Nonlinear optical functionalized Cu(II) coordination complexes with chiral ligands: design, structural elucidation, and theoretical investigation

Abstract

This study investigates the design, synthesis, and comprehensive characterization of five chiral ligands derived from L-phenylalanine and halogen-substituted salicylaldehydes (F, Cl, Br, and I) and their copper(II) coordination complexes. All of these ligands and their complexes are fully characterized. The crystallographic studies reveal that these complexes are in the Cc space group and the central Cu(II) has a distorted square-pyramidal geometry, and noncentrosymmetric packing, which are advantageous for improving nonlinear optical (NLO) properties. Complementary computational analyses, including density functional theory (DFT) calculations, emphasized the significant impact of halogen substitution on the electronic characteristics of both the ligands and their copper complexes. These substitutions from H, F, Cl, Br, and I significantly reduced the HOMO–LUMO gaps, increasing the electron density and improving charge transfer characteristics gradually and respectively. Comparing with ligands, their copper complexes exhibited enhanced linear and nonlinear optical properties, among which complex-4 shows the most significant NLO performance, particularly in second-harmonic generation and electro-optic responses. The relationship between the structure and NLO properties has been understood based on experimental and computational investigations. This work can help in designing functionalized coordination complexes with enhanced NLO properties.

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Introduction

High-performance materials with enhanced nonlinear optical (NLO) properties have become more and more in demand in recent years because of their potential uses in data processing, medical imaging, telecommunications, and photoelectric technologies.^1–3 Among various NLO materials, those operating in a broad spectral range, from ultraviolet to far-infrared, have garnered significant attention.^4,5 In particular, deep-ultraviolet lasers (wavelengths below 200 nm) play vital roles in high-resolution spectroscopy, photochemical synthesis, laser micromachining, and other cutting-edge scientific applications.^6,7 One class of compounds that has emerged as particularly interesting in this context is metal complexes derived from Schiff bases. Formed by the condensation of primary amines and carbonyl compounds, Schiff bases offer great structural flexibility and tunable electronic properties, making them versatile ligands in coordination chemistry. Their ease of synthesis, thermal and chemical stability, and relatively low cost have made them attractive not only for catalytic applications, such as oxidation of alcohols and alkenes, but also for various biological activities including DNA binding and antioxidant, antiviral, and anticancer effects.^8,9 Recent work has highlighted how the careful engineering of SB metal complexes leads to novel functional materials with tailored band gaps and NLO behavior suitable for semiconducting and sensor applications.^10–13 Among metal complexes, those based on copper have gained considerable attention. Copper complexes are more affordable and easier to synthesize than precious metal analogues, while still providing tunable electronic properties critical for NLO behavior. Improved nonlinear optical responses are a result of copper ions’ extensive d-orbital interactions, charge transfer mechanisms, and coordination flexibility. Interestingly, copper(I) complexes frequently display metal-to-ligand charge transfer (MLCT), while copper(II) complexes usually display ligand-to-metal charge transfer (LMCT) and d–d transitions, both of which affect their optical characteristics.^14,15

Schiff bases derived from naturally occurring amino acids, such as L-phenylalanine, add another layer of functional diversity by introducing chirality and additional donor sites through their aromatic and carboxylate groups.¹⁶ Motivated by these advantages, in this study, we prepared a series of five Schiff base ligands using L-phenylalanine as the amine source and various halogen-substituted salicylaldehydes (fluoro, chloro, bromo, and iodo) as the carbonyl components. This substitution strategy was designed to probe how electronic and steric effects influence the ligand structure and coordination behavior. In our previous study, we reported Schiff base Cu(II) coordination complexes derived from L-glutamine.¹⁷ This work serves as a continuation of our ongoing research, expanding the scope to L-phenylalanine-based Schiff bases and their copper complexes to further investigate their structural, electronic, and nonlinear optical properties. The synthesized ligands are as follows: (E)-2-((2-hydroxybenzylidene)amino)-3-phenylpropanoic acid (H₂L¹), (E)-2-((4-fluoro-2-hydroxybenzylidene)amino)-3-phenylpropanoic acid (H₂L²), (E)-2-((4-chloro-2-hydroxybenzylidene)amino)-3-phenylpropanoic acid (H₂L³), (E)-2-((4-bromo-2-hydroxybenzylidene)amino)-3-phenylpropanoic acid (H₂L⁴), and (E)-2-((2-hydroxy-4-iodobenzylidene)amino)-3-phenylpropanoic acid (H₂L⁵) (Scheme 1). The five novel Cu(II) complexes [Cu(L¹)]_n (1), [Cu(L²)]_n (2), [Cu(L³)]_n (3), [Cu(L⁴)]_n (4), and [Cu(L⁵)]_n (5), respectively, were successfully synthesized, as illustrated in Scheme 2.

Scheme 1

Scheme 2

The ligands were thoroughly characterized using ¹H and ¹³C NMR spectroscopy, ESI-mass spectrometry, FT-IR, and UV-vis titration, which confirmed their structures and purity. The halogen substituents not only influence the electronic density but also create opportunities for halogen bonding, potentially affecting molecular recognition and supramolecular arrangement. After synthesizing the ligands, five copper(II) complexes were successfully prepared. Copper, with its biological significance, redox activity, and flexible coordination, was chosen as the ideal metal center for this study. Among these complexes, those derived from bromo- and iodo-substituted ligands formed single crystals suitable for X-ray diffraction analysis, providing profound insights into their coordination geometry, ligand conformation, and packing interactions. Thermogravimetric analysis (TGA) was employed to assess the thermal stability and decomposition pathways of these complexes.

To supplement the experimental work, density functional theory (DFT) calculations were conducted at the B3LYP level. Key molecular descriptors, including frontier molecular orbitals (HOMO–LUMO), molecular electrostatic potential (MEP), density of states (DOS), non-covalent interaction (NCI) plots, quantum theory of atoms in molecules (QTAIM), and natural bond orbital (NBO) analyses, were used to gain deeper insights into bonding characteristics, electron delocalization, and intra/intermolecular interactions within both the free ligands and copper complexes. A key focus of the study was assessing the nonlinear optical (NLO) properties. Since molecules with strong donor–acceptor characteristics and extended conjugation typically exhibit enhanced NLO responses, we examined the linear polarizability (α), first hyperpolarizability (β), and second hyperpolarizability (γ) of these Schiff bases and their copper complexes. The results demonstrate that both the type of halogen substituents and the metal coordination with the Cc space group, and noncentrosymmetric packing significantly influence the NLO behavior, highlighting their potential applications in photonics, electro-optic devices, and optical signal processing.

Results and discussion

Crystal structure description of complex-4

The crystal structure of complex-4, synthesized from L-phenylalanine and 4-bromo-salicylaldehyde, reveals significant details about its coordination environment and crystal packing (Table 1). In Fig. 1(a), the molecular structure shows that the Cu(II) center (Cu01) is coordinated by a Schiff base ligand, where the copper ion is bound to nitrogen (N009) and three oxygen atoms (O003, O004, O005) from the ligand, with the bromine atom (Br) positioned away from the metal center. The geometry around the copper is distorted square pyramidal, a common arrangement for Cu(II) complexes with this type of Schiff base ligand.^18–20 The space group of the crystal structure is Cc, which indicates a noncentrosymmetric arrangement. This feature is crucial, as noncentrosymmetric crystals often exhibit significant non-linear optical (NLO) properties that are important for applications in optical switching, frequency doubling, and other NLO phenomena.^21,22 The noncentrosymmetric structure of complex-4 indicates its strong potential for NLO applications, as it can facilitate second-harmonic generation (SHG) and other nonlinear effects that are typically challenging to achieve in centrosymmetric materials.

Fig. 1 (a) ORTEP molecular structure of complex-4; (b) 2D packing arrangement showing the interaction network in the crystal lattice; (c) 3D packing arrangement of the complex; and (d) view of the Cu(II) coordination environment, illustrating the distorted square pyramidal geometry.

Table 1 Crystallographic data of complexes 4 and 5

Complex	4	5
Formula	C18H19BrCuNO5	C18H18CuINO5
M (mol^−1)	472.79	518.77
Crystal system	Monoclinic	Monoclinic
Space group	Cc	Cc
T (K)	296.15	296.15
α (°)	90	90
β (°)	99.534(5)	99.858(2)
γ (°)	90	90
a (Å)	10.6897(15)	10.9097(8)
b (Å)	27.411(4)	27.347(2)
c (Å)	6.6883(9)	6.7079(5)
Z	4	4
V (Å^3)	1932.7(5)	1971.7(3)
ρ (calculated)(g cm^−3)	1.625	1.748
F(000)	952	1020
2θ range (°)	4.14 to 63.174	5.016 to 56.708
GOF on F^2	0.985	0.99
R int	0.0277	0.0452
R 1[I > 2σ(I)]	0.0329	0.0413
wR2[I > 2σ(I)]	0.0712	0.0745
R 1(all data)	0.0485	0.0671
wR2(all data)	0.0773	0.0852
Residuals (e Å^−3)	0.47, −0.32	0.45, −0.51
CCDC	2415583	2415691

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In the crystal packing images which are shown in Fig. 1(b and c), the molecules are arranged in a regular, three-dimensional network. The ligands effectively bridge the copper centers, creating a repeating pattern in the crystal lattice. This packing arrangement highlights the stability of the complex in the solid state and emphasizes the structural integrity that contributes to the noncentrosymmetric nature of the crystal. The well-organized packing may play a role in enhancing the material's NLO properties, as the alignment of the molecules in the crystal lattice can facilitate the desired optical effects. In Fig. 1(d), the coordination environment of Cu(II) is further illustrated. The distorted square pyramidal geometry is clearly depicted, with the copper ion coordinated by four donor atoms from the Schiff base ligand. This arrangement is crucial for the stability of the complex and for its potential reactivity in various applications, including catalysis and coordination chemistry. The Cu–N bond was measured to be 1.9362 Å, and the Cu–O bond lengths range from 1.9160 Å to 2.2832 Å. These bond lengths are consistent with those typically observed in Cu(II) complexes with Schiff base ligands, where the copper ion exhibits coordination to both nitrogen and oxygen donor atoms. The oxygen atoms are likely involved in strong donor–acceptor interactions with the Cu(II) center, helping stabilize the metal in the square pyramidal arrangement. The O–Cu–N bond angles, which are close to 90°, further confirm the complex's geometry, typical for copper complexes with such ligands.

Despite the presence of a metal–ligand interaction, hydrogen bonding is not observed in this crystal structure. The lack of hydrogen bonding is likely due to the specific nature of the Schiff base ligand and its coordination with the copper ion.^23,24 The ligand's rigid structure and the coordination of the copper to the nitrogen and oxygen atoms leave little room for intermolecular interactions that would lead to hydrogen bonding. Furthermore, the presence of the bromine atom may also sterically hinder potential hydrogen bond formation, as it could prevent close contact with neighboring molecules that would typically be required for hydrogen bonds to form. The absence of hydrogen bonding does not negatively impact the crystal stability but rather reinforces the specific type of coordination and packing that contributes to the complex's unique properties. Complex-4 crystallizes in the noncentrosymmetric Cc space group, which positions it as a strong candidate for nonlinear optical (NLO) applications because of its ability to produce nonlinear optical effects. The lack of hydrogen bonding in the crystal structure highlights the unique characteristics of this complex, where the metal–ligand coordination and crystal packing play a more significant role in determining its stability and optical properties. This structure is important for the design of NLO materials, as it demonstrates how molecular symmetry and the absence of certain interactions, such as hydrogen bonds, can influence the properties of coordination compounds. Complex-5, included in the SI (Fig. S19), is an isostructural counterpart to complex-4, which has been discussed in detail within the manuscript. Although both complexes share similar structural features, they exhibit distinct nonlinear optical (NLO) properties, highlighting the influence of subtle differences in their composition on their optical behavior.

Powder X-ray diffraction analysis

The PXRD spectra of the complexes (complex-1 to complex-5) reveal information on their phase purity and crystallinity (Fig. S20). A comparison of the experimental and simulated spectra highlights that the PXRD patterns of complex-4 and complex-5 closely match with their respective simulated patterns, confirming their crystallinity and the correct phase. For complexes 1, 2, and 3, the experimental PXRD patterns show similar peaks to those of complex-4 and complex-5, suggesting they likely adopt the same structural framework, although these complexes did not yield single crystals for X-ray diffraction analysis. The small discrepancies observed in the peak intensities can be attributed to differences in crystallite size, preferred orientation, or scattering effects from the halogen substituents. Nevertheless, the PXRD data confirm the structural integrity and phase purity of all the complexes, providing strong evidence for their isostructural nature, especially for complex-4 and complex-5. This analysis supports the conclusion that the halogen substitution plays a role in the crystal packing and overall structure of these complexes.

Hirshfeld surface analysis

The Hirshfeld surface analysis of complex-4 and complex-5 offers important information on the molecular packing and intermolecular interactions within both complexes. In Fig. 2 for complex-4, the dnorm surface (a) reveals the distribution of interatomic interactions, where red areas indicate close contacts, and blue areas highlight less significant interactions. The surface of complex-4 shows a balanced distribution of red and blue regions, suggesting a well-ordered packing with appropriate interatomic distances. The shape index (b) displays the curvature of the surface, where regions with positive curvature are shown in blue, and negative curvature regions are in red. This helps to understand the molecular shape and how it influences the packing in the crystal lattice. The curvedness surface (c) further supports this by showing the degree of surface distortion, with regions of high curvature appearing in green, indicating potential areas for interaction or strain. HS analysis further provides information about the intermolecular interactions for these complexes through 2D fingerprint plots (Fig. 2(d and e)), showing a significant contribution from H–H, C–H, O–H, and Br–H interactions, with the highest density of points indicating the prominence of these interactions in the crystal structure. The H–H interactions dominate, comprising 44.9% of the total intermolecular interactions, followed by O–H (9.1%) and C–H (8.5%) interactions. The presence of Br–H interactions (8.0%) is noteworthy due to the incorporation of a bromine atom in the structure, highlighting its role in the molecular packing and lattice stabilization.

Fig. 2 Hirshfeld surface mapped over dnorm (a), shape index (b), curvedness (c), 2D fingerprint plots (d) and percentage of various interactions (e) for complex-4, and dnorm (f), shape index (g), curvedness (h), 2D fingerprint plots (i) and percentage of various interactions (j) for complex-5, respectively.

For complex-5, the dnorm surface (f) shows a similar distribution to complex-4, with regions of close contact (red) and areas of weaker interaction (blue). However, complex-5 exhibits more pronounced areas of red, which could indicate stronger intermolecular interactions or more closely packed molecules. In the shape index (g), the molecular surface shows a relatively smooth distribution of positive and negative curvatures, which may suggest a more symmetrical packing compared to complex-4. Finally, the curvedness surface (h) indicates a more uniform distribution of curvature, suggesting less molecular strain and a more stable packing arrangement compared to complex-4. The 2D fingerprint plot for complex-5 confirms that the introduction of iodine leads to a different balance of interactions, with I–H interactions being a prominent feature, contributing to the overall molecular packing and structural stability. The H–H interactions still contribute a significant portion (43.1%), but the crystal structure is also influenced by I–H interactions (9.2%) due to the presence of the iodine atom. This unique I–H interaction, absent in complex-4, suggests that iodine plays a distinct role in the packing and stability of the complex. The C–H (8.2%) and O–H (7.8%) interactions are also significant, similar to complex-4.

The dnorm, shape index, and curvedness surfaces are useful to investigate how molecular geometry and intermolecular interactions influence the overall structural stability and packing of the complexes in the crystalline state. The detailed 2D fingerprint plots are represented in Fig. S22 for complex-4 and Fig. S23 for complex-5. This detailed analysis can be useful for understanding the properties of these complexes, especially in terms of their potential for NLO applications and other crystallographic properties.

UV-vis titration

The UV-vis titration spectra of the synthesized Schiff base ligands (H₂L¹, H₂L², H₂L³, H₂L⁴, and H₂L⁵) upon complexation with Cu(NO₃)₂·xH₂O are shown in Fig. 3. The titration shows the interaction between the Schiff base ligands and copper ions, as reflected in the shifting and intensity changes of the absorbance peaks with varying concentrations of copper nitrate. In each graph (a–e), we observe a distinct pattern in the absorbance changes as the copper nitrate solution is added. Initially, when no copper is present, the Schiff base ligands exhibit characteristic absorption peaks in the UV region. As copper is gradually introduced, notable changes in the absorption spectra are observed. For each ligand, the peak intensities evolve, and some peaks shift, indicating coordination between the ligand and the metal center.

Fig. 3 UV-vis absorption spectra of H₂L¹ (a), H₂L² (b), H₂L³ (c), H₂L⁴ (d), and H₂L⁵ (e) with Cu²⁺ in ethanol at a 2.5 × 10⁻⁵ M concentration.

For H₂L¹ (Fig. 3a), we observe the initial absorption peaks around 200 nm, which shift slightly toward longer wavelengths with increasing copper concentrations. This shift is indicative of the ligand's coordination to copper ions, resulting in altered electronic transitions. Similarly, for H₂L² (Fig. 3b), noticeable shifts in the absorbance bands around 300 nm suggest a strong interaction between the ligand and Cu²⁺ ions. The gradual increase in absorbance intensity further supports the formation of the metal–ligand complex. In the case of H₂L³ (Fig. 3c), significant changes are observed, with the peaks at around 250 nm and 300 nm becoming more pronounced as copper is added, indicating complexation between copper and the Schiff base ligand. The absorption peaks at higher wavelengths (around 400 nm) suggest further shifts in electronic transitions due to the coordination of copper. Fig. 3d and e, corresponding to H₂L⁴ and H₂L⁵, show similar trends, where both ligands undergo noticeable shifts in their absorption profiles as the concentrations of Cu(NO₃)₂ increase. These shifts are accompanied by changes in peak intensities, indicating the formation of stable Cu(II) complexes with the Schiff bases. For all ligands, the changes in absorbance and peak positions as copper concentrations increase suggest the formation of stable copper-Schiff base complexes. These results reflect the strong coordination between the metal center and the ligand, which is essential for the stability and functionality of the resulting complexes. The optical band gaps of the Cu(II) complexes were determined using the Tauc plot method from the UV-vis spectra (Fig. S24). The calculated band gaps indicate the consistent change in the optical bandgap trend. These values are important to determine the electronic structure and potential applications of the complexes in areas such as photocatalysis and optoelectronics.

Computational study

Geometry optimization

The B3LYP functional was used to optimize the structures of Schiff base ligands (H₂L¹ to H₂L⁵) (Fig. S25). The halogen atoms are attached to the aromatic rings and influence the electronic properties of the ligands.

Similarly, the Cu(II) coordination complexes (complex-1 to complex-5), synthesized from L-phenylalanine-based Schiff base ligands, were optimized using the B3LYP/LANL2DZ level of theory (Fig. S26). The presence of various halogen atoms (F, Cl, Br, and I) in the ligand structures influences the geometry and electronic properties of the complexes. These compounds were further used for DFT calculations.

Frontier molecular orbital analysis

The highest occupied molecular orbitals (HOMOs) reflect a molecule's electron-donating ability due to their electron-rich nature, while the lowest unoccupied molecular orbitals (LUMOs) represent regions capable of accepting electrons.²⁵ Additionally, the band gap, calculated as the energy difference between the LUMO and HOMO (E_g = E_LUMO − E_HOMO), plays a crucial role in determining the nonlinear optical (NLO) properties of the materials being studied. As shown in Fig. S27, the energy gap (ΔE) decreases from 4.90 eV in H₂L¹ to 4.45 eV in H₂L⁵ as the halogen atom changes from fluorine to iodine, with fluorine causing the largest gap and iodine the smallest. This trend is linked to the influence of halogen substituents on the frontier orbitals: the HOMO retains significant electron density across the entire ligand, but heavier halogens, especially iodine, increase electron donation to the aromatic ring, stabilizing the molecule. The LUMO, mainly localized on the aromatic ring and C=N group, becomes progressively stabilized with larger halogens, thus lowering the LUMO energy and narrowing the gap. This narrowing of the gap suggests that an iodo-substituted ligand (H₂L⁵) is more reactive and polarizable, making it more prone to electron transfer and coordination to metals, while a fluoro-substituted ligand (H₂L²) remains more stable and less reactive.^26,27 These electronic changes also influence their optical properties, with the iodine-substituted ligand likely showing red-shifted absorption due to the smaller ΔE, indicating lower energy transitions.

The L-phenylalanine derived Schiff base ligands (H₂L¹–H₂L⁵) show a dramatic contraction of their HOMO–LUMO energy gaps upon coordination to Cu(II). In the free ligands, the gaps are relatively large (on the order of 4.90–4.45 eV), whereas in the Cu(II) complexes (complex-1 to 5) (Fig. 4) these gaps shrink to roughly half their original size (∼2.32–2.11 eV). For instance, the unsubstituted ligand H₂L¹ has a gap of ∼4.90 eV, which drops to about 2.14 eV in its Cu(II) complex, and a similar gap reduction is seen for each halogenated ligand (e.g.H₂L⁵ falls from ∼4.45 eV to 2.11 eV upon complexation). Such coordination-induced narrowing of the frontier orbital gap indicates greatly enhanced intramolecular charge transfer (ICT) character in the complexes−the metal's involvement effectively delocalizes electron density and brings the HOMO and LUMO closer in energy. As a result, the electron cloud becomes much more easily polarizable, meaning the complex can redistribute or shift charge under an external field with less energy input. This increased electronic polarizability directly benefits nonlinear optical behavior: a smaller HOMO–LUMO gap correlates with stronger NLO responses, since the ease of electron excitation and movement (ICT) boosts the material's nonlinear polarization.²⁸

Fig. 4 The frontier molecular orbital (FMO) diagram for complex-1 (a), complex-2 (b), complex-3 (c), complex-4 (d), and complex-5 (e) computed using the B3LYP/LANL2DZ level of theory.

The calculated HOMO–LUMO gaps for our complexes were found to be 2.14, 2.26, 2.32, 2.23, and 2.11 eV respectively, which are in agreement with previously reported crystals studied for NLO applications (Table 2). In particular, the observed larger HOMO–LUMO gaps suggest enhanced NLO response, which is beneficial for optoelectronic devices. These results underscore the role of halogen substitution in tuning the electronic properties of Schiff base complexes for effective NLO performance.

Table 2 Comparison of optical bandgaps of complexes with reported NLO crystals

Crystals	Bandgap (eV)	Ref.
DAST	2.32	29
DSTMS	1.7	30 and 31
PAB	2.7	32
PAC	2.65	32
Complex 1	2.14	Present work
Complex 2	2.26	Present work
Complex 3	2.32	Present work
Complex 4	2.23	Present work
Complex 5	2.11	Present work

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Density of states

DOS provides insights into how molecular fragmentation influences the characteristics of the HOMO and LUMO orbitals.^33,34 The results from DOS studies complement findings from frontier molecular orbital (FMO) analysis. The DOS plots (Fig. S28) compare the electronic states of the ligands H₂L¹ to H₂L⁵ (red for aldehyde, green for amino acid) with the Cu(II) complexes (Fig. 5) (blue for Cu²⁺ and black for the total complex). These plots illustrate how the electronic distribution and energy states of the systems evolve upon coordination with Cu(II), offering a deeper understanding of the stability, reactivity, and bonding characteristics of the complexes. The graphs show the relative intensity of the electronic states of the aldehyde and amino acid components and their combined total states across the energy range from −15 eV to +5 eV.

Fig. 5 PDOS profiles for complex-1 (a), complex-2 (b), complex-3 (c), complex-4 (d), and complex-5 (e) computed using the B3LYP/LANL2DZ level of theory.

Notably, there is also a clear substituent-dependent trend across the halogen series F, Cl, Br, and I. In particular, the iodine-substituted ligand H₂L⁵ has the lowest gap among the free ligands (∼4.45 eV) and its Cu(II) complex likewise exhibits the smallest gap (∼2.11 eV) of the series. In contrast, lighter halogens (like F or Cl) have a somewhat lesser effect on gap reduction. The large, diffusive orbitals of the heavier halides participate in conjugation and ICT more strongly, thereby amplifying the gap-narrowing effect.³⁵ Finally, coordination with Cu(II) roughly halves the HOMO–LUMO gaps of these Schiff bases, and this pronounced reduction – especially potent with heavier halogen substituents – enhances their NLO performance by fostering greater intramolecular charge transfer and higher electronic polarizability.³⁶

From the plots, we can observe how the individual components of the Schiff base (the aldehyde and amino acid groups) contribute to the overall electronic structure. The aldehyde group (red curve) typically shows significant peaks in the negative energy region, which suggests the presence of electronic states associated with the C=O bond and related interactions. On the other hand, the amino acid (green curve) contributes to peaks primarily in the positive energy range, reflecting the influence of nitrogen and hydrogen bonding within the ligand structure. As we move from H₂L¹ to H₂L⁵, the influence of halogen substitutions on the salicylaldehyde group becomes evident in the shifts and modifications of these peaks. The halogen atoms (F, Cl, Br, and I) in the aromatic ring introduce electronic perturbations that alter the overall distribution of the density of states. These changes in the DOS indicate that halogenation modulates the electronic properties of the Schiff base ligands, likely influencing their coordination behavior and reactivity. The combined DOS (black curves) reveals the overall electronic structure, which is a result of the interactions between the aldehyde and amino acid components. The relative intensities and shifts in the total DOS reflect the overall stability and electronic character of each ligand.

The DOS spectra for the Cu(II) complexes synthesized from L-phenylalanine based Schiff base ligands, shown in Fig. 5(a)–(e), illustrate the contributions of the aldehyde (red), amino acid (green), Cu(II) ion (blue), and the total complex (black) to the electronic states across a range of energy levels. The comparison with the earlier Schiff base ligands (H₂L¹ to H₂L⁵) highlights the influence of the Cu(II) coordination on the overall electronic structure. In the Cu(II) complexes, the Cu(II) ion exhibits distinct features in the DOS spectra, with its electronic states contributing significantly to the energy levels, particularly in the positive energy range. The interactions between the aldehyde and amino acid components also shift with the introduction of the Cu(II) ion, causing notable changes in the distribution of electronic states, particularly around the Fermi level. These shifts in the DOS spectra are indicative of the metal–ligand interactions and the modulation of the electronic properties by halogen substitutions (F, Cl, Br, and I) in the salicylaldehyde group.

Molecular electrostatic potential study

In the MEP color map, electron-deficient regions appear blue, indicating areas of positive electrostatic potential that are prone to electrophilic attack, while electron-rich regions are shown in red, corresponding to negative electrostatic potential favorable for nucleophilic attack. The MEP map follows a color gradient from blue to red representing increasing electrostatic potential: blue < green < yellow < orange < red, with green indicating regions of neutral (zero) potential. The MEP maps for the ligands H₂L¹ to H₂L⁵ (Fig. S29) and complexes 1 to 5 (Fig. 6) were simulated using the B3LYP functional, applying the LANL2DZ basis set for copper atoms and 6-31G* for C, H, N, O, F, Cl, Br, and I atoms. These maps were visualized in three dimensions over the full electron density surface. Examining the electrophilic (electron-deficient) and nucleophilic (electron-rich) areas on these maps provides profound observations into the compound's reactivity.³⁷ In the MEP images, electron-rich (negatively charged) regions are shown in red, electron-deficient (positively charged) areas are depicted in blue, and neutral regions appear white.³⁸ In both complexes studied, the oxygen atom was located at the core of an electronegative, electron-rich region, making it a likely target for electrophilic attack. Fig. S29 shows, in H₂L¹ (a), the electron density is concentrated around the –OH group, making it a strong donor site. H₂L² (b) shows a more electrophilic character at the fluorine site, while H₂L³ (c) exhibits a similar but slightly weaker electron-withdrawing effect. H₂L⁴ (d) further reduces the electron density, and H₂L⁵ (e) exhibits the largest shift, increasing the electrophilic nature at the iodine site. These variations influence the ligands’ reactivity, stability, and coordination with metals, with the halogen substitution potentially enhancing their application in nonlinear optics (NLO) by modulating electron delocalization and electron transfer properties.

Fig. 6 The molecular electrostatic potential (MEP) maps of complex-1 (a), complex-2 (b), complex-3 (c), complex-4 (d), and complex-5 (e) drawn using an isovalue of 0.02 where blue and red colors indicate positive and negative potential maxima.

When the ligands coordinate with Cu(II), the electron density is redistributed due to the interaction between the metal and the ligand, significantly altering the electrostatic environment (Fig. 6). In complex-1 (a), the Cu(II) ion pulls electron density from the ligand, resulting in a more positively charged region near the Cu center, as reflected in the red−orange regions. This shift in electron density, especially around the metal and coordinating oxygen atoms, contrasts with the free ligand H₂L¹, where the electron density is more localized around the –OH and C=N groups. In the case of complex-2 (b), the electron-withdrawing effect of the fluorine atom on the ligand is magnified when coordinated to Cu(II), making the C–F bond more electrophilic. Similarly, for complex-3 (c), the electron density is pulled towards the chlorine atom and the Cu–O interactions, creating an electrophilic region near the chlorine.

In complex-4 (d), the Cu(II) ion further alters the electrostatic potential, causing more extensive redistribution of electron density, particularly around the C–Br bond, where a more diffuse electron cloud is observed. complex-5 (e) shows the most pronounced changes, with iodine's large atomic radius resulting in a significantly more polarizable environment around the iodine atom. The C–I bond exhibits a less localized charge density compared to the free ligand, reflecting the lower electronegativity of iodine.

Non-covalent interactions

The NCI studies^39,40 include analysis of three-dimensional (3D) iso-surfaces and two-dimensional (2D) RDG plots for ligands H₂L¹ to H₂L⁵ (Fig. S30) and the complexes (Fig. 7). The 3D isosurface models utilize three distinct color patterns to represent different types of bonding interactions. Repulsive forces are shown in red, while attractive forces appear in blue and green, with the gradient from blue to green indicating varying strengths of attraction. Specifically, green regions highlight non-covalent interactions. These interactions are also represented in 2D RDG plots, where the electron density values (sign(λ₂)ρ) are plotted along the x-axis and the RDG values along the y-axis. The sign(λ₂)ρ values help confirm the nature of the interactions: values less than zero indicate weak attractive forces, whereas values greater than zero correspond to repulsive forces.

Fig. 7 The pictorial representation of NCI diagrams of complex-1 (a), complex-2 (b), complex-3 (c), complex-4 (d), and complex-5 (e) computed using the B3LYP/LANL2DZ level of theory.

As illustrated in Fig. S30, the RDG analysis visualizes attractive (red), repulsive (blue), and weak (green) interactions, essential for understanding the ligands’ stability and reactivity. The 3D isosurface highlights electron density around key atoms like oxygen, nitrogen, and halogens, showing significant attractive interactions in H₂L¹, which are confirmed by the RDG plot. As we move to H₂L²–H₂L⁵, the electron distribution shifts due to halogen substitutions, with larger halogens like iodine causing increased repulsive interactions. This variation influences the strength of metal–ligand interactions and stability of the complexes.

Similar to the ligands, the Cu(II) complexes exhibit distinct electron density distributions and non-covalent interactions (Fig. 7). In the 3D isosurface structures of the Cu(II) complexes, the spatial distribution of electron density around key atoms, such as oxygen, nitrogen, and halogen atoms, is clearly visible. The corresponding 2D RDG plots reveal a similar pattern of attractive (red), repulsive (blue), and weak (green) interactions. However, the Cu(II) complexes show more significant electron density shifts due to the central metal ion's coordination with the Schiff base ligands. This results in a stronger concentration of interactions around the Cu center, as evidenced by increased attractive interactions (red regions), particularly between the Cu ion and the oxygen or nitrogen atoms in the ligands. The RDG plots of the Cu(II) complexes show more pronounced red regions, indicating stronger attractive interactions due to the metal coordination.

QTAIM analysis of complexes

The QTAIM⁴¹ analysis was carried out to explore various types of interactions within the complexes. Key topological parameters, such as electron density (ρ) and the Laplacian of electron density (∇²ρ), are used to characterize these interactions. A bond critical point (BCP) is defined as the location within a molecule where the electron density between two connected atoms reaches a maximum. The BCPs identified for complexes 1 through 5 are illustrated in Fig. 8. The properties of these bond critical points are evaluated using the following parameters.

Fig. 8 The QTAIM analysis of complex-1 (a), complex-2 (b), complex-3 (c), complex-4 (d), and complex-5 (e) computed using the B3LYP/LANL2DZ level of theory. The numbers shown in black are the bond critical points (BCPs).

In computational chemistry, QTAIM analysis utilizes several parameters, such as the Laplacian of electron density (∇²ρ), kinetic electron density (G_r), potential electron density (V_r), and their ratio (−V_r/G_r), to evaluate the nature of interactions and bond strength. Strong covalent bonds are characterized by negative values for ∇²ρ, a −V_r/G_r ratio less than 1, total energy density (H_r) below 0 atomic units, and an electron density (ρ) greater than 0.1 atomic units. In contrast, van der Waals interactions are indicated by positive ∇²ρ values, a −V_r/G_r ratio greater than 1, H_r above 0 atomic units, and ρ less than 0.1 atomic units. When the total energy density (H_r) is negative but the Laplacian (∇²ρ) is positive, complexes display a mixture of covalent and electrostatic interactions.^17,42 The bond critical points (BCPs) for complexes 1 through 5 are illustrated in Fig. 8, with their corresponding values listed in Tables S17–S21. In the case of complex-1 to complex-5, the electron density at the bond critical points reveals the nature of the interactions between the Cu(II) center and the ligand atoms. For instance, in complex-1, the interaction between Cu21 and O23 (BCP 82) has a significant electron density of 1.22 × 10⁻¹, indicating a strong Cu–O coordination bond. Similarly, in complex-2, the Cu21–N9 interaction (BCP 79) shows strong bonding characteristics with ρ = 1.16 × 10⁻¹ and a relatively high −V_r/G_r value of 1.27 × 10⁻¹, which points to a robust interaction between copper and nitrogen atoms. Moreover, the hydrogen bonding interactions, such as the H–O interactions, also play a role in the stability and structure of these complexes. For instance, in complex-4, the H34–O4 interaction (BCP 53) shows a moderate electron density and potential energy density, suggesting a relatively weaker interaction, while Cu–O and Cu–N bonds dominate the overall structure. The ratio −V_r/G_r is used to further confirm the stability of the interactions. Most interactions exhibit a favorable ratio, highlighting the existence of stable and strong interactions. Particularly, in complex-5, the Cu2–O3 bond (BCP 62) shows a strong interaction, with an electron density of 1.00 × 10⁻¹, and a high −V_r/G_r ratio of 1.07, which indicates the strength of the copper–oxygen coordination bond.

Nonlinear optical properties

Linear polarizability (frequency dependent)

An excess of electrons in a system is a key factor in enhancing nonlinear optical (NLO) properties, as it leads to a reduction in the HOMO–LUMO energy gap.^43,44 Therefore, it is expected that complexes, calculated using the B3LYP level of theory, will exhibit greater polarizability compared to their corresponding ligands. The linear polarizability results of Schiff base ligands (H₂L¹ to H₂L⁵) and their corresponding Cu(II) coordination complexes are shown in Fig. 9, showcasing the frequency-dependent behavior of both isotropic and anisotropic polarizabilities. The graphs depict the changes in polarizability (α_iso and α_aniso) across varying frequencies (ω), revealing the electronic properties of the ligands and complexes.

Fig. 9 A representation of the frequency dependent isotropic polarization (α_iso) of ligands (a) and complexes (c) and anisotropic polarization (α_aniso) of ligands (b) and complexes (d).

In the case of the Schiff base ligands (H₂L¹ to H₂L⁵), the isotropic polarizability (α_iso) generally increases with frequency, with H₂L⁵ showing the highest values across all frequencies (Fig. 9(a)). This trend suggests that H₂L⁵ has the greatest ability to polarize under an electric field, likely due to its electron-donating characteristics, which are supported by the high nucleophilicity index (NI) (Table S22). The anisotropic polarizability (α_aniso) also follows a similar pattern, where H₂L⁵ exhibits the highest values, indicating its strong directional dependence of polarizability, which can be beneficial for nonlinear optical (NLO) applications (Fig. 9(b)).

When transitioning to the Cu(II) coordination complexes (complex-1 to complex-5), we observe a noticeable increase in both isotropic and anisotropic polarizabilities compared to the ligands (Fig. 9(c & d)). Complex-5, in particular, continues to exhibit the highest values of both a_iso and a_sniso, reinforcing the idea that the coordination of Cu(II) enhances the polarizability of the system. This increase is attributed to the influence of the metal ion, which facilitates the redistribution of electron density, thereby enhancing the polarizability of the complex. The trend in the complexes mirrors that of the ligands, but with a more pronounced increase, especially at higher frequencies, indicating the strong impact of the Cu(II) center on the electronic behavior.

First hyperpolarizability (frequency dependent)

The frequency-dependent second-order nonlinear optical (NLO) properties of the ligands and complexes were evaluated by analyzing their total hyperpolarizability (β_total) values. These values include contributions from the electro-optic Pockels effect (EOPE), represented by β(−ω; ω, 0), and second-harmonic generation (SHG), represented by β(−2ω; ω, ω).⁴⁵ The frequency dispersion was examined at ω values of 0.02, 0.04, 0.06, 0.08, 0.10, and 0.12 a.u. to provide a detailed investigation of the nonlinear dynamics. The resulting graphs illustrate the frequency-dependent first hyperpolarizabilities (β^EOPE and β^SHG) of the Schiff base ligands (H₂L¹ to H₂L⁵) (Fig. 10(a & b)) and their Cu(II) coordination complexes (complex-1 to complex-5) (Fig. 10(c & d)). β^EOPE and β^SHG are key indicators of the nonlinear optical (NLO) properties of these compounds, which measure the response to an applied electric field. As the frequency increases, both β^EOPE and β^SHG show a general upward trend for all compounds, indicating an enhancement in NLO response at higher frequencies.

Fig. 10 A graphical representation of the frequency dependent electro-optic Pockels effect (EOPE) of ligands (a) and complexes (c) and second-harmonic generation (SHG) of ligands (b) and complexes (d).

Looking first at the Schiff base ligands (H₂L¹ to H₂L⁵), β^EOPE increases progressively with frequency for all ligands, with H₂L⁵ showing the highest values at higher frequencies, indicating that it has the strongest response to an electric field. This trend is similar for β^SHG, but H₂L⁴ exhibits the most significant increase in values at higher frequencies, suggesting that it has the highest nonlinear optical (NLO) performance among the ligands.

When comparing the Cu(II) coordination complexes (complexes 1 to 5), a dramatic increase in β^EOPE and β^SHG values is observed, with complex-4 exhibiting the highest values, far surpassing the Schiff base ligands. The metal coordination enhances the NLO properties, likely due to the redistribution of electron density upon metal binding, making these complexes much more responsive to external fields, especially at higher frequencies. The complexes demonstrate a stronger β^SHG signal compared to β^EOPE, with complex-4 again showing the most significant contribution, indicating that the Cu(II) coordination substantially boosts the frequency-doubling capabilities of the system. To validate these results and to enhance the reliability of the outcome, additionally we have studied first hyperpolarizability coefficients of the complexes using CAM-B3LYP and ωB97X-D levels of theory, which are tabulated in Table 3 and Table S23.

Table 3 Comparison of the second-harmonic generation (SHG) of synthesized complexes computed using CAM-B3LYP and ωB97X-D levels of theory

Frequency (ω)	β(−2ω; ω, ω)
	SHG (a.u.)
	CAM-B3LYP					ωB97X-D
	1	2	3	4	5	1	2	3	4	5
0.02	374.53	409.35	881.33	998.41	55 736.5	449.2	484.86	914.35	913.29	873.94
0.04	368.38	551.58	1134.37	1308.91	46 425.27	421.33	597.59	1223.2	1232.36	1050.18
0.06	64 886.74	49 256.77	4749.49	109 597.04	11 587.66	29 578.43	30 848.58	6369.51	8599.64	3922.79
0.08	20 778.38	36 231.59	17 174.87	12 292.26	141 988.51	26 961.55	34 901.58	9048.54	62 927	3009.78
0.10	17 491.28	20 210.19	30 213.42	798 197.02	489 308.2	112 475.57	4383.86	65 617.19	367 321.4	320 973.15
0.12	252 391.46	65 777.93	105 166.17	1 541 359.76	1 366 725.47	42 098.55	14 198.19	97 228.96	435 633.68	336 618.13

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Comparing these results with those from the L-glutamine based Schiff base ligands and their complexes¹⁷ (previously published from our research group), similar trends can be observed. In both systems, the Cu(II) coordination enhances the β^EOPE and β^SHG values, suggesting that the presence of the metal ion in the complex plays a crucial role in increasing the NLO response. However, the Schiff base ligands derived from L-phenylalanine tend to exhibit slightly higher β^SHG values than the L-glutamine based complexes, particularly in complex-4, suggesting that the type of amino acid plays a significant role in tuning the NLO properties. Furthermore, both systems show that halogen substitution in the Schiff base ligands increases the NLO responses, but the L-phenylalanine derivatives exhibit a more pronounced enhancement, especially at higher frequencies.

The higher SHG response of complex-4 can be attributed to several factors. Firstly, the presence of bromine in the ligand significantly enhances the molecular polarizability by influencing the electron density distribution within the Schiff base ligand.⁴⁶ Bromine, being a heavier halogen, contributes to stronger intermolecular interactions, and it is well-known for stabilizing the charge transfer in NLO materials.⁴⁷ Furthermore, the coordination of the Cu(II) center to the ligand amplifies this effect, further reducing the HOMO–LUMO gap and facilitating better electron delocalization. This enhanced electronic structure in complex-4 increases its potential to undergo second-harmonic generation, thus leading to higher SHG values.

In contrast, complex-5 (which contains iodine) shows slightly lower SHG values, likely due to the different electronic effects induced by iodine compared to bromine. While iodine's larger atomic size and lower electronegativity influence the electronic structure, it may not enhance the polarizability and charge transfer in the same way as bromine does in complex-4, resulting in a somewhat reduced NLO performance.

Second hyperpolarizability (frequency dependent)

In addition to the first hyperpolarizability, the second hyperpolarizability coefficients of the ligands (H₂L¹ to H₂L⁵) and complexes (1 to 5) were calculated using B3LYP functionals across six different frequencies. Third-order nonlinear optical (NLO) properties describe a material's response to an applied electric field raised to the third power, commonly characterized by the third-order susceptibility, γ.⁴⁸ This property controls phenomena like the dc-Kerr effect (γ(−ω; ω, 0, 0)) and electric field-induced second harmonic generation (ESHG, γ(−2ω; ω, ω, 0)). Fig. 11 shows the results for the Kerr effect (γ^dc-Kerr) for ligands (a) and for complexes (c), as well as the enhanced field-induced second harmonic generation (γ^ESHG) for ligands (b) and for complexes (d).

Fig. 11 A graphical representation of the frequency dependent dc-kerr effect of ligands (a) and complexes (c) and enhanced second-harmonic generation (ESHG) of ligands (b) and complexes (d).

Looking at the γ^dc-Kerr values, we observe that for both the ligands and complexes, H₂L⁵ and complex-5 show the highest values, but at the higher frequencies (ω = 0.12) complex-4 shows highest values, possibly due to the heavier bromine atom, indicating strong electro-optic properties. This trend is consistent with our previous published findings on L-glutamine based Schiff base ligands, where the introduction of larger halogens like bromine and iodine also led to higher γ^dc-Kerr values. In L-phenylalanine based Schiff bases, the substitution with halogens, particularly bromine, enhances the γ^dc-Kerr response, supporting the notion that halogen substituents increase the nonlinear optical response.

The γ^ESHG results, also show a significant increase at higher frequencies for H₂L⁴ and complex-4, with complex-4 once again demonstrating the highest values at ω = 0.12. This indicates that the Cu(II) coordination not only enhances the electro-optic properties but also greatly improves the frequency-doubling capabilities of these complexes. When comparing these results to the L-glutamine based complexes, the L-phenylalanine based complexes exhibit even higher γ^ESHG values, particularly in complex-4, suggesting that the choice of amino acid has a pronounced effect on the NLO performance, with L-phenylalanine yielding stronger second harmonic responses. To validate these results and to enhance the reliability of the outcome, additionally we have studied second hyperpolarizability coefficients of the complexes using CAM-B3LYP and ωB97X-D levels of theory, which are tabulated in Table 4 and Table S24.

Table 4 Comparison of the second hyperpolarizability coefficient (dc-kerr) of the synthesized complexes computed using CAM-B3LYP and ωB97X-D levels of theory

Frequency (ω)	γ (−ω; ω, 0, 0)
	(dc-kerr) × 10^4 (a.u.)
	CAM-B3LYP					ωB97X-D
	1	2	3	4	5	1	2	3	4	5
0.02	2.009	2.01	3.26	2410	3.08	1.81	1.83	3.02	6.03	2.94
0.04	2.307	2.27	3.65	1460	3.62	2.12	2.07	3.39	5	3.42
0.06	6.033	16.5	11.2	1500	5.93	10.2	9.75	5.83	6.35	11.9
0.08	2.944	2.68	4.79	39 400	3.38	2.77	2.63	4.27	10.3	2.56
0.10	43.93	69.9	102	57 300	78.9	38.7	60.1	320	318	205
0.12	10 930	2990	90.6	79 200	16 900	119	168	362	6180	4000

---

Overall, complex-4 stands out as the most promising compound in both the γ^dc-Kerr and γ^ESHG tests, surpassing the Schiff base ligands. This emphasizes the potential of Cu(II)-based complexes, particularly with halogen-substituted Schiff bases, in NLO applications, suggesting their suitability for future advancements in nonlinear optical technologies.

Conclusions

This study reports the synthesis and detailed characterization of Schiff base ligands derived from L-phenylalanine and their corresponding copper(II) complexes, focusing on their nonlinear optical (NLO) properties. Ligands were prepared using halogen-substituted salicylaldehydes, and their structural, electronic, and optical behaviors were extensively investigated using experimental and computational techniques, including NMR, FT-IR, ESI-MS, and UV-Vis spectroscopy, and DFT calculations. The copper(II) complexes, notably those bearing bromo and iodo substituents, showed significantly reduced HOMO–LUMO gaps upon metal coordination i.e. 2.23 and 2.11, respectively, enhancing polarizability and facilitating intramolecular charge transfer, resulting in improved NLO responses such as increased first hyperpolarizability (β) and second hyperpolarizability (γ). Halogen substituents, particularly bromine, notably amplified these NLO effects by modulating electronic density and polarizability. Structural analyses revealed noncentrosymmetric packing arrangements with the Cc space group and the central Cu(II) having a distorted square-pyramidal geometry and robust metal–ligand interactions, crucial for enabling effective second-harmonic generation (SHG). Further computational analyses, including molecular electrostatic potential and density of states, supported the experimental findings, emphasizing electron redistribution upon copper coordination. Thermogravimetric analysis indicated good thermal stability of the complexes, particularly complex-5. Complex-4 demonstrated superior second- (1 260 754.11 at ω = 0.12) and third-order (5 334 967 × 10⁴ at ω = 0.12) nonlinear optical responses, underscoring its potential in advanced photonic applications. Overall, this work emphasizes the impact of the ligand structure and metal coordination with halogen substitution in tuning NLO properties, providing key takeaways for the design of high-performance materials for optical switching, electro-optic modulation, and photonics. This study contributes to the growing body of knowledge on Schiff base Cu(II) complexes and their potential in nonlinear optics, paving the way for further exploration and application in diverse technological fields.

Author contributions

Nagesh Manurkar: conceptualization, methodology, investigation, writing – original draft preparation, data curation, resources, visualization, formal analysis, and project administration. Mubashar Ilyas: software, investigation, formal analysis, and data curation. Faiza Arshad: formal analysis, validation, and writing – reviewing & editing. Prasanna Patil: formal analysis and writing – reviewing & editing. Qamar-Un-Nisa Tariq: writing – reviewing & editing. Shahzad Khan: writing – reviewing & editing. Maroof Ahmad Khan: writing – reviewing & editing. Hui Li: conceptualization, visualization, resources, funding acquisition, and supervision.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the SI: Fig. S1–S30 and Tables S1–S25. See DOI: https://doi.org/10.1039/d5dt01513h.

CCDC 2415583 and 2415691 contains the supplementary crystallographic data for this paper.^49a,b

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21071018, 21271026, and 21471017). We are thankful for the scholarship and support from the Chinese Scholarship committee and the Analysis & Testing Centre of the Beijing Institute of Technology.

References

1. V. Snigirev, A. Riedhauser, G. Lihachev, M. Churaev, J. Riemensberger, R. N. Wang, A. Siddharth, G. Huang, C. Möhl and Y. Popoff, Ultrafast tunable lasers using lithium niobate integrated photonics, Nature, 2023, 615, 411–417.
2. S. Aithal, P. Aithal and A. Bhat, Advancements in Nonlinear Optical Materials: Paving the Way for Future Photonic Devices, Poornaprajna Int. J. Basic Appl. Sci., 2024, 1, 1–58.
3. H. Wang, M. Mutailipu, Z. Yang, S. Pan and J. Li, Computer–Aided Development of New Nonlinear Optical Materials, Angew. Chem., 2025, 137, e202420526.
4. E. Hong, Z. Li, T. Yan and X. Fang, Surface-tension-dominant crystallization of 2D perovskite single crystals for vertically oriented hetero-/homo-structure photodetectors, Nano Lett., 2022, 22, 8662–8669.
5. S.-P. Guo, Y. Chi and G.-C. Guo, Recent achievements on middle and far-infrared second-order nonlinear optical materials, Coord. Chem. Rev., 2017, 335, 44–57.
6. Y. Xu, Y. Huang, X. Cui, E. Razzoli, M. Radovic, M. Shi, G. Chen, P. Zheng, N. Wang and C. Zhang, Observation of a ubiquitous three-dimensional superconducting gap function in optimally doped Ba0. 6K0. 4Fe2As2, Nat. Phys., 2011, 7, 198–202.
7. B. Dalai and S. K. Dash, Deep-ultraviolet (DUV) nonlinear optical (NLO) crystals: An application in photonic technologies, Opt. Mater., 2023, 143, 113909.
8. M. Galini, M. Salehi, M. Kubicki, M. Bayat and R. E. Malekshah, Synthesis, structural characterization, DFT and molecular simulation study of new zinc-Schiff base complex and its application as a precursor for preparation of ZnO nanoparticle, J. Mol. Struct., 2020, 1207, 127715.
9. A. A. Adeleke, S. J. Zamisa, M. S. Islam, K. Olofinsan, V. F. Salau, C. Mocktar and B. Omondi, Quinoline functionalized schiff base silver(I) complexes: interactions with biomolecules and in vitro cytotoxicity, antioxidant and antimicrobial activities, Molecules, 2021, 26, 1205.
10. H. Kargar, M. Fallah-Mehrjardi, M. Moghadam, S. Yarahmadi, A. Omidvar, H. R. Zare-Mehrjardi, N. Dege, M. Ashfaq, K. S. Munawar and M. N. Tahir, Structural and electrochemical properties of a Cu(I) Schiff-base complex: Catalytic application to the synthesis of tetrahydropyrimidine derivatives, Inorg. Chim. Acta, 2024, 570, 122160.
11. P. Kumari, M. Choudhary, A. Kumar, P. Yadav, B. Singh, R. Kataria and V. Kumar, Copper(II) Schiff base complexes: Synthetic and medicinal perspective, Inorg. Chem. Commun., 2023, 158, 111409.
12. D. Majumdar, S. Roy, J. E. Philip, B. Tüzün and S. Hazra, In situ Salen-type ligand formation-driven of a heterometallic Cu(II)-Hg(II) complex: synthetic update, crystallographic features, DFT calculations, and unveil antimicrobial profiles, Inorg. Chem. Commun., 2024, 160, 111933.
13. D. Majumdar, B. Gassoumi, A. Dey, S. Roy, S. Ayachi, S. Hazra and S. Dalai, Synthesis, characterization, crystal structure, and fabrication of photosensitive Schottky device of a binuclear Cu(II)-Salen complex: a DFT investigations, RSC Adv., 2024, 14, 14992–15007.
14. B. J. Coe, Switchable nonlinear optical metallochromophores with pyridinium electron acceptor groups, Acc. Chem. Res., 2006, 39, 383–393.
15. D. Ramakrishna, Metal-centric organic compounds: boon to third-order nonlinear optical applications, Rev. Inorg. Chem., 2024, 44, 135–158.
16. A. Arunadevi and N. Raman, Biological response of Schiff base metal complexes incorporating amino acids–a short review, J. Coord. Chem., 2020, 73, 2095–2116.
17. N. Manurkar, M. Ilyas, F. Arshad, P. Patil, H. Shah, M. A. Khan, W. Hussain and H. Li, Exploring the first and second hyperpolarizabilities of l-glutamine-based Schiff base ligands and their Cu (ii) coordination complexes, New J. Chem., 2025, 49, 5200–5212.
18. M. P. Kumar and D. Ayodhya, Novel copper(II) binary complexes with N, O-donor isoxazole Schiff base ligands: Synthesis, characterization, DPPH scavenging, antimicrobial, and DNA binding and cleavage studies, Results Chem., 2023, 5, 100845.
19. G. Venkatesh, P. Vennila, S. Kaya, S. B. Ahmed, P. Sumathi, V. Siva, P. Rajendran and C. Kamal, Synthesis and spectroscopic characterization of Schiff base metal complexes, biological activity, and molecular docking studies, ACS Omega, 2024, 9, 8123–8138.
20. T. L. Yusuf, S. D. Oladipo, S. Zamisa, H. M. Kumalo, I. A. Lawal, M. M. Lawal and N. Mabuba, Design of new Schiff-Base Copper(II) complexes: Synthesis, crystal structures, DFT study, and binding potency toward cytochrome P450 3A4, ACS Omega, 2021, 6, 13704–13718.
21. Q. Zhu, Y. Tao, C. Yang, J. Gou, Y. Zhu, X. Wang and Q. Wu, Unveiling Noncentrosymmetric Pyridine Carboxylates from Centrosymmetric Templates through Motif Configuration Modulation in Zero-Dimensional System, Inorg. Chem., 2024, 63, 22620–22627.
22. B. Ivanova, Linear and Nonlinear Optical Properties of Non-Centrosymmetric Crystals of Substituted Aliphatic Secondary Amines, 2025.
23. N. Kordestani, H. A. Rudbari, A. R. Fernandes, L. R. Raposo, A. Luz, P. V. Baptista, G. Bruno, R. Scopelliti, Z. Fateminia and N. Micale, Copper(II) complexes with tridentate halogen-substituted Schiff base ligands: synthesis, crystal structures and investigating the effect of halogenation, leaving groups and ligand flexibility on antiproliferative activities, Dalton Trans., 2021, 50, 3990–4007.
24. A. Khalaji, M. Gholinejad and S. Triki, The copper(II) complexes with tetradentate Schiff base ligands: Synthesis, crystal structures, and computational studies, Russ. J. Coord. Chem., 2013, 39, 209–213.
25. N. T. Anh, Frontier orbitals: a practical manual, John Wiley & Sons, 2007.
26. N. Mahieu, Synthesis, redox chemistry, and electronic structure investigation of lanthanide complexes with aromatic ligands of various sizes and bulkiness, Institut Polytechnique de Paris, 2023.
27. G. Berger, J. Soubhye and F. Meyer, Halogen bonding in polymer science: from crystal engineering to functional supramolecular polymers and materials, Polym. Chem., 2015, 6, 3559–3580.
28. R. Bano, M. Asghar, K. Ayub, T. Mahmood, J. Iqbal, S. Tabassum, R. Zakaria and M. A. Gilani, A theoretical perspective on strategies for modeling high performance nonlinear optical materials, Front. Mater., 2021, 8, 783239.
29. C. Karthikeyan, A. H. Hameed, J. S. A. Nisha and G. Ravi, Spectroscopic investigation on the efficient organic nonlinear crystals of pure and diethanolamine added DAST, Spectrochim. Acta, Part A, 2013, 115, 667–674.
30. Y. Li, J. Zhang, G. Zhang, L. Wu, P. Fu and Y. Wu, Growth and characterization of DSTMS crystals, J. Cryst. Growth, 2011, 327, 127–132.
31. T. Wang, L. Cao, D. Zhong, J. Liu, F. Teng, S. Ji, S. Sun, J. Tang and B. Teng, Growth, electrical and optical studies, and terahertz wave generation of organic NLO crystals: DSTMS, CrystEngComm, 2019, 21, 2754–2761.
32. X. Feng, J. Ma, K. Xu, Y. Wu, Y. Zhai, F. Xuan, D. Zhai, L. Cao and B. Teng, Design, growth, and characterization of novel schiff base organic nonlinear optical crystals PAC and PAB, J. Mol. Struct., 2025, 1327, 141200.
33. T.-C. Chang, Y.-T. Lu, C.-H. Lee, J. K. Gupta, L. J. Hardwick, C.-C. Hu and H.-Y. T. Chen, The effect of degrees of inversion on the electronic structure of spinel NiCo2O4: a density functional theory study, ACS Omega, 2021, 6, 9692–9699.
34. M. W. Iqbal, M. Asghar, N. Noor, H. Ullah, T. Zahid, S. Aftab and A. Mahmood, Analysis of ternary AlGaX2 (X = As, Sb) compounds for opto-electronic and renewable energy devices using density functional theory, Phys. Scr., 2021, 96, 125706.
35. S. Ghosh and L. Manna, The many “facets” of halide ions in the chemistry of colloidal inorganic nanocrystals, Chem. Rev., 2018, 118, 7804–7864.
36. M. Huang, L. Jiao, J. Yang, D. Ning, J. Xu, Q. Wu and Z. Weng, Regulating effect of the halogen atoms on the cofacial dinuclear Schiff base complexes: Synthesis, spectroscopy, electrochemistry, and DFT calculations, Appl. Organomet. Chem., 2024, 38, e7536.
37. S. Muhammad, A. Bibi, S. Bibi, A. G. Al-Sehemi, H. Algarni and F. Sarwar, Exploring the quinoidal oligothiophenes to their robust limit for efficient linear and nonlinear optical response properties, Chem. Pap., 2022, 76, 4273–4288.
38. S. Muhammad, F. Sarwar, S. Bibi, R. Nadeem, M. W. Mushtaq, A. G. Al-Sehemi, S. S. Alarfaji and S. Hussain, Insighting the functionally modified C60 fullerenes as an efficient nonlinear optical materials: A quantum chemical study, Mater. Sci. Semicond. Process., 2022, 141, 106421.
39. E. R. Johnson, S. Keinan, P. Mori-Sánchez, J. Contreras-García, A. J. Cohen and W. Yang, Revealing noncovalent interactions, J. Am. Chem. Soc., 2010, 132, 6498–6506.
40. M. Ilyas, M. A. Khan, N. Manurkar, M. Ilyas, M. Abbas, H. M. Zohaib, T. Baig, A. Saba and H. Li, Unravelling the impact of octahedral distortion and non-covalent interactions on the hyperpolarizabilities of Co(II) and Ni(II) coordination complexes with uridine nucleotide, Mater. Today Chem., 2025, 47, 102818.
41. A. Malloum and J. Conradie, QTAIM analysis dataset for non-covalent interactions in furan clusters, Data Brief, 2022, 40, 107766.
42. M. Ilyas, M. A. Khan, L. Xiong, L. Zhang, M. Lauqman, M. Abbas, H. M. Zohaib, N. Manurkar and H. Li, Enhancements of the first and second hyperpolarizability of a GMP coordination polymer: crystal structure and computational studies, Dalton Trans., 2025, 54, 5921–5934.
43. Y. Liu, R. Sakamoto, C.-L. Ho, H. Nishihara and W.-Y. Wong, Electrochromic triphenylamine-based cobalt(II) complex nanosheets, J. Mater. Chem. C, 2019, 7, 9159–9166.
44. W.-M. Sun, L.-T. Fan, Y. Li, J.-Y. Liu, D. Wu and Z.-R. Li, On the potential application of superalkali clusters in designing novel alkalides with large nonlinear optical properties, Inorg. Chem., 2014, 53, 6170–6178.
45. F. L. Gu, Y. Aoki, M. Springborg and B. Kirtman, Calculations on nonlinear optical properties for large systems: The elongation method, Springer, 2014.
46. J. Xu, M. Huang, L. Jiao, H. Pang, X. Wang, R. Duan and Q. Wu, Supramolecular Dimer as High-Performance pH Probe: Study on the Fluorescence Properties of Halogenated Ligands in Rigid Schiff Base Complex, Int. J. Mol. Sci., 2023, 24, 9480.
47. K. Othman, Y. Azeez, R. Omer and R. Kareem, Condensed Matter and Interphases, Energy, 2024, 19, 21.
48. R. R. Tykwinski, U. Gubler, R. E. Martin, F. Diederich, C. Bosshard and P. Günter, Structure− property relationships in third-order nonlinear optical chromophores, J. Phys. Chem. B, 1998, 102, 4451–4465.
49. (a) N. Manurkar, M. Ilyas, F. Arshad, P. Patil, Q.-U.-N. Tariq, S. Khan, M. Ahmad Khan and H. Li, CCDC 2415583 (4): Experimental Crystal Structure Determination, 2025,  DOI:10.5517/ccdc.csd.cc2m2m1d; (b) N. Manurkar, M. Ilyas, F. Arshad, P. Patil, Q.-U.-N. Tariq, S. Khan, M. Ahmad Khan and H. Li, CCDC 2415691 (5): Experimental Crystal Structure Determination, 2025,  DOI:10.5517/ccdc.csd.cc2m2qjz.

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