Sequential effects of two cations on the fluorescence emission of a coordination polymer with Zn₄O core in node

Distinct changes in the fluorescence emissions of free ligand 5-(1,3-dioxo-1H-benzo[de]isoquinolin-2(3H)-yl)isophthalic acid (H₂NAPHISO) than a 2D-zinc-coordination polymer of it, caused by sequential interactions with different sets of binary cations were observed. The coordination polymer having unsymmetrical Zn₄O core of tetranuclear zinc-node could be dispersed in dimethylformamide without its degradation. The coordination polymer had an emission peak at 435 nm (quantum yield = 0.082) which was selectively quenched by adding Fe²⁺ ions. Based on this quenching, the Fe²⁺ ions in aqueous solution could be detected with a detection limit 42.57 nM. The metal ions such as Li⁺, Na⁺, Cd²⁺, Hg²⁺, Al³⁺ did not interfere in the detection; but each of these ions together with Fe²⁺ ions showed characteristic shift of the emission spectra. The H₂NAPHISO in dimethyl formamide was non-fluorescent, but showed emission at 452 nm upon addition of Cd²⁺ or Zn²⁺ ions. This new emission of H₂NAPHISO caused by zinc or cadmium ions was not quenched by Fe²⁺ ions. Various cations had affected the emission of the H₂NAPHISO with Zn²⁺ which was much different from the corresponding changes caused by the same ion on the emission of the coordination polymer. For example, the Mn²⁺ and Zn²⁺ ions together in a solution of the ligand showed a broad emission spectrum spreading over 380-450 nm, but ions Sn²⁺ and Zn²⁺ together had showed emission at a shorter wavelength (380 nm). These allowed to modulate the emission of the ligand by binary combination of metal ions.