Alkyloxy azo-cinnamate ester based thermotropic liquid crystals and their photophysical investigations

Article abstract of DOI:10.1080/15421406.2017.1301859

Two series of rod shaped Schiff base containing azo-cinnamate thermotropic liquid crystalline compounds were synthesized and characterized. These molecules have four different alkyl spacers (n = 6, 8, 10, and 12) at one end and a dodecyloxy chain at anoth

Full text of DOI:10.1080/15421406.2017.1301859

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Alkyloxy azo-cinnamate ester based thermotropic
liquid crystals and their photophysical
investigations
Chinnaiyan Selvarasu & Palaninathan Kannan
To cite this article: Chinnaiyan Selvarasu & Palaninathan Kannan (2017) Alkyloxy azo-cinnamate
ester based thermotropic liquid crystals and their photophysical investigations, Molecular Crystals
and Liquid Crystals, 648:1, 77-87, DOI: 10.1080/15421406.2017.1301859
To link to this article: http://dx.doi.org/10.1080/15421406.2017.1301859
Published online: 28 Jun 2017.
Submit your article to this journal
View related articles
View Crossmark data
Full Terms & Conditions of access and use can be found at
http://www.tandfonline.com/action/journalInformation?journalCode=gmcl20
Download by: [The UC San Diego Library]
Date: 29 June 2017, At: 03:01

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –

https://doi.org/./..
Alkyloxy azo-cinnamate ester based thermotropic liquid
crystals and their photophysical investigations
Chinnaiyan Selvarasu and Palaninathan Kannan
Department of Chemistry, Anna University, Chennai, India
ABSTRACT
KEYWORDS
Two series of rod shaped Schiff base containing azo-cinnamate ther-
motropic liquid crystalline compounds were synthesized and charac-
terized. These molecules have four different alkyl spacers (n = 6, 8, 10,
and 12) at one end and a dodecyloxy chain at another end (n = 12)
which influence their liquid crystalline properties. Liquid crystallinity
commence from six methylene spacer onwards in the series exhibit-
ing with Nematic to Smectic-C mesophases. Analytical data confirm the
molecular structures of homologues series of compounds. The charac-
teristic texture of liquid crystalline phases obtained using polarizing-
light microscope with a heating stage. The structural transformation of
these liquid crystals was confirmed by differential scanning calorimetry.
All the compounds exhibited various calamitic mesophases with wide
mesomorphic temperature ranges. The relationship between structure
and mesomorphic properties was discussed in framework of geomet-
rical configuration of central unit, linking part, and length of alkoxy
chains. Photosensitive azobenzene group undergoes photoisomeriza-
tion under UV light and monitored by UV-Visible spectroscopy.
Azo-cinnamate; Nematic;
photoisomerization;
Smectic-C; thermotropic
liquid crystal
1
. Introduction
Azo compounds are exceedingly colored and studied broadly because of their excellent ther-
mal and optical properties. These compounds are linked with two core groups having stepped
core structure and maintaining molecular linearity, allowing mesophase formation and higher
thermal stability [1,2]. Azobenzene based liquid crystalline materials are potentially interest-
ing class of compounds due to their distinctive mechanical properties [3, 4]. The reversible
cis-trans photoisomerization enables them promising candidate towards variety of applica-
tions involving photonic devices, [5] nonlinear optics, [6] optical data storage, [7, 8] photo-
switching materials [9], liquid crystal display [10, 11], polarization holography [12], nanogels
for drug delivery [13], and optical modulation [14]. Azobenzene functionalized photorespon-
sive materials have been widely explored, for instance surface photoalignment [15], photoin-
duced mass transport [16], and phase transition [17]. The liquid crystals obtained from the
molecules possessing Schiff base linkages particularly attracted profound attention ascribed to
their rich mesomorphism. Moreover, the Schiff base linkages are easily achievable under mild
reaction conditions yielding molecules with high thermal stability, high purity and calamitic
CONTACT Palaninathan Kannan
 , India.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
 Taylor & Francis Group, LLC
pakannan@annauniv.edu
Department of Chemistry, Anna University, Chennai

©

7
8
C. SELVARASU AND P. KANNAN
liquid crystalline texture [18, 19]. The mesomorphic nature of azo-cinnamate was found to be
strongly depended on the nature of substituent in the terminal alkyl position. The presence of
substituent at different alkyl positions on benzene ring (ortho-, meta-, and on attached alkyl
chain) have influenced the liquid crystalline structure [20] along with crystal-mesophase and
mesophase-isotropic transition temperatures as compared with their unsubstituted counter-
part. The liquid crystalline behavior of the material is also governed by chemical nature of rod
like mesogen attached to azobenzene. In a significant work, the extent of increasing temper-
ature depends on the size and morphology of lateral substituents present in the molecules. It
is proved that using appropriate combinations of these chemical parameters, one can modu-
late technological importance of liquid crystalline phases [21]. The complete understanding
of structure property correlation of azo-cinnamate based esters is still challenging as the del-
icate modifications in structure of these molecules create an extreme impact on the liquid
crystalline behavior [22–24].
The necessity in designing liquid crystalline compounds possessing double bond with
an additional functional group in their molecular architecture brings mesomorphic behav-
ior by revelation. The double bond of cinnamic acid has already been the keen area of
research in the field of liquid crystals [25]. Thus upon bringing a double bond in skeletal
framework enables electro-optical switching to mesomorphic properties [26], such as pho-
toisomerization of molecules in liquid crystalline state [27].The terminal substituents are of
prime importance to obtain liquid crystals since compounds with flexible chain have intrinsic
tendency to form a mesophase. A number of homologous series of mesogenic esters with
terminal substituents of azobenzene and branched terminal alkyl chain have been reported
previously. Recent work on cinnamate ester with terminal substituents of azobenzene by
Kannan et al has demonstrated that they are of wide Nematic range, high Nematic-isotropic
transition temperature and low melting points. Thus, molecules with azo core attract lot of
interest as they form liquid crystalline (Nematic and Smectic) textures depending upon the
terminal substituents attached to an azobenzene [28]. In a significant work by Yang et al.
have demonstrated that self-assembled azobenzene substituted compounds bearing six or
eleven methylene segments exhibited oily, streaky, and cholesteric phases [29]. Furthermore,
Sadashiva et al. reported synthesis and mesomorphic property studies on certain esters of
trans-4-n-alkoxycinnamic acid and trans-4-n-alkoxy-a-methylcinnamic acid with branched-
chain alkyl tails exhibit ferroelectric and antiferroelectric phases [30]. Vora and Rajput [31]
reported a binary mixture of cinnamate esters indicates a wide range of Nematic and Smectic
mesophases.
In this article, we report the study on mesophase stabilities of Schiff base containing azo
and ester linkages in the molecular frameworks. The initially obtained results encouraged us
to synthesize compounds of similar family (azo and ester) facilitating further understanding
of the effect central linkage and spacer length on mesomorphism. The series of compounds
obtained were based on higher spacer length (n = 12) on one end of the cinnamate ester and
with tuned azobenzene and ester linkages possessing different terminal chain lengths (n = 6,
8
, 10, and 12) on the other end. We have obtained an improved liquid crystalline property,
being geometrically closely related to known rod shaped molecules, upon introduction of
dodeclyoxycinnamate group, containing azo and ester linkage seemed to be particularly eco-
nomic and straightforward. The mesomorphic behavior was primarily controlled mainly by
length of alkyl chain and character of olefinic group bearing cinnamate group. Structural
parameters of different calamitic phases were carefully analyzed and compared. Furthermore,
this observation provides an in-sight on significance from the fact that introduction of an azo
linkage in homologues series involve a new dimension to this field, known as photochromism.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
79
The photophysical behavior (trans-cis isomerization) in azo group functionalized molecules
within tetrahydrofuran medium was also explored.
2
. Experimental section
2
.1. Materials
4
-Hydroxycinnamic acid (Sigma-Aldrich, India), triethylamine, trifluoroacetic acid (TFA),
hydrochloric acid, thionyl chloride (Merck, India), 4-hydroxybenzaldehyde, and potassium
dichromate were used as received. Solvents such as benzene, ethanol are purified by reported
procedure [32].
2.2. Instrumentation
The crude samples were purified by column chromatography using silica gel (400 mesh). Thin
layer chromatography (TLC) was performed on aluminum sheets pre-coated with silica gel
(
Merck, Kiesinger 60 and F254). The infrared spectra of compounds were measured using KBr
1
13
disk in Perkin Elmer FT-IR spectrometer. H (400 MHz) and C NMR (100 MHz) measure-
ments were performed with Bruker spectrometer using TMS as an internal standard. Com-
positions of compounds were determined by Heraeus CHN elemental analyzer. The sequence
of phases and phase transition temperatures were determined from optical textures and their
changes were observed under a polarizing optical microscope (Linkem HFS91). Samples were
placed in between two thin glass cover slips and melted with heating and cooling at the rate of
−1
1
°C min . The photographs were taken with a Canon 1000D camera. Differential scanning
calorimetry (DSC) was carried out for the selected compounds by employing a Perk-Elmer
Pyris Diamond calorimeter. Samples of about 2–5 mg, hermitically sealed in aluminum pan,
were placed in N atmosphere working place. The calorimeter data were calibrated on extrap-
2
olated onset temperatures and enthalpy changes of water, indium and zinc. The measurements
−1
were performed on heating/cooling run at a rate of 5 °C min . UV absorption spectra were
recorded on a UV-1650PC UV-visible spectrophotometer (Shimadzu). The sample dissolved
−5
in tetrahydrofuran was prepared with concentration of 1×10 M.
2
.3. Synthesis of 4-formylphenyl (E)-3-(4-(dodecyloxy)phenyl)acrylate (5)
The dodecyloxycinnamoyl chloride (2) prepared by the reported procedure [26] (5 g, 3.37
mmol) was added drop wise to the reaction mixture containing 4-hydroxybenzaldehyde
(
2
4.12 gm, 3.37 mmol) and triethylamine (3.35 mmol) dissolved in dry THF and stirred for
4 hr. After completion of reaction, triethylamine hydrochloride was precipitated out and
removed by filtration. The crude product was subjected to column chromatography (hex-
ane:ethylacetate (95:5%)) to obtain colorless compound (5) (7.21 g, 78%). M.p: 174 °C.
1
H NMR (400 MHz, CDCl ) δ/ppm:9.90 (s, 1H, CHO), 7.85–6.83 (d, 16H, J = 7.3 Hz,
3
Aromatic-H) 7.43–7.75 (d, J = 13.2 Hz, 1H, olefinic- H), 6.41–6.37 (d, J = 13.4 Hz, 1H,
olefinic- H), 3.92–3.89 (t, J = 7.6 Hz, 2H, -OCH -), 1.73–1.18 (m, 20H, -CH -), 0.78 (m, 3H,
2
2
13
aliphatic-CH ). C-NMR (100 MHz, CDCl ) δ/ppm: 13.10, 21.67, 24.97, 28.11, 28.33, 28.61,
3
3
3
1
0.90, 67.21, 112.59, 113.97, 121.42, 125.38, 129.17, 130.16, 132.80, 146.31, 154.74, 160.63,
63.95, 189.90.Selected IR bands (cm ): ν 2856, 2921, 1727, 1608, 1494, 1255, 1023, 1102.
−1
Anal.Calcd for C H O C, 77.03%; H, 8.31%. Found: C, 76.67%; H, 7.89%.
28
36
4

8
0
C. SELVARASU AND P. KANNAN
2
.4. General procedure for the preparation of Schiff base compounds (6a–d and 7a–d)
The synthesis of Schiff base compounds was performed by condensation reaction of appropri-
ate aldehyde (5) with (E)-4-((4-(alkyloxy)phenyl)diazenyl)aniline (3a–d) or 4-aminophenyl
4
-(alkyloxy)benzoate (4a–d) taken in 1:1 molar ratio. The reagents were dissolved in ethanol
to form a 15%–20% solution and reaction mixture heated and stirred for 6 hr, in an inert
atmosphere. After cooling, the crystalline product was separated by filtration and purified by
recrystallization. Finally, the compounds dried overnight in vacuum at 60 °C. Pure, Schiff
base compounds were obtained as yellowish crystals with yields of 68%–74%. Their structure
1
was confirmed by FT-IR and H-NMR spectroscopy.
... (E)--((E)-((-((E)-(-(hexyloxy)phenyl)diazenyl)phenyl)imino)methyl)phenyl

-(-(dodecyloxy)phenyl)acrylate
1
6a: Yield: 70%. H NMR (400 MHz, CDCl ) δ/ppm: 8.70 (s, Ar-CH = N-, 1H), 7.92–7.89
3
(
d, J = 7.7 Hz, aromatic-H, 2H), 7.94 (d, J = 7.8 Hz, aromatic-H, 6H), 7.51–7.35 (m, 5H,
aromatic-H), 7.19 (d, J = 7.7 Hz, aromatic-H, 2H), 7.15–7.17 (d, J = 7.9 Hz, aromatic-H, 2H),
.48 (d, J = 12.7 Hz, olefinic-H, 1H), 6.61 (d, J = 11.6 Hz, olefinic-H, 1H), 4.07–3.95 (t, 4H,
7
J = 8.1 Hz, -OCH ), 1.86–1.74 (m, 4H, aliphatic-CH ), 1.31–1.06 (m, aliphatic-CH , 24H),
2
2
2
13
0
2
1
1
.91–0.86 (t, 6H, J = 6.8 Hz, aliphatic-CH ). C NMR (100 MHz, CDCl )δ/ppm: 14.1, 22.8,
3
3
6.3, 28.7, 29.2, 31.6, 69.6, 115.2, 121.7, 123.2, 124.5, 128.6, 129.2, 131.3, 134.5, 144.7, 147.6,
−1
51.4, 155.2, 159.9, 162.3, 163.0, 168.1. Selected IR bands (cm ): ν 2852, 2913, 1726, 1604,
472, 1476, 1253, 1024, 1137.Anal.Calcd for C H N O . C, 77.17%; N, 5.87%; H, 8.02%.
46
57
3
4
Found:C, 76.39%; N, 5.27; H, 7.59.
1
2
.4.2. 6b: Yield: 69%. H NMR (400 MHz, CDCl ) δ/ppm: 8.71 (s, Ar-CH = N-, 1H), 7.93–
3
7
.94 (d, J = 7.5 Hz, aromatic-H, 2H), 7.93 (d, J = 7.7 Hz, aromatic-H, 6H), 7.58–7.25 (m, 5H,
aromatic-H), 7.15 (d, J = 7.6 Hz, aromatic-H, 2H), 7.21–7.19 (d, J = 7.29 Hz, aromatic-H, 2H),
.48 (d, J = 12.3 Hz, olefinic-H, 1H), 6.52 (d, J = 11.9 Hz, olefinic-H, 1H), 4.05–3.96 (t, 4H,
7
J = 8.3 Hz, -OCH ), 1.89–1.72 (m, 4H, aliphatic-CH ), 1.37–1.03 (m, aliphatic-CH , 28H),
2
2
2
13
0
2
1
1
.96–0.94 (t, 6H, J = 6.8 Hz, aliphatic-CH ). C NMR (100 MHz, CDCl )δ/ppm: 14.1, 22.8,
3
3
6.3, 28.7, 29.1, 31.5, 69.6, 115.4, 121.7, 123.2, 124.4, 128.6, 129.2, 131.3, 134.4, 144.6, 147.6,
−1
51.4, 155.2, 159.8, 162.3, 163.1, 168.0. Selected IR bands (cm ): ν 2849, 2915, 1729, 1607,
474, 1502, 1252, 1024, 1139.Anal.Calcd for C H N O . C, 77.49%; N, 5.65%; H, 8.26%.
48
61
3
4
Found: C, 76.83%; N, 5.15%; H, 7.68%.
1
2
.4.3. 6c: Yield: 71%. H NMR (400 MHz, CDCl ) δ/ppm: 8.70 (s, Ar-CH = N-, 1H), 7.92 –
3
7
.90 (d, J = 7.5 Hz, aromatic-H, 2H), 7.93 (d, J = 7.3 Hz, aromatic-H, 6H), 7.42–7.31 (m, 5H,
aromatic-H), 7.16 (d, J = 7.5 Hz, aromatic-H, 2H), 7.15–7.17 (d, J = 7.5 Hz, aromatic-H, 2H),
.48 (d, J = 12.2 Hz, olefinic-H, 1H), 6.49 (d, J = 11.8 Hz, olefinic-H, 1H), 4.06–3.97 (t, 4H,
7
J = 8.4 Hz, -OCH ), 1.86–1.79 (m, 4H, aliphatic-CH ), 1.35–1.09 (m, aliphatic-CH , 32H),
2
2
2
13
0
2
1
1
.98–0.96 (t, 6H, J = 6.8 Hz, aliphatic-CH ). C NMR (100 MHz, CDCl )δ/ppm:14.02, 22.8,
3
3
6.4, 28.6, 29.1, 31.6, 69.5, 115.4, 121.5, 123.4, 124.4, 128.7, 129.2, 131.4, 134.6, 144.7, 147.6,
−1
51.4, 155.2, 159.8, 162.4, 163.1, 168.1. Selected IR bands (cm ): ν 2848, 2917, 1726, 1603,
474, 1502, 1254–1023, 1138.Anal. Calcd for C H N O C, 77.78%; N, 5.44%; H, 8.49%.
50
65
3
4.
Found: C, 77.18%; N, 5.01%; H, 8.11%.
1
2
.4.4. 6d: Yield: 68%. H NMR (400 MHz, CDCl ) δ/ppm: 8.71 (s, Ar-CH = N-,
.
3
1
6
7
H), 7.94–7.92 (d, J = 7.8 Hz, aromatic-H, 2H), 7.90 (d, J = 7.54 Hz, aromatic-H,
H), 7.52 (m, 5H, aromatic-H), 7.11–7.08 (d, J = 7.43 Hz, aromatic-H, 2H), 7.07–
.06 (d, J = 7.43 Hz, aromatic-H, 2H), 7.52 (d, J = 12.5 Hz, olefinic-H, 1H), 6.46
(
d, J = 11.7 Hz, olefinic-H, 1H), 4.01–3.98 (t, 4H, J = 8.2 Hz, -OCH ), 1.77–1.72
2

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
81
(
m, 4H, aliphatic-CH ), 1.38–1.25 (m, aliphatic-CH , 36H), 0.99–0.97 (t, 6H, J = 6.4 Hz,
13
2
2
aliphatic-CH ). C NMR (100 MHz, CDCl )δ/ppm:14.01, 22.9, 26.4, 28.7, 29.0, 31.6, 69.6,
3
3
1
1
15.3, 121.7, 123.3, 124.5, 128.6, 129.1, 131.3, 134.5, 144.7, 147.6, 151.5, 155.2, 159.9, 162.4,
63.0, 168.0. Selected IR bands (cm ): ν 2850, 2918, 1724, 1602, 1473, 1503, 1251, 1025,
−1
1
147.Anal Calcd for C H N O .C, 78.06%; N, 5.25%; H, 8.69%.Found: C, 77.86%; N, 5.01%;
52
69
3
4
H, 7.42%.

... Synthesis of -(((E)--(((E)--(-(hexyloxy)phenyl)acryloyl)oxy)benzylidene)amino)
phenyl -(hexyloxy)benzoate (a)
1
Yield: 71%. H NMR (400 MHz, CDCl ) δ/ppm: 8.70 (s, Ar-CH = N-, 1H), 8.12–8.04 (d, J =
3
7
.6 Hz, aromatic-H, 2H), 7.52 (d, J = 7.5 Hz, aromatic-H, 6H), 7.37–7.32 (m, 5H, aromatic-
H), 7.15 (d, J = 7.5 Hz, aromatic-H, 2H), 7.18–7.08 (d, J = 7.6 Hz, aromatic-H, 2H), 7.43
d, J = 11.7 Hz, olefinic-H, 1H), 6.74–6.71 (d, J = 11.5 Hz, olefinic-H, 1H), 4.02–3.98 (t, 4H,
(
J = 8.2 Hz, OCH ), 1.74–1.72 (m, 4H, aliphatic-CH ), 1.52– 1.21 (m, aliphatic-CH , 24H),
2
2
2
13
1
2
1
2
7
.02–0.98 (t, 6H, J = 6.7 Hz, aliphatic-CH ). C NMR (100 MHz, CDCl )δ/ppm:14.1, 22.8,
3
3
6.5, 28.6, 29.1, 31.5, 69.6, 114.6, 115.3, 119.2, 121.7, 122.7, 123.6, 128.6, 129.0, 131.2, 132.3,
−1
34.5, 135.8, 144.6, 146.4, 150.4, 155.2, 163.9, 167.1, 168.1. Selected IR bands (cm ): ν 2847,
922, 1729, 1606, 1494, 1254–1028, 1127.Anal Calcd for C H NO .C, 76.02%; N, 2.16%; H,
.00%. Found: C, 75.32%; N, 2.01%; H, 6.48%.
41
45
6
1
2
.4.6. 7b:Yield: 69%. H NMR (400 MHz, CDCl ) δ/ppm: 8.68 (s, Ar-CH = N-, 1H), 8.12–
3
8
.08 (d, J = 7.6 Hz, aromatic-H, 2H), 7.7 (d, J = 7.4 Hz, aromatic-H, 6H), 7.36–7.31 (m, 5H,
aromatic-H), 7.16 (d, J = 7.6 Hz, aromatic-H, 2H), 7.19–7.14 (d, J = 7.4 Hz, aromatic-H, 2H),
7
4
1
2
1
2
7
.47 (d, J = 11.7 Hz, olefinic-H, 1H), 6.72–6.70 (d, J = 11.4 Hz, olefinic-H, 1H), 4.02–3.98 (t,
H, J = 8.4 Hz, OCH ), 1.76–1.72 (m, 4H, aliphatic-CH ), 1.50–1.24 (m, aliphatic-CH , 28H),
2
2
2
13
.02–0.98 (t, 6H, J = 6.7 Hz, aliphatic-CH ). C NMR (100 MHz, CDCl )δ/ppm: 14.2, 22.7,
3
3
6.6, 28.5, 29.2, 31.4, 69.5, 114.6, 115.3, 119.2, 121.7, 122.7, 123.4, 128.5, 129.1, 131.3, 132.2,
−1
34.5, 135.6, 144.6, 146.5, 150.4, 155.2, 163.8, 167.2, 168.1. Selected IR bands (cm ): ν2856,
925, 1731, 1606, 1499, 1254, 1026, 1102.Anal Calcd for C H NO .C, 76.42%; N, 2.07%; H,
.31%. Found: C, 76.02%; N, 1.76%; H, 6.82%.
43
49
6
1
2
.4.6. 7c: Yield: 73%. H NMR (400 MHz, CDCl ) δ/ppm: 8.69 (s, Ar-CH = N-, 1H), 8.12–
3
8
.05 (d, J = 7.4 Hz, aromatic-H, 2H), 7.51 (d, J = 7.5 Hz, aromatic-H, 6H), 7.37–7.34 (m, 5H,
aromatic-H), 7.16 (d, J = 7.6 Hz, aromatic-H, 2H), 7.19–7.12 (d, J = 7.6 Hz, aromatic-H, 2H),
7
.51 (d, J = 11.4 Hz, olefinic-H, 1H), 6.71–6.68 (d, J = 11.4 Hz, olefinic-H, 1H), 4.02–3.98 (t,
H, J = 8.4 Hz, OCH ), 1.74–1.70 (m, 4H, aliphatic-CH ), 1.51– 1.21 (m, 32H, aliphatic-
4
2
2
13
CH ), 1.0–0.98 (t, 6H, J = 6.5 Hz, aliphatic-CH ). C NMR (100 MHz, CDCl )δ/ppm:14.1,
2
3
3
2
1
2.6, 26.5, 28.6, 29.1, 31.5, 69.6, 114.5, 115.4, 119.2, 121.5, 122.8, 123.4, 128.4, 129.1, 131.4,
−1
32.3, 134.2, 135.6, 144.6, 146.4, 150.3, 155.2, 163.7, 167.2, 168.1.Selected IR bands (cm ):
ν 2854, 2928, 1725, 1608, 1494, 1262, 1016, 1108. Anal Calcd for C H NO . C, 76.78%; N,
45
53
6
1
.99%; H, 7.59%. Found: C, 75.67%; N, 1.27%; H, 6.85%.
1
2
.4.7. 7d: Yield: 72%. H NMR (400 MHz, CDCl ) δ/ppm: 8.71 (s, Ar-CH = N-, 1H), 8.11–
3
8
.09 (d, J = 7.5 Hz, aromatic-H, 2H), 7.54 (d, J = 7.6 Hz, aromatic-H, 6H), 7.35–7.35 (m, 5H,
aromatic-H), 7.17 (d, J = 7.74 Hz, aromatic-H, 2H), 7.16–7.06 (d, J = 7.52 Hz, aromatic-H,
H), 7.42 (d, J = 11.6 Hz, olefinic-H, 1H), 6.75–6.71 (d, J = 11.6 Hz, olefinic-H, 1H), 4.01–3.99
t, 4H, J = 8.3 Hz, OCH ), 1.76–1.71 (m, 4H, aliphatic-CH ), 1.51–1.23 (m, 36H, aliphatic-
2
(
2
2
13
CH ), 1.01–0.99 (t, 6H, J = 6.4 Hz, aliphatic-CH ). C NMR (100 MHz, CDCl )δ/ppm:14.0,
2
3
3
2
1
2.9, 26.4, 28.7, 29.0, 31.6, 69.6, 114.7, 115.3, 119.1, 121.7, 122.9, 123.6, 128.6, 129.0, 131.1,
32.3, 134.5, 135.8, 144.7, 146.4, 150.5, 155.1, 163.9, 167.1, 168.0. Selected IR bands (cm ):
−1

8
2
C. SELVARASU AND P. KANNAN
Scheme . Synthesis of liquid crystalline compounds.
ν 2853, 2929, 1729, 1608, 1502, 1258, 1029, 1107. Analysis Calcd for C H NO . C, 76.78%;
47
57
6
N, 1.99%; H, 7.59%. Found: C, 75.56%; N, 1.37%; H, 6.67%.
. Results and discussion
.1. Synthesis and characterization
3
3
Scheme 1 illustrates the procedure employed for the synthesis of two series of azo and ester
linkage of Schiff base containing chromophores with both end containing different alkoxy
1
13
cinnamate esters. The synthesized compounds were characterized by means of FT-IR, H, C
NMR, and elemental analysis. The Fourier transform infrared (FT-IR) spectroscopic analy-
1
13
ses, H and C nuclear magnetic resonance (NMR) spectroscopy confirmed the synthesis
of compounds. The compound 6d displays intensive characteristic absorption band around
−
1
1
622–1629 cm attributed to azomethine bond. The chemical shift of protons was detected
1
in the H NMR spectrum, ratio values of integrals perfectly agree with calculated ones for the
proposed structure. The FT-IR, H, and C-NMR spectra of compounds were provided in
Supporting Information.
1
13
3
.2. Thermotropic behavior
The compounds 6a–d and 7a–d displayed thermotropic calamitic mesophases on heating and
cooling cycles in the DSC and POM investigations.
The POM textures of 6d and 7d are exhibited in Figures 1 and 3 The DSC curves of 6d
−1
indicated a crystalline melting endotherm at 164.4 °C (enthalpy changes δH = 22.6 kJ mol )
−1
and Nematic–isotropic phase transition at 269.2 C (1.3 kJ mol ) on heating, as well as an
isotropic-Nematic and Nematic – Smectic-C transition at 238.4 °C and 178.5 °C (−0.7 kJ
−
1
−1
mol ). Finally, the crystallization at 131.5 °C (−33.5 kJ mol ) on cooling, as displayed in
Figure 2. Taking into account on the observations, DSC exothermic peaks were assigned to

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
83
Figure . POM textures (X) of compound 6d (a) Nematic texture on heating to . °C; (b) Schlieren tex-
ture of Nematic on heating to  °C; (c) Nematic droplets which form by coalescence Schlieren texture on
cooling to . °C; (d) Smectic C phase on cooling at  °C.
Figure . DSC curves of compounds 6d and 7d.
Figure . POM textures (X) of Schiff base azo compound d (a) oily streak texture on cooling to  °C; (b)
broken fan-shaped texture at  °C; (c) Smectic-C texture on cooling to  °C.

8
4
C. SELVARASU AND P. KANNAN
−
Table . Phase transition temperatures (T/°C) and enthalpies (δH/J g ) obtained from DSC scans of 6a–d
and 7a–d.
Compound
Heating
Cooling cycle




a
b
c
Cr . (.) N . (.) I
Cr . (.) N . (.) I
Cr . (.) N . (.) I
Cr . (.) N . (.) I
Cr . (.) N . (.) I
Cr . (.) N . (.) I
Cr . (.) N . (.) I
Cr . (.) N . (.) I
I . (−.) N . (.) SmA . (−.) Cr . (−.)
I . (−.) N . (.) SmA . (−.) Cr . (−.)
I . (−.) N . (.) SmC . (−.) Cr . (−.)
I . (−.) N . (.) SmC . (−.) Cr . (−.)
I . (−.) N . (−.) SmA . (−.) Cr . (−.)
I . (−.) N . (−.) SmA . (−.) Cr . (−.)
I . (−.) N . (−.) SmC . (−.) Cr . (−.)
I . (−.) N . (−.) SmC . (−.) Cr . (−.)
d




a
b
c
d
Cr = Crystal, N = Nematic, SmA = Smectic A, SmC = Smectic C, I = Isotropic.
Isotropic–Nematic, Nematic–Smectic-C, and Smectic-C–crystalline transitions, respectively
(
Table 1). In the first heating scan, compound 6d of POM observation did not reveal signifi-
cant changes corresponding to first endothermic peak, while at the temperature correspond-
ing to second endothermic peak, a clear, fluid Nematic texture displayed (Figure 1a and b),
which further transformed into an isotropic liquid (at a temperature corresponding to high-
est temperature endothermic peak). Based on these observations, first endothermic peak
was ascribed to a solid–solid transition, second endothermic peak to a Nematic–isotropic
transition.
In the first cooling scan, Nematic droplets appeared from isotropic liquid (Figure 1c),
forming by coalescence an unstable Schlieren texture which very rapidly transformed into
a Smectic-C texture (Figure 1d). The DSC thermogram in Figure 2 exhibited a crystalline
−1
melting at 136.2 °C (H = 23.3 kJ mol ) and Nematic-isotropic phase at 275.4 °C (0.7 kJ
mol ) on heating for 7d, as well as an isotropic-Nematic and Nematic-Smectic-C transition
−1
−1
−1
at 225.6 °C and 146.7 °C (−1.3 kJ mol ) finally crystallization at 60.4 °C (−18.3 kJ mol ) on
cooling. The first heating/cooling cycle is similar to compound 7d, the only difference being
the weaker first endothermic peak (7d) compared with compound 6d, attributed to differ-
ence between crystalline state obtained by crystallization from molten state and recrystalliza-
tion from solvent, respectively. The enthalpy values recorded by DSC are in accordance with
values usually observed for characteristic crystalline–Nematic, Nematic–Isotropic, Nematic–
Smectic-C, and Smectic-C–Crystalline transitions, verifying the presence of a Nematic and
Smectic-C mesophase [33].
On cooling, isotropic liquids of series I and II compounds with n = 6 and 8 formed small
droplets that coalesced to Schlieren texture of SmA phase while, n = 10 and 12 derivative
exhibited only focal conic texture characteristic of Smectic-C mesophase when cooled from
isotropic liquid. The remaining lower members of compound 6a–6b and 7a–7b exhibit from
above similar observation of Nematic and Smectic-A mesophase occur with wide transition
temperature. This is attributed to flexibility of increased alkyl chain length whereas the phase
transition sequence from isotropic from cooling as Isotropic–Nematic–Smectic-A–Crystal.
The 7c and 7d alone exhibit Smectic-C texture because of increasing alkyl chain length (n = 10
and 12). It is noticed that crystal mesophase transition temperatures decreases with increase
in length of terminal alkoxy chain length. The Nematic-isotropic transition temperatures also
decreases with increase in number of carbon atoms in the alkoxy chain, whereas it exhibits
a tendency for rising Smectic-C–Nematic transition temperature in ascending series, which
levels off slightly in the higher homologues, when terminal alkoxy chain attractions are more
prominent.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
85
Figure . Absorption spectrum of compound 7a.
The molecules of series I and II have cinnamate connected to Schiff base with azo and
ester linkages have addition of double bond in the system which increases length and polar-
izability of rod like molecules. The presence of this double bond system (cinnamate group)
increases the length of polarizability of rod like molecules and enhances mesophase thermal
stability. Therefore, greater mesophase thermal stability of the present series I and II may be
explained in terms of greater molecular length and polarizability of molecule resulting from
additional –CH═CH– units in central linkage. The overall polarizability is enhanced with
increase of alkyl chain length as the series is ascended. The ratio of terminal attractions to
lateral attractions decreases as the series is ascended; the net result is that the lateral attrac-
tions gain dominance over the terminal attractions. The molecules under such conditions will
observe, strictly maintaining layered structure, although they might partially attain a fluid
condition. It revealed that alkyl chain length from tenth member onwards; purely Smectic-C
mesophase is exhibited. The overall polarizability of the molecules is enhanced as the length
of alkyl chain at one end increases, contributing to exhibit Smectic-C textures. However, the
ratio of terminal to lateral attractions is also low at certain stage the thermal vibration break
down the orientation of molecules without giving them a chance for adopting Nematic ori-
entation. Thus, Smectic-C mesophase alone prevails from dodecyloxyderivative.
3
.3. Photophysical properties
Photophysical properties of synthesized compounds have been investigated using UV-visible
spectroscopy of the compound containing azo moiety in the diluted solutions, to have an
insight into the electronic structure generated by chromophores. Absorption spectrum of
compound were obtained from its absorption band is summarized in Table S6 and a spec-
trum is shown in Figure 4.
The absorption spectra of compounds were carried out in tetrahydrofuran solution The
.
azo linkage containing compounds was irradiated with UV light (365 nm), the band corre-
∗
∗
sponding to π–π and n–π transitions decreases with increasing irradiation time [28]. In
Figure 4, the band corresponding to isomerized product increases by increasing the irradiat-
ing time, which confirmed photoisomerization at various time intervals has occurred in the

8
6
C. SELVARASU AND P. KANNAN
range of 385–550 nm. The UV-vis absorption spectrum of compound in solution is character-
ized by three absorption bands: an absorption band at longer wavelength 260–440 nm range
∗
corresponding to π–π transition of the mesogen. The 7a irradiated at 365 nm, E–Z isomer-
ization, resulting with a dramatic decrease in the absorption band from 379 nm is ascribed to
∗
π–π transition of the trans (E) isomer in azo group. The further very weak absorption band
∗
also found in the visible region around 487 nm, appropriate to n–π transition of cis (Z) iso-
mer in the azo group was gradually increased by reaching photostationary state 80 sec. After
8
0 sec illumination, there is no change in absorption spectrum confirms photo-saturation
of E-Z (trans-cis) isomerization process. The isobestic points were observed at 340 and 450,
corresponding to trans-cis isomerization as these suggesting that only two isomers obtained.
In Figure 4 for another band absorption exhibit the compound containing cinnamate unit in
double bond undergo isomerization around at 273 nm. The absorption spectra are similar
for all the compounds because of their similar molecular structure; the only difference is in
the alkyl chain, which does not alter the electronic transitions [6, 34]. At the same time, we
have tried for reversible photo isomerization in the dark condition for several min for 7a, with
same concentration of tetrahydrofuran solution. The transformed cis isomer (Z) was not con-
verted into trans form (E). The transformed cis isomer (Z) is not converted into trans form
(
E) ascribed to the imine (−CH═N–) group restricts electron delocalization on cinnamate
moiety and chemical structure of chromophores attained the anisotropic interaction between
the liquid crystals and unchanged trans isomer of azo group.
4
. Conclusion
Novel thermotropic liquid crystalline rod shaped molecules containing Schiff base of azo and
ester linkages have been synthesized and characterized. The mesophase range of present series
I and II for those of structurally related compounds that have been depend on rod shaped
and attributed to high polarizability of the molecule. The structural aspects strongly enhance
phase transition behavior of Schiff’s base compounds. The mesophases of rod like molecules
have very high thermal stability and isotropic temperature of >275 °C depending on the pres-
ence of olefinic double bond enhances the length of polarizability. Interestingly, change in ori-
entation of outer ester group in lengthening of arm in the compounds in I series led to decrease
in clearing temperature. The effect of terminal alkoxy containing azobenzene moiety, central
linkage with spacer attached also plays a crucial role in the formation of mesophase. The
duration required for exhibiting photoisomerization phenomena mainly related with chemi-
cal structure of compounds.
References
[
[
[
1] Sie, T. H., Yeap, G., & Boey, P. L. (2009). Aust. J. Basic. Applied. Sci., 3(4), 3417–3422.
2] Shukla, H. M., Shah, A. I., Shah, P. J., & Raj, D. S. (2010). J. Chem. Pharm. Res., 2(5), 169–176.
3] Rahman, M. L., Hegde, G., Yusoff, M. M., Fazli, M. N., Malek, A., Srinivasa, H. T., & Kumar, S.
(
2013). New. J. Chem., 37, 2460–3467.
[
[
[
[
[
4] Wang, G., Zhang, M., Zhang, T., Guan, J., & Yang, H. (2012). RSC. Advan., 2, 487–493.
5] Smela, E. (2003). Advan. Mater., 15, 481–494.
6] Vijay Srinivasan, M., & Kannan, P. (2011). J. Mater. Sci., 46, 5029–5043.
7] Hegde, G., Alla, R. A., Matharu, A., & Komitov, L. (2013). J. Mater. Chem. C., 1(22), 3600–3605.
8] Matsumori, M., Takahashi, A., Tomioka, Y., Hikima, T., Takata, M., Kajitani, T., & Fukushima, T.
(
2015). Appl. Mater. Inter., 7(21), 11074–11078.
[
9] Vijay Srinivasan, M., Kannan, P., & Roy, A. (2013). New. J. Chem., 37, 1584–1590.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
87
[
[
[
10] Fukuhara, K., Nagano, S., Hara, M., & Seki, T. (2014). Natur. Commun., 5, 3320.
11] Chung, C. M., Roh, Y. S., Cho, S. Y., & Kim, J. G. (2004). Chem. Mater., 16(21), 3982–3984.
12] Kawatsuki, N., Yamashita, A., Kondo, M., Matsumoto, T., Shioda, T., Emoto, A., & Ono, H. (2010).
Polymer., 51, 2849–2856.
[
[
[
[
13] Nagata, A., Sakaguchi, T., Ichihashi, T., Miya, M., & Ohta, K. (1997). Macro. Rapid. Commun., 18,
1
91–196.
14] Chaplanova, Z. D., Murauski, A. A., Rogachev, A. A., Agabekov, V. V., & Gracheva, E. A. (2013).
J. Appl. Spect., 80, 658–662.
15] Seki, T., Sakuragi, M., Kawanishi, Y., Tamaki, T., Fukuda, R., Ichimura, K., & Suzuki, Y. (1993).
Langmuir, 9, 211–218.
16] (a) Viswanathan, N. K., Kim, D. Y., Bian, S., Williams, J., Liu, W., Li, L., Samuelson, L., Kumar,
J., & Tripathy, S. K. (1999). J. Mater. Chem., 9, 1941. (b) Seki, T. (2007). Curr. Opin. Solid. State.
Mater. Sci., 10, 241–248.
[17] Sung, J. H., Hirano, S., Tsutsumi, O., Kanazawa, A., Shiono, T., & Ikeda, T. (2002). Chem. Mater.,
1
4, 385–391.
[18] Rusu, G. I., Airinei, A., Rusu, M., Prepelita, P., Marin, L., Cozan, V., & Rusu, I. I. (2007). Acta.
Mater., 55, 433–442.
[
[
[
[
[
[
[
[
19] Marin, L., Cozan, V., Bruma, M., & Grigoras, V. C. (2006). Eur. Polym. J., 42, 1173–1182.
20] Singh, S. K., Vikram, K., & Singh, B. (2011). Liq. Cryst., 38(9), 1117–1129.
21] Czajkowski, M., Dradrach, K., Bartkiewicz, S., & Galewski, Z. (2013). Opt. Mater., 35, 2449–2455.
22] Selvarasu, C., & Kannan, P. (2016). J. Mol. Struct., 1125, 234–240.
23] Gray, G. W., & Harrison, K. J. (1971). Mol. Cryst. Liq. Cryst., 13, 37–60.
24] Dave, J. S., & Vora, R. A. (1974). Mol. Cryst. Liq. Cryst., 28, 269–273.
25] Kihara, H., & Tamaoki, N. (2006). Macro. Rapid. Commun., 27, 829–834.
26] Pintre, I. C., Serrano, J. L., Blanca Ros, M., Martinez-Perdiguero, J., Alonso, I., Ortega, J., Folcia,
C. L., Etxebarria, J., Alicante, R., & Villacampa, B. (2010). J. Mater. Chem., 20, 2965–2971.
27] (a) Yang, P. C., & Liu, J. H. (2008). J. Poly. Sci. Part A: Poly. Chem., 46, 1289–1304. (b) Bobrovsky,
A., Shibaev, V., Hamplova, V., Kaspar, M., Novotna, V., Glogarov´a, M., & Pozhidaev, E. (2009).
Liq. Cryst., 36, 989–997.
[
[
[
[
[
[
28] Selvarasu, C., & Kannan, P. (2015). J. Mol. Struct., 1092, 176–186.
29] Yang, P. C., Li, C. Y., Wu, H., & Chiang, J. C. (2012). J. Taiwan. Inst. Chem. Eng., 43, 480–490.
30] Sadashiva, B. K., Ksthuraiah, N., Krishnaprasad, S., & Nair, G. G. (1998). Liq. Cryst., 24, 639–645.
31] Vora, R. A., & Rajput, S. J. (1991). Mol. Cryst. liq. Cryst., 209, 265–277.
32] Perin, D. D., & Aramarego, W. L. F. (1998). Purification of Laboratory Chemicals, Pergamon Press:
New York.
[33] Marin, L., Destri, S., Porzio, W., & Bertini, F. (2009). Liq. Cryst., 36, 21–32.
34] Rahman, L., Kumar, S., Tschierske C., Israel, G., Ster, D., & Hegde, G. (2009). Liq. Cryst., 36(4),
[
3
97–407.