Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 383–388

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Crystal growth, characterization and theoretical studies of alkaline earth metal-doped tetrakis(thiourea)nickel(II) chloride R. Agilandeshwari, K. Muthu, V. Meenatchi, K. Meena, M. Rajasekar, A. Aditya Prasad, SP. Meenakshisundaram ⇑ Department of Chemistry, Annamalai University, Annamalainagar 608 002, India

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Influence of strontium(II)

incorporation on the properties of TTNC is investigated.  First-order molecular hyperpolarizability is estimated.  Optimized molecular structure of TTNC is reported.  Dopant incorporation into the host crystal is established.

a r t i c l e

i n f o

Article history: Received 30 July 2014 Received in revised form 20 August 2014 Accepted 24 August 2014 Available online 3 September 2014 Keywords: Crystal growth NLO material Vibrational spectroscopy Hyperpolarizability TG/DTA

a b s t r a c t The influence of Sr(II)-doping on the properties of tetrakis(thiourea)nickel(II) chloride (TTNC) has been described. The reduction in the intensity observed in powder X-ray diffraction of doped specimen and slight shifts in vibrational frequencies of doped specimens confirm the lattice stress as a result of doping. Surface morphological changes due to doping of the Sr(II) are observed by scanning electron microscopy. The incorporation of metal into the host crystal lattice was confirmed by energy dispersive X-ray spectroscopy. Lattice parameters are determined by single crystal XRD analysis. The thermogravimetric and differential thermal analysis studies reveal the purity of the materials and no decomposition is observed up to the melting point. The nonlinear optical properties of the doped and undoped specimens were studied. Theoretical calculations were performed using the Density functional theory (DFT) method with B3LYP/LANL2DZ as the basis set. The molecular geometry and vibrational frequencies of TTNC in the ground state were calculated and the observed structural parameters of TTNC are compared with parameters obtained from single crystal X-ray studies. The atomic charge distributions are obtained by Mulliken charge population analysis. The first-order molecular hyperpolarizability, polarizability and dipole moment were derived. Ó 2014 Elsevier B.V. All rights reserved.

Introduction Nonlinear optical materials are the materials in which light waves can interact with each other [1]. They have an impact on fre⇑ Corresponding author. Tel.: +91 4144 221670. E-mail address: [email protected] (SP. Meenakshisundaram). http://dx.doi.org/10.1016/j.saa.2014.08.089 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

quency conversion, optoelectronic, laser, telecommunication, optical computing and data storage technology [2,3]. It is well known that thiourea is capable of forming a number of coordination compounds with various metals. The NLO properties of metal complexes of thiourea have attracted significant attention in the last few years, because both inorganic and organic compounds contribute specifically to the process of second harmonic generation

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(SHG). Tetrakis(thiourea)nickel(II) chloride is a promising semiorganic nonlinear optical material and it belongs to the tetragonal system with noncentrosymmetric space group I4 [4]. Its structure, growth from an aqueous solution and characterization studies have been reported in the literature [5–8]. In our previous investigation, the influence of alkali (potassium) and alkaline earth metal (magnesium) doping on the growth and the SHG efficiency of TTNC [9,10] has been demonstrated. Alkaline earth metal impurity in the crystalline matrix influences the physical properties of the host crystal [11–20]. Strontium was found to be a efficient dopant to influence the optical, electrical, mechanical, magnetic and thermal properties of bis(thiourea)zinc(II) chloride (BTZC) [20], tris(thiourea)zinc(II) sulfate (ZTS) [21], barium hydrogen phosphate [22], BaCoO3 [23] and calcium tartrate crystals [24]. Recently, we have investigated the influence of Sr(II)-doping and molecular structure, spectroscopic (FT-IR, FT Raman, UV–vis), NBO, thermochemistry analysis of bis(thiourea)zinc(II) chloride crystals [20,25]. As far as we know, there is no reference in literature regarding the Sr(II)-doping and theoretical studies on TTNC. In the present study, the effect of Sr(II)-doping on TTNC has been studied using X-ray diffraction, FT-IR, SEM, EDS, thermal, optical, Kurtz and Perry powder SHG technique and compared with theoretical studies. Experimental Synthesis and crystal growth Tetrakis(thiourea)nickel(II) chloride was synthesized according to the reported method [4] using AR grade nickel chloride and thiourea in a stoichimetric ratio 1:4.

NiCl2  6H2 O þ 4ðSCðNH2 Þ2 ރƒƒ!NiCl2 ½SCðNH2 Þ2 4

Fig. 1. Photo images of as-grown TTNC crystals. (a) Sr(II)-doped and (b) pure TTNC.

The purity of the synthesized material was increased by successive recrystallization processes. Crystals were grown by slow evaporation solution growth technique. Doping of 10 mol% strontium in the form of strontium chloride (Merck) was done during the crystallization process. The crystallization took place within 30– 35 days and the defect free crystals were harvested. Photo images of as-grown undoped and Sr(II)-doped crystals are shown in Fig. 1.

the Los Alamos National Laboratory 2-Double-Zeta (LANL2DZ) [29] basis set as implemented in the GAUSSIAN 09W [30] program package with the default convergence criteria without any constraint on the geometry [31]. By the use of the GAUSSVIEW 5.0 molecular visualization program the optimized structure of the molecule has been visualized.

Characterization techniques

Results and discussion

The FT-IR spectra were recorded using an AVATAR 330 FT-IR by KBr pellet technique in the range of 400–4000 cm1. The grown crystal was subjected to single crystal X-ray diffraction using Bruker AXS (Kappa APEXII) X-ray diffractometer. The powder X-ray diffraction was performed by using Philips X’pert Pro Triple-axis X-ray diffractometer at room temperature at a wavelength of 1.540 Å with a step size of 0.008°. The SEM images were taken using a JEOL JSM 5610 LV scanning electron microscope. Thermogravimetric (TG) and differential thermal analysis (DTA) were carried out using a NETZSCH STA 409C thermal analyzer in nitrogen atmosphere, at a heating rate of 10 °C min1. 0.5 g of sample taken in an Al2O3 crucible is placed on top of a thermocouple resting on a balance and the system is sealed into a chamber and heated with a constant heating rate. The second harmonic generation test on the crystals was performed by the Kurtz and Perry powder SHG method [26].

Molecular geometry

Computational details All calculations have been performed by employing the Becke three-parameter hybrid functional [27] with the correlation functional of Lee, Yang, and Parr [28] (B3LYP) in combination with

The structural analysis of Sr(II)-doped TTNC was carried out by single crystal X-ray diffraction analysis and it belongs to tetragonal system with noncentrosymmetric space group I4. The cell parameters are, a = b = 9.536(01) Å, c = 9.023(01) and V = 821.3(01) Å3. The observed unit cell parameters are very close to the corresponding reported values of TTNC [32] and nominal changes observed could be due to alkaline earth metal doping. The optimized molecular structure of TTNC is shown in Fig. 2 and it closely resembles previously reported ORTEP diagram [7]. The important structural parameters of the TTNC along with XRD data is listed in Table 1. As seen from Table 1, slight variations in the bond lengths and angles are observed because the molecular states are different during experimental and theoretical processes. The atomic charge distributions of the atoms present in the TTNC obtained by Mulliken population analysis [33] is shown in Fig. 3 and the corresponding charge values are listed in Table 2. Natural atomic charges have an important role in the application of quantum mechanical calculation to molecular system. From the listed atomic charge values, the amino nitrogen (0.570/ 0.547 e) and chlorine (0.524 e) in TTNC had a large negative

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R. Agilandeshwari et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 383–388 Table 2 Mulliken atomic charges of TTNC.

Fig. 2. Optimized molecular geometry of TTNC.

Table 1 Important structural parameters of TTNC [bond lengths (Å) and angles (°)].

a

Parameter

Single crystal XRD dataa

Theoretical data

Bond length NiACl NiAS SAC CAN

2.40 & 2.52 2.46 1.73 1.34 & 1.32

2.50 2.76 1.75 1.37 & 1.35

Bond angle ClANiAS NiASAC NACAN SACAN

96.7 & 83.3 113.9 120.8 116.9 & 122.3

97.89 & 82.11 115.35 117.39 118.57 & 124.02

Ref. [4].

Atomic No.

Atoms

Atomic charge (e)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

C C C C N N N N N N N N S S S S Zn H H H H H H H H H H H H H H H H Cl Cl

0.001 0.001 0.001 0.001 0.570 0.547 0.547 0.570 0.570 0.547 0.547 0.570 0.093 0.093 0.093 0.094 0.551 0.304 0.348 0.297 0.387 0.297 0.387 0.304 0.348 0.304 0.348 0.297 0.387 0.297 0.387 0.304 0.348 0.524 0.524

Fig. 3. Natural atomic charge distribution of TTNC.

charge and behaved as electron donors. The carbon (0.001 e) and sulfur (0.093 e) atoms in TTNC have a negligible negative charge. The zinc (0.551 e) and the hydrogen (0.304/0.348/0.297/0.387 e) atoms exhibit positive charge, which are acceptor atoms and it could be due to the attachment of more electronegative atoms (Cl, S and N). Fig. 4 shows the calculated molecular electrostatic potential map of TTNC. The different values of the electrostatic potential at the surface are represented by different colors. Potential increases in the order red < orange < yellow < green < blue. The color code of these maps is in the range between 7.279 a.u. (deepest red) to 7.279 a.u. (deepest blue) in compound, where blue indicates the strongest attraction and red indicates the strongest repulsion. The negative regions of V(r) were related to electrophilic reactivity

Fig. 4. Total electron density surface mapped with electrostatic potential of TTNC.

and the positive ones to nucleophilic reactivity. As seen from Fig. 4, in TTNC, the regions having negative potential are around the central metal atoms and the regions having the positive potential are over amino hydrogen atoms. The predominance of the light green region over the MEPs surface corresponds to a potential halfway between the two extremes red and dark blue color. Powder X-ray diffraction analysis The as-grown undoped and doped TTNC crystals were finely powdered and subjected to powder X-ray diffraction analysis.

R. Agilandeshwari et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 383–388

100

(a)

101

80

222

20

3367

(a)

1396

1018

80 1101

2853 2922 3449

3185 3373

543

1439

40

716

1484

479

60

1397

1615

3276

4000

130

2000

3000

6000

1387

4000

701 813 483

3350

200

(c)

8000 10

20

(b)

1000

1100

1651

1514

510

240 204

2000

3541

400 141 123

202

1619

3276

477

0

103 112

3181

538

1483

3448

20

600

400

40

3307

510

% Transmittance

400 141 303 240 204

(b)

222

112 121

800

002

101

Counts

200

1103

1438

123

400

716

60

130 103

600

121

002

800

3649

386

30

40

4000

50

3000

Position [° 2 Theta] Fig. 5. Powder XRD patterns of (a) Sr(II)-doped and (b) pure TTNC.

The observed peaks are in good agreement with the JCPDS file [32]. No new peaks or phases were observed by doping but variations in intensities are observed. The well defined Bragg’s peaks at specific 2h angles show high crystallinity of specimens. The indexed powder patterns of undoped and doped specimens are shown in Fig. 5.

1000

0

Fig. 6. FT-IR spectra of (a) Doped specimen (b) pure TTNC (Experimental) and (c) pure TTNC (Theoretical).

Table 3 Observed vibrational bands of thiourea, TTNC and Sr(II)-doped TTNC. Assignments

ds(CASAN)

ms(C@S) ms(CAN) mas(C@S)

FT-IR analysis FT-IR spectra of undoped and doped TTNC crystals were recorded in the spectral range of 400–4000 nm (Fig. 6a and b). FT-IR spectra reveal that doping results in small shifts in some of the characteristic vibrational frequencies of host specimen and it could be due to lattice strain developed. The symmetric and asymmetric stretching frequencies of CS in thiourea (740 and 1417 cm1 ) [27] are shifted to lower frequencies (716 and 1397 cm-1) and this lowering can be attributed to the reduced double bond character of the C@S bond on coordination. Similarly, CN stretching frequencies of thiourea (1089 and 1472 cm1) are shifted to higher frequencies (1103 and 1483 cm1) and such higher shifts of vibration indicating that the presence of greater double bond of the carbon and nitrogen bond on complex formation. The broad envelope positioned in between 2750 and 3500 cm1 corresponds to the symmetric and asymmetric stretching modes of ANH2 group. These observations suggest that metal coordinate with thiourea through sulfur atom. The theoretical vibrational frequencies are also calculated for TTNC molecule by B3LYP/LANL2DZ as basis set (Fig. 6c). An empirical scaling factor of 0.9608 was used to offset the systematic error caused by neglecting anharmonicity and electron correlation. The observed theoretical (only for pure TTNC) and experimental vibrational bands of thiourea, TTNC and Sr(II)-doped specimens are listed in Table 3.

2000

Wavenumbers (cm -1)

d(NH2)

ms(NH2) mas(NH2) a

Thioureaa

469 740 1089 1417 1627 3167, 3280 3376

Pure TTNC Experimental

Theoretical

479 716 1101 1484 1615 3185, 3276 3373, 3449

483 700 1100 1514 1651 3350 3541, 3649

Sr(II)-doped TTNC 477 716 1103 1483 1619 3181, 3276 3367, 3448

Ref. [34].

Thermal analysis The simultaneous TG/DTA curve of doped TTNC is shown in Fig. 7. Absence of water of crystallization in the molecular structure is indicated by the absence of weight loss around 100 °C. Thermal stability of the doped specimen is slightly increased by doping with Sr(II) (225 °C) in comparison with the thermally stable temperature reported earlier for pure TTNC (211 °C) [8]. No decomposition up to the melting point ensures the suitability of the material for application in lasers where the crystals are required to withstand high temperatures. SEM and EDS The effect of the influence of Sr(II)-doping on the surface morphology of TTNC crystal faces reveals structure defect centers as seen in Fig. 8. Incorporation of strontium dopant into the

387

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100200300400500600700

Table 4 The calculated b components, btot (1033 esu), dipole moment (in D) and polarizabilty (a in esu) values of TTNC.

4

100 3 2 60 1 40

225 °C

First-order molecular hyperpolarizability

DTA/(mW/mg)

TG/%

80

0 20 0

100

200

300

400

500

600

700

-1

Temperature/°C Fig. 7. TG/DTA curve of Sr(II)-doped TTNC.

bxxx bxxy bxyy byyy bxxz bxyz byyz bxzz byzz bzzz btot

0.0000003 0.0000001 0.0000003 0.0000009 1.5600091 284.0207757 1.6140189 0.3193035 0.0679156 1.1294919 10.60645

Dipole moment

lx ly lz l

0.0332 0.5498 0.5760 0.7970

Polarizability

axx axy ayy axz ayz azz a

228.5476836 0.0181622 202.3610746 0.0000083 0.0000046 228.906027 32.59485

crystalline matrix was observed by EDS (Fig. 9). It reveals that the accommodating capability of the host crystal is limited and only a small quantity is incorporated into the crystalline matrix. SHG efficiency In order to confirm the influence of doping on the NLO properties, Sr(II)-doped specimen was subjected to SHG test with an input radiation at 2.9 mJ/pulse. The output SHG intensities of the doped specimens give the relative NLO efficiencies. The doubling of frequency was confirmed by the green color of the output radiation whose characteristic wavelength is 532 nm. Green color emission indicates that the doped material exhibits second order NLO effect. Increasing the doping level by 10 times enhances the SHG efficiency significantly (output intensity increase from 2.24 mV to 2.6 mV) and hence Sr(II) is a useful dopant. The efficient SHG demands specific molecular alignment of the crystal facilitating nonlinearity. Fig. 8. SEM micrographs of TTNC. (a) Sr(II)-doped and (b) pure TTNC.

First-order molecular hyperpolarizability The calculated first-order hyperpolarizability (b), polarizability (a) and dipole moment (l) characteristic of TTNC are listed in Table 4. As seen from the listed values, the maximum b is due to the behavior of nonzero l values. Noncentrosymmetric structure of TTNC was not disturbed and NLO character is not affected by doping. Nonlinearity is sustained at the macro level. Conclusions

Fig. 9. EDS spectrum of Sr(II)-doped TTNC.

Single crystals of pure and strontium doped tetrakis(thiourea)nickel(II) chloride were grown from an aqueous solution by slow evaporation solution growth technique. The single crystal X-ray diffraction revealed that the crystal belongs to tetragonal system with noncentrosymmetric space group I4 and nominal changes in the cell parameters are observed by doping with Sr(II). Powder XRD indicates good crystallinity and intensity variations

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are observed because of doping. Comparison of FT-IR of pure and doped specimens indicates slight shifts in vibrational pattern. The theoretically obtained FT-IR spectrum resembles the experimental pattern and the results are in close agreement. Mulliken charges of the molecule was calculated and interpreted. TG/DTA studies show that doped specimen has higher thermal stability than host material. SEM images show morphological changes in the doped specimen. Incorporation of strontium into the crystalline matrix of TTNC is evidenced by EDS. Emission of green light (532 nm) indicates the NLO characteristic of the material. Acknowledgements The authors thank the Department of Science and Technology (DST), New Delhi, for the financial support through research Grant No: SR/S2/LOP-0025/2010. One of the authors, K. Muthu is thankful to CSIR, New Delhi, for the award (no. 9/3(0009)2K11-EMR-I) of a Senior Research Fellowship. References [1] P. Gter (Ed.), Nonlinear Optical EÒects and Materials, Springer, 2000. [2] N.B. Singh, T. Henningsen, E.P.A. Metz, R. Hamacher, E. Cumberledge, R.H. Hopkins, Mater. Lett. 12 (1991) 270. [3] D.S. Chemla, J. Zyss, Nonlinear properties of organic molecules and Crystals, Academic Press, London, 1987. [4] C.R. Hare, C.J. Ballhausen, J. Chem. Phys. 40 (1964) 788. [5] A. Lopez–Castro, M.R. Truter, J. Chem. Soc. (1963) 1309. [6] K. Ambujam, P.C. Thomas, S. Aruna, D. Prem Anand, P. Sagayaraj, Cryst. Res. Technol. 41 (2006) 1082. [7] K. Ambujam, P.C. Thomas, S. Aruna, D. Prem Anand, P. Sagayaraj, Mater. Manu. Process 22 (2007) 346. [8] A. Bhaskaran, C.M. Ragavan, R. Sankar, R. Mohankumar, R. Jayavel, Cryst. Res. Technol. 42 (2007) 477.

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