# Growth Aspects, Structural and Optical Properties of 2-aminopyridinium 2,4 Dinitrophenolate Single Crystal

S Reena Devi, B Valarmathi, R Mohan Kumar

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S Reena Devi, B Valarmathi, R Mohan Kumar. Growth Aspects, Structural and Optical Properties of 2-aminopyridinium 2,4 Dinitrophenolate Single Crystal. Mechanics, Materials Science & Engineering Journal, 2017, 9, 10.2412/mmse.6.55.34. hal-01500567

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# Growth Aspects, Structural and Optical Properties of 2-aminopyridinium 2,4 Dinitrophenolate Single Crystal

S. Reena Devi<fnref n="1" />, B. Valarmathi<fnref n="1" />, R. Mohan Kumar<fnref n="1" />, a

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

DOI 10.2412/mmse.6.55.34 provided by Seo4U.link

**Keywords:** Organic compounds, solution growth, Laser damage threshold, Nonlinear optic materials.

**ABSTRACT.** Organic single crystal of 2-aminopyridinium 2,4-dinitrophenolate single crystal was grown by slow evaporation technique. The cell parameters and space group (P̅1) were determined from single X-ray diffraction analysis. HRXRD studies ascertained the crystalline quality. UV-Visible and PL spectral studies revealed the emission in red region, transparency (75%) cutoff wavelength around 440 nm respectively. The laser damage threshold of grown crystal was estimated by using Nd:YAG laser beam and these results were mutually related with specific heat capacity of the grown crystal. The third-order nonlinear optical parameters were estimated by Z-scan technique which is useful for optical applications.

**Introduction.** Organic nonlinear optical materials have many advantages over inorganic nonlinear optical materials due to their potential applications in scientific and technical fields like high density optical data storage, ultra compact laser and electro-optical amplitude modulation [1]. Generally, organic materials are having donor and acceptor groups located at either end of a required conjugation path and the organic backbone delocalized π-electron determines high molecular polarizability and thus exhibiting third order optical nonlinearity [2]. 2-aminopyridine, a heterocyclic molecule contains two nitrogen atoms which are used to understand the nucleic acid bases. Nitrophenolate derivatives are interesting NLO candidates and phenolic OH favors the formation of salts with large hyperpolarizabilities. 2-aminopyridine compound intercalated with 2,4-dinitrophenol forms hydrogen bonds which are having charge transfer complexes of donor π-acceptor structure. The structure of 2-aminopyridinium 2,4-dinitrophenolate in stoichiometry ratio was reported [3]. In the present investigation, 2-aminopyridinium 2,4-dinitrophenolate single crystal was grown by low temperature solution method and its structural, UV-visible, laser damage threshold, specific heat and third order nonlinear optical properties have been studied.

## Experimental

### Material Synthesis and Crystal Growth.

The 2AP24DNP compound was synthesized from 2-aminopyridine and 2,4-dinitrophenol starting materials and they were purchased from (Sigma-Aldrich 97%) and (AR grade 90%) respectively. They were taken in equimolar ratio and dissolved in mixed solvent of water and ethanol. The solution was stirred about 4 hr to obtain uniform density solution and it was filtered by using whatman filter paper. The saturated solution was allowed to evaporate at room temperature and after three weeks

<footnote n="1">1 – Department of Physics, Presidency College, Chennai, India</footnote>

<footnote>a – mohan66@hotmail.com</footnote>
period a good quality of 2AP24DNP crystal was harvested. The photograph of as grown 2AP24DNP crystal is shown in Fig.1(a).

## Results and Discussion

**X-ray Diffraction Analyses.** The grown crystal was subjected to single crystal XRD analysis and the lattice parameter values were found to be a = 7.6303 Å, b = 9.3142 Å, c=17.2518Å, α = 90.339° β = 99.468° γ = 99.556° and the volume of the unit cell V = 1191.91 Å³. Single crystal XRD data confirms that the grown crystal belongs to triclinic crystal system with the space group (P1). From the Fig.1(b), the HRXRD diffraction curve contains a single peak and represents that the 2AP24DNP crystal is free from structural grain boundaries. The full width at half maximum of the curve was found to be 6 arc sec which is somewhat more than that expected from plane wave theory of dynamical X-ray diffraction for an ideally perfect crystal [4]. This rocking curve clearly indicates that the crystal possess significantly interstitial defects instead of vacancy defects. However, these interstitial defects may be due to the self interstitials, impurity atoms including solvent atoms or molecules in the crystalline material. In this study, the single diffraction peak with low FWHM indicates that the crystalline perfection is fairly good.

<figure><img src="image_3.png" /><figcaption>Fig. 1. (a) Photograph of as grown crystal of 2AP24DNP, (b) HRXRD diffraction curve of 2AP24DNP crystal.</figcaption></figure>

**UV-visible and Photoluminescence studies.** UV-Vis spectrum of grown 2AP24DNP single crystal was recorded by using Perkin-Elmer Lambda spectrometer in the wavelength range 400-800 nm. In the UV-visible studies, transmission range and cut off wavelength are important criterion. 2AP24DNP single crystal showed 75 % transmission with lower cut off wavelength around 440 nm as shown in Fig.2(a).

The optical absorption coefficient (α) can be calculated with transmission (T) value by using the relation,

$$ \alpha = \frac{2.3026}{t} \log (1/T) \qquad (1) $$

where t is the thickness of the sample. The optical band gap determined from transmission spectrum and absorption coefficient (α) is given by:
$$ (\alpha h\nu)^2 = A (E_g - h\nu) \qquad (2) $$

$$ E_g = \frac{1240}{\lambda} eV \qquad (3) $$

where $\lambda$ is the lower cut off wavelength, $E_g$ is the optical band gap of the material, $A$ is the constant, $h$ is the Planck's constant and $\nu$ is the incident photon frequency. The optical band gap of 2AP24DNP crystal was estimated by plotting $(\alpha h\nu)^2$ against $h\nu$ as shown in Fig. 2(b). The band gap value was evaluated by extrapolating the linear part. It was found to be 2.8 eV which is in good agreement with theoretical value of optical band gap.

Photoluminescence is an important optical property for investigating the crystalline quality and fine structure of exciton which is obtained by monitoring emission at a fixed wavelength while varying the excitation wavelength. PL study was carried out by spectrofluorometer and the recorded spectrum of 2AP24DNP crystal is shown in Fig. 2(c). A broad emission peak was observed at wavelength of 645 nm with an excitation wavelength of 435 nm and it covered the red region of the visible spectrum which may be attributed to the $\pi^*\to n$ transition. Thus, the PL spectrum revealed the electronic transition and band gap energy of the grown 2AP24DNP crystal.

<figure><img src="image_4.png" /><figcaption>Fig. 2. (a) UV-Visible transmittance spectrum, (b) Plot of $(\alpha h\nu)^2$ Vs photon energy and (c) Photoluminescence spectrum of 2AP24DNP crystal.</figcaption></figure>

**Laser-induced damage threshold and specific heat capacity.** A Q-switched Nd:YAG laser of 1064 nm fundamental beam with pulse width 10 ns and 10 Hz repetition rate was used as the source. The cut and polished sample was placed at the focus of a plano convex lens of focal length 30 cm and multiple shot mode LDT measurement was made on well polished crystal. An attenuator was used to vary the energy of the laser pulses with a polarizer and a half wave plate. The laser damage threshold value was measured using combination of a phototube and an oscilloscope.

The surface damage threshold of the crystal was calculated using the relation:

$$ \text{Powder density } P(d) = \frac{E}{\pi r^2 \tau} \text{ W/cm}^2 \qquad (4) $$

where $E$ is the energy (mJ), $\tau$ is the pulse width (ns) and $r$ is the radius of the spot (mm).
The laser damage threshold value of 2AP24DNP crystal was found to be 10.54 GW/cm². The specific heat of solid is one of the factors that determine the laser damage threshold value of crystal, the threshold intensity is directly proportional to square root of specific heat of the material. The specific heat capacity of the 2AP24DNP crystal was measured by using DSC thermal analysis in the temperature range 30°C – 95°C at a heating rate of 3.5°C/minute. Fig. 3 shows the dependence of the specific heat of 2AP24DNP with different temperatures.

When the laser radiation penetrated into the 2AP24DNP crystal, it absorbed thermal energy and caused damage on the surface of the crystal. At low temperature gradient, the crystal possessed high specific heat capacity. Hence the laser damage threshold is high for the crystal possessing low temperature gradient. The measured specific heat capacity of the compound is compatible with laser damage threshold value of 2AP24DNP crystal.

<figure><img src="image_5.png" /><figcaption>Fig. 3. Plot of specific heat capacity vs. temperature of 2AP24DNP crystal.</figcaption></figure>

**Z-scan studies.** Z-scan technique is a simple and accurate method for determining third order nonlinear optical response of nonlinear refractive index (n₂) and nonlinear absorption coefficient (β). The favorable advantage of this method is used to measure both the sign and magnitude of nonlinear refractive index and nonlinear absorption coefficient using Z-scan plots measured in closed aperture and open aperture respectively. Z-scan experiment was performed using Nd:Yag laser with 532 nm as an excitation source and its propagation along Z-scan axis.

Fig. 4(a) shows the distinctive closed aperture Z-scan curve of the title crystal with normalized transmittance for the incident intensity. The closed curve was depicted by placing an aperture in front of the detector and sample moved towards the focus resulting higher transmittance (peak) due to increasing intensity. The sample moved away from the focus produced minimum transmittance (valley) due to decreasing intensity. Hence the peak followed by a valley from closed aperture Z-scan of normalized transmittance is the signature of negative refractive nonlinearity (n₂ < 0). The negative nonlinear refractivity indicated the self defocusing effect which is in result of phase distortion transformation of the propagating beam. The value of ‘n₂’ was obtained from the result of the difference between the normalized peak and valley transmittance (ΔTₚ-ₖ) and it was calculated using the relation,

$$ \Delta T_{p-v} = 0.406(1-S)^{0.25}|\Delta\Phi_0| \quad (5) $$

where ΔΦ₀ is the on-axis phase shift at the focus, S is the linear transmittance aperture and it was calculated using the relation,
$$S = 1 - \exp(-\frac{2r_a^2}{\omega_a^2}) \qquad (6)$$

where, $r_a$ is the radius of the aperture and $\omega_a$ is the diameter of the spot size at nearest position of the aperture. The nonlinear refractive index ($n_2$) was determined using the closed aperture Z-scan data using the relation,

$$n_2 = \frac{\Delta\Phi_0}{K I_0 L_{eff}} \qquad (7)$$

where k is the wave number (=2π/λ), $I_0$ is the intensity of laser beam at the focus (z=0) and $L_{eff}$ = {[1- exp(-αL)]/α} is the effective thickness of the sample, where, α is the linear absorption and L is the thickness of the sample [5].

Fig. 4(b) shows the normalized transmission observed in open aperture Z-scan mode. For this measurement the aperture was removed, making the scan insensitive to nonlinear refraction. The intensity distribution of a Gaussian laser beam could be symmetric around the focus (Z=0) where it has minimum transmittance. The nonlinear absorption co-efficient (β) was estimated from the open aperture Z-scan data.

$$\beta = \frac{2\sqrt{2} \Delta T}{I_0 L_{eff}} \qquad (8)$$

where ΔT is the valley point at the open aperture Z-scan data. The value of 'β' could be positive sign for two photon absorption and negative sign for saturable absorption which is equal to,

$$\Delta T = 1 - T_v$$

The enhanced transmission near the focus is indicative of saturable absorption at high intensity. The real and imaginary parts of the third order nonlinear optical susceptibility ($\chi^{(3)}$) were estimated using the relations,

$$R_e \chi^3 (esu) = \frac{10^{-4} (\epsilon_o c^2 n_o^2 n_2)}{\pi} (\text{cm}^2/\text{w}) \qquad (9)$$

$$I_m \chi^3 (esu) = \frac{10^{-2} (\epsilon_o c^2 n_o^2 \lambda \beta)}{4 \pi^2} (\text{cm/w}) \qquad (10)$$

where $\epsilon_o$ is the vacuum permittivity, c is the velocity of light in vacuum, $n_o$ is the linear refractive index of the sample and $\lambda$ is the wavelength of laser beam.

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

$$\chi^{(3)} = \sqrt{(R_e \chi^{(3)})^2 + (I_m \chi^{(3)})^2} \qquad (11)$$
From the above analysis, the nonlinear refractive index n₂ = 7.32 x 10⁻⁸ cm²/W, nonlinear absorption coefficient, β = 0.37 x 10⁻⁴ cm/W and third order nonlinear susceptibility, χ⁽³⁾ = 8.437 x 10⁻⁶ esu were determined by Z-scan technique.

<figure><img src="image_6.png" /><figcaption>Fig.4. Z-scan plot of 2AP24DNP crystal measured in (a) closed aperture mode and (b) open aperture mode</figcaption></figure>

**Summary.** Third order nonlinear organic single crystal of 2AP24DNP with 11x7x4 mm³ dimension was grown by slow evaporation technique. From the single crystal and HRXRD studies, it was observed that 2AP24DNP crystal belongs to triclinic crystal system with the space group (PÎ) and shows good crystalline infallibility. UV-Vis transmittance studies imparted the electronic transition mechanism of ions, crystal transparency, cut-off wavelength and band gap energy of the grown crystal. The laser damage threshold of grown crystal was found and those results were mutually related with specific heat capacity of the grown crystal. The third-order nonlinear optical parameters estimated for grown crystal by Z-scan technique could be useful for optical applications.

**References**

[1] K. Sathishkumar, J. Chandrasekaran, B. Babu, C. I. Sathish, Y. Matsushita, Synthesis, growth and characterization of bis (potassium) 2, 4-dinitrophenolate monohydrate (BPDNP): a new third harmonic generation material. Appl. Phys. A, Vol. 119, pp. 1355-1364, 2015, DOI: 10.1007/s00339-015-9103-6.

[2] I. P. Bincy, R. Gopalakrishnan, Synthesis, growth and characterization of new organic crystal: 2-Aminopyridinium p-Toluenesulfonate for third order nonlinear optical applications, J. Cryst. Growth, Vol. 402, pp. 22-31, 2014 DOI: 1016/j.jcrysgro.2014.03.024.

[3] S. Reena Devi, R. Akilan, R. Mohan Kumar, T. Ganesh, G. Chakkaravarthi. 2-Aminopyridinium2, 4-dinitrophenolate, IUCr Data, Vol. 9, pp. 1,x161489, 2016, DOI: org/10.1107/S2414314616014899.

[4] B.W. Betterman, H. Cole, Dynamical diffraction of X-rays by perfect crystals, Rev. Mod. Phys., Vol. 36, pp. 681-717, 1964, DOI: org/10.1103/RevModPhys.36.681.

[5] P. Nagapandiselvi, C. Baby, R. Gopalakrishnan, Self-assembled supramolecular structure of N,N, N', N'-tetramethylethylenediammonium-bis-(4-nitrophenolate): synthesis, single crystal growth and photo physical properties, RSC Adv., Vol. 4, PP. 22350-22358, 2014, DOI: 10.1039/C4RA02393E.

Cite the paper
S. Reena Devi, B. Valarmathi, R. Mohan Kumar, (2017). *Growth Aspects, Structural and Optical Properties of 2-aminopyridinium 2,4 Dinitrophenolate Single Crystal*. Mechanics, Materials Science & Engineering, Vol 9.
doi [10.2412/mmse.6.55.34](https://doi.org/10.2412/mmse.6.55.34)