Imine Linkage Isomerism in Layer-Entangled Three-Dimensional Covalent Organic Frameworks Modulates Fluorescent...

Three-dimensional covalent organic frameworks (3D COFs) constructed from entangled two-dimensional layers represent a rare yet intriguing structural class; however, the influence of variations in linkage structure within such architectures remains unexplored. Herein, we investigate a linkage-isomeric pair of isoreticular layer-entangled 3D COFs with opposite imine bond orientations (3D-An-CN-COF and 3D-An-NC-COF). Remarkably, although these two frameworks share nearly identical composition, topology, and porosity, the subtle difference in C═N bond orientation leads to pronounced differences in their optoelectronic properties and fluorescent sensing behaviors. In particular, 3D-An-CN-COF exhibits nearly an order-of-magnitude higher sensitivity toward 2,4,6-trinitrophenol (TNP) in aqueous media than 3D-An-NC-COF, with a quenching constant of 3.27 × 10⁶ M^-1 and a detection limit of 9.5 nmol L^-1, placing it among the most sensitive COF-based fluorescent sensors for TNP reported to date. These results establish imine linkage isomerism as an effective structural parameter for reprogramming the optoelectronic properties and sensing behavior of layer-entangled 3D COFs, offering new insight into the rational design of high-performance fluorescent sensing materials.