Exploring superior nonlinear optical properties of copper complexes with π-conjugated ligands: experimental and theoretical investigation

This study explores the nonlinear optical (NLO) properties of two synthesized copper complexes, {[Cu(dmp)₂(H₂O)]·(PA)₂} (complex 1) and {[Cu(dmp)₂]·(PA)} (complex 2), where dmp represents 2,9-dimethyl-1,10-phenanthroline (neocuproine) and PA refers to picric acid. Employing both experimental and theoretical approaches, we comprehensively analyzed their NLO properties. Experimentally, third-order NLO parameters were determined using the Z-scan technique with a 520 nm continuous-wave diode laser, yielding values of nonlinear refractive index (<i>n</i> ₂), nonlinear absorption coefficient (<i>β</i>), and third-order nonlinear optical susceptibility (<i>χ</i> ⁽³⁾) on the order of 10^-7 cm² W^-1, 10^-3 cm W^-1, and 10^-6 esu, respectively. These findings underscore their significant optical nonlinearity and suitability for intensity-dependent NLO applications, as evidenced by their high values of negative refractive index. Density Functional Theory (DFT) investigations were performed at the M062X/(6-31G(d,p)/LanL2DZ)) level to evaluate key reactivity descriptors, including polarizability, hyperpolarizability, and dipole moment, which align with the experimental results. Conceptual DFT analysis reveals that complex 1 is more stable, while complex 2 exhibits higher reactivity. The strong hyperpolarizability and remarkable third-order NLO properties of these copper complexes demonstrate their potential for advanced photonic and optoelectronic applications.