Experimental and computational studies on optical properties of a promising N-benzylideneaniline derivative for non-linear optical applications

2.1 Material synthesis

All the reagents of n-phenyl-p-phenylenediamine and p-nitrobenzaldehyde required for synthesizing PNBPDA compound was purchased from sigma Aldrich and used without further purification. The saturated solution of the deep-red mixture of n-phenyl-p-phenylenediamine (184.2 mg, 1.001 mmol) and p-nitrobenzaldehyde (151.2 mg, 1.001 mmol) prepared in adequate quantity of ethanol has undergone vigorous stirring for about 6 h at room temperature to obtain a homogeneous solution of the resultant compound. The solvent was removed incorporating rotary evaporation and the obtained precipitate was again dissolved in chloroform for the crystallization process. Then the obtained solution was filtered off using Whatmann filter paper (Grade I) and kept in a dust free atmosphere for evaporation. Small red coloured prismatic shaped seed crystals of p-nitrobenzylidene-p-phenylamineaniline were observed within two weeks after successive recrystallization using methanol and chloroform.

2.2 Solubility of the material

After the synthesis of the title compound, the solubility was determined in the temperature range of 30 °C to 65 °C by maintaining the solution in a temperature controlled water bath of ±0.01 °C. The recrystallized sample of PNBPDA was first dissolved in 100 ml of ethanol and water by continuous stirring at 30 °C and the resultant homogeneous saturated solution was filtered. About 5 ml of the solution was pipetted out to a petri dish and the amount of solvent present in the solution was calibrated gravimetrically. From the data, the accurate quantity of PNBPDA sample dissolved in 100 ml of the two solvents at 30 °C was determined. The procedure was repeated for 35, 40, 45, 50, 55, 60 and 65 °C and the corresponding amount of PNBPDA dissolved at respective temperatures was noted. The variation in solubility for two solvents as a function of temperature is portrayed in Figure 2a. In accordance with the solubility data, we inferred that the material holds a positive temperature co-efficient exhibiting an increasing variation in solubility of PNBPDA with an increase in temperature.

Figure 2:

(a) Solubility data of PNBPDA crystal (b) Grown single crystals of PNBPDA.

2.3 Crystal growth

The obtained red prismatic seed crystals via re-crystallization using the mixture of methanol and chloroform were successfully grown by the conventional slow evaporation solution growth technique using ethanol as the solvent. For the growth of bulk single crystals of PNBPDA, the recrystallized red prismatic seed crystals were dissolved in ethanol by maintaining a temperature of 30 °C. The obtained saturated homogeneous solution was filtered and covered using perforated polythene sheet for ensuring the controlled slow evaporation process. Thus bulk crystals of PNBPDA of prismatic shape with significant dimension were harvested within a period of one week and it was then used for further investigations. The obtained red prismatic single crystal was as shown in Figure 2b.

2.4 Characterization techniques

Single crystal X-ray diffraction (XRD) analysis was performed using ENRAF NONIUS CAD4 diffractometer for identifying the cell parameters and Powder X-ray diffraction (PXRD) spectrum was recorded via Cary 630 with ATR– Agilent technologies to confirm the crystalline nature of the obtained PNBDPA single crystal. Fourier Transform Infrared spectrum (FTIR) was examined through Rigaku Miniflex 600 set-up for determining the various functional groups present in the grown crystal. Thermo gravimetric and Differential Scanning Calorimetric (TG&DSC) curves were generated by employing NETZSCH STA 449F5 STA449FSA-0231-M thermal analysis system to examine the thermal stability of the material. Ultra-violet–visible–near infra-red (UV–Vis–NIR) and fluorescence spectra were found by Varian Carry 5000 spectrophotometer and Shimadzu Spectrofluorophotometer respectively to detect the optical transparency and luminescence behaviour of the grown crystal. LEITZ WETZLER micro hardness tester with diamond indenter was used to confirm the mechanical stability of the material, dielectric studies were performed by HIOKI 3532 LCR HITESTER set-up and the second and third order non-linear property of the material was evaluated and confirmed using Kurtz Perry powder technique and Z-Scan technique.

2.5 Quantum computational calculations

The PNBPDA molecule was subjected to computational studies to investigate various electronic and molecular structural parameters as well as the non-linear optical properties to check the practical applicability of the material for various optoelectronic applications. The study was performed choosing hybrid functional density theory with Lee–Yang–Parr correlation density functional (B3LYP: Becke-3-Lee–Yang–Parr) and ab initio time dependent Hartree-Fock (TD-HF) with 6−31 ++ G^*d(p) basis set. The optimization of molecule to ground state (GS) equilibrium geometry, calculation of various structural properties like Frontier Molecular Orbital (FMO) analysis, molecular electrostatic potential analysis and non-linear optical parameters like dipole moment, polarizability and hyperpolarizability were executed at B3LYP method utilizing Gaussian 09 W quantum chemistry package as the software program. The 2D structure of the title compound obtained from the Crystallograhic Information File (CIF) of the reported literature was used for the optimization of the molecular geometry of PNBPDA.

3.1 Crystallographic studies

The grown single crystals of PNBPDA (C₁₉H₁₅N₃O₂) were subjected to XRD studies to determine the lattice parameters and the crystal structure of the obtained compound. The result was analysed using ENRAF NONIUS CAD-4 diffractometer. The diffractogram spots represent the intensities corresponding to the X-rays reflected from the lattice site of the crystal [9]. It was found that the crystal crystallizes in the monoclinic system with non-centrosymmetric space group Cc. The obtained data via X-ray diffractometer was verified with the already reported details and a comparative study was shown in the Table 1 as below;

Powder XRD analysis has been carried out to identify the crystallinity and the purity of the grown PNBDPA single crystals. The sample was scanned over the angular range of 5°–70° with a step increment of 0.02° using CuKα (λ = 1.5148A°) radiation and Bragg’s peaks corresponding to particular 2θ angle was plotted (Figure 3). The existence of sharp Bragg’s peaks at specific 2θ angle confirms the crystalline nature of the obtained compound.

Figure 3:

Powder XRD spectrum of PNBPDA single crystal.

Powder XRD analysis helps us to determine the crystalline nature of the grown crystal by exploiting various parameters like crystallite size, degree of crystallinity, dislocation density and the effect of microstrain within the grown crystal of PNBPDA.

The crystallite size of PNBPDA was evaluated using Debye–Scherrer’s formula;

where k = 0.9, λ = 1.5405 A° & β corresponds to the peak width of the reflection at half intensity. Here the maximum intensity was observed to be at 2θ = 21.029°. Hence the size of the crystal was found to be 142 A°. This was then verified from the Hall Williamson plot which is depicted in figure. The dislocation density of the crystal lattice was determined via the relation;

The presence of microstrain with the lattice was obtained using the Hall Williamson equation;

It was also in good agreement with the slope of the linear fitted region from the Hall–Williamson plot (Figure 4) [10]. The crystalline nature of the grown crystal was then strictly confirmed by calculating the percentage of crystallinity embedded within the PNBPDA sample. The determined structural parameters are tabulated in Table 2.

Figure 4:

Hall Williamson Plot of βCosθ Vs Sinθ.

3.2 FTIR analysis

In vibrational spectroscopy, various functional groups present within a material would be identified by observing the absorption spectral bands recorded in the FTIR. Depending on the nature of the material, distinct frequencies will be absorbed by different functional groups for carrying out the stretching, wagging, bending and rocking vibrations within the material. The various functional groups and the corresponding vibrational assignments will provide us limited information about the molecules present within the obtained compound. Here the vibrational spectrum was recorded at room temperature in the range of 4000 cm⁻¹ to 500 cm⁻¹ using KBr beam splitter with a He–Ne laser source. In the observed FTIR spectrum (Figure 5), a sharp band identified at 3406 cm⁻¹ could be attributed to N–H asymmetric stretching and 2873 cm⁻¹ band has been assigned to aromatic C–H asymmetric stretching. A medium weak band occurred at 2350 cm⁻¹ were the characteristic of N–H… O stretching whereas a sharp peak identified at 1561 cm⁻¹ and 1508 cm⁻¹ could be due to the effect of NO₂ asymmetric stretching and C=C symmetric stretching. A medium strong band was predicted at 1316 cm⁻¹, 1220 cm⁻¹ and 1146 cm⁻¹ confirmed the occurrence of C–H bending, C–C asymmetric stretching and C–O bending vibrations. At higher frequency ranges, around 1092 cm⁻¹ and 836 cm⁻¹, a sharp intensity peak was appeared and it is contributed by the stretching and bending modes of N–C–N and C–C functional groups. Later on, the rocking of CH₂ group and the out of plane bending vibrations of C–H groups are confirmed by the absorption bands viewed at 740 cm⁻¹ and 684 cm⁻¹. Similarly, the sharp absorption frequencies represented by 538 cm⁻¹ and 436 cm⁻¹ represents the bending vibrations of C–O group and the rocking vibrations of NO₂ group in the compound respectively.

Figure 5:

FTIR spectrum of PNBPDA single crystal.

3.3 Thermal exploration

The specimen was subjected for thermal characterization, which provides elaborative information about the suitability of the material for non-linear applications. The thermo gram obtained via thermo gravimetric (TG) and differential scanning calorimetric (DSC) analysis helps us to determine the melting point and the decomposition process involved within the material. The TG/DSC profile was obtained between 35 °C and 500 °C with a heating rate of 10  K/min using Al₂O₃ crucible and the formed pattern was illustrated in Figure 6. From the TG curve, it can be inferred that there is no significant weight loss up to 300 °C which reflects the non-existence of water molecule within the crystal and a major mass loss was observed from 300 °C to 350 °C, which might be due to the liberation of moieties as volatile gaseous products like CO, CO_2, and nitro groups [10]. Hence, the melting temperature of the sample was found to be 300 °C. But in the temperature range 360 °C to 500 °C, there exists a consistent decrease in mass within which the residual product undergoes complete decomposition and the entire mass will be collapsed to zero. DSC profile exhibits a sharp endothermic peak and a sharp exothermic peak at 180 °C and 340 °C respectively and confirms the starting stage of the decomposition of the sample before melting and the exothermic peak which coincides with the TG data reveals the elimination of functional groups which results in the decaying of the compound [11], [12]. The sharpness of the peak and the higher melting point reveals the crystallinity of the material and its suitability for various optoelectronic applications up to 300 °C.

Figure 6:

TG/DSC profile of the grown crystal of PNBPDA.

3.4 UV–VIS–NIR spectrum analysis

Optical transmission spectral analysis provides us some basic information about the electronic band structure, localized states and the presence of defects and deformations within the crystal lattice. Such factors strongly influence the optical properties like scattering, transmittance etc. which can adversely affect the practical usability of the material. i. e. crystal with excellent optical transparency and defect free lattice can only be exploited for promising NLO applications [13], [14]. The UV–visible spectrum of the obtained compound was recorded in the wavelength range of 200 to 800 nm and is shown in Figure 7. The spectrum shows significant transmission near the fundamental and second harmonic generation (SHG) signal wavelengths and a small percentage of absorption observed in UV and Visible region could be assigned to the transfer of electrons into the p and n orbitals from the GS to the higher excited states [15], [16] and it can be ignored by the prominent transmittance in those region which can confirm the suitability of the material for laser frequency conversion in optoelectronic applications[17].

Figure 7:

UV–Vis–NIR spectrum of the grown PNBPDA crystal.

3.5 Fluorescence studies

The emission spectrum of an optical material corresponding to a specified excited state helps us to understand the structural arrangement of electrons and its chemical behaviour [24].The luminescence property will be efficiently shown by compounds having defect free lattices and a prolonged conjugated double bond in aromatic rings [25]. Here the fluorescence spectrum was recorded with the excitation wavelength of 276 nm in the wavelength range of 300 to 360 nm and a high intensity sharp emission was found near 330 nm, which reveals that the grown crystal exhibits UV emission and is illustrated in Figure 8. The band gap energy corresponding to the obtained spectra was then calculated using the expression;

where ‘h’ is the Planck’s constant and ‘c’, velocity of the light, ’λ_max’, the maximum wavelength corresponding to the emission spectrum. Energy gap was determined to be 3.76 eV, which demonstrates a decrease in the energy value while emitting the absorbed radiation and is attributed to a significant loss in energy during the electronic transitions.

Figure 8:

PL emission spectrum of grown PNBPDA single crystal.

3.6 Microhardness analysis

The mechanical properties like resistance, brittleness index, stiffness constant and yield strength entangled with hardness of the crystal has to be strictly verified during device fabrications [26]. The hardness of a crystal can be defined as the resistance offered by the material to the localized plastic distortions and deformations arising within the lattice due to the scattering and indentation. Hence the hardness of the material can be found out using Vicker’s micro hardness tester by subjecting a well-polished crystal of PNBPDA upon different loads ranging from 20 to 100 gm. The mechanical stability of any material depends on the elasticity of the crystal lattice which would withstand any deformation caused by the impression by producing an internal stress, and this elastic limit can be achieved by a regular packing arrangement of atoms within the crystal lattice which can pave the way for reverse indentation size effect [27]. But beyond an applied load, the elasticity of the material will be lost resulting in the formation of micro cracks within the crystal and this maximum limit up to which a material can withstand stress produced by indentation process is defined as the hardness number related by;

where ‘H_ν’ is the Vickers hardness number, ‘P’ is the applied load in gm and ‘d’ is the average diagonal length of the indentation marked by the pyramidal indenter in mm. The hardness number of the material with the applied load is graphically shown by Figure 9 (Table 4).

Figure 9:

Dependence of Hardness number (H_V) upon the applied load (P).

According to Onitsch [28] and Hanneman [29], the applied load (P) and the diagonal length of indentation (d) is linked by the relation;

where ‘a’ is the proportionality constant and ‘n’ is the Mayer’s index number. The above equation can be significantly rearranged as;

Meyer’s index number ‘n’ can be determined from the graph plotted between ‘Log P’ on y-axis and ‘Log d’ in μm on x-axis as shown in Figure 10. The slope of the graph was found as the Meyer’s index number ‘n’ and was calculated to be 2.99 which is greater than 2, satisfying Onitsch concept and thus confirms that the grown crystal of PNBDPA belongs to soft material category.

Figure 10:

Determination of Meyer's index of the grown PNBPDA crystal.

Mechanical parameters like stiffness constant and yield strength can be determined using the following relations and the variation in those parameters with the applied load is shown in Figure 11a and b.

Figure 11:

a) Variation of stiffness constant (C11). b) Variation of yield strength (σ_y) with the applied load (P).

3.7 Dielectric studies

Dielectric property of a material provides some basic knowledge about the lattice dynamics which serves as a backbone during the fabrication of crystals for non-linear optical applications. It mainly depends on space charge, orientation, ionic and electronic polarization generated due to the electronic displacement occurred within the crystal lattice as a result of the exchange of ions in the crystal for the applied electric field [23], [30].

From the calculated value of parallel capacitance (C_p), the dielectric constant can be found out using the expression;

where d is the thickness of the sample, ε_0, the free space permittivity and A describes the area of the sample.

The variation of dielectric constant with that of applied frequency is shown in Figure 12 at three different temperatures (33 °C, 50 °C and 70 °C). Generally, at low frequency region dielectric constant attains a maximum value influenced by space charge polarization and the value eventually got decreased and reaches at a constant with the increase of frequency thereby indicating the occurrence of less energy dissipation as heat [31]. The lower dielectric constant value at high frequency seems to be an admirable property of a material holding defect free lattice and optically good quality crystals, which paves the way for an enhanced NLO behaviour in optical crystals.

Figure 12:

Variation of dielectric constant with the log of the applied frequency.

3.8 Laser damage threshold study

While selecting a material for non-linear optical applications in photonic and optoelectronic devices, one must ensure its optical surface damage tolerance or the ability of the material to withstand high power laser beams [32]. For this, Laser Damage Threshold (LDT) study was carried out in order to estimate the stability of PNBPDA crystal when subjected to highly intensified laser power. Generally, the threshold value depends on the deformations and defects on the crystal lattice of the grown material which reduces the strength of interatomic bonding between the molecules [33]. The study was performed under Q-Switched Nd-YAG laser source of 532 nm wavelength, pulse width of 9 ns with pulse repetition rate of 10 Hz. The measurement was done on the flat surface of PNBPDA crystal of thickness 0.8 mm by placing the sample at the focal point of the converging lens of focal length, 10 cm. The energy with which crystal got damaged was observed to be at 3.7 mJ/pulse and the radius of the circular spot size was found. The damage threshold value was then computed using the standard power density formula;

where E is the input laser energy in millijoules, τ is the nanosecond pulse width and A is the area of the circular spot (πr²) impinched on the crystal under study. The evaluated value is compared with some standard values as in Table 6.

The obtained LDT value for PNBPDA crystal is 7.342 GW/cm² which is very much greater than the threshold value of KDP and urea crystals. Thus the present non-linear optical material of PNBPDA crystal confirms its candidature in laser assisted NLO applications.

3.9 Second harmonic generation studies

Kurtz Perry powder technique was explored in order to know the frequency conversion efficiency of the title compound because second harmonic generation (SHG) materials with a good transparency window in the UV–Visible region find better practicability in various non-linear optical fields [8], [32]. In this experimental study, the powdered crystalline sample was packed in a capillary tube and exposed to Q-switched Nd-YAG pulsed laser of fundamental wavelength 1064 nm with input laser beam parameters of 0.69 J, 10 Hz, 10 ns as the beam energy, pulse rate and pulse width. Then a frequency doubling output of 532 nm was detected by the photomultiplier diode and the power meter records the energy corresponding to the output pulse and it was found to be 0.18 V for the PNBDPA compound. Here KDP sample is used as the reference material to compare the SHG signals to determine the efficiency of the compound for SHG and the output signal was found to be 30 mV. The level of NLO response is greatly influenced by the structural behaviour of the material which implies that the NLO output can be enhanced by the promising occurrence of CT between the individual reactants involved within the synthesis procedure [35]. The strength of SHG signal was examined to be 6 times higher than that of the standard KDP, which strongly recommends the material to be used as a potential candidate for optoelectronic and photonic applications. The obtained SHG value of PNBPDA crystal was compared with some organic derivatives as exhibited in Table 7 [4], [6], [27], [36], [37].

3.10 Z-Scan measurement

Third order non-linear optical properties of PNBPDA crystal were evaluated employing the most effective and sensitive Z-scan technique proposed by sheik Bahae et al. The analysis was performed using continuous wave Nd–YAG laser with wavelength of 532 nm. For the measurement of non-linear absorption co-efficient by open aperture Z-Scan method, the sample is translated through the region of focus and the intensity corresponding to each z position is recorded using the photo multiplier detector which is placed in the far field and the respective laser beam energy is obtained from the digital power meter. The intensity profile is demonstrated with normalized transmittance as a function of z positions. The valley curve obtained via open aperture study reveals the reverse saturable behaviour of the sample which promotes to the optical limiting nature of PNBPDA crystal. The transmission equation of two photon absorption (TPA) process to which the experimental data through open aperture has to be fitted is given by;

Where,

The sample had a linear transmission of about 83% at the excitation wavelength and it got reduced to 70% of the laser intensity when PNBPDA sample was placed at the beam focus of Z-Scan. Since the sample had high linear transmission, Two photon absorption process was assumed to be the reason for optical limiting behaviour of the material. But a best fit was obtained only when saturable absorption (SA) was also added as an effect along with multi photon absorption processes like two photon and three photon absorption, for the origin of non-linear properties within crystals. Hence contributions from both, saturation of the ground state (GS) absorption and absorption when excited at resonant wavelengths have to be considered as the predominant reasons for the occurrence of non-linearity within the material [38]. Usually the excited state absorption is encountered by an entity called saturation intensity (I_s) [39].

Therefore an effective intensity dependent non-linear absorption co-efficient (α(I)) can be represented as;

where α₀ is the unsaturated linear absorption co-efficient, β, TPA or excited state absorption co-efficient. Thus the non-linear parameters β and I_s can be numerically found out by fitting the experimental open aperture Z-Scan curve (Figure 13) holding the transmission data with the non-linear pulse propagation equation;

where z^| indicated the propagation distance within the sample.

The numerically estimated values of β and I_s are 0.7 ∗ 10–11 m/W and 12 ∗ 1012 W/m².

Figure 13:

Open aperture Z-Scan curve of PNBPDA crystal.

Various parameters involved in Z-Scan experiment

1. Laser used: Q-Switched Nd YAG laser (Continuum, Minilite)

2. Laser beam energy: 100 µJ

3. Wavelength: 532 nm (frequency doubled output)

4. Pulsewidth: 5 ns

5. Pulse Repetition rate used – approximately 1 pulse in 5 s

6. Focal length of Plano Convex lens used: 6.3 cm

7. Focal spot radius: 13 μm

8. Cuvette path length: 1 mm

3.11 Computed molecular geometry

3.11.1 DFT calculations

The geometry of PNBPDA crystal was optimized by density functional theory (DFT) B3LYP/6-31++ G × d(p) method using Gaussian 09 W software and visualized by Chemcraft software. FMO energy parameters and Molecular Electrostatic Potential mapping of the PNBPDA were also found out by DFT method with same basis set. Using DFT– B3LYP functions with 6−31 ++ G ∗ d(p) basis set, the most stable structure of the PNBPDA compound was computed and is illustrated in Figure 14a.

Figure 14:

(a) Optimized geometry. (b) FMO analysis. (c) Graphical representation of MESP.