Solvatochromic and electrochemical properties of new thermally stable azo-azomethine dyes with N2S2O2 donor set of atoms

Article abstract of DOI:10.1016/j.molliq.2012.10.035

A new series of azo-azomethine dyes with N₂S₂O ₂ donor set of atoms have been synthesized via condensation reaction of α,α'-bis(2-aminoethylthio)-1,3-xylene and substituted azo-coupled salicylaldehyde. The dyes were characterized by IR, UV-Vis and ¹H NMR spectroscopic methods as well as elemental analysis. The solvatochromic behavior of the dyes was probed by studying their UV-Vis spectra in four pure organic solvents of different polarities and a meaningful correlation was observed. The electrochemical properties of the dyes have been also studied by cyclic voltammetry in DMSO. Moreover, thermal properties of the prepared dyes were examined by thermogravimetric analysis and results indicated that the framework of the dyes is stable up to 260 °C.

Full text of DOI:10.1016/j.molliq.2012.10.035

Journal of Molecular Liquids 177 (2013) 198–203
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Solvatochromic and electrochemical properties of new thermally stable
azo–azomethine dyes with N2S2O2 donor set of atoms
⁎
Hamid Khanmohammadi , Fatemeh Khodam
Department of Chemistry, Faculty of Science, Arak University, Arak 38156-8-8349, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 August 2012
Received in revised form 8 October 2012
Accepted 29 October 2012
Available online 20 November 2012
A new series of azo–azomethine dyes with N2S2O2 donor set of atoms have been synthesized via condensa-
tion reaction of α,α′-bis(2-aminoethylthio)-1,3-xylene and substituted azo-coupled salicylaldehyde. The
dyes were characterized by IR, UV–Vis and 1H NMR spectroscopic methods as well as elemental analysis.
The solvatochromic behavior of the dyes was probed by studying their UV–Vis spectra in four pure organic
solvents of different polarities and a meaningful correlation was observed. The electrochemical properties
of the dyes have been also studied by cyclic voltammetry in DMSO. Moreover, thermal properties of the pre-
pared dyes were examined by thermogravimetric analysis and results indicated that the framework of the
dyes is stable up to 260 °C.
Keywords:
Azo–azomethine
Dye
© 2012 Elsevier B.V. All rights reserved.
Solvatochromism
Thermal properties
Substituent effects
1. Introduction
However, photo-physical and photo-chemical properties of polyhy-
droxy azo–azomethine dyes are in turn strongly influenced by several
Azo compounds are receiving continuous attention primarily due
to their applications in synthesis of dyes, pigments and functional ma-
terials [1–6]. Attention was devoted also to the optical and physico-
chemical properties of these compounds. For example, azo-containing
photochromic organic compounds with liquid crystalline character
and azo-conjugated metal complexes have been attracting much atten-
tion because of their possible applications in the area of photon-mode
high density information storage, photo-switching devices and optical
computing [7–9]. On the other hand, it is well known that the spectral
and physico-chemical properties of azo dyes depend not only on the
azo groups, but also on the presence of other coupling groups. Among
many different types of coupled azo compounds, the azo–azomethine
dyes have attracted particularly great researchers because of their
structural properties and uses [10–13]. Azo–azomethine dyes are rela-
tively robust and chemically stable and their spectral properties depend
on the combination of electron-donating and -withdrawing moieties in
the molecules [14–16].
factors including temperature, substituent groups and solvent polarity
[23–27]. Because of this, our interest has been focused in preparing
new o-hydroxy azo–azomethine derivatives with various donor and
acceptor groups [16,28,29]. We report here the synthesis and character-
ization of new acyclic azo–azomethines, 1a–5a, by condensation reac-
tion of azo-coupled salicylaldehyde derivatives, 1–5, with α,α′-bis(2-
aminoethylthio)-1,3-xylene, a (Fig. 1). All the prepared multifunctional
compounds are air stable solids, intensely colored, soluble in DMF and
DMSO and exhibited dye characteristic since the molar extinction coef-
ficients (ε) were over 11,000 M−1 cm−1 in DMSO [30]. The solvato-
chromic behavior and substituent effect of the potentially polydentate
prepared compounds in four pure organic solvents of different polari-
ties were evaluated. Also, the thermal properties of the prepared com-
pounds were examined by thermogravimetric analysis. The results
indicated that the framework of the dyes is stable up to 260 °C.
2. Experimental details
Recently, considerable attention has been paid to the synthesis and
spectral studies of azo–azomethine dyes containing hydroxyl groups
[10,17–20]. The prototropic properties of hydroxyl groups of those
dyes have important consequences on their electronic structure which
can be exploited for their thermochromic and/or photochromic behav-
ior [21] and also for the design of molecular electronic devices [22].
2.1. Materials
All of the reagents and solvents involved in synthesis were of analyt-
ical grade and used as received without further purification. Salicylal-
dehyde, 4-nitroaniline, 4-chloroaniline, 4-ethylaniline, 4-bromaniline
and 4-flouroaniline were obtained from Aldrich and Merck. Azo-
coupled salicylaldehyde precursors (4, 5) were prepared according to
the well-known literature procedure [16,28,31].
⁎
Corresponding author. Tel.: +98 861 2777401; fax: +98 861 4173406.
E-mail address: h-khanmohammadi@araku.ac.ir (H. Khanmohammadi).
0167-7322/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molliq.2012.10.035

H. Khanmohammadi, F. Khodam / Journal of Molecular Liquids 177 (2013) 198–203
199
S
S
(a)
NH2
S
S
H2N
H
H
H
N
N
O
OH
HO
R
N
N
OH
N
N
N
N
R
1
2
3
4
5
4-NO2
4-Et
4-Cl
4-Br
4-F
R
R
1a-5a
Fig. 1. Synthesized azo–azomethine dyes.
2.2. Instrumentation
carbonate for a period of 30 min at 0 °C. The resulting solution was
added slowly to a solution of diazonium chloride (10 mmol) in
water at 0–5 °C. The reaction mixture was stirred for 1 h at 0 °C and
then allowed to warm slowly to room temperature. The product
was collected by filtration and washed with 100 mL of NaCl solution
(10%) under vacuum. The obtained solid was dried under vacuum at
80 °C overnight.
The structure of all synthesized compounds was confirmed by 1H
NMR spectra, recorded on a Bruker Avance 300 MHz spectrometer.
FT-IR spectra were recorded as pressed KBr disks, using Unicom Galaxy
Series FT-IR 5000 spectrophotometer in the region of 400–4000 cm−1
.
Melting points were determined on an Electrothermal 9200 apparatus.
Electrochemical measurements were recorded on an Autolab 30 V
potentiostat/galvanostat from the Eco Chemie company. All readings
were taken using three electrode potentiostatic systems in DMSO with
0.1 mol cm−3 tetrabutylammonium bromide (TBAB) as supporting
electrolyte. A three-electrode assembly composed of a platinum work-
ing electrode, a platinum auxiliary electrode, and calomel reference
2.3.2.1. 1-(3-Formyl-4-hydroxyphenylazo)-4-bromobenzene, 4. Yield:
2.77 g (91%), m.p. 206–207 °C. 1H NMR (DMSO-d6, 300 MHz, ppm):
δ 7.19 (d, 1H, J=8.86 Hz), 7.78 (br, s, 4H), 8.09 (dd, 1H, J=8.86 Hz
and 2.41 Hz), 8.18 (d, 1H, J=2.41 Hz), 10.36 (s, 1H), 11.61 (br, 1H).
IR (KBr, cm−1); 1668 (CHO), 1620 (C_C), 1572 (phenol ring), 1475
(N_N), 1286 (C\O), 1167, 829 and 719.
electrode was used with sample concentrations of 1×10−3 molL−1
.
Thermal analyses were performed on TGA V5.1A DuPont 2000 and
Perkin-Elmer thermogravimetric analyzer TG/DTA 6300 instruments.
C. H. N. analyses were performed on a Vario EL III elemental analyzer.
Electronic spectral measurements were carried out using Perkin-Elmer
Lambda spectrophotometer in the range 200–700 nm.
2.3.2.2. 1-(3-Formyl-4-hydroxyphenylazo)-4-flourobenzene, 5. Yield:
2.27 g (93%), m.p. 156–158 °C. 1H NMR (DMSO-d6, 300 MHz, ppm):
δ 7.17 (d, 1H, J=8.76 Hz), 7.37 (br, t, 2H), 7.88 (m, 2H), 8.04 (dd,
1H, J=8.76 and 2.22 Hz), 8.14 (d, 1H, J=2.22 Hz), 10.35 (s, 1H),
11.48 (br, 1H). IR (KBr, cm−1); 1672 (CHO), 1620 (C_C), 1595 (phe-
nol ring), 1574, 1501, 1479 (N_N), 1287 (C\O), 1226, 1152, 839 and
767.
2.3. Synthesis
2.3.1. Preparation of 1 α,α′-bis(2-aminoethylthio)-1,3-xylene, a
Sodium metal (0.92 g, 40 mmol) was dissolved in degassed absolute
ethanol (50 mL) under N2 and the solution was refluxed for 10 min. A
solution of 2-aminoethanethiol hydrochloride (2.27 g, 20 mmol) in
warm degassed absolute ethanol (20 mL) was then added, whereupon
a white solid (NaCl) separated immediately. The reaction mixture
was stirred at 70 °C for 30 min and a solution of 1,3-bis-(bromomethyl)
benzene (2.64 g, 10 mmol) in degassed absolute ethanol (20 mL) was
added dropwise, with stirring, over a period of 1 h. The reaction mixture
was then refluxed for 6 h and left at room temperature overnight. The
white solid was filtered off, the filtrate was concentrated to a volume
of about 10 mL, and diethyl ether was added. After this the mixture
was stored in a refrigerator. The separated yellow solid was filtered,
washed with diethyl ether, and dried. Yield; 1.61 g (63%). 13C NMR
(D2O, ppm): δ 31.27, 32.33, 38.69, 127.61, 130.80 and 136.05. IR (KBr,
cm−1); 1668 (CHO), 1620, 1572, 1476 (N_N), 1287 (C\O), 1167,
829 and 719.
2.3.3. General procedure for the synthesis of azo–azomethine compounds,
1a–5a
A solution of diamine, a (1 mmol) in absolute EtOH (10 mL) was
added to a stirring solution of azo-coupled precursors, 1–5, (2 mmol)
in absolute EtOH (50 mL) for a period of 10 min at 60–70 °C. The mix-
ture was heated in a water bath for 4 h at 80 °C with stirring. The mix-
ture was filtered while hot and the obtained solid was washed with hot
ethanol (three times) and then with diethyl ether. The resulting product
was dried in air.
2.3.3.1. 1,3-Bis(2-[2-sulfanyl-ethylimino)-methyl]-4-(4-nitro-phenylazo)-
phenol)methylenbenzene, 1a. Yield: 0.61 g (80%), m.p. 144–146 °C. 1H
NMR (DMSO-d6, 300 MHz, ppm) δH: 2.77 (br, 4H, J=5.58 Hz), 3.80
(br, 8H), 6.76 (d, 2H, J=9.61 Hz), 7.25 (m, 4H), 7.91 (m, 6H), 8.05 (s,
2H), 8.33 (d, 4H, J=8.53 Hz), 8.66 (s, 2H), 13.80 (vbr, 2H). IR (KBr,
cm−1); 1634 (C_N), 1614 (C_C), 1587 (phenol ring), 1516 (NO2),
1491 (N_N), 1341 (NO2), 1288 (C\O), 1105, 856. Anal. calcd. for
2.3.2. General procedure for the synthesis of azo-coupled precursors, 4, 5
Salicylaldehyde (10 mmol) was dissolved in water (20 mL) con-
taining 10 mmol of sodium hydroxide and 40 mmol of sodium
C38H34N8O6S2: C, 59.83; N, 14.69; H, 4.49; S, 8.41. Found: C, 59.74; N,
14.82; H, 4.53; S, 8.61%. λmax (nm) (ε (M−1 cm−1)): 258 (6789), 417
(67,963) and 470 (6843) in DMSO.

200
H. Khanmohammadi, F. Khodam / Journal of Molecular Liquids 177 (2013) 198–203
Table 2
2.3.3.2. 1,3-Bis(2-[2-sulfanyl-ethylimino)-methyl]-4-(4-ethyl-phenylazo)-
phenol)methylenbenzene, 2a. Yield: 0.37 g (51%), m.p. 78–80 °C. 1H
NMR (DMSO-d6, 300 MHz, ppm) δH: 1.21 (t, 6H, J=7.47 Hz), 2.66 (q,
4H, J=7.47 Hz), 2.76 (t, 4H, J=6.21 Hz), 3.77 (br, 8H), 6.89 (d, 2H,
J=9.04 Hz), 7.26 (m, 4H), 7.65 (d, 4H, J=7.99 Hz), 7.73 (d, 4H, J=
7.99 Hz), 7.90 (dd, 2H, J=9.04 Hz and 1.77 Hz), 7.99 (d, 2H, J=
1.77 Hz), 8.64 (s, 2H), 14.08 (br, 2H). IR (KBr, cm−1); 1636 (C_N),
1603 (C_C), 1582 (phenol ring), 1489 (N_N), 1287 (C\O), 1049, 845.
Anal. calcd. for C42H44N6O2S2: C, 69.20; N, 11.53; H, 6.08; S, 8.80. Found:
C, 69.38; N, 11.64; H, 5.94; S, 8.74%. λmax (nm) (ε (M−1 cm−1)): 272
(7451), 367 (13,323) and 419 (2586) in DMSO.
Thermal analyses data for 1a–5a.
Compounds
Tda, °C
Temp. of
steps, °C
Mass loss
% found
Probable
% calcd.
Nature of
fragment lost
1a
265
225–300
300–510
510–730
730–900
210–300
300–510
510–900
250–290
290–500
500–900
50–150
150–310
310–800
210–290
290–480
480–800
23.00
12.60
10.60
25.20
28.50
16.28
12.10
28.61
20.50
9.90
22.06
12.96
10.71
26.06
28.30
16.00
11.78
28.80
19.80
9.81
C8H8S2
C2H7NO2
C3H6N
C6H4NO2
C6H6N2O2S2
C7H7N2
2a
3a
4a
5a
267
277
277
274
C5H4N
C5H6N2O2S2
C5H5N4O2S2
C4H5N
1 mol C2H5OH
C6H6N2O2S2+Br2
C10H6N2
C6H15N2O2S2
C6H4N2
C6H8N
2.3.3.3. 1,3-Bis(2-[2-sulfanyl-ethylimino)-methyl]-4-(4-chloro-phenylazo)-
phenol)methylenbenzene, 3a. Yield: 0.47 g (63%), m.p. 108–110 °C. 1H
NMR (DMSO-d6, 300 MHz, ppm) δH: 2.79 (t, 4H, J=6.49 Hz), 3.77 (br,
8H), 7.07 (d, 2H, J=8.80 Hz), 7.27 (m, 4H), 7.46 (d, 4H, J=8.55 Hz),
7.81 (d, 4H, J=8.55 Hz), 7.87 (s, 2H), 7.96 (dd, 2H, J=8.80 Hz and J=
2.08 Hz), 8.35 (s, 2H), 13.85 (br, 2H). IR (KBr, cm−1); 1636 (C_N),
1605 (C_C), 1586 (phenol ring), 1576, 1491, 1481 (N_N), 1287
(C\O), 1090, 839. Anal. calcd. for C38H34N6O2S2Cl2: C, 61.35; N, 11.33;
H, 4.62; S, 8.65. Found: C, 61.22; N, 11.48; H, 4.72; S, 8.76%. λmax (nm)
(ε (M−1 cm−1)): 262 (11,620), 375 (11,475) and 428 (6563) in DMSO.
5.61
5.53
48.70
18.70
27.72
19.00
10.35
48.00
18.20
27.30
18.80
10.12
a
Thermal decomposition temperature.
prepared multifunctional Schiff bases are thermally stable up to
260 °C, air stable solids, intensely colored and soluble in DMF and
DMSO. The existence of \N_N\ and \C_N\ moieties in the struc-
ture of the prepared dyes has caused them to show specific spectral
properties. All of the new Schiff bases showed high molar extinction co-
efficients. Also, the presence of various coordination sites makes them
very versatile species, which could potentially act as multidentate
ligands.
2.3.3.4. 1,3-Bis(2-[2-sulfanyl-ethylimino)-methyl]-4-(4-bromo-phenylazo)-
phenol)methylenbenzene, 4a. Yield: 0.61 g (73%), m.p. 84–86 °C. 1H NMR
(DMSO-d6, 300 MHz, ppm) δH: 2.76 (t, 4H, J=6.06 Hz), 3.77 (br, 8H),
6.86 (d, 2H, J=9.14 Hz), 7.27 (m, 4H), 7.72 (br, 8H), 7.90 (dd, 2H, J=
9.14 Hz and 1.76 Hz), 8.00 (d, 2H, J=1.76 Hz), 8.64 (s, 2H), 14.09 (br,
2H). IR (KBr, cm−1); 1636 (C_N), 1607 (C_C), 1582 (phenol ring),
1571, 1491, 1478 (N_N), 1287 (C\O), 1109, 1069, 839. Anal. calcd. for
C
38H34N6O2S2Br2. CH3CH2OH: C, 54.80; N, 9.59; H, 4.60; S, 7.31. Found:
3.1. Infrared spectra
C, 55.06; N, 10.24; H, 4.03; S, 7.58%. λmax (nm) (ε (M−1 cm−1)): 268
(8148), 382 (11,757) and 420 (8147) in DMSO.
The positions of some of the prominent bands in the IR spectra of
1–5 and 1a–5a and their assignments based on extensive data avail-
able for related compounds [12,31,32] are given in Table 1.
2.3.3.5. 1,3-Bis(2-[2-sulfanyl-ethylimino)-methyl]-4-(4-flouro-phenylazo)-
phenol)methylenbenzene, 5a. Yield: 0.42 g (59%), m.p. 119–120 °C. 1H
NMR (DMSO-d6, 300 MHz, ppm) δH: 2.76 (t, 4H, J=6.10 Hz), 3.79 (br,
8H), 6.88 (d, 2H, J=8.97 Hz), 7.27 (m, 4H), 7.38 (d, 4H, J=7.28 Hz),
7.85 (m, 6H), 7.99 (s, 2H), 8.65 (s, 2H), 14.09 (br, 2H). IR (KBr, cm−1);
1636 (C_N), 1593 (phenol ring), 1501, 1484 (N_N), 1288 (C\O),
1236, 1109, 1049, 845. Anal. calcd. for C38H34N6O2S2F2: C, 64.39; N,
11.86; H, 4.83; S, 9.05. Found: C, 64.53; N, 11.96; H, 4.67; S, 8.89%.
λmax (nm) (ε (M−1 cm−1)): 256 (11,642), 367 (11,692) and 415
(5686) in DMSO.
A strong band observed in the IR spectra of 1–5 in the region of
1655–1675 cm−1 can be assigned to the υ(C_O) group. The total ab-
sence of the υ(C_O) band in the IR spectra of the prepared dyes
together with the appearance of new υ(C_N) band in the range of
1634–1636 cm−1 clearly indicated that new Schiff-base compounds
had formed in each case. The symmetric N_N stretching mode, in
the IR spectra of 1a–5a leads to a medium band at 1478–1491 cm−1
,
while the υ(C\O) stretching mode gives an intense band within the
range 1287–1288 cm−1. The IR spectra of 1 and the related azo–
azomethine dye, 1a, show strong bonds at 1341–1346 and 1516–
1525 cm−1 assigned to the NO2 groups [33].
3. Result and discussion
New azo–azomethine compounds were synthesized via condensa-
tion reaction of 5-(4-X-phenyl)-azo-salicylaldehyde (X=NO2, Et, Cl,
Br and F) with α,α′-bis(2-aminoethylthio)-1,3-xylene in ethanol
which were easily identified by their IR and 1H NMR spectra. All
3.2. 1H NMR spectra
The 1H NMR spectra of 1–5 and 1a–5a, obtained at ambient tem-
perature in DMSO-d6, display a group of signals corresponding to
Table 1
Tentative assignments of some selected IRa frequencies (cm−1) and UV–Vis data of the prepared azo–azomethine dyes and their azo precursors.
Compounds
ν(C\O)
ν(C_O)
ν(C_C)
ν(C_N)
ν(NO2)
ν(phenol ring)
ν(N_N)
λmax (nm) (ε (M−1 cm−1)) in DMSO
1
2
3
4
1288
1277
1286
1286
1286
1288
1287
1287
1287
1288
1666
1653
1668
1668
1672
–
1608
1602
1622
1620
1620
1614
1603
1605
1607
1600
–
1525, 1346
–
1575
1581
1572
1572
1595
1587
1582
1586
1582
1593
1479
1479
1477
1475
1479
1491
1489
1481
1478
1484
246 (4500), 382 (12,700), 551 (1850)
260 (7360), 347 (16,314), 447 (3120)
256 (6490), 410 (7850), 466 (9345)
260 (9320), 349 (18,700), 456 (7049)
260 (14,650), 341 (25,253), 461 (5022)
258 (6789), 417 (67,963), 470 (68,430)
272 (7451), 363 (13,323), 419 (2586)
262 (11,620), 375 (11,475), 428 (6563)
268 (8148), 378 (11,757), 420 (8147)
256 (11,642), 362 (11,692), 415 (5686)
–
–
–
–
–
5
–
–
1a
2a
3a
4a
5a
1634
1636
1636
1636
1636
1516, 1341
–
–
–
–
–
–
–
–
a
KBr disks.

H. Khanmohammadi, F. Khodam / Journal of Molecular Liquids 177 (2013) 198–203
201
signals at δH 7.78–7.86 ppm were simplified after irradiation of an-
other doublet at 8.67–8.73 ppm. The Δυ/J ratios (Δυ is the difference
in resonance frequencies for two sets of doublets) of 1a, 2a, 3a and 5a
are 14.28, 13.32, 12.14 and 19.16, respectively. However, the ratio for
4a is equal to zero. This situation is encountered with equal resonance
frequencies of two sets of para-substituted aromatic ring protons in
4a. Therefore, the AA′XX′ spin system in 1a, 2a, 3a and 5a changes
to A4 spin system in 4a.
3.3. Thermal properties
In order to give more insight into the structure of the prepared
dyes, the thermal studies of the prepared dyes have been carried
out using thermogravimetry techniques. In the present investigation,
the heating rates were suitably controlled at 10 °C min−1 under N2
atmosphere and the weight loss was measured from 25 °C up to
900 °C.
Fig. 2. Electronic absorption spectra of 1a–5a in DMSO.
The initial decomposition and inflection temperatures have been
used as an indication of the thermal stability of the prepared dyes. The
TG results indicate that the framework of 1a–5a is stable up to 260 °C.
The thermal stability increases in the order 3a=4a>5a>2a>1a,
Table 2. This fact should be related with the structure of the dyes and in-
dicates that the 4-chloro- and 4-bromo-substituted dyes, 3a and 4a are
more thermally stable than others. The presence of water or solvent
molecules in the lattice of the dyes is supported by the results of
thermogravimetric analysis. The first stage of mass loss of 4a is observed
the hydrogen atoms of each molecule. The slightly broad signal at δH
11.47–11.61 ppm in the 1H NMR spectra of 1–5 is assigned to the OH
protons, as was confirmed by deuterium exchange when D2O was
added to d6-DMSO solution. The OH protons of 1a–5a exhibit a slight-
ly broad resonance at δH 13.68–14.10 ppm. The CH_N protons of 1a–
5a exhibit a singlet resonance at δH 8.35–9.04 ppm. Also, the 1H NMR
spectra of 1a–5a show two doublets at δH 7.78–7.86 and 8.67–
8.73 ppm assigned to the para-substituted aromatic ring. The doublet
S
S
H
H
N
N
Azo-iminophenol
OH
HO
N
N
N
N
R
R
S
S
H
H
N
N
Iminohydrazone
O
O
N
N
HN
NH
R
R
S
S
H
H
NH
HN
Azo-enaminone
O
O
N
N
N
N
R
R
Fig. 3. Structures of azo, hydrazone and enaminone tautomers for 1a–5a.

202
H. Khanmohammadi, F. Khodam / Journal of Molecular Liquids 177 (2013) 198–203
between 50 and 150 °C, which along with the endothermic peak in the
DTA curve, is in agreement with the calculated values for the removal of
ethanol molecule from the dye. Furthermore, above 400 °C, the TG
curves of all dyes show continuous significant weight loss up to
900 °C (see Supplementary materials).
3.4. Electronic absorption spectra and substituent effects
The electronic absorption spectra of 1a–5a, recorded in DMSO at
room temperature display mainly three bands, Fig. 2. The first band lo-
cated at 256–272 nm can be assigned to the moderate energy (π→π*)
transition of the aromatic rings while the second band at 362–417 nm is
due to low energy (π→π*) transition involving the π-electrons of the
azo and azomethine groups [34]. Examination of the results reported
in Table 1, reveals that the position of the π→π* band is highly
influenced by the nature of the para-substituent, R. It acquires a red
shift in λmax along with the increase in the electron-accepting ability
of substituent in the sequence of NO2>Br>Cl>F≈CH2CH3. This be-
havior can be interpreted on the basis of the expected high delocaliza-
tion of the electrons in the para-substituted dye with nitro group
which enhance the delocalization of the conjugated system [26]. Fur-
thermore, the low energy π→π* band of azo–azomethine dyes is
more red shifted than the related azo-coupled precursors, 1–5, because
of the extended π-resonance system. In o-hydroxy substituted azo–
azomethine compounds, this may be inferred from (i) selective solva-
tion, (ii) the ability of the solvent to form stronger intermolecular
H-bonds with a particular tautomeric form (Fig. 3) and (iii) the intra-
molecular hydrogen bonding between C_N or N_N and OH groups
Fig. 4. Electronic spectra of 1a in various solvents.
in the hydrogen bonding strength in polar solvents [36]. The absorption
spectra of 1a in various solvents are shown in Fig. 4 (for other dyes, see
Supplementary materials).
4. Conclusion
New thermally stable azo–azomethine compounds have been
synthesized via condensation reaction of 5-(4-X-phenyl)-azo-salicyl-
aldehyde (X=NO2, Et, Cl, Br and F) with α,α′-bis(2-aminoethylthio)-
1,3-xylene in ethanol which were easily identified by their IR and 1H
NMR spectra. All prepared compounds are soluble in DMF and DMSO
and have dye character because of their extraordinary coloring proper-
ties and their high molar extinction constant. Also, the prepared dyes
show positive solvatochromic behavior as the polarity of solvent was
increased.
[12,26]. The very broad band located at
a longer wavelength,
415–470 nm, corresponds to an intramolecular charge transfer involv-
ing the whole molecule [34].
3.5. Solvatochromism behavior
It has been accepted that the electronic transitions of azo–
azomethine compounds strongly depend on the nature of media [35].
For this purpose, the electronic absorption spectra of 4×10−5 molL−1
solution of the prepared dyes in four organic solvents with different
polarities were measured at room temperature.
Acknowledgments
The polarity and hydrogen bonding properties of the solvents used
in the present investigation, indicated by the Kamlet–Taft parameters
(α, β, and π*) [35], are summarized in Table 3. β is a scale of solvent
hydrogen-bond acceptor (HBA) basicities which refers to the accep-
tance of the proton of a hydrogen bond. α is a scale of solvent hydrogen
bond donor (HBD) acidities which refers to the donation of the proton.
π* is a scale of solvent dipolarity/polarizability which refers to the abil-
ity of a solvent to stabilize a charge or a dipole by virtue of its dielectric
effect. The π* value is correlated with the solvatochromic behavior
values. The solvatochromic behavior of a dye is the shift of the π→π*
band wavelength due to the presence of solvent with different polari-
ties, which is due to the interaction between the solute and solvent mol-
ecules [35]. We found that the absorption band at 362–417 nm
generally shows a bathochromic shift (positive solvatochromism) as
the polarity of the solvent was increased. The influence of solvents for
the prepared dyes increases in the order DMSO>DMF>CHCl3≈THF.
This positive solvatochromism exhibited by the dyes may be due to
the effect of dipole moment changes of the excited state and/or changes
We are grateful to the Arak University for financial support of this
work. We also would like to thank to Dr. A. Babaei, M. Erfantalab and
M. Sohrabi for their assistance on electrochemical studies.
Appendix A. Supplementary data
Supplementary materials associated with this article can be ob-
tained free of charge via http://www.araku.ac.ir/~h_khanmohammadi/
Supportinginformation-8.doc. Supplementary data associated with
this article can be found, in the online version, at http://dx.doi.org/10.
1016/j.molliq.2012.10.035.
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