CHAPTER 4 GROWTH AND CHARACTERIZATION OF AN ADDUCT: 4-AMINOBENZOIC ACID WITH NICOTINIC ACID

Transcription

1 108 CHAPTER 4 GROWTH AND CHARACTERIZATION OF AN ADDUCT: 4-AMINOBENZOIC ACID WITH NICOTINIC ACID 4.1 INTRODUCTION In recent years, the interest of researchers with various organic materials has increased considerably. Current interest in device fabrication has stimulated the need for newer and efficient organic crystals. In organic molecular crystals, the energy of van der Waals interactions or even hydrogen bonds are responsible for the intermolecular cohesion of the lattice, that is orders of magnitude lower than that of intramolecular chemical bonds (Gnanasambandam et al 2010, Zhang et al 1944 and Jebas and Balasubramanian 2006). An organic crystal has innumerable potential application in telecommunications, frequency doubling, optical computing and also acts as a source in electro-optics. However most of the current devices are made from inorganic crystals and their processing technology is also well established Adduct An adduct is a product of a direct addition of two or more distinct molecules resulting in a single reaction product containing all atoms of all components. The resultant is considered as a distinct molecular species. Examples include the adduct between hydrogen peroxide and sodium carbonate.

2 Aminobenzoic Acid The molecule consists of an aromatic ring, an NH 2 substituent which is electron donating and a COOH substituent which is electron withdrawing. This molecule has both hydrogen bonding accepting and donating abilities (three hydrogen-bond donors and three hydrogen-bond acceptors) (Sandra Gracin and Åke C. Rasmuson 2004) Nicotinic Acid Nicotinic acid is 3-pyridine carboxylic acid. Wright and King (1953) reported the bond angle and the nature of intermolecular hydrogen bonds. It has a pyridine ring with a carboxyl group attached to it. The molecules are linked in chains parallel to the b-axis by a hydrogen bond. In this chapter the growth, optical, thermal, dielectric, mechanical, vibrational and etching properties of an adduct (2:1) 4-aminobenzoic acid - nicotinic acid (AMN) single crystal are presented. The molecular structure of the title compound is shown in Figure 4.1. Figure 4.1 (2:1) 4-aminobenzoic acid - nicotinic acid

3 EXPERIMENTAL PROCEDURE Commercially available 4-aminobenzoic acid and nicotinic acid were purified by repeated recrystallization using ethanol as solvent. The adduct of 4-aminobenzoic acid and nicotinic acid was synthesized by dissolving in 2:1 ratio using the mixed solvents of deionized water and ethanol solvent. Figure 4.2 As grown AMN single crystal The seed crystals were grown by slow cooling technique with spontaneous nucleation. The supersaturated solution of 4-aminobenzoic acid and nicotinic acid was kept at 34 o C for two days and the temperature was reduced at a lowering rate of 0.1 o C/day. Good optical quality crystals with regular shape and size were grown. The harvested seed crystal was inserted into the mother solution at 34 o C and the solution was allowed for slow evaporation. The approximate growth rate was 0.1 mm/day. Figure 4.2 shows the grown crystal of the title compound of an adduct (2:1) 4-aminobenzoic acid and nicotinic acid. The reaction mechanism is given below 2(H 7 NC 7 H 4 O 2 ) + C 5 H 5 NCO 2 2(C 7 H 7 NO 2 ).C 6 H 5 NO 2

4 RESULTS AND DISCUSSION XRD Analysis An adduct of 4-aminobenzoic acid and nicotinic acid, 2(C 7 H 7 NO 2 ).C 6 H 5 NO 2 (AMN) has 4-aminobenzoic acid and nicotinic acid as the reacting species. The structure is stabilized by a network of N-H---O, O-H---O and O-H---N hydrogen bonds. Unit cell parameters of the grown crystal were obtained using single crystal X-ray diffractometer and the values are presented in Table 4.1. The experimental lattice constants of AMN single crystal and the unit cell volume agree well with the reported value (Jebas and Balasubramanian 2006). The grown crystal belongs to the monoclinic crystal system with the space group Cc. Table 4.1 Crystallographic data of the grown AMN crystal Lattice constants Present Value ( Jebas and Balasubramanian 2006) a (Å) (6) (4) b (Å) (3) (7) c (Å) (9) (8) 99 (5) 99 (4) HRXRD Figure 4.3 shows the High Resolution X-ray diffraction curve recorded for a typical AMN single crystal specimen using (010) diffracting planes in symmetrical Bragg geometry by employing the multicrystal X-ray diffractometer with MoK 1 radiation as mentioned in section The

5 112 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 98 arc sec away from the main peak. This additional peak depicts an internal structural low angle of α > 1 arc min of arc but less than a degree (Lal and Bhagavannarayana 1989) whose tilt angle (misorientation angle, α) between the two crystalline regions on both sides of the structural boundary is 98 arc sec from its adjoining region. For both the main peak and the low angle boundary the FWHM value is 138 arc sec. Though the specimen contains a low angle boundary, the relatively low angular spread of the diffraction curve and the low FWHM value shows 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 (Bhagavannarayana 2005). Figure 4.3 High Resolution X-ray diffraction curve recorded for a typical AMN single crystal

6 FT-IR Studies The functional groups were analyzed using FT-IR spectrophotometer in the entire region of cm -1 using KBr pellet technique (Silverstein et al 1981). The presence O-H stretch of COOH and N-H stretching of CH 2 grouping produce a group of peaks between 2550 and 3500 cm -1. This broadening is due to hydrogen bonding. C=O stretch of COOH group gives its characteristic peak at 1663 cm -1 splitting in this region is equal to nicotinic acid and 4-aminobenzoic acid carboxyl groupings. The ring skeletal vibrations show their ring at 1600, 1518, 1500, 1442 and 1420 cm -1. The C-N vibration of 4-aminobenzoic acid is assigned to the peak at 1291 cm -1 and 1256 cm -1 is assigned to COO vibrations. The O-H bending vibration of COO group appears at 965 cm -1. The peak at 1173 cm -1 is due to C-O vibration. The 1-4 substitution in 4-aminobenzoic acid is confirmed by its CH 2 bending vibration at 870 cm -1, the peaks at 772, 793 are due to CH bending mode of nicotinic acid. This is the characteristic vibration to establish the presence of nicotinic acid in the adduct. Figure 4.4 shows the FT-IR spectrum of the title compound.

7 114 Figure 4.4 FT-IR spectrum of AMN crystal Thermal Properties The thermogravimetric analysis of an adduct, 4-aminobenzoic acid and nicotinic acid was carried out at a heating rate of 10 o C/min in the nitrogen atmosphere. The powdered sample of about 9 mg of AMN crystal was used for analysis. The TG trace of the adduct is illustrated in the Figure 4.5. There is a sharp weight loss at about 150 o C, which indicates the subliamble nature of the crystal. The crystal can be used up to 150 o C. The result of DTA shows a sharp endothermic peak at 160 o C. The sharp endothermic starts just below o C, it coincides with the sublimation temperature as noted in TG. There is a weight loss between 180 and 240 o C. The weight loss corresponds to 90% leaving a residue. There occurs a simultaneous weight loss and an endothermic peak.

8 115 Figure 4.5 TG/DTA trace of AMN crystal UV-Visible NIR Studies The UV-Visible NIR spectrum shows the characteristic peak of a compound which occurs due to the electronic transitions of the molecule. The UV-Visible spectrum of the AMN single crystal was recorded using a 2 mm thick crystal in the region nm. The characteristic absorption band occurs below 450 nm and the absorption band lies between nm hence the crystal is transparent in the UV-Visible-NIR region. Electronic transitions are usually classified according to the orbitals engaged or to specific parts of the molecules involved. Common types of electronic transitions in organic compounds are -*, n-* and *(acceptor)- (donor). Observed band at 250 nm is due to the -* transition. The less intense band centered at 259 nm is due to partly forbidden n-* transition. The more intense band observed at 334 nm is ascribed to an allowed -*

9 116 transition. The UV-visible absorption spectrum of the grown crystal is shown in Figure 4.6. Figure 4.6 UV-Visible spectrum of AMN crystal Hardness Measurement Vickers microhardness test was carried out using a tester attached to a metallographic microscope on (010) face of the grown crystal at room temperature to determine the mechanical strength of the grown crystal. The indentation time was kept constant and the hardness was measured between the loads ranging from g. The average diagonal length was measured at each time. Microhardness values were calculated using the Equation (1.7). Figure 4.7 (a) shows the variation of hardness number with load. The hardness increases on increasing the load which indicates the reverse indentation size effect. When the applied load is less, the indenter point will not penetrate into the surface of the crystal, on the other hand the indenter will penetrate the surface of the crystal while applying higher loads, which is

10 117 restricted by the movement of dislocations and also the bond strength between the neighboring molecules. The relation between P and d is given by Meyer s relation (2.2). The value of n determines the work hardening coefficient. From the linear fit the value of Meyer s index n is 2.2. This shows that the AMN single crystal is a moderately hard one. H v should increase with P if n > 2, decreases with P if n < 2. From the hardness value H v, the yield strength of the material can be found out using the Equation 2.3. Figure 4.7 (b) shows the variation of elastic stiffness constant plotted with load is shown. The elastic stiffness constant gives an idea about the nature of bonding between neighboring atoms (Rani Christhu Dhas 1994). This is the property of the material by virtue of which it can absorb maximum energy before fracture occurs. For various loads the elastic stiffness constant is calculated using Wooster s empirical Equation (2.3). The plot of load dependent yield strength is shown in Figure 4.7 (c). Figure 4.7 (a) Variation of hardness number with respect to load

11 118 Figure 4.7 (b) Stiffness constant vs load (c) Yield strength vs load Dielectric Studies The variation of dielectric permittivity and dielectric loss as a function of frequency is studied and presented here. It is clear from Figure 4.8 (a and b) that both dielectric permittivity and dielectric loss decrease gradually with increasing frequency. The decrease of dielectric permittivity with increasing frequency is a normal dielectric behaviour and can be explained on the basis of polarization mechanism. There are four primary mechanisms of polarization exists in materials. At low frequencies, all the mechanisms of polarization contribute to the dielectric permittivity and

12 119 with the increase in frequency, the contributions from different polarizations start decreasing. For example, at very high frequencies (10 15 Hz), only electronic polarization contributes to the dielectric permittivity, while ionic polarization takes place at IR frequencies (10 13 Hz). The high rise of dielectric permittivity at lower frequencies may be attributed to space charge polarization and also due to crystal lattice defects (Austin and Mott 1969). In organic crystal, the dielectric response is good in the lower frequency region, hence, the experiments were performed in the lower frequency region only The gradual decrease in dielectric permittivity and dielectric loss with frequency suggests that the grown crystals have varying relaxation times. The variation of ac conductivity with frequency at different temperatures is shown in Figure 4.8 (c). The conductivity increases with increasing frequency and can be expressed in Equation (1.9). It is observed that at a given temperature, the magnitude of conductivity is high at higher frequencies, thereby supporting the small polaron hopping model (Varma et al 1983). The characteristic of low dielectric loss with high frequency for a given sample as evident, from Figure 4.8 (b) suggested that the sample possesses good optical quality with lesser defects (Balarew et al 1984 and Narang et al 1974) and this parameter is of vital importance for nonlinear optical applications (Miller 1961). Using DFT theory, the calculated tensorial components were ε xx = , ε yy = , ε zz = and ε xz = 6.37.

13 120 Figure 4.8 Dielectric behaviour of AMN single crystal (a) Variation of dielectric permittivity (b) Variation of dielectric loss (c) Variation of a. c conductivity with log frequency Etching Studies The simplest technique of etching can be best employed to study the defect structure of a single crystal. However, the success of this technique lies in the efficiency of a chemical etchant to sense the dislocation or non-dislocation sites selectively. Etching studies were performed on as-grown (010) face. The crystal was etched using ethanol as etchant, the etched samples were dried using filter papers and subsequently examined in reflection mode of an optical

14 121 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, became flat bottomed rectangular elongated and eventually disappear leaving a large pits or otherwise a clear surface (Sangwal 1987). Figure 4.9 (A) shows the encircled picture of the crystal surface before etching. Figure 4.9 (B, C, D, E) shows encircled boundary of etch patterns with rectangular elongated boundary with the etch time of 5 sec. When the etch time was increased to 10, 15, 20, 25 sec the size of etch pits increases. The formation of etch pit lies parallel to the surface of the crystal. This may be due to the presence of dislocation caused by thermal stresses which is imposed on the growth surface (Ravi et al 1994). A B C D E Figure 4.9 Etch patterns observed using ethanol as etchant

15 Laser Damage Threshold Studies The laser damage threshold studies were carried out for AMN single crystal. Well polished crystal samples were chosen for the present study. Laser damage threshold measurements were carried out using a Nd:YAG laser of 1064 nm, for a pulse duration of 10 ns, frequency repetition rate of 10 Hz with a vibration isolation support. Experiments were performed by keeping the crystal fixed at a particular fixed position and the laser pulse energy was increased. The beam was passed on the (010) of the grown crystal. Figure 4.10 shows the laser damage pattern. Figure 4.10 Laser damage observed on AMN crystal surface The output from the laser was rendered to the test sample, at the near focus of the converging lens. During laser irradiation, the power meter records the energy density of the input laser beam for which the crystal gets damaged. When the intensity (in mj) of the laser beam increases a dot, crack and heavy damage occurred on the surface of grown AMN crystal. The crystal withstands up to 59 mj, after that the damage was observed. Laser

16 123 damage threshold depends on chemical impurities and growth imperfection (Alexandru et al 2003) Group Theoretical Analysis An adduct of 4-aminobenzoic acid and nicotinic acid (AMN) crystallizes in the monoclinic system with space group Cc, ( c 4 s ) of site symmetry. The factor group analysis of AMN crystal was carried out by following the procedure outlined by Rousseau et al (1981). The primitive unit cell contains four molecules (Z = 4). The total possible irreducible modes of vibration 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 both Raman and infrared active. Table 4.2 gives the results of factor group analysis. The unit cell has 48 atoms 2(C 7 H 7 NO 2 ).C 6 H 5 NO 2. Hence , total of 288 modes of vibrations of which there exist 3 acoustical modes (2A + A). Thus it has 285 optical modes of vibrations. The irreducible representation of the 285 modes can be classified as 285 = 142A A. The total external modes of vibrations are classified as rotational (3A + 3A) and translational (A + 2A) in addition to 276 internal modes. Table 4.3 summarizes the factor group analysis. Each irreducible representation splits into A(X,Y) 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

17 xx ' xy yy zz '' xz yz Table 4.2 Factor group analysis - Summary Factor group symmetry C 1 Site Symmetry Ext Int C H N O Optical modes Acoustical modes Total C s A T, 3R A 2T, 3R Total 3T, 6R Table 4.3 Correlation scheme of AMN Sl.No Factor group symmetry IR Active Raman Active 1 A X,Y α xx, α yy, α zz, α xy 2 A Z α xz, α yz 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

18 125 molecule is stacked in columns with the packing stabilized by N-H-O and C-H-O hydrogen bonds and π - π* stacking interactions External vibrations These modes are due to translational and rotational vibrations of the molecule. The low wavenumber bands of hydrogen bond are found to be weak and asymmetric. The lattice modes are very intense in Raman spectra 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 presented in Table Theoretical calculations of First Order Hyperpolarizability () of AMN Crystal The first order hyperpolarizability was calculated using the Equation (1.12). The 6-31G(d) basis set has been employed. The calculated first order hyperpolarizability value is esu. The calculated first order hyperpolarizability () values are presented in Table 4.4. The theoretical calculation seems to be more helpful in the determination of particular components of tensors than in establishing the real values of. Domination of particular components indicates a substantial delocalization of charges in those directions. It is noticed that in the zzz direction the value of hyperpolarizability is biggest; hence subsequently delocalization of electron cloud is more in that direction. The maximum value of may be due to п electron cloud movement from donor to acceptor which can make the molecule highly polarized and electric dipoles may enhance,

19 126 oppose or at least bring the dipoles out of the required net alignment necessary for NLO properties such as total values. Table 4.4 The hyperpolarizability value of AMN β Components Value (esu) β xxx β xxy β xyy β yyy β xxz β xyz β yyz β xzz β yzz 40.2 β zzz β tot CONCLUSION Single crystals of adduct 4-aminobenzoic acid with nicotinic acid (AMN) were grown by solution growth technique. The crystallographic data of AMN were confirmed by single crystal X-ray diffraction analysis. The High Resolution X-ray diffraction studies substantiate the crystal quality is reasonably good. The occurrence of active modes was ascertained by theoretical factor group analysis and FT-IR spectrum. The lower cutoff

20 127 wavelength was determined from the UV-Visible-NIR spectrum. The dielectric permittivity, loss and ac conductivity data of AMN crystal establish the normal behavior of organic compounds. Hardness study indicates that the hardness increases with increasing load. From the mechanical studies yield strength, stiffness constant were calculated. The chemical etching study shows the elongated rectangular etch patterns. On increasing the etch period the shape of etch pits also increases.