Nonlinear optical functionalized Cu( <scp>ii</scp> ) coordination complexes with chiral ligands: design, structural elucidation, and theoretical investigation

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 <i>Cc</i> 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.