Symmetric bent-core liquid crystals of some schiff bases containing azo linkage

Article abstract of DOI:10.5562/cca2150

The paper presents the synthesis and liquid crystalline properties of some new symmetric bentcore liquid crystals containing a 1,3-dihydroxybenzene central core disubstituted with mesogenic units consisting of three aromatic rings connected through esteric, azomethinic and azo groups. The wings are ended with terminal acyloxy or alkyloxy chains containing 6 to 10 carbon atoms. Compounds were obtained by the condensation reactions between some 4-((4- alcanoyloxyphenyl)azo)benzaldehydes or 4-((4- alkyloxyphenyl)azo)benzaldehydes with the 1,3-phenylene bis(4-aminobenzoate) core. The new synthesized compounds were characterized from structural point of view (¹H NMR, ¹³C NMR, MS spectroscopy) and liquid crystalline properties (polarizing optical microscopy and differential scanning calorimetry). All the banana shaped compounds presented enantiotropic liquid crystalline properties, with a wideranging stability of mesophases both on heating and cooling cycles. The synthesized compounds presented a good thermal stability in the existence range of the mesophases as evidenced by thermogravimetric studies.

Full text of DOI:10.5562/cca2150

CROATICA CHEMICA ACTA
CCACAA, ISSN 0011-1643, e-ISSN 1334-417X
I h k d ^ROATICA
1926^1
I HIM!'. A
Croat. Chem. Acta 87 (1) (2014) 7-16.
http://dx.doi.org/!0.5562/cca2150
Original Scientific Article
Symmetric Bent-core Liquid Crystals of Some Schiff Bases
Containing Azo Linkage
Catalina Ionica Ciobanu, Irina Carlescu, Gabriela Lisa, and Dan Scutaru*
Department ofNatural and Synthetic Polymers, Faculty o f Chemical Engineering and Environmental Protection,
“Gheorghe Asachi ” Technical University ofIasi, Bd. D. Mangeron 71A, 700050 Iasi, Romania
RECEIVED JULY 27,2012; REVISED OCTOBER 14,2013; ACCEPTED OCTOBER 21, 2013
Abstract. The paper presents the synthesis and liquid crystalline properties of some new symmetric bent-
core liquid crystals containing a 1,3-dihydroxybenzene central core disubstituted with mesogenic units
consisting of three aromatic rings connected through esteric, azomethinic and azo groups. The wings are
ended with terminal acyloxy or alkyloxy chains containing 6 to 10 carbon atoms. Compounds were ob-
tained by the condensation reactions between some 4-((4-alcanoyloxyphenyl)azo)benzaldehydes or 4-((4-
alkyloxyphenyl)azo)benzaldehydes with the 1,3-phenylene bis(4-aminobenzoate) core. The new synthe-
sized compounds were characterized from structural point of view ('H NMR, 13C NMR, MS spectrosco-
py) and liquid crystalline properties (polarizing optical microscopy and differential scanning calorimetry).
All the banana shaped compounds presented enantiotropic liquid crystalline properties, with a wide-
ranging stability of mesophases both on heating and cooling cycles. The synthesized compounds present-
ed a good thermal stability in the existence range of the mesophases as evidenced by thermogravimetric
studies, (doi: 10.5562/cca2150)
Keywords: Schiff Bases, symmetric bent-core compounds, mesomorphic properties, azo linkage
INTRODUCTION
Bi, B2 and B6 are the most commonly observed on ba-
nana shaped compounds.1114 The smectic liquid crystal-
line phase separates from isotropic liquid in a rich varie-
ty of forms, from classical “batonnets” to filamentary
shapes or helical coils in lyotropic systems. From these,
more frequently are elongated structures consisting of
focal-conic domains.
Probably the most interesting mesophases exhibit
the bent-core compounds containing the thermally sen-
sitive azomethine group. Among rod-shaped mesogens,
the Schiff bases tend to order into smectic phases. Oth-
erwise the first smectic mesophases of banana shaped
molecules had azomethine connecting group between
aromatic rings.15
The first reported bent-core compounds containing
azo linkage into structure has been reported since 2001
and contains six aromatic rings.16 Since then, some
other bent shaped molecules containing azobenzene
moieties have been synthesized, the majority of these
molecules presenting interesting liquid crystalline prop-
erties.16-24
The goal of the paper is to extent the classes of
bent shaped molecules containing the azo photo-
isomerisable group. The paper presents the synthesis of
some bent-core liquid crystals based on a 1,3-dihydroxy-
In the last period, bent-core molecules attracted consid-
erable interest because of their capability to generate
new mesophases with non-conventional properties.
Compared to classical calamities, structure-property
relationships in banana shaped liquid crystals are more
difficult to predict. The main characteristic is the non-
linear (bent-shaped) structure achieved by connecting
two mesogenic groups to a central core such as 1,3-
disubstituted benzene, 2,7-disubstituted naphthalene,
2,5-disubstituted [l,3,4]oxadiazole, 2,5-disubstituted
thiophene, 2,6-disubstituted pyridine etc. Several hun-
dred of compounds having a bent molecular shape and
exhibiting characteristic mesophases have been reported
so far.
Up to now, eight phases formed by bent-core mol-
ecules and designated as Br B8 have been found and
most of them present additional sub-phases.7,8 The
shape of banana achiral mesogens cause steric interac-
tions that led to unique phases of special complexity
from columnar (B,) to smectic (B2, B3, Bć and B7) up to
nematic ordering.9 These “banana type phases” are
specific phases different from conventional phases
found in calamitic mesogens.10Among them, the phases
* Author to whom correspondence should be addressed. (E-mail: dscutara@ch.tuiasi.ro)

C. I. Ciobanu et al., Bent-core Liquid Crystals
8
diciclohexyl urea was filtered off and the solution was
concentrated. The product was purified by column
chromatography silicagel/dichloromethane. Yield: 5 g
(62 %), m.p. 185 °C. 'H NMR (CDC13) 8 / ppm: 8.38 (m,
8H, Ar), 7.55 (m, 1H, Ar), 7.25 (d, 1H, Ar, J= 1.96 Hz),
7.23 (dd, 2H, Ar, J, = 2.16 Hz, J2= 2.15 Hz), ,3C NMR
(DMSO-d6) 8 / ppm: 162.95, 151.08, 151.05, 134.55,
131.38, 130.32, 123.84, 119.50, 115.47 (-O -C O +
8 C, aromatic).
benzene core with two symmetric wings containing each
three benzene rings connected via esteric, azomethinic
and azo groups. One of the series ends with terminal
acyloxy chains while the other one contains alkyloxy
ones. The introduction of the -N=N—linkage contrib-
utes to the extension of conjugation between the aro-
matic rings and may induce photo-responsive properties
due to the capacity to generate trans-cis isomerization
under UV-VIS irradiation. All the obtained compounds
presented enantiotropic liquid crystalline properties,
mainly with smectic type textures.
1.3- phenylene bis(4-aminobenzoate) (2)
1 g (2.45 mmol) 1,3-phenylene bis(4-nitrobenzoate) (1)
was dissolved under reflux in 50 mL anhydrous ethanol
and 5.54 g (12.25 mmol) SnCl2-2H20 (as a solid) was
added in several portions. The mixture was heated under
stirring and reflux for 6 hours. After cooling, the reac-
tion mixture was poured in a beaker containing ice wa-
ter and treated with a solution of 5 % NaOH until the
pH = 8. The fine organic suspension was extracted sev-
eral times with ethyl acetate, the collected extracts were
washed several times with water, dried up with MgS04
and then concentrated. The purification of the product
was made by recrystallization from diethyl ether. Yield:
0.6 g (60 %), m.p. 192 °C. ]H NMR (DMSO-d6) 8 /
ppm: 7.82 (d, 4H, Ar, J= 8.70 Hz), 7.48 (m, 1H, Ar),
7.13 (m, 3H, Ar), 6.66 (d, 4H, Ar, J = 8.71 Hz), 6.20
(s, 4H, 2* -NH2), 13C NMR (DMSO-d6) 8 / ppm: 164.2,
154.14, 151.33, 131.81, 129.48, 119.04, 116.14, 114.08,
112.63 (—O—C=0 + 8 C, aromatic).
EXPERIMENTAL
Materials
Analytical reagents such as resorcinol, p-nitrobenzoic
acid, DCC, DMAP, were provided by Aldrich (Germa-
ny) or by Merck (Germany) and were used without
further purification. Synthesis of 3a-3e and 4a-4e com-
pounds was previously reported.25 Silica gel 60 (Merck)
was used for column chromatography (CC). The chro-
matography (TLC) was performed on silica gel plates
(Merck, silica gel F254).
Instrumentation
Confirmation of the structures of the intermediates and
final products was obtained by ‘H NMR and l3C NMR,
using a Bruker Avance DRX 400 MHz spectrometer
(Rheinstein, Germany) with tetramethylsilane as inter-
nal standard. Mass spectra were recorded on a quadru-
p le time-of-flight mass spectrometer equipped with an
electrospray ion source (Agilent 6520 Accurate Mass
Q-TOF LC=MS (Santa Clara, CA,USA).
Transition temperatures were determined using a
Linkam heating stage and Linksys 32 temperature con-
trol unit in conjunction with a Axioscop 40 Zeiss polar-
izing optical microscope and Qimaging/Retiga-IOOOR
camera for image capture, the transitions being con-
firmed by DSC analysis (Mettler Toledo DSC1). All
thermal analyses were performed on a Mettler-Toledo
TGA SDTA851e derivatograph in an N2 atmosphere,
with a flow rate of 20 mL min-1 and a heating rate of
10 °C min-1 from 25 °C to 900 °C. In order to obtain
comparable data, constant operational parameters were
kept for all samples.
General procedure for the synthesis of the SchiffBases
5a-5e and 6a-6e
1.3- Phenylene bis(4-aminobenzoate) (2) and the corre-
sponding 3a-3e or 4a-4e aldehydes were refluxed un-
der stirring, for two hours, in anhydrous ethanol con-
taining a few drops of glacial acetic acid as catalyst. The
obtained suspension was filtered while hot and washed
twice with ethanol. The resulting products were purified
by recrystallization from ethanol.
1.3- phenylene bis(4-(4-((4-hexanoyloxyphenyl)azo)ben-
zylideneamino)benzoate) (5a)
Quantities: 0.25 g (0.72 mmol) 1,3-phenylene bis(4-
aminobenzoate) (2) in 15 mL ethanol, 0.46 g (1.44
mmol) 4-((4-hexanoyloxyphenyl)azo)benzaldehyde (3a)
in 15 mL ethanol, glacial acetic acid (1-2 drops), yield:
0.35 g (51 %), liquid crystal: 169 °C (Ki/K2), 202 °C
(K2/LC), 315 °C (LC/I), 307 °C (I/LC), 193 °C (LC/K,),
166 °C (K,/K2). 'H NMR (CDC13) 8 / ppm: 8.53 (s, 2H,
2* -CH=N-), 8.25 (d, 4H, Ar, J = 8.55 Hz), 8.08 (d,
4H, Ar, J= 8.33 Hz), 8.01 (d, 4H, Ar, J= 8.77 Hz), 7.98
(d, 4H, Ar, J - 8.77 Hz), 7.48 (m, 1H, Ar), 7.29 (m, 9H,
Ar), 7.20 (dd, 2H, Ar, Jx = 8.33 Hz, J2 = 8.11 Hz),
2.58 (t, 4H, 2* -0-CO-CH2-), 1.80 (qv, 4H, 2* -CH2-),
1.43 (m, 8H, 4* -CH2-), 0.95 (t, 6H, 2* -CH3),
,3C NMR (CDCI3) 8 / ppm: 171.73, 164.46, 160.77,
156.88, 154.72, 153.53, 151.81, 150.42, 137.87, 131.65,
130.03, 128.78, 126.99, 124.35, 123.37, 122.33, 120.97,
Synthesis
1,3-phenylene bis(4-nitrobenzoate) (1)
To a solution of 2 g (18.18 mmol) resorcinol in 30 mL
anhydrous CH2C12, 6.07 g (36.36 mmol) 4-nitrobenzoic
acid and a catalytic amount of DMAP as catalyst were
added under stirring. Over the mixture a solution of 8.25
g (40 mmol) DCC in 50 mL anhydrous CH2C12 was
added. After stirring overnight at room temperature, the
Croat. Chem. Acta 87 (2014) 7.

C. I. Ciobanu et al., Bent-core Liquid Crystals
9
119.17, 115.86 (2*-0-0=0 + -CH=N- +16 C, aroma-
tic), 34.51, 31.34, 24.64, 22.30, 13.81 (5C, aliphatic),
mlz: 959.96 [M]+.
4H, Ar, J = 8.55 Hz), 7.99 (m, 8H, Ar), 7.47 (m, 1H,
Ar), 7.28 (m, 9H, Ar), 7.20 (dd, 2H, Ar, J, = 8.12 Hz, J2
= 8.33 Hz), 2.58 (t, 4H, 2* -0-C O -C H 2-), 1.78 (qv,
4H, 2* -CH2-), 1.36 (m, 20H, 10* -CH2-), 0.90 (t, 6H,
2* -CHj), 13C NMR (CDCIj) 5 / ppm: 171.74, 164.44,
160.75, 156.87, 154.73, 153.54, 151.83, 150.43, 137.91,
131.64, 130.03, 129.77, 127.00, 124.34, 123.36, 122.32,
120.97, 119.15, 115.84, (2* -0 -C = 0 + -CH=N- +16
C, aromatic), 34.55, 31.83, 29.22, 29.17, 29.11, 24.97,
22.62, 13.97 (8C, aliphatic), m/z: 1044.49 [M]+.
1.3- phenylene bis(4-(4-((4-heptanoyloxyphenyl)azo) ben-
zylideneamino)benzoate) (5b)
Quantities: 0.25 g (0.72 mmol) 1,3-phenylene bis(4-
aminobenzoate) (2) in 15 mL ethanol, 0.49 g (1.44 mmol)
4-((4-heptanoyloxyphenyl)azo)benzaldehyde (3b) in 15
mL ethanol, glacial acetic acid (1-2 drops), yield: 0.33 g
(46.5 %), liquid crystal: 164 °C (K,/K2), 209 °C (K2/
LC), 304 °C (LC/I), 302 °C (I/LC), 200 °C (LC/K,),
158 °C (K,/K2). 'H NMR (CDCIj ) S / ppm: 8.53 (s, 2H,
2* -CH=N-), 8.24 (d, 4H, Ar, J = 8.11 Hz), 8.07 (d,
4H, Ar, J = 7.89 Hz), 7.99 (m, 8H, Ar), 7.48 (m, 1H,
Ar), 7.29 (m, 9H, Ar), 7.20 (dd, 2H, Ar, J, = 8.33 Hz,
J2= 8.11 Hz), 2.58 (t, 4H, 2* -O-CO-CHj-), 1.78 (qv,
4H, 2* -CH2-), 1.40 (m, 12H, 6* -CH2-), 0.92 (t, 6H,
2* -CHj), I3C NMR (CDCIj) 8 / ppm: 171.74, 164.45,
160.77, 156.84, 154.67, 153.49, 151.78, 150.38, 137.87,
131.63, 130.02, 129.78, 126.95, 124.34, 123.36, 122.32,
120.97, 119.16, 115.85, (2* -0 -C = 0 + -CH=N- +16
C, aromatic), 34.53, 31.46, 28.80, 24.92, 22.47, 13.92
(6C, aliphatic), mlz: 987.72 [M]+.
1.3- phenylene bis(4-(4-((4-decanoyloxyphenyl)azo)ben-
zylideneamino)benzoate) (5e)
Quantities: 0.25 g (0.72 mmol) 1,3-phenylene bis(4-
aminobenzoate) (2) in 15 mL ethanol, 0.55 g (1.44
mmol) 4-((4-decanoyloxyphenyl)azo)benzaldehyde (3e)
in 15 mL ethanol, glacial acetic acid (1-2 drops), yield:
0.3 g (39 %), liquid crystal: 148 °C (K,/K2), 195 °C
(K2/LC), 287 °C (LC/I), 285 °C (I/LC), 185 °C (LC/K,),
178 °C (K,/K2). 'H NMR (CDCIj) 6 / ppm: 8.53 (s, 2H,
2* -CH=N-), 8.25 (d, 4H, Ar, J = 8.33 Hz), 8.08 (d,
4H, Ai,J= 8.55 Hz), 8.01 (d, 4H, Ar, J= 8.77 Hz), 7.98
(d, 4H, Ar, J= 8.77 Hz), 7.48 (m, 1H, Ar), 7.28 (m, 9H,
Ar), 7.20 (dd, 2H, Ar, J, = 8.20 Hz, J2= 2 Hz), 2.58 (t,
4H, 2* -0-C O -C H 2-), 1.78 (qv, 4H, 2* -CH2-), 1.39
(m, 24H, 12* -CH2-), 0.89 (t, 6H, 2* -CH3), l3C NMR
(CDCIj) 5 /ppm: 171.76, 164.45, 160.79, 156.85, 154.66,
153.49, 151.76, 150.37, 137.86, 131.64, 130.02, 129.79,
126.93, 124.34, 123.37, 122.32, 120.97, 119.18, 115.85,
(2* -0 -C = 0 + -CH=N— +16 C, aromatic), 34.52,
31.88, 29.43, 29.27, 29.25, 29.15, 24.96, 22.65, 14.02
(9C, aliphatic), m/z: 1071.01 [M-l]+.
1.3- phenylene bis(4-(4-((4-octanoyloxyphenyl)azo)ben-
zylideneamino)benzoate) (5c)
Quantities: 0.25 g (0.72 mmol) 1,3-phenylene bis(4-
aminobenzoate) (2) in 15 mL ethanol, 0.51 g (1.44
mmol) 4-((4-octanoyloxyphenyl)azo)-benzaldehyde (3c)
in 15 mL ethanol, glacial acetic acid (1-2 drops), yield:
0.38 g (52 %), liquid crystal: 146 °C (K,/K2), 207 °C
(K2/LC), 285 °C (LC/I), 277 °C (I/LC), 190 °C (LC/K,).
'H NMR (CDCIj)
8 / ppm: 8.52 (s, 2H, 2*
1.3- phenylene bis(4-(4-((4-hexyloxyphenyl)azo)benzyl-
ideneamino)benzoate) (6a)
—CH=N—), 8.24 (d, 4H, Ar, J = 7.90 Hz), 8.07 (d, 4H,
Ar, J = 7.89 Hz), 8.00 (d, 4H, Ar, J= 8.98 Hz), 7.98 (d,
4H, Ar, J = 8.99 Hz), 7.47 (m, 1H, Ar), 7.29 (m, 9H,
Ar), 7.19 (d, 2H, Ar, J = 7.90 Hz), 2.58 (t, 4H, 2*
-0-C O -C H 2-), 1.78 (qv, 4H, 2* -CH2-), 1.44 (m, 16H,
8* -CH2-), 0.91 (t, 6H, 2* -CHj), 13C NMR (CDCIj) 6 /
ppm: 171.74, 164.44, 160.77, 156.84, 154.67, 153.49,
151.78, 150.38, 137.87, 131.63, 130.02, 129.78, 126.94,
124.34, 123.36, 122.32, 120.97, 119.16, 115.85, (2*
-0 -C = 0 + -CH=N—+ 16 C, aromatic), 34.52, 31.67,
29.11, 28.90, 24.96, 22.58, 13.97 ( 7C, aliphatic), m/z:
1015.48 [M-l]+.
Quantities: 0.25 g (0.72 mmol) 1,3-phenylene bis(4-
aminobenzoate) (2) in 15 mL ethanol, 0.44 g (1.44
mmol) 4-((4-hexyloxyphenyl)azo)benzaldehyde (4a) in
15 mL ethanol, glacial acetic acid (1-2 drops), yield:
0.41 g (61.2 %), liquid crystal: 159 °C (K,/K2), 181 °C
(K2/LCi), 215 °C (LC,/LC2), 294 °C (LC2/I), 293 °C
(I/LC,), 202 °C (LC,/LC2), 197 °C (LC2/K,), 149 °C
(K,/K2). 'H NMR (CDCIj) 8 / ppm: 8.52 (s, 2H, 2*
-CH=N-), 8.24 (d, 4H, Ar, J = 8.33 Hz), 8.05 (d, 4H,
Ar, J= 8.33 Hz), 7.97 (d, 4H, Ar, J = 8.55 Hz), 7.94 (d,
4H, Ar, J = 8.99 Hz), 7.47 (m, 1H, Ar), 7.29 (d, 4H, Ar,
J= 8.55 Hz), 7.24 (m, 1H, Ar), 7.19 (dd, 2H, Ar, Jx=
8.12 Hz, J2 = 8.11 Hz), 7.01 (d, 4H, Ar, J= 8.99 Hz),
4.06 (t, 4H, 2* -0-C H 2-), 1.83 (qv, 4H, 2* -CH2-),
1.50 (qv, 4H, 2* -CH2-), 1.36 (m, 8H, 4* -CH2), 0.92
(t, 6H, 2* -CHj). 13C NMR (CDCIj) d / ppm: 164.47,
162.44, 160.94, 157.01, 155.06, 151.82, 147.28, 137.23,
131.64, 130.01, 129.76, 126.88, 125.18, 123.08, 120.97,
119.16, 115.85, 115.01, (-0-C = 0 + -CH=N- + 16 C,
aromatic), 68.64 (-0-C H 2), 31.60, 29.25, 25.73, 22.58,
13.91 (5C, aliphatic), m/z: 932.30 [M]+.
1.3- phenylene bis(4-(4-((4-nonanoyloxyphenyl)azo)ben-
zylideneamino)benzoate) (5d)
Quantities: 0.25 g (0.72 mmol) 1,3-phenylene bis(4-
aminobenzoate) (2) in 15 mL ethanol, 0.53 g (1.44
mmol) 4-((4-nonanoyloxyphenyl)azo)benzaldehyde (3d)
in 15 mL ethanol, glacial acetic acid (1-2 drops), yield:
0.4 g (53 %), liquid crystal: 148 °C (K,/K2), 202 °C
(K2/LC), 288 °C (LC/I), 285 °C (I/LC), 190 °C (LC/K,),
86 °C (K,/K2). ‘H NMR (CDCIj) <5/ ppm: 8.52 (s, 2H,
2* -CH=N-), 8.24 (d, 4H, Ar, J = 8.55 Hz), 8.07 (d,
Croat. Chem. Acta 87 (2014) 7.

10
C. I. Ciobanu et al., Bent-core Liquid Crystals
1.3- phenylene bis(4-(4-((4-heptyloxyphenyl)azo)benzyl- 8.33 Hz), 8.05 (d, 4H, Ar, J= 8.33 Hz), 7.97 (d, 4H, Ar,
ideneamino)benzoate) (6b)
J= 8.33 Hz), 7.94 (d, 4H, Ar, J = 8.77 Hz), 7.48 (m,
1H, Ar), 7.30 (d, 4H, Ar, J = 8.55 Hz), 7.24 (m, 1H,
Ar), 7.19 (dd, 2H, Ar, J, = 8.12 Hz, J2= 8.11 Hz), 7.01
(d, 4H, Ar, J = 8.99 Hz), 4.06 (t, 4H, 2* -0 -C H 2-),
1.83 (qv, 4H, 2* -CH2-), 1.49 (qv, 4H, 2* -CH2-), 1.34
(m, 20H, 10* -CH2), 0.89 (t, 6H, 2* -CH3), 13C NMR
(CDCI3) 8 / ppm: 164.49, 162.44, 160.95, 156.99, 155.05,
151.80, 147.25, 137.21, 131.63, 130.01, 129.78, 126.85,
125.17, 123.07, 120.98, 119.17, 115.87, 115.01 (- 0 -
C=0 + -CH=N- + 16 C, aromatic), 68.63 (-0-C H 2),
31.90, 29.54, 29.40, 29.28, 29.25, 26.07, 22.66, 14.00
(8C, aliphatic), m/z: 1016.58 [M]+.
Quantities: 0.25 g (0.72 mmol) 1,3-phenylene bis(4-
aminobenzoate) (2) in 15 mL ethanol, 0.47 g (1.44
mmol) 4-((4-heptyloxyphenyl)azo)benzaldehyde (4b) in
15 mL ethanol, glacial acetic acid (1-2 drops), yield:
0.37 g (53.6 %), liquid crystal: 151 °C (K,/K2), 181 °C
(K2/LCi), 213 °C (LCi/LC2), 278 °C (LC2/I), 274 °C
(I/LC), 201 °C (LC/K,), 145 °C ( K i/ K 2) . *H NMR
(CDC13) 3 / ppm: 8.52 (s, 2H, 2* -CH=N-), 8.24 (d, 4H,
Ar, J= 8.33 Hz), 8.05 (d, 4H, Ar, J= 8.33 Hz), 7.97 (d,
4H, Ar, J = 8.55 Hz), 7.94 (d, 4H, Ar, J= 8.99 Hz), 7.48
(m, 1H, Ar), 7.30 (d, 4H, Ar,J = 8.33 Hz), 7.24 (m, 1H,
Ar), 7.20 (dd, 2H, Ar, J, = 8.30 Hz, J2 = 2.20 Hz), 7.01
(d, 4H, Ar, J = 8.99 Hz), 4.06 (t, 4H, 2* -0-C H 2-),
1.83 (qv, 4H, 2* -CH2-), 1.49 (qv, 4H, 2* -CH2-), 1.30
(m, 12H, 6* -CH2), 0.91 (t, 6H, 2* -CH3), 13C NMR
(CDCI3) d / ppm: 164.49, 162.45, 160.94, 157.01, 155.07,
151.82, 147.27, 137.23, 131.63, 130.01, 129.77, 126.87,
125.18, 123.08, 120.98, 119.16, 115.87, 115.02, (-0 -
C=0 + -CH=N- +16 C, aromatic), 68.65 (-0-C H 2),
31.80, 29.29, 29.05, 26.04, 22.59, 13.97 (6C, aliphatic),
m/z: 960.05 [M]+.
1,3-phenylene bis(4-(4-((4-decyloxyphenyl)azo)benzyl-
ideneamino)benzoate) (6e)
Quantities: 0.25 g (0.72 mmol) 1,3-phenylene bis(4-
aminobenzoate) (2) in 15 mL ethanol, 0.52 g (1.44
mmol) 4-((4-decyloxyphenyl)azo)benzaldehyde (4e) in
15 mL ethanol, glacial acetic acid (1-2 drops), yield:
0.31 g (41.3 %), liquid crystal: 152 °C (K,/K2), 177 °C
(K2/LC), 259 °C (LC/I), 257 °C (I/LC), 166 °C (LC/K,),
149 °C (K,/K2). 'H NMR (CDC13) d / ppm: 8.52 (s, 2H,
2* -CH=N-), 8.24 (d, 4H, Ar, J = 8.33 Hz), 8.06 (d,
4H, Ar, J = 8.33 Hz), 7.97 (d, 4H, Ar, J= 8.33 Hz), 7.94
(d, 4H, Ar,J = 8.77 Hz), 7.48 (m, 1H, Ar), 7.30 (d, 4H,
Ar, J= 8.33 Hz), 7.25 (m, 1H, Ar), 7.19 (dd, 2H, Ar, Jx=
8.12 Hz, J2 = 8.11 Hz), 7.01 (d, 4H, Ar, J = 8.77 Hz),
4.06 (t, 4H, 2* -0-C H 2-), 1.83 (qv, 4H, 2* -CH2-),
1.49 (qv, 4H, 2* -CH2-), 1.35 (m, 24H, 12* -CH2),
0.89 (t, 6H, 2* -CH3). 13C NMR (CDC13) 3 / ppm:
164.48, 162.43, 160.96, 156.98, 155.03, 151.77, 147.22,
137.19, 131.63, 130.01, 129.78, 126.83, 125.17, 123.07,
120.98, 119.17, 115.87, 114.99 (-0-C = 0 + -C H =N -+
16 C, aromatic), 68.62 (-0-C H 2), 31.92, 29.58, 29.56,
29.40, 29.31, 29.27, 26.07, 22.67, 14.02 (9C, aliphatic),
m/z: 1043.32 [M-l]+.
1.3- phenylene bis(4-(4-((4-octyloxyphenyl)azo)benzyl-
ideneamino)benzoate) (6c)
Quantities: 0.25 g (0.72 mmol) 1,3-phenylene bis(4-
aminobenzoate) (2) in 15 mL ethanol, 0.49 g (1.44
mmol) 4-((4-octyloxyphenyl)azo)benzaldehyde (4c) in
15 mL ethanol, glacial acetic acid (1-2 drops), yield:
0.31 g (43.6 %), liquid crystal: 135 °C (K,/K2), 201 °C
(K2/LC), 267 °C (LC/I), 263 °C (I/LC), 185 °C (LC/K,),
130 °C (K,/K2). 'H NMR (CDCI3) 6 / ppm: 8.51 (s, 2H,
2* -CH=N-), 8.24 (d, 4H, Ar, J = 8.33 Hz), 8.05 (d,
4H, Ar, J= 8.33 Hz), 7.97 (d, 4H, Ar, J = 8.55 Hz), 7.94
(d, 4H, Ar, J= 8.99 Hz), 7.47 (m, 1H, Ar), 7.29 (d, 4H,
Ar,J = 8.55 Hz), 7.24 (m, 1H, Ar), 7.19 (dd, 2H, Ar, J,
= 8.12 Hz, J2= 8.33 Hz), 7.01 (d, 4H, Ar,J = 8.99 Hz),
4.06 (t, 4H, 2* -0-C H 2-), 1.83 (qv, 4H, 2* -CH2-),
1.49 (qv, 4H, 2* -CH2-), 1.33 (m, 16H, 8* -CH2), 0.91
(t, 6H, 2* -CH3), 13C NMR (CDC13) 6 / ppm: 164.48,
162.44, 160.93, 156.99, 155.06, 151.81, 147.26, 137.21,
131.63, 130.01, 129.77, 126.86, 125.17, 123.07, 120.98,
119.16, 115.86, 115.01 (-O -C O + -CH=N- + 16 C,
aromatic), 68.64 (-0-C H 2), 31.83, 29.36, 29.27, 29.22,
26.08, 22.64, 13.99 (7C, aliphatic), m/z: 988.79 [M]+.
RESULTS AND DISCUSSION
Synthesis
The bent-core Schiff Bases were obtained by the bis-
condensation reactions between the 1,3-phenylene
bis(4-aminobenzoate) core (2) and the corresponding
mesogenic aldehydes 3a-3e or 4a-4e (Scheme 1).
Compounds 2 and 3a-3e or 4a-4e were refluxed in
1.3- phenylene bis(4-(4-((4-nonyloxyphenyl)azo)benzyl- anhydrous ethanol in the presence of glacial acetic acid
ideneamino)benzoate) (6d)
as catalyst. The core compound 2 was obtained in two
steps. In the first step, resorcinol was reacted with
4-nitrobenzoic acid in the presence of DCC / DMAP.
The obtained 1,3-phenylene bis(4-nitrobenzoate) (1)
was reduced to 1,3-phenylene bis(4-aminobenzoate) (2)
by adding in several portions solid SnCl2 • 2H20. Com-
pounds 3a-3e were synthesized by treating 4-((4-
hydroxyphenyl)azo)benzaldehyde with the correspond-
Quantities: 0.25 g (0.72 mmol) 1,3-phenylene bis(4-
aminobenzoate) (2) in 15 mL ethanol, 0.5 g (1.44 mmol)
4-((4-nonyloxyphenyl)azo)benzaldehyde (4d) in 15 mL
ethanol, glacial acetic acid (1-2 drops), yield: 0.38 g
(52 %), liquid crystal: 195 °C (K/LC), 260 °C (LC/I),
257 °C (I/LC), 169 °C (LC/K). 'H NMR (CDC13) d /
ppm: 8.52 (s, 2H, 2* -CH=N-), 8.24 (d, 4H, Ar, J =
Croat. Chem. Acta 87 (2014) 7.

C. I. Ciobanu et al., Bent-core Liquid Crystals
11
COOH
Scheme 1. Synthesis ofthe Schiffbases based on a 1,3-phenylene bis(4-aminobenzoate) core
ing M-aliphatic carboxylic acids in the presence of DCC
and DMAP.14 The synthesis of intermediates 4a-4e
involved the Williamson etherification of 4-((4-hydroxy-
phenyl)azo)benzaldehyde with the corresponding /7-alkyl
bromides in ethanol in the presence of KOH.5
heating and cooling cycles. Figure la presents, as an
example, the DSC curves of compound 5e.
In the case of the first two compounds of the se-
ries, 5a and 5b, the Tonset values are very close to the
clearing points and Table 1 presents the isotropisation
temperatures observed during the POM investigations.
For compound 5a the mesophase started on heating at
202 °C with a smectic like texture which maintained up
to isotropisation at 315 °C (Figure 2a). Upon cooling
from the isotropic liquid, a nematic phase appeared for a
wide temperature interval of about 78 °C with character-
istic Schlieren nematic texture (Figure 2b). On further
cooling, the Schlieren texture transformed into a smectic
one that maintained down to 193 °C when crystalliza-
tion occurred.
Liquid Crystalline Properties
Polarized optical microscopy (POM) and differential
scanning calorimetry (DSC) investigations evidenced for
all the synthesized compounds enantiotropic liquid crys-
talline properties, mainly with different smectic or nemat-
ic textures. The domains of the existence of the
mesophases are quite wide for both 5a-5e and 6a 6e
classes (between 63 and 114 °C, Table 1). Thermogravi-
metric studies evidenced a good thermal stability for all the
obtained compounds, in all the cases the r onset values being
superior to the isotropisation temperatures (Table 1.).
Although the compound 5b presented similar val-
ues of the temperature transitions as 5a, it has not
showed nematic textures on cooling, only smectic order-
ing being evidenced for an interval of about 100 °C
(Figure 2c).
The 5a-5e Schiff bases contain an esteric group in
the terminal chain. DSC investigations evidenced for all
the 5a-5e compounds three transitions, both on the
Croat. Chem. Acta 87 (2014) 7.

12
C. I. Ciobanu et al., Bent-core Liquid Crystals
Table 1. Transition temperatures (expressed in °C) and transition enthalpies (expressed in Jg ') for compounds 5a-e and 6a-e
T
(a) / op
Compound
n = 5
k ,/k 2
k 2/k 3
k 3/l c
LC/I
LCj/K,
k ,/k 2
I/LC
LQ/LQ,
1 onset
'
169
[-3]
202
[-13]
166
[2]
193
[14]
307W
5a
315(b)
304(b)
324
-
-
-
-
-
-
-
-
-
-
164
209
[-13]
200
[17]
158
[3]
5b
5c
n
n
=
6
7
302(b)
329
328
[-2]
146
[-30]
207
[-14]
285
[-20]
277
[12]
190
[14]
=
-
148
[-29]
202
[-17]
288
[-20]
285
[19]
190
[17]
86
[29]
5d
5e
6a
6b
6c
n = 8
n = 9
n = 6
n = l
325
329
148
[-27]
195
[-17]
287
[-18]
285
[18]
185
[16]
78
[24]
159
294
[-17]
181
[-4]
215
[-15]
293
[17]
202
[15]
197
[0.5]
149
[H ]
335
350
348
355
345
[-5]
151
181
[-6]
213
[-18]
276
[-17]
274
[17]
201
[18]
145
[6]
-
-
-
-
[-5]
136
[-0.8]
202
[-16]
267
[-19]
263
[15]
185
130
[1]
n
n
=
8
-
-
-
[15]
195
[-20]
260
[-22]
257
[22]
169
[20]
6d
6e
=
9
-
-
152
177
[-21]
259
[-20]
257
[20]
166
[21]
149
[0.8]
n = 10
[-1]
(a)Toma- temperature at which thermal degradation begins.
(b)Data obtained from POM investigations.
Abbreviations: K-crystalline, LC-liquid crystal, I-isotropic
185
120
160
200
240
280
77 °C
Figure 1. DSC curves: compound 5e (a): 1-first heating; 2- first cooling; 6a (b): 1-second heating; 2- first cooling.
The increasing of the length of the terminal
acyloxy chain resulted in the slight decreasing of both
melting and clearing temperatures for compounds 5c-
5e. An interesting behavior showed compound 5c both
on heating and cooling cycles. During the first heating,
at 207 °C ordering in oily streaks like appearance be-
gan (Figure 3a) which changed near the isotropisation
temperature to a focal conic texture (Figure 3b). On
the first cooling from the isotropic liquid, lath-like and
circular smectic type textures were evidenced (Figures
3c, 3d). The smectic ordering maintained on the sec-
ond heating but the second cooling debuted with a
short nematic Schlieren phase (Figure 3e) that changed
to a smectic one at 263 °C.
Croat. Chem. Acta 87 (2014) 7.

C. I. Ciobanu et al., Bent-core Liquid Crystals
13
Figure 2. Optical textures: 5a, 303 °C, first heating (a); 5a, 274 °C, first cooling (b); 5b, 219 °C, first cooling (c).
Figure 3. Optical textures of compound 5c: 234 °C, first heating (a); 282 °C, first heating (b); 277 °C, first cooling (c); 196 °C,
first cooling (d); 262 °C, second cooling (e).
The 6a-6e class of Schiff bases containing the less
polar alkyloxy terminal chain evidenced a similar liquid
crystalline behavior with the 5a-5e series. The transi-
tion temperatures have slightly lower values if com-
pared with the 5a-5e series. The stability ranges of the
mesophases maintains on a wide interval, between 63
and 82 °C on heating and 73 and 96 °C on cooling.
The last two compounds of the series showed similar
thermal behavior and textures. On heating compounds 5d
and 5e ordered in an unidentified mesophase as thin fila-
ments (Figure 4a). On cooling, a nematic phase appeared
for a short time accompanied by fan-shaped textures (Fig-
ure 4b) that grew up into an unidentified mesophase that
maintained down for about 100 °C (Figure 4c).
Croat. Chem. Acta 87 (2014) 7.

14
C. I. Ciobanu et ah, Bent-core Liquid Crystals
Figure 4. Optical textures: 5e, 272 °C, first heating (a); 5d, 249 °C, first cooling (b); 5e, 250 °C, first cooling (c).
Figure lb presents as an example the DSC curves
for compound 6a.
Compound 6c showed similar characteristic fan-
like textures on cooling from the isotropic state which
finally developed into unidentified texture that main-
tained up to crystallization (181 °C) (Figure 5c).
Compounds 6d and 6e presented similar textures
on heating but different ones during the cooling cycles.
In the case of compound 6d, a lath-like textures with
stripes running across could be evidenced (Figure 6a),
while for compound 6e, around 167 °C, near the crystal-
lization point, some circular and elongated domains
On heating, compound 6a evidenced an unspecific
smectic texture; on cooling from the isotropic liquid a
lath-like texture that developed into a dense texture
(Figure 5a). On cooling, the DSC curve evidenced at
202 °C a new LC/LC transition which could not be seen
on POM investigations and that maintained up to 197
°C when crystallization took place. Similar textures
appeared for compound 6b (Figure 5b).
Figure 5. Optical textures: 6a, 200 °C, second cooling (a); 6b, 274 °C, second cooling (b); 6c, 217 °C, second cooling (c).
Figure 6. Optical textures: 6d, 229 °C, second cooling (a); 6e, 209 °C, second cooling (b); 6e, 167 °C, second cooling (c).
Croat. Chem. Acta 87 (2014) 7.

15
C. I. Ciobanu et al., Bent-core Liquid Crystals
♦
Series 5, heating
Series 5, cooling
Series 6, Heating
Series 6, cooling
■
A
------ Linear (Series 5, heating)
-------Linear (Series 5, cooling)
-------Linear (Series 6, cooling)
-------Linear (Series 6, Heating)
Figure 7. Domains of the mesophases stabilities for 5a-5e and 6a-6e series.
Acknowledgements. This work was supported by CNCS -
UEFISCDI (Romania), project number PNII - IDEI356/2008.
grew up from the smectic texture, which are similar
with the arrangements observed for the B7phase.
By comparing the mesophase stability for the two
series of banana Schiff Bases, it has been noted that
acyloxy derivatives 5a-5e generally showed a wider
mesophase domain both on heating and cooling stages
(Figure 7).
The presence of the more polar esteric linking
groups between the aromatic rings and the terminal
flexible chain in compounds 5a-5e slightly increases
the physical interactions between the molecules in the
liquid crystalline state. The mesophase stability of
acyloxy compounds 5a-5e may be explained by the
extended conjugated system combined with the
strengthening of the lateral interactions too.
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Croat. Chem. Acta 87 (2014) 7.

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