Polymer Network-Confined Purely Organic Material with Long-Lived Delayed Emission for Aqueous Iron(III) Ion Sensing

Luminescent sensing in aqueous media using organic small-molecule emitters is often constrained by water-induced fluorescence quenching and indicator leakage. In this study, a polymer-confined thermally activated delayed fluorescence (TADF) material, poly-BrTPPz, was synthesized by covalently copolymerizing a donor-acceptor monomer into a polyacrylamide network. Density functional theory calculations indicate spatial frontier orbital separation. The polymer matrix restricts intramolecular motion, while the polar amide microenvironment provides a solid-state solvation effect, decreasing the reverse intersystem crossing barrier to activate delayed luminescence with a lifetime of 303 μs and a photoluminescence quantum yield of 69.1% in the solid state. In aqueous environments, the material exhibits a selective quenching response toward iron(III) ions (Fe³⁺) through a mechanism involving the inner filter effect and pyrazine-coordinated static quenching. To mitigate potential secondary environmental contamination, a transmembrane diffusion model was evaluated by encapsulating the polymer within a semi-permeable membrane, which limits indicator leakage while permitting analyte permeation. This work outlines a design approach for environment-responsive luminescent devices in closed aquatic systems.