Spectroscopic investigation on the efficient organic nonlinear crystals of pure and diethanolamine added DAST

Article abstract of DOI:10.1016/j.saa.2013.06.060

4-N,N′-dimethylamino-N-methyl-4-stilbazolium toyslate (DAST) and diethanolamine (DEA) added DAST crystals are grown by slow cooling method. The corresponding powder samples are examined by characterization studies such as XRD, FT-IR, FT-Raman, UV-Vis-NIR

Full text of DOI:10.1016/j.saa.2013.06.060

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 115 (2013) 667–674
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Spectroscopic investigation on the efficient organic nonlinear crystals
of pure and diethanolamine added DAST
C. Karthikeyan a, A.S. Haja Hameed a, , J. Sagaya Agnes Nisha a,b, G. Ravi c
⇑
a PG and Research Department of Physics, Jamal Mohamed College (Autonomous), Tiruchirappalli 620 020, Tamil Nadu, India
b Department of Physics, Bishop Heber College (Autonomous), Tiruchirappalli 620 017, Tamil Nadu, India
c School of Physics, Alagappa University, Karaikudi 630 003, Tamil Nadu, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
The strongest vibrational modes
contributing to the electro optic effect
were identified.
The energy of HOMO and LUMO
explained the eventual charge
transfer interaction within the
molecule.
The density of states of the system
showed the energy levels available to
be occupied by electrons.
The strong peak at 609 nm indicated
the yellow–orange emission.
500
DAST
B1
B2
B3
B4
B5
B6
peak sum
400
B3
ꢀ
300
200
100
0
B6
ꢀ
ꢀ
B4
B1
B2
B5
550
600
650
700
750
800
Wavelength (nm)
a r t i c l e i n f o
a b s t r a c t
4-N,N0-dimethylamino-N-methyl-4-stilbazolium toyslate (DAST) and diethanolamine (DEA) added DAST
crystals are grown by slow cooling method. The corresponding powder samples are examined by charac-
terization studies such as XRD, FT-IR, FT-Raman, UV–Vis–NIR and photoluminescence studies. From the
powder X-ray diffraction, their lattice parameter values are found out. Since the vibrational spectra of the
molecules are considerably contributed to their linear and nonlinear optical effects, Infrared and Raman
spectroscopic studies are carried out for the samples. The UV–Vis–NIR absorption spectra of the samples
are used to find the nature of transitions occurred in the samples. Using the density functional theory,
highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) analyses
are done in order to explain the transition and density of states (DOS). The first order hyperpolarizability
is calculated by HF and B3LYP/6-311 G(d,p) basis sets for the DAST molecule. From the photolumines-
cence (PL) spectral studies, the strong excitation emissions are observed.
Article history:
Received 5 March 2013
Received in revised form 17 May 2013
Accepted 13 June 2013
Available online 27 June 2013
Keywords:
DAST
DFT
FT-IR
FT-Raman
UV–Vis–NIR Spectra
HOMO–LUMO
Ó 2013 Elsevier B.V. All rights reserved.
Introduction
effects. Good quality NLO crystals are required for their applica-
tions in fabricating second harmonic generating (SHG) elements
Organic materials have large second-order nonlinear optical
(NLO) susceptibilities with very good frequency conversion [1,2].
The optical molecular crystalline materials are composed of stable
chromophoric molecules with large molecular hyperpolarizabili-
ties with an optimized orientation for large macroscopic NLO
and electro-optical (EO) devices [3]. Among the promising organic
NLO materials, 4-N,N0-dimethylamino-N-methyl-4-stilbazolium
toyslate (DAST) is one of the best materials for very pronounced
bulk quadratic NLO activity and the powder second harmonic gen-
eration (SHG) efficiency of DAST is 1000 times that of urea as a ref-
erence at 1907 nm laser emission. The largest nonlinear
coefficients are obtained when all the molecules are aligned in
the same direction [4]. The static first hyperpolarizability, bo is
⇑
Corresponding author. Tel.: +91 431 2331135/2332235; fax: +91 431 2331435.
E-mail address: hajahameed2001@gmail.com (A.S. Haja Hameed).
1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2013.06.060

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C. Karthikeyan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 115 (2013) 667–674
Fig. 1. Synthesizing scheme of DAST material.
evaluated to be 364 ꢁ 10ꢂ30 e.s.u. from the hyper-Rayleigh scatter-
ing investigation and the electro-optic coefficient r11 is measured
to be 160 50 pm/V. The largest second-order NLO coefficient,
d11 = 1010 110 pm/V at 1318 nm, and the high band width of
140 GHz have rendered DAST crystal to be an efficient candidate
for the ultra-fast electro optical modulation [5,6].
In the present study, diethanolamine (DEA: [CH2 (OH) CH2]2
NH) is selected for adding with DAST. The reason is that the amino
group of DEA is having more energy to give the charge transfer (CT)
for electronic absorption corresponding to the transition from the
ground to the first excited state. Pure and DEA added DAST crystals
are grown by slow cooling method. The powder samples are char-
acterized by XRD, FT-IR, FT-Raman, UV–Vis–NIR and photolumi-
nescence studies. Using density functional theory, HOMO–LUMO
analysis is carried out. The first order hyperpolarizability is also
calculated by HF and B3LYP/6-311 G(d,p) basis sets for the DAST
molecule.
Experimental procedure
DAST was synthesized by the condensation of 4-methyl-N-
methyl pyridinium toyslate and 4-N-N-dimethylamino benzalde-
hyde in the presence of piperdine [7–9]. The 4-methyl-N-methyl
pyridinium tosylate used in the condensation reaction was pre-
pared from equimolar quantities of 4-picoline and methyl-para-
toluene sulfonate. The entire synthesis process was conducted in
dry nitrogen atmosphere to avoid the formation of the orange hy-
drated form of DAST. The scheme route to synthesis DAST material
is shown in Fig. 1. The resulting DAST material was further purified
by recrystallization from methanol. The recrystallized DAST crys-
tals were used for the preparation of two equal amounts of satu-
rated DAST solutions. 0.1 mol of DEA was added into one of the
DAST solutions. DEA added DAST solution was stirred till DEA
was dissolved completely in it. Then, both solutions were kept at
the saturated temperature for 6 h to attain homogenization. There-
after, the solution temperature was reduced at a cooling rate of
Fig. 2. Grown crystals of (a) pure DAST and (b) DEA added DAST.
0.5 °C/day. After a week, spontaneously grown crystals were ob-
tained. The grown crystals of pure and DEA added DAST are shown
in Fig. 2. The crushed powder of the grown pure and DEA added
DAST crystals were used for the present studies.

C. Karthikeyan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 115 (2013) 667–674
669
Stilbazolium
N+
CH3
CH3
H3C
N
Toyslate
O
S
O-
CH3
O
(a)
(b)
(c)
(d)
(e)
Fig. 3. The chemical and molecular structure of DAST (a and b). The projections of two unit cells of the DAST crystal along principal crystallographic axes (c) a-axis, (d) b-axis
and (e) c-axis.
The powder X-ray diffraction data were collected for finely
crushed powder of both pure and DEA added DAST crystals sub-
jected to intense X-rays of Cu Ka radiation using X-pert PRO PAN-
alytical powder diffractometer. The scan angle was varied from 10°
to 80°. The Infra-Red spectra of pure and DEA added DAST samples
were recorded in the range 400–4000 cmꢂ1 using Perkin–Elmer
spectrometer using KBr pellet technique. FT-Raman spectra were
recorded using Burker RFS 25 spectrometer. The absorption spectra
of pure and DEA added DAST samples were studied in the range
200–1100 nm by Lamda 35 spectrometer. The photoluminescence
spectra were recorded in the wavelength from 540 to 800 nm using
JASCO luminescence spectrometer. The density functional theory
(DFT) was used for HOMO–LUMO analysis, by means of the hybrid
functional DFT/B3LYP with the 6-311 G(d,p) basis set available in
(b)
(a)
10
15
20
25
30
35
2θ (deg)
Fig. 4. X-ray diffraction pattern of (a) DAST and (b) DEA added DAST crystals.

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C. Karthikeyan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 115 (2013) 667–674
Gaussian 09 Program [10–12]. For the calculation of first order
hyperpolarizability of DAST molecule, HF and B3LYP/6-311 G(d,p)
basis sets are used.
respectively. The largest contribution of electro-optic effect is
caused from the aromatic ring deformations and CH3. The aromatic
ring deformations at A and B modes are active in both IR and Ra-
man spectra because of the symmetry lowering of the molecule.
Such deformations change the bond length between the carbon
atoms, which build the charge transfer axis. The vibration mode
C is due to CH3 umbrella vibration. This symmetric bending modes
yield a large positive charge localized on the hydrogen, which fur-
ther supports the presence of hyperconjugation. Furthermore, the
symmetrical umbrella mode of the methyl groups can contribute
in the electro-optic effect of DAST. The CH3 umberlla vibration
can be understood since the in-phase displacement of the three
H atoms changes the electronic environment for the donor and
acceptor of the stilbazolium chromophore and therefore the polar-
izability is modified [15]. The strongest vibration mode D takes
part in collective stretching and shrinking of the skeletal C@C
and CAC intra-ring and inter-ring bonds. This is mainly involved
Results and discussion
X-ray diffraction analysis
DAST crystal belongs to the non-centrosymmetric monoclinic
space group Cc, with Laue symmetry of 2/m. The cell dimensions
are a = 10.3650 Å, b = 11.3220 Å, c = 17.8930 Å;
a = c = 90° and
b = 92.24 with V = 2098.18 Å3 [4]. The molecular structure of the
cation in DAST consists of two aryl rings adopting the expected
trans arrangement about the ethylenic linkage. The chemical and
optimized molecular structures of DAST are shown in Fig. 3a and
b. The projections of two unit cells of the DAST crystal along prin-
cipal crystallographic axes are shown in Fig. 3c–e. The crystals
have highly oriented stilbazolium chromophores aligned at an an-
gle of less than 20° with the respect to polar axis a. These fully con-
jugated nonlinear active chromophores are forced to align parallel
to one another by tosylate counter ions, leading to extremely large
electro-optical anisotropic properties [13,14].
in the delocalization of the
p electrons. The strongest vibrational
modes at 1577 cmꢂ1 are resulted from in-phase symmetric
stretching vibrations of single CC bonds and shrinking of CC double
bonds,
t (C@C/CAC). This vibration spreads over the whole p-con-
jugated parts of the molecule. This vibration involves the intermo-
lecular charger transfer from the donor and acceptor and gives rise
to a large variation in the dipole moment [16]. This vibration repro-
duces the evolution from an aromatic to a quinonoid structure in
the ethylenic bridge, which carries out the phenomenon of the
From the X-ray powder diffraction pattern of pure and DEA
added DAST crystals shown in Fig. 4a and b, the lattice parameter
values are calculated using 2h values of peaks corresponding to the
(hkl) planes using the monoclinic crystallographic equation given
below.
electron/phonon coupling in the conjugated material [17]. The
p-
electron movement from donor to acceptor can make the molecule
highly polarized through the single-double path when it changes
from the benzenoid form (ground state) into the quinonoid form
(first excited state). These vibrational modes are contributing to
electro-optic effect of DAST crystals. There is small shifts in the po-
sition of spectrum obtained for the DEA added DAST as compared
to that of pure DAST.
!
1
1
h2 k2 sin2 b l2 2hl cos b
¼
þ
þ
ꢂ
ð1Þ
d2 sin2 b
b2
a2
c2
ac
V ¼ abc sin b
ð2Þ
The calculated lattice parameter values of pure and DEA added
DAST crystals are shown in Table 1. The lattice parameter values
of DAST crystals are found to be slightly varied in DEA added DAST
crystals due to the influence of DEA molecules in the DAST crystal
lattice.
Fig. 5c–f shows all the vibration modes, the higher number re-
gion spectra due to symmetric and asymmetric stretching modes
of NH for pure and DEA added DAST samples. The NH stretching
is observed at 3201 cmꢂ1 from the IR spectra of DAST sample.
The ring CAH stretching vibrations appear to be very weak, which
is due to the steric interaction that induces effective conjugation
and charge carrier localization resulting in phenyl ring twisting
[18]. This vibration is observed at 3168 cmꢂ1 from the IR spectra
of DAST sample. The aromatic CAH stretching vibrations are found
at 3016 and 3070 cmꢂ1 from the IR spectra of pure and DEA added
DAST samples. The symmetric NACH3 stretching appears at
2887 cmꢂ1 in the IR spectra of DAST. The combined overtones
are observed at 2792 and 2475 cmꢂ1 from the spectra of DAST
and DEA added DAST samples. The CAN stretching and C@C
stretching are found at 2358 cmꢂ1 and 1619 cmꢂ1 respectively in
the case of DAST sample. The trans stilbene appears at
1580 cmꢂ1 from the IR spectra of DAST sample. From the Raman
spectra of DAST and DEA added DAST samples, the trans stilbene
C@C stretching is observed at 1577 and 1586 cmꢂ1. The CAH in
bending vibrations is observed at 1215 and 1212 cmꢂ1 respectively
in DAST and DEA added DAST samples.
Infrared and Raman spectroscopic studies
The direction of the charge-transfer axis of the stilbazolium is
defined by the two nitrogen atoms. The two molecules are dis-
played as they are packed in the crystal structure as shown in
Fig. 3c–e where the projections of two unit cells of the DAST crystal
are along the principal crystallographic axes. It is quite necessary
to interpret the Infrared and Raman spectra of DAST samples while
considering the optical effects in the crystals. In particular, for con-
tribution to the linear electro-optic effect, a certain vibrational
mode has to be Raman and Infrared active. It is found that the
Infrared and Raman spectra show the strongest modes in DAST
and DEA added DAST samples as shown in Fig. 5a and b. The
description of the modes is given in Table 2.
There are four strongest modes which are infrared as well as
Raman active. The four strong vibrational modes A, B, C and D
are contributing to linear electro-optic effect of DAST crystals
and from the vibrational spectra, the values corresponding to the
modes are at 1161 cmꢂ1, 1181 cmꢂ1, 1346 cmꢂ1 and 1577 cmꢂ1
The sulfonate functional group is characterized by IR absorp-
tions at 1161 and 1172 cmꢂ1. The corresponding wave numbers
are found at 1175–1181 cmꢂ1 and 1161–1167 cmꢂ1 from the
Raman spectra of pure and DEA added DAST samples. From the
IR and Raman spectra, SO3 symmetric stretching vibration appears
at 1000–1027 cmꢂ1 and 1040–1070 cmꢂ1 respectively in the case
of pure and DEA added samples. The 1,4-distribution aromatic ring
vibration appears at 820 and 848 cmꢂ1 from the IR spectra of both
samples. The Cis orientation of the substitute at the olefinic double
bond is found at 677 and 714 from the respective IR spectra of the
Table 1
Lattice parameter values of pure and DEA added DAST crystals.
Sample
a (Å)
b (Å)
c (Å)
V (Å)3
DAST [4]
10.3660
10.4023
11.3223
11.3242
17.8910
17.8971
2085.81
2088.02
DAST + DEA
samples. SOꢂ3 deformation vibration mode is observed at 501 cmꢂ1
.

C. Karthikeyan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 115 (2013) 667–674
671
(a)
(b)
1000
1200
1400
1600
1800
1000
1200
1400
1600
1800
Wavenumber (cm-1)
Wavenumber (cm-1)
(c)
(d)
(e)
(f)
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber (cm-1)
500
1000
1500
2000
Wavenumber (cm-1)
Fig. 5. Expanded optically active region of interest in Infrared and Raman vibrational spectra for (a) DAST and (b) DEA added DAST. Full IR spectra of (c) DAST and DEA added
DAST and Full Raman spectra of (e) DAST and (f) DEA added DAST.
imum absorption at 478 nm and 477 nm corresponding to the en-
ergy difference between the ground state and the first excited state
of the chromophores. The absorption wavelength is observed in the
visible region. The optical band-gap is estimated as 2.32 eV for the
Table 2
The four strongest vibrational modes of pure and DEA added DAST samples.
DAST
DAST + DEA
Mode of
vibrations
Assignments
DAST sample and the respective transition is p–
pꢃ. The absorption
1577
1586
D
Typical CAC@CAC in plane
stretching frequency (carbon
backbone between the aromatic
rings of the stilbazolium
chromophore)
CH3 umbrella deformation
In plane aromatic ring
deformations, as typical for para
substituted benzenes
peak value (474 nm) is found to be a good agreement with the
DAST sample reported by Marder et al. [19]. But there is a small
shift in the absorption peak of DEA added DAST sample with an in-
crease in the optical band gap, which is found as 2.33 eV due to the
effect of DEA. The increase in the band gap energy is due to the fact
that NH group of DEA is giving more energy for the charge transfer
(CT) of electronic transition from the ground to the first excited
state.
1346
1181 and
1161
1345
1181 and
1167
C
B and A
The electronic absorption corresponding to the transition from
the ground state to the first excited state is mainly described by
one electron excitation from the highest occupied molecular orbi-
tal (HOMO) to the lowest unoccupied molecular orbital (LUMO)
[20,21]. HOMO represents the ability to donate an electron
UV–Vis–NIR spectroscopic studies
The absorption spectra (Fig. 6a and b) of pure and DEA added
DAST samples dissolved in methanol show broad peaks with max-

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C. Karthikeyan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 115 (2013) 667–674
2.5
DAST
DAST+DEA
(a)
(b)
E
g = 2.32 eV
E
g = 2.33 eV
2.0
1.5
1.0
0.5
0.0
2.20 2.25 2.30 2.35 2.40 2.45 2.50 2.55 2.60
2.25
2.30
2.35
2.40
2.45
2.50
2.55
Photon energy (h
υ)
Photon energy (eV)
400
600
800
1000
400
600
800
1000
Wavelength (nm)
Wavelength (nm)
Fig. 6. UV–Vis–NIR absorption spectra of (a) DAST and (b) DEA added DAST samples.
Fig. 7. The atomic orbital compositions of the frontier molecular orbital of DAST.
whereas LUMO represents the ability to obtain an electron. The
HOMO is delocalized over the whole C–C backbone of the -conj-
overlap of two orbitals in the middle region of the p-conjucated
systems, which is a prerequisite to allow an efficient CT transition
p
ucated phenyl ring, ethylenic bridge and over the dimethylamino
group (N (CH3)2). The LUMO is located on the pyridine ring and
the vinyl group. Consequently, the HOMO–LUMO transition im-
plies an electron density transfer from, the more aromatic part of
[22].
The frontier molecular orbitals of DAST are shown in Fig. 7. The
energy of HOMO, LUMO and HOMO–LUMO gap are ꢂ0.20137 a.u,
ꢂ0.09947 a.u and ꢂ0.30084 a.u., respectively.
the
p-conjucated system including the electron donor group to
The N(CH3)2 group closer to the donor group results from the
charge transfer interaction of the phenyl ring and the amino group
in the electron donor side of the NLO chromophores. The existence
its more quinonoid side and mainly to the electron withdrawing
end. Moreover, the HOMO and LUMO topologies show certain

C. Karthikeyan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 115 (2013) 667–674
673
Fig. 8. Density of states (DOS) of DAST molecules.
of two methyl groups provides the additional negative charges to
Photoluminescence studies
the amino nitrogen atom. It gives that HOMO–LUMO energy gap
of the charge transfer is negative. The calculated Self Consistent
Field (SCF) energy of DAST is ꢂ1625.77502121 a.u. at B3LYP/6-
311 G(d,p). The energy of HOMO and LUMO explains the fact that
eventual charge transfer interaction is taking place within the mol-
ecule. The HOMO–LUMO energy value of DAST is found to be con-
sistent with the value reported by Vijayakumar et al. [16] through
normal mode analysis performed using the Becke’s three-parame-
ter B3P86 exchange–correlation functional together with 6-31G (d)
basis set. The density of states (DOS) of DAST is shown in Fig. 8. The
DOS of the system describes the number of states per interval of
energy at each energy level that are available to be occupied by
electrons. The density distributions are not discrete like a spectral
density but they are continuous. A high DOS at a specific energy le-
vel indicates many states that are available for occupation.
Fig. 9 shows the photoluminescence spectra of DAST sample. A
good fit of six peaks B1, B2, B3, B4, B5 and B6 is made using Gauss-
ian function. The solid lines represent the linear combination of six
Gaussian peaks. B1 has the lowest wavelength and B6 has the high-
est wavelength. In the PL spectra of DAST, the six peaks are ob-
served in the visible region.
The peaks B1, B2, B3, B4, B5 and B6 are represented the green,
yellow, yellow–orange, orange, orange and red emissions at
548 nm (2.2625 eV), 575 nm (2.156 eV), 609 nm (2.0359 eV),
641 nm (1.9342 eV), 674 nm (1.8395 eV) and 705 nm (1.7586 eV),
respectively.
The B1 and B2 bands (548 and 575 nm) are expected to occur
from the dimethylamino group (N(CH3)2) present in DAST crystal.
The B3 band at 609 nm is supposed to appear from another chro-
mophore (sulfonate group) through molecule exhibiting twisted
intermolecular charge transfer (TICT) process. The TICT molecule
on electronic excitation forms a moderately nonpolar state, and
this nonpolar excited state undergoes an intermolecular transfer
of an electron from donor (dimethylamino group) to an acceptor
(sulfonate group) connected through flexible single bond about
which rotation is free. Here electron transfer occurs between two
molecules through ionic interaction between dimethylamino
(„N+) and sulfonate (ASOꢂ3 ) groups. The B4 and B5 bands at
641 nm and 675 nm indicate the olefinic double bond. The B6 band
appears at 705 nm for the benzene ring. A slight variation in the
peak positions and band gap energy values is observed in the case
of DEA added DAST samples. The NLO response of the push–pull
NLO chromophores consisting of the stilbazolium cations and the
toluene sulfonate (tosylate) anions involves in the attainment of
excellent optical properties of the samples.
500
DAST
B1
B2
B3
400
B3
B4
B5
B6
300
peak sum
200
B6
B4
B1
100
B2
B5
Determination of hyperpolarizability
0
The mean polarizability (a), anisotropy of polarizability (Da)
550
600
650
700
750
800
and average value of the first order hyperpolarizability (btot) can
be calculated using the equations given in the literature [23].
The first order hyperpolarizability are calculated by HF and
Wavelength (nm)
Fig. 9. Photoluminescence spectra of DAST sample.

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C. Karthikeyan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 115 (2013) 667–674
B3LYP/6-311 G(d,p) basis sets for the DAST molecule. The values of
first hyperpolarizability btot of the DAST molecule are estimated as
130.130296 ꢁ 10ꢂ31 e.s.u. and 155.658135 ꢁ 10ꢂ31 e.s.u. from HF
and B3LYP/6-311 G(d,p) calculations respectively. These values
are found to be consistent with the reported values [24].
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The synthesis of 4-N,N0-dimethylamino-N-methyl-4-stilbazoli-
um toyslate (DAST) was carried out at dry nitrogen atmosphere.
Pure and DEA added DAST crystals were grown by slow cooling
method. The powder samples from the grown crystals were used
for XRD and spectroscopic studies. From the X-ray powder diffrac-
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lattice parameter values of DEA added DAST crystals was observed
due to the effect of DEA molecules. The strongest vibrational
modes contributing to the electro optic effect were identified from
the Infrared and Raman spectroscopic analyses. The optical band
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One of the authors (ASH) is grateful to University Grants Com-
mission (UGC), New Delhi, India for sanctioning the financial assis-
tance (UGC MRP F. No. 38-97/2009(SR)) to execute the major
research project.