Experimental and theoretical investigation of optical nonlinearities in (nitrovinyl)-1H-pyrazole derivative

Article abstract of DOI:10.1016/j.saa.2012.12.060

This work reports on the optical nonlinearities of a newly synthesized pyrazole derivative, namely (E)-1-(4-chlorophenyl)-4-(2-nitrovinyl)-1H-pyrazole. The Z-scan technique with femtosecond laser pulses was used to determine the two-photon absorption (2PA

Full text of DOI:10.1016/j.saa.2012.12.060

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 483–487
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Experimental and theoretical investigation of optical nonlinearities in
(nitrovinyl)-1H-pyrazole derivative
Y. Dwivedi a, L. de Boni a, P.J. Gonçalves b, L.M. Mairink c, R. Menegatti c, T.L. Fonseca b, S.C. Zilio a,
⇑
a Instituto de Física de São Carlos, Universidade de São Paulo, Caixa Postal 369, 13560-970 São Carlos, SP, Brazil
b Instituto de Física, Universidade Federal de Goiás, Caixa Postal 131, 74001-970 Goiânia, GO, Brazil
c Faculdade de Farmácia, Universidade Federal de Goiás, Caixa Postal 131, 74001-970 Goiânia, GO, Brazil
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
Reaction mechanism to achieve (nitrovinyl)-1H-pyrazole structure exhibiting 58 and 67 GM absorption
cross-section at 520 and 690 nm wavelengths, respectively and first hyper polarizability
45 ꢀ 10ꢁ30 cm5/esu.
"
"
"
"
"
Nonlinear optical properties of
newly synthesized (nitrovinyl)-1H-
pyrazole explored.
fs Z-scan technique used for 2PA
cross-section spectrum in 460–
780 nm range.
2PA exhibits two bands with cross-
sections of 58 (520 nm) and 67 GM
(690 nm).
Experimental bHRS (45 ꢀ 10ꢁ30 cm5/
esu) estimated by ps hyper-Rayleigh
scattering.
Theoretical calculations (MP2/6-
311+G(d) yield similar value
40 2 ꢀ 10ꢁ30 cm5/esu.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 October 2012
Received in revised form 19 December 2012
Accepted 21 December 2012
Available online 2 January 2013
This work reports on the optical nonlinearities of a newly synthesized pyrazole derivative, namely (E)-1-
(4-chlorophenyl)-4-(2-nitrovinyl)-1H-pyrazole. The Z-scan technique with femtosecond laser pulses was
used to determine the two-photon absorption (2PA) cross-section spectrum, which presents a maximum
of 67 GM at 690 nm. We have combined hyper-Rayleigh scattering (HRS) experiments and second-order
Møller–Plesset perturbation theory (MP2) calculations to study the first hyperpolarizability (bHRS). It was
found that the MP2/6-311+G(d) model, taking into account solvent and dispersion effects, provides the
bHRS value of 40 ꢀ 10ꢁ30 cm5/esu for the compound, in good agreement with the experimental result of
45 2 ꢀ 10ꢁ30 cm5/esu.
Keywords:
Two-photon absorption
Quantum chemical calculations
Z-scan technique
Ó 2012 Elsevier B.V. All rights reserved.
Hyper Rayleigh scattering
First hyperpolarizability
Introduction
oxazoles and thiazoles [4]. It contains a ring structure composed
of three carbon and two nitrogen atoms, whose presence as a lone
Pyrazole is an important heterocyclic compound whose deriva-
tives are used in a number of applications in medicine, going from
analgesia to antibacterial activities [1–3]. Such molecule is a mem-
ber of the azole family that also includes imidazoles, oxadiazoles,
pair produces a conjugated system that points to the possibility of
molecular nonlinear optical (NLO) activity. Currently, new pyrazole
derivates has been designed and synthesized in order to explore
their central pharmacological activity, in vivo hypnotic and analge-
sic profile and potential antipsychotic properties [5–7]. Addition-
ally, optical properties of a number of pyrazole derivatives were
already reported in literature [8–13]. The UV absorption spectra
⇑
Corresponding author. Tel./fax: +55 16 3373 8085.
E-mail address: zilio@ifsc.usp.br (S.C. Zilio).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.12.060

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Y. Dwivedi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 483–487
and dissociation constants of some 1-phenyl-5-substituted pyra-
zoles were studied by Tabak et al. [13], while Ovejero et al. inves-
tigated the luminescence characteristics of pyrazole ligands
bonded to Au(I) metal centers or AuL metal fragments (L = Cl,
PPh3, Hpz) [14]. In this study we focus on determining the nonlin-
ear optical (NLO) properties of a novel derivative of pyrazole,
which appear to be promising for NLO applications.
The nonlinear behavior of these molecules is important for sev-
eral applications and can be adjusted, to some extent, by changing
the asymmetric polarization through the inclusion of different do-
be named as compound (2), was carried out through the reaction
between aldehydes and nitromethane, and catalyzed by ammo-
nium acetate, at reflux temperature, for 12 h [23]. The typical pro-
cedure to produce it is as follows: in a 50 mL round-bottom flask,
5 mmol
of
1-(4-chlorophenyl)-1H-pyrazole-4-carbaldehyde,
named as compound (1), were added to a stirred solution of
ammonium acetate 1.25 mmol in dry nitromethane 25 mL. The
reaction mixture was refluxed for 12 h, then was allowed to cool
and was poured into water. The precipitate was filtrated to vac-
uum, dried and the crude product was purified by chromatography
on silica, using 50:50 n-hexane-CH2Cl2 as mobile phase. The com-
pound (2) was obtained in 72% of yield, as a yellow solid, with a
melting point of 146 °C and retardation factor Rf = 0.92 (n-hex-
ane/ethyl acetate 7:3). The main infrared bands and NMR data ob-
tained are briefly summarized as follows: IRmax (KBr) cmꢁ1: 3139
(CAH), 1552 and 1325 (NO2), 970 (C@C trans); 1H NMR
(500.13 MHz) CDCl3 ():8.17 (1H, s, H-5), 8.00 (1H, d, J = 13.5 Hz,
H-b), 7.96 (1H, s, H-3), 7.65 (2H, d, J = 8.9 Hz, H-20 and 60), 7.52
nor–acceptor groups at the terminal positions of a
p-conjugated
bridge [15–17]. Indeed, Miller et al. have studied NLO properties
of various bisubsituted pyrazol derivatives and reported large first
hyperpolarizabilities values for 1,3 and 1,4 substitutions, and dra-
matically lower for 1,5-substituted derivatives [8]. In this case the
NLO activity is analyzed using the push–pull strategy, where the
conjugated segment is capped by an electron-donor group on
one side and an electron-acceptor group on the other side. Other
NLO properties such as fluorescence following one- and two-pho-
ton absorption (2PA) in 3-(1,1-dicyanoethenyl)-1-phenyl-4,5-dihy-
dro-1H-pyrazole crystal using 1064 and 532 nm wavelengths [18].
One of the reasons for the search of materials with large 2PA is the
wide range of potential applications such as two-photon fluores-
cence imaging, optical limiting, optical data storage, phototherapy
and up-conversion lasing [9,19–22]. These also show that the prep-
aration of pyrazole-based materials with large NLO could be impor-
tant for several technological applications.
The present investigation reports on the synthesis of a novel
pyrazole derivative, which contain a ring structure, composed of
three carbon and two nitrogen atoms, whose presence as a lone
pair produces a conjugated system that points to the possibility
of molecular NLO activity. The NLO characterization in dimethyl
sulfoxide (DMSO) solvent by means of femtosecond 2PA and hyper
Rayleigh scattering (HRS) with picoseconds pulse trains. The first
technique provides the imaginary part of the third order nonlinear
optical susceptibility while the second provides the first hyperpo-
larizability, a second order effect of the optical molecular polariz-
ability. In this case, in order to make an appropriate comparison
between theoretical and experimental results, correlated first
hyperpolarizability calculations have been performed accounting
for solvent and frequency dispersion effects.
(1H, d, J = 13.5 Hz, H-
a
), 7.47 (2H, d, J = 8.9 Hz, H-30 and 50); 13C
NMR (125.76 MHz) CDCl3 (): 140.7 (C-3), 137.3 (C-10), 135.9 (C-
a
), 133.3 (C-40), 129.8 (2 C-30 and 50), 129.2 (C-b), 128.3 (C-5),
120.4 (2 C- 20 and 60), 115.4 (C-4).
NLO measurements
Dimethyl sulfoxide (DMSO) of spectroscopic grade was used as
solvent for compound (2). Solutions with different concentrations
were prepared and ultrasonicated for 10 min. before performing
the experiments. Transparent solutions of light pale yellow and
yellow/red colors were achieved. The absorption spectra in the
UV/Vis region were measured with a Shimadzu UV-1800 spec-
trometer, with sample concentration of 6 ꢀ 1018 molecules per
cm3 (1 ꢀ 102 mol Lꢁ1). Both linear and nonlinear measurements
were done in fused silica cuvettes with a 2 mm optical path.
The 2PA cross-section spectrum was obtained with the open-
aperture Z-scan technique [24,25] using 120-fs pulses from an
optical parametric amplifier pumped by a commercial Ti:Sapphire
chirped pulse amplifier (775 nm and 1 kHz repetition rate), allow-
ing wavelength tuning from 450 to 800 nm. To improve the Gauss-
ian profile of the laser beam spatial filtering was used. The beam
waist at the focus was measured to be 15–17 lm, and the laser
pulse energies range from 50 to 100 nJ, depending on the excita-
tion wavelength. The details of the Z-scan experimental setup were
already discussed in Ref. [26]. The experimental Z-scan curves
Experimental section
Synthesis of (nitrovinyl)-1H-pyrazoles
High purity reagents and solvents, including ammonium ace-
tate and nitromethane, were purchased from Acros Organics Com-
pany and distilled prior using. The melting point was determined
with a MarteÒ 284594 apparatus. The E configurations of double
bonds were determined through NMR data, recorded by using a
Bruker Avance III spectrometer, operating at 500.13 MHz to proton
frequency. The samples for NMR measurements were prepared in
CDCl3 containing 1% tetramethylsilane as an internal standard.
Splitting patterns are designated as follows: s, singlet; d, doublet.
The infrared (IR) spectra was obtained with a Perkin–Elmer spec-
trometer 400 N FT-IR in KBr plates. The progress of the reaction
was monitored by thin-layer chromatography (TLC), which was
performed on 2.0 ꢀ 6.0 cm aluminum sheets precoated with silica
gel 60 (Merck) to a thickness of 0.25 mm. The developed chromato-
grams were observed under ultraviolet light (254–265 nm) and
treated with iodine vapor. For column chromatography, we used
Merck silica gel (70–230 mesh).
As illustrated in Fig. 1, the synthesis of (E)-1-(4-chlorophenyl)-
4-(2-nitrovinyl)-1-phenyl-1H-pyrazole, which for simplicity will
Fig. 1. Synthesis of (E)-1-(4-chlorophenyl)-4-(2-nitrovinyl)-1-phenyl-1H-pyrazole
(2).

Y. Dwivedi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 483–487
485
computational cost and reliability for a number of organic mole-
cules. Both electronic properties and geometry optimization calcu-
lations were performed using the GAUSSIAN 09 electronic structure
package [30].
were fitted using the expression for the normalized energy trans-
mittance, T(z), given as [24,25]:
Z
1
1
2
TðzÞ ¼
ln½1 þ q0ðz; tÞeꢁt ꢂdt;
ð1Þ
pffiffiffi
p
q0ðz; 0Þ
ꢁ1
À
Á
where q ðz; tÞ ¼ bI0ðtÞL 1 þ z2=z2 ꢁ1, L is the optical path, z0 is the
Results and discussion
0
0
Rayleigh length, z is the sample position, and I0 is the pulse irradi-
ance. By fitting the Z-scan curve with Eq. (1) one obtains the 2PA
coefficient b. The 2PA cross-sections can be determined using the
Two-photon absorption
relationship d = hmb/N, where hm is the excitation energy, and N is
As shown in Fig. 2, the one-photon absorption (1PA) spectrum
of compound (2) contains two absorption bands at 258 and
350 nm. The values of absorbance and the concentration were used
the number of molecules per cm3. Usually, d is expressed in Goep-
pert-Mayer (GM) units, being 1 GM = 1 ꢀ 10ꢁ50 cm4 s moleculeꢁ1
-
photonꢁ1 [27].
to estimate the ground-state absorption cross-section,
rg, from the
For the measurement of first hyperpolarizability, HRS experi-
ments were carried out with picosecond infrared (1064 nm) pulse
trains composed by approximately 20 pulses of 70 ps separated by
13 ns, delivered by a Q-switched and mode-locked Nd:YAG laser.
To improve the detection, the scattering point is located between
a spherical mirror and a telescope, and the signal is collected by
a photomultiplier with a response time of 3 ns at FWHM. A refer-
ence signal for the laser intensity is collected by a PIN detector
with a response time of 1 ns. The value of the first hyperpolariz-
ability was determined by using the external approach with p-
NA (solute) in DMSO (solvent) as reference. Detailed description
of the experimental arrangement can be found in Ref. [28].
relation g = N g, where ag is the ground-state absorption coeffi-
a
r
cient and N is the molecular concentration. The value of absorption
cross-section obtained for compound (2) was found to be
5.94 ꢀ 10ꢁ16 cm2 at 354 nm.
TD-CAM-B3LYP calculations indicate that the absorption bands
originate from
p ?
pꢄ transitions. In gas-phase, the TD-CAM-B3LYP
results give the lower transition energy of 4.14 eV (299 nm) and
the higher one of 5.39 eV (230 nm). In DMSO, the IEFPCM-TD-
CAM-B3LYP model predicts transition energies of 3.78 eV
(328 nm) and of 5.25 eV (236 nm), respectively. These findings
show that the solvent effects lead to very large red shift of
ꢁ0.36 eV (29 nm) for the lower energy band when going from
gas-phase to DMSO. For the higher energy band the corresponding
red shift is smaller and is predicted to be ꢁ0.14 eV (6 nm).
Quantum chemical calculations
For the compound (2), the CAM-B3LYP functional predicts an
absolute energy value that is underestimated in 22 nm, as com-
pared with experimental result of 350 nm (lower energy band) in
DMSO. Our TD-DFT calculations also indicate that the lower energy
band corresponds to HOMO ? LUMO excitation whereas the high-
er one originates from a HOMO ? LUMO + 1 excitation. The shape
of the frontier orbitals involved in the electronic transitions is illus-
trated in Fig. 3. One can see that the HOMO orbital is delocalized
over the compound but its major part is localized on the benzene
and pyrazole rings. Upon excitation, the shape of the LUMO orbital
is characterized by an intramolecular charge transfer from benzene
ring (with a contribution of the Cl atom) to NO2 group, including
some atoms of the pyrazole ring. The LUMO + 1 orbital is delocal-
ized over the benzene and pyrazole rings and NO2 group.
Initially, the ground-state geometry was fully optimized in gas-
phase and in DMSO without any symmetry constraint using the
second-order Møller–Plesset perturbation theory (MP2) method
with the 6-311G(2d) basis set. To calculate the solvent effects we
have employed the self-consistent reaction field (SCRF) approach
with the polarizable continuum model within the integral equation
formalism (IEFPCM) [29], as implemented in the GAUSSIAN 09
package [30]. DMSO is an aprotic polar solvent and is expected that
IEFPCM method provides an appropriated description of solvent
effects.
The excitation energies were obtained using the TD-DFT ap-
proach [31] in combination with the CAM-B3LYP (CAM-B3LYP:
Coulomb-attenuating method applied to B3LYP) of Yanai et al.
[32]. This long-range corrected functional improves the asymptotic
behavior of the exchange interaction, which provides a reliable
description of the charge-transfer excited-states. Following a pre-
vious work [33], we have adopted in the TD-DFT calculations the
6-311+G(2d,p) basis set.
The 2PA cross-section spectrum of compound (2) in the wave-
length range of 510–750 nm is also depicted in Fig. 2. They were
The static and dynamic components of the first hyperpolariz-
ability were calculated analytically using the time-dependent Har-
tree–Fock (TDHF) approach [34].
A standard wavelength of
1064 nm was considered in dynamic property calculations. At the
MP2 level, the calculations of the static properties were performed
numerically through the finite field (FF) method using field
strengths of order 0.001 a.u. To estimate the correlated dynamic
results, we have employed the multiplicative correction scheme
used in the literature [35]:
bTDHFðꢁ2w; w; wÞ
bMP2ðꢁ2w; w; wÞ ꢃ bMP2ð0; 0; 0Þ ꢀ
ð2Þ
bTDHFð0; 0; 0Þ
The first hyperpolarizability reported here is the quantity related to
the hyper-Rayleigh scattering intensity for plane polarized incident
light, given by [36]:
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
bHRSðꢁ2w; w; wÞ ¼ hb2ZZZi þ hbZ2ZZ
i
ð3Þ
The first hyperpolarizability calculations were performed with the
Fig. 2. Linear absorption spectrum (solid line) and 2PA cross-section (open circles)
spectra of compound (2) in DMSO solution.
6-311+G(d) basis set that provides
a compromise between

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Y. Dwivedi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 483–487
extracted by fitting the Z-scan normalized transmittance curves
with Eq. (1) for several excitation wavelengths, as shown by the
dashed lines of Fig. 4a. These Z-scan signatures present a decrease
in the normalized transmittance on the z position, which indicates
the existence of a 2PA process.
Comparing the 1PA and 2PA spectra depicted in Fig. 2, one can
see that the absorption bands appear at nearly the same energy.
Similar to the 1PA spectrum, 2PA also exhibits two bands with
cross-section of ꢅ58 and 67 GM (1 GM = 1 ꢀ 10ꢁ50 cm4 s pho-
tonꢁ1) at 520 and 690 nm, respectively. Recalling the symmetry
selection rule, in case of molecules without center of symmetry,
i.e. noncentrosymmetric molecules, the parity of the electronic
states is not well-defined and the same transition is simulta-
neously allowed for 1PA and 2PA. This is distinct from centrosym-
metric molecules, in which the selection rules are exclusive for 1PA
and 2PA. The allowed transitions are shown in Fig. 4b.
First hyperpolarizability
As already mentioned, the first order hyperpolarizability was
measured in picosecond hyper Rayleigh scattering experiments.
In HRS, the intensity of the scattered light is proportional to the
quadratic of the incident light intensity, according to the expres-
sion [37]:
À
Á
Ið2
x
Þ ¼ G ꢆ
q
svb2sv
þ
q
stb2st I2ð
x
Þ
ð4Þ
where I(
x
) and I(2
x
) are, respectively, the irradiance of the incident
light and the light scattered at the double of the frequency of the
incident light. The molecular density ( ) and first hyperpolarizabil-
ity (b) of the solvent and solute are represented as qsm qst and bsv
q
,
,
bst, respectively. Keeping the solvent density constant and varying
the solute density, a linear behavior with angular coefficient Gb2st,
obtained by plotting I(2x x) against qst, is expected.
)/I2(
Fig. 4. (a) Experimental open aperture Z-scan curves of (2) obtained upon
excitation with different wavelengths at identical power. The dashed lines are the
theoretical fits obtained by using Eq. (1). (b) Energy level diagram for compound (2)
and pathways for optical transitions.
Since the molecule under study partially absorbs at 532 nm, the
light scattered should be corrected by the factor 10A/b, where A is
absorbance at 532 nm and b is the optical path of the sample.
The HRS data acquired for compound (2) in DMSO solution, given
by Eq. (4), and the behavior of I(2x x) as a function of the con-
)/I2(
centration are depicted in Fig. 5.
The first hyperpolarizability can be calculated by taking the ra-
tio between the angular coefficient ( ) of the reference and com-
a
pound (2), and using the known first hyperpolarizability of the
reference (which for pNA in DMSO is 26.2 ꢀ 10ꢁ30 cm5/esu)
according to:
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
asample
areference
bsample
¼
breference
ð5Þ
Using Eq (5), we obtained the first hyperpolarizability of compound
(2) in DMSO ass (45 2) ꢀ 10ꢁ30 cm5/esu.
We compare the MP2/6-311+G(d) results for the static and dy-
namic first hyperpolarizabilities of compound (2) obtained in the
gas-phase and in DMSO with the experimental value. In the gas-
phase, the static result for bHRS is of 12.40 ꢀ 10ꢁ30 cm5/esu. This va-
lue represents around of 28% of the experimental result in DMSO,
indicating that the MP2 result obtained in gas-phase for bHRS is
only in qualitative agreement with the measured value. It is found
here that the addition of the solvent polarization effects have a
marked influence on the first hyperpolarizability and is crucial to
Fig. 3. Representation of the molecular orbitals for compound (2) in DMSO.

Y. Dwivedi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 483–487
487
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In summary, a novel organic compound namely (E)-1-(4-chloro-
phenyl)-4-(2-nitrovinyl)-1H-pyrazole was synthesized and its
nonlinear optical properties were investigated using 2PA and
HRS measurements, as well ab initio calculations at the MP2 level
with the 6-311+G(d) basis set. It is found that this molecule is non-
linear active with 2PA cross-sections of ꢅ58 and 67 GM at 520 and
690 nm, respectively. The MP2 results show that the role of the sol-
vent is crucial to meet the concordance with experiment. Disper-
sion effects increase the static bHRS value around 7%. The solution
dynamic MP2 result for bHRS is estimated to be 40 ꢀ 10ꢁ30 cm5/
esu. This theoretical value of bHRS is in good agreement with the
measured value of 45 2 ꢀ 10ꢁ30 cm5/esu.
Acknowledgements
The authors are grateful to Fundação de Amparo à Pesquisa do
Estado de São Paulo (FAPESP) and Conselho Nacional de Desen-
volvimento Científico e Tecnológico (CNPq) for financial support.