Synthesis, structural elucidation, solvatochromism and spectroscopic properties of some azo dyes derived from 6-chloro-4-hydroxyquinoline-2(1H)-one

Article abstract of DOI:10.1016/j.molstruc.2015.12.001

Malondianilide (I) derived from p-chloroaniline was cyclized to 6-chloro-4-hydroxyquinoline-2(1H)-one (II) in moderately good yield using polyphosphoric acid as catalyst. This compound was then coupled with some diazotized aromatic amines to give the corr

Full text of DOI:10.1016/j.molstruc.2015.12.001

Accepted Manuscript
Synthesis, structural elucidation, solvatochromism and spectroscopic properties of
some azo dyes derived from 6-chloro-4-hydroxyquinoline-2(1H)-one
E.O.Moradi Rufchahi, A. Ghanadzadeh Gilani, Reyhane Karimi, Nasim
Ramezanzade, V. Taghvaei
PII:
S0022-2860(15)30489-0
10.1016/j.molstruc.2015.12.001
MOLSTR 22034
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 9 October 2015
Revised Date: 30 November 2015
Accepted Date: 1 December 2015
Please cite this article as: E.O.M. Rufchahi, A.G. Gilani, R. Karimi, N. Ramezanzade, V. Taghvaei,
Synthesis, structural elucidation, solvatochromism and spectroscopic properties of some azo dyes
derived from 6-chloro-4-hydroxyquinoline-2(1H)-one, Journal of Molecular Structure (2016), doi:
10.1016/j.molstruc.2015.12.001.
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ACCEPTED MANUSCRIPT
CN
CN
Ha
O
O
N
N
Cl
N
O
Cl
N
Ha
N
N
O
Hb
Hb
T2
T1
Hb
Ha
ppm
Graphical Abstract
1

ACCEPTED MANUSCRIPT
Highlights
6-chloro-4-hydroxyquinoline-2(1H)-one was used as a coupling component > solvatochromism
of the azo dyes was studied > acidic dissociation constants were measured and correlated with
Hammet substitution constants
2

ACCEPTED MANUSCRIPT
Synthesis, structural elucidation, solvatochromism and spectroscopic properties of
some azo dyes derived from 6-chloro-4-hydroxyquinoline-2(1H)-one
a
E.O. Moradi Rufchahi , A. Ghanadzadeh Gilanib, Reyhane Karimia, Nasim Ramezanzadea,
V. Taghvaeib
aDepartment of Chemistry, Faculty of Science, Lahijan Branch, Islamic Azad University, Lahijan, Iran
bDepartment of Chemistry, Faculty of Science, University of Guilan, Rasht, Iran
Corresponding author: Tel.: +981333515849; fax: +981412229081.
E-mail address:ena_moradi@yahoo.com, moradierufchahi1350@gmail.com,
moradierufchahi@liau.ac.ir (E. Moradi)
3

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Abstract
Malondianilide (I) derived from p-chloroaniline was cyclized to 6-chloro-4-hydroxyquinoline-
2(1H)-one (II) in moderately good yield using polyphosphoric acid as catalyst. This compound
was then coupled with some diazotized aromatic amines to give the corresponding azo disperse
dyes 1-12. A systematic study of the effect of solvent, acid, base and pH upon the electronic
absorption spectra of the dyes 1-12 was carried out. In DMSO, DMF, CH3CN, CHCl3, EtOH and
acidic media (CH3COOH, acidified EtOH) these dyes that theoretically may be involved in azo-
hydrazone tautomerism have been detected only as hydrazone tautomers T1 and T2. The acidic
dissociation constants of the dyes were measured in 80 vol% ethanol-water solution at room
temperature and ionic strength of 0.1. The results were correlated by the Hammett-type equation
using the substituent constants σx.
Key words: azo dye, tautomerism, pH, solvatochromism, pKa, hydrazone
1. Introduction
Dyes can be classified as azo, anthraquinone, arylmethane, acridine, cyanine, phthalocyanine,
nitro, nitroso, quinoneimine, thiazole or xanthene dyes according to the type of chromophore
group on the dye molecule. Azo dyes contain one or more azo groups (N=N) having aromatic
rings. They are the largest dye class of synthetic dyes [1-3]. Azo dyes have been extensively used
in various fields, such as dyeing of textile fibers, colouring of different materials, advanced
applications in organic synthesis and many non textile applications [4-6]. Striking examples in
this respect are those of containing quinolone moieties that deserve special attention due to their
applications as disperses dyes [7, 8]. Moreover, the hydroxyl-quinolone compounds are very
useful as enol-type precursors for the synthesis of some heterocyclic dyestuffs in color chemistry
[9-11].
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The presence of one hydroxyl group in conjugation to the azo bridge makes these compounds
the main source of azo-hydrazone tautomeric structural requirement [12, 13]. In addition, the
spectral data generally led to the conclusion that in some solvents, the tautomeric equilibrium of
the arylazoquinolone dyes is a mixture of two hydrazone-keto forms [14-17].
Although several studies concerning the azo-hydrazone tautomeric equilibrium of dyes having
azo-enol moieties have been reported, very few investigations have been devoted on the
structures, precise characterization and spectroscopic properties of substituted 3-arylazo-4-
hydroxyquinoline-2(1H)-ones.
Herein, some 3-aryl azo-quinolone dyes were prepared by coupling 6-chloro-4-
hydroxyquinoline-2(1H)-one with diazotized aniline derivatives. The visible absorption spectra
of these dyes were evaluated with respect to the influence of solvents and substituents on the
phenyl ring of the diazo components. The effects of acid and base on the visible absorption
maxima of the dyes are reported. The pH effects along with the acid dissociation constants of the
dyes are also included with the aim of elucidating the absorption behaviors of the compounds
under investigation. The structures of coupling component and prepared dyes are depicted in
Schemes 1 and 2.
2. Experimental
2.1. Materials and instrumentation
All the chemicals used in dye synthesis were of laboratory grade and were used without
further purification. Elemental analyses were performed at the Microanalytical Center using
CHNS- EL III Vario Elemental Analyzers and the results were within the accepted range (±0.40)
of the calculated values. All melting points were measured on a Barnstead Electrothermal 9100
melting point apparatus and were uncorrected. IR spectra were recorded for KBr disc on a
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PerkinElmer FT-IR spectrometer. Electronic absorption spectra were recorded on Cary UV–Vis
1
double-beam spectrophotometer (Model 100). H NMR spectra were measured on a FT-NMR
(500 MHz) Brucker apparatus in CDCl3 or DMSO as a solvent, using TMS as an internal
standard, chemical shifts are expressed as δ/ppm, and J values are given in Hz. Thin-layer
chromatography (TLC) analysis was carried out on plates coated with silica gel 60 F254.
O
O
NH2
HN
NH
O
O
Reflux
2EtOH
+
+
2
EtO
OEt
Cl
Cl
Cl
I
Cl
NH2
OH
O
Cl
HN
Cl
PPA
150-160 oC, 5 h
+
N
H
O
N
H
O
Cl
II
I
Scheme1. Preparation of 6-chloro-4-hydroxyquinolin-2(1H)-one.
2.2. Preparation of N, N′ -bis (p-chlorophenyl) malonamide (I)
A mixture of p-chloroaniline (100 mmol, 12.80 g) with diethylmalonate (50 mmol, 7.62 ml) was
refluxed in a round bottom flask equipped with an air condenser for 2 h so that the ethanol
evolved and escaped from the top of the condenser. After cooling, the precipitate was filtered by
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suction and recrystallized from ethanol giving the malonamide (I) (13.25g, 82%) as white
crystals, m.p. 218–220 °C (reported 217 °C [10]).
2.3. Synthesis of 6-chloro-4-hydroxyquinolin-2(1H)-one (II)
N, N′-bis (p-chlorophenyl) malonamide (I) (3.23 g, 10.0 mmol) at 80°C was thoroughly mixed
with 22 g polyphosphoric acid (PPA, sp.gr. 2.08) and then heated in an oil bath for 5 h at 150-
160°C. The dark viscous solution was allowed to cool, water was added and the resultant gum
solidified on prolonged standing. The precipitate so formed was filtered off, washed several
times with water and dried in air. The crude product was dissolved in 100 ml of sodium
hydroxide solution (0.5 molꢀL-1) and undissolved material was filtered. The filtrate was carefully
neutralized with concentrated hydrochloric acid and the precipitate was recrystallized from
ethanol to afford 6-chloro-4-hydroxyquinolin-2(1H)-one (II) (1.47 g, 75%) as white powder:
mp> 350 °C, FT-IR (KBr): ν (cm-1): 3400-3150 (OH, NH), 1654(C=O), 1600(C=C), 1116(C-O),
1H NMR(500 MHz,DMSO-d6, 25°C): δ= 5.8 (s, 1H), 7.69(s, 1H), 7.26 (dd, J=8.6, 4.8
Hz,1H),7.51(d, J=8.6Hz, 1H), 11.35(NH).
2.4. Preparation of arylazo quinolone dyes 1-12
A cold solution of aryldiazonium salt (2.0 mmol) was prepared by adding a solution of
NaNO2 (2.2 mmol, 0.15 g into 1.0 mL H2O) to a cold solution of arylamine hydrochloride (2.0
mmol of arylamine in 1.5 mL conc. HCl). The resulting solution of aryl diazonium salt was
added drop wise to a mixture of compound (II) (2.0 mmol, 0.43g) in water (15 mL) containing
NaOH (25 mmol, 1.0 g) at 0- 5°C. The pH of the reaction mixture was maintained at 9-10 by
adding 2.5 % sodium hydroxide solution. Then, the reaction mixture was stirred for 2 h at the
same temperature and the precipitate was filtered off and the resulting solid was washed with
cold ethanol and dried at 50 °C in an oven. The crude product was purified using the
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recrystallization method as mentioned in Table 1. Under the same reaction conditions, a series
substituted arylazo quinolone dyes were obtained in satisfactory yields (Table 1, Scheme 2).
N2 Cl-
+
OH
OH
X
N
Cl
Cl
N
0- 5°C
+
N
H
O
N
H
O
X
(1):X= p-NO2
(5):X= m-CF3
(9):X= p- I
(2):X= m-NO2
(6):X= p-F
(10): X= -H
(3):X= p -CN
(7): X= p-Cl
(11):X= p-CH3 (12):X= p-OCH3
(4):X= p-CF3
(8):X= p-Br
Scheme 2. Structures of dyes 1-12.
3. Results and discussion
3.1. Synthesis and characterization
As depicted in Scheme 1, malonamide (I) was prepared by refluxing 4-chloro aniline with
diethyl malonate (2:1 molar ratio) in good yield. N, N′- di(4-chlorophenyl)malonamide (I) was
then heated
in polyphosphoric acid (PPA) at 150-160°C to afford the 6-chloro-4-
hydroxyquinolin-2(1H)-one (II) [10, 18]. The crude products were purified by recrystallization
1
from ethanol–water and were characterized by the FT-IR and H NMR spectral analysis. For
compound (II), the peak at 5.83 ppm (=CH) confirmed the formation of the desired quinolone
ring.
The arylazo-quinolone dyes 1–12 were synthesized by coupling reaction of 6-chloro-4-
hydroxyquinolin-2-(1H)-one with diazotized p-substituted aniline derivatives in alkaline
solution. The purified products can be obtained through recrystallization of the crude dyes in hot
DMF/water (Table 1, Scheme2). The chemical structures of these dyes were confirmed by FT-
IR, NMR and UV-vis spectroscopic methods and elemental analysis. The infrared spectra of all
the compounds (in KBr) showed intense carbonyl bands at 1691–1655 cm-1, broad hydroxyl (-
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OH) and amide (NH-C=O) bands at 3500–3350 cm-1 and 3291–3193 cm-1, respectively. The
aromatic C-H νmax values at 3097–3051 cm-1 were also recorded. The actual tautomeric forms of
1
the azo dyes 1-12 were experimentally examined by their UV-vis absorption and H NMR
spectroscopy. It can be assumed that the prepared azo dyes may exist in one or more of seven
tautomeric structures T1-T7 (Scheme3). The absence of a methine (>CH-) proton signal at about
6.4 ppm precludes the existence of structures T4 andT6 [19].
Table 1
The physical properties of the dyes synthesized in this study.
Yield
(%)
Dye
Color
Mp(˚C) Recrystalization
%C
%H
%N
Calcd. Found
Calcd. Found
Calcd. Found
2.69 16.25 16.33
2.67 16.25 16.36
2.83 17.25 17.32
2.49 11.43 11.55
2.52 11.43 11.56
2.89 13.23 13.74
2.74 12.57 12.66
2.38 11.10 11.25
327-329
305-307
325-327
288-290
320-322
288-290
315-317
348-350
278-280
286-288
295-297
288-290
DMF
52.26 52.71 2.63
52.26 52.69 2.63
65
65
70
70
75
77
75
70
68
74
80
85
Dark red
Reddish brown
Orange
1
2
3
4
5
6
7
8
9
DMF/H2O
DMF/H2O
DMF/H2O
DMF/H2O
DMF/H2O
DMF/H2O
DMF/H2O
DMF/H2O
DMF/H2O
DMF/H2O
DMF/H2O
59.18 58.96
52.26 52.54
52.26 52.58
56.71 56.89
53.91 54.13
47.58 47.71
42.33 42.56
60.11 61.35
61.25 61.64
58.28 58.63
2.79
2.47
2.47
2.86
2.71
2.40
2.13
3.36
3.86
3.67
Dark orange
Clear yellow
Brown
Gold
Yellow
Dark brown
Clear brown
Dark orange
Orange
2.15
9.87 10.06
3.29 14.02 14.34
3.84 13.39 13.48
3.70 12.74 12.73
10
11
12
The existence of tautomeric forms T3-T5 involving quinolone NH proton rearrangement can
1
be eliminated by studying the H NMR spectrum of the compounds in the appropriate region.
The 1H NMR spectrum of the dyes in DMSO-d6 showed two singlets at δ 11.56–11.33ppm and δ
11.51–11.12ppm. The presence of these two broad singlets provides firm evidence for presence
of the amide (–NH–C=O) bonds and is related to amide protons of two types of tautomeric forms
T1 and T2. In addition, the 1H NMR spectra of the compounds show the significantly downfield-
shifted resonances in range between 14 to 17 ppm corresponding to protons that are engaged in
strong hydrogen bonding. According to results previously published in literature [13, 20], the
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hydrazone NH proton resonance appears approximately between 13.0 and 16.0 ppm in o- and p-
1
hydroxyazo dyes. H NMR spectra measured in DMSO-d6 at 25 °C are given in Table 2. In
addition to known aromatic and aliphatic protons, all the prepared azo dyes showed two broad
peaks at δ 15.05–14.95ppm and δ 15.53–14.78ppm assigned to hydrazone proton signals (=N–
NH–). These signals can be attributed to the hydrazone NH proton resonance related to
hydrazone-keto forms T1 and T2 and evident that these azo dyes do not exist in tautomeric form
T7. This downfield position of the NH proton is evidence for intramolecular chelation with the
carbonyl group (Scheme 4). For example, the 1H NMR spectrum of dye 10 (Fig. 1) in DMSO-d6
at room temperature showed two peaks at 15.90 ppm and 15.16 ppm for hydrazone NH protons
and 11.47 ppm and 11.31ppm for amide NH protons. It can be validly concluded that azo dyes
1–12 are present in hydrazone structures which can exist as a mixture of two tautomeric forms,
namely the hydrazone-keto (T1) and hydrazone-keto (T2) as shown in Scheme 3.
ppm
Fig. 1. Partial 1H NMR spectra of dye 10 (X=-H) in DMSO in which peaks are assigned to the
presence of the two isomers of T1 and T2.
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Further evidence for this assignment was provided by the observation that the quinolone ring
NH was extremely affected by solvent species, but the hydrazone NH was not affected so much.
1
For example, H NMR spectrum of dye 12 was measured in CDCl3. It was observed that the
quinolone ring NH peak appears at higher field in chloroform than in DMSO. The quinolone
rings NHs of dye 12 were observed at δ 8.38ppm and δ 8.04ppm in CDCl3, whereas the
corresponding protons in other dyes were observed at δ 11.54ppm to δ 11.07ppm in DMSO-d6.
The downfield chemical shift of the NH proton signal in DMSO-d6 is larger than in CDCl3,
because of the intermolecular hydrogen bonding between the NH and DMSO. Inspection of the
hydrazone NHs chemical shifts of those azo dyes in both CDCl3 and DMSO-d6 showed that there
was no significant difference in the chemical shifts in the two solvents, provided that the
molecules are involved in a strong intramolecular hydrogen bond as shown in Scheme 4.
Ar
Ar
Ar
OH
N
N
O
HN
N
O
HN
N
Cl
Cl
Cl
N
H
O
N
H
O
N
OH
T3
T2
T1
Ar
Ar
Ar
Ar
Cl
O
N
N
O
N
N
O
N
OH
N
N
Cl
Cl
N
Cl
N
H
OH
N
OH
N
O
N
OH
H
T4
T5
T6
T7
Scheme 3. Possible tautomeric forms for the synthesized azo dyes.
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Table 2. Characterization data of azo dyes 1-12.
Dye
IR(KBr, cm-1)
1H NMR (δ, ppm)
N-H
Other
functional
groups
Hydrazone amide
vO-H
vN-H
vC=O
Aro. and Aliph.-H's
vc=c
3415
3266
1690
1610
1529 and
1348(NO2)
8.32(2H, d, J= 9.2 Hz), 7.86(2H, d, J= 9.2 Hz), 7.84 14.78
11.07
11.44
1
2
(1H, d, J= 2.8 Hz), 7.70-7.65(2H,m, overlapped)
15.07
3417
3416
3291
3257
1691
1690
1612
1607
1531 and
1347(NO2)
8.52(1H, s),8.12-8.07(2H,m, overlapped), 7.85(1H,
d, J=2.4 Hz), 7.76(1H,t,J=8 Hz), 7.67(1H, dd, J=
8.6, 2.6 Hz), 7.21(1H, d, J=8.8Hz)
14.92
15.47
11.41
11.53
2229(CN)
7.92(2H,d, J=8.4 Hz), 7.85-7.83(3H,m,
overlapped), 7.66( 1H, dd, J= 9, 2.4 Hz), 7.20(1H,
d, J=9Hz)
14.82
15.34
11.35
11.54
3
3418
3415
3288
3321
1694
1685
1606
1609
1316(C-F)
1317(C-F)
7.91-7.76 (5H, m, overlapped), 7.58(1H, dd, J= 8.8
, 2.4Hz), 7.15 (1H, d, J=8.8 Hz)
14.93
15.50
11.34
11.54
4
5
8.01(1H, s), 7.94(1H, d, J=5.2 Hz), 7.80(1H,s),
7.70(1H, t, J= 8 Hz), 7.64-7.60 (2H, m,
overlapped), 7.19(1H, d, J=8.8Hz)
8.31(2H, d, J= 9.2 Hz), 7.84-7.18(5H, m,
overlapped)
7.86(1H, d, J= 2Hz), 7.70(2H, d, J=7 Hz),
7.58(1H,dd, J= 8.4, 2.4 Hz), 7.49(2H, d, J= 7 Hz),
7.20(1H, d, J=8.8Hz)
15.53
14.95
11.16
11.34
3413
3417
3246
3227
1655
1689
1600
1614
1231(C-F)
823(C-Cl)
15.12
15.87
15.15
15.89
11.45
11.55
11.10
11.34
6
7
3414
3272
1626
1591
534(C-Br)
7.89(1H, d, J= 2.4Hz), 7.59(2H, d, J=8.8 Hz),
7.50(2H,d, J= 8.8Hz), 7.33(1H, dd, J= 5.6, 2.4 Hz),
7.10(1H, d, J=8.8Hz)
14.80
15.48
10.26
10.43
8
3418
3413
3203
3161
1657
1630
1610
1595
510(C-I)
-
7.84(2H, d, J= 8.4 Hz), 7.73-7.17(5H, m,
overlapped)
7.85(1H, s), 7.69-7.63 (3H, m, overlapped),
7.50(2H, t, J= 7.6 Hz), 7.32-7.22 (2H, m,
overlapped)
15.00
15.68
15.16
15.91
11.31
11.49
11.31
11.47
9
10
3415
3421
3193
3244
1675
1689
1611
1609
2921(C-H)
1253(C-O)
7.86(1H, d, J=2.4Hz), 7.65(1H, dd, J= 8.8, 2.4 Hz),
7.58(2H, d, J=8.5 Hz), 7.31(2H, d, J=8.0 Hz),
7.21(1H, d, J=8.4 Hz), 2.50 (3H, s, -CH3)
8.02 (1H, d, J=2.4Hz), 7.62( 2H, d, J= 9.0 Hz),
7.45( 1H, dd, J=8.0, 2.4 Hz),7.44-7.40 (3H, m,
overlapped), 3.80 (3H,s, -OCH3)
15.23
16.05
11.09
11.30
11
12a
a
15.40
16.02
8.04
8.38
Recorded in CDCl3
H
N
Ar
O
O
Cl
N
O
Ar
Cl
N
N
H
N
H
N
H
O
T1
T2
Scheme4. Intramolecular hydrogen bonding in tautomers T1 and T2.
12

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3.2. UV-Visible Spectroscopic Study
3.2.1. Solvatochromism
The UV-Vis absorption spectra of dyes 1-12 were measured in four polar aprotic (DMSO, DMF,
CH3CN and CHCl3) and two protic (EtOH and AcOH) solvents in a concentration range of 10−8
to 10−6 mol. L−1 in the wavelength range 300-600 nm. The dyes have a very low solubility in all
used solvents except DMSO and DMF. Therefore, each dye was dissolved in 100% DMSO to
create stock solutions of 10−3 mol. L−1. Then the necessary amounts of dyes were removed from
its DMSO stock and dilutions were made in six above mentioned solvents for absorption
measurements in the visible region. The results are given in Table 3.
The UV-vis absorption spectra of all the dyes showed two absorption bands in the visible region
(Table 3). It can be suggested that the prepared azo dyes may exist as a mixture of two
tautomeric forms in various solvents. These results are in accordance with NMR conclusions and
can be assigned to the hydrazone tautomeric forms T1 and T2 (Scheme 4). It was observed that,
although slightly positive solvatochromism is evident, the absorption spectra of dyes 1-12 did not
change significantly in all the employed solvents and the absorption maxima did not correlate
with the polarity of the solvent. This little absorption changes in λmax strongly suggest the
intramolecular hydrogen bonding in the compounds. As an example, dye 10 showed two
absorbances at 391 nm and 430 nm (in DMSO), 392 and 422 (in DMF), 390 and 420 (in
acetonitrile), 382 and 429 (in chloroform), 392 and 429 (in ethanol), and 389 and 431 (in acetic
acid). The relatively small difference observed is assumed to be caused by solute - solvent
interaction. Fig. 2 shows the electronic absorption spectra of dye
prepared, in the used solvents.
4, as a typical example of series
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Fig.2. Absorption spectra of dye
4
in different solvents.
The difference in the
λmax values for the dye solutions can be explained by electron withdrawing
and donating properties of the substituents in the benzenoid ring. The wavelengths of maximum
absorption spectra in the visible region of the synthesized arylazo-quinolone dyes containing
electron-accepting and electron-donating substituents are shown in Table 3. As it is apparent, the
introduction of electron donating methyl and/or methoxy group in the benzene ring resulted in
bathochromic shifts in all solvents with respect to electron-accepting nitro-, cyano,
trifluoromethyl and halogen groups (e.g. for dye 12 ꢁλ=+36 nm relative to dye
1
,
ꢁλ = +30 nm
relative to dye
2
,
ꢁλ = +58 nm relative to dye
ꢁλ=+37 nm relative to dye
to dye 9, ꢁλ=+31nm relative to dye 10, ꢁλ=+22nm relative to dye 11 in chloroform). The
3
,
ꢁλ = +30 nm relative to dye
4, ꢁλ = +39 nm
relative to dye
5
,
6
,
ꢁλ=+24nm relative to dye
7, ꢁλ=+27nm relative
introduction of electron-donating methoxy and methyl groups in the benzene rings resulted in
bathochromic shifts in all solvents in comparison with unsubstituted dye 10. Fig. 3 shows the
absorption spectra of some of the dyes with different substituents in chloroform.
14

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Table3. Absorption maxima (in nm) of dyes 1–12 in different solvents.
Dye
1
2
X
DMSO
DMF
CH3CN
CHCl3
406, 424s
392, 430s
EtOH
400s, 418
396s, 440
CH3COOH
403s , 425
395s, 440
p-NO2
m-NO2
p-CN
404s, 421 402s, 417 404s, 416
399s, 439 401s, 437 402s, 433
403s ,418 403s ,414 395s , 408 388s , 402
397s , 418 393s , 420
3
p-CF3
m-CF3
p-F
p-Cl
p-Br
p-I
p-H
p-CH3
p-OCH3
397, 432s 396, 431s 391, 427s
407, 433s 402, 427s 398, 422s
320s, 413 340s, 400 340s, 400
392s ,432 391s ,414 388s ,423
407s ,458 406s ,454 393s , 449 396s , 460
401s ,434 402s ,432 393s , 430 378s , 433
391s ,430 392s ,422 390s , 420 382s , 429
401s ,438 395s ,438 394s , 432 400s , 438
404s ,454 403s ,454 388s , 449 389s , 460
375, 430s
400, 421s
357, 423s
384s , 436
396, 431s
392s, 408
340s, 416
388s , 434 384s , 433
396s , 455 394s , 434
390s , 434 383s , 432
392s , 429 389s , 431
396s , 438 398s , 438
389s , 459 389s , 455
396, 430s
391s , 417
335s, 429
4
5
6
7
8
9
10
11
12
s
=shoulder
Fig 3. The effects of substituents on the absorption spectra of some of the prepared azo dyes.
3.2.2. Determination of pKa value of the dyes in ethanol-aqueous buffer solution
In order to investigate the role of pH of the media on the spectral changes in the structure of the
studied dyes, their absorption characteristics as a function of pH in the pH range 1.1-13.6, in
ethanol-water solution were studied. Buffer standard solutions with different pH values were
15

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prepared with traditional procedure from distilled deionized water and ethanol with HCl, NaOH,
H3PO4, KHC8H4O4, KH2PO4, H2B4O7, NaHCO3, KCl, and Na2CO3. In most cases, the pH
needed to be adjusted using a pH meter and the drop wise addition of either 1.0 mol.L-1 HCl or
1.0 mol.L-1 NaOH to 1.0 L of solution. The accurate pH for each buffer solution was measured
with a Jenway model 3505 digital pH meter.
It was observed that the absorption curves of the dyes were not sensitive to acid but were very
sensitive to base. Absorption spectra of the compounds in 80 vol% ethanol-water solution did not
change significantly when 0.1 mol.L-1 HCl solution was added, being nearly the same as those
observed in high proton donating solvents such as ethanol and acetic acid. In contrast, the
λmax of
the dyes 1–
12 shifted considerably when a few drops of 0.1 mol·L-1 NaOH solution were added
to their ethanolic solutions. In the ethanolic solution of the dyes containing a trace amount of
NaOH, both a shoulder and a maximum in the longer wavelength region disappeared and a new
maximum at short wavelength region was observed. A typical example is shown in Fig. 4. These
hypsochromic shifts in basic media arise from the deprotonation of the enolic OH or hydrazone
NH of the dye molecules, which lead to anionic forms of dyes with the predominant azo
structure A- as is shown in Scheme 5.
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Fig. 4. The UV-Vis absorption spectra of the dye
3
in acidic and basic solutions.
H
O
N
N
X
Cl
N
H
O
pH>6
O
N
N
X
O
N
N
X
Cl
Cl
N
H
O
N
H
O
-
A
-
H
Scheme 5. Deprotonation of the studied azo dyes.
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The absorption spectra of the studied azo dyes are very sensitive to pH values in the range of
6–10.The absorbance of a solution of each dye in 80 vol% ethanol-water was measured as the
solution's pH was varied from acidic to basic conditions. There was no significant change in the
spectra of all the synthesized dyes in acidic to medium (pH<6.0) and the absorption curves are
resembled those in high proton donating solvents such as ethanol and acetic acid. As the solution
pH was changed to basic, both a shoulder and a maximum in the longer wavelength region
disappeared and a new maximum at shorter wavelength region was observed. These
hypsochromic shifts in basic solvents are due to deprotonation of dye molecules, which lead to
anionic forms of dyes (Scheme 5).
The acid dissociation constants for the hydroxy-azo quinolones dyes were determined
o
spectrophotometrically at 25 C in 80% ethanol-water mixture (v/v) [17]. Each compound
exhibits two bands in the region of 353- 460 nm and 350–419 nm for molecular and the anionic
species, respectively. As the pH value of the solution increases, the height of the former band
decreases and simultaneously that of the latter band increases (Fig.5 for dye 10). From the
optical spectra, in each case, the isobestic points indicate that two species are in equilibrium. For
all the compounds under study, the dependence of the absorbance on the pH value gives sigmoid
curves. Typical absorbance-pH curve is shown in Fig. 6. For all of these dyes, the changes of
absorbance were extremely sharp and clear in the pH range of 6-10. The value of pKa was
calculated from each measurement using the equation:
pKa = pHi +log[(Ab -Ai )/(Ai − Aa )]
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Where Ai is the absorbance of the solution at intermediate pHi and Aa and Ab are the
absorbance values of the strongest acidic and alkaline buffer solutions of each compound. The
pKa values of the dyes were estimated by analysis of the spectral changes and listed in Table 4.
Sensitive acid–base property of the compounds can be explained by the formation of a
π system
within the whole dye structures. Since the electron density on the aromatic rings can be reduced
by the electron withdrawing groups such as -NO2, -CN and -CF3, the pKa values show that the
acidity of the hydroxyl group increases (6.90 for dye
1, 7.77 for dye 3, 7.70 for dye 4 and 7.57
for dye ). While -OCH3 group attached to phenyl ring of the dye 12 decrease the acidic
5
character, due to its electron donating ability which destabilizes anionic forms of the azo dyes
(Scheme 5).
Table 4. Absorption maxima (
λ
max) in different pH values and acidic dissociation constants
(pKa) of prepared azo dyes.
pH
1.56 2.56 4.36
6.32
8.67
9.54
10.58 12.61 13.63
-a
Dye
X
σp
σa
λmax(nm)
pKa
6.90
7.83
7.77
7.70
7.57
7.84
7.91
8.15
8.74
8.43
8.64
9.63
1
2
3
4
5
6
7
p
-NO2
1.27
-
0.78
0.71
0.66
0.54
0.43
425 426 425 360, 413
372 372 391 353, 427
422 422 424 357, 423
416 416 411 354, 439
394 394 395 360, 398
427 427 427 354, 427
438 440 446 360, 441
441 442 443 355, 436
419
414
351
418
351
396
376
352
351
353
353
385
350
351
380
414
351
353
351
353
351
352
354
351
350
351
353
419
352
352
389
352
351
352
350
352
350
352
352
m
-NO2
-CN
-CF3
-CF3
-F
351
p
1.00
0.65
-
351
393
p
351
372
m
355
352
p
-0.03 0.06
350
350
p
-Cl
0.23
0.25
0.27
0.0
0.23
0.23
0.23
0.0
352
352
8
9
p
-Br
354
355
p
-I
372 374 374
430 430 430
359
430
352
355, 425
350
351
10
11
p
-H
350, 435
p
-CH3
-0.17 -0.17 395 395 395 355, 431 351, 457
428
12
p
-OCH3 -0.24 -0.28 434 460 450
355,452 351, 435
a
Taken from Ref.[22]
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Fig. 5. The dependence of the spectra of the dye 10 by the pH of its 2.0×10-5 mol. L-1 solutions in
80 vol% EtOH-water at ionic strength 0. 1 mol. L-1 NaCl at 25 ºC.
Fig. 6. Effect of pH on the maximum absorption intensity of 3-(4-trifluoromethylphenylazo)-6-
chloro-4-hydroxyquinolin-2-(1H)-one (dye 4) in different80% ethanol-water buffer solutions.
Correlation of the pKa data with the Hammett substituent constants σx resulted in the following
equations:
pKa= 8.56-1.66
σ
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Plotting these pKa data against σx values yields the graph shown in Fig. 7. It is obvious that all
the substituents fall nearly on one correlation except the substituents with resonance effect such
as p-OCH3 and p-NO2, which are capable of direct interaction with the negatively charged sites
in H− and A− (Scheme 5). Since the electron density on the aromatic rings can be reduced by the
electron withdrawing groups such as -NO2, -CN and -CF3, the pKa values show that the acidity
of the hydroxyl group increases. While -CH3 and -OCH3 groups attached to the compounds 11
and 12 will decrease the acidic character. This can be explained by the fact that the electron
density on the aromatic rings can be increased by the electron releasing groups such as -OCH3
due to their electron donating ability which destabilizes anionic forms of the dyes.
Fig. 7. Plot of the pKa values against the Hammett constant (
4. Conclusion
In conclusion, 6-chloro-4-hydroxyquinolin-2-(1
σ) for the prepared azo dyes.
H
)-one was prepared by thermal
cyclocondensation reaction of
N
,
N
´-di(4-chlorophenyl) malonamide in polyphosphoric acid.
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Then, this compound was applied as a coupling component for preparation of 3-arylazo-6-
chloro-4-hydroxyquinolin-2-(1H)-ones in good yield. The structures of the newly synthesized
compounds 1-12 were confirmed by spectral and elemental analyses data.
The investigation of the effect of solvent, acid and alkali addition, along with pH upon the
electronic absorption spectra of the dyes
determination were performed. The acid dissociation constants (pKa) of these dyes showed a
good correlation with Hammett -constants of substituents on the diazo components. These
1-12 , together with spectrophotometric pKa values
σ
results agreed with those from our previous study showing that the 6-chloro substituted aryl-azo
quinolone dyes are more acidic than our previously published reports [17] due the negative
inductive effect of chlorine, which reduces the negative charge, located on the oxygen or
nitrogen of the anionic forms of the azo dyes.
The results have proved that in all the used solvents (DMSO, DMF, CHCl3, CH3CN and EtOH)
and in acidic media (AcOH or acidified EtOH) the structures of dyes correspond to hydrazone
tautomers T1 and
T2, whilst by increasing of pH or in alkalinized EtOH, these can be in
equilibrium with deprotonated azo structures A-.
1
Additionally, both H NMR spectral data and absorption spectroscopy in the visible region of
the newly synthesized azo dyes indicate collectively that compounds
1 to 12 exist predominantly
in hydrazo-keto structure T1 and T2.
5. Acknowledgment
The authors are very grateful to Islamic Azad University of Lahijan Branch for providing a part
of financial support for this project.
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