Lanthanide MOFs and their composite gel glasses: Central metal ion engineering of nonlinear optical responses

Nonlinear optical (NLO) materials capable of efficient frequency conversion, optical switching, and all-optical signal processing are fundamental building blocks for modern photonics and quantum technologies. Herein, we report on lanthanide-based metal-organic framework (Ln-MOF) composite gel glasses, employing central metal ion engineering as an efficient strategy for tailoring their third-order NLO responses. Three isostructural Ln-MOFs based on La 3 + (4 f⁰), Ce 3+ (4 f¹), and Pr 3+ (4 f²) are synthesized and uniformly incorporated into transparent silica gel glass matrices via sol-gel processing. Combined morphological, structural, and optical investigations demonstrate the successful preparation of Ln-MOFs and their integration into composite gel glasses. Open-aperture Z-scan measurements reveal a distinct and counterintuitive trend in the nonlinear absorption coefficients (i.e., La 3+ > Ce 3+ > Pr 3+ ), which mainly originates from the central-ion-dependent charge transfer polarizability within the MOFs. The NLO performance of Ln-MOF@SiO 2 gel glasses is enhanced compared with that of Ln-MOFs owing to the matrix effect. Furthermore, the Ln-MOFs and Ln-MOF@SiO 2 gel glasses exhibit wavelength-dependent transitions from reverse saturable absorption to saturable absorption. The resulting composite gel glasses combine the molecular designability of Ln-MOFs with the optical transparency, mechanical robustness, and processability of inorganic glasses, offering an integrable solid-state platform for NLO device applications. Our work not only elucidates the dominant role of charge transfer processes in governing the NLO properties of Ln-MOF materials but also provides theoretical insights and a robust platform for the design of high-performance, integrable solid-state nonlinear optical devices.