The elemental analysis data (Table 1) for the all Schiff base compounds are well agreed with the proposed molecular formulae.

The FT-IR spectroscopy is a powerful tool for the assignments of fundamental functional group determinations of organic compounds. In the title compounds frequencies of the functional groups viz., ν_>C=N-, ν_ph-OH, ν_ph-C-O, ν_ph-N=C- and ν_-N=CH-/ν_-N=C(CH3)- are of great importance in the infrared spectra. The infrared spectra of all the four Schiff bases (Figure 1) exhibited a strong band in the 1692 - 1642 cm⁻¹ region due to the characteristic of azomethine n(C=N) group. All the title compounds were displayed a band in the region 3399 - 3308 cm⁻¹, which could be due to ν(ph-OH). A strong band observed at 1373 - 1362 cm⁻¹ in

the four compounds has been assigned to phenolic C-O stretching and a band due to ph-N=C- of aminophenol group exhibited in the region 1274 - 1296 cm⁻¹. Further the spectra of all the Schiff bases showed a weak band in the 3020-3069 cm⁻¹ region due to -N=CH-/_-N=C(CH₃)- groups of NA, SA, VA and AA [14] .

Theoretically all the fundamental vibrations were active in IR. The results showed that the DFT (B3LYP) method applied in this work leads to vibrational wavenumbers which are in good agreements with the experimental data. It is noteworthy that the very important role of vibrational frequencies salen compounds, based on experimental data as well as theoretical calculations, the computed vibrational frequencies in Table 2 and Figure 1 for SA, AA, VA and NA at the DFT level of theory are in acceptable agreement with the experimental data.

The UV-Visible spectra of the salen compounds (Figure 2) exhibit mainly two bands. The first band on the higher energy side, the range l_max= 350 - 385 nm, due to the excitation of the p-electrons (p®p^* transitions) of the aromatic azo-

methine group. The second band, l_max = 450 - 480 nm range is assigned to an intramolecular charge transfer (ICT transition) involving the salen compounds. This band observed in salicylaldimine compounds of salen derivatives is facilitated by the presence of intramolecular hydrogen bonding between the -OH group and the azomethine nitrogen [26] .

The formations of Schiff base were conveniently monitored by peak ratios in the ¹H-NMR spectra. ¹H-NMR spectra of all the four compounds (Table 3 and Figure 3) were taken in DMSO.d₆ solvent. The aromatic region was a set of multiplets in the range 6.6 - 8.4 ppm for all the Schiff bases, while the azomethine proton of SA, VA and NA compounds were observed in the range 9.0 - 9.5 ppm. The phenolic-OH protons of all the four Schiff bases were observed as a singlet in the region 13.8-15.6 ppm. ¹H-NMR spectra exhibited a strong peak at 3.8 and 3.5 ppm for -OCH₃ protons of VA and -CH₃ protons of AA salen compounds.

¹³C-NMR spectral data (Table 3 and Figure 3) were consistent with ¹H-NMR spectral data. The methyl carbon of aliphatic substituents, azomethine (>C=N) for all the Schiff bases and for Ph-C-CH₃ of AA and Ph-OCH₃ of VA compounds were 45 and 56 ppm respectively. The resonance observed at 108 - 178 ppm was assigned to the phenyl group carbon of salen compounds.

At the minimum energy conformations (Figure 4) obtained from the energy scan, further geometry optimization was performed with the B3LYP/6-311G (d,p) basis set. No geometrical parameter constraint was imposed during the optimization, except those favoring the stabilizing effects due to hydrogen bonding between two adjacent -OH groups. The optimized structures of the most stable conformers of neutral form of salen compounds are shown in Figure 5.

The optimized geometrical parameters are shown in Table 4 (Supplement materials). From the data of bond distances and bond angles, it can be seen that no significant geometrical change has been observed for all the compounds. From the data of dihedral angle in Table 4, it can also be seen that compounds with o-hydroxyl group in A-ring are completely planar, while others have some

degree of deviation from the planarity due to the torsion between A-ring and the plane of aminophenol system B-ring (Scheme 2).

In Table 5 the dipole moment of four compounds at B3LYP/6-311G(d,p) level of computation have been reported. The Dipole moment values lie in the range of 1.7675D to 4.8823D for salen compounds. These values are rather high, reflecting the numerous polarized hydroxyl or carbonyl functions distributed over the structures. The three structures of salen compounds differ only by the presence of methyl at C7, methoxy at C3 and naphthyl groups in the A-ring and there is a little difference in the dipole moment value. The molecular dipole moment is used to measure bond polarities and charge densities in a molecule.

Hardness,.

Bond polarity is one of the factors that determine the physicochemical property of molecules. The calculated values of the total dipole moments, which signifies the relatively polarized nature of the systems and they are soluble in polar solvents like CH₃CN, DMSO, CHCl₃, etc.

The computed NMR chemical shifts for SA, AA, VA and NA at the DFT level of theory are in acceptable agreement with the experimental data. Differences between the calculated and measured values may be a result of solvent interactions.

The ¹³C-NMR chemical shifts of selected carbons were calculated on the optimized structures of SA, AA, VA and NA using GIAO/DFT method with B3LYP/6-311(d,p) basis set for all atoms. Calculated and measured ¹³C chemical shifts of selected atoms are numbered in Figure 6.

The HOMO-LUMO energy (Figure 7 and Table 6) which characterizes the ability of electron-giving is appropriate to represent the free radical scavenging efficiency of phenolic compounds because the process to inhibit auto-oxidation may include the electron-transfer besides the abstraction of the H-atom [12] . The HOMO (−0.2070, −0.2040, −0.2160 and −0.2040 Hartree) and the LUMO (−0.0590, −0.0440, −0.0750 and −0.0790 Hartree) for the compounds SA, AA, VA, NA respectively, are p-like orbitals confirmed by UV-Vis spectra and they are delocated in the whole molecule of all the salen compounds. Considering the disposition of HOMO and LUMO orbitals, the -OH group of 2 and 9-position is easily attacked by either the electrophilic or nucleophilic agents, such as radicals, metal ions and O₂. Indeed, HOMO and LUMO orbitals for four compounds are localized on A and B ring, conjugated double bond nature in the phenyl and naphthyl moiety and on the A and B conjugated ring salen compounds [27] [28] .

The spin density is often considered to be a more realistic parameter which provides a better representation of the reactivity [6] [9] of salen moiety. It should be pointed out that the more delocalized the spin density in the salen compound is, the easier the radical is formed [5] . In the Figure 8, the spin densities of all the radicals mainly distribute on the phenolic oxygen atom and the phenyl ring A

and B. Comparing all the radicals, we find that the radicals have almost the same spin density distribution with each other, indicating the presence of additional hydroxyl or o,o-dihydroxyl group on A and B-ring has almost no influence on the spin density distribution [27] [28] .

Electrostatic potential surfaces are mainly used to study the reactive species of electrophilic or nucleophilic attacks/substitution in the chemical reactions, biological process, catalysis and also molecular modeling. Electrostatic potential mapped surface displays the molecular size, shape and potential values. In this study, 3 dimensional surfaces of mapped onto the constant electron density surface is as shown in Figure 9. Different values of electrostatic potential at the surface are represented by different colors [29] .

The NLO response calculation was performed on the optimized geometry using the same level of theory. The first static hyperpolarizability is a third rank tensor that can be described by a 3 × 3 × 3 matrix. The 27 components of the 3D matrix

can be reduced to 10 components due to the Kleinman symmetry [27] [30] (β_xyy = β_yxy = β_yyx, β_yyz = β_yzy = β_zyy, ..., likewise other permutations also take same value). It can be given in the lower tetrahedral format. The output from Gaussian 03W provides 10 components of this matrix as β_xxx, β_xxy, β_xyy, β_yyy, β_xxz, β_xyz, β_yyz, β_xzz, β_yzz and β_zzz respectively. Many types of hyperpolarizabilities have been discussed in the literature [31] . When reporting a single value of β, one of the common formats is to simply treat the three independent values for β as a quasi-Pythagorean problem and solve for the average β by Equation (5):

The complete equation for calculating the magnitude of the total first statichyperpolarizability from Gaussian 03W output is given as Equation (6):

Since these β values of the first static hyperpolarizability (β) tensors of the output file of Gaussian 03W are reported in atomic units (a.u.), the calculated values were converted into electrostatic units (1 a.u. = 8.6393 × 10⁻³³ esu). The first static hyperpolarizability (β) values of these Schiff bases were calculated under static electronic field by the DFT (B3LYP/(6-311G(d,p)) method. From Table 7, compounds NA, VA and AA have larger the first static hyperpolarizability (β_tot) values than those of compound SA. As mentioned above, the DFT results show that the LUMO orbitals of all compounds were obtained from the linear combination of the orbitals of phenyl moiety, the compounds AA, VA and NA with an electron-donating and withdrawing group (CH₃, OCH₃ and naphthyl) have a larger β_tot value compare with compound SA. Hence the compound VA was predicted to have larger NLO property. The energy difference (∆E_gap) between the HOMO and the LUMO orbitals 3.5506, 4.3538, 3.8368, 3.4014 eV for the compounds SA, AA, VA, NA respectively has a larger influence on the β_tot value [32] [33] . To understand the relationship between the static hyperpolarizabilities and the HOMO-LUMO energy gap, viz. the compound VA (3.4014 eV) lower the HOMO-LUMO energy gap show that the larger the β_tot (13.455 a.u) value.