“Synthesis and Study of Azo based Liquid Crystals: Effect of the lateral bromo and terminal alkoxy side chain on Thermal, Mesomorphic and Optical properties”

Article abstract of DOI:10.1080/15421406.2019.1627091

An azoester based calamitic shaped LCs material has been synthesized and studied their optical and thermal properties. The present synthesized derivatives having terminally substituted –OR and –OC₆H₁₃ (n) groups on both the end side including ester and azo linking groups. All the synthesized derivatives displayed enantiotropical nematic and SmC phase with good temperature range which is influenced by the substitution of the alkyl chain on alkoxy group. The observed texture images of present synthesized compounds are fan, needle and droplets type which were measured by POM equipped with heating stage. All this compounds were characterized by FT-IR, ¹H NMR analysis. The mesomorphism is measured by POM, DSC and the molecular packing is further confirmed by XRD technique. The relationship between structure and mesomorphic properties was discussed in framework of geometrical configuration of central linking unit and molecular length with addition of lateral substituted bromo group which affect the interaction of the molecules. Photosensitive azobenzene group undergoes photoisomerisation under UV light.

Full text of DOI:10.1080/15421406.2019.1627091

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: https://www.tandfonline.com/loi/gmcl20
“Synthesis and Study of Azo based Liquid Crystals:
Effect of the lateral bromo and terminal alkoxy
side chain on Thermal, Mesomorphic and Optical
properties”
Upendra.H. Jadeja, Vinay S. Sharma, Varsha Prajapat, Akshara Shah & Anuj
S. Sharma
To cite this article: Upendra.H. Jadeja, Vinay S. Sharma, Varsha Prajapat, Akshara Shah &
Anuj S. Sharma (2019) “Synthesis and Study of Azo based Liquid Crystals: Effect of the lateral
bromo and terminal alkoxy side chain on Thermal, Mesomorphic and Optical properties”, Molecular
Crystals and Liquid Crystals, 680:1, 46-64, DOI: 10.1080/15421406.2019.1627091
To link to this article: https://doi.org/10.1080/15421406.2019.1627091
Published online: 20 Aug 2019.
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MOLECULAR CRYSTALS AND LIQUID CRYSTALS
019, VOL. 680, NO. 1, 46–64
https://doi.org/10.1080/15421406.2019.1627091
2
“
Synthesis and Study of Azo based Liquid Crystals: Effect of
the lateral bromo and terminal alkoxy side chain on
Thermal, Mesomorphic and Optical properties”
a
a
a
b
Upendra H. Jadeja , Vinay S. Sharma , Varsha Prajapat , Akshara Shah , and
Anuj S. Sharma
c
a
b
Chemistry Department, President Science College, Ahmedabad, Gujarat, India; Department of
c
Chemistry, Mumbai University, Santacruz, Mumbai, India; Department of Chemistry, School of Science,
Gujarat University, Ahmedabad, Gujarat, India.
ABSTRACT
KEYWORDS
Azoester; Mesomorphism;
Smectic; Nematic;
Thermotropic
An azoester based calamitic shaped LCs material has been synthesized
and studied their optical and thermal properties. The present synthe-
sized derivatives having terminally substituted –OR and –OC6H13 (n)
groups on both the end side including ester and azo linking groups.
All the synthesized derivatives displayed enantiotropical nematic and
SmC phase with good temperature range which is influenced by the
substitution of the alkyl chain on alkoxy group. The observed texture
images of present synthesized compounds are fan, needle and drop-
lets type which were measured by POM equipped with heating stage.
1
All this compounds were characterized by FT-IR, H NMR analysis. The
mesomorphism is measured by POM, DSC and the molecular packing
is further confirmed by XRD technique. The relationship between
structure and mesomorphic properties was discussed in framework of
geometrical configuration of central linking unit and molecular length
with addition of lateral substituted bromo group which affect the
interaction of the molecules. Photosensitive azobenzene group under-
goes photoisomerisation under UV light
CONTACT
vinaysharma3836@gmail.com
aksh_shah91@yahoo.com
Department of Chemistry, Faculty of Basic
and Applied Science, Madhav University, Aburoad, Sirohi, Rajasthan, India.
ß 2019 Taylor & Francis Group, LLC

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
47
1
. Introduction
An unique physical state of matter termed as liquid crystal (LC) or mesomorphic state
1] which is neither fully crystalline nor fully isotropic liquid are useful in the manufac-
[
ture of electronic display devices, light emitting materials. Azoester dyes are equally use-
ful as normal dyes including changing their colours with changing frequency of exposed
light. LC materials are useful in pharma industry also [2–8]. This phenomenon arises
from the reversible trans–cis isomerization that is induced by light irradiation, resulting
in the molecular orientation of nematic liquid crystal to higher orientational
entropy [9].
The trans form of the rod-like aromatic compounds based on azo group stabilizes the
mesophase structure of the synthesized LC, whereas its bent cis isomer destroys the
phase structure. As a significance importance, the LC to isotropic phase transition tem-
perature of the LC mixture with the cis isomer is lower than as compare to trans one.
Thus, the reversible trans–cis isomerization in such materials induced by light irradi-
ation will decrease clearing point of the LCs [10].
On the other hand, the aromatic esters are known for their thermal stability, easily
synthesis and relative resistance to the hydrolysis; in addition, the conjugation interac-
tions of ester groups in between the two phenyl rings [11–12]. It is well established that
the stability of a mesophase in a liquid crystalline compound depends primarily on the
anisotropy of intermolecular forces. As a result of lateral attractions, rod-shaped mole-
cules tend to adopt a parallel alignment [13]. Thus, the stability of a mesophase should
be greater, the greater the lateral adhesion of linear molecules which in turn would be
augmented by an increase of the polarity and/or polarizability of the mesogenic portion
of the molecule [14]. In this respect, terminal substituents would affect the polarizability
of the aromatic ring to which they are attached. This is attributed to increased intermo-
lecular attractions with increasing polarity and polarizability of the molecule [15–16].
Alternatively, lateral substitution increases the breadth of the laterally neat molecule,
thus reducing the strength of intermolecular lateral attraction [17]. In literature, various
calamitic LCs based on azo group inbuilt with ester and schiff base linking
unit [18–22].
Naoum et al. prepared eight homologues series based on ester and azo linking unit
having methyl and fluoro group on lateral position exhibited smectic C and nematic
mesophase [23]. Thaker et al. reported various liquid crystals based on azo–ester, azo-
vinylester and azo-schiff base derivatives [24–27]. Yoon and his coworkers studied
the optical and electrical switching properties of cholesteric liquid crystals containing
azo linking group [10]. He et al. synthesized the chiral azo-liquid crystalline terpoly-
mer containing the cyano substituents and also studied its optical behaviour by
photo-induced isomerization behaviour [28]. Recently, Kannan et al. synthesized
alkoxy based azo-cinnamate based liquid crystals and studied their photophysical
behaviours. They found the smectic C and nematic type nature in synthesized liquid
crystalline azo-cinnamate based liquid crystalline compounds [29]. Yahya et al. syn-
thesized azo-ester based liquid crystalline acrylate compounds and study the effect of
terminal substituents groups like (-F, -Cl, -Br, -OCH , -OC H and –OC H ) on
3
2
5
3
7
liquid crystalline properties [30].

4
8
U. H. JADEJA ET AL.
In present investigation, we planned to synthesized azo-ester based molecules and
view to understand and establish the effect of changing the molecular structure by
substitution of lateral and terminal groups on liquid crystalline properties respectively.
In present article, we have prepared total five derivatives based on two linking unit
and terminally substituted hexyloxy side chain. Proposed investigation will consist of
synthesis of six azo-ester based two phenyl rings inbuilt with one napthyl ring and
varying n-alkoxy left terminal group and fixed n-OC H13 (para) tailed group with
6
presence of lateral bromo group. The results obtained for present synthesized com-
pounds is also compared with some structurally similar series to getting more infor-
mation regarding the effect of substituents on terminal and lateral side on
LC properties.
2
. Experimental
.1. Materials
For present synthesized compounds required materials: 4-hydroxy benzoic acid and
-hydroxy 3-bromo acetanilide, 1-naphthol was purchased from (Sigma Aldrich).
2
4
Alkyl bromides (R-Br) were purchased from (S.R.L, Mumbai). N, N-dimethyl amino
pyridine (DMAP) and Dicyclohexyl carbodiimide (DCC) was purchased from (Fluka
Chemie, Switzerland). The solvents were dried and purified by standard method prior
to use.
2.2. Measurements
Melting points were taken on Opti-Melt (Automated melting point system). The FT-IR
ꢀ
1
spectra were recorded as KBr pellet on Shimadzu in the range of 3800-600 cm .
Microanalysis was performed on Perkin-Elmer PE 2400 CHN analyser. The texture
images were studied on a trinocular optical polarising microscope (POM) equipped
1
with a heating plate and digital camera. H NMR was recorded on a 400 MHz in
Bruker Advance in the range of 0.5 ppm-16 ppm using CDCl solvent. The phase transi-
3
tion temperatures were measured using Shimadzu DSC-50 at heating and cooling rates
ꢁ
ꢀ1
of 10 C min . Texture image of nematic phase were determined by miscibility
method. Thermodynamic quantities enthalpy (DH) and entropy (DS ¼ DH/T) are quali-
tatively discussed. For the POM measurement, the newly synthesized compound is
sandwiched between glass slide and cover slip and heating and cooling rate is (2˚C/
min) respectively.
2
.3. Reaction scheme
2
.4. Synthesis of compounds in present series
2
.4.1. 4-n-alkoxy benzoic acid (A)
4-n-alkoxy benzoic acid is prepared by using previously established method [31].

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
49
Scheme 1. Chemical and reagents; (i) R-Br, KOH, EtOH, reflux; (ii)C H Br, anhydrous K CO , dry acet-
6
13
2
3
one, reflux 1 hr; (iii) Conc.HCl, H O, hydrolysis, 6 hr, reflux; (iv) NaNO , HCl, 0-5˚C, 1-Naphthol, NaOH;
2
2
(v) DCC, DMAP, DCM, 2h hour stirring at RT, 24 hr.
2.4.2. Preparation of 4-hexyloxy 3-bromo acetanilide from 4-hydroxy 3-bromo acet-
anilide (B)
4
-hexyloxy 3-bromo acetanilide (0.1 mole), anhydrous K CO (0.15 mole) and n-hexyl
2 3
bromide (0.15 mole) mixed in acetone (50 ml), reflux the reaction mixture at 3 to
hours. The whole mass was then added to water and extracted with ether. Finally, off
white colour product of 4-pentoxy 3-bromo acetanilide is obtained [32].
4
2
.4.3. Preparation of 4-hexyloxy 3-bromo aniline from 4-hexyloxy 3-bromo acetanilide (C)
-hexyloxy 3-bromo acetanilide (0.146 mole) taken with water (40 ml) and concentrated
HCl (30 ml) were stirred for 6 to 7 hour at 90˚C to 95˚C and then cooled to room
4

5
0
U. H. JADEJA ET AL.
temperature. The reaction mixture was made alkaline by addition of 50% NaOH at
0˚C. The oily product of 4-hexyloxy 3-bromo aniline is obtained [32].
2
2
.4.4. Diazotization of 4-hexyloxy 3-bromo aniline
-hexyloxy 3-bromo aniline (0.005 mole) was taken in a beaker contain water (10 ml). It
was then cooled to 0˚C to 5˚C with ice in an ice bath. Later on, conc.HCl (0.03 mole,
.4 ml) was added and the reaction mixture was stirred for 1 hour. A solution of
4
3
NaNO (0.005 mole, 0.35 g) in water (5 ml) added drop wise to the solution. The reac-
2
tion mixture was stirred for 1 hour. At this stage, congo red paper turns blue and starch
iodide paper also turns blue. It showed the positive test. The final diazonium salt was
obtained as a clear solution, which was used for subsequent further coupling with 1-
naphthol [32].
2
.4.5. Preparation of 4-hydroxy benzoyloxy naphthyl azo 4’-hexyloxy 3’-bromo ben-
zene (D)
To well-stirred solution of 1-naphthol (0.005 mole, 0.81 g) in NaOH solution was added
to the diazonium chloride solution of 4-hexyloxy 3-bromo aniline for 1 hour at 0-5˚C.
The pH (7.0) was maintained by the addition of NaOH solution (10% w/v). The mix-
ture was stirred for another 3 to 4 hours for complete separation and the orange colour
dye was isolated by vacuum filtration which is further washed by water, dried and puri-
fied via column chromatography performed on silica-gel (60 mesh) by using eluent of
chloroform/methanol (6:4) [32].
2
.4.6. Preparation of 4-n-alkoxy benzoyloxy-naphthylazo-4-hexyloxy 3-bromo ben-
zene (D)
The compound has been prepared by esterification of the appropriate 4-n-alkoxy ben-
zoic acid (1.02 mmol) and azo dye (0.246g, 1.02 mmol), dicyclohexylcarbodiimide
(DCC) (0.457 g, 1.22 mmol) and dimethyl amino pyridine (DMAP) in catalytic amount
(
0.002 g, 0.2 mmol) in dry CH Cl (DCM) (30ml) was stirred at room temperature for
2
2
24 h. The white precipitate of DCU is obtained which was insulated by filtration and
then remove it. The resultant crude was purified by column chromatography on silica
gel eluting with dichloromethane and recrystallization is carried out by using chloro-
form until to getting clearly transition temperatures were observed [32].The synthetic
route to a series is mentioned in above Scheme-1.
2
.5. Selected data of synthesized compounds
21
IR (KBr) in cm
for Hexyloxy (E ), Hexadecyloxy (E ), Heptyloxy (E ),
6 14 7
Derivatives:
Hexyloxy (E ): 806 Poly methylene group of (-CH -)n in –OC H , 885 (–C-H- def.
6
2
6
13
di-substituted on para side), 981 (–C-H- def. hydrocarbon), 1205(-C-O-) Str, 1296,
423, 1493 (-C-O str in –(CH -) alkyl chain group), 1580 (-C-H- def. in -CH unit in
1
2
n
2

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
51
alkyl chain), 1390 (-N ¼ N-) group, 1660 (carbonyl -C ¼ O group), 1750 (-COO- ester
group), 2870 and 3072 and (-C-H str in CH ).
3
Hexadecyloxy (E ): 706 and 806 Polymethylene (-CH -)n of –OC H and
1
4
2
16 33
–
OC H , 883(–C-H- def. di-substituted-Para), 989 (–C-H- def. hydrocarbon), 1008,
6 13
1
056, (-C-O-) Str, of -OC H , 1276 and 1392 and 1373, 1246 (-C-O str in –(CH )n
5 11 2
alkyl chain group, 1490(-C-H- def. in -CH groups), 1402 (-N ¼ N-) group, 1610 (car-
2
bonyl -C ¼ O group), 1740 (-COO- ester group), 2984 and 3071 (-C-H str in CH ).
3
Heptyloxy (E ): 761 and 806 Polymethylene (-CH -)n of –OC H and –OC H , 885
7
2
7
15
6
13
(
(
(
–C-H- def. di-substituted-Para), 988 (–C-H- def. hydrocarbon in alkyl chain), 1061
-C-O- Str.), 1290 (-C ¼ O str in –(CH -)n alkyl chain), 1579 (-C-H- def. in CH ), 1390
2
2
-N ¼ N-)str, 1660 (-C ¼ O group), 1735 (-COO- ester group), 2870 and 3072 and (-C-
H str in CH ).
3
1
HNMR (300 MHz) in CDCl (d ppm):
3
Dodecyloxy (E ): 0.90(t, 6H, of –OC H & -OC H ), 1.26-1.28 (m, 16H, n-poly
1
2
12 25
6
13
methylene groups of –OC H and –OC H ), 1.76 (p, 4H, –OC H & -OC H ),
1
2
25
6
13
12 25
6
13
4
7
6
.06 (t, 4H, -O-CH -CH - of –OC H & -OC H ), 7.19 (d, 2H, third phenyl ring),
.92 (s, 1H, third phenyl ring), 7.08 and 8.24 (d, 4H, p-di substituted first phenyl ring),
.84 (d, 2H of middle phenyl ring), 8.01 (d, 2H, middle phenyl ring), 7.67 (s, 2H, mid-
2 2 12 25 6 13
dle phenyl ring).
Hexyloxy (E ): 0.90 (t, 6H, of –OC H & -OC H ), 1.26-1.28 (m, 8H, n-poly
6
6
13
6
13
methylene groups of –OC H and –OC H ),1.74 (p, 4H, –OC H & -OC H ), 4.08
6
13
6
13
6
13
6
13
(
t, 4H, -O-CH -CH - of –OC H & -OC H ), 7.19 (d, 2H, third phenyl ring), 7.92 (s,
2 2 6 13 6 13
1
2
H, third phenyl ring), 7.08 and 8.24 (d, 4H, p-di substituted first phenyl ring), 6.84 (d,
H of middle phenyl ring), 8.01 (d, 2H, middle phenyl ring), 7.67 (s, 2H, middle phe-
nyl ring).
Butyloxy (E ): 0.90(t, 4H, of –OC H & -OC H ), 1.26-1.28 (m, 16H, –OC H
10
10 21
6
13
10 21
&
-OC H ), 1.76 (p, 4H, –OC H & -OC H ), 1.47 (sext, 4H, -OC H
10 21
&
6
13
10 21
5
11
-
OC H ), 4.06 (t, 4H, -O-CH -CH - of –OC H & -OC H ), 7.19 (d, 2H, third phe-
6 13 2 2 10 21 6 13
nyl ring), 7.94 (s, 1H, third phenyl ring), 7.08 and 8.24 (d, 4H, p-di substituted first
phenyl ring), 6.87 (d, 2H of middle phenyl ring), 8.01 (d, 2H, middle phenyl ring), 7.63
(s, 2H, middle phenyl ring).
Octyloxy (E ): 0.89(t, 6H, -CH -CH ,–OC H & -OC H ) , 1.26-1.28 (m, 12H, n-
8
2
3
8
17
6
13
poly methylene groups of –OC H and –OC H ), 1.74 (p,4H, -CH -CH -of –OC H
17
8
17
6
13
2
2
8
&
-OC H ), 4.06 (t, 4H, of –OC H & -OC H ), 7.19 (d, 2H, third phenyl ring), 7.92
6 13 8 17 6 13
(
(
s, 1H, third phenyl ring), 7.08 and 8.27 (d, 4H, p-di substituted first phenyl ring), 6.81
d, 2H of middle phenyl ring), 8.08 (d, 2H, middle phenyl ring), 7.61 (s, 2H, middle
phenyl ring).
3. Result and Discussion
3.1. Mesomorphic behaviour study
Here in present investigation, we have synthesized calamitic rod shaped biphenyl based
liquid crystalline compounds having two linking group joint via two phenyl rings. We
have introduced total five liquid crystalline derivatives based on azo-ester linking group
and substitution of variable side alkoxy group on left side and fixed hexyloxy group on

5
2
U. H. JADEJA ET AL.
end groups respectively. A newly chalconyl-vinyl ester linking group based homologous
series synthesized by the esterification reaction of 4-n-alkoxy benzoic acid (A) and azo
dye (D). The thermotropic mesophase behaviours of all the synthesized compounds
were primarily investigated by using polarising optical microscope (POM) and further
confirmed by differential scanning calorimetry (DSC) analysis. The presence of lamellar
type of arrangement layer arrangement in SmC phase is confirmed by further high tem-
perature XRD technique at the observed transition temperature which was preliminary
obtain from POM and DSC investigation. We have totally prepared five compounds,
out of five compounds, comp. E E observed SmC and nematic phase while comp. E ,
6
,
8
10
E12 and E14 displayed only smectic C phase with higher mesophase stability. The
observed mesophase in all the compounds are enantiotropically in nature.
3.1.1. POM study
The transition temperature observed by POM of synthesized compounds is listed in
Table 1. Compounds E and E displayed smectic C and nematic phase while com-
6
8
pound E12 and E14 showed only smectic C phase. It can be noted that, the higher side
spacer compounds showed lamellar type arrangement of molecules which on further
heating do not passing on sliding layer and directly converted into isotropic phase.
Mesophase formation was found to be dependent on the number, position and the
molecular length of the side alkyl chain which increases the flexibility of molecules and
also affected by the polar lateral group. As shown in Table 1, lower member compounds
of present series shows higher transition temperatures as compare to higher members.
The presences of bromo group affect the polarity and dipole-moment of the molecules
and also take part in the dipole-dipole interaction to induce mesomorphism.
The phase diagram of present series is shown in Figure 1 which indicates the phase
behaviour of Cr-SmC, SmC-N and SmC/N-I phase transition curve respectively. It can
be seen that, as the alkyl side chain increases on left terminal part from lower to higher
alkyl side spacers which affect the transition temperatures observed in POM respect-
ively. The entire compounds showed decreasing tendency with exhibition of enantio-
tropical mesogenic behaviour. The decreasing tendency of the transition temperature of
the compounds could be due to the lowering the vander waals interactions between aro-
matic phenyl cores and higher degree of flexibility due to presence of long alkyl chain
in alkoxy group at left terminal part. Figure 2 indicates the bargraph of the synthesized
compounds which is observed in POM study.
3
.1.2. Textural study
The crystalline compounds placed on clear glass slide sheltered by coverslip were heated
ꢁ
to the isotropic state and heating and cooling rate is (2 C/min) respectively and observ-
ing mesophase texture image. Microphotographs of the textures observed for some of
the compounds like E , E , E and E18 are mention in Figure 3 respectively. The tex-
6
8
10
ture images of liquid crystalline compounds were observed on cooling condition.
Compound E shows nematic droplets type texture image of nematic phase at 146.0 ˚C
6
on heating and cooling condition. Further, Compound E shows same texture pattern
8

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
53
Figure 1. Phase diagram of present synthesized compounds.
Figure 2. Bar-graph of present synthesized compounds (E -E ).
6
14
of nematic phase at 138.0 ˚C on heating and cooling condition. Compound E10 and E14
displayed typical texture pattern of smectic C phase at 114.0 ˚C and 101.0 ˚C on cool-
ing condition respectively. The texture pattern of other derivatives from the present ser-
ies is look like as threaded and broken fan type which were observed on heating and
cooling condition.
3
.1.3. Comparative study
Figure 4 indicates the molecular structure of present synthesized series-1 which is com-
pared with structurally similar reported series by our research group. The phenyl ring
which is substituted by variable alkoxy side chain at left side and fixed hexyl group with

5
4
U. H. JADEJA ET AL.
Figure 3. Optical photomicrographs on cooling; (a) nematic droplets (comp.E ) at 146 ˚C; (b) nematic
6
droplets (comp.E ) at 138 ˚C; (c) texture pattern of smectic C (comp.E ) phase at 114 ˚C; (d) texture
8
10
pattern of smectic C phase (comp.E ) at 101 ˚C.
14
Figure 4. Molecular structure of similar series; (a) series-1; (b) series-2.
addition of bromo substituent on third phenyl ring is connected through azo and ester
linking units respectively. The presence of polar flexible methylene part in variable
alkoxy chain and also presence of lateral group on the central phenyl core increases the

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
55
Figure 5. Ball stick model of similar series; (a) series-1; (b) series-2.
polarity and flexibility of molecule to induce mesomorphism. Figure 5 indicates the
ball-stick model of present synthesized series-1 and series-2 [33].
The presence of the lateral substituents leads to disruption of molecular packing and,
therefore, reduces liquid crystalline phase stability. This disruption of the molecular
packing is particularly advantageous for the mesomorphic and physical properties
required for specific applications. In general, the depression of the clearing point
expressed as TN-I by a lateral substituent is directly proportional to the substituent size,
irrespective of its polarity [34].
3
.2. DSC study
The phase transition temperatures and enthalpies were measured by carrying out DSC
thermal analysis. All compounds are thermally stable as confirmed by the reproducibil-
ity of thermograms on first heating and cooling cycles. The DSC thermograms of
comp.E , E , E , E , E and E18 are shown in Figure 6. Compounds E -E traced three
7
6
10 12 16
6
8
endothermic peaks on first heating and cooling condition which is corresponding to
solid- SmC, SmC-N and N-I phase while compound E -E showed two endothermic
10
14
peaks which is belongs to solid-SmC and SmC-I phase transition respectively. Comp.E6
exhibited three endothermic peaks at 126.1 ˚C, 141.2 ˚C and 164.6 ˚C on heating cycle
and on applying cooling it exhibited at 119.8 ˚C, 139.7 ˚C and 163.1 ˚C which is corre-
sponding to the presence of Cr-SmC, SmC-N and N-I phase. Similarly, comp. E shows
8
ꢁ
three endothermic peaks at 121.6 C, 128.7 ˚C and 161.9 ˚C on heating and 119.8 ˚C,
1
1
26.5˚C and 162.8 ˚C on cooling cycle. Comp. E12 displayed two endothermic peaks at
06.4 ˚C and 138.5 ˚C on heating and reappeared at 94.8 ˚C and 136.1 ˚C on cooling
condition which is revealed with the result obtain by POM analysis. In addition,
Comp.E14 exhibited two endothermic peaks at 97.8 ˚C and 137.1 ˚C on heating cycle
while on cooling cycle, these two exothermic peaks traced at 93.2 ˚C and 139.1 ˚C
which confirm the presence solid to smectic C phase type arrangement of molecular
organization. Comp.E10 showed two significant endothermic peaks at 112.4 ˚C and
1
1
56.3 ˚C on heating condition while on cooling condition, it observed at 110.8 ˚C and
53.4 ˚C which is further confirm by using POM analysis. It can be noted that, higher

5
6
U. H. JADEJA ET AL.
Figure 6. DSC thermograms of compound comp.E12 (a); comp.E14 (b); comp.E (c) comp.E (d) on
6
8
second heating and cooling cycles.
alkyl chain substituted compounds showed lamellar type molecular organization in layer
type structure pattern of smectic C phase without showing nematic type texture pattern.
The phase transition temperatures, enthalpy and entropy values for the present synthe-
sized series are summarized in Table 2.
3
.3. XRD analysis
To investigate the structure of the observed smectic and nematic phase in present syn-
thesized compounds, we have performed high temperature XRD to correlate with the
results obtained from DSC and POM analysis. The XRD data of comp. E , E , E and
6
8
12
E14 are shown in Figure 7. According to the graphical proposed molecular arrangement
in smectic C as well nematic phase, the molecular are arranged in lamellar order,
although, the long axes of the molecules are tilted to the layers planes shown in Figure
8. Figure 7 indicates the X-ray pattern shows the diffraction peaks in the small angle
diffraction region corresponded to the smectic arrangement. The reflections for the
small-angle area correspond to a smectic layer structure. The d-spacing of the reflec-
tions observed for E at 2h ¼ 9.12, 14.21, 24.61 are 9.71 Å, 6.27 Å, 3.69 Å. Compound
8
E
at 2h ¼ 7.61, 11.98, 24.12 showed d-spacing at 6.23 Å and 5.12 Å. Similarly, com-
1
0
pound E showed two reflections at 9.8 and 14.1 having corresponding d-spacing value
1
2

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
57
Figure 7. XRD traces of compound E (a); E10 (b); E12 (c); E14 (d) measured at transition temperature.
8
Figure 8. Representation of molecular packing of smectic C and nematic phase in present series.

5
8
U. H. JADEJA ET AL.
Figure 9. XRD traces of compound E (a) at 126 ˚C; compound E (b) at 141 ˚C.
6
6
Figure 10. Absorption spectrum of compound E (a); E10 (b); E12 (c); E14 (d).
8
9
.04 Å and 6.32 Å. Compound E14 showed two reflection peaks at 10.3 and 15.8 which
having d-spacing value 8.61 Å and 5.65 Å respectively. The presence of alkyl chain in
alkoxy tail group in compounds E12 and E14 may increase the arrangement of molecules
in ordering layered and formation of molecules in smectic mesophase. Figure 9 showed
the XRD diffractogram of compound E at two different temperatures, the results indi-
6
cates the arrangement of molecules may be just a smectic organization which is further
revealed with POM and DSC results.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
59
3.4. Optical properties
Figure- shows the UV–vis absorption spectra of compounds E , E , E and E14 are
8
10
12
recorded in dichloromethane solution. The azo linkage containing compounds was irra-
ꢂ
ꢂ
diated with UV light, the band corresponding to P-P and n-P transitions which is
decreases with increasing irradiation time [29]. From figure 10, the band corresponding
to isomerized product increases by increasing the irradiation time, which confirmed the
photoisomerisation at various time intervals has occupied in the range of 381-550 nm. It
can be observed that, all the spectra of four corresponding compounds are characterised
by the two absorption peaks with the maxima at 291 and 383 nm. The low intense short
wavelength absorption maxima appeared at nearly 291 and 288 nm, due to the presence
ꢂ
of P-P transition phenyl rings respectively. However, the high intense longer wave-
length absorption maxima observed at 381 and 384 nm, this is due to the presence of a
ꢂ
P-P transition involving the p-electronic system throughout the whole mesogenic por-
tion, with a considerable charge transfer character [29]. Compound E irradiated at
8
3
3
83 nm, E-Z isomerization taking place which is decrease in the absorption band from
66 nm is ascribed to P-P transition of the trans (E) isomer observed in azo group
ꢂ
respectively. The further very weak absorption band found in the visible region around
at 481 nm which is represents to n-P transition of cis (Z) isomer in the azo group was
gradually increased by reaching photo stationary state at 80 sec. After that, no change
observed in the absorption spectrum which is further confirms the photo-saturation of
ꢂ
E-Z (trans-cis) isomerization process. Similarly, Compound E irradiated at 383 nm, E-Z
8
ꢂ
isomerization taking place which is further decrease at 366 nm is ascribed to P-P
transition of the trans (E) isomer and very weak absorption band found in the visible
ꢂ
region around at 478 nm which is represents to n-P transition of cis (Z) isomer.
Compound E12 and E14 irradiated at 383 nm and 387 nm which is decreased due to for-
mation of E-Z isomerization at 361 nm and 368 nm due to P-P transition of the trans
isomer (E) and also exhibiting the weak absorption peak at 481 nm and 482 nm, this is
ꢂ
ꢂ
due to the formation of n-P transition of cis (Z) isomer respectively. These obtained
results may be ascribed to the fact that the presence of Br atom on lateral position on
the third phenyl rings tends to provide extra electron density on the ester linkage via
the resonance effect of the phenyl rings which could reduce the HOMO-LUMO energy
gaps of the molecules [35]. Figure 11 shows the trans and cis isomer of azo-ester based
present synthesized derivatives which clearly indicated that the photoisomerisation is
taking place.
3
.5. Frontier molecular orbital (FMO) distribution
The calculated frontier molecular orbital (FMO) distributions of comp.D, comp.C (ser-
6
ies-1) and comp.E are depicted in Figure 12 and Figure 13. For clear understanding,
6
we have compare compound C and E with respect to compound D. The band gap of
6
6
azo based compound D is 3.002 eV which is higher as compare to azo-ester based com-
pound C and E respectively. From Figure 12, A close examination of these orbitals
6
6
reveal that the HOMO is predominantly localized on the first phenyl ring with addition
to phenyl ring inbuilt with azo linking unit while the LUMO is mostly concentrated on
azo group based third phenyl ring; both the terminal side group does not affect the

6
0
U. H. JADEJA ET AL.
Figure 11. Schematic representation of the trans-cis transition of the azo group.
Figure 12. FMO distribution of comp.D (azo-dye).
significant influence on the LUMO orbitals. The band gap of the compound C (series-
6
1
) is found to be 2.521 eV which is higher as compare to compound E . In Figure 13,
6
the HOMO and LUMO is mostly localized on the first phenyl ring joint with ester and
azo group. In can be clearly noted that the distribution of HOMO and LUMO are con-
trasted as increasing in alkoxy side chain at left tail part. The presence of bromine
group on third phenyl ring provides more electron density and lateral charge transfer
electron and also reduce the resonance effect cause by phenyl core, as a results the
HOMO-LUMO energy gap or band gap (Eg) is reduced in addition of ester linking

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
61
Figure 13. FMO distribution of comp.C (HOMO, series-1) (a); comp.C (LUMO, series-1) (b); comp.E
6
6
6
(
HOMO) (c); comp.E (LUMO).
6
group. Thus, it can be concluded that the electronic properties of present synthesized
compounds could have originated from the phenylic-azo group rather than other link-
ing unit.
3.6. Thermal stability and temperature range of mesophase
Thermal stability of present novel synthesised series is listed in Table 3. From that, it
clearly understands that present synthesized series-1 display higher thermal stability of
crystal to smectic C and smectic C to nematic phase and nematic to isotropic with
good mesophase temperature range respectively. The geometrical structures of both the
series are same expect terminal group and also differ from lateral group. Series-1 con-
tain –OC H group on right terminal end while bromo group on third phenyl ring
6
13
which effect on the polarity and linearity of the molecules causes higher mesophase
thermal stability as compare to series-1 which having no any lateral group interactions.
In series-2, mesophase commences from C homologue and continue to display LCs
4
properties till last C18 derivatives with lower thermal stability. The temperature range of
series-1 is higher as compare to series-2.
Figure 14 represents the proposed mechanism to indicate the arrangement of lamellar
packing in molecules to shows SmC and on further heating transform into nematic
phase. In addition, we have compare the structural arrangement of lower side alkyl
chain substituted compounds which showed nematic mesophase and higher alkyl chain
substituted compounds that exhibited SmC phase. All the compounds showed meso-
phase on heating and cooling condition with addition of good photophysical behaviour
as well.

6
2
U. H. JADEJA ET AL.
Figure 14. Propose mechanism of present series-1.
ꢁ
Table 1. Transition Temperature in C by POM
Transition Temperatures in(˚C)
Comp.
n-alkyl chain C
n
H
2nþ1
SmC
N
I
1
2
3
4
5
E
E
E
E
E
6
128.0
119.0
114.0
108.0
101.0
146.0
138.0
–
–
–
173.0
162.0
158.0
142.0
138.0
8
10
12
14
(
SmC ¼ Smectic C, N ¼ Nematic; I ¼ Isotropic)
ꢁ
ꢀ1
ꢀ1 ꢀ1
Table 2. Transition temperature ( C) and enthalpy (J g ) and entropy change (J g
k
) by DSC
measurement.
ꢁ
ꢁ
Comp.
Transition
Heating scan( C)
Cooling scan( C)
DH (Jg-1)
DH (Jg-1)
DS (J g-1k-1)
DS (J g-1k-1)
E
6
8
Cr-SmC
SmC-N
N-I
Cr-SmC
SmC-N
N-I
Cr-SmC
SmC-I
Cr-SmC
SmC-I
Cr-SmC
SmC-I
126.1
141.2
164.6
121.6
128.7
161.9
112.4
156.3
106.4
138.5
97.8
119.8
139.7
163.1
119.8
126.5
162.8
110.8
153.4
94.8
7.93
11.87
6.74
5.81
10.95
4.34
8.73
12.74
1.32
12.21
15.67
7.42
6.86
12.64
9.43
0.0198
0.0286
0.0154
0.0147
0.0272
0.0099
0.0226
0.0296
0.0034
0.0296
0.0422
0.0180
0.0036
0.0429
0.0216
0.0227
0.0416
0.0264
0.0151
0.0339
0.0231
0.0456
0.0531
0.0223
E
8.92
16.63
11.54
5.82
14.46
8.47
18.71
19.43
9.23
E
10
E
12
E
14
136.1
93.2
139.1
137.9
(
Cr-SmC ¼ solid to SmC; SmC-I ¼ SmC to isotropic; Cr-N ¼ solid to nematic; N-I ¼ nematic to isotropic)
4. Conclusion
A new calamitic rod-shaped azo-ester derived LCs compounds based on two phenyl
and one naphthyl rings with variable side chain on left end and fix –OC H group on
6
13

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
63
ꢁ
Table 3. Average thermal stability in C
Series
Series-1
Series-2
SmC-I
Cr-SmC
N-I
146.9
114.4
154.6
147.6
142.5
99.77
119.21
141.4
SmC-N
(
SmC-I ¼ smectic C to isotropic; Cr-N ¼ solid to nematic;
Cr-SmC ¼ solid to smectic C; N-I ¼ nematic to isotropic)
right terminal end is synthesized and well characterized. The liquid crystalline proper-
ties of the synthesized mesogens are carried out by POM, DSC and high temperature
XRD techniques. The lower member of the series showed smectic C with addition of
nematic phase while higher members of the series displayed only smectic C mesophase.
The observed mesophase in present series are enantiotropically in nature. The thermal
stability and mesophase temperature range is influenced by the lateral substituted
bromo group on third phenyl ring. The group efficiency order derived on the basis of
(a) mesophase thermal stability, (b) early or late commencement of mesophase, (c)
Temperature range of mesophase. The HOMO-LUMO calculations indicated the inter-
molecular charge transfer interactions and revealed the electronic properties of present
synthesized newly compounds were originated from first and second phenyl ring inbuilt
with ester and azo group and also studied the effect of lateral bromo on electron charge
transfer and reactivity. The duration required for exhibiting photoisomerisation phe-
nomena mainly related to the chemical structure of the target compounds.
Acknowledgement
UJ and VS acknowledge thanks to President Science College, Gujarat University, and Ahmedabad for
providing research lab and other facility. One of the authors (A. Shah) is thankful to the University
Grants Commission, New Delhi, India for providing financial support under the MANF (Maulana
Azad National Fellowship). VS also acknowledge thanks to Department of Chemistry, Faculty of Basic
and Applied Science, Sirohi, Rajasthan for providing lab facility. Authors are also thankful to NFDD
Centre Rajkot and MNIT Jaipur for providing analytical and spectral services.
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