Dependence of thermotropic mesomorphism on molecular flexibility of changing tail group

Article abstract of DOI:10.1080/15421406.2016.1159786

A novel chalconyl ester homologous series of liquid crystals has been synthesized and studied with a view to understanding and establishing the effect of molecular structure on liquid crystalline properties. Novel series consisted of 13 members (C₁–C₁₈) of a series. C₁–C₄ member of a series is nonliquid crystals and, the rest of the homolog members (C₅–C₁₈) are enantiotropically nematogenic with absence of smectogenic property. None of the homolog is monotropically mesomorphic. Transition and melting temperatures were determined by an optical microscope equipped with a heating stage. Textures of nematic phase are threaded or schlieren. Spectral and analytical data of selected members of a series confirms the molecular structures of homologues. Cr-N and N-I transition curves of a phase diagram behaved in normal manner with exhibition of odd-even effect of very short range as observed for N-I transition curve. Thermal stability for nematic is 122.0°C and the nematogenic mesophase ranges from 7.0°C to 38.0°C. Liquid crystal properties of a present series are compared with the structurally similar known homologous series.

Full text of DOI:10.1080/15421406.2016.1159786

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Dependence of thermotropic mesomorphism on
molecular flexibility of changing tail group
Ravindra Solanki, Vinay Sharma & Roshan Patel
To cite this article: Ravindra Solanki, Vinay Sharma & Roshan Patel (2016) Dependence of
thermotropic mesomorphism on molecular flexibility of changing tail group, Molecular Crystals
and Liquid Crystals, 631:1, 107-115, DOI: 10.1080/15421406.2016.1159786
To link to this article: http://dx.doi.org/10.1080/15421406.2016.1159786
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Date: 18 July 2016, At: 16:33

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
http://dx.doi.org/./..
Dependence of thermotropic mesomorphism on molecular
flexibility of changing tail group
Ravindra Solanki, Vinay Sharma, and Roshan Patel
Chemistry Department, K K Shah Jarodwala Maninagar Science College, Gujarat University, Ahmedabad, India
ABSTRACT
KEYWORDS
Liquid crystal;
A novel chalconyl ester homologous series of liquid crystals has been
synthesized and studied with a view to understanding and establishing
the effect of molecular structure on liquid crystalline properties. Novel
series consisted of 13 members (C1–C18) of a series. C1–C4 member of a
series is nonliquid crystals and, the rest of the homolog members (C5–
C18) are enantiotropically nematogenic with absence of smectogenic
property. None of the homolog is monotropically mesomorphic. Tran-
sition and melting temperatures were determined by an optical micro-
scope equipped with a heating stage. Textures of nematic phase are
threaded or schlieren. Spectral and analytical data of selected members
of a series confirms the molecular structures of homologues. Cr-N and N-
I transition curves of a phase diagram behaved in normal manner with
exhibition of odd-even effect of very short range as observed for N-I
transition curve. Thermal stability for nematic is 122.0°C and the nemato-
genic mesophase ranges from 7.0°C to 38.0°C. Liquid crystal properties
of a present series are compared with the structurally similar known
homologous series.
mesomorphism; nematic;
odd-even effect; smectic
Introduction
The whole world has accepted the liquid crystalline (LC) state of a substance [1] in the benefit
of the mankind in various fields of applications [2–12] in thermotropic or lyotropic condi-
tions. The present investigation is planned with a view to
(i) Synthesize novel substances, which can be useful for the applications as thermotropic
LC or Lyotropic LC, which possess biological activity, toward prevention or curing
diseases;
(ii) to understand and establish the effects of molecular structure on LC properties of ther-
motropic LCs;
(iii) to evaluate some LC data and to be compared with structurally similar homologous
series of novel substances; and
(iv) to provide the source of novel substances to the researchers working using LC materials
to continue and extend their research activities [13–15].
However, our present investigation will be limited to novel thermotropic LC materials only.
Number of homologous series have been reported till the date [16–22] by various groups of
researchers. Present novel thermotropic LCs consisted of three phenyl rings linked through
–
–
–
–
=
–
two central bridges COO and CO CH CH and two flexible terminal end groups, viz.,
CONTACT Ravindra Solanki
ravindrasolanki@gmail.com
© Taylor & Francis Group, LLC

108
R. SOLANKI ET AL.
Scheme . Synthetic route of the series.
–
varying –OR and fixed OC18H37(C1–C18) and their LC behaviors are discussed and inter-
preted in terms of molecular rigidity and flexibility [23–26].
Experimental
Synthesis
4-Hydroxy benzoic acid was alkylated using suitable alkylating agent (R-X) to convert it into 4-
n-alkoxy benzoic acids(A) by modified method of Dave and Vora [27], α-4-Hydroxy benzoyl
β-4’-octadecyloxy phenyl ethylene(B) was prepared by usual established method [28]. Acid
chloride component of each 4-n-alkoxy benzoic acids (A) were condensed with 4-hydroxy
derivative of chalcone (B) in dry cold pyridine by usual established method [29] carefully.
Thus, chalconyl ester homologue derivatives were finally decomposed, filtered, washed, dried,
and purified till constant transition temperatures obtained using an optical polarizing micro-
scope equipped with a heating stage. 4-Hydroxy benzoic acid, alkyl halides, thionyl chloride,
MeOH, EtOH, KOH, H2SO4, acetone, pyridine, 4-hydroxy acetophenone, 4-hydroxy ben-
zaldehyde, octadecyl halide, etc., required for synthesis were used as received except solvents

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
109
Table . Elemental analysis for methoxy, pentyloxy, decyloxy, octadecyloxy derivatives.
Elements found%
Elements calculated%
Sr. no.
Molecular formula
CHO
C
H
C
H

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.



C 
HO
CHO
H O
C
.
which were dried and distilled prior to use. The synthetic route to the series is shown below
as Sch. 1.
Characterization
Representative homologs of a series were characterized by elemental analysis, mass spectrog-
1
raphy, Infrared spectroscopy, H NMR spectra, IR spectra were recorded by Perkin-Elmer
spectrum GX, 1H NMR spectra were recorded on Bruker using CDCl3 as solvent. Microanal-
ysis was performed on Perkin-Elmer PE 2400 CHN analyzer (Table 1). Transition tempera-
ture and LC properties (Textures) were determined using an optical polarizing microscopy
equipped with heating stage. Textures of nematic phase determined by a miscibility method
(Table 2).
Analytical data
IR Spectra in cm−1 for decyloxy, Hexadecyloxy, and octadecyloxy derivatives
–
–
–
–
– – –
– – –
Decyloxy: 717 Polymethylene ( CH2 )n of OC10H21, 840( C H def. m di-substituted-
–
–
Para), 771 Polymethylene ( CH2 ) of OC18H37, 941 ( C H def. hydrocarbon), 1056 and
– – – – –
1018( C O ) Str, 1373 and 1303 and 1249, 1165 ( C O str in –(CH2)n chain, 1427 and 1465
– – – – =
– = –
–
–
( C H def. in CH2)1512 ( C C )str, 1604 and 1681 ( C O group), 1735 ( COO ester
– –
group), 2854 and 2916 ( C H str in CH3).
–
–
–
–
–
Hexadecyloxy: 740 Polymethylene ( CH2 )n of OC18H37, 648 polymethylene ( CH2 )n
– – – – – –
of C16H33 817( C H def. m di-substituted-Para), 972 ( C H def. hydrocarbon), 1064
– – – – – – – –
( C O ) Str, 1365 and 1249, 1165 ( C O str in –(CH2)n chain, 1365 and 1427 ( C H def.
– = – – =
–
–
in CH2),1504 C C )str, 1604 and 1681 ( C O group), 1735 ( COO ester group), 2854
– –
and 2924( C H str in CH3).
Octadecyloxy:763 Polymethylene ( CH2 )n of OC18H37, 833( C H def. m di-
– – – – – –
–
–
–
– – –
substituted-Para), 941 ( C H def. hydrocarbon), 1056 ( C O ) Str, 1373 and 1303 and
– – – – –
1249, 1165 ( C O str in –(CH2)n chain, 1388 and 1468 ( C H def. in CH2),1512
– = –
– =
–
–
( C C )str, 1604 and 1681 ( C O group), 1735 ( COO ester group), 2854 and 2916
– –
( C H str in CH3).
Table . Textures of nematic phase by miscibility method for C, C, C, and C.
Sr. no.
Homolog
Texture

C
C
C
C
Threaded
Threaded
Schlieren
Schlieren




110
R. SOLANKI ET AL.
Table . Transition temperatures in °C for Homologous series:
α--(’-n-alkoxybenzoyloxy)benzoyl-β-”-octadecyloxyphenyl ethylenes.
Transition temperatures (˚C)
Compound no.
n-Alkyl chain CnHn+
Smectic
Nematic
Isotropic













C
C
C
C
C
C
C
C
C
C
C
C
C
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
1H NMR spectra in CDCl3 in δ ppm for heptyloxy and tetradecyloxy derivative: hepty-
–
–
loxy: 0.88(t, CH3 of –C7H15), 1.25-1.20(m, n-poly methylene groups of OC7H15),1.50
–
–
–
–
(m, n-poly methylene groups of OC18H37),3.4-3.8(s, OCH2 CH2 ofOC18H37),4.0-
–
–
–
–
–
=
4.2(s, OCH2 CH2 of OC7H15),7.2-7.3(s, CO CH CH),8.8-8.9 (s, p-disubstituted phenyl
–
–
–
ring). Tetradecyloxy: 0.87(t, CH3 of –C18H37), 0.98 (t, CH3 of OC14H29), 1.2–1.4 (m, n-
poly methylene groups of OC18H37), 1.5-1.8 (m, n-poly methylene groups of OC14H29), 2.6
(s, OCH2), 3.4 (s, OCH2 CH2 of OC14H29),4.0-4.2(s, OCH2 CH2 of OC18H37),6.9-7.2(s,
CO CH CH),7.5-7.7(s, p-substituted phenyl ring).
–
–
–
–
–
–
–
–
–
–
–
=
Results and discussion
Dimerization of 4-n-alkoxy benzoic acid disappears on esterification process by linking it with
4-hydroxy chalconyl derivative (m.p. is 74°C and yield is 69%). Mesomorphism commences
from C5 homologue as enantiotropic nematic phase with absence of smectogenic mesophase,
that is, C5–C18 homolog derivatives are only nematogenic whose melting and/or transition
temperatures vary between 84.0°C and 139.0°C. Thus, lowering of transition temperatures as
compared to corresponding n-alkoxy benzoic acids, in spite of increasing length or size of the
novel molecules with alternation of transition temperatures. The phase transition tempera-
tures (Table 3) of the homologs are plotted versus the number of carbon atom present in n-
alkoxy chain bonded to phenyl ring through oxygen atom. Smooth curves are drawn through
the like or related points (N-I and Cr-N) in a phase diagram showing phase behaviors of series
(Fig. 1).
The Cr-N transition curve adopts a zigzag path of rising and falling with overall descend-
ing tendency as series is ascended. N-I transition curve exhibit very narrow ranged odd-
even effect between C5 and C10 homologs. It descends from C5 to C8 and then rises for
6 to 8.0°C at C10 and C12 and finally descended up to C16, but abnormally ascended by
10°C at C18 homolog. N-I transition curves for odd and even members are extrapolated to

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
111
Figure . Phase behavior of series.
C3 and C4 homolog to intensify the odd-even effect, following the trend of N-I transition
curve by extrapolation method [30, 31, 32]. Thus, Cr-N/I transition curve behaved in nor-
mal manner, but N-I transition curve behaved predominantly in normal manner with partly
negligible abnormal manner. The thermal and mesomorphic properties from homolog to
homolog vary in the same series. Thermal stability for nematic in average is 122.0°C with
total mesophase length range 7.0–38.0°C. Hence, the presently investigated novel homologous
series is predominantly nematogenic and partly nonmesogenic and middle ordered melting
type.
Disappearance of dimerization and the lowering of transition temperatures of novel homo-
logues as compared to corresponding 4-n-alkoxy benzoic acids are attributed to breaking of
hydrogen bonding between two molecules of acid and subsequent esterification process. The
nonmesogenic property of C1–C4 homologs is attributed to their high crystallizing tendency
which arises as a result of unsuitable magnitude of combined effect of molecular rigidity and
flexibility induced by low dipole–dipole interactions and low magnitudes of dispersion forces

112
R. SOLANKI ET AL.
Figure . Structurally similar series.
by the interactions between instantaneous dipoles produced by the spontaneous oscillations of
electron clouds of the molecules inducing high crystallizing tendency, which causes inability
to resist or withstand exposed thermal vibrations. Hence, the molecules of C1–C4 homologs
sharply transform into isotropic liquid prior to exhibition of liquid crystal state.
The absence of lamellar packing of molecules in their crystal lattices (C1–C18) due to
unsuitable magnitudes of intermolecular anisotropic forces of lateral attractions which hin-
ders to maintain sliding layered arrangement of molecules in floating condition on the sur-
face under exposed thermal vibrations. The exhibition of only nematic mesophase forma-
tion for definite range of temperature by C5–C18 homolog is attributed to the suitable magni-
tudes of anisotropic forces of intermolecular attractions due to fittest magnitudes of molecu-
lar polarity and polarizability, permanent dipole-moment across long molecular axis, disper-
sion forces, etc. as a consequence of favorable molecular rigidity and flexibility which facili-
tated statistically parallel orientational order of molecules in floating condition on the surface
to cause nematic mesophase formation under thermal energy. The negligible abnormality
shown by few homologues is due to the unexpected status of the n-alkyl chains (C10, C12,
and C18) causing hindrance in the magnitudes of molecular flexibility. Variations observed
in transition temperatures from homolog to homolog in the same series are attributed to the
–
–
sequential addition of methylene ( CH2 ) unit or units in a left n-alkoxy terminal end group
keeping right-sided end group unchanged. Odd-even effect in N-I transition curve disap-
pears from and beyond C10 homolog because longer n-alkoxy chain from C10 to C18 may
coil or bend or flex or couple to lie with major axis of the core structure. Thus, N-I tran-
sition curve adopt a path of single transition curve or with the missing of odd-even effect.
Some mesomorphic properties of presently investigated homologous series are compared
with the structurally similar known homologous series-X [33] and Y [34] in the following
Fig. 2.
Homologous series of present investigation-1 and the homologous series selected for com-
parison purpose are identical with respect to three phenyl rings and the two central bridges
–
–
–
–
=
–
COO and CO CH CH contributing to the total molecular rigidity and the left n-alkoxy
–
terminally situated varying end group ( OR) for the same homolog from series to series, But
they [series-1,X,Y] differ with respect to terminally situated fixed right-handed flexible tail or
–
–
–
end groups, viz., OC18H37, Cl, and OCH3 contributing partly to the total molecular flex-
–
ibility, which added to the common part of the flexibility contributed by left n-alkoxy [ OR]
end group. Thus suitable or unsuitable magnitudes of anisotropic forces of intermolecular
end to end and lateral attractions occurred as a consequence of combined effect of molecular
rigidity and flexibility from homolog to homolog in the same series or for the same homolog
from series to series vary with the changing magnitudes of permanent dipole-moment across
the long molecular axis, dipole–dipole interactions and dispersion forces, etc.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
113
Table . Average thermal stabilities in °C.
Series→
–
–
–
( OCH)
X( Cl)
Y( OCH)
Smectic-isotropicSmectic-nematicCommencement of smectic phase
Nematic-isotropicCommencement of nematic phase
Total mesophase (Sm+N) length in °CFrom Ci to Cj
–
–
.(C–C)C
.(C–C)C .(C–C)C .(C–C)C
–C–C –C–CC –C–C
Thus, the mesomorphic properties and the degree of mesomorphism vary with the chang-
ing part of molecule, that is, in present novel series variations in LC behaviors will depend
–
–
–
upon polarity of changing tail group OC18H37, Cl, and OCH3, which remains unchanged
for same individual series. Some LC properties of present series like, thermal stabilities, com-
mencement of mesophase or mesophase length range, odd-even effect, etc. are recorded in
the following (Table 4) as shown below.
From Table 4, it is seen that
• Presently investigated a series-1 of chalcone derivatives is thermotropically nematogenic
and the structurally similar homologous series-X and Y are only nematogenic as well as
nematogenic in addition to smectogenic, respectively.
• Smectic phase for series-Y commences from C7 homolog, whereas it does not commence
till the last homolog of the series-X and 1.
• Nematic mesophase commences from C5 homolog of the series 1, X, and Y.
• Smectic thermal stability of series-Y is 118.0, whereas smectic mesophase destabilized
in series 1 and X, till the last member.
• Nematic thermal stabilities of series 1, X, and Y are in increasing order (122, 149.4, 157.0)
respectively.
• Total mesophase length range is the highest for a series-Y and the lowest for a series-X,
whereas it is intermittent for present series-1, between the series-X and series-Y.
• Odd-even effect is exhibited by N-I and/or Sm-N transition curves in the respective
phase diagram.
The observed difference among the series under comparative study of series 1, X, and Y
for the mesogenic properties and the degree of mesomorphism, which depend, upon the tail
–
–
–
groups, OC18H37, Cl, and OCH3 for the same homolog from series to series. The individ-
ual group polarity of tail groups is different. The group polarity of any group is a vector sum
of all bond polarities in a functional group.
Similarly, molecular polarity of a molecule is the vector sum of all bonds. Therefore, molec-
–
–
ular polarity of series-1, X, and Y vary with the changing tail groups OC18H37, Cl, and
OCH3.
–
The changing molecular polarity causes variations in permanent dipole moment across
the long molecular axis, dipole–dipole interactions, dispersion forces, related to end to end,
and lateral attractions for the same homolog from series to series as a consequence of chang-
ing molecular rigidity and flexibility. Internal stored energy of any thermodynamic system
of a substance depends upon the quantity (mass) of the substance and its characteristics or
thermodynamically heat of formation (ꢀH) at constant pressure and at a given tempera-
ture as a consequence of molecular rigidity, which varies from homolog to homolog in the
same series and for the same homolog from series to series. Therefore, energy associated
(ꢀH) with a molecule, depending upon molecular rigidity and flexibility induces thermal
resistivity or thermal stability towards exposed thermal vibrations to facilitate the ordered
molecular arrangement in floating condition either as sliding layers or/and statistically par-
allel orientational order for definite range of temperature to induce either smectic or/and

114
R. SOLANKI ET AL.
nematic mesophase formation. Thus, the magnitudes of thermal stability indicates resistiv-
ity toward exposed thermal vibrations in terms of transition temperature and the mesophase
length range in terms of degree of mesomorphism for the same homolog from series to series.
The increasing order of thermal stability values from series-1 to X to Y are related to highly
–
–
–
polar OCH3 tail group to compared weaker polar Cl and OC18H37 groups. Weakening of
polarity of C18 homolog is attributed to the cancellation of vector sum of bond polarity by
–
–
two n-alkoxy ( OR and OC18H37) groups bonded to first and last phenyl ring in opposite
directions.
The magnitudes of which go on reducing from C1 to C16 and the molecular polar-
ity reduced to zero by tail groups, because both tail (left and right) ends are same, that
–
is, OC18H37 situated in opposite directions. Moreover, phenyl rings are nonpolar. Thus,
abnormality of C18 homolog is attributed to its molecular polarity and rigidity or thermal
–
–
–
–
=
–
resistivity values which it exhibited by only two central bridges COO and CO CH CH
which is the lowest in present series-1 and the highest for the series-Y and intermediate for
series-X as observed in Table 4. Smectic mesophase formation in series-Y indicates lamel-
lar packing of molecules in its crystal lattices, which forms sliding layered arrangement in
floating condition, but absence of preoccupied layered structure in the molecules of series-
1 and X disallows smectic mesophase formation. Narrow Odd-even effect observed com-
monly for the series under comparative study is attributed to the sequentially added even
and odd number of methylene units and the late commencements of mesophase. Hence,
the variation in LC properties for the same homolog from series to series or from homo-
logue to homologue in the same series is attributed to changing molecular rigidity or/and
flexibility as immerged in form of enthalpy change (ꢀH) at a given temperature at constant
pressure.
Conclusions
–
• Chalconyl ester novel derivatives of 13 homologs (C1 C8,C10,C12,C14,C16,C18) have been
synthesized and their thermotropic LC properties, evaluated and compared with the
structurally similar known homologous series with a view to understand the relation
between LC property and molecular structure depending on changing tail groups of dif-
ferent polarities.
• Group efficiency order derived on the basis of (a) thermal stability, (b) commencement
of mesophase, and (c) mesophase-length range for smectic and nematic are as under.
–
–
–
–
–
–
–
–
–
Smectic: OCH3 > Cl = OC18H37
(a)
–
Nematic: OCH3 > Cl > OC18H37
–
(b) Smectic: OCH3 > Cl = OC18H37
–
Nematic: OCH3 = Cl = OC18H37
–
–
–
(c) Smectic Nematic: OCH3 > OC18H37 > Cl
–
• OCH3 tail group is a stronger and highly polar-polarizable, which induces smectic and
nematic phase by suitable magnitudes of lateral and end-to-end intermolecular attrac-
–
–
tions whereas Cl and OC18H37 are relatively less polar and polarizable, fail to induce
smectic phase, but induces only nematic mesophase.
• Molecular rigidity and flexibility are the main operators of mesomorphism.
• Chalconyl ester derivatives being biologically active molecules may be useful in pharma-
ceutical and medicinal field, including study of binary systems.
• A phenomenon of mesomorphism is very sensitive and susceptible to molecular struc-
ture of a substance.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
115
Acknowledgments
The authors acknowledge thanks to Dr. R. R. Shah, principal of K. K. Shah Jarodwala Maninagar Science
College, Ahmedabad. The authors are also thankful to Dr. A. V. Doshi, Ex-principal of M.V.M. Science
College, Rajkot for his constant support, inspirations, and help, as and when needed during the course
of present investigation. The authors thank to NFDD Centre for the analytical services and spectral
services.
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