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 mm³ obtained in 28 days is shown in Fig. 2.

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 C₁₄H₁₄NO₃. PTSA crystallizes in monoclinic crystal system with space group P2₁/n which is a centro-symmetric. 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) ų.

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 NH₂ 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 sp² 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.

In the crystal structure, intra as well as intermolecular hydrogen bonds could be observed. The intra-molecular hydrogen bonds are formed by the H atom of the alcoholic hydroxyl group as the donor and the carboxylic O atom of the carboxylic acid group as the acceptor in the salicylate anion (O3–H3A…O2), which generates an S(6) ring motif [15]. This motif is also observed in the crystal structure of 2-aminopyridinium salicylate [16]. The intermolecular N–H…O hydrogen bond’s interaction links the molecules into an infinite one-dimensional ribbon structure extending along the b-axis (Fig. 4.). The crystallographic data and structural refinement detail of PTSA crystal are given in Table 1. The atomic coordinates and equivalent isotropic displacement parameters for PTSA crystal are presented in Table 2. The bond length, bond angles and torsion angles are given in Tables 3, 4, respectively. The anisotropic displacement parameters for PTSA crystal are given in Table 5. The hydrogen coordinates and isotropic displacement parameters are presented in Table 6. The hydrogen bond interaction involved in the PTSA crystal is shown in Table 7.

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 (NH₃⁺) 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.

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,

(1)

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,

(αhν)² = A (E_g – hν), (2)

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 (PL) spectral investigation is one of the most competent mechanisms which impart comparatively 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 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,

(3)

tan δ= ε_rD (4)

where C is the capacitance, t is the thickness of the crystal, ε₀ 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.

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,

σ_ac = 2πfε₀ε_rtan δ, (5)

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,

ρ = 1/2πfε₀ε_rtan δ, (6)

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.

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 (n₂) 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,

ΔT_p-v = 0.406(1-S)^0.25|φ|, (7)

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,

(8)

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,

|φ| = kn₂L_effI₀, (9)

where L_eff = (1 – e^–αL)/α, L stands for the length of the used sample, I₀ 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,

(10)

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 (χ³) were evaluated from the experiment data of ‘n₂’and ‘β’ values.

(11)

(12)

where, ε₀ 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

(13)

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.