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As one of most problematic radionuclides, technetium‐99, mainly in the form of anionic pertechnetate (TcO4−), exhibits high environmental mobility, long half‐life, and radioactive hazard. Due to low charge density and high hydrophobicity for this tetrahedral anion, it is extremely difficult to recognize it in water. Seeking efficient and selective recognition method for TcO4− is still a big challenge. Herein, a new water‐stable cationic metal‐organic framework (ZJU‐X8) was reported, bearing tetraphenylethylene pyrimidine‐based aggregation‐induced emission (AIE) ligands and attainable silver sites for TcO4− detection. ZJU‐X8 underwent an obvious spectroscopic change from brilliant blue to flavovirens and exhibited splendid selectivity towards TcO4−. This uncommon fluorescent recognition mechanism was well elucidated by batch sorption experiments and DFT calculations. It was found that only TcO4− could enter into the body of ZJU‐X8 through anion exchange whereas other competing anions were excluded outside. Subsequently, after interaction between TcO4− and silver ions, the electron polarizations from pyrimidine rings to Ag+ cations significantly lowered the energy level of the π* orbital and thus reduced the π–π* energy gap, resulting in a red‐shift in the fluorescent spectra.

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99 Tc is one of the most abundant radiotoxic isotopes in used nuclear fuel with a high fission yield and a long half-life. Effective removal of pertechnetate (TcO 4 − ) from an aqueous solution is important for nuclear waste separation and remediation. Herein, we report a series of facilely obtained benzene-linked guanidiniums that could precipitate TcO 4 − and its nonradioactive surrogate ReO 4 − from a high-concentration acidic solution through self-assembly crystallization. The resulting perrhenate and pertechnetate solids exhibit exceptionally low aqueous solubility. The benzene-linked guanidiniums hold one of the highest TcO 4 − removal capacities (1279 mg g −1 ) among previously reported materials and possess a removal percentage of 59% for ReO 4 − in the presence of Cl − over 50 times. The crystallization mechanism was clearly illustrated by the single-crystal structures and density functional theory calculations, indicating that TcO 4− is captured through a charge-assisted hydrogen bonding interaction and stabilized by π−π stacking layers. In addition, the removal process is easily recycled and no toxic organic reagents are introduced. This work provides a green approach to preliminarily separate TcO 4− from high-level nuclear wastes.