Synthesis, crystal growth, structural, thermal and optical properties of naphthalene picrate an organic NLO material

Article abstract of DOI:10.1016/j.saa.2008.01.036

Crystalline substance of naphthalene picrate (NP) was synthesized and single crystals were grown using slow evaporation solution growth technique. The solubility of the naphthalene picrate complex was estimated using different solvents such as chloroform and benzene. The material was characterized by elemental analysis, powder X-ray diffraction (XRD), nuclear magnetic resonance (NMR) and fourier transform-infrared (FT-IR) techniques. The electronic absorption was studied through UV-vis spectrophotometer. Thermal behavior and stability of the crystal were studied using thermogravimetric (TG) and differential thermal analysis (DTA) techniques. The second harmonic generation (SHG) of the material was confirmed using Nd:YAG laser.

Full text of DOI:10.1016/j.saa.2008.01.036

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Spectrochimica Acta Part A 71 (2008) 755–759
Synthesis, crystal growth, structural, thermal and optical properties
of naphthalene picrate an organic NLO material
A. Chandramohana, R. Bharathikannanb, V. Kandaveluc,∗
,
J. Chandrasekaranb, M.A. Kandhaswamya
a Department of Chemistry, Sri Ramakrishna Mission Vidyalaya College of Arts and Science, Coimbatore 641020, Tamil Nadu, India
b Department of Physics, Sri Ramakrishna Mission Vidyalaya College of Arts and Science, Coimbatore 641020, Tamil Nadu, India
c School of Chemistry, Bharathidasan University, Tiruchirappalli 620024, Tamil Nadu, India
Received 1 March 2007; accepted 28 January 2008
Abstract
Crystalline substance of naphthalene picrate (NP) was synthesized and single crystals were grown using slow evaporation solution growth
technique. The solubility of the naphthalene picrate complex was estimated using different solvents such as chloroform and benzene. The material
was characterized by elemental analysis, powder X-ray diffraction (XRD), nuclear magnetic resonance (NMR) and fourier transform-infrared
(FT-IR) techniques. The electronic absorption was studied through UV–vis spectrophotometer. Thermal behavior and stability of the crystal were
studied using thermogravimetric (TG) and differential thermal analysis (DTA) techniques. The second harmonic generation (SHG) of the material
was confirmed using Nd:YAG laser.
© 2008 Elsevier B.V. All rights reserved.
PACS: 34.70.+e; 42.70. Mp; 77.84.Jd; 81.10.Dn
Keywords: Charge transfer; Solution growth; Naphthalene picrate; Nonlinear optical (NLO) materials
1. Introduction
chromophores but in most of the inorganic materials the lacking
of ␲ electron delocalization makes them to possess only mod-
erate optical nonlinearities. The major difficulty in inorganic
materials is how to impart the acentric packing with large χ2
value. Alternatively, the structural flexibility of the organic chro-
mophores permits to easily modify the chemical composition in
a precise manner to induce acentric packing which results in
large hyperpolarizability (β) and remarkable second order NLO
activity (χ2) [6,7].
In our present work an attempt has been made to grow a new
class of organic NLO crystal of NP which is an intermediate
product of naphthalene and picric acid. The wide transparency
window and lower cutoff wavelength in the visible region of
this material attract greater attention for extensive investigations.
The crystals which are composed of aromatic molecules with ␲
electron donor and acceptor substitutions normally exhibit inter-
molecular charge transfer and leads to the required property
of noncentrosymmetry and makes them good frequency con-
version material. In this paper the synthesis, solubility, growth
aspect, linear, nonlinear, optical and thermal properties of the
title compound have been reported. It belongs to the monoclinic
system with unit cell dimensions a = 16.248(5), b = 6.871(2),
Nonlinear optical crystals have been a great deal of inter-
est in recent years due to their potential use in the fields like
laser technology, optical communication, optical data storage
and optical signal processing [1–3]. It is also contributing num-
ber of applications in the domain of optoelectronics and photonic
technologies [4,5]. This induces the physicists, chemists, mate-
rial scientists and crystal engineers to identify a new class of
NLO materials which should fulfill the needs of the above fields.
The search for finding the new and better NLO materials having
high transparent window in visible region, high optical dam-
age threshold and good optical frequency conversion efficiency
have been engaged by several scientists. In this context, vari-
ety of organic and inorganic NLO materials has been proposed.
Normally organic chromophores exhibit high and fast nonlinear-
ities than their inorganic counterparts. This is because of their
tendency to induce delocalization of ␲ electron in their organic
∗
Corresponding author. Tel.: +91 431 2407053; fax: +91 431 2407045.
E-mail address: vkands@gmail.com (V. Kandavelu).
1386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2008.01.036

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A. Chandramohan et al. / Spectrochimica Acta Part A 71 (2008) 755–759
◦
3
˚
˚
c = 14.306(5) A, β = 96.62(5) , V = 1586.47 A and space group
P21/a have been already reported [8].
2. Experimental
2.1. Material synthesis
AnalaR grade naphthalene and picric acid were obtained
from Merck, India. The title compound was synthesized by dis-
solving naphthalene (C10H8) and picric acid (PA) in benzene
separately, in the ratio of 1:1 and the resulting solution was
stirred well, a yellow precipitate of naphthalene picrate (NP)
(C6H3N3O7·C10H8) crystalline substance was obtained. The
purity of synthesized compound was improved by successive
recrystallization process. The reaction for the synthesis of NP is
illustrated in Scheme 1.
Fig. 1. Solubility temperature gradiant for naphthalene picrate in chloroform
and in benzene.
almost increases linearly with temperature. Hence chloroform
has been selected as solvent. The solubility curves are shown in
Fig. 1.
2.2. Solubility
To grow good quality single crystals of considerable size the
selection of the solvent is very important. For choosing the best
solvent for crystal growth the valuable information are obtained
through solubility test [9]. To carryout the crystal growth by
solution method the solubility estimation has been studied by
dissolving NP in different solvents. The solvents which dis-
solve NP were examined using chloroform and benzene. The
solubility was measured by adding excess amount of NP in
the solvent at constant temperature (30 ◦C) and it was contin-
uously stirred using magnetic stirrer to achieve homogeneous
concentrationovertheentirevolumeofthesolution. Onreaching
saturation the content of the solution was analyzed gravimet-
rically. This process was repeated for different temperatures
(30–50 ◦C) with the interval of 5 ◦C for chloroform and benzene.
It is observed that NP exhibits high positive solubility tem-
perature gradient in chloroform than in benzene and solubility
2.3. Crystal growth
In accordance with the estimated solubility curve a satu-
rated solution of NP in chloroform was prepared. The saturated
solution was thoroughly stirred using magnetic stirrer for about
30 min to achieve homogeneous mixing of solvent. Suspended
impurities from the solution were removed using high qual-
ity filter paper. This is kept in clean beaker for crystal growth.
Throughout the experiment the supersaturation was maintained
at the required level by selecting the room temperature i.e., about
30 ◦C as saturation temperature. This minimizes the temperature
gradientofthesolventandalsopropercarewastakentofreefrom
the mechanical shocks. Then, the saturated chloroform solution
of the synthesized complex has been kept for crystal growth
by slow evaporation technique. In about 11 days well-defined,
bright, transparent, good morphological yellow coloured crys-
tals were harvested. The average size of the grown crystal is
8 mm × 4 mm × 2 mm. The photograph of the grown crystals is
shown in Fig. 2.
2.4. Analyses
The elemental analysis was carried out using a PerkinElmer
2400 CHN instrument. The powdered sample had been sub-
jected to X-ray diffraction studies using Shimadzu XRD
6000 diffractometer. The 1H NMR spectrum was recorded
using AMX 400 spectrometer in CHCl3 with tetramethylsi-
lane as internal reference. FT-IR spectrum was recorded by
employing PerkinElmer FT-IR spectrometer using KBr pel-
let technique. Electronic absorption spectrum was recorded
using Schimadzu 1601 UV–vis spectrophotometer. The ther-
mal stability was identified by thermogravimetric (TG) and
differential thermal analyses (DTA). The thermal analyses
were carried out using Seiko Instruments between the tem-
peratures 30–800 ◦C at a heating rate of 20 ◦C/min in air
atmosphere.
Scheme 1.

A. Chandramohan et al. / Spectrochimica Acta Part A 71 (2008) 755–759
757
Table 1
X-ray powder diffraction data of naphthalene picrate crystals
Peak
2θ (◦)
d-Value
I value
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
11.0111
11.8000
12.0323
12.4697
13.2975
15.7281
17.5200
17.6560
20.6710
21.9435
22.2800
23.1211
26.5210
27.1433
27.4172
8.0287
7.4937
7.3495
7.0927
6.6530
5.6299
5.0579
5.0192
4.2934
4.0472
3.9867
3.8437
3.3582
3.2836
3.2504
420
361
1140
1051
807
434
327
977
571
269
271
514
680
1385
2005
cific 2θ angles confirm the crystalline nature of the synthesized
compound.
1
From the H NMR spectrum (Fig. 4), the chemical shifts
obtained at δ = 7.6 and 7.9 ppm confirm the presence of two dif-
ferent kinds of protons in the naphthalene moiety and the signal
observed at δ = 7.4 ppm indicates the presence of two protons
of the same kind in picric acid moiety. The peak at δ = 9.3 ppm
is assigned to the O–H proton of picric acid in NP whereas it
was observed at δ = 11.94 ppm [11] in the case of picric acid.
This upfield shift is due to shielding of O–H protons by the ␲
electrons of the naphthalene ring system confirming the charge
transfer mechanism in this adduct.
The characteristic vibrational frequencies of the functional
groups in the crystal lattices of NP are identified from the FT-
IR technique and those are tabulated in Table 2. The formation
of charge transfer complex during the reaction of naphthalene
with picric acid is strongly evidenced through the realization
of important bands of donor and acceptor in the resultant com-
plex spectrum (Fig. 5). It is generally expected in acid–base
interaction that there is a transfer of proton from the donor
(acid) to acceptor (base) [10]. A similar activity is liable to
expect in naphthalene and picric acid reaction also. This assump-
tion is strongly evidenced through the appearance of a band
Fig. 2. Photograph of single crystals of naphthalene picrate.
3. Results and discussion
The purity of the synthesized compound was checked by the
analysis of carbon, hydrogen and nitrogen present in them. The
percentage composition of the elements present in NP are estab-
lished as C = 52.83% (53.79%), H = 3.22% (3.10%), N = 12.37%
(11.76%) and O = 31.62% (31.35%). It is observed that the cal-
culated values given in parentheses are in good agreement with
experimentally observed values. This result indicates that the
NP was free from impurities also, confirm the absence of water
molecules. It confirms the stoichiometry, hence the molecular
formula of the complex crystal.
The powder X-ray diffraction pattern of the synthesized com-
pound is shown in Fig. 3. The observed intensity of the peaks
and its d-values are given in Table 1. The base peak appears at
2θ values of 12.03◦, 12.47◦, 17.66◦, 26.52◦ and 27.14◦. Many
low intensity lines are also present. Among those the promis-
ing low intensity lines are at 13.30◦, 15.73◦, 21.94◦, 22.28◦
and 23.12◦. These sharp and well-defined Bragg peaks at spe-
Fig. 4. 1H NMR spectrum of naphthalene picrate.
Fig. 3. Powder XRD patterns of naphthalene picrate crystals.

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A. Chandramohan et al. / Spectrochimica Acta Part A 71 (2008) 755–759
Table 2
Characteristic infrared frequencies (cm−1) and assignments for the naphthalene
picrate crystals
Infrared frequencies (cm−1
)
Assignments
3300
3103
O–H stretching
Asymmetric C–H stretching due to
the aromatic ring
2850
Symmetric C–H stretching due to the
aromatic ring
1632, 1543, 1436
C C stretching due to the aromatic
ring
1543
1311
1280
Asymmetric stretching of NO2 group
Symmetric stretching of NO2 group
C–O stretching
Fig. 6. TG and DTA thermograms of naphthalene picrate crystals.
at 3300 cm−1 This is due to the O–H stretching absorption
.
which is normally expected at 3433 cm−1 for picric acid. This
reduction in absorption has been attributed to the attraction of
O–H protons by the ␲ electron system of naphthalene moiety
leading to an increase in O–H bond length. This confirms the
expected symmetry and change in electronic structure through
the formation of C–T complex. The absorptions at 1543 and
1311 cm−1 are confirming the anti-symmetrical and symmetri-
cal stretching vibrations of NO2 group, respectively. Usually the
anti-symmetrical stretching vibration νas(NO2) is sensitive for
polar influences and the electronic states of the nucleus. There-
fore one can realize that the shift to lower frequency of νas(NO2)
vibration (1543 cm−1) in the complex spectrum compared with
the free picric acid (1607 cm−1) [11] is due to the large electron
density on the picrate as a result of charge transfer interaction in
the complex. The presence of C C stretching vibration of aro-
matic ring is revealed from the absorption peaks at 1632, 1543
and 1436 cm−1 [12].
Scheme 2.
there is no significant absorption at 532 nm indicating that this
is suitable material for second harmonic generation.
From the thermogravimetric curve (Fig. 6) it is noted that the
decomposition of the title compound took place in two stages
while the compound was heated from 189 ◦C to 413 ◦C. It is
observed that the complex is very stable up to 151 ◦C after which
the first decomposition starts and the second begins at 265 ◦C.
The above reaction scheme (Scheme 2) is the most reasonable
to account for the weight losses in the decomposition process.
The absence of weight loss around 100 ◦C confirms that there
is no crystallization of water in the molecular structure. C10H8 is
a mixture of different hydrocarbon gases which are evolved. Fur-
ther, it is important to note that the compound has no phase tran-
sition till the material reaches the melting point which enhances
the temperature range of the crystal for NLO applications.
The SHG property of as grown naphthalene picrate crys-
tal was examined through modified Kurtz and Perry powdered
technique. In this method powdered sample of randomly ori-
ented crystallite particles were packed as a cell by sandwiching
between glass slides. The sample was then subjected to the
output of Q-switched Nd:YAG laser emitting a wavelength of
1064 nm with power 20 MW. The beam was well focused. It is
observed that a signal of wavelength 532 nm was generated by
the naphthalene picrate crystal. This reduction in wavelength of
input radiation by half confirms the frequency doubling which
strongly evidences the second harmonic generation of the naph-
thalene picrate crystal.
From the electronic absorption spectrum (not shown), the
absorption maximum of naphthalene is at 312 nm and that for
picric acid is at 210 nm. Whereas, a new distinct absorption
maximaisobservedat380 nmforthecomplex. Thelongerwave-
length (red shift) shift of complex from the reactants is the clear
indication of charge transfer activity of the complex. As well,
4. Conclusions
The NP complex was synthesized and single crystals were
grown by slow evaporation solution growth technique. The sol-
ubility of synthesized compound was tested using benzene and
chloroform. It was found that the complex compound was more
soluble in chloroform than in benzene. The SHG nature of
the crystal was tested through preliminary NLO testing which
attributes to the noncentrosymmetry nature of the NP crystals.
The formation of charge transfer complex has been confirmed
1
through UV–vis spectrum. Elemental analysis, FT-IR and H
Fig. 5. FT-IR spectrum of naphthalene picrate.

A. Chandramohan et al. / Spectrochimica Acta Part A 71 (2008) 755–759
759
NMR spectral studies confirm the molecular structure of NP
crystals. The thermal analysis establishes the molecular formula
of the compound and its thermal stability which indicates the
suitability of the material for NLO applications.
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One of the authors (A. Chandramohan) thanks to University
Grants Commission (UGC) for the financial assistance provided
under faculty development programme.
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