Imine Linkage Isomerism in Layer‐Entangled Three‐Dimensional Covalent Organic Frameworks Modulates Fluorescent Sensing

ABSTRACT 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 × 10 6 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.