Growth and characterization studies of l -threonine phosphate (LTP) a new semiorganic NLO crystal

Accepted Manuscript Title: Growth and Characterization studies of L-threonine phosphate (LTP) a new semiorganic NLO crystal Author: P. Christuraj S. Anbarasu P.S. Joseph D. Prem Anand PII: DOI: Reference:

S0030-4026(15)01146-8 http://dx.doi.org/doi:10.1016/j.ijleo.2015.09.090 IJLEO 56295

To appear in: Received date: Accepted date:

10-10-2014 8-9-2015

Please cite this article as: P. Christuraj, S. Anbarasu, P.S. Joseph, D.P. Anand, Growth and Characterization studies of L-threonine phosphate (LTP) a new semiorganic NLO crystal, Optik - International Journal for Light and Electron Optics (2015), http://dx.doi.org/10.1016/j.ijleo.2015.09.090 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Growth and Characterization studies of L-threonine phosphate (LTP) a new semiorganic NLO crystal P. Christuraj1, S. Anbarasu2, P.S. Joseph3 and D. Prem Anand2* 1

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Department of Physics, St. Joseph’s College (Autonomous), Tiruchirappalli - 620 002, Tamilnadu, India. 2

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Department of Physics, St. Xavier’s College (Autonomous), Palayamkottai - 627 002, Tamilnadu, India.

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Corresponding Author: [email protected]

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Abstract:

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Department of Physics, Caauvery College of Engineering & Technology, Perur, Tiruchirappalli - 639 103, Tamilnadu, India

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A semiorganic NLO material L-threonine phosphate (LTP) has been synthesized. Good quality single crystal of LTP of dimension 17 x 4 x 3 mm3 was successfully grown by slow evaporation method. The unit cell dimensions were determined from single crystal X-ray diffraction analysis confirming that LTP crystal belongs to the orthorhombic system with noncentrosymmetric space group Pna21. Orientation of hkl planes were identified by Powder XRD technique. The presence of functional groups was estimated qualitatively by FTIR analysis. 1H1 NMR spectral analysis confirmed the presence of hydrogen bonded network in the grown crystal. The optical cut-off wavelength of LTP was found to be 230 nm. The variations of dielectric constant (εr) and dielectric loss (D) with frequency at different temperatures were studied. Thermal stability was studied through TG/DSC measurements. SHG efficiency of the LTP crystal is 1.6 times that of KDP determined by Kurtz-Perry powder test.

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Key-words: L-threonine phosphate (LTP), XRD, FTIR, UV-vis-NIR, Dielectric, TG/DSC, SHG 1. Introduction

Second order nonlinear optical (SONLO) materials are technologically powerful tools that underpin laser generation typically advancing in photonics and electronics industries [1], optical computers [2], isotope separation [3], plasma accelerators [4], information processing [5] space communication [6] detectors [7] medicinal biology [8] etc. These SONLO materials are of organic and inorganic crystalline molecules lacking a centre of symmetry. The investigation on organic compounds of acentric molecules shows highly delocalized π – electron system exhibits a high value of second order polarizability [9] due to electrically ground state charge assymmetrization [10] attained by interaction between electron donor and acceptor groups. But the weak Van -der walls impedes their chemical, mechanical and thermal stabilities. Developing new SONLO crystal fabrication technology that overcomes the limitations found in organic

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crystals is necessary, by designing new class materials called semiorganic crystals where the inorganic crystalline flexible features of chemical, thermal and mechanical stabilities can be incorporated into those organic host moieties. Alpha amino acids, the organic compounds are featured by characteristic homochirality [11], zwitterionic nature, transparency window in UVvis region are suitable for NLO applications, but poor stability damages its features. Phosphorylation makes good changes in chemical, thermal stability of organic molecules and excels the NLO efficiency. Phosphorylated amino acid crystals have excellent chemical, thermal, flexibilities and electro-optic properties. Excellent SHG efficiency greater than quartz, higher laser damage threshold and critical phase matching was observed in L-arginine phosphate [1214]. Phosphorylation plays an important role in structural modulation of bio-proteins [15] and some amino acids [16]. Phosphorylated L-alanine molecules [17] crystallize in noncentrosymmetric space group P21. Motivated by these findings we propose a new source material composed of an organic amino acid L-threonine and inorganic phosphoric acid. Lthreonine is the only alpha amino acid with secondary hydroxyl in polarizable uncharged side chain and chiral beta carbon. Beside these, it has dipolar ions (acidic COO- and basic NH3+) giving good electro-optic parameters [18]. G. Ramesh kumar et al [19-20] investigated the growth, thermal and nonlinear properties of L-threonine single crystals and it reveals that it has SHG efficiency 1.2 times greater than KDP and decomposes at 270 oC. The UV cut-off wavelength of L-threonine was found to be 230 nm [21]. The variance in optical and dielectrical properties of L-threonine with respect to isoelectric pH values was studied [22]. As a dopenant, L-threonine excels the SHG efficiency in KDP crystals [23]. In our present work we aim to synthesize the new semi organic compound L-threonine phosphate (LTP) through phosphorylation of L-threonine and grow in solution growth technique. The structural, physicchemical and optical properties of LTP were characterized by Single crystal XRD, Powder XRD, FTIR, proton NMR, UV-vis-NIR, Dielectric measurement TG/DSC, NLO Test,

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2. Experimental Techniques 2.1. Chemicals

Commercially available high purity L-threonine (99.9%) and ortho phosphoric acid were purchased from E-Merck Co Ltd. 2.2. Synthesis of L-threonine phosphate Aqueous solution of L-threonine of 100 mL was taken in a 250 mL borosil beaker. orthophosphoric acid was added drop by drop into aqueous solution of L-threonine in the appropriate stoichiometric ratio. The resultant solution was stirred for three hours at the temperature of 40oC and then cooled to room temperature. A white colour crystalline salt was obtained at the bottom of the beaker and it was filtered. The synthesized salt of LTP was purified by a repeated recrystallization process. The reaction scheme was shown in Fig.1. 2.2. Solubility studies

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The synthesized salt was used to measure the solubility of LTP crystal in doubly deionized water. A 250 ml borosil glass beaker filled with 100 ml doubly deionized water was placed inside a constant temperature bath. An acrylic sheet with a circular hole at the middle was placed over the beaker through which a spindle from an electric motor, placed on the top of the sheet was introduced into the solution. A Teflon paddle was attached at the end of the rod for stirring the solution. The synthesized salt was added in small amounts with doubly deionized water and stirring was continued till the formation of precipitate, which confirmed the supersaturation of the solution. A 20 ml of the saturated solution was withdrawn by means of a warmed pipette and the same was poured into a clean, dry and weighed Petri dish. The solution was kept in a heating mantle for slow evaporation till the whole of the solution got evaporated and the mass of the LTP salt in 20 ml of solution was determined by weighing the Petri dish with salt and hence the solubility, i.e quantity of salt in gram dissolved in 100 ml of the solvent was determined. The solubility of LTP crystals in doubly deionized water solvent was determined for five different temperatures (30, 35, 40, 45 and 50 oC) by adopting the same procedure. The resulting solubility curve of pure LTP is shown in Fig.2. 2.3. Growth of L-threonine phosphate

3. Characterization

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3.1. Single crystal XRD

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The as obtained crystalline salt of LTP dried at room temperature was used to prepare saturated solution with doubly deionized water as the solvent. By slow evaporation at room temperature optically transparent crystals of dimension 17 x 4 x 3 mm were harvested in a period of 25 to 30 days. Fig.3 shows the as grown LTP single crystal by slow evaporation method.

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X-ray diffraction data for the structure analysis was collected by a single crystal X-ray diffractometer (Model; Brucker - Nonius K alpha Apex II CCD). Single crystal XRD analysis confirms that the grown LTP single crystal belongs to orthorhombic system with the space group P and the lattice parameters are a= 5.159 Å b = 7.759 Å, c = 13.659 Å, α = 90°, β = 90°, γ = 90° and V = 546.751 Å3.

3.2. X-ray powder diffraction (XRPD) studies The crystallinity and purity of the as grown LTP single crystal was assured by X-ray powder diffraction analysis. Powder XRD pattern was recorded using XPERT Powder diffractometer by scanning the powdered sample using CuKα radiation of wavelength λ = 1.5418 Å over the range of 10-80o with a scan speed of 0.2o/sec and the diffractogram is shown in Fig.4.

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The lattice parameter values obtained from XRD were applied to simulate the hkl value and the corresponding d values were calculated. Using the simulated hkl values and the experimental set of d values, the hkl index of the corresponding reflecting planes were enumerated by manual indexing [24]. A major orientation of (100) planes is present while other orientations like (110) and (210) are also seen comparatively lower intensities.

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3.3. FTIR

3.4. NMR spectral analysis

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The FTIR spectrum was recorded in the range 400–4000 cm-1 employing Brukker model IFS 66 V FTIR spectrometer by KBr pellet technique. Fig.5 shows the characteristic vibrational frequencies of the functional groups observed in FT-IR spectrum of LTP. A broad band of peaks in the region 3100-2600 cm-1 correspond to NH3+ stretching and multiple combination and overtone bands extend this absorption to about 2000 cm-1 [25]. The peak at 3031 cm-1 is attributed to N-H stretching in -NH3+ group. The combination -NH3+ overtone extends to 2048 cm-1. The sp3 hybridization is present in C-H absorption peak at 2864 cm-1. The NH3+ ion group absorbs nearly 1660-1610 cm-1 and 1550-1485 cm-1 resulting respectively from asymmetrical and symmetrical N-H bending. They were observed at 1631 and 1485 cm-1 respectively. The carboxylate ion group presents its peak at 1417 by COO- symmetric stretching . The presence of phosphate ion is confirmed by [PO4]3- stretching observed at 1112 and 1038 cm-1. The peaks at 930 and 872 cm-1 correspond to P-OH stretching. The peak observed at 767 cm-1 is assigned to P-O stretching [26]. All other peaks and their corresponding assignments are represented in Table.1. Thus the presence of functional groups phosphate ion group and zwitterions (COO- and NH3+) are confirmed by their characteristic vibration modes.

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The proton NMR spectral analysis of LTP crystalline sample was carried in deuterated chloroform (CDCl3) using BRUKER ARX 300 spectrometer at 300 K. Fig.6 shows 1H1 NMR spectrogram indicating the chemical shift (δ) values for various types of protons in different groups. The existence of hydrogen atoms in different chemical environments in LTP molecule is represented. The multiple signals in the range 0.93-1.71ppm is pertained to the 3H of the methyl group annexed with the beta Carbon. The hydroxyl group (-OH) protons present its resonance in the range 2.03-2.17ppm. The protons attached with α, β carbons (-CH-) resonate in the range 4.34.41ppm. The signal at 7.4 shows the solvent (CDCl3) peak. The triplet at 8.0 is assigned to the resonance of protons in NH3+ [26]. 3.5. Optical Absorption Study The optical absorbance behavior of LTP single crystal in the range 200-1200 nm at room temperature was measured using UV-vis-NIR spectral analysis employing Varian Carry 5E UVvis-NIR spectrometer. Fig.7(a). shows the recorded UV-vis-NIR spectrum of LTP NLO single crystal. From the curve it is observed that LTP has UV- cut off wavelength at 230 nm. The optical transmittance window was found to be 420-1100 nm. The bathochromic shift was

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identified in the crystal transparency, due to the less polarizable nature of L-threonine cation compared to molecular equivalent. Unlike the inorganic materials, where the large transparency is combined with weaker susceptibilities, [27] This LTP semiorganic crystal has transparency limit with large quadratic susceptibilities. LTP is suitable for SHG laser radiation of 1064 nm and also for other blue laser applications. Fig. 7(b) shows the Tauc’s plot for direct band gap energy. It is find that LTP single crystal has the value of Eg = 3.2 eV which is sustainable for LED fabrication and laser generation. 3.6. Dielectric Studies

3.7. TG/DSC Analyses

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Dielectric property is very important to determine the suitability for NLO activities. LTP crystal was subjected to dielectric measurement using HIOKI 3532-50 LCR HITESTER. The crystal sample was cut and polished by paraffin oil and fine grade alumina powder. Silver paint was coated on the crystal faces ensuring good electrical contact between the crystal and electrodes. Then the prepared sample was mounted between copper platforms and electrodes. A sinusoidal potential was applied to the sample through the electrodes for varying frequencies ranging from 5 Hz - 5 MHz at various temperatures (40, 50, 60 and 70 oC ). Fig.8 (a & b) illustrate the variation of dielectric constant and dielectric loss respectively as a function of frequency. Form the Fig. 8 (a), it is found that the dielectric constant increases with frequency increases in the lower frequency region up to 100 Hz and attain the maximum value at all temperatures. This is due to the electronic, ionic, dipolar and space charge polarizations [28]. Then the dielectric constant is inversely proportional to frequency in the higher frequency region because space charge polarizability vanishes at higher frequency region. The variance of dielectric loss as shown in Fig. 8 (b) is also similar to dielectric constant. This behavior enhances LTP as the potential candidate for photonic, laser and other NLO devices

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The thermal properties of LTP NLO single crystal was studied by measuring its thermal stability and decomposition nature through Thermogavimetry (TG) and Differential Scanning Calorimetry (DSC) analyses. The quantitative information about weight change was obtained in TG study. The heat change associated with the physico-chemical transitions occurred in the material which is under varying temperature was detected by DSC. TG and DSC were simultaneously carried out employing NETZSCH STA 409C thermal analyzer in the temperature range 40-1000 oC in nitrogen atmosphere with heating rate of 5 oC/min. Fig.9. shows the TG/DSC curve of LTP single crystal. It was observed from TG curve that LTP crystalline compound of 3.524 mg initially taken kept its weight with negligible loss till 200 oC. It reveals the absence of inclusion of water in the crystal lattice, which was used as a solvent for crystallization [29]. Major weight loss starts from 210 oC and continues up to 400 oC. The final residue left at 950 oC is 0.5%. The TGA draws a single stage decomposition curve for LTP single crystal. From the DSC curve, it is observed that a single endothermic peak at 252.75 oC, the melting point of LTP. which is greater than most of the organic molecules.

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3.8. NLO Test

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SHG measurement of LTP was tested using Kurtz-Perry powder technique. The optical source Nd: YAG laser having fundamental radiation of 1064nm with an input power 0.68 J directed on to the powdered sample through a visible blocking filter. The 532nm radiation was collected by a monochromator after spreading the 1064nm pump beam with an infrared blocking filter. Powder SHG efficiency was measured by a photomultiplier tube. A sample of KDP was used as a reference material for the present measurement. It is found that the LTP single crystal has an efficiency of 1.6 times that of KDP. This highlights that the LTP crystal has potential SHG applications.

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4. Conclusion:

References

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L-threonine phosphate (LTP), a new semiorganic crystalline material was synthesized and successfully grown by using a slow evaporation technique at room temperature. The solubility studies have been carried out in the temperature range 30-50 oC. Structural analysis was carried out by XRD and XRPD techniques. The crystal belongs to orthorhombic system with noncentrosymmetric space group. The functional groups were confirmed by FTIR and the chemical shifts in 1H1 spectrum confirm the formation hydrogen bonded network in the LTP crystal. The optical band gap energy (Eg) is 3.2 eV calculated through optical absorption study. The normal dielectric behavior of LTP crystal was established by dielectric measurements. By a modified Kurtz-Perry method, the powder SHG efficiency was found to be 1.6 times that of KDP. LTP has wide transparency enables for frequency doubling owing to its SHG efficiency. The observed properties of LTP with large photon absorption band gap, low dielectric constant at higher frequencies, excellent thermal stability reveal the potential application of this crystal towards photonic, electro-optic and SONLO devices.

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[12]. Yu.I. Smolin, A.E. Lapshin, G.A. Pankova, Crystal structure of L-alanine phosphate, Crystallography Reports,48(2) (2003) 318-320.

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[20]. G. Ramesh Kumar, S. Gokul Raj, Amit Saxena, A.K. Karnal, T. Raghavalu, R. Mohan, Deutration effects on structural, thermal, linear and nonlinear properties of L-threonine single crystals, Materials Chemistry and Physics 108 (2008) 359-363.

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[21]. Structural characteristics and second harmonic generation in L-threonine crystals, M.R. Suresh Kumar, H.J. Ravindra, A. Jayarama, S.M. Dharmaprakash, J. Cryst. Growth 286 (2006) 451-456.

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[22]. G. Ramesh Kumar, S. Gokul Raj, R. Mohan, R. Jayavel, Influence of isoelectric pH on the Growth Linear and Nonlinear Optical and Dielectric Properties of L-threonine Single Crystals, Cryst. Growth & Design 6(6), (2006) 1308-1310.

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[23]. S.K. Kushwaha, M. Shakir, K.K. Maurya, A.L. Shah, M.A. Wahab, G. Bhagavannarayana, Remarkable enhancement in crystalline perfection, second harmonic generation efficiency, optical transparency and laser damage threshold in potassium dihydrogen phosphate crystals by L-threonine doping, J. Appl. Physics 108 (3) (2010) 033506.

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[24]. R. Hesse, Indexing powder photographs of tetragonal, hexagonal and orthorhombic crystals, Acta Cryst.1 (1948), 200-207].

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[25]. R.M. Silvestein and F.X. Webster, Spectrometric Identification of Organic compounds, 6th edCompounds, sixth ed., 2005, Wiley; India.

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[26]. D.L.Pavia, G.M. Lampman, G.S.Kriz, J.A.Vyvyan, Introduction to Spectroscopy, 4th ed., CengageLearning, USA, 2009. [27]. S. Manivannan, S. Dhanuskodi, K. Kirschbaum, and S. K. Tiwari, Design of an Efficient Solution Grown Semiorganic NLO Crystal for Short Wavelength Generation: 2-Amino-5nitropyridinium Tetrafluoroborate, CRYSTAL GROWTH & DESIGN 2005 5(4) (2005) 14631468. [28]. B. Viswanarthan, Structure and Properties of Solid State materials, Narosa Publishing House, New Delhi, India, 2009. [29]. K. Sethuraman, R. Ramesh Babu, R. Gopalakrishnan, P. Ramasamy, Synthesis, Growth and Characterization of a new semiorganic nonlinear optical crystal: L-alaline sodium nitrate (LASN), Crystal growth and Design, 8(6) (2008) 1863-1869. List of Figures:

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Fig.1. Reaction scheme of L-threonine phosphate Fig.2. Solubility curve of LTP Fig.3. Photograph of as grown LTP NLO single crystals by slow evaporation method

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Fig.4. Powder X-ray diffractogram LTP NLO single crystal Fig.5. FTIR spectrum of LTP NLO single crystal

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Fig.6. Proton NMR of LTP NLO single crystal

Fig. 8 (a) Dielectric Constant of LTP NLO single crystal

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Fig. 8(b) Dielectric Loss of LTP NLO single crystal

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Fig.7(a). UV-vis-NIR spectrum (b) Tauc’s plot for direct band gap energy for LTP NLO single crystal

Fig.9. TG-DSC curve of LTP NLO single crystal

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List of Tables:

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1. Vibrational assignments of LTP single crystal

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Figures: OH

NH2 OH

O HO

OH

O

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HO

P

O P

OH

-H2O L-Threonine

OH O

O

L-Threonine phosphate

ortho-phosphoric acid

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Fig.1. Reaction scheme of L-threonine phosphate

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Solubility (g/10ml)

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NH2

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10 5 30

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Temperature ( C)

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Fig.2. Solubility curve of LTP

Fig.3. Photograph of as grown LTP NLO single crystals by slow evaporation method

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(100)

12000

6000

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Intensity

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(210)

(014)

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Angle 2 (theta)

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Fig.4. Powder X-ray diffractogram LTP NLO single crystal

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4000

2500

488

767

1500

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2000

1343 1112 1038

2864

3031

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1631 1485 1417

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2048

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Transmittance (%)

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wavenumber (cm )

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Fig.5. FTIR spectrum of LTP NLO single crystal

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Fig.6. Proton NMR of LTP NLO single crystal

2.0 1.8

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1.2 1.0

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0.2

200

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wavelength (nm)

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(h)

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Absorbance

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Photon Energy (eV)

Fig.7(a). UV-vis-NIR spectrum (b) Tauc’s plot for direct band gap energy for LTP NLO single crystal

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6 5

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Dielectric co

70 C o 60 C o 50 C o 40 C

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95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0

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Fig. 8 (a) Dielectric Constant of LTP NLO single crystal

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Fig. 8(b) Dielectric Loss of LTP NLO single crystal

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Fig.9. TG-DSC curve of LTP NLO single crystal

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Table.1. Vibrational assignments of LTP NLO single crystal mode of vibrations

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C-C torsional modes

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P-OH deformation

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N-H torsional mode

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C-C-N stretching

1348

CH3 bending

3169

-OH stretching

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wavenumber

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