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, 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 pi* orbital and thus reduced the pi-pi* energy gap, resulting in a red-shift in the fluorescent spectra.