Imine-Orientation and Confinement-Engineered Programming of Polyiodide Speciation in Nitrogen-Rich COFs for Tailored...

Achieving precise control over the aggregation state of volatile iodine (I₂) within porous adsorbents is critical for developing high-performance materials that go beyond mere capacity metrics. Herein, we report a rational design strategy for covalent organic frameworks (COFs) in which polyiodide speciation is programmed through meticulous manipulation of nitrogen site environments and spatial confinement. Two nitrogen-enriched COFs, Py-Trz-COF-1 and Py-Trz-COF-2, are constructed with deliberate imine orientation and interlayer slippage. Despite nearly identical topologies and comparably high iodine uptake (5.0 vs. 4.7 g·g^-1), they exhibit distinctly different confined iodine chemistry. Comprehensive spectroscopic analyses reveal that Py-Trz-COF-1 stabilizes a higher proportion of I₃ ^- species, whereas Py-Trz-COF-2 favors I₅ ^- formation. Density functional theory calculations attribute this divergence to site-specific electronic modulation: in Py-Trz-COF-1, localized electron density at the terminal imine nitrogen enhances charge transfer and stabilizes I₃ ^-, while in Py-Trz-COF-2, enhanced π-delocalization around the pyridine-triazine cavity, coupled with larger confinement space, promotes the evolution toward I₅ ^-. This work demonstrates that polyiodide distribution in COFs can be deliberately engineered through structural precision at the molecular level, offering a new design paradigm for tailoring iodine chemistry in porous materials.