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Unveiling the Uncommon Fluorescent Recognition Mechanism Towards Pertechnetate Using a Cationic Metal-Organic Framework Bearing N-heterocyclic AIE Molecules.

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, we reported a new water-stable cationic metal-organic framework (ZJU-X8) 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-. Such uncommon fluorescent recognition mechanism was well elucidated by the batch sorption experiments and density functional theoretical (DFT) calculations. It is 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.