An Amide-Functionalized Pillar-Layered Metal–Organic Framework Based on Mixed Linker Systems with Enhanced Third-Order Nonlinear Optical Performance

The development of novel nonlinear optical (NLO) materials is increasingly focused on crystal structures that offer both high efficiency and laser damage thresholds, particularly for appliances in optical switching and communications. Herein, an experimental inquiry is described into the third order NLO response of an amide-functionalized pillar-layered metal-organic framework (MOF) encompassing cobalt ions and 4,4'-oxybis(benzoic acid) and N,N'-bis(4-pyridylformamide)-1,4-benzenediamine linkers. The introduced MOF possesses a complex porous structure defined by a 3,4,5T94 topology. The π-conjugated structure and nitrogen-containing derivatives with lone electron pairs on the nitrogen atoms endow this MOF with strong intramolecular charge-transfer capabilities, thereby facilitating the formation of donor-acceptor molecules. The pillared Co-MOF offers a significant nonlinear optical profile, with an NLR index (n₂) at a range of (6.6-19.1) × 10^-8 cm²/W and an NLA coefficient (β) of (2.92-14.27) × 10^-3 cm/W. The rationale explanation for the NLO response in pillared Co-MOFs, could be d-d transitions of Co²⁺ ions, π-π and LMCT/MLCT transitions, charge-transfer interactions between metal nodes and organic linkers, structural anisotropy, and pillar-induced electron delocalization. The Z-scan findings highlight the potential of the pillared MOFs with mixed ligands as promising alternatives for photonics devices, optical switches, and optical power-limiting technologies.