Synthesis and characterization of biphenyl-based azo liquid crystals and its optical properties: effect of lateral and tail group

Article abstract of DOI:10.1080/15421406.2021.1933685

An azo ester-based biphenyl substituted rod-shaped azo-based material has been synthesized and well-characterized and further studied their mesogenic, optical, and thermal properties. All the synthesized derivatives displayed enantiotropical nematic and SmC phases with good temperature range of mesophase which is further influence by the variation of alkyl chain. The present synthesized derivatives having left terminally substituted phenyl ring by -OR inbuilt with ester azo group and right terminally dodecyloxy tail (-OC₁₂H₂₅) at the right terminal end. The mesomorphism is measured by using POM, DSC, and high-temperature XRD technique, and the photophysical behavior was measured by UV-Vis spectroscopy.

Full text of DOI:10.1080/15421406.2021.1933685

Molecular Crystals and Liquid Crystals
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/gmcl20
Synthesis and characterization of biphenyl-based
azo liquid crystals and its optical properties: effect
of lateral and tail group
Akshay Thakor, Durgesh J. Dwivedi, Vipul Desai, Upendra H. Jadeja, Vinay S.
Sharma & R. B. Patel
To cite this article: Akshay Thakor, Durgesh J. Dwivedi, Vipul Desai, Upendra H. Jadeja, Vinay S.
Sharma & R. B. Patel (2021): Synthesis and characterization of biphenyl-based azo liquid crystals
and its optical properties: effect of lateral and tail group, Molecular Crystals and Liquid Crystals,
DOI: 10.1080/15421406.2021.1933685
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MOLECULAR CRYSTALS AND LIQUID CRYSTALS
https://doi.org/10.1080/15421406.2021.1933685
Synthesis and characterization of biphenyl-based azo liquid
crystals and its optical properties: effect of lateral and
tail group
a
a
a
b
Akshay Thakor , Durgesh J. Dwivedi , Vipul Desai , Upendra H. Jadeja ,
c
a
Vinay S. Sharma , and R. B. Patel
a
Department of Chemistry, K.K. Shah Jarodwala Maninagar Science College, Gujarat University,
b
Ahmedabad, Gujarat, India; Department of Chemistry, President Science College, Gujarat University,
Ahmedabad, Gujarat, India; Department of Chemistry, Faculty of Basic and Applied Science, Madhav
c
University, Sirohi, Rajasthan, India
ABSTRACT
KEYWORDS
Azo; liquid crystal; nematic;
smectic C; thermotropic
An azo ester-based biphenyl substituted rod-shaped azo-based
material has been synthesized and well-characterized and further
studied their mesogenic, optical, and thermal properties. All the syn-
thesized derivatives displayed enantiotropical nematic and SmC
phases with good temperature range of mesophase which is further
influence by the variation of alkyl chain. The present synthesized
derivatives having left terminally substituted phenyl ring by -OR
inbuilt with ester azo group and right terminally dodecyloxy tail
(
-OC H ) at the right terminal end. The mesomorphism is measured
12 25
by using POM, DSC, and high-temperature XRD technique, and the
photophysical behavior was measured by UV-Vis spectroscopy.
GRAPHICAL ABSTRACT
1. Introduction
A unique physical state of matter termed as liquid crystal (LC) or mesomorphic or
mesogenic state [1] which is neither fully solid crystalline nor fully isotropic liquid is
useful in the manufacture of electronic display devices, light-emitting materials, and
CONTACT Akshay Thakor
akkythakor2901@gmail.com
Department of Chemistry, K.K. Shah Jarodwala Maninagar
Science College, Gujarat University, Ahmedabad-380009, Gujarat, India.
ß 2021 Taylor & Francis Group, LLC

2
A. THAKOR ET AL.
other electronic applications. Azo dyes are equally useful as normal dyes including chang-
ing their colors with changing frequency of exposed light. LC materials are useful in the
pharma industry also [2–7]. This phenomenon arises from the reversible trans–cis isomer-
ization that is induced by light irradiation, resulting in the molecular orientation of nem-
atic LC to higher orientational entropy [8]. On the other hand, the aromatic esters are
known for their thermal stability, easy synthesis, and relative resistance to the hydrolysis;
in addition, the conjugation interactions of ester groups in between the two phenyl rings
[9–11]. Thus, the stability of a mesophase should be greater, the greater the lateral adhe-
sion 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 [12, 13]. In this respect,
terminal substituents would affect the polarizability of the aromatic ring to which they are
attached. This is attributed to increased intermolecular attractions with increasing polarity
and polarizability of the molecule [14, 15]. Alternatively, lateral substitution increases the
breadth of the laterally neat molecule, thus reducing the strength of intermolecular lateral
attraction [16]. In literature, various calamitic LCs based on azo group inbuilt with ester
and Schiff base linking unit were well reported [17–21].
Naoum et al. prepared eight homologous series based on ester and azo linking unit
having methyl and fluoro group on lateral position exhibited smectic C and nematic mes-
ophase [22]. In continue to this, Thaker and his coworkers reported various LCs based
on azo ester, azo vinyl ester, and azo Schiff base derivatives [23–26]. Yoon and his cow-
orkers studied the optical and electrical switching properties of cholesteric LCs containing
azo linking group [9]. He et al. synthesized the chiral azo liquid crystalline terpolymer
containing the cyano substituents and also studied its optical behavior by photo-induced
isomerization behavior [27]. Recently, Kannan et al. synthesized alkoxy substituted azo
cinnamate-based mesogens and further studied their photophysical behavior. In addition
to this, they found the smectic C and nematic type nature in synthesized liquid crystalline
azo cinnamate-based liquid crystalline compounds [28]. Yahya et al. prepared azo ester-
based liquid crystalline acrylate compounds and study the effect of terminal substituents
groups on liquid crystalline properties [29]. Ahmed et al. reported four-ring linked six
derivatives and reported their liquid crystalline properties based on azo/ester/Schiff base
group and further studied their photophysical and Density Functional Theory (DFT)
study [30]. Titinchi et al. reported azo benzothiazole-based LCs and studied the lateral
substituents on liquid crystalline properties. They studied the effect of various substituents
like (-H, -OH, -CH , -Cl, -F) group at 3-position of the central benzene core ring [31].
3
After that, Naoum et al. reported four ring-based mesogens linked by azo ester azo group
and two side substituted terminal alkoxy side groups with different side lengths and pro-
portions. The investigation suggested the reported compounds exhibited higher meso-
morphic temperature range [32]. In addition to this, Salleh and his groups prepared azo
ester-based mesogens and studied the effect of lateral methyl and terminal substituents on
thermal, mesomorphic, and optical properties [33]. Wang and his group reported two ser-
ies of cholesteric LC polymers CPQ and CPZ series based on chiral azo LC polymethylsi-
loxanes with isosorbide units [34]. Very recently, El-Atawy et al. prepared two
homologous series based on lateral nitro and azomethine linkage group [35].
In the present study, we planned to synthesize two linking group-based materials and
investigated their liquid crystalline properties with understanding the effect of changing

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
3
the molecular structure by substitution at lateral and left/right terminal groups. In add-
ition, we have also studied the optical properties of synthesized azo functionalized mate-
rials. The proposed investigation consists of the synthesis of six azo group-based
mesogens with biphenyl ring and varying n-alkoxy left terminal group and fixed dodecyl
(
-OC H ) tailed group at right end with the presence of nitro group on the third phe-
12 25
nyl ring. The results obtained for the present synthesized compounds are also compared
with some structurally similar series reported by our group to gain more information
regarding the effect of substituents on terminal and lateral side on mesomorphic prop-
erties respectively.
2
. Experimental
.1. Materials
For the present synthesized compounds required materials: 4-hydroxy benzoic acid and
-hydroxy acetanilide, 1-naphthol was purchased from Merck. Alkyl bromides (R-Br)
2
4
were purchased from S.R.L., India. N,N-dimethyl aminopyridine (DMAP) and dicyclo-
hexyl carbodiimide (DCC) were purchased from Finar Chemicals, India. 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 . The
texture images were studied on a trinocular polarizing optical microscope (POM)
1
equipped with a heating plate and digital camera. H NMR was recorded on a 400 MHz
in Bruker Advance in the range of 0.5–16 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 images of nematic phase were determined by the 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 coverslip and heating and cooling rate is (2 C/min)
respectively. The X-ray diffraction (XRD) measurements were performed on a Rigaku-
Ultima IV powder diffractometer equipped with a Cu ka source (k ¼ 1.5418 A˚ and
1.6 kW, X-ray tube with applied voltage and current values as 40 kV and 30 mA power)
and also Philips X’PERT MPD. The absorption spectra was studied by using JASCO V-
70 UV-visible recording spectrophotometer with a variable wavelength between 200
and 800 nm.
5
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 the previously established method [36].

4
A. THAKOR ET AL.
2.4.2. Preparation of 4-hexyloxy 3-nitro acetanilide from 4-hydroxy acetanilide (B)
4
-Hexyloxy acetanilide (5 g) dissolves in 5 ml of glacial acetic acid and cooled it, add
ꢁ
ꢁ
nitrating mixture and maintain the temperature up to 30 C to 40 C for 1 h continuing
stirring. The whole mass was then added to ice-cold water and extracted with ethyl
acetate. Finally, light yellow color product of 4-hydroxy 3-nitro acetanilide is obtained,
filter it and crystallize in methanol solvent [37].
2
.4.3. Preparation of 4-dodecyloxy 3-nitro acetanilide from 4-hydroxy 3-nitro acetanilide (C)
4
-Hexyloxy 3-nitro acetanilide (0.1 mole), anhydrous K CO (0.15 mole) and n-dodecyl
2
3
bromide (0.15 mole) mixed in dry acetone (50 ml), reflux the reaction mixture at 3–4 h.
The whole mass was then added to water and extracted with ether. Finally, off-white
color product of 4-dodecyloxy 3-nitro acetanilide is obtained [37].
2
.4.4. Preparation of 4-dodecyloxy 3-nitro aniline from 4-dodecyloxy 3-nitro acetanilide (D)
4
-Dodecyloxy 3-nitro acetanilide (0.146 mole) taken with water (40 ml) and concen-
ꢁ
ꢁ
trated HCl (30 ml) were stirred for 6–7 h at 90 C to 95 C and then cooled to room
temperature. The reaction mixture was made alkaline by the addition of 50% NaOH at
ꢁ
2
0 C. The oily product of 4-dodecyloxy 3-nitro aniline is obtained [37].
2
.4.5. Diazotization of 4-dodecyloxy 3-nitro aniline
4
-Dodecyloxy 3-nitro aniline (0.005 mole) was taken in a beaker containing 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, 3.4 ml) was added and the reaction mixture was stirred for 1 h. A solution of
NaNO (0.005 mole, 0.35 g) in water (5 ml) was added dropwise to the solution. The
2
reaction mixture was stirred for 1 h. 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 in the presence of basic medium of aqueous NaOH [37].
0
0
2
.4.6. Preparation of 4-hydroxy benzoyloxy naphthyl azo 4 -dodecyloxy 3 -nitrobenzene (E)
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-dodecyloxy 3-nitro aniline for 1 h 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 3–4 h for complete separation and the orange color dye was
isolated by vacuum filtration which is further washed by water, dried, and purified via
column chromatography performed on silica gel (60 mesh) by using the eluent of
chloroform/methanol (6:4) [37].
0
0
2
.4.7. Preparation of 4-n-alkoxy benzoyloxy-naphthylazo-4 -dodecyloxy-3 -nitroben-
zene (F)
The compound has been prepared by esterification of the appropriate 4-n-alkoxy ben-
zoic acid (1.02 mmol) and azo dye (0.246 g, 1.02 mmol), DCC (0.457 g, 1.22 mmol), and
DMAP in catalytic amount (0.002 g, 0.2 mmol) in dry CH Cl (DCM) (30 ml) was
2
2

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
5
ꢁ
Scheme 1. Chemical and reagents: (i) R-Br, KOH, ethanol, reflux; (ii) Nitrating mixture; 50–60 C, 2 h;
ii) C H Br, anhydrous K CO , dry acetone, reflux 10 h; (iii) Conc.HCl, H O, hydrolysis, 6 h, reflux; (iv)
(
1
2
25
2
3
2
ꢁ
NaNO , HCl, 0–5 C, 1-Naphthol, NaOH; (v) DCC, DMAP, DCM, 8-h stirring at RT.
2
stirred at room temperature for 24h. The white precipitate of Dicyclohexylurea (DCU) is
obtained which was insulated by filtration and then remove. The resultant crude was
purified by column chromatography on silica gel eluting with dichloromethane and
recrystallization is carried out by using chloroform until to getting clearly transition tem-
peratures were observed [37]. The synthetic route to a series is mentioned in Scheme 1.
2
.5. Selected data of synthesized compounds
ꢀ
1
IR (KBr) in cm for Hexyloxy (F ), Octyloxy (F ), Dodecyloxy (F ) and Tetradecyloxy
6
8
10
(
F ) derivatives:
14

6
A. THAKOR ET AL.
Hexyloxy (F ): 881 Poly methylene group of (-CH -)n in -OC H , 880 (-C-H- def.
6
2
6
13
di-substituted on para side -OC H ), 981 (-C-H- def. hydrocarbon), 1210 (-C-O-) Str,
1
2
25
1
296, 1420, 1491 (-C-O str in -(CH -) alkyl chain group), 1580 (-C-H- def. in -CH
2
n
2
unit in alkyl chain), 1410 (-NO group), 1490 (-N ¼ N-) group, 1660 (carbonyl -C ¼ O
2
group), 1740 (-COO- ester group), 2876 and 3070 (-C-H str in CH ).
3
Octyloxy (F ): 771 and 804 Polymethylene (-CH -)n of -OC H and -OC H ,
8
2
8
17
12 25
881(-C-H- def. di-substituted-Para), 980 (-C-H- def. hydrocarbon), 1128, 1056, (-C-O-)
Str, of alkyl chain, 1373, 1246 (-C-O str in -(CH )n alkyl chain group), 1410 (-NO2
2
group), 1470 (-N ¼ N- group) group, 1610 (carbonyl -C ¼ O group), 1740 (-COO- ester
group), 2906 and 3036 (-C-H str in CH ).
3
Dodecyloxy (F ): 741 and 806 Polymethylene (-CH -)n of -OC H and -OC H
12 25
1
2
2
12 25
group, 881 (-C-H- def. di-substituted-Para), 987 (-C-H- def. hydrocarbon in alkyl
chain), 1057 (-C-O- Str.), 1290 (-C ¼ O str in -(CH -)n alkyl chain), 1576 (-C-H- def.
2
in CH ), 1410 (-NO group), 1490 (-N ¼ N- group), 1640 (-C ¼ O group), 1740 (-COO-
2
2
ester group), 2876 and 3070 and (-C-H str in CH ).
3
Tetradecyloxy (F ): 760 and 871 Polymethylene (-CH -)n of -OC H and
1
4
2
14 29
-
OC H , 806 (-C-H- def. di-substituted-Para side), 981 (-C-H- def. hydrocarbon in
12 25
alkyl chain), 1060 (-C-O- Str.), 1210 (-C ¼ O str in -(CH -)n alkyl chain), 1579 (-C-H-
2
def. in CH ), 1410 (-NO group), 1490 (-N ¼ N- group), 1640 (-C ¼ O group), 1750
2
2
(
-COO- ester group), 2824 and 3072 and (-C-H str in CH ).
3
1
13
H NMR (300 MHz) in CDCl (d ppm) and C NMR (100 MHz):
3
Dodecyloxy (F ): 0.88–0.90(t, 6H, of -OC H and -OC H ), 1.26–1.28 (m, 24H,
1
2
12 25
12 25
n-poly methylene groups of -OC H and -OC H ), 1.76 (p, 4H, -OC H and
1
2
25
12 25
12 25
-
OC H ), 4.06 (t, 4H, -O-CH - of -OC H and -OC H ), 7.11 (d, 2H, phenyl ring),
12 25 2 12 25 12 25
8
6
1
2
.12 (d, 4H, phenyl ring), 8.62 (d, 4H, phenyl ring), 7.51 and 7.94 (d, 4H, phenyl ring),
13
.84 (d, 2H, phenyl ring), 8.01 (d, 2H, phenyl ring). C NMR (100 MHz): 165.2, 164.6,
53.1, 144.4, 135.8, 130.9, 127.9, 126.0, 125.6, 122.5, 116.6, 109.7, 68.7, 53.8, 31.7, 26.1,
6.0, 14.2.
Hexyloxy (F ): 0.88 (t, 6H, of -OC H and -OC H ), 1.26–1.28 (m, 16H, n-poly
6
12 25
6
13
methylene groups of -OC H and -OC H ), 1.73 (p, 4H, -OC H and -OC H ),
1
2
25
6
13
12 25
6
13
4
.04 (t, 4H, -O-CH - of -OC H and -OC H ), 7.13 (d, 2H, phenyl ring), 8.01 (d, 4H,
2 12 25 6 13
phenyl ring), 8.57 (d, 4H, phenyl ring), 7.01 and 7.68 (d, 4H, phenyl ring), 6.87 (d, 2H,
1
3
phenyl ring), 6.89 (d, 2H, phenyl ring). C NMR (100 MHz): 167.2, 164.3, 149.4, 144.0,
135.6, 127.8, 126.6, 121.4, 119.4, 114.3, 109.6, 68.8, 29.8, 24.6, 15.6.
Decyloxy (F ): 0.88–0.90(t, 6H, of -OC H and -OC H ), 1.26–1.28 (m, 22H, n-poly
10
12 25
10 21
methylene groups of -OC H and -OC H ), 1.76 (p, 4H, -OC H and -OC H ), 4.04
12
25
10 21
12 25
12 25
(
t, 4H, -O-CH - of -OC H and -OC H ), 7.09 (d, 2H, phenyl ring), 8.21 (d, 4H, phenyl
2 12 25 10 21
ring), 8.57 (d, 4H, phenyl ring), 7.08 and 7.91 (d, 4H, phenyl ring), 6.73 (d, 2H, phenyl
13
ring), 8.05 (d, 2H, phenyl ring). C NMR (100 MHz): 167.2, 164.4, 149.4, 141.7, 136.4,
131.2, 125.6, 122.5, 121.6, 116.6, 114.2, 109.8, 68.7, 53.8, 31.8 26.1, 26.0, 15.2.
Octyloxy (F ): 0.90(t, 6H, of -OC H and -OC H ), 1.26–1.28 (m, 18H, n-poly
8
12 25
8
17
methylene groups of -OC H and -OC H ), 1.73 (p, 4H, -OC H and -OC H ),
1
2
25
8
17
12 25
8
17
4
.04 (t, 4H, -O-CH - of -OC H and -OC H ), 7.09 (d, 2H, phenyl ring), 8.10 (d, 4H,
2 12 25 8 17
phenyl ring), 8.23 (d, 4H, phenyl ring), 7.61 (d, 4H, phenyl ring), 6.89 (d, 2H, phenyl
ring), 8.03 (d, 2H, phenyl ring).

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
7
1
3
C NMR (100 MHz): 167.2, 164.8, 153.1, 141.7, 135.4, 130.6, 125.6, 122.5, 121.0,
19.7, 116.6, 114.2, 108.7, 68.7, 53.8, 31.7, 26.1, 26.0, 15.2.
1
3. Result and discussion
3.1. Mesomorphic behavior study
We have prepared calamitic rod-shaped optically active three phenyl ring-based azo
materials and investigated their liquid crystalline properties and further compared with
structurally similar homologous series. We have prepared a total of five homologous
based on two linkers and one lateral group at third phenyl ring with variable side
alkoxy chain on left side and fixed dodecyloxy side chain on terminal phenyl ring. At
present, newly series based on ester and azo group have been synthesized by the reac-
0
0
tion of 4-n-alkoxy benzoic acid (A) and 4-hydroxy biphenyl azo 3 -nitro 4 -dodecyloxy
phenyl (azo dye) (D) by well-reported method [37]. The thermotropic mesophase
behaviors of all the azo ester-based materials were primarily investigated by using POM,
differential scanning calorimetry (DSC) analysis, and the molecular packing arrange-
ments were further confirmed by high-temperature XRD pattern at the observed transi-
tion temperature which was observed from POM and DSC analysis. We have totally
prepared five mesogens in which comp. F showed only nematic phase while comp.F8
6
displayed both Smectic C and nematic phase respectively. In higher chain substituted
compounds (F , F , and F ) showed only smectic C phase with higher mesophase sta-
1
0
12
14
bility and mesophase range. All the compounds showed mesogenic properties in heating
as well as cooling conditions.
3
.1.1. POM study
The transition temperatures of all six derivatives were observed by POM analysis listed
in Table 1. All six derivatives showed liquid crystalline properties with higher meso-
phase range and stability. From the observation of the mesogenic behavior in all differ-
ent materials, it can be seen that the higher alkyl side spacer substituted materials
displayed lamellar type arrangement of molecules which on further heating do not pass
on sliding layer and directly converted into isotropic phase without showing any nem-
atic type molecular arrangements [11]. The formation of liquid crystalline state in mate-
rials is 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 and various kinds of forces acting on that [11].
ꢁ
Table 1. Transition temperature of series-F in C by POM.
ꢁ
Transition temperatures ( C)
Comp.
n-alkyl chain C
n
H
2n þ 1
SmC
N
I
1
2
3
4
5
F
F
6
8
–
102.6
149.7
–
–
–
171.4
160.1
159.6
142.4
134.6
110.8
104.6
101.2
96.8
F
10
F
12
F
14
SmC ¼ smectic C, N ¼ nematic; I ¼ isotropic.

8
A. THAKOR ET AL.
Figure 1. Mesomorphic behavior as a function of the number of carbon atoms (n) in the terminal
alkoxy chain for series-F.
Figure 2. Bar graph of present synthesized compounds (F –F ) on heating (a) and cooling (b).
6
14
The mesomorphic behavior of present series-F with respect to transition temperature
versus number of variable alkoxy side chain (-OR) is shown in Fig. 1 which indicates
the variation in alkyl side spacer affects the phase transition temperatures of respected
compounds. Figure 1 clearly indicates the decreasing tendency with respect to lower
homologous to higher homologous on heating and cooling region which basically
depends on the molecular properties and thermal behavior. The decreasing tendency of
the transition temperature of the compounds could be due to the lowering of the Van
der Waals interactions between aromatic phenyl cores and higher degree of flexibility
due to the presence of long alkyl chain in alkoxy group at left terminal part. Figure 2
indicates the bar graph of the synthesized compounds on heating and cooling condition,

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
9
ꢁ
Figure 3. Optical photomicrographs on heating condition; (a) smectic C (comp.F ) at 110.8 C; (b)
8
ꢁ
schlieren type pattern of nematic phase (comp.F ) at 149.7 C.
6
which indicates all azo mesogens showed well-order mesophase with higher tempera-
ture range.
3.1.2. Textural study
The azo-based material was sandwiched between a glass slide and a coverslip that was
heated from the solid crystalline state and further cooled from the isotropic state. The
POM texture image observed for compound F at two different temperatures is shown
6
in Fig. 3, respectively. The texture images of liquid crystalline compounds were observed
in both heating and cooling conditions. Compound F shows typical texture pattern of
6
ꢁ
smectic phase at 110.8 C on heating condition and on applying further heating up to
49.7 C, the texture pattern is transformed into schlieren type texture pattern of nem-
ꢁ
1
atic phase. Further, Compound F showed threaded type texture pattern of smectic C
6
ꢁ
ꢁ
phase at 102.6 C on heating and 98.3 C on cooling condition. Further, compound F
and F12 showed needle-type texture pattern of smectic C phase at 104.6 C and 101.2 C
on heating and at 101.2 C and 98.6 C on cooling condition. The texture pattern of
other derivatives from the present series-F showed threaded, needle and fan type texture
pattern of smectic C type phase on heating and cooling condition.
1
ꢁ
0
ꢁ
ꢁ
ꢁ
3
.1.3. Comparative study
Figure 4 indicates the molecular structure of presently synthesized series-F and previ-
ously reported structural similar series by our research group [38, 39]. The first phenyl
ring is substituted at left side by variable alkoxy side chain and fixed alkoxy group on
right terminal end side with the addition of lateral substituent on third phenyl ring. All
the series are similar with respect to phenyl core and similar linking group and induce
smectic and nematic mesophase. The presence of polar flexible methylene part in vari-
able alkoxy chain and also the presence of lateral group on the central phenyl core
increases the polarity and flexibility of molecule to induce mesomorphism. The presence
of the lateral substituents on third phenyl ring makes disruption and hindrance which

1
0
A. THAKOR ET AL.
Figure 4. Molecular structure of present and similar series; series-F (a); series-1 (b); series-2 (c).
affect molecular packing arrangement and also various forces acting simultaneously
which further affect mesomorphic properties. This disruption of the molecular packing
is particularly advantageous for the mesomorphic and physical properties required for
specific applications.
3.2. DSC study
The phase transition temperatures, enthalpies (DH), and entropy (DS) were measured
by using DSC thermal analysis. All compounds are thermally stable as confirmed by the
reproducibility of thermograms on second heating and cooling cycles. The DSC thermo-
grams of comp.F , F , F , and F are shown in Fig. 5. Compounds F traced two
8
10
12
14
6
endothermic peaks on second heating and cooling condition which is corresponding to
solid-N and N-I phase while compound F showed three endothermic peaks which
8
belong to Cr-SmC, SmC-N and SmC-I phase transition. Comp.F displayed two endo-
1
0
ꢁ
ꢁ
thermic peaks at 101.1 C, and 145.9 C on heating cycle and on applying cooling, it
exhibited at 99.3 C and 143.7 C which is corresponding to the presence of two-phase
transition. Similarly, comp. F shows two endothermic peaks at 94.2 C and 152.8 C
on heating and 91.7 C and 156.6 C on cooling cycle with associated enthalpy 16.25 J/g
and 12.75 J/g respectively. Comp. F showed two endothermic peaks at 92.6 C and
31.6 C on heating condition and on cooling, two peaks again reappeared at 97.7 C
and 136.3 C which is further confirmed with the result obtained by POM analysis. It
can be observed that the associate enthalpy of second phase transition is higher as com-
pare to first phase transition due to the formation of crystal to mesophase in all azo-
ꢁ
ꢁ
ꢁ
ꢁ
1
2
ꢁ
ꢁ
ꢁ
1
4
ꢁ
ꢁ
1
ꢁ

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
11
Figure 5. DSC thermograms of compound comp. F (a); comp. F10 (b); comp. F12 (c) comp. F14 (d) on
8
second heating and cooling cycles.
ꢁ
ꢀ1
ꢀ1 ꢀ1
Table 2. Transition temperature ( C), enthalpy (J g ), and entropy change (J g
F6–F14 by DSC measurement.
k
) of compound
ꢁ
ꢁ
ꢀ1
ꢀ1
ꢀ1 ꢀ1
ꢀ1 ꢀ1
Comp. Transition Heating scan ( C) Cooling scan ( C) DH (J g
)
DH (J g
)
DS (J g
k
)
DS (J g
k )
F
6
Cr-N
N-I
107.4
172.3
104.3
142.1
163.9
101.1
145.9
94.2
101.7
169.3
102.9
138.2
157.3
99.3
143.7
91.7
156.6
97.7
136.3
14.78
11.21
7.54
10.58
8.23
9.45
11.86
10.76
8.23
12.96
9.86
14.68
12.54
8.52
0.0388
0.0316
0.0251
0.0199
0.0254
0.0188
0.0252
0.0163
0.0442
0.0299
0.0286
0.0228
0.0243
0.0216
0.0315
0.0229
0.0394
0.0300
0.0233
0.0177
0.0527
0.0342
F
8
Cr-SmC
SmC-N
N-I
Cr-SmC
SmC-I
Cr-SmC
SmC-I
Cr-SmC
SmC-I
F
10
F
12
F
14
6.83
16.25
12.75
10.48
9.26
152.8
92.6
131.6
7.64
19.54
14.02
Cr-SmC ¼ solid to SmC; SmC-I ¼ SmC to isotropic; Cr-N ¼ solid to nematic; N-I ¼ nematic to isotropic.
based derivatives which confirm the liquid crystalline properties with good stability as
well. The phase transition temperatures, enthalpy, and entropy values for the present
synthesized series are summarized in Table 2.
3
.3. XRD analysis
To investigate the molecular arrangement in azo-based mesogens for further confirm-
ation to observed smectic and nematic phase, we have performed to perform a high-

1
2
A. THAKOR ET AL.
Figure 6. XRD traces of compound F (a); F10 (b); F12 (c); F14 (d) measured at transition temperature.
8
temperature XRD to correlate with the results obtained from DSC and POM analysis.
The XRD data of comp. F , F , F , and F are shown in Fig. 6. According to the
8
10
12
14
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 Fig. 7. Figure 6 clearly indicates the X-ray pattern
of compound F showed the diffraction peaks in the small-angle diffraction region cor-
8
responded to the smectic arrangement. The reflections for the small-angle area corres-
pond to a smectic layer type arrangement of present synthesized azo-based mesogens.
ꢁ
ꢁ
ꢁ
The reflection and d-spacing observed for compound F at 2h ¼ 8.8 , 11.6 , 22.6 are
8
9
.71 Å, 6.27 Å, 3.42 Å which confirms the lamellar packing of molecules in smectic type
ꢁ
ꢁ
arrangement. Compound F10 at 2h ¼ 8.3 , 11.4 , 23.1 showed d-spacing at 10.67 Å,
ꢁ
7
1
1
.79 Å, and 3.43 Å. Further, compound F showed mainly two reflections at 8.1 and
1
2
ꢁ
ꢁ
0.9 in small and one trace peak at 24.6 having corresponding d-spacing value
0.92 Å, 8.14 Å, and 3.43 Å, respectively. Similarly, compound F showed two main
reflection peaks at 7.4 , 9.8 , and 23.4 with d-spacing value as 11.96 Å, 9.04 Å, and
.42 Å. The presence of variable alkyl chain in alkoxy terminal side group in octyl (F8),
dodecyl (F ), and tetradecyl (F ) may increase the arrangement of molecules in order-
14
ꢁ
ꢁ
ꢁ
3
1
2
14
ing layered and formation of molecules in smectic mesophase. The reflection peak
appeared at higher angle region is due to the presence of alkyl chain interaction.
Further, the observed d-spacing length of respected compounds is nearly matched with

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
13
Figure 7. Representation of molecular packing in smectic and nematic mesophase in present series-F.
the calculated length of the molecules respectively. The XRD results indicate the
molecular arrangement of molecules may be just a smectic organization which is further
revealed with POM and DSC results.
3.4. Optical properties
Figure 8 represents the UV-Vis absorption spectra of presently synthesized azo-based
materials (F , F , F , and F ) are recorded in dichloromethane solution. The azo-based
6
8
10
14
ꢂ
mesogens were irradiated with UV light, the band corresponding to both p–p and
n–p transitions which decrease with increasing irradiation time period [28]. The
ꢂ
observed bands corresponding to isomerize product increase by increasing the irradi-
ation time, which confirmed the presence of photoisomerization effect at various time
intervals has occupied in the range of 364–551 nm. One can see that all the spectra of
four corresponding compounds are characterized by the two absorption peaks with the
maxima at 294 and 374 nm. The appearing of low intense short-wavelength absorption
ꢂ
maxima appeared at nearly 294 and 281 nm due to the presence of p–p transition of
phenyl rings. However, the highly intense longer wavelength absorption maxima
ꢂ
observed at 374 and 388 nm due to the presence of a p–p transition involving the
p-electronic system throughout the whole mesogenic portion, with a considerable
charge transfer character [28]. Compound F irradiated at 378 nm, for the study of E-Z
6
ꢂ
isomerization, the decrease in the absorption band to 363 nm is ascribed to p–p transi-
tion of the trans (E) isomer basically observed in azo (-N ¼ N-) group. In higher region
with time interval, a weak absorption band found in the visible region around 484 nm

1
4
A. THAKOR ET AL.
Figure 8. Absorption spectrum of compound F (a); F (b); F (c); F14 (d).
6
8
10
ꢂ
which represents to n–p transition of cis (Z) isomer in the azo group was gradually
increased by reaching photo stationary state at 80 s. After that, no change observed in
the absorption spectrum which is further confirmed the photo-saturation of E-Z
(trans–cis) isomerization process. In addition to this, compound F8 irradiated at
3
76 nm, again E-Z isomerization is taking place which further decreases up to 361 nm is
ꢂ
ascribed to p–p transition of the trans (E) isomer and very weak absorption band
found in the visible region around at 479 nm which confirms the appearance of n–p
transition of cis (Z) isomer. Further, compound F10 and F14 irradiated at 383 nm and
ꢂ
3
3
88 nm which is decreased due to the formation of E-Z isomerization at 357 nm and
ꢂ
63 nm due to p–p transition of the trans isomer (E) and also exhibiting the weak
ꢂ
absorption peak at 483 nm and 489 nm, this is due to the formation of n–p transition
of cis (Z) isomer, respectively. Figure 9 shows the trans and cis isomer of azo ester-
based present synthesized derivatives which clearly indicated that the photoisomeriza-
tion is taking place.
3
.5. Frontier molecular orbital (FMO) distribution
The distribution of FMO of final target ester azo group-based comp.F (series-F) and
6
comp.F14 are shown in Fig. 10. In this, we have compared the effect on band gap over

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
15
Figure 9. Schematic representation of the trans–cis transition of the azo-based mesogens (five
derivatives).
Figure 10. FMO distribution of comp. F (HOMO, series-F) (a); comp. F (LUMO, series-F) (b); comp.
6
6
F14 (HOMO, series-F) (c); comp. F14 (LUMO, series-F).
the substitution on azo dye (E) to achieve final liquid crystalline materials (F , F )
6
14
(Fig. 10). A close examination of these orbitals reveals that the HOMO (highest occu-
pied molecular orbital) is predominantly localized on the third phenyl ring over azo

1
6
A. THAKOR ET AL.
ꢁ
Table 3. Average thermal stability in C.
Series
Series-F
Series-1
Series-2
SmC-I
Cr-SmC
N-I
145.5
103.3
165.7
149.7
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.
linking unit while the LUMO (lowest unoccupied molecular orbital) is mostly concen-
trated on second phenyl and third phenyl ring over azo group. It clearly indicates that
the substitution of flexible alkyl chain on both the terminal side group does not affect
influence on the HOMO–LUMO orbital distributions. The band gap calculated for both
the target compounds (F , F ) is found to be lower than one which clearly indicates
6
14
the higher reactivity of the compounds. The presence of nitro group on third phenyl
ring provides more electron density, polarity which provide more charge transfer of
electron over the central azo functionalized group further reduces the resonance effect
caused by phenyl core, as a result the HOMO–LUMO energy gap or band gap (Eg) is
reduced with respect to other similar reported azo ester-based mesogenic materials.
Thus, it can be concluded that the electronic properties of present synthesized com-
pounds could have originated from the phenylic-azo group rather than other link-
ing units.
3
.6. Thermal stability and temperature range of mesophase
Thermal stability of present novel synthesized series-F is further compared with struc-
turally similar series-1 and series-2 are listed in Table 3, respectively. The present syn-
thesized series-F displayed higher thermal stability of nematic to isotropic and smectic
C to nematic phase with good mesophase temperature range as compared to series-1
and series-2. The geometrical structures of all three series are the same except terminal
and lateral groups to induce smectic and nematic phases. In present series-F which con-
tains azo ester linking group and fixed -OC H group on right terminal end with the
1
2
25
presence of nitro group which affect its polarity, polarizability and also affect the linear-
ity of the molecules causes higher mesophase thermal stability as compared to series-1
and series-2. Previously reported series-2 showed lower stability and mesophase
temperature range as compared to other series due to its planarity structure without
any lateral substituent groups, as a result, the mesophase commences early from
C homologue and continue to exhibit liquid crystalline properties till the last C
4
18
homologue [38, 39].
Fig. 11 represents the proposed mechanism to indicate the arrangement of azo-based
mesogens in both lamellar packing (SmC) and on further heating transform into nem-
atic phase (N) before passing to the isotropic phase transition. All the derivatives
showed good E-Z isomerization with good temperature and stability of mesophase. In
addition, we have compared the structural arrangement of lower side alkyl chain substi-
tuted compounds which showed nematic mesophase and higher alkyl chain substituted
compounds that exhibited SmC phase. All the compounds showed mesophase on heat-
ing and cooling conditions with the addition of good photophysical behavior as well.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
17
Figure 11. Propose mechanism of present series-F.
4. Conclusion
A new optically active azo ester-based mesogens based on two phenyl and one napthyl
cores with variable alkoxy side chain and lateral nitro group has been synthesized and
well characterized by various spectroscopic techniques. The liquid crystalline properties
of the synthesized mesogens are carried out by POM, DSC, and high-temperature XRD
techniques, and the photophysical behaviors are studied by UV-Vis spectroscopy tech-
nique. In the present nitro-based azo functionalized series, lower members showed
smectic C and nematic phase while higher members displayed only smectic C meso-
phase. The observed mesophase in the present series is enantiotropically in nature. The
thermal stability and mesophase temperature range are influenced by the lateral substi-
tuted nitro group in series-F which further compares with well-reported azo-based hom-
ologous series. The group efficiency order derived on the basis of (a) observed
mesophase thermal stability, (b) early or late commencement of mesophase, (c) tem-
perature range of mesophase. The HOMO–LUMO calculations indicate the lower band
gap of target compounds which showed good reactivity and lower stability with addition
by intermolecular charge transfer interactions and revealed the electronic properties of
present synthesized azo mesogens were originated from first and second phenyl ring
inbuilt with ester and azo group and also studied the effect of lateral nitro on electron
charge transfer and reactivity. The duration required for exhibiting photoisomerization
phenomena mainly related to the chemical structure of the target compounds.
Acknowledgments
A.T., D.D., V.D., and R.B.P. acknowledge thanks to K.K. Jarodwala Maninagar Science College,
Gujarat University, Ahmedabad for providing research lab and other facilities. V.S. also acknowl-
edges thanks to Department of Chemistry, Faculty of Basic and Applied Science, Sirohi,

1
8
A. THAKOR ET AL.
Rajasthan for providing some instrument and lab facility. The authors are also thankful to NFDD
Centre Rajkot, IIT Mandi, Sourashtra University, MLSU University, and MNIT Jaipur for provid-
ing spectral and other services.
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