Responsive, di-metallic lanthanide complexes of a piperazine-bridged bis-macrocyclic ligand: modulation of visible luminescence and proton relaxivity

Abstract

The synthesis of a new functionalised bis-macrocyclic ligand (L1) is described together with the corresponding Ln^III complexes, Ln₂-L1 (Ln = Gd^III, Eu^III). Phosphorescence measurements on Gd₂-L1 at 77 K allowed the ligand-centred triplet state (³π–π*) to be estimated at ca. 28500 cm⁻¹. Steady state and time-resolved measurements confirmed emission from the f-centred excited state (⁵D₀) for Eu₂-L1. ¹H NMRD profiles revealed the longitudinal proton relaxivity (r₁) of Gd₂-L1 to be 8.3 mM⁻¹s⁻¹(30 MHz, 25 °C). The interaction of Cu^II and Hg^II with the lanthanide complexes was probed using luminescence and relaxivity measurements. Addition of Cu^II (10 eq.) resulted in quenching of the Eu^III emission, but no increase in r₁ of the Gd^III dimer. Addition of Hg^II (10 eq.) caused changes to the hypersensitive emission bands of Eu^III together with an increase in r₁ of Gd₂-L1 to be 10.3 mM⁻¹s⁻¹(30 MHz, 25 °C) suggesting a net increase in hydration at the Gd^III centres.