Dimensional Isomerism in COFs Enabled by Spirobifluorene Substitution for Enhanced Photocatalytic CO₂ Reduction

The concept of dimensional isomerism-covalent organic frameworks (COFs) constructed from identical molecular precursors but forming structures of different dimensions-remains largely unexplored. Here, we report a pair of isomeric COFs derived from the same spirobifluorene core, wherein substitution at either the C2/C7 or C3/C6 positions leads to the formation of a planar two-dimensional (2D) or a nonplanar three-dimensional (3D) framework, respectively. This substitution-controlled strategy enables precise modulation of π-conjugation and framework dimensionality without altering the molecular identity of the building block. Comparative photocatalytic experiments reveal that the 2D C2/C7-substituted COF exhibits significantly superior CO₂ reduction activity under visible light, attributed to its extended conjugation and efficient charge transport. These findings underscore the critical role of molecular connectivity in determining the structure-function relationships of COFs and propose a generalizable approach for designing high-performance photocatalytic materials through topological isomerism.