Substitution Site‐Controlled Dimensional Isomerism in Covalent Organic Frameworks

ABSTRACT Dimensional isomerism, whereby identical central building blocks give rise to frameworks with different topological dimensionalities, remains largely underexplored in covalent organic frameworks (COFs). Herein, we report a pair of isomeric COFs constructed from bicarbazole and spirobifluorene building units. When substitution occurs at the C2/C7 positions of the building units, the framework preferentially forms a planar 2D structure, whereas shifting the substitution to the C3/C6 positions leads to a nonplanar 3D framework. This site‐engineering strategy enables precise control over framework dimensionality and π‐conjugation connectivity simply by altering the substitution position, without changing the molecular backbone or chemical composition of the building units. Benefiting from the extended π‐conjugated framework enabled by the combined bicarbazole and spirobifluorene building units, the C2/C7‐substituted 2D CZ‐2D‐COF exhibits markedly enhanced visible‐light‐driven photocatalytic performance and achieves more efficient H ₂ O ₂ production. These findings highlight the critical role of substitution sites in determining COF dimensionality and function, and provide a generalizable strategy for designing high‐performance photocatalytic COFs through dimensional isomerism.