Experimental and theoretical studies on monoazo dye including diphenylamine and N-methyldiphenylamine derivatives

Article abstract of DOI:10.1016/j.molstruc.2020.128449

In this manuscript, two series of azo dyes preparing by coupling diphenylamine and N-methyl diphenylamine and diazotized substituted anilines bearing various substituents were designed and synthesized. These azo dyes were characterized by spectroscopic techniques such as ¹H/¹³C NMR and HRMS or LCMS. Photophysical, NLO and protonation properties were investigated by experimentally and theoretically. The absorption maxima were observed in long wavelength in DMSO. The most stable tautomeric form of dyes which have tautomeric forms, were determined using model compounds. The results showed that the azo dyes are stable in azo form in all solvents used. For the usability as optic dye of the synthesized dyes, thermal property of all dyes was also determined. The all compounds are thermally stable and T_d values are bigger than 220 °C.

Full text of DOI:10.1016/j.molstruc.2020.128449

Journal of Molecular Structure 1217 (2020) 128449
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Experimental and theoretical studies on monoazo dye including
diphenylamine and N-methyldiphenylamine derivatives
a
b
c
ꢀ
ꢀ
Tugçe Aksungur , Fatma Erol, Assit.Prof.Dr. , Nurgül Seferoglu, Prof.Dr. ,
a, *
ꢀ
Zeynel Seferoglu, Prof.Dr.
a Department of Chemistry, Gazi University, Ankara, 06560, Turkey
b Technical Sciences Vocational School, Gazi University, Ankara, 06374, Turkey
c Department of Advanced Technologies, Graduate School of Natural and Applied Sciences, Gazi University, Ankara, 06560, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
In this manuscript, two series of azo dyes preparing by coupling diphenylamine and N-methyl diphe-
nylamine and diazotized substituted anilines bearing various substituents were designed and synthe-
sized. These azo dyes were characterized by spectroscopic techniques such as 1H/13C NMR and HRMS or
LCMS. Photophysical, NLO and protonation properties were investigated by experimentally and theo-
retically. The absorption maxima were observed in long wavelength in DMSO. The most stable tauto-
meric form of dyes which have tautomeric forms, were determined using model compounds. The results
showed that the azo dyes are stable in azo form in all solvents used. For the usability as optic dye of the
synthesized dyes, thermal property of all dyes was also determined. The all compounds are thermally
stable and Td values are bigger than 220 ꢀC.
Received 4 April 2020
Received in revised form
10 May 2020
Accepted 13 May 2020
Available online 18 May 2020
Keywords:
Azo dyes
Push-pull chromophores
Proton sensitive dyes
Low pH sensor
NLO
© 2020 Elsevier B.V. All rights reserved.
DFT
1. Introduction
[11], optical data storage [12], non-linear optics (NLO) [13], and dye-
sensitized solar cells (DSSC) [14]. Aromatic azo derivatives, espe-
Azo dyes are among the most important classes of synthetic
dyes due to the fact that they are synthesized easily and have broad
color spectrum [1]. Although a number of azo dyes have been re-
ported to be poisonous, a lot of them are being used in drugs and
food [2]. Azo dyes are known essential compounds [3,4] and are
applicable not only for dyeing [5e7] but also for high-technology
applications [7e9]. For the fact that they are organic dyes bearing
auxochromes such as amino, alkylamino, dialkylamino, hydroxy,
alkoxy, and nitro groups, they are usually solid. These auxochromes
can induce strong polarity into a molecule and then increase its
intermolecular interactions. Additionally, they also make the dye
cially those having intramolecular D-
phores have exhibited excellent photophysical properties [15e17]
because they possess extensive -systems delocalized between the
acceptor and donor units across the azo linkage. Moreover, azo dyes
have different applications that range from textile dyeing [18],
leather dyeing [19], coloring of plastics and polymer [20] to
advanced usages such as liquid crystal displays [21], in biological
and medical investigations and advanced application in organic
synthesis [22].
In recent years, a great effort has been exerted in the synthesis of
new materials with large second-order optical non-linearity as a
result of their potential usage in applications such as high bit rate
long-distance optical communications, and more generally, for
optical information processing [23]. Again, due to their advanced
functional properties such as optical storage capacity, optical
switching, holography and non-linear optical (NLO) properties,
molecules with azo moieties show promising capability as photo-
active materials [24e26]. NLO materials have attracted great in-
terest among several researchers at present time due to their
significant applications in optoelectronic and photonic
p-A charge-transfer chromo-
p
molecule often large and having
p-electrons to exhibit strong
disperse forces and interactions. (see Figs. 9 and 10)
p-p
Azo-functionalized dyes appended with aromatic heterocyclic
moieties [10] have shown increasing attention recently as a result
of having varieties of applications in metallochromic indicators
* Corresponding author.
ꢀ
E-mail addresses: nurguls@gazi.edu.tr (N. Seferoglu), znseferoglu@gazi.edu.tr
ꢀ
(Z. Seferoglu).
https://doi.org/10.1016/j.molstruc.2020.128449
0022-2860/© 2020 Elsevier B.V. All rights reserved.

2
T. Aksungur et al. / Journal of Molecular Structure 1217 (2020) 128449
technologies [27e30]. The principal advantages of organic second-
order NLO molecules are their high NLO activity, chemical stability,
and are easily processed [31]. Organic molecules that possess large
NLO responses are needed for lots of applicability ranging from the
development of photonic devices [32] to biological or medical ap-
plications [33].
0e5 ꢀC with stirring. The mixture was stirred for an additional 1 h
while maintaining at temperature of 0e5 ꢀC. Excess nitrous acid
was destroyed by addition of urea. After that, N,N-diphenylamine or
N-methyldiphenylamine (2.0 mmol, 0.338 g) was dissolved in
acetic acid/propionic acid (4 mL, ratio 3:1) and cooled to 0e5 ꢀC in a
salt/ice bath. The cold diazonium salt solution was added to this
cooled solution over 1 h with vigorous stirring in a dropwise
manner and with the addition of diluted potassium hydroxide so-
lution, pH was mainted in the range of 4e6. The mixture was
further stirred for 1 h at 0e5 ꢀC and resulting solid was filtered,
washed with cold water and dried in air. The crude product was
crystallized by using ethanol.
In our previous study [34], diphenylamine based azo dye named
as ((E)-1-(4-((4-(phenylamino)phenyl)diazenyl)phenyl)ethanone,
DPA was synthesized and investigated its photophysical and NLO
properties. This compound showed good photophysical and NLO
properties therefore, it can be a good candidate for optic application
as NLO dyes. Thus, in this current study, two new additional series
of azo dyes bearing various substituents and coupling components
designed and synthesized. Photophysical, NLO and protonation
properties of the compounds were investigated by using experi-
mentally and theoretically. In first series (I-VI), the compounds can
be shown azo-hydrazone tautomerism due to the transfer of
hydrogen of the eNH of diphenylamine. For the determination of
the most stable tautomeric form of the first serie of compounds, the
second serie of compounds (I′-VI′) having no tautomeric forms
because of blocking NH proton by methyl substituted were used as
model compounds. However, the thermal property of the com-
pounds was determined to estimate their usability as an optical
dye.
(E) -1-(4 -((4-(phenylamino)phenyl)diazenyl)phenyl)ethan-1-
one (I)
Yellow solid; m.p: 184e186 ꢀC; yield: % 80, 1H NMR (DMSO‑d6,
300 MHz): dH 9.00 (s, 1H), 8.10 (d, J ¼ 8.54 Hz, 2H), 7.9 (m, 4H), 7.38
(m, 2H), 7.21 (m, 4H), 7.05 (m, 1H), 2.60 (s, 3H) ppm, 13C-APT
(DMSO‑d6, 75 MHz): dC 197.7; 155.3; 148.7; 145.3; 141.5; 137.5;
129.9; 129.8; 129.5; 125.9; 122.7; 122.4; 120.1; 117.1; 115.1;
27.34 ppm, 1H NMR (CHCl3-d1, 300 MHz): dH 8.11 (q, J ¼ 8.59 Hz,
2H), 7.86 (m, 4H), 7.39 (m, 2H), 7.25 (m, 4H), 7.12 (m, 1H), 6.10 (s,
1H), 2.65 (s, 3H) ppm, HR-MS (ESI, CH3CN, m/z): C20H18N3O [MþH]þ
found: 316.1450 calcd.: 316.1436.
2. Experimental
(E) -(4-((4-(phenylamino)phenyl)diazenyl)benzoic acid (II)
2.1. Materials and physical measurements
Yellow solid; m.p: 246e248 ꢀC; yield: % 50, 1H NMR (DMSO‑d6,
300 MHz): dH 9.00 (s,1H), 8.10 (d, J ¼ 6.81 Hz, 2H), 7.85 (m, 4H), 7.35
(m, 2H), 7.21 (m, 4H), 7.02 (t, J ¼ 7.25 Hz, 1H) ppm, 13C-APT
(DMSO‑d6, 75 MHz): dC 167.5; 154.9; 148.5; 145.3; 141.5; 133.3;
130.9; 129.8; 125.8; 122.6; 122.2; 120.0; 117.1; 115.2 ppm, HR-MS
(ESI, CH3CN, m/z): C19H15N3O3 [MþH]þ found: 318.1243 calcd.:
318.1235.
All commercially available chemicals were reagent grade and
used without further purification. Thin-layer chromatography (TLC)
was used for monitoring the reactions using precoated silica gel 60
F254 plates. NMR spectra were measured on Bruker Avance 300
(1H: 300 MHz, 13C: 75 MHz) spectrometers at 25 ꢀC (298 K).
Chemical shifts (d) are given in parts per million (ppm) using the
residue solvent peaks as a reference relative to TMS. Coupling
constants (J) are given in hertz (Hz). Signals are abbreviated as
follows: broad. br; singlet. s; doublet. d; doublet-doublet. dd;
doublet-triplet dt; triplet. t; multiplet. m. High-resolution mass
(HRMS) spectra were recorded at Gazi University Faculty of Phar-
macy using electron ionization (EI) mass spectrometry (Waters-
LCT-Premier-XE-LTOF (TOF-MS) instruments; in m/z (rel. %). The
liquid chromatography mass spectrometry (LCMS, Waters Micro-
mass ZQ connected with Waters Alliance HPLC (Waters Corpora-
tion, Milford, MA, USA), using the ESI (þ) method) spectra were
recorded at Ankara University Faculty of Pharmacy. The uncorrec-
ted melting points were measured using Electrothermal IA9200
apparatus. Absorption spectra were recorded on a Shimadzu 1800
spectrophotometer. Thermal analyses were performed with a Shi-
madzu DTG-60H system, up to 700 ꢀC (10 ꢀC minꢁ1) under a dy-
namic nitrogen atmosphere (15 mL minꢁ1).
(E) -(4-((4-phenlamino)phenyl)diazenyl)benzonitrile (III)
Orange solid; m.p: 186e187 ꢀC; yield: % 75, 1H NMR (DMSO‑d6,
300 MHz): dH 9.01 (s, 1H), 8.10 (d, J ¼ 8.64 Hz, 2H), 7.88 (m, 4H), 7.36
(m, 2H), 7.21 (m, 4H), 7.05 (t, J ¼ 7.28 Hz, 1H) ppm, 13C-APT
(DMSO‑d6, 75 MHz): dC 155.1; 149.2; 145.2; 141.3; 134.1; 129.9;
129,6; 126.2; 123.0; 122.9; 120.3; 120.1; 119.1; 117.1; 115.1;
112.0 ppm, HR-MS (ESI, CH3CN, m/z): C19H15N4 [MþH]þ found:
299.1302 calcd.: 299.1297.
(E) -4-((4-(nitrophenyl)diazenyl)-N-phenylalanine (IV)
Dark red solid; m.p: 157e159 ꢀC; yield: %72, 1H NMR (DMSO‑d6,
300 MHz): dH 9.02 (s, 1H), 8.45 (d, 2H), 8.00 (m, 4H), 7.45 (m, 2H),
7.30 (m, 4H), 7.12 (t, J ¼ 7.27 Hz, 1H) ppm, 13C-APT (DMSO‑d6,
75 MHz): dC 156.4; 149.5; 147.7; 145.3; 141.2; 129.9; 126.5; 125.5;
123.2; 123.1; 120.5; 115.1 ppm, HR-MS (ESI, CH3CN, m/z):
2.2. Synthesis and characterization of compounds
C
18H15N4O2 [MþH]þ found: 319.1194 calcd.: 319.1195.
(E) -4-((4-(chlorophenyl)diazenyl)-N-phenylalanine (V)
Yellow solid; mp:125e127 ꢀC; yield: %80, 1H NMR (DMSO‑d6,
Carbocyclic amines were diazotized with HCl and NaNO2. A
typical reaction condition of diazotizing and coupling is described
below using aniline derivatives. The obtained compounds were
purified by crystallization using ethanol and then analyzed.
300 MHz): dH 9.00 (s,1H), 7.82 (m, 4H), 7.61 (d, J ¼ 8.69 Hz, 2H), 7.35
(d, 2H), 7.21 (d, 4H), 7.01 (t, J ¼ 7.27 Hz,1H) ppm, 13C-APT (DMSO‑d6,
75 MHz): dC 151.4; 148.3; 145.1; 141.7; 134.8; 129.8; 125.5; 124.1;
122.5; 121.8; 119.9; 115.3 ppm, HR-MS (ESI, CH3CN, m/z):
2.3. General synthetic procedures of I-VI and I0-VI0
2.0 mmol aniline derivative is dissolved in hydrochloric acid
(1.5 mL conc. HCI in 4 mL water). The solution was then cooled to
0e5 ꢀC with stirring. Sodium nitrite (0.15 g, 2.0 mmol) in water
(3 mL) was gradually added to this solution over 15 min period at
C
18H15N3Cl [MþH]þ found: 308.0955 calcd.: 308.0955.
(E) -4-((4-(bromophenyl)diazenyl)-N-phenylalanine (VI)

T. Aksungur et al. / Journal of Molecular Structure 1217 (2020) 128449
3
Light orange solid; m.p:150e151 ꢀC; yield: %40, 1H NMR (DMSO-
CDCI3-d1, 300 MHz): dH 7.85 (d, J ¼ 8.84 Hz, 2H), 7.77 (d, J ¼ 8.76 Hz,
2H), 7.62 (d, J ¼ 8.76 Hz, 2H) 7.38 (m, 2H), 7.24 (m, 4H), 7.11 (m, 1H),
6.02 (s, 1H) ppm, 13C-APT (DMSO‑d6, 75 MHz): dC 151.7; 148.3;
145.1; 141.7; 132.8; 129.8; 125.6; 124.4; 123.6; 122.5; 119.9;
115.3 ppm, HR-MS (ESI, CH3CN, m/z): C18H15N3Br [MþH]þ found:
352.0457 calcd.: 352.0449.
J ¼ 9.06 Hz, 2H) 3.45 (s, 3H) ppm, 13C NMR (DMSO- d6, 75 MHz): dC
152.1; 151.7; 147.4; 144.4; 132.8; 130.4; 126.1; 125.9; 125.2; 124.3;
123.5; 114.9, HR-MS (ESI, CH3CN, m/z): C19H16BrN3 [MþH]þ found:
366.0600 calcd.: 366.0606.
2.4. Computational details
(E) -1-(4-((4-(methyl(phenyl)amino)phenyl)diazenyl)phenyl)
All calculations were carried out using the Density Functional
Theory (DFT) approach at the Becke-3-LeeeYangeParr (B3LYP)
functional [35e37] at 6e31þ(G)(d,p) level within the Gaussian 09
program [38]. The ground state of the compounds were obtained in
gas phase and various solvents. The confirmation of the conver-
gence to minima on the potential energy surface was made from
the vibrational analysis for each molecule. The absorption spectra
of the compounds were obtained from TD-DFT calculations at the
same level in various solvents. The integral equation formalism of
the polarizable continuum model (PCM) [39,40] was employed to
evaluate the solvent effects for calculation in solvents. The NLO
properties of the compounds were calculated by using finite field
method at the same level in gas phase.
ethan-1-one (I′)
Orange solid; m.p: 142 ꢀC; yield: %49, 1H NMR (DMSO‑d6,
300 MHz): dH 8.01 (d, J ¼ 8.59 Hz, 2H), 7.91 (d, J ¼ 8.39 Hz, 2H), 7.83
(d, J ¼ 9.08 Hz, 2H) 7.50 (m, 1H), 7.32 (m, 4H), 6.92 (d, J ¼ 9.11 Hz,
2H), 3.45 (s, 3H) ppm, 13C NMR (DMSO‑d6, 75 MHz): dC 155.1; 152.7;
147.1; 144.4; 134.1; 130.5; 126.5; 126.3; 125.8; 123.0; 119.1; 114.6;
111.9 ppm, LC-MS (ESI, CH3CN): C21H19N3O [MþH]þ 313.9, HR-MS
(ESI, CH3CN, m/z): C21H19N3O [MþH]þ 313.1298.
(E) -2-(4-((4-(methyl(phenyl)amino)phenyl)diazenyl)benzoic acid
(II0)
Orange solid; m.p: 241e242 ꢀC; yield: % 49, 1H NMR (DMSO‑d6,
300 MHz): dH 13.02, (s, 1H) 8.10 (d, J ¼ 6.77 Hz, 2H), 7.85 (m, 4H)
7.47 (m, 1H) 7.29 (m, 4H), 6.92 (d, J ¼ 9.17 Hz, 2H), 3.45 (s, 3H) ppm,
13C NMR (DMSO‑d6, 75 MHz): dC 167.3; 155.3; 152.3; 147.2; 144.6;
131.8; 130.9; 130.4; 126.2; 126.1; 125.8; 125.4; 122.3; 114.8 ppm,
HR-MS (ESI, CH3CN, m/z): C20H18N3O2 [MþH]þ found: 332.1395
calcd.: 332.1399.
3. Results and discussion
3.1. Synthesis and photophysical properties of azo dyes
The azo dyes (I-VI and I′-VI′) were prepared by coupling
Diphenylamine and N-Methyldiphenylamine with diazotized some
carbocyclic amines having different substituents in hydrochloric
acid/water and sodium nitrite (Scheme 1). The synthesized dyes
were obtained generally in good yields (91-77%); the structure of
these dyes was confirmed by spectroscopic techniques such as
1H/13C NMR and HRMS or LCMS (Supporting Information, Figs. S1-
S38).
As seen in Scheme 2, the azo dyes prepared from diphenylamine
(I-VI) may exist potentially in two tautomeric forms which is
named as azo and hydrazone forms. After deprotonation of two
tautomers, the common anion is obtained. The UVevis spectra of
the synthesized dyes (I-VI) were measured in different solvents
with various polarities at a concentration of 3 ꢂ 10ꢁ5 M. The
observed results were compared with their corresponding model
dyes (I′-VI′). In this study, the dyes synthesized from N-methyl-
diphenylamine (I′-VI′) were used as model dyes for determination
of stable tautomer of dyes I-VI. The photophysical results of dyes in
solvents used were given in Table 1. The absorption maxima of all
dyes showed bathochromic shifts by increasing polarity of solvents
except in proton-donating acetic acid. The lmax values of the all
dyes prepared showed the largest bathochromic shifts in highest
polar solvent DMSO, as shown in Table 1. However, the dyes I and I
′-III′ showed higher lmax values or shoulder in acetic acid while
comparing with in DMSO. This result may shown that the forma-
tion hydrogen bonds between dye and acetic acid or partial pro-
(E) -4-((4-(methyl(phenyl)amino)phenyl)diazenyl)benzonitrile
(III′)
Orange solid; m.p: 143 ꢀC; yield: % 74, 1H NMR (DMSO‑d6,
300 MHz): dH 8.10 (d, J ¼ 8.49 Hz, 2H), 7.85 (m, 4H) 7.47 (m, 1H) 7.29
(m, 4H), 7.10 (d, J ¼ 7.06 Hz, 2H), 3.45 (s, 3H) ppm, 13C NMR (DMSO-
d6, 75 MHz): dC 155.1; 152.7; 147.1; 144.4; 134.1; 130.5; 126.4;
125.8; 123.0; 119.1; 114.6; 111.9 ppm, HR-MS (ESI, CH3CN, m/z):
C
20H17N4 [MþH]þ found: 313.1459 calcd.: 313.1453.
(E) -N-methyl-4-((4-nitrophenyl)diazenyl)-N-phenylalanine (IV
′)
Dark red; m.p: 170 ꢀC; yield: % 80, 1H NMR (DMSO‑d6, 300 MHz):
dH 8.37 (d, J ¼ 9.11 Hz, 2H), 7.96 (d, J ¼ 9.92 Hz, 2H), 7.85 (d,
J ¼ 9.16 Hz, 2H) 7.50 (m, 1H), 7.32 (m, 4H), 6.91 (d, J ¼ 9.22 Hz, 2H)
3.45 (s, 3H) ppm, 13C NMR (DMSO‑d6, 75 MHz): dC 156.4; 152.9;
147.7; 147.0; 144.6; 130.5; 129.7; 126.6; 126.5; 126.0; 125.5; 123.2;
120.5; 114.6 ppm, HR-MS (ESI, CH3CN, m/z): C19H17N4O2 [MþH]þ
found: 333.1354 calcd.: 333.1352.
(E) -4-((4-chlorophenyl)diazenyl)-N-methyl-N-phenylaniline (V
′)
tonation azo group via b-N in azo bridge. The excited state of the
dyes may be stabilized by this case (Figs. 1 and 2 and, Figs. S39-S40
in Supporting Information) (see Table 2).
Yellow solid; m.p: 135 ꢀC; yield: % 16, 1H NMR (DMSO‑d6,
300 MHz): dH 7.80 (m, 4H), 7.60 (d, J ¼ 8.75 Hz, 2H) 7.45 (m, 1H),
7.26 (m, 4H), 6.96 (d, J ¼ 9.12 Hz, 2H) 3.45 (s, 3H) ppm, 13C NMR
(DMSO‑d6, 75 MHz) dC 152.0; 151.4; 147.4; 144.4; 134.7; 130.4;
129.8; 126.1; 125.9; 125.1; 124.0; 114.9 ppm, HR-MS (ESI, CH3CN, m/
z): C19H16ClN3 [MþH]þ found: 322.1084 calcd.: 322.1111.
The dyes I-VI theoretically may be shown azo-hydrazone tau-
tomers in especially high polar solvents such as DMSO (Scheme 2).
For determination of the most stable tautomeric form of the dyes (I-
VI) in solvents, the model dyes (I′-VI′) which are in only azo form in
all solvents, were used. While compare lmax values of the both
series of dyes, it was observed the dyes (I-VI) are stable in azo
tautomeric form. However, the small hypsochromic and bath-
ochromic shifts in lmax values of the dyes were observed in these
solvents because of solute-solvent interactions.
(E) -4-((4-bromophenyl)diazenyl)-N-methyl-N-phenylaniline (VI0)
Orange solid; m.p: 137 ꢀC; yield: %25, 1H NMR (DMSO‑d6,
300 MHz): dH 7.75 (m, 6H), 7.46 (m, 1H) 7.25 (m, 4H) 6.90 (d,

4
T. Aksungur et al. / Journal of Molecular Structure 1217 (2020) 128449
Scheme 1. Synthetic pathway for obtaining of I-VI and I′-VI’.
Scheme 2. Possible tautomeric forms and common anion of I-VI.
Table 2
Table 1
The absorption wavelength values (nm) of the dyes in acidic and basic media in
DCM.
The absorption wavelengths (nm) of dyes in various solvents.
Compounds
HOAC
DMSO
DCM
CHCl3
MeOH
Compounds
DCM
DCM þ TFA
DCM þ Piperidine
I
II
III
IV
V
544
432
437
458
413
414
444.551(s)
439.550(s)
443.553(s)
473
459
439
464
493
435
438
458
444
458
489
430
432
432
427
431
452
409
410
446
440
446
476
421
423
437.535
425
431
451
408
409
446
439
446
474
419
421
443
432
444
467
419
420
441
428
441
464
416
417
I
II
III
IV
V
432
427
431
452
409
410
446
440
446
464
421
432
550
541
538
538
546
552
546
549
545
547
554
552
432
425
431
452
409
410
446
440
446
464
421
432
VI
I0
VI
I0
II0
III0
IV0
V0
VI0
II0
III0
IV0
V0
VI0
416
418
s: shoulder.
In this type functional dye, dialkylamino groups use as strong
donor, dicyanomethylene, mono/dicyano and nitro groups use as
strong acceptor [41].
3.2. Substituent effect on absorption spectra of dyes in various
solvents
In our current study, while we choose diphenylamine fragment
as strong donor, nitro, cyano etc. groups were choosed as strong
In the push-pull dyes generally include strong donor and
acceptor groups in conjugate system.

T. Aksungur et al. / Journal of Molecular Structure 1217 (2020) 128449
5
Fig. 1. UVevis spectra of IV in various solvents (c ¼ 3 ꢂ 10ꢁ5 M).
Fig. 3. The absorption spectra of IV (c ¼ 3 ꢂ 10ꢁ5 M) upon addition of TFA (1 M) in
DCM.
investigated using TFA as proton source. Upon addition of TFA to a
solution of all dyes in DCM, they exhibited clear color changes from
yellow to purple with appearing of a new absorption band in long
wavelength. For instance, the maximum absorption wavelength of
IV shift the long wavelength and one isosbestic point located at
485 nm, as a result of the addition of 20 equiv of TFA to IV (Sup-
porting Information, Table S1). Upon addition TFA to the solution of
all dyes in DCM, b-nitrogen of azo group protonated. The major
color changes were observed after protonation from yellow to
purple with well-defined isosbestic points at the range of
450e482 nm for the all dyes synthesized (Figs. 3 and 4 and Table S1
in Supporting Information) (Schemes 3 and 4). The obtained the
positive charge on b-nitrogen is delocalized in push-pull system of
dyes and large bathochromic shift of lmax values with dominant
color change for all dyes in comparison with deprotonated form.
The obtained red shift after monoprotonation of the azo group can
be explained by an increased charge transfer from the diphenyl-
amine/N-methyldiphenylamine to the protonated azo group. To
Fig. 2. UVevis spectra of IV′ in various solvents (c ¼ 3 ꢂ 10ꢁ5 M).
acceptor groups. The UVeVis spectra of azodiphenylamine dyes
bearing electron-accepting and donating substituents are seen in
Table 1. While the strong electron-accepting groups such as eNO2,
eCN in the para position of the phenyl ring were attached, signif-
icant bathochromic shifts were observed in all solvents used. For
example, lmax values of dyes for IV and IV’ bearing nitro group were
shifted bathochromic in all solvents used relative to other sub-
stituents such as eCN, eCOOH, eCOOCH3 (for IV lmax ¼ 458 nm
bearing NO2; for III lmax ¼ 437 nm bearing CN). In addition, the lmax
values of both series of dyes were seen in longer wavelength in the
most polar solvent DMSO relative to other solvents.
3.3. Protonation ability properties
The dyes using as optical pH sensor after protonation (in low pH
values) and deprotonation (in high pH values) show various
changes of their photophysical properties in absorbance or emis-
sion, such as hypsochromic or bathochromic shifts and hyper or
hypochromic effects. And they may show color and fluorescence
changes that can be seen easily with naked eye. Because of these
properties these type dyes can be used optical pH sensor in po-
tential application in clinical and environmental analysis [42].
In this study, the protonation ability of the all dyes in DCM were
Fig. 4. The absorption spectra of IV’ (c ¼ 3 ꢂ 10ꢁ5 M) upon addition of TFA (1 M) in
DCM.

6
T. Aksungur et al. / Journal of Molecular Structure 1217 (2020) 128449
Scheme 3. Protonation of azo group and possible azonium-ammonium forms of dyes.
Scheme 4. Color change after protonation of dye III’.
Fig. 5. Absorption spectra of IV (3x10-5 M) in the absence and presence of TFA and
upon addition of piperidine into the acidic DCM (black: IV, red: IV þ TFA, blue:
IV þ TFA þ piperidine).
Fig. 6. Absorption spectra of IV′ (3x10-5 M) in the absence and presence of TFA and
upon addition of piperidine into the acidic DCM (black: IV′, red: IV’þTFA, blue:
IV’þTFA þ piperidine).

T. Aksungur et al. / Journal of Molecular Structure 1217 (2020) 128449
7
Fig. 7. The optimized geometries of compounds II-VI.
Table 3
The total and relative energies (E in a.u,
D
E in kcal/mol) for the azo (a) and hydrazone (h) forms of II-VI in gas phase and different solvents.
Gas
CHCl3
HOAC
DCM
MeOH
DMSO
II
E
a
h
a
h
a
h
a
h
a
h
a
h
a
h
a
h
a
h
a
h
ꢁ1047.81266
ꢁ1047.79305
0.00
ꢁ1047.82315
ꢁ1047.80523
0.00
ꢁ1047.73063
ꢁ1047.80654
0.00
ꢁ1047.82532
ꢁ1047.80784
0.00
ꢁ1047.82733
ꢁ1047.81029
0.00
ꢁ1047.82758
ꢁ1047.81060
0.00
D
E
E
E
E
E
12.30
11.24
11.11
10.97
10.69
10.65
III
IV
V
E
ꢁ951.470865
ꢁ951.450045
0.00
ꢁ951.481298
ꢁ951.462769
0.00
ꢁ951.485609
ꢁ951.468327
0.00
ꢁ951.483414
ꢁ951.465473
0.00
ꢁ951.485369
ꢁ951.468012
0.00
ꢁ951.485609
ꢁ951.468327
0.00
D
E
13.06
11.63
10.84
11.26
10.89
10.84
ꢁ1063.73582
ꢁ1063.71592
0.00
ꢁ1063.74642
ꢁ1063.729
0.00
ꢁ1063.74752
ꢁ1063.73043
0.00
ꢁ1063.74861
ꢁ1063.73185
0.00
ꢁ1063.75065
ꢁ1063.73455
0.00
ꢁ1063.75091
ꢁ1063.73488
0.00
D
E
12.48
10.93
10.73
10.52
10.10
10.05
ꢁ1318.82018
ꢁ1318.79778
0.00
ꢁ1318.82713
ꢁ1318.8066
0.00
ꢁ1318.82789
ꢁ1318.8076
0.00
ꢁ1318.82865
ꢁ1318.8086
0.00
ꢁ1318.83008
ꢁ1318.81053
0.00
ꢁ1318.83026
ꢁ1318.81078
0.00
D
E
14.05
12.88
12.73
12.58
12.27
12.23
VI
ꢁ3430.35258
ꢁ3430.33034
0.00
ꢁ3430.35954
ꢁ3430.33917
0.00
ꢁ3430.36031
ꢁ3430.34017
0.00
ꢁ3430.36107
ꢁ3430.34118
0.00
ꢁ3430.36251
ꢁ3430.34311
0.00
ꢁ3430.36269
ꢁ3430.34335
0.00
D
13.96
12.79
12.64
12.48
12.18
12.14
test reversibility property of protonated dyes, same equivalent
piperidine was added acidic solution of dyes in DCM. The absorp-
tion curve in long wavelength disappeared and a new absorption
maximum was observed at shorter wavelength which is same
values of absorption maxima of neutral form for all dyes (Figs. 5 and
6 and Figs. S41-S44 in Supporting Information).
numbering in Fig. 7. The NeN bond length has a double bond
character in all structures and a single bond character in hydrazone
form of I-VI. In all structures, the phenyl and diphenylamine groups
are planar along the -N]N- azo bridge. However, the geometrical
parameters for azo forms of I-VI are compatible with their model
dyes (I′-VI′).
The stability of the azo and hydrazone forms of II-VI were
determined with the energy comparison of each compounds. In
Table 3, the total and relative energy (Et and DEt) in gas phase and
3.4. Computational results
studied solvents (CHCl3, HOAC, DCM, MeOH and DMSO) were
shown for azo and hydrazone forms of each one. As depicted in
table, the azo form was more stable than hydrazone form in gas
phase and its stability was not affected by solute-solvents in-
teractions. However, it should be noted that the presence of
different electron withdrawing group at para position of phenyl
ring was not affected the geometrical parameters in two series and
The optimized geometries of compounds II-VI and I′-VI′ ob-
tained by performing DFT calculations with B3LYP method and
6e31þg(d,p) basis set. The all data for I was taken from Ref. 34, in
which compound I refers as DPA, for integrity and easy comparison.
The obtained geometries and geometry parameters were presented
in Fig. 7 Fig. S45 and Table S2-S3 (in Supporting Information). The
data of geometry parameters were given in according to the

8
T. Aksungur et al. / Journal of Molecular Structure 1217 (2020) 128449
Table 4
The calculated absorption maxima (lmax), oscillator strengths (f) and relevant transitions and CI coefficient for I-VI and I′-VI′ in used solvents. f values are given in parentheses.
Solvent
lmax, f
Transitions, CI
lmax, f
Transitions, CI
I [34]
HOAC
DMSO
DCM
CHCl3
MeOH
HOAC
DMSO
DCM
CHCl3
MeOH
HOAC
DMSO
DCM
CHCl3
MeOH
HOAC
DMSO
DCM
CHCl3
MeOH
HOAC
DMSO
DCM
CHCl3
MeOH
HOAC
DMSO
DCM
471 (1.2857)
479 (1.3027)
475 (1.3011)
472 (1.3021)
475 (1.2796)
484 (1.1420
493 (1.2176
489 (1.0512)
485 (1.1842)
489 (1.1605)
484(1.2542)
492(1.2697)
488(1.2713)
485 1.2754)
488(1.2447)
553(0.9988)
569(0.9902)
559 1.0068)
551(1.0288)
564(0.9638)
459 (1.2098)
465 (1.2120)
462 (1.1995)
459 (1.2311)
462 (1.1483)
461 (1.2525)
467 (1.2551)
464 (0.8625)
461 (1.2742)
464 (1.2224)
H/L, 0.7062
H/L, 0.70554
H/L, 0.70604
H/L, 0.70605
H/L, 0.70604
H/L, 0.68029
H/L, 0.70197
H/L, 0.65133
H/L, 0.68683
H/L, 0.69219
H/L, 0.70387
H/L, 0.70594
H/L, 0.70531
H/L, 0.70400
H/L, 0.70585
H/L,0.70692
H/L,0.70718
H/L,0.70696
H/L,0.70670
H/L,0.70725
H/L,0.70469
H/L,0.70467
H/L,0.69861
H/L,0.70460
H/L,0.69271
H/L, 0.70430
H/L, 0.70462
H/L, 0.58154
H/L, 0.70425
H/L, 0.70191
I′
482 (1.1981)
489 (1.2020)
485 (0.9595)
482 (1.2187)
486 (0.9509)
477 (1.2063)
485 (1.0673)
481 (1.1979)
478 (1.2247)
481 (1.1326)
476 (1.2512)
483 (1.2110)
480 (1.2626)
477 (1.2692)
479 (1.2381)
546 (1.0021)
562 (0.9964)
553 (1.0105)
544 (1.0299)
557 (0.9710)
452 (1.1660)
459 (1.0796)
456 (1.1738)
453 (1.1850)
455 (1.1438)
455 (1.2068)
462 (0.8411)
458 (1.2058)
455 (1.2263)
457 (1.1589)
H/L, 0.69869
H/L, 0.69764
H/L, 0.62324
H/L, 0.69916
H/L, 0.62537
H/L, 0.70370
H/L, 0.65938
H/L, 0.69835
H/L, 0.70358
H/L, 0.68670
H/L, 0.70580
H/L, 0.69140
H/L, 0.70571
H/L, 0.70565
H/L, 0.70595
H/L, 0.70705
H/L, 0.70737
H/L, 0.70715
H/L, 0.70685
H/L, 0.70737
H/L, 0.70426
H/L, 0.67598
H/L, 0.70371
H/L, 0.70422
H/L, 0.70270
H/L, 0.70298
H/L, 0.51526
H/L, 0.69983
H/L, 0.70305
H/L, 0.69367
II
II′
III
III′
IV′
V′
IV
V
VI
VI′
CHCl3
MeOH
also stability of azo tautomer of II-VI. The same results were re-
ported in Ref. 34 for I.
and NLO response. In addition, dipole moment is an indicator of
charge separation within the molecule. In such systems, the large
molecular dipole moment, high polarizability and high hyper-
polarizability are observed due to donor and acceptor strengths and
increasing conjugations.
The quantum chemical calculations by various software pack-
ages are often used to predict the efficiency of NLO properties of
molecule. In a molecular system, the effects of an applied electro-
The absorption spectra were calculated using TD-DFT methods
at B3LYP/631þg(d,p) level in used solvents. The calculated ab-
sorption maxima (lmax), oscillator strength (f) and associated with
its orbital contributions were given in Table 4. The substituent ef-
fects on absorption spectra were consistent with experimental re-
sults. The largest absorption maxima were found for IV and IV′
including strong electron withdrawing group (-NO2) and the lowest
ones were for V, VI and their model compounds V′ and VI′ con-
taining less electron withdrawing groups (i.e. eBr and eCl). The
solvent effects were seen for IV and its model compound IV’ as
bathochromic shifted about 18 nm and no significant changes for
other compounds from CHCl3 to DMSO. However, the absorption
maxima for all studied compounds mainly occurred to the elec-
tronic transition from the highest occupied molecular orbital
(HOMO) to the lowest unoccupied molecular orbital (LUMO) (Fig. 8
and S46).
magnetic field are characterized by the polarizabilities (
order hyperpolarizabilities ( ). In the calculations, the output pro-
xx, xy, yy, xz, yz and zz) and
ijk (i, j, k ¼ x, y and z) and also three
my mz). Thus, using the calculation
outputs, the dipole moment of a molecule (
a) and first
b
vides six compounds of
eighteen components of
a
b
(
a
a
a
a
a
a
compounds of dipol moment (mx
,
,
m) is defined as follows:
m
¼ ðmx
þ
my
þ
mz
Þ
(1)
The average polarizability (
a) can be calculated by the following
equation:
To confirm the protonation via
a and b nitrogens atom for all
ꢀ
compounds in DCM, the protonated forms were optimized and
obtained their total and relative energy values were given in
Table 5. The comparison of the protonated forms of all compounds
a
¼ 1 3ðaxx
þ
ayy
þ
azz
Þ
(2)
The magnitude of the static first hyperpolarizability can be
confirmed that the
b
-protonated structure was more stable than
nitrogen atom was
calculated using the following equations:
btot ¼ ðb2x þ b2y þ b2zÞ
where,
the -protonated structure, so protonation via
a
b
more likely as depicted in experimental results.
(3)
3.5. Non-linear optical (NLO) properties
bx ¼ ðbxxx
by ¼ ðbyyy
bz ¼ ðbzzz
þ
bxyy
bxxy
bxzz
þ
þ
bxzz
Þ
(4)
(5)
(6)
NLO response of a molecular system containing electron-donor
and electron-acceptor units linked through a -conjugated bridge
depends on the conjugated electronic systems, i.e., molecule length
and shape as well as the character of donor and acceptor groups. In
conjugated organic molecular systems, the reduction of the energy
gap between HOMO and LUMO leads to an increase in polarization
p
þ
byzz
Þ
þ
þ
byzz
Þ

T. Aksungur et al. / Journal of Molecular Structure 1217 (2020) 128449
9
Fig. 8. The frontier orbitals of II-VI.
The obtained values of
m
,
a
and
b
from Eqs. (1)e(6) and their
hyperpolarizability values were observed as the largest value
components obtained from the calculation output are listed in
Table 6 and Table S4. The largest dipole moment was 10.13 D and
11.56 D for IV and its model compound IV0 due to the presence of
eNO2 group when the compared with the other compounds. In
case of V/VI and V’/VI′ including the weakest electron-acceptor eCl
and eBr, the dipole moment was the smallest one as 4.65 D and
6.03e6.05 D, respectively. Similarly, the presence of the strong
electron-accepting group decreased the energy gap between
HOMO and LUMO for IV and IV’ (Table 7). Thus, the
b
¼ 270.4 ꢂ 10ꢁ30 esu for IV and
b
¼ 239.8 ꢂ 10ꢁ30 esu for IV′ and
the smallest values
for VI and
Also, the comparison of b values of the studied compounds with the
b
¼ 86.7 ꢂ 10ꢁ30 esu for V,
b
¼ 89.9 ꢂ 10ꢁ30 esu
b
¼ 69.9 ꢂ 10ꢁ30 esu for V′,
b
¼ 73.1 ꢂ 10ꢁ30 esu for VI’.
value of prototype molecule urea (
b
¼ 0.38 ꢂ 10ꢁ30 esu) shows that
compounds have higher values than urea as 230e720 times for I-
IV, 186e639 times for I′-VI’. From these results, it can be concluded
that these compounds may be recommended for future NLO
studies.
Fig. 9. TGA curves of II-VI.

10
T. Aksungur et al. / Journal of Molecular Structure 1217 (2020) 128449
Fig. 10. TGA curves of I′-VI’.
Table 5
Total energy and relative energy values of the protonated I-VI and I′-VI′ via
a
and
b
nitrogens atom in DCM.
E (a.u)
DE (kcal/mol)
E (a.u)
DE (kcal/mol)
I
a
b
a
b
a
b
a
b
a
b
a
b
ꢁ1012.320732
ꢁ1012.329766
ꢁ1048.2516375
ꢁ1048.2608409
ꢁ951.9079033
ꢁ951.917298
5.67
0.00
5.78
0.00
5.90
0.00
6.36
0.00
4.57
0.00
4.61
0.00
I′
a
b
a
b
a
b
a
b
a
b
a
b
ꢁ1051.626359
ꢁ1051.63582
ꢁ1087.557375
ꢁ1087.566902
ꢁ991.213668
ꢁ991.22345
5.94
0.00
5.98
0.00
6.14
0.00
6.59
0.00
4.89
0.00
4.93
0.00
II
II′
III
IV
V
III′
IV′
V′
ꢁ1064.171183
ꢁ1064.181324
ꢁ1319.257229
ꢁ1319.264506
ꢁ3430.789672
ꢁ3430.797021
ꢁ1103.477065
ꢁ1103.48757
ꢁ1358.56266
ꢁ1358.570448
ꢁ3470.095064
ꢁ3470.102916
VI
VI′
3.6. Thermal characterization
analysis (TGA) was used. The change in weight of the compounds
was measured as a function of temperature. Thermal stability of the
dyes is shown in Figs. 9 and 10 and S47-S48 (in Supporting Infor-
mation). The absence of weight loss in the range of 0e100 ꢀC in-
dicates that the solids have no water molecule. The initial
The determination of thermal property of dyes is very important
for their usability in electro optic materials. To determine the
thermal stability of the all synthesized dyes, thermogravimetric
Table 6
The electric dipole moment (
The components are in a.u.
m
), the polarizability (
a
) and the first hyperpolarizability (btot) and their components calculated at the B3LYP/6e31þg(d) level in gas phase for II-VI.
II
III
IV
V
VI
mx
my
mz
ꢁ2.240265
ꢁ0.103851
ꢁ0.1244186
5.71
ꢁ3.5100455
0.712242
0.0117641
9.10
ꢁ3.9228564
0.7061897
0.0027141
10.13
ꢁ1.7732323
0.4419206
ꢁ0.0122946
4.65
ꢁ1.7841925
0.4056969
ꢁ0.0162391
4.65
m
(D)
axx
axy
ayy
axz
ayz
azz
687.9063553
9.7716061
234.8155804
10.6368353
0.1349964
141.7226519
52.6
700.2876797
ꢁ3.3590893
224.8876855
9.5805577
0.7319935
139.6625451
52.6
717.3529512
4.5433864
235.3579422
10.314842
0.7257733
139.1882058
53.9
645.9534372
4.2425266
224.7463849
9.7373303
1.272439
673.490449
9.2826867
230.2223446
9.8932406
0.9590457
145.4654532
51.8
139.6522411
49.9
a
(x10¡24) esu
bxxx
bxxy
bxyy
byyy
bxxz
bxyz
byyz
bxzz
20506.60706
ꢁ1004.05995
ꢁ366.2875454
35.7059622
ꢁ48.6285327
ꢁ45.5815701
ꢁ14.4866225
ꢁ94.8141671
ꢁ11.2704899
ꢁ11.1734596
173.4
18897.80037
ꢁ1067.492224
ꢁ296.3945059
37.9067562
ꢁ93.4413814
ꢁ38.8077455
ꢁ18.163339
ꢁ92.3695529
ꢁ1.3008396
ꢁ6.0117274
160.2
31695.49868
ꢁ1905.616902
ꢁ327.4595435
55.1323719
ꢁ127.8017402
ꢁ51.1717838
ꢁ15.7054165
ꢁ125.0026333
1.8416816
ꢁ6.6821093
270.4
10347.99727
ꢁ429.19933
ꢁ258.6161253
8.1333901
ꢁ46.0716367
ꢁ16.951617
ꢁ20.7693166
ꢁ62.6655028
ꢁ7.3431915
ꢁ4.0113568
86.7
10679.14287
ꢁ230.5849848
ꢁ266.6068514
ꢁ11.1401944
ꢁ43.2126488
ꢁ28.1880899
ꢁ19.6526534
ꢁ13.7111023
ꢁ15.2871649
ꢁ6.0364116
89.9
byzz
bzzz
btot (x10¡30) esu

T. Aksungur et al. / Journal of Molecular Structure 1217 (2020) 128449
11
Table 7
The energy gap between HOMO and LUMO (
DE) and the energies of HOMO and LUMO (EHOMO and ELUMO) for I-IV and I′-IV’.
I
II
III
IV
V
VI
EHOMO (a.u)
ELUMO (a.u)
ꢁ0.20911
ꢁ0.09989
2.97
ꢁ0.20963
ꢁ0.09973
2.99
ꢁ0.21355
ꢁ0.10441
2.97
ꢁ0.21674
ꢁ0.11602
2.74
ꢁ0.20558
ꢁ0.09043
3.13
ꢁ0.20564
ꢁ0.09091
3.12
D
E (eV)
I′
II′
III′
IV′
V′
VI′
EHOMO (a.u)
ELUMO (a.u)
ꢁ0.20568
ꢁ0.09555
3.00
ꢁ0.20624
ꢁ0.09528
3.02
ꢁ0.21027
ꢁ0.09983
3.01
ꢁ0.21364
ꢁ0.11216
2.76
ꢁ0.20202
ꢁ0.08561
3.17
ꢁ0.20207
ꢁ0.08611
3.16
D
E (eV)
decomposition temperatures (Td) of dyes showed that the dyes
have enough thermal stability while using as optic dyes in electro
optic (EO) material [43].
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.molstruc.2020.128449.
4. Conclusions
References
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diphenylamine and N-methyl diphenylamine and diazotized
substituted anilines bearing various substituents. The compounds
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response in gas phase: the first hyperpolarizability of I-IV (I′-VI′)
were higher than urea as 230e720 (186e639) times. The both
compounds series are thermally stable and Td values are bigger
than 220 ꢀC. Therefore, they can be offered as potential candidates
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Declaration of competing interest
The authors declare that they have no known competing
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