ABSTRACT An integrated computational and experimental investigation was carried out on a series of diimine–diphosphine ligands (L1–L5) and their Ru(II), Zn(II), and Ni(II) complexes (C1–C8) to elucidate the structure–property relationships governing third‐order nonlinear optical (NLO) activity. Geometry optimization, frontier molecular orbital (FMO) analysis, and TD‐DFT calculations were employed to evaluate electronic transitions, intramolecular charge‐transfer (CT) pathways, and static second hyperpolarizabilities ( γ ). These theoretical insights were correlated with single‐beam Z‐scan measurements performed under continuous‐wave (CW) excitation to quantify nonlinear refraction ( n 2 ) and reverse saturable absorption (RSA). The results reveal that third‐order NLO behavior is strongly dictated by π‐conjugation length, electron‐donor/acceptor topology, and metal–ligand electronic coupling. Ruthenium complexes display the largest γ and n 2 values, consistent with efficient metal‐to‐ligand charge transfer (MLCT) and reduced HOMO–LUMO gaps, while Zn(II) and Ni(II) systems exhibit comparatively modest responses governed mainly by ligand‐centered π–π* transitions. Across the series, azo‐linked and extended aromatic ligands significantly enhance ICT and electronic delocalization, resulting in superior γ – n 2 correlations and high third‐order susceptibility. The strong agreement between DFT predictions, Z‐scan measurements, and QSPR models provides a unified framework for understanding tunable NLO behavior in metal complexes and establishes MLCT‐active Ru(II) architectures as promising candidates for photonic and optical‐limiting applications.
Jillani et al. (Mon,) studied this question.