Molecular flexibility operated mesomorphism by end groups in azoester series

Article abstract of DOI:10.1080/15421406.2016.1263098

A novel azoester homologous series, RO?C₆H₄?COO?C₆H₄?N = N?C₆H₄?OC₇H₁₅(n) (para) has been synthesized and studied with a view to understanding and establishing the effects of molecular structure on mesomorphic properties with reference to tthe flexible terminal chain. The novel azoester series consists of thirteen (C₁–C₁₈) members. Mesomorphism commences from C₅ homologue. C₁–C₄ homologues are nonmesomorphic, C₅ homologue is only nematogenic and C₆–C₁₈ homologues are smectogenic and nematogenic. All mesogenic homologues are enantiotropically mesomorphic. Transition temperatures and textures were determined by polarizing optical microscopy (POM) and a heating stage. Cr-M/I transition curve follows a zigzag path of rising and falling values and behaves in a normal manner. The Sm-N and N-I transition curves of a phase diagram behave in the normal established manner except for a small deviation for the higher homologues (C₁₄ and C₁₆). An odd-even effect is exhibited by Sm-N and N-I transition curves. Analytical spectral and thermal data support the molecular structures. Smectic and nematic thermal stabilities are 93.62°C and 114.3°C, respectively. Smectic and nematic mesophas elengths range from 12.0°C to 18.0°C and 11.0°C to 31.0°C. The series is predominantly nematogenic and partly smectogenic and of middle order melting type. Group efficiency order derived for smectic and nematic. Smectic and nematic: ?OCH₃ > ?OC₄H₉ > ?OC₇H₁₅(n).

Full text of DOI:10.1080/15421406.2016.1263098

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Molecular flexibility operated mesomorphism by
end groups in azoester series
B. B. Jain, U. H. Jadeja & R. B. Patel
To cite this article: B. B. Jain, U. H. Jadeja & R. B. Patel (2017) Molecular flexibility operated
mesomorphism by end groups in azoester series, Molecular Crystals and Liquid Crystals, 652:1,
1-9, DOI: 10.1080/15421406.2016.1263098
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Date: 18 October 2017, At: 07:04

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
https://doi.org/./..
Molecular flexibility operated mesomorphism by end groups
in azoester series
B. B. Jain, U. H. Jadeja, and R. B. Patel
Chemistry Department, K.K. Shah Jarodwala Maninagar Science College, Gujarat University, Ahmedabad,
Gujarat, India
ABSTRACT
KEYWORDS
Azoester; liquid crystals;
mesogenic; nematic; smectic
–
–
–
–
A novel azoester homologous series, RO C6H4 COO C6H4 N =
–
–
N C6H4 OC7H15(n) (para) has been synthesized and studied with a view
to understanding and establishing the effects of molecular structure
on mesomorphic properties with reference to tthe flexible terminal
chain. The novel azoester series consists of thirteen (C1–C18) members.
Mesomorphism commences from C5 homologue. C1–C4 homologues
are nonmesomorphic, C5 homologue is only nematogenic and C6–C18
homologues are smectogenic and nematogenic. All mesogenic homo-
logues are enantiotropically mesomorphic. Transition temperatures and
textures were determined by polarizing optical microscopy (POM) and
a heating stage. Cr-M/I transition curve follows a zigzag path of rising
and falling values and behaves in a normal manner. The Sm-N and N-I
transition curves of a phase diagram behave in the normal established
manner except for a small deviation for the higher homologues (C14
and C16). An odd-even effect is exhibited by Sm-N and N-I transition
curves. Analytical spectral and thermal data support the molecular
structures. Smectic and nematic thermal stabilities are 93.62°C and
114.3°C, respectively. Smectic and nematic mesophas elengths range
from 12.0°C to 18.0°C and 11.0°C to 31.0°C. The series is predominantly
nematogenic and partly smectogenic and of middle order melting type.
Group efficiency order derived for smectic and nematic. Smectic and
–
–
–
nematic: OCH3 > OC4H9 > OC7H15(n).
Introduction
This study on novel liquid crystals of the thermotropic type [1] is planned with a view
to understanding and establishing the relationship between molecular structure and LC
properties [2–6] by altering the moiety, geometrical shape, size, aromaticity, thermal and/or
lateral functional groups or central group or groups in isomeric and/or nonisomeric
molecules or by changing or exchanging place of the position or positions on same or dif-
ferent phenyl or alicyclic or heterocyclic ring, etc. Such information can play an important
role in predicting the temperature or temperature range and the suitability for LC and other
applications such as, light emitting materials, agriculture prediction, pharmaceuticals, etc. in
the benefit of mankind. Azoester dyes including LC properties which are useful and applica-
ble as equal as ordinary and other normal dyes and for many other application [7–11]. This
CONTACT U. H. Jadeja
upenjadeja@gmail.com
College, Gujarat University, Ahmedabad, Gujarat, , India.
Chemistry Department, K.K. Shah Jarodwala Maninagar Science
©  Taylor & Francis Group, LLC

2
B. B. JAIN ET AL.
investigation focuses on the synthesis of compounds with three phenyl rings bonded through
–
= –
–COO and -N N central bridges and varying and fixed terminal end groups of –OR and
–OC7H15(n), respectively. Evaluated thermometric data after due characterization will be dis-
cussed and interpreted in terms of molecular rigidity and flexibility [12–15]. Many azoesters
and esters have been reported to date [16–21].
Experimental
Synthesis
4-Hydroxy benzoic acid was alkylated using suitable alkylating agents (R-X) to convert it
into 4-n-alkoxy benzoic acids by a modified method of Dave and Vora [22]. Alkylation of
Paracetamol using an alkylating agent (R-X) is carried out to form 4-n-alkoxy acetanilide
and then hydrolyzed to give 4-n-heptyloxy aniline by a reported method [23(a)], The azo
dye was prepared by a known method [23(b)]. The azoesters were synthesized by a litera-
ture method [24]. Thus, the azo-ester homologue derivatives were filtered and washed with
sodium bicarbonate solution following distilled water and dried and purified until constant
transition temperatures obtained, using an optical polarizing microscopy 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 shown in Scheme 1.
Characterization
Representative homologues of the series were characterized by elemental analysis, mass spec-
trography, infrared spectroscopy, and 1H NMR spectra. IR spectra were recorded on a Perkin-
Elmer spectrum GX, 1H NMR spectra were recorded on a Bruker using CDCl3 as solvent.
Microanalysis was performed on a Perkin-Elmer PE 2400 CHN analyzer (Table 1). Transi-
tion temperatures and LC properties (Textures) were determined using an optical polarizing
microscopy equipped with heating stage. Texture images of nematic phase were determined
by a miscibility method (Table 2).
Analytical data
IR spectra in cm− for octyloxy and decyloxy derivatives
–
– –
Octyloxy: 694 Polymethylene ( CH2-)n of –OC8H17, 844 (–C H def. m di-substituted-
–
– –
Para), 759 Polymethylene ( CH2-) of –OC7H15, 968 (–C H def. hydrocarbon) 1083 and
– – – – – – – –
1015 ( C O ) Str, 1168, 1246, and 1307 ( C O str in –(CH2)n chain, 1427 and 1462 ( C H
– = –
–
–
def. in CH2), 1512 ( C C )str, 1604(N = N group) and 1728 ( COO ester group), 2850 and
– –
–
2924 ( C H str in CH3).
Decyloxy: 640 Polymethylene ( CH2-)n of –OC10H21, 840 (–C H def. m di-substituted-
– –
–
– –
–
Para), 756 Polymethylene ( CH2-) of –OC7H15, 968 (–C H def. hydrocarbon), 1083 and
– – – – – – – –
1022 ( C O ) Str, 1172,1246 and 1307 ( C O str in –(CH2)n chain, 1458 ( C H def. in
– = –
–
–
CH2), 1527 ( C C ) str, 1604 (N = N group) and 1728 ( COO ester group), 2850 and
– –
2924 ( C H str in CH3).

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
3
Scheme . Synthetic route to the series.
Table . Elemental analysis for heptyloxy, decyloxy, tetradecyloxy, and hexadecyloxy derivatives.
%Elements found
H
%Elements calculated
H
Molecular
formula
Sr. no.
C
N
C
N

CHON
CHON
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.



C
HON
HON
C
HNMR spectra in CDCl in δ ppm for hexyloxy & dodecyloxy derivative
–
–
Hexyloxy: 0.86 (t, CH3 of –C6H13), 1.18–1.28(m, n-poly methylene groups of OC6H13),
–
–
–
1.43 (m, n-poly methylene groups of OC7H15), 3.9 (s, OCH2 CH2-of OC6H13), 4.0
(s, OCH2 CH2-of OC7H15), 6.79–7.28 (s, phenyl ring), 7.8–8.1 (s, p-disubstituted phenyl
–
–
ring).

4
B. B. JAIN ET AL.
Table . Texture of smectic and nematic phase of C, C, C, C by miscibility method.
Texture
Sr. no.
Homologue
Smectic
Nematic

C
Sm-A
Sm-A
Threaded
Threaded
Schlieren
Schlieren



C
C
C
–
Sm C
–
Sm C
α--(/-n-alkoxybenzoyloxy) phenyl azo-/- heptyloxy benzene
–
Dodecyloxy: 0.898 (t, CH3 of –C12H25), 1.09–1.34 (m, n-poly methylene groups
–
–
–
–
of OC12H25), 1.48 (m, n-poly methylene groups of OC7H15), 3.51 (s, OCH2 CH2-
ofOC12H25), 4.16 (s, OCH2 CH2-of OC7H15), 6.9-7.4 (s,phenyl ring), 7.9–8.1 (s, p-
–
–
disubstituted phenyl ring).
Results and discussion
Novel homologous series of azoesters was synthesized by condensation of dimeric 4-n-Alkoxy
benzoic acids and 4-hydroxy phenyl azo-4/- heptyloxy benzene (m.p. –89°C, yield 81.22%).
Dimerization of 4-n-alkoxy benzoic acids disappears on esterification and mesomorphism
commences from C5 homologue in enantiotropic manner. C5 homologue is only nematogenic
and the rest of the LC homologues are (C6–C18) smectogenic plus nematogenic. Transition
temperatures including textures were determined by polarizing optical microscopy equipped
with a heating stage (POM). Transition temperatures (Table 3) were plotted versus the num-
ber of carbon atom present in n-alkyl chain ‘-R-’ of –OR terminal group and on linking, like
or related points, the transition curves Cr-M/I, Sm-N, and N-I are obtained as shown in a
phase diagram (Fig. 1) showing phase behaviors of series. Cr-M/I transition curve follows a
zigzag path of rising and falling values with overall descending tendency. Sm-N transition
curve initially rises passes through maxima at C10 homologue and then descended up to last
homologue of a series with exhibition of sharp narrow and shorter odd-even effect. N-I transi-
tion curve descended as series is ascended with exhibition of sharp and short odd-even effect,
with negligible deviation at C14 and C16 and overall descending tendency. Thus, all the transi-
tion curves of a phase diagram behaved in normal manner except negligible deviation in N-I
Table . Transition temperature table in°C.
Transition temperatures in°C
Sr. no
R = n-alkyl group
Smectic
Nematic
Isotropic













C
C
C
C
C
—
—
—
—
—
.
.
.
.
.
.
.
.
—
—
—
.
.
.
.
.
.
.
.
.
.
.
.
.
—
.
.
.
.
.
.
.
.
.
C
C
C
C
C
C
C
C

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
5
Figure . Phase behaviors of series.
curve for C14 and C16 homologue. Analytical, thermal and spectral data supported molec-
ular structure of homologues. Thermal stabilities for smectic and nematic are 93.62°C and
114.33°C with total mesophase length varying from 23.0°C to 44.0°C. Mesomorphic proper-
ties and behaviors vary from homologue to homologue in a same series including textures of
homologues.
Disappearances of dimerization of n-Alkoxy benzoic acids and the lowering of transition
temperatures of corresponding azoester homologues are due to the breaking of hydrogen
bondings between two molecules of n-alkoxy benzoic acids by esterification process. In exhi-
bition of mesomorphism, either in reversible or irreversible manner by C1, C2, C3, and C4
homologues is attributed to their high crystallizing tendency which arises from unsuitable
magnitudes of anisotropic forces of intermolecular end to end or lateral cohesion as a conse-
quence of unfavorable molecular rigidity and flexibility, due to low magnitudes of dispersion
forces and low magnitudes of dipole-dipole interactions, which hinders the disalignments
of molecule under the influence of exposed thermal vibrations as a result of their low ther-
mal resistivity. Exhibition of mesomorphism by C5 to C18 homologues either as enantiotropic
nematic and/or smectic is attributed to the disalignments of molecules at an angle ninety

6
B. B. JAIN ET AL.
Figure . Structurally similar analogous series.
degree or/and less than ninety degree with the plane of the floating surface, which resisted
exposed thermal vibrations to float with statistically parallel orientational order excluding
or including sliding layered molecular arrangement for definite range or ranges of temper-
ature in reversible manner. The exhibition of odd-even effect by Sm-N and N-I transition
curves of a phase diagram is due to the sequentially added methylene unit or units at n-alkyl
chain ‘-R-’ of –OR group up to C8 or C9 homologue respectively. Beyond that (C8 or C9),
a single transition curve is followed, i.e. odd-even effect disappears from and beyond merg-
ing of transition curves indicating odd-even effect for higher homologues of longer n-alkyl
–
chain ‘R’ of left OR group. Because, longer n-alkyl chain may coil or bend or flex or cou-
ple to lye with major axis of core structures of a molecule. The negligible deviation in N-I
transition curve observed for C14 and C16 homologues is attributed to the unpredictable and
unexpected or unusual status of left and right –R and –OC7H15(n) hydrocarbon chains of –
OR and –OC7H15(n) terminally situated end groups. The extrapolation of Sm-N transition
curve at C5 [25] predicts a value (LTT) at 83°C but it is unrealizable due to high crystalline
tendency. The variations in mesogenic properties and behaviors from homologue to homo-
logue in the same series is attributed to the variations of number of carbon atoms or –CH2-
unit, which causes variations in the length to breadth ratio, ratio of the molecular polarity to
polarizability, permanent dipole moment across the long molecular axis, magnitudes of the
molecular rigidity and flexibility, intermolecular dispersion forces, etc. required to facilitate
and stabilize a phenomena of mesomorphism as smectic and/or nematic in either reversible or
irreversible manner. The mesogenic (some) properties of presently investigated novel series-1
are compared with structurally similar analogous series-X [26] and Y [27] as mentioned in
Fig. 2.
Above Fig. 2 indicates that, homologous series-1 of present investigation and the homol-
ogous series X and Y chosen for comparative study are identical with respect to molecular
rigidity through aromaticity of three phenyl rings and the identically same central bridges
–
–
– = –
COO and N N from series to series as well as homologue to homologue in the same
series. Left flexible n-alkoxy terminal end group are identical for the same homologue from
series to series but, varies from homologue to homologue in the same series. Tailed terminal
–
–
end groups OC7H15 (n) and OCH3 which differ in the magnitudes of thermal property for
the same homologue from series to series but, remains unaltered for, all the members of each
same series. Thus, variations in mesomorphic properties and their magnitudes depend upon

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
7
Table . Relative thermal stability in°C.
–
–
–
Series
Series-( OCH (n)) Series-X ( OCH (n))
Series-Y ( OCH)
Sm-N or Sm-I Commencement of Smectic phase
N-I Commencement of Nematic phase
Total mesophase length range in°C
. (C–C) C
. (C–C) C
. (C–C) C
. (C–C) C
. (C–C) C
. (C–C) C
.°C to .°C C C .°C to .°C C C .°C to .°C C C
the varying combined effects of molecular rigidity plus flexibility or upon the differing fea-
tures of individual molecular structure for the same homologue from series to series and from
homologue to homologue in the same series. FollowingTable 4 represents some thermometric
properties of series 1, X and Y in comparative manner.
Careful observation of Table 4 suggest that,
r
All the homologues of series 1, X and Y under comparative study are enantitropically
smectogenic plus nematogenic.
Thermal stabilities foe smectic and nematic are in increasing order from series-1 to
series-X to series-Y.
Early commencement of smectic phase is in decreasing order from series-1 to series-X
to series-Y.
Early commencement of nematic phase is the highest for series-1 (C1) and the lowest for
series-X (C6) and intermittent for series-1 (C5).
r
r
r
r
Upper and lower total mesophase lengths increases from series-X to series-1 to series-Y.
Exhibition of smectic plus nematic mesophase formations are obvious, because of the pres-
–
ence of varying –OC7H15 (n), OC4H9 (n) and –OCH3 tailed ended functional groups of
linear, lath like geometrical shape, their group polarity, size, end to end as well as lateral cohe-
sions and closeness, dispersion forces, etc. within the molecules of each same series and for
the same homologue from series to series, which creates possibilities to facilitate an envi-
ronmental situation to induce smectic and/or nematic phase early or late as a consequence
of suitable differing magnitudes of anisotropic forces of intermolecular attractions and com-
bined effects of differing molecular rigidity plus flexibility. The increasing order of thermal
stabilities for smectic and nematic is attributed to the decreasing length of flexible tailed n-
–
alkoxy end groups –OC7H15(n), OC4H9(n), and –OCH3 which decreases possibilities of
uncertainty in the status of n-alkyl chain –C7H15(n), C4H9, and –CH3 commonly bonded
–
through oxygen atom with phenyl ring which affects magnitudes of molecular flexibility in
favor of increasing thermal stabilities at constant molecular rigidity. Early or late commence-
ment of mesophase depended upon the extent of molecular noncoplanarity and the degree of
uncertainty in the status of n-alkyl chain of flexible end groups. The total mesophase lengths
are depended upon the thermal resistivity offered by a molecule depending upon the internal
energy stored by the molecules of different characteristics of varying structures. Thus, upper
and lower total mesophase lengths followed the same order of comparison as early commence-
ment of mesophase or mesophases.
Conclusions
r
Novel azoester homologous series is predominantly nematogenic and partly smectogenic
with its middle ordered melting type.
As tailed ended n-alkyl chain become shorter the early commencement of nematic
mesophase and total mesophaselengths are favored positively.
r

8
B. B. JAIN ET AL.
r
The group efficiency order derived for smectic and nematic on the basis of (i) thermal
stabilities (ii) early commencement of mesophase and (iii) total mesophaselengths are as
under.
–
–
–
(i) Smectic: OCH3 > OC4H9(n) > OC7H15(n)
–
–
–
Nematic: OCH3 > OC4H9(n) > OC7H15(n)
–
–
–
(ii) Smectic: OC7H15(n) > OC4H9(n) > OCH3
–
–
–
Nematic: OCH3 > OC7H15(n) > OC4H9(n)
(iii) Total mesophaselengths
–
–
–
Upper and lower: OCH3 > OC7H15(n) > OC4H9(n)
Molecular structure of a molecule is very sensitive and susceptible to mesomorphism or
vice versa as a result of molecular rigidity and/or flexibility.
Present investigation very well supports and raises credibility to the conclusions drawn
earlier.
r
r
Acknowledgments
Authors acknowledge thanks to Dr. R. R. Shah, principal of K. K. Shah Jarodwala Maninagar Science
College, Ahmedabad. Authors are thankful to Dr. A. V. Doshi, Ex-principal of M.V.M. Science and
Home Sci. College - Rajkot for his constant support, inspirations, valuable comments, suggestions, and
help, as and when needed during the course of present investigation. Authors Acknowledge thanks to
NFDD Centre for the analytical and spectral services.
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