Second harmonic generation in pyrazoline derivatives of dibenzylideneacetones and chalcone: A combined experimental and theoretical approach

Article abstract of DOI:10.1016/j.jphotochem.2019.112147

In this work, we investigate theoretically and experimentally second harmonic generation (SHG) in three pyrazoline compounds, being two derivatives of dibenzylideneacetone (DBA) (C₂₃H₂₀N₂ and C₂₅H₂₄N₂O₂) and one derivative of chalcone (C₂₁H₁₈N₂). The compounds were synthesized after two steps employing a Claisen-Schmidt condensation followed by an addition-elimination reaction with phenylhydrazine. All compounds were characterized using NMR, FT-IR, UV-Vis, and XRD. We calculated the first-order hyperpolarizabilities of these molecules using program packages based on the time-dependent Hartree-Fock (TDHF) and density functional theory (DFT). SHG was characterized by Kurtz and Perry's powder method. We observed that these organic crystals present SHG efficiencies up to 5 times larger than the KDP, and we associated these values to their molecular structure and crystalline arrangements. The values obtained experimentally and theoretically evidence that these compounds have good potential for application in electronic devices based on second-order nonlinear responses.

Full text of DOI:10.1016/j.jphotochem.2019.112147

Journal Pre-proof
Second harmonic generation in pyrazoline derivatives of
dibenzylideneacetones and chalcone: A combined experimental and
theoretical approach
´
ˆ
Raiane Sodre de Arau´jo, Aline Moreira de Alcantara, Luis M.G.
˜
ˆ
˜
Abegao, Yago Pereira de Souza, Antonio Carlos Brandao Silva,
´
´
Rogerio Machado, Jose Joatan Rodrigues Jr, Josefredo Rodriguez
´
Pliego Ju´nior, Francesco d’Errico, Marcelo Siqueira Valle, Marcio
´
Andre Rodrigues Cavalcanti de Alencar
PII:
S1010-6030(19)31072-X
DOI:
https://doi.org/10.1016/j.jphotochem.2019.112147
JPC 112147
Reference:
To appear in:
Journal of Photochemistry & Photobiology, A: Chemistry
Received Date:
Revised Date:
Accepted Date:
21 June 2019
12 September 2019
6 October 2019
ˆ
˜
Please cite this article as: de Arau´jo RS, de Alcantara AM, Abegao LMG, de Souza YP,
˜
Brandao Silva AC, Machado R, Joatan Rodrigues J, Rodriguez Pliego J, d’Errico F, Siqueira
Valle M, de Alencar MARC, Second harmonic generation in pyrazoline derivatives of
dibenzylideneacetones and chalcone: A combined experimental and theoretical approach,
Journal of Photochemistry and amp; Photobiology, A: Chemistry (2019),
doi: https://doi.org/10.1016/j.jphotochem.2019.112147

This is a PDF file of an article that has undergone enhancements after acceptance, such as
the addition of a cover page and metadata, and formatting for readability, but it is not yet the
definitive version of record. This version will undergo additional copyediting, typesetting and
review before it is published in its final form, but we are providing this version to give early
visibility of the article. Please note that, during the production process, errors may be
discovered which could affect the content, and all legal disclaimers that apply to the journal
pertain.
© 2019 Published by Elsevier.

Second harmonic generation in pyrazoline derivatives of dibenzylideneacetones
and chalcone: A combined experimental and theoretical approach
Raiane Sodré de Araújo a* marca.ufs@gmail.com, Aline Moreira de Alcântara b, Luis M. G. Abegãoc,
Yago Pereira de Souza b, Antônio Carlos Brandão Silva a, Rogério Machado a, José Joatan Rodrigues Jr a,
Josefredo Rodriguez Pliego Júnior b, Francesco d'Erricoc,d, Marcelo Siqueira Valle b, Márcio André
Rodrigues Cavalcanti de Alencar a
a Departamento de Física, Universidade Federal de Sergipe, 49100-000, São Cristóvão, SE, Brasil
b Departamento de Ciências Naturais, Universidade Federal de São João del Rei, 36301-160, São João
del-Rei, MG, Brasil
c Department of Radiology & Biomedical Imaging, Yale University – School of Medicine, 330 Cedar St,
New Haven, CT 06520, USA
dUniversità degli Studi di Pisa, Scuola di Ingegneria, Largo Lazzarino 1, I-56126 Pisa, Italy
Graphical abstract
1

Highlights




Second-order nonlinear response of three pyrazoline compounds was investigated.
The inclusion of pyrazole group increases the first-order hyperpolarizability.
Pyrazolines powdered crystals are up to 5 times more efficient than KDP for SHG.
Minor changes in molecular and crystalline structures influence strongly the SHG.
ABSTRACT
In this work, we investigate theoretically and experimentally second harmonic generation (SHG) in three
pyrazoline compounds, being two derivatives of dibenzylideneacetone (DBA) (C23H20N2 and
C25H24N2O2) and one derivative of chalcone (C21H18N2). The compounds were synthesized after two steps
employing a Claisen-Schmidt condensation followed by an addition-elimination reaction with
phenylhydrazine. All compounds were characterized using NMR, FT-IR, UV-Vis, and XRD. We
calculated the first-order hyperpolarizabilities of these molecules using program packages based on the
time-dependent Hartree-Fock (TDHF) and density functional theory (DFT). SHG was characterized by
Kurtz and Perry’s powder method. We observed that these organic crystals present SHG efficiencies up to
5 times larger than the KDP, and we associated these values to their molecular structure and crystalline
arrangements. The values obtained experimentally and theoretically evidence that these compounds have
good potential for application in electronic devices based on second-order nonlinear responses.
Keywords: Pyrazoline, Second harmonic generation, First hyperpolarizability, Hartree-Fock, density
functional theory.
-------------------------------------------------------------------------------------------------------------------------------
1. Introduction
Organic molecules (OM) have been systematically investigated as promising materials for
nonlinear optics applications [1,2]. In comparison with inorganic compounds, OM present
various advantages, including its low production cost, ease of synthesis, the possibility of device
integration, high damage threshold, wide transparency range and intense optical nonlinear
2

responses [3,4]. The origin of the large optical nonlinearities in OM is related to the presence of
π-electrons in their structure and the strong charge delocalization along the length of the
conjugate system that they create [5].
Among the multitude of nonlinear effects, there was massive interest in the development of
inorganic [6–8] and organic [9–13] materials with efficient second-order optical nonlinear
responses, such as second harmonic generation (SHG). Indeed, since 1970s, a considerable
increase on the SHG from organic compounds was observed exploiting dipolar and multipolar
molecules [14–16].
The microscopic origin of nonlinear optical effects can be described in terms of the
induction of a dipole moment due to the interaction of a molecule with an intense optic field. In
this case, this dipole moment presents a nonlinear dependence with the excitation electric field
described as
훍 = μ0i + αij 퐄퐣 + βijk 퐄퐣퐄퐤 + γijkl 퐄퐣퐄퐤퐄퐥 + ⋯.
(1)
In this equation, the subscripts i, j, k refer to the spatial coordinates of the molecular systems;
Ej , Ek , El are the vector components of the applied electric field; μ0i is the molecule’s
permanent dipole moment; αij is the linear polarizability; βijk is the first hyperpolarizability,
which is associated with the second-order nonlinear effects; and γijkl is the second
hyperpolarizability, that is related to third-order nonlinear phenomena [17].
Nevertheless, for nonlinear optical applications in which solid state materials, such as
macroscopic organic crystals, are required, the medium response due to the interaction with an
intense optical field is described by the nonlinear polarization
( )
ij
( )
2
ijk
퐏 = P + χ 퐄퐣 + χ 퐄퐣퐄퐤 + χ( ) 퐄퐣퐄퐤퐄퐥 + ⋯.
(2)
1
3
ijkl
0
( )
Here, P is the static dipole moment, χ is the linear susceptibility, χ( ) corresponds to the
1
2
0
second-order nonlinear susceptibility, χ( ) is the third-order nonlinear susceptibility.
3
(
Naturally, equations (1) and (2) are linked. The components of tensor χ ) are related to
N
their molecular polarizability and hyperpolarizabilities counterparts through the molecules’
organization in the crystalline structure. In summary, the susceptibilities values depend on the
3

way the molecules pack to form the crystal lattice and on the internal geometry of the individual
molecule. For second-order nonlinear optical phenomena, such as SHG, the tensor elements of
the macroscopic second-order susceptibility are represented by a sum of the molecules’ first
hyperpolarizabilities over the crystal unit cell
1
( )
2
푠
푖푗푘 cos 휃퐼푠푖 cos 휃퐽푗 cos 휃퐾푠푘 β ,
∑ ∑
χIJK= 푓 푓 푓퐾 ×
퐼
퐽
푠
ijk
푉
(3)
where V is the volume of the unit cell, 푓 , 푓퐾 and 푓퐾 are tensors related to the local field
퐼
corrections; the indexes 퐼, 퐽, 퐾 correspond to the crystallographic axes, while the molecular
axes are labeled as 푖, 푗, 푘; 푠 denotes the index of an equivalent position of a molecule in the
lattice; 휃퐼푠푖 is the angle between the crystallographic axis 퐼 and the molecular axis 푖푠 [1,14].
By enhancing the NLO response at the molecular level, it is possible to achieve better
macroscopic nonlinear properties through appropriate molecular engineering. Typically,
enhanced molecular NLO can be achieved exploiting organic dipolar systems whenever strong
electron donor (D), and acceptor (A) groups are inserted at the end of a 휋-conjugated bridge.
These "push-pull" (D-휋-A) organic molecules allow charge transfer to occur from one end to
the other of the molecular structure, contributing to achieving higher values of the first-order
hyperpolarizability [10,11,15]. However, dipolar molecular structures usually form
centrosymmetric crystals, and this has been one of the much-reported problems of active SHG
materials synthesis [9].
Branched molecular structures have been synthesized as an alternative molecular design to
achieve a better performance of second-order nonlinear optical properties. These systems have
quadrupolar (A- 휋-D-휋-A or D- 휋-A-휋-D), octupolar ((D-π-A)3) and dendritic structures ((D-π-
A )푛), and efficient SHG can be obtained due to the formation of non-centrosymmetric crystals
[5,9,10,16,18–20].
Nowadays, the adjustment of molecular structures has been one of the most popular
approaches to the design of SHG organic materials [21–24]. In summary, the ideal material for
4

that purpose should display high values of the first-order hyperpolarizability, asymmetric load
distribution, high degree of electronic delocalization, transparency in the region of emission of
SHG and favorable crystalline packing for efficient SHG [9,25–29].
Several classes of organic compounds present the abovementioned properties. Among them,
pyrazolines are a class of organic compounds with a still poorly exploited potential aiming
nonlinear optical applications. They contain C=N-N group in their structures and are related to
ketones and aldehydes [30,31]. Pyrazolines are reduced dihydro [4H, 5H] pyrazole compounds
in which have two adjacent nitrogen atoms in a ring of five members. This class of organic
compounds is a versatile moiety that exhibits a wide variety of biological activities as
antimicrobial [32], antioxidant [33], antidepressant [34,35], anti-inflammatory and analgesic
[36,37], anticancer [38], antitubercular [39], antimalarial [40], antidiabetic [41], antiviral
[30,42], anti-amoebic [43] and anticonvulsant [44,45]. Due to their structural versatility and
capability to accept and donate electrons throughout the length of the 휋-conjugated systems,
these compounds’ second and third order nonlinear optical responses can be improved and are
promising materials in optoelectronics and photonics technologies [46–55].
In this work, we investigated theoretically and experimentally the nonlinear optical response
of three pyrazoline compounds. Two compounds are derivatives of dibenzylideneacetone
(DBA), while the other one is a chalcone derivative. As a strategy to optimize their nonlinear
responses, we theoretically calculate the value of the first-order hyperpolarizabilities of these
molecules and analyzed the influence of: (i) changing the functional group C=O by C=N-N with
the addition of the phenyl group linked to the nitrogen atom sp3 hybridized and (ii) the inclusion
of donor group OCH3 in the para-position of the pyrazoline derivative from DBA. Crystals of
these compounds were synthesized and characterized by NMR, FTIR, XRD and UV-VIS
absorption. We experimentally evaluated the SHG efficiency of these organic crystals in powder
form.
2. Computational study
5

The molecules’ dipole moment, as well as static and dynamic first-order
hyperpolarizabilities, were calculated using time-dependent Hartree-Fock (TDHF) and density
functional theory (DFT). For TDHF calculations, MOPAC 2016 and GAMESS software were
employed (the TDHF calculations with GAMESS were done on the geometries obtained with
the ORCA program), while DFT was implemented using the Gaussian16 program package [56–
59]. Different functionals and basis set were exploited in this work. Applying this methodology,
we intend to predict theoretically the influence of different molecular structures to the
compounds’ electrical properties, independently the computational method employed.
In this approach, the tensorial nature of the molecular dipole moment and
hyperpolarizabilities is considered, and all tensor elements are provided. However, we will
focus the analysis on the total values of these quantities, which have a direct relation with the
respective tensor elements. The molecule’s total dipole moment was calculated using the
relation [1,60]
μtot
=
μ2 + μ2 + μ2 ,
(4)
√
x
y
z
while the total first hyperpolarizability was evaluated as
ퟐ
ퟐ
ퟐ
√
훃퐭퐨퐭
=
(훃퐱퐱퐱 + 훃퐱퐲퐲 + 훃퐱퐳퐳) + (훃퐲퐲퐲 + 훃퐲퐳퐳 + 훃퐲퐱퐱) + (훃퐳퐳퐳 + 훃퐳퐱퐱 + 훃퐳퐲퐲) . (5)
In these equations, 휇푖 and 훽푖푗푘 are the tensor elements of the dipole moment and first
hyperpolarizability in the molecule’s reference system.
In the present study, the molecules were drawn on the ChemDraw software [61] and pre-
optimized using Avogadro program [62]. From Avogadro’s results, we extracted the input files
to perform a new structural optimization. In the following procedures, all quantities were
calculated considering the molecules in the gas phase.
Using the MOPAC 2016 software, the geometry optimization, as well as the entire
calculation of the electric properties of all compounds, were done with the PM7 Hamiltonian.
Then, the molecule’s total dipole moment, 휇, static first hyperpolarizability, β(0; 0,0), and
6

dynamic first hyperpolarizability, β(−2ω; ω, ω), were achieved with the TDHF method. The
command POLAR (퐸 = 1.16526 eV) was used in the calculation of the dynamic first
hyperpolarizability, which is equivalent to consider that the wavelength of the fundamental
optical field is 1064 nm. Such a procedure allows the comparison of the theoretical results with
the experimental characterization, which was performed using a Nd:YAG laser.
With ORCA software, geometry optimizations for all the molecules were done with the
PBE functional and the def2-SVP basis set using the resolution of identity approximation.
Following this step, the calculation of electric properties of molecules was done with the
GAMESS program also using the TDHF method with the double-zeta basis set augmented with
d polarization functions on the heavy atoms (DZ+P(d)).
Using Gaussian16 program package [58], the equilibrium geometries of the model
molecules were optimized using three distinct functionals, B3LYP, CAM-B3LYP and M06-2X,
and the same basis set, 6-311++G(2d,p). Then, the same functionals and basis set levels of
theory were employed to calculate the abovementioned electrical properties of the optimized
structures, using DFT method.
3. Experimental
The chalcone and dibenzylideneacetones were synthesized by a Claisen-Schmidt
condensation and they were converted pyrazolines by an addition-elimination reaction with
phenylhydrazine on the carbonyl carbon. This is schematically shown in figure 1. The
characterization of SHG exhibited by these compounds was performed employing the Kurtz and
Perry powder method. The other characterization techniques provided us with relevant
information regarding the studied crystals, such as absorption wavelengths, energy gap, and
XRD patterns.
7

Figure 1. Scheme of the synthesis of pyrazolines H-1, H-2 and H-3.
3.1. Procedures to the synthesis of the investigated compounds
3.1.1. Procedure for the preparation of dibenzalacetones DBA and DBA-OCH3
To a solution of acetone (0.74 mL, 10.0 mmoles) and NaOH (880 mg, 22.0 mmoles) in
ethanol/water 1:1 (50 mL) was added a solution of benzaldehyde (2.24 mL, 22 mmoles) in
methanol (10 mL) at 0°C. The reaction mixture was then gradually allowed to reach room
temperature under constant stirring. After 4 h, the formed solids were filtered, washed with a
cold mixture of ethanol/water to yield the respective dibenzalacetone derivative in 89% yield
(yellow crystals, 2.07 g, 8.8 mmoles). The same procedure was carried out for the synthesis of
DBA-OCH3, replacing benzaldehyde with 4-methoxybenzaldehyde (2.24 mL, 22.0 mmoles) to
furnish it in 92% yield (yellow crystals, 2.70 g, 9.2 mmoles).
3.1.2 Procedure for the preparation of chalcone
To a solution of acetophenone (1.17 mL, 10.0 mmoles) and NaOH (440 mg, 11.0 mmoles) in
ethanol/water 1:1 (50 mL) was added a solution of benzaldehyde (1.12 mL, 11.0 mmoles) in
methanol (10 mL) at 0°C. The reaction mixture was then gradually allowed to reach room
8

temperature under constant stirring. After 4 h, the formed solids were filtered, washed with a
cold mixture of ethanol/water to yield the respective chalcone derivative in 95% yield (yellow
crystals, 2.00 g, 9.6 mmoles).
3.1.3 Procedure for the preparation of H-1, H-2 and H-3
To a solution of DBA (1.17 g, 5.0 mmoles) or DBA-OCH3 (1.47 g, 5.0 mmoles) or chalcone
(1.04 g, 5.0 mmoles) in 10 mL in ethanol was added phenylhydrazine (0.59 mL, 1.2 mmoles).
To this solution was added, dropwise, a solution containing H2SO4 (98.1%, 0.27 mL, 5.0
mmoles) in 5.0 mL of ethanol. The reaction was kept at 60°C. After the reaction was over 5
hours, the solid formed was filtered and dried in a desiccator. The product was obtained as
yellow crystals in 52% (0.84 g, 2.18 mmoles, H-1), 88% (1.70 g, 4.42 mmoles, H-2) and 82%
(1.22 g, 4.09 mmoles, H-3) yields.
3.2. FT-IR analysis
Fourier-transform infrared (FT-IR) spectra were recorded on a Bomem FTR MB-102
spectrometer in the region of 4000-400 cm-1 with KBr pellet, with 4 cm-1 spectral resolution and
a mean of 128 scans. Only significant peaks were recorded.
3.3. NMR analysis
1H and 13C NMR spectra were recorded on a BRUKER 500 MHz Advance Spectrometer,
located at the DQ-UFJF. As internal reference tetramethylsilane (TMS) or the residual signal of
the CDCl3 solvent was used. The chemical shift values (δ) were given in parts per million (ppm)
and the coupling constants given in Hertz (Hz).
3.4. UV-Vis optical absorption
The compounds were dissolved in N, N-dimethylformamide (DMF) at a concentration equal
to 10−5 mol/L−1. The solution samples were conditioned in a glass cuvette with a 1 mm optical
path, and the measurements were performed using a spectrophotometer UV-Vis (Agilent Cary
9

100 Scan, Varian). From the spectra, we calculated the energy band gap of the investigated
pyrazolines crystals using Tauc relation [63].
3.5. Crystalline structure
The XRD powder patterns of the crystals was obtained using Bruker D8 Advance
diffractometer with voltage 40 KV, current of 40 mA and a source of radiation Cukα (λ =
1.54 A). The powder samples of the pyrazolines crystals were recorded in a range of 3-35
degrees 2-theta, with a stepsize of 0.02° and a steptime of 0.4 s per step with a solid-state
detector comprising 192 channels of measurement.
3.6. Second-order nonlinear optical characterization
The experimental determination of optical susceptibilities in crystals is a laborious task. It
usually requires the growth of a single crystal with good optical quality and large size, which
can be time-consuming and costly. Aiming fast verification of SHG active materials, the method
developed by Kurtz and Perry [64] is used as an alternative to measure the efficiency of new
materials. In this methodology, the nonlinear optical materials were investigated in powder
form.
This method is widely used to evaluate the potential of new materials for application based
on SHG [15,22,25,27,65]. It consists of comparing the SHG intensity emitted by the
investigated compound with the emission from a reference sample. As the emission from
powdered samples is strongly affected by the particles size distribution, the thickness of the
analyzed samples and light scattering, the measurements must be performed under the same
experimental conditions.
Moreover, the investigation of SHG as a function of particle size is a very useful tool, which
allows classifying the nonlinear material into two distinct classes, crystals that present phase-
matching and those that do not have phase-matching [22,25,27]. According to Kurtz and Perry
[64], when the material presents phase-matching, the SHG intensity increases with the increase
in particle size reaching an approximately constant value when the powder particles are much
10

larger than the coherence length. While materials that do not have phase-matching, the SHG
emission signal has a maximum when the particle size of the powder is equal to the coherence
length.
In the present study, a Nd:YAG Q-Switched laser, emitting at 1064 nm, with 8 ns pulse
width and 10 Hz repetition rate was used as the excitation source. A set of half-wave plate and
polarizer was employed to allow continuous control of the laser power. The powdered samples
were placed on the surface of a glass plate. The incident light beam was directed to the surface
of the powder with an incident angle of 45o. The light emitted normally in respect to the sample
surface was collected by a set of lenses and coupled to an optical fiber connected to a compact
CCD spectrometer. Optical filters were used to eliminate the presence of the fundamental beam.
The light intensity emitted at 532 nm was measured. As a reference material, we have used
powdered potassium dihydrogen phosphate (KDP).
As a result of the synthesis method, the organic crystals were already produced in a
powdered form. However, for NLO measurements, the organic crystals and KDP were
pulverized into a fine powder and sieved in different particle size ranges (< 75, 75-106, 106-150
and > 150 μm). The grain samples were standardized in different sizes. The powdered materials
were compacted in a small disk and positioned in the sample holder and exposed to laser
radiation. The SHG efficiency results were obtained by calculating the ratio between the
emission intensity from pyrazoline samples and KDP.
4. Results and discussion
4.1. FT-IR spectra
The FT-IR spectra of DBAs and chalcone showed all characteristic absorption bands of their
structures, as the stretch of the carbonyl group (C=O, 1655-1651 cm-1), the ,-unsaturated
system trans disubstituted alkene (HC=CH, 970-990 cm-1) and pyrazolines exhibit characteristic
absorption bands spectra related to C=N and C–N moieties at 1510-1600 and 1310-1330 cm-1 ,
respectively (Supporting Information, Spectra 1, 4, 7, 10, 13 and 16).
11

4.2. NMR spectra
Nuclear Magnetic Resonance spectra allowed the unambiguous elucidation of the
compounds molecular structure. The data obtained by the NMR analysis of the DBAs and
chalcone, as well as the H-1, H-2 and H-3 pyrazolines, are in agreement with those described in
the literature [66–70] and are presented in the Supporting Information. For example, we will
describe the signals related to pyrazoline H-2.
The 1H NMR spectrum of H-2 (Spectrum 17) showed two doublet of doublets (dd) relative
to the diastereotopic hydrogens CH2 of the ring pyrazoline at chemical shift in 3.00 and 3.69
ppm, which have coupling constant 3J = 6.6 and 16.8 Hz, respectively. Two singlets in δ 3.8 and
3.85 ppm are observed, signals corresponding to the homotopic methyl hydrogens that are part
of the methoxyl group. The aromatic hydrogens were identified in the range of δ 6.75 to 7.45
ppm. In this same frequency range, we can also find two doublets at 6.63 and 7.12 ppm the
olefinic hydrogens (CH=CH) with a coupling constant of 3J = 16.3 Hz, confirming the trans
isomerism present in the structure. The chemical shift of these hydrogens occurs at higher
frequencies than expected because they are especially disjointed because of the magnetic field
induced by double-bond electronic currents and proximity to the aromatic ring, a phenomenon
called magnetic anisotropy [71]. In the 13C NMR spectrum of H-2 (Spectrum 18), the 19 signals
corresponding to all chemically non-equivalent carbons present in the structure were observed.
Signals at δ 55.5 ppm are attributed to the carbons of the methoxy groups and those occurring at
42.6, 63.8 and 159.9 ppm are part of the pyrazoline ring (CH2, CH, and C = N, respectively). On
the other hand, the signals of the aromatic and olefinic carbons appear in the range of δ 113.5 to
159.2 ppm.
4.3. UV-Vis absorption
UV-Vis spectra of the pyrazoline’s solutions are shown in Fig. 2. It is observed that these
exhibit bands characteristic of the electronic transitions of the studied chromophores, that can be
associated with transitions between states n → π and π → π∗ [27,29,72]. The molecules H-1,
12

H-2, and H-3 exhibited very good transparency in the visible region, in the range of 450-800
nm. Their absorption bands are mainly in the ultraviolet region, presenting peaks centered at
385 nm, 386 nm and 363 nm for the samples H-1, H-2, and H-3 respectively. Their energy gap
was also estimated. For samples H-1 and H-2, the energy gap was 2.93 eV and 2.92 eV,
respectively. Sample H-3, on the other hand, presented a slightly higher energy gap, equal to
3.06 eV. In addition, these crystals present low molar absorptivity in the region of 532 nm. It
must be emphasized that an efficient SHG material should have a broad transparency window
around the emission wavelength [5,9]. Therefore, these results indicate that SHG will not be
significantly affected by reabsorption processes when an optical beam from a Nd:YAG interacts
with these crystals [27,73].
30000
H-1
H-2
H-3
l
max=386
25000
20000
15000
10000
5000
0
l
max=363
l
cutoff=452
l
exc=532 nm
l
cutoff=420
300
400
500
600
700
800
Wavelength (nm)
Figure 2. UV-Visible spectra of the pyrazoline compounds.
4.4. X-rays diffraction
The XRD powder patterns of the three samples are shown in figure 3. Such patterns
presented a high degree of crystallinity, which indicates that well-defined crystalline structures
were produced. After an extensive search in the crystallographic main databases, such as ICSD
and CSD from Cambridge, we did not find a perfect match for molecules H-1 and H-2. This
searching comprised stoichiometry, 2D and 3D structural search as well.
13

Nevertheless, for the H-3 sample, the obtained XRD pattern presented a very good
agreement with a previously reported structure for the same compound (CCDC 145482). This is
a non-centrosymmetric structure, which belongs to P21 space group. Using Rietveld analysis
tools from TOPAS V.5 software, it was possible to refine sample H-3 XRD data and determine
that this crystalline material has cell parameters a=8.9199 Å, b=5.8915 Å, c=15.5999 Å and
β=93.44o, very close to the reported. This result is showed in figure S1 of the supplementary
material.
H-1
H-2
H-3
5
10
15
20
25
30
35
2q
Figure 3. Powder XRD graphs of pyrazoline crystals.
4.5. Nonlinear optical properties
4.5.1. Molecular geometry
The optimized molecular structures and their corresponding geometric parameters, obtained
using distinct functionals and basis sets, are summarized in the Supporting Information Tables
S1-S3. It was observed that despite the employed functional and basis sets, all optimized
molecules presented a C1 point group symmetry. Additionally, the insertion of the pyrazoline
nucleus causes a shift in structural planarity in phenyl 2, conserving the planarity of the
14

remaining molecules in all investigated compounds and level of theory. These results can be
easily visualized in figure 4, where it is presented the planar, lateral and front views of the three
compounds structures calculated with CAM-B3LYP functional and 6-311++G(2d,p) basis set.
It should be mentioned that a molecular structure for the H-3 sample was proposed
previously by Foces et al. [74]. Analyzing only the bonds related to the carbon linked to the
pyrazoline group, the calculated results using M06-2X and PBE functionals present the best
agreement with the structure reported in ref. [74].
Front view
H-1
Planar view
Side view
H-2
H-3
Figure 4: Molecular structure of chromophores pyrazolines H-1, H-2 and H-3 obtained with the CAM-
B3LYP functional using the 6-311++G(2d,p) basis set.
4.1.1 Calculation of the first hyperpolarizability
15

A summary of the calculated electrical properties for the pyrazoline compounds using
TDHF and DFT methods is presented in Tables 1 and 2. It should be noticed that for different
functional and basis set, the calculated values of the molecules’ dipole moment differs slightly,
with the exception of H-2 sample. For this compound, the calculated values of dipole moment
using Hartree-Fock (HF) and M06-2X were roughly two times smaller than the results obtained
using other functionals. This difference can be attributed to the discrepancies among the
optimized molecular structures obtained by distinct functionals, as well as the differences
among the implemented Hamiltonians in each method.
Table 1: Total dipole moment (in units of Debye) of the pyrazoline compounds calculated using distinct
functionals and basis set in gas phase.
NLO Pyrazolines
Functional
B3LYP
H-1
H-2
H-3
2.59
5.17
5.26
2.56
2.54
4.82
2.57
2.60
2.54
2.82
2.73
CAM-B3LYP 2.74
M06-2X
HF
2.63
2.89
2.72
PM7
Firstly, we compare the H-1 sample with its DBA molecule counterpart, investigated by
Sudha et al. [24]. Using the theory level HF/6-31G (d,p) and the density functional theory
(DFT), they obtained a total dipole moment of 4.12 D, which is ~ 1.6 times larger than the H-1
values calculated in this present work. This indicates that the functional group modification
changes the molecular dipole moment significantly.
A similar analysis was performed, comparing molecule H-3 and its chalcone base structure,
reported by Abegão et al. [26]. The calculated values of H-3 dipole moment were similar to the
previously reported value of the original chalcone molecule, 2.54 D [26]. Such values are
16

suggesting that the change of the functional group C=O by C=N-N-Phenyl affects very little the
molecule dipole moment in this case.
However, owing to the discrepancies amongst the dipole moment of H-2 molecules
calculated by distinct methods, a direct comparison with previous works and H-1 molecule
should not be undertaken.
Table 2: Theoretically calculated static and dynamic first-order hyperpolarizability of the pyrazolines
compounds in gas phase, expressed in units of 10-30 esu. The dynamic β was evaluated for λexc=1064 nm.
NLO Pyrazolines
H-1
H-2
H-3
Functional
B3LYP
β (0;0,0)
28.9
β(-2ω;ω,ω) β (0;0,0)
β(-2ω;ω,ω) β (0;0,0)
β(-2ω;ω,ω)
33.5
114.7
35.9
44.0
26.5
52.4
15.7
6.3
7.4
4.1
7.8
77.0
19.2
24.5
13.4
44.1
11.8
6.3
6.8
5.1
7.8
CAM-B3LYP 14.9
12.9
M06-2X
TDHF
PM7
16.7
11.3
9.7
15.0
9.3
35.1
As shown in table 2, regarding the differences between the employed theoretical models, it
can be observed that the investigated molecules present relatively large calculated values of
static and dynamic first-order hyperpolarizability. These results are comparable with the
response of other promising organic molecules reported in literature, such as chalcones
[26,29,75,76], DBAs [24,25,27,77,78], and different pyrazoline-derivatives [48,51,79–82].
Although different functional and basis sets led to distinct values of the molecules’ static
and dynamic first hyperpolarizabilities, general behaviors can be detected. For instance, in
comparison with its DBA molecule counterpart investigated by Sudha et al. [24]. The values of
the calculated first-order hyperpolarizabilities of H-1 pyrazoline are larger than the reported
values for the base structure [20]. This result suggests that the change of the functional group
17

C=O by C=N-N-Phenyl increases the electronic delocalization along the molecule, providing
larger second-order optical nonlinear responses.
An analogous result was achieved when a comparison between molecule H-3 and its
chalcone base structure, reported by Abegão et al. [26]. In this previous investigation, they
calculated the static and dynamic first hyperpolarizabilities of the chalcone molecule in gas
phase and within a specific solvent. They reported static and dynamic first-order
hyperpolarizabilities equal to 5.3 ×10-30 esu and 9.1 ×10-30 esu in gas phase. Again, the
compound with C=N-N-Phenyl functional group presents slightly higher calculated
hyperpolarizabilities values, despite the level of theory employed.
Nevertheless, a definitive conclusion cannot be made comparing sample H-2 with its
corresponding DBA. Indeed, Ravindra et al. [25] and Shettigar et al. [77] have previously
investigated the second-order nonlinear response of this DBA para-disubstituted by the OCH3
electron donor group (p-methoxy dibenzylideneacetone). Using MOPAC 2007 with PM6
Hamiltonian, the calculated value of this molecule’s dynamic first-order hyperpolarizability was
22 ×10-30 esu, in gas phase [25]. On the other hand, its experimental value was estimated as
9.9 ×10-30 esu [77]. Analyzing only the theoretical result obtained with the functionals B3LYP,
M06-2X, and PM7, the change of the functional group increased the nonlinear response of H-2
compound. On the other hand, the conclusion was the opposite whenever CAM-B3LYP and
TDHF level of theory were employed.
Comparing the DBA-derivatives molecules, all results indicate that the first-order
hyperpolarizabilities of sample H-1 are larger than the values exhibit by sample H-2. This
indicates that the inclusion of the OCH3 group caused a significant decrease in the value of the
first order hyperpolarizabilities. This indicates that insertion of the OCH3 group on both sides of
this pyrazoline compound is not a good strategy to enhance its nonlinearity, which is the
opposite behavior of the observed in the corresponding DBAs base structures [25]. This
suggests that the branched molecular structure of pyrazoline, in other words, the addition of
18

C=N-N-Phenyl, affects the charge delocalization in a different fashion than the C=O group of
the corresponding DBA system [25].
Another interesting common result observed in all calculations is that the chalcone
derivative pyrazoline, H-3, exhibit smaller first hyperpolarizabilities values than H-1 and H-2
DBA- derivatives compounds. Comparing the pyrazolines H-1 and H-3, the only difference
between them is the presence of a double bond and an additional single bond in the DBA-
derivative compound. This result suggests that, at the molecular level, the DBA-derivative
pyrazoline is more likely to present better values of the first-order hyperpolarizability in
comparison with the chalcone-derivative pyrazoline. We believe that the extent of the conjugate
chain affects the charge transfer (CT) and determines the efficiency of SHG. The larger the
conjugate chain, the higher is the first-order hyperpolarizabilities of the pyrazolines [25,77].
4.5.2. Study of SHG powder efficiency
The SHG response of a material depends inherently on its structural and electronic
properties [5,9–12]. Thus, to verify the efficiency of macroscopic SHG, one should consider
factors such as average crystal size, crystal packing, transparency in the region of interest, and
excitation laser intensity.
Firstly, we performed the measurement of the SHG signal with the powders as prepared.
Thus, the samples consisted of particles with a wide range of sizes, and all of them contributed
to the observed SHG signals. In Fig. 5, we present the emission spectra obtained for the samples
H-1, H-2, and H-3, comparing with the KDP results. The emission of light with a narrow
spectral width, precisely at half the wavelength of excitation at 1064 nm, was observed. Under
the same excitation and signal collection conditions, compounds H-1 and H-3 showed higher
SHG intensities than that obtained for the reference sample, while the compound H-2 emission
of second harmonic intensity was lower than that emitted by the KDP. These results indicate
that all samples must exhibit at least one phase with a non-centrosymmetric crystalline
structure. Indeed, samples H-1 and H-2 showed well shape diffractograms, but no structure
determination was possible based on the XRD results. Nevertheless, based on the SHG results,
19

we believe H-1 and H-2 powders must present a non-centrosymmetric crystalline structure or, at
least, a mix of phases in which at least one of them must be non-centrosymmetric. Further
investigation must be carried out aiming an accurate crystalline elucidation of these materials.
700
6000
KDP
H-1
a)
b)
KDP
H-2
600
500
400
300
200
100
0
5000
4000
3000
2000
1000
0
h = 1,61
580
h = 0.02
580
480
500
520
540
560
600
480
500
520
540
560
600
Wavelenght (nm)
Wavelength (nm)
1200
1000
800
600
400
200
0
c)
KDP
H-3
h = 2.72
560 580
480
500
520
540
600
Wavelenght (nm)
Fig. 5. Comparative powder SHG intensity spectrum of pyrazolines crystals with KDP. For all
measurements, the excitation laser average power was 28 mW.
We defined the sample’s relative SHG efficiency (η) as the ratio between the intensity
emitted by this compound and the second harmonic intensity emitted by the reference sample
under the same experimental conditions. From the results presented in Fig. 5, it is observed that
the compounds H-1 e H-3 were more efficient for SHG than KDP. H-1 is approximately 1.6
20

times more efficient than KDP, while of H-3 presented a SHG efficiency about 2.72 times
higher than the reference sample.
As the intensity of the SHG signal obtained by the powder method depends on the size of
the particles, for a more accurate evaluation of SHG efficiency, the crystals were pulverized and
separated into four sets of samples, each one containing particles with distinct size distributions,
that is, particles with sizes < 75 μm, within the ranges of 75-106 μm and 106-150 μm, and larger
than 150 μm. We measured the SHG and its corresponding efficiency for each set, in
comparison with powdered KDP response with the same size distribution, under the same
excitation and light collection conditions. The results are presented in Figure 6.
KDP
H-1
(a)
2,5
2,0
1,5
1,0
0,5
0,0
2000
1500
1000
500
0
(b)
H-1 X KDP
> 150
75-106
106-150
>150
75-106
106-150
Particle Size (mm)
Particle size (mm)
0,12
0,10
0,08
0,06
0,04
0,02
0,00
7000
6000
5000
4000
3000
2000
1000
0
(d)
KDP
H-2
H-2 X KDP
(c)
21
< 75
75-106
106-150
>150
106-150
< 75
75-106
>150
Particle size (mm)
Particle size (mm)

3000
2500
2000
1500
1000
500
(f)
H-3
KDP
(e)
H-3 X KDP
5
4
3
2
1
0
0
< 75
75-106
Particle size (mm)
106-150
>150
106-150
Particle size (mm)
< 75
>150
75-106
Figure 6. Particle size dependence of SHG intensity of pyrazolines powder and efficiency of SHG.
Excitation laser average power = 52 mW.
As we can see in Fig. 6, the compound H-1 showed SHG signal and efficiency higher than
the KDP for all the size ranges explored in this study. We also verified that the SHG signal of
the crystals of H-1 increases as the particle size is raised. This result suggests that the
combination of the relatively high hyperpolarizability of the molecule, associated with favorable
crystallization contributed to these crystals’ good second-order nonlinear response.
Similarly, to the sample H-1, the crystalline powder of H-2 exhibited larger SHG signal for
larger crystals’ size. However, their SHG intensity and efficiency were lower than the results
presented by KDP and H-1 samples, throughout the particle size range explored in our work.
Although a less efficient SHG was expected for compounds H-2, based on the first order
hyperpolarizability calculations, this is not enough to explain this huge reduction in the
experimental crystal response. These results indicate that the inclusion of the electron donor
group OCH3 at para positions of the phenyl groups located at the ends of the base molecule H-
1, combined with the crystalline structure formed, resulted in the strong suppression of the SHG
observed in the crystals of H-2.
22

Although its calculated hyperpolarizability value was slightly lower than that of H-1, the
crystalline powder of the compound H-3 presented the highest efficiency of SHG among the
materials investigated in this study. In fact, its SHG efficiency was about 5 times greater than
the KDP when particles of a smaller size than 75 μm were used. We believe this is related to the
distinct crystalline structures and molecular packing of the investigated compounds.
Additionally, the observed SHG signal for H-3 presented an apparent discrete maximum for
particles of size comprised between 106 and 150 µm. In principle, this result would suggest that
these crystals could be classified as phase-matchable [64]. However, a more detailed study must
be performed with particles discriminated in narrower size ranges and ranges with larger
particles included, before we can safely state in what class of nonlinear medium these organics
crystals belong.
A direct comparison between the experimental SHG of the investigated samples with other
similar groups of organic crystals cannot be easily done, owing to the limitations of the
employed experimental techniques. In Table 3, it is summarized the results obtained in the
present work and the reported values for chalcone and DBAs base systems. It can be noticed
that no SHG efficiency was reported for DBA and chalcone base structures. On the other hand,
DBA derivative compound substituted by OCH3 showed a measurable SHG, with an efficiency
of up to 15.5 and 6 times larger than the response of urea [25,77]. From these results, we
observed that the inclusion of C=N-N-Phenyl produced crystals with much better SHG
responses than the DBA [65] and chalcone [15] base structures. Nevertheless, without further
information concerning the experimental conditions of these previous works, it is not possible to
compare accurately the results among H-2 its corresponding base structure [25,77].
Table 3: Experimental SHG and UV-vis.
Compound
SHG efficiency
Reference
λcut-off (nm)
H-1
H-2
2.27a ± 0.02
0.06a ± 0.00
442
442
This work
This work
23

H-3
2.62a ± 0.30
420
440
434
435
This work
DBA
0
Vanchinathan et al. [65] and Rajasekar et al. [83]
Ravindra et al. [25] and Shettigar et al. [77]
Zhao et al. [15]
DBA 4-OCH3
Chalcone
15.5-6b
0
a In comparison with KDP powder. The experiments were carried out with pyrazolines and KDP particles size within
the range from 75 to 106µm.
b In comparison with urea.
5 Conclusions
We investigated experimentally and theoretically the optical properties of three pyrazoline
compounds derived from two DBAs and a chalcone. It was observed that these molecules are
NLO active materials and present excellent macroscopic and microscopic efficiency for second
harmonic generation. Based on the theoretical analysis, it was verified that the branched
molecular structure of the investigated pyrazolines compounds enhances the molecules first
hyperpolarizabilities in comparison with the linear molecular structure exhibited by the
chalcone and DBAs counterparts. We also verified that the introduction of the donor group
OCH3 at para positions in the compound H-2 reduces molecular second-order nonlinear
response in comparison to the compound H-1. Additionally, the comparison of H-1 and H-3
first hyperpolarizabilities results suggests that increasing the conjugation length enhances the
molecular nonlinear response. From SHG macroscopic measurements, it was verified that H-2
and H-3 samples presented the lowest and the highest SHG efficiencies respectively. The huge
difference among the SHG efficiencies among H-1 and H-2 samples and the best performance
exhibited by H-3 crystalline powder cannot be explained based only on their molecular
geometry and their first hyperpolarizabilities. Indeed, this indicates that the formed crystalline
structure is playing an important role on the macroscopic SHG exhibited by these compounds.
The results strongly suggest that these compounds are potential sources for new NLO materials,
which can be designed to present enhanced optical properties aiming applications in nonlinear
second-order optical devices.
24

Declaration of interests
The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
The authors declare the following financial interests/personal relationships which may be
considered as potential competing interests:
Acknowledgements
Financial support from CNPQ (Conselho Nacional de Desenvolvimento Científico e
Tecnologico) and FAPITEC/SE (Fundação de Apoio à Pesquisa e à Inovação Tecnológica do
Estado de Sergipe) are acknowledged. This work was financed in part by the Coordenação de
Aperfeiçoamento de Pessoal de Nível Superior (CAPES) - Brasil - and was performed in the
framework of the National Institute of Photonics (INCT de Fotônica), Grant Number
465763/2014-6, MCTI/CNPq/FACEPE. We also thank the DQ-UFJF for the analysis of FT-IR
and NMR.
25

References
[1]
[2]
[3]
[4]
[5]
[6]
D.S. Chemla, Nonlinear optical properties of organic molecules and crystals, Elsevier,
2012.
C.W. Lee, O.Y. Kim, J.Y. Lee, Organic materials for organic electronic devices, J. Ind.
Eng. Chem. 20 (2014) 1198–1208. doi:10.1016/j.jiec.2013.09.036.
R. Tomas, Principles and applications of nonlinear optical materials, Springer, 2002.
doi:10.1016/0143-8166(93)90051-l.
J. Clark, G. Lanzani, Organic photonics for communications, Nat. Photonics. 4 (2010)
438–446. doi:10.1038/nphoton.2010.160.
J. Zyss, Molecular nonlinear optics: Materials, phenomena and devices - Preface,
Academic press, 1999.
K. Iliopoulos, D. Kasprowicz, A. Majchrowski, E. Michalski, D. Gindre, B. Sahraoui,
Multifunctional Bi2ZnOB2O6 single crystals for second and third order nonlinear
optical applications, Appl. Phys. Lett. 103 (2013) 231103.
[7]
[8]
B. Kulyk, B. Sahraoui, O. Krupka, V. Kapustianyk, V. Rudyk, E. Berdowska, S.
Tkaczyk, I. Kityk, Linear and nonlinear optical properties of ZnO/PMMA
nanocomposite films, J. Appl. Phys. 106 (2009) 93102.
B. Sahraoui, R. Czaplicki, A. Klöpperpieper, A.S. Andrushchak, A. V Kityk,
Ferroelectric AgNa (NO 2) 2 crystals as novel highly efficient nonlinear optical material:
Phase matched second harmonic generation driven by a spontaneous and electric field
induced polarizations, J. Appl. Phys. 107 (2010) 113526.
[9]
T.P. Radhakrishnan, Molecular structure, symmetry, and shape as design elements in the
fabrication of molecular crystals for second harmonic generation and the role of
molecules-in-materials, Acc. Chem. Res. 41 (2008) 367–376. doi:10.1021/ar7002072.
[10] X. Liu, Z. Yang, D. Wang, H. Cao, Molecular structures and second-order nonlinear
optical properties of ionic organic crystal materials, Crystals. 6 (2016) 158.
26

doi:10.3390/cryst6120158.
[11] H. Chen, Q. Ma, Y. Zhou, Z. Yang, M. Jazbinsek, Y. Bian, N. Ye, D. Wang, H. Cao, W.
He, Engineering of Organic Chromophores with Large Second-Order Optical
Nonlinearity and Superior Crystal Growth Ability, Cryst. Growth Des. 15 (2015) 5560–
5567. doi:10.1021/acs.cgd.5b01216.
[12] D.R. Kanis, M.A. Ratner, T.J. Marks, Design and construction of molecular assemblies
with large second-order optical nonlinearities. Quantum chemical aspects, Chem. Rev.
94 (1994) 195–242. doi:10.1021/cr00025a007.
[13] L.M.G. Abegão, R.D. Fonseca, F.A. Santos, J.J. Rodrigues, K. Kamada, C.R. Mendonça,
S. Piguel, L. De Boni, First molecular electronic hyperpolarizability of series of π-
conjugated oxazole dyes in solution: an experimental and theoretical study, RSC Adv. 9
(2019) 26476–26482. doi:10.1039/C9RA05246A.
[14] K.Y. Suponitsky, T. V Timofeeva, M.Y. Antipin, Molecular and crystal design of
nonlinear optical organic materials, Russ. Chem. Rev. 75 (2006) 457–496.
[15] B. Zhao, W.-Q. Lu, Z.-H. Zhou, Y. Wu, The important role of the bromo group in
improving the properties of organic nonlinear optical materials, J. Mater. Chem. 10
(2000) 1513–1517. doi:10.1039/A909757K.
[16] R.D. Fonseca, M.G. Vivas, D.L. Silva, G. Eucat, Y. Bretonnière, C. Andraud, L. De
Boni, C.R. Mendonça, First-Order Hyperpolarizability of Triphenylamine Derivatives
Containing Cyanopyridine: Molecular Branching Effect, J. Phys. Chem. C. 122 (2018)
1770–1778. doi:10.1021/acs.jpcc.7b05829.
[17] R.W. Boyd, Nonlinear Optics 3rd ed (New York: Academic), 2008.
[18] W. Wu, J. Qin, Z. Li, New design strategies for second-order nonlinear optical polymers
and dendrimers, Polymer (Guildf). 54 (2013) 4351–4382.
doi:10.1016/j.polymer.2013.05.039.
[19] H.C. Jeong, M.J. Piao, S.H. Lee, M. Jeong, K.M. Kang, G. Park, S. Jeon, B.R. Cho,
Octupolar dendrimers with large first hyperpolarizability, Adv. Funct. Mater. 14 (2004)
64–70. doi:10.1002/adfm.200304478.
27

[20] F. Liu, H. Xiao, Y. Yang, H. Wang, H. Zhang, J. Liu, S. Bo, Z. Zhen, X. Liu, L. Qiu,
The design of nonlinear optical chromophores exhibiting large electro-optic activity and
high thermal stability: The role of donor groups, Dye. Pigment. 130 (2016) 138–147.
doi:10.1016/j.dyepig.2016.03.003.
[21] J. Indira, P.P. Karat, B.K. Sarojini, Growth, characterization and nonlinear optical
property of chalcone derivative, J. Cryst. Growth. 242 (2002) 209–214.
doi:10.1016/S0022-0248(02)01306-4.
[22] E.D. Dsilva, D. Narayan Rao, R. Philip, R.J. Butcher, Rajnikant, S.M. Dharmaprakash,
Synthesis, growth and characterization of novel second harmonic nonlinear chalcone
crystal, J. Phys. Chem. Solids. 72 (2011) 824–830. doi:10.1016/j.jpcs.2011.04.003.
[23] J.M.F. Custodio, F.G. Santos, W.F. Vaz, C.E.P. Cunha, R.G. Silveira, M.M. Anjos,
C.E.M. Campos, G.R. Oliveira, F.T. Martins, C.C. Silva, C. Valverde, B. Baseia, H.B.
Napolitano, Molecular structure of hybrid imino-chalcone in the solid stateꢀ: X-ray
diffraction , spectroscopy study and third-order nonlinear optical properties, J. Mol.
Struct. 1157 (2018) 210–221. doi:10.1016/j.molstruc.2017.12.023.
[24] S. Sudha, N. Sundaraganesan, K. Vanchinathan, K. Muthu, S.P. Meenakshisundaram,
Spectroscopic (FTIR, FT-Raman, NMR and UV) and molecular structure investigations
of 1, 5-diphenylpenta-1, 4-dien-3-one: A combined experimental and theoretical study, J.
Mol. Struct. 1030 (2012) 191–203. doi:10.1016/j.molstruc.2012.04.030.
[25] H.J.Ravindra, A.John Kiran, S.M. Dharmaprakash, N.Satheesh Rai, K.Chandrasekharan,
B. Kalluraya, F. Rotermund, Growth and characterization of an efficient non-linear
optical D-π-A-π-D type chalcone single crystals, J. Cryst. Growth. 310 (2008) 4169–
4176. doi:10.1016/j.jcrysgro.2008.06.045.
[26] L.M.G. Abegão, R.D. Fonseca, F.A. Santos, G.B. Souza, A.L.B.S. Barreiros, M.L.
Barreiros, M.A.R.C. Alencar, C.R. Mendonça, D.L. Silva, L. De Boni, J.J. Rodrigues,
Second- and third-order nonlinear optical properties of unsubstituted and mono-
substituted chalcones, Chem. Phys. Lett. 648 (2016) 91–96.
doi:10.1016/j.cplett.2016.02.009.
28