An ultra-stable vinylene-linked covalent organic framework for the olefination of fluorene under green light

Vinylene-linked covalent organic frameworks (COFs) have attracted significant attention for their exceptional chemical stability and optoelectronic properties. However, the large-scale synthesis of vinylene-linked COFs remains challenging, as base- or acid-mediated solvothermal methods often result in uncontrolled crystal growth and poorly ordered frameworks. In this study, we present a method for the large-scale synthesis of a highly crystalline vinylene-linked COF (TMT-DMTD) via solid-state Knoevenagel condensation using a flux-mediated approach, with benzoic acid and benzoic anhydride acting as activators. The resulting two-dimensional (2D) COF exhibits remarkable properties, including high crystallinity, a substantial surface area (1035 m² g^-1), exceptional chemical stability in acidic and basic media for up to 30 days, and excellent light-harvesting capability. Leveraging these properties, TMT-DMTD COF was employed as a reusable, metal-free, heterogeneous photocatalyst for the olefination of fluorene under mild conditions. Unlike conventional approaches that rely on precious-metal homogeneous catalysts operating at elevated temperatures (130-140 °C), this methodology operates at room temperature under green-light irradiation with low catalyst loading, affording a broad range of olefins in good to excellent yields. The protocol demonstrates wide functional-group tolerance, truly mild reaction conditions, and significant catalyst recyclability, representing a meaningful advancement over existing methodologies.