Synthesis, growth and characterization of 4-bromo-4′-nitrobenzylidene aniline (BNBA): A novel nonlinear optical material with a (3+1)-dimensional incommensurately modulated structure

Article abstract of DOI:10.1039/c3ce26663j

The organic nonlinear optical material of 4-bromo-4′-nitrobenzylidene aniline (BNBA, C₁₃H₉BrN₂O₂), a new derivative of benzylideneaniline family, was synthesized and purified by repeated recrystallization. Singl

Full text of DOI:10.1039/c3ce26663j

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Synthesis, growth and characterization of 4-bromo-
49-nitrobenzylidene aniline (BNBA): a novel nonlinear
optical material with a (3+1)-dimensional
Cite this: CrystEngComm, 2013, 15,
2474
incommensurately modulated structure3
Ashokkumar Subashini,a Sundararaman Leela,a Kandasamy Ramamurthi,*b
e
f
cd
Alla Arakcheeva,*cd Helen Stoeckli-Evans, Vaclav Petrıcek, Gervais Chapuis,
ˇ´ˇ
Philip Pattisondg and Philip Rejih
The organic nonlinear optical material of 4-bromo-49-nitrobenzylidene aniline (BNBA, C13H9BrN2O2), a
new derivative of benzylideneaniline family, was synthesized and purified by repeated recrystallization.
Single crystals of BNBA were grown in acetone solvent by slow evaporation at room temperature. Its (3+1)-
dimensional incommensurately modulated structure was elucidated from single crystal X-ray diffraction
data obtained at 173 K. The non-centrosymmetric structure was solved and refined in the monoclinic
superspace group A2(a0c)0; the unit cell parameters are a = 10.5217(10) Å, b = 16.2535(16) Å, c =
7.4403(7) Å, b = 110.709(7)u, modulation vector q = 0.0658(1)a*–0.2658(1)c*. At 290 K, the structure is
partially disordered, however, the local structure of domains is identical to that at 173 K. The 1H and 13C
NMR and Fourier Transform Infrared and FT-Raman spectroscopic studies confirmed the molecular
structure. The Kurtz powder second-harmonic generation (SHG) test reveals that the SHG efficiency of
BNBA is about 9.4 times higher than that of potassium dihydrogen phosphate. According to the
thermogravimetric, differential thermal and differential scanning calorimetric analyses, BNBA is a stable
phase starting from room temperature and up to the melting point, 436 K. The optical nonlinearity of
BNBA was investigated at 532 nm using 5 ns laser pulses, employing the open aperture Z-scan technique.
Received 11th October 2012,
Accepted 14th January 2013
DOI: 10.1039/c3ce26663j
www.rsc.org/crystengcomm
Second-order nonlinear optical materials find excellent appli-
cations in laser frequency conversion, optical parameter
Introduction
oscillator (OPO), signal communication,5,6 high-speed infor-
mation processing and optical data storage.7–11 In the case of
third order nonlinearities, two photon absorber (TPA) materi-
als find applications in upconversion lasing,12 information
technology,13 three dimension fluorescence imaging,14,15
Over the past two decades, there has been growing interest in
the elaboration of p-conjugated organic compounds due to
their applications in various fields such as molecular wires,
liquid crystals, electronic and optoelectronic devices, non-
linear optical (NLO) and two photon absorption material.1–4
aCrystal Growth and Thin Film Laboratory, School of Physics, Bharathidasan
University, Tiruchirappalli – 620024, India. E-mail: viji.suba@gmail.com
bDepartment of Physics and Nanotechnology, SRM University, Kattankulathur-603
203, Kancheepuram Dt., India. E-mail: krmurthin@yahoo.co.in;
Electronic supplementary information (ESI) available: Refinement details of
3
the (3+1) dimensional structure at 173 K. Discussion of the C–H...O and C–H...Br
interactions. Details of the room temperature single crystal XRD analysis. Table
S1, details of XRD experiments and structure refinements at 173 and 290 K.
Tables S2 and S3, positional and ADP parameters of atoms in the rigid BNBA
molecule at 173 K. Table S4, modulations of the rotation and displacement
parameters of the rigid molecule at 173 K. Tables S5 and S6, interatomic
distances and angles. Table S7, candidates for the C–H...O and C–H...Br
interactions aperiodically appear between molecules. Tables S8 and S9,
positional and ADP parameters of atoms in the rigid BNBA molecule at 290 K.
Fig. S1, the h0l and h2l experimental patterns of the reciprocal space at 173 K.
Fig. S2, the most probable C–H...O and C–H...Br interactions in the
incommensurately modulated BNBA structure. Fig. S3, split BNBA molecule at
290 K. Fig. S4, the calculated and experimental powder X-ray diffraction patterns
of BNBA at room temperature. CCDC 900429 and 900430. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c3ce26663j
ramamurthi.k@ktr.srmuniv.ac.in
c
´
Phase Solution Sarl, ch. des Mesanges 7, Lausanne CH-1012, Switzerland.
E-mail: allaarakcheeva@gmail.com
d
´
´ ´
Crystallography Competence Center, Ecole Polytechnique Federale de Lausanne,
1015 Lausanne, Switzerland. E-mail: gervais.chapuis@epfl.ch
e
ˆ
ˆ
Institute of Physics, University of Neuchatel, CH-2000 Neuchatel, Switzerland.
E-mail: helen.stoeckli-evans@unine.ch
fInstitute of Physics ASCR v.v.i., Department of Structure Analysis, 182 21 Praha 8,
Czech Republic. E-mail: petricek@fzu.cz
gSwiss–Norwegian Beam Lines at ESRF, Grenoble BP-220, 38043, France.
E-mail: pattison@esrf.fr
hLight and Matter Physics Group, Raman Research Institute, C. V. Raman Avenue,
Sadashivanagar, Bangalore 560080, India. E-mail: reji@rri.res.in
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microfabrication,16 optical power limiting, optical pulse
shaping and processing and laser mode locking.17
Benzylideneanilines are Schiff bases, which have been widely
used in coordination, medicinal and biological chemistry.
Further, benzylideneanilines show significant anticancer and
anti-inflammatory activities and in stereo selective organic
synthesis they may serve as reagents.
Scheme 1
A literature survey shows that many researchers have
reported on the synthesis and growth of crystals of some of
the benzylideneaniline derivatives and on their structural and
other properties. For example, 4-nitro-49-methylbenzylidene
aniline (NMBA),18 4-nitro-49-methoxybenzylidene aniline
(NMOBA)19 and 4-methoxy-49-dimethylaminobenzylidene ani-
line (MDMABA)20 crystallize in the monoclinic crystal system.
The powdered second harmonic generation efficiency of
NMBA is 16 times larger than that of urea18 and NMOBA is
1.3 times than that of potassium dihydrogen orthophosphate
(KDP).19 Employing the Z-scan technique, third order non-
linear optical properties were studied for centrosymmetric
compounds such as 4-chloro-49-dimethylaminobenzylidene
aniline [CDMABA]21 and 4-bromo-49-chlorobenzylidene aniline
[BCBA]22 and centrosymmetric MDMABA23 compound. In the
present work, we report on the synthesis, growth and
characterization of one such derivative, 4-bromo-49-nitroben-
zylidene aniline (BNBA, C13H9BrN2O2), for the first time. The
structure of BNBA appears as incommensurately modulated at
173 K. The structure solution has been obtained using the
superspace concept based on consideration of higher dimen-
sional periodicity of a crystal lattice.24,25 The recent review on
organic molecular compounds with modulated crystal struc-
tures shows that structures with higher dimensional periodi-
city are quite common in the field of organic materials.26
NMR spectroscopy
The 1H NMR and 13C NMR spectral analyses were made on the
BNBA sample in chloroform-D (CDCl3) using a Bruker AC 200-
NMR spectrometer.
FTIR and FT-Raman spectroscopy
The Fourier transform infrared (FTIR) and Fourier transform
Raman (FT-Raman) spectra were recorded in the wavelength
range 400–4000 cm21. FTIR spectrum was recorded using a
Perkin-Elmer-Paragon 500 spectrometer using a potassium
bromide (KBr) pellet, and the FT-Raman spectrum was
recorded using a Varian FT-Raman spectrometer.
Single crystal and powder XRD
The single crystal X-ray diffraction data were collected at 173 K
on a Stoe Mark II-Image Plate Diffraction System equipped
with a two-circle goniometer and using Mo Ka graphite
Experimental section
Synthesis and growth of BNBA
BNBA was synthesized by reacting p-bromoaniline (p-BA) and
p-nitrobenzaldehyde (p-NB) in a molar ratio of 1 : 1. A
calculated amount of p-BA was dissolved in ethanol containing
the appropriate amount of p-NB. The solution was refluxed for
6 h and then cooled to ambient temperature, yielding BNBA as
a yellow solid. The reaction mechanism is depicted in
Scheme 1.
The yellow solid was dissolved in acetone and a successive
recrystallization process was adopted to improve the purity of
the salt. Single crystals of BNBA were grown by slow
evaporation at room temperature from an acetone solution
in a beaker covered by parafilm. A transparent crystal of
dimension 4 6 2 6 1 mm3 was grown in a period of 20 days
and is shown in Fig. 1a. The morphology of the grown crystal is
given in Fig. 1b and the different crystallographic planes of the
crystal were identified from single crystal X-ray diffraction
study.
Fig. 1 Harvested crystal of BNBA.
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monochromated radiation (l = 0.71073 Å). Image plate
distance 130 mm, v rotation scans 0–180u at Q 0u, and 0–
123u at Q = 90u, step Dv = 1.0u, exposures of 2 minutes per
image, 2h range 1.76–52.59u, dmin–dmax = 23.107–0.802 Å. The
images showed clearly satellite reflections indicating an
using an energy meter placed after the sample. For this study
the sample was placed in a 1 mm path length glass cuvette
(Helma GmBH) which was mounted on a stepper motor
controlled linear translation stage. The optical density of the
prepared solution is low at 532 nm, and its linear transmission
is high (82%). The laser pulse energy reaching the sample is 90
mJ. The sample was translated in the Z direction in steps of 200
mm, and the transmitted energy was measured for each
position, using a pyroelectric energy meter (RjP 735 Laser
Probe Inc.). The experiment was automated using the program
LabVIEW. The interval between two successive laser pulses was
always kept sufficiently large (typically more than 2 s), to
enable the complete thermal relaxation of the sample before
the arrival of the following pulse. The open aperture
transmission normalized to the linear transmission of the
sample (normalized transmittance) was then plotted against
the sample position measured relative to the beam focus.
Nonlinear absorption was indicated by a smooth valley shaped
curve, which is symmetric about the focal (Z = 0) position. The
Z-scan of pure chloroform was run separately to ensure that
the solvent shows no nonlinearity under identical excitation
conditions.
incommensurately modulated structure.
A unit cell was
obtained for the strong non-satellite reflections and the
average structure was solved by direct methods with
SHELXS.27 Least-squares refinement was attempted with
SHELXL27 but the solution was of course not satisfactory.
The description and refinement of the solution of this
incommensurately modulated structure is described later.
The room temperature (290 K) data set was collected on the
Swiss–Norwegian Beamline BM01A at the ESRF, Grenoble.
Diffraction patterns were collected using a MAR Research
MAR345 image plate detector with a rotation angle of 2u per
image. A focussed beam with a wavelength of 0.7000 Å was
selected with a Si(111) monochromator. Integration of the
images and indexation of the diffraction reflections were
performed with the CrysAlis software package from Agilent. All
structure analysis calculations were performed using the
program JANA 2006.28 The main crystallographic and refine-
ment details are given in Table S1 of the ESI.
3
Thermal analyses
X-ray powder diffraction pattern was obtained with Cu Ka (l
= 1.54060 Å) radiation from the crushed powder of a BNBA
crystal. The sample was scanned over the 10–80u 2h range at a
Thermogravimetric (TG) and differential thermal (DT) analyses
were recorded using an S. T. A – 1500 Simultaneous Thermo
Analytical system in the temperature region 40–940 uC at a
heating rate of 10 K min21 under nitrogen atmosphere. 13.79
mg of the BNBA material was taken for TG-DTA measure-
ments. The differential scanning calorimetry (DSC) experiment
was performed using a TA instrument, Model Q20 V24.2 Build
107 differential scanning calorimeter. 5.02 mg of BNBA sample
was taken in a sealed aluminium pan with a perforated lid and
subjected to DSC analysis under an inert nitrogen atmosphere
(50 ml min21). BNBA crystalline powder was heated from 5 uC
scan rate of 2u min21
.
Fluorescence study
The emission spectrum for BNBA was recorded using a Perkin-
Elmer LS55 Luminescence spectrometer in the range 400–600
nm.
Kurtz powder test
The second harmonic generation (SHG) conversion efficiency
of BNBA was determined by the powder technique developed
by Kurtz and Perry.29 The crystal was powdered and the fine
powdered sample was inserted in a microcapillary tube and
then subjected to a Q-switched Nd:YAG laser emitting 1064 nm
radiation at the pulse energy of 4.0 mJ. The frequency
doubling was confirmed by the emission of green radiation
of wavelength 532 nm collected by a monochromator after
separating the 1064 nm pump beam with an IR-blocking filter.
A detector connected to a power meter was used to detect the
second harmonic intensity. A powdered sample of potassium
dihydrogen phosphate (KDP) crystal of the same size was used
as a reference material.
to 165 uC at a rate of 5 uC min21
.
Results and discussion
Confirmation of the BNBA molecule formation
1
NMR spectra. The H NMR spectrum provides information
about the number of different types of protons and also
regarding the nature of the immediate environment of each of
them. 1H NMR spectrum of BNBA shows five important
absorption peaks named A, B, C, D and E in Fig. 2. They point
to the different hydrogen atoms present in five different
chemical environments in the BNBA molecule. The chemical
shift values of protons in different environments correspond-
ing to peaks A, B, C, D and E, respectively are: a one proton
singlet (C–H) at d 8.88 ppm, a two proton doublet at d 8.44
ppm, a two proton doublet at d 8.24 ppm, a two proton doublet
at d 7.70 ppm and a two proton doublet at d 7.38 ppm. The
singlet signal A at d 8.88 ppm corresponds to the proton of
imine group,30 which confirms the formation of Schiff base
compounds. The other adjacent four doublet protons are due
to aromatic ring protons. In the NMR spectrum of BNBA, the
ratio of steps obtained are A : B : C : D : E = 1 : 2 : 2 : 2 : 2.
Z-Scan
In order to study the third order optical nonlinearity, the non-
linear optical absorption of BNBA dissolved in chloroform was
measured using 532 nm, 5 ns laser pulses from a frequency
doubled Nd:YAG laser (Continuum Minilite), employing the
open aperture Z-scan technique. The laser beam was focused
using a convex lens and the sample was translated along the
beam axis (z-axis) through the focal region. The focal point is
taken as z = 0. As the sample is translated along the z-axis, it
sees different laser intensity at each position. The position
(and hence the intensity) dependent transmission is measured
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Fig. 2 1H-NMR spectrum of BNBA.
Fig. 4 FTIR and FT-Raman spectra of BNBA.
Thus, the number of protons associated with the signals is A –
1H and B, C, D, E – 2H.
The 13C NMR is an important tool in chemical structure
elucidation in organic chemistry.31 The 13C NMR spectrum of
BNBA, presented in Fig. 3, contains nine signals. The signal at
A is attributed to the carbon of imine group, which confirms
the formation of Schiff base compounds. The presence of the
signals at B, C, D and I are attributed to the singlet carbons of
the aromatic ring. The doublet signals at E, F, G and H are due
to the aromatic ring carbon. The three-line signal at J from the
solvent corresponds to the single carbon atom of deutero-
chloroform, CDCl3; it appears as a triplet because of the
interaction with the deuterium atom. Molecular structure of
BNBA is thus confirmed from the NMR spectra.
Similarly in the FT-Raman spectrum asymmetric stretching
vibrations of the nitro group occurs at 1516 cm21 and
symmetric stretching vibrations of the nitro group occurs at
1339 cm21. Benzylidene anilines display their CLN stretching
as a band at 1600 cm21 32
Thus the band obtained at y1594
.
cm21 and y1595 cm21 in the FTIR and FT-Raman spectrum,
respectively, of BNBA is due to the formation of imine group
(CLN) as a result of the condensation reaction between
aldehyde and amine. Para-di- substituted benzenes show C–
H deformation vibration in the region 840–800 cm21 and in
FTIR the C–H deformation vibration appears at y838 cm21. In
the FTIR spectrum the band at y532 cm21 is due to aromatic
C–Br stretching. Thus the FTIR and FT-Raman spectral
analyses confirm the formation of BNBA.
FTIR and FT-Raman spectra. The functional groups present
in the BNBA sample were analysed by FTIR and FT-Raman
spectral analyses and are shown in Fig. 4.
Aromatic nitro compounds can be recognized from two
strong bands in the region 1570–1500 cm21 and 1370–1300
Crystal structure of BNBA
cm .
21 19 In the FTIR spectrum the peak at y1516 cm21 and at
Crystal structure at 173 K. The diffraction patterns contain
strong satellite reflections, which cannot be indexed using
y1353 cm21 represents the asymmetric and symmetric modes
of vibration of the nitro group (–NO2), respectively.
three indexes hkl (Fig. 5 and S1, ESI ).
3
The presence of satellites points to an incommensurately
modulated structure, which has been solved in the non-
centrosymmetric monoclinic (3+1)-dimensional superspace
group A2(a0c)0, (crystallographic data,{ lattice parameters
and the q-vector are given in Table S1, ESI ). A reliable
3
solution of this structure (twinned by a mirror plane normal to
the c axis) has been reached with twin components 0.504(3)
and 0.496(3); the Flack-parameter is equal to 0.01(1). A single
molecule has been considered as a rigid unit. The structural
modulations appear as a result of relative shifts and tilts of the
molecules. Details of the structure refinement are given in
Table S1, ESI. Positional and anisotropic displacement
3
parameters of atoms in the BNBA molecule and information
{ Crystallographic data of BNBA, C13H9BrN2O2 at 173 K: the (3+1)D group of
symmetry is A2(a0c)0; the modulation vector q = 0.0658(1)a* 20.2658(1)c*; a =
10.5217(10), b = 16.2535(16), c = 7.4403(7) Å, b = 110.709(7); V = 1190.2(2) Å3; Z =
4; dx = 1.7024 g cm21. Crystallographic data of BNBA, C13H9BrN2O2 at 290 K:
space group A2; a = 10.6408(09), b = 16.2706(10), c = 7.5669(7) Å, b = 110.783(8); V
Fig. 3 13C-NMR spectrum of BNBA.
= 1224.8(2) Å3; Z = 4; dx = 1.6542 g cm21
.
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Fig. 5 Satellite reflections (indicated by red circles) in the h0l section of the
reciprocal space. The satellite reflections are split owing to the twin mirror plane
normal to the c axis.
on the molecule modulation are presented in the ESI (Tables
3
S2–S4) and in the CIF file. The nine crystallographically
independent H atoms (Table S2, ESI ) are in perfect agreement
3
with the NMR spectra of BNBA.
The crystal packing of BNBA is illustrated in Fig. 6, along
with the details of a single molecule in Fig. 7, which confirm
the expected molecular connectivity (Scheme 1). For refine-
ment purposes, each BNBA molecule has been constrained as
two flat units, (L)N–C6H4–Br (BB) and NO2–C6H4–CH(L) (NB).
The angle between the units was refined to 18.1(8)u. No
restrictions were applied in the final refinement. However, H
atoms were constrained at 0.96 Å from the respective parent
C-atoms. A complete listing of bond distances and angles are
Fig. 6 ac projection of a portion of the structure and a representative layer of
the incommensurately modulated structure of BNBA. In the layer, the dashed
…
…
…
lines indicate H A bonds with the distances H Br ¡ 3.6 Å, H O ¡ 2.6 Å and
…
angles C–H A ¡ 140u.
temperature, however, diffuse scattering pointed to a partially
disordered crystal structure. This 290 K structure was success-
fully refined using the average structural model obtained for
the incommensurately modulated structure at 173 K. This
model corresponds to split molecules (Table S8 and Fig. S3 in
available in the ESI (Tables S5 and S6) and CIF file. The
3
following distances characterize the NB moiety: C–C = 1.39(3)
¡ 0.03 Å in the phenyl rings, Car–C(LN) = 1.47(2) Å, Car–NO2 =
1.47(2) Å, N–O = 1.23(2) ¡ 0.02 Å in NO2. The following
distances characterize the BB moiety: C–C = 1.39(3) ¡ 0.03 Å
in the phenyl rings, Car–N = 1.43(2) Å, Br–Car = 1.89(1) Å. The
CLN distance is equal to 1.25(2) ¡ 0.02 Å. In BNBA, all angles
between closest atoms have been refined as 120(1) ¡ 5u (Table
the ESI ) and implies that the local domain structure at room
3
temperature is identical to that at 173 K, whereas a long-range
order of the domains is missing at room temperature. The
main difference between the molecule at 290 and 173 K relates
to the atomic displacement parameters (ADP), which are
unusually large for all atoms at 290 K (Table S9 in the ESI ).
3
S6 in ESI ). According to the high-dimensional symmetry, the
3
Splitting of the molecules and the large ADP atomic
molecules tilt and shift following the structure modulation
(Table S4 in ESI ).
3
…
The displacements and rotations are imposed by C–H
…
O
and C–H Br interactions occurring between molecules (Fig. 6,
8 and S2 in the ESI ). These interactions do not satisfy
3
periodicity in the 3-dimensional bulk, only the [010] direction
remains periodic (Fig. 6 and 8).
…
…
The H O (H Br) average distances vary from 2.47(5) to
…
…
2.63(5) Å (from 2.88(5) to 3.82(3) Å) and C–H O (C–H Br)
average angles vary from 148(4) to 165(2)u (from 132(3) to
165(2)u). Full details are given in Table S7, ESI.
3
Crystal structure at 290 K. No satellite reflections were
Fig. 7 A single molecule of BNBA with displacement ellipsoids drawn at the
50% probability level.
observed in the diffraction patterns obtained at room
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Fig. 9 Powder XRD pattern of BNBA.
The BNBA sample was excited at 325 nm and the
fluorescence emissions were observed at y469 nm (Fig. 10)
which are due to the presence of aromatic rings in the BNBA
compound. The broad fluorescence maxima is probably due to
the modulated nature of BNBA.
BNBA as a nonlinear optical material
The Kurtz Powder Test showed that the reference KDP
powdered crystal produced 6.4 mV as output beam voltage,
while the BNBA powdered crystal of the same size has
produced about 60 mV. Hence, the BNBA material has an
NLO efficiency of about 9.4 times higher than that of KDP.
The open-aperture Z-scan curve obtained for BNBA is shown
in Fig. 11.
Fig. 8 View along the [010] direction and perspective view of the typical
…
…
connection of the BNBA molecules by C–H O and C–H Br interactions
(dashed lines). Brown, blue, red, black and pink colours correspond to Br, N, O, C
and H atoms, respectively.
An increase in absorption is observed as the laser intensity
is increased, indicating nonlinear optical absorption. Since the
linear transmission of the sample at the excitation wavelength
is high at 82%, the observed nonlinear absorption might have
contributions from genuine two-photon (2PA) or three-photon
parameters confirmed the molecule modulations in local
domains. Absence of satellite reflections on the diffraction
patterns indicates an absence of coherency of the modulations
between the domains. Despite the almost identical molecular
structure at both temperatures, the correct structure solution
is available only for the lower temperature structure owing to
the presence of the satellite reflections.
The recorded powder X-ray diffraction pattern of BNBA is
shown in Fig. 9, and is compatible to that calculated using the
room temperature single crystal CIF file (see Fig. S4 in the
ESI ). CCDC 900430 and 900429 contain the crystallographic
3
data for the BNBA structure at 173 K and 290 K, respectively.
Fluorescence of BNBA
Fluorescence finds wide applications in the branches of
biochemical, medical and chemical research fields for analyz-
ing organic compounds.33 Fluorescence generally occurs in
compounds containing aromatic functional groups with low-
energy p A p* transition levels. Compounds containing
aliphatic and alicyclic carbonyl structures or highly conjugated
double-bond structures exhibit fluorescence, but the number
of such compounds is small compared with the number of
aromatic systems.34
Fig. 10 Fluorescence spectrum of BNBA.
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Fig. 11 The open aperture Z-scan curve measured for BNBA dissolved in
chloroform. Open circles are experimental data points while the solid curve is
the numerical fit for a two-photon absorption process, calculated using the non-
linear transmission equation.35 Tnorm is the normalized transmittance.
Fig. 12 The TG (blue) and DTA (black) diagrams of BNBA.
sharpness of the peak reveals the good crystallinity of the
synthesized compound.
(3PA) absorption, as well as from excited state absorption
(ESA). We tried to fit the data to both two-photon and three-
photon absorption equations, and the best fit was obtained for
a two-photon process described by the non-linear transmis-
sion equation:35
Conclusions
Single crystals of the Schiff base 4-bromo-49-nitrobenzylide-
neaniline (BNBA) have been grown from acetone solution by
the slow evaporation technique at room temperature. BNBA is
a new novel nonlinear optical material. Its relative powder
SHG efficiency was measured to be 9.4 times higher than that
of KDP. The effective two-photon absorption coefficient b is
!
?
ð
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
ð1{RÞ expð{aLÞ
T~
ln 1zq0expð{t2Þdt
pffiffiffiffiffiffiffi
pq0
{?
where T is the intensity dependent transmittance of the sample; L
is the length of the sample; R is its surface reflectivity; a is the
linear absorption coefficient; q0 is given by b(1 2 R)I0Leff, where b
is the two-photon absorption coefficient and I0 is the peak
intensity, Leff is given by [1 2 exp(2aL)]/a.
estimated to be 5.95 6 10211 m W21
The molecular structure and the functional groups are
confirmed by NMR, FTIR and FT-Raman spectra.
.
The noncentrosymmetric structure of the grown crystal is
incommensurately modulated (A2(a0c)0 superspace group) at
In the present case, the quantity b is the effective 2PA
coefficient, which has contributions from both resonant and
non-resonant nonlinear processes (ESA and 2PA, respec-
tively).36 The resonant term is given by the expression bESAI =
sN, where s is the ESA cross-section and N(I) is the intensity-
dependent excited state population density. By determining
the best-fit curve for the Z-scan data, the effective two-photon
…
…
173 K. The C–H O and C–H Br interactions occurring between
molecules in structural layers force the structure modulations.
…
The C–H Acceptor interactions do not exhibit periodicity in the
3-dimentional bulk of the crystal, only the [010] direction is
periodic. The layers subject to interactions are at the origin of a
strong preferred orientation in a powder sample using to record
powder XRD pattern. At room temperature, the structure is
partially disordered, however, the local structure of domains is
identical to that at 173 K. The model obtained for the
incommensurately modulated structure at 173 K is necessary
to solve the room temperature structure.
The fluorescence emissions of BNBA observed at y469 nm
is due to the presence of aromatic rings in its structure.
The melting point of the BNBA material was found to be
y436 K and no phase transformation has been observed
before melting.
absorption coefficient b is calculated to be 5.95 6 10211
W21
m
.
Thermal stability of BNBA
TG-DTA of BNBA is shown in Fig. 12. In TGA a single stage
weight loss was observed and the major weight loss starts at
200 uC due to decomposition of BNBA. The decomposition of
BNBA leads to zero residue. The shape of the TGA curve
indicates that the melting is immediately followed by decom-
position with the formation of the volatile reaction products of
the sample.37 A sharp endothermic peak at y163 uC (436 K)
corresponds to the melting point of the material in DTA.
In DSC a sharp endothermic peak was observed at y163 uC,
which represents the melting point of the BNBA material and
there is no phase transformation before the melting point. The
melting point of the BNBA is in good agreement with the DTA
result. The BNBA material is stable up to its melting point. The
Acknowledgements
One of the authors (AS) thanks the University Grant
Commission, New Delhi for providing UGC: Meritorious
2480 | CrystEngComm, 2013, 15, 2474–2481
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Maughon, J. Qin, H. Rockel, M. Rumi, X. Wu, S. Marder
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Fellowship [File No. 4-1/2008 (BSR)] and also thanks Remya R
Mohan and Athira Mohanan for help with Z-scan measure-
ments. AS and RP are grateful to Dr. Priya Rose for useful
discussions. The authors AS, SL, KR and PR thank the
University Grant Commission, Government of India [File No.
32-37/2007 (SR)] and the Department of Science and
Technology (DST), Government of India for financial assis-
tance. VP thanks Praemium Academiae of the Czech Academy
of Sciences for support. HSE thanks the XRD Application
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