Thermal, optical, electrical and structural study of new symmetrical azomethine based on poly(1,4-butanediol)bis(4-aminobenzoate)

Article abstract of DOI:10.1016/j.molstruc.2009.10.031

The synthesis, characterization and mesomorphism of new thermotropic imine prepared via condensation of biphenyl-4-carboxaldehyde with poly(1,4-butanediol)bis(4-aminobenzoate) was reported. The structure of imine was characterized by means FTIR, ¹H, ¹³C NMR spectroscopy and elemental analysis; the results show an agreement with the proposed structure. The differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction (WAXRD, SAXRD) were employed to evaluate their phase transitional behavior. The imine exhibited smectic A (SmA) and smectic B (SmB) mesophases. Liquid crystalline properties of the azomethine were studied additionally via UV-vis spectroscopy in the function of temperature (UV-vis(T)). The compound has one emission band at 360 nm and thermoluminescence emission occurs at about 500-600 nm wavelengths. Current-voltage measurements were performed on ITO/AZ/Alq₃/Al device with three different thickness of the film.

Full text of DOI:10.1016/j.molstruc.2009.10.031

Journal of Molecular Structure 963 (2010) 175–182
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Thermal, optical, electrical and structural study of new symmetrical
azomethine based on poly(1,4-butanediol)bis(4-aminobenzoate)
b
c
c
c
Agnieszka Iwan a, , Pawel Bilski , Henryk Janeczek , Bozena Jarzabek , Marian Domanski ,
*
Patrice Rannou d, Andrzej Sikora a, Damian Pociecha e, Bozena Kaczmarczyk c
a Electrotechnical Institute, Division of Electrotechnology and Materials Science, M. Sklodowskiej-Curie 55/61 Street, 50-369 Wroclaw, Poland
b Institute of Nuclear Physics, Polish Academy of Sciences, Radzikowskiego 152 Street, 31-342 Krakow, Poland
c Centre of Polymer and Carbon Materials, Polish Academy of Sciences, 34 M. Curie-Sklodowska Street, 41-819 Zabrze, Poland
d Laboratoire d’Electronique Moléculaire, Organique et Hybride, UMR5819-SPrAM (CEA-CNRS-Univ. J. FOURIER-Grenoble I), INAC Institut Nanosciences & Cryogénie, CEA-Grenoble,
17 Rue des Martyrs, 38054 Grenoble Cedex 9, France
e University of Warsaw, Department of Chemistry, Zwirki i Wigury 101, 02-089 Warsaw, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 August 2009
Received in revised form 20 October 2009
Accepted 20 October 2009
Available online 23 October 2009
The synthesis, characterization and mesomorphism of new thermotropic imine prepared via condensa-
tion of biphenyl-4-carboxaldehyde with poly(1,4-butanediol)bis(4-aminobenzoate) was reported. The
structure of imine was characterized by means FTIR, 1H, 13C NMR spectroscopy and elemental analysis;
the results show an agreement with the proposed structure. The differential scanning calorimetry, polar-
izing optical microscopy and X-ray diffraction (WAXRD, SAXRD) were employed to evaluate their phase
transitional behavior. The imine exhibited smectic A (SmA) and smectic B (SmB) mesophases. Liquid crys-
talline properties of the azomethine were studied additionally via UV–vis spectroscopy in the function of
temperature (UV–vis(T)). The compound has one emission band at 360 nm and thermoluminescence
emission occurs at about 500–600 nm wavelengths. Current–voltage measurements were performed
on ITO/AZ/Alq3/Al device with three different thickness of the film.
Keywords:
Azomethines
Imines
Liquid crystals
Photoluminescence
Thermoluminescence
Current–voltage measurements
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
conjugated compounds is the presence of basic centres associated
with imine nitrogen. This implies that acid–base chemistry can be
The development of new materials based on self-organizing
systems has had a great deal of attention due to their potential in
the construction of well-defined supramolecular nanostructures.
Self-assembling molecules, which include liquid crystals (LC), block
copolymers, hydrogen bonded complexes and coordination
polymers are widely studied for their great potential as advanced
functional materials [1–8]. Liquid crystals, made up of rod-like mol-
ecules, exhibit novel supramolecular architecture and have found
novel use in optoelectronic applications i.e., flat panel displays
(FPDs). The azomethines called also Schiff bases or imines are widely
investigated as liquid crystalline materials. LC, rod shaped, symmet-
rical azomethines, being also under consideration in the present
paper, have been examined in the following papers [9–21].
used for the modification of the compound properties.
Inspired by the above described findings we have undertaken a
detailed study of the structure–spectroscopic–thermal properties
in a new azomethine (abbreviated hereinafter as AZ). In this work
we will present the thermal (POM, DSC), optical (UV–vis, photo-
and thermoluminescence), electrical (current–voltage) measure-
ments and structural (AFM, X-ray) characterizations of calamitic
azomethine. The LC behavior as well as photo- and thermolumi-
nescence properties of the symmetrical azomethine based on the
structure poly(1,4-butanediol)bis(4-aminobenzoate) were not
investigated so far.
In this paper, we will present full characterization of the LC azo-
methine of great interest for the emerging field of (supra)molecu-
lar electronics and for their uses as active layers in optoelectronic
devices (LEDs, FETs, Solar Cells, Lasers).
Azomethines i.e., compounds containing CH@N– groups in the
main chain, being isoelectronic with the corresponding p-pheny-
lene vinylene, exhibit interesting electronic, luminescent and
non-linear optical properties [8,12,20]. The principal feature of
the azomethines which distinguishes them from the majority of
2. Experimental section
2.1. Synthesis
* Corresponding author.
E-mail address: a.iwan@iel.wroc.pl (A. Iwan).
All chemicals and reagents were used as received from Aldrich.
0022-2860/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2009.10.031

176
A. Iwan et al. / Journal of Molecular Structure 963 (2010) 175–182
2.1.1. Synthesis of 4,40-(butane-1,4-diylbis(oxy)bis(butane-4,1-diyl)-
bis(4-(biphenyl-4-ylmethyleneamino)benzoate) (AZ)
Current–voltage characteristics were detected using electrome-
ter Keithey 6715. Current–voltage measurements were performed
on ITO/AZ/Alq3/Al device. Alq3 layer was vacuum deposited on top
of the ITO/AZ layer at a base pressure of ca. 10ꢁ6 Torr and then Al
electrode was vacuum deposited at the same pressure. The area of
the diodes was 9 mm2.
The surface morphology investigation of the AZ was performed
in air using a commercial Innova company Veeco Co. Atomic Force
Microscopy (AFM) in the contact and uncontact mode. Measure-
ments were done in Tapping Mode and Phase Imaging. Also Local
Contrast was made.
The phase transitions and mesogenicity were studied by differ-
ential scanning calorimetry (DSC) and polarizing microscope
observations (POM). DSC were measured on a TA-DSC 2010 appa-
ratus using sealed aluminium pans under nitrogen atmosphere.
The textures of their mesophases were observed with a POM
setup composed of: (i) a LEICA DMLM Microscope working in both
transmission and reflexion modes and equipped with a set of
lenses allowing magnification by 2.5ꢀ, 5ꢀ, 10ꢀ, 20ꢀ and 50ꢀ,
(ii) a LINKAM LTS350 (ꢁ196 °C till +350 °C) hot plate and a LINKAM
CI94 temperature controller, (iii) a JVC Numeric 3-CCD KYF75 cam-
era (resolution: 1360 ꢀ 1024).
Poly(1,4-butanediol)bis(4-aminobenzoate) (PBBA, mp. 56 °C) (1
mmol) and biphenyl-4-carboxaldehyde (mp. 57–59 °C) (2 mmol)
were added in a glass reactor fitted with stirrer. The reaction mix-
ture was purged with nitrogen for 30 min, and then the tempera-
ture was raised to 170 °C and kept on for 24 h under positive
pressure of nitrogen. The mixture was then cooled to room tem-
perature, scraped and powdered. The crude product was washed
three times with methanol (3 ꢀ 500 ml) and next two times with
acetone (2 ꢀ 350 ml) to remove unreacted compounds. Finally
the azomethine was dried at 60 °C under vacuum for 24 h to give
the AZ (88%).
1H NMR (300 MHz, CDCl3, TMS) [ppm] d 8.47 (s, 2H, 2 ꢀ CH@N);
8.07–8.10 (d, 4H, orto position to –O–C@O); 7.97–7.99 (d, 4H,
4 ꢀ HAr); 7.64–7.74 (m, 8H, 8 ꢀ HAr); 7.45–7.50 (m, 2H, 2 ꢀ HAr);
7.30–7.39 (m, 4H, 4 ꢀ HAr); 7.24–7.26 (m, 4H, 4 ꢀ HAr, orto position
to N@CH); 4.36 (m, 4H, 2 ꢀ CH2–O–CO); 3.46–3.51 (m, 8H,
4 ꢀ CH2–O); 1.64–1.87 (m, 12H, (CH2)6).
13C NMR (75 MHz, CDCl3, TMS) [ppm] d 166.37 (–O–CO); 161.19
(CH@N); 156.19 (–CAr–N@); 140.09 (CAr–CAr–); 131.55 (CAr–CH@
N–); 130.84; 130.27 (CAr–, orto position to O–CO–); 129.53;
129.00; 128.92; 128.46; 128.02; 127.68; 127.62; 127.50; 127.35;
127.18 (CAr– in Ph–Ph); 120.69 (CAr– at orto position to –N@CH);
70.72; 70.35; 70.24 (–CH2–O–); 64.79; 64.16 (–CH2–O–CO–);
26.49; 26.39; 25.66 (–CH2–CH2–).
Small angle X-ray diffraction (SAXRD) experiments were per-
formed with Bruker NanoStar setup (Cu Ka radiation, crossed-cou-
pled Goebel mirrors, Vantec 2000 area detector). Temperature of
the sample, placed in thin walled glass capillary (1 mm), was con-
trolled using MRI TCPU H heating stage. Wide angle X-ray diffrac-
tion (WAXRD) was conducted on Bruker D8 GADDS system (Cu Ka
radiation, Goebel mirror, Hi-Star area detector). Sample was pre-
pared as a droplet on a heated surface.
UV–vis spectra in different temperatures were measured for
thin film on the quartz (film cast from dichloroethane) by JASCO
V-570 UV–vis-NIR spectrometer using a temperature-controlled
optical cell in a temperature range from the room temperature to
clearing point in the heating process.
Thermoluminescent (TL) measurement was realized using the
RA’94 TL Reader/Analyzer. It is equipped with a platinum planchet
heater and a photomultiplier with a bialkali photocathode. The AZ
sample was irradiated with a test dose of 23 Gy Cs-137 gamma-
rays. Measurements were performed with a linear heating ramp
at a rate of 1 K/s up to temperature of 420 K. At this temperature
sample was totally liquidized.
FTIR (KBr)/cmꢁ1: 3055 (
2862 (ms CH2), 2804 (ms CH2), 1712 (
1595( Ph), 1561( Ph), 1520 ( Ph), 1487 (d CH2), 1471 (d CH2),
1450 ( Ph), 1411 ( Ph), 1372 (d CH2), 1364 (d CH2), 1339 (
CH2), 1306 (
m
CHr), 3035 (
m
CHr), 2939 (mas CH2),
m
C@O), 1625 ( C@N),
m
m
m
m
m
m
x
x
@CHr), 1287 (
s
CH2), 1253 (
m
(C@O)–O), 1191 (
m
Ph–N@), 1169 (d CHr), 1123 (d CH/
1100 (d CHr), 1047 (d CH), 1007 (d CHr/
CHr), 891 ( CHr/ COC), 853 ( CH2), 834 (
759 ( Ph), 732 ( CHr/skel. CH2), 719 (
Ph), 668 (d Ph) where:
mation, – out-of-plane deformation, Ph – phenyl,
vibrations, d – in-plane deformation, – out-of-plane deformation,
– wagging, – rocking, – twisting, skel. – skeletal vibrations,
m
COC), 1114 (d CH/
m
COC),
m
CO), 987 (
c
CHr), 972 (
c
c
m
q
c
CHr), 771 (
c
CHr),
m
c
c
Ph), 698 (c Ph), 688 (
c
m
– stretching vibrations, d – in-plane defor-
c
m
– stretching
c
x
q
s
Ph – phenyl, CHr – CH group in phenyl ring.
Tg, 23.8 °C (10 deg/min); Anal. Calcd. for C52H52N2O6 (800.979):
C, 77.97%; H, 6.54%; N, 3.50%. Found: C, 77.64%; H, 6.46%; N, 3.75%.
2.2. Instruments
3. Results and discussion
Synthesized compound was characterized by 1H NMR and ele-
mental analysis. Compound was also characterized by Fourier
transform infrared (FTIR) and ultraviolet–visible (UV–vis) absorp-
tion spectroscopy. NMR was recorded on a Bruker AC 200 MHz.
Chloroform-d (CDCl3) containing TMS as an internal standard
was used as solvent. Elemental analyze (C, H and N) was recorded
on a 240C Perkin-Elmer analyzer. Infrared spectrum (FTIR) was ac-
quired on a DIGILAB FTS-40A Fourier transform infrared spectrom-
eter in the range of 4000–400 cmꢁ1 at a resolution of 2 cmꢁ1 and
for an accumulated 32 scans. Sample was analyzed as pellets in
potassium bromide.
3.1. Synthesis and characterization
In general, the condensation of amines with aldehydes efficiently
takes place in different solvents in the presence of the catalyst. The
condensation of poly(1,4-butanediol)bis(4-aminobenzoate) (PBBA)
with biphenyl-4-carboxaldehyde, did not proceed in these reaction
conditions (see Table 1). In our opinion it is caused by low reactivity
of the PBBA.
It is well known that the azomethine carbon has poor electro-
philicity and has the tendency of enolizable imines and imine
derivatives [22]. Additionally, there are many factors that can be
altered in order to drive the reaction forward such as solvent, con-
centration, pH and temperature. For an imine bond formation (it
means dynamic reaction) the change in the free energy during
Solution (THF) UV–vis absorption spectra were recorded in situ
during analytical size-exclusion chromatography (SEC) analyses
using the diode array detector (DAD) of a Hewlett–Packard 1100
Chemstation equipped with a 300 ꢀ 7.5 mm Polymer labs PLgel
Mixed-D 5
l
m/104 Å column, a DAD detector and a refractive index
the reaction must be favourable, i.e.,
DG° in equation DG° =
(RI) detector.
D
H° ꢁ T
D
S° must be less than zero. Moreover, many external con-
Solution photoluminescence (PL) spectra were recorded at RT
on a Hitachi F-4500 spectrometer using diluted THF solution show-
ing an absorption level of. ca. 0.1 absorbance unit at the wave-
length chosen for their excitation.
siderations, including steric and electronic factors can influence on
the progress of the reaction.
Azomethine described in this paper was prepared via high tem-
perature melt condensation at 170 °C. Substrates were introduced

A. Iwan et al. / Journal of Molecular Structure 963 (2010) 175–182
177
Table 1
Optimization of the reaction conditions of the azomethine.
Solvent
Catalyst
Temperature (°C)
Time (h)
Yield (%)
Chloroform
DMA
NMP
m-krezol
–
PTS, LiCl, DABCO, DMAP, DABCO/TiCl4
PTS, LiCl, DABCO, DMAP, DABCO/TiCl4
PTS, LiCl, DABCO, DMAP, DABCO/TiCl4
PTS, LiCl, DABCO, DMAP, DABCO/TiCl4
–
b.p.
12, 24, 48
12, 24, 48
12, 24, 48
10, 24, 48
24
0
0
0
0
100, 120, 150, 170
100, 120, 150, 170
100, 120, 150, 170
170
88
b.p., boiling point of the solvent, PTS, p-toluenesulfonic acid; DABCO, 1,4-diazobicyclo[2,2,2]octane; DMAP, 4-dimethylamino-pyridin; DMA, N,N-dimethylacetamide; NMP,
1-methyl-2-pyrrolidinone.
into a 20-ml, two-necked, round-bottomed flask equipped with an
overhead stirrer and a nitrogen inlet. The crude product was
washed three times with methanol and next two times with ace-
tone to remove unreacted diamine and aldehyde. Additionally,
the compound was characterized by thin layer chromatography
(TLC) (developing solvent dichloroethane/ethyl acetate, 50/50).
Analytical thin layer chromatography (TLC) was performed on sil-
ica gel plates from E. Merck (silica gel F254). Additionally, the mass
dispersion of the AZ was examined by SEC method (polystyrene
standards were used). A single narrow SEC band found for the AZ
clearly indicated a monodispersive distribution of the imine
molecular masses and thereby additionally proved height purity
of the compound synthesized. Azomethine yield was 88%. The
compound was soluble at room temperature in chloroform, THF,
DMA and DMF. The azomethine was characterized by FTIR, NMR
spectroscopy and elemental analysis. The synthetic pathway along
with chemical structure of the azomethine synthesized in this re-
search is presented in Fig. 1 whereas their principal spectroscopic
and molecular characteristics are collected in Experimental sec-
tion. The spectral data were in accordance with the expected
formula.
spectrum in THF solution (1.25 ꢀ 10ꢁ3 M) at room temperature
was recorded for the 296 and 325 nm excitation wavelength. Inde-
pendently on the excitation wavelength the azomethine AZ exhib-
ited one emission band at 360 nm (Fig. 2). The Stokes shift for the
AZ was 66 nm. These spectroscopic data ascertained the fluores-
cence properties of this symmetrical azomethine.
3.3. Current–voltage characteristics
Current–voltage measurements were performed on ITO/AZ/
Alq3/Al device. The azomethine solution (1 w/v% in dichloroeth-
The absence of the residual amino (NH2) and aldehyde (CHO)
groups together with the appearance of band typical for azome-
thine bonds (HC@N–) was confirmed by FTIR and NMR spectra.
The spectra have assessed their purity and confirmed their molec-
ular structure. In particular the signal about 161 ppm, present in
the 13C NMR spectrum of the compound, confirms the existence
of the azomethine group carbon atoms. In proton NMR spectrum
of the investigated compound the imine proton signal at
8.47 ppm was observed. Additionally, in FTIR spectrum the imine
vibration at about 1625 cmꢁ1 was observed.
3.2. Optical properties
The photophysical properties of AZ were investigated using UV–
vis and photoluminescence spectroscopy. The UV–vis absorbance
spectrum of the AZ in THF solution displayed one absorption max-
imum at 296 nm and hump at 350 nm. The fluorescence (emission)
Fig. 2. Absorption (UV–vis) and emission (photoluminescence) spectra of the AZ in
THF solution.
NH2
O
C
O
O
O
H
O
O
C
O
+
2
C
PBBA
H2N
1700C
O
O
C
O
N
H
N
H
O
O
C
O
C
C
AZ
Polar spacer
Fig. 1. Synthetic route and chemical structure of the azomethine.
Mezogen
Mezogen

178
A. Iwan et al. / Journal of Molecular Structure 963 (2010) 175–182
ane) was spin-cast onto ITO-covered glass substrate at room tem-
perature in order to produce a thin hole transporting layer (HTL).
Residual traces of solvent were removed by heating the film in a
vacuum. The green emitting Alq3 layer was vacuum deposited on
top of the HTL layer at a base pressure of ca. 10ꢁ6 Torr and then
Al electrode was vacuum deposited at the same pressure. The area
of the diodes was 9 mm2. Current–voltage (I–V) curves of ITO/AZ/
Alq3/Al devices investigated in room temperature are shown in
Fig. 3. Differences were found along with change the thickness of
the film from 150 to 250 nm. It can be seen that for the thickness
of the film about 150 nm current rapidly increases with applied
voltage increase. The turn-on voltage of this device was observed
at about 2 V in room temperature.
about 12 V in room temperature. Additionally, for these devices we
did not observed quick current increases along with applied voltage
increase. This behavior confirmed the influence of the thickness of
the layer on the electrical properties of the compound. The differ-
ences found in the I–V characteristic of the imine confirm the differ-
ence planarity of the imine structure and different conformations of
the compound in film with various thicknesses.
3.4. AFM analysis
AZ film on the typical microscopic glass substrate was obtained
by dissolving at room temperature the compounds in chloroform
to form a homogenous solution. Residual solvent was removed
by heating the film. Film of the AZ obtained by casting from chlo-
roform solution shows a very interesting morphology, characteris-
tic of systems capable of forming organized supramolecular
structures. The surface morphology investigation of the AZ was
performed in air using an Atomic Force Microscope (AFM) in the
intermittent contact mode. The Inclination transformation of the
topography was made in order to provide better view of the struc-
tures. Additionally, the surface potential imaging (SP) was per-
formed in order to obtain information about homogeneity of
electrical properties of the surface. Obtained images of the AZ in
different range of scan (1000 and 500 nm, respectively) are pre-
sented in Fig. 4 in order they were mentioned above.
The film of the AZ obtained by casting from chloroform solution
on glass substrate showed ordered, planar-oriented morphology
(see Fig. 4). AFM images of the AZ, revealed terraces-like organiza-
tional features as we could clear seen in the Inclination transforma-
tion. One can note the ‘‘steps” of few nanometers (5–12 nm) in
height, creating main relief of the topography. After careful study
of the picture (4b), one can see also a number of edges of very small
terraces covering all surface of the sample. The height of single
layer is about 0.15 nm. It reveals layered structure of the sample.
On the other hand the turn-on voltage of the devices with thick-
ness of the film about 200–250 nm was similar and was observed at
0.00035
200 nm
250 nm
150 nm
0.00030
0.00025
0.00020
0.00015
0.00010
0.00005
0.00000
0
2
4
6
8
10
12
14
Voltage [V]
Fig. 3. Current–voltage curves of device ITO/azomethine/Alq3/Al (thickness about
150, 200 and 250 nm).
Fig. 4. AFM images of the AZ coated from chloroform solution. Left side: AFM topography images, middle transformation Inclination, right side: surface potential (a) surface
pattern 20ꢂ20
l
m2, Rms: 50 nm, (b) surface pattern 0.5ꢂ0.5 m2, Rms: 9.95 nm.
l

A. Iwan et al. / Journal of Molecular Structure 963 (2010) 175–182
179
The slopes of large terraces are as a matter of fact made of a large
group of elementary layers placed very close to each other.
3.5. Liquid crystalline properties
Liquid crystal properties of the AZ were investigated mainly
with the help of differential scanning calorimetry (DSC), polarizing
optical microscope (POM) and X-ray diffraction. The tentative mes-
ophases identifications and the sequence of phase transitions re-
lated to the AZ are based on the identification of textures
appearing in two reference textbooks for liquid crystals [23,24]
and on repeated POM and DSC experiments. Liquid crystalline
properties of the azomethine AZ were additionally studied by
UV–vis spectroscopy in different temperatures (UV–vis(T)) and
thermoluminescence (TL).
3.5.1. DSC and POM study
Upon DSC analysis the AZ exhibited two enantiotropic transi-
tions such as crystal-to-mesophase (Cr/M) and mesophase-to-iso-
tropic state (M/I). DSC curves of the AZ during second cooling
and heating scan (1 deg/min), along with the DSC curves of the
AZ during first heating scan (10 deg/min) and heating scan after
quenching (15 deg/min) are presented in Fig. 5a and b and 5c,
respectively. Compound AZ showed a liquid crystal to isotropic
transition at 149.8 °C with a
DH = 8.7 J/g on the heating rate
1 deg/min (Fig. 5a). On cooling from the isotropic state to room
temperature, an exothermic peak appeared at 147.2 °C with a
D
H = 8.9 J/g, corresponding to the transition from the isotropic
state to the liquid crystalline phase (Fig. 5a). Moreover, it should
be note that during heating of the sample (after rapid cooling) so
called ‘‘cold” crystallization was found at 88.9 °C (Fig. 5a). This
behavior was not observed for the AZ during first heating scan
(Fig. 5b). Details of the transition temperatures and associated en-
thalpy change of the AZ, as determined by DSC are summarized in
Table 2.
The POM measurements revealed textures which could indicate
SmA and SmB mesophases. Details of transition temperatures of
the compound AZ as determined by POM are summarized in Table
2 together with their phase variants.
The microphotographs of the smectic phases obtained for the
compound AZ are shown in Fig. 6.
The SmA mesophase of the AZ showed a focal conic fan texture
(Fig. 6). We did not manage to catch a POM image (observed during
a cooling scan) at the SmA > SmB transition with the typical so-
called transition bars. Nevertheless we are quite confident that
SmB > SmA transition and SmA > SmB occurred at 125 °C and at
120 °C during heating and cooling scans, respectively. These obser-
vations were confirmed by DSC study of the sample during the first
heating scan at 10 deg/min and heating rate 15 deg/min after
quenching (Table 2). On DSC thermogram presented in Fig. 5b
two endotherms at 124.6 °C and at 154.9 °C before isotropisation
were found during heating of the sample at 10 deg/min. Similar
behavior was observed during heating of the sample at 15 deg/
min after quenching (see Table 2 and Fig. 5c). These observations
are in quite good agreement with DSC measurement of the sample
during cooling of the sample at 1 deg/min (Fig. 5a). After a first
exotherm with a maximum at 147.2 °C another exotherm is slug-
gishly developing from ca. 115 °C to its maximum at ca. 99.2 °C.
We suppose that this last exotherm is in fact consisting of 2 con-
secutive exotherms characteristics of a SmA > SmB transition fol-
lowed by a SmB > Frozen SmB and/or Cr transition.
Fig. 5. DSC curves of the AZ obtained on (a) the heating and the cooling rate 1 deg/
min under N2 atmosphere. The temperature range from 40 to 160 °C, (b) the heating
rate 10 deg/min under N2 atmosphere. The temperature range from 0 to 200 °C and
(c) the heating rate 15 deg/min after quenching under N2 atmosphere. The
temperature range from 12 to 160 °C.
or heating scans. However, at the bottom left corner of POM images
more white parts were observed for the focal conic fans in Fig. 6b
than in Fig. 6a.
While a SmA mesophase show a 1D (lamellar) organization
(with no positional order within a layer), going to a more ordered
orthogonal smectic phase could lead to two closely related SmB
mesophases which show the same POM textures and that can only
be distinguished by X-ray: the so-called hexatic smectic B meso-
phase (SmBHex: an orientational order of local crystallographic
The paramorphotic (i.e., because its look is depending on the
existence of a previous focal conic fan texture of a SmA mesophase)
focal conic fan texture of the SmB mesophase (see Fig. 6) of AZ is
quite difficult to differentiate with the one of its SmA mesophase
as no typical transitory transition bars appeared during cooling

180
A. Iwan et al. / Journal of Molecular Structure 963 (2010) 175–182
Table 2
with long-range positional correlations of molecular mass centres).
In the above-mentioned proposed scenario of phase transitions, we
just indicated that the second mesophase observed on cooling from
the isotropisation state is a SmB.
Phase transition temperature (°C) of the compound detected by POM and DSC along
with enthalpy values, H (J/g).
D
Phase transitions, cooling, POM
Cr (or frozen SmB) 97.5, SmB 120, Sm A 156.5, I
Phase transitions, heating, POM
Cr (or frozen SmB) 105, SmB 125.5, Sm A 158.5, I
3.5.2. X-ray(T) study
X-ray diffraction revealed lamellar structure of mesophases of
studied compound, in agreement with proposed designation of
SmA and SmB phases. Layer spacing in smectic phases, d ꢃ 27 A,
only weakly depends on temperature, interestingly there is no pro-
nounced increase of layer thickness on transition from SmA to SmB
phase (Fig. 7). However, this transition is accompanied by clear
changes of high-angle signal, reflecting molecular ordering within
the smectic layer – broad diffused peak characteristic for short-
range correlations in SmA becomes much narrower in SmB phase
showing increase of in-plane correlation length (Fig. 8). Width of
the high-angle signal observed at 100 °C suggest that the meso-
phase is hexatic smectic B phase rather than a crystal smectic B.
In crystalline phase the basic periodicity of the structure is dou-
bled, as evidenced by subharmonic of the main small angle signal.
The tentative scenario of phase transition (during a cooling
scan: I > SmA > SmB > Cr) was fully confirmed and supported by
DSC, POM and XRD.
Phase transitions, cooling, DSC, [
Cr (or frozen SmB) 99.2 [5.5], SmB 115, Sm A 147.2 [8.9], I
D
H]*
Phase transitions, heating, DSC, [
H]**
‘‘cold” Cr 88.9 [51.8], SmB 126.1 [56.5], Sm A 149.8 [8.7], I
Phase transitions, heating, DSC, [
H]***
mp. 104.4, SmB 124.6, SmA 154.9 [11.0], I
Phase transitions, heating, DSC, [
H]****
‘‘cold” Cr 89.2 [19.3], SmB 125.9, SmA 146.4 [6.6], I
Phase transitions, heating, DSC, [
H]*****
D
D
D
D
‘‘cold” Cr 45.3 [3.4], mp. 97.9 [5.0], SmB 123.7 [1.3], SmA 142.6 [2.9], I
Cr, SmA, SmB, and I indicate crystal, smectic A, smectic B, and isotropic phases,
respectively.
*
Cooling 1 deg/min.
Heating 1 deg/min.
Heating 10 deg/min.
Heating 10 deg/min after slow cooling.
**
***
****
*****
Heating 15 deg/min after quenching, mp, melting point.
3.5.3. UV–vis(T) study
UV–vis spectra in different temperatures were measured for
thin film on the quartz (film cast from dichloroethane) by using a
Fig. 6. Photomicrographs of the optical textures of mesophases obtained for the AZ
(a) SmA, focal conic (seen at 135 °C) and (b) SmB, paramorphotic focal conic (seen at
104.9 °C).
axes, called Bond Orientational Order, develops within the layer,
without long-range positional order) and the so-called crystal B
mesophase (SmBCryst: a hexagonal lattice develops within the layer
Fig. 7. (a) Temperature dependence of layer spacing for AZ on cooling, (b) small
angle X-ray diffraction patterns at T = 135 °C (SmA), 100 °C (SmB) and 80 °C (Cry).

A. Iwan et al. / Journal of Molecular Structure 963 (2010) 175–182
181
0.12
25°C
105°C
120°C
126°C
159°C
0.10
0.08
0.06
0.04
0.02
200
250
300
350
400
450
500
wavelength [nm]
Fig. 9. Temperature dependence of UV–vis spectra of the AZ at the following
temperatures: 25, 105, 120, 126, 159 °C.
Table 3
UV–vis absorption characteristic of the AZ.
UV–vis
Temp. (°C) k* (nm) A1 (a.u.) k2 (nm) A2 (a.u.) k* (nm) A3 (a.u.)
1
3
25
105
120
126
159
230
231
231
231
231
0.089
0.066
0.058
0.055
0.053
311
301
298
295
295
0.079
0.061
0.055
0.053
0.049
–
–
368
365
360
360
0.048
0.041
0.039
0.035
*
Position of bands obtained by the second derivative method.
The changes of the absorption spectra along with increase the
temperature from 25 to 159 °C can be explained by the geometric
isomers (E and Z) as shown in Fig. 10 and by the aggregation of the
AZ at this temperature range to induce tautomerizm in the ground
state as proposed by Ogawa et al. [26,27]. In the case of the imine
structure two isomers are possible (E and Z), even though the Z
(trans) structure is always supposed to be thermodynamically
more stable. Similar behavior was found for the salicylideneani-
lines and polyazomethines [8,28]. The changes of the absorption
properties of the imine along with change the temperature con-
firmed the conformational changes in the conjugating backbone
structure of the compound.
Fig. 8. 2-D wide angle X-ray diffraction patterns at (a) 80 °C, Cry; (b) 100 °C, SmB
and (c) 135 °C, SmA.
temperature-controlled optical cell. Fig. 9 shows the UV–vis spec-
tra of the AZ in a temperature range from room temperature to
clearing point in the heating process.
UV–vis absorption bands in the azomethine thin film spectrum
at ambient temperature were about 40 nm blue shifted in compar-
ison with the one detected in THF solution. This seems to indicate
that the conformation of the molecule in solution and in film is dif-
ferent. Moreover, probably this is the case when the transition di-
pole moments of the molecules are parallel (k) and leading to the
formation of so-called H-aggregates, characterized by strongly
hipsochromic (blue) shifted UV–vis spectra [25].
3.5.4. Thermoluminescence study
An example of a typical thermoluminescent glow-curve is illus-
trated in Fig. 11. The glow-curve consists of a weak single peak
located at about 385 K. Its shape was fitted with a first order kinet-
In UV–vis spectra of the AZ blue shift of the second absorption
band along with increase the temperature was observed (Table 3).
The second absorption band around 311 nm at 25 °C was about
10 nm hipsochromically shifted along with increase the tempera-
ture to 105 °C. The absorption band corresponding to the azome-
thine bond was broader but well defined and was clearly
observed to temperature of isotropisation of the AZ appeared at
195 °C. Along with increase the temperature from 25 to 159 °C
the second absorption band was 15 nm blue shift along with de-
creased the absorption intensity (hypochromic effect). The first
and third absorption bands were not well defined and exhibited
very low intensity (Fig. 9). To find the positions of this band the
second derivatives method has been used (i.e., minimum of the
second derivative of absorption corresponds to the absorption
maximum).
isomer E (cis)
N
H
C
N
C
H
isomer Z (trans)
Fig. 10. Schematic structure of the two possible geometric isomers of imine bond.

182
A. Iwan et al. / Journal of Molecular Structure 963 (2010) 175–182
and thermal properties of such compounds with imines bonds.
50
40
30
20
10
0
For the practical point of view this compound could be applied in
organic light-emitting devices, which operating temperature could
be higher than room temperature. Additionally, obtained azome-
thine could be used in mixture with other LCs for LC displays or
could be applied as thermo-detectors.
E=0.88 eV
10 s-1
s=2.3 10
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4. Conclusion
The first example of poly(1,4-butanediol)bis(4-aminobenzo-
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AZ was found to exhibit an enantiotropic smectic A and B phases.
The present investigation may be useful for the better understand-
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