Synthesis, structural, spectroscopic and optical studies of charge transfer complex salts

Article abstract of DOI:10.1016/j.saa.2012.08.086

New charge transfer molecular complex adducts of picric acid (C ₆H₃N₃O₇) with triethylamine (C ₆H₁₅N) and dimethylformamide (HCON(CH₃) ₂) were synthesized successfully

Full text of DOI:10.1016/j.saa.2012.08.086

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 178–183
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, structural, spectroscopic and optical studies of charge transfer
complex salts
Maruthappan Manikandan a, Thaiyan Mahalingam b, Yasuhiro Hayakawa c, Ganesan Ravi a,c,
⇑
a Department of Physics, Alagappa University, Karaikudi 630 003, India
b Department of Physics, Karunya University, Coimbatore 641 114, India
c Research Institute of Electronics, Shizuoka University, Hamamatsu 432 8011, Japan
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
"
TEAP and DMFP were synthesized
for the first time.
"
Functional groups and modes of
vibrations were identified by FTIR
analysis.
"
"
Optical band gap of TEAP and DMFP
was 3.585 eV and 3.236 eV
respectively.
SHG efficiency of TEAP and DMFP
was 1.62 and 0.72 times greater than
KDP.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 June 2012
Received in revised form 27 August 2012
Accepted 31 August 2012
Available online 12 September 2012
New charge transfer molecular complex adducts of picric acid (C6H3N3O7) with triethylamine (C6H15N)
and dimethylformamide (HCON(CH3)2) were synthesized successfully for the first time. Chemical compo-
sition and stoichiometry of the synthesized complex salts were verified by CHN elemental analysis.
Solubility of the complex salts have been determined by gravimetric method and single crystals of two
new salts were grown by low temperature solution growth technique. Crystal system, crystalline nature
and cell parameters of the grown crystals were determined by single crystal X-ray diffraction (SXRD) and
powder X-ray diffraction (PXRD) analyses. The formations of the charge-transfer complex, functional
groups and the modes of vibrations have been confirmed by Fourier transform infrared (FTIR) spectros-
copy. In order to know the linear and nonlinear optical suitability for device fabrication, UV–Vis (UV)
spectral analysis and relative second harmonic generation (SHG) efficiency test were performed for the
grown crystals.
Keywords:
A1. Characterization
A1. X-ray diffraction
A1. Crystal structure
A2. Single crystal growth
B1. Organic compounds
B2. Nonlinear optic materials
Ó 2012 Elsevier B.V. All rights reserved.
Introduction
applications in photonic devices, the NLO properties of molecules
and their hyperpolarizabilities have become an important area of
Engineering of new nonlinear optical (NLO) materials, struc-
tures and devices with enhanced figures of merit has developed
over the last two decades as a major force to drive nonlinear optics
from the laboratory to real applications. Because of their potential
extensive research and a lot of experimental [1,2] and theoretical
efforts [3,4] are focused on the bulk NLO properties as well as their
dependence on the hyperpolarizabilities of molecules. An organic
^
molecule should have large second-order hyperpolarizability (a)
to exhibit good nonlinear optical properties [5]. The hyperpolariz-
ability can be enhanced by increasing intra-molecular charge
⇑
Corresponding author at: Department of Physics, Alagappa University, Karaik-
udi 630 003, India. Tel.: +91 04565 225206; fax: +91 04565 225202.
transfer interaction by extending
p-conjugated system [6]. The
term charge transfer gives a certain type of complex resulting from
E-mail addresses: gravicrc@gmail.com, raviganesa@rediffmail.com (G. Ravi).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.08.086

M. Manikandan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 178–183
179
interactions of donor and acceptor with the formation of weak
bonds [7,8]. Picric acid forms crystalline picrates of various organic
assessed as a function of temperature in the temperature range
30–45 °C in steps of 5 °C. Synthesized TEAP and DMFP salts was
dissolved in methanol and kept in the constant temperature bath
with accuracy of 0.02 °C. On reaching saturation, the equilibrium
concentration of the solute was determined by gravimetric meth-
od. Fig. 2 shows the solubility curves of TEAP and DMFP. We infer
from Fig. 2 that both title compounds exhibits good solubility and a
positive temperature solubility gradient (direct solubility) in meth-
anol solvent and found to be appropriate for bulk crystal growth by
solution growth method.
molecules through ionic and hydrogen bonding and
p–p interac-
tions [9]. Bonding of electron donor/acceptor picric acid molecules
strongly depends on the nature of the partners [10]. Picric acid
derivatives are interesting candidates, as the presence of phenolic
OH favors the formation of salts with various organic bases
[11,12]. The conjugated base, picrate, thus formed has increased
molecular hyperpolarizability because of the proton transfer. In
the present investigation, we have synthesized the molecular com-
plex adduct of triethylamine and dimethylformamide with picric
acid involving charge transfer from donor to acceptor followed
by proton transfer from the acceptor. Single crystals were grown
by low temperature solution growth technique. Elemental, struc-
tural, functional, linear and nonlinear optical properties of the
grown crystals were characterized.
Crystal growth
Saturated solutions of TEAP and DMFP in methanol at 40 °C
were prepared in accordance with the determined solubility data
using the recrystallized salts and then stirred for half an hour for
the homogenous solution mixture. The solution was filtered using
a Whatman filter sheet in order to remove the suspended impuri-
ties from the solution. The filtered solution was transferred to a
100 ml beaker having uniform perforated closure and the beaker
was placed in the constant temperature bath. By employing sol-
vent evaporation method, the nucleated crystals were allowed to
grow for a definite period and then harvested in a growth period
of thirty-five days from the mother solution. The grown single
crystals of TEAP and DMFP from the methanol are shown in Fig. 3.
Material synthesis
AnalaR grade of picric acid, triethylamine and dimethylformam-
ide were used for the synthesis process. The picric acid is less sol-
uble in water. For the synthesis of complex salt triethylamine
picrate (TEAP), equimolar quantities of the parent compounds pic-
ric acid and triethylamine were dissolved in methanol separately
and mixed together, then stirred well for about half an hour. When
a proton is transferred from the electron-donor group of an acid to
the electron acceptor group of a base, it results in increase of
hyperpolarizability of the resultant compound. The picric acid nec-
essarily protonates the amino group of the triethylamine resulting
in the formation of the yellow colored precipitation of the charge
transfer complex salt TEAP. The same procedure was followed for
the synthesis of complex salt dimethylformamide picrate (DMFP)
where dimethylformamide was used instead of triethylamine in
the above process. The yellow precipitations of both complex salts
were filtered off and further purified using methanol by recrystal-
lization process. The purified material was then used as a raw
material for the growth process. The reaction mechanism of TEAP
and DMFP is shown in Fig. 1.
CHN analysis
For ascertaining the constituents, purity and compositions of
the synthesized material, analysis of carbon, hydrogen and nitro-
gen was carried out for the recrystallized materials of TEAP and
DMFP. The percentage of composition of the elements present in
the TEAP and DMFP salts was C = 43.34% (43.59%), H = 4.796%
(5.45%), N = 17.22% (16.95%) and C = 35.15% (35.74%), H = 2.616%
(3.30%), N = 20.11% (18.53%). The experimental and calculated (gi-
ven in parentheses) values of C, H, and N agree with each other and
indicates that TEAP and DMFP are free from impurities. This study
also confirms the stoichiometry of our complex crystals.
Structural analyses
Solubility studies
Single crystal X-ray diffraction analysis (SXRD)
Selection of suitable solvent is very definitive for the growth of
good quality single crystals. The equilibrium solubility and its tem-
perature dependence are essential for solution growth. The data
from the solubility curve will suffice to start growing fair quality
single crystals. The solubility of TEAP and DMFP in methanol was
To confirm the structural identity of the synthesized complex
salts, single-crystal XRD was carried out for the TEAP and DMFP
single crystals. Good quality crystals of TEAP and DMFP were cho-
sen and data was collected using MACH 3 Nonius CAD – 4 X-ray
Fig. 1. Reaction mechanism of TEAP and DMFP.

180
M. Manikandan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 178–183
transfer complex, a proton transfer from donor to the acceptor is ex-
pected to take place which is strongly supported by the appearance
of a new band of medium intensity in the spectrum of TEAP and
DMFP. However, the bands of donor and acceptor were shifted
and this shift owes to the changes in the electronic structure on
the formation of charge transfer complex. In order to analyze
qualitatively the presence of functional groups in TEAP and DMFP,
the FT-IR spectrum was recorded using a Thermo Nicolet 380
FT-IR spectrometer by the KBr pellet technique in the range of
400–4000 cmꢀ1. The FT-IR spectra of TEAP and DMFP are shown
in Fig. 5. The observed vibrational frequencies and their corre-
sponding assignments are presented in Table 1 for TEAP and DMFP.
The bands observed in the spectra of the complex salts TEAP
and DMFP arise from the internal vibrations of the picric acid
(comprises nitro group vibration and OH vibration), triethylamine
(encompass the methyl group vibration, ethyl group vibration) in
the case of TEAP and dimethylformamide (comprises methyl group
vibration) in the case of DMFP.
Vibration of N+–H group
Fig. 2. Solubility curve of TEAP and DMFP.
In TEAP and DMFP, the amino N atom of triethylamine and
dimethylformamide cation form N–H bond with the picrate anion.
Intermolecular hydrogen bonding between the donor and the
acceptor molecules is the root cause for the NLO property of the
picrate materials [13]. This intermolecular hydrogen bonding exist
in the charge transfer complex is expected around 3400 cmꢀ1. The
peak observed at around 3408 cmꢀ1 for TEAP and 3422 cmꢀ1 for
DMFP are the evidence for hydrogen bonding in the title com-
pounds. In the IR spectrum of TEAP and DMFP, asymmetric N+–H
deformation modes are observed at 1629 cmꢀ1 and 1625 cmꢀ1
and the bending vibrations of N–H are found at 714 cmꢀ1 and
708 cmꢀ1 respectively.
diffractometer with Mo K
XRD revealed the lattice parameters of TEAP and DMFP as
a radiation (k = 0.7107 Å). Single crystal
a = 6.947 Å, b = 20.735 Å, c = 21.941 Å,
a = 10.026 Å, b = 11.103 Å, c = 21.334 Å,
tively. Hence, both the complex salts TEAP and DMFP crystallizes
in the orthorhombic system and the cell volumes were 3161 Å3
and 2375 Å3 for TEAP and DMFP, respectively.
a
= b =
c = 90.00° and
a
= b = = 90.00° respec-
c
Powder X-ray diffraction analysis (PXRD)
Powder X-ray diffraction studies were also carried out for the
complex salts to demonstrate the crystallinity using PANalytical
model X’PERT-PRO X-ray diffractometer system. The K radiations
a
Vibration of NO2 group
0
from a copper target (k = 1.5418 ÅA) was used. The single crystals of
TEAP and DMFP were ground to fine powder and these powders
were spread over a square centimeter area and placed in a beam
of monochromatic X-rays. The mass of powder was rotated in all
possible axes. From the h value for each peak, the spacing d was ob-
tained. The indexed X-ray diffraction patterns for these two sam-
ples are shown in Fig. 4. Appearances of sharp and strong peaks
confirm the good crystallinity of the grown crystals. The lattice
parameters of TEAP and DMFP crystals were calculated theoreti-
cally using the powder XRD data and it is in good agreement with
the values obtained from single crystal XRD.
The asymmetric vibration of NO2 group is observed at
1553 cmꢀ1 and 1558 cmꢀ1 for TEAP and DMFP respectively. The
absorption at 1331 cmꢀ1 and 1335 cmꢀ1 are due to the NO2 sym-
metric vibration of the TEAP and DMFP. Usually for the free picric
acid NO2 vibration occurs at 1607 cmꢀ1 [14]. Charge transfer inter-
action in the complex salts TEAP and DMFP, NO2 vibration is
shifted to lower frequency 1553 cmꢀ1 and 1558 cmꢀ1 due to the
increased electron density of the picric acid. The rocking modes
of NO2 group are identified at 529 cmꢀ1 and 532 cmꢀ1 for the TEAP
and DMFP. The NO2 scissoring vibrational modes appear in the
spectra of TEAP and DMFP at 787 cmꢀ1 and 794 cmꢀ1 respectively.
The band observed at 913 cmꢀ1 and 908 cmꢀ1 is due to the C–NO2
stretching vibration in TEAP and DMFP.
Spectroscopic analysis
Fourier transform infrared analysis (FTIR)
Vibration of phenolic and phenoxy group
In the charge transfer interaction of TEAP and DMFP, picric acid
necessarily protonates the phenolic O vibration which produces
peaks at 1156 cmꢀ1 and 1158 cmꢀ1 [15]. The absorption peak at
Infrared spectroscopy is used to identify the functional groups
and modes of vibration of the synthesized complex salts. In charge
Fig. 3. As grown crystals of (a) TEAP and (b) DMFP.

M. Manikandan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 178–183
181
Table 1
Tentative frequency assignment of TEAP and DMFP.
Vibration
TEAP
DMFP
(cmꢀ1
)
Assignment
(cmꢀ1
)
N+–H
group
3408
3422
Intermolecular hydrogen bonding N+–H
asymmetric stretching
R2 N+–H deformation mode
Bending of N+–H
1629
714
1625
708
NO2
group
1553
1558
Asymmetric stretching vibration of NO2
group
1331
1335
Symmetric stretching vibration of NO2
group
787
529
913
1156
794
532
908
1158
Scissoring of NO2
Rocking of NO2
Stretching vibration of C–NO2
Phenolic O vibration
Phenolic
group
Phenoxy
group
Methyl
group
1271
2746
1497
1268
2782
1483
C–O vibration
Symmetric C–H stretching vibration of
methyl group
Asymmetric C-H deformation of methyl
group
1073
3031
1082
3078
CH3 rocking
Asymmetric stretching vibration of
methyl group
Fig. 4. Powder X-ray diffraction patterns of TEAP and DMFP.
Ethyl
1443
–
CH2 deformation
group
Fig. 5. FTIR spectra of TEAP and DMFP.
1271 and 1268 can be ascribed to the C–O vibration of the TEAP
and DMFP complex salt, respectively.
Fig. 6. Absorption spectra of TEAP and DMFP.
Vibration of CH3 group
Internal vibration of the cations in the title compounds arises
from the functional group CH3. The peak at 3031 cmꢀ1 for TEAP
and 3078 cmꢀ1 for DMFP is attributed to asymmetric stretching
vibration of C–H in methyl group. The symmetric stretching vibra-
tion of C–H in the methyl group is observed at 2746 cmꢀ1 and
2782 cmꢀ1 for TEAP and DMFP respectively. The asymmetric C–H
deformation of methyl group occurs near 1497 cmꢀ1 for TEAP
and 1483 cmꢀ1 for DMFP. The peaks at 1073 cmꢀ1 and
1082 cmꢀ1 indicate the rocking vibration of methyl group.
in Fig. 6. Strong absorption was observed at 362 nm and 420 nm
for TEAP and DMFP respectively, which is attributed to
pꢁ tran-
p
–
sition of picrate ion. It is seen from the spectrum that the crystal is
transparent in the entire range (450–1100 nm) without any inter-
mediate absorption peak due to the charge transfer of the electron
from the donor to the acceptor, which is an essential parameter for
NLO crystals and used as SHG material in the visible range.
Good optical transmittance and lower UV cutoff wavelength are
two important required properties for NLO crystals. To determine
the transmission range and to know the suitability of TEAP and
DMFP single crystals for optical applications, the UV–Vis transmit-
tance spectrum was recorded for the grown crystals in the range
200–1100 nm (Fig. 7). The UV cutoff wavelength of TEAP and DMFP
are 361 nm and 426 nm, respectively. These spectra again confirm
the suitability of these crystals for optoelectronic applications and
Vibration of CH2 group
The CH2 deformation mode in TEAP appears at 1443 cmꢀ1
.
UV–Vis–NIR spectral analysis
The optical absorption spectra of TEAP and DMFP in solution
mode were recorded in the range 200–1100 nm and are shown

182
M. Manikandan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 178–183
Fig. 7. Transmission spectra of TEAP and DMFP.
that the SHG efficiency of the title compounds TEAP and DMFP
Table 2
are 1.62 and 0.702 times than that of the standard KDP crystal.
The extent of charge transfer across the NLO chromophore deter-
mines the level of SHG output of the material, the greater the
charge transfer, the larger the SHG output. The presence of strong
intermolecular interactions can extend the level of charge transfer
into the supramolecular realm, thereby enhancing the SHG re-
sponse [16,17]. The comparative properties of the charge transfer
complex salts are given in the Table 2.
Comparative properties of TEAP and DMFP.
Parameters
TEAP
DMFP
Molecular formula
C
12H18N4O7
C10H10N4O8
302.2
Molecular weight
330.3
System
Orthorhombic
6.947 Å
20.735 Å
21.941 Å
90.00°
Orthorhombic
10.026 Å
11.103 Å
21.334 Å
90.00°
a
b
c
a
= b = c
Cell volume
Transmittance edge (nm)
Band gap (eV)
3161 Å3
361
2375 Å3
426
3.237
Conclusions
3.585
SHG efficiency (than KDP)
Solubility (gm/100 ml)
1.62 times
21.29 g (at 45 °C)
0.703 times
9.48 g (at 45 °C)
The organic charge transfer molecular complex adducts of tri-
ethylamine and dimethylformamide with picric acid was synthe-
sized and purified by recrystallization process. The single crystals
of the title compounds were grown by slow evaporation method.
The composition and stoichiometry of the synthesized salts were
verified by CHNS elemental analysis. The single crystal X-ray dif-
fraction study revealed that both TEAP and DMFP crystallize into
orthorhombic crystal system. Vibrational frequencies assigned
from FTIR spectral analysis confirm the presence of functional
groups and prevalent charge transfer activity in the complex salts.
The UV–Vis–NIR spectrum of TEAP and DMFP in solution exhibits a
wide transparency in the visible region between 450 and 1100 nm
second-order harmonic generation of the Nd:YAG laser (1064 nm).
In order to determine the band gap of the grown crystals, extrapo-
2
lation of the straight line in the plot of (
done for TEAP and DMFP (Fig. 7) where
cient and h the photon energy. The band gap energies of TEAP and
ahm
)
versus h
m
, has been
a
is the absorption coeffi-
m
DMFP were calculated as 3.585 eV and 3.236 eV, respectively.
Second harmonic generation study
due to the p–
pꢁ transition of picrate ion in the complex salts. The
band gap energy of TEAP and DMFP was estimated from the UV–
Vis spectrum. The relative SHG activity in the complex salts was
confirmed by employing Kurtz and Perry method using high inten-
sity Nd:YAG laser as a source.
The relative second harmonic generation behavior of the charge
transfer complex salts TEAP and DMFP was tested using the Kurtz
and Perry method [18]. The grown single crystals of TEAP, DMFP
and KDP were ground into fine powdered sample to a particle size
of 150 lm and then filled into the micro capillary tube. Then high-
intensity Nd:YAG laser (k = 1064 nm) with a pulse duration of
10 ns was passed through the micro capillary tube. The emission
of bright green radiation (k = 532 nm) from the crystals confirm
the generation of second harmonics. The second harmonic signal
of 30 mV for TEAP and 13 mV for DMFP was obtained for an input
energy of 5.3 mJ/pulse. For KDP crystal the SHG value gives a signal
of 18.5 mV/pulse for the same input energy. Thus, it is observed
Acknowledgements
One of the authors, MR. Manikandan gratefully acknowledges
the DST-PURSE, India for providing SRF to carry out this work.
The authors acknowledge Prof. C. Sekar, Head, Department of
Bioelectronics and Biosensors, Alagappa University, Karaikudi for
his valuable suggestions.

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183
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