# Growth and Characterization of Organo-metallic Single Crystals of (HCLPTM) Heptachloro (L-Proline) TetraMercury (II)

V Revathi Ambika, D Shalini, R Usha, N Hema, D Jayalakshmi

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V Revathi Ambika, D Shalini, R Usha, N Hema, D Jayalakshmi. Growth and Characterization of Organo-metallic Single Crystals of (HCLPTM) Heptachloro (L-Proline) TetraMercury (II). Mechanics, Materials Science & Engineering Journal, 2017, 9, 10.2412/mmse.42.99.188. hal-01503631

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# Growth and Characterization of Organo-metallic Single Crystals of (HCLPTM) Heptachloro (L-Proline) TetraMercury (II)

V. Revathi Ambika¹, D.Shalini¹, R. Usha¹, N. Hema¹, D. Jayalakshmi¹,ᵃ

1. PG & Research Department of Physics, Queen Mary's College, Chennai -600 004, India

a. djayalakshmi2016@gmail.com

<figure><img src="image_2.png" /></figure>

DOI 10.2412/mmse.42.99.188 provided by [Seo4U.link](https://seo4U.link)

**Keywords:** Solution growth, Single XRD, UV, FTIR, microhardness, NLO.

**ABSTRACT.** A new promising organo-metallic nonlinear optical material, was synthesized and single crystals of HCLPTM were grown from solution by slow evaporation technique. Single-crystal X-ray diffractometer was utilized to measure unit cell parameters and to confirm lattice parameter. The optical transmittance window and the lower cutoff wavelength of the HCLPTM have been identified by UV-Vis-NIR studies. The modes of vibration of different molecular groups present in the sample were identified by the FTIR spectral analysis. The mechanical strength of the crystal was analyzed by Vickers hardness test. The second harmonic generation (SHG) efficiency was estimated using the modified Kurtz-Perry powder test and was found to be 2.5 times that of KDP.

**Introduction.** Amino acids are the building blocks of proteins that are considered to be the very important for most of the processes in living organisms [1] and their complexes belong to a class of organic materials find immense nonlinear optical (NLO) applications [2]. The presence of acidic carboxyl and basic amino group paves the way for the formation of zwitter ion or dipolar ion as a result of internal neutralization reaction. This dipolar nature of amino acids helps to improve the crystal's hardness [3] and the physical and chemical properties also making them as a peculiar candidates in nonlinear optical field [4]. The organic materials have excellent properties compared to the conventional inorganic solids which show ultrafast response times, lower dielectric constants, better process ability characteristics and enhanced NLO responses [5]. But the practical applications of the organic nonlinear optical materials are limited because of their poor physicochemical stability and low mechanical strength [6]. For overcoming the above difficulties of the organic NLO crystals, in recent years the organic materials were mixed with inorganic materials to improve their chemical stability, physico-chemical properties, mechanical strength and nonlinear optical coefficients, which are the important parameters for an NLO crystal [7,8]. According to Dewar-Chatt-Duncanson model, the binding between metal and ligand is due to the interplay of donor and acceptor contributions [9]. Yukawa et al.[10] have already reported the structure of heptachloro(l-proline)tetramercury(II) and it belongs to monoclinic crystal system. In the present work we report synthesis, crystal growth, spectral, optical, and dielectric properties of metal coordinated complex of HCLPTM.

The asymmetric unit of the title compound, [Hg₄Cl₈-(C₅H₉NO₂)₂]n, consists of four HgCl₂ units and two L-prolineligands in the zwitterionic form. In each HgCl₂ unit, the HgII ion is strongly bonded to two Cl atoms, and the HgII ions intwo of the HgCl₂ units are chelated by O atoms of two Cl atoms of the ligands, with one strong and one weak Hg—O bond. In the crystal structure, HgCl₂ and L-proline units are linked to form an extended chain along the a-axis. The chain structure is further stabilized by N—H···Cl hydrogen bonds, and the chains are arranged in layers parallel to the ab plane.
The structure of the title compound was originally determined by Ehsan, Malik & Haider [(1996). J. Bangladesh Acad. Sci. 20, 175] but no three-dimensional coordinates are available.

## Experimental Procedure

**Material Synthesis.** The title complex crystal was grown from an aqueous solution employing the slow solvent evaporation solution growth technique at room temperature. The chemicals used for synthesis are of analytical grade and are commercially available. The starting material of L-proline and HgCl₂ was taken in 1:2 stoichiometric ratio to synthesis heptachloro (l-proline) tetramercury (II). The reaction scheme is shown in Eqn. 1.

$$2(\mathrm{C}_{5}\mathrm{H}_{9}\mathrm{NO}_{2}) + 4\mathrm{HgCl}_{2} \rightarrow [\mathrm{Hg}_{4}\mathrm{Cl}_{8}(\mathrm{C}_{5}\mathrm{H}_{9}\mathrm{NO}_{2})_{2}] \quad (1)$$

The calculated amount of mercury chloride was first dissolved in deionized water. L-proline was then added to the solution slowly by stirring. The prepared solution was allowed to dry at room temperature and the salt was obtained by slow evaporation technique. The purity of the synthesized salt was improved by successfully recrystallization. After 25 days of growth, transparent single crystal were obtained by slow evaporation technique and grown crystals are shown in Fig. 1.

<figure><img src="image_3.png" /><figcaption>Fig. 1. Photograph of the HCLPTM single crystal.</figcaption></figure>

## Characterization Technique

**Single Crystal X-Ray Diffraction Studies.** Single crystal X-ray diffraction study was carried out on the as grown HCLPTM single crystal. HeptaChloro(L-Proline)TetraMercury(II) crystal belongs to triclinic system. a = 7.31 Å, b = 9.47 Å, c = 10.50 Å, α = 108.48, β = 107.36, γ = 97.24 and volume V = 638 Å³.

**UV Analysis.** The transmittance spectrum of grown single crystal was recorded using Varian Cary 5E UV-vis-NIR spectrometer in the range of 200-900 nm. From the transmission spectrum (Figure 2), it is observed that the maximum transparency of 88% and UV cut-off wavelength 238nm were observed for the LPDMC single crystal. The very high transmission in the entire visible and nearIR region and short cut-off wavelength facilitates it to be a potential NLO material for second harmonic frequency doubling [11].
<figure><img src="image_4.png" /><figcaption>Fig. 2. UV-transmittance spectrum of HCLPTM single crystal.</figcaption></figure>

**FTIR Analysis.** The powdered specimen of HCLPTM has been subjected to FT-IR analysis by using PERKIN ELMER RX1 Fourier transform infra-red spectrophotometer. Using KBr pellet technique in the wavelength range between 400 and 4000 cm⁻¹ carried out the FTIR analysis of HCLPTM. The recorded Fourier transform infra-red (FT-IR) spectrum of HCLPTM is shown in Figure 4.

<figure><img src="image_5.png" /><figcaption>Fig. 3. FTIR spectrum of HCLPTM single crystal.</figcaption></figure>

The transmission is due to the carboxylate group (COO⁻) of free proline is normally observed in the region 460.04 cm⁻¹, 614.35 cm⁻¹ and 1400.38 cm⁻¹. Where as in the case of HCLPTM, these peaks were shifted to wagging and symmetric stretching respectively. Wagging of CH₂ occurs at 1337.69 cm⁻¹. The symmetric stretching vibrations of C-H are observed at 1036.78 cm⁻¹ and 1163.13 cm⁻¹ C-N symmetric stretching. The CH₂ rocking vibration mode occurs at 837.14 cm⁻¹ and 960.59 cm⁻¹. The low intensity sharp peaks appear at 3160.50 cm⁻¹ and 3522.17 cm⁻¹ N-H asymmetric and symmetric stretching vibrations respectively. The very small intensity peaks appear at 662.58 cm⁻¹ C-Cl occurs. The vibrations at 1619.31 cm⁻¹ and 2763.15 cm⁻¹ is due to NH² in-plane deformation. This observation confirms that proline mercuric chloride exists in the zwitterionic form and the
involvement of NH<sup>2+</sup> in hydrogen bonding is evident by the fine structure of the band in the lower energy region.

**Microhardness Studies.** Hardness is an important mechanical property required for the fabrication of electronic and optical devices. To get accurate result of hardness of the grown crystal, several indentations were made on the sample for different applied loads from 10g to 100g and mean diagonal lengths were measured.

The Vickers hardness number was calculated Using the following expression [12].

$$Hv = (1.8544 P/d^2) \text{ kg/mm}^2 \qquad (2)$$

where P is the applied load in kg and d is the average diagonal length of the indentation in mm.

The hardness value of HCLPTM is found to increase with the applied load. Work hardening coefficient n, a measure of the strength of the crystal, is computed from the logp-logd plot (Fig. 4). It brings forth the fact that the HCLPTM crystals are found to have been improved in mechanical strength. The plot of logp-logd yields a straight line and its slope, and the work hardening index, n are found to 1.33. Hence it is concluded that HCLPTM belongs to hard materials.

<figure><img src="image_6.png" /><figcaption>Fig. 4. Log P Vs Log D of HCLPTM.</figcaption></figure>

**SHG Measurement.** The SHG of the crystal was checked using the powder SHG technique developed by Kurtz and Perry [13]. The grown single crystal was crushed to fine powder and then packed in a micro capillary of uniform bore and exposed to laser radiations. The 532 nm radiation was collected by a monochromater after separating the 1064 nm pump beam with an infra-red blocking filter. The second harmonic radiation generated by randomly oriented micro crystals was focused by a lens and detected by a photo multiplier tube (Hamamatsu R2059). The second harmonic generation is confirmed by the emission of green light and its efficiency is found to be 2.5 times greater than that of KDP crystal.

**Conclusion.** Single crystals of HCLPTM were successfully synthesized and the single crystals have been grown by solution growth technique. From XRD data, it is observed that the grown crystal belongs to triclinic structure. The various functional groups in HCLPTM have been identified from the Fourier transform infrared (FT-IR) analysis. The grown crystal has good transmission window in the visible region between 238 and 900 nm suitable for NLO applications. The mechanical behavior was discussed using Vickers hardness test. The SHG test confirms the second harmonic conversion
efficiency of the crystal and it is found to be 2.5 times better than that of potassium dihydrogen phosphate (KDP).

Acknowledgement

One of the authors (D.JAYALAKSHMI) is grateful to the university grants commission for financial support under minor research project scheme.

References

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Cite the paper
V. Revathi Ambika, D. Shalini, R. Usha, N. Hema, D. Jayalakshmi (2017). *Growth and Characterization of Organo-metallic Single Crystals of (HCLPTM) Heptachloro (L-Proline) TetraMercury (II)*. *Mechanics, Materials Science & Engineering*, Vol 9. doi 10.2412/mmse.42.99.188