Metastable and Stable States in Covalent Organic Frameworks Controlled by Synthetic Selection Pressure.

The synthesis of covalent organic frameworks (COFs) has long followed a thermodynamics-driven approach, aiming to obtain highly ordered stable crystalline materials while overlooking metastable structures with unique dynamic characteristics. Inspired by the biological concept of "selection pressure," we propose that the harshness of synthetic conditions serves as a key selection pressure governing the final state of COFs. A model COFs system comprising flexible tetrazine and rigid pyrene units is designed, and differential selection pressures are applied by adjusting reaction temperature. Under conditions of lower temperature (lower selection pressure), flexible conformations based on non-covalent interactions are preserved, yielding a metastable AA-stacking material (Tz-COF-AA) with high flexibility and environmental sensitivity, which could further transform into an interlayer-shifted phase (Tz-COF-AA'). Under conditions of higher temperature (higher selection pressure), these flexible conformations are disrupted; the system generates a rigid AB-stacking stable material (Tz-COF-AB) dominated by covalent bonds. Iodine adsorption experiments demonstrated that the metastable materials exhibited variability in adsorption performance due to framework flexibility, whereas the stable material possessed performance reproducibility. This selection pressure strategy provides a new example for the controllable construction of metastable COFs and offers insights for the development of porous crystalline materials with tailored dynamic behavior or high stability.