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 (fluoro, chloro, bromo, and iodo) and their copper(II) coordination complexes. All of these ligands and their complexes are characterized fully. The crystallographic studies reveals these complexes with Cc space group and the central Cu(II) having a distorted square-pyramidal geometries, both of 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, 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, in 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. The works can help to design the enhancing NLO functionalized coordination complexes.