Synthesis, spectral analysis, optical and thermal properties of new organic NLO crystal: N,N′-Diphenylguanidinium Nitrate (DPGN)

Article abstract of DOI:10.1016/j.saa.2014.04.059

A new organic NLO material N,N′-Diphenylguanidinium Nitrate (DPGN) single crystal was grown by slow evaporation technique using methanol as solvent. Single crystal X-ray diffraction and powder X-ray diffraction experiments were carried out in order to confirm the structure and crystalline nature of DPGN crystal. Wide band gap of 3.9 eV with transmittance of 57% up to 800 nm is observed for the grown crystal using UV-vis spectral analysis. The chemical bonding and presence of various functional groups were confirmed by the FT-IR and FT-Raman spectral studies. The thermal behavior of DPGN crystal was analyzed by simultaneous TG-DTA studies. The second harmonic generation (SHG) nonlinearity of the grown crystal was measured by Kurtz and Perry powder technique and was found to be comparable with that of the standard reference material potassium dihydrogen phosphate (KDP) crystal.

Full text of DOI:10.1016/j.saa.2014.04.059

Accepted Manuscript
Synthesis, Spectral analysis, Optical and Thermal properties of new organic
NLO crystal: N,N’-Diphenylguanidinium Nitrate (DPGN)
G. Saravana Kumar, P. Murugakoothan
PII:
S1386-1425(14)00632-5
http://dx.doi.org/10.1016/j.saa.2014.04.059
SAA 12023
DOI:
Reference:
Spectrochimica Acta Part A: Molecular and Biomo-
lecular Spectroscopy
To appear in:
Received Date:
Revised Date:
Accepted Date:
20 November 2013
6 March 2014
7 April 2014
Please cite this article as: G. Saravana Kumar, P. Murugakoothan, Synthesis, Spectral analysis, Optical and Thermal
properties of new organic NLO crystal: N,N’-Diphenylguanidinium Nitrate (DPGN), Spectrochimica Acta Part A:
Molecular and Biomolecular Spectroscopy (2014), doi: http://dx.doi.org/10.1016/j.saa.2014.04.059
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers
we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and
review of the resulting proof before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Synthesis, Spectral analysis, Optical and Thermal properties of new organic
NLO crystal: N,N’-Diphenylguanidinium Nitrate (DPGN)
Saravana Kumar.G, Murugakoothan. P*
MRDL, PG & Research Department of Physics, Pachaiyappa’s College, Chennai – 600 030,
India
Abstract
A new organic NLO material N,N’-Diphenylguanidinium Nitrate (DPGN) single crystal
was grown by slow evaporation technique using methanol as solvent. Single crystal X-ray
diffraction and powder X-ray diffraction experiments were carried out in order to confirm the
structure and crystalline nature of DPGN crystal. Wide band gap of 3.9 eV with transmittance of
57% up to 800 nm is observed for the grown crystal using UV–vis spectral analysis. The
chemical bonding and presence of various functional groups were confirmed by the FT-IR and
FT-Raman spectral studies. The thermal behavior of DPGN crystal was analyzed by
simultaneous TG-DTA studies. The second harmonic generation (SHG) nonlinearity of the
grown crystal was measured by Kurtz and Perry powder technique and was found to be
comparable with that of the standard reference material potassium dihydrogen phosphate (KDP)
crystal.
Keywords: Powder X-Ray diffraction, Spectral analysis, nonlinear optical material.
*Corresponding author. Tel.: 91-944 444 7586.
E-mail address: murugakoothan03@yahoo.co.in (P.Murugakoothan).
1

1. Introduction
Nonlinear optical materials find a number of applications, like frequency conversion,
light modulation, optical switching and optical memory storage. Recent researches reveal that the
organic nonlinear optical materials are used for developing relatively low power laser-driven
nonlinear optical system. They exhibit less optical response time but high second harmonic
generation (SHG) efficiency compared to that of inorganic materials [1, 2]. The development of
optical devices, such as photonic integrated circuitry, depends strongly on the design of highly
efficient nonlinear optical (NLO) materials. Among the different types of NLO materials,
organic materials are shown to be superior to their inorganic counter parts in terms of synthesis,
crystal fabrication, potential to create large devices and much faster optical nonlinearities [3].
Guanidinium family complexes have been chosen for study as potential material for nonlinear
optics. Guanidine is an important compound that has many biological, chemical and medical
applications [4]. Guanidine as well biguanide, can be employed in material engineering of a
promising class of NLO compounds based on salts combining a cation derived from a
polarizable organic molecule with an anion capable of forming hydrogen bonded crystal
structures [5]. Thus the attention of researchers is focused especially on the group of hydrogen
bonded solids that exhibits potential non-linear optical properties. Some potential guandinium
based NLO materials synthesized with L-tartaric acid via strong hydrogen bond interactions are
aminoguanidinium (1+) hydrogen L-tartrate monohydrate [6], guanidinium L-tartrate hydrate
[7]. Keeping this in mind we have focused on the substituted guanidine family, i.e.,
N,N’-Diphenylguanidine (DPG) which is also known as melaniline that have been widely used
as a primary and secondary accelerator in the vulcanization of rubber. In this context, herein we
2

report on the synthesis, spectral analysis, optical and thermal properties of new organic NLO
crystal: N,N’-Diphenylguanidinium Nitrate (DPGN).
2. Experimental Procedure
2.1 Material synthesis
The title compound was synthesized by mixing equimolar portions of
N,N’-Diphenylguanidine and nitric acid using methanol as solvent. At first the starting material,
nitric acid was dissolved in methanol and then further N,N’-Diphenylguanidine was added to this
solution, by continuous stirring, until complete neutralization occurred resulting in a pale brown
colored solution. The solution was made to stir for 6 hours using a magnetic stirrer in order to
achieve homogeneity at room temperature and the resulting solution was filtered in a 500 mL
clean borosil beaker through Whatmann filter paper. The beaker was covered with aluminium
foil in order to control the rate of evaporation. The purity of the synthesized salt was further
improved by successive recrystallization. The reaction scheme is given in Fig.1. A well defined
pale brown colored transparent single crystal of DPGN was harvested after a period of 46 days
and the photograph of the as grown DPGN crystal is shown in Fig.2.
2.2 Characterization studies
The single crystal X-ray diffraction data of N,N’-Diphenylguanidinium Nitrate was obtained
using ENRAF NONIUS CAD-4 single crystal X-ray diffractometer. X-ray powder diffraction
analysis of DPGN crystal was carried out using Rich Seifert diffractometer with CuKα
(λ = 1.5405 Å) radiation over the range of 10–70o at a scanning rate of 1o/min. The optical
transmission spectrum was recorded in the wavelength region 200–800 nm using Varian Carry
3

SE model spectrometer. FT-IR and FT- Raman spectra were recorded to elucidate the chemical
bonding and molecular structure of DPGN. The FT-IR spectrum was recorded in the range
400–4000 cm-1 by PERKIN-ELMER spectrometer using the KBr pellet technique. FT- Raman
spectrum was recorded using FT-RAMAN spectrometer (Perkin-Elmer GX 2000) in the range
4000-400 cm-1. Thermo gravimetric and differential thermal analysis was carried out using
NETZSCH STA 409 thermal analyzer instrument in nitrogen atmosphere at a heating rate of
10oC/min. in the temperature range of 30 – 500 oC. The second harmonic generation nonlinearity
was measured using Kurtz and Perry technique.
3. Results and Discussions
3.1 X-ray diffraction analysis
The single crystal X-ray diffraction analysis of DPGN was carried out to evaluate the
crystal system, space group and the cell parameters. It is observed that the DPGN belongs to
orthorhombic crystal system with space group Pna21 and the measured cell parameters were
a = 16.97 Å, b = 13.89 Å, c = 5.79 Å, V = 1364 Å3and α = β = γ = 90o. The obtained cell
parameters and cell volume are found to be in good agreement with the reported value [8].
X-ray powder diffraction pattern of DPGN is shown in Fig. 3. The presence of prominent Bragg
peaks at specific 2θ angles in the powder X-ray diffraction confirms the perfect crystalline nature
of DPGN crystal.
3.2 UV-vis-spectral analysis
UV-vis spectral study gives important structural information because absorption of UV
and visible light involves promotion of the electrons in п and σ orbital from the ground state to
higher energy states [9]. The UV-vis transmission spectrum of DPGN crystal sample was
recorded in the range of 200 – 800 nm using a polished crystal sample of thickness 1.5 mm.
4

Title compound is found to be active in the UV region having a transmittance of about 57% with
the lower cut-off wavelength of 310 nm. The recorded optical transmittance spectrum is shown
in Fig.4. It is well known that the parameters, such as optical transmittance range and lower
cutoff region are important to tailor the material for optical applications. The spectrum shows
that there are no significant absorptions in the range of 310 – 800 nm. The transmission window
in the visible region enables good optical transmission of the second harmonic frequencies of
Nd:YAG laser.
The measured transmittance was used to calculate the absorption coefficient (α) using the
relation:
2.3026
(1)
1
log
α =
(
)
T
t
where T is the transmittance and t is the thickness of the crystal sample.
In the high photon energy region, the energy dependence of absorption coefficient
suggests the occurrence of direct band gap of the crystal obeying the following equation for high
photon energies hν [10]
(
)
2
αhν = A E − hν
(2)
(
)
(
)
g
where α is the absorption coefficient, h is the Planck’s constant, A is a constant, ν is the
frequency of the incident photon and Eg is the optical band gap.
2
The Tauc’s graph [11] plotted between
and the photon energy
is shown in
)
hν
αhν
(
(
)
Fig.5. The band gap of the crystal was evaluated by extrapolating the linear part of the graph to
the energy axis and is found to be 3.9 eV. The wide band gap of DPGN crystals confirms the
large transmittance in the visible region.
5

3.3 Spectral analysis
The chemical bonding of the grown compound were analyzed by recording FT-IR and
FT-Raman spectra. The spectral analysis of DPGN was carried out on the characteristic group
vibrations of guanidinium groups, nitrate ions and phenyl rings. The FT-IR and FT-Raman
spectra of DPGN compound were presented in Fig.6 and Fig.7. The vibrational frequency of
various functional groups of DPGN and the frequency assignments are presented in Table 1.
Hydrogen Bond
The very broad nature of the spectrum in the region above 3000 cm-1 in IR clearly shows
the strong hydrogen bond interactions between the anions and cations. The hydrogen bond
interaction paves way for the second harmonic generation.
Guanidinium vibrations
The high frequency absorption peak at 3352 cm-1 and 3223 cm-1 in IR are assigned to NH2
asymmetric and symmetric stretching mode respectively. The characteristic band observed at 995
cm-1 in the Raman spectrum is assigned to the symmetric C-N stretching vibrations. The IR
counterpart is observed as very weak intensity at 1002 cm-1. The band observed in IR spectrum
at 1645 cm-1 is assigned to the asymmetric C-N stretching mode. In Raman spectrum a weak
band corresponding to this type of vibration is observed at 1662 cm-1. The very strong band at
1582 cm-1 in the IR spectrum arises from the NH2 bending vibrations. The Raman counter parts
for these vibrations appear at 1547 cm-1. The bands at 1092 cm-1 and 1087 cm-1 in the IR and
Raman spectra corresponds to the NH2 rocking in-plane vibrations.
6

Phenyl ring vibrations
The phenyl ring is a cyclic group of atoms with formula C6H5. Phenyl groups have six carbon
atoms bonded together in a hexagonal planar ring, five of which are bonded to individual
hydrogen atoms, with the remaining carbon bonded to a substituent. The phenyl group vibrations
are assigned on the basis of C-C, C-H vibrations. Though the title compound has two mono
substituted phenyl rings there are no separate vibrations for the both. The C-H stretching
frequencies of the monosubstituted benzene are expected in the region 3010 – 3120 cm-1 [12].
The very strong band at 3070 cm-1 in the IR spectrum is attributed to the C-H stretching of the
phenyl rings. The Raman counter part of this vibration occurs at 3071 cm-1. The strong intensity
band at 1158 cm-1 in IR spectrum is assigned to the in-plane C-H bending of monosubstituted
benzene ring. The Raman counterpart is found to occur at 1159 cm-1with strong intensity. The
allowed vibrational frequencies arising from the C-H out of plane bending vibrations of the
monosubstituted benzene ring are expected in the region 1000 – 650 cm-1 [12] . The medium
band observed at 883 cm-1 in IR is assigned to the C-H out of plane bending vibrations. The
Raman counterpart arising from this vibration is found to occur with a strong intensity at
884 cm-1. The C-C stretching modes of vibrations are expected in the range 1300 cm-1 [12].
The strong intensity band at 1306 cm-1 in Raman is assigned to the C-C stretching mode of
vibration. The IR counterpart for this mode is found to occur at 1307 cm-1 with very strong
intensity.
Nitrate ions
The presence of nitrate anions in the title compound can be confirmed by the presence of
characteristic vibrations arising due to N-O stretching and the in-plane and out of plane bending
7

vibrations of nitrate ions. The N-O stretch of nitrate groups appears as an intense band in the
region between 1400 and 1340 cm-1. The in-plane and out of plane bending vibrations of the
nitrate groups appear in the region around 720 cm-1 and 840 – 810 cm-1[13]. The very strong
intensity band at 1385 cm-1 in IR spectrum is assigned to the N-O stretching. The very strong IR
intensity at 754 cm-1 and weak band at 754 cm-1 is assigned to the N-O in plane bending
vibrations. The strong band occurring at 825 cm-1 is arising due to N-O out of plane bending
vibrations. The IR and Raman spectrum of DPGN consists of the characteristic vibrations of both
the cation and anion and thus it can be claimed that the title compound are not separate entities
but it is N,N’-Diphenylguanidinium Nitrate.
3.4 Thermal Properties
The thermal stability of DPGN was studied by thermo gravimetric (TG) and
Differential thermal analyses (DTA). The DPGN sample weighing 10.672 mg was analysed and
the thermogram is depicted in Fig.8. The DTA curve indicates the same changes as shown in TG
curve. From the TG curve, it is evident that the material is stable up to 196 °C and moisture free.
The DTA curve shows an endothermic peak at 196 °C, no weight loss in TG, which can be
attributed to the melting point of the sample. The compound starts to decompose around,
200 °C. The TG curve shows two stages weight loss pattern when the material was heated from
34 to 500 °C. The first major weight loss was occurred between the temperatures 200 and 258 °C
with the elimination of 40.18 % of the material into gaseous products as seen in the low
temperature region. The second stage weight loss noticed between the temperatures 268 and
498 °C experiences a weight loss of about 27.95%. The residue left out at the end is about
31.87% by weight at 498.1 °C. This may be due to the residual carbon mass at the end of the
8

decomposition reactions as the theoretical elemental analysis shows that the percentage of carbon
in the title compound is nearly 56.93%. From the above discussions it can be concluded that the
DPGN crystals can be used for NLO applications till 196 °C.
3.5 Nonlinear optical study
The effectively valuable tool for screening materials for second harmonic generation is
the Kurtz-Perry powder technique [14]. This technique enables to measure the second harmonic
generation non-linearity of the materials with respect to standard reference material Potassium
dihydrogen phosphate (KDP). A fundamental beam of wavelength 1064 nm with pulse duration
of 10 ns and frequency repetition of 10 Hz from Q-switched Nd:YAG laser was used as the
source and passed through the powder sample. The SHG behavior was confirmed from the
output of the laser beam which had bright green emission (λ = 532 nm) from the powder sample.
The SHG output was converted into electrical signal and was displayed on a digital storage
oscilloscope. The optical signal incident on photomultiplier tube was converted into voltage
output. The second harmonic signal of 14.2 mV was obtained for DPGN powder crystal sample
for an input energy of 5 mJ/pulse, while the standard potassium dihydrogen phosphate (KDP)
powder crystal sample gave a SHG signal of 15.4 mV for the same input energy. It shows that
the SHG effective nonlinearity of DPGN is 0.92 times that of standard NLO material KDP.
4. Conclusions
A new organic nonlinear optical DPGN single crystal has been grown by slow
evaporation technique at room temperature using methanol as solvent. The unit cell parameters
have been evaluated by single crystal XRD technique. The good crystalline nature of DPGN
powdered sample was confirmed by powder XRD analysis. The DPGN crystal exhibits good
9

optical transparency in the entire visible region of UV–vis spectrum. The optical band gap of the
grown crystal has been calculated as 3.9 eV. The chemical bonding and the presence of various
functional groups was elucidated using FT-IR and FT-Raman spectral analyses. The thermal
stability of DPGN has been analyzed by simultaneous TG-DTA studies and it reveals that the
title compound can be used for NLO applications till 196°C. The powder SHG analysis reveals
that the nonlinearity of this crystal is 0.92 times that of KDP. The promising crystal growth,
characteristics and properties of DPGN crystal nominate it as a potential material for photonics,
electro-optics and SHG devices applications.
Acknowledgements
The authors acknowledge Prof. P. K. Das, Department of Inorganic and Physical
Chemistry, Indian Institute of Science, Bangalore for extending the laser facilities for the SHG
measurement. Authors also acknowledge SAIF, IIT, Chennai for providing the analytical
instrument facilities.
References
1. M. Kitazawa, R. Higuchi, M. Takahashi, Appl. Phys. Lett. 64 (1994) 2477-2479.
2. W.S. Wang, M.D. Aggarwal, J. Choi, T. Gebre, A.D. Shields, B.G. Penn, D.O. Frazier,
J. Cryst.Growth 198 (1999) 578-582.
3. G. Jordon, T. Kobayashi, WJ. Blau, S. Pfeiffer, H.H. Horhold, Adv. Func. Mater.
13 (2003) 751–754.
4. T. Arumanayagam and P. Murugakoothan, Mater. Lett. 65 (2011) 2748-2750.
5. C.B. Aakeroy, P.B. Hitchcock, B.D. Moyle, and K.R. Seddon, J. Chem. Soc. Chem.
Commun. 2 (1993) 152-156.
6. Z. Machova, I. Nemec, K. Teubner, P. Nemec, P. Vanek, Z. Micka, J. Mol. Struct. 832
(2007) 101– 107.
10

7. W. Krumbe, S. Haussühl, R. Fröhlich, Zeitschrift fur Kristallographie 187 (1989) 309–
318.
8. J. A. Paixao, P. S. Pereira Silva, A. Matos Beja, M. Ramos Silva and L. Alte da Veiga,
Acta Cryst. C54 (1998) 805-808.
9. S. Masilamani, A. Mohamed Musthafa, Microchemical Journal 110 (2013) 749–752.
10. A. Ashour, N. El-Kadry, S.A. Mahmoud, Thin Solid Films 269 (1995) 117–120.
11. J. Tauc, R. Grigorovici, A. Vancu, Phys. Status Solidi B 15 (1966) 627–637.
12. C. James, V.S. Jayakumar, I. Hubert Joe, J. Mol. Struct. 830 (2007) 156-166.
13. B.C. Smith, Infrared Spectral interpretations: A systematic Approach, CRC press, LLC,
1999.
14. S.K. Kurtz and T.T. Perry, J. Appl. Phys. 39 (1968) 3798–3813.
Figure captions
Fig.1. Chemical reaction scheme
Fig.2. Photograph of as grown DPGN single crystal
Fig.3. Powder X-ray diffraction pattern of DPGN
Fig.4. Optical transmittance spectrum of DPGN crystal
2
Fig.5. Plot of αhν vs. photon energy of DPGN crystal
(
)
Fig.6 FT-IR spectrum of DPGN
Fig.7. FT-Raman spectrum of DPGN
Fig.8. TG-DTA thermogram of DPGN
Table captions
Table.1. Vibrational band assignments of DPGN
11

Fig.1. Chemical reaction scheme
12

Fig.2. Photograph of as grown DPGN single crystal
13

Fig.3. Powder X-ray diffraction pattern of DPGN
14

Fig.4. Optical transmittance spectrum of DPGN crystal
15

2
Fig.5. Plot of αhν vs. photon energy of DPGN crystal
(
)
16

Fig.6. FT-IR spectrum of DPGN
17

Fig.7. FT-Raman spectrum of DPGN
18

Fig.8. TG-DTA thermogram of DPGN
19

Table.1. Vibrational band assignments of DPGN
______________________________________________________________________________
FT-IR
FT-Raman
Vibrational assignments
_____________________________________________________________________________
3352
3223
3070
1645
1582
1385
1307
1158
1092
1002
883
-
NH2 Asymmetric stretching
-
NH2 Symmetric stretching
3071
1662
1547
-
C-H stretching of the phenyl rings
Asymmetric C-N stretching
NH2 bending vibrations
N-O stretching of Nitrate
1306
1159
1087
995
884
-
C-C stretching of Phenyl rings
in-plane C-H bending of Phenyl ring
NH2 rocking in-plane vibrations
C-N symmetric stretching
out-of plane C-H bending of Phenyl ring
N-O out of plane bending of Nitrate
N-O in plane bending of Nitrate
825
754
754
______________________________________________________________________________
20

Graphical abstract
21

Highlights
• High quality DPGN crystal was grown.
• DPGN crystal exhibit good thermal stability
• Nonlinear optical efficiency almost equivalent to KDP
22