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

The synthesis of a new functionalised bis-macrocyclic ligand (L1) is described together with the corresponding Ln(III) complexes, Ln(2)-L1 (Ln = Gd-III, Eu-III). Phosphorescence measurements on Gd-2-L1 at 77 K allowed the ligand-centred triplet state ((3)pi-pi*) to be estimated at ca. 28500 cm(-1). Steady state and time-resolved measurements confirmed emission from the f-centred excited state (D-5(0)) for Eu-2-L1. H-1 NMRD profiles revealed the longitudinal proton relaxivity (r(1)) of Gd-2-L1 to be 8.3 mM(-1)s(-1)(30 MHz, 25 degrees 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 r(1) 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 r(1) of Gd-2-L1 to be 10.3 mM(-1)s(-1)(30 MHz, 25 degrees C) suggesting a net increase in hydration at the Gd-III centres.