# Studies on the Growth, Thermal and Optical Properties of p-methyl Anilinium Malate Single Crystal

S Kalaiyarasi, S Suresh, R Mohan Kumar

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S Kalaiyarasi, S Suresh, R Mohan Kumar. Studies on the Growth, Thermal and Optical Properties of p-methyl Anilinium Malate Single Crystal. Mechanics, Materials Science & Engineering Journal, 2017, 8, 10.2412/mmse.83.31.489. hal-01502431

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# Studies on the Growth, Thermal and Optical Properties of p-methyl Anilinium Malate Single Crystal

S. Kalaiyarasi¹, S. Suresh¹, R. Mohan Kumar¹

1 – Department of Physics, Presidency College, Chennai, India

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

DOI 10.2412/mmse.83.31.489 provided by [Seo4U.link](https://link.springer.com/10.2412/mmse.83.31.489)

**Keywords:** organic compound, solution growth, photoluminescence, nonlinear optical studies.

**ABSTRACT.** Single crystal of a novel p-methyl anilinium malate (PTM) was grown by slow evaporation method. Single crystal and powder X-ray diffraction studies confirm that PTM belongs to monoclinic system with centro-symmetric space group P2₁/c. FTIR spectral analysis showed the presence of functional groups in PTM compound. Thermal studies exhibit that PTM crystals are stable up to 166°C. UV-visible study showed the good transmission region, cut-off wavelength (206 nm) and band gap energy (5.8 eV) and photoluminescence studies explored its efficacy towards device fabrication. The third order nonlinear optical parameters such as the nonlinear refractive index (n₂) = 3.41 × 10⁻⁸ cm²/W, nonlinear absorption coefficient (β) =0.03 × 10⁻⁴ cm/W and third order nonlinear susceptibility (χ⁽³⁾) = 3.77 × 10⁻⁶ esu of PTM crystal were estimated by using Z-scan measurement.

**Introduction.** Recently, much attention has been paid on the development of a novel nonlinear optical (NLO) materials because of their optical applications, such as optical data storage, electro-optical modulation, optical switching, optical frequency doubling and optical communication. The organic compounds are having high nonlinear optical susceptibility (χ) than inorganic materials. The organic materials contain proton acceptor and donor groups positioned at either end of a suitable conjugation path. The efficient optical switching behaviour of third order nonlinear optical organic materials was investigated in recent years. The aim for designing the molecules with high third-order nonlinearity is to incorporate them into device applications. 4-methylaniline contains a proton acceptor amino (NH₂) group, which can create a strong hydrogen bond with organic acids and forms N-H--O, an anilinium group [1]. DL-malic acid one of the simplest chiral dicarboxylic acids, is a suitable building block in crystal engineering and it is used to create two-dimensional anionic networks held together by hydrogen bonds [2]. The structure of the p-methyl anilinium malate compound has been reported [3]. The systematic investigation has been carried on the growth aspects of PTM crystal. The spectral, optical, thermal properties of PTM crystal were studied by using various characterization techniques and results are reported.

**Material synthesis and crystal growth.** p-methyl anilinium malate compound was synthesized from high pure p-toluidine (sigma-Aldrich 99.6%) and malic acid. Equimolar amounts of reactants were fully dissolved in deionized water. The solution was continuously stirred for obtaining homogeneous state and the solution was allowed for evaporation by using a constant temperature bath. After the period of 30 days, a good quality of single crystal was harvested with dimension 14x3x2 mm³ as shown in Fig. 1.
<figure><img src="image_3.png" /><figcaption>Fig. 1. Photograph of PTM crystal.</figcaption></figure>

## Results and discussion

**X-ray diffraction studies.** The single crystal X-ray diffraction study was performed using MoKα radiation from X-ray diffractometer. The estimated cell parameters values of PTM crystal are a = 7.4849 Å, b = 16.1306 Å, c = 10.4904 Å, α = γ = 90°, β = 109.23°, V = 1195.9 Å³ and Z=4. It was found that the grown crystal belongs to monoclinic system with space group P2₁/c. The powder X-ray diffraction of the grown crystal was recorded from 10° to 50° by using CuKα radiation of wavelength 1.5406 Å (Fig.2). The hkl values of prominent planes were indexed.

<figure><img src="image_4.png" /><figcaption>Fig. 2. Powder X-ray diffraction pattern of PTM crystal.</figcaption></figure>

**FTIR spectral studies.** Fourier transform infrared spectrum of PTM was recorded in the range 400-4000 cm⁻¹ by KBr pellet method (Fig.3). The presence of functional groups in the synthesized compound was ascertained and corresponding frequency assignments are given in Table 1. FT-IR spectrum of PTM crystal shows a band at 3432 cm⁻¹ which is assigned to N-H stretching vibrations. The asymmetric and symmetric stretchings observed at 2990 and 2605 cm⁻¹ are due to C-H vibrations. The presence of carboxylate ions confirmed through the asymmetric and symmetric stretching vibrations of COO⁻ at 1591 and 1422 cm⁻¹ respectively. The deformation and wagging vibrations of N-H group yielded peaks at 1243 and 1084 cm⁻¹. The infrared bands appeared at 878 and 803 cm⁻¹ is attributed to C-C stretching vibrations. The absorption occurred at 597, 548 and 484 cm⁻¹ is
characteristics of COO⁻ wagging mode and supports the protonation and the deprotonation of title compound.

<figure><img src="image_5.png" /><figcaption>Fig. 3. Infrared spectrum of PTM.</figcaption></figure>

**TG-DSC analysis.** From the TG-DSC curves (Fig.4), it was observed that, the title compound is thermally stable upto 166°C and the DSC thermal study confirms that the PTM crystal melts at 167°C. There was no major weight loss occured before 167° C. The weight loss started at 167°C due to the liberation of volatile substances such as CO, CO₂ and hydrocarbons. The final stage of decomposition started at 215°C and it prolonged upto 315°C. From these results, it was concluded that the PTM crystal is capable to function at temperature upto 166°C which could be useful in optical applications.

<figure><img src="image_6.png" /><figcaption>Fig. 4. TG-DSC Thermogram of PTM.</figcaption></figure>

**UV-Visible transmission studies.** From the UV-Visible optical studies, the transmission range, transparency, absorption coefficient band gap energy were estimated which are the important parameters for optical applications. UV-vis spectrum of PTM showed good transparency about 72% with lower cut-off wavelength 206 nm. The optical band gap energy (E_g) was estimated using (Eqn.1), and it was found to be 5.8 eV as shown in Fig.5(a) and (b).
$$ (\alpha h\nu)^2 = A (E_g - h\nu) \qquad (1) $$

<figure><img src="image_7.png" /><figcaption>Fig. 5. (a) UV-Visible transmission spectrum (b) Tauc's plot of PTM crystal.</figcaption></figure>

**Photoluminescence spectral studies.** Photoluminescence spectrum was recorded for PTM crystal at room temperature with an excitation wavelength of 250 nm as shown in Fig.6. The sharp spectrum showed a peak centered at 355 nm and no other visible emission peak has been observed. In the present study, a very strong intense emission peak observed at 355 nm ($E_g$ = 3.4 eV) corresponds to near band-edge exitons of as-grown crystal. It may be occurred due to the $n \to \pi^*$ transition. Therefore, the PTM crystals might be suitable for UV filters and optoelectronic laser devices [4].

<figure><img src="image_8.png" /><figcaption>Fig. 6. PL spectrum of PTM crystal with an excitation wavelength of 250 nm.</figcaption></figure>

**Nonlinear optical studies.** Third order nonlinear optical property of PTM crystal has been investigated by Z scan technique and it is an exact method to find the sign and magnitude of nonlinear refractive index ($n_2$) and nonlinear absorption coefficient ($\beta$) of the sample. It is the single beam
method, which utilizes self focusing or self defocusing phenomena in optical nonlinear materials [5]. The Z-scan measurement traces in closed aperture mode and open aperture are shown in Fig. 7(a) and Fig. 7(b) respectively.

The third order nonlinear optical susceptibility was calculated using the relation,

$$ \chi^{(3)} = \sqrt{(R_e\chi^{(3)})^2 + (I_m\chi^{(3)})^2} \quad (2) $$

The third-order nonlinear refractive index ($n_2$) = $3.41 \times 10^{-8}$ cm$^2$/W, nonlinear absorption coefficient ($\beta$) = $0.03 \times 10^{-4}$ cm/W and third order non-linear susceptibility ($\chi^{(3)}$) = $3.77 \times 10^{-6}$ esu were estimated by Z-scan technique.

<figure><img src="image_9.png" /><figcaption>Fig. 7 (a) Z-scan plot of PTM crystal in closed aperture (b) Z-scan plot of PTM crystal in open aperture.</figcaption></figure>

**Summary.** Third-order nonlinear optical PTM single crystal with 14x3x2 mm$^3$ dimension was grown by slow evaporation technique. Single crystal X-ray diffraction studies reveal that the grown PTM crystal belongs to monoclinic system with P2$_1$/c space group. The functional groups present in PTM were confirmed by FTIR spectral studies. TG-DSC thermogram revealed the thermal stability of PTM crystal. UV-visible study showed the good transmission region and the cut-off wavelength, band gap energy were found to be 206 nm and 5.8 eV respectively. Photoluminescence spectral analysis suggests that PTM could be used in UV filters and optoelectronic devices. Z-scan measurements revealed the values of third-order nonlinear refractive index, nonlinear absorption coefficient and third order non-linear susceptibility.

References

[1] J.V. Jovita, K. Boopathi, P. Ramasamy, A. Ramanand, P. Sagayaraj, Synthesis, growth and characterization of 4-methyl anilinium phenolsulfonate single crystal, J. Cryst. Growth, Vol. 380, pp. 218-223, 2013, DOI: 10.1016/j.jcrys gro.2013.06.027.

[2] A. Senthil, P. Ramasamy, Synthesis, growth and characterization of strontium bis (hydrogen l-malate) hexahydrate bulk single crystal: a promising semi-organic nonlinear optical material, J. Cryst. Growth, Vol. 312, pp. 276-28, 2010, DOI: 10.1016/j.jcrys gro.2009.10.021.
[3] S. Kalaiyarasi, S. Reena Devi, R. Akilan, R. Mohan Kumar, G. Chakkaravarthi, 4-Methylanilinium 3-carboxy-2-hydroxypropanoate, IUCrData, Vol.1(9), pp.1,x161525, 2016, DOI:10.1107/S241431461601525X.

[4] S. Sudhahar, M. KrishnaKumar, A. Silambarasan, R. Muralidharan, R. Mohan Kumar, Studies on structural, spectral, and optical properties of organic nonlinear optical single crystal: 2-Amino-4,6-dimethylpyrimidinium p-Hydroxybenzoate, J. Mater., 2013, DOI: org/10.1155/2013/539312

[5] V. Subashini, S. Ponnusamy, C. Muthamizhchelvan, Synthesis, growth, spectral, thermal, mechanical and optical properties of piperazinium (meso) tartrate crystal: A third order nonlinear optical material, J. Cryst. Growth, Vol. 363, pp. 211-219, 2013, DOI: 10.1016/j.jcrysgro.2012.10.045.

Cite the paper

S. Kalaiyarasi, S. Suresh, R. Mohan Kumar, (2017). *Studies on the Growth, Thermal and Optical Properties of p-methyl Anilinium Malate Single Crystal*. Mechanics, Materials Science & Engineering, Vol 9. DOI: 10.2412/mmse.83.31.489