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 metal–organic framework
(MOF) materials with the 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 a quenching agent and 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 conjugated
auxiliary ligand Bpy causes fluorescence quenching. Time-dependent
density functional theory was employed to understand the photoinduced
electron transfer process and explain the origins of fluorescence
quenching. Using this strategy, the prepared MOF material can perform
as a fluorescence “off–on” probe for highly sensitive
detection of Al3+ in aqueous media. The hydroxyl group
plays a crucial role in sensing as it can selectively chelate Al3+, which is directly related to the dissociation of nonfluorescent
MOF and consequent activation of the AIE process.