Coumarin-based luminescent ligand that forms helicates with dicationic metal ions

The potentially hexadentate ligand L, which contains two terminal coumarin fluorophores, forms dinuclear double-stranded helicates with dicationic metal ions, giving species of the generic form [M₂L₂]⁴⁺. In solution the free ligand was fluorescent with emission attributed to the coumarin fluorophores (λ_em = 437 nm). The luminescent properties of the corresponding dimetallic helicates complexes were examined and revealed that the Zn²⁺ complex demonstrates enhanced emission when compared to the parent ligand, whereas Co²⁺, Cu²⁺, Cd²⁺ and Hg²⁺ induce varying degrees of fluorescence quenching. In particular, comparative luminescence measurements at 77 K and room temperature showed that the quenching mechanism for [Cu₂L₂]⁴⁺ can be attributed to a photoinduced electron transfer. ESI-MS selectivity studies showed that in the presence of a mixture of metal dications no preference for any one metal ion was observed.