A ruthenium-based metallostar: synthesis, sensitized luminescence and 1H relaxivity

Abstract

Ru-based metallostars Na₄{Ru[Ln₂bpy-DTTA₂(H₂O)₄]₃} (Ln = Y, Gd, and Eu) have been self-assembled in aqueous solution and their relaxivity and optical properties unravelled. The synthesis and the purification of the new, highly stable heptametallic entities have been optimized for the diamagnetic Y³⁺ complex and followed by ¹H NMR. The europium(III) ruthenium-based metallostar {Ru[Eu₂bpy-DTTA₂(H₂O)₄]₃}⁴⁻ displays sensitized ⁵D₀→⁷F_J luminescence upon excitation of the tris(2,2′-bipyridyl)ruthenium(II) unit in the ultraviolet around 293 nm, as well as in the visible around 450 nm (¹MLCT state). NMRD profiles at two temperatures (25 °C and 37 °C) were performed on {Ru[Gd₂bpy-DTTA₂(H₂O)₄]₃}⁴⁻. NMRD profiles of the ruthenium-based {Ru[Gd₂bpy-DTTA₂(H₂O)₄]₃}⁴⁻ and the iron-based {Fe[Gd₂bpy-DTTA₂(H₂O)₄]₃}⁴⁻ metallostars were fitted with SBM theory coupled to the model-free Lipary-Szabo method for internal motion as well as with the modified Florence approach. Comparison of both fitting methods shows that the Florence approach is able to fit NMRD profiles up to 100 MHz, fails however at higher frequencies because it does not account for internal motion. Overall, the results detailed point to the heptametallic self-assembled edifices being potential relaxivity and luminescence bimodal bioprobes for use in animal models.