Growth, structural, thermal, mechanical, optical and third order nonlinear optical studies of 3-hydroxy 2-nitropyridine single crystal

Optical Materials 86 (2018) 562–570

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Growth, structural, thermal, mechanical, optical and third order nonlinear optical studies of 3-hydroxy 2-nitropyridine single crystal

T

P. Justina, K. Anithaa,∗, S.S.R. Inbanathanb, G. Giesterc, M. Fleckc a

School of Physics, Madurai Kamaraj University, Madurai-625021, Tamil Nadu, India Post Graduate and Research Department of Physics, The American College, Madurai 625002 Tamil Nadu, India c Institute of Mineralogy and Crystallography, University of Vienna, Althanstrabe 14, A-1090, Vienna, Austria b

ARTICLE INFO

ABSTRACT

Keywords: Single crystal growth Organic compounds Optical properties Nonlinear optical materials

A Novel pyridine derivative 3-hydroxy 2-nitropyridine (3H2NP) was crystallized using slow evaporation solution growth technique (SEST). The single crystal XRD analysis was carried out to elucidate the structure of this organic material. 3H2NP was crystallized in the monoclinic crystal system with centrosymmetric space group P21/c. The functional groups and its vibrational assignments were identified by FT-IR analysis. The UV spectrum specified that 3H2NP crystal had wide transparency in visible regions with lower cut off wavelength and optical band gap of 3.7 eV was obtained from Tauc's plot. The photoluminescence study confirmed the luminescence property and defect level present in the 3H2NP crystal. The mechanical hardness and the stability were evaluated by Vickers micro hardness test and the result showed 3H2NP was a hard material. The electronic properties, dielectric constant and dielectric loss of 3H2NP were recorded at room temperature for various frequencies. Thermogravimetric analysis confirmed the thermal stability of 3H2NP crystal and the melting point of the crystal was found to be 144 °C. The third order nonlinear optical properties of 3H2NP has been investigated by employing Z-scan technique. The title compound was computed using quantum mechanical calculations and the first order hyper polarizability value of 3H2NP was found to be 4.9 times that of the urea which settles NLO activity. The calculated HOMO and LUMO energies showed that the charge transfer occurred within the molecule.

1. Introduction In search of new organic materials with third order nonlinear optical properties is a directive research due to their potential applications in high speed optical switching and electro optic devices which are commonly used [1,2]. Among all classified materials organic materials investigated numerously because of their compatibility and indispensable NLO properties owing to their large NLO susceptibilities and nonlinear optical coefficients [3,4]. In particular, pyridines are the important class of organic NLO chromophores which attracts the researchers for its bio-activity, industrial applications [5], pharmaceutical significance [6], protonation [7] and complex formation with coordinating compounds like metal ions [8]. The structure of pyridine is similar to benzene with the presence of nitrogen atom in place of carbon-hydrogen ring. Compounds related to pyridine structure can be found naturally in some vitamins and drugs such as Niacin, Pyridoxal, Isoniazid, Azines and Nicotine [9,10]. Characteristically, nitro pyridines are heterocyclic aromatic

∗

compounds exhibit distinctive properties namely large intermolecular association, high polarity, large dipole moment, high polarizability, and tendency to accept protons and donate electrons [11–13]. The 3-Hydroxy-2-Nitropyridine (3H2NP) is an anatomy of nitro pyridine compound used in the synthesis of some novel sulfonates and act as potential inhibitors of cell proliferation, tubulin polymerization, and active against cancer cells. It is chemically stable under normal conditions [14]. Notably, the donor moiety and acceptor moiety are located in sides of aromatic ring which act as donor-acceptor chromophore and the intramolecular charge transfer between this donor-acceptor chromophore express high NLO response [15] and with the formation of centrosymmetric crystal structure adopts third order nonlinearity. In addition, the presence of strong electron acceptors like NO2 in the aromatic ring system leads to large molecular first-order hyperpolarizabilities [16]. On the basis of NLO chromophore, centrosymmetric system, molecules exhibiting large hyperpolarizabilities, 3H2NP is likely to be a good candidate for third order nonlinear materials. Considering the above motivating factors 3H2NP has been crystallized

Corresponding author. E-mail address: [email protected] (K. Anitha).

https://doi.org/10.1016/j.optmat.2018.10.047 Received 24 July 2018; Received in revised form 22 October 2018; Accepted 22 October 2018 0925-3467/ © 2018 Elsevier B.V. All rights reserved.

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and characterised for NLO applications. This aromatic heterocyclic compound has been extensively studied under theoretical and experimental perspectives [17,18] for probable chemical structures towards innovative drug designs in pharmaceutical fields, thermodynamic stabilities, vibrational modes, molecular interactions and chemical reactivity. However, no crystallographic studies and physical investigation has been done for this title compound. In this report, this novel organic nitro compound 3H2NP has been crystallized using slow evaporation solution growth technique and is subjected to various characterisation techniques for analysing its optical, electrical, thermal and mechanical stability and the in depth physicochemical properties of 3H2NP. To investigate the third order nonlinear properties of this centrosymmetric crystal 3H2NP, Z-scan technique was carried out and the nonlinear absorption coefficient and refractive index were evaluated. Using abinito calculations the HOMO-LUMO analysis, dipole moment and the hyperpolarazability were calculated to show the eminent NLO activity of 3H2NP in the outlook of optical applications.

Table 1 Crystal data and details of the refinement for 3H2NP.

2. Experimental 2.1. Crystal growth

Formula

C5H4N2O3

Mr Crystal system Space group a (Å) b (Å) c (Å) α (°) β (°) γ (°) V (Å3), Z Dcalcd (g cm−3) T (K) μ(MoKα) (cm−1) F(000) hkl range Refl. measured/unique Data with Fo > 4σ(Fo) Rint Parameters refined R1(F)∗(for Fo > 4 σ (Fo)) wR2(F2)∗ (all reflections) Δρfin (max/min) (e Å−3)

140.10 Monoclinic P21/c 10.4045(6) 13.6378(8) 8.0087(5) 90 95.381(3) 90 1131.38(12), 8 1.645 200(2) 0.139 576 ± 16, −21/20, ± 12 42319/4355 3581 0.0313 186 0.0405 0.1119 0.455/-0.251

*R1 = Σ||Fo|-|Fc||/|Fo||, wR2 = [Σw(Fo2-Fc2)2/ΣwFo4]1/2, [σ2(Fo2)+(a × P)2+b × P], P = (Fo2+2Fc2)/3.

3-hydroxy-2 nitropyridine was purchased from Alfa Aesar and used for synthesis without further purification. Crystals of 3H2NP were grown by dissolving the title compound in ethanol and water mixture (1:1). After dissolution, the solution was filtered using Whatman filter paper in a 50 mL glass beaker and covered with perforated polythene sheets. It was kept in a vibration free area to evaporate. The first nucleation was observed within 15 days and single crystals were collected after a period of 35 days. The collected crystals were recrystallized for 3 times to attain good quality crystals. Goldish yellow colour crystals of dimensions 7 × 4 × 2 mm were obtained, and one specimen is shown in Fig. 1.

w = 1/

structure was solved by direct methods and subsequent Fourier and difference Fourier syntheses, followed by full-matrix least-square refinements on F2, using the program SHELX [21]. All the non-hydrogen atoms have been refined with anisotropic displacement parameters, the hydrogen atoms were partially located from the difference Fourier map, partially placed on calculated positions and refined in riding mode. The displacement parameters of the hydrogen atoms were refined at values of 1.2 times of the equivalent isotropic displacement parameter of the parent atom. The crystallographic data as well as details of the measurement are listed in Table 1. Further the crystallographic data have been deposited with the Cambridge Crystallographic Data Centre and can be obtained free of charge via HYPERLINK "http://www.ccdc.cam. ac.uk/conts/retrieving.html" \o " (or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033), citing the title of this paper and the CCDC no. 1586089. The functional groups and vibrational assignments present in this organic crystal were identified using FT-IR spectrum. The grown 3H2NP crystal contains 14 atoms and 36 modes of vibrations all the vibrations were active. The FT-IR spectrum was recorded from the region 4000-400 cm−1 using KBr pellet technique at room temperature by FTIR-8400, CE-Shimadzu model spectrometer with a resolution of 1 cm−1. The optical absorption and transmission spectrum was recorded using Shimadzu UV-2450 spectrometer carried within 200–900 nm. The photoluminescence spectra were recorded in the wavelength range 400–800 nm with Jobin Yvon Fluorolog-3 spectrofluorometer using xenon lamp (450 W) as an excitation source. The mechanical stability of the 3H2NP crystal was studied at room temperature using MATSUZAWA MMTX- 7 series load – B type hardness tester fitted with Vickers pyramidal intender. The thickness of the sample used for hardness measurements is 2.2 mm.

2.2. Characterisation techniques Single crystal X-ray diffraction intensity data of 3-hydroxy-2 nitropyridine were collected at 200 K on a Bruker-Nonius APEX II diffractometer, which was equipped with a CCD area detector and employing graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). For the measurement, suitable single crystal samples were mounted on thin glass needles with laboratory grease. The reflection data were collected and processed using the Bruker-Nonius program suites COLLECT and SAINT and related analysis software [19,20]. The

2.3. Computational studies The computations were performed at DFT/B3LYP [22] method with 6–311++G (d, p) [23] basis set. The molecular structure of 3H2NP has been optimized using Guassian 09 [24] program and visualized using Gauss view. The total first order hyperpolarazability (β), dipole moment (μ), and HOMO-LUMO molecular orbital analysis has been calculated to investigate the NLO activity of 3H2NP.

Fig. 1. Grown 3H2NP single crystal. 563

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Fig. 2. Molecular structure of 3H2NP.

3. Results and discussion 3.1. Crystal structure of 3H2NP

Fig. 3. Packing diagram of 3H2NP, viewed along [001]. The molecules A are located in the middle (viewed side-on), the molecules B are located at the borders at the side of the unit cell (left and right).

The crystal structure comprises two crystallographically different molecules of 3H2NP (labelled as A and B). The molecular geometry is similar, the only major difference being the conformation of the nitrogroup (Fig. 2). A list of intramolecular distances and angles is compiled in Table 2. These values are within the range of the usual values. The molecular structure is in very good agreement with the data from the FT-IR experiment. However, the orientation of the two molecules within the crystal structure is clearly different: The molecules labelled as A are located in the middle of the unit cell (Fig. 3), stacked on top of each other, slanted to the left and to the right, alternatively. The molecules designated as B are located at the borders of the unit cells, all oriented parallel to each other, more or less parallel to the (001) plane.

The molecules are connected to each other via the only possible hydrogen bond, namely the one extending from the O-H-group towards the nitrogen atom of the pyrimide ring. There are two bonds, one for each molecule, viz. O1A-H1A···N1B and O1B-H1B⋯N1A, with O-N distances of 2.7256 (11) Å and 2.7432 (11) Å, respectively. The corresponding O-H⋯N angles are 162.9 (2) ° and 168.4 (2) °. By these bonds, the molecules are connected to form a one-dimensional chain, more or less along [301] (Fig. 4).

Table 2 Distances and angles in 3H2NP (in Å and °) with Comparison of optimized geometry and XRD. Bond length (Å) Parameter

C1A-C2A C2A-C3A C3A-C4A C5A-C1A C5A-N1A C4A-N1A N2A-C5A N2A-O2A N2A-O3A C1A-O1A C1B-C2B C2B-C3B C3B-C4B C4B-N1B N1B-C5B C5B-N2B N2B-O3B N2B-O2B O1B-C1B

Bond angle (◦) Values

Parameter

B3LYP/6–311++G(d,p)

XRD

1.4005(13) 1.3785(15) 1.3888(14) 1.3958(12) 1.3266(12) 1.3300(13) 1.4689(12) 1.2215(11) 1.2179(12) 1.3365(12) 1.4015(13) 1.3762(14) 1.3888(15) 1.3292(13) 1.3277(11) 1.4717(12) 1.2194(11) 1.2197(11) 1.3377(11)

1.4126 1.3819 1.4073 1.4128 1.3396 1.3342 1.4615 1.2857 1.2517 1.3523 1.4072 1.3833 1.4075 1.3376 1.3411 1.4609 1.2925 1.2491 1.3599

C1A-C2A-C3A C2A-C3A-C4A C5A-C1A-C2A C1A-C5A-N2A C5A-N1A-C4A N1A-C4A-C3A N1A-C5A-N2A O1A-C1A-C5A O1A-C1A-C2A O3A-N2A-O2A O3A-N2A-C5A O2A-N2A-C5A N1A-C5A-C1A O1B-C1B-C5B O1B-C1B-C2B C5B-C1B-C2B C3B-C2B-C1B C2B-C3B-C4B N1B-C4B-C3B C5B-N1B-C4B N1B-C5B-C1B N1B-C5B-N2B C1B-C5B-N2B O3B-N2B-O2B O3B-N2B-C5B O2B-N2B-C5B

564

Values B3LYP/6–311++G(d,p)

XRD

119.97(8) 119.21(9) 115.44(8) 120.32(8) 117.92(8) 122.12(9) 114.38(7) 120.98(8) 123.57(8) 124.75(9) 117.55(8) 117.69(8) 125.27(8) 121.30(8) 123.09(8) 115.60(8) 120.16(9) 118.90(9) 122.33(9) 118.13(8) 124.77(8) 114.15(7) 121.08(8) 124.32(9) 117.63(8) 118.04(8)

120.46 119.06 115.79 121.51 119.59 121.40 114.80 128.65 115.52 123.19 116.44 120.35 123.67 125.68 117.62 116.68 119.72 119.48 121.25 119.40 123.41 115.77 120.79 122.83 116.89 120.26

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Fig. 4. One-dimensional chain in the crystal structure of 3H2NP.

Fig. 6. Raman spectrum of 3H2NP.

Fig. 5. Optimized geometry of 3H2NP.

3.2. Optimized geometry

Fig. 7. FT-IR spectrum of 3H2NP.

The optimized structure of 3H2NP was shown in Fig. 5. The bond lengths and bond angles of optimized geometry of 3H2NP was compared with experimental values obtained by X-ray diffraction method and listed in Table 2. Strong bonds are formed between N2A-O3A, N2AO2A, N2B-O3B, N2B-O2B because they have shorter bond lengths compared to others [25]. All the experimental values are in good agreement with the calculated values the average deviation is 0.017 Å. The calculated N2A-O2A & N2B-O3B bond lengths are elongated to 1.285 Å and 1.292 Å and C5B-N2B bond length are shortened to 1.460 Å. A slight variation is observed between calculated geometrical parameters and the experimental values because the experimental values are calculated in crystalline form (solid phase) whereas the theoretical parameters are calculated for isolated molecule (gas phase) [25].

Raman peaks at 1260 cm−1 & 1216 cm−1 are due to C-OH and C-C vibrations. The low frequency FT-IR peaks at 876 cm−1 & 811 cm−1 and sharp Raman peaks of 878 cm−1 & 831 cm−1 denotes NO2 bending. The characteristic band emerges from CH and C-NO2 stretching vibrations appeared in FT-IR spectrum at 701 cm−1 and 776 cm−1 and single Raman peak at 679 cm−1. All the experimental, calculated frequencies using 6–311++G (d, p) basis set and their vibrational assignments of 3H2NP were compared and tabulated in Table 3 [26,27]. The calculated frequencies found disagreement with experimental frequencies to minimize the mismatch, selective scaling has been calculated using the formula [28].

C=

3.3. FT-IR and Raman spectra

(

i

i) 2 i

(1)

Where C is the scaling factor, i & i is the experimental and theoretical frequencies. The Calculated scaling factors are 0.95 and 0.92 respectively. The characteristic higher frequency is stretching vibrations and lower frequencies contain overlapped bands.

The Raman and FT-IR spectrum of 3H2NP crystal was shown in Figs. 6 and 7. Vibrational analysis is an important factor to understand the total vibrational assignments, placement of functional groups, and their bonding to the formation of molecular structure of a crystal. Combination of Raman and FT-IR spectroscopic analysis gives the complete vibrational study of the material. In FT-IR and Raman spectra, sharp peaks signify the crystalline nature and purity of 3H2NP. The higher energy frequency peaks at 3085 cm−1, 3055 cm−1 of FT-IR peaks and Raman peak at 3074 cm−1 corresponds to CH symmetry stretching. FT-IR sharp peaks at 1613 cm−1, 1573 cm−1 and 1543 cm−1 and Raman peaks at 1611 cm−1 and 1566 cm−1 attributed to NO2 stretching and OH bending vibrations. The symmetric stretching vibrations of C-OH and C-NO2 appeared at 1467 cm−1 and 1475 cm−1 in both spectrum. The double peak at 1368 cm−1 and 1323 cm−1 and the single Raman peak at 1362 cm−1 assigned to the combination of CH & OH bending vibrations. In FT-IR spectrum the recorded broad band at 1262 cm−1 & 1213 cm−1 and

3.4. UV-Visible study Due to the absorption of UV light transition between the energy states which provides information about the optical properties of the materials. The optical constants and its parameters can be inferred by UV- Visible spectroscopy analysis. Considering these optical properties which afford details about the electronic band structure, localized states, and defect peaks present in the material [29]. The absorption spectrum of 3H2NP crystal is shown in Figs. 8 and 9. The grown 3H2NP crystal shows two absorption peaks at 208 and 358 nm in UV and visible regions. This two peaks are contributed n→ π*, π→π* transitions. Many aromatic compounds like benzene, pyridine usually show strong bands at 202 nm due to π→π* transition. For 565

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Table 3 Experimental and calculated frequencies and their corresponding vibrational assignments of 3-hydroxy 2-nitropyridine. Observed Fundamentals (cm−1)

Calculated Wavenumber B3LYP/6–311++G(d,p)

υIR

υRaman

3085

3074

υcal (cm−1) 3359 3070 3063 3054 1596 1558 1537 1442 1414 1352 1308 1257 1225 1172 1094 1063 1024 965 851 810 792 735 704 678 658 564 435 272 127

3055 1613 1573 1543 1467 1368 1323 1262 1213 1151 1100 1067 876 811 776 701 672 582

1611 1566 1475 1362 1260 1216 1142 1068 878 831

679 566 410 264 154

Assignments νOH νCH

ν (ring) + ν(NO2) + δ(CH) + δ(OH) ν(C-OH) + ν(C-NO2) + δ(CH) δ(CH) + δ(OH) + ν(ring) ν(NO2) + ν(CC) + δ(CH) + δ(OH) δ(CH) + δ(OH) δ(CH) + δ(OH) + ν(C-OH) ν(CC) + δ(CH) + δ(OH) ν(ring) + δ(CH) δ(ring) + δ(OH) ν(CC) + δ(CH) ν(ring) γ(CH) δ(ring) + δ(NO2) + ν(C-OH)

Fig. 9. a) Transmittance spectrum of 3H2NP b) Taucs plot of 3H2NP. 1

=

2.303 log( T ) t

(2)

Where T is the transmittance and t is the sample thickness. Using the absorption coefficient value, the optical band gap for direct transition was calculated using the relation

γ(CH) γ(ring) γ(CH) + γ(C-NO2) δ(ring) + δ(C-NO2) γ(OH) δ(ring) + δ(C-NO2) + δ(C -OH) δ(C-NO2) + δ(C-OH) ρr(ring) + δ(C-NO2) ρr(ring) out of plane

(αhυ)2 = A (Eg – hυ)

(3)

Where ‘A’ is a constant, ‘Eg’ is the optical band gap, ‘h’ is the plank's constant and ‘υ’ is the frequency of the incident photons. From the Tauc's plot between (αhυ)2 and photon energy (hυ) the band gap energy of 3H2NP was found to be 3.53 eV as shown in Fig. 9. 3.5. Photo-luminescence studies

ν: stretching; δ: bending; γ: out of plane bending ρr: rocking; τ: torsion.

The emission of photons due to the excitation of electronic states by certain wavelength give rise to photoluminescence spectrum [31]. Photoluminescence studies are able to identify the luminescence property, recombination mechanisms, deeper defects and dislocations present in the grown crystal. Aromatic organic compounds contain localized π-electrons which are the main objective of the luminescence property and they are highly stable in nature and exhibit fluorescence [32]. The photoluminescence of 3H2NP aromatic compound was recorded at room temperature with an excitation wavelength of 355 nm which was a cut-off measured from UV spectrum. The photoluminescence of 3H2NP is shown in Fig. 10. Emission peak was obtained at 495 nm with bad gap energy Eg = 2.50 eV. The appearance of broad peak may be due to the overlapping of emission peaks and charge transfer transitions takes place in the system [33]. The high intense peak at 714 nm corresponds to second order excitation. The emission peak at 495 nm contributes to blue light emission which denotes 3H2NP

Fig. 8. UV-Visible absorbance spectra of 3H2NP.

3H2NP it has been shifted and appeared at 208 nm because of the presence of nitro and hydroxyl chromophores in the structure an absorption peak rises at 358 nm due to n→π* transition. The absorption spectrum of 3H2NP was found to be active in UV region which may be useful for higher harmonic light generation using infrared lasers [30]. The optical parameters such as absorption coefficient (α), optical band gap (Eg), extinction coefficient has been calculated from the transmittance spectrum of 3H2NP crystal using Tauc's relation. Good quality crystal of thickness 1 mm was used for analysis. The grown 3H2NP crystal has 85% transparency in the visible region of lower cutoff wavelength which qualifies it to be a good material for optoelectronic applications

Fig. 10. Photoluminescence spectrum of 3H2NP crystal. 566

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Fig. 11. TG-DTA spectrum of 3H2NP. Fig. 12. a) plot of Vickers hardness against the load b) log d vs log p.

may be used as blue light emission source in WLED's applications [34].

analysis concludes 3H2NP has good mechanical strength with increased hardness number by increasing the load.

3.6. Thermal studies

3.8. Dielectric studies

The grown 3H2NP crystal was subjected to TG-DTA analysis to study its thermal stability and melting point. TG-DTA measurements of 3H2NP crystal are presented in Fig. 11. Powdered crystals of a mass 1.64 mg is used for analysis. The energy change and the weight loss of the sample were measured. The figure shows that 3H2NP is stable upto 83 °C prior to this temperature no weight loss occurs. After 83 °C a major weight loss and single stage decomposition of 3H2NP is observed. The onset of decomposition starts at 83 °C and extends up to 145 °C. A significant sharp endothermic peak appears at 144 °C indicates the melting point of the title compound. A small DTA peak at 73 °C corresponds to the dehydration of water molecules present in the molecule. Good thermal stability and high degree of crystallinity of 3H2NP was observed from the sharp peaks and absence of peaks after the melting point from TG-DTA analysis.

Dielectric studies shows detailed electronic property of the material which is correlated with the electro optic property [40]. High value of dielectric constant and dielectric loss is usually observed at lower frequency is owing to space, charge and ionic polarisation and decreased dielectric constant and loss at higher frequencies occurs due to the reversal of electrical field. The dielectric constant and loss relay on the crystal structure, nonlinear optical behaviour and its imperfections. Fig. 13 shows the dielectric constant and dielectric loss of 3H2NP recorded at room temperature with varying frequencies. Both dielectric constant and dielectric loss of 3H2NP was found to be increased at lower frequencies and decreased at higher frequencies which agrees the NLO property of 3H2NP which can be used for photonic applications. The overall electronic studies infer the defect free nature of the grown 3H2NP crystals.

3.7. Hardness analysis

3.9. Z-scan measurement

The mechanical property of the crystal can be validated by subjecting the crystal to Vickers indentations to measure its hardness value. Vickers hardness test is a non-destructive technique gives detailed information about the mechanical strength of the crystal [35–37] Good mechanical crystals with greater mechanical stability are used for fabrication of devices. The mechanical properties of 3H2NP were assessed by the Vickers hardness test at room temperature. Good quality crystals with crack free surfaces were chosen for the indentation. By applying the loads and increasing from 3 to 50 g. After that the cracks are formed in the crystal which indicate the hardness limit of the crystal. The hardness number (Hv) was calculated using the following relation

Hv = 1.8544 ( p d2 ) Kg / mm2

Z-scan technique is used to measure the third order optical nonlinearity and its parameters of nonlinear absorption and nonlinear refractive index of the material based on the principle of spatial beam distortion [41,42]. The third order non-linear refractive index (n2) and absorption coefficient (β) of 3H2NP were evaluated using 25 MW, Spectra physics Model no 127 He–Ne laser of wavelength 632 nm was used as an excitation source. The laser beam was focused using an

(4)

where Hv is the Vickers hardness number, p is the applied load in Kg, and d is an average diagonal length in mm. 1.8544 is a constant geometrical factor for the diamond pyramid [38]. Fig. 12 shows the relation between the applied load and Vickers hardness number and graph between log p and log d. The grown 3H2NP crystal exhibits reverse indentation size effect which can be observed by increased hardness number by increasing the load from the graph [39]. The Meyer index value can be obtained by using least square fitting method from the plot between log p and log d. The Meyer index value (n) which differentiates the material category according to Onitsch and Hanneman rule. From the graph the Meyer index value (n) of 3H2NP was found to be 1.1 which comes under the hard material category. The mechanical

Fig. 13. a) Dielectric constant of 3H2NP b) Dielectric loss of 3H2NP. 567

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n2 =

0

(7)

KIO Leff

where k is the wave number, Leff is the effective thickness of the sample calculated from the formula Leff = [1- exp(- αL)/α,] IO is the intensity of the laser beam, ‘α’ is the linear absorption coefficient and ‘L’ is the thickness of the material. From the open aperture Z- scan plot, lens is used instead of aperture so is called open aperture. It resembles valley denotes reverse saturation effect and positive nonlinear absorption. The non-linear absorption co-efficient β can be estimated from the formula

2 2 T IO Leff

=

Where, ΔTv is one valley value at the open aperture from the Z-scan curve. The real part of third-order susceptibility is proportional to nonlinear refractive index (n2) and the imaginary part of third order optical susceptibility is proportional to the nonlinear absorption coefficient (β). The third order nonlinear optical susceptibility was calculated using these real and imaginary parts using the following relations [48,49].

Fig. 14. Open aperture z-scan of 3H2NP.

(3)

Re

Im

18.5 cm focal length lens with the beam waist was found to be 36.78 μm. The magnitude of nonlinear absorption coefficient and refractive index of a nonlinear material can be measured by the changes in transmittance and refractive index of the same material with respect to the intensity [43,44] which was determined by open and closed aperture experiments. The crystal of thickness 1 mm is placed in different positions with respect to the focused laser beam the corresponding normalized transmission of 3H2NP is measured. Figs. 14 and 15 depicts the open and closed aperture data, from the closed aperture data, the valley followed by the peak was observed denotes the selffocusing behaviour or positive nonlinear refractive index of the material. When the sample is displaced from -Z to + Z direction produces convergence and divergence beam to the aperture. It causes reduced and larger transmittance at the detector is responsible for the valley and peak formation. The aperture linear transmittance S is calculated using the relation

S= 1

exp( 2

=

Tp 0.406(1

(3) (esu)

(3)

(3) ) 2

=

(esu) =

10

10

+ (Im 4

( o C2 n2o n2) 2

(9)

(3) )2

( o C 2no2 4 2

(cm2/w)

(10)

(cm2/ w )

(11)

Laser damage threshold studies plays important role in finding the suitability and laser stability of the crystal for device fabrications and laser applications [51]. The Q switched Nd: YAG laser operating at 1064 nm radiation was used for LDT measurement. The laser was operated at the repetition rate of 10 Hz with the pulse width 6 ns. For the LDT measurements, 1 mm diameter transparent 3H2NP crystal was used,Initially 10 mJ energy was applied for 6 ns on the surface no damage was observed. And the energy was increased from 10 to 40 mJ, after 40 mJ damage started, 3H2NP crystal withstands upto 42.2 mJ. The laser damage threshold value of 3H2NP crystal was found to be 42.2 mJ. This specifies that 3H2NP had good laser stability and reconfirms the hard material nature of 3H2NP which we observed in Table 4 3H2NP Crystal measurement details of Z-scan experiment and obtained parameters.

(5)

v

s )0.25

(Re

3.10. Laser damage threshold studies

where ‘ra’ is the radius of aperture and ‘ωa’ is the beam radius at the aperture, the difference between the peak and valley transmission (ΔTpv) in closed aperture is written in terms of axis phase shift at the focus ( 0) 0

=

where εo is the vacuum permittivity, c is the velocity of light in vacuum, no is the linear refractive index of the sample, and λ is the wavelength of laser beam. The nonlinear susceptibility and its parameters were effectively calculated and tabulated in Table 4. From the table the calculated parameters also signify similar results of positive nonlinear absorption and refractive index. The 3H2NP crystal holds self-focusing behaviour and possess multiphoton absorption it may be useful material for optical limiting applications [50].

Fig. 15. Closed aperture Z-scan of 3H2NP.

ra2 ) 2 a

(8)

(6)

The nonlinear refractive index (n2) was calculated using the relation [45–47]. 568

Laser beam wavelength (λ)

632.8 nm

Lens focal length (f) Optical path distance (Z) Spot-size diameter in front of the aperture (ωa) Aperture radius (ra) Effective thickness (Leff) Nonlinear refractive index (n2) Nonlinear absorption coefficient (α) Real part of third order susceptibility [Re (χ3)] Imaginary part of third order susceptibility [Im (χ3) ] Third order nonlinear susceptibility (χ3)

18.5 cm 115 cm 10 mm 2 mm 1.858 mm 3.984 × 10−11 cm2/W 0.1725 × 10−4 cm/W 1.791 × 10−7 esu 4.2472 × 10−5 esu 4.2483 × 10−5 esu

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P. Justin et al.

3.12. HOMO – LUMO analysis

Table 5 The calculated β components of 3H2NP using B3LYP/ 6–311++G(d,p) method. PARAMETERS

6-311++G(d,p)

βXXX βXXY βYYZ βYYY βXXZ βXYZ βZZZ βXZZ βXYY βYZZ

31.8318 45.9203 −5.5929 40.1634 −16.2378 −18.1567 −7.7810 −12.5512 −11.6188 −1.6643

HOMO-LUMO analysis is the quantification of one electron transition and charge transfer mechanism between the highest occupied molecular orbital and the lowest unoccupied molecular orbital through π-conjugated path [53]. An electronic band of allowed transition appears due to this electron density transfer between the donor and acceptor groups. HOMO-LUMO analysis affords essential information about electronic band, chemical reactivity, ionisation potential, and intermolecular interactions of the molecular system [54]. The HOMO and LUMO analysis, shows positive and negative phase of the molecule represented by red and green colour respectively. In optimized 3H2NP molecular system the HOMO states are localized at A molecule of 3H2NP with molecular orbital energy EHOMO = −0.24837 eV and the LUMO states are localized at B molecules of 3H2NP with orbital energy ELUMO = −0.13651 eV was shown in Fig. 16. And the HOMO-LUMO energy gap of 3H2NP was found to be ΔE = 0.11186 eV. The lower energy gap evidences the strong intermolecular hydrogen bond between the cation and anion. In 3H2NP molecule the HOMO is located over the A moiety and LUMO is mainly spread over B moiety this shows that charge transfer from A moiety to B moiety through the (N1A-H-O1B) hydrogen bond which enhances the NLO activity [55]. The energy gap of 3H2NP is very low which signifies the higher reactivity, greater polarizability and stability of the title compound [56] and this analysis also confirms that the charge transfer interaction takes place between the molecule. This small HOMO-LUMO gaps of 3H2NP compound pronounces the material as exceptionally useful for optoelectronic properties and electronic applications. 4. Conclusion Third order nonlinear organic crystal of 3- hydroxy 2- nitropyridine was successfully grown by slow evaporation solution growth technique at room temperature. Single crystal XRD analysis shows that 3H2NP crystallizes in monoclinic system. Raman and FT-IR analysis confirms the functional groups and their vibrational assignments constituting 3H2NP. Good transparency and the optical band gap of 3.75 eV was revealed by UV-Visible spectrum. Photoluminescence spectrum shows blue emission at 495 nm and suggested that it could be used in light emitting diodes. TG-DTA analysis elucidated the decomposition and confirms the melting point of 3H2NP as 144 °C. Reverse indentation size effect and hard material nature was exposed by Vickers micro hardness study. According to Z-scan measurements, the third order nonlinear optical susceptibility of 3H2NP crystal was found to be (χ3) = 4.2483 × 10−5 esu and it discloses the self-focusing behaviour and reverse saturation absorption of 3H2NP crystal. Laser induced damage threshold value of grown crystal was 42.2 mJ. Quantum chemical calculations performed on 3H2NP showed the hyper polarizability value of 3H2NP was 4.9 times larger than that of the urea and the intramolecular charge transfer within the molecule was explained by HOMO-LUMO analysis. Hence the abovementioned studies attest that 3H2NP is a potential candidate for NLO device fabrication and electrooptic applications.

Fig. 16. HOMO-LUMO compositions of 3H2NP molecule.

mechanical studies. 3H2NP shows good mechanical strength and advances the stability and suitability towards higher order nonlinear optical device applications. 3.11. First-order hyperpolarizability The first order hyperpolarizability calculation is the measure of the polarizability of molecule in the presence of electric field. It pronounces the electro-optic coefficient, NLO activity, intermolecular charge transfer, dipole moment, anisotropy of polarizability and the second order electrical susceptibility of the material [52]. Using B3LYP/ 6–311++G (d,p) level of basis set total hyperpolarizability (β) of 3H2NP was calculated from ten tensor components (βx,βy, βz) which is the reduced form of 27 components in 3D matrix. The first order hyper polarizability of 3H2NP was calculated using the following equation tot

=

(

2 x

+

2 y

+

2 z

1

)2

References

(12)

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All the calculated hyperpolarizability components using DFT have been tabulated in Table 5. The Gaussian output units have been converted into electronic units (esu) (1 a.u. = 8.6393 × 10−33 esu). The calculated dipole moment and hyperpolarizability of 3H2NP is found to be 9.2642 Debye and 7.75720 × 10−31 esu. The hyperpolarizability value of 3H2NP is compared with the urea (1.5841 × 10−31 esu) and is found to be 4.9 times larger than that of the urea. This large hyperpolarizability value designates 3H2NP as a good NLO material for optical applications. 569

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