Structural, thermal, linear and nonlinear optical studies of an organic optical limiter based on reverse saturable absorption

Article abstract of DOI:10.1016/j.molstruc.2016.04.052

A new derivative of chalcone, 3-(4-bromophenyl)-1-(pyridin-4-yl) prop-2-en-1-one (4BP4AP), crystallizing in centrosymmetric structure has been synthesized using the Claisen-Schmidt condensation reaction method. The FTIR and FT-Raman spectral studies were

Full text of DOI:10.1016/j.molstruc.2016.04.052

Journal of Molecular Structure 1119 (2016) 167e176
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Structural, thermal, linear and nonlinear optical studies of an organic
optical limiter based on reverse saturable absorption
,
Anthoni Praveen Menezes a *, S. Raghavendra b, A. Jayarama c, H.P. Sarveshwara a,
S.M. Dharmaprakash d
a Department of Physics, Mangalore Institute of Technology & Engineering (MITE), Moodabidri, 574225, India
b Department of Physics, Adichunchanagiri Institute of Technology, Jyothinagaar, Chikmagalore, 577102, India
c Department of Physics, Sahyadri College of Engineering & Management, Adyar, 575007, India
d Department of Studies in Physics, Mangalore University, Mangalagangotri, 574199, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A new derivative of chalcone, 3-(4-bromophenyl)-1-(pyridin-4-yl) prop-2-en-1-one (4BP4AP), crystal-
lizing in centrosymmetric structure has been synthesized using the ClaiseneSchmidt condensation re-
action method. The FTIR and FT-Raman spectral studies were carried out on 4BP4AP for structural
conformation. The single crystals were grown using slow evaporation solution growth technique. The
single crystal XRD of the crystal shows that the crystal system of 4BP4AP is triclinic with space group P-1.
Scanning electron microscope images enunciate the surface smoothness and the two dimensional
growth mechanisms in the crystal. The crystal is transparent in the entire visible region as indicated by
the UV-VIS-NIR spectrum. The thermal stability and phase transition of the compound was studied by
thermogravimetric and differential scanning calorimetric analysis and found to be stable up to 200 ꢀC. By
performing the open aperture z-scan experiment, nonlinear absorption and optical limiting behavior of
the crystal were studied. The crystal can be used for optoelectronic application due to its excellent photo-
physical properties.
Received 25 January 2016
Received in revised form
16 April 2016
Accepted 18 April 2016
Available online 26 April 2016
Keywords:
Crystal growth
Characterization
Thermal analysis
X-ray diffraction
Nonlinear optical materials
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
one can tune one of the aromatic rings of chalcones as donor and
the other as acceptor by incorporating suitable functionalities on
Currently nonlinear optical (NLO) materials exhibiting strong
two photon absorption (TPA) are of great interest due to their wide
range of applications. Among the variety of molecular and bulk
materials studied for NLO properties, organic materials are of
particular importance due to their optical and electronic properties,
which are tunable through structural modifications [1]. Organic
materials with large third order NLO coefficients are attractive for
various photonic applications such as optical communication,
signal processing, optical modulation, optical disc data storage,
electro-optic modulation, optical switching and optical limiting
[2,3]. Chalcone and its derivatives are one of the important classes
of optically nonlinear organic materials. These are open chain fla-
vonoids with two aromatic rings joined by a carbonyl group and
the aromatic rings so that the charge transfer takes place from the
donor to the acceptor through the charge transfer axis [5,6]. Due to
the overlapping of
p orbital in these molecules, delocalization of
electronic charge distribution increases the mobility of the electron
density which in turn enhances the optical nonlinearity [7].
Heterorings such as furan, pyridine and thiophene, due to their
lower aromatic stabilization energy than benzene have been re-
ported to provide more effective
p-conjugation between donors
and acceptors, resulting in larger nonlinearities [8]. Recently our
research group has reported a number of pyridine based chalcone
molecules with efficient NLO response [9e14]. Among them BPP
[14] showed a SHG efficiency of 1.4 times that of urea. In this
compound a bromo group is substituted at the para position of
phenylene moiety seeing that para substitution is more effective in
forming noncentrosymmetric crystal structures [13]. Moreover, BPP
is designed by just replacing the benzene ring of 4-bromochalcone
(4BC) by the pyridine ring in which a nitrogen atom is placed at the
meta position to the carbonyl group and this results in the
two a,b-unsaturated carbon atoms [4]. The presence of extensive
conjugation in these molecules provides a charge transfer axis. Also
* Corresponding author.
E-mail address: mapraveen.80@gmail.com (A.P. Menezes).
http://dx.doi.org/10.1016/j.molstruc.2016.04.052
0022-2860/© 2016 Elsevier B.V. All rights reserved.

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A.P. Menezes et al. / Journal of Molecular Structure 1119 (2016) 167e176
improved SHG efficiency [15]. In continuation of our study on
pyridine based substituted chalcones, we synthesized a new
molecule called 3-(4-bromophenyl)-1-(pyridin-4-yl) prop-2-en-1-
one (4BP4AP), in which a nitrogen atom is placed at the para po-
sition of the carbonyl group (Fig. 1). Both 4BC and BPP crystallized
in noncentrosymmetric crystal form with good SHG efficiency but
interestingly, the molecule 4BP4AP crystallizes in centrosymmetric
structure and thus the possibility of SHG is ruled out. The linear
optical and thermal studies revealed that this crystal is transparent
down to the blue region with very good thermal stability. Therefore,
in order to explore the usability of the crystal for device applica-
tions, its third order NLO studies were also carried out.
In recent times, a number of chalcone derivatives have been
investigated for their third order NLO properties such as nonlinear
absorption (NLA), nonlinear refraction (NLR) etc [16e21]. Materials
with large reverse saturable absorption are found to exhibit good
optical limiting behavior and such materials find applications in
protective devices for human eyes and also for various optical
sensors [22]. The third order NLO properties of chalcones can be
explored by single beam z-scan technique using which the third
order NLO behavior of 4BP4AP is explored [23]. In this article, we
report the growth of a chalcone derivative 4BP4AP by employing
slow evaporation solution growth technique and the experimental
results on its structural, thermal, linear and nonlinear optical
properties.
Fig. 2.
The solubility studies of the compound in various solvents were
carried out to select a suitable solvent for the growth of the single
crystals. To grow large single crystals of 4BP4AP, we used N, N-
dimethyl formamide (DMF) due to the suitable solubility in this
solvent and crystals were grown by the slow evaporation of the
solvent at room temperature [13]. A saturated solution of 3-(4-
bromophenyl)-1-(pyridin-4-yl) prop-2-en-1-one was prepared in
DMF in a 100 ml beaker at 30 ꢀC and the solution was filtered to
remove suspended particles. The beaker was covered with a poly-
thene sheet and kept in a dust free atmosphere. Transparent single
crystals of 4BP4AP were harvested from the growth solution after
allowing them to grow to the maximum possible dimensions
(4 ꢁ 2.5 ꢁ 1 mm3). High quality single crystals were selected for
further study. A photograph of the grown crystals is presented in
Fig. 3a. The morphology of crystal simulated by WinXMorph [24]
computer program is shown in Fig. 3b. The point group, lattice
parameters and (h k l) values obtained from single crystal x - ray
diffraction study were used as data input. By looking at the actual
morphology of the crystal, the well developed faces were indexed
by editing the distance from the centre of the model to its face. The
crystal is elongated slightly along the b e direction due to a higher
growth rate along this axis compared to the other two directions.
3. Results and discussion
2. Experimental procedure: synthesis, solubility and crystal
growth
3.1. CHN and EDAX analysis
Chalcone derivatives are synthesized by the ClaiseneSchmidt
condensation reaction [9]. It is the reaction of a substituted acetyl
pyridine with substituted benzaldehyde in the presence of an alkali.
The compound 4BP4AP was synthesized by stirring a mixture of
Analytical Reagent (AR) grade chemicals 4-acetyl pyridine
(0.01 mol) and 4-bromobenzaldehyde (0.01 mol) in methanol
(50 ml) without further purification. Sodium hydroxide (5 ml, 20%)
was then added drop wise to the solution, and the mixture was
stirred for 2 h. The mixture is stirred by maintaining the solution
within the temperature range of 27e30 ꢀC. The contents of the flask
were poured into a beaker containing ice-cold water. The crude
product was collected by filtration, washed with excess water and
dried. The synthetic scheme for the title compound is shown in
The percentage composition of the elements present in 4BP4AP
was confirmed by performing carbon, hydrogen, nitrogen (CHN)
analysis. The Elementar Vario EL III CHN analyzer was used for the
purpose. The result of the analysis is presented in Table 1. Theo-
retical values of CHN were calculated from the molecular formula
C14H10BrNO. The micro analysis of the sample showed a good
agreement with the calculated values, confirming the formation of
the compound. Energy dispersive Analysis of X rays (EDAX) of the
crystal was carried out using JEOL JSM-6380LA analytical scanning
electron microscope (SEM) system. The resulted spectrum over a
certain area is given in Fig. 4. The characteristic peaks in the
spectrum reveal the presence of various elements in the grown
crystal.
Fig. 1. Scheme for the structure of some chalcone derivatives.

A.P. Menezes et al. / Journal of Molecular Structure 1119 (2016) 167e176
169
Fig. 2. Synthetic scheme of 4BP4AP.
Fig. 3. [a] Photograph of 4BP4AP single crystals. [b] Simulated Crystal Morphology.
Table 1
CHN Analysis data of 4BP4AP.
3.2. Fourier transform infrared (FT-IR) and FT-Raman spectral
studies
Element
Experimental (%)
Computed (%)
To investigate the presence of functional groups in 4BP4AP and
their vibrational modes, FT-IR and FT-Raman spectral analysis was
carried out. The FTIR spectrum was recorded between 500 and
Carbon
Hydrogen
Nitrogen
58.61
3.38
4.72
58.36
3.50
4.86
Fig. 4. EDAX spectrum of 4BP4AP.
3500 cmꢂ1 by KBr pellet technique using the Thermo Nicolet,

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A.P. Menezes et al. / Journal of Molecular Structure 1119 (2016) 167e176
Avatar 370 FT-IR spectrometer equipped with KBr beam splitter and
a DTGS detector, with a spectral resolution of 4 cmꢂ1 and the
spectrum is shown in Fig. 5. The FT-Raman spectrum was recorded
using the BRUKER RFS 27, Stand alone FT-Raman Spectrometer,
having a spectral resolution of 2 cmꢂ1 with an exciting wavelength
of 1064 nm from an Nd: YAG laser source. Fig. 6 displays the FT-
Raman spectrum. The characteristic IR absorption and Raman
bands observed are consistent with the functional groups present
in the compound and the corresponding assignments are recorded
in Table 2.
3.3. Surface morphology
The surface morphology of the 4BP4AP single crystal was
studied by using a JOEL JSM-6380LA analytical scanning electron
microscope (SEM). SEM was operated at an accelerating voltage of
20 kV and probe current of 1 nA by keeping the sample in high
vacuum. Due to the non-conducting behavior of the organic ma-
terials, gold coating is done by the sputtering technique using JOEL
JFC-1600 auto fine coater before subjecting the surface of 4BP4AP
crystal to electron beam. The SEM images of 100ꢁ and 5000ꢁ
magnification are shown in Fig. 7. It can be observed from the figure
that the surface appears smooth with some micro crystallites
grown on the surface. These inclusions are formed during the
growth of the crystal and influenced by the growth conditions.
Fig. 6a indicates a layered growth pattern which confirms the two
dimensional growth mechanisms [14].
Fig. 6. FT-Raman spectrum of 4BP4AP.
Single-crystal XRD data shows that the crystal is triclinic in
structure with centrosymmetric space group P-1. The asymmetric
unit of the title chalcone derivative contains two independent
molecules A and B of similar geometry adopting an E configuration
about the C7A]C8A [1.326 Å] and C7B]C8B [1.342 Å] double
bonds. The dihedral angle between the benzene and the pyridine
rings is 9.41ꢀ in the molecule A and it is 41.49ꢀ in the molecule B.
Thus, the molecule A has an almost planar structure where as the
structure of molecule B is twisted. The mean plane of phenyl ring
(C9eC14) in molecule A and molecule B is deviated by an angle of
8.170 and 18.78ꢀ respectively from a plane C6eC9. Similarly the
plane through pyridine rings of A and B molecules are deviated
from the plane C6eC9 by 10.09ꢀ and 25.11ꢀ respectively. The
thermal ellipsoid plot of the molecule at 60% probability is shown
in Fig. 8. In the crystal structure, it is observed that the molecules
are packed in anti parallel layers where the molecules are inter-
connected in a head to tail fashion (Fig. 9) and the structure is
stabilized by the hydrogen bonds (Fig. 10).
3.4. Single crystal X - ray diffraction study
The grown crystal was subjected to single-crystal X-ray
diffraction (XRD). A Supernova Dual diffractometer with Atlas de-
tector [Agilent (2012)] using the mirror monochromated Cu K
a
radiation of wavelength 1.54184 Å was used for data collection. The
cell refinement and data reduction was done using CrysAlis PRO
[Agilent, 2012]. The crystal structure was solved by the direct
method and refined by the full matrix least squares method using
SHELXL-97 [25] in the WinGx package suite [26]. The X-seed [27]
and the Mercury [28] softwares were used for molecular
graphics. The details of the crystal data and refinement are given in
Table 3.
3.5. Thermal studies
The Differential Scanning Calorimetric (DSC) data was recorded
using Mettler Toledo DSC 822e thermal analyzer on a sample
weight of 0.9460 mg in the temperature range 25 ꢀCe300 ꢀC, at a
heating rate of 10 ꢀC per minute in the nitrogen atmosphere. The
thermogravimetric analysis (TGA) and differential thermal analysis
(DTA) was done using a Perkin Elmer TG/DTA analyzer to study the
rate of change of weight of the sample with respect to temperature.
About 1.62 mg of powdered compound was used for this analysis.
The sample was analyzed under the nitrogen atmosphere at a
heating rate of 10 ꢀC per minute in the temperature range of
25 ꢀCe630 ꢀC. The results of the analysis are presented in Fig. 11.
The sharp peak at 200 ꢀC in the DSC curve corresponds to the
melting point of the crystal. The sharpness of the peak indicates the
good crystallinity of 4BP4AP [29]. It is evident from the TG plot that
the mass of the sample remains unchanged till 160 ꢀC after which
there is about 4.8% weight loss due to dehydration. The decom-
position of the material starts at around 215 ꢀC and continues up to
305 ꢀC. A major weight loss of 92.8% was observed in the temper-
ature range. The steep weight loss around 265 ꢀC is because of the
boiling of the melt. After heating the sample beyond 305 ꢀC, the
residual weight is approximately 1.5% of the initial weight. The
Fig. 5. FTIR spectrum of 4BP4AP.

A.P. Menezes et al. / Journal of Molecular Structure 1119 (2016) 167e176
171
Table 2
Assignment of vibrational frequencies.
Wave number (cmꢂ1
Infrared lines
)
Assignments
Raman lines
3039.79
2972.76, 2922.65
1675.28
3064.1
e
aromatic CeH stretching vibration
CeH stretching vibration
C]O stretching vibration
e
1597
1552.08
1485.22
1409.6
1350.57
1218.7, 1068.83
810.98
1583.9
1559.9
1492.7
1402.1
1346.1
1176.9, 1076.7
894
aromatic ring vibration
C]C stretching vibrations (chalcone)
aromatic C]C stretching vibrations
CeH bending vibration
CeN stretching vibration
aromatic CeH in-plane bending vibrations
CeBr stretching vibration
677.8
664.9
Out of plane CeH aromatic bending or wagging vibrations
Fig. 7. SEM images of 4BP4AP crystal.
differential thermal gravimetric (DTG) curve coincides well with
the TGA curve. Furthermore, the high melting point of the crystal is
due to the presence of Br group in the crystal structure as Br group
improves the melting points of the compounds [30]. The high
thermal stability of the crystal and its ability of not getting
decomposed till the melting temperature may be advantageous in
device fabrication.
The molar enthalpy of fusion DfusHm of the single crystal 4BP4AP
was obtained from the DSC curve using the area integration
method and is found to be 159.03 J/g. The molar entropy of fusion
DfusSm is calculated using the thermodynamic equation [9]
transparency range extending into the entire visible and the IR
region (cut off 417 nm). By converting the observed UV-VIS spec-
trum in to Tauc's plot (as shown in the Fig. 13a), the optical energy
gap of the crystal is estimated [31]. The linear behavior exhibited by
the Tauc's graph can be considered as an evidence of direct tran-
sition [32]. The intercept of linear fitting line of (ahy
)2 and the zero
absorption coefficient line gives the direct optical band gap and the
value thus obtained is 3.09 eV (Fig. 13 (b). This value is very close to
the value obtained from the cutoff wavelength (lC) of the crystal
using the relation Eg ¼ hc/lC which is equal to 2.98 eV.
From the UV-VIS transmittance data, the extinction coefficient
(K) can be calculated using the equation,
DfusSm
¼
DfusHm/Tm, where Tm is the melting point of the crystal.
The calculated molar entropy of fusion is 0.336 J/Kg.
la
K ¼
(1)
3.6. Linear optical studies
4
p
The transmittance (T) is related to the reflectance (R) through
the equation [32],
Organic NLO materials having optical absorption edge in the
400 nm region are used for blueeviolet frequency conversion. It is
necessary that there is no absorption in the visible region for
4BP4AP crystal since it has to be exploited for NLO applications at
room temperature. UVeVISeNIR absorption spectrum of the
4BP4AP crystal was recorded using a Varian, Cary 5000 UVeVIS
spectrophotometer in the wavelength range of 200e1000 nm. A
solution of 4BP4AP in acetone was placed in a 1 cm thick cuvette for
measurement. The recorded spectrum is shown in Fig. 12. It is
2
a
t
ð1 ꢂ RÞ eꢂ
T ¼
(2)
1 ꢂ R2eꢂ2
at
where
a
¼ ln(Tꢂ1)/t, the optical absorption coefficient and t is the
thickness of the sample. When the absorption is high, the refractive
index (n) of the sample can be calculated from the reflectance data
using equation [33,34],
evident from the spectrum that the crystal has
a wider

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A.P. Menezes et al. / Journal of Molecular Structure 1119 (2016) 167e176
Table 3
Crystal data of 4BP4AP Crystal.
Molecular formula
C14 H10 Br N O
Formula weight/g molꢂ1
Crystal shape and colour
Crystal System
288.14
plate, yellow
triclinic
Space group
P-1
Unit cell parameters
a ¼ 5.9544(4) Å
b ¼ 7.8143(6) Å
c ¼ 24.7471(13) Å
Cell Angles (deg)
a
¼ 83.329(5),
b
¼ 84.815(5),
g
¼ 82.092(6).
Z ¼ 4
Volume
1129.55(13)Å3
Radiation
Cu Ka radiation
Radiation Wavelength
Temperature
Density
1.54184 Å
T ¼ 100.0(2) K
1.694 Mg/m3
4.796
Absorption Coefficient (m )
/mmꢂ1
F (000)
576
qrange for data collection (deg)
3.61e75.30
h
min,max, kmin,max,lmin,max
(-7 6), (ꢂ9 9), (ꢂ31 26)
Max. and min. Transmission
Refinement method
0.8313 and 0.4472
Full-matrix least-squares on F^2
No. of measured reflections
No. of independent reflections
11,951
4672
No of reflections with I > 2
No. of parameters
s
(I)
4138
307
No. of restraints
1
Goodness-of-fit on F^2
1.12
Final R indices [I > 2
R indices all data
Drmin, max/e Åꢂ3
s
(I)]
R1 ¼ 0.0986, wR2 ¼ 0.2655
R1 ¼ 0.1087, wR2 ¼ 0.2725
ꢂ1.595, 2.917
Fig. 8. ORTEP of 4BP4AP. Thermal ellipsoids were drawn at 60% probability level.
pffiffiffi
ðn ꢂ 1Þ2
ðn þ 1Þ2
1 þ R þ
1 ꢂ R
R
R ¼
⇔n ¼
(3)
Fig. 9. Packing of molecules in anti parallel layers where molecules in a layer are
aligned in head to tail fashion in the crystal structure of 4BP4AP when viewed down a-
axis. Hydrogen atoms are omitted for clarity.
In the present case, the optical absorption is minimum and
hence the refractive index can be calculated from the transmittance
data using the equation [35],
pffiffiffiffiffiffiffiffiffiffiffiffiffiffi
[36].
2n
n2 þ 1
1 þ 1 ꢂ T2
T ¼
⇔n ¼
(4)
T
n2 ꢂ K2 ꢂ ε0
cc
¼
(5)
Refractive index of 4BP4AP is almost a constant in its trans-
mission window as shown in the Fig. 14a. The value of n for the
crystal is 1.8 in the range 460e1000 nm. From these values of n and
K, the electric susceptibility (cc) can be calculated using the relation
4
p
whereε0 is the dielectric constant when there is no contribution
from the free carriers. The electric susceptibility of 4BP4AP crystal

A.P. Menezes et al. / Journal of Molecular Structure 1119 (2016) 167e176
173
at 1000 nm is found to be 0.2615.
Further, real part (εr) and imaginary part (εi) of dielectric con-
stant can be calculated from the relations [36],
εr ¼ n2 ꢂ K2 and εi ¼ 2nK
(6)
The values of εr and εi at 1000 nm are 3.285 and 4.828 ꢁ 10ꢂ6
respectively.
The optical conductivity (
s) of the crystal is related to the optical
absorption coefficient ( ) through the relation,
a
s
¼ nc /4 , where c
a p
is the velocity of light [37]. A plot between the optical conductivity
versus photon energy for 4BP4AP single crystal is shown in Fig. 14b.
It is clear from the graph that the optical conductance remains
constant up to the photon energy of 2.7 eV beyond which it in-
creases with increase in the photon energy.
3.7. Nonlinear optical studies
The Kurtz and Perry powder SHG efficiency test was performed
to check the second order NLO activity of the 4BP4APcrystal [38]. A
Fig. 10. Packing of 4BP4AP viewed down the a-axis showing hydrogen bond
interactions.
Fig. 11. Thermal plots of 4BP4AP single crystal.
Q-switched, 8 ns Nd: YAG laser beam of wavelength 1064 nm, with
beam energy of 3.3 mJ/pulse was used. The powdered 4BP4AP
crystal was packed in a micro-capillary of uniform bore and
exposed to laser radiations. As expected, no green light was
generated. This indicates that the crystal is SHG inactive due to its
centrosymmetric crystal structure.
To explore the third order nonlinearity of the crystal, the open
aperture z-scan experiment was performed. A 0.02 M solution of
4BP4AP in DMF was taken in a 1 mm quartz cuvette and exposed to
a Q-switched Nd: YAG laser beam of wavelength 532 nm having a
repetition rate of 10 Hz and pulse width of 5 ns. Cuvette containing
the sample solution was moved along the z-axis with the help of a
stepper motor, controlled by a computer. The laser beam was
focused with the help of a 20.5 cm convex lens and the normalized
transmittance data was collected using RJP 735 pyroelectric de-
tector. The experimental open z-scan aperture trace of 4BP4AP for a
laser input energy of 50 mJ is shown in the Fig. 15.
To calculate the nonlinear absorption (NLA) coefficient (beff) the
experimental open aperture z-scan data was fitted with the theo-
retical open aperture z-scan equation [23]:
Fig. 12. Optical absorption and transmittance spectrum of 4BP4AP.

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A.P. Menezes et al. / Journal of Molecular Structure 1119 (2016) 167e176
Fig. 13. Tauc's plots of 4BP4AP.
Fig. 14. Plots of (a) Refractive Index as a function of wavelength, (b) Optical Conductivity Versus photon energy.
0
@
1
A
I0Leff beff
pffiffiffi
1
TðzÞ ¼ 1 ꢂ
(7)
ꢀ
1 þ z2
2
2
z02
In the above equation, I0 is the on-axis peak irradiance at focus, z is
the distance of the sample from the focus, z0 ¼
p l is the Rayleigh
w20/
length of the sample ( is the wavelength andw0 is the beam waist
l
at the focus), Leff is the effective thickness of the sample given
a
byLeff ¼ [1ꢂeꢂ L]/
a, a being the linear absorption coefficient. It is
clear from the z-scan trace (Fig. 14) that at focus the transmission is
least and hence there is maximum absorption. This implies that
4BP4AP has positive NLA coefficient. The NLA in a medium may be
due to excited state absorption, two photon absorption (TPA), free
carrier absorption and reverse saturable absorption (RSA) [17]. The
relation between the NLA coefficient (b) and the molecular TPA
coefficient (s2) is
b
¼
s2NAC ꢁ 10ꢂ3, where C is the concentration
Fig. 15. Open aperture Z escan curve of 4BP4AP. The solid line is the theoretical fit of
the experimental data in the nanosecond excitation regime.
(mole/litre), NA is Avogadro number. The molecular TPA cross sec-
tion (s12) can be calculated from the equation, s21
¼
s2hn; where hn is
the photon energy. The excited state absorption (sex) can be
determined by fitting normalized open aperture z-scan data with
the equation [17]:

A.P. Menezes et al. / Journal of Molecular Structure 1119 (2016) 167e176
175
Br group being strong electron donor pushes the electron towards
the acceptor groups and hence there is an effective charge transfer
taking place across the molecule. The molecules are held together
À
ꢀ
Á
ln 1 þ q0 1 þ x2
ꢀ
TðzÞ ¼
(8)
q0 1 þ x2
…
…
in the crystal structure by a network of CeH
O and CeBr
N
hydrogen bonds (Fig. 10). These hydrogen bonds offer a charge
asymmetry in the molecule which is a necessary condition for NLO
property [46]. The asymmetric unit of the compound contains a
planar molecule with dihedral angle between the benzene and
pyridine of 9.41ꢀ. This kind of planar structure increases the degree
where x ¼ z/z0, z is the distance of the sample from the focus, z0 is
the Rayleigh length. q0 can be calculated from the relation,
psexF0Leff a
q0 ¼
(9)
h
u
of
p-conjugation which in turn increases the extent of charge
Here F0 is the on-axis fluence at focus,
is the linear absorption coefficient of the sample. The ground state
u is the angular frequency, ‘a’
transfer across the molecule thus contributing to the nonlinearity.
Further, as can be seen in the packing diagram of the crystal (Fig. 9),
the molecules are arranged in anti parallel layers and in each layer
there is a head to tail molecular arrangement. Thus, observed third
order nonlinearity in 4BP4AP may be attributed to its molecular
structure and packing of molecules in the crystal structure.
absorption is given by sg
sex, and sg are 3.2 cm/GW, 2.66
¼
a
/NAC. The calculated values ofꢂb4e7ff
, , ,
s21
s2
cm4S,
ꢂ28
cm4/W, 9.92
4.05 ꢁ 10ꢂ18 cm2 and 1.5 ꢁ 10ꢂ20 cm2 respectively. The value of sex
is 270 times greater than that of sg and hence the condition for RSA
is satisfied [39].beff of 4BP4AP is compared with some of the re-
ported NLO materials and found that the effective beta of 4BP4AP is
greater than many of the chalcone derivatives (Table 4), it is greater
than that of benzilidine derivatives [40] and hydrazone derivatives
[8]. Further, beff of 4BP4AP is 2.1 times that of terphenyl chalcone, 3-
(4-bromophenyl)-1-(4,400-difluoro-50-methoxy-1,10:30,100-ter-
phenyl-40-yl)prop-2-en-1-one [41]. This shows that 4BP4AP is an
efficient new entrant for NLO device applications.
3.8. Optical power limiting studies
Optical limiters are the devices which have a high transmittance
for low intensity signals but a low transmittance for high intensity
signals such as for laser beams. Optical limiters (OL) are used to
protect the optical sensors from high intensity laser beams. They
are also used to protect the human eye. For nanosecond laser pul-
ses, molecules with reverse saturable absorption (RSA) exhibit
strong optical limiting behavior. RSA occurs in materials where the
ground state absorption cross section is lesser than that of the
excited state [47]. Since sex is greater than sg for 4BP4AP, the optical
limiting property is achievable in this molecule. Optical limiting
property of the 4BP4AP molecule is extracted from the open
aperture z-scan data [48]. Fig. 16 shows the variation of normalized
transmittance of 4BP4AP molecule with laser power density. It is
clear that the transmittance decreases with increase in the input
laser intensity. At lower irradiance, 4BP4AP responds linearly to the
input intensity and obeys Beer's law. But as the input intensity in-
creases, the transmittance of the sample decreases and this varia-
tion of transmittance indicate the optical limiting behavior of
4BP4AP.
The molecular static and frequency dependent first hyper-
polarizabilities of the crystal 4BP4AP were calculated using the
semiempirical computer program molecular orbital package,
MOPAC 2012 [44]. For calculations PM7 Hamiltonian was used [45].
The geometry obtained from the single crystal XRD study of the
molecule was used as an input to this program. To optimize the
geometry the default geometry optimizer, Eigenvector Following
(EF) geometry optimizer was used. The Time-Dependent Har-
treeeFock (TDHF) theory was used to compute the molecular
hyperpolarizabilities. The keyword “PRESICE” was used as the
convergence criterion for PM7. The computed static (bo) and fre-
quency dependent (b1064nm) first molecular hyperpolarizabilities of
4BP4AP are 3.5 ꢁ 10ꢂ30 esu and 7.02 ꢁ 10ꢂ30 esu respectively.
The compound under investigation 4BP4AP, possesses a donor
e
p e acceptor (DepeA) type pushepull structure. In this com-
pound, both pyridine and carbonyl group act as electron acceptors,
whereas the bromo phenyl group acts as an electron donor. The
intramolecular charge transfer thus takes place from one end of the
molecule to other through the conjugated eCH]CHedouble bond.
4. Conclusion
The NLO material, 4BP4AP was synthesized and single crystals of
Table 4
Third order NLO Coefficients of Some Chalcone
derivatives.
Crystal
b (cm/GW)
4MSTP [17]
CDAC [18]
TTCP [42]
CTDMP [21]
3bisMPC [43]
MPNP [43]
4BP4AP*
1.1
1.3
2.463
3.05
3.25
2.65
3.2
CDAC - 1-(4-chlorophenyl)-3-(4-dimethylaminophenyl)
prop-2-en-1-one.
4MSTP
trimethoxyphenyl)prop-2-en-1-One.
TTCP
1-(20-thiophen)-3-(2,3,5-trichlorophenyl)-2-
1-[4-(methylsulfanyl)phenyl]-3-(2,4,5-
-
propen-1-one.
3bisMPC - 1,3-bis(4-methoxyphenyl)prop-2-en-1-one.
MPNP-1-(4-methoxyphenyl)-3-(3-nitro- phenyl)prop-2-
en-1-one.
CTDMP
-
1-(5-chlorothiophen-2-yl)-3-(2,3-
dimethoxyphenyl)prop-2-en-1-one.
*present work.
Fig. 16. Optical limiting behavior of 4BP4AP crystal.

176
A.P. Menezes et al. / Journal of Molecular Structure 1119 (2016) 167e176
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this compound were successfully grown by the solution growth
technique at room temperature using DMF as the solvent. The
molecular structure of the grown crystals was confirmed by EDAX,
FTIR, FT-Raman and single crystal XRD analysis. 4BP4AP is highly
transparent both in the visible and in the IR region with a cutoff
wavelength of 417 nm. Using the approximate formulae the various
optical parameters were evaluated from the spectrophotometric
measurement. The optical energy band gap obtained from Tauc's
plot is 3.09 eV. The SEM studies showed a two dimensional growth
pattern with smooth surface morphology. The crystal is thermally
stable up to its melting point. From the open aperture z-scan
experimental studies, it was found that the crystal has a positive
nonlinear absorption coefficient. The decrease in nonlinear trans-
mission with the increase in the input laser intensity indicates that
4BP4APsingle crystal can be used as an optical limiter.
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