Nagesh
Manurkar
a,
Mubashar
Ilyas
a,
Faiza
Arshad
b,
Prasanna
Patil
c,
Qamar-Un-Nisa
Tariq
d,
Shahzad
Khan
a,
Maroof Ahmad
Khan
e and
Hui
Li
*a
aKey Laboratory of Clusters Science of Ministry of Education, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing, 100081, P. R. China
bSchool of Science, Harbin Institute of Technology (Shenzhen), Shenzhen, 518055, China
cSchool of Food and Health, Beijing Technology and Business University, No. 11, Fucheng Road, Beijing 100048, China
dInternational Collaborative Laboratory of 2D Materials for Optoelectronics Science and Technology of Ministry of Education, Institute of Microscale Optoelectronics, Shenzhen University, Shenzhen, 518060, China
eState Key Laboratory of Marine Resource Utilization in South China Sea, Hainan University, Haikou 570228, China
First published on 7th August 2025
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.
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.
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.
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 (Å3) | 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 F2 | 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 |
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.
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.
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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.
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.
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
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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.
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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 CO 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.
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 CN 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.
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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.
In computational chemistry, QTAIM analysis utilizes several parameters, such as the Laplacian of electron density (∇2ρ), kinetic electron density (Gr), potential electron density (Vr), and their ratio (−Vr/Gr), to evaluate the nature of interactions and bond strength. Strong covalent bonds are characterized by negative values for ∇2ρ, a −Vr/Gr ratio less than 1, total energy density (Hr) 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 −Vr/Gr ratio greater than 1, Hr above 0 atomic units, and ρ less than 0.1 atomic units. When the total energy density (Hr) 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 −Vr/Gr 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 −Vr/Gr 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 −Vr/Gr ratio of 1.07, which indicates the strength of the copper–oxygen coordination bond.
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Fig. 9 A representation of the frequency dependent isotropic polarization (αiso) of ligands (a) and complexes (c) and anisotropic polarization (αaniso) of ligands (b) and complexes (d). |
In the case of the Schiff base ligands (H2L1 to H2L5), the isotropic polarizability (αiso) 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 (αaniso) 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 aiso and asniso, 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.
Looking first at the Schiff base ligands (H2L1 to H2L5), βEOPE 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 βSHG, 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 βEOPE and βSHG 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 βSHG signal compared to βEOPE, 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.
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![]() |
449.2 | 484.86 | 914.35 | 913.29 | 873.94 |
0.04 | 368.38 | 551.58 | 1134.37 | 1308.91 | 46![]() |
421.33 | 597.59 | 1223.2 | 1232.36 | 1050.18 |
0.06 | 64![]() |
49![]() |
4749.49 | 109![]() |
11![]() |
29![]() |
30![]() |
6369.51 | 8599.64 | 3922.79 |
0.08 | 20![]() |
36![]() |
17![]() |
12![]() |
141![]() |
26![]() |
34![]() |
9048.54 | 62![]() |
3009.78 |
0.10 | 17![]() |
20![]() |
30![]() |
798![]() |
489![]() |
112![]() |
4383.86 | 65![]() |
367![]() |
320![]() |
0.12 | 252![]() |
65![]() |
105![]() |
1![]() ![]() |
1![]() ![]() |
42![]() |
14![]() |
97![]() |
435![]() |
336![]() |
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 βEOPE and βSHG 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 βSHG 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.
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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 γdc-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 γdc-Kerr values. In L-phenylalanine based Schiff bases, the substitution with halogens, particularly bromine, enhances the γdc-Kerr response, supporting the notion that halogen substituents increase the nonlinear optical response.
The γESHG 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 γESHG 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.
Frequency (ω) | γ (−ω; ω, 0, 0) | |||||||||
---|---|---|---|---|---|---|---|---|---|---|
(dc-kerr) × 104 (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![]() |
3.38 | 2.77 | 2.63 | 4.27 | 10.3 | 2.56 |
0.10 | 43.93 | 69.9 | 102 | 57![]() |
78.9 | 38.7 | 60.1 | 320 | 318 | 205 |
0.12 | 10![]() |
2990 | 90.6 | 79![]() |
16![]() |
119 | 168 | 362 | 6180 | 4000 |
Overall, complex-4 stands out as the most promising compound in both the γdc-Kerr and γESHG 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.
CCDC 2415583 and 2415691 contains the supplementary crystallographic data for this paper.49a,b
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