6-Substituted benzothiazole based dispersed azo dyes having pyrazole moiety: Synthesis, characterization, electrochemical and DFT studies

Article abstract of DOI:10.1016/j.molstruc.2019.126959

In the present work discusses synthesis, structural characterization and electrochemical investigation on some azo dyes derived from substituted benzothiazoles. The conventional diazo-coupling method was adopted to afford the heterocyclic azo dyes of benz

Full text of DOI:10.1016/j.molstruc.2019.126959

Journal of Molecular Structure 1199 (2020) 126959
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
6-Substituted benzothiazole based dispersed azo dyes having pyrazole
moiety: Synthesis, characterization, electrochemical and DFT studies
,
,
M.R. Maliyappa a, J. Keshavayya a *, Mallappa Mahanthappa b c, Y. Shivaraj b,
K.V. Basavarajappa a
a Department of PG Studies and Research in Chemistry, Kuvempu University, Jnanasahyadri, Shankaragatta, 577451, Shivamogga, Karnataka, India
b Department of Chemistry, Government Science College, Bengaluru, 560001, Karnataka, India
c Department of Chemistry, School of Applied Sciences, REVA University, Bengaluru, 560001, Karnataka, India
a r t i c l e i n f o
a b s t r a c t
Article history:
In the present work discusses synthesis, structural characterization and electrochemical investigation on
some azo dyes derived from substituted benzothiazoles. The conventional diazo-coupling method was
adopted to afford the heterocyclic azo dyes of benzothiazoles having pyrazole moiety. The obtained azo
dyes were characterized by FT-IR, UVevisible, fluorescence, 1HNMR and, mass spectrometric techniques.
The quantum chemical studies (DFT) was also used to interpret the structural properties of the com-
pounds by using B3LYP program at 6e311þþ (d, p) basis set. Further, the cyclic voltammetry was used to
study the electrochemical behavior of the synthesized azo dyes in 0.1 M H2SO4 solution on a glassy
carbon electrode at different scan rates.
Received 28 April 2019
Received in revised form
7 August 2019
Accepted 19 August 2019
Available online 21 August 2019
Keywords:
Benzothiazole
Azo dyes
© 2019 Elsevier B.V. All rights reserved.
DFT
Cyclic voltammetry
1. Introduction
very useful in the further evaluation of molecular and pharmaco-
logical studies [21]. Several research articles have been published
Azo dyes are the colored organic substances and are obtained by
simple diazotization of primary aromatic amines followed by
coupling with the heterocyclic systems like pyrazole [1,2], thiazole
[3], thiophene, benzothiazole [4], etc. under suitable experimental
conditions. From the literature, it is observed that, azo dyes have
been extensively used in various fields viz, nonlinear optics (NLO)
[5], coloring agents for foodstuff [6], textiles [7], dye-sensitized
solar cells [8], liquid crystal displays [9] and electro-optical de-
vices [10]. Recently, benzothiazole based heterocyclic compounds
are found to be quite interesting because of their wide range of
on the synthesis of azo dyes that exhibit electrochemical behavior
in aqueous H2SO4 by using GCE [22,23].
Very recently, we have published an article wherein we have
discussed the synthesis and characterization of dispersed azo dyes,
derived from benzothiazole and its derivatives. These compounds
were investigated by theoretical and experimental spectroscopic
techniques [24]. Tempted by the above work, in the present paper
we have reported the preparation of four novel benzothiazole
based azo dyes derived from pyrazole by electrophilic substitution
at 0e5 ꢀC and are characterized by adopting various spectroscopic
and analytical techniques. Further, the synthesized compounds
were subjected to fluorescence studies. However, the electro-
chemical response of azo dyes at GCE with different scan rates was
also explored.
applications in electro phosphorescent emitters [11], a-glucosidase
inhibitors [12], chemosensors [13], optics [14], structure-activity
relationships [15], pharmacology and biology [16]. Besides, ben-
zothiazole and pyrazole-based azo dyes also used in a wide range of
applications such as NLO [17], redox properties [18], biological
properties [19] and pharmacological pathogens [20].
2. Experimental
From the literature, it is revealed that the theoretical and
experimental studies of these heterocyclic azo dyes are found to be
2.1. Materials and methods
All the chemicals and solvents used for the synthesis of azo
molecules are of analytical grade (AR) and are purchased from
Spectrochem, Sigma-Aldrich, SD fine and Avra chemical companies.
* Corresponding author.
E-mail address: jathikeshavayya1959@gmail.com (J. Keshavayya).
https://doi.org/10.1016/j.molstruc.2019.126959
0022-2860/© 2019 Elsevier B.V. All rights reserved.

2
M.R. Maliyappa et al. / Journal of Molecular Structure 1199 (2020) 126959
Melting points of the synthesized azo compounds were recorded in
an open capillary on the electrothermal melting point apparatus.
The percentage composition of the elements was recorded using
Thermofinnigan by K factor calibration method. The FT-IR spectra
were recorded on Bruker Alpha-T Attenuated Total Reflection-
Fourier Transform Infrared (ATR-FTIR) spectrometer between the
frequency range 4000e400 cmꢁ1. The electronic absorption (exci-
tation) spectra were recorded using a UVeViseNIR [USB 4000,
Ocean optics USA], spectrophotometer at room temperature with a
concentration of 10ꢁ6 M. The emission fluorescence spectra were
measured in various solvents at room temperature with an F-2700
FL spectrophotometer. The 1H NMR spectra were recorded on
Bruker avance II 400 MHz nuclear magnetic resonance spectro-
photometer, with DMSO‑d6 as a solvent and Tetramethyl silane
(TMS) as an internal standard. The molecular geometry of the
synthesized compounds was optimized by DFT/B3LYP level at
6e311þþ (d, p) basis set using HyperChem Rel. 8.0 software.
for C18H15N5O2S: C, 59.16; H, 4.14; N, 19.17, Found: C, 59.02; H, 4.10;
N, 19.09.
2.2.4. 4-[(6-chloro-1,3-benzothiazol-2-yl)diazenyl]-5-methyl-2-
phenyl-2,4-dihydro-3H pyrazol-3-one (4d)
Red solid, yield 85%, m. p: 214e216 ꢀC. IR (cmꢁ1): 3067 (Ar-CH),
1732 (C¼O), 1661 (C¼N), 1542 (N¼N), 1438 (Ar-C¼C). 1H NMR
d
(ppm): 8.02e8.04 (d, J ¼ 7.9 Hz, 1H). 7.95e7.97 (d, J ¼ 8.0 Hz, 1H),
7.62e7.63 (d, J ¼ 1.7 Hz, 2H), 7.47 (s, 2H), 7.29e7.33 (q, J ¼ 7.3, 2H),
7.15e7.17 (dd, J ¼ 1.9 Hz, 1H), 7.02e7.06 (t, J ¼ 7.2 Hz, 1H), 2.35 (s,
3H, CH3), LC-MS: m/z (%)
¼
370 [MþH]þ. Anal. Calcd for
C
17H12ClN5OS: C, 55.21; H, 3.27; N, 18.94, Found: C, 55.31; H, 3.15;
N, 18.17.
2.3. Computational details
The Density Functional Theory (DFT) is one of the most impor-
tant and versatile theoretical method used to understand the
chemical structure in correlation with the experimental results
using different quantum parameters. The DFT calculations were
carried out by hybrid B3LYP (Becke's three Lee-Yang-Parr functions)
level performed with quantum chemical package (Hypercube.inc,
USA) using 6e311þþG (d, P) as a basis set. The theoretical IR ab-
sorption values have been predicted theoretically by using the
above program. Further, the evaluated HOMO-LUMO energies are
2.2. General procedure for the synthesis of 6-substituted
benzothiazole-based disperse azo dyes (4ae4d)
2-Amino-6-substituted benzothiazoles (1a-1d) (2 mmol) were
dissolved in a mixture of glacial acetic acid and propionic acid
(9 mL) in the ratio of 2:1. This solution was added dropwise to a
well-cooled solution of nitrosylsulfuric acid (2 mmol NaNO2 in 2 mL
of H2SO4) at 0e5 ꢀC and the reaction mixture was stirred for 2 h at
the same temperature. The resulting diazonium salt solution (2a-
2d) was added in portions with vigorous stirring over 30 min to a
well-stirred solution of 5-methyl-2-phenyl-2,4-dihydro-3H-pyr-
azol-3-one (Pyrazole) (3) (2 mmol) in 10 mL acetic acid at 0e5 ꢀC. A
saturated solution of sodium hydrogen carbonate was added to the
reaction mixture with constant stirring until the pH was attained
6e7 which results in the formation of a colored product. The
precipitated colored product was filtered off, washed several times
with distilled water, dried and recrystallized from ethanol. The
general route for the synthesis of azo dyes (4ae4d) is outlined in
Scheme 1.
used to evaluate the global parameters like electronegativity (
c),
electrophilicity index ( ), the chemical potential ( ), chemical
u
m
hardness (ƞ) and softness (1/ƞ). The dipole moments, atomic gra-
dients, molecular point group and, energy gradient, were also
calculated at B3LYP/6-311G (d, p) level. Finally, the structures were
optimized by using the DFT HyperChem PolakRibiere optimizer
[25].
2.4. Cyclic voltammetry
Cyclic voltammetric (CV) measurements were performed by
using a CH-Electrochemical analyzer (CHI 6039E, USA) connected
with a three-electrode cell. In this study GCE, platinum wire and the
saturated calomel electrode (SCE) were used as working electrode
(WE), the counter electrode (CE) and a reference electrode (RE),
respectively.
2.2.1. 4-[1,3-benzothiazol-2-yldiazenyl]-5-methyl-2-phenyl-2,4-
dihydro-3H-pyrazol-3-one (4a)
Yellow solid, yield 84%, m. p: 202e204 ꢀC. IR (cmꢁ1): 3034 (Ar-
CH), 1655 (C¼O), 1603(C¼N), 1436 (Ar-C¼C), 1551 (N¼N). 1H NMR
d
(ppm): 7.92e7.94 (d, J ¼ 7.9 Hz, 2H), 7.77e7.83 (q, J ¼ 8.1 Hz, 2H),
3. Results and discussion
7.42e7.45 (m, 3H), 7.32 (s, 1H), 7.21e7.25 (t, J ¼ 7.3 Hz, 2H), 2.37 (s,
3H, CH3). LC-MS: m/z (%)
¼
336 [MþH]þ. Anal. Calcd for
The route for the synthesis of four 6-substituted benzothiazoles
containing azo compounds 4-[1,3-benzothiazol-2-yldiazenyl]-5-
methyl-2-phenyl-2,4-dihydro-3H-pyrazol-3-one (4a), 5-methyl-4-
[(6-methyl-1,3-benzothiazol-2-yl)diazenyl]-2-phenyl-2,4-dihydro-
3H pyrazol-3-one (4b), 4-[(6-methoxy-1,3-benzothiazol-2-yl)dia-
zenyl]-5-methyl-2-phenyl-2,4-dihydro-3H-pyrazol-3-one (4c) and
4-[(6-chloro-1,3-benzothiazol-2-yl)diazenyl]-5-methyl-2-phenyl-
2,4-dihydro-3H pyrazol-3-one (4d) is depicted in scheme-1. These
compounds were synthesized by conventional diazotization reac-
tion between the various 2-amino-6-substituted benzothiazoles
(1a-1d) and 5-methyl-2-phenyl-2,4-dihydro-3H-pyrazol-3-one (3)
as coupling component at 0e5 ꢀC in good yields. All the synthesized
azo molecules were characterized by various spectroscopic and
analytical techniques. The obtained data and theoretical values of
all the synthesized molecules were in good agreement with the
proposed molecular structure.
C
17H13N5OS: C, 60.88; H, 3.91; N, 20.88, Found: C, 59.25; H, 3.21; N,
21.10.
2.2.2. 5-methyl-4-[(6-methyl-1,3-benzothiazol-2-yl)diazenyl]-2-
phenyl-2,4-dihydro-3H pyrazol-3-one (4b)
Orange solid, yield 79%, m. p: 215e217 ꢀC. IR (cmꢁ1): 3171 (Ar-
CH), 1688 (C¼O), 1651 (C¼N), 1585 (N¼N), 1371 (Ar-C¼C). 1H NMR
d
(ppm): 7.92e7.94 (d, J ¼ 8.4 Hz, 2H), 7.77e7.83 (q, J ¼ 7.6 Hz, 2H),
7.42e7.45 (m, 3H), 7.32 (s, 1H), 7.21e7.25 (t, J ¼ 7.6 Hz, 2H), 2.42 (s,
3H, CH3), 2.37 (s, 3H, CH3). LC-MS: m/z (%) ¼ 350 [MþH]þ. Anal.
Calcd for C18H15N5OS: C, 61.87; H, 4.33; N, 20.04, Found: C, 61.22; H,
3.92; N, 19.55.
2.2.3. 4-[(6-methoxy-1,3-benzothiazol-2-yl)diazenyl]-5-methyl-2-
phenyl-2,4-dihydro-3H-pyrazol-3-one (4c)
Orange solid, yield 82%, m. p: 220e222 ꢀC. IR (cmꢁ1): 3069 (Ar-
CH), 1663 (C¼O), 1610 (C¼N), 1543 (N¼N), 1468 (Ar-C¼C). 1H NMR
3.1. Electronic spectral data
d
(ppm): 7.88e7.90 (d, J ¼ 7.2 Hz, 2H), 7.60 (s, 1H), 7.46e7.44 (d,
J ¼ 6.0 Hz, 2H), 7.21 (s, 1H), 7.05e7.06 (d, J ¼ 6.4 Hz, 2H), 3.80 (s, 3H,
The UVeVis absorption spectra of the dyes (4a-4d) were
OCH3), 2.32 (s, 3H, CH3). LC-MS: m/z (%) ¼ 366 [MþH]þ. Anal. Calcd
recorded in nine different (protic and aprotic) solvents in 10ꢁ6 M

M.R. Maliyappa et al. / Journal of Molecular Structure 1199 (2020) 126959
3
Scheme 1. The synthetic pathway of azo dyes 4a-4d.
concentration at room temperature and the results are displayed in
Table 1 and the recorded spectra are given in Figs. 1e4. The elec-
tronic absorption spectra of all the synthesized azo dyes in various
solvents exhibited two absorption maxima in the range of
This is due to the interaction of the protic and aprotic solvents with
the synthesized azo dyes having lone pair of electrons on atoms like
nitrogen, sulfur, and oxygen.
250e300 nm and 350e450 nm and are assigned to n/
p* and p
3.2. Fluorescence spectral data
/p* transitions respectively. From Table 1, it is revealed that all the
synthesized azo molecules showed absorptions in the wavelength
range from 396 to 458 nm in various protic solvents. Further, in two
polar aprotic solvents, DMF and DMSO the compounds exhibited
maximum bathochromic shift towards the longer wavelength in
the range of 471e481 nm which is a relatively higher shift when
compared to the shift observed in the protic and borderline aprotic
solvents. This observation was in good agreement with the litera-
ture, wherein such bathochromic shifts have been observed for
various azo dyes with increasing the polarity of the solvents [26].
The photophysical properties of the synthesized azo dyes (4a-
4d) have been explored in various protic and aprotic solvents in the
spectral range from 450 to 700 nm. The fluorescence emission
spectra of all the dyes (4a-4d) were recorded in the concentration
of 10ꢁ6 M and at room temperature. The obtained results are pre-
sented in Table 1 and the characteristic spectra of all the dye
molecules are shown in Figs. 5e8. The compounds 4b and 4c
exhibited higher fluorescence intensities and quantum yields that
of the compounds 4a and 4d in all the solvents. This is due to

4
M.R. Maliyappa et al. / Journal of Molecular Structure 1199 (2020) 126959
Table 1
Electronic absorption and fluorescence data of the synthesized azo dyes (4a-4d).
Solvents
DMSO
4a
4b
4c
4d
lex
Log ε
lem
f
lex
Log ε
lem
f
lex
Log ε
lem
f
lex
Log ε
lem
f
lex
Log ε
lem
f
lex
Log ε
lem
f
lex
Log ε
lem
f
lex
Log ε
lem
f
464
5.665
585
0.006
481
5.329
593
0.008
428
4.000
555
0.100
425
5.531
562
0.013
441
5.079
525
0.035
426
5.518
552
0.015
423
5.279
578
0.022
425
5.71
528
473
4.845
545
0.156
471
5.775
545
0.021
397
5.548
543
0.031
396
5.598
534
0.137
398
5.609
540
0.090
396
5.684
541
0.049
398
5.572
525
0.070
396
5.755
547
472
5.472
545
0.038
473
5.496
540
0.086
406
5.386
532
0.093
407
5.486
564
0.022
407
5.124
551
0.055
405
5.247
544
0.037
409
5.308
549
0.017
405
5.612
551
477
5.427
596
0.026
476
5.392
595
0.027
420
5.271
588
0.056
438
5.322
592
0.045
458
5.491
551
0.039
440
5.322
552
0.002
431
5.271
540
0.012
431
5.360
568
DMF
1,4 dioxane
THF
Methanol
Ethyl acetate
DCM
Chloroform
Ethanol
0.042
443
5.556
544
0.021
405
5.348
525
0.011
412
4.970
540
0.003
440
5.315
566
lex
Log ε
lem
f
0.009
0.005
0.016
0.024
Fig. 1. Electronic spectra of azo dye 4a in different solvents.
presence electron donor groups like methyl and methoxy which are
directly attached to the benzene ring of the benzothiazole based
azo dyes. Based on the literature, the observed high quantum yields
are associated with the delocalization of pi electrons forming long-
range conjugation when the substitution of electron donors
attached at the para position of the synthesized organic molecules
[27]. The quantum yield (QY) of the synthesized azo molecules
were calculated by using the formula,

M.R. Maliyappa et al. / Journal of Molecular Structure 1199 (2020) 126959
5
Fig. 2. Electronic spectra of azo dye 4b in different solvents.
Fig. 3. Electronic spectra of azo dye 4c in different solvents.
compared with the theoretical values and the results are found to
be in good agreement with each other. In present work, the FT-IR
vibrational frequency values are calibrated with the default
scaling factor of 1.0 at 6e311þþG (d, P) level. The molecules (4a-
4d) consist of 37, 40, 41 and 37 atoms and expected to have 105, 114,
117 and 105 normal fundamental (3N-6) vibrational modes. Theo-
retical and experimental FT-IR spectra of all the compounds were
shown in Figs. S1eS4 and the obtained frequencies were listed in
Table S1 of the supplementary material.
h2
h2ref
I
Aref
A Iref
QY ¼ QYref
(1)
where, the ‘QYref’ is the quantum yield of the reference or standard
(Rhodamine B, the quantum yield is 0.7 at room temperature) and
‘I’ is referred as the integrated fluorescence intensity or area under
the curve. ‘A’ is the absorbance at the excitation wavelength. ‘ƞ’ and
‘ƞref’ are the refractive indexes of test solvents and reference solvent
respectively. In the present study, various protic and aprotic (nine)
solvents are the test solvents and ethanol is the reference solvent.
3.4. The theoretical and experimental calculation of frontier
molecular orbitals
3.3. Vibrational analysis
The combinations of highest occupied molecular orbital with
the lowest unoccupied molecular orbital are also known as frontier
The experimentally obtained IR spectral data have been

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M.R. Maliyappa et al. / Journal of Molecular Structure 1199 (2020) 126959
Fig. 4. Electronic spectra of azo dye 4d in different solvents.
Fig. 5. Fluorescence spectra of azo dye 4a in different solvents.
molecular orbitals (FMO) [28]. FMOs play an important role in the
interactions between the electromagnetic radiations with the
synthesized organic molecules account for the electronic transition
in a molecule. During the electronic-transition, the energy gap is
produced within a molecule. The molecules having a large energy
gap are known as hard molecules and those having a small energy
gap are referred to as soft molecules. The literature revealed that
soft molecules are more polarizable than the hard ones because of
the small energy gap helps the excitation of an electron from
HOMO to LUMO more easily than that of hard molecules [29].
Further, by using DFT/B3LYP level at 6e311þþG (d, p) basis set, the
structure optimization, HOMO-LUMO energies, dipole moments,
molecular point group, energy gradient, and the global reactivity
parameters have been calculated.
appropriate method for a broad range of applications in physical
chemistry. Also, cyclic voltammetry is the most important method
for evaluation of various energies and it easily differentiates the
energy bands like HOMO-LUMO of organic molecules. Moreover,
HOMO e LUMO energies are required for calculating the various
global energy parameters such as electronegativity, electro-
chemical potential, electrophilicity index, hardness and softness
[30]. The energy required to eject an electron in the outermost
orbital of a molecule (referred to as HOMO) is calculated by the
oxidation process. Similarly, the energy needed to add an electron
to the orbital in a molecule (LUMO) it is achieved by the reduction
process. In the present case, we calculated LUMO by using the
reduction peak of the cyclic voltammogram [31]. The values of
ELUMO and EHOMO are calculated by using the Bredas empirical
equations (2)e(4).
The cyclic voltammetry is a well-known technique and its

M.R. Maliyappa et al. / Journal of Molecular Structure 1199 (2020) 126959
7
Fig. 6. Fluorescence spectra of azo dye 4b in different solvents.
Fig. 7. Fluorescence spectra of azo dye 4c in different solvents.
molecules are displayed in Table 2. The energy gradients of the
synthesized azo molecules are displayed in Table S2. The optimized
structures and HOMO-LUMO energy level diagram of the dyes (4a-
4d) were shown in Figs. S5 and S6 of the supplementary material.
From the above data, both experimental and theoretical values are a
good agreement with each other.
h
i
E
E
¼ Eonset þ 4:4
(2)
(3)
ðLUMOꢂ
ðLUMOꢂ
red
¼ E½HOMOꢂ þ Eg
hc
lonset
Eg
¼
(4)
3.5. Cyclic voltammetry
Where, h, c, and Eg are the Planck constant, speed of light and
optical band gap of the compounds. Similarly, ꢁ4.4 eV is the
3.5.1. Electrochemical reduction of azo dyes at glassy carbon
electrode (GCE)
reference energy level of ferrocene.
All other energies and global parameter are calculated by using
equations S1-S7.
Electrochemical reduction of azo dyes was carried out by using
cyclic voltammetry. Fig. 9 represents the cyclic voltammograms of
azo dyes (4a-4d) at the scan rate (
H2SO4 which acts as a supporting electrolyte. The above plots
The theoretical and cyclic voltammetric calculated energy
y
) of 50 mVsꢁ1 on GCE in 0.1M
(HOMO-LUMO) and
a global parameter of the synthesized

8
M.R. Maliyappa et al. / Journal of Molecular Structure 1199 (2020) 126959
Fig. 8. Fluorescence spectra of azo dye 4d in different solvents.
Table 2
The theoretical and experimental energy parameters of the dyes 4a-4d.
Energy parameters
Theoretical (Experimental)
4a
4b
4c
4d
EHOMO (eV)
ELUMO (eV)
EHOMO - ELUMO gap (eV)
Chemical hardness (ƞ)
Chemical potential (
Electronegativity (
Electrophilicity index (
Ionization energies (I)
Electron affinities (A)
ꢁ6.271 (ꢁ6.956)
ꢁ2.952 (ꢁ4.376)
3.318 (2.575)
1.659 (1.289)
ꢁ4.611 (ꢁ5.666)
4.611 (5.666)
6.409 (12.452)
6.271 (6.956)
2.952 (4.376)
0.301 (0.387)
ꢁ6.184 (ꢁ6.872)
ꢁ2.872 (ꢁ4.332)
3.311 (2.520)
1.655 (1.270)
ꢁ4.528 (ꢁ5.603)
4.528 (5.603)
6.193 (12.359)
6.184 (6.872)
2.872 (4.332)
0.301 (0.395)
ꢁ6.019 (ꢁ6.716)
ꢁ2.792 (ꢁ4.369)
3.226 (2.347)
1.613 (1.173)
ꢁ4.405 (ꢁ5.543)
4.405 (5.543)
6.015 (13.096)
6.019 (6.716)
2.792 (4.369)
0.309 (0.426)
ꢁ6.360 (ꢁ6.726)
3.055 (ꢁ4.328)
3.304 (2.397)
1.652 (1.198)
ꢁ4.708 (ꢁ5.275)
4.708 (5.275)
6.707 (11.613)
6.360 (6.726)
3.055 (4.328)
3.302 (0.417)
m)
)
c
u)
Chemical Softness (1/2ƞ)
in the potential range ꢁ0.13 and ꢁ0.61 V, for compound 4b ꢁ0.03
and ꢁ0.47, for compound 4c ꢁ0.05 and ꢁ0.52 V and compound
4d ꢁ0.18 and ꢁ0.54 V, which correspond to breaking of an azo
group (-N¼N-). The absence of an anodic peak in CV plot confirms
the irreversible nature of the electrode process.
3.5.2. Effect of scan rate
The influence of scan rate on the electrochemical reduction of
azo compounds (4a-4d) at GCE in 0.1 M H2SO4 was recorded and as
shown in Fig.10. As sweep rate increased from 25 to 250 mV sꢁ1, the
reduction peak currents of azo dyes were increased significantly
according to Randles-Sevick's relation [32].
To confirm the electrode reaction process, a graph was plotted
by peak current (Ipa) vs. scan rate (n) and a straight line was
observed [The calculated values are displayed in Table S3 of the
supplementary materials] as shown in Fig. 11(A). Similarly, peak
½
current (Ipa) vs. square root of scan rate (
n
) was also plotted as
shown in Fig. 11(B) by which a linear regression equation was ob-
tained as Ipa
(
m
A) ¼ ꢁ1.259 þ (ꢁ8.642)
n
1/2 (Vs
)
,
ꢁ1 1/2, r2 ¼ 0.9893; Ipa
(ꢁ10.27) n1/2 (Vsꢁ1 1/2
)
r2 ¼ 0.9969; Ipa
Fig. 9. Overlaid cyclic voltammograms of the synthesized azo dyes [4a-4d] on GCE in
0.1 M H2SO4 (supporting electrolyte) at 50 mV sꢁ1
(
(
(
m
m
m
A)
A)
¼
ꢁ0.293
þ
(ꢁ18.23) n1/2 (Vsꢁ1 1/2
)
,
r2 ¼ 0.9503 and Ipa
.
¼
1.486
þ
A) ¼ 2.556 þ (ꢁ22.58) n1/2 (Vs
)
ꢁ1 1/2, r2 ¼ 0.9386 for the com-
pounds (4a), (4b), (4c) and (4d), respectively. Thus, linear correla-
(Fig. 9) reveal the presence of two reduction peaks for all the
compounds. For compound 4a the reduction peaks were observed
tions were observed between peak current (Ipa) and the square root
of the scan rate (
½
n
) which confirms the diffusion-controlled

M.R. Maliyappa et al. / Journal of Molecular Structure 1199 (2020) 126959
9
Fig. 10. Overlaid CV's of Compounds (4ae4d) at various scan rates from 25 to 250 mV sꢁ1 in 0.1 M H2SO4 (supporting electrolyte) at GCE.
Fig. 11. The plot of (A) peak current vs.y
and (B) peak current vs.y1/2 of azo dye compounds (4a), (4b), (4c) and (4d).
electrode process. Also, the plot of log Ipavs. log
supplementary materials] by which a linear regression equation
n
[Fig. S7 of the
were close to 0.5 and this further confirms the electrode process is a
diffusion-controlled under experimental conditions [33].
A linear relationship between peak potential (Ep) and logarith-
mic scan rate (lnn) can be expressed by the following regression
equations as given in Table S4. The Ep for an irreversible electrode
process is given by Laviron's relation [34],
was obtained as log Ipa
(
m
A) ¼ ꢁ5.04 þ 0.39 log
n
(Vsꢁ1),
r2 ¼ 0.9948; log Ipa
(
m
A) ¼ ꢁ4.98 þ 0.46 log
n
(Vsꢁ1), r2 ¼ 0.9978; log
Ipa
(
m
A) ¼ ꢁ4.78 þ 0.57 log
n
(Vsꢁ1), r2 ¼ 0.9583 and log Ipa
(m
A) ¼ ꢁ4.67 þ 0.61 log
n
(Vsꢁ1), r2 ¼ 0.9251 for the compounds
(4a), (4b), (4c) and (4d), respectively. The calculated slope values

10
M.R. Maliyappa et al. / Journal of Molecular Structure 1199 (2020) 126959
Scheme 2. The possible irreversible electrochemical reduction mechanism of the compounds (4a-4d).
4. Conclusions
ꢀ
ꢁ
ꢀ
ꢁ
ꢀ
ꢁ
RT
RTk0
RT
nF
The present study deals with the synthesis of four benzothiazole
based dispersed azo dyes derived from 5-methyl-2-phenyl-2, 4-
dihydro-3H-pyrazol-3-one via diazo coupling electrophilic substi-
tution reaction at 0e5 ꢀC. The structural characterization of all the
synthesized azo compounds was performed by both theoretical as
well as experimental techniques. Moreover, the electrochemical
behavior of the synthesized azo dyes (4a-4d) was investigated by
the cyclic voltammetric method in aqueous 0.1 M H2SO4 with glassy
carbon electrode at a range of scan rates 25e250 mV sꢁ1. The re-
sults of the study informed that the overall electrode process is
diffusion-controlled and irreversible.
Ep ¼ E0 þ
ln
þ
ln
n
(5)
an
anF
a
Where E0 is formal potential, k0 is the standard heterogeneous
reaction rate constant and other notations have their usual mean-
ings. The value of
an was calculated from the slope of Ep vs. ln
y as
seen in Table S4. Generally,
a
was assumed to be 0.5 in an irre-
versible electrode process. Therefore, the total number of electrons
(n) transferred during the electrochemical reduction of azo moiety
were calculated to be z 4 and it involves four-protons and four-
electrons at GCE as reported in the literature [35,36] and the
respective mechanism was shown in Scheme 2. Based on the
literature, the azo dyes undergo irreversible electrochemical
reduction process involving 2eꢁ, 2Hþ which occurs in neutral and
basic media in a single step which leads to the formation of a stable
hydrazo compound. In the present study, the voltammogram con-
sists of two irreversible cathodic peaks which indicate that the
electrochemical reduction occurs in two steps. In the first step, a
hydrazo compound is formed which involves 2eꢁ, 2Hþ whereas in
the second step the hydrazo compound undergo further reduction
utilizing 2eꢁ, 2Hþ resulting in the formation of amines [37]. Hence,
the whole process involves 4eꢁ, 4Hþ.
Acknowledgments
One of the authors Mr. Maliyappa M R is grateful to the Uni-
versity Grant Commission (UGC), New Delhi for awarding UGC-BSR
fellowship (letter No.F.7- 229/2009/BSR/dated:06-08-2014). The
authors are grateful to the Department of Chemistry, Jnanasahydri,
Kuvempu University for providing laboratory facility. The authors
are thankfully and acknowledged to the SAIF, Panjab University and
Department of Physics, M. S. Ramaiah Institute of Technology,

M.R. Maliyappa et al. / Journal of Molecular Structure 1199 (2020) 126959
11
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Hojat
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