CHAPTER - IV SYNTHESIS, GROWTH AND CHARACTERIZATION OF PICRATE SINGLE CRYSTALS

CHAPTER - IV SYNTHESIS, GROWTH AND CHARACTERIZATION OF PICRATE SINGLE CRYSTALS 4.1 INTRODUCTION E 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 secondorder 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

P a g e 110 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].

P a g e 111 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,Ndimethyl 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].

P a g e 112 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

P a g e 113 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].

P a g e 114 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.

P a g e 115 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.

P a g e 116 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.

P a g e 117 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.

P a g e 118 Table 4.1 Elemental composition of the TEAP material Elements Theoretical TEAP 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 Å 3. 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

P a g e 119 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

P a g e 120 Table 4.2 Lattice parameter values of TEAP crystal Sample a (Å) b (Å) c (Å) β Cell Volume (Å 3 ) 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).

P a g e 121 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

P a g e 122 Vibration of NO 2 group The asymmetric vibration of NO 2 group is observed at 1553 cm -1 for TEAP. The absorption at 1331 cm -1 is due to the NO 2 symmetric vibration of TEAP. Usually for the free picric acid NO 2 vibration occurs at 1607 cm -1 [37]. Charge transfer interaction in the complex salt TEAP, NO 2 vibration is shifted to lower frequency at 1553 cm -1 due to the increased electron density of the picric acid. The rocking modes of NO 2 group are identified at 529 cm -1 for the TEAP. The NO 2 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-NO 2 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 CH 3 group Internal vibration of the cations in the TEAP arises from the functional group of CH 3. 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.

P a g e 123 Vibration of CH 2 group The CH 2 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 NO 2. The peak at 1271.09 cm -1 confirms C-O vibration of the crystal. C-NO 2 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 NO 2 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.

P a g e 124 Table 4.3 Frequency assignments of TEAP Vibration TEAP (cm -1 ) Assignments N + - H group Intermolecular hydrogen bonding N + -H asymmetric 3408 Stretching 1629 R 2 N + - H deformation mode 714 Bending of N + - H 1553 Asymmetric stretching vibration of NO 2 group 1331 Symmetric stretching vibration of NO 2 group NO 2 group 787 Scissoring of NO 2 529 Rocking of NO 2 913 Stretching vibration of C- NO 2 Phenolic group 1156 Phenolic O vibration Phenoxcy group 1271 C-O vibration 2746 Symmetric C-H stretching vibration of methyl group Methyl group 1497 Asymmetric C-H deformation of methyl group 1073 CH 3 rocking 3031 Asymmetric stretching vibration of methyl group Ethyl group 1443 CH 2 deformation

P a g e 125 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 NO 2 1271 C-O vibration 1141 Phenolic O vibration 1022 CH 3 rocking 915 Stretching of C-NO 2 753 Bending N + -H 541 Rocking of NO 2

P a g e 126 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

P a g e 127 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.

P a g e 128 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

P a g e 129 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 (ε r ) was calculated. Frequency dependence of dielectric constant (ε r ) and dielectric loss of TEAP crystals at different temperatures are shown in figure 4.10 and 4.11 respectively. Both the dielectric constant (ε r ) 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 (ε r ) and D.

P a g e 130 Figure 4.10 Frequency dependence of dielectric constant of TEAP crystal Figure 4.11 Frequency dependence of dielectric loss of TEAP crystal

P a g e 131 Continuous gradual decrease in D as well as (ε r ) suggests that TEAP crystal is like any normal dielectric, may have domains of different sizes and varying relaxation times. The high value of (ε r ) 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.

P a g e 132 Figure 4.12 TG and DTA spectra of TEAP

P a g e 133 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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