–
– =
–
= CH COO and N CH with flexible terminal and lateral groups. Then to characterize
these novel materials through thermal, analytical and spectral studies, and evaluate and dis-
cuss the thermometric data in terms of molecular rigidity and flexibility [11–14] in compari-
– = –
son with known structurally-similar materials. Several ester homologues series with N N ,
–
= – –
–
–
–
–
–
CH N , CO CH = CH or CH = CH CO groups have been reported to date [15–21].
CONTACT U. C. Bhoya
India
drucbhoya@gmail.com
DepartmentofChemistry, SaurashtraUniversity, Rajkot, Gujarat,
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
©  Taylor & Francis Group, LLC

160
P. K. RAKHASIA ET AL.
Scheme . Synthetic route to the novel series.
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 [22]. The resulting 4-n-alkoxybenzaldehydes were reacted
with malonic acid (1.2 equiv.) in the presence of 1–2 drops piperidine as catalyst and
pyridine as solvent to yield corresponding trans-4-n-alkoxy cinnamic acids (A) [23]. 4-
Hydroxy amino benzene 3ꢀ, 4ꢀ-dimethyl benzal(B) was prepared by an established method
[24] M.P.170–172°C, Yield- 76.5%. Coupling of compound A and B was carried out by a
Steglich esterification to yield 4-(4ꢀ-n-alkoxy cinnamoyloxy) amino benzene 3ꢀꢀ, 4ꢀꢀ-dimethyl
benzal [25].
The synthetic route to the novel homologous series of Schiff’s base cinnmoyl esters is shown
in Scheme 1.
Characterization
Some members of the novel series were characterized by elemental analysis (Table 1), infrared
1
spectroscopy, H NMR spectra and mass spectroscopy. Microanalysis was performed on

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
161
Table . Elemental analysis for () Propyloxy () Octyloxy () Dodecyloxy derivatives.
Elements %Found
H
Elements %Calculated
H
Sr. No.
Molecular formula
C
O
C
O



CHNO
CHNO
HNO
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
C
EuroEA Elemental Analyzer. IR spectra were recorded on Shimadzu FTIR Model -IRAffinity-
1S (MIRacle 10), 1H NMR spectra were recorded on Bruker spectrometer using CDCl3 as a
solvent and mass spectra were recorded on a Shimadzu GC-MS Model No.QP-2010. The liq-
uid crystal behavior and the type of textures were determined by a miscibility method on
microscopic observations.
Analytical data
Spectral data
H NMR in ppm for ethyloxy derivative
–
–
1.35–1.38 (t,3H, CH3 of OC2H5 group), 3.98–4.03 (m,2H,CH3-CH2-O), 7.44–7.48 (d,1H,
–
=
–
–
–
=
–
–
–
= –
CH CH COO ), 6.41–6.45 (d,1H, CH CH COO ), 8.27 (s,1H, CH N ), 3.26 (s, 6H,
–
two CH3 groups attached to benzene), 6.84–6.88 & 7.87–7.90 (4H, phenyl ring containing
ethoxy chain), 7.46–7.48 & 7.76–7.80 (4H, middle phenyl ring), 7.02–7.80 (3H, phenyl ring
containing two methyl ring). The NMR data are reliable with the molecular structure.
H NMR in ppm for pentyloxy derivative
–
–
– –
0.85–0.88 (t,3H, CH3 of –OC5H11 group), 1.29–1.32 (m,2H,CH3-CH2 (CH2)3 O )1.37–
–
– – – –
–
–
1.39 (p,2H,-CH2-CH2 CH2 O ), 1.70–1.75 (p,2H,-CH2-CH2 O ), 3.91–3.94 (t,2H, CH2
O ), 7.45–7.47 (d,1H, CH CH COO ), 6.40–6.44 (d,1H, CH CH COO ), 8.27 (s,1H,
CH N ), 2.19–2.21, (s, 6H, two CH3 groups attached to benzene), 7.02–7.04 & 7.75–7.78
(4H, phenyl ring containing ethoxy chain), 7.45–7.47 & 7.75–7.79 (4H, middle phenyl ring),
6.88–7.89 (3H, phenyl ring containing two methyl ring). The NMR data are reliable with the
molecular structure.
–
–
=
–
–
–
–
=
–
–
–
= –
IR in cm− for butyloxy derivative
–
–
–
–
3020 (C H str. of alkene disubstituted), 2924 & 2866 (C H str. of ( CH2 )n group of –
=
=
OC4H9), 1726 (C O str. of carbonyl carbon of ester group), 1629 (C C str. of alkene), 1598
& 1508 (C C str. of aromatic ring), 1309 & 1265 (C H bending of alkene), 1203 (C O str. of
ether linkage), 1132 (C O str. of ester group), 999, 970 & 931 (C H bending of alkene). The
=
–
–
–
–
IR data are consistent with the molecular structure.
IR in cm− for octyloxy derivative
–
–
–
–
3037 (C H str. of alkene disubstituted), 2920 & 2850 (C H str. of ( CH2 )n group of –
=
=
OC8H17), 1720 (C O str. of carbonyl carbon of ester group), 1629 (C C str. of alkene), 1600,
1571 & 1506 (C C str. of aromatic ring), 1394 & 1319 (C H bending of alkene), 1246 (C O
str. of ether linkage), 1118 (C O str. of ester group), 991, 968 & 923 (C H bending of alkene).
=
–
–
–
–
The IR data are consistent with the molecular structure.

162
P. K. RAKHASIA ET AL.
Table . Transition temperatures in°C.
Compound No.
n-alkyl chain CnHn+ (n)
Sm
N
Isotropic
























—
—
—
—
—
(.)
(.)
.
.
.
.
.
—
—
—
—
—
.
.
—
—
—
.
.
.
.
.
.
.
.
.
.
.
.
—
—
Sm = Smectic, N = Nematic, () indicate monotropy
Mass spectra of methyloxy derivative
m/z (rel.int%): 385 (M)+, 224, 180, 161, 133, 103, 77
Mass spectra of heptyloxy derivative
m/z (rel.int%): 469(M)+, 343, 260, 245, 224, 181, 147, 119, 91, 65
Result and discussion
The homologues from C1 to C5 members of the novel series are nonmesomorphic. However,
nematogenic and smectogenic mesomorphism commences from the C6 homologue, but only
smectogenic mesomorphism is exhibited by the C8 to C16 homologues in an enantiotropic
manner. Transition and mating temperatures as determined by POM (Table 2) were plotted
to give Cr –Sm/I, Sm – N/I, N–I transition curves, showing phase behaviors of the series in a
phase diagram (Fig. 1). The Sm–I transition curve exhibits an odd–even effect, which appears
up to the C12 homologue. The N–I curves for odd and even members were extrapolated up to
the C12 homologue to show the odd–even effect for the N–I transition curve. The transition
curve for odd members occupies the upper position as compared to transition curve for even
membered homologues. The Sm–N or Sm–I transition curve initially rises, passes from max-
ima at C12 homologue and then descends in the usual established manner. The imaginary N–I
transition curve showing the odd–even effect is shown by an extrapolation method [26–29].
The Cr–M or Cr–I transition curve adopts a zigzag path of rising and falling values with an
overall descending manner of values. Thus transition curves of a phase diagram behaved in
expected normal manner. Odd–even effect disappears from and beyond C12 homologue. The
mesogenic property varies from homologue to homologue in the present series.
The lowering of transition temperatures of novel homologues as compared to correspond-
ing dimeric n-alkoxy acids are attributed to the breaking of hydrogen bonding by esterification
process. The lack of smectic or/and nematic mesophases either monotropically or enatiotropi-
cally by the C1 to C5 homologues is attributed to their high crystallizing tendency which arises
from low magnitudes of dispersion forces and low magnitudes of dipole-dipole interaction.
The mesophase formation by the C6 to C16 homologues is attributed to the disalignment of
molecules, perpendicular or at an angle less than ninety degree to the floaling surface, depend-
ing upon thermal resistivity causes by suitable magnitudes of intermolecular dipole–dipole
interaction and dispersion forces which induced by favorable end to end attractions or / and
lateral attractions of suitable magnitudes as a consequence of their molecular rigidity and

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
163
Figure . Phase behavior of series.
flexibility, which maintains mainly sliding layered molecular arrangement for all the meso-
genic homologues as well as maintain statistically parallel orientational order of molecules
under floated condition for C6 and C7 homologues. Thus, C6 to C16 mesogenic homologues
showed smectogenic character, whereas C6 and C7 homologues shows appearance of smec-
togenic and nematogenic mesophase formation, of different textures and of different range
of temperatures. All molecules of each mesogenic homologues individually, randomly orient
in all possible directions from and beyond isotropic temperature, which on cooling the same
from and below isotropic temperature reappears smectic or/and nematicmesophase forma-
tion in reversible manner for a temperature range which was appeared on heating. Thus, C6 to
C16 homologues are enantiotropically mesogenic. Odd–even effect observed for a novel series
is due to the sequential addition of methylene unit or units at n-alkyl chain “R” bonded to
first phenyl ring through oxygen atom. Decreasing and normal tendency of transition curves
observed is attributed to the shortening but suitable molecular length or their permanent
dipolemoment across the molecules of which longer n-alkyl chain “R” of –OR group coil or
bend or flex or couple to lie with major axis of core structure in expected manner and causes
disappearance of –odd–even effect from and beyond C12 homologue. The variations in meso-
genic behavior, properties, degree of mesomorphic from homologue to homologue in present
novel series are attributed to the changing number of carbon atom or atoms in n-alkyl chain
–
“R” of left n-alkoxy, OR group, keeping the rest of the molecular part unaltered of fixed
molecular part unaltered of fixed molecular rigidity and changing total molecular flexibility
ꢀꢀ ꢀꢀ
–
due to left OR group and fixed flexibility, polarity and polarizability of 3 , 4 -dimethyl termi-
nal and lateral groups at the other end. Some mesogenic behaviors and properties of presently
investigated novel series 1 are compared with structurally similar analogous series X [30] and
Y [31] as mentioned below in Fig. 2.
Homologous series 1 and X are identical with respect to three phenyl ring and two cen-
–
tral bridges contributing to the total molecular rigidity and left n-alkoxy OR terminal con-
tributing partly to the total molecular rigidity for the same homologue from series to series.
ꢀꢀ ꢀꢀ
–
–
However they differ with respect to 3 ,4 dimethyl and OCH2 CH3 terminal and/or lateral

164
P. K. RAKHASIA ET AL.
Figure . Structurally similar series.
groups for the same homologue from series to series, contributing partly to the total flexibility.
Homologous series 1 and Y are isomeric to each other, but their molecular rigidity may dif-
– =
–
–
= –
fer by very small or of negligible magnitudes due to N CH as well as CH N inversion
of central bridge linking middle and third phenyl rings for the same homologues affecting
molecular rigidity. The molecular flexibility and the molecular polarizability also differ from
each other (1 and Y) due to positional difference of 3ꢀꢀ,4ꢀꢀ-dimethyl and 2ꢀꢀ,5ꢀꢀ-dimethyl group
for the same homologue from series to series. Therefore, the variations in mesogenic proper-
ties and the degree of mesomorphism among the series 1, X and Y depend upon the differing
features and their magnitudes for the same homologue from series to series and homologue to
homologue in the same series. Table 3, represents some thermotropic LC properties for series
1, X and Y in comparative manner, as under.
Form Table 3, it is clear that,
• Homologous series 1, X and Y are smectogenic in addition to nematogenic.
• Mesomorphism of smectic type commences from C6 homologue of the series 1 and Y
but it commences late from C12 homologue in case of series X.
• Nematogenic mesomorphism commences from C6, C1 and C2 homologue of the series
1, X and Y respectively.
• Thermal stability for smectic and nematic of present series 1 is more than an isomeric
series Y, but less than series X.
Table . Relative thermal stabilities in°C.
Series  (ꢀꢀ, ꢀꢀ-dimethyl)
Series X ( OCH CH)
Series Y (ꢀꢀ, ꢀꢀ-dimethyl)
–
–
Series →
Sm—N or Sm—I
Commencement of smectic
phase
.
(C—C)
C
.
(C— C)
C
.
(C— C)
C
N—I
.
.
.
Commencement of nematic
phase
Total mesophase length in°C
(C— C)
(C— C)
(C— C)
C
C
C
. to .
CC
. to .
CC
. to .
CC

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
165
• Upper and lower degree of mesomorphism of present novel series 1 is lower than a series
X and more than a series Y.
The suitable magnitudes of anisotropic force of dispersion vary with the linearity or non-
linearity performing to the geometrical shape of molecules. Aromaticity and central bridges
are identically of series 1, X and Y are same. The molecules of series X are more linear than
series 1 and X without lateral substitution. Therefore, intermolecular distance parameter pre-
dominated against polarizability faster, affecting the magnitudes of the forces of cohesion for
–
–
series X, Y and 1. Thus, presence of lateral one CH3 group series 1 or two CH3 as lat-
eral group affects the possibility and suitability of magnitudes of intermolecular end to end
and/or lateral cohesion which causes more or less magnitudes of facilitating and maintain-
ing either only statistically parallel orientational order of molecules at the cost of maintaining
sliding layered molecular arrangement or facilitating statistically parallel orientational order
of molecular arrangement in addition to sliding layered molecular arrangement like series
1, X and Y of different temperature ranges for the same homologue from series to series; to
induce either only smectic or only nematic or both together one after another mesophases
or in general mesophases. The molecules of different moieties and of different geometrical
shape and size are formed from their atoms of different elements by breaking of odd bonds
and forming of new bonds in standard state or at a particular temperature. The change of
energy involved in all this process as heat of reaction and heat of formation (ꢀH) from atoms
of elements in standard state, which differs from homologue to homologue in the same series
and for the same homologue from series to series. Therefore, the resistivity towards exposed
thermal vibrations for the same homologue from series to series of mesogenic homologue
vary in transition temperature, mesophase lengths due to extant of molecular noncoplanarity
and thermal stabilities for smectic and nematic. Thus smectic and nematic thermal stabilities
early or late commencement of mesophase as well as upper and lower mesophase lengths are
in decreasing order as series X > series 1 > series Y. Hence, magnitudes of molecular rigidity
and flexibility as depended upon thermodynamic quantities and molecular structure play an
important role in mesogenic tendency of individual molecule of same series or of structurally
similar analogous isomeric or non isomeric series.
Conclusions
r
Ester homologous series of azomethan central bridge is predominantly smectogenic and
partly nematogenic and of middle-ordered meeting type with considerable mesophase
lengths.
The group efficiency order derived for smectic and nematic based on (i) thermal stability
(ii) early commencement of mesophase (iii) mesophase lengths are as under.
r
I. Smectic:
ꢀꢀ ꢀꢀ
ꢀꢀ ꢀꢀ
4ꢀꢀ OCH2 CH3> 3 ,4 -dimethyl > 2 ,5 -dimethyl
–
–
Nematic:
ꢀꢀ ꢀꢀ
ꢀꢀ ꢀꢀ
4ꢀꢀ OCH2 CH3> 3 ,4 -dimethyl > 2 ,5 -dimethyl
–
–
II. Smectic:
3ꢀꢀ,4ꢀꢀ-dimethyl = 2ꢀꢀ,5ꢀꢀ-dimethyl > 4ꢀꢀ OCH2 CH3
Nematic:
–
–
4ꢀꢀ OCH2 CH3> 2 ,5 -dimethyl > 3 ,4 -dimethyl
ꢀꢀ ꢀꢀ
ꢀꢀ ꢀꢀ
–
–
III. Total mesophase lengths:
ꢀꢀ ꢀꢀ
ꢀꢀ ꢀꢀ
Upper and lower: 4ꢀꢀ OCH2 CH3 > 3 ,4 -dimethyl > 2 ,5 -dimethyl
–
–

166
P. K. RAKHASIA ET AL.
r
r
r
r
– =
–
–
= –
Inversion of central group N CH to CH N in isomeric series can cause variations
in mesogenic tendency.
Study of binary systems of present investigation can lower transition temperatures
nearby 50°C
Mesogenictendancy of a substance is very sensitive and susceptible to the molecular
structure of a substance.
Present investigation raises and supported the conclusion drawn earlier.
Acknowledgments
The authors thank the Department of Chemistry (DST-FIST Funded & UGC-SAP Sponsored), Saurash-
tra University, Rajkot, for research work. The 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 inves-
tigation 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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