Crystal Growth and Characterization of a New NLO Material: p-Toluidine p-Toluenesulfonate

Single crystals of p-Toluidine p-Toluenesulfonate (PTPT), an organic nonlinear optical (NLO) material, have been grown by slow evaporation method at room temperature using ethanol as solvent. The crystal system was confirmed from the single crystal X-ray diffraction analysis. The functional groups were identified using FTIR spectroscopy. UV-Vis-NIR spectrum showed that the UV cut-off wavelength of PTPT occurs at 295 nm and it has insignificant absorption in the wavelength region of 532–800 nm.The SHG efficiency of PTPT was measured by employing Kurtz and Perry powder technique using a Q-switched mode locked Nd: YAG laser emitting 1064 nm for the first time and it was found to be 52% of standard KDP.Thermal and mechanical properties of PTPT were examined by TG/DTA and Vickers microhardness test, respectively.


Introduction
Materials with large nonlinear optical susceptibilities are of current interest in the area of harmonic generation and optical modulation.In recent years, some polar organic crystals, which form a noncentrosymmetric structure which exhibit second-order nonlinear optical properties that far surpassed those of the conventional materials, have led to the synthesis and evaluation of a wide range of potentially useful solids [1].Materials showing high optical nonlinearity have potential applications in signal transmission, data storage, optical switching, laser printing, displays, inflorescence, photolithography, remote sensing, chemical and biological species detection, high resolution spectroscopy, medical diagnosis, and underwater monitoring and communication [2].Different types of molecular and bulk materials have been examined for nonlinear optical properties.Organic nonlinear materials are attracting a great deal of attention, as they have large optical susceptibilities, inherent ultrafast response times, and high optical thresholds for laser power as compared with inorganic materials [3].Organic molecules with significant nonlinear optical activity generally consist of a -electron conjugated structure.The conjugated electron moiety provides a pathway for the entire length of conjugation under the perturbation of an external electric field.Fictionalization of both ends of the -bond systems with appropriate electron donor and acceptor group can increase the asymmetric electronic distribution in either or both the ground and excited states, thus leading to an increased optical nonlinearity [4][5][6][7].In the present investigation, we report the growth and characterization especially the SHG efficiency of p-Toluidine p-Toluenesulfonate (PTPT) for the first time.The grown crystals have been subjected to the characterizations like XRD, FTIR, UV-vis, thermal analysis, powder SHG, and microhardness studies.

Material and Methods
2.1.Crystal Growth.The title compound was obtained by the addition of p-toluenesulfonic acid (0.02 mol) to a solution of p-Toluidine (0.02 mol) in ethanol, in the stoichiometric ratio 1 : 1.Within a week, tiny crystals were formed due to the spontaneous nucleation.Optically transparent good quality  crystals were used as seeds for further growth experiments.For growth, seed crystals were just immersed into the prepared solution.Good quality crystals with characteristic shape and size of 10 × 5 × 2 mm 3 were obtained for 10 days.Figures 1 and 2 show the photograph of the grown crystals and molecular structure of PTPT, respectively.

Characterization.
In order to confirm the grown crystal, single crystal -ray diffraction studies were carried out using ENRAF NONIUS CAD4-F single -ray diffractometer with MoK ( = 0.71073 Å) radiation.The FTIR spectrum of the sample was recorded in the range 4000-450 cm −1 using Jasco Spectrometer (FTIR, model 410) by KBr pellet technique.The UV-Vis-NIR analysis was carried out between 200 and 800 nm covering the entire ultraviolet, visible, and near infrared regions using the UV-1700 Shimadzu spectrometer.Vicker's microhardness tester was used to assess the mechanical properties of the grown crystal for various loads ranging from 25 to 100 g.The thermal behavior of the grown crystal was studied using simultaneous TG/DTA in the temperature range of 20 ∘ C-600 ∘ C analyses using SEIKO TG/DTA6200 analyzer in nitrogen atmosphere at the heating rate of 20 ∘ C/min.The SHG efficiency was measured by employing Kurtz and Perry powder technique using a Qswitched mode locked Nd: YAG laser emitting 1064 nm.

Results and Discussion
3.1.Single Crystal XRD Studies.Using the single crystal ray diffraction analysis, the cell parameters of PTPT were obtained from least-squares refinement of the setting angles of 25 reflections.The XRD study reveals that the crystal belongs to monoclinic system with lattice parameters of  = 5.778(6) Å,  = 9.010(8) Å and  = 13.320(7)Å,  = 2, and space group is P2 1 , which is in agreement with those of reported values [8].

Density Measurement.
The density of the crystal was determined by the floatation technique (also called the sink or swim technique).For the measurement of crystal density, carbon tetrachloride of density 1.59 gm/cm 3 was used as denser liquid and xylene of density 0.89 gm/cm 3 was used as lower density liquid.The density of the single crystal of title compound was determined as 1.341(1) gm/cm 3 using this floatation technique and the theoretically expected density is 1.298 gm/cm 3 .

FTIR Analysis.
The FTIR spectrum recorder for the PTPT is shown in Figure 3 and its tentative assignments of vibrational frequencies are given in the Table 1.

UV-Vis-NIR Studies.
To analyze the optical properties of grown PTPT crystal, UV-Vis transmittance was recorded.For this, polished sample of 1 mm thick from the grown crystal was used.The recorded UV-Vis-NIR spectrum of PTPT is shown in Figure 4.The optical absorption study shows that the UV cut-off wavelength of PTPT occurs at 295 nm.It is well known that the efficient NLO crystal has an optical transparency at lower cut-off wavelength between 200 and 400 nm [9].There is no significant absorption in the entire visible region which reveals that it can find applications in the  optoelectronic devices.Also, the very low absorption around 532 nm signifies the resistance of the grown crystal to laser induced damage [10].

Thermal Studies.
TGA/DTA curve of PTPT is given in Figure 5.The DTA curve of PTPT indicates that the material has two sharp exothermic peaks at 202.3 ∘ C and 320.3 ∘ C, which represent the melting and decomposition points, respectively.The TGA curve of PTPT indicates that the sample is stable from ambient up to 202 ∘ C. The major weight loss between 202 and 320 ∘ C is due to the decomposition.The total weight loss of the sample is 96% at 320 ∘ C.

Microhardness Studies.
Hardness is one of the important mechanical properties to determine the plastic nature and strength of a material.The well polished PTPT crystal was placed on the platform of the Vicker's microhardness tester and the loads of different magnitudes were applied over a fixed interval of time.The hardness number was calculated using the relation  V = (1.8544)/( 2 ) kg/mm 2 , where  is the applied load in kg and  is the diagonal length of the indentation impression in micrometer.The relation between hardness number ( V ) and load () for PTPT is shown in Figure 6.The hardness increases gradually with the increase in load and above 100 g cracks were developed on the plane of the crystal due to the release of internal stresses generated locally by indentation.Meyer's index  was calculated from the graph (Figure 7) plotted against log  versus log .The slope of the line obtained from graph predicts that the value of  is greater than 2.  V should increase with the increase of  if  > 2 and decrease if  < 2 [11].Thus, the  value satisfies the observed result.

Second Harmonic Generation.
Since PTPT has crystallized in a noncentrosymmetric space group, a preliminary study of the powder SHG conversion efficiency was carried.A Q-switched Nd: YAG laser beam of 1064 nm wavelength was used with an input power of 4.4 mJ pulse −1 , pulse width of 10 ns, and the repetition rate of 10 Hz.The crystals of PTPT were ground to a uniform particle size of about 125-150 m and then packed in capillaries of uniform bore and exposed to the laser radiation.A powder of potassium dihydrogen orthophosphate (KDP), with the same particle size, was used as the reference.The output from the sample was  monochromated to collect only the second harmonic ( = 532 nm) eliminating the fundamental, and the intensity was measured using a photomultiplier tube.Second harmonic signal of 5.58 mV was obtained.The standard KDP crystals gave a SHG signal of 11 mV for the same input energy.From this measurement, it is found that the SHG efficiency of PTPT is 52% of standard KDP.

Conclusion
Single crystals of p-Toluidine p-Toluenesulfonate (PTPT) were grown from ethanol solution by slow evaporation of the solvent at room temperature.The crystal system was confirmed from the single crystal -ray diffraction analysis.The functional groups of the compound were determined from FTIR spectrum.Optical absorption studies show that the sample has minimum absorption in the entire visible region.The melting point of PTPT was found to be 202.3∘ C from the DTA curve.The SHG efficiency of PTPT was found to be 0.52 times than that of standard KDP.
S. No. Wave number (cm −1 )T e n t a t i v ea s s i g n m e n t