Shedding light on molecular structure, spectroscopic, nonlinear optical and dielectric properties of bis(thiourea) silver(I) nitrate single crystal: A dual approach

2.1 Synthesis

The synthesis of bis(thiourea) silver(I) nitrate (BTSN) was successfully done by chemical reaction between commercially available thiourea and silver nitrate purchased from Sigma Aldrich by taking their equimolar ratio (2:1). Primarily the calculated amount of thiourea (CH₄N₂S) and Silver nitrate (AgNO₃) was dissolved separately in double distilled water and filtered, then was mixed slowly in another beaker and again stirred well for a long time using a temperature controlled magnetic stirrer above the room temperature for proper chemical reaction and to yield a homogeneous mixture of solution. The reaction mechanism involved in the synthesis of BTSN is as follows: $$\underset{\text{Thiourea}}{2({\text{CH}}_4{\text{N}}_2\text{S})}+\underset{\text{Silver Nitrate}}{{\text{AgNO}}_3}\to \underset{\text{Bis}(\text{thiourea})\text{silver nitrate}}{\text{Ag}{[{\text{CH}}_4{\text{N}}_2\text{S}]}_2.{\text{NO}}_3}$$

Then the prepared solution was evaporated at room temperature to yield the crystalline powder salt of Bis(thiourea) silver nitrate (BTSN).

2.2 Crystal growth

By using the synthesized materials the good quality single crystal of BTSN was grown by slow evaporation solution technique from the aqueous solutions. Saturated solution of BTSN was prepared at high temperature ∼35 °C. The continuous stirring was done for more than 72 h to get transparent and homogeneous solution of the titled compound. The prepared solution was then filtered twice in another flat surface beaker using good quality filter paper and covered with a perforated lid and was housed in a constant temperature bath with temperature stability ±0.01 °C at constant temperature (35 °C). After one day we have started to reduce the temperature up to room temperature and left for crystal growth. Good quality and optically transparent single crystals were harvested after a span of 15 days from the beaker as shown in Fig. 1.

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Figure 1 As grown single crystals of BTSN.

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2.3 Characterization

The homogeneous powder of the grown crystals of BTSN was prepared for performing the powder X-ray diffraction analysis on Shimadzu X-600 Japan powder X-ray diffractometer (PXRD) having Cu Kα radiation (40 kV, 30 mA, $\lambda$  = 0.1543 nm). The specimen was scanned at the scan rate of 0.002°/s in the angular range of 2θ (5–90°) to find the crystal structure and lattice parameters of BTSN.

The surface topography of the samples was investigated by using scanning electron microscope (JSM 6360 LA, Japan) equipped with energy dispersive analytical X-ray unit (EDAX). EDAX was used to find out the elemental composition of the samples. Before SEM examination, the samples were sputter coated with 10 nm of platinum to eliminate charge buildup.

FT-Raman spectrum was recorded for BTSN single crystal to study the vibrational modes. To record the spectrum the laser beam was made to incident normally on the most prominent surfaces of the grown crystal and scattered intensity was collected at 300 K using THERMO SCIENTIFIC, DXR FT-RAMAN spectrometer coupled with microscope using 532 nm laser of power up to 10 mW, full range grating (3500–5 cm⁻¹), at estimated resolution 5.1–8.3 cm⁻¹ and the size of the aperture pinhole was kept 50 μm.

For optical testing BTSN single crystal of thickness ∼1.2 mm and its powder in a holder of thickness 1.5 cm were subjected to JASCO V-570 UV–vis.-NIR and Shimadzu UV–VIS-NIR spectrophotometer model (UV-3600) spectrophotometer to measure its optical absorbance and diffuse reflectance (DR) in wavelength range of 190–2300 nm at 300 K, respectively.

The dielectric studies were carried out in the higher frequency range i.e. 1 kHz–10 MHz at constant temperature under the dark and UV light using a KEITHLEY 4200-SCS and indigenously designed and developed UV light system.

2.4 Computational details

All the calculations have been performed using Gaussian suit of programs (Frisch et al., 2009). The equilibrium geometry of BTSN was successfully achieved corresponding to the true minimum on the potential energy surface (PES) by solving self-consistent field equation. The density functional theory (DFT) method has been used to optimize the molecular geometry of BTSN at B3LYP and hybrid Gen basis set. The hybrid Gen basis consists of explicitly two types of basis sets i.e. 6-31G∗ and LANL2DZ. For instance, 6-31G∗ has been implemented on carbon, nitrogen, oxygen and hydrogen atoms while LANL2DZ basis set has been applied for silver metal atom. The LANL2DZ basis set LANL (Los Alamos National Laboratory) provides effective core potential (ECP) for heavy atoms (Ag) and the double zeta (DZ) type basis set is utilized as valence basis set (Wadt and Hay, 1985). The use of ECP for the core electrons simplifies the problem by eliminating the need for the core basis functions, which requires large computation, and at the same time the accuracy is also maintained. This basis set takes into account possible relativistic effects due to the presence of the metal atoms. In addition to B3LYP, highly correlated method MP2 has also used to optimize the initial molecular geometry for comparison of results with B3LYP method. The optimized structural parameters of BTSN were used to characterize all stationary points as minima using IR and Raman frequency calculations. The IR and Raman spectra of BTSN were calculated by taking the second derivative of the energy that is computed analytically. Similarly, all other electronic properties including dipole moment, polarizability, first hyperpolarizability of BTSN were calculated at the B3LYP/6-31G∗ level of theory. The time dependent density functional theory (TD-DFT) has been used to calculate excitation energies with relatively larger basis set of 6-31+G∗ in place of 6-31G∗ basis set. The static first hyperpolarizability (β_tot) and its components for BTSN were calculated by finite field (FF) approach at B3LYP/6-31G∗ level of theory. The FF method has been broadly applied to investigate the NLO properties of organic materials because this methodology can be used in concert with the electronic structure method to calculate β values. Recently, several β_tot amplitudes calculated by this method are found to be in a semi quantitative agreement with experimental structure property relationship (Muhammad et al., 2010, 2015). In FF method, a molecule is subjected to a static electric field (F), and the energy (E) of the molecule is expressed by the following equation:

3.1 Structural analysis

3.1.1

3.1.1 Powder X-ray diffraction analysis

Powder X-ray diffraction technique is one of the best techniques to analyze the crystalline nature as well as crystal structure of any material. For better understanding and comparison between thiourea and BTSN we have subjected both for powder X-ray diffraction analysis (PXRD). The recorded PXRD pattern of pure thiourea as well as BTSN is shown in Fig. 2(a). From the PXRD patterns (Fig. 2(a)), it is clearly visible that the pattern of BTSN is totally different than that of thiourea, which confirms its formation. Moreover, the XRD pattern of current work with previously reported (Sivakumar et al., 2014) is also shown in Fig. 2(b). It is clear from Fig. 2(b) that the PXRD pattern is similar to the reported one (CCDC# 913226) which confirms the similar structure of the currently grown crystals. The obtained X-ray data were used to confirm the crystal structure and to calculate the lattice parameters using ‘POWDERX’ and ‘CHECKCELL’ software’s. The crystal system, lattice parameters, space group, etc., are presented in Table 1 and were found in close covenant with the earlier report (Sivakumar et al., 2014). From figure it is clear that the peak intensities are very high which suggest good crystallinity of the grown crystals and its usefulness.

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Figure 2 (a) Powder X-ray diffraction patterns of BTSN with thiourea and (b) BTSN with earlier reports (CCDC#913226).

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Table 1 Lattice parameters of BTSN crystal.

Lattice parameters	Reported work (Sivakumar et al., 2014)		Current work (powder XRD)
	Single crystal XRD	Powder XRD	POWDERX	CHECK CELL
a (Å)	33.3455(6)	33.3440	33.3547	33.3459
b (Å)	45.2957(7)	45.2916	45.2536	45.2858
c (Å)	20.3209(5)	20.3064	20.3565	20.3249
V (Å)3	30692.8(10)	30,666	30729.038	30692.545
α (°)	90	90	90	90
β (°)	90	90	90	90
γ (°)	90	90	90	90
System	Orthorhombic	Orthorhombic	Orthorhombic	Orthorhombic
S.G.	C2221	C2221	C2221	C2221

3.2 Scanning Electron Microscope (SEM) and energy dispersive X-ray spectroscopy (EDXS) analysis

SEM/EDXS is found to be a very effective technique to analyze the qualitative (surface morphology/inclusions) as well as quantitative (the presence of elements in any crystalline matrix) of single crystals (Shkir et al., 2014a, 2014b).

Different crystals of BTSN and its various places were chosen to record the SEM micrographs and EDXS patterns. The captured SEM micrographs and recorded EDXS patterns of BTSN crystal along prominent plane are shown in Fig. 3. The recorded EDXS pattern reveals the elemental or atomic percentage of C (9.50%), N (43.06%), O (26.98%), S (13.64%), Ag (6.24%) and Pt (0.57% due to Pt coating). The energy (keV), mass %, error % and atomic % of all the elements present in BTSN are presented in Table 2 with theoretical values. As clear from the EDX and theoretically calculated data available in Table 2 that there is a difference in both the values of each element composition. The difference in experimental and theoretical elemental composition is may be due to inability of EDX to detect lighter atoms such as H in the system (Srinivasan and Narvekar, 2016). However, the experimental data obtained from EDX study are very useful and supportive as it is a good technique to identify the elemental composition in any complex (Kalaivani et al., 2016). SEM micrographs were captured at different resolution 27×, 30× and 650× and show that the surface of grown crystal is of good quality.

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Figure 3 EDX spectra and SEM images of BTSN single crystals.

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3.3 Molecular geometry

In the present investigation, we have adopted a tetrahedral coordination of thiourea with silver atom as in it has been suggested in some previous studies including single crystal analysis of BTSN (Sivakumar et al., 2014). The initial geometry of monomeric molecule BTSN has been adopted from its experimental crystallographic coordinates (see Fig. S1 of supporting information) and fully optimized to get the lowest energy global minimum structure. The optimized geometrical structure of monomeric BTSN has been illustrated in Fig. 4 as calculated at B3LYP/6-31G∗ level of theory. The geometrical parameters including bond lengths and bond angles are calculated at two different levels of theory as collected in Table 3. Table 3 shows that the B3LYP and MP2 methods in conjugation with hybrid Gen basis sets (as explained in computational details section) are used to study the geometrical parameters of BTSN. A comparison of results from Table 3 shows that the Ag–S bond lengths as at B3LYP method are in reasonable agreement with highly correlated MP2 method and also available experimental parameters. The slight variations among all four Ag–S bond lengths from average Ag–S bond length (2.751 Å) are perhaps due to the steric orientations of thiourea ligands. The nitrate ion has been stabilized through hydrogen bonding with two of the thiourea ligands. The hydrogen bond distances between thiourea and nitrate ion might be considered in medium (1.5–2.2 Å) range of hydrogen bonds (Scheiner, 1997). It is important to mention here that due to the gas phase optimization of BTSN monomeric structure and its structure relaxation from polymer to monomer, its nitrate ion configuration (hydrogen bond angles) has been changed or became irrelevant (for comparison) from its original experimental positions as shown in Fig. S1 of supporting information, while the other details of different bond angles have been remain relevant and shown in Table 3 for comparison purpose.

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Figure 4 Optimized molecular geometry of monomeric BTSN at B3LYP/6-31G∗ level of theory.

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3.4 Experimental and theoretical IR and FT-Raman spectroscopic analysis

To characterize the organic, inorganic and organic-inorganic hybrid materials infrared (IR) and Raman spectroscopy plays a vital role. These techniques help us to identify the presence of functional groups as well as to understand different molecular conformations and reaction mechanism. Raman Spectra of BTSN have been recorded in the wavenumber range of 3500–50 cm⁻¹ as shown in Fig. 5(a). There is no theoretical report available so far on the titled compound; therefore, the IR and Raman spectra were calculated theoretically for the first time by DFT method as explained in methodology section and shown in Figs. 5(b) and 6. To compare the theoretically calculated IR and Raman spectra with experimental we have used a scaling factor 0.9777 (Andrade et al., 2008). It is considerable that several chemical groups can be formed in titled compound due to the coordination of metal with thiourea and that may be possible either through bonding to the nitrogen or sulfur of the thiourea (Selvakumar et al., 2007; Kannan et al., 2004). Some previous reports have already explained the bond formation patterns in the titled compound (Sivakumar et al., 2014). The vibrations of thiourea and silver ion coordinated tetrahedral bonds show its Raman peaks in the lower frequency region i.e. 900–50 cm⁻¹. It is clear from the molecular geometry of BTSN molecule (Fig. 4) that a tetrahedral arrangement forms as the silver atom coordinates to sulfur atom from thiourea and to oxygen atoms from nitrate group. For pure thiourea generally NH₂ symmetric stretching vibration occurs in the range of 3400–3100 cm⁻¹ (Selvakumar et al., 2007; Angeli Mary and Dhanuskodi, 2001). Experimentally, the symmetric and asymmetric stretching vibrations of NH₂ occur at 3220 and 3343 cm⁻¹, while theoretically these bands are observed at 3062, 3106, 3176, 3194 and 3238 respectively. The asymmetric and symmetric stretching vibrations of C–N in BTSN are observed at 1510 and 1055 cm⁻¹, while theoretically these bands are observed at 1504 and 1082 cm⁻¹ respectively.

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Figure 5 (a) Experimental and (b) theoretically calculated Raman spectra of BTSN.

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Figure 6 Calculated IR spectrum of BTSN at B3LYP/6-31G∗ level of theory.

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The asymmetric and symmetric vibrations of C–S stretching vibrations are observed at 1410 and 722 cm⁻¹ experimentally, while theoretically at 1381 and 712 cm⁻¹ due to the formation of BTSN complex as thiourea is bonded to silver (Ag⁺). The N–C–N vibration band is observed at 477 cm⁻¹ in experimental Raman spectrum, while theoretically at 466 cm⁻¹ in metal complex BTSN crystals. The stretching vibration band of AgS occurs at 177 cm⁻¹ (Bowmaker et al., 2009; Sivakumar et al., 2014) while theoretically at 193 cm⁻¹. Spectral features associated with N–C–S, and S–M vibrations are observed due to the delocalization of lone pairs of S atoms (Jayalakshmi et al., 2006). The experimental Raman vibration bands are in agreement with earlier report (Sivakumar et al., 2014). In Fig. 5(a) and (b) the band occurs at 1639 cm⁻¹ in experimental Raman spectrum is due to NH₂ bending vibrations while this vibration occurs at 1627 cm⁻¹ in theoretically calculated Raman spectrum. The band at 1636 cm⁻¹ in IR spectrum (Fig. 6) was also observed due to above said vibrations. The low intensity vibration band at ∼1480 cm⁻¹ in experimental Raman and at 1487 cm⁻¹ in theoretically calculated IR spectrum occurs due to C–N asymmetric stretching vibration. The two bands appeared in the lower energy region at ∼1082 and 712 cm⁻¹ in theoretical spectrum may be assigned to C–N symmetric and C–S stretching vibrations, respectively (Kumari et al., 2009; Sivakumar et al., 2014). The asymmetric bending vibrations of N–C–S, N–C–N are observed at 624 and 475 cm⁻¹ respectively which are found to be shifted in comparison of pure thiourea (Kumari et al., 2009), indicating the formation of thiourea metal complex.

3.5 Optical studies

The study of optical light absorbance of the grown crystal is key parameter to understand the suitability of the grown crystal of titled material for optical applications. The UV–Vis–NIR spectra of BTSN crystal were recorded as shown in Fig. 6 as we can get crucial information about its structure because the absorption of ultra violet and visible light involves the promotion of electron in σ and π orbital from the ground to higher energy states. A nonlinear optical material can be of great practical use only if it has wide transparency window; therefore, it is essential to study the ultraviolet spectra of any NLO crystal. The grown single crystal has UV cutoff wavelength around 315 nm as clear from Fig. 7(a), which also shows that the grown crystal has very low absorbance or high transmittance i.e. ∼80% in visible region which indicates its use in optical window for SHG laser radiation applications in blue region (Rao, 1974). The optical transparency of the grown crystal is higher than many other materials.

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Figure 7 (a) Absorbance spectra and (b) optical band gap plot of BTSN.

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We have calculated the optical band gap of the grown crystal from absorbance data as it will give very profound and good insight about its optical performance and application in optoelectronic devices. Optical band gap (E_g^opt) of the BTSN crystal was calculated according to the following steps:

1. Initially we have calculated optical absorption coefficient (α) from absorbance data using the following relation:

1. Then E_g^opt was evaluated from the following relation:

Diffuse reflectance (DR) is a destructive technique for surface analysis using a mirror like reflection from the sample using an integrating sphere attachment. The ratio of the light scattered from an infinitely thick layer and from an ideal non-absorbing reference sample is measured as a function of the wavelength in diffused reflectance spectrum measurement. Illumination of the powdered samples by incident radiation leads to diffuse illumination of the samples and the incident light is partially absorbed and scattered. The scattered radiation, coming from the sample is collected in an integration sphere and detected (Weckhuysen and Schoonheydt, 1999). In general the Kubelka-Munk theory is used for diffuse reflectance spectra analysis from the weakly absorbing samples. DR of the titled crystal has been measured for the first time. For diffuse reflectance measurements we have crushed the grown crystals in homogeneous powder form and then loaded into the deep sample holder. The recorded DR spectra of BTSN crystal are shown in the Fig. 8(a). The measure data were used to evaluate the band gap.

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Figure 8 (a) Diffuse reflectance and (b) optical band gap from DR data.

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At any wavelength the Kubelka-Munk equation is given by the following:

Eq. (2) can be written in terms of F(R) as follows:

For calculating the optical band gap from DR data the Eq. (2) can be written as follows:

Time dependent density functional theory (TD-DFT) is considered to be a reliable quantum chemical technique for studying the UV–Visible absorption spectra (corresponds to vertical electronic transitions) of different organic compounds (Jacquemin et al., 2004, 2005; Cossi and Barone, 2001). Furthermore, the comparison of calculated and experimental absorption wavelengths has also been made as per suggestion of respected reviewer. For instance, different TD-DFT methods have been applied to study the absorption spectrum of BTSN on its ground state optimized geometry in the framework of B3LYP/Gen level of theory. The experimental electronic absorption spectrum of title BTSN molecule is showing maximum absorption at ∼315 nm. The absorption spectra of BTSN at five different TD-DFT methods have been shown in Fig. 9 and its experimental spectrum is also illustrated through the inset of Fig. 9. It can be seen from Fig. 9 that all the TD-DFT methods (except TD-CAM-B3LYP) have reasonably reproduced the experimental absorption spectrum of BTSN molecule.

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Figure 9 The calculated absorption spectra of BTSN at different levels of theory while inset is its experimental absorption spectrum and shape of grown crystal.

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A careful analysis of Fig. 9 shows that the absorption energies calculated with TD-B3LYP and TD-M06 are closer to experimental wavelength, which represents their better accuracy in case of BTSN molecule. Calculations of optimized molecular orbital geometry of BTSN show that the visible absorption maxima are of the electron transition between frontier orbitals such as transition from HOMO to LUMO which will be explained in next Section 3.6. The maximum absorption band observed in UV region 315 nm (Exp.) and 300 nm (Theoretical) could be attributed to the redistribution of intramolecular electronic charge involving HOMO and LUMO orbitals.

3.6 Frontier molecular orbital (FMO) analysis

The frontier molecular orbitals (FMOs) play a very crucial role in the reactivity of any molecule. Among FMOs, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are very important. These FMOs of a molecule determine the way that it interacts with other species. The spatial distributions of the molecular orbitals for BTSN molecule have been shown in Fig. 10 as calculated at B3LYP/6-31G∗ level of theory. It can be seen from Figure that the HOMO and LUMO orbitals are localized on different components of BTSN molecule. The HOMO-LUMO transition indicates the redistribution of electronic density from nitrate ion fragment to main silver thiourea part. As the transition does not involve any significant intramolecular charge transfer within the BTSN molecule so the transition energy is relatively larger and lie in UV region, which give us an advantage of significant transparency. Based on energies of FMOs, we have also calculated the global reactivity parameters as given in Table 4. The global reactivity parameters play a very crucial role in the reactivity of any molecule and also determine the way that it interacts with other species.

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Figure 10 FMOs involve in critical transition in BTSN molecule calculated at B3LYP/6-31G∗ level of theory.

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3.7 Density of states (DOS)

The total density of states (TDOS) and partial density of states (PDOS) are investigated for BTSN molecule using AOMix wave function analysis program (Gorelsky and Lever, 2001). The total density of states (TDOS) and partial density of states (PDOS) are calculated to understand the role of individual molecules into the bonding properties of BTSN molecule. We generate the total and partial density-of-states plots by dividing BTSN molecule into three fragments including Ag, nitrate and thiourea fragments as shown in Fig. 11. The TDOS and PDOS plots show population analysis per orbital and demonstrate the modest view of the makeup of the molecular orbitals in a certain energy range while PDOS plot shows percentage contribution of each group to each molecular orbital in the final molecule. A careful analysis of Fig. 11 shows that thiourea contributes more to the total number of states per interval of energy as compared with other fragments. The most of the high energy occupied states and lower energy unoccupied states around the band gap are composed of thiourea fragment. Similarly the contribution of Ag is significant over energy range of −9 to −8 eV and a further contribution is at lower conduction bands around −1 to 6 eV. Additionally, it can be seen that the contributions of nitrate are between −14 to −12 eV and −8 to −5 eV in valance bands; and 0 to −1 eV and 5 to 7 eV in conduction bands.

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Figure 11 Total density of state (TDOS) and partial density of state (PDOS) plots for BTSN molecule.

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3.8 Static polarizability and first hyperpolarizability (β)

In our present investigation, we have calculated the electronic dipole moment, molecular polarizability and first hyperpolarizability. For a molecule, its dipole moment (μ) is defined as follows:

The average polarizability ( ${\alpha }_0$ ) can be calculated by following equations:

For anisotropy of polarizability (Δα)

Similarly, the amplitude of first static hyperpolarizability (β₀) is defined as

Similarly, the total first hyperpolarizability (β_tot) can also be calculated using following equation:

The second-order polarizability (β) is a third rank tensor that can be described by a 3 × 3 × 3 matrix. According to Kleinman symmetry (β_xyy = β_yxy = β_yyx, β_yyz = β_yzy = β_zyy,…likewise other permutations also take same value), the 27 components of the 3D matrix can be reduced to 10 components (Kleinman, 1962). The details of 10 components have been shown in Table 5. It is well acknowledged fact that the importance of polarizability and hyperpolarizabity of a molecular system is dependent on the electronic communication of two different parts of a molecule. The calculated values of dipole moment (a.u.), average polarizability (α₀), anisotropy of polarizability (Δα) and hyperpolarizability (β) are given in Table 4.

Table 5 shows the calculated value of total dipole moment (14.20D) for BTSN molecule. The highest component of dipole moment is μ_x having a value of −13.89 D, where the negative sign implies that it is almost entirely directed along negative x-axis as shown in Fig. 12. Fig. 12 clearly indicates that the dipole moment of BTSN molecule is entirely directed from nitrate ion toward tetrakis(thiourea) silver(I) along the negative x-axis as shown in Fig. 10. As the electronic dipole moment is usually directed from negative to positive point charges, the Milliken population analysis indicates a significantly negatively charged Nitrate ion and positively charged silver(I) present along x-axis. A similarly but somewhat different analysis can also be performed for polarizability values i.e. average polarizability (α₀), anisotropy of polarizability (Δα) and hyperpolarizability (β_tot) of BTSN molecule, which have nonzero amplitudes of 34.24 × 10⁻²⁴, 90.94 × 10⁻²⁴ and 5.01 × 10⁻³⁰ esu, respectively. The nonzero value of β_tot shows that the titled molecule possesses microscopic first static hyperpolarizability. The first hyperpolarizability value of BTSN molecule is 27 times larger than that of urea as calculated in the present investigation at the same B3LYP/6-31G∗ level of theory, which shows that BTSN can be considered as a potential candidate for NLO applications having a further advantage of its good transparency in visible region as discussed in UV–Visible spectra of BTSN in next section.

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Figure 12 The representation of partial charges (relative color format) and dipole moment vector calculated using Milliken population analysis at B3LYP/6-31G∗ level of theory for BTSN.

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3.9 Frequency dependent polarizability and first hyperpolarizability

In addition to static first hyperpolarizability values, we have also calculated the dynamic frequency dependent first hyperpolarizabilities that are corresponding to electric field induced second harmonic generation (EFISHG) ${\beta }_{\omega }^{\text{SHG}}(-2\omega \text{;}\omega \text{,}\omega )$ and the electro-optical Pockels effect (EOPE) ${\beta }_{\omega }^{\text{OP}}(-\omega \text{;}0\text{,}\omega )$ . The EFISHG and EOPE are usually approximated as complement to the experimental frequency dependent first hyperpolarizability values. The frequency dependent coupled-perturbed Kohn-Sham (CPKS) method with the B3LYP functional has been applied to calculate the dynamic first hyperpolarizability values. In case of EFISHG experiments, the measurement provides information on the projection of the vector part of β on the dipole moment vectors as given by following equation

3.10 Molecular electrostatic potential

To have molecular level understanding, we have calculated 3-D plots of molecular electrostatic potential of BTSN molecule as shown in Fig. 13. The MEP is the measurement of electrostatic potential on constant electron density surface. The 3-D plots of MEP surface overlap on the top of total energy density. The MEP is helpful property to investigate the reactivity of molecular species by predicting that the approaching nucleophile is attracted to a positive region of molecule. In MEP plot, the maximum positive region that is preferred site for nucleophilic attack is indicated as blue color, while a maximum negative region is preferred site for electrophilic attack is indicated as red surface. The MEP of BTSN has been drawn in Fig. 13 to get simultaneous information about its molecular size, shape along with its positive, negative and neutral electrostatic potential regions in terms of color grading. The red color on MEP surface shows highest negative potential while blue color shows highest positive potential. The green color represents the slight positive or neutral potential over the thiourea atoms indicating their non-reactive nature toward any substitutions. The MEP is a real physical property, which is an observable in experimental diffraction method.

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Figure 13 Molecular electrostatic potential (MEP) plot of BTSN molecule.

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3.11 Dielectric studies

As per the concern over storage and dissipation of electric and magnetic energy the investigation of dielectric properties of any material is a necessary task (Von Hippel, 1954; von Hippel and Morgan, 1955). It is well known that the dielectric properties play a key role in explaining the various phenomena in electronics, optics, and solid-state physics. There is clear evidence in the literature that the dielectric and electro-optical properties of a crystal (Boomadevi and Dhanasekaran, 2004; Shakir et al., 2010a, 2010b, 2010c, 2010d) are interlocked, especially for non-conducting materials. Generally, a larger voltage is required to the material which has high value of dielectric constant needs in order to polarize the dipoles, due to which it may suffer changes in its refractive index. But as per our study on the title material it has low value of dielectric constant as well as dielectric loss, which eliminates the need of poling during maintaining the refractive index.

The grown crystals were subjected to dielectric measurements in the high frequency range from 3 kHz to 10 MHz under dark and high intensity UV light and the plots of variation of dielectric constant ( ${\epsilon }_r$ ), dielectric loss (tan δ) are shown in Fig. 14(a) and (b). As per the available literature the value of dielectric constant founds to be higher in the lower-frequency region and then it decreases and becomes stable at higher frequencies, the reason behind this behavior is well explained (Shakir et al., 2009a, 2010b, 2010c, 2010d; Xue and Kitamura, 2002). By keeping this reason in mind we have done our dielectric measurement in higher frequency range. As clear from Fig. 14(a) the values of ${\epsilon }_r$ are found to be stable in the entire testing range and based on the fact that the electric field, the dipole does not follow the alternating field beyond a certain frequency (Batra et al., 2005; Hill, 1969). The value of dielectric constant was found to be almost doubled under UV light which suggests its use in UV region to get good out from the device. The value of dielectric loss (Fig. 14(b)) also follows the similar path as that of dielectric constant. The value of dielectric loss depends on several factors such as size of crystal, defects and can be controlled as well (Przeslawski et al., 1995; Shakir et al., 2010b). As the value of dielectric is found to be very low which suggests the grown crystals are free from any major defects and high crystalline perfection, therefore it may be useful in electro-optic modulation applications.

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Figure 14 Plots of variation of (a) dielectric constant and (b) dielectric loss measured on BTSN crystal.

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