Ionically self-assembled terephthalylidene-bis-4-n-alkylanilines/n-decanesulfonic acid supramolecules: Synthesis, mesomorphic behaviour and optical properties

Article abstract of DOI:10.1016/j.saa.2008.08.014

This paper describes the preparation, mesomorphic and photophysical studies of a two type of calamitic molecules derived from azomethines, N,N′-(1,4-phenylenebis(methan-1-yl-1-ylidene)bis(4-pentylbenzenamine) (LCBAZ1) and N,N′-(1,4-phenylenebis(methan-1-y

Full text of DOI:10.1016/j.saa.2008.08.014

Spectrochimica Acta Part A 72 (2009) 72–81
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Ionically self-assembled terephthalylidene-bis-4-n-alkylanilines/n-
decanesulfonic acid supramolecules: Synthesis, mesomorphic
behaviour and optical properties
Agnieszka Iwana,∗, Henryk Janeczeka, Patrice Rannoub
a Centre of Polymer and Carbon Materials, Polish Academy of Sciences, 34 M Curie-Sklodowska Street, 41-819 Zabrze, Poland
b Laboratoire d’Electronique Moléculaire, Organique et Hybride, UMR5819-SPrAM (CEA-CNRS-University J. FOURIER-Grenoble I), DRFMC,
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:
Received 9 May 2008
Received in revised form 19 August 2008
Accepted 20 August 2008
This paper describes the preparation, mesomorphic and photophysical studies of a two type of calamitic
molecules derived from azomethines, N,Nꢀ-(1,4-phenylenebis(methan-1-yl-1-ylidene)bis(4-pentyl-
benzenamine) (LCBAZ1) and N,Nꢀ-(1,4-phenylenebis(methan-1-yl-1-ylidene)bis(4-decylbenzenamine)
(LCBAZ2) before and after protonation with the n-decyl sulfonic acid (DSA). The lengths of the outer
spacers are four or nine methylene units connected with the imine group by phenyl ring in the para posi-
tion. Liquid crystal properties of the undoped and doped azomethines are studied by differential scanning
calorimetry (DSC) and polarizing optical microscopy (POM). Wide-angle X-ray diffraction (WAXD) tech-
nique is used to probe the structural properties of the azomethines as well as its complexes. The lengths
of the outer flexible spacers have an effect on the mesomorphic properties of the azomethines. The com-
pound LCBAZ2 with nine methylene units exhibit smectic phases (Sm C and Sm X), while the LCBAZ1
with four methylene units exhibit nematic and smectic phases (N and Sm C). The effects of protonation
on the phase transitions of the azomethines are investigated. The structure formation of (LCBAZx)1(DSA)2
complexes are discussed on the basis of FTIR spectroscopy. Additionally, the azomethines before and after
protonation with DSA are investigated by UV–vis and photoluminescence (PL) spectroscopy. With the
exception of LCBAZ1 in its undoped state, the chloroform solution of the doped or undoped azomethines
exhibit greenish fluorescence when the solutions are subjected to 400 nm excitation wavelength. It are
concluded that the combination of the molecular and supramolecular engineering concepts stabilized the
smectic phase.
Keywords:
Azomethines
Protonation
Liquid crystalline
Photoluminescence
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
(–N N–) bonds and aliphatic spacers. All the tetramers exhibited an
enantiotropic nematic phase. While Smith et al. [13] obtained the
Various kinds of molecular interactions such as hydro-
gen bonds, halogen bonds and electrostatic interaction are
widely used to assemble the ordered supramolecular structures
[1–4]. Interactions between the polymer (oligomer) chains and
dopant (i.e. Lewis and Brönsted acids) influence changes of the
electrical, optical, thermal and mechanical properties in compar-
ison with the undoped compound and also improve solubility
[1].
Azomethines (polyazomethines) belong to the classic type of
molecules with liquid crystalline (LC) properties [1,5–15]. For
example, Henderson et al. [10] characterized a series of semi-
flexible LC tetramers with the two azomethine (HC N–), two azo
azomethines based on fluorinated carbon chains exhibited nematic
and smectic A phase transitions.
Less attention is paid to the LC oligomers with the azomethine
bond after doping, especially after protonation [16,17]. So far, one
important property of the azomethines is fact that lone electrons
pair of nitrogen atom in the imine group (HC N–) is able to form
supramolecular interactions with many electrophiles (dopants)
[1–4]. Pucci et al. [16] studied the liquid crystalline azomethine
before and after protonation with 4-decyloxybenzoic acid, being
able to form H-bond with pyridyl moiety of the azomethine. The
azomethine complexes exhibited nematic and smectic C phase
transitions.
Also a lot of works is dedicated to investigate the liquid crys-
talline properties of the terephthalylidene-bis-4-n-alkylanilines
[18–31] especially by X-ray technique. However, in accordance with
the best our knowledge the LC behaviour as well as photolumines-
∗
Corresponding author. Tel.: +48 32 2716077; fax: +48 32 2712969.
E-mail address: agnieszka.iwan@cmpw-pan.edu.pl (A. Iwan).
1386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2008.08.014

A. Iwan et al. / Spectrochimica Acta Part A 72 (2009) 72–81
73
cence properties of terephthalylidene-bis-4-n-alkylanilines after
protonation were not investigated so far.
1598, 1562, 1499, 1467, 1419, 1364, 1303, 1192, 1171, 1116, 1013, 970,
884, 852, 839, 812, 728, 591, 566, 551 cm−1
.
The main goal of this paper is obtain ionically self-organized
thermotropic liquid-crystalline comb-shaped supramolecules via
protonation of the imine sites by special protonating agent. All com-
pounds after protonation with n-decyl sulfonic acid (DSA) formed
smectic phase. This is probably the first example of the azomethine-
based derivatives exhibiting LC properties after protonation with
the DSA. Their fluorescent properties were also examined as poten-
tial application in the opto(electronics).
2.2.2. N,Nꢀ-(1,4-Phenylenebis(methan-1-yl-1-ylidene)bis(4-
decylbenzenamine)
(LCBAZ2)
Yellow powder, yield: 84%. 1H NMR (300 MHz, CDCl3, TMS)
[ppm]: ı = 8.53 (s, 2H, 2× HC N); 7.99 (4H, s, 4× HAr); 7.17–7.24 (m,
8H, 8× HAr); 2.60–2.65 (m, 4H, 2× CH2); 1.61–1.63 (m, 4H, 2× CH2);
1.27–1.31 (m, 28H, 14× CH2); 0.86–0.90 (m, 6H, 2× CH3). 13C NMR
(75 MHz, CDCl3, TMS) [ppm]: ı = 158.57, 149.24, 141.38, 138.62,
129.15, 128.98, 120.87, 35.52, 31.89, 31.52, 29.61, 29.60, 29.51, 29.32,
29.29, 22.68, 14.12. Anal. Calcd. for C40H56N2: C, 85.05%; H, 9.99%;
N, 4.96%. Found: C, 85.05%; H, 9.82%; N, 5.10%. mp: 70 ◦C. FTIR (KBr):
3031, 2955, 2916, 2847, 1623, 1600, 1594, 1498, 1467, 1415, 1358,
1303, 1261, 1197, 1170, 1116, 1100, 1013, 971, 938, 906, 888, 848, 831,
2. Experimental
2.1. General
All chemicals and solvents were of reagent grade obtained
from Aldrich Chemical Co., and all solvents were dried by stan-
dard techniques. The synthesized compounds in the undoped state
were characterized by 1H and 13C NMR and elemental analysis.
Undoped compounds were also characterized by Fourier trans-
form infrared (FTIR) and ultraviolet-visible (UV–vis) absorption
and photoluminescence (PL) spectroscopy. Azomethines doped
with DSA were subjected to UV–vis and PL spectroscopy. NMR
was recorded on a Varian Inova 300 MHz Spectrometer. Chloro-
form d (CDCl3) containing TMS as an internal standard were used
as solvent. Elemental analyses (C, H, N) were carried out by the
240C PerkinElmer analyser. FTIR spectra of the azomethines were
recorded on a PerkinElmer paragon 500 spectrometer (wavenum-
ber range: 400–4000 cm−1; resolution: 2 cm−1). Solution (CHCl3)
UV–vis absorption spectra were recorded on Hewlett-Packard
8452A spectrophotometer. Photoluminescence measurements of
the azomethines were carried out on a Jobin-Yvon HR 460
monochromator equipped with a CCD silicium detector cooled at
140 K. Excitation was performed with an argon laser at 400 nm
(1.25 × 10−3 M). Melting point and liquid crystal properties of the
synthesized compounds were determined by differential scan-
ning calorimetry (DSC) on a TA-DSC 2010 apparatus using sealed
aluminium pans under nitrogen atmosphere. X-ray diffraction pat-
terns were recorded using powder or film samples on a wide-angle
HZG-4 diffractometer working in typical Bragg geometry. Cu K␣
radiation was applied.
819, 783, 720, 557, 529 cm−1
.
2.3. Protonation of azomethines
Protonation of the azomethines with the DSA was carried out at
room temperature using chloroform as a solvent. DSA was added to
the chloroform solution of the azomethines studied in the 1:1 ratio
with respect to the imine nitrogens.
3. Results and discussion
3.1. Synthesis and characterization
The synthetic route for the azomethines is quite straightforward,
and each reaction step is a relatively well-known type. The most
common method for the imine synthesis is the reaction of car-
bonyl group in the aldehyde or ketone with the amine being usually
acid catalysed. The synthetic pathway for the imines is summa-
rized in Scheme 1. This reaction was first described by Schiff in
1864 and because of this imines are often referred to as Schiff bases
[32]. If Rꢀꢀ is a hydrogen atom, i.e. the reacting group is the alde-
hyde one product of the condensation sometimes is called aldimine
or azomethine while in the case when Rꢀꢀ exhibits alkyl or aryl
structure, i.e. the reacting group is the ketone—the corresponding
compound after condensation with the amine is called ketimine or
ketanil.
Imine bond formation is a simple reaction which involves loss
of H2O within a single molecule, or between two molecules con-
taining amino and carbonyl groups, such that a C N double bond is
formed either intra- or inter-molecularly [33]. The reaction, which
is acid-catalysed, is executed typically by refluxing the starting
materials under azeotropic conditions. Many external considera-
tions, including solvent, concentration, pH and temperature, as well
as steric and electronic factors, can influence the equilibrium shown
in Scheme 1. As such, there are many parameters that can be altered
in order to drive the reaction forward—or indeed backwards. For
a dynamic reaction, like imine bond formation, to proceed in the
direction of products, i.e. imines, the change in the free energy dur-
2.2. General synthetic procedure of LCBAZx
A detailed procedure is following: a solution of terephthaldicar-
boxaldehyde (TA) (1.0 mmol) in 5 ml of N,N-dimethylacetamide
(DMA) was added to a solution of amine (2.0 mmol) in DMA
(5 ml) with 0.06 g of p-toluenesulfonic acid (PTS). The mixtures
were introduced into a 20-ml, two-necked, round-bottomed flask
equipped with an overhead stirrer, a reflux condenser and an argon
inlet. The mixture was refluxed with stirring for 10 h. Then the com-
pound was filtered, washed and dried at 60 ◦C under vacuum for
12 h, and finally crystallised from a methanol.
2.2.1. N,Nꢀ-(1,4-Phenylenebis(methan-1-yl-1-ylidene)bis(4-
pentylbenzenamine)
(LCBAZ1)
Yellow powder, yield: 88%. 1H NMR (300 MHz, CDCl3, TMS)
[ppm]: ı = 8.52 (s, 2H, 2× HC N); 7.98 (4H, s, 4× HAr); 7.19–7.21
(m, 8H, 8× HAr); 2.60–2.63 (m, 4H, 2× CH2); 1.32–1.36 (m, 4H,
2× CH2); 1.61–1.64 (m, 4H, 2× CH2); 0.90–0.92 (m, 6H, 2× CH3).
13C NMR (75 MHz, CDCl3, TMS) [ppm]: ı = 158.55, 149.24, 141.46,
138.61, 129.98, 128.96, 120.87, 35.46, 31.46, 31.18, 22.53, 14.03. Anal.
Calcd. for C30H36N2: C, 84.86%; H, 8.55%; N, 6.60%. Found: C, 84.92%;
H, 8.61%; N, 6.80%. mp: 150 ◦C. FTIR (KBr): 2951, 2923, 2854, 1623,
Scheme 1. Schematic reactions of the imine structure formation.

74
A. Iwan et al. / Spectrochimica Acta Part A 72 (2009) 72–81
Scheme 2. Chemical structure of the synthesized azomethines.
ing the reaction must be favourable, i.e. ꢀG◦ in Eq. (1) must be less
than zero:
the 1:1 ratio with respect to the imines nitrogen in order to
assure full protonation of the compound. Azomethines doped with
the DSA were subjected to DSC, POM, FTIR, UV–vis and PL spec-
troscopy.
ꢀG◦ = ꢀH◦ − TꢀS◦
(1)
This paper reports the first example of the use of DSA
to modified liquid crystalline properties as well spectroscopic
(FTIR) and optical (UV–vis, PL) properties of the liquid crystalline
azomethines with odd- and even-aliphatic chain. In our previous
paper [36] we described the spectroscopic and optical proper-
ties, mainly UV–vis and FTIR of doped with DSA polyketimine
obtained from 3,8-diamino-6-phenylphenanthridine and trans-
1,2-dibenzoylethylene (PK1). The absorption spectrum of the doped
with DSA PK1 exhibited the small red shift (5 nm) in the position
of ketimine band in UV–vis spectrum in DMA solution. In FTIR
spectrum of doped PK1 the area of band attributed to stretching
vibrations of ketimine group diminishes a little, while the band at
about 1625 cm−1 ascribed to stretching vibrations of phenantridine
ring disappears. This indicates that DSA in the case of PK1 interact
mainly with nitrogen lone electron pair of phenantridine ring [36].
The synthesis and characteristic of the DSA will be present in
the separate paper [37].
Azomethines described in this paper were prepared from the
terephthaldicarboxaldehyde (TA) and two amines via high tem-
perature solution condensation in N,N-dimethylacetamide (DMA)
at 160 ◦C (see Section 2). Substrates were introduced into a
20-ml, two-necked, round-bottomed flask equipped with an
overhead stirrer, a reflux condenser and an argon inlet. Azome-
thines yield was in the range of 84–88% in dependence on the
amines structures (see Section 2). The compounds were solu-
ble at room temperature in chloroform, THF, DMA, and HMPA.
The azomethines were characterized by FTIR, proton and carbon
NMR spectroscopy and elemental analysis. The synthetic pathway
along with chemical structures of the azomethines synthesized
in this research is presented in Scheme 2 whereas their principal
spectroscopic and molecular characteristics are collected in Sec-
tion 2. The spectral data were in accordance with the expected
formula.
In Section 2 NMR data concerning two compounds investigated
are collected. In particular the signal about 159 ppm, present in
the 13C NMR spectra of the compounds, confirms the existence of
the azomethine group carbon atoms. In proton NMR spectra of the
investigated compounds the imine proton signal at 8.52 ppm was
observed. 1H and 13C NMR spectra of the LCBAZ1 and LCBAZ2 are
present in Fig. 1.
3.2. Protonation of azomethines
The main aim of this paper is preparation new types of lumines-
cent, amphiphilic, supramolecular compounds in which the lateral
groups are bonded to the ␲-conjugated main chain not by covalent
bonds as it has been done to date but by ionic-type bonds. This
ionically bonded lateral group must serve as dopant anions which
strongly influence not only the processibility but also the band
structure, and by consequence the spectroscopic properties, of the
compounds. This approach requires a careful design of the dopant
structure which promotes supramolecular ordering of the sys-
tems by molecular recognition phenomena. We propose an unique
approach in which the processibility, supramolecular structure and
by consequence selected physical properties of the electroactive
compounds will be tuned via specific interactions with specially
designed functional dopant capable of inducing a controled order in
the host compound. Proposed schematic model for the interaction
between the azomethines and the protonating agent are presented
in Scheme 3.
Protonation of the azomethines with the DSA was carried out
at room temperature using chloroform as a solvent. DSA was
added to the chloroform solution of the azomethines studied in
Fig. 1. (a) 1H NMR and (b) 13 C NMR spectra of the azomethines.

A. Iwan et al. / Spectrochimica Acta Part A 72 (2009) 72–81
75
the frequencies of the bands due to the CH2 antisymmetric and
symmetric modes of the alkyl chain usually appear at 2915 and
2850 cm−1, corresponding to highly ordered hydrocarbon chains
with an all-trans conformation [34,35]. All of the CH2 antisym-
metric vibration in the case of the LCBAZ1 appeared at a relatively
higher frequency, indicating the existence trans and cis isomers in
the hydrocarbon chains. While for the LCBAZ2 only the hydrocarbon
chains with an all-trans conformation were found.
FTIR spectroscopy provides a facile method of elucidating the
effect of Brönsted acid protonation on the molecular structures of
the azomethines. LCBAZ1 and LCBAZ2 with the DSA (dopant) at
1:2 ratio was dissolved in chloroform and after evaporating of the
solvent at vacuum FTIR spectra were recorded. In the region charac-
teristic for the stretching vibrations of HC N– group FTIR spectra of
the dopedsamples investigateddiffer from this oftheundoped ones
(Fig. 2b–d). It can be mentioned that chosen protonation agent have
no bands in that region. The most evident changes can be noticed
for the imine band of the azomethines doped with the DSA. In that
case the broadening of the imine band along with shift of absorption
to shorter wavelength is recorded. Simultaneously the other bands
corresponding to stretching vibrations of the imine band decrease
their areas (Fig. 2d). On the other hand changes are also observed for
the bands corresponding to the stretching vibrations of the aliphatic
spacers of the doped compounds. The band at 2923 cm−1 asso-
ciated with the aliphatic group asymmetric stretching vibrations
at LCBAZ1 doped with the DSA spectrum is shift to the shorter
wavelength for the doped LCBAZ1. The band at 2854 cm−1 ascribed
to symmetric stretching vibrations of the aliphatic group of the
undoped LCBAZ1 is shift to the shorter wavelength for the doped
LCBAZ1 (see Fig. 2b). On the other hand the band at 2847 cm−1
(undoped LCBAZ2) is shift to the longer wavelength for the doped
LCBAZ2, while the bands at 2916 and 2955 cm−1 are not change for
the doped compound (see Fig. 2c).
3.4. Mesomorphic properties
The mesomorphic behaviour of the compounds was studied by
DSC and POM. The phase transitions and thermodynamic data of the
undoped and doped compounds are summarized in Tables 1 and 2.
The mesomorphic results indicated that the compound LCBAZ1
showed nematic and smectic mesophases, while the compound
LCBAZ2 showed only smectic mesophases. As can be seen from
Tables 1 and 2, the formation of the mesophases observed was
strongly dependent on the length of the side chains. Upon DSC
analysis all compounds exhibited typical transition behaviour, and
two enantiotropic transitions, crystal-to-mesophase (Cr/M) and
mesophase-to-isotropic (M/I), were observed. Both the melting
temperatures and the clearing temperature slightly decreased with
increase the length of the side chains, i.e. 48 (LCBAZ1) < 51 (LCBAZ2)
and 230.6 (LCBAZ1) > 181.5 (LCBAZ2) on cooling cycle. Also the tem-
perature range of the mesophases was decreased with increase the
length of side chains. The DSC analysis of the compounds LCBAZ1
and LCBAZ2 showed four an endothermic clearing transition on
the second heating cycle. When cooled from the isotropic liquid
state to room temperature, the compounds exhibited three or five
exotherms corresponding to the transition from the isotropic liquid
to crystalline phase (see Fig. 3).
Scheme 3. Proposed schematic model for the interaction between the azomethine
and protonating agent.
3.3. FTIR spectroscopy
In the FTIR spectra, the absorption band at about 1623 cm−1 for
the LCBAZ1 and LCBAZ2, which is characteristic of the –HC N–
group, confirms the presence of the azomethine group. In addi-
tion to the –HC N– stretching band, a band at 1596 cm−1 can be
distinguished ascribed to the C C stretching deformations in the
aromatic ring. Small changes are observed for the vibrations that
originated from the alkyl chain. The stretching vibration bands of
the alkyl spacer appeared at 2923 and 2854 cm−1 for the LCBAZ1
and at 2916 and 2847 cm−1 for the LCBAZ2 (see Fig. 2a).
Compound LCBAZ2 showed a crystal to isotropic liquid tran-
sition at 183.4 ◦C with a ꢀH = 13 J/g on the second heating cycle.
On cooling from the isotropic state to room temperature, a small
exothermic peaks appeared at 181.5 and at 146.7 ◦C with a ꢀH = 13
and 8.6 J/g, respectively, corresponding to the transition from the
isotropic liquid to the liquid crystalline phase. Compound LCBAZ1
has isotropisation at higher temperature being observed at 230.8
and at 230.5 ◦C on heating and cooling of the sample, respectively.
The vibration bands of the alkyl spacer can be assigned to
the antisymmetric and symmetric CH2 vibration. It is known that

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A. Iwan et al. / Spectrochimica Acta Part A 72 (2009) 72–81
Fig. 2. FTIR spectra of: (a) undoped azomethines in the range of 600–1650 cm−1 and 2800–3100 cm−1, (b) doped LCBAZ1 in the range of 500–1650 cm−1 and 2700–3100 cm−1
,
(c) doped LCBAZ2 in the range of 500–1650 cm−1 and 2700–3100 cm−1 and (d) doped azomethines in the range of 1500–1650 cm−1 (azomethine band).
Under the polarized optical microscope both compounds dis-
played fan-shaped textures (shown in Fig. 4). In the POM and DSC
study, the compound LCBAZ1 exhibited stable enantiotropic smec-
tic C (SmC) and nematic (N) liquid crystal phases (Fig. 4a and b),
while the compound LCBAZ2 exhibited stable enantiotropic smec-
tic C (SmC) and smectic X (SmX) liquid crystal phases (Fig. 4d and
e). According to the thermodynamic sequence of the phases, nor-
mally between the isotropic liquid and SmC there are two choices
Table 1
Identified mesophases and thermal parameters of the undoped and doped azomethines determined by POM
Phase transition behaviour, cooling, detected by POM
LCBAZ1
I 237 ◦
C
N 237 ◦C till 216 ◦
SmX 185 ◦C till 150 ◦
SmC 132 ◦C till 30 ◦
C
SmC 216 ◦C till 155 ◦
SmC 150 ◦C till 70 ◦
Cr < 30 ◦
C
Cr < 160 ◦
C
LCBAZ2
I 185 ◦C
C
C
Cr < 50 ◦
C
LCBAZ1/DSA (1:2)
I 132 ◦
C
C
C
LCBAZ2/DSA (1:2)
I 110 ◦C
SmC 108 ◦C till −5 ◦
C
Cr < −5 ◦
C
I, isotropic; Cr, crystalline; Sm, smectic; SmX is an unidentified order smectic phase.

A. Iwan et al. / Spectrochimica Acta Part A 72 (2009) 72–81
77
Table 2
Thermal parameters of the azomethines and their doped analogs determined by DSC
Phase transition behaviour detected by DSC
Code
Phase transitions (◦C) (corresponding enthalpy changes) (J/g)
Heating
Cooling
LCBAZ1
LCBAZ2
LCBAZ1/DSA
LCBAZ2/DSA
230.8 (3.4), 210.0 (1.9), 147.2 (8.7), 69.0 (42.0)
183.4 (13.0), 149 (8.5), 70.3 (30.0), 66.9 (53.0)
131.0 (3.7), 123.6 (5.8), 102.0 (7.5)
230.5 (2.6), 209.8 (1.8), 146.5 (8.6), 60.7 (2.9), 47.5 (36.0)
181.5 (13.0), 146.7 (8.6), 51.2 (68.0)
128.8 (6.6), 121.3 (6.0), 90.3 (8.5)
107.9 (28.0), 94.0 (12.0), 56.1 (4.9), 31.0 (14.0)
104.3 (35.0), 68.2 (3.7)
The temperature range from 25 to 225 ◦C.
either N or SmA. However to clearly confirmed the type of the
mesophase X-ray diffraction is necessary. The transition temper-
atures of the azomethines detected by POM are summarized in
Table 1.
Nematic phase of the LCBAZ1 shows Schlieren texture as is
shown in Fig. 4a, while smectic C phase of the LCBAZ1 exhibit
sanded texture (see Fig. 4b). The mosaic and sanded textures were
observed under POM for the smectic X and smectic C phases of the
compound LCBAZ2 (Fig. 4d and e).
The homeotropic texture was observed by using clean glass slide
and coverslip (sample sandwiched in between two glass plates). In
our case homeotropic texture for the smectic X (probably Sm A)
of the LCBAZ2 was observed. It is know that, in the homeotropic
texture, the molecules are aligned with an average perpendicu-
lar orientation with respect to the layers which lie parallel to the
surfaces. Additionally, the presence of two nitrogen atoms in the
azomethine structure which are able to interact with glass substrate
and as a result induce the molecules to readily self-assemble to be
perpendicular to the glass substrate and then to form homeotropic
alignment can not to be excluded.
Doping with the DSA change the mesophases of the LCBAZ1 and
LCBAZ2 (see Tables 1 and 2). To investigate effect of the dopant
on stability of the mesophase, the phase behaviour of the doped
compounds were compared with undoped ones, as shown in Fig. 5.
As it was said previously the smectic phases (Sm C and Sm X) were
identified by observation of their textures in polarized light optical
microscopy. It was not surprise that, addition of the DSA to the
azomethine was effective to reduce the clearing temperatures in
the range of 107.9–131.0 ◦C (see Table 2). Compounds doped with
the DSA exhibited enantiotropic Sm C phase lover than observed
for the undoped compounds on slow cooling from the isotropic
liquid. The distance between the doped and undoped Sm C phase
were found in the range of 42–104 ◦C. The biggest distance of the
undoped and doped azomethines of Sm C transition for the LCBAZ1
was observed.
Smectic C phase of the doped LCBAZ1 shows Schlieren and bro-
ken focal-conic fan textures as is shown in Fig. 4c.
It should be stress that the doped compounds exhibited low
crystallization temperatures observed by DSC and POM. The DSC
thermograms present in Fig. 3 were obtained in the temperature
range 25–225 ◦C. To detect the crystallization temperatures of the
doped compounds by DSC the temperature from −100 to 130 ◦C
were used. For the compound LCBAZ2 the crystallization at −3.3 ◦C
with ꢀH = 17.7 J/g during cooling of the sample from isotropic
state was found (see Fig. 6). For the compound LCBAZ1 crystal-
lization was found during heating of the sample from frozen state
(from −100 ◦C) of the sample at 14.6 ◦C with ꢀH = 17.2 J/g (Fig. 3c,
inset).
The stabilization of the smectic phase in the doped state (seen
in the disappearance of the nematic phase and very low crystal-
lization temperatures) probably was caused by the supramolecular
arrangement of these molecules in a columnar phase. But possi-
bly, due to this exact arrangement the order of the phase increased
so much as it caused a huge drop in the clearing temperatures
of the doped azomethines as well as in their low crystallization
temperatures.
Fig. 3. DSC thermogram of LCBAZ1 (a), LCBAZ2 (b) obtained on second heating and
second cooling, the heating rate 1 ◦C/min under N2 atmosphere, and of the doped
LCBAZ1 (c) obtained on second heating and cooling, the heating rate 0.5 ◦C/min
under N2 atmosphere.
The phase transition behaviour detected by POM is in good
agreement with the phase transition temperatures detected by
DSC.

78
A. Iwan et al. / Spectrochimica Acta Part A 72 (2009) 72–81
Fig. 4. Polarized light micrograph of the undoped LCBAZ1 (a and b), undoped LCBAZ2 (d and e) and doped LCBAZ1 (c), doped LCBAZ2 (f) (cross-polarization position).
crystallites is observed which showed crystalline pattern of both
compounds. The WAXD patterns for the undoped compounds show
very sharp diffraction peaks with several weak diffractions in
smaller angles (2ꢂ scanning) have several peaks, indicating that a
highly ordered crystalline structure exists in the azomethines, espe-
cially in the LCBAZ1. For the doped LCBAZ1 and LCBAZ2 (detected on
the glass as a thin film cast from dichloroethane) one broad diffrac-
tion peak of diffusion type centered at 23.26◦ (2ꢁ) was observed
in plot present in Fig. 7. Doped azomethines are semi-crystalline
as determined by the X-ray diffraction pattern. The diffractions
arising from the crystallites are observed at 6.67◦ and 10.21◦ for
LCBAZ2/DSA and at 5.47◦, 6.23◦, 8.34◦, 9.18◦, 9.84◦, 16.67◦ and 19.92◦
for doped LCBAZ1. The WAXD patterns for the doped LCBAZ1 show
more sharp diffraction peaks in the range 5–20◦, indicating that
a higher ordered crystalline structure exists in the doped LCBAZ1
than in doped LCBAZ2. The following two factors may be responsi-
ble for this effect: (i) the close molecular packing of the LCBAZ2/DSA
complex transforms the nonplanar conformation of the LCBAZ2
into the planar conformation and (ii) the fairly strong interaction
between the azomethine nitrogen atoms and dopant. This part of
our work needs more investigations.
Fig. 5. Graphs showing the phase behaviour of the undoped and doped compounds.
3.5. Wide-angle X-ray diffraction (WAXD)
The wide-angle X-ray diffraction patterns of the undoped and
doped compounds over the 2ꢁ range of 5◦–60◦ are shown in Fig. 7.
For the LCBAZ1 and LCBAZ2 WAXD in the solid state as a pow-
der was investigated (see Fig. 7). The diffraction arising from the
3.6. Optical properties
The UV–vis absorption and PL spectra of the compounds in
CHCl3 are presented in Fig. 8. The data of ꢃmax peaks of UV–vis
absorption and PL spectra in CHCl3 are listed in Table 3. The
absorption spectra of the azomethines in chloroform solution are
characterized by two well overlapping bands, one near 272 nm and
the other at around 360 nm. The former band can be assigned to
␲–␲* transition whereas the second band is characteristic to the
imine group in the LCBAZx and its position and intensity are not
dependent on the chemical constitution of the compound.
Electronic spectra of the azomethines after protonation change
dramatically in comparison with the undoped analogues (Fig. 8).
Both absorption bands in UV–vis spectra of the doped LCBAZx are
not well defined and exhibit lower intensity than absorption bands
of the undoped azomethines. Additionally, it should be emphasized
that the imine band in UV–vis spectra of the doped azomethines
is a little blue shift after protonation with the DSA and exist as a
shoulder (see Fig. 8).
Fig. 6. DSC thermogram of the doped LCBAZ2. The heating rate 6 ◦C/min under N2
atmosphere. The cooling rate 5 ◦C/min under N2 atmosphere. The temperature range
from −100 to 130 ◦C.
Additionally, we have studied the concentration behaviour
of the azomethines using UV–vis spectroscopy, monitoring the

A. Iwan et al. / Spectrochimica Acta Part A 72 (2009) 72–81
79
Fig. 7. WAXD diffractograms intensity vs. Bragg angle graph for the undoped and doped azomethines.
changes in the ␲–␲* and n–␲* transitions (see Fig. 9). UV–vis spec-
tra of the azomethines are exemplified by the undoped and doped
LCBAZ1 and shown the evolution of the absorption spectra for the
LCBAZ1 upon increasing the compound concentration at chloro-
form solution.
We found changes in the UV–vis spectra of the undoped and
doped LCBAZ1 in chloroform solution along with increase the con-
centration and consequently along with increase the absorbance
from 1 to 3.00 (see Fig. 9). All of the spectra shown in Fig. 9 were
recorded also after 24 h and 48 h. No time-dependent in UV–vis
absorption spectra of the compound was observed.
The fluorescence under 400 nm excitation wavelength exhibits
only the azomethine LCBAZ2 in chloroform solution and is observed
at 538 nm (=2.30 eV). LCBAZ1 under 400 nm excitation wavelength
exhibits very low fluorescence, cased probably by differences in
the length of the aliphatic chain and the effect of parity of the outer
flexible spacer (odd–even effect).
Protonation of the azomethine LCBAZ2 in the chloroform solu-
tion causes a blue shift of the maximum of emission band (507 nm,
2.44 eV). The photoluminescence properties of the doped LCBAZ1
are observed at about 502 nm (=2.47 eV), and are a little blue shifted
in comparison to the LCBAZ2. Stokes shift being the difference
between emission and absorption wavelengths indicates differ-
ences in the energy loss which occurred during transition from S0
to S1. The values of the energy loss increased in both the azome-
thines after protonation. The Stokes shift was calculated according
to the following equation:
ꢀ
ꢁ
1
ꢃabs.
1
ꢄabs. − ꢄemis.
=
−
× 107(cm−1
)
(2)
ꢃemis.
The Stokes shift for the undoped LCBAZ2 was 9313 cm−1 while
for the doped one was 8249 cm−1. For the doped LCBAZ1 the Stokes
shift was 8446 cm−1. Table 3 shows absorption and photolumi-
nescence of the azomethines before and after protonation with
the DSA. PL spectra of the undoped and doped azomethines are
presented in Fig. 8. Additionally, the observed photoluminescence
shifts after protonation with the DSA clearly indicate that this prop-
erty is principally governed by local conformation of the compound.
Table 3
UV–vis absorption and photoluminescence characteristics of the undoped and doped azomethines
Code
UV–vis
Photoluminescence
b
ꢃabs. (nm)/(eV)
ꢃemis. (nm)/(eV)
Stokes shifta (cm−1
)
ꢀꢃemis. (nm)
LCBAZ1
LCBAZ2
LCBAZ1/DSA
LCBAZ2/DSA
272/4.55, 360/3.44
272/4.55, 358/3.46
300/4,13, 355/3.49
300/4,13, 355/3.49
&
−
9313
8249
8446
−
−
538/2.30
502/2.47
507/2.44
−
−31
a
Calculated according to equation ꢄabs. − ꢄemis. = (1/ꢃabs. − 1/ꢃemis.) × 107 (cm−1).
b
Shift of undoped and doped azomethines in solution, “−” means blue shift; &, very low intensity.

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A. Iwan et al. / Spectrochimica Acta Part A 72 (2009) 72–81
Fig. 8. Chloroform solution UV–vis absorption and emission spectra of the undoped and doped azomethines.
Fig. 9. Concentration-dependent solution UV–vis spectra of the undoped (a) and doped (b) LCBAZ1 in chloroform solution.
4. Conclusions
agent on the optical and thermal properties was investigated. The
mesophase of the two azomethines have been designated to be a
Sm C phase, additionally for the LCBAZ1 the nematic (N) phase was
observed, while for the LCBAZ2 the Sm X phase was found. Dop-
ing with the DSA change the mesophases of the azomethines and
In conclusion, two symmetrical, liquid crystal azomethines were
obtained and characterized by FTIR, NMR, DSC, POM, WAXD, UV–vis
and photoluminescence spectroscopy. The effect of the protonating

A. Iwan et al. / Spectrochimica Acta Part A 72 (2009) 72–81
81
additionally decrease the phase transition temperatures. The doped
compounds were semi-crystalline, while the undoped azomethines
were crystalline.
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The photoluminescence properties under 400 nm excitation
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After protonation with the DSA PL spectrum of the azomethine
LCBAZ2 is blue shift in comparison with the undoped azomethine.
Protonation induced PL properties of the LCBAZ1. In conclusion we
hope that more attention will be paid to exploit the potential use
of the protonation in materials science.
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The research was carried out within the PAN/CNRS-Direction
des Relations Internationales collaboration program No. 14494. The
authors thank MS. M. Domanski for WAXD measurements. The
authors thank Dr. S. Martins for the protonating agent synthesis.
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