Programming Migration Energy Landscapes in Isoreticular Hydrogen‐Bonded Organic Framework Nanochannels for Kinetic Cs ⁺ /Sr ²⁺ Separation

ABSTRACT Precise separation of Cs + and Sr 2+ remains a critical challenge in nuclear waste remediation, where subtle variations in migration energetics under sub‐nanometer confinement limit separation fidelity. Here, we demonstrate migration energy‐landscape programming in an isoreticular series of hydrogen‐bonded organic framework (HOF) nanochannels to achieve kinetic Cs + /Sr 2+ separation. To overcome the intrinsic processability limitations of hydrogen‐bonded assemblies, we develop an interfacial chemical reaction‐mediated confined assembly strategy that suppresses stochastic nucleation and yields continuous, defect‐minimized crystalline HOF membranes. This isoreticular platform preserves identical channel geometry while enabling systematic modulation of pore‐wall nitrogen density as an independent chemical variable, effectively decoupling structural confinement from chemical regulation. Multiscale simulations and temperature‐dependent transport measurements reveal that nitrogen enrichment selectively amplifies the translocation energy barrier for Sr 2+ while maintaining low‐barrier hopping pathways for Cs + . The resulting migration‐barrier asymmetry transforms structurally equivalent nanochannels into precise kinetic discriminators. Under competitive and electrically assisted conditions, the optimized membrane achieves a record‐high Cs + /Sr 2+ selectivity of 155.5. This work establishes programmable migration energy landscapes in crystalline nanochannels as a general strategy for engineering kinetic ion separations beyond conventional size‐ or valence‐governed limits.