Solvatochromic, acid-base features and time effect of some azo dyes derived from 1,3-benzothiazol-2-ylacetonitrile: Experimental and semiempirical investigations

Article abstract of DOI:10.1016/j.saa.2009.11.051

The solvatochromism and other spectroscopic properties of seven azo dyes were studied, with a particular respect to the role of the solvent basicity, and interpreted with the aid of experimental findings and semiempirical data. The electronic absorption spectra of the dyes examined in different solvents combined with theoretical calculations showed that most of the investigated compounds coexist in the hydrazone and/or azo-enamine-common anion equilibrium or in the solely anionic form depending upon the nature of the solvent employed. These interesting features open up possibilities for the use of these compounds in analytical chemistry as acid-base indicators. Furthermore, both of intermolecular and intramolecular charge transfer equilibria have been reflected by experimental absorption spectra of compounds 4 and 5. The enthalpies of formation predicted at PM6 (COSMO) and PM6/CI (COSMO) for the ground (S₀) and excited (S₁) states, respectively have been successfully used for the explanation of the observed bathchromic shift in non-polar solvents. The effect of time on the longer wavelength visible band of compound 7 has been thoroughly investigated.

Full text of DOI:10.1016/j.saa.2009.11.051

Spectrochimica Acta Part A 75 (2010) 760–768
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Solvatochromic, acid–base features and time effect of some azo dyes derived
from 1,3-benzothiazol-2-ylacetonitrile: Experimental and semiempirical
investigations
Y.H. Ebeada,∗, M.A. Selima, S.A. Ibrahimb
a South Valley University, Faculty of Science, Chemistry Department, 83523 Qena, Egypt
b Assiut University, Faculty of Science, Chemistry Department, 71516 Assiut, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 26 October 2009
Accepted 19 November 2009
The solvatochromism and other spectroscopic properties of seven azo dyes were studied, with a par-
ticular respect to the role of the solvent basicity, and interpreted with the aid of experimental findings
and semiempirical data. The electronic absorption spectra of the dyes examined in different solvents
combined with theoretical calculations showed that most of the investigated compounds coexist in
the hydrazone and/or azo-enamine-common anion equilibrium or in the solely anionic form depend-
ing upon the nature of the solvent employed. These interesting features open up possibilities for the use
of these compounds in analytical chemistry as acid–base indicators. Furthermore, both of intermolecular
and intramolecular charge transfer equilibria have been reflected by experimental absorption spectra of
compounds 4 and 5. The enthalpies of formation predicted at PM6 (COSMO) and PM6/CI (COSMO) for
the ground (S0) and excited (S1) states, respectively have been successfully used for the explanation of
the observed bathchromic shift in non-polar solvents. The effect of time on the longer wavelength visible
band of compound 7 has been thoroughly investigated.
Keywords:
Azo dyes
Solvatochromism
Acid–base properties
Semiempirical calculations
Time effect
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
colorants [10,14,15]. Recently, the demand for such group of com-
pounds especially those having sufficient large non-linear optics
Electronic absorption spectroscopy is one of the most powerful
tools available to chemists and widely used for the identifica-
tion and determination of myriad inorganic and organic species,
particularly for determining environmental pollutants [1,2]. Con-
sequently, it is probably more widely used in chemical and clinical
laboratories – because many substances can be selectively con-
verted into a color derivative – throughout the world than any other
single procedure [3]. However, synthesis of new azo compounds
and studying their physicochemical properties are important sub-
jects due to their fundamental role in biological, pharmaceutical
and colorimetric chemosensors for metal ions [4–9]. Of particular
interest are their widespread commercial applications in a textile
dyeing industries as well as food, cosmetic and ink-jet printers
[10,11]. Nevertheless, azo dyes are not renowned for recalcitrant
to biodegradation and consequently they contribute to mutagenic
activity of ground and surface waters pollution [12,13]. Considering
their significant biological activity, pharmacological and physico-
chemical properties, azo dyes are extremely important. In a broader
sense, the azo dyes constitute the largest group of all the synthetic
(NLO) have increased as a result of several new technological appli-
cations that have been found [16–18]. Furthermore, azo dyes have
received considerable attention in the field of solvatochromic stud-
ies and in the design of polarity scale [19–21]. It is well-known
that benzothiazole derivatives show a broad spectrum of biological
activities [22–24]. Additionally, benzothiazole and its derivatives
were investigated in azo push–pull systems [25]. In continuation
of our earlier work on azo and other biologically active compounds
[26–29], the present work focuses on the synthesis, spectroscopic
properties of some novel azo dyes derived from 1,3-benzothiazol-
2-ylacetonitrile using UV–vis and semiempirical molecular orbital
PM3, PM6 methods. Furthermore, the effect of time on the low
energy band of compound 7 appearing in the basic solvent (DMF)
has been also investigated. A wide variety of organic solvents have
been employed, aiming to evaluate which tautomeric form of these
compounds is spectroscopically active, considering their possible
applications as indicators or spectral probes.
2. Experimental
2.1. Synthesis of the azo compounds 1–7 (Scheme 1)
∗
The azo compounds under investigation were prepared accord-
ing to the procedure previously described in the literature [30,31].
Corresponding author. Tel.: +20 108928365; fax: +20 96/5213383.
E-mail address: ebeadhassan88@yahoo.com (Y.H. Ebead).
1386-1425/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2009.11.051

Y.H. Ebead et al. / Spectrochimica Acta Part A 75 (2010) 760–768
761
Thus, to an ice cooled stirred solution of the proper primary aro-
matic amine (0.01 mol) dissolved in diluted HCl a cold solution of
NaNO2 (0.01 mol) was added gradually with stirring for 20 min. To
a solution of 1,3-benzothiazole-2-ylacetonitrile (0.01 mol) [32] in
ethanol in the presence of sodium acetate (0.01 mol), a solution
of diazotized amine was added dropwise with vigorous stirring
at 0–5 ◦C for 35 min. After the addition, stirring was continued for
1 h and the obtained precipitate was filtered off and washed with
water and cold ethanol. The solid products were recrystallized from
ethanol and dried in vaccuo over silica gel. The chemical struc-
tures of compounds were assigned on the basis of their elemental
analysis and spectral data (Table 1).
2.2. Measurements
The chemical composition of the products was proved by ele-
mental analysis using Carlo-Erba (model EAGER 200) instrument.
Melting points were determined by Electro-thermal melting point
apparatus. Spectral measurements in the UV and visible regions
were recorded on a Shimaduz 2401 PC spectrophotometer within
the wavelength range 200–700 nm using 1-cm matched quartz cell.
1H NMR (DMSO-d6) spectra were taken on a Varian Mercury VX-
300 MHz spectrometer. All solvents (methanol, ethanol, acetone,
acetonitrile, carbon tetrachloride, chloroform, methylenechloride,
dimethylesulfoxide and dimethyformamide) used in this investiga-
tion were analytically pure or spectroscopic grade and were used as
purchased. Infrared spectra of the solids were performed on a FT-
IR Bruker Vector 22 using the KBr technique. All the measurements
were carried out at room temperature (25 ◦C).
2.3. Quantum chemical calculations
The fully geometry optimization of the isolated molecules 1–7
(Scheme 1) in ground (S0) and first excited (S1) states were opti-
mized using the semiempirical PM6 and PM6/CI methods [33].
From which some physicochemical and structural parameters in
gaseous phase for each tautomer were estimated. Solvent effects
on tautomers were also investigated using PM6 (COSMO) model
[33] as implemented in MOPAC 2007. PM3 method was employed
for comparison when it was necessary [34]. The geometry data
were obtained from MOPAC 2007—free license on internet [35].
The wavelengths of the electronic transitions in absorption spec-
tra were calculated at AM1 with a 10 × 10 CI singly excited matrix
[36]. Heats of formation, dipole moments and other parameters
were extracted directly from the data files following the geome-
try optimizations. All calculations were done on a Pentium IV PC
computer.
3. Results and discussion
3.1. Structural and physicochemical features of the molecules in
the ground and excited states
Three different structures could be written corresponding to the
molecular composition of each compound namely, azo, hydrazone
and azo-enamine (Scheme 1). This is due to the migration of the
hydrogen atom attached to C10 to the benzothiazole nitrogen atom
or to N13. A further tautomeric structure is also possible in the case
of compound 7 (R OH) as a result of transfer of the phenolic OH
proton to N12. The latter tautomer is excluded from our discussion
due to its high heat of formation. Azo, hydrazone and azo-enamine
as well as the anionic forms have been geometrically optimized
in the gaseous phase and in different media using PM6 and PM6
(COSMO), respectively. PM6 which has been recently released as
a modification on NDDO formalism gave satisfactory estimates of

762
Y.H. Ebead et al. / Spectrochimica Acta Part A 75 (2010) 760–768
Scheme 1. The compounds investigated with the number of atoms indicated: 1—1,3-benzothiazol-2-yl[(E)-phenyldiazenyl]acetonitrile; 2—1,3-benzothiazol-2-yl[(E)-
(4-methylphenyl)diazenyl]acetonitrile (p–CH3); 3—1,3-benzothiazol-2-yl[(E)-(4-methoxyphenyl)diazenyl]acetonitrile (p–OCH3); 4—1,3-benzothiazol-2-yl[(E)-(4-carboxli-
cyphenyl)diazenyl]acetonitrile (p–COOH); 5—1,3-benzothiazol-2-yl[(E)-(4-nitophenyl)diazenyl]acetonitrile (p–NO2); 6—1,3-benzothiazol-2-yl[(E)-(4-chlorophenyl)
diazenyl]acetonitrile (p–Cl); 7—1,3-benzothiazol-2-yl[(E)-(4-hydroxyphenyl)diazenyl]acetonitrile (p–OH).
several molecular and physical properties such as heats of forma-
tion and electronic dipole polarizability and many other various
properties [37,38]. Analysis of the optimized molecular geometries
reveals that the azo and hydrazone tautomers are essentially non-
planar in the ground and excited electronic states since a significant
torsion angles are detected between the plane of the azo linkage
and the carbons C2 and C10 (Fig. 1, Table 2). In contrast, our calcula-
tions on the other forms considered here gave structures in which
the heterocyclic moiety is essentially coplanar with the rest of the
molecule (Table 2).
In the light of our data obtained from PM6 in the gaseous phase,
the azo tautomers were found to be less thermodynamically sta-
ble than the other ones (Table 2). Moreover, the energy difference
between both hydrazone and azo-enamine tautomers in ꢁHf of
each dye was ranging from 1 to 2 kcal/mol. The highest energy dif-
ference – 3 kcal/mol – was found only for compound 5 and the
calculated values of heats of formation of various forms in non-
aqueous media predicted are presented in Table 3. Calculations
combining COSMO in semiempirical calculations demonstrate that
the inclusion of a continuum dielectric representing a particu-
lar solvent provides lower values of enthalpies of formation in
respect with the ones estimated at the gaseous phase. Furthermore,
hydrazone and azo-enamine tautomers are still thermodynami-
cally preferred compared to the corresponding azo tautomer and
can coexist in equilibrium as a consequence. It is worthwhile men-
tioning that the differences in the enthalpies of formation of the
relevant excited and ground state forms remain comparable and
only slightly affected by the medium properties. In addition, the
thermodynamically preferred tautomers in the ground state are
also favorable in the excited state for all compounds investigated.
Changes in the dipole moment as a result of electronic excitation
(ꢂS1 − ꢂS0) for each tautomeric form are quite clear. The dipole
moment changes accompanying electronic excitation in the case of
azo-enamine forms are not very high but always negative, though
they are mainly positive in the hydrazone tautomers. This implies
that the positions of the long-wavelength bands in the absorption
spectra of the mentioned tautomers should depend closely on the
solvent polarity. Thus, substantial changes to the position of the
absorption bands that occur upon changing media are due to spe-
cific interactions of the tautomeric forms with the solvent. In view
of these facts, the hydrazone tautomers should absorb at longer
wavelength relative to that of the azo-enamine ones. Therefore,
azo-enamine–hydrazone transformation should be accompanied
by a bathochromic shift of the longer wavelength absorption band.
In fact, compound 5 reflected such phenomena whereas the pre-
dicted spectra show an opposite behavior (Fig. 2, Tables 4 and 5).
Based on the electronic absorption transitions predicted for
the tautomers investigated here (Table 5), in the visible region
the hydrazone tautomers should show bands at 340–379 nm and
the azo-enamine forms absorb strongly at 419–435 nm depending
Table 2
Physicochemical and structural parameters predicted in the gaseous phase at the PM6 (S0 state) and PM6/CI (S1 state) levels of theory.
Comp.
Form
ꢁf,298H0 (kcal/mol)
ꢂ (D)
ꢂS (D)
ꢂS1 –ꢂ (D)
Energy
HOMO
ꢃ
1
LUMO
1
Azo
Hydrazone
Azo-enamine
157.53 (229.34)
145.91 (220.58)
144.84 (204.20)
4.99
4.37
7.50
5.14
4.99
5.61
0.15
0.62
−1.89
−9.20
−8.95
−8.48
−1.27
−1.15
−1.40
−144.18
174.39
−179.97
2
3
4
5
6
7
Azo
Hydrazone
Azo-enamine
146.82 (198.45)
135.46 (211.43)
134.36 (194.64)
5.81
4.35
7.50
4.08
4.26
6.08
−1.73
−0.09
−1.42
−9.15
−8.90
−8.34
−1.15
−1.06
−1.32
−147.70
174.04
−179.99
Azo
Hydrazone
Azo-enamine
115.22 (169.24)
104.57 (180.81)
103.22 (163.47)
5.39
4.70
5.89
2.80
4.99
4.90
−2.59
−9.11
−8.88
−8.26
−1.10
−1.03
−1.28
−143.64
0.29
175.08
−0.99
−179.85
Azo
Hydrazone
Azo-enamine
72.08 (153.34)
58.57 (130.93)
56.47 (113.88)
3.87
4.69
7.55
5.03
8.55
5.89
1.16
3.86
−1.66
−9.53
−9.13
−8.74
−1.33
−1.62
−1.77
−99.26
175.80
179.97
Azo
Hydrazone
Azo-enamine
155.78 (237.15)
142.82 (213.26)
139.23 (201.61)
7.10
7.58
11.16
8.30
13.23
8.98
1.20
5.65
−2.18
−9.64
−9.27
−9.03
−1.65
−1.99
−1.81
−99.17
176.42
179.93
Azo
Hydrazone
Azo-enamine
149.94 (231.20)
136.95 (212.22)
134.83 (193.13)
3.93
4.62
7.88
4.97
6.52
5.40
1.04
1.90
−2.48
−9.42
−9.05
−8.58
−1.37
−1.27
−1.56
−114.49
−174.04
−179.92
Azo
Hydrazone
Azo-enamine
111.57 (164.43)
100.83 (175.70)
99.28 (159.23)
4.37
4.08
5.99
2.49
5.19
4.22
−1.88
−9.17
−8.95
−8.39
−1.23
−1.10
−1.35
−143.23
173.69
179.97
1.11
−1.77
ꢁf,298H0—enthalpy of formation in the S0 and S1 (in parentheses) states, ꢂ, ꢂS —dipole moments (Debye) in the S0 and S1 states respectively. ꢃ–dihedral angle between
1
C2C10N12N13; HOMO and LUMO, in ev, indicate the energies of the highest occupied and lowest unoccupied molecular orbitals.

Y.H. Ebead et al. / Spectrochimica Acta Part A 75 (2010) 760–768
763
Table 3
Enthalpies of formation (in kcal/mol) predicted at the PM6 (COSMO) (S0 state) and PM6/CI (COSMO) (S1 state) levels of theory.
Comp.
Form
Medium
C2H5OH
DMSO
DMF
CH3CN
CH2Cl2
CHCl3
1
2
3
4
5
6
7
Azo
146.46 (228.24)
131.16 (202.87)
130.03 (195.69)
33.08a (104.67)
146.12 (229.59)
130.49 (222.10)
129.35 (194.17)
124.66 (154.13)
146.17 (227.97)
146.19 (227.99)
130.75 (202.14)
129.64 (194.49)
125.31 (154.69)
147.87 (230.11)
133.07 (205.09)
132.18 (193.45)
149.46 (232.11)
135.42 (209.51)
134.45 (198.47)
Hydrazone
Azo-enamine
Ionized form
154.46a (209.66)
146.14 (192.94)
125.27 154.68
Azo
135.20 (226.67)
120.89 (221.45)
119.44 (183.95)
19.74 (103.17)
134.91 (223.80)
120.18 (203.26)
118.67 (183.16)
17.74 (101.57)
134.90 (225.88)
144.92a (206.25)
136.60 (181.95)
18.34 (102.41)
134.91 (226.82)
120.43 (203.56)
118.98 (183.50)
18.40 (102.05)
136.59 (221.51
122.93 (204.81)
121.56 (186.14)
138.31 (220.69)
125.37 (204.67)
123.91 (188.67)
Hydrazone
Azo-enamine
Ionized form
azo
101.66 (191.70)
89.48 (155.90)
86.31 (147.20)
−13.41 (69.82)
101.15 (191.36)
88.89 (154.96)
85.91 (146.85)
−15.47 (67.67)
101.31 (192.33)
89.05 (155.19)
85.86 (146.77)
−14.88 (68.67)
101.33 (191.84)
89.05 (155.34)
85.88 (146.78)
−14.75 (68.90)
103.26 (191.15)
91.48 (158.73)
88.37 (149.20)
105.15 (190.84)
93.94 (161.72)
90.88 (151.75)
Hydrazone
Azo-enamine
Ionized form
Azo
53.44 (109.59)
38.34 (107.65)
35.27 (103.63)
−66.94 (10.15)
52.77 (108.73)
37.67 (107.02)
34.42 (102.79)
−67.33 (10.23)
53.00 (109.17)
37.85 (107.24)
34.68 (103.07)
−66.78 (10.60)
52.81 (134.79a)
37.85 (107.31)
34.69 (103.17)
−66.70 (10.67)
55.47 (111.54)
40.59 (110.07)
37.93 (106.78)
58.01 (115.20)
43.36 (112.88)
41.21 (109.62)
Hydrazone
Azo-enamine
Ionized form
Azo
137.40 (201.68)
122.08 (190.93)
119.09 (182.77)
17.88 (65.72)
136.82 (202.87)
121.45 (189.59)
118.25 (182.24)
15.99 (63.68)
136.96 (202.38)
121.55 (190.11)
118.46 (182.42)
16.53 (64.16)
136.97 (202.51)
121.57 (190.14)
118.48 (182.38)
16.59 (64.27)
139.48 (208.64)
124.39 (193.57)
121.30 (191.80)
142.05 (224.93)
127.17 (196.06)
125.25 (186.76)
Hydrazone
Azo-enamine
Ionized form
Azo
136.99 (218.63)
122.42 (206.20)
119.90 (184.81)
19.50 (102.51)
136.63 (204.50)
121.73 (205.51)
119.24 (184.15)
17.63 (101.33)
136.74 (204.04)
121.63 (205.73)
119.44 (184.38)
18.19 (101.75)
136.72 (218.28)
121.95 (205.69)
119.45 (184.37)
18.23 (101.88)
138.28 (220.42)
124.50 (208.17)
122.05 (184.95)
139.93 (194.64)
126.91 (210.09)
124.37 (189.36)
Hydrazone
Azo-enamine
Ionized form
Azo
Hydrazone
Azo-enamine
Monoanionic form
Dianionic form
96.86 (189.35)
82.70 (165.80)
96.09 (188.61)
81.93 (165.07)
80.31 (144.31)
−20.60 (62.58)
−105.30 (−45.64)
96.49 (189.02)
82.14 (165.23)
80.60 (144.62)
−20.04 (62.96)
−103.81 (−44.28)
96.50 (189.03)
82.17 (165.19)
80.69 (144.72)
−19.98 (62.94)
−103.69 (−44.17)
98.66 (190.68)
85.34 (168.28)
83.56 (147.68)
100.63 (192.16)
88.12 (170.77)
86.36 (150.66)
81.21 (154.24a)
−18.49 (62.78)
−100.14 (−40.83)
a
These inharmonic values have been checked several and were also tested by PM3 and the same tendency has been found.
on the substituents (R). Besides, many weaker transitions can be
expected for the azo-enamine tautomers in the range of 300 nm
and for the hydrazone forms between 250 and 280 nm. The results
given in Table 5, the calculations – based on AM1 geometries –
predicted a very small intensity (f) for the longest wavelength
transition bands in the case of the azo tautomers compared to
␲–␲* one within the heterocyclic moiety. In the hydrazone, azo-
enamine tautomers with acceptor substituents predicted to absorb
light at longer wavelength compared to derivatives with donor
substituents. Consequently, positive solvatochromism of the vis-
ible band should be noted on transferring from very strong donor
substituents to poor acceptor ones. Generally, the above findings
are in consistent with the experimental UV–vis spectra to some
extent and the slight deviation may in part be due to the solvation
effects (Table 4). Further information provides values of energies
of the first HOMO and LUMO orbitals showing that the difference
between HOMO and LUMO depends markedly on the substituents
(Table 2).
This assignment is in accordance with the literature data and it is
sustained by the slight effect on these transitions by the nature of
the substituent on the phenyl ring [31,39]. The weak band appeared
at around 300 nm can be attributed to n–␲* of the –N N– linkage.
Evidence supporting this assignment comes from the disappear-
ance of this band in the acidic medium. Obviously, the main band
in all compounds investigated appears in the range 400–425 nm
with a maximum absorption (ꢄmax) depending largely on their
molecular structure. This band can be regarded as a ␲–␲* transition
involving the ␲-electronic system throughout the whole molecule
with a considerable charge transfer (CT) character. This CT orig-
inates mainly from the aryl moiety as a donor to the heterocyclic
benzothiazole ring [31,40,41]. The CT nature of this band is achieved
from its broadness [42], the sensitivity of its ꢄmax to the type of the
substituent attached to the phenyl and the nature of the solvent
(Table 4 and Fig. 3). Generally, it acquires blue shift in its ꢄmax when
R is an electron acceptor group whereas an electron releasing sub-
stituent results in a red shift (Table 4). This behavior in compounds
2, 3, and 7 can be interpreted on the basis of the expected low exci-
tation energy for the intramolecular CT band. This is a result of the
increased delocalization of the ␲-electronic system relative to the
former ones. The discrepancy of compound 5 from this trend can
be explained either by the antagonizing effect by the nitro group
or under the acceptor character of this group and consequently the
CT transitions may take place in the opposite direction [26].
3.2. Electronic absorption spectra in ethanol
The values of ꢄmax and the excitation coefficients (log ε) of
the characteristic absorption bands of the dyes 1–7 in various
organic solvents are listed in Table 4. Only the longer wavelength
absorption bands are tabulated as the shorter wavelength band
remained virtually unaltered in different solvents. Fig. 3 represents
the absorption spectra of the studied compounds (1–7) in ethanol.
The bands observed in the range 205–265 nm are due to ␲–␲* tran-
sitions within the ␲-electronic system of the heterocyclic moiety.
In particular, compound 5 exhibits a special behavior that is a
new band at longer wavelength (525 nm) has appeared. This extra
visible band exceeds by far the polarity effect of the solvent and
could be attributed to the absorption by the ionized form even

764
Y.H. Ebead et al. / Spectrochimica Acta Part A 75 (2010) 760–768
Table 4
Experimental electronic absorption spectral data of compounds 1–7.
Compound
C2H5OH
CH3OH
DMF
DMSO
CH3CN
Acetone
CH2Cl2
CHCl3
CCl4
ꢄmax
log ε
ꢄmax
log ε
ꢄmax
log ε
ꢄmax
log ε
ꢄmax
log ε
ꢄmax
log ε
ꢄmax
log ε
ꢄmax
log ε
ꢄmax
log ε
1
2
3
399
231
4.49
4.89
398
218
4.51
4.47
448
4.50
420
4.40
460sh
404
4.05
4.40
462sh
399
3.77
3.78
400
4.47
402
4.47
404
4.48
407
232
4.51
4.89
405
233
4.51
4.89
450
452
4.57
4.60
422
445
4.44
4.48
409
424
4.47
4.47
405
417
4.47
4.48
410
421
4.49
4.47
411
422
4.48
4.48
412
424
4.46
4.42
418
301
232
4.48
3.83
4.90
416
301
232
4.49
3.86
4.89
4
5
402
271
209
4.56
4.20
4.53
400
265
215
4.59
4.23
4.65
480
392sh
4.73
3.94
481
412
4.50
4.34
467
400
4.17
4.46
475
399
3.70
4.54
401
4.49
403
4.57
404
4.55
525
418sh
406
301
211
3.68
4.54
4.57
3.68
4.52
519
418sh
406
298
213
4.03
4.52
4.54
3.92
4.56
548
375
4.76
3.80
552
420
4.52
4.25
539
419
405
4.47
4.28
4.30
543
418sh
406
4.11
4.54
4.40
421
409
4.61
4.66
420
410
4.60
4.63
418sh
407
4.58
4.62
6
7
399
301
249
224
205
4.46
3.72
4.11
4.28
4.22
398
299
249
223
206
4.48
3.76
4.14
4.33
4.25
456
450
4.58
4.52
450
435
4.39
4.44
457
408
4.23
4.37
465sh
399
3.68
4.45
402
424
4.45
4.46
404
425
4.46
4.44
405
425
4.45
4.42
423
308sh
264
4.41
3.76
4.06
4.64
420
308sh
262
4.44
3.77
4.09
4.74
425
4.42
425
4.45
212
218
ꢄmax—position of band maxima in absorption spectra (nm), ε—absorption coefficients (M−1 cm−1) and sh—shoulder.
though the basicity character of ethanol is quite low. This find-
ing occurs on the basis of the well-known acid–base equilibrium
(HA + S ꢀ A− + HS+) of other azo dyes which was discovered by an
examination of their absorption spectra [28,43]. Such phenomenon
is much more pronounced in this derivative due to the presence
of the nitro group (a very strong electron withdrawing group).
Consequently, the introduction of strongly acceptor group at C17
in the phenyl moiety facilitates the deprotonation process. Addi-
tionally, we have to state here that the spectra of this dye in all
solvents except DMF and DMSO contain elements characteristic of
both hydrazone and azo-enamine forms (Scheme 1, Fig. 2). Such
tautomerization ability is demonstrated by the composite nature
of the main visible band, where a clear shoulder can be observed
in the longer wavelength side. Therefore, it is possible to assign
Table 5
Wavelengths (nm) and oscillator strengths (in parentheses) of electronic absorption transitions predicted in gaseous phase at AM1/CI level of theory.
Form
Azo
Compound
1
2
3
4
5
6
7
353 (0.06)
295 (0.12)
264 (0.11)
249 (0.95)
228 (0.24)
215 (0.73)
352 (0.06)
305(0.20)
264 (0.10)
289 (0.95)
229 (0.30)
215 (0.81)
352 (0.06)
324 (0.06)
310 (0.22)
263 (0.10)
249 (0.89)
218 (0.84)
352 (0.06)
298 (0.13)
247 (0.88)
224 (0.24)
215 (0.42)
211 (1.20)
351 (0.06)
300 (0.13)
257 (0.14)
245 (1.08)
226 (0.30)
211 (0.77)
352 (0.06)
304 (0.18)
262 (0.08)
251 (0.10)
248 (0.93)
214 (0.91)
352 (0.06)
326 (0.10)
315 (0.24)
264 (0.10)
249 (0.92)
231 (0.28)
214 (0.73)
Hydrazone
352 (0.06)
332 (0.30)
317 (0.13)
280 (0.12)
270 (0.31)
265 (0.12)
244 (0.29)
218 (0.28)
215 (0.68)
375 (0.81)
356 (0.76)
280 (0.37)
270 (0.78)
247 (0.17)
228 (0.62)
219 (0.25)
213 (0.55)
207(0.39)
347 (0.56)
293 (0.09)
279 (0.07)
273 (0.20)
246 (0.23)
222 (0.25)
220 (0.31)
215 (0.19)
208 (0.23)
341 (0.43)
275 (0.32)
260 (0.14)
243 (0.25)
233 (0.09)
228 (0.09)
221 (0.17)
218 (0.42)
213 (0.70)
344 (0.48)
272 (0.40)
267 (0.07)
255 (0.10)
241 (0.39)
230 (0.22)
227 (0.13)
216 (0.29)
209 (0.50)
344 (0.53)
321 (0.08)
286 (0.32)
266 (0.11)
241 (0.28)
225 (0.19)
220 (0.24)
215 (0.43)
210 (0.37)
379 (0.83)
356 (0.07)
283 (0.36)
270 (0.08)
247 (0.19)
229 (0.45)
219 (0.24)
214 (0.46)
209 (0.47)
Azo-enamine
419 (0.97)
264 (0.22)
260 (0.31)
249 (0.31)
233 (0.12)
217 (0.49)
207(0.47)
424 (1.01)
305(0.18)
261 (0.37)
249 (0.30)
234 (0.16)
219 (0.51)
209(0.39)
426 (1.04)
309 (0.11)
262 (0.32)
249 (0.30)
235 (0.20)
220 (0.56)
209 (0.40)
428 (1.04)
306 (0.14)
262 (0.49)
247 (0.28)
233 (0.16)
225 (0.31)
205 (0.97)
435 (1.08)
301 (0.10)
262 (0.44)
246 (0.29)
234 (0.22)
229 (0.28)
208 (0.53)
426 (1.02)
307 (0.11)
262 (0.43)
248 (0.30)
234 (0.16)
229 (0.21)
209 (0.25)
427 (1.04)
309 (0.11)
262 (0.38)
249 (0.30)
235 (0.20)
220 (0.53)
209 (0.37)

Y.H. Ebead et al. / Spectrochimica Acta Part A 75 (2010) 760–768
765
Fig. 1. The PM6 optimized structures of the tautomers of compounds 1, 3 and 5 in ground (S0) and excited state (S1) as a representative example (Scheme 1).
the shorter wavelength band to a transition within the hydra-
zone form, while the longer band to a similar transition within the
azo-enamine tautomer. Similar acid–base equilibrium is also rec-
ognized in the other derivatives and it might be responsible for the
disappearance of the prototropic equilibrium in these compounds.
Since the tautomeric equilibrium should be influenced by the pos-
sibility of interaction with the solvent through charge transfer from
solute to solvent. This effect, however, could not enable us to see dif-
ferent forms in the other investigated compounds. Detailed study
of the tautomerism in these compounds may provide interesting
information. Examination of this problem was not the main target
of the present piece of work.
3.3. Effect of solvents and acid–base equilibrium
The electronic absorption spectra of compounds 1–7 were stud-
ied in organic solvents with different polarities to investigate their
solvatochromic behavior. Comparison of absorption maxima of the
dyes investigated revealed a blue shift in ꢄmax of the main visi-
ble band as the polarity of the solvent increases. This shift can
be explained on the basis that the excited states of these dyes
are less polar than the ground states. As a consequence, the non-
polar solvents stabilize these dyes more than polar ones and this
result is generally very close to the calculated dipole moments for
azo-enamine form (Table 2).

766
Y.H. Ebead et al. / Spectrochimica Acta Part A 75 (2010) 760–768
Fig. 4. Visible absorption spectra of 4.0 × 10−5 M solution of compounds 1–3, 5 and
7 in DMF.
Fig. 2. Absorption spectra of 4.0 × 10−5 M solution of compound 5 (p–NO2) in
organic solvents. EtOH (i); DMF (ii); acetonitrile (iii); acetone (iv); CH2Cl2 (v); CHCl3
(vi) and CCl4 (vii).
rium in the polar solvents is obtained from two important features
namely:
Moreover, our results demonstrate that compounds 1–3 and
6, 7 exist principally in the ionized form in DMSO and DMF due
to their low ionization potential and high hydrogen bond accept-
ing character [44]. Moreover, the spectra of compounds 4 and 5
indicate that they coexist in acid–base equilibrium, (representa-
tive Fig. 2). Furthermore, dissolving compounds 1 and 6 in acetone
and acetonitrile showed a clear shoulder at a longer wavelength.
This finding makes one also propose without any doubt that dif-
ferent structures are involved in equilibrium. Thus, it is possible to
assign the shorter wavelength visible band displayed by the neutral
form HA in compounds 1 and 4–6 to an intramolecular CT within
the azo-enamine and/or hydrazone forms of the molecule. While
the longer wavelength visible one can be attributed to a similar
transition within anionic form (A−). It is anticipated that the CT
interaction occurs more easily for the A− form than the HA one.
It is of considerable interest to emphasize that the appearance of
the absorption band due to the ionized form in the polar solvents
is because of the high basicity (pKs) of these solvents compared to
the non-polar ones [44,45]. Strong evidence verifying such equilib-
The position of ꢄmax of a solution containing few drops of 1.0N
NaOH is the same as in pure organic solvents, while in a solution
containing 1.0N HCl only the shorter wavelength band appears.
The color of the dye solution containing NaOH is exactly the
same as in pure solvents and widely different from that in solvents
containing HCl.
Also it is markedly clear that the electron donating nature con-
tributes to the stabilization of the neutral form and hence the above
equilibrium has not been observed in compounds 2, 3 and 7.
3.4. Spectra in mixed organic solvents
The visible spectra of compounds 1–3, 5 and 7 in DMF–CCl4,
DMF–CHCl3, DMF–acetone and DMF–C2H5OH binary solvent mix-
tures were undertaken to obtain an insight into the origin of the
interaction between these dyes and DMF. This is performed to find
Fig. 5. Visible spectra of 4.0 × 10−5 M solution of compound 1 (p–H) in DMF–EtOH
mixture solvents. (a) 1.30 M DMF; (b) 2.59 M DMF; (c) 3.89 M DMF; (d) 5.19 M DMF;
(e) 6.49 M DMF; (f) 7.78 M DMF; (g) 9.08 M DMF; (h) 10.38 M DMF and (i) 11.67 M.
DMF.
Fig. 3. Electronic absorption spectra of 4.0 × 10−5 M solution of compounds 1–7 in
ethanol.

Y.H. Ebead et al. / Spectrochimica Acta Part A 75 (2010) 760–768
767
Table 6
Values of Kion and −ꢁG for the ionization process for some azo derivatives from the investigated compounds.
System
Compound
1
2
3
5
7
Kion
−ꢁG
Kion
−ꢁG
Kion
−ꢁG
Kion
−ꢁG
Kion
−ꢁG
DMF–acetone
DMF–C2H5OH
DMF–CHCl3
DMF–CCl4
5.13
8.13
8.51
6.76
0.97
1.24
1.26
1.13
6.03
8.91
9.55
8.32
1.07
1.30
1.33
1.25
5.13
7.94
9.77
9.12
0.96
1.23
1.35
1.30
4.57
3.89
6.03
4.68
0.90
0.80
1.06
0.91
5.75
1.03
1.25
1.36
1.49
8.32
10.00
12.30
how these derivatives, dissolved in highly proton acceptor (DMF),
would respond to the presence of successfully increasing amounts
of CCl4, CHCl3, acetone and ethanol. It is evident that in pure DMF
the dyes exhibit only one band in the visible region at wavelengths
depending upon the molecular structure (Table 4, Fig. 4). When the
second solvent is added into the solution of the dye in dimethylefor-
mamide, the band which previously assigned to the absorption by
the ionized form is shifted to a lower wavelength. With the increase
of the mole fraction of the added solvent to the binary solvent mix-
ture, the absorption band by neutral form develops gradually and
keeps almost constant at very low amount of DMF. The spectra
recorded in all mixed solvents show a fine isosbestic point which
indicates the establishment of equilibrium between the ionized
and non-ionized forms of the molecule (Fig. 5). This behavior also
indicates that DMF molecules have a greater tendency to form ion-
ized forms with the solute molecules comparable to acetone and
ethanol. The value of the ionization constant (Kion) was determined
from the variation of absorbance obtained by decreasing the DMF
concentration at a constant wavelength using the following equa-
tion [46].
ꢀ
ꢁ
A − Amin
Amax − A
Log CDMF = log Kion + log
(1)
where Amin, absorbance in a low polarity solvent; Amax absorbance
in a high polarity solvent (DMF); A absorbance in the mixed solvent.
Gibbs free energy (ꢁG) values are obtained by applying Eq. (2):
−ꢁG = RT ln Kion
(2)
The values of ꢁG and Kion of the ionization equilibrium existing in
solution are cited in Table 6. The negative values of the free energy
change of the ionization process indicate that this process is spon-
taneous. In general, the experimental results presented in Table 6
show that the values of Kion and ꢁG of the compounds investigated
in mixed solvent are dependent upon the substituents and increase
in the following sequence:
7(p–OH) > 3(p–OCH3) > 2(p–CH3) > 1(p–H) > 5(p–NO2)
This order is in harmony with the increase in the donating char-
acter of the substituent.
3.5. Time effect
The most striking behavior in these compounds is the visible
spectrum of compound 7 in DMF where the main band with ꢄmax
at 450 nm varies apparently with time. This can be considered as
further evidence that this band corresponds to an association pro-
cess through a solute–solvent interaction. The two possible mono-
and dianionic structures exhibit low thermodynamic values in both
the ground and excited electronic states (Table 3). Accordingly, both
mono- and dianionic forms of 7 can exist in DMF. As can be readily
seen from Fig. 6, the intensity of the former band (due to absorp-
tion by dianionic form) decays gradually with time, while a shorter
wavelength visible band (ꢄmax = 397) starts to appear as a shoul-
der and its absorbance increases gradually. After a long time the
extinction of the shorter wavelength band becomes higher and
the longer wavelength band completely disappears. This behav-
ior denotes that the rate of dissociation – the concentration of the
equilibrium HA− + S ꢁ A−− + HS+ – shifted predominantly toward
the backward direction in such equilibrium. This confirms the exis-
tence of the compound in acid–base equilibrium and supports our
previous discussion.
4. Conclusion
We have investigated solvatochromism, acid–base equilibrium,
preferred tautomers of seven azo compounds using UV–vis spectra
and quantum—mechanical calculations at the semiempirical levels
(AM1 and PM6). Our calculated results give rise to the following
findings:
Fig. 6. Time effect of 4.0 × 10−5 M solution of compound 7 in DMF at (a) 1 h; (b) 48 h;
(c) 96 h; (d) 216 h; (e) 288 h; (f) 360 h; (g) 432 h; (h) 528 h; (i) 624 h; (j) 744 h; (k)
816 h; (l) 936 h; (m) 984 h; (n) 1056 h; (o) 1176 h; (p) 1272 h and (q) 1368 h.

768
Y.H. Ebead et al. / Spectrochimica Acta Part A 75 (2010) 760–768
•
Two of the plausible tautomers – azo and hydrazone – are
essentially non-planar in the ground and excited electronic
states.
The azo tautomers are characterized by higher values of
ꢁHf which means that compounds 1–7 are not likely to
exist.
The predicted electronic absorption spectra have some consider-
able shifts compared to the experimental spectral data.
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•
The well-known strong hydrogen bond acceptor character of
DMF.
Its low ionization potential (9.12 ev) [44,45].
•
Nevertheless, recent crystallographic data concerning com-
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are energetically favored. Compound 7 is considerably affected
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