Studies on growth, thermal, optical, vibrational properties and hyperpolarizability of a complex orthonitroaniline with picric acid

Journal of Crystal Growth 312 (2010) 3292–3299

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Studies on growth, thermal, optical, vibrational properties and hyperpolarizability of a complex orthonitroaniline with picric acid S. Anandhi, T.S. Shyju, R. Gopalakrishnan n Crystal Research Lab, Department of Physics, Anna University Chennai, Chennai 600 025, India

a r t i c l e in f o

a b s t r a c t

Article history: Received 9 February 2010 Received in revised form 3 August 2010 Accepted 9 August 2010 Communicated by M. Fleck Available online 14 August 2010

The present article reports the growth of single crystals of a complex Orthonitroaniline with picric acid (2[C6H6N2O2]  C6H2(NO2)3OH) (ONAP) by solution growth (slow evaporation) method at room temperature. Single crystal XRD, UV–vis spectral analysis and TGA/DTA studies were carried out. FT-IR and Raman spectra were recorded to explore information of the functional groups. The highresolution X-ray diffraction curve reveals the internal structural low angle boundaries. The PL spectrum of the title compound shows green emission. Dielectric behaviour was investigated at 33 and 70 1C. The dipole moment and first-order hyperpolarizability (b) values were evaluated by using Gaussian 98 W software package with the help of B3LYP the density functional theory (DFT) method. The possible modes of vibrations are theoretically predicted by factor group analysis. The mechanical stability of the grown crystal was tested with Vicker’s microhardness tester and the work hardening coefficient of the grown material was estimated. & 2010 Elsevier B.V. All rights reserved.

Keywords: A1. Growth from solution A1. Dielectric polarization A2. X-ray diffraction B1. Organic compounds

1. Introduction Nonlinear optics plays a major role in the field of photonics, comprising fiber optic communication, optical computing, data storage and optical switching [1–3]. Some organic crystals are highly polar, which form noncentrosymmetric crystal structure. Despite the appealing possibilities for both fundamental and applied research, photonic crystals with band gap in the visible range have not been realized yet, which is rather puzzling. The solution to this mystery is linked to three basic properties of a photonic crystal, which are the refractive index contrast, the crystal symmetry and the lattice spacing. A major problem to the development of many organic materials is their mechanical property and thermal stability. L-Asparaginium picrate [3], L-valinium picrate [4] and L-prolinium picrate [5] prove to be good candidates for nonlinear optical applications. This article elucidates the vibrational modes both by experimental and theoretical aspects in addition to thermal, mechanical and dielectric properties. Nitroaniline is easily polarizable with internal charge transfer between electron withdrawing nitro groups and electron donating amino groups, which makes nitroaniline a good choice for nonlinear optical applications based on the intermolecular properties [3–7]. Also hydrogen bonds

n Corresponding author. Tel.: +91 44 2235 8710/91 44 2235 8707; fax: + 91 44 2235 8700. E-mail addresses: [email protected], [email protected] (R. Gopalakrishnan).

0022-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2010.08.007

in nitroaniline involves in charge redistribution and hence results in a change in hyperpolarizability value. The intermolecular interactions between pairs of orthonitroaniline and picric acid have already been reported and it is well established that picric acid forms crystalline picrates with various organic molecules through ionic and hydrogen bonding and p–p interaction. The title material is a complex, which crystallizes in the monoclinic crystal system with a space group Cc [6]. The orthonitroaniline acts as p and n-donor. The possession of intramolecular and intermolecular hydrogen bonds link in a material makes it an eligible material for further studies [2,7–9,11]. The results on growth and characterization are presented in detail.

2. Synthesis, solubility and growth The title compound was synthesized by reacting orthonitroaniline and picric acid in the 2:1 ratio in ethanol. The reaction scheme involved in the formation of complex compound is 2C6H6N2O2 + C6H2(NO2)3OH-(2[C6H6N2O2]  C6H2(NO2)3OH) 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 42 1C in steps of 3 1C. The solubility at different temperatures is shown in Fig. 1. From the solubility diagram it is found that the solubility of the title compound increases on increasing the temperature.

S. Anandhi et al. / Journal of Crystal Growth 312 (2010) 3292–3299

The growth solution was prepared in accordance with the solubility data. The solute was dissolved in 100 ml of ethanol, stirred for 35 min and filtered in a 250 ml beaker. The solution is optimally closed and kept in a constant temperature bath with a control accuracy of 70.01 1C. Good-quality single crystals were harvested in a period of 11 days. The bright red-coloured single crystal of the complex (21  2  4 mm3) is shown in Fig. 2a. The molecular structure of the title compound is shown in Fig. 2b. The picric acid molecule has three nitro groups attached to the phenyl ring, with an intramolecular hydrogen bond. When the interatomic distances are less than the sum of the van der Waals radii it is regarded as hydrogen bond. Intramolecular hydrogen bonding is formed by the adjacent hydroxyl and nitro groups [12]. The effect of hydrogen bonding between the phenolate and nitro group enhances the hyperpolarizability value, which is the basic requirement for a system to exhibit nonlinear optical property. In

3293

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 [6]. The effect of hydrogen bonding between the picric and anilne group tends to form a complex.

3. Results and discussion 3.1. X-ray diffraction studies The grown crystal of ONAP was subjected to single crystal XRD analysis using ENRAF NONIUS CAD4/MAC4 X-ray diffractometer ˚ radiation and it is confirmed that the with MoKa (l ¼0.71073 A) title compound crystallizes in the monoclinic crystal system with space group Cc. The obtained lattice parameters are tabulated in Table 1 and are found to be in good agreement with that reported in the literature [6]. 3.2. High-resolution X-ray diffraction analysis The high-resolution X-ray diffraction curve (DC) was recorded for a typical single crystal specimen of ONAP using (1 1 0) diffracting planes in symmetrical Bragg geometry by employing the multicrystal X-ray diffractometer, with MoKa1 radiation [13]. 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 arcsec away from the main peak. This additional peak as in Fig. 3 depicts an internal structural low angle boundary whose tilt angle a is 182 arcsec. For both the main peak and the low angle boundary, the FWHM (full-width at Table 1 Single Crystal XRD data of ONAP.

Fig. 1. Solubility diagram of ONAP.

Lattice parameters

Present work

Reported work [6]

a

10.366 (3)A˚ 15.139 (7)A˚

10.364 A˚ 15.139 A˚

14.091 (2)A˚ 106.761

14.104 A˚ 106.971

b c

b

Fig. 2. (a) As-grown single crystal using ethanol as solvent. (b) Molecular structure of the grown complex.

Fig. 3. High-resolution X-ray diffraction curve recorded for the single crystal in (1 1 0) diffracting plane.

S. Anandhi et al. / Journal of Crystal Growth 312 (2010) 3292–3299

Vibrational spectral analysis is an important tool to understand the chemical bonding and provides useful information in studying the properties of materials. It also provides evidence for the presence of reacting species in the title compound. Vibrational analysis gives the details of the presence of OH, NH, NH2, NO2, C¼ C 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. [17], each internal modes split into two components (A0 and A00 ). 3.4. FT-IR spectrum Spectroscopic methods were used to elucidate the functional groups of the complex. The FT-IR spectrum of the title complex was recorded using Perkin Elmer Fourier Transform Infrared spectrometer (model: SPECTRUM RXI) in the range 4000–400 cm  1. The sample was mixed with KBr in the ratio of 1:10 and subjected to IR radiation. Fig. 4 shows the recorded FT-IR spectrum of ONAP and the assignments made are listed in Table 2. The peak at 3580 cm  1 is due to O–H stretching vibration of picric acid, NH2 vibrations give their peaks at 3377 and 3486 cm  1. Aromatic ring CH stretching vibrations occur at 3090 cm  1. The NH bending vibrations are assigned to the

3.5. Raman spectrum Raman spectrum was recorded using R3000 model Laser Raman spectrometer with the input source of 532 nm in the range of 400–3500 cm  1 in anti-stokes region. The Raman spectrum obtained is shown in Fig. 5. The observed wavenumber and the assignments made are listed in Table 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. 3.6. Group theoretical analysis The title compound crystallizes in the monoclinic system with space group Cc (cs4 ). The factor group analysis of ONAP crystal is carried out using the character table for the point group C1(2). The primitive unit cell contains four molecules (Z ¼4). The total possible irreducible modes of vibrations can be divided into two factor group species such as A0 and A00 . The species A0 and A00 are rich in dipole moment along the Z, X and Y crystal axes. Hence they are active both in Raman and infrared. Table 3 gives the results of factor group analysis. The factor group analysis was performed by following the procedure outlined by Rousseau et al. [16]. The unit cell of 2[C6H6N2O2]  C6H3N3O7 has 51 atoms hence, 3  2  51 a total of 306 modes of vibration of which there exist 3 acoustical modes (2A0 + A00 ). Thus it has 303 internal modes of vibrations. The irreducible representation of the 303 internal modes can be classified as G303 ¼151A0 +152A00 . The total external modes of

Fig. 4. FT-IR spectrum of ONAP.

750 919 1248 1346 1493 1620 3090 3377 3486 3580

688 822 1076 1367 1570 1644 – 3090 3258 3367

C–H bending C–H bending Phenolic CO stretching NO2 symmetric stretching NO2 asymmetric stretching N–H bending C–H symmetric stretching NH2 vibrations NH2 vibrations O–H stretching

1270

600 400

1076

1 2 3 4 5 6 7 8 9 10

800

822

Assignments

886

Raman (cm  1)

1000

577 688

FT-IR (cm  1)

1200

416

SI. No

1400 Raman intensity (a. u)

Table 2 The vibrational assignments of FT-IR and Raman spectra – ONAP.

1367

1600

3367

3.3. Vibrational spectral analysis

intense peak at 1620 cm  1. The aromatic ring skeletal vibrations occur at 1451 cm  1. The asymmetric stretching of NO2 occurs at 1493 cm  1 and its symmetric stretching at 1346 cm  1. The peak at 1541 cm  1 is due to aromatic ring skeletal vibration. The phenolic CO stretching occurs at 1248 cm  1 [14,15]. The aromatic ring CH 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 NH2 group of orthonitroaniline. It is due to less basic nature of the NH2 group of this compound. Since the nitro group at the second position is an electron withdrawing group, the amino group NH2 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 Fig. 2b.

3090 3258

half-maximum) value is 264 arcsec. Though the specimen contains a low angle boundary, the relatively low angular spread of around 800 arcsec 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.

1570 1644

3294

200 500

1000

1500

2000

2500 -1

Raman shift (cm ) Fig. 5. Raman spectrum of ONAP.

3000

3500

S. Anandhi et al. / Journal of Crystal Growth 312 (2010) 3292–3299

vibrations are classified as rotational (3A0 + 3A00 ) and translational (A0 + 2A00 ). Table 4 summarizes the factor group analysis. Each irreducible representation splits into A0 (X,Y) and A00 (Z), which are IR active and A0 (axx, ayy, azz, axy) and A00 (axz, ayz) are Raman active [17]. The polarizability tensors are of the form 0 1 0 1 axx 0 0 0 0 axz B C B C Au ¼ @ axy ayy 0 A, A00 ¼ @ 0 0 ayz A 0

0

azz

0

0

the relation between the product of absorption coefficient and the incident photon energy (ahu)1/2 with the photon energy hu at room temperature. From equation (1) it is clear that band gap depends on the variation in the absorption coefficient. The optical energy band gap (Eg) of the title compound is estimated by extrapolation of the linear portion of the curve to a point (ahu)1/2 ¼0. The optical energy band gap of the crystal is found to be 1.9 eV from the plot shown in Fig. 6(b). The value of energy band gap shows its suitability for photonic and optoelectronic applications [18]. It is also concluded that there is a red shift in its optical absorption edge, owing to internal transition between the nitro group and the aniline group.

0

3.6.1. 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 p–p stacking interactions. The vibrations that occur in the title compound can be correlated with Table 2.

3.8. Photoluminescence The photoluminescence spectrum was recorded using Jobin Yvon-Spex Spectrofluorometer (Fluorolog version-3; Model FL3–11).

3.6.2. 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 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 highfrequency region when compared to translational modes. The possible external modes are tabulated in Table 4.

0.35

Absorbance (a.u)

0.30

3.7. UV–vis 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 using Lambda 35, model no. 101N6122007, Perkin Elmer UV WinLab spectrophotometer in the range 300–800 nm. The lower cutoff occurs at 528 nm. Fig. 6a shows the absorbance spectrum of the title compound. The peak below 400 nm is attributed to the presence of p–p* transition. ðahuÞ ¼ AðhuEg Þn

0.25 0.20 0.15 0.10 0.05 0.00 300

400

500 600 700 Wavelength (nm)

800

ð1Þ

αhυ)1/2

where A is a constant and Eg is the optical band gap. The exponent n has the value 1/2 for direct allowed transition, for direct forbidden transition n ¼3/2, for indirect allowed transition n ¼2 and finally for indirect forbidden transition n ¼3. Fig. 6b shows

)

Table 3 Results of factor group analysis. A0

SI.No

Factor group

1

External modes (i) Translational (ii) Rotational

2

3295

Internal modes Total

A00

1 3

2 3

147 151

147 152

hυ Fig. 6. (a) UV–vis absorbance spectrum (b) plot of (ahu)1/2 with photon energy.

Table 4 Factor group analysis—summary. Factor group symmetry Cs

0

A A00 Total

C1Site symmetry Ext.

Int.

T, 3 R 2 T, 3 R 3 T, 6 R

147 147 294

C

H

N

O

Optical modes

Acoustical modes

Total

54 54 108

45 45 90

21 21 42

33 33 66

153 153 306

2 1 3

151 152 303

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S. Anandhi et al. / Journal of Crystal Growth 312 (2010) 3292–3299

The sample compartment module equipped with a Xenon lamp operated at 450 W was used. This high-pressure Xenon lamp is typically used in fluorescence instruments because they provide a continuous output from 200 to 700 nm. The sample was excited at 270 nm. The emission spectrum was recorded between 520 and 640 nm. The PM tube is frequently used for detecting the output signal. A broad emission band observed in the range 550–570 nm shows the presence of green emission. Fig. 7 shows the recorded PL spectrum for the title material.

point. From this it is identified that there is no phase transition up to its melting point and this enables the suitability of the crystal for NLO applications. From the TGA curve (Fig. 8a) the absence of solvent (ethanol) entrapment during the crystallization process is noted. 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 at around 324 1C in DTA curve owing to the decomposition of the compound.

3.9. Thermal analysis

3.10. Mechanical properties

3.9.1. Melting point measurement The melting point of the title compound is found to be 80 1C using melting point apparatus (VEEGO VMP-PM model). The endothermic peak in the DTA curve at 80 1C gives a sharp melting point of the material (Fig. 8b).

The microhardness studies were carried out on the (1 1 0) 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. Fig. 9a shows the variation in Hardness number with the applied load. The Vickers hardness number Hv is calculated using the relation

3.9.2. TG/DTA study Fig. 8(a and b) shows the TGA/DTA spectra recorded for the grown ONAP crystal with a heating rate of 20 1C/min in the temperature range up to 500 1C in N2 atmosphere. The endothermic peak at 80.23 1C in the DTA curve (Fig. 8b) shows the melting

Hv ¼ 1:8544P=d2 ðkg=mm2 Þ

12000

140 120 Hardness number H v

10000 8000 6000 4000

100 80 60 40 20

2000 500

0

520

540

560 580 600 Wavelength (nm)

620

20

30

10

20

30

40 50 Load (g)

60

70

5000

Fig. 7. PL spectrum of ONAP.

Fig. 8. TGA/DTA spectra of ONAP.

10

640

Stiffness constant (Pa)

PL Intensity

ð2Þ

4000

3000

2000

1000

40 50 Load (g)

60

70

Fig. 9. Mechanical behaviour: (a) dependence of hardness on load and (b) dependence of stiffness constant on load.

S. Anandhi et al. / Journal of Crystal Growth 312 (2010) 3292–3299

where P is the applied load in kg and d the mean diagonal length in mm. From the plot it is found that on increasing the load the hardness increases, which reveals that the crystal exhibits reverse indentation size effect (RISE) [19]. The hardness increases with increase in load up to 70 g and on further increasing the load the crystal cracks. The relationship between P and d is given by P ¼ Kdn :

ð3Þ

where P is the applied load, d the diagonal length and n the Meyer index. It is possible to find the value of n by taking log P¼n log d. Onitsch [20] and Hanneman [21] pointed out that n lies between 1 and 1.6 for hard materials and if n is more than 1.6 it is classified as soft material. The value of n obtained for the complex is 3.8, which reveals that it is softer than its parent material picric acid (n¼ 2.07) [14]. The 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 [22] 7=4

C11 ¼ Hv

ð4Þ

The variation in stiffness constant plotted with load is shown in Fig. 9b.

1.8

33°C 70°C

1.6

dielectric loss

1.4 1.2 1.0 0.8 0.6 0.4 5.4

5.6

5.8 6.0 log freq.

6.2

800

6.4

The dielectric study was carried out using HIOKI 3532 LCR unit in the frequency range of 100 Hz–5 MHz at 33 and 70 1C. Goodquality crystals were selected and polished by a soft polishing pad with fine grade alumina powder. The (1 1 0) face of single crystal was cut into rectangular shape and well polished, so that it behaves as a parallel plate capacitor. Silver paste was used for making the electrode plates on these surfaces of the crystal. The samples were dried after electrode preparation to remove moisture. The capacitance of the crystal was measured to find out the relative dielectric constant. The sample was placed inside the dielectric cell. Fig. 10(a and b) shows the dependence of dielectric constant and dielectric loss on frequency. The dielectric constant and the dielectric loss increase with increase in temperature [3]. The large value of dielectric constant 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 ac conductivity s is calculated using the relation

s ¼ oeo er tan d

500 400 300 200 100

ð5Þ

where eo is the permittivity of free space, er the dielectric constant and tan d the dielectric loss. Fig. 11 shows the frequency dependence of conductivity. It is found that the conductivity increases with increase in frequency and temperature owing to an increase in the density of states and the results obtained are similar to other organic materials [23]. The variation in capacitance with frequency is depicted in Fig. 12. It is observed that the capacitance decreases with increase in frequency and this is due to charge redistribution. The residuals present in the title compound act as mesoscopic capacitors that can acquire multiple charges of either sign. 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 [24] C ¼ Aer eo =d

ð6Þ

0.0030

conductivity (S/cm)

600

dielectric constant

3.11. Dielectric studies

33 °C 70 °C

700

3297

0.0025 o

33 C o 70 C

0.0020 0.0015 0.0010 0.0005

0 4.0

4.5

5.0

5.5

6.0

6.5

log freq. Fig. 10. (a) Dependence of dielectric constant with log frequency. (b) Dependence of dielectric loss with log frequency.

0.0000 4.0

4.5

5.0

5.5

6.0

log freq. Fig. 11. Dependence of conductivity with log frequency.

6.5

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S. Anandhi et al. / Journal of Crystal Growth 312 (2010) 3292–3299

1.00E-010

33 °C 70 °C

Capacitance Fd

8.00E-011

6.00E-011

4.00E-011

2.00E-011

0.00E+000 4.0

4.5

5.0

5.5

6.0

6.5

log freq. Fig. 12. Capacitance with log frequency.

where er is the dielectric constant, C the capacitance value of the crystal, A the area of the crystal under investigation, d the thickness of the sample used and eo the permittivity of free space. Using the Gaussian basic 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. The dielectric constant is a second-order tensor. The components for a monoclinic system are exx, eyy, ezz and exz [25]. The calculated polarizability tensors are exx ¼239, eyy ¼234, ezz ¼218, exz ¼  5.46. From the density functional theory, with the local density approximation, the four independent components of dielectric tensor can be determined. From the calculated dielectric tensor the values can be shown as exx 4 eyy 4 ezz 4 exz, that is, in decreasing order which indicates the dielectric anisotropies of the grown crystal [26]. 3.12. Theoretical calculation of first-order hyperpolarizability First-order hyperpolarizability (b) was calculated by utilizing high-accuracy density functional theory (DFT) using Gaussian 98 W. The designing of the system with inter- and intra-molecular hydrogen bonding between the ligands will lead to a very large value of b. The NLO response of this class of second-order NLO complex is dominated by the hydrogen bonding. The existence of strong hydrogen bonding in a noncentrosymmetric molecular environment is the key to NLO activity. The first criterion can be satisfied by a polarizable molecular system having an asymmetric charge distribution. 3.12.1. Computational method Density functional theory (DFT) is used to calculate molecular properties, such as ground state geometries, force constants, molecular energies and optical transitions. However the application of DFT for calculation of polarizabilities has been limited. Density function results are more reliable than Hartee–Fock calculations. Hyperpolarizability and geometry optimization were performed using Gaussian 98 W on Intel pentimum IV (500 MHz processor with 230 V, RAM, Windows Microsoft XP) as the operating system. The geometry is converged using Gaussview software packages [27] and the output of Gaussview was given as

the input for Gaussian 98 W. The geometry optimization was carried out with the DFT method at B3LYP level in two steps. The semi-empirical method Austin model 1 (AM1) is parameterized for hydrogen bonds and it is applied to study the hydrogenbonded complex. Analysis of this type of calculation indicates improved geometries. The AM1 methods in molecular orbital have also had finite-field polarizability and hyperpolarizability procedures applied to a variety of organic molecules with encouraging results. Using Gaussian 98 W, this loosely converged geometry was extended towards full optimization with HF/3-21G for C, H, O and N. The calculation of first-order hyperpolarizability using Gaussian 98 W output provides 10 components of 3  3  3 matrix as bxxx, bxxy, bxyy, byyy, bxxz, bxyz, byyz, bxzz, byzz and bzzz, respectively, from which x, y, z components of b were calculated [28]. The 27 components of 3D matrix can be reduced to 10 components. The calculated b value for urea is found to be 0.1947  10  30 esu. In the case of polyatomic molecule, b is enriched due to the non-zero m value. The electric dipole moment enhances NLO properties. These electric dipole moments of an organic molecule having donor–acceptor substituents increase hyperpolarizability b. Table 5 shows the hyperpolarizability and dipole moment. The main criteria to run this program are the geometry optimization. The geometry was converged using Gaussview software. When the geometry is optimized, there will not be imaginary frequency modes. The HF/631G (d,p) basis set has been employed. A positive value of b (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 [29]. DFT calculation shows a better description about the structural characteristics for NLO application. The first-order hyperpolarizability (b) and dipole moment (m) for the 2:1 complex of orthonitroaniline and picric acid derived from DFT calculations are presented in Table 5. From the tabulated values it is noticed that in the byzz direction, hyperpolarizability is more due to the delocalization of charge cloud. Theoretical values represent that b component is dominant in the (y z z) direction. The maximum b value is due to intermolecular hydrogen bonds and p–p stacking interactions. The obtained maximum b value indicates the displacement of charge cloud is more in that particular direction. The complete equation for the first-order hyperpolarizability calculation using Gaussian 98 W output is given as [30] btot ¼



bxxx þ bxyy þ bxzz

2

 2  2 1=2 þ byyy þ byzz þ byxx þ bzzz þ bzxx þ bzyy

ð7Þ Table 5 The hyperpolarizability value of ONAP.

bxxx bxxy bxyy byyy bzxx bxyz bzyy bxzz byzz bzzz btot

mx my mz mtot

 10993.44  6261.47  1073.23 2349.98 6829.93 243.21  6157.94 1043.03 11427.46  18425.87 1.918  1.988 4.925  1.454 5.507

Dipole moment (m) in debye, hyperpolarizability b(  2o,o,o) in 10  29 esu.

S. Anandhi et al. / Journal of Crystal Growth 312 (2010) 3292–3299

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References

Table 6 Comparative data of hyperpolarizability values.

btot (esu)

Organic species

Ref.

9.72  10  30 4.8152  10  29 1.347  10  30 1.918  10  29

l-Valinium Picrate l-Asparaginium Picrate 3-nitroaniline 2-nitroaniline: Picric acid

Srinivasan et al. [32] Srinivasan et al. [31] Nagalakshmi et al. [30] Present work

btot of this system is calculated using 6-31G(d) basis set based on finite-field approach. btot is found to be 1.918  10  29 esu. In Table 6 comparative data of some of the hyperpolarizability values are presented.

4. Conclusion Single crystals of the complex 2:1 orthonitroaniline and picric acid were grown by slow evaporation solution growth technique. Single crystal XRD confirms the crystal system of the title compound. HRXRD reveals the low angle boundary present in the crystal. UV–vis studies show the cutoff at 528 nm and the optical band gap is found to be 1.9 eV. PL study shows the green emission. TG/DTA gives a single stage weight loss. The hardness study enumerates that the crystal is found to be moderately hard. The stiffness constant of the title material is evaluated. Dielectric studies give information about dielectric loss, dielectric constant and ac conductivity. Capacitance measurements as a function of frequency show the normal behaviour of organic materials. Factor group analysis enumerates the possible theoretical modes of vibration for the title compound. The irreducible representation of these 303 modes can be classified as G303 ¼151A0 + 152A00 . 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. Based on these studies it is concluded that the material stands as a candidate for optical applications.

Acknowledgement The authors thank Dr. Hubert Joe for rending his help towards theoretical calculation. We also thank all the members of Radiation Safety Division, IGCAR, Kalpakkam for permitting us to carry out PL studies. The authors also thank Dr. G. Bhagavannarayana, NPL, for providing the HRXRD facility.

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