Crucial role of molecular planarity on the second order nonlinear optical property of pyridine based chalcone single crystals

Article abstract of DOI:10.1016/j.molstruc.2015.02.011

An efficient nonlinear optical material 2E-3-(4-bromophenyl)-1-(pyridin-3-yl) prop-2-en-1-one (BPP) was synthesized and single crystals were grown using slow evaporation solution growth technique at room temperature. Grown crystal had prismatic morphology

Full text of DOI:10.1016/j.molstruc.2015.02.011

Accepted Manuscript
Crucial role of molecular planarity on the second order nonlinear optical prop-
erty of pyridine based chalcone single crystals
Anthoni Praveen Menezes, A. Jayarama, Seik Weng Ng
PII:
S0022-2860(15)00096-4
DOI:
http://dx.doi.org/10.1016/j.molstruc.2015.02.011
MOLSTR 21309
Reference:
Journal of Molecular Structure
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Received Date:
Revised Date:
Accepted Date:
28 November 2014
21 January 2015
5 February 2015
Please cite this article as: A.P. Menezes, A. Jayarama, S.W. Ng, Crucial role of molecular planarity on the second
order nonlinear optical property of pyridine based chalcone single crystals, Journal of Molecular Structure (2015),
doi: http://dx.doi.org/10.1016/j.molstruc.2015.02.011
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Crucial role of molecular planarity on the second order nonlinear
optical property of pyridine based chalcone single crystals
Anthoni Praveen Menezes1*, Jayarama A2, Seik Weng Ng3
1Department of Physics, Mangalore Institute of Technology & Engineering (MITE), Moodabidri-
574225, India.
2Department of Physics, Sahyadri College of Engineering & Management, Adyar, 575007, India.
3Department of Chemistry, University of Malaya, 50603, Kuala Lumpur, Malaysia.
*Corresponding author E-mail: mapraveen.80@gmail.com; PH: +91 9880223265
Abstract
An efficient nonlinear optical material 2E-3-(4-bromophenyl)-1-(pyridin-3-yl) prop-2-en-
1-one (BPP) was synthesized and single crystals were grown using slow evaporation solution
growth technique at room temperature. Grown crystal had prismatic morphology and its structure
was confirmed by various spectroscopic studies, elemental analysis, and single crystal X- ray
diffraction (XRD) technique. The single crystal XRD of the crystal showed that BPP crystallizes
in monoclinic system with noncentrosymmetric space group P21 and the cell parameters are a =

5.6428(7) Å, b = 3.8637(6) Å, c = 26.411(2) Å, β= 97.568(11) deg and v = 575.82(12) Å3. The
UV-Visible spectrum reveals that the crystal is optically transparent and has high optical energy
band gap of 3.1 eV. The powder second harmonic generation efficiency (SHG) of BPP is 6.8
times that of KDP. From thermal analysis it is found that the crystal melts at 139o C and
decomposes at 264o C. High optical transparency down to blue region, higher powder SHG
efficiency and better thermal stability than that of urea makes this chalcone derivative a
promising candidate for SHG applications. Furthermore, effect of molecular planarity on SHG
efficiency and role of pyridine ring adjacent to carbonyl group in forming noncentrosymmetric
crystal systems of chalcone family is also discussed.
Keywords
Nonlinear optical
Crystal growth
X-ray diffraction
Second harmonic generation
Thermal analysis
1. Introduction
The demonstration of second harmonics in quartz single crystal by Franken et al led to
the development of an exhilarating field of modern times known as nonlinear optics (NLO)
which has plethora of applications in the field of opto-electronics [1-3]. The most desirable
properties for a material to be the best NLO material are high power conversion efficiency, large
transparency extending down to UV region, high melting point, high mechanical stability, high
chemical inertness, high laser damage threshold, phase matching etc [4,5]. Among the various
classes of materials investigated for NLO properties, organic materials received unique
consideration. The optical nonlinear behavior in this class of materials is electronic in origin and
can be tuned through structural modifications [6]. These are characterized by an extremely
extended π-conjugated chain with strong electron acceptor-donor pairs at the ends (D–π–A) [4,
7]. Organic NLO materials are very often formed by hydrogen bonds and Van der Waals forces
and hence posses high degree of delocalization. Due to the overlap of π orbits, delocalized
electronic charge distribution leads to high mobility of the electron density. The optical
nonlinearity can be increased by functionalizing the ends of the π bond system with apt electron
donor and acceptor groups [8]. Hence, the search for new NLO materials had never ceased.

Chalcone derivatives are an interesting type of organic NLO materials which can be
tuned to match the requirements. In these systems, two aromatic rings have to be substituted with
suitable electron donor or acceptor groups like -OCH3, -SCH3, -Cl, -Br etc to increase the
asymmetric charge distribution in either or both ground state and excited states, giving rise to an
enhanced optical nonlinearity [9,10]. The substitution of electron acceptors/donors on either of
the benzene rings of chalcones significantly influences the noncentrosymmetric crystal packing
structures requisite for second harmonic generation (SHG) [11]. Thus, chalcones can be
considered as cross-conjugated molecules that possess two independent hyperpolarizable parts to
have a two-dimensional β character [12]. Recently several chalcone derivatives have been
reported where the phenyl moiety adjacent to carbonyl group was replaced with pyridine moiety
[13-19]. The results show that these derivatives exhibit good second order NLO response.
Among these, the chloro substituted chalcone [17] showed a SHG efficiency of 3.68 times that of
urea and methoxy group substituted chalcone MPP [13] showed a SHG efficiency of 4.7 times
that of urea. This high SHG response is attributed to the donor group substituted at para position
of the phenylene moiety and the zigzag head-to-tail alignment of molecular dipoles in the crystal
structure. This shows that a para substituent on the phenyl ring and the zigzag head-to-tail
alignment of molecular dipoles connected through strong intermolecular hydrogen bonds in the
crystal structure is necessary for enhancing the SHG activity of organic materials [12, 20]. It is
well known that for a material to have highly efficient NLO properties, the constituting
molecules must exhibit large molecular hyperpolarizabilities (β). Further, β increases with the
increase in the electron donating strength of the substituent [21]. The electronagativity of Br
group is 2.8 (Pauling value) [22] and hence its electron donating ability is greater than that of
chloro and methoxy groups. Also it is reported that the Br substitution in chalcones results in
non-centrosymmetric crystal structure, required for a crystal to exhibit second order optical
nonlinearity [23]. With the anticipation of an efficient NLO material, a pyridine based bromo
substituted donor- π-acceptor (D-π-A) type chalcone derivative was designed. The present article
describes the synthesis, crystal growth and characterization of a nonlinear optical material, 2E-3-
(4-bromophenyl)-1-(pyridin-3-yl) prop-2-en-1-one (BPP).
2. Experimental procedure
2.1. Material synthesis and single crystal growth
BPP was synthesized by the Claisen–Schmidt condensation reaction [24]. A mixture of
Analytical Reagent grade chemicals, 3-acetyl pyridine (0.01mol) and 4-bromobenzaldehyde
(0.01mol) in methanol (50 ml) was prepared without further purification. Aqueous solution of

sodium hydroxide (5 ml, 20%) was then added drop wise, and the mixture is stirred for 2 hours.
The temperature of the solution was maintained within the range of 26-300C throughout the
stirring process. The contents of the flask were poured into a beaker containing ice-cold water
and kept aside for 24hrs. The crude product was then collected by filtration, washed with excess
of water and dried. The crude product was used for the single crystal growth without further
purification. The synthetic scheme of the product is shown in Fig. 1.
Single crystals of BPP were grown by slow evaporation solution growth technique using
N,N-dimethyl formamide (DMF) as the solvent at room temperature. A saturated growth solution
was prepared by dissolving the synthesized compound in DMF in a 100 ml beaker at room
temperature; the solution was filtered to remove suspended particles. The solvent was allowed to
evaporate slowly by covering the beaker with a polythene sheet with few pin holes in it. Single
crystals of BPP were harvested from the growth solution after 45-50 days. Good quality single
crystals were selected for further study. A photograph of the grown crystals is shown in Fig. 2a.
The morphology of the single crystal of BPP was simulated using the software WinXMorph [25,
26]. Data obtained from the single crystal XRD study were used as the input to index the faces in
the crystal and is shown in Fig. 2b. The crystal has a higher growth rate along crystallographic b-
axis compared to other two axes which results in the elongation of the crystal along this
direction. Morphology of BPP may change if it is crystallized by changing the solvent.
Fig. 1. Scheme for synthesis of BPP.

Fig. 2. (a) Photograph of the BPP crystals. (b) Simulated Morphology of the crystal.
2.2. Characterization methods
In order to establish that the material (BPP) obtained is certainly that of the desired
compound, energy dispersive X-ray analysis (EDAX) was carried out on the sample. The grown
crystal was subjected to EDAX analysis using JEOL JSM-6380LA analytical scanning electron
microscope (SEM) system. The recorded spectrum of BPP is shown in Fig. 3. The peaks in the
EDAX spectrum confirm the presence of C, N, O and Br in the compound. The EDAX elemental
analysis confirms the formation of the compound.

Fig. 3. EDAX spectrum of BPP
The surface morphology of the BPP 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 20 kV and probe current was 1 nA 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 BPP crystal to electron beam.
The FT-Raman and FT-IR spectral analysis of BPP was carried out to identify the
presence of functional groups and their vibrational modes. A BRUKER RFS 27, Stand alone FT-
Raman Spectrometer, having spectral resolution of 2 cm-1 with an exciting wavelength of 1064
nm from an Nd: YAG laser source was used to record the FT-Raman spectrum. The FT-IR
spectrum was obtained by mixing the compound with KBr pellets. The spectrum was recorded
between 500 cm-1 and 3500 cm-1 by a Thermo Nicolet, Avatar 370 FTIR spectrometer outfitted
with KBr beam splitter and a DTGS detector, with a spectral resolution of 4 cm-1.
A Bruker Avance III FT-NMR spectrometer was used to record the 1H NMR spectrum of
BPP with CDCl3 as the solvent and tetramethylsilane (TMS) as the internal standard. The grown
crystal was subjected to single-crystal X-ray diffraction (XRD) studies. A Bruker SMART
APEX diffractometer with CCD area detector using graphite monochromated MoKα radiation of
wavelength 0.71073 Å was used to solve the three dimensional structure. Crystal data were
collected with a scan width of 1°. The crystal structures were solved by direct method and refined
by full matrix least squares method using SHELXL-97 [27] in the WinGx package suite [28]
with isotropic temperature factors for all the atoms. For molecular graphics X-seed [29] and
Mercury [30] software was used. A transmission spectrum is very important for any NLO
materials as a non linear material can be used for practical applications such as frequency
doubling if it has wider transparency window [31]. In this view UV–VIS–NIR absorption
spectrum of the BPP crystal was recorded using a Varian, Cary 5000 UV–VIS spectrophotometer
in the wavelength range of 200–1100 nm.
To study the thermal stability of the crystal BPP, thermo gravimetric (TG)/differential
thermal (DT) analysis and differential scanning calorimetric (DSC) analysis was carried out.
TG/DT analysis was done using a Perkin Elmer simultaneous TG/DTA analyzer. Powder sample
of the crystal was taken for this purpose and the analysis was carried out under the nitrogen
atmosphere at a heating rate of 100 C per minute in the temperature range from 300 C to 3500 C.
The DSC plot for the crystal BPP was obtained using Mettler Toledo DSC 822e thermal analyzer
on a sample weight of 1.188 mg in the temperature range 50o C to 190o C, at a rate of 10o C /min,

in nitrogen atmosphere. Kurtz and Perry Technique [32] of powder SHG efficiency measurement
enables one to measure SHG efficiency of a new material relative to standard KDP or urea. A
Q-switched 8 ns Nd: YAG laser beam delivering energy of 4.4 mJ/pulse at a wavelength of 1064
nm with 10 Hz repetition rate is used as light source in the experiment. A fine powder of BPP
crystal was tightly packed in a micro-capillary of uniform bore and exposed to laser radiations.
The output from the sample was monochromated and the intensity of 532 nm component was
collected. The emission of green light confirms the generation of second harmonics. The
electrical signal corresponding to the SHG output was displayed on a storage oscilloscope for
measurement. Urea and KDP, powdered to the same particle size as that of the experimental
sample, were used as indication materials for the present measurement.
3. Results and Discussion
The SEM images of 70x and 5000x magnification are shown in Fig. 4a and 4b
respectively. From the Fig. 4a, it is clear that the surface of the crystal appears smooth with some
micro-crystals on the surface. These inclusions are formed during the crystal growth and are
influenced by the growth conditions. Fig. 4b shows a layered growth pattern which is an
indication of two dimensional growth mechanisms [33]. The uneven darker and brighter regions
seen on the surface are might be due to the inclusion of solvent, a common event in solution
grown crystals.
Fig. 4. SEM images of BPP
FTIR and FT-Raman spectra are shown in Fig. 5 and Fig. 6 respectively. The
characteristic frequencies and corresponding assignments confirming the presence of various
functional groups in the material are tabulated in the Table 1. The FT-IR and FT-Raman spectral
study ascertains the various functional groups present in the compound.

Table 1
Assignments of vibrational frequencies:
Wave number (cm-1)
Assignment
FT-IR lines
FT Raman lines
3081.9
3064
aromatic C-H stretching
C=O stretching vibration
aromatic ring vibrations
chalcone C=C stretching vibrations
aromatic C=C stretching vibrations
C-N stretching vibration
C-H bending vibration
1658.1
1658.8
1583.9
1484.4
1401.4
1351.3
-
1588.9
1481.9
1396.6
-
1317.7
1233.18, 1186.49
1039.59
970.25
1242.8, 1176.2
1027.2
976.4
aromatic C-H in-plane bending
vibrations
CH=CH stretching vibration
C-Br stretching vibration
Out of plane C – H aromatic bending
vibrations
799.62
-
691.5, 614.64
702.5, 627.7

Fig.5. FTIR spectrum of BPP.
Fig.6. FT-Raman spectrum of BPP crystal.

1
The H NMR spectrum displayed in Fig.7 shows two doublets centered at δ=7.761 ppm
and 7.496 ppm with a coupling constant of 15.6Hz each corresponds to –C8-H8 and –C7-H7
protons of -CH=CH- moiety. The estimated strong coupling constant J = 15.6 Hz represents the
trans coupling of the adjacent protons, and this confirms the trans geometry of the double bond
of chalcone. A multiplet signal resonated at 7.564-7.590 ppm integrates for three protons (2H, -
C2-H2 & -C6-H6 of 4-bromophenyl ring and 1H, -C13-H13 of pyridine ring). The peak at 7.26
ppm is due to the solvent CDCl3. The other characteristic peaks observed in the spectrum are
assigned as given below. Thus the 1H NMR spectral study confirm the functional groups present
in the title compound BPP and hence the molecular structure.
δ (ppm): 9.223-9.228(dd, 1H, -C10-H10 of pyridine ring), 8.805-8.821(td, 1H, -C12-H12
of pyridine ring), 8.273-8.302(td, 1H, -C14-H14 of pyridine ring), 7.513-7.503(d, 2H, -C3-H3 &
-C5-H5 of 4-bromophenyl ring).
Fig. 7. NMR spectrum of BPP
From single-crystal XRD studies, it has been found that the compound crystallizes in the
monoclinic crystal system with a space group P21, which suggests that the crystal is
noncentrosymmetric in nature, thus satisfying one of the essential requirements for the SHG
activity of the crystal. An ORTEP plot of the molecule at 50 % probability is shown in Fig. 8.
The details of the crystal data and refinement are given in Table 2.

The molecule of the title chalcone derivative exists in E configuration with respect to the
-C7=C8- double bond [1.336 Å]; the torsion angle C6-C7-C8-C9 is 176.28o. The dihedral angle
between the pyridine and 4-bromophenyl rings is 19.96o and therefore the molecule is almost
planar. The carbonyl group is also planar with respect to the adjacent pyridine unit showing a
torsional angle O1-C6-C1-C5 of 5.65o. The degree of planarity of molecule correlates with the
degree of π-conjugation throughout the molecule. This increases the donor-acceptor interactions
across the molecule which in turn enhances the degree of molecular charge transfer and degree
of nonlinearity [32]. The bromine atom is displaced from C9 - C14 mean plane by 0.006 Å.
Atoms of the propenone bridge (C7, C8, C9 and O1) lie almost on the same plane and the torsion
angle O1-C6-C7-C8 is -10.11o. The mean plane through this bridge makes dihedral angles of
7.32o and 19.37o with the planes of pyridine and benzene rings respectively. The packing
diagram displayed in Fig. 9 shows that the molecular dipoles are aligned in zigzag head to tail
fashion stacked along b axis. The crystal structure is stabilized by weak intermolecular C-H…O
and C-H…N hydrogen bonds (Fig. 10) which forms an infinite molecular chain along a axis. It is
interesting to note that the arrangement of molecules in the crystal structure of BPP is very
similar to that in CPP [17].
Fig.8. ORTEP diagram of BPP. Thermal ellipsoids were drawn at 50 % probability level.

Fig. 9. Packing diagram viewed down the a–axis. Red arrow indicates the molecular dipole
calculated from MOPAC 2012 semi empirical package and green arrows indicate the alignment
of the dipoles in the crystal structure.
TABLE 2
Crystal data of BPP Crystal
Empirical formula
Formula weight
Crystal Size
C14 H10 Br NO
288.14 g mol-1
0.3 x 0.10 x 0.03 mm3
monoclinic
Crystal System
Space group
P 21
Unit cell parameters
a = 6.0063(7) Å
b = 5.0134(6) Å
c = 20.246(2) Å
α =90, β= 97.568(11), γ = 90 (deg)
Z = 2
Cell Angles
Volume
604.35(12) Å3
Mo Kα radiation
0.71073 Å
Radiation
Radiation Wavelength
Temperature
Density
T = 295(2) K
1.606 kg/m3
Absorption Coefficient
3.383 μ/mm-1
F(000)
296

θrange for data collection
3.04 - 27.55 (deg)
Limiting indices
-7<=h<=7, -6<=k<=6, -26<=l<=26
Max. And min. Transmission
Refinement method
0.9053 and 0.4302
Full-matrix least-squares on F^2
No. of measured reflections
Measured No. of unique reflections
No. of independent reflections
No. of parameters
3030
2166
1443
154
Goodness-of-fit on F^2
No. of restraints
0.984
1
R_all, R_obs.
0.0938, 0.0604
0.1483, 0.1237
wR2_all, wR2_obs.
Δρmin, max/e Å-3
-0.638, 0.528
Fig. 10. Partial packing diagram of BPP viewed down the b-axis. The only hydrogen atoms
involved in bonding are shown for clarity.

The recorded UV–VIS-NIR spectrum is shown in Fig. 11. The spectrum shows that the
crystal possesses a wide transparency from 419 nm to 1100 nm, 419 nm being the lower cut-off
wavelength. The absence of absorbance and overtones due to electronic transitions above this
wavelength is an essential parameter for frequency doubling process using solid- state and diode
lasers [34]. The inset of Fig.8 shows a reflectance of more than 85% in the entire visible region.
Thus the crystal can be used to obtain SHG in the visible region.
The optical band gap, which is responsible for the sharpness of the absorption edge of the
single crystals, can be found out by the mechanism of band to band transitions. The optical
absorption coefficient (α) has been evaluated from the recorded absorbance (A)-wavelength data
(
2.303/ d × A
where‘d’ is the thickness of the crystal. This
)
of the crystal using the relation, α =
value is used to determine optical band gap of the grown crystal following the Tauc’s relation,
αhυ = (hυ−Eg)n. Here h is Planck’s constant; υ is the frequency of the incident radiation and Eg is
the optical energy band gap [35]. A plot of (αhυ)2 versus hυ (Fig. 12), gives the direct band gap
of the crystal. The band gap value (3.1 eV) is obtained as the intercept of the linear fit of (αhυ)2
with the abscissa (Fig. 12(b)).
Fig. 11. Optical absorption spectrum of BPP. Inset figure shows the reflectance spectrum of
the crystal

Fig. 12. Tauc plot of the crystal BPP
The results of the thermal analysis are shown in the Fig. 13. The DTA curve shows that
the compound undergoes an irreversible endothermic transition at 122.4o C where melting
begins. The peak of the endothermic, which represents the temperature 139.78o C at which
melting terminates corresponds to the melting point of the crystal and the same was also
observed in the DSC plot. The sharp endotherm in the DSC response curve represents solid state
transition of relatively pure material [36]. In the TG curve, no weight loss was observed at the
melting point. Below the melting temperature, no endothermic or exothermic peaks were
observed which indicates the absence of any phase transitions and its thermal stability in this
region. The resistivity of the crystal against thermal crack is clearly evident as the DSC curve is
smooth up to the melting point. In the TG curve, weight loss begins to occur at about 175o C
which is due to the volatilization of the compound without decomposition. A weight percentage
of 96.83% at this temperature may be attributed to the loss of water. A major weight loss of 87%
was observed in the TG curve between 198o C and 279o C and is due to the decomposition of
BPP. The peak at 264.5o C in the DTA plot shows the complete dissociation of the compound
into gaseous products. Thermal stability of BPP is better than that of standard urea (130o C) and
do not get decomposed till the melting temperature. This ensures the firmness of the crystal for
NLO applications involving laser where the crystal has endure very high temperatures.

From the DSC curve, the molar enthalpy of fusion Δ fus Hm of the crystal BPP was
estimated adopting the area integration method and is found to be 81.47 Jg-1. The molar entropy
of fusion Δ fus Sm was calculated using the thermodynamic equation [18]Δ fus Sm = Δ fus Hm /Tm ,
where Tm is the melting point of the crystal. The calculated molar entropy of fusion is 0.197 JK-
1g-1.
Fig.13. TG/DTA/DSC response curves of BPP
Nonlinear Optical Property: Structure-Property Relationship.
The SHG conversion efficiency of BPP crystal measured by the classical powder SHG
efficiency test was found to be 6.8 times that of KDP and 1.4 times that of urea. SHG efficiency
of BPP is nearly 7 times greater than that of 3-(2,3-dimethoxyphenyl)-1-pyridin-2-yl)prop-2-en-
1-one [15], twice that of 1-(4-chlorophenyl)-3-(4-chloroyphenyl)-2-propen-1-one [8], 1.5 times
greater than that of 1,3-bis-(4-methoxyphenyl)prop-2-en-1-one [37], 3-(2,3-dichorophenyl)-1-
pyridin-2-yl)prop-2-en-1-one[16] and 1-(4-methylphenyl)-3-(4-N,N-dimethylaminophenyl)-2-
propen-1-one[38] and comparable to that of 2,4-dichloro-4 -bromochalcone[39].
Using semiempherical computer program molecular orbital package, MOPAC 2012 [40]
the molecular static and frequency dependent first and second hyperpolarizabilities of BPP

crystal were calculated using PM7 Hamiltonian [41]. The geometry obtained from the single
crystal XRD study of the BPP molecule was used as an input to the MOPAC 2012 program. The
geometry was optimized using the default geometry optimizer, Eigenvector Following (EF)
geometry optimizer. Using Time-Dependent Hartree–Fock (TDHF) theory, the molecular
hyperpolarizabilities were calculated. As a convergence criterion for PM7 calculation of
geometries and the NLO properties, the keyword “PRESICE” was used. The computed static and
frequency dependent first (β , β1064nm) and second (γ1064nm) molecular hyperpolarizabilties of BPP
o
are 5.54x10-30 esu, 11.88x10-30 esu and 25.36x10-32 esu respectively. The molecular first
hyperpolarizabilties at 1064 nm of some of the reported chalcones along with other properties are
presented in Table 3.
Table 3.
Properties of some chalcone derivatives.
Compound
R
C
R1
β
SHG
Melting
Dipole
λ
1064nm
cutoff
(10-30esu) (×Urea)point (0C)
moment (D) (nm)
1[23]
2[13]
3[17]
4[23]
5*
OCH3 46.70
8.5
4.7
3.7
7
73
81
2.45
1.53
1.23
2.12
1.16
440
410
417
435
419
o-N OCH3 35.62
m- N
C
Cl
Br
Br
11.57
86.40
11.88
133
125
139
m-N
1.4
*Present work
The title molecule BPP has a D-π-A push pull structure formed by the electron donating
Br group at one end and the carbonyl group along with pyridine ring acting as electron accepting
moiety being at the other end of the molecule. This means that the direction of the contributing
charge transfer or the dipole moment is form carbonyl to bromo group (Fig. 9). As result there is
delocalization of the electronic charge within the molecule. The donor-acceptor groups are
interconnected by conjugated –C=C- double bond which forms the pathway for the effective
charge transfer across the molecule.
Chalcone derivatives, such as CPP[17], 4NP3AP[18], MPP[13] and PDBA[42] reported
earlier by our group, showed very good SHG efficiency and is ascribed to the arrangement of

molecular dipoles in the crystal structure. The zigzag head–to-tail alignment of dipoles stacked in
to parallel columns and the fashion in which they are interconnected through the hydrogen bond
interactions in these crystals resulted in a net polarization due to the reckoning of individual
dipole contributions. This kind of head–to-tail configuration was also observed for p-methoxy
chalcone [43] and MMONS [44] crystals possessing a very high SHG of 8.5 and 1259 times that
of urea respectively. In the present case, the molecular dipoles are stacked in parallel columns
along the crystallographic b - axis in a zigzag fashion as shown in Fig. 9. The C-H…O and C-
H…N hydrogen bond interactions shown in the partial packing diagram (Fig. 10) consolidate the
structure in enantiomorphic form and controls the orientation of molecules. This establishes the
observed high SHG efficiency of BPP.
Table 4.
Hyperpolarizabilities, space group and dipole moments of trimethoxy substituted
chalcone derivatives.
Sl. No.
R R1
R2
R3
R4
R5
Space
Group
β
Dipole
1064nm
(10-30esu)
moment (D)
6 [45]
7 [46]
8 [47]
9 [48]
C
N
H
OCH3 OCH3 OCH3
OCH3 OCH3 OCH3
H
P21/c
Pna21
P21/c
Fdd2
12.9
21.7
13.7
23.3
2.50
2.76
5.54
5.01
H
H
C OCH3
N OCH3
H
H
OCH3
OCH3
H
H
OCH3
OCH3
For a material to exhibit strong macroscopic second order nonlinearity, the constituting
molecules must exhibit large molecular hyperpolarizabilities (β), which is evident from Table 3.
Further, the macroscopic SHG depends on the orientation of molecules in the crystal structure.
Since acentric crystal structure is necessary for strong SHG, the molecules should possess small
ground state dipole moments which lead to a favorable molecular orientation in the crystal by
minimizing the dipole-dipole interaction. Zhao et al [23] and Fichou et al [49] suggested the
bulky bromo group as an effective substituent at the para position of the phenylene moiety of the
chalcones for obtaining noncentrosymmetrics crystal structures. As can be seen from table 3, the

replacement of benzene ring near to the carbonyl group by pyridine ring brings down the dipole
moment and results in the crystallization of the compounds in acentric style. The table 4 provides
further evidence for this argument. The expected SHG efficiency in compounds 6 and 8 is zero,
since these compounds crystallize in centrosymmetric space group P21/c [20]. But compounds 7
and 9 where in a pyridine ring is substituted adjacent to carbonyl group, crystallizes in
noncentrosymmetric crystal system which is a prerequisite condition for SHG activity. Thus, in
addition to the bromo group, the pyridine moiety at the benzoyl arm instead of phenyl moiety is
also an effective replacement. But this resulted in decreased β value and hence the SHG of
compounds 2 and 5 in comparison with that of compounds 1 and 4. This decrease in SHG can be
attributed to the variation in the extent of charge transfer across the molecule due to the
difference in the planarity of the molecules and electron donating abilities of methoxy and bromo
groups. The degree of π-conjugation depends on the degree planarity of molecule. The planar
structure increases the extent of intramolecular charge transfer across the molecule and hence the
degree of nonlinearity and any deviation from planarity results in its reduction [17]. The degree
of planarity in compound 2 is high as indicated by the dihedral angle between phenyl and
pyridine rings of 2.040. The molecule of compound 3 is almost planar as indicated by the
dihedral angle of 19.530. In the present case (compound 5) the dihedral angle is 19.960 due to
which the crystal BPP shows a lower SHG efficiency. Thus, though Br group is a better electron
donor compared to Cl group, the lower SHG of BPP when compared to compound 3 may be due
an extra twist of small angle between phenyl and pyridine rings in BPP. Moreover, pyridine ring
improves the thermal stability and optical transparency of the material.
4. Conclusion
An efficient NLO material 3-(4-bromophenyl)-1-(pyridin-3-yl) prop-2-en-1-one was
synthesized and single crystals were grown at room temperature from DMF by the solution
growth technique. The functional groups in the compound and hence its structure was established
by FTIR and FT-Raman spectral analysis. EDAX elemental analysis and 1H NMR spectral study
confirms the purity of the sample. The crystals are optically transparent in the region above 419
nm and optical energy gap of the material is found to be 3.1 eV. Material is nonlinear optically
active with a powder SHG efficiency of 6.8 times that of KDP. Thermal study by TG/DTA and
DSC analysis showed that the compound does not get decomposed till its melting temperature of
1390C. Surface features of the crystals were studies by SEM analysis. The study showed that
BPP crystal may be a promising candidate for frequency doubling and other technological
applications involving second order NLO property. The investigation of structure – property

relationship of BPP in comparison with other reported compounds reveals pyridine ring as an
effective replacement for benzene ring in forming the enantiomorphic crystal structures needed
for second order nonlinearity. Further, small deviation from molecular planarity results in a
drastic change in the SHG response of the chalcones.
Appendix A. Supplementary information
The crystallographic information file for this paper containing 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 has been deposited by us in the Cambridge structure database
(CCDC 1026995). These data can be obtained free of charge from The Cambridge
Crystallographic Data Center via http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the
Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK).
Acknowledgement
The authors would like to thank Dr. P.K. Das, IPC Department, Indian Institute of
Science, Bangalore for NLO studies. The authors also acknowledge the SAIF-STIC, Cochin
University, SAIF, IIT Madras, and SAIF, IIT Bombay for providing experimental facilities.
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Crucial role of molecular planarity on the second order nonlinear
optical property of pyridine based chalcone single crystals
Anthoni Praveen Menezes*, Jayarama A, Seik Weng Ng
GRAPHICAL ABSTRACT
HIGHLIGHTS
• Efficient NLO single crystals are grown by slow evaporation solution growth technique.
• Scanning Electron Microscope image reveals that the crystal has smooth surface
morphology.
• Crystal melts at 139o C and decomposes at 264o C.
•
Molecular dipoles are stacked in zigzag head-tail fashion in the crystal structure
consolidated by hydrogen bonds.
•
The pyridine ring plays a significant role in forming enantiomorphic crystal system.