Two-dimensional fluorinated nanomaterials (2DFs) possess tunable band structures and unique optoelectronic properties, but their controlled synthesis remains challenging due to the lack of mild and scalable methods. Here, we report a gram-scale, room temperature synthesis of ultra-thin 2DF sheets (1–2 nm) with micrometer-scale lateral dimensions. The approach involves nucleophilic substitution of fluorine atoms in perfluoro monomers with alkynyl groups, generating intermediates that undergo in situ [2 + 2+2] cyclotrimerization to form porous 2D networks. Experiments and computations reveal periodic pores with fluorine atoms adopting flipped conformations at pore centers. The resulting 2DFs show broad visible-near-infrared absorption (Eg ≈ 2.4 and 3.2 eV) and strong fluorescence (300–550 nm), with quantum yields up to 66%. Their emission is selectively quenched by trace PFAS, enabling rapid detection in drinking water at subpicomolar concentrations.