Role of n-alkoxy terminal end groups on mesomorphism

Article abstract of DOI:10.1080/15421406.2016.1190135

A homologous series RO-C₆H₄-CH:CH-COO-C₆H₄-CO-CH:CH-C₆H₄-OC₆H₁₃ (n) of novel liquid crystals (LC) has been synthesized and studied with a view to understanding and establishing the relation between molecular structure of a substance and the LC properties or behavior of thermotropic LC materials. Novel series consist of 11 homologues. C₁ and C₂ homologue derivatives are non-LCs and the rest of homologues (C₃ to C₁₆) are nematogenic without exhibition of smectic property. Transition temperatures were determined by an optical polarizing microscopy equipped with a heating stage. Transition curve Cr-N/I of a phase diagram behaved in normal manner. N/I transition curve partly deviated at C₆ and C₁₀ homologues from normal descending behavior, and exhibited very narrow and sharp odd-even effects. Textures of nematic phase are threaded or Schlieren. Thermal, analytical, and spectral data confirmed the molecular structures of novel chalconyl ester derivatives. Thermal stability for nematic is 95.77°C and the upper and lower mesophase lengths vary from 35 to 16°C at C₅ and C₈ homologues respectively. Group efficiency order derived for nematic on the basis thermal stability is -OC₁₀H₂₁ (n) > -OC₈H₁₇(n) > -OC₆H₁₃(n).

Full text of DOI:10.1080/15421406.2016.1190135

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Role of n-alkoxy terminal end groups on
mesomorphism
N. R. Muniya & V. R. Patel
To cite this article: N. R. Muniya & V. R. Patel (2016) Role of n-alkoxy terminal end
groups on mesomorphism, Molecular Crystals and Liquid Crystals, 633:1, 129-136, DOI:
10.1080/15421406.2016.1190135
To link to this article: http://dx.doi.org/10.1080/15421406.2016.1190135
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Date: 26 August 2016, At: 03:47

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –

http://dx.doi.org/./..
Role of n-alkoxy terminal end groups on mesomorphism
N. R. Muniya and V. R. Patel
Chemistry Department, Sheth P. T. Arts & Science College, Gujarat University, Godhra, Gujarat, India
ABSTRACT
KEYWORDS
A
homologous series RO-C H -CH:CH-COO-C H -CO-CH:CH-C H -
Liquid crystal; chalconyl
ester; smectic; nematic;
mesomorphism
6
4
6
4
6
4
OC H (n) of novel liquid crystals (LC) has been synthesized and studied
6
13
with a view to understanding and establishing the relation between
molecular structure of a substance and the LC properties or behavior
of thermotropic LC materials. Novel series consist of 11 homologues. C1
and C homologue derivatives are non-LCs and the rest of homologues
2
(C to C ) are nematogenic without exhibition of smectic property.
3 16
Transition temperatures were determined by an optical polarizing
microscopy equipped with a heating stage. Transition curve Cr-N/I of a
phase diagram behaved in normal manner. N/I transition curve partly
deviated at C and C homologues from normal descending behavior,
6
10
and exhibited very narrow and sharp odd-even effects. Textures of
nematic phase are threaded or Schlieren. Thermal, analytical, and
spectral data confirmed the molecular structures of novel chalconyl
ester derivatives. Thermal stability for nematic is 95.77°C and the
upper and lower mesophase lengths vary from 35 to 16°C at C and C8
5
homologues respectively. Group efficiency order derived for nematic
on the basis thermal stability is -OC H (n) > -OC H (n) > -OC H (n).
1
0
21
8
17
6
13
Introduction
The study of liquid crystalline (LC) state [1] has a strong impact on the minds of scientists and
technologist to the benefits of humanity [2–10]. Therefore, present investigation is planned
with a view to understanding and establishing the effects of molecular structure [11–16] of
LC properties with reference to tail group and terminally situated end group through syn-
thesis of novel chalconyl ester substances. Chalconyl ester derivatives of thermotropic variety
can be useful for the manufacture of LC devices to be operated at desired room tempera-
ture or as desired. The chalconyl derivatives can be useful for the manufacture of pharma-
ceutical preparations due to their bioactivity. Thus, the dual role of chalconyl derivatives has
been attracted for the planning of present investigation of synthesizing novel thermotropic
LC substances through chalconyl ester series. Thus, the proposed investigation will consist of
synthesis, characterization, evaluation of thermotropic data, their interpretation, and discus-
sion in terms of molecular rigidity and flexibility [17–20] and derivation of group efficiency
order from comparative study with structurally similar analogous series. The study of bio-
logical activity and the application part of study with novel substances is left for scientific
and technological researchers working on LC state with views, aims, and objects other than
CONTACT N. R. Muniya
University, Godhra , Gujarat, India.
 Taylor & Francis Group, LLC
nikitamuniya@gmail.com
Chemistry Department, Sheth P.T. Arts & Science College, Gujarat
©

1
30
N. R. MUNIYA AND V. R. PATEL
Scheme . Synthetic route to series .
present view of investigation. A number of ester homologous series of LC substances have
been reported till date [21–30].
Experimental
Synthesis
4-Hydroxy benzaldehyde was alkylated by an established method [31a]. 4-N-alkoxy benzalde-
hydes were treated with malonic acid to convert them into trans 4-n-alkoxy cinnamic acids
ꢀ
(
A) [31b]. N-alkoxycinnamic acids were condensed with α-hydroxy benzoyl β -4 hexyloxy
phenyl ethylene [32] [B] (m.p. = 103°C) by usually established method. Components A and
B were condensed [33] to give final products. Synthetic route to series is mentioned below as
Scheme 1. Final products were individually decomposed, filtered, washed, dried, and purified
till the constant transition temperatures were obtained.
R=C H2n+1, n = 1, 2, 3, 4, 5, 6, 8, 10, 12, 14, 16.
n
The chemicals 4-hydroxyl benzaldehyde, malonic acid, alkyl halides, 4-
hydroxyacetophenone, 4-hexyloxybenzaldehyde, potassium hydroxide, hydrochloric acid,
etc. required for synthesis were used as received except solvents which were dried and
distilled prior to use.
Characterization
1
Some selected members of the titled series were characterized by elemental analysis, H NMR
spectra, and IR spectra. Textures of mesophases were characterized by the miscibility method.
Microanalysis for CHN elements was performed on Perkin Elmer PE 2400 analyzer (Table 1).
1
H NMR spectra were obtained on Bruker spectrometer using CDCl as solvent. IR spectra
3

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
131
Table . Elemental analysis for hexyloxy, octyloxy, and decyloxy derivatives.
Elemental percentage found (compared with percentage calculated)
Molecular formula
C
H
CH O
. (.)
. (.)
. (.)
. (.)
. (.)
. (.)

CH O

CHO
were recorded on a Perkin Elmer spectrum GX. Transition temperatures and LC properties
were investigated using an optical polarizing microscope with heating stage.
Analytical data
1
H NMR: in ppm. For octyloxy homologue.
Ethylenes: (200 MHz) δ (CDCl ) (ppm) 0.75 (-CH -CH of -C H ), 1.20 (long -CH -
3
3
2
8
17
2
chain), 3.2 OCH of (-OCH of C H ), 6.81 and 6.80, 8.20 and 8.05 (p-sub. benzene rings).
2
2
8
17
1
H NMR: in ppm. For decyloxy homologue.
Ethylenes: (200 MHz) δ (CDCl ) (ppm) 0.90 (-CH -CH of -C H ), 1.19 (long -CH -
3
3
2
8
17
2
chain), 3.86 -OCH of (-OCH of C H ), 6.69 and 6.89, 8.16 and 8.21 (p-sub. benzene rings).
2
2
8
17
−1
IR in cm
IR Spectrum: For octyloxy homologue
Ethylenes: (vmax/cm ): 2928, 2835, 1456, and 1389 (-C-H, aliphatic), 1739, 1260 (ester
−1
group), 1720 (>C=O group), 1561 (-C=C-, aromatic), 1059, 1253 (ether group), 846.2
(
p- sub. benzene ring), 1620, 1498, 1456 (Aromatic ring).
IR Spectrum: For decyloxy homologue
−1
Ethylenes: (vmax/cm ): 2920, 2839.1, 1432, and 1382 (-C-H, aliphatic), 1736, 1239 (ester
group), 1730 (>C=O group), 1523 (-C=C-, aromatic), 1063, 1253 (ether group), 849 (p-sub.
benzene ring), 1622, 1483.2, and 1463 (aromatic ring).
Results and discussion
ꢀ
Trans 4-n-alkoxy dimeric cinnamic acids on condensation with α-4-hydroxy benzoyl β-4 -
hexyloxy phenyl ethelene yielded enantiotropic nematogenic thermotropic chalconyl ester
homologues without exhibition of smectic properties. The C and C homologues are non-
1
2
LC homologues and the rest of the nine homologues (C to C ) are nematogenic. Transi-
3
16
tion and melting temperatures were determined through an optical polarizing microscopy
POM) equipped with a heating stage. Textures of a nematic phase are threaded or Schlieren.
(
Transition temperatures (Table 2) are plotted against the number of carbon atoms present
in n-alkyl chain “R” of -OR group, and then Cr-N/I and N-I transition curves are obtained
by linking similar or related points as depicted in a phase diagram (Figure 1). Cr-N/I tran-
sition curve adopted zigzag path of rising and falling with overall descending tendency and
behaved in normal manner. The N-I transition curve initially rises and descends at C homo-
6
logue and then it again rises and descends at C and C homologues, and finally rises at
8
10
C derivative and descends up to C homologue with exhibition of very sharp and narrow
12
16
odd-even effect. Thus, the N-I transition curve partly deviates from the normal descend-
ing tendency at C and C homologues. The odd-even effect diminishes from and beyond
6
10

1
32
N. R. MUNIYA AND V. R. PATEL
Table . Transition temperatures in °C.
Compound No.
n-Alkyl (C H
) group
Smectic
Nematic
Isotropic
n
n+

C
C
C
C
C
C
C
C
C
C
C
–
–
–
–
–
–
–
–
–
–
–
–
–
.
.
.
.
.
.
.
.
.
.
.









.
.
.
.
.
.
.
.
.



C: Threaded nematic
C: Threaded nematic
C: Schlieren nematic
nearby C or C homologue and then N-I transition curve prolonged as a single transi-
6
7
tion curve. Variations in mesogenic thermometric LC properties from homologue to homo-
logue in the same series are observed, keeping right terminal -OC H (n) and the rest of
6
13
the molecular part intact throughout the entire series. Thermal stability for nematic is 95.77
and the nematogenic mesophase lengths vary from minimum to maximum are 16.0°C to
3
4.0°C at the C and C homologues. Thus, novel series is a lower middle-ordered melting
8 5
type.
ꢀ
ꢀꢀ
Figure . Homologous series: α--[ -n-alkoxy cinnamoyloxy] β-benzoyl- -hexyloxy phenyl ethylenes.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
133
Lowering of transition temperatures of novel chalconyl esters as compared to correspond-
ing trans n-alkoxycinnamic acids is attributed to the breaking of hydrogen bonding between
two molecules by esterification process. The nematic mesophase formation is exhibited after
disappearance of dimerization of corresponding n-alkoxycinnamic acid. The molecules of
novel nematogenic esters are attributed to suitable magnitudes of anisotropic forces of inter-
molecular end-to-end cohesion and closeness because of favorable molecular rigidity and
flexibility. This molecular rigidity and flexibility resist the exposed thermal vibrations to sta-
bilize, maintain, and facilitate the molecules of C to C homologues to float on the sur-
3
16
face with statistically parallel orientational order to induce nematic mesophase. However, the
magnitude of intermolecular forces of attractions and closeness is insufficient to facilitate and
organize any of the molecules to float with sliding layered arrangement to exhibit smectogenic
mesophase formation even in the monotropic condition. The non-mesomorphicity of C and
1
C homologues is attributed to their high crystallizing tendency. On cooling the C and C
2
1
2
homologues, no mesophase is generated. However, the nematogenic mesophase formation for
C and C homologues reappears exactly at a temperature at which it had appeared reversibly
3
16
on heating. The exhibition of odd-even effect in N-I transition curve is due to the sequen-
tially added methylene unit at the n-alkyl chain “R” of “-OR .” The disappearance of odd-even
effect from and beyond merging of N-I transition curves for higher homologues of longer
alkyl chain is attributed to the coiling or bending or flexing, or coupling of longer n-alkyl
chain “R” of “-OR” and -OC H (n) terminals with the principal axis of core structure. Thus,
6
13
unusual and uncertainty in the status of longer n-alkyl chains “R” of “-OR” induces deviating
trend in N-I transition curve, affecting molecular length, length-to-breadth ratio, permanent
dipole moment across long molecular axis, magnitudes of intermolecular dispersion forces,
molecular rigidity and flexibility, and molecular polarity and polarizability from homologue
to homologue in the same series. Thus, mesomorphic (LC) properties vary from homologue
to homologue in the same series by changing number of carbon atoms in n-alkyl chain “R”
of “-OR” group keeping rest of the molecular part uncharged. The thermotropic properties of
presently investigated novel series 1 is compared with structurally similar analogous series X
[
33] and Y [34] as shown below in Figure 2.
Homologous series 1 of the present investigation and series X and Y chosen for comparison
are identically similar with respect to three phenyl rings and two central bridges -CH=CH-
COO- and -CO-CH=CH- contributing to total molecular rigidity. The left n-alkoxy -OR
groups are identically the same for the same homologue from series to series, contributing
partly to total flexibility, but its magnitude differs from homologue to homologue in the same
series. Moreover, the flexible nature of right-sided end groups -OC H (n), -OC H , and
6
13
8
17
-
OC H (n), which partly contributes to the total flexibility, differs from series to series.
10 21
Thus, changing trend in mesogenic properties and the degree of mesomorphism from series
to series for the same homologue or from homologue to homologue in the same series depend
upon the differing features of molecular structure from series to series or from homologue to
homologue in the same series based on differing magnitudes of combined effects of molecular
Figure . Structurally similar analogous series.

1
34
N. R. MUNIYA AND V. R. PATEL
Table . Thermal stability in °C.
→
Series
Series  [-OC H (n)]
Series  [-OC H (n)]
Series  [-OC H (n)]




 
Sm-I or Sm-N
Commencement of smectic phase
N-I
–––––— –-––––––––––––––––––––––––––––––
–
–––––— –-––––––––––––––––––––––––––––––
–
.
–
.
.
Commencement of nematic phase
Total mesophase lengths in °C
(C –C ) C
. to .
(C –C ) C
(C –C ) C
. to .
CC









. to .
C C
CC

 
rigidity plus molecular flexibility. Table 3 represents some thermotropic evaluated data for
series 1, X, and Y under comparative study.
Homologous series 1, X and Y under comparative study are as follows:
•
•
•
•
•
Only nematogenic with absence of smectic property.
Nematic thermal stability is in increasing order.
Upper mesophase, lengths are in increasing order.
Lower mesophase lengths are in decreasing order from series Y to 1 to series X.
Commencement of nematic phase identically occurs from C homologue.
3
All the molecules of three analogous series have identical molecular rigidity by three phenyl
rings and the same two central bridges containing conjugated double bonds. However, the
total molecular flexibilities vary from homologue to homologue in the same series and for
the same homologue from series to series. Thus, variations in observed mesogenic behaviors
from series to series and homologue to homologue in the same series can be linked with the
changing magnitudes of molecular flexibility. The exhibition of only nematogenic character by
series 1, X, and Y is attributed to the suitable magnitude of anisotropic forces of intermolecu-
lar cohesion and closeness as occurred in molecules through end-to-end attractions because
of favorable constant molecular rigidity and changing molecular flexibility. However, inter-
molecular cohesion by lateral attraction through individual varying molecular polarity and
polarizability causes unsuitable magnitudes of forces of intermolecular attractions, which dis-
allow the formation of sliding layered molecular arrangement prior to appearance of nematic
phase in floating condition under the influence of exposed thermal vibrations. The increasing
order of thermal stability from series 1 to X to Y is attributed to the increasing number of two
methylene units from -OC H (n) to -OC H (n) to -OC H (n) tail ends of series under
6
13
8
17
10 21
comparative study. Thus, increasing number of methylene unit or units sequentially increases
molecular polarity and polarizability as well as length to breadth ratio for the same homologue
from series to series. Thus, stabilization of nematogenic mesophase formation is favored by
adding two methylene units at the tailed n-alkoxy terminal end group in a sequential manner.
As tailed n-alkoxy end group is gradually lengthened, the resistivity toward exposed thermal
vibration also gradually raises, and hence the upper nematogenic mesophase lengths acquire
increasing order from series 1 to X to Y irrespective of order of lower mesophase lengths. The
commencement of nematogenic mesophase identically takes place from C homologue for all
3
the series of the present comparative study because the extent of molecular non-coplanarity
of analogous series remains almost equivalent with very negligible difference in magnitude,
which maintains mesophase commencement in equal order.
Conclusions
•
Novel homologous series of vinyl carboxychalconyl derivatives are only nematogenic
with absence of smectogenic property whose mesophase lengths are relatively shorter
and low-order melting-type series.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
135
•
Group efficiency order derived for nematic based on (a) thermal stability, (b) commence-
ment of mesophase, and (c) upper mesophase lengths are as under.
(
a) Nematic
-
OC H (n) > -OC H (n) > -OC H (n)
10
21
8
17
6
13
(
b) Nematic
-
OC H (n) = -OC H (n) = -OC H (n)
6
13
8
17
10 21
(
c) Nematic
Upper: -OC H (n) > -OC H (n) > -OC H (n)
10
21
8
17
6
13
Lower: -OC H (n) > -OC H (n) > -OC H (n)
Present study may be useful for the study of binary systems and bioactivity of novel com-
pounds.
10
21
6
13
8
17
•
•
•
•
Mesomorphism is very sensitive and susceptible to molecular structure.
Chalconyl group is generally nematogenic like vinyl carboxy group.
Present investigation supports and raises credibility to the conclusions drawn earlier.
Acknowledgments
Authors acknowledge thanks to Dr. N. N. Vyas, head of chemistry department, and principal, Dr. M. B.
Patel of the college, for their supportive view and providing research facility for present investigation.
Authors are also thankful to Dr. A. V. Doshi, ex. principal, M. V. M. Science and Home. Sc. College,
Rajkot, for his valuable suggestions, cooperation, and support as and when needed throughout this
investigation. Authors acknowledge thanks to L. M. Pharmacy Collage, Ahmedabad for analytical and
spectral services.
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