Tailoring the Fluorescence of AIE Active Metal-Organic Frameworks for Aqueous Sensing of Metal Ions.

A hydroxyl functionalized ligand was designed for the construction of MOF materials with aggregation-induced emission (AIE) feature, in which the fluorescence can be deliberately tailored: quenching the fluorescence to an "off" state by the decoration with heterocyclic auxiliary ligand 4,4'-bypyridine (Bpy) in the framework as quenching agent; triggering the enhanced fluorescence to an "on" state by removal of Bpy through the metal competitive coordination substitution strategy. Our study shows that the occurrence of exciton migration between the AIE linker and the conjugated auxiliary ligand Bpy cause the fluorescence quenching. Time-dependent density functional theory (TD-DFT) was employed to understand the photo induced electron transfer process, and explained the origins of fluorescence quenching. Using this strategy, the prepared MOF material can perform as fluorescence "off-on" probe for highly sensitive detection of Al3+ in aqueous media. The hydroxyl group play crucial role in the sensing as it can selectively chelate with Al3+, which directly relate to the dissociation of non-fluorescent MOF and consequent activation of AIE process.