Dependence of LC state on molecular flexibility

Article abstract of DOI:10.1080/15421406.2016.1146935

A novel azoester homologous series of liquid crystals (LCs) viz. RO?C₆H₄COOC₁₀H₆-N?N(ortho)-C₆H₄?OC₄H₉(para) has been synthesized and studied with a view to understanding and establishing the effect of molecular structure on LC properties. Homologous Series consists of thirteen members (C₁ to C₁₈). C₁ to C₃ members are nonliquid Crystals and the rest of the homologues are LC in an enantiotropic manner. C₇ to C₁₈ are smectogenic in addition to nematogenic whereas C₄, C₅, and C₆ are only nematogenic. The Sm-N and N?I transition curves behave in a normal manner with the usual exhibition of an odd–even effect. The Cr-M/I curve also behaves in a normal manner. Analytical and spectral data confirm the molecular structures of homologues. The average smectic and nematic thermal stabilities are 60.31°C and 79.6°C, respectively, with total mesophase length varying minimum to maximum is 21°C to 57°C at the C₆ and C₁₄ homologue, respectively. Thus, present novel azoester homologous series is partly smectogenic and predominantly nematogenic with low ordered melting type and useful to construct LC devices workable at low temperatures or room temperature.

Full text of DOI:10.1080/15421406.2016.1146935

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Dependence of LC state on molecular flexibility
U. H. Jadeja, Vinay S. Sharma, B. B. Jain & R. B. Patel
To cite this article: U. H. Jadeja, Vinay S. Sharma, B. B. Jain & R. B. Patel (2016) Dependence of
LC state on molecular flexibility, Molecular Crystals and Liquid Crystals, 630:1, 144-153, DOI:
10.1080/15421406.2016.1146935
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Date: 21 July 2016, At: 23:24

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
http://dx.doi.org/./..
Dependence of LC state on molecular flexibility
U. H. Jadeja, Vinay S. Sharma, B. B. Jain, and R. B. Patel
Chemistry Department, K. K. Shah Jarodwala Maninagar Science College, Gujarat University, Ahmedabad,
Gujarat, India
ABSTRACT
KEYWORDS
Azoester; liquid crystal;
mesomorphs; smectic;
nematic
A novel azoester homologous series of liquid crystals (LCs) viz.
–
=
–
RO C6H4COOC10H6-N N(ortho)-C6H4 OC4H9(para) has been synthe-
sized and studied with a view to understanding and establishing the
effect of molecular structure on LC properties. Homologous Series con-
sists of thirteen members (C1 to C18). C1 to C3 members are nonliquid
Crystals and the rest of the homologues are LC in an enantiotropic man-
ner. C7 to C18 are smectogenic in addition to nematogenic whereas C4,
–
C5, and C6 are only nematogenic. The Sm-N and N I transition curves
behave in a normal manner with the usual exhibition of an odd–even
effect. The Cr-M/I curve also behaves in a normal manner. Analytical
and spectral data confirm the molecular structures of homologues. The
average smectic and nematic thermal stabilities are 60.31°C and 79.6°C,
respectively, with total mesophase length varying minimum to maxi-
mum is 21°C to 57°C at the C6 and C14 homologue, respectively. Thus,
present novel azoester homologous series is partly smectogenic and
predominantly nematogenic with low ordered melting type and useful
to construct LC devices workable at low temperatures or room temper-
ature.
Introduction
The study of the liquid crystalline (LC) state [1] has attracted scientists and technologists for
its unique applications in electronic display devices [1–4] and biological activity [5–9] from
thermotropic or lyotropic LC substances. Hence, every scientist and technologist needs novel
LC substances to continue and extend their research programs. The present investigation
is planned with a view to understanding and establishing the effects of molecular structure
[10–13] on LC properties by synthesizing novel LC substances through homologous series of
novel unexploited moieties of varying geometrical shape, size, polarities, polarizability, ortho-
meta-para substitution, or lateral, and/or terminal substitution etc. of molecule which causes
variations in molecular rigidity and/or flexibility as well as thermodynamical properties from
one molecular structure to other molecular structure in crystalline or solution state [14–18].
The present proposed homologous series planned to synthesize novel azoester series consist-
ing of naphthyl ring, two phenyl rings bonded to central bridges and two tail groups. Number
of ester series have been reported till the date [19–23].Thermometric properties will be deter-
mined after characterization of novel homologues and then the data will be recorded and
CONTACT U. H. Jadeja
upenjadeja@gmail.com; R. B. Patel
K. K. Shah Jarodwala Maninagar Science College, Gujarat University, Ahmedabad , Gujarat, India.
roshanpatel@gmail.com
Chemistry Department,
©  Taylor & Francis Group, LLC

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
145
interpreted in terms of molecular rigidity and flexibility including derivation of group effi-
ciency order on the basis of thermal stability, Commencement of LC state and the degree of
mesomorphism.
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 [24], Alkylation of
Paracetamol using alkylating agent n-C4H9Br is carried out to form 4-n-butyloxy acetanilide,
which on hydrolysis converted to 4-n-butyloxy aniline is form by usual establish method.
Azo dye (B) 2-hydroxy naphthyl azo 4ꢀ butyloxy benzene (m.p. 80°C, yield 72%) was pre-
pared by well-known azotization method, Final azoester products were synthesized by con-
densation of (A) and (B) [25]. Thus, the azo-ester homologue derivatives were filtered, washed
with sodium bicarbonate solution followed by distilled water, dried and purified till constant
transition temperatures obtained using an optical polarizing microscope equipped with a
heating stage. 4-hydroxy benzoic acid, Alkyl halides, Paracetamol, dicyclohexyl carbodimide,
Dimethyl amino pyridine, DCM, MeOH, Acetone required for synthesis were used as received
except solvents which were dried and distilled prior to use. The synthetic route to the series
is mentioned in Scheme 1.
Characterization
Representative homologues of a series were characterized by elemental analysis, Infrared
spectroscopy, 1H NMR spectra, IR spectra were recorded on Perkin–Elmer spectrum GX,1H
NMR spectra were recorded on Bruker using CDCl3 as solvent. Microanalysis was performed
on Perkin-Elmer PE 2400 CHN analyzer (Table 1). Transition temperature and LC proper-
ties (Textures) were determined using an optical polarizing microscopy equipped with heat-
ing stage (POM). Texture images of nematic phase were determined by miscibility method
(Table 2), thermodynamic quantities enthalpy(ꢀH) and entropy(ꢀS = ꢀH/T) are qualita-
tively discussed.
Analytical data
IR Spectra in cm−1 for Octyloxy & Dodecyloxy Derivatives:
–
–
–
– – –
– – –
Octyloxy : 669 Polymethylene ( CH2 )n of OC8H17, 840 ( C H def. m di-substituted-
–
–
–
– –
=
Para), 759 Polymethylene ( CH2 ) of OC4H9, 1105 and 1024 ( C O ) Str 1163,1247( C O
– – –
– = –
str in –(CH2)n chain, 1423 and 1469 ( C H def. in CH2), 1494 ( C C )str, 1604(N N
group) and 1735 ( COO ester group), 2848 and 2918 ( C H str in CH3).
–
–
– –
–
–
–
– – –
Dodecyloxy : 661 Polymethylene ( CH2 )n of OC12H25, 844 ( C H def. m di-
–
–
–
– – –
substituted-Para), 769 Polymethylene ( CH2 ) of OC4H9, 948 ( C H def. hydrocarbon),
– – – – – – – –
1085 and 1031 ( C O ) Str, 1165,1255 and 1307 ( C O str in –(CH2)n chain, 1465 ( C H
– = –
=
–
–
def. in CH2), 1566 ( C C )str, 1604(N N group) and 1737 ( COO ester group), 2848 and
2916 ( C H str in CH3).
– –
1HNMR spectra in CDCl3 in δ ppm for Hexyloxy & Octadecyloxy Derivative:

146
U. H. JADEJA ET AL.
Scheme . Synthetic route to the series-.
–
–
Hexyloxy: 0.88 (t, CH3 of –C6H13), 1.18-1.37(m, n-poly methylene groups of OC6H13),
–
–
–
–
1.48 (m, n-poly methylene groups of OC4H9), 3.8(s, OCH2 CH2 ofOC6H13), 3.99
(s, OCH2 CH2 of OC4H9), 6.9-7.1(s,naphthalene ring), 7.7-7.99 (s, p-disubstituted phenyl
–
–
–
ring).
–
Octadecyloxy: 0.82 (t, CH3 of –C18H37), 1.1-1.3(m, n-poly methylene groups
–
–
–
–
–
of OC18H37), 1.51 (m, n-poly methylene groups of OC4H9), 3.3-3.6(s, OCH2 CH2 of
–
–
–
OC18H37), 4.0(s, OCH2 CH2 of OC4H9), 7.0-7.2(s,naphthalene ring), 7.9-8.1 (s, p-
disubstituted phenyl ring).

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
147
Table . Elemental analysis for pentyloxy, octyloxy, decyloxy, hexadecyloxy, derivatives.
%Elements found
H
%Elements calculated
H
Sr. No.
Molecular formula
C
N
C
N

CHON
CHON
CHON
CHON
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.



.
Result and discussion
Smectogenic and nematogenic mesomorphism is induced on condensing dimeric 4-n- alkoxy
benzoic acids and an azo dye (m.p. 80°C) derived from 2-napthol and 4- butyloxy aniline. The
transition temperatures of novel azoester homologues alternates and are relatively lower than
the corresponding dimeric n-alkoxy acids. Nematogenic mesophase formation and Smecto-
genic mesophase formation commences from C4 and C7 homologues, respectively, in an enan-
tiotropic manner upto last C18 homologue with absence of LC Phase in C1,C2, and C3. Tran-
sition temperatures (Table 3) as determined from an optical polarizing microscopy equipped
with a heating stage are plotted versus the number of carbon atoms present in left n-alkoxy (-
–
OR) group. A Phase diagram consisted of Cr-M/I, Sm-N and N I transition curves is obtained
by linking like or related transition points, showing phase behaviors of a novel series as shown
in Fig. 1. Cr-M/I transition curve follows a zigzag path of rising and falling with overall
descending tendency and behaved in normal manner.
The Sm-N transition curve initially rises and passes through maxima at the C14 homo-
logue and then descended with exhibition of odd–even effect from C7 to C9 derivative. An
–
N I transition Curve initially descends and then rises from C7 homologue to maxima at C14
Table . Texture of nematic phase of C, C, C, C by miscibility method.
Texture
Sr. No.
Homologue
Smectic
Nematic

C
C
C
C
Sm-A
Sm-A
Sm-C
Sm-C
Threaded
Threaded
Schlieren
Schlieren



Table . Transition temperatures in °C for series-.
Transition Temperatures in(˚C)
Compound No
n-alkyl chain CnHn+
Smectic
Nematic
Isotropic













C
C
C
C
C
C
C
C
C
C
C
C
C
—
—
—
—
—
—
—
—
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
—
.
.
.
.
.
.
.

148
U. H. JADEJA ET AL.
Figure . Phase diagram of series .
homologue and then descended as series is ascended upto C18 homologue with exhibition of
odd –even effect from C4 to C7 derivative. Odd-even effect for Smectic and Nematic appears
upto C9 and C7 homologue, respectively, and then disappears for higher homologues of longer
–
n-alkyl chain of left n-alkoxy group. Thus, Cr-M/I, Sm-N and N I transition curve behaved
in normal manner with negligible abnormality. Sm-N transition curve is extrapolated to C6
homologue [27–29] to determine the absence of smectogenic character in C6 homologue. Tex-
tures of a Nematic phase are threaded or schlieren and that of smectogenic homologues are
Table . Relative thermal stability in °C.
–
–
–
Series
( OCH)
X( COCH)
Y( CH)
Smectic-Isotropic Or
Smectic-Nematic
Commencement of
Smectic phase
Nematic–Isotropic
Commencement of
Nematic phase
Total mesophase length
range (Sm+N)
Ci to Cp
.
–
(C C)
—
—
C
.
.
.
–
–
–
(C C)
(C C)
(C C)
C
C
C
. to °C
. to
.°C
C C
. to
.°C
C C
C C

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
149
focal conic of the type Smectic A or C. Analytical and Spectral data confirms the molecular
structures of homologues.
Disappearance of dimerization and the lowering of transition temperatures of novel homo-
logues are attributed to the breaking of hydrogen bonding between two molecules of 4-n-
alkoxy benzoic acids through esterification process. The absence of LC property in C1, C2,
and C3 homologues is attributed to the unsuitable magnitudes of anisotropic forces of inter-
molecular attractions and closeness occurred as a consequence of unfavorable molecular
rigidity and flexibility, which induced high crystallizing tendency due to their low dipole–
dipole interactions and dispersion forces by interactions between instantaneous dipoles pro-
duced by spontaneous oscillations of electron clouds of the molecules. Thus, crystal lattices
of molecules of C1 to C3 derivatives breaks abruptly and directly transform into isotropic liq-
uid from solid crystalline state under the influence of exposed thermal vibrations, without
exhibition of LC state. The exhibition of LC state by C4 to C18 is attributed to disalignment of
molecules at an angle ninety or < 90° by suitable magnitudes of anisotropic forces of inter-
molecular attractions and closeness as a consequence of a favorable molecular rigidity and
flexibility caused by resultant permanent dipole moment across the long molecular axis and
dispersion forces. The molecules of C3 to C18 homologues adopted either only statistically par-
allel orientational order of molecules (C3 to C6) or after exhibition of Smectic phase (C7 to C18)
under floating condition due to favorable end to end intermolecular cohesion, within definite
range of temperature, which, shows nematic type of textures under microscopic (POM) exam-
inations. However, the molecules of C7 to C18 homologues exhibited smectogenic character
prior to nematic mesophase formation for lower mesogenic temperature range by maintain-
ing sliding layered molecular arrangement under floating conditions with showing texture,
–
either smectic-A or smectic C.
All the mesogenic transitions are enantiotropic and none of them is monotropic. The odd
member’s transition temperatures being higher than even members up to the end of odd-even
effect at (C7) homologue. Thus, odd–even effect disappeared from and beyond C7 homologue
for longer n-alkyl chain from C8 to C18 homologues, because, longer n-alkyl chain may coil
or bend, or flex or couple to lye with the principal axis of a molecular core structure. Hence,
–
–
addition of CH2 units becomes an important. Odd-even effect observed due to odd and
even number of methylene units present in n-alkyl chain. Sometimes uncertainly in the sta-
tus of longer n-alkyl chain causes deviations in the normal behavior of a transition curve or
curves of a phase diagram. The variations in the mesomorphic behaviors or LC properties
from homologue to homologue in the same series are attributed to the increasing molecular
–
–
length due to progressive and sequential addition of CH2 unit or units at the left n-alkoxy
terminal end group, contributing to suitable magnitudes of intermolecular anisotropic forces
of adhesion and closeness. The changing trend in left n-alkoxy terminal, keeping rest of the
molecular part intact in the same series causes variation in molecular characteristics. The
properties of present thermotropic LC series are compared with structurally similar known
series X [30] and series Y [31] as shown below in Fig. 2.
Homologous series1, X and Y under comparison contain identically two phenyl rings
–
= –
and one naphthyl ring bonded through –COO and –N N central bridges on identical
position which contribute to almost identical molecular rigidity and common (-OR) left n-
alkoxy terminal end group for the same homologue from series to series. However, they dif-
–
– –
–
–
–
fer with respect to right ended uncommon terminal OC4H9(n)/ O CH2 CH2 CH2 CH3,
COCH3 or CH3 polar groups; from series to series. Thus, left terminal OR vary from
–
–
–
homologue to homologue in the same series, keeping right handed terminal unchanged, as
well as right handed terminal end group changes for the same homologue from series to series

150
U. H. JADEJA ET AL.
Figure . Structurally similar series.
keeping rest of the molecular part unchanged. Therefore, changing trend in LC properties and
the degree of mesomorphism will depend upon the differing magnitudes of intermolecular
anisotropic forces occurring as a consequence of changing molecular rigidity and molecular
flexibility due to respective molecular structure of individual homologue of same series or for
the same homologue from series to series. The extraploted Sm-N transition curve matches
–
with N I transition point of C6 homologue, which rejects the possibility of smectogenic char-
acter in C6 homologue. Table 3 represents some LC properties of thermotropic present novel
series-1 and structurally similar known series X and Y chosen for comparison.
From Table 4, it is clear that,
r
Homologous novel series-1 of present investigation is smectogenic plus nematogenic, but
homologous series X and series Y selected for comparative study are only nematogenic.
Smectogenic mesophase commences from C7 homologue of a series-1, whereas, it does
not commence till the last homologue of series X and series Y.
Nematogenic mesophase commences from C4, C6, and C1 for series-1, X and Y respec-
tively.
Smectic thermal stability is 60.31 for series-1 whereas, it does not stabilize for series X
and Y.
Thermal stability for nematic are 79.6, 144.0, and 127.27 for series-1, X and Y, respec-
tively.
The upper and lower mesophase lengths are decreasing from series-X to series-1 to
series-Y.
r
r
r
r
r
The molecular rigidity due to phenyl rings and some central bridges with same geometri-
cal molecular shape being equivalent from homologue to homologue in the same series and
for the same homologue from series to series in case of present comparative study (Series 1,
X,Y). However, the molecular flexibility is varied from homologue to homologue in the same
series and for the same homologue from series to series depending upon the difference of
group polarities of left and right sided end groups as well as molecular polarizability differ-
ence due to geometrical shapes of molecules. Thus, magnitudes of dispersion forces related

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
151
to permanent dipole moment across the long molecular axis, dipole–dipole and/ or elec-
tronic interactions and hence the molecular flexibility vary from homologue to homologue
in the same series or from series to series; because, the vector sum of the bond polarities of
–
OC4H9(n),
–
–
COCH3 and CH3 are different from each other for the same homologue from series to
series and from homologue to homologue in the same series. The function group
–
CH3 of series Y is less polar than –COCH3. Therefore molecular polarity and polariz-
ability of series-X will be higher than a series-Y for corresponding homologue of each other.
– –
–
–
–
–
But the terminal end group OC4H9 or O CH2 CH2 CH2 CH3 is longer than –COCH3
–
and CH3 which induces lamellar packing of molecules in rigid crystal lattices which causes
sliding layered molecular arrangement in floating condition under the influence of exposed
thermal vibrations prior to commencement of nematic phase relatively at lower temperature
up to below 45°C to 40°C and stabilizes smectogenic character in addition to nematogenic
character. The commencement of mesophase either smectic or nematic depends upon the
extent of molecular noncoplanarity which differs from series to series due to magnitudes of
group polarity and polarizability difference and reflects to early or late commencement of
mesophase formation. Lower or higher transition temperatures resulting low or high aver-
age thermal stability depend upon the resistivity offered by homologue or homologues of an
individual series constituting enthalpy change (ꢀH) value which is related with magnitudes
of resistivity towards the exposed thermal vibrations to facilitate molecular arrangement in
floating condition maintaining either of only nematic phase or smectic phase or both, one
–
after another. Thus, COCH3 terminal being relatively more polar and polarizable causes
highest nematic thermal stability, but fails to facilitate lamellar molecular packing in their
– –
crystal lattices due to relatively shorter –C C linkage with third phenyl ring and does not
–
exhibit smectogenic character throughout a series. Same in the case with CH3 terminal end
group which bears shortest –C C linkage with third phenyl ring of series-Y. However, a
– –
terminal end group which is relatively longer and bonded through oxygen atom by phenyl
– –
–
–
–
ring O CH2 CH2 CH2 CH3 bears etherial linkage with the last phenyl ring, facilitated
smectic mesophase formation. Hence, type of linkage between tail group and last phenyl
ring can also affect (ꢀH) value of a molecular structure and hence the molecular flexibil-
ity or thermal stability through molecular polarity and polarizability as well as the degree of
mesomorphism.
Conclusions
r
Two phenyl rings and Naphthyl ring containing substances are not necessarily be always
nematogenic, but may be smectogenic in addition to nematogenic, if proper tail groups
at proper position are substituted.
Nonlinear azoester homologous series of present investigation is partly smectogenic and
predominantly nematogenic with low ordered melting type and of sufficient range of
liquid crystallity.
The group efficiency order derived for smectic and nematic on the basis of (1) thermal
stability (ll) early commencement of mesophase (lll) total upper mesophase length in
case of azoester series are as under.
r
r
(i)Smectic
–
–
–
OC4H9(n) > COCH3 = CH3
Nematic
–
–
–
COCH3> CH3> OC4H9 (n)

152
U. H. JADEJA ET AL.
(ii) Smectic
–
–
–
OC4H9 (n) > COCH3 = CH3
Nematic
–
–
–
CH3> OC4H9 (n) > COCH3
(iii) Total Mesophase length
–
–
–
COCH3> OC4H9 (n) > CH3
r
The magnitudes of molecular flexibility of a molecule depends upon the difference of
vector sum of bond polarities of two end groups or tail group polarities, contributing to
total molecular polarities and polarizability.
Suitable magnitudes of molecular rigidity and flexibility can induce LC property.
Molecular rigidity and flexibility are very sensitive and susceptible to molecular struc-
ture.
The LC state of homologue of present series stabilizes at low as 40 to 45°C which can be
brought to low as 20 to 25°C or at room temperature by the study of binary systems.
Room temperature LCs are useful for the manufacture of LC devices to be operated
between 20°C to 50°C.
Present investigation very well support and raises the credibility to the conclusions
drawn earlier.
r
r
r
r
r
Acknowledgments
The authors acknowledge thanks to Dr. R.R.Shah, principal and management 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 and Dr. H.R. Prajapati of C.U. Shah Science College, Ahmedabad
for their constant support, inspirations and help, as and when needed during the course of present
investigation. The authors thank the NFDD Centre for the analytical and spectral services.
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