Growth and characterization of an organic single crystal: 2-[2-(4-Diethylamino-phenyl)-vinyl]-1-methyl-pyridinium iodide

Article abstract of DOI:10.1016/j.saa.2014.01.109

Optically transparent crystal of the organic salt DEASI (2-[2-(4-Diethylamino-phenyl)-vinyl]-1-methyl-pyridinium iodide) has been synthesized by using knoevenagel condensation reaction method. The synthesized material has been purified by successfully recrystallization process. Single crystals of DEASI have been grown by slow evaporation technique at room temperature. The solubility of the title material has been determined at different temperature in acetonitrile/methanol mixture. The cell parameters and crystallinity of the title crystal were determined by single crystal XRD. The powder diffraction was carried out to study the reflection plane of the grown crystal and diffraction peaks were indexed. The presence of different functional groups in the crystal was confirmed by Fourier transform infrared (FTIR) analysis. ¹H NMR spectrum was recorded to confirm the presence of hydrogen nuclei in the synthesized material. The optical property of the title crystal was studied by UV-Vis-NIR spectroscopic analysis. The melting point and thermal property of DEASI were studied using TGA/DSC technique. The Vicker's hardness (H_v) was carried out to know the category. The dielectric constant and dielectric loss of the compound decreases with an increase in frequencies. Chemical etching studies showed that the DEASI grows in the two dimensional growth mechanisms. The Kurtz-Perry powder second harmonic generation (SHG) test has done for title crystal.

Full text of DOI:10.1016/j.saa.2014.01.109

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 125 (2014) 335–344
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Growth and characterization of an organic single crystal:
2-[2-(4-Diethylamino-phenyl)-vinyl]-1-methyl-pyridinium iodide
⇑
K. Senthil, S. Kalainathan , A. Ruban Kumar
Centre for Crystal Growth, School of Advanced Sciences, VIT University, Vellore 632 014, 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
ꢀ
Highly good transparent single
crystals with size up to
10 ꢁ 7 ꢁ 3 mm3 have been harvested
by slow evaporation technique.
1H and 13C NMR confirms the
molecular structure of the DEASI
crystal.
ꢀ
ꢀ
ꢀ
Crystal has good transparency in
visible–NIR region.
From the TGA curve of this sample
shows that material is highly stable
up to 262 °C.
ꢀ
Refractive index, mechanical,
dielectric, etching studies were
carried out.
a r t i c l e i n f o
a b s t r a c t
Article history:
Optically transparent crystal of the organic salt DEASI (2-[2-(4-Diethylamino-phenyl)-vinyl]-1-methyl-
pyridinium iodide) has been synthesized by using knoevenagel condensation reaction method. The syn-
thesized material has been purified by successfully recrystallization process. Single crystals of DEASI have
been grown by slow evaporation technique at room temperature. The solubility of the title material has
been determined at different temperature in acetonitrile/methanol mixture. The cell parameters and
crystallinity of the title crystal were determined by single crystal XRD. The powder diffraction was carried
out to study the reflection plane of the grown crystal and diffraction peaks were indexed. The presence of
different functional groups in the crystal was confirmed by Fourier transform infrared (FTIR) analysis. 1H
NMR spectrum was recorded to confirm the presence of hydrogen nuclei in the synthesized material. The
optical property of the title crystal was studied by UV–Vis–NIR spectroscopic analysis. The melting point
and thermal property of DEASI were studied using TGA/DSC technique. The Vicker’s hardness (Hv) was
carried out to know the category. The dielectric constant and dielectric loss of the compound decreases
with an increase in frequencies. Chemical etching studies showed that the DEASI grows in the two dimen-
sional growth mechanisms. The Kurtz–Perry powder second harmonic generation (SHG) test has done for
title crystal.
Received 20 November 2013
Received in revised form 13 January 2014
Accepted 22 January 2014
Available online 5 February 2014
Keywords:
Chemical synthesis
Optical materials
FT-IR
Crystal growth
Mechanical studies
Ó 2014 Elsevier B.V. All rights reserved.
Introduction
The wide range of organic nonlinear optical crystals has been
investigated due to of their potential applications in signal process-
ing, optoelectronics, laser technology, bioimaging, optical data
⇑
Corresponding author. Tel.: +91 416 2202350; fax: +91 416 2243092.
E-mail address: kalainathan@yahoo.com (S. Kalainathan).
http://dx.doi.org/10.1016/j.saa.2014.01.109
1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

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K. Senthil et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 125 (2014) 335–344
storage for the development of technologies in telecommunication
and data processing [1–4]. Organic materials exhibit large nonlin-
ear response when compared to inorganic materials because of the
presence of g-conjugated system linked by appropriate electron
releasing groups (ERGs) and electron withdrawing groups (EWGs)
at their two ends, which can enhance the asymmetric electronic
charge distributions in the system. The asymmetric charge distri-
butions between such groups leading to capable of generating sec-
ond harmonic frequency, and which is playing the key step in the
field of photonics [5–7]. In recent years, the efforts have been
growth technique is commonly used for the growth of organic
and inorganic crystals.
In the present investigation, we report the synthesis, growth of a
stilbazolium derivative, 2-[2-(4-Diethylamino-phenyl)-vinyl]-1-
methyl-pyridinium iodide single crystals. These crystals were suc-
cessfully grown by the slow evaporation technique. The grown title
crystal has been subjected to different characterizations such as sin-
gle crystal X-ray diffraction studies, powder XRD, FTIR, 1H NMR, 13
C
NMR, UV–Vis NIR, refractive index, thermal, dielectric studies,
chemical etching and Vicker’s microhardness measurements.
investigated towards the synthesis of novel
p conjugated organic
molecules because of their second and third-order nonlinear opti-
cal activities, large optical damage threshold, low frequency dis-
persion and fast response time [8–10]. Organic stilbazolium
crystals have a relatively high probability because of their highly
aligned and stable orientation of NLO chromospheres in the crystal
system [11]. For the past few years, the ionic organic material is a
particular interest, because of their thermal stability and photo-
chemical stability, and its mechanical properties in the field of
photonics [12,13]. The development of organic highly polar ionic
stilbazolium crystals is based on the presence of strong coulomb
interactions [13]. Recently stilbazolium family crystals are of con-
siderable attention due to their attractive large nonlinear optical
properties, a large electrooptic coefficient and low dielectric con-
stant. One of the main advantages of organic materials is that the
structure can be altered by suitable substituted electron donor
Experimental procedure
Material synthesis
The title material was synthesized by Knoevenagel condensa-
tion of 1,2-Dimethylpyridinium iodide, which was prepared from
2-methylpyridine and methyl iodide and 4-(N, N-diethyl amino)-
benzaldehyde in the presence of piperidine. The overall synthesis
process and molecular structures are given in Scheme 1.
1,2-Dimethylpyridinium iodide (A)
The equimolar ratio of 2-methylpyridine (8 ml, 80 mmol) and
methyl iodide (5.2 ml, 80 mmol) was added in dried acetone
(50 ml). The whole mixture was taken in
a two-neck flask
(250 ml) fitted with thermometer and condenser. Then the mixture
was refluxed at 60 °C for 2 h until it pale white precipitate crystal-
lized. The resulting salts were collected by filtration, and the salt
was washed with diethyl ether to removing of unreacted starting
and electron acceptor groups in their
p-conjugated system com-
pared to inorganic materials [8,14]. There are different methods
for crystal growth. It is known that the slow evaporation solution
Scheme 1. Synthesis process of DEASI.

K. Senthil et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 125 (2014) 335–344
337
materials. The pale white salt dried at the oven, and it had a melt-
ing point at 155 °C.
2-[(E)-4-(Diethyl amino) styryl]-1-methyl-pyridinium iodide (B)
The title material (B) was prepared by previously reported
method [15]. The equimolar ratio of A (7.052 g, 30 mmol) and 4-
(N, N-diethyl amino)-benzaldehyde (5.317 g, 30 mmol) in a dried
methanol (50 ml) was placed in round bottom flask. After the addi-
tion of the above mixture, the piperidine was added as a catalyst
and the mixture solution refluxed for 4 h under nitrogen atmo-
sphere. The resulting red precipitate suspension was cooled and
then precipitate was filtered off and finally washed with diethyl
ether to remove the starting material. The final product was then
successively recrystallized using mixture solvent methanol–aceto-
nitrile (1:1).
Solubility
Thermodynamically, the chemical potential of the purified salt
is equal to the chemical potential of the same solute in the satu-
rated solution. The determination of solubility measurement is
one of the decisive factors for crystal growth technique since it de-
cides estimation of solubility amount of material and temperature.
The solubility of DEASI was investigated in different solvents such
as methanol, ethanol, acetone, methanol–acetonitrile, methanol–
acetone, and acetonitrile–ethanol mixed solvent. Interestingly the
solubility of DEASI is very high in methanol–acetonitrile mixed sol-
vent system, and hence that solvent system was used as a growth
process. The solubility of DEASI salt was determined for different
temperatures from 30 to 45 °C by dissolving the solute in 100 ml
of methanol–acetonitrile (1:1) solvent mixture in an air tight con-
tainer maintained at a constant temperature with continuous stir-
ring to get the saturated solution. A solution of 10 ml was taken in
a Petri dish and dried up till to get rid of solvents. By measuring the
dried salt in the dish, the solubility at a relevant temperature was
calculated by using the formula [16].
Fig. 1. Solubility curve of DEASI crystal in methanol–acetonitrile solvent system.
wtðsoluteÞ
Solubility ðWt%Þ ¼
ꢁ 100:
Wtðsolute þ solventÞ
Fig. 2. As grown crystal of DEASI.
The solubility of the title compound at different temperatures as ex-
posed in Fig. 1. From the graph, it is seen that the solubility linearly
increases with the increase of temperature.
Fig. 3. Ortep diagram of – [2-(4-Diethylamino-phenyl)-vinyl]-1-methyl-pyridinium iodide.

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K. Senthil et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 125 (2014) 335–344
Table 1
Single crystal XRD data of DEASI.
Empirical formula
C18 H23 I N2
Formula weight
394.28
Crystal system
Space group
Monoclinic
P2(1)/c
a (Å)
b (Å)
c (Å)
a
7.8282(2) Å
20.3830(4) Å
11.0259(2) Å
90°
b
c
92.4780(10)°
90°
1757.67(6)
Volume (Å3)
Molecules per unit cell (Z)
Calculated density (Mg/m3)
Absorption coefficient (mmꢂ1
4
1.490
1.818
)
Crystal growth
Crystal growth was performed by slow evaporation solution
growth technique. Based on the solubility data, a saturated solu-
tion of DEASI was dissolved in methanol–acetonitrile mixed sol-
vent system (3.08 g in 50 ml) at 35 °C. The solution was filtered
using fine pore Whatman filter paper. The filtered solution was
kept in a 100 ml beaker, and it was tightly covered with aluminum
foil with few holes for slow evaporation rate of the solvent, and it
was kept at 35 °C in a constant temperature water bath. After a
period of 10 days of solvent evaporation, good transparent single
crystals with dimension up to 10 ꢁ 7 ꢁ 3 mm3 have been har-
vested. The as grown single crystals are shown in Fig. 2.
Fig. 4. Morphology of DEASI.
Characterization studies
Single crystal X-ray diffraction study was examined to confirm
the crystal system and the lattice parameter values of the DEASI
single crystal using Bruker Kappa APEX II diffractometer with Mo
Ka radiation (k = 0.71073 Å). The powder XRD pattern was carried
out on a BRUKER X-ray diffractometer with the Cu Ka radiation
(k = 1.5406 Å) in the range of 10–80°, at the scanning rate of
0.02° Sꢂ1 (2h). The functional groups vibration of DEASI crystals
were recorded in the range of 4000–400 cmꢂ1 using SHIMADZU
IRAFFINITY spectrometer by the pressed KBr pellet technique.
The formation of molecular structure of DEASI crystal has been
confirmed by using BRUKER instrument operating with 400 MHz
model FT-NMR spectrometer, and DMSO-d6 was used as a solvent.
The optical transparency of DEASI single crystal was checked using
ELICO SL 218 double beam UV–Vis–NIR spectrometer in the range
of 190–1100 nm. The TGA/DSC study was carried out for the title
crystal using Netzsch Sta-449F3 analyzer, and in the temperature
range of 35 °C to 500 °C at a heating rate of 10 °C/min in a nitrogen
atmosphere. The mechanical strength of grown crystal was deter-
mined using MH-112 Viker’s microhardness diamond indenter at
ambient temperature. The dielectric response of the grown crystal
was analyzed using HIOKI 3532 50 LCR HTESTER instrument for
different temperatures at various frequencies. Etching studies were
analyzed on the (010) plane of the DEASI crystal using Carl Zeiss
optical microscope.
Fig. 5. Indexed powder XRD pattern of DEASI.
Table 2
Powder XRD data for DEASI.
0
Pos [2Th.]
Height [cts]
Area
4898.9
FWHM
hkl
d-Spacing d (ÅA)
15.385
20.631
21.789
26.076
27.15
5.7545
333.5
238.8
811.4
672.3
265.6
745.3
429.1
166.5
805.7
542.9
404.9
283.2
412.2
159.1
186.7
170.7
157.1
74.6
0.2953
0.3134
0.2557
0.2695
0.3585
0.3218
0.3856
0.3538
0.4186
0.331
0.4819
0.355
0.4333
0.4735
0.266
031
112
050
151
ꢂ113
ꢂ160
ꢂ232
ꢂ251
ꢂ322
182
4.30169
4.07566
3.41453
3.28175
3.08646
2.9006
2.72756
2.29254
2.18807
2.02206
1.94145
1.82449
1.76693
1.68886
1.55674
1.4494
3770.7
10138.1
8911.7
4739.4
11664.3
8339.9
3019.6
17000.6
8879.2
9716.3
5030.1
8960.1
3863.7
2496.8
3305.2
3745.9
2516.6
2402.4
28.905
30.801
32.809
39.267
41.225
44.785
46.752
49.947
51.692
54.272
59.315
64.209
69.055
75.376
ꢂ361
Result and discussions
290
ꢂ372
1111
0120
ꢂ434
143
Single crystal X-ray diffraction analysis
0.385
Single crystal X-ray diffraction analysis was carried out to
determine the crystal structure and to determine lattice parameter
values. The structure was solved and refined by full matrix least
square technique using the SHELX97 program. It is observed that
0.4771
0.6524
0.3682
1.35903
1.25997
244
302
134

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339
Powder X-ray diffraction
To confirm the crystalline nature of the grown crystals, powder
X-ray diffraction analysis of the sample were carried out. The ob-
served powder XRD pattern has been indexed by POWDER X soft-
ware program. The indexed powder X-ray diffraction pattern for
DEASI is given in Fig. 5. The observed sharp and well defined
Bragg’s peaks at specific 2h angle shows the good crystallinity of
DEASI crystal. The Table 2 shows the powder X-ray diffraction data
of title material.
FTIR spectral analysis
The recorded FTIR spectrum of the DEASI is shown in Fig. 6. The
peaks at 3001.24 and 3061.01 cmꢂ1 are attributed to the aromatic
CAH stretching vibrations. The obtained stretching band at
1629.85 cmꢂ1 due to the C@C stretch vibration in the DEASI com-
pound. The observed band between 1527.62 and 1591.27 cmꢂ1 is
attributed to the stretching vibrations in the aromatic rings. The
peak at 1456.26 cmꢂ1 is corresponds to the CH2 bending mode.
The band found at 1373.32, and 1334.74 cmꢂ1 are assigned to
the CH3 bending absorption and CAN stretching mode. The peak
at 956.69 cmꢂ1 is corresponds to the 1, 2 substituted pyridinium
ring. The observed peaks that are between 500–700 cmꢂ1 are ex-
pected to the out–plane of ring bending modes, and in–plane ring
deformation modes are observed between 1100 and 1200 cmꢂ1
[18,19]. Thus, the FTIR studies confirm the presence of the key
functional group of DEASI.
Fig. 6. FTIR spectrum of DEASI.
the compound crystallizes in the monoclinic crystal system with
space group P2(1)/c, and the obtained lattice parameters values,
a = 7.8282 (2) Å, b = 20.3830 (4) Å, c = 11.0259 (2) Å, V = 1757.67
(6) Å3 and Z = 4. The obtained single crystal XRD data of the present
work confirmed the structure of the grown DEASI crystal (Fig. 3-
Ortep diagram). Thus, the obtained results are in good agreement
with the literature [15]. The single crystal data of DEASI is pre-
sented in Table 1. Morphology of the grown crystal was indexed
using WinXMorph program [16,17]. The indexed morphology of
DEASI crystal is shown in Fig. 4.
Fig. 7. 1H NMR spectrum of DEASI.

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1H and 13C NMR spectral analysis
1H NMR and 13C NMR spectral studies have been carried out to
confirm the formation of the synthesized materials and their
molecular structure of the grown crystal. From the 1H NMR spec-
trum of DEASI, the triplet at 1.136 is due to the six N-(CH2CH3)2
hydrogens. The quartet peak at 3.436 is due to the four hydrogens
of the N-(CH2CH3)2. The singlet peak at 4.292 is ascribing to the
three hydrogens of N-CH3. The chemical shift (d) at 3.373 is due
to the water molecule. The quintet observed at 2.50 is due to the
solvent ((CD3)2S@O). The two doublet signals at 7.212 and 7.910
are due to the two olefinic hydrogens (HC@CH). The two doublet
peak lie at 6.746 and 7.691 are ascribed to the four hydrogens of
the N-(CH2CH3)2@C6H4 aromatic ring. The two doublet peaks of a
pyridinium ring produced at 8.447 and 8.745. The triplet observed
around 8.329 is due to the sixth position hydrogen of the pyridini-
um ion of DEASI. So, the formation the title compound was con-
firmed by proton NMR spectral analysis and it is shown in Fig. 7.
The recorded 13C NMR spectrum of DEASI is shown in Fig. 8. The
peak at 12.45 ppm and 43.85 ppm are attributed to the methyl
group, and 45.62 ppm is assigned to the methylene group in the ti-
tle material. The pyridinium ring carbon is observed from the
peaks at 131.12 ppm, 142.90 ppm and 149.70 ppm. There are
peaks nine found at, 109.86, 112.22, 121.56, 122.56, 123.41,
142.90, 144.18, 145.16, 153.26 ppm, which are corresponds to
the formation of vinyl carbon and aromatic ring.
Fig. 9. UV–Vis NIR spectrum of DEASI.
in methanol solution, and the obtained spectrum is as shown in
Fig. 9. The absorption spectrum of the DEASI crystal and its solu-
tion phase are shown in the same figure for the comparison. The
absorption spectrum of DEASI crystal in methanol shows two dif-
ferent absorption peaks and the appearance of this kind of peaks
were reported by Kumar et al. and Jagannathan and Kalainathan
[20,21]. The minor absorption peak at 268 nm corresponds to the
UV–Vis NIR optical absorption spectral analysis
The optical absorption spectrum for the grown DEASI crystals
was recorded in the crystal (thickness 2 mm was used) as well as
Fig. 8. 13C NMR spectrum of DEASI.

K. Senthil et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 125 (2014) 335–344
341
n–
in the crystal system, and main absorption peak at 478 nm corre-
sponds to the
p* transitions of the extended conjugated system
(stilbazolium chromophore) which causes the material absorbs
light in the Vis–NIR range.
p
* transition which is indicating to the pyridinium group present
that material is highly stable up to 262 °C, and there is no phase
transition up to 262 °C, and it is also indicating that the absence
of the water molecule in the material. The major weight loss of
about 57.37% in the temperature range 262–268 °C due to the
complete decomposition of the stilbazolium ion. After that, the
remaining 35.56% molecules decomposed and volatilization above
312 °C. A very broad peak observed at 310.8 °C attributed to the
change in the physical state of the crystal. Thus, the TGA and
DTA analysis confirm the stability of the material, and which
makes a potential material for laser applications and fabrication
of any optical devices below its melting point.
p–
Refractive index
The refractive index measurement of the crystal is one of the
useful parameters for optical studies. The refractive index of the ti-
tle crystal was measured using Abbe refractometer (ATAGO, NAR-
4T, Japan) connected by digital thermometer with thermistor cable
connector jack, and the instrument was designed use at the wave-
length of the sodium D line (589.3 nm). The Abbe refractometer
was pre-calibrated by measuring the refractive indices of toluene
and benzene. Crack free and optically clear surface of the grown
crystal was chosen for the study. Few drops of methylene iodide
were allowed on both the prism surface and crystal surface which
make close contact between crystal and prism. The crystal was
placed between two prisms (measuring and illuminating). Light
passes through the crystal from the illuminating prism (sodium
lamp) gets reflects on the bottom surface of reflector prism. The
telescope was used to evaluate the location of the margin between
dark and light region. The refractive index of DEASI crystal was
measured at room temperature, and the measured refractive index
was found to be 1.647.
Mechanical properties
Good optical quality of the crystals and mechanical properties
are playing a main role in device applications. Mechanical strength
of the crystal depends on the various parameters such as lattice en-
ergy, Debye temperature, interatomic spacing composition of the
crystalline solids. The transparent well polished smooth surface
of the grown crystal was selected, and it was subjected to static
indentation tests made at ambient temperature and loads varied
from 10 to 100 g were used for making indentations with indenta-
tion time of 5 s in all cases. The Vicker’s micro hardness (Hv) profile
of crystal is calculated from the standard equation.
Hv ¼ 1:8544ðP=d2Þ kg=mm2;
ð1Þ
Here, Hv is the Vickers hardness number in kg/mm, P is the applied
load in kg and d is the mean diagonal length in mm of the indenta-
tion impression. The variation of hardness number (Hv) with the ap-
plied load (P) is shown in Fig. 11. It is evident from the curve that
increase in microhardness gradually with an increase in applied
loads up to 100 g, and further increase crack develop on the smooth
surface of the grown crystal due to the release of internal stresses
generated locally by indentation [22]. The Meyer’s index number
was calculated from the Meyer’s law, which relates the applied load
(P) and indentation diagonal length (d) as P = k1dn
Thermo gravimetric analysis (TGA) and differential scanning
calorimetry (DSC) studies
The thermo gravimetric analysis (TGA) and differential scanning
calorimetric analysis (DSC) are very useful characterization tech-
nique to study the phase transition, different stages of decomposi-
tions and thermal stability of the materials. The powdered crystal
sample 1.769 was used for the initial analysis. The final residual
mass of the experiment was 5.31% of the initial mass at 498.1 °C.
The resulting TGA and DSC spectrum of DEASI is shown in
Fig. 10. From the figure, the DSC curve shows a sharp endothermic
peak at 276.6 °C. The sharpness of the endothermic peak confirms
the melting point of the specimen and good crystalline perfection
of the synthesized compound. The TGA curve of this sample shows
log P ¼ log k þ n log d
ð2Þ
where k is the material constant and n is the Meyer’s index. In order
to find the value of ‘n’, the graph is plotted against logP vs. logd
(Fig. 12), which provides a straight line. The slope of this straight
Fig. 10. TGA/DSC curve of DEASI.

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K. Senthil et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 125 (2014) 335–344
Fig. 13. Variation of dielectric constant with log frequency at different tempera-
tures for DEASI single crystal.
Fig. 11. Variation of hardness number (Hv) with load (P) for DEASI crystal.
where C is the capacitance of the crystal, t is the thickness of the
crystal,
eo is the absolute permittivity of the free face
(8.854 ꢁ 10ꢂ12 F/m), A is the area of the crystal and
er is the dielec-
tric constant of the crystal. Fig. 13 shows the variations of dielectric
constant (er) with log frequency. From the graph, the value of
dielectric constant decreases with the increase in frequency for all
temperature and then remains constant at higher frequencies. The
high value of dielectric constant of DEASI crystal at low frequencies
may be attributed to the contribution of polarizations, namely,
space charge, orientational, electronic and ionic polarizations and
its low value at higher frequencies may be due to the loss of signif-
icance of these polarizations gradually. From the graph, the dielec-
tric constant is seen to increase with an increase of temperature,
and this ascribed to space charge polarization near the grain bound-
ary interfaces, which depends on the purity and perfection of the
crystal [26]. The very low value of dielectric constant at higher fre-
quencies is important for the fabrication of materials towards pho-
tonic, electro-optic devices and NLO applications.
Fig. 14 shows the variation of dielectric loss of the grown crystal
with frequency at different temperature. It is seen from the figure
the dielectric loss values are found to be high at low frequencies
and further decreases with an increase in high frequencies. The
behavior of low value dielectric loss with high frequency for DEASI
crystals suggests that the crystal possesses enhanced optical qual-
ity with lesser defects, which is the desirable property for the fab-
rication of nonlinear optical materials in their field of applications
[27]. Thus, the results suggest that the dielectric constant and loss
strongly depend on the frequency and the temperature of the
crystal.
Fig. 12. Graph between LogP and Logd.
line gives the Mayer’s index number ‘n’. The calculated value of n is
5.28. Hv should increase with the increase of P if n > 2 and decrease
if n < 2. According to the concept of Onitsch ‘n’ should lie between 1
and 1.6 for harder material and above 1.6 for softer materials
[23,24]. Thus, DEASI crystal belongs to the soft material category.
Dielectric studies
Dielectric properties are correlated with the lattice dynamics in
the crystal and electro-optic property of the crystals particularly
when they are non conducting materials [25]. Hence, the promi-
nent (010) faces of defect free and transparent DEASI single crys-
tals of 3 mm thickness were subjected to dielectric studies at
different temperatures for various frequencies ranging from
50 Hz to 5 MHz. The surface of the sample was electroded on both
opposite faces with high-grade silver paste to promote good elec-
trical conduction. The dielectric constant of the crystal can be cal-
culated using the following relation,
Etching studies
The chemical etching studies were carried out to analyse crystal
growth mechanism and crystal defects. The (010) faces of DEASI
have been completely dipped at room temperature in the mixed
etchant of methanol and acetonitrile for a period of 5, 10 and
15 s, and then surface etchant was removed gently using tissue pa-
per. Afterwards, the feature of the crystal surface was examined
under an optical microscope reflection mode which is shown in
the Fig. 15a–d. Fig. 15(a) shows the surface features of the DEASI
as grown single crystal before etching. From the Fig. 15b–d, it is
seen that the number of low angle grain boundaries and etch pits
er ¼ Ct=A
e
o
ð3Þ

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343
SHG measurement
Kurtz and Perry technique
The second harmonic generation test of DEASI crystals was
examined by Kurtz and Perry powder method [30]. A high density
Q-switched Nd: YAG laser emitting radiation of wavelength
1064 nm with pulse width an input repetition rate of 10 Hz and en-
ergy 4.6 mJ/pulse was allowed to fall onto the powdered sample in
a Capillary tube using visible blocking filter. This experiment
shows the lack of greenish emission of radiation for DEASI (zero
second order NLO response). So, this test reveals that the centro-
symmetry nature of the crystal, but no symmetry restrictions are
there for to fulfill the third order NLO applications [31].
Conclusion
Bulk crystals of a stilbazolium derivative 2-(4-Diethylamino-
phenyl)-vinyl]-1-methyl-pyridinium iodide were synthesized,
and the crystals were grown by slow evaporation technique at
35 °C. Single crystal X-ray diffraction study has been performed
to determine the lattice parameter and the crystal system. The
grown crystal belongs to the monoclinic crystal system with cen-
trosymmetric space group P2(1)/c. The functional groups and for-
mation of the electronic structure were confirmed from FT-IR, 1H
NMR and 13C NMR analysis. The UV–Vis NIR spectrum shows, its
absorption takes place in UV region and transmittance nature in
the Vis–NIR region. The refractive index (n) value was found at
1.647. Thermal behavior of DEASI was studied by TGA/DSC analysis
and its reveals that the material is decomposes at 276.6 °C.
Mechanical properties of the grown crystal were studied by using
Fig. 14. Variation of dielectric loss with log frequency at different temperatures for
DEASI single crystal.
have been identified which are clearly visible. The low angle grain
boundaries occurred may be due to segregation of impurities on
the surface of the grown crystal [28]. Thus, the observed etch pits
suggest that DEASI crystal is in a layer growth, confirm the two
dimensional (2D) growth mechanisms with increasing size of the
pattern with increasing the etching time, and fewer dislocations
[29].
Fig. 15. Chemical etching time of DEASI single crystal on (010) face with methanil and acetonitrile mixed solvent as an etchant (a) as grown surface, (b) 5 s, (c) 10 s and
(d)15 s.

344
K. Senthil et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 125 (2014) 335–344
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Vicker’s micro hardness technique, and DEASI crystal falls in a soft
material category. Dielectric studies revealed that dielectric con-
stant and dielectric loss of the grown crystal decrease with in-
crease of frequencies. The chemical etching studies shows that
the perfections and quality of grown crystal is good. All these prop-
erties show that this compound may be a promising material for
applications in nonlinear optical area.
Acknowledgements
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The authors wish to thank to SAIF IIT madras for providing a
single crystal XRD analysis. The authors also would like to thank
to Defence Research and Development Organization (DRDO), Gov-
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ment, Vellore for their constant support and encouragements.
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