Optical and relaxometric properties of monometallic (EuIII, TbIII, GdIII) and heterobimetallic (ReI/GdIII) systems based on a functionalized bipyridine-containing acyclic ligand

Abstract

A series of lanthanide complexes of [LnL(H₂O)]²⁻ composition where Ln = Eu^III, Tb^III or Gd^III has been studied for determining their photophysical and relaxometric properties in aqueous solution. The bifunctional ligand L (H₅BPMNTA) is an acyclic chelator based on a central functionalized 2,2′-bipyridine core and two iminodiacetate coordinating arms. The mono-aqua Eu^III and Tb^III complexes display attractive spectroscopic properties with an excitation wavelength at 316 nm, similar excited state lifetimes and overall quantum yields (in the ranges 0.5–0.6 ms and 10–13%, respectively) in Tris buffer (pH 7.4). The proton longitudinal relaxivity, r₁, of the Gd^III complex is 4.4 mM⁻¹ s⁻¹ at 20 MHz and 310 K, which is comparable to that of the clinically used Gd-DTPA (Magnevist®). Interestingly, the water exchange rate between the coordination site and the bulk solvent is very fast (K_ex = 2.6 × 10⁸ s⁻¹ at 310 K). The ability of the complex to bind non-covalently to human serum albumin (HSA) was also examined by relaxometric measurements. We also report the synthesis and properties of a bimetallic complex based on Gd-BPMNTA and Re^I(bpy)(CO)₃ components. In this system, the Re core exhibits interesting photophysical properties (λ_em = 588 nm, Φ = 1.4%) and the Gd-BPMNTA core displays improved relaxivity (r₁ = 6.6 mM⁻¹ s⁻¹ at 20 MHz and 310 K), due to an increase of the rotational correlation time. Besides these appealing optical and relaxometric properties, the presence of a reactive function on the structure proposes this potential dual imaging probe for conjugation to biomolecules or nanomaterials.