CHAPTER - IV - Shodhgangashodhganga.inflibnet.ac.in/bitstream/10603/35211/12/12_chapter 4.pdf ·...

CHAPTER - IV

SYNTHESIS, GROWTH AND CHARACTERIZATION OF PICRATE

SINGLE CRYSTALS

4.1 INTRODUCTION

ngineering of new nonlinear optical (NLO) materials, structures and

devices with enhanced figure of merit has developed over the last two

decades as a major force to help drive nonlinear optics from the laboratory to real

applications. Because of their potential applications in photonic devices, the NLO

properties of molecules and their hyperpolarizabilities have become an important area

of 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 to exhibit good nonlinear optical properties [5]. The

hyperpolarizability can be enhanced by increasing intramolecular charge transfer

interaction by extending π-conjugated system [6]. The term charge transfer gives a

certain type of complex resulting from interactions of donor and acceptor with the

formation of weak bonds [7, 8].

Molecular complexes in which extensive charge transfer interactions between

electron donors and acceptors molecules are generally expected to have high NLO

properties. Picric acid forms crystalline picrates of various organic molecules through

ionic and hydrogen bonding and π-π interactions. Bonding of electron donor/acceptor

E

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CHAPTER-IV | Synthesis, Growth and Characterization of Picrate Single Crystals

picric acid molecules strongly depends on the nature of the partners. Picric acid

derivatives are interesting candidates, as the presence of phenolic OH favors the

formation of salts with various organic bases. The conjugated base, picrate formed has

increased molecular hyperpolarizability because of the proton transfer.

In the present investigation, we attempted to synthesis the molecular complex

adduct of triethylamine with picric acid involving charge transfer from donor to

acceptor followed by proton transfer from the acceptor. The solubility of complex

salts in methanol has been determined gravimetrically. Single crystals were grown by

low temperature solution growth technique. Material formation and purity was

confirmed by CHN analysis. The structural properties of the grown crystals were

characterized by single crystal and powder X-ray diffraction techniques. Fourier

transform infrared spectroscopic analysis, UV-Vis-NIR analysis, TG/DTA and second

harmonic generation measurements were also carried out.

4.2 REVIEW OF LITERATURE

Graham Smith et al (2004) have reported that the monoclinic polymorph of

anilinium picrate shows a three dimensional hydrogen bonded polymer with strong

primary interspecies interactions involving the proximal phenolate and adjacent nitro

group O-atom acceptors and separate anilinium H-atom donors in two cyclic

associations [9]. P.Srinivasan et al (2006) have been grown good quality single

crystals of L-asparaginium picrate by a low temperature solution growth technique

[6].

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A comparative study of infrared and Raman spectra of DL-valine DL-valinium

picrate (DL-VVP) and DL-methionine DL-methioninium picrate (DL-MMP) at the

room temperature in 4000 - 50 cm−1 range helps to determine the effect of hydrogen

bonds in these crystals. The existence of the zwitterion and the protonated form in

both the crystals have been observed by M.Briget Mary et al (2006) [10].

A.Chandramohan et al (2007) have synthesized the crystalline substance of N,N-

dimethyl anilinium picrate (DMAP) and the single crystals were grown by slow

evaporation solution growth technique at room temperature [11].

R.Bharathikannan et al (2008) have been investigated the charge transfer

complex adduct of 2 nitro aniline with picric acid. The needle shaped crystals were

grown by slow evaporation method [12]. A.Chandramohan et al (2008) have been

synthesized the acenaphthene picrate material and single crystals were grown and

fundamental studies were characterized [13]. T.Uma Devi et al (2008) have

investigated the synthesis of glycine picrate material and have grown single crystals.

The cell parameters and functional groups present in the material were studied [14].

A.Chandramohan et al (2008) reported the synthesis, growth and

characterization of caffeinium picrate (CAFP) material [15]. P.Srinivasan et al

(2008) reported the Z scan determination of the asparaginium picrate crystals. The

magnitude of third order susceptibility and non-linear refractive index were also

determined [16]. S.A.Martin Britto Dhas et al (2008) characterized the vallinium

picrate single crystals. Structural, functional and mechanical studies were performed

for the grown crystals [17].

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P.Srinivasan et al (2008) reported the synthesis, growth, crystal structure

determination, and hyperpolarizability studies of L-argininium-4-nitro phenolate

monohydrate (LARP) single crystals. First order hyperpolarizabilty of LARP has been

computed using density functional theory [18]. T.Uma Devi et al (2008) have

investigated the synthesis and growth of prolinium picrate crystals. The cell

dimensions were obtained by single crystal X-ray diffraction study. FTIR, UV-Vis-

NIR and fluorescence spectral analyses were carried out for the grown crystals.

Thermo gravimetric study was also carried out to determine the thermal properties of

the grown crystal [19].

A.Chandramohan et al (2008) have synthesized the crystalline substance of

naphthalene picrate (NP) and single crystals were grown using slow evaporation

solution growth technique [20]. G.Anandha Babu et al (2010) have reported the

growth of single crystal of dimethylammonium picrate (DMAP). High resolution X -

ray diffraction study was carried out for the grown crystals. The optical, dielectric and

mechanical studies were also performed [21]. S.Natarajan et al (2010) have been

grown the organic nonlinear optical (NLO) crystal from the amino acid family, viz., l-

threoninium picrate (LTHP) by solvent evaporation technique from aqueous solution

[22].

B.Dhanalakshmi et al (2010) have been grown the bulk single crystals of L-

histidine-4-nitrophenolate 4-nitrophenol (LHPP) using slow evaporation solution

growth technique at room temperature [23]. M.Magesh et al (2011) have grown

single crystal of dimethyl ammonium picrate (DMAP) by slow evaporation solution

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technique (SEST) and subsequently by Sankaranarayanan Ramasamy (SR) method

using acetone as solvent [24]. G.Anandha Babu et al (2011) reported the growth of

1,3-dimethylurea dimethylammonium picrate crystal. The crystal structure of the

grown material has been determined [25].

S.Anandhi et al (2011) have reported the synthesis and growth of organic

crystal of imidazolium picrate (IP) by the slow cooling solution growth method using

ethanol and acetone as solvents. The structural, thermal, optical and mechanical

properties were studied for the grown crystal [26]. A.Antony Joseph et al (2011)

have grown glycine mixed L-valine picrate (GVP) from saturated aqueous solution by

slow evaporation method [27]. S.Gowri et al (2011) have investigated the spectral,

thermal and optical properties of L-tryptophanium picrate [28]. K.Muthu et al (2011)

have studied the proton transfer complex of 2,4,6-trinitrophenol as an electron

acceptor with p-toluidine as electron donor [29].

G.Bhagavannarayana et al (2011) have grown the single crystals of L-

leucine L-leucinium picrate (LLLLP) by the slow evaporation solution technique and

fundamental characterizations were analysed [30]. S.Gowri et al (2012) have reported

the synthesis and growth of adenosinium picrate crystals by solution growth

technique. Fundamental characterizations of the grown crystals have been studied

[31]. Mohd.Shkir et al (2012) have studied the growth, spectroscopic, relative second

harmonic generation (SHG) efficiency and thermal analysis of 2-aminopyridinium

picrate (2APP) [32]. T.Chen et al (2012) have grown the good-quality single crystals

of L-histidinium-4-nitrophenolate 4-nitrophenol (LHPP) by slow cooling method [33].

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N.Sudharsana et al (2012) have been determined the centrosymmetric crystal

structure of hydroxyethylammonium picrate (HEAP) crystals by single crystal X-ray

diffraction analysis [34]. Preparation, crystal growth and molecular structure as well

as vibrational spectra of the crystal L-alanine L-alaninium picrate monohydrate were

described by V.V.Ghazaryan et al (2012) [35].

4.3 EXPERIMENTAL DETAILS

4.3.1 Material synthesis

Analar grade of picric acid, triethylamine and methanol were used for the

synthesis process. The picric acid is less soluble in water. For the synthesis of

complex salt triethylamine picrate (TEAP), equimolar quantities of the parent

compounds picric 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

necessarily protonates the amino group of the triethylamine resulting in the formation

of the yellow colored precipitation of the charge transfer complex salt TEAP. The

yellow precipitation of the complex salt was filtered off and then further purified

using methanol by recrystallization process. The purified material was then used as a

raw material for the growth process. The reaction involved in the synthesis process is

illustrated in figure 4.1.

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Figure 4.1 Synthesis Scheme of TEAP material

4.3.2 Solubility measurement

The equilibrium solubility and its temperature 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 in methanol was assessed as a function

of temperature in the temperature range 30 - 45 °C in steps of 5 °C. Synthesized

TEAP salt was dissolved in methanol in an airtight container and kept in the constant

temperature bath (CTB). On reaching saturation, the equilibrium concentration of the

solute was determined by gravimetric method. Figure 4.2 shows the solubility curve

of TEAP. We infer that the TEAP exhibits good solubility compared to other organic

materials and a positive solubility temperature gradient (direct solubility) in methanol

solvent. Hence this material is appropriate for bulk crystal growth by solution growth

method.

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Figure 4.2 Solubility curve of TEAP in methanol

4.3.3 Crystal growth

Saturated solutions of TEAP in methanol at 40 °C were prepared in accordance

with the determined solubility data using the recrystallized salt and then stirred for

half an hour for the homogenous solution mixture. The solution was filtered by

Whatman filter sheet in order to remove the suspended impurities from the solution.

The filtered solution was transferred into 100 ml vessel having uniform perforated

closure and this vessel was placed in a constant temperature bath (CTB) having a

controlling accuracy of ± 0.02 °C for solvent evaporation. By employing solvent

evaporation method, the nucleated crystals were allowed to grow for a definite period

and then harvested from the mother solution. The grown single crystal of TEAP from

methanol is shown in figure 4.3.

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Figure 4.3 As grown TEAP single crystal

4.4 CHARACTERIZATION OF TEAP CRYSTAL

4.4.1 CHNS analysis

The purity and percentage compositions of the constituent elements present in

the synthesized compound were examined by CHN analysis using Elemental vario

micro CHNS analyzer. The percentage of elements present in the synthesized TEAP

material is given in table 4.1. It shows that the C, H and N values are fairly in good

agreement with the theoretically calculated values. The result further indicates that

TEAP is free from impurities and devoid of the water molecules in any form. From

the result, the stoichiometry and hence the molecular formula of the synthesized

material is confirmed.

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Table 4.1 Elemental composition of the TEAP material

TEAP

Elements

Theoretical Experimental

C 43.59 % 43.34 %

H 5.45 % 4.79 %

N 16.95 % 17.22 %

4.4.2 Single crystal X-ray diffraction (SXRD) analysis

Single crystal X-ray diffraction is by far the most popular method for the

identification of substances for the investigation of crystal structure and degree of

crystalline perfection. In this study, good quality crystal of TEAP was chosen and the

cell parameters of the grown crystal was estimated using MACH 3 Nonius CAD - 4 X-

ray diffractometer with Mo Kα radiation (λ = 0.7107 Å). From the results, TEAP

crystallizes into orthorhombic crystal system. Cell parameter values of TEAP

determined from the single crystal X-ray diffraction analysis are given as;

a = 6.947 Å, b = 20.735 Å, c = 21.941 Å and β = 90° and cell volume V = 3161 Å3.

4.4.3 Powder X-ray diffraction (PXRD) analysis

Powder X-ray diffraction studies were also carried out for the complex salt

TEAP to demonstrate the crystallinity using PANalytical model X’PERT PRO X-ray

diffractometer system. The Kα radiations from a copper target (λ = 1.5406 Ǻ) was

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used. The single crystals of TEAP were ground into fine powder and then powdered

samples were spread over a square centimeter area and placed in a beam of

monochromatic X-rays. The mass of powder was rotated about all possible axes. From

the θ value for each peak, the spacing d was obtained. The diffraction peaks were

indexed by least square fitting and X-ray diffraction pattern of TEAP is shown in

figure 4.4. Appearance of sharp and strong peaks confirm the good crystalline nature

of the crystals. The lattice parameters of TEAP crystal were calculated theoretically

using the powder XRD data (table 4.2) and it is in good agreement with the values

obtained from single crystal XRD.

Figure 4.4 Powder X-ray diffraction pattern of TEAP

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Table 4.2 Lattice parameter values of TEAP crystal

Sample a (Å) b (Å) c (Å) β Cell Volume (Å3)

TEAP

(SXRD) 6.947 20.735 21.941 90° 3161

TEAP

(PXRD) 6.894 20.797 21.973 89° 3160

4.4.4 Fourier transform infrared (FTIR) spectral analysis

Infrared spectroscopy is used to identify the functional groups and modes of

vibration of the synthesized complex salts. In charge transfer complex, a proton

transfer from donor to the acceptor is expected to take place which is strongly

supported by the appearance of a new band of medium intensity in the spectrum of

TEAP. 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, the FTIR

spectrum was recorded using a Thermo Nicolet 380 FTIR spectrometer by the KBr

pellet technique in the range of 400 - 4000 cm-1. The FT-IR spectrum of TEAP is

shown in Figure 4.5. The bands observed in the spectra of the complex salt TEAP

arises from the internal vibrations of the picric acid (comprises nitro group vibration

and OH vibration), triethylamine (encompass the methyl group and ethyl group

vibrations).

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Vibration of N+- H group

In TEAP, the amino N atom of triethylamine 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 [36]. This

intermolecular hydrogen bonding exist in the charge transfer complex is expected at

around 3400 cm-1. The peak observed at around 3408 cm-1 is the evidence for

hydrogen bonding in the TEAP. In the IR spectrum of TEAP, asymmetric N+-H

deformation modes are observed at 1629 cm-1 and the bending vibrations of N-H are

found at 714 cm-1 respectively.

Figure 4.5 FTIR spectrum of TEAP

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Vibration of NO2 group

The asymmetric vibration of NO2 group is observed at 1553 cm-1 for TEAP.

The absorption at 1331 cm-1 is due to the NO2 symmetric vibration of TEAP. Usually

for the free picric acid NO2 vibration occurs at 1607 cm-1 [37]. Charge transfer

interaction in the complex salt TEAP, NO2 vibration is shifted to lower frequency at

1553 cm-1 due to the increased electron density of the picric acid. The rocking modes

of NO2 group are identified at 529 cm-1 for the TEAP. The NO2 scissoring vibrational

modes appear in the spectra of TEAP at 787 cm-1. The band observed at 913 cm-1 is

due to the C-NO2 stretching vibration in TEAP.

Vibration of Phenolic and Phenoxcy groups

In the charge transfer interaction of TEAP, picric acid necessarily protonates

the phenolic O vibration which produces peaks at 1156 cm-1 [38]. The absorption peak

at 1271 cm-1 can be ascribed to the C-O vibration of the TEAP complex salt.

Vibration of CH3 group

Internal vibration of the cations in the TEAP arises from the functional group

of CH3. The peak at 3031 cm-1 for TEAP is attributed to asymmetric stretching

vibration of C-H in methyl group. The symmetric stretching vibration of C-H in the

methyl group is observed at 2746 cm-1 for TEAP. The asymmetric C-H deformation of

methyl group occurs near 1497 cm-1 for TEAP. The peaks at 1073 cm-1 and 1082 cm-1

indicate the rocking vibrations of methyl group.

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Vibration of CH2 group

The CH2 deformation mode in TEAP appears at 1443 cm-1. The

observed vibrational frequencies and their corresponding assignments are presented in

table 4.3.

4.4.5. Laser Raman study

The grown single crystal of TEAP was subjected to laser Raman spectral study

using a laser Raman Spectrometer model (R3000) with 532 nm as the operating

source in the region 3500 - 400 cm-1. The recorded laser Raman spectrum is shown in

figure 4.6. The sharp and broad peaks obtained are due to hydrogen bonding. The

peak at 1356 cm-1 is assigned to symmetric stretching of NO2. The peak at

1271.09 cm-1 confirms C-O vibration of the crystal. C-NO2 stretching is assigned at

915.99 cm-1. The peaks at 753.64 and 541.96 cm-1 are attributed to N+-H bending and

NO2 rocking respectively. The absorption peak obtained at 1156 cm-1 in the spectrum

representing phenolic ‘O’ vibration of the crystal. The presence of this band in both

FTIR and Raman spectra confirms the formation of TEAP salt. The laser Raman

spectral assignments are given in table 4.4.

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Table 4.3 Frequency assignments of TEAP

Vibration TEAP

(cm-1)

Assignments

3408 Intermolecular hydrogen bonding N+-H asymmetric

Stretching

1629 R2 N+- H deformation mode

N+- H group

714 Bending of N+- H

1553 Asymmetric stretching vibration of NO2 group

1331 Symmetric stretching vibration of NO2 group

787 Scissoring of NO2

529 Rocking of NO2

NO2 group

913 Stretching vibration of C- NO2

Phenolic group 1156 Phenolic O vibration

Phenoxcy

group 1271 C-O vibration

2746 Symmetric C-H stretching vibration of methyl group

1497 Asymmetric C-H deformation of methyl group

1073 CH3 rocking Methyl group

3031 Asymmetric stretching vibration of methyl group

Ethyl group 1443 CH2 deformation

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Figure 4.6 Laser Raman spectrum of TEAP

Table 4.4 Assignments of Raman spectrum of TEAP

Raman (cm-1) Assignments

1356 Symmetric stretching of NO2

1271 C-O vibration

1141 Phenolic O vibration

1022 CH3 rocking

915 Stretching of C-NO2

753 Bending N+-H

541 Rocking of NO2

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4.4.6 UV-Vis-NIR analysis

The optical absorption spectrum of TEAP was recorded in the range 200 - 1100

nm which is shown in figure 4.7. Strong absorption was observed at 362 nm for

TEAP, which is attributed to π- π* transition of picrate ion. It is seen from the

spectrum that the crystal is transparent in the range 450 to 1100 nm without any

intermediate absorption peak due to the charge transfer of the electron from the donor

to the acceptor. This is an essential parameter for NLO crystals and can be used as

SHG material in the visible range.

Figure 4.7 Absorption spectrum of TEAP

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Figure 4.8 Transmittance spectrum of TEAP

To determine the transmission range, the UV-Vis transmittance spectrum was

recorded for the grown crystal in the range 200 - 1100 nm (Figure 4.8). The UV

cutoff wavelength of TEAP was observed at 361 nm. This spectrum again confirms

the suitability of this crystal for optoelectronic applications and second- order

harmonic generation of the Nd:YAG laser (1064 nm). In order to determine the band

gap of the grown crystals, extrapolation of the straight line in the plot of (αhν) 2 versus

hν, has been done for TEAP (Figure 4.9) where α is the absorption co efficient and hν

is the photon energy. The band gap energy of TEAP was calculated as 3.55 eV.

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Figure 4.9 Plot of energy versus (αhν)2 for TEAP

4.4.7 Second harmonic generation measurement

The relative second harmonic generation behaviour of the charge transfer

complex salt TEAP was tested using the Kurtz and Perry method [39]. The grown

single crystal of TEAP was grounded into fine powder with uniform particle size and

then filled into the micro capillary tube. Then high-intensity Nd:YAG laser (λ =1064

nm) with a pulse duration of 10 ns was passed through the micro capillary tube. The

emission of bright green radiation (λ = 532 nm) from the samples confirm the

generation of second harmonics. The second harmonic signal of 30 mV for TEAP was

obtained for an input energy of 5.3 mJ/pulse. The SHG value of reference KDP

samples gives a signal of 18.5 mV/pulse for the same input energy. Thus, it is

observed that the SHG efficiency of the title compound TEAP was 1.62 times than

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that of the standard KDP crystal. The extent of charge transfer across the NLO

chromophore determines 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 response [40, 41].

4.4.8 Dielectric study

In order to carry out the dielectric measurements, carefully selected samples of

TEAP single crystal were cut and later polished to obtain a good surface finish.

Dielectric study was carried out from 35-50 °C at different frequencies range from

100 Hz to 100 kHz. The capacitance and the dielectric loss were measured at different

temperatures for TEAP crystal and then subsequently the dielectric constant (εr) was

calculated. Frequency dependence of dielectric constant (εr) and dielectric loss of

TEAP crystals at different temperatures are shown in figure 4.10 and 4.11

respectively.

Both the dielectric constant (εr) and the dielectric loss (D), are inversely

proportional to the frequency. This can be understood on the basis that the mechanism

of polarization was similar to that of the conduction process. The electronic exchange

of number of ions in the crystal give local displacement of electrons in the direction of

the applied field, which in turn gives rise to polarization. As the frequency increases, a

point will be reached where the space charge cannot sustain and comply with the

external field and hence the polarization decreases, giving rise to diminishing values

of (εr) and D.

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Figure 4.10 Frequency dependence of dielectric constant of TEAP crystal

Figure 4.11 Frequency dependence of dielectric loss of TEAP crystal

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Continuous gradual decrease in D as well as (εr) suggests that TEAP crystal is

like any normal dielectric, may have domains of different sizes and varying relaxation

times. The high value of (εr) at lower frequencies may be due to the presence of all the

four polarizations, namely space charge, orientational, electronic and ionic

polarizations and its low value at higher frequencies may be due to the loss of

significance of these polarizations gradually. The low value of dielectric loss with

high frequency for these samples suggests that the samples possess enhanced optical

quality with lesser defects and this parameter is of vital importance.

4.4.9 Thermal analysis

Thermal stability and physiochemical changes of the grown TEAP crystal has

been identified in powder form by recording TG/DTA curve in the temperature range

0 and 600 °C using NETZSCH STA 449 F3 analyzer under nitrogen atmosphere at a

rate of 10 °C/min. Figure 4.12 shows the thermal properties of the TEAP crystal

carried out by TG/DTA. In the differential thermogram, sharp exothermic peak was

found at 155.3 °C. This exothermic is assigned to the melting point at which no

weight loss from TG has been noticed. The sharp endothermic reaction observed at

around 252.3 °C may be possibly due to some complex formation. There is steep loss

of weight starting around 252.3 °C and after complex formation the weight loss is

gradually decreased.

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Figure 4.12 TG and DTA spectra of TEAP

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4.5 CONCLUSION

The organic charge transfer molecular complex salt triethylaminium picrate

was synthesized and purified by recrystallization process using methanol. Solubility of

TEAP in methanol was determined by gravimetric method. The single crystals of

TEAP were grown by slow evaporation method using methanol as a solvent.

Elemental analysis data confirm the purity, stoichiometry and molecular formula of

TEAP crystal. As grown single crystal of TEAP was characterized by single crystal

X-ray diffractogram, which reveals that TEAP crystallizes into orthorhombic crystal

system. From the powder XRD pattern the various planes of reflections have been

identified and reconfirmed the lattice parameters and crystal system of TEAP. FTIR

and laser Raman spectral studies established the molecular structure of TEAP and also

bring forth the evidence for the prevalent charge transfer activity in the complex salt.

The UV-Vis-NIR spectrum of TEAP in solution mode exhibits a wide transparency in

the visible region between 450 and 1100 nm due to the π- π* transition of picrate ion

in the complex salt. The band gap energy of TEAP was estimated from the UV -Vis

spectrum. The relative SHG activity in the complex salt was confirmed by employing

Kurtz and Perry method. The result reveals that SHG efficiency of TEAP is 1.62 times

greater than that of KDP. Dielectric constant and loss of TEAP decreases with

increase in frequency. The very high value of dielectric constant at lower frequencies

may be due to the presence of all the four polarizations and its low value at higher

frequencies may be due to the loss of these polarizations gradually. Thermal stability

of the grown TEAP was confirmed by TGA and DTA analyses.

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