The mesomorphic properties of chalcone dimer derivatives

Article abstract of DOI:10.1080/15421406.2014.953720

A homologous series of symmetrical dimers viz bis (4-n-alkoxy benzoyloxy) -phenyl β-benzoyl-ethylenes was synthesized and studied with a view to understanding the relation between molecular structure and the mesomorphic behavior. Twelve members of the dimeric series were synthesized, and the methoxy to butyloxy homologues are not liquid crystals, but the pentyloxy to hexadecyloxy are liquid crystals. The pentyl to heptyl dimers are only nematogenic, but the higher homologues show smectogenic mesomorphism prior to nematogenic mesophase formation. The phase diagram of phase transition curves behaves in a normal manner except for the smectic to nematic transition curve which behaves in an unexpected manner. The smectic and nematic thermal stabilities are 103°C and 131.7°C, respectively. The smectogenic and nematogenic phase lengths vary between 17.4°C to 28.3°C and 2.1°C to 29.9°C, respectively. The liquid crystal properties of presently investigated novel chalcone dimers are compared with known monomeric chalcones.

Full text of DOI:10.1080/15421406.2014.953720

Mol. Cryst. Liq. Cryst., Vol. 608: pp. 116–124, 2015
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2014.953720
The Mesomorphic Properties of Chalcone
Dimer Derivatives
V. C. KOTADIYA AND U. C. BHOYA∗
Department of Chemistry, Saurashtra University, Rajkot, Gujarat, India
A homologous series of symmetrical dimers viz bis (4-n-alkoxy benzoyloxy) α-phenyl β-
benzoyl-ethylenes was synthesized and studied with a view to understanding the relation
between molecular structure and the mesomorphic behavior. Twelve members of the
dimeric series were synthesized, and the methoxy to butyloxy homologues are not liquid
crystals, but the pentyloxy to hexadecyloxy are liquid crystals. The pentyl to heptyl dimers
are only nematogenic, but the higher homologues show smectogenic mesomorphism
prior to nematogenic mesophase formation. The phase diagram of phase transition
curves behaves in a normal manner except for the smectic to nematic transition curve
which behaves in an unexpected manner. The smectic and nematic thermal stabilities
are 103◦C and 131.7◦C, respectively. The smectogenic and nematogenic phase lengths
vary between 17.4◦C to 28.3◦C and 2.1◦C to 29.9◦C, respectively. The liquid crystal
properties of presently investigated novel chalcone dimers are compared with known
monomeric chalcones.
Keywords Liquid crystal; mesogen; mesophase; nematic; smectic
Introduction
Liquid crystalline materials of conventional and unconventional molecules have shown a
great deal of scientific technological, pharmaceutical, and medicinal or biological potential
in applications [1–5]. There are several potential applications of symmetrical and non-
symmetrical LC dimers. Among the dimers, chalcone derivatives play a significant role
in prevention of various degenerative diseases, and human ailments [6]. Chalcones and
their derivatives contribute in their role as biogenetic precursors, anti-inflammatory [4],
anticancer, and antioxidant agents [7], as well as for protective efficacy against radiation
induced oxidative stress [8]. The simplest oligomer is known as a dimer in which two
mesogenic units are bonded through a flexible spacer. The proposed present investigation
is aimed to synthesize a symmetrical chalcones dimer derivatives with two 4-n-alkoxy
benzoyloxy units bonded through α-4-hydroxy phenyl β-4ꢀ-hydroxy benzoyl ethylene. The
flexible section of the dimers will consist of alkoxy terminal chains CnH2n+1
O
(where,
n = 1–8, 10, 12, 14, and 16). Four phenyl rings bonded through two carboxy groups and one
central chalconyl group act as the rigid core of a molecule without the attachment of any
lateral groups. The present investigation will include the synthesis of chalconyl homologue
∗Address correspondence to U. C. Bhoya, Department of Chemistry, Saurashtra University,
Rajkot 360005, Gujarat, India. E-mail: drucbhoya@gmail.com
Color versions of one or more of the figures in the article can be found online at
www.tandfonline.com/gmcl.
116

The Mesomorphic Properties of Chalcone Dimer Derivatives
117
dimer derivatives, characterization of some selected derivatives by elemental analysis, infra
1
red [IR], HNMR, mass spectroscopy, thermography by polarizing microscopy equipped
with a heating stage, and the texture determination of mesophases by a miscibility method.
Finally, the comparative study of the LC properties of the present series will be compared
with other known series. Thus, present study is planned with a view to understanding and
establishing the relation between molecular structure [9–11] and the mesogenic behavior of
chalconic novel dimers on the basis of the molecular rigidity and flexibility [12–15]. Doshi
et al. have reported number of conventional and unconventional chalconic (α, β ethylene
derivatives) homologous series [16–20].
Experimental
Synthesis
4-n-Alkoxy benzoic acids were synthesized by alkylating, 4-hydroxy benzoic acid with a
suitable alkylating agent by a modified method of Dave and Vora [21] and component-A,
component-B, α-4-hydroxy phenyl β-4ꢀ- hydroxyl benzoyl ethylene (m.p. 202◦C–203◦C
yield 80%) were prepared by the usual established method [22, 23] using 4-hydroxy
benzaldehyde and 4-hydroxy acetophenon. Components A (2 moles) and B (one mole)
were condensed in dry cold pyridine through the corresponding acid chlorides. [24, 25].
The final chalconic ester dimeric homologue derivatives were individually decomposed,
filtered, washed, dried, and recrystallised until constant transition temperatures obtained.
4-Hydroxy benzoic acid, alkyl halides, MeOH, KOH, EtOH, 4-hydroxy benzaldehyde,
4-hydroxy aceetophenone, thionyl chloride, pyridine, etc. required for synthesis were used
as received except solvents which were dried and purified prior to use. The synthetic route
to the novel dimeric chalconic series is shown in Scheme 1.
Characterization
Some of members of a novel series were characterized by elemental analysis (Table 1), IR
1
spectroscopy, HNMR spectra, mass spectrometry, and textural identification. Elemental
analysis was performed on EuroEA Elemental Analyzer. FTIR data were acquired with a
Shimadzu FTIR-8400 spectrophotometer in the frequency range of 4000–400 cm−1 with
1
samples embedded in KBr pellets. HNMR spectra were recorded in DMSO-d6 using a
BRUKER Avance-III (400 MHz) spectrometer and mass spectra were recorded on Shi-
madzu GC-MS Model No.QP-2010 instrument. The phase transition temperatures and the
type of textures were measured by a miscibility method on microscopic observations, the
sample being prepared as a thin film sandwiched between a glass slide and a cover.
Analytical Data
Elemental Analysis
Spectral Data.
NMR in ppm for Hexyloxy Derivative
0.82–0.86 (t, 6H, CH3, alkyl chain), 1.25–1.76 (m, 16H, (CH2)4 , alkyl chain), 3.95–3.98
(t, 4H, CH2 O, alkoxy chain), 6.89–8.08 (m,16H, aromatic proton), 7.75, and 8.03

118
V. C. Kotadiya and U. C. Bhoya
Scheme 1. Synthetic route of novel series.
(d, 2H, olelfinic proton( CH CH )). The NMR data are consistent with the molecular
structure.
NMR in ppm for Octyloxy Derivative
0.82–0.84 (t, 6H, CH3, alkyl chain), 1.23–1.77 (m, 24H, (CH2)6 , alkyl chain), 3.96–4.00
(t, 4H, CH2 O, alkoxy chain), 6.90–8.10 (m,16H, aromatic proton), 7.47, and 8.04 (d,
2H, olelfinic proton( CH CH )). The NMR data are consistent with the molecular
structure.

The Mesomorphic Properties of Chalcone Dimer Derivatives
119
Table 1. Elemental analysis for (1) ethyloxy, (2) pentyloxy, and (3) tetradecyloxy
derivatives
Elements % found
Elements % calculated
Sr. no.
Molecular formula
C
H
O
C
H
O
1
2
3
C33H28O7
C39H40O7
C57H76O7
73.82
75.51
78.55
5.29
6.45
8.67
20.90
18.03
12.78
73.87
75.46
78.40
5.26
6.50
8.77
20.87
18.04
12.83
IR in cm−1 for Decyloxy Derivative
2929, 2843 (C H Str. of polyethylene group), 1726 (C O Str. of ester group), 1510, 1604
(C C Str. of aromatic ring), 1417 (aliphatic C H bond), 1068, 1261 (C O Str. of ester
group), and 42 (para substituted phenyl ring). The IR data are consistent with the molecular
structure.
IR in cm−1 for Dodecyloxy Derivative
2926 (C H Str. of aromatic ring), 2856 (C H Str. of polyethylene group), 1728 (C O
Str. of ester group), 1506, 1618 (C C Str. of aromatic ring), 1464 (aliphatic C H bond),
1022, 1166, 1288 (C O Str. of ester group), and 848 (para-substituted phenyl ring). The
IR data are consistent with the molecular structure.
Mass Spectra of Butyloxy Derivative
m/z (rel.int%): 592(M)+
Mass Spectra of Heptyloxy Derivative
m/z (rel.int%): 676(M)+
Texture Determination by Miscibility Method
• Pentyloxy homologue: Nematic Threaded type
• Heptyloxy homologue: Nematic Schlieren type
• Octyloxy homologue: Smectic-A
• Tetradecyloxy homologue: Smectic-C
Result and Discussion
The resently synthesized novel series Bis (4-n-alkoxy benzoyloxy) α-phenyl β-benzoyl-
ethylenes consists of 12 homologue dimer derivatives of chalcones in which variable
n-alkyl chains bonded to first and last phenyl rings in para position through ether link-
ages as a flexible unit. Mesomorphism commences from the fifth dimer of a series. The
pentyl, hexyl, and heptyl dimers are only nematogenic, whereas the octyl, decyl, dode-
cyl, tetradecyl, and hexadecyl homologue dimers possess smectogenic character prior to
nematogenic character. The remaining homologues are nonmesogenic. The transition and
melting temperatures were determined by an optical polarizing microscope equipped with a
heating stage (Table 2). Transition temperatures of liquid crystal (LC) and nonliquid crystal
(NLC) dimers were plotted against the number of carbon atoms present in n-alkyl chain
bonded to phenyl rings through oxygen atom as terminal end groups. Transition curves,
viz., Solid-isotropic/mesomorphic, smectic-nematic, and nematic-isotropic are drawn by
linking like or related points to depict a phase diagram showing phase behavior of a novel

120
V. C. Kotadiya and U. C. Bhoya
Table 2. Transition temperatures in ◦C of series-1
Transition temperature in oC
Compound no.
R n-alkyl group CnH2n+1
Sm
N
Isotropic
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
10
12
14
16
—
—
—
—
—
—
—
—
—
136.4
113.8
160.6
102.1
96.6
91.9
108.3
116.1
170.5
161.2
145.2
149.9
162.5
143.7
185.6
121.8
108.5
102.4
111.5
118.2
—
—
80.5
71.3
74.5
86.0
73.4
9
10
11
12
Sm, Smectic; N, Nematic.
series (Fig. 1). Solid-isotropic/mesomorphic transition curve follows a zigzag path of ris-
ing and falling values with overall descending tendency as the series is ascended except
for the tetradecyloxy and hexadecyloxy derivatives. The nematic-isotropic transition curve
adopts a descending tendency as the series is ascended with the exhibition of an odd-even
effect and behaves predominantly in normal established manner except for the tetradecyl
Figure 1. Phase behavior of series.

The Mesomorphic Properties of Chalcone Dimer Derivatives
121
and hexadecyl derivatives. The smectic-nematic transition curve is descended from the
octyloxy dimer to the lowest dodecyl derivative, passes through minima and ascends until
the last hexadecyloxy homologue without exhibition of an odd-even effect. Analytical and
spectral data confirmed the molecular structures of the novel dimers. The nematic-isotropic
transition curve is extrapolated [26–28] for nonnematogenic behavior of the propyloxy and
butyloxy homologues to predict their nematic-isotropic transition temperatures keeping
in view of the trend of the concerned curves. The curves for odd and even members of
the dimers merge into each other at the third homologue of the series after extrapolation.
The total mesophase length (Sm + N or only N) vary between 25.3◦C at the hexadecyloxy
dimer. The average thermal stability for smectic and nematic phases are 103◦C and 131.7◦C,
respectively. Thus the novel dimer series is predominantly nematogenic and partly smecto-
genic of a middle ordered melting type and relatively short degree of mesomorphism. The
transition and melting temperatures generally alternate in a decreasing order. The variations
in mesomorphic behavior from homologue to homologue are observed with the number
of carbon atoms present in n-alkyl chains of left and right flexible terminal end groups.
The two mesogenic units bonded to two phenyl rings connected by two carboxy group
strengthens the molecular rigidity. The first to fourth members of the series do not have the
suitable combination of anisotropic forces of intermolecular attractions as a consequence
of combined effect of equivalent molecular rigidity required to induce mesomorphisam, not
even in the monotropic condition. Thus, they have a strong crystal tendency and sharply
melt to the isotropic liquid without passing through a liquid crystal phase. The rest of the
novel dimers do have molecules with suitable magnitudes of intermolecular anisotropic
forces of attractions which resist exposed thermal vibrations for some temperature differ-
ence during which they disalign at an angle less than 90◦ with the floating surface. The
molecules of pentyl, hexyl, and heptyl dimers are floated with only statistically parallel
orientational order. Whereas the octyl, decyl, dodecyl, tetradecyl, and hexadecyl dimers
have a sliding layered ordered arrangement initially and then show a statistically parallel
orientational ordered arrangement. The appearance of the nematogenic mesophase forma-
tion is attributed to the suitable magnitudes of end-to-end intermolecular attractions for
definite temperature range; whereas appearance of smectogenic mesophase formation is
attributed to the presence of lamellar packing of molecules in their crystal lattices caus-
ing sliding layered network arrangement with maintenance of some degree of temperature
range against thermal treatment through polarizing microscopy. The mesophase disappears
above the isotropic temperature of LC homologues. The molecules under the microscopic
examination are randomly oriented in all possible direction with high order of disorder or
high magnitudes of entropy (ꢀS ꢀH/T), but on cooling the same (enantiotropic homo-
logue) mesophase reappears at the same temperature at which the mesophase was formerly
appeared, i.e., reversal of transition temperature reappears in case of enantiotropic dimer
homologue. Changing trend of mesomorphic properties from homologue to homologue in
the same dimer series is attributed to the sequential addition of methylene unit or units
at the n-alkyl chain bonded to phenyl ring through ethereal linkage. The extrapolated
nematic-isotropic transition curve [29, 30] for odd and even homologues merge into propyl
and butyl dimers which matches to solid-isotropic point of propyl and butyl dimers, thus
the nonmesomorphicity of propyl and butyl derivatives are very well supported by appro-
priate extrapolation method. The mismatching or deviating trend from normal behavior
of smactic-nematic and nematic-isotropic transition curve is attributed to the presence of
two mesogenic units with two identical flexible end group units which may contribute
equitable or identical rigidity and flexibility from both ends of a single dimer molecule.
The mesomorphic behavior of presently investigated novel homologous dimer series-1 is

122
V. C. Kotadiya and U. C. Bhoya
Figure 2. Structurally similar homologous series.
compared with other monomeric known identical series of chalcones-X [31] as shown in
Fig. 2.
The presently investigated chalcone dimer series and a monomeric chalcones series
chosen for comparative study are identical with respect to three phenyl rings bonded
through one of the COO and only one CH CH CO central bridges linking first
and second as well as second and third phenyl rings, respectively, including one of the left
n-alkoxy terminal end group. However, they differ with respect to the presence and absence
of a OOC C6H4 OR mesogenic unit in series-1 and a series-X, respectively. There-
fore, observed difference in mesogenic behavior between dimer series-1 and monomeric
series-X are attributed to their differing features as presence and absence of one mesogenic
unit as shown above. Differing mesogenic unit OOC C6H4 OR contain COO cen-
tral group, one phenyl ring, and flexible unit OR, i.e., CnH2n+1. Thus, absence of
mesogenic unit of series-X reduces molecular length, aromaticity, and ratio of the molec-
ular polarity to polarizability, length to breath ratio, and many other factors conducive
to these effects. One of these effects is suitable magnitudes of anisotropic forces of in-
termolecular end-to-end and lateral attractions as a consequence of resultant molecular
rigidity and flexibility, which are main causes of inducing mesomorphism and extend or
contract degree of mesomorphism, etc. Thus, following Table 3 indicates some differ-
ence of mesomorphic behaviors between series-1 and X depending upon their structural
variations.
Table 3 indicates that both series [1andX] under comparative study,
• Do exhibit smectogenic and nematogenic characters.
Table 3. Average thermal stabilities in ◦C
Series
[1]
[X]
Smectic-isotropic or
Smectic-nematic
103.6
[C8-C16]
C8
106.6
[C10-C16]
C10
Commencement of smectic phase
Nematic-isotropic
Commencement of nematic phase
131.7
[C5-C16]
C5
119.8
[C8-C16]
C8

The Mesomorphic Properties of Chalcone Dimer Derivatives
123
• Smectic thermal stability of dimer series-1 is lower than the series-X, chosen for
comparison.
• Nematic thermal stability of dimer series-1 is higher than the series-X.
• Smectic and nematic mesophases of series-1 commences (C8 and C5) earlier than a
series-X (C10 and C8) under comparison.
• In general, mesomorphism commences from fifth member of dimer series-1, whereas
it commences late from octyloxy chalcone derivative of series-X.
Early or late commencement of mesophase depends upon the suitable magni-
tudes of anisotropic force of intermolecular attractions as a consequence of a favor-
able molecular rigidity and flexibility as well as the extent of noncoplanarity caused
by a molecule. The presences of one more mesogenic unit in a series of dimer deriva-
tives facilitate lamellar packing of molecules in their crystal lattices favorably earlier.
Moreover, presence of one more mesogenic unit, increases molecular length, polar-
ity, polarizability, rigidity, flexibility, dispersion forces, end-to-end and lateral attrac-
tions depending upon molecular closeness which resist or withstand exposed thermal
vibrations relatively of greater magnitudes than a molecule of series-X. Thus, nematic
mesophase formation facilitated to stabilize more strongly, than in the molecule of series-
X and hence, the nematic thermal stability of series-1 is greater than a series-X. The
early commencement of smectic mesophase is attributed to the favorable intermolecu-
lar forces arising from additional mesogenic unit which build up lamellar packing of
molecules in the crystal lattices of dimer series molecules earlier than the molecules of
series-X.
Conclusions
• A dimer homologous series of chalcones is predominantly nematogenic and partly
smectogenic with middle ordered melting type.
• As numbers of methylene units are increasing in n-alkyl chain of flexible units, the
nematic character of a substance is gradually reduced by smectogenic character.
i.e., lamellar packing of molecules in the crystal lattices depends upon the length of
n-alkyl chain, of a terminal unit.
• The behavior of smectic-nematic or smectic-isotropic transition curve depdnded
upon number of msogenic units of a dimer.
• As molecular rigidity or number of aromatic rings are increased in a molecule,
the thermal stability for smectic decreases and the thermal stability for nematic
increases.
• As aromaticity of a molecule increased, the commencements of smectic and nematic
mesophase formation tend to pass from higher number of a homologue, i.e., early or
late commencement of smectic and nematic mesophase is related to aromaticity of
a molecule or with the length of n-alkyl chain of terminal end group or end groups
of chalconic dimer molecule.
• The mesomorphic behaviors of chalconic dimer derivatives are susceptible and
sensitive to number of mesogenic unit or units present in a molecule.
• Mesophase formation efficiency order derived for chalcones on the basis of (a)
thermal stabilities (b) early commencement of mesophase, are as under (a) smectic:
Two mesogenic units < one mesogenic unit.
• Nematic: One mesogenic unit < Two mesogenic units.
• (b) smectic: Two mesogenic units > one mesogenic unit.
• Nematic: Two mesogenic units > one mesogenic unit.

124
V. C. Kotadiya and U. C. Bhoya
Acknowledgments
The authors thank the Department of Chemistry (DST-FIST Funded & UGC-SAP Spon-
sored) 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 valu-
able cooperation during present investigation as and when needed. Also thanks are due
to the National Facility for Drug Discovery through New Chemical Entities (NCE’s), for
analysis of samples.
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