Mesomorphism dependence on heterocyclic end group

Article abstract of DOI:10.1080/15421406.2015.1068972

An ester homologous series containing a heterocyclic end group was synthesized and studied with a view to understanding and establishing the effect of molecular structure on liquid crystal behavior. The novel series consists of 13 homologues. All members of the series are enantiotropically smectogenic without exhibition of nematogenic character. The textures of smectogenic homologues are of the type smectic A or C. The transition temperatures were determined by an optical polarizing microscope equipped with a heating stage. The Sm-I transition curve in a phase diagram behaves partly in an abnormal manner with the exhibition of an odd-even effect. The average thermal stability for smectic is 141.45°C and the mesomorphic phase length ranges from 8.1°C to 30.3°C. Analytical and spectral data confirm the molecular structures. The liquid crystal properties of present series are compared with the structurally similar known homologous series.

Full text of DOI:10.1080/15421406.2015.1068972

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Mesomorphism dependence on heterocyclic end
group
G. N. Bhola & U. C. Bhoya
To cite this article: G. N. Bhola & U. C. Bhoya (2016) Mesomorphism dependence
on heterocyclic end group, Molecular Crystals and Liquid Crystals, 625:1, 11-19, DOI:
10.1080/15421406.2015.1068972
To link to this article: http://dx.doi.org/10.1080/15421406.2015.1068972
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Date: 24 February 2016, At: 03:33

MOL. CRYST. LIQ. CRYST.
, VOL. , –

http://dx.doi.org/./..
Mesomorphism dependence on heterocyclic end group
G. N. Bhola and U. C. Bhoya
Department of Chemistry, Saurashtra University, Rajkot, Gujarat, India
ABSTRACT
KEYWORDS
An ester homologous series containing a heterocyclic end group was
synthesized and studied with a view to understanding and establishing
the effect of molecular structure on liquid crystal behavior. The novel
series consists of 13 homologues. All members of the series are enan-
tiotropically smectogenic without exhibition of nematogenic character.
The textures of smectogenic homologues are of the type smectic A or C.
The transition temperatures were determined by an optical polarizing
microscope equipped with a heating stage. The Sm-I transition curve in
a phase diagram behaves partly in an abnormal manner with the exhi-
bition of an odd-even effect. The average thermal stability for smectic
is 141.45°C and the mesomorphic phase length ranges from 8.1 °C to
Enantiotropy; liquid crystal;
mesomorphic; nematogenic;
smectic
30.3 °C. Analytical and spectral data confirm the molecular structures.
The liquid crystal properties of present series are compared with the
structurally similar known homologous series.
Introduction
The study of mesomorphism has proven its importance since its discovery in 1888 [1] for its
unique combination of molecular ordering in a liquid phase. Various research groups in differ-
ent branches of science and technology have exploited the liquid crystal phase for the benefit
of mankind [2–12]. The present investigation is planned with a view to understanding and
establishing the effects of molecular structure on mesomorphism of a substance through the
synthesis of novel homologous series. Several homologous series have been reported [13–20]
of varying moieties by changing molecular rigidity and flexibility [21–24] through variation
of the number of phenyl, heterocyclic or hydrocarbon rings, the central groups, the terminal
or lateral groups, which change the shape, size, length, polarity, and polarizibility, etc. [25–27].
Mesomorphic properties were evaluated in terms of molecular rigidity and flexibility. Thus,
with the knowledge of the effects of molecular structure the wider research community can
benefit from this novel research.
Experimental
Synthesis
4
-n-Alkoxybenzaldehydes were synthesized by refluxing 4-hydroxybenzaldehyde (1 equiv.)
with corresponding n-alkyl bromides (1 equiv.) in the presence of potassium carbonate
CONTACT G. N. Bhola
versity, Rajkot , Gujarat, India.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
 Taylor & Francis Group, LLC
drucbhoya@gmail.com, drucbhoya@sauuni.ernet.in
Department of Chemistry, Saurashtra Uni-
©

1
2
G. N. BHOLA AND U. C. BHOYA
Scheme . Synthetic route to the novel series.
1 equiv.) and acetone [28]. The resulting 4-n-alkoxybenzaldehydes were reacted with mal-
(
onic acid (1.2 equiv.) in the presence of 1–2 drops piperidine as catalyst and pyridine as
solvent to yield corresponding trans 4-n-alkoxy cinnamic acids (A) [29]. 3-(2-furanyl)-1-
(
4-hydroxyphenyl) 2-propen-1-one (B) was prepared by an established method [30] M.P.:
1
58 °C–160 °C, Yield: 69.3%. Coupling of compound A and compound B was done by Steglich
esterification to yield α-4(4’-n-alkoxy cinnamoyloxy) benzoyl-β-(2-furanyl) ethylene [31].
The synthetic route to the novel homologous series of ethylene derivatives is shown in
Scheme 1.
Characterization
Some of the members of novel series were characterized by elemental analysis (Table 1),
1
infrared spectroscopy, H NMR spectra, and mass spectroscopy. Microanalysis was per-
formed using a EuroEA Elemental Analyzer. IR spectra were recorded on Shimadzu FTIR
1
Model -IRAffinity-1S (MIRacle 10), HNMR spectra were recorded on a Bruker spectrome-
ter using CDCl as a solvent, and mass spectra were recorded on Shimadzu GC-MS Model
3
No.QP-2010. The liquid crystal behavior and the type of textures were determined by a mis-
cibility method on microscopic observations.
Table . Elemental analysis for () ethyloxy, () octyloxy, and () dodecyloxy derivatives.
Elements %found
H
Elements %calculated
H
Sr. no.
Molecular formula
C H O
C
O
C
O



.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.




C H O
 
C HO



MOL. CRYST. LIQ. CRYST.
13
Analytical data
Elemental analysis (Table 1)
Spectral data
HNMR in ppm for pentyloxy derivative
0
.92–0.95 (t,3H,-CH of –OC H group), 1.36–1.48 (m,4H,CH -CH -CH -), 1.73–1.83
3
5
11
3
2
2
(
p,2H,-CH -CH -O-), 3.97–3.99 (t,2H,-CH -O-), 7.26–7.32 (d,1H, -CH=CH-COO-), 6.46–
2
2
2
6
.52 (d,1H, -CH=CH-COO-), 7.53–7.54 (d,1H, -CO-CH=CH-), 7.82–7.86 (d,1H, -CO-
CH=CH-), 6.87–7.62 (3H, furfural ring),7.47–7.54 & 8.08–8.10 (4H, middle phenyl ring),
6
.91–6.93 & 7.58–7.62 (4H, phenyl ring with alkoxy chain). The NMR data are reliable with
the molecular structure.
HNMR in ppm for decyloxy derivative
0
.86–0.90 (t,3H,-CH of –OC H group), 1.27–1.32 (m,12H,CH -CH -CH -CH -CH -
3
10 21
3
2
2
2
2
CH -CH -), 1.42–1.45 (p,2H,-CH -CH -CH -O-), 1.76–1.83 (p,2H,-CH -CH -O-), 3.97–
2
2
2
2
2
2
2
4
.01 (t,2H,-CH -O-), 7.47–7.54 (d,1H, -CH=CH-COO-), 6.47–6.52 (d,1H, -CH=CH-COO-
2
)
, 7.59–7.62 (d,1H, -CO-CH=CH-), 7.82–7.86 (d,1H, -CO-CH=CH-), 6.91–8.11 (3H, fur-
fural ring), 7.47–7.54 & 8.08–8.11 (4H, middle phenyl ring), 6.91–6.93 & 7.59–7.62 (4H,
phenyl ring with alkoxy chain). The NMR data are reliable with the molecular structure.
-
IR in cm for heptyloxy derivative
3
1
1
1
056 (C-H str. of alkene disubstituted), 2927 & 2856 [C-H str. of (-CH -) group of –OC H ],
2
n
4
9
720 (C=O str. of carbonyl carbon of ester group), 1653 (C=O str. of α, β unsaturated ketone),
653 & 1629 (C=C str. of alkene), 1593, 1571, & 1504 (C=C str. of aromatic ring), 1332 &
288 (C-H bending of alkene disubstituted), 1251 (C-O str. of ether linkage), 1130 (C-O str.
of ester group), 999, 972, & 867 (C-H bending of alkene). The IR data are consistent with the
molecular structure.
-
IR in cm for tetradecyloxy derivative
052 (C-H str. of alkene disubstituted), 2918 & 2850 (C-H str. of (-CH -) group of –
3
2
n
OC H ), 1724 (C=O str. of carbonyl carbon of ester group), 1654 (C=O str. of α, β unsatu-
12
25
rated ketone), 1629 (C=C str. of alkene), 1597, 1573, & 1514 (C=C str. of aromatic ring), 1336,
1
303, & 1288 (C-H bending of alkene disubstituted), 1255 (C-O str. of ether linkage), 1132
(
C-O str. of ester group), 972, 831, & 817 (C-H bending of alkene). The IR data are consistent
with the molecular structure.
Mass spectra of propyloxy derivative
+
m/z (rel.int%): 402 (M) ,189, 147, 119, 91, 65
Mass spectra of hexyloxy derivative
+
m/z (rel.int%): 444(M) , 231, 214, 147, 127, 119, 91, 65

14
G. N. BHOLA AND U. C. BHOYA
Table . Transition temperatures in °C.
Compound no.
n-alkyl chain C H
(n)
Sm
N
Isotropic
n
n+














.
.
.
.
.
.
.
.
.
.
.
.
.
–
–
–
–
–
–
–
–
–
–
–
–
–
.
.
.
.
.
.
.
.
.
.
.
.
.
















Sm, Smectic; N, Nematic.
Results and discussion
4
-n-Alkoxy cinnamic acids are dimeric, which on condensation with 3-(2-furanyl)-1-(4-
hydroxyphenyl) 2-propen-1-one in pyridine yielded C –C , C , C , C , C , and C smec-
1
8
10
12
14
16
18
togenic homologues with absence of nematogenic mesophase formation. Their transition and
melting temperatures are between 115.0 °C and 158.9 °C. Dimerization of trans n-alkoxy cin-
namic acids disappears on esterification. Transition temperatures (Table 2) of homologues
as determined from a hot stage polarizing microscopy were plotted versus the number of
carbon atoms in the n-alkoxy chain and then, like or related points were linked to draw the
Cr-Sm and Sm-I transition curves of the phase diagram (Fig. 1). The Cr-Sm transition curve
adopts a zigzag path of rising and falling values with an overall falling tendency as the series is
ascended. The Sm-I transition curve descends with about five degrees Celsius difference in the
transition temperatures of the odd and even members; and then passes through a maxima at
the C homologue and finally descends through the C , C , and C homologues. The tran-
12
14
16
18
sition curves for odd and even homologues merge at the nonyloxy (C ) homologue and then
9
continue as a single curve up to the C member. Thus, the Cr-Sm transition curve behaves
18
in a usual established manner and the Sm-I transition curve behaves partly in normal man-
ner. The average thermal stability for smectic is 141.45 °C and the mesophase length ranges
from 8.1 °C to 30.3 °C. Thus, the present novel series is middle ordered melting type with
short range of liquid crystallinity of smectic type. The mesomorphic properties commence
Figure . Phase behavior of series.

MOL. CRYST. LIQ. CRYST.
15
from the very first member of the novel series and vary from homologue to homologue due
to the changing length of the left n-alkoxy terminal end group. None of the homologues show
monotropic mesomorphism and nonmesomorphic behaviors.
The absence of nematogenic mesophase formation and the exhibition of only smectic
mesophase formation from very first member of a present series is attributed to the unsuitable
magnitudes of anisotropic forces of intermolecular end-to-end attractions as a consequence
of combined effects of unfavorable molecular rigidity and flexibility which are not sufficiently
strong enough to maintain statistically parallel orientational order of molecules in floating
condition to induce nematic property. However, the favorable molecular rigidity displayed by
the central bridges which link the phenyl or heterocyclic ring through at least one conjugated
double or multiple bond as well as overall sufficient molecular polarizibility which favors the
formation of lamellar packing of molecules in their crystal lattices from very first member (C1)
of the series to its last member (C ). Thus, the molecules of C –C homologues with suitable
18
1
18
magnitudes of intermolecular anisotropic forces are sufficient to facilitate originally layered
order in their crystal lattices which acquires sliding layered molecular arrangement under
exposed thermal vibrations for certain definite range of temperatures; depending upon molec-
ular resistivity toward heat or exposed thermal vibrations or the forces holding the planes of
the layers together in the rigid crystal which are weakened relative to the attractions among
the molecules within a given layer at solid mesomorphic transition temperature and the lay-
ers may become free to slide and rotate over one another giving rise to smectic mesophase
formation and then on heating the smectic phase to a higher temperature the intermolecular
forces between the slides of the molecules weakened to such an extent that resultant inter-
molecular forces do not permit molecules to slide out of the layer and statistically parallel
orientational order is not maintained at all; but entire sample of a substance transform into
isotropic liquid. Conversely on cooling the isotropic melt, the smectic phase reappears exactly
at a temperature at which mesophase was disappeared in a reversible manner in case of all the
homologues (C –C ) of the present novel series. The shorter n-alkoxy terminal end group
1
18
lies with the major axis of the core, and the end-to-end contact ultimately differs for odd and
even homologues through C –C . Therefore, an odd-even effect is observed. However, the
1
8
odd-even effect disappears from C because the longer n-alkyl chain may coil, flex, or bend
9
to lie with the major axis of the core structure and the end-to-end contact would then be the
same for odd and even homologues. Variations in degree of mesomorphic properties from
homologue to homologue in the same series is attributed to the sequentially added methy-
lene unit to the left n-alkoxy chain. The sequential addition of a methylene unit contributes
toward molecular flexibility and the resultant combined effects of molecular rigidity and flex-
ibility, which vary from homologue to homologue causing changing trend in mesomorphic
behaviors.
Some mesomorphic properties such as thermal stability, commencement of mesophase,
mesophase length range, and odd-even effect of the presently investigated novel series-1 are
compared with the structurally similar known homologues series-X [32] and -Y [33] as shown
below in Fig. 2.
Homologous series-1, -X, and -Y are identical with respect to two or three phenyl rings
and two central bridges linking three phenyl rings or two phenyl rings and a heterocyclic
ring. Moreover, they are identical with respect to left n-alkoxy terminal end group for the
same homologue, but they differ from homologue to homologue in the same series. However,
homologous series-1 differs from series-X and -Y with respect to ring system as terminal
end groups. Series-1 contain a heterocyclic five-membered furan ring system, series-X
contain aromatic unsubstituted phenyl ring system and series-Y contain monoatomic chloro

1
6
G. N. BHOLA AND U. C. BHOYA
Figure . Structurally similar series.
para)-substituted phenyl ring. Therefore, the mesomorphic properties and the degree of
(
mesomorphism vary with changing part of the molecular structures of series-X and -Y
chosen for comparison in Fig. 2; because molecular rigidity and flexibility vary with the
changing molecular structures. Table 3 shows comparative data for series-1, -X, and -Y.
From Table 3 the following points emerge:
r
Presently investigated homologous series-1 is only smectogenic, series-X is only nemato-
genic, and series-Y is smectogenic in addition to nematogenic.
r
Smectogenic property commences from very first member of a series-1, but it com-
mences late from eighth member of a series-Y and it does not commence till the last
(
C ) member of a series-X.
1
6
r
Nematic property commences from fifth (C ) member of the series-X and -Y, but it does
5
not commence till the last member of a present series-1.
r
r
r
r
Thermal stability for smectic of series-1 and -Y are 141.45 °C and 132.6 °C, respectively.
Thermal stability for nematic for series-X and -Y are 155.4 °C and 156.7 °C, respectively.
Total mesophase length is in increasing order from series-1 to series-X to series-Y.
All the three homologous series under comparison exhibited odd-even effect in their
transition curves of respective phase diagram.
Exhibition of mesophase formation, the type of mesophase formation, either smectic or
nematic, and the total mesophase length range of a series depends upon its suitable magni-
tudes of anisotropic forces of intermolecular end-to-end and/or lateral attractions as a con-
sequence of favorable molecular rigidity and/or flexibility as immerged from the permanent
dipole moment across the long molecular axis, dipole-dipole and electronic interactions, dis-
persion forces, etc. Homologous series of present investigation contain five-membered ring
with oxygen as hetero atom, which withdraws electron from carbon with absence of any flex-
ible functional group bonded to terminal ring system, except left n-alkoxy group. Therefore,
molecular rigidity of series-1 predominates over its molecular flexibility forming lamellar
packing of molecules in their crystal lattices giving rise to sliding layered arrangement of
molecules in floating condition under the influence of exposed thermal vibrations favorably
to cause smectic phase due to heterocyclic terminal. However, on further heating the same
Table . Average thermal stabilities in °C.
Series →

X
Y
Smectic-sotropic
Commencement of smectic mesophase
Nematic-isotropic
Commencement of nematic phase
Total mesophase length range (Sm + N)
. (C –C )
–
–
. (C –C )




C
–
–
C
. (C –C )
. (C –C )




C
C
. (C ) to . (C )
. (C ) to . (C )
. (C ) to . (C )







MOL. CRYST. LIQ. CRYST.
17
smectic phase, the end-to-end attractions are weakened to such an extent that statistically par-
allel orientational order of molecules is not maintained in floating condition to cause nematic
phase. Series-X possesses phenyl ring as terminal instead of heterocyclic ring, but it is homo-
cyclic ring with absence of hetero atom like oxygen. Therefore, combined effect of molecular
rigidity and flexibility causes suitable magnitudes of end-to-end intermolecular attractions,
which maintains only statistically parallel orientational order of molecules to cause nematic
phase, but it fails to form lamellar packing of molecules and hence the sliding-layered molecu-
lar arrangement which is missing under thermal treatment; showing absence of smectic phase.
Homologous series-Y contains polarizable monoatomic functional -Cl group substituted at
para position of third phenyl ring, which increases molecular length, polarity and polariz-
ibility, flexibility in addition to equivalent rigidity of series-1 and -X, the intermolecular end-
to-end and lateral attractions are suitable and sufficient to maintain sliding-layered molecu-
lar arrangement, and then to maintain statistically parallel orientational order of molecules
beyond smectic phase under exposed thermal vibrations in floated condition. Thus, smectic
phase in addition to nematic phase, one after another appeared for different ranges of tem-
perature from homologue to homologue in the same series and for the same homologue from
series to series with varying degree or range and the type (Sm or/and N) of mesomorphism
which are resulted and appeared through an optical polarizing microscopy equipped with
a heating stage or thermogram from differential scanning calorimeter or thermal analysis.
Early commencement of smectic mesophase as compared to series-Y and -X is attributed to
the less noncoplanarity of the molecules, i.e., molecules of series-1 are relatively less non-
coplaner than the corresponding same homologue of series-Y and -X due to heterocyclic end.
However, nematic mesophase commences from C member of series-X and -Y, but it does not
5
commence till the last member of a series-1, because the end-to-end attractions are stronger
for the series-X and -Y as compared to series-1 due to chloro substitution at para position and
polarizibility difference of phenyl ring and a heterocyclic ring system. The thermal stability for
smectic of series-1 is greater than a series-Y under comparison is due to the variation in the
molecular rigidity and flexibility associated with the respective molecular structure whose
energy content (ꢀH) differs. Thermal resistivity and thermal withstanding capacity of the
same homologue molecules from series-1 to series-Y may be in decreasing order for smectic
which reflected in facilitating the stabilization of smectic phase throughout the series-1, but
partly stabilized smectic phase in case of series-Y. The highest thermal stability for nematic
of series-Y as compared to series-X and -1 is attributed to the difference of end-to-end inter-
molecular attractions occurred due to differing permanent dipole moment across the long
molecular axis, dipole-dipole and electronic interactions, etc., which hinders the stabilization
of nematic phase in series-1 but favors in case of series-X and -Y. Odd-even effect is observed
for transition curve or curves in the phase diagrams of series of present investigation and
the series-X and -Y chosen for comparison. Thus, liquid crystal behavior and properties from
homologue to homologue in the same series and for the same homologue from series to series
depend upon the changing part of the molecules as a consequence of changing molecular
rigidity and flexibility due to varying magnitudes of heat content (ꢀH) of different molecular
structures under comparative study.
Conclusions
r
Presently investigated homologous series is entirely smectogenic without exhibition of
nematic property with middle-ordered melting type and shorter phase length range.

1
8
G. N. BHOLA AND U. C. BHOYA
It consisted with three-ring system bonded through -CH=CH-COO- central bridge
between two phenyl rings and another central bridge -CO-CH=CH- linking a phenyl
ring and a heterocyclic furan ring.
r
Group efficiency order derived on the basis of (i) thermal stability, (ii) early commence-
ment of mesophase, and (iii) total (Sm + N) mesophase length range for smectic and
nematic are as under.
(
i) Smectic
Nematic
(ii) Smectic
Nematic
(iii) Smectic + Nematic
r
A molecule bearing heterocyclic ring is smectogenic irrespective of nematogenic central
bridge -CH=CH-COO-.
r
A phenomenon of mesomorphism and the degree of mesomorphism are very sensi-
tive and susceptible to the molecular structures as a consequence of favorable molecular
rigidity and flexibility.
Present investigation of smectogenic mesophase may be useful for the study of binary
systems and for the devices operated between 115.0 °C and 158.9 °C or below about
r
2
0.0 °C (i.e., 100.0 °C) or smectic LC are valuable for increasing the sensitivity of the
color response of modified nematic.
r
Present homologue derivatives may be useful to act as antimalarial, anticancer, antibac-
terial, anti-inflammatory, antifungal, etc. if their biological or microbiological activity is
properly studied by the researchers or biologists.
r
Mesomorphism depends upon molecular structure.
Acknowledgment
The authors acknowledge thanks to the Department of Chemistry (DST-FIST Funded & UGC-SAP
Sponsored), 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 valuable cooperation
during present investigation as and when needed. They are also thankful to the National Facility for
Drug Discovery through New Chemical Entities (NCE’s) for analysis of samples.
References
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MOL. CRYST. LIQ. CRYST.
19
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