Spectroscopic characterization and binding interaction of heavy metal onto the surface receptor of the azobenzene: DFT and experimental approach

Article abstract of DOI:10.1016/j.molstruc.2021.130962

The azobenzene 1-arylazo-2-naphthol has been synthesized and characterized by elemental analysis, ¹H NMR, IR and UV–Vis spectroscopies using both experimental and theoretical methods. The MEP, NPA and FMOs have also been performed at the DFT/B3LYP-D3 level with different solvents in order to investigate the solvent's effect. The energetic behavior of the compound has been examined in the gas phase and in solvent media using the integral equation formalism polarizable continuum model (IEF-PCM). Solvents had sligth significant effect on NPA values whereas both molecules hardness and stability decrease along with increasing of the solvent's polarity. Vibrational and absorption analysis showed no significant azo±hydrazone tautomerism. Theorically, the title molecule calculated in Acetonitrile solvent is the most reactive. The observed red shifts are explained by the large extension of the π conjugated system and the blue shifts confirm the appearance of several tautomeric forms of the studied azo-benzene which lead to a marked improvement in photochemical stability. The cation binding properties of azo benzene as absorption sensor for Cu²⁺, Mg²⁺, Ni²⁺ and Zn²⁺ cations were reported. The highest adsorption energy in both gas and liquid phase corresponds to the complexes Azoic/Ni²⁺ then in Azoic/Cu²⁺ and somewhat in Azoic/Zn²⁺complex. The lowest adsorption energy is given to Azoic/Mg²⁺ complex. After cations chemisorption on azoic molecule a bathochromic shift is observed in the optical spectra, particularly for Azoic/Ni²⁺ complex, which is the proof of interaction between azoic compound and mentioned cations. The electronic proprieties and the vibrational spectra can therefore be used to detect the presence of different cations and the azobenzene shows appropriate properties to constitute good cations (Ni²⁺ and Cu²⁺) sensors.

Full text of DOI:10.1016/j.molstruc.2021.130962

Journal of Molecular Structure 1244 (2021) 130962
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstr
Spectroscopic characterization and binding interaction of heavy metal
onto the surface receptor of the azobenzene: DFT and experimental
approach
Marwa Chaabenea,∗, Soumaya Agrenb, Jamal El Haskouric, Abdul-Rahman Allouched,
Lahcini Mohamede,f, Rafik Ben Chaâbanea, Mohamed Hassen V Baouabb
a Laboratory of Advanced Materials and Interfaces (LIMA), Faculty of Sciences of Monastir, University of Monastir, Avenue of the Environment, 5000
Monastir, Tunisia
b Research Unit Materials and Organic Synthesis (UR17ES31), Preparatory Institute for Engineering Studies of Monastir, University of Monastir, Avenue of the
Environment, 5000 Monastir, Tunisia
c Instituto de Ciencias de Los Materiales de la Universitad de Valencia, Calle Catedratico José Beltran 2, 46980 Paterna, Valencia, Spain
d Institute of Light and Matter, UMR5306 University of Lyon 1-CNRS, University of Lyon, 69622 Villeurbanne cedex, France
e Laboratory of Organometallic and Macromolecular Chemistry-Composites Materials, Faculty of Sciences and Technologies, Cadi Ayyad University, Avenue
Abdelkrim Elkhattabi, B.P. 549, 40000 Marrakech, Morocco
f Mohammed VI Polytechnic University, Lot 660, Hay Moulay Rachid, 43150 Ben Guerir, Morocco
a r t i c l e i n f o
a b s t r a c t
The azobenzene 1-arylazo-2-naphthol has been synthesized and characterized by elemental analysis, 1H
NMR, IR and UV–Vis spectroscopies using both experimental and theoretical methods. The MEP, NPA and
FMOs have also been performed at the DFT/B3LYP-D3 level with different solvents in order to investi-
gate the solvent’s effect. The energetic behavior of the compound has been examined in the gas phase
and in solvent media using the integral equation formalism polarizable continuum model (IEF-PCM). Sol-
vents had sligth significant effect on NPA values whereas both molecules hardness and stability decrease
along with increasing of the solvent’s polarity. Vibrational and absorption analysis showed no significant
azo hydrazone tautomerism. Theorically, the title molecule calculated in Acetonitrile solvent is the most
reactive. The observed red shifts are explained by the large extension of the π conjugated system and
the blue shifts confirm the appearance of several tautomeric forms of the studied azo-benzene which
lead to a marked improvement in photochemical stability. The cation binding properties of azo benzene
as absorption sensor for Cu2+, Mg2+, Ni2+ and Zn2+ cations were reported. The highest adsorption energy
in both gas and liquid phase corresponds to the complexes Azoic/Ni2+ then in Azoic/Cu2+ and some-
what in Azoic/Zn2+complex. The lowest adsorption energy is given to Azoic/Mg2+ complex. After cations
chemisorption on azoic molecule a bathochromic shift is observed in the optical spectra, particularly for
Azoic/Ni2+ complex, which is the proof of interaction between azoic compound and mentioned cations.
The electronic proprieties and the vibrational spectra can therefore be used to detect the presence of
different cations and the azobenzene shows appropriate properties to constitute good cations (Ni2+ and
Cu2+) sensors.
Article history:
Received 1 March 2021
Revised 6 June 2021
Accepted 21 June 2021
Available online 23 June 2021
Keywords:
Azobenzene
MEP
NPA
FMOs
Solvent effects and adsorption energy
© 2021 Elsevier B.V. All rights reserved.
1. Introduction
fect and it is hard to eliminate because of its persisting nature. The
cycling of organic materials and the exchange of energy, resulting
With the expansion of the modern industry, agriculture and
transport, heavy metal pollution has become a serious problem
and an important factor in environmental pollution. Heavy metal
contamination is a widespread contamination with a long-term ef-
in the transmission and enrichment of heavy metals through food
chains, which ultimately endangers human health [1]. Recent in-
vestigations have found labile zinc to be severely involved in the
human pathophysiology and neurology. For particular, failure in
zinc homeostasis is implicated in the progression of prostate can-
cer as well as Alzheimer’s disease [2,3]. Cu2+ is known to have
a toxic effect on cells due to cellular apoptosis due to Cu2+ in-
duced hydroxyl radicals [4]. It has also been documented that a
∗
Corresponding author.
E-mail address: marwachaaben0@gmail.com (M. Chaabene).
https://doi.org/10.1016/j.molstruc.2021.130962
0022-2860/© 2021 Elsevier B.V. All rights reserved.

M. Chaabene, S. Agren, J.E. Haskouri et al.
Journal of Molecular Structure 1244 (2021) 130962
significantly elevated level of Cu2+ is damaging to various organs,
comprising the liver, kidneys, and brain [5] which leads to organ
disfunction and neuro-degenerative diseases. Mercury, can cause
to organisms and environment even at low concentrations [6,7].
When cobalt achieves a certain level in the human body, it causes
and stimulates vasodilation, flushing and cardiomyopathy, blood
and kidney adverse effects and other respiratory and central ner-
vous system disorders. Simultaneously, animals deprived of cobalt
show growth retardation, anemia, a loss of appetite and decreased
lactation [8]. In addition, Nickel compounds are believed to cause
cancer [9]. Some new investigations have shown that Pb2+ can
enter the human body from wastewater and waste residues and
cause damage to the haematopoiesis, nervous system and kidneys,
resulting in chronic poisoning [10–12]. A lack of Magnesium can
damage more than 300 enzyme systems that regulate various bio-
chemical reactions in the body. These reactions involve proteins
synthesis, muscle and nerve activity [13]. The Detection of chem-
ical species is of major concern in a variety of fields, comprising
environmental, medical and biological domains, and security. It is
well recognized that recently, the increasing pressure from envi-
ronmental authorities has led to the establishment of release stan-
dards, which in turn necessitate the effective use of chemical re-
mediation and purification processes [14]. The design as well as
the development of novel sensitive and specific chemosensors for
the environmentally and biologically important heavy and transi-
tion metal ions have been strongly focused on over last few years
[15–20]. These conventional approaches suffer greatly from the ex-
pensive cost of the instruments, complicated sample pre-treatment
processes and rigorous experimental requirements [21]. On the
other hand, it is required to transmit selectivity to the ion sen-
sor for a material that has a great affinity for a certain metal ion
but has a low sensitivity to other metals. In deed, the synthesis of
azobenzene was shown to be easy and fast [22]. Azo compounds
have attracted much attention thanks to their versatile properties
and their photosensitivity [23] as well as their ability of bonding
through N = N groups [24] which make them favorable for use
in fiber dyeing [25], analytical chemistry and in biological reac-
tions, in particular, sensing and recognition of cations, anions and
biomolecules [23]. Furthermore, metals and their ions especially
toxic metals such as Cu2+, Mg2+,Ni2+, etc. play an important role
in several biological, environmental processes and they are quite
significant in human health [23,26,27]. In addition, the determina-
tion of cations in a real sample is quite important with simpler,
cheaper and faster methods than conventional techniques. Until
now, the determination of cations can be attained by several an-
alytical methods using spectroscopic techniques. Though, simpler
methods for practical applications of the detection of any analyte
of interest are wanted. Fluorimetric and colorimetric chemosensors
can be alternative techniques instead of the traditional analytical
methods for the determination of cations in real sample. Conse-
quently, the development of chromogenic chemosensors for cations
recognition has become an attractive research field and the synthe-
sis of new additional chemosensor is still required.
the characterization and cation binding properties of azobenzene
as an adsorption sensor for Cu2+ [28], Mg2+ [29], Ni2+ [30] and
Zn2+ [31,32] cations. Furthermore, the coordination and the struc-
tural features of the cations complexes of azobenzene have been
demonstrated by computational calculations at DFT and TD-DFT
levels, where the presence of a number of heteroatoms (-N,-O) in
azoic molecule makes it suitable for the recognition of different
cations [33].
2. Experimental section
2.1. Materials and methods
1H NMR spectra were measured on Bruker 300 MHz NMR spec-
trometer. FTIR spectrum was measured on a Perkin Elmer spec-
trum two ranging from 350 to 8300 cm−1 with DTGS as a detec-
tor. UV–vis absorption spectra were obtained on a SPECORD PLUS
ranging from 190 to 1100 nm.
2.2. Quantum chemical calculation
Chemical calculations were performed by means of the Gaus-
sian09 D.01 package [34] using the DFT method with the B3LYP
functional, as it is the most suitable for reactivity studies. The re-
sults were analyzed using the graphical interfaces Gabedit [35] and
gaussview software [36]. 6–311++G (d, p) base were used for
the carbon (C), oxygen (O), hydrogen (H) and azote (N) atoms,
while the def2tzvpp one was investigated for Ni2+, Cu2+, Mg2+
and Zn2+. All the reported computations were performed using
the dispersion-including B3LYP-D3 method to adequately describe
the dispersion interactions between a particle and its neighbors
in a given radius, via a simple pair-wise force field summed to
the pure DFT energy [37]. The 1H calculations used the Gauge-
Independent Atomic Orbital (GIAO) [38] method. In all the cases
1H for the test molecules were corrected with a chemical shift
(δ/ppm) relative to simulated values for tetramethylsilane (TMS),
computed at the same level. Calculations of vibrational IR frequen-
cies of Azoic molecule before and after the adsorption of different
cations have been carried out with the same method as geome-
try optimization in gas phase. It has been confirmed that these
structures correspond to the characteristics of the local minimum
by the absence of an imaginary mode. Natural population anal-
ysis (NPA) calculations and the theoretical electronic absorption
spectra have been simulated by using B3LYP-D3 and CAM/B3LYP-
D3 method respectively. NPA and the theoretical and experimental
UV–vis absorption measurements were carried out at room tem-
perature in seven kinds of solvent (Acetonitrile, Chloroform, Cyclo-
hexane, Dichloromethane, Ethanol, THF and Toluene), also in the
gas phase in order to evaluate the solvent effect to the atomic
charge distribution over the chemical bonding, total energy, gap
energy, HOMO and LUMO energies, chemical reactivity descriptors
and dipole moment of the studied azo-benzene compound. The
computations were done with 6–311++G (d, p) basis set using the
polarizable continuum model (PCM) [39].
In this paper, we focus on the synthetical, structural, vibrational
and optical analysis of azobezene for the first time using both
experimental and theoretical methods in different solvents (Ace-
tonitrile, Chloroform, Cyclohexane, Dichloromethane, Ethanol, THF,
Toluene and gas phase). The solvation treatment was selected as
follows : The solvent’s choice was done according to their pro-
ticity and polarity as well as its predictible effect on the tau-
tomeric forms of the studied compound. In deed, we used cyclo-
hexan, toluen and chloroform as non-polar aprotic solvents. THF
was chosen as a polar aprotic solvent and ethanol was employed
for its obvious polarity and proticity. The concentration of the pre-
pared samples is maintained to be 10−5 M. Moreover, we report
In addition, the adsorption energy in the gas phase (Ead/gas) cal-
culated was based on the following expression:
Ead/gas = EComplex − − EAzoic + ECations + EBSSE (1)
(
)
Where Ecomplex is the total energy of cations adsorbed on Azoic
molecule. EAzoic is the total energy of isolated Azoic molecule, and
Ecations is the total energy of isolated cations Ni2+, Cu2+, Mg2+ and
Zn2+. The counterpoise method is used to correct the basis su-
perposition error (EBSSE) [40]. We have also calculated the solva-
tion energy (Eads/solv), using a Hess cycle, as the optique detection
of all cations by the azoic molecule is investigated in an aqueous
2

M. Chaabene, S. Agren, J.E. Haskouri et al.
Journal of Molecular Structure 1244 (2021) 130962
Scheme 1. General scheme for the synthesis of Azoic molecule.
medium. Eads/solv is calculated from Eq. (2) [41]
Ead/solv = Eads/gas + Esolv Azoic + cations
(
)
−Esolv cations − Esolv Azoic
(2)
(
)
(
)
Where Eads/gas is the adsorption energy in gas, Esolv (azoic),
Esolv (cations) and Esolv (Azoic+cation) are the solvation energies of
azoic molecule, cations, and complexes, respectively. Negative en-
ergy adsorption indicates a favorable interaction (the most stable
structure relates to the largest negative value of energy), whereas
positive energy adsorption indicates an unstable system [42].
The NPA analysis were performed again to investigate the di-
rection of the charge transfer between our synthesized compound
(Azoic) and the different cations in the complexe.
The UV–vis spectra of isolated Azoic molecule and Azoic/cations
complexe are calculated at TD-DFT/CAM-B3LYP-D3/6–311++ G (d,
p) level of theory [43].
Fig. 1. Optimized structure of the Azoic molecule along with numbering of atoms.
Furthermore, by using the frontier molecular orbitals (High-
est Occupied Molecular Orbital (HOMO) and Lowest Unoccupied
Molecular Orbital (LUMO)) different global reactivity parameters
have been determined such as electronic gap energy (Eg), chemi-
cal potential (μ), global hardness (η) and global softness (σ) men-
tioned in [44]:
8 mL of chilled water has been added drop by drop to the cold
amine solution with constant stirring for 2 h by maintaining the
reaction mixture 0–5 0C to achieve the diazotization reaction. This
obtained diazonium salt solution was involved in the coupling re-
action. A 0.02 mmol of the naphtol solution was prepared by dis-
solving 2.88 g in the 16 mL of aqueous NaOH and 100 mL of chilled
water keeping the temperature at 0–5–0 C. Then the diazonium
chloride solution was added dropwise into the nucleophilic com-
pound within one hour of stirring condition. The orange color evo-
lution of the reaction mixture is the primary indicator that our di-
azonium salt has turned into a diazo compound. The non-evolution
of the shade of the azobenzene obtained and followed by CCM
marks the reaction’s end. The resultant product was filtered off,
washed a number of times with chilled water and air dried for a
night.
Eg ≈ ELU MO − EHOMO
(3)
(4)
(5)
(6)
μ = − EHOMO + ELU MO /2
[
]
η = ELU MO − EHOMO /2
[
]
σ = 1/η
The chemical potential (μ) characterizes the escaping tendency
of electron from a stable system. The chemical hardness (η) mea-
sures the resistance of a molecule to charge transfer. The global
softness (σ) is the capacitance of the molecule to acquire charge.
When we assess the sensitivity of the sensor, the shift of the
HOMO-LUMO energy gap (Eg) is obtained as follows:
3. Results and discussion
3.1. Molecular structure
The geometry parameters of 1-arylazo-2-naphthol were ex-
amined in different solvents using DFT calculation by B3LYP-D3
method with 6–311++ G (d, p) basis set in order to find the most
stable geometry in the most suitable solvent. The choice of the sol-
vent for this investigation was mainly based on the value of the
calculated total energy. These values were calculated in different
solvents (Acetonitrile, Chloroform, Cyclohexane, Dichloromethane,
Ethanol, THF, Toluene and gas phase) using PCM approach and
reported in Table 1. It is clearly observed that the highest value
found with Acetonitrile to its efficiency.
ꢀEg = Eg2 − Eg1 /Eg1 ∗ 100 %
[(
(7)
)
]
Where Eg1 and Eg2 are the values of the Eg for the isolated
Azoic molecule and after detection of different cations, respec-
tively.
2.3. Synthesis and characterization
The investigated azo compound; 1-arylazo-2-naphthol has been
obtained through the well-known diazotization process reported
in literature (Scheme 1) [22]. This azoic dye is prepared as fol-
lows, 0.02 mol (1.9 g) of aniline has been dissolved in a mixture
of Chlorohydric Acid (12 M) and distilled water (6 mL) at room
temperature. The obtained solution was magnetically stirred and
then cooled to 0–5 0C using a cold solution of an ice bath. A pre-
viously prepared 0.02 mol solution of sodium nitrite (1.37 g) in
In this part, all the calculations were made in Acetonitrile sol-
vent, since the azoic is slightly the most stable in this solvent. The
optimized structure of 1-arylazo-2-naphthol with the atomic num-
bering scheme is shown in Fig. 1.
Its correspondent energy and its dipole moment were found to
be −801.870799 Hartree and 2.641 D, respectively. The calculated
optimized geometrical parameters of the Azoic molecule in various
3

M. Chaabene, S. Agren, J.E. Haskouri et al.
Journal of Molecular Structure 1244 (2021) 130962
Table 1
The calculated electronic and quantum chemical parameters for the Azoic molecule in different solvents at DFT/B3LYP-D3/6–311++ G (d, p).
Solvents
Acetonitrile
Chloroform
Cyclohexane
Dichloromethane
Ethanol
THF
Toluene
Gas
ET (a, u)
Eg (eV)
−801.870799
2.799
−801.868522
2.770
−801.865988
2.732
−801.86968
2.785
−801.870626
2.797
−801.869403
2.781
−801.866571
2.741
−801.863047
2.683
EHOMO (eV)
ELUMO (eV)
μ (eV)
−5.891
−3.092
4.492
−5.829
−3.059
4.444
−5.763
−3.031
4.397
−5.860
−3.075
4.468
−5.886
−3.090
4.488
−5.852
−3.071
4.462
−5.778
−3.037
4.408
−5.700
−3.017
4.359
η (eV)
σ (eV−1
1.400
1.385
1.366
1.393
1.398
1.391
1.371
1.342
)
0.715
0.722
0.732
0.718
0.715
0.719
0.730
0.745
μD (D)
2.641
2.451
2.230
2.548
2.627
2.526
2.282
1.953
Table 2
˚
Selected bond distances (A) and bond angles (°) of the Azoic molecule calculated in various solvents by DFT method: compared with available theoretical and experimental
results (first and second lines respectively), for similar compound.
Solvents
Acetonitrile
Chloroform
Cyclohexane
Dichloromethane
Ethanol
THF
Toluene
Gas
Litera.
˚
Bond length (A)
O17–H20
C8–O17
0.988
1.359
1.434
1.437
0.987
1.357
1.434
1.437
0.986
1.355
1.435
1.437
0.988
1.358
1.434
1.437
0.988
1.359
1.434
1.437
0.987
1.358
1.434
1.437
0.986
1.356
1.434
1.437
0.985
1.353
1.435
1.437
0.68 [44]
1.34 [44]
1.39 [44]
1.402 [45]
1.417 [46]
1.397 [45]
1.394 [46]
1.261 [45]
1.276 [46]
1.414 [45]
1.415 [46]
1.406 [45]
1.396 [46]
—-
C3–C8
C2–C3
C2–N18
1.413
1.276
1.416
1.406
1.413
1.276
1.416
1.406
1.413
1.276
1.416
1.406
1.413
1.276
1.416
1.406
1.413
1.276
1.416
1.406
1.413
1.276
1.416
1.406
1.413
1.275
1.416
1.406
1.413
1.275
1.416
1.405
N18–N19
N19–C23
C22–C23
C22–H28
1.082
1.731
1.082
1.734
1.082
1.738
1.082
1.733
1.082
1.732
1.082
1.733
1.082
1.737
1.082
2.741
N18-H20
1.67 [44]
Bond angle (°)
C8-O17-H20
C3-C8-O17
C2-C3-C8
107.4
122.3
124.3
116.7
117.3
107.4
122.4
124.4
116.8
117.3
107.4
122.6
124.4
116.8
117.3
107.4
122.3
124.4
116.8
117.3
107.4
122.3
124.4
116.8
117.3
107.4
122.3
124.4
116.8
117.3
107.4
122.4
124.4
116.8
117.3
107.4
122.5
124.4
116.8
117.3
—-
123.1 [44]
—-
C3-C2-N18
C2-N18-N19
—-
116.2 [45]
113.3 [46]
114.6 [45]
113.5 [46]
124.7 [45]
125.2 [46]
—-
N18-N19-C23
N19-C23-C22
C23-C22-H28
115.9
125.7
120.2
115.9
125.8
120.1
115.9
125.8
120.1
115.9
125.7
120.2
115.9
125.7
120.2
115.9
125.7
120.2
115.9
125.8
120.1
115.9
125.9
120.1
solvents are enumerated in Table 2 and compared with theoreti-
cal and experimental data available in literature for similar com-
pound [45–47]. These optimized structures are helpful in comput-
ing a variety of parameters further. The optimized bond lengths of
C2–C3, C2–N18 , N19–C23 and C22–H28 are the same in different sol-
˚
vents with the value of 1.437, 1.413, 1.416 and 1.082 A respectively.
The bond angles of (C8–O17–H20), (C2–N18–N19 ) and (N18–N19–C23
)
are not changed in the different solvents. Computed values of most
of the bond lengths and bond angles have shown very close values
in these solvents. Therefore, it can be noticed that the solvent has
a very small effect on optimized geometrical parameters values. It
is evident to mention that the majority of the experimental val-
ues of bond lengths and bond angles are slightly shorter than the
theoretical ones. This discrepancy may be explained by the fact of
hydrogen bonding interactions in the solid phase that are not taken
into consideration in the calculations [48,49]. As shown in Table 2,
most of the calculated values have shown good agreement with
theoretical and experimental values available in literature.
Fig. 2. The total electron density mapped with electrostatic potential of Azoic
molecule in Acetonitrile solvent.
Molecular electrostatic potential (MEP) mapping is very use-
ful in the investigation of the molecular structure with its physio-
chemical property relationships [50,51]. It is obtained at the B3LYP-
D3/6–311++G (d, p) optimized geometry. The negative (red) re-
gions are related to electrophilic reactivity and the positive (blue)
regions to nucleophilic reactivity sites as shown in Fig. 2. The color
code maps for the Azoic compound were predicted in the range
between −0.04 a.u and 0.04 a.u. The MEP is the best suitable
tool for the identification of inter and intramolecular interaction
sites [52]. Intramolecular O17–H20ꢀN18 stabilizes the title molecule.
The molecular electrostatic potential map indicates that the region
around the oxygen atoms (O17 ) linked with carbon through the
double bond was the most negative potential region (red). More-
over, a negative electrostatic potential zone occurs on the imine
4

M. Chaabene, S. Agren, J.E. Haskouri et al.
Journal of Molecular Structure 1244 (2021) 130962
Table 3
Experimental and theoretical calcula-
tion NMR of 1H for of the Azoic
molecule.
Atom
δ
exp(ppm)
δtheo(ppm)
H7
8.55
8.53
7.76
7.76
7.43
6.90
15.70
7.57
7.96
7.93
7.55
7.36
8.72
8.55
7.97
7.80
7.85
7.53
15.81
7.90
8.15
8.51
8.09
7.99
H10
H11
H14
H15
H16
O17H20
H27
H28
H29
H30
H31
N19 and N18 . Positive MEP results are on hydrogen atoms of the
title molecule. Negative and positive electrostatic potential sites
give information about the possible sites having hydrogen bond-
ings. Therefore, Fig. 2 verifies intramolecular O17–H20ꢀN18 interac-
tions.
Fig. 3. Comparison of experimental and theoretical FT-IR spectra for the Azoic
molecule.
get the subject of the spectroscopic signature of the studied azoic
dye: 1-arylazo-2-naphthol. The comparative experimental and the-
oretical FT-IR spectra are shown in Fig. 3. The experimentally-
observed IR bands and the calculated frequencies as well as their
assignments are summarized in Table 4. The scaled frequencies
are also provided in Table 4 along with unscaled frequencies
[57] where the theoretical harmonic frequencies were scaled down
uniformly by a factor of 0.987 (for wave numbers under 1800
cm−1) and 0.961 (for those over 1800 cm−1) for B3LYP-D3/6–
311++G(d, p) level of theory, which accounts for systematic errors
caused by basis set incompleteness, vibrational anharmonicity and
neglect of electron correlation [58].
3.2. NMR spectrum analysis
Multinuclear magnetic resonance spectroscopy is a very advan-
tageous tool for the study of organic compounds stucture elucida-
tion. The recorded and computed proton chemical shifts in DMSO
solution with respect to TMS are gathered in Table 3. Theoreti-
cal chemical shifts were computed using DFT/GIAO methodology
as well as 6–311++G (d, p). Experimental Chemical shifts were
also reported in parts per million relative to TMS. The proton
atom’s positions were numbered as in Fig. 1. The analysis of mag-
netic properties of 1-arylazo-2-naphthol was identified by com-
bined use of 1H NMR spectroscopy and quantum chemical calcula-
tions. The theoretical 1H chemical shifts have been compared with
the recorded data. Azoic (a); δ = 15.7 ppm (s, J = 1.00, 1H, OH),
6.9 ppm (d, J = 1.804, 2H), 7.43–8.55 ppm (td, dd, J = 6.135, 6H),
7.43–7.36–7.93 ppm (m, J = 5.404, 5H).
The assignments for the studied molecular system are essen-
tially proposed on the basis of frequency agreement between sim-
ulated and experimental fundamentals. As a general overview, the
experimental spectrum complements the simulated spectrum and
both FT-IR spectra confirm the proposed structure.
The hydroxyl proton exhibits a singlet resonance in the region
of 15.7 ppm and 15.81 ppm for the experimental and theoreti-
cal spectra, respectively. The high hydroxyl band observed in this
range is assigned to a strong intramolecular charge transfer inter-
action [53] involving the whole studied molecule. This trend is
especially observed for azo groups [54]. The aromatic ring pro-
tons of the investigated compound were observed from 6.90 to
8.55 ppm for the experimental spectra and from 7.53 to 8.72 ppm
for the theoretical data. The corresponding protons appear at lower
field by comparison of those cited in literature [46] because of
the strong electron delocalization of benzen rings. The resonance
signals for H30 and H27 were hardly assigned. Mesomeric effect
produces an electron excess which causes dishielding in the ortho
position sush as H27 relatively-low resonance [55]. In the light of
these results, we clearly notice that the calculated 1H NMR chem-
ical shifts for the title compound were in a satisfactory agree-
ment with the experimental results. These findings indicate that
the employed density functional method provides valuable infor-
matuion for our title compound charecterization. This computa-
tional method was also employed in our recent studies [56] and
shows that the predicting results values are coinciding very well
with the observed frequencies.
The 1-arylazo-2-naphthol FT-IR spectra exhibited characteristic
bands along with their appropriate shifts due to several bond’s for-
–
mation. The studied compound displayed the typical C H as well
as the skeletal vibrations C = C stretching and bending vibrations.
–
The benzen ring modes involve C C bonds and the vibrational fre-
quencies are related to stretching modes of vibration of phenyl
–
ring. The C C stretching vibrations are estimated in the range from
800 to 1700 cm−1 [46]. These frequencies are not significantly in-
fluenced by the nature of the substituents [59] and are very much
important and highly characterized by the aromatic ring itself [60].
These corresponding values agree with the literature data pub-
–
lished for comparable azo dyes structures [61]. The C H in-plane
bending vibration generally occurs in the range of 1050 to 1520
cm−1 [46], while the C H out of plane bending vibrations for sub-
–
stituted benzenes are in the region 500–1000 cm−1 [62]. Generally,
–
the C H out-of-plane bending with the highest frequencies have a
weaker intensity than those absorbing at lower frequencies [62].
In the present work, the absorption peaks in the experimental
FT-IR spectrum observed in the range of 1034–1668 cm−1 were as-
–
certained to the C H in plane bending vibrations. This theoretical
–
C H in plane bending vibrations were also observed in the same
range from 1041 to 1644 cm−1. Similarly, The FT-IR peaks occur-
3.3. Vibrational analysis
ring at 526, 540, 584, 653, 683, 752, 782, 841 and 984 cm−1 are
–
assigned to the C H out of plane bending vibrations and showed
In the present study, we have performed a detailed frequency
a good concurrence with theoretical observed frequencies at 511,
552, 572, 644, 691, 746,773, 821 and 981 cm−1 respectively. We
calculation using DFT computed by B3LYP/6–311++G in order to
5

M. Chaabene, S. Agren, J.E. Haskouri et al.
Journal of Molecular Structure 1244 (2021) 130962
Table 4
B3LYP-D3/6–311++G (d, p) calculated vibrational frequencies (cm−1) in gas phase.
a
νcalc Unscaled
νcalc Scaled
ν
Vibration∗
Exp
–
518
511
526
CH oopb + CC ipb+ CN ipb+ C O-H twisting
–
559
552
540
CH oopb + CC ipb+ CN ipb+ C OH str
–
580
572
584
CH oopb+ CC str+ CC ipb+ CN ipb +C OH str
–
652
644
653
CH oopb + CC oopb+CN oopb+C OH out of plane deformation
700
691
683
CH oopb + CC oopb +CN oopb
CH oopb+ OH oopb+ CC oopb
CH +oopb+CN oopb+ CC oopb
756
746
752
783
773
782
–
832
821
841
CH oopb + CN oopb+ C O-H twisting
994
981
984
CH oopb
1055
1108
1189
1206
1226
1260
1296
1352
1388
1400
1473
1480
1553
1639
1666
3176–3217
3354
1041
1094
1174
1190
1210
1244
1279
1334
1370
1382
1453
1460
1533
1618
1644
3052–3092
3223
1034
1073
1142
1207
1228
1255
1269
1341
1389
1400
1448
1500
1558
1638
1668
3035–3331
3431
CH ipb+ CC str+ CO str+ OH ipb
CH ipb+ CC str+ OH ipb
CH ipb+ CC str+ OH ipb
CH ipb+ CC str+ CN str
CC str+ CH ipb+ CN str+ CO str
CC str+ CH r+ CO str+ CN str
CH r+ CC str+ CO str+ OH b+ CN ipb+ NN str+ CN str
CH ipb+ CC str+ NN str + CN b
CC str+ CH ipb+ CO str+ CN ipb+ NN str+ CN str
CC str+ CO str+ OH ipb+ CH ipb+ CN ipb+ NN str+ CN str
CC str+ CH ipb+ CO str+ OH ipb+ NN str+ CN str
CC str+ NN str+ CN str+ CO str+ CH r+ OH ipb
CC str+ CH ipb+ CO str+ OH ipb+ NN str
CC str+ NN str+ CH ipb+ OH ipb+ CN ipb
CC str+ OH ipb+ CO b+ CH ipb+ NN str
CH str
OH str
str: stretching; r: rocking; b: bending; oopb: out of plane bending; oop: out of plane; ipb: in plane bending.
∗
The majority of harmonic modes were composed of a variety of local modes. Only the local modes that have the
most important contribution were listed in the Table.
3559–3200 cm−1 region [67]. In the case of an unsubstituted phe-
nol, the peak observed at 3657 cm−1 has been assigned to an hy-
droxyl stretching vibration in the gas phase [66]. Based on these
facts, the medium band observed at 3431 cm-1 in the experimen-
tal spectrum is assigned to O-H stretching vibration. A comparison
of this band with the theoretical value at 3223cm-1 shows an evi-
dent deviation of ~200 cm-1. The difference between the computed
and the experimental value may be essentially due to the presence
of strong intramolecular hydrogen bonding [64]. The O-H in-plane
bending vibration in phenolic compounds lies in the region 1150-
1270 cm-1 [61] and is not affected due to OH hydrogen bonding
unlike the stretching and out-of-plane bending frequencies [68]. In
our case, the bands observed between 1034 and 1142 cm−1 and at
1400–1668 cm−1 are attributed to O-H in-plane bending vibration.
The theoretically computed value at 1041–1174 cm−1 and at 1382–
1644 cm-1 shows good agreement with our experimental obser-
vation. The O-H out-of-plane bending vibration for similar struc-
tures is around 290-320 cm-1 for non hydrogen bonded O-H and
at 517-710 cm−1 for associated O-H [69]. In both inter-molecular
and intra-molecular hydrogen bonds, the frequency is at a higher
value than free O-H. In the same line of thought, the wavenum-
ber increases with hydrogen bond strength because of the large
amount of the required energy to the O-H bond twisting [70].
In the present study, the theoretical wavenumber of O-H in-
plane bending vibration which were recorded between 1279 and
1644 cm-1 shows an excellent agreement with the experimen-
tal wavenumber at 1269 and 1644 cm-1 respectively. As a gen-
eral trend, all OH vibration’s modes appeared in the characteris-
tic range and they are well approved. The observed bands in both
experimental and theoretical spectra in the region of 1200–1700
can notice from Table 4 that the theoretical frequencies will match
well with the experimental ones, after scaling.
–
The C H out of plane bending vibrations are apparently not
pure and contain important contributions of other modes sush out
–
of plane C N bending vibration [61]. Additionally, the assignment
–
of C N stretching frequency is a tough task since a large num-
–
ber of coupling interactions between rings of C N stretching vi-
–
brations take place in this region [63]. C N stretching absorption
is usually observed in the region of 1200–1520 cm−1for azoic dyes
–
[13,11] and C N bending vibrations appear in the range of 600–
1510 cm−1 for example, the bands observed at 1207–1500 cm−1 in
–
FT-IR are assigned as C N stretching vibrations. The theoretically
calculated wavenumbers at 1190–1460 cm−1 are assigned to C N
–
–
stretching vibrations. We noted also that the C N bending vibra-
tions gives rise to several characteristic bands at 526–841 cm−1
and at 1341–1638 cm−1 covering the spectral range. The bands
were confirmed by their corresponding theoretical values appear-
ing at 511–821 cm−1 and at 1334–1618 cm−1
.
The N = N stretching vibrations are expected in the range from
1490 to 1525 cm−1. This band is not significantly influenced by
the substituents and the environment’s nature [61]. In our case,
the experimental N = N stretching vibration appeared in the range
of 1269–1668 cm−1 and shows a good harmony with the theoreti-
cal calculated values and similar published structures [61]. The hy-
droxyl group gave rise to three vibrations specifically stretching,
in-plane and out-of plane bending vibrations.
The OH group vibration, one of the most sensitive bonds to the
environment modification, shows obvious shifts in the hydrogen
bonded species’ spectra. The hydroxyl stretching vibrations are ob-
served in the region around 3500 cm-1 [61-66]. The comparison
of this band with the theoretical value shows a good harmony to
the experimental data prediction that there is a descrepancy, that
may be due to the fact that there is a strong intramolecular hydro-
gen bonding. The usual intra-molecular hydrogen bonding present
in a six-membered ring would reduce the OH stretching band to
cm−1 confirm the presence of C OH in plane bending vibrations.
–
These bands cover a wider range due to the extent of ᴨ conjugua-
–
tion and charge delocalization [53,56]. The O H strtching vibra-
tion occurs generally in region 3700–3550 cm−1 [71]. The presence
–
of a stronger intra-molecular hydrogen shifts the O H stretching
6

M. Chaabene, S. Agren, J.E. Haskouri et al.
Journal of Molecular Structure 1244 (2021) 130962
Table 5
NPA Charges of the important atoms of the Azoic molecule in different solvents.
Atom
H20
O17
C8
C3
C2
C1
H7
N18
N19
C23
C22
H28
Acetonitrile
Chloroform
Cyclohexane
Dichloromethane
Ethanol
0.489
0.488
0.486
0.488
0.489
0.488
0.487
0.485
−0.716
−0.710
−0.703
−0.713
−0.715
−0.712
−0.705
−0.695
0.303
0.306
0.309
0.305
0.304
0.305
0.308
0.312
0.014
0.015
0.017
0.015
0.014
0.015
0.017
0.018
0.084
0.085
0.085
0.084
0.084
0.084
0.085
0.086
−0.178
−0.178
−0.177
−0.178
−0.178
−0.178
−0.177
−0.176
0.232
0.231
0.230
0.231
0.232
0.231
0.230
0.228
−0.261
−0.261
−0.260
−0.261
−0.261
−0.261
−0.260
−0.259
−0.182
−0.181
−0.180
−0.181
−0.182
−0.181
−0.181
−0.180
0.087
0.089
0.091
0.088
0.087
0.088
0.090
0.093
−0.198
−0.196
−0.195
−0.197
−0.197
−0.197
−0.195
−0.193
0.222
0.221
0.220
0.221
0.222
0.222
0.220
0.219
THF
Toluene
Gas
bond to 3550–2000 cm−1 region [70]. In the present study, the
wavenumbers observed at 3441 cm−1 and 3223 cm−1 in the exper-
imental FT-IR spectrum and theoretical simulated spectrum. 200
cm−1 deviation from experimental to theoretical values is due to
the presence of remarkable unharmonicity in this vibration as well
3.5. Photophysical properties
3.5.1. Absorption properties
Ultraviolet and visible light absorption spectroscopy is a mea-
sure of a beam’s reduction after it has passed through a stud-
ied sample or reflected from a sample surface [75,76]. To explore
the solvent’s impact on the absorption spectra and the photophysi-
cal properties of the synthesized azo dye: I-phenyl azo-2-naphthol,
we have recorded its electronic absorption spectra (Fig. 5(a)) over
the wavelength range 210–485 nm in six different solvents; Ace-
tonitrile, Ethanol, THF, Toluene and cyclohexane. The spectroscopic
analyses of the studied molecule are investigated by not only ex-
perimental means (at 10−6 M concentration and at room temper-
ature) but also theoretically in the above-mentioned media and in
Dichloromethane solvent. The theoretical calculations of the ab-
sorption spectra were done by TD-DFT method at CAM-B3LYP-
D3/6–311++G (d, p) level of basis set in gas and other solvents
which are represented in Fig. 5(b) in comparison with the experi-
mental one. The calculated and experimental wavelengths from ab-
sorption peak, oscillator strength and (%) contribution from each
transition of the studied azobenzene in various solvents are pre-
sented in Table 6.
–
as the O H may have participated in a strong intermolecular hy-
drogen bonding in the title compound [72].
The harmonic frequencies by DFT calculations are usually
higher than the corresponding experimental ones. This discrepan-
cies between the experimental and the computed values may be
caused by several factors. The environment is the first cause while
the second factor seems to be the electron correlation approximate
treatment, anharmonicity effects (where the experimental vibra-
tional value is an unharmonic frequency whereas the calculated
vibrational value is a harmonic frequency) and basis set deficien-
cies, etc. [73]. Furthermore, the interactions between the molecules
make the frequencies shift to the lower frequencies.
a
Obtained from the wave numbers calculated at B3LYP-D3/6–
311++G (d, p) using scaling factors 0.987 (for wave numbers under
1800 cm−1) and 0.961 (for those over 1800 cm−1).
The orange shade of the investigated dye wasn’t influenced by
the solvent nature which makes it suitable for industrial appli-
cation [77]. The 1-arylazo-2-naphthol dyes can show significant
azo hydrazone tautomerism [77]. Thus, their visible absorption
spectra sometimes show two absorption bands attributable to the
two tautomers, and the ratio of the tautomers at equilibrium de-
pends both on substituents in the dye and the nature of the sol-
vent. Two factors are responsible for the tautomeric shift, i) se-
lective solvatation depending on the two and three dimensional
structures of the solvent [78], ii) the ability of the solvent to form
strong intermolecular hydrogen bonds with particular tautomeric
form.
3.4. Natural population analysis
The atomic charges of the Azoic molecule were calculated by
natural population analysis (NPA) using B3LYP-D3/6–311++G (d, p)
level of basis set. It is used to understand the atomic charge dis-
tribution over the chemical bonding. The variation of bond length
between atoms depends on the allocation of charges. The atomic
charges on every atoms in the molecule gives an effective informa-
tion about the electronic structure, acidic-basic nature and molecu-
lar polarizability [74]. The NPA analysis of the title compound were
performed in different solvent using PCM model and gathered in
Table 5. The plot of natural atomic charges for Azoic molecule is
shown in Fig. 4(a). The O17 (−0.716; −0.695) atom has more nega-
As a general overview, the 1-arylazo-2-naphthol spectra exhib-
ited similar and narrow absorption bands as well as comparable
band intensities in all the solvents except for Acetonitrile solvent.
As shown in Table 4, the experimental maximal wavelengths coin-
cident reasonably with the calculated results. It is clear that there
is no correlation between the polarity and proticity of the used
solvents with the main absorption wavelength except for the Ace-
tonitrile. Thanks to the hydroxyl substantial strength of the intra
and intermolecular hydrogen bondings, electron delocalization and
solvent interactions are limited which is confirmed from the theo-
retical calculations [45]. A relatively remarkable red shift for some
bands and an increasing in the band’s intensity is observed in cy-
clohexane spectrum (then in Toluene solvent) while a blue shift
and a deacreasing in the band’s intensity is observed in Acetoni-
trile spectrum and theoretical findings can support this viewpoint.
According to several studies, the red shift is associated with the
large extension of the π conjugated system [53,56,79,80]. Thus, the
blue shift may indicate the appearance of several tautomeric forms
of 1-arylazo-2-naphthol and the disappearence of the hydroxyl
–
tive charge than other atoms due to the formation of O H…N hy-
drogen bonding. The carbon atoms attached to the nitrogen and
oxygen atoms C2, C3, C8 and C23 are positive thanks to the elec-
tron with drawing nature of electronegative atoms, other carbon
atoms presented in the phenyl ring and naphthalene shows neg-
ative charges. The N19 atom shows (−0.182; −0.180) less negative
charge compared with N18 (−0.261; −0.259) confirmed the forma-
–
tion of C H…N hydrogen bonding. All hydrogen atoms show posi-
tive charge in which, H20 (0.489; −0.485) is more positive because
of the electron with drawing nature of oxygen atom in the hy-
droxyl group. As shown in Table 5, we can notice that the solvent
has a very small effect on NPA values. Indeed, the maximum shift
between charges calculated with solvent and those obtained in gas
phase is about 0.021 a.u observed for O17
For this reason, we chose to put the NPA analysis in Acetonitrile
solvent.
.
7

M. Chaabene, S. Agren, J.E. Haskouri et al.
Journal of Molecular Structure 1244 (2021) 130962
Fig. 4. NPA distribution of Azoic compound before (a) and after Ni capture (b) calculated at B3LYP-D3/6–311++G (d, p) level of theory in Acetonitrile solvent.
Fig. 5. The solvent effect on UV–Vis spectra of the title compound in Acetonitrile, Chloroform, Cyclohexane, Dichloromethane, Ethanol, THF, Toluene and gas phase.
donor group at certain wavelengths. It is well recognized that the
intramolecular binding of hydrogen in a 1-arylazo-2-naphthol will
frequently lead to a marked improvement in photochemical stabil-
ity [81].
fect, absorption maxima in Acetonitrile occur at lower wavelengths
than in the less polar other solvents. The experimental absorption
bands of the studied azobenzene recorded in acetonitrile showed
five main characteristic absorption peaks at 210, 234, 268, 416
and 476 nm, respectively. These bands show good agreement with
measured theoretical bands at 201, 258, 307, 417 and 452 nm, re-
spectively. The peaks at 200 and 300 nm range are attributed to
the n→ π∗ transition of benzene and naphthalene ring, respec-
tively. The peak located around 300 nm can be assigned to the
n → π∗ electronic transition of N = N group [83]. The peaks in the
range of 420 nm and the at 485 nm corresponds to the π → π∗
transitions of the hydrazone form and azo, respectively [83,84]. The
absorption peaks in all solvents exhibited smaller variations than
in gas possibly due to the solvent effect.
The ortho–hydroxyl substituted azo-azomethine compounds are
distinguished by the possibility to be inferred from, selective sol-
vation, the ability of the solvent to form stronger intermolecu-
lar H-bonds with a particular tautomeric form along with the in-
–
tramolecular hydrogen bonding between C N or N = N and OH
groups which generates several tautomeric forms as shown in
Fig. 4(a). These diffrent tautomeric forms are generally instable
[82]. Regarding the spectral properties of the studied molecule, we
noticed that the maximal absorption bands emerged at the same
optical range for all the solvents except for some bands for the
spectrum record in Acetonitrile.
In contrast, in the protic solvent ethanol obvious anomalies
were observed, and the title dye with intramolecular hydrogen
bonding, absorb at longer wavelengths in ethanol than in Acetoni-
We also notice that the shorter wavelength UV band suffers a
small shift on changing the solvent. Among studied solvent’s ef-
8

M. Chaabene, S. Agren, J.E. Haskouri et al.
Journal of Molecular Structure 1244 (2021) 130962
Table 6
Calculated and experimental electronic transition parameters of the Azoic compound by a CAM-B3LYP-D3/6–311G++
method in different solvents.
Solvents
λ
(nm)
λ
(nm)
f (a.u)
Major and minor contributions (%)
Exp
Cal
Acetonitrile
210
234
277
416
476
217
264
308
421
484
222
259
318
424
485
—-
201
258
307
417
452
202
263
309
423
455
202
264
311
428
457
202
259
308
420
454
202
258
307
418
452
202
259
308
420
454
202
264
310
428
457
200
258
302
429
455
0.313
0.250
0.230
0.299
0.157
0.460
0.243
0.273
0.283
0.178
0.481
0.254
0.283
0.257
0.183
0.412
0.234
0.260
0.293
0.172
0.333
0.248
0.236
0.299
0.160
0.416
0.236
0.256
0.290
0.169
0.504
0.278
0.298
0.263
0.193
0.322
0.217
0.322
0.218
0.100
H-1 →L + 2 (25%) H-2 →L + 1 (25%) H-3 →L + 1 (22%)
H →L + 2 (50%) H-2 →L (30%) H →L + 1 (29%)
H-1 →L (40%) H →L + 2 (21%) H-2 →L (36%)
H →L (58%) H-1 →L (17%) H-3 →L (25%)
H →L (58%)
Chloroform
Cyclohexane
Dichloromethane
Ethanol
H-2 →L + 1 (39%) H-1 →L + 1 (21%) H →L + 2 (13%)
H →L + 1 (54%) H →L + 2 (28%) H-2 →L (25%)
H-1 →L (43%) H →L + 2 (19%) H-2 →L (34%)
H →L (57%) H-1 →L (18%) H-3 →L (30%)
H →L (41%)
H-2 →L + 1 (44%) H →L + 2 (11%) H-1 →L + 1 (20%)
H →L + 1 (52%) H →L + 2 (30%) H-2 →L (26%)
H-1→L (43%) H-2 →L (36%) H →L + 1 (10%)
H →L (56%) H-1 →L (18%) H-3 →L (34%)
H →L (42%)
H-2 →L + 1 (34%) H-1 →L + 1 (23%) H-3 →L + 1 (25%)
H →L + 2 (48%) H →L + 1 (34%) H-2 →L (28%)
H-1 →L (43%) H-2 →L + 1 (12%) H-2 →L (34%)
H →L (57%) H-3 →L (29%) H-1 →L (18%)
H →L (40%)
—-
—-
—-
—-
212
259
305
420
483
212
247
280
419
482
224
263
304
433
485
—-
H-2 →L + 1 (27%) H-1 →L + 1 (25%) H →L + 2 (14%)
H →L + 2 (50%) H →L + 1 (30%) H-2 →L (30%)
H-1 →L (41%) H-2 →L (36%) H →L + 1 (10%)
H →L (58%) H-3 →L (26%) H-1 →L (18%)
H →L (40%)
THF
H-2 →L + 1 (34%) H-1 →L + 1 (23%) H →L + 2 (14%)
H →L + 2 (48%) H →L + 1 (33%) H-2 →L (28%)
H-2 →L (42%) H-2 →L (35%) H →L + 2 (20%)
H →L (57%) H-3 →L (29%) H-2 →L (18%)
H →L (40%)
Toluene
H-2 →L + 1 (44%) H-1 →L + 1 (19%) H →L + 2 (12%)
H →L + 1 (51%) H →L + 2 (32%) H-2 →L (27%)
H-1 →L (44%) H-2 →L (34%) H →L + 1 (10%)
H →L (55%) H-3 →L (34%) H-1 →L (19%)
H →L (42%)
Gas
H-1 →L + 3 (34%) H-2 →L + 1 (26%) H-2 →L + 3 (10%)
H →L + 3 (44%) H-2 →L (32%) H →L + 3 (25%)
H-1 →L (52%) H-2 →L (29%) H-3 →L (26%)
H →L (60%) H-3 →L (28%) H-1 →L (17%)
H →L (40%)
—-
—-
—-
—-
trile. The deviation between experiment and theory may be re-
sulted from solvent effects, where the solvent makes the molecule
chemical environment simulation become very complex [46].
havior of chemical systems and is associated with the stability
and reactivity of a chemical system [85]. The compound’s hardness
value is 1.400 eV in Acetonitrile. Accordingly, the molecule’s hard-
ness and stability decrease along with the increasing of the sol-
vent’s polarity in gas phase (1.342 eV) and in Cyclohexane solvent
(1.366 eV). Global softness (σ) is a molecule’s property that mea-
sures the extent of chemical reactivity [86]. Among the different
solvents, 1-arylazo-2-naphthol have the greatest softness values in
the gas phase than in Cyclohexane solvent while it has the weakest
value of softness in the Acetonitrile solvent.
3.5.2. Frontier molecular orbital analysis
We have carried out calculations in seven different solvents
(Acetonitrile, Chloroform, Cyclohexane, Dichloromethane, Ethanol,
THF and Toluene) and in gas phase in order to evaluate the sol-
vent’s polarity effect on the energy gap, HOMO and LUMO en-
ergies, chemical reactivity descriptors and dipole moment of the
studied Azoic dye. The computations were done by the CAM-
B3LYP/6–311++G (d, p) level using IEF-PCM model and the results
are given in Table 1. Hard molecules have a large HOMO–LUMO
energy gap whereas soft molecules have a small HOMO–LUMO en-
ergy gap. As shown in Table 1, the small HOMO–LUMO energy gap
corresponds to the title compound in gas phase (2.683 eV) then in
Cyclohexane solvent (2.732 eV) which means small excitation en-
ergies. Therefore, hard molecules, with a large gap, correspond to
the studied molecule in Acetonitrile solvent (2.799 eV), where their
electron density changed more hardly than a soft molecule [56].
Electronic chemical potential is defined as the negative of elec-
tronegativity of a molecule [53]. The reactivity increases along with
the increase of the chemical potential. Thus, the title molecule cal-
culated in Acetonitrile solvent is more reactive than in other sol-
vents. Global hardness is a useful concept to understand the be-
We can conclude from a close observation of Table 1 that the
HOMO–LUMO energy gaps, the molecule’s chemical potential and
global hardness, have the highest values in acetonitrile and these
values decrease along with the increasing of the solvent polar-
ity (which are cyclohexane solvent and gas phase). The frontier
molecular orbitals of azobenzene molecule before (Fig. 6(a)) and
after Ni capture (Fig. 6(b)) in acetonitrile solvent were calculated
with CAM-B3LYP-D3 method. Red and green colours represented
the positive and negative phases of HOMO and LUMO respectively.
It can be seen from this figure, that HOMO is localized mainly on
the naphthalene ring and it slightly shifted to the π conjugated
carbon atoms of the phenyl ring while the LUMO is mainly lo-
calized on the phenyl ring to which N = N- core is attached. The
studied azoic molecule shows strong delo-calization of both HOMO
and LUMO on the π-conjugation bridgecan. The qualitative picture
9

M. Chaabene, S. Agren, J.E. Haskouri et al.
Journal of Molecular Structure 1244 (2021) 130962
Fig. 6. Calculated frontier molecular orbitals of azobenzene molecule before (a) and after Ni capture (b) in acetonitrile solvent with CAM-B3LYP-D3 method.
Fig. 7. The optimized structures of the complexes formed between Azoic compound and Cu2+ (a), Mg2+ (b), Ni2+ (c) and Zn2+ (d) cations.
proves a flow of electron density (intramolecular charge transfer)
from the electron-donor group byway of the p-electron bridge to
the electron acceptor group (except for the hydroxyl group) [46].
will be discussed in this section. Structural and energetic parame-
ters in both gas and liquid phases are gathered in Table 7.
As represented in the previous figure, we notice that the best
adsorption mode of these different cations is located around the
strong hydrogen bonding zone. In particular, they are closer to
4. Cation binding studies
the nitrogen atom (N19 ) with a N19–Cu2+, N19 Mg2+, N19–Ni2+
−
and N19 -Zn2+ distances of 2.011, 2.093, 1.877 and 2.411 A respec-
˚
4.1. Structural, energetic and electronic analysis
tively in gas phase and of 1.996, 2.264, 1.857 and 2.211 respectively
in Acetonitrile solvent. It’s clear that the distances N19-cations
are smaller in the acetonitrile solvent than in the gas phase ex-
The optimized structures of azoic complexes (Azoic/Cu2+ (a),
Azoic/Mg2+ (b), Azoic/Ni2+ (c) and Azoic/Zn2+ (d)) are shown in
Fig. 7, where only complexes with the largest adsorption energy
cept for the distance N19 Mg2+ which is greater in this phase.
−
10

M. Chaabene, S. Agren, J.E. Haskouri et al.
Journal of Molecular Structure 1244 (2021) 130962
Table 7
˚
Distances of N19 -cations (in A), adsorption energies (Ead) for different complexes, HOMO, LUMO energies, gap energy (Eg), ꢀEg indicates the change of Eg after the cations
adsorption and dipole moment in Debye of metal cations on Azoic molecule in both gas and liquid phases.
dad, gas
dad, solv
(N19 -cations)
(N19 -cations)
ꢀEad, gas
(Kcal.mol−1
ꢀEad, solv
(Kcal.mol−1
EHOMO, solv
(eV)
ELUMO, solv
(eV)
˚
˚
Azoic
(A)
(A)
)
)
Eg
(eV)
ꢀEg
μD,solv (D)
solv
solv
Ni2+
Cu2+
Mg2+
Zn2+
1.877
2.011
2.093
2.411
1.857
1.996
2.264
2.211
−330.374
−309.335
−173.437
−242.196
−119.743
−79.854
−8.979
−7.140
−7.424
−6.391
−6.486
−5.278
−4.547
−3.980
−4.172
1.862
2.877
2.411
2.314
−33.476
2.787
10.118
7.043
−12.790
−17.328
26.440
20.862
−18.900
Table 8
Calculated charge by natural population analysis (QNBO) of some atoms of azoic complexes using B3LYP-D3/6–
311++G (d, p).
Atom
H7
C1
C2
N18
N19
C23
C24
H29
Cation32
Isolated Azoic
Azoic/Ni2+
Azoic/Cu2+
Azoic/Mg2+
Azoic/Zn2+
0.232
0.293
0.126
0.234
0.241
−0.178
−0.375
−0.089
−0.228
−0.236
0.084
0.219
0.111
0.126
0.129
−0.261
−0.214
−0.133
−0.206
−0.179
−0.182
−0.159
−0.109
−0.171
−0.126
0.087
0.100
0.046
0.095
0.091
−0.173
−0.138
−0.061
−0.128
−0.127
0.224
0.238
0.111
0.217
0.225
—-
1.126
0.463
1.017
1.075
Azoic/Ni2+, Azoic/Cu2+, Azoic/Zn2+ and Azoic/Mg2+ respectively in
theorectical spectra (Fig. 8(b)). The Changes in the position of this
band in experimental and theoretical azo benzene-complexe’s ab-
sorption spectra reffers essentially to metallic electronic structure
changes, geometry (valence orbital’s spatial arrangements), coordi-
nation numbers, effective charge and size, in addition to charge
distribution, azoic molecule orbital’s suitability to bonding to a
metal ion, the ligand steric demands and chelating effect, diffrent
metal–azo benzene interaction bond types as well as charge distri-
bution along the metal–azo benzene bond. All these propositions
also arise when the coordination object is not a separate metal-
lic ion or numerous combined ions [87]. Moreover, this red shift
clearly provides a very interesting index for detecting these cations
at sensor devices especially for the detection of Ni2+ cation. Be-
sides, we can notice from Table 9 that the most intense peak in the
theoretical spectra corresponds to the highest oscillator strength
(for example the most intense peak in the theoretical absorption
spectra of the isolated molecule Azoic and of the Azoic / Ni2+ com-
plex is located at 201 and 429 nm respectively with an important
oscillator forces (f) which are 0.313 and 0.503 a. u respectively).
Consequently, the oscillator strength is directly related to photon
absorption (absorption coefficient) and is inversely proportional to
the carrier lifetime. The oscillator strength is given by transition
moment integral that relates to the electronic dipole - allowed
transition.
According to MEP maps, this corresponds to an electrophilic at-
tack of the different cations on N19 atom (nucleophilic region).
Upon cations adsorption, atomic charge on N19 (see Table 8) de-
creases from about −0.182 in isolated azoic compound to −0.109
a.u. in azoic/Cu2+while all cations get positively charged and the
most positively charged cation is Ni2+ (1.126 a.u) (Fig. 4(b)). Thus,
we can say that the adsorption process of cations occurs through
metal coordination, i.e. σ bonding formation between N19-cations
moieties. This bond formation is made possible by an electron
transfer from the cations (Cu2+, Mg2+, Ni2+ and Zn2+) to the
azoic compound. Here, we developed the azoic dye as chemosen-
sor containing the hydroxyl and N = N- core as an electron ac-
ceptor group. While the naphthalene and the phenyl moiety act
as a donor site. Consequently, all cations are close to the strong
hydrogen-bonding zone through relatively clear chemisorption pro-
cess. The planarity of the azoic molecule is deformed, for each
Azoic/cations complex, where the presence of different cations
specially the Ni2+ close to the donating azotes atoms of the azoic
group of the studied compound tends to increase the charge den-
sity and the pre-organization of the benzen rings.
Data listed in Table 7 indicate that Azoic/cations complexations
are exothermic, as all calculated adsorption energies are negative.
For each, adsorption energies corrected for BSSE at the B3LYP-
D3/6–311++G (d, p) level of theory were simulated. The values of
adsorption energies indicate that the solvation effect is very impor-
tant, especially the solvation energies of cations are important and
decrease the interaction energy significantly from gas phase to the
liquid one. Calculations show that the most stable complex, in both
gas and liquid phase, is Azoic/Ni2+ where the adsorption energy is
found to be −330.374 kcal/mol and −119.743 kcal/mol respectively.
Azoic is consequently the most reactive compound with Ni2+ then
Cu2+ respectively.
Furthermore, the energy gap of HOMO-LUMO is an impor-
tant value which serves as a stability index. In our case, the en-
ergy gap of azoic compound was reduced after the adsorption of
Ni2+, Mg2+ and Zn2+ cations and it was slightly increased af-
ter the adsorption of Cu2+ cation (Table 7). It implies that the
Azoic/Ni2+, Azoic/Mg2+and Azoic/Zn2+ complexes become more
conductive (with lower molecular stability and high reactivity in
chemical reactions) compared to isolated azoic molecule, although
Azoic/Cu2+complex become more stable compared to isolated azoic
compound in the acetonitrile solvent. We can also note that the
adsorption of these cations does not change the nature of transi-
tion which remains a π → π∗ transition. The maximum absorp-
tion wavelength corresponds to the electronic transition from the
HOMO to LUMO with a contribution of about 50–60% for all com-
plexes.
4.2. UV–Vis absorption spectra
The optical absorption spectra of Azoic/cations complexes are
studied experimentally and theoretically by using TD-DFT/CAM-
B3LYP-D3/6–311G++ (d, p) method. Fig. 8 shows spectra for azoic
compound before and after cations adsorption in the acetonitrile
solvent and their wavelengths are given in Table 9. One can no-
tice that after the adsorption of different cations on the azoic
compound, the last peak is red shifted in both experimental and
4.3. Vibrational analysis
theoretical spectra by about 12, 4, 3 and 3 nm for Azoic/Ni2+
,
Azoic/Cu2+, Azoic/Zn2+ and Azoic/Mg2+ respectively in experimen-
As the intensities and the vibrational frequency positions are
very sensitive to changes in the molecular structure, the IR spec-
tal spectra (Fig. 8(a)) and by about 208, 153, 65 and 59 nm for
11

M. Chaabene, S. Agren, J.E. Haskouri et al.
Journal of Molecular Structure 1244 (2021) 130962
Fig. 8. Calculated absorption spectrum of Azoic compound before and after cations adsorption at 6–311++ G (d, p) basis set: (a) experimental and (b): theoretical.
Table 9
The experimental and theoretical of the azo molecule before and after the adsorption of the different cations in the acetonitrile solvent.
Isolated Azoic
Azoic/Ni2+
Azoic/Cu2+
Azoic/Zn2+
Azoic/Mg2+
λ
λ
λ
λ
λ
λ
λ
λ
λ
λ
Cal
Exp
Exp
Exp
Exp
Exp
Cal
Cal
Cal
Cal
(nm)
(nm)
f (a.u)
(nm)
(nm)
f (a.u)
(nm)
(nm)
f (a.u)
(nm)
(nm)
f (a.u)
(nm)
(nm)
f (a.u)
210
234
277
416
476
201
258
307
417
452
0.313
0.299
0.230
0.250
0.157
230
282
318
423
488
272
347
429
511
660
0.069
0.220
0.503
0.030
0.050
230
281
310
421
480
368
378
398
471
605
0.052
0.249
0.032
0.135
0.122
230
265
311
422
479
214
257
316
380
517
0.689
0.106
0.350
0.057
0.240
230
282
311
421
479
213
275
319
385
511
0.315
0.129
0.257
0.017
0.326
tra can be used to detect the adsorption of Cu2+, Mg2+, Ni2+ and
Zn2+ cations on azoic dye. We present in Fig. 9 the calculated IR
spectra before and after adsorption of these cations. One can ob-
serve an important difference between the spectra of isolated azoic
and azoic/cations. The frequency of (CH oopb+OH oopb+ CC oopb)
at 756 cm−1 is red-shifted after the adsorption of these cations.
The (CH oopb+ CN oopb+ COH twisting) at 832 cm−1 is the same
before and after cations adsorption but its intensity grows a lit-
tle from 128 km/mol to 171 km/mol. Upon cations adsorption, the
mixed CH ipb+ CC str+ CN str mode of azoic dye at 1206 cm−1
is slightly red-shifted by about 6 cm−1 (1200 cm−1) while its in-
tensity grows from 15 km/mol to 285, 257, 90 and 25 Km/mol
for azoic/Ni2+, azoic/Cu2+, azoic/Zn2+ and azoic/Mg2+ respectively
(Fig. 9(a)). The (CC str+CH r+ CO str+ CN str) mode is blue-shifted
12

M. Chaabene, S. Agren, J.E. Haskouri et al.
Journal of Molecular Structure 1244 (2021) 130962
Fig. 9. IR spectra of azobenzene compound before and after Cations adsorption with an enlargement in both area (a) and (b) using B3LYP-D3/6–311++G (d, p) method.
from 1260 cm−1 to 1279, 1267,1271 and 1266 cm−1 with an inten-
sity enhancement from 88 to 333, 160, 290 and 219 km/mol af-
ter the adsorption of Ni2+, Cu2+, Zn2+ and Mg2+ respectively. The
band at 1400 cm−1 for azoic dye due to several modes (Table 4)
and we notice an increase in intensity after cations adsorption,
where the intensity grows from 70 km/mol for isolated azoic
azoic dye to 37, 259, 35 and 99 Km/mol for azoic/Ni2+, azoic/Cu2+
,
azoic/Zn2+ and azoic/Mg2+. The infrared spectra of azoic dye and
their complexes with different cations, clearly show a significant
effect on the vibrational spectra after adsorption of these cations
for all complexes studied here. These frequencies’ change of fre-
quency can be related to the interaction between the title cations
and the hydrogen bonding zone.
dye to 104, 601, 171 and 230 Km/mol for azoic/Ni2+, azoic/Cu2+
,
azoic/Zn2+ and azoic/Mg2+ respectively. The band at 1480 cm−1
occurs because of several modes in this region of the spectrum.
It is slightly blue-shifted (11–8 cm−1) after adsorption of different
cations. The band at 1553 cm−1, blue-shifted after adsorption is
also due to several active modes. This important change in the IR
spectrum constitutes a useful index to detect the binding of cations
on azoic dye. The band at 3176–3217 cm−1, red-shifted after the
adsorption of all cations. The peak which appears at 3354 cm−1, is
blue-shifted after the adsorption of Ni2+, Zn2+ and Mg2+ by about
180, 200 and 292 cm−1 respectively, while it is red-shifted after
the adsorption of Cu2+ by about 42 cm−1 (Fig. 9(b)). The intensi-
ties of these bands decrease in a remarkable way after the adsorp-
tion of these cations, where they pass from 26 Km/mol for isolated
5. Conclusion
In summary, we have described the synthesis of I-phenyl azo-2-
naphthol and we have studied the optoelectronic properties of the
mentioned compound experimentally and theoretically in differ-
ent solvents. The MEP, NPA and NBO were computed using B3LYP-
D3/6–311++G (d, p) method to understand the atomic charge
distribution of the azoic dye and to identify the way in which
the molecules interact with each other. The most stable geome-
try was characterized for the azobenzene in Acetonitrile. The vi-
brational frequencies of the fundamental modes of this molecule
have been accurately assigned and investigated in gas phase. The
13

M. Chaabene, S. Agren, J.E. Haskouri et al.
Journal of Molecular Structure 1244 (2021) 130962
absorption spectra were carried out by TD-DFT method in gas
and other solvents and compared with the experimental one. In
order to assess the potential role of the synthesized azoic dye
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Mg2+, Ni2+ and Zn2+ adsorbed complexes. A clear sigma bond-
ing chemisorption of cations on N19 atom is observed for all com-
plexes, followed by a charge transfer from the cations (Cu2+, Mg2+
,
Ni2+ and Zn2+) to the strong hydrogen bonding zone. Adsorp-
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tween I-phenyl azo-2-naphthol and different cations. Results in-
dicate that the azoic/Ni2+ is the most favorable complex in both
gas and liquid phase, where the adsorption energy is found to
be −330.374 kcal/mol and −119.743 kcal/mol respectively. Further-
more, after cations chemisorption on azoic compound, a remark-
able red shift is observed in the optical spectra, especially with
azoic/Ni2+ complex, offering thus a very good index to detect the
binding of Ni2+. The IR spectra analysis also shows a significant
change in the signal strengths and vibrational frequencies. The op-
tical spectra, energy gap and the vibrational spectra can therefore
be used to detect the presence of different cations and azoic dye
show appropriate properties to constitute good Ni2+ sensors.
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Declaration of Competing Interest
[17] Y.-.F. Sun, J.-.J. Li, F. Xie, Y. Wei, M. Yang, Ruthenium-loaded cerium diox-
ide nanocomposites with rich oxygen vacancies promoted the highly sensi-
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The authors declare that they have no conflict of interest.
CRediT authorship contribution statement
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Marwa Chaabene: Data curation, Software, Writing - original
draft. Soumaya Agren: Investigation, Software, Writing - original
draft. Jamal El Haskouri: Data curation, Software. Abdul-Rahman
Allouche: Formal analysis, Resources. Lahcini Mohamed: Fund-
ing acquisition, Validation. Rafik Ben Chaâbane: Methodology,
Validation. Mohamed Hassen V Baouab: Project administration,
Methodology.
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Simultaneous detection of trace Ag(I) and Cu(II) ions using homoepitaxially
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Acknowledgements
The authors greatly acknowledge financial support between co-
operation Tunisia-Morocco for funding the work through the re-
search group N° 17/TM10.
[22] S. Benkhaya, S. M’rabet, A. El Harfi, Classifications, properties, Recent
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