Unlocking Giant Optical Nonlinearity in Rare-Earth MOFs

The escalating demand for reliable protection against high-fluence laser pulses across industrial, defense, and biomedical fields has driven intensive research into advanced optical limiting materials. A critical challenge remains the often-high operational threshold of such devices, which limits their sensitivity and practical deployment. Herein, we report the rational design and synthesis of a series of isostructural Ln³⁺-porphyrinic metal-organic frameworks (MOFs, Ln³⁺ = Gd, Tb, Er) that exhibit exceptional reverse saturable absorption (RSA). Remarkably, the erbium-based analogue (Er-TCPP) exhibits a giant third-order nonlinear absorption coefficient of 4.86 × 10^-4 m/W and an ultralow starting threshold of 1.42 mJ/cm² at 532 nm using the Z-scan technique. A synergistic mechanism is unveiled through combined femtosecond transient absorption spectroscopy and density functional theory (DFT + U) calculations. The giant third-order nonlinear response of Er-TCPP is likely driven by the combined effects of spin-orbit coupling and f-state-mediated excited-state interactions, together with the extensive electronic delocalization within the 18π-electron conjugated framework of the TCPP ligand, all of which collectively prolong excited-state carrier lifetimes and substantially enhance excited-state absorption. This study not only reports a state-of-the-art MOF with exceptional optical limiting performance but also establishes a general design principle based on targeted metal-ligand electronic synergy for developing advanced nonlinear optical materials.