Engineered Fluorescent Ionic Liquids with Tunable Cationic Architecture for Charge Delocalization-Driven Al3+ Recognition and Real-Sample Analysis

Abstract

Fluorescent ionic liquids (FILs) provide a versatile platform for metal ion sensing; however, the role of cationic structure in governing sensing efficiency remains underexplored. Herein, we report two 8-hydroxyquinoline-based FIL probes incorporating imidazolium (compound 1) and pyridinium (compound 2) frameworks for selective detection of Al3+ and Zn2+. Compound 1 exhibited significantly enhanced intramolecular charge transfer (ICT), resulting in a ∼47-fold fluorescence enhancement for Al3+ compared to ∼12-fold for compound 2. UV–vis titration revealed the formation of a LMCT/ICT band (∼367 nm) with a distinct isosbestic point, confirming a well-defined complexation process. Computational analysis indicated a higher dipole moment, improved charge delocalization, and favorable HOMO–LUMO alignment in compound 1, correlating with increased electronic softness and stronger binding affinity. Mechanistically, Al3+ coordination induces structural rigidification, coordination-induced rigidification and stabilization of the emissive ICT state and nonradiative decay pathways, and stabilizes the emissive ICT state, while fluoride-mediated masking enables selective discrimination over Zn2+. The probe exhibited high selectivity, reversibility, and a 1:2 binding stoichiometry with enhanced fluorescence lifetime. Real sample analysis demonstrated excellent linear response in soil extracts (R2 = 0.993) through spiking experiments, and the method was successfully extended for quantification of Al3+ in pharmaceutical formulations. Furthermore, a paper-based sensing device enabled rapid visual detection with smartphone-assisted quantification, highlighting its potential for low-cost, on-site Al3+ monitoring.