Mesomorphism of Novel Symmetric Dimers with Changing Alkoxy Terminal End Group

Article abstract of DOI:10.1080/15421406.2015.1017299

A novel homologous series of symmetric dimers has been synthesized. It consists of thirteen (C1 to C8, C10, C12, C14, C16 and C18) dimer ester derivatives from trans cinnamic acid with pyrocatechol. Mesomorphic property commences from C7 homologue as smectic. C1 to C6 members of the series are nonmesomorphic. C7 is monotropic smectic and the rest of the homologues from C8 to C18 are enantiotropic smectic. The nematic mesophase is totally absent. Textures and transition temperatures of the homologues are determined by an optical polarizing microscope equipped with a heating stage. IR, ¹H NMR spectra, mass spectra, and differential scanning calorimetry of some members is included. The spectroscopic analyses support the molecular structures. Mesomorphic properties of the novel symmetric dimer series are compared with structurally similar series. The novel series is partly nonmesomorphic and predominantly smectogenic with textures of the A or C type, without the exhibition of the nematic phase.

Full text of DOI:10.1080/15421406.2015.1017299

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Mesomorphism of Novel Symmetric Dimers with
Changing Alkoxy Terminal End Group
G. N. Bhola, R. H. Maheta & U. C. Bhoya
To cite this article: G. N. Bhola, R. H. Maheta & U. C. Bhoya (2015) Mesomorphism of Novel
Symmetric Dimers with Changing Alkoxy Terminal End Group, Molecular Crystals and Liquid
Crystals, 623:1, 129-138, DOI: 10.1080/15421406.2015.1017299
To link to this article: http://dx.doi.org/10.1080/15421406.2015.1017299
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Date: 01 February 2016, At: 08:48

Mol. Cryst. Liq. Cryst., Vol. 623: pp. 129–138, 2015
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2015.1017299
Mesomorphism of Novel Symmetric Dimers with
Changing Alkoxy Terminal End Group
G. N. BHOLA, R. H. MAHETA, AND U. C. BHOYA∗
Department of Chemistry, Saurashtra University, Rajkot, Gujarat, India
A novel homologous series of symmetric dimers has been synthesized. It consists of
thirteen (C1 to C8, C10, C12, C14, C16 and C18) dimer ester derivatives from trans cin-
namic acid with pyrocatechol. Mesomorphic property commences from C7 homologue
as smectic. C1 to C6 members of the series are nonmesomorphic. C7 is monotropic smec-
tic and the rest of the homologues from C8 to C18 are enantiotropic smectic. The nematic
mesophase is totally absent. Textures and transition temperatures of the homologues are
determined by an optical polarizing microscope equipped with a heating stage. IR, 1H
NMR spectra, mass spectra, and differential scanning calorimetry of some members is
included. The spectroscopic analyses support the molecular structures. Mesomorphic
properties of the novel symmetric dimer series are compared with structurally similar
series. The novel series is partly nonmesomorphic and predominantly smectogenic with
textures of the A or C type, without the exhibition of the nematic phase.
Keywords Liquid crystal; Monotropic; Nematic; Smectic; Symmetric dimers
Introduction
The liquid crystalline (LC) state of matter was first reported in 1888 [1]. Many researchers
of various disciplines in science and technology have shared their keen interest with dif-
ferent views, aims and objectives in order to benefit mankind. Chemists have synthesized
and reported many novel LC substances [2–5] useful other research groups. The chemists
have synthesized novel conventional and unconventional LC and characterized them and
further studied the effects of molecular structure on LC property [6, 7] by comparing LC
behaviors with other series. Vorlander described LC properties in ‘V’-shaped compounds
[8, 9]. Over the past 30 years, chemists have shared their increasing interest in unconven-
tional symmetric and asymmetric LC dimers [10]. The present investigation is planned
to synthesize symmetric dimers in which, two identical mesogenic units are bonded to
a spacer at an angle 60 degrees through a 1,2-dihydroxy benzene (pyrocatechol) unit to
construct ‘V’ shaped novel LC dimers to understand and establish the relations between
molecular structure and liquid crystallinity on the basis of molecular rigidity and flexibility
[11–15]. Thus, the present study is aimed at fundamental aspects of liquid crystals rather
than directed at applications [16, 17].
∗Address correspondence to U. C. Bhoya, Department of Chemistry, Saurashtra University,
Rajkot 360005, Gujarat, India. E-mail: drucbhoya@sauuni.ernet.in
Color versions of one or more of the figures in the article can be found online at
www.tandfonline.com/gmcl.
129

130
G. N. Bhola et al.
Scheme 1. Synthetic route to the series.
Experimental
Synthesis
4-Hydroxy benzaldehyde was alkylated by suitable alkylating agents R-X by refluxing
for 4-5 hr with acetone and K2CO3 through an established method [18]. 4-n-Alkoxy ben-
zaldehydes were individually condensed with malonic acid in pyridine in the presence of
few drops of piperidine to obtain trans 4-n-alkoxy cinnamic acids. Transition temperatures
of dimeric 4-n-alkoxy cinnamic acids matched with the reported values. Trans n-alkoxy
cinnamic acids were individually treated with pyrocatechol (ortho dihydroxy benzene)
in the presence of DCC, 5 mol% DMAP and CH2Cl2 in the usual manner [19]. Final
products were washed with methanol under vacuum filtration, then recrystallized until con-
stant transition temperatures were obtained. The synthetic route to the series is shown in
scheme-1.
The chemicals 4-hydroxy benzaldehyde, R-X (alkyl halides), K2CO3, acetone, DCC,
DMAP, CH2Cl2, pyrocatechol etc. were used as received except solvents which were dried
and distilled prior to use.
Characterization
Some selected members of a series were characterized by elemental analysis (Table 1),
1
infrared spectroscopy, H NMR spectra, mass spectroscopy, and DSC analysis (Table 2).
Table 1. Elemental analysis for (1) Butyloxy (2) Octyloxy (3) Octadecyloxy derivatives
Molecular
formula
Elements%
Found
Elements%
Calculated
Sr. No.
C H O
C H O
1
2
3
C32H34O6
C40H50O6
C60H90O6
74.75 6.62 18.63
76.71 8.02 15.27
79.38 10.02 10.60
74.69 6.66 18.65
76.65 8.04 15.31
79.42 10.00 10.58

Mesomorphism of Novel Symmetric Dimers with Changing Alkoxy Terminal 131
Table 2. DSC data for C14 and C16 homologues at the heating rate of 5◦C per minute
Transition
Absolute
Temperature
temperature
◦C
(t)
◦K
Homologue
C14
Transition
ꢀH Jg−1
T = t + 273 ꢀs = ꢀH/T
Cr-Sm
Sm-I
Cr-Sm
Sm-I
70.57
94.21
87.37
94.57
1.77
8.78
154.38
11.02
343.57
367.21
360.37
367.57
0.00515
0.02391
0.4284
C16
0.02998
Microanalysis was performed on a EuroEA Elemental Analyzer. IR spectra were recorded
on a Shimadzu FTIR-8400 spectrometer. 1H NMR spectra were recorded on Bruker Avance-
III (400 MHz) spectrometer using CDCl3 as a solvent. Mass spectra were recorded on
Shimadzu GC-MS Model No.QP-2010. Differential scanning calorimetery (DSC) was
performed by Shimadzu DSC-60 at scanning rate of 5◦C min−1. The liquid crystal behavior
and the type of textures were determined by a miscibility method based on microscopic
observations.
Analytical Data
Elemental analysis (Table 1)
Spectral Data
1HNMR in ppm for Ethyloxy Derivative. 1.38–1.44 (t,6H,-CH3 of both –OC2H5
group), 4.01–4.06 (m,4H,CH3-CH2-O- of both phenyl rings containing alkoxy
chain), 6.42–6.46(d,2H, -OOC-CH = CH- of both phenyl rings containing alkoxy
chain), 7.59–7.63(d,2H, -OOC-CH = CH- of both phenyl rings containing alkoxy chain),
7.27–7.45 (4H, middle phenyl ring), 6.82–7.84 (8H, both phenyl ring with alkoxy chain),
The NMR data is consistent with the molecular structure.
1HNMR in ppm for Decyloxy Derivative. 0.86–0.89 (t,3H,-CH3 of –OC10H21 group), 1.27
(m,24H,CH3-{CH2}6- of both phenyl rings containing alkoxy chain), 1.40–1.44(p,4H,-
CH2-CH2-CH2-O- of both phenyl rings containing alkoxy chain), 1.70–1.84 (4H,-
CH2-CH2-O- of both phenyl rings containing alkoxy chain), 4.07–4.13 (t,4H,-CH2-O-
of both phenyl rings containing alkoxy chain), 6.42–6.62(d,2H, -OOC-CH = CH- of
both phenyl rings containing alkoxy chain), 7.42–7.44(d,2H, -OOC-CH = CH- of both
phenyl rings containing alkoxy chain), 7.25–7.29 (4H, middle phenyl ring), 6.83–7.81 (8H,
both phenyl ring with alkoxy chain), The NMR data are consistent with the molecular
structure.
IR in cm−1 for Hexyloxy Derivative. 3045(C-H str. of aromatic), 2929, 2920 &2852 (C-H
str. of (-CH2-)n group of –OC6H13), 1730 (C=O str. of carbonyl carbon of ester group),
1697–1641 (C=O str. of α,β unsaturated ketone), 1641 (C=C str. of alkene), 1573, 1535
&1510 (C=C str. of aromatic ring), 1450(C-H bending of –CH2 alkane group), 1377 &
1342 (C-H bending of alkene), 1301, 1246, & 1224 (C-O str. of ester group), 991 &

132
G. N. Bhola et al.
Figure 1. Phase behavior of series.
698 (C-H bending of alkene disubstituted). The IR data is consistent with the molecular
structure.
IR in cm−1 for Dodecyloxy Derivative. 3066 (C-H str. of aromatic), 2955, 2929, & 2856
(C-H str. of (-CH2-)n group of –OC10H21), 1739 (C=O str. of carbonyl carbon of ester
group), 1676 (C=O str. of α,β unsaturated ketone), 1639 (C=C str. of alkene), 1577 &
1510 (C=C str. of aromatic ring), 1479 (C-H bending of –CH2 alkane group), 1388 & 1332
(C-H bending of alkene), 1296, 1286, & 1282 (C-O str. of ester group), 995 & 835 (C-H
bending of alkene disubstituted). The IR data are consistent with the molecular structure.
Mass Spectra of Propyloxy Derivative
m/z (rel.int%): 486(M)+, 412, 329, 287, 231, 204, 189 (base peak), 162, 147, 119, 91
Mass Spectra of Heptyloxy Derivative
m/z (rel.int%): 598(M)+, 337, 245(base peak), 215, 189, 147, 119, 91, 57, 41
Differential Scanning Calorimetry
The transition temperatures in ◦C and ◦K as associated value of enthalpy (ꢀH) changes in
Jg−1 of C14 and C16 homologues from DSC measurements are summarized in following
Table 2. C14 and C16 homologues are enantiotropically smectogenic which undergo Cr-Sm-
I transition recorded during heating at the rate of 5◦C per minute. DSC thermo gram of C16
(Fig.-2) bearing texture of smectic-C confirmed by miscibility method.

Mesomorphism of Novel Symmetric Dimers with Changing Alkoxy Terminal 133
Figure 2. DSC spectra of tetradecyloxy and heptadecyloxy homologues.
Results and Discussion
Trans 4-n-alkoxy cinnamic acids are dimeric, but their dimerization disappears on esterifica-
tion on linking with pyrocatechol and yields novel symmetric ester dimers with transition

134
G. N. Bhola et al.
Table 3. Transition temperatures in ◦C
Compound No.
n-alkyl chain CnH2n+1 (n)
Sm
N
Isotropic
1
2
3
4
5
6
7
8
C1
C2
C3
C4
C5
C6
C7
C8
C10
C12
C14
C16
C18
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
148.4
159.3
148.1
152.6
137.5
133.8
82.6
103.4
118.6
94.9
—
(69.8)
84.5
85.7
80.8
70.5
87.4
81.8
9
10
11
12
13
89.0
96.3
93.5
() indicates monotropy.
temperatures relatively lower than the corresponding trans p-n-alkoxy cinnamic acids.
Methoxy (C1) to hexyloxy (C6) homologue derivatives of dimers are nonmesomorphic and
the heptyloxy (C7) to octadecyloxy (C18) dimers exhibit smectogenic character without the
exhibition of a nematogenic mesophase. The C7 homologue is monotropic smectic and the
rest of dimers C8 to C18 are enantiotropic smectic. The transition temperatures (Table 3)
as determined using an optical polarizing microscopy equipped with a heating stage were
plotted against the number of carbon atoms present in the alkoxy terminal end group
(Figure 1). Cr-I or Cr-Sm and Sm-I transition curves are obtained by linking like or related
points showing the phase behavior of the novel dimers in a phase diagram. A Cr-I or Cr-Sm
transition curve follows a zigzag path of rising and falling values with an overall descending
tendency as the series is ascended. The Sm-I transition curve exhibits an odd-even effect.
It initially rises for odd and even members of the series and passes through a maxima at the
decyloxy (C10) dimer and then descends as the series is ascended. Thus, the Cr-I or Cr-Sm
and Sm-I transition curves behave in a normal established manner. The Sm-I transition
curves for the odd and even members merge into each other at the decyloxy (C10) dimer
and then it follows as a single transition curve for the higher homologues. Mesomorphic
properties of the dimers vary from homologue to homologue depending upon the variation
in molecular structure. Thermal stability (average) for smectic phase is 95.95◦C. Thus, the
present ‘V’-shaped dimer molecules of the series are smectogenic without the exhibition
of any nematogenic property.
Two identical mesogenic units are bonded to a rigid spacer to form symmetric dimers
of ‘V’-shaped geometry. The two identical units individually containing chemical bonds
have equal vector sum of bond polarities but due to the ‘V’ shape, the vector sum of
all bonds of a molecule as a whole have some resultant value which generates the dipole
moment across the long molecular axis of a molecule generating dipole–dipole interactions,
dispersion forces, suitable or unsuitable magnitudes of anisotropic forces of intermolecular
end to end and/or lateral attractions as consequence of the molecular rigidity and flexibility.
Intermolecular anisotropic forces of cohesion are unsuitable for the C1 to C6 dimers which
are unable to withstand exposed thermal vibrations exposed upon them. Hence, the crystal

Mesomorphism of Novel Symmetric Dimers with Changing Alkoxy Terminal 135
Figure 3. Structurally similar series.
lattices abruptly break and sharply melt into the isotropic state without the exhibition of a
mesomorphic state. However, on increasing the length of the two terminal alkoxy chains,
the combined effect of molecular rigidity and flexibility become favorable for generating
lamellar packing of molecules in a sliding layered arrangement of floating layers. Hence the
homologues from C7 to C18 exhibit smectic mesomorphism. The absence of nematogenic
character is attributed to the relatively weaker end to end attractions. The Sm-I curve for
the even numbered dimers is extrapolated [20, 21, 22] to the C6 homologue dimer to
determine the latent transition temperature (LTT). However, the mesophase formation
is unrealizable due to the LTT position being far below the isotropic temperature. The
disappearance of odd-even effect from and beyond C10 member of the dimer series is
attributed to the coiling or bending or flexing or coupling of longer n-alkyl chain with
the principal axis of the core molecular structure. Variations in mesogenic properties from
homologue to homologue in same dimer series is attributed to the sequentially added
methylene unit in the terminal alkoxy chains, which causes variations in permanent dipole
moment across the core structure of a molecule. Thus, the presently investigated novel dimer
series is predominantly smectogenic and partly nonmesomorphic, whose mesomorphic
phase length varies between 8.9◦C to 32.9◦C and is of a middle ordered melting type.
Some mesomorphic properties of present symmetric dimer series-1 are compared with
structurally similar known Series-X [23] as shown in Fig. 3.
The symmetric dimer series-1 of the present investigation and a dimer series-X as
chosen for comparison are identical with respect to the terminal alkoxy flexible chains.
Therefore molecular flexibility contribution due to this similarity may be considered as
equivalent. However, the molecular part linking two RO-C6H4- parts differ from each
other with respect to molecular rigidity and flexibility as well as their shapes and size
or with respect to suitable magnitudes of anisotropic forces of intermolecular cohesive
energy and closeness due to their varying permanent dipole moment across the long
molecular axis, dipole–dipole interactions, electronic interaction etc. for the same ho-
mologue from series to series under comparative study. Thus, the phenomena of mesomor-
phism, the type and the degree of mesomorphism depended upon changing molecular parts
-CH = CH-COO-C6H4-OOC-CH = CH- and
respectively

136
G. N. Bhola et al.
Table 4. Average thermal stabilities in ◦C
Dimer series →
Smectic–Isotropic or
Smectic–Nematic
Commencement of Smectic
phase
Nematic–Isotropic
Commencement of Smectic
phase
1
X
95.95◦C
(C8-C18)
129.0◦C
(C6-C10)
C6
C7
–
–
147.7◦C
(C6-C16)
C6
Total mesophase length
8.9–32.9◦C
(only smectic)
18–56◦C
Sm:12–34◦C/N: 16–32 oC
for dimer series-1 and X. The differing mesogenic molecular part of series-1 contains an
aromatic phenyl ring linked to two vinyl carboxylate (-CH = CH-COO-) units. Whereas,
the differing mesogenic part of series-X contain two methyl groups bonded to carbon (or
n-propane) and two aromatic carboxylate (-C6H4-COO-) units bonded to remaining two
valences of a carbon atom (Bisphenol-A). Thus, changing molecular part of series-1 contain
only one phenyl ring and conjugated -CH = CH- two units which contributes molecular
rigidity; whereas in case of series-X, two phenyl rings bonded to aliphatic
unit,
which contributes partly to molecular rigidity and partly to molecular flexibility. Thus,
suitable magnitudes of anisotropic forces of intermolecular end to end and lateral cohesive
forces differs for the same homologue from series-1 to series-X and differ from homologue
to homologue in the same series. Table 4 represents some mesomorphic properties like
thermal stabilities of mesophase or mesophases, commencement of mesophase, mesophase
length, trend of transition temperatures etc. as mentioned below.
Table 4 indicates that,
• Presently investigated series-1 is smectogenic without exhibition of nematic prop-
erty, whereas homologous series-X is partly smectogenic and partly nematogenic.
• Average thermal stability for smectic of series-1 is lower (95.95◦C) than series-X
(129.0◦C).
• Average thermal stability for nematic of a series-X is 147.7◦C, whereas present
series-1 does not exhibit nematogenic property i.e. nematic mesophase formation is
not stabilized till the last member of a series-1, irrespective of the presence of vinyl
carboxylate units, which has preferred tendency to form nematic phase.
• Smectogenic mesophase commences from C7 member of a series-1, whereas smectic
and nematic mesophase formation commences from C6 member of a series-X.
• Smectogenic mesophase length stabilities and ranges minimum of 8.9◦C to maxi-
mum of 32.9◦C; whereas the same smectogenic mesophase length ranges between
12 oC and 34 oC.
• Isotropic transition temperatures of ‘V’ shaped nonlinear dimer series-1 ranges
between 82.6◦C to 159.3◦C; whereas it ranges between 108◦C and 226◦C for entire
series, but of course, it ranges between 108◦C and 160◦C for smectogenic dimers
only or Sm-N transition temperatures ranges between 92◦C and 150◦C.

Mesomorphism of Novel Symmetric Dimers with Changing Alkoxy Terminal 137
Each homologue of a homologous series-1 contain overall three phenyl rings bonded
through two -CH = CH-COO- central bridges with common and same n-alkoxy terminal
end groups for the same homologue. But the molecules of dimer series-X contain four
phenyl rings bonded through –COO- central bridges via
with identical n-alkoxy
terminal end groups. Therefore dimers of series-1 are less aromatic but contain conjugation
in central bridges with two -CH = CH- units. Dimers of series-X are more aromatic, but
contain relatively shorter central bridges -COO- via aliphatic
unit. Therefore suit-
able magnitudes of anisotropic forces of intermolecular cohesion depending on molecular
shape and of almost equal molecular length for the same homologue from series to series
induces relatively stronger end to end attraction and molecular polarizibility in addition to
attractions needed for lamellar packing of molecules in crystal lattices of series-X. Thus,
the molecules of series-1 exhibited and stabilized smectic mesophase only with low smectic
thermal stability and late commencement of smectic phase from and beyond C7 homologue.
Whereas, dimer series-X exhibited and stabilized smectic and nematic phase with a little
bit higher thermal stability and early commencement of smectic phase from C6 homologue.
The depression of transition temperatures of series-1 reflects the low resistivity toward
exposed thermal vibrations as compared to corresponding homologue resistivity series-X.
Hence, the variations in mesomorphic behavior of the same homologue from series to series
depend upon differing molecular part of a substance.
Conclusions
• Presently investigated ‘V’-shaped dimer series is smectogenic only.
• Molecular rigidity and flexibility of the dimers depend upon the rigidity offered by
the spacer unit linking two identical mesogenic units of a symmetric dimer.
• Two arms of a ‘V’-shaped dimers with their two arms at an angle sixty degree are
generally exhibit smectic polymesomorphism or smectic and nematic mesophase
formation. This statement is very well supported by present investigation and thus,
it raises the credibility to the conclusions drawn earlier.
• Presently investigated novel ester series of thirteen dimers (C1 to C18) are smec-
togenic and nonmesomorphic from C1 to C6, whose transition curves behaved in
normal manner in a phase diagram with exhibition of an odd-even effect for Sm-I
transition curve.
• Thus, ‘a phenomenon of mesomorphism is very sensitive and susceptible to molec-
ular structure.’
Acknowledgments
The authors acknowledge thanks to the Department of Chemistry (DST-FIST Funded &
UGC-SAP Sponsored), Saurashtra 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 investigation as and when needed. Also
thanks to the National Facility for Drug Discovery through New Chemical Entities (NCE’s)
for analysis of samples.

138
G. N. Bhola et al.
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