Crystal, spectroscopic and quantum mechanics studies of Schiff bases derived from 4-nitrocinnamaldehyde

Two Schiff bases, (E)-1-(4-methoxyphenyl)-N-((E)-3-(4-nitrophenyl)allylidene)methanamine (compound 1) and (E)-N-((E)-3-(4-nitrophenyl)allylidene)-2-phenylethanamine (compound 2) have been synthesized and characterized using spectroscopic methods; time of flight MS, 1H and 13C NMR, FT-IR, UV–VIS, photoluminescence and crystallographic methods. The structural and electronic properties of compounds 1 and 2 in the ground state were also examined using the DFT/B3LYP functional and 6-31 + G(d,p) basis set, while the electronic transitions for excited state calculations were carried out using the TD-DFT/6-31 + G(d,p) method. The Schiff base compounds, 1 and 2 crystallized in a monoclinic crystal system and the P21/c space group. The emission spectra of the compounds are attributed to conjugated π-bond interaction while the influence of the intra-ligand charge transfer resulted in a broad shoulder for 1 and a double emission peak for 2. The calculated transitions at 450 and 369 nm for 1 and 2 respectively are in reasonable agreement with the experimental results. The higher values of dipole moment, linear polarizability and first hyperpolarizability of 1, suggest a better optical property and better candidate for the development of nonlinear optical (NLO) materials.


1
H and 13 C spectra, while TMS was used as the internal standard. The 2D NMR spectra were also collected and recorded on the same NMR instrument. Data of the single crystal were collected on a Bruker SMART APEX2 area detector diffractometer. All the synthesis and analysis were carried out at Catalysis and Peptide Research Unit, School of Health Sciences and School of Chemistry and Physics, University of KwaZulu-Natal, Durban South Africa.

Synthesis of the Schiff base compounds. Synthesis of (E)-1-(4-methoxyphenyl)-N-((E)-3-(4-nitrophe-
nyl)allylidene)methanamine (1). 4-Methoxybenzylamine (230 mg; 1.70 mmol) was added to a solution of 4-nitrocinnamaldehyde (300 mg; 1.70 mmol) in ethanol (15 ml). The mixture was stirred for 48 h in a 100 ml roundbottom flask at room temperature. The resulting precipitates were filtered and washed with cold ethanol and then dissolved in hot ethanol and allowed to stand. Single crystals of the compound 1 (Scheme 1), suitable for X-ray diffraction study, were obtained as the ethanol evaporated slowly. The following properties are obtained: ((C 17 Figure S1). See Table 4 and Table S1 for other spectroscopic data. (2). Compound 2 (Scheme 2) was synthesized under the same experimental condition as 1. The following properties are obtained: (C 17 Table 4 and Table S1 for other spectroscopic data.

Synthesis of (E)-N-((E)-3-(4-nitrophenyl)allylidene)-2-phenylethanamine
X-ray determination of the compounds. Single rod-shaped crystals of 1 and single plank-shaped crystals of 2 were obtained by recrystallization from ethanol and methanol respectively. Suitable crystals had dimensions of 0.38 × 0.22 × 0.14 mm 3 for compound 1 and 0.32 × 0.24 × 0.13 mm 3 for 2. These were selected and mounted on a suitable support on the single crystal X-ray instrument. The crystals were kept at a steady temperature (t = 100(2) k) during data collection. The structures were resolved with the shelxs-2013 34 structure solution program using the intrinsic phasing solution method and by using olex2 34 as the graphical interface. The models were refined with version 2016/6 of shelxl 35 using least squares minimisation. CCDC-1998746 and CCDC-1998749 contain the supplementary crystallographic data information for 1 and 2 respectively. This data Scheme 1. Synthesis of (E)-1-(4-methoxyphenyl)-N-((E)-3-(4 nitrophenyl)allylidene)methanamine (1).  Table 1, bond lengths and angles are given in Tables 2 and 3.
Computational methods. Molecular chemical stability is a measure of the difference in HOMO and LUMO energies 36 . This energy gap between HOMO and LUMO can be used to determine molecular electrical transport properties. The electrophilicity and electronegativity index, chemical hardness and softness of a molecule were calculated using the HOMO and LUMO energy values as follows 37 : where A is the electron affinity and I, the ionization potential. A = − E LUMO , I = − E  . Thermodynamic energy parameters of compounds were also calculated using B3LYP/6-31G(d,p) method. The total static dipole moment (µ) and the average linear polarizability ( α ) for the Schiff base compounds were calculated using the B3LYP/6-31G(d,p) method (Eqs. 5-7), the first hyperpolarizability (β) were calculated using the Kleinmann's symmetry 42,43 . β value is a measure of the second harmonic generation efficiency 44 : Natural bond orbital (NBO). NBO analysis is an effective approach to understand the intra and intermolecular bonding and interaction among bonds, and information of charge transfer or conjugative interactions in molecular system 45,46 . The associated electron donor orbital, acceptor orbital and the interacting stabilization Electrophilicity index 7) β = β xxx + β xyy + β xzz 2 + β yyy + β xxy + β yzz 2 + β zzz + β xxz + β yyz  www.nature.com/scientificreports/ energy were derived from the second-order micro-disturbance theory. For each donor (i) and acceptor (j), the stabilization energy E (2) associated with delocalization is approximated as 47 : where q j is the donor orbital occupancy, ε i and ε j are diagonal matrix elements and F i, j is the off-diagonal Fock matrix element. B3LYP functional with 6-31 + G(d,p) basis was used for NBO calculation. All calculations were carried out using Gaussian16 48 with the default convergence criteria, without any constraint on the geometry.

Results and discussion
Crystallographic study. The two compounds crystalized in the monoclinic crystal system and P2 1 /c space group, the ORTEP view of 1 and 2 are shown in Fig. 1. The crystal data and refinement details for the compounds are presented in Table 1. The compounds lie on centre of symmetry. The bond lengths in the compounds both experimental and computed are within the expected range. From the data in Tables 2 and 3, the bond distance of equivalent atoms in the compounds are equal, the C7=C8 length is 1.3383 (16) Table 2). The dihedral angles of the atoms (C10-N2-C9-C8) in 1 and 2 are found to be 178.70° and 179.50° respectively (Table 3).
Infrared spectra. The FT-IR spectra of the compounds 1 and 2 are presented in Figure S2 and S3. The strong peaks at 1593 and 1591 cm −1 for 1 and 2 correspondingly are assigned to the azomethine ѵ(CH=N) vibrations which indicate the formation of the Schiff bases. The peaks around 3088 (1) and 3031 (2) cm −1 are diagnostics of aromatic C-H vibrational stretching frequency 51 . While the bands at 2947 (1) and 2926 (2) cm −1 are due to aliphatic C-H stretching vibrations in the respective compounds. The peak at 1242 cm −1 in 1 spectrum is assigned to C-O stretching vibration but absent in the spectrum of 2. While those at 1341 and 1443 cm −1 in 1, and 1347 and 1506 cm −1 in 2 spectrum are assigned to the -NO 2 symmetric and antisymmetric stretching vibrational frequency respectively 52 . The FT-IR assignments for the compounds are presented in Table 4.
Electronic spectra. The UV-VIS electronic absorption spectra recorded in chloroform for compounds 1 and 2 are presented in Figure S4-S5. A broad absorption band in the range 250-370 nm with peaks at 322 and 320 nm corresponding to 3.86 and 3.89 eV for 1 and 2 respectively are associated with π-π* transitions in the compounds 53 . The calculated energy gap for 1 and 2 was 3.73 and 3.70 eV respectively. However, the molar absorptivity (ε) of 1 (1.1 × 10 6 M −1 cm −1 ) was greater than that of 2 (3.2 × 10 5 M −1 cm −1 ); an indication of the influence of the electron donating effect of the p-methoxy substituent in 1.  www.nature.com/scientificreports/ 127.71 ppm corresponding to the proton at 7.59 ppm which was assigned H-5/9, while the signal at 8.21 ppm was assigned H-6/8. Hence, by elimination, the signal at 7.00 ppm was assigned to H-3. All quaternary carbons and methylene carbons were differentiated from methine and methyl carbons with the aid of 13 C attached proton test (APT) experiment. The methoxy proton resonance recorded at 3.79 ppm in 1 serves as a diagnostic structural difference between the two compounds. This observation is corroborated by the presence of C-O stretching band at 1242 cm −1 in compound 1, but absent in 2. The NMR spectra of compound 2 was elucidated similarly, the discussion is presented in the Supplementary Material.

Photoluminescence (spectroscopic analysis).
The photoluminescence study of 1 and 2 was carried out in chloroform at room temperature. The excitation and emission spectra of the compounds 1 and 2 as seen in Figures S11 and S12 showed that the excitation bands were in the UV region at 302 nm with a shoulder at 345 nm in 1, and at 345 nm with a shoulder at 360 nm in 2. However, the emission bands were in the visible region centred at 486 nm with a shoulder at 446 nm in 1, while a double peak was observed at 455 and 478 nm in  www.nature.com/scientificreports/ 2. The main band of the excitation spectra could be associated with π-π* intra-ligand transition and the shoulder suggested intra-ligand charge transfer band. The emission spectra of the compounds are attributed to conjugated π-bond interaction while the influence of the intra-ligand charge transfer resulted in a broad shoulder in 1 and a double emission peak in 2 27,51 . Electronic excited state calculations obtained using TD-DFT in chloroform as a solvent phase are presented in Table 5. Since 1 and 2 are conjugated systems with π bonds and aromatic rings, thus, it allows π-π* transitions in the UV-vis region with high extinction coefficients. The calculated transitions at 450 and 369 nm for 1 and 2 respectively agree reasonably with the experimental results. Table 6, 1 has higher values for dipole moment, linear polarizability and first hyperpolarizability; suggesting that 1 has a higher tendency to interact with an external field. The reason for this could be that the presence of methoxy group (-O-CH 3 ) in 1 induces more polar and multipolar components than in 2, with the β values of both molecules being dominated by the β xxx tensor component, this is indicative that there is delocalization of charges in the β xxx direction (Table S2). The optical properties of 1 and 2 were compared with the properties of urea and L -leucine nitrate ( Table 6). Urea is one of the prototypical molecules applied in the study of the NLO properties of molecular systems. It was observed that both compounds in this study are better NLO materials based on their dipole moment and β values. According to the magnitudes of the polarizability (α) and the first hyperpolarizability (β) values of the investigated Schiff compounds, they may be useful in the development of NLO materials.

Nonlinear optical parameters. From the results in
HOMO-LUMO analysis. The energy gap, chemical hardness, electrophilicity index, electronegativity and softness of the compounds are listed in Table 7. The energy gap of the one electron excitation from HOMO to LUMO for 1 and 2 are 2.76 and 3.57 eV, respectively. The lower energy gap of 1 indicates that it is more reactive and softer than 2, the implication is that electrons are more easily migrated from the donor part to the acceptor part, it is also more polarizable and needs smaller energy for excitation than 2. The 3D plots for the HOMO and LUMO of 1 and 2 are shown in Fig. 2. The variation of energy gap with dipole moment, polarizability and hyperpolarizability agrees with the expected trend. The lower the energy gap, the higher the dipole moment,  www.nature.com/scientificreports/ polarizability and hyperpolarizability 26,56 . Analysis of the result showed that compound 1 has lower energy gap, higher dipole moment, higher polarizability and hyperpolarizability compared to 2. Chemical hardness has a direct significance as it is derived from the energy gap, as seen in Eq. 1 while chemical softness increases as the energy gap reduces (Eq. 4). These parameters help in predicting excitation properties through electron transport. A soft molecule is characterized by low LUMO-HOMO gap which favours better chemical reactivity, and this is a measure of the polarizability and hyperpolarizability character of a compound 54 . 1 is termed a softer molecule with more chemical reactivity based on the descriptors indicating that higher electron transition occurs in 1 compared to 2.
The thermodynamic parameters of 1 and 2 were obtained and listed in Table S3. Thermodynamics properties such as zero point energy (ZPE), enthalpy, Gibbs free energy (G 0 ) are essential to establish the stability, structural and reactivity of systems 57,58 . The high values of G 0 and ZPE (Table S3) suggests thermodynamic stability of compounds.
Natural bond orbital (NBO) analysis. The larger the E (2) value, the more intensive is the interaction between electron donors and electron acceptors, i.e. the more donating tendency from electron donors to electron acceptors and the greater the extent of conjugation of the entire system 59 . NBO calculations were performed on the molecules at the DFT/B3LYP/6-31G(d,p) level in order to explain the intra molecular rehybridization and delocalization of electron density within the studied molecules.
The second-order perturbation energies E (2) associated with the delocalization of donor and acceptor bonds were presented in Table 8. The intramolecular hyperconjugative interactions formed by the orbital overlap between π*(C21-C25) and π*(C18-C19) bond orbitals of 1 coupled with a substituted methoxy at the gamma position resulted in a huge intramolecular charge transfer leading to stabilization of ∼217.77 kJ/mol of the system. With respect to 2, as it has only a conjugated ring formed overlap between π(C27-C29) and π*(C32-C34) with a corresponding stabilization energy of 21.25 kJ/mol. Hydrogen bonding interactions was observed between the nitrogen lone pair and C-H antibonding orbital (LP (1)N33 → σ*C16-H36, LP(1)N18 → σ*C16-H21) with the energetic contribution (12.99 and 13.21 kJ/mol) for 1 and 2, respectively. Electron intramolecular delocalization is obvious in the resonating of the nitrogen and oxygen lone pairs (LP (3) O13 → σ*N11 of both molecules given a close stabilization energy of 163.35 and 163.31 kJ/mol. Molecular electrostatic potential analysis. The molecular electrostatic potential (MEP), V(r) at a specified point r(x, y, z) within the environs of a molecule is given in terms of the interaction energy between a test positive charge (a proton) located at point r and an electrical energy which is generated from the molecule electrons and nuclei. The V(r) values for the studied systems are calculated by the Eq. (9) 60 Figure 2. 3D plots of HOMO and LUMO for the compounds. www.nature.com/scientificreports/ where Z A is the charge of nucleus A located at R A in the vicinity of the system, ρ(r′) is the electronic density function of the molecule, and r′ is the dummy integration variable 61 . The MEP at the B3LYP/6-31G(d,p) optimized geometry of the compounds were calculated in order to predict the sites of nucleophilic and electrophilic attack on the Schiff base compounds. MEP is determined by electron density and is a reliable indicator of sites of possible nucleophilic and electrophilic attack as well as sites for hydrogen-bonding interaction on a compound. From the MEP map of the compounds as shown in Fig. 3, electrophilic and nucleophilic attack could likely take place on the O1 and N1 as they are the most electronegative and electropositive sites on the maps.

Conclusion
Compounds of (E)-1-(4-methoxyphenyl)-N-((E)-3-(4-nitrophenyl)allylidene)methanamine and (E)-N-((E)-3-(4-nitrophenyl)allylidene)-2-phenylethanamine have been synthesized and characterized using 1 H and 13 C NMR, UV, FT-IR spectroscopy, Time of flight MS, X-ray crystallographic methods. The structural and photophysical properties of the studied systems were rationalized via DFT and TD-DFT calculations. The structural analysis from the crystallographic data shows that the compounds were obtained as monoclinic crystals. The structural difference between the two compounds were established from the NMR study and confirmed by the single crystal X-ray crystallographic data. Bond lengths and bond angles obtained experimentally closely agree with the theoretical values. Similarly, the energy gaps from the electronic spectra associated with π-π * transitions in the compounds are comparable with those obtained theoretically. The photoluminescence properties of the compounds were investigated, and the emission spectra obtained are attributed to conjugated π-bond interaction characterised by high intramolecular charge transfer and leading to the stabilization of the studied systems. The low ∆E, high polarizability (α) and the first hyperpolarizability (β) values obtained in this study suggests that the studied compounds are good condidatesfor the development of NLO materials.