Investigation on nucleation, growth and physical properties of low soluble 4-N, N-dimethylamino-4-N’-methylstilbazolium 4-aminotoluene-3-sulfonate crystal – A potential NLO material

Article abstract of DOI:10.1016/j.molstruc.2021.130669

Terahertz generation requires materials with excellent nonlinear optical (NLO) properties and environment stability. 4-N, N-dimethylamino-4-N’-methylstilbazolium 4-aminotoluene-3-sulfonate (DAAS) an organic ionic compound is found to match these conditions but it is one of the sparingly soluble compounds among the stilbazolium derivatives causing difficulty in crystal growth. Therefore, in this article, the nucleation and growth kinetics of DAAS in different solvents are examined in order to understand and improve its crystal growth. The structural properties like symmetry, hydrogen bonding that contribute to crystal growth and morphology are studied and discussed. As nonlinear optical properties in organic compound is affected by vibration of the molecules, the existence of functional groups and various modes of vibration present in the molecule are identified and categorized by Fourier transform infrared (FT-IR) and FT-Raman analyses. Linear optical properties of the DAAS at molecular level, liquid and solid phases are investigated using density functional theory and UV-Visible spectroscopic technique. The NLO properties of DAAS molecule like the first order hyperpolarizability and mean polarizability is predicted using density function theory and the second harmonic generation efficiency is determined using Kurtz powder test. The thermal stability of DAAS is compared with other highly nonlinear optical stilbazolium derivatives.

Full text of DOI:10.1016/j.molstruc.2021.130669

Journal of Molecular Structure 1241 (2021) 130669
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstr
Investigation on nucleation, growth and physical properties of low
soluble 4-N, N-dimethylamino-4-N’-methylstilbazolium
4-aminotoluene-3-sulfonate crystal – A potential NLO material
S. John Sundarama,∗, A. Antony Rajb, R. Jerald Vijayb,∗, M. Jaccobc, P. Sagayarajd
a Department of Physics, Sacred Heart College (Autonomous), Tirupattur, 635 601, India,
b Department of Physics, St. Joseph’s College (Autonomous), Trichy, 600 002, India,
c Department of Chemistry, Loyola College, Chennai – 600 034, Tamil Nadu, India & Computational Chemistry Laboratory, Loyola Institute of Frontier Energy
(LIFE), Loyola College, Chennai, 600 034, Tamil Nadu, India
d Department of Physics, Loyola College, Chennai, 600 034, Tamil Nadu, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Terahertz generation requires materials with excellent nonlinear optical (NLO) properties and environ-
ment stability. 4-N, N-dimethylamino-4-N’-methylstilbazolium 4-aminotoluene-3-sulfonate (DAAS) an or-
ganic ionic compound is found to match these conditions but it is one of the sparingly soluble com-
pounds among the stilbazolium derivatives causing difficulty in crystal growth. Therefore, in this article,
the nucleation and growth kinetics of DAAS in different solvents are examined in order to understand and
improve its crystal growth. The structural properties like symmetry, hydrogen bonding that contribute to
crystal growth and morphology are studied and discussed. As nonlinear optical properties in organic com-
pound is affected by vibration of the molecules, the existence of functional groups and various modes of
vibration present in the molecule are identified and categorized by Fourier transform infrared (FT-IR) and
FT-Raman analyses. Linear optical properties of the DAAS at molecular level, liquid and solid phases are
investigated using density functional theory and UV-Visible spectroscopic technique. The NLO properties
of DAAS molecule like the first order hyperpolarizability and mean polarizability is predicted using den-
sity function theory and the second harmonic generation efficiency is determined using Kurtz powder
test. The thermal stability of DAAS is compared with other highly nonlinear optical stilbazolium deriva-
tives.
Received 4 December 2020
Revised 21 April 2021
Accepted 6 May 2021
Available online 15 May 2021
Keywords:
Ionic organic crystal
Stilbazolium derivative
crystal growth from solution
Spectroscopy
© 2021 Elsevier B.V. All rights reserved.
1. Introduction
Based on these considerations, researchers are aiming at the de-
velopment of new stilbazolium derivatives by effectively varying
THz region of the electromagnetic spectrum offers diverse ap-
plications such as wireless communications, inspection of drugs,
detection of explosives and metals, analysis of DNA, gas sensing,
spectroscopy and imaging [1,2]. Ionic organic crystals are emerg-
ing as THz emitters than the commonly used semiconductors or
inorganic electro-optic emitter due to their attractive characteris-
tics such as high second nonlinear optical properties and broad-
band THz emission [3-5]. 4 -N, N-dimethylamino-4-N’-methyl stil-
bazolium tosylate (DAST) is one such material that is widely dis-
cussed and also commercialized. DAST crystal is composed of stil-
bazolium cation and tosylate anion [6]. The advantages and lim-
itations of DAST have been discussed in detail elsewhere [3,7,8].
the counter anion of DAST. Tan et al reported that so far 72 stil-
bazolium derivatives with different counter anions has been syn-
thesized and only 29% were grown in non-centrosymmetic struc-
ture with only 15 % showing second harmonic generation (SHG)
efficiency comparable with DAST. Among them only DSTMS, DASC
and DSMOS which is 4% of the crystals were grown as high-quality
single crystals and these materials are demonstrated to have either
comparable or even better optical and physicochemical properties
than DAST crystal [3,9-12]. The major difficulty in the remaining 7
% of crystals is the low solubility of these salts in most solvents.
Here we report yet another organic stilbazolium single crystal
4 -N, N-dimethylamino-4-N’-methyl stilbazolium 4-aminotoluene
3-sulfonate (DAAS), which is environmentally and thermally sta-
ble and possess acentric crystal structure for NLO applications but
shows poor crystal growth properties [13]. One of the salient fea-
tures is that, DAAS is not forming hydrated centrosymmetric struc-
∗
Corresponding author.
E-mail
addresses:
johnantonathan@gmail.com
(S.J.
Sundaram),
jeraldramaclus@gmail.com (R.J. Vijay).
https://doi.org/10.1016/j.molstruc.2021.130669
0022-2860/© 2021 Elsevier B.V. All rights reserved.

S.J. Sundaram, A.A. Raj, R.J. Vijay et al.
Journal of Molecular Structure 1241 (2021) 130669
ture even when it is crystallized in water or water containing sol-
vent. Therefore, DAAS offers the freedom from air-humidity dam-
age and thus avoids strict protecting conditions if employed as
crystal-based devices. The research group of Sun et al in their
maiden attempt successfully synthesized DAAS and have grown
crystals of few micrometers by temperature lowering method. It is
interesting to find that DAAS could not be grown into large crystal
and hence attracts that attention to investigate further the possible
reasons.
iodide (DMSI) and sodium salt of 4-aminotoluene-3-sulfonate. The
DMSI cation was synthesized by the condensation reaction method
employing the same procedure reported by our research group
[16]. The metathesization reaction was carried out as follows: Ini-
tially, DMSI (0.7324, 2 mmol) was dissolved in 100 ml of Mil-
lipore water with continuous heating. Aqueous solution of 4-
aminotoluene-3-sulfonate sodium salt was prepared by dissolving
equimolar ratio of 4-aminotoluene sulfonic acid (0.375 g, 2 mmol)
and NaOH (0.08 g, 2 mmol) in 50 ml of Millipore water and the
content was mildly heated. This solution was thoroughly mixed
with the already prepared DMSI solution and further heated for an
hour at 70°C with continuous stirring. The resulting solution was
cooled down to room temperature and then kept undisturbed for
a day. A red colour precipitate was obtained due to the cation and
anion exchange reaction. The left out aqueous solution of sodium
iodide was later separated by vacuum filtration. The scheme for
synthesizing DAAS is illustrated in Fig. 1.
DAAS. Yield: 76%. 1H-NMR (500 MHz, CD3OD): δ(ppm) = 8.51
(d, 2H, J =6.5 Hz, C5H4N), 7.98 (d, 2H, J =7 Hz, C6H4N), 7.862 (d,
1H, J = 16 Hz, C6H3SO3−), 7.639 (d,2H, J = 9 Hz,C6H4-N), 7.495
(d,1H,J= 2Hz, C6H3SO3−), 7.119 (d, 1H, J = 16 Hz, C6H3SO3−), 7.002
(d, 1H, J = 10 Hz, CH), 6.82 (d, 1H, J = 9Hz, CH), 6.696 (d, 2H,
J = 8Hz, C6H4-N), 4.64(s, 2H, NH2), 4.232 (s, 3H, NCH3), 3.088 (s,
6H, NC2H6), 2.223 (s, 3H, CH3). Anal. Calcd. For C23 H27 N3 O3 S:
C, 64.92; H, 6.39; N, 9.87. Found: C, 64.69; H, 6.31; N, 9.83.
Therefore, an effort has been made to grow large single crystals
of this low soluble DAAS in a mixed solvent system by exploit-
ing the Oswald ripening process using the slow evaporation tech-
nique [3,14-16]. It is believed that the use of mixed solvent of wa-
ter and methanol along with prolonged period for Oswald ripen-
ing have promoted the growth of crystal with improved size. This
method can be adopted for other low soluble materials to grow
larger crystals of reasonable size suitable for various applications.
Along with growth, linear, nonlinear optical and detailed spectral
properties of DAAS are investigated. In this article, we have car-
ried out the growth experiment with the vision to further improve
the size of the crystal and did a systematic study on various phys-
ical chemical properties. To the best of authors knowledge Hirsh-
feld surface analysis, molecular orbital analysis, vibrational spec-
troscopy and photoluminescence of DAAS are reported for the first
time.
2. Experimental procedure
2.2. Solubility
2.1. Material preparation
In the present work, several solvents in pure or mixture forms
were used to determine the solubility of DAAS. It is noted that sol-
vents like methanol, water and mixed solvent like water-methanol
showed reasonable solubility compared to other solvents. In water-
The title compound (DAAS) was prepared via metathesization
reaction between 4-N, N-dimethylamino-4-N’-methyl stilbazolium
Fig. 1. Preparation method of DAAS.
2

S.J. Sundaram, A.A. Raj, R.J. Vijay et al.
Journal of Molecular Structure 1241 (2021) 130669
Fig. 2. Solubility curve for DAAS in methanol and water-methanol
mixed solvent.
Fig. 3. Nucleation mechanism of DAAS in methanol.
3

S.J. Sundaram, A.A. Raj, R.J. Vijay et al.
Journal of Molecular Structure 1241 (2021) 130669
methanol mixed solvent system, the material showed positive sol-
ubility coefficient. In a 250 ml beaker, 100 ml of water-methanol
(1:1) was taken and kept in a water bath. The beaker was closed
with appropriate sheet and the initial temperature was set as 35
◦C. By adding the powdered sample, the solution was stirred con-
tinuously. The amount of sample in the solution was analysed with
saturation level by gravimetric analysis. In a warmed pipette, 10
ml of saturation solution was taken and poured in to a cleaned
and dried Petri dish. By gentle heating, the solution was allowed
to evaporate completely. The mass of DAAS in 10 ml of solution
was determined by weighing the Petri dish with salt. With this
method, the solubility of DAAS was measured for various tempera-
tures (35, 40, 45, 50 and 55 ◦C) for the mixed solvent system of
water: methanol (1:1) and Fig. 2 depicts the solubility curve of
DAAS. The same procedure was used to determine the solubility
of DAAS in methanol solvent.
drop from the saturated solution of DAAS in methanol at 30°C
was cast on a glass slide and the nucleation mechanism were
recorded using a polarizing microscope. It is observed from Figs.
3 and 4 that multinucleation results in precipitation of DAAS into
several microcrystals rather than single crystals. It is the spon-
taneous multinucleation that leads to the precipitation of DAAS
in methanol. Rapid evaporation of methanol is also another fac-
tor: faster nucleation resulting in few tiny crystals within 5 sec-
onds and several crystals with complete evaporation of solvent
within 17 seconds. The same experiment when performed in
water-methanol solvent yielded spherulites of a different kind. It
can also be observed that the growth terminates once the solution
is deficient of solute. At first, nucleation takes place in the form
of a cluster and then growth of this cluster into a spherulite. It
took 20 seconds for first crystals to appear after nucleation and the
growth progress slowly when compared to methanol system. An-
other, feature in this system is the growth of crystal occurring at
the edge of the droplet and progressing inwards in contrast to the
methanol system. Rapid multinucleation is the reason for not ob-
taining single crystals of sufficient size in the methanol solvent. In
the water-methanol solvent system, the growth of spherulite signi-
fies that this solvent system would yield single crystals. From the
nucleation experiments, it is observed that the ability of solute to
2.3. Nucleation and growth mechanism
In the present work, the growth habit was investigated by nu-
cleation mechanism. In order to further understand why DAAS
grows rarely in methanol but reasonably good in water-methanol,
the nucleation mechanism in these solvents were investigated. A
Fig. 4. Nucleation mechanism of DAAS in water-methanol.
4

S.J. Sundaram, A.A. Raj, R.J. Vijay et al.
Journal of Molecular Structure 1241 (2021) 130669
Fig. 5. Growth mechanism of DAAS spherulites in water-methanol.
grow into a spherulite is an indication of the tendency to grow
bulk single crystals at optimized conditions [17]. The evolution of
morphology of DAAS spherulite in water-methanol begins with a
point nucleus, later growing of needles from this point. These nee-
dles grow in two opposite directions first and then other needles
fill the space thus leading to a flat sphere. Fig. 5 is a rough in-
dication of how a single needle branches and then the separate
branches grow in 3 dimensions to form a spherulite. But for most
purposes, we can treat a spherulite as composed of needles radiat-
ing from the centre. The slow growth axes all club into their neigh-
bors, while the fast axis becomes the radial growth direction. Due
to poor solubility of DAAS in water, its nucleation study was not
performed.
2.4. Crystal growth
Crystal growth of DAAS was achieved by adopting slow evap-
oration along with Oswald ripening technique. For crystal growth
experiment, equal amount of water: methanol (1:1) was taken, in
a 100 ml of mixed solvent; 0.4 g of DAAS was dissolved by stir-
ring vigorously. The content was heated up to 50 ◦C and stirred
continuously until the material is completely dissolved in the sol-
Fig. 6. a) Photograph of DAAS crystals grown mixed solvent of
Water-Methanol and b) its microscopic view.
5

S.J. Sundaram, A.A. Raj, R.J. Vijay et al.
Journal of Molecular Structure 1241 (2021) 130669
vent. The filtered saturated solution was shifted to a Teflon beaker.
The beaker was closed with a suitable cap and maintained at con-
stant temperature without any disturbance. The growth solution
was prepared at 50°C and the temperature was reduced 1 ◦C per
day. When the temperature of the solution reaches 35 ◦C, the so-
lution was allowed to Oswald ripening process and the concentra-
tion would be sufficient to drive the nucleation and growth. Dur-
ing this isothermal period, only those crystals that are larger in
size will grow at the expense of smaller ones and the number of
crystals available to grow further therefore reduces with the so-
lute remaining in the solution. Within a few days, the growth of
tiny needle like crystals could be observed at different parts of the
beaker. The solution was then slowly allowed to evaporate, after
25-35 days, crystals with size up to 3-5 × 1-2 × 1-3 mm3 were
harvested. The size of the crystal is found to be 6 times larger than
the previously reported size [13]. The crystals appeared reddish in
colour (Fig. 6a) with moderately smooth surface. The quality of the
surface of the crystals can be further improved by adopting slow
cooing and slope nucleation techniques.
molecular structure of DAAS was subjected to ground state opti-
mization without any constraints on the molecular system using
Density Functional Theory (DFT) at B3LYP (Becke three parameter
Lee Yang Parr) /6-311+G(d,p) level [18,19]. Subsequently, frequency
calculations were also performed to confirm that each stationary
point was at energy minima. From the ground state optimized
geometries, Time Dependant-Density Functional Theory (TD-DFT)
was employed to compute the absorption properties for the 20
lowest singlet vertical excitations. In order to examine the solvent
effect, self-consistent reaction field-polarisable continuum model
(SCRF-PCM) was utilized in methanol medium. NLO properties and
the entire computational calculations were performed using Gaus-
sian 09 suite of program [20].
4. Results and Discussion
4.1. Structural analysis
XRD data of DAAS crystal were collected and presented in Ta-
ble (T1-supplementary material). It confirms the triclinic structure
of DAAS crystal with acentric space group of P1. The single crys-
tal XRD data for DAAS crystal collected in this work is in close
3. Characterization
The grown single crystal of DAAS was subjected to the fol-
lowing studies. The cell parameters and the crystal structure
were confirmed by single crystal XRD data which was col-
agreement with the earlier work [13]. It is found that the volume
3
˚
of crystal structure in the present study is smaller (534.22(3) A )
3
˚
when compared to the crystals of Sun et. Al., (543.92(54) A ) hence
lected from
a
Bruker X8 KAPPA APEX II X-ray diffractometer
˚
the density has increased slightly to 1.323 g/cm3 when compared
3
˚
with MoK (λ = 0.71073 A) radiation. Powder XRD was per-
to 1.299 g/cm . This has resulted in a stronger 1.928 A, N−H•••
O
α
˚
formed with BRUKER X-Ray diffractometer with the CuK radiation
type hydrogen bonds between anions and 2.433 A, C−H•••O type
bonds between cations (Fig. 7). Hydrogen bonds play a major role
in stabilizing the structure and Hirshfeld surface analysis reveals
that nearly 22 % of the interactions in the crystal are hydrogen
bonds (Fig. S1 and S2-supplementary material). Fig. 8 shows the
Hirshfeld surfaces of anion and cation molecules of DAAS. The opti-
mized crystal structure of DAAS was computed at B3LYP / 6-311+G
(d, p) level of quantum theory and the optimized structure with
its Lateral view are shown in Fig. 9. The bond length between the
molecules are measured and the bond length between the N1-C20
α
˚
(λ=1.540598 A). The vibrational assignments were determined by
the FT-IR spectrum and it was recorded in the region 400-4000
cm−1 with a Perkin-Elmer FT-IR spectrometer, employing the KBr
pellet technique. FT Raman spectrum was recorded with a Bruker
RFS 27: standalone FT-Raman spectrometer using the Nd: YAG laser
at 1064 nm with 470 mW output as the excitation source. The
proton NMR spectrum was recorded using a Bruker ADVANCE III
500 MHz FT-NMR spectrometer by dissolving the sample in deuter-
ated methanol. The optical absorption spectrum of the sample was
recorded using the Cary 5000UV-Vis Spectrometer. The thermal
behavior was investigated by thermogravimetric (TG-DTA) tech-
nique using the Perkin-Elmer STA 6000. The dielectric constant
and dielectric loss were measured at different temperatures. The
photo conducting property of the crystal was investigated with a
Keithley 485 picoammeter at room temperature. The second har-
monic generation test was conducted by Kurtz and Perry tech-
nique using a Q-switched mode locked Nd: YAG laser (1064 nm)
with a beam of energy 10 ns pulse width and 0.68 J power. The
˚
in pyridinium ring is found to be 1.457 A, which is slightly higher
than that dimethyl amino ring, where the distance between C11-
˚
N2 is determined as 1.362 A. Since, DAAS is a π-conjugated crys-
tal; the bond length of the π-bridge was also measured. The bond
˚
length of C7-C8 and C13-C15 are measured as 1.431 and 1.424 A,
respectively. The planarity of the crystal plays an important role
in microscopic and macroscopic NLO properties like hyperpolariz-
ability, mean polarizability and higher order NLO properties [21].
The bond angles of C22 – S1-O1, C22 – S1-O2 and C22 – S1-O3 are
Fig. 7. (a) ORTEP diagram of DAAS molecule and (b) Strong hydrogen bonds present in DAAS molecule.
6

S.J. Sundaram, A.A. Raj, R.J. Vijay et al.
Journal of Molecular Structure 1241 (2021) 130669
Fig. 8. Hirshfeld surface of anion and cation molecules of DAAS.
O
Fig. 10 b. It is clearly evident that (0 0 1) plane is the most
104.96, 105.74 and 106.44
respectively; the compression of the
˚
dominant face of the crystal. The strong 1.928 A hydrogen bond
angles is attributed to the Thorpe-Ingold effect [22].
between the anions along the a-axis causes the (001) face grow
much faster than the other faces. Also, along the b-axis, between
the chromophores, the acceptor of the molecule, (N1CH3) and the
donor of the adjacent molecule, N(CH3)2 are bound by Van der
4.2. Morphology analysis
˚
Waals forces with distances of 2.413, 2.576, and 2.795 A between
In order to find the morphology, crystalline nature and pu-
rity, crystalline powder XRD of DAAS was performed [23]. At room
temperature the diffracted pattern was recorded and plotted be-
tween10 to 45° range. Various planes at specific 2θ angles were
observed from the reflections of DAAS sample and the peaks were
indexed. The simulated pattern was produced from the single crys-
tal XRD data and compared with the experimentally observed
powder XRD pattern (Fig. 10 a). It is confirmed that both the exper-
imental and simulated XRD patterns match well for DAAS crystal.
The good crystalline nature of the DAAS crystal is confirmed with
the sharp well-defined Bragg’s peaks at the respective 2θ angles
[12]. The simulated crystal morphology predicted from powder (h
k l) data of DAAS is depicted in
the CH— and —HC atoms [Fig. S3-supplementary material] [24].
4.3. Molecular Vibrational Analysis
The vibrational spectral analysis (both FT-IR and FT-Raman) of
the sample was done based on the attributing vibrations of the
vinyl group, phenyl ring, methyl group, amino group, sulfonate and
skeletal modes. Fig. 11 and 12 represented the FT-IR and FT-Raman
spectra of the DAAS sample. The observed IR and Raman band po-
sitions with their corresponding assignments are presented in Ta-
ble T2 (see supplementary material).
Vinyl group vibrations
In the conjugated systems, the alkene bond stretching vibra-
tions without a center of symmetry is probably to have two C=C
stretching bands at around 1650 and 1600 cm−1 [25]. The bands
observed at 1644.68 cm−1 in the IR spectrum and another band
at 1617.68 cm−1 in Raman spectrum can be ascribed to the C=C
stretching vibrations. The aliphatic C–H stretching for stilbazolium
derivatives is expected in the region between 3050 and 3000 cm−1
[26]. The band positioned at 3039.68 cm−1 in the IR spectrum is
attributed to the aliphatic C–H stretching.
Phenyl and pyridinium ring vibrations
The stilbazolium cation is made up of p-disubstituted benzene
and pyridine ring, connected by ethylenic bridge; by these con-
stituents of stilbazolium cation the ring mode vibrations in DAAS
can be analyzed. C–H in-plane and out-of-plane bending vibrations
generally occur in the region 1300-1000 cm−1 and 1000-675 cm−1
[25]. The bands at 1208.38, 1179.95, 1160.16 and 1070.81 cm−1 in
IR and 1210.57, 1179.22 and 1159.05 cm−1 in Raman spectrum are
assigned to C–H in-plane bending vibrations. In the IR spectrum
the C–H out of plane bending vibrations is observed at 984.90 and
880.29 cm−1. The phenyl ring mode C–C–C deformation vibrations
are noticed at 699.90, 625.06 and 530.42 cm−1 in the IR spectra
[27].
Methyl group vibrations
Two kinds of methyl groups are present in the DAAS molecule,
one type
(C–CH3) is directly connected with the benzene ring, while the
other type (N–CH3) is attached with pyridine ring [26]. The asym-
metric and symmetric bending vibrations for methyl groups are
seen near 1450 and 1375 cm−1 respectively. The medium band ob-
served at 1500.55 cm−1 (IR) is assigned to the asymmetrical bend-
ing vibration. The bands positioned at 1339.69 and 1375.69 cm−1
(IR), and at 1377.47 and 1341.95 cm−1 (Raman) are attributed to
the symmetrical bending vibrations. For CH3 group, the rocking
mode usually appears in between 1070 and 1020 cm−1 [28]. This
mode appears at 1045.32 cm−1 in IR and at 1046.97 cm−1 in the
Fig. 9. (a) Optimized structure of DAAS at B3LYP / 6-311+G(d,p) level of quantum
theory, (b) Optimized structure showing significant dihedral angles
(in ), (c) Lateral view of the optimized structure.
7

S.J. Sundaram, A.A. Raj, R.J. Vijay et al.
Journal of Molecular Structure 1241 (2021) 130669
Fig. 10. (a) and (b) Powder X-ray patterns of DAAS crystal and Simulated morphology for DAAS crystal.
Raman spectra respectively. The CH3 torsion vibrations are identi-
fied as very weak band at 76.91 cm−1 in the Raman spectrum [25].
Primary amine group vibrations
(Raman) correspond to the C–N stretching mode. The NH2 wag-
ging mode for aromatic primary amine is observed at 777.06 and
723.19cm−1 in the Raman spectra.
For aromatic primary amines (RNH2), the asymmetrical N–H
stretch and the symmetrical N–H stretch will be observed in the
regions of 3380-3415 cm−1 and 3460-3510 cm−1, respectively [29].
For DAAS crystal, the N–H asymmetric and symmetric stretching
vibrations are found at 3334.43 and
Sulfonate and skeletal mode vibrations
The SO2 group gives rise to asymmetric and symmetric stretch-
ing vibrations in the infrared spectrum for the range 1420-1000
cm−1 with high intensity peak.
In DAAS crystal, the strong bands present at 1179.95 and
1160.16 cm−1 in IR, and at 1179.22 and 1159.05 cm−1 in Raman
spectra could be attributed to the
3452.55 cm−1 in the IR spectrum, respectively. A shoulder band
seen at
3223.39 cm−1 is due to the overtone of the N–H bending vibra-
tion for primary amine group. The NH2 bending mode for aromatic
primary amine is seen to overlap with the C=C stretching band at
1644.68 cm−1 in the IR and at 1617.68 cm−1 in the Raman spec-
tra, respectively. The C–N stretching mode for aromatic primary
amine is expected in the region between 1340 and 1250 cm−1 [29].
Thus, the bands observed at 1318.12 cm−1 (IR) and at 1318.29 cm−1
SO2 asymmetric stretching modes. The bands positioned at
1045.32 (IR) and
1046.97 cm−1 (Raman) correspond to the SO2 symmetric
stretching. The band at 500.18 cm−1 in the IR spectrum and the
one observed at 500.26 cm−1 in the Raman spectrum may be at-
tributed to the SO3 deformation. The observed result matches well
with an earlier report [30]. The skeletal mode vibrations arising
Fig. 11. FT-IR spectrum of DAAS.
8

S.J. Sundaram, A.A. Raj, R.J. Vijay et al.
Journal of Molecular Structure 1241 (2021) 130669
Fig. 12. FT-Raman spectrum of DAAS.
Table 1
from C–N and C–C are usually measured in the region 1150-850
Computationally calculated absorption maxima (λmax
) and their cor-
cm−1 The band observed at 880.29 cm−1 in the IR spectrum cor-
.
responding oscillator strengths (f0), major contribution, Light Harvest-
ing Efficiency (LHE), nature of transition and Transition assignment for
DAAS molecule.
responds to the C–N stretching.
4.4. Linear Optical Properties
Molecule
DAAS
Type of Transition
λmax (nm)
S0 S1
470.5
The optical transmission window and cut-off wavelength of the
crystal can be obtained by optical transmittance study. Also, it is
very useful to understand about its electronic transition states. The
optical absorption spectrum (Fig. 13) of DAAS crystal was carried
out in solid phase. The cut-off wavelength of the sample is found
to be 610 nm and below which the material shows complete ab-
sorption due to the bonding and anti-bonding transitions. The ma-
terial is transparent with very low absorption beyond 610 to 1700
nm. The absence of absorption in this region paves way for NLO
Oscillator Strength f
LHE
1.257
0.94
Major Contribution
Nature of Transition
Dipole Moment (Debye)
Energy gap (eV)
HOMO - LUMO (100%)
π - π∗
14.76
2.07
applications [31]. The energy band gap was calculated using the
relation:
hc
Eg =
λ
Where, h - Planck’s constant, c - Velocity of light and λ - cut off
wavelength. The optical energy band gap of DAAS crystal is found
to be 2.03 eV.
4.4.1. Frontier molecular orbital analysis
To evaluate the molecular level energetic behaviour of the π-
conjugated DAAS crystal, the HOMO-LUMO analysis was carried
out in a methanol medium and calculations were done using DFT
method at the B3LYP /6-311+G (d, p) basis set level using Gaus-
sian program (Table 1). This frontier molecular orbital gap is con-
nected with the parameters like high chemical reactivity, large po-
larizable, low kinetic stability, and soft nature of the molecule.
HOMO depicts the donor of electron and LUMO is the acceptor of
the electron. In the present case, dimethylamino moiety acts as an
electron donor and pyridinium moiety plays as an electron accep-
tor. The transmission of the electronic absorption by one electron
excitation from ground state (highest occupied molecular orbital
(HOMO)) to the first excited state (lowest unoccupied molecular
Fig. 13. UV-Vis absorption spectrum of DAAS crystal.
9

S.J. Sundaram, A.A. Raj, R.J. Vijay et al.
Journal of Molecular Structure 1241 (2021) 130669
Table 2
First order hyperpolarizability (β) and mean polarizability (α) of DAAS com-
puted at B3LYP /6-311+G(d,p) level of theory.
First Order Hyperpolarizability (esu)
Mean Polarizability (esu)
Tensors
Value
Tensors
Value
βxxx
βxxy
βxyy
βyyy
βxxz
βxyz
βyyz
βxzz
βyzz
βzzz
βTotal
-6086.49186
7377.32022
αxx
αxy
αyy
αxz
αyz
αzz
-
551.210787
-158.213120
-7205.17206
6811.48768
517.608083
6.80565563
13.0421883
13.0177819
70.8535186
232.199970
-169.065192
-39.7391084
-37.6497728
129.263797
167.963×10−30 e.s.u.
-
-
-
-
-
-
-
αTotal
6.42703×10−23e.s.u.
ꢂ
ꢃ
1/2
2
)
2
2
)
ꢀα = 2−1/2
α
xx − −αyy + αyy − −αzz + αzz − −αxx
(
(
)
(
The first order hyperpolarizability (β) and mean polarizabil-
ity (αtotal) values for DAAS are listed in Table 2. DAAS crystal is
found to have the first order hyperpolarizability and mean po-
larizability values of 167.96×10−30 esu and 6.42703×10−23 esu
respectively. The calculated β of DAAS is comparable with the
other well-known organic stilbazolium family crystals like DAST
(159.05×10−30 esu) and DASC (165.05×10−30 esu).
4.5.2. SHG efficiency test
The second harmonic generation test was done on the DAAS
sample by Kurtz and Perry technique [34] using a Q-switched
mode locked Nd: YAG laser (1064 nm, Quanta ray series, Spec-
tra Physics) with a beam of 10 ns pulse width and 0.68 J power.
The sample was made into a very fine powder (particle of size in
the range 100-120 μm) and then closely packed in a microcapil-
lary tube. The powder sample was then mounted in the path of
a Q-switched Nd:YAG laser. The generated SHG signal at 532 nm
is split from the fundamental frequency using an IR separator. A
detector connected to power meter is used to detect second har-
monic intensity and also to read the input and output. The SHG ef-
ficiency of the material was compared with microcrystalline pow-
der sample of urea. The SHG signal recorded for the DAAS sample
and it was found to be 145 mV. It is possible to achieve much
higher SHG efficiency for DAAS with suitable laser arrangement
since it is reported to have SHG efficiency of 0.83 times that of
DAST crystal [13].
Fig. 14. Frontier Molecular Orbitals (HOMO and LUMO) of DAAS from ground state
optimized geometries.
orbital (LUMO)) was calculated. DAAS molecule is found to have
a HOMO value of -4.905 eV, while the LUMO value is -2.826 eV
(Fig. 14) and the energy separation gap between the HOMO and
LUMO is found to be 2.07 eV. The value of dipole moment is 14.76
Debye. This value is much better than that of well-known refer-
ence compound urea (1.3732 D). There is no complete transmission
of electronic absorption from HOMO to LUMO but intramolecular
charge transfer has occurred in the molecule, making it responsible
for the large NLO property [32].
4.5. Nonlinear Optical Properties
4.6. Thermal analysis
4.5.1. Molecular Hyperpolarizability
At molecular level, the first order hyperpolarizability tensors
of the DAAS was computed and calculated, by density functional
theory (DFT) using the hybrid functional B3LYP /6-311+G (d, p)
[33-35]. The donor–acceptor system, nature of substituents, π-
conjugated system and planarity are the parameters that can make
an impact on hyperpolarizability. NLO properties such as first order
hyperpolarizability (β), mean polarizability (α) and static dipole
moment have been calculated computationally using the formula:
The thermal behavior of DAAS was investigated by thermogravi-
metric
(TG-DTA) and differential scanning calorimetric (DSC) tech-
niques using the Perkin-Elmer DSC-7 and TGA-7, respectively. From
the TG curve (Fig. 16), it is inferred that DAAS starts decomposing
at 274 ◦C, after which it undergoes continuous weight loss up to
800 ◦C with three stages of decomposition. In the first stage, a ma-
jor weight loss of 54.84% takes place between 274 and 370 ◦C and
this may be due to the complete decomposition of stilbazolium
cation. In the second stage, a loss of 13.28% is observed due to
the removal of SO2 molecule and in the third stage around 28.08%
of the material decomposes, which may be due to the removal of
aminotoluene compound. The DTA traces show a sharp endother-
mic peak at 287 ◦C, the sharpness in the peak confirms the good
degree of crystallinity and purity of the sample. Interestingly, the
observed melting point of the sample is not only higher than DAST
crystal (256 ᵒC) [14] but also better than a few other highly NLO
active stilbazolium derivatives such as DSSS (277 ᵒC) [3], DSTMS
ꢀ
ꢁ
1/2
2
2
2
βtot = βx + βy + βz
Where
2
2
βx = βxxx + βxyy + βxzz
,
(
)
2
2
)
βy = βyyy + βyzz + βyxx
(
and
2
2
)
βz = βzzz + βzxx + βzyy
(
10

S.J. Sundaram, A.A. Raj, R.J. Vijay et al.
Journal of Molecular Structure 1241 (2021) 130669
Fig. 16. TG-DTA traces of DAAS crystal.
(258 ᵒC) [10], DASC (282 ᵒC) [15], DSMOS (264 ᵒC) [11] and also
some other extended molecules like DACSC [271 ᵒC] [33], DCSSS
[237.8 ᵒC] [35] and DACSI (262.81 ᵒC) [36].
CRediT authorship contribution statement
S. John Sundaram: Conceptualization, Writing - original draft,
Investigation. A. Antony Raj: Writing - original draft, Methodology.
R. Jerald Vijay: Validation, Writing - review & editing. M. Jaccob:
Software, Validation. P. Sagayaraj: Supervision, Funding acquisition.
5. Conclusion
The single crystal of DAAS was grown by slow evaporation tech-
nique coupled with Oswald Ripening process. This process reduced
the forming of several number of tiny crystals and yielded single
crystals with improved size. By adopting this method, the crys-
tals with low solubility can be grown into larger size. The X-ray
diffraction study ascertained the triclinic structure of DAAS with
space group P1. The characteristic vibrations of the vinyl group,
phenyl ring, methyl and amino groups and the sulfonate and skele-
tal modes were confirmed by FT-IR and FT-Raman spectral studies.
DAAS molecule is found to have a HOMO value of -4.905 eV, while
the LUMO value is -2.826 eV and the energy gap 2.07 eV is calcu-
lated, experimentally the optical energy band gap of DAAS crystal
is found to be 2.03 eV in solid phase and the low absorption in
the region 1000 to 1600 nm suggests the suitability of the material
for photonics and electro-optic applications in the infrared region.
DAAS is found to have the first order hyperpolarizability and mean
polarizability values of 167.96×10−30 esu and 6.42703×10−23 esu,
respectively which is comparable with DAST and its derivatives.
DAAS showed increased thermal stability when compared to DAST
as well as a few other highly nonlinear optical stilbazolium deriva-
tives. This projects the crystal to be used as a promising THz gener-
ation material at high temperatures. Hence the compound may be
considered as potential material for photonic device fabrications.
Acknowledgement
The authors acknowledge the DST-SERB, India for funding this
research work (SR/S2/LOP-29/2013). The authors acknowledge SAIF,
IIT Madras for providing Single crystal XRD analysis.
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