Effect of azo and ester linkages on rod shaped Schiff base liquid crystals and their photophysical investigations

Article abstract of DOI:10.1016/j.molstruc.2016.06.081

Two new series of rod shaped Schiff base containing liquid crystal compounds with azo and ester linkages have been synthesized and characterized respectively. The rod like molecules containing cinnamate linkages with four different alkyl spacers (n?=?6, 8

Full text of DOI:10.1016/j.molstruc.2016.06.081

Accepted Manuscript
Effect of azo and ester linkages on rod shaped Schiff base liquid crystals and their
photophysical investigations
Chinnaiyan Selvarasu, Palaninathan Kannan
PII:
S0022-2860(16)30668-8
10.1016/j.molstruc.2016.06.081
MOLSTR 22700
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 13 April 2016
Revised Date: 28 June 2016
Accepted Date: 28 June 2016
Please cite this article as: C. Selvarasu, P. Kannan, Effect of azo and ester linkages on rod shaped
Schiff base liquid crystals and their photophysical investigations, Journal of Molecular Structure (2016),
doi: 10.1016/j.molstruc.2016.06.081.
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ACCEPTED MANUSCRIPT
Effect of azo and ester linkages on rod shaped Schiff base liquid crystals and their
Photophysical investigations
Chinnaiyan Selvarasu and Palaninathan Kannan*
Department of Chemistry, Anna University, Chennai, 600 025, India.
Graphical Abstract

ACCEPTED MANUSCRIPT
Effect of azo and ester linkages on rod shaped Schiff base liquid crystals and their
Photophysical investigations
Chinnaiyan Selvarasu and Palaninathan Kannan*
Department of Chemistry, Anna University, Chennai, 600 025, India.
Abstract
Two new series of rod shaped Schiff base containing liquid crystal compounds with azo and
ester linkages have been synthesized and characterized respectively. The rod like molecules
containing cinnamate linkages with four different alkyl spacers (n = 6, 8, 10 and 12) and
influence of linking group have been elucidated. Considerable changes in mesomorphic
properties were noticed starting from Nematic to Smectic-C on changing of azo and ester
linkages along with different terminal alkyl chain lengths. The mesomorphic properties of
both series are compared. Photosensitive azobenzene group undergoes photoisomerization
under UV light and monitored by UV-Visible spectroscopy.
Keywords: Cinnamate ester, calamitic liquid crystal, azobenzene, photoisomerization,
Schiff base
*Corresponding author. Tel: +91 44 22358666, fax: +91 44 2220 0660.
E-mail address: pakannan@annauniv.edu (P. Kannan).

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1. Introduction
Azobenzene based liquid crystalline material have been frequently discussed for their
sensitivity of chromophoric group towards light and found to exhibit interesting optical
properties, enable us to study the materials in holography, optical storage device, optical
switching and establish widespread applications in display technology [1–5]. By virtue of
their fluidity and translational periodicity, liquid crystal molecules are more easily induced
into a new arrangement when triggered by an electric field, [6,7] temperature, [8–10] and
light [11–16]. Photo-induced phenomenon is one of the research domain which include
incident light brings about molecular ordering and disordering of liquid crystalline system
[17]. The designing of liquid crystalline compounds containing double bond as an additional
functional group in their molecular structure brings special features. The double bond present
in cinnamic acid has already been of research interest in the domain of liquid crystals [18].
Thus upon bringing a double bond in the skeletal frame work enables the compound to have
mesomorphic properties, [19] photoisomerization and photocrosslinking in the liquid
crystalline state [20].
The rigid skeleton of rod like azobenzene structure and ester linkage aligns
themselves as suitable candidates for exhibiting liquid crystal properties [21-24].
Nevertheless, the inimitable characteristics of azobenzene molecules provide prospect of
molecular motion in response to heat or light and thus offer many chances in photonic
applications. The azo moiety is usually incorporated into cinnamate ester backbone to form
liquid crystal depending upon the terminal substituents of azobenzenes [25]. Azobenzene
molecules are known to exhibit reversible photoisomerization transformations upon
irradiation with UV-visible light [26]. Azo linkage based low and high molecular weight
liquid crystalline materials have received attention ascribed to their unique photo-switchable
properties induced by light [27–30]. This reveals that azo-cinnamate ester with Schiff base
linkage in the skeleton of compound have advantages over substances with other linkages
such as ester and silyl group. Several studies reported on ester with Schiff base containing
benzene and naphthalene core on liquid crystals. Dave and Prajapati [31] have synthesized
rod like liquid crystal materials for number of Schiff base homologous series containing
naphthalene moiety. Recently, Prajapati et al have also reported a few mesogenic
homologous series of Schiff base esters [32] and azo-esters [33] based rod like mesomorphic
nature of naphthalene group.

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In the present work, on attempt has been made to study the mesophase stabilities of
azomethine-azo and azomethine-ester as part of the frame work which hitherto unreported in
the literature. This led us to prepare such compounds that would help further in understanding
the effect of central linkage and spacer length on mesomorphism. Herein, the Schiff base
containing two series of molecules consist of linking group azo and ester respectively
connected to terminal alkyl chains with even number of carbon atoms (6, 8, 10 and 12).
Furthermore, this observation assumes significance from the fact that introduction of azo
linkage in the first series involve in a new dimension to this field, namely photochromism.
These cinnamate ester based azobenzene groups undergo photochemical process in
tetrahydrofuran also discussed. The two series have rod shaped cinnamate molecules bearing
one side alkyl chain length with azomethine-azo and ester- azomethine linkages respectively
with different chain length. Being geometrically closely related to known rod shaped
molecules, upon introduction of cinnamate group, containing azo and ester linkages, these
systems provided improved liquid crystalline property.
The mesomorphic behaviour was controlled mainly by length of alkyl chain and olefinic
character containing cinnamate group. The amine containing azo and ester linking group
connected to central core of cinnamate linkage to form Schiff base and changes mesomorphic
textures with unique sequence of Nematic and Smectic phases have been noticed. Structural
parameters of different calamitic phases were analysed and compared. The synthesis of rod
shaped liquid crystal compounds is depicted in Scheme 1.

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Scheme 1: Synthesis of rod shaped liquid crystal compounds.
2. Experimental
2.1 Techniques and Materials
All the chemicals and solvents were obtained from commercial sources. The crude samples
were purified by column chromatography using silica gel (400 mesh). Thin layer
chromatography was performed on aluminium sheets pre-coated with silica gel (Merck,
Kieselge 60 and F254). The infrared spectra of compounds were recorded using KBr disk in
1
Perkin Elmer FT-IR spectrometer. H (400 MHz) and 13C NMR (100 MHz) spectra were
performed on a Bruker spectrometer using TMS as an internal standard. Compositions of the
compounds were determined by Heraeus CHN elemental analyzer. The enthalpies of
transitions were determined from thermogram recorded on a differential scanning calorimeter
on heating/cooling run at a rate of 5 °C min-1. The transition temperatures for the compounds
were determined using a Linkem HFS91 hot stage and controlled processor in conjunction
with Linkem HFS91polarizing optical microscope with a TP-93 temperature programmer
connected to 1000D camera. Samples were placed in between two thin glass cover slips and
melted with heating and cooling at the rate of 2 °C min-1. UV absorption spectra were

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recorded on a UV-1650PC UV-Vis absorption spectrophotometer (Shimadzu). The
chloroform solution was prepared with concentration of 1x10-5 M.
2.2 Synthesis of 4-formylphenyl cinnamate (2)
The cinnamoyl acid chloride (1) (5 gm, 3.37 mmol) was added drop wise to the reaction
mixture containing 4-hydroxybenzaldehyde (4.12 gm, 3.37 mmol) and triethylamine (3.35
mmol) dissolved in dry THF and stirred for 24 h. After completion of reaction, triethylamine
hydrochloride was precipitated out and removed by filtration. The crude product was
subjected to column chromatography (hexane:ethylacetate (95:5%) to obtain colorless
compound 2. Yield: 77%. M.p: 120 °C. Anal. Calcd for C16H12O3: C, 76.18; H, 4.79. Found
(%): C 75.72, H 4.21. FT-IR (KBr pellet, cm-1): 2856–2921 cm−1 (C–H str. aliphatic), 1727
cm−1 (C=O str. of ester), 1608 cm−1 (–C=C– str. vinyl group of cinnamate), 1494 cm−1 (C=C
str. of aromatic), 1255 – 1023 cm−1 (C–O–C str. of alkoxy), 1102 cm−1 (C–O str. of ester).1H
NMR (400 MHz, CDCl3) δ 9.90 (s, 1H, CHO), 7.86(d, 2H J = 8.6 Hz, aromatic-H), 7.84 (d,
2H, J = 8.6 Hz, aromatic-H), 7.83 (d, 2H, J = 8.3 Hz, aromatic-H), 7.79 (d, 2H, J = 8.5 Hz,
aromatic-H), 7.31- 7.28 (d, 1H, J = 8.1 Hz, aromatic-H), 7.51 (d, J = 14.2 Hz, 1H, olefinic-
H), 6.56- 6.52 (d, J = 14.4 Hz, 1H, olefinic- H). 13C NMR (100 MHz, CDCl3) δ 115.54,
121.38, 127.39, 128.03, 130.19, 132.88, 146.51, 154.54, 163.59, 189.92.
2.3 Synthesis of 4-(((E)-4-(cinnamoyloxy)benzylidene)amino)phenyl 4-(hexyloxy)
benzoate (4a)
4-Aminophenyl 4-(hexyloxy)benzoate (3)(0.1 gm, 0.39 mmol) and 4-formylphenyl
cinnamate (2, 0.15 gm, 117 mmol) were mixed with ethanol (20 ml), one drop of TFA was
added and refluxed for 24 h. The solid obtained after evaporation of chloroform was
recrystallized from ethanol. The similar procedure was adapted to synthesis other alkyl
compounds (4b, 4c and 4d; Scheme 1) with yield: 57 %. Anal. Calcd for C34H33N3O3: C,
76.81; N, 7.90; H, 6.26. Found (%): C, 75.72; N, 7.07; H, 5.69. FT-IR (KBr pellet, cm-1):
2852–2914 cm−1 (C–H str. aliphatic), 1732 cm−1 (C=O str. of ester), 1605 cm−1 (–C=C– str.
vinyl group of cinnamate), 1472 cm−1 (N=N), 1496 cm−1 (C=C str. of aromatic), 1254 – 1027
cm−1 (C–O–C str. of alkoxy), 1103 cm−1 (C–O str. of ester). 1H NMR (400 MHz , CDCl3): δ
8.48 (s, Ar-CH=N-, 1H), 7.92 – 7.89 (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), 7.48 (d, J = 12.7 Hz, olefinic-H, 1H), 6.61 (d, J =
11.6 Hz, olefinic-H, 1H),4.07 – 3.95 (t, 2H, J = 8.1 Hz, -OCH2), 1.86 – 1.74 (d, 2H, aliphatic-

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CH2), 1.31 – 1.06 (m, aliphatic-CH2, 8H), 0.91 – 0.86 (t, 3H, J = 6.6 Hz, aliphatic-CH3).13C
NMR (100 MHz, CDCl3): 14.3, 22.5, 26.6, 28.6, 29.2, 31.7, 69.7, 115.3, 121.7, 123.5, 124.4,
128.4, 129.3, 131.6, 144.4, 147.6, 151.4, 155.7, 162.5, 163.2, 168.4.
4b: Yield: 61 %. Anal. Calcd for C36H37N3O3: C, 77.25; N, 6.66; H, 7.51. Found (%): C,
76.83; N, 6.11; H, 6.82. FT-IR (KBr pellet, cm-1): 2849–2914 cm−1 (C–H str. aliphatic), 1729
cm−1 (C=O str. of ester), 1607 cm−1 (–C=C– str. vinyl group of cinnamate), 1478 cm−1
(N=N), 1502 cm−1 (C=C str. of aromatic), 1252 – 1025 cm−1 (C–O–C str. of alkoxy), 1108
cm−1 (C–O str. of ester). 1H NMR (400 MHz , CDCl3): δ 8.48 (s, Ar-CH=N-, 1H), 7.93 – 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), 7.48 (d, J = 12.3 Hz, olefinic-H, 1H), 6.52 (d, J = 11.9 Hz, olefinic-H, 1H), 4.05 – 3.96
(t, 2H, J = 8.3 Hz, -OCH2), 1.89 – 1.72 (d, 2H, aliphatic-CH2), 1.37 – 1.03 (m, aliphatic-CH2,
12H), 0.96 – 0.94 (t, 3H, J = 6.8 Hz, aliphatic-CH3).13C NMR (100 MHz, CDCl3): 14.2, 22.5,
26.6, 28.4, 29.2, 31.5, 69.7, 115.3, 121.6, 123.4, 124.7, 128.3, 129.8, 131.4, 144.8, 147.4,
151.2, 155.3, 162.7, 163.3, 168.2.
4c: Yield: 63 %. Anal. Calcd for C38H41N3O3: C, 77.65; N, 7.15; H, 7.03. Found (%): C,
77.15; N, 7.01; H, 6.81. FT-IR (KBr pellet, cm-1): 2847–2919 cm−1 (C–H str. aliphatic), 1739
cm−1 (C=O str. of ester), 1603 cm−1 (–C=C– str. vinyl group of cinnamate), 1477 cm−1
(N=N), 1498 cm−1 (C=C str. of aromatic), 1254 – 1025 cm−1 (C–O–C str. of alkoxy), 1108
cm−1 (C–O str. of ester).1H NMR (400 MHz , CDCl3): δ 8.52 (s, Ar-CH=N-, 1H), 7.92 – 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), 7.48 (d, J = 12.2 Hz, olefinic-H, 1H), 6.49 (d, J = 11.8 Hz, olefinic-H, 1H), 4.06 – 3.97
(t, 2H, J = 8.4 Hz, -OCH2), 1.86 – 1.79 (d, 2H, aliphatic-CH2), 1.35 – 1.09 (m, aliphatic-CH2,
16H), 0.98 – 0.96 (t, 3H, J = 6.8 Hz, aliphatic-CH3).13C NMR (100 MHz, CDCl3): 14.1, 22.8,
26.6, 28.6, 29.2, 31.5, 69.4, 115.6, 121.4, 123.4, 124.6, 128.3, 129.2, 131.5, 144.5, 147.3,
151.6, 155.4, 162.6, 163.2, 168.2.
4d: Yield: 61 %. Anal. Calcd for C40H45N3O3: C, 78.02; N, 6.82; H, 7.37. Found (%): C,
77.85; N, 5.62; H, 7.12. FT-IR (KBr pellet, cm-1): 2848–2915 cm−1 (C–H str. aliphatic), 1735
cm−1 (C=O str. of ester), 1601 cm−1 (–C=C– str. vinyl group of cinnamate), 1473 cm−1
(N=N), 1499 cm−1 (C=C str. of aromatic), 1250 –1022 cm−1 (C–O–C str. of alkoxy), 1106
cm−1 (C–O str. of ester). 1H NMR (400 MHz , CDCl3): δ 8.51 (s, Ar-CH=N-, 1H), 7.94 – 7.92

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(d, J = 7.8 Hz, Aromatic-H, 2H), 7.91 (d, J = 7.54 Hz, Aromatic-H, 6H), 7.44 - 7.33 (m, 5H,
aromatic-H), 7.17 (d, J = 7.43 Hz, Aromatic-H, 2H), 7.13 - 7.11(d, J = 7.43 Hz, Aromatic-H,
2H), 7.50 (d, J = 12.5 Hz, olefinic-H, 1H), 6.60 (d, J = 11.7 Hz, olefinic-H, 1H), 4.02 – 3.99
(t, 2H, J = 8.2 Hz, -OCH2), 1.81 – 1.75 (d, 2H, aliphatic-CH2), 1.32 – 1.00 (m, aliphatic-CH2,
20H), 0.99 – 0.97 (t, 3H, J = 6.4 Hz, aliphatic-CH3).13C NMR (100 MHz, CDCl3): 14.0, 22.9,
26.4, 28.7, 29.0, 31.6, 69.6, 115.5, 121.7, 123.3, 124.5, 128.6, 129.1, 131.3, 144.7, 147.7,
151.5, 155.1, 162.4, 163.0, 168.0.
2.4 Synthesis of 4-((E)-(4-(cinnamoyloxy)benzylidene)amino)phenyl 4-(hexyloxy)
benzoate (5a): A similar synthetic procedure (4a) was employed in the second series for the
preparation of 4-((E)-(4-(cinnamoyloxy)benzylidene)amino)phenyl 4-(hexyloxy)benzoate
(5a) by changing the reactant to 4-aminophenyl 4-(hexyloxy)benzoate (3a). The similar
procedure was adapted to synthesis other alkyl compounds (5b, 5c and 5d; Scheme 1) with
yield: 57 %. Anal. Calcd for C35H33NO5: C, 76.76; N, 2.56; H, 6.07 %; Found (%): C, 76.22;
N, 2.02; H, 5.58. FT-IR (KBr pellet, cm-1): 2847–2922 cm−1 (C–H str. aliphatic), 1729 cm−1
(C=O str. of ester), 1606 cm−1 (–C=C– str. vinyl group of cinnamate), 1494 cm−1 (C=C str. of
aromatic), 1254 – 1028 cm−1 (C–O–C str. of alkoxy), 1106 cm−1 (C–O str. of ester).1H NMR
(400 MHz , CDCl3): δ 8.70 (s, Ar-CH=N-, 1H), 8.12 – 8.04 (d, J = 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, 2H, J = 8.2 Hz, OCH2),
1.74 – 1.72 (d, 2H, aliphatic-CH2),1.52– 1.21 (m, 6H, aliphatic-CH2), 1.02 – 0.98 (t, 3H,
J = 6.7 Hz, aliphatic-CH3).13C NMR (100 MHz, CDCl3): 14.1, 22.8, 26.5, 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, 134.5, 135.8,
144.6, 146.4, 150.4, 155.2, 163.9, 167.1, 168.1.
5b: Yield: 62 %. Anal. Calcd for C37H37NO5: C, 77.19; N, 2.43; H, 6.48 %. Found (%): C,
76.82; N, 2.16; H, 6.12. FT-IR (KBr pellet, cm-1): 2856–2925 cm−1 (C–H str. aliphatic), 1731
cm−1 (C=O str. of ester), 1606 cm−1 (–C=C– str. vinyl group of cinnamate), 1499 cm−1 (C=C
str. of aromatic), 1254 – 1026 cm−1 (C–O–C str. of alkoxy), 1102 cm−1 (C–O str. of ester). 1H
NMR (400 MHz , CDCl3): δ 8.68 (s, Ar-CH=N-, 1H), 8.12 – 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), 7.47 (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, 2H, J = 8.4 Hz,
OCH2), 1.76 – 1.72 (d, 2H, aliphatic-CH2), 1.50– 1.24 (m, 10H, aliphatic-CH2), 1.02 – 0.98

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(t, 3H, J = 6.7 Hz, aliphatic-CH3). 13C NMR (100 MHz, CDCl3): 14.2, 22.7, 26.6, 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, 134.5, 135.6,
144.6, 146.5, 150.4, 155.2, 163.8, 167.2, 168.1.
5c: Yield: 57 %. Anal. Calcd for C39H41NO5: C, 77.59; N, 2.32; H, 6.84 %. Found (%): C,
77.25; N, 1.67; H, 6.22. FT-IR (KBr pellet, cm-1): 2854 –2928 cm−1 (C–H str. aliphatic), 1725
cm−1 (C=O str. of ester), 1608 cm−1 (–C=C– str. vinyl group of cinnamate), 1494 cm−1 (C=C
str. of aromatic), 1262 – 1016 cm−1 (C–O–C str. of alkoxy), 1108 cm−1 (C–O str. of ester). 1H
NMR (400 MHz , CDCl3): δ 8.69 (s, Ar-CH=N-, 1H), 8.12 – 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, 2H, J = 8.4
Hz, OCH2), 1.74 – 1.70 (d, 2H, aliphatic-CH2), 1.51– 1.21 (m, 14H, aliphatic-CH2), 1.0
– 0.98 (t, 3H, J = 6.5 Hz, aliphatic-CH3). 13C NMR (100 MHz, CDCl3): 14.1, 22.6, 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, 132.3, 134.2,
135.6, 144.6, 146.4, 150.3, 155.2, 163.7, 167.2, 168.1.
5d: Yield: 54 %. Anal. Calcd for C41H45NO5: C, 77.94; N, 2.22; H, 7.18 %. Found (%): C,
77.23; N, 1.87; H, 7.02. FT-IR (KBr pellet, cm-1): 2853 – 2929 cm−1 (C–H str. aliphatic),
1729 cm−1 (C=O str. of ester), 1608 cm−1 (–C=C– str. vinyl group of cinnamate), 1502 cm−1
(C=C str. of aromatic), 1258 – 1029 cm−1 (C–O–C str. of alkoxy), 1107 cm−1 (C–O str. of
1
ester). H NMR (400 MHz , CDCl3): δ 8.71 (s, Ar-CH=N-, 1H), 8.11– 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, 2H), 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, 2H,
J = 8.3 Hz, OCH2), 1.76 – 1.71 (d, 2H, aliphatic-CH2), 1.51 – 1.23 (m, 18H, aliphatic-CH2),
1.01 – 0.99 (t, 3H, J = 6.4 Hz, aliphatic-CH3). 13C NMR (100 MHz, CDCl3): 14.0, 22.9, 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, 132.3,
134.5, 135.8, 144.7, 146.4, 150.5, 155.1, 163.9, 167.1, 168.0.
3. Results and discussion
3.1 Molecular structural characterization
4-(((E)-4-(cinnamoyloxy)benzylidene)amino)phenyl 4-(alkyloxy)benzoate (series I)
and 4-((E)-4-(cinnamoyloxy)benzylidene)amino)phenyl 4-(alkylxoy)benzoate (series II)

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comprising azo-Schiff base and ester-Schiff base as a central linkage respectively were
synthesized to investigate the influence of functional groups on Schiff base. In both series,
alkoxy chain is attached in one side in the terminal position. Their chemical structures are
1
confirmed by spectroscopic studies. H-NMR spectrum of compound 2 revealed that
aldehyde proton signal at 9.98 ppm and imine proton in the Schiff base (4d) in signal in the
range of 8.38–8.45 ppm were observed. 13C NMR spectrum of compound 2, the aldehyde
exhibits a resonance signal at 189.92 ppm and in the Schiff base 4d; the imine exhibits a
resonance signal at 163.96 ppm. The presence of intermediate compound having aldehyde
and imine group of Schiff base compound in the similar structure of 5d were confirmed by
Fourier transform infrared (FT-IR) spectroscopy, since in each case the band characteristic of
-CH=N- stretching deformations was detected. The exact position of this band varies in the
spectral range of 1601–1620 cm-1 (see experimental details). In accordance with NMR and
FT-IR data collected in the experimental section, spectral data obtained are in agreement with
data predicted from chemical formulae of synthesized compounds. Purity of all imines
compounds was confirmed by elemental analysis. Elemental analysis indicated quite good
agreement with calculated values of carbon, nitrogen and hydrogen in the compounds.
3.2 Mesomorphic properties
The mesomorphic properties of all the synthesized compounds have been
characterized by differential scanning calorimetry and polarizing optical microscopy attached
with a hot stage. Thin-film samples were obtained by sandwiching them between a glass
slides with cover slip. All the compounds of I and II series exhibited mesomorphism.
On cooling, isotropic liquids of I and II series compounds with n= 6 formed small droplets
that coalesced to classical schlieren texture of Nematic phase while, n= 8 and 10 derivatives
exhibited focal–conic texture, characteristic of Nematic phases. On increasing chain length
(n=12), they display Nematic and focal conic texture characteristic of Smectic-C mesophase
when cooled from isotropic liquid. The Nematic mesophase prevails ascribed to terminal
alkoxy chain attractions are more pronounced. The lower homologous members exhibit
purely Nematic mesophase. The I and II series have cinnamoyloxy connected with azo and
ester central linkage in addition to double bond that increases length and polarizability. The
presence of double bond leads to form rod like molecules and enhances mesophase thermal
stability. Therefore the greater mesophase thermal stability of the present I and II series can
be explained in terms of greater molecular length and polarizability of molecule resulting
from additional –CH=CH- units in the central linkage.

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It is inferred that as the series is ascended, the overall polarizability is enhanced with
increase in alkyl chain length [34]. The ratio of terminal attractions to lateral attractions
decreases as the series is descended; the net result is that the lateral attractions gain
prevalence over the terminal attractions. The molecules under such conditions will observe,
strictly maintaining the layered structure, although they might partially attain a fluid
condition. Under such situations, the layered Smectic texture is maintained, giving rise to a
Smectic mesophase. As the temperature is further increased, the strict performance of
molecules in the layered form is weakened ascribed to increased thermal vibrations. The
molecules slowly acquire a more fluid nature, although they do not abruptly attain any
randomness. At a definite temperature, the layered structure breaks down, but end-to-end
parallel orientation is still maintained, giving rise to more fluidity and a Nematic threaded
texture to the molecules. Smectic-C mesophase is exhibited from decyloxy derivative
onwards. The overall polarizability of the molecules is enhanced as the alkyl chain length at
one end increases, contributing to the exhibition of Smectic texture [35]. However, ratio of
terminal to lateral attractions is also low at a certain stage, leading to thermal vibration break
down the orientation of molecules without giving them a chance for adopting Nematic
orientation. Thus, only a Smectic-C mesophase prevails in dodecyloxy derivative. DSC is a
valuable tool for detection of phase transition temperature. It yields quantitative results;
therefore conclusions may be drawn concerning the nature of phases which participate in the
transition. The 4a-4d and 5a-5d compounds consist of aromatic ring depend upon the I - II
series including cinnamate as central core for the rod shaped molecule.
Figure 1. Microphotographs texture observed for different phases for compound 4a.
a) Schlieren texture of Nematic phase at 249 °C for heating (b & c) Nematic droplet and
Nematic phase on cooling at 275 °C and 254 °C.
The DSC data of 4d, 5d were recorded in the first heating cycle as a representative example
for each series and investigated in detail. The phase transition temperatures and
corresponding enthalpy changes of compounds were determined using DSC. The data

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obtained from DSC analysis are summarized in Table 1, which helps us to further confirm
mesophase type. The calamitic liquid crystal textures of compounds were observed on
polarized optical microscope in the range of 35 – 295 °C upon heating and cooling cycle at
20X magnification.
Figure 2. Microphotographs of texture observed for compound 4d. (a) Schlieren texture of
Nematic phase on heating at 245 °C. (b & c) Cholesteric fingerprint region and Sm-C phases
at 255 and 186 °C upon cooling.
The peak temperatures were consistent with those inferred from optical experiments.
Normally, calamitic liquid crystals of lower alkyl chain are Nematic, the medium length of
alkyl chain exhibit Nematic and Smectic phases; on the other hand, higher members are
entirely Smectic-C in nature [36]. Upon heating and cooling 4a exhibits two transitions, in
which transitions from crystal to Nematic and Nematic to isotropic occurs while heating to
285 °C and reverse transitions occur on cooling to 169 °C, respectively as displayed in Fig.1.

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Figure 3. Microphotographs texture observed for liquid crystalline textures for compound 5a.
(a & b) Nematic phase at 249 and 266 °C on heating (c & d) Nematic droplet and Nematic
phase on cooling at 271 °C and 260 °C.
The 4d revealed two phases on heating which corresponds to crystal-Nematic and Nematic–
isotropic phase to 275 °C while cooling from isotropic, Nematic droplet was observed and
further cooling, Cholesteric finger print region were exhibited at 255 °C and promote cooling
a typical fan shaped texture for Smectic-C phase was observed at 186 °C as exhibited in
Fig.2.
Table 1. Phase transition temperatures (T/°C) and enthalpies (∆H/J g−1) obtained from
DSC scans of compounds 4a-4d and 5a-5d.
Compound
Heating
Cooling cycle
Cr 143.9 (15.5) N 285.4 (2.21) I
Cr 140.1 (21.75) N 281.7 (2.13) I
Cr 137.7 (29.54) N 278.1 (1.62) I
Cr 135.4 (32.51) N 275.2 (1.36) I
Cr 152.1 (19.75) N 296.6 (0.29) I
Cr 148.3 (21.42) N 289.9 (0.38) I
Cr 145.8 (26.14) N 279.3 (0.54) I
Cr 139.2 (30.73) N 275.4 (0.73) I
I 281.7 (-0.2) N 169.9 (-21.5) Cr
4a
4b
4c
4d
5a
5b
5c
5d
I 277.2 (-0.1) N 162.6 (-25.7) Cr
I 273.7 (-0.5) N 152.4 (-29.2) Cr
I 270.4 (-0.4) N 266.6 (2.45) SmC 148.7 (-33.5) Cr
I 282.5 (-0.1) N 172.6 (-37.4) Cr
I 283.3 (-0.6) N 169.2 (-27.5) Cr
I 276.2 (-0.4) N 158.5 (19.5)Cr
I 270.4 (-0.3) N 249.6 (-19.7) SmC 151.7 (18.3) Cr
Abbreviations: Cr = Crystal, SmA= Smectic A, N = Nematic, Sm-C = Smectic-C, I = Isotropic

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The compound 5a revealed two endo and exo peaks during first heating and cooling. On
cooling the phase sequence found as Crystal–Nematic and Nematic–isotropic while reverse
phase occur on cooling as evidenced in Fig. 3 (a–d).
Figure 4. Microphotograps of texture observed for compound 5d. (a & b) Nematic phases
exhibited on heating at 268 °C (b & c) Cholesteric fingerprint region and SmC phases at 256
and 196 °C upon cooling.
The 5d exhibited Nematic phase for heating at 275 °C as indicated in Fig 4 (a & b). On
cooling from isotropic exhibits Nematic to Cholesteric finger print region and Smectic-C
phase were observed in Fig.4 (c & d) and crystallized at 122 °C. The remaining lower
members of 4b, 4c and 5b, 5c exhibit from similar observation of Nematic mesophase with
wide transition temperature ascribed to flexibility of increased alkyl chain length whereas the
phase transition sequence from cooling as Iso–Nematic–Cr. The 5d exhibit Smectic-C texture
because of increased alkyl chain length (n=12) and revealed that crystal mesophase transition
temperatures decrease with increase in length of terminal alkyl chain. The Nematic-isotropic
transition temperatures also decrease 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.

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3.3 Photophysical property
Absorption spectra of compound were obtained and their absorption band summarized in
Table ST1 and Fig. 5. The absorption spectra of compounds was carried out in
tetrahydrofuran solution (C = 1.1×10-3 mol L-1). The compounds were irradiated with UV
light (365 nm), the band corresponding to π–π* and n–π* transitions decreases with increase
in irradiated time. [25] The band corresponding to isomerized product increases by increasing
irradiating time and exhibited in Fig.5. The photoisomerization has confirmed and occurred
in the range of 380–550 nm.
Figure 5. Absorption spectrum of compound 5a
The 5a irradiated at 365 nm, E–Z isomerization resulting with a dramatic decrease in
absorption band from 382 nm ascribed to π–π* transition of trans (E) isomer in azo group. In
addition, very weak absorption band is found in the visible region around 505 nm,
appropriate to n–π* transition of cis (Z) isomer in azo group which gradually increased by
reaching photo stationary state within 25 s. After 25 s 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 470 nm, corresponding to trans-cis isomerization as
these suggesting that only two isomers obtained. In Fig.5 another band absorption exhibit
compound containing cinnamate unit (_CH=CH-COO-) in double bond undergo
isomerization around 263 nm. At the same time, we have tried for reversible photo
isomerization under dark condition for several minutes in 5a with same concentration in
tetrahydrofuran solution. The transformed cis isomer (Z) is not converted into trans form (E)

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due to the imine (-CH=N-) group restricts electron delocalization on cinnamate (_CH=CH-
COO-) moiety.
3.4 Comparison of mesophase and thermal property
The azo and ester linkage in the Schiff base at para position in the I and II series has a
pronounced effect on mesophase transition temperatures. Series I where the central core is
connected to ester linkage demonstrates decrease in stability of the system when compared to
azo linkage in series II. Moving on further level study, ester linkage consist of ketonic bond
where electron density pulled by ester system in the central core that leads to decrease in
transition temperatures. On the other hand, in series II, the azo linkage in the central core
provides rich electron density towards entire system that enhances transition temperature. The
mesomorphic properties of entire rod like molecules exhibit good liquid crystalline texture
relative on alkyl chain length. The four-aromatic ring containing azo linkage compounds
have faster photo isomerization. This may be attributed to the nature of linking group in the
azobenzene as well as flexibility of ester moiety. The time taken for photoisomerization
depends on nature of linking group and size of substituent in the azobenzene moiety.
4. Conclusion
We have reported the synthesis and characterization of rod shaped liquid crystalline
Schiff base containing compounds with azo and ester linkages. The mesophase range of
present series I is higher than series II for those of structurally related compounds possess
ester and azo linkage and attributed to high polarizability of the molecules. The effect of
terminal alkoxy containing azobenzene moiety, central linkage with or without spacer
attached also plays a crucial role in formation of mesophase. The presence of olefinic double
bond enhances length of polarizability of rod shaped molecules and increases thermal
stability with high polarizability resulting from additional olefinic unit in the linkage. The
results provided the understanding of structure–property relationships in liquid crystals. The
structural aspects strongly enhance phase transition behaviour of Schiff’s base compounds.
Very high isotropic transition temperature of >296 °C was observed for azomethine
containing cinnamate esters. Interestingly the change in orientation of ester group in
lengthening the arm in compounds of series II led to decrease of both melting and clearing
temperatures. The azobenzene containing ester molecules involves isomerisation. The

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duration required for exhibiting the phenomena of photoisomerization mainly depends on
nature of the linking group in the azo moiety.
Acknowledgements
C.S and P.K sincerely acknowledge to University Grants Commission, New Delhi,
Government of India [39-691/2010 (SR)] for the financial support. Instrumentation facilities
provided under DST-FIST, UGC-DRS to Department of Chemistry, Anna University are
gratefully acknowledged.
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Research Highlights
ꢀ We have synthesized and characterized new rod shaped liquid crystalline
compounds.
ꢀ Azo and ester linkage affected the mesomorphism.
ꢀ All the compounds exhibited good LC mesophase depend upon the alkyl
chain length.
ꢀ Azo-Schiff base compounds exhibited photoisomerization.