A VIE-Based Fluorescent Probe for the Detection of Zn²⁺/Cd²⁺/Cu²⁺

N, N’-diphenyldihydrodibenzo a, c phenazines (DPACs) are a class of vibration-induced emission (VIE)-active molecules, which exhibit either a bent or planar configuration at the excited-state, leading to unique blue and orange-red dual fluorescence emission features. The emission color of DPAC molecules is tuned from red to deep blue by varying the degree of inhibition of the planarization processes. Based on this principle, a new fluorescent probe, DPAC-H-COOH, was designed and synthesized in this work by asymmetrically introducing metal-ion coordination groups—aniline N, N’-diacetic acid and phenoxyacetic acid ligands—onto the two “arms” of a DPAC core for chelating metal ions. DPAC-H-COOH was characterized by ¹H-NMR (nuclear magnetic resonance), ¹³C-NMR, and HRMS (high resolution mass spectrum), and the test results indicate that DPAC-H-COOH binds selectively to Zn²⁺/Cd²⁺/Cu²⁺ through tetra-coordination, forming intramolecular complexes in a stoichiometric ratio of 1∶1. The coordination with Zn²⁺/Cd²⁺ limits the planarization process of the excited state of DPAC core resulting in an enhancement in blue fluorescence and decrease in orange-red fluorescence. While DPAC-H-COOH exhibits a grayish green fluorescence when coordinated with Cd²⁺, chelation with Zn²⁺ leads to a stronger enhancement in the blue fluorescence and a more thorough decrease in the red fluorescence of the probe. Moreover, due to the ligand-metal charge transfer (LMCT) when bound with paramagnetic Cu²⁺, the blue and orange-red dual fluorescence of DPAC-H-COOH are quenched simultaneously. The detection limit of the probe for Zn²⁺, Cd²⁺ and Cu²⁺ is 80.2 nmol/L, 0.506 μmol/L and 0.263 μmol/L, respectively. This work develops a single molecular probe suitable for the detection of multiple metal ions based on the VIE mechanism.