Synthesis, geometry optimization, spectroscopic investigations (UV/Vis, excited states, FT-IR) and application of new azomethine dyes

Article abstract of DOI:10.1016/j.molstruc.2017.07.036

In the present work, the quantum theoretical calculations of the molecular structures of the four new synthesized azomethine dyes such as: 1. (E)-N-(4-butoxybenzylidene)-4-((E)-phenyldiazenyl)aniline (PAZB-6),2. (E)-N-(4-(benzyloxy)benzylidene)-4-((E))-ph

Full text of DOI:10.1016/j.molstruc.2017.07.036

Accepted Manuscript
Synthesis, geometry optimization, spectroscopic investigations (UV/Vis, excited
states, FT-IR) and application of new azomethine dyes
Siyamak Shahab, Masoome Sheikhi, Liudmila Filippovich, Rakesh Kumar, Evgenij
Dikusar, Hooriye Yahyaei, Mehrnoosh Khaleghian
PII:
S0022-2860(17)30969-9
10.1016/j.molstruc.2017.07.036
MOLSTR 24063
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 3 June 2017
Revised Date: 15 July 2017
Accepted Date: 16 July 2017
Please cite this article as: S. Shahab, M. Sheikhi, L. Filippovich, R. Kumar, E. Dikusar, H. Yahyaei, M.
Khaleghian, Synthesis, geometry optimization, spectroscopic investigations (UV/Vis, excited states,
FT-IR) and application of new azomethine dyes, Journal of Molecular Structure (2017), doi: 10.1016/
j.molstruc.2017.07.036.
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Graphical Abstract
R
R1
NH2
CHO
N
N
H
R1
R
N
2
N
MeOH, AcOH
N
1
PAZB-6
PAZB-7
PAZB-8
PAZB-9

ACCEPTED MANUSCRIPT
Title Page Information
Sub: Submission of manuscript for full paper in Journal of Molecular Structure
Dear Sir,
Please find herewith the enclosed manuscript:
Article title:
Synthesis, Geometry Optimization, Spectroscopic Investigations (UV/Vis, Excited States, FT-IR)
and Application of New Azomethine Dyes
The name of journal
Journal of Molecular Structure
The author’s affiliations with full postal address
Rakesh Kumar
Department of Chemistry, DAV University, Jalandhar -144012 (Punjab) INDIA
Email: rinkumeister@gmail.com
Siyamak Shahab
Institute of Physical Organic Chemistry
National Academy of Sciences of Belarus, 13 Surganov Str., Minsk 220072
E-mail: siyamak.shahab@yahoo.com
Liudmila Filippovich
Institute of Physical Organic Chemistry
National Academy of Sciences of Belarus, 13 Surganov Str., Minsk 220072
E-mail: luda1977@list.ru
Evgenij Dikusar
Institute of Physical Organic Chemistry
National Academy of Sciences of Belarus, 13 Surganov Str., Minsk 220072
E-mail: dikusar@ifoch.bas-net.by
Masoome Sheikhi
Young Researchers and Elite Club, Gorgan Branch, Islamic Azad University, Gorgan, Iran
E-mail: m.sheikhi2@gmail.com
Hooriye Yahyaei
Department of Chemistry, Zanjan Branch, Islamic Azad University, Zanjan, Iran
E-mail: Hooriye_Yahyaei@yahoo.com
Mehrnoosh Khaleghian
Department of Chemistry, Islamshahr Branch, Islamic Azad University, Islamshahr, Iran
E-mail: mehr_khaleghian@yahoo.com
1

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Synthesis, Geometry Optimization, Spectroscopic Investigations (UV/Vis, Excited States,
FT-IR) and Application of New Azomethine Dyes
Siyamak Shahaba,b, Masoome Sheikhic, Liudmila Filippovicha,b, Rakesh Kumard, Evgenij Dikusara,
Hooriye Yahyaeie, Mehrnoosh Khaleghianf
aInstitute of Physical Organic Chemistry, National Academy of Sciences of Belarus,13 Surganov Str., Minsk 220072
bInstitute of Chemistry of New Materials, National Academy of Sciences of Belarus, 36 Skarina Str., Minsk 220141
cYoung Researchers and Elite Club, Gorgan Branch, Islamic Azad University, Gorgan, Iran
dDepartment of Chemistry, DAV University, Jalandhar - 144012 (Punjab) India
eDepartment of Chemistry, Zanjan Branch, Islamic Azad University, Zanjan, Iran
fDepartment of Chemistry, Islamshahr Branch, Islamic Azad University, Islamshahr, Iran
Abstract
In the present work, the quantum theoretical calculations of the molecular structures of the four
new synthesized azomethine dyes such as:
1.(E)-N-(4-butoxybenzylidene)-4-((E)-phenyldiazenyl)aniline (PAZB-6),
2. (E)-N-(4-(benzyloxy)benzylidene)-4-((E))-phenyldiazenyl)aniline (PAZB-7),
3. 4-((E)-4-((E)-phenyldiazenyl)phenyl)imino)methyl)phenol (PAZB-8),
4. (E)-N-(4-methoxybenzylidene)-4-((E))-phenyldiazenyl)aniline (PAZB-9)
have been predicted using Density Functional Theory in the solvent Dimethylformamide. The
geometries of the azomethine dyes were optimized by PBE1PBE/6-31+G* level of theory. The
electronic spectra of the title compounds in the solvent DMF was carried out by TDPBE1PBE/6-
31+G* method. FT-IR spectra of the title compounds are recorded and discussed. Frontier
molecular orbitals, molecular electrostatic potential, electronic properties, natural charges and
Natural Bond Orbital (NBO) analysis of the mentioned compounds were investigated and
discussed by theoretical calculations. The azomethine dyes were synthesized after quantum
chemical modeling for optical applications. A new study of anisotropy of thermal and electrical
conductivity of the colored stretched PVA-films have been undertaken
Keywords: Azomethine dye, DFT calculation, UV/Vis spectrum, polarizing film, anisotropy of thermal, electrical
conductivity.
Corresponding author: m.sheikhi2@gmail.com (M. Sheikhi); Hooriye_Yahyaei@yahoo.com (H. Yahyaei)
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1. Introduction
Some organic compounds can cause electro-optic effects and produce nonlinear optical
responses due to the large delocalization of π electrons compared with the inorganic compounds
[1]. In the other words, the organic compounds including certain functional groups
(chromophores) can absorb ultraviolet and visible radiations. In order to produce the organic
nonlinear optical (NLO) molecules are used donor and/or acceptor substituents such as azo- (-
N=N-), amino- (-NH2), nitro- (NO2), cyano- (-CN), hydroxy- (-OH) and etc. One of the most
important organic dyes are compounds containing nitrogen as the azo (-N=N-) group [2]. The
azo dyes use as colorants in the textile, paper, cosmetics, drugs, foods coloring and other
consumer goods. Azomethines have many applications, for example, they are used in
photoluminescence materials [3], optical materials and devices [4], organic light-emitting diodes
[5], photovoltaic cells [6] and polarizing films [4,7,8]. The uniaxially oriented polymer sheets
containing dichroic material such as azomethines are used in the development of polarizing films
[7,8]. These structures are beneficial for achieving higher dichroism due to their linear structure
and anisotropic transition moment. Azomethines also widely used for photonic applications like
lasers, spectroscopic analysis, liquid crystal devices and optoelectronic systems [3-8].
Theoretical quantum chemistry methods based on HF and DFT are widely used for the
calculation of optimized geometry, absorption spectrum, UV, IR and NMR spectra of the organic
molecules [3,6-15]. In recent years, computational chemistry has become an important tool for
chemists and a well-accepted partner for experimental chemistry. DFT method is a major tool in
the methodological arsenal of computational organic chemists. Also Time-Dependent Density
Functional Theory (TD-DFT) used for predicting the absorption spectra of the compounds [16-
18]. Geometry, vibrational spectra and excited states of the platinum-olefin complexes and
benznidazole have been investigated using the DFT method in [19,20]. We have investigated the
molecular geometry optimizations and absorption bands in UV/Vis spectrum of Azure A
chloride by DFT and TD-DFT methods and it has been concluded that experimental data are in
excellent agreement with theoretical results [12]. We also have previously reported the results of
TD-DFT calculations for the oxazine dyes [13,14]. In the current study, we designed four new
azomethine dyes and modeled their geometrical parameters, UV/Vis and IR spectra by
DFT/PBE1PBE method with 6-31+G* basis set. Frontier molecular orbitals, detail of quantum
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molecular descriptors, molecular electrostatic potential, natural charge and natural bond orbital
analysis of the new compounds were calculated.
2. Equipment and methods
2.1. Synthesis of the compounds PAZB-6, PAZB-7, PAZB-8, PAZB-9
R
NH2
R1
CHO
N
H
R1
R
N
2
N
MeOH, AcOH
N
N
1
PAZBꢀ6
PAZBꢀ7
PAZBꢀ8
PAZBꢀ9
Fig. 1. Scheme of synthesis of the compounds PAZB-6, PAZB-7, PAZB-8, PAZB-9
PAZB-6: A mixture of 4-aminoazobenzne (1) (2.5 mmol) and aldehyde (2) (2.8 mmol) in 34 ml
methanol was prepared. Then 2 ml of glacial acetic acid was added to the mixture and was
refluxed for 1 h. The hot solution was filtered through a paper filter, cooled and left for 15 h. at
0-5 °C. The solid was dissolved in cold methanol and washed with water and dried at 19-21 °C.
PAZB-7: A mixture of 4-aminoazobenzne (1) (2.6 mmol) and aldehyde (2) (2 mmol) in 38 ml
methanol was prepared. Then 3 ml of glacial acetic acid was added to the mixture and was
refluxed for 2 h. The hot solution was filtered through a paper filter, cooled and left for 10 h. at
0-5°C. The solid was dissolved in cold methanol and washed with water and dried at 22-24 °C.
PAZB-8: A mixture of 4-aminoazobenzne (1) (2.8 mmol) and aldehyde (2) (2.6 mmol) in 34 ml
methanol was prepared. Then 3 ml of glacial acetic acid was added to the mixture and was
refluxed for 4 h. The hot solution was filtered through a paper filter, cooled and left for 7 h. at 10
°C. The solid was dissolved in cold methanol and washed with water and dried at 23-25 °C.
PAZB-9: A mixture of 4-aminoazobenzne (1) (2.0 mmol) and aldehyde (2) (2.9 mmol) in 31 ml
methanol was prepared. Then 2 ml of glacial acetic acid was added to the mixture and was
refluxed for 4 h. The hot solution was filtered through a paper filter, cooled and left for 7 h. at 10
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°C. The solid was dissolved in cold methanol and washed with water and dried at 27-29 °C (Fig.
1).
2.2. Computational details
The entire quantum chemical calculations were performed using the Gaussian 09W software
programs [21] on a Pentium IV/4.28 GHz personal computer. The geometry of the title
molecules: PAZB-6, PAZB-7, PAZB-8, PAZB-9 was fully optimized using Gaussian 09W
program by gradient geometry optimization. The frequencies and structural parameters of the
four new azomethine compounds were calculated using functional PBE1PBE (One-parameter
hybrid PBE functional incorporating 25% HF exchange) with 6-31+G* basis set for the
optimized geometries in the solvent DMF. Without any constraint on the geometry the energies
of the mentioned molecules were minimized, whole intramolecular forces were brought to be
zero. We were also used TD-DFT for computing the electronic transitions of the new azomethine
structures. The Polarized Continuum Model (PCM) [15] was used for calculations of the solvent
effect. The theoretical absorption spectrum of all the molecules in the solvent DMF was
calculated using TDPBE1PBE/6-31+G* method. Also, we has calculated the theoretical IR
spectra of the optimized structures using PBE1PBE/6-31+G* method. The electronic properties
such as dipole moment (ꢀD), point group, EHOMO, ELUMO, HOMO-LUMO energy gap and natural
charges were calculated [16]. The optimized molecular structures, HOMO and LUMO surfaces
were visualized using GaussView 05 program [22]. The electronic structure of the mentioned
compounds was studied by using Natural Bond Orbital (NBO) analysis [16] at the PBE1PBE/6-
31+G* level of energy in order to understand hyperconjugative interactions and charge
delocalization.
2.3. Reagent and apparatus
All chemicals used were of analytical reagent grade. PVA used in this work «Mowiol 28-99»
was purchased from Hoёchst Akiengesllschaft Co. (Germany). The experimental UV absorption
spectra of the title molecules were recorded on UV-Visible Spectrophotometer Cary 300 (Varian,
USA). Experimental IR-spectra of films was recorded in the frequency region 400-4000 cm-1 on
a Spectrophotometer of Protégé 460 (Nicolet, US). The experimental FT-IR spectrum was
recorded by KBr pellet method with spectral resolution of 2 cm-1.
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Thermal and electrical conductivity of films was measured on the complex equipment LC – 201
(Alfa Laval Group, Sweden) using indicator method of determining the thermal and electrical
conductivity of polymer materials and thin films.
2.4. Preparation of PVA-films
The PVA-films were prepared from (wt. %) 10 PVA solution containing 0.01-0.03 PAZB-6,
PAZB-7, PAZB-8, PAZB-9 (0.10-0.30 in film), 0.01 boric acid (H3BO3), 6.0 ethyl alcohol
(C2H5OH) and water. An initial composition was prepared by dissolving PVA in distilled water
and ethyl alcohol. The composition was mixed at temperature 85-90 °C. The compounds PAZB-
6, PAZB-7, PAZB-8, PAZB-9 and additives were added after 3 h. after starting of heating PVA
solution at intervals 20 min. The mixture was heated for 3 h. The hot solution was filtered
through two layers of technical nylon. Deaeration was occurred during 12-15 h. The composition
was cast on the polished glasses and dried in the closed box at temperature 20-22 °C. Uniaxial
orientation was done in the 4 % boric acid solution at temperature 42-45 °C. The washed film
was dried for 30 min. at temperature 60-63 °C. The value of Stretching Degree (Rs) was
determined as the ratio between length of the films after and before (laft / lbef) uniaxial
orientation. The thickness of the resulting films was between 50 to 55 µm. The film thickness
was measured with a micrometer with an accuracy of ± 5µm (GS SSSR 6507-90).
Accuracy of measurements in this research limits the accuracy of the results:
- Sample of the starting composition for preparation of solutions and films for casting taken on
an analytical balance accurate to 0.0005 g;
- Temperature measurement error was:
± 2.0 °C in the preparation of polymer solutions with additives,
± 1.0 °C in the chemical treatment of the films,
± 3.0 °C at thermal fixation and drying films;
- Relative error of spectrophotometric analysis did not exceed 1.0 % and thermal conductivity
0.1 %.
3. Results and discussion
3.1. Optimized structure of the compounds PAZB-6, PAZB-7, PAZB-8, PAZB-9
We applied conformational analysis for four molecules PAZB-6, PAZB-7, PAZB-8, PAZB-9
and reach to the most stable conformations. Then, we fully optimized the most stable
conformations. We have carried out quantum chemical calculations and have optimized structure
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of the four compounds: PAZB-6, PAZB-7, PAZB-8, PAZB-9 using DFT (PBE1PBE) method
with 6-31+G* basis set by the Gaussian 09W program package. The calculations were performed
in the solvent DMF using the IEFPCM (Integral Equation Formalism PCM) method coupled to
UAKS radii. It employs a molecule shaped cavity composed of spheres centered on the nuclei,
while the reaction field is modeled by placing charges on the cavity surface. The Integral
Equation Formalism PCM, by Cances, Mennucci and Tomasi is the most popular PCM version.
The optimized molecular structures and parameters of the studied structures are shown in Fig. 2
and Tables 1S,2S,3S,4S.
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Fig. 2. Optimized molecular structures of the compounds PAZB-6, PAZB-7, PAZB-8, PAZB-9 calculated
by PBE1PBE/6-31+G* method.
3.2. Electronic Structure and Excited States of the compounds PAZB-6, PAZB-7, PAZB-8,
PAZB-9
We used the Time-Dependent Density Functional Theory (TD-DFT) for predicting the
absorption spectra of the compounds PAZB-6, PAZB-7, PAZB-8, PAZB-9. The theoretical
absorption spectra of the optimized compounds were calculated in the solvent DMF using the
TDB3LYP/6-31G, TDB3LYP/6-31G*, TDB3LYP/6-31+G* and TDB3LYP/6-31++G*,
TDPBE1PBE/6-31G, TDPBE1PBE/6-31G*, TDPBE1PBE/6-31+G*, TDPBE1PBE/6-31++G*
methods. The results of calculations were showed that the TDPBE1PBE/6-31+G* method is better
correlated with the experimental data. We considered 20 excited states and the computational
equations were performed using the IEFPCM (Integral Equation Formalism PCM) method
coupled to UAKS radii. The accurate value of absorption wavelength is obtained using the TD-
DFT method. 20 excited states for the compounds PAZB-6, PAZB-7, PAZB-8, PAZB-9 is
reported in Table 1. In addition, Fig. 3 show molecular orbitals involved in formation of
absorption spectrum of the compounds PAZB-6, PAZB-7, PAZB-8, PAZB-9 at λmax
.
Table 1. Electronic absorption spectrum of the compounds PAZB-6, PAZB-7, PAZB-8, PAZB-9
calculated by TDPBE1PBE/6-31+G*.
PAZB-6
Configurations Composition
(corresponding transition orbitals)
Excited Wavelength
Excitation
Energy (eV)
2.64
Oscillator
Strength (f)
0.00
State
S1
S2
(nm)
469
411
333
0.67(H-2→L) + 0.15(H-2→L+1)+ 0.13(H-1→L)
0.69(H→L)
- 0.12(H-2→L) + 0.65(H-1→L) + 0.19(H→L+1)
0.10(H-6→L+1) + 0.10(H-3→L) - 0.22(H-1→L) -
0.14(H-1→L+1) + 0.61(H→L+1)
0.20(H-6→L) - 0.31(H-4→L) + 0.55(H-3→L) -
0.10(H→L+1) - 0.15(H→L+2)
- 0.19(H-6→L) + 0.53(H-4→L) + 0.38(H-3→L)
+ 0.12(H→L+4)
0.14(H-7→L) - 0.11(H-7→L+1) + 0.57(H-6→L)
+ 0.24(H-4→L) + 0.21(H→L+1)
0.61(H-5→L) - 0.22(H-5→L+1) - 0.11(H-1→L+3) -
0.22(H→ L+3)
3.01
3.72
1.65
0.04
S3
S4
S5
S6
S7
S8
308
301
299
286
277
4.01
4.12
4.14
4.33
4.47
0.02
0.02
0.00
0.17
0.00
- 0.18(H-6→L) - 0.14(H-6→L+1) - 0.11(H-5→L) -
0.12(H-4→L+1) + 0.59(H-1→L+1) +0.15(H→L+1)
- 0.15(H-2→L) + 0.67(H-2→L+1) - 0.10(H-2→L+5)
0.31(H-5→L) + 0.26(H-5→L+1) + 0.16(H-1→L+3) +
0.16(H→L+2) + 0.50(H→L+3)
S9
269
265
253
250
4.60
4.67
4.89
4.95
0.24
0.00
0.06
0.01
S10
S11
S12
0.55(H-7→L) - 0.16(H-6→L) - 0.12(H-1→L+1) -
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0.17(H→L+1) - 0.20(H→L+2) + 0.14(H→L+3)
- 0.12(H→L+4) + 0.10(H→L+5)
0.22(H-7→L) - 0.16(H-4→L) + 0.21(H-4→L+1) -
0.11(H-1→L+1) + 0.53(H→L+2) - 0.14(H→L+3)
+ 0.17(H→L+4)
0.12(H-6→L+1) - 0.13(H-3→L) - 0.12(H-3→L+1) -
0.10(H-3→L+5) - 0.16(H-1→L+4) - 0.22(H→L+2)
+ 0.57(H→L+4)
S13
S14
248
238
5.01
5.21
0.08
0.03
0.23(H-7→L) + 0.18(H-7→L+1) + 0.54(H-4→L+1) -
0.18(H-3→L+1) + 0.16(H-1→L+1) + 0.16(H→L+2)
0.67(H-2→L+2) - 0.15(H-2→L+3) + 0.12(H-1→L+2)
0.13(H-7→L) + 0.17(H-7→L+1) + 0.56(H-6→L+1)
- 0.25(H-4→L+1) + 0.12(H-1→L+1) - 0.15(H→L+4)
0.69(H-2→L+4) + 0.12(H-1→L+4)
0.48(H-5→L+1) + 0.28(H-1→L+3) - 0.38(H→L+3)
0.14(H-4→L+1) + 0.60(H-3→L+1) + 0.12(H-1→L+2)
- 0.18(H→L+4) - 0.15(H→L+5)
S15
S16
S17
235
234
232
5.26
5.29
5.34
0.01
0.00
0.00
S18
S19
230
229
5.40
5.42
0.00
0.04
S20
221
5.60
0.00
PAZB-7
Configurations Composition
(corresponding transition orbitals)
Excited Wavelength
Excitation
Energy (eV)
2.64
Oscillator
Strength (f)
0.00
State
S1
S2
(nm)
470
410
330
- 0.41(H-2→L) + 0.55(H-1→L)+ 0.12(H-1→L+1)
0.69(H→L)
3.02
3.75
1.73
0.02
S3
0.51(H-2→L) + 0.39(H-1→L) - 0.20(H→L+1)
0.10(H-8→L+1) - 0.10(H-6→L) - 0.11(H-5→L) +
0.20(H-2→L) + 0.15(H-1→L) + 0.60(H→L+1)
0.18(H-8→L) - 0.32(H-6→L) + 0.55(H-5→L) -
0.12(H→L+1) + 0.15(H→L+4)
- 0.17(H-8→L) + 0.53(H-6→L) + 0.38(H-5→L)
+ 0.12(H→L+5)
0.14(H-9→L) - 0.12(H-9→L+1) + 0.55(H-8→L)
+ 0.16(H-7→L) + 0.23(H-6→L) + 0.22(H→L+1)
- 0.19(H-8→L) + 0.59(H-7→L) - 0.21(H-7→L+1)
- 0.21(H→ L+3)
S4
S5
S6
S7
S8
308
301
300
286
276
4.02
4.11
4.13
4.32
4.48
0.02
0.02
0.00
0.18
0.00
S9
S10
272
270
4.56
4.58
- 0.70(H-3→L)
- 0.69(H-4→L)
0.00
0.00
- 0.10(H-9→L) + 0.15(H-8→L) + 0.15(H-8→L+1) +
0.13(H-7→L) + 0.12(H-6→L+1) + 0.44(H-2→L+1) +
0.38(H-1→L+1) - 0.14(H→L+1)
S11
267
4.64
0.30
S12
S13
266
252
4.65
4.91
- 0.43(H-2→L+1) - 0.13(H-1→L) + 0.52(H-1→L+1)
- 0.13(H-9→L) + 0.29(H-7→L) + 0.25(H-7→L+1) -
0.12(H-2→L+3) + 0.50(H→L+3)
0.00
0.07
0.55(H-9→L) - 0.18(H-8→L) - 0.14(H-8→L+1) +
0.15(H-2→L+1) + 0.10(H-1→L+1) + 0.21(H→L+4)
- 0.10(H→L+5) + 0.11(H→L+7)
S14
250
4.96
0.01
- 0.19(H-9→L) + 0.16(H-6→L) + 0.23(H-6→L+1) -
0.11(H→L+3) + 0.54(H→L+4) - 0.17(H→L+5)
- 0.18(H-2→L+2) - 0.14(H-1→L+2) + 0.65(H→L+2)
0.13(H-8→L+1) - 0.17(H-5→L) + 0.13(H-5→L+1)
- 0.17(H-5→L+7) + 0.56(H-1→L+5) + 0.25(H→L+4)
+ 0.12(H→L+5) + 0.15(H→L+6)
S15
S16
247
242
5.02
5.12
0.08
0.00
S17
237
5.22
0.04
0.23(H-9→L) + 0.17(H-9→L+1) + 0.54(H-6→L+)
- 0.17(H-5→L+1) - 0.13(H-2→L+1) -0.10(H-1→L+1)-
0.16(H→L+4)
S18
S19
235
234
5.26
5.29
0.02
0.00
0.11(H-2→L+3) -0.42(H-2→L+4) - 0.13(H-1→L+3) +
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0.53(H-1→L+4)
0.14(H-9→L) + 0.17(H-9→L+1) +0.53(H-8→L+1) +
0.15(H-7→L+1) - 0.24(H-6→L+1) - 0.10(H-2→L+1) -
0.18(H→L+5)
S20
232
5.33
0.00
PAZB-8
Configurations Composition
(corresponding transition orbitals)
Excited Wavelength
Excitation
Energy (eV)
2.64
Oscillator
Strength (f)
0.00
State
S1
(nm)
470
408
326
0.68(H-1→L) + 0.15(H-1→L+1)
S2
3.04
3.80
0.70(H→L)
1.61
0.02
S3
0.63(H-2→L) + 0.24(H→L+1)
0.10(H-5→L+1) - 0.10(H-4→L)- 0.14(H-3→L) -
0.29(H-2→L) - 0.12(H-2→L+1)- 0.58(H→L+1)
- 0.19(H-5→L) + 0.32(H-4→L) + 0.55(H-3→L)
+ 0.13(H→L+1) - 0.16(H→L+3)
- 0.17(H-5→L) + 0.53(H-4→L) - 0.38(H-3→L)
- 0.13(H→L+4)
0.13(H-7→L) - 0.11(H-7→L+1) + 0.10(H-6→L)
+ 0.58(H-5→L) + 0.22(H-4→L) + 0.22(H→L+1)
0.60(H-6→L) - 0.22(H-6→L+1) - 0.12(H-5→L)
- 0.13(H-2→L+2) - 0.22(H→L+2)
- 0.16(H-1→L) + 0.68(H-1→L+1) - 0.10(H-1→L+5)
0.13(H-7→L) - 0.10(H-7→L+1) - 0.13(H-6→L)
- 0.15(H-5→L) - 0.17(H-5→L+1) - 0.14(H-4→L+1)
+ 0.57(H-2→L+1) + 0.15(H→L+1)
0.32(H-6→L) + 0.25(H-6→L+1) + 0.16(H-2→L+2)
+ 0.53(H→L+2)
0.53(H-7→L) - 0.6(H-5→L) - 0.13(H-5→L+1)
- 0.20(H-2→L+1) + 0.26(H→L+3) + 0.12(H→L+4)
+ 0.12(H→L+5)
- 0.24(H-7→L) + 0.16(H-4→L) + 0.23(H-4→L+1)
+ 0.16(H-2→L+1) + 0.52(H→L+3) + 0.15(H→L+4)
- 0.13(H-5→L+1) - 0.14(H-3→L) + 0.15(H-3→L+1) -
0.10(H-3→L+5) - 0.14(H-2→L+4) - 0.21(H→L+3)
+ 0.56(H→L+4)
S4
S5
S6
S7
307
301
299
286
4.03
4.12
4.13
4.33
0.01
0.02
0.00
0.14
S8
S9
275
266
4.51
4.65
0.00
0.00
S10
S11
S12
S13
S14
264
251
249
246
237
4.69
4.94
4.98
5.03
5.22
0.23
0.07
0.03
0.07
0.04
0.23(H-7→L) + 0.15(H-7→L+1) + 0.55(H-4→L+1)
+ 0.16(H-3→L+1) + 0.16(H-2→L+1) - 0.17(H→L+3)
- 0.10(H-1→L+2) + 0.69(H-1→L+3)
0.16(H-7→L) + 0.17(H-7→L+1) + 0.55(H-5→L+1)
- 0.21(H-4→L+1) + 0.15(H-2→L+1) + 0.17(H→L+4)
0.70(H-1→L+4)
S15
S16
S17
235
234
239
5.27
5.29
5.16
0.02
0.00
0.08
S18
S19
S20
229
228
221
5.40
5.42
5.59
0.00
0.03
0.01
0.46(H-6→L+1) + 0.31(H-2→L+2) - 0.39(H→L+2)
- 0.15(H-4→L+1) + 0.62(H-3→L+1) -0.20(H→L+4)
PAZB-9
Excited Wavelength
Excitation
Energy (eV)
2.63
Configurations Composition
Oscillator
Strength (f)
0.00
State
S1
(nm)
470
410
330
(corresponding transition orbitals)
0.68(H-2→L) + 0.15(H-2→L+1)
0.69(H→L)
S2
3.02
3.75
1.64
0.03
S3
0.65(H-1→L) + 0.20(H→L+1)
0.10(H-6→L+1) - 0.11(H-4→L) + 0.12(H-3→L)
- 0.24(H-1→L) - 0.13(H-1→L+1)+0.60(H→L+1)
0.18(H-6→L) - 0.31(H-4→L) + 0.55(H-3→L)
- 0.12(H→L+1)+ 0.13(H→L+3)
- 0.18(H-6→L) + 0.53(H-4→L) + 0.38(H-3→L)
S4
308
4.02
0.02
S5
S6
301
300
4.12
4.13
0.02
0.00
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+ 0.12(H-→L+4)
0.14(H-7→L) - 0.11(H-7→L+1) + 0.56(H-6→L) +
0.13(H-5→L) + 0.22(H-4→L)+ 0.22(H→L+1)
- 0.15(H-6→L) + 0.60(H-5→L) - 0.21(H-5→L+1) +
0.17(H→L+2)- 0.13(H→L+3)
S7
S8
286
277
4.33
4.48
0.16
0.00
0.12(H-7→L) -0.16(H-6→L) - 0.15(H-6→L+1)
- 0.12(H-5→L) - 0.12(H-4→L+1) + 0.58(H-1→L+1) +
0.15(H→L+1)
- 0.15(H-2→L) + 0.67(H-2→L+1) - 0.10(H-2→L+5)
0.13(H-7→L) - 0.29(H-5→L) - 0.26(H-5→L+1)
- 0.11(H-1→L+1)+ 0.13(H-1→L+2) - 0.10(H-1→L+3)
+ 0.45(H→L+2) - 0.25(H→L+3)
S9
267
266
252
4.64
4.66
4.91
0.25
0.00
0.07
S10
S11
0.55(H-7→L) - 0.17(H-6→L) - 0.13(H-6→L+1)
- 0.19(H-1→L+1) + 0.23(H→L+3) - 0.10(H→L+4) +
0.11(H→L+5)
- 0.19(H-7→L) + 0.16(H-4→L) + 0.22(H-4→L+1)
+ 0.12(H-1→L+1)+ 0.31(H→L+2) + 0.45(H→L+3)-
0.17(H→L+4)
0.13(H-6→L+1) - 0.14(H-3→L) + 0.14(H-3→L+1)
- 0.10(H-3→L+5)- 0.15(H-1→L+4) + 0.15(H→L+2) +
0.15(H→L+3) + 0.57(H→L+4)
S12
S13
S14
250
247
237
4.95
5.02
5.22
0.01
0.09
0.04
0.22(H-7→L) + 0.18(H-7→L+1) + 0.54(H-4→L+1)
- 0.17(H-3→L+1) + 0.17(H-1→L+1)- 0.13(H→L+2) -
0.12(H→L+3)
S15
S16
S17
235
234
232
5.26
5.29
5.34
0.02
0.00
0.01
0.43(H-2→L+2) + 0.56(H-2→L+3)
0.14(H-7→L) + 0.17(H-7→L+1) + 0.54(H-6→L+1) +
0.11(H-5→L+1) - 0.25(H-4→L+1) + 0.13(H-1→L+1)
- 0.17(H→L+4)
S18
S19
230
228
5.40
5.43
0.70(H-2→L+4)
0.47(H-5→L+1) - 0.23(H-1→L+2) + 0.17(H-1→L+3)
+0.31(H→L+2) - 0.25(H→L+3)
0.00
0.04
0.15(H-4→L+1) + 0.61(H-3→L+1) - 0.20(H→L+4)
- 0.11(H→L+5)
S20
222
5.59
0.01
*H-HOMO, L-LUMO
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Fig. 3. MO involved in formation of absorption spectrum of the compounds PAZB-6 at λmax = 411 nm,
PAZB-7 at λmax = 410 nm, PAZB-8 at λmax = 408 nm, PAZB-9 at λmax = 410 nm.
According to theoretical calculations, the strong peak at λmax = 411 nm and the oscillator strength
f = 1.65 in electronic absorption spectrum of the compound PAZB-6 is due to Charge-Transfer
(CT) of one electron into the excited singlet state S0→S2, which it describes by a wave function
corresponding to configuration for one-electron excitation [(HOMO)→(LUMO)] (Fig. 3).
Excitation of one electron at 269 nm (f = 0.24) belonged to the transition into the state S0→S9
and describes by a wave function corresponding to a superposition of six configurations for one-
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electron excitations [(H-6→L), (H-6→L+1), (H-5→L), (H-4→L+1), (H-1→L+1), (H→L+1)].
Excitation of an electron from (H-1) to (L+1) molecular orbital (MO) gives the main contribution
to the formation of the absorption band at 269 nm.The other excited states of the title compound
have very small intensity (f ≈ 0) that are nearly forbidden by orbital symmetry considerations
(Table 1). The calculated UV/Vis spectrum of the compound PAZB-6 in the solvent DMF and
experimental spectrum at concentration 4.0·10–4M and in the solvent DMF is observed in Fig.
4a,b. The calculated and experimental values of maximum wavelength (λmax) are 411 nm (f =
1.65, Fig. 4a) and 371 nm (D = 0.938, Fig. 4b), respectively.
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Fig. 4. UV/Vis spectrum of the PAZB-6, PAZB-7, PAZB-8, PAZB-9 in the solvent DMF: a) calculated
by TDPBE1PBE/6-31+G* method and b) experimental at concentration of dye: 4.0·10–4M.
The maximum wavelength with the highest oscillation of the compound PAZB-7 is observed at
410 nm and f = 1.73 (Table 1). Calculations show transition [(HOMO)→(LUMO)] is the most
important Charge-Transfer in the compound PAZB-7 with the absorption peakat 410 nm (Fig. 3)
that belong to the transition of one electron into the excited singlet state S0→S2. Excitation of one
electron at 267 nm (f = 0.30) belonged to the transition into the state S0→S11 and describes by a
wave function corresponding to a superposition of eight configurations for one-electron
excitations [(H-9→L), (H-8→L), (H-8→L+1), (H-7→L), (H-6→L+1), (H-2→L+1), (H-
1→L+1), (H→L+1)]. Excitation of an electron from (H-2)→(L+1)MO gives the main
contribution to the formation of the absorption band at 267 nm. The calculated UV/Vis spectrum
of the compound PAZB-7 in the solvent DMF and experimental spectrum at concentration
4.0·10–4M and in the solvent DMF is observed in Fig. 4c,d. The calculated and experimental
values of maximum wavelength (λmax) are 410 nm (f = 1.73, Fig. 4c) and 368nm (D = 2.839, Fig.
4d), respectively.
According to Table 1, the highest oscillationof the compound PAZB-8 observed at λmax = 408
nm and f = 1.61. Theoretical calculations show transition [(HOMO)→(LUMO)] is the most
important Charge-Transfer in the compound PAZB-8 with the absorption band at 408 nm (Fig.
3) that belong to the transition of one electron into the excited singlet state S0→S2. Excitation of
one electron at 264 nm (f = 0.23) belonged to the transition into the state S0→S10 and describes
by a wave function corresponding to a superposition of eight configurations for one-electron
excitations [(H-7→L), (H-7→L+1), (H-6→L), (H-5→L), (H-5→L+1), (H-4→L+1), (H-
2→L+1), (H→L+1)]. Excitation of an electron from (H-2)→(L+1) MO gives the main
contribution to the formation of the absorption band at 264 nm. The other excited states of the
title compound have very small intensity (f ≈ 0) and are nearly forbidden by orbital symmetry
considerations (Table 1). The maximum wavelength (λmax) in experimental UV/Vis spectrum of
the compound PAZB-8 is observed at 367 nm with D = 2.239 (Fig. 4e). Comparison of
calculated and experimental absorption spectra of the compound PAZB-8 shows high accuracy
of the constructed model calculations (Fig. 4e,f).
The maximum wavelength with the highest oscillation of the compound PAZB-9 is observed at
410 nm and f = 1.64 (Table 1). Calculations show transition [(HOMO)→(LUMO)] that is the
15

ACCEPTED MANUSCRIPT
most important Charge-Transfer in the compound PAZB-9 with the absorption peak at 410 nm
(Fig. 3) that belong to the transition of one electron into the excited singlet state S0→S2.
Excitation of one electron at 267 nm (f = 0.23) belonged to the transition into the state S0→S9
and describes by a wave function corresponding to a superposition of seven configurations for
one-electron excitations [(H-7→L), (H-6→L), (H-6→L+1), (H-5→L), (H-4→L+1), (H-
1→L+1), (H→L+1)]. Excitation of an electron from (H-1) to (L+1) MO gives the main
contribution to the formation of the absorption band at 267 nm. The other excited states of the
title compound have very small intensity (f ≈ 0) and are nearly forbidden by orbital symmetry
considerations (Table 1). The calculated UV/Vis spectrum of the compound PAZB-9 is obtained
in the solvent DMF (Fig. 4g) and experimental spectrum at concentration 4.0·10–4M and in the
solvent DMF is observed in Fig. 4h. The calculated and experimental values of maximum
wavelength (λmax) are 410nm (f = 1.64) and 368 nm (D = 0.839), respectively.
From the investigation of the absorption spectra of the compounds PAZB-6, PAZB-7, PAZB-8
and PAZB-9 it was found that the compound PAZB-6 has the highest λmax (experimental = 371
nm, calculated = 411 nm) and the compound PAZB-8 has the lowest λmax (experimental = 367
nm, calculated = 408 nm). We believe the dipole moment parameter can be an effect on the
amount of λmax of the title compounds. The dipole moment parameter is used for investigation of
intermolecular interactions. The high values of dipole moment lead to stronger intermolecular
interaction [16]. Also, the dipole moment is a good measurement for the study of asymmetric
nature of compounds. The values of dipole moment for the compounds PAZB-6, PAZB-7,
PAZB-8, and PAZB-9 are listed in Table 2. The magnitude of the dipole moment is related to the
composition and dimensionality of the 3D compounds.
The point group of the title compounds is C1, which refers to their high asymmetry. The
maximum absorption in a molecule with the higher dipole moment is moving to the longer
wavelength is observed. As can be seen from Table 5, the highest value of dipole moment is
observed for the compound PAZB-6 (6.2028 Debye). On the other hands, the compound PAZB-
6 is the most polar with the highest wavelength (371 nm), whereas the compound PAZB-8 with
the lowest polarity (5.2812 Debye) has the lowest wavelength (367 nm). The high value of
dipole moment for PAZB-6 is due to its asymmetric character. In the compound PAZB-6, the
16

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atoms are irregularly arranged which leads to the increasing dipole moment rather than other
compounds.
Table 2. The experimental and calculated values of λmax and dipole moment of the compounds using
TDPBE1PBE/6-31+G* method.
Compound
PAZB-6
PAZB-7
PAZB-8
PAZB-9
λmax, exp., nm
371
λmax, cal., nm
411
Dipole moment, Debye
6.2028
Point group
C1
C1
C1
C1
368
367
368
410
408
410
5.8709
5.2812
5.7678
3.3. Frontier molecular orbital analysis of the compounds PAZB-6, PAZB-7, PAZB-8,
PAZB-9
Quantum chemical methods are important for obtaining information about molecular structure
and electrochemical behavior of organic molecules. The frontier molecular orbitals (FMO)
analysis plays an important role in theelectronic and optical properties, UV/Vis spectrum and
chemical reactions. The highest occupied molecular orbital (HOMO) can act as an electron donor
and the lowest unoccupied molecular orbital (LUMO) can act as the electron acceptor. The high
value of EHOMO shows the tendency of the compound to donate an electron to acceptor compound
with low energy, whereas the low value of ELUMO indicates that the compound accepts electrons.
The FMO results of the title compounds such as electronic properties are summarized in Table 3.
Table 3. The calculated electronic properties of the compounds using TDPBE1PBE/6-31+G* method
.
Property
PAZB-6
-6.23
-2.62
3.61
PAZB-7
-6.26
-2.62
3.64
PAZB-8
-6.28
-2.63
3.65
PAZB-9
-6.25
-2.62
3.63
EHOMO (eV)
ELUMO (eV)
Energy gap (eV)
Ionisation potential I (eV)
Electron affinity A (eV)
Electronegativity (χ)
Global hardness (η)
Chemical potential (ꢀ)
Global electrophilicity (ω)
Chemical softness S (eV-1)
6.23
2.62
4.42
1.80
-4.42
5.43
0.555
6.26
2.62
4.44
1.82
-4.44
5.42
0.549
6.28
2.63
4.45
1.82
-4.45
5.44
0.549
6.25
2.62
4.43
1.81
-4.43
5.42
0.552
Also, the HOMO-LUMO energy gap is an important parameter in determining molecular
electrical transport properties and reactivity of the molecules. The increase of the HOMO-
17

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LUMO energy gap decreases the reactivity of the compound that leads to increasing in the
stability of the compound [23].
Total electronic densities of states (DOSs) [10] of the compounds PAZB-6, PAZB-7, PAZB-8,
PAZB-9 were computed (Fig. 5).
Fig. 5. Calculated DOS plots of the compounds PAZB-6, PAZB-7, PAZB-8, PAZB-9 by PBE1PBE/6-
31+G* method.
The DOS analysis indicates that the HOMO–LUMO energy gaps of the compounds PAZB-6,
PAZB-7, PAZB-8, PAZB-9 are about 3.61 eV, 3.64 eV, 3.65 eV and 3.63 eV at the PBE1PBE/6-
31+G* method, respectively. The energy gap of the compound PAZB-8 has the highest value
(3.65 eV) therefore it is less reactive compared with other structures. It implies that the electronic
transfer in the molecules PAZB-6, PAZB-7,PAZB-9 is easier. Substituting butyl, benzyl and
methyl groups in the compounds PAZB-6, PAZB-7 and PAZB-9 respectively, instead of the
hydrogen atom at H-O group increase energy of the HOMO (EHOMO) compared with the
18

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compound PAZB-8 (-6.28 eV). As can be seen from Table 3, the compound PAZB-6 has the
highest energy of HOMO value (-6.23 eV). The butyl group in PAZB-6 increase EHOMO further
compared with benzyl and methyl groups in PAZB-7 (-6.26 eV) and PAZB-9 (-6.25 eV)
respectively.
As mentioned, the molecular orbitals of the HOMO and LUMO have an important role in
theoptical properties and UV/Vis spectrum. In all four compounds, PAZB-6, PAZB-7, PAZB-8,
PAZB-9, the electron density of the HOMO is mainly focused on the azo group (-N=N-) and
LUMO is mainly focused on double bonds (-C=C-) of three aryl rings and azo group (-N=N-)
(Fig. 3). Therefore the electronic transitions from the HOMO to LUMO are due to the
contribution of pi (π) bands.
The energy of HOMO is directly related to the ionization potential (I =-EHOMO), while the energy
of LUMO is related to the electron affinity (A=-ELUMO). Also the global hardness (η=I -A/2),
electronegativity (χ=I+A/2), electronic chemical potential (µ=-(I+A)/2) and electrophilicity
(ω=µ2/2η) [16], chemical softness (S=1/η) [10] are calculated and is reported in Table 3. The
global hardness (η) corresponds to the HOMO-LUMO energy gap. A molecule with a small
energy gap has a high chemical reactivity, low kinetic stability and is a soft molecule, while a
hard molecule has a large energy gap [10]. The global hardness (η) of the compounds PAZB-6,
PAZB-7, PAZB-8, PAZB-9 is 1.80 eV, 1.71 eV, 1.82 eV and 1.81 eV, respectively. From these
values can understand that the compound PAZB-8 has the higher global hardness, therefore it is
a hard molecule rather than the other compounds. Electronegativity (χ) is a measure of the power
of an atom or a group of atoms to attract electrons and the chemical softness (S) describes the
capacity of an atom or a group of atoms to receive electrons. These values for structures PAZB-
6, PAZB-7, PAZB-8, PAZB-9 are summarized in Table 3.
3.4. Molecular electrostatic potential (MEP) of the compounds PAZB-6, PAZB-7, PAZB-8,
PAZB-9
Molecular electrostatic potential (MEP) maps show the electronic density and are useful in
recognition of the negative and positive electrostatic potentials sites for electrophilic attack and
nucleophilic reactions as well as hydrogen bonding interactions [17,18]. The difference of the
electrostatic potential at the surface is shown by different colors. The negative regions (red,
orange and yellow color) of MEP with the high electron density were related to electrophilic
reactivity, the positive regions (blue color) with the low electron density ones to nucleophilic
19

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reactivity and the green color is neutral regions. The MEPs of the compounds PAZB-6, PAZB-7,
PAZB-8, PAZB-9 were checked out by theoretical calculations using the PBE1PBE/6-31+G*
level of energy (Fig. 6).
Fig. 6. Molecular electrostatic potential (MEP) map of the compounds PAZB-6, PAZB-7, PAZB-8,
PAZB-9 calculated using the PBE1PBE/6-31+G* level of energy.
As shown in Fig. 6, the negative region (red color) of the compounds PAZB-6, PAZB-7, PAZB-
9 is mainly focused on aryl rings and the N15 atom. Therefore, these regions are suitable for
electrophilic attack. According to MEP map of the compounds PAZB-7 and PAZB-9, hydrogen
atoms of ethyl and methyl groups respectively, are the positive potential sites and these are
suitable sites for nucleophilic activity (blue color). The parts of the title compounds are pale red
or pale blue. These areas are sites with weak interaction. Also, the regions with green color
indicate areas with zero potential. As can be seen in Fig. 6, the compound PAZB-8 is
approximately neutral molecule because MEP map of the title compound has the green color.
However in the compound PAZB-8, the hydrogen atom of the O-H group is a positive potential
site and it is suitable for nucleophilic activity (blue color).
3.5. Natural charge analysis of the compounds PAZB-6, PAZB-7, PAZB-8, PAZB-9
The atomic charges play an important role in molecular polarizability, dipole moment, electronic
structure and related properties of molecular systems [24]. We calculated the charge distributions
for equilibrium geometry of the compounds PAZB-6, PAZB-7, PAZB-8, PAZB-9 by the NBO
(natural charge) charges [16] using the PBE1PBE/6-31+G* level of energy. The calculated
natural charges are listed in Table 4 (Atoms labeling is according to Fig. 2).
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Table 4. Natural Charges (NBO charges, e) of the compounds PAZB-6, PAZB-7, PAZB-8, PAZB-9
calculated using the PBE1PBE/6-31+G* level of energy.
PAZB-6
Natural
PAZB-7
PAZB-8
PAZB-9
Atoms
Atoms
Natural
Atoms
Natural
Atoms
Natural
charge
-0.226
-0.258
-0.237
-0.253
-0.241
0.071
-0.211
-0.200
0.060
-0.211
-0.281
0.142
-0.250
-0.230
-0.481
0.125
-0.173
-0.186
-0.338
0.346
-0.294
-0.190
-0.534
-0.135
-0.522
-0.501
-0.717
0.267
0.266
0.264
0.265
0.271
0.269
0.266
0.267
0.272
0.213
0.265
0.267
0.270
0.270
0.230
0.256
0.256
0.252
0.249
0.249
0.251
0.244
0.244
charge
-0.226
-0.258
-0.237
-0.253
-0.241
0.071
-0.210
-0.200
0.060
-0.211
-0.280
0.141
-0.249
-0.230
-0.480
0.125
-0.177
-0.173
-0.344
0.341
-0.290
-0.191
-0.532
-0.146
-0.088
-0.233
-0.255
-0.250
-0.254
-0.233
0.267
0.266
0.264
0.265
0.271
0.269
0.266
0.267
0.273
0.213
0.266
0.270
0.271
0.271
0.246
0.245
0.261
0.264
0.263
0.263
charge
-0.226
-0.258
-0.237
-0.253
-0.241
0.071
-0.210
-0.200
0.060
-0.211
-0.280
0.141
-0.249
-0.230
-0.479
0.125
-0.171
-0.188
-0.329
0.331
-0.302
-0.186
-0.706
0.267
0.266
0.265
0.265
0.271
0.269
0.266
0.267
0.273
0.214
0.266
0.268
0.271
0.271
0.531
charge
-0.226
-0.258
-0.237
-0.253
-0.241
0.071
-0.210
-0.200
0.060
-0.211
-0.280
0.142
-0.249
-0.230
-0.480
0.125
-0.171
-0.186
-0.339
0.341
-0.291
-0.191
-0.533
-0.360
0.267
0.265
0.265
0.265
0.271
0.269
0.266
0.267
0.273
0.213
0.265
0.271
0.271
0.271
0.253
0.229
0.229
C1
C2
C3
C4
C5
C6
N7
N8
C1
C2
C3
C4
C5
C6
N7
N8
C1
C2
C3
C4
C5
C6
N7
N8
C9
C1
C2
C3
C4
C5
C6
N7
N8
C9
C9
C9
C10
C11
C12
C13
C14
N15
C16
C17
C18
C19
C20
C21
C22
O23
C24
C25
C26
C27
H28
H29
H30
H31
H32
H33
H34
H35
H36
H37
H38
H39
H40
H41
H42
H43
H44
H45
H46
H47
H48
H49
H50
C10
C11
C12
C13
C14
N15
C16
C17
C18
C19
C20
C21
C22
O23
C24
C25
C26
C27
C28
C29
C30
H31
H32
H33
H34
H35
H36
H37
H38
H39
H40
H41
H42
H43
H44
H45
H46
H47
H48
H49
H50
C10
C11
C12
C13
C14
N15
C16
C17
C18
C19
C20
C21
C22
O23
H24
H25
H26
H27
H28
H29
H30
H31
H32
H33
H34
H35
H36
H37
H38
C10
C11
C12
C13
C14
N15
C16
C17
C18
C19
C20
C21
C22
O23
C24
H25
H26
H27
H28
H29
H30
H31
H32
H33
H34
H35
H36
H37
H38
H39
H40
H41
21

ACCEPTED MANUSCRIPT
H51
0.261
22

ACCEPTED MANUSCRIPT
Fig. 7. Natural charges distribution of the compounds PAZB-6, PAZB-7, PAZB-8, PAZB-9 calculated by
PBE1PBE/6-31+G* method.
23

ACCEPTED MANUSCRIPT
The total charge of the investigated compounds is equal to zero. Also Fig. 7 shows results of
natural charges in a graphical form. The natural charge (NBO) analysis of the title compounds
shows that carbon atoms have both positive and negative charges magnitudes. According to
results in all four compounds, the positive carbons are observed for the carbons atoms
attachment to the electron-withdrawing nitrogen and oxygen atoms (C6, C9, C12, C16, C20).
The other carbon atoms of the phenyl rings have a negative charge. The oxygen and nitrogen
atoms in all four structures have a negative charge. As can be seen in Table 4 and Fig. 7, the
negative charge of the N15 atom in all four molecules is the higher than N7 and N8 atoms. The
highest negative charge in the compounds PAZB-6, PAZB-7, PAZB-8, PAZB-9 is observed for
the O23 atom such as -0.534e, -0.532e, -0.706e and -0.533e, respectively. According to Natural
charge’s plot (Fig. 7) all hydrogen atoms have the positive charge. In the compound PAZB-8, the
H38 atom of the O-H group has the highest positive charge (0.531e) compared with other
hydrogen atoms due to the electron-withdrawing nature of the O23, therefore it is acidic
hydrogen.
3.6. NBO analysis of the compound PAZB-6
Natural bond orbital (NBO) analysis is an important method for studying intra- and inter-
molecular bonding and interaction between bonds in molecular systems [25]. Electron donor
orbitals, acceptor orbitals and the interacting stabilization energy (E(2)) resulting from the
second-order micro disturbance theory are reported in Table 5.
Table 5. Significant donor–acceptor interactions and second order perturbation energies of the compound
PAZB-6 calculated using the PBE1PBE/6-31+G* level of energy.
Donor (i)
Occupancy
1.61964
Acceptor (j)
Occupancy
E(2)a
kcal/mol
19.97
20.15
21.57
22.90
19.81
20.67
21.55
10.81
10.61
23.29
19.50
20.27
25.42
16.90
30.19
32.62
E(j)-E(i)b
a.u.
F(i , j)c
a.u.
π(C1-C6)
π*(C2-C3)
π*(C4-C5)
π*(N7-N8)
π*(C1-C6)
π*(C4-C5)
π*(C1-C6)
π*(C2-C3)
π*(C1-C6)
π*(C9-C10)
π*(N7-N8)
π*(C11-C12)
π*(C13-C14)
π*(C9-C10)
π*(C13-C14)
π*(C1-C6)
π*(C9-C10)
0.31923
0.28988
0.22753
0.37810
0.28988
0.37810
0.31923
0.37810
0.39017
0.22753
0.37807
0.26854
0.39017
0.26854
0.37810
0.39017
0.30
0.30
0.25
0.29
0.30
0.29
0.30
0.42
0.42
0.25
0.29
0.30
0.29
0.30
0.05
0.05
0.070
0.071
0.068
0.074
0.070
0.071
0.072
0.065
0.064
0.070
0.068
0.072
0.077
0.066
0.062
0.063
π(C2-C3)
π(C4-C5)
π(N7-N8)
π(C9-C10)
1.65348
1.67700
1.91269
1.61388
π(C11-C12)
π*(N7-N8)
1.61815
0.22753
24

ACCEPTED MANUSCRIPT
π*(C19-C20)
σ(C1-C2)
0.38533
1.97970
π*(C21-C22)
σ*(C1-C6)
σ*(C2-C3)
σ*(C3-H30)
σ*(C6-N7)
σ*(C3-C4)
σ*(C2-C3)
σ*(C4-C5)
σ*(C6-N7)
σ*(C1-C6)
σ*(C4-C5)
σ*(C6-N7)
σ*(C5-C6)
σ*(N8-C9)
σ*(C12-C13)
σ*(C12-N15)
σ*(C18-C19)
σ*(C19-C20)
σ*(C21-C22)
σ*(C1-C6)
0.26992
0.02176
0.01575
0.01185
0.02941
0.01615
0.01575
0.01445
0.02941
0.02176
0.01445
0.02941
0.03407
0.02871
0.02559
0.02363
0.01269
0.03050
0.01209
0.02176
0.03407
0.02175
0.01253
0.01560
0.02176
0.01445
0.01583
0.02941
0.02175
0.03449
0.03246
0.37807
0.02559
0.03070
0.03734
0.03050
0.02333
0.02296
0.01398
0.38533
0.02093
0.02296
0.01398
0.01373
203.24
2.35
2.27
2.45
3.60
2.08
2.10
2.28
4.87
3.43
2.28
0.72
1.37
4.47
2.44
1.59
1.04
0.69
1.38
0.62
9.06
1.07
0.52
0.62
1.10
0.55
0.64
0.50
0.70
9.25
7.03
6.67
0.65
3.00
12.99
8.51
0.93
2.48
1.52
35.36
6.12
3.01
0.50
0.69
0.01
1.30
1.31
1.22
1.17
1.30
1.21
1.32
1.17
1.30
1.32
1.17
1.37
1.25
1.46
1.38
1.55
1.51
1.56
1.01
1.00
1.00
1.04
0.92
1.00
1.03
0.92
0.87
1.00
1.00
0.93
0.39
0.93
0.87
0.80
1.13
1.12
1.00
1.02
0.36
0.74
0.77
0.78
0.75
0.084
0.049
0.049
0.049
0.058
0.047
0.047
0.049
0.067
0.060
0.049
0.026
0.039
0.067
0.054
0.042
0.036
0.029
0.042
0.022
0.085
0.029
0.021
0.022
0.030
0.021
0.022
0.019
0.024
0.086
0.074
0.049
0.023
0.047
0.093
0.088
0.029
0.045
0.035
0.106
0.063
0.045
0.019
0.021
σ(C2-C3)
σ(C3-C4)
1.98119
1.98128
σ(C4-C5)
σ(C5-C6)
1.97861
1.97827
σ(C6-N7)
σ(N15-C16)
σ(C20-O23)
1.98067
1.98694
1.99224
n1(N7)
n1(N8)
1.95134
1.95026
σ*(C5-C6)
σ*(C9-C10)
σ*(C13-C14)
σ*(C14-H36)
σ*(C1-C6)
σ*(C4-C5)
σ*(C5-H32)
σ*(C6-N7)
σ*(C9-C10)
σ*(C9-C14)
σ*(C11-C12)
π*(C11-C12)
σ*(C12-C13)
σ*(C16-C17)
σ*(C16-H37)
σ*(C19-C20)
σ*(C20-C21)
σ*(C24-H42)
σ*(C24-H43)
π*(C19-C20)
σ*(C24-C25)
σ*(C24-H42)
σ*(C24-H43)
σ*(C25-C26)
n1(N15)
1.87760
n1(O23)
n2(O23)
1.96196
1.82276
a E(2) Energy of hyperconjucative interactions,
b Energy difference between donor and acceptor i and j NBO orbitals,
c F(i, j) is the Fock matrix element between i and j NBO orbitals.
The electron delocalization from filled NBOs (donors) to the empty NBOs (acceptors) describes
a conjugative electron transfer process between them. For each donor (i) and acceptor (j), the
stabilization energy E(2) associated with the delocalization i→j is estimated [24]:
25

ACCEPTED MANUSCRIPT
(1)
where qi is the donor orbital occupancy, εj and εi are diagonal elements and F(i,j) is the off
diagonal NBO Fock matrix element. The resonance energy (E(2)) detected the quantity of
participation of electrons in the resonance between atoms. The larger E(2) value, the more
intensive is the interaction between electron donors and acceptor, i.e. the more donation tendency
from electron donors to electron acceptors and the greater the extent of conjugation of the whole
system [25]. Delocalization of electron density between occupied Lewis-type (bond or lone pair)
NBO orbitals and formally unoccupied (antibonding or Rydberg) non-Lewis NBO orbitals
correspond to a stabilization donor–acceptor interaction. NBO analysis has been performed for
the compound PAZB-6 the PBE1PBE/6-31+G* level of energy in order to elucidate the
intramolecular, rehybridization and delocalization of electron density within the compound
PAZB-6. The strong, moderate and weak intramolecular hyperconjugative interactions of the
title compound such as π→π*, π*→π*, σ→σ*, n→σ* and n→π* transitions are presented in
Table 5. According to NBO analysis, the σ(C4-C5)→σ*(C6-N7) transition has the highest
resonance energy (4.87 kcal/mol) compared with other σ→σ* transitions of the compound
PAZB-6. The σ(C1-C2) orbital in phenyl ring participates as donor and the anti-bonding σ*(C1-
C6), σ*(C2-C3), σ*(C3-H30) and σ*(C6-N7) orbitals act as acceptor with resonance energies
(E(2)) of is 2.35 kcal/mol, 2.27 kcal/mol, 2.45 kcal/mol and 3.60 kcal/mol, respectively. These
values indicate that σ(C1-C2)→σ*(C6-N7) transition has the highest resonance energy (3.60
kcal/mol) compared with σ(C1-C2)→σ*(C1-C6), σ(C1-C2)→σ*(C2-C3) and σ(C1-C2)→σ*(C3-
H30).
The intramolecular hyperconjugative interactions of the π→π* transitions have the most
resonance energy (E(2)) compared with σ→σ* transitions. The important intramolecular
hyperconjugative interaction of the π→π* transitions in phenyl ring that lead to a strong
delocalization are such as C2-C3→C1-C6, C9-C10→N7-N8 and C11-C12→C9-C10 with the
strong resonance energies (E(2)) 22.90 kcal/mol, 23.29 kcal/mol and 25.42 kcal/mol, respectively.
The π*→π* transitions have the highest resonance energies compared with other interactions of
the title compound. The highest resonance energy of the title compound isobserved for the C19-
C20→C21-C22 transition with resonance energy (E(2)) 203.24 kcal/mol respectively, that lead to
stability of the title compound. The strongest n→σ* interactions are due to n1(N15)→σ*(C16-
26

ACCEPTED MANUSCRIPT
H37) and n2(O23)→π*(C19-C20) interactions with stabilization energies of 12.99 kcal/moland
35.36 kcal/mol, respectively.
The results of NBO analysis such as the occupation numbers with their energies for the
interacting NBOs [interaction between natural bond orbital A and natural bond orbital B (A-B)]
and the polarization coefficient amounts of atoms in the title compound are presented using the
PBE1PBE/6-31+G* method is summarized in Table 6 (Atoms labeling is according to Fig. 2).
Table 6. Calculated natural bond orbitals (NBO) and the polarization coefficient for each hybrid in
selected bonds of the compound PAZB-6 using the PBE1PBE/6-31+G* level of energy.
Occupancy
(a.u.)
Bond (A-B)a
Energy
(a.u.)
EDA (%)
EDB (%)
NBO
S(%)
(A)
S(%)
(B)
P(%)
(A)
P(%)
(B)
1.97970
σ(C1-C2)
σ(C1-C6)
-0.72434
-0.72329
50.26
48.97
49.74
51.03
0.7089 (sp1.82) + 0.7053 (sp1.88
0.6998 (sp1.94) + 0.7143 (sp1.85
)
)
35.44
34.02
34.65 64.39
35.11 65.94
64.53
64.86
1.97652
1.61964
1.98119
1.97827
1.98067
1.98691
1.91269
1.98079
1.97612
1.98199
1.98694
π(C1-C6)
σ(C2-C3)
σ(C5-C6)
σ(C6-N7)
σ(N7-N8)
π(N7-N8)
σ(N8-C9)
σ(C9-C10)
σ(C12-N15)
σ(N15-C16)
-0.26809
-0.72260
-0.72497
-0.80158
-0.99947
-0.39167
-0.80836
-0.72358
-0.80967
-0.90398
47.93
50.12
49.05
41.52
49.93
50.37
58.52
50.89
41.41
59.54
52.07
49.88
50.95
58.48
50.07
49.63
41.48
49.11
58.59
40.46
0.6923 (sp1.00) + 0.7216 (sp1.00
0.7079 (sp1.87) + 0.7063 (sp1.87
0.7004 (sp2.00) + 0.7138 (sp1.73
0.6444 (sp2.55) + 0.7647 (sp2.12
0.7066 (sp2.23) + 0.7076 (sp2.22
0.7097 (sp1.00) + 0.7045 (sp1.00
0.7650 (sp2.08) + 0.6440 (sp2.51
0.7133 (sp1.86) + 0.7008 (sp1.93
0.6435 (sp2.48) + 0.7654 (sp2.02
0.7716 (sp1.58) + 0.6361 (sp2.05
)
)
)
)
)
)
)
)
)
)
0.00
34.88
33.34
28.12
30.94
0.00
0.00
99.95
99.98
65.08
63.36
67.90
68.90
99.70
71.43
65.87
66.78
67.15
34.88 65.09
36.61 66.62
32.02 71.74
30.97 68.94
0.00
99.71
32.45
34.98
29.11
38.74
28.46 67.47
34.09 64.98
33.05 70.83
32.79 61.05
1.90508
1.97559
π(N15-C16)
σ(C17-C18)
-0.35130
-0.72178
61.31
51.24
38.69
48.76
0.7830 (sp99.99) + 0.6220 (sp99.99
)
0.54
33.82
0.49
34.68 66.15
99.22
99.39
65.29
0.7158 (sp1.96) + 0.6983 (sp1.88
)
1.99224
1.98381
1.97961
1.98243
1.95134
1.95026
1.87760
1.96196
1.82276
σ(C20-O23)
σ(C16-H37)
σ(C25-H44)
σ(C3-H30)
n1(N7)
n1(N8)
n1(N15)
n1(O23)
n2(O23)
-0.95241
-0.55764
-0.50394
-0.54105
-0.43020
-0.42942
-0.36828
-0.56702
-0.33613
32.64
61.64
62.96
63.31
-
-
-
-
-
67.36
38.36
37.04
36.69
-
-
-
-
-
0.5713 (sp2.95) + 0.8207 (sp1.94
)
25.28
31.79
23.25
30.40
37.61
37.12
27.67
35.90
0.03
34.01 74.49
65.92
0.7851 (sp2.14) + 0.6193 (s)
100
68.15
76.69
69.55
62.33
62.82
72.19
64.04
99.88
-
-
-
-
-
-
-
-
0.7935 (sp3.30) + 0.6086 (s)
0.7957 (sp2.29) + 0.6057 (s)
100
100
sp1.66
sp1.69
sp2.61
sp1.78
sp99.99
-
-
-
-
-
a A-B is the bond between atom A and atom B. (A: natural bond orbital and the polarization coefficient of atom; A-B: natural bond orbital and
the polarization coefficient of atom B).
The size of polarization coefficients shows the importance of the two hybrids in the formation of
the bond in a molecular system. The differences in electronegativity of the atoms involved in the
bond formation are reflected in the larger differences in the polarization coefficients of the atoms
(C-O, C-H C-N bonds). The calculated bonding orbital for the σ(C20-O23) bond is the σ=0.5713
(sp2.95)+0.8207(sp1.94) with high occupancy 1.99224a.u. and low energy -0.95241a.u. The
polarization coefficients of the C20= 0.5713 and the O23= 0.8207 shows the importance of the
O23 in forming σ(C20-O23) bond compared with the C20 atom. The calculated bonding orbital
27

ACCEPTED MANUSCRIPT
for the σ(N7-N8) bond is σ=0.7066(sp2.23)+0.7076(sp2.22) with low energy -0.99947 a.u. and high
occupancy 1.98691 a.u. The polarization coefficients of N7=0.7066 and N8=0.7076 show low
difference in polarization coefficients N7 and N8 atoms in N7-N8 bond and importance of two
atoms in forming the bond. According to NBO analysis, the natural hybrid orbital n2(O23) with
occupancy 1.82276 a.u. and high energy -0.33613 a.u. has p-character (99.88%), whereas
n1(O23) occupy a lower energy orbital (-0.56702 a.u) with p-character (64.04%) and high
occupation number (1.96196 a.u). The pure p-type lone pair orbital n2(O23) participates the
electron donation to π*(C19-C20) forn 2(O23)→π*(C19-C20) interaction with high resonance
energies (E(2)) 35.36 kcal/mol in the title compound (see Table 5). Also the natural hybrid orbital
n1(N15) with occupancy 1.87760 a.u. has p-character (72.19%), whereas n1(N7) and n1(N8)
have p-character (62.33%) and (62.82%). From Table 5, it is found that the p-type lone pair
orbital n1(N15) participates the electron donation to σ*(C16-H35) forn1(N15)→σ*(C16-H36)
interaction with high resonance energies (E(2)) 12.99 kcal/mol.
3.7. Vibrational frequencies of the compounds PAZB-6, PAZB-7, PAZB-8, PAZB-9
In order to confirm the accuracy of our findings, IR spectroscopy was used. The IR experimental
spectra of the compounds PAZB-6, PAZB-7, PAZB-8, PAZB-9 is shown in the file
supplementary (Fig. 1S-8S). The vibrational frequencies of the optimized compounds PAZB-6,
PAZB-7, PAZB-8, PAZB-9 were calculated using the PBE1PBE/6-31+G* method. The
vibrational frequencies assignments were made using the GaussView 05 program. The
theoretical frequencies calculated by DFT method are usually higher than the corresponding
experimental data due to the approximate treatment of the electron correlation, anharmonicity
effects and basis set deficiencies [26]. The important calculated and experimental vibrational
frequencies of the compounds PAZB-6, PAZB-7, PAZB-8, PAZB-9 are summarized in Tables
S5,S6,S7,S8 respectively. The azo group (N=N) stretching vibration in azo compounds shows
important bands in the region 1400-1600 cm-1 [27]. The C-N stretching vibrations are expected
in the range 1306-1439 cm-1 [28]. The C=N stretching mod in imines appears in the region 1640-
1690 cm-1. Also the C=C aromatic ring stretching vibrations are expected in the range 1200-1650
cm-1 [29]. The C-H aromatic ring bending vibrations (out of plane) are expected in the region
667-900 cm-1 [30]. The symmetric C-H stretching vibrations of CH2 and CH3 are expected in the
range 2900-3050 cm-1 [31,32]. These vibrations appear in IR spectra of the compounds PAZB-6,
PAZB-7, PAZB-8. Also the C-O stretching vibration in ethers shows important bands in the
28