Emergence of a Fluorochromic Covalent Organic Framework for Charge-Transfer-Driven Radionuclide Detection

Volatile radioiodine species, particularly methyl iodide (CH₃I), present radiological hazards due to their high volatility, chemical inertness, and weak interactions with conventional sorbents, underscoring the urgent need for effective detection strategy. Here, we report for the first time a fluorochromic approach for radioiodine monitoring, realized through a nitrogen-rich, vinylene-linked covalent organic framework (COF-TT), which functions as a highly selective and sensitive sensor for trace-level CH₃I. COF-TT features an extended π-conjugated network with abundant Lewis basic sites, facilitating strong charge-transfer interactions with CH₃I that produce pronounced, concentration-dependent fluorochromic shifts. When embedded in an epoxy-resin membrane (COF-TT@ER), the material achieves a low detection limit of 63 ppb, while demonstrating minor interference from I₂ and exceptional stability under humid conditions. Integration of COF-TT@ER into a compact RGB-based device enables portable, real-time, and instrument-free CH₃I sensing, establishing a versatile platform for environmental radiological surveillance. Mechanistic studies associated with density functional theory calculations, reveal that upon adsorption, CH₃I dissociates with electron transfer from COF-TT, leading to enhanced π-π stacking, modulation of the HOMO-LUMO gap, and ultimately the observed fluorochromic response. This work pioneers the application of fluorochromic sensing for radioiodine detection, opening a new avenue for rapid and sensitive radionuclide monitoring.