Liquid-crystalline polymorphism of symmetrical azobananas: Bis(4-(4-alkylphenyl)azophenyl) 2-nitroisophtalates

Article abstract of DOI:10.1080/15421400903065754

In this paper we present a series of novel compounds, bis(4-(4-alkylphenyl) azophenyl) 2-nitroisophtalates, which exhibit nematic and banana-type liquidcrystalline phases. The alkyl chain length varies from 1 to 18 carbons. The first ten members of this series exhibit nematic phase. The last eleven compounds exhibit banana-type liquid crystalline phases. The propyl and pentyl derivatives have extra second type of banana mesophase. Copyright Taylor & Francis Group, LLC.

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Liquid-Crystalline
Polymorphism of Symmetrical
Azobananas: bis(4-(4-
alkylphenyl)azophenyl) 2-
nitroisophtalates
K. Żygadło a , D. Dardas b , K. Nowicka b , J.
Hofmann b & Z. Galewski a
a Faculty of Chemistry, University of Wrocław ,
Wrocław, Poland
b Institute of Molecular Physics, PAS , Poznań,
Poland
Published online: 05 Oct 2009.
To cite this article: K. Żygadło , D. Dardas , K. Nowicka , J. Hofmann & Z. Galewski
(2009) Liquid-Crystalline Polymorphism of Symmetrical Azobananas: bis(4-(4-
alkylphenyl)azophenyl) 2-nitroisophtalates, Molecular Crystals and Liquid Crystals,
509:1, 283/[1025]-291/[1033], DOI: 10.1080/15421400903065754
To link to this article: http://dx.doi.org/10.1080/15421400903065754
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Mol. Cryst. Liq. Cryst., Vol. 509, pp. 283=[1025]–291=[1033], 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400903065754
Liquid-Crystalline Polymorphism of Symmetrical
Azobananas: bis(4-(4-alkylphenyl)azophenyl)
2-nitroisophtalates
1
2
2
2
˙
K. Zygadło , D. Dardas , K. Nowicka , J. Hofmann ,
and Z. Galewski1
1Faculty of Chemistry, University of Wrocław, Wrocław, Poland
2
´
Institute of Molecular Physics, PAS, Poznan, Poland
In this paper we present a series of novel compounds, bis(4-(4-alkylphenyl)
azophenyl) 2-nitroisophtalates, which exhibit nematic and banana-type liquid-
crystalline phases. The alkyl chain length varies from 1 to 18 carbons. The first
ten members of this series exhibit nematic phase. The last eleven compounds
exhibit banana-type liquid crystalline phases. The propyl and pentyl derivatives
have extra second type of banana mesophase.
Keywords: 2-isophtalic acid; antiferroelectric switching; banana mesophases;
bent-core; liquid crystals; nematic
INTRODUCTION
Since the time of their discovery liquid crystals have been very
extensively investigated. The exploration of liquid crystals included
molecules with different shapes (rod-like, disc-like, sanidic). Also there
are different possibilities of substitution and combination of linking
units in designing new molecular structures. From 1997 [1] much
attention was dedicated to bent-core molecules because of their
unusual properties. Built of achiral modules these compounds show
properties typical for chiral molecules having tilted smectic phases
with ferroic features. The banana-type mesophases for bent-core mole-
cules have been denoted as Bx[2], where the subscript ‘x’ describes the
Research described in this paper is supported by the grant N N204 217034 of the
Polish Science Foundation KBN.
˙
Address correspondence to K. Zygadło, Faculty of Chemistry, University of Wrocław,
Joliot-Curie 14, Wrocław 50-383, Poland. E-mail: ly@eto.wchuwr.pl
283=[1025]

˙
K. Zygadło et al.
284=[1026]
specific type of mesophase. Up till now eight type of banana phases
were identified [2–6]. Ferroic properties are described in the second
type notation as the SmCP, where ‘‘P’’ means that the mesophases
has ferroic properties.
Most common bent-core liquid crystals are resorcine esters, because
of simplicity of their synthesis. Molecules with reversed linking
groups at the central phenyl ring, namely isophthalic acid derivatives,
are often ignored. Nguyen et al in their review paper [7] described
20 types of structures with the isophthalic acid units. So small amount
of published papers about these types of molecules was results from
their high phase transition temperatures [7]. Decreasing of these
temperatures is possible by introducing a substituent in the central
benzene ring.
The aim of this paper is the description of the new series of bent-
type compounds, bis(4-alkylphenyl-40-hydroxyazophenyl) 2-nitroiso-
phtalates, and detection of all present mesophases with special atten-
tion to the switching banana-type mesophases.
2. EXPERIMENTAL
The main root of synthesis is shown on Scheme 1.
2.1. General Information
All the solvents (DCM, toluene, ethanol) were used without puri-
fication. For the diazotation reaction distilled water was used.
2,6-dimethylnitrobenzene was obtained from Aldrich. NMR spectra
were recorded using a Bruker Avance 500 spectrometer with TMS
as internal standard. Purity of all compounds was confirmed by thin
layer chromatography and elemental analysis, Perkin-Elmer 2400
CHN. Microphotographs of mesophases textures were taken using
BX-60 Olympus microscope with Motic M2000 digital camera att-
ached. Transition temperatures and enthalpies were determined
using Perkin-Elmer DSC7 calorimeter at a rate 10ꢀC=min and with
the indium as the standard of caloric effects.
2.2. Synthesis
4-Alkylanilines
were synthesized in a classical manner by acylation of benzene, reduc-
tion of thus formed keton with hydrazine and subsequent acylation
with acetyl chloride. 4-Alkylphenylmethyl ketons were transformed

Liquid-Crystalline Polymorphism of Symmetrical Azobananas 285/[1027]
SCHEME 1 Scheme of synthesis of bis(4-(4-alkylphenyl)azopheny) 2-nitro-
isophtalates.
into anilines in Schmidt reaction. All the compounds were purified
by vacuum distillation.
4-Alkylphenyl-40-Hydroxyazobenzenes
ꢁ0.5 mole of 4-alkylaniline was dissolved in 200 cm3 of 12% solution
of hydrochloric acid. Solution was cooled to 0ꢀC in an ice bath. Then
0.52 mole of sodium nitrite(III) in 80 cm3 of distilled water was added
within 30 minutes. Solution of diazoted amine was put into the refri-
gerator. 0.5 mole of phenol was dissolved in 10% solution of sodium
carbonate. Resulting mixture was cooled and placed in the ice bath.

˙
K. Zygadło et al.
286/[1028]
After cooling it to 0ꢀC the solution of diazo compound was added
dropwise into it, with mechanical stirring. After 60 minutes yellow
precipitate was filtered off on a Buechner funnel and dried on air.
2-Nitroisophtalic Acid [8]
ꢁ15.8 g (0.104 mol) of 2,6-dimethylnitrobenzene was placed in a
flask containing 800 cm3 of distilled water and 6.4 g (0.16 mole) of
sodium hydroxide. The mixture was heated to 90–95ꢀC and 66 g
(0.42 mol) of kalium permanganate was added within 90 minutes.
It was left overnight with stirring. Next it was cooled to room tempera-
ture and brown precipitate was filtered off. The clear solution was
cooled below 5ꢀC and then it was acidified with concentrated solution
of hydrochloric acid. White precipitate was filtered off and dried on air.
It was used in next step without further purification. Analysis: Calc.
–45.51 C%, 2.39 H%, N 6.63%; Found – 45.39 C%, 2.10 H%, 6.62 N%.
Bis(4-(4-dodecylphenyl)azophenyl) 2-nitroisophtalate
ꢁ0.144 g (0.95 mmol) of 2-nitroisophtalic acid was placed in the
100 cm3 round-bottom flask together with 0.4 cm3 of thionyl chloride
and 50 cm3 of toluene was added. The mixture was heated for three
hours. Then the toluene was removed on the rotatory evaporator.
Oily remaining was dissolved in fresh portion (20 cm3) of dry toluene.
0.5 g of 4-dodecyl-40-hydroxyazophenyl was placed in a 250 cm3
round-bottom flask and dissolved in 150 cm3 of toluene. The flask with
azophenol was placed in an ice bath. The solution of acid chloride was
poured into flask with phenol while stirring. The mixture was cooled
below 3ꢀC. To this was added slowly a solution of triethylamine in
10 cm3 of toluene from pipette. The reaction mixture was left over-
night. Next it was heated till boiling for 2 hours and filtered hot.
Toluene was removed on the rotator evaporator. Light orange solid
was dissolved in DCM and chromatographated on silica gel (mesh
60) with DCM as eluent. Only the first fraction was collected. DCM
was evaporated. The solid was recrystallized from 1:1 mixture of
absolute ethanol and acetone, filtered off and dried on air. Analysis:
Calc. –; 74.06 C%, 7.66 H%, 7.71 N% Found –74.28 C%, 7.80 H%,
7.80 N%. The rest of compounds was synthesized in a similar way.
3. RESULTS AND DISCUSSION
All the synthesized compounds exhibit mesomorphic behavior.
Compounds with 1–2 carbons in alkyl chain show only nematic phase.
The elongating of chain up to 8 carbon atoms leads to dimorphic
compounds – nematic and Bx phases appear. Only one derivative,

Liquid-Crystalline Polymorphism of Symmetrical Azobananas 287/[1029]
the pentyl one, is trimorphic with nematic and two banana type meso-
phases. The nematic phase vanishes in the derivative with 8 carbons
in the alkyl chain. Further elongating it from 9 carbons to 18 gives
only monomorphic derivatives. The dependence of transitions
temperatures against alkyl chain length is shown on Figure 1. Transi-
tion temperatures have been confirmed by differential calorimetry
studies presented in Table 1.
The nematic phase was denoted as such after careful observations.
On isotropic-nematic phase transition very discrete formation of
homeotropic texture was observed, with almost indistinguishable
difference from liquid phase. The nematic was confirmed by flashing,
when subjected to a shearing stress (Fig. 2a). Other confirmation was
given by microscopic observations on oriented sample. The typical
observed textures of the presented mesophases are shown in
Figure 2. As the sample was chosen bis(4-(4-pentylphenyl)azophenyl)
2-nitroisophtalate (5-2N).
This derivative (5-2N) has extraordinary properties. Only this one
has three types of mesophases. Typical thermo-optical scan is shown
in Figure 3. In both scans, in the heating and cooling modes, there
are three mesophases distinguishable. As was shown in Figure 2 each
mesophase has characteristic texture and with the change of texture
FIGURE 1 Dependence of transition temperatures against alkyl chain
length.

˙
288/[1030]
K. Zygadło et al.
ꢀ
TABLE 1 Transition Temperatures (in C) and Enthalpies (in kJ=mol) in
Rectangular Brackets
n
Cr
Bx2
Bx1
N
Is
1
.
232.6
[55.25]
202.2
[49.02]
192.0
[28.77]
163.5
[26.09]
162.7
[24.53]
153.7
[24.75]
148.0
[25.86]
137.7
[23.64]
133.0
[21.66]
129.0
[22.27]
126.9
[21.84]
124.9
[21.51]
119.8
[22.65]
115.2
[29.5]
115.2
.
240.2
[0.20]
219.3
[0.45]
215.67
[0.33]
202.4
[0.35]
202.1
[0.47]
192.3
[0.43]
200.9
[0.12]
198.0
[0.32]
.
2
3
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
(179.3)
[0.24]
4
.
.
.
.
.
.
.
.
.
.
.
.
165,3
[6.08]
182.1
[7.18]
185.3
[9.17]
192.5
[13.07]
195.0
[15.49]
197.2
[14.07]
197.5
[15.33]
197.5
[14.94]
198.4
5
(138.5)
[1.33]
6
7
8
9
10
11
12
14
16
18
[10.67]
196.0
[14.91]
191.0
[15.82]
185.7
[27.95]
[11.47]
the change of the light intensity coming through the microscope is
observed. We have detected four phase transitions. The same were
observed in calorimetric measurements (DSC). Figure 4 shows four
peaks of caloric anomalies, which are connected with the phase transi-
tions. The temperatures read at DSC scan are the same as in the
thermo-optic measurements.
Particularly interesting are enthalpies of the phase transitions. The
melting process has almost 4 times higher enthalpy than the phase
transition BX1-N. It is typical for the mesophases with some transla-
tional disorder. The small value of the phase transition BX1-BX2 shows
significant similarity of both banana-mesophases.

Liquid-Crystalline Polymorphism of Symmetrical Azobananas 289/[1031]
FIGURE 2 Microphotographs of the textures of bis(4-(4-pentylphenyl)
azophenyl) 2-nitroisophtalate. A) nematic phase, b) BX1 mesophase, c) phase
transition BX1-BX2, d) BX2 mesophase, e) recrystallization process.
FIGURE 3 Thermo-optical analysis of bis(4-(4-pentylphenyl)azophenyl)
2-nitroisophtalate (5-2N), heating and cooling 10ꢀC=min.

˙
K. Zygadło et al.
290/[1032]
FIGURE 4 DSC traces of bis(4-(4-pentylphenyl)azophenyl) 2-nitroisophtalate
(5-2N), heating and cooling 10ꢀC=min.
Unexpectedly low value of BX-N phase transition enthalpy was
observed in the propyl derivative. It suggests that this banana
mesophase is rather Bx2 than BX1. The proper identification will be
done on the base of X-ray experiment.
FIGURE 5 Ferroic switching of tetradecyl derivative.

Liquid-Crystalline Polymorphism of Symmetrical Azobananas 291/[1033]
In this group of compounds we have tried to observe the switching
effects. Our preliminary investigations show that for banana
mesophases in the hexyl and tetradecyl derivatives such effect was
observed. An example of ferroelectric switching is shown in Figure 5
(tetradecyl derivative).
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