# Growth Aspects, Structural, Thermal and Optical Properties of an Organic Single Crystal: 4-(Dimethylamino)Pyridinium 4-Amino Benzonate Dihydrate

A Thirunavukkarsu, T Sujatha, P R Umarani, A Chitra, R Mohan Kumar

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A Thirunavukkarsu, T Sujatha, P R Umarani, A Chitra, R Mohan Kumar. Growth Aspects, Structural, Thermal and Optical Properties of an Organic Single Crystal: 4-(Dimethylamino)Pyridinium 4-Amino Benzonate Dihydrate. Mechanics, Materials Science & Engineering Journal, 2017, 9, 10.2412/mmse.5.65.404. [hal-01500562](https://hal.science/hal-01500562)

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# Growth Aspects, Structural, Thermal and Optical Properties of an Organic Single Crystal: 4-(Dimethylamino)Pyridinium 4-Amino Benzonate Dihydrate

A. Thirunavukkarsu¹,³, T. Sujatha¹, P.R. Umarani²,³, A. Chitra¹, R. Mohan Kumar³,ª

1 – Department of Physics, S.I.V.E.T College, Chennai-600073, India

2 – Directorate of Collegiate Education, Chennai-600006, India

3 – Department of Physics, Presidency College, Chennai-600005, India

a – mohan66@hotmail.com

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

DOI 10.2412/mmse.5.65.404 provided by Seo4U.link

**Keywords:** Z-scan technique, X-ray diffraction, photoluminescence, mechanical properties, optical properties.

**ABSTRACT.** 4-(dimethylamino) pyridinium 4-aminobenzoate dihydrate (DMAPAB) single crystal was grown by employing the solution growth technique. The grown crystal was characterized by using single X-ray diffraction analysis and found that the title compound crystallizes in triclinic space group P\bar{1}. From the FT-IR spectral analysis, the presence of functional groups of synthesized compound was identified. The transmission spectrum of DMAPAB crystal showed no absorption in the visible region. The fluorescence spectra of the title compound were analyzed to evaluate the excitation states. The thermal stability and decomposition stages of DMAPAB were elucidated from the TGA-DTA studies. Z-scan technique was employed to determine the third-order nonlinearity of grown crystal.

**Introduction.** The development of nonlinear optical materials has been the subject of numerous investigations by both theoreticians and experimentalists in recent years due to their potential applications in optical signal processing. In recent years, researchers have shown much interest in the pyridine family crystals due to their wide transparency window, extended thermal stability and high NLO coefficient. There is much interest in organic nonlinear optical crystals due to their potentially high non-linearity and rapid response in electro-optic devices compared to inorganic NLO materials. The organic NLO crystals play important role in third harmonic generation (THG), frequency mixing, electro-optic modulation and optical parametric oscillation [1]. Pyridine and benzoic acid mixed analog NLO crystals are found to show extended optical and physical properties and hence they can be extensively used for molecular engineering application. Recently, DMAPAB possess the medium strength N—H···O and O—H···O hydrogen bonds connected by the adjacent anions and cations, involving water molecules into three dimensional framework. In continuation with the zeal of developing new organic centric crystal, 4-(dimethylamino)pyridinium 4-aminobenzoate dihydrate (DMAPAB) crystal has been synthesized from aqueous solution. The present investigation deals with the synthesis, growth and characterization of 4-(dimethylamino)pyridinium 4-aminobenzoate dihydrate single crystal.

**Materials and methods.** The starting materials were of analytical grade reagents and the synthesis, growth process were carried out by dissolving 4-Dimethylaminopyridine and 4-aminobenzoic acid in the stoichiometric ratio of 1:1 in aqueous solution. Single crystal of 4-(dimethylamino) pyridinium 4-aminobenzoate dihydrate (DMAPAB) was grown by the slow evaporation solution technique. The crystalline precipitate was formed by proton transfer reaction, where a proton can be transferred from the electron donor group of 4-aminobenzoic acid to the electron acceptor group of 4-Dimethylaminopyridine. The saturated solution was kept at 32°C with
moisture and dust free environment. The repeated recrystallization process improved the purity of the compound. The high pure synthesised compound was used to grow the bulk crystal by slow evaporation method. The reaction scheme and photograph of grown single crystal of DMAPAB are shown in Fig. 1.

<figure><img src="image_3.png" /><figcaption>Fig. 1. (a) Synthesis scheme and (b) Photograph of DMAPAB crystal.</figcaption></figure>

**Experimental.** A Bruker Kappa Apex II single crystal X-ray diffractometer with MoKα (λ = 0.71013 Å) radiation was used to measure cell parameters of DMAPAB crystal with a typical cell dimension of about 0.36 x 0.32 x 0.30 mm³. Powder X-ray diffraction pattern of the grown crystal was recorded by using BRUKER AXS CAD 4 with 1.5406 Å CuKα radiation. The vibrational spectrum was recorded for DMAPAB compound by using FTIR-4100 type spectrometer at a resolution of 4 cm⁻¹ in the range 400-4000 cm⁻¹. UV-Vis transmission region of DMAPAB crystal was ascertained in the range 190-1100 nm by using LABINDIA T90+ UV-Vis spectrophotometer.TG-DTA experiment was carried out by using a NETZSCH STA 409 instrument with a heating rate of 10°C/min ranging from 30 to 500 °C. Z-scan technique was employed to determine the third-order nonlinearity of grown crystal by using 632.8 nm He-Ne laser source.

**Results and discussion.**

**X-ray diffraction studies.** Single crystal X-ray diffraction analysis shows that the title compound crystallizes in the triclinic space group P\line{1}, with cell parameters of a = 9.3402(7) Å, b = 9.7999 (7) Å, c = 10.2132 (8) Å, α = 65.755 (3)°, β = 69.983 (2)°, γ = 89.212 (3)°, V = 792.08 (10) Å³, and Z = 2 [2]. The sharpness of defined Bragg peaks in the powder X-ray diffraction pattern confirmed its crystallinity. X-ray powder pattern of DMAPAB is shown in Fig. 2. The prominent Bragg peaks in the powder X-ray diffraction pattern were indexed. The peak corresponding to (200), (212), (012) planes have maximum intensity counts at 2θ values of 20.10, 22.70 and 18.50 respectively. The strongest diffraction peaks revealed the crystalline nature of the title compound.

**FT-IR analysis.** The infrared spectral analysis is effectively used to understand the chemical bonding and it provides information about the molecular structure of the synthesized compound. The formation of charge transfer complex during the acid: base interaction of 4-Dimethylaminopyridine with 4-aminobenzoic acid is strongly evidenced through the realization of important bands of donor and acceptor in the resultant spectrum of the complex saltas shown in Fig. 3. The bands observed at 3451 and 3350 cm⁻¹ may be due to the asymmetric and symmetric stretching modes of primary amine. A strong broad NH⁺ stretching band observed at 3241 cm⁻¹. The vibrational frequency occurred at 2927 and 2875 cm⁻¹ are due to asymmetry and symmetry stretching vibrations of methylene group. The peaks arose at 2061, 1965 and 1877 cm⁻¹ are due to aromatic overtones. The absorption peak noted at 1602 cm⁻¹ establishes the presence of carbonyl stretching frequency. The band yielded at 981 cm⁻¹ is due to C-N in-plane bending vibration. The broad band observed at 796 cm⁻¹ is due to C-H-N bending vibration. The peak observed at 623 cm⁻¹ is due to the N-H out-of-plane bending [3]. The recorded FT-IR spectrum clearly showed the formation of crystal lattice of DMAPAB.
<figure><img src="image_4.png" /><figcaption>Fig. 2. Powder XRD pattern of DMAPAB crystal. Fig. 3. FT-IR spectrum of DMAPAB.</figcaption></figure>

**UV-Vis transmission studies.** Optical characteristics provide a good way of examining the properties of crystalline materials. Particularly, measurement of absorption coefficient for various energies gives information about band gap of the material. The recorded UV-Vis-NIR transmittance of DMAPAB is shown in Fig.4. From the UV-visible transmittance analysis, it is clear that the grown crystal exhibit good transparency of about 60% in the visible region with the lower cut-off wavelength 318 nm. The optical band gap (Eg) was evaluated from the transmission spectrum and the optical absorption coefficient (α) near the absorption edge was evaluated from the relation:

$$ (\alpha h\nu) = A (E_g - h\nu)^{1/2} \quad (1) $$

where A is a constant, Eg is the optical band gap, h is the Planck's constant and v is the frequency of the incident photons.

The Tauc's plot [4] drawn between the product of absorption coefficient and the incident photon energy (αhν)² with the photon energy (hv) (Fig.5) shows a linear behavior which is considered as evidence for direct transition. The Eg value obtained by extrapolating the curve was found to be 3.63eV. The wide optical band gap of the grown crystal confirms that the crystal possess large transmittance in the visible region.

<figure><img src="image_5.png" /><figcaption>Fig. 4. UV-Vis transmission spectrum of DMAPAB.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Fig. 5. Tauc's plot drawn between $(ahv)^2$ vs photon energy (hν).</figcaption></figure>

**Photoluminescence studies.** Photoluminescence is a contactless, non-destructive method of probing the electronic structure of materials. Fluorescence may be expected generally in molecules that are aromatic or containing multiple conjugated double bond switches for a high degree of resonance stability. The sample was excited at 300 nm and the emission spectrum was measured in the range of 300–650 nm. From the recorded photoluminescence spectrum (Fig. 6), it was observed that the material exhibits a high intense emission peak at 350 nm. Hence, the photoluminescence analysis concludes that DMAPAB crystal exhibits violet fluorescence. The maximum intensity which appears at 350 nm is attributed to $n-π*$ transition with N-H and O-H functional group molecules.

**Thermal analysis.** The thermal analysis of DMAPAB was carried out in nitrogen atmosphere and thermograms of DMAPAB crystal areshown in Fig. 7. In TGA, the first weight loss stage observed at 123°C is due to the presence of water molecules in the synthesized salt. It is seen, that the major weight loss started at 214°C and it continued up to 290°C. It indicates that this huge weight loss is due to the decomposition of functional group molecules in the form of NH and CH₃ evaporation. The synthesized compound DMAPAB has thermal stability upto214°C. In the DTA, the weight loss occurred due to the water molecules; also the sharp endothermic peak observed at 180°C is due to decomposition or melting point of compound of DMAPAB crystal. The sharpness of endothermic peaks observed in DTA indicates the good degree of crystallinity of the sample.

<figure><img src="image_7.png" /><figcaption>Fig. 6. PL spectrum of DMAPAB. Fig. 7. TG/DTA thermogram of DMAPAB.</figcaption></figure>

**Z-Scan Measurement.** The Z-scan study is needed to examine nonlinear optical behavior which is used to find the third-order nonlinearity, changes in the nonlinear refractive index and variation in absorption [5]. The Z-scan plots measured in open and closed aperture modes are shown in Fig. 8.
From the Z-scan data, the difference between the valley and peak transmittance (ΔT<sub>v-p</sub>) was evaluated in terms of the on-axis phase shift at the focus.

$$ \Delta T_{v-p} = 0.40(1-S)^{0.25}|\Delta\Phi_o| \quad (3) $$

where S is the linear transmittance aperture and it was calculated using the relation,

$$ S = 1 - \exp\left(\frac{-2r_a^2}{\omega_a^2}\right) \quad (4) $$

where r<sub>a</sub> is the radius of aperture and ω<sub>a</sub> is the beam radius at the aperture. The nonlinear refractive index (n<sub>2</sub>) was calculated by using closed aperture Z-scan data [6, 7].

$$ n_2 = \frac{\Delta\Phi_o}{KI_o L_{eff}} \quad (5) $$

The third order nonlinear optical susceptibility was calculated using the relation [8],

$$ \chi^{(3)} = \sqrt{(\mathrm{Re}\,\chi^{(3)})^2 + (\mathrm{Im}\,\chi^{(3)})^2} \quad (6) $$

The calculated third order nonlinear optical parameters are given in Table 1. Z-scan studies confirm that the DMAPAB crystal can be a promising NLO material for optical device applications such as optical modulators and optical limiters.

<table><caption>Table 1. Optical parameters of DMAPAB measured in Z-scan experiment.</caption><tr><td>Effective thickness (L<sub>eff</sub>)</td><td>0.9974 mm</td></tr><tr><td>Nonlinear refractive index (n<sub>2</sub>)</td><td>0.9584 ×10<sup>-10</sup>cm<sup>2</sup>/W</td></tr><tr><td>Nonlinear absorption coefficient (β)</td><td>2.4283 ×10<sup>-3</sup> cm/W</td></tr><tr><td>Third-order nonlinear optical susceptibility (χ<sup>(3)</sup>)</td><td>1.8594 ×10<sup>-10</sup>esu</td></tr></table>
<figure><img src="image_8.png" /><figcaption>Fig. 8. Z-scan traces observed in (a) Open aperture and (b) closed aperture modes for DMAPAB Crystal.</figcaption></figure>

**Summary.** Organic single crystal of DMAPAB was grown by the slow evaporation solution growth technique. The crystal structure of DMAPAB was determined by single crystal XRD studies. From the UV-Vis spectral analysis, it was found that the DMAPAB crystal is transparent in the entire visible region and the cut-off wavelength was found to be 318 nm. The photoluminescence analysis concludes that DMAPAB crystal exhibits violet fluorescence. From the TG-DTA curve, it is clearly observed that the synthesised material is stable up to 214 °C. The third order nonlinear optical susceptibility, nonlinear refractive index and nonlinear absorption coefficient were estimated by Z-scan studies.

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Cite the paper
A. Thirunavukkarsu, T. Sujatha, P.R. Umarani, A. Chitra, R. Mohan Kumar, (2017). *Growth Aspects, Structural, Thermal and Optical Properties of an Organic Single Crystal: 4-(Dimethylamino)Pyridinium 4-Amino Benzonate Dihydrate*. Mechanics, Materials Science & Engineering, Vol 9. Doi 10.2412/mmse.5.65.404