Synthesis, characterization and mesomorphic properties of new unsymmetrical azomethine-type liquid crystals derived from 4-biphenyl carboxaldehyde

Article abstract of DOI:10.1016/j.molliq.2009.10.015

Synthesis, characterization and thermal properties of a series of four azomethines (1)-(4) derived from 4-biphenyl carboxaldehyde were described. The structures of the azomethines were characterized by means FTIR, ¹H, ¹³C NMR spectroscopy and elemental analysis; the results show an agreement with the proposed structure. The methods of differential scanning calorimetry (DSC) and polarizing optical microscopy (POM) were used to describe the liquid-crystalline properties of the azomethines. The mesomorphic properties of the compounds depend on the amine used in the synthesis of imines. In particular, three of them prepared from 4-hexadecylaniline, 4-decylaniline or 4-((4-aminophenyl)diazenyl)-N,N-dimethylbenzenamine (1)-(3) showed smectic A and smectic X mesophases, while the last one obtained from 4-(heptadecafluoroctyl)aniline (4) exhibited SmX2 and nematic mesophases. Current-voltage measurements were performed on ITO/compound/Alq₃/Al device with two different thicknesses of the film. Additionally, we compared the mesomorphic behavior of the azomethines presented in this work with another azomethines.

Full text of DOI:10.1016/j.molliq.2009.10.015

Journal of Molecular Liquids 151 (2010) 30–38
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Synthesis, characterization and mesomorphic properties of new unsymmetrical
azomethine-type liquid crystals derived from 4-biphenyl carboxaldehyde
b
b
c
Agnieszka Iwan a, , Henryk Janeczek , Marian Domanski , Patrice Rannou
⁎
a
Electrotechnical Institute, Division of Electrotechnology and Materials Science, M. Sklodowskiej-Curie 55/61 Street, 50-369 Wroclaw, Poland
Centre of Polymer and Carbon Materials, Polish Academy of Sciences, 34 M. Curie-Sklodowska Street, 41-819 Zabrze, Poland
Laboratoire d'Electronique Moléculaire, Organique et Hybride, UMR5819-SPrAM (CEA-CNRS-Univ. J. FOURIER-Grenoble I),
b
c
INAC Institut Nanosciences & Cryogénie, CEA-Grenoble, 17 Rue des Martyrs, 38054 Grenoble Cedex 9, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Synthesis, characterization and thermal properties of a series of four azomethines (1)–(4) derived from
4-biphenylcarboxaldehyde weredescribed. The structures of the azomethines were characterized bymeans FTIR,
1H, 13C NMR spectroscopy and elemental analysis; the results show an agreement with the proposed structure.
The methods of differential scanning calorimetry (DSC) and polarizing optical microscopy (POM) were used
to describe the liquid-crystalline properties of the azomethines. The mesomorphic properties of the compounds
depend on the amine used in the synthesis of imines. In particular, three of them prepared from 4-hexadecylaniline,
4-decylaniline or 4-((4-aminophenyl)diazenyl)-N,N-dimethylbenzenamine (1)–(3) showed smectic A and smectic
X mesophases, while the last one obtained from 4-(heptadecafluoroctyl)aniline (4) exhibited SmX2 and nematic
mesophases. Current–voltage measurements were performed on ITO/compound/Alq3/Al device with two different
thicknesses of the film. Additionally, we compared the mesomorphic behavior of the azomethines presented in
this work with another azomethines.
Received 24 September 2009
Accepted 27 October 2009
Available online 4 November 2009
Keywords:
Azomethines
Imines
Liquid crystals
Mesomorphic behavior
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
voltage measurements were performed on ITO/compound/Alq3/Al
device.
Synthesis and thermal characterizations of new mesogens are one
of the important areas for materials research community. A lot of
compounds, polymers and dendrimers with different shapes, sym-
metries and consequently with different types of mesogens are
synthesized and investigated as liquid-crystalline materials [1–13].
Among new mesogens, azomethines have a special interest [14].
These compounds exhibit dependently from the structure mono- or
polymorphism and are very sensitive to different acids and have
photochromic properties [14]. Additionally, these compounds are
thermostable and create complexes with different types of metals
[14]. Therefore these mesogenic compounds are very perspective
materials for technical and technological applications.
Additionally, the aim of the paper was a comparison of the thermal
properties of: (i) two azomethines obtained from 4-(heptadecafluor-
octyl)aniline and two different aldehydes such as 4-biphenyl carbox-
aldehyde and 4-octadecyloxybenzaldehyde (compare (3) and (A3))
and (ii) four azomethines obtained from 4-((4-aminophenyl)diazenyl)-
N,N-dimethylbenzenamine and 4-hexadecylaniline and two different
aldehydes such as 4-(4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-heptadeca-
fluoroundecyloxy)benzaldehydeand 4-biphenyl carboxaldehyde (com-
pare (2) and (F2) and (4) and (F4)).
2. Experimental section
The azomethines that exhibited different liquid-crystalline proper-
ties depend from the numbers of HC=N bonds in the structure. For this
reason we could divide symmetrical and unsymmetrical azomethines
for: (a) azomethines having one azomethine (HC=N) bond [15–23] and
(b) azomethines with two HC=N bonds in the structure [24–34].
Here we presented a series of unsymmetrical azomethines with
different end-groups and their thermal properties. The transition
temperatures were studied by DSC and POM techniques. Current–
2.1. Materials
Acetone, ethanol, N,N-dimethylacetamide (DMA) (Aldrich) and
4-biphenyl carboxaldehyde (Aldrich), 4-hexadecylaniline (Aldrich),
4-decylaniline (Aldrich), 4-((4-aminophenyl)diazenyl)-N,N-dimethyl-
benzenamine (Aldrich) and 4-(heptadecafluoroctyl)aniline (Fluka)
were used without any purification.
2.2. Characterization techniques
All synthesized compounds were characterized by NMR and FTIR
⁎
Corresponding author.
E-mail address: a.iwan@iel.wroc.pl (A. Iwan).
spectroscopy. NMR was recorded on a Bruker AC 200 MHz. Chloroform-d
0167-7322/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.molliq.2009.10.015

A. Iwan et al. / Journal of Molecular Liquids 151 (2010) 30–38
31
(CDCl3) containing TMS as an internal standard were used as solvent.
FTIR spectra of the compounds were recorded on a Perkin-Elmer
paragon 500 spectrometer (wavenumber range: 400–4000 cm−1; reso-
lution: 2 cm− 1). Current–voltage measurements were performed on
ITO/compound/Alq3/Al device. The azomethine solution (1 w/v%
in dichloroethane) was spin-cast onto ITO-covered glass substrate at
room temperature. Residual solvent was removed by heating the film
in a vacuum. Alq3 layer was prepared on the polymer film surface by
vacuum deposition at a pressure of 10− 6 Torr and then Al electrode
was vacuum deposited at the same pressure. The area of the diodes
was 9 mm2. Current–voltage characteristics were detected using
electromether Keithey 6715.
(4): Yield: 82%. FTIR: νmax in cm−1 2881, 2825, 1636 (HC=N–), 1597,
1558, 1516, 1488, 1445, 1403, 1361, 1313, 1299, 1286, 1236,
1149, 1145, 1074, 1007, 946, 889, 821, 762, 736, 723, and 963. 1H
NMR (200 MHz, CDCl3, TMS) [ppm]: δ 8.57 (s, 1H, HC=N–); 8.02,
7.99, 7.93, 7.91, 7.90, 7.87, 7.74, 7.71, 7.68, 7.67, 7.65, 7.48, 7.47,
7.45, 7.41, 7.36, 7.33, 7.25, 6.78, 6.75, 3.09, 3.03, 2.91, 2.80, and
1.57 (H2O from CDCl3), (as in Fig. 2). 13C NMR (50 MHz, CDCl3,
TMS) [ppm]: δ 159.72 (–HC=N–), 131.96, 130.72, 124.82,
124.10, 123.25, 121.53, 114.72, and 111.53.
3. Results and discussion
The phase transitions and mesogenicity were studied by differential
scanning calorimetry (DSC) and polarizing microscope observations
(POM). DSC were measured on a TA-DSC 2010 apparatus using sealed
aluminium pans under nitrogen atmosphere at a heating/cooling
rate 0.5 °C/min in a temperature range from −20 °C to over the clearing
point.
The textures of the liquid-crystalline phase were observed with a
Polarized Optical Microscopy (POM), set-up composed of: i) LEICA
DMLM Microscope (magnification: 2.5×, 5×, 10×, 20× and 50×)
working in both transmission and reflexion modes, ii) LINKAM LTS350
(−196 °C till +350 °C) Hot plate and LINKAM CI94 temperature
controller, and iii) JVC Numeric 3-CCD KYF75 camera (resolution:
1360×1024).
3.1. Synthesis and characterization
Azomethines described in this paper were prepared from 4-biphenyl
carboxaldehyde and four amines via high temperature solution
condensation in N,N-dimethylacetamide (DMA) at 160 °C. Chemical
structures of the azomethines (abbreviated hereinafter as (1)–(4)) are
presented in Fig. 1.
3.2. NMR study
Their expected chemical constitution is confirmed by spectroscopic
studies. In the Experimental section NMR data concerning all imines
investigated are collected. In particular the signals in the range of 159–
161 ppm, present in the 13C NMR spectra of all compounds, confirm the
existence of the azomethine group carbon atoms. The changes in the
chemical shift, observed upon the modification of the chemical
constitution of the amine originating sub-unit are clearly observed.
For example, in the case of (3), the presence of fluoro atoms in the chain
results in an up-field shift of the line related to the azomethine carbon
atom in comparison with another azomethines. On the other hand, the
presence of azo group (4) induces an up-field shift of the line related to
the azomethine carbon atom in comparison with (3) and results in a
very small down-field shift of the line related to the azomethine carbon
atom in comparison with (1) and (2).
2.3. General synthetic procedure of (1)–(4)
A mixture of aldehyde (1.0 mmol) and amine (1.0 mmol) in N,N-
dimethylacetamide (DMA) solution, with the presence of p-toluenesul-
fonic acid (PTS) (0.06 g) was refluxed with stirring for 10 h. The reaction
was conducted in an argon atmosphere and the condenser was fitted with
a Dean-stark trap. After cooling, the mixture was precipitated with 100 ml
of ethanol. The crude product was washed with methanol (500 ml) and
next with acetone (350 ml) to remove the unreacted monomers. Then
the compound was dried at 60 °C under vacuum for 12 h.
Most signals were assigned based on well-known proton NMR
chemical shift displacements resulting from electron shielding/
deshielding of the hydrogen nuclei by the inductive effects or from
the diamagnetic anisotropy of various neighboring groups. In the Ex-
perimental section 1H NMR data concerning all compounds were
investigated while in Fig. 2a 1H NMR spectra of the imines are present.
In particular in proton NMR spectra of the investigated compounds
the azomethine proton signal was observed in the range of 8.48–
8.57 ppm as it was expected (Fig. 2). For example, in the imine (4) the
signal from the imine group at 8.57 ppm was observed and was down-
field shift in comparison with another compounds. On the other hand
the presence of the azo group in (3) caused significant up-field shift of
the imine proton signals to 8.48 ppm with respect of the line related
to imine proton in comparison with another azomethines (see Ex-
perimental section). No chemical shift of the imine group proton and
carbon was observed along with an increase in length of the aliphatic
chain from 10 to 16 (compare (1) and (2)).
(1): Yield: 90%. FTIR: νmax in cm−1 3031, 2960, 2954, 2918, 2849,
1623 (HC=N–), 1599, 1580, 1559, 1502, 1486, 1467, 1450,
1406, 1363, 1312, 1210, 1179, 1115, 1006, 973, 889, 834, 819,
759, 719, 687, 558, and 533. 1H NMR (200 MHz, CDCl3, TMS)
[ppm]: δ 8.52 (s, 1H, HC=N–); 7.98, 7.95, 7.72, 7.69, 7.67, 7.66,
7.64, 7.47, 7.45, 7.23, 7.21, 7.19, 2.63, 1.63, 1.61, 1.56 (H2O from
CDCl3), 1.27, and 0.88 (as in Fig. 2). 13C NMR (50 MHz, CDCl3,
TMS) [ppm]: δ 159.06 (–HC=N–), 140.99, 129.16, 129.11,
128.87, 127.84, 127.41, 127.16, 120.81, 35.51, 31.89, 31.54,
29.61, 29.51, 29.32, 22.67, and 14.10.
(2): Yield: 92%. FTIR: νmax in cm−1 3031, 2954, 2917, 2849, 1624
(HC=N–), 1599, 1580, 1560, 1502, 1465, 1450, 1407, 1364, 1312,
1169, 1115, 1006, 973, 889, 834, 759, and 719. 1H NMR (200 MHz,
CDCl3, TMS) [ppm]: δ 8.52 (s, 1H, HC=N–); 7.98, 7.95, 7.72, 7.69,
7.67, 7.66, 7.64, 7.49, 7.47, 7.45, 7.26, 7.23, 7.21, 7.19, 2.63, 1.59
(H2O from CDCl3), 1.26, and 0.88 (as in Fig. 2). 13C NMR (50 MHz,
CDCl3, TMS) [ppm]: δ 159.09 (–HC=N–), 149.58, 143.85, 141.00,
140.31, 135.31, 130.28, 129.16, 129.12, 129.01, 128.87, 128.46,
127.85, 127.69, 127.41, 127.16, 120.82, 35.51, 35.07, 31.92, 31.83,
31.56, 29.69, 29.60, 29.52, 29.36, 29.31, 22.68, and 14.13.
(3): Yield: 89%. FTIR: νmax in cm−1 3030, 2875, 1628 (HC=N–),
1600, 1505, 1450, 1415, 1370, 1300, 1201, 1172, 1153, 1136,
1104, 1093, 1048, 979, 918, 886, 848, 766, 723, 699, 653, 562,
and 529. 1H NMR (200 MHz, CDCl3, TMS) [ppm]: δ 8.48 (s, 1H,
HC=N–); 8.01, 7.97, 7.75, 7.72, 7.68, 7.67, 7.65, 7.64, 7.63, 7.61,
7.31, 7.28, 7.26, and 1.57 (H2O from CDCl3), (as in Fig. 2). 13C
NMR (50 MHz, CDCl3, TMS) [ppm]: δ 161.64 (–HC=N–),
156.28, 144.34, 140.79, 134.60, 129.56, 128.94, 128.07,
127.55, 127.20, and 120.96.
3.3. FTIR study
The presence of the imine group was also confirmed by FTIR
spectroscopy since in each case the band characteristic of the HC=N–
stretching deformations was detected. The exact position of this band
varies in the spectral range 1624–1636 cm−1 shifting towards higher
wavenumbers for the compound with the azo group in the structure.
Particular bands appearing in FTIR spectra of the imines investigated are
shown in the Experimental section. In addition to the –HC=N– stretching
band, a band in the range 1597–1600 cm−1 can be distinguished to the
C=C stretching deformations in the aromatic ring. As an example FTIR
spectra of (1) and (3) are shown in Fig. 2b. Absorption at shorter

32
A. Iwan et al. / Journal of Molecular Liquids 151 (2010) 30–38
Fig. 1. Synthetic route and chemical structure of the synthesized azomethines.
Fig. 2. (a) 1H NMR spectra of all azomethines and (b) FTIR spectra of the azomethines (1) (left side) and (3) (right side) in the range 500–1700 cm−1
.

A. Iwan et al. / Journal of Molecular Liquids 151 (2010) 30–38
33
wavelength being in the range of 1623–1624 cm−1 was observed for
Table 1
Phase transition temperature (°C) of the compounds detected by POM.
the azomethines (1) and (2) (see Experimental section). The position of
the azometine band is shifting towards higher wavenumbers for the
imines (3) and (4) and shows an intense absorption band at 1628 and
1636 cm−1, respectively. Higher wavenumber value indicates that the
bond between carbon and nitrogen atoms in the imine group is shorter
Code
Phase transitions [°C], cooling, POM
Ref.
(1)
SmX 124, SmA 129, I
This work
This work
This work
This work
[23]
(2)
(3)
(4)
(A3)
(F2)
(F4)
Cr 98, SmX 109, SmA 114, I
Cr 115, SmX 137, SmA 201, I
Cr 160, SmX 165, N 213, I
Cr 87, SmA 89, I
Cr 107, SmC 129, SmA 131, I
Cr 115, Sm X1 240, SmA 290, I
[22]
[22]
Cr, Sm and I indicate crystal, smectic and isotropic phases, respectively.
that means that the π-electrons in the double bond are less involved in
conjugation. The lower wavenumber of the imine group absorption
indicates the better conjugation of π-electrons caused by influence of
adjusted group to the azomethine linkage.
In accordance with the NMR and FTIR spectra collected in the ex-
perimental part the spectral data obtained are in agreement with the
spectral data predicted from the chemical formulae of the synthesized
compounds.
3.4. Current–voltage experiment
Current–voltage curves of ITO/compound/Alq3/Al are shown in
Fig. 3. It can be seen that current increases with applied voltage
increase and less or more depended from thickness of the film being
150 nm and 200 nm. The turn-on voltage of the devices was observed
at about 2 V in room temperature for the compound (4), at about 4 V
for the compound (3) and at about 9 V for the compound (2) when
thickness of the film was 200 nm.
The turn-on voltage of the devices with thickness of the film about
150 nm was observed at about 5 V in room temperature for the
compound (4), at about 4 V for the compound (3) and at about 14 V for
the compound (2). Value of conductivity of the investigated azo-
methines (3) and (4) was about 10−7 [S/m], while for the azomethine
(2) was about 10−8 [S/m], what categorized probe materials between
semiconductors and dielectrics [35–42].
Different types of conjugated polymers such as poly(acetylene),
poly(p-phenylene), poly(p-phenylenevinylene), poly(aniline), poly
(pyrrole) and poly(thiophene) have been developed and intensively
investigated as organic semiconductors [35–42]. Concerning the elec-
trical properties of the poly(azomethine)s it should be stressed that the
conductivities of undoped polymers are low (10−16–10−11 S/m), but
doping causes an increase of the conductivity even to 10−9 S/m [14].
The turn-on voltage of the device (F2) was observed at about 6 V
Table 2
Transition temperatures and enthalpies of the azomethines detected by DSC.
Code
Phase transitions [°C] (corresponding enthalpy changes) Ref.
[J/g], DSC
Heating
Cooling
(1)
(2)
(3)
78.1 (3.2), 95.0 (96.7), 120.6
(8.5), 125.7 (8.6)
107.1 (113.3), 112.2 (2.8)
73.1 (92.8), 119.9 (8.6), This work
124.2 (7.9)
105.2 (12.1), 97.2, 96.2 This work
(105.5)
100.1 (0.7), 132.9 (33.9),
192.3 (9.3), 199.8 (1.5)
100.0 (0.5), 111.7
(30.8), 191.2 (8.1),
199.3 (1.4)
This work
(4)
54.5 (8.4), 125.8 (4.9),
177.5 (13.5), 236.6 (3.66)
46.8 (6.0), 61.8, (12.1), 87.3
(46.1)
108.3 (15.08), 128.9 (2.68),
131.9 (13.00)
77.3 (1.1), 134.3 (14.1) This work
(A3)
(F2)
(F4)
32.1 (4.7), 48.6 (9.9),
85.6 (46.4)
[23]
[22]
[22]
106.9 (14.55), 128.7
(2.62), 131.5 (12.70)
93.7 (5.58), 200.5
(1.59), 296.9 (8.51)
118.7 (11.17), 202.2 (4.77),
297.4 (8.59)
Fig. 3. Current–voltage curves of devices: (a) ITO/(2)/Alq3/Al, (b) ITO/(3)/Alq3/Al and
(c) ITO/(4)/Alq3/Al, thickness about 150 nm (curve (2)) and 200 nm (curve (1)).

34
A. Iwan et al. / Journal of Molecular Liquids 151 (2010) 30–38
in room temperature [22] similar as for poly(azomethine)s with
triphenylamine core (PAZ-TPA) in the main chain devices [43]. On the
other hand for the azomethine obtained from heptadecafluorounde-
cyloxy benzaldehyde and pyren-1-amine the turn-on voltage of the
device ITO/azomethine/Alq3/Al was observed at about 8 V in room
temperature [22].
We found differences between I–V characteristic of the azomethines
presented in this paper and azomethines based on heptadecafluor-
oundecyloxy benzaldehyde [22]. For example, the turn-on voltage of
the device (F2) was observed at about 6 V in room temperature, while
for the azomethine (2) was 9 V. This behavior confirmed the influence
of the aldehyde used in the synthesis of the azomethines on the elec-
trical properties of the compounds.
appearing in two reference textbooks for liquid crystals [44,45] and on
repeated POM and DSC experiments.
The POM and DSC observations revealed that all of the compounds
were enantiotropic liquid crystals. Azomethines (1)–(3) exhibited
two mesophases such as SmA and SmX (probably SmB). In contrast,
the azomethine (4) exhibited two totally different mesophases in
comparison with (1)–(3) such as SmX2 (probably SmG) and nematic
mesophases, which implies a significant effect of the azo group (N=N)
on the mesomorphic properties.
Details of transition temperatures of the compounds as deter-
mined by POM are summarized in Table 1 together with their phase
variants. The transition temperatures and enthalpies of the azomethines
(1)–(4) determined by DSC are presented in Table 2.
The azomethine (1) has an enantiotropic smectic A and X (probably
SmB) mesophases. During cooling scans from 129.0 °C to 123.5 °C and
during heating scans from 123.8 °C to 130 °C, the development of the
typical colorful focal conic fan texture colorful of a SmA mesophase (see
Fig. 4a) was observed. Additionally, homeotropically aligned SmA
domains of (1) were found (Fig. 4a).
Exactly at 123.5 °C during cooling scans a sharp transition consisting
in the appearance of transitory (quick appearing and disappearing) bars
within the focal–conic fans texture of the SmA mesophase of (1) was
observed, which unambiguously indicate the transition between a SmA
3.5. Mesomorphic behavior
Our investigations showed that the all azomethines (1)–(4) have
mesomorphic behavior. All azomethines were studied by differential
scanning calorimetry (DSC) and Polarized Optical Microscopy (POM)
method to determine their mesomorphic properties. The tentative
mesophase identifications and the scenario of phase transitions
related to all compounds are based on the identification of textures
Fig. 4. Photomicrographs of the optical textures of mesophases obtained for (1): (a) SmA-
focal conic fan and homeotropic alignment area, (seen at 123.5 °C) and (b) SmX mosaic
texture and focal conic fan texture (seen at 100 °C).
Fig. 5. Photomicrographs of the optical textures of mesophases obtained for (2): (a) SmX
mosaic texture, (seen at 107 °C) and (b) SmA texture (seen at 112.5 °C).

A. Iwan et al. / Journal of Molecular Liquids 151 (2010) 30–38
35
and a SmX mesophase (Fig. 4b). Indeed, as the arcs, which suddenly
appeared within the fan of the SmA mesophase, did not persist below
123 °Cin the formof fineor blurred lines. Alsoa homeotropically aligned
SmX (probably SmB) mesophase of (1) can also (as a SmA) give a black
POM image. Additionally, by cooling the sample slowly enough to
develop its SmX mesophase under 123 °C it was possible to obtain
Fig. 6. Photomicrographs of the optical textures of mesophases obtained for (3): (a) SmX
—colored mosaic with rectangular platelets, (seen at 133.1 °C), (b) SmA— conic focal fan
and homeotropicalignment area, (seen at159.9 °C) and (c) SmA— homeotropic alignment
area with two defects, (seen at 137 °C).
Fig. 7. Photomicrographs of the optical textures of mesophases obtained for (4): (a) SmX2
—dendritic growth of mosaic texture and splinters, (seen at 190 °C), (b) nematic droplets
and small schliren, (seen at 230 °C) and (c) nematic Schliren, (seen at 239.7 °C).

36
A. Iwan et al. / Journal of Molecular Liquids 151 (2010) 30–38
Fig. 8. DSC traces of the: (a) azomethine (1) and (b) azomethine (3) obtained on the second heating and cooling, under N2 atmosphere.
colorful POM picture in which the mosaic and focal–conic fan textures
were simultaneously present (Fig. 4b).
almost exclusively to the homeotropic one. It was not a surprise for
us that, the temperature at which the SmA is transformed to SmX
(probably SmB) transition occurred for (1) and (3) at 123.0 °C and
136.0–137.0 °C, respectively what confirmed the replacement of n-
For the azomethine (2) all transitions detected by POM were in
good agreement with the DSC analyses. SmA mesophase showed a 2D
(lamellar) organization (with orientational order but no positional
order within a layer). On the other hand more ordered X-type
mesophase could lead to two closely related SmB mesophases which
show the same POM textures and that can only be distinguished
by X-ray diffraction. In the above-mentioned proposed scenario of
phase transitions, we just indicated that the second mesophase
observed on cooling from the isotropisation state was a SmX (probably
SmB) (Fig. 5a).
The SmA mesophase of (2) exhibited a black texture (homeotropically
aligned SmA mesophase) perturbed by a myriad of small and colorful
defects (leaf-like and small fan-like) which reflect areas of the POM
sample which are in a conventional SmA planar texture (see Fig. 5b).
The fact that the azomethine (3) has perfluorinated tails is con-
siderably increasing the stability of the SmA mesophase of this LC,
allowing a clearing point of ca. 203 °C. Concomitantly, the perfluori-
nated tails of (3) molecules are driving the texture of SmA mesophase
C10H21 (1) by n-C8F17 (3). During cooling scans from 203.0 °C to
136.0–137.0 °C and during heating scans from 136.0–137.0 °C to
202.0 °C the development of the typical colorful focal conic fan texture
of a SmA mesophase (see Fig. 6b) was observed. Moreover, home-
otropically aligned SmA areas with 2 focal conic fan defects were found
(Fig. 6c). Contrary to (1), for (3) we did not observed a sharp transition
consisting in the appearance of transitory bars within the focal–conic
fans texture of the SmA mesophase of (1) which could have
unambiguously indicated the transition in between a SmA and a SmX
(probably SmB) mesophase. Indeed, careful and slow heating of a
crystallized (3) sample over its melting point (118 °C) allowed us to
image mosaic texture (with some rectangular-like platelets) of its SmX
mesophase from 133 °C onwards (see Fig. 6a).
Theazomethine(4) has two enantiotropic mesophases such asSmX2
(probably SmG) and N. Transitions observed by POM are in very good
agreement with the DSC analyses. Unfortunately, the compound (4) was

A. Iwan et al. / Journal of Molecular Liquids 151 (2010) 30–38
37
Fig. 9. Chemical structure of the selected azomethines with different types of side groups.
simultaneously going to its isotropic state and is partly decomposed. It is
very difficult during POM experiments to accurately determine the
clearing point of this LCs. Moreover vapor phase transport of this deeply
yellow/orange compounds on the windows of hot plate at temperature
higher than 250 °C prevents an easy determination of the clearing point.
The observed nematic (droplet, Schlieren, thread-like) textures of (4)
(Fig. 7b–c) were strongly destabilized (without completely disappearing)
at a temperature of ca. 260 °C, but this does not mean that the transition of
the N to isotropisation occurred at this temperature. During cooling scans
from 265–270 °C to 200 °C–195 °C and during heating scans from 212.5 °C
to 260 °C, the development of colorful and quickly moving birefringent
textures was observed that can be related to nematic mesophases such as
droplet texture, Schlieren texture, droplet and small Schlieren textures
and Schlieren and thread-like texture (see Fig. 7b–c). When looking at the
molecular structure of (4), it is no surprise that this rod-like mesogen is
giving rise to a nematic mesophase. Nevertheless, the fact that it contains
an azo group (which can quickly isomerize at high temperature under
white-light illumination) can explain why it was quite difficult to get nice
POM images. Shape fluctuation and modification of shape-persistency
could be the reason for the difficulty we encountered to capture good POM
images of this N mesophase over large area. During cooling scans from
200–195 °C to 165–155 °C and during heating scans from 170 °C to
210 °C, the developments of very peculiar birefringent textures that can
be related to a SmX2 (probably SmG) mesophase were found: (a)
dendritic growth of a mosaic texture if which elongated platelets with
side-splinters are merging from black (homeotropically aligned) domains
and (b) mosaic texture (see Fig. 7a). It is quite difficult to differentiate a
SmB mesophase from a SmG mesophase based solely on the observation
of their textures by POM as the latter mesophase is a titled version of the
former one. Nevertheless few tips are suggesting that the mesophase
appearing below the N one of (4) during cooling scans is indeed a SmG
one. The fact that we were able to image the dendritic growth of a mosaic
texture in which elongated platelets with side-splinters are emerging
from black (homeotropically aligned) domains is strongly in favour of its
attribution to a SmG mesophase.
Tables 1–2 reveal that the compounds (3)–(4) exhibited a clearing
point temperature higher than 200 °C and broad thermal range of the
mesophase. For example, the thermal range of SmX phase of the com-
pound (3) exceeds 64 °C and the SmX2 mesophase of the azomethine
(4) exceeds 48 °C (see Table 1). On the other hand the compounds
(1) and (2) have a clearing point temperature at about 120 °C and narrow
thermal range of the mesophase (see Table 1). This behavior indicates the
role of the side chain in creating their mesomorphic properties.
Because in our previous work we investigate azomethines obtained
from different aldehydes ((A3), (F2), and (F4)) and the same amines
[22,23] we have possibility to compare few of them and show not only
influence of amine but also aldehyde on the mesomorphic properties
of the compounds. For these reason we presented on Fig. 9 the structure
of the responsible azomethines, while in Tables 1 and 2 we presented
the thermal characteristic of the compounds.
Considering the temperature of isotropisation the following scenario
was observed: 86 °C (A3) N 199 °C (3), 96 °C (2) N 132 °C (F2) and
134 °C(4) N 297 °C (F4). It wasnot a surprise thatthe introduction of the
long aliphatic chain as in the case of (A3) influenced the decrease of
the temperature of isotropisation in comparison with (3). Contrary, the
presence of alkoxysemiperfluorinated chain as in the case of (F2) and
(F4) influence the increase of the temperature of isotropisation
(compare (2) and (F2) and (4) and (F4)). Different aldehydes used in
the synthesis of the azomethines influence also on the kind of
mesophase (Table 1). Compound (A3) obtained from 4-octadecylox-
ybenzaldehyde exhibited only SmA mesophase while (3) has addition-
ally SmA and also SmX mesophase. For (F2) two mesophases such as
SmA and SmC were found while for the azomethine (2) instead SmC
mesophase SmX was observed. Totally different kinds of mesophases
were found for (4) and (F4) (Table 1). This behavior could be explained
by the fact that in (F4) additionally to azo bond there exists also
alkoxysemiperfluorinated chain (Fig. 9).
4. Conclusion
DSC thermograms of the azomethines (1) and (3) obtained on the
second heating and the second cooling, under N2 atmosphere are shown
in Fig. 8, as examples.
In summary, we prepared a series of liquid-crystalline unsymmet-
rical azomethines by the condensation of 4-biphenyl carboxaldehyde
with aliphatic and aromatic amines. The thermal properties of these

38
A. Iwan et al. / Journal of Molecular Liquids 151 (2010) 30–38
[18] T. Narasimhaswamy, K.S.V. Srinivasan, Liquid Cryst. 31 (2004) 1457.
[19] C.V. Yelamaggad, I.S. Shashikala, G. Liao, D.S.S. Rao, S.K. Prasad, Q. Li, A. Jakli, Chem.
Mater. 18 (2006) 6100.
[20] O. Stamatoiu, A. Bubnov, I. Tarcomnicu, M. Iovu, J. Mol. Struct. 886 (2008) 187.
[21] J. Godzwon, M.J. Sienkowska, Z. Galewski, J. Mol. Struct. 844–845 (2008) 259.
[22] A. Iwan, H. Janeczek, B. Jarzabek, M. Domanski, P. Rannou, Liq. Cryst. 36 (2009) 873.
[23] A. Iwan, H. Janeczek, Mol. Cryst Liq. Cryst. (in press).
[24] D. Ribera, A. Mantecon, A. Serra, Macromol. Chem. Phys. 202 (2001) 1658.
[25] D. Ribera, A. Mantecon, A. Serra, J. Polym. Sci. Part A: Polym. Chem. 40 (2002) 4344.
[26] P.A. Henderson, C.T. Imrie, Macromolecules 38 (2005) 3307.
[27] K. Kishikawa, N. Muramatsu, S. Kohmoto, K. Yamaguchi, M. Yamamoto, Chem.
Mater. 15 (2003) 3443.
compounds were investigated via DSC and POM techniques. All
azomethines showed the smectic mesomorphism. The mesophases of
the azomethines (1)–(3) were designated to be SmA and SmX phases.
Azomethine (4) showed smectic X2 and N phases. Preliminary in-
vestigations of the current–voltage characteristics for device such as
ITO/azomethine/Alq3/Al confirmed their semiconductivity properties
of the organic thin film and showed that the thickness of the film
influences less or more for the electrical properties of the investigated
compounds.
[28] Y. Naito, R. Ishige, M. Itoh, M. Tokita, J. Watanabe, Chem. Lett. 37 (2008) 880.
[29] S. Sudhakar, T. Narasimhaswamy, K.S.V. Srinivasan, Liquid Cryst. 27 (2000) 1525.
[30] V. Cozan, M. Gaspar, E. Butuc, A. Stoleriu, Eur. Polym. J. 37 (2001) 1.
[31] K. Sabdham, V. Srinivasan, T. Padmavathy, Macromol. Symp. 199 (2003) 277.
[32] A. Iwan, H. Janeczek, P. Rannou, Spectrochim. Acta Part A: Mol. Biomol. Spectrosc. 72
(2009) 72.
[33] A. Iwan, H. Janeczek, B. Jarzabek, P. Rannou, Materials, Special Issue of Liquid
Crystals 2 (2009) 38.
[34] A. Iwan, H. Janeczek, P. Rannou, R. Kwiatkowski, J. Mol. Liq. 148 (2009) 77.
[35] A. Pron, P. Rannou, Prog. Polym. Sci. 27 (2002) 135.
References
[1] M. Hird, Chem. Soc. Rev. 36 (2007) 2070.
[2] J.M. Frechet, Prog. Polym. Sci. 30 (2005) 844.
[3] C. Tschierske, Chem. Soc. Rev. 36 (2007) 1930.
[4] C.T. Imrie, P.A. Henderson, Chem. Soc. Rev. 36 (2007) 2096.
[5] J. Barbera, B. Donnio, L. Gehringer, D. Guillon, M. Marcos, A. Omenat, J.L. Serrano,
J. Mater. Chem. 15 (2005) 4093.
[6] D. Demus, Liquid Cryst. 5 (1989) 75.
[7] W. Ong, R.L. McCarley, Org. Lett. 7 (2005) 1287.
[8] A.G. Cook, U. Baumeister, C. Tschierske, J. Mater. Chem. 15 (2005) 1708.
[9] B. Donnio, S. Buathong, I. Bury, D. Guillon, Chem. Soc. Rev. 36 (2007) 1495.
[10] M. Marcos, R.M. Rapun, A. Omenat, J.L. Serrano, Chem. Soc. Rev. 36 (2007) 1889.
[11] I.M. Saez, J.W. Goodby, J. Mater. Chem. 15 (2005) 26.
[12] V. Percec, Phil. Trans. R. Soc. A 364 (2006) 2709.
[13] V. Percec, M.R. Imam, M. Peterca, D.A. Wilson, R. Graf, H.W. Spiess, V.S.K.
Balagurusamy, P.A. Heiney, J. Am. Chem. Soc. 131 (2009) 7662.
[14] A. Iwan, D. Sek, Prog. Polym. Sci. 33 (2008) 289.
[15] J.A. Smith, R.A. DiStasio, N.A. Hannah, R.W. Winter, T.J.R. Weakley, G.L. Gard, S.B.
Rananavare, J. Phys Chem B. 108 (2004) 19940.
[16] I.K. Grabtchev, V.B. Bojinov, I.T. Moneva, J. Mol. Struct. 471 (1998) 19.
[17] H. Hioki, M. Fukutaka, H. Takahashi, M. Kubo, K. Matsushita, M. Kodama, K. Kubo,
K. Ideta, A. Mori, Tetrahedron 61 (2005) 10643.
[36] M. O'Neil, S.M. Kelly, Adv. Mater. 15 (2003) 1135.
[37] U. Mitschke, P. Bauerle, J. Mater. Chem. 10 (2000) 1471.
[38] H. Shirakawa, E.J. Louis, A.G. Mac-Diarmid, C.K. Chiang, A. Hegger, J. Chem. Soc.
Chem. Commun. 16 (1977) 578.
[39] K.Y. Jen, G.C. Miller, R.L. Elsenbaumer, J. Chem. Soc., Chem. Commun. 17 (1986) 1346.
[40] R.L. Elsenbaumer, K.Y. Jen, R. Oboodi, Synth. Met. 15 (1986) 169.
[41] Y. Elsworth, R. Howe, G.R. Isaak, C.P. McLeod, R. New, Nature (1990) 536.
[42] Y. Cao, P. Smith, A.J. Heeger, Synth. Met. 48 (1992) 91.
[43] D. Sek, A. Iwan, B. Jarzabek, B. Kaczmarczyk, J. Kasperczyk, Z. Mazurak, M.
Domanski, K. Karon, M. Lapkowski, Macromolecules 41 (2008) 6653.
[44] D. Demus, L. Richter, Textures of Liquid Crystals, 1st Ed, Verlag Chemie, Leipzig,
1978, p. 1.
[45] G.W. Gray, J.W. Gooby, Smectic Liquid Crystals—Textures and Structures, Leonard
Hill, Glasgow, 1984, p. 1, and Londow.