Solvent‐Directed Modulation of Stacking Isomers in Single‐Crystal One‐Dimensional Covalent Organic Frameworks

Stacking isomerism in covalent organic frameworks (COFs) has long remained elusive, with no unambiguous single‐crystal structure confirmation to date. Herein, we present the first definitive structural resolution of stacking isomers in one‐dimensional COFs, enabled by a solvent‐directed modulation strategy. Two distinct stacking isomers were obtained: one exhibiting AA′‐stacking (synthesized in mesitylene) and the other featuring an ABCD quadruple stacking mode (synthesized in anisole/n‐butanol). Their atomic structures were determined by continuous rotation electron diffraction (cRED) at resolutions of 0.98 and 1.02 Å, respectively, with all non‐hydrogen atoms directly located. Compared to the AA′‐stacked isomer, the ABCD‐stacked counterpart displays broader optical absorption, a narrower bandgap, and approximately twofold higher charge‐separation efficiency, resulting in significantly enhanced visible‐light photocatalytic activity for H 2 O 2 production and N‐benzylbenzaldimine synthesis. Mechanistic studies reveal that the ABCD stacking configuration narrows the bandgap, weakens electron localization, and substantially increases the dipole moment, thereby inducing strong interlayer charge transfer and polarization effects.