Synthesis, Growth, Thermal and Third‐Order Nonlinear Optical Studies of Bisglycine Lithium Perchlorate Single Crystal

The synthesis as well as the results of the investigations on the structure as well as properties ‐ thermal, mechanical, surface and nonlinear optical of semiorganic bisglycine lithium perchlorate (BGLP) single crystals are herein recorded. BGLP single crystal with dimensions 14× 3 × 2 mm3 has been synthesized and its structure and lattice parameters have been identified with the aid of X‐ray diffraction. The vibrational spectrum of BGLP has been investigated making use of FT‐IR and FT‐Raman spectra. Optical transmittance and absorption is examined by UV–vis spectroscopy with thickness 2 mm and it is shown that the crystal is highly transparent with a lower cut‐off of 287 nm. The optical bandgap is evaluated as 5.7 eV. The dielectric constant is found to show variations with frequency as well as temperature, which remains constant at high frequencies. Dielectric loss is found to decrease with the applied electric field. BGLP is manifested to be thermally stable up to 256 °C as revealed from thermogravimetric analyses. The 2D nucleation mechanism in the growth of BGLP crystals is understood from etching studies. By implementing Z‐scan studies with a He─Ne laser as a source, the nonlinear absorption coefficient, susceptibility, and third order nonlinear refractive index are all computed.


Introduction
The discovery of incredibly efficient NLO materials has increased research efforts in several areas, including optical communication, harmonic generation, frequency mixing, laser lithography, attracted significant interest due to their ease of crystallization in huge symmetries and shapes, which can pretty well pave the way for quick changes in the physical and chemical properties of the compounds, and have therefore become a highly investigated field. [7,8]The properties of both organic and inorganic materials, such as a high laser damage threshold, optical transparency, and high efficiency, are shared by semi-organic materials, which are currently being investigated.All amino acids, except for glycine, have chiral carbon atoms and may crystallize in the non-centrosymmetric space group, making complexes of amino acids with inorganic salts of interest as materials for optical second harmonic generation (SHG). [9][12] Glycine has been proved to combine with lithium complexes such as glycine lithium chloride, [13] glycine lithium bromide, [14,15] glycine lithium nitrate, [16] glycine lithium sulfate, [17][18][19][20] and glycine lithium chromate and glycine lithium molybdate. [21].We have recently published the 3D crystal structure of the new alkali metal halogenides compound bis glycine lithium perchlorate (BGLP). [22]This manuscript reports the crystal growth, structural, thermal, optical, mechanical, dielectric, photoluminescence, antimicrobial and etching studies of BGLP.

Crystal Growth
Bisglycine Lithium Perchlorate was synthesized starting from the dissolution of glycine with Lithium perchlorate in 2:1 m ratio in double distilled water which was homogenized and single crystal of bisglycine lithium Perchlorate was grown via slow evaporation solution growth.Repeated recrystallization was done for yielding crystals of good purity.14 × 3 × 2 mm 3 sized needle like crystals of BGLP crystals were harvested in 15 days as given in Figure 1.

Characterization Techniques
The FT-IR spectrum of BGLP was recorded in the region 4000−400 cm −1 on a BRUKER IFS 66 V spectrophotometer using the KBr pellet technique.FT-Raman spectrum was recorded in the range 4000 ̶ 50 cm −1 with BRUKER RFS 27 spectrometer.The optical transmittance and absorbance measurements were performed using a Perkin Elmer Lambda 35 double beam UV-Vis-NIR spectrophotometer in the wavelength range of 190-1100 nm.The Photoluminescence spectrum was recorded using the Cary eclipse spectrophotometer in the range of 200-900 nm of the BGLP.Dielectric studies have been performed on BGLP with HIOKI 3532 50 LCR HITESTER at 50 Hz to 2 MHz frequency.Polovision high-resolution optical microscope fitted with Motic camera has been utilized for the etching analysis of BGLP crystal.

Single Crystal X-Ray Diffraction Analysis
Single crystal X-ray diffraction has been performed to understand the unit cell dimensions and molecular structure of BGLP crystal with the aid of BRUKER KAPPA APEX II CCD Xray diffractometer.The analysis brings out the crystallization of BGLP in a centrosymmetric space group, a monoclinic system with Z = 4.The ORTEP diagram of the BGLP crystal is given in Figure 2. The asymmetric unit contains two glycinium units, one Li cation, and a perchlorate anion.Both glycine molecules exhibit a zwitterionic structure, as evident from the bond lengths involving the carboxylate atoms (Figure 2) and the protonation of the N atoms of the glycine molecules.In (I), the torsion angle N1A-C2A-C1A-O1A in one of the glycinium is 0.18 (19)°, while the corresponding angle is 20.75 (18)°in the other glycinium.The superposition of these two glycine molecules involving non-hydrogen atoms reveals a high degree of similarity with an r.m.s.d.value of 0.13A°, the maximum deviation (0.19 A °) being observed at the C (C2A and C2B) atom.In the crystal, the Li cation is coordinated by four carboxylate oxygen atoms of the glycine molecules.One oxygen atom from each glycine molecule is incorporated in the Li coordination sphere with Li─O distances ranging from 1.906 (3) to 2.015 (3) A°. [22] The geometry around the Li cation is distorted tetrahedral, as discernible from the angles around the Li cation (Figure 2a).The lithium coordination is extended as a layer that runs parallel to the b-axis.The distance between two adjacent Li ions is 3.270 (13) A°.In a closely related structure of the complex bis(glycine) lithium nitrate (Baran et al., 2009), the Li cation is surrounded by four carboxylate oxygen atoms in a distorted tetrahedral geometry as in (I).The distance between two adjacent Li ions is 5.034 A°.Lattice parameter values are presented in Table 1.

Powder X-Ray Diffraction Analysis
The powder X-ray diffraction (PXRD) pattern of BGLP is recorded with the aid of RICH SEIFERT diffractometer employing Cu K ( = 1.54058Å) radiation in the 10˚-80˚2 range at 0.5˚min −1 .Sharp peaks of the PXRD pattern point toward the good crystallinity of the material.Figure 4 illustrates the PXRD pattern of BGLP, and the reflection peaks have been indexed with AUTOX software.

Vibrational Spectral Analysis
FT -IR spectrum of BGLP has been recorded with the aid of a Perkin Elmer FTIR instrument by KBr pellet technique at wavelength 400-4000 cm −1 and FT-Raman spectrum with the aid of BRUKER RFS 27 spectrometer at 50-4000 cm −1 and are given in Figures 5 and 6 respectively.The observed vibrational   wavenumbers measured infrared and Raman band positions, and their tentative assignments are presented in S1 (Supporting Information).The peak at 3575 cm −1 in the FT-IR spectrum corresponds to O-H symmetric stretching vibration. [23]The asymmetric NH 2 stretching vibrations observed band between the range 3380−3350 cm −1 , while the symmetric stretch will appear between 3310 and 3280 cm −1 . [24,25]The protonation of the NH 2 group can produce a shift in band position toward the range 3300-3100 cm −1 for asymmetric stretch and 3100-3000 cm −1 for symmetric stretch, respectively, as appeared in glycine derivatives. [24,25]The NH 3 + asymmetric stretching observed as an intense broad band in IR at 3118 cm −1 and a medium intense band in Raman at 3013 cm −1 corresponds to NH 3 + symmetric stretching vibrations.The CH 2 asymmetric vibrations are observed as a strong band in the Raman spectrum at 2973 cm −1 and a shoulder band in the IR at 2929 cm −1 assigned to CH 2 symmetric vibrations.The NH 3 + asymmetric deformation mode is observed band in the Raman spectrum at 1602 cm −1 .The NH 3 + symmetric deformation mode occurs in the IR spectrum at 1558 cm −1 .The bands corresponding to scissoring, wagging, and rocking vibrations of CH 2 appear at 1444, 1322, and 1127 cm −1 in the Raman spectrum, and corresponding IR bands are observed in the same region.The COO bending, wagging, and rocking vibrations are observed as intense bands in the IR spectrum in the expected regions. [26]at 681, 580, and 515 cm −1 , respectively.

Optical Transmittance Spectral Studies
The UV-vis-NIR spectrum of BGLP is measured with Lambda 35 UV-vis-NIR Spectrophotometer at room temperature for 190-1100 nm wavelengths, which is given in Figure 7.The spectrum shows that BGLP crystal is optically transparent in the whole Vis-NIR region and that its lower cut-off wavelength is 287 nm.The bandgap of BGLP is determined from the fundamental absorption.Absorption coefficient () and transmittance (T) are related as  = (1/t) 2.303log (1/T), t represents the crystal thickness and h the photon energy.Tauc's relation. [27] = A(h − E g ) 1/2 / h, in which A represents a material dependent constant, has been employed to determine the direct bandgap (Eg).h versus (h) 2 plot is presented in Figure 8. Eg is determined to be 5.7 eV for the BGLP crystal.

Dielectric Studies
Defect free, flat crystals have been selected for electrical measurements.Opposite faces of the crystal are silver coated to ensure conduction, which is then positioned at the center of two copper electrodes, forming a parallel plate capacitor whose capacitance has been evaluated by varying the AC field at several temperatures.Dielectric constant (ɛ r ' ) can be calculated with the equation, ɛ r = C P d/(Aɛ o ) , in which C represents the capacitance, d the thickness, and A the area of the crystal.Dielectric loss (ɛ r '' ) can be determined as ɛ r '' = ɛ r × ɛtan.The dielectric loss is directly obtained from the instrument.ɛ r versus, frequency plot is given in Figure 9, from which we observe a decrease in the value of dielectric constant with an increase in frequency for all temperatures, which becomes constant at high frequencies.The high dielectric constant of BGLP for small frequencies arises from the contribution of polarization from dipolar orientation.For large frequencies, the dielectric constant is seen to drop gradually because of the loss of their significance in the above said polarizations. [28]From the graph, the dielectric constant decreases rapidly at low frequency regions, and this is ascribed to space charge polarization near the grain boundary interfaces which depends on the purity and perfection of the crystal. [28]igure 10 shows the variation of dielectric loss with applied frequency and it was observed that the dielectric loss was reduced at higher frequencies.The low values of dielectric constant and loss at higher frequencies reveal the good optical quality of the grown crystals with less defects, which is the desirable property of the materials to be used for various optical and communication devices. [28]

TGA-DTG Analysis
Thermogravimetric analysis (TGA) of BGLP has been done in the range 30-900 °C in a nitrogen atmosphere at 10 °C min −1 heating rate using TGA instrument Pyris 6 TGA V11.0.0.0449 thermal analyzer.BGLP sample (5.560 mg) was used for the experiment.TGA brings out that BGLP is stable up to 256 °C after which weight loss occurs (Figure 11).Further, the purity of BGLP crystal is revealed from the sharpness of the thermogram.

Etching Analysis
Symmetry, defects, growth patterns, growth mechanism, and surface features of crystals can be identified through etching which involves selective dissolution in a desirable solvent. [29]The grown BGLP crystal has been etched for five s in double distilled water.The wet crystal is dried using tissue paper.Etching the crystal, the top surface is removed and the fresh layer is viewed through the microscope.Figure 12a shows the recorded micrograph with 20X magnification.Again, the crystal is then etched for another 10 s.The etch patterns recorded with 40X magnification are shown in Figure 12b.The etch pattern shows growth hillocks in the surface micrograph of the crystal which are seen to be rectangular in shape with step growth pattern.The development of rectangular etch pattern points to 2D nucleation mechanism in BGLP crystal growth.

Z-Scan Analysis of BGLP
The Z-scan technique developed by Sheik-Bahae et.al. [30,31]is employed to study the nonlinear refractive index (n 2 ) as well as nonlinear absorption coefficient () of BGLP.632.8 nm He─Ne laser, focused by a 12 cm focal length lens has been used.A digital power meter has been employed for measuring the transmitted beam through the aperture.
Closed aperture and open aperture Z-scan curves of BGLP are illustrated in Figures 13 and 14 respectively.Peak and valley separation (ΔT p-v ) and difference in normalized transmittance value at peak and valley positions was used to determine the on-axis nonlinear phase shift at focus (Δϕ) using a set of Equations.in which S represents the aperture linear transmittance of incident Gaussian beam, specified as In the above relation, r a represents the aperture radius, and  a the linear beam radius at the aperture.Nonlinear refractive index (n 2 ) is evaluated as For the incident laser wavelength () k represents wave number (k = 2/) and I o is the laser beam intensity at focal position (Z = 0).L eff shows the effective propagation length in the material given by the relation.Here L stands for sample thickness and  for linear absorption coefficient. is determined directly using open aperture Z-scan data (Figure 12) by using the following relation (6)   in which ΔT represents the peak value from the open aperture Z-scan curve.Finally, the real and imaginary part of the third order nonlinear optical susceptibility can be evaluated from the experimental measurable value of  and n 2 as where n 0 stands for the sample's linear refractive index.The third order nonlinear optical susceptibility is determined as The third order nonlinear parameters determined using the above relations are given in Table 2.The results confirm that BGLP crystal is capable of transmittance in addition to large beam divergence through the far field aperture, a crucial attribute for applicability in optical switching. [28]9.Photoluminescence Study Photoluminescence (PL) spectroscopy characterizes the defects, vacancies, and imperfections in grown crystals.[32] The photoluminescence spectrum of BGLP crystal, measured with Carry Eclipse fluorescence spectrometer employing the excitation source of 450 W high pressure Xenon lamps, at 252-500 nm, is illustrated in Figure 15, A broad emission peak was observed at 419 nm.A very weak peak is also observed at 448 nm. Photluminescence spectral analysis reveals that the crystal exhibits blue green light emission.

Morphological Analysis
The surface morphology of BGLP crystal, has been examined through SEM analysis.Five and 20 μm magnifications have been used to scan the transparent growth plane of BGLP crystal, which  are given in Figure 16a,b.The image reveals the surface which is more or less smooth and devoid of cracks with the exception of a few microcrystals which might have been formed as a consequence of the growth conditions.FEI QUANTA 200F spectrometer was used to record the Energy dispersive X-ray analysis (EDAX) spectrum for obtaining the crystalline information.The EDAX spectrum given in Figure 16b and Table 3 show very close weight percentages of C, N, and O experimentally which substantiates the growth of BGLP.

Antimicrobial Studies
Single crystal bisglycine lithium perchlorate was tested against gram positive as well as gram negative pathogens employing disc diffusion method. [33]The stock solution has been made by us-   of sterile Whatman filter paper (6 mm diameter) after which the sterile disc has been dried at room temperature.The antibacterial activity of BGLP has then been analyzed using Mueller Hinton agar medium.Bacterial pathogen cultures of Escherichia coli and staphylococcus aureus grown in peptone broth at room temperature for 24 h have been spread over the Mueller Hinton agar plates with the aid of sterile cotton swabs maintained at 37 °C for 24 h.The inhibition zone formation is measured in millimeters from which BGLP is observed to show more antibacterial activity against E.Coli than against staphylococcus aureus.Plates showing the zone of inhibition of the bacterial species are presented in Figure 17.Antibacterial activities of BGLP against the gram positive and negative bacteria are shown in Table 4.

Conclusion
Slow evaporation yielded transparent single crystals of bis glycine lithium perchlorate from an aqueous solution.Single crystal XRD study reveals BGLP to be crystallizing in the monoclinic system.UV-vis-NIR spectrum reveals that the absorption edge occurs at 247 nm.FT-IR and FT-Raman spectra authenticate the functional groups in grown BGLP crystals.The dielectric constant, as well as dielectric loss of BGLP, are found to be decreasing with increasing frequency at various temperatures.Z-scan study reveals the nonlinear refractive index value of 1.17102, nonlinear absorption coefficient value of 0.362618 and brings out the suitability of BGLP in optical switching applications.Photoluminescence spectrum exhibits violet emission.BGLP is found to exhibit antibacterial activity against E. coli to a greater degree than against staphylococcus aureus.
title salt is stabilized by a network of intermolecular N-H … O, N-H … Cl and C-H … O interactions.Overall, the crystal structure of the title salt can be described as alternate layers of perchlorate anions and Li glycine cations (Figure 2); these layers extend along the c-axis direction.In the crystalline state, each of the zwitterionic glycine molecules is arranged differently.The first glycine, molecule A (shown in grey), forms double arrays that run parallel to the b-and c-axis directions.In the array parallel to the b-axis, the molecules are oriented in opposite directions, as shown in Figure 3.The first glycine molecule also forms arrays running parallel to the b-axis.The second glycine molecules (shown in orange) and the perchlorate anions are sandwiched between adjacent arrays formed by the first glycinium molecules (Figure 3).The Crystallographic information file has been deposited by us in the CCDC No.: 1 886 641.

Figure 3 .
Figure 3. Second glycinium molecules (orange) and perchlorate anions are sandwiched between arrays of the first glycinium molecules (grey).

Figure 9 .
Figure 9. Dielectric Constant versus log f at various temperatures.

Figure 10 .
Figure 10.Dielectric loss versus log f at various temperatures.

Figure 12 .
Figure 12. a) Surface of as grown BGLP crystal etched for 5s and b) BGLP crystal etched for 10 s.

Table 1 .
Unit cell parameters of BGLP single crystals.

Table 2 .
Comparative NLO studies of Amino acids complexes.

Table 3 .
Elemental analysis of BGLP single crystal.
ing finely powdered BGLP single crystal dissolved in double distilled water at different molar concentrations.100 μL of BGLP extract has been taken from the solution and filtered with the aid