Study of mesomorphism dependence on molecular flexibility of an azoester series containing a napthyl unit

Article abstract of DOI:10.1080/15421406.2016.1190501

A novel azoester homologous series of liquid crystalline (LC) compounds: RO?C₆H₄-COO?C₁₀H₆-N:N-C₆H₄?OC₄H₉(n) without lateral substitution has been synthesized and studied with a view to understanding and establishing the effects of molecular structure on thermotropic LC substances with reference to tailed-end group. The novel homologous series consists of 13 homologs (C₁ to C₁₈) whose nematogenic and smectogenic mesomorphism commences enantiotropically from C₆ and C₁₂ members of the series, respectively. The C₁₂–C₁₈ homologs are smectogenic and C₆–C₁₈ are nematogenic, of which C₁₂–C₁₈ homologs are smectogenic plus nematogenic. The C₁–C₅ homologs are nonmesogenic. Transition temperatures and the textures of the homologs were determined and identified by an optical polarizing microscope (POM) equipped with a heating stage. Textures of a nematic phase are threaded or Schlieren and that of the smectic phase are of the type A or C. Transition curves Cr-M/I, Sm-N and N-I of a phase diagram behaved in normal manner except N-I transition temperature of C₁₀ homolog which deviated by 9°C–10°C from normal behavior. N-I transition curve exhibited odd-even effect. Analytical, spectral, and thermal data confirms the molecular structures of homologs. Thermal stability for smectic and nematic are 115.5°C and 138.5°C, respectively whose corresponding mesophaselengths are varied from 10.0°C to 16.0°C and 13.0°C to 24.0°C, respectively. Group efficiency order for smectic and nematic are derived from comparative study of structurally similar analogous series; as smectic: ?OC₄H₉ (n) > ?CH₃ > ?H; Nematic: ?H > ?OC₄H₉ (n) > ?CH₃

Full text of DOI:10.1080/15421406.2016.1190501

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Study of mesomorphism dependence on
molecular flexibility of an azoester series
containing a napthyl unit
U. H. Jadeja & R. B. Patel
To cite this article: U. H. Jadeja & R. B. Patel (2016) Study of mesomorphism dependence on
molecular flexibility of an azoester series containing a napthyl unit, Molecular Crystals and
Liquid Crystals, 637:1, 10-18, DOI: 10.1080/15421406.2016.1190501
To link to this article: http://dx.doi.org/10.1080/15421406.2016.1190501
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Date: 11 November 2016, At: 00:18

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –

http://dx.doi.org/./..
Study of mesomorphism dependence on molecular flexibility
of an azoester series containing a napthyl unit
U. H. Jadeja and R. B. Patel
Chemistry Department, K. K. Shah Jarodwala Maninagar Science College, Gujarat University, Ahmedabad,
Gujarat, India
ABSTRACT
KEYWORDS
A novel azoester homologous series of liquid crystalline (LC) com-
pounds: RO–C H -COO–C H -N:N-C H –OC H (n) without lateral
Azoester; liquid crystal; LTT;
Nematic; Smectic
6
4
10
6
6
4
4
9
substitution has been synthesized and studied with a view to under-
standing and establishing the effects of molecular structure on
thermotropic LC substances with reference to tailed-end group. The
novel homologous series consists of 13 homologs (C to C ) whose
1
18
nematogenic and smectogenic mesomorphism commences enan-
tiotropically from C and C members of the series, respectively. The
6
12
C –C homologs are smectogenic and C –C are nematogenic, of
1
2
18
6
18
which C –C homologs are smectogenic plus nematogenic. The C –C5
1
2
18
1
homologs are nonmesogenic. Transition temperatures and the textures
of the homologs were determined and identified by an optical polar-
izing microscope (POM) equipped with a heating stage. Textures of a
nematic phase are threaded or Schlieren and that of the smectic phase
are of the type A or C. Transition curves Cr-M/I, Sm-N and N-I of a phase
diagram behaved in normal manner except N-I transition temperature
of C10 homolog which deviated by 9°C–10°C from normal behavior.
N-I transition curve exhibited odd-even effect. Analytical, spectral, and
thermal data confirms the molecular structures of homologs. Thermal
stability for smectic and nematic are 115.5°C and 138.5°C, respectively
whose corresponding mesophaselengths are varied from 10.0°C to
1
6.0°C and 13.0°C to 24.0°C, respectively. Group efficiency order for
smectic and nematic are derived from comparative study of structurally
similar analogous series; as smectic: –OC H (n) > –CH > –H; Nematic:
–
4
9
3
H > –OC H (n) > –CH
4 9 3
1
. Introduction
Our continuous efforts on the study of thermotropic liquid crystals[1] is aimed at understand-
ing and establishing the effects of molecular structure[2–5] on liquid crystal (LC) properties
with reference to terminal and/or lateral group or groups; and on the number of phenyl rings,
different functional groups and bridging groups. The LC materials are useful in the benefit
of mankind for LC devices workable at low or desired temperature, light emitting diodes and
devices of semiconductor materials, optical imaging, etc.[6–10]. Azoester LC dyes find equal
importance as useful as other dyeing LC material. Proposed investigation involve synthesis of
an azoester homologous series consisting of three phenyl rings bonded through two central
CONTACT R. B. Patel
lege, Gujarat University, Ahmedabad , Gujarat, India.
 Taylor & Francis Group, LLC
roshanpatel@gmail.com
Chemistry Department, K. K. Shah Jarodwala Maninagar Science Col-
©

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
11
bridges viz. –COO– and –N=N– and two terminal end groups –OR and –OC H (n) where
4
9
R is varied from C to C in P’ and P” positions. The thermometric data will be evaluated
1
18
using an optical polarizing microscopy (POM) equipped with a heating stage and then will
be interpreted in terms of molecular rigidity and flexibility[11–14]. The group efficiency
order for nematic and smectic will be derived from comparative study of structurally similar
analogous series. Number of ester homologous series are reported till the date[15–21].
2
. Experimental
2.1. Synthesis
4
4
-Hydroxy benzoic acid was alkylated using suitable alkylating agent (R-X) to convert it into
-n-alkoxy benzoic acids(A) by modified method of Dave and Vora[22], Alkylation of Parac-
etamol using alkylating agent n-C H Br is carried out to form 4-n-butyloxy acetanilide, which
4
9
on hydrolysis converted to 4-n-butyloxy aniline is formed by usual establish method. Azo dye
B) 4-hydroxy naphthyl azo 4”- butyloxy benzene (m.p. 132°C, yield 71%) was prepared by
(
well-known azotization method[23]. Final azoester products were synthesized by condensa-
tion of (A) and (B)[24]. Thus, the azoester homolog derivatives were filtered, washed with
sodium bicarbonate solution followed by distilled water, dried and purified till constant tran-
sition temperatures obtained using an optical polarizing microscope equipped with a heating
stage. 4-Hydroxy benzoic acid, Alkyl halides, Paracetamol, 1-Napthol dicyclohexyl carbod-
imide, Dimethyl amino pyridine, K CO , NaOH, NaHCO , NaNO , KOH, DCM, MeOH,
2
3
3
2
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 inScheme 1.
2
.2. Characterization
Representative homologs of the series were characterized by elemental analysis, Infrared spec-
1
1
troscopy, H NMR spectra. IR spectra were recorded on Perkin-Elmer spectrum GX, H NMR
spectra were recorded on Bruker using CDCl as solvent. Microanalysis was performed on
3
Perkin-Elmer PE 2400 CHN analyzer (Table 1). Transition temperatures and LC properties
(
textures) were determined using an optical polarizing microscopy equipped with heating
stage (POM). Texture images of nematic, smectic of selected homologs phase were determined
by miscibility method (Table 2).
2
2
.3. Analytical data
−1
.4. IR Spectra in cm for tetradecyloxy & dodecyloxy derivatives
Tetradecyloxy: 653 Polymethylene (–CH -)n of –OC H , 813 (–C–H- def. di-
2
14 29
substituted-Para), 769 Polymethylene (–CH -) of –OC H , 1139 and 1062 (–C–O–)
2
14 29
Str 1163,1247(–C–O str in -(CH )n chain, 1377 and 1467 (–C–H- def. in CH ), 1502
2
2
(
–C=C–)str, 1598, 1604(N=N group), 1645 (C=O of ester group), and 1735 (–COO– ester
group), 2848 and 2918 (–C–H str in CH ).
3
Dodecyloxy: 640 Polymethylene (–CH -)n of –OC H , 837 (–C–H- def. di-substituted-
2
12 25
Para), 761 Polymethylene (–CH -) of –OC H , 1085 and 1031 (–C–O–) Str, 1165,1255 and
2
4
9
1
307 (–C–O str in -(CH )n chain, 1465 (–C–H- def. in CH ), 1566 (–C=C–)str, 1602 (N=N
2
2
group), 1625 (C=O of ester), and 1737 (–COO– ester group), 2848 and 2922 (–C–H str in
CH ).
3

1
2
U. H. JADEJA AND R. B. PATEL
Scheme . Synthetic route of series .
1
2
.5. HNMR spectra in CDCl in δ ppm for hexyloxy & hexadecyloxy derivative
3
Heptyloxy: 0.88 (t,–CH of –C H ), 1.18–1.37(m, n-poly methylene groups of–OC H ),
3
7
15
7
15
1
(
.48 (, n-poly methylene groups of –OC H ),3.8(s,–OCH –CH -of –OC H ), 3.99
4 9 2 2 7 15
s,–OCH –CH -of –OC H ), 6.9–7.1(s,naphthalene ring), 7.7–7.99 (s, p-disubstituted phenyl
2 2 4 9
ring).
Hexadecyloxy: 0.82 (t,–CH3 of –C H ), 1.1–1.3(m, n-poly methylene groups
16
33
of–OC H ), 1.51 (m, n-poly methylene groups of–OC H ), 3.3–3.6(s,–OCH –CH -
of OC H ), 4.0(s,–OCH –CH -of –OC H ), 7.0–7.2(naphthalene ring), 7.9–8.1 (s, p-
16
33
4
9
2
2
16
33
2
2
4
9
disubstituted phenyl ring).
4
-[4’-n-Alkoxy benzoyloxy]-naphthylazo-4”-butyloxy benzenes

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
13
Table . Elemental analysis for hexyloxy, octyloxy, dodecyloxy, hexadecyloxy derivatives.
%
Elements found
%Elements calculated
H
Sr. No.
Molecular formula
C H O N
C
H
N
C
N

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

   



C HON

C HON

C HON
.
.


Table . Texture of nematic phase of C , C , C , C by miscibility method.


 
Texture
Sr. No.
Homolog
Smectic
Nematic

C
C
C
C
Sm-C
Sm-A
Sm-A
Sm-C
Threaded
Threaded
Threaded
Schlieren



3. Results and discussion
The novel homologous series is derived from condensation of dimeric 4-n-alkoxy benzoic
acids and 4-hydroxy napthyl azo 4’-n-butyloxy benzene (m.p. 132°C, yield 71%) azo dye. The
transition temperatures of novel mesomorphic azoester homologs are relatively lower than
corresponding n-alkoxy acids. C –C homologs are nonmesomorphic. C –C homologs are
1
5
6
18
enantiotropically mesomorphic. C –C homologs are only nematogenic, whereas C –C
6
10
12
18
homologs are smectogenic in addition to nematogenic. Transition temperatures of homologs
Table 2) were determined by POM. Transition temperatures of homologs were plotted versus
(
the number of carbon atoms present in n-alkyl chain ‘R’ of –OR terminal group. Transition
curves, Cr-M/I, Sm-N, N-I were obtained by linking like or related points as depicted in a
phase diagram (Fig. 1) showing their phase behaviors for present series. Cr-M/I transition
curve adopts a zigzag path of rising and falling tendency with overall descending tendency
and behaves in normal expected manner. The Sm-N transition curve continuously descends
as the series is ascended without exhibition of odd-even effect, and behaves in normal manner.
The N-I transition curve initially rises and descends abnormally at C homolog and then rises
10
Table . Transition temperatures in°C for series .
Transition Temperatures in(˚C)
Compound No
n-alkyl chain C H
Smectic
Nematic
Isotropic
n
n+

C
C
C
C
C
C
C
C
C
C
C
C
C
–
–
–
–
–
–
–
–
–
.
.
.
.
–
–
–
–
.
.
.
.
.
.
.
.
.
.
.
.
.












–
.
.
.
.
.
.
.
.





14
U. H. JADEJA AND R. B. PATEL
Figure . Phase behaviors of series.
through C and C and descends in normal manner with exhibition of odd-even effect from
12
14
C to C homolog. N-I transition curves for odd and even members from C to C homologs
6
9
6
9
merged into each other at C homolog and then it disappears from and beyond C for higher
9
9
homologs of longer n-alkyl chain ‘R’ of –OR group. N-I transition curve is extrapolated to
the left of the curves for[25–28] odd and even members to predict and determine the latent
transition temperature (LTT) for nematic for C and C nonmesomorphic homologs; as 122°C
4
5
and 124°C, respectively. Thermal stabilities for smectic and nematic are 115.5°C and 138.5°C,
respectively whose total mesophase length are ranging from 13.0°C to 36.0°C. Textures of a
nematic phase are threaded or Schlieren and that of the smectic are A or C. Analytical and
spectral data supported molecular structures of homologs. The mesomorphic properties of
homologs varied from homolog to homolog in present novel series.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
15
The lowering of transition temperatures of novel homologs as compared to the correspond-
ing dimeric n-alkoxy benzoic acids is attributed to the breaking of hydrogen bonding by ester-
ification process. Alternations of transition temperature and exhibition of odd-even effect by
N-I transition curve observed in a phase diagram is due to the presence of odd and even
numbered carbon atom in n-alkyl chain ‘R’ of left –OR group. The exhibition of mesogenic
characteristic commencing from C homolog is attributed to the disalignment of molecules at
6
an angle less than ninety degree and/or perpendicular to the plane of floating surface depend-
ing upon thermal resistivity against exposed thermal vibrations as a consequence of favorable
magnitudes of molecular rigidity and flexibility, induced by suitable magnitudes of disper-
sion forces and dipole-dipole interactions. The disalignment of molecules perpendicular to
the plane of floating surface under the influence of exposed thermal vibrations organized
molecules in lamellar fashion in their crystal lattices and occupy layered molecular arrange-
ment which under the influence of exposed heat to float with two dimensional ordered sliding
layered molecular arrangement within definite temperature range to show exhibition of smec-
togenic character, from C to C homolog. However the group of homologs which exhibited
12
18
smectogenic character have residual end to end intermolecular attractions of suitable mag-
nitudes with their disalignment less than ninety degree which facilitate the floating of the
molecules of same C –C homologs with statistically parallel orientational order for another
12
18
higher range of temperature to show nematogenic mesophase formation of higher entropy
ꢀS). Then the same molecules on continued heating from and beyond isotropic tempera-
(
ture randomly orient in all possible directions with high order of disorder or high entropy
in reversible manner. The molecules of C –C homologs having their only disalignment less
6
10
than ninety degree with floating surface, float With only statically parallel orientational order
of suitable magnitudes of end-to-end attractions facilitating formation of nematic phase only
for definite range of temperature excluding formation of smectic phase in reversible manner.
The nonmesomorphicity of C –C homolog is due to the low magnitudes of dispersion forces
1
5
and low magnitudes of dipole-dipole interactions leading to high crystallizing tendency. The
diminishing tendency of odd-even effect from and beyond C homologs of longer in alkyl
9
chain ‘R’ of higher homologues may coil or bend or flex or couple to lye with major axes of core
structures of a molecule. Therefore, the uncertainty in the status of n-alkyl chain may modify
permanent dipole moment across the long molecular axes, end-to-end dispersion forces or
molecular rigidity and/or flexibility and can induce abnormality in the behaviors of transi-
tion curve to more or less extent depending upon the magnitudes and type of modifications
in the status of longer n-alkyl chain R beyond merging of curves for odd-even members of
mesogenic homologs, as observed for C homolog of N-I transition curve of present investi-
10
gation. The magnitudes of intermolecular cohesion and closeness vary in present series which
depends upon difference of molecular polarity and polarizability for –OR and –OC H (n)
4
9
end groups keeping rest of the molecular part other than R remain unaltered from homolog
to homolog in the same series. Thus, changing molecular length, permanent dipole moment
across long molecular axis, polarity and polarizability of terminal or lateral groups, molec-
ular rigidity and/or flexibility, suitable magnitudes of end-to-end or/and or lateral cohesion
etc., which vary from homolog to homolog in the same series altering magnitudes of meso-
morphism or the degree of mesomorphism. The extrapolated N-I transition curve for C and
4
C members are not realizable due to their high crystallizing tendency. The changing trend
5
in mesomorphic behaviors of present novel series 1 are compared with structurally similar
analogous series X[29] and Y[30] selected for the study; as shown below in the Fig. 2.
Above homologous series 1, X, and Y under comparative study are identical with respect
to two phenyl rings and one (middle) naphthyl ring as well as two central bridges –COO– and

1
6
U. H. JADEJA AND R. B. PATEL
Figure . Structurally similar analogous series.
–
N=N– which contributes to the total molecular rigidity. Left n-alkoxy groups are identically
same for all the three series for the same homolog from series to series but differ from homolog
to homolog in the same series which contributes partly to the total molecular flexibility. The
tailed groups –O–CH –CH –CH –CH , –CH and –H differs from series to series for the
2
2
2
3
3
same homolog but remain unaltered from homolog to homolog in the same respective series,
which contributes partly to the total molecular flexibility. Thus, changing trends in mesogenic
properties and the degree of mesomorphism will depend upon the magnitudes of changing
features among the series 1, X, and Y. Thus, the degree of mesomorphism will depend upon
the magnitudes of changing features among the series 1, X, and Y under comparative study as a
consequence of effective molecular rigidity, flexibility, and thermal resistivity against exposed
thermal vibrations. Following Table 3 represents some thermometric data of series-1, X, and
Y in comparative manner as mentioned below.
Table 3 involving comparative data indicates that,
•
•
•
A homologous series 1 of present investigation is smectogenic plus nematogenic whereas
homologous series X and Y chosen for comparative study are nematogenic only with
absence of smectic property.
Commencement of mesophase takes place from C homolog of a present series 1,
6
whereas it commences earliest from very first member C of the series X and Y under
1
comparison.
Thermal stability for smectic of present novel series 1 is 115.5, but it does not stabilize
till the last homolog of the series X and Y.
Table . Relative thermal stabilities in°C.
Series →
Series-(–OC H )
Series-X(–CH)
—
Series-Y(–H)
—


Sm-N Or Sm-I
Commencement of Smectic phase
.
(C –C )


C
N-I
.
(C –C )
.
.
Commencement of nematic phase
(C –C )
(C –C )






C
C
C
Total mesophase length in°C
.–.
C C
.–.
C C
.–.
C C

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
17
•
•
Thermal stability for nematic and the total lower mesophase length is in increasing order
from series X to series 1 to series Y.
Upper total mesophase lengths are in increasing order from series 1 to series X to series
Y under comparison.
The lack of smectogenic property by homologous series X and Y indicates absence of lamel-
lar packing of molecules. This effect may be linked to decreasing order of polarity and polar-
izability of tail-ended terminal groups –OCH –CH –CH –CH , –CH , –H, respectively, for
2
2
2
3
3
series 1, X and Y. Late commencement of mesophase formation of series-1 from C homolog
6
as compared to series X and Y in which mesophase commences from very first member (C1)
of the series is attributed to the extent of molecular noncoplanarity caused by lateral or termi-
nal or central groups and their magnitudes of polarity as well as polarizability. The tail groups
of series 1 is –CH –CH2–CH –CH which is longer by three (Series X) and four (Series Y)
2
2
3
–
CH - groups in X and Y homologous series respectively under comparison; which causes
2
difference of the extent of molecular noncoplanarity and magnitudes of molecular linearity
or nonlinearity, resulting into differing magnitudes of intermolecular cohesion and closeness.
Now regarding thermal stability which is related with the energy stored in a molecule as heat
of formation (ꢀH) due to its mass and characteristics, which is higher for present novel series
as compared to series X and Y, whose thermal resistivity toward exposed thermal vibrations
relates with upper mesophase length, as consequence of differing molecular rigidity and flex-
ibility of individual series under comparison. Thus, an azoester series containing napthyl unit
under comparison is generally or predominantly nematogenic, but introduction ofn-butyloxy
group as tail-end group partly induces smectogenic property.
4
. Conclusions
•
•
Novel azoester homologous series of thermotropic liquid crystal is predominantly
nematogenic and partly smectogenic of considerable phase length and it is of middle
ordered melting type.
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 lengths are as
under.
(
i) Smectic
–
O–CH –CH –CH –CH > –CH = –H
2
2
2
3
3
Nematic
–
H > –O–CH –CH –CH –CH > –CH
2 2 2 3
3
(
ii) Smectic
–
O–CH –CH –CH –CH > –CH = –H
2
2
2
3
3
Nematic
–
CH = –H > –O–CH –CH –CH –CH
3
2
2
2
3
(iii) Total mesophaselengths
Lower: –H > –O–CH –CH –CH –CH > –CH
3
2
2
2
3
Upper: –H > –CH > –O–CH –CH –CH –CH
3
2
2
2
3
•
A homologous series containing napthyl unit are generally nematogenic, but introduc-
tion of n-alkyl terminal tail group may induce smectic property.
Mesomorphism is very sensitive and susceptible to the molecular structure.
Combined effects of effective molecular rigidity and flexibility of suitable magnitudes
can induce mesomorphism.
•
•

1
8
U. H. JADEJA AND R. B. PATEL
•
•
Present investigation may be useful for the manufacture of LC devices, the study of light
polarization and in the ester needed vegetable plants and trees.
Present investigation supported and raised the credibility to the conclusions drawn ear-
lier.
Acknowledgments
Authors acknowledge thanks to Dr. R.R.Shah, principal and management of K. K. Shah Jarodwala Mani-
nagar Science College, Ahmedabad. Authors are also thankful to Dr. A.V.Doshi, Ex-principal of M.V.M.
Science and Home Sci. College-Rajkot for his constant support, inspirations and help and valuable com-
ments and suggestions as and when needed during the course of present investigation. Authors also
thank to NFDD Centre for the spectral services.
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