Dependence of thermotropic mesomorphism on flexible tail group

Article abstract of DOI:10.1080/15421406.2016.1146933

A novel homologous series RO-C₆H₄-COO-C₆H₄-CO-CH:CH-C₆H₄-OC₁₂H₂₅ of chalcones have been synthesized and studied with a view to understand and establish the effects of molecular structure on liquid crystal (LC) properties of a substance. Liquid crystal properties of a novel series commences from C₆ homologue and continue upto the last C₁₈ homologue derivative of a series, as enantiotropically nematogenic without exhibition of smectogenic characteristic. The rest of the homologues (C₁ to C₅) are nonliquid crystals (NLC). Transition temperatures of homologues (C₁ to C₁₈) are determined by an optical polarizing microscopy equipped with a heating stage. Textures of a nematic phase are threaded or schlieren. Transition curves of a phase diagram behaved in normal manner except for C₁₈ member of a series, which drastically behaved with abnormal manner. N-I transition curve exhibits odd–even effect. Average thermal stability for nematic is 110.25°C and mesophase length ranges from 10°C to 39°C. Analytical and spectral data supported the molecular structures of homologues. LC properties of present novel series are compared with structurally similar other known homologous series. Thus, present novel series of chalcones is partly nematogenic and of middle-ordered melting type.

Full text of DOI:10.1080/15421406.2016.1146933

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Dependence of thermotropic mesomorphism on
flexible tail group
Priya K. Patel & R. R. Shah
To cite this article: Priya K. Patel & R. R. Shah (2016) Dependence of thermotropic
mesomorphism on flexible tail group, Molecular Crystals and Liquid Crystals, 630:1, 130-138,
DOI: 10.1080/15421406.2016.1146933
To link to this article: http://dx.doi.org/10.1080/15421406.2016.1146933
Published online: 01 Jul 2016.
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Date: 02 July 2016, At: 03:12

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –

http://dx.doi.org/./..
Dependence of thermotropic mesomorphism on flexible tail
group
Priya K. Patel and R. R. Shah
Chemistry Department, K. K.Shah Jarodwala Maninagar Science College, Gujarat University, Ahmedabad,
Gujarat, India
ABSTRACT
KEYWORDS
Liquid Crystal;
mesomorphism; nematic;
odd-even effect; smectic
A
novel homologous series RO-C H -COO-C H -CO-CH:CH-C H -
6
4
6
4
6
4
OC H of chalcones have been synthesized and studied with a view
1
2
25
to understand and establish the effects of molecular structure on
liquid crystal (LC) properties of a substance. Liquid crystal properties
of a novel series commences from C homologue and continue upto
6
the last C18 homologue derivative of a series, as enantiotropically
nematogenic without exhibition of smectogenic characteristic. The rest
of the homologues (C to C ) are nonliquid crystals (NLC). Transition
1
5
temperatures of homologues (C to C ) are determined by an optical
1
18
polarizing microscopy equipped with a heating stage. Textures of a
nematic phase are threaded or schlieren. Transition curves of a phase
diagram behaved in normal manner except for C18 member of a series,
which drastically behaved with abnormal manner. N-I transition curve
exhibits odd–even effect. Average thermal stability for nematic is
1
10.25°C and mesophase length ranges from 10°C to 39°C. Analytical
and spectral data supported the molecular structures of homologues.
LC properties of present novel series are compared with structurally
similar other known homologous series. Thus, present novel series of
chalcones is partly nematogenic and of middle-ordered melting type.
Introduction
The study of an intermediate state between crystalline solid and isotropic liquidous state,
which is called as anisotropic liquid or liquid crystalline (LC) state [1] has become popular
among the scientific and technological community to carry out research with different aims,
objects and views at different angles including applications [2–5]. Every field of researchers
always need novel LC materials for their novel investigation, either as thermotropic LC or
lyotropic LC. Present investigation is planned with a view to understand and establish the
effects of molecular structure on LC properties [6–9] of thermotropic LC, excluding the bio-
logical study of novel LCs in lyotropic condition. However, the present study will include
the synthesis of novel chalconyl ester derivatives, whose, molecular structures will be char-
acterised and to evaluate thermotropic mesogenic properties. Then, the mesogenic proper-
ties of proposed novel series will be compared with structurally similar series with reference
to changing flexibility of tail end groups, keeping molecular rigidity unchanged. Results of
the novel investigation will be recorded and discussed on the basis of molecular rigidity and
CONTACT Priya K. Patel
lege, Gujarat University, Ahmedabad, Gujarat,India.
 Taylor & Francis Group, LLC
pp@gmail.com
Chemistry Department, K. K. Shah Jarodwala Maninagar Science Col-
©

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
131
flexibility [10–14]. Number of ester derivatives have been reported till the date [15–21] but
present novel series of chalconyl derivatives are important as biologically active compounds
which may be studied separately as lyotropical by pharmacist and medicinal chemist [22–26].
Experimental
Synthesis
4
4
- Hydroxy Benzoic acid was alkylated using suitable alkylating Agent (R-X) to convert it into
-n alkoxy benzoic acids by modified method of Dave and Vora [27].
α-4-hydroxyl benzoyl β-4 -dodecyloxy phenyl ethylene (B) was prepared by usual estab-
ꢀ
lished method [28]. Acid chlorides component of each 4-n-alkoxy benzoic acid (A) were con-
ꢀ
densed with α-4 -hydroxyl benzoyl β-4-dodecyloxy phenyl ethylene (B) in dry cold pyridine
by usual established method [29] carefully. Thus, chalconyl ester homologue derivatives were
finally decomposed, filtered, washed, dried and purified till constant transition temperature
obtained using an optical polarising microscope, equipped with a heating stage. 4-Hydroxy
benzoic acid, Alkyl halides, Thionyl chloride, MeOH, EtOH, KOH, Acetone, Pyridine, 4-
Hydroxy acetophenone, 4- Hydroxy benzaldehyde, Dodecyl halide etc., required for synthesis
were used as received except solvents which were dried and distilled prior to use. The synthetic
route to the series is mentioned below as Scheme 1.
Characterization
1
Selected representative homologues of a titled novel series were characterised by IR, HNMR
technique and elemental analysis (Table 1). IR spectra were recorded on Perkin Elmer spec-
1
trum GX and HNMR spectra were recorded using CDCl as solvent. Liquid Crystal prop-
3
erties i.e. transition and melting temperatures of homologues were investigated by polarising
optical microscopy with heating stage. Textures of the novel homologues were determined by
miscibility method (Table 2). Thermodynamic quantities enthalpy (ꢀH) and entropy (ꢀS)
are qualitatively discussed.
Analytical data
−
IR spectra in cm : For octyloxy and octadecyloxy derivatives
Octyloxy: 763(polymethylene (–CH –) of –OC H , 839 (–OH def., m,p– di substituted),
2
n
8
17
9
(
54 (–CH def. hydrocarbon), 1024 & 1060 (–C–O stre.), 1116,1164,1253 (–C–O stre. in
–CH ) chain),1500 & 1590 (–C–C stre.), 1676 (–C = O group), –COO ester group, 2856
2
n
&
2923 (–CH str. in –CH ).
3
Octadecyloxy:707, 759 (polymethylene (–CH –) of –OC H , 842 (–OH def., m,p – dis-
2
n
18 37
ubstituted), 939 (–C–H def. hydrocarbon), 1097 & 1058 (–C–O stre.), 1305, 1253 & 1176
(
–C–O stre. in (–CH ) chain), 2921, 2858, 2696 (–C–H stre.), 3087 ( = C–H stre.)
2
n

H– NMR spectra in CDCl in δ ppm: For pentyloxy and decyloxy derivatives

Pentyloxy: 0.61, 0.67, 0.83 (t, –CH of –OC H ), 1.08 (m, n-polymethylene group of
3
5
11
–
OC H ), 2.00 (s, –COCH group), 2.39 (s, –CH group), 3.46 to 3.78 (s, –OCH –CH of
5 11 3 3 2 2
–OC H ), 4.3 to 4.4 (–CH ), 8.30 (s, p-disubstituted phenyl ring).
18
37
3

1
32
P. K. PATEL AND R. R. SHAH
Scheme . Synthetic route of the series-.
Decyloxy: 0.99 (t, –CH of –C H ), 1.14 to 1.00 (m, n-polymethylene group of
10 21 18 37
3
10 21
–
OC H ), 1.91(m, n-polymethylene group of –OC H ), 2.82 to 2.08 (Ar–C–H), 11.29 (s,
p-disubstituted phenyl ring).
ꢀ
ꢀꢀ
Homologous series: α-4 [4 -n-alkoxy benzyloxy] benzoyl β-4 dodecyloxy phenyl
ethylenes.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
133
Figure . Phase Behaviours of Series.
Results and discussion
ꢀ
α-4-Hydroxy benzoyl β-4 -Dodecyloxy phenyl ethylene is a nonliquid crystal (M.P. is 92°C
and yield is 65%) component, which on condensation with dimeric 4-n-alkoxy benzoic
acids yielded nematogenic mesomorphs from C homologue to C homologue with rela-
6
18
tively lower transition, than corresponding 4-n-alkoxy benzoic acids. Transition tempera-
tures (Table 1) of homologues were plotted against the number of carbon atoms present in
n-alkyl chain bonded to first phenyl ring through oxygen atom. A phase diagram (Fig. 1)
consisted of Cr-N/I and N-I transition curves which are formed on linking, like or related
points showing phase behaviours of series. Cr-N/I transition curve adopts a zigzag path of
rising and falling with overall descending tendency and behaved in normal manner. N-I tran-
sition curve descended from C to C homologues and behaved in normal manner, but it
6
16
drastically rises for C and showed abnormal behaviour. N-I transition curve exhibited late
18
odd-even effect in very short range of temperature. Transition curves (N-I) for odd and even
members of a series merged into each other at the octyloxy (C ) homologue and then it prop-
8
agated as a single curve. The mesomorphic properties are absent from C to C homologues.
1
5
N-I transition curve for odd members occupy lower position than even member’s N-I transi-
tion curve. N-I transition curve is extrapolated from C to C to determine its nonmesogenic
7
5
behaviour. The variations in mesogenic properties are observed from homologue to homo-
logue in the same series. Lowering of transition temperatures of novel chalconyl homologue

1
34
P. K. PATEL AND R. R. SHAH
derivatives and the disappearance of dimerization of 4-n-alkoxy benzoic acids are attributed
to the breaking of hydrogen bonding between two molecules of corresponding n-alkoxy ben-
zoic acid by esterification process, through their acid chlorides. The nonmesogenic behaviour
of C to C homologues is attributed to their high crystallising tendency, which arises from
1
5
low dipole-dipole interactions and the low magnitudes of dispersion forces through inter-
actions between instantaneous dipole produced by the spontaneous oscillations of electron
clouds of the molecules. Hence, the combine effect of molecular rigidity and flexibility fail to
induce mesomorphic property but abruptely breaks the crystal lattices and sharply transform
rigid crystals into isotropic state without passing through LC state. The exhibition of nemato-
genic character by C to C homologues is attributed to the suitable magnitudes of end to
6
18
end anisotropic forces of intermolecular attractions and closeness caused as a consequence
of favorable molecular rigidity and flexibility which maintained the floating of the molecules
on the surface with statistically parallel orientational order only within definite range of tem-
perature under exposed thermal vibrations. However on further continued heating from and
above the isotropic temperature, the molecules are randomly oriented in all possible direc-
tions with high order of disorder or high entropy [ꢀS = ꢀH/T]. But on cooling the same with
proper care, the molecules of a homologue under microscopic examination reversibly adopt
reappearance of nematic mesophase formation from and below isotropic temperature. How-
ever none of the nonmesomorphic (C to C ) homologues did show exhibition of nematic
1
5
mesophase in irreversible manner as monotropic mesophase. The exhibition of odd–even
effect is observed due to odd and even number of methylene units present in homologues at
left n-alkoxy terminal end group. The disappearance of odd-even effect from and beyond C8
homologue in N-I transition curve is due to the coiling or bending or flexing or coupling of
longer n-alkyl chain with the principal axis of the core structure of a molecule. The abnormal
behaviour of C homologue may be attributed to the uncertainity in the status of the longest
18
n-alkyl chain of the left n-alkoxy end group. The extrapolation of N-I transition curve [30–32]
from C to C which matches to isotropic temperature of C indicating that, there is no possi-
7
5
5
bility of nematic mesophase formation either in monotropic or enantiotropic condition. The
variations in mesomorphic properties or behaviour from homologue to homologue in the
same series is attributed to the changing member of carbon atoms which increases gradu-
ally the molecular length, length to breadth ratio, permanent dipole moment across the long
molecular axis, unsuitable or suitable magnitudes of intermolecular end to end anisotropic
forces or dispersion forces of cohesion and closeness, molecular polarity and polarizability
as a consequence of resultant molecular rigidity and flexibility etc., keeping the rest of the
molecular part unchanged. The absence of smectogenic property in presently investigated
novel series may be due to the absence of lamellar packing of molecules in crystal lattices of
rigid crystals which hinders the sliding layered molecular arrangement under floating condi-
tion on the surface. The extent of molecular noncoplanarity occured due to longer -OC H
12
25
right sided end group, which do not facilitate layered structure in crystal lattices of any of the
homologues of present series. Thus, smectogenic mesophase does not occure. The mesogenic
behaviours of presently investigated series-1 are compared with the structurally similar other
known homologous series as shown in following figure-2.
Homologous series 1 of present investigation and the series X [33] and Y [34] selected
for comparative study are identical with respect to three phenyl rings bonded through two
central groups –COO– and –CO–CH = CH– and left n-alkoxy terminal end group (–OR)
for the same homologue from series to series. But they differ from each other with respect to
changing right terminal tail end group –OC H , –H and m,p-dimethoxy groups for the same
12
25
homologue from series to series. Thus, variations in mesogenic properties and the degree of

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
135
Figure . structurally similar series.
Table . Elemental analysis for butyloxy, pentyloxy, hexadecyloxy, and octadecyloxy.
Element % found
Element % calculated
Molecular formula
C HO
C
H
C
H
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.


C H O





C H O
 
C H O
 
mesomorphism will depend upon changing tail groups from series to series. Table 3 represents
some thermometric properties of series 1, X and Y as mentioned below.
Table 4 indicates that,
•
•
•
•
Homologous series-1 of present investigation and a series- X are only nematogenic and
a series-Y is smectogenic in addition to nematogenic.
The smectogenic mesophase formation does not commence till the last member of series
1
and X, but it commence from C member of a series-Y.
6
Nematic mesophase commences from C member of the Series 1 and X, but it com-
6
mences early from second member (C ) of series-Y.
2
Nematic thermal stability of the series under comparative study are in the decreasing
order as series-X > Y > 1. i.e. thermal stability for nematic is the lowest for presently
investigated series.
•
•
Smectic mesophase facilitated and stabilized by series-Y only.
Total mesophase length range wonderfully varies from maximum to minimum between
C to C or C last homologues for all the series under comparison. However, numer-
10
16
18
ical values of total phase length ranges differ from series to series.
The observed difference in the mesogenic or physical properties for the same homologue
from series to series among the series 1, X and Y are attributed to the flexibility difference of
tail end groups –OC H , –H and m,p-dimethoxy groups, whereas the difference in meso-
12
25
genic or physical properties from homologue to homologue in the same series is attributed to
the changing molecular flexibility due to sequentially added methylene unit or units at n-alkyl
chain of left terminal keeping tail end groups unchanged for each structurally similar series
Table . Textures of nematic phase of C, C, C, C by miscibility method.
Sr. No.
Homologue
Texture

C
C
C
C
Schlieren
Threaded
Threaded
Schieren




136
P. K. PATEL AND R. R. SHAH
Table . Transition temperatures in°C.
Transition Temperatures in°C
Homologue(n-alkyl
Compound No.
chain)
Smectic
Nematic
Isotropic

C
C
C
C
C
C
C
C
C
C
C
C
C
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
















.
because molecular rigidity from series to series and homologue to homologue in the same
series are constant throughout due to three phenyl rings and two central bridges. The chang-
ing magnitudes of molecular flexibility depends upon changing tail end group polarities and
polarizability or the difference of group polarities of terminal, including lateral groups, atom-
icity of an individual tail group, and the vector sum of its bond polarities, dipolarities of phenyl
ring-tail group, status of tail end group under exposed thermal vibrations, modes of vibrations
depending upon the number of degrees of freedom (3n–5) or (3n–6), where n is total number
of atoms of flexible specy, etc. m,p-Dimethoxy group are highly polar and polarizable (series-
Y) groups, the one of them meta substituted methoxy group is laterally substituted group
which induces polarizability of a molecule and the para substituted methoxy group strengthen
and induces end to end attractions. Thus the suitable magnitude of anisotropic forces of cohe-
sion and closeness commences from C homologue derivative as nematogenic masophase and
2
from C homologue induces smectogenic mesophase formation in case of series-Y. The lat-
6
eral methoxy group contributes to layered or sliding layered molecular arrangement under
floating condition to induce smectogenic character in addition to nematogenic character. The
absence of lateral substitution at ortho or meta position like series-Y do not induce suitable
magnitudes of intermolecular polarizable nature of attractions except suitable magnitudes of
end to end cohesion and closeness, which hinders the smectogenic mesophase formation but
favours to induce only nematogenic mesophase formation identically to commence from C6
member of the series 1 and X. The lowest value of nematic thermal stability of present novel
series-1 is attributed to uncertainity arising in the status of longest n-alkyl chain (-C H ) of
12
25
tail end group bonded to phenyl ring through oxygen atom. The highest thermal stability for
Table . Thermal stabilities in°C.
Series →
(-OC H )
X(-H)
—
Y(-m,p- dimethoxy)
.(C -C )C

 
Smectic-
—



IsotropicOrSmectic-
NematicCommencement of
Smectic phase
Nematic-Isotropic
commencement of nematic
phase
.(C -C )C
.(C -C )C
.(C - C )C
  






Total mesophase length
Range in°C t to t c to c
j
. to .C C
.–.C C
.–.C C
i
j
i

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
137
nematic of series-X is attributed to the absence of tail end group (or -H) in which molecular
flexibility in addition to molecular rigidity are sufficient to induce nematogenic mesophase
formation relatively at higher transition temperatuters as compared to series 1 and Y. Vector
sum of the bond polarities, strengthens the molecular rigidity or enthalpy (ꢀH) value due
to absence of tail end group (or by Hydrogen atom) in case of series-X under comparison.
Thus, inter molecular cohesion and closeness vary for the same homologue from series to
series with changing tail end group which consequently causes variations in molecular rigid-
ity, flexibility, polarity and polarizibility, dispersion forces, permanent dipole moment across
the long molecular axis, dipole—dipole, and electronic interactions etc. Thus, present novel
chalconyl ester homologous series is partly nematogenic with low degree of mesomorphism
and middle-ordered melting type.
Conclusions
•
Chalconyl ester derivatives of thermotropic liquid crystals are nematogenic without
exhibition of smectogenic character, with low degree of mesomorphism and low ther-
mal stability.
•
Smectic and nematic group efficiency order derived on the basis of (a) thermal stability
(b) early commencement of mesophase and (c) the total mesophase length with respect
to tail end group.
(
a) Smectic
m, p-dimethoxy > –H = –OC H
1
2
25
Nematic
–H > m, p dimethoxy > –OC H
12
25
(
b) Smectic
m, p-dimethoxy > –H = –OC H
1
2
25
Nematic
m, p-dimethoxy > –H = –OC H
12
25
(
c) Total mesophase length range – (Sm + N)
–H > m, p-dimethoxy > –OC H
12
25
•
•
Molecular flexibility operates mesomorphism, if molecular rigidity is constant.
The phenomina of mesomorphism is very sensitive and susceptible to a molecular struc-
ture.
•
•
Thermotropic chalconyl ester derivatives may be useful in the manufacture of LC devices
using study of binary systems. Chalconyl ester derivatives are antibacterial, antimalarial,
anticancer etc. which may be studied by biologists through lyotropic mesomorphism.
Present investigation of novel substances has raised the reliability and credibility to the
conclusions drawn earlier.
Acknowledgments
Authors are thankful to the management of K. K. Shah Jarodwala Maninagar Science College, Ahmed-
abad and Head of Chemistry Department Dr. R. B. Patel. Authors are also thankful to Dr. A.V. Doshi,
Ex-Principal, M. V. M. Science and Homesci. College, Rajkot and Dr. Hemant R. Prajapati, Assi. Prof.,
C. U. Shah Science college, Ahmedabad for their whole hearted support and co-operation as and when
needed during course of this investigation. Thanks are due to NFDD Centre, Saurashtra University,
Rajkot for Analytical Services.

1
38
P. K. PATEL AND R. R. SHAH
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