A butterfly-shaped AIEgen with excited-state intramolecular proton transfer effect for colorimetric and fluorescent detection of copper and zinc ions

Multipurpose aggregation-induced emission (AIE) fluorescence probes for metal ion detection have drawn extensive interest in recent years. Here, a butterfly-shaped AIE luminogen (TOPD) was synthesized from 2-hydroxy-5-(1,2,2-triphenylethenyl)-benzaldehyde and o-phenylenediamine via a one-step Schiff-base condensation for flexibly detecting copper and zinc ions. The aggregated state of TOPD revealed a large Stokes shift of 263 nm due to the excited-state intramolecular proton transfer (ESIPT) effect benefiting from the planar cis-configuration and enol-keto tautomerization of TOPD molecules confirmed by the modeling investigation. TOPD showed remarkable colorimetric and high selective fluorescence turn-off response to Cu2+ in phosphate buffer saline/tetrahydrofuran (PBS/THF, 9:1, v/v) with a low detection limit (0.14 μM) and quick response time (<30 s). TOPD realized the Zn2+ sensing via turn-on and blue-shifted fluorescence in PBS/THF (8:2, v/v) with a detection limit of 0.93 μM. The significant color changes can be directly discriminated by the “naked eye”. Experiments and DFT theoretical calculations strongly suggested that sensing mechanisms were via 1:1 TOPD-metal complexes. The strategy of tunable AIE fluorescence could provide new ideas for potential applications in the detection of multiple ions in environmental fields.