Synthesis, growth and characterization of amino acid based semiorganic crystal bis-thiourea l-lysine monohydrochloride

INTRODUCTION

The search for new non-linear optical materials for tailor made device application had been increasing from the last few decades because of their potential industrial applications especially in the field of photonics for optical switching, optical modulators, data storage devices, telecommunication and high density optical data storage.^1,2 The importance of amino acids and their salts belongs to a family of organic materials that have wider NLO applications.^3-5 Amino acids contain a deprotonated carboxylic acid group (COO^-) and protonated amino group (NH₃⁺). This dipolar nature exhibits peculiar physical and chemical properties in amino acids, thus make them ideal candidate for NLO applications.⁶

Thiourea molecule is an interesting inorganic matrix modifier due to its large dipole moment and its ability to form an extensive network of hydrogen bonds⁷. The nonlinear optical properties of some of the complexes of thiourea, such as bis-thiourea cadmium chloride (BTCC), bis-thiourea zinc chloride (BTZC), and potassium thiourea bromide (PTB) have gained significant attention in the last few years because both organic and inorganic components in it contribute to the process of second harmonic generation.^8,9 The centrosymmetric thiourea molecule, when combined with inorganic salt yield non-centrosymmetric complexes which has the nonlinear optical properties.¹⁰

Owing to this basic nature L-lysine monohydrochloride form a number of salts with different organic and inorganic salts, which also showed NLO properties.¹¹ Hence L-arginine, L-histidine, L-threonine, L-alanine and L-valine have been subjected for the formation of salts with different inorganic acids. As a result very good semiorganic materials such as L-arginine phosphate monohydrate¹², L-histidine hydrochloride¹³, L-alanine cadmium chloride¹⁴, L-valine hydrochloride¹⁵ are some of example which proved very suitable material for NLO applications. In the organic class α-amino acids exhibit some specific features such as molecular chirality, weak vanderwaals, hydrogen bonds and the absence of strongly conjugated bonds, wide transparency ranges in visible and UV spectral region and zwitterionic nature of the molecule which favours crystal hardness.¹⁶ The present research article focused on synthesis, growth and characterization of Bis-thiourea L-lysine monohydrochloride crystal by solution growth method.

METHODS

Synthesis of BTLM

BTLM single crystals were synthesized using Bis-thiourea (AR Grade, Merck) and L-lysine monohydrochloride (AR grade, Merck) in deionized water by stoichiometric incorporation of Bis- thiourea and L-lysine monohydrochloride in the ratio 2:1 according to the following chemical reaction,

2(CS (NH₂) ₂) + H₂N (CH₂)₄CH (NH₂) CO₂H.HCl ➢ 2(CS (NH₂)₂) H₂N (CH₂)₄CH (NH₂) CO₂H.HCl

Bis-thiourea + L-lysine monohydrochloride ➢ BTLM

The mother solution was thoroughly stirred using magnetic stirrer to form a homogeneous solution. Since thiourea has the coordinating capacity to form different phases, the mixture of the reactants had to be stirred well to avoid co-precipitation of multiple phases. Purity of the synthesized salt was improved by successive recrystallization process. The molecular structural diagram of BTLM is shown in Figure 1.

Crystal growth of BTLM

The prepared BTLM solutions were stirred vigorously for 8 hours using magnetic stirrer. High degree of purity of synthesized salts was achieved by successive recrystalization process and filtration. Whatmann filter paper of micron pore size was used for filtration. The filtered solutions were poured into a beaker and covered with porous paper and housed for slow evaporation of the solvent at room temperature. After a time span of 40 to 45 days, good quality BTLM crystals of size 7mm × 4mm × 2mm were harvested. The photography of as grown crystal of BTLM is shown in Figure 2.

RESULTS

Characterization

Single crystal X-ray diffraction analysis of BTLM crystal

A tiny crystal of BTLM was subjected to single crystal X-ray diffraction analysis using ENRAF NONIUS CAD 4-F single X-ray diffractometer with MOK_α(λ=0.717Å) radiation. The calculated lattice parameter values are a = 14.41 Å, b = 5.89 Å and c = 15.52 Å, α = 90.00º, β = 90.13º, γ = 89.98º and Volume V = 1317.258 ų, which reveals that the grown BTLM crystal crystallizes in orthorhombic crystal system with non-centrosymmetric space group P2₁2₁2₁.

Powder X-ray diffraction analysis of BTLM crystal

A powder sample of Bis-thiourea L-lysine monohydrochloride (BTLM) was analyzed using BRUCKER Germany (Model D8 advance) X-ray diffractrometer CuKα (λ=1.5405 Å) radiation. The powder XRD pattern is shown in Figure 3. From the powder XRD pattern, a well-defined Bragg’s peak confirms the crystalline nature of the grown BTLM crystal. The (hkl) values are indexed for corresponding intensity value using INDX software.

FTIR spectral analysis

A freshly crushed powder of BTLM crystal was subjected to FTIR studies using thermo Nicolect v-200 FTIR spectrometer by KBr pellet method in the range 500-4000 cm^-1. The presences of functional groups are identified by FTIR spectrum which is shown in Figure 4. The absorbed frequencies and their assignment of 2APKHP crystals are shown in the Table 1. The broad band in the higher energy region around 3381 cm^-1 is due to NH₂ asymmetric stretching. The strong but broad peaks observed at 3108 cm^-1 due to presence of C-H symmetric stretching. The C-N stretching and bending identified at 2016 cm^-1 and respectively. The absorption band at 1575, 1525 and 914 cm^-1 is due to the N-C-N stretching. The peak at 1144 cm^-1 is attributed to C=H bending. The C-C-N symmetric stretching is found at 791 cm^-1. The band appearing at 736 cm^-1 infers the C-O-H stretching. The COO^- symmetric stretching was observed at 653 cm^-1. The absorption peak at 555 cm^-1 is due to the C-Cl stretching. The S-C-N symmetric stretching was found at 478 cm^-1. The assignments confirm the presence of various functional groups present in the material.

Photoluminescence study

The excitation and emission spectra of 2APKHP was recorded using Cary Eclipse spectrophotometer. The PL study finds wide applications in the field of medical, biochemical and chemical research fields for analyzing compounds. Photoluminescence in solids is the phenomenon in which electronic states of solids are excited by light of particular energy and the excitation energy is released as light. The photon energies reflect the variety of energy states that are present in the material. Figure 5 shows PL emission spectrum recorded in the range of 280–500 nm with an excitation wavelength of 260 nm. The highest emission peak from the spectrum was observed to be at 484.85 nm. Other peaks observed are due to anionic and cationic nature of the sample. From this wavelength it is concluded that BTLM emits blue fluorescence.

Nonlinear optical study

The powder sample of BTLM was subjected to KURTZ and PERRY techniques. A Q-switched Nd: YAG laser emitting 1.06 µm with power density up to 1 GW/cm² was used as a source to illuminate the powdered sample. The sample of good graded crystalline powder with average particle size of about 90µm sand witched between two glass slides using copper spices of 0.4mm thickness. A laser was produced as a continuous laser pulses with repetition rate of 10Hz. The input power was fixed at 0.68 J and the output power was measured as 4.4mJ, which was compared to output 8.8 mJ of standard KDP. The diffusion of bright green radiation of wave length λ=532 nm (P₂ω) by the sample confirms second harmonic generation (SHG). The powder SHG efficiency of Bis-thiourea L-lysine monohydrochloride crystal was about 0.55 times of KDP. The good second harmonic generation efficiency indicates that the BTLM crystals can be used as a suitable material for non-linear optical devices.

Vicker’s microhardness test

Mechanical strength of the materials plays a key role in device fabrication. According to Jiang et al.¹⁷, during an indentation process, the external work applied by the indentor is converted to a strain energy component which is proportional to the volume of the resultant impression. The hardness of a material is influenced by various parameters such as the lattice energy, Debye temperature, and heat of formation and inter atomic spacing. The Vickers hardness indentations were made on the cut and polished samples of BTLM of the crystals grown by slow evaporation method. At room temperature, the load was varied as, 25, 50 and 100 g and the several indentations were made for each load and the diagonal lengths (d) of the indented impressions were measured using Vickers hardness tester (LEITZ WETZLER) fitted with Vickers diamond indenter and attached to an incident light.. Vicker’s hardness number was determined using the formula Hv=1.8544 P/d2 Kg/mm².^18,19 The variation of hardness Hv with load P for BTLM crystal are shown in Figure 6. A plot between log p vs. log d for the grown crystal is shown in Figure 7.

The plot between log p and log d yields a straight line graph, and its slope gives the work hardening index n, which is found to be n = 1.7 for grown BTLM crystal. According to Onitsch, the value of ‘n’ should lie below 1.6 for comparatively hard materials, whereas it is above 1.6 for softer ones.²⁰ The microhardness study indicates that the crystal belongs to the class of soft materials.

Dielectric studies

Dielectric properties were highly correlated with the electro-optic property of the crystals.²¹ The magnitude of the dielectric constant depends on the degree of polarization (charge displacement) in the crystals. The dielectric experiment is carried out within the frequency range of 50 Hz to 7 MHz at various temperatures using HIOKI 3532-50 LCR HITESTER. The cut and polished rectangular shaped BTLM crystal is subjected to dielectric studies. Silver coating is provided on the electrode and the two phases of the crystal to make firm electrical contact. The capacitance (C_cryst) across the sample and the dielectric loss (tan δ) is measured for various temperatures. The frequency dependence of the dielectric constant at different temperatures is shown in Figure 8. It is observed that the dielectric constant has high values at lower frequencies and further decreases with increase in frequency and become independent at higher frequencies. The dielectric constant of the materials is due to the contribution of electronic, ionic, dipolar or orientational and space charge polarizations which high relay upon on the frequencies.²² The higher dielectric constant at lower frequencies is due to all active polarizations. The space charge polarization is generally active at lower frequencies and high temperatures.²³ The variation of dielectric loss with logarithmic frequency (Figure 9) indicates the low value of dielectric loss at higher frequency which suggests that the sample has enhanced optical quality which is of vital importance for non-linear optical materials.

CONCLUSION

A good quality, optically transparent BTLM crystal has been grown successfully by slow evaporation solution growth technique at room temperature. The unit cell parameters are calculated using Single crystal X-ray diffraction analysis. The grown BTLM crystal gy crystallizes in orthorhombic crystal system with non-centrosymmetric space group P2₁2₁2₁. Powder XRD shows good crystalline nature of the as grown crystal. The presences of various functional groups in the grown crystal are identified by FT-IR spectrum. PL emission spectrum recorded in the range of 280–500 nm with an excitation wavelength of 260 nm. The powder SHG efficiency analysis shows that the efficiency of BTLM crystal is 0.55 times than that of KDP. The mechanical behavior reveals that the BTLM crystal belongs to soft category. The dielectric studies show that the dielectric constant and dielectric loss of the crystal decreases exponentially with increase in frequency and same trend were observed for different temperatures.

Funding: No funding sources

Conflict of interest: None declared

Ethical approval: The study was approved by the institutional ethics committee

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