Synthesis, growth, structural, optical and electrical properties of novel organic single crystal: p-toluidinium salicylate

A novel organic single crystal of p-toluidinium salicylate (PTSA) was grown by slow evaporation solution growth technique. The structure of grown crystal was determined from the single crystal X-ray diffraction analysis. The functional groups existing in PTSA crystal were accomplished by using Fourier transform infrared analysis. The optical transparency and band gap energy were estimated by utilizing the UV-Visible spectrum. Photoluminescence spectral studies revealed the photon excitation. The dielectric behavior of PTSA was investigated for different frequencies in a room temperature (308 K) environment. Third-order nonlinear optical susceptibility (χ3) of PTSA crystal was elucidated by Z-scan measurements.


Introduction
In the modern crystal science and technology, organic materials find wide application in telecommunication, optical limiting, optical data storage, ultrafast signal processes [1].The donor-acceptor based intermolecular charge transfer organic molecular complex has high nonlinear optical properties due to the presence of delocalized π-electron system which enhance the asymmetric polarizability of the material.The molecular level organic compounds are expected to exhibit large values of molecular hyperpolarizability, if they possess polarizable electrons, for example, π-electrons spreading over a large distance.
It has been extended π-conjugated system with terminal donor-acceptor substituent to exhibit large values.Organic materials are known for their potential application in semiconductors, superconductors and nonlinear optical devices [2][3].In addition, organic molecules also have a great attention owing to their potential application in the frontier areas such as nonlinear optics and light emitting diodes.Thus, the potential use of organic materials in optoelectronics has now become a serious matter.Organic single crystals with large π-electron delocalization are preferred for second and third order nonlinear optical applications.Because, optical nonlinearity depends on the crystal symmetry as the control of molecular orientation in supramolecular structure is tedious and identified as a major obstacle in material's design.Many attempts have been made to control the structure of solids through charge transfer, hydrogen bond and π-π interactions.Generally, organic materials possess both donor and acceptor groups with a suitable conjugation path which leads to large third order nonlinearities.Due to the weakly base character of aniline complexes, p-toluidine (C 7 H 9 N) accepts the proton forming p-Toluidinium cation to enchance the NLO properties.The p-toluidine based crystals 4-methylanilinium p-toluenesulfonate, p-toludinium picrate have been reported [4][5].Salicylic acid also known as 2-hydroxybenzoic acid is widely used as plant growth regulators and preservative in food products.It has antiseptic and anti-fungal properties and used widely in organic synthesis.The derivatives of salicylic acid compound have recently become attractive to experimentalists as well as theoreticians since their structure are some biological significance particularly in medical and enzyme chemistry [6].The salicylic acid (C 7 H 6 O 3 ) donates the proton to the base and enhances the nonlinear property.The 3,5-dinitro salicylic acid monohydrate [7], Ammonium dihydrogen phosphate and Ammonium salicylate [8], Lithium salicylate [9], L-valinium salicylate [10] have been synthesized and their optical properties have been studied.In the crystal structure of the title compound: p-toluidinium saliylate (C 7 H 10 N + ×C 7 H 5 O 3 -) the cation and anion are linked via C-H-N-and C-H-O-interactions.In the present report, organic p-toluidinium salicylate (PTSA) was synthesized and crystal structure was solved for the first time.PTSA crystal was grown by slow evaporation solution growth technique and the structural and optical properties of the grown crystal were characterized by single crystal X-ray diffraction, Fourier transform infrared spectroscopic analysis, UV-Vis transmission spectral analysis, dielectric measurements and Z-scan studies.

Material synthesis and crystal growth
PTSA was crystallized by slow evaporation process from p-toluidine (LOBA 99%) and salicylic acid (MERCK 99%) taking in equimolar ratio and the chemical reaction scheme of PTSA is illustrated in Fig. 1.The estimated amounts of the reactants were dissolved in double distilled water and solution was stirred for 6 h by using a magnetic stirrer to establish homogenous concentration in the solution.To improve the purity of the solution, Whatman filter papers were used to filter the solution and then it was transferred to a crystal growing utensil.The product was further purified by repeated recrystallization.The growth solution was prepared using the purified PTSA salt and it was kept in a constant temperature bath which was maintained at 36 °C.The colorless transparent single crystal of size about 11×5×3 mm 3 obtained in 28 days is shown in Fig. 2.

Single crystal X-ray diffraction studies
The crystal structure of PTSA was determined by single crystal X-ray diffraction studies.The intensity data were collected using Bruker kappa APEXII single crystal X-ray diffractmeter with a graphite monochromated MoK α radiation (λ = 0.71073 Å) at 293 K [11].The structure was solved by the direct method and refined by the full matrix least-squares technique on F2 employing the SHELXL 97 program package [12].The crystallographic data of PTSA crystal is listed in Table 1.The asymmetric unit of the title compound comprises of a p-toluidine cation and a salicylate anion as shown in the ORTEP diagram (Fig. 3).The packing diagram of p-toluidine salicylate is shown in Fig. 4. The molecular formula of the crystal is C 14 H 14 NO 3 .PTSA crystallizes in monoclinic crystal system with space group P2 1 /n which is a centro-symmetric.The p-toluidine cation and salicylate anion are essentially planar, having a maximum deviation of 0.001(3) Å for atom C19 and 0.004(4) Å for atom C7, respectively.The dihedral angle between these two planes is about 36.3(2)Å, indicating that they are nearly parallel to each other.The C-N bond distance of NH 2 group i.e., N1-C12 is 1.454 (3) Å, which is short for a C-N single bond, but still not quite as contracted as one would expect for a fully  established C═N.These bond length features are consistent with amino resonance form as it is commonly found for C-N single bonds involving sp 2 hybridized C and N atoms [13][14].The bond distances of C1-O2, C1-O1 are 1.262(3) and 1.270(3) Å, respectively.It is clearly indicated the presence of C═O double bonds, including those generated through resonance.

FTIR spectral studies
The FTIR spectrum of PTSA crystal was recorded in the range 4000 -400 cm -1 by using KBr pellet technique and the presence of various functional groups in the grown PTSA has been elucidated as shown in Fig. 5.The characteristic vibrational frequencies were assigned and presented in Table 8.The functional groups of PTSA such as asymmetric and symmetric stretchings of N-H in the amino (NH 3 + ) observed at the wave numbers 3496 and      3354 cm -1 , respectively.The presence of methyl groups was confirmed through the symmetric and asymmetric stretching vibrations of C-H at 2927 and 1429 cm -1 respectively.The sharp peaks observed at 2849 cm -1 is due to the stretching of C-H.The peak observed at 1751 cm -1 is due to the stretching of C═C in the aromatic ring and the peak noted at 1919 cm -1 is due to C═N stretching vibration.The C-C stretching and COO -wagging peaks observed at 910 and 613 cm -1 respectively [17][18].The carboxyl group (COOH) of PTSA was confirmed at the wave number 2552 cm -1 .Hence, the presence of functional groups confirmed the formation title compound.

Optical transmittance studies
UV-Vis spectral analysis provides an essential structural knowledge due to UV and visible light absorption and it is also associated with the shifting of the electrons from the ground to the higher energy states of π-and σ-orbitals [2].UV-Vis transmission spectrum recorded in the range 200-800 nm is shown in Fig. 6.It is seen from the spectrum that PTSA crystal has a very low cut-off wavelength around 320 nm with transparency around 65%.The absorption coefficient (α) can be determined from the transmission spectrum based on the relation, where T is the transmittance and 't' is the thickness of the crystal.The optical band gap was estimated from the transmission spectrum and the optical absorption coefficient (α) near the absorption edge was calculated using the relation, where E g is the optical band gap of the crystal and A is a constant.The variation of (αhν) 2 with 'hν' in the fundamental absorption region was plotted as shown in Fig. 7.
The band gap of PTSA crystal estimated by extrapolation of the linear part of the graph is 3.2 eV.The band gap and transmittance in the entire visible region enables the crystal suitable for optoelectronic and photonics applications [19].

Photoluminescence studies
Photoluminescence (PL) spectral investigation is one of the most competent mechanisms which impart compara-tively direct knowledge regarding the materials' physical properties at the molecular level, which also includes the defects in deep and shallow level mechanisms and energy gap states.The emission occurs because of the radioactive recombination of electron and hole pair, that is specially necessary for the mechanism of laser in the visible range and it is meant for chromophores in the case of organic crystals [2]. Figure 8 shows the   room temperature PL spectrum of PTSA crystal and the broad photoluminescence emission (PLE) peak seen in the spectrum at 440 nm is due to interaction between the electronic system of the luminescent center and the vibrations of ions.Such a broad PL emission caused by diverse electronic transitions occurring in different energy levels is due to deep or shallow holes with in the band gap [20].The deep holes are origin states for the green, yellow, orange and red PL emissions, while the shallow holes are responsible for the violet and blue emissions [21].The broad PL emission observed at 440 nm violet emission is an indicative of the charge transfer process as well as the trapping of electrons and it is due to the contribution of the shallow holes than the deep holes [22].Thus, PLE spectrum reveals that PTSA crystal exhibits blue shift emission, which is most useful for luminescent applications.

Dielectric studies
Dielectric properties are correlated with the electro-optic property of the crystals [23].The dielectric constant and dielectric loss of the grown single crystal was measured as a function of frequency from 50 Hz to 3 MHZ.The polished PTSA crystal faces were coated with silver paste and mounted between the two copper electrodes to form a parallel plate capacitor.The dielectric constant (ε r ) of the sample was calculated using the relations, r 0 where C is the capacitance, t is the thickness of the crystal, ε 0 is the permittivity of free space, D is the dissipation factor and A is the area of the crystal.The variation of dielectric constant as a function of frequency at room temperature is shown in Fig. 9.The high value of dielectric constant at low frequencies are due to the presence of electronic, ionic, dipolar and space charge polarizations, and its low value at high frequencies may be due to loss of these polarizations gradually [24].The dielectric loss observed as a function of frequency at room temperature is shown in Fig. 10.The dielectric loss value decreased at high frequencies.The low values of dielectric constant with minimized dielectric loss at higher frequencies of the grown single crystal shows enhanced optical quality with lesser defects and it suggests that the PTSA single crystals are useful for devices applications.

Study of AC conductivity and resistivity
The effect of frequency and temperature on AC conductivity offers a lot of information about the bound electric charge carriers.This leads to good explanation and understanding of the electric behavior of organic semiconductors [25].The AC conductivity was calculated using the relation, Photon energy(eV)   where, f is the frequency of applied field.The plot of AC conductivity versus frequency at room temperature is shown in Fig. 11.It shows that at a given temperature, the magnitude of conductivity is high at high frequencies which is a normal dielectric behavior.The electrical conduction in dielectrics is mainly a defect controlled process in the room temperature environment.It is clear that the AC conductivity increases and resistivity decreases with increase of frequency.Fig. 12 shows the resistivity of material for different frequency measured at room temperature.It shows that there is a relationship between the relative charge in temperature with the two electrical factors; conductivity and resistivity.The resistivity was calculated using the relation, where, ρ is the resistivity and ω is angular frequency of applied electric field.However, the electrical conductivity varied in the opposite direction of resistivity.The PTSA single crystal showed good optical quality with lesser defects and it suggests that the PTSA crystal will be useful for device applications.

Z-scan measurement
The third order nonlinear optical characteristics of PTSA were accomplished by means of the process of Z-scan.This approach facilitates the concurrent measurement of magnitude as well as sign of the nonlinear refractive index (n 2 ) and the nonlinear absorption (β) using the open and closed configurations of Z-scan.In the present investigation, Nd-YAG laser was used to assess the third order nonlinearity of PTSA.
From the obtained data of Z-scan, the difference between the normalized valley and peak transmittance ΔT p-v can be evaluated by using the relation, where, |φ| signifies the on-axis phase shift at the focus and S denotes the aperture linear transmittance, which can be estimated using the relation, where r a indicates the aperture radius and ω a represents the radius at the aperture.
The on-axis phase shift |φ| is given by, where L eff = (1 -e -αL )/α, L stands for the length of the used sample, I 0 denotes the laser intensity at focus z = 0, α indicates the linear absorption coefficient and k is the wave number (k = 2π/λ).
The nonlinear absorption is reckoned by utilizing the data obtained from the open aperture Z-scan and is given by,    ΔT is the maximum value of the open aperture normalized transmittance obtained from the Z-scan plot.The nonlinear absorption coefficient (β) exhibits a negative value for saturation absorption and positive in the case of two photon absorption.The real and imaginary part of nonlinear optical susceptibility (χ 3 ) were evaluated from the experiment data of 'n 2 'and 'β' values.
( ) ( ) where, ε 0 denotes the vacuum permittivity and C represents the velocity of light in vacuum.The absolute third order nonlinear optical susceptibility |χ (3) | is given by ( ) ( ) it is observed that the closed aperture Z-scan curve of PTSA discloses the peak to valley configuration as well as it is an evidence for negative nonlinearity as illustrated in Fig. 13.This is also represented as self-defocusing effect which takes place due to the dependence of refractive index with temperature [26] and finds application in the domain of fabricating optical sensors [27].
The open aperture configuration of Z-scan is displayed in Fig. 14.The nonlinear absorption coefficient (β) is found to be 0.02 × 10 -4 cm/W and it signifies the process of saturable absorption and it is widely used for the application of optical power limiting process [26].
The obtained Z-scan data measurement precisely illustrates that PTSA shows the third order nonlinear optical characteristics and enumerated in Table 9.The third order susceptibility of PTSA was compared with other reported acentric crystals and listed in Table 10.
The nonlinear refraction and absorption are attributed to self-defocusing nature and two photon absorption process of PTSA respectively.

Conclusion
A novel organic material p-toluidinium salicylate was synthesized and single crystal of PTSA was grown by slow evaporation method.The molecular structure was analyzed by using single crystal X-ray diffraction studies.The single crystal data indicates that the PTSA crystallizes in monoclinic crystal system with centro-symmetric space group P2 1 /n.The existence of diverse functional groups was identified from the FTIR spectral studies.The transmittance in the visible region had the lower cutoff wavelength of 320 nm and the band gap energy of PTSA was estimated to be 3.2 eV.The photoluminescence spectral study disclosed the electron excitation of PTSA.The dielectric constant and dielectric loss of PTSA crystal were studied with different frequency at room temperature.The nonlinear optical parameters were evaluated by closed and open Z-scan signature technique.Thus, the various characterization of PTSA crystal proved its suitability for the future photonic and optoelectronic device fabrication.9.39 × 10 -8 cm 2 /W Nonlinear absorption coefficient (β) 0.02 × 10 -4 cm/W Real susceptibility (χ R (3) ) 10.13 × 10 -6 esu Imaginary susceptibility (χ I (3) ) 0.12 × 10 -6 esu Absolute susceptibility (χ (3) The calculated cell parameters are a = 14.1940(16)Å, b = 4.6883(6) Å, c = 19.684(2) Å, α = γ = 90°, β = 90.613(3)°and volume V = 1309.8(3)Å 3 .

3 Figure 3 .
Figure 3.The molecular structure of atom with labels and displacement ellipsoids are drawn at the 50% probability level.In the anion (O3-H3A…O2), which generates an S(6) ring motif.

Figure 4 .
Figure 4.The packing of the title compound, viewed extending along the b axis.Intermolecular hydrogen bonds are shows as dashed lines.H atoms not involved in hydrogen bonding have been omitted.

Figure 9 .
Figure 9. Plot of dielectric constant vs. log frequency of PTSA crystal.

Figure 10 .
Figure 10.Plot of Dielectric loss vs. log frequency of PTSA crystal.

Figure 11 .
Figure 11.Plot of AC Conductivity vs. log frequency of PTSA crystal.

Figure 12 .
Figure 12.Plot of resistivity vs. log frequency of PTSA crystal.

Figure 13 .
Figure 13.Z-scan curve traced in closed aperture mode for PTSA crystal.

Figure 14 .
Figure 14.Z-scan curve traced in open aperture mode for PTSA crystal.

Table 1 .
Crystal data and structure refinement for PTSA crystal.

Table 8 .
FT-IR spectral assignment of PTSA crystal.

Table 9 .
Third order nonlinear optical parameters of PTSA crystal.

Table 10 .
Comparison of χ(3)values of PTSA with other NLO materials.