Mesomorphism dependence on molecular rigidity

Article abstract of DOI:10.1080/15421406.2015.1068993

A novel ester homologous series of thermotropic liquid crystal (LC) has been studied with a view to understanding and establishing the relation between LC property and the molecular structure. The series consists of 11 members, the CCand Cmembers of the series are nonliquid crystals. LC properties commence from the Chomolog and continue up to the Chomolog as enantiotropic nematic and smectic in addition to nematic. Transition temperatures of the homologs were determined by an optical polarizing microscope equipped with a heating stage. Textures of nematic phase are threaded or Schlieren and that of smectic is focal conic of the type-A. Analytical and spectral data confirm the structures of homologs. Thermal stability for nematic is 93 °C and 136°C, respectively. The N-I and Sm-N transition curves of phase diagram do not exhibit odd-even effect. The N-I transition curve partly behaves in an abnormal manner. The Cr-I and Sm-N transition curves behave in normal manner. The LC behavior of the present series is compared with structurally similar known homologous series.

Full text of DOI:10.1080/15421406.2015.1068993

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
M. L. Chauhan, B. K. Patel, S. N. Pandya & A. V. Doshi
To cite this article: M. L. Chauhan, B. K. Patel, S. N. Pandya & A. V. Doshi (2016) Mesomorphism
dependence on molecular rigidity, Molecular Crystals and Liquid Crystals, 625:1, 47-54, DOI:
10.1080/15421406.2015.1068993
To link to this article: http://dx.doi.org/10.1080/15421406.2015.1068993
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Date: 05 March 2016, At: 22:23

MOL. CRYST. LIQ. CRYST.
, VOL. , –

http://dx.doi.org/./..
Mesomorphism dependence on molecular rigidity
a
b
c
d
M. L. Chauhan , B. K. Patel , S. N. Pandya , and A. V. Doshi
a
b
Sheth P. T. Arts & Science College, Godhra, Gujarat, India; Shri P. N. Pandya Arts, M. P. Pandya Sci. & Smt. D. P.
c
d
Pandya Comm. College, Lunawada, Gujarat, India; J & J Science College, Nadiad, Gujarat, India; Matushri
Virbaima Mahila Sci. & Home Sci. College, Rajkot, Gujarat, India
ABSTRACT
KEYWORDS
Enantiotropy;
mesomorphism; nematic;
smectic liquid crystal
A novel ester homologous series of thermotropic liquid crystal (LC) has
been studied with a view to understanding and establishing the relation
between LC property and the molecular structure. The series consists of
1
1 members, the C –C and C members of the series are nonliquid crys-
1 5 16
tals. LC properties commence from the C homolog and continue up to
6
the C14 homolog as enantiotropic nematic and smectic in addition to
nematic. Transition temperatures of the homologs were determined by
an optical polarizing microscope equipped with a heating stage. Tex-
tures of nematic phase are threaded or Schlieren and that of smectic is
focal conic of the type-A. Analytical and spectral data confirm the struc-
tures of homologs. Thermal stability for nematic is 93 °C and 136 °C,
respectively. The N–I and Sm–N transition curves of phase diagram do
not exhibit odd-even effect. The N–I transition curve partly behaves in
an abnormal manner. The Cr–I and Sm–N transition curves behave in
normal manner. The LC behavior of the present series is compared with
structurally similar known homologous series.
Introduction
Liquid crystalline (LC) state of a substance [1] has contributed and inspired scientists and
technologists to a very great extent in the benefit of mankind throughout the world [2–5].
Therefore, researchers working in LCs with different aims and objectives and disciplines
require novel LC substances to extend and continue their research activities. Therefore, we
being chemists, have planned to synthesize novel substances with a view to understand-
ing the effect of molecular structure on LC properties and to establish the relation between
them [6–8] through varying molecular rigidity and, or flexibility [9–12] of different moi-
ety and changing terminal or/and lateral or central bridge or bridges or number of phenyl
rings or cyclic other ring, etc. Many LC series have been reported [13–16] to date. The
present novel series consists of two phenyl rings bonded through –COO– central bridge
and n-alkoxy varying as well as n-alkyl (hexyl) unchanging left and right terminal end
groups. LC properties are evaluated and compared with structurally similar other known
series.
CONTACT M. L. Chauhan
India.
chauhanmukesh@yahoo.com
Sheth P. T. Arts & Science College, Godhra , Gujarat,
©
 Taylor & Francis Group, LLC

4
8
M. L. CHAUHAN ET AL.
Scheme . Synthetic route to the series
Experimental: (Synthesis)
r
p-n-Alkoxy benzoic acids were prepared by the modified method of Vora and
Dave(1970) [17], using suitable alkylating agents (R-X)
r
p-n-Alkoxy benzoyl chlorides were prepared by refluxing the corresponding alkoxy ben-
zoic acids with freshly distilled [18] thionyl chloride.
r
p-Hydroxy n-hexyl cinnamate was prepared by reacting p-hydroxy cinnamic acid with
freshly distilled and dry n-hexyl alcohol in presence of concentrated sulfuric acid [19].
r
Acid chlorides were directly condensed [Doshi and Lohar (1992,1993)] [12] with the p-
n-hydroxy n-hexyl cinnamates dissolved in ice-cooled dry pyridine in portions with stir-
ring reaction mixture. Products were decomposed, filtered, washed, dried, and purified,
until the constant transition temperatures are obtained. 4-Hydroxy benzoic acid, alkyl
halides (R-X), methanol, KOH, thionyl chloride, 4-Hydroxy cinnamic acid, n-propyl
alcohol, con. H SO , pyridine, etc. are required for synthesis were used as received except
2
4
solvents which were dried and distilled prior to use. The synthetic route to the series is
shown below in Scheme 1. The elemental analysis is shown in Table 1.
Characterization
Analytical data support the title homologous series 4-(4’-n-alkoxy benzoyloxy) n-propyl cin-
namates.
Result and discussion
Novel homologs series of vinyl ester is formed by condensing dimeric 4-n-alkoxy benzoic
acids and common component a 4-hydroxy n-hexyl cinnamate. The dimerization of n-alkoxy

MOL. CRYST. LIQ. CRYST.
49
Table . Elemental analysis.
Sr.
R = n-alkyl
Chain
Molecular
Formula
Calculated%
Observed%
No.
C
H
C
H

Butyl
Hexyl
Decyl
C H O
  
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.




C H O
  
C HO

Tetradecyl
C H O




Textured by miscibility method
Nematic: Threaded – C
Smectic: Smectic-A – C , C , C

 
Homolog
IR Spectra (cm 
−
)
−



., ., and. cm →Confirms alkyl group
−
., ., and . cm →Confirms –COO- group
−

Decyl
. cm →Confirms ether
−






. cm →Confirms trans –CH=CH– group
−

. cm →Confirms p-sub. benzene ring
−

. cm →Confirms polymethylene of C H
 
−

., ., and  cm →Confirms alkyl group
−

., ., and . cm →Confirms –COO- group
−
Tetradecyl
. and. cm →Confirms gem dimethyl group
−




. cm →Confirms ether
−

. cm →Confirms trans –CH=CH- group
−

. cm v→Confirms p-sub. benzene ring
−


. cm →Confirms polymethylene of C H
 
Homolog
NMR Spectra δ(ppm)

. (-CH of –C H )
  


. (-CH of –C H )
  
. (-O-CH -CH of –C H )


 
Hexyl
. (-O-CH of –C H )
  


. and . (Two p-sub. benzene ring)
. and .

.
. (-CH of –C H )


 

. (-CH - of –C H )
  

. (-O CH -CH of –C H )


 
Octyl
. (-O-CH of –C H )
  


. and . (Two p-sub. benzene ring)
. and .

.
benzoic acids disappears on esterification process. C –C and C homologs are nonliquid
1
5
16
crystals. C homologs are only enantiotropically nematogenic and C –C homologs are enan-
6
8
14
tiotropically smectogenic in addition to nematogenic. Transition temperatures (Table 2) of the
homologs were plotted versus the number of carbon atoms present in n-alkyl chain bonded
to first phenyl ring through oxygen atom. Smooth curves are drawn through like or related
points, which form Cr–I/M, Sm–N, and N–I transition curves showing phase behaviors of
series in a phase diagram (Fig. 1). Cr–M/I transition curve follows a zigzag path of rising and
falling manner with overall descending tendency. Sm–N transition curve commences with
maxima and then descended as series is ascended with negligible deviation. N–I transition
curve initially falls from C to C through C homologs. Thus, Cr–I/M and Sm–N transi-
6
14
12
tion curves behave abnormally for its EDF path in realistic manner. However, the hypothetical

5
0
M. L. CHAUHAN ET AL.
Table . Transition temperatures.
Series-: -(’-n-alkoxy benzoyloxy) n-hexyl Cinnamates
Compound No.
R = n-alkyl group
Transition temperature in °C
Smectic
Nematic
Isotropic








.
.
.
.
.
.
.
.
.
Methyl C
Ethyl C
Propyl C
Butyl C
Pentyl C
Hexyl C
Octyl C
Decyl C
–
–
–
–
–
–
–
–
–
–
.
.
.
.
.
–
.
.
.
.
.
.
.
.
.
.
.
–
.
.
.
.
–

Dodecyl C
Tetradecyl C
Hexadecyl C


.
.
unrealistic path represented by ET as a part of N-I curve is an ideal expected curve through
which ideal descending behavior represented. Therefore, area within the EDT, indicates mag-
nitudes of abnormality in mesomorphic behavior. N-I transition curve is extrapolated [20–22]
to C homolog to predict its probable transition temperature. Similarly, Sm-N transition curve
4
is also extrapolated to C and C homolog to predict their transition temperature. N–I and
6
16
Sm–N transition curves fails to exhibit odd-even effect.
Figure . Phase behavior of -(’-n-alkoxy benzoyloxy) n-hexyl cinnamates.

MOL. CRYST. LIQ. CRYST.
51
Disappearance of dimerization of n-alkoxy benzoic acids is due to the breaking of hydro-
gen bonding between two molecules of corresponding n-alkoxy acids by esterification pro-
cess. The absence of any sort of mesophases formation by C –C and C is attributed to
1
5
16
their inability to withstand exposed thermal vibration arising from unsuitable magnitudes of
anisotropic forces of intermolecular end-to-end attraction, which facilitate breaking of crys-
tal lattices abruptly. Therefore, a nonmesomorphic homolog sharply transform into isotropic
state without passing through LC state. At this stage, molecules under microscopic obser-
vation randomly orient in all possible direction with high order of disorder or entropy
(
ꢀS=ꢀH/T). On super cooling the isotropic melt, of nonmesogenic homologs, which trans-
form into crystalline solid state without exhibition of monotropic LC state due to their high
crystallizing tendency arising from unsuitable magnitudes of molecular rigidity and flex-
ibility induced by low dipole-dipole interactions and low dispersion forces by interaction
between instantaneous dipoles produced by the spontaneous oscillation of electron clouds
of the molecules which can induce high crystallizing tendency hindering mesophase forma-
tion in monotropic condition. The molecules of the C –C members of the series resisted
6
14
exposed thermal vibration, which disalign at an angle 90° or less than 90° with the float-
ing surface. The suitable magnitudes of end-to-end intermolecular attractions as molecules
make possible to arrange the molecule to float with statistically parallel orientational molecu-
lar order for C –C homologs via smectic (except C ) phase from solid state. The molecule of
6
14
6
C –C homologs exhibit smectic phase through their occupancy of sliding layered arrange-
8
14
ment of molecules for definite range of temperatures depending upon their angle of disalign-
ment with floating surface offering resistivity toward exposed thermal vibrations. Thus, smec-
tic and/or nematic mesophase formation occur. The disappearance of odd-even effect in N-I
and Sm-N transition curves is attributed to the late commencement of mesophase forma-
tion due to presence of longer (C ) n-alkyl chain at the tail end group and C –C or C –C ,
6
1
5
6
16
shorter or longer respectively n-alkyl chain at the left n-alkoxy end groups which may coil
or bend or flex or couple to lie with the principal axis of the core structure inducing uncer-
tainty in the status of molecular length, permanent dipole moment across the long molecu-
lar axis, dispersion forces, molecular rigidity and flexibility, polarity, and polarizability, etc.
Interferes to fulfill the requirement to induce mesomorphism and degree of mesomorphism
to the fullest extent. Thus, observe abnormality (C –C ) in N-I transition temperature and
10
12
transition curve behaviors can be assigned as explained in above para. The variations in meso-
morphic (LC) properties from homolog to homolog in presently investigated novel series is
attributed to the sequentially added methylene unit or units at the left n-alkoxy terminal end
group keeping -C H (n) tail group unchanged throughout entire series. Thus, present series
6
13
is partly smectogenic and partly nematogenic with middle-ordered melting type. The extrap-
olated N–I transition curve to C homolog merges into isotropic temperature of C which
4
4
approves nonliquid crystal behaviors of C member. Similarly, nonmesomorphicity of C
4
16
homolog is approved by extrapolation of Sm–N transition curve to C . The extrapolation of
16
Sm–N transition curve to C homolog predicts monotropic transition temperature for smec-
6
tic as 95 °C which is not realizable in actual practice due to its high crystallizing tendency
arising for the reasons as discussed earlier in the result and discussion part this manuscript.
LC properties of present homologous series are compared with structurally similar other
known homologous series-X [23] as shown in the following figure (Fig. 2).
Homologous series-X chosen for comparison and the presently reported homologous
series-1 are identical with respect to two phenyl rings, tail group, –CH=CH–COOCH (n)
13
and left n-alkoxy group for the same homolog from series to series. However, both series under

5
2
M. L. CHAUHAN ET AL.
Figure . Structurally similar series.
comparative study differ with respect to central bridge –COO–/and –CH=CH–COO– link-
ing first and second phenyl rings. Therefore, differing features in LC properties and the degree
of mesomorphism under comparison can be attributed to the molecule rigidity contributed
by carboxy (–COO–) and vinyl carboxy (–CH=CH–COO–) central bridges.
Following Table 3 represents some LC properties of the present series-1 as compared to
homologous series-X which differs with respect to molecular rigidity through their central
bridges, viz., –COO– and –CH=CH–CH-COO–
r
Homologous series-1 and -X are smectogenic in addition to nematogenic with different
mesophase length range
r
Thermal stabilities for smectic and nematic for series-X is relatively higher than present
series-1.
r
Smectic and nematic mesophases commences earlier for series-X (C and C ) than
7
3
present series-1 (C and C ).
8
6
r
Maximum total phase length range is exactly same of both series-1 and -X, but minimum
values differs.
r
Odd-even effect is not exhibited by Sm–N and N–I transition curves of series-1 but it is
exhibited by series-X under comparison.
Series-1 and -X have their differing part at central linking –COO– and–CH=CH–COO.
This difference bring in with it, other differences in the display of molecular forces. The over-
all length of the molecules of series-1 is decreased as compared to series-X. Such difference
causes differences in length-to-breadth ratio. Simultaneously, the pi electron density due to the
delocalized electrons on account of added double bond which is absent for the series-1 and
present for series-X which affects the overall molecular polarizability. Thus, shorter length
and lesser polarizability of series-1 would cause the LC properties to undergo change for the
same homolog from series-1 to series-X. In all other respects except the central bridge which
is –CH=CH–COO– in case of series-X and –COO– in case of series-1 under comparative
study, both series are closely similar, but central bridge vinyl carboxylate –CH=CH–COO–
has greater length and causes more noncoplanarity due to the twist obtained as the oxygen
Table . Average thermal stabilities.

X
Series
–COO–
–CH=CH–COO-
Smectic–isotropic
.
.
Or smectic-nematic
Commencement of smectic phase
Nematic-isotropic
(C –C )
(C –C )




C
C
.
.
Commencement of nematic
Phase
(C –C )
C
—
C C
(C –C )
C
–
C C




Total mesophase length in °C (Sm + N)
Cn Cn



MOL. CRYST. LIQ. CRYST.
53
atoms of the vinyl carboxylate group bump into the nonbonded adjacent hydrogen atoms of
the aromatic phenyl ring. On account of these differences, the Sm–N and N–I thermal sta-
bility of present series 1 are lower than the series-X. Moreover, enhanced molecular length
of –CH=CH–COO– unit and length which increases molecular rigidity and lateral attrac-
tions as compared to –COO– which do not link the phenyl rings through multiple bond or
hydrogen-bonded ring system. Consequently the stereochemistry of the molecules, results
in less thermally stable mesophase or mesophases. Thus, the difference of molecular rigid-
ity, keeping the molecular flexibility intact, causes variations in LC properties for the same
homolog from series-1 to series-X.
Conclusions
r
ꢀ
Homologous series 4(4 -n-alkoxybenzoyloxy) n-hexyl cinnamates is partly smectogenic
and partly nematogenic and middle-ordered melting type.
r
The group efficiency order derived for smectic and nematic on the basis of (i) thermal
stability (ii) early commencement of mesophase, and (iii) total mesophase length range
with respect to central bridge are as under.
(
i) Smectic: –CH=CH–COO– > –COO–
i. Nematic: –CH=CH–COO– > –COO–
ii. Smectic: –CH=CH–COO– > –COO–
iii. Nematic:–CH=CH–COO– > –COO–
iv. (iii) Sm + Nm: –CH=CH–COO– > –COO–
r
Mesomorphic tendency of a substance is dependent upon combined effects of molecular
rigidity and flexibility of a molecular structure.
r
LC properties of a substance are very sensitive and susceptible to molecular structure.
Present investigation may be useful for the study of binary systems between 72.0 °C and
15.0 °C for the devices to be operated between 52 °C and 95 °C. Also, it may be useful
for agricultural products growth.
r
1
Acknowledgments
The authors are thankful to the management of M. V. M. Science and Home Science College, Rajkot
and Dr. N. D. Jadav (Retd. Prof.) of Faculty of Technology and Engineering, The M.S. University of
Baroda, Vadodara, for their valuable co-operation in the work. Authors are also thankful to scientists
of National Salt and Marine Research Institute, Bhavnagar for the analysis of sample.
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