Mesomorphism dependence on molecular rigidity with reference to -CH=CH- unit of central bridge

Article abstract of DOI:10.1080/15421406.2015.1069004

A novel ester homologous series of rich mesomorphism and low temperatures with unexpected phase behaviors of eleven homologues was synthesized and studied with a view to understanding and establishing the relation between mesomorphic behaviour and the molecular structure of a series 4-(4′-n-alkoxybenzoyloxy)-4″-chlorobenzyl cinnamates. All the members of the novel series are enantiotropically smectogenic and the octyloxy (C₈) to hexadecyloxy (C₁₆) homologues are enantiotropically nematogenic, in addition to smectogenic. Odd-even effect is observed for Sm-I/Sm-N transition curve but it is absent for N-I transition curve. Textures of nematic phase are threaded or Schlieren and that of the smectic phase are fan shaped or batonates of smectic-A type phase or Smectic-C type for C₁₆ homologues as judged directly from a heating stage of an optical polarizing microscopy. Analytical and spectral data confirmed the molecular structures of novel homologues. Mesomorphic properties of present series are compared with the structurally similar other known series. The average smectic and nematic thermal stabilities are 92.78°C and 100.8°C, respectively. Mesophase length minimum to maximum for smectic and nematic are 21.0°C to 31.1°C and 8.4°C to 42.6°C respectively. Thus, the present novel series is partly nematogenic and fully smectogenic with considerable degree of mesomorphism and low melting type.

Full text of DOI:10.1080/15421406.2015.1069004

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Mesomorphism dependence on molecular rigidity
with reference to ‒CH˭CH‒ unit of central bridge
R. B. Marathe & A. V. Doshi
To cite this article: R. B. Marathe & A. V. Doshi (2016) Mesomorphism dependence on
molecular rigidity with reference to ‒CH‒CH‒ unit of central bridge, Molecular Crystals and
Liquid Crystals, 625:1, 81-92, DOI: 10.1080/15421406.2015.1069004
To link to this article: http://dx.doi.org/10.1080/15421406.2015.1069004
Published online: 19 Feb 2016.
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Date: 15 March 2016, At: 09:53

MOL. CRYST. LIQ. CRYST.
, VOL. , –

http://dx.doi.org/./..
Mesomorphism dependence on molecular rigidity with
reference to –CH=CH– unit of central bridge
a
b
R. B. Marathe and A. V. Doshi
a
b
Huntsman International India Pvt. Ltd, Umraya village, Taluka: Padra, Vadodara, Gujarat, India; Dr K. N. Modi
University, Plot No.INS-, RIICo Industrial Area Ph-II, Newai, Dist. Tonk, Rajasthan, India
ABSTRACT
KEYWORDS
A novel ester homologous series of rich mesomorphism and low tem-
peratures with unexpected phase behaviors of eleven homologues was
synthesized and studied with a view to understanding and establish-
ing the relation between mesomorphic behaviour and the molecular
Liquid crystals; mesogen;
mesomorphism; nematic;
smectic
ꢀ
ꢀꢀ
structure of a series 4-(4 -n-alkoxybenzoyloxy)-4 -chlorobenzyl cinna-
mates. All the members of the novel series are enantiotropically smec-
togenic and the octyloxy (C ) to hexadecyloxy (C ) homologues are
8
16
enantiotropically nematogenic, in addition to smectogenic. Odd–even
effect is observed for Sm–I/Sm–N transition curve but it is absent for
N–I transition curve. Textures of nematic phase arethreaded or Schlieren
and that of the smectic phase are fan shaped or batonates of smectic-
A type phase or Smectic-C type for C16 homologues as judged directly
from a heating stage of an optical polarizing microscopy. Analytical and
spectral data confirmed the molecular structures of novel homologues.
Mesomorphic properties of present series are compared with the struc-
turally similar other known series. The average smectic and nematic
thermal stabilities are 92.78°C and 100.8°C, respectively. Mesophase
length minimum to maximum for smectic and nematic are 21.0°C to
31.1°C and 8.4°C to 42.6°C respectively. Thus, the present novel series is
partly nematogenic and fully smectogenic with considerable degree of
mesomorphism and low melting type.
Introduction
The mesogenic tendency of liquid crystalline (LC) substances [1] has been proven its ability to
benefit mankind since 1888 [2–10]. The present investigation is planned with a view to under-
standing and establishing the effects of molecular structure on LC properties and the degree of
mesomorphism through synthesis of novel LC substances by homologous series. The present
investigation is aimed (i) to synthesize novel homologous series including their characteriza-
tion, (ii) to evaluate thermal data in terms of thermal stability, commencement of mesophase,
and mesophase length range, (iii) to compare the LC behaviors of present series with struc-
turally similar other series, (iv) to interpret and discuss normal or abnormal behaviours in
terms of molecular rigidity and flexibility [11–16] with reference to presence and absence of
–CH=CH– unit in central bridge, and (v) conclude the effects of molecular structure on LC
CONTACT R.B.Marathe
Vadodara, Gujarat, India.
maratharaj@gmail.com
HuntsmanInternationalIndiaPvt.Ltd.,Umrayavillage,Taluka,Padra,
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

8
2
R. B. MARATHE AND A. V. DOSHI
properties [17–20] including suggested possibility of application of novel substances to LC
researchers working with aims, object and views other than present investigation. Number
of homologous series of LC have been reported to date [21–25] but present novel series con-
sists of three phenyl rings, two central bridges –COO– and –CH=CH–COO–CH - with two
2
terminal end groups –OR and –Cl.
Experimental
Synthesis
4
-Hydroxy benzoic acid was alkylated by suitable alkylating agents R-X to form n-alkoxy
benzoic acid by a modified method of Dave and Vora [26] as main component of a
ꢀ
series. 4-Hydroxy-4 -chlorobenzyl cinnamate (m.p.: 98°C, Yield: 80%) was prepared from 4-
chlorobenzyl bromide and 4-hydroxycinnamic acid by applying the method of a US patent No.
0
053964, European patent and the modified method of Doshi, Patel and Marathe [27]. The
detailed process includes preparation of sodium salts of 4-hydroxy cinnamic acid by dissolv-
ing in the solvent N,N-dimethyl formamide and NaHCO at room temperature and sodium
3
salt of 4-hydroxycinnamic acid is condensed with 4-chlorobenzyl bromide by dissolving in
N,N-dimethyl formamide at room temperature. The solution was added to the solution of 4-
hydroxycinnamic acid slowly at room temperature. The reaction mass was stirred for 24 hr at
room temperature after addition of 4-chlorobenzyl bromide. After heating to 70–80°C for 2–
3
hr the mass was allowed to cool at room temperature. The inorganic salt (NaBr) isolated by
filtration. The filtrate containing the product decomposed in cold water under vigorous stir-
ring. The off-white precipitate formed and the N,N-dimethyl formamide dissolved in excess
water. The precipitated product was separated by a simple filtration method. The product was
washed with excess of water to remove the salts and solvents. The product was dried and
purified in toluene until constant melting point was obtained.
And n-Alkoxy benzoic acids were condensed through their corresponding acid chlorides
ꢀ
with 4-hydroxy-4 -chlorobenzyl cinnamate in dry pyridine to form final products by a usual
established method [28]. Final esterified products were individually decomposed, filtered,
washed dried, and purified until constant transition temperatures were obtained.
The required chemicals for synthesis 4-hydroxycinnamic acid, 4-hydroxy benzoic acid,
alkyl halides, methanol, KOH, 4-chlorobenzyl bromide in alcohol or N,N-dimethyl for-
mamide, NaHCO , HCl, Dry Pyridine, Thionyl chloride, Fresh Toluene, EtOH, etc. All these
3
were used as received, except solvents which were dried and purified prior to synthesis. Syn-
thetic route to the series is mentioned below (on page 17) as Scheme 1.
Characterization
The selected members of a novel homologues series were characterized and analyzed by ele-
1
mental analysis and the structure elucidation by Infra-red spectra, HNMR spectra and mass
spectra. Textures and transition temperatures of homologues as well as of related materials
were determined by an optical polarizing microscope, equipped with a heating stage. Ele-
mental analysis was performed on Perkin-Elmer PE 2400 C,H,N analyzer (Table 1). The per-
centage halide content is determined by usual analytical method. IR spectra were recorded
1
on Shimadzu Spectrum analyzer. HNMR spectra were determined on Bruker spectrometer
using CDCl solvent. Textures of smectic mesophase of some homologues were recognized
3
either directly from the microscopic observations or by miscibility method. Table 1

MOL. CRYST. LIQ. CRYST.
83
Scheme . Synthetic route to the series-.
Table . Elemental analysis for ethoxy, propyloxy and hexyloxy derivatives.
Sr. No.
Molecular formula
% Elements calculated (Experimental%)
H
C
Cl



C H ClO
. (.)
. (.)
. (.)
. (.)
. (.)
. (.)
. (.)
. (.)
. (.)




C H ClO
  
C H ClO





8
4
R. B. MARATHE AND A. V. DOSHI
Analytical data
Spectral data
HNMR in ppm for the dodecyloxy derivative
0
6
.86–1.83 (–CH of C H ), 3.99–4.05 (–CH – of –C H ), 5.19–5.22 (–CH=CH–), 6.29–
3
12 25
2
12 25
.99 (p-substituted phenyl ring), 7.33–7.40 (p-substituted benzene).
HNMR in ppm for the tetradecyloxy derivative
0
.85–0.89 (–CH of C H ), 1.15–1.50 (–CH – of –C H ), 3.97–4.05 (–O–CH –), 5.04–5.21
3
14 29
2
14 29
2
(
–CH=CH–), 6.20–6.24 (p-substituted phenyl ring), 6.26–6.97 (p-substituted benzene).
NMR data confirm the structure.
IR in cm  for butyloxy derivative
−
6
90–771 polymethylene of C H , 840 para-substituted phenyl ring, 1083, 1253, 1508 ester
4
9
linkage of (–COO–C H ), 921–943 Trans of (–CH=CH–) 1161 ether linkage of (C H –O–
4
9
4
9
of phenyl), 1014 Aromatic –Cl linkage, 758–775 (C–Cl stretch).
IR in cm  for pentyloxy derivative
−
7
1
25 poly methylene of C H , 802–833 para substituted benzene, 981 Trans (–CH=CH–),
5
11
012 Aromatic –Cl linkage, 1220, 1315, 11676, 1728 (–COO– ester), 1074, 1157 ether linkage
of (C H –O–C H ), 758 (para substituted– Cl), 3437 (H– bonding of OH), 2872, –COOH.
5
11
6
4
IR data confirm the structure.
Mass spectra
Mass Spectra for Hexyloxy Derivative:
Molecular weight: Calculated: 422. Molecular formula: C H ClO
24
19
5
Experimental: 422.
Texture of phase by miscibility method
Octyloxy (C ) derivative ———————-→ Threaded nematic
8
Tetradecyloxy (C ) derivative ———————–→ Schlieren nematic
14
Ethoxy (C ) derivative———————–→ Battonate Smectic-A
2
Hexyloxy (C ) derivative———————–→ Fan shaped Smectic–A
6
Tetradecyloxy (C ) derivative———————–→ Smectic-C
14
Results and discussion
The non-mesomorphic component 4-Hydroy-4 -chlorobenzylcinnamate (m.p.-98°C, Yield:
0%) is linked to the dimeric 4-n-alkoxy benzoic acid, whose dimerization disappears with
ꢀ
8
lowering and alternation of transition temperatures of resulted novel homologues. Mesomor-
phic behaviours are promoted by increasing molecular length through –CH –C H –Cl unit.
2
6
4
The unusual pattern of order of the mesophase occurrence observed for a present series,
because most of the ester or other homologous series follows usual pattern of mesophase
sequence. But in present series C –C are with smectic–isotropic transition temperatures,
1
6

MOL. CRYST. LIQ. CRYST.
85
Table . Transition temperatures of series- in °C.
Transition temperature in °C
Compound no.
R=C H
(n)
Sm
N
Isotropic
n
n+

C
C
C
C
C
C
C
C
C
C
C
.
.
.
.
.
.
.
.
.
.
.
–
–
–
–
–
.
.
.
.
.
.
.
.
.
.
.










–
.
.
.
.
.


Sm: Smectic
N :Nematic
without nematic mesophase formation, whereas C –C homologues give rise to form smec-
8
16
tic to isotropic phase change via nematic mesophase formation with alternation of isotropic
transition temperatures of C , C , C , C , C homologues. Thus, the sequential order
8
10
12
14
16
of mesophase occurrence in any series normally should be first nematic then smectic and
nematic and finally smectic-isotropic in the most of the homologous series. Generally, the
initial members may either nematic or isotropic or smectic. The odd–even effect may be
observed for N–I or / and Sm–N / Sm–I transition curve, but in case of presently investi-
gated novel series, the smectic-isotropic transition appears first and then smectic–nematic–
isotropic transition temperatures practically observed. Hence, the sequence of mesophase pat-
tern in present series is not maintained or followed in unexpected sequential ordered pattern
or it followed in unusual or unexpected ordered of mesophase pattern for nematic–isotropic
transition of higher homologues (C to C ); with longer n-alkyl chain bonded to first phenyl
8
16
ring through oxygen atom. Transition temperatures (Table 2) are plotted versus the num-
ber of carbon atoms present in n-alkyl chain of left n-alkoxy terminal end group. Smooth
curves are drawn through like or related points to draw phase transition curves as depicted
in a phase diagram (Fig. 1) showing the phase behavior of present series. Smectic-isotropic or
smectic–nematic transition curve initially descended and then rises from C homologue, then
8
passes through maxima at C12 homologue and finally descends till C through C homo-
16
14
logue. Thus, Sm–I or Sm–N transition curve behaves in normal manner. Cr–Sm transition
curve follows zigzag path of rising and falling with overall descending tendency and behaved
in expected normal manner. N–I transition curve adopts wave like path as shown by dotted
wave, in which dotted curve alternate its path C to C to C to C to C , i.e. N–I points of
8
10
12
14
16
C , C , C on linking show a N–I transition curve which descends as series is ascended. Sim-
8
12
16
ilarly on linking N–I points of C and C homologue constitute N–I transition curve with
10
14
descending tendency as series is ascended. These two hypothetical N–I curves mentioned
above are just like odd–even effect curves. But here they are curves related to even–even mem-
bers which may be assigned the term even–even effect with difference of four homologue, i.e.
phase behaviors in present series alternates with every two higher even numbered homologues
and repeats behavior at every four homologues. Thus, N–I transition curve like electromag-
netic wave appearance does not match with the normal behavior, as observed in the most
of the homologous series. The N–I transition as observed abnormally in higher homologue
C –C instead of in the lower homologues as odd–even effect. The consistency of transition
8
16
temperatures of homologues of present investigation were repeatedly confirmed on an optical

8
6
R. B. MARATHE AND A. V. DOSHI
Figure a. Phase behavior of Series-.
Figure b. Phase behavior of Series-X.

MOL. CRYST. LIQ. CRYST.
87
Figure c. Phase behavior of Series-Y.
polarizing microscopy equipped with a heating stage (POM) and the same values were exper-
imentally noted. Thus, mesomorphic properties of thermotropic liquid crystals depending
upon transition temperature which varies from homologue to homologue in the same present
series. Hence, present series is fully smectogenic and partly nematogenic whose Sm–I or N–I
transition temperatures vary between 90.3°C and 145.9°C. Cr–Sm transition temperatures
vary between 42.8°C and 89.3°C and corresponding Sm–I, Sm–N, N–I, and total (Sm + N)
mesophase length ranges between 21.7 (C ) and 63.9 (C ), 21.0 (C ) and 31.1 (C ), 8.4 (C )
6
2
14
10
10
and 42.6 (C ), 21.7 (C ) and 72.2 (C ), respectively.
8
6
8
Disappearance of dimerization of n-alkoxy acids is attributed to the breaking of hydrogen
bonding by the process of esterification, between two acid molecules. Alternation of transi-
tion temperatures is due to the even and odd number of methylene unit or units present in
n-alkyl chain bonded to first phenyl ring through oxygen atom. The disappearance of dimer-
ization reduces the rigidity of aromatic acid molecule and hence the transition temperatures
of final esterified compounds are lowered as compared to the corresponding n-alkoxy acids.
Moreover, the non-mesomorphicity of 4-methoxy benzoic acid and 4-ethoxy benzoic acids
ꢀ
disappears due to linking of non-mesogenic component 4-hydroxy-4 -chlorobenzyl cinna-
mate with methoxy and ethoxy benzoic acids by increasing molecular length, polarity, and
polarizability as a consequence of favorable molecular rigidity and flexibility, the magnitudes
of which results into lamellar packing of molecules in the crystal lattices of all members of a
novel series including methoxy and ethoxy benzoic acids derivatives which provides an envi-
ronment to induce smectogenic property. The exhibition of only smectogenic property from

8
8
R. B. MARATHE AND A. V. DOSHI
Figure . Structurally similar homologous series.
C to C suggest that, the magnitudes of intermolecular anisotropic forces of end to end attrac-
1
6
tions and closeness are unsuitable as a consequence of unfavorable magnitudes of molecular
rigidity and flexibility which hinders the statistically parallel orientational order of molecules
in floating condition on the surface; but the same molecules of pre-occupied lamellar packing
of molecules in their crystal lattices which exhibited smectic phase and now directly trans-
form into the isotropic liquid, in which molecules are randomly oriented in all possible direc-
tions with high order of disorder or high entropy from and beyond isotropic temperatures,
by missing of formation of nematic mesophase. The predominancy of intermolecular lateral
attractions caused more as compared to end to end attractions which arranges molecules to
float with sliding layered arrangement to show ordered molecular arrangement on heating
and subsequent cooling, respectively. Thus, molecules of C to C are unable to resist exposed
1
6
thermal vibrations beyond temperature of maintaining smectic phase formation because of
the low dipole–dipole interactions and the low magnitudes of dispersion forces by the inter-
actions between instantaneous dipoles produced by the spontaneous oscillations of electron
clouds of molecules. The exhibition of nematic mesophase between C and C of the even
8
16
numbered homologues is due to the suitable magnitudes of anisotropic forces of end to end
cohesion of the molecules. The intermolecular residual forces of end to end attractions is
now from and beyond C homologue is sufficient to maintain statically parallel orientational
8
molecular order in floating condition on the surface to exhibit nematogenic character, i.e.
isotropic mesophase appears via nematic mesophase from smectic phase. Odd–even effect
observed for Sm-I/Sm-N transition curve is due to the sequentially added methylene unit or
units present in n-alkyl chain of left n-alkoxy terminal end group. The absence of odd–even
effect in N–I transition curve is attributed to the late commencement of nematic phase from
C homologue and it continues for higher homologues of longer (C , C , C , C , C ) n-
8
8
10
12
14
16
alkyl chain with only even numbered methylene units in n-alkyl chain. Abnormal behavior of
Sm-N transition curve is attributed to the uncertainty in the status of the n-alkyl chain which
may coil or bend or flex or couple to lye with the principle axis of the core structure affecting
molecular rigidity and flexibility, due to the variations in the length, size, polarity and polariz-
ability, permanent dipole moment across the long molecular axis etc. Sm–I/Sm–N transition
curve for odd members of the present series occupies partly lower position (C and C ) and
1
3
vice versa than a curve for even members in unusual manner. The molecular polarizability is
2
attributed to the size of the mono-atomic −Cl tail group bonded to sp carbon of third phenyl
ring and –CH=CH– unit of vinyl carboxy group involving –CH – unit. The mesogenic prop-
2
erties of present novel series-1 are compared with structurally similar other known series-X
[
29] and Y [30], as shown in following Fig. 2:
Homologous series-1 of present investigation and the Series-X and Y chosen for compar-
ison are structurally similar. Homologous series-1 and X are identical with respect to three
phenyl rings and a central bridge –COO– linking first and middle phenyl rings which partly

MOL. CRYST. LIQ. CRYST.
89
Table . Thermal stability in °C.
Series→

X
Y
Smectic–Isotropic
Or
Smectic–Nematic
Commencement of Smectic phase
Nematic–Isotropic
Commencement of
Nematic phase
.
.
(C –C )
C
–
.
(C –C )
(C –C )






C
C
–
.
(C –C )

C

Mesophaselength range in °C
Sm-
Nm-
Sm:-  to .
Nm:- . to .
Total: .–.
C C
Total: . to .
Total: . to .
Total-
contributes to molecular rigidity and flexible –Cl tail end group common for the homologues
of same series. The left n-alkoxy terminal end group which vary from homologue to homo-
logue for same series but remain unaltered for the same homologue from series to series.
However, a central bridge –CH=CH–COO–CH – and –COO–CH – linking middle and
2
2
third phenyl ring differs from series-1 to series-X, which contributes to the total molecular
rigidity. Thus, combined effect of molecular rigidity and flexibility which operates the phe-
nomenon of mesomorphism can vary accordingly. Homologous series-1 and Y are isomeric
to each other. They are identical with respect to three phenyl rings which partly contributes
to total molecular rigidity and two flexible terminal end groups –OR and –Cl which con-
tribute to total molecular flexibility for the same homologue from series to series but differs
from homologue to homologue in same series. However the central bridge –COO– of series-1
linking first and middle phenyl rings is replaced by –CH=CH–COO– in series-Y. Similarly
a central bridge linking middle and third phenyl ring –CH=CH–COO–CH – of series-1 is
2
replaced by –COO–CH – in series-Y. The replacement of central bridges in case of isomeric
2
series-1 and Y causes difference due to –CH=CH– unit which displaces its place nearer or
away from tailed –Cl common terminal end group. –CH=CH– unit in case of series-1 and
Y contributes equally to the total molecular rigidity. But the difference in molecular rigidity
arises due to the difference in the distance of its place from tailed end group –Cl, (or –OR),
as affected by induced electronic environmental change on –CH=CH– unit. Thus, molecular
flexibility of flexible units –Cl and –OR for the same homologue from series to series remains
all almost unaltered by the replacement of central bridges but the magnitudes of molecular
rigidity between isomeric series-1 and Y differs from each other. The terminal end groups –Cl
and –OR being commonly present in all the three series under comparative study, the molec-
ular flexibility for the same homologue from series to series remains unchanged but differs in
molecular rigidity for the same homologue from series-1 to series-X to series-Y due to their
part of dissimilarity. Thus, the variations in mesomorphic properties and the degree of meso-
morphism observed amongst the molecules of homologues in the same series or for the same
homologue from series to series depend upon changing molecular rigidity (ꢀH), keeping
molecular flexibility almost unchanged. The phenomenon of mesomorphism occurs by the
combined effect of molecular rigidity and flexibility, but in case of present comparative study,
the variations in the mesomorphism will arise by varied molecular rigidity by replacement
of –CH=CH– unit difference (series-1 and X) and the displacement of place (series-1 and
Y) of –CH=CH– unit. Hence, the mesomorphism dependence of molecular rigidity plays an
important role in present comparative study. Following Table 3 represents some mesomorphic
properties for the series under comparison.
From the Table 3, it is clear that,

90
R. B. MARATHE AND A. V. DOSHI
r
r
Homologous series of present investigation-1 is fully or predominantly smectogenic and
partly nematogenic whereas; homologous series-X and y chosen for comparison are only
smectogenic without exhibition of nematogenic property.
Thermal stability for smectic and nematic for series-1 are 92.78 and 100.8, respectively,
whereas, smectic thermal stability of series-X and Y are 124.53 and 128.09, respectively,
i.e. smectic thermal stability is in increasing order from series-1 to X to Y.
r
Smectic mesophase commences from very first (C ) member of series-1, X and Y under
1
comparative study and it continues till the last member (C ) member of each series.
16
r
Nematic mesophase commences from octyloxy homologue (C ) of a series-1 and con-
8
tinue till hexadecyloxy homologue (C ) of the series. But nematic mesophase does not
16
commence till the last member (C ) of the series-X and Y.
16
r
r
Total mesophase length range is lowest and highest for the series-X and Y whereas, it is
an intermediate valued for present series-1.
Phase diagrams of series under comparison (series-1, X, Y) differ from each other as
shown in Figs. 1a–c.
Exhibition of only smectic property in absence of nematic property in series-X and Y indi-
cate that, the forces holding the planes of the layers together in the pre-occupied lamellar
packing of molecule in the rigid crystal lattices are weaken relatively to the intermolecular
attractions among the molecules within a given layer at solid-mesomorphic transition tem-
perature and the layers may become free to slide and rotate over one another inducing smectic
phase. Then on heating the smectic mesophase to a higher temperature, the intermolecular
forces between the sides of the molecules further weakened to such an extent that, resultant
intermolecular forces do not permit molecules to slide out of the layer and statistically paral-
lel orientational molecular order in floating condition does not maintain at all. Thus, absence
of nematic property and the exhibition of only smectic property for series-X and Y become
feasible. However, exhibition of nematic phase in addition to smectic phase become feasi-
ble in series-1 by suitable magnitudes of residual anisotropic forces which resists exposed
thermal vibrations to maintain statistically parallel orientational order of molecules to induce
nematic phase after the smectic phase in series-1 as a consequence of added molecular rigid-
ity through –CH=CH– unit which situated nearest to flexible –Cl tail end group in presently
investigated series-1, but it is either absent in series-X or it is far away from –Cl tail end group
in series-Y the causes hindrance of the possibility of inducing nematic property. The com-
mencement of smectic phase of series-1, X and Y occurs from the very first member (C ) of
1
each series. Early or late commencement smectic phase of series is related to the extent of
non-coplanarity of molecules. The early commencement of smectic phase is related with less
molecular non-coplanarity. Thus, planarity of the molecules of all the series under compar-
ison are reduced by common −Cl mono-atomic tailed end group for the same homologue
from series to series as well as due to the presence of multiple bond in –CH=CH– unit. The
extent of non-coplanarity and lateral attractions due to the central bridge being almost the
same for the molecules of all the series under comparative study, the added polarizability due
to tailed end group in present comparison is almost zero, because all the series have common
flexible –Cl tail. Moreover, the added polarizability due to –CH=CH– unit of central bridge
increases intermolecular attractions as a consequence of differing molecular rigidity related
to energy stored in unit mass and characteristics at constant pressure (ꢀH). Therefore, the
resistivity towards exposed thermal vibrations varies for the same homologue from series to
series and from homologue to homologue in the same series. Thus, varying thermal resistivity
results into variations in the degree of mesomorphism of smectic/nematic or total mesophase
length range and thermal stabilities for smectic and/or nematic. Hence, increasing order of

MOL. CRYST. LIQ. CRYST.
91
smectic thermal stability is observed for the series-1, X and Y. Missing of facilitating nematic
mesophase in series X and Y operates higher values of their smectic thermal stability and the
lowest value of smectic thermal stability of series-1 attributed to occurred at the cost of the
exhibition of added nematic mesophase formation. Now looking to the phase diagrams of iso-
meric series-1 and Y both series resembles to each other with respect to abnormal transition
curves obtained for C , C , C , C , C homologues, which appears like an electromag-
8
10
12
14
16
netic wave and the positions of curves for odd and even members of the series-1 and Y. Thus,
exchange of central bridge involving –CH=CH– unit facilitated to induce nematic phase in
present series-1, but the same –CH=CH– nearer to same tail group facilitated and induce
smectic phase for C , C , C , C and C homologues. In case of homologous series-X, the
8
10
12
14
16
transition curves for odd-even members, resemble to series-1, but shapes of curves from C8
to C differs abnormal behaviors due to absence of –CH=CH– unit.
16
Conclusions
r
Homologous series of present investigation is fully smectic and partially nematic of low
thermal stability and low melting type with considerable mesophase length.
The group efficiency order derived for smectic and nematic on the basis of (i) thermal
stability or rigidity (ii) early commencement of mesophase and (iii) mesophase length
as under
r
(i) Smectic
Series-Y > Series-X > Series-1
Nematic
Series-1 > Series-X = Series-Y
(
ii) Smectic
Series-1 = Series-X = Series-Y
Nematic
Series-1 > Series-X = Series-Y
(
iii) Mesophaselength (total)
Series-Y > Series-1 > Series-X
r
Exchange of central bridges in the isomeric series (1 and Y) bearing same tailed end
group can give rise to similar abnormal behavior and shapes of transition curve, irre-
spective of smectic or nematic phase for higher (C to C ) homologues.
8
16
r
r
r
r
Replacement of vinyl carboxy central bridge to carboxy central bridge raises smectic
thermal stability but lowers mesophase length range.
–
CH=CH– unit increases molecular rigidity, thermal stability, and the mesophase
length range.
Mesomorphism and the degree of mesomorphism are very sensitive and susceptible to
molecular structure.
Study of binary system with C , C , C , and C homologue can give rise to LC mixtures
6
8
10
16
workable at room temperature which may be useful to manufacture articles related to
mesomorphism.
r
Novel LC materials of present series may be useful to continue or extended research of
researchers belonging to fields other than chemistry.
The generalization that, sequential pattern mesophase formation in a homologous series
as only nematic in initial homologue, then smectic plus nematic mesophase formation
for middle homologue and finally the only smectic mesophase formation for higher
homologue. “Above generalization is not universal”
r

92
R. B. MARATHE AND A. V. DOSHI
Acknowledgments
Authors acknowledge thanks to the Green Circle Inc. Laboratory for providing research facilities ser-
vices as and when needed. Authors acknowledge thanks to Dr. Keyur Trivedi of Solaris Industries Ltd.,
Vadodara, for providing the free sample of 4-chlorobenzyl bromide. Authors are also thankful to Dr.
N. N. Vyas, Dr. Vipul Patel, and Dr. M. L.Chauhan, P.T. Arts and Science College, Godhara, for their
valuable helping hand and microscopic facility. Also thanks are due to the Sophisticated Analytical
Instrumentation Facility, Punjab University, (Chandigarh) for extending their help for analytical ser-
vices.
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