Studies on growth and characterization of nonlinear optical material 4-chloro-4′methoxy benzylideneaniline: A Schiff base organic material

2.1 Preparation and growth of CMOBA

CMOBA was synthesized by the condensation reaction between p-chlorobenzaldehyde (p-CB) and p-methoxyaniline (p-MOA) in equimolar ratio (Ren et al., 2008). The schematic representation for the synthesis of CMOBA is depicted in Scheme 1. The reaction mixture was refluxed for about 8 h and the solution was filtered using the Whatman filter paper and the resulting product of 4-chloro-4′methoxy benzylideneaniline was obtained. The purity of the synthesized salt was improved by successive recrystallization process in ethanol. Activated charcoal was added during the recrystallization process for removing colored impurities and the purified sample was shinny. Thin layer chromatography (TLC) studies confirmed that the synthesized material consists of single compound. Important factor that influences the habit of growing crystal is the polarity of the solvents (Lide, 1999). Hence, in this study a few organic solvents were employed to identify the reasonable solvent. Various solvents having different dipole moments such as methanol, ethylacetate and acetonitrile and their mixed solvents were employed to study their influence on the growth habits of CMOBA crystal. The mixed solvent of ethanol and ethyl acetate yielded relatively good transparent crystal. Hence the single crystals of CMOBA have been grown from saturated solution of the synthesized salt employing the mixed solvent of ethanol and ethyl acetate by the slow evaporation at room temperature. Transparent single crystal of dimension 6 × 4 × 2 mm³ was grown in a growth period of twenty one days and is shown in Fig. 1.

Close

Scheme 1 Schematic diagram for the synthesis of CMOBA.

Export to PPT

Close

Figure 1 Harvested crystal of CMOBA.

Export to PPT

3.1 FTIR spectral analysis

The Fourier transform infrared (FTIR) spectrum recorded for the CMOBA material using a Perkin Elmer-Paragon – 500 FTIR spectrometer is given in Fig. 2. The spectrum was recorded by the KBr pellet technique between the range of 400 and 4000 cm⁻¹ and the presence of various functional groups was confirmed. Benzylideneaniline displays the C⚌N stretching at 1613 cm⁻¹ (Kemp, 1993) and the imine C—H in-plane bending is observed at 1362 cm^-1. Para-di-substituted benzenes show the C—H deformation vibration in the region 840–800 cm⁻¹. In this work the C—H deformation vibration appears at 833 cm⁻¹. C—H in-plane and out-of-plane bending vibrations of the phenyl ring are observed at 1025, 1094, 1176, 1247 and 1297 cm⁻¹ and 761 and 833 cm⁻¹, respectively (Silverstein et al., 1981). The absence of the vibrational frequency of the N—H and C⚌O groups in the FTIR spectrum also confirms the formation of the imine group (Kemp, 1993). Thus the FTIR spectral analysis confirms the formation of the CMOBA.

Close

Figure 2 FTIR spectrum of CMOBA.

Export to PPT

3.2 NMR spectral analyses

In the present study, Nuclear Magnetic Resonance (NMR) spectral analysis, a powerful tool to derive the structural information from the synthesized compound, was carried out on the purified CMOBA sample. The ¹H and ¹³C NMR spectra of the CMOBA were recorded by employing a Bruker AC 400 MHz NMR spectrometer in CDCl₃. In the ¹H NMR spectrum, the singlet at 8.423 ppm confirms the formation of Schiff base compounds containing the imine (CH⚌N) group. The adjacent four doublet protons are due to the aromatic rings proton. The signal at 3.820 ppm corresponds to three protons of the methoxy group of the aniline phenyl ring moiety. The presence of protons in the CMOBA material is shown in Fig. 3a. In the NMR spectrum of CMOBA, the ratio of steps obtained is as follows, $$\begin{array}{cccccc}\text{A}:\hfill & \text{B}:\hfill & \text{C}:\hfill & \text{D}:\hfill & \text{E}:\hfill & \text{F}\hfill \\ 1:\hfill & 2:\hfill & 2:\hfill & 2:\hfill & 2:\hfill & 3\hfill \end{array}$$

Close

Figure 3a ¹H NMR spectrum of CMOBA.

Export to PPT

Thus, the number of protons associated with the signals are A-1H; B, C, D and E-2H; and F-3H, respectively. The CMOBA peaks are compared with 4-nitro-4′methoxy benzylideneaniline (NMOBA) crystal. Substitution of the chloro group at the para-postion of benzaldehyde causes shielding effect of imine, methoxy and aromatic ring protons when compared to NMOBA. For NMOBA two proton doublet (ortho to nitro) at 8:1 ppm and CMOBA chemical shift at 7.8. Generally the nitro group has strong electron withdrawing nature when compared to the chloro group so chemical shift slightly changed.

The common range of energy absorption for ¹³C is wide (δ 0-200) relative to tetramethylsilane (TMS). In the ¹³C NMR spectrum (proton decoupled) each magnetically non-equivalent carbon gives a single unsplit peak. The spectrum (Fig. 3b) shows the placement of carbon atoms in the CMOBA material. The imine group carbon atom (C⚌N) at 156.65 ppm (signal B) appears as singlet peak and C⚌N peak position well coincide with the reported literature value (Neuvonen et al., 2006) which confirms the formation of Schiff base compound. The chemical shift at 55.49 ppm is attributed to the methoxy (OCH₃) (signal K) group. The peak at 158.51 ppm is due to = C—OCH3 which belongs to the aromatic ring carbons. The signals at A, C, D, E, F, G, H and I are due to aromatic ring carbon. The signal at J is due to the solvent. Thus the molecular structure of CMOBA is confirmed by proton and carbon NMR analyses.

Close

Figure 3b ¹³C NMR spectrum of CMOBA.

Export to PPT

3.3 Single crystal X-ray diffraction

Single crystal X-ray diffraction study was carried out using a single crystal diffractometer CAD4/MACH 3 which shows that CMOBA crystallizes in the orthorhombic crystal system. The recorded unit cell parameters are a = 6.102 Å (6.155 Å), b = 7.357 Å (7.3392 Å) and c = 27.555 Å (27.469 Å) and these values compare well with the corresponding values of the earlier work (Ren et al., 2008) given in the parentheses.

3.4 Linear optical property

3.4.1

3.4.1 UV–vis-NIR transmittance

In order to understand the optical transparency in the UV–vis-NIR region of the electromagnetic spectrum, the optical transmittance study was carried out for the CMOBA crystal of thickness ∼2 mm. In this study the transmittance of the sample, which is the descriptive result of absorption, was recorded at room temperature. The lower cutoff wavelength of the CMOBA starts at 390 nm and it shows the wider range of the optical transparency. The crystal is transparent in the visible region of 400–1100 nm wavelength as shown in Fig. 4.

Close

Figure 4 UV–vis-NIR Transmission spectrum of CMOBA.

Export to PPT

3.5 Nonlinear optical properties

3.5.2

3.5.2 Z-scan studies

The nonlinear absorption of CMOBA was studied using the single beam Z-scan studies. To measure the optical nonlinearity, open aperture Z-scan measurements were done at 532 nm using 5 ns laser pulses from a frequency-doubled Nd:YAG laser (Quanta Ray-Spectra Physics). For this measurement the sample was dissolved in chloroform and excited with laser and the pulse energy used is 90 μJ. The laser beam is focused using a lens, and the sample is translated along the beam axis (z-axis) through the focal region over a distance several times that of the diffraction length. At each position z the sample sees different laser intensities, and the position dependent (i.e., intensity-dependent) transmission is measured using an energy meter placed after the sample. Fig. 5 shows the open aperture Z-scan of CMOBA. The Z-scan curves obtained are numerically fitted to the nonlinear transmission equation for a two-photon absorption process (Sutherland, 1996) given by $$T=\frac{1}{\sqrt{\pi }{q}_0}{\int }_{-\infty }^{+\infty }\ln (1+q_0{e}^{-t^2})\mathit{dt}$$ where α is the linear absorption coefficient. q₀ is given by β(1 − R)I₀L_eff, where β is the nonlinear absorption coefficient and I₀ is the on-axis peak intensity. L_eff is given by [1 − exp(−αL)]/α, where L is the sample length. Measured two-photon absorption coefficient of CMOBA is 3 × 10⁻¹² m/W. Under similar excitation conditions, one of the benzylideneaniline derivative materials 4-bromo-4′dimethylamino benzylideneaniline (Subashini et al., 2013) exhibits the effective TPA coefficient value in the range of about 10⁻¹² m/W.

Close

Figure 5 Open aperture Z-scan of CMOBA.

Export to PPT

3.6 Thermal analyses

The thermogravimetric and differential thermal analyses (TG/DTA) of CMOBA was carried out between 30 and 1200 °C at a heating rate of 20 °C/min using TA instruments Model SDT Q 600 in the nitrogen atmosphere and is given in Fig. 6a. The material is stable up to ∼177 °C and the major weight loss (97.78%) is observed in the temperature range of 178–285 °C. Single stage weight loss is observed in the TG curve. A sharp endothermic peak at ∼125 °C corresponds to the melting point in DTA. The sharpness of the peak confirms the good crystallinity of the synthesized compound. The differential scanning calorimetry (DSC) study was performed with 3.033 mg of CMOBA using TA instruments Model SDT Q 600 in the temperature range of 27–1200 °C at a heating rate of 20 °C/min in the nitrogen atmosphere. The CMOBA material melts at ∼125 °C (Fig. 6b) and the value agrees well with the DTA.

Close

Figure 6a TGA/DTA curve of CMOBA.

Export to PPT

Close

Figure 6b DSC curve of CMOBA.

Export to PPT

3.7 Dielectric studies

Dielectric properties are correlated with the electro-optical property of the crystals. The measurement of dielectric properties provides information on materials (Hiremath and Venkataraman, 2003). Carefully discerned sample of CMOBA was cut and polished and coated by the ohmic contact to make the sample as a parallel plate capacitor. Agilent Model LCR meter was used to measure the capacitance and dielectric loss (tan δ) of the sample as a function of frequency (100 Hz to 1 MHz) in the temperature range of 60–100 °C. The dielectric constant was calculated using the relation $${\epsilon }_{\text{r}}=\left\{[{\text{C}}_{\text{crys}}-{\text{C}}_{\text{air}}(1-{\text{A}}_{\text{cry}}/{\text{A}}_{\text{air}})]/{\text{C}}_{\text{air}}]\right\}\text{x}[{\text{A}}_{\text{air}}/{\text{A}}_{\text{cry}}]$$

where C_crys and C_air are the capacitance of the crystal and air respectively, and A_cry and A_air are the area of the crystal and air respectively. The maximum dielectric constant measured at 100 °C for the CMOBA crystal is 82 for 1 KHz and this value decreases to 55 for 1 MHz as shown in Fig. 7a. It is observed that the dielectric constant of the crystal at higher frequencies is almost constant. At lower frequencies, the dielectric constant was considerably higher than that at higher frequencies. This can be attributed to the interfacial polarization in which the mobile charge carriers are impeded by a physical barrier that inhibits producing a localized polarization of the material. This type of dielectric response revealed is of great interest in applications in which materials with high dielectric constant values are good candidates for heating devices (Sankar et al., 2007). The magnitude of dielectric constant (ε_r) depends on the degree of polarization. The dielectric constant of CMOBA crystal measured at 100 °C is 130 for 1 KHz and this value decreases to 93 for 1 MHz as shown in Fig. 7b. These curves suggest that the dielectric constant and dielectric loss strongly depend on the frequency of the applied field (Hill et al., 1969; Rao and Smakula, 1965).

Close

Figure 7a Dielectric constant and loss vs. frequency of CMOBA.

Export to PPT

Close

Figure 7b Dielectric constant and loss vs. temperature of CMOBA.

Export to PPT