Spectroscopic behavior, FMO, NLO and substitution effect of 2-(1H-Benzo[d]imidazole-2-ylthio)-N-o-substituted-acetamides: Experimental and theoretical approach

Article abstract of DOI:10.1016/j.dyepig.2019.107742

The benzimidazole-based derivatives 2a-c were designed and synthesized via C–N coupling reaction and experimentally characterized by infrared spectroscopy (FT-IR), nuclear magnetic resonance (¹H NMR) spectroscopy and mass spectrometry (MS). The

Full text of DOI:10.1016/j.dyepig.2019.107742

DyesandPigments171(2019)107742
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Spectroscopic behavior, FMO, NLO and substitution effect of 2-(1H-Benzo
[d]imidazole-2-ylthio)-N-o-substituted-acetamides: Experimental and
theoretical approach
Ali Muhammad Arifa, Afifa Yousafa, Hong-liang Xua,∗, Zhong-Min Sua,b,∗∗
a Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal University, Changchun, 130024, Jilin, People's Republic of China
b School of Chemistry and Environmental Engineering, Changchun University of Science and Technology, Changchun, Jilin, People's Republic of China
A B S T R A C T
The benzimidazole-based derivatives 2a-c were designed and synthesized via C–N coupling reaction and experimentally characterized by infrared spectroscopy (FT-
IR), nuclear magnetic resonance (1H NMR) spectroscopy and mass spectrometry (MS). The observed and calculated FT-IR frequencies correspond to C]O stretching
of amide group are depicted at 1657, 1672 and 1682, 1689 and 1699, 1682 for 2a, 2b and 2c respectively and are in good agreement. The synthesized compounds
2a-c exhibit non-linear optical response with the first hyperpolarizability (β0) at 783, 1550 and 694 au, respectively. Due to the presence of electron withdrawing
–NO2 on the primary amine of 2b, the β0value is estimated at 1550 au, which reduces the energy barrier hence increasing the β0 value, with maximum UV–vis
absorption at 224 nm with TD-DFT method. The opposite behavior is demonstrated by the electron donor –OCH3 substituent. This study of NLO responses would be
beneficial to the development of high-performance NLO materials.
1. Introduction
component of Vitamin B12 [13]. The structural alteration and en-
hancement of the existing abilities of the bioactive species are highly
challenging in material chemistry. To achieve potential NLO response
and efficient bioactivity of benzimidazole derivatives, exploitation can
be carried out via functional group substitution. Recently, various such
materials were extensively explored for their potential NLO applica-
tions as well as biological applications. Muhammad et al., successfully
tuned the hyperpolarizabilities of benzimidazole derivatives by proton
abstraction [14]. Yu et al., also synthesized the hybrid benzimidazole
derivatives with efficient NLO responses [15]. Yadav et. al., demon-
strated the synthesis and biological evaluation of 2-(1-benzoyl-1H-
benzo[d]imidazole-2-ylthio)-N-substituted acetamides as antimicrobial,
antitubercular and anticancer agents [16]. Similarly, Mao and co-
workers also reported some bioactive derivatives with –COO at ortho
position as protein methyltransferase 5 (PRMT5) inhibitors [17].
Mavrova et. al., also reported the biological activities of 5(6)-(un)sub-
stituted-1H-benzimidazol-2-ylthioacetylpiperazine derivatives [18].
However, there is no sufficient literature available which highlight both
theoretical and experimental studies of these type of donor-acceptor
benzimidazole-based derivatives. Whereas, there are recent analogues
studies have been reported by Muhammad and fellow collaborators on
chalcone, glycinium picrate, thiazole, and benzothiazole derivatives
[8,19,20]. The combination of both theoretical and experimental
Since the past few decades, the organic nonlinear optical (NLO)
materials gain the attention of experimental and theoretical in-
vestigators because of their potential applications in modern-day de-
vices [1,2]. There are several NLO materials have been designed, syn-
thesized and characterized both theoretically and experimentally.
Among these studied NLO compounds, the organic materials have
certain advantages over other inorganic or hybrid materials, such as
vast structural diversity, a broad range of optical susceptibilities and
higher NLO thresholds. The modulation of molecular hyperpolariz-
abilities is the fundamental property to characterize any class of NLO
materials [3]. There are several structural modification techniques used
to modulate the NLO response such as donor-π-acceptor configurations,
metal doping, bond length alteration, functional group replacements
and so forth [4–10].
The heterocyclic compounds especially 2-substituted benzimida-
zoles derivatives exhibit a broad range of biological, pharmacological
and agricultural activities such as antibacterial, antiviral, anti-ulcer,
anti-inflammatory, anti-cancer, anti-HIV and anti-corrosion activities
[11,12]. Organic compounds with both nitrogen and sulfur reflect
better activity against several biological diseases. Benzimidazole, being
an important constituent of several synthetic drugs, is also the major
∗ Corresponding author.
∗∗ Corresponding author. Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal University, Changchun, 130024, Jilin, People's
Republic of China.
E-mail addresses: hlxu@nenu.edu.cn (H.-l. Xu), zmsu@nenu.edu.cn (Z.-M. Su).
https://doi.org/10.1016/j.dyepig.2019.107742
Received 7 June 2019; Received in revised form 11 July 2019; Accepted 18 July 2019
Availableonline19July2019
0143-7208/©2019ElsevierLtd.Allrightsreserved.

A.M. Arif, et al.
DyesandPigments171(2019)107742
techniques provides a better understanding of compound behavior at
the molecular level. Therefore, in order to determine the structural as
well as spectroscopic abilities of benzimidazole derivatives, this field
attracts several theoretical and model chemists [17,18,21,22].
From the literature review, there is no such work has been found so
far on these type of molecules. Therefore, to enhance the applications of
these types of benzimidazole derivatives and obtain detailed structural,
spectroscopic as well as nonlinear optical properties of these bioactive
species, we combined the experimental and theoretical methods.
Herein, we report the synthesis, characterization and optical properties
of 2-(1H-Benzo[d]imidazole-2-ylthio)-N-substituted acetamides, pri-
marily comparing their experimental as well as theoretical IR and 1H
NMR spectroscopic studies and theoretically estimated FMOs, and non-
linear optical properties.
The 2-(1H-Benzo[d]imidazole-2-ylthio)-N-substituted acetamides
2a-c have been obtained upon hydrolysis of thiazolo[3,2-a]benzimi-
dazol-3(2H)-ones 1 with an ortho-substituted primary amine in ethanol
under reflux conditions. The ortho-substituted primary amine, with
electron-rich –CH3, –OCH3 and electron deficient –NO2 group, was se-
lected to predict the change in chemical behavior and optical properties
of synthesized compounds. In this work, theoretical studies have been
performed on synthesized compounds to manipulate the effect of
electron donor/acceptor substituents on electronic structures, hence,
the detailed analysis of the geometric structures and appropriate non-
linear optical properties has been carried out and reported.
2-(1H-Benzo[d]imidazole-2-ylthio)-N-o-tolylacetamide
(2a).
Melting point: 165–167 °C; Yield: 47%; IR (KBr, cm−1): 3288, 3045,
(NH stretching), 1657 (C]O stretching); 1H-NMR (DMSO‑d6, δ ppm):
2.19 (s, 3H, CH3), 4.23 (s, 2H, CH2), 7.04 (m, 4H, benzimidazole), 7.38
(m, 4H, phenyl ring), 9.85 (s, 1H, imidazole NH), 12.69 (s, 1H, aryl
NH); MS (70 eV), m/z (rel. int.) (%): 297 (M+, 16), 223 (4), 191 (27),
164 (100), 150 (16), 131 (53), 119 (16), 107 (45), 83 (21), 65 (5), 51
(3).
2-(1H-Benzo[d]imidazole-2-ylthio)-N-(2-nitrophenyl)acet-
amide (2b). Melting point: 210–212 °C; Yield 46%; IR (KBr, cm−1):
3171, 3140, (NH stretching), 1672 (C]O); 1H-NMR (DMSO‑d6, δ ppm):
4.31 (s, 2H, CH2), 7.36 (m, 4H, benzimidazole), 7.75 (m, 4H, phenyl
ring), 10.97 (s, 1H, imidazole NH), 12.66 (s, 1H, aryl NH); MS (70 eV),
m/z (rel. int.) (%): 328 (M+, 18), 191 (13), 164 (100), 150 (14), 131
(44), 118 (14), 92 (6), 63 (4).
2-(1H-Benzo[d]imidazole-2-ylthio)-N-(2-methoxyphenyl)acet-
amide (2c). Melting Point: 165–167 °C; Yield: 38%; IR (KBr, cm−1):
3182, 3045, (NH stretching), 1682 (C]O); 1H-NMR (DMSO‑d6, δ ppm):
3.67 (s, 3H, OCH3), 4.20 (s, 2H, CH2), 6.98 (m, 4H, phenyl ring), 7.15
(m, 4H, benzimidazole), 9.93 (s, 1H, imidazole NH), 12.71 (s, 1H, aryl
NH); MS (70 eV), m/z (rel. int.) (%): 314 (M+, 8), 282 (5), 191 (59),
164 (100), 150 (25), 131 (97), 123 (100), 108 (42), 92 (19), 80 (17), 65
(21), 52 (12).
2.3. Quantum chemical details
2. Methodology
In order to achieve profound insight into the distribution of elec-
trons and geometrical configuration of the studied compounds 2a-c,
DFT calculations were carried out using Gaussian 09 package. Firstly,
geometric estimations and vibrational frequencies were at B3LYP and
PBE0 with 6–31G(d) basis set and the obtained results are summarized
in Table S1 & Table S2 respectively. The optimized geometric structure
with atom labeling for 2a is expressed in Fig. 2. To make sure the op-
timized geometries are real minima, all frequency calculations were
cross-checked and these are found to be positive values, as no ima-
ginary frequencies present on the related potential energy surfaces.
Hence, B3LYP optimized geometry is selected for further estimations
because the results obtained with this method are more consistent with
some reported benzimidazole derivatives [14,24,25]. From the ground
state optimized geometries, the vibrational frequencies and 1H NMR
chemical shifts of the studied molecules were estimated in the ethanol
as a solvent. To explore the origin of second-order NLO response, TD-
DFT calculations were performed to estimate the nature of the excited
state with CAM-B3LYP/6-31 + G* method [5,26]. Moreover, to de-
termine the effect of substitution on nonlinear optical properties, the
static first hyperpolarizability βo was obtained by the finite field (FF)
method at CAM-B3LYP/6-31 + G* level. The FF approach was devel-
oped by Kurtz et al., has been widely used to estimate the first hy-
perpolarizabilities because it provides a reasonable agreement with
experimental NLO responses. In this FF method, a static electric field (F)
is applied and the energy (E) of the molecule is expressed by the fol-
lowing equation:
2.1. Instruments
All reagents and solvents were used as obtained from the suppliers,
or recrystallized or redistilled as necessary. Melting points were taken
on a Fisher-Johns melting point apparatus and are uncorrected. IR
spectra (KBr disks) were run on Shimadzu (Japan) Prestige-21 FT-IR
spectrometer. 1H NMR spectra were recorded in C2D6SO on Bruker
(Rhenistetten-Forchheim, Germany) AM 300 spectrometer operating at
300 MHz, using tetramethylsilane (TMS) as an internal standard. Proton
chemical shifts are reported in δ (ppm) whereas coupling constants in
Hz. Electron impact (EI) mass spectra were recorded on JEOL MS Route
mass spectrometer. Thin layer chromatography using glass plates
coated with Silica gel 60 GF254 (E.Merck) was carried out to monitor
the progress of the reactions and purity of the products. The spots were
visualized under The UV-light at 254 and 366 nm and iodine vapors
were used to visualize the spots The structural optimization, Nonlinear
optical properties and UV–vis absorption estimations for these mole-
cules were performed at B3LYP/6-31G(d) and CAM-B3LYP/6-
31 + G(d) level, respectively, using the Gaussian 09 program package
[14,23].
2.2. Synthesis
The synthetic route to achieve target compounds 2a-c is shown in
Scheme 1 and structures of synthesized molecules are presented in
Fig. 1. The syntheses of three 2-(1H-Benzo[d]imidazole-2-ylthio)-N-
substituted acetamides 2a-c are carried out via hydrolysis of thiazolo
[3,2-a]benzimidazole-3(2H)-one 1 in ethanol with an o-substituted
primary amine, with a yield of 46%, 47%, and 38%, respectively. The
final products are characterized by standard spectroscopic techniques.
To a hot stirred solution of thiazolo[3,2-a]benzimidazole-3(2H)-one (1)
(0.001 mol) in ethanol (5 ml) containing a few drops of acetic acid was
added an appropriate amine (0.001 mol) dissolved in ethanol (5 ml).
The resultant mixture was then refluxed for 2–6 h and the solid formed
during heating under reflux was filtered hot. The desired products were
obtained by thorough washing with hot ethanol in the pure form.
Hence, low yields of synthesized compounds could be increased via
column chromatography and further purification.
1
2
1
6
1
24
E = E(0) − μ F1 − αij F Fj − βijk F Fj Fk −
γ
i
ijkl
F Fj Fk F − …
i
i
l
1
(1)
Here, E(0) represents the total energy of molecule in the absence of an
electric field, μ is the vector component of the dipole moment, α is the
linear polarizability, β and γ are the second and third-order polariz-
abilities, respectively, while x, y and z label the i, j, k components, re-
spectively. It can be seen from eq. (1) that differentiating E with respect
to F obtains the μ, α, β, and γ values. In this investigation, we have
calculated the electronic dipole moment, molecular polarizability, and
molecular first hyperpolarizability. For a molecule, its dipole moment
(μ) is defined as follows:
μ = (μx2 + μy2 + μ2)
(2)
z
2

A.M. Arif, et al.
DyesandPigments171(2019)107742
Scheme 1. The synthetic pathway of the studied molecules 2a-c.
The average polarizability (α0) can be calculated by the following
equation:
1
3
α0 = (αxx + αyy + αzz)
(3)
Similarly, the magnitude of the total static first hyperpolarizability
can be calculated using the following equations:
βtot = (βx2 + βy2 + βz2)12
(4)
in which
Fig. 2. The optimized structure of studied molecule 2a with atom numbering.
βx = βxxx + βxyy + βxzz
,
,
,
βy = βyyy + βxxy + βyzz
The second-order polarizability (β), that is a third rank tensor and
can be described by 3 × 3 × 3 matrix. According to Kleinman sym-
metry (βxyy = βyxy = βyyx, βyyz = βyzy = βzyy, …likewise other permu-
tations also take the same value), the 27 component of the 3D matrix
can be reduced to ten components. All reported β values are expressed
in atomic units [1 a.u. of β = 3.62 × 10−42 m4 V−1 = 3.2063 × 10−53
C3 m3 J−2 = 8.641 × 10−33 esu] within the T convention.
βz = βzzz + βxxz + βyyz
while the static first hyperpolarizability (β0) is also calculated using the
following equation:
3
β0 =
(β )
tot
(5)
5
Fig. 1. Chemical structures of the synthesized compounds 2a-c.
3

A.M. Arif, et al.
DyesandPigments171(2019)107742
3. Results and discussions
Table 1
The experimental and computed 1H NMR chemical shifts of all synthesized
compounds 2a-c.
3.1. Geometry optimization
2a
2b
2c
The geometry optimization of all studied molecules is summarized
in Table S3. As there is no crystallographic data of this molecule has
been reported, different synthesized compounds of thiazolo-benzimi-
dazole and its derivatives were considered to evaluate the optimized
geometry of the synthesized molecules. As the –OCH3 and –NO2 sub-
stituents are present on the far ends, the structural parameters of the
optimized geometries are moderately affected. Table S3 revealed that
C7–N12 is 1.31 Å, and C7–N13 is 1.37 Å resembling C]N (1.306 Å)
and C–N (1.364 Å), the bond of benzimidazole [27] indicates that
C7–N12 is as possible as double bond and C7–N13 is more likely a
single bond [27]. Whereas, N18–C24 is the longest C–N bond exists in
the studied molecules with a bond length of 1.41 Å, evidencing that
N18–C24 is a free single bond of o-toluidine [28]. The length of the
C23–N32 bond is 1.45 Å, hinting that it is a single bond of –NO2 sub-
stitution at the ortho position of o-toluidine. The bond lengths of
(Ph)C–N in this molecule are ranging between 1.3779 and 1.3823 Å,
revealing their single character with some p ingredients. As reported
earlier, the lengths of (Ph)C–N bonds are 1.372, 1.395 Å and 1.379,
1.388 Å for benzimidazole and 5-amino-1-methyl-1H-benzimidazole
[29,30], respectively. The bond distance of C7–S15, S15–C16 are about
1.76 Å and 1.83 Å respectively, which deviates from normal C–S bond
length 1.80 Å [31], may be the cause of pulling interactions of the
benzimidazole moiety on C7. The dihedral angles of benzimidazole and
o-toluidine parts are almost equal to 180 or 0. On the other hand, the
bond lengths of C23C33 1.51, C23N32 1.45 and C23O33 1.36 corre-
spond to C–CH3, C–NO2, and C–OCH3 of different substitutions.
Whereas, C24C23C33 121.6, C24C23N32 123.1 and C24C23O33 114.7
correspond to bond angles of these substituted groups. These differ-
ences in geometric structures are the source to produce different NLO
responses.
Atom Exp (ppm) Calca
(ppm)
Exp (ppm) Calca
(ppm)
Exp (ppm) Calca
(ppm)
8-H
9-H
7.04
7.04
7.06
7.06
7.06
7.06
9.68
7.31
7.31
7.32
7.32
4.28
4.26
10.54
2.23
2.23
7.36
7.36
7.36
7.36
7.75
12.66
7.75
7.75
7.75
4.31
4.31
7.34
7.34
7.35
7.34
8.09
10.71
7.78
7.77
7.77
4.34
4.35
7.15
7.15
7.15
7.15
9.93
7.43
6.98
6.98
6.98
4.20
4.20
12.71
7.18
7.18
7.18
7.18
9.68
7.38
7.07
7.02
7.11
4.24
4.24
10.46
10-H 7.04
11-H 7.04
14-H 9.85
19-H 7.33
26-H 7.38
27-H 7.38
28-H 7.38
30-H 4.23
31-H 4.23
32-H 12.69
34-H 2.19
35-H 2.19
36-H
10.97
9.95
3.67
3.67
3.67
3.62
3.62
3.63
37-H
a
Calculated with B3LYP/6-31G(d) method.
The C–C stretching vibrations of the phenyl rings often observed around
1450, 1500, 1580 and 1600 cm−1. The C–C and C–H stretching vibra-
tions of the phenyl ring of benzimidazole are reported between the
range of 1588–1620 cm−1 and 2096-3062 cm−1, respectively, and the
vibrations under 1000 cm−1 correspond to C–H out-of-plane bending
vibrations of the phenyl ring of benzimidazole [31–33]. The peaks
around 3288 cm−1, 3045 cm−1 assigned to stretching vibrations of
benzimidazole hydrogen and NH hydrogen of respective amine re-
spectively, whereas peak around 1657 cm-1 corresponds to intense C]
O stretching.
3.2. Vibrational assignments
3.3. Nuclear magnetic resonance
The experimental and calculated IR spectra are presented in Fig. 3.
In general, the DFT methods provide excellent vibrational wave-
numbers for organic molecules. For comparison with experimental re-
sults, a proper scaling factor is required to compensate the over-
estimation of the vibrational frequencies obtained with quantum
calculations (0.9613 for B3LYP method [29]). The recorded and cal-
culated vibrational frequencies are in good consistency, supports the
reliability of the used method.
The vibrations C–H and C–C of aromatic rings provide obvious
characteristics. The C–H stretching vibrations of phenyl rings usually
observed at 3030-3100 cm−1. The bending vibrations of C–H often
absorb in the range of 1000–1300 cm−1, whereas, the C–H vibrations
around 650-900 cm−1 correspond to out-of-plane bending vibrations.
The values of NMR collected from the recorded spectra are ex-
pressed as follows:
1H-NMR (DMSO‑d6, δ ppm): 4.31 (s, 2H, CH2), 7.36 (m, 4H, ben-
zimidazole), 7.75 (m, 4H, phenyl ring), 10.97 (s, 1H, imidazole NH),
12.66 (s, 1H, aryl NH) for studied molecule 2b.
The observed and estimated chemical shift values are in concert
with peak assignments and summarized in Table 1. The 1H NMR spectra
of these systems were recorded using DMSO‑d6 as a solvent with TMS as
an internal standard. The quantum chemical estimations have been
performed with the B3LYP/6-31G(d) method through gauge-including
atomic orbital (GIAO) approach. The Obtained chemical shift values are
converted to the TMS scale by subtracting the calculated absolute
chemical shielding of TMS (δ = Σo - Σ), where δ is the chemical shift, Σ
Fig. 3. Experimental and calculated FT-IR spectra of synthesized compounds (a) 2a, (b) 2b and (c) 2c.
4

A.M. Arif, et al.
DyesandPigments171(2019)107742
Fig. 4. Correlation graphs of observed and estimated chemical shifts of synthesized compounds (a) 2a, (b) 2b and (c) 2c.
is the absolute shielding and Σo is the absolute shielding of TMS, whose
value is 31.64 at B3LYP/6-311 + G (2d,p) level.
For a fair comparison with observed values, the correlation graphs
are presented in Fig. 4, whereas, the correlation coefficient 0.9106 is
also considered. A good agreement has been found between the esti-
mated and recorded proton chemical shifts values.
Table 3
The calculated first hyperpolarizabilities (β0) along with their individual com-
ponents for all synthesized compounds obtained with CAM-B3LYP/6-31 + G(d)
method.
β components
2a
2b
2c
βxxx
βxxy
βxyy
βyyy
βxxz
βxyz
βyyz
βxzz
βyzz
βzzz
βx
1084
0
−39
0
−145
0
−35
167
0
−306
727
0
−292
783
−539
476
−539
1963
2
7
−10
−7
−94
−1
−651
1407
−6
−1027
134
−208
−250
3
5
−1
91
−53
−3
3.4. Dipole moment, first hyperpolarizability and frontier molecular orbital
analysis
The fundamental electronic responses of the studied compounds are
ground state dipole moments and the first hyperpolarizability values
are summarized in Table 2 and Table 3, respectively. Usually, the di-
rection of the ground state dipole moment is from negative to positive
part of the molecule. The strong electron withdrawing moiety (–NO2)
substituted at the ortho position of the primary amine enhances the
charge separation within the molecule, resulting in the highest ground
state dipole moment for 2b. Alternatively, the –CH3 and –OCH3 sub-
stitution on the same part of the compounds 2a and 2c, respectively,
resulting in less charge separation and lower ground state dipole mo-
ments. The main charge transfer axis in all synthesized compounds is
the y-axis, becoming the major component of dipole moment. This
supports the fact that the ground state dipole moment can be tuned by
swapping the donor/acceptor substituents.
−687
−101
−1
βy
βz
β0
1550
694
The large nonlinear optical response as a structure-property re-
lationship can be best estimated by quantum chemical calculations
[34–37]. The nonlinear optical susceptibilities of the all synthesized
compounds are characterized by the first hyperpolarizability. In this
work, the static (β0) first hyperpolarizabilities have been calculated
according to eq. (5) and presented in Table 3.The graphical re-
presentation of static first hyperpolarizability (β0) estimated at CAM-
B3LYP and PBE0 is given in Fig. 5. From Table 3, it can be depicted that
the 2c has the smallest amplitude of β0 694 a.u., which reflects the
electron donating behavior of –OCH3 substitution resulting in less
charge separation supports the lowest βo values for this system.
Whereas, the replacement of this functional group with –CH3 and
–NO2, the electron donating/withdrawing effect of these functional
groups show interesting changes in these derivatives. The –NO2 sub-
stitution of 2b significantly enhances the charge separation because of
Fig. 5. Relationship between the first hyperpolarizability and –CH3, –NO2 and
–OCH3 substituents with two density functional methods.
its electron accepting ability, hence, increasing the βo amplitude to
1550 a.u. which is about twice than that of compound 2c. While in 2a,
the substitution of –CH3 functional group moderately affects the βo
value up to 783 a.u. This is due to the smaller electron donating ability
of –CH3 as compared to –OCH3, still causing the intermolecular charge
transfer and modulating the first hyperpolarizability value in this
system. The values of first hyperpolarizabilities of 2b is significantly
higher than the standard NLO molecule p-nitroaniline (PNA = 908 a.u.
at CAM-B3LYP/6-31 + G* level of theory), whereas, β0 amplitude of
Table 2
The calculated ground state dipole moment (μtot) of all synthesized compounds
2a-c.
Components
2a
2b
2c
μx
μy
μz
−0.866
2.581
−0.001
2.723
1.041
0.958
3.664
−0.089
3.788
−4.249
−0.196
4.379
μtot(Debye)
5

A.M. Arif, et al.
DyesandPigments171(2019)107742
Fig. 7. The total electron density mapped with the MEP surface for 2a, 2b, and
2c (Red regions designate negative charge, while blue regions stand for a po-
sitive charge). (For interpretation of the references to color in this figure legend,
the reader is referred to the Web version of this article.)
be depicted form Fig. 7, that the molecular charges of compound 2a and
2c are uniforms, with negative potential is on carbonyl group and ne-
gative charge is on the nitrogen atom of benzimidazole part. Moreover,
the MEP of 2b represents a more charge separation because of negative
charge is seen on the carbonyl group as well as –NO2 functional group
whereas positive charge density on the nitrogen atom of benzimidazole
moiety. The MEP plots are consistent with the above mentioned elec-
tronic dipole moments and first hyperpolarizabilities. The –NO2 sub-
stitution of 2b compound act as an electron acceptor by withdrawing
the electron density form the benzimidazole part towards primary
amine as seen in the potential map. These MEP results are also in line
with some similar reported systems [38–40].
Fig. 6. HOMO-LUMO representation of the studied molecule 2b at CAM-
B3LYP/6-31 + G (d) level.
ρ(r′)
za
v (r) =
−
dr′
∑
∫
r − Ra
r − r′
(6)
a
2a, 2b and 2c is 57, 113 and 58 times larger than urea (14 a.u. at CAM-
B3LYP/6-31 + G* level of theory), which is also used in literature as
standard NLO molecule. The higher NLO response of 2b can be ex-
plained by the strong electron-withdrawing effect of –NO2 which may
be reducing the energy barrier with substantial frontier orbital overlaps
as explained in the next section. Conclusively, it can be stated that the
NLO responses of the organic systems can be modulated by the sub-
stituting the electron donating/withdrawing functional groups in these
kinds of organic molecules.
3.5. Comparative analysis of first hyperpolarizability
As mentioned earlier there has been no literature available com-
bining both experimental and theoretical studies of these type of mo-
lecules. From the literature review, we have sorted out some organic
systems bearing similar terminal functional groups to make a compar-
ison with our studied systems. It is worth mentioning that all the β0
values reproduced in Table 4 are theoretically reported earlier using
almost the same methodologies as in present work. From this com-
parative analysis, it can be seen that the first hyperpolarizability values
(β0)of our synthesized systems are significantly higher than other re-
ported systems bearing the same terminal functional groups
[19,41–44]. Especially the β0 value of 2b is roughly 85–185 folds larger
than other systems represented in Table 4. This analysis support that
our studied compounds might be used as potential NLO materials.
Frontier molecular orbitals (FMOs) provide significant information
to explain the spectral behavior and fluorescence abilities. Moreover,
the kinetic stability, chemical reactivity and optical polarizability of
any molecule can be described from the HOMO-LUMO energy gap. The
HOMO-LUMO energy gap for compound 2b is found to be 3.04eV
supports its higher stability and optical response, Fig. 6. The inter-
molecular charge transfer can be characterized by the excitation of an
electron from the occupied orbital (HOMO-i) to unoccupied orbital
(LUMO + i) where i = 0,1,2 …, and can be depicted from the graphical
representation of the HOMO(s) and LUMO(s) orbitals for all studied
molecules presented in Table S4. When comparing with the LUMO,
frontier molecular orbital diagrams of studied systems revealed that the
electron density of the respective HOMO is more extended at the
electron-rich benzimidazole part which is located farther away from the
respective phenyl-amine except for 2c which is due to electron donating
ability of –OCH3 substituent at the primary amine.
In order to achieve further insight into the dipole moment, we have
also performed the molecular electrostatic potential (MEP) analysis,
which has been carried out by the molecular charge density using the
following equation:where Za represents a charge of the α-th nucleus at
position Ra. The origin of the dipole moment can be obtained by distant
charge separation. MEP represents the electrophilicity or nucleophili-
city of the molecule. MEP maps are presented in Fig. 7. For example, the
red surface contains a maximum negative charge and is the preferred
site for electrophilic attack. While the blue color represents the max-
imum positive region and is preferred site for nucleophilic attack. It can
3.6. TD-DFT studies
In order to explain the second-order NLO responses, TD-DFT
method was used to simulate the UV–visible absorption spectra of all
studied systems and presented in Table 5. The absorption spectra are
calculated using the TD-DFT method on the ground state geometries at
CAM-B3LYP/6-31 + G* theory level and the estimated absorption
spectra of the studied molecules is shown in Fig. 8. The substitution of
electron donor/acceptor functional groups in these compounds is ob-
vious from the change in absorption wavelengths and oscillator
strengths. The electron donor functional groups decrease the absorption
wavelengths to 202 nm and 203 nm for –CH3 (2a) and –OCH3 (2c)
substituted systems, respectively, whereas the absorption at 224 nm
corresponds to electron withdrawing –NO2 (2b) substituted system.
As reported in the literature, a strong bathochromic shift leads to
the enhanced first hyperpolarizability, from Fig. 8, it is obvious that
synthesized compound 2b has an obvious red-shift, which supports its
higher dipole moment and second-order NLO response.
6

A.M. Arif, et al.
DyesandPigments171(2019)107742
Table 4
Comparison of theoretically estimated β0 amplitudes of some recently reported compounds bearing similar terminal functional groups.
Compound
β0 (a.u.)
Methodology
Ref./Year
2-(1H-Benzo[d]imidazole-2-ylthio)-N-o-tolylacetamide
2-(1H-Benzo[d]imidazole-2-ylthio)-N-(2-nitrophenyl)acetamide
2-(1H-Benzo[d]imidazole-2-ylthio)-N-(2-methoxyphenyl)acetamide
(2E)-1-(2,5-dimethylthiophen-3-yl)-3-(2-methoxyphenyl)prop-2-en-1-one)
((E)-2-(3-(2,4-dimethoxyphenyl)acryloyl)
783
1550
694
467
8.35
CAM-B3LYP/6-31 + G*
CAM-B3LYP/6-31 + G*
CAM-B3LYP/6-31 + G*
LCwPBE/6-311G**
This work
This work
This work
[39]/2016
[6]/2018
BMK/6-311G**
-3H-benzo[f]chromen-3-one
glycine glycinium picrate
427
B3LYP/6-31G*
[17]/2018
Conflicts of interest
Table 5
TD-DFT results of all studied compounds including their total dipole moments
(μtot), transition energies ΔE (eV), oscillator strengths (f0) and major orbital
contributions.
There are no conflicts of interest to declare.
Acknowledgment
Compounds
μtot (Debye)
ΔE (eV)
f0
Major contribution
% C·I.
2a
2b
2c
2.723
4.379
3.788
6.119
5.541
6.113
0.760
0.479
0.847
H-2→L+7
H-2→L+1
H-2→L+1
26
72
12
The authors gratefully acknowledge financial support from the
National Science Foundation of China (NSFC) (21473026), the Science
and Technology Development Planning of Jilin Province
(20140101046JC), and H.-L. Xu appreciates the support from Project
funded by the China Postdoctoral Science Foundation (2014M560227).
Thirteen Five-Year Sci-tech Research Guideline of the Education
Department of Jilin Province, China.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.dyepig.2019.107742.
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