Topological Engineering of Metal–Organic Architectures for Tunable Ultrafast Nonlinear Optical Responses

Advances in nonlinear optical (NLO) materials are central to the development of ultrafast photonic and optoelectronic technologies, where macroscopic responses depend strongly on chromophore geometry and orientational order. Here, mechanically non‐trivial metal–organic architectures are used as a molecular engineering strategy to control and stabilize three‐dimensional chromophore arrangements, enabling systematic tuning of second‐ and third‐order NLO properties. Two classes of topologically complex architectures, [2]catenates ([2]C) and trefoil knots (TK), based on Zn(II) and Cd(II), are investigated in guest–host PMMA thin films and nanoconfined environments. Pyrene‐functionalized Zn(II) systems show strong second‐ and third‐harmonic generation (SHG, THG), with first and second hyperpolarizabilities reaching β = 43.66 × 10 − 3 8 m 4 V − 1 and γ = 102.67 × 10 − 4 7 m 5 V − 2 , respectively, reflecting the influence of extended π‐conjugation and intramolecular charge transfer. Z‐scan analysis indicates self‐focusing behavior and architecture‐dependent refractive index modulation. Incorporation into nanoporous alumina membranes significantly enhances SHG compared to thin films, attributed to nanoscale confinement and partial orientational ordering. These findings demonstrate that molecular topology, metal coordination, and ligand‐centered electronic structure collectively govern ultrafast NLO responses in such systems.