Mesomorphism and Molecular Structure of Ester Homologous Series Containing Carboxy and Vinyl Carboxy Linking Groups

Article abstract of DOI:10.1080/15421406.2015.1036494

A novel homologous series 4-(4′-n-alkoxy benzoyloxy)-isopentyl cinnamates consisting of eleven homologs is reported. Mesomorphism commences from the hexyloxy homolog up to the hexadecyloxy homolog. The hexyloxy and hexadecyloxy derivatives are only enantiotropically nematogenic while, but the octyl, decyl, dodecyl, and tetradecyl homologs exhibit enantiotropic nematogenic and smectogenic mesophases. The rest of the homologs are nonmesogenic. The textures of nematic phase are threaded or Schlieren and that of the smectic mesophase is focal conic smectic A or C type. The transition curves of the phase diagram behave in a normal manner without the exhibition of an odd-even effect. Analytical and spectral data confirm the molecular structure of the homologs. Overall mesogenic phase temperature range varies from 11.0°C to 59.0°C. The average thermal stabilities of smectic and nematic are 122.25°C and 139.0°C, respectively. The series is predominantly nematogenic and partly smectogenic with a middle-ordered melting type and a considerable range of liquid crystallinity. Melting points and transition temperatures were determined using an optical polarizing microscope equipped with a heating stage. The mesomorphic properties are compared with structurally similar series.

Full text of DOI:10.1080/15421406.2015.1036494

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Mesomorphism and Molecular Structure of Ester
Homologous Series Containing Carboxy and Vinyl
Carboxy Linking Groups
M. L. Chauhan & A. V. Doshi
To cite this article: M. L. Chauhan & A. V. Doshi (2015) Mesomorphism and Molecular Structure
of Ester Homologous Series Containing Carboxy and Vinyl Carboxy Linking Groups, Molecular
Crystals and Liquid Crystals, 623:1, 166-175, DOI: 10.1080/15421406.2015.1036494
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Date: 09 February 2016, At: 03:38

Mol. Cryst. Liq. Cryst., Vol. 623: pp. 166–175, 2015
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2015.1036494
Mesomorphism and Molecular Structure of Ester
Homologous Series Containing Carboxy and Vinyl
Carboxy Linking Groups
M. L. CHAUHAN1,∗ AND A. V. DOSHI2
1Sheth P. T. Arts & Science College, Godhra, India
2Matushri Virbaima Mahila Science & Home Science College, Saurastra
University, Rajkot, Gujarat, India
A novel homologous series 4-(4ꢀ-n-alkoxy benzoyloxy)-isopentyl cinnamates consisting
of eleven homologs is reported. Mesomorphism commences from the hexyloxy homolog
up to the hexadecyloxy homolog. The hexyloxy and hexadecyloxy derivatives are only
enantiotropically nematogenic while, but the octyl, decyl, dodecyl, and tetradecyl ho-
mologs exhibit enantiotropic nematogenic and smectogenic mesophases. The rest of the
homologs are nonmesogenic. The textures of nematic phase are threaded or Schlieren
and that of the smectic mesophase is focal conic smectic A or C type. The transition
curves of the phase diagram behave in a normal manner without the exhibition of an
odd-even effect. Analytical and spectral data confirm the molecular structure of the
homologs. Overall mesogenic phase temperature range varies from 11.0◦C to 59.0◦C.
The average thermal stabilities of smectic and nematic are 122.25◦C and 139.0◦C,
respectively. The series is predominantly nematogenic and partly smectogenic with a
middle-ordered melting type and a considerable range of liquid crystallinity. Melting
points and transition temperatures were determined using an optical polarizing micro-
scope equipped with a heating stage. The mesomorphic properties are compared with
structurally similar series.
Keywords Liquid crystals; mesogen; mesomorphs; nematic; smectic
Introduction
Liquid crystal materials find importance in various fields of life in the present era [1,2,3].
Therefore, liquid crystal materials of suitable range of temperature or composition are
required to manufacture various articles [4] at economical cost. Hence, the present inves-
tigation is planned to synthesize novel LC materials through homologous series of “iso”
linking terminal end group, which may yield LC materials of lower transition temperatures
with a considerable range of liquid crystallinity. Thus, the present investigation is restricted
to a novel ester homologous series of cinnamic acids [5,6] with —OR and – C5H11(iso)
terminal end group as flexible part of a novel series and two end phenyl rings bridged
through – COO- central bridge as rigid core of a molecule. Hence, proposed present work
is aimed to understand and establish the effects of molecular structure on LC behavior
∗Address correspondence to M. L. Chauhan, Sheth P. T. Arts & Science College, Godhra 389 001,
India. E-mail: chauhanmukesh2@yahoo.com
Color versions of one or more of the figures in the article can be found online at
www.tandfonline.com/gmcl.
166

Novel Mesomorphic Compounds and Study of Their Mesomorphism
167
of novel substances which may be suitable for applications at an economical cost. The
variation of the combined effects of molecular rigidity and flexibility [7,8,9] can yield LC
materials of different mesogenic behavior.
Experimental
Synthesis
4—Hydroxy benzoic acid was alkylated with suitable alkylating agents (R-X) by a modified
method of Dave and Vora [10]. 4–Hydroxy isopentyl cinnamate was synthesized by the
usual method of esterification using conc. sulfuric acid [11]. Alkoxy benzoic acids were
converted to the corresponding acid chlorides using thionyl chloride and then directly
condensed with 4–hydroxy isopentyl cinnamate in dry cold pyridine [12,13]. The final ester
homolog derivatives were individually decomposed, filtered, washed, dried, and purified
until constant transition temperatures were obtained. A synthetic route to the novel series
is mentioned below in Scheme 1.
Scheme 1. Synthetic route to the series.
4–Hydroxy benzoic acid, alkyl halides (R-X), methanol, KOH) 4–hydroxy cinnamic
acid, sulfuric acid, isopentyl alcohol, thionyl chloride, pyridine ethanol, etc. required for
synthesis were used as received except solvents which were dried and purified prior to use.

168

169

170
M. L. Chauhan and A. V. Doshi
Table 2. Transition temperature in ◦C
Transition temperature in ◦C
Sr. No.
R = n-alkyl group
Smectic
Nematic
Isotropic
1.
2.
3.
4.
5.
6.
7.
8.
Methyl
Ethyl
Propyl
Butyl
Pentyl
Hexyl
Octyl
Decyl
Dodecyl
Tetradecyl
Hexadecyl
—
—
—
—
—
—
—
—
—
—
107.0
114.0
124.0
129.0
122.0
114.0
130.0
180.0
123.0
141.0
116.0
152.0
155.0
144.0
132.0
126.0
125.0
—
96.0
97.0
75.0
85.0
—
9.
10.
11.
Characterization
Representative members of the series were characterized by elemental analysis through
Perkin Elmer PE 2400 CHN analyzer, infrared spectra were recorded on a Perkin Elmer
Spectrum GX, 1HNMR spectra on a Bruker spectrometer using CDCI3 as solvent and by
mass spectra (Table 1). The transition temperature and the liquid crystal properties were
determined by a miscibility method through an optical polarizing microscope equipped
with a heating stage.
Results and Discussion
4–Hydroxy isopentyl cinnamate is a nonmesomorphic vinyl ester. However on linking it
with 4–n–alkoxy benzoic acids through their acid chlorides induces mesomorphism as
nematic (Hexyloxy and hexadecyloxy) or as smectic in addition to nematic for mesogenic
homologs. The rest of the homolog derivatives, viz., methoxy to pentoxy are nonmesogenic.
Transition temperatures of the novel homologs are relatively lower than the corresponding
n-alkyl benzoic acids. Transition temperatures (Table 2) of the homologs are plotted as
the numbers of carbon atoms in n-alkyl chain of the left n-alkoxy terminal end group. A
phase diagram (Fig. 1) showing phase behavior of a novel series is obtained by linking like
or related points. The solid mesomorphic transition curve adopts a zigzag path of rising
and falling as the series is ascended in normal manner. The smectic-nematic transition
curve initially rises, passes through maxima and then descends as the series is ascended
without the exhibition of an odd-even effect and behaves in normal established manner.
The smectic-nematic transition curve is extrapolated [14,15] for butyloxy, hexyloxy, and
hexadecyloxy homologs on the basis of the trend of the curve to predict the latent transition
temperature (LTT) for smectic of the homologs concerned which are 92.0◦C, 102◦C, and
103.0◦C, respectively. The mesomorphic behavior of series varies homolog to homolog.
Nematic and smectic mesophase length varies from 45◦C to 03◦C and 54◦C to 18◦C,
respectively. Isotropic transition temperature of LC homologs varied between 155◦C and
125◦C. Thus, present novel series is predominantly nematogenic and partly smectogenic
and of middle ordered melting type.

Novel Mesomorphic Compounds and Study of Their Mesomorphism
171
Figure 1. 4-(4ꢀ-n-alkoxy benzoyloxy) isopentyl Cinnamates.
The nonmesomorphic behavior of methoxy to pentyloxy homologs is observed for
their inability to resist exposed thermal vibrations as a consequence of unfavorable molec-
ular rigidity and flexibility, induced by low dipole–dipole interaction and low magnitudes
of dispersion forces by the interaction between instantaneous dipoles produced by sponta-
neous oscillations of the electron clouds of the molecules which induced high crystallizing
tendency of a molecules with absence of mesomorphism. Mesomorphic homologs from and
beyond hexyloxy homolog which resist exposed thermal vibrations and disabling at an an-
gle less then 90◦ on the floating surface. The molecules of all the mesomorphic homologs
arrange themselves with statistically parallel orientational order on the floating surface
and showing threaded or schlieren texture of nematic type mesophase, whereas octyloxy,
decyloxy, dodecyloxy, and tetradecyloxy homolog molecules exhibited sliding layered ar-
rangement on floating surface due to having lamellar packing of molecules in their crystal
lattices which facilitated smectogenic mesophase formation prior to nematic mesophase
formation within definite range of temperatures. Thus, heating process of a homolog against
cohesive forces gradually reduces the intermolecular end to end and lateral attractions and a
sample under examination as soon as acquires suitable magnitudes of anisotropic forces of
intermolecular attractions to facilitate only nematic or smectic prior to the nematic phase.
The absence of an odd-even effect in N-I and Sm-N transition curve is attributed to the
absence of mesomorphism in the sorter homologs. Variations observed from homolog to
homolog in the same series are due to the sequentially changing methylene unit in the left
n-alkoxy (-OR) terminal end group. The variations observed from series to series for the
same homolog is attributed to the common fixed group of each series, which are present
at the other end of the molecule {viz., –CH=CH-COOC5H11 (iso), -COOC5H11(n), and
–OC5H11(n)}.
Extrapolation of nematic-isotropic transition curve to the left side at the butyloxy
homolog suggests that solid-isotropic and nematic-isotropic transition temperatures merges
into each other giving single temperature 140.0◦C, i.e., there is no possibility of nematogenic
phase formation for butyloxy homolog and its LTT for smectic should be 82.0◦C, but is
not actually realizable due to its high crystallization tendency. Similarly predicted LTT for

172
M. L. Chauhan and A. V. Doshi
Figure 2. Structurally similar homolog series.
hexyloxy and hexadecyloxy homolog (102◦C and 103◦C) below solid-nematic transition
temperatures are also not realizable due to their high crystallizing tendency.
Mesomorphism or LC property of present series-1 are compared with the structurally
similar homolog series-1, series-X [16], and series-Y [17] as shown below in Fig. 2.
Homolog series-1, X, and Y are identically similar with respect to rigid core, and consist
of two phenyl rings bonded through a –COO- central bridge and a flexible unit of the left
n-alkoxy terminal end group. However, they differ by the flexible terminal group bonded
from series to series at other end of the molecules, which are –CH CH-COOC5H11 (iso),
-COOC5H11(n), and –OC5H11(n), respectively, for series-1, X, Y. Therefore, combined
effects of varying molecular rigidity and flexibility operates the variations in mesomorphic
properties and the degree of mesomorphism [18].

Novel Mesomorphic Compounds and Study of Their Mesomorphism
173
Table 3. Average thermal stabilities in oC
Series
Series-1
Series-X
Series-Y
Smectic nematic
Smectic isotropic
Commencement of smectic phase
Nematic isotropic
Commencement of nematic phases
122.25
(C8–C14)
C8
139.3
(C6–C16)
C6
51.5
(C6–C12)
C5
71.0
(C10–C14)
C10
109.0
(C5–C8)
C5
—
Following Table 3 represents average thermal stabilities for smectic and nematic,
commencement of smectic and nematic mesophase or phases, exhibition of mesomorphism
from homolog to homolog, etc. for a presently investigated series-1 and series-X and series-
Y chosen for comparison.
Table 3 indicates that
• Smectic thermal stability of ester homologs series-1 is higher than ester homolog
series-X of the lowest thermal stability and are etheroester homologous series-Y
• Smectic mesophase formation commences earliest from pentoxy homolog of series-
X, while, it commences from octyloxy and decyloxy homologs for series-1 and Y,
respectively.
• Nematic thermal stability of series-1 is greater than series-X and Y in which nematic
mesophase formation is totally absent for a series-X.
• Nematic mesophase commences from pentyloxy homolog for a series Y and it com-
mences from hexyloxy homolog of a series-1 with absence of nematic mesophase
in series-X.
• Homologous series-1 and Y are partly smectogenic and partly nematogenic, whereas
homologous series-X is smectogenic only.
The highest thermal stability for smectic and nematic of series-1 is attributed to the
iso linkage present in Pentyl hydrocarbon chain, and the presence of conjugated double
bond in vinyl carboxy group which increases molecular rigidity as compared to –COO-
group of series-X and Y. Moreover presence of iso linking (series-1) enhances molecular
polarizability which contributed more than the intermolecular attraction which occurred
by intermolecular cohesion. Moreover the part of a terminal end group –CH=CH-COO-
has relatively greater length than –COO- group present as a part of a terminal group of
series-X, and ether group –O- of series-Y. Therefore, vinyl group of series-1 causes more
noncoplanarity due to the twist obtained as the oxygen atoms of the vinyl carboxy group
bump into the non bonded adjacent hydrogen atoms of the aromatic rings. On account of
above effects, the suitable magnitudes for anisotropic forces of intermolecular attractions
as a consequence of differing molecular rigidity and flexibility, the thermal stability, and
commencement of smectic and nematic mesophases are higher than series chosen for
comparison X and Y. Thus, variations in suitable magnitudes of anisotropic forces of
intermolecular attractions causes variation in disalignment of molecules at an angle less
than 90◦ on floating surface, presence or absence of lamellar packing of molecules in their
respective crystal lattices, extent of molecular noncoplanarity, type of maintenance of two
dimensional order of molecules in floating a condition, etc. which result into variations in
mesogenic behavior for the same homolog from series to series.

174
M. L. Chauhan and A. V. Doshi
Conclusion
(1) Presently investigated novel homologous series is predominantly nematogenic and
partly smectogenic with middle-ordered melting type and considerable degree of
mesomorphism.
(2) Group efficiency order derived on the bases of (a) thermal stability and (b) early
commencement of mesophase for smectic and nematic are as under
(a) Smectic: –CH=CH-COOC5H11 (iso) > –OC5H11(n) > -COOC5H11(n) Ne-
matic: –CH=CH-COOC5H11 (iso) > –OC5H11(n) > -COOC5H11(n)
(b) Smectic: -COOC5H11 (n) > –CH=CH-COOC5H11 (iso) > –OC5H11(n) Ne-
matic: –OC5H11(n) > –CH=CH-COOC5H11 (iso) > -COOC5H11(n)
(3) Molecular rigidity and flexibility are the important factor to induce mesomorphism.
(4) Suitable magnitudes of anisotropic forced of intermolecular attractions emerging
as a consequence of favorable molecular rigidity and flexibility are very sensitive
and susceptible to molecular structure.
Acknowledgments
Authors are thankful to the management of M. V. M. Science and Home Science College,
Rajkot and Dr. N. D. Jadav (Retd. Prof.) of Faculty of Technology and Engineering, M.S.
University of Baroda, Vadodara, for their valuable cooperation in the work. Authors are
also thankful to scientists of National Salt and Marine Research Institute, Bhavnagar for
the analysis of sample. Authors are also thankful to the management of Sheth P.T. Arts &
Science College, GODHRA for their provided laboratory work in facilities
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