Managing Lattice Strain Enables Irreversible Single‐Crystal‐to‐Single‐Crystal Transformation of Covalent Organic Frameworks

ABSTRACT Covalent organic frameworks (COFs) typically rely on reversible covalent chemistry to achieve crystallinity, whereas irreversible and rigid linkages are generally required to achieve enhanced robustness and extended π‐conjugation. Single‐crystal‐to‐single‐crystal (SCSC) transformation offers a promising route to access such frameworks; however, irreversible bond reconfiguration imposes permanent mechanical strain that often disrupts lattice order. Here, we demonstrate sulfur‐assisted SCSC transformation of imine‐linked COFs into rigid benzothiazole‐linked frameworks with experimentally resolved atomic structures. Time‐resolved structural analyses uncover two distinct stress‐accommodation pathways‐cooperative lattice adaptation and stress‐driven transient domain reconstruction that enable irreversible bond fusion while preserving long‐range crystallographic order. The resulting single‐crystalline thiazole‐linked COFs exhibit enhanced chemical stability, rigidified pore architectures, and improved optoelectronic performance. This study establishes a mechanistic framework for managing lattice strain during irreversible covalent transformation and provides a general design principle for constructing structurally robust crystalline polymers.