GROWTH, THERMAL, OPTICAL, VIBRATIONAL PROPERTIES AND HYPERPOLARIZABILITY OF PICRATES : ORTHONITROANILINE WITH PICRIC ACID AND IMIDAZOLIUM PICRATE

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1 39 CHAPTER 2 GROWTH, THERMAL, OPTICAL, VIBRATIONAL PROPERTIES AND HYPERPOLARIZABILITY OF PICRATES : ORTHONITROANILINE WITH PICRIC ACID AND IMIDAZOLIUM PICRATE 2.1 INTRODUCTION Nonlinear optics plays a major role in the field of photonics, comprising fiber optic communication, optical computing, data storage and optical switching etc. Some organic crystals are highly polar, which form noncentrosymmetric crystal structure. The novel functional organic material can replace the traditionally used materials and also bring out some new applications (Zyss and Oudar 1982, Chemla and Zyss 1987 and Oudar and Hierle 1977). Organic materials are molecular materials that consist of chemically bonded molecular units interacting in the bulk media through weak van der Waals interactions. L-Asparaginium picrate (Srinivasan et al 2006), L-Valinium picrate (Anitha et al 2004) and L-Prolinium picrate (Uma Devi et al 2008) prove to be good candidates for nonlinear optical applications Picric Acid Picric acid (whose chemical name is 2,4,6-trinitrophenol) is a water soluble chemical that is highly sensitive to heat and shock. Picric acid complexes are formed with aromatic amines having electron withdrawing

2 40 substitutions. Picric acid (PA) is a strong organic acid, attacks common metals (except tin and aluminium), creates explosive salts. Usually aromatic amines are less basic, lone pair of electron on nitrogen either donate electrons or accept protons from others (Srikrishnan et al 1980 and Srinivasan et al 2006) Hydrogen Bonding in Picrates A hydrogen bond is the attractive force that arises between the donor covalent pair D-H in which a hydrogen atom H is bound to a more electronegative atom D (donor), and other non-covalently bound nearest neighbour electronegative (acceptor) atom. An important feature of hydrogen bond is that they are highly directional. The strongest hydrogen bonds are those in which donor, hydrogen and acceptor atoms are collinear. During the formation of salts and complexes with PA, intermolecular and intramolecular hydrogen bonds like long and short hydrogen bonds N - H...O, C - H...O have been formed. It is reported that due to hydrogen bonding, several sub maxima have been formed in the region cm -1 in the spectra of picrates. As expected, the hydrogen bonding vibrations appear in the above mentioned frequency region. The bands centered at around 3435 cm -1 in the spectra of picrate salts could be attributed to O...H hydrogen bond stretching Orthonitroaniline - Picric Acid The complex crystallizes in the monoclinic crystal system with a space group Cc (Saminathan and Sivakumar 2007). The orthonitroaniline acts as and n-donor. The possession of intramolecular and intermolecular hydrogen bonds link in the material makes it an eligible material for further studies (Durbin and Feher (1991), Koshima Wang et al (1996) and Algra et al (2005). Nitro compounds are strongly basic and easily polarizable with

3 41 internal charge transfer between electron withdrawing nitro groups and electron donating amino groups (Margaret C. Etter 1990) Imidazolium Picrate The strong N...H...N hydrogen bond which is formed in the crystal structure of imidazole is of interest due to proton migrations. Imidazole chemistry currently attracts considerable attention, where the imidazole derivatives are widely applied as N-ligands coordinating transition metals. In the imidazolium phosphate (Robert H. Blessing and Edward McGandy 1972), imidazolium moiety acts as a mediating proton transfer reaction. The thermal and luminescence properties of many imidazole derivatives have been reported. The application of imidazoles in medicinal chemistry or chemistry of natural products/alkaloids or of 1,3-disubstituted imidazole salts as ionic liquids (Soriano-García et al 1990 and Herbstein and Kapon 1991). The results on growth and characterization are presented in detail. 2.2 GROWTH AND CHARACTERIZATION OF A COMPLEX ORTHONITROANILINE - PICRIC ACID The title compound was synthesized by reacting orthonitroaniline and picric acid (ONAP) in the ratio 2:1 in ethanol. The reaction scheme involved in the formation of complex compound is 2 C 6 H 6 N 2 O 2 + C 6 H 2 (NO 2 ) 3 OH (2[C 6 H 6 N 2 O 2 ].C 6 H 2 (NO 2 ) 3 OH) The synthesized complex was purified by repeated recrystallization. Solubility test was performed by dissolving the purified salt in 100 ml of ethanol from 30 to 42C in steps of 3C. The solubility at different temperatures is shown in Figure 2.1. From the solubility diagram it is found

4 42 that the solubility of the orthonitroaniline-picric acid (ONAP) increases on increasing the temperature. Figure 2.1 Solubility curve of ONAP The growth solution was prepared in accordance with the solubility data. The solute was dissolved in 100 ml of ethanol, stirred for 35 minutes and filtered in a 250 ml beaker. The solution is optimally closed and kept in a constant temperature bath with a control accuracy of ± 0.01C. Good quality single crystals were harvested in a period of 11 days. The obtained bright red colored single crystal of the complex ( mm 3 ) is shown in Figure 2.2 (a). The molecular structure of ONAP is shown in Figure 2.2 (b). The picric acid molecule has three nitro groups attached to the phenyl ring, with an intramolecular hydrogen bond. Intramolecular hydrogen bonding is formed by the adjacent hydroxyl and nitro groups (Hong Zhang et al 2008 and Bharathikannan et al 2008). The effect of hydrogen bonding between the

5 43 phenolate and nitro group enhances the hyperpolarizability value, which is the basic requirement for a system to exhibit nonlinear optical property. Figure 2.2 (a) As grown single crystal using ethanol as solvent (b) Molecular structure of the grown complex (Saminathan and Sivakumar 2007) In this context the complex formation of ONAP, three intramolecular hydrogen bonds found in the complex in addition to that intermolecular N-H-O and C-H-O hydrogen bonds link the molecules of adjacent columns as per the report of the structure of Saminathan and Sivakumar et al (2007). The effect of hydrogen bonding between the picric acid and aniline group tends to form a complex.

6 X-RAY DIFFRACTION STUDIES The grown crystal of ONAP was subjected to single crystal XRD analysis with MoK ( = Å) radiation and it is confirmed that ONAP crystallizes in the monoclinic crystal system with space group Cc. The obtained lattice parameters are tabulated in Table 2.1 and are found to be in good agreement with the values reported in the literature (Saminathan and Sivakumar 2007). Table 2.1 Single crystal XRD data of ONAP Lattice parameters Present work (Saminathan and Sivakumar 2007) a (3) Å (17) Å b (7) Å (19) Å c (2) Å (15) Å (7) (11) 2.4 HIGH RESOLUTION X-RAY DIFFRACTION ANALYSIS The High Resolution X-ray diffraction curve was recorded for a typical single crystal specimen of ONAP using (110) diffracting planes in symmetrical Bragg geometry by employing the multicrystal X-ray diffractometer with MoK 1 radiation. The solid line (convoluted curve) is well fitted with the experimental points represented by the filled circles. On deconvolution of the diffraction curve, it is clear that the curve contains an additional peak, which is 182 arc sec away from the main peak. This

7 45 additional peak as in Figure 2.3 depicts an internal structural low angle boundary whose tilt angle is 182 arc sec. For both the main peak and the low angle boundary the FWHM (full width at half maximum) value is 264 arc sec. Though the specimen contains a low angle boundary, the relatively low angular spread of around 800 arc sec of the diffraction curve and the low FWHM values show that the crystalline perfection is reasonably good. Thermal fluctuations or mechanical disturbances during the growth process could be responsible for the observed low angle boundary. Figure 2.3 High Resolution X-ray diffraction curve recorded for the single crystal in (110) diffracting plane 2.5 VIBRATIONAL SPECTRAL ANALYSIS Vibrational spectral analysis is an important tool to understand the chemical bonding and provides useful information about the properties of materials. It also provides evidence for the presence of reacting species in the synthesized compound. Vibrational analysis gives the details of the presence

8 46 of O-H, N-H, NH 2, NO 2, CC groups. The classification of total fundamental modes predicts 294 internal modes and 9 external modes such as 3 Translational and 6 Rotational. From the correlation scheme given by Fateley et al (1972), each internal modes split into two components (A and A) FT-IR spectrum Spectroscopic methods were used to elucidate the functional groups of the complex. The FT-IR spectrum (Figure 2.4) of ONAP was recorded in the range cm -1. The sample was mixed with KBr in the ratio of 1:10 and subjected to IR radiation. Figure 2.4 shows the recorded FT-IR spectrum of ONAP and the assignments made are listed in Table 2.2. The peak at 3580 cm -1 is due to O-H stretching vibration of picric acid, NH 2 vibrations give their peaks at 3377 and 3486 cm -1. Aromatic ring C-H stretching vibrations occur at 3090 cm -1. The NH bending vibrations is assigned to the intense peak at 1620 cm -1. The aromatic ring skeletal vibrations occur at 1451 cm -1. The asymmetric stretch of NO 2 occurs at 1493 cm -1 and its symmetric stretching at 1346 cm -1. The phenolic C-O stretching occurs at 1248 cm -1. The aromatic ring C-H bending vibrations give peaks at 919 and 750 cm -1. Hence from this IR spectrum, it is established that picric acid proton is not transferred to the NH 2 group of orthonitroaniline. It is due to less basic nature of the NH 2 group of this compound. Since the nitro group at the second position is an electron withdrawing group. The amino group N 2 is not much available for protonation, in other words amino nitrogen lone pair is delocalized over the nitro group via the aromatic ring. Rejection of proton transfer of picric acid to orthonitroaniline is also verified from the ORTEP diagram shown in Figure 2.2 (b).

9 47 Figure 2.4 FT-IR spectrum of ONAP Raman Spectrum Raman spectrum was recorded using R3000 model Laser Raman spectrometer with the input source of 532 nm in the range of cm -1. The Raman spectrum obtained is shown in Figure 2.5. The observed wavenumber and the assignments made are listed in Table 2.2. The number of modes assigned from the experimentally recorded FT-IR and Raman spectra are less when compared to the assignments predicted by theoretical factor group analysis. The entire modes of vibrations can be obtained by polarized Raman spectrum.

10 48 Figure 2.5 Raman spectrum of ONAP Table 2.2 The vibrational assignments of FT-IR and Raman spectra - ONAP FT-IR (cm -1 ) Raman (cm -1 ) Assignments C-H bending C-H bending Phenolic C-O stretching NO 2 symmetric stretching NO 2 asymmetric stretching N-H bending NH 2 vibrations NH 2 vibrations O-H stretching

11 GROUP THEORETICAL ANALYSIS ONAP crystallizes in the monoclinic system with space group Cc ( c 4 s ). The factor group analysis of ONAP crystal is carried out using the character table for the point group C 1 (2). The primitive unit cell contains four molecules (Z = 4). The total possible irreducible modes of vibrations can be divided into two factor group species such as A and A. The species A and A are rich in dipole moment along Z, X and Y crystal axes. Hence they are active both in Raman and infrared. Table 2.3 gives the results of factor group analysis. Table 2.3 Results of factor group analysis A A Sl. No Factor group External modes 1 (i) Translational 1 2 (ii) Rotational Internal modes Total The factor group analysis was performed by following the procedure outlined by Rosseau et al (1981). The unit cell of 2[C 6 H 6 N 2 O 2 ].C 6 H 3 N 3 O 7 has 51 atoms hence, a total of 306 modes of vibration of which there exist 3 acoustical modes (2 A + A). Thus it has 303 internal modes of vibrations. The irreducible representation of the 303 internal modes can be classified as 303 = 151 A A (Table 2.4). The total external modes of vibrations are classified as rotational (3 A + 3 A) and translational (A + 2 A). Each irreducible representation splits into A (X,Y)

12 50 and A (Z) which are IR active and A (α xx, α yy, α zz, α xy ) and A (α xz, α yz ) are Raman active. The polarizability tensors are of the form 0 0 xx ' xy yy zz '' xz yz Table 2.4 Factor group analysis Summary Factor C 1 Site group symmetry Symmetry Ext Int C H N O Optical modes Acoustical modes Total C s A T, 3R A 2T, 3R Total 3T, 6R Internal Vibrations The presence of hydrogen bonding is often found in organic materials. The influence of hydrogen bonding is difficult to predict in organic materials. It may be intermolecular or intramolecular hydrogen bonding. The molecule is stacked in columns with the packing stabilized by N-H-O and C-H-O hydrogen bonds and π - π stacking interactions. The vibrations occur in the complex can be correlated with Table External Vibrations These modes are due to translational and rotational vibrations of the molecules. The low wavenumber bands of hydrogen bond are found to be

13 51 weak and asymmetric. The lattice modes are very intense in Raman spectrum when compared to other modes in the high wavenumber region. The rotational modes occur in the high frequency region when compared to translational modes. The possible external modes are tabulated in Table UV-VISIBLE SPECTRAL STUDY The study of optical absorption of a material is important for NLO applications. In order to find the suitability of this material for optical application, the absorbance spectrum was recorded. The lower cutoff occurs at 528 nm. Figure 2.6 (a) shows the absorbance spectrum of ONAP. The peak below 400 nm is attributed to the presence of -* transition. (αh) = A(h-E g ) n (2.1) where A is a constant and E g is the optical band gap. The exponent n has the value ½ for direct allowed transition, for direct forbidden transition n = 3/2, for indirect allowed transition n = 2 and finally for indirect forbidden transition n = 3. From Equation (2.1) it is clear that bandgap depends on the variation in the absorption coefficient. Figure 2.6 (b) shows the relation between the product of absorption coefficient and the incident photon energy (αh) 1/2 with the photon energy h at room temperature. The optical energy bandgap (E g ) of ONAP is estimated by extrapolation of the linear portion of the curve to a point (αh) 1/2 = 0. The optical bandgap of the crystal is found to be 1.9 ev from the plot shown in Figure 2.6 (b). The value of energy bandgap shows its suitability for photonic and optoelectronic applications (Ren et al 2000 and Kaid and Ashour 2007).

14 52 Figure 2.6 (a) UV-Visible spectrum (b) plot of (h) 1/2 with h It is also concluded that there is a red shift in its optical absorption edge, owing to internal transition between the nitro group and aniline group. 2.8 PHOTOLUMINESCENCE OF ONAP Photoluminescence (PL) measurements can be employed as a powerful and sensitive tool to study the effects of purification and contamination during crystal growth process. The sample was excited at 270 nm. The emission spectrum was recorded between nm. A broad emission band observed in the range of nm, shows the presence of green emission. The peak broadening suggested intermolecular interactions. The appearance of single absorption band illustrating a single excited state for fluorescence emission. It also suggested a large gap between energy states and excitation to any of the vibrational level of the first electronic excited state. After a vibrational cascade the excited species could reach the ground state vibrational level of the first electronic excited state and emit fluorescence. Figure 2.7 shows the recorded PL spectrum of ONAP. The absence of the additional peaks reveals the structural perfection of the grown single crystals.

15 53 Figure 2.7 Photoluminescence Spectrum of ONAP 2.9 THERMAL ANALYSIS Melting Point Measurement The melting point of ONAP is found to be 80 C using melting point apparatus (VEEGO VMP-PM model). The endothermic peak in the DTA curve at 80 C gives a sharp melting point of the material (Figure 2.8 (b)) TG/DTA Study Figure 2.8 (a, b) shows the TG/DTA trace recorded for the grown ONAP crystal with a heating rate of 20 C/min in the temperature range up to 500 C in N 2 atmosphere. The endothermic peak at C in the DTA trace (Figure 2.8 (b) shows the melting point. From this it is identified that there is no phase transition up to its melting point and this enables the suitability of

16 54 the crystal for NLO applications. From the TGA trace (Figure 2.8 (a)) it is noted that the absence of solvent (ethanol) entrapment during crystallization process. It is confirmed by the absence of weight loss at TGA curve. TGA infers that there is a single stage weight loss which is due to the liberation of volatile substances. There is a small hump around 324 C in DTA curve owing to the decomposition of the compound. Figure 2.8 (a) TG (b) DTA trace of ONAP 2.10 MECHANICAL PROPERTIES The microhardness studies were carried out on the (110) plane of the grown crystal for various loads ranging from 10 to 70 g. The diagonal lengths (d) of the indented impressions obtained for various loads were measured using a micrometer eyepiece. Indentations were made twice in each load and the average value of the diagonal lengths of the indentation mark in each trial was calculated. Figure 2.9 (a) shows the variation of hardness

17 55 number with the applied load. The Vickers hardness number H v is calculated using the Equation (1.7). From the plot it is found that on increasing the load the hardness increases which reveals that the crystal exhibits reverse indentation size effect (Mythili et al 2007). The hardness increases with increase of load up to 70 g and on further increasing the load the crystal cracks. The relationship between P and d is given in the Equation (2.2) P = Kd n (2.2) where P is the applied load, d is the diagonal length, n is the Meyer s index. Figure 2.9 Mechanical behaviour: (a) Dependence of hardness on load (b) Dependence of stiffness constant on load If n lies between 1 to 1.6 the material comes under hard material category and if n is more than 1.6 it is classified as soft material (Onitsch 1956). The value of n obtained for the complex is 3.8 which reveals that it is softer than its parent material picric acid (n = 2.07).

18 56 The elastic stiffness constant gives an idea about the nature of bonding between neighbouring atoms. This is the property of the material by virtue of which it can absorb maximum energy before fracture occurs. For various loads the stiffness constant is calculated using Wooster s empirical relation (Susmita Karan and Sen Gupta 2005) C 11 = H v 7/4 (2.3) Figure 2.9 (b). The variation of stiffness constant plotted with load is shown in 2.11 DIELECTRIC STUDIES The dielectric study was carried out at 33 C and 70 C. The (110) face of single crystal was cut into rectangular shape and well polished. The experiment was done as discussed in section Figure 2.10 (a) Dependence of dielectric permittivity with log frequency (b) Dependence of dielectric loss with log frequency

19 57 The sample was placed inside the dielectric cell. Figure 2.10 (a, b) shows the dependence of dielectric permittivity and dielectric loss with log frequency. The dielectric permittivity and the dielectric loss varies with increasing temperature. The large value of dielectric permittivity at low frequency is due to the presence of space charge polarization. The low value of dielectric loss reveals that the crystal has less defects. The a.c conductivity a.c is calculated using the Equation (1.9) Figure 2.11 shows the frequency dependence of conductivity. It is found that the conductivity increases with increase in frequency and temperature, owing to the increase in the density of states and the results obtained are similar to other organic materials (Srinivasan et al 2006 and Krishnamurthy et al 2000). The variation of capacitance with frequency is depicted in Figure It is observed that the capacitance decreases with increase in frequency and this is due to charge redistribution. The residuals present in ONAP act as mesoscopic capacitors that can acquire multiple charges of either sign. Figure 2.11 Dependence of conductivity with log frequency

20 58 At low frequency the residual charges more readily redistribute to the positive side of the applied field and become negatively charged, while the residues close to the negative side of the applied field become positively charged since the capacitance of the parallel plate capacitor is inversely proportional to the applied electric field. As the frequency increases the capacitance decreases and the charges no longer have time to rearrange in response to the applied voltage (Vasudevan et al 1998). By the use of Gaussian basis sets the calculation of electronic polarizability of the organic molecules at the self consistent field (SCF) level is tailored. The main goal is to reduce substantially the size of the particle without the loss in the calculation accuracy at the SCF level. Dipole polarizabilities are very important to understand the polarization of an electronic medium and naturally relate several molecular properties. In the present study the dipole polarizabiltiy was calculated using density functional theory (DFT) with some semi empirical models. Figure 2.12 Capacitance with log frequency

21 59 The dielectric permittivity is a second order tensor. The components for a monoclinic system are xx, yy, zz and xz (Soscun et al 2004). The calculated polarizability tensors are xx = 239, yy = 234, zz = 218 and xz = From the density functional theory with the local density approximation the four independent components of dielectric tensor can be determined (George Maroulis 2004). From the calculated dielectric tensor the values can be shown as xx > yy > zz > xz that is, in decreasing order which indicate the dielectric anisotropies of the grown crystal (Gracia et al 2006) THEORETICAL CALCULATION OF FIRST ORDER HYPERPOLARIZABILITY First order hyperpolarizability (β) was calculated by utilizing high accuracy density functional theory (DFT) using Gaussian 03. The designing of the system with inter and intra molecular hydrogen bonding between the ligands will lead to a very large value of β. The above criterion can be satisfied by a polarizable molecular system having an asymmetric charge distribution as mentioned in section (1.14) Computational Approach A positive value of β (i.e., when the hydrogen bond dipole moment is oriented antiparallel to the dipole moment) indicates that the applied field polarizes the hydrogen bond in the same direction as the electron donation along the hydrogen bond (Poornima et al 2003). The first order hyperpolarizability (β) for the 2:1 complex of orthonitroaniline and picric acid derived from DFT calculations are presented in Table 2.5. Theoretical values represent that β component is dominant in (yzz) direction. The obtained maximum β value indicates the displacement of charge cloud is more in that particular direction. β tot of this system is calculated using 6-31G(d) basis set

22 60 based on finite field approach. β tot is found to be esu. In Table 2.6 comparative data of some of the hyperpolarizability values are presented. Table 2.5 The hyperpolarizability value of ONAP β Components Value (esu) β xxx β xxy β xyy β yyy β zxx β xyz β zyy β xzz β yzz β zzz β tot (esu) Hyperpolarizability β(-2,, ) in esu Table 2.6 Comparative data of hyperpolarizability values β tot (esu) Organic species Reference L-Asparaginium picrate Srinivasan et al nitroaniline Ortho nitroaniline: Picric acid (2:1) Krishnakumar and Nagalakshmi 2008 Present work

23 ETCHING STUDIES OF ONAP It is essential to study the growth features of the grown single crystal. Chemical etching is the process used to observe growth hillocks, etch spirals, rectangular etch pits etc., on the crystal surface. In the present case acetone and ethanol were used as etchant. The surface of as grown ONAP crystal (Figure 2.13 (A)) was polished before etching studies. Etching was carried out at room temperature for 5 and 10 seconds on (110). The etched surface was dried by gently pressing them between the filter papers and crystal surface was photographed using an optical microscope (Leitz wetzler) in reflection mode. Well defined etch pits were observed for the etching time of 5 sec (Figure 2.13 (B). The striations indicate the controlled growth rates of the faces. Increasing of etching time to 10 sec yielded etch pits as shown in Figures 2.13 (B) and (D), respectively for acetone and ethanol etchant. The size of the etch pits was found to increase with the time of etching. However, Sangwal (1987) points out that they are produced when the supersaturation at some points on the growing surface of a crystal is higher than at other parts of it. Etch pits spread on the growing surfaces. Further, the pits get vanished from the location of the crystal face.

24 62 Figure 2.13 Etch pattern (A, B) Acetone as etchant, (C, D) Ethanol as etchant 2.14 GROWTH AND CHARACTERIZATION OF IMIDAZOLIUM PICRATE Synthesis and Growth of Imidazole - Picric Acid The imidazole ring bonds with picric acid molecule and forms a salt of Imidazolium picrate. The moieties are arranged in a zig-zag way. Imidazolium picrate was synthesized by dissolving equimolar quantities of imidazole and picric acid in 25 ml of acetone as shown in Figure The purity of the synthesized compound was improved by successive recrystallization process. The solubility was estimated in ethanol, methanol, chloroform and acetone. Among the four solvents used, acetone was found to

25 63 be the best solvent for the growth of Imidazolium picrate. The recrystallized salt of Imidazolium picrate was used as starting material to prepare the solution. The solution of Imidazolium picrate was prepared at 35 o C and the solution was optimally covered with perforated sheet and housed in the constant temperature bath (accuracy of 0.01 o C) with continuous stirring to ensure homogeneous temperature. The temperature was reduced by 0.3 o C per day. The ph of the solution was found to be in the acidic medium. Transparent single crystal of size mm 3 was harvested after a period of 25 days as shown in Figure 2.15 (a). The ethanol grown crystal of Imidazolium picrate is shown in Figure 2.15 (b). Table 2.7 shows the variation of morphology with solvent. Figure 2.14 Reaction scheme of Imidazolium picrate Table 2.7 Habit change with solvent Solvent Habit Visual quality a) Acetone plate good b) Ethanol plate good

26 64 Figure 2.15 Grown crystals of Imidazolium picrate (a) Acetone as a solvent (b) Ethanol as a solvent 2.15 RESULTS AND DISCUSSION Single Crystal X-ray Diffraction The grown single crystal was subjected to single crystal X-ray diffraction analysis. The obtained crystallographic data are given in Table 2.8. From the single crystal X-ray diffraction data it is confirmed that the grown crystal belongs to orthorhombic system with space group Pbca. The obtained lattice parameters are in good agreement with the reported values (Soriano-García et al 1990). Table 2.8 Crystallographic data of Imidazolium picrate Lattice constants a (Å) b (Å) c (Å) Present work (4) (8) (6) (Soriano-García et al 1990) (2) (5) (7)

27 Structural Perfection Analysis High Resolution X-ray Diffraction Figure 2.16 shows the High Resolution X-ray diffraction curve recorded for a typical Imidazolium picrate single crystal specimen using (100) diffracting planes in symmetrical Bragg geometry by employing the multicrystal X-ray diffractometer with MoK 1 radiation (Lal and Bhagavannarayana 1989). The solid line is well fitted with the experimental points represented by the filled circles. On deconvolution of the diffraction curve, it is clear that the curve contains an additional peak which is 250 arc sec away from the main peak. This additional peak corresponds to an internal structural low angle boundary whose tilt angle is 250 arc sec from the adjoining main crystal block. The FWHM of the main peak is 60 arc sec and that of the low angle boundaries is 286 arc sec. A broad peak with FWHM value of 410 arc sec, which represents the mosaic blocks mis-oriented randomly with a broad range of tilt angles few arc sec to few arc min. Though the specimen contains low angle structural boundaries and mosaic blocks, the relatively low angular spread of around 1000 arc sec of the diffraction curve and the low FWHM values show that the crystalline perfection is reasonably good.

28 66 Figure 2.16 High Resolution X-ray diffraction curve recorded for a typical single crystal of Imidazolium picrate Thermal Properties The thermal behaviour of Imidazolium picrate was studied by thermogravimetric analysis (TGA) and differential thermal analysis (DTA) with the material mass of mg. A thermal analyzer was employed at a heating rate of 10C/min in the N 2 atmosphere. Figure 2.17 shows the TGA and DTA curves recorded in the temperature range of C. The TGA trace illustrates a major weight loss at 235C due to decomposition. But the decomposition gives a residue of 33%. DTA trace shows an endothermic peak at 205C followed by an exothermic peak. This exothermic peak matches with the decomposition stage of a TGA trace. As the compound melts at 205C the optical applicability is limited to 205C, in contrast to 235C as discussed in

29 67 TGA analysis. The sharpness of this peak at the melting point shows a good degree of crystallinity of the grown crystal. Figure 2.17 TG/DTA trace of Imidazolium picrate UV-Visible Spectral Analysis The optical absorption spectrum of the IP crystal was recorded using UV-Visible spectrometer within the visible range and it is found that there is no absorption peak between 520 to 800 nm. The scanned spectrum is displayed in the Figure From the spectrum it is clear that the crystal is transparent in the visible region and absorption takes place between 470 to 520 nm. The optical bandgap, extinction coefficient and refractive index are governed by the interaction of the crystal with the electromagnetic wave. Optical absorption spectrum provides information on optically induced electronic transitions and band structure as well as the energy gap in crystalline and non-crystalline materials. The Tauc s plot (Equation 2.1)

30 68 between the product of absorption coefficient and the incident photon energy (h) 1/2 with the photon energy (h) at room temperature shows a linear behaviour that can be considered as evidence of the direct transition. 0.5 Absorbance (a.u) Wavelength (nm) Figure 2.18 UV-Visible spectrum of Imidazolium picrate Hence, assuming a transition between valence and conduction bands, the optical bandgap (E g ) is estimated by extrapolation of the linear portion of the curve to a point (h) 1/2 = 0. Using this method, the optical bandgap of the IP crystal is found to be 2.4 ev. The reflectance in terms of the absorption coefficient can be derived from the Equation (2.4). The refractive index can be determined from the reflectance (R) data (Bhuvana et al 2007) using relation (2.5). 1 1 exp( t) exp( t) R 1 exp( t) (2.4)

31 69 n 2( R 1) 2 ( R 1) ( 3R 10R 3) (2.5) Figure 2.19 (a and b) shows the obtained optical bandgap and the variation of refractive index with energy respectively. By tailoring the optical parameters the suitability of materials for device fabrications can be obtained. Figure 2.19 (a) Optical bandgap of Imidazolium picrate (b) Energy dependent Refractive Index Photoluminescence Studies The Photoluminescence spectrum was recorded for the title sample with an excitation wavelength of 270 nm. The emission spectrum was recorded between nm. The Photomultipler (PM) tube is used for detecting the output signal. A broad emission band observed in the range nm, shows the presence of green emission. Figure 2.20 shows the recorded PL spectrum. Upon irradiation at room temperature the sample shows a green emission with a sharp edge at 515 nm, corresponding to 2.4 ev. The presence of a shoulder peak at 590 nm is due to the shallow traps.

32 70 Figure 2.20 PL spectrum of Imidazolium picrate Fourier Transform Infrared Spectrum Thin pellet was subjected for analysis. The presence of O-H, NO 2, N-H and C-H is confirmed with the assigned wavenumbers from the spectrum. The FT-IR spectrum (Figure 2.21) of the compound shows the presence of hydrogen bond with nitrogen or oxygen atom (N-H-O, O-H-O) owing to symmetrical interactions. The strong O-H stretching occurs at 3342 cm -1. The results obtained by both Raman and IR spectral studies were compared and tabulated. The presence of imidazole is assigned to the wavenumber 789 cm -1 and the nitro group (NO 2 ) in picric acid is exhibited at 1610 cm -1 (Silverstein et al 1981)

33 71 Figure 2.21 FT-IR spectrum of Imidazolium picrate Raman Spectrum The grown single crystal of Imidazolium picrate of thickness 1 mm was subjected to Raman spectral studies as mentioned in section The recorded Raman spectrum is shown in Figure The sharp and broad peaks obtained are due to hydrogen bonding. The Raman spectral assignments are given in Table 2.9.

34 72 Figure 2.22 Raman spectrum of Imidazolium picrate Table 2.9 Assignments of IR and Raman spectra FT-IR Wave number (cm -1 ) Raman shift ( cm -1 ) Assignments O-H stretching N-H stretching C-H stretching Asymmetric NO Aromatic ring vibration C-H deformation C-O stretch C-H wagging Five membered imidazole ring C-H bending

35 Factor Group Analysis A group theoretical analysis predicts the possible modes of vibrations of the materials. Imidazolium picrate (IP) crystallizes in 15 orthorhombic crystal system with space group Pbca ( D ). The crystal has Z = 8 with eight molecules per unit cell. The unit cell has 28 atoms which gives rise to 672 (3 28 8) fundamental modes of vibrations. The factor group analysis of the compound gives raise to 672 normal modes of vibrations distributed as 624 internal 78 (A g + B 1g + B 2g + B 3g + A u + B 1u + B 2u + B 3u ) along with 3 acoustical (1A g + 2A u ) and 45 external modes. The A g species are only Raman active and B 1g, B 2g, B 3g species are both Raman and IR active. Group theoretical analysis carried out by following the procedure outlined by Rosseau et al (1981) shows 3 acoustical, 21 Translational and 24 Rotational modes. Table 2.10 gives the summary of factor group analysis. Each internal mode of Imidazolium picrate splits into B 1g (Z), B 2g (Y), B 3g (X), B 1u (Z), B 2u (Y) and B 3u (X) which are IR active and A g (XX, YY, ZZ), B 1g (XY), B 2g (XZ) and B 3g (YZ) are Raman active. Table 2.11 shows the correlation scheme obtained by following the procedure of Fateley et al (1972). The polarizability tensors are of the form 2h xx 0 0 Ag 0 yy zz B1 g 0 xy B 3g yz B2 g 0 0 xz

36 74 Table 2.10 Factor group analysis Summary Site Factor group symmetry Symmetry Ext Int C H N O Optical modes Acoustical modes Total A g 2T 3R B 1g 3T 3R B 2g 3T 3R B 3g 3T 3R A u T 3R B 1u 3T 3R B 2u 3T 3R B 3u 3T 3R Total 21T 24R Table 2.11 Correlation scheme Factor group symmetry IR active Raman active 83 A g - xx, yy, zz 84 B 1g Z xy 84 B 2g Y xz B 3g X yz 82 A u B 1u Z - 84 B 2u Y - 84 B 3u X -

37 Vibrational Analysis of Imidazolium Picrate Vibrational studies reveal the presence of the possible bondings that are present in the synthesized compound. The vibrations present are classified as the lattice vibrations and internal vibrations such as C-C, N-H, NO 2. The bands occuring between 4000 to 400 cm -1 correspond to the internal modes of vibrations Internal vibrations The internal vibrations are mainly due to the presence of C, H, N and O. The vibrations of the above said elements are strongly coupled among themselves. The internal vibrations exhibited are found to be IR and Raman active. These vibrations occur due to the combination of C, H, N and O External vibrations The bands are due to translational and rotational modes. The rotational modes are more intense in Raman spectra and the translational modes are more intense in IR spectra. There exist 45 external modes apart from 3 acoustical modes. External modes can be attributed as A g (2T + 3R), B 1g (3T + 3R), B 2g (3T + 3R), B 3g (3T + 3R), A u (T + 3R ), B 1u (3T + 3R), B 2u (3T + 3R) and B 3u (3T + 3R) Hardness Studies of Imidazolium Picrate To investigate the mechanical strength Vickers pyramidal indenter was used which is attached to a metallurgical microscope. The (100) plane was used for the present study at room temperature. The indentation time was kept constant for all loads varying from 10 to 80 g. Microhardness values are calculated using the relation (1.7).

38 76 The indented surfaces were examined under microscope in reflection immediately after hardness measurements to check the shapes of the indentation impressions and to measure the length of cracks, if any, around the indentations to investigate thoroughly the mechanical behaviour of the sample (Stephens et al 2003). The hardness value increases on increasing the load, which indicates the reverse indentation size effect (Susmita Karan and Sen Gupta 2005). Figure 2.23 (a) shows the variation of hardness value with load. The relation between log P and log d is represented by Meyer's law. The n represents the capacity of work hardening. However, n is less than 2, H v decreases with increasing load. The graph between log P vs log d is depicted in Figure 2.23 (b). From the n value obtained the compound is categorized as a soft material. The elastic stiffness coefficient (C 11 ), which is the reciprocal of the elastic coefficient, was estimated using Wooster s empirical relation as mentioned in Equation (2.3) that gives an idea about the bonding between the nearby atoms (Vasudevan et al 1998). The plot between stiffness constant and the load is shown in Figure 2.23 (c). From the hardness value, the yield strength ( y ) can be determined using the following Equation (2.6). Figure 2.23 (d) shows the relation between yield strength and load for the grown single crystal. From the hardness value H v, the yield strength of the material can be found using the relation (2.6). For n > 2, then H v 12.5( n 2) y [1 ( n 2)] ( n 2) n2 (2.6) if n < 2, then the above equation reduces to Hv y (2.7) 3

39 77 Figure 2.23 (a) Variation of hardness number with load and inset of an indentation mark on (100) (b) Plot of log P vs log d (c) Stiffness constant vs load (d) Variation of yield strength with load Dielectric Studies The dielectric permittivity (ε r ), dielectric loss (tan δ), a.c conductivity of the IP crystal were studied. The dielectric measurements were made in the frequency range 100 Hz - 5 MHz at temperatures 35, 80 and 130C as mentioned in section (1.11.6). The variation of dielectric

40 78 permittivity (ε r ), dielectric loss and conductivity with frequency also shows a similar trend for other organic materials (Rao and Smakula 1965). The space charge, ionic, orientational and atomic polarizations are all active at low frequency. At low frequencies the dielectric loss and dielectric permittivity values depend on the excitation of bound electrons, lattice vibrations, dipole orientations, and space charge polarization. The contribution of space charge depends upon the purity and perfection of the crystal, its influence will be mainly noticeable at the low frequency region. The high values at low frequencies are due to space charge polarization, which are as shown in Figure 2.24 (a) along the three components ε xx, ε yy and ε zz. Tensorial components were measured along a, b and c axes. In normal dielectric behaviour, the dielectric tensorial components decrease with increasing frequency and reach a constant value, depending on the fact that beyond a certain frequency of the electric field, the dipole does not follow the alternating field. Therefore the polarization decreases and exhibits the reduction in the value of ε r (Varma et al 1983). The dielectric loss is also studied as a function of frequency at different temperatures along the three components xx, yy and zz (Figure 2.24 (b). These results suggest that the dielectric loss strongly depends on the frequency of the applied field, similar to what commonly happens with the dielectric behaviour in the ionic system. The rapid raise in conductivity during heating indicates that the free charge carriers are available for conduction. These carriers leave the lattice, when they are thermally activated. As a result there is a rise in conductivity at higher temperature (Figure 2.24 (c) along the three components xx, yy and zz (Balarew and Duhlew 1984). The variations of dielectric permittivity ε r with temperature and frequency are shown in Figure 2.25 (i, ii and iii), indicating the temperature independence property of ε r, owing to immobilization of local charge carriers (Austin and Mott 1969 and Arora et al 2004).

41 79 The dielectric permittivity (ε r ) of a crystal is expressed by a second order tensor. An orthorhombic system has three crystallographic axes of unequal length. The accessible face for the measurement includes all the three direction (Tavazzi et al 2004). By the density functional theory with the local density approximation the three independent components of dielectric tensor can be determined. The components are xx, yy and zz. The calculated polarizability tensorial components are xx = , yy = and zz = This shows the dielectric anisotropies of the grown crystal (Fapeng Yu et al 2010). Experimentally dielectric permittivity has been measured along a, b and c axes. As it is mentioned earlier such high values of the dielectric permittivity at low frequencies arise due to space charge polarization. Dielectric tensor ε ij (three axes) for the grown single crystal was determined as a function of frequency and temperature. The variation of dielectric permittivity values along the three axes is due to the variation of dielectric polarization which occurs when an electric field is applied. Figure 2.24 (a) Variation of dielectric permittivity with log frequency along (i) ε xx (ii) ε yy and (iii) ε zz

42 80 Figure 2.24 (b) Variation of tan δ with log frequency along (i) xx (ii) yy and (iii) zz Figure 2.24 (c) Variation of ac conductivity with log frequency along (i) xx (ii) yy and (iii) zz

43 81 Figure 2.25 Variation of ε r with temperature along (i) ε xx (ii) ε yy and (iii) ε zz Etching of Imidazolium Picrate Etching studies were performed on as-grown (100) face. The crystal was etched, dried using filter papers and subsequently examined in reflection mode of an optical microscope. It was observed that the morphology of etch pits strongly depends on the nature of etchants. Arrays of etch pits were observed. These etch patterns were recorded at a lower magnification of 100x. The use of solvent etchant provides a more direct indication of bulk defect concentration. The small etch pits rapidly expand, and eventually disappear leaving a large pits on an otherwise a clear surface. Figure 2.26 (A) shows the etch pit observed for 5 sec for a crystal surface using ethanol as etchant. Figure 2.26 (B) shows the etch pattern with elongated boundary with the etch time of 10 sec. When the etch time was increased, the size of etch pits increases. This may due to the presence of dislocation caused by thermal stresses which is imposed on the growth surface (Sangwal 1987).

44 82 Figure 2.26 Etch patterns observed using ethanol A) 5 sec and B) 10 sec Laser damage threshold studies Laser damage in crystals mainly occurs due to the focus of multiphoton at a particular point where the temperature raises which may lead to the cause (Montgomery and Milanovich 1990). Figure 2.27 Image of Laser damage threshold on (100) as grown single crystal of IP

45 83 Crystal was exposed to laser beam on the (100) plane. During laser radiation the power meter was on, which gives the measure of the energy density for which the laser beam destroys the surface of the sample. The grown crystal was exposed to multiple shot laser beam as discussed in section The laser damage pattern on the crystal surface is shown in Figure Catastrophic damage occurred when the fluence level was 56.7 mj/cm 2 in the IP crystal. During laser irradiation, localized heating due to absorption by the inclusion results in its vaporization, followed by damage to the crystal through local melting as well as fracture from thermal stresses (Nakatani et al 1988). The sample shows distinct signs of laser irradiated region, owing to increase in the temperature at that particular point. The energy density is calculated using the relation 2.8 InputEnergy( GW ) Energy density = 2 Area( cm ) (2.8) where the input is measured in milli Joule (mj) and A is the area of the spot. These results show that this can be applicable for high power laser applications Computational Method for Hyperpolarizability of IP The first order hyperpolarizability (β) derived from DFT calculations using 6-31G(d) basis set are presented in Table From the tabulated values it is noticed that in β zzz direction, hyperpolarizability is more due to the delocalization of charge cloud. Theoretical values represent that β component is dominant in (zzz) direction. The obtained maximum β value indicates the displacement of charge cloud is more in that particular direction.

46 84 The complete equation for the first order hyperpolarizability calculation using Gaussian 03 output is given by the Equation After the energy of the molecular system is obtained the static response, polarizability, hyperpolarizability of the molecular system are calculated as a derivative of energy, upon calculating the static components, hyperpolarizability can be obtained using the Equation Calculated β total is found to be esu. Table 2.12 The hyperpolarizability value of IP β Components Value (esu) β xxx β xxy β xyy β yyy β zxx β xyz β zyy 6.89 β xzz β yzz β zzz β total CONCLUSION Single crystals of the complex 2:1 orthonitroaniline - picric acid were grown by slow evaporation solution growth technique. UV-Visible

47 85 spectrum and PL emission were recorded. TG/DTA gives a single stage weight loss. The hardness study enumerates that the crystal is found to be moderately hard. Dielectric studies give information about dielectric loss, dielectric permittivity and conductivity. Factor group analysis enumerates the possible irreducible representation of these 303 modes can be classified as 303 = 151A + 152A. Density functional calculations were performed to evaluate the first order hyperpolarizability. It is found to be very high owing to the presence of intermolecular hydrogen bonding. The surface of the grown single crystal was observed after etching using acetone and ethanol as etchants. Single crystals of imidazolium picrate (IP) were grown by slow cooling solution growth technique. The lattice parameters were confirmed using single crystal X-ray diffraction analysis. HRXRD reveals the crystalline perfection of the grown crystal. The functional groups were ascertained by FT-IR and Raman studies. The thermal behaviour of the grown IP was studied using TG/DTA. The theoretical factor group analysis of IP predicts 672 vibrational optical modes that decompose into total = 78(A g + B 1g + B 2g + B 3g + A u + B 1u + B 2u + B 3u ) modes with three acoustical modes acou = 1A g +2A u and 45 external modes. From the UV-Visible spectrum optical bandgap and refractive index were calculated as a function of energy. The PL spectrum shows green emission in the crystal. From the hardness measurements, the yield strength and elastic stiffness constant were estimated. Dielectric tensorial components were determined along the three axes for different temperatures. Etching study shows the elongated etch patterns. The measured laser damage threshold value for the grown crystal is 6.5 GW/cm 2 for (100). Density functional calculations were performed to evaluate the first order hyperpolarizability.