Role of direction of charge transfer on the nonlinear optical behavior of pyridine substituted chalcone derivatives

Article abstract of DOI:10.1016/j.molstruc.2014.06.095

High quality single crystals of an efficient novel nonlinear optical (NLO) chalcone derivative 3-(4-nitrophenyl)-1-(pyridine-3-yl) prop-2-en-1-one (4NP3AP) that are transparent in the entire visible and infrared region are grown by the slow evaporation so

Full text of DOI:10.1016/j.molstruc.2014.06.095

Accepted Manuscript
Role of direction of charge transfer on the nonlinear optical behavior of pyridine
substituted chalcone derivatives
Anthoni Praveen Menezes, A. Jayarama
PII:
S0022-2860(14)00708-X
DOI:
http://dx.doi.org/10.1016/j.molstruc.2014.06.095
MOLSTR 20776
Reference:
To appear in:
Journal of Molecular Structure
Received Date:
Revised Date:
Accepted Date:
22 March 2014
29 June 2014
30 June 2014
Please cite this article as: A.P. Menezes, A. Jayarama, Role of direction of charge transfer on the nonlinear optical
behavior of pyridine substituted chalcone derivatives, Journal of Molecular Structure (2014), doi: http://dx.doi.org/
1
0.1016/j.molstruc.2014.06.095
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Role of direction of charge transfer on the nonlinear optical behavior of pyridine
substituted chalcone derivatives
a
b
Anthoni Praveen Menezes *, A. Jayarama
a
Department of Physics, Mangalore Institute of Technology & Engineering (MITE), Moodabidri,
5
74 225, India.
b
Department of Physics, Sadguru Swami Nithyananda Institute of Technology (SSNIT),
Kanhangad 671315, India.
*
E-mail:mapraveen.80@gmail.com; PH: +919880223265
Abstract
High quality single crystals of an efficient novel nonlinear optical (NLO) chalcone
derivative 3-(4-nitrophenyl)-1-(pyridine-3-yl) prop-2-en-1-one (4NP3AP) that are transparent in
the entire visible and infrared region are grown by the slow evaporation solution growth method
at ambient temperature. The single crystal XRD studies reveal that the crystal belongs to
1
monoclinic system with the noncentosymmetric space group P2 . The presence of functional
groups in the compound is confirmed by different spectroscopic techniques. The surface
morphology of the crystal is studied by scanning electron microscopy. The second harmonic
generation (SHG) efficiency of 4NP3AP determined by Perry and Kurtz method is 56% that of
potassium dihydrogen phosphate (KDP) single crystals. The static and frequency dependent
molecular hyperpolarizabilities were computed using MOPAC 2012. Thermal analysis confirms
0
that the crystal is thermally stable up to 177 C. The origin of nonlinearity and the role of charge
transfer direction on the nonlinearity of the chalcone have been discussed in detail.

Graphical Abstract
Keywords:
Nonlinear optical material
Crystal Growth
X-ray diffraction
Second Harmonic Generation
Thermogravimetric analysis
1
. Introduction
Rapid development in the field of photonics has increased the demand for new nonlinear
optical (NLO) materials. The development of optical devices, such as photonic integrated
circuitry, optical data storage and optical information processing depends on the design of
efficient nonlinear optical (NLO) materials [1]. The selection of materials for industrial
applications depend on their physical properties such as transparency, laser damage threshold,
power conversion efficiency, temperature and chemical stability and phase matching along with
large molecular first hyperpolarizability. From the viewpoint of applications to optical devices,

organic materials are considered as the ‘‘materials for future” due to their unique features like
processability, ultrafast response, cost effectiveness, high damage threshold, wider transparency
down to the blue region, large nonlinearity, and the ease with which the structure of these
materials can be modified by suitable functional substitution to get large crystals of high optical
quality and desired optical properties [2 - 4]. The generation of the blue–violet from IR laser
radiation by second order organic nonlinear optical materials can be used to realize many
applications such as high density data storage, laser printing, displays, remote sensing,
inflorescence, medical diagnosis, photolithography, biological species detection, high resolution
spectroscopy, underwater monitoring and communication [5, 6].
Therefore the search for suitable novel organic NLO materials exhibiting an ensemble of
optical and functional properties is still in progress. In this context, simple organic materials like
chalcones can be attractive due to their versatile design strategy, easy crystal growth, ultrafast
and large optical nonlinearities and high laser damage resistance [3, 7]. These materials possess
high tendency to crystallize in noncentrosymmetric structure, excellent second harmonic
generation efficiency, and optical limiting behavior with nanosecond laser pulse at 532nm
wavelength [8]. These compounds consist of two phenyl ring connected through a conjugation
bridge. By substituting suitable electron donor or acceptor groups on the ends of this conjugated
structure the asymmetric electronic distribution in either or both ground and excited states can be
enhanced leading to an increased optical nonlinearity [9]. Considering these theoretical aspects
many chalocone derivatives like 1-(4-methoxyphenyl)-3-(2,3,5-trichlorophenyl)prop-2-en-1-one
(
MPTCPP) [4], N’-[(Z)-(4-methylphenyl) methylidene]-4-nitrobenzohydrazide (NMPMN) [6],
-(4- methylthiophenyl)-3-(4-methoxyphenyl) prop-2-en-1-one (MMPP) [10],
1
dimethylammonium picrate (DMAP) [11], 4-hydroxy benzaldehyde-N- methyl 4-stilbazolium
tosylate (HBST) [12] and L-Valine Hydrobromide (LVB) [13] are reported for their NLO
properties (Fig.1a).
Recently Ganapayya et al [14, 15] and Jayarama et al [16 -18] reported few pyridine
based chalcone derivatives. Among these, 3-(2,3-dichlorophenyl)-1-(pyridin-2-yl)prop-2-en-1-
one (DCPP) [14], 3-(2,3-dimethoxyphenyl)-1-(pyridin-2-yl)prop-2-en-1-one (DMPP) [15] and 3-
(4-Methoxyphenyl)-1-(pyridin-2-yl) prop-2-en-1-one (MPP) [16] in which nitrogen atom is
substituted at the ortho position of the pyridine ring and with suitable functional groups at the
phenylene ring resulted in materials with good SHG efficiency (Fig.1b) But 3-[4-
(Dimethylamino)phenyl]-1-(3-pyridyl)prop-2-en-1-one
(DMAPP)
[17]
and
3-(3,4-
Dimethoxyphenyl)-1-(3-pyridyl)-prop-2-en-1-one monohydrate (DMPM) [18] with nitrogen

atom substituted at the meta position of the pyridine ring are found to crystallize in
centrosymmetric space groups and hence showed zero SHG efficiency (Fig.1c). Further it is
reported that single electron donor group substituted at the para position of benzene ring is most
effective in forming a non-centrosymmetric crystal structure [19]. But even though
dimethylamino group in DMAPP is a strong electron donor compared to methoxy group in MPP
[
20], MPP showed a high SHG efficiency of 4.7 times urea whereas DMAPP is nonlinearly
inactive. In both the compounds, carbonyl group (–C=O-) along with pyridine ring acts as
electron acceptor [16] whereas the phenylene moiety containing functional group attached to the
para position acts as electron donors and hence the charge transfer is from phenylene moiety to
carbonyl group through –CH=CH- bond (A- π -D). This implies that a shift in the position of the
nitrogen atom in the ring alters the extent of charge transfer across the molecule and this in turn
affects the nonlinearity. Moreover it is observed by Kiran et al [7] that an interchange of donor
and acceptor components in a molecule results in a considerable change in the
hyperpolarizability of the molecule. Considering the above facts, a pyridine based chalcone is
designed by placing a strong electron withdrawing nitro group at the para position of the
benzene ring. The molecule contains three electron accepting groups namely pyridine, carbonyl
group and the nitro group. Since nitro group is the strongest electron acceptor with electron
accepting ability greater than that of carbonyl group [21], it will pull the electrons towards it and
as a consequence pyridine will donate electrons to some extent. Hence charge transfer takes
place from pyridine moiety to phenylene moiety containing nitro group through –CH=CH- bond.
This strategy resulted in a new material crystallizing in noncentrosymmetric crystal structure
with good SHG efficiency. In this paper we describe our experimental results on the crystal
growth and characterization of a new nonlinear optical crystal 3-(4-nitrophenyl)-1-(pyridine-3-
yl) prop-2-en-1-one (4NP3AP).

Fig.1. Scheme for the structure of few chalcone derivatives
2
2
. Experimental
.1. Synthesis of the compound
Commercially available analytical reagent (AR) grade chemicals 3-acetyl pyridine and 4-
nitrobenzaldehyde are used to synthesize the chalcone, 3-(4-nitrophenyl)-1-(pyridine-3-yl) prop-
-en-1-one (4NP3AP) by adopting Claisen–Schmidt condensation reaction method [22]. A 0.01
2
mol solution of 3-acetyl pyridine in methanol (15 ml) is mixed with 0.01 mol of 4-
nitrobenzaldehyde in methanol (15 ml) and the mixture was treated with an aqueous solution of
sodium hydroxide (4 ml, 20%). The mixture was stirred well for 45 minutes. The contents of the
flask were poured into ice cold water (1000 ml) and kept aside for 24 hrs. The resulting crude
solid mass was collected by filtration and dried. The reaction scheme for the title compound is
shown in Fig. 2.
Fig.2. Scheme for the synthesis of 4NP3AP
2.2. Solubility and Crystal growth
Growth of organic single crystals with good optical quality and well defined faces
depends on the selection of suitable solvent. Solvents offering moderate solubility for a material
and yielding prismatic growth habits are considered to be best solvents for growing single

crystals of that material. The solubility of the compound was checked in water, polar organic
solvents such as methanol, ethanol, benzene, acetone and mixture of these solvents. A known
amount of the compound was added to a known volume of a given solvent till it is dissolved
completely. The solubility of the compound was found to be suitable in a 1:1 mixture of ethanol
and acetone. Crystals were grown by the slow evaporation solution growth technique at room
temperature [23] using mixture of acetone and ethanol (1:1) as the solvent. A saturated solution
of 4NP3AP was prepared and is filtered in to a beaker in order to remove the suspended
particles. The beaker was covered with a polythene sheet containing few pin holes, and kept
undisturbed in a dust free atmosphere. Tiny crystals were appeared after 3 days which were
allowed grow further to the maximum possible dimensions. Good quality single crystals of
3
maximum size 10× 2×1 mm were obtained in 15 days of duration. A photograph of the
harvested golden coloured single crystals of 4NP3AP is shown in Fig. 3a. The morphology of the
single crystal of 4NP3AP was simulated using the software WinXMorph [24, 25] where the data
obtained from the single crystal XRD study were used as input to index the well-developed faces
in the crystal and is shown in Fig. 3b. The growth rate is found to be more along crystallographic
a-axis compared to other two axes and as a result the crystal is elongated along this axis. It can
be noted the (010) plane has larger area due to the reduced growth rate along crystallographic b-
direction compared to other directions. Morphology of 4NP3AP may change if it is crystallized
from other solvents.

Fig.3. (a) A photograph of the grown 4NP3AP crystals and (b) Simulated Morphology of
NP3AP crystal
4
2
.3. Characterization
In order to ascertain that the material (4NP3AP) obtained is surely that of the desired
compound, the grown crystal was subjected to Energy dispersive Analysis of X rays (EDAX)
analysis using JEOL JSM-6380LA analytical scanning electron microscope (SEM) system. The
recorded spectrum over a certain area is given in supporting information S0. The compositional
ratios, corresponding to the elements present in the crystal, as calculated from chemical formula
and EDAX analysis, are given in Table 1. From the table, it can be found that there is a small
deviation of about 12% in the stoichiometric values (atom %) calculated from the chemical
formula and those obtained from EDAX analysis. When the experiment is repeated at a point,
approximately the same result was obtained (supporting information S1 and supporting
information T1). The EDAX analysis confirms the presence of carbon, nitrogen and oxygen and
hence the formation of the compound.
Table 1
EDAX Analysis data of 4NP3AP
Element
From EDAX at. %
From Chemical Formula at. %
C
N
O
42.12
4.85
9.36
48.23
6.89
10.34
The surface morphology of the 4NP3AP single crystal was studied by scanning electron
microscopy (SEM). A JOEL JSM-6380LA analytical scanning electron microscope was used for
the analysis. SEM was operated at an accelerating voltage of 20kV and probe current of 1nA and
the sample was kept in high vacuum. Due to the non-conducting behavior of the organic
materials, gold coating is done by sputtering technique before subjecting the surface of 4NP3AP
crystal to electron beam.
The presence of functional groups and their vibrational modes are investigated by
carrying out FT- Raman and FTIR spectral analysis of 4NP3AP crystal. A BRUKER RFS 27,
-
1
Stand alone FT-Raman Spectrometer, having spectral resolution of 2 cm with an exciting
wavelength of 1064 nm from an Nd: YAG laser source at 100 mW was used to obtain the FT-
Raman spectrum of the title compound. A Thermo Nicolet, Avatar 370 FT-IR spectrometer

-
1
equipped with a DTGS detector and KBr beam splitter, with a spectral resolution of 4 cm was
used to record the FTIR spectrum of 4NP3AP. The compound was mixed with KBr pellets and
-
1
-1
the spectrum is recorded in the range between 500 cm and 4000 cm .
The recorded FT-Raman spectrum of 4NP3AP is shown in the Fig.4. The FTIR spectrum
-
1
-1
is shown in supporting information S2. The weak peaks at 3084.4 cm and 3055 cm are due to
-1
aromatic C-H stretching vibrations. The weak intensity peak at 1661.5 cm corresponds to the
C=O stretching vibration. The peaks corresponding to asymmetric and symmetric stretching of
-
1
-1
-1
2
NO group appear at 1597.7 cm and 1348 cm respectively. The peak at 1477.3 cm implies
-
1
-1
the chalcone C=C stretching vibration, the peaks at 1206.9 cm and 1028 cm indicates the
aromatic C-H in-plane bending vibrations. The peak pertaining to C-N stretching vibration
-
1
appears at 1348 cm . The assignments of all the peaks in the FT-Raman and absorption bands in
the FTIR spectrum of 4NP3AP are summarized in the supporting information T2.
Fig.4. FT-Raman spectrum of 4NP3AP single crystal
1
H NMR spectrum of 4NP3AP was recorded using a Bruker Advance III FT-NMR
spectrometer. The spectrum was recorded at 400 MHz by dissolving the sample in deuterated
1
chloroform (CDCl
3
) and tetramethylsilane (TMS) as the internal standard. The H NMR
spectrum of 4NP3AP and is shown in supporting information S3. The spectrum shows a doublet
of doublet at δ: 9.25 for C15-H of pyridine ring. A doublet of doublet centered at δ: 8.84 (J=4.8
Hz) corresponds to C11-H. The signal corresponding to C7-H and C8-H protons of chalcone

moiety resonates as separate doublets centered at δ: 7.87 and δ: 7.60. The strong coupling
constant of J=16 Hz for each signal shows the trans coupling of the adjacent protons and this in
turn confirms the trans geometry of the double bond of chalcone. A doublet at δ: 7.81 (J=7.2 Hz)
integrates for two protons which are meta to nitro group in the phenyl ring. A multiplet at δ:
8.29-8.30 corresponds to three protons, two protons which are ortho to nitro group in the phenyl
ring and C13-H of pyridine ring. The C12-H proton of pyridine ring resonates at δ: 7.48-7.51 as
1
a quartet. The peaks observed in the H NMR spectrum reconfirm the structure of 4NP3AP.
3
A single crystal of 4NP3AP of dimensions 0.50×0.30×0.30 mm was chosen for X-ray
diffraction study. A Bruker Kappa Apex II CCD diffractometer with graphite monochromated
Mo Kα radiation of wavelength 0.71073 Å from a fine-focus sealed tube x-ray source was used
for single crystal XRD data collection. Crystal data was collected with a scan width of 1°. The
data reduction was done by SAINT/XPREP [26] and cell refinement by APEX2/SAINT [26].
The crystal structures were solved by direct methods using SIR92 [27] and refined by full matrix
least squares method using SHELXL-97 [28] in the WinGx package suite [29] with isotropic
temperature factors for all atoms. Carbon-bound H-atoms were placed in calculated positions and
were included in the refinement in the riding model approximation. 173 parameters were refined
with 2312 unique reflections and this saturated the residuals to R1= 0.0353. For molecular
graphics Mercury [30] and ORTEP3 (31) software was used.
Thermogravimetric (TG)/differential thermal (DT) analysis experiment was carried out
using a Perkin Elmer simultaneous TG/DTA analyzer. The analysis was carried out on powder
0
sample under the nitrogen atmosphere at a heating rate of 10 C per minute in the temperature
0
0
range from 50 C to 370 C. The differential scanning calorimetric (DSC) plot for the crystal was
0
obtained using Mettler Toledo DSC 822e thermal analyzer in the temperature range 50 C to 230
0
0
C, at a rate of 10 C /min, in nitrogen atmosphere.
3. Results and Discussion
The SEM images of 100x and 500x magnification showing the surface morphology are
presented in Fig. 5a and Fig. 5b respectively. From the Fig. 5a it is clear that the surface of the
crystal appears smooth with a few micro-crystals grown on the surface. These inclusions are
formed during the crystal growth and are influenced by the growth conditions. Fig. 5b shows a
layered growth pattern which is an indication of two dimensional growth mechanisms [32]. The
uneven darker and brighter regions seen on the surface are might be due to the inclusion of
solvent, which is common in solution grown crystals.

Fig. 5. SEM images of 4NP3AP crystal
From single-crystal XRD studies, it has been found that the title compound crystallizes in
1
the monoclinic crystal system with a space group P2 suggesting that the crystal is non-
centrosymmetric in nature. This supports the observed second order nonlinear property of the
crystal as this a basic precondition for a potential second harmonic generation material [33, 34].
1 2
The final R values of the refinement are R = 0.0353, wR = 0.1024 for 2-theta maximum of
0
5
6.68 . As small organic molecules generally refine to better than R = 0.05 [35], there is a good
agreement between observed and calculated intensities in the present case. Also, the low value of
R indicates the purity of the grown single crystal [36]. An ORTEP plot of the molecule at 50%
probability is shown in Fig. 6. The details of the crystal data and refinement are given in
supporting information T3.
Fig.6. ORTEP plot of the molecule with thermal ellipsoids drawn at 50% probability
.
The molecular structure consists of a benzene ring and a pyridine ring linked by prop-en
one chain at the 3, 1 – position. The olefenic double bond -C7=C8- (1.320 Å) is in E-

0
configuration; the torsion angle C6-C7-C8-C9 is 176.43 . The phenylene ring is twisted relative
0
to pyridine by an angle of 17.77 and therefore the molecule exhibits planar structure. The
carbonyl group is also planar with respect to the adjacent pyridine moiety showing a torsional
0
angle of O1-C9-C10-C11 of 2.27 . This kind of planar structure would increase the electron
mobility within the π - conjugated molecular systems through increased donor-acceptor
interactions across the molecule which in turn increases the degree of molecular charge transfer
and degree of nonlinearity [4, 37]. Atoms of the propenone bridge (C9, C7, C8 and O3) lie
0
almost on the same plane and the torsion angle is O3-C9-C8-C7 is -10.03 . The mean plane
0
0
through this bridge makes dihedral angles of 15.81 and 6.97 with the planes of benzene and
pyridine rings respectively. Further, the crystal structure of 4NP3AP is stabilized by weak C-
…
…
H N and C-H O intermolecular interactions in the crystalline lattice (Table 2). These weak
intermolecular interactions help the molecules to pack in parallel layers and in zigzag fashion
along the crystallographic a-axis. The partial packing diagrams of the crystal viewed down the a-
axis and b-axis are shown in Fig.7 and Fig. 8 respectively.
Fig.7. Packing diagram of 4NP3AP showing the zigzag alignment of molecular dipoles
viewed down the a- axis. The molecular dipole of 4NP3AP computed using
semiemperical package MOPAC2012 is shown in the inset.

…
…
Fig.8. Packing diagram of 4NP3AP viewed along the b axis showing C-H O and C-H N
interactions.
Fig.9. Packing diagram of 4NP3AP single crystal viewed down the a–axis. The red arrow
marks shows the dipole moment of individual molecules and the green arrow represents the
effective dipole moment in a unit cell and is along crystallographic b –axis.

Table 2
Geometries of Intermolecular Interactions observed in the 4NP3AP crystal.
_
_______________________________________________________________________
Interactions X-H [Å] H-D [Å] X-D [Å] X-H-D[deg] symmetry
_______________________________________________________________________
_
…
C15-H16 O3
0.930
0.930
0.930
0.930
0.930
1.467
2.719
2.592
2.559
2.645
2.567
2.927
3.573
3.415
3.356
3.383
3.228
3.326
59.9
x,-1+y, 1+z
…
C11-H11 N2
147.78
144.00
136.78
128.46
89.48
1-x, -1/2+y,1-z
1-x, -1/2+y,1-z
2-x, -1/2+y, -z
1-x, -1/2+y,1-z
-1+x, y, z
…
C5-H5 O2
…
C7-H7 O2
…
C2-H2 O3
…
C3-N1 O1
_
_______________________________________________________________________
To examine the thermal stability of the crystal 4NP3AP and its thermal decomposition
characteristics, thermo gravimetric (TG)/differential thermal (DT) analysis and differential
scanning calorimetric (DSC) analysis was carried out. The weight loss of the compound with
respect to temperature is measured in TGA and the change in the energy with temperature is
monitored in DTA studies. The thermograms of the crystal are presented in Fig.10 and Fig.11
respectively. As can be seen from TGA plot the compound looses a weight of 2.8% due to the
0
loss of water by hydration which is indicated by an endothermic peak at 136.5 C in the DTA
curve. The DTA curve also shows that the sample undergoes an irreversible endothermic
0
0
transition at 165.2 C where melting begins. The sharp endothermic peak at 178.4 C at which no
weight loss is observed corresponds to the melting point of the crystal. As there is no
decomposition of the material at the melting temperature, the crystal is thermally stable beyond
the melting temperature. The melting of the sample at this temperature is further confirmed by a
0
sharp peak in the DSC curve observed at 177.5 C. The peak at 140°C in the DSC plot indicates
the existence of a reversible solid to solid phase transition [38]. Below the melting temperature
of 4NP3AP, no endothermic or exothermic peaks were observed in the TG curve, which
indicates the absence of any phase transitions and the thermal stability in this region. The weight
loss of 83.47% due to decomposition of the compound takes place in the temperature range 190
0
0
C – 305 C. The result of DTA matches well with the weight loss observed in the TGA plot.
0
4
NP3AP has better thermal stability than standard urea (130 C) and many of the reported
organic NLO materials with significant SHG efficiencies [39, 40] (Table 3). The high thermal

stability of 4NP3AP can be attributed to the presence of nitro group which helps the molecules to
form strong intra and inter molecular interactions and hydrogen bonds [41]. Based on the thermal
analysis, the maximum recommended temperature for the NLO applications of the crystal
0
without affecting its stability is limited to 177 C.
The molar enthalpy of fusion Δ H of the single crystal 4NP3AP was obtained from the
fus
m
-1
DSC curve adopting the area integration method and is found to be 85.95 Jg . The molar entropy
of fusion Δ S was calculated using the thermodynamic equation [42]Δ S = Δ H /T ,
fus
m
fus
m
fus
m
m
-
where T
m
is the melting point of the crystal. The calculated molar entropy of fusion is 0.191 JK
1
-1
g .
Fig.10. TG/DTA plot of 4NP3AP crystal

Fig.11. DSC thermogram of 4NP3AP
Table 3
Melting points of selected organic NLO materials.
_
______________________________________________________________
ο
Crystal
Melting Point ( C)
_
_______________________________________________________________
DMPP [15]
MPP [16]
CMC [7]
79
81
104
131
141
146
152
154
166
174
177
PDBA [43]
CDAC [44]
DCPP [14]
MMP [45]
TMBC [8]
4
N4MSP [46]
MNC [47]
NP3AP*
4
_
________________________________________________________________

CMC- 1-(4-chlorophenyl)-3- (4-methoxyphenyl)prop-2-en-1-one; PDBA- p-methoxy
dibenzylideneacetone; CDAC- 1-(4-chloro phenyl)-3-(4-dimethylamino phenyl) prop-2-en-1-
one;
MMP-
1-[4-({(E)-[4-(methylsulfanyl)phenyl]methylidene}amino)phenyl]ethanone;
TMBC- 2,4,5-trimethoxy-4′-bromochalcone; 4N4MSP- 3-[4-(methylsulfanyl)phenyl]-1-(4-
nitrophenyl)prop-2-en-1-one (4N4MSP); MNC- 4-methoxy-40-nitrochalcone (MNC);
*
Present work
A good optical transmittance is very essential for an NLO crystal to be useful in
NLO applications since any absorption in the material near the fundamental or the second
harmonics will lead to the loss of conversion efficiency at those wavelengths. The optical
absorption by the sample in the ultraviolet, visible and near infrared regions was recorded using a
Shimadzu 1800 UV–Vis-NIR spectrophotometer in the wavelength range of 200 nm–1100 nm
and is shown in the supporting information S4. The spectrum reveals that crystal is transparent in
the visible region and the absorption is only in the UV region. As the entire visible region does
not bear any absorption peak, it may be exploited for NLO applications at the room temperature.
g
The energy gap (E ), in general can be determined by electric process via conductivity
method and optical process via absorption method. In the present case, energy gap is determined
2
by the optical process. The variation of (αhυ) versus incident photon energy (hυ) using the
Tauc’s relation [48] in the fundamental absorption region is plotted and is given in supporting
information S5. Here the absorption coefficient (α) is obtained from the relation
2
.303
1
1
α =
log10 ( ) where‘d’ is the thickness of the crystal and log10 ( ) is defined as the
T
T
d
absorbance (A) of the material [49]. The energy band gap (E
g
) is obtained by extrapolation of the
linear portion of the curve to the x-axis. The optical energy band gap of the crystal 4NP3AP is
found as 3.35 eV. Due to this wide band gap, the crystal under study is expected to possess high
laser threshold and a large transmittance in the visible region [50, 51]. The absence of absorption
bands in the visible region and the wide band gap shows the suitability of the grown crystal for
photonic applications such as optical switches, optical attenuators and other optical devices [52].
The second harmonic generation efficiency (SHG) of the grown 4NP3AP crystal
was measured by the Kurtz and Perry technique [53] using a fundamental beam of wavelength
1064nm from a Q-switched 8 ns laser beam from Nd: YAG. Pulse of energy of 3.3 mJ/pulse and
width of 10 ns with a repetition rate of 10 Hz was used for the measurement. The fundamental
beam was filtered by using IR filter. A photo-multiplier tube (Hamamatsu-R 2059) was used as a
detector. Urea and KDP were used as the reference materials, and the output intensity of

4
NP3AP was observed. Second harmonic signal obtained was monochromated to eliminate the
fundamental wavelength and 532 nm component was collected. Thus, the SHG efficiency of
4
NP3AP is nearly 0.15 times that of urea and 0.56 times that of KDP.
The molecular static and frequency dependent first and second hyperpolarizabilities of
the crystal 4NP3AP were calculated using the semiempherical computer program molecular
orbital package, MOPAC 2012 [54]. For calculations PM7 Hamiltonian was used [55]. The
geometry obtained from the single crystal XRD study of the molecule was used as input to
MOPAC 2012. To optimize this geometry the default geometry optimizer, Eigenvector
Following (EF) geometry optimizer was used. The Time-Dependent Hartree–Fock (TDHF)
theory was used to compute the molecular hyperpolarizabilities. The keyword “PRESICE” was
used as a convergence criterion for PM7 calculation of geometries and the NLO properties. The
o
computed static and frequency dependent first (β , β1064nm) and second (γ1064nm) molecular
-
30
-30
-32
hyperpolarizabilties of 4NP3AP are 1.7x10 esu, 3.4x10 esu, and 11.5x10 esu respectively.
The observed non linear optical response of the title compound can be understood from
its structure (Fig. 7). The 4NP3AP molecule contains a strong electron acceptor nitro group
attached to the para-position of benzene ring. A charge transfer axis is formed in the molecule by
the electron transfer from the pyridine moiety to the nitro group via the conjugation path way
formed by carbon – carbon double bond. Thus 4NP3AP molecule possesses a push-pull structure
where charge transfer is from one end of the molecule to the other. This ensures the
delocalization of the electronic charge within the molecule which helps the molecule to possess
optical nonlinearity.
In SHG-active crystals, it is interesting to note that the arrangement of molecules inside a
crystal is critical to yield large effective nonlinearity, besides the noncentrosymmetric crystal
structure. The hydrogen bond interactions play a significant role in the crystal packing. The
hydrogen atom bonds two atoms having higher electro negativity (such as O and N) than
hydrogen. As π -electrons are more delocalized than σ-electrons; they tend to be less tightly
bound and respond more freely to an applied optical field and thus give rise to enhanced linear
and nonlinear optical responses. Hydrogen bonds, parallel stacking and π – π interaction favors
the second harmonic generation efficiency of the nonlinear materials [6]. In the present case, the
…
…
intermolecular C-H O, and C-H N hydrogen bonds [Fig. 8] create a delicate balance between
the molecular and supramolecular charge transfer processes by creating an enantiomorphic
structure. These weak intermolecular hydrogen bonds help molecules to stack in parallel layers
in a head–to-tail fashion along the crystallographic b-axis. Systems with this type of packing are

more likely to exhibit large SHG efficiency [16, 56]. In addition, an optimized alignment of the
molecular dipoles with θ =0 (where θ is the angle between the charge transfer axis and the
b
b
crystallographic b-axis) will establish a net macroscopic second order polarization in the crystal
by adding the dipole moments of each molecule and maximize the SHG efficiency. The
molecular dipole moment of 4NP3AP computed using MOPAC 2012 [54] semi-empirical
package is found to be 5.004 Debye (Fig. 7). Though the dipole moment contribution by
individual molecules is quite large, the net dipole moment in a unit cell is small since the
molecular dipoles are aligned at larger angles to the crystallographic b-axis (Fig. 9). A low SHG
efficiency of 0.8 times urea was reported for a chalcone derivative CMC [7]. This low SHG
efficiency was attributed to the orientation of the molecules/the charge transfer axis at larger
angles with respect to the crystallographic c-axis. Hence in spite of the crystal packing stabilized
by the hydrogen bonds with planar molecules aligned in zigzag passion, the 4NP3AP crystal
shows low SHG efficiency of only 0.15 times that of urea due to an un-optimized alignment of
molecular dipoles in the crystal structure.
4. Conclusion
In summary, a new chalcone derivative 4NP3AP was synthesized and the single crystals
have been grown. The structure of the compound was confirmed by various spectroscopic
studies. From single crystal XRD, the crystal structure, space group and the crystal parameters
were found out. The powder SHG efficiency of 4NP3AP is 56% that of KDP. The SHG response
of this crystal is discussed in terms the molecular arrangement in the unit cell and is inherently
dependent upon its structural attributes. A strong electron accepting nitro group attached at the
phenylene moiety ensures the charge transfer from carbonyl group to nitro group which results in
noncentrosymmetric crystal structure and nonlinearity. The UV–Visible characteristic shows that
the crystal has sufficient transmission in the entire visible region and a wide optical band gap of
3
.35 eV. Thermal study reveals that the crystal is thermally stable in nature and has high melting
0
point of 177 C. The study indicates that the 4NP3AP crystal seems to be a promising candidate
for optoelectronic applications such as second order frequency conversion, optical switches,
optical attenuators and in other photonic devices due to its relatively good SHG efficiency, large
optical band gap and good thermal stability.

Appendix A. Supplementary material
CCDC 992923 contains the supplementary crystallographic data for this paper
comprising full details of the X-ray data collection and final refinement parameters including
anisotropic thermal parameters and full list of the bond lengths and angles. These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the
Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK).
Acknowledgements
The authors would like to express their thanks to Dr. P.K. Das, IPC Department, Indian
Institute of Science, Bangalore for NLO studies. We are grateful to Praveen Mugali, Dept of
chemistry, Alva’s College, Moodbidri for NMR discussions and Dr. H.J. Ravindra for fruitful
discussions. The authors also acknowledge the SAIF-STIC, Cochin University for single crystal
XRD, SAIF, IIT Madras for FT-Raman spectrum and SAIF, IIT Bombay for TG/DTA analysis.
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HIGHLIGHTS
•
•
New efficient NLO single crystals are grown by slow evaporation technique.
A change in the direction of charge transfer resulted in noncentrosymmetric structure and
nonlinearity.
•
The molecular dipoles aligned in zigzag layered fashion results in good SHG efficiency.