Spectral, optical, thermal and mechanical studies on l-Histidine oxalate crystals

2.1 Crystal growth

Single crystals of l-Histidine oxalate (LHO) were grown by slow evaporation method from a saturated aqueous solution containing l-Histidine and oxalic acid in the equimolar ratio. Repeated recrystallization process was employed so as to improve the purity of the solution. Optically clear and well-shaped good quality crystals suitable for usage as seed crystals were obtained in the course of one week. Bulk crystals were grown using the seeds from a saturated aqueous solution of LHO in a crystallizer using the submerged-seed solution growth method. Transparent crystals of size: 4.0 × 2.0 × 2.0 mm³ were obtained in a period of four weeks (Fig. 1).

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Figure 1

Photograph of LHO crystals.

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2.2 Characterization

The grown crystals were subjected to single crystal X-ray diffraction using Nonius CAD-4/MACH 3 Diffractometer, with MoKα radiation (λ = 0.71073 Å). The cell data were obtained from the least-squares refinement of the setting angles of 25 reflections. Density of LHO was found to be 1.61 kg/m³ using the floatation method. The nonlinear optical conversion efficiency was tested using a modified setup of (Kurtz and Perry, 1968). A Q-switched Nd:YAG laser beam of wavelength 1064 nm was used with an input power of 3.5 mJ and pulse width of 10 ns, the repetition rate being 10 Hz. The crystals of LHO were ground to a uniform particle size of about 125–150 μm and packed in a capillary of uniform bore and exposed to laser radiations. The second harmonic signal generated in the crystalline sample was confirmed from the emission of green radiation (λ = 532 nm) from the sample. The intensity of the green light was measured using a photomultiplier tube. The FTIR spectra of the sample were recorded in the frequency region of 400–4000 cm⁻¹ using a BRUKER alpha FTIR Spectrometer at a resolution of 4 cm⁻¹ and with a scanning speed of 2 mm/s. The FT-Raman spectrum of the title crystal was recorded using BRUKER FRA 106 FT-Raman spectrometer for a range of 500–4000 cm⁻¹. Simultaneous thermogravimetric (TG) analysis and differential thermal analysis (DTA) were carried out for the crystals, using a NETZSCH-Gerätebau STA 409 PC thermal analyzer. A powder sample was used for the analysis in the temperature range of 26–800 °C with a heating rate of 10 °C/min. The crucible used was of alumina (Al₂O₃), which served as a reference for the sample. The transmission spectrum was recorded using a UV–Vis–NIR spectrophotometer (UV 2400 PC Series) in the range of 200–1100 nm covering the entire near-ultraviolet, visible and NIR regions. The microhardness of the grown crystals was measured using a Shimadzu microhardness tester (Model No. HMV2T) with a diamond indenter. Well-polished crystals were mounted on the platform of the microhardness tester and loads of different magnitudes (10, 25, and 50 g) were applied over a fixed interval of time. The indentation time was fixed as 15 s.

3.1 Single crystal X-ray diffraction analysis

It was found that the crystals belong to the orthorhombic system with four molecules in the unit cell of dimensions: a = 5.523(2) Å, b = 6.793(1) Å, c = 26.852(3) Å and the space group is P2₁2₁2₁. These values agreed well (within the standard deviations) with those reported in the structural investigation (Ben Ahmed et al., 2011). The valence electron plasma energy, ℏω_p is given by

3.2 Second harmonic generation

A second harmonic signal of 330 mV/pulse was obtained for LHO while the standard KDP crystal gave a SHG signal of 530 mV/pulse for the same input energy. The efficiency varies with the size of the particle and the orientation of the crystallites.

The level of SHG response of a given material is inherently dependent upon its structural features. On a molecular scale, the extent of charge transfer (CT) across the NLO chromophore determines the level of SHG output, i.e. greater the CT, larger the SHG output. The presence of strong intermolecular interactions, such as hydrogen bonds, can extend this level of CT into the supramolecular realm, owing to their electrostatic and directed nature, thereby enhancing the SHG response (Zyss and Oudar, 1982; Oudar, 1977). In LHO, the histidine cation and the oxalate anion are linked through O–H…O hydrogen bonding. The amino N atom of the l-Histidine cation forms N–H…O hydrogen bonds with the oxygen atoms of the oxalate anion. In addition, an N atom in the five membered ring makes N–H…O hydrogen bond with the oxygen atom of the carboxyl group of the other histidine cation. The five membered rings of histidine are arranged as parallel arrays (Fig. 2) with a distance of separation of 3.25 Å, which falls in the category of π–π stacking. The existence of several hydrogen bonds, the parallel stacking of molecules and closer distance between the parallel arrays are the contributing factors for the SHG efficiency of this material.

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Figure 2

The infinite chain of LHO molecule along the b axis (black – carbon, red – oxygen, blue – nitrogen).

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3.3 Spectroscopic studies

3.3.1

3.3.1 FTIR analysis

The recorded FTIR spectra (Fig. 3) were compared with the standard spectra of the functional groups (Socrates, 1980; Silverstein et al., 1991). The peak at 3131 cm⁻¹ is due to asymmetric stretching modes of – ${\text{NH}}_3^{+}$ and the C–H stretching appears at 2915 cm⁻¹. The presence of strong acid C⚌O stretching is evident from the peak at 1714 cm⁻¹. The medium peak at 1597 cm⁻¹ is due to asymmetric bending of ${\text{NH}}_3^{+}$ and also the strong peak at 1502 cm⁻¹ is because of symmetrical bending of ${\text{NH}}_3^{+}$ . The peak at 1340 cm⁻¹ is due to the medium C–H deformation. The presence of O–H…O bending is evident from the wave number 1120 cm⁻¹. Ring symmetric stretching is identified at 832 cm⁻¹.The peak at 708 cm⁻¹ represents C–C deformation. The frequency of ring deformation is observed at 626 cm⁻¹. The corresponding C–C–O deformation is positioned at 522 cm⁻¹ .The sharp peak at 481 cm⁻¹ is assigned to the presence of C–C deformation.

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Figure 3

FTIR spectra of LHO.

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3.3.2

3.3.2 FT-Raman spectral analysis

In the recorded FT-Raman spectra (Fig. 4), at the higher energy region there is an intense peak at 3154 cm⁻¹ showing the presence of ${\text{NH}}_3^{+}$ asymmetric stretching and the peak at 2999 cm⁻¹ is due to strong CH₂ asymmetric stretching. C–H stretching is assigned at 2971 cm⁻¹and weak C⚌O stretching is represented by the peak at 1715 cm⁻¹. The corresponding strong ${\text{NH}}_3^{+}$ symmetric bending is positioned at 1523 cm⁻¹. Based on the peak at 1470 cm⁻¹, COO⁻ symmetric stretching is identified. The presence of strong CH₂ twist is evident from the peak at 1362 cm⁻¹.Strong C–C stretching and C–O stretching appear at 1223 cm⁻¹ and the peak at 1186 cm⁻¹ represents weak C–O stretching. Medium C–H in plane bending is observed at 1149 cm⁻¹ and the peak at 1093 cm⁻¹ denotes weak O–H…O in-plane bending. Ring asymmetric stretching is seen at 991 cm⁻¹ and at 959 cm⁻¹. The peak at 888 cm⁻¹ is because of C–C–O symmetric stretching and at 817 cm⁻¹ is due to medium ring symmetric stretching. C–C deformation is identified at 705 cm⁻¹. Weak ring deformation occurs at 621 cm⁻¹. Weak C–N and C–C–O deformations are assigned at 525 cm⁻¹. The peak at 483 cm⁻¹ may be for the deformation of C–C.

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Figure 4

FT-Raman spectra of LHO.

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3.3.3

3.3.3 UV–Vis–NIR studies

The UV–Vis–NIR spectrum (Fig. 5) contains a strong absorption occurring at 252 nm, as found in most of the amino acid complexes and also it is transparent in the entire visible and near infrared region. The absence of strong absorption in the region between 252 and 1100 nm shows that this crystal can be used for the fundamental and second harmonic generation of Nd:YAG laser. This is an important requirement for a material to be nonlinear.

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Figure 5

UV–Vis–NIR spectra of LHO.

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3.4 Thermal analysis

In the TG/DTA curves (Fig. 6), an endothermic peak observed at 248 °C (in DTA) corresponds to the melting point of LHO. The melting of the crystal and the decomposition of the molecules occur simultaneously. The decomposition is taking place in three stages. In the first stage, two CO₂ molecules from the oxalic acid get liberated leading to 43.29% of weight loss between the temperature regions of 248 °C and 260 °C. Immediately, one more CO₂ molecule from the histidine molecule gets liberated very slowly in the temperature range of 260–482 °C, leading to a weight loss of 28.35%, in the second stage. After that, in the third stage, all the remaining molecules get liberated simultaneously before 630 °C without any residues.

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Figure 6

TG/DTA of LHO.

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3.5 Microhardness studies

Microhardness is one of the important mechanical properties of materials and it plays a vital role in device fabrication. It can be used as a suitable measure for the plastic properties and strength of a material (Selvaraju et al., 2007). The Vickers microhardness of LHO was calculated as 46.0 kg/mm² at 50 g, using the relation H_v = 1.8544 P/d² (kg/mm²), where P is the applied load in kg and d is the average diagonal length of the indentation in mm. When the load was increased to above 50 g cracks developed on the smooth surface of the crystals.