Dependence of liquid crystallinity on molecular rigidity

Article abstract of DOI:10.1080/15421406.2015.1068994

A novel homologous series of liquid crystals (LC) of chalconyl derivatives is synthesized and studied with a view to understanding and establishing the effects of molecular structure on liquid crystal properties. The novel series consists of 12 homologues. Nematogenic LC property commences from C₃ and continues up to C₁₄ and the smectogenic mesophase is exhibited from C₄ to C₁₆. The C₃ homologue is a monotropic nematic and the rest of the homologues are enantiotropically smectic and nematic or only smectic (C₁₆). Transition curves Cr-Sm, Sm-N, Cr-I, and N-I behave in normal manner with exhibition of odd-even effect, showing phase behaviors of the series.

Full text of DOI:10.1080/15421406.2015.1068994

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Dependence of liquid crystallinity on molecular
rigidity
P. K. Rakhasia & U. C. Bhoya
To cite this article: P. K. Rakhasia & U. C. Bhoya (2016) Dependence of liquid crystallinity
on molecular rigidity, Molecular Crystals and Liquid Crystals, 625:1, 55-62, DOI:
10.1080/15421406.2015.1068994
To link to this article: http://dx.doi.org/10.1080/15421406.2015.1068994
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Date: 05 March 2016, At: 22:25

MOL. CRYST. LIQ. CRYST.
, VOL. , –

http://dx.doi.org/./..
Dependence of liquid crystallinity on molecular rigidity
P. K. Rakhasia and U. C. Bhoya
Department of Chemistry, Saurashtra University, Rajkot, Gujarat, India
ABSTRACT
KEYWORDS
A novel homologous series of liquid crystals (LC) of chalconyl deriva-
tives is synthesized and studied with a view to understanding and
establishing the effects of molecular structure on liquid crystal prop-
erties. The novel series consists of 12 homologues. Nematogenic LC
property commences from C and continues up to C and the smec-
Enantiotropic; liquid crystal;
nematic; smectic;
thermotropic
3
14
togenic mesophase is exhibited from C to C . The C homologue is a
4
16
3
monotropic nematic and the rest of the homologues are enantiotropi-
cally smectic and nematic or only smectic (C ). Transition curves Cr-Sm,
1
6
Sm-N, Cr-I, and N-I behave in normal manner with exhibition of odd-
even effect, showing phase behaviors of the series.
Introduction
Applicability of thermotropic liquid crystal (LC) substances in liquid crystal devices and chal-
conyl substances in pharmaceutical and medicinal treatment have inspired the synthesis of
novel chalconyl substances of liquid crystal property, which may be useful to mankind in both
the ways [1]. Many thermotropic [2,3] and lyotropic [4–9] substances have been reported to
date. The present investigation is planned with a view to synthesize novel LC substances and
to understand and establish the relations between molecular structure and thermotropic LC
properties [10–16]. The novel LCs consist of three phenyl rings, two central bridges, viz.,
–
COO– and –CH=CH–CO– with two flexible end groups –OR and –Cl (R = C H
). The
n
2n+1
results of the novel investigation are evaluated and compared with a known structurally sim-
ilar homologous series and interpreted in terms of molecular rigidity and flexibility [17–20].
Evaluated results can be useful to the other groups of researchers who are working with LC
substances because every researcher in LCs requires novel substances [21–23].
Experimental
Synthesis
4
-Hydroxy benzoic acid was alkylated by suitable alkylating agents by the modified method
of Dave and Vora [24]. β-4-Hydroxy phenyl α-4’-chlorobenzoyl ethylene was prepared by a
well-known method [25]. 4-n-Alkoxy benzoic acids were converted to corresponding acid
chlorides using freshly distilled thionyl chloride and condensed with an ice cold solution of
CONTACT U. C. Bhoya
India.
drucbhoya@gmail.com
Department of Chemistry, Saurashtra University, Rajkot, Gujarat ,
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

5
6
P. K. RAKHASIA AND U. C. BHOYA
chalcone in pyridine [26]. Final products were individually decomposed in a 1:1 hydrochloric
acid aqueous solution, filtered, washed, dried, and purified until the homologue gave constant
transition temperatures.
4
-Hydroxy benzoic acid, alkyl halides, KOH, thionyl chloride, methanol, ethanol, pyridine,
4
-hydroxy benzaldehyde, 4-chloro acetophenone, etc., were used as received except solvents
that were dried and purified prior to use. The synthetic route of the novel homologous series
is shown in Scheme 1.
Scheme . Synthetic route to the novel series.
Characterization
Some members of the novel series as the representative members were characterized by ele-
1
mental analysis (Table 1), infrared spectroscopy, H NMR spectra, and mass spectroscopy.
Microanalysis was performed on a EuroEA Elemental Analyzer. IR spectra were recorded on
Table . Elemental analysis for () ethyloxy, () hexyloxy, and () tetradecyloxy derivatives.
Elements% found
H
Elements% calculated
H
Sr. no.
Molecular formula
C H ClO
C
O
C
O



.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.



C H ClO
  
C H ClO





MOL. CRYST. LIQ. CRYST.
57
1
a Shimadzu FTIR Model -IRAffinity-1S (MIRacle 10), H NMR spectra were recorded on
a Bruker spectrometer using CDCl as a solvent, and mass spectra were recorded on a Shi-
3
madzu GC-MS Model No. QP-2010. The liquid crystal behavior and the type of textures were
determined by a miscibility method on microscopic observations.
Analytical data
Spectral data
H NMR in ppm for propyloxy derivative
1
.04–1.08 (t, 3H,–CH of –OC H group), 1.81–1.89 (m, 2H, Ar–O–CH –CH ), 3.93–4.02
3
3
7
2
2
(
2H, t, Ar–O–CH ), 6.96–8.15 (4H of phenyl ring with n-propoxy group), 7.26–7.84 (4H
2
–
of middle phenyl ring), 7.68–7.84 (4H of phenyl ring with chloro group), 7.96–7.98 (d, 1H,
–
CH=CH–CO–), 7.44–7.48 (d, 1H, –CH=CH–CO–). The NMR data are consistent with the
molecular structure.
H NMR in ppm for pentyloxy derivative
0
1
4
.93–0.96 (t, 3H,–CH of –OC H group), 1.35–1.37 (m, 2H, CH –CH –(CH ) –O–), 1.39–
3
5
11
3
2
2 3
.41(m, 2H, CH –CH –CH –(CH ) –O–), 1.79–1.86 (m, 2H, CH –(CH ) –CH –CH –O–),
3
2
2
2 2
3
2 2
2
2
.02–4.06 (t, 2H, CH –(CH ) –CH –O–), 6.96–8.15 (4H of phenyl ring with n-pentyloxy
3
2 3
2
group), 7.26–7.84 (4H of middle phenyl ring), 7.69–7.84 (4H of phenyl ring with chloro
group), 7.96–7.98 (d, 1H, –CH=CH–CO–), 7.44–7.47 (d, 1H, –CH=CH–CO–). The NMR
data are consistent with the molecular structure.
IR in cm  for butyloxy derivative
−
3
070 (=C–H Str. of phenyl nucleus), 2956 and 2870 (CH sym. and asym. stra.), 1720 (aro-
2
matic C=O Str.), 1656 (aromatic C=C str.), 1506 (C–C aromatic str.), 1471 (C–H bend alka-
nes), 1321 (aromatic and aliphatic C–O str.), 1166 (Ar–O str.), 848 (C–H oop. phenyl ring),
8
06 (C–Cl str.), 758 (C–H oop. bending of phenyl ring). The IR data are consistent with the
molecular structure.
IR in cm  for decyloxy derivative
−
3
070 (=C–H Str. of phenyl nucleus), 2920 and 2850 (CH sym. and asym. stra.), 1722 (aro-
2
matic C=O Str.), 1656 (aromatic C=C str.), 1512 (C–C aromatic str.), 1469 (C–H bend alka-
nes), 1328 (aromatic and aliphatic C–O str.), 1174 (Ar–O str.), 848 (C–H oop. phenyl ring),
8
06 (C–Cl str.), 759 (C–H oop. bending of phenyl ring). The IR data are consistent with the
molecular structure.
Mass spectra of hexyloxy derivative
+
m/z (rel.int%): 462 (M) ,257,223,205,165,139,121,93,65
Mass spectra of octyloxy derivative
+
m/z (rel.int%): 490(M) ,315,233,194,165,139,121,111,93,65

5
8
P. K. RAKHASIA AND U. C. BHOYA
Table . Transition temperatures in °C.
Transition temperatures in oC
N
Compound no.
R=n-alkyl chain C H
Sm
Isotropic
n
n+













–
–
–
–
–
.
.
.
.
.
.
.
.
.
.
.
.











(.)
.
.
.
.
.
.
.
.
–
.
.
.
.
.
.
.
.
.



Sm, Smectic; N, Nematic; () indicate monotropy.
Result and discussion
β-4-Hydroxy phenyl α-4’-chlorobenzoyl ethylene (M.P. 178 °C; Yield 77.8%) is a nonliquid
crystal component. However on linking it with 4-n-alkoxy benzoic acids yielded LC com-
pounds from C number of a series. C homologue is a monotropic nematic. C homologue is
3
3
16
enantiotropic smectic only and the rest of the LC homologues (C to C ) are enantiotropically
4
14
smectic in addition to nematic. C and C members are nonliquid crystals (NLC). Transition
1
2
temperatures (Table 2) were plotted versus the number of carbon atoms present in n-alkyl
chain of the last n-alkoxy terminal end group. Transition curves showing phase behaviors are
obtained on linking like or related points as depicted in a phase diagram (Fig. 1). Cr-I or Cr-
Sm transition curve descends continuously with exhibition of minor odd-even effect as series
is ascended and behave in normal manner. Sm-N or Sm-I transition curve initially rises and
passes through maxima at the C homologue and then descends as series is ascended with
12
exhibition of odd-even effect. N-I transition curve initially descends and then rises up to C14
homologue with exhibition of odd-even effect. Thus, Sm-N or Sm-I transition curve behaved
in expected normal manner, but N-I (or vice versa) transition curve behaved in abnormal
manner from and beyond C homologue. Curves for odd and even members are merging into
6
Figure . Phase behavior of series.

MOL. CRYST. LIQ. CRYST.
59
each other at the C members of a series and then curves (N-I and Sm-N/I) propagated as a
6
single transition curve from and beyond C homologues. Thus, odd-even effect disappears for
6
higher homologues from and beyond C number of a series. LC properties vary from homo-
6
logue to homologue in the present same series in usual manner. Thus, series under present
discussion is partly smectogenic and partly nematogenic with middle ordered melting type.
The exhibition of mesomorphic tendency from C homologue to C homologues is due to
3
16
increase in molecular length by linking 4-hydroxyl chalconyl derivative with 4-n-alkoxy ben-
zoic acids, which increases length to breadth ratio, ratio of the polarity to polarizability, molec-
ular rigidity and flexibility, permanent dipole moment across the long molecular axis, dipole–
dipole interactions, dispersion forces, etc., to disalign the molecules perpendicular or/and at
an angle less than 90° due to a suitable magnitude of anisotropic forces of intermolecular end
to end and lateral attractions on floating surface under the influence of exposed thermal vibra-
tions. Thus, mesomorphic homologues resisted thermal vibrations for certain degree of tem-
perature for exhibition of only smectic or only nematic or smectic and nematic mesophase
formation. C to C members of a present series resisted exposed thermal vibrations and
4
14
arranged or sliding layered organization to show smectic phase and then at higher tempera-
ture the same molecules arrange themselves or statistically parallel orientation ordered orga-
nization of higher entropy value under floating condition on the surface. The C homologue
16
does not exhibit the nematic phase due to poor end to end attractions of the molecules. Thus,
smectic and nematic mesophases appear for the C to C homologues and the C16 homo-
4
14
logue shows solely a smectic phase. The C homologue transforms into isotropic state with-
3
out an exhibition of a mesomorphic state, but on supercooling the isotropic state melts and
a nematic mesophase is generated in a monotropic manner. Non-mesomorphicity of C and
1
C is attributed to their high melting points due to low flexibility of the short alkyl chains.
2
Exhibition of an odd-even effect in the alternation of transition temperature is attributed to
the sequentially added methylene unit at the left n-alkyl chain. Disappearance of odd-even
effect from and beyond C- number of a novel series is attributed to the coiling, bending, flex-
6
ing, or coupling of longer n-alkyl chain with the major axes of the core structure of higher
homologues. Thus, transition curves in a phase diagram for higher homologues prolong as a
single curve, from and beyond C homologue. The deviation from normal behavior of N–I
6
transition curve is attributed to the polarity and polarizability induced by a tail group –Cl
and the uncertainty in the molecular length and the status of longer n-alkyl chain of left end
group, which disturbs the suitable magnitudes of molecular flexibility; operating N–I transi-
tion temperatures. N–I transition curve is extrapolated to C homologue to predict its N–I
16
transition temperature, which is not realizable in actual practice due to insufficient end to end
intermolecular cohesion. Similarly Sm–N transition curve is also extrapolated for C [27–30]
3
to determine its latent transition temperature (L.T.T.) for smectic, but it is not realizable due
to its high crystallizing tendency. Variations in LC properties from homologue to homologue
in the same series is attributed to the changing molecular length by a –CH unit, which affects
2
the suitable magnitudes of anisotropic forces of end to end and lateral attraction or cohesive
energy.
Some LC properties of presently investigated homologous series-1 are compared with the
other structurally similar known homologous series-X [31] and Y [32] as shown in Fig. 2.
Homologous series-1 of present investigation and the homologous series-X and Y chosen
for comparison are identical with respect to three phenyl rings, central bridge –COO– link-
ing first and middle phenyl rings, para-substituted flexible tail group –Cl as well as the left
n-alkoxy terminal end group for the same homologue from series to series. But, they differ
with respect to central bridges linking middle and third phenyl rings as –CH=CH–CO–,

6
0
P. K. RAKHASIA AND U. C. BHOYA
Figure . Structurally similar series.
–
CO–CH=CH–, and –N=N–. Therefore, observed difference in LC properties and the
degree of liquid crystallinity can be attributed to the differing features (central bridges or
molecular rigidity) of the molecules under comparative study. Table 3 represents some LC
properties of present series-1 and the series-X and Y chosen for comparison.
Table 3 indicates that,
r
Homologous series-1 and Y are smectogenic in addition to nematogenic character,
whereas series-X that is an isomeric with present series-1 is only nematogenic.
r
Thermal stability for smectic of present series-1 is lower than a series-Y by 6.14 and
smectic mesophase is destabilized till the last member of a series-X.
r
Nematic thermal stability of series-1 is higher than an isomeric series-X and the lower
than an azoester series-Y.
r
Smectic mesophase commences earlier for series-1 (C ) than a series-Y (C ) but it does
4
6
not commence till the last member of a series-X.
r
r
r
Total mesophase (Sm+N) of present series-1 is more than a series-X but it is less than a
series-Y under comparison.
Nematic mesophase commences from presenting series (C ) than a series X (C ) but it
3
5
commences later than a series-Y (C1).
Odd-even effect is exhibited by all the homologous series under comparative study.
Homologous series-1 of present investigation and an isomeric series-X differ only by dis-
placement of –CO– group between middle and third phenyl ring as para-substituted –Cl
group. The displacement of –CO– group bonded directly to either middle phenyl ring or last
third phenyl ring, which affects the extent of molecular noncoplanarity. Therefore, the suit-
able magnitudes of anisotropic forces of intermolecular end to end and lateral attractions as a
consequence of varying molecular rigidity and flexibility undergo variations. Thus, lamellar
Table . Average thermal stabilities in °C.
Series →

X
Y
Smectic-Nematic or Smectic-Isotropic
.
—
.
(
C -C )
(C -C )




Commencement of smectic mesophase
Nematic-Isotropic
C
—
C
.
.
.
(
C -C )
(C -C )
(C -C )






Commencement of nematic phase
Total Mesophase
C
C
C
.(C)
.(C)
. (C)
length range (Sm+N)
to .(C )
to . (C)
to . (C)



MOL. CRYST. LIQ. CRYST.
61
packing of molecules in the crystal lattices is favored for the novel, present series-1, while it
hinders for an isomeric series-X. Hence, variations arise in degree of molecular planarity due
to positional difference of same –CO– group at the end phenyl ring or middle phenyl ring
causing difference in the formation at smectic and nematic phase one after another or the for-
mation at only nematic phase excluding formation of smectic phase. The degree of molecular
planarity in case of series-Y favors sliding layered molecular arrangement, simultaneously
prior to statistically parallel orientational order of molecules under floating condition on the
surface resisting exposed thermal vibrations for definite range of temperature like present
series-1 to emerge smectic and nematic phase one after another.
Difference in molecular resistivity due to very minor change in molecular structure
by changing molecular rigidity through central bridges, –CH=CH–CO–, –CO–CH=CH–,
–
N=N– linking middle and third phenyl ring with para-substituted –Cl group, causes differ-
ence in permanent dipole moment across the long molecular axis, dispersion forces, electronic
and dipole interactions, extent of planarity, etc., which results into the suitable magnitudes of
thermal resistivity (ꢀH) toward exposed thermal vibrations of different range of temperature
(including zero degree difference at temperature), which facilitate or hinder the mesophase
stabilization and formation of either one or both type at LC state or states. Thus, based on the
suitable magnitudes of thermal resistivity against thermal vibrations with relevant molecu-
lar arrangement in floating condition can stabilize more or less either of the mesophase with
different degree of thermal stability. Highest thermal stability of nematic of series-Y and the
lowest at series-X as compared to series-1 is attributed to the rigidity played and contributed
by uncommon central bridges. Commencement of mesophase formation early or late depends
upon magnitudes of intermolecular end to end and/or lateral attractions that vary for the same
homologue from series to series. Thus, present investigation novel series is partly smectic and
partly nematic with middle ordered melting type.
Conclusion
•
Novel chalconyl ester derivatives are synthesized, which are thermotropically liquid
crystalline. In which –CO– group position for –CH=CH–CO– from middle phenyl
ring induces smectic and nematic mesophase and the –CO– group attached at the mid-
dle phenyl ring direction (–CO–CH=CH–) hinders the smectic mesophase formation.
The group efficiency order derived on the basis of (i) thermal stabilities, (ii) early
commencement of mesophase, and (iii) total mesophase length range for smectic and
nematic are as under.
•
(
i) Smectic: –N=N– > –CH=CH–CO– > –CO–CH=CH–
ii) Nematic: –N=N– > –CH=CH–CO– > –CO–CH=CH–
iii) Smectic: –CH=CH–CO– > –N=N– > –CO–CH>CH–
iv) Nematic: –N>N– > –CH=CH–CO– > –CO–CH=CH–
(
(
(
(
v) Smectic + Nematic: –N=N– > –CH=CH–CO– > –CO–CH=CH–
•
•
Liquid crystal properties and the degree of liquid crystallinity of substances are very
sensitive and susceptible to the molecular structure.
The novel compounds may be useful for devices to be operated between 62.0 °C and
8
2.0 °C by the study of binary systems. Chalconyl LC substances are biologically active
compounds, which may be pharmaceutically and medicinally useful.
•
Molecular rigidity and flexibility vary from substance to substance depending on
molecular structure in which central bridge and position can play an important role.

6
2
P. K. RAKHASIA AND U. C. BHOYA
Acknowledgments
The authors acknowledge thanks to the Department of Chemistry (DST-FIST Funded & UGC-SAP
Sponsored), Saurashtra University, Rajkot, for research work. The authors are also thankful to Dr. A. V.
Doshi, Ex. Principal M.V.M. Science and Home Science College Rajkot, for his valuable co-operation
during present investigation as and when needed. Also thanks to the National Facility for Drug Discov-
ery through New Chemical Entities (NCE’s) for analysis of samples.
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