Structural characterization, thermal investigation, and liquid crystalline behavior of 4-[(4-chlorobenzyl)oxy]-3,4′-dichloroazobenzene

Article abstract of DOI:10.1007/s10973-010-0899-1

Using the Williamson method, a new dye 4-[(4-chlorobenzyl)oxy]-3,4′- dichloroazobenzene (CODA) with liquid crystalline properties was synthesized. The structure and the thermal behavior of CODA were investigated by means of nuclear magnetic resonance, X-r

Full text of DOI:10.1007/s10973-010-0899-1

J Therm Anal Calorim (2010) 102:1079–1086
DOI 10.1007/s10973-010-0899-1
Structural characterization, thermal investigation, and liquid
crystalline behavior of 4-[(4-chlorobenzyl)oxy]-3,40-
dichloroazobenzene
•
Anca Moant¸a Catalina Ionescu P. Rotaru
•
•
˘ ˘ ˘
Margareta Socaciu Ana Harabor
•
˘ ˘
Received: 7 March 2010 / Accepted: 17 May 2010 / Published online: 4 June 2010
´
´
Ó Akademiai Kiado, Budapest, Hungary 2010
Abstract Using the Williamson method, a new dye
4-[(4-chlorobenzyl)oxy]-3,40-dichloroazobenzene (CODA)
with liquid crystalline properties was synthesized. The
structure and the thermal behavior of CODA were investi-
gated by means of nuclear magnetic resonance, X-ray
diffraction, differential scanning calorimetry, and light
polarized optical microscopy techniques. The thermophys-
ical processes were monitored by heating–cooling cycles,
but the formation of liquid crystal phases were exhibited
only for small values of the cooling rates. For the first
heating–cooling cycle, the melting and the solidification
processes, thus the characteristic temperatures, are shifted
to higher values when compared to the following cycles.
From the point of view of possible applications (e.g., liquid
crystals for non-linear optics applications or for dye lasers)
[15–17], azomonoethers are a large interest class of dyes.
We have reported the synthesis, spectral characterization
[18–20], and thermal stability [21–29] of some synthetic
azoic dyes, which have a 4-[(4-chlorobenzyl)oxy]azoben-
zene derived structure. These compounds were synthesized
using the Williamson etherification method [18, 19], by
condensation of 4-chloromethyl chlorobenzene with
sodium salts of 4-(phenylazo)phenols. One of the com-
pounds we have synthesized is 4-[(4-chlorobenzyl)oxy]-
3,40-dichloroazobenzene (CODA). In this article, the
thermal behavior and the structure of CODA are reported,
using differential scanning calorimetry (DSC), nuclear
magnetic resonance (NMR), X-ray diffraction (XRD),
and light polarized optical microscopy (LPOM) tech-
niques. For the CODA compound the liquid crystalline
properties were exhibited in the characteristic the temper-
ature ranges and the thermophysical parameters were
determined.
Keywords Azomonoethers ꢀ DSC ꢀ NMR ꢀ LPOM ꢀ
Liquid crystals ꢀ Phase transitions ꢀ Thermal analyses
Introduction
Thermal analyses of new compounds used as precursors for
nanostructured materials synthesis or designed for tem-
perature controlled applications like dyes exhibiting liquid
crystalline nature [1–8] are a real need and an advanta-
geous pointer before trying to functionalize them [9–14].
Experimental
Synthesis of 4-[(4-chlorobenzyl)oxy]-3,40-
dichloroazobenzene
To a one-necked round-bottomed flask equipped with a
mechanical stirrer, thermometer, and condenser, 2-chloro-
4-(4-chloro-phenylazo)-phenol, NaOH (Fluka pellets
100%) and C2H5OH (Fluka absolute)–C6H6 (Fluka 99%)
mixture (3:1, in volumes) were added. 2-chloro-4-(4-
chloro-phenylazo)-phenol was synthesized by coupling the
diazonium salt of 4-chloro-aniline (Fluka 97%) with
2-chloro-phenol (Fluka 98%). The reaction mixture was
˘
A. Moan¸ta ꢀ C. Ionescu
Faculty of Chemistry, University of Craiova, Calea Bucuresti
Str., Nr. 107 I, Craiova, Romania
˘ ˘
P. Rotaru (&) ꢀ M. Socaciu ꢀ A. Harabor
Faculty of Physics, University of Craiova, A.I. Cuza Str., Nr. 13,
Craiova, Romania
e-mail: protaru@central.ucv.ro; petrerotaru@yahoo.com
123

˘
A. Moan¸ta et al.
1080
stirred 2 h at 343 K, until the 2-chloro-4-(4-chloro-pheny-
lazo)-phenol has reacted with sodium hydroxide. By the
distillation of azeotropic mixture ethanol–benzene–water,
the reaction water was removed. 4-chloromethyl chloro-
benzene (Fluka 98%) was added to anhydrous azophenoxide
(Fluka 99%) and the reaction mixture was stirred 4 h at 323–
328 K. After cooling at room temperature, the solid product
was filtered, washed with water in order to remove the
sodium chloride and dried further in a heating chamber at
378 K. The reaction product was recrystallized from
C2H5OH (Fluka absolute) to C6H5CH3 (Fluka 99%) (2:1, in
volumes), providing 77.5% yield of CODA.
Results and discussion
Structural characterization by NMR and XRD
techniques
The unconventional numbering system (Fig. 1) of CODA
was chosen for an easier understanding of the signals in the
NMR spectra.
The proton spectrum of this molecule contains one
singlet, one AMX system, and two AA0BB0 systems due,
respectively, to the methylene group, the ring A, and the
two para-disubstituted benzene rings B and C (Fig. 2).
The two doublets at 8.02 (J = 2.4 Hz) and 7.06 ppm
(J = 8.8 Hz) belong obviously to H2 and H5, respectively.
The COSY spectrum (Fig. 3) shows that the third signal of
the AMX system is at 7.83, mixed with the AA0 part of the
Characterization methods and techniques
All NMR spectra were recorded at 293 K on a Bruker DRX-
400 spectrometer working at 400.13 MHz for 1H and
AA0BB0 system with the largest DmAB
.
100.62 MHz for 13C. The chemical shifts (d) of 1H and 13
C
The moiety at 7.83 ppm of this AA0BB0 system belongs
1
spectra are reported in ppm/TMS, with the H and 13C
chloroform signals at 7.26 and 77.00 ppm, respectively.
The coupling constants (J) are reported in Hz. 1D spectra
(1H-and 13C-APT) and 2D spectra of homonuclear (COS-
YGP) and inverse heteronuclear (HMQCGP) correlations
were recorded with the standard BRUKER sequences.
XRD measurements were performed on powder sam-
ples, with a Shimadzu XRD-6000 X-ray diffractometer,
equipped with a vertical goniometer and a scintillation
detector. The functioning parameters of the X-ray tube
(A40-Cu type) were set to 40 kV and an electric current
intensity of 30 mA. A continuous scan measurement has
been chosen as operating mode in a geometry (h/2h) setting
a scan rate of 1° 2h min-1 and a scan range from 2 to 37.5°
2h. Divergence slit was of 1.0000°, scattering slit was of
1.0000°, and receiving slit of 0.1500 mm.
0
to Ha and Ha which are ortho to the azo group, known to
be a more ortho deshielding group than chlorine [30]. The
other half of this system being at 7.47 ppm, it is almost an
AA0XX0 system at 400 MHz.
On the contrary, ring C is substituted with two groups of
similar substituent effects [30] and the DmAB of the AA0BB0
system is small, even at 400 MHz. Thus, the system was
simulated with gNMR [31] to obtain the correct chemical
shifts shown in Table 1. It may be noticed that the high-
frequency moiety of the AA0BB0 system is slightly
b
c
a
d
7
9
10
8
5
6
2
Cl
N
C
Cl
B
1
4
N
A
O
a'
b'
c'
d'
3
Cl
DSC studies of CODA compound were carried out in
dynamic air atmosphere (150 cm3 min-1), under combined
non-isothermal linear and isothermal regimes. A horizontal
‘‘Diamond’’ Differential/Thermo gravimetric Analyzer
from PerkinElmer Instruments was used during the mea-
surements. The enthalpic calculations were performed with
the specialized software ‘‘Pyris.’’
Fig. 1 Structure of CODA and employed numbering system
CH2
a
a'
6
Optical microstructural observations were made with a
polarizing microscope LEICA DM 2500 P, equipped
with a video-recorder camera and a hot thermostated
stage TMS 94 (Linkam Scientific Instruments Ltd.)
connected to the temperature programmer. The new
synthesized compound was introduced between two par-
allel glass plates (a slide and a coverslip) without any
particular care and was examined between crossed po-
larisers under a polarizing microscope [29]. The heating
and cooling cycles were performed with the rates of
2 K min-1 each.
b
b'
c
d
c' d'
2
5
*
8.05 8.00 7.95 7.90 7.85 7.80
7.50 7.45 7.40 7.35 7.10 7.05 5.25 5.20 5.15
Fig. 2 1H-NMR spectrum of 4-[(4-chlorobenzyl)oxy]-3,40-dic-
hloroazobenzene in CDCl3
123

Structural characterization, thermal investigation, and liquid crystalline behavior of CODA
1081
ppm
7.0
7.1
7.2
7.3
7.4
7.5
7.6
7.7
7.8
7.9
8.0
ppm
5.15
5.20
5.25
5.30
8.1
8.1 8.0 7.9 7.8 7.7 7.6 7.5 7.4 7.3 7.2 7.1 7.0 ppm
7.50
7.45
7.40
7.35
ppm
Fig. 3 COSY (1H–1H) spectrum of CODA (aromatic region only)
Fig. 4 COSY spectrum of CODA. Expansion of the cross-peak
between the AA0BB0 system of ring C and the CH2 group
Table 1 1H- and 13C-NMR parameters of CODA
1H
13C
–
–
C1
146.76a
123.24
124.20a
156.12a
113.08
124.90
150.69a
136.77b
134.31b
134.02b
123.99
129.31
128.39
128.87
70.14
H2
8.017
–
C2
–
C3
–
–
C4
H5
7.063
7.832
–
C5
H6
C6
–
C7
–
-–
C8
135
ppm
131 130 129 128 127ppm
–
–
C9
–
–
C10
Ca and Ca
0
0
Ha and Ha
7.830
7.474
7.427
7.388
5.210
0
0
Hb and Hb
Cb and Cb
0
0
Hc and Hc
Hd and Hd
CH2
Cc and Cc
Cd and Cd
CH2
160
150
140
130
120
110
100
90
80
70 ppm
0
0
Fig. 5 13C-APT spectrum of CODA in CDCl3
a
These assignments were realized on the basis of calculations using
additive substituent effects [38]
b
These assignments may be interchanged
ppm
broadened by a long range coupling constant with the ortho
0
112
114
116
118
120
122
124
126
128
130
CH2 group, allowing its assignment to the pair HcꢁHc :
This scalar interaction is confirmed by the COSY spectrum
(Fig. 4).
For 13C analysis, a 13C-APT experiment was recorded,
based on the JMOD pulse sequence which separates the
signals of primary and tertiary atoms from the signals of
secondary and quaternary atoms by giving them different
phases (Fig. 5).
The CH carbon signals were assigned using the inverse
2D heteronuclear correlation spectrum HMQC (Fig. 6) and
the results are shown in Table 1.
132
ppm
8.1 8.0 7.9 7.8 7.7 7.6 7.5 7.4 7.3 7.2 7.1
Fig. 6 Heterocorrelation (1H–13C) spectrum of CODA (tertiary
atoms region only)
The XRD analysis is a good method to evaluate the long
range structural ordering or periodicity of the material [32].
123

˘
A. Moan¸ta et al.
1082
–10
–8
–6
–4
–2
0
600
500
400
300
200
100
0
2
4
6
8
10
333 343 353 363 373 383 393 403 413 423 433 443 453
Temperature/K
5
10
15
20
2q/°
25
30
35
Fig. 8 DSC heating–cooling cycles of CODA for 10 K min-1
much empirical approach of Jones will provide the best
means for making allowance for the dimensions of the
samples. In this last case, by using the integral widths BI
when KI = 1.333, we have obtained for the particles
dimensions of CODA the mean value of about 1.21 nm.
Fig. 7 XRD patterns of CODA in 7–37.5° (2h) range
Table 2 Main XRD peaks of CODA
˚
d/A
Relative intensity/a.u.
2h/°
hkl
Thermal study of CODA by differential scanning
calorimetry
100
67
40
33
20
18
16
16
15
5.46009
3.35018
4.97040
5.58152
3.094639
2.47705
3.19223
2.97739
9.95619
16.222
26.854
17.815
15.865
28.800
36.260
27.945
30.015
8.875
200
301
020
111
031
023
212
131
010
In order to determine the thermal stability domain and to
identify the phase transitions of the investigated compound,
heating–cooling cycles were imposed by alternating non-
isothermal linear temperature programs of 10 K min-1
with 10 min isothermal regimes at the starting (343 K) and
ending (443 K) temperatures. Beginning with the forth
cycle, only the melting has been realized, the temperature
increasing with 10 K min-1 until 600 °C. The compound
is stable until 200 °C; afterward, it decomposes entirely in
two steps up to 580 °C.
In the case of CODA, the Bragg diffraction patterns
˚
obtained for k = 1.54059 A Cu Ka1 X-ray radiation,
indicate a good crystallinity (Fig. 7).
The DSC curve of the heating–cooling cycles for
10 K min-1 is shown in Fig. 8.
Our results have been compared with those in the
JCPDS database at the International Centre for Diffraction
Data. For indexing the diffraction patterns of this new
material, we were guided by a orthorhombic similar
structure which may be found in the cards 20-1271 [33]
and 47-2210 [34].
For the part of the experiment which corresponds to the
heating, an endothermic effect representing the melting of
the compound is observed, while for the cooling one, the
exothermic peaks are characteristic to its solidification. In
the first cycle the compound is stabilized, the transforma-
tion peaks being different from the other cycles (for the
melting and for the solidification as well). The thermo-
physical parameters of the melting and solidification of
CODA, obtained by means of the DSC heating–cooling
cycles (at 10 K min-1) are shown in Table 3.
Table 2 shows for the main XRD peaks of CODA: the
relative intensity, the interplanar distance (d), the diffrac-
tion angles (2h), and the Miller indexes (hkl) of the dif-
fraction planes.
The unit cell of CODA material was indexed as ortho-
rhombic structure and the calculated experimental lattice
The melting temperature in the first cycle is with 6 K
greater than for the following cycles, while the solidifica-
tion temperature in the first cycle is with about 1.5 K
greater than in the other cycles. The displacement of the
melting process to higher temperatures in the first heating
cycle can be explained by the supplementary strength that
the crystalline structure introduces. The enthalpy variations
of the melting (endothermic) and solidification (exother-
mic) processes are not much different from one cycle to the
˚
parameters were: a = 10.920 A, b = 9.956 A, and
˚
˚
c = 8.567 A; this proves a long range structural organi-
zation degree.
Concerning the average crystallite sizes when using the
Scherrer formula [32, 35], there are some approximation
methods to define the peak width (B), resulting different
values for the K constant in the Scherrer formula. When
assuming ellipsoidal particles, it was found [35] that the
123

Structural characterization, thermal investigation, and liquid crystalline behavior of CODA
1083
Table 3 Thermophysical parameters of CODA for the DSC heating–cooling cycles at 10 K min-1
Cycle
Process
Temperature range/K
DH/kJ kg-1
T
max/K
Process
Temperature range/K
DH/kJ kg-1
T
max/K
First
Melting
Melting
Melting
Melting
403.2–425.7
396.3–428.0
394.7–427.7
393.1–417.5
97.96
102.73
103.43
100.71
408.6
402.8
402.6
402.3
Solidification
Solidification
Solidification
–
386.0–3374.0
384.2–372.7
384.5–372.4
–
-80.42
-80.19
-80.33
–
383.6
382.1
382.0
–
Second
Third
Fourth
other. Only for the first cycle, the melting peak is sharper
and the thermal effect is smaller, being enhanced by the
actual higher temperatures. After stabilization, the com-
pound properties repeat rigorously in the other cycles.
Since the temperature variation rates of 10 K min-1 are
too large, the characteristic liquid crystalline mesophases
do not appear. The additional phase transitions can be
observed only for smaller variation rates of the tempera-
ture, when thermophysical processes would separate.
Therefore, the cooling processes should be further inves-
tigated, these transformations happening in quite narrower
temperature ranges.
382.0 K, the transition to the solid crystalline phase is
produced.
At cooling in the second cycle (Fig. 11), the thermal
behavior of CODA is different from the one in the first
cycle, because the solidification process is displaced with
temperature. In Fig. 11, the thermal effect of the liquid
crystal formation is apparently bigger than the thermal
effect of the solidification. This statement is not true, since
from 384.1 to 381.3 K, the solidification takes place
simultaneously with the liquid crystal phase transformation
(with maxima at 383.9 and 381.9 K). Between 381.3 and
378.0 K, the (re)crystallization of CODA follows.
This kind of behavior is expected, since for the first
heating–cooling cycle, the powder used was the one
obtained straight from the organic synthesis, which has a
rather different microstructure (than the solid after the first
The second experiment used a temperature program of
two complete heating–cooling cycles (rates of 2 K min-1
)
in the temperature range of 323–433 K, intercalated by
10 min maintaining the temperature to the initial (323 K)
and final (433 K) values. Further with the third cycle, after
melting the investigated compound, the temperature con-
tinued to increase by 2 K min-1 up to 873 K.
1
378.8 K
2
3
4
5
6
7
8
383.0 K
382.0 K
The DSC curve (heat flow curve) and the imposed
temperature variation of the sample in time are shown in
Fig. 9.
382.7 K
At lower values of temperature variation rates (for
example, as here for 2 K min-1), the liquid crystal prop-
erties of CODA can exhibited by the DSC measurements.
In the first cycle, at heating, the melting is produced and
the isotropic liquid phase is obtained. For the first cycle as
well, but at cooling (Fig. 10), the liquid crystal phase is
generated at 383.0 K in the isotropic liquid and from
9
10
11
12
100
381.8 K
104
101
102
103
105
106
107
Time/min
Fig. 10 DSC cooling curve of CODA for the first cycle at 2 K min-1
1
–1
0
873
823
773
723
673
623
573
523
473
T
378.0 K
2
3
4
5
6
7
8
384.1 K
381.3 K
381.9 K
2
4
6
DSC
8
381.1 K
10
423
373
323
283
12
14
383.9 K
233
Time/min
0
50 100 150 200 250 300 350 400 450 500 545
230
231
232
234
235
236
237
Time/min
Fig. 9 Heat flow and temperature of the sample versus time
(b = 2 K min-1
Fig. 11 DSC cooling curve of CODA for the second cycle at
2 K min-1
)
123

˘
A. Moan¸ta et al.
1084
Table 4 Thermophysical parameters of CODA at 2 K min-1
P
Cycle
Process
Temperature
range/K
DH/kJ kg-1
T
max/K
Process
Temperature
range/K
DH/kJ kg-1
Tmax/K
First
Melting
Melting
Melting
404.5–412.6
397.6–405.8
396.5–405.3
92.03
87.56
83.99
407.8
401.5
401.3
Phase transition
383.0–382.0
382.0–378.0
384.1–381.3
381.3–378.0
–
-79.72
-80.77
–
382.7
381.8
383.9
381.1
–
Solidification ? crystallization
Phase transition ? solidification
Crystallization
Second
Third
–
Fig. 12 Small droplets in the homeotropic isotrope liquid matrix
(cooling, 383.1 K)
Fig. 14 Batonnets shape, as simple cylinders with only one straight
disclination line (cooling, 382.0 K)
Fig. 13 Growth of the batonnets’ texture together with the droplets
texture (cooling, 382.7 K)
Fig. 15 Coalescence and shape reorganization of the batonettes
(cooling, 380.0 K)
recrystallization in the DSC) and has retained small
amounts of solvent.
thermophysical processes are therefore suggested to
occurred, with only the solidification one shifting to higher
or lower temperatures.
The thermophysical parameters obtained by means of
the DSC analysis in the heating–cooling cycles at
2 K min-1 are shown in Table 4. As in the first experiment
at higher temperature variation rate, the melting point is
about 6–6.5 K displaced to higher temperature values.
For the cooling part of the second experiment, the
solidification can occur together with the crystallization—
first cycle, or together with the liquid phase transition—
second cycle (Table 4), but the total enthalpy variation is
constant for each cycle (RDH & 80 kJ kg-1); the same
These processes can be also observed using an optical
microscope with polarized light, confirming the results
obtained by the two DSC experiments. At cooling in first
cycle, from the isotropic melt a few droplets (spherulitic
domains) appear near the clearing point, as in Fig. 12.
Furthermore, the formation of batonnets follows (Fig. 13).
For most droplets the spherical symmetry is reduced to
revolution symmetry by a focal domain attached at the
center [36]. In its initials state, the batonnets may be simple
123

Structural characterization, thermal investigation, and liquid crystalline behavior of CODA
1085
In Fig. 17, it is shown that the image of CODA crystals
at cooling in the second cycle, with batonnets which coa-
lesce in exotic texture.
The crystals in Fig. 17 appearing at cooling, should
finally look like the solid paramorphe texture with the
small homeotropic domains of isotropic phase (Fig. 18),
image which was obtained at heating in the second cycle,
just before the clearing point.
Conclusions
Fig. 16 Reticular or fan shaped texture of the batonnets (cooling,
376.8 K)
A new azomonoetheric dye CODA with liquid crystalline
properties has been synthesized. Detailed 1H- and 13C-
NMR study (400 MHz) of CODA compound using mono
and bidimensional NMR spectra was reported. The results
are in agreement with the considered molecular structure.
By XDR analysis, the main diffraction maxima of the
CODA compound have been identified since it has exhib-
ited a good crystallinity and the size of crystallite was
established. CODA compound has been indexed as crys-
tallizing in orthorhombic symmetry. The thermophysical
processes were monitored by heating–cooling cycles, but
the formation of liquid crystal phases were observed only
for small values of the cooling rates, when they would
separate. When realizing heating–cooling cycles with the
temperature variation rate of 10 K min-1, mesomorphic
transitions do not appear in the isotropic liquid phase and
all processes are gathered under the same DSC peak. For
the first heating–cooling cycle, the melting and the solidi-
fication processes are shifted to higher values when com-
pared to the following cycles. The crystalline structure
influences this first cycle, the melting temperature being
with 6 K greater than for the following cycles, while the
solidification temperature in the first cycle is with
approximately 1.5 K greater than in the other cycles. For
the 2 K min-1 experiment, in the cooling part, the liquid
crystalline phase transition was identified. In this part of
the experiment, solidification of CODA can occur together
with the crystallization—first cycle, or together with the
liquid phase transition—second cycle. This temperature
displacement of the solidification is highlighted by the total
enthalpy variation which preserves its value for each cycle
(RDH&80 kJ kg-1) and by optical microscopy images
collected at precise temperatures during cooling.
Fig. 17 The batonnets (cooling, 378.0 K)
Fig. 18 Solid paramorphe texture, with small homeotropic domains
of isotropic phase, before clearing point (heating, 398.6 K)
cylinder with only one straight disclination line, as in
Fig. 14. During its growth, two or more batonnets meet and
they may coalesce, sometimes textures of exotic shape
resulting [37] as in Fig. 15. During further growth, the
batonnets often coalesce and may reorganize its shape until
a final fan shaped or reticular texture like in Fig. 16 is
established.
Acknowledgements The authors are grateful to Andrei Rotaru
(INFLPR Bucharest/University of St Andrews), Prof. Alain Fruchier
´
(Ecole Nationale Superieure de Chimie, Montpellier), and Diego
Carnevale and Karen Johnston (University of St Andrews) for
their useful discussions and the help they provided in writing this
article.
123

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1086
from 2-amino-5-nitrothiazole. Thermochim Acta. 2007;453(1):
52–6.
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