Rapid and Selective Fluorescent Sensing of Histidine Driven by Metal-Specific Displacement in AIE-Based Complexes

The rational control of ligand substitution in metal complexes remains underexplored for turn-on fluorescent probes. Here, varying the metal center (Co 2+, Ni 2+, or Cu 2+ ) in AIE-based complexes M(L) 2 (L = tetraphenylethylene-based β-diketone) enables precise tuning of metal–ligand bond dissociation, dictating competitive ligand exchange with histidine (His). Single-crystal X-ray diffraction confirmed the structures. All complexes are nonemissive in the aggregated state due to paramagnetic quenching. Upon His addition, only Ni(L) 2 undergoes rapid and complete ligand displacement, generating a strong fluorescence turn-on signal within 10 min at room temperature. Co(L) 2 reacts much more slowly (>560 min), while Cu(L) 2 shows no change. DFT calculations based on crystal structures quantify Gibbs free energy changes: spontaneous displacement in Ni(L) 2 correlates with favorable ligand dissociation energetics, whereas Cu(L) 2 is thermodynamically trapped. Exploiting this metal-dependent reactivity, Ni(L) 2 serves as a highly selective and rapid fluorescent probe for His, discriminating against 20 other amino acids with a detection limit of 4.66 nM. This work establishes a coordination chemistry strategy for designing small-molecule probes by modulating metal-dependent dissociation energetics in AIE complexes, moving beyond conventional PET mechanisms.